Process operation performance of PDMS membrane pervaporation coupled with fermentation for efficient bioethanol production☆

Senqing Fan,Jingyun Liu,Xiaoyu Tang,Wenguo Wang,Zeyi Xiao,*,Boya Qiu,Yuyang Wang,Shizhao Jian,Yangmei Qin,Yinan Wang

1 School of Chemical Engineering,Sichuan University,Chengdu 610065,China

2 Biogas Institute of Ministry of Agriculture,Chengdu 610041,China

Keywords:Pervaporation Membranes Biofuel Ethanol recovery Fermentation broth

A B S T R A C T There would be strong product inhibition on ethanol fermentation process if ethanol is not removed in situ from broth.PDMS membrane pervaporation coupled with fermentation is a promising process for efficient bioethanol production since ethanol inhibition is relieved or eliminated.From the perspective of process operation,membrane separation performance,ethanol fermentation performance and the subsequent processing on membrane downstream are the three key issues.This review aims at contributing a comprehensive overview on the operation performance of the integrated process.The state-of-the-art of the three key issues related to the operation performance is focused.Finally,the tentative perspective on the possible future prospects of the integrated process is briefly presented.?2018 The Chemical Industry and Engineering Society of China,and Chemical Industry Press Co.,Ltd.All rights reserved.

1.Introduction

The transportation sector worldwide is almost entirely dependent on petroleum-based fuels and it is responsible for over than 50%of the world oil consumption[1].It is predicted that motor vehicles account for more than 19%of global carbon dioxide emissions and 70%of global carbon monoxide emissions [1,2]. Bioethanol with carbon neutral,derived from a renewable biomass,is considered an important way of progress for improving air quality,limiting greenhouse gas emissions,and finding new energetic resources.As a transportation fuel,ethanol has a higher octane number,higher flame speeds,broader flammability limits,and higher heats of vaporization[1,2].Ethanol could be combined and blended with petrol within unmodified spark-ignition engines or burned in its pure form within modified spark-ignition engines[3].

Batch,fed-batch and continuous operations are three fermentation modes[4].Batch fermentation is the simplest operation mode.During the process,nothing is added after inoculation except possibly acid or alkali for culture pH control. The yeast cells works in initial high substrate concentration and final high ethanol concentration. Yeast cells work at low substrate concentration with an increasing ethanol concentration during the course of fed-batch fermentation process.The major advantage of fed-batch over batch fermentation is the ability to prolong culture life time,reach the maximum viable cell concentration and allow a higher concentration of accumulative fermentation product.Continuous fermentation can be performed in different kinds of bioreactors with stirred tank reactors in single or series.In the process,feed containing substrate,culture medium and other required nutrients, is pumped continuously into an agitated vessel. Continuous fermentation can give a higher productivity than batch fermentation and the highest productivities can be achieved at low dilution rates.

During the above fermentation processes,the accumulation of ethanol in the broth inhibits the cell growth and fermentation,leading to low ethanol productivity,low ethanol concentration in the broth,and large amount of wastewater treatment. In the subsequent process, a lot of energy is consumed for product recovery from the broth.Ethanol fermentation is a biochemical reaction process controlled by product concentration,and ethanol accumulated in the broth has a serious inhibition effect on fermentation.During conventional fermentation process without ethanol in situ removal from broth,the cell metabolic activity would be weaken leading to the decrease of ethanol productivity with ethanol accumulation. The inhibition of ethanol on yeast began to appear, if ethanol concentration in the broth reaching 60 g·L-1-80 g·L-1. The final ethanol concentration can hardly reach 140 g·L-1during batch fermentation[5].Several separation technologies,including gas stripping,extraction, adsorption, distillation and pervaporation, have been explored for ethanol removal in situ from the broth during fermentation to meet the requirement of reducing the cost of ethanol recovery[6,7].

