The application of membrane technology has been aimed at the design of mem­branes that allow the recovery of either ethanol from water (as in the case of

technology Continuous fermentation coupled with stripping	Bioagent/unit operation Saccharomyces cerevisiael ethanol stripping with CO2			references Taylor et al (1996, 1998, 2000)
S. uvarumlethanol stripping with CO2 Kluyveromyces marxianus/ Pichia stipitis/ethanol stripping with CO2 S. cerevisiae or Zymomonas mobilislethanol stripping with CO2
ALSA MSCRS	Glucose Lignocellulosic biomass (oat hulls)	Gong et al. (1999) Dale and Moelhman (2001)
MSCRS	Gelatinized starch	Dale (1992)
Source: Modified from Cardona, C. A., and O. J. Sanchez. 2007. Bioresource Technology 98:2415-2457. Elsevier Ltd. Note: ALSA = Airlift reactor with a side arm, MSCRS = Multistage continuous reactor-separator.

membrane modules coupled to fermenters) or water from ethanol (as in the opera­tion of pervaporation during ethanol dehydration). Membranes make possible the removal of ethanol from a culture broth, which neutralizes the inhibitory effect of ethanol on microorganisms. Most of integrated schemes of this kind correspond to membrane modules coupled to fermenters. The use of ceramic membranes located inside the fermenter has been proposed, although most of these systems have been studied only on a laboratory scale. These laboratory configurations have shown interesting results, but their implementation at an industrial scale can be very dif­ficult. The utilization of ceramic membranes has been proposed for the filtration of cell biomass and the removal of ethanol during the fermentation (Ohashi et al., 1998). The removed ethanol is distilled and the obtained bottoms are recycled back to the culture broth resulting in a drastic reduction of generated wastewater. This configuration uses a stirred ceramic membrane reactor (SCMR). In the same way, immobilized cells can be used in order to allow an easier separation of etha­nol and the recirculation of distillation bottoms to the reactor (Kishimoto et al., 1997). Kobayashi et al. (1995) developed a mathematical model for optimization of temperature profiling during the batch operation of a fermenter coupled with a hollow-fiber module. The temperature was kept initially at 30°C descending later to 20°C and attaining higher ethanol concentration and productivity. However, it is necessary to analyze the scalability of these configurations due to their complexi­ties (immobilization, presence of membranes, recirculation, repeated batches) and taking into account that no mathematical description has been presented (Cardona and Sanchez, 2007). The utilization of liquid membranes (porous material with an organic liquid) in schemes involving the extraction of ethanol by the organic phase and the reextraction with a liquid stripping phase used as an extractant (perstrac- tion or membrane-aided solvent extraction) or gaseous stripping phase have been also coupled to the fermentation process showing the increased effectiveness of the latter configuration (Cardona and Sanchez, 2007; Christen et al., 1990). Some of these configurations are summarized in Table 9.8.

Pervaporation has offered new possibilities for integration, as evidenced in Table 9.8. The coupling of fermentation with the pervaporation allows the remov­ing of produced ethanol (Figure 9.10), reducing the natural inhibition of the cell growth caused by high concentrations of ethyl alcohol (Cardona and Sanchez, 2007). Nomura et al. (2002) observed that the separation factor of silicalite zeolite membranes used for continuous pervaporation of fermentation broth was higher than the corresponding value for ethanol-water mixtures due to the presence of salts that enhance the ethanol selectivity. Ikegami et al. (2003, 2004) employed this same kind of membrane coated with two types of silicone rubber or covered with a silicone rubber sheet as a hydrophobic material for obtaining concentrated solutions of ethanol. The coupling of T. thermohydrosulfuricum that directly converts uncooked starch into ethanol with pervaporation has also been tested obtaining ethanol concentrations in the permeate of 27 to 32% w/w (Mori and Inaba, 1990).

O’Brien et al. (2000) employed process simulation tools (Aspen Plus) for evaluating the costs of the global process involving fermentation-pervaporation

Reaction-Separation Integration for Alcoholic Fermentation Processes through Ethanol Removal by Using Membranes

