Sizing with mist eliminator

IV.2.2 Sizing vapor/liquid separator with mist eliminator [2]... 

The reservoir may be either pressurized or atmospherie. It must have suffieient eapaeity to eontain all oil during drain-baek or shutdown. It must be equipped with an oil level indieator, a low-level alarm switeh, safety relief valve, a pump for oil makeup during operation, drain valve, heater, mist eliminator, strainers, and required valves. Expander reservoirs must be designed and eonstrueted in aeeordanee with applieable ASME eodes. Reservoir retention time is typieally between 5-18 min depending on turboexpander size and manufaeturer s sizing eriteria. This is an area where the owner/purehaser should ask for the manufaeturer s assistanee. 

Screens, mist eliminators, packing support and hold-down plates, bed limiters, and vortex breakers have the correct opening size. The author is familiar with one experience in which a mist eliminator with excessively large openings was installed and was ineffective for checking entrainment. 

One of the problems associated with sulfuric acid is its tendency to form mists. The liquid particles in mists are by definition 10 xm or less in diameter, and sulfuric acid mists range down into the submicron sizes. Particles larger than about 3 p,m are collected efficiently by inertial impact, but Brownian motion becomes the predominant mechanism below 1 p,m. Wire-mesh and other common impingement-type mist eliminators therefore lose their efficiency below 1 or 5 pm. 

The highest gas velocities and the best mist removal efficiencies are obtained with horizontal gas flow (vertically configured mist eliminator profiles). Removal of droplets down to 8 to IS microns in size is possible, although 20 microns is mote typical. The higher pressure drop associated with the high velocities used with this type mist eliminator increases operating cost. 

Vessel sized for liquid surge drum and vapor/liquid separation by gravity settling method can be used for final design. Vessel siz for vapor/liquid separation by mist eliminator method should be checked by the mist eliminator vendor to confirm its size. Vessel sized for Hquid/liquid separation by surge volume method or with coalescer pad is a rough preliminary sizing. Its size should be checked by vendor or an expert. 

The mechanical performance of a mist eliminator is measured by two curves the collection efficiency versus particle size, and the pressure drop versus the vapour load. To operate at high efficiency, a mist eliminator should have a fibre or mesh pad with a high surface area to volume ratio. 

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