(572a) Improving the Energy Efficiency of Carbon Capture Processes – Combining Enzyme Accelerated Solvent Systems and Improved Contacting Equipment

In order to achieve the ambitious targets specified by the United Nations Framework Convention on Climate Change in 2011 and reduce GHG emission in the EU by 2020 at least 20% below those in 1990 carbon dioxide (CO₂) emissions have to be reduced significantly. Fossil-fired power plants are identified as major targets for carbon capture processes, since they are responsible for 40 % of the total global CO₂ emissions (Mondal et al., 2012). Efficient post-combustion capture of CO₂is one of the most promising options to face this crucial challenge and to achieve at least the specified short-term targets (Rochelle, 2009).

As a result, a wide range of different fluid separation processes, including (reactive) absorption, adsorption, cryogenic distillation and gas permeation, have been intensively studied in recent years (Mathias et al., 2013). Nevertheless, the current state-of-the-art process for CO₂ capture is the reactive absorption in packed columns using amines. Especially primary amines, like Monoethanolamine (MEA), are often used because of the covalent bonding of the CO₂, resulting in high absorption rates (Kenig & Gorak, 2005). However, the recovery of the solvent, which is required for a continuous operation, becomes more challenging the stronger the CO₂is bound (Yildirim et al., 2015).

Therefore, the best absorption process will represent an optimal trade-off between fast absorption rates and energy efficient solvent recovery. The latter causes up to 80 % of the total capture costs for the MEA process and results in a significant loss in overall power plant efficiency (Notz et al., 2011). Consequently, solvent recovery represents the bottleneck of the reactive absorption/desorption process, which is practically not considered for large scale industrial application (Mondal et al., 2012). In order to overcome this restriction a lot of research effort is carried out with a focus on more energy efficient solvents. While tertiary amines or potassium carbonate require less energy in the regeneration step, application of these solvents is restricted by their relatively low absorption rate (Kunze et al., 2015).

Process intensification (PI) can help to overcome this restriction in different ways, which are investigated in this study. In order to compensate for the low absorption rates of tertiary amines enzymes are integrated as efficient biocatalysts to the solvent system. Here, especially the enzyme Carbonic Anhydrase (CA) is considered as the fastest catalyst for the hydration of CO₂, which is the prevailing CO₂consuming reaction in tertiary amine systems (Gundersen et al., 2014). A significant enhancement of the absorption performance was observed in previous studies with a wetted wall column set-up, validating the direct influence on the reaction rate (Kunze et al., 2015). However, besides the possible improvement, the application of carbonic anhydrase as a biocatalyst also introduces additional restrictions to the process due to sensitivity towards temperature and pH (Gundersen et al., 2014).

In addition to an intensification of the solvent system, combining high capacity solvents with the efficient biocatalyst, the mass transfer can be further improved by means of intensified contacting devices (ICD), other than making use of conventional columns. The use of a membrane contactor provides a well-defined interfacial area, with packing densities being a magnitude higher as in conventional equipment (Zhao et al., 2016). Due to the fully separated liquid and vapour flow, membrane contactors also allow operational modes different from conventional equipment, which are limited by flooding or loading. The modular nature and operational flexibility make membrane contactors a promising alternative to conventional equipment. However, mass transfer resistance might be increased due to the membrane and the operational window of the membrane contactor is limited as well, such that especially transmembrane pressure differences have to be controlled in order to avoid breakthrough of one of the two phases, which will lead to undesired phase dispersion.

Another ICD that can improve mass transfer in ab- and desorption is the rotating packed bed (RPB) technology. In RPB processes centrifugal forces are exploited to increase acceleration of the liquid. In a counter-currently operated RPB, gas is pushed through the torus-shaped rotating packing from the outside to the middle of the rotor. Liquid is sprayed into the eye of the rotor and flows counter-currently to the gas outwards, driven by the centrifugal forces. RPBs offer mass transfer improvements due to high turbulences and extend the operating window, having the rotational speed as an additional degree of freedom. These advantages result in high capacity combined with compactness (Sudhoff et al., 2015). Yet, application of RPBs is still restricted due to lack of reliable data on the performance efficiency.

While both, ICD and the use of an improved solvent system with CA as catalyst, have the potential to significantly intensify the CO₂capture process, literature studies and research on the combination of both is scarce. In order to investigate if the advantages of the different PI techniques can be exploited and if a combination is feasible, first investigations are performed in order to characterize the operating windows of such intensified processes and to evaluate the potential benefits for an actual implementation. In preliminary work (Kunze et al., 2015), the tertiary amine Methyldiethanolamine (MDEA) has shown promising results to be more energy-efficient compared to MEA and is therefore chosen as solvent (30 wt.-%) for this work. Absorption experiments in a column and the different ICD are performed to determine the operating windows and compare separation efficiency. Subsequently CA is added to the solvent system in order to investigate the potential synergetic intensification of ICD and CA versus the acceleration by CA in a conventional packed column set-up. The results enable the discrimination between the different means for process intensification depending on the determined operating windows and separation performances. Finally, the obtained results are evaluated and benchmarked against the current state-of-the-art process on a theoretical basis. Future work will also investigate the application of the different means for process intensification to the desorption step in order to provide a complete picture for a potential industrial application.

REFERENCES

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Kenig, E.Y., Górak, A., 2005, Integrated Chemical Processes: Synthesis, Operation, Analysis, and Control, pp. 265-311.

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ACKNOWLEDGEMENT

The research leading to these results has received funding from the European Union Seventh Framework Programme FP7/2007-2013 under grant agreement n° 608535.