United States       Office of Research and    EPA/625/R-97/002
          Environmental Protection   Development       December 1997
          Agency         Washington DC 20460

          Technology Transfer
x>EPA    Capsule Report
          Sources and Air Emission
          Control Technologies at
          Waste Management
          Facilities

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Technology Transfer	EPA/625/R-97/002	
Capsule Report

Sources and Air Emission
Control Technologies at Waste
Management Facilities
 December 1997
      U.S. Environmental Protection Agency
      Off ice of Research and Development
         Cincinnati, Ohio 45268
                             Printed on Recycled Paper

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                           Disclaimer
The information in this capsule report has been funded by the U.S. Environmental
Protection Agency and the U.S. Department of Energy. It has been subjected to
the EPA's peer and  administrative  review, and it has been approved for
publication as an  EPA document. Mention of trade  names  or commercial
products does not constitute endorsement or recommendation for use by EPA.

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                             Foreword
The U.S. Environmental  Protection Agency is charged  by Congress with
protecting the Nation's land, air, and water resources. Under a mandate of
national environmental  laws, the Agency strives to  formulate and implement
actions leading toacompatible balance  between human activities and the ability
of natural systems to support  and  nurture life. To meet this mandate, EPA's
research program is providing data andtechnical support for solving environmental
problems today and building a science knowledge base necessary to  manage
our ecological resources wisely,  understand  how pollutants affect our health,
and prevent or reduce environmental risks in the future.

       The National Risk Management Research Laboratory is the Agency's
center for  investigation of technological and management approaches for
reducing risks from threats to human health and  the environment. The  focus of
the Laboratory's research program is on methods for the prevention and control
of pollution to air,  land, water and subsurface resources;  protection of water
quality in public water systems; remediation of contaminated sites and ground
water; and prevention  and control of indoor air pollution.  Thegoal of this research
effort is to catalyze development and implementation of innovative, cost-
effective environmental technologies;  develop scientific and  engineering
information needed by EPA to support regulatory and policy decisions; and
provide technical support and information transfer to ensure effective
implementation of  environmental regulations  and strategies.

       This publication has been produced as part of  the Laboratory's  strategic
long-term research plan. It  is published and made available by EPA's Office of
Research and Development to assist the user community and to link researchers
with their clients.

                       E.  Timothy Oppelt,  Director
                       National Risk Management  Research  Laboratory

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                                Contents

   Figures	    vi
   Acknowledgments	   v'i'
   introduction  	   1
   Sources  and Control Technologies	  1
   Sources  of Air Emissions	   1
       Surface  Impoundments	   1
       Tanks	    -2
           Fixed Roof Tanks	   3
           Floating  Roof Tanks	   4
           Pressure Tanks	   -^
       Containers  	   7
'       Treatment Devices	    10
           Distillation	1°
           Solvent  Extraction 	    11
           Air Stripping	    11
           Steam Stripping	    11
           Thin-Film  Evaporation	   12
           Waste  Incineration	    13
   Control Devices	    13
       Adsorption	    13
       Condensation	    15
       Absorption	    15
       Combustion   Equipment	   15
       Thermal  Incineration	    16
       Catalytic  Incineration 	    17
       Existing  Boilers and Process Heaters	   17
       Unconventional   Technologies	   17
   References  	    20

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                              Figures

1.   Emissions from impoundments and other open
    liquid  surfaces	    1
2.   Major factors  affecting emissions	  .2
3.   Fate of organics in a surface impoundment:
    emissions, effluent, biodegradation, sludge	 .2
4.   Typical fixed roof tank	   .3
5.   Tankworking losses	   .3
6.   Covered tanks (working losses)	  .3
7.   Tank breathing  losses	   4
a.   Covered tanks (breathing  losses)	  .4
9.   Closed-vent system and control device	 .4
10. Floating roof	   -4
11. Internal floating  roof	   .5
12. External floating roof	   .5
13. External floating roof tank	   -6
14. IFR mechanical shoe seal	   .6
15. IFR liquid-mounted seal.	   .7
16. IFR vapor-mounted seal with secondary seal	 .7
17. IFR fittings that require controls	  -8
18. EFR mechanical shoe seal with secondary  seal	8
19. EFR liquid-mounted  seal with  secondary seal	 .9
20. EFR fittings that require controls	  -9
21. Splash loading  method	   9
22. Submerged fill pipe	    9
23. Bottom loading  reduces  liquid turbulence and
    vapor and/or  liquid contact	   10
24. Typical bottom  loading  system with vapor collection	 10
                                   VI

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                           Figures  (continued)
 25. Emission  sources  for distillation	 11
 26. Emissions from  solvent extraction	  11
 27. Schematic diagram of an air stripping system	12
 26. Steam stripper for ethylene dichloride (EDC)/vinyl chloride	12
 29. Preliminary  treatment prior to stripping	12
 30. Flow path of thin-film evaporator	  13
 31. Carbon canisters	   13
 32. General process flow diagram of an adsorption process for
     organic recovery	   14
 33. Fixed-bed  regenerative carbon  adsorption system process
     flow diagram and potential  emission sources	  14
 34.  Schematic  diagram of a contact condenser	  15
 35. Schematic diagram of a shell-and-tube  surface condenser	 15
/ 36. Packed tower for gas absorption	  16
 37. Steam-assisted elevated flare system	  16
 36. Thermal vapor incinerator	   17
 39. Schematic diagram of a catalytic incinerator system	  18
 40. Fluid-bed  catalytic incinerator	   18
 41. Typical rotary carousel system	   18
 42. Carousel system with incinerator	  19
 43. Schematic of an open  single-bed biofilter system	  19
                                    VII

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                      Acknowledgments
Thiscapsule reportwasdeveloped  from technical material prepared by Research
Triangle Institute (RTI) for a seminar publication on air emissions control at
waste management facilities.  The original  seminar  publication  was prepared
under contract to the  U.S. EPA Office of Research  and Development  (ORD),
through Eastern Research Group, Inc., and the Office of Air Quality Planning and
Standards (OAQPS).

