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1 NRC Publcatons Archve (NPArC) Archves des publcatons du CNRC (NPArC) Process Energy Effcency n Pervaporatve and Vacuum Membrane Dstllaton Separaton of 2,3-Butanedol Shao, Pngha; Kumar, Ashwan Publsher s verson / la verson de l'édteur: Canadan Journal of Chemcal Engneerng, 89, 5, pp , Web page / page Web Access and use of ths webste and the materal on t are subject to the Terms and Condtons set forth at READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. L accès à ce ste Web et l utlsaton de son contenu sont assujetts aux condtons présentées dans le ste LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D UTILISER CE SITE WEB. Contact us / Contactez nous: nparc.cst@nrc-cnrc.gc.ca.

2 Process Energy Effcency In Pervaporatve And Vacuum Membrane Dstllaton Separaton Of 2,3-Butanedol Pngha Shao and Ashwan Kumar* Insttute of Chemcal Process and Envronmental Technology, Natonal Research Councl Canada, M-12, 1200 Montreal Road, Ottawa, Ontaro, Canada K1A 0R6 Ths work focused on the energy aspects of the pervaporatve separaton of 1-butanol/2,3-butanrdol. A numerc model was developed to smulate the mass and energy balance of the pervaporaton process. It was found that the dstrbuton of evaporaton heat requrement over the membrane area s asymmetrc, and more than 85% of the heat was consumed n the 60% of the membrane area. It was also revealed that recyclng the permeate havng hgher than 5% w/w 2,3-butanedol can mprove the recovery of 2,3-butanedol, and thus enhance the process energy effcency. Two recyclng strateges (the sngle or multple pont admsson of permeate to the retentate flow) were explored. The specfc energy requrement (the heat requred by generatng 1 kg 99.5% w/w 2,3-butanedol as product) was proposed to evaluate the process energy effcency of both the pervaporaton and vacuum membrane dstllaton, and t was shown that pervaporaton can brng about nearly four tmes energy savngs over the vacuum membrane dstllaton. Ce traval état axé sur les aspects énergétques de la séparaton pervaporatve de 1-butanol/2,3-butanedol. Un modèle numérque a été créé pour smuler le blan massque et énergétque du processus de pervaporaton. On a découvert que la dstrbuton de la consommaton calorfque de l évaporaton sur la surface de la membrane est asymétrque et plus de 85% de la chaleur état consommée dans les 60% de la surface de la membrane. On a également révélé que le recyclage du perméat supéreur à 5% p/p 2,3-butanedol peut amélorer la récupératon de 2,3- butanedol et ans accroître l effcacté énergétque du processus. Deux stratéges de recyclage (l admsson unque ou multple du perméat au flux de rétentat) ont été explorées. Le beson énergétque partculer (la chaleur requse par la producton de 1 kg 99,5% p/p 2,3-butanedol comme produt) a été proposé pour évaluer l effcacté énergétque du processus à la fos de la pervaporaton et de la dstllaton de membrane sous vde et l a été démontré que la pervaporaton peut occasonner presque quatre fos les économes d énerge par rapport à la dstllaton de membrane sous vde. Keywords: pervaporaton, vacuum membrane dstllaton, process energy effcency, numercal model, product recovery, permeate recyclng INTRODUCTION As a fermentaton product, 2,3-butanedol has long been produced by dstllaton (Larson and Stormer, 1973; Magee and Kosarc, 1987; Garg and Jan, 1995; Lee, 1999; Jansen and Tsao, 2007). Because of the low concentraton of 2,3- butanedol n the fermentaton broth, and the stronger hydrogen bondng nteracton between water molecules, recovery of 2,3- butanedol by dstllaton s a very energy-ntensve process (Neel and Huang, 1991; Shao and Huang, 2007). As an attempt, an ntegrated process comprsng of solvent extracton and pervaporaton (PV) has been proposed