EPA450/3-90-006b
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
QAQPS Control Cost Manual (Fourth Edition): Supplement 2
5. REPORT DATE
October 1992
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Radian; W. Barbour, R. OOmmen, G.Shareef
EPA: W. Vatavuk
s. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME ANO ADDRESS
Radian Corporation
P.O. Box 13000
Research Triangle Park, NC 27709
'HUI
10. PROGRAM ELEMENT NO.
. TEXAf
11. CONTRACT/GRANT NO.
EPA-68-D1-0117 (W. A. 20)
12. SPONSORING AGENCY NAME AND ADDRESS |
Environmental Protection Agency *•
Office of Air and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This is the second supplement to the OAQPS Control Cost
Manual (Fourth Edition). The supplement consists of a new Manual
chapter, Chapter 9 ("Gas Absorbers"). Like the other chapters in
the Manual, Chapter 9 is self-contained. It discusses: (1) the
typfes and applications of packed column gas absorbers used in air
pollution control; (2) the theory underlying their operation and
design; (3) basic sizing procedures; and (4) current data and
procedures for estimating study-level (± 30%-accurate) capital
and annual costs. In .particular, the chapter contains 1991
column and packing costs, which are correlated with appropriate
sizing parameters (e.g., column height and diameter). Finally,
Chapter 9 includes: a comprehensive example problem that
illustrates the sizing and costing procedures; three appendices;
a table of contents; and a list of references.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Stationary emission sources
Costs
Control techniques
Control device design/sizing
Gas absorbers
Packed columns
Packing
Cost estimation
Capital costs
Equipment, installation cbsts
Annual costs (direct, indirect)
Operating and maintenance costs
"Add-on" controls
18. DISTBIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (Tins Report!
Unclassified
21. NO. OF PAGES
67
20. SECURITY CLASS (Tliispage)
Unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
-------
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17. KEY WORDS AND DOCUMENT ANALYSIS
(a) DESCRIPTORS - Select from the Thesaurus of Engineering and Scientific Terms the proper aullmri/cd terms that identity the major
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Chapter 9
GAS ABSORBERS
Wiley Barbour
Roy Oommen
Gunseli Sagun Shareef
Radian Corporation
Research Triangle Park, NC 27709
William M. Vatavuk
Standards Development Branch, OAQPS
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
October 1992
*
Contents
9.1 Introduction 9-3
9.1.1 System Efficiencies and Performance 9-4
9.2 Process Description 9-5
9.2.1 Absorber System Configuration 9-5
9-1
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9.2.2 Types of Absorption Equipment 9-6
9.2.3 Packed Tower Internals 9-7
9.2.4 Packed Tower Operation 9-11
9.3 Design Procedures 9-13
9.3.1 Step 1: Determining Gas and Liquid Stream Conditions 9-14
9.3.2 Step 2: Determining Absorption Factor 9-22
9.3.3 Step 3: Determining Column Diameter 9-23
9.3.4 Step 4: Determining Tower Height and Surface Area . . 9-26
9.3.5 Step '5: Calculating Column Pressure Drop 9-29
9.3.6 Alternative Design Procedure 9-29
9.4 Estimating Total Capital Investment 9-31
9.4.1 Equipment Costs for Packed Towers 9-32
9.4.2 Installation Costs 9-36
9.5 Estimating Annual Cost 9-36
9.5.1 Direct Annual Costs 9-36
9.5.2 Indirect Annual Costs 9-40
9.5.3 Total Annual Cost 9-40
*
9.6 Example Problem #1 9-41
9.6.1 Required Information for Design 9-41
9.6.2 Step 1: Determine Gas and Liquid Stream Properties . 9-41
,9.6.3 Step 2: Calculate Absorption Factor 9-45
9.6.4 Step 3: Estimate Column Diameter 9-45
9-2
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9.6.5 Step 4: Calculate Column Surface Area 9-47
9.6.6 Step 5: Calculate Pressure Drop 9-48
9.6.7 Equipment Costs 9-48
9.6.8 Total Annual Cost 9-50
9.7 Example Problem #2 9-54
9.8 Acknowledgements 9-54
Appendix 9A - Properties of Pollutants 9-56
Appendix 9B - Packing Characteristics 9-58
Appendix 9C - Minimum Wetting Rate Analysis 9-63
9C.1 Overview of the Approach 9-63
9C.2 Example Problem Calculation 9-64
References 9-66
9.1 Introduction
Gas absorbers are used extensively in industry for separation and purification
of gas streams, as product recovery devices, and as pollution control devices.
This chapter focuses on the application of absorption for pollution control on
gas streams with typical pollutant concentrations ranging from 250 to 10,000
ppmv. Gas absorbers are most widely used to remove water soluble inorganic
contaminants from air streams.[1, 2]
Absorption is a process where one or more soluble components of a gas
mixture are dissolved in a liquid (i.e., a solvent). The absorption process
can be categorized as physical or chemical. Physical absorption occurs when
the absorbed compound dissolves in the solvent; chemical absorption occurs
when the absorbed compound and the solvent react. Liquids commonly used
as solvents include water, mineral oils, nonvolatile hydrocarbon oils, and
aqueous solutions.[1]
9-3
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9.1.1 System Efficiencies and Performance
Removal efficiencies for gas absorbers vary for each pollutant-solvent system
and with the type of absorber used. Most absorbers have removal efficiencies
in excess of 90 percent, and packed tower absorbers may achieve efficiencies
as high as 99.9 percent for some pollutant-sol vent systems.[1, 3]
The suitability of gas absorption as a pollution control method is generally
dependent on the following factors: 1) availability of suitable solvent; 2)
required removal efficiency; 3) pollutant concentration in the inlet vapor;
4) capacity required for handling waste gas; and, 5) recovery value of the
pollutant(s) or the disposal cost of the spent solvent.[4]
Physical absorption depends on properties of the gas stream and sol-
vent, such as density and viscosity, as well as specific characteristics of the
pollutant(s) in the gas and the liquid stream (e.g., diffusivity, equilibrium
solubility). These properties are temperature dependent, and lower temper-
atures generally favor absorption of gases by the solvent.[1] Absorption is also
enhanced by greater contacting surface, higher liquid-gas ratios, and higher
concentrations in the gas stream. [1]
The solvent chosen to remove the pollutant(s) should have a high solu-
bility for the gas, low vapor pressure, low viscosity, and should be relatively
inexpensive. [4] Water is the most common solvent used to remove inorganic
contaminants; it is also used to absorb organic compounds having relatively
high water solubilities. For organic compounds that have low water solu-
bilities, other solvents such as hydrocarbon oils are used, though only in
industries where large volumes of these oils are available (i.e., petroleum
refineries and petrochemical plants).[5]
Pollutant removal may also be enhanced by manipulating the chemistry
of the absorbing solution so that it reacts with the pollutant(s), e.g., caustic
solution for acid-gas absorption vs. pure water as a solvent. Chemical ab-
sorption may be limited by the rate of reaction, although the rate limiting
step is typically the physical absorption rate, not the chemical reaction rate.
9-4 . . -
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9.2 Process Description
Absorption is a mass transfer operation in which one or more soluble com-
ponents of a gas mixture are dissolved in a liquid that has low volatility
under the process conditions. The pollutant diffuses from the gas into the
liquid when the liquid contains less than the equilibrium concentration of the
gaseous component. The difference between the actual concentration and the
equilibrium concentration provides the driving force for absorption.
A properly designed gas absorber will provide thorough contact between
the gas and the solvent in order to facilitate diffusion of the pollutant(s).
It will perform much better than a poorly designed absorber. [6] The rate
of mass transfer between the two phases is largely dependent on the surface
area exposed and the time of contact. Other factors governing the absorption
rate, such as the solubility of the gas in the particular solvent and the degree
of the chemical reaction, are characteristic of the constituents involved and
are relatively independent of the equipment used.
9.2.1 Absorber System Configuration
Gas and liquid flow through an absorber may be countercurrent, cross-
current, or cocurrent. The most commonly installed designs are countercur-
rent, in which the waste gas stream enters at the bottom of the absorber col-
umn and exits at the top. Conversely, the solvent stream enters at the top and
exits at the bottom. Countercurrent designs provide the highest theoretical
removal efficiency because gas with the lowest pollutant concentration con-
tacts liquid with the lowest pollutant concentration. This serves to maximize
the average driving force for absorption throughout the column. [2] Moreover,
countercurrent designs usually require lower liquid to gas ratios than cocur-
rent and are more suitable when the pollutant loading is higher.[3, 5]
In a crosscurrent tower, the waste gas flows horizontally across the column
while the solvent flows vertically down the column. As a rule, crosscurrent
designs have lower pressure drops and require lower liquid-to-gas ratios than
both cocurrent and countercurrent designs. They are applicable when gases
are highly soluble, since they offer less contact time for absorption.[2, 5]
In cocurrent towers, both the waste gas and solvent enter the column at
9-5 - "
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the top of the tower and exit at the bottom. Cocurrent designs have lower
pressure drops, are not subject to flooding limitations and are more efficient
for fine (i.e., submicron) mist removal. Cocurrent designs are only efficient
where large absorption driving forces are available. Removal efficiency is
limited since the gas-liquid system approaches equilibrium at the bottom of
the tower. [2]
9.2.2 Types of Absorption Equipment
Devices that are based on absorption principles include packed towers, plate
(or tray) columns, venturi scrubbers, and spray chambers. This chapter fo-
cuses on packed towers, which are the most commonly used gas absorbers for
pollution control. Packed towers are columns filled with packing materials
that provide a large surface area to facilitate contact between the liquid and
gas. Packed tower absorbers can achieve higher removal efficiencies, handle
higher liquid rates, and have relatively lower water consumption requirements
than other types of gas absorbers.[2] However, packed towers may also have
high system pressure drops, high clogging and fouling potential, and exten-
sive maintenance costs due to the presence of packing materials. Installation,
operation, and wastewater disposal costs may also be higher for packed bed
absorbers than for other absorbers.[2] In addition to pump and fan power re-
quirements and solvent costs, packed towers have operating costs associated
with replacing damaged packing.[2]
Plate, or tray, towers are vertical cylinders in which the liquid and gas
are contacted in step-wise fashion on trays (plates). Liquid enters at the
top of the column and flows across each plate and through a downspout
(downcomer) to the plates below. Gas moves upwards through openings in
the plates, bubbles into the liquid, and passes to the plate above. Plate
towers are easier to clean and tend to handle large temperature fluctuations
better than packed towers do.[4] However, at high gas flow rates, plate towers
exhibit larger pressure drops and have larger liquid holdups. Plate towers
are generally made of materials such as stainless steel, that can withstand
the force of the liquid on the plates and also provide corrosion protection.
