New Directions in Fermenter Configuration and Design
There are a number of good fermenter designs adoptable for cel — lulosic ethanol production. Nevertheless, each of these has their advantages and disadvantages. Simple designs, easy operation and low maintenance costs are good features of a reactor design. Then there are some recent developments in novel fermenter designs that can achieve easy separation of the fermented ethanol from feedstocks. In one example, Viola and coworkers reported a two-chamber reactor in order to improve the bioethanol production from lignocellulosic materials via enzymatic hydrolysis and fermentation [31]. The design of this two-chamber configuration is shown in Figure 9.3. This reactor consists of two chambers kept at different temperatures and separated by a porous medium through which the solutes can diffuse. The reactor was tested using previously steam-exploded and detoxified wheat straw as substrate. The yields of cellulose hydrolysis and glucose fermentation obtained using this reactor were compared to those obtained by simultaneous enzymatic saccharification and fermentation (SSF) carried out in only one vessel. The results showed that a significant increase in the ethanol yield (20%) can be achieved by using this bioreactor. An additional advantage of the reactor is the confinement of the solid lignin in one chamber, allowing a simplified separation process between broth and unreacted residue. Polysaccharides are depolymerized by enzymes at higher temperature (left chamber); the released sugars diffuse through the filter in the chamber kept at lower temperature (right) where the microorganisms metabolize them. Unreacted biomass and residue lignin remain in the chamber where they were loaded, and lignin residue can be removed from this chamber without contaminating the fermentation process [31].
Furthermore, Viola and coworkers found that higher ethanol concentrations can be achieved by using the two-chamber reactor in a fed-batch procedure. The reduction in yield under high percentage solid loading in bioreactors is well documented in the literature [32-34]. In using the two chamber bioreactor in the fed-batch mode they have been able reduce this drawback by removing the solid lignin [31].
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Figure 9.3 The operating principle of two-chamber bioreactor with separate chambers for saccharification and fermentation. (Reprinted with permission from reference [31]; copyright 2013 Elsevier). |
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