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V(VabXa)

xpapB ^(VabXa)

Thus, the composition of the vapour is conveniently found from that of the liquid by use of the relative volatilities of the components. Examples 11.14-11.17 which follow illustrate typical calculations using multicomponent systems. such solutions are now computerised, as discussed further in Volume 6.

Example 11.14

A mixture of ortho, meta, and para-mononitrotoluenes containing 60, 4, and 36 mole per cent respectively of the three isomers is to be continuously distilled to give a top product of 98 mole per cent ortho, and the bottom is to contain 12.5 mole per cent ortho. The mixture is to be distilled at a temperature of 410 K requiring a pressure in the boiler of about 6.0 kN/m2. If a reflux ratio of 5 is used, how many ideal plates will be required and what will be the approximate compositions of the product streams? The volatility of ortho relative to the para isomer may be taken as 1.70 and of the meta as 1.16 over the temperature range of 380-415 K.

Solution

As a first estimate, it is supposed that the distillate contains 0.6 mole per cent meta and 1.4 mole per cent para. A material balance then gives the composition of the bottoms.

For 100 kmol of feed with D and W kmol of product and bottoms, respectively and xdo and xwo the mole fraction of the ortho in the distillate and bottoms, then an overall material balance gives:

An ortho balance gives:

The compositions and amounts of the streams are then be obtained as:

Component

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