625377009B
Controlling Mlution
from the Manufacturing
and Coating
off Metal Roducts
Solvent Metal Cleaning Air Pollution Control
Do not WEED. This document
should be retained in the EPA
Region 5 Library Collection.
zPA Technology Transfer Seminar Publication
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EPA-625/3-77-009
CONTROLLING POLLUTION
FROM THE MANUFACTURING
& COATING OF METAL PRODUCTS
SOLVENT METAL CLEANING
AIR POLLUTION CONTROL-II
U.S. ENVIRONMENTAL PROTECTION AGENCY
Environmental Research Information Center • Technology Transfer
MAY 1977
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ACKNOWLEDGMENTS
This seminar publication contains materials prepared for the U.S. Envi-
ronmental Protection Agency Technology Transfer Program and has been
presented at Technology Transfer design seminars throughout the United
States.
The technical information in this publication was prepared by Chuck
Marshall and Uday Potankar of JACA Corporation.
The mention of trade names or commercial products in this publication is
for illustration purposes, and does not constitute endorsement or recommen-
dation for use by the U.S. Environmental Protection Agency.
US. Environments! F/ctcsticn Agency
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CONTENTS
Page
Chapter I-INTRODUCTION 1
Chapter II-METAL CLEANING METHODS 2
Solvent Cleaning 3
Non-Solvent Cleaning 7
Chapter III-SOLVENT CLEANING EMISSIONS 9
Solvent Cleaning Emission Sources 9
Comparisons of Solvent Cleaning Systems Emissions 14
Emission Rates 17
Solvent Vapor Concentrations 19
Solvent Ventilation Systems 20
Applicable Regulations 21
Chapter IV-PROCESS CHANGES FOR REDUCING SOLVENT
CLEANING EMISSIONS 24
Cleaning Changes 24
Solvent Substitutions 24
Solvent Degreasing Equipment Substitutions 25
Solvent Degreasing Operating Improvements 29
Distillation or Reclamation 30
Combinations of Process Changes 31
Chapter V-TECHNIQUES FOR TREATING SOLVENT
CLEANING EMISSIONS 33
Characteristics of Solvent Emissions 33
Techniques for Controlling Solvent Emissions 34
Chapter VI—COMPARISONS OF PROCESS CHANGES AND
CONTROL TECHNIQUES 58
APPENDIX A—Form For Calculation of Carbon Adsorption Cost 61
BIBLIOGRAPHY 68
111
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FIGURES
Figure Page
1 Open Top Vapor Degreaser 5
2 Open Top Vapor Degreaser with Lip Exhaust 5
3 Cross-Rod Conveyorized Degreaser 6
4 Monorail Conveyorized Degreaser 6
5 Contribution of Solvent Metal Cleaning Hydrocarbon
Emissions to Other Hydrocarbon Emissions, 1972 10
6 Emission Sources in Open Top Degreasers 11
7 Schematic Representation of Degreaser with
Cold Trap Installed 28
8 Adsorption Cycle 38
9 Sizes of Carbon Pores Effective in Adsorption 41
10 Desorption Cycle 43
TABLES
Table Page
1 Comparison of Process Changes for Reducing
Hydrocarbon Emissions 32
2 Working Bed Capacities 36
3 Comparison of Process Changes and a Treatment Technique
for Reducing Hydrocarbon Emissions 59
IV
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CHAPTER I
INTRODUCTION
Hydrocarbon emissions, generated by industry and vehicles, contribute to the
photochemical atmospheric formation of oxidants that is known as smog. Vehicle emis-
sion control is under the jurisdiction of the U.S. Environmental Protection Agency
(EPA); industrial emissions currently fall under state jurisdiction, as authorized in
the EPA's State Implementation Plans. Twenty-two states now have controls, and
their number is increasing. Emissions from new or modified industrial sources fall
under control of the federal government, which will shortly be addressing the problem
of solvent metal cleaning.
This publication outlines for plant operators practical and proven techniques for
controlling hydrocarbon emissions from metal cleaning operations, along with cost
data for the various techniques. It is not our intent to present a best practicable re-
duction control technology. Rather, the purpose is to give information on the various
ways that representatives of the industry have reduced and controlled solvent emis-
sions in their own operations.
Solutions must generally be plant-specific, and therefore the material should be
regarded as background information rather than as an overall prescription.
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CHAPTER II
METAL CLEANING METHODS
Plant personnel who are responsible for metal cleaning, newly subject to emission
(air) or effluent (water) standards, must now become familiar with the techniques and
equipment available for reducing pollution from cleaning operations. As background to
a discussion of these techniques, however, we will briefly review the standard metal
cleaning methods.
Before metal becomes a finished product, it goes through many steps—e.g.,
stamping, cutting, tapping, drawing, quenching, assembly, and finishing. These op-
erations often deposit substances on the metal surfaces that must be removed before
assembly or finishing operations such as enameling or painting.
We will focus here on the removal of greases for those metal finishing processes
that require a surface ranging from relatively clean to very clean. The greases in-
clude lubricants, coolants, waxes, and lapping/buffing compounds.
Metal can be effectively degreased only by wet processes; dry processes, such as
sand or grit blasting, are effective only for smaller batches where surface finish or
dimensions are not critical. There are two major wet metal cleaning methods: water-
based washing and solvent cleaning. Practical considerations usually make the choice
between them obvious. These same considerations often make it impractical and un-
economic to switch from solvent to water-based cleaning to reduce hydrocarbon emis-
sions, even though this is technically feasible. In the future, however, the economics
of metal cleaning by both methods will change; effluent limitations will be imposed on
wastewater from washing systems, and emission limitations may be placed on new sol-
vent cleaning operations. The effect on relative costs, and on choice of systems, is
yet to be seen.
The selection of a metal cleaning system depends on:
• The type of metal;
• The material to be removed; and
• The subsequent metal operation.
Materials to be removed are either organic (lubricating and cooling oils and waxes)
or inorganic (buffing compounds, metallic oxides, and metal chip and dirt adhering to
the organic substances). Lightweight mineral-oil-based lubricants and coolants are
more easily removed than heavier lubricants and coolants with extreme-pressure
agents like sulfur and chlorine. Buffing compounds whose emulsifiers have evaporated
pose a difficult problem with both cleaning methods. Water-based lubricants and
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coolants are more readily cleaned by washing methods than by solvent cleaning. Oil-
based lubricants and coolants are more readily dissolved by solvents, but some light
mineral oils can be cleaned with water-based solutions containing emulsifiers.
The metal itself dictates cleaning methods. Water-based cleaners may be the
wrong choice for electronic parts and for ferrous parts susceptible to rust.
Temperature-sensitive parts may be damaged by the relatively high temperatures of
solvent-vapor degreasing. Similarly, precision parts with different coefficients of
expansion cannot be exposed to boiling solutions.
Several of the final metal operations require very clean metal, which also dictates
the cleaning method. Assembly of precision-cut or formed parts may be hindered by
corrosive water residues from water-based washing. To achieve the cleanest possible
surfaces for electroplating and enameling, parts must either be exposed to a solvent
distillate vapor or a clean or deionized water rinse.
SOLVENT CLEANING
The major advantage of solvent-based cleaners is their ability to dissolve oils and
greases. Two types of solvents are effective: the low-cost flammable petroleum-
based solvents, and the more expensive nonflammable halogenated hydrocarbon solvents.
Solvent cleaning is performed by boiling halogenated solvents (vapor degreasing),
or by cleaning at room temperatures with either petroleum or halogenated solvents
(cold cleaning). The best cleaning action is achieved by vaporizing solvents and ex-
posing the metal to the pure vapors. However, because of the flammability of petro-
leum solvents, this method is confined to the halogenated solvents. Both petroleum
and halogenated solvents can be used in cold cleaning.
COLD CLEANING
Cold cleaning may consist of hand wiping, spraying, or dipping, or may take place
in closed conveyorized systems that use tumble, dip, and spray operations. Fine
sprays of petroleum solvents should be avoided in open areas because of flammability
and inhalation problems. Hence, Stoddard solvents (with high boiling points) are often
used for spraying a petroleum solvent. Air pollution regulations for cold cleaning are
most likely to apply to tanks for dipping and spraying and to cabinets with conveyor
systems. Pre-regulatory studies of air pollution regulations for new sources have not
focused on hand wiping or spraying performed in a booth.
VAPOR DEGREASING
The best cleaning action is achieved by vaporizing solvents and exposing the metal
to the pure vapors.
The two basic types of equipment for vapor degreasing are open top tanks and
closed-conveyorized systems. Of approximately 18,000 vapor degreasing units used
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in the manufacturing sector, 85 percent are open top tank and 15 percent are conveyor-
ized systems. Open top systems are often used intermittently, while conveyor systems
are generally used in mechanical production lines. Where air pollution regulations are
applied, both systems are likely to be affected.
Sketches of two open top tanks and two conveyorized systems are included here as
Figures 1 through 4. The difference between the two open top systems shown is the
lip exhaust in the second one (Figure 2), which keeps vapors away from workers. The
difference between the two conveyorized systems is that the monorail system (Figure 4)
can be conveniently used in a plant equipped with a monorail on its production line.
Three cleaning stages may take place in open top tank or conveyorized vapor de-
greasing systems: vapor boiling, immersion, and warm immersion. The heat sources
for these systems are low-temperature steam, electricity, or gas. Heavy cleaning re-
quirements may justify the use of ultrasonic transducers in the liquid stages or of
sprays in the vapor zone. Much higher-pressure spraying can be used in the conveyor-
ized systems because emissions will not escape as readily as from an open top tank.
Five halogenated solvents are used in vapor degreasing:
Solvent Boiling Point
Trichloroethylene 188° F
1,1,1-Trichloroethane 165° F
Perchloroethylene 250° F
Methylene chloride 104° F
Trichlorotrifluoroethane 118° F
Emission differences may be less of a factor in selecting a solvent, however, than
differences in cleaning properties. High-melting grease deposits on metal may require
the high-temperature cleaning power of perchloroethylene, which has the highest boiling
point—250°F—of the halogenated solvents. (The use of perchloroethylene will require
high-temperature steam instead of the more common low-temperature steam.)
Temperature-sensitive metals may require methylene chloride (104° F boiling point) or
trichlorotrifluoroethane (118°F boiling point). On the other hand, methylene chloride
may be too harsh for plastics and elastomers. Energy considerations favor the low-
boiling solvents. Removing water, among other things, from metal parts would re-
quire the only halogenated solvent with a boiling point above water, i.e., perchloro-
ethylene, or the azeotropes of the other solvents.
Differences in Threshold Limit Value (TLV)—an arbitrary level of a substance
(expressed in ppm) at which workers may presumably be safely exposed—may also
affect a choice of solvents. The TLVs of methylene chloride (500), trichlorotrifluo-
roethane (1000), and 1,1,1-trichloroethane (350) are higher, for instance, than those
of trichloroethylene (100) and perchloroethylene (100).
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Safety Thermostat
Condensing Coils
Temperature
Indicator
Cleanout Door
Solvent Level Sight Glass
Heating Elements
Work Rest And Protective Grate
Figure 1. Open Top Vapor Degreaser
Freeboard
Water Jacket
Condensate Trough
Water Separator
Vent
Lip Exhaust
Figure 2. Open Top Vapor Degreaser with Lip Exhaust
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Figure 3. Cross-Rod Conveyorized Degreaser
Figure 4. Monorail Conveyorized Degreaser
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The advantages of solvent cleaning relative to water washing are:
• Vapor degreasing systems consume significantly less energy than water systems;
• Solvents are highly effective for removing soluble oils and greases and therefore
less expensive;
• Solvent cleaning is usually more effective in cleaning precision parts and is
safer for rust-sensitive metals; and
• Solvent cleaning is effective for small parts.
NON-SOLVENT CLEANING
There are many combinations of water-based solutions for cleaning and many
methods of using them. Water-based solutions usually have a pH greater than 7.0 and
thus are alkaline washing systems. Since water is cheaper than solvents, these are
the first cleaning methods considered in figuring costs.
Water by itself cannot perform all necessary cleaning of metal parts. It must be
assisted by mechanical action (agitation), cleaning compounds, and heat.
The washing action for alkaline systems may include soaking, spraying, tumbling,
electrolytic action, flushing, and handwiping. These cycles can be housed within auto-
mated systems using moving belts, monorails, or spirals, or they can be carried out
in open top tanks.
Cleaning compounds include caustics, carbonate salts, phosphates, silicates, bo-
rates, soaps, and surfactants. Concentrations of cleaning compounds in washers
range from 0.5 to 12 ounces per gallon of water, but agitation reduces the concentra-
tions required. The combination of cleaning compounds and mechanical agitation
enables the following cleaning functions:
• Wetting—Loosening soil through use of an active agent that lessens surface
tension of the metal;
• Emulsifying—Dispersing two immiscible liquids after wetting occurs, one
usually an oil or grease;
• Saponification—Forming soap (which may then be removed by abrasives) by
reaction of an organic oil containing fatty acids with free alkali;
• Deflocculation—Breaking the soil into fine particles that are dispersed in the
cleaning media and prevented from agglomerating; and
• Sequestration—Tying up metal ions, such as calcium and magnesium, so that
they cannot react with and precipitate oils.
Alkaline cleaning compounds can be liquids or solids; low-temperature cleaning to
save energy often requires liquid compounds.
