真空膜蒸馏机理研究
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摘要
膜分离是一门新型的高分离、浓缩、提纯及净化技术。膜蒸馏技术是膜分离技术中的一个重要分支，它是指利用一疏水性的多孔膜，膜热侧与待处理液直接接触，待处理溶液中的水在膜热侧汽化后，气体在压强差的推动下，通过膜孔传递到膜冷
侧，冷凝后收集或除去的一种分离技术。
真空膜蒸馏（VMD）技术是膜蒸馏技术的一种，其研究主要集中于工艺过程及其影响因素、过程机理及过程应用三大方面。VMD与其它膜蒸馏过程的最根本区别在于有真空系统提供增强驱动力，同时又具有气隙膜蒸馏的特点。
本文在评述真空膜蒸馏技术研究现状的基础上，对膜蒸馏过程热侧、膜内及冷侧的质量传输和热量传输行为进行了深入的分析与推导。理论推导与计算表明：膜的导热性对真空膜蒸馏过程的影响可以忽略，膜蒸馏过程膜内质量传输机制是努森流与粘性流共同作用的混合流机制。
本文进行了气体渗透实验、蒸馏水的VMD实验及含铬水溶液的VMD实验。实验结果的分析表明：在VMD过程中，其过程主要阻力来自于热侧汽化和膜内的质量传输；热量传输与质量传输是相互影响、相互制约的；其过程中的浓度极化和温度极化现象是膜蒸馏过程进行时的必然现象。
实验表明，随进料温度、进料流速、冷侧真空度的增大，膜通量增大；但进料流速增大到60L.h-1之后，通量随进料流速的增长变缓；膜通量随孔径较快地增大，但当膜平均孔径达到0.5m时，截留率迅速降低，已无实际意义。
在上述理论推导及实验分析的基础上，本文将热侧溶剂传输与溶质传输、膜蒸馏过程热量传输与质量传输相关联，并引入一个能够反映膜内质量传输特征的“贡献系数”，将努森流模型与粘性流模型结合起来，从而提出了新的混合流模型。
为验证所建模型的实用性与正确性，利用VB6.0编制了一个能够进行循环迭代的计算程序，计算出模型的理论值。结果显示，模型计算值与实验值在较大范围有良好的一致性。
Membrane separation is a kind of new technology which can be used for separation, concentration, purification and cleanse. Membrane distillation is an important branch of membrane separation technology which employees a hydrophobic microporous membrane for the separation process. During MD process, feed contacts with one side of membrane, after water evaporates on hot side, vapor transfers to cold side under pressure drive, where vapor is condensed on cooling wall and then collected or removed.
Vacuum membrane distillation (VMD) which possesses of vacuum pump to provide enhanced drive force and air gap similar to air gap membrane distillation is a kind of separation technology of membrane distillation.
A brief introduction of VMD process is presented. Mass and heat transfer behavior through hot side, membrane as well as cold side during VMD process is analyzed and derived. Theory calculation and derivation demonstrate that the heat conduction of membrane exerts little influence on performance. The characteristics of mass transfer through membrane are mixed flow including Knudsen flow and Poiseuille flow, which moreover, vary with operational variables.
Air permeation experiment, VMD of distilled water and VMD of solution containing Cr3+ are carried out. The results indicate that the dominant resistance during membrane process comes mainly from evaporation of water on hot side and mass transfer through membrane, and that mass transfer and heat transfers during VMD process are interdependent and interactive; The results also indicate that the phenomenon of temperature polarization and concentration polarization is inevitable during VMD process.
Results of VMD experiments indicate that flux increases when feed temperature, feed rate and vacuum degree increase. But the increase of flux becomes slowly when feed rate reaches 60L.h-1; Flux increases with pore size rapidly, but rejection approaches zero when pore size reaches 0.5m.
On the basis of above theoretical derivation and analysis on experimental results, by correlating solvent (water) transfer with solute (salt) transfer on hot side and correlating mass transfer with heat transfer during VMD and introducing a contribution coefficient which can reflect the characteristics of mass transfer through membrane, a new model of mixed flow is presented.