TABLE 9.8 technology Bioagentuunit operation feedstock/Medium remarks references Continuous fermentation coupled with filtration Inhibitor-tolerant Saccharomyces cerevisiae/ cross flow microfiltration unit with stirring Undetoxified dilute acid spruce hydrolyzate Supplementation with complete mineral medium; 90% cell recirculation; microaerobic cond.; productivity up to 1.44 g/(L. h); 96 h of operation Brandberg et al. (2005) Continuous membrane — filtration bioreactor S. cerevisiae /internal ceramic tubes inside the fermentor Wood hydrolyzate High cell retention; EtOH conc. 58.8-76.9 g/L; yield 0.43 g/g; productivity 12.9-16.9 g/(L. h); 55 h of operation Lee et al. (2000) Batch fermentation coupled with continuous perstraction S. bayanus/Teflon sheet soaked with isotridecanol Glucose-containing medium Water was used as an extractant; EtOH conc. in the broth 75-61 g/L, in the extractant 38 g/L; yield 0.46; aver. productivity 1.2 g/(L. h) Christen et al. (1990) Continuous fermentation coupled with continuous perstraction Immobilized S. cerevisiae in alginate/membrane of the type of artificial kidneys Glucose-containing medium Tri-n-butylphosphate was used as an extractant; glucose feed conc. 506 g/L; aver. EtOH conc. in broth 67 g/L, in the extractant 53 g/L; productivity 48 g/ (L. h); up to 430 h of operation Matsumura and Markl (1986) Batch fermentation coupled with distillation Immobilized S. cerevisiae in Ca alginate/distillation Glucose-containing medium Distillation was carried out periodically; recycling of distillation bottoms; 500 h of cultivation; yield 92%; EtOH conc. 10-80 g/L; reduced wastewater Kishimoto (1997)

Free or immobilized S. cerevisiae/distillation

Glucose-containing medium

Batch fermentation coupled with continuous pervaporation

Batch co-fermentation coupled with continuous pervaporation

Fed-batch fermentation coupled with pervaporation

Continuous fermentation coupled with pervaporation

S. cerevisiae/silicalite zeolite membrane

Pichia stipitisl polytetrafluoro-ethylen membrane

Immobilized S. cerevisiae in Ca alginate/microporous polypropylene membrane S. cerevisiae! commercial polydimetylsiloxane membranes

Immobilized S. cerevisiae on beads of PAAH gel coated with Ca alginate/ membrane of silicone composite on a polysulfone support

High cellular retention by ceramic membrane; recycling of distillation bottoms; 100 h of cultivation; without wastewater; productivity 13.1-14.5 g/ (L. h); EtOH cone. 20-50 g/L For 4.6 wt.% EtOH in the broth, EtOH in permeate reaches 81.7 wt.%; separation factor of membrane 88; up to 48 h of operation For 10 g/L EtOH in the broth, EtOH in permeate reaches 50 g/L; yield 0.43 g/g; 100 h of operation 72 h cultivation; EtOH cone. 50 g/L; yield 0.49 g/g; productivity 2.9 g/(L. h); 61.5% reduction in wastewater Aspen Plus simulation based on fermentation-pervaporation lab experiments; EtOH cone, in permeate 420 g/L; recycling of retentate to fermenter, reduction of cost associated with fermentation by 75%

For 4 wt.% EtOH cone, in the broth, EtOH in permeate reaches 12-20% wt.%; yield 0.36-0.41 g/g; productivity 20-30 g/(L. h); over 40 d of operation

Reaction-Separation Integration for Alcoholic Fermentation Processes through Ethanol Removal by Using Membranes

TABLE 9.8 (Continued) technology Bioagentuunit operation feedstock/Medium remarks references Continuous SSF coupled with pervaporation S. cerevisiae + Trichoderma reesei cellulases/silicate membrane Cellulose Modeling based on kinetic approach; yield 0.44 g/g; EtOH conc. 248.3 g/L in permeate and 4.1 g/L in broth; reduced product inhibition effect; residence time of 72 h; 60-99% substrate conversion Sanchez et al. (2005) Batch fermentation coupled with membrane distillation S. cerevisiae/capillary polypropylene membrane Sucrose-containing medium 2-3 d cultivation; periodic flow of broth through membrane distillation module during 5-6 h per day or continuous coupling to bioreactor; yield 0.47-0.51 g/g; EtOH conc. 50 g/L in broth; productivity 2.5-5.5 g/(L. h) Gryta (2002, 2001) Gryta et al. (2000) Continuous fermentation coupled with membrane distillation S. cerevisiae and S. uvarum/ polypropylene and poly(tetrafluoro-ethylene) membranes Glucose and molasses solutions 430-695 h cultivation; EtOH conc. 60 g/L in broth and 200-400 g/L in cold trap; high concentrated medium (316 g/L molasses) Calibo et al. (1989)

HFMEF	S. cerevisiae/hydrophobic microporous hollow fibers/ oleyl alcohol or dibutyl phtalate	Glucose	Yeast cells are immobilized on the shell side; solvent flows in fiber lumen; feed glucose conc. 300 g/L; productivity 31.6 g/(L. h)	Kang et al. (1990)
CMFS	S. cerevisiae/membrane bioreactor with continuous removal of ethanol by pervaporation/coupling with cell separator	Not specified	Modeling study; higher dilution rates and productivity (up to 13.5 g/(L. h); recycle ratio 0-2.0; pervaporation factor 0-2.5 h-1; EtOH conc. 10^7 g/L; cell conc. increased from 1.9 to 14.6 g/L due to recycle and pervaporation.	Kargupta et al. (1998)

Source: Modified from Cardona, C. A., and O. J. Sanchez. 2007. Bioresource Technology 98:2415-2457. Elsevier Ltd.

