United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park NC 27711
EPA-450/3-91-027
January 1992
Air
&EPA Assessment of VOC Emissions
and Their Control from Baker's
Yeast Manufacturing Facilities
control
technology center
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ASSESSMENT OF VOC EMISSIONS AND THEIR CONTROL
FROM BAKER'S YEAST MANUFACTURING FACILITIES
Control Technology Center
Sponsored by:
Emission Standards Division
Office of Air Quality Planning and Standards
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
Air and Energy Engineering Research Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
January 1992
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EPA No. 450/3-91-027
January 1992
ASSESSMENT OF VOC EMISSIONS AND THEIR CONTROL
FROM BAKER'S YEAST MANUFACTURING FACILITIES
By
Midwest Research Institute
401 Harrison Oaks Boulevard
Suite 350
Gary, North Carolina 27513
EPA Contract Nos. 68-D1-0115,
68-DO-0137, and 68-02-4379
Project Lead
" Martha Smith
Office of Air Quality Planning and Standards
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
Prepared for:
Control Technology Center
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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NOTICE
This report was prepared by Midwest Research Institute,
Gary, North Carolina. It has been reviewed for technical
accuracy by the Emission Standards Division of the Office of Air
Quality Planning and Standards and the Air and Energy Engineering
Research Laboratory of the Office of Research and Development,
U. S. Environmental Protection Agency, and approved for
publication. Mention of trade names or commercial products is
not intended to constitute endorsement or recommendation of use.
ii
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ACKNOWLEDGEMENT
This report was prepared for EPA's Control Technology Center
(CTC) by Robin Barker and Maresa Williamson of Midwest Research
Institute. The Work Assignment Manager was Martha Smith of EPA's
Chemicals and Petroleum Branch (CPB). Also participating on the
project team were Bob Blasczcak, CTC; Chuck Darvin, EPA Air and
Energy Engineering Research Laboratory; and K.C. Hustvedt, CPB.
111
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PREFACE
The Control Technology Center (CTC) was established by the
U. S. Environmental Protection Agency's (EPA's) Office of
Research and Development and Office of Air Quality Planning and
Standards to help State and local air pollution control agencies
implement their air toxics and other pollution programs. Three
levels of assistance can be accessed through the CTC. First, a
CTC Hotline has been established to provide telephone assistance
on matters relating to air pollution control technology. Second,
more in-depth engineering assistance can be provided when
appropriate. Third, the CTC can provide technical guidance
through publication of technical guidance documents, development
of personal computer software, and presentation of workshops on
control technology matters.
The major objectives of this document are to provide a
general overview of the baker's yeast production process, to
summarize available data on VOC emissions from baker's yeast
manufacturing facilities, and to evaluate potential emission
control options. This work was initiated by a State of Maryland
request for technical support from the CTC. The State is
concerned that while VOC concentrations in the flue gas from the
yeast fermentors at a facility in Baltimore, Maryland, are low,
the large volume of flue gas indicates a high mass emission rate.
Thus, the State of Maryland is looking for guidance on
controlling these dilute, high-volume gas streams.
IV
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TABLE OF CONTENTS
Page
1.0 INTRODUCTION 1
1.1 OVERVIEW 1
2.0 INDUSTRY PROFILE .... 3
2.1 EVOLUTION OF THE INDUSTRY 3
2.2 NUMBER AND LOCATION OF FACILITIES 3
2.3 OZONE NONATTAINMENT STATUS 5
2.4 PRODUCTION STATISTICS AND GROWTH PROJECTION ... 5
3.0 PROCESS DESCRIPTION 5
3.1 RAW MATERIALS 7
3.2 FERMENTATION 10
3.2.1 General 10
3.2.2 Fermentation Sequence 10
3.2.3 Laboratory stage, or Flask Stage (Fl) . . 12
3.2.4 Pure Culture Stages (F2 and F3) 12
3.2.5 Main Fermentation Stages (F4-F7) 13
3.2.6 Harvesting and Packaging 15
3.2.7 Production of Dry Yeast 17
4.0 EMISSION ESTIMATIONS 17
4.1 SOURCES OF EMISSIONS 18
4.2 EMISSION DATA AND PROCESS EMISSION FACTORS ... 19
4.3 NATIONWIDE EMISSION LEVELS 23
4.4 MODEL EMISSION STREAM 23
5.0 EMISSION CONTROL TECHNIQUES 24
5.1 COMPUTERIZED PROCESS CONTROL MEASURES 24
5.2 ENGINEERING CONTROLS 27
5.2.1 Wet Scrubbers (Gas Absorption) 27
5.2.2 Carbon Adsorption 30
5.2.3 Incineration 30
5.2.4 Condensation 33
5.2.5 Biological Filtration 35
5.3 IMPACT ASSESSMENT OF CONTROL OPTIONS 37
6.0 CONCLUSIONS 41
7.0 REFERENCES 44
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LIST OF FIGURES
Page
Figure 1. Process flow diagram for producing baker's
yeast
Figure 2. Fermentation sequence used in producing
baker's yeast
Figure 3. Emissions profile of a typical trade
fermentation
Figure 4. Schematic of a packed tower absorber . . . .
Figure 5. Schematic of a biofiltration system . . . .
LIST OF TABLES
TABLE 1. YEAST MANUFACTURING PLANTS
TABLE 2. OZONE NONATTAINMENT STATUS
TABLE 3. VOC EMISSIONS FROM YEAST FERMENTATION . . .
TABLE 4. VOC EMISSIONS FROM A TYPICAL YEAST
FERMENTATION FACILITY
TABLE 5. SUMMARY OF THE COST AND ENVIRONMENTAL
IMPACTS OF THE CONTROL OPTIONS
TABLE 6. CAPITAL COSTS OF ADD-ON CONTROL OPTIONS . .
TABLE 7. ANNUAL COSTS OF ADD-ON CONTROL OPTIONS . . .
TABLE 8. COSTS FOR CONDENSER CONTROL OPTION
11
25
28
36
4
6
20
22
38
39
40
42
VI
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1.0 INTRODUCTION
The objectives of this study were to obtain information on
baker's yeast fermentation processes, to determine the number and
locations of yeast plants, to estimate the potential emissions
from the process, and to evaluate potential emission control
options. The information contained in this report includes an
industry profile, descriptions of the production process,
available emission data, descriptions and technical evaluations
of the control options, the impacts (cost and environmental) of
each option, and the results of an evaluation to determine the
most effective and least costly technology options.
This document is organized into the following sections:
Section 2.0 provides a brief characterization of the baker's
yeast manufacturing industry, including the number, locations,
and production capacities of facilities; Section 3.0 describes
the baker's yeast fermentation process, including the equipment
used, feed materials and process requirements, and typical
process yields and limitations; Section 4.0 presents the emission
sources and pollutant types, available emission test data,
process emission factors, nationwide emission levels, and model
emission stream data; Section 5.0 presents the available
information on potential emission controls for the pollutants and
emission points identified in Section 4.0; Section 6.0 summarizes
the conclusions derived from this investigation; and Section 7.0
provides references for this report.
1.1 OVERVIEW
Currently 13 facilities produce baker's yeast in the United
States. The facilities are widely dispersed and can be found in
10 different States. Ten of the facilities are located in ,ozone
nonattainment areas. The majority of yeast manufacturing
facilities employ a moderate degree of process control to reduce
the amount of volatile organic compounds (VOC's) generated.
However, only one facility has applied an air pollution control
system to control emissions from the process. Because of the
location of some facilities in nonattainment areas, controlling
yeast production VOC emissions may help bring these areas into
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compliance with the national ambient air quality standard (NAAQS)
for ozone.
Baker's yeast is produced by a fermentation process in which
large quantities of ethanol and acetaldehyde are generated and
emitted to the atmosphere. Based on test data from three
facilities, the VOC mass emission rate from a typical facility is
estimated at 82 megagrams per year (Mg/yr) (90 tons per
year [tons/yr]). Ethanol is approximately 80 to 90 percent of
the emissions generated, and the remaining 10 to 20 percent
consists of other alcohols and acetaldehyde. The VOC
concentration from a typical trade fermentor varies over a range
from 5 to 600 parts per million by volume (ppmv). The
fluctuation in the VOC concentration is attributable primarily to
the variation in feed rates to the fermentor, variation in the
airflow rate, and the design of the fermentor's air sparger and
agitation systems. Based on the number of yeast facilities and
the typical VOC emission levels, the total annual VOC emissions
from this source category are estimated to range from 780 to
1,060 Mg/yr (860 to 1,170 tons/yr).
The VOC emission alternatives that were evaluated during
this study were process control measures to reduce the formation
of VOC emissions as well as wet scrubbers, carbon adsorbers,
incinerators, condensers, and biological filters to control VOC
emissions. Of these approaches, it appears that process control
measures, catalytic incinerators, or a combination of add-on
control techniques (e.g., wet scrubbers followed by an
incinerator or a biological filter) are the most feasible
approaches for controlling yeast process emissions. Based on the
results of this study, the control efficiency associated with the
add-on control systems is estimated to be 95 to 98 percent.
