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 drum's diameter), use Souders-Brown equation:

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

where:
Vmax = maximum 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)

BEGIN QUOTE:
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. Typically use one-half of the above K values for approximate sizing of vertical separators without mesh pads.
3. For compressor suction scrubbers and expander inlet separators, multiply K by 0.7 – 0.8 (END QUOTE)

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 section 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.