With Ponchon-Savarit method

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. 

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. 

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-... 

For the Ponchon-Savarit method, a pinch occurs when an operating line coincides with a tie line, thus preventing further stepping of theoretical stages. This is shown in Fig. S.3-12, and normally the pinch occurs in the vicinity of the feed stage. 

The major difference between the McCabe-Thiele method and the Ponchon-Savarit method is that the liquid and vapor flow rates in the latter method are not assumed constant throughout the column. Because of this, the Ponchon-Savarit method is a more general and accurate method. Since the method is based on enthalpy values of fluid mixtures throughout the column, an enthalpy-concentration diagram is used in conjunction with a vapor-liquid equilibrium plot to determine the number of theoretical plates required for a specific separation. Units based on either moles (and mole fractions) or mass (and mass fractions) are acceptable as long as they are consistent. 

The procedure in the stripping section is very similar. An equilibrium tie-line relates the reboiler concentration Xb with the vapor concentration 3 above the reboiler liquid. A line drawn from A 2 through establishes the liquid concentration Xi on the first plate in the bottom of the column. This procedure is repeated until the principal operating line is reached. Generally, only a part of a theoretical plate is required for the last step. This will have to be counted as a theoretical plate since partial plates are not practical in a real column. The separation called for in Example 6.8 and solved with the aid of the Ponchon-Savarit method shows a requirement of 4 theoretical plates in the rectifying section and 5 theoretical plates in the stripping section. The results are identical to those determined when the McCabe-Thiele method was utilized in Example 6.8 because the partial plates in both methods were counted as full plates. 

With the aid of a phase equilibrium diagram to obtain the vapor-liquid tie lines, the number of theoretical plates may be stepped off according to the Ponchon-Savarit method outlined previously in this chapter. The number of theoretical plates for the desired separation, as shown in Fig. 6.24, is six plates. 

Rework Problem 6.22 using the Ponchon-Savarit method with the enthalpy concentration data for nitrogen-oxygen mixtures as presented in Table 6.3. 

The separation analysis by established methods of McCabe -Thiele or Ponchon Savarit are compared with a flow sheeting program ASPEN+. It will be demonstrated, that a reliable scale -up is possible combining all these methods. 

Then, a first simplified determination of the number of theoretical plates can be carried out, using well known chemical engineering methods like McCabeThiele or Ponchon-Savarit in. This analysis is carried out on a quasi binary system (key-components) or with pseudo-components and provides essential insight into the operation of the separation column or the separation machines. 

Methods of dealing with these problems are considered following the McCabe-Thiele method and are equally applicable to Ponchon-Savarit calculations. 

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