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20 Sep 2007

Design of vapor-liquid separators

A vapor-liquid separator drum is a vertical vessel into which a liquid and vapor mixture (or a flashing liquid) is fed and wherein the liquid is separated by gravity, falls to the bottom of the vessel, and is withdrawn. The vapor travels upward at a design velocity which minimizes the entrainment of any liquid droplets in the vapor as it exits the top of the vessel.

The size a vapor-liquid separator drum (or knock-out pot, or flash drum, or compressor suction drum) should be dictated by the anticipated flow rate of vapor and liquid from the drum. 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 somewhat below the vessel's half-full level).

Calculate the vessel diameter by the Souders-Brown equation to determine the maximum allowable vapor velocity:

V = (k) [ (dL - dV) /dV) ]^0.5

V = maximum allowable vapor velocity, m/sec
dL = liquid density, kg/m^3
dV = vapor density, kg/m^3
k = 0.107 m/s (when the drum includes a de-entraining mesh pad)

Then A, the cross-sectional area of the drum:

A, in m^2 = (vapor flow rate, in m^3/s) / (vapor velocity V, in m/s)

and D, the drum diameter:

D, in m = ( 4 A / 3.1416 )^0.5

The GPSA Engineering Data Book recommends the following k values for vertical drums with horizontal mesh pads (at the denoted operating pressures):

0 barg: 0.107 m/s
7 barg: 0.107 m/s
21 barg: 0.101 m/s
42 barg: 0.092 m/s
63 barg: 0.083 m/s
105 barg: 0.065 m/s

GPSA Notes:
1. K = 0.107 at 7 barg; subtract 0.003 for every 7 bar above 7 barg
2. For glycol or amine solutions, multiply above k values by 0.6 – 0.8.
3. Typically use one-half of the above k values for approximate sizing of vertical separators without mesh pads.
4. For compressor suction scrubbers and expander inlet separators, multiply k by 0.7 – 0.8

The drum 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 pad within the drum such that the vapor must pass through that mesh before it can leave the drum. Depending upon how much liquid flow you expect, the liquid outlet line should probably have a level control valve.

Below is a schematic diagram of typical vapor-liquid separator:


As for the mechanical design of the drum (i.e., materials of construction, wall thickness, corrosion allowance, etc.), use the same methodology as for any pressure vessel.

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