Vacuum Fermentation
Studies on vacuum fermentation have been conducted in Great Britain, the United States,
and elsewhere. Work by the W. S. Atkins Group at Manchester University has centered
on reducing energy use and fermentation time through vacuum fermentation of
concentrated sub strafes with high yeast levels. Substrates such as molasses and sac
charified cassava are fed directly to the fermenter, and a portion of the fermenting mass is
recirculated through a flash separator to remove alcohol as it is formed.
Figure 19. Profile view of the experimental unit showing the flow patterns maintained
during fermentation. (E. Wick)
Figure 20. Continuous fermentation system. (K. Rosen)
Work on vacuum fermentation by Ramalingham and Finn at Cornell University indicates
that a threefold higher sugar concentration can be fermented in one-third of the time
needed in a conventional process.
Preliminary process studies by Cysewski and Wilke at the University of California
indicate that ethanol plant capital costs for vacuum operation may be reduced up to 71
percent over batch processing.
Extractive Fermentation
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Rolz at ICAITI (Guatemala) has developed an extractive fermentation system based on
sugarcane. In this process, whole cane from the fields is cleaned and cut, with the foliage
retained for fuel use. The cane is then chipped and placed in a fermenter, where water and
yeast are added. The sucrose from the cane is extracted and fermented simultaneously in
the same equipment. When the fermentation is finished, the extracted chips are separated
and the solution is charged to a batch of fresh chips. After this second fermentation, the
solution (containing about 5 percent ethanol) is distilled to recover the alcohol.
Alcohol Recovery
In examining the energy consumption of alternative recovery systems, Vergara has
summarized the status and efficiency of several approaches as shown in Table 8.
Other possibilities for low-energy alcohol recovery include the use of reverse osmosis,
molecular sieves, solvent extraction, and supercritical carbon dioxide.
In studies comparing these techniques at Battelle Pacific Northwest Laboratories (USA),
a solvent extraction method looks most promising. In this process, a proprietary solvent
mixture is contacted with the ethanol-water mix derived from fermentation and extracts
the ethanol. This is done under slight pressure, so that when the pressure is subsequently
reduced, the bulk of the solvent flashes off and is recovered for reuse.
Battelle also examined a carbon dioxide extraction process developed by Arthur D. Little
(USA) in which liquid CO2 is used to extract ethanol and then depressurized to flash off
the CO2. Results of these tests are compared with other conventional and
nonconventional techniques in Table 9.
TABLE 8 Energy Efficiency of Alternative Distillation Systems
System
Live steam
Energy Used
Status
(kg steam/ liter alcohol)
4.5-5.0
Commercial
Reboiler, feed preheating, optimization
3.5
Commercial
3.7
Pilot
3.6
Pilot
1.6
Laboratory
Atmospheric distillation, CaO dehydration 1.3
Laboratory
Vacuum fermentation, atmospheric
distillation
Differential pressure fermentation,
atmospheric distillation
Differential pressure fermentation, vacuum
distillation
Atmospheric distillation, cellulose
dehydration
1.0
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Laboratory
An alternative to the final distillation step used to produce anhydrous alcohol has been
examined by Maiorella at the University of California. Using the process developed by
the Mobil Oil Corporation for the conversion of methanol to gasoline, 85 percent ethanol
is first partially converted to ethylene and ethyl ether and then catatyfically converted to a
synthetic gasoline. The reactions and conceptual design are shown in Figures 21 and 22.
By-Product Recovery and Waste Treatment
The principal by-products in ethanol production are carbon dioxide, fusel oils, and
stillage.
Carbon Dioxide
This gas is produced during the fermentation step and is usually vented to the
atmosphere. In most cases, recovery of carbon dioxide and its compression for use as a
solid refrigerant ("dry ice") or use in promoting growth in controlled environment
agriculture (greenhouses) is uneconomic: about 575 kg of carbon dioxide are produced
for each 1,000 liters of ethanol.
TABLE 9 Energy Requirements of Ethanol Separation Processes
Ethanol
(%)
Energy
Type of Separation Initial Final
Complete
Process
10
100
Conventional distillation
Needed
Btu/gal)
27,400a
10
100
CO2 extraction
8,00010,000b
To azeotrope
Azeotropic to
complete
10
100
Solvent extraction
3,600
10
100
Vacuum distillation
37,000c
10
95
Conventional distillation
18,000
10
95
Vapor recompression
6,400b
10
95
Multi-effect vacuum
7,200d
95
100
Conventional distillation
9,400
95
100
Dehydration by adsorption
1,200e
95
100
Low-temperature gasoline
blending
3,000f
95
100
Molecular sieve
4,7006,270
Other
5
10
Reverse osmosis
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500
a
Sum of distillation to azeotrope and azeotropic distillation
Thermal energy required for process mechanical energy
c
Single-column distillation
d
Three-column distillation
e
For drying with CaO
f
Azeotrope blended with gasoline and cooled to - 30° C
SOURCE: Battelle-Northwest, Richland, Washington, USA.
b
Figure 21. Ethanol can be converted to gasoline components by a route similar to one
used for methanol to gasoline. (B. Maiorella)
Figure 22. Conceptual design of a two-stage system to convert ethanol to gasoline
hydrocarbons. (B. Maiorella)
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