Process control measures would limit the amount of ethanol
formed by the process. This can be accomplished by incrementally
feeding the molasses mixture (the principal source of carbon for
yea'st growth) and by supplying sufficient oxygen to the
fermentors. Although these process control strategies are
routinely employed at yeast production facilities for economic
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reasons (i.e., to optimize the use of raw materials), these steps
are not optimized to limit VOC emissions. Implementing and
optimizing more stringent process control, especially during the
early stages of fermentation where close process control is
usually not required, would reduce the formation of VOC emissions
by 75 to 95 percent.
2.0 INDUSTRY PROFILE
Information on the evolution of the baker's yeast industry,
the number and location of facilities, the ozone nonattainment
status of facility locations, yeast production statistics, and
growth projections for the industry is presented below.
2.1 EVOLUTION OF THE INDUSTRY
The production of baker's yeast is a process that has been
developed over centuries. The earliest means of producing
baker's yeast was to inoculate fresh dough with a portion of the
preceding fermented dough. Yeast will grow indefinitely by this
means, and this method is still used today in producing sour
dough breads. Before the 1800's, the top-fermenting yeast from
breweries was used for baking bread. However, the yeast yields
were very low and the ethanol yield was very high. Over the last
100 years, grain mashes were replaced with molasses as the
principal carbon source for producing yeasts, fermentation
process tanks were equipped with air.supply and incremental feed
systems to reduce the formation of ethanol and increase the
quantities of yeast produced. At this point, the yeast
production process became independent of the brewery process and
has remained virtually unchanged since the 1920's.
2.2 NUMBER AND LOCATION OF FACILITIES
Six major companies manufacture baker's yeast in the United
States. These companies are Universal Foods (Red Star Yeast),
Fleischmanns, Gist-brocades, Lallemand (American Yeast), Minn-
dak, and Columbia. There are a total of 13 manufacturing plants
owned by these companies in the United States. Table 1 lists the
locations of the plants by manufacturer.
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TABLE 1. YEAST MANUFACTURING PLANTS
Lallemand (American Yeast)
Columbia
Fleischmanns
Gist-brocades
Minn-dak
Universal Foods Corp. (Red
Star Yeast)
Baltimore, Maryland
Headland, Alabama
Gastonia, North Carolina
Memphis, Tennessee
Oakland, California
Sumner , Wash ington
Bakersfield, California
East Brunswick, New Jersey
Wahpeton, North Dakota
Baltimore, Maryland
Dallas, Texas
Milwaukee, Wisconsin
Oakland, California
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2.3 OZONE NONATTAINMENT STATUS
The baker's yeast manufacturing plants in the United States
are located in both attainment and nonattainment areas for ozone.
This ranking is according to the latest update of the cities
exceeding the ozone limit, made in 1988.^ The ozone air quality
standard calls for no more than 1 hour in a year during which any
monitor in an area records an O3 level in excess of 0.12 ppm.1
Table 2 presents the locations of all the yeast manufacturing
plants located in the United States and whether or not they are
located in ozone nonattainment areas.
2.4 PRODUCTION STATISTICS AND GROWTH PROJECTION
In 1989, only 12 yeast plants were in operation. The total
U.S. production of baker's yeast in 1989 was 223,500 Mg
(245,000 tons). Of this total, approximately 85 percent of the
yeast was compressed or cream yeast, and the remaining 15 percent
was dry yeast.
Between 1990 and 1991, two additional facilities were opened
and are currently in production, and one facility was closed
down. A foreign manufacturer of baker's yeast, Minn-dak, has
recently opened a new yeast plant in Wahpeton, North Dakota.
Fleischmanns also has opened a plant in Memphis, Tennessee, but
closed its plant in St. Louis, Missouri. The opening of these
two facilities in 1990 is expected to increase production of
baker's yeast by approximately 5 to 10 percent.
3.0 PROCESS DESCRIPTION
Two main types of baker's yeast are produced: compressed
yeast and active dry yeast (ADY). Compressed yeast is a
perishable commodity and must be refrigerated or frozen at all
times. Refrigerated compressed yeast remains useful for several
weeks before molds begin to develop. Frozen compressed yeast can
be stored and used for up to a month, but some softening of the
yeast cake occurs. Active dry yeast has a lower bake activity
than compressed yeast, but it can be stored for 1 to 2 years
without refrigeration before the bake activity is lost.
Compressed yeast is sold mainly to wholesale bakeries, whereas
ADY is sold mainly for home baking needs and, to a limited
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TABLE 2. OZONE NONATTAINMENT STATUS
Plant location
Milwaukee, WI
Oakland, CA
Baltimore, MD
Dallas, TX
Bakersfield, CA
East Brunswick, NJ
Sumner , WA
Memphis , TN
Gastonia, NC
Headland, AL
Wahpeton , ND
Company
Universal Foods Corporation
(UFC)
UFC
Fleischmanns
Lallemand (American Yeast)
UFC
UFC
Gist-brocades
Gist-brocades
Fleischmanns
Fleischmanns
Fleischmanns
Columbia
Minn-dak
Nonatt a inment
for ozone
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
Yesa
No
No
aGastonia is not formally recognized by EPA as an ozone
nonattainment area, but exceedances for ozone occur, and the
area will most likely be nonattainment for ozone when the 1990
Clean Air Act Amendments are fully implemented.
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extent, to bakeries. Compressed yeast and ADY are produced in a
similar manner, but ADY is dried after processing and is
developed from a different yeast strain. Another dry yeast
product, instant dry yeast (IDY), is dried after processing and
is produced from a faster-reacting yeast strain than that used
for ADY. The main difference between ADY and IDY is that IDY
does not have to be dissolved in warm water prior to usage,
whereas ADY does. The following discussion is directed towards
compressed yeast manufacturing, although a brief discussion of
the production of dry yeast is also presented (Section 3.2.7).
A variety of processes are used in producing baker's yeast.
Most processes, however, are a variation on the Zulauf process,
which was introduced in the early 1900's. This report provides a
general description of the Zulauf production process.
Figure 1 presents a process flow diagram for the production
of baker's yeast. The first stages of production consist of
growing the yeast from the pure yeast culture in a series of
fermentation vessels. The yeast is then recovered from the final
fermentor using .centrifugal action to concentrate the yeast
solids. The yeast product is subjected to one or more washings
in the centrifugal separator. The yeast solids are then filtered
by a filter press or a rotary vacuum filter to further
concentrate the yeast. Next, the yeast filter cake is blended in
mixers with small amounts of water, emulsifiers, and cutting
oils. After mixing, the mixed press cake is extruded and cut.
The yeast cakes are then either wrapped for shipment or dried to
form dry yeast.
3.1 RAW MATERIALS
The principal raw materials used in producing baker's yeast
are the pure yeast culture and molasses. The yeast strain used
in producing compressed yeast is Saccharomyces cerevisiae.2 Cane
and beet molasses are used as the principal carbon source to
promote yeast growth. Molasses contains 45 to 55 percent
fermentable sugars by weight in the forms of sucrose, glucose,
and fructose. This sugar by-product is the least expensive
source of sugar known. Other sources, such as corn grits,
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Raw Materials
Fermentation
Ethanol, CO2
Filtration
Blending
Extrusion & Cutting
Packaging
Shipment of packaged yeast
Figure 1. Process flow diagram for producing baker's yeast,
8
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raisins, or sugar-containing wastes of the confectionary industry
are also effective, but for various reasons (mostly economic),
these alternatives were found to be unsuitable as carbon and
energy substrates for baker's yeast production.^
The amount and type of cane and beet molasses used depends
on the availability of the molasses types, cost, and the presence
of inhibitors and toxins. Usually, a blend consisting of both
cane and beet molasses is used in the fermentations. Once
blended, the molasses mixture is clarified to remove any sludge.
Prior to clarification, the pH of the molasses mixture is
adjusted because too high a pH will promote bacterial growth.
Bacterial growth occurs under the same conditions as yeast
growth, making pH monitoring a very important factor. The
clarified molasses mixture is then sterilized with high-pressure
steam. After sterilization, it is diluted with water and held in
holding tanks until it is needed by the fermentation process.
Other required raw materials are a variety of essential
nutrients and vitamins. Mineral requirements include nitrogen,
potassium, phosphate, magnesium, and calcium. Nitrogen is
normally supplied through the addition of ammonium salts, aqueous
ammonia, or anhydrous ammonia to the feed stock.4 The molasses
normally provides sufficient quantities of potassium and calcium.
Phosphates and magnesium are added in the form of phosphoric acid
or phosphate salts and magnesium salts.^ Iron, zinc, copper,
manganese, and molybdenum are also required in trace amounts.
Several vitamins are required for yeast growth (biotin,
inositol, pantothenic acid, and thiamine). Yeast will not grow
in the absence of biotin.6 Thiamine is not required for yeast
growth but is normally added to the feed stock because it is a
potent stimulant for fermenting doughs. Both cane and beet
molasses usually provide enough inositol and pantothenic acid for
yeast growth. However, if beet molasses, which is deficient in
biotin, is used, biotin must be added or a mixture of cane and
beet molasses is required.
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3.2 FERMENTATION
Yeast cells are grown in a series of fermentation vessels.
A typical fermentation process is shown in Figure 2. The process
begins when a pure yeast culture is grown in the laboratory.
