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Hydrolysis and Fermentation Technologies for Alcohols
with improved unit operations, liquid–liquid extraction with biocompatible organic
solvents, distillation under vacuum, and selective adsorption on the solids have demonstrated the technical feasibility of the extractive fermentation concept. Finally,
membrane separation processes that decrease the biocompatibility constraints have
been proposed, which include dialysis [58] and reverse osmosis [43].
More recently, the concept of supported liquid membranes has been reported.
This method minimizes the amount of organic solvents involved and permits simultaneous realization of the extraction and recovery phases. Enhanced volumetric productivity and high substrate conversion yields have been reported [59] via the use of a
porous “Teflon” sheet soaked with isotridecanol as support for the extraction of ethanol during semicontinuous fermentation of Saccharomyces bayanus. This selective
process results in ethanol purification and combines fermentation, extraction, and reextraction (stripping). Such a novel process idea can further accomplish maximized
alcohol production and energy savings, and reduce the cost in production.
9.4.5 lignin ConverSion
In the United States, about 250 billion pounds per year of lignin is produced largely
as a byproduct of paper and pulp industry. Lignins are complex amorphous phenolic
polymers that are not sugar based and fermentable. The phenol in lignins may be either
a guaiacyl or a syringyl unit. These units are bonded by alpha- or beta-ether linkages.
A variety of C–C linkages may also be present and these are less common [2]. The distribution of linkages in lignin is random and highly resistant to chemical, enzymatic,
and microbial hydrolysis due to extensive cross-linking. Lignin protects cellulose and
needs to be removed to carry out hydrolysis and fermentation of cellulose. Lignin
monomer units are similar to gasoline that has high octane number. The removal of
oxygen and the breaking down of lignin molecules make it a suitable transportation
fuel. Hydrotreating of lignin will produce a mixture of phenolic and hydrocarbon compounds, which can then be converted to methyl aryl ether by reaction with methanol.
The conversion of lignin can be carried out by dual function catalysts. Metals such
as molybdenum and molybdenum/nickel catalyze deoxygenation and acidic alumina
support promote carbon–carbon bond cleavage. Lignin chemicals have applications in
drilling muds, binders for animal feeds, base for artificial vanilla, and surfactants for
oil recovery [60]. For the last usage, lignosulfonates are blended with tallow amines
and conventional sulfonates. Lignin can react with hydrogen or carbon monoxide
to form new class of chemicals called lignin phenols. These phenols are soluble in
organic solvents but not in water, and they are good candidates for further conversion
to produce chemicals that may be useful in enhanced oil recovery.
9.4.6 CoProduCTS oF CelluloSiC eThAnol TeChnology
Potential coproducts for cellulosic ethanol technology include hemicellulose hydrolyzate (xylose), cellulose hydrolyzate (glucose of mixed sugars), cell mass, enzymes,
soluble and insoluble lignins, lignin-derived chemicals and fuels, solid residues, and
so on. Other valuable coproducts include xylitol (which is sugar alcohol sweetener)
and is produced by hydrogenation of xylose (an aldehyde) into a primary alcohol.