(735c) An Experimental and Modeling Study on Gas Solubility and Transport in Ionic Liquids Employed As Sweep Solvents for Membrane Contactor Reactor Applications

Methanol (MeOH) is an important compound finding applications today as fuel and an important raw material in the chemical industry. Methanol is produced industrially by the catalytic conversion of syngas (a mixture of CO/CO₂/H₂) produced either through the steam reforming of natural gas or the gasification of coal or biomass. Methanol synthesis (MeS) is a highly exothermic, equilibrium-limited process, technically challenged by both low per-pass carbon conversions (typically, ~25%) and difficulties with heat management during reactor operation.

In place of the conventional MeS processes, our team recently proposed a novel reactive separation technology that makes use of a membrane contactor reactor (MCR)^1,2. The process employs a non-volatile, high-temperature resistant ionic liquid (IL) as a sweep fluid to remove in situ the MeS products (H₂O and MeOH). As a result, significantly improved per-pass carbon conversions and MeOH selectivity are obtained.

For optimal reactor design and operation, it is essential to have effective models for estimating the solubility and transport properties of the MeS reaction components, including CO/CO₂/H₂/H₂O/MeOH (both individually and in the mixture) in the IL (in this case EMIM-BF₄) at the relevant high pressure and temperature MeS process conditions. In the present study, the solubilities of the MeS components in EMIM-BF₄ were measured experimentally using a pressure-volume-temperature (PVT) system. To model and analyze the experimental behavior, the Peng-Robinson and Soave-Redlich-Kwong equation of states (EOS) were applied to account for the non-ideality of the gas phase, while the UNIQUAC and UNIFAC activity coefficient models were used to calculate the fugacity of the liquid phase in equilibrium with the gas phase. The modeling results are compared with our experimental solubility data, and the applied models are shown to be capable of predicting the experimental behavior. In addition, the sensitivity of MeS-MCR model predictions to the accuracy of the thermodynamic models is investigated. CO₂ diffusivities were measured both in a PVT cell (constant volume diffusion experiments) and also in a high-pressure TGA (thermogravimetric analysis) system.

The measured thermodynamic and transport properties are used to optimize the performance of the membrane reactor system. These experimental data are integrated in our in-house MeS-MCR models¹ and are shown capable of predicting the experimental behavior. Currently, we are looking at other IL candidates to optimize the final sweep solvent choice based on performance, cost, chemical stability, safety, and environmental considerations.

References:

1. Zebarjad, S.F., Gong, J., Li, Z., Jessen, K., Tsotsis, T.T., “Simulation of Methanol Synthesis in a Membrane-Contactor Reactor,” In Press J. Membrane Science, 2

2. Zebarjad, F., Hu, S., Li, Z., and Tsotsis, T.T., “Experimental Investigation of the Application of Ionic Liquids to Methanol Synthesis in Membrane Reactors,” Ind. Eng. Chem. Res., 58. 11911, 2019.