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Category: Equipment Design
Question: How do I design a vapor-liquid separator or a flash drum?
Keywords: knockoutpotdesignvaporliquidseparator
Answer: The size of a vapor-liquid separator should be dictated by the anticipated flow rate of vapor and liquid from the vessel. The following sizing methodology is based on the assumption that those flow rates are known.Use a vertical pressure vessel with a length-to-diameter ratio of about 3 to 4, and size the vessel to provide about 5 minutes of liquid inventory between the normal liquid level and the bottom of the vessel (with the normal liquid level being at about the vessel's half-full level).For the maximum vapor velocity (which will set the vessel's diameter), use this equation:Vmax = (k) [ (dL - dV) / dV ]0.5where:Vmax = maximum vapor velocity, ft/secdL = liquid density, lb/ft3dV = vapor density, lb/ft3k = 0.35 (when the vessel includes a de-entraining section)The vessel should have a vapor outlet at the top, liquid outlet at the bottom, and feed inlet at somewhat above the half-full level. At the vapor outlet, provide a de-entraining mesh section within the vessel such that the vapor must pass through that mesh before it can leave the vessel. Depending upon how much liquid flow you expect, the liquid outlet line should probably have a level control valve.As for the mechanical design of the vessel (i.e., materials of construction, wall thickness, corrosion allowance, etc.), use the same methodology as for any pressure vessel.Also see the following references:1. "Design Two-Phase Separators Within the Right Limits", W.Y. Svrcek and W. D. Monnery, Chemical Engineering Progress, October 19932. "Successfully Specify Three-Phase Separators", same authors, Chemical Engineering Progress, September, 1994.3. Vapor-Liquid Separator Design Spreadsheet (Linked below)
Links: ChE Plus Subscribers - Vertical-Liquid Separator Sizing





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