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The Marangoni effects also have an influence on heat-
and mass-transfer processes. The induced convection in
the liquid increases gradients and therefore enhances
transfer rates. Reviews on this subject are available.7
2.3. Foaming in Columns. Numerous foaming case
histories of columns have been reported in the literature
(see work by Kister4). These suggest the following
symptoms for foaming problems: (1) premature flooding
and massive entrainment [a foaming condition or ag-
gravation of it is indicated by (a) a sudden increase in
the differential pressure, (b) a differential pressure
exceeding 50% of the tray spacing, or exceeding 8 10
mbar/m for packed beds, (c) an erratic differential
pressure]; (2) nonreproducible pressure drop measure-
ments or flood points; (3) beginning of flooding from
steady-state conditions for no apparent reason; (4) flood
problem sensitive to temperature; (5) antifoam addition
leading to an increase in throughput; (6) abnormal
temperature profiles due to reactions occurring on
higher stages of the column (e.g., in amine absorbers).
Important for this work is the fact that foams can be
wall-stabilized (cellular foams) as reported by Kister4
for small and pilot-size columns as used in Chapter 4.
If this type of foam occurs, the transferability on
industrial-scale columns is questionable.
The reasons for foaming can be numerous and some-
times astonishing. A few chemicals that can be respon-
sible for foaming are given by Kister:
4 high-molecular-
weight organic solvents, corrosion inhibitors, reaction
products of solvents and materials of filters or similar
equipment, finely suspended solids, oils or greases, and
even leached-off additives from plastic packings.
3. Experimental Assessment of Foam Stability
Assessment of foamability in a small and easy to use
test cell can be done prior to experiments in the pilot
plant to obtain a general rough estimate of the likeliness
of foaming in the column. Different test methods are
reported in the literature, which have been applied with
various success. To obtain significant results, the test
should be made at operating conditions (pressure,
temperature, and composition). However, this will
always be a compromise between effort and detailed
replication of operating conditions.
Two different types of methods are given in the
literature: dynamic methods and static methods.In
dynamic methods, the foam is in a state of dynamic
equilibrium between rates of formation and collapse. In
static methods, the foam is once formed and then
allowed to collapse. No regeneration by input of me-
chanical energy is done. Static methods are used for
foams of high stability, whereas dynamic methods are
applicable for foams with a low foamability.8
A simple and easy to apply method is the bottle shake
test. A closed bottle is strongly shaken up and down and
then set on a table. The foam height and the time taken
for the foam to collapse are measured. Foaminess4 is
indicated with settling times greater 5 s.
3.1. Experimental Procedure. A preliminary as-
sessment of the questioned solution is done with the
“bottle shake test”. Short settling times in the range of
10 s are observed, suggesting that the solution has only
small foaminess behavior. A more detailed dynamic
method is chosen to obtain hydrodynamic mechanisms
similar to those of the real column and because of the
low foamability of the chemical system of the AS-
circulation scrubbing. In this case a method proposed
by Bikerman3 is used. In this pneumatic method, the
foam height is measured while nitrogen flows through
a sintered frit producing small bubbles in the liquid