Portions of this pure culture are placed in the first fermentor
along with the other feed materials and are allowed to grow.
Yeast is propagated when the entire yeast mixture (or a portion
of the mixture from the preceding fermentor) is placed into the
next fermentor, which is equipped for batch or incremental
feeding of the molasses malt. The process continues until the
yeast mixture reaches the final fermentation vessel.
3.2.1 General
Yeast fermentation vessels are operated under aerobic
conditions (free oxygen present, or excess air) because under
anaerobic conditions (limited or no oxygen available) the
fermentable sugars are consumed in the formation of ethanol and
carbon dioxide, which results in low yeast yields.7 Yeast yields
under anaerobic conditions are often less than 10 percent-by-
weight of fermentable sugars, whereas yeast yields of up to
50 percent-by-weight of fermentable sugars are obtained under
aerobic conditions. Therefore, to maximize yeast yields, it is
important to supply enough oxygen for the dissolved oxygen
content in the liquid surrounding the yeast cells to be at an
optimal level. In practice, oxygen transfer rates are often
inadequate, and under such conditions, some ethanol is formed.
In addition, it is also important to control the amount of
fermentable sugars present in the fermentor, so that the sugar is
assimilated by the yeast as fast as it is added. This balance is
accomplished by using an incremental feed system in the final
fermentation stages.
3.2.2 Fermentation Sequence
Compared with subsequent fermentation stages, in the first
stages of yeast propagation, the medium is richer in nutrients,
and there is less aeration. Consequently, the fermentor liquor
contains more alcohol and yields of yeast are lower. The lower
yields in the first stages are not necessarily a drawback,
10
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Air
F2
Mr
F3
F1 • Laboratory fermentation
F2/F3 • Pure culture fermentations
F4 - Intermediate fermentation
FS - Stock fermentation
F6 • Pitching fermentation
F7 • Trade fermentation
F4
t
Air
Molasses,
Nutrients
*
Molasses,
Nutrients
Molasses,
Nutrients
*
PS
1
*
F6
I
»
F6
.
Figure 2. Fermentation sequence used in producing baker's yeast.
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because the overall economy of the operation depends on the yield
from the final trade fermentation stage. The following sections
describe each stage in the fermentation sequence.
3.2.3 Laboratory Stage, or Flask Stage (Fl)
The first fermentation stage takes place in the laboratory
when a portion of the pure yeast culture is mixed with the
molasses malt in a sterilized Erlenmeyer or Pasteur flask. The
total contents of the flask are typically less than 5 liters (L)
(1.3 gallons [gal]), and the yeast is allowed to grow in the
flask for 2 to 4 days.8 The entire flask contents are then used
to inoculate the second fermentation stage.
3.2.4 Pure Culture Stages (F2 and F3)
Generally, this stage consists of two pure culture
fermentations. The capacities of the fermentation vessels used
in this stage range from 1,140 L (300 gal) to 26,500 L
(7,000 gal). The pure culture fermentations are batch
fermentations where the yeast is allowed to grow for 13 to
24 hours. The contents of the fermentor from the first pure
culture stage (F2) is added to the next fermentation vessel,
which already contains the nutrient-rich molasses malt. The pure
culture fermentations are basically a continuation of the flask
fermentation, except that the pure culture fermentations have
provisions for sterile aeration and aseptic transfer to the next
stage. The yeast yield in the pure culture fermentations is
approximately 27 kilograms (kg) (60 pounds [lb]) in the first
fermentor and 600 kg (1,300 lb) in the second fermentor.
The critical factor in the pure culture operation is
sterility. Rigorous sterilization of the fermentation medium
prior to inoculation is conducted by heating the medium under-
pressure or by boiling it at atmospheric pressure for extended
periods. If a sterile environment is not provided, contaminating
microorganisms can easily outgrow the yeast.
The need for process control in the pure culture medium is
limited. However, microbiological testing of the medium before,
during, and after each fermentation is essential. The malt
concentration of the initial fermentor is often standardized to
12
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between 5 and 7.5 percent sugar.9 Once the pure culture
fermentation is started, the only controllable parameters are the
temperature and the degree of aeration. However, control of
aeration is not critical because of the excess sugar present, and
control of temperature is not critical because the fermentation
operates over a broad temperature range.
3.2.5 Main Fermentation Stages (F4-F7)
The majority of the yeast yield grows in the final
fermentation stages. The main fermentation stage takes place in
two to four fermentation vessels. These yeast fermentors vary
considerably in size. The volumes of fermentation vessels in the
F4 through F7 stages range from 37,900 L (10,000 gal) to over
283,900 L (75,000 gal). The vessels have diameters in excess of
7.0 meters (m) (24.5 feet [ft]) and heights up to 14 m (45 ft).
The larger vessels are associated with the final fermentation
stages (F6 and F7). The fermentation vessels are typically
operated at a temperature of 30°C (86°F).
The fermentors are usually constructed of stainless steel
and are equipped with an incremental feed system. This
incremental feed system may be a pipe or a series of pipes that
distribute the molasses over the entire surface of the fermentor
liquid. The rate at which the molasses is fed is critical and
may be controlled by a speed controller connected to a pump or by
a valve on a rotameter, which delivers a certain volume of
molasses at regulated time intervals. Nutrient solutions of
vitamins are kept in small, separate tanks and are charged
through rotameters into the fermentor, but the rate of feed is
not as critical as with molasses. However, if ammonia is used as
a nitrogen source, additions must be made in a manner that avoids
sudden pH changes. The nitrogen salts and phosphates may be
charged in a shorter period of time than the molasses feed.
Fermentors must also be equipped with heat exchangers to remove
the heat produced from the production process and to cool the
fermentor. The type of heat exchanger system used depends on the
size of the fermentation vessel. Because large volumes of air
are supplied to the fermentation vessels during this stage of
13
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production, the fermentor size influences the type of aeration
system selected. The different types of aeration systems include
horizontal, perforated pipes; compressed air and mechanical
agitation; and a self-priming aerator.
In the horizontal, perforated pipe system, air is blown
through a large number of horizontal pipes that are placed near
the bottom of the fermentor. With this aeration system, the only
agitation of the fermentor liquid is carried out by the action of
the air bubbles as they rise to the surface. Typically, this
type of aeration system requires from 25 to 30 ra (880 to
1,060 ft3) of air to produce 0.45 kg (1 Ib) of yeast.10
The efficiency of aeration with a given volume of air can be
greatly increased by mechanical agitation. In a compressed
air/mechanical agitation aeration system, air under pressure is
supplied to a circular diffuser pipe. Directly above the air
outlets, a horizontal turbine disc provides mechanical agitation,
which distributes the air bubbles uniformly. Agitation systems
have baffles to keep the fermentor liquid from rotating in the
direction of the motion of the disc. This uniform distribution
of air bubbles reduces the volume of air needed to grow the
yeast. In an agitated system, only 10 to 15 m3 (350 to 530 ft3)
of air are required to produce 0.45 kg (1 Ib) of yeast.10
The self-priming aerator operates with a turbine that draws
air through a hollow, vertical shaft into the fermentor liquid.
Because air is drawn through the shaft of the turbine without a
compressor, the pressure of the air at the outlets is not very
high and the depth to which the turbine can be submerged is
limited.
When using the four-stage fermentation series, the pure
culture stage is followed by an intermediate stage (F4) of yeast
growth without incremental feeding. The entire fermentor
contents from the intermediate stage are then pumped into a tank
that is equipped for incremental feeding and that has good
aeration. This stage (F5) is often called stock fermentation
because, after fermentation is completed, the yeast is separated
from the bulk of the fermentor liquid by centrifuging, producing
14
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a stock (pitch) of yeast for the next stage. The third stage
(F6) is usually carried out in fermentors as large as those used
for the trade fermentation or final fermentation. Aeration is
vigorous, and molasses and other nutrients are fed incrementally.
The fermentor liquor from this fermentor (F6) is usually divided
into several parts for pitching the final trade fermentation
(adding the yeast to start fermentation). Alternately, the yeast
may again be separated by centrifuging and stored for several
days prior to its use in the final trade fermentations. The
final trade fermentation (F7) has the highest degree of aeration,
and molasses and other nutrients are fed incrementally. Due to
the large air supplies required during the final trade
fermentations, these vessels are often started in a staggered
fashion to reduce the load or size of the air compressors
required. The duration of each of the final fermentation stages
ranges from 11 to 15 hours. The amount of yeast growth increases
in each stage and is typically 120 kg (265 Ib) in the first
stage, 420 kg (930 Ib) in the second stage, 2,500 kg (5,510 Ib)
in the third stage, and 15,000 to 100,000 kg (33,070 to
220,460 Ib) in the fourth stage.11
When using the two-stage final fermentation series, the only
fermentations are the stock fermentation and the trade
fermentation (F5 and F7, respectively). About half of the
13 yeast manufacturing facilities use the four-stage final
fermentation series and the other half use the two-stage process.
After all of the required molasses has been fed into the
fermentor, the liquid is aerated for an additional 0.5 to
1.5 hours. This permits further maturing of the yeast and
results in a yeast that is more stable in refrigerated storage.
