1 (9) Heating and cooling requirements Assume Heat loss is relatively small, column itself is essentially adiabatic ( 绝热 ).Assume Heat loss is relatively small, column itself is essentially adiabatic ( 绝热 ). Heat effects of the entire unit are confined to the condenser and the reboiler.Heat effects of the entire unit are confined to the condenser and the reboiler. Average molal latent heat change and total sensible heat change in liquid streams is small.Average molal latent heat change and total sensible heat change in liquid streams is small. Heat added at reboiler (q r )Heat added at reboiler (q r )

2 Control surface 1 Control surface 2 Feed Condenser Accumulator Heat out Overhead product Bottoms product Reboiler Reflux Heat in [Fig.21.8(p.649)] T2 T1

3 Steam consumption （蒸汽消耗量）Steam consumption （蒸汽消耗量） (21.32) =heat added at reboiler =steam consumption =V b, vapor rate from reboiler =latent heat of steam =molal latent heat of mixture Bottoms product Reboiler Heat in

4 Water consumption/Cooling water flow rateWater consumption/Cooling water flow rate (21.33) If condensate is not subcooled,If condensate is not subcooled, heat removed in condenser:heat removed in condenser: =heat removed in condenser =water consumption =temperature rise of cooling water Condenser Heat out T2 T1 (V=V a )

5 *[For enthalpy balances in detail, see pp ]( 自学 )*[For enthalpy balances in detail, see pp ]( 自学 ) EXAMPLE 21.2 [(c)(d)]EXAMPLE 21.2 [(c)(d)]

6 (10) Minimum number of plates 最小理论板 数 /Total reflux 全回流 When total reflux,When total reflux, (3) Operating lines then both coincide 重合 with the diagonal, Effect of reflux ratio on number of plates:Effect of reflux ratio on number of plates:

7 Total reflux flow

8 Method for ComputingMethod for Computing (1) Constructing method: constructing steps on an x-y diagram between compositions x D and x B, using 45°line as the operating line for both sections of the column.(1) Constructing method: constructing steps on an x-y diagram between compositions x D and x B, using 45°line as the operating line for both sections of the column. N min is the ideal plates needed, not including a reboiler.N min is the ideal plates needed, not including a reboiler. (2)Analytic method for ideal mixtures (pp )(2)Analytic method for ideal mixtures (pp )

9 Equation (21.42) is the Fenske equation （芬斯克方程）,Equation (21.42) is the Fenske equation （芬斯克方程）, Which applies when is constant. If the change in the value of from the bottom of the column to the top is moderate, a geometricmean of the extreme values is recommended for.Which applies when is constant. If the change in the value of from the bottom of the column to the top is moderate, a geometricmean of the extreme values is recommended for.

10 (11) Minimum reflux （最小回流比） /Infinite number of plates （理论板无穷多） When the intersection of the two operating lines falls on the equilibrium line (point d),When the intersection of the two operating lines falls on the equilibrium line (point d), Fig Minimum reflux ratio 最小回流比

11 When the intersection of the two operating lines falls on the equilibrium line (point d),When the intersection of the two operating lines falls on the equilibrium line (point d), =minimum reflux ratio All actual columns operates at a reflux ratio between the minimum and infinity （无穷大）.All actual columns operates at a reflux ratio between the minimum and infinity （无穷大）.

12 For the normal type of equilibrium curve,For the normal type of equilibrium curve,

13 Eq.(21.44) cannot be applied to all systems.Eq.(21.44) cannot be applied to all systems. For example, ethanol-water system, abnormal equilibrium curve.For example, ethanol-water system, abnormal equilibrium curve.

14 In such a situation the minimum reflux ratio can be computed from the slope of the operating line ac that is tangent to the equilibrium curve.In such a situation the minimum reflux ratio can be computed from the slope of the operating line ac that is tangent to the equilibrium curve. a c

15 Question: Is it the only way that the minimum reflux ratio must be found by drawing the operating line that is tangent to the equilibrium curve for abnormal equilibrium curve?Question: Is it the only way that the minimum reflux ratio must be found by drawing the operating line that is tangent to the equilibrium curve for abnormal equilibrium curve? Factors influencing R Dm include: (1)Equilibrium relation (curve); (2)x D, x F, x B ; (3)q (Thermal condition parameter of feed). Therefore, it is not the only way for determining R Dm by drawing the operating line that is tangent to the equilibrium curve for abnormal equilibrium curve.Factors influencing R Dm include: (1)Equilibrium relation (curve); (2)x D, x F, x B ; (3)q (Thermal condition parameter of feed). Therefore, it is not the only way for determining R Dm by drawing the operating line that is tangent to the equilibrium curve for abnormal equilibrium curve. For example, …For example, …

