CO2 Absorption and Desorption Simulation with Aspen HYSYS

Abstract

The last years it has been an increasing global interest to reduce emissions of greenhouse gases to the atmosphere. One of the most important greenhouse gases is CO2. To reduce CO2 emissions carbon capture and storage (CCS) is the most realistic approach. With today’s technology absorption by an amine solution is the most developed and applicable method for post-combustion CO2 capture. But this technology is very energy demanding. To reduce the energy demand this technology must be optimized to realize this process as a beneficial method for large scale CO2 capture.

This thesis considers three different configurations for absorption by an amine mixture aimed to reduce the energy demand. The different configurations are the standard absorption process, a vapour recompression and a lean split with vapour recompression. Aspen HYSYS has been used as the simulation tool. To compare the different models equally the CO2 removal efficiency was kept at 85% and the minimum temperature approach in the lean/rich heat exchanger was 5K. Kent-Eisenberg was used as the thermodynamic model for the aqueous amine solution and Peng-Robinson for the vapour phase.

All configurations were evaluated due to the energy cost. The lean split with vapour recompression had the lowest energy cost with 81 MNOK/year. However, the vapour recompression had only a slightly higher cost equal to 85 MNOK/year. The standard absorption process was simulated to have an energy cost of 120 MNOK/year. At these values 1.15 M ton CO2/year are removed.

A capital cost estimation of the configurations has also been conducted. This capital cost estimation has considered equipment, engineering and installation cost. The standard absorption process was estimated to have the lowest capital cost by 514 MNOK. The two other modifications were more expensive. The biggest difference was due to the extra compressor. The lean split with vapour recompression had a cost of 768 MNOK, while the vapour recompression had a cost of 832 MNOK.

Some sensitivity calculations have also been conducted, especially for the vapour recompression. Under these conditions the following parameter values were optimal: CO2 removal efficiency of 84-86%, flash tank pressure at 110-120 kPa, 14-16 stages in the absorption column.

More research should be done to verify values due to uncertainties in the models and cost estimates.