Figure P-3 will be useful as a guide in selecting a satisfactory type of mist…
Vertical Knockout Drum – Vane Mist Eliminators
Vane Mist eliminators (sometimes referred as chevron design mist eliminators) are best used when liquid loading is high and/or the liquid is viscous or has fouling characteristics (such as paraffins, hydrates). They are not recommended for use as the sole mist elimination device for compressor suction service due to their high particle size cut-off point. The expected particle size cutoff (98% efficiency) is about 15 micron, resulting in a considerable amount of aerosols carried through with the gas.
The ability of the vane mist eliminator to handle large volumes of liquid at relatively high vapor velocity is one of its key benefits. As long as the actual vapor velocity is restricted to proper design K values, reasonable liquid scrubbing efficiency is achieved with minimum pressure drop and fouling.
When determining a vessel diameter for vane mist eliminators, the actual velocity is calculated using the same Souders Brown equation previously discussed in Section P1.2, Determining Vessel Diameter. The recommended K values for vane mist eliminators in vertical vessels are:
• Minimum K value = 0.12
• Maximum K value = 0.54
• K value 0.60 and above = Re-entrainment
A maximum K value of 0.54 is recommended for vertical vessels. This is approximately 120% of design rate to protect against carryover during upsets and unusual operating conditions. A minimum K value of 0.12 is a turndown limitation to prevent an insufficient velocity condition such that aerosols tend to flow around the vanes in laminar gas flows, as opposed to impinging on the vane surfaces.
Another benefit of the vane mist eliminator is its stout physical structure. In corrosive services, a wire mesh pad can be relatively short lived compared to a vane mist eliminator. In this circumstance, a combination of wire mesh and vanes can be of further benefit. In either case, a conservative corrosion allowance is recommended when separating corrosive liquid/gas vapors.
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