The Effects of Intraparticle Diffusion Phenomena on Dimethyl Ether Direct Synthesis

Introduction

Dimethyl ether (DME) is widely used as propellant and it is considered a valid alternative fuel. It can be employed as alternative to diesel fuel, having higher cetane number (55-60) and less pollutants emission, or as alternative to liquefied petroleum gas (LPG), because of the similar physical properties (-24°C boiling point; liquefied at 5 bar pressure at ambient temperature). Traditionally DME is produced from syngas in a two-step process consisting in methanol synthesis and dehydration (indirect synthesis) [1]. To improve the process efficiency, many efforts have been made in the last decades to develop the one-step direct synthesis, combining methanol (Cu/ZnO/Al₂O₃) and dehydration catalyst (zeolites or γ-Al₂O₃). The two catalysts can be either mechanically mixed as different pellets in a random fixed bed or intimately coupled by producing hybrid pellets [2][3]. Core-shell pellet configuration has also been proposed [4]. The intraparticle diffusion phenomena have a strong influence on the reactants and products profiles inside the pellets, affecting the reaction kinetics and the catalyst efficiency. As a consequence the thermal behavior and the performances of the reactor are also affected, especially under industrial relevant conditions. However, these effects have not been deeply investigated in literature.

Methods

In order to compare the different configurations (bed of mixed pellets vs. bed of hybrid pellets vs. core-shell pellets), two different kind of simulation have been performed. First, single pellet 1D models for each configuration have been used in order to evaluate the effects of diffusion in the different cases analyzed considering same boundary conditions. The models account the i-species concentration gradients inside the catalyst pellet assuming isothermal conditions. A reaction scheme including methanol (MeOH) synthesis from CO₂, Reverse Water Gas Shift (RWGS) and MeOH dehydration has been adopted with kinetics taken from the literature [5][6][7]. Then, in order to evaluate the effect of diffusion on industrially relevant conditions, a heterogeneous 2D model of a single tube of a multi tubular fixed bed technical reactor for the direct DME synthesis has been used. The models consist of i-species mass, energy and momentum balances for the gas-phase, coupled with i-species mass and energy balances for the catalyst phases (one solid phase for the hybrid and core-shell pellets, two solid phases for the mixed pellets) accounting for concentration gradients (1D) in isothermal pellets. An externally cooled multi tubular reactor of 6 m long and 1 inch internal diameter tubes, filled with 4.8 mm diameter spherical catalyst pellets, has been considered in the simulations. The model equations have been implemented in gPROMS^® commercial software for the numerical resolution of the boundary value problem.

Results

The simulations have been performed considering different feed compositions (consisting of H₂, CO, CO₂, CH₄). The results show that the performances of the hybrid pellet configuration in terms of conversion and DME yield are higher compared to the mechanical mixture. So, as consequence of the exothermicity of the reaction involved, the reactor loaded with hybrid pellets has a more difficult thermal control, that is shown by the higher hot-spot temperature, and this can represent an issue considering the possibility of catalyst deactivation. These results can be explained considering that, in the hybrid catalyst pellet, the DME synthesis reaction consumes the methanol produced by the MeOH synthesis, while WGS removes H₂O produced by both MeOH and DME syntheses. This results in a synergistic effect on the conversion rate with respect to the mechanical mixture, where, due to intraparticle diffusion limitation, the catalyst efficiencies decrease both in the MeOH catalyst pellets, because of the equilibrium approach, and in the DME catalyst pellets due to combined effect the lower methanol concentration and the higher water content.

However, the coupling of the two catalysts in hybrid pellets can cause the deactivation of the support, due to the bad interaction between the two catalytic functions [8], with a consequent decrease in performances. The core-shell configuration can be a good compromise in order to guarantee an advantageous contact of the two catalyst functions while avoiding unwanted deactivation effects. The core-shell configuration is at present under analysis.

References

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Acknowledgements This contribution has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 727600.