Development of a System of MEA Solutions Activated by Novel Poyamines for Natural Gas Sweetening Mass Transfer in Absorber and Desorber Columns

Abstract

This study focused on finding new and efficient activators to be blended with methyldiethanolamine (MDEA) for natural gas processing. Polyamines were highlighted in the open literature to have high CO2 loading as well as high absorption rate. As branched polyethyleneimine (PEI-B) (MW ≈ 800 with almost 16 amino groups) showed a superior performance in adsorption applications for capturing CO2, it was important to test its performance in absorption process. Tetraethylenepentamine (TEPA) is another polyamine (five amino groups) that was indicated in both adsorption and absorption as an excellent solvent; therefore, it was included in this study. PEI-B and TEPA were tested individually and as activators in MDEA blends. Furthermore, the performance of PEI-B and TEPA was evaluated against the performance of the standard activator of MDEA which is piperazine (PZ). Theses activators were screened using a simple absorption and desorption set up. In the screening experiments, the absorption and desorption temperatures were 40 and 100 ○C respectively. A dilute concentration for single polyamine (0.01 M) was applied at 100 mol% CO2 to investigate the maximum absorption rate at low viscosity and resistance in liquid and gas phases respectively. Also, higher amine concentrations of 0.1, 0.3, and 0.5 M were tested at 50 mol% CO2 to investigate their performance when both the solution viscosity and the gas resistance increased. Furthermore, in the screening step, these activators were tested in MDEA blends. The concentration of MDEA was kept constant at 3 M (30 wt%) while the concentration of the activators (PZ, TEPA, PEI-B) was varied at 0.1, 0.2 and 0.3 M. The performance of these activators individually and in MDEA blends was evaluated in terms of rich loading, lean loading, cyclic capacity, absorption rate and desorption rate. The results showed that PEI-B gave the highest performance individually and in MDEA blends followed by TEPA while PZ exhibited the lowest efficiency in all absorption and desorption parameters. Moreover, MDEA blends with PZ, TEPA and PEI-B were also investigated in a small pilot plant. Two blending ratios (0.1 and 0.3 M activator to 3 M MDEA) were selected to be tested in the pilot plant. Four CO2 contents of 20, 50, 70 % (the balance is N2) as well as 100 mol% were used in the gas feed. Again, PEI-B blends showed the highest absorption and desorption efficiencies followed by TEPA blends and then PZ blends. Also, the overall gas mass transfer coefficient (KGav) as well as the overall liquid mass transfer coefficient (KLav) were calculated to represent the mass transfer of CO2 in absorber and in desorber respectively. In both trends of KGav and KLav, PEI-B blends were recorded the best followed by TEPA blends and then PZ blends while non-activated MDEA gave the lowest mass transfer coefficients. Two models were applied for KLav and for KGav to predict their experimental values. The experimental values of KLav and KGav showed good fit with the models with acceptable average absolute deviations of 4.1 and 14.5 %, respectively.