(203m) COMT?CAMD: Simultaneous Process and Solvent Design Using PCP-SAFT Applied to CO2 Pre-Combustion Capture

Holistic process design should consider the process units as well as the required process fluids such as solvents. Today, process design and the design of the appropriate solvent are treated separately and independently from each other. A fully integrated process and solvent design problem is prohibitively complex for a true deterministic solution procedure. Any practical approach requires the decomposition of the full problem. In previous studies, the decomposition was done by pre-selecting the solvent using auxiliary, typically heuristic objective functions, such as maximal CO₂ solubility. The true process objective function, in contrast, should capture the full process-level information such as cost or energy.

Here, a new method is proposed for achieving a more closely integrated design procedure. The method is illustrated for the simultaneous process and solvent optimization for physical absorption of CO₂ as employed by pre-combustion capture processes.

In the so-called CoMT-CAMD method (‘Continuous Molecular Targeting - Computer Aided Molecular Design’ ^[1]), any molecule is represented by a set of molecular parameters of a physically-based thermodynamic model (here: the PCP-SAFT model ^{[2], [3]}). In model-based process optimization, these molecular parameters are treated as continuous variables and optimized alongside with the process variables (e.g. desorption pressure). This kind of optimization methodology (‘Continuous Molecular Targeting’) leads to both a hypothetical target solvent and a set of process variables that minimize the process objective function. The objective function is defined as process costs, taking into account various environmental, product and process impacts. In a subsequent step (‘mapping step’) the best real solvent is efficiently identified from a database. For an analysis of the optimization problem in the vicinity of the optimum, we use the second-order Taylor series in the molecular parameters.  

The integrated process and solvent problem is thus decomposed into a ‘continuous molecular targeting’ step and a ‘mapping’ step, avoiding any auxiliary objective function and any a prioriexclusion of substances from the pool of solvent candidates through heuristics. The approach allows for a direct access of molecular characteristics in the process optimization. Due to the strong molecular picture underlying PCP-SAFT, high quality of the mapping of the target parameters to real fluids is achieved. The suggested CoMT-CAMD approach thereby closely approximates the truly integrated process and solvent optimization problem.

References:

[1] Bardow, A.; Steur, K.; Gross, J. Continuous-molecular targeting for integrated solvent and process design. Ind. Eng. Chem. Res.,2010, 49, 2834–2840.

[2] Gross, J.; Sadowski, G. Perturbed-chain SAFT: An equation of state based on a perturbation theory for chain molecules. Ind. Eng. Chem. Res. 2001, 40, 1244–1260.

[3] Gross, J.; Vrabec, J. An equation-of-state contribution for polar components: Dipolar molecules. AIChE J. 2006, 52, 1194–1204.