to deal wth ths recovery. In the ntegrated process, 2,3-butanedol n the fermentaton broth was frst extracted usng 1-butanol, and the extracted 2,3-butanedol was further enrched by a PV membrane process. We recently nvestgated the PV separaton of 1-butanol/2,3-butanedol usng polydmethylsloxane (PDMS) and PDMS flled wth ZSM-5 zeolte membranes, the results showed that the ZSM-5-flled PDMS membrane appears very promsng for the separaton (Shao and Kumar, 2009a,b, 2010). As a contnued effort, ths current work s dedcated to the energy aspects of the PV process, wth the NRCC No Author to whom correspondence may be addressed. E-mal address: ashwan.kumar@nrc-cnrc.gc.ca Can. J. Chem. Eng. 9999:1 11, Canadan Socety for Chemcal Engneerng DOI /cjce Publshed onlne n Wley Onlne Lbrary (wleyonlnelbrary.com). VOLUME 9999, 2011 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 1

3 emphass gven to demonstratng that for conductng recovery of 2,3-butanedol the proposed PV (the ntegrated process) s better than the energy-ntensve dstllaton. Compared wth the tradtonal column-tray dstllaton, membrane dstllaton (MD) exhbts some advantages, ncludng lower captal nvestment, flexble processng capablty, more vapor lqud contact area n per unt volume of the separator, and less energy-ntensveness (reflux s generally not consdered n MD). Accordng to lterature (Quresh et al., 1994; Izquerdo- Gl et al., 1999, 2000; Huang et al., 2002; El-Bouraw et al., 2006; Cerneaux et al., 2009), the mass transport drvng force for MD can be created ether by mantanng a temperature dfference across the porous membrane, or by usng a sweepng gas stream or usng a vacuum operaton to remove all the transportng speces on the downstream face of the membrane. A bref theoretcal analyss remnds that the vacuum MD tends to gve the hghest process selectvty because of easy avalablty of the low permeate pressure. Therefore n ths work the vacuum MD process was selected as the reference framework for evaluatng the energy effcency of the proposed PV process. To compare the energy effcency of the two processes, the energy balance of the PV and vacuum MD were modelled. Some mportant parameters, whch characterse the process heat transfer (e.g., dstrbuton of heat transfer over the membrane area, and total heat flow needed for PV separatons) were nvestgated. It s found that permeate-recyclng s an effcent approach for ncreasng the product recovery, and the permeate-recyclng mproves the energy effcency of PV though t ncreases the total energy requrement. The process energy effcency s evaluated n terms of the specfc energy requrement, whch s defned as the total evaporaton heat needed for producng one klogram of 99.5% w/w 2,3-butanedol. Results showed that the process energy effcency of PV s much better than that of vacuum MD as t s drectly used for recoverng 2,3-butanedol from the fermentaton broth. THEORETICAL A Numercal Model for Evaluatng Mass and Energy Balance of PV Accordng to Fck s frst law, the mass transfer through the membrane results from dffuson of a transportng speces n the membrane, therefore the membrane flux of a transportng speces can be expressed as: df da = D dc m dl where F s the mass flow rate of the feed component (1-butanol or 2,3-butanedol), A s the membrane area, D s the dffuson coeffcent, C m s the concentraton n the membrane phase, and l s the locaton varable. Based on the soluton dffuson theory (Neel and Huang, 1991), the concentraton of a transportng speces n the upstream membrane surface s n equlbrum wth ts feed concentraton, namely: C m /C f = K, where K s the partton coeffcent. Snce the concentraton of the transportng speces at the downstream membrane face can be neglected n PV due to the vacuum operaton. Equaton (1) can be rewrtten as Equatons (2) and (3): df da = DK l C f (2) (1) Fgure 1. The mass transport and balance across a typcal membrane sector for llustratng the numercal smulaton of the pervaporaton process. df da = P (X) F l 2 (F =1 / ) where P(X) = DK, the permeablty of the membrane for the feed component, whch s usually dependent upon the feed composton, X s the feed mass fracton of 1-butanol, and s the specfc densty of the component. Based on Fgure 1, transformng Equaton (3) nto a dfference equaton gves: F (k) F (k 1) = P (X (k 1) ) F (k 1) A (k) l 2 =1 (F(k 1) / ) The mass fracton of 1-butanol n the feed comng nto the membrane sector (k) from the prevous one s determned by: X (k 1) = F(k 1) 1-butanol 2 =1 F(k 1) Based on mass balance, the mass fracton of component n permeate of the membrane sector (k) s acheved by: Y (k) = F (k 1) F k 2 =1 (F(k 1) (3) (4) (5) F k ) (6) The boundary condtons (F (0),X (0) ) of Equaton (4) are known, therefore all the process quanttes ncludng F (k),x (k), and Y (k) (1 k K) can be obtaned by selectng an approprate membrane area A (k) for each of the membrane sectors. The crteron for selectng the membrane area for the membrane sector s such that the stage cut for each of the components never exceeds 3%. The total membrane area needed for the separaton s dependent upon the objectve purty (1 X) of the product, as the targeted purty s reached, the total membrane area can be estmated by: A total = K A (k) (7) k=1 The percentage of membrane area A(L) from membrane sector 1 through L s defned as: A(L) = 1 A total L A (k) 100% (8) k=1 In case permeate of membrane sectors (from L to L and 1 L<L K) s gathered for recyclng, the mean mass fracton of 2 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 9999, 2011

4 Table 1. Physcal propertes of lquds used n process smulaton Feed components Molecular weght (Da) Bolng pont ( C) Vapor pressure a (mm Hg b ) Evaporaton enthalpy c (kj/mol) Specfc evaporaton heat (kj/g) Permeablty a (cm 2 /s) 1-Butanol E 8exp(1.93X) 2, E 10exp(4.23X) Butanedol Water a The value of the permeablty at 50 C. b 1mmHg = Pa. c Data from Eusebo et al. (2003); Sharma et al. (1996); and Glan et al. (2006). component n the accumulated permeate s obtaned by: Y = F L FL 2 1 (FL FL ) The recovery R of 2,3-butanedol (denoted as dol n the subscrpt) s defned as: R = F(K) dol F (0) dol (9) (10) The heat flow rate needed for evaporatng component n membrane sector k s obtaned by: Q k = (F k Fk 1 ) H evaporaton (11) where H s the evaporaton enthalpy. The heat transferred over membrane sector k s: E(k) = 2 Q k (12) =1 The cumulatve heat transferred over membrane sectors (from 1toL) can be wrtten as: E cumulatve (L) = 2 =1 L Q k (13) k=1 When L = K, the cumulatve heat s equal to the total heat Q total transferred throughout the membrane separator. Consderng the fact that vapor permeate n PV needs to be condensed. Fgure 2. The dependence of permeate and retentate 2,3-butanedol mass fracton upon the percentage of membrane area. The ntal feed mass flow rate s 360 kg/h, and the ntal mass fracton of 2,3-butanedol s VOLUME 9999, 2011 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 3

5 Theoretcally ths porton of energy consumpton equals the total evaporaton heat, therefore the total energy consumpton of the PV process s: Q PV = 2 Q total (14) The process energy effcency s charactersed by the energy needed for producng 1 kg of 2,3-butanedol havng the objectve purty (99.5% w/w), and t can be wrtten as: Eff. PV = QPV R F (0) dol = QPV F (K) dol (15) Snce both the apparent heat and the mxng heat, and the vacuum operaton cost nvolved n the separaton s much less than the evaporaton heat that s needed to drve the separaton, all these mnor energy components are not consdered n ths modellng. An Analytcal Model Descrbng Mass Balance