Foam prevention in amine gas treating (gas sweetening)

Raw natural gas and biogas cannot be used immediately after extraction, but must first undergo a refining process. Among other substances, the gases contain hydrogen sulfide (H₂S) and carbon dioxide (CO₂), which are referred to as acidic gases because of the formation of weak acids in aqueous solution. These must be removed from the gas mixture for several reasons:

• They accelerate the corrosion of pipelines and other plant components.
• They reduce the efficiency of heat energy generation.
• When H₂S is combusted, more acidic sulfur oxides are formed, which pose a significant environmental hazard. These substances must also be removed from flue gases produced by fossil fuels or waste incineration (flue-gas desulfurization).

To remove the gases CO₂ and H₂S, they are absorbed in a process known as amine gas treating in an aqueous solution of various amine compounds. Because the gas is no longer chemically acidic after passing through this step, the process is also known as amine sweetening. The elaborate procedure is carried out cyclically to prepare the amines for reuse and to isolate the previously bound acidic gases.

The long-established process may play an even more important role in the near future if it is used on a large scale in the capture and storage of CO₂ from industrial combustion processes of all kinds.

Foaming – a side effect with consequences

Foaming severely impairs the amine treating process and has significant negative effects, ranging from serious operational disruptions to a temporary complete shutdown of the plant. Pure amine solutions, however, do not tend to foam. Foam is only formed when they are contaminated with higher-quality hydrocarbons and their compounds. These impurities reduce the surface tension of the liquid due to a layer on the surface, which is the fundamental cause of foaming.

Different sources of contamination, different effects

Aminetreating is a counter-current process, with the gas flowing upwards in a tower while the absorbent solution flows downwards over a series of permeable trays. If the gas is the source of the contamination, e.g. due to insufficient filtering, the foam will initially form at the bottom of the tower and then swell from tray to tray, continuing to contaminate the solution as it rises. At least this is noticeable when monitoring the pressure along the tower, so there is still time to react before the foam reaches the top. However, the quality of the neutralized gas may suffer before this, since H₂S in particular is less bound by a foaming amine solution.

A pre-contamination of the amine solution flowing down can even be more serious. In this case, the foaming starts further up in the tower and it can quickly lead to a flashover of the mixture (amine carryover) into other parts of the plant. This not only means a significant disruption of the process, but can even endanger people who are exposed to the possibly leaking hazardous substances.

Ironically, an often unrecognized source of contamination lies with the antifoam agents added to the amine mixture. If these are added in too high a dosage, they can stabilize foam lamellae instead of destroying them as intended. This is a classic “solution as a problem”, because the seemingly obvious measure of adding more of the agent only makes the situation worse.

Targeted foam analysis to tackle the cause

Laboratory-scale investigations are useful for identifying foam formation in advance, rather than just counteracting it during the process. Within the framework of meaningful measurements, the process of amine treating can be simulated realistically and the extent of foam formation can be recorded with solid figures.

Measuring foamability

Many plant operators already use manual foamability measurements for process monitoring. However, the reproducibility of such tests often leaves room for improvement, because not only the specified parameters, in particular the gas pressure, are adjusted manually. The foam height is also usually read visually and the decay time is measured using a stopwatch. This is problematic because the time span between acceptable and overly long decay times is often in a range of a few seconds.

When foamability is analyzed using a Dynamic Foam Analyzer measuring instrument, the gas passes through a filter bottom with a defined pore size into the liquid contained in a standardized glass measuring column. The volume flow is electronically controlled and can be precisely specified over a wide range. An LED bar and an opposing row of photodetectors measure the foam height over time along the column with high resolution. The reproducibility of the recorded and digitally stored measurement curves is excellent.

Instead of foaming with air as a standard procedure, an external gas connection can be used and measurements can also be taken at the actual process temperatures in order to transfer the results to the conditions during amine treating as closely as possible. 

Trials designed accordingly reveal the source of the foaming:

• Foaming of the amine mixture intended for the process with clean gas to test the liquid for contamination. In many cases, tests with air are sufficient to detect the tendency to foam formation.
• Foaming of a clean model mixture with the actual gas mixture to detect surface-active impurities in the gas.

This makes it possible to determine whether and to what extent the components involved – the incoming gas or the absorbing solution – are responsible for the foaming in each case.

Evaluation of foam decay

Whether and to what extent foam becomes a disruptive factor depends not only on its formation, but also on its decay dynamics. In the case of very unstable foams, an equilibrium often occurs between formation and decay, so that the amount of foam no longer increases. The longer the foam lives, the greater the risk of accumulation, which spreads from tray to tray in the tower. This risk increases with rising gas pressure.

In the foam analyses described, the foam height is recorded with high temporal resolution even after the end of the foaming phase, and the decay dynamics are characterized with meaningful key figures, such as the decay half-life. The question of whether and how quickly the amount of foam is increasing can also be answered by cyclic measurements – a programmed sequence of foam and decay measurements.

Overall, foam analysis offers a wide range of options for detecting the causes of foam formation and preventing it.