Lignin Conversion

image44 Подпись: Extracted Phase Подпись: Permeation Supported Liquid Membrane Condensed Permeate
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Lignin is produced in large quantities, approximately 250 billion pounds per year in the United States, as a by-product of the paper and pulp industry. Lignins are complex amorphous phenolic polymers that are not sugar-based,

FIGURE 4.13

Extractive fermentation system: (1) fermenter; (2) permeation cell; (3) supported liquid mem­brane; (4) extracted phase; (5) gaseous stripping phase; (6) cold trap; (7) condensed permeate. (Modified from Christen, Minier, and Renon, 1990. Enhanced extraction by supported liquid membrane during fermentation, Biotechnol. Bioeng., 36: 116-123.)

Подпись:Подпись:

Подпись: а в Y Phenylpropane Unit

Guaiacyl

Syningyl

FIGURE 4.14

Monomer units in lignin. (Modified from Wright, 1988. Ethanol from biomass by enzymatic hydrolysis, Chem. Eng. Prog., 84: 62-74.)

hence, they cannot be fermented into ethanol. Lignin is a random polymer made up of phenyl propane units, where the phenol unit may be either a guaiacyl or syringyl unit (Figure 4.14). These units are bonded together in many ways, the most common of which are a — or в-ether linkages. A vari­ety of C-C linkages are also present, but are less common (Figure 4.15). The distribution of linkage in lignin is random because lignin formation is a free radical reaction that is not under enzymatic control. Lignin is highly resistant to chemical, enzymatic, and microbial hydrolysis due to extensive crosslinking. Therefore, lignin is frequently removed simply to gain access to cellulose.

Lignin monomer units are similar to gasoline, which has a high octane number; thus, breaking the lignin molecules into monomers and removing the oxygen makes them useful as liquid transportation fuels. The process for lignin conversion is mild hydrotreating to produce a mixture of phenolic and hydrocarbon materials, followed by reaction with methanol to produce methyl aryl ether. The first step usually consists of two principal parts: hydro­deoxygenation (removal of oxygen and oxygen-containing groups from the phenol rings) and dealkylation (removal of ethyl groups or large side chains from the rings). The major role of this stage is to carry out these reactions

Подпись: — C —Подпись:Подпись:image50"Подпись:Подпись: CH3Oі

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0 I

a-a’ Bonding

FIGURE 4.15

Ether and C-C bonds in lignin. (Modified from Wright, 1988. Ethanol from biomass by enzy­matic hydrolysis, Chem. Eng. Prog, 84: 62-74.) to remove the unwanted chains without carrying the reaction too far; this would lead to excessive consumption of hydrogen and produce saturated hydrocarbons, which are not as effective as octane enhancers as are aromatic compounds. Furthermore, excessive consumption of hydrogen would repre­sent additional cost for the conversion process. Catalysts to carry out these reactions have dual functions. Metals such as molybdenum and molybde- num/nickel catalyze the deoxygenation, and the acidic alumina support pro­motes the carbon-carbon bond cleavage.

Although lignin chemicals have applications in drilling muds, as bind­ers for animal feed, and as the base for artificial vanilla, they have not been previously used as surfactants for oil recovery. According to Naee [87], lig­nin chemicals can be used in two ways in chemical floods for enhanced oil recovery. In one method, lignosulfonates are blended with tallow amines and conventional petroleum sulfonates to form a unique mixture that costs about 40% less to use than chemicals made solely from petroleum or petro­leum-based products. In the second method, lignin is reacted with hydrogen or carbon monoxide to form a new class of chemicals called lignin phenols. Because they are soluble in organic solvents, but not in water, these phenols
are good candidates for further conversion to produce chemicals that may be useful in enhanced oil recovery (EOR).