Pressure and Type of Condenser

Now consider the more general case of the synthesis of all possible ordinary distillation sequences for a multicomponent feed that is to be separated into P final products, which are nearly pure components and/or multicomponent mixtures. The components in the feed are ordered by volatility, with the first component being the most volatile. This order is almost always consistent with that for normal boiling point if the mixture forms nearly ideal liquid solutions, such that Eq. (7.3) applies. Assume that the order of volatility of the components does not change as the sequence proceeds. Furthermore, assume that any multicomponent products contain only components that are adjacent in volatility. For example, suppose that the previously cited mixture of benzene, toluene, and biphenyl is to be separated into toluene and a multicomponent product of benzene and biphenyl. With ordinary distillation, it would be necessary first to produce products of benzene, toluene, and biphenyl, and then blend the benzene and biphenyl. 

An equation for the number of different sequences of ordinary distillation columns, A(, to produce a number of products, P, can be developed in the following manner. For the first separator in the sequence, P — 1 separation points are possible. For example, if the desired products are A, B, C, D, and E in order of decreasing volatility, then the possible separation points are 5 — 1 = 4, as follows A-B, B-C, C-D, and D-E. Now let j be the number of final products that must be developed from the distillate of the first column. For example, if the separation point in the first column is C-D, then7 = 3 (A, B, C). Then P - j equals the number of final products that must be developed from the bottoms of the first column. If N-, is the number of different sequences for i final products, then, for a given separation point in the first colunm, the number of sequences is NjNp-j. But in the first separator, P - 1 different separation points are possible. Thus, the number of different sequences for P products is the following sum  

Application of Eq. (7.9) gives results shown in Table 7.2 for sequences producing up to 10 products. As shown, the number of sequences grows rapidly as the number of final products increases. 

Number of Products, P Number of Separators in the Sequence Number of Different Sequences, 

Ordinary distillation is to be used to separate the ordered mixture C, C j, Cj, I-C4, /1C4 into the three products C2 (C 3,1-C J) (Cj, nC4). Determine the number of possible sequences. 

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To initiate the calculational procedure for the determination of the product distribution for specified reflux and distillate rates, a number of plates between the two pinches is selected. (As discussed in a subsequent section, too few plates but not too many plates may be selected.) Next L/V and temperature profiles for the plates between and including the two pinches as well as the distillate and bottoms temperatures are selected. Next the components of the feed are classified according to the above criteria. Since it is supposed that the complete definition of the feed, the reflux and distillate rates, as well as the column pressure and type of condenser are specified, the component-material balances can be solved for the component-flow rates throughout the column. The component-material balances may be simplified by taking advantage of the unique characteristics of the three classes of components, the distributed components, the separated lights, and the separated heavies. 

Example 12.1. Determine column operating pressures and type of condenser for the debutanizer of Fig. 12.3. 

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