3.2.6 Harvesting and Packaging
Once an optimum quantity of yeast has been grown, the yeast
cells are recovered from the final trade fermentor by centrifugal
yeast separators. The separators used in this process are
continuous dewatering centrifuges.12 After the first pass
through the separators, a yeast solids content of 8 to 10 percent
can be acquired from a fermentor liquor containing 3.5 to
15
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4.5 percent solids.12 Next, the yeast is washed with water and
passed through the separators a second time. The second pass
usually produces concentrations of 18 to 21 percent solids.12
If the concentration of yeast solids recovered from the second
pass is below 18 percent, then another washing and a third pass
through the separators is normally required. The yeast cream
resulting from this process can be stored for several weeks at a
temperature slightly above 0°C (32°F). After storage, the yeast
cream can be used to propagate yeast in other trade fermentations
or can be further dewatered by filtration.
The centrifuged yeast solids are further concentrated by
pressing or filtration. Two types of filtering systems are used:
filter presses and rotary vacuum filters. In the filter press,
the filter cloth consists of cotton duck or a combination of
cotton duck and synthetic fibers so tightly woven that no filter
aid is necessary. Filter presses having frames of 58 to
115 centimeters (cm) (24 to 48 inches [in.]) are commonly "used,
and pressures between 860 to 1,030 kiloPascals (125 to 150 pounds
per square inch) are applied.12 Yeast yields between 27 and
32 percent solids may be obtained by pressing.1^ Rotary vacuum
filters are used for continuous feed of yeast cream. Generally,
the filter drum is coated with yeast by rotating the drum in a
trough of yeast cream or by spraying the yeast cream directly
onto the drum. The filter surface is coated with potato starch
containing some added salt to aid in drying the yeast product.
The filter drum rotates at a rate of 15 to 22 revolutions per
minute (rpra).12 As the drum rotates, blades at the bottom of the
drum remove the yeast. After a filter cake of yeast is formed
and while the drum continues to rotate, excess salt is removed by
spraying a small amount of water onto the filter cake. From this
process, filter cakes containing approximately 33 percent solids
are formed.
The filter cake is blended in ribbon mixers with small
amounts of water, emulsifiers, and cutting oils. Emulsifiers are
added to give the yeast a white, creamy appearance and to inhibit
water-spotting of the yeast cakes. A small amount of oil,
16
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usually soybean or cottonseed oil, is added to help extrude the
yeast. The mixed press cake is then extruded through open-
throated nozzles to form continuous ribbons of yeast cake. The
ribbons are cut and the yeast cakes are wrapped with wax paper.
The wrapped cakes are cooled to below 8°C (46°F), at which time
they are ready for shipment in refrigerated trucks.
3.2.7 Production of Dry Yeast
In yeast manufacturing, two types of dry yeast are produced:
(1) ADY and (2) IDY. Active dry yeast is produced from a yeast
strain identified as No. 7752 in the American Type Culture
Collection.13 This strain gives better yields than that used in
producing compressed yeast and is commonly referred to as the
"dry yeast strain." Instant dry yeast is produced from a yeast
strain different from that used for ADY. The ADY and IDY are
produced through the same process as that described for
compressed yeast. After filtration, the dry yeast product is
sent to an extruder, where emulsifiers and oils different from
those used for compressed yeast are added to texturize the yeast
and aid in extruding the yeast. After the yeast is extruded in
thin ribbons and cut, the yeast is dried in either a batch or a
continuous drying system. Fluidized bed dryers can be used to
dry the extruded yeast. The extruded yeast strands are fed into
the drying chamber of a fluidized bed dryer. Heated air blown
into the bottom of the dryer suspends the yeast particles into a
fluid bed and dries them. The drying time varies from 0.5 to
4 hours (hr).14 The humidity in the dryers is continuously
monitored to determine when the drying cycle is complete.
Following drying, the yeast is vacuum-packed or packed under
nitrogen gas before heated sealing. The shelf life of ADY and
IDY is 1 to 2 years at ambient temperature.15
4.0 EMISSION ESTIMATIONS
The following sections present a composite of the available
emissions data, process emission factors, nationwide emissions
estimates, and the documentation on the development of a model
process emission stream.
17
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4.1 SOURCES OF EMISSIONS
The VOC emissions are generated as by-products of the
fermentation process. The two major by-products are ethanol,
which is formed from acetaldehyde, and carbon dioxide. Other by-
products consist of other alcohols and organic acids such as
butanol, isopropyl alcohol, 2,3 butanediol, and acetate. These
by-products form as a result of excess sugar present in the
fermentor or an insufficient oxygen supply to the fermentor.
Under these conditions, anaerobic fermentation occurs and results
in the excess sugar being broken down to form alcohols and carbon
dioxide. When anaerobic fermentation occurs, 2 moles of ethanol
and 2 moles of carbon dioxide are formed from 1 mole of glucose.
Under anaerobic conditions, the ethanol yield is increased
and yeast yields are decreased. In producing baker's yeast it is
essential to suppress ethanol formation in the final fermentation
stages by incremental feeding of the molasses mixture and by
supplying sufficient oxygen to the fermentor.
The rate of ethanol formation is higher in the earlier
stages (pure culture stages) than in the final stages of the
fermentation process. The earlier fermentation stages are batch
fermentors, where excess sugars are present and less aeration is
used during the fermentation process. These fermentations are
not controlled to the degree that the final fermentations are
controlled, because the majority of yeast growth occurs in the
final fermentation stages and, therefore, there is no economical
reason for equipping the earlier fermentation stages with the
necessary process control equipment.
Another potential emission source is the wastewater
treatment system used to treat process wastewaters. If the
facility does not employ an anaerobic biological treatment
system, significant quantities of VOC could be emitted from this
stage of the process. For more information on wastewater
treatment systems as an emission source of VOC's, please refer to
an earlier CTC document on industrial wastewater treatment
systems entitled "Industrial Wastewater Volatile Organic
18
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Emissions—Background Information for BACT/LAER
Determinations."16
4.2 EMISSION DATA AND PROCESS EMISSION FACTORS
Emission test data were received from three yeast
manufacturing facilities.17'21 From a combination of these three
facilities, emission test data were available for the last four
fermentation stages (F4-F7).
During the fermentation process, ethanol and acetaldehyde
are not formed at constant rates; therefore, over the course of
the fermentation, the concentrations of these compounds vary
significantly depending upon the amount of excess sugars present
and the combined effectiveness of the aeration and agitation
systems to supply sufficient oxygen throughout the fermentor
volume. A review of the emission test data showed that the VOC
concentrations did vary significantly for each fermentation stage
and between different fermentors at a given stage. This
variation in emissions was expected between facilities because of
the differences in the feed systems and the size of the
fermentors. However, even within a given facility, the emission
data vary from fermentor to fermentor because of the differences
in the design of the air sparger system and the placement of
baffles and mechanical agitators within the fermentors.
Table 3 presents the VOC emission levels measured during
batch cycles for each type of fermentation. The emission test
data were converted from total VOC concentrations to ethanol
concentrations since ethanol is the primary VOC compound emitted.
Ethanol is approximately 80 to 90 percent of the emissions
generated, and the remaining 10 to 20 percent consists of other
alcohols and acetaldehyde.
A review of the emission data in Table 3 reveals the
significant variation in VOC emission levels. To obtain more
meaningful data, the process emission factors presented in
Table 3 were developed in an effort to normalize the fluctuations
in emission data between facilities and fermentors. However,
there was still a significant variation in the process emission
factors. Upon reviewing process information obtained from these
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TABLE 3. VOC EMISSIONS FROM YEAST FERMENTATION17'2la
Emission profile
Concentration range
, ppmv
Average concentration, ppmv
Maximum concentration, ppmv
Batch emissions, kg (Ib)
Process emission factors
VOC's emitted per volume of
fermentor operating capacity, kg/L
(Ib/gal)
VOC's emitted per amount of yeast
produced, kg/ 1,000 kg (Ib/ 1,000 Ib)
F4 (Intermediate)
900-4,600
1,900-2,400
3,000-4,600
24-71 (53-156)
0.0011-0.0014
(0.009-0.012)
12-49 (12-49)
Fermentation stages
F5/F6 (Stock/pitch)
2-1,350
50-700
200-1,350
6-821 (13-1,810)
0.0001-0.003
(0.0008-0.025)
0.5-40 (0.5-40)
F7 (Trade)
5-600
200
600
4.5-154(10-340)
0.000036-0.0006
(0.0003-0.005)
0.19-4.7(0.19-4.7)
aTotal VOC emissions as ethanol.
20
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facilities, it was concluded that the low end of the data range
is attributable to facilities that have implemented a greater
degree of process control or have improved fermentor designs over
those facilities that represent the high end of the data range.
Therefore, the typical emission levels and process emission
factors presented in Table 4 were developed to represent a
typical facility with a moderate degree of process control.
Based on process control information obtained from yeast
facilities, it is believed that the majority of yeast
manufacturing facilities fall within the emission ranges
presented in Table 4.
The typical emission levels for each fermentation stage
reveal the process control changes between fermentation stages.
