Ethanol can be removed from the culture broth through absorption employing a stripping gas. This makes possible the increase of sugar concentration in the feed stream entering the fermenter. This process has been studied in the case of corn mashes obtained by the dry-milling process. Taylor et al. (1998) stud­ied this integrated process in the case of the dry-milling ethanol process in a pilot plant integrating a 30 L fermenter with a 10 cm packed column for ethanol removal by the CO2 (stripping gas) released during the fermentation. A simpli­fied scheme of this process is presented in Figure 9.9 where two circulation loops are employed. In this scheme, concentrated solutions of the product are obtained

from the condensation of ethanol. The model proposed by these authors showed that ethanol inhibition influences especially the cell yield reaching a value of 60 g/L of ethanol in the broth above which the inhibition is very strong. The authors point out that the values of kinetic parameters depend in a high degree on the type of fermentation: batch or continuous. Later, Taylor et al. (2000) employed a saccharified corn mash containing high levels of suspended solids as a feed and compared the results obtained with a state-of-the-art dry-milling process using Aspen Plus. Savings of US0.8 cents/L of ethanol can be attained in comparison with the state-of-the-art process for which saccharification and fermentation are carried out separately (Cardona and Sanchez, 2007).

Other variants of this type of integrated configuration have been proposed as evidenced in Table 9.7. Gong et al. (1999) report the simultaneous variant of the fermentation-stripping process using an air-lift reactor with a side arm (external loop) that improves liquid circulation and mass transfer. A more complex configu­ration integrating the fermentation and stripping was developed by Bio-Process Innovation, Inc. (West Lafayette, IN, USA). A pilot plant was designed and built for ethanol production from lignocellulosic biomass using a 130 L multistage, con — tinuous-stirred, reactor separator (MSCRS) for the SSF of cellulose and hemicel — lulose (Dale and Moelhman, 2001). The MSCRS consists of a series of six stirred

TABLE 9.6 Reaction-Separation Integration for alcoholic Fermentation Processes through ethanol removal by Bioagent/unit Vacuum Technology operation feedstock/Medium remarks references Continuous vacuum fermentation Saccharomyces cerevisiae/vacuum system Glucose-containing medium 50 mm Hg; with and without cell recycling; sparging of oxygen; 33.4 % glucose feed; productivity 40-82 g/(L x h) Cysewski and Wilke (1977) Continuous fermentation coupled with vacuum flashing S. cerevisiae/extractive vacuum flash chamber Sugarcane molasses Modeling based on kinetic approach; 4-5.33 kPa; recycling of liquid stream from flash; cell recycling; 98% conversion; 23-26.7 g/(L x h) productivity Costa et al. (2001) da Silva et al. (1999) Source: Modified from Cardona, C. A., and O. J. Sanchez. 2007. Bioresource Technology 98:2415-2457. Elsevier Ltd.

stages in which the SSF of biomass is carried out. Each stage has a stirred tank for the reaction and a gas-liquid separation contactor. In the three upper stages, SSF of cellulose is carried out at 42°C using a thermotolerant K. marxianus, while in three lower stages, the fermentation of xylose is achieved using the yeast P. stipi — tis at 30°C. In addition, part of the broth containing enzymes is recirculated from the last stage to the first upper stage in order to favor the reaction with the fresh pretreated biomass. Reaction-reaction integration is implemented because com­mercial cellulases were added for the saccharification of pretreated biomass. This defines the process temperature helping the generation of ethanol vapors. The broth overflowing from one stage into the next stage is contacted with a stripping stream of CO2 that entraps the ethanol. A gas stream passes across the reactor and then through an absorption tower where water is used for removing ethanol vapors. CO2 is again recirculated to the reactor. In this way, reaction-separation integration is verified through the in situ removal of ethanol produced in each stage. The same company has installed a pilot plant reactor using MSCRS tech­nology (Dale, 1992) in the plant of Permeate Refining, Inc., located in Hopkinton, IA (USA), that produces 11.36 million liters per year of ethanol from starch. This unit has been in operation since September 1995 and employs starch dextrins, thought its use for lignocellulosic biomass has been proposed. Unfortunately, no reports are available describing the modeling and performance of this type of configuration (Cardona and Sanchez, 2007).