Foaming in Acid Gas Removal Units: Operations Challenge, Diagnosis, and Remedies

by Mourad BOUMOSBAH

The foaming in acid gas removal units (AGRU) is a recurring operational challenge. In this critical process in LNG production, we remove corrosive and toxic components like hydrogen sulfide (H2S) and carbon dioxide (CO2) from the natural gas feed. This article will focus on what causes foaming, how to diagnose it, and what you can do to remedy it.

Understanding the Foaming Phenomenon:

Foaming in AGRUs occurs when the amine solution, the heart of the process, forms a stable layer of bubbles that inhibits efficient gas-liquid contact.

This can happen for a few reasons:

• Contaminants: Liquid hydrocarbons: Think of these as surfactants – they reduce surface tension and make it easier for bubbles to form. Heat stable salts (HSS): These are byproducts of amine degradation, and they increase viscosity, making the foam stick around longer. Solids: Fine particles like iron sulfide or rust can act as nucleation sites for foam. Well chemicals: Corrosion inhibitors and other additives can introduce surfactants.

• Operating conditions: High amine concentration: A thicker amine solution is more prone to foaming, as it's harder for the bubbles to drain. High gas flow rates: More gas means more bubbles, and more opportunity for foaming. Lean amine temperature: If the lean amine is too hot, it can lead to increased amine degradation and HSS formation.

Diagnosing Foaming:

Now that you know what causes foaming, how do you know if it's happening in your unit? Here are some telltale signs:

• Increased differential pressure across the absorber column: The foam acts like a barrier, making it harder for gas to flow through.

• Liquid level fluctuations: Foam displaces liquid, leading to erratic level readings.

• Amine carryover: Foam can entrain amine solution into downstream equipment.

• Reduced sweet gas production: Inefficient gas-liquid contact reduces the AGRU's efficiency, as a result the H2S & CO2 breakthrough.

• Increased antifoam consumption: Repeated antifoam injections may indicate a persistent problem, and the antifoam itself if it is accumulated can cause foaming.

Remedies for Foaming:

Addressing foaming requires a multi-pronged approach:

1. Contamination control : Effective filtration: Install and maintain high-quality filters to remove solids and hydrocarbons, generally, between 10% to 20% of the flow is filtered to avoid contaminants accumulation in the amine inventory. Activated carbon beds: Use carbon beds to adsorb organic contaminants and reduce surfactant levels. Amine reclaiming: Regularly reclaim amine solution to remove HSS and other contaminants. Minimize well chemical ingress: Use appropriate well treatment practices to minimize contaminants.

2. Operational optimization: Control amine concentration: Maintain the optimal amine concentration for efficient operation. Adjust gas flow rates: Optimize flow rates to minimize bubble generation. Monitor lean amine temperature: Maintain the recommended temperature range to minimize degradation.

3. Antifoam use: Use with caution: While antifoam can temporarily break the foam, it does not address the root cause. Select appropriate antifoam: Use a compatible antifoam and ensure proper dosing.

4. SigmaPure™ process: Consider advanced technologies: Technologies like the SigmaPure™ process can remove foam-causing agents from the amine circulation, providing long-term control.  

Conclusion:

Foaming in AGRUs is a common operational challenge but can be effectively addressed with proper understanding, diagnosis, and a combination of preventive measures and corrective actions. By implementing the above strategies, LNG industry workers can ensure the reliable and efficient operation of AGRUs, minimizing operational downtime, optimizing production, and ensuring safe plant operations.