Justice A. Manning, Center for Environmental Research Information, ORD,
coordinated the original seminars with assistance from ERG, Inc. Peer reviewers
who provided the final  review of this capsule report are Michele Aston, OAQPS,
and John 0. Burckle and Scott  R. Hedges,  ORD, National Risk Management
Research Laboratory.  The tireless efforts of Robert. A. Zerbonia of RTI were
greatly appreciated.
                                 VIII

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Introduction
  The  chemicals  processed  during waste  management
operations  can volatilize  into  the atmosphere  and cause
carcinogenic or other toxic effects or contribute to ozone
formation. Regulations have been developed to control air
emissions from these operations. The  EPA has  promul-
gated standards under the authority of  Section 3004(n) of
the Hazardous and Solid  Waste Amendments  to  the Re-
source Conservation and  Recovery  Act (RCRA) that limit
organic  air emissions from  waste  management  units  at
hazardous waste treatment, storage, and disposal  facilities
(TSDF); the rules also apply to 90-day  accumulation units
at hazardous waste generator facilities.  In June 1990,  EPA
promulgated standards for process vents  and equipment
leaks;  additional RCRA standards  were promulgated on
December 6,  1994,  (effective  December  6,  1996) and
amended on November 25, 1996, to limit air emission from
tanks, surface  impoundments,  and containers used in man-
aging hazardous wastes.  Implementation of air  emission
controls  on many types  of waste management operations
are required by these RCRA air rules.
  This capsule report focuses on the  major sources and
controls  of air  emissions  at waste  management facilities,
how these emissions occur,  and how they can  be  con-
trolled. The major  sources  that  are  discussed  in  detail
include surface impoundments, the very broad and diverse
category  of tanks and ancillary equipment,  containers, and
treatment devices.  As each  source is  described,  controls
that are inherent to that source or commonly found on that
particular source  are presented.  In addition,  details  are
provided  on the  basic  mechanisms by which  emissions
occur and the major factors that affect  the emissions.
  After the discussion of  sources and  their inherent con-
trols,  air pollution  control  devices  that  may be  generally
applicable to any enclosed or vented source (i.e., add-on
controls)  are described.  The discussion of control devices
focuses  on their applicability, control performance, and the
major factors affecting performance. Organic removal (i.e.,
pretreatment) and destruction processes are also discussed
as  a means of controlling air emissions and reducing or
eliminating  the emission potential. This  discussion  de-
scribes  processes  that remove  or destroy  the  organics in
the waste, which may eliminate the  need to  control subse-
quent waste processing steps.
 Sources and Control  Technologies
   The types of sources found  at waste management facili-
 ties,  inherent  controls that  are  typically  part of the con-
 struction  and  operation  of the sources,  and  emission
 mechanisms are discussed in this chapter. As each source
 is discussed,  covers  and enclosures that  are specifically
 applicable to the source are described,  as well as simple
 work practices that reduce emissions. Other emission con-
 trols  that  are  broadly applicable to  many of the  individual
 sources  are discussed  collectively in the  last part of the
 chapter. These controls  include traditional air pollution
 control  devices, processes  that remove the organics be-
 fore  the waste  is placed in  units  with  a  high  emission
 potential,  and waste  incineration.
Sources  of Air Emissions
  The  discussion  of emission sources is divided into four
categories:  surface  impoundments,  the  very  broad and
diverse group of  tanks and  ancillary equipment,  contain-
ers, and treatment devices. The major focus is on the first
three  categories because they  are the most  directly im-
pacted by the RCRA air emission  regulations.
Surface Impoundments
  A surface  impoundment is defined under  RCRA  as
"a  natural  topographical  depression,  man-made excava-
tion, or diked  area  formed primarily of earthen materials
(although it may be lined with man-made materials) which
is designed to hold an  accumulation  of liquid wastes or
wastes containing free liquids and which is not an injection
well. Examples of surface impoundments are holding, stor-
age, settling, and aeration pits, ponds, and lagoons."
   Impoundments are simply ponds  and lagoons that are
used primarily  for managing aqueous wastes and sludges.
Specific uses  include storage,  equalization,  neutralization,
evaporation, solids settling, and biodegradation.
  A surface  impoundment is  below  grade,  usually  has
berms  with sloping sides to  contain wastes,  and  has a
liquid surface that is exposed to the atmosphere. It may be
operated as a flowthrough system with liquid flowing in at
one point and  out at the same time at another point, or the
liquid may be  pumped out or evaporated, leaving behind a
sludge. Figure 1 illustrates emissions from impoundments
and other open liquid surfaces as  caused by diffusion and
wind flow across the surface.
   Figure 2 lists several of the factors that affect emissions
from impoundments. These same factors are applicable to
emissions from open tanks. The constituent's volatility has
a direct effect  on emissions from impoundments and other
sources with exposed liquid surfaces.  Highly volatile com-
pounds such  as  benzene are readily emitted  from  open
sources, whereas  relatively nonvolatile compounds such
as  phenol tend to stay in the water.
  The residence time in the impoundment has  an obvious
effect on emissions: longer residence times result in higher
emissions.  If the waste is in the impoundment long enough,
even  relatively nonvolatile compounds  are evaporated.  For
impoundments with relatively short residence times, a higher
percentage  of the  organics  may  be  removed  with the
effluent and emitted later in  other  units in  the treatment
sequence.
                     From surface to air
   Wind
      Flow
      in —
                   Diffusion through liquid
Flow
Out -
  Figure  1.    Emissions from impoundments and other open
              liquid surfaces.

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  • Constituent volatility
  • Residence time
  • Surface area
  • Turbulence  (aeration,  agitation)
  • Windspeed  and  temperature
  • Extent of competing mechanisms (such as biodegradation)


Figure 2.    Major factors affecting emissions.
  Many impoundments and tanks are agitated for mixing,
air stripping, or biodegradation. Agitation and aeration in-
crease emissions by creating turbulent zones and increase
contact between the waste and air. There are a highly
turbulent area and water spray around the agitators used
in mechanically aerated units. Essentially all of the highly
volatile compounds can be emitted when the impound-
ment  is mechanically agitated.
  Approximately half of the impoundments used to treat
hazardous waste are aerated or agitated.
  As  Figure 3 illustrates, organic removal from impound-
ments also occur from mechanisms other than air emis-
sions resulting from wind  blowing across the exposed
surface of the waste. Organics can also be removed by
biodegradation, adsorption  onto sludge,  or removed with
the effluent.  Emission  models have  been developed  to
estimate the extent of each of these different removal
mechanisms.
  The models developed  for open liquid surfaces are
applicable to  impoundments and open tanks.  These mod-
els can account for the emissions from relatively calm
surfaces or from the turbulence created by aeration or
agitation. The emissions are modeled as two mass trans-
fer steps in series: (1) diffusion through the liquid, and (2)
mass  transfer from the  surface of the liquid to the air. The
approach can  account for removal in  flowthrough  systems
and removal in units designed for disposal  or evaporation.
The extent of biodegradation, if any, can also be esti-
mated.
  One of the controls demonstrated for impoundments is
an air-supported structure, which uses fans to maintain a
positive pressure to inflate the structure. For effective
control, the air vented from the structure  must be sent to a
control device, such as a carbon adsorber. Air-supported

                         Emissions
  Wind

  Flow-
  in
Biodegradation


         Sludge
                                  Flow
                                  in

                                  Sludge
                                  out
Figure 3.
Fate of organics in a surface impoundment:
emissions, effluent, biodegradation,  sludge.
                                            structures have been used as enclosures for conveyors,
                                            open top tanks,  and storage piles,  as well as impound-
                                            ments.
                                              An air-supported structure  and control device has been
                                            installed on a l-acre aerated lagoon that is used for
                                            biodegradation at a  pharmaceutical  manufacturing  facility.
                                            The cover material is PVC-coated polyester with a Tedlar
                                            backing. An agitator system in the structure provides oxy
                                            gen and keeps carbon  and biomass  suspended.  In this
                                            application,  the exhaust from the structure is vented to a
                                            carbon  adsorber. Very few leaks were found around the
                                            structure; consequently,  the control efficiency is deter-
                                            mined primarily by  how well the control device works;  it
                                            may exceed 95 percent.  This plant's experience with the
                                            air-supported structure has  found that corrosion can be
                                            accelerated  inside the structure, condensation  may occur,
                                            temperatures may be high, and  special worker safety pre-
                                            cautions are  needed.
                                               Floating  membrane covers are another control option
                                            and have been demonstrated on various types of im-
                                            poundments, including water reservoirs in the western
                                            parts of the United States. For proper operation as a
                                            control  technique  for organic compounds, the membrane
                                            must provide a seal at the edge of the impoundment and
                                            provisions made to  remove rainwater. If gas is generated
                                            under the cover, vents and a control device may be needed.
                                            In addition,  if sludge accumulates, some means for peri-
                                            odic sludge removal may be required, such as a  sludge
                                            pump.
                                               Emission control depends  primarily on the type of mem-
                                            brane, its thickness,  and the individual  organic compounds
                                            in the waste. Theoretical estimates based on diffusion
                                            through the  membrane indicate worst-case control effi-
                                            ciencies of  50 to  over 95 percent. Laboratory studies indi-
                                            cate that the cover is an  efficient control for some organic
                                            compounds,  but, for specific compounds that permeate
                                            the membrane, the control efficiency  is lower.
                                              The floating membrane cover has been demonstrated
                                            on  an impoundment that is used as an anaerobic digester.
                                            The impoundment is about 7 acres in size with a depth of
                                            approximately 14 feet. The membrane material is 100-mil
                                            high-density polyethylene. The  cover  is anchored  over a
                                            concrete ring wall that extends above grade level around
                                            the  perimeter of the impoundment. The membrane ex-
                                            tends over the concrete wall  and is covered with backfill  to
                                            anchor and seal  it.  Punctures or tears in the membrane
                                            can  be  patched.  This installation has been in operation  for
                                            over 4 years, and the company supplying the membrane
                                            offered  a 20-year warranty on the  life of the material.
Tanks
  The  most diverse group of waste management  sources
falls into the category of tanks,  which is broadly  defined
under RCRA. If the unit is not a  land disposal source, it is
probably a tank. A tank is defined as "a stationary device,
designed to contain an accumulation of hazardous waste,
which is constructed primarily  of nonearthen materials
(e.g., wood, concrete,  steel, plastic) that provide structural
support." A tank system is defined as a tank  and its
ancillary equipment, where ancillary equipment  includes
such devices as piping, fittings, flanges, pumps,  and valves.

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  The category  of tanks and tank systems  includes two
broad classes, open top tanks and covered tanks. The
latter class is divided into fixed roof tanks,  floating roof
tanks,  and pressure tanks. Fixed roof tanks  are the most
common type of storage tank found  at hazardous waste
facilities. Emissions occur through the tank's vent, which
may be open to the atmosphere, equipped  with a pres-
sure-vacuum relief valve, or vented to a pollution control
device.

Fixed Roof Tanks
  The fixed roof may have several openings  in addition to
the vent, such as a manhole for tank entry, a hatch used
for measuring the liquid level, or an overflow pipe (Fig-
ure 4). The pressure-vacuum relief valve is  also called a
conservation vent, which permits small changes in the
liquid level without expelling the tank's vapors. If the tank
has a conservation vent or is vented to a control device, it
is important that the other openings on the  tank be kept
closed and sealed for the emission controls to be  effective.
  As  an emission control option, fixed  roofs can be retrofit-
ted to open tanks, or a fixed roof tank can be used to
replace an open tank or impoundment. Compared to an
open tank, a  fixed roof tank can provide additional control
of 66  to 99 percent, depending on the waste volatility and
the operating characteristics of the tank openings. If the
fixed  roof tank is constructed to withstand pressures of
2.5 psig, an additional control of 20 to 45 percent can  be
obtained. (Most tanks are not  designed and constructed to
withstand this^pressure.)
  Emissions from fixed roof tanks occur primarily from
working! (loading) losses and, to a lesser extent, from
breathing losses. The quantity emitted is most directly
affected by the rate at which vapors are expelled from the
tank and the  volatility of the tank's contents.  These emis-
sions  are increased by heating or aeration. Working  losses
occur  when waste is pumped into the tank and vapors are
expelled by the rising level  of liquid (Figures 5 and 6).
Breathing  losses occur when the volume of  vapor in the
tank is increased because of changes in temperature or
pressure (Figures 7 and  8).
  Equations developed by the American Petroleum Insti-
tute (API)  are used to estimate emissions for  organic
liquids. The basic form of the equation,  which  can  be used
for  other types of wastes, estimates the volume of vapor

 Pressure/vacuumvalve
 (for venting)
 Manhole
H Nozzle
 (for submerged
H fill or drainage)
                  Figure 5.   Tank working losses.
                                      Working losses ,
                                      due to loading  J
                  Volume of
                  displaced -
                  vapors
                                     Vapor space
                                    : Liquid waste :
                                           New
                                           liquid
                                          . level

                                           Original
                                           liquid
                                           level
                                                      >•* £3- Liquid in
Figure 4.    Typical fixed roof tank.
                 Figure 6.    Covered  tanks (working  losses).
expelled from the tank based on the amount of liquid
pumped in and on the vapor concentration. The concen-
tration of constituents in the vapor can be measured  or
estimated from volatility data. One error in using the API
tank equations for aqueous wastes is to estimate the
concentration in  the vapor from the mole fraction of the
compound in  the  liquid, which  significantly  underestimates
concentration. Henry's law constant should be used  for
dilute aqueous wastes. Breathing losses are usually very
low compared to working losses. Note that if the tank is
operated at a constant liquid level,  as some separators
and collection tanks are, very little vapor is displaced and
working losses are small.
  Two primary emission control approaches exist for fixed
roof tanks. The vapor space above the liquid can  be
eliminated by keeping the liquid level at the roof line (or
vice versa), or the vapor space can  be vented to a control
device. An alternative to keeping the tank at a constant
liquid level is to install a floating roof, as discussed as
follows. Figures 9 and 10 illustrate the .application of a
closed vent system with control device and two arrange-
ments of  floating  roof tanks.

