Process Description
Simtronics’ Ethanol Plant simulator enables comprehensive training for ethanol plant operators. The simulator consists of two connected processing units: Biomass Fermentation and Ethanol Distillation.
Biomass Fermentation Unit
The simulated Biomass Fermentation process is representative of new processes that employ Simultaneous Saccharification and Co-Fermentation (SSCF). The simulated process includes three fermenters which are used to:
– Grow cellulase (an enzyme that converts feed cellulose to sugars)
– Grow the inoculum Z. mobilis (a seed that converts sugars to ethanol)
– Simultaneously convert cellulose to sugars and produce ethanol
These three fermenters allow operators to develop a good understanding of fermentation principles and operations that would be applicable to commercial ethanol production facilities using a biomass feedstock.
Ethanol Distillation Unit
The simulated Ethanol Distillation unit is representative of the basic two-stage distillation process to conventionally concentrate the ethanol in a beer feed to approximately 95 liquid volume %. To make a fuel-grade ethanol product, the distillation section is followed by a drying unit to eliminate most of the remaining water in the product ethanol produced by distillation of the feed beer. The simulated process includes the following main equipment:
– Beer Column (to do the primary separation of ethanol from the beer)
– Rectification Column (to eliminate most of the water from the ethanol)
– Stillage Flash Drum (to remove ethanol from produced stillage)
– Ethanol Drying Unit (to eliminate additional water from the ethanol)
– Fusel Oil Decanter Unit (to remove byproduct fusel oils)
The conventional two-column distillation process allows operators to develop a good understanding of ethanol distillation principles and operations that would be applicable to commercial ethanol production facilities. This includes developing and understanding of the thermodynamic limit of ethanol concentration using conventional distillation.
Simulator Application
A full range of operations can be learned and practiced on the Ethanol Plant simulator. These include normal, startup, shutdown, and emergency shutdown procedures. The fermenters can be operated in either batch or continuous mode. This helps the operator understand why commercial processes employ many parallel trains that operate on a staged schedule so as to produce a near-continuous, high ethanol concentration product.
Biomass Fermentation Process Overview
The Biomass Fermentation process converts a cellulosic, biomass-derived feed into a solution of ethanol and water and smaller amounts of byproducts and solids. This solution can be distilled into fuel-quality ethanol. There are three main processing sections:
- Production of cellulase (enzymes) which are used to break down feed cellulose into sugars (saccharification)
- Production of a biological seed for converting sugars to ethanol (seed fermentation)
- Simultaneous saccharification of cellulose and fermentation of sugars to produce ethanol
All three sections use an acid-treated feed derived from ground wood chips as a main reactant/carrier. Cellulase is produced the first section and is used in both of the other sections. Saccharification of cellulose and fermentation of sugars occur simultaneously in the process. Therefore, the process is characterized by simultaneous saccharification and co-fermentation (SSCF).
Biomass Feedstock (Hydrolyzate)
The biomass feed (called hydrolyzate) is a water-based slurry produced from\ hydrolysis of ground wood chips using a dilute sulfuric acid solution. As a result of the acid hydrolysis, the feed contains exposed cellulose and sugars which can be converted to ethanol. The main sugars are glucose, xylose and arabinose. The feed has also been neutralized and detoxified to remove compounds that will inhibit the organisms used in the process.
The feed contains 16.2 weight % insoluble solids. The solids are mainly cellulose and lignin. Total sugar content is 5.6 weight %. Other soluble impurities amount to 1.8 weight % and the balance is water.
The hydrolyzate feed is pumped and split into three streams. About 5% of the total feed, on average, is used for cellulase production; about 10%, on average, is used for fermentation seed production; and the balance is sent to the main fermenter for ethanol production. Holdup tanks are provided for each fermenter to hold the product from each fermenter to buffer the fermenters from each other. In batch operations this provides feed scheduling flexibility.
Cellulase Production Unit Process Description
Cellulase Production Overview
A portion of the hydrolyzate feed is used as an environment to grow enzymes, known as cellulase, which are used to hydrolyze the exposed cellulose in the pretreated feed to form sugars such as glucose and xylose. To produce the enzymes, hydrolyzate, ammonia, water and air are combined in an agitated aerobic bioreactor vessel called the Cellulase Fermenter. Cellulase seed organisms (Trichoderma reesei) grown from smaller batch fermenters are also added to the Cellulase Fermenter. These organisms will continue to grow in the Cellulase Fermenter and produce the desired collection of enzymes. While growing enzymes, the Cellulase Fermenter operates in batch mode with air blown into the vessel continuously. Heat produced from the bioreactions is removed using cooling water which circulates through the coils in the vessel.
After the cellulase has reached the desired concentration, it is transferred from the Cellulase Fermenter to the Cellulase Hold Tank where it can be pumped to the SSCF Seed Fermentation Section and the Fermentation Section.
Note that in an industrial operation, the processing section for cellulase production may consist of several parallel cellulase fermentation vessels which operate in various stages: standby, supplying finished enzymes, and charging and fermenting. For purposes of training, only one vessel is simulated. The Cellulase Hold Tank is sized large enough where additional batches of cellulase can be produced in the Cellulase Fermenter while supplying cellulase to the other sections of the process without disruption.
F-301 Cellulase Fermenter
The Cellulase Fermenter F-301 is designed to produce large quantities of cellulase from a feed stream of cellulase seed containing seed organisms grown in smaller batch reactors. F-301 is charged with:
- Hydrolyzate feed (via FIC-301)
- Recycled water (via FIC-302)
- Ammonia (via FIC-303)
- Cellulase seed (via FIC-304)
Hydrolyzate for all three fermenters is pumped from battery limit by pump P-301. P-301 is a rotary lobe type of pump to handle the solids present in the hydrolyzate. The pump is designed to nominally move 1,580 GPM of hydrolyzate.
Each of the non-hydrolyzate feed flows to F-301 can be controlled in a ratio to the hydrolyzate feed flow during charging of the Cellulase Fermenter. To expedite training, the simulated process runs about 25 times faster than commercial processes, since the fermenters may take on the order of a day or days to conduct charging and reaction.
