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 a significant
problem for a 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. A knockout drum with mist
eliminator can achieve an overall efficiency of 99.99% liquid removal.
Knockout Drum Configurations
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 reentrainment (depending on the liquid load and on the
height).
A horizontal entrainment
separation vessel can also be designed to operate as a droplet coalesce. 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.