In order to examine the model, a computerizing program with witch can realize cycle calculation is programmed in VB6.0, The comparison between experimental results and theoretical calculations indicates that the computerizing results agree with experimental results well in a large scope.
侧，冷凝后收集或除去的一种分离技术。
真空膜蒸馏（VMD）技术是膜蒸馏技术的一种，其研究主要集中于工艺过程及其影响因素、过程机理及过程应用三大方面。VMD与其它膜蒸馏过程的最根本区别在于有真空系统提供增强驱动力，同时又具有气隙膜蒸馏的特点。
本文在评述真空膜蒸馏技术研究现状的基础上，对膜蒸馏过程热侧、膜内及冷侧的质量传输和热量传输行为进行了深入的分析与推导。理论推导与计算表明：膜的导热性对真空膜蒸馏过程的影响可以忽略，膜蒸馏过程膜内质量传输机制是努森流与粘性流共同作用的混合流机制。
本文进行了气体渗透实验、蒸馏水的VMD实验及含铬水溶液的VMD实验。实验结果的分析表明：在VMD过程中，其过程主要阻力来自于热侧汽化和膜内的质量传输；热量传输与质量传输是相互影响、相互制约的；其过程中的浓度极化和温度极化现象是膜蒸馏过程进行时的必然现象。
实验表明，随进料温度、进料流速、冷侧真空度的增大，膜通量增大；但进料流速增大到60L.h-1之后，通量随进料流速的增长变缓；膜通量随孔径较快地增大，但当膜平均孔径达到0.5m时，截留率迅速降低，已无实际意义。
在上述理论推导及实验分析的基础上，本文将热侧溶剂传输与溶质传输、膜蒸馏过程热量传输与质量传输相关联，并引入一个能够反映膜内质量传输特征的“贡献系数”，将努森流模型与粘性流模型结合起来，从而提出了新的混合流模型。
为验证所建模型的实用性与正确性，利用VB6.0编制了一个能够进行循环迭代的计算程序，计算出模型的理论值。结果显示，模型计算值与实验值在较大范围有良好的一致性。
Membrane separation is a kind of new technology which can be used for separation, concentration, purification and cleanse. Membrane distillation is an important branch of membrane separation technology which employees a hydrophobic microporous membrane for the separation process. During MD process, feed contacts with one side of membrane, after water evaporates on hot side, vapor transfers to cold side under pressure drive, where vapor is condensed on cooling wall and then collected or removed.
Vacuum membrane distillation (VMD) which possesses of vacuum pump to provide enhanced drive force and air gap similar to air gap membrane distillation is a kind of separation technology of membrane distillation.
A brief introduction of VMD process is presented. Mass and heat transfer behavior through hot side, membrane as well as cold side during VMD process is analyzed and derived. Theory calculation and derivation demonstrate that the heat conduction of membrane exerts little influence on performance. The characteristics of mass transfer through membrane are mixed flow including Knudsen flow and Poiseuille flow, which moreover, vary with operational variables.
Air permeation experiment, VMD of distilled water and VMD of solution containing Cr3+ are carried out. The results indicate that the dominant resistance during membrane process comes mainly from evaporation of water on hot side and mass transfer through membrane, and that mass transfer and heat transfers during VMD process are interdependent and interactive; The results also indicate that the phenomenon of temperature polarization and concentration polarization is inevitable during VMD process.
Results of VMD experiments indicate that flux increases when feed temperature, feed rate and vacuum degree increase. But the increase of flux becomes slowly when feed rate reaches 60L.h-1; Flux increases with pore size rapidly, but rejection approaches zero when pore size reaches 0.5m.
On the basis of above theoretical derivation and analysis on experimental results, by correlating solvent (water) transfer with solute (salt) transfer on hot side and correlating mass transfer with heat transfer during VMD and introducing a contribution coefficient which can reflect the characteristics of mass transfer through membrane, a new model of mixed flow is presented.