The following flow rates are used to charge F-301:
Stream Flow Rate
Hydrolyzate, FIC-301 70.0 GPM
Recycled water, FIC-302 87.7 GPM
Ammonia (gas), FIC-303 112.1 SCFM
Cellulase seed, FIC-304 9.94 GPM
At these rates it takes about one hour to charge F-301 to about 90% full on the simulator.
F-301 is a cylindrical vessel 30 feet in diameter and 60 feet high. The liquid contents are internally stirred with an agitator system that keeps conditions as well-mixed as possible, considering that materials in all three phases (liquid, solid and vapor) participate in the reactions in F-301. A sparger system in the lower part of the fermenter distributes air evenly throughout the vessel.
The cellulase seed stream contains the active organisms which will produce the cellulase (enzymes for converting cellulose to sugars) in F-301. The hydrolyzate and water feeds provide a fertile environment for the growth of the enzymes. As cellulase is produced, it converts some of the hydrolyzate to sugars. Cellulase is aerobically (using oxygen) grown from these sugars. Ammonia is provided for pH control as well as for providing nitrogen as a nutrient for growing the cellulase.
Both the saccharification reaction and the cellulase growth reaction produce heat which must be removed. F-301 normally operates at 84 DEG F. Operation at significantly higher temperatures results in destruction of the seed organisms while lower temperatures result in slow reaction rates. F-301 is outfitted with internal cooling coils that allow the heat to be carried away by cooling water. The agitator system also imparts heat to the contents of F-301 through the shear forces of the impellers.
Air is fed to F-301 at a rate of 22,271 SCFM. A small portion of the oxygen in the air is used to produce cellulase. Carbon dioxide (CO2) is produced and is carried away along with some water that evaporates by the air stream passing through F-301. The air, CO2 and water vapor are routed from the top of F-301 to an atmospheric vent.
In batch mode, the cellulase concentration will reach about 2.1 weight % in about an hour under proper conditions of feed ratios, air flow, temperature, and agitation. The reactor may also be operated in continuous mode where feeds are added while product cellulase is simultaneously being drawn off to the Cellulase Hold Tank, T301. In this case, the cellulase concentration will only reach about 1.5 weight %.
In commercial operations the fermenters are normally operated in batch mode since there are parallel fermentation trains operating in a staggered schedule so that a near-continuous product of highest possible ethanol concentration is produced. The simulator allows operation of a single train in either batch or continuous mode. The final ethanol concentration from a single train will naturally be lower when running in continuous mode, but a higher continuous production rate will result.
A cleaning system is provided for F-301 to wash away toxic organisms that would inhibit cellulase growth. A specially designed spray system ensures all internal parts of F-301 are thoroughly washed. The cleaning solution is routed to the Cellulase Hold Tank T-301 and/or a disposal system as needed.
K-301 Air Blower
Air for the production of cellulase is compressed from ambient conditions by Air Blower K-301. The increase in air pressure causes the discharge temperature to be too high for direct introduction to F-301. The air is cooled using cooling water in heat exchanger E-301 prior to introduction to F-301. In case of low flow operation to F-301, an automatic vent to atmosphere will open to ensure enough air flow through K-301 so it is not damaged by surge.
T-301 Cellulase Hold Tank
The Cellulase Hold Tank T-301 is used to store the cellulase product from the Cellulase Fermenter F-301 for feed to SSCF Seed Fermenter F-311 and the SSCF Fermenter F-321. The Cellulase Hold Tank allows the flexibility to clean F-301 and ready it for additional cellulase production while feeding the other two fermenters.
Cellulase product is pumped from the Cellulase Fermenter F-301 by the Cellulase Transfer Pump P-302 to the Cellulase Hold Tank T-301. P-302 is a rotary lobe type of pump so it can handle the solids present in F-301 that come from the hydrolyzate feed. The cellulase from P-302 can also be routed to the disposal system in case of cleaning F-301 or if the cellulase batch needs to be dumped because of a quality problem.
T-301 is sized identically to F-301. T-301 also has an agitator system that keeps the solids of the cellulase product suspended. The cellulase product in T-301 is pumped to the two other fermenters by Cellulase Pump P-303 which is also a rotary lobe type pump. T-301 is vented to atmosphere.
P-302 and P-303 are sized to nominally pump 166 GPM of liquid.
T-301 is cleaned using cleaning solution from F-301 after it has been cleaned. The contents of T-301 are then routed to fermenters F-311 and F-321 to clean the lines. F-311 and F-321 have wash and disposal capabilities like F-301.
SSCF Seed Fermentation Process Description
SSCF Seed Fermentation Overview
This section produces a biological seed for the fermentation of sugars to produce ethanol. The seed is produced by growing an inoculum (Zymomonas mobilis) in the SSCF Seed Fermenter. The inoculum requires sugars and other nutrients to grow.
Hydrolyzate is fed to the agitated SSCF Seed Fermenter vessel along with corn steep liquor, inoculum and cellulase from the Cellulase Fermenter. The corn steep liquor is a nutrient for growing the inoculum. The SSCF Seed Fermenter normally operates in batch mode. Heat produced from anaerobic fermentation of the inoculum and from saccharification of cellulose is removed by cooling coils in the vessel.
The produced mixture is called the SSCF Seed and after reaching the desired concentration it is transferred to the SSCF Seed Hold Tank and then charged to the Fermentation Section to produce ethanol. The SSCF Seed Hold Tank is sized large enough to provide sufficient flow of the SSCF Seed to the Fermentation Section while another batch of SSCF Seed is prepared. In an industrial operation, several parallel trains of fermentation vessels would be in operation. For training purposes, only one vessel is simulated.
F-311 SSCF Seed Fermenter
Hydrolyzate destined for the SSCF Seed Fermenter F-311 is pumped from P-301 and cooled in heat exchanger E-311 using cooling water. Corn steep liquor, inoculum and cellulase from the Cellulase Hold Tank T-301 are also fed to F-311. Cellulase is added to convert the cellulose in the hydrolyzate feed to sugars so that the inoculum seed will grow.
