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1、1,(16) Use of Murphree efficiency默弗里板效率/Determining the number of actual plates实际板 pp.692-693,When the Murphree efficiency is known, it can readily be used in the McCabe-Thiele diagram. Fig.21.33p.693,2,Fig.21.33,3,Triangle acd represents the ideal plate and triangle abe the actual plate. The actual
2、 plate, instead of enriching the vapor from yn+1 to y*n, accomplished a lesser enrichment yn- yn+1.,4,To apply a known Murphree efficiency to an entire column, it is necessary only to replace the true equilibrium curve ye versus xe by an effective equilibrium curve ye versus xe, whose ordinates are
3、calculated from the equation (21.70).,5,How to plot the effective equilibrium curve? Giving xn, find y*n from equilibrium curve and yn+1 from operating line, calculate yn from equation (21.70), then plot point (xn, yn) ,How to determine the number of actual plates? The reboiler is not subject to a d
4、iscount for plate efficiency, and the true equilibrium curve is used for the last step in the stripping section.,6,How many actual plates are needed in the left construction?,7,How many actual plates are needed in the left construction?,No fractional actual plate, then,8,Murphree efficiency can be d
5、ifferent from individual plates, and this is more accurate to calculate the number of actual plates than by overall efficiency of a column全塔效率.,9,10,There are many cases where batch distillation间歇精馏 is preferred, particularly in the food and pharmaceutical industries. Batch distillation is used when
6、 small amounts of product are made in a pilot plant to provide samples for product sampling or testing.,Features: 1)Unsteady state; 2)Without reflux; 3)One-stage partial vaporization process. The vapor leaving the still at any time is in equilibrium with the liquid in the still. y is in equilibrium
7、with x.,11,12,(2)Rayleigh equation（瑞利方程）,Heater,Eq.(21.83) is known as the Rayleigh equation. dx/(y-x) can be integrated graphically or numerically using equilibrium data (yx equilibrium relationship).,13,(3) For an ideal mixture,That is,Because,or,14,After integration between limits,15,Number of mo
8、les of product: D,EXAMPLE 21.9.,Average concentration of product:,16,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
9、 more operator attention, uses more energy, and because it is a dynamic process, is harder to control and model.,17,2)Batch Distillation with reflux间歇精馏/有回流的间歇蒸馏 pp.702-703 In many cases, a rectifying column with reflux is used to improve the performance of the batch still. Flow diagram. Features: U
10、nsteady state; Only rectifying section in the column. Product concentration differs from different operations.,18,Two basic operation methods:,(a). Keep the top composition (xD) constant by increasing the reflux ratio as the composition of the liquid in the reboiler changes.,(b). Fix the reflux rati
11、o 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) value.,19,Calculation and analysis for the operation of a distillation column.,The plate column has been established, and
12、 the following data are given: Equilibrium relationship; xF; q; N and M (Nactual).,In order to obtain overhead and bottom products with required mole fractions of xD and xB, the reflux ratio RD and the feed plate position must be determined.,Calculation method: Trial and error（反复试验）.,20,Attentions o
13、f Operation analysis of distillation column:,(a) Suitable feed plate position;,(b) Keep material balance: If D and B changed, xD and xB will be changed too.,(c) Reflux ratio is the key measure of controlling product quality. For example, if N is given, RDxD, xB .,(d) If xF changed, feed plate positi
14、on should be changed too.,21,4. AZEOTROPIC AND EXTRACTIVE DISTILLATION pp.131（恒沸精馏和萃取精馏）,When relative volatility（相对挥发度） near or equal to unity, the separation of components is difficult by conventional distillation, or impossible because of azeotrope恒沸混合物 formation.,Basic principles of azeotropic a
15、nd extractive distillation: Adding a third component(solvent) to increase the relative volatility of the original components, so that the mixtures with relative volatility near or equal to unity can be separated by conventional distillation.,22,(1) Azeotropic distillation恒沸精馏 Materials called entrai
16、ners(夹带剂)(third component) are added to the original mixture, and usually a new azeotrope with lowest-boiling points有最低恒沸点的恒沸物 is formed. The new azeotrope will of course contain one or more of the feed components, and leaving behind component(s) which may be recovered in the pure state.,Example: Re
17、covery and purification of ethanol and water. Flow diagram,Selection of entrainers.,23,(2) Extractive distillation萃取精馏 A solvent is added to the distillation tower to increase relative volatility of key components in the feed mixture. The effective extractive solvent萃取剂will selectively interact with one (or more) of the components, thereby increasing relative volatilities.,