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Figure 7.    Tank breathing  losses.
              Breathing losses due
              to ambient pressure
              and temperature
              fluctuations
         Vapor phase concentration
         in equilibrium with
         waste liquid
               Liquid waste
  Head space
- volume
  decreases as
  temperature
  increases
                                                           Figure 9.    Closed-vent  system and control device.
                                                           Figure  10.   Floating roof.
Figure 8.     Covered tanks (breathing losses).
Floating Roof Tanks
  Floating  roof tanks are  common at petroleum refineries
and  gasoline marketing facilities  for the  storage of volatile
liquids. The floating roof can be installed internally in a
fixed roof tank or externally without a  fixed roof.  Figures 11
and  12 show internal floating  roof  and external floating
roof tanks in cross-section. The roof floats on the liquid
and  moves with changes in the  liquid level, thus reducing
vapor displacement and controlling  working losses.  The
major requirement  for a floating roof is an effective seal
between the roof and the tank walls. Emissions from a
properly maintained floating roof are very low and occur
from standing  losses and  withdrawal losses.
  Figure 13 shows the equipment associated with an ex-
ternal floating roof. Standing losses occur at the deck
seals and at openings for fittings in the floating roof.
  If  retrofitted  to  a  hazardous waste  tank, the  floating roof
materials must  be compatible with  the waste; also  floating
roofs cannot be used in hazardous waste treatment tanks
with  surface mixers or aeration  equipment. The emission
                   reductions achieved by a floating roof relative to a fixed
                   roof have  been evaluated for volatile organic liquids by
                   using empirical  models.  Depending on the type of deck
                   and  seal system selected, emission reductions of 93  to 97
                   percent can be  obtained. For the smaller size tanks and
                   varieties of wastes found  at  hazardous waste facilities,
                   reductions of 74 to 82 percent can be obtained relative to
                   a fixed roof. Convening an  open top tank to  a floating roof
                   tank is estimated to reduce  emissions by 96 to 99 percent.
                      Internal and external floating roofs have different design
                   and  operating  requirements.  Typical  EPA  requirements
                   relating to  roof seals and fittings for floating  roof tanks are
                   discussed as follows.
                     An internal floating roof must be equipped with either a
                   mechanical shoe  seal, or  a  continuous liquid-mounted
                   seal, or double-seal system. Figures 14, 15, and 16 illus-
                   trate the different  kinds of seals.  Mechanical shoe  seals
                   (Figure 14} operate through a scissors design with a
                   weighted leg to  maintain pressure between the seal and
                   the tank wall. A  flexible envelope  covers the  area between

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                                                                                     Gauge
                                                                                     float system
F i g . 1 1 .  Internal floating roof.
                                                                                  Gauge float
                                                                                  system
 Mechanics*
 shoe   	
 seal
                 Automatic
             Aleeder
                 vent
Emergency
roof drain
 Figure 12.   External  floating roof.
the roof deck and the seal. Liquid-mounted seals (Figure
15) maintain pressure between the roof and the tank wall
by means of a liquid-filled tube that rides  between the two.
The term "liquid-mounted" refers to the fact that the seal  in
this case contacts the liquid directly. A scuff band, sealed
to the roof, protects the tube from abrasion against the
wall. A variation of this seal uses a resilient foam ring
instead of a liquid-filled tube. Double-seal systems are
illustrated by a vapor-mounted seal with secondary seal
(Figure 16). This seal is similar to the liquid-mounted seal
with two exceptions.  The liquid is replaced by foam and a
flexible rim-mounted seal is added to the top of the floating
deck.  The term "vapor-mounted" refers to the fact that the
seal does not come into direct contact with the liquid.
                          Fittings that typically require controls to achieve emis-
                       sion reductions are shown in Figure 17. A seal is required
                       around each of these vents, wells, or hatches.  Further,
                       each opening must project below the liquid surface. Ac-
                       cess openings and gauge wells must be equipped with
                       gasketed covers equipped with bolts, and the cover must
                       be closed and bolted except when in use. Ladder wells
                       must have a gasketed sliding cover. Other requirements
                       include a gasketed sliding cover or flexible fabric seal for
                       column wells and slitted fabric covers that cover at least
                       90 percent of the opening for sample wells. Automatic
                       bleeder vents must  be  gasketed and must be closed
                       except when the internal floating roof is floated off or
                       landed. Rim space vents must be gasketed, and open only

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                                                             Main drain
             Pontoon manhole
                                                                                Seal envelope
Figure 13.   External floating  roof tank
       ' Tank wall
                           Envelope
Figure 14.   IFR mechanical  shoe seal.
when the roof is floated off or as directed by the  manufac-
turers for proper and safe operation.
   External  floating  roof (EFR) requirements  are  similar in
that the EFRs must use a primary seal with continuous,
roof rim-mounted secondary seal. This specification means
that the EFR must have a primary and a secondary seal.
The primary seal can be either liquid-mounted or  equipped
with a mechanical shoe seal.
   Figure 18 is an illustration of the  EFR mechanical shoe
seal with secondary seal. This seal is identical to the
internal floating roof seal except for the addition of a
flexible secondary seal mounted to the top of the  deck rim.
Figure 19  shows EFR liquid-mounted seals with added
secondary seals at the deck rim. These seals also are
identical to their internal floating roof counterparts except
for the added secondary seals.
   The EPA rules that apply to storage tanks have very
specific requirements for the EFR fittings.  The EFR fittings
requiring controls are shown in  Figure 20.  Access open-
ings, sampling wells,  and gauge wells must  project below
the liquid surface, their covers must be gasketed, and the
cover must be closed and sealed except when in use.
Emergency roof drains, must be equipped with  a slotted
membrane fabric cover that covers at least 90 percent of
the opening. Automatic bleeder vents must be  gasketed
and closed except when the EFR is floated off or landed.
The rim space vents must be gasketed and may be open
only when the roof  is floated off.

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                  • Tank wall
                   /IT
   • Tank wall
                                                                            Floating roof

                                                                            Seal fabric
Figure 15.   IFR liquid-mounted seal.
         -Tank wall
                         L
                          Rim-mounted
                          secondary  seal
                            Floating roof



                            Scuff band

                            Foam filled  tube


                            Vapor space
                            	-A
Figure 16.    IFR vapor-mounted seal with secondary seal.
Pressure Tanks
   Pressure tanks are designed to operate safely at inter-
nal pressures  above atmospheric  pressure.  Consequently,
these tanks can  often be operated as closed systems and
do not have emissions at normal storage conditions or
during routine loading and withdrawal. Pressure-relief valves
on the tanks open  only in  the event of  improper operation
or an emergency to  relieve excess pressure. These tanks
are most common for the storage  of gases;  however, they
can also  be used to  store  liquids.
   Low pressure tanks can be defined as those operating
at up to  2  atm, and  high-pressure tanks are those operat-
ing at more than 2 atm. However, these values are not
specified in the  RCRA air rules;  pressure tanks are de-
fined in the rules as designed not to vent to the  atmo-
sphere as a result  of compression of the vapor headspace
in the tank during filling of the tank to its  design capacity.
   In summary, tank  emissions primarily occur due to am-
bient temperature changes  and tank filling  operations. Con-
trol of tank emissions is accomplished  by reducing vapor
displacement or by venting vapors to closed systems.
Tank controls include use of floating roofs with specific
seal types and fittings or  installation of leak-tight closed-
vent systems and control devices  that achieve  95 percent
or more control.