The SSCF Seed Fermenter F-311 is designed to produce large quantities of SSCF Seed from a feed stream of inoculum containing seed organisms grown in smaller batch reactors. F-311 is charged with:
- Hydrolyzate feed (via FIC-311)
- Inoculum (via FIC-312)
- Corn Steep Liquor (via FIC-313)
- Cellulase (via FIC-314)
Each of the non-hydrolyzate feed flows to F-311 can be controlled in a ratio to the hydrolyzate feed flow during charging of the SSCF Seed Fermenter. To expedite training, the simulated process runs about 25 times faster than commercial processes, since the fermenters may take on the order of a day or days to conduct charging and reaction. The following flow rates are used to charge F-311:
Stream Flow Rate
Hydrolyzate, FIC-311 145.8 GPM
Inoculum, FIC-312 0.481 GPM
Corn Steep Liquor, FIC-313 2.47 GPM
Cellulase, FIC-314 15.63 GPM
F-311 is a cylindrical vessel 30 feet in diameter and 45 feet high. The liquid contents are internally stirred with an agitator system that keeps conditions as well-mixed as possible in F-311. F-311 is vented to an exhaust gas scrubber for odor control before being discharged to the atmosphere.
The growth of Zymomonas mobilis occurs anaerobically (without oxygen) in F-311 and generates heat along with the saccharification of the hydrolyzate. F-311 normally operates at 88 DEG F. Operation at significantly higher temperatures results in destruction of the inoculum while lower temperatures result in slow growth rates. F-311 is outfitted with internal cooling coils that allow the heat to be carried away by cooling water. The agitator system also imparts heat to the contents of F311 through the shear forces of the impellers.
In batch mode, the SSCF seed concentration will reach about 0.44 weight % in about an hour under proper conditions of feed ratios, temperature and agitation. The reactor may also be operated in continuous mode where feeds are added while product cellulase is simultaneously being drawn off to the SSCF Seed Hold Tank, T311. In this case, the SSCF seed concentration will only reach about 0.32 weight %.
A cleaning system is provided for F-311 to wash away toxic organisms that would inhibit seed growth. A specially designed spray system ensures all internal parts of F311 are thoroughly washed. The cleaning solution is routed to the SSCF Seed Hold Tank T-311 and/or a disposal system as needed.
T-311 SSCF Seed Hold Tank
The SSCF Seed Hold Tank T-311 is used to store the SSCF seed product from the SSCF Seed Fermenter F-311 for feed to the SSCF Fermenter F-321. The SSCF Seed Hold Tank allows the flexibility to clean F-311 and ready it for additional SSCF seed production while feeding F-321.
SSCF Seed product is pumped from the SSCF Seed Fermenter F-311 by the SSCF Seed Transfer Pump P-311 to the SSCF Seed Hold Tank T-311. P-311 is a rotary lobe type of pump so it can handle the solids present in F-311 that come from the hydrolyzate feed. The SSCF Seed from P-311 can also be routed to the disposal system in case of cleaning F-311 or if the SSCF seed batch needs to be dumped because of a quality problem.
T-311 is sized identically to F-311. T-311 also has an agitator system that keeps the solids of the SSCF seed product suspended. T-311 is vented to an exhaust gas scrubber for odor control before being discharged to the atmosphere.
The SSCF seed product in T-311 is pumped to the SSCF Fermenter F-321 by SSCF Seed Pump P-312 which is also a rotary lobe type pump. P-311 and P-312 are sized to nominally pump 160 GPM of liquid.
T-311 is cleaned using cleaning solution from F-311 after it has been cleaned. The contents of T-311 are then routed to SSCF Fermenter to clean the line. F-321 has wash and disposal capabilities like F-311.
SSCF Fermentation Process Description
SSCF Fermentation Overview
The SSCF Fermentation section simultaneously converts cellulose to sugars (mainly glucose and xylose) and ferments these sugars to ethanol. The resulting mixture has an ethanol content high enough to be concentrated to fuel grade ethanol using distillation.
The bulk of the hydrolyzate feed is first cooled in heat exchanger E-321 using cooling water and fed to the SSCF Fermenter which is a large cylindrical tank. Cellulase from the Cellulase Hold Tank is charged to the SSCF Fermenter along with SSCF Seed from the SSCF Hold Tank. Ammonia and corn steep liquor are added as nutrients for the bacteria for cellulose conversion (saccharification) and for ethanol fermentation.
The SSCF Fermenter operates anaerobically and normally in batch mode. Any gases produced from fermentation are routed to a scrubber for odor control and ethanol recovery prior to venting to atmosphere. As enzymatic saccharification and fermentation simultaneously proceed, heat is given off from the mixture in the SSCF Fermenter. To keep the temperature at an optimal value, the mixture (called beer) is circulated through the SSCF Fermenter Cooler and returned to the SSCF Fermenter.
After most of the sugars have been converted to ethanol, the beer is transferred to the Beer Storage Tank and is then sent to the Beer Column for distillation of the ethanol from the water.
F-321 SSCF Fermenter
Hydrolyzate destined for the SSCF Fermenter F-321 is pumped from P-301 and cooled in heat exchanger E-321 using cooling water. Corn steep liquor, SSCF seed from the SSCF Seed Hold Tank T-311, cellulase from the Cellulase Hold Tank T-301, and ammonia are also fed to F-321. Cellulase is added to convert the cellulose in the hydrolyzate feed to sugars. SSCF seed is the active agent that converts the sugars to ethanol.
F-321 is charged with:
- Hydrolyzate feed (via FIC-321)
- Cellulase (via FIC-322)
- SSCF seed (via FIC-323)
- Corn Steep Liquor (via FIC-324)
- Ammonia (gas) (via FIC-325)
Each of the non-hydrolyzate feed flows to F-321 can be controlled in a ratio to the hydrolyzate feed flow during charging of the SSCF Fermenter. To expedite training, the simulated process runs about 25 times faster than commercial processes, since the fermenters may take on the order of a day or days to conduct charging and reaction. For commercial SSCF Fermenters, the hold time of the batch is on the order of several days to a week.
The following flow rates are used to charge F-321:
Stream Flow Rate
Hydrolyzate, FIC-321 1,364 GPM
Cellulase, FIC-322 149 GPM
SSCF seed, FIC-323 161 GPM
Corn Steep Liquor, FIC-324 4.23 GPM
Ammonia, FIC-325 15.63 SCFM
F-321 is a cylindrical tank 72 feet in diameter and 36 feet high. The liquid contents are internally stirred with an agitator system that keeps conditions as well-mixed as possible in F-321. F-321 is vented to an exhaust gas scrubber for odor control and ethanol recovery before being discharged to the atmosphere.
