(Body of report)
In this plant, saccharification will occur separately from fermentation, although
simultaneous saccharification and fermentation does occur in the fermentors since the
cellulose enzyme is not removed between steps. Arranging the process in this manner
has several advantages. First, separate reactors allow for the use of separate reaction
conditions. Because fermentation requires a whole organism to convert useable sugars
into the product, while saccharification requires only the active enzyme, the
saccharification reactor can operate at more extreme temperatures and product
concentrations. Operating these reactors at higher temperature utilize Arrhenius kinetics
and optimize the rate of hydrolysis without degrading the enzyme.Aden This reduces both
the holding time of each reactor and the amount of enzyme required. A second
advantage of independent saccharification and fermentation is that smaller reactors are
required than if the processes were to occur simultantously. This is especially
advantageous with process control. If a reactor were to become contaminated, for
example, a large investment would be required to disinfect large reactors necessary for
simultaneous saccharification and fermentation. However, if the two processes are
separated, then potential contaminants would be easier to contain and less down-time
would be required to restart the process. Finally, large reactors that would be required
for simultaneous saccharification and fermentation would introduce problems with mass
transfer. For example, large reactor volumes require more enzyme and substrate to
maintain the same concentrations that allow for optimal reaction rates. They would also
require much larger impellers to agitate the fluid and evenly distribute the substrate. This
is especially problematic since large impellers introduce significant operating costs. All of
these factors greatly outweigh the lower capital costs that result from fewer reactors.
Thus, separate saccharification and fermentation is optimal for this process.
Saccharification will take place in a train of five 500,000 gallon reactors, each
operating at 65°C in fed batch mode. As mentioned before, having multiple reactors is
beneficial because one can control contamination and product output. The holding time
for each reactor will be 36 hours – 24 hours of reaction time and 12 hours of turn-around
time. Tanks are maintained at a constant temperature by continuously pumping reactor
contents through a heat exchanger cooled by cooling water. The cellulase required is
12.0 filter paper units per gram of hydrolyzed sugar.Aden Assuming pretreatment is
capable of producing 0.75 tons of utilizable sugars per ton of feedstock (the remainder
consisting primarily of lignin), and saccharification conditions allow 71% conversion to
monosaccharides,Qu then the amount of cellulase required is 12.8 GFPU per day. The
reactors reach concentrations of about 5.1 kFPU/gal cellulase and 0.4 kg/gal sugar. One
problem that may arise, however, is large pumping costs due to Non-Newtonian
behavior of polysaccharide solutions.
To grow up the yeast necessary to ferment the saccharification slurry, 10% of the
product is sent to two trains of five seed fermentors with an inoculum volume of 10%.
The reactors operate at 41°C in fed batch mode with a holding time of 36 hours,
including 12 hours for turn-around time. Each stage scales up by a factor of 10 until the
seed volume is capable of fermenting the slurry. Based on the desired production rate,
fermentor sizes will be 20, 200, 2000, 20,000, and 200,000 gallons in size. As with the
saccharification reactors, the seed fermentors are cooled with cooling water in a heat
exchanger. The yeast will also need nutrients to grow to the desired level. Sufficient
growth occurs with corn steep liquor levels of 0.5% and diammonium phosphate levels
of 2.5 g/gal.Aden The remaining slurry is then fed to a train of large-scale fermentors,
which are roughly twice the size of the saccharifiers to account for the nutrients that
need to be added.
(Conclusion)
Developing countries like Taiwan will likely struggle to satisfy their energy needs
as industrialization continues to take effect. This is especially true for transportation
fuels, which will increase in demand as more and more people begin to purchase
automobiles. In a time of increasing energy costs and decreasing fossil fuel reserves,
researchers are forced to find an alternative to conventional energy sources. Corn-toethanol plants are a promising step in the transition to renewable energy. However,
before these plants can be implemented feasibly, several improvements need to be
made in the process. Research must be conducted to improve yields of the
saccharification and fermentation reactions. Also, more economical methods must be
developed to make the cellulosic biomass accessible to enzymatic hydrolysis. Finally,
and perhaps most importantly, the price of conventional gasoline will have to increase
significantly before ethanol can become a competitive transportation fuel. Nonetheless,
corn-to-ethanol plants will likely play a large role in the industrialization of developing
countries, especially those which have an abundant supply of crop residues which
otherwise would go to waste.
(References)
Aden, et. al. (already listed under other references)
Qu, et. al., State Key Laboratory of Microbial Technology, Studies on Cellulosic Ethanol
Production for Sustainable Supply of Liquid Fuel in China, Biotechnology Journal,
pp. 1235-1240, 2006.