Page 109 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 85

recycle, and reuses the enzyme which is further use in hydrolysis proce-
dure and was established as best recovering procedure.
Membrane filtration, which is widely used in this process, is the most
used organic membrane made of cellulose acetate (CA), polysulfone (PS),
nylon (NY), polyethersulfone (PES). The membrane module that is usu-
ally adopted for this process includes hollow fiber, spiral wound, and flat
sheet membrane. MF can remove remaining biomass after hydrolysis,
whereas UF is used to recover soluble enzyme from hydrolysis solution.
Permeate flux for MF is higher than UF due to large pore size of MF
membrane [194]. Sedimentation followed by MF and UF methods can
recover and recycle 75% of the enzyme used in active catalytic form and
exhibits a low-cost ethanol produced from pretreated ground yellow pop-
lar. MF membranes were made of CA, PS, and NY, whereas UF mem-
branes were made of PS (PSf) and PES. In combined sedimentation and
cross flow UF method, sedimentation using inclined settler removes lig-
nocelluloses particles larger than 50 μm in length, and UF transmits
fermentable sugar, while retaining remaining biomass particle and cellulase
2
enzyme. The permeate flux from UF is 64 6 5 L/(m h) when feed con-
2
sists of 0.22 w/v% cellulose, and it increases up to 130 6 20 L/(m h)
when feed consists of 10 wt.% lignocelluloses and settler overflow from
mixture of 0.22 w/v% cellulose. As cellulose binds to the lignocellulosic
particle during enzymatic hydrolysis and filtration, it prevents the enzyme
from fouling the membrane, and thus increasing the permeate flux [195].
High conversion rate of cellulose and low consumption of cellulase
can be achieved in a continuously or semicontinuously operated attrition
bioreactor conjugated with a membrane filter which retains cellulase and
remaining cellulose in reactor and allows sugar to pass through it as prod-
uct [196]. Hydrothermal pretreated wheat straw for ethanol production
was varied with Celluclast supplemented with BGL at different concentra-
tions and subjected to hydrolysis followed by fermentation using yeast
strain. After fermentation, cellulase was recovered by passing the liquid
phase through 0.22 μm PES membrane followed by concentration and
NaAc buffer exchange in a tangential UF system. Pellicon XL membrane
with a 10 kDa cutoff PES membrane achieves 80% recovery of soluble
enzyme, 70% recovery of activity from liquid phase providing high con-
version degree. Increasing the hydrolysis temperature and enzyme loading
drop the recovery of cellulose from liquid phase about 40% [197].
UF with PES10 membrane was studied to recycle 73.9% cellulase
present in the hydrolyzate suspension of steam-exploded wheat straw,