The intermediate stage (F4) that follows pure culture
fermentations is either batch or fed-batch. The degree of
process control is not as stringent for this type of fermentation
as it is for trade fermentation because the yeast production
output from this fermentor is not as critical as that from the
final trade fermentation. As a result, the emission levels for
the intermediate fermentation are much higher than those for the
trade fermentation. However, total batch emissions from the
intermediate fermentation stage are lower than those from the
trade fermentation stage due to the smaller fermentors used and
the lower production rate. The final three fermentations (F5-F7)
are typically carried out in the same fermentors. The tighter
process control measures used during these fermentations result
in the lower emission levels.
The annual VOC emission rates presented in Table 4 were
developed based on the batch emissions from each fermentor and
the typical number of batches produced per year. As shown in
Table 4, the majority of emissions are associated with trade
fermentation as a result of the number of batches produced per
year. Trade fermentations account for 80 to 90 percent of the
emissions generated from -a given facility.
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TABLE 4. VOC EMISSIONS FOR A TYPICAL YEAST
MANUFACTURING FACILITY9
F4 (Intermedia
Typical ethanol emission levels
Fermentation stages
te) F5/F6 (Stock/pitch) F7 (Trade)
Concentration range, ppmv 900-4,600 2-400 6-600
Average concentration, ppmv 1,900 200 250
Maximum concentration, ppmv 3,000-4,600 200-400 500-600
Batch emissions, kg (Ib) 36.3(80) 49.9(110) 63.5(140)
Tvpical process emission factors
VOC's emitted per volume of fermentor 0.0012(0.010) 0.0004(0.0035) 0.0005(0.0045)
operating capacity, kg/L (Ib/gal)
VOC's emitted per amount of yeast 15 (15) 5.0 (5.0) 3.5 (3.5)
produced, kg/ 1,000 kg (Ib/ 1,000 Ib)
Operating time
No. of batches per week
No. of weeks per year
3 4 20
52 52 52
Annual VOC emissions rate, Mg/yr 5.6 (6.2) 8.5 (9.4) 66 (73)
(tons/yr)
*Total VOC's as ethanol.
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4.3 NATIONWIDE EMISSION LEVELS
Based on the process emission factor of 0.0005 kilogram of
VOC's emitted per liter per batch (0.0045 pound per gallon per
batch) of fermentor operating capacity, the typical size
(117,260 L [31,000 gal]) of trade fermentors, the number of
batches processed per year (1,040) at each facility, and the
number of yeast manufacturing facilities (13), the nationwide VOC
emissions from manufacturing baker's yeast is estimated at 860
Mg/yr (950 tons/yr).
Based on the process emission factor of 0.0035 kilogram of
VOC's emitted per kilogram (0.0035 pound per pound) of yeast
produced and the current nationwide annual production of baker's
yeast (220 million kilograms [490 million pounds]), the
nationwide VOC emissions from yeast manufacturing is estimated at
780 Mg/yr (860 tons/yr).
4.4 MODEL EMISSION STREAM
A model emission stream was developed in order to evaluate
control device performance at yeast manufacturing facilities.
The model emission stream was developed based on the combined
emission levels from five 117,260-L (31,000-gal) trade
fermentors. Trade fermentation was selected because the majority
of emissions are generated from this stage of the process. In
addition, the majority of the final fermentations (F5-F7) are
carried out in the same fermentors. Therefore, any control
technique applied to the trade fermentors would also control
emissions from the stock and pitching fermentation stages. Five
117,260-L (31,000-gal) trade fermentors were selected because
this was the average number and capacity of trade fermentors at
all yeast manufacturing facilities. Each trade fermentor was
assumed to have an air flow rate of 159 m3/min (5,600 ft3/min),
which was based on air flow rate data for actual trade
fermentors. Therefore, the model emission stream has an air flow
rate of 790 m3/min (28,000 ft3/min) and an average VOC
concentration of 200 to 300 ppmv. The average VOC concentrations
were determined based on the emission levels from actual trade
23
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fermentors. An emission profile for this fermentation stage is
presented in Figure 3.
5.0 EMISSION CONTROL TECHNIQUES
Only one yeast manufacturing facility employs an add-on
pollution control system to reduce VOC emissions from the
fermentation process. The pollution control system at this
facility consists of a wet scrubber followed by a biological
filter. However, all of the yeast manufacturers suppress ethanol
formation through varying degrees of process control.
The process control measures consist of incrementally
feeding the molasses mixture to the fermentors so that excess
sugars are not present and of supplying sufficient oxygen to the
fermentors to optimize the dissolved oxygen content of the liquid
in the fermentation vessel. The following sections provide a
more detailed discussion of the process control measures to
reduce ethanol formation and provide information on the
feasibility of implementing add-on control devices to reduce or
eliminate ethanol emissions from yeast fermentation vessels.
5.1 COMPUTERIZED PROCESS CONTROL MEASURES
Traditionally, yeast manufacturing plants have implemented
incremental feed systems on the final fermentation vessels in an
effort to optimize yeast yields and suppress ethanol formation.
However, these systems were established to add a given amount of
molasses and nutrients over specified time intervals. This
practice does reduce ethanol formation beyond that achieved under
a total batch condition; however, it does not minimize ethanol
formation to the highest degree possible. A greater degree of
control can be achieved by implementing a continuous monitoring
system.
Experimental studies have shown that the ethanol production
rate is a function of the yeast growth rate, and both of these
parameters are related to the residual sugar concentration.22 It
is therefore important that the actual sugar concentration in the
fermentor be maintained at a low but optimal value at all times.
In order to achieve this, the fermentation process must be
continuously monitored with the aid of a computer to anticipate
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500-
i*^
300-
to
200-
100
0
i r
1 2 3 4 5 67 8 9 10 11 12 13 14 15
TIME, H
Figure 3. Emissions profile of a typical trade fermentation.
-------
the precise demand for sugar. By continuously adding only the
exact amount of molasses required by the fermentation, conditions
of excess sugar are eliminated, thus minimizing ethanol
formation.
The demand for molasses depends on the cell concentration,
specific growth rate, and cell yield. Since no sensors are
available that can quickly and reliably give a direct measurement
of cell mass, computer-aided material balance techniques can be
used to calculate continuously the cell concentration, specific
growth rate, sugar consumption rate, and other growth-related
parameters.^ A computer can process information taken from
direct measurement of airflow, carbon dioxide production, oxygen
consumption, and ethanol production to anticipate the demand for
sugar by the system. The result is an indirect method for
monitoring yeast production that is regulated by a computer. The
computer continuously controls the addition of molasses, thereby
achieving optimum productivity with minimal ethanol production.
However, this type of process control system is extremely
difficult to refine and implement because of the time delays
between ethanol formation in the fermentor, its detection in the
stack, and the computer adjustments to the feed rates.
Another process measure that can reduce ethanol formation is
in the equipment design of the aeration and mechanical agitation
systems installed in each fermentor. The distribution of oxygen
by the air sparger system to the malt mixture is critical in
minimizing ethanol formation. If oxygen is not being transferred
uniformly throughout the malt, then ethanol will be produced in
the oxygen-deficient areas of the fermentors. The type and
position of baffles and/or a highly effective mechanical agitator
in the fermentors also ensures proper distribution of oxygen
throughout the malt mixture.
Facilities that are able to implement feed rate controls
through stack gas monitoring and that have efficient aeration and
mixing systems can optimize yeast production and suppress ethanol
formation to the highest degree. Based on available emission
test data, it is anticipated that a reduction of 75 to 95 percent
26
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can be achieved through the combination of feedback controls and
optimizing fermentor design.
5.2 ENGINEERING CONTROLS
The most common add-on control devices for controlling VOC
emissions are wet scrubbers (gas absorbers), carbon adsorbers,
incinerators, and condensers. These types of controls are widely
used in a variety of industries to control VOC emissions. The
following sections present a description of these control
techniques, operating parameters that affect their performance,
and the feasibility of implementing these controls to reduce
ethanol emissions from yeast manufacturing processes. In
addition to these traditional VOC controls, a section on
biological filtration is also presented below. Biological
filtration is a relatively new control technique for reducing VOC
emissions, although the engineering concept has been used for
over 50 years in treating process wastewaters.
5-2.1 Wet Scrubbers^ (GasAbsorption)
Wet scrubbers can control ethanol emissions from yeast
fermentation vessels. In this type of system, the contaminant is
absorbed in an absorbing liquid. Absorption techniques require
large liquid surface areas for the incoming gas stream to make
good contact with the absorber liquid. Good gas/liquid contact
is increased by using hydraulic sprays, impingement trays, bubble
cap trays, sieve trays, packing (modular and dump-type), grids,
or a combination of devices to create a high liquid surface area
while minimizing volume. These scrubbers can be divided into two
types: (1) packed/tray towers or (2) spray towers.
Figure 4 presents a schematic of a packed tower. In
packed/tray towers, the incoming gas stream enters the base of
the scrubber and passes up through the trays or packing
countercurrent to the absorbing liquid, which flows down through
the packing or tray media. In the packing section of the
scrubber, the contaminant in the gas stream is absorbed by the
absorbing liquid, and the cleaned gas stream flows up and out the
27
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LIQUID IN [J
GAS OUT
I . . I
PACKING
GAS IN
LIQUID OUT
Figure 4. Schematic of packed tower absorber.
28
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top of the unit. The absorbing liquid flows down and out the
bottom of the unit.
