Conversion of hardwoods to ethanol: design and economics of delignification and enzyme recycling
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The objective of this study was to investigate the possibility of recycling enzymes during saccharification of cellulose for the production of ethanol from woodchips. To make enzyme recycling feasible and economical when woodchips are processed for ethanol, the lignin in the wood is to be removed before the enzymes are added. Since enzymes constitute a major part of the input costs, second only to the feedstock, the ability to reuse the enzymes could lead to a considerable decrease in the production cost of ethanol. Tulip poplar woodchips were selected as the feedstock. Different delignification methods with recovery of byproducts were investigated. Alkali extraction, using dilute NaOH for the removal of lignin after steam pretreatment, was used as the base case against which all other processes were compared. Recovery of furfural and methanol, produced during the pretreatment of the woodchips, for sale as byproducts was one modification to the alkali extraction process that was investigated. The conversion of xylose to furfural and the recovery of the furfural as a byproduct was the third case explored. Solvent extraction using a 50:50 ethanol-water mixture instead of extraction with NaOH was the fourth case examined. Process flow sheets were then developed to recycle the enzymes during the hydrolysis and fermentation of this prehyrolyzed and delignified wood. Two reactor setup schemes were examined for enzyme recycling. One scheme involved a single train of reactors, with the whole pretreated slurry flowing from one reactor to the next, whereas, in the other scheme, the slurry was split among parallel trains of reactors. The activity loss of the enzymes was modeled such that a part of the enzymes entering the reactor lost all their activity. The loss of activity in multiple steps, with enzymes losing only some of their activity, was also modeled. Here the enzymes entering the reactor constituted a mixture with different activities instead of all the enzymes having the same activity like in the previous single step model. Recovering methanol and furfural reduced the minimum ethanol selling price. High temperature ethanol water pretreatment and lignin extraction reduced the minimum ethanol selling price compared to the base case of steam pretreatment followed by alkali extraction. Enzyme recycling also reduces the minimum ethanol selling price. The magnitude of the price reduction depends on the recycling scheme selected and the rate of enzyme deactivation, which has not been measured.