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.When the Murphree efficiency is known, it can readily be used in the McCabe-Thiele diagram. Fig.21.33[p.693]Fig.21.33[p.693]

2. 2 Fig.21.33

3. 3 Triangle acd represents the ideal plate and triangle abe the actual plate.Triangle acd represents the ideal plate and triangle abe the actual plate. The actual plate, instead of enriching the vapor from y n+1 to y* n, accomplished a lesser enrichment y n - y n+1.The actual plate, instead of enriching the vapor from y n+1 to y* n, accomplished a lesser enrichment y n - y n+1.

4. 4 To apply a known Murphree efficiency to an entire column, it is necessary only to replace the true equilibrium curve y e versus x e by an effective equilibrium curve y’ e versus x e, whose ordinates are calculated from the equation (21.70).To apply a known Murphree efficiency to an entire column, it is necessary only to replace the true equilibrium curve y e versus x e by an effective equilibrium curve y’ e versus x e, whose ordinates are calculated from the equation (21.70).

5. 5 How to plot the effective equilibrium curve?How to plot the effective equilibrium curve? Giving x n, find y* n from equilibrium curve and y n+1 from operating line, calculate y n from equation (21.70), then plot point (x n, y n ) …Giving x n, find y* n from equilibrium curve and y n+1 from operating line, calculate y n from equation (21.70), then plot point (x n, y 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 efficiency, and the true equilibrium curve is used for the last step in the stripping section.The reboiler is not subject to a discount for plate efficiency, and the true equilibrium curve is used for the last step in the stripping section.

6. 6 How many actual plates are needed in the left construction?

7. 7 No fractional actual plate, then

8. 8 Murphree efficiency can be different 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.Murphree efficiency can be different 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. 9 3. BATCH DISTILLATION （间歇精馏 ） [pp.700-703] 1)Simple Distillation 简单蒸馏 ( 自学 ) (1)Flow diagram (Refer to Fig.21.37)(1)Flow diagram (Refer to Fig.21.37) Batch Feed 间歇进料 Condenser Cooling water Product receiver Still

10. 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 small amounts of product are made in a pilot plant to provide samples for product sampling or testing.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 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 with x.]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 with x.]

11. 11 Batch Feed Condenser Cooling water Product receiver

12. 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 (y~x equilibrium relationship).

13. 13 (3) For an ideal mixture, That is, Because or

14. 14 After integration between limits

15. 15 Number of moles of product: D EXAMPLE 21.9. Average concentration of product:

16. 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 more operator attention, uses more energy, and because it is a dynamic process, is harder to control and model.

17. 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.In many cases, a rectifying column with reflux is used to improve the performance of the batch still. Flow diagram.Flow diagram. Features:Unsteady state; Only rectifying section in the column. Product concentration differs from different operations.Features: Unsteady state; Only rectifying section in the column. Product concentration differs from different operations.

18. 18 Two basic operation methods:Two basic operation methods: (a). Keep the top composition (x D ) constant by increasing the reflux ratio as the composition of the liquid in the reboiler changes.(a). Keep the top composition (x D ) constant by increasing the reflux ratio as the composition of the liquid in the reboiler changes. (b). Fix 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 ) value.(b). Fix 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 ) value.

19. 19 Calculation and analysis for the operation of a distillation column.Calculation and analysis for the operation of a distillation column. The plate column has been established, and the following data are given: Equilibrium relationship; x F ; q; N and  M (N actual ).The plate column has been established, and the following data are given: Equilibrium relationship; x F ; q; N and  M (N actual ). In order to obtain overhead and bottom products with required mole fractions of x D and x B, the reflux ratio R D and the feed plate position must be determined.In order to obtain overhead and bottom products with required mole fractions of x D and x B, the reflux ratio R D and the feed plate position must be determined. Calculation method: Trial and error （反复试验）.Calculation method: Trial and error （反复试验）.

20. 20 Attentions of Operation analysis of distillation column:Attentions of Operation analysis of distillation column: (a) Suitable feed plate position; (b) Keep material balance: If D and B changed, x D and x B will be changed too. (c) Reflux ratio is the key measure of controlling product quality. [For example, if N is given, R D  x D , x B .] (d) If x F changed, feed plate position should be changed too.

21. 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.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 and 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.Basic principles of azeotropic and 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. 22 (1) Azeotropic distillation 恒沸精馏 Materials called entrainers( 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.Materials called entrainers( 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: Recovery and purification of ethanol and water. Flow diagram…Example: Recovery and purification of ethanol and water. Flow diagram… Selection of entrainers.Selection of entrainers.

23. 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.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. Example:Use of furfural 糠醛 to permit the separation of butadiene 丁二烯 from a mixture containing butane 丁烷 and butenes 丁烯. Flow diagram…Example:Use of furfural 糠醛 to permit the separation of butadiene 丁二烯 from a mixture containing butane 丁烷 and butenes 丁烯. Flow diagram… Selection of extractivesolvents.Selection of extractive solvents.