The saccharification and fermentation reactions generate heat which must be removed. The agitator system also imparts heat to the contents of F-321 through the shear forces of the impellers. F-321 normally operates at 93 DEG F. Operation at significantly higher temperatures results in destruction of the seed resulting in lower yields while lower temperatures result in low production rates.
F-321 is externally cooled with chilled water using a pumparound system. To ensure sufficient circulation two SSCF Circulation & Transfer Pumps (P-321A/B) are normally operating. The SSCF Fermenter Cooler E-322 uses chilled water. The normal circulation flow through E-322 is 1,125 GPM. The outlet temperature of the beer from E-322 is normally 65 DEG F. The circulation flow through E-322 should be reduced if the temperature of F-321 (as indicated on the suction of the circulation pumps) becomes too low.
In batch mode, the ethanol concentration will reach about 5.40 weight % in about two to three hours on the simulator under proper conditions of feed ratios, temperature and agitation. The fermenter may also be operated in continuous mode where feeds are added while product beer is simultaneously being drawn off to the Beer Storage Tank, T-321. In this case, the ethanol concentration will only reach about 3.24 weight %.
A cleaning system is provided for F-321 to wash away toxic organisms that would saccharification and/or ethanol production. A specially designed spray system ensures all internal parts of F-321 are thoroughly washed. The cleaning solution is routed to the Beer Storage Tank T-321 and/or a disposal system as needed.
T-321 Beer Storage Tank
The Beer Storage Tank T-321 is used to store the beer product from the SSCF Fermenter F-321 for feed to the Beer Column (off plot) where it will be distilled and the ethanol will be concentrated into a fuel grade product. The SSCF Seed Hold Tank allows the flexibility to clean F-321 and ready it for additional beer production while feeding the Beer Column.
Beer (SSCF Fermenter product) is pumped from the SSCF Fermenter F-321 by the SSCF Circulation & Transfer Pumps P-321A/B to the Beer Storage Tank T-321. P321A/B are rotary lobe types of pumps so they can handle the solids present in F321 that come from the hydrolyzate feed. The beer from P-321A/B can also be routed to the disposal system in case of cleaning F-321 or if the SSCF batch needs to be dumped because of a quality problem.
T-321 is a round vessel that is 40 feet in diameter and 40 feet high. T-311 also has an agitator system that keeps the solids of the beer product suspended. F-321 is vented to an exhaust gas scrubber for odor control before being discharged to the atmosphere.
The beer product in T-321 is pumped to the Beer Column by Beer Pump P-322 which is also a rotary lobe type pump. T-301 is vented to atmosphere.
P-321A/B are sized to nominally pump 1,378 GPM of liquid each. P-322 is sized to nominally pump 1,630 GPM of liquid.
T-321 is cleaned using cleaning solution from F-321 after it has been cleaned. The contents of T-321 are then routed to Beer Colum (off plot) to clean the line.
Ethanol Distillation Unit Process Description
Ethanol Distillation Process Overview
The Ethanol Distillation process distills a dilute solution of ethanol in the feed to a vapor ethanol stream of about 95 volume percent concentration with the balance being water vapor. This distilled vapor stream is sent to a drying unit to remove water and produce a liquid fuel-grade ethanol product, suitable for blending with gasoline. The main processing equipment are:
- T-501 Beer Column
- T-502 Rectification Column
- D-501 Stillage Flash Drum
- D-502 Reflux Drum
- X-501 Ethanol Drying Unit
- X-502 Fusel Oil Decanter
- E-501 Beer Preheater No. 2
- E-502 Beer Preheater No. 1
- E-503 Condenser
- EJ-501 Stillage Ejector
The feed to the unit is called beer and contains approximately 3.24 weight percent ethanol. The beer also contains non-volatile compounds such as unconverted sugars and fine solids from the fermentation process used to produce the beer. These compounds amount to about 16.8 weight percent of the feed. The balance of the beer consists of water and a small concentration of fusel oils generated in the fermentation process that produces the beer.
The beer is preheated in two heat exchangers (E-501 and E-502) using warm process streams prior to entering the Beer Column T-501. Vapor rich in ethanol flashes from the warm beer near the top of T-501. The balance of the ethanol in the beer is stripped out using direct steam injection into the bottom of the Beer Column.
Water, dissolved sugars and solids (known as stillage) are removed from the bottom of the Beer Column and sent to the Stillage Flash Drum D-501 to remove any ethanol that was not stripped out in the Beer Column. Stillage Flash Ejector EJ-501 pulls vacuum on the stillage in the Stillage Flash Drum. The vacuum conditions in D501 ensure very little ethanol is present in the final stillage, which is sent from D-501 to storage. Heat is recovered from the stillage by preheating beer in Beer Preheater No. 2 E-501.
Ethanol concentrates to about 32.5 weight percent in the overhead vapor from the Beer Column. The balance of the vapor is water vapor and a small amount of fusel oils from the beer feed. This vapor is sent to the base of the Rectification Column T502.
The Rectification Column concentrates the vapor from the Beer Column to the maximum practicable concentration for a mixture of ethanol and water by refluxing overhead condensate back to the Rectification Column. The overhead vapor from T502 is first cooled by preheating beer feed in Beer Preheater No. 1 E-502 and then is partially condensed using cooling water in Condenser E-503.
Vapor from E-503 contains about 88.6 weight percent ethanol and is sent off to the Ethanol Drying Unit X-501 where water is removed. The product ethanol is essentially water free and is taken off to storage. A recycle stream containing a low concentration of ethanol is produced from X-501 because a portion of the dried ethanol is used to regenerate the dryer beds in X-501 and picks up water in the process.
The liquid from E-503 is collected in Reflux Drum D-502 and is completely pumped back to the Rectification Column. The concentration of ethanol in the reflux is about 85.9 weight percent. Reflux to the Rectification Column results in concentration of ethanol in the vapor phase which results in a corresponding concentration of water in the liquid phase at the bottom of T-502. The liquid phase at the bottom of the Rectification Column contains only about 4.3 weight percent ethanol. This liquid is collected and pumped back (refluxed) to the Beer Column.