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For alkaline washing of metal to be effective, the soils build-up in the cleaning
solution must be limited, and rinsing, which can be done with clean alkaline solution,
tap water, or deionized water, must be thorough. If the rinsing is not thorough, a
soil residue will remain on the metal surface.
Drying is often necessary after alkaline washing to ready parts for the subsequent
metal operation. Drying is not needed in vapor degreasing units where the metal part
becomes hot enough to evaporate the solvent when it is withdrawn from the vapor zone.
The advantages of alkaline washing, relative to solvent degreasing, are:
• Alkaline solutions have a lower cost per gallon than solvents, especially the
halogenated solvents. However, some of this cost advantage is offset by the
lower tolerances of soils build-up in the cleaning solution, necessitating more
frequent changes.
• Alkaline washing systems are compatible with subsequent operations that use
wet processes like plating and phosphating. The energy cost of drying the parts
is eliminated in these cases, although overall energy costs for alkaline washing
are high.
• Alkaline systems are particularly effective in cleaning buffing compounds and
solid dry lubricants whose soluble oils have volatilized. Chemical reactions,
emulsifiers, agitation, and heat provide the cleaning action.
• Alkaline systems can tolerate more agitation and higher-pressure sprays for
better cleaning. However, the additional agitation and higher-pressure spray-
ing in solvent vapor degreasing disturbs the vapor zone and increases hydro-
carbon emissions.
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CHAPTER III
SOLVENT CLEANING EMISSIONS
The solvents used in solvent cleaning are one of many industrial sources of hydro-
carbon emissions. Figure 5 shows the contribution of solvent cleaning emissions,
broken down by vapor and cold cleaning, to annual hydrocarbon emissions in the United
States. The figure also gives data for surface coating operations, related in many
plants to solvent cleaning operations.
SOLVENT CLEANING EMISSION SOURCES
Solvent hydrocarbon emissions are caused by evaporation of the petroleum or hal-
ogenated solvents. Evaporation rates are a function of vapor pressure, which is di-
rectly proportional to temperature. At the temperatures prevailing in most plants,
solvent vapor pressures are sufficient for evaporation. When evaporated solvent and
a given volume of air mix, they each exert a partial pressure; at 20°C, the total at-
mospheric pressure is 760mm of mercury. Solvent evaporation will occur until, at a
given temperature, a maximum partial pressure is reached, a condition of equilibrium
or saturation.
For example, trichloroethylene has a partial pressure of 60mm mercury at 20°C.
In a given volume of air, this solvent will evaporate until its partial pressure is reached.
d (\
The volume it occupies is in the ratio of its partial pressure to air, — ; or 7.9 per-
cent of the total volume. This percentage of trichloroethylene in air would be 79,000
ppm at equilibrium. Because the TLV of trichloroethylene for worker exposure is
100ppm, control or dilution is needed to offset the natural tendencies of this solvent to
evaporate.
OPEN TOP TANK VAPOR DECREASING
An earlier figure (Figure 1) depicting an open top vapor degreaser is here repro-
duced as Figure 6, and marked to show how hydrocarbon emission occurs.
Solvent is heated at the bottom of the tank by gas, electric, or steam coils. The
heating coils may cover the entire degreaser bottom or be confined to a separate boil-
ing chamber within the degreaser. The hot vapors generated are heavier than air and
displace the air inside the degreaser tank. As boiling continues, the vapor volume in-
creases. The bulk of the vapor is prevented from escaping the tank, by condenser coils
attached to one or more walls of the tank near the top. In small tanks, the coils might
be at one end; in larger tanks, the coils may surround the entire tank. Open tank de-
greasers of all sizes usually have water jackets with flowing water around the entire
tank (above the coils) for further condensing action.
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TOTAL HYDROCARBON
EMISSIONS
STATIONARY HYDROCARBON
EMISSIONS
48.0%
14.5x 106 Tons
Stationary
6.6%
2.0 x 106 Tons
Surface Coating
2.4%
1.0 x 106 Tons
Solvent Cleaning
Other
81.2
52.0%
15.7 x 106 Tons
Mobile
Surface
Coating
13.8%
Vapor
Degreasing
1.9%
l I
Cold
Cleaning
3.0%
Data Sources: TRC - The Research Corporation of New England and EPA Office of Air Quality Planning and Standards
Figure 5. Contribution of Solvent Metal Cleaning Hydrocarbon Emissions to Other Hydrocarbon Emissions, 1972
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#1
Idling
Convection
#3
Dragout
#5
Sump
Evaporation
#4
Spraying
#6
Lift Out/
Piston Effect
#2
Drafts
Freeboard
Water Jacket
Condensate Trough
Water Separator
Temperature
Indicator
Cleanout Door
Solvent Level Sight Glass
Heating Elements
\
Work Rest And Protective Grate
Figure 6. Emission Sources in Open Top Degreasers
Condensation of vapor on the condensing coils prevents further rise into the vapor
zone when the degreaser heat-input and condenser heat-outflow are balanced. Even
then, some solvent vapors will escape through diffusion of the uncondensed fraction.
Because the amount of a solvent vapor escaping depends on its partial pressure at
the condenser temperature, one would expect different emission rates for the various
solvents. However, the industry uses a general rule of thumb for the evaporation rate
of all halogenated solvents—.05 Ibs/ft2/hr of open top area.
Above the condensing coils are additional degreaser walls, called freeboard, that
protect the top of the vapor zone from being disturbed by drafts (Figure 6 - #2). Sol-
vents are emitted when drafts cause air and solvent to mix at the top of the vapor zone.
Denser vapors, such as trichlorotrifluoroethane (6.5 density, with air equal to 1.00)
and perchloroethylene (5.73), are less disturbed by drafts than, for instance, methylene
chloride, which has a density of only 2.93.
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in open top degreasers, the object to be cleaned is lowered into the vapor zone.
The hot vapor condenses on the cold metal, dissolves the soluble greases, and flows
off. It is replaced by clean solvent. The solvent, greases, and soils return to the
boiling liquid on the bottom of the tank. The high boiling points of the greases and
soils prevent them from being vaporized and contaminating the solvent vapor zone,
ensuring high-quality cleaning.
The metal is held in the vapor zone until the metal temperature reaches that of the
vapor and condensation has stopped. A solvent film will, however, remain on the metal
surface. When condensation on the metal has stopped, the part is slowly removed. As
the object is removed, the heat of the metal evaporates the remaining solvent film.
Where evaporation is completed in the upper zone of the tank, most of this solvent is
recovered by the condensing coils and water jacket. Solvent evaporated outside the
tank is termed "drag-out" (Figure 6 - #3).
Some tanks have hoses with nozzles for spraying the part while it is suspended in
the vapor zone, which enhances the cleaning action. Separate chambers that collect
uncontaminated solvent from the condenser water separators are frequently used to
feed the sprayer. Some degreasers also have liquid solvent chambers where the part
can be immersed for additional cleaning or immersed in the boiling solvent for cleaning
by the boiling action.
Improper spraying can increase hydrocarbon emissions in two ways: Vapors cre-
ated when spraying is performed above the vapor level will be subject to drafts and
dispersion from the cleaning unit; spraying near the top of the vapor zone can cause
relatively uncontaminated air from above the zone to mix with and force out contami-
nated zone air (Figure 6 - #4). Very fine sprays or aspirated sprays cause greater
disturbance than low-pressure flush spraying.
When the vapor degreaser is not in operation, the solvent on the bottom will evap-
orate (Figure 6 - #5). Emissions will occur unless the top of the tank is covered during
these shutdown times. If the surface area of the metals being lowered into the zone is
greater than 50 percent of the open tank area, emissions will occur as the load is low-
ered and again when it is lifted out. Such emissions are referred to as piston-effect
emissions, with the tank acting as the cylinder and the bulky load the piston.
The three most significant emission sources in open tank top vapor degreasing are
those from:
• Drag-out;
• Solvent Disposal; and
• Improper Spraying.
CONVEYORIZED VAPOR DEGREASING
Conveyorized vapor degreasers are usually larger and more expensive than open
tank degreasers and therefore fewer are normally used in a single plant. The cleaning
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principles of conveyorized systems are the same as those for open top tank degreasers.
The main difference between the two systems is in materials handling. Metal objects
are transported through the liquid or vapor zone of closed systems by monorails,
chains, mesh belts, or moving spiraling trays. Monorails are expedient where the
parts move by monorail in other parts of the plant. Parts can be carried in baskets
or in rotating perforated cylinders.
All conveyorized systems use either hot liquid immersion or solvent spraying,
combined with movement of the part through a vapor zone. Drag-out air pollution
occurs to a lesser degree than in open top systems because the conveyor speed can be
regulated so that evaporation takes place in the enclosed system. Close-fitting enclo-
sures at the degreaser entrance and exit decrease vapor loss from plant air movement.
Piston-effect solvent emissions do not occur in conveyorized systems because the car-
rier size and spacing can be designed to avoid it. Some idling convection and sump
evaporations occur, but not as much as in open top tanks.
Because the emission rate per unit of metal cleaned is lower than in open tanks,
the use of closed conveyor systems can be considered as a control device.
COLD CLEANING
There have been relatively few cold cleaning conveyorized systems installed. The
main difference in emissions between conveyorized vapor and conveyorized cold clean-
ing methods is the rate of drag-out, which occurs because the metal is not heated dur-
ing cleaning and the solvent is not vaporized inside the closed system. An attempt can
be made to dry the metals inside the conveyor by forced air or with ultraviolet light
(printed circuit board cleaners), but forced air will increase cold cleaning emissions.
In addition to drag-out effects, cold cleaning tank emissions are the result of spray
evaporation, surface evaporation, and draft-induced surface evaporation. Traditionally,
it has been assumed that open-top cold cleaning produces more emissions than open top
vapor degreasing. However, a set of recent tests at Presolite Corp. showed 47 percent
fewer emissions when the tank was operated cold.
SOLVENT WASTE
Hydrocarbon emissions from disposal of spent solvents make up a significant
portion of the nation's total solvent hydrocarbon emissions. Some solvent disposal
methods are considered as part of air emissions because ultimately the solvent evapo-
rates. These methods include flushing and landfilling, whether done by a manufacturer
or a disposal service. Disposal of solvents through a reclaimer or by proper inciner-
ation are not methods that contribute to hydrocarbon air pollution.
Approximately 35 percent of all solvents used by plants with more than 19 employ-
ees require disposal. Half of the disposed amounts evaporate. Therefore, about one-
sixth of yearly hydrocarbon emissions from these plants result from disposal practices.
For plants with 19 employees or less, utilizing primarily cold cleaning systems, the
residues are a higher percentage of total solvent use, and a lower percentage of solvent
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is reclaimed. Therefore, the figure of one-sixth (disposal emissions as a fraction of
solvent use) used above for plants of 20 or more employees increases significantly
when smaller plants are included.
Calculations for compliance purposes frequently include disposed solvents as part
of the plant's hydrocarbon emissions load. In jurisdictions where air pollution control
regulations apply to solvent degreasers, emission limits are expressed in pounds per
hour or pounds per 24-hour day. One method of determining the uncontrolled emission
rate is through materials-balance calculations in which the amount of solvent fed to the
degreaser over a period of time is recorded and not reduced by the amounts disposed
of, unless these go to a reclaimer or are burned.
The economics of whether to sell spent solvent to a reclaimer or to develop on-site
recovery will depend on the amount and type of solvent consumed. For the more expen-
sive halogenated solvents, it will frequently be economical to reprocess the solvent on-
site, using a small distillation unit. However, for the cheaper hydrocarbons, a high
volume is necessary to justify on-site distillation. Further, with petroleum-derived
solvents, measures must be taken against flammability.
In most industrialized areas, used solvents can be sold to specialty reclaiming
firms. The income may be minor, but disposal problems are avoided. Only in remote
areas, or in the case of small solvent volumes or badly contaminated or mixed solvents,
should distillation or resale be ruled out.
Large degreasers are often equipped with their own attached stills for separating
the solvent from the greases and soils. Separate stills can be centrally located in a
plant to reclaim solvents from several degreasers.
Open top degreasers without separate stills collect some solvent condensate in
troughs under the condensing coils, and contamination concentration in the disposed
solvent can be increased in this way to 20-40 percent. Stills can recover enough sol-
vent vapor from used solvents to increase the soil and grease concentration to 60-85
percent before final disposal of the solvent. Specially designed stills may leave only
5 percent solvent in the residue.
Properly designed stills can be operated without emitting their own air pollution.
COMPARISONS OF SOLVENT CLEANING SYSTEMS EMISSIONS
There is no survey that thoroughly compares hydrocarbon emission rates for vari-
ous solvent cleaning systems, probably because of the difficulty of finding representative
systems that clean the same types of parts. The information available, however, sug-
gests the following ranking of solvent metal cleaning options by emission rates, from
lowest to highest, considering equivalent wasteloads:
1. Conveyorized vapor degreasing
2. Conveyorized cold cleaning
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3. Open top vapor degreasing
4. Open top cold cleaning with petroleum solvents
5. Open top cold cleaning with halogenated solvents.
Exceptions to this ranking are easily found. Further, we are not suggesting that
the systems are interchangeable and that a plant could reduce emissions or achieve
compliance by switching to a higher-ranked, lower-emission system. Other factors
(discussed in Chapter II) would first have to be evaluated, including characteristic dif-
ferences between petroleum and halogenated solvents, large and small volumes, and
open top and conveyorized systems. The effectiveness of vapor exposure vs. dipping
in partially contaminated liquids would also have to be considered. The overall find-
ings might well preclude switching.
