Coupled Pervaporation-Reaction Distillation Process for the Production of n-Bromopropane

TSINGHUA SCIENCE AND TECHNOLOGYISSN 1007-0214 07/22pp466 - 470Volume 7，Number 5，October 2002Coupled Pervaporation- Reaction Distillation Process forthe Production of n- BromopropaneMAO Chengyu (毛澄宇)，YU Lixin (余立新)** , GUO Qingfeng (郭庆丰), XI Chunguang (席春光)Department of Chemical Engineering，Tsinghua University， Beijing 100084， ChinaAbstract :The reaction of n-C.H,OH + HBr = n-C3H,Br + H2O was used to experimentally study a coupledpervaporation (PV ) reaction distillation (RD) process. The results show that polyvinyI alcohol (PVA) is asuitable membrane material for water removal. The typical separation properties of PVA polyacrylonitrile(PAN) composite membranes are a highest flux of 780g/(m2●h) and a separation factor of 840 for the C.H,OHconcentration in the original feed of 95% at 90 C and below 3300 Pa(abs). Reaction distillation produced then- bromopropane from the disillation colum as a ternary azeotropic liquid mixture of CHOH, H2O and C.H,Br ,with a product concentration of about 92%. The coupled PV RD membrane reactor experiment shows that theBrPr yield can reach 92%，much higher than that for reaction distilation without pervaporation.Key words: pervaporation; membrane; polyvinyI alcohol (PVA); polyacrylonitrile (PAN); PVA/PANcomposite membrane ; reaction distillationhydrobromic acid + 1 propanol-Introductionl-bromopropane + water.. There are several industrializable membraneThe derived product C.H,Br can be obtained as an .separation techniques， such as reverse osmosisoverhead product from a distillation column in the(RO)，nanofiltration (NF ), ultrafiltration (UF),form of a ternary azeotrope mixture of C3H,Br，dialysis (D), electric dialysis (ED), gas separationC3H,OH and water. Furthermore， water can be(GS) and pervaporation (PV )41-3. In recent years,continuously removed from the bottom by means ofanother application of membrane technology tcpervaporation. A water selective membrane iscoupled reaction-separation processes has receivedchosen to accomplish the dehydration.special attention from academic circles and1 Pervaporationindustrialenterprises[+-7l.Whenseparationmembranes are incorporated into a reversible1. 1 Membrane material and structurereaction system controlled bythermodynamicequilibrium to remove the product continuously ，The separation properties of a membrane arethe original chemical equilibrium shifts and thecontrolled by many factors including the selectionconversion ratio of reactants and the yield of mainof polymer materials ，modification， film formationproducts increase remarkably.and operation conditions. To be used in practicalThe following reaction is taken as an example toseparation and_ reactionprocesses，an idealstudy the coupled PV-reaction distillation (RD)per中国煤化工should not only haveprocess:goMHC N M H Gppropriate permeationflux，but also have high thermal stability, solventresistance and mechanical stability.The selective separation of the water/C.H,OHReceived: 2001-04-18; revised: 2001-10-08 .mixtureby membranes canrealized* *Towhom correspondence should be addressed.E- mailmail. tsinghua. edu. cn;incorporating functional groups, such as hydroxylTel: 86-10-62788777groups，into a membranel8.Polyvinyl alcoholMAO Chengyu (毛澄宇) et al: Coupled Pervaporation- Reaction Distillation Process ....467(PVA) has many hydroxyl groups and excellentfilm-forming ability. However， being water-soluble and having high crystallinity， pure PVAcannotbe easily usedforpervaporation.Therefore，certain modification of PVA chains正-2should be carried out. So far， PVA has proved to. be the most suitable membrane material for thedehydration of organic solventsL9l. In China，twokinds of PVA membranes are available. One is theformaldehyde treated PVA membrane， and theother is the maleic acid crosslinked PVAmembrane. .。