Packed columns are preferred to plate towers when acids and other corrosive
materials are involved because tower construction can then be of fiberglass,
polyvinylchloride, or other less costly, corrosive-resistant materials. Packed
towers are also preferred for columns smaller than two feet in diameter and
9-6 - , -
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when pressure drop is an important consideration.[3, 7]
Venturi scrubbers are generally applied for controlling participate mat-
ter and sulfur dioxide. They are designed for applications requiring high
removal efficiencies of submicron particles, between 0.5 and 5.0 micrometers
in diameter. [4] A venturi scrubber employs a gradually converging and then
diverging section, called the throat, to clean incoming gaseous streams. Liq-
uid is either introduced to the venturi upstream of the throat or injected
directly into the throat where it is atomized by the gaseous stream. Once
the liquid is atomized, it collects particles from the gas and discharges from
the venturi.[1] The high pressure drop through these systems results in high
energy use, and the relatively short gas-liquid contact time restricts their
application to highly soluble gases. Therefore, they are infrequently used for
the control of volatile organic compound emissions in dilute concentration. [2]
Spray towers operate by delivering liquid droplets through a spray dis-
tribution system. The droplets fall through a countercurrent gas stream un-
der the influence of gravity and contact the pollutant(s) in the gas.[7] Spray
towers are simple to operate and maintain, and have relatively low energy
requirements. However, they have the least effective mass transfer capability
of the absorbers discussed and are usually restricted to particulate removal
and control of highly soluble gases such as sulfur dioxide and ammonia. They
also require higher water recirculation rates and are inefficient at removing
very small particles.[2, 5]
9.2.3 Packed Tower Internals
A basic packed tower unit is comprised of a column shell, mist eliminator,
liquid distributors, packing materials, packing support, and may include a
packing restrainer. Corrosion resistant alloys or plastic materials such as
polypropylene are required for column internals when highly corrosive sol-
vents or gases are used. A schematic drawing of a countercurrent packed
tower is shown in Figure 9.1. In this figure, the packing is separated into
two sections. This configuration is more expensive than designs where the
packing is not so divided.[5]
The tower shell may be made of steel or plastic, or a combination of these
materials depending on the corrosiveness of the gas and liquid streams, and
the process operating conditions. One alloy that is chemical and temperature
9-7
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f GuOut
liquid In —
n/
7'i'" ^ "
Miat Siminator
Liquid Oiatnbutor
^ Spray Nozzt*
Packing
Raatrainar
•Shall
Random
Packing
Liquid fla-dlatnbutor
Packing Support
Gas In
UquttOut
Figure 9.1: Packed Tower for Gas Absorption
9-8 -
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resistant or multiple layers of different, less expensive materials may be used.
The shell is sometimes lined with a protective membrane, often made from a
corrosion resistant polymer. For absorption involving acid gases, an interior
layer of acid resistant brick provides additional chemical and temperature
resistance. [8]
At high gas velocities, the gas exiting the top of the column may carry
off droplets of liquid as a mist. To prevent this, a mist eliminator in the
form of corrugated sheets or a layer of mesh can be installed at the top of
the column to collect the liquid droplets, which coalesce and fall back into
the column.
A liquid distributor is designed to wet the packing bed evenly and
initiate uniform contact between the liquid and vapor. The liquid distributor
must spread the liquid uniformly, resist plugging and fouKng, provide free
space for gas flow, and allow operating flexibility. [9] Large towers frequently
have a liquid redistributor to collect liquid off the column wail and direct it
toward the center of the column for redistribution and enhanced contact in
the lower section of packing.[4] Liquid redistributors are generally required
for every 8 to 20 feet of random packing depth.[5, 10]
Distributors fall into two categories: gravitational types, such as orifice
and weir types, and pressure-drop types, such as spray nozzles and perfo-
rated pipes. Spray nozzles are the most common distributors, but they may
produce a fine mist that is "easily entrained in the gas flow. They also may
plug, and usually require high feed rates to compensate for poor distribution.
Orifice-type distributors typically consist of flat trays with a number of risers
for vapor flow and perforations in the tray floor for liquid flow. The trays
themselves may present a resistance to gas flow.[9] However, better contact
is generally achieved when orifice distributors are used. [3]
Packing materials provide a large wetted surface for the gas stream
maximizing the area available for mass transfer. Packing materials are avail-
able in a variety of forms, each having specific characteristics with respect to-
surface area, pressure drop, weight, corrosion resistance, and cost. Packing
life varies depending on the application. In ideal circumstances, packing will
last as long as the tower itself. In adverse environments packing life may be
as short as 1 to 5 years due to corrosion, fouling, and breakage.[11]
Packing materials are categorized as random or structured. Random
packings are usually dumped into an absorption column and allowed to settle.
9-9
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Pall Ring
Tellerecte
Incalox Saddle
Berl Saddle
Raschig Ring
Figure 9.2: Random Packing Materials
Modern random packings consist of engineered shapes intended to maximize
surface-to-volume ratio and minimize pressure drop.[2] Examples of different
random packings are presented in Figure 9.2. The first random packings
specifically designed for absorption towers were made of ceramic. The use of
ceramic has declined because of their brittleness, and the current markets are
dominated by metal and plastic. Metal packings cannot be used for highly
corrosive pollutants, such as acid gas, and plastic packings are not suitable
for high temperature applications. Both plastic and metal packings are gen-
erally limited to an unsupported depth of 20 to 25. At higher depths the
weight may deform the packing.[10]
Structured packing may be random packings connected in an orderly
arrangement, interlocking grids, or knitted or woven wire screen shaped
into cylinders or gauze like arrangements. They usually have smaller pres-
sure drops and are able to handle greater solvent flow rates than random
9-10
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packings.[4] However, structured packings are more costly to install and may
not be practical for smaller columns. Most structured packings are made
from metal or plastic.
In order to ensure that the waste gas is well distributed, an open space
between the bottom of the tower and the packing is necessary. Support
plates hold the packing above the open space. The support plates must
have enough strength to carry the weight of the packing, and enough free
area to allow solvent and gas to flow with minimum restrictions.[4]
High gas velocities can fluidize packing on top of a bed. The packing
could then be carried into the distributor, become unlevel, or be damaged.[9]
A packing restrainer may be installed at the top of the packed bed to
contain the packing. The packing restrainer may be secured to the wall so
that column upsets will not dislocate it, or a "floating" unattached weighted
plate may be placed on top of the packing so that it can settle with the bed.
The latter is often used for fragile ceramic packing.
9.2.4 Packed Tower Operation
As discussed in Section 9.2.1, the most common packed tower designs are
counter current. As the waste gas flows up the packed column it will experi-
ence a drop in its pressure as it meets resistance from the packing materials
and the solvent flowing down. Pressure drop in a column is a function of
jthe gas and liquid flow rates and properties of the packing elements, such as
surface area and free volume in the tower. A high pressure drop results in
high fan power to drive the gas through the packed tower, and consequently
high costs. The pressure drop in a packed tower generally ranges from 0.5 to
1.0 in. H20/ft of packing.[7]
For each column, there are upper and lower limits to solvent and vapor
flow rates that ensure satisfactory performance. The gas flow rate may be-
come so high that the drag on the solvent is sufficient to keep the solvent from
flowing freely down the column. Solvent begins to accumulate and blocks the
entire cross section for flow, which increases the pressure drop and prevents
the packing from mixing the gas and solvent effectively. When all the "free
volume in the packing is filled with liquid and the liquid is carried back up
the column, the absorber is considered to be flooded.,'4] Most packed towers
operate at 60 to 70 percent of the gas flooding velocity, as it is not practical to
9-11
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operate a tower in a flooded condition. [7] A minimum liquid flow rate is also
required to wet the packing material sufficiently for effective mass transfer
to occur between the gas and liquid.[7]
The waste gas inlet temperature is another important scrubbing param-
eter. In general, the higher the gas temperature, the lower the absorption
rate, and vice-versa. Excessively high gas temperatures also can lead to
significant solvent loss through evaporation. Consequently, precoolers (e.g.,
spray chambers) may be needed to reduce the air temperature to acceptable
levels.[6]
For operations that are based on chemical reaction with absorption, an
additional concern is the rate of reaction between the solvent and pollu-
tant^). Most gas absorption chemical reactions are relatively fast and the
rate limiting step is the physical absorption of the pollutant(s) into the sol-
vent. However, for solvent-pollutant systems where the chemical reaction is
the limiting step, the rates of reaction would need to be analyzed kinetically.
Heat may be generated as a result of exothermal chemical reactions. Heat
may also be generated when large amounts of solute are absorbed into the
liquid phase, due to the heat of solution. The resulting change in temper-
ature along the height of the absorber column may damage equipment and
reduce absorption efficiency. This problem can be avoided by adding cooling
coils to the column.[7] However, in those systems where water is the solvent,
adiabatic saturation of the gas occurs during absorption due to solvent evap-
oration. This causes a substantial cooling of the absorber that offsets the
heat generated by chemical reactions. Thus, cooling coils are rarely required
with those systems.[5] In any event, packed towers may be designed assuming
that isothermal conditions exist throughout the column.[7]
The effluent from the column may be recycled into the system and used
again. This is usually the case if the solvent is costly, i.e., hydrocarbon oils,
caustic solution. Initially, the recycle stream may go to a waste treatment
system to remove .the pollutant(s) or the reaction product. Make-up solvent
may then be added before the liquid stream reenters the column. Recircula-
tion of the solvent requires a pump, solvent recovery system, solvent holding
and mixing tanks, and any associated piping and instrumentation.
9-12 -
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9.3 Design Procedures
The design of packed tower absorbers for controlling gas streams containing
a mixture of pollutant(s) and air depends on knowledge of the following
parameters:
1. Waste gas flow rate;
2. Waste gas composition and concentration of the pollutant(s) in the gas
stream;
3. Required removal efficiency;
4. Equilibrium relationship between the pollutant(s) and solvent; and
5. Properties of the pollutant(s), waste gas, and solvent:
• Diffusivity,
• Viscosity,
• Density, and
• Molecular weight.
The primary objectives of the design procedures are to determine column
surface area and pressure drop through the column. In order to determine
these parameters, the following steps must be performed:
Step 1: Determine the gas and liquid stream conditions entering and exiting
the column.
Step 2: Determine the absorption factor (AF).
Step 3: Determine the diameter of the column (D).
Step 4: Determine the tower height (H^ower] and surface area (S).
Step 5: Determine the packed column pressure drop (A/7).
9-13
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To simplify the sizing procedures, a number of assumptions have been
made. For example, the waste gas is assumed to comprise a two-component
waste gas mixture (pollutant/air), where the pollutant consists of a single
compound present in dilute quantities. The waste gas is assumed to behave
as an ideal gas and the solvent is assumed to behave as an ideal solution.
Heat effects associated with absorption are considered to be minimal for the
pollutant concentrations encountered. The procedures also assume that, in
chemical absorption, the process is not reaction rate limited, i.e., the reaction
of the pollutant with the solvent is considered fast compared to the rate of
absorption of the pollutant into the solvent.
