372693 A Process Design and Operability Framework for Post-Combustion CO2 Capture Systems and Solvents: Integrated Assessment and Selection
Post-combustion CO2 capture technologies are predominantly based on the use of solvents that enhance the separation efficiency in absorption/desorption systems by selectively dissolving CO2. A large part of published research is focused on the identification of new solvents with improved thermodynamic, reactivity and sustainability characteristics  to help reduce the high operating costs and environmental impacts associated with absorption/desorption systems. As solvents are inherent components of the separation systems in which they are utilized, the consideration of different solvents in the optimization of absorption/desorption characteristics such as process structure and size, topology of recycle streams and solvent feed flowrates , to name a few, is receiving significant attention in CO2 capture research to further improve the process economics. However, CO2 capture processes are essentially dynamic environments susceptible to variations in operating parameters (e.g., temperature, pressure, and composition, solvent degradation and losses) and influence from exogenous disturbances (e.g., input flue gas composition). Absorption/desorption systems designed for optimality assuming constant operating conditions will divert from optimal operation if any of these parameters vary in time, as is often the case in industrial practice. This issue has received very limited attention in CO2 capture research, where the effects of variability are largely overlooked during absorption-desorption design as it is generally assumed that a properly designed control system will eventually compensate for such effects. On the other hand, the effects of different CO2 capture solvents in process design under operating variability that explicitly considers the controllability properties and the operability performance of the overall process system have yet to be addressed systematically.
The current work addresses the economic performance simultaneously with the static and dynamic controllability properties of amine based CO2 capture processes. Amine solvent candidates are introduced into a unified framework that enables the optimal absorption/desorption design while considering variations in process parameters and disturbances in process operation. The proposed framework consists of a process design and a nonlinear sensitivity analysis stages that evaluate process design and control structure configurations under the influence of multiple simultaneous parameter variations and/or exogenous disturbances. The absorption/desorption design stage involves a process synthesis approach employing reactive and non-reactive separation column modules of sufficient modelling detail and flexibility acquired through the implementation of an orthogonal collocation on finite elements (OCFE) approximation technique . The nonlinear sensitivity analysis stage investigates the steady-state and dynamic effects process parameters and disturbances impose on the system for a given control structure. The control scheme categorizes control objectives and preferences in the utilization of manipulated variables in a multivariable centralized fashion . As a result, an extended sensitivity surface around the optimal operating points is developed that reflects the variability of process manipulated and controlled variables in response to changes in process parameters and disturbances. It is within this framework that alternative solvents of different chemical structure and physical properties are introduced in order to study their effects in optimal design and operability performance under operating conditions variations. Each solvent constitutes an additional design feature in the formulated optimization problem, demonstrating alternating process design directions under the imposed variability. The performed sensitivity analysis generates useful insights regarding the control structure selection and the range of parameter variations within which the solvents demonstrate optimum performance. Furthermore, it allows the adaptive modification of the process design characteristics, inclusive of the available solvent options, in order to enhance process operability.
The proposed developments are illustrated in case studies considering amine solvents such as Monethanolamine (MEA), 2-amino-2methyl-1-propanol (AMP) and Diethanolamine (DEA) which are commonly used for CO2 capture. The thermodynamic behavior of the CO2-water-amine mixtures is modelled through pressure-loading relationships extracted from the Statistical Associating Fluid Theory for potentials of variable range (SAFT-VR) [4, 5]. Different absorption/desorption structures are evaluated including recycle split streams, muti-pressure cascades etc. in view of the considered solvents.
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.
 Papadopoulos A.I., S. Badr, A. Chremos, E. Forte, T. Zarogiannis, P. Seferlis, S. Papadokonstantakis, C. S. Adjiman, A. Galindo, G. Jackson, 2014, Efficient screening and selection of post-Combustion CO2 capture solvents, Chemical Engineering Transactions, 39, ISBN 978-88-95608-30-3; ISSN 2283-9216.
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