The reasons that conveyorized systems using halogenated solvents (vapor or cold)
cause fewer emissions than open top systems are:
• Elimination of losses from plant air movements;
• Capturing of more drag-out evaporation by the enclosure;
• Greater efficiency of automation and high-production operations in solvent use;
and
• Reduction of waste solvent at the source, with distilling systems often built into
the larger conveyorized systems.
The primary reason that conveyorized vapor systems cause fewer emissions than
conveyorized cold systems is less drag-out. The unheated metal in a cold system
means that more solvent evaporation occurs outside the enclosure than in vapor sys-
tems. As noted earlier, if forced air drying is used inside the enclosure to reduce
cold-system drag-out, the disturbance could increase exhaust emissions and would
tend to offset any decrease in drag-out.
One report describes two cases where similar parts were being vapor-degreased
by an open top tank at one plant and by a conveyorized cross-rod system at another.
Very careful production and solvent consumption records were kept at each plant. The
solvent consumption per ton of metal cleaned in the open top tank was 99 pounds using
trichloroethylene, and 80 pounds using 1,1,1-trichloroethane. The solvent consump-
tion rate in the conveyorized system using trichloroethylene was 35 pounds with carbon
adsorption and was estimated at 44 pounds without the adsorption system. ^
Another case involved replacement of an open top tank vapor degreaser with a par-
tially enclosed conveyorized vapor degreaser equipped with a still. Emissions per
weight of metal cleaned were 40 percent less with the partially enclosed system.
Doubling the production load for metal cleaning facilitated the decision to switch to the
more expensive conveyorized system.^
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There are fewer emissions with open-top vapor degreasing than open top cold
cleaning, with either halogenated or petroleum solvents. The primary reasons are:
• The use of condensation systems (water jackets, cooling coils using water, or
refrigerated coils) to prevent solvent emissions; and
• The reduction of drag-out emissions because the heated metal causes condensate
evaporation in the vapor zone.
There are other equipment and operating factors that cause higher emissions from
cold cleaning systems. These include:
• Frequent use of mechanical agitation (spraying, air agitation, and mixers) to
improve cleaning, which increases the evaporation of solvents;
• Lower soil concentration tolerances in cold cleaning (5-10 percent for cold
cleaning vs. 15-25 percent for vapor degreasing), which increase the need for
distillation or disposal; and
• The difficulties and cost of distilling petroleum solvents, which make emission
rates dependent on how a plant disposes of its waste solvent.
On the other hand, petroleum solvents themselves cause fewer emissions than
halogenated hydrocarbons. The difference in evaporation rates is shown below:
Relative Evaporation Rates
(1,1,1-trichloroethane = 1.00)
Stoddard .09
Perchloroethylene . 25
Heptane . 40
Trichloroethylene . 63
1,1,1-trichloroethane 1.00
Acetone 1.40
Methylene Chloride 3.50
Trichlorotrifluoroethane 3.85
However, benefits from the lower emission rates of petroleum solvents are de-
pendent upon disposal practices. Because of the flammability problem previously de-
scribed, petroleum distillation is very expensive, and any disposal practice leading to
evaporation will significantly increase the emission calculations.
16
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EMISSION RATES
There are essentially two ways to quantify emissions:
• Concentration (weight or volume) of solvent in the air emitted from the de-
greaser; and
• Solvent consumed by the degreaser over a time period (materials-balance data).
The first method is useful for the design of a vapor recovery system such as car-
bon adsorption. However, concentration measurements are not valid for determining
compliance with air pollution control regulations, because even though concentrations
can readily be converted to pounds per hour (to conform to typical regulatory specifi-
cations), this does not account for solvent disposal, sump evaporation and leakage
losses. Therefore, solvent consumed by the degreaser is a more accurate measure
for compliance-determination purposes.
The common solvent consumption measurement consists simply of recording all
solvent added to the solvent cleaning system to maintain a constant level and dividing
this by system-operating hours. This yields an average emission in pounds per hour
and includes all solvent losses. Because of errors in determining the liquid levels
when adding solvent, it is preferable to develop materials-balance data over several
weeks or months. For a more precise measurement, the degree of contamination of
the solvent in the degreaser at the last addition must also be known. Because the
grease generally has a higher boiling point than the solvent, an estimate of the grease
content can be obtained by evaporating a known quantity of the dirty solvent in a weighted
dish to determine the weight of solvent loss.
A sample materials-balance calculation is given below:
Given: A vapor degreaser of 4ft x 5ft surface dimensions with a liquid depth of
2ft. The solvent used is trichloroethylene. The degreaser is used 7 hours
per day on a 5-day per week basis and is covered at other times.
Procedure:
1. Clean the degreaser and fill to the 2-ft level with trichloroethylene.
2. Maintain records on additions of makeup solvent over a selected period
of time, e.g., 4 weeks. For calculation purposes, assume that addi-
tions of 5, 6, 5, 8, 7, 6, 5, and 4 gallons were added to the degreaser
during the 20 operating days.
3. At the end of the 4-week period, carefully make up the solvent level to
a 2-ft depth. Assume that an additional 2.5 gallons was required for
this purpose.
4. Collect a suitably sized sample of the degreaser solution and place in
a dish in a drying oven at a temperature slightly below the boiling point
of trichloroethylene. Assume that a 100ml sample is used. The
17
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sample and dish weigh 212.4 gm. After drying at 175°F to remove the
solvent, the cooled dish and contents weigh 88.7gm.
Calculations:
1. Total solvent added from steps 2 and 3 above = 5+6 + 5 + 8 + 7+6 +
5+4+2.5 gallons = 48.5 gallons.
2. Solvent displaced by grease =
Volume solvent evaporated, ml\ * n i . . -,
1 ———r-^- x Volume of Solvent in degreaser
100ml /
• Weight loss in dish
= 212.4gm - 88.7gm
= 123. 7 gm
• Volume less
= 123.7 gm
~ 1.456gm/ml
= 85.0ml
• Volume of solvent in degreaser
= 4ft x 5ft x 2ft x 7.48gal/ft3
= 299.2 gal
• Thus, solvent displaced
I 85.0ml\
~\l 100ml j^99'"1
= 44.9 gallons
_ „ . Total Solvent Loss (in Ibs)
3. Emission rate = — — ——
Operating Hours
= (48.5gals +44.9gals) x (1.456x 8.341b/gal)
4 weeks x 5 days x 7 hrs/day
= 8.1 Ibs/operating hour
= 56.71bs/day
The result represents the total diffusion, convection, and drag-out losses during
the 4-week period. The only error would result from solvent displacement in the tank
by the accumulation of solid soil (metal chips, dirt, etc.), and possibly from water in
the degreaser. These materials would not mix with the solvent and would not be present
in the selected sample used for drying. If these factors are significant, it would be
necessary to drain the tank and obtain an estimate of the volume of water and soil at the
end of the test period. This volume of additional contaminants would then be added to
the other solvent losses in step 3 of the calculation above.
18
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A materials-balance calculation stated in weight per unit of time is useful in meet-
ing regulatory requirements, but not for the design of vapor recovery systems such as
carbon adsorption. The concentration of inlet gases to a carbon adsorption system is
a critical design factor. Materials-balance figures, although mathematically convert-
ible to concentrations, would not yield an accurate inlet concentration. The converted
figures would be too high because they include drag-out losses, waste-solvent disposal,
and shutdown evaporation. None of these emission sources would be ventilated to a
carbon bed.
Some emission data are available for solvent cleaning operations, derived from
materials-balance calculations and expressed in concentrations of ppm or in pounds per
hour. Rule-of-thumb emission rates, derived from materials-balance calculations for
open top tank vapor degreasers, are from .25 - .501bs/hr/ft2 of open top tank area.
(The rule of thumb for convection losses during idling is .05 lbs/hr/ft2, or 10 - 20 per-
d
cent of the total emission rates. ) Since applicable regulations usually allow 8 pounds
of emissions per hour, the estimated tank size for regulated emissions cannot exceed
16ft2 or 32ft2. These calculations and emission rates include assumptions that:
• The tank is covered at all times during shutdown;
• Condensing coils and a water jacket are used; and
• The vapor degreaser has minimal freeboard.
Again, it must be remembered that materials-balance emission rates represent an
average per hour. They do not reflect either peak concentration during heavy workloads
or concentrations that leave through the degreaser opening.
No rule-of-thumb emission rates have been established for conveyorized solvent
cleaning systems.
SOLVENT VAPOR CONCENTRATIONS
Concentrations from the solvent cleaning openings have been found to vary from
50ppm to 1000ppm, the low end of the range occurring during idling and the high end
during cleaning .
At present, there is no federal government reference on sampling hydrocarbon
solvent concentrations and no preferred method. There are, however, methods for
measuring sample concentrations of halogenated solvents. These sampling methods
can measure the ventilated air from a solvent cleaner or the breakthrough point of a
carbon adsorption system, i.e., the jump in emissions after the bed is saturated.
Available sampling equipment can measure general organic compounds or specific
compounds. Some systems provide on-site results; others require remote laboratory
analysis. Gas chromotography is one on-site sampling system for specific organic
compounds. The cost and maintenance of gas chromotography facilities is high for
small plants, but it is possible for air samples to be taken and analyzed by this method
at an outside laboratory.
19
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For air pollution purposes, the need for sampling specific compounds is minimal;
the more useful measure is total organic concentrations. Direct-reading, on-site total
organic sampling systems are available, based on coulometry, ionization, thermal
conductivity, combustion, ultraviolet and IR spectroscopy, chemiluminescence, elec-
tric conductivity, or flame photometry.
Most suppliers of solvents and carbon adsorption systems have technicians who can
perform all emission measurements for customers, and where multiple adsorption beds
are used, suppliers can also design the carbon bed switching systems. Switching is
typically accomplished by timers. When the beds are first installed, the supplier
measures the time to breakthrough, applies a safety factor, and establishes an auto-
matic timing cycle.
SOLVENT VENTILATION SYSTEMS
Ventilation of solvent cleaners is often necessary to carry vapors away from
workers and plant equipment. It is also important where the effectiveness of various
controls depends on solvent vapor concentrations.
However, excessive ventilation increases the solvent loss rate. The objective is
to attain only that ventilation rate commensurate with worker safety and plant equip-
ment protection.
Where worker safety is not involved, the desired ventilation rate from a vapor de-
greaser is a maximum of 50cfm/ft2 of opening. As ventilation rates increase from
this level, so does solvent vapor loss. In some cases, worker safety requirements
may call for a minimum ventilation rate of 100 cfm/ft2 of opening. At this rate, there
is significant impairment of the various emission reduction techniques (to be described
later) that can be used inside the degreaser.
Conventional open top vapor degreasers have a lip or slot ventilation arrangement
running laterally along the top of the degreaser below the freeboard. However, this is
not always sufficient to keep the room air solvent concentrations at safe levels, and it
may be necessary to enclose the entire cleaning and drying area.
For conveyorized systems, a centrally located air intake can be used, at the rec-
ommended ventilation rate of 75 cfm/ft2 of inlet and outlet openings. When drag-out
losses are high from conveyorized systems, a grille top downdraft hood can be used.
The recommended exhaust rate is 100 cfm/ft2 of grille top.
In some cases, filters should be used with the ventilation system to remove dust
generated by other operations in the plant. The filters will prevent adsorption-type
recovery systems from clogging.
20
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APPLICABLE REGULATIONS
Air pollution regulations specifically affecting solvent cleaning operations are
under dual regulatory control: state authority sets the rules and standards for existing
solvent cleaning operations and the EPA is responsible for establishing rules and crite-
ria for emissions from new sources of pollution and modified solvent cleaning systems.
EXISTING SOURCES
As of June 30, 1976, there were air pollution control regulations for hydrocarbons
in 22 states, although some regulations cover only a local area. Only three states have
specific regulations for solvent operations, and in each case these are local: Arizona
(for Maricopa County); California (for Los Angeles, San Diego, Sacramento Valley,
San Joaquin Valley, and San Francisco); and Texas (for Houston-Galveston). In all
other regulations, solvent cleaning operations are affected as a part of the hydrocarbon
or surface coating category.
There are two types of applicable regulations, specifying:
• Allowable pounds per hour; or
• Allowable pounds per hour as a function of a designated photochemical reactivity
of the solvent.
The pounds-per-hour limits, as expressed in Appendix B of the August 14, 1976
Federal Register guidelines and criteria for the adoption of State Implementation
Plans, were no more than 3 pounds per hour or 15 pounds per day, unless 85-percent
controlled. Common solvents used in vapor degreasers, except trichloroethylene,
were exempted from this regulation. Therefore, plants affected by these regulations
switched from trichloroethylene, often to 1,1,1-trichloroethane.
The second type of regulation is exemplified by the benchmark "Rule 66" of Los
Angeles County Air Pollution Control District (later to become Southern California Air
Pollution Control District Rule 442) which states: "For the purpose of this rule, a
photochemically reactive solvent is any solvent with an aggregate of more than 20 per-
cent of its total volume composed of the chemical compounds classified below, or,
which exceeds any of the individual percentage composition limitations referred to the
total volume of solvent:
(1) A combination of hydrocarbons, alcohols, aldehydes, esters, ethers or
ketones, having an olefinic or cycloolefinic type of unsaturation: 5 percent;
(2) A combination of aromatic compounds with eight or more carbon atoms to the
molecule except ethyl-benzene: 8 percent;
(3) A combination of ethyl-benzene, ketones having branched hydrocarbon struc-
tures, trichloroethylene, or toluene: 20 percent."
