Preventing foaming in acid gas removal units

Operators of amine-based acid gas removal units have been preventing foaming, a longstanding problem that causes loss of capacity, by working with Shell Global Solutions to adopt global best practices.

“When foaming occurs in the acid gas removal unit, it causes solvent to carry over into the downstream units, which can cause a trip,” says Guillaume Fournier, Process Engineer, Gas Processing, Shell Global Solutions. “Sometimes, an operator may choose to mitigate the foaming by reducing the gas throughput. Either way, it affects the plant’s bottom line, so there is considerable interest in preventing it.”

Foaming is caused by the presence of foampromoting impurities in the amine system. Liquid hydrocarbons, particulates, oil, grease, corrosion inhibitors and degradation products can all initiate it. In fact, anything that influences the solvent’s surface tension is a possible contributor to foaming. However, it should not be confused with flooding constraints caused by increased throughput or operating conditions that deviate from the original design.

“We have been working with operators around the world on this issue and compiling the lessons learned,” Fournier explains. “We have licensed more than 700 amine plants, so we have developed an extensive knowledge base.”

This information has enabled Shell Global Solutions to develop a set of guidelines that it follows in its design process and a set of start-up and operating best practices that help to guard against foaming.

“In order to give fit-for-purpose recommendations, we need detailed information on the assets and their specific operating conditions. But in general terms, keeping the upstream unit’s operation steady to prevent carryover of liquid contaminants can often help, as can frequent skimming of the liquid hydrocarbons. Likewise, maintaining the correct water content in the solvent may help some customers, as the foaming tendency increases with a higher water level.”

Antifoaming agents can be effective, but, says Fournier, in some circumstances, they may also cause foaming. “Generally, they should be used in small quantities and only in response to specific episodes. It may not be advisable to use them continuously or as a preventive measure. Moreover, we often suggest that operators should only apply the approved antifoaming product in the operating manual, and that they test its suitability with the plant solvent in the laboratory before adding it to a system.”

Shell Global Solutions has also devised foaming incident response procedures. The recommended response depends on the severity of the incident, Fournier says. “Again, it depends on the operator’s specific circumstance but for a minor incident, we might recommend the injection of an antifoaming agent into the liquid stream to the column.

For major incidents, reducing the gas load, either the feed gas to the absorber or the heat into the reboiler, or both, can be effective in certain situations.” He adds, “In principle, foaming can happen in any amine unit at any time, but there are ways to reduce its likelihood and to mitigate its impact when it does occur.”

Five key steps that may help to avoid foaming

• Keep the unit clean, both before start-up and during operation.
• Monitor and maintain solvent cleanliness, and take measures to improve solvent quality when it is required.
• Prevent contaminants from entering the unit by using the appropriate mechanical devices and active carbon filters, and maintain them well.
• Check the antifoaming agent in the laboratory before adding it to the system.
• Operate within the unit’s design parameters and avoid rapid changes in feed gas flow, temperature and composition.