How Gas Coolers and Aftercoolers Create a New Mist Eliminator Duty
A gas stream can enter a cooler with no visible liquid droplets and leave with a significant condensate load.
This happens because cooling changes the amount of vapor the gas can hold.
Once the gas crosses its saturation condition, vapor condenses into liquid.
The equipment downstream of the cooler now sees a new gas-liquid separation duty.
This is why:
- gas coolers;
- compressor aftercoolers;
- intercoolers
are frequently followed by:
- knockout drums;
- mist eliminators;
- coalescing separators.
The separator is not removing upstream entrainment alone.
It is removing liquid newly created by cooling.
Cooling Changes Phase, Not Just Temperature
Suppose warm gas contains water vapor.
Before cooling, all of that water may remain in the gas phase.
A mist eliminator upstream would have nothing to capture.
The gas then passes through an aftercooler.
As temperature decreases, the saturation capacity of the gas decreases.
Part of the water vapor becomes liquid condensate.
The stream now contains:
- gas;
- newly formed droplets.
A downstream separator is required.
Hydrocarbons Can Condense Too
The condensed liquid may not be water alone.
In process gas, cooling can condense:
- hydrocarbons;
- solvent vapor;
- other condensable components.
The separator must therefore be designed for the actual liquid mixture.
Its:
- density;
- viscosity;
- surface tension
may differ substantially from water.
This affects:
- droplet capture;
- coalescence;
- drainage.
Heat Exchanger Geometry Can Generate a Broad Droplet Distribution
Condensation may first form as a film on cooler surfaces.
Gas flow can then shear liquid from the surface.
Droplets are created downstream of:
- tubes;
- fins;
- channels.
The resulting size distribution depends on equipment geometry and gas velocity.
Some liquid may leave as large drops.
Another fraction can be much finer.
Therefore, condensate mass alone does not completely define the separator duty.
A Knockout Drum and Demister Perform Different Functions
After cooling, a large amount of liquid may enter the separator vessel.
Gravity and vessel disengagement should remove:
- bulk liquid;
- large droplets.
The mist eliminator then removes smaller remaining droplets.
If all condensed liquid is forced directly into fine mesh without adequate bulk separation, the pad can become flooded.
The system should separate the duties:
- collect bulk condensate;
- polish the gas.
Compressor Aftercoolers Make This Particularly Important
Compressed gas is often hot.
After compression, it is cooled.
Water or hydrocarbons then condense.
If this liquid is not removed effectively before the next:
- compressor stage;
- dryer;
- process unit,
it can cause operating problems.
The mist separator therefore becomes part of the compressor train's liquid-management system.
Condensate Load Changes With Ambient and Cooling Conditions
A cooler does not always generate the same amount of liquid.
Condensation depends on:
- inlet temperature;
- outlet temperature;
- gas composition;
- pressure;
- cooling-water condition.
A colder cooling-water temperature can create more condensate.
Seasonal operation can therefore change separator liquid loading even when gas flow remains constant.
The maximum condensate case should be considered.
Pressure Can Strongly Affect Condensation
In compressed-gas systems, pressure affects:
- vapor partial pressures;
- condensation behavior.
The separator should use the actual process condition after the cooler.
The gas density at this point may also be high.
This changes the allowable separator velocity.
An atmospheric demister rule should not automatically be applied to a pressurized aftercooler separator.
Liquid Can Continue Forming Downstream
A common mistake is to locate the separator immediately after partial cooling and then allow the gas to cool further in downstream piping.
More liquid condenses after the separator.
The downstream equipment becomes wet.
Operators blame the demister.
But the separator could only remove droplets that existed at its own location.
Where possible, the final separator should be positioned after the relevant condensation step.
Piping Layout Can Re-Entrain Collected Condensate
Condensate can collect in:
- low points;
- horizontal piping.
High gas velocity can pick it up again.
The downstream separator then receives intermittent liquid slugs rather than steady fine mist.
Good piping drainage is therefore part of the overall separation system.
The vessel should not be expected to compensate for poorly drained upstream lines indefinitely.
Fine Mesh May Be Useful for Clean Condensate
If gas is relatively clean and the liquid is nonfouling, wire mesh can provide effective final polishing.
The large internal area encourages small droplets to:
- impact;
- coalesce;
- drain.
But mesh density should still be selected for:
- gas velocity;
- liquid loading.
A condensate separator is not automatically a “use the finest mesh available” application.
Dirty Gas Changes the Choice
If the gas contains:
- solids;
- compressor oil;
- sticky contamination,
the wet demister can trap these materials.
The pad begins to foul.
A more open vane separator or staged system may provide better lifecycle performance.
The selection should consider what the condensate carries with it.
Drain Capacity Must Match Peak Condensation
The separator vessel and drain system need enough capacity to remove condensate continuously.
If the drain is undersized or blocked:
- liquid level rises;
- disengagement space falls;
- demister loading increases.
A separator can therefore show liquid carryover because the vessel's condensate-removal system is inadequate.
How to Estimate the Separator Duty
A thermal/process calculation can estimate how much vapor condenses between:
- cooler inlet;
- outlet.
Useful inputs include:
- gas composition;
- flow;
- inlet temperature;
- outlet temperature;
- pressure.
This gives the bulk condensate estimate.
Droplet-separation design then requires additional consideration of:
- vessel geometry;
- expected mist distribution.
What Should Be Included in an RFQ?
Useful data includes:
- gas flow at cooler outlet conditions;
- pressure;
- outlet temperature;
- gas composition;
- expected condensate rate;
- condensate properties;
- solids or oil content;
- vessel diameter;
- downstream carryover requirement.
The flow should be stated at the actual post-cooler condition.
Final Engineering Perspective
Gas coolers and aftercoolers create mist by changing phase equilibrium.
The separator downstream is therefore handling liquid that may not have existed upstream.
Reliable design requires coordination between cooler duty, condensate generation, bulk-liquid separation, mist polishing, vessel drainage, and downstream temperature profile.
The separator should be located and sized around where the liquid actually forms.