There are two side-stream draw lines from the Rectification Column to the Fusel Oil Decanter X-502. This system separates an immiscible layer of fusel oil in the draw streams from T-502. Fusel oil is drawn off, washed with fresh water to remove any dissolved ethanol and sent to storage. The aqueous streams from X-502 are combined and returned to T-502 under gravity flow.
Overview of Ethanol Distillation Unit Operating Conditions
Beer normally contains 3.42 weight percent ethanol and is fed in at a rate of 1,630 GPM. Direct low pressure (LP) steam injection to T-501 is normally 40,000 lb/h and motive steam for the Stillage Flash Ejector is 50,000 lb/h. The produced rate of stillage from the Stillage Flash Drum is 1,755 GPM. It exceeds the beer feed rate because the steam injected into the Beer Column is condensed and refluxed via the Rectification Column, so it ends up in the stillage. The stillage product has very low ethanol content. All the sugars and solids in the beer feed end up in the stillage. The overhead vapor from T-501 contains about 32.5 weight percent ethanol with the balance being water and a small concentration of fusel oils.
The ethanol concentration of the vapor leaving the top of Rectification Column T-502 is 86.4 weight percent. After partial condensation in E-502 and E-503, the net vapor produced from distillation has an ethanol concentration of 88.6 weight percent.
The net vapor from distillation is dried and condensed in X-501. The recovery ratio of ethanol in X-501 is assumed to be 95%. The balance is taken off as a recycle stream to the fermentation section at battery limits. The Ethanol Drying Unit produces 63.6 GPM of fuel grade ethanol (assumed to contain no water) and 9.4 GPM of recycle ethanol/water mixture.
The Ethanol Distillation Unit overhead runs at atmospheric pressure (14.7 PSIA). The two distillation columns run at slightly higher pressures owing to the pressure drop across the columns and heat exchangers in the unit. The Stillage Flash Drum D-501 normally runs at 10.1 PSIA.
T-501 Beer Column
The beer feed is pumped from battery limits to the Ethanol Distillation Unit and is preheated in two heat exchangers (E-501 and E-502) using T-502 overhead vapor and stillage to storage, respectively. The warm beer enters the Beer Column T-501 two trays from the top tray of T-501. The Beer Column consists of 30 special, high capacity distillation trays designed to handle solids contained in the liquid phase (baffle tray design). This type of tray helps minimize solids in the beer from accumulating within the distillation column. Ethanol and some water vapor flash from the warm beer as it enters T-501. The feed flash vapor combines with ethanol
and water vapors rising from the bottom of T-501. Reflux from the bottom of T-502 is pumped to the top tray of T-501. The top two trays of T-501 serve as a wash section to keep any entrained beer from reaching the outlet vapor line from the top of T-501. The top vapor from T-501 is routed to the bottom of Rectification Column T-502.
The liquid beer remaining from the feed flash combines with reflux falling from the top of T-501 and falls down through the baffle trays of T-501. The liquid mixture contacts rising vapors generated by steam injection to the bottom of the Beer Column. The steam strips out the ethanol from the falling beer. There are two sources of steam: direct low pressure (LP) live steam injection and exhaust steam from the Stillage Flash Ejector EJ-501. The ejector also compresses flash vapor from Stillage Flash Drum D-501 (mainly water vapor and a small amount of ethanol) which is routed into the bottom of T-501 along with the motive steam for EJ-501. The liquid that accumulates in the bottom of T-501 is taken off to D-501 by a combination of gravity and differential pressure developed by EJ-501.
D-501 Stillage Flash Drum
The Stillage Flash Drum D-501 receives stillage (water, dissolved sugars and solids) along with a small concentration of ethanol from the bottom of the Beer Column T501. The pressure in the D-501 is lower than the pressure in the Beer Column T-501 because of the compression of stillage flash vapors by Stillage Flash Ejector EJ-501. This lower pressure ensures that very little ethanol will be present in the flashed stillage liquid.
Stillage Flash Ejector EJ-501 pulls vacuum on the stillage in D-501 using medium pressure (MP) steam as the motive fluid for the ejector. The flash vapors combine with the ejector motive steam and are sent to the bottom of the Beer Column.
Stillage collected in D-501 is pumped by Stillage Pumps P-501A/B which are rotary lobe types of pumps so they can handle the solids present in the stillage. The Stillage Pumps are driven by electric motor. Normally only one pump is in operation.
The stillage from P-501A/B is routed to Beer Preheater No. 2 E-501 to cool down the stillage and heat the beer feed prior to sending the stillage to storage facilities at battery limits.
T-502 Rectification Column
The Rectification Column T-502 concentrates the vapor from the Beer Column using all the overhead condensate from the Reflux Drum D-502 as reflux. In lieu of a reboiler as would be found on a conventional distillation column, the Rectification Column relies on the heat content of the vapor from Beer Column to drive the separation of water and ethanol. T-502 consists of 35 valve trays. Reflux is received on the topmost tray. Net liquid from the bottom tray is collected in the bottom of T-502.
Overhead vapor from T-502 is routed to Beer Preheater No. 1 E-502. Some of the ethanol and water is condensed in E-502 while it heats up the beer feed from battery limits. The warm beer feed continues on to Beer Preheater No. 2 E-501. The partially condensed mixture from E-502 flows into Condenser E-503.
Liquid collected in the bottom of T-502 is pumped to the top of the Beer Column T-501 by Rectification Column Bottom Pumps P-502A/B. These are motor-driven centrifugal pumps. Normally, only one pump is in operation.
In order to prevent accumulation of fusel oils in T-502, two side draw lines are provided to continuously purge the oils from the column by routing them to the Fusel Oil Decanter X-502. Fusel oils are produced in the fermentation process that produces the beer feed. Because the boiling points of these oils are often close to that of the temperatures in the Rectification Column, they can end up getting trapped in the column, resulting in a buildup of liquid in the middle of the column which can lead to erratic operation of the Rectification Column. Normally, the lower purge line is in service. Either line draws a fraction of the liquid entering the tray they serve.
The Fusel Oil Decanter X-502 is simulated as a black box in the simulator. It is assumed that all the feed oil to the decanter system is separated and that all the ethanol is recovered by wash water in the secondary separator. The following equipment is assumed to be employed in X-502:
- Primary separator that provides enough residence time to allow the oils to separate from the ethanol/water phase.