Pervaporation is one of the most promising approaches for the recovery of alcohols from fermentation broths[8].It is simple,nontoxic to cells, and potentially less energy consuming than distillation [9].The feed or fermentation broth is forced flowing on one side of the membrane,and a gaseous phase permeate would be released.In order to supply a driven force between the membrane upstream and downstream sides, a vacuum or sweep gas less common is exerted at the other side of the membrane.Polydimethylsiloxane(PDMS)is the current benchmark hydrophobic material for ethanol pervaporation,since it has good stability and nice tolerance to the organic solvent[10,11].Ethanol in situ removal by pervaporation can reduce ethanol concentration in the broth,relieve inhibition,prolong culture time and enhance fermentation process.In recent years,several reviews have provided the state-of-the-art of the membrane material and fabrication for bioalcohol pervaporation[8,12].The operation performance of the integrated process is of great importance,since it could provide significant information for process design and regulation.However,so far there is no review focused on the operation performance of the integrated process.The main objective of this review is to provide the recent insights about the operation performance of PDMS membrane pervaporaiton coupled with ethanol fermentation for bioethanol production,which includes the separation performance of PDMS membrane,ethanol fermentation performance with in situ removal and subsequent processing on the downstream of the membrane.

2.PDMS Membrane Separation for Ethanol Recovery

2.1.Mass transfer steps and model

The mass transfer of ethanol from the upstream of the membrane to the downstream of the membrane during pervaporation could be included four steps, as shown in Fig. 1:(1) transferred through a liquid boundary layer by convective mass transfer,and ethanol concentration in liquid bulk(cb)was decreased to the concentration in liquid boundary layer(cbm)at interface of liquid boundary layer/membrane;(2)sorpted into the upstream surface of the membrane,and ethanol concentration was increased to the concentration in membrane(cm1)at interface of liquid boundary layer/membrane; (3) transferred through pervaporation membrane by diffusion,and ethanol concentration in membrane was decreased to the concentration on the downstream side of the membrane (cm2); 4) desorption of permeating molecules to vapor phase on the downstream of the membrane.With regard to pervaporation process,the desorption step is usually ignored since it happens extremely fast.

The mass transfer resistances are mainly existed in the second and the third step. Therefore, the mass transfer resistances during pervaporation for ethanol recovery are mainly composed of mass transfer resistance during convective mass transfer on the upstream of the membrane and the mass transfer resistance during ethanol diffusion across the membrane.The Sherwood correlation related the Reynolds Number and Schmidt Number can be used to describe the convective mass transfer performance on the upstream of the membrane [13].From this dimensionless correlation,it could be obtained that the convective mass transfer on the upstream of the membrane is related to the component property and flowing performance.Higher convective mass transfer coefficient and ethanol concentration in liquid boundary could be obtained,under the condition of higher feed velocity on the upstream of the membrane[13].Moreover,there is a linear relationship between the convective mass transfer coefficient on the upstream of membrane and the exponential function of Reynolds Number.

Several mathematical models (pore flow model, pseudo phase change model and solution-diffusion model)have been developed for the description of ethanol mass transfer from membrane upstream to membrane downstream and the solution-diffusion model is widely accepted[14-18].It is often assumed that the equilibrium of solution is achieved at the interface of liquid and membrane during sorption process. The Flory-Huggins theory is usually used to calculate the activity during solution.Strong interaction that occurred between permeates and PDMS material is assumed by Flory-Huggins theory. It is commonly accepted that the diffusion of ethanol through PDMS networks has occurred owing to the passage of ethanol through the voids and intermolecular spacing between PDMS chains[8].The free volume theory is usually used to describe ethanol diffusion across the PDMS membrane.A mass transfer model for the aqueous solution permeating through the PDMS membrane can be developed based on solutiondiffusion theory combined with the free volume and UNIFAC group contribution theory[19].The simplified mathematical model can be developed to describe ethanol mass transfer by the solution-diffusion theory,with the assumption that the diffusion behavior of various components is only depending on the total volume fraction of all the solvent species in the membrane[20].In general,membrane flux is negatively related to the thickness of the active layer of PDMS and the membrane resistance towards the diffusion increases at a higher PDMS layer thickness since the feed components has to diffuse a longer path[21].

During the mathematical model development,the concentration polarization on the upstream of the membrane is neglected[14,15,20,22].Thus,ethanol concentration in the fluid bulk is used for the calculation of the mathematical models and the boundary layer effect is usually ignored[23].As a matter of fact,ethanol concentration at the interface between fluid and membrane should be applied for model calculation.Thus,the concentration polarization effect should not be ignored during pervaporation process[24].For high selectivity membrane,the boundary layer effect is expected to be more serious[25].An early study of concentration polarization has been conducted with the demonstration that the influence of concentration polarization depends not only on the membrane permeability and fluid dynamics conditions,but also on the selectivity of the membrane[24].Some authors have also emphasized the importance of considering boundary layer resistance in studying the transport of organic solutes through membranes with high selectivity and/or permeability during pervaporation behavior [25].