and Energy Consumpton of Vacuum MD Process for Drect Recovery of 2,3-Butanedol From Its Fermentaton Broth Consder a fermentaton broth stream consstng of water and 2,3- butanedol, and the flow rate of the components are m and n, respectvely. Assume that the lqud and the vapor are always n equlbrum throughout evaporaton across the membrane area (refer to Fg. 1, where the membrane s non-porous). Suppose that dm of water and dn of 2,3-butanedol are locally evaporated, and that mass transport through the porous dstllaton membrane s predomnated by the vscous flow regme, thus the mole fracton of water n the permeate phase s determned by: y = dm/m dm/m + dn/n (16) where M, and N represent the molecular weght of water and 2,3- butanedol, respectvely. By neglectng the small losses of dm and dn, the mole fracton of water n feed s: x = m/m m/m + n/n (17) Assumng the feed s an deal soluton, based on the vapor lqud equlbrum, the mole fracton of water at the vapor lqud nterface s: y = x p water x p water + (1 x) p dol (18) where p water and p dol are the saturated vapor pressure of water and 2,3-butanedol, respectvely. Recallng the vscous flow assumpton, the mole fracton of water n permeate should be equal to that at the vapor lqud nterface, therefore nsertng Equatons Fgure 3. The plot of the percentage of feed component passng through the pervaporaton membrane versus the percentage of membrane area. 4 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 9999, 2011

6 (16) and (17) nto Equaton (18) yelds: dm/m dm/m + dn/n = (m/m) p water (19) (m/m) p water + (n/n) p dol Elmnatng m between Equatons (22) and (23) gves: ( ) 1/ˇ 1 n0 X n = n 0 (24) m 0 1 X Rearrangng Equaton (19) gves: dm m = dn n ( ) p water p dol (20) Wth a set objectve purty (1 X) of 2,3-butanedol, the mass flow rate (m and n) n the product stream can be estmated, and the recovery (R(X)) of 2,3-butanedol of the vacuum MD process can be obtaned as: Integratng Equaton (20) from the ntal feed quanttes (M 0, n 0 ) to a random ntermedate state (m, n) gves Equatons (21) and (22): m m 0 dm m = ˇ n ( )ˇ m n = m 0 n 0 n 0 dn n (21) (22) where ˇ = p water /p dol, the relatve volatlty, whch s a constant at a temperature. Note m and n n the mxture can be correlated wth the mass fracton (X) of 1-butanol by: R(X) = n n 0 100% = ( ) 1/ˇ 1 n0 X 100% (25) m 0 1 X Agan, consderng the vapor condensaton, the total energy requred by the vacuum MD process can be evaluated by: Q MD = 2 [(m 0 m) H water + (n 0 n) H dol ] (26) Note n Equaton (26), the energy consumed for condensng vapor permeate s also consdered. Smlarly, the process energy effcency s charactersed by the specfc energy demand by: Eff. MD = Q MD R(0.995) n 0 (27) m n = X 1 X (23) Usng Equatons (15) and (27), the process energy effcency of PV and vacuum MD can be calculated and compared. Fgure 4. The dstrbutons of evaporaton heat over membrane area. The ntal flow rate s 360 kg/h, and the ntal mass fracton of 2,3-butanedol s VOLUME 9999, 2011 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 5

7 RESULTS AND DISCUSSION Smulaton Condtons and Membrane Performance Fermentaton s generally a product-nhbtng process; the economcal concentratons of 2,3-butanedol n the fermentaton broth are n the range of 4 6% w/w (Shao and Kumar, 2009b). A mean value of 5% w/w was consdered as the ntal concentraton of 2,3-butanedol n the broth when MD was employed to enrch 2,3-butanedol drectly from the fermentaton broth. In the case of an ntegrated solvent extracton and PV process as a substtute for the MD, accordng to our earler work (Shao and Kumar, 2009a), as the ntal concentraton of 2,3-butanedol n the fermentaton broth s 5% w/w, the concentraton of 2,3-butanedol n the extract phase s very close to 5% w/w as 1-butanol s employed as the