Biomedical Engineering Reference
In-Depth Information
2. Hydrolysis (conversion into sugar).
3. Fermentation (conversion of the sugar into ethanol).
4. Distillation (removal of water and solids).
5. Dehydration (final drying).
The second step is different for the two biochemical processes. All other
steps are the same.
11.5.4.3 Feed Preparation
This step prepares the biomass for processing. It involves cleaning and then
pretreatment. Unlike food grain (e.g., corn and wheat), lignocellulose feed-
stock often comes mixed with dirt and debris. These must be cleaned from
the delivered biomass, which is then shredded into small particles. In pre-
treatment, the hemicellulose/lignin sheath that surrounds the cellulose in
plant material is disrupted. To make the cellulose more accessible to the
hydrolysis process, one could adopt physical, chemical, or biological pre-
treatment as described below.
Physical methods: grinding, milling, shearing (energy intensive), and
steam explosion (to produce some inhibitory compounds).
Chemical methods: treatment with acid (for pH neutralization and recov-
ery of chemicals), treatment with alkalis (for pH adjustment and recycling
of chemicals), and treatment with organic solvents (solvent removal and
recycling is expensive).
Biological method:
enzymatic
treatment
of
the
cellulose
(time
consuming).
11.5.4.4 Hydrolysis
Acid hydrolysis uses dilute acid at high temperature and pressure.
Concentrated acid at lower temperature and pressure may be used, but this
produces a toxic by-product
that
inhibits fermentation and so must be
removed.
In enzymatic hydrolysis, cellulose chains are broken into glucose mole-
cules by cellulose enzymes, in a process similar to what occurs in the stom-
ach of a cow to convert grass or fodder cellulose into sugar. Xylanose and
hemicellulose enzymes can convert many cellulosic agricultural residues into
fermentable sugars. These residues include corn stover, distiller grains, wheat
straw, and sugarcane bagasse, as well as energy crops such as switchgrass.
Lignin is difficult to convert into sugar, so it is discarded as waste.
Figure 11.10
shows a process based on cellulose hydrolysis.
11.5.4.5 Fermentation of Hemicellulosic Sugars
Through a series of biochemical reactions, bacteria convert xylose and other
hemicellulose and cellulose sugars
into ethanol. The yeast or other
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