2.2.Effect of membrane characteristics on pervaporation

In order to increase the mechanical strength of the PDMS membrane,several polymer materials (Cellulose acetate (CA), Polyamide (PA),Polysulfone(PSF),Polyetherimide(PEI)and ceramics)can be used as the supported layer for PDMS composite membranes[9,12,26,27].The separation performances of these composite membranes for ethanol recovery are illustrated in Table 1.It can be seen that the membrane flux was in the range of 500-1200 g·m-2·h-1with the corresponding separation factor in the range of 5-10, under the condition of feed temperature in the range of 30-35 °C. The tensile strength of these above polymer materials is in the range of 40-80 MPa, which makes them being suitable to be used as supported layer material.The instruction layer between the PDMS layer and the supported layer could be existed,if the PDMS is filled into the pores of the supported layer during the preparation of the membrane. The intrusion layer could significantly increase the membrane thickness and lead to higher resistance[43].Therefore,it is necessary to avoid the intrusion layer between the active layer and the support layer.Filling the pores in the supported layer with water or other liquid is a practical method to avoid the intrusion layer[27,44].

Table 1Separation performances of PDMS membranes for ethanol recovery

The performance of PDMS can be further enhanced by incorporation of fillers to form mixed matrix membranes(MMMs).The structure of the MMMs can be classified as:concrete-like structure and sandwich-like structure,as shown in Fig.2.The concrete-like structure MMMs can be achieved by dispersing the fillers homogeneous into the polymer matrix[8].The sandwich-like structure membrane are generally obtained by lay by layer self-assembly methods,which mainly includes five approaches:immersive assembly,spin assembly,spray assembly,electromagnetic assembly and fluidic assembly[45].Recently,the ultra-thin reverse osmosis membrane with polyamide material has been successfully fabricated by electrospraying deposition based on 3D printing principle[46].In our opinion,this emerging technology would also be advantage assembly method to realize the sandwich-like structure membrane. The zero dimensional nano-particles,such as nano-silica(ONS)and hydrophobic zeolite,are generally filled into PDMS to improve the membrane separation performance [47,48]. It has been reported that increasing the amount of the fumed silica resulted in significantly enhanced ethanol permeability of the membranes.When the content of the fumed silica in the PDMS skin layer is 20 wt%, ethanol permeability increased to nearly twice that of the unfilled PDMS-PA composite membrane[44].One dimensional fillers such as carbon nanotubes have also gained extensive attentions in preparing the MMMs with high performance for alcohol separation[49].Moreover,metal organic frameworks(MOFs)are a new class of porous materials that have been considered as excellent fillers for the preparation of MMMs[50].It has been reported that compared to unfilled PDMS membranes,MMMs loaded with 20 wt%a subclass of MOFs, zeolitic imidazole framework (ZIF-67), showed an increase in flux and a doubled separation factor[39].Apart from filler introduction for membrane preparation,surface patterning has been an emerging and effective method to break through the trade-off phenomenon and improve separation efficiency for pervaporation processes.Surface patterning could increase the effective membrane separation surface and enhance the separation efficiency.It has been reported that the membrane flux of patterned membranes cross linked with tetraethyl orthosilicate(TEOS)reached 977.73 g·m-2·h-1(over 2 times as high as that of non-patterned one)for 2 wt%ethanol solution recovery with the feed temperature of 45°C[51].

2.3.Effect of feed composition on pervaporation

Fig.2.Structure of the mixed matrix membranes.Left:Concrete-like structure;right:Sandwich-like structure[8].

In general, the ethanol flux is almost proportional to the feed concentration since the partial pressure in permeate on the downstream of the membranes is very low under the vacuum condition[9].The presence of aroma compounds produced during ethanol fermentation can decrease the permeability coefficient and separation factor of ethanol from those in ethanol model solution, while affects little on the fluxes[52].The effect of ethanol feed concentration on mass transfer of each compound is much related to the solubility properties of the compound in ethanol and water.Little interactions exist in pervaporation process of diluted solution containing these aroma compounds within 1000 ppm[52].In general,the separation performance of PDMS membrane for ethanol recovery from model solution is better than that from fermentation broth.One contribution to the reduction of the membrane flux in the experiment of ethanol fermentation coupled with pervaporation would be the yeast cell and the byproducts of the yeast cell metabolism accumulated in the fermentation broth[53].It has been reported that at the end of the fermentation process,about 100 g·L-1of the total byproducts(acetic acid,lactic acid,propionic acid,citric acid,succinic acid and glycerol) accumulated in the residual broth of the pervaporation membrane bioreactor were obtained [54]. It has been also demonstrated that sugar and salts in the fermentation broth increased the membrane performance but 2,3-butanediol decreased the ethanol flux and selectivity,and glycerol exhibited no effect,during ethanol recovery from the fermentation broth by PDMS membrane[55].

