Mechanisms of Phase Separation for Dispersions in Continuous Flow

Abstract

The literature pertaining to droplet hydrodynamics and
coalescence affecting the separation of primary liquid-liquid
dispersions has been reviewed.

The behaviour of droplets forming a flocculation zone has
been studied in specially constructed 3, 6 and 9 inch diameter
vertical glass gravity settlers. Characteristics of flocculation
zones, namely hold-up aid the variation of drop sizes and
residence times, have been determined and correlated with zone
heights for 4 different systems.

Difficulty was encountered initially in obtaining
reproducible results due to the effects of interface scum and
colloidal impurities. This was attributable to system ageing
accelerated by sunlight and was minimised by blacking out the
transparent sections of the equipment.

Initial experiments were performed with the system
toluene-water to find the minimum practical settler diameter.
Significant differences, due to wall effects, were observed in
columns below 6 inches in diameter and therefore work was not
pursued in the 3 inch column. The effect of countercurrent |
phase flow on zone height was found to be insignificant, hence subsequent work employed a stationary continuous phase.

Experimental techniques included high speed still and
normal speed cine photography and the injection of dyed droplets.
A novel attempt was also made to encapsulate drops in a close
packed swarm.

Flocculation zones were observed to comprise 3 distinct
sections i.e. an inlet section wherein droplets form a close
packed arrangement, a larger mid-section where inter—drop
coalescence and droplet distortion occurs, and an exit section
bounded by the interface. The differences in vertical hold-up
profiles between these sections was measured. Total zone
height has been correlated by equations of the form,

H=a.VdbDpcd12e

Deviations occurred with drop sizes in the lower end of the
range studied, i.e. 0.7 mm to 1.0 mm.

A flocculation zone model is postulated leading to a
relationship between drop residence time and hold-up which is in
agreenent with the experinental results. Consideration is also
given to conditions under which a modified fluidised bed model
is applicable.

Date of Award	1972
Original language	English