Biomedical Engineering Reference
In-Depth Information
most vital consideration for technology integration and process optimization is to
minimize the water usage of the process without significantly compromising the
performance of the enzymes and microorganisms. Taking the COFCO-SINOPEC-
Novozyme second generation fuel ethanol project with an annual production
capacity of 62 million liters as an example, the overall process involves feedstock
handling, size reduction, pretreatment, substrate conditioning, enzymatic hydro-
lysis, fermentation, ethanol distillation, residue dewatering and biogas production.
The prior feedstock for the project is corn stover, which contains 10-15%
moisture under field-dried conditions and detectable impurities. After a primary
size reduction by a shredder, the feedstock is screened to remove dirt and grit and
passed through a magnetic separator to remove tramp metals. It is then further
reduced by the secondary shredder to 20-50 mm. Steam explosion is used for the
biomass pretreatment, and solid contents are controlled at 30-40% during the
pretreatment process. The feedstock is pre-heated by the flash vapor, which not
only saves energy consumption, but also reduces condensed water to ensure the
high solid content. The temperature and residence time can be controlled in the
ranges of 130-220C and 5-120 min, depending on the feedstocks and the size
reduction. A small amount of acid is supplemented to accelerate the hydrolysis of
hemicelluloses to deconstruct the LCCs more efficiently and enhance the acces-
sibility of cellulases to the surface of cellulose. In addition, the addition of acid can
also lessen the severity of the pretreatment conditions, reduce degradation of
sugars and enhance pentose recovery.
The pretreated substrate is transferred into the hydrolysis reactor with initial
dry matter content of 20-25% after neutralization by alkalis such as lime, sodium
hydroxide and ammonia. The mixing of the substrate with enzymes presents
challenges due to the high viscosity and poor fluidity of the slurry at the early stage
of the hydrolysis. Laboratory trials and scaling-up practice indicated that feeding
substrate and enzymes in a fed-batch mode can improve the mixing performance
and facilitate the enzymatic hydrolysis. The temperature and pH are set at 50C
and 5.0 respectively under the optimal conditions for the cellulases (Cellic CTec2)
developed by Novozyme for the pre-hydrolysis of the pretreated feedstock, fol-
lowed by the co-fermentation of the hexose and pentose sugars by the genetically
engineered S. cerevisiae developed by Dr. Nancy Ho at Purdue University and
licenced to COFCO. The yeast seed is cultivated with the hydrolysate supple-
mented with CSL. Due to the high concentrations of inhibitors and low content of
nutrients in the hydrolysate, an extended time is required for the seed culture, and
much higher inoculation is needed to initiate the fermentation, which is completed
within 96-120 h.
The broth containing 5-7% (v/v) ethanol is then distilled for ethanol recovery.
It is worth noting that this unit operation is more energy-intensive than that for
ethanol production from sugar- and starch-based feedstocks. The distillage dis-
charged from the distillation system is filtered to separate lignin residues
remaining after the fermentation, and the filtrate is digested anaerobically for
biogas production, while the cake is dewatered. Both biogas and lignin residues
can be co-fired to generate steam.
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