(681a) Generalized Modeling Framework for the Optimal Synthesis of Solvent-Based Post-Combustion CO2 Capture Processes
Solvent-based chemical absorption/desorption systems are the most mature among industrial CO2 capture applications because the technology is well established, the capture conditions are relatively mild, and the process can be easily retrofitted onto existing plants. Despite this maturity, a major challenge hindering their widespread adoption in industry is the very high cost that is introduced to the operation of CO2 emitting plants, mainly due to the thermal demands for solvent regeneration. Reported research findings indicate that modifications in the structural and operating characteristics of conventional absorption/desorption flowsheets may result in significant energetic and efficiency improvements. Most reported CO2 capture process design approaches focus on the use of rigorous absorption/desorption simulation models supporting the identification of realistic design solutions, but mainly rely on empirically identified flowsheet configurations . Empirical knowledge is clearly useful, yet the proposed improvements are bound to be limited unless an extensive number of structural and operating parameters are taken into account in optimum flowsheet design. This requirement may be well supported by process synthesis methods which combine generic representations of process layouts with optimization algorithms to systematically investigate an enormous range of potentially optimum process options. However, the utilization of such methods in CO2 capture process design is very limited. Their systematic consideration with rigorous process models to support the identification of both realistic and highly optimum solutions has yet to be considered.
This work proposes the implementation of a generic process modeling framework  in the context of a flexible and inclusive synthesis model to support the optimum design of absorption/desorption CO2 capture flowsheets. The framework serves as a mathematical tool able to reproduce any potentially favorable representation of solvent based CO2 capture processes supported by an underlying superstructure. The proposed superstructure consists of modules representing generic process tasks (e.g., reactive and nonreactive separation, heat transfer, mixing) and interconnecting streams emulating material flows. Each module may be independently assigned with a process model representing a particular task, the type of equipment utilized, the desired operating conditions and so forth. Many different modules may be connected in the same flowsheet using an inclusive set of streams (e.g., recycle, bypasses and so forth). The proposed generic tasks account for (a) reaction, mass and heat exchange options between different phases within each module, and (b) stream mixing and splitting options to enable distribution of materials among different modules. Each module may be independently diversified in the same flowsheet into one of the three structural blocks considered in this work; namely the column section, the heater associated with phase change and the heat exchanger.
In particular, the behavior of column sections for reactive or non-reactive separation is described using an equilibrium based (EQ) model, where orthogonal collocation on finite elements approximation techniques are employed (OCFE). EQ/OCFE model formulation enables a robust and flexible representation of the column section that accommodates high resolution in describing the occurring phenomena in a compact in terms of number of required modeling equations form. Furthermore, OCFE formulation offers a competitive advantage by treating the column section height, a decision variable in the design problem, as a continuous variable thus greatly facilitating the numerical solution in non-linear optimization. In this way complex column configurations with multiple side feed and product draw streams are effectively represented and simulated. Column section size is deduced by the aggregate effect of multiple finite elements with adjustable length towards the achievement of separation targets set by the design specifications.
Using the proposed synthesis approach a number of novel absorption/desorption flowsheet configurations is generated and evaluated systematically. Such flowsheets are designed using various combinations of recycle split streams acting as multiple feed and product draw streams towards different column sections, aiming at the maximization of the driving forces inside the separation columns. Optimal design using the modular flowsheet supported by the EQ/OCFE model enables the calculation of the main design variables such as the heat duties for the required condenser and reboiler units in the stripping columns, the operating temperatures and pressures as well as structural features such as column section packing heights, and heat exchanger area for heat integration. Process design is based on the minimization of a suitable objective function comprised of the sum of the total capital and process costs in each flowsheet. Several solvents are considered including Monethanolamine (MEA), 2-amino-2methyl-1-propanol (AMP), and Diethanolamine (DEA). Vapor-liquid equilibrium calculations are performed using statistical associating fluid theory for potentials of variable range (SAFT-VR) [3-4].
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.
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