258045 Experimental and Theoretical Process Analysis of Membrane-Assisted Reactive Distillation
Experimental and Theoretical Process Analysis of Membrane-Assisted Reactive Distillation
Johannes Holtbruegge, Sebastian Heile, Philip Lutze, Andrzej Górak
TU Dortmund University, Laboratory of Fluid Separations
Emil-Figge-Strasse 70, D-44227 Dortmund, Germany
Phone: +49 (0) 231 755-2342, E-Mail: email@example.com
Keywords: Process Intensification, Hybrid Process, Reactive Distillation, Membrane Separation, Modeling, Experimental Investigation
Due to decreasing fossil fuel reserves, the development of innovative process concepts that lead to economical and ecological benefits is absolutely necessary. Promising concepts to increase sustainability are integrated and intensified processes. Those processes integrate different phenomena or operations within one apparatus or allow for strong interactions between unit-operations with different separation characteristics in different apparatuses. Two examples are reactive distillation which superimposes chemical reaction and distillation in one apparatus as well as membrane-assisted hybrid separation processes that make use of a membrane to overcome thermodynamic limitations. As reactive distillation (RD) is nowadays the front-runner to overcome chemical equilibrium-limited reaction systems the combination of RD with a membrane separation is applicable for equilibrium-limited systems that exhibit strong thermodynamic non-ideality thus having a high potential for process improvement. To establish membrane-assisted reactive separation processes in industry, detailed experimental and model-based investigations of hybrid processes have to be performed. Thus it is possible to get on the one hand important process know-how and based on this on the other hand process design tools for membrane-assisted reactive separation processes. One of those tools is a process analysis tool which supports the user to the decision whether the process window of the considered phenomena for integration may happen at the same time and place as well as which process parameters are the sensitive ones to be intensified/improved to reach a defined process improvement.
In this work, both the process analysis tool as well as the experimental results in a pilot plant for the development of detailed models will be highlighted for a transesterification process.
Therefore, the transesterification of propylene carbonate (PC) with two molecules of methanol (MeOH) to form the target product dimethyl carbonate (DMC) and the byproduct 1,2-propanediol (PDO) in a chemical equilibrium-limited reaction is chosen as case-study.
Reaction kinetics and chemical equilibrium for this chemical system were determined using sodium methoxide as homogeneous catalyst since no suitable heterogeneous catalyst has been found. The chemical equilibrium constant was found to be very low (Keq = 0.2 at T = 343 K) yielding in low reactant conversions. By process analysis on the reaction, it was observed that by ensuring a molar excess of methanol or selective removal of one product species an almost complete propylene carbonate conversion can be obtained. To establish this while also having low recycle streams, a reactive distillation process is suggested for this system. With a molar excess of methanol in the feed stream it is possible to reach a complete propylene carbonate conversion while obtaining an azeotropic mixture consisting of dimethyl carbonate and large amounts of methanol in the upper part of the column. To overcome this azeotrope the integration with a membrane at the top of the column is identified. Hence, this mixture is fed to a vapor permeation membrane that separates unreacted methanol from the mixture so that dimethyl carbonate is enriched on the front side of the membrane. Unreacted methanol can be recycled afterwards to the RD column to guarantee a high methanol excess. A flowsheet of the membrane-assisted reactive distillation process is shown in Figure 1.
Figure 1: Simplified flowsheet of the membrane-assisted reactive distillation process
The integrated process is determined to be promising enabling the need for detailed models. Hence, experimental investigations of reactive distillation were performed in a pilot-scale RD-column with an inner diameter of 0.05 m that was equipped with Sulzer BX packing elements. The total packing height was 5.2 m and the total feed flow rate was 4 kg/h. The achieved composition and temperature profiles along the column were used successfully to validate the rate-based simulation model.
The vapor permeation was investigated in a lab-scale plant equipped with a flat-sheet membrane module. The separation characteristics of the dense, hydrophilic membrane Sulzer PERVAPTM 1255 were determined by investigating the influence of feed composition, feed pressure and feed temperature on the transmembrane flux and membrane selectivity.
For the process analysis, a detailed simulation model comprising the rate-based model for reactive distillation and an empirical solution-diffusion model was set up. The model parameters for the membrane separation were determined from the lab-scale permeation experiments.
A process analysis on the detailed simulation model is used to show the influence of important operational parameters like reflux ratio and reactant ratio on the propylene carbonate conversion, the product purities and the necessary membrane area.
The financial support of the German Federal Ministry of Education and Research for the project “Energy Efficiency Management and Benchmarking in the Process Industry” is gratefully acknowledged.