21
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Rule 66 (now Rule 442) bases its emission regulations on this designated photo-
chemical reactivity. The Rule states:
I!
(a) A person shall not discharge organic materials into the atmosphere from
equipment in which organic solvents or materials containing organic solvents
are used, unless such emissions have been reduced by at least 85% or to the
following:
(1) Organic materials that come into contact with flame or are baked, heat
cured or heat polymerized, are limited to 1.4 kilograms (3.1 pounds) per
hour not to exceed 6.5 kilograms (14.3 pounds) per day.
(2) Organic material emitted into the atmosphere from the use of photochem-
ically reactive solvents are limited to 3.6 kilograms (7.9 pounds) per
hour, not to exceed 18 kilograms (39.6 pounds) per day, except as pro-
vided in subsection (a) (1). All organic materials emitted for a drying
period of 12 hours following their application shall be included in this
limit.
(3) Organic materials emitted into the atmosphere from the use of non-
photochemically reactive solvents are limited to 180 kilograms (396
pounds) per hour not to exceed 1350 kilograms (2970 pounds) per day,
except as provided in subsection (a) (1). All organic materials emitted
for a drying period of 12 hours following their application shall be in-
cluded in this limit.
(b) (omitted here because it is not germane to solvent cleaning.)
(c) Emissions of organic materials into the atmosphere required to be controlled
by subsection (a) shall be reduced by:
(1) Incineration, provided that 90 percent or more of the carbon in the or-
ganic material being incinerated is oxidized to non-organic materials or
(2) Incineration, provided that the concentration of organic material following
incineration is less than SOppm, calculated as carbon and with no dilution,
or
(3) Adsorption, or
(4) Processing in a manner determined by the Air Pollution Control Officer
to be not less effective than (1) or (3) above."
The rule also specifies an allowable disposal of no more than 1.5 gallons per day
of reactive solvent.
Any regulated solvent cleaning emission source that cannot be exempted by substi-
tuting compliance solvents constitutes a serious control problem. Data in Chapters IV
and V indicate that the effectiveness of emission reduction and control systems for sol-
vent cleaning ranges up to 60 percent. Even though a device like carbon adsorption is
over 90 percent effective, it cannot control the substantial drag-out solvent waste and
sump evaporation losses that escape the ventilation systems.
22
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Therefore, the maximum controllable emission rate any source can have is approx-
imately 20 pounds per hour (8 + (1 - .60)).
Within the next few years, it is anticipated that more existing hydrocarbon sources,
including solvent metal cleaning, will be required to reduce solvent consumption and/or
emissions. Many states have not achieved the prima^ air quality standard for photo-
chemical oxidants, the air pollutants related to hydrocarbons. EPA has required these
states to change their implementation plans to achieve more control of hydrocarbon
sources. States have discretion in choosing which sources they want to control, i.e.,
mobile vs. stationary, refineries vs. surface coating, etc.
EPA will be issuing guidelines to the states on reasonably available control tech-
nology for solvent metal cleaning. These guidelines can be used by the states that must
change their implementation plans to include existing solvent metal cleaning sources.
NEW SOURCES
The Environmental Protection Agency is now considering regulating all new solvent
cleaning systems; these regulations are known as New Source Performance Standards
(NSPS). A new source can be either a new system or a modified existing degreaser
costing more than 50 percent of a new source. Increases in emissions from an existing
source due to fuel conversion or increases in production rates or operating hours are
not, in most cases, governed by NSPS.
NSPS regulations call for new sources to install the best available control technol-
ogy. The standards are usually industry-specific and are generally expressed in per-
formance terms such as weight per standard cubic foot of air flow per minute. This
gives maximum engineering options to the plant engineer. Where standards are design-
specific, such as to height of freeboard vs. width of tank or temperature of refrigerated
chillers, the control options are limited. The NSPS, though still under consideration
by EPA and not yet finalized, may use design-specific standards because of the diffi-
culty of measuring concentrations and the extensive record keeping involved in material
balances, and because controlled emission rates vary greatly according to the amount
and type of wasteloads.
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CHAPTER IV
PROCESS CHANGES FOR REDUCING
SOLVENT CLEANING EMISSIONS
Techniques for reducing solvent emissions from solvent cleaning operations (re-
ferred to as process changes) include:
• Cleaning changes;
• Solvent substitutions;
• Solvent degreasing equipment substitutions;
• Solvent degreasing equipment modifications;
• Solvent degreasing operating improvements;
• Distillation or reclamation; and
• Combinations of process changes.
CLEANING CHANGES
Water-based and solvent-based metal cleaning methods are theoretically inter-
changeable . However, the soil or grease to be removed, the type of metal, and the
ensuing operations often dictate the choice of cleaning.
For many applications, for instance, water-based methods are uneconomical be-
cause of the rinsing and drying operations they necessitate. However, when air pollu-
tion control costs are added to the equation, a reassessment may be necessary, based
on a comparison of the added cost of water pollution control with the cost of controlling
air pollution from solvent cleaning.
SOLVENT SUBSTITUTIONS
In some instances, compliance with present state hydrocarbon pollution control
regulations can still be met through solvent substitutions. For vapor degreasing, this
virtually always means replacing trichloroethylene with 1,1,1-trichloroethane, per-
chloroethylene, methylene chloride, or trichlorotrifluoroethane.
Trichloroethylene is considered by the Los Angeles Air Pollution Control District
and the Environmental Protection Agency to be a reactive pollutant. Regulations en-
acted to date exempt all other halogenated solvents in common use as nonphotochemi-
cally reactive. More-recent information from the Environmental Protection Agency
indicates further refinements in the designations.
24
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Trichloroethylene and the petroleum solvents have moderate photochemical reac-
tivity in the short run, usually considered one day. Perchloroethylene is relatively
unreactive the first day, but significantly reactive beyond that. Methylene chloride,
1,1,1-trichloroethane, and trichlorotrifluoroethane are considered unreactive.
Although solvent substitution is a widely accepted method of meeting regulations
for existing sources, it is too early to predict what EPA may allow for New Source
Performance Standards. EPA has several alternative control measures for NSPS for
solvent cleaning operations, including both solvent substitution and equipment changes.
While solvent substitution has traditionally been considered a good control strategy,
there is some evidence that suggests otherwise. A recent EPA report states that sol-
vent cleaning equipment and operating changes may be required with solvent substitu-
tions. ^ For example, when methylene chloride or trichlorotrifluoroethane are substi-
tuted, the freeboard ratio must be increased to .75. Further, these solvents require
less heat to vaporize, but more coolant is required to condense the vapors.
SOLVENT DECREASING EQUIPMENT SUBSTITUTIONS
Chapter HI compared the relative emission rates from various solvent cleaning
operations and noted that conveyorized systems emitted fewer solvent vapors than open
top tank systems for the same workload. Further, vapor degreasing in conveyorized
systems caused fewer emissions than cold cleaning, which was also generally true for
open top tank systems.
It may be worthwhile to substitute a conveyorized system for two or more open top
tank degreasers. Because there are fundamental differences in cleaning quality be-
tween vapor degreasing and cold cleaning, cleaning specifications and solution soils
concentration tolerances are critical considerations in any change from vapor to cold
cleaning, in either an open top tank or in a conveyorized system.
AUTOMATIC OR MANUALLY OPERATED COVERS
This discussion applies to open top tank vapor or cold cleaning equipment only.
Actual metal cleaning takes place during only about 25 percent of a working shift.
It is typical, even though covers come with the equipment, for open top systems to be
left open during the extensive idling and shutdown time.
It is economical to cover tanks when they are not in use. Covers may be canvas,
plastic, fiberglass, or metal and can be supplied by solvent cleaning equipment manu-
facturers. For large tanks, pedal-activated covers can be used. Automatic covers
can also be equipped with timers that close them after a degreasing cycle. If non-
horizontal covers are considered, precaution should be taken that the vapor zone is not
disturbed by air movements. All covers should function below the lip exhaust.
25
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Covers are highly effective, particularly during idling time, when emission rates
are anywhere from .05 to . 101bs/ft2/hr; with covers, nearly all emissions can be pre-
vented. Covers are even more necessary where freeboard height is minimal and where
drafts exist, making idling emission as high as .37 Ibs/ft2/hr.
Tests that measured the effectiveness of covers on halogenated solvent consumption
rates showed savings ranging from 24 to 50 percent. * The same equipment and solvents
were used in the comparisons; the cover was the only variable.
The cost of covers is dependent on square footage. For retrofit situations and for
automatic covers, the costs range from $100/ft2 for 15ft2 to $25/ft2 for 200ft2.
For a 4' x 12' open top system, the solvent emission savings with halogenated sol-
vents should pay for the covers in 6 months for a three-shift operation, or in about 15
months for a single-shift operation. For cold cleaning in an open tank, the payback
will take slightly longer.
For a 2 1/2' x 6' open top system, the payback period would range from 13 months
to 2 1/2 years. Thus, covers appear to be an attractive investment even without the
consideration of air pollution.
Where a plant already has covers for its tanks that employees fail to use properly,
an automatic feature may help. Clearly marked instructions will also help the plant
achieve the full emissions control benefits of covers.
If a cover cannot be used, for example over the weekends, it is recommended that
the solvent be pumped out, stored, and re-entered at the next working shift.
INCREASED FREEBOARD
Greater freeboard height will reduce solvent emissions from open top tanks per-
forming either vapor degreasing or cold cleaning. The current Occupational Safety and
Health Act requires the freeboard height to be .50 times the width of the tank, but it
need not be higher than 36". This ratio applies to the use of all solvents. It has been
the practice of the industry, however, to have a ratio of .75 with methylene chloride
and trichlorotrifluoroethane.
Increases above this height can further reduce solvent emissions. In addition to
blocking drafts, the added height also permits the lip exhaust ventilation rates to be
less disruptive to the vapor zone. In one set of tests, increasing the freeboard ratio
from .50 to .75 of the tank width reduced solvent consumption by 27 percent. With
slight drafts occurring, the effectiveness in the same tests was 55 percent. Further
ratio increases to 1.00 did not improve emission reduction efficiencies. Efficiencies
for methylene chloride and trichlorotrifluoroethane would only improve if freeboard
were increased above .75, the ratio in common use; however, no tests were run on
this.
26
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The current ASTM recommendation is a freeboard ratio for all solvents of .75, to
a maximum 48"; the EPA's recommendation for new sources is expected to be about
the same.
Increases in freeboard can easily be made by plant personnel or degreaser equip-
ment manufacturers. However, increased heights can pose an operational problem
because they may reduce materials handling space. For adequate handling, a pit may
have to be dug to lower the degreaser. Increased freeboard may also require that the
standing area around the degreaser be raised to enable operating personnel to handle
the material or to spray. Spray hoses may require lengthening for spraying in the
vapor zone.
The payback period for increasing freeboard from .50 to .75, or to a maximum of
48", should be less than one year, excluding any pit or worker-platform costs. Savings
with cold cleaning open top equipment, where the freeboard has traditionally been
smaller, should also be considerable.
REFRIGERATED CHILLERS
One method of controlling solvent emissions from vapor degreasers is to create a
cold air blanket over the vapor zone that inhibits the vapor from rising. The blanket
is created by a second set of condensing coils above the traditional condensing coils
and water jacket. The added condensers, referred to as refrigerated chillers, are
generally made of copper and are finned to obtain more contact area. Figure 7 shows
a chiller and the effect it creates. A refrigerated chiller patent operates from 32°F
down to -20°F. Other refrigerated chillers are operated outside the patent coverage,
at temperatures of 34°F - 40° F.
When methylene chloride or trichlorotrifluoroethane is used, the condensing
coils are operated at a 40°F-50°F temperature range and can be cooled by chilled
water, possibly from a well. Degreaser equipment manufactures and other suppliers
sell refrigerated chillers. Chillers operating below 32 °F (The Cold Trap Model) have
a removal capacity of 100 Btu/ft/hr per coil. Because two parallel coils are typical,
the total removal capacity is 200 Btu/ft/hr. The above-freezing-temperature refrig-
erated chillers have total removal capacities of up to 500 Btu/ft/hr. New York State
requires that the blanket temperature be 70 percent lower than the boiling point of the
degreasing solvent and that the refrigerant temperature not be warmer than 0°F.
The energy requirement of chillers is only about 5 percent more than the vapor de-
greaser energy requirement.
One problem encountered with refrigerated chillers is frost on the coils. During
the defrost cycle, water can enter the vapor zone; when this happens, it depletes sol-
vent stabilizers and may corrode equipment. Degreasers using refrigerated chillers
should be equipped with water separators, not shown in Figure 7. Water that collects
on refrigerated chillers will extract the alcohol and acetone co-solvents from
trichlorotrifluoroethane.
27
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COLD TRAP
(chiller)
SLOT EXHAUST
L
REFRIGERATION
COILS
COOLING WATER
COILS
PARTS SCREEN
BOILING SOLVENT
IN STEAM OUT'
Figure 7. Schematic Representation of Degreaser with Cold Trap Installed"*
One set of chiller tests showed an emission reduction range of 16-60 percent with
an average of 40 percent.1 Cost recovery, as expected, was better for multiple-shift
operations and for the higher-priced halogenated solvents. Large and/or square vapor
degreasers showed greater savings than small or narrow units.