The design procedures presented here are complicated, and careful atten-
tion to units is required. Table 9.1 is a list of all design variables referred
to in this chapter, along with the appropriate units. A key is provided to
differentiate primary data from calculated data.
9.3.1 Step 1: Determining Gas and Liquid Stream
Conditions
Gas absorbers are designed based on the ratio of liquid to gas entering the
column (L,/G,), slope of the equilibrium curve (m), and the desired removal
efficiency (77). These factors are calculated from the inlet and outlet gas and
•liquid stream variables:
Waste gas flow rate, in actual cubic feet per minute (acfm), entering
and exiting column (G, and G0, respectively);
Pollutant concentration (Ib-moles pollutant per Ib-mole of pollutant
free gas) entering and exiting the column in the waste gas (Yt and Y0,
respectively);
Solvent flow rate, in gallons per minute (gpm), entering and exiting the
column (L, and L0, respectively); and
Pollutant concentration (Ib-moles pollutant per Ib-mole of pollutant
free solvent) entering and exiting the column in the solvent (Xt and
X0, respectively).
9-14
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Table 0.1: List of Design Variables
Variable
> Surface to volume ratio of
packing
Cross-sectional area of ab-
sorber
Abscissa value from plot of
generalized pressure drop cor-
relation
Absorption factor
Diameter of absorber
t> Diffusivity of pollutant in gas
> Diffusivity of pollutant in liq-
uid
> Flooding factor
> Packing factor
t> Waste gas flow rate entering
absorber
Waste gas flow rate exiting ab-
sorber
Waste gas molar flow rate en-
tering absorber
Molar flow rate of pollutant
free gas
Waste gas superficial flow rate
entering absorber
Height of gas transfer unit
Height of liquid transfer unit
Height of overall transfer unit
Height of packing
Height of absorber
Pressure drop constants
Liquid rate entering absorber
. Liquid rate exiting absorber
Liquid molar flow rate enter-
ing absorber
Molar flow rate of pollutant
free solvent
Symbol
a
A
ABSCISSA
AF
D
DC
DL
f
Fp
Gt
G0
Gmol
G,
G,fr,i
Ho
HL
Htu
Hpack
IT
tower
ku,ki,k-i,k3,k4
Lt
Lo
mol,i
L,
Units
ft2/ft3
ft2
—
—
feet
ft2/hr
ft2/hr
—
—
acfm
acfm
Ib-moles/hr
lb*- moles /lir
lb/sec-ft2
feet
feet
feet
feet
feet
—
gpm
gpm
Ib-moles/hr
Ib-moles/hr
9-15
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Table 0.1: List of Design Variables (Continued)
Variable
Symbol
Units
MWR
Niu
ORDINATE
5
T
Liquid superficial flow rate en- L^ ,
tering absorber
Slope of equilibrium line m
> Molecular weight of the gas MW Molecular weight of the liquid
stream
t> Minimum wetting rate
Number of overall transfer
units
Ordinate value from plot of
generalized pressure drop cor-
relation
Surface area of absorber
> Temperature of solvent
Mole fraction of pollutant en- z,
tering absorber in liquid
Mole fraction of pollutant ex- x0
iting absorber in liquid
Pollutant concentration enter- .Y,
ing absorber in liquid
Maximum pollutant concen- X*
tration in liquid phase in equi-
librium with pollutant enter-
ing column in gas phase
Pollutant concentration exit- X0
ing absorber in liquid
Mole fraction of pollutant en- y,
tering absorber in waste gas
Mole fraction of pollutant in y*
gas phase in equilibrium with
mole fraction of pollutant en-
tering in the liquid phase
Mole fraction of pollutant ex- y0
iting scrubber in waste gas
lb/hr-ft2
Ib/lb-mole
Ib/lb-mole
ft2/hr
ft2
K
Ib-mole of pollutant
Ib-mole total liquid
Ib-mole of pollutant
Ib-mole total liquid
Ib-moles pollutant
Ib-moles pollutant free solvent
Ib-moles pollutant
Ib-moles pollutant free solvent
Ib-moles pollutant
Ib-moles pollutant free solvent
Ib-moles pollutant
Ib-moles total gas
Ib-moles pollutant
Ib-moles total gas
Ib-moles pollutant
Ib-moles total gas
9-16
-------
Table 9.1: List of Design Variables (Continued)
Variable
Symbol
Units
Mole fraction of pollutant in
gas phase in equilibrium with
mole fraction of pollutant ex-
iting in the liquid phase
> Pollutant concentration enter-
ing scrubber in waste gas
Pollutant concentration enter-
ing scrubber in equilibrium
with concentration in liquid
phase
Pollutant concentration exit-
ing scrubber in waste gas
Pollutant concentration exit-
ing scrubber in equilibrium
with concentration in liquid
phase
t> Pollutant removal efficiency
"> Density of waste gas stream
I> Density of liquid stream
t> Viscosity of waste gas
t> Viscosity of solvent
Ratio of solvent density to wa-
ter density
Pressure drop
t> Packing factors
PC
PL
AF
Ib-moles pollutant
Ib-moles total gas
Ib-moles pollutant
Ib-moles pollutant free gas
Ib-moles pollutant
Ib-moles pollutant Free gas
Ib-moles pollutant
Ib-moles pollutant free gas
Ib-moles polhitant
Ib-moles pollutant Tree gas
lb/ft3
lb/ft:l
Ib/ft-hr
Ib/ft-hr
inches H2O/feet of packing
t> Denotes required input data.
9-17
-------
This design approach assumes that the inlet gas stream variables are
known, and that a specific pollutant removal efficiency has been chosen as
the design basis; i.e., the variables (?,, Yj, and if are known. For dilute
concentrations typically encountered in pollution control applications and
negligible changes in moisture content, (7, is assumed equal to G0. If a
once-through process is used, or if the spent solvent is regenerated by an air
stripping process before it is recycled, the value of Xf will approach zero.
The following procedures must be followed to calculate the remaining stream
variables Y0, Li (and L0), and X0. A schematic diagram of a packed tower
with inlet and outlet flow and concentration variables labeled is presented in
Figure 9.3.
The variable Y0 may be calculated from TJ using the following equation:
The liquid flow rate entering the absorber, £, (gpm), is then calculated
using a graphical method. Figure 9.4 presents an example of an equilibrium
curve and operating line. The equilibrium curve indicates the relationship
between the concentration of pollutant in the waste gas and the concentra-
tion of pollutant in the solvent at a specified temperature. The operating line
indicates the relation between the concentration of the pollutant in the gas
and solvent at any location in the gas absorber column. The vertical distance
between the operating line and equilibrium curve indicates the driving force
for diffusion of the pollutant between the gas and liquid phases. The mini-
mum amount of liquid which can be used to absorb the pollutant in the gas
stream corresponds to an operating line drawn from the autlet concentration
in the gas stream (Y0) and the inlet concentration in the solvent stream (Xt)
to the point on the equilibrium curve corresponding to the entering pollu-
tant concentration in the gas stream (Yt). At the intersection point on the
equilibrium curve, the difiusional driving forces are zero, the required time
of contact for the concentration change is infinite, and an infinitely tall tower
results.
The slope of the operating line intersecting the equilibrium curve is equal
to the minimum L/G ratio on a. moles of pollutant-free solvent (Z,) per moles
of pollutant-free gas basis ( do not
include the moles of pollutant in the liquid and gas streams. The values of L,
and (j, are constant through the column if a negligible amount of moisture
is transferred from the liquid to the gas phase. The slope may be calculated
9-18
-------
G0
mol, o
Y
y i
mol,
s
mol, o
Figure 9.3: Schematic Diagram of Countercurrent Packed Tower Operation
9-19
-------
01
m
0
"3
«
o
ffl
I
Moles of Pollutant/Mole of Solvent
Figure 9.4: Minimum and Actual Liquid-to-Gas Ratios
9-20
-------
from the following equation:
(£
mm
where X* would be the maximum concentration of the pollutant in the liquid
phase if it were allowed to come to equilibrium with the pollutant entering the
column in the gas phase, Yi. The value of X* is taken from the equilibrium
curve. Because the minimum L,/Ga ratio is an unrealistic value, it must
be multiplied by an adjustment factor, commonly between 1.2 and 1.5, to
calculate the actual L/G ratio: [7]
_i = -i x (adjustment factor) (9.3)
act
The variable G, may be calculated using the equation:
MW0(1 + 1")
where 60 is the conversion factor from minutes to hours, MW<; is the molec-
ular weight of the gas stream (Ib/lb-mole), and pc is the density of the
gas stream (lb/ft3). For pollutant concentrations typically encountered, the
molecular weight and density of the waste gas stream are assumed to be
equal to that of ambient air.
The variable L, may then be calculated by:
L, = (£•} x G3 (9.5)
\tr»/ act
The total molar flow rates of the gas and liquid entering the absorber (Gmo[ j
and Lmoii) are calculated using the following equations:
= G,(l + Yt) (9.6)
= L,(l + X,) (9.7)
The volume flow rate of the solvent, L,, may then be calculated by using the
following relationship:
7.48 Lmoi l MWL
where 60 is the conversion factor from minutes to hours, MW/v is the molec-
ular weight of the liquid stream (Ib/lb-mole), pi is the density of the liquid
9-21
-------
stream (lb/ft3), and 7.48 is the factor used to convert cubic feet to gallons.
If the volume change in the liquid stream entering and exiting the absorber
is assumed to be negligible, then £, = L0.
Gas absorber vendors have provided a range for the Li/Gi ratio for acid
gas control from 2 to 20 gpm of solvent per 1000 cfm of waste gas. [12] Even
for pollutants that are highly soluble in a solvent (i.e., HC1 in water), the
adjusted Li/Gi ratio calculated using Equations 9.2 to 9.8 would be much
lower than this range, because these equations do not consider the flow rate
of the solvent required to wet the packing.
Finally, the actual operating line may be represented by a material bal-
ance equation over the gas absorber: [4]
XiLa + YiG3 = X0L, + Y0G, (9.9)
Equation 9.9 may then be solved for X0:
X, = fc£ + x< f9-10'
(fc)
9.3.2 Step 2: Determining Absorption Factor
The absorption factor (AF) value is frequently used to describe the rela-
tionship between the equilibrium line and the liquid-to-gas ratio. For many
pollutant-solvent systems, the most economical value for AF ranges around
1.5 to 2.0.[7] The following equation may be used to calculate AF:[4, 7]
AF = ™t (9.11)
m Gmol,i
where m is the slope of the equilibrium line on a mole fraction basis. The
value of TO may be obtained from available literature on vapor/liquid equilib-
rium data for specific systems. Since the equilibrium curve is typically linear
in the concentration ranges usually encountered in air pollution control, the
slope, m would be constant (or nearly so) for all applicable inlet and out-
let liquid and gas streams. The slope may be calculated from mole fraction
values using the following equation: [4]
__ an Vi fn^'^\
m = - (9.12)
9-22
-------
where y* and y* are the mole fractions of the pollutant in the vapor phase in
equilibrium with the mole fractions of the pollutant entering and exiting the
absorber in the liquid, x, and x0, respectively. The slope of the equilibrium
line in Figure 9.4 is expressed in terms of concentration values A",-, X0, Yf,
and Y*. These values may be converted to x,, i0, y,*, and j/* using the
equations:
(9.13)
(9.14)
(9.15)
(9.16)
where the units for each of these variables are listed in Table 9.1.
