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Distillation Overhead Compressor

(i) Reflux-overhead heat exchanger. This exchanger superheats the column overhead..while subcooling the reflux. Subcooling the reflux reduces the vapor flash generated upon expansion of the reflux stream. The lower vapor flash reduces vapor recycle through the compressor and therefore lowers compressor power consumption. This power reduction is somewhat offset by the higher compressor suction temperature and lower suction pressure (because of exchanger pressure drop). The higher compressor suction- temperature may be advantageous if a compressor discharge cooler is used.

Depending on the temperature-entropy diagram, ' in some cases it may be necessary to superheat column overheads in order to avoid condensation in the compressor (24-26,31). In hydrocarbon systems, superheating is needed when compressing butane or heavier alkanes (31).

Compressor suction knockout drum is often used to eliminate liquid droplets from damaging compressor blades.

Intermediate discharge and an interreboiler or preheater. In refrigerated systems, this can be a major energy saver (10). The intermediate discharge reduces compressor power consumption by having a portion of the vapor compressed only to an intermediate pressure rather than compressing it to the full compressor discharge pressure.

A number of variations of the intermediate discharge arrangement are often used. A common variation (10,16,17,41) is to use a two-stage compressor and take the intermediate discharge from between these stages. Another variation (10) is to connect the overhead vapor from the flash separator (item (vii) below) to the intermediate or interstage vapor line, thus reducing vapor recycle through the compressor; this variation, however requires locating the interreboiler above the flash separator, or bypassing the interreboiler liquid around the flash drum. One other variation is to use a 2-stage compression system without an interreboiler (16,17,41); in this case, the interstage discharge reboils the column while the compressor discharge is condensed by a trim condenser. This variation is attractive if there is an incentive to lower column pressure beyond the point of permitting trim-condensation (item (iv) below) with readily available cooling media.

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