传质与分离工程精.ppt

传质与分离工程1第1页，本讲稿共23页Fig.21.33Fig.21.332第2页，本讲稿共23页Triangle acd represents the ideal plate and triangle abeabe the actual plate.The actual plate,instead of enriching the vapor The actual plate,instead of enriching the vapor from yfrom yn+1n+1 to y*n,accomplished a lesser enrichment yn-y-yn+1n+1.3第3页，本讲稿共23页To apply a known Murphree efficiency to an entire To apply a known Murphree efficiency to an entire column,it is necessary only to replace the true column,it is necessary only to replace the true equilibrium curve yequilibrium curve ye e versus x versus xe by an effective by an effective equilibrium curve yequilibrium curve ye versus xe,whose ordinates,whose ordinates are calculated from the equation(21.70).are calculated from the equation(21.70).4第4页，本讲稿共23页How to plot the effective equilibrium curve?Giving xGiving xn n,find y*,find y*n n from equilibrium curve and y from equilibrium curve and yn+1n+1 from from operating line,calculate yoperating line,calculate yn n from equation(21.70),then plot from equation(21.70),then plot point(xpoint(xn n,y,yn n)How to determine the number of actual plates?How to determine the number of actual plates?The reboiler is not subject to a discount for plate efficiencyThe reboiler is not subject to a discount for plate efficiency,and the true equilibrium curve is used for the last step in the and the true equilibrium curve is used for the last step in the stripping section.stripping section.5第5页，本讲稿共23页How many How many actual plates actual plates are needed in are needed in the left the left construction?construction?6第6页，本讲稿共23页How many actual plates are needed in the left construction?No fractional actual plate,then7第7页，本讲稿共23页Murphree efficiency can be different from individual Murphree efficiency can be different from individual platesplates,and this is more accurate to calculate the number of actual plates than by overall efficiency of a column全塔效率全塔效率全塔效率全塔效率.8第8页，本讲稿共23页3.BATCH DISTILLATION（间歇精馏间歇精馏间歇精馏间歇精馏 ）pp.700-7031)Simple Distillation简单蒸馏简单蒸馏(自学自学)(1)Flow diagram(Refer to Fig.21.37)(1)Flow diagram(Refer to Fig.21.37)Batch FeedBatch Feed间歇进料间歇进料间歇进料间歇进料CondenserCondenserCooling waterCooling waterProduct Product receiverreceiverStillStill9第9页，本讲稿共23页 There are many cases where There are many cases where batch distillationbatch distillation间歇精馏间歇精馏间歇精馏间歇精馏 is is preferred,particularly in the preferred,particularly in the food and pharmaceutical food and pharmaceutical industries.industries.Batch distillationBatch distillation is used when is used when small amounts of small amounts of productproduct are made in a pilot plant to provide samples for are made in a pilot plant to provide samples for product sampling or testing.product sampling or testing.Features:1)Unsteady state;2)Without reflux;3)One-stage Features:1)Unsteady state;2)Without reflux;3)One-stage partial vaporization process.partial vaporization process.The vapor leaving the still at The vapor leaving the still at any time is in equilibrium with the liquid in the still.any time is in equilibrium with the liquid in the still.y y is is in equilibrium with in equilibrium with x x.10第10页，本讲稿共23页Batch FeedBatch FeedCondenserCondenserCooling waterCooling waterProduct Product receiverreceiver11第11页，本讲稿共23页(2)Rayleigh equation（瑞利方程）（瑞利方程）（瑞利方程）（瑞利方程）HeaterHeaterEq.(21.83)is known as the Rayleigh equation.Eq.(21.83)is known as the Rayleigh equation.dx/(y-x)dx/(y-x)can be integrated graphically or numerically using can be integrated graphically or numerically using equilibrium data(equilibrium data(yxyx equilibrium relationship).equilibrium relationship).12第12页，本讲稿共23页(3)For an ideal mixture,That is,Becauseoror13第13页，本讲稿共23页After integration between limits14第14页，本讲稿共23页Number of moles of product:DNumber of moles of product:DEXAMPLE 21.9.EXAMPLE 21.9.Average concentration of product:15第15页，本讲稿共23页Advantages.The advantages of batch distillation are that several products can be made from a single unit,and can effectively handle sludges and solids.Disadvantages.For a given product rate,the equipment is larger.It requires more operator attention,uses more energy,and because it is a dynamic process,is harder to control and model.16第16页，本讲稿共23页2)Batch Distillation with reflux间歇精馏间歇精馏/有回流的有回流的有回流的有回流的间歇蒸馏间歇蒸馏间歇蒸馏间歇蒸馏 pp.702-703In many cases,a rectifying column with reflux is used to improve the performance of the batch still.Flow diagram.Features:Features:Unsteady state;Unsteady state;Only rectifying section in the Only rectifying section in the column.column.Product concentration differs from different Product concentration differs from different operations.operations.17第17页，本讲稿共23页Two basic operation methods:Two basic operation methods:(a).