Minimum vapor throughput

If the chemical plant must operate over a wide range of throughputs, the distillation columns in the plant must be able to turn down to low throughputs. Hydrauhc limitations will be unavoidably encountered, so the control structure must be able to avoid these limitations. In this chapter, we assume that the minimum vapor rate is the limitation. 

The objective in this chapter is to avoid minimum vapor flow rate limitations. If feed rates to a distillation column are reduced due to a reduction in plant throughput or a diversion of the feed stream to a more economical product (i.e., hydrogen gas to combustion turbine during peak power demand instead of methanol), the distillation control structure needs to be robust enough to handle these disturbances while maintaining product specifications. Three alternative control structures are explored to obtain this objective. 

Tray efficiency is as high as for bubble caps and almost as high as sieve trays. It is higher than bubble caps in some systems. Performance indicates a close similarity to sieve trays, since the mechanism of bubble formation is almost identical. The real point of concern is that the efficiency falls off quickly as the flow rate of vapor through the holes is reduced close to the minimum values represented by the dump point, or point of plate initial activation. Efficiency increases as the tray spacing increases for a given throughput. 

These checks will use all the parameters in the layout summary in Sec. 6.5.3 (Table 6.11, third trial). The minimum throughput calculations will be performed for stages 8 and 9 in the depropanizer, where the loads are lowest. Since the example requires column turndown to 60 percent of the expected design loads, the vapor and liquid loads shown in Table 6.10 for stages 8 and 9 are multiplied by 0.6 for the turndown checks. 

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