solvent, and ths value was used as the ntal feed concentraton of 2,3-butanedol for PV separaton. The expermentally obtaned permeablty and some mportant thermodynamc data for estmatng the energy demand of both PV and MD are summarsed n Table 1. Snce the process energy effcency s theoretcally ndependent of the feed flow rate, only one feed flow rate (360 kg/h) s consdered n the smulaton. In order to determne the permeaton rate (P/l) of a composte membrane for 1-butanol and 2,3-butanedol, a representatve value of 5 m s assgned to the thckness of the supported thn flm. The target purty of 2,3-butanedol s set at 99.5% w/w, all the process parameters (such as product recovery and energy effcency) for both PV and MD were evaluated wth respect to ths purty. Dstrbuton of Mass Transfer Over Membrane Area The total membrane area needed for enrchng 2,3-butanedol from the ntal concentraton of 5% w/w to the antcpated purty of 99.5% w/w was m 2. In order to have an overvew of the mass transfer occurrng over the entre membrane area, the mass fractons of 2,3-butanedol n both permeate and retentate streams were plotted aganst the percentage of membrane area n Fgure 2. Clearly the total membrane area can be dvded nto two dstnct sectons. Membrane n Secton I (0 60%) removes essentally pure 1-butanol from the retentate because of the sem-permeable property of the membrane and more mportantly the dlute retentate concentraton of 2,3-butanedol. Membrane n Secton I s thus responsble for pre-concentratng 2,3-butanedol. By contrast membrane n the Secton II (60 100%) deals wth hgher concentratons of 2,3-butanedol, and the permeate concentraton of 2,3-butanedol starts gettng ncreasngly hgher as the separaton progresses. Ths porton of the membrane serves to refne 2,3-butanedol to the targeted product purty. It can be conceved that due to the permeablty dfference between the components, the permeate flux should change from one place to another across the membrane n a PV separator. The dependence of 1-butanol and 2,3-butanedol (n percentage) passng through membrane upon the percentage of total membrane area used s llustrated n Fgure 3, and the local mass transport ntensty (LMTI) of the membrane for each of the components can be obtaned as the slope of the curves shown n Fgure 3. LMTI = [ ] d(mass(%)) d(area(%)) (28) Clearly LMTI s an ndex measurng the quantty of a component transportng over 1% of the total membrane area. Evdently, Fgure 5. The mass fracton of 2,3-butanedol n permeate and the accumulated permeate versus the mass fracton of 2,3-butanedol n retentate at 50 C. 6 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 9999, 2011

8 Fgure 6. The flowchart of pervaporaton wthout (A) and wth (B) 1-pont-permeate-recyclng at 50 C, the ntal feed flow rate s 360 kg/h. and the ntal mass fracton of 2,3-butanedol s the membrane at the feed nlet has the hghest LMTI. It can also be found that the LMTI decreases slowly before the percentage of membrane area reaches 60%, and ths mld LMTI declne n Secton I s replaced wth a sharp decrease n Secton II, where the quantty of the remanng 1-butanol n the retentate s qute lmted, partcularly when the percentage of membrane area s hgher than 80%. Clearly, the lowest LMTI s at the retentate outlet, and the majorty of mass transport takes place n Secton I. Dstrbuton of Heat Transfer Across Membrane Area A phase change from lqud to vapor s nvolved n PV separaton. As dscussed prevously, the LMTI vares from one locaton to another, t s thus practcally mportant to know how much evaporaton heat should be offered to each unt membrane area at dfferent locatons of the separator. Ths nformaton allows us to desgn an effcent PV process by provdng the rght amount of evaporaton heat at dfferent locatons so that the separaton