- Wash water mixer – oils drawn off the primary separator are mixed and washed with fresh water to remove any ethanol that may have dissolved in the fusel oils.
- Secondary separator that separates the oils and the wash water. The fusel oil byproduct is taken off to storage at battery limits. The water phase from the secondary separator is combined with the ethanol/water phase from the primary separator and returned to a lower tray in the Rectification Column by gravity.
Ethanol/Water Azeotrope
NOTE
Because of the non-ideal relationship of vapor pressure versus ethanol concentration for ethanol/water mixtures, it is impossible to achieve an overhead ethanol concentration exceeding 95.6 weight percent using conventional distillation at roughly atmospheric pressure. Unlike more ideally behaving mixtures, the vapor pressure of solutions of ethanol and water reaches a maximum at this ethanol concentration. Such a mixture is called an azeotrope, which derives from Greek meaning “no change in boiling”. Pure ethanol is more volatile than water, so that mixtures containing lesser concentrations of ethanol will more readily boil off ethanol than water. However, as the concentration of ethanol in the liquid increases and approaches the azeotropic composition, the relative volatility of ethanol and water become the same, owing to non-ideal interactions between the two compounds. Thus, boiling off more vapor when the azeotropic point is reached will result in no more concentration of ethanol in the vapor. In fact, the vapor produced by additional boiling of the mixture will have the same exact composition as the liquid. At atmospheric pressure, an azeotropic mixture of ethanol and water will boil at 173.1 DEG F.
As a consequence, the maximum achievable concentration of ethanol in the overhead vapor from the Rectification column is the azeotropic composition. In theory, it would take a large number of trays (and cost) in a distillation column to reach the azeotropic composition. In practice, commercial ethanol distillation columns are designed to produce ethanol concentrations of about 89 weight percent ethanol (corresponding to 95 liquid volume percent or 190 proof). Downstream drying units (as in this simulator) or specially designed distillation processes that use a circulating solvent are commercially employed to remove the water which cannot be separated using conventional distillation. To qualify as fuel-grade ethanol the water content typically must be lower than 1.0 volume percent (about 98 weight percent ethanol). It is commercially preferable that the water content be even lower than this specification.
Condenser & Drying Section
The partially condensed mixture from E-502 flows into Condenser E-503 where additional condensation of ethanol and water occurs by heat exchange with cooling water. Not all the vapor is condensed in E-503 because the concentrated ethanol stream will be taken off to the Ethanol Drying Unit X-501 which uses molecular sieves operating in the vapor phase to remove the residual water. The amount of condensate and vapor produced in E-503 will depend on how much heat is removed by the cooling water.
Reflux Drum D-502 separates the vapor and liquid from E-503 and collects the liquid condensate. Normally all the vapor is taken off to X-501. In case of an upset to X-501, the vapor from D-502 can be vented to atmosphere to maintain a safe pressure in the process equipment.
Collected liquid in D-502 is pumped by Reflux Pumps P-503A/B to the top of Rectification Column T-502. P-503A/B are motor-driven centrifugal pumps. Normally, only one pump is in operation. In case of upsets which might cause overfilling of D-502, it is possible to send some or all of the reflux (wet ethanol) to storage.
Ethanol Drying Unit X-501 is simulated as a black box because drying units are highly automated and normally require very little operator attention. The product stream flow rates from the dryer are calculated assuming a constant recovery ratio of ethanol in the vapor feed to the unit (95%). The recycle stream flow rate is computed by material balance. The drying unit is assumed to employ the following equipment:
- Blower/compressor to move the vapor from the Reflux Drum to the dryer beds
- Dryer beds (molecular sieves)
- Valves and lines for automatic bed switching and regeneration
- Product ethanol condenser, receiver and product pump
- Regeneration gas heater
- Regeneration gas condenser, receiver and recycle pump
Instrumentation
Cellulase Fermentation
The motor for P-301 is operated by switch HS-301. The hydrolyzate flow rate to F-301 is controlled by controller FIC-301. The water, ammonia and cellulase feed flows are controlled by controllers FIC-302, FIC-303 and FIC-304, respectively. Each of these controllers can be operated in cascade/ratio control to the flow rate of FIC-301 by placing them into cascade mode.
Note that a selector switch, HS-301R, is present on page 2 of the group and trend displays. When this switch is in the AUTO position, the ratio of FIC-302, FIC-303 and FIC-304 will be automatically set when these controllers are not in cascade mode to provide bumpless transfer when switched into cascade mode. If HS-301R is in the MAN position, the operator must adjust the ratio manually from detail display of the controller before placing the controller into cascade mode, or leave the controllers in automatic mode and adjust the setpoints manually.
The flow rate of cleaning solution to F-301 is controlled by FIC-300. When cleaning F-301, the solution should initially be routed to the disposal facilities using HIC-302. Once F-301 is clean, the solution from F-301 can be pumped to T-301 via FIC-307 for cleaning the downstream equipment and lines.
The level of F-301 is indicated on LI-301. The operator must manually adjust inflows and outflows of F-301 to avoid overfilling it or to avoid operation of P-302 with a low liquid level in F-301. Overfilling F-301 will result in liquid carryover to the atmospheric vent line and should be avoided. Operation of P-302 with a low liquid supply level can result in damage to the pump.
The motor of the agitator for F-301 is operated by switch HS-304. It should not be operated whenever there is not a measurable liquid level in F-301.
The temperature of F-301 is controlled by controller TIC-303 which adjusts the flow of cooling water through the coils within F-301.
The concentration of the cellulase enzymes in F-301 is indicated on AI-301.
The motor of Air Blower K-301 is operated by switch HS-306. The outlet temperature of Air Cooler E-301 is controlled by TIC-308 which adjusts the cooling water flow through E-301.
The flow rate of air to F-301 is controlled by FIC-305. The air flow through K-301 is kept from going below a minimum flow of 18,000 SCFM by controller FIC-306.
The motor of Cellulase Transfer Pump P-302 is operated by switch HS-302. The cellulase transfer flow rate to T-301 is controlled by FIC-307. The manual control valve from P-302 to the disposal facilities is controlled by HIC-302. The flow in this line is not metered.
