Topic : Bio-EthanolAdvisor : Prof. Jo-Shu Chang

NURHAYATI / 林海亞N36017011

PAPER REVIEW

Due to date : 14 December 2012

Title : A novel integrated biological process for cellulosic ethanol production featuring high ethanol productivity, enzyme recycling and yeast cells reuse

Authors : Mingjie Jin, Christa Gunawan, Nirmal Uppugundla, Venkatesh Balan and Bruce E. Dale

Journal : Journal of Energy and Environmental ScienceYear : 2012Impact Factor : 9.61

Supporting Papers : -

Discussion

This study aim to overcome several major issues in commercial biochemical production of cellulosic ethanol. This paper exhibits a novel integrated biological process to solve high enzyme loading requirements, slow xylose fermentation and low ethanol productivity problems.

Approaching to some subsequent cycles both in hydrolysis and fermentation period to allow enough residence time for each processes.

Experimental DesignThe following figure exhibits the flow chart of the SHF and SScF process. In this process, the digestible biomass was hydrolyzed first proper to the recalcitrant parts of biomass was hydrolyzed due to avoid long-time period requirement for hydrolysis. Then, the remaining unhydrolyzed solids with some enzymes adsorbed were recycled to next cycle to complete hydrolysis, hence the enzyme requirements was reduced for next step.

The high xylose fermentation was achieved by employed high inoculum level (OD = 20) and completed fermentation within 24 h.

Discussion

To avoid the inhibition of cells growth at high ethanol concentration level, the liquid part of the hydrolysis and fermentation mixture was harvested and subjected to separation process (i.e. membrane distillation) immediately. The unhydrolyzed solid (recalcitrant part) were repeatedly transferred to the next hydrolysis/fermentation cycle to allow sufficient hydrolysis time. After 5 cycles (in this study) of hydrolysis/fermentation, the accumulated recalcitrant residual solids were further hydrolyzed in the last step. Since some enzymes were recycled by this approached and part of the inhibitory degradation products were removed along with liquid during the five cycles, the hydrolysis of total solids (both digestible and recalcitrant part) will reach optimum level, hence this approach then can overcome low ethanol productivity problem.

Optimization of Fermentation ConditionsThis figure shows the effect of initial OD (a), pH (b) and temperature (c) on xylose consumption, ethanol production and cell viability (d) during the first cycle 24 hour SScF with 7%w/w glucan loading.

Based on this data, it is clear that by increasing the OD cells, more xylose was consumed and more ethanol was produced. Therefore, an initial OD of 20, pH 5.5 and the temperature of 32oC were chosen as the optimum condition for SScF. Also, the same condition were employed for the SHF process.

Discussion

SHF ProcessThis figure exhibits the effect of each fermentation cycle to sugar concentration, fermentation performance, sugar consumption and ethanol metabolic yield during both fermentation and hydrolysis period.

Consistently high sugar concentration were produced during cycle 1-5 period by applying the enzyme loadings strategy as mentioned in the previous discussion.

Ethanol produced for each cycle was always approximately to 40 g/L and the OD was increasing from cycle to cycle, which proved the fast xylose fermentation.

The ethanol metabolic yield also was maintained at around 90%.

This figure also shows the overall glucan and xylan conversion were only 68.3% and 37.7% respectively.

Discussion

SScF ProcessThis figure shows the effect of enzyme loading profile on ethanol production, sugar loading to ethanol conversion, and viable density during SScF process.

The ethanol concentration for each cycle largely met criterion of 40 g/L for all cases. Glucose to ethanol conversions were around 75% to 80%. However, the xylose to ethanol conversions were quite low, around 40% to 50%.

The viable cell density could be one problem in this system. Decreasing viable cell densities were observed from cycle to cycle.

The solid residue concentrations in cycles 1 to 6 of this process were around 88, 125, 153, 170, 185, and 185 g/L, respectively.

Discussion

Comparisons of Process Ethanol Productivity and Ethanol Yield of Different ProcessFigure below depicts the SHF and SScF processes achieved much higher process ethanol productivities compared to conventional SHF or SScF processes, probably due to the time saved by fast hydrolysis and fast fermentation.

For the ethanol yield, 7% glucan loading was less effective than a 6% glucan loading using conventional processes. This could be due to the inhibitory effect of degradation products on both enzymatic hydrolysis and fermentation.

The SScF process achieved higher ethanol yield compared to the SHF process probably because of the additional 24 h hydrolysis during fermentation even though the hydrolysis was not performed at its optimal condition.

Critical Thinking

There are no special new things from this study, they only tried to manipulate several strategies to obtain higher ethanol production regarding to enzyme loading, xylose fermentation and ethanol productivity. But, the principles exhibited common strategies which had been employed by previous researchers already.