膜蒸馏技术分析课件.ppt

1、膜蒸馏技术膜蒸馏技术MEMBRANE DISTILLATIONMEMBRANE DISTILLATION引言膜蒸馏简介文献汇报引言引言蒸馏（蒸馏（D DISTILLATION）蒸馏：一种分离液体混合物的方法 蒸馏的历史蒸馏的历史蒸馏的原理蒸馏的原理 利用液体混合物中各组分挥发度的差别，使液体混合物部分汽化并随之使蒸气部分冷凝，从而实现其所含组分的分离。最早蒸馏器蒸馏实验室基本装置 膜蒸馏膜蒸馏膜蒸馏（MD）是膜技术与蒸发过程相结合的膜分离过程。原理：以微孔疏水膜将两种不同温度的水溶液分开，膜两侧温度差造成两侧蒸气压差，使易挥发组分的蒸气分子通过膜孔从高温侧向低温侧扩散，并冷凝。MD的条件：所用

2、膜必须是疏水微孔膜；膜两侧要有一定的温度差存在，以提供传质所需的推动力。膜膜蒸蒸馏馏示示意意图图传递过程步骤：1）高温侧蒸发；2）蒸气分子通过疏水膜孔进行传递；3）低温侧冷凝。膜蒸馏所用膜材料膜蒸馏所用膜材料应满足疏水性和多孔性两个要求，以保证水不会渗入到微孔内和具有较高的通量。近年来，膜蒸馏过程的膜材料的研究开发集中于三种膜材料，即：v聚四氟乙烯（聚四氟乙烯（PTFEPTFE）v聚偏氟乙烯（聚偏氟乙烯（PVDFPVDF）v聚丙烯（聚丙烯（PPPP）膜蒸馏的分类膜蒸馏的分类v根据操作方式或透过水蒸汽的冷凝方式可分为：直接接触式膜蒸馏(DCMD)气隙式膜蒸馏(AGMD)真空式膜蒸馏(VMD)吹扫

3、式膜蒸馏(SGMD)渗透膜蒸馏(OMD)直接接触膜蒸馏：用于渗透物为水的场合，如脱盐或浓缩水溶液；气隙膜蒸馏：用于直接接触膜适用的场合或除去水溶液中的痕量挥发性成分；有关气体吹扫膜蒸馏的使用研究较少；渗透膜蒸馏主要用于去除稀水溶液中的挥发组分，优点是通过膜的热传导损失可忽略不计。渗透膜蒸馏 上世纪80年代提出了渗透膜蒸馏概念，采用疏水性微孔膜，膜两侧也是不润湿膜孔的水溶液，且也是利用膜两侧的蒸汽压差将料液侧水蒸汽传递到膜的另一侧。值得注意的是，膜蒸馏两侧有压差是由膜两侧的温度差产生的，而渗透蒸馏两侧的压差是由膜两侧溶液的渗透压产生的（也称为等温膜蒸馏）。渗透膜蒸馏的原理渗透膜蒸馏的原理渗透膜蒸

4、馏3个连续过程：被处理物料中易挥发组分的汽化；易挥发组分选择性通过疏水性膜；透过疏水性膜的易挥发组分被提取剂所吸收。因此，影响渗透膜蒸馏因素有：膜两侧提取相和物料相表观渗透压差；进料流速；浓缩度。提高进料流速，透水速率提高，而随浓缩度的提高透水速率降低。提取剂满足几个条件：不会腐蚀设备；价格便宜，而且可以循环使用；渗透活度尽量低，即提取剂中水的蒸汽压尽量低；满足提取剂的一些其它工业条件。常用的提取剂：CaCl2、甘油、NaCl提取剂提取剂渗透蒸馏膜渗透蒸馏膜渗透蒸馏膜必须满足两个最基本的条件：v被处理物料及提取剂的液相不能通过渗透蒸馏膜的膜孔；v被处理物料中易挥发组分汽化后能以气体分子的形式顺

5、利通过渗透蒸馏膜的膜孔。为了满足对渗透蒸馏膜的要求，选用的膜材料应是具有尽量低的表面自由能的非极性高聚物以保证膜的强疏水性及一定的机械强度。渗透蒸馏膜组件渗透蒸馏膜组件渗透蒸馏膜组件有平板式、卷式和中空纤维式。但由于渗透蒸馏膜组件不仅要提供被处理物料的通道，同时还要提供提取剂（盐水溶液）的通道，因此渗透蒸馏膜组件与其它膜过程存在一定的区别。平板式渗透蒸馏膜组件主要用于渗透蒸馏过程的实验研究。卷式渗透蒸馏膜组件既能用于实验研究，亦可用于工业化生产。中空纤维渗透蒸馏膜组件是一种最适用于渗透蒸馏工业化的渗透蒸馏膜组件。膜蒸馏过程的特征膜蒸馏过程的特征所用的膜为微孔膜；只有蒸汽能通过膜孔传质；膜不能被