Note: CMFS = continuous membrane fermentor-separator, HFMEF = hollow-fiber membrane extractive fermentor, PAAH = polyacrylamide hydrazide, SCMR = stirred ceramic membrane reactor.

Liquid

retentate PERVAPORATION

UNIT

Conc.

Gaseous EtOH permeate

To distillation
step

in comparison with the conventional batch process from starch. Fermentation — pervaporation was simulated based on experimental data from tests carried out during more than 200 h using commercial membranes of polydimethylsiloxane. Performed simulations revealed costs slightly higher for the coupled fermenta — tion-pervaporation process due to the capital and membrane costs. Nevertheless, fermentation costs were reduced 75% and distillation costs decreased signifi­cantly. Sensitivity analysis indicated that few improvements in membrane flux or selectivity could make this integrated process competitive (Cardona and Sanchez, 2007). Wu et al. (2005) have investigated the mass transfer coefficients for this type of membrane in the case of pervaporation of fermentation broths showing that active yeast cells were favorable for ethanol recovery. Kargupta et al. (1998) carried out the simulation of continuous membrane fermenter separator (CMFS) removing ethanol by pervaporation in a membrane reactor, which is coupled with a cell separator in order to increase the concentration of cells inside the reactor by recycling them. The models predicted an increase in productivity because this system could be operated at high dilution rates as a consequence of in situ product removal and higher cell concentrations.

Besides pervaporation, membrane distillation has been studied (see Table 9.8). In this type of distillation, aqueous solution is heated for the formation of vapors, which go through a hydrophobic porous membrane favoring the pass of vapors of ethanol (which is more volatile) over the vapors of water. The process’s driv­ing force is the gradient of partial pressures mainly caused by the difference of temperatures across the membrane (Cardona and Sanchez, 2007). Gryta et al. (2000) implemented a batch fermenter coupled with a membrane distillation
module leading to the ethanol removal from culture broth diminishing the inhibi­tion effect and obtaining an increase in ethanol yield and productivities. Gryta (2001) points out that when a tubular fermenter working in a continuous regime is coupled with the membrane distillation module, higher increases in ethanol productivity can be achieved (up to 5.5 g/(L x h)). This author determined that the number of yeast cells that are deposited on the membrane is practically zero dur­ing the operation of these modules (Gryta, 2002). Calibo et al. (1989) also dem­onstrated the possibility of coupling the continuous fermentation with membrane distillation. They used a column fermenter, a cell settler, and a membrane mod­ule. This system operated during almost 700 h with a feed of molasses. Garcia — Payo et al. (2000) studied the influence of different parameters for the case of air gap membrane distillation based on the model of temperature polarization. It was observed that permeate flux increases in a quadratic way when ethanol con­centration increases in the membrane distillation module. Similarly, Banat and Simandl (1999) indicate that the effects of concentration and temperature polar­ization should be accounted for during the modeling of this process and highlight the need for optimizing it with respect to feed stream temperature. Banat et al. (1999) also analyzed different models based on Fick’s law and on the solution of Maxwell-Stefan equations for this type of distillation. Likewise, the characteris­tics of the vacuum membrane distillation (Izquierdo-Gil and Jonsson, 2003) and direct contact membrane distillation have been studied for the concentration of aqueous solutions of ethanol (Fujii et al., 1992a, 1992b). Without a doubt, these studies are of great interest considering the simulation of these integrated con­figurations (Cardona and Sanchez, 2007).

In the case of the bioethanol production from sugarcane, the integration of fermentation with pervaporation or vacuum membrane distillation can allow the recovery of a valuable product: the fructose. For this, mutant strains of yeasts without the capacity of assimilating this monosaccharide should be used. Thus, continuous ethanol removal through the membranes coupled to the fermenter makes possible the accumulation of fructose in the culture medium that can be recovered in an extraction column (Cardona and Sanchez, 2007). According to Di Luccio et al. (2002), the simulation of this process based on experimental data and semiempiric models for the evaluation of the required membranes area allowed performing of a preliminary economic analysis. This analysis showed that vari­able costs involving membrane area influence in a higher degree the viability of the process. The process is viable only if the cost of membranes is not greater than US$550/m2 for a new plant or US$800/m2 for an adapted plant considering an internal return rate of 17%.