Comparison between SSF and SHF Processes
SSF systems offer large advantages over SHF processes, thanks to their reduction of end product inhibition of the cellulase enzyme complex. The SSF process shows ahigher yield (88% vs. 73%) and greatly increases product concentrations (equivalent glucose concentration of 10% vs. 4.4%). The most significant advantage is the enzyme loading, which can be reduced from 33 to 7 IU/g cellulose, and this cuts down the cost of ethanol appreciably. With constant development of low-cost enzymes, the comparative analysis of the two processes is in flux. A comparative study of the approximate costs of the two processes was reported in Wright痴 article.45 The results show that based on the estimated ethanol selling price from a production capacity of 25,000,000 gallons per year, SSF is more cost-effective than SHF by a factor of 1:1.49, i.e., $SHF/$SSF = 1.49. It has to be clearly noted that the number quoted here is the ratio of the two prices, not the direct dollar value of the ethanol selling price.
From the very same process for economic reasons, it is anticipated that a hybrid hydrolysis and fermentation (HHF) process configuration is going to be widely accepted as a process of choice for production of lignocellulosic fuel ethanol, which begins with a separate hydrolysis step and ends with simultaneous saccharification(hydrolysis) and fermentation (SSF) step. In the first stage of hydrolysis,higher-temperature enzymatic cellular saccharification takes place, whereas in the second stage of SSF, mesophilic enzymatic hydrolysis and biomass sugar fermentation occur simultaneously.
As xylose accounts for 30 to 60% of the fermentable sugars in hardwood and herbaceous biomass, it becomes an important issue to ferment it to ethanol. The efficient fermentation of xylose and other hemicellulose constituents is essential for the development of an economically viable process to produce ethanol from biomass. Xylose fermentation using pentose yeasts has proved to be difficult, owing to the requirement for O2 during ethanol production, acetate toxicity, and production of xylitol as a by-product. Xylitol (or xyletol) is a naturally occurring low-calorie sugar substitute with anticariogenic (preventing production of dental caries) properties.
Other approaches to xylose fermentation include the conversion of xylose to xylulose (a pentose sugar that is a part of carbohydrate metabolism and is found in the urine in pentosuria50) using xylose isomerase prior to fermentation by Saccharomyces cerevisiae and the development of genetically engineered strains.
The method of integrating xylose fermentation into the overall process is shown in Figure 11.11. The liquid stream is neutralized to remove any mineral acids or organic acids liberated in the pretreatment process and then sent to xylose fermentation. Water is added before the fermentation, if necessary, so that organisms can make full use of the substrate without having the yield limited by end-product inhibition. The dilute ethanol stream from xylose fermentation is then used to provide the dilution water for the cellulose-lignin mixture entering SSF. Thus, the water that enters during the pretreatment process is used in both xylose fermentation and the SSF process.
The conversion of xylose to ethanol by recombinant E. coli has been investigated in pH-controlled batch fermentations. Relatively high concentrations of ethanol (56 g/l) were produced from xylose with excellent efficiencies. In addition to xylose, all other sugar constituents of biomass can be efficiently converted to ethanol by recombinant E. coli.4 Neither oxygen nor strict maintenance of anaerobic conditions is required for ethanol production by E. coli. However, the addition of base to prevent excessive acidification is essential. Although less base is needed to maintain lowpH conditions, poor ethanol yields and slower fermentations are observed below a pH of 6. Also, the addition of metal ions stimulates ethanol production. In general, xylose fermentation does not require precise temperature control, provided the broth temperature is maintained between 25 and 40ｰC. Xylose concentrations as high as 140 g/l have been positively tested to evaluate the extent to which this sugar inhibits the growth and fermentation. Higher concentrations considerably slow down growth and fermentation.
Ethanol Extraction during Fermentation
In spite of the considerable efforts given to the fermentative alcohols, industrial applications have been delayed because of the high cost of production, which depends primarily on the energy input to the purification of dilute end products and on the low productivities of cultures. These two points are directly linked to inhibition phenomena.
Along with the conventional unit operations, liquid僕iquid extraction with biocompatible organic solvents, distillation under vacuum, and selective adsorption on the solids have demonstrated the technical feasibility of the extractive fermentation cocept. Of late, membrane separation processes, which decrease compatibility constraints, have been proposed. These include dialysis, and reverse osmosis.More recently, the concept of supported liquid membranes has also been reported. This method minimizes the amount of organic solvents involved and permits simultaneous realization of the extraction and recovery phases. Enhanced volumetric productivity and high substrate conversion yields have been reported,7 using a porous Teflon sheet as support (soaked with isotridecanol) for the extraction of ethanol during semicontinuous fermentation of Saccharomyces bayanus. This selective process results in ethanol purification and combines three operations: fermentation, extraction, and reextraction (stripping).