Foaming in Amine Units - part 1

-A foam is basically a dispersion of gas within a liquid.

Foam consists of bubbles, i.e. gas with entrained liquid.

-Foams form when bubbles rise to the surface of a liquid and persist without coalescing ,or without rupture.

-A foam has been defined as having a certain height (>10cm) and a certain collapse time( >7 Sec.).

Cause of Foaming

-Presence of Surface active agent, called surfactant.

-Lack of filtration package efficiency.

-Presence of liquid hydrocarbon in amine solvent.

-Amine degradation products, impurities.

-Formation of Heat stable salts.

Excess dosing or low dosing of anti-foam.

-High concentration of solvent.

-Higher solvent and / or gas flow.

-Design fault of the equipment.

What is Surface active agent?

-Surfactants could be soluble or insoluble and were surface active,i.e.,lowered solutions surface tension at extremely low concentrations.

-Common surfactants are : carboxylic acids, sulfonates, sulfates and some alcohols.

-Source of Surfactants: well treatment chemicals, corrosion inhibitors, amine degradation products, liquid hydrocarbon, make-up water, etc.

Cause of amine degradation

 -High temperature in reboiler.

-Presence of oxygen in the solvent. Antifoam tank and MDEA tank are potential source of oxygen ingress.

-Carboxylic acids ( oxalic, formic, acetic) can be formed in presence of oxygen.

-Presence of Primary or Secondary amines in the MDEA solution.

Formation of heat stable salts(HSS)

-Heat stable salts such as oxalate, acetate, thiocyanate, sulfate and chloride decrease the amine solutions acid-gas carrying capacity.

-HSS are not regenerated in the steam stripping and left in the amine solvent .

-Presence of HSS make solvent high viscous and a potential source of foaming.

-It also cause of high corrosion problems in amine system.

Effects of foaming

-Poor mass transfer in amine contractor.

-A layer formation in the vapor- liquid interface which will reduce the contact are of vapor – liquid.

- Loss of solvent by carry - over.

-Higher acid gas content of contractor outlet.

-Reduction in absorption capacity due to higher pressure drop and poor gas/liquid contact. Liquid flow instability and loss of columns liquid levels.

-Hydraulic flooding in column. Circulating liquid restrictions.

Symptoms of foaming

•  Unexplained solvent loss.
•  Unexplained low level in contractor.
•  High foaming tendency and high break time.
•  Higher contractor diff. pressure.
•  Liquid HC in the Regen. Overhead cooler or reflux sample.
•  Unexplained low flash drum level.
•  Increased liquid level in the contractor overhead knockout drum.
•  Decreasing contractor diff. pressure with the addition of small volume of antifoam.
• Constant gas pockets in the rich solvents filters and/or activated carbon vessel.
• Solvent carry over to the fuel gas system.

Why contractor diff. pressure increase?

-When surfactants concentration increase in the solvent, a film form in the vapor- liquid interface.

If the film is stable enough it will persists and form a foam plug that will carryover into the next higher tray, or create a restriction that floods the tower at that point.

When foam is stable enough it will fill the entire vapor space above the tray and restrict both gas and liquid flow.

This restriction to flow causes an increase in tower diff. pressure.

Where foam can form

 -Foam can form anywhere gas and liquid coexist in a turbulent environment.

-Overhead side of the mist eliminator, where surfactants will concentrate and carry with clean solvent. Normally this liquid is reintroduced in the contractor ,cause foaming. System differential pressure at these points would be low , making difficult to identify as foaming source.

-Flash drum: Where pressure is reduced to flash dissolved gas extremely small bubbles are formed in the solvent.

-Stripping tower have a lower tendency of foaming due to higher temperature and lower pressure. At higher temperature and lower pressure reduce the solvent’s surface tension which reduce the filming tendency of the surfactants.

Types of Anti-Foam 

 Three types of antifoam used in chemical industries:

 1-Silicon based Polydimethylsiloxane ( PDMS) - can overdosage but cause foaming.

 2-Polyglycols.

 3-Polyalcohols( long chain alcohols).

Polyglycol can work better and less prone to over dosage.

Long chain alcohols were sometimes effects but need to dosage continuous ( 100 ppm level).

Polyglycol nor polyalcohol have no tendency to cause foaming by itself.

A combination of polyglycol and silicon based antifoam gives better result.

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