Page 108 - Lignocellulosic Biomass to Liquid Biofuels

84 Lignocellulosic Biomass to Liquid Biofuels

(UF), and reverse osmosis. Most of the membrane that is used in different
chemical processes is made polymer. But nowadays, the use of inorganic
and hybrid membrane is common comprising inorganic nanoparticle and
also polymer. The commonly used membrane configurations are hollow-
fiber module that is currently being used vastly. In bioethanol industry,
membrane is generally used for downstream purification to concentrate
the final product. But it can be sufficiently used in cellulase enzyme recy-
cling, cell recycling, and removal of inhibitory by-products as well.

3.3.3.1 Membrane-assisted enzymatic hydrolysis
Fermentation of lignocellulosic biomass to fuel-grade ethanol challenges
two major difficulties: (1) crystalline structure of cellulose which resists
enzymatic hydrolysis of cellulose and (2) lignin-cellulose association that
impedes enzymatic attack on cellulose [188]. Dilute acid catalyzed
hydrolysis of cellulose uses high pressure and temperature that leads to the
destruction of monomer glucose to hydroxymethylfurfural (HMF), FA,
levulinic acid (LA), including side reaction that cause disruption of cellu-
lose leading to the formation of nonreactive material [189]. On the other
hand, this chemical process, enzymatic hydrolysis of cellulose, requires
relatively low energy as well as normal temperature and pressure.
Cellulolytic enzyme containing CBHs, EG and BGL obtained from dif-
ferent bacteria or fungi can sufficiently hydrolyze pretreated lignocellulosic
biomass to fermentable sugar. But the cost of the enzyme constitutes 20%
of the total ethanol production cost and 50% of the total cost of whole
hydrolysis process [190].
One way to reduce the manufacture cost of ethanol from lignocellu-
losic biomass is to recycle and reuse of enzyme as well as the specific
activity of enzyme should be recovered, retained. Enzyme recycling is
predisposed by various reasons such as nature of biomass, process para-
meters and origin, and activity of enzyme [191]. After hydrolysis, cellulase
can either appear in solution as free enzyme or can be adsorbed in
remaining biomass [192]. Recovery and recycling of bound enzyme after
initial hydrolysis require separation and desorption method [193], but the
catalytic activity gradually decreases during hydrolysis steps.
Recovery of filter paper actions decreases from 91% to 22% by this
method. But the use of alkali or surfactant increases enzyme activity to
some point. Membrane-based process somehow adequately recovers the
enzyme from hydrolysis solution, retains its catalytic activity, satisfactorily