Graphical Method for Continuous Distillation Design

There are 2 types of problems in distillation: the design problem and the rating (operational) problem.

In a design problem the desired separation is set, and the design objective is to determine the number of trays to be put into a new column in order to separate a given feed into products of desired purities. For a binary distillation, we would usually specify the mole fraction of the MVC in the distillate and bottom products. The column pressure, feed flow rate, thermal condition (e.g. temperature, fraction vapour) and composition is also specified. In addition, the (external) reflux ratio is usually specified. An alternative specification commonly used is the split, or fractional recovery, i.e. the fraction of a component (e.g. the MVC) recovered in the distillate and/or bottoms.

[ see p.97 - 99, "Equilibrium Staged Operations", P.C. Wankat for more examples ]

The rating problem is to determine the performance of an existing column with a fixed number of trays. The column's suitability under different conditions is to be evaluated.

There are a number of different types of rating problems. All of them involve iterative (trial-and-error) solutions. A guess is made of one or more unknown variables, and a design solution procedure is carried out using the values guessed. The results are compared with the column specifications, and if they do not match, new guesses are made and the solution is repeated.

There are 2 primary method of column design: The Ponchon-Savarit Method and the McCabe-Thiele Method. The main differences are presented below:

1. Ponchon-Savarit Method

Rigorous and capable of handling all distillations, but requires detailed enthalpy data for its application
Simultaneous material and energy balance calculations
Detailed calculations of equilibrium conditions
More accurate, but difficult to use
Largely superseded by rigorous computer simulation calculations

2. McCabe-Thiele Method

Less rigorous, enthalpy data not required.
Adequate for many applications, more commonly use because of its simplicity
Uses graphical solution for binary mixture on equilibrium diagram (x-y plot).
Provides the number of theoretical (ideal) trays required for a given separation.
Pressure is assumed constant throughout the entire column.

The development of computers has also made the McCabe-Thiele graphical technique obsolete for most detailed design calculations. One of the main disadvantages of the McCabe-Thiele Method is that it is not very useful for analyzing the distillation of mixtures containing more than 2 components.

However, the method is still useful as a teaching tool and a conceptual tool. The graphical procedure is easy to understand and presents a very clear visual picture of the design problem. The graphs are also very useful in helping to determine what the effect of changing variables will be.

This site is restricted to the use of the McCabe-Thiele Method.

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