In order to examine the model, a computerizing program with witch can realize cycle calculation is programmed in VB6.0, The comparison between experimental results and theoretical calculations indicates that the computerizing results agree with experimental results well in a large scope.
引文
[1] 郑领英，王学松.膜技术.北京.化学工业出版社.2000.11:15-56.
[2] 王学松编著.膜分离技术及其应用.北京.科学出版社.1992:126-138.
[3] 徐又一，项慧，陈元胜等.膜蒸馏研究的新进展.膜科学与技术.1998.18(6):1-7.
[4] 朱小惠译.膜蒸馏技术.化工新型材料.1998.6(5):3-8.
[5] Kevin W. Lawson， Douglas R.Lloyd. Membrane Distillation.II.Direct contact MD. J.Memb.Sci.1996.120:123-133.
[6] 陈翠仙，余立新，戴猷元.新膜及膜过程的研究现状及发展动向.水处理技术.1996.22(6):307-313.
[7] 孔瑛，吴庸烈，杨金荣等.膜蒸馏用聚偏氟乙烯微孔膜的结构控制I.铸膜液中溶胀剂对膜形态结构的影响.水处理技术.1992.18(1):11-16.
[8] 孔瑛，吴庸烈，杨金荣.用于膜蒸馏的疏水性聚偏氟乙烯不对称微孔膜的制备及其形态结构的研究.膜科学与技术.1990.10(3):19-26.
[9] 孔瑛，吴庸烈，杨金荣等.膜蒸馏用聚偏氟乙烯微孔膜的结构控制III.制膜条件的选择和膜蒸馏性能.水处理技术.1992.18(3):167-172.
[10] 吴庸烈，孔瑛，林晓等.亲水膜的表面改性及膜蒸馏中的应用.功能高分子学报.1991.4(3):199-206.
[11] R.A.Peterson， M.A.nderson， C.G.Hill Jr. Development of TiO2 membranes for gas phase nanofiltration. J.Memb.Sci 1994.94:103-109.
[12] Roda G. C.， Cristiana B.， Saavedra A.， Sarti G. C. Heat and mass transfer boundary layers in radial creeping flow. Int. J.Heat Mass Transfer.1994.
37(14):2145-2153.
[13] J.M.Ortiz-Zarate ，F.Garcia Lopez，Ｊ.Ｉ.Mengual. Non-isothermal water transport through membranes J.Memb.Sci 1991 56:181-194.
[14] M.C.Garcia-Payo.Air gap membrane distillation of aqueous alcohol solutions. J.Memb.Sci. 2000.169: 61-80.
[15] FAHMI ABU AL-RUB，FAWZIA.BANAT Theoretical assessment of dilute acetone removal from aqueous streams by membrane distillation Sept Sci &Tech 1999.34(14):2817-2836.
[16] Khayet， Mohamed ET. Nature of flow on sweeping gas membrane
distillation
J. Memb. Sci 2000. 170:243-255.
[17] Mohamed Khayet，Paz Godino，Juan I.Mengual.Theory and experiments on
sweeping gas membrane distillation . J. Memb. Sci .2000. 165):261-272.
[18] Serena Bandini.C.Gostoli and G.C.Sarti. Separation efficiency in vacuum
membrane distillation J.Memb.Sci 1992.73:217-229.
[19] Serena Bandini Aldo Saavedra and Giulio Cesare Sarti Vacuum membrane
distillation：Experiments and modeling J.AIChE.1997 43(2):398-408.
[20] Kevin W. Lawson，Douglas R. Lloyd Membrane Distillation. I. Module design and performance evaluations using vacuum membrane distillation J.Memb Sci 1996.120:111-121.
[21] Bandini S.， Sart G. C. Heat and mass transport resistances in vacuum membrane distillation per drop.J. AIChE .1999. 45(7): 1422- 1433.
[22] 刘茂林，霍中心，陈定江等.冷侧真空度对减压膜蒸馏过程影响的研究.膜科学与技术.1997.17(3):65-67.
[23] 毛尚良.减压膜蒸馏法的研究.水处理技术. 1994.20(5):267-270.
