Ponchon separation method

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-... 

A mixture of ethane and propane is to be separated by distillation at 475 psia. Explain in detail how a series of isothermal flash calculations using the Soave-Redlich-Kwong equation of state can be used to establish y-x and H-y-x diagrams so that the Ponchon-Savarit method can be applied to determine the stage and reflux requirements. 

Two common graphical procedures used to calculate the number of theoretical plates for a separation system involving binary mixtures are the McCabe-Thiele and the Ponchon-Savarit methods. The former method is based on the assumptions of constant vapor and liquid flow rates, or constant molal overflow, throughout the column and negligible heat losses, whereas the latter method does not assume constant molal overflow. Before developing the appropriate material balances for either procedure, some standard notation and definitions are necessary. 

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. 

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. 

Established methods like McCabe-Thiele or Ponchon-Savarit may be used to compute the number of theoretical stages for a given separation. Both methods are limited to a binary... 

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 ... 

One hundred pound-moles per hour of a mixture of 60 mole% methanol in water at 30°C and 1 atm is to be separated by distillation at the same pressure into a distillate containing 98 mole% methanol and a bottoms product containing % mole% water. The overhead condenser will produce a subcooled reflux at 40°C. Determine by the Ponchon method ... 

The method of Ponchon and Savarit [2.61] is a graphical determination of the number of theoretical separation stages in counterflow columns. It is particularly applicable to binary mixtures in rectification processes. Exact results are obtained when the enthalpy-concentration diagram of Pon-... 

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. 

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