Flash/Knock Out Drum

KPE manufactured depressurizing and vapor/liquid separation devices may be used in several industries.

A knockout drum with a mist eliminator is common whenever a process requires entrained droplets to be separated from a vapor stream. A simple knockout drum (no mist eliminator) will remove droplets larger than about 380 microns by gravity settling M. Generally, gravity settling removes more than 90% of the liquid entering the vessel. However, the remaining droplets smaller than 380 microns can be significant problem for downstream unit. A mist eliminator in the top of the knockout drum will remove the remaining droplets down to a diameter of 6 microns or less, depending on the type of mist eliminator (1). A knockout drum with mist eliminator can achieve an overall efficiency of 99.99% liquid removal.

Knockout drums may be oriented vertically or horizontally. In both types, the mist eliminator may also be oriented vertically or horizontally. For a vertical mist eliminator (horizontal vapor flow), the drainage flow is cross-current, whereas for vertical upflow the drainage flow is counter-current. Because cross-current flow results in less liquid holdup, a vertical mist eliminator can be operated at a higher vapor loading without re-entrainment (depending on the liquid load and on the height).

A horizontal entrainment separation vessel can also be designed to operate as a droplet coalescer. In this case, the mist eliminator operates beyond the reentrainment load. Large, coalesced droplets blow off the down stream side of the mist eliminator and either settle by gravity or are collected by a vane type mist eliminator.

A preliminary analysis may suggest that a horizontal knockout vessel may reduce cost. In the final analysis, however, many factors should be evaluated to arrive at the decision between a horizontal versus a vertical vessel.


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