267448 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
*email@example.com, **firstname.lastname@example.org, ***email@example.com, ****firstname.lastname@example.org
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 Pa0.5 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.
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.