can be operated properly. The heat demand of each of the membrane sectors and the cumulatve heat demand were depcted n Fgure 4. As expected, a good large part (85%) of the evaporaton heat s consumed by the membrane n Secton I (0 60% membrane area), where accordng to the LMTI, the majorty of 1-butanol s removed. Suggestng that n practcal operaton more powerful heat exchangers should be deployed n Secton I to enable the separaton. Permeate-Recyclng for Improved Product Recovery and Process Energy Effcency The nterdependence of the permeate and retentate concentraton of 2,3-butanedol s plotted n Fgure 5. It s shown that the mass fracton of 2,3-butanedol n permeate ranges from 0.00 to Apparently recyclng the permeate contanng hgher concentratons of 2,3-butanedol s a feasble approach to mprove ts recovery. The recyclng s conducted n such a way that all the permeate havng a 2,3-butanedol mass fracton hgher than 0.05 (.e., the ntal mass fracton) s accumulated, and the accumulated permeate s admtted to the retentate stream at a pont, where the composton s theoretcally the same as that of the accumulated permeate, whle all the other permeate (<0.05 n mass fracton) s collected and sent back as solvent to extract 2,3-butanedol. The flow rate, and composton of the recycled permeate are obtaned by teraton, and the detaled data are specfed on the flowchart shown n Fgure 6, whch are 18.2 kg/h and 11.4% w/w, respectvely. As shown n Fgure 7A, wth the permeate recyclng, the fnal product recovery s enhanced from 64.6% to 74.7%. It s also noted that due to the ncreased processng duty wth the permeate recyclng the total membrane area for the processng s ncreased from to m 2. Consderng the tremendous long-term energy savngs by elevatng the concentraton of 2.3-butanedol from 5 to 11.4% w/w n the retentate, the extra captal nvestment n membrane area appears nsgnfcant. VOLUME 9999, 2011 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 7

9 Fgure 7. The dependence between the product purty and product recovery of the pervaporaton process wth and wthout consderng the 1-pont permeate recyclng (A), comparson between 1-pont and 3-pont permeate recyclng (B) at50 C. Accordng to thermodynamcs, the beneft of permeaterecyclng can be further enhanced by recyclng permeate at multple ponts n the PV process. The dsadvantage of sngle pont recyclng s that 2,3-butanedol n a hgher concentraton of permeate has to mx wth that of a low concentraton of permeate. Obvously ths knd of mxng wastes the energy requred for gettng them separated, a mult-pont permeate-recyclng strategy tends to reduce ths type of energy waste. Here a 3-pont-recyclng was attempted, and the three permeate streams are obtaned by collectng the permeate n three dfferent ranges of 2,3-butanedol concentraton: the stream I took all the permeate n the range of (5 10% w/w), stream II (10 60% w/w), and stream III (60 82% w/w) (Fg. 8). The composton and the flow rate of these three streams are summarsed n Table 2. Just lke the 1-pont-recyclng, the compostons of the retentate at the three ponts are exactly the same as those of the permeate streams I, II, and III. Accordng to Fgure 7B, the resultng mprovement n recovery relatve to the 1-pont-recyclng s not so sgnfcant. Ths could be due to the fact that the flow rates of hgher concentraton permeates (e.g., permeate streams II and III) are too small to brng a bg effect. And 1-pont-recyclng s therefore adequate for mprovng the process performance of PV (Fg. 6). 8 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 9999, 2011

