Back

Question

Submit an Answer

A CO2 removal unit with MDEA solvent has experienced severe foaming issue in the regenerator which has lead to solvent loss to the regenerator vent. When foaming happens in the regenerator, observed high/ fluctuating DP across the packed bed and wash water tray (bubble cap trays) on the regenerator section. On the absorber side, the DP maintain stable and no solvent carry over to downstream vessel during foaming incident. However, the foaming in regenrator has led to inefficient solvent regeneration and caused high CO2 breakthrough at the absorber overhead (treated gas).

The process gas to the absorber is mainly coming from the syngas produced from upstream Steam Methane Reformer unit with Natural Gas feed. The Process Gas to absorber mainly composed of CH4, CO, CO2 (7-8 mol%) and H2.

Analysis on the cause of the foaming incident in the MDEA regenerator is suspected due to solvent contamination with particulate matters as contamination due to long chain hydrocarbon is not possible in this process. MDEA solvent has been analyzed with the TSS has been observed between 3 - 10 mg/mL. The solvent appearance remains clear without any coloration which would indicate solvent degradation. Lab analysis has also shown low HSS and no signs of solvent degradation.

Hence, the way forward to avoid the foaming in the regenerator is to replace the filter element of the side stream filter from 10 micron nominal to 5 micron absolute. This is to ensure small particulate matters are sufficiently filtered during normal operation with 5 micron absolute filter.

However, would like to check if following parameters can also cause amine foaming inside the regenerator:
* Can over stripping from the regenerator reboiler caused turbulence and foaming especially at the rich amine inlet to the regenerator feed gallery?
* Rich amine at inlet of regenerator is located below of the wash water trays (bubble cap trays). Some amine would expected to be entrained with CO2 to the bubble cap trays. Should the antifoam be injected at the regenerator reflux line to break down the foam which could build up at the bubble cap tray section?

Appreciate your feedback/ thoughts on this.

Mar-2024

Answers

Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

Regarding the first question, I understand that over stripping is related to superheating of the steam fed to the reboiler of the regenerating tower. In this case, the main effect or risk is the thermal degradation of the amine, some references quote that the liquid temperature in the reboiler should be kept below 125 oC with a low pressure steam temperature close to 150 oC. I understand that a possible overheating can cause turbulence which favors the foam formation, but in this case the thermal degradation of the amine solution is a more significant risk. Furthermore, the stripping columns have a lower tendency of foam formation due to the operating conditions, the lower pressure and higher temperature which reduces the surface tension of the amine solution minimizing the filming tendency of the surfactants compounds.

Another key point to be verified is regarding the presence of oxygen in the amine solution which can lead to the formation of carboxylic acids raising the rate of chemical degradation of the amine solution, the antifoam and amine tanks can be a possible point of oxygen to the amine system.

About the second question, it seems that it can be a good strategy but it's necessary to consider if this will be an additional antifoam injection or if the point of antifoam injection will change for this point. In this case, it's possible to reach an over injection of antifoam which can cause more foam. Another key point which is not mentioned in the question is related to the operating temperature of the absorbing tower, the literature recommends that the temperature of the amine solution should be 5 to 10 oC above the inlet temperature of the gas. A very good article about this topic was published in the September 2007 Issue of PTQ Magazine - Control of foaming in amine systems, by Mr. Stephen A. von Phul and Mr. Arthur L. Cummings, which you can VIEW HERE

Mar-2024