(687f) Optimal Design of Solvent-Based Post-Combustion CO2 Capture Plants Using Phase-Change Solvents

Solvent-based CO₂ capture using chemical absorption is a well-established and mature technology with broad applications in the industrial sector. However, as the cost introduced for solvent regeneration is rather high, resent research efforts focus on the identification of alternative solvents as well as flowsheet modifications that can reduce the energetic demands of the overall process without compromising the efficiency of the capture plant. The use of new phase-change fluids as alternative CO₂ capture solvents appears appealing and with a high potential for energy reduction [1]. Biphasic solvents include lipophilic amines which exhibit a liquid–liquid phase separation upon heating. As a result two liquid phases emerge including a concentrated organic phase containing the largest part of the amine solvent as well as the chemically bound CO₂ and an aqueous phase comprised mainly of water which can be easily separated. The non-thermal extraction of the aqueous phase prior to desorption using decanter drums enables stripping of the rich solution at much lower temperature than the 120 ^oC used in conventional regeneration. The low regeneration temperature of often less than 90 ^oC, together with the high cyclic CO₂ loading capacity considerably reduce the energetic desorption requirements [2]. Furthermore, stripper operation is enabled at elevated pressure, considerably increasing the desorption efficiency. However, as the number of phases may vary within the process due to thermodynamic conditions, tracking of the phase boundary is essential to determine the existence of the second liquid phase. This is challenging because the existence of the third phase causes discontinuities due to the alternating number of phases and the subsequent change in the model structure.

To address the above challenges we combine a phase boundary tracking method with the approximating properties of orthogonal collocation on finite elements (OCFE) model [3, 4] formulation to develop an accurate and reliable process model for three-phase reactive separation columns. Phase boundary tracking is greatly facilitated by the OCFE formulation as it is approached with the help of an efficient phase stability algorithm at the finite element breakpoints. The adaptive placement of the element breakpoint on the phase transition boundary enables the use of suitable modules that account explicitly for all phases that appear at certain regions within the column. Therefore, the associated mathematical discontinuity is properly handled and the model accuracy is greatly improved.

The energetic gains obtained from the use of a phase change solvent are further enhanced by the consideration of systematic structural and operating modifications imposed on a reference absorption/desorption flowsheet. Such modifications are realized with the help of a generalized process design framework [5]. These modifications include a plethora of choices such as a number of unconventional stream topologies, use of intercooler circuits, cascades of desorption columns as well as multi-pressure operation in the regeneration step. The employed module-based framework encompasses “building blocks” emulating the operation of basic processes, including chemical reactions, mass and heat exchange, as well as stream mixing and splitting. The location of equipment such as side-coolers or liquid-liquid separators as well as the existence and location of potential side feed/draw streams is implicitly determined through design optimization in the sense of minimizing the total process cost.

The proposed developments are illustrated considering an aqueous solution of hexylamine as the phase change solvent [2] in a case study addressing the effective treatment of an industry-derived CO₂-containing flue gas stream. A number of unconventional process flowsheets aiming towards the maximization of the driving forces within the separation columns is employed and used in design optimization studies. Vapor-liquid-liquid data are obtained for the hexylamine-water-CO₂ mixtures using the statistical associating fluid theory for potentials of variable range (SAFT-VR) [6].

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] Svendsen H.F., Hessen E.T., Mejdell T. (2011). Carbon dioxide capture by absorption, challenges and possibilitites. Chem. Eng. J., 171, 718-724.

[2] Zhang J., Qiao Y., Agar D.W. (2012). Intensification of low temperature thermomorphic biphasic amine solvent regeneration for CO2 capture. Chem. Eng. Res. Des., 90, 743–749.

[3] Swartz C.L.E., Stewart W.E. (1987). Finite-element steady state simulation of multiphase distillation. AIChE J., 33, 1977.

[4] Damartzis T., Seferlis P., (2010). Optimal Design of Staged Three-Phase Reactive Distillation Columns Using Nonequilibrium and Orthogonal Collocation Models. Ind. Eng. Chem. Res., 49(7), 3275–3285.

 [5] Damartzis T., Papadopoulos, A.I., Seferlis P. (2014). Optimum synthesis of solvent-based post-combustion CO₂ capture flowsheets through a generalized modeling framework. Clean. Techn. Environ. Policy, 16(7), 1363-1380.

 [6] Mac Dowell N., Pereira F.E., Llovell F., Blas F.J., Adjiman C.S., Jackson G., Galindo A., (2011). Transferable SAFT-VR models for the calculation of the fluid phase equilibria in reactive mixtures of carbon dioxide, water, and n-alkylamines in the context of carbon capture, J. Phys. Chem. B, 115, 8155-8168.