10 Fgure 8. The flowchart of pervaporaton separaton process consderng the 3-pont-permeate-recyclng at 50 C. The ntal flow rate s 360 kg/h, and the ntal mass fracton of 2,3-butanedol s Table 2. The flow rate and composton of the recycled permeate n the three permeate streams Permeate stream Permeate flow rate (kg/h) Conc. of 2,3-butanedol (% w/w) Stream I Stream II Stream III Comparson of PV Wth MD n Process Energy Consumpton and Process Energy Effcency Dstllaton remans so far the key ndustral means for separatng 2,3-butanedol from ts aqueous fermentaton broth. Some researchers also attempted the vacuum MD as a substtute (El- Bouraw et al., 2006). Phase change occurs n both PV and MD, but the key permeate n PV and MD s 1-butanol, and water, respectvely. Accordng to the thermodynamc data summarsed n Table 1, the specfc evaporaton heat of water s over three tmes hgher than that of 1-butanol, mplyng that the drect MD s a more energy-ntensve process than PV. The heat flow requred Fgure 9. The heat flow needed for enrchng the feed from the ntal mass fracton of 2,3-butanedol to a hgher mass fracton by pervaporaton or vacuum membrane dstllaton. VOLUME 9999, 2011 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 9

11 Fgure 10. The specfc energy needed for enrchng the feed from the ntal mass fracton of 2,3-butanedol to a hgher mass fracton by pervaporaton and vacuum membrane dstllaton. for enrchng 2,3-butanedol from the ntal feed composton to a hgher purty by both PV and the MD s llustrated n Fgure 9. The total heat flow needed for enrchng 2,3-butanedol from 0.05 to by PV s nearly four tmes lower than that by MD. Therefore recoverng 2,3-butanedol by the proposed ntegrated process can save a lot of energy. In order to compare the process energy effcency, the specfc energy demand for producng 1 kg of 2,3-butanedol of a hgher purty are descrbed n Fgure 10, and once agan t s ndcated that PV excels the MD. At the targeted product purty of 0.995, the energy demand for PV and MD are 38 and 148 MJ/kg, respectvely. Meanng that f MD s employed for processng the fermentaton broth drectly the process energy effcency would be nearly four tmes less than PV. In addton, Fgure 10 showed that the mpact of permeate-recyclng on enhancng the energy effcency of PV s qute remarkable. Permeate-recyclng s thus an attractve measure for mprovng both the product recovery and process economcs for PV. CONCLUSIONS Mathematcal models (the numercal and analytcal one) for descrbng both mass and energy balances between the feed and permeate sde of PV and MD were developed. The process smulaton showed that the dstrbutons of mass and heat transport over the membrane area are asymmetrcal, and the majorty of mass and heat transfer was takng place n the 60% of the membrane area from the feed nlet. Therefore more powerful heat exchangers should be deployed for ths energy-ntensve membrane secton. It was shown that the strateges of both 1- and 3-pont permeaterecyclng enhanced the product recovery and reduced the specfc energy demand of the PV process. Further results showed that 1-pont-recyclng was adequate for enhancng product recovery and process energy effcency. Comparson of PV and MD revealed that PV was a more energy-effcent process for enrchng the 2,3- butanedol product. It was possble to save nearly four tmes of the specfc energy demand by usng PV n place of the vacuum MD. NOMENCLATURE A membrane area (m 2 ) E energy dstrbuton Eff. energy effcency of the process (MJ/kg) F mass flow rate of the feed component (g/s) H evaporaton enthalpy (kj/mol) subscrpt representng the feed component k superscrpt representng the number of the numbered membrane sector K number of the membrane sectors l thckness of the actve thn flm of the composte membrane (cm) L random number of the numbered membrane sector m mass flow rate of water n MD (g/s) M molecular weght of water (Da) LMTI local mass transfer ntensty n mass flow rate of 2,3-butanedol n the MD (g/s) N molecular weght of 2,3-butanedol (Da) P membrane permeablty (cm 2 /s) Q evaporaton energy flow (kj/h) R product recovery (%) x mass fracton of 1-butanol n the feed for PV 10 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 9999, 2011