The composition of the fermentative broth influences on the separation and the use of different substrates leads to the need of reevaluation of the process,even if it is already well established.It has been reported that the use of the microfiltration process coupled to the pervaporation in order to recover ethanol from the fermentative broth produced from the lignocellulosic residues used in the banana cultivation showed itself as a very attractive alternative[56].The breakdown of lignocellulosic biomass by pretreatment and the fermentation of the resulting sugars lead to a variety of byproducts,if lignocellulosic biomass is used as substrate for ethanol fermentation.These byproducts are mainly divided into carboxylic acids,furans and phenolics,which could influence the separation performance of the membranes.Pervaporation carried out with three different lignocellulosic fermentation broths(barley straw with concentrated acid pretreatment,willow wood chips with mild alkaline pretreatment and barley straw with mild alkaline pretreatment)reduced the membrane performance by 17%-20%as compared to a base case containing only 3 wt%ethanol in water[57].

2.4.Effect of operation conditions on pervaporation

Temperature and feed flowrate are two key parameters related to the separation performance of the pervaporation[27,58].The flux of the membrane could be increased at the higher feed temperature[59].This phenomenon should be attributed to the increase of the mass transfer driving force and reduction of mass transfer resistance across the membrane,since both the mass transfer driving force and resistance across the membrane would be affected by the feed temperature.It is accepted that the driving force of pervaporation is the vapor pressure difference between the up and downstream of the membrane.The vapor pressure on the downstream of the membrane is much lower under the vacuum condition.Therefore,the driving force of the mass transfer would be assumed being equal to the saturation pressure of the feed.The saturation pressure of both water and ethanol would be increased with the increase of the feed temperature[36].Therefore,both ethanol and water flux are increased with the feed temperature increase.Feed temperature increase could lead to higher temperature of the membrane,according to the heat transfer theory.The thermal mobility of the membrane polymer chains would be enhanced and extra free volume within the membrane matrix may be created at a higher temperature,which would reduce the mass transfer resistance across the membrane and improve the flux.During pervaporation,the temperature dependency of permeate would follow the Arrhenius formula[59].The apparent activation energy value for water is higher than that for ethanol,which implied that water flux is more sensitive than ethanol flux.Therefore,water flux would improve more obvious than ethanol flux and the ethanol separation factor would be decreased with the increase of temperature.

The flux of the membrane could be increased at the higher feed velocity.It could be known that higher flowrate increased the velocity of the feed on the upstream of the membrane and the concentration polarization would be weakened [60]. Therefore, thinner of the mass transfer layer on the upstream of the membrane could be achieved and the convective mass transfer could be enhanced under the condition of higher flowrate [60]. Enhanced convective mass transfer on the upstream of the membrane could reduced the mass transfer resistance of pervaporation and improve the separation performance of the membrane with higher ethanol flux achieved [27]. According to the resistance-in-series model,the mass transfer resistance of membrane separation included the convective mass transfer resistance on the upstream of the membrane and the mass transfer resistance across the membrane[61].The convective mass transfer resistance is related to the flowing performance of the feed while the mass transfer resistance across the membrane is related to the membrane property,independent of the flowing performance on the upstream of the membrane.The convective mass transfer resistance could be predominant under the condition of lower flowrate and the membrane flux could be increased obviously with the increase of the flowrate. However, the mass transfer resistance across the membrane could be predominant under the condition of higher flowrate and the membrane flux could not be increased obviously with the increase of the flowrate.Therefore,the membrane flux should be increased asymptotically with the increase of the flowrate [27]. As ethanol is the component having the greatest affinity with the PDMS membrane,the reduction of concentration polarization means that ethanol concentration near the membrane surface is close to ethanol concentration in the bulk of the fermentation broth, which would increase the driving force of ethanol. Therefore,a higher flux of the membrane is obtained at higher flowrate.