The absorption factor will be used to calculate the theoretical number of
transfer units and the theoretical height of a transfer unit. First, however,
the column diameter needs to be determined.
9.3.3 Step 3: Determining Column Diameter
Once stream conditions have been determined, the diameter of the column
may be estimated. The design presented in this section is based on select-
ing a fraction of the gas flow rate at flooding conditions. Alternatively, the
column may be designed for a specific pressure drop (see Section 9.3.6.). Eck-
ert's modification to the generalized correlation for randomly packed towers
based on flooding considerations is used to obtain the superficial gas flow
rate entering the absorber, G^ ,• (lb/sec-ft2), or the gas flow rate per cross-
sectional area based on the Lmon/Gmon ratio calculated in Step 2.[10] The
cross-sectional area (A) of the column and the column diameter (D) can then
be determined from G3tr ,.
Figure 9.5 presents the relationship between G3tr z- and the Lmoi i/Gmoi l
ratio at the tower flood point. The abscissa value (X axis) in the graph is
9-23
-------
-------
expressed as:[10]
ABSCISSA = s (9.17)
\Gmol,
The ordinate value (Y axis) in the graph is expressed as:[10]
0.2
ORDINATE = (9.18)
PLPG9c
where Fp is a packing factor, gc is the gravitational constant (32.2), m is the
viscosity of the solvent (Ib/ft-hr), 2.42 is the factor used to convert Ib/ft-hr to
centipoise, and # is the ratio of the density of the scrubbing liquid to water.
The value of Fp may be obtained from packing vendors (see Appendix 9B,
Table 9.8).
After calculating the ABSCISSA value, a corresponding ORDINATE
value may determined off the flooding curve. The ORDINATE may also
be calculated using the following equation: [10]
ORDINATE = in"1'668"1'085^08 ABSCISSA)-°-297(los ABSCISSA)2
Equation 9.18 may then be rearranged to solve for G3tr i:
G'fr,i =
0.2
The cross-sectional area of the tower (ft2) is calculated as:
Gmo, i MWC
A — moi'1 C
~ Qfinnr* / (
*WvG,fr,if
where / is the flooding factor and 3600 is the conversion factor from hours to
seconds. To prevent flooding, the column is operated at a fraction of G 3tr j .
The value of / typically ranges from 0.60 to 0.75. [7]
The diameter of the column (ft) can be calculated from the cross-sectional
area, by:
D = J-A (9.22)
"
9-25
-------
If a substantial change occurs between inlet and outlet volumes (i.e., moisture
is transferred from the liquid phase to the gas phase), the diameter of the
column will need to be calculated at the top and bottom of the column. The
larger of the two values is then chosen as a conservative number. As a rule
of thumb, the diameter of the column should be at least 15 times the size of
the packing used in the column. If this is not the case, the column diameter
should be recalculated using a smaller diameter packing. [10]
The superficial liquid flow rate entering the absorber, L^ ,• (lb/hr-ft2),
based on the cross-sectional area determined in Equation 9.21 is calculated
from the equation:
For the absorber to operate properly, the liquid flow rate entering the
column must be high enough to effectively wet the packing so mass transfer
between the gas and liquid can occur. The minimum value of L^ ,- that is
required to wet the packing effectively can be calculated using the equation: [7,
13]
a (9.24)
where MWR is defined as the minimum wetting rate (ft2/hr), and a is the
surface area to volume ratio of the packing (ft2/ft3). An MWR value of
0.85 ft2/hr is recommended for ring packings larger than 3 inches and for
structured grid packings. For other packings, an MWR of 1.3 ft2/hr is
recommended. [7, 13] Appendix 9B, Table 9.8 contains values of a for common
packing materials.
If i>sfTii (the value calculated in Equation 9.23) is smaller than (L3tr j)min
(the value calculated in Equation 9.24), there is insufficient liquid flow to wet
the packing using the current design parameters. The value of G3tr ,-, and A
then will need to be recalculated. See Appendix 9C for details.
9.3.4 Step 4: Determining Tower Height and Surface
Area
Tower height is primarily a function of packing depth. The required depth of
packing (Hpacf..) is determined from the theoretical number of overall transfer
9-26
-------
units (Niv) needed to achieve a specific removal efficiency, and the height of
the overall transfer unit (/fjtt):[4]
Hpock = N* Htu (9-25)
The number of overall transfer units may be estimated graphically by step-
ping off stages on the equilibrium-operating line graph from inlet conditions
to outlet conditions, or by the following equation:[4]
IYJK -m*i\ / , x 1 I '
AF
where In is the natural logarithm of the quantity indicated. The equation
is based on several assumptions: 1) Henry's law applies for a dilute gas
mixture; 2) the equilibrium curve is linear from x, to xa\ and 3) the pollutant
concentration in the solvent is dilute enough such that the operating line can
be considered a straight line. [4]
If z, —» 0 (i.e., a negligible amount of pollutant enters the absorber in the
liquid stream) and 1/AF —»• 0 (i.e., the slope of the equilibrium line is very
small and/or the Lmoi/Gmoi ratio is very large), Equation 9.26 simplifies to:
Ntu = In fi (9.27)
There are several methods that may be used to calculate the height of
the overall transfer unit, all based on empirically determined packing con-
stants. One commonly used method involves determining the overall gas
and liquid mass transfer coefficients (Ko, KL). A major difficulty in using
this approach is that values for KG and KL are frequently unavailable for
the specific pollutant-solvent systems of interest. The reader is referred to
the book Random Packing and Packed Tower Design Applications in the
reference section for further details regarding this method.[14]
For this chapter, the method used to calculate the height of the overall
transfer unit is based on estimating the height of the gas and liquid film
transfer units, HL and HO, respectively:[4j
J_
AF
Htu = HG + ~HL (9.28)
9-27
-------
The following correlations may be used to estimate values for HI and
Hc,=
' (3600/C^)'
Ot -, r^v
(9.29)
(9.30)
The quantity fi/pD is the Schmidt number and the variables a, /?, 7, <£,
and 6 are packing constants specific to each packing type. Typical values for
these constants are listed in Appendix 9B, Tables 9.9 and 9.10. The advan-
tage to using this estimation method is that the packing constants may be
applied to any pollutant-solvent system. One packing vendor offers the fol-
lowing modifications to Equations 9.29 and 9.30 for their specific packing:[15]
a
01
(9.31)
-4.2S5
(9.32)
where T is the temperature of the solvent in Kelvin.
After solving for flpodb usmg Equation 9.25, the total height of the column
may be calculated from the following correlation:[16]
Htower = 1-40 Hpack + 1.02 D + 2.81
(9.33)
Equation 9.33 was developed from information reported by gas absorber ven-
dors, and is applicable for column diameters from 2 to 12 feet and packing
depths from 4 to 12 feet. The surface area (S) of the gas absorber can be
calculated using the equation:[16]
+ f
Equation 9.34 assumes the ends of the absorber are flat and circular.
(9-34)
9-28
-------
9.3.5 Step 5: Calculating Column Pressure Drop
Pressure drop in a gas absorber is a function of G-i, ^ and properties of the
packing used. The pressure drop in packed columns generally ranges from
0.5 to 1 inch of HjO per foot of packing. The absorber may be designed
for a specific pressure drop or pressure drop may be estimated using Leva's
correlation: [7, 10]
(i_L'fr,i\ (fr \*
AP = C10U^J!/^. (9.37)
(pL - pG)Pc9c
For a calculated ABSCISSA value, a corresponding ORDINATE value at
each pressure drop can be read off Figure 9.6 or can be calculated from the
following equation:[10]
ORDINATE = exp [k,, + As, (In ABSCISSA) + Jb2(ln ABSCISS A)2 +
A;,(ln ABSCISSA)'' + A. (In ABSCISSA)1] (9.38)
The constants &„, A;(, A2> ^i an(l ^i are shown below for each pressure drop
value.
9-29
-------
in
O
£ ••
C3
ex
~O .
o. .,
4P-1.5
4P-10
iP - 0.50"
• 025-
4P-0.1-
iP-O.OS
lLmn\, \
\°-s
pG|
N
N
Figure 9.6: Generalized Pressure Drop Correlations 110]
9-30
-------
Constants for Each Pressure Drop Correlation
AP
(inches water/
ft packing)
0.05
0.10
0.25
0.50
1.00
1.50
ko
-6.3025
-5.5009
-5.0032
-4.3992
-4.0950
-4.0256
*i
-0.6080
-0.7851
-0.9530
-0.9940
-1.0012
-0.9895
k2
-0.1193
-0.1350
-0.1393
-0.1698
-0.1587
-0.0830
*3
-0.0068
0.0013
0.0126
0.0087
0.0080
0.0324
*4
0.0003
0.0017
0.0033
0.0034
0.0032
0.0053
Equation 9.37 can be solved for (?,&. j.'
G*fr,i =
\
(PL - pc)*tfc(ORDINATE)
(9.39)
The remaining calculations to estimate the column diameter and Latr ^ are the
same as presented in Section 9.3.3, except the flooding factor (/) is not used
in the equations. The flooding factor is not required because an allowable
pressure drop that will not cause flooding is chosen to calculate the diameter
rather than designing the diameter at flooding conditions and then taking a
fraction of that value.
9.4 Estimating Total Capital Investment
This section presents the procedures and data necessary for estimating cap-
ital "costs for vertical packed bed gas absorbers using countercurrent flow
to remove gaseous pollutants from waste gas streams. Equipment costs for
packed bed absorbers are presented in Section 9.4.1, with installation costs
presented in Section 9.4.2.
Total capital investment, TCI, includes equipment cost, EC, for the entire
gas absorber unit, taxes, freight charges, instrumentation, and direct and
indirect installation costs. All costs are presented in third quarter 1991
dollars. The costs presented are study estimates with an expected accuracy
of ± 30 percent. It must be kept in mind that even for a given application.
9-31
-------
design and manufacturing procedures vary from vendor to vendor, so costs
vary. All costs are for new plant installations; no retrofit cost considerations
are included.