Containers
   Containers are defined  under RCRA as, "any portable
device in which a material is stored, transported, treated,
disposed of, or otherwise handled." Examples of typical
containers are  drums, dumpsters or roll-off bins, and tank
trucks. Emissions  occur from loading  these  containers,
from uncovered containers during  storage or transport,
from treatment operation losses, and from spills.
   Sources  of emissions  associated with drums include air
emissions  from the evaporation of leaks and  spills; and
poor housekeeping practices that can make spill  detection
and cleanup difficult. If the  drums are well  maintained on a
diked  pad,  emissions from  spills or ruptures can be identi-
fied by routine inspection procedures and  promptly cleaned
up. Dumpsters or  roll-off  bins can  be  a  source of emis-

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                                                                                 Gauge
                                                                                 float system
           Ladder
           well
                                                  ff
Access
hatch
Bleeder
vent
                           s^_MM
 Column
/well
Sample
well
Figure  17.   IFR fittings that require controls.
              Tank wall
                             Rim-mounted
                             secondary seal
                                 Envelope



                                 Floating roof

                                 Shoe
Figure  18.   EFR mechanical shoe seal with  secondary
            seal.
                sions if they are left uncovered with the surface of the
                waste exposed to the  atmosphere.
                  Emissions from containers also occur when they are
                loaded,  and emissions are greatest  when splash filling
                (shown in Figure 21) is used. When splash filling, the
                vapors displaced from the  container by loading can quickly
                become  saturated with volatiles caused by the splashing.
                  Submerged filling (shown in Figure 22) uses an influent
                pipe that is below the liquid surface, which reduces splash-
                ing  and  the degree of saturation of  the displaced vapors.  A
                study of submerged filling of tank trucks indicates that
                emissions are reduced by about 65 percent relative to
                splash filling.
                  A third method of filling containers, bottom loading,  is
                shown in Figure 23. In this case, a tight connection  is
                made between the fill pipe or hose and the container.
                Also, the container hatches remain closed and all vapors
                vented  from the container can be routed to a control
                device. A typical  bottom loading system with vapor collec-
                tion is shown in Figure 24. Advantages for bottom loading
                include improved safety, faster loading,  reduced labor costs,
                and  lower emissions.
                  Other  basic controls for containers  include  using simple
                covers during storage, placing containers used for treat-
                ment operations in an enclosure vented to a control device
                during treatment, or  ensuring that appropriate transport
                and  routine housekeeping  practices are followed, with daily
                inspections and prompt cleanup of spills. For enclosures
                vented to a control device,  EPA has provided criteria for
                design and operation  of the enclosure (40 CFR 52.571,
                Appendix B). Efficiency of the system  depends  on the type
                and  operation of both the control  device and the enclo-
                sure.

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                   - Tank wall
                                   Rim-mounted
                                   secondary seal
                                                               -Tank Wall
                                         Rim-mounted
                                         secondary seal
Figure 19.   EFR liquid-mounted  seal  with  secondary seal.
                 Automatic
                 bleeder
                 vent
Emergency
roof drain
                                                                      Guide-pole well
Figure 20.   EFR fittings that require controls.
                                                                                            Gauge float
                                                                                            svstem
                                                                                                    1
                                           Fill pipe
          Organic vapor emissions
                                                                                                          Fill pipe
                                               Hatch
                                               cover
                                                Cargo
                                                tank
                                    Organic vapor emissions
                                                                           Hatch
                                                                           cover
                                                                            Cargo
                                                                            tank
Figure 21.   Splash  loading  method.
                         Figure 22.   Submerged fill  pipe.

-------
          Vapor vent
          to recovery
          or atmosphere
                                Hatch closed
    Waste
                                               Cargo
                                               tank
Figure 23.
                                              Fill pipe
Bottom loading reduces liquid turbulence  and
vapor and/or liquid contact.
 Treatment Devices
  Several treatment technologies may be used to remove
organic compounds from waste. The most common of
these treatments include steam stripping, air stripping,
thin-film evaporation, solvent extraction,  distillation, and
waste  incineration.
  Features common  to all the  technologies include  avoid-
ing  the need for controls  on  subsequent processes (hence
pretreatment), removal efficiency dependant on waste con-
stituents and process design, removal of essentially 100
percent of highly volatile compounds, and applicability to
many wastes and compounds.
  The  control efficiency achieved by waste treatment de-
pends  on several factors including the following: percent
removed  from waste  and  emissions from removal process,
e.g., emissions from a steam stripping operation. Overall,
a 98 to 99 percent removal efficiency can be obtained. For
example,  benzene  being  steam stripped from wastewater
would typically achieve a 98  to 99 percent  removal.

Distillation
  One of the most commonly used treatment process for
organic liquids  is distillation. This separation process is
based  on the differences in  volatility of components, and
can be performed  as continuous or batch operations at
atmospheric  pressure,  under vacuum, or at greater than
atmospheric  pressure.  Batch distillation is  the most com-
mon of the three for TSDF sites. For each type of distilla-
                Level sensor
                                                           Island wiring
                                                                     \


Figure  24.   Typical bottom loading system with  vapor  collection.
                                                                                             Product
                                                                                             supply
                                                                            To vapor
                                                                            processor
                                                      10

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         Vapors
  Waste
  in


sun
                                  Vent

                                    t
                         Condenser
                       Residual
                       out
Vent

 t
                                         Receiver
Figure 25.    Emission sources for distillation.
tion operation, the  organics are transferred from the liquid
waste to the vapor phase, then condensed and collected
as a separate liquid.
  Emissions from distillation  typically  occur from the over-
head venting system, the collection tank vents, or the
vacuum system. The primary condenser is  an inherent
control for these emissions, but pollution control  devices
can be added to the still vents to provide additional con-
trol. Examples include a second condenser or an activated
carbon canister. Figure 25 is a schematic showing emis-
sion  sources for the distillation process.

Solvent  Extraction
  Solvent extraction is another type of separation  process
used for organic liquids. In this process, the constituent to
be removed  preferentially dissolves in a solvent chosen for
its high capacity for dissolving the constituent combined
with  easy physical  separation from the remainder of the
treated waste.  The  solvent in the extract is typically recov-
ered by distillation, while the still bottoms containing the
constituent are decanted for  reclamation  or disposal. How-
ever, this solvent recovery stage is associated with emis-
sions as shown in Figure 26  at the still vent. Any collection
tanks associated with the unit may also have emission
points. Although 80 to 100 percent .of the target organics
can be removed from  the waste by the solvent, the overall
control efficiency is probably less. Also, waste residuals
usually require  further treatment.
  In common industrial applications, solvent  extraction is
used to  remove phenols, acetic acid,  hydroxy aromatic
acids, and petroleum  oils. Extraction  is also  applicable for
organic sludges  such as benzene  from  petroleum  refinery
sludge.

