(554f) Performance Assessment of Solvent-Based CO2 Capture Processes: Design of Complex Flowsheets with Different Solvents

Targeted process modifications on conventional process design have shown a positive impact on the efficiency of the classic absorption-stripping loop used in solvent-based post-combustion CO₂ capture processes. These modifications can either be structural, with the main focus on the redistribution of the material and energy streams to the separation columns used in the process [1], and operational, referring to the conditions imposed on the unit operation to meet the design specifications. Selection of the most appropriate solvent for the CO₂ capture [2] sets the fundamental structural and operational constraints for the process system. Solvent selection is usually associated with the need for increased CO₂ capture capacity as well as the low energy demand for regeneration in the stripping column. Furthermore, solvent selection guides the generation of flowsheet alternative configurations aiming at ensuring the maintenance of strong average driving forces within the unit operations (absorption/desorption, heat exchange), thus leading to a highly intensified operation and ultimately to a reduction in the overall process investment and operational costs.

This work focuses on the investigation, development and evaluation of several combinations of amine solvents and alternative flowsheet structures and for the efficient separation of CO₂ from a flue gas stream. The developed flowsheets are generated using a generalized design framework which incorporates a robust and compact modeling scheme that accounts for all the physicochemical phenomena taking place within the process, coupled with a nonlinear optimization algorithm [3]. Evaluation is performed on the basis of minimization of a suitable objective function representing the total annualized cost of the process. The proposed flowsheet configurations have been chosen for their capabilities to exploit the connectivity possibilities of the process streams in an effort to enhance the main driving forces in the separation columns and thus, to intensify the process itself by the appropriate combination and redistribution of existing and external process streams (material, heat) as well as process units (separators, heat exchangers, etc.). Alternative column configurations are implemented through material redistribution achieved with the aid of multiple feed side-streams and product draw streams. Similarly, energy distribution in the system is imposed through the incorporation of appropriate heat exchange units such as reboilers and heat exchange equipment for heat injection or removal in suitably selected recycled streams. Furthermore, the manipulation of the operating conditions such as column pressure mandates the inclusion of vapor compression units in order to favorably adjust the vapor-liquid equilibrium and other properties of the process streams. An example of a proposed design involves an absorption column of enhanced performance where the cooling of the liquid phase is implemented using a number of coolers at selected points in the column. In this way, a fraction of the liquid phase is by-passed through a heat exchanger, cooled to a target temperature and returned to the column. This way, not only the chemical equilibrium of the exothermic reactions is improved, allowing for the more efficient capture of CO₂, but the physical absorption in the aqueous solution is enhanced as well. Similar flowsheets additionally including different stream topologies and cascades of desorption columns, to name a few, are among the investigated options of potentially improved performance.

The proposed developments are implemented in the optimal design of a CO₂ capture process for a quicklime plant. Such plants are characterized by high CO₂ concentration flue gas streams due to CO₂ generated from the calcination process in addition to CO₂ generated from fuel combustion, ranking amongst the highest industrial CO₂ emitters. A number of commercial solvents, namely monoethanolamine (MEA), diethanolamine (DEA) and 2-amino-2-methyl-1-propanol (AMP), each one representing a class of solvents (primary, secondary and sterically hindered amines respectively) have been used for the evaluation of the generated flowsheets. The selected solvents exhibit a wide range of physicochemical properties (i.e. absorption capacity, boiling point, heat of absorption/regeneration etc.) which is expected to diversify their overall performance in each flowsheet. Furthermore, through the successful evaluation of each flowsheet-solvent combination, the interaction between the process and solvent on the design level is revealed.

Acknowledgements

The authors would like to thank Prof. Claire Adjiman, Prof. Amparo Galindo, Prof. George Jackson, and Dr. Alexandros Chremos, of Imperial College London for providing the SAFT-VR thermodynamic models. Funding from the European Commission under grant FP7-ENERGY-2011-1-282789-CAPSOL is gratefully acknowledged.

Cited References

[1] Ahn H., Luberti M., Liu Z, Brandani S., 2013, Process configuration studies of the amine capture process for coal-fired plants, International Journal of Greenhouse Gas Control, 16:29-40.

[2] Neveux T., Le Moullec Y., Corriou J.P., Favre E., 2013, Energy performance of CO₂ capture processes: Interaction between process design and solvent, Chemical Engineering Transactions, 35:337-342.

[3] Damartzis T., Papadopoulos A.I., Seferlis P., 2014, Optimum synthesis of solvent-based post-combustion CO₂ capture flowsheets through a generalized modeling framework, Clean Technologies and Environmental Policy, DOI: 10.1007/s10098-014-0747-2.