Distillation enthalpy-concentration method

The McCabe-Thiele constructions described in Chapter 8 embody rather restrictive tenets. The assumptions of constant molal overflow in distillation and of interphase transfer of solute only in extraction seriously curtail the general utility of the method. Continued use of McCabe-Thiele procedures can be ascribed to the fact that (a) they often represent a fairly good engineering approximation and (b) sufficient thermodynamic data to justify a more accurate approach is often lacking. In the case of distillation, enthalpy-concentration data needed for making stage-to-stage enthalpy balances are often unavailable, while, in the Case of absorption or extraction, complete phase equilibrium data may not be at hand. 

FRACTIONAL DISTILLATION USING ENTHALPY-CONCENTRATION METHOD... 

Sec. 11.6 Fractional Distillation Using Enthalpy-Concentration Method... 

EXAMPLE 11.6-2. Distillation Using Enthalpy-Concentration Method A liquid mixture of benzene-toluene is being distilled using the same conditions as in Example 11.4-1 except that a reflux ratio of 1.5 times the minimum reflux ratio is to be used. The value o R = 1.17 from Example 11.4-2 will be used. Use enthalpy balances to calculate the flow rates of the liquid and vapor at various points in the tower and plot the curved operating lines. Determine the number of theoretical stages needed. 

Distillation of Benzene-Toluene Feed Using Enthalpy-Concentration Method. A... 

Himmelbau (1995) or any of the general texts on material and energy balances listed at the end of Chapter 2. The Ponchon-Savarit graphical method used in the design of distillation columns, described in Volume 2, Chapter 11, is a further example of the application of the lever rule, and the use of enthalpy-concentration diagrams. 

The lower part of the column is covered by stepping off stages in a fashion similar to that in the upper part of the column, and the final conni of theoretical stages is then determined. The Ponchon-Savarit method may be used for many situations more complex lhan the simple one just described mixed vapor-liquid distillate product, side draw streams, multiple feeds, and so on. Standard unit operations textbooks should be consulted for more dentils on this methnd. As mentioned, it suffers from a need for enthalpy-concentration data, but even a crude approximation based on linear variation of enthalpy with concentration can be better than the McCabe-Thiele approach if there is a very large difference in the latent heats of vaporization of the iwo components being distillnd. 

Example 10.1. One hundred kilogram-moles per hour of saturated n-hexane-n-octane vapor containing 69 moIe% hexane is separated by distillation at atmospheric pressure into product containing 90 mole% hexane and bottoms containing 5 mole% hexane. The total condenser returns 42.5 mole% of the condensate to the column as saturated liquid. Using the graphical Ponchon method and the enthalpy-concentration data of Fig. 10.8 calculate ... 

The Ponchon-Savarit method is summarized in Fig. 5.3-8. The method involves an enthalpy-concentration diagram, and the enthalpies of the saturated liquid and vapor are first plotted on the diagram. Next, the equilibrium tie lines are added, based on phase equilibria. Compositions of feed, distillate, and bottoms are then located on the diagram (in the example shown the feed is mixed vapor-liquid and the distillate and bottoms are saturated liquids). A reflux ratio is chosen, and the enthalpy of the reflux is located as the top difference point, Ag- (The reflux ratio is equal numerically to the vertical distance from the difference poim to the value of y/v> divided by the vertical distance from y to Xg.)... 

Introduction. In Section 11.4B the McCabe-Thiele method was used to calculate the number of theoretical steps or trays needed for a given separation of a binary mixture of A and B by rectification or fractional distillation. The main assumptions in the method are that the latent heats are equal, sensible heat differences are negligible, and constant molal overflow occurs in each section of the distillation tower. In this section we shall consider fractional distillation using enthalpy-concentration data where the molal overflow rates are not necessarily constant. The analysis will be made using enthalpy as well as material balances. 

The phase equilibrium diagrams shown thus far have been in terms of T, P, and concentrations however, other thermodynamic functions can be used in place of these. In graphical methods for distillation, for instance, it is sometimes convenient to work in terms of enthalpy rather than temperature, because the diagram can then be used to show heat addition or removal as well as composition changes. 

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