Eight other chiller tests reported emission reduction ranges of 38-77 percent,
with an average of 57 percent. These tests were not, however, verified by experimen-
tal control.
The actual payback periods for chillers vary considerably and are generally longer
than those for covers and increased freeboard. Although the emission-reduction effec-
tiveness of all three equipment modifications is similar, refrigerated chillers are the
most expensive. The payback periods when trichloroethylene was used ranged from a
loss for a one-shift operation using a low horsepower compressor to a payback of 2
years for a three-shift operation. Possible corrosion costs were not part of these pay-
back calculations. The payback periods were shorter when the more expensive tri-
chlorotrifluoroethane was used. No tests were made with cold cleaning operations, but
significant control effectiveness can be expected with volatile solvents.
Refrigeration in primary condensing coils is a variation of the chiller concept.
No test data is available on this process, and the costs are very high. As previously
noted, condensing coils are standard equipment with methylene chloride and trichloro-
trifluoroethane solvents.
28
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The lip exhaust of the degreaser should have a shut-off device to prevent disturbing
the cold air blanket when the chiller is operating.
DRAINAGE BOARDS
Drag-out from cold cleaning operations may contribute to dripping outside the tank.
Solvent emissions can be prevented by catching the drippings with boards that drain
back into the degreaser. No tests are reported for the emission reduction and cost
effectiveness of this equipment modification.
SOLVENT DECREASING OPERATING IMPROVEMENTS
The following suggestions have been presented by people in the metal cleaning field
as practical, cost-effective means of reducing hydrocarbon emissions:
• Ventilation rates on all systems should be kept as low as possible, commensu-
rate with health considerations (See OSHA).
• Parts should not be moved either vertically or horizontally through the vapor
zone at speeds greater than llfpm (3.3 meters per minute).
• All solvent spraying should be performed in the vapor zone with low nozzle
pressures (less than lOpsig).
• If a solvent spray cold cleaning method is used, the stream should be solid,
that is, a flush, at a pressure not to exceed 10 psig.
• The opening(s) to closed conveyorized systems should not have more than 6
inches of space around the metals.
• Metal parts should be arranged in baskets and on hooks to avoid solvent puddles
and assure that solvent does not drain from one part onto another.
• Where wipe-cleaning is the only way of cleaning, rags should not be allowed to
drip and should be wrung or centrifuged after use.
• Cold cleaning containers and solvent spray reservoir temperatures should be
at least 30°F-36°F less than the boiling point of the solvent, and in all cases
no greater than 120°F, to avoid excessive evaporation.
• Cold cleaning solvent agitation should not be done with air when low-boiling-
point solvents are used; mixers or recirculating pumps should be used instead.
• Routine inspections should be made for leaks.
• In cold cleaning systems where the solvent is immiscible with water and con-
siderably denser, a 4" (10cm) water seal can be vised.
• Porous or adsorbent materials, such as cloth or rope, must not be cleaned or
be part of the hoist or basket systems.
• The EPA suggests that the metal parts (the workload) should not occupy an area
greater than 50 percent of top tank areas, to avoid piston effects, and ASTM
suggests 75 percent.
29
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• All vapor degreasers should be equipped with a safety-vapor thermostat that
detects vapor rises and shuts off the sump heat source.
• Controls should also be used in vapor degreasers to prevent the primary coolant
from overheating.
• Fans elsewhere in the plant should not be directed at a degreaser opening.
• Heavy loads with large surface areas should not stay too long in the freeboard
area, since strong thermal updrafts from the hot work will draw vapors from
below and emit them.
• All operators should be thoroughly familiar with all ways of reducing emissions,
and reminder signs or labels should be posted where appropriate.
• Proper solvent cleaning start-up, shutdown, and maintenance procedures, as
specified by ASTM,5 should be followed.
Although no quantitative data is available on the effectiveness Df these measures,
they are highly recommended by those who have implemented them.
DISTILLATION OR RECLAMATION
For cold cleaning operations, perhaps the single most important process change to
control emissions is solvent recovery, either by distillation at the plant or reclamation
by a contractor. Distillation is also an effective emission control for vapor degreasing
operations, though control is less important because waste volumes are lower in vapor
systems.
Distillation is commonly used in plants that have closed conveyorized systems and
many open top tanks, but not in those with one or two open top tank vapor degreasers
or in petroleum-based cold cleaning systems. The options are single continuous stills
connected to conveyorized units, continuous-feed centralized stills serving many de-
greasers, and separate batch stills.
Manufacturers sell long-lasting, stainless-steel stills with a capacity of 60 gallons
per hour at a cost of $2500-$3500.-'- These units take up little floor space. Cost com-
putations based on trichloroethylene show that a break-even figure can be obtained by
a plant that distills 350 gallons per year. For petroleum-based solvents, the break-
even point is from 2100 to 4200 gallons per year. Two major obstacles push the cost
of petroleum solvent distillation much higher: the low value of recovered solvent rel-
ative to halogenated solvents and the flammability of petroleum solvents, which re-
quires many equipment safety features.
Stills can reduce the solvent volume content from the 70 to 90 percent found in
vapor degreasing waste to 20 percent. Some disposed petroleum solvents can be re-
used in oil furnaces as fuel; simple filtration will prepare the solvents for burning.
Disposed solvents containing lubricants with chlorine or sulfur additives are not useful
in oil furnaces because of air pollutants in the additives.
30
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Companies that decide not to burn fuel or buy stills can contract with reclaimers
that perform distillation and return the solvent. The cost is approximately half the
market value.
Another disposal option is to sell the solvent to a reclaimer, who usually offers
low prices for disposal. In this case, the reclaimed solvent is not sold back to the
company.
COMBINATIONS OF PROCESS CHANGES
Combining some of the process changes described above, increased freeboard,
refrigerated chillers, and distillation equipment, can increase their effectiveness in
emission control. Results of these combinations are shown in Table 1.
It should be noted that distillation equipment is compatible with all the process
changes listed in this chapter. When process changes are made, there will be the
same or slightly greater amounts of contaminated solvents to dispose of; the greater
amounts, where they occur, will result from the loss of stabilizers. When emission
losses occur normally, the makeup solvent contains fresh stabilizers. When emission
losses are curtailed, less makeup is required, and the stabilizers are consumed more
rapidly. This requires either more frequent disposal or the addition of stabilizing
solvents.
The combined effectiveness of covers and increased freeboard is estimated at 5-15
percent greater than the effectiveness of covers alone. The reason this figure is less
than the sum of their individual effects is that freeboard and covers overlap in prevent-
ing solvent emission from drafts. The combined effectiveness of chillers and covers
is 16-43 percent greater than the effectiveness of the chiller alone, the higher percen-
tage being for the more volatile methylene chloride. The reason chiller and cover ef-
fects are not completely cumulative is that they share the effect of preventing convection
idling losses; however, their dual effect is increased because chillers do not prevent
sump shutdown evaporation losses.
There are no test data available on emission reduction with combinations of in-
creased freeboard and chillers.
31
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Table 1
Comparison of Process Changes for Reducing Hydrocarbon Emissions
Emission
Reduction
Technique
Covers
Increased
Freeboard
Refrigerated
Chillers
Distillation
Solvent(a)
Reduction
Effectiveness
24-50%
35%
27-55%
30%
16-6 0%(b)
40%
35-77%
57%
80% of
disposed
solvent
Profitability
Generally profitable
Generally profitable
with the exception of
methylene chloride and
trichlorotrifluoroethane
where freeboards are
already increased
Generally profitable
with the exception
of small tank only
one-shift operation
Profitable above 350
gallons per year of
halogenated solvent
Effectiveness When Combined With
Other Process Changes
With Freeboard: 5-15% greater than cover
With Chillers: 16-43% greater than chillers
With Distillation: Additive
With Covers: 5-15% greater than cover
With Chillers: No data
With Distillation: Additive
With Covers: 16-43% greater than chiller(c)
With Freeboard: No data
With Distillation: Additive
Can be combined with all other process
changes.
Combined effectiveness is additive.
CO
to
(a) upper sets of numbers is range, lower set is expected efficiency
(b) two sets of test results
(c) 16% for 1,1,1-trichloroethane and 43% for methylene chloride
-------
CHAPTER V
TECHNIQUES FOR TREATING
SOLVENT CLEANING EMISSIONS
Current techniques for treating solvent emissions, used in conjunction with process
changes that reduce them (Chapter IV), can bring solvent metal operations into line with
air pollution standards.
Certain features of solvent emissions discussed below are relevant to treatment
techniques.
CHARACTERISTICS OF SOLVENT EMISSIONS
The characteristics of solvent cleaning emissions that most affect pollution control
treatment are:
• Percent of ventilation airflow that can be captured;
• Solvent emission concentration in the airflow;
• Rate of airflow;
• Temperature of airflow;
• Number of compounds present in the emissions; and
• Range of variation in the above characteristics.
The effectiveness of any treatment device in reducing solvent consumption depends
on the percentage of escaping emissions that can be entrapped and channeled into the
device. For instance, considerable amounts of emissions come from drag-out; most
of these evaporate beyond the capture point of the ventilation system.
It is estimated that from 25 to 55 percent of all solvent emissions cannot be cap-
tured for treatment. For example, carbon adsorption systems, to be described later,
reduce solvent consumption by an average of 40 percent; the highest tested achieved a
65-percent reduction. Yet carbon adsorbers are 90-95 percent effective in controlling
solvent emissions drawn into them. A 90-percent effective, properly designed carbon
adsorber that reduces solvent consumption for a total cleaning system by only 40 per-
cent is receiving less than half the solvent cleaner emissions. Where the solvent con-
sumption reduction reaches 65 percent, the carbon adsorber is receiving no more than
three-quarters of the emissions.
33
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Solvent concentrations in the airflow from solvent cleaners will vary considerably
due to intermittent workloads. For example, open top tank degreasers may operate
only 25 percent of a shift, during which time the exhaust fan is always on. The ex-
pected concentration variation is from 50 ppm to 1000 ppm, with the average between
100-500 ppm.
Air flow rates range from 50 to 125 cfm/ft2 of opening area.
Temperatures of solvent degreasing emissions range from room temperature to
125°F.
Where single-solvent cleaners are used, the compound present in the airflow will
be the same as the solvent. With petroleum solvents and other solvent blends, the dif-
ferences in hydrocarbon compounds will affect the efficiency of some treatment devices,
notably carbon adsorption.
TECHNIQUES FOR CONTROLLING SOLVENT EMISSIONS
The choice of techniques to control solvent cleaning emissions is relatively limited,
and includes condensation, absorption, incineration, and carbon adsorption.
CONDENSATION
Where low efficiency is acceptable, condensation of vapors is possible. The entire
airflow must be chilled to a point where the partial pressure of the solvent at that tem-
perature will result in a vapor concentration that is substantially less than the vapor
concentration in the air stream. An amount of solvent equivalent to the difference in
vapor concentrations can then condense on a cold surface. In actuality, something less
than the theoretical amount will be removed because of equilibrium rate factors, the
heat evolved, and droplet losses. Where airflow rates are low, vapor concentrations
high, and the solvent expensive, condensation may be economical. In solvent cleaning,
however, flows are normally too high and vapor concentrations too low for economical
chilling operations.
ABSORPTION
Absorption is a common industrial technique for separating vapors from a carrier
gas stream. For it to be effective, a scrubbing liquid (absorbent) must be available in
which the solvent is either soluble or will react to form a less volatile compound. The
absorbent in itself must not release undesirable vapors under operating conditions, and
the solvent must be separable from the absorbent unless the mixture can be economi-
cally and safely wasted. For the halogenated hydrocarbons typically used in solvent
cleaning, there are no known absorbent materials that meet the above criteria. At-
tempts have been made to use mineral oil to collect trichloroethylene vapors, but
emissions from the mineral oil itself have been excessive.
34
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INCINERATION
Heating the air stream to temperatures of 1200°F - 1500°F will destroy solvent
vapors. Carbon and hydrogen will be converted to CO2 and water. However, the hal-
ogens will burn to compounds such as phosgene, hydrochloric acid, and hydrofluoric
acid, and unless these are removed in a second-stage collector, they may become a
more serious source of air pollution than the original solvent vapors. Additional dis-
advantages of incineration are the loss of recovered-solvent benefits and the high fuel
requirement for burning air streams with low concentrations of combustible vapors.
Incineration, therefore, is generally too expensive for the low solvent concentrations
from degreasers and even more so for secondary control of incinerator halogen emis-
sions. It is not often a serious contender for degreaser emission control.
CARBON ADSORPTION
Adsorption, almost universally using activated carbon beds, is the most effective
control system for halogenated and petroleum solvent cleaning emissions. Because of
its importance, we will discuss it here in detail.
The carbon adsorption technique uses regenerative activated carbon to remove
gaseous molecules from an air stream in three distinct phases: adsorption, desorption,
and disposal or recovery of the adsorbed material. (Note that the term adsorption can
refer to the entire process or the first phase.) "Regenerative" means that the carbon
can be used repeatedly after the captured gases are removed. "Activated" refers to
the treatment that gives the carbon its adsorptive properties: the carbon is heated in
the absence of oxygen, removing tars present in the pores, and subjected to high-
pressure steam to create a network of micro-capillaries that will collect organic
molecules.