ThermometerTo get an ample water flux， the membrane0-2. Feed outlet3. Feed inletshould be as thin as possible. Thus， a PVA/4. Upstream half-cll5. Rubber ringpolyacrylonitrile (PAN )composite membrane is6. Membranechosen. The composite membrane has three1-7. Porous metal support and metal ring8. Downstream half-celllayers. A PVA active layer， with a thickness of9. Clamping screws10. Permeate outletabout several microns， is supported by a PAN2-11.Vacuum pumpmicrofiltration membrane, and the PAN layer is on12.Liquid nitrogen cold trapthe top of a nonwoven cloth.To lose less propanol in the pervaporationseparation, the PV A/PAN composite membrane'sFig.1 Schematic of the pervaporation cellselectivity should be ashigh as possible.Furthermore，the literature indicates that the1.2.2 Experimental resultsperformance of membranes is best when thePervaporation experiments were carried out bycrosslinking degree is 1%10]. Therefore， themaintaining a mixture of water-C3H;OH on onePVA/PAN composite membranes crosslinked withside of the membrane. The feed mixture was addedmaleic acid were used in theseresearchon the upper side of the membrane at a constantexperiments.temperature.To simulate the actual reactionsystem and determine the actual concentration in1. 2 Separation properties of PVA/PANthe bottom of the reaction distillation column, acomposite membranesseries of experiments were carried out. Th1.2.1 Experimental apparatusconcentration of C3H,OH in the feed was 20%The experimental pervaporation data were obtained40%，60%，80%，and 95% (mass fraction), andwith a standard laboratory setup consisting of athe experimental temperature was 20，40，60，andthermo - regulated storage tank and recycling lines.90C. At givenThe concentration in the feed tank was keptconcentration of water in permeation， and theconstant. For all experiments the feed wasseparation factor, a， are changing withrecycled with a centrifugal pump at a constanttemperatures shown in Figs. 2 - 4.velocity. The pervaporation cell， schematically3000represented in Fig. 1，consisted of an upstream2500f - ←40%half-cell and a downstream one. The liquid feed十-60%.80%flowed through the upstream side bringing the-+95%百1500upper side of the membrane in contact with theliquid. The pressure at the permeate side was kept只1000-below 3300 Pa by a vacuum pump. The composite中国煤化工membrane with an effective membrane area of 20YHCNMHGcm2 was stuck on a porous stainless steel support102030405060708090100plate using a rubber ring. The water vapor wasTemperature (°C)cooled down and gathered in a liquid nitrogen coldtrap. .Fig. 2 Flux changing with temperature at differentC;H;OH concentrations468Tsinghua Science and Technology, October 2002，7(5): 466- 470s 0.99100C for two reasons. First, the reaction rate wasrequired to be as fast as possible. Secondly，a0.98-reasonable vapor flowrate must be maintained in0.97the column to obtain enough product at the top of-20the column. In this experiment, the height of the ..96-packed column was 50 cm and the diameter was 5τ 60%0.95cm. The molar ratio of C3H;OH to hydrobromic★- 95%acid in feed was around one. A ternary azeotropicS 0.94-liquid mixture consisting of C.H,OH， H2O， and102030405060708090100C.H,Br was obtained at the top, whose boiling pointTemperature(°C)was 62C and in which the concentration of C.H,Brwas about 92%. The process is shown in Fig. 5.Cooling 4Fig.3 Concentration of water in permeation changingwithtemperature atdifferent CH,OHconcentrationsReflux900800Packing -CH,BrHBr400CH,OH30002030405060708090100Temperature (°C)Fig. 5 Reaction distillation processFig.4 Relationship between separation factor andExperiments were performed at three originaltemperature at C;H,OH concentration of 95%molar ratios of C.H,OH to HBr: A, C.H,OHThese figures show that both flux and separationHBr; B, CH,OH=HBr; and C, C.H,OH> HBr.The experimental results are shown in Fig. 6.factor increased with operation temperature at acertain feed concentration and the separation factor0.8-reached a high value of 840 at the temperature of0.7-90C when the concentration of C.H,OH was 95%.To study the performance of membranes in品0.5-actual systems， hydrobromic acid was fed to the= 0.4fpervaporationequipment.Wefound that0.3丰hydrobromic acid cannot permeate through the0.2PVA/PANcompositemembraneat the low0.1temperature (40C)， but when the temperature is0 : 20406080100T20high enough， for example, 90C，the membraneTime (min)will be gradually damaged.2 Reaction DistillationFig. 