3.Fermentation with Ethanol Removal in situ from Broth

3.1.Enhanced fermentation performance

The schematic diagram of fermentation coupled with PDMS membrane pervaporation is illustrated in Fig.3.Compared with conventional batch fermentation or continuous stirring tank reactor(CSTR)operation,there are five advantages for ethanol fermentation coupled with pervaporation:increase of ethanol productivity,increase of cell density,continuous operation for a long time,reduction of the amount of wastewater treatment and energy consumption required for subsequent purification[36,62].Ethanol fermentation performance with pervaporation for ethanol removal is illustrated in Table 2.

Fig.3.Schematic diagram of fermentation coupled with pervaporation for bioethanol production.

Ethanol fermentation is controlled by the products in broth and ethanol is the main product that has strong inhibition effect on cell growth and ethanol fermentation.During batch fermentation or CSTR operation process without ethanol removal,the cell growth and ethanol productivity would be decreased gradually,owing to ethanol accumulated in the broth.In general,ethanol concentration can hardly reach 100 g·L-1.Above this point,the cells can be hardly survived,leading to the fermentation process terminated. The tolerance of the cell to ethanol would be improved some extent by genetic modification,and ethanol concentration may reach about 120 g·L-1in this case.Ethanol recovery from broth by pervaporation during ethanol fermentation could reduce ethanol concentration in broth,relieve ethanol inhibition and therefore increase ethanol productivity.

Table 2Ethanol fermentation performance with ethanol in situ removal by pervaporation

During ethanol fermentation,ethanol producing is directly related to cell growth.The nutrients in the fermentation broth transported into cell are converted into ethanol and other products under the action of intracellular enzymes.Then the metabolites are transported into fermentation broth. During the batch or CSTR fermentation, ethanol weakens the enzymes'activities and limits the cytoplasmic flow.During ethanol fermentation coupled with pervaporation,ethanol is removed in situ from the broth and therefore ethanol concentration in cytoplasm is reduced.It has been reported that about 40 g·L-1of cell concentration could be achieved during ethanol fermentation coupled with pervaporation [54]. Cell concentration could be improved further if cell is immobilized during fermentation[35].During ethanol fermentation coupled with pervaporation,several obvious stages of cell growth could be distinguished.Rapid growth stage,inhibition growth stage,second growth stage,stationary stage and decline stage are observed,as shown in Fig.4.The yeast cell could grow quickly owing to the sufficient substrate and low ethanol concentration during the initial fermentation period.The cell growth could be inhibited with ethanol accumulated in the broth.The cell growth could be performed continuously after ethanol is removed in situ from the broth with PDMS membrane.The cell concentration could be kept as a stationary stage and declined with byproducts accumulated in the broth and cell lysis.The second growth stage(inhibited by ethanol)would not be performed if ethanol fermentation is coupled with pervaporation at the initial period[54].

Fig. 4. Cell growth performance during ethanol fermentation coupled with PDMS membrane pervaporation.1 rapid growth stage;2 inhibition growth stage;3 s growth stage;4 stationary stage;5 decline stage[62].

During ethanol fermentation in batch process,the process has to be terminated when ethanol in the broth accumulated to a critical point.In general, when ethanol fermentation process lasted for 50-80 h, the broth could be discharged from the fermentor for ethanol further separation.The fermentor could be cleaned and sterilized,before the next batch fermentation,which leads to low utilization of equipment.During ethanol fermentation coupled with pervaporation,the process could last for hundreds of hours since ethanol is removed from the broth,eliminating ethanol inhibition on fermentation[35,62,63].Long time of fermentation with high ethanol productivity can reduce the time of equipment repose and improve utilization of equipment.During the long time of fermentation,adaptive evolution of the cell is discovered[69].During fermentation coupled with pervaporation by the screened strain,the ethanol productivity,ethanol yield on biomass and specific ethanol productivity are 1.63 g·L-1·h-1, 194.3 g·g-1and 0.385 1 h-1,which are increased by 10.13%,14.97%and 19.57%compared to the fermentation with the parent strain[70].

