Knockout drums with demisters

A knockout drum or demister separator may be either a vertical or a horizontal vessel. A vertical vessel is generally preferred because its efficiency does not vary with the liquid level. Alternatively, a horizontal... 

Knockout drums handling gas with particulates are usually better off without a demister, unless there is a provision to backwash the demister. The demister must be made out of a corrosion-resistant material. In many refinery services, a high-molybdenum-content (high-moly) stainless steel is sufficient. 

Cyclones (Figure 4-3). These are robust and not susceptible to fouling or wax. Multicyclones are compact and are reasonably efficient for foam, but not for slugs. The efficiency of a cyclone decreases with increasing diameter, and cyclones are not applicable above 5 ft. diameter. They possess a good efficiency for smaller flow rates. However, cyclones are expensive to operate, especially under vacuum conditions, because they have a higher pressure drop than either a knockout drum or a demister separator. 

Knockout drums equipjjed with demisters, according to what is written in other books, can tolerate K values 30 to 50 p>ercent higher than can ordinary KO drums. More importantly, demisters definitely do scrub out much smaller droplets from a flowing gas stream than could settle out by gravity settling alone. The most common example... 

Knockout drums handling gas with particulates are usually better off without a demister, unless there is a provision to back-wash the demister. 

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