Cellulase Hold Tank
The level of T-301 is indicated on LI-302. The operator must manually adjust inflows and outflows of T-301 to avoid overfilling it or to avoid operation of P-303 with a low liquid level in T-301. Overfilling T-301 will result in liquid carryover to the atmospheric vent line and should be avoided. Operation of P-303 with a low liquid supply level can result in damage to the pump.
The temperature of T-301 is indicated on TI-304. Unexpected increase of this temperature likely indicates reaction(s) are occurring in T-301 (e.g. saccharification) or there are excessive shear forces on the agitator.
The motor of the agitator for T-301 is operated by switch HS-305. It should not be operated whenever there is not a measurable liquid level in T-301.
The motor of Cellulase Pump P-303 is operated by switch HS-303. The flow rate through the pump is set by flow controllers FIC-314 at SSCF Seed Fermenter F-311 and FIC-322 at SSCF Fermenter F-321.
The concentration of the cellulase enzymes in the discharge line of P-303 is indicated on AI-302.
SSCF Seed Fermentation
The hydrolyzate flow rate to F-311 is controlled by controller FIC-311. The inoculum, corn steep liquor and cellulase feed flows are controlled by controllers FIC-312, FIC-313 and FIC-314, respectively. Each of these controllers can be operated in cascade/ratio control to the flow rate of FIC-311 by placing them into cascade mode.
Note that a selector switch, HS-311R, is present on page 6 of the group and trend displays. When this switch is in the AUTO position, the ratio of FIC-312, FIC-313 and FIC-314 will be automatically set when these controllers are not in cascade mode to provide bumpless transfer when switched into cascade mode. If HS-311R is in the MAN position, the operator must adjust the ratio manually from detail display of the controller before placing the controller into cascade mode, or leave the controllers in automatic mode and adjust the setpoints manually.
The temperature of the hydrolyzate feed to the SSCF Seed Fermenter is controlled by TIC-312 which adjusts the cooling water flow through Feed Cooler E-311.
The flow rate of cleaning solution to F-311 is controlled by FIC-310. When cleaning F-311, the solution should initially be routed to the disposal facilities using HIC-311. Once F-311 is clean, the solution from F-311 can be pumped to T-311 via FIC-315 for cleaning the downstream equipment and lines.
The level of F-311 is indicated on LI-311. The operator must manually adjust inflows and outflows of F-311 to avoid overfilling it or to avoid operation of P-311 with a low liquid level in F-311. Overfilling F-311 will result in liquid carryover to the exhaust gas scrubber and should be avoided. Operation of P-311 with a low liquid supply level can result in damage to the pump.
The motor of the agitator for F-311 is operated by switch HS-313. It should not be operated whenever there is not a measurable liquid level in F-311.
The temperature of F-311 is controlled by controller TIC-313 which adjusts the flow of cooling water through the coils within F-311.
The concentration of the SSCF seed in F-311 is indicated on AI-311.
The motor of SSCF Seed Transfer Pump P-311 is operated by switch HS-311. The cellulase transfer flow rate to T-311 is controlled by FIC-315. The manual control valve from P-311 to the disposal facilities is controlled by HIC-311. The flow in this line is not metered.
SSCF Seed Hold Tank
The level of T-311 is indicated on LI-312. The operator must manually adjust inflows and outflows of T-311 to avoid overfilling it or to avoid operation of P-312 with a low liquid level in T-311. Overfilling T-311 will result in liquid carryover to the exhaust gas scrubber and should be avoided. Operation of P-312 with a low liquid supply level can result in damage to the pump.
The temperature of T-311 is indicated on TI-314. Unexpected increase of this temperature likely indicates reaction(s) are occurring in T-311 (e.g. saccharification) or there are excessive shear forces on the agitator.
The motor of the agitator for T-311 is operated by switch HS-314. It should not be operated whenever there is not a measurable liquid level in T-311.
The motor of SSCF Seed Pump P-312 is operated by switch HS-312. The flow rate through the pump is set by flow controller FIC-323 at SSCF Fermenter F-321.
The concentration of the SSCF Seed in the discharge line of P-312 is indicated on AI-312.
SSCF Fermentation
The hydrolyzate flow rate to F-321 is controlled by controller FIC-331. The cellulase, SSCF seed, corn steep liquor and ammonia feed flows are controlled by controllers FIC-322, FIC-323, FIC-324 and FIC-325, respectively. Each of these controllers can be operated in cascade/ratio control to the flow rate of FIC-321 by placing them into cascade mode.
Note that a selector switch, HS-321R, is present on page 9 of the group and trend displays. When this switch is in the AUTO position, the ratio of FIC-322, FIC-323, FIC-324 and FIC-325 will be automatically set when these controllers are not in cascade mode to provide bumpless transfer when switched into cascade mode. If HS-321R is in the MAN position, the operator must adjust the ratio manually from detail display of the controller before placing the controller into cascade mode, or leave the controllers in automatic mode and adjust the setpoints manually.
The temperature of the hydrolyzate feed to the SSCF Fermenter is controlled by TIC-321 which adjusts the cooling water flow through Feed Cooler E-321.
The flow rate of cleaning solution to F-321 is controlled by FIC-320. When cleaning F-321, the solution should initially be routed to the disposal facilities using HIC-321. Once F-321 is clean, the solution from F-321 can be pumped to T-321 via FIC-327 for cleaning the downstream equipment and lines.
The level of F-321 is indicated on LI-321. The operator must manually adjust inflows and outflows of F-321 to avoid overfilling it or to avoid operation of P-321A/B with a low liquid level in F-321. Overfilling F-321 will result in liquid carryover to the exhaust gas scrubber and should be avoided. Operation of P-321A/B with a low liquid supply level can result in damage to the pump.
The motor of the agitator system for F-321 is operated by switch HS-323. It should not be operated whenever there is not a measurable liquid level in F-321.
The temperature of F-321 is indicated by TI-322 on the line to the suction of pumps P-321A/B. The circulating beer return temperature is controlled by TIC-323 which adjusts the flow of chilled cooling water through the heat exchanger E-322. If the temperature of F-321 becomes too low or too high, adjust the circulating beer flow rate through E-322 using FIC-326.
The concentration of the ethanol in F-321 is indicated on AI-321.