The principle of carbon adsorption is based on the attraction between gaseous
molecules and the molecules forming a solid surface. The attractive or capturing
force, known as van der Waals force, occurs at the point where the gas and the solid
meet. It takes place when a fluctuating dipole moment in a non-polar molecule induces
the same in another non-polar molecule, whereupon they interact. The force holds the
molecule to the solid surface. As more gaseous molecules come in contact with the
surface, a monomolecular and then a multimolecular layer builds up. The molecules
can then be removed by the application of heat.
The term "force" is used to differentiate a physical attraction from a chemical
reaction, which can also occur at the interface of a solid and a gas. When a chemical
reaction occurs, the process is called chemisorption and can be detected by a chemical
change in the gaseous molecule. In other words, if the gaseous molecule is removed
from the solid surface by attraction and its identity has been retained, then the van der
Waals force prevailed. A chemical change in the removed molecule indicates chemi-
sorption, which usually requires more energy for separation.
Among the best adsorbers are activated carbon and silica gel, the former being
used much more extensively, since silica gel prefers to adsorb water (polar) over
35
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organics (non-polar). The unique characteristics of both these solids is the large num-
ber of micro-capillaries that provide large surface collection areas. The range of
solid particles used in adsorption is from .5 inches down to 200 microns.8
Carbon particles are carefully selected to yield a porous but strong granule. Once
activated, the carbon is packed loosely as granules or glued together into chambers
called beds. Surface areas of activated carbon are typically 1100 m2/gram expressed
in weight or 2 x 108ft2/ft3(2) expressed as a volume ratio.
The absolute retention capacity of a given adsorbent for a particular solvent will
depend on the solvent, the material from which the carbon is made, the manufacturing
process, and the conditions of use. For activated carbon, manufacturers specify max-
imum capacities based on specified solvents and test conditions. Published values for
useful carbons range from about 0.4 (40%) to approximately 1.0 (100%) grams of sol-
vent adsorbed per gram of carbon.
Because it is impractical to remove all the captured solvent before the solvent bed
is put back on stream after regeneration, the actual working capacity of the bed after
use is considerably lower than the maximum stated capacity. Table 2 shows typical
working bed adsorption capacity for some common solvents :
Table 2
Working Bed Capacities^9
Solvent
% of Carbon Bed Weight
-------
A discussion of some more specific aspects of carbon adsorption (listed below)
will give readers the practical information they need.
• Airflow conditioning for carbon adsorption
• The adsorption phase
• The desorption phases and disposal/recovery
• Adsorption effectiveness
• Adsorption equipment
• Adsorption costs
Airflow Conditioning for Carbon Adsorption
Extraneous materials such as dust, tars, and mineral oil will clog the capillary
pores of carbon beds and reduce adsorption capacity. Other materials, particularly
certain metallic compounds, can cause undesirable chemical reactions in the bed.
Excessive moisture, if condensation occurs, can physically block the carbon pores.
When these substances are present in harmful amounts, precleaning or conditioning of
the gas stream may be necessary.
Filters are commonly used with carbon adsorbers to eliminate dust or paint spray
contaminants. Moisture is not an insurmountable problem because activated carbon
adsorbs organic matter in preference to water. However, although activated carbon is
less sensitive than silica gel to high gas-moisture content, its adsorption capacity may
be lower with a dry gas stream. For excessively humid air, the temperature of the
stream can be increased to lower the relative humidity, a demister can be used, or
the moisture can be condensed by cooling.
The Adsorption Phase
Certain variable factors affect the efficiency of carbon adsorption. Several of the
more important ones are gas velocity and direction, preferential adsorption, and
temperature.
Gas Velocity and Direction
In most adsorption units, the gas enters the activated carbon bed from top to bot-
tom (Figure 8). Downflow allows the use of higher gas velocities; in upflow, the ve-
locity must be limited to a value that will not lift the carbon, because this would result
in reentrainment of the adsorbent.6 Some units, however, have been deliberately con-
structed as fluidized bed systems, with upflow velocities sufficient to suspend but not
remove the carbon from the bed. This prevents the clogging of the bed by contaminants
or the breakdown of granules to small sizes during regeneration. However, the accel-
erated carbon attrition caused by the impact between granules results in high carbon-
replacement costs.
37
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Solvent-Laden
Air Inlet
Activated Carbon
Bed
Exhaust
Figure 8. Adsorption Cycle
Downward velocities through the bed are limited by the crushing effects of the gas
on the activated carbon and the length of the Mass Transfer Zone (MTZ). The crushing
velocity limit for each type or size of activated carbon is specified by the manufacturers.
The MTZ is the zone of partial saturation between the fully saturated zone adjacent
to the entrance to the bed and the nonsaturated zone at the exit of the bed. For best op-
eration, it is important to have a short mass transfer zone in the adsorber (usually
about 2"). Initially, collection occurs at the upstream face and the MTZ moves toward
the outlet side. The operator usually stops the adsorption in a bed when the MTZ
reaches the outlet of the bed (commonly when there is one percent of inlet concentra-
tion). The capacity of the bed is therefore greater with a short MTZ. Because the
MTZ of a bed lengthens with increased velocity, it is desirable to keep low velocities.
38
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Preferential Adsorption
When the properly conditioned, hydrocarbon-laden gases enter the carbon bed in a
downward direction at the preferred velocity, adsorption occurs between the hydrocar-
bon molecules and the activated carbon. However, activated carbon will adsorb organic
molecules in a preferential manner. Non-polar organic molecules are preferentially
adsorbed by activated carbon over polar organic molecules. Although both molecules
will be adsorbed by the activated carbon (which is non-polar), a non-polar molecule
can replace a polar molecule. Therefore, if both types of molecules are present in the
inflow, it is likely that the emissions will consist of polar molecules at breakthrough.
Higher boiling point (or molecular weight) molecules are also preferentially ad-
sorbed over lower boiling point molecules, meaning that the more volatile, low boiling
point hydrocarbon molecules will be present at breakthrough; if the adsorption capacity
along their path is saturated and there are no lower weight molecules to replace, they
will exit the bed. Moisture is also adsorbed by activated carbon, but most of it is pref-
erentially replaced by organic molecules. Exceptions occur in which moisture con-
denses and physically clogs the capillaries.
Temperature
Adsorption, an exothermic process, releases heat to the activated carbon bed.
For typical degreasing concentrations, the temperature of the bed may increase by
only 10° F -20°F. This temperature increase will slightly reduce the adsorption ca-
pacity of the bed, but does not disrupt the collection process. With high concentration
vapor streams, some form of cooling will be required. This may involve cooling the
entire air stream before it reaches the bed or embedding cooling coils in the charcoal.
The latter approach is more common, but complicates carbon replacement.
Selection of an Adsorbent
In addition to temperature and gas velocity, discussed above, there are a number
of other controllable factors that influence the adsorption process. These factors en-
able activated carbon to be tailored to adsorb many different chemical compounds at
different efficiencies. Adsorption-equipment manufacturers work closely with
activated-carbon suppliers, who have considerable information on the effects of these
design variables. Manufacturers of adsorbers can provide quotations based on infor-
mation given them about cfm, temperatures, solvents to be adsorbed, operating hours,
and concentrations.
Important features in the design of an adsorbent are:
• Carbon Surface Area. As noted, adsorbent capacity is directly proportional to
the carbon surface area. The maximum surface area for a given carbon bed is
a function of the starting material, the production method, and the efficiency of
activation. Production techniques are generally considered proprietary by the
manufacturer.
39
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• Pore Size. For a given use, this may also be a significant factor. At low sol-
vent concentrations, the smallest pore size through which the adsorbate can
enter is the most efficient, while at higher concentrations larger pores are
more efficient. This principle is illustrated in Figure 9. At very high pres-
sures, capillary condensation takes place within the pore, and total micropore
volume determines effectiveness rather than pore size.
• Physical Toughness. Resistance to crushing and attrition is also important in
an adsorbent. This property is most dependent on the starting material, but
can be significantly influenced by the production method. Coatings and form of
the carbon (glued slab vs. granules) also influence strength.
There are a number of other variables—some of them plant operating factors—that
should be considered in selecting or designing an adsorbent for a specific use, including:
• Adsorbent particle sizes;
• Adsorbent bed depth;
• Adsorbate concentration;
• Pressure effects;
• Bed geometry;
• Decomposition or polymerization of the adsorbate; and
• Intermittent operations.
For a definitive discussion of these factors, the reader can consult any of the spe-
cialty texts on adsorption. However, the principal points are summarized below.
Adsorbent Particle Size
"The dimension and shape of the carbon particle size affects both the pressure
drop through the adsorbent bed and the diffusion rate into the particles. The pressure
drop is lowest when the adsorbent particles are spherical and uniform in size. "^
Pressure drop is directly proportional to velocity and inversely proportional to particle
size. Adsorbent beds consisting of smaller particles require greater pressure drops
but have better collection efficiency. Small particle beds are characterized by a
sharper and smaller MTZ.
Adsorbent Bed Depth
The first consideration in bed size is that the depth of the bed should be greater
than the length of the MTZ, otherwise breakthrough will occur too quickly after initial
adsorption. An adsorbent bed should be sized to the maximum depth allowed by pres-
sure drop considerations. Bed depths of 1 1/2 - 3 feet are common.
40
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o. 2.0
1.5
1.0
.5
Benzene 20°C (68°F)
10
15 20
Concentration
25
30
35
Figure 9. Sizes of Carbon Pores Effective in Adsorption
Adsorbate Concentration
The solvent retention capacity of carbon is directly proportional to the concentra-
tion of the solvent. Thus, everything else being equal, a deeper bed will be required
to collect a low concentration of contaminant than to collect the same contaminant at
higher concentrations.
Pressure Effects
The saturation adsorption capacity of a bed increases in proportion to increasing
pressure within the bed. Where a long bed is used, the pressure drop across the bed
will cause a significant difference in adsorption capacity between the upstream and
downstream face.
Adsorbent Bed Geometry
Bed volume is the key design factor in adsorption removal efficiency. For a given
volume, the depth and diameter must be balanced to achieve acceptable face velocities
(which affect pressure drop, crushing, and the length of the MTZ) and a reasonable
depth (which retards the tendency to short circuit and affects the average saturation
concentration obtainable). Reducing the bed volume theoretically reduces only the cycle
time, but in practice it also tends to reduce the efficiency of collection.
Decomposition and Polymerization of the Adsorbate
Organic molecules may decompose or polymerize because of temperatures of the
gas stream, stripping temperature, or the presence of catalysts. Decomposition or
41
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polymerization of molecules can change the overall adsorption rate and reduce the
capacity of the system. Severe polymerization may require either periodic high-
temperature reactivation of the bed or frequent replacement of the carbon. However,
this is not a problem with typical solvent cleaning vapors.
Intermittent Operations
Short, intermittent operation cycles generally do not affect the overall capacity of
the adsorption system if the bed depth equals several MTZ lengths. Long periods of
intermittent operation, particularly in small systems, will lower capacity because of
the longer MTZs produced by solvent migration during inactive periods. The continued
circulation of the carrier gas without contaminants also causes the adsorbate to diffuse
through the bed by desorption into the carrier gas and readsorption. Thus, when no
contaminant is being emitted, it is preferable to shut off the blower.
The Desorption and Recovery Phases
Once an activated carbon bed is saturated, it can either be discarded or regener-
ated by removing the hydrocarbons (desorption) and then returning the bed to service.
These choices are determined more by economics than by the nature of the hydrocar-
bons. In most solvent degreasers using activated carbon, it is cheaper to regenerate
the beds.
Regeneration requires heat for raising the partial pressure of the solvents and
causing them to revolatilize. The larger the percentage of captured solvent that can be
removed from the bed, the greater its capacity for further service. (The solvent not
removed is called the heel.) Although higher temperatures and longer application of
heat increase solvent removal, there is an economic limit.
Desorbing (stripping) and recovering or disposing the captured hydrocarbons from
the bed may involve a number of factors:
• Steam
• Hot air
• Hot inert gas
• Settling
• Distillation
• Incineration
• Readsorption
• Condensation.
The commonest form of desorption uses a counter-current flow of low temperature
steam (5-10 psig) through the bed after the bed has been closed off to pollutant flow
(Figure 10). The incoming steam condenses and heats the bed and then acts as a flush
42
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Condenser
Water
Separator
Steam Source
u
Figure 10. Desorption Cycle
gas to strip solvent from the bed. The steam and solvent can be routed to a water-
cooled condenser for liquification. If the steam-to-solvent ratio does not greatly ex-
ceed a minimum of 3-5 pounds of steam per pound of solvent, simple decantation is
sufficient to separate immiscible solvents from the water. However, distillation may
be required for mixtures with larger water-to-solvent ratios or miscible solvents.
The stripped carbon bed is then cooled, usually by dry air, and returned to use.
If temperature increases during adsorption are a problem, enough water can be left
in the bed to lower them. The moisture remaining will not affect adsorption, since
the moisture is preferentially replaced by organic molecules.
