(801d) Simulation of a Phase Change System for CO2 Capture

Chemical absorption is the most studied and applied process for carbon capture. However, it is yet an expensive process, mainly due to the regeneration section. Monoethanolamine (MEA) is still the benchmark solvent for CO₂ capture; nevertheless, new solvents and plant design could reduce the operational costs and make the process cheaper and industrially feasible. For instance, Rochelle et al. (2011) showed a new plant configuration that uses a two-stage flash to regenerate an aqueous piperazine (PZ) solution loaded with CO₂.

Following the trend of finding new solvents or solvent blends, an aqueous blend of Diethylaminoethanol (DEEA) and N-methyl-1,3-propanediamine (MAPA) at certain concentrations, herein called the blended system, was found to split into two liquid phases upon CO₂ loading. The upper phase is rich in DEEA and lean in CO₂ while the lower phase is rich in MAPA, CO₂ and water. Preliminary screening tests showed a high initial CO₂ absorption rate and a high capacity for a mixture of 5 M DEEA and 2 M MAPA.

Liebenthal et al. (2012) showed a simulation of a post combustion CO₂ capture process that uses the blended DEEA/MAPA system as a solvent in an integrated overall process. The results of the simulations showed a potential for reducing the reboiler temperature and duty compared to MEA. Moreover, the system was able to operate at higher desorber pressures than the conventional process which can benefit the compression section.

A simulation of a full capture process based on a blended system is presented in this work. The model was implemented in CO2SIM, an in-house SINTEF/NTNU simulator for CO₂ capture and a full pilot plant was simulated. The column modules are rate-based. This implementation was successfully applied in Knuutila et al. (2011) and Kvamsdal et al. (2011) where more information on the implementation of CO2SIM and CO2SIM dynamics can be found.

The model is based on VLE, kinetics and physical property data from the laboratory. The equilibrium behavior of the systems is based on a developed “soft” model which calculates total pressure and partial pressure of CO₂ as a function of temperature and loading. The model was validated by experimental data.  A similar correlation was presented by Brúder et al. (2011). The “soft” model approach is a fast way to evaluate novel solvent systems and determine the best operational conditions for pilot plants (Kvamsdal et al., 2011).

To validate the simulation, a pilot campaign in the NTNU facilities was run with the blended system. The pilot was previously used for testing single phase amine and amino acids solutions (Aronu et al., 2010; Knuutila et al., 2011; Tobiesen et al., 2007). However, after minor changes, the plant was able to run the phase change system.

Since the pilot operates in a closed setup, after the injection of a desired amount of CO₂, the stripped CO₂ from the desorber was used as feed gas while the reboiler duty was varied. Additional injections of CO₂ were done so the amount of CO₂, and therefore, the loading was varied during the campaign.

Samples from the lean amine entering the absorber, rich amine samples after the separator and lean amine after the reboiler were withdrawn. After being centrifuged, to speed up the separation between the phases, the samples were analyzed for amine and CO₂ content. Additionally, the volume ratios (upper phase/lower phase) were measured.

The campaign was able to generate overall mass transfer coefficients values(Kg overall), CO₂ capture efficiencies, temperatures and pressure profiles for both the absorber and the desorber in addition to the heat duties required.

In this work, the model implemented in CO2SIM, after validation with the existing experimental data, was used to simulate a complete industrial plant for CO₂ capture. The model will be compared to experimental data for heat requirement, absorption rates and temperature profiles in the absorber and stripper over different operational conditions. Based on the validation energy consumption of a full scale plant will be simulated and the main advantages and limitations of blended, two phase systems are discussed. Moreover, the best estimated operational conditions will be presented and compared with a 30 weight % MEA campaign.

References

Aronu, U.E., Svendsen, H.F., Hoff, K.A., Knuutila, H., 2010. Pilot Plant Study of 3-(methylamino)Propylamine Sarcosine for Post-combustion CO₂ Capture, in: Farid, B., Fadwa, E. (Eds.), Proceedings of the 2nd Annual Gas Processing Symposium. Elsevier, Amsterdam, pp. 339-348.

Brúder, P., Grimstvedt, A., Mejdell, T., Svendsen, H.F., 2011. CO₂ capture into aqueous solutions of piperazine activated 2-amino-2-methyl-1-propanol. Chemical Engineering Science 66, 6193-6198.

Knuutila, H., Aronu, U.E., Kvamsdal, H.M., Chikukwa, A., 2011. Post combustion CO₂ capture with an amino acid salt. Energy Procedia 4, 1550-1557.

Kvamsdal, H.M., Haugen, G., Svendsen, H.F., Tobiesen, A., Mangalapally, H., Hartono, A., Mejdell, T., 2011. Modelling and simulation of the Esbjerg pilot plant using the Cesar 1 solvent. Energy Procedia 4, 1644-1651.

Liebenthal, U., Pinto, D.D.D., Monteiro, J.G.M.-S., Svendsen, H.F., Kather, A., 2012. Overall Process Analysis and Optimisation for CO₂ Capture from Coal Fired Power Plants based on Phase Change Solvents Forming Two Liquid Phases, GHGT11. Energy Procedia, Kyoto, Japan.

Rochelle, G., Chen, E., Freeman, S., Van Wagener, D., Xu, Q., Voice, A., 2011. Aqueous piperazine as the new standard for CO2 capture technology. Chemical Engineering Journal 171, 725-733.

Tobiesen, F.A., Svendsen, H.F., Juliussen, O., 2007. Experimental validation of a rigorous absorber model for CO₂ postcombustion capture. AIChE Journal 53, 846-865.