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What Is a Demister (Mist Eliminator)? | Process Engineering Glossary
What Is a Demister (Mist Eliminator)?
A demister, also called a mist eliminator, is an internal device installed in process vessels, columns, and scrubbers to remove entrained liquid droplets from a gas stream by providing a surface on which small droplets coalesce into larger drops that then drain back by gravity. Demisters are typically constructed as woven wire mesh pads, vane packs, or fiber bed elements installed across the gas flow path near the gas outlet of a separator, distillation column, or other gas-liquid separation vessel.
Effective mist elimination prevents liquid carryover into downstream equipment, protecting compressors from liquid slugs, preventing catalyst contamination in catalytic reactors, maintaining product purity in overhead condensing systems, and reducing emissions from atmospheric vents.
Applications
Separator and Knockout Drum Service
Demisters in separators and knockout drums capture liquid droplets entrained in the gas phase that gravity settling alone cannot remove, particularly the fine mist that remains suspended even with adequate vessel residence time.
Distillation Column Overhead
Demisters in distillation column overhead sections prevent liquid entrainment from reaching the overhead condenser system, improving separation efficiency and product quality by keeping liquid reflux from being carried out with the vapor.
Compressor Suction Protection
Demisters in suction knockout drums ahead of compressors provide critical protection against liquid carryover that can cause immediate, severe mechanical damage to compressor internals.
Benefits
Captures droplets too small for gravity separation. Demisters extend separation capability to droplet sizes well below what gravity settling can achieve, typically removing droplets down to three to ten microns depending on the demister type.
Protects downstream equipment from liquid damage. Effective mist elimination prevents the compressor damage, catalyst contamination, and product quality problems that liquid carryover causes in downstream equipment.
Low pressure drop in normal operation. Well-designed wire mesh and vane pack demisters operate with very low pressure drop across the gas flow range, adding effective mist removal without significant compression energy penalty.
Limitations
Demisters can flood at excessive gas velocity. Gas velocity above the demister’s design capacity prevents captured liquid from draining, causing liquid to accumulate and re-entrain, a condition called flooding that eliminates the demister’s separation function.
Fouling and plugging reduce effectiveness. Demisters in dirty or fouling services can plug with accumulated solids, restricting gas flow and increasing pressure drop, requiring periodic inspection during turnaround activities.
Demister type must match the application. Wire mesh, vane pack, and fiber bed demisters have different capacity ranges, droplet capture efficiency, and fouling resistance, requiring selection based on the specific service conditions.
FAQ
How does demister sizing relate to process design?
Demister sizing is based on the maximum gas velocity at the demister cross-section, calculated from the process flow rates and fluid properties documented in the process design basis.
How is a demister shown on process documentation?
Demisters are shown as internal devices within the vessel on the P&ID, with the demister type and specification referenced on the vessel datasheet.
How does demister condition relate to mechanical integrity?
Demister condition is assessed during vessel internal inspections within the mechanical integrity program, checking for fouling, physical damage, and displacement that would compromise separation performance.
How does demister failure relate to process safety?
Demister failure allowing liquid carryover to compressors is evaluated during process hazard analysis as a potential cause of compressor trip or damage scenarios.
Reference
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