Heat Duty, Cyclic Capacity and Rate of Desorption for MDEA Activated by Novel Polyamines for Natural Gas Sweetening

Abstract

This study focused on finding suitable activators to be blended with aqueous MDEA for use in natural gas sweetening. Polyamines are promising amines for capture of CO2 efficiently as they contain more than one amino group in their structures. Polyethylenimine (PEI-B) and Tetraethylenepentamine (TEPA) were chosen and screened to investigate their performance as efficient activators for MDEA. PEI-B and TEPA were compared with piperazine (PZ) which is currently used as the standard activator of MDEA. The screening experiments were carried out using a simple absorption and desorption setup at atmospheric pressure. This setup was validated using 5 M MEA with 100 mol% CO2 and the equilibrium rich loading was in a good agreement with the literature (Shen and Li, 1992) with a deviation of 5%. Single polyamines (PZ, TEPA and PEI-B) were tested at dilute concentration (0.01 M) using 100 mol% CO2 to investigate their performance at low viscosity in liquid phase as well as at zero resistance in the gas phase. The absorption and desorption temperatures were 40 and 100 ◦C respectively. Also, the individual single polyamines were tested at higher concentrations (0.1, 0.3 and 0.5 M) using a feed gas with 50% CO2 content (balance is N2) to investigate their capacity at higher viscosity and higher gas resistance in the liquid and gas phases, respectively. Moreover, the activators (PZ, TEPA and PEI-B) were blended with MDEA and screened using 100 mol% CO2. In MDEA blends, the concentration of MDEA was kept constant at 3 M (34 wt%) whereas the concentration of the activators was varied as 0.1, 0.2 and 0.3 M. The reproducibility of the results in the screening set up was within 1%. The screening experiments focused on absorption and desorption parameters which include rich loading, lean loading, cyclic capacity, desorption rate and heat duty. In the results of screening experiments, PEI-B performed the best as an individual amine as well as in blends with MDEA as compared with TEPA or PZ in all absorption and desorption parameters. Also, PEI-B gave the lowest heat duty using individual single solutions and in blends with MDEA as compared with TEPA or PZ. Furthermore, MDEA blends with PZ, TEPA and PEI-B were also investigated in a bench-scale pilot plant. The pilot plant was validated using 5 M MEA at 15 mol% CO2 and the rich loading was compared with the standard rich loading of 5 M MEA. The deviation obtained was 3%. Two blending ratios of 0.1 M/3 M and 0.3 M/3 M (activator/MDEA) were chosen to be used in the small pilot plant studies. The CO2 contents of 20, 50, 70 and 100% were applied in the pilot plant. The absorber column was operated at a pressure of 1 bar while the regenerator as well as the reboiler were operated at 2 bar. The results also showed a superior performance of PEI-B blends in all absorption and desorption parameters followed by TEPA blends and then PZ blends. The reproducibility of the experimental results obtained from the pilot plant was within 8%. PEI-B blends displayed the lowest heat duty among all tested blends. Furthermore, the NMR analysis was used to investigate the possible products at different CO2 loadings in 0.1 M PEI-B and 0.1 M PZ single amines as well as in blends with 3 M MDEA. Also, a heat duty model was developed based on the results obtained from the pilot plant and the structural properties of the amines. The model showed an acceptable average absolute deviation (7%). Moreover, a preliminary cost analysis was performed to evaluate the total cost (capital and operating costs) for PZ blends and PEI-B blends at 20, 50 and 70 mol% CO2. This economic analysis showed that PEI-B blends exhibited lower total costs than PZ blends.