(397c) Derivation and Validation of a Short-Cut Model for the Absorption of CO2 in an OCM Mini-Plant

Derivation and Validation
of a Short-Cut Model for the Absorption of CO₂ in an OCM Mini-Plant

Erik Esche*, David Müller**, Steffen Stünkel***,
Günter Wozny****

Chair of Process Dynamics
and Operation, Berlin Institute of Technology, Sekr. KWT-9, Str. des 17. Juni 135, D-10623 Berlin, Germany

*erik.esche@tu-berlin.de,
**david.mueller@tu-berlin.de, ***steffen.stuenkel@tu-berlin.de, ****guenter.wozny@tu-berlin.de

The Oxidative Coupling of
Methane (OCM) presents a possibility of catalytically turning methane into
longer hydrocarbons, especially ethylene and ethane. Hence, it could be an
opportunity for replacing oil with methane from natural or biogas for producing
base chemicals such as polymers, ethylene oxide etc.

As part of the Cluster of Excellence
?Unifying Concepts in Catalysis?, funded by the German Research Foundation, a
mini-plant has been built at Berlin Institute of Technology (Technische Universität
Berlin). The mini-plant features several types of reactors, e.g. fixed-bed,
membrane, and fluidized-bed reactors, thus implementing the OCM process. For
the subsequent product gas separation an absorption-desorption process for the
extraction of carbon dioxide (CO₂), two gas separation membranes,
and an adsorption-desorption unit are installed.

It is rather difficult to
find the optimal combination and operation conditions of these reaction and
separation units only through experimental investigations. On top of that,
there are fluctuations in all mass and energy flows, which have to be taken
into regard to ensure a safe and reliable operation.

Consequently, simulative
optimization under uncertainty of the entire superstructure is proposed. Using
rigorous models for all plant components would create a large-scale MINLP with
complex non-linearities for example caused by reaction kinetics. Preliminary,
deterministic optimization studies of separate reactors already showed
convergence difficulties with the number of variables for a conventional
packed-bed membrane reactor surpassing 130,000. To mitigate the size,
complexity, and convergence time validated short-cut models should be
introduced for all units in the entire mini-plant.

As a first step, the
stand-alone optimization of the CO₂ absorption process is dealt
with. Extensive experimental data for both monoethanolamine (MEA) and
piperazine-activated methyldiethanolamine (aMDEA) for synthetic OCM product gas
in the mini-plant described above exist and simulations have been set-up in
Aspen Plus mimicking the plant behavior. The rate-based model in Aspen Plus
also shows convergence difficulties, which highlights the necessity for
simpler, more stable short-cut models.

The absorption desorption
process is modeled in AMPL using an equilibrium-based approach for each
theoretical plate of the packed columns. The number of components appearing in
the reaction mechanism for the chemical equilibrium is reduced using dummy
species representing the left out components. The heat of absorption is modeled
similarly. Furthermore, basic equations for heat loss, pressure drop, and
efficiencies for each theoretical plate are introduced. Preliminarily, AMPL and
Aspen Plus simulations are compared to assess the behavior of the absorption in
the short-cut model using the rigorous model as a reference case.

The derived model is
compared to own experimental data for gases containing 15 to 25 vol% of CO₂,
gas and liquid load factors of 0.3 to 0.7 Pa^0.5 and 20 to 40 m³/m²h
respectively at pressures from 10 to 32 bar. The parameters for heat transfer,
pressure drop, and efficiencies are fitted accordingly.

To prepare for the
introduction of the model for the CO₂ absorption process into the
whole superstructure, the stand-alone operation is optimized. The OCM product
gas with a predefined composition is fed to the absorption column, which is
specified to remove 90% of the CO₂ contained in the feed flow. The
objective function for the stand-alone operation is the minimization of the
energy required for the separation of CO₂, which is mainly the heat
needed for regenerating the scrubbing fluid in the desorption column.

The optimal operating conditions
are then applied to the mini-plant and validated by testing the plant behavior
over several hours and observing the required adherence to the specifications.
The optimality of the results is checked with the help of an experimental
sensitivity analysis.

As a next step
uncertainties observed in the experimental data will be added to feed streams
etc. of the absorption process and the optimization problem will be
reformulated for optimization under uncertainty.

Acknowledgements

This
work is part of the Cluster of Excellence ?Unifying Concepts in Catalysis?
coordinated by the Berlin Institute of Technology (Technische Universität
Berlin). Financial support by the Deutsche Forschungsgemeinschaft (DFG) within
the framework of the German Initiative for Excellence is gratefully
acknowledged.