9.4.1 Equipment Costs for Packed Towers
Gas absorber vendors were asked to supply cost estimates for a range of
tower dimensions (i.e., height, diameter) to account for the varying needs
of different applications. The equipment for which they were asked to pro-
vide costs consisted of a packed tower absorber made of fiberglass reinforced
plastic (FRP), and to include the following equipment components:
• absorption column shell;
• gas inlet and outlet ports;
• liquid inlet port and outlet port/drain;
• liquid distributor and redistributor;
• two packing support plates;
• mist eliminator;
• internal piping;
• sump space; and
• platforms and ladders.
The cost data the vendors supplied were first adjusted to put them on a
common basis, and then were regressed against the absorber surface area (5).
The equation shown below is a multivariant regression of cost data provided
by six vendors.[16, 12]
Total Tower Cost($) = 115 S - (9.40)
where 5 is the surface area of the absorber, in ft'2.
Figure 9.7 depicts a plot of Equation 9.40. This equation is applicable
for towers with surface areas from 69 to 1507 ft2 constructed of FRP. Costs
9-32
-------
200,000
180,000
160,000
£
a
140,000
I
~ 120.000
100,000
80,000
•5 60,000
er
HI
40,000
20,000
200
400
600 800 1,000
Surface Area of Tower (ft2)
1.200
1,400
1.600
Figure 9.7: Packed Tower Equipment Cost[16]
9-33
-------
for towers made of materials other than FRP may be estimated using the
following equation:
TTCA/ = CF x TTC (9.41)
where TTC;i/ is the total cost of the tower using other materials, and TTC
is the total tower cost as estimated using Equation 9.40. The variable CF
is a cost factor to convert the cost of an FRP gas absorber to an absorber
fabricated from another material. Ranges of cost factors provided by vendors
are listed for the following materials of construction:[12]
304 Stainless steel = 1.10 - 1.75
Polypropylene = 0.80 - 1.10
Polyvinyl chloride = 0.50 - 0.90
Auxiliary costs encompass the cost of all necessary equipment not in-
cluded in the absorption column unit. Auxiliary equipment includes packing
material, instruments and controls, pumps, and fans. Cost ranges for various
types of random packings are presented in Table 9.2. The cost of structured
packings varies over a much wider range. Structured packings made of stain-
less steel range from $45/ft3 to $405/ft3, and those made of polypropylene
range from $65/ft:l to $350/ft3.[17]
Similarly, the cost of instruments and controls varies widely depending
on the complexity required. Gas absorber vendors have provided estimates
ranging from $1,000 to $10,000 per column. A factor of 10 percent of the
tower cost will be used to estimate this cost in this chapter. Design and cost
correlations for fans and pumps will be presented in a chapter on auxiliary
equipment elsewhere in this manual. However, cost data for auxiliaries are
available from the literature (see reference [18], for example).
The total equipment cost (EC) is the sum of the component equipment
costs, which includes tower cost and the auxiliary equipment cost.
EC = TTC -|- Packing Cost + Auxiliary Equipment (9.42)
The purchased equipment cost (PEC) includes the cost of the absorber
with packing and its auxiliaries (EC), instrumentation (0.10 EC), sales tax
(0.03 EC), and freight (0.05 EC). The PEC is calculated from the following
factors, presented in Chapter 2 of this manual and confirmed from the gas
absorber vendor survey conducted during this study:[12, 19]
PEC = (14- 0.10 + 0.03 4- 0.05) EC = 1.18 EC (9.43)
9-34
-------
Table 9.2: Random Packing Costs0
Nominal
Diameter
(inches)
1
1
1
2
2
3.5
3.5
Construction
Material
304 stainless steel
ceramic
polypropylene
ceramic
polypropylene
304 stainless steel
polypropylene
Packing Type
Pall rings, Raschig rings, Bal-
last rings
Raschig rings, Berl saddles
Tri-pack®, Pall rings, Ballast
rings, Flexisaddles
Berl saddles, Raschig rings
Tri-pack®, Lanpac®, Flexir-
ing, Flexisaddle, Tellerette®
Ballast rings
Tri-pack®, Lanpac®, Ballast
rings
Packing cost ($/ff'J)
< 100 ft3
70-109
33-44
14-37
13-32
3-20
30
6-14
> 100 ft:!
65-99
26-36
12-34
10-30
5-19
27
6-12
"Provided by packing vendors.[17]
^Denotes registered trademark.
9-35
-------
9.4.2 Installation Costs
The total capital investment, TCI, is obtained by multiplying the purchased
equipment cost, PEC, by the total installation factor:
TCI = 2.20 PEC (9.44)
The factors which are included in the total installation factor are also listed
in Table 9.3.[19] The factors presented in Table 9.3 were confirmed from the
gas absorber vendor survey.
9.5 Estimating Annual Cost
The total annual cost (TAG) is the sum of the direct and indirect annual
costs.
9.5.1 Direct Annual Costs
Direct annual costs (DC) are those expenditures related to operating the
equipment, such as labor and materials. The suggested factors for each of
these costs are shown in Table 9.4. These factors were taken from Chapter 2
of this manual and were confirmed from the gas absorber vendor survey. The
annual cost for each item is calculated by multiplying the number of units
used annually (i.e., hours, pounds, gallons, kWh) by the associated unit cost.
Operating labor is estimated at 1/2-hour per 8-hour shift. The super-
visory labor cost is estimated at 15 percent of the operating labor cost.
Maintenance labor is estimated at 1/2-hour per 8-hour shift. Maintenance
materials costs are assumed to equal maintenance labor costs.
Solvent costs are dependent on the total liquid throughput, the type of
solvent required, and the fraction of throughput wasted (often referred to as
blow-down). Typically, the fraction of solvent wasted varies from 0.1 percent
to 10 percent of the total solvent throughput.[12] For acid gas systems, the
amount of solvent wasted is determined by the solids content, with bleed
off occurring when solids content reaches 10 to 15 percent to prevent salt
carry-over. [12]
9-36
-------
Table 9.3: Capital Cost Factors for Gas Absorbers[19]
Cost Item Factor
Direct Costs
Purchased equipment costs
Absorber+packing+auxiliary equipment0, EC As estimated, A
Instrumentation6 0.10 A
Sales taxes 0.03 A
Freight 0.05 A
Purchased equipment cost, PEC B = 1.18 A
Direct installation costs
Foundations & supports 0.12 B
Handling & erection 0.40 B
Electrical 0.01 B
Piping 0.30 B
Insulation 0.01 B
Painting 0.01 B
Direct installation costs 0.85 B
Site preparation As required, SP
Buildings As required, Bldg.
Total Direct Costs, DC 1.85 B + SP + Bldg.
Indirect Costs (installation)
Engineering 0.10 B
Construction and field expenses 0.10 B
Contractor fees 0.10 B
Start-up 0.01 B
Performance test 0\01 B
Contingencies 0.03 B
Total Indirect Costs, 1C 0.35 B
Total Capital Investment = DC + 1C 2.20 B + SP + Bldg.
"Includes the initial quantity of packing, as well as items normally not in-
cluded with the unit supplied by vendors, such as ductwork, fan, piping, etc.
^Instrumentation costs cover pH monitor and liquid level indicator in sump.
9-37
-------
Table 9.4: Suggested Annual Cost Factors for Gas Absorber Systems
Cost Item
Factor
Direct Annual Costs, DC
Operating labor"
Operator
Supervisor
Operating materials6
Solvent
Chemicals-
Wastewater disposal
Maintenance"
Labor
Material
Electricity
Fan
Pump
Indirect Annual Costs, 1C
Overhead
Administrative charges
Property tax
.Insurance
Capital recovery0
Total Annual Cost
1/2 hour per shift
15% of operator
Application specific
(throughput/yr) x (waste fraction)
Based on annual consumption
(throughput/yr) x (waste fraction)
1/2 hour per shift
100% of maintenance labor
All electricity equal to:
(consumption rate) x
(hours/yr) x (unit cost)
60% of total labor and material costs
2% of Total Capital Investment
1% of Total Capital Investment
1% of Total Capital Investment
0.1315 x Total Capital Investment
DCT+1C
"These factors were confirmed by vendor contacts.
6If system does not use chemicals (e.g., caustic), this quantity is
equal to annual solvent consumption.
cAssuming a 15 year life at 10%. See Chapter 2.
9-38
-------
The total annual cost of solvent (Ca) is given by:
n . / annual \ / , , \
0mml A- I / solvent \ tn AK\
- operating . , (9.45)
hr I , I \ unit cost / v '
\ hours / v '
where WF is the waste (make-up) fraction, and the solvent unit cost is ex-
pressed in terms of $/gal.
The cost of chemical replacement (Cc) is based on the annual consumption
of the chemical and can be calculated by:
/Ibs chemical used\ [ ann" \ ( chemical \ ,- ...
Cc = operating . . (9.46)
I hr / I i. / V umt cost /
v ' \ hours / v '
where the chemical unit cost is in terms of $/lb.
Solvent disposal (€„,„,) costs vary depending on geographic location, type
of waste disposed of, and availability of on-site treatment. Solvent disposal
costs are calculated by:
cn . / annual \ / , , \
r> T M/p60"1"1 I ..- \ I solvent \ , .
Cww = LiWF—~ operating ,. . I (9.47)
hr I , / V disposal cost / v '
\ hours /
where the solvent disposal costs are in terms of $/gal of waste solvent.
The electricity costs associated with operating a gas absorber derive from
fan requirements to overcome the pressure drop in the column, ductwork, and
other parts of the control system, and pump requirements to recirculate the
solvent. The energy required for the fan can be calculated using Equation
9.48:
1.17 x 10-' d AP
Energy/ttn = (9.48)
where Energy (in kilowatts) refers to the energy needed to move a given
volumetric flow rate of air (acfm), G, is the waste gas flow rate entering
the absorber, A.P is the total pressure drop through the system (inches of
H20) and e is the combined fan-motor efficiency. Values for e typically range
from 0.4 to 0.7. Likewise, the electricity required by a recycle pump can be
calculated using Equation 9.49:
(0.746) (2.52 x 10-') I.(pressure)
Energypttmp = i ^ e ' / „ l (9.49)
- 9-39-
-------
/
C, = Energy/,, + w operating (9.50)
v '
where 0.746 is the factor used to convert horsepower to kW, pressure is
expressed in feet of water , and e is the* combined pump-motor efficiency.
The cost of electricity (Ce) is then given by:
/ annual
operatii
\ hours
where cost of electricity is expressed in units of $/KW-hr.
9.5.2 Indirect Annual Costs
Indirect annual costs (1C) include overhead, taxes, insurance, general and
administrative (G&A), and capital recovery costs. The suggested factors for
each of these items also appear in Table 9.4. Overhead is assumed to be
equal to 60 percent of the sum of operating, supervisory, and maintenance
labor, and maintenance materials. Overhead cost is discussed in Chapter 2
of this manual.