During the batch fermentation or CSTR process,ethanol concentration in broth is low,which leads to large amount of wastewater.For example, in order to achieve 1 L of fuel grade ethanol, 12.5 L of fermentation broth must be treated,in the case of fermentation broth with 80 g·L-1of ethanol.During ethanol fermentation coupled with pervaporaiton, the amount of ethanol produced by the continuous separation of ethanol is greatly improved and the highest amount of ethanol production can reach 750 g·L-1[54].Therefore,the amount of fermentation broth is greatly reduced when fermentation is finished.Distillation is the common approach for ethanol purification.The energy required for ethanol purification is related to ethanol concentration in broth.During batch of CSTR process,ethanol concentration in broth is lower and a large amount of energy should be required for ethanol purification.During ethanol fermentation coupled with pervaporation,ethanol concentration on the downstream of pervaporation membrane could reach 30 wt%. Higher ethanol concentration in permeate can reduce energy consumption required for purification.

3.2.Byproduct inhibition

Complex metabolic pathways exist in the yeast cells and ethanol is the end product of the metabolisms during ethanol fermentation from substrate.Therefore,several byproducts would be produced during the process of ethanol fermentation.Compared with ethanol production,trace of these byproducts could be obtained.During ethanol fermentation coupled with pervaporation,ethanol can be in situ removed from the fermentation broth but these byproducts could hardly penetrate the membranes and would be accumulated in the broth. It has been found that in the fermentation broth,concentration of these byproducts from high to low is in order:glycerol,lactic acid,acetic acid,propionic acid,succinic acid,citric acid[71].During the fermentation process,the amount of each individual byproduct in the broth is nearly proportional to ethanol production. This indicated that a corresponding constant amount of each individual byproduct would be accumulated in the fermentation broth with per unit ethanol produced in the coupled process.Furthermore,in our previous study it has reported that 45.3×10-3mol of glycerol,10.1×10-3mol of lactic acid,4.5×10-3mol of acetic acid,2.8 × 10-3mol of propionic acid, 1.6×10-3mol of succinic acid and 0.5×10-3mol of citric acid could be accumulated in the broth respectively,with 1 mol of ethanol produced in the coupled process[71].

The byproducts would be accumulated to a critical point where the fermentation process could be inhibited.The synergetic inhibition effect of the byproducts accumulated in the broth of the coupled process can be studied in shake flask experiments.It has been found that with increasing concentration of the byproducts in the medium,obvious inhibition effect could be observed with a reduction of cell production,a reduction of ethanol production and longer lag time[71].Considering the byproducts observed during the coupled process,all of the compounds were weak acids except the glycerol. The undissociated forms of weak acids are liposoluble and can diffuse through the plasma membrane.These undissociated acids penetrate into the cytoplasm and may subsequently dissociate intracellularly,leading to the acidification of the cytoplasm.In order to maintain intracellular pH in an optimal metabolic range,cells pump out the surplus protons at the expense of additional metabolic energy.Therefore,an increased diversion of energy to export protons results in decreased biomass yield with respect to glucose and a reduction of cell specific growth rate.In the coupled process,higher cell concentration and ethanol production can be achieved under the condition of higher pH[54].

Lignocellulosic biomass is known for its recalcitrant to biological degradation.For biological conversion process of lignocellulosic biomass to ethanol, a pretreatment step before enzymatic hydrolysis is required to facilitate the access of the enzymatics to its substrates and then the conversion of carbohydrates into fermentable sugars.Several degradation compounds would be generated during the pretreatment process and these compounds would inhibit cell growth during fermentation.If these inhibition compounds could not be removed by PDMS membrane,they could be accumulated in the broth,leading to the inhibition effect more and more serious. Therefore,the pretreatment process should be optimized to obtain biomass and hydrolysates with higher digestibility and fermentability with less inhibition compounds generated[72].It is critical to identify the main inhibition compounds and better understand the inhibition mechanism.The resulting hydrolyzates contain substances inhibitory to fermentation depending on both the raw material and the pretreatment methods applied[73].Different inhibition compounds would be generated by different pretreatment methods and different lignocellulosic biomasses.It has been reported that acetic acid,furfural and 5-hydroxymethylfurfural are the main inhibition compounds formed during the steam explosion pretreatment for wheat straw[74].Two approaches would be applied to relive the inhibition resulting from pretreatment: removal of the inhibition compounds before fermentation or development of the cells by adaptive evolution for the inhibition compounds.

