In every process involving contact between liquid and flowing gas, tiny mist droplets are carried away with the gas. This phenomenon is called entrainment.
Beginning about 1947, special devices were developed to remove mist from gas streams. Now known as mist eliminators, or demister pads (Demister is a registered trademark of koch-Glitsch) these devices provide a large surface area in a small volume to collect liquid without substantially impeding gas flow. Unlike filters, which hold particles indefinitely, mist eliminators coalesce (merge) fine droplets and allow the liquid to drain away. Gas typically flows upward through a horizontal mist eliminator.
More recently, advances in technology have enabled substantial progress in mist eliminator designs, materials, and application expertise. New products and methods of use have been found highly effective for many purposes, especially the following:
• Increasing throughput
• Downsizing new vessels
• Improving product purity
• Cutting operating costs
• Reducing environmental pollution
• Reducing downstream corrosion
• Increasing recovery of valuable liquids
In today’s era of higher expectations of mist eliminators, achieving such benefits requires better knowledge on the part of users. It is no longer adequate for a designer simply to indicate “mist eliminator” in a drawing. The results will depend on proper specification of mist eliminator type (or combination of types), orientation, thickness, internal details, support and spacing in the vessel, vapor velocity and flow pattern, and many other considerations.
Despite the advances that have been made, mist eliminator and demister pads (Demister is a registered trademark of koch-Glitsch) specification is still as much art as science. For all but the most experienced users, proper application depends on consultation with a manufacturer’s engineers. Such help should be considered for every new mist eliminator application as well as every upgrade or debottlenecking of existing applications.
TO MAKE THE MOST of a mist eliminator investment, the designer should become familiar with the considerations and possibilities involved. Due to the numerous variables involved in specifying mist eliminators, designers and purchasers should consult with Amistco’s separation specialists before making a final decision.