[24] 张贵清，张启修，周康根.减压膜蒸馏浓缩硫酸溶液的探索研究.第三届全国膜和膜过程学术报告会论文集.1999:396-400.
[25] 刘茉娥，膜分离技术.北京.化学工业出版社.1998:108-120.
[26] 孙彦.生物分离工程.北京.化学工业出版社.1998:124-125.
[27] 骆广生，峡芳.高分子膜浸润性能及溶胀.膜科学与技术.1998.18(5):38-41.
[28] R.Rautenbach and F.P.Helmus. Some considerations on mass-transfer resistances in solution-diffusion-type membrane process. J.Memb.Sci 1994.87:171-181.
[29]Xianshe.Feng， Robert， Y.M.Huang.Concentration polarization separation processes J.Memb.Sci 1994.92:201-208.
[30] G.A.Denisov.Theory of concentration polarization in cross-flow ultrafilation: gel-layer model and osmotic-pressure model. J.Memb.Sci 1994.91:173-187.
[31] L.MARTINEZ，M.IVAZQUEZ. Temperature Polarization Coefficients in Membrane Distillation.J.Sept Sci and Tech.1998.33(6):787-789.
[32] 秦允豪.热学.北京，高等教育出版社.1999:42-112.
[33] A.M.Urtiaga. G.Ruiz， I.Ortiz.Kinetic analysis of vacuum membrane distillation of chloroform from aqueous solutions.J.Memb.Sci. 2000.165:99-110.
[34] R.W.Schofield， A.G.Fane and C.J.D.Fell. Heat and mass transfer in membrane distillation.J..Memb.Sci. 1987. 33:299-313.
[35] 邝喜旺，陈洪钫.渗透蒸发膜极其传质研究进展.膜科学与技术.1995.15(3):1-8.
[36] [德] Rautenbach R.著.王乐夫译.膜工艺-组件和装置设计基础.北京.化学工业出版社.1998:10-22.
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[49] R.W.Schofield，A.G.Fane and C.J.D.Fell.Gas and vapor transport through microporous membranes. J.Memb.Sci. 1990. 53:159-171.
[50] 蒋维均，余立新，刘茂林.膜蒸馏过程传质传热机理研究.水处理技术.1992.18(4):230-233.
[51] 毛尚良.减压膜蒸馏法的研究.水处理技术. 1994.20(5):267-270.
[52] 杜军，刘作华，陶长元，黄德新，徐楚韶.减压膜蒸馏用于Cr3+废水处理的实验研究.水处理技术.1992.18(4):230-233.
[2] 王学松编著.膜分离技术及其应用.北京.科学出版社.1992:126-138.
[3] 徐又一，项慧，陈元胜等.膜蒸馏研究的新进展.膜科学与技术.1998.18(6):1-7.
[4] 朱小惠译.膜蒸馏技术.化工新型材料.1998.6(5):3-8.
[5] Kevin W. Lawson， Douglas R.Lloyd. Membrane Distillation.II.Direct contact MD. J.Memb.Sci.1996.120:123-133.
[6] 陈翠仙，余立新，戴猷元.新膜及膜过程的研究现状及发展动向.水处理技术.1996.22(6):307-313.
[7] 孔瑛，吴庸烈，杨金荣等.膜蒸馏用聚偏氟乙烯微孔膜的结构控制I.铸膜液中溶胀剂对膜形态结构的影响.水处理技术.1992.18(1):11-16.
[8] 孔瑛，吴庸烈，杨金荣.用于膜蒸馏的疏水性聚偏氟乙烯不对称微孔膜的制备及其形态结构的研究.膜科学与技术.1990.10(3):19-26.
[9] 孔瑛，吴庸烈，杨金荣等.膜蒸馏用聚偏氟乙烯微孔膜的结构控制III.制膜条件的选择和膜蒸馏性能.水处理技术.1992.18(3):167-172.
[10] 吴庸烈，孔瑛，林晓等.亲水膜的表面改性及膜蒸馏中的应用.功能高分子学报.1991.4(3):199-206.