4.Subsequent Processing of Ethanol on Membrane Downstream

4.1.Purification for fuel grade ethanol

In general,ethanol concentration on the downstream of the PDMS membrane is in the range of 20 wt%and 40 wt%[41].Ethanol concentration on membrane downstream could not meet the requirement of fuel grade ethanol,since ethanol concentration must be over 99.5 wt%for fuel grade ethanol. Distillation would be the common approach to increase ethanol concentration to the azeotropic point(95.5 wt%).However,distillation is energy extensive since liquid evaporation and vapor condensation are necessary.Fractional vapor condensation with cheap running water at the room temperature on the downstream of the membrane could enhance the pervaporation process separation factor and higher ethanol concentration could be achieved[75].In general,fractional condensation is achieved by multi-stage condensation at different decreasing cold temperatures.The second condenser in the series is operated at a lower temperature than the first condenser.An ethanol depleted condensate is obtained at the first condenser and an ethanol enriched condensate is achieved at the second condenser[76].

In order to collect the permeate vapor on the downstream of the membrane under the vacuum condition,“cold traps”related to refrigerating system or liquid nitrogen are required.“Cold traps”are complex and energy extensive,which could impart its application.The saturation pressure of the permeate vapor can be increased by mechanical vapor compression and the vapor enriched with ethanol can be condensed at room temperature [36,77]. Using mechanical vapor compression can eliminate the cold traps required for vapor condensation under the vacuum condition. Ethanol fermentation coupling pervaporaiton with permeate fractional condensation and mechanical vapor compression is illustrated in Fig.5.It has been reported that ethanol concentration in condensate of condenser C1 could be increased if ethanol concentration in condensate of condenser C2 is increased[75].However,the permeate recovery rate and the ethanol recovery rate could be reduced.Moreover,the condensate of condenser C1 with high ethanol concentration feeding back to the broth may cause inhibition on fermentation.Ethanol concentration in condensate of condenser C2 should be controlled at about 70 wt% by fractional condensation [75]. Ethanol concentration of over 90 wt% at the top of the dephlegmator and ethanol concentration of less than 5 wt% at the bottom of the dephlegmator could be obtained, if a dephlegmator fractional condenser could be used for permeate vapor purification during the pervaporation process.

Fig.5.Schematic diagram of pervaporation process with fractional permeate condensation for bioethanol production[75].1,fermentor;2,circular pump;3,membrane module;4,condenser C1;5,vacuum pump;6 condenser C2;A,feed;B,fermentation broth;C,retentate;D,permeate;E,permeate enriched with ethanol;F and G,condensates;H and I,vent;a,thermostat;b,running water;c,air.

The concentration of ethanol from the top of the fractional condenser could not exceed the azeotropic point. The remaining water mixed in ethanol could be removed by azeotropic rectification with benzene added into the mixture to break the azeotropic point.Recently,adsorption dehydration by molecular sieves has been also applied to realize the fuel grade ethanol.The molecular sieves used for dehydration should be regenerated for reuse. It has been reported that the cost of molecular sieve regeneration via continuous heating by hightemperature nitrogen can stand comparison with other purify methods[78].The most significant recent competition to molecular sieve dehydration is the membrane based technologies of pervaporation and vapor permeation.For ethanol dehydration,the membrane should be hydrophilic and selectively transportation water.The integrated process for fuel grade ethanol production with PDMS membrane pervaporation are illustrated in Fig.6.The integrated process is mainly composed of fermentation unit,PDMS membrane module,fractional condenser and dehydration unit with pervaporation or vapor permeation.In the integrated process,the liquid depleted with ethanol collected at the bottom of the fractional condenser is fed back to the fermentor,which could reduce the amount of waste water treatment.

Fig.6.Schematic diagram of pervaporation process with fractional permeate condensation and dehydration for bioethanol production.1,fermentor;2,circular pump;3,membrane module;4,condenser C1;5,vacuum pump;6 dehydration by pervaporation or vapor permeation;7,vacuum pump;8,condenser C2;A,feed;B,fermentation broth;C,retentate;D,permeate;E,permeate with 95 wt%ethanol;F,condensates;G,fuel grade ethanol;H,permeate enrich with water;I,vent;J,liquid enriched with water;K,vent;a,thermostat;b,running water;c,air.

