Number of Real Stages

Because the real steps are shorter, it will take more of them to achieve the same separation. The ratio of the number of ideal to real stages required will be less than one and is called the  

While this efficiency is easy to use, the value of the efficiency is hard to determine. A more fundamental quantity is the 

On an xy diagram, this represents the height of a real step divided by the height of an ideal step for the same Alternatively, one could also define efficiency based on the change in liquid composition (rather than vapor composition)  

This definition is based on the width of a step (involves changes in x) rather than based on the height of a step (changes iny) as in (84). 

Some typical values of plate efficiencies are given in Table 18-4 (taken from Perry s 6 edition). 

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Size the column diameter, number of real stages. 

In some methods, the Murphree plate efficiencies can be incorporated into the procedure for calculating the number of stages and the number of real stages determined directly. 

Using O Connell s correlation, estimate the overall column efficiency and the number of real stages required for the separation given in Example 11.5. 

In addition, in the case of cascades of stages operated in counter-current flow, an over-all fractional stage efficiency Eo is defined as the ratio of the number of ideal stages n< to which the cascade is equivalent divided by the number of real stages nr ... 

TABLE 13-5 Key Components and Typical Number of (Real) Stages Required to Perform the Separation for Distillation Processes of Industrial Importance... 

Suppose we have a value for or Ej y, how do we use it to determine the number of real stages required The procedure is similar to that for ideal stages ... 

From these results, we conclude that the plate design is appropriate from the pont of view of pressure drop, entrainment, weeping, and plate efficiency. The next step in the design is to determine the number of real stages required. Since the plate efficiency does not change much from one end of the tower to the other, we will assume a constant value taken as the arithmetic average of the efficiency values at the tower ends Emce = (0.77 + 0.73)/2 = 0.75. 

In the approach based on equilibrium stages the number of real stages can be found by dividing the number of theoretical plates Nj by an overall efficiency E ... 

Equilibrium data are in Table 12-1. Use a Murphree vapor efficiency of 40%. Find oudet vapor composition and number of real stages needed. 

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