[11] R.A.Peterson， M.A.nderson， C.G.Hill Jr. Development of TiO2 membranes for gas phase nanofiltration. J.Memb.Sci 1994.94:103-109.
[12] Roda G. C.， Cristiana B.， Saavedra A.， Sarti G. C. Heat and mass transfer boundary layers in radial creeping flow. Int. J.Heat Mass Transfer.1994.
37(14):2145-2153.
[13] J.M.Ortiz-Zarate ，F.Garcia Lopez，Ｊ.Ｉ.Mengual. Non-isothermal water transport through membranes J.Memb.Sci 1991 56:181-194.
[14] M.C.Garcia-Payo.Air gap membrane distillation of aqueous alcohol solutions. J.Memb.Sci. 2000.169: 61-80.
[15] FAHMI ABU AL-RUB，FAWZIA.BANAT Theoretical assessment of dilute acetone removal from aqueous streams by membrane distillation Sept Sci &Tech 1999.34(14):2817-2836.
[16] Khayet， Mohamed ET. Nature of flow on sweeping gas membrane
distillation
J. Memb. Sci 2000. 170:243-255.
[17] Mohamed Khayet，Paz Godino，Juan I.Mengual.Theory and experiments on
sweeping gas membrane distillation . J. Memb. Sci .2000. 165):261-272.
[18] Serena Bandini.C.Gostoli and G.C.Sarti. Separation efficiency in vacuum
membrane distillation J.Memb.Sci 1992.73:217-229.
[19] Serena Bandini Aldo Saavedra and Giulio Cesare Sarti Vacuum membrane
distillation：Experiments and modeling J.AIChE.1997 43(2):398-408.
[20] Kevin W. Lawson，Douglas R. Lloyd Membrane Distillation. I. Module design and performance evaluations using vacuum membrane distillation J.Memb Sci 1996.120:111-121.
[21] Bandini S.， Sart G. C. Heat and mass transport resistances in vacuum membrane distillation per drop.J. AIChE .1999. 45(7): 1422- 1433.
[22] 刘茂林，霍中心，陈定江等.冷侧真空度对减压膜蒸馏过程影响的研究.膜科学与技术.1997.17(3):65-67.
[23] 毛尚良.减压膜蒸馏法的研究.水处理技术. 1994.20(5):267-270.
[24] 张贵清，张启修，周康根.减压膜蒸馏浓缩硫酸溶液的探索研究.第三届全国膜和膜过程学术报告会论文集.1999:396-400.
[25] 刘茉娥，膜分离技术.北京.化学工业出版社.1998:108-120.
[26] 孙彦.生物分离工程.北京.化学工业出版社.1998:124-125.
[27] 骆广生，峡芳.高分子膜浸润性能及溶胀.膜科学与技术.1998.18(5):38-41.
[28] R.Rautenbach and F.P.Helmus. Some considerations on mass-transfer resistances in solution-diffusion-type membrane process. J.Memb.Sci 1994.87:171-181.
[29]Xianshe.Feng， Robert， Y.M.Huang.Concentration polarization separation processes J.Memb.Sci 1994.92:201-208.
[30] G.A.Denisov.Theory of concentration polarization in cross-flow ultrafilation: gel-layer model and osmotic-pressure model. J.Memb.Sci 1994.91:173-187.
[31] L.MARTINEZ，M.IVAZQUEZ. Temperature Polarization Coefficients in Membrane Distillation.J.Sept Sci and Tech.1998.33(6):787-789.
[32] 秦允豪.热学.北京，高等教育出版社.1999:42-112.
[33] A.M.Urtiaga. G.Ruiz， I.Ortiz.Kinetic analysis of vacuum membrane distillation of chloroform from aqueous solutions.J.Memb.Sci. 2000.165:99-110.
[34] R.W.Schofield， A.G.Fane and C.J.D.Fell. Heat and mass transfer in membrane distillation.J..Memb.Sci. 1987. 33:299-313.
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