(a).Keep the top composition(xD)constant by increasing the reflux ratioincreasing the reflux ratio as the composition of the as the composition of the liquid in the reboiler changes.liquid in the reboiler changes.(b).Fix the reflux ratioFix the reflux ratio and let the overhead product purity vary with time,stopping the distillation when the amount of product or the average concentration in the total product reaches a certain(某个某个某个某个)value.value.18第18页，本讲稿共23页Calculation and analysis for the operation of a distillation column.The plate column has been established,and the following data are given:Equilibrium relationship;xF;q;N and MM(N(Nactual).In order to obtain overhead and bottom products In order to obtain overhead and bottom products with required mole fractions of xwith required mole fractions of xD and xB,the the reflux ratio Rreflux ratio RD D and the feed plate position must be determined.Calculation method:Calculation method:Trial and error（反复试验）（反复试验）（反复试验）（反复试验）.19第19页，本讲稿共23页 Attentions of Operation analysis of distillation Attentions of Operation analysis of distillation column:column:(a)Suitable feed plate position;(a)Suitable feed plate position;(b)Keep material balance:If D and B changed,x(b)Keep material balance:If D and B changed,xD D and x and xB B will be changed too.will be changed too.(c)(c)Reflux ratio is the key measure of controlling product Reflux ratio is the key measure of controlling product quality.quality.For example,if N is given,R For example,if N is given,RD Dx xD D ,x,xB B .(d)If x(d)If xF F changed,feed plate position should be changed too.changed,feed plate position should be changed too.20第20页，本讲稿共23页4.AZEOTROPIC AND EXTRACTIVE 4.AZEOTROPIC AND EXTRACTIVE DISTILLATION pp.131DISTILLATION pp.131（恒沸精馏和萃取精馏恒沸精馏和萃取精馏恒沸精馏和萃取精馏恒沸精馏和萃取精馏）When When relative volatilityrelative volatility（相对挥发度）（相对挥发度）（相对挥发度）（相对挥发度）near or equal to unitynear or equal to unity,the,the separation of components is difficult by conventional separation of components is difficult by conventional distillation,or impossible because of distillation,or impossible because of azeotropeazeotrope恒沸混恒沸混恒沸混恒沸混合物合物合物合物 formation.formation.Basic principles of azeotropic and extractive Basic principles of azeotropic and extractive distillationdistillation:Adding a third component(:Adding a third component(solventsolvent)to)to increase the relative volatilityincrease the relative volatility of the original of the original components,so that the mixtures with relative components,so that the mixtures with relative volatility near or equal to unity can be separated by volatility near or equal to unity can be separated by conventional distillation.conventional distillation.21第21页，本讲稿共23页(1)(1)Azeotropic distillationAzeotropic distillation恒沸精馏恒沸精馏恒沸精馏恒沸精馏 Materials called Materials called entrainersentrainers(夹带剂夹带剂夹带剂夹带剂)(third component)(third component)are added to the original mixture,and usually a new are added to the original mixture,and usually a new azeotrope with lowest-boiling pointsazeotrope with lowest-boiling points有最低恒沸点的恒沸物有最低恒沸点的恒沸物有最低恒沸点的恒沸物有最低恒沸点的恒沸物 is formed.The new azeotrope will of course contain is formed.The new azeotrope will of course contain one or more of the feed components,and leaving one or more of the feed components,and leaving behind component(s)which may be recovered in the behind component(s)which may be recovered in the pure state.pure state.Example:Recovery and purification of ethanol and water.Example:Recovery and purification of ethanol and water.Flow diagramFlow diagram Selection of Selection of entrainers.entrainers.22第22页，本讲稿共23页(2)(2)Extractive distillationExtractive distillation萃取精馏萃取精馏萃取精馏萃取精馏 A solvent is added to the distillation tower to increase A solvent is added to the distillation tower to increase relative volatility of key components in the feed mixture.relative volatility of key components in the feed mixture.The effective The effective extractive solventextractive solvent萃取剂萃取剂萃取剂萃取剂will selectively will selectively interact with one(or more)of the components,interact with one(or more)of the components,thereby increasing relative volatilities.thereby increasing relative volatilities.Example:Use of Example:Use of furfuralfurfural糠醛糠醛糠醛糠醛 to permit the separation of to permit the separation of butadienebutadiene丁二烯丁二烯丁二烯丁二烯 from a mixture containing from a mixture containing butanebutane丁烷丁烷丁烷丁烷 and and butenesbutenes丁烯丁烯丁烯丁烯.Flow diagram.Flow diagram Selection of Selection of extractiveextractive solventssolvents.23第23页，本讲稿共23页