A high-purity product containing 98-mol% benzene is required at the rate of 200 lb-mol/h. The balance of the benzene is to be recovered as a liquid at a purity of 90 mol%. Toluene is to be recovered at 95 mol% purity. A total overhead condenser is to be used.

Cooling water is at a premium in the plant, but a warmer process stream is available for intercondensing. This stream's available heat is 2.8 million Btu/h, and its temperature is such that it is best for condensing vapor containing 0.7 mole-fraction benzene.

The column is designed to operate at an LjV ratio 25 percent above the minimum.

How many stages are required, and where should the feed streams, the product stream and the mtercondenser be located?

solution The column is shown in Fig. 2.15a.

1. An overall mass and component balance for determining Pl (which is equal to -F^ and B yields Px = 252 and B = 548.

2. Via Eq. (2.42) and assuming the latent heat is 14,000 Btu/lb-mol, F3 (which is equal to -P3) = (2.8 x 106)/14,000 = 200 lb-mol/h.

3. Construct the equilibrium curve. Construct all g-lines by drawing a line of slope q/(q - 1) through the composition point of each feed (Fig. 2.156).

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