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1、DistillationPrinciples of Chemical Engineering-CDUT*1.Equilibrium Distillation【Definition】Also known as flash distillation.It is a continuous,single-stage and steady-state operation of distillation.【Seperation Principle】(1)The bubble point of liquid mixture increaseswith the increase of pressure,and
2、 the lower the pressure is,the lower thebubble point is;(2)The high-pressure and high-temperature liquid mixture is decompressedto reduce its bubble point and enter the flash tank(separation tank).At thistime,the temperature of the liquid mixture is higher than the bubble point underthis pressure;(3
3、)The fluid is rapidly boiled and vaporized in the flash tank,and the two-phase equilibrium occurs and the two-phase separation is carried out.Simple Distillation and Equilibrium DistillationPrinciples of Chemical Engineering-CDUT*Separation tankproduct at the top of the toweryAxAHeaterRaw materialli
4、quidproduct at the bottom of the towerQThrottle valveOperation of equilibrium distillation(1)The separated mixture is heated by aheater so that the temperature is higherthan the bubble point of the liquid underthe pressure of the separation tank;(2)To reduce the pressure to the specifiedvalue throug
5、h the pressure reducing valve(throttle valve)and enter the separationtank.(3)The superheatedliquid mixture ispartially vaporized inthe separator,and theequilibrium vaporand liquid phases areextracted from thetop and bottom of thes e p a r a t o rrespectively.Principles of Chemical Engineering-CDUT*O
6、perating Instructions for Equilibrium Distillation【Application】The liquid mixture was separated preliminarily.【Instruction】(1)The equipment for gasifying the liquidmixture is not a flash tank,but a pressurereducing valve.(2)The function of flash tank is to providespace for rapid vaporization of liqu
7、id mixture andseparation of vapor and liquid.Principles of Chemical Engineering-CDUT*The Heat Balance of DistillationTo calculate the total material balance.What enters is equal to what goes out,that is:The material balance of volatile components is:So the flow rate of the gas phase product is:Princ
8、iples of Chemical Engineering-CDUT*The Heat Balance of DistillationDefine the liquefaction rate of raw material liquid:The gasification rate is 1-q,which is equal to:By substituting the balance equation,the relation between vapor-liquid phasecomposition y and x in equilibrium distillation is obtaine
9、d:Principles of Chemical Engineering-CDUT*The Heat Balance of DistillationThe heat load Q of the heater is obtained by calculating theheat balance of the heater and ignoring the heat loss.The separator is used for heat balance,when ignores the heat loss,the latent heat needed for vaporization of the
10、 material isprovided by the sensible heat of the raw material liquid itself.Then the equilibrium temperature te in the separator should be:Principles of Chemical Engineering-CDUT*2.Simple Distillation【Definition】Also known as differential distillation,is an intermittent,single-stage,unsteady distill
11、ation operation for the preliminary separation ofliquid mixtures.【Operation】(1)The raw material liquid is added to the distillation kettle atone time;(2)Partially vaporized by indirect heating,the resulting steam is condensedinto the condenser,and the condensate is discharged into the receiver as ad
12、istillate product(with high concentration of light components).(3)After the completion of distillation,the unvaporized kettle residue isdischarged at one time.*(1)The raw materialliquid is added to thedistillation kettle atone time;(2)Partially vaporized byindirect heating,the resultingsteam is cond
13、ensed into thecondenser,and the condensate isdischarged into the receiver as adistillate product(with highco n cent r at io nofl ightcomponents).(3)After the completion of distillation,the unvaporized kettle residue is discharged at one time.*t/C0 x1y2xfy1yfx2x3Heating temperatureCharacteristics of
14、simple distillationy3Dew point lineBubble point lineLiquid phase composition xVapor phase composition ytt1t2t3(1)The vapor and liquid in any instantaneous kettle are in equilibrium with each other.(2)With the increasing temperature of steam and liquid phase in the kettle,thecomposition of steam and
15、liquid phase x and y also change,which is a kind ofintermittent,single-stage and unsteady distillation operation.Principles of Chemical Engineering-CDUT*Calculation of Simple DistillationTo set at a certain moment t,the liquid volume of the kettle is nL(Kmol),the composition isx,after dt time,the li
16、quid volume of thekettle is changed to(nL-dnL),the composition is(x-dx),and theamount of gas vaporized is dnD,with a composition of y.The materialbalance is carried out for the dt time process,and the total materialbalance is calculated as:The volatile component balance is calculated as:nLx=(nL-dnL)
17、(x-dx)+ydnDPrinciples of Chemical Engineering-CDUT*Calculation of Simple DistillationTwo simultaneous equations are obtained by omitting the second-orderinfinitesimal:To replace the initial quantity of the boundary condition and the final amount of the constituent nF，xF;and the constituent nW,xW.integrating:The vapor-liquid equilibrium relation is substituted into the relationbetween nFandnw.Principles of Chemical Engineering-CDUT*Calculation of Simple DistillationMaterial balance for a batch of operations:Average composition of distillate: