Separation of Phases

Vapor + Liquid

Vapor-Liquid Separation

• Vapor and liquid volumes from thermodynamics

• Remove liquid drops from vapor, vapor bubbles from liquid

• Avoid foaming

• Key processes:– Gravity settling– Impingement

Gas Liquid Separators

Vessel volume = time forsettling

Mesh pads = impingement

Gravity Settling

V

gD

Ct

p l g

g

4

3

'

Terminal settling velocity of droplets in vapor:

C’=f(Re)

Souders-Brown Approximation

V Kt

l g

g

K = factor for sizing separatorsHorizontal 0.12-0.15 m/sVertical 0.05 to 0.11 m/sUlrich recommends K=0.06 m/s- use 1/2 of these values when no demisters

Approximation for 125 m droplets:

Liquid/Liquid Separations

• Called decanters or coalescers

• Low terminal velocities due to small • Coalescence of droplets is important

• Typically design from time and L/D

• Horizontal to allow maximum interfacial area

• e.g. Hydrocarbon water - 3-5 min

Lecture Problem

Gas-liquid and liquid-liquid separators are very common in the process industries. These vessels are given many different names, but they all operate on similar principles of gravity-driven fluid separation.

What do the following vessels do? How much of each fluid phase (as a fraction of vessel volume) would be present in each at normal operation?Compressor knock-out drumSteam drum API SeparatorReflux accumulator Slug catcherSteam trap Flash tank

Fluid-Fluid Separators