Mist Extractor

Key Takeaways

• Wire mesh mist extractor performance is characterized by the K-factor (also called the Souders-Brown coefficient), which is an empirical constant relating the maximum allowable gas velocity through the mesh pad to the vapor-liquid density difference, with higher K-factor values indicating higher capacity mesh designs that can handle greater gas throughput at the same mist removal efficiency: the design equation is Vmax = K * sqrt((rho_L - rho_G) / rho_G), where Vmax is the maximum superficial gas velocity through the mesh pad in feet per second, rho_L is the liquid density in lb/ft3, rho_G is the gas density in lb/ft3, and K is the Souders-Brown coefficient typically ranging from 0.10 to 0.35 ft/s for standard wire mesh designs depending on the mesh density, wire diameter, and surface area; exceeding Vmax causes re-entrainment of the coalesced droplets from the mesh surface as the gas velocity is high enough to strip the liquid film from the wires before it can drain downward, significantly reducing the mist extraction efficiency and potentially causing liquid carryover; the gas velocity through the mesh pad drops below Vmax during periods of low production, and very low velocities (below approximately 20-30% of design velocity) may also reduce efficiency because the droplets have insufficient inertial momentum to impinge on the wire surfaces rather than following the gas streamlines around the wires.
• Vane-type mist extractors (chevron vane packs) provide higher liquid handling capacity than wire mesh pads and are preferred in high liquid loading applications where the wire mesh would flood and re-entrain: the chevron vane geometry forces the gas to make a series of 90 to 180 degree direction changes as it passes through the closely spaced corrugated vane passages, and the inertia of the liquid droplets prevents them from following the gas through the sharp bends, causing them to impinge on the vane surfaces and coalesce into drainage films that run down the vane surfaces into collection troughs or pockets at the bottom of the vane pack; vane mist extractors are rated for higher gas velocities (K-factors of 0.25 to 0.50 ft/s) and higher liquid loading rates (up to several gallons per minute per square foot of vane pack face area) than standard wire mesh pads, at the cost of somewhat lower efficiency for very fine droplets (below approximately 20 microns) that follow the gas streamlines through the vane passages without impacting the vane surfaces; vane mist extractors with internal drainage hooks or pockets on each vane surface (designed to capture the coalesced drainage film before gas flow can re-entrain it) extend the effective operating envelope to higher gas velocities and liquid loadings than simple smooth chevron vanes.
• Cyclonic mist extractors (tube-type centrifugal separators) provide the highest gas velocities and smallest footprint among the three principal mist extractor types and are used where space is constrained, gas throughput is high, and the separated liquid can drain continuously from the centrifugal tubes without accumulating to flood levels: a cyclonic mist extractor consists of a bundle of small-diameter (typically 1 to 4 inch) tubes with tangential inlet slots or a fixed-pitch helical swirl element at the tube inlet that imparts angular momentum to the entering gas, generating centrifugal forces that throw the liquid droplets radially outward to the tube wall where they form a draining film; the liquid film drains downward within the tube and is collected at the tube outlet through a liquid discharge port that is separate from the gas outlet at the top of the tube; the centrifugal separation factor (the ratio of centrifugal acceleration to gravitational acceleration) in a small-diameter cyclonic tube is typically 50 to 200 times gravity, compared to 1 g in a gravity separator, enabling separation of droplets down to approximately 5 to 10 microns that would not be removed by gravity settling within practical vessel dimensions; cyclonic mist extractors are not suitable for high liquid loading applications because liquid accumulation in the tube reduces the centrifugal separation efficiency and can cause flooding and liquid re-entrainment at the gas outlet.
• Mist extractor fouling and plugging in gas processing and production service requires preventive maintenance and monitoring to ensure that the mist extractor continues to provide adequate liquid removal throughout the operating life of the separator or scrubber: wire mesh pads are susceptible to plugging by hydrate formation (when the gas contains water vapor and the temperature drops below the hydrate formation point, ice-like hydrate crystals can form on the wire surfaces and progressively reduce the open area until the pad pressure drop increases to the point of separation loss), by wax deposition from heavy hydrocarbon condensates at low temperatures, by scale and corrosion product accumulation in produced water service, and by glycol contamination in glycol dehydration units where the regenerated glycol contains residual hydrocarbon liquids that wet and foul the mesh; vane packs are susceptible to corrosion (particularly in acid gas service with CO2 and H2S present) that weakens the vane material and causes structural failure or loss of vane pack integrity; the periodic inspection and cleaning of mist extractors (using high-pressure water jets, solvent flushing, or physical replacement of fouled mesh elements) is an important part of the preventive maintenance program for gas processing facilities, and the differential pressure across the mist extractor is monitored as a key performance indicator that detects fouling before it causes liquid carryover.
• Mist extractor sizing in gas production separators must account for the full range of operating conditions including turndown (low flow) and upset (high liquid slugging) scenarios in addition to the design operating point, because separator performance under off-design conditions determines the reliability of the downstream gas quality during the variations in production rate and liquid loading that occur throughout the producing life of an oil and gas field: at low gas rates (early in well life before gas cap breakthrough, or during reduced production periods), the gas velocity through the mist extractor drops and the separation efficiency may decline if the velocity falls below the minimum effective range for the mist extractor type; the solution for maintaining adequate mist extraction efficiency across a wide gas rate range includes multi-stage mist extraction (coarse separation in the first stage, fine mist extraction in the second stage, with the first stage reducing the liquid loading on the second stage to improve overall efficiency), variable-geometry mist extractors with adjustable vane spacing or mesh area, or parallel mist extractor elements that can be isolated at low flow rates to maintain adequate gas velocity through the active elements; the liquid slug handling capacity of the mist extractor system must be evaluated against the expected slug sizes from pipeline pigging operations, well restart after shut-in, and terrain-induced slug flow in undulating gathering systems that can deliver large liquid slugs to the separator inlet at rates far exceeding the design continuous liquid rate.