Sweetening of sour gases with high-CO2 content using MDEA

For sour gas with high CO₂ content, Sulfinol [1] or similar proprietary solvents are typically used. Sulfinol is a mixture of Sulfolane (a physical solvent), water and either DIPA (Diisopropanolamine) or MDEA (with or without Piperazine, an accelerator). Sulfinol has a high CO₂ absorption capacity and has been designed for CO₂ removal. Generic MDEA (N-methyldiethanolamine) is typically only used for stripping H₂S gases from the sour gas based on selective absorption. MDEA is not typically used for gases with high-CO₂ content due its slow reaction rate with CO₂.

BRE [2] suggests that challenges for using MDEA for treating gases with high-CO₂ content could be overcome by using proper operating temperatures and designing the absorber trays to provide adequate residence time.

This article investigates the impact of varying lean amine temperature on CO₂ removal. Typically, lean amine temperature is kept 5°C above the inlet gas temperature to prevent hydrocarbon condensation and avoid the problem of a second liquid phase [4]. While this rule of thumb is useful, it can lead to poor CO₂ absorption when using MDEA. CO₂ reaction with MDEA is kinetically controlled such that the higher the temperature the faster the reaction rate. An operator could vary the lean amine temperature to achieve the desired CO₂ in the treated gas.

Case Study

 A sour gas feed with 6 mole% CO₂ is considered. The amine plant is to be designed to achieve the sales gas specification of less than 2 mole% CO₂. To study the impact of lean amine temperature on CO₂ removal a case study is performed using Aspen HYSYS by varying MDEA solution temperature from 25°C (same as the feed gas temperature) to 55°C.

The following table summarizes the feed conditions:

The results of this study are shown below:           

Figure 1: Contactor column performance with 6% CO₂ in the inlet

The results indicate how CO₂ removal in the amine absorber increases by increasing the lean amine temperature while increasing the H₂S pickup. The rule of thumb suggested a minimum 5°C approach, which is fine for the design of the trim cooler but could possibly lead to unnecessarily oversized equipment. Going against this rule leads to significant reduction in CO₂ in the treated gas. These results agree with results presented in [5].

However, there should be a limit on the higher lean amine temperature. Lean amine temperature should be limited to avoid corrosion issues [6], higher lean amine temperature also increases amine and water losses. In this case, amine losses increase 2-3 times when increasing the MDEA temperature from 25°C to 55°C.   

The use of MDEA for bulk CO₂ removal is difficult and requires considering the impact of lean amine temperature, circulation rate, stream stripping, and liquid residence time on trays [2]. If deeper CO₂ cleaning is required, the best solution is to switch the solvent from MDEA to one of the solvents designed specifically for CO₂ removal rather than using generic MDEA.

References

• Shell Global Solutions Technology Portfolio  https://www.shell.com/content/dam/shell-new/global/downloads/pdf/interactive-gas-processing-portfolio-final.pdf
• Bullin, J and Polasek, J., “The Use of MDEA and Mixtures of Amines for Bulk CO₂ Removal”  https://www.bre.com/PDF/The-Use-of-MDEA-and-Mixtures-of-Amines-for-Bulk-CO2-Removal.pdf
• Addington, L. and Ness, C., “An Evaluation of General “Rules of Thumb” in Amine Sweetening Unit Design and Operation”,  https://www.bre.com/PDF/An-Evaluation-of-General-Rules-of-Thumb-in-Amine-Sweetening-Unit-Design-and-Operation.pdf
• Arnold K. and M. Stewart, Design of Gas Handling Systems and Facilities, Surface Production Operations, Volume 2, 2nd Ed., Gulf Publishing, Houston, Texas 1989
• Lunsford, K. and J.A. Bullin, “Optimization of Amine Sweetening Units”, Proceedings of the 1996 AIChE Spring National Meeting, New York, NY.
• Gas Processors Suppliers Association (GPSA) Engineering Data Book, 2017, 14^th Edition, Section 21 Hydrocarbon Treating