3. Any liquid will produce froth or foam if gas is
introduced into the liquid faster than the two phases
disengage.
The “disengagement”, or “break time” differentiates
froths from foams in amine treating processes.
The free liquid will drain from around the gas bubble
until the gas pressure inside the bubble is greater than
the liquid wall’s surface or interfacial surface tension.
At this point the bubble will either break or combine
with other surrounding bubbles (coarsening)
4. • Amine process towers with trayed internals are
designed to produce froth by violently bubbling
gas through the treating solution contained on
the trays.
• The bubble walls in the froth act as mass transfer
area for the removal of the gas contaminants.
• The froth bubbles break quickly enough for the
treating liquid to pass down to the next tray in
the column before the gas flow is restricted.
5. How does froth turn into foam?
If anything chemical or environmental increases
the gas/liquid disengagement time, the froth
will remain in the vapor space of the tray, and
the gas flow will be restricted.
This hydraulic restriction is detected as a
differential pressure
This condition is described
as solution foaming
6.
7.
8. How does froth turn into foam?
• Increases in surfactant species and
concentration chemically stabilize foams.
• Increases in solution viscosity inhibit drainage;
therefore increase stability.
• Small solid particles increase solution viscosity.
• In the case of amines, higher solution
concentration favors stability.
9. How does froth turn into foam?
• Surface active chemical compounds
(surfactants) naturally adsorb to gas/liquid
interfaces due to polar and non-polar
chemical groups at each end of the molecule.
See figure 2&3.
• Surfactants reduce the surface tension, and
increase the elasticity of the amine
solution/gas interface; thereby producing a
more stable interface than an uncontaminated
amine solution of the same chemistry and
strength
10. Figure 2 Surfactant molecules stabilize froth into foam by coating the gas – liquid interface
13. How does froth turn into foam?
Surfactants may enter recirculating amine
systems in two ways:
•By amine degradation, i.e., organic acids.
•With fluids like feed gas, make up water, and
chemical additives
14.
15. Surfactant concentration
This common scenario is played
out in amine plants every day,
control room operator asks
outside operator to inject a
chemical antifoam.
Note:
Adding antifoam to the system
broke the foam, but had no
effect on the surfactants
dissolved in the solution.
If the foaming tendency of the
solution increase , the solution
will foam again.
An over injection of AFA can
affect the towers ability to
generate froth mass transfer
area.
16. Filter change out
Changing filters affects solution
foaming in both positively and
negatively.
First, solids increase the
solution’s tendency to foam by
inhibiting liquid from draining
from the foam structure.
Mechanical filters can also make
antifoams less effective by
coalescing the small AFA
droplets into larger ones, or
removing it completely from the
flow stream.
17. Solution strength
1. Lower strength solutions
can increase the solution
solvency which can increase
its surfactant concentration
which favors foaming.
2. Higher strength solutions
have higher viscosities which
inhibit foam drainage, and
therefore favor foaming.
Activated carbon adsorbs dissolved
organic molecules, insoluble
antifoam and antifoam-solids
agglomerates.
Activated carbon change out
18.
19. Antifoam -WHAT IS IT?
1. Non-polar oils, like minerals, silicones
2. Polar oils, such as fatty alcohols, fatty
acids, alkyl amines, alkyl amides
3. Hydrophobic solids, i.e., treated silica,
aluminum oxide, polypropylene
Antifoams are intended to facilitate gas and
liquid disengagement by weakening the cell
structure of the bubbles
20. The most common antifoam types in gas
processing plant:
1. Silicones, mixtures of non-polar siloxane oils
and hydrophobic silica solids,
2. Glycols or high molecular weight alcohols,
mixtures of polar oils and other high molecular
weight surfactants.
21. The silicone family of antifoams have
unquestioned antifoaming ability, but there
have been serious concerns with their use in gas
processing systems:
1. Their tendency to exhaust in service.
2. Their potential impact on heat transfer and
corrosion inhibitor function due to their
surface wetting or coating tendency
23. Overdosing silicone antifoam has gained the
reputation of actually creating foam.
In fact, studies show that at
elevated concentrations the
antifoam gets tied up in
micelle structures in the
bulk solution. Once tied up
in micelles, it can no longer
spread on the interface,
form lenses, and be
incorporated in the foam’s
structure.
24. Factors that affect antifoam performance
1. Solubility – Most antifoams exhibit extremely
low solubility in aqueous solutions.
2. Droplet size – The entry force required to
allow the antifoam droplet to enter the bubble
wall increases as antifoam droplets become
smaller.
3. Presence of hydrophobic solids – Liquid/solid
mixtures are usually more effective than either
component used alone.
25. 4. Environmental shear – Some antifoams are
inactivated due to solid/liquid separation, and
antifoam droplets being too small to bridge the
bubble wall.
Easy to de-foaming Hard to de-foaming
Can’t bridge the
bubble wall
Factors that affect antifoam performance
26. Factors that affect antifoam performance
5. Repeated exposure to foaming –some
antifoams gradually lose their antifoaming
ability due to the separation of hydrophobic
solids and/or reduced droplet size ( see no.4)
6. Surfactant concentration – Higher surfactant
concentrations tend to reduce antifoam
effectiveness
7. Dissolved salt species and concentration –
The presence of high valence metal ions
reduces antifoam effectiveness.
27. Why does antifoam appear to
become less effective over time?
• Activated carbon and tight mechanical filters
can remove antifoam from the amine stream
• Antifoams, being surfactants themselves
One recent study that included antifoam as a
surfactant showed a 55% reduction in MDEA
efficiency.
• AFA droplets will coalesce with each other as
they touch, particularly at high concentrations
28. • Antifoams are also known to become ineffective
due to agglomeration with suspended solids.
These antifoam/solids agglomerates not only
inactivate antifoam, but also prematurely plug
activated carbon beds and mechanical filters.
29. Removal of Foam Causing Agents
from Amine Systems
The Sigma Pure™ process controls amine
foaming by taking advantage of the fact that the
causative agents of foaming are incorporated in
the foam’s structure. Thus, if the amine is made
to foam and the foam separated from the
solution, the contaminants that caused or
enhanced the foaming are also removed.
