Separators without mist extractors are not frequently utilized. The most common application of a vapor-liquid separator that does not use a mist extractor is a flare knockout drum. Mist extractors are rarely used in flare knockout drums because of the potential for plugging and the serious implications this would have for pressure relief. is typically a horizontal vessel that utilizes gravity as the sole mechanism for separating the liquid and gas phases. Gas and liquid enter through the inlet nozzle and are slowed to a velocity such that the liquid droplets can fall out of the gas phase. The dry gas passes into the outlet nozzle and the liquid is drained from the lower section of the vessel.
To design a separator without a mist extractor, the minimum size diameter droplet to be removed must be set. Typically this diameter is in the range of 300 to 2,000 microns (1 micron = 10-4 cm or 0.00003937 inch).
The length of the vessel required can then be calculated by assuming that the time for the gas flow from inlet to outlet is the same as the time for the liquid droplet of size Dp to fall from the top of the vessel to the liquid surface. Eq 7-10 then relates the length of the separator to its diameter as a function of this settling velocity (assuming no liquid retention):
If the separator is to be additionally used for liquid storage, this must also be considered in sizing the vessel.
Example 7-1—A horizontal gravity separator (without mist extractor) is required to handle 60 MMscfd of 0.75 specific gravity gas (MW = 21.72) at a pressure of 500 psig and a temperature of 100°F. Compressibility is 0.9, viscosity is 0.012 cp, and liquid specific gravity is 0.5. It is desired to remove all entrainment greater than 150 microns in diameter. No liquid surge is required.