The cost of steam for desorption is an important factor in the total cost of an ad-
sorption system, making accurate assessment of steam requirements an essential part
of the design process. As noted earlier, the working adsorption capacity of a carbon
bed is the difference between the theoretical adsorption capacity and the saturation
concentration of solvent remaining after desorption. For a given solvent and carbon,
the saturation value is a function of the solvent concentration in the carrier gas and
43
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the operating temperature. At a working bed temperature in the normal range of
105°F-120°F, the saturation capacity decreases rapidly with decreasing concentration
in the air. Desorption is also more difficult at low solvent-to-carbon ratios. Thus,
the working capacity of the bed decreases and the steam requirements for regeneration
increase with lower incoming vapor concentrations. For inlet vapor concentrations
above 1000 ppm, from 3 to 5 pounds of steam will be required per pound of solvent re-
covered. This will increase to about 30 pounds of steam per pound of solvent at an
inlet vapor concentration of 10 ppm.^
When steam desorption requirements rise to the level of 5 -10 pounds of steam
per pound of solvent removed, the solvent in the mixture produced by condensation is
dilute. Decantation for separation is difficult, and for slightly soluble solvents sewer
losses may be excessive. Under these conditions, hot-gas desorption may be more
economical. Air, flow gas, or an inert gas may be as effective as steam, pound-for-
pound, in stripping solvent vapors from carbon, although the initial heating of the bed
is slower because there is no condensation heat gain. This may be offset by using a
regeneration gas temperature of 300°F or higher, if decomposition, polymerization,
or fire does not constitute a problem. Hot gas is considerably cheaper than steam,
but the cost of secondary recovery is generally higher except in large steam-to-solvent
ratios. Recovery of the solvent from the hot gas may be accomplished by:
• Passing the gas through a small carbon bed, which may be conventionally re-
generated with steam on a low pounds-steam to pounds-solvent basis;
• Cooling the gas stream to cause partial condensation of solvent and passing the
remainder of the contaminated gas to the on-line adsorber; or
• For petroleum hydrocarbons, burning the solvents in a fume incinerator.
Steam desorption of petroleum-sol vent blends with boiling points above 300° F is
difficult. Sometimes superheated steam (343°C) is required, with a significant cost
increase over low temperature steam.
Steam desorption creates small amounts of organic water pollution.
ADSORPTION EFFECTIVENESS
Recent tests for carbon adsorption based on five adsorbers show overall solvent
usage reductions of 20- 65 percent. Carbon adsorption systems with average inlet
concentrations of 500 ppm can achieve 95-percent control, or an average outlet con-
centration of 25 ppm. The carbon bed itself is more than 90-percent effective in re-
moving pollutants from gas streams that enter it; however, the drag-out source of
emissions prevents many vapors from reaching the bed. Thus, minimizing drag-out
emissions and capturing most of the vapors is critical to the overall effectiveness of
carbon adsorption systems.
The lower efficiencies reported were, in part, attributable to intermittent produc-
tion rates. During slack intervals, the low inlet concentrations and moving air cause
44
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hydrocarbons to migrate deeper into the bed and cause more rapid breakthroughs. An-
other cause of low efficiencies was high fan speeds, leading to excessive emissions
from drafts and rapid breakthrough.
The five adsorbers tested all used steam stripping. The tests showed that recov-
ered solvents often require re stabilization by additions of stabilizing solutions. The
acetone and alcohol cosolvents used with trichlorotrifluoroethane are lost during bed-
regeneration by steam because these stabilizers react with the water.
Although petroleum solvent blends were not a part of these tests, recovered
amounts would be of different composition than the original blend in the degreaser.
Therefore, it may not be possible to put recovered petroleum solvent blends directly
back into the cold cleaning degreaser.
The use of carbon adsorbers with automatic covers is somewhat self-defeating,
because when the cover is closed the previously captured solvent will migrate in the
bed with additional air movement. Controls can be designed to shut off the adsorbing
fan when the cover is closed.
The construction materials for the adsorption bed housing should be mild steel
with a phenolic resin coating where steam regeneration is used, because high temper-
atures and the affinity of carbon for water can cause formation of hydrochloric acid
from disassociation of the chlorinated solvents. In other operations, materials with
less corrosion resistance can be used where there is low moisture content. The car-
bon adsorbers are commonly connected by ductwork to the lip exhaust system. It is
important for plants to control fan speeds to minimize ventilation rates. This is true
for either a package-unit fan or a plant exhaust fan.
ADSORPTION EQUIPMENT
For batch or noncontinuous degreasing operations with low emission concentrations,
a single adsorbing bed can be used for the entire system. The bed is sized to operate
during a degreasing cycle, or perhaps a single shift. On completion of the cycle, there
must be sufficient downtime to allow regeneration of the bed before further degreasing.
Beyond a certain bed size or cycle time, economics dictates multiple beds to pro-
vide continuous operation. Most small degreasing operations use two beds; larger op-
erations may have three. One bed can be collecting while a second bed is regenerating
and the third bed is drying. The first bed can collect to saturation, and the clean efflu-
ent gas can be passed through the drying bed for quicker drying. When the first bed is
saturated, the dried bed is ready for collecting. The first bed is then regenerated and
the bed that was regenerating enters the drying phase. In this way, the cycle goes on.
Often beds are not connected in series, and the effluent gas from the adsorbing bed
goes to the atmosphere. When the bed reaches breakpoint or the allowable emission
rate, the adsorption process is switched by dampers to a second bed. This arrange-
ment is more common and is standard design practice for package adsorption units up
to 10,000 cfm capacity. The manufacturer runs tests to determine the typical amount
45
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of time for the bed to reach breakpoint. A safety time factor is applied and bed switch-
ing occurs at preset time intervals. Timers are used rather than monitoring devices,
because reliability of the latter is questionable over long periods.
Carbon adsorption systems require considerable maintenance to keep them func-
tioning properly. Special attention should be given to:
• Reliable valve and damper functioning;
• Changes in carbon bed switching time as a function of changing production rates;
and
• System leaks.
ADSORPTION COSTS
This section will give the results of cost analyses performed for existing adsorp-
tion systems and will present a form that plant personnel can use to estimate their own
adsorption costs.
As noted in Chapter IV, the use of covers and increased freeboard will generally
return a profit, as will the use of refrigerated chillers. The overall profitability of
carbon adsorption systems is dependent upon the emission concentration and the market
value of the recovered solvent. Cost calculations for trichloroethylene show that car-
bon adsorbers can be operated on a profitable basis when bed inlet emission concentra-
tions are above 200 ppm. However, there are no data on the distribution of emission
concentrations from various sources. Therefore, general statements—such as those
made for certain process changes—cannot be made about the use of carbon adsorption
systems by the industry.
Because of the complexities of calculating the costs of carbon adsorption systems,
a form has been developed to help plant personnel estimate the capital and operating
costs.
The following example illustrates the use of the form. A blank cost calculation
form is included as Appendix A to allow plant personnel to estimate their adsorption
costs.
46
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General Design Data
Gas Flow, Q = 3000 scfm
Solvent: Trichloroethylene
Solvent Concentration, S = 500 ppm
Solvent Molecular Weight, MW = 135.5
Solvent Value, V = $2.15/gallon
Operating Hours, H = 2080 per year
Adsorber Type: 2 tank, automatic
Adsorber Unit Price, U = $4.67/scfm
(a) Package Adsorber
The adsorber equipment cost is found by multiplying the adsorber unit price (U)
and the gas flow (Q). The installed cost of the adsorber would be the sum of the equip-
ment and installation costs. It is convenient to use a factor called the installation cost
factor (ICF) to obtain installed cost, C1:
C1 = equipment cost x ICF
The value of the ICF will vary with the complexity of the job, space available, and
modifications needed to the process before the adsorber can be added. Here a value of
2.0, a value typical of many installations, is chosen for the installation cost factor.
To determine a year's amortization, this cost is multiplied by a factor called the
capital recovery factor (CRF). The value of the CRF depends on the interest rate and
the useful life of the equipment, and is arrived at by the following formula:
CRF -
where: i = interest rate, expressed as a fraction
n1 = useful life of the adsorber in years
In this example, it is assumed that the interest rate is 10% (i - 0.1) and the useful
life (n1) is 15 years. The capital recovery factor is:
CRF = 0.1315
The annual package adsorber capital cost is given by the formula:
$
U —7— x Q scfm x ICF x CRF
scfm
47
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(b) Space
This cost is the yearly value of the space taken by the adsorber installation, and
is given by
A sq' ft' x Q scfm x CA - ^- x CRF
scfm sq. ft.
where A = unit area occupied by the adsorber
CA = cost per sq. ft. of space
The value for A is typically 0.045 sq. ft. /scfm. CA is assumed here to be $40/
sq. ft. The CRF for the space cost is found by assuming a 25-year useful building
life (n2) and 10% interest rate (i = 0.1).
CRF = 0.1102
(c) Steam
This cost is determined by multiplying the cost of steam (Cs , $/lb steam) and the
Ibs. of steam needed (Ps ) and is given by the formula:
Ps - (78 + 0.04 S 1085 ) x 10"3 x Q x H
The cost of steam Cs is assumed here to be $1.50 per 1000 Ibs. , or $0.0015/lb.
steam.
(d) Water
This cost represents the cost of water (Cw, $/lb. water) times the amount of water
needed to condense the steam (Pw, Ibs.). It is assumed here that the cost of water is
15 cents per 1000 gallons, or
x - - 1.8 x 10-s $/lb. water
inn
1000 gallons 8.37 Ibs.
A typical value for the amount of cooling water needed is 30 Ibs. water per Ib. of steam,
and will be assumed here. Thus
Pw = 30 x Ps Ibs.
The value of Ps is determined from the formula given in (c) above.
48
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(e) Electricity
The cost of electricity is found by multiplying the cost per kwh (CK, $/kwh) and
the kwh consumed by the fan. The kwh consumed by the fan is given by:
kw
F —— x Q scfm x H hrs
scfm
kw
where: F = fan power per scfm, and is typically 0.003
The cost per kwh is assumed here to be 4 cents per kwh, or
CK =-- $0.04/kwh
(f) Carbon Replacement
This cost is the price of carbon (Cc, $/lb) times the amount of carbon replaced
per year (P(, Ibs). Carbon replacement needs depend on such factors as fouling and
loss due to abrasion. It is assumed here that 300 Ibs. of carbon will have to be re-
placed every 5 years per 1000 scfm of adsorber capacity. Thus:
^c 300 Ibs. 1
P = 1000 scfm X Q Scfm X 5
300 x 3000 x ±
1000 ~ ---- 5
= 180 Ibs. annually
The price of carbon, Cc, is assumed to be $1.00/lb.
(g) Operation
For manual systems, this cost is found by multiplying hourly labor cost (CL, $/hr.)
and the number of labor hours needed annually for steam regeneration (HL).
The hours needed for regeneration will depend on the solvent concentration: the
higher the concentration, the less time needed to regenerate. HL is given by the fol-
lowing empirical formula:
gO.7
HL = — — x H (for manual systems)
For automated systems, this cost is also calculated by multiplying hourly labor
cost, C1 , and the labor hours needed, HL.
49
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However, in this case the labor hours are calculated by assuming that 15 minutes
of labor is needed per 8-hour shift. Thus:
TJ i -i e • .. ! hour 1 TT
HL = 15 minutes x — :—-— x — x H
60 minutes 8
= 0.03125 x H (for automated systems)
The labor cost, CL, is assumed here to be $10/hr.
(h) Maintenance, Insurance and Taxes
These costs can be conveniently expressed as percentages of installed cost, C1.
Typical values are:
maintenance = 5% of C1
insurance = 1% of C1
taxes = 1% of C1
Installed cost is given, as in (a), by:
C1 = Equipment Cost x ICF
- U x Q x ICF
(i) Solvent Credit
Solvent credit is value of solvent (V, $/gal.) times solvent recovered (R, gallons).
90% recovery is assumed here:
R = 0.9 x solvent recoverable
= 0.9 x 1.67 x 10~7 x S x MW x Q x H x ^ gallons
= 1.5 x 10"7 x S x MW x Q x H x —
where D is the solvent density in Ibs/gallon.
The following pages show how costs were calculated for the data used in the above
example, using the cost calculation form in Appendix A.
50
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EXAMPLE CALCULATION - ADSORPTION COSTS
General Design Data
Solvent: Trichloroethvlene Gas Flow, Q = 3Qnn scfm
Solvent Concentration, S = 5QQ pprc Operating Hours, H = 208Q Per year
Solvent Molecular Weight, MW = 135.5 Adsorber Type: 2-tank, automatiq
Solvent Value, V = $2.15 per gallon Adsorber Unit Price, U = $4.67 /scfm
Annualized Capital Costs:
a) Package Adsorber
• additional data
installation cost factor, ICF = 2.0
useful life of adsorber, n* = 15 years
interest rate, i = IQ per cent = Q i (expressed as a fraction)
• capital recovery factor, CRF
CRF = id + i)n B 0.1(1+0.1]15 = 0.1315
(I**)" -1 (1+0.I)15 - 1
• annualized package adsorber cost
= U $_ x Q scfm x ICF x CRF
scfm
= 4.67 x 3000 x 2.0 x 0.1315
= $3684.63
b) Space
• additional data
unit area occupied by adsorber, A = Q.Q45sq.ft./scfm
cost per square foot of space, C^ = $40 /sq.ft.
useful building life, n^ = 25 years
51
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• capital recovery factor, CRF
n2
CRF = i(l + i\ = °.l (1
• annualiied space cost
= A sg. ft. x Q scfra x C $ x CRF
scfm sq.ft.
= 0.045 x 3000 x 40 x 0.1102
= $ 595.08
Operating Costs
c) Steam
• additional data
5
cost of steam, C = $ 0.0015/lb. steam
5
c Ibs. of steam needed, P
= (78 + 0.04 s1'085) 10"3 x Q x H
= (78 + 0104 x (500)1'085) x 10"3 x 3000 x 2080
= (78 + 33.92) x 10~3 x 3000 x 2080
= 698.374 Ibs.
