| One way to produce bioethanol by straw is by using the acid hydrolysis – fermentation process. However, the hydrolys is catalyzed by concentrated acid has many disadvantages such as huge amount of acid consumption, difficult y for acid recovery and high cost of reagents; unfavorable factors of dilute acid hydrolysis is also remarkable such as the generation of quantities of by-products, low sugar yield and high energy consumption. Besides, the toxicity of by-products and low yield of sugar influence the fermentation efficiency and ethanol yield. In this study, catalytic hydrolysis of straw by using sulfuric acid of medium concentration(10~30 wt%) under mild conditions(at atmospheric pressure and below 100?) was performed, and multiple-effect membrane distillation(MEMD) was applied to the detoxification and concentration of the hydrolysate.Straw component content was determined, and influences of sulfuric acid concentration, reaction temperature, reaction time, particle size and liquid-solid ratio on sugar yield in the acid hydrolysis process were studied. The experimental results showed that sugar yield increased with the increase of sulfuric acid concentration and reaction temperature, with the decrease of particle size; with the increase of reaction time, sugar yield increased in the first stage and then decreased; sugar yield was hardly affected by liquid-solid ratio. The impact of these factors on the sugar yield was in the following order: reaction temperature > sulfuric acid concentration > reaction time > liquid-solid ratio. The optimal hydrolysis condition was at the sulfuric acid concentration of 30 wt%, reaction temperature of 100?, reaction time of 10 h, liquid-solid ratio of 5:1, under which the sugar yield could reach 72.12%. The acid-sugar mixtures in the acidic hydrolysate could be effectively separated by acid retardation process, so that acid could be reused as catalyst.MEMD was then used to remove volatile inhibitors from the simulated hydrolysate and meanwhile enrich the sugar solution. The effects of different operating variables were tested on the performance of MEMD process ind icated by permeation flux J, gained-output-ratio(GOR), rejection rate of glucose Rg and removal rate of volatile inhibitors Dv. The experimental results showed that the maximum J and GO R obtained were 6.17 L·m-2·h-1 and 13.57, respectively. Rg was hardly affected by these factors and kept above 99.99% during the whole process. The removal rates of formic acid, acetic acid, levulinic acid and furfural in simulated acidic hydrolysate of straw were 87.76%, 90.62%, 4.25% and 76.52%, respectively. During the detoxification process by membrane method,neither new impurities nor sugar loss was observed, and meanwhile the glucose in the hydrolysate was concentrated. In deep-concentrating process, 20 g·L-1 aqueous glucose solution could be concentrated at least 28 times. When the glucose in the feed solution was concentrated to 561 g·L-1, GOR could still reach 5.03, which provided a similar thermal efficiency of a conventional 7-effect evaporator. MEMD showed good stability during the long-term stability experiment, there was no significant decrease of permeation flux, gained-output-ratio and rejection rate of glucose.This study showed that MEMD process could remove volatile inhibitors from the hydrolysate and meanwhile enrich the sugar solution effectively, which contributed to the increase of subsequent fermentation efficiency and yield of ethanol. The combination of hydrolysis catalyzed by sulfuric acid of moderate concentration and MEMD process could be effectively applied to bioethanol production. This study provides some basic data for the realization of the industrialization of bioethanol. |
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