6、所处理的液体润湿；在膜孔内没有毛细管冷凝现象发生；膜至少有一面与所处理的液体接触；所用膜不能改变所处理液体中所有组分的气液平衡；对于任何组分该膜过程的推动力是该组分在气相中的分压差。膜蒸馏的优点膜蒸馏的优点v常压操作常压操作，设备简单，操作方便；v只有挥发性物质可透过膜只有挥发性物质可透过膜，用于制水过程水质纯净；v可处理极高浓度的水溶液，达到过饱和、析出结晶；v无需加热到沸点，可利用廉价、低品质能源；v组件特殊的结构设计，潜热回收，提高能源利用率提高能源利用率。膜蒸馏缺点膜蒸馏缺点有相变，汽化潜热降低了能量利用率;通量仍较小;与亲水膜相比，疏水微孔膜材料和制备工艺有局限性。膜蒸馏的应用膜蒸馏

7、的应用海水或苦咸水的淡化强酸、强碱中水的分离甘蔗榨汁液的浓缩人参露和洗参水的分离浓缩分离提纯天然盐水中的食盐和芒硝超纯水的制备废水的处理共沸混合物及有机溶液的分离化学物质的浓缩和回收膜蒸馏的应用前景膜蒸馏的应用前景操作条件非常温和(常温、常压)，使用自然能源或废热便可以实现操作；存在热回收率的提高与膜蒸馏推动力的降低的矛盾。膜的价格若有大幅度降低，膜蒸馏将具有更明显的竞争力；膜蒸馏过程与其它膜过程耦合使用，发挥各自的优势，是今后膜蒸馏技术应用的一个重要方向。文献汇报文献汇报Experiments were performed using PTFE hydrophobic membranes o

8、f different nominal pore diameter.Red grape juice solution and concentrated calcium chloride solution(50 wt.%)were used as feed and stripping solutions,respectively.The permeate flux,the initial and the final juice concentration,the total phenolic content(TPC)and the antioxidant activity(TEAC)were m

9、easured for each single experiment.1.INTRODUCTIONFruits and vegetables contain various bioactive compounds such as vitamins A,C and E,phenolic compounds and other important food ingredients.Polyphenols,the natural antioxidants present in plant extracts play a key role in antioxidative mechanisms in

10、biological systems.Polyphenols show antioxidant and germicidal properties,decrease cholesterol level and possess possibility of cancer prevention coronary heart disease inhibition.Grapes are known to be phenol-rich plants.The main polyphenols in red grapes are anthocyanins（花青素）（花青素）and flavonols（黄酮醇

11、）（黄酮醇）.The concentration step is usually carried out in evaporators at high temperature,which causes valuable constituent losses(for example aroma compounds)and decreases the final product quality.几种浓度技术的比较几种浓度技术的比较A good alternative to the thermal methods of juice concentration is osmotic membrane

12、distillation.OMDa membrane contactor technique applying a hydrophobic porous membrane；allowing the concentration of solutions at constant temperature under atmospheric pressure；also used to remove alcohol from wine or hydroalcoholic(水醇的)solution.the relationship between transmembrane mass flux and t

13、he driving force is：J=K(PbwF-PbwSS)（1）Assuming the overall isothermal conditions in the syste（TbF=TSSb=const）PbwX=P*wabwX so Eq.(1)can be written as：J=KP*w(abwF-abwSS)（2）2.MATERIAL AND METHODSWater activity of the red grape juice and calcium chloride solutions was measured by using a NOVASINA IC-500

14、 AW-LAB Hygrometer；The initial and final concentration of the red grape juices and CaCl2 solutions were measured by using the Abbe refractometer；The TPC of red grape juice was determined by using the FolinCioclteus method；The antioxidant capacity of the food sample can be determined by using several

15、 methods，in this study the antioxidant capacity was measured by TEACat the beginning of the experiment the permeate flux was decreasing rapidly,reaching the pseudo-stationary state after ca.3050 min.3.RESULTS AND DISCUSSION the permeate fluxes through the investigated membranes,which have different

16、pore size,were very alike.the permeate fluxes were practically independent on the membrane pore size,except 0.2m;the permeate fluxes of higher porosity membranes are lowerthe increase of the grape juice concentration is linear during OMD regardless of the membrane pore size.The final juice concentra

17、tion was proportional to the time of the dehydration process and the initial juice concentration.It can be seen that the OMD process does not affect the antioxidant activity and the TEAC index increasedlinearly with the red grape juice concentration.4.CONCLUSIONThe results obtained in this study sho

18、w the feasibility of OMD for red grape juice concentration;Important physicochemical properties for the quality of the concentrated juice are not affected during the concentration by OMD with porous hydrophobic membranes;One of the drawbacks in the industrial application of OMD can be the re-use of the diluted stripping solution generated during the process;OMD can be integrated as a pre-step for fruit juice concentration or milk concentration.