(135a) 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