In spray towers, the absorbing liquid used in the tower is
sprayed countercurrent to the gas flow using high-pressure sprays
to create a uniform distribution of very small droplets of the
absorbent within the absorber. As in packed towers, the
contaminated gas stream enters the base of the unit and flows up
the tower, where it mixes with the liquid droplets in the unit.
This mixing of the gas and liquid streams results in the
contaminant's being absorbed by the liquid. The cleaned gas
stream then flows up and out the top of the unit, and the liquid
stream flows out the bottom of the unit.
Factors affecting the design and the performance of packed
towers include the VOC concentration and temperature of the inlet
gas stream, the type of absorbent used, the size and type of
packing material or trays used, the liquid-to-gas ratio, the
particulate loading, and the distribution of the liquid across
the packing media. The first parameters given depend on the
process being controlled. The next three parameters are design
parameters for the unit and depend on the type and concentration
of the contaminant to be removed. The contaminant must be highly
soluble in the absorbent selected. The particulate loading also
affects the performance of the scrubber: if the gas stream has a
moderate-to-high particulate content, the packing media and spray
nozzles will become clogged. Therefore, a filter needs to be
placed before the absorber to remove any particulates in the gas
stream prior to entry into the absorber. The distribution of
liquid across the packing media is critical for adequate contact
between the contaminated gas stream and the absorbing liquid.
For spray towers, the factors affecting performance are the
concentration and temperature of the inlet gas stream, the type
of absorbent selected, the liquid-to-gas ratio, the particulate
loading, and the uniform distribution and droplet size of the
absorber liquid. The first four factors are the same as those
for packed towers and affect the design of the unit in a similar
manner. The last two parameters are very important because if
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the absorbing liquid is not distributed uniformly across the
absorber volume and the droplet sizes are too large, the
performance level of the absorber will be adversely affected.
Applying wet scrubbers to control VOC emissions from yeast
manufacturing is feasible because both ethanol and acetaldehyde
are extremely soluble in water. Using water as the absorbing
liquid, a control device efficiency of better than 90 percent can
be achieved. The only adverse factor associated with using a
scrubber system is the amount of wastewater generated from the
scrubber system and its associated treatment. The wastewater
treatment procedure used to treat scrubber effluent should be an
anaerobic treatment system that prevents or minimizes VOC
emissions from the treatment process. If this type of treatment
system is not used, then the VOC emissions may not be reduced but
transferred to a different source at the plant site.
5.2.2 Carbon Adsorption
Carbon adsorption has been used for the last 50 years by
many industries to recover a wide variety of solvents from
solvent-laden air streams.23 Carbon adsorbers reduce VOC
emissions by adsorbing organic compounds onto the surface of
activated carbon. The high surface-to-volume ratio of activated
carbon and its preferential affinity for organics make it an
effective adsorbent of VOC's.23 The organic compounds are
subsequently desorbed from the activated carbon and recovered.
Carbon adsorbers do not apply to the low-VOC-concentration
gas streams from the final fermentation because of the low
adsorbtivity of ethanol at levels less than 500 ppmv. In
addition, one carbon adsorber vendor stated that the acetaldehyde
present in the gas stream is very reactive with the carbon and
would break down the carbon, resulting in low VOC removal
efficiencies.24 Therefore, carbon adsorption was not considered
to be a viable control option for yeast manufacturing facilities.
5.2.3 Incineration
Incineration is the oxidation of organic compounds by
exposing the gas stream to high temperatures in the presence of
oxygen and sometimes a catalyst. Carbon dioxide and water are
30
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the oxidation products. Incineration is often used in industries
when solvent recovery is not economically feasible or practical
such as at small plants or at plants using a variety of solvent
mixtures.25 The two types of incinerators used to control VOC
emission are thermal and catalytic. Both designs may use primary
or secondary heat recovery to reduce energy consumption.
5.2.3.1 Thermal incinerators. Thermal incinerators are
usually refractory-lined oxidation chambers with a burner located
at one end. In these units, part of the solvent-laden air is
passed through the burner along with an auxiliary fuel. The
gases exiting the burner that are blended with the by-passed
solvent-laden air raise the temperature of the mixture to the
point where the organics are oxidized. With most solvents,
oxidization occurs in less than 0.75 second at a temperature of
870°C (1600°F),26
The interrelated factors important in incinerator design and
operation include:
1. Type and concentration of VOC's;
2. Solvent-laden airflow rate;
3. Solvent-laden air temperature at incinerator inlet;
4. Burner type;
5. Efficiency of flame contact (mixing);
6. Residence time;
7. Auxiliary fuel firing rate;
8. Amount of excess air;
9. Firebox temperature; and
10. Preheat temperature.
The first three parameters are characteristics of the
fermentation process. The next three parameters are
characteristics of the design of the incinerator. The auxiliary
fuel firing rate is determined by the type and concentration of
VOC's, the solvent-laden airflow rate, the firebox temperature,
and the preheat temperature. The auxiliary fuel firing rate, the
amount of excess air, the firebox temperature and the preheat
temperature are operating variables that may affect the
performance of the incinerator. Well-designed and -operated
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incinerators in industry have achieved VOC destruction
efficiencies of 98 percent or better.25
Applying thermal incinerators to reduce VOC emissions from
the fermentation process should result in destruction
efficiencies of better than 98 percent. The costs associated
with operating a thermal incinerator are presented in
Section 5.3.
5.2.3.2 Catalytic incinerators. Catalytic incinerators use
a catalyst to promote the combustion of VOC's. The solvent-laden
air is preheated by a burner or heat exchanger and then brought
into contact with the catalyst bed, where oxidation occurs.
Common catalysts used are platinum or other noble metals on
supporting alumina pellets or ceramic honeycomb. Catalytic
incinerators can achieve destruction efficiencies similar to
those of thermal incinerators while operating at lower
temperatures, i.e., 315° to 430°C (600° to 800°F). Thus,
catalytic incinerators can operate with significantly lower
energy costs than can thermal incinerators that do not practice
significant heat recovery.26 The materials of construction may
also be less expensive because of the lower operating
temperatures.
Factors important in designing and operating catalytic
incinerators include the factors affecting thermal incinerators
as well as the operating temperature range of the catalyst and
the presence of constituents in the gas stream that could foul
the catalyst. The operating temperature range for the catalyst
sets the upper VOC concentration that can be incinerated. For
most catalysts on alumina, catalyst activity is severely reduced
by exposure to temperatures greater than 700°C (1300°F).26
Consequently, the heating value of the inlet stream must be
limited. Typically, inlet VOC concentrations must be less than
25 percent of the lower explosive limit (LEL). The typical VOC
concentrations emitted from yeast manufacturing facilities are
less than 25 percent of the LEL.
As with thermal incinerators, catalytic incinerators are a
viable control option for reducing VOC emissions from the
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fermentation process, having typical destruction efficiencies
greater than 98 percent. The costs associated with operating
catalytic incinerators are presented in Section 5.3.
5.2.4 Condensation
Condensation is a process in which all or some portion of
the volatile components in the vapor phase are transformed into
the liquid phase. This process can be accomplished through
several different methods. Increasing the system pressure at a
constant temperature, reducing the temperature, or a combination
of increasing pressure and reducing temperature are possible
methods. However, the most widely used condensation method is
decreasing the temperature at a constant pressure.
In a two-component vapor stream, where one of the components
is noncondensable, condensation occurs when the partial pressure
of the condensable component becomes equal to the component's
vapor pressure. At these conditions, the liquid begins to form.
As the temperature of the stream is further reduced, condensation
continues until the partial pressure of the vapor is equal to the
vapor pressure of the liquid phase at the lower temperature. The
amount of the compound that can remain as a vapor at a given
temperature is directly related to the volatility of the
compound. The more volatile the compound, the greater the amount
that will remain as a vapor. The type of coolant needed for the
condensation process depends on the temperature required for
condensation to occur.
Condensers are most effective on streams that are saturated
or nearly saturated with condensable VOC. When a gas stream is
dilute, as is the case with baker's yeast manufacturing,
extensive cooling is required just to bring the stream to the
saturation point. Furthermore, additional cooling is then needed
to actually condense the VOC. Condensers are not effective on
gas streams that contain low-boiling-point VOC's (i.e., highly
volatile compounds) or for gas streams that have a high flow rate
of noncondensables, such as carbon dioxide, nitrogen, or air.
The VOC's with low boiling points exert a high vapor pressure and
hence are more difficult to condense totally under normal
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condenser operating conditions. High flow rates of
noncondensable gases in the stream dilute the stream and reduce
condenser efficiency by increasing the heat load that must be
removed from the gas stream.
There are two major types of condensers: surface condensers
and contact condensers. Surface condensers are usually shell and
tube heat exchangers. The coolant flows through the tube, and
the vapor condenses on the outside, or shell-side, of the tube.
The condensate forms a film or masses of droplets on the tube and
drains into a collection tank for storage or disposal. Surface
condensers usually require more auxiliary equipment than contact
condensers, but solvent recovery is possible since the coolant
and the condensate are kept separate. Also, the coolant cannot
become contaminated in a surface condenser. Equipment typically
needed for surface condensers includes dehumidification
equipment, a shell-and-tube heat exchanger, a refrigeration unit,
a recovery tank for the condensate, and a pump to discharge
recovered VOC to storage or disposal.