4.2.Heat recovery of the permeate vapor

During pervaporation for ethanol recovery,heat is required to provide the latent heat for feed evaporation from the membrane upstream to the downstream.The vapor condensation for permeate collection on the membrane downstream could release heat.If the heat released from the permeate vapor could be used to heat the feed for evaporation,the energy consumption of pervaporation process would be reduced significantly.Ideally,the heat released from the permeate vapor during condensation could be used to heat the liquid feed.However,this could not be achieved due to the heat transfer resistances and the difference between the temperatures of the liquid feed and the permeate condensate necessary to maintain a permeate pressure driving force[6].Heat integration with heat pumps could be employed to upgrade the heat released from the vapor during the condensation and then the heat released from the permeate vapor could be utilized.Heat integration is generally achieved by using a closed-cycle heat pump.During the pervaporation process with a closed-cycle heat pump,the cold side of the heat pump is linked to the condensers of the system and the hot side of the heat pump is linked to the feed liquid heaters of the system[75].

Fig.7.Schematic diagram of pervaporation process with fractional permeate condensation and dehydration for bioethanol production.1,fermentor;2,circular pump;3,membrane module;4,condenser C1;5,vacuum pump;6 dehydration by pervaporation;7,vacuum pump;A,feed;B,fermentation broth;C,retentate;D,permeate;E,permeate with 95 wt%ethanol;F,condensates;G,fuel grade ethanol;H,permeate enrich with water vapor;I,vent;J,liquid enriched with water;a,running water.

Different from the standard closed-cycle heat pump,there is no expansion process or refrigerant in the mechanical vapor compression heat pump with less heat transfer units involved. Mechanical vapor compression heat pump could compact the system and increase energy efficiency of the system.The energy availability and the temperature of the vapor would be increased after the permeate vapor is compressed by the mechanical vapor compression heat pump.The low energy availability of the permeate vapor on the downstream side of the membranes would be utilized.Great amount of the available energy could be obtained at the expense of less energy required for vapor compression.It has been reported that the Coefficient of Performance(COP)value of the mechanical vapor compression heat pump could be reached 7-9[75].This implies that 7-9 MJ of the available energy could be obtained with 1 MJ of the energy inputted into the vacuum pump.The compressed vapor with high available energy and high temperature could be used to heat the feed on the upstream side of the membranes.Therefore,the energy consumed during ethanol fermentation coupled with pervaporation process can be reduced further.Bioethanol production system with PDMS membrane pervaporation and mechanical vapor compression heat pump for heat integration is illustrated in Fig. 7. The designed system would be attracting and promising for bioethanol production,since high ethanol productivity,less amount of waste water treatment and energy efficient could be achieved.

5.Conclusions and Perspectives

PDMS membrane pervaporation coupled with fermentation is a promising process for bioethanol production.Solution-diffusion model is the most common theory for the description of ethanol mass transfer from the upstream to the downstream of the membrane.In order to increase the mechanical strength of the PDMS membrane,several polymer and inorganic materials,such as Cellulose acetate(CA),Polyamide(PA), Polysulfone (PSF), Polyetherimide (PEI) and ceramics can be used as the supported layer for PDMS composite membranes. The performance of PDMS can be further enhanced by incorporation of fillers to form MMMs.Yeast cells and the byproducts accumulated on the upstream of the membrane could reduce the flux of the membrane.Higher feed temperature can increased the mass transfer driving force and reduce mass transfer resistance.Higher velocity on the upstream of the membrane can enhance the flowing performance of the feed and improve the convective mass transfer on the upstream of the membrane.

Increase of ethanol productivity,increase of cell density,continuous operation for a long time,reduction of the amount of wastewater treatment and energy consumption required for purification are the advantages for ethanol fermentation coupled with pervaporation.Trace of byproducts can be accumulated in the fermentation broth with ethanol in situ removal by pervaporation.These accumulated byproducts would acidified the cytoplasm and inhibit fermentation process.At higher pH in the medium,the byproducts inhibition can be relieved.Enhancement of the pervaporation process separation factor could be achieved by permeate fractional condensation with high ethanol concentration be achieved.The saturation vapor pressure can be increased by mechanical vapor compression and the vapor can be condensed at room temperature.The COP value of the mechanical vapor compression heat pump could be reached 7-9,which lead the process more energy efficient.

Despite great progresses having been achieved,there are still several effective strategies could be employed for the further development of PDMS membrane pervaporation coupled with ethanol fermentation.More kinds of substrate material should be used in the integrate process and the appropriate pretreatment and hydrolysis methods should be applied,since several byproducts during pretreatment and hydrolysis may be accumulated in the fermentation broth;Membrane with better separation performance and stability should be developed,and the cost of the membrane should be reduced,since high membrane material current impart the application of pervaporaiton for ethanol recovery;The process scaled up should be integrated with more functional unit with higher efficiency and less energy consumption.