12 X mole fracton of water n the feed for MD Y mass fracton of 1-butanol n permeate of PV Y mole fracton of water n the permeate of MD ˇ relatve volatlty of water to 2,3-butanedol densty of the feed component (g/cm 3 ) ACKNOWLEDGEMENTS The fnancal supports of AAFC s Agrculture Boproducts Innovaton Program and NRC s Natonal Boproducts Program are gratefully acknowledged. REFERENCES Cerneaux, S., I. Struzynska, W. M. Kujawsk, M. Persn and A. Larbot, Comparson of Varous Membrane Dstllaton Methods for Desalnaton Usng Hydrophobc Ceramc Membranes, J. Membr. Sc. 337, (2009). El-Bouraw, M. S., Z. Dng, R. Ma and M. Mhayet, A Framework for Better Understandng Membrane Dstllaton Separaton Process, J. Membr. Sc. 285, 4 10 (2006). Eusebo, M. E., A. J. L. Jesus, M. S. C. Cruz, M. L. P. Letao and J. S. Rednha, Enthalpy of Vaporaton of Butanedol Isomers, J. Chem. Thermodyn. 35, (2003). Garg, S. K. and A. Jan, Fermentatve Producton of 2,3-Butanedol: A Revew, Boresour. Technol. 51, (1995). Glan, H. G., G. Khat and A. K. Hagh, Lqud-Lqud Equlbrum of (Water, 2,3-Butanedol, 2-Ethyl-1-Hexanol) at Several Temperatures, Flud Phase Equlb. 247, (2006). Huang, R. Y. M., P. Shao, X. Feng and W. Anderson, Separaton of Ethylene Glycol Aqueous Solutons Usng Sulfonated Poly(ether ether ketone) (SPEEK) Pervaporaton Membranes: Membrane Relaxaton and Separaton Performance Analyss, Ind. Eng. Chem. Res. 41, (2002). Izquerdo-Gl, M. A., M. C. Garca-Payo and C. Fernandez-Pneda, Ar Gap Membrane Dstllaton of Sucrose Aqueous Solutons, J. Membr. Sc. 155, (1999). Izquerdo-Gl, M. A., M. C. Garca-Payo and C. Fernandez-Pneda, Ar Gap Membrane Dstllaton of Aqueous Alcohol Solutons, J. Membr. Sc. 169, (2000). Jansen, N. and G. T. Tsao, Boconverson of Pentoses to 2,3-Butanedol by Klebsella Pneumona Laboratory of Renewable Resources Engneerng (Onlne Book Chapter), Purdue Unversty, West Lafayette, IN, USA (2007), pp Larson, S. H. and C. F. Stormer, Dacetyl (Aceton) Reductase From Aerobacter Aerogenes-Knetc Mechansm and Regulaton by Acetate of the Reversble Reducton of Aceton to 2,3-Butanedol, Eur. J. Bochem. 34, (1973). Lee, J., The Converson of 2,3-Butandol to Methyl Ethyl Ketone Over Zeoltes, PhD Thess, Purdue Unversty, USA (1999). Magee, R. J. and N. Kosarc, The Mcrobal Producton of 2,3-Butanedol, Adv. Appl. Mcrobol. 32, (1987). Neel, J. and R. Y. M. Huang, Eds., Introducton to Pervaporaton, Pervaporaton Membrane Separaton Processes, Elsever, Amsterdam (1991). Quresh, N., M. M. Meagher and R. W. Hutjns, Recovery of 2,3-Butanedol by Vacuum Membrane Dstllaton, Sep. Sc. Technol. 291, (1994). Shao, P. and R. Y. M. Huang, (Revew) Polymerc Membrane Pervaporaton, J. Membr. Sc. 287, (2007). Shao, P. and A. Kumar, Recovery of 2,3-Butanedol From Water by a Solvent Extracton and Pervaporaton Separaton Scheme, J. Membr. Sc. 28(1 2), (2009a). Shao, P. and A. Kumar, Separaton of 1-Butanol/2,3-Butanedol Usng ZSM-5 Zeolte-Flled Polydmethylsloxane Membranes, J. Membr. Sc. 339, (2009b). Shao, P. and A. Kumar, Pervaporatve Enrchment of 2,3-Butanedol From 1-Butanol and 2,3-Butanedol Usng a Polydmethylsloxane and ZSM-5 Mxed Matrx Membrane: Effect of Ethanol as an Impurty, Sent to Chem. Eng. Process. (2010). Sharma, S., G. Pandy, T. Chakrart and P. Khanna, Lqud Lqud Equlbrum for Water, 2,3-Butanedl, 3-Methyl-1-Butanol Wth Sodum Sulfate and Sodum Carbonate at K, J. Chem. Eng. Data 41, (1996). Manuscrpt receved July 22, 2010; revsed manuscrpt receved September 24, 2010; accepted for publcaton September 27, VOLUME 9999, 2011 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 11