Carbon dioxide capture using phase changing solvents - A comparison with state-of-the-art MEA technologies
Examensarbete för masterexamen
Innovative and sustainable chemical engineering (MPISC), MSc
In order to combat climate change Carbon Capture and Storage (CCS) has been suggested as an important tool to reduce the emissions of the potent greenhouse gas carbon dioxide (CO2). CCS can be used for large point source emissions of CO2, like power plants using fossil fuels, where it removes CO2 from the flue gases. This is most commonly done through the use of chemical absorption of CO2 in an amine solvent called monoethanoloamine (MEA). After the absorption the solvent is regenerated and the captured CO2 is released in a stripping column, a process which is both expensive and energy consuming. The energy used for the regeneration reduces the output from the power plants imposing an energy penalty of up to almost 30%. Reducing the energy requirement in the carbon capture process is crucial in order to make CCS commercially viable, and one possible way to do this is to use so-called phase changing solvents. This new family of molecules are amines that in the presence of water and CO2 exhibit a triple phase vapour-liquid-liquid equilibrium. The liquid mixture separates into two liquid phases where one has a high CO2 and amine content while the other phase consists mainly of water. The two liquids can be separated without the addition of energy in a decanter so that only a part of the flow is sent to the stripper. This opens up new possibilities for the design of absorption/desorption flowsheets for solvent based CCS that may reduce the total energy requirement of the process. Creating flowsheets that can be used for future reference and identifying important aspects and challenges of simulating CCS processes with phase-changing solvents is of importance. Apart from this the aim of this project is to achieve a regeneration energy requirement below 2.0 GJ/- tonne CO2 captured and reduce the operating cost by 80% compared to an MEA reference process. The project consisted of three parts where the first part was a literature study to identify and select solvents that exhibit the desired phase changing properties followed by property estimation of these solvents in Aspen Plus®. The estimated properties were also validated using reference values. Secondly, different flowsheets were created identifying different layout possibilities and a sensitivity analysis was performed investigating the impact of the biggest uncertainties in assumptions made as well as differences in operating conditions. The third part was an economic and environmental assessment of the process and a comparison with an MEA reference process. In a previous study ten potential phase changing solvents, referred to as D1-D10, had been identified. From this list the solvents that were chosen for this project v were (3-[1-(dimethylamino)propan-2-yl]aminopropyl)dimethylamine, called D6, and 2-[2-(methylamino)ethyl]aminoethan-1-ol, called D9, based on their phase equilibrium. Of these two solvents, D9 overall showed the most promising results. A base case flowsheet layout and two different variations were considered, where both variations reduced the energy demand compared to the base case. Flowsheet Layout 1 where the decanter placement was changed from after to before the heat exchangers showed the largest decrease in energy consumption. The best result was thus obtained from D9 using Layout 1 which has an operating cost of 30.10 EUR/tCO2 captured, a reduction of over 37% compared to the MEA process. The regeneration energy demand for this process was 1.46 GJ/tCO2 captured, which is well below the target. The three environmental indicators used (Cumulative Energy Demand, Global Warming Potential and ReCiPe) all point to the processes using D9 being more environmentally friendly than MEA. The results for D6 are not as promising as for D9, which is mostly due to the high reboiler duty of the D6 processes. When using flowsheet Layout 1 the reboiler duty is decreased also for D6 bringing all environmental metrics to levels lower than for MEA, although not as low as D9. However the operating cost is still high which is due to the high price of D6 offsetting the benefits of reduced heat demand.
Energi , Hållbar utveckling , Energiteknik , Energy , Sustainable Development , Energy Engineering