High Efficiency Carbon Capture Technology Utilizing Advanced Micro-Structured Surfaces

Solvent-based absorption systems appear to have the greatest potential for post-combustion carbon capture for the foreseeable future. The most commonly used solvents are amine-based solutions. A major focus area of numerous research groups is to minimize the energy required to regenerate the rich amine solution. This is of particular significance to thermal power plants, wherein lowered energy generation efficiency will result in a higher cost of electricity. At the same time, it is essential to utilize a solvent that has good reaction characteristics with carbon dioxide. Apart from the solvent, the other aspect of carbon capture that requires consideration is the gas-liquid contacting equipment. Common designs include bubble columns, concurrent packed columns, countercurrent packed columns, horizontal and coiled tube reactor and vertical tube reactor. The system design influences the gas-liquid interfacial area and mass transfer coefficient. Microscale systems have been known to improve transport phenomena in various processes. The inherent high surface area to volume ratio of a microchannel can substantially enhance the mass transfer performance, thereby yielding significant reductions in the overall equipment size. Additionally, the increased interfacial area can potentially permit the use of less reactive liquid solvents that have low regeneration energy requirements. This paper reports a review of efforts undertaken at the University of Maryland to test the performance of microchannel technology for post-combustion carbon capture. The absorption of carbon dioxide mixed with nitrogen into aqueous diethanolamine was investigated. The first stage of testing was performed with a single microchannel having various values of hydraulic diameter. The channel length was also varied in order to study the effect of residence time. This was subsequently scaled up to a multiple channel reactor that was fabricated and tested. While both these stages only tested the absorption performance, the next step involved developing a bench-scale setup that performed simultaneous absorption and regeneration processes. The performance of the reactor was characterized with respect to the absorption efficiency and mass transfer coefficient. Various regimes including slug, slug-annular, annular and churn flow patterns were observed and mapped with respect to the gas and liquid phase superficial velocities. High levels of absorption efficiency, close to 100%, were observed under optimum operating conditions. The mass transfer coefficient was found to be enhanced at reduced channel lengths and the same was attributed to the improved utilization of the absorption capacity of the amine solution for a given reactor volume. Reduced channel diameters were also found to enhance the absorption performance. The liquid-side volumetric mass transfer coefficients obtained for 254 and 508 mm channels were on an average 256% and 53.2% higher than that obtained with a 762 mm channel. On the whole, mass transfer coefficients as high as 620 s^-1 were achieved, and this is between 2-3 orders of magnitude higher than that associated with most conventional technologies. The same was considered an indication of the high level of process intensification and enhancement that can be achieved by the use of microscale technologies for gas-liquid absorption applications such as carbon capture. Further, sequential transitioning during the testing phase from single channel to multiple channel systems demonstrated the scalability of the technology. Future work will focus on improving and optimizing the reactor design using innovative fluid feed systems that result in enhanced convective mixing and modest levels of pressure drop. Additionally, state-of-the-art liquid solvents will also be tested with respect to their absorption and regeneration performances.

Keywords: Carbon capture, microchannel, microscale, diethanolamine, mass transfer coefficient