Two-Column System with Excess

As determined in a later section of this chapter, the total capital investment of this one-column process is 358,300 and the annual energy cost is 122,800 per year, giving a total annual cost (TAC) of 242,300 per year. 

In this section B is selected as the excess component. If for some reason reactant A were chosen, the flowsheet would be different. This case is considered in Section 4.4. The selection of which reactant to keep in excess depends on kinetics, relative volatilities, heats of vaporization, and the costs of the two reactants. In either flowsheet, the excess reactant and one of the products are vaporized only once, so there is no obvious inherent energy difference between the two flowsheets. 

The sizing relationships and econotnie parameters presented in Chapter 3 are used to determine the optimum design of the two-eolumn system. The presence of the second column adds additional design optimization variables. To simplify the optimization, we assume that the reactive column has the same numbers of trays in each section as in the one-column process and operates at the same pressure. These appear to be reasonable assumptions because the TAC is fairly insensitive to the number of trays chosen in each section as long as the number of trays is kept fairly large (well above the minimum). 

NEAT OPERATION VERSUS USING EXCESS REACTANT 

The pressure in the recovery column is fixed at 1 bar, which permits the use of cooling watCT in the condenser (about 330 K in the reflux drum with mostly B). The number of trays in this column is varied to find the minimum TAC. Energy consumption is fairly insensitive to feed tray location in the recovery column, so the feed is introduced into the middle of the column. 

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