Contact condensers cool the vapor by spraying a liquid,
usually at ambient temperature or slightly chilled, into the gas
stream. The result is intimate mixing of the gas stream and the
cooling medium. In some instances, contact condensers act as
scrubbers in that they collect noncondensables that are miscible
with the cooling medium. Contact condensers are simple in design
and relatively inexpensive to install. Although contact
condensers have advantages, their application is limited because,
like wet scrubbers, the VOC-contaminated coolant cannot normally
be reused directly.
The most obvious area to use condensers in the baker's yeast
manufacturing process is during the early stages of yeast growth
(pure culture stages). In these early stages, the concentration
of ethanol is expected to be at its highest and the airflow rates
are lowest. Although no emissions test data were available,
information supplied from yeast manufacturing facilities
indicates that the airflow rate from pure culture fermentation is
typically 10 m3/min (400 ft3/min) with an average VOC
34
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concentration of 5,000 to 10,00.0 ppmv. At these levels,
condensers should achieve better than 90 percent emission
reduction at condensation temperatures below -40°C (-40°F).
5.2.5 Biological Filtration
Figure 5 presents a simplified schematic of a biofiltration
system. A biological filter is basically a compost bed that has
been inoculated with aerobic microorganisms. The filter
eliminates VOC emissions by passing the VOC-laden gas stream
through the compost bed. As the gas stream passes through the
bed, the contaminants are removed through adsorption, absorption,
and chemical degradation. Portions of the contaminants are
adsorbed by the compost material while others are absorbed by the
water in the bed. These contaminants are then metabolized by the
microorganisms and are converted to carbon dioxide and water.
Ethanol conversion, in the case of yeast fermentation, is
accomplished in two stages. First, one type of microorganism
consumes the alcohol and converts it to organic acids. In the
second stage, another microorganism converts the organic acids
into carbon dioxide and water. A delicate balance between the
two microorganisms must be maintained to ensure proper operation
of the biological filters. After the gas stream passes through
the bed, the cleaned gas is vented out stacks located at the top
of the filtration unit. Volatile organic compound efficiencies
of better than 90 percent have been obtained when biofiltration
units have been installed to control emissions from yeast
fermentation vessels.27
The critical parameters for a biofiltration system are the
VOC concentration in the inlet gas stream, the pH and moisture
content of the bed, and the bed temperature. The bed temperature
must be maintained above 10°c (50°F) or the microorganisms in the
compost will become dormant. The pH and moisture content of the
bed should be in the range of 6.5 to 7.0 and 65 to 70 percent,
respectively. Water spray lines are located at the top of the
biofiltration unit to help maintain the moisture content at the
appropriate level. The inlet VOC concentration is also critical
to the performance of the biofiltration system. The
35
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Vents for
purified
gas stream
Water spray
nozzles
o\
Inlet VOC-laden
gas stream
Compost bed and
microorganisms
Figure 5. Schematic of a biofiltration system.
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biofiltration system is designed to handle dilute VOC streams
with fixed concentrations. Fluctuations in the concentration or
high VOC concentrations result in an imbalance between the two
controlling microorganisms. The microorganism that converts
alcohols to organic acids assimilates the alcohols at a rate that
decreases the pH of the bed, making the bed acidic. A low pH
will kill the other type of microorganism, the bed will no longer
operate efficiently, and incomplete conversion of the alcohols
will occur. For this reason, biofiltration systems were not
designed for batch processes but for continuous, dilute VOC
streams. However, a control system (i.e., wet scrubber) located
upstream of the biofiltration system that could control the peak
concentration levels would result in a fairly constant dilute VOC
stream to the biofiltration system. This combination of a
scrubber plus a biofiltration system would be a practical
approach to controlling emissions from a batch process.
5.3 IMPACT ASSESSMENT OF CONTROL OPTIONS
Based on the technical evaluation of traditional add-on VOC
control techniques, the most promising options for controlling
VOC emissions from the final fermentations appear to be wet
scrubbers, thermal incinerators, and catalytic incinerators.
Therefore, these control systems were evaluated further to
determine their associated cost and environmental impacts. The
impacts of the control options were determined based on the
effectiveness of the control systems to handle the model emission
stream presented in Section 4.4.
Table 5 summarizes the cost and environmental impacts of
each control system. To determine the cost impacts of the
control options, cost algorithms were developed based on standard
EPA methods.28'29 Tables 6 and 7 give details of the annual and
capital costs associated with each control device. The
environmental impacts were derived as a function of the design
and size of the control system. As shown in Table 5, catalytic
incinerators appear to be the most cost-effective control system
for this application.
37
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TABLE 5. SUMMARY OF THE COST AND ENVIRONMENTAL IMPACTS OF THE CONTROL OPTIONS
Control technique
Wet scrubbers*1
Thermal incinerators*
Catalytic incinerators
Capital cost, $
312,000
610,000
883,000
Annual cost,
$/yr
346,000
519,000
294,000
VOC
emissions
reduction,
Mg/yr
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TABLE 6. CAPITAL COSTS OF ADD-ON CONTROL OPTIONS*
Cost item
Direct costs, DC
1
Factor in
Cost, $
"hermal Catalytic
regen. incinerator Wet scrubber
;inerator
Purchased equipment costs
Control device
Auxiliary equipment"
Subtotal, A
Instrumentation
Sales taxes
Freight
0.1 A*
0.03A*
0.05A*
Total purchased equipment cost, B
Direct installation cost, C
[Foundation and supports
0.30B*
, handling
286,840 426,350 34,340
34.000 38.190 130.020
320,840 464,540 164,360
32,080 46,450 16,440
9,630 13,940 4,930
16.040 23.230 8.220
378,590 548,150 193,940
113,580 164,450 58,180
and erection, electrical, piping,
insulation, and painting]
Indirect costs (installation), 1C 0.3 IB*
117.360 169.930 60.120
[Engineering, construction and field
expenses, contractor fees, start-up,
performance test, and contingencies]
TOTAL CAPITAL INVESTMENT, Sum of B, C, 1C
609,500 882,500 312,200
aNumbers may not add exactly due to independent rounding.
^or incineration, auxiliary equipment consists of ductwork, stack, and fan. For wet scrubbers, auxiliary
equipment consists of ductwork, stack, fan, pump, platform and ladders, and packing.
cRounded to nearest $100.
*Source: OAQPS Control Cost Manual. Fourth Edition. EPA 450/3-90-006. January 1990. Chapter 3.
39
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TABLE 7. ANNUAL COSTS OF ADD-ON CONTROL OPTIONS8
Cost item
Direct annual costs. DAC
Operator labor
Operator
Supervisor
Maintenance
Labor
Material
Catalyst replacement
Waste disposal
Utilities
Water
Natural gas
Electricity
TOTAL DAC
Indirect annual costs. IAC
Overhead
Administration
Property taxes
Insurance
Capital recovery6
TOTAL IAC
TOTAL ANNUAL COST
Factor Unit cost
0.5 hr/shift* 12.96/hr*
15% of operator*
0.5 hr/shift* $14.26/hr*
100% of maintenance
labor*
100% catalyst $650/ft3 for
(2-yr life)* metal oxide*
$2/1, 000 gal**
$0.3/1, 000 gal**
$3 .30/1, 000 ft3*
$0.059/kWh*
60% of sum of
operator, supervisor,
labor, and materials*
2% TCI6*
1 % TCI*
1 % TCI*
CRF (TCI)*
Sum of DAC, IAC
Thermal regen. Catalytic
incinerator incinerator
3,590 3,590
540 540
3,950 3,950
3,950 3,950
42,220
-
349,280 32,050
26.570 29.190
387,870 1 15,480
7,210 7,210
12,190 17,650
6,100 8,830
6,100 8,830
99.200 136.210
130,790 178,720
518,700 294,200
Wet scrubber
3,290
490
3,620
3,620
-
217,650
32,650
15.190
276,510
6,610
6,250
3,120
3,120
50.820
69,920
346,400
'Numbers may not add exactly due to independent rounding.
**Total capital investment.
cThe capital recovery cost factor, CRF, is equal to 0.163 for a 10-year equipment life and a 10 percent interest rate. For catalytic
incineration, capital recovery is equal to 0.163(TCI-1.08x catalyst replacement).
•Source: OAQPS Control Cost Manual. Fourth Edition. EPA 450/3-90-006. January 1990. Chapter 3.
"Source: Organic Chemical Manufacturing Volume 5: Adsorption, Condensation, and Absorption Devices. EPA
No. 450/3-80-027. December 1980. Appendix B.
Report
40
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The cost of control could be reduced further, however, by
the use of a combination of control systems. A process using a
wet scrubber followed by an incinerator or a biological filter
could be conceptualized with the expectation that the annual cost
would be reduced. The water from the scrubber would not be
considered "wastewater" but would be recycled within the process
and used to continually generate an emission stream with a
constant concentration of ethanol at a reduced air volume. The
generation of a low-volume constant ethanol concentration stream
would allow the use of a smaller incinerator system or the use of
a biological filtration system, which could conceivably result in
lower control costs. Because of the complexity of such a system
and the need to consider plant specific conditions, a cost
analysis is not included in this report.