The motor of SSCF Circulation & Transfer Pumps P-321A/B are operated by switches HS-321A/B. The beer transfer flow rate to T-321 is controlled by FIC-327. The manual control valve from P-321A/B to the disposal facilities is controlled by HIC-321. The flow in this line is not metered.
Beer Storage Tank
The level of T-321 is indicated on LI-322. The operator must manually adjust inflows and outflows of T-321 to avoid overfilling it or to avoid operation of P-322 with a low liquid level in T-321. Overfilling T-321 will result in liquid carryover to the exhaust gas scrubber and should be avoided. Operation of P-322 with a low liquid supply level can result in damage to the pump.
The temperature of T-321 is indicated on TI-324. Unexpected increase of this temperature likely indicates reaction(s) are occurring in T-321 (e.g. saccharification and/or ethanol production) or there are excessive shear forces on the agitator.
The motor of the agitator for T-321 is operated by switch HS-334. It should not be operated whenever there is not a measurable liquid level in T-321. The motor of Beer Pump P-322 is operated by switch HS-322. The flow rate through the pump is set by flow controller FIC-328. The concentration of the ethanol in the discharge line of P-322 is indicated on AI-322.
Beer Column
The beer feed to the unit from battery limits is controlled by FIC-328 in the fermentation section. It is also indicated on FI-501 in the distillation section. The beer ethanol concentration is indicated on AI-501 and the temperature is indicated on TI-501. After passing through E-502 and E-501, the preheated beer temperature is indicated on TI-503 prior to entering the Beer Column T-501.
The top pressure of T-501 is indicated on PI-501 and the overhead vapor temperature is indicated TI-504. Tray 20’s temperature is indicated on TI-505. The differential pressure across the trays of the Beer Column is indicated on PDI-503. The bottom level of stillage in T-501 is controlled by LIC-501 which adjusts the flow of stillage to the Stillage Flash Drum D-501. The stillage temperature from T-501 is indicated on TI-506. Direct LP steam flow to the bottom of T-501 is controlled by FIC-503. The supply pressure of LP steam is indicated on PI-503.
The level of stillage in the Stillage Flash Drum D-501 is controlled by LIC-502 which adjusts the flow of stillage to storage at battery limits. The flow rate of MP steam to the Stillage Flash Ejector EJ-501 is controlled by FIC-504. The supply pressure of MP steam is indicated on PI-504. The pressure of the Stillage Flash Drum is indicated on PI-502. This pressure is affected by the net vapor compression capacity of EJ-501 which is, in turn, affected by the MP steam flow to the ejector. Therefore, adjustments to the MP steam flow will affect the pressure in D-501. Keep in mind that any MP steam flow adjustments will also affect the performance of the distillation columns (e.g. more steam will eventually increase the ethanol concentration in the vapor to the drying unit and will increase the reflux in the two columns). The motors for Stillage Pumps P-501A/B are operated by switches HS-501A/B, respectively. The warm stillage temperature leaving P-501A/B is indicated on TI-507. The temperature of stillage after cooling in E-501 is indicated on TI-508. The flow rate of stillage to storage at battery limits is indicated on FI-502.
Rectification Column
The ethanol concentration of the vapor feed to the Rectification Column T-502 from the Beer Column T-501 is indicated on AI-502. The level of the bottom of T-501 is controlled by LIC-511 which adjusts the setpoint of the column bottoms liquid flow controller FIC-511. Rectification Column bottoms is reflux for the Beer Column T-501. The T-502 bottoms liquid temperature is indicated on TI-515. The ethanol composition of the bottoms liquid is indicated on AI-511.
The motors for Rectification Column Bottoms Pumps P-502A/B are operated by switches HS-502A/B, respectively. The flow of column bottoms from P-502A/B to Beer Column T-501 is controlled by FIC-511.
The differential pressure across the trays of T-502 is indicated on PDI-512. The flow of the upper side draw to the Fusel Oil Decanter X-502 is controlled by HIC-511. The upper draw tray temperature is indicated on TI-513. The flow of the lower side draw to the Fusel Oil Decanter X-502 is controlled by HIC-512. The lower draw tray temperature is indicated on TI-514. The wash water flow to X-502 is controlled by FIC-512. The net produced fusel oil product flow to battery limits is indicated on FI-513.
The temperature of tray 28 is controlled by TIC-512 which adjusts the cooling water flow through Condenser E-503. Increasing the cooling water flow will produce more condensate from E-503 which, in turn, will cause the reflux flow to the top of T-502 to increase. As the reflux flow increases, the temperature of tray 28 will decrease because the tray’s ethanol concentration will increase at the expense of the water concentration.
The pressure at the top of T-502 is indicated on PI-511. The overhead vapor temperature entering Beer Preheater No. 1 E-502 is indicated on TI-511. The temperature of the partially condensed vapor leaving E-502 is indicated on TI-516. The beer feed temperature leaving E-502 is indicated on TI-502.
Condenser & Drying Section
The process side outlet temperature of Rectification Column Condenser E-503 is indicated on TI-521. The cooling water flow through E-503 is controlled by tray 28 temperature controller TIC-512. Note that TV-512 has a minimum stop of 10% to prevent complete stoppage of cooling water flow through E-503.
The pressure of Reflux Drum D-502 is normally controlled by PIC-521B which controls the flow of concentrated ethanol vapor to the Ethanol Drying Unit X-501. The flow of vapor to X-501 is indicated on FI-523 and the ethanol content of the vapor is indicated on AI-521. PIC-521A will automatically vent the ethanol vapor from D-502 if the pressure gets too high. Its setpoint is 20 PSIA.
The flow of product ethanol from X-502 is indicated on FI-524. The flow of recycle ethanol is indicated on FI-525.
The level of D-502 is normally controlled by LIC-521A which adjusts the setpoint of the Rectification Column reflux flow controller FIC-521. In case the level in D-502 gets too high, LIC-521B will divert wet ethanol from D-502 to storage at battery limits. The setpoint of LIC-521B is 90%.
The motors for Reflux Pumps P-503A/B are operated by switches HS-503A/B, respectively. The flow of reflux (wet ethanol) P-503A/B to Rectification Column T-502 is controlled by FIC-521. The concentration of ethanol in the reflux is indicated on AI-522 and the temperature of the reflux is indicated on TI-522. The flow rate of wet ethanol to storage at battery limits is controlled by FIC-522.