• annual steam cost
= r^ x P^
Vj A I
= 0.0015 x 698374
= $ 1047.56
52
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d) Water
» additional data
cost of water, CW = 15 £/1000 gallons
cooling water needed = 30 Ibs/lb. steam
• cost of water in $/lb.
= $ 0.15 _ * _ L_
1000 gallons 8.37 Ibs,
= $1.79xlO"hb.
W
Ibs. of cooling water needed, P
5
= 30 Ibs. water x P Ibs. steam
= 30 x 698574
Ibs. steam
= 20.95x10 Ibs. water
• annual water cost
W W
= C x P
1.79 x 10~ x 20.95 x 106
= $375.47
e) Electricity
• additional data
fan power per scfm, F = Q.QQ3 kw/scfm
electrical cost, C^ = $ Q.Q4 /kwh
• annual electricity cost
= CKxFxQxH
= 0.04 x 0.003 x 3000 x 2080
= $748.80
53
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f) carbon replacement
• additional data
C
price of carbon, C - $i.QQ /lb.
carbon replaced - 300 Ibs. per 1000 scfm every 5 (n ) years
C
o annual carbon replacement, P
= 50Q Ibs. x Q scfm x I
1000 scfm 3
- ToW x 300° * I
= 180 lbs-
e annual carbon replacement cost
= CC x PC
= 1.00 x 180
= $180.00
g) operation
For Manual Systems
« additional data
hourly labor cost, C = $
• labor hours needed for steam regeneration, H
= S°'7
125 X H
hrs.
54
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annual operation cost, manual systems
* CL x HL
= $
For Automated Systems
additional data
hourly labor cost, C = $ ip /hr.
labor needed = 15 minutes per 8-hour shift
Labor hours needed for steam regeneration, H
= 15 minutes x _!_ hour x H
8 hours 60 minutes
= i| x Ij
65 hrs.
• annual operation cost, automated systems
= CLxHL
= 10 x 65
= $650.00
h) Maintenance, Insurance and Taxes
• additional data
Maintenance = _g percent of installed cost, C
Insurance = _J percent of installed cost, C*
Taxes = _1 percent of installed cost, C*
• installed cost, C (from a)
= U x Q x ICF
= 4.67 x 3000 x 2.0
= $ 28,020
55
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e annual maintenance, insurance and tax costs
= (_5 +_1 + 1 ) x 1 x C1
100
7 x 28020
100
= $ 1961.40
i) Solvent Credit
• additional data
solvent recovery, R = gp percent
solvent density, D = 12.islbs./gallon
e solvent recovered, R
= RS x 1.67 x 10"7 x S x MK x Q x H x ^-
100 U
_ _90. x 1.67 x 10" x 500 x 135.5 x 3000 x 2080 x
~ 100
= 5238.3 gallons
• annual solvent credit
= V x R
= 2.15 x 5238.3
= $11262.38
56
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SUMMARY
Annualized Cost
Capital Costs:
a) Package Adsorber = $3684.63
b) Space = 595.08
Total Annualized Capital Cost = $4279.71
Operating Costs: Annual Cost
c) Steam = $ 1047.56
d) Water = 375.47
e) Electricity = 780.80
f) Carbon Replacement = 180.00
g) Operation = 650.00
h) Maintenance, Insurance S Taxes = 1961.40
i) Solvent Credit Total Annual Operating Cost = $ 4995.23
TOTAL ANNUAL COST = $ 9274.94
= $11262.38
NET ANNUAL PROFIT (COST) = $ 1987.44
Profit (Cost) Per Operating Hour = $ 1987.44 /H
= 1987.44
2080
=$0.96
57
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CHAPTER VI
COMPARISON OF PROCESS CHANGES
AND CONTROL TECHNIQUES
This section compares selected process changes discussed in Chapter IV (covers,
increased freeboard, refrigerated chillers, and distillation) and one treatment tech-
nique from Chapter V (carbon adsorption). The process changes selected were those
with test data available, and they in no way constitute all the options for change. The
comparisons are made on the basis of:
• Emission reduction effectiveness of each alternative, as measured by solvent
consumption;
• Profitability of each alternative; and
• Effectiveness of combined process changes and/or treatment techniques in re-
ducing solvent consumption.
The effectiveness of covers in conjunction with other control alternatives is depen-
dent on the frequency of vapor degreaser usage. As usage is increased from the typical
25 percent of a work shift, the effectiveness will be diminished. Further, the usage of
covers and carbon adsorption requires coordination. When covers are closed, the car-
bon adsorber exhaust fan should be shut off to minimize the solvent's migration in—and
escape from—the carbon bed.
Carbon adsorption appears to be less complimentary to process changes than some
of the process changes are to each other. The reason is the dependency of carbon ad-
sorption collection and efficiency on exhaust systems, which pull airflow from the sol-
vent cleaners. Process changes operate more effectively under conditions where air-
flow is minimized.
Table 3 (originally presented in part as Table 1) shows the effectiveness of vari-
ous process changes and a treatment technique (carbon adsorption), singly and in com-
bination, in reducing hydrocarbon emissions. No combination of the process changes
listed will meet an overall 85-percent efficiency. Only the combination of one or more
process changes with carbon adsorption could have a chance of meeting this figure.
For example, under the best conditions, a refrigerated chiller might achieve a
60-percent emission reduction. Again, under ideal conditions a cover added to the
chiller may achieve an additional 40-percent reduction of the remaining 40 percent of
the emissions, i.e., 16 percent. The total emission reduction of 76 percent is still
below 85 percent.
58
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Table 3
Comparison of Process Changes and a Treatment Technique for Reducing Hydrocarbon Emissions
Emission
Reduction
Technique
Covers
Increased
Freeboard
Refrigerated
Chillers
Distillation
Carbon
Adsorption
Solvent(a)
Reduction
Effectiveness
24-50%
35%
27-55%
30%
16-60%(b)
40%
35-77%
57%
80% of
disposed
solvent
20-65%
40%
Profitability
Generally profitable
Generally profitable
with the exceptions of
methylene chloride and
trichlorotrifluoroethane
where freeboards are
already increased
Generally profitable
with the exception of
small tank on one shift
operation
Profitable above 350
gallons per year of
halogenated solvent
Profitable when inlet
concentration greater
than 200 ppm,
depending upon
solvent value
Effectiveness When Combined With
Other Process Changes
With Freeboard: 5-15% greater than cover
With Chillers: 16-43% greater than chillers(c)
With Distillation: Additive
With Adsorption: Slightly additive
With Covers: 5-15% greater than cover
With Chillers: No data
With Distillation: Additive
With Adsorption: Slightly additive
With Covers: 16-43% greater than chiller s(c)
With Freeboard: No data
With Distillation: Additive
With Adsorption: Not additive
Can be combined with all other changes.
Combined effectiveness is additive.
With Covers: Slightly additive
With Freeboard: Slightly additive
With Chillers : Not additive
With Distillation: Additive
(a) Upper set of numbers is range, lower is expected efficiency. Overall effectiveness used except where noted.
(b) Two sets of test results.
(c) 16% for 1,1,1-trichloroethane and 43% for methylene chloride.
-------
The difficulties in achieving control efficiencies of 85 percent or better, along with
the difficulties in sampling solvent emissions, are among the reasons that EPA is not
considering a similar regulation for new sources; for these, the Agency is studying
equipment specifications and solvent substitutions, or combinations thereof.
The process changes all have a positive—and in most cases, an attractive—return
on investment and should be considered for that reason irrespective of pollution control.
Since any gain is due to solvent recovery, changes should be implemented on a step-by-
step basis, with compliance evaluations after each step.
The suggested steps for achieving the greatest emission reduction possible (where
solvent substitution is not an alternative) would be in the following order:
1. Improve solvent cleaning operating procedures.
2. Distill spent solvents.
3. Make process changes:
• Chillers,
• Chillers and covers, and
• Chillers, covers and freeboard.
4. Install a carbon adsorption system.
60
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APPENDIX A
FORM FOR
CALCULATION OF CARBON ADSORPTION COST
General Design Data
Solvent:
Gas Flow, Q =
scfm
Solvent Concentration, S = ppm Operating Hours, H = per year
Solvent Molecular Weight, MW = Adsorber Type:
Solvent Value, V = $ per gallon Adsorber Unit Price, U = $
/scfm
Annualized Capital Costs:
a) Package Adsorber
• additional data
installation cost factor, ICF =
useful life of adsorber, n* =
interest rate, i = per cent =
capital recovery factor, CRF
CRF = —/
• annualized package adsorber cost
= U $_ x Q scfm x ICF x CRF
scfm
years
(expressed as a fraction)
= $
b) Space
• additional data
unit area occupied by adsorber, A
cost per square foot of space, C^
useful building life, n^ =
= $
_sq.ft./scfm
/sq.ft.
years
61
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• capital recovery factor, CRF
CRF =
(1+i)"2 - 1
• annual! zed space cost
q. ft. x Q scf
scfm sq.ft.
= A sq. ft. x Q scfra x C $ x CRF
= $
Operating Costs
c) Steam
• additional data
c
cost of steam, C = $ _ /lb. steam
• Ibs. of steara needed, P
1
= (78 + 0.04 S1< ) lO"" x Q x H
Ibs.
annual steara cost
- CSxPS
- $
62
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d) Water
• additional data
cost of water, C =
cooling water needed = _
cost of water in $/lb.
= $ x 1
1000 gallons
= $ /lb.
-------
f] carbon replacement
• additional data
price of carbon, C = $ /lb.
carbon replaced = Ibs. per 1000 scfm every (n ) years
• annual carbon replacement, P
= Ibs. x Q scfm x 1
1000 scfm 3
Ibs.
annual carbon replacement cost
f f
= C x PL
= $
g) operation
For Manual Systems
• additional data
hourly labor cost, C = $
labor hours needed for steam regeneration, H
- S°'7
' S x H
125
hrs.
64
-------
• annual operation cost, manual systems
« CL x HL
= $
For Automated Systems
• additional data
hourly labor cost, C = $ /hr.
labor needed = minutes per 8-hour shift
• Labor hours needed for steam regeneration, H
= minutes x _l_ hour x H
8 hours 60 minutes
hrs.
annual operation cost, automated systems
= CLxHL
= $
Maintenance, Insurance and Taxes
• additional data
Maintenance = percent of installed cost, C
Insurance = percent of installed cost, C*
Taxes = percent of installed cost, C*
• installed cost, C (from a)
= U x Q x ICF
= $
65
-------
• annual maintenance, insurance and tax costs
= ( + + ) x 1 x C1
100
= $
i) Solvent Credit
• additional data
solvent recovery, Rs= percent
solvent density, D = Ibs./gallon
• solvent recovered, R
= RS x 1.67 x 10"7 xSxMWxQxHx-i-
100 U
gallons
annual solvent credit
= V x R
= $
66
-------
SUMMARY
Annualized Cost
Capital Costs:
a) Package Adsorber = $
b) Space =
Total Annualized Capital Cost = $
Operating Costs: Annual Cost
c) Steam = $
d) Water =
e) Electricity »
f) Carbon Replacement =
g) Operation =
h) Maintenance, Insurance 5 Taxes =
i) Solvent Credit Total Annual Operating Cost = $
TOTAL ANNUAL COST = $
- $
NET ANNUAL PROFIT (COST) = $
Profit (Cost) Per Operating Hour = $ /H
67
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BIBLIOGRAPHY
1. DOW Chemical Company, Study To Support New Source Performance Standards For Solvent
Metal Cleaning Operations. Prepared for Office of Air Quality Planning, U.S. Environmental
Protection Agency, June 30, 1976.
2. EPA Office of Air Quality Planning and Standards, Control of Photochemical Oxidants -
Technical Basis and Implications of Recent Findings. EPA-450/2-75-005, July 1975.
3. Perry, J. H., and Chilton, G. H., Chemical Engineers'Handbook, 5th Ed., Part 3. New York:
McGraw-Hill Book Co., 1973, p. 250.
4. Rekstead, G. M., "Upheaval In Vapor Degreasing." Factory, Jan. 1974, pp. 27-32.
5. Handbook of Vapor Degreasing. American Society For Testing and Materials, STP 310A,
April 1976.
6. Danielson, John (ed.), Air Pollution Engineering Manual, U.S. Environmental Protection Agency,
AP-40, 2nd Ed., May 1973.
7. Kovach, J. Louis, "Principles of Adsorption." Control of Gaseous Emissions, Air Pollution
Training Institute, U.S. Environmental Protection Agency, Jan. 1973.
8. Cheremisinoff, Paul N., P.F., "Control of Gaseous Air Pollutants." Pollution Engineering,
May 1976, p. 30.
9. Manzone, R. R., and Oakes, D. W., "Profitability of Recycling Solvents From Process Systems."
Pollution Engineering, Vol. 5, No. 10, Oct. 1973, pp. 23-24.
U.S. Environmental Protection Agency
Region 5, Library (PI-12J)
77 West Jackson Boulevard. 12th Floor
Chicago, II 60604-3590
*US GOVEmtWEHTPSINTINGOfFICE 1977—757-056/6562
68
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U.S ENVIRONMENTAL PROTECTION AGENCY
ENVIRONMENTAL RESEARCH INFORMATION CENTER • TECHNOLOGY TRANSFER
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