The system capital recovery cost, CRC, is based on an estimated 15-year
equipment life. (See Chapter 2 of this manual for a discussion of the capital
recovery cost.) For a 15-year life and an interest rate of 10 percent, the
capital recovery factor is 0.1315. The system capital recovery cost is then
estimated by:
CRC = 0.1315 TCI (9.51)
G&A costs, property tax, and insurance are factored from total capital
investment, typically at 2 percent, 1 percent, and 1 percent, respectively.
9.5.3 Total Annual Cost
Total annual cost (TAG) is calculated by adding the direct annual costs and
the indirect annual costs.
TAG = DC 4- 1C (9.52)
. 9-40
-------
9.6 Example Problem
The example problem presented in this section shows how to apply the gas
absorber sizing and costing procedures presented in this chapter to control
a waste gas stream consisting of HC1 and air. This example problem will
use the same outlet stream parameters presented in the thermal incinerator
example problem found in Chapter 3 of this manual. The waste gas stream
entering the gas absorber is assumed to be saturated with moisture due to
being cooled in the quench chamber. The concentration of HC1 has also been
adjusted to account for the change in volume.
9.6.1 Required Information for Design
The first step in the design procedure is to specify the conditions of the gas
stream to be controlled and the desired pollutant removal efficiency. Gas and
liquid stream parameters for this example problem are listed in Table 9.5.
The quantity of HC1 can be written in terms of Ib-moles of HC1 per Ib-moles
of pollutant-free-gas (¥<) using the following calculation:
0.001871
•*!
1 - 0.001871
Ib-moles HC1
= 0.00187
Ib-mole pollutant free gas
The solvent, a dilute aqueous solution of caustic, is assumed to have the same
physical properties as water.
9.6.2 Step 1: Determine Gas and Liquid Stream Prop-
erties
Once the properties of the waste gas stream entering the absorber are known,
the properties of the waste gas stream exiting the absorber and the liquid
streams entering and exiting the absorber need to be determined. The pol-
lutant concentration in the entering liquid (Xt) is assumed to be zero. The
pollutant concentration in the exiting gas stream (}„) is .calculated using
9-41 -
-------
Table 9.5: Example Problem Data
Parameters
Values
Stream Properties
Waste Gas Flow Rate Entering Absorber
Temperature of Waste Gas Stream
Pollutant in Waste Gas
Concentration of HC1 Entering Absorber in Waste Gas
Pollutant Removal Efficiency
Solvent
Density of Waste Gas"
Density of Liquid [7]
Molecular Weight of Waste Gas"
Molecular Weight of Liquid [7]
Viscosity of Waste Gas"
Viscosity of Liquid [7]
Minimum Wetting Rate[7]
Diffusivity of HC1 in Air
Diffusivity of HC1 in Water
Packing fype
Packing factor: Fp
Packing constant: a
Packing constant: (3
Packing constant: 7
Packing constant:
Packing constant: b
Surface Area to Volume Ratio
Pollutant Properties*
Packing Properties'
21,377 scfm (22,288 acfm)
100°F
HC1
1871 ppmv
99% (molar basis)
Water with caustic in solution
0.0709 lb/ft3
62.4 lb/ft3
29 Ib/lb-mole
18 Ib/lb-mole
0.044 Ib/ft-hr
2.16 Ib/ft-hr
1.3 ft2/hr
0.725 ft2/hr
1.02 x 10~4 ft2/hr
2-inch ceramic Raschig rings
65
3.82
0.41
0.45
0.0125
0.22
28
"Reference [7], at 100°F.
* Appendix 9A.
"Appendix 9B.
9-42
-------
Equation 9.1 and a removal efficiency of 99 percent.
(99 \
1 - —J = 0.0000187
The liquid flow rate entering the column is calculated from the //.,/(?,
ratio using Equation 9.2. Since Yi, Y0, and Xi are defined, the remaining un-
known, X*, is determined by consulting the equilibrium curve. A plot of the
equilibrium curve-operating line graph for an HCl-water system is presented
in Figure 9.8. The value of X* is taken at the point on the equilibrium curve
where Yi intersects the curve. The value of Y[ intersects the equilibrium curve
at an X value of 0.16.
The operating line is constructed by connecting two points: (Xi, Y0) and
(X*, Yi). The slope of the operating line intersecting the equilibrium curve,
(L,/G,)min,iS:
'0.00187 -0.0000187\
0.16-0 / ~
The actual Ly/G, ratio is calculated using Equation 9.3. For this example,
an adjustment factor of 1.5 will be used.
= (0.0116)(1.5) = 0.0174
zt
The value of Ga may be calculated using Equation 9.4.
(60 min/hr)(0.0709 Ib/ft3)(22,288 acfm)
3 ~ (29 lb/lb-mole)(l + 0.00187)
The flow rat'e of the solvent entering the absorber may then be calculated
using Equation 9.5.
L, = 0.0174 3, 263 = 56.8
\ hr
The values of Gmoi z- and Lmon are calculated using Equations 9.6 and 9.7,
respectively:
Gmoll = (3,263^^) (1 + 0.00187) = 3,269
V hr y
Ib-moles
hr
9-43
-------
0.002
0.0002 -
T
0.06 0.08 0.1 0.12
Ib-moies HCi/lb-moles Solvent
0.14
T
0.16
0.18
Figure 9.8: Equilibrium Curve-Operating Line for HC1-Water System[7]
9-44
-------
r _ ItM s lb'moles'\ n _L m - « « lb'mc
^mo^t = I 56-8 j^ 1 (1 + 0) = 56.8 —
-moles
The pollutant concentration exiting the absorber in the liquid is calcu-
lated using Equation 9.10.
_ 0.00187 - 0.0000187 0.106 Ib-moles HC1
° ~ 0.0174 ~ Ib-mole solvent
9.6.3 Step 2: Calculate Absorption Factor
The absorption factor is calculated from the slope of the equilibrium line and
the Lmoiii/Gmoiii ratio. The slope of the equilibrium curve is based on the
mole fractions of z,-, z0, y,*, and y0*, which are calculated from A",-, X0, Yf,
and Y* from Figure 9.8. From Figure 9.8, the value of Y* in equilibrium with
the X0 value of 0.106 is 0.0001. The values of Yi* and Xt are 0. The mole
fraction values are calculated from the concentration values using Equations
9.13 through 9.16.
The slope of the equilibrium line from x, to x0 is calculated from Equation
9 12-
0-0001 ~ 0 n nn n
= °-»0104
Since HC1 is very soluble in water, the slope of the equilibrium curve is very
small. The absorption factor is calculated from Equation 9.11.
0.0174 _
9.6.4 Step 3: Estimate Colunjn Diameter
Once the inlet and outlet stream conditions are determined, the diameter of
the gas absorber may be calculated using the modified generalized pressure
9-45
-------
drop correlation presented in Figure 9.5. The abscissa value from the graph
is calculated from Equation 9.17:
ABSCISSA = 0.0174 W = 0-000364
Since this value is outside the range of Figure 9.5, the smallest value (0.01)
will be used as a default value. The ordinate is calculated from Equation
9.19.
ORDINATE =
= 0.207
The superficial gas flow rate, Gsfr ^ is calculated using Equation 9.20. For
this example calculation, 2-inch ceramic Raschig rings are selected as the
packing. The packing factors for Raschig rings are listed in Appendix 9B.
G'fr,i = \
(0.207)(62.4 Ib/ft3)(0.0709 Ib/ft3)(32.2 ft/sec2)
(65)(1)(0.893)°-2
= 0.681 lb/sec-ft2
Once G.f-.- is determined, the cross-sectional area of the column is cal-
J I
culated using Equation 9.21.
(3,263 lb-mol/hr)(29 Ib/lb-mol) 2
(3600 sec/hr)(0.681 Ib/sec-ft2)(0.7) "
The superficial liquid flow rate is determined using Equation 9.23.
(56.8 lb-mol/hr)(18 Ib/lb-mol) „...,, ,2
Lstr ,• = 2 = lo.b Ib/nr-it
At this point, it is necessary to determine if the liquid flow rate is sufficient
to wet the packed bed. The minimum value of L3tr , is calculated using
Equation 9.24. The packing constant (a) is found in Appendix 9B.
(L*fr,i)min = (1-3 ft2/hr)(62.4 Ib/ft3)(28 ft2/ft3) = 2,271 lb/hr-ft2
The L3fr j value calculated using the L/G ratio is far below the minimum
value needed to wet the packed bed. Therefore, the new value, (£,~.
9-46
-------
will be used to determine the diameter of the absorber. The calculations
for this revised diameter are shown in Appendix 9C. Appendix 9C shows
that the cross-sectional area of the column is calculated to be 60 ft2, Lmo\ t-
is 7572, and G,^ is 0.627 lb/sec-ft2. The diameter of the column is then
calculated using Equation 9.22:
V 7T
The value of X0 is then:
0.00187 - 0.0000187
7^72
3,26?
= °-0008
Expressed in terms of mole fraction:
• °-0008
The value of y0 in equilibrium with x0 cannot be estimated accurately. How-
ever, the value will approach zero, and the value of AF will be extremely
large:
7,572
9.6.5 Step 4: Calculate Column Surface Area
Since x, = 0 and AF is large, Equation 9.26 will be used to calculate the
number of transfer units:
= in (Q
°-00187^=4.61
0000187/
The height of a transfer unit is calculated from AF, HL, and HG- The
values of HG and HI are calculated from Equations 9.29 and 9.30:
3.82[(3,600)(0.7)(Q.627)]°-11 / Q.044
G 2,271°-15 V(0.725)(0.0709)
,271V'"22 / 2l6~
(O.a00.102)(62.4)
9-47
-------
The height of the transfer unit is calculated using Equation 9.28:
Hiu = (2.24 ft) + —(1.06 ft) = 2.24 ft
oo
The depth of packing is calculated from Equation 9.25.
Hp*ck = Niu x Htu = (4.61)(2.24 ft) = 10.3 ft
The total height of the column is calculated from Equation 9.33:
Htower = 1.40(10.3) + 1.02(8.74) + 2.81 = 26.1 ft
The surface area of the column is calculated using Equation 9.34:
5 = (3.14)(8.74)(26.1 + 8.74/2) = 836 ft2
9.6.6 Step 5: Calculate Pressure Drop
The pressure drop through the column is calculated using Equation 9.35.
AP . (0.24) 10*^^ m-Wgr)!'
= 0.83 inches water/foot packing.
The total pressure drop (through 10.3 feet of packing) equals 8.55 inches of
water.
9.6.7 Equipment Costs
Once the system sizing parameters have been determined, the equipment
costs can be calculated. For the purpose of this example, a gas absorber
constructed of FRP will be costed using Equation 9.40.
