Ponchon-Savarit method

Figure 8-42A-E. Performance analysis of unequal molal overflow for binary systems using Ponchon-Savarit Method.

Unequal Molal Flow, 63 Ponchon-Savarit Method, 63 Example 8-21 Ponchon Unequal Molal Overflow, 65 Multicomponent Distillation, 68 Minimum Reflux Ratio — Infinite Plates, 68 Example 8-22 Multicomponent Distillation by Yaw s Method, 70 Algebraic Plate-to-Plate Method,... 

This method is one of the most important concepts in chemical engineering and is an invaluable tool for the solution of distillation problems. The assumption of constant molar overflow is not limiting since in very few systems do the molar heats of vaporisation differ by more than 10 per cent. The method does have limitations, however, and should not be employed when the relative volatility is less than 1.3 or greater than 5, when the reflux ratio is less than 1.1 times the minimum, or when more than twenty-five theoretical trays are required(13). In these circumstances, the Ponchon-Savarit method described in Section 11.5 should be used. 

The Ponchon-Savarit method, using an enthalpy-composition diagram, may also be used to handle sidestreams and multiple feeds, though only for binary systems. This is dealt with in Section 11.5. 

Distillation to 90-volZ ethanol requires multistage operation. The process calculations were performed according to the Ponchon-Savarit method.(12) Assuming the concentration of the ethanol bottoms stream to be 1 volZ, and a reflux ratio of 1.5 times the minimum, the heat input to the reboiler corresponds to an energy requirement of 3.2 kWh/gal of 90-volZ product as distillate, or about 2.7 times that of a CCRO process employing the hypothetical, improved membrane. 

The Ponchon-Savarit method is summarized in Fig. 5.3-8, The methud involves an enlhalpy-coucen-traiion diagram, and tha enthalpies of the saturated liquid and vapor are first plotted on the diagram. Next, the equilibrium lie 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 cbosen. and the enthalpy of die reflux is located as the lop difference point. A0. (The reflux ratio is equal numerically to the vertical distance from the difference point to the value of yN, divided by the vertical distance from to xD.)... 

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. 

Graphical Multistage Calculations by the Ponchon-Savarit Method... 

In this chapter, the Ponchon-Savarit graphical method for making stage-to-stage calculations is applied to distillation and extraction. Like the McCabe-Thiele procedure, the Ponchon-Savarit method is restricted to binary distillation and ternary extraction systems. The method does, however, obviate the need for the constant phase-ratio flow assumptions. 

Figure 10.5 demonstrates the basic concept behind the Ponchon-Savarit method. In the McCabe-Thiele construction, material balance equations are plotted on an x-y phase equilibrium diagram, the stages being calculated by alternate use of the material balance and equilibrium relationships. The Ponchon diagram embodies both enthalpy and material balance relationships as well as... 

As indicated in the sketch below, a mixture of propane and n-pentane is to be separated by distillation at lOOpsia. Two designs are to be made by the Ponchon-Savarit method. In the first design, a total overhead condenser and a partial bottoms reboiler are to be used with saturated reflux at a flow rate of twice the minimum value. In the second design, a total overhead condenser, an inter-... 

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