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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 a device installed within a vessel’s vapor space designed to remove entrained liquid droplets from a gas stream before it exits. Demisters typically consist of a mesh pad, a set of closely spaced vanes, or specialized fiber elements that provide a large surface area for droplets to impact, coalesce into larger drops, and fall back into the liquid phase under gravity, rather than being carried out with the vapor.
Demisters supplement, rather than replace, adequate vessel disengagement space, providing an additional, targeted mechanism for capturing entrained liquid that simple gravity settling alone might not fully remove, particularly for finer droplet sizes.
Applications of Demister
Flash Drum and Separator Vessels
Flash drums and other separator vessels commonly include a demister pad near the vapor outlet to improve vapor-liquid disengagement beyond what vessel geometry alone provides.
Distillation Column Overhead Systems
Distillation columns sometimes include demisters in the overhead vapor line or reflux drum to capture entrained liquid before it reaches downstream equipment like compressors or condensers.
Compressor Suction Protection
Demisters protecting compressor suction lines help prevent liquid carryover into the compressor, which can cause significant mechanical damage if not adequately controlled.
Benefits of Knowing Demister
Improves vapor purity beyond gravity settling alone. Demisters capture finer entrained droplets that simple gravity-based disengagement might not fully remove, improving overall vapor stream quality.
Protects downstream equipment from liquid carryover. Effective demister design helps protect compressors, instrumentation, and other downstream equipment from the damage liquid carryover can cause.
Can allow more compact vessel design. Incorporating an effective demister can sometimes allow a smaller vessel disengagement space than would otherwise be needed to achieve the same vapor purity through gravity settling alone.
Limitations to Consider
Can foul or plug over time. Demister elements can accumulate solids, polymers, or other fouling material over time, degrading performance and potentially increasing pressure drop if not properly maintained.
Effectiveness depends on proper sizing for the specific service. A demister sized incorrectly for the actual vapor velocity and droplet size distribution won’t perform as intended, making proper sizing calculation important rather than generic selection.
Adds pressure drop to the vapor path. Demister elements introduce additional pressure drop compared to an open vapor space, a factor that needs to be accounted for in overall system pressure balance calculations.
Demister FAQ
How does a demister relate to vapor-liquid disengagement in vessel design?
A demister supplements the basic vapor-liquid disengagement achieved through vessel geometry alone, providing additional droplet capture for a more complete separation, relevant also to decanting and gravity-based separation more broadly.
Why is demister performance relevant to annular flow and entrainment behavior?
Understanding flow regimes like annular flow helps predict the entrainment conditions a demister needs to address, since higher entrainment rates require more effective mist elimination capability, informed by broader fluid mechanics.
How does demister design relate to a flash calculation and downstream equipment protection?
The vapor and liquid split determined by a flash calculation informs expected entrainment levels a demister needs to handle, directly protecting downstream heat exchanger design and other equipment, including a can pump, from liquid carryover damage, relevant also to critical flow conditions downstream.
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