TTC ($) = 115(836) = $96,140
•The cost of 2-inch ceramic Raschig rings can be estimated from packing
cost ranges presented in Section 9.5. The volume of packing required is
calculated as:
9-48
-------
Volume of packing = (60 ft2)(10.3 ft) = 618 ft3
Using the average of the cost range for 2-inch ceramic packings, the total
cost of packing is:
Packing cost = ($20/ft3)(618 ft:') = $12,360
For this example problem, the cost of a pump will be estimated using
vendor quotes. First, the flow rate of solvent must be converted into units of
gallons per minute:
= 272 gpm
The average price for a FRF pump of this size is $16/gpm at a pressure of 60
ft water, based on the vendor survey. [12] Therefore, the cost of the recycle
pump is estimated as:
= (272 gpm)($16/gpm) = $4,350
For this example, the cost for a fan (FRP, backwardly-inclined centrifugal)
can be calculated using the following equation:[18]
= 57,9dl-3S
where d is the impeller (wheel) diameter of the fan expressed in inches. For
this gas flow rate and pressure drop, an impeller diameter of 33 inches is
needed. At this diameter, the cost of the fan is:
The cost of a fan motor (three-phase, carbon steel) with V-belt drive,
belt guard, and motor starter can be computed as follows: [18]
CmoioT = 104 (hp)0'82'
As will be shown in Section 9.6.8, the electricity consumption of the fan is
32.0 kW. Converting to horsepower, we obtain a motor size of 42.6 hp. The
cost of the fan motor is:
Cmotor = 104(42.6)(um =52,260
9-49
-------
The total auxiliary equipment cost is:
$4,350 + $7,210 + $2,260 = $13,820
The total equipment cost is the sum of the absorber cost, the packing
cost, and the auxiliary equipment cost:
EC = 96,140 + 12,360 + 13,820 = $122,320
The purchased equipment cost including instrumentation, controls, taxes,
and freight is estimated using Equation 9.43:
PEC = 1.18(122,320) = $144,340
The total capital investment is calculated using Equation 9.44:
TCI = 2.20(144,340) = $317,550 » $318,000
9.6.8 Total Annual Cost
Table 9.6 summarizes the estimated annual costs using the suggested factors
and unit costs for the example problem.
Direct annual costs for gas absorber systems include labor, materials,
utilities, and wastewater disposal. Labor costs are based on 8,000 hr/year of
operation. Supervisory labor is computed at 15 percent of operating labor,
and operating and maintenance labor are each based on 1/2 hr per 8-hr shift.
The electricity required to run the fan is calculated using Equation 9.48
and assuming a combined fan-motor efficiency of 70 percent:
, (U7xl.-Ha.2g8X8.55) =
The energy required for the liquid pump is calculated using Equation 9.49.
The capital cost of the pump was calculated using data supplied by vendors
9-50
-------
Cost Item
Table 9.6: Annual Costs for Packed Tower Absorber
Example Problem
Calculations
Cost
Direct Annual Costs, DC
Operating Labor
Operator
Supervisor
Operating materials
Solvent (water)
Caustic Replacement
Wastewater disposal
Maintenance
Labor
Material
Electricity
Total DC
Indirect Annual Costs, 1C
Overhead
Administrative charges
Property tax
Insurance
Capital recovery"
Total 1C
0.5 hr v shift v 8,000 hr $15.64
UuTT x FhT x yr x ~hT~
15% of operator = 0.15 x 7,820
KDm x
gpm x
1
07T6
Ib-
Hr
6°.min x 8'000 hr x SO-20 ,
hr x yr x 10QO gal
3.06 Ib-mole 62 Ib 8,000 hr ton
Ib-mole x yr x 2000 IU' x
$300
TorT
,.,.„.„
shift ., 8,000 hr $17.21
shift " FhF x yr x hr
100% of maintenance labor
36.4 kW x yr
60% of total labor and maintenance material:
= 0.6(7,820 + 1,170 + 8,610 + 8,610)
2% of Total Capital Investment = 0.02(8317,550)
1% of Total Capital Investment = 0.01(8317,550)
1% of Total Capital Investment = 0.01(8317,550)
0.1315 x $317,550
Total Annual Cost (rounded)
$7,820
1,170
690
299,560
13,060
8,610
8,610
13,420
$352,940
* 15,730
6,350
3,180
3,180
41,760
$70,200
8423,000
"The capital recovery cost factor, CRF, is a function of the absorber equipment life and
the opportunity cost of the capital (i.e., interest rate). For this example, assume a 15-year
equipment life and a 10% interest rate.
9-51
-------
for a pump operating at a pressure of 60 feet of water. Assuming a pressure
of 60 ft of water and a combined pump- motor efficiency of 70 percent:
- (0^46) (2.52 x 10^) (272)(60)(1) _
The total energy required to operate the auxiliary equipment is approxi-
mately 36.4 kW. The cost of electricity, Ce, is calculated using Equation 9.50
and with the cost per kWh shown in Table 9.6.
Cc = (36.4 kW)(8,000 h/yr)($0.0461/kWh) = $13,420/yr
The costs of solvent (water), wastewater disposal, and caustic are all
dependent on the total system throughput and the fraction of solvent dis-
charged as waste. A certain amount of solvent will be wasted and replaced
by a fresh solution of water and caustic in order to maintain the system's
pH and solids content at acceptable levels. Based on the vendor survey, a
maximum solids content of 10 percent by weight will be the design basis for
this example problem. [12] The following calculations illustrate the procedure
used to calculate how much water and caustic are needed, and how much
solvent must be bled off to maintain system operability.
From previous calculations, l>mo\i = 7,572 Ib-moles/hr. The mass flow
rate is calculated as:
is * = 136,300 -
,
hr / \ lb-mole/ hr
With Gmoi i at 3,263 Ib-moles/hr, the mass flow rate of the gas stream is
calculated as:
= 3, 263 29 -_ = 94,800
The amount of HC1 in the gas stream is calculated on a molar basis as follows:
C - ft Pfi?lb"mole>\ (v. 371 ppmv ^ fl10lb-molHCl
Gmol,HCl - (3,263 -^-J (1871 ^^J = 6.12 — - -
On a mass basis:
_ / fi12lb-mole HC1W Ib \ Ib HC1
Gma3S,HCl ~ (6.12 - - - j (36.5 -— = 223.4
9-52
-------
For this example problem, the caustic is assumed to be Na20, with one
mole of caustic required for neutralizing 2 moles of HCL. Therefore, 3.06
Ib-moles/hr of caustic are required.
The unit cost of a 76 percent solution of Na2<3 is given in Table 9.6. The
annual cost is calculated from:
C -
Gc -
lb W8,OOOhrW ton \f 1 \/$30
flowrate \^ ' hr / \0.1 lb NaCl/ \8.34 lb ww/ \60 miny
= 7.16 gpm
where 8.34 is the density of the wastewater.
The cost of wastewater disposal is:1
hr\ / $3.80 \
Cm = (7,6 gpm) 3,000
The cost of solvent (water) is:
'Because the wastewater stream contains only NaCl, it probably will not require pre-
treatment before discharge to a municipal wastewater treatment facility. Therefore, the
wastewater disposal unit cost shown here is just a sewer usage rate. This unit- cost
($3.80/1000 gal) is the average of the rates charged by the seven largest municipalities
in North Carolina.[20] These rates range from approximately S2 to $6/1000 gal. This wide
range is indicative of the major differences among sewer rates throughout the country.
9-53
-------
Indirect annual costs include overhead, administrative charges, property
tax, insurance, and capital recovery. Total annual cost is estimated using
Equation 9.52. For this example case, the total annual cost is estimated to
be $423,000 per year (Table 9.6).
9.7 Example Problem #2
In this example problem the diameter of a gas absorber will be estimated
by defining a pressure drop. A pressure drop of 1 inch of water per foot
of packing will be used in this example calculation. Equation 9.38 will be
used to calculate the ordinate value relating to an abscissa value. If the
^mol i/^mol i rati° ^s known, the abscissa can be calculated directly. The
ordinate value is then:
ORDINATE = exp [-4.0950 - 1.0012 ln(0.0496) - 0.1587(ln 0.0496)2 +
0.0080(ln 0.0496)3 + 0.0032(ln 0.0496)']
= 0.084
The value of G3fr is calculated using Equation 9.39
G'fr,i =
\
(62.4 - 0.0709)(0.0709)(32.2)(0.084)
65(0.893)°
.1
= 0.43 Ib/ft2-sec
The remaining calculations are the same as in Section 9.3.4, except the flood-
ing factor is not used in the equations.
9.8 Acknowledgements
The authors gratefully acknowledge the following companies for contributing
data to this chapter:
• Air Plastics, Inc. (Cincinnati, OH)
• Airpol, Inc. (Teterboro, _NJ)
9-54 - '
-------
• Anderson 2000, Inc. (Peachtree City, GA)
• Calvert Environmental (San Diego, CA)
• Ceilcote Air Pollution Control (Berea, OH)
• Croll-Reynolds Company, Inc. (Westfield, NJ)
• Ecolotreat Process Equipment (Toledo, OH)
• Glitsch, Inc. (Dallas, TX)
• Interel Corporation (Englewood, CO)
• Jaeger Products, Inc. (Spring, TX)
• Koch Engineering Co., Inc. (Wichita, KS)
• Lantec Products, Inc. (Agoura Hills, CA)
• Midwest Air Products Co., Inc. (Owosso, MI)
• Monroe Environmental Corp., (Monroe, MI)
• Norton Chemical Process Products (Akron, OH)
9-55
-------
-------
Appendix 9A
Properties of Pollutants
9-56
-------
-------
Table 9.7: Physical Properties of Common Pollutants0
Pollutant
Ammonia
Methanol
Ethyl Alcohol
Propyl Alcohol
Butyl Alcohol
Acetic Acid
Hydrogen Chloride
Hydrogen Bromide
Hydrogen Fluoride
Molecular
Weight
( lb )
VlbTmole;
17
32
46
60
74
60
36
36
20
Diffusivity in
Air
at 25° C
(cm2/sec)
0.236
0.159
0.119
0.100
0.09
0.133
0.187
0.129
0.753
Diffusivity in
Water
at 20° C
(cm2/sec)xl05
1.76
1.28
1.00
0.87
0.77
0.88
2.64
1.93
3.33
'Diffusivity data taken from Reference [7, 21].
9-57
-------
-------
Appendix 9B
Packing Characteristics
-9-58
-------
-------
Table 9.8: Packing Factors for Various Packings[3, 7, 10, 13]
Packing
Type
Raschig rings
Raschig rings
Pall rings
Pall rings
Berl saddles
Intalox saddles
Tri- Packs®
Construction
Material
ceramic
metal
metal
polypropylene
ceramic
ceramic
plastic
Nominal
Diameter
(inches)
1/2
5/8
3/4
1
1 1/2
2
3
1/2
5/8
3/4
1
1 1/2
2
3
5/8
1
1 1/2
2
3 1/2
5/8
1
1 1/2
2
1/2
3/4
1
1 1/2
2
1/2
3/4
1
1 1/2
2
3
2
3 1/2
F?