Air  Stripping
  Air stripping is a common treatment  process used for
aqueous wastes. The stripper may  be  a spray tower,  a
packed column, or simply an  aerated tank used to  provide
contact between the waste  and air. Strippers are most
commonly used to remove parts per million or lower levels
of volatiles from dilute aqueous wastes. Many strippers are
simply vented  directly to the atmosphere, while others are
controlled  by carbon adsorption  or incineration. Condens-
ers on air strippers are uncommon and are generally inef-
fective because of low vapor phase concentrations and
high volumetric flow rates.  Figure  27 is a schematic dia-
gram of an air stripping system and shows the system
elements and vents that are emission  points.

Steam Stripping
  Steam stripping is another method used to treat aque-
ous wastes with concentrations on the order of hundreds
of parts per million or higher. Steam is injected directly into
the wastewater, the  overhead vapors are condensed, or-
ganics are separated from the condensed water, and the
decanted water is returned to the feed stream. Emissions
occur from the vent  on the condenser/decanter and from
collection tank vents. Continuous steam  strippers  operate
with  either trays or packing and require  a low solids con-
tent in the feed to  prevent plugging and fouling.
  Figure 28 is a schematic of an  actual steam stripping
system and illustrates the use of a  heat exchanger to
preheat the feed (to recover energy) and to cool the bot-
toms stream from the stripper before  additional wastewater
treatment. This particular system has a  high level of or-
ganic recovery because both a primary  and much colder
secondary condenser are used. Overall emission control
should be  excellent  because  noncondensibles  are  vented
to an incinerator.
  For  continuous steam strippers,  pretreatment as  shown
in Figure 29 may sometimes be required  to adjust pH or to
remove solids, which can foul the column packing or trays
and cause  plugging problems. Any  separate organic phase
that can be decanted from the wastewater is removed  prior
to stripping.
                    Solvent
             Waste in

1
idr^ ^b
Solvent
extractor
Emissions
Extracted
Extracted
organics
f
and solvent J \ _. .... .. \

Solvent
recycle
Figure 26.   Emissions from solvent  extraction.
                                                       11

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                                                        Overhead vapors
                                                              Liquid distributor
                     Vent
                                                                         Control device  —>• Vent air
                                              I
                                     Control device residue
                                     (e.g., spent  carbon)
              Waste in
Figure  27.   Schematic diagram of an air stripping system.
                                     Feed
                                                                                       To incinerator
                                                                           Water to feed tank
                                                                   EDO
                                Bottoms
Figure  28.   Steam stripper for ethylene  dichloride (EDC)/vinyl chloride.
                Additives      Vent

                   1	t
Wastewater -
                     Decanter
Organics
                              Decanted
                              water
                      Sludge

Figure  29.    Preliminary treatment prior to stripping.
  Emissions from  steam stripping  come from the con-
denser/decanter vent  and collection tanks.  The primary
condenser provides an inherent control for this process,
but additional control may be obtained by installing  control
devices on the equipment vents.

Thin-Film  Evaporation
  Thin-film evaporation typically is  used for viscous  liq-
uids,  sludges,  and slurries that  often cannot  be treated
with other technologies. A thin layer of waste is  spread
over a moving or wiped surface that is heated to volatilize
organics.  This configuration  makes the technology  adapt-
able to  many physical  forms  and waste compositions.
  The vertical thin-film evaporator (TFE) looks like  a distil-
lation column.  In the TFE, the vapors with volatile organics
are removed overhead to a condenser,  and the treated
waste is discharged from the bottom as shown in  Figure
30. As with distillation units, emissions occur  from vents
on  condensers, decanters, collection tanks,  and vacuum
systems if used. The  primary condenser is an inherent
control, but additional  control may be obtained from con-
trol  devices on  the  system  vents.
                                                       12

-------
      Drive
      systemi
                               vapor \  To condenser/
                               outlet /  decanter
           Modular
           heating
           bodies
Adsorption
   In the adsorption process,  organics are selectively col-
lected on the surface  of a porous solid adsorbent (acti-
vated carbon, silica gels,  molecular sieves). Activated
carbon is the  most commonly used adsorbent because it
has high internal surface area, low cost, and insensitivity
to water. One gram of activated carbon can have a sur-
face area equal to that of a football field and typically can
adsorb up to half its weight in organics. The adsorber will
remove essentially all of the target volatiles from the vented
vapors until breakthrough, which is when the volatiles are
first detected in the cleaned vapor leaving the adsorption
bed. Carbon  adsorbers can achieve control efficiencies of
at least 95 percent, and control levels of 97 to 99 percent
have  been demonstrated  in  many applications. The two
common types of adsorbers are carbon canisters and
regenerable  fixed beds.
   Carbon canisters, shown in Figure 31, are used for low
vent flows, usually less than 100 ft3/min, and are not
regenerated onsite. They  are usually discarded or re-
turned to the  supplier. The canisters are fairly compact
units  and can be removed easily and fresh canisters in-
stalled.  However,  their compact  size implies limited capac-
ity and a possible requirement for frequent changes.
   Fixed-bed  carbon adsorbers that can be regenerated
(shown as a general process flow diagram in Figure 32
and in more detail in Figure  33) are used for controlling
continuous vent streams, with  flows exceeding 100,000 ft3/
min, and can  handle a wide  range of organic concentra-
                   Product outlet
Figure 30.   Flow path  of thin-film evaporator.
Waste   Incineration
  Waste incineration is also an emission control option
that can  be used instead of processing the waste in units
with a high emission potential. Waste incineration is com-
monly used for wastes that were previously land-disposed.
The technology provides  destruction of  99.9 percent or
higher as demonstrated in many units and is applicable to
organic  wastes and  sludges. This technology should  not
be confused with vapor incineration, which is discussed in
a later section.

Control  Devices
  Conventional, or commonly used, air  pollution control
technologies applicable to  waste  management  operations
are discussed in  this section. The conventional air pollu-
tion control devices  are divided into two types:  recovery
devices,  e.g.,  adsorption,  condensation,  and absorption;
and combustion  devices,  e.g.,  flares, thermal  incineration,
catalytic  incineration, and boilers or process heaters. The
recovery  devices recover organics for reuse when they  are
of value, while the combustion devices oxidize the organ-
ics to destroy them.
                                                                  . For vent flows less than 100 CFM
                                                                  . Cannot be regenerated  in canister
                                      Activated
                                      carbon
Figure 31.   Carbon  canisters.
                                                      13

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                 Organic-
                 containing
                 gas stream
                                                  On-line.
                                                             Sorbent
                                                                   •Clean gas
    Organfci
    ,sensor
                               Regenerating
                               gas       -
                                                  .Qff-jjne.
   Sorbent
   bed
Organic + regenerating
gas
                                                                            Organic (liquid)


                                                                            Byproducts
Figure 32.    General process flow diagram of an adsorption process for organic recovery.
                                                                          Exhaust vent
      Vent
                   Filter   Blower
tream /^ \ \
=>y>=i_r^ =n
f- j
leal exchanger rvmiinn
°P*>nal> dSng9
air




