[ gas-liquid separation ] to accumulate additional liquid under surge conditions.
Regarding green hydrogen applications, there is often a design compromise that ensures sufficient volume for reliable electrolyte recycle flow, but with a minimised vessel footprint. allowable liquid carryover and the separator orientation. Towler and Sinnott have specified K = 0.07 for approximate sizing calculations when densities are given in kg / m ³ and maximum vapour velocity is in m / s. If a demister pad is not installed, a safety factor of 0.15 should also be applied to allow for surges in flow.³
The disengagement of entrained liquid droplets from the gas stream occurs due to gravity. If the gravity force and the drag force acting upon the droplet are equated, then the terminal settling velocity, U T
, can be calculated for a specified droplet size as follows:²
Further experimentally derived values for K are available based upon the target droplet size for separation. For example, the American Petroleum Institute provides values in Table 1, which should allow all liquid droplets larger than 10 microns to be separated by gravity, with a wire mesh mist extractor fitted. 4
For a vertical separator, the minimum allowable diameter that will slow the gas sufficiently for the droplets to settle out is then calculated from U max by:
If U V
< U T
, then the liquid droplets( and those heavier) will settle out. However, in practice, predicting the droplet size and drag coefficient for a system can be difficult.² Therefore, the maximum gas velocity in the disengagement section can be approximated using empirically determined relationships. Once calculated, the maximum gas velocity defines the minimum vessel diameter for effective separation.¹
The vessel height is then specified to provide the required liquid holdup volume, accounting for the location of inlet and outlet nozzles.
For a horizontal separator, the cross-sectional area for disengagement is the vapour headspace of the tank, which is typically taken as 20 % of the vessel volume:
Where U max is the maximum vapour velocity, ρ L and ρ v are the liquid and vapour densities, and
K is the separator sizing factor. This expression is commonly referred to as the Souders-Brown equation. The value of K is dependent upon the
The length of the vessel is then determined from the required holdup volume and the calculated minimum diameter:¹
Table 1. Separator sizing K-factors from API 12J, 8th Edition, 2008( Table C. 1) converted to SI units 4 Separator orientation Height or length( m) Typical K range( m / s)
Vertical
1.52 3.05
Horizontal 3.05
0.037 – 0.073 0.055 – 0.107
0.122 – 0.152 0.122 – 0.152 ×( L / 3.05) 0. 56
Hydrogen Tech World | Issue 22 | June 2025 31