Hybrid Processing for CO2 Capture – A Model Based Approach for Process Synthesis

Hybrid processing for CO₂ capture – A model based approach for process synthesis

A. Kunze, P. Lutze, A. Górak

TU Dortmund University, Laboratory of Fluid Separations

Emil-Figge-Strasse 70, D-44227 Dortmund, Germany

Phone: +49 (0) 231 755-3034, E-Mail: anna-katharina.kunze@bci.tu-dortmund.de

In general, separation of gaseous components is an important process step in chemical processes. But process synthesis for this application is a challenge, because there is a huge variety of possible unit operations to fulfill the same separation task. A restriction to a small number of possible unit operations for gas separations is not that easy, because there is no as dominant unit operation as for example distillation for liquid separations. This leads on the one hand to the question, how to decide which unit operation should be used in each individual separation task and operates best at different process conditions. On the other hand, due to the large number of possible unit operations, hybrid processes might be an opportunity here to gain process intensification. Hybrid processes are defined as combinations of at least two separation techniques in one operation by using the synergy between them ^[1].

Gas separation processes are often developed based on knowledge ^[2]. This might end in fixed decision pathways. A model based approach connected to a superstructure analysis will be presented to obtain a higher flexibility and a more general procedure in process synthesis for gas separations. In contrast to purely knowledge based approaches, this method offers the opportunity to take more process configurations into account and evaluate them. A scheme of this multi-step method is shown in Figure 1.

Figure 1: Method scheme for model based process synthesis

Therefore, the first step after defining a separation task is to use a technology database to summarize suitable unit operations. This database contains information about various separation technologies for gaseous mixtures including further reactions and necessary recycle runs. Not only industrial mature technologies can be taken into account, but also promising new technologies like membrane contactors. For each technology the process windows, structural and logical constraints are implemented to restrict the number of process options. The next step is a superstructure analysis based on short cut models of each separation step. If necessary mixed integer non-linear programming (MINLP) has to be used to gain a number of reasonable process options. Those options will be ranked following the objective function, for example energy consumption and the most promising process options are set. In the last step, rigorous modeling can be used to proof the results of the short cut analysis and define the process configurations in a more detailed way.

To figure out a method to find advantageous process configurations, CO₂ capture is used as a case study as a first step. CO₂ can for example be separated with different absorption methods, like washing with aqueous monoethanolamine or piperazine promoted potassium carbonate as well as different absorption technologies such as conventional columns and membrane contactors. But also membrane and adsorption techniques might be suitable to separate CO₂ from flue gas stream. Another possible application of the developed method at a later stage could be the adjustment of biogas upgrading plants to the process conditions of each individual place of action. Therefore, the developed method has to be generally applicable and easily adaptable to other, more complex separation tasks, which include for example more components.

Literature

[1]    A. Stankiewicz et al., Re-Engineering the Chemical Processing Plant. Industrial & Engineering Chemistry Research. New York: Marcel Dekker, Inc., 2003

[2]    S.D. Barnicki et al., “Separation system synthesis: a knowledge-based approach. 2. Gas/vapor mixtures”. Industrial & Engineering Chemistry Research 1992, 31 (7), 1679–1694