Ambient
air intake for
cooling/drying I"
L
Filter Blower
.^tnnm 	 1

E— t*fl-







-j-

T
H>
I










Z]

;

L
i







*•
^


1







absorber

Steam


Carbon
absorber







/


^St=
«-cSj-







N








l|
/\ <2>
Vent Vlnt


Condenser
~
\3/

Organ; phase-
o recovery


                                                                                                0
                                                                                             Aqueousphase
                                                                                            • to disposal or
                                                                                             treatment
Figure 33.   Fixed-bed regenerative carbon adsorption system process flow diagram and  potential emission  sources.
tions. A common procedure is to have dual beds with one
desorbing  while the other is adsorbing. The carbon capac-
ity for organics is affected by the concentration of organics
in the vapor. Carbon manufacturers generally have equi-
librium data for  specific compounds and  specific carbons.
The bed design  is  important and must be deep enough to
prevent rapid  breakthrough, yet not so deep as to cause
excessive  pressure drop. Flow rate is important in  the bed
design and in determining  carbon capacity requirements.
Humidity has an  adverse effect when water occupies some
of the adsorption sites. For a relative humidity of 50 per-
cent or  more, dehumidification  or dilution may be neces-
sary to lower the relative humidity of the inlet stream. The
bed's operating temperature affects capacity, and some
compounds such as aldehydes and  ketones may generate
heat in adsorbers. For these special cases, some means
of removing  the excess  heat may be necessary.
   For effective emission control by adsorption, it is neces-
sary either to monitor for breakthrough or to replace the
carbon periodically before breakthrough  occurs (based on
design and operating experience). In addition, any emis-
sions from the disposal or regeneration of the carbon
should be controlled. It is of little value to control emis-
sions from a vent stream only to emit the collected organ-
                                                       14

-------
ics later in the wastewater treatment associated with re-
generation.


Condensation
  One of  the most common devices for  organics recovery
is the condenser. Condensers work by cooling the vented
vapors to  the dew point and removing the organics as a
liquid. Typical coolants  include  cooling  tower  water, refrig-
erated water, brines, and glycols. The efficiency of a con-
denser is determined by the vapor phase concentration of
the  specific organics  and  the condenser  temperature. Two
common types of condensers  are contact condensers  and
surface condensers (also called non-contact or  shell and
tube condensers).
  The contact condenser (shown in Figure 34 ) is cheap
and efficient. However, the cooling  liquid that directly con-
                                     -vtet
        Vapojt inlet  —»
                     "^         ^  Distribution  tray

                                  Liquid  level

               Liquid outlet (water and VOCs)


 Figure 34.    Schematic  diagram of a contact condenser.
 tacts the vented vapors can present a disposal problem.
  For example,  if the coolant is water that is sent to waste-
 water treatment, the volatiles may be emitted in open
 tanks.
    The shell and tube condenser (Figure 35) does not
  allow contact  between the vented vapors and the cooling
  medium. In  this type of condenser, a concentrated organic
  liquid can be recovered for recycle or other use.

 Absorption
    Absorption also is used as an air pollution  control tech-
  nology. In absorption, the organics in. the vent gas are
 dissolved in a liquid by direct contact as shown for the
 packed tower  absorber in Figure 36. The  contact between
 the absorbing liquid and the vent gas is accomplished in
 spray towers or  packed or plate columns. Some common
 solvents that  may be  useful for volatile organics include
 water, mineral oils,  or other nonvolatile petroleum oils.
 Absorption efficiency is affected  by temperature of the gas
 and sorbent (higher temperatures  give poorer absorption),
 pressure (higher pressures increase solubility), solubility of
 the constituent in the  sorbent (higher solubilities require
 less sorbent and allow higher efficiencies at  equilibrium),
 and reaction  kinetics (for reactions that can effectively
  remove the constituent from the surface of the absorbing
  liquid).  Absorption efficiencies of  60 to 96 percent  have
 been reported for organics.  For example, methylene  chlo-
  ride removal from vented vapors  has been measured at
 87 percent using water as the absorbing liquid. Commonly,
 absorbers also are  used to remove inorganic gases such
 as SO,, H2S, HCI, and NH,.  When choosing a solvent, it
 must be  compatible with the target constituent at the
 operating  conditions of the absorber.
    The material removed from the  absorber may present a
 disposal or separation problem.  For example,  organics
  must be removed from the water  or nonvolatile oil without
 losing them as emissions during  the solvent recovery or
 treatment  process.

 Combustion Equipment
    Vapor combustion is  another control technique for vented
 vapors. The destruction of organics can be accomplished
                                   Coolant inlet
                     Cooling tower or
                     refrigeration unit
                                   Coolant  outlet
Vapor  outlet
Vapor  inlet
 Condensed  VOC
 (to decanter or receiving tank)
Figure 35.   Schematic diagram of a shell-and-tube surface condenser.
                                                      15

-------
                               3 c> Cleaned gas
                                   to final control
                                   device
Absorbing
liquid in
                                           Organic
                                           laden
                                           gas in
           Absorbing liquid with organics to disposal
           or organic solvent  recovery
                                     in flares; thermal oxidizers, such as incinerators, boilers,
                                     or process heaters; and  in catalytic oxidizers.
                                       Flares are an open combustion process in which oxygen
                                     is supplied by the air surrounding the flame. Surrounding
                                     the  flame with steam improves combustion. Flares are
                                     operated either at ground level (usually with enclosed
                                     multiple burner heads) or are elevated.  Elevated flares
                                     often use steam injection to improve combustion  by  in-
                                     creasing mixing or turbulence and by pulling in additional
                                     combustion air. Properly operated flares can achieve de-
                                     struction efficiencies of at least 98 percent. Figure 37 is a
                                     schematic of the basic components of a flare system. The
                                     EPA has  developed  regulations for the design and  opera-
                                     tion of flares that include tip exit velocities for different
                                     types of flares and  different gas stream heating values
                                     (above 200 Btu/scf). These design and operating criteria
                                     were established because  of the  difficulty and cost associ-
                                     ated with  measuring flare emissions.
Figure 36.   Packed tower for gas absorption.
                                     Thermal Incineration
                                       Thermal incineration is a common air pollution control
                                     technology used to destroy organic  vapors. Thermal incin-
                                                Steam
                                                nozzles
                                                   Gas
                                                   barriers
                                         Helps  prevent flashback
                                                   Pilot
                                                   burners
                                                    Rare
                                                    stack'
                 Gas collection  header
                   and
 ollection header n^
 I transfer line    A

 £3Il_r-"-i
	  I
Knock-out_w.
drum     ^^
                                          seal

     Steam
 — line

f  -    Ignition
        device
                                                                           i  f -  -  Air line
                                                                             	Gas line
                                   Drain
Figure 37.   Steam-assisted  elevated flare system.
                                                      16