In addition, impacts for condensers were also evaluated
based on the control of emissions from the pure culture
fermentations. Table 8 gives details on the costing associated
with using condensers for controlling emissions from the pure
culture fermentors. These costs were developed based on a
typical airflow rate from pure culture fermentation of 10 m3/min
(400 ft3/min) at a VOC concentration of 7,500 ppmv. Based on a
95 percent control requirement, the condensation temperature of
the condenser would be -47°C (-53°F). The capital cost per
fermentor is estimated at $250,000. The annual operating cost is
estimated at $lll,000/yr. Based on an emission reduction of
27 Mg/yr (29 tons/yr), the average cost-effectiveness for
condensers is $4,200/Mg ($3,800/ton).
6.0 CONCLUSIONS
The typical yeast manufacturing facility emits approximately
82 Mg/yr (90 tons/yr) of VOC emissions. The primary constituents
in the emission stream are ethanol and acetaldehyde, with ethanol
comprising approximately 80 to 90 percent of the emissions and
acetaldehyde comprising the remaining 10 to 20 percent of
emissions. The primary emission sources are the final trade
fermentations, which account for 80 to 90 percent of the total
facility emissions.
41
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TABLE 8. COSTS FOR CONDENSER CONTROL OPTION3
Cost item Factor
Refrigeration cost, Ref
Total systems cost, TSC 1.25 (Ref)*
TOTAL CAPITAL INVESTMENT, TCI 2. 1 8 (TSC)
Direct annual costs, DAC
Labor
Operator 0.5 hr/shift*
Supervisor 1.15 (operator)*
Maintenance
Labor 0.5 hr/shift*
Materials 100% of labor*
Electricity $0.059/kwh*
TOTAL DAC
Indirect annual costs, IAC
Unit cost Cost, $
91,840
114,800
249,700
$15.64/hr* 8,560
9,850
$17.21/hr* 9,420
9,420
8,560
45,810
Overhead 60 % of sum of operator and 22,350
supervisor labor, and
maintenance labor and
materials*
Administration, taxes, insurance 0.04 (TCI)*
Capital recovery11 CRF (TCI)*
TOTAL IAC
TOTAL ANNUAL COSTS Sum of DAC, IAC
9,990
32,840
65,180
111,000
aNumbers may not add exactly due to independent rounding.
"The capital recovery cost factor, CRF, is equal to 0.1315.
*Source: OAQPS Control Cost Manual. Fourth Edition. EPA 450/3-90-006. January 1990. Draft Chapter.
42
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The two types of control measures that are currently
employed at yeast manufacturing facilities are (1) process
control and (2) add-on controls. The majority of yeast
manufacturers use a moderate degree of process control in the
final fermentation stages to reduce ethanol formation. However,
these process control measures can be enhanced by implementing
computer-based feed rate controls and improving fermentor
designs. Implementing a computer-based feed rate control system
and improved fermentor design can potentially suppress ethanol
formation by 75 to 95 percent.
One yeast manufacturer has applied a combination wet
scrubber and biofiltration system for controlling VOC emissions.
Performance data from this unit suggests an emission control
efficiency of better than 90 percent.27 Other add-on control
techniques that could potentially be applied to the yeast
fermentation process are incinerators. The control technology
evaluation suggests that a catalytic incinerator is the most
cost-effective approach for reducing VOC emissions, if the use of
a single control device is applied to the emission stream.
However, a combination -system such as that described above or a
combination of a wet scrubber and incinerator could result in
lower control costs and relatively equivalent emission
reductions.
At present, no process control measures or add-on pollution
control systems are currently being used to reduce VOC emissions
from the pure culture fermentations. However, based on the
information supplied by yeast manufacturing facilities, it
appears that adding process control measures to the pure culture
fermentations or applying condensers could potentially reduce VOC
emissions from this stage of the process by better than
90 percent.
43
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7.0 REFERENCES
1. Clean Air-Ozone. Press release, Associated Press. May 3,
1988.
2. Chen, S. L., and M. Chiger. Production of Baker's Yeast.
Comprehensive Biotechnology. Vol. 20. NY, Pergamon Press, p.
430.
3. Reed, G., and H. Peppier. Yeast Technology. Wesport, CT, Avi
Publishing Company. 1973. pp. 54-55.
4. Reference 2, p. 433.
5. Reference 3, p. 58.
6. Reference 3, pp. 57-58.
7. Reference 3, p. 60.
8. Reference 2, pp. 441-442.
9. Reference 2, p. 442.
10. Reference 2, pp. 74-75.
11. Reference 2, p. 80.
12. Reference- 2, p. 84-85.
13. Reference 2, p. 90.
14. Reference 2, p. 91-92.
15. Reference 2, p. 96.
16. Industrial Wastewater Volatile Organic Emissions—Background
Information for BACT/LAER Determinations. Control
Technology Center. U. S. Environmental Protection Agency,
Research Triangle Park, NC. EPA No. 450/3-90-004. January
1990.
17. Fermentor Emissions Test Report. Universal Foods
Corporation, Baltimore, MD. Prepared by Gannett Fleming,
Inc., Baltimore, MD. October 1990.
18. Fermentor Emissions Test Report. Gist-brocades Food
Ingredients, Inc., East Brunswick, NJ. Prepared by Trace
Technologies, Inc., Bridgewater, NJ. April 1989.
19. Fermentor Emissions Test Report. American Yeast, Baltimore,
MD. Prepared by Gannett Fleming, Inc., Baltimore, MD.
August 1990.
44
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20. Fermentor Emissions Test Report. Universal Foods, Inc.,
Baltimore, MD. Prepared by Universal Foods, Inc.,
Milwaukee, WI. 1990.
21. Fermentor Emissions Test Report. Universal Foods, Inc.,
Baltimore, MD. Prepared by Environmental Technology and
Engineering Corporation, Elm Grove, WI. May 1989.
22. Wang, H. Y., et al. Computer Control of Baker's Yeast
Production. Biotechnology and Bioengineering, Volume 21.
1979. pp. 975-995.
23. U. S. Environmental Protection Agency. Polymeric Coating of
Supporting Substrates - Background Information for Proposed
Standards. EPA-450/3-85-022a. Research Triangle Park, NC.
April 1987. p. 4-10.
24. Telecon. Hale, C., MRI, to Wersal, D., VIC Manufacturing,
Minneapolis, MN. September 3, 1990. Information regarding
the use of carbon adsorbers for controlling VOC emissions
from yeast fermentation processes.
25. Reference 23, p. 4-30.
26. Reference 23, p. 4-31.
27. Memo from Barker, R., and M. Williamson, MRI, to Smith, M.,
ESD/CPB. Trip report for Gist-brocades, East Brunswick, NJ.
August 1991.
28. Control Technologies for Hazardous Air Pollutants. U. S.
Environmental Protection Agency, Research Triangle Park, NC.
EPA/625/6-86/014. September 1986.
29. OAQPS Control Cost Manual. U. S. Environmental Protection
Agency, Research Triangle Park, NC. EPA 450/3-90-006.
January 1990.
45
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO. 2.
EPA 450/3-91-027
4. TITLE AND SUBTITLE
Assessment of VOC Emissions and Their Control
From Baker's Yeast Manufacturing Facilities
7. AUTHOR(S)
Robin Barker and Maresa Williamson
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Midwest Research Institute
401 Harrison Oaks Boulevard
Suite 350
Carv. NC 27513
12. SPONSORING AGENCY NAME AND ADDRESS
Control Technology Center
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
3. RECIPIENT'S ACCESSION NO.
5. REPORT DATE
January 1992
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-01-0115
13. TYPE OF REPORT AND PERIOD COVEHED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The Environmental Protection Agency's (EPA's) Control Technology Center
(CTC) conducted a study to obtain information on the baker's yeast manufacturing
industry. Baker's yeast La produced by a fermentation process that generates
large quantities of ethanol and acetaldehyde. Currently, 13 facilities produce
baker's yeast in the United States. The volatile organic compound (VOC) emission
rate from a typical facility is estimated at 82 megagrams per year (90 tons per
year). The majority of these emissions occurs in the final trade fermentations.
The VOC emission alternatives that were evaluated during this study were process
control measures to reduce the formation of VOC emissions as well as wet
scrubbers, carbon adsorbers, incinerators, condensers, and biological filters to
control VOC emissions. Of these approaches, it appears that process control
measures, catalytic incinerators, or a combination of add-on control techniques
(e.g., wet scrubbers followed by an incinerator or a biological filter) are the
most feasible approaches for controlling yeast process emissions. Based on the
results of this study, the control efficiency associated with the add-on control
systems is estimated to be 95 to 98 percent. This report contains information on
the baker's yeast fermentation process, the number and locations of yeast plants,
the potential emissions from the process, and an evaluation of potential emission
control options.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI E-ield/Group
Baker's yeast
yeast
fermentation
active dry yeast
compressed yeast
cream yeast
instant dry yeast
yeast manufacturing
ethanol
acetaldehyde
VOC emissions
VOC controls
process control
biological filtration
18. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
53
20. SECURITY CLASS (This page I
Unclassified
22. PRICE
EPA Form 2220-1 (R«v. 4-77) PREVIOUS EDITION is OBSOLETE
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