Design of liquid-vapor separator or flash drum or knockout drum

[mbeychok]

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(or a flashing liquid) is fed and wherein the liquid is separated by gravity, falls to the bottom of the vessel, and is withdrawn.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 at about the vessel's half-full level).

 

For the maximum design vapor velocity (which will set the drum's diameter), use the Souders-Brown equation:

 

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

 

where:

Vmax = maximum design vapor velocity, ft/sec

dL = liquid density, lb/ft3

dV = vapor density, lb/ft3

k = 0.35 (when the drum includes a de-entraining mesh pad)

 

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

 

0 psig: 0.35

300 psig: 0.33

600 psig: 0.30

900 psig: 0.27

1500 psig: 0.21

 

GPSA Notes:

1. K = 0.35 at 100 psig; subtract 0.01 for every 100 psi above 100 psig

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 or 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.

 

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.

[insane_alien]

w00t someone else who knows something about chemical engineering. btw you should probably use SI units as these are used everywhere except america cos they're arrogant and want to screw up everyones calculations by introducing conversion factors.