-------
eration requires high  temperatures,  good mixing, sufficient
oxygen, and adequate resident time for proper operation
and complete combustion of the organics. Auxiliary fuel is
needed if the heating value of the waste gas is less than
50 Btu/scf. Thermal vapor incinerators  are available in a
wide range of sizes; capacity from 200 to 50,000 scfm are
available.  Destruction efficiency of at least 98 percent is
achievable for an  inlet concentration  greater than
2,000 ppm. Below 2,000 ppm, EPA studies have found
that combustion or destruction efficiency decreases as
concentration decreases below  2,000 ppmv. A heat recov-
ery unit, such  as a  steam generator, may be used to
recover some of the energy from  a thermal incinerator.
Figure 38  is a  depiction of the major  components of a
thermal  vapor  incinerator.
  The two types of thermal  incinerators are recuperative
and regenerative. Recuperative incinerators  have an  exte-
rior shell and tube type heat recovery system  and process
gases with heats of combustion sufficiently high to sustain
high temperatures without auxiliary fuel. Regenerative in-
cinerators have internal heat recovery by means of two or
more, usually ceramic, gas paths through the incinerator.
While gases are burned and heating one side,  the previ-
ously  heated side is  used to heat the incoming gas. The
two sides are switched to maintain an  operating equilib-
rium.  Regenerative  incinerators are effective for waste
gases with low concentrations of the  constituent to be
destroyed.

Catalytic Incineration
  Catalytic incinerators provide oxidation at temperatures
lowe*1han those required by thermal incinerators (typically
350°C to 500°C versus 750°C to 1 ,000°C). Design consid-
erations are important because the catalyst may  be  ad-
versely affected  by  high temperatures, high  concentrations
of organics, fouling from  particulate matter or polymers,
and deactivation by halogens or certain metals. Destruc-
       tion efficiencies in catalytic  incinerators are on the order of
       98 percent. The  basic components of a catalytic oxidizer
       are shown in Figure 39 and  are similar to those of a
       thermal unit except that a  catalyst bed is used. Sizes of
       catalytic incinerators  range from large, high-volume units
       to small, low-volume  packaged  units.  A  fluid-bed catalytic
       incinerator is shown in Figure 40. The energy require-
       ments (and therefore, operating costs) of a catalytic oxi-
       dizer are lower than those of a thermal unit because of the
       lower operating temperatures.  However,  capital  costs are
       higher.

       Existing Boilers and Process Heaters
         The organics in  vented vapors  can also be destroyed
       with a high level  of efficiency in existing boilers or process
       heaters. In this case, no dedicated or new control equip
       ment is required. In these  devices, vapors with halogens
       or sulfur are sometimes  avoided because of potential  cor-
       rosion problems when acid gases  are formed. These de-
       vices recover the heating  value of the vent stream and
       also  offer the advantage of using existing equipment to
       control emissions. Vent streams are added as fuel, as
       secondary  combustion air, or as diluent. Other advantages
       include recovery  of the heating value of vent stream  and
       destruction efficiencies of 98 percent or higher.

       Unconventional Technologies
         New, unconventional technologies are  being used more
       frequently in the United  States.  One of these technologies,
       a modified two-stage system,  combines adsorption with
       incineration or further adsorption.  The first stage collects
       the target constituent  and concentrates it in a regenerated
       gas from less than 100 ppm up to more than 2,000 ppm.
       The second  stage  involves more  conventional treatment
       and depends on  the concentration and chemical charac-
       teristics of the constituent.  For example, if the constituent
                 Waste gas
                Auxiliary
                fuel burner
                (discrete)
                                                                                          Stack
                                  Mixing
                                  section
Combustion section
optional
Heat
Recovery
Figure 38.   Thermal vapor incinerator.
                                                     17

-------
can be reused, fixed bed adsorption would be used to
collect it. Otherwise, the constituent  would be thermally  or
catalytically incinerated. The most common of the two-
stage systems with  rotary carousels is shown in  Figure 41.
In this system either activated carbon or  zeolite is used as
the first-stage sorbent. While one segment of the  wheel  is
adsorbing, another segment is being desorbed. While the
second stage shows  incineration or  recovery in the figure,
the specific  process that is chosen depends on site  condi-
tions.
  A carousel system  in line  with an  incinerator is shown  in
Figure 42.  In this configuration, combustion gas  is used  to
produce hot desorption gas by means of  a heat exchanger.
Many of these units are  in operation in Japan  and Europe,
and several  have recently begun operation in the United
States. Their use is driven by air standards and air toxics
rules. These systems do not tolerate particulate  loadings
well  and may require  filtration of the incoming gas stream.
                        Another nonconventional  device that is finding increased
                      use for air pollution control is biofiltration. Vapors are
                      vented through biologically active material where organics
                      are digested  to carbon dioxide and water by microorgan-
                      isms. This technology,  similar to compost in  a garden, has
                      been successfully applied in Europe to control odors,  vola-
                      tile  organic compounds, and air toxic  emissions. However,
                      the process has not been  used extensively in the United
                      States.  One  of the properties  of biofiltration is a  general
                      limitation to organic concentrations of 1,000 ppm  or  less,
                      although new systems are reportedly treating higher con-
                      centrations.  Control efficiencies greater than 90  percent
                      are achieved in various applications, and the literature
                      indicates  low operating  costs  that provide an economic
                      advantage over other technologies. Figure 43 is  a sche-
                      matic of an open single-bed biofilter system. In practice,
                      industrial  applications typically involve multiple closed or
                      covered beds.
                                                                Emission source
                                 Supplementary
                                 Tuei      Catalytic incinerator
                                       Preheater
                                                                             Stack
                                                      - Catalyst bed1
                                                                            Heat
                                                                            exchanger
                                                                            (optiona
Figure 39.   Schematic diagram of a catalytic  incinerator system.
                        t Outlet

                           Stack
                             Transition
     H eatexchanger
            r
    Process
    gas
    inlet
   Catalyst bed

,.  Temperature
T  indicating
   controller
                       Process
                       air in
                                                            Concentrate
                                          Blower
Desorptionflow
                              —1(|  Incinerator
                                                                         Recovery
                   Preheat  burner
                                                        •» Return
                                                          to process
                                                                          Solvent
Figure 40.   Fluid-bed  catalytic incinerator.
                      Figure 41.   Typical rotary carousel system.
                                                        18

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                                            Mist removal filters
                                                                                 Adsorption cylinder
                     Fan
                                         Preheater
                     Catalytic  combustion
Figure 42.   Carousel system with incinerator.
                                                                                         Hot
                                                                                         desorbing
                                                                                         air
                     Ducting
                 Raw  gas
                              Blower
                                                                                      Clean  gas
                                                                                           tJ
                                                                                    Or               o
                                             Humidifier       Drainage



Figure 43.   Schematic of an open single-bed biofilter system.
                                                                                 Filter material
                                                                                 VVVVVSA*
                                                                                 xVVVVVVV
                                                                              Air distribution system


                                                                                    Biofilter
                                                        19

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