640
380
255
160
95
65
37
410
290
230
137
83
57
32
70
48
28
20
16
97
52
32
25
240
170
110
65
45
200
145
98
52
40
22
16
12
a
111
100
80
58
38
28
118
72
57
41
31
21
131
66
48
36
110
63
39
31
142
82
76
44
32
190
102
78
60
36
48
38
9-59
-------
Table 9.9: Packing Constants Used to Estimate J5TC[1, 3, -7, 13]
Packing
Type
Raschig Rings
Berl Saddles
Partition Rings
LanPac®
Tri-Packs®
Size
(inches)
3/8
1
1
1 1/2
1 1/2
2
1/2
1/2
1
1 1/2
3
2.3
2
3 1/2
Packing Constants
a
2.32
7.00
6.41
1.73
2;58
3.82
32.4
0.81
1.97
5.05
640.
7.6
1.4
1.7
ft
0.45
0.39
0.32
0.38
0.38
0.41
0.30
0.30
0.36
0.32
0.58
0.33
0.33
0.33
7
0.47
0.58
0.51
0.66
0.40
0.45
0.74
0.24
0.40
0.45
1.06
-0.48
0.40
0.45
Applicable Range"
<**
200-500
200-800
200-600
200-700
200-700
200-800
200-700
200-700
200-800
200-1,000
150-900
400-3,000
100-900
100-2,000
V
500-1,500
400-500
500-4,500
500-1,500
1,500-4,500
500-4,500
500-1,500
1,500-4,500
400-4,500
400-4,500
3,000-10,000
500-8,000
500-10,000-
500-10,000
'Units of lb/hr-ft2
9-60
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Table 9.10: Packing Constants Used to Estimate HL[1, 3, 13]
Packing
Type
Raschig Rings
Berl Saddles
Partition Rings
LanPac®
Tri-packs®
Size
(inches)
3/8
1
1 1/2
21/2
2
1/2
1
1 1/2
3
2.3
3.5
2
3 1/2
Packing Constants
^
0.00182
0.00357
0.0100
0.0111
0.0125
0.00666
0.00588
0.00625
0.0625
0.0039
0:0042
0.0031
0.0040
6
0.46
0.35
0.22
0.22
0.22
0.28
0.28
0.28
0.09
0.33
0.33
0.33
0.33
Applicable Range
Tn
L3fr
400-15,000
400-15,000
400-15,000
400-15,000
400-15,000
400-15,000
400-15,000
400-15,000
3,000-14,000
500-8,000
500-8,000
500-10,000
500-10,000
'Units of lb/hr-ft2
9-61
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Table 9.11: Packing Constants Used to Estimate Pressure Drop[l, 7, 13]
Packing
Type
Raschig rings
Raschig rings
Pall rings
Berl saddles
Intalox saddles
Construction
Material
ceramic
metal
metal
ceramic
ceramic
Nominal
Diameter
(inches)
1/2
3/4
1
1 1/4
1 1/2
2
5/8
1
1 1/2
2
5/8
1
1 1/2
2
1/2
3/4
1
1 1/2
1/2
3/4
1
1 1/2
c j
3.1
1.34
0.97
0.57
0.39
0.24
1.2
0.42
0.29
0.23
0.43
0.15
0.08
0.06
1.2
0.62
0.39
0.21
0.82
0.28
0.31
0.14
0.41
0.26
0.25
0.23
0.23
0.17
0.28
0.21
0.20
0.135
0.17
0.16
0.15
0.12
0.21
0.17
0.17
0.13
0.20
0.16
0.16
0.14
9-62
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Appendix 9C
Minimum Wetting Rate
Analysis
As explained in the design procedures, the liquid flow rate entering the col-
umn must be high enough to effectively wet the packing. If the liquid flow
rate, as determined theoretically in Equation 9.23, is lower than the flow rate
dictated by the minimum wetting rate, calculated in Equation 9.24, then the
packing will not be wetted sufficiently to ensure mass transfer between the
gas and liquid phases. The minimum liquid flow rate should then be used as
a default value. The superficial gas flow rate, G^ ^ and cross-sectional area
of the column must then be recalculated to account for the increased liquid
flow rate. The approach necessary to recalculate these variables is explained
in Section 9C.1 of this Appendix. The calculation of these variables using
the results from Example Problem #1 are presented in Section 9C.2 of this
Appendix.
9C.1 Overview of the Approach
1. The value of Lmon must be recalculated from the value of (-£5A.J
using the equation:
r _ v <•>•>•> mm
moi'1 ~ MWL
9-63
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The value of A (the cross-sectional area of the absorber column) is the
only unknown in the equation.
2. The ABSCISSA value is calculated in terms of A by substituting the
new Lmon into Equation 9.17.
3. The value of G3fr ^ is recalculated by rearranging Equation 9.21, with
A as the only unknown.
4. The ORDINATE value is calculated in terms of A from the new G3fr;i
using Equation 9.18.
5. An iterative process is used to determine A, ABSCISSA, and ORDI-
NATE. Values of A are chosen and the ABSCISSA and ORDINATE
values are calculated. The ORDINATE value corresponding to the AB-
SCISSA value is determined from Figure 9.5 (or Equation 9.19), and
this value is compared to the ORDINATE value calculated using Equa-
tion 9.18. This process is continued until both ORDINATE values are
equal.
9C.2 Example Problem Calculation
Step 1: The first step is to recalculate the liquid flow rate. The liquid molar
flow rate may be calculated using Equation 9.23.
Lmol>i = (2,271 lb/hr-ft2) (-^ ^
= (126.2 lb-mole/hr-ft2)A
Step 2: The abscissa value from Figure 9.5, and presented in Equation 9.17, is
calculated as:
(126.2 lb-mole/hr-ft2)^ /18
3,263 Ib-mole/hr \29~y V 62.4
= 8.09 x 10~4A (9.53)
Step 3: The value of Gstr j is then recalculated in terms of the cross-sectional
area of the column.
_ (3,263 Ib-mole/hr)(29 Ib/lb-mole) _ 37.6
sfr ~ (3600 sec/hr)(0.7).4= ~A~
9-64
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Step 4: The ordinate value from Figure 9.5, and presented in Equation 9.18, is
calculated as:
ORDINATE =
(62.4)(0.0709)(32.2)
- f <••«>
Step 5: At this point the simplest solution is an iterative approach. Choose
a value for A, calculate the ABSCISSA value using Equation 9.53,
and find the corresponding ORDINATE value off the flooding curve in
Figure 9.5 (or use Equation 9.19 to calculate the ORDINATE value).
Compare the calculated ORDINATE value from Equation 9.54 to the
value obtained from the graph or from Equation 9.19. By continuing
this process until the ORDINATE values converge the value of A is
determined to be 60 ft2. The following table illustrates the intermediate
steps in the calculational process.
Assumed
Value
of A
65
62
60
ABSCISSA
Calculated
From Eqn. 9.53
0.0526
0.0502
0.0485
ORDINATE
Calculated
From Eqn. 9.19
0.1714
0.1740
0.1757
ORDINATE
Calculated
From Eqn. 9.54
0.1493
0.1642
0.1752
The value of Gstr is then:
Gsfr=^ = 0.627 Ib/sec-ft2
The liquid molar flow rate is:
Lmol,i = (126.2)(60) = 7,572 Ib-mole/hr
The diameter and height of the column using the results of this calculation
are presented in Example Problem ^1.
9-65
-------
-------
References
[1] Control Technologies for Hazardous Air Pollutants, Office of Research
and Development, U.S. Environmental Protection Agency, Research Tri-
angle Park, North Carolina, Publication No. EPA 625/6-91-014.
[2] Mclnnes, R., K. Jameson, and D. Austin, "Scrubbing Toxic Inorganics",
Chemical Engineering, September 1990, pp. 116-121.
[3] Letter from Jose L. Bravo of Jaeger Products, Inc., to William M.
Vatavuk, U.S. Environmental Protection Agency, June 8, 1992.
[4] Treybal, Robert E., Mass Transfer Operations (Third edition), McGraw-
Hill Book Company, New York, 1980.
[5] Letter from Jack D. Brady of Anderson 2000, Inc., to William M.
Vatavuk, U.S. Environmental Protection Agency, June 9, 1992.
[6] Letter from S. Raymond Woll of Air Products, Inc., to William M.
Vatavuk, U.S. Environmental Protection Agency, June 25, 1992.
[7] Perry, R.H. and C.H. Chilton, Eds., Chemical Engineers' Handbook
(Sixth edition), McGraw-Hill Book Company, New York, 1984.
[8] Crowe, Charles R., and D. Cooper, "Brick/Membrane Linings Pass the
Acid Test", Chemical Engineering, July 1988, pp. 83-86.
[9] Harrison, Mark E., and John J. France, "Distillation Column Trou-
bleshooting, Part 2: Packed Columns", Chemical Engineering, April
1989, pp. 121-128.
[10] Coker, A.K., "Understanding the Basics of Packed-Column Design'',
Chemical Engineering Progress, November 1991, -pp. 93-99.
[11] Telephone conversation between Roy Oommen, Radian Corporation and
Gerald Nealon, Norton Process Equipment, April 4, 1992.
* 9-66
-------
[12] Gas absorber questionnaire responses from nine gas absorber vendors to
Radian Corporation, August-December, 1991.
[13] Buonicore, A. J., and L. Theodore, Industrial Control Equipment for
Gaseous Pollutants, Volume I, CRC Press, Inc., Cleveland, Ohio, 1975.
[14] Strigle, Ralph F., Random Packings and Packed Towers, Design Appli-
cations, Gulf Publishing Company, Houston, Texas, 1987.
[15] Questionnaire response from K. C. Lang of Lantec Products, Inc. to
R.V. Oommen, Radian Corporation, August 23, 1991.
[16] Memorandum from Vatavuk, W.M. of U.S. Environmental Protection
Agency to Martha Smith, U.S. EPA, March 27, 1992.
[17] Packing vendor questionnaire responses from seven packing vendors to
Radian Corporation, August, 1991-January, 1992.
[18] Vatavuk, W.M., "Pricing Equipment for Air-Pollution Control", Chem-
ical Engineering, May 1990, pp. 126-130.
[19] Vatavuk, W.M., and R.B. Neveril, "Estimating Costs of Pollution Con-
trol Systems, Part II: Factors for Estimating Capital and Operating
Costs", Chemical Engineering, November 3, 1980, pp. 157-162.
[20] Telephone conversation between William M. Vatavuk, U.S. Environmen-
tal Protection Agency, and Cindy Kling, City of Raleigh, N.C., July 16,
1992.
[21] "Air Pollution Engineering Manual" (AP-40), (Second Edition), Daniel-
son, John A., Los Angeles County Air Pollution Control District, CA,
May 1973.
9-67
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