6Yield changing with reaction time in batchAs stated above，when a pervaporation process is .中国煤化工: different original ratiosused，the water can be removed continuously fromJHCNMHGthe reaction system. Only if the other product,Figure 0 shuws llidl lIe .yield reached 80% afterC.H,Br，can also be continuously gathered fromtwo hours. As a comparison, this experiment wasthesystem，can the reactants beaddedconducted in an ordinary reactor without,continuously. Then， the whole process will bedistillation section. After enough reaction time，alimited equilibrium conversion (X) or equilibrium .continuou丙方数据The RD apparatus was operated at about 8(yield (Y) of less than 65% was obtained.MAO Chengyu (毛澄宇) et al: Coupled Pervaporation- Reaction Distillation Process ....469performance during the reaction period. In Figs. 83Continuous Reaction/ Distillationand 9，the flux increased and the permeated waterPervaporation Processconcentration descended with time. The membraneBased on the above pervaporation and reactionarea was not large enough to remove all the waterdistillation results, a set of pervaporation- reactiongenerated inside thereactor.Thus,waterdistillation coupled membrane reactor was built toaccumulated in the reactor.investigate the effect of the coupled reaction-鱼0.97separation process on equilibrium shift (Fig. 7).E 0.96-Cooling 4waler_0.95-Reflux Ig 0.94-0.93-Packing tδ一十2言4方6t/hC,H,BrFig.9 Relationship between permeated water_HBconcentration and reaction timeC,H,OHPervaporationequipment」With the coupled PV /RD membrane reactor, theyield of bromopropane reached 92% (Fig. 10).H2O1.0r0.8-Fig.7 Schematic of the reaction distillation/pervaporation processBecause hydrobromic acid can damage the PVA/PAN composite membrane at high temperatures(about 80 - 90C)，the bottom of the distillation.2-reactor was filled with C.H,OH at the beginning.The mixture of hydrobromic acid and C;H,OH at ai254568molar ratio of 1 : 1 was continuously fed into thereactor.' I he reaction temperature was around80C. The water produced was continuouslyremoved by a PV unit. C.H,Br could be obtainedFig. 10 Yield changing with reaction time in coupledreactor .from the top of the distillation column.In the PV unit, the pressure at the permeate sidewas kept below 3300 Pa by a vacuum pump. The4 Conclusionscomposite membrane's effective area was 20 cm2.. The reaction of n-C:H,OH and HBr was used toFigures 8 and 9 show the PV membraneexperiment on the coupled pervaporation-reaction900rdistillation process. A PVA/PAN compositemembrane crosslinked with maleic acid was foundto be a suitable membrane for the water removal ithis system. The tvpical water flux was up to 780 .中国煤化工aration factor was 840。700-2(MHCNMHGat90Cand 3300 Pa(abs ).1 ne imelurane s major shortcoming was600-its poor stability at high temperatures and in the2京45一6presence of HBr. T he reaction distillation was alsocarried out and a ternary azeotropic liquid mixtureof CgH,OH, H2O and C.H,Br was obtained at theFig. 8 Relationship between flux and reaction timetop of the column，in which the concentration of470Tsinghua Science and Technology. Oclober 2002. 7(5): 466 - 470 .CH,Br was about 92%. When the distllation5]Lipnizki F, FieldR w, Ten P.reaction was coupled with pervaporation, the yieldhybrid. process: A review of process design,of CH,Br reached 92%，which was much higherapplication and economics. Journal of Membranethan. that obtained when only the distillation[6]Science, 1999, 153: 83- 210. .reaction was used.WangLefu，Li Xuehui.Studypervaporation-esterificationcoupledReferencesmembrane process. Joural of South ChinaUniversity of T'echnology. 1998. 26(11): 46-51.[1]Rapin J L. The betheivlle pervaporationn unit-The[7] Zhu Y, Minet G，Tsotsis T T. 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