Of the four major components of a separator that were discussed in a previous section, the mist extractor has the most impact on separated gas quality with respect to carried over liquid content. The sizing equations and parameters provided in the mist extraction section size the mist extractor itself, not the actual separation vessel. The gas capacities of the various types of mist extractors is generally inversely related to the amount of entrained liquid that the mist extractor is required to remove.
• the amount of liquid in the separator feed gas.
• the inlet flow condition of the feed, i.e. multiphase flow pattern, .
• the type of inlet device used.
• the sizing/dimensions of the gas gravity separation section of the separator.
For lightly liquid loaded separation applications, e.g. less than 10-15 BBL/MMSCF (gas scrubbing application), the mist extractor performance will be controlling and will generally dictate the cross-sectional area requirements of the gas gravity separation section. For a vertical separator, this will determine the vessel diameter. A horizontal vessel would not typically be used in a lightly loaded gas-liquid separation application. Good flow distribution to the mist extractor is still required.
In more heavily liquid loaded separation applications, a vertical or horizontal separator configuration may be chosen depending on the specific conditions. In either case, some “pre-conditioning” of the gas phase, specifically reduction of the gas entrainment loading, may be required ahead of the final mist extraction element.