Environmental Engineering Reference
In-Depth Information
products will also be taken into account. These
by-products have several different effects to the
process including degradation of sugar molecules,
to inhibiting the fermentation process, as well as
requiring additional steps to recycle or neutralize
harmful agents. All of these will be taken into
consideration for this criterion and will account
for 10 percent of the selection process.
Concentrated Acid Hydrolysis
The low acid solution in the previous method
requires higher temperatures and pressures as
well as longer processing time to break down the
cellulase. This processing time was one of the
factors causing sugar degradation and furfural
production. One method to address these downfalls
was the concentrated acid hydrolysis. This pro-
cess uses a 70% sulfuric acid solution at only 38°
and only requires a processing time of 2-6 hours
(Graf and Koehler, 2000). This process results
in a higher ethanol yield of 85-90% due to less
sugar degradation (U.S. Department of Energy,
n.d.). Now because of the increased concentration
of acid, this process requires an additional step
of acid recovery to recycle and reuse the acid in
the system.
This process requires additional costs and
deals with harsh chemicals that require a signifi-
cant level of control. As shown in Figure 2, this
process still requires acid neutralization as well
as the acid re-concentration. This process is still
very mature and has been in production for several
years. It was first developed in USDA's Peoria
Lab in the 1940's and then again further refined
by TVA in the 1980's, however this process has
reached the limits of process improvements and
cost reductions in its current form (U.S. Depart-
ment of Energy, n.d.). The produced ethanol is
chemically identical to the dilute acid process.
Ethanol Product Processes Available
Dilute Acid Hydrolysis
This process of cellulose hydrolysis has been in use
since at least 1898 where it was used in Germany
(U.S. Department of Energy, n.d.). The biomass
material can be broken into cellulose through use
of an acid solution under high temperatures and
pressures. This process uses ~1% sulfuric acid
solution at 215°C (Graf and Koehler, 2000). This
process takes several hours, and each sugar mol-
ecule reacts differently. The hemicelluloses reacts
to the acid first, but will then degrade during the
remaining cellulose hydrolysis process. This sugar
degradation will ultimately reduce the potential
ethanol yield. Another by-product of this diluted
acid hydrolysis can be furfural, which will also
reduce the ethanol yield. This initially resulted in
a 50% efficiency rating (Graf and Koehler, 2000),
but has since been altered to include two separate
steps. The first step creates the hemicelluloses (C
5
sugars) and the second step creates the cellulose
(C
6
sugars). Each step is followed by a separation
step to remove the sugar molecules
This process modification has reduced the
occurrence of sugar degradation and reduced the
production of furfural, which has increased the
overall ethanol yield to almost 90% (U.S. Depart-
ment of Energy, n.d.). Even with this reduction
of sugar degradation and furfural production, this
process still requires neutralization of the acid
through the use of lime. This adds cost to the
process by adding extra steps.
Direct Microbial Conversion Hydrolysis
This is the newest hydrolysis method which at-
tempts to separate the cellulase, perform cellulose
hydrolysis, and ferment the sugars in one step using
micro-organisms (Wyman, n.d.). Because all of
the steps are compressed into one single operation,
there is significant potential for cost savings and
low operational costs (Graf and Koehler, 2000).
Currently two bacteria are required to perform the
hydrolysis, but they also produce several other
by-products which reduce the ethanol output.
The current yield is low and the potential yield is
