Air emissions management

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:

Inlet temperature
25°C
Feed Flow
10 e3m3/d
CO2 in Feed
6 mole%
H2S in Feed
0.01 mole%
Amine Circulation Rate
15 USGPM

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.



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