A demister pad, is a device used for removing entrained liquid droplets from a gas stream. As the name suggests the demister is used for removal of the mist from gaseous phase. Demister pad is often fitted just below the top vapor outlet of a vapor liquid separator, as shown in the figure.
The demister pads work by coalescing smaller liquid droplets by obstructing their path. Obstruction of path causes increased collisions among the liquid droplets. Most of these droplets stick together and form bigger droplets which are too heavy to rise with the gas stream. Thus the bigger liquid drops drop down in the pool of liquid below. Gas stream is not affected by the obstruction in the path and escapes through top vapor outlet. The obstruction in the path of liquid droplets can be achieved by a variety of geometries. Demister pad may be a mesh type coalescer, vane pack or other structure intended to aggregate the mist into droplets that are heavy enough to separate from the vapor stream.
Demister pads assist the empty drums by increasing the gas liquid separation efficiency. Empty drums rely on high residence time for separating liquid from vapor and require high sizes. By using demister pads, the degree of separation can be achieved using smaller vessels. Demister pads are often necessary to be used where any liquid entrainment in vapor stream is not acceptable, for example - compressor suction lines.
Vertical gas liquid separators are sized based on a design velocity for the gas phase which is described by the following equation,
ρL and ρG are the densities of liquid and gas phase respectively.
K is a proportionality constant which depends on a number of factors which include liquid viscosity, surface tension, entrainment loading, and the content of dissolved and suspended solids. Recommended 'K' values are also highly dependent geometry of the demister and the vessel. The 'K' values for empty drum separator vessels can be improved 10 folds by using the appropriate demister pad. High 'K' value means higher allowable gas velocities. Thus, for same design gas flow rate, vessel size (vessel diameter) can be reduced.