16 (12)Invariant zone 夹紧区 / 恒浓区 Pinch point Invariant zone: At minimum reflux ratio, near pinch point 夹紧点, where there is no change in either liquid or vapor concentrations from plate to plate,soInvariant zone: At minimum reflux ratio, near pinch point 夹紧点, where there is no change in either liquid or vapor concentrations from plate to plate,so

17 (13)Optimum reflux ratio 适宜回流比 Effect of Reflux ratio on distillation: Effect of Reflux ratio on distillation: Fixed charges on reboiler and condenser  Fixed charges on reboiler and condenser  Steam and cooling-water costs 

18 Figure R Dopt R Dm

19 The total cost reaches a minimum at a definite reflux ratio, that is, optimum reflux ratio R Dopt.The total cost reaches a minimum at a definite reflux ratio, that is, optimum reflux ratio R Dopt. The optimum reflux ratio depends on the cost of energy; it will be closer to R Dm when energy costs are relatively high, and farther from R Dm when distillation equipment is made of expensive alloys.The optimum reflux ratio depends on the cost of energy; it will be closer to R Dm when energy costs are relatively high, and farther from R Dm when distillation equipment is made of expensive alloys. R Dopt =(1.1~2.0)R DmR Dopt =(1.1~2.0)R Dm [EXAMPLE 21.3][p.670][EXAMPLE 21.3][p.670]

20 (14)Nearly pure products When either the bottom or overhead product is nearly pure, a single diagram covering the entire range of concentrations is impractical as the steps near x=0 and x=1 become small.When either the bottom or overhead product is nearly pure, a single diagram covering the entire range of concentrations is impractical as the steps near x=0 and x=1 become small. Method 1: Auxiliary diagrams on a large scaleMethod 1: Auxiliary diagrams on a large scale [calculations are usually done by computer].[calculations are usually done by computer].

21 Method 2: Using Eq.(20.27) or (20.28).Method 2: Using Eq.(20.27) or (20.28). [At each end of the equilibrium curve, both the equilibrium and the operating lines are straight ， so Eq.(20.27) can be used, and no graphical construction is required ][At each end of the equilibrium curve, both the equilibrium and the operating lines are straight ， so Eq.(20.27) can be used, and no graphical construction is required ] [Basis: Raoult’s law applies to the major （主要 的） component and Henry’s law to the minor （次要的） component at each end of the equilibrium curve. ]?[p.671][Basis: Raoult’s law applies to the major （主要 的） component and Henry’s law to the minor （次要的） component at each end of the equilibrium curve. ]?[p.671] [EXAMPLE 21.4.][EXAMPLE 21.4.]

22 (15) Some special cases of distillation a side-stream draw off ( 自学 ):a side-stream draw off ( 自学 ): a) 3 operating lines; 2 q-lines (feed lines); b) Equations of operating lines: Equations of operating lines of top and bottom sections are the same as that of the rectifying and stripping sections discussed before. The equation of operating line in the middle section of the column has to be derived.

23 S Fig. for Ex Assuming constant molal overflow in this column. By material balance:

24 Assuming constant molal overflow in this column, equation for the operating line of middle section is The upper operating line is given by Eq.(21.13) The lower operating line is given by Eq.(21.16)

25 The operating lines for the 3 sections are shown in the following diagram. Dashed lines （虚线） are for R Dm =(L/D) min.

26 How to determine the number of ideal plates?

27 (b) Direct steam heating 直接蒸汽加热 Live steam is injected directly below the bottom plate.Live steam is injected directly below the bottom plate. Equation of operating line of the rectifying section is the same as that discussed before.Equation of operating line of the rectifying section is the same as that discussed before. The equation of operating line in the stripping section of the column has changed.The equation of operating line in the stripping section of the column has changed.

28 Assuming constant molal overflow in this column. By material balance: Steam

29 The rectifying section operating line is given by Eq.(21.13) The stripping section operating line is given by the following equation

30 Fig. Live steam is injected directly below the bottom plate. How to determine the number of ideal plates?

31 which cases will live steam be used for in distillation?which cases will live steam be used for in distillation? Water soln （水溶液）. Bottom product nearly pure water.Water soln （水溶液）. Bottom product nearly pure water.