Performance assessment of a packed bed microstructured reactor - heat exchanger for methanol synthesis from syngas

Abstract

About 25% of the world’s proven natural gas reserves are located offshore with no present economic feasibility to be produced, shipped and sold. Utilization of these resources calls for developing new technologies, which enable conversion of “remote natural gas” to transportable fuels and chemicals at lower production capacities. On this context, investigation of compact methanol synthesis from synthesis gas over Cu based catalysts in a multi-slit Integrated Micro Packed Bed Reactor-Heat Exchanger (IMPBRHE) has been the purpose of the present work through experimental and modelling approaches.

A well equipped experimental setup was built and successfully operated. The main characteristics of the IMPBRHE for methanol synthesis over three catalysts; a commercial Cu/ZnO/support and two home-made Cu/ZnO/Al2O3catalysts were investigated. These include productivity, thermal behaviour, mass transfer properties and fluid flow. A wide range of industrial operating conditions were applied; 50 and 80 bar, 488–543 K and contact time of 50-500 ms g/ml. The results show near equilibrium CO conversion per pass is achievable at contact time of ~300 ms g/ml for the commercial catalyst and ~470 ms g/ml for the home made catalysts at 80 bar and 528 K. However, catalyst deactivation observed in experiments remains to be explained.

Superior thermal behaviour of the IMPBRHE was investigated through temperature measurements inside the middle reaction slit and at outer reactor skin. The results indicate a gradient of 1-2 K along the slit axis regardless of the rate of heat generation by the methanol synthesis at different conditions, i.e. high or low productivity when using a CO rich syngas. Moreover, the controlling factor of the slit temperatures was found to be the temperature of the cooling medium (heat transfer oil). Varying the feed gas temperature had no significant effect on the CO conversion. It was shown that the reduction procedure could be replaced by applying the syngas directly, achieving similar activity and under isothermal conditions in the IMPBRHE and hence shortening of the reaction pre-treatment. Results also revealed minor sintering effects caused by exothermic methanol synthesis over a long time on stream.

The possibility of film diffusion limitations was experimentally investigated. Variation of the total pressure and change of inert gas was applied at Reynolds numbers ~1 to alter the diffusivities of reactants in the gas mixture by dilution, while keeping reactant flow and partial pressure constant. It was shown that IMPBRHE operates in a regime with negligible external diffusion limitations. To evaluate possible internal mass transfer effects, experiments were performed with three different particle sizes and the results revealed the negligible pore diffusion limitation effects on the performance of the IMPBRHE under the relevant conditions applied.

Results showed that the reaction media in the slits is isobaric (up to 8 mbar pressure drop over the IMPBRHE). According to the simulation results, the packed slit with the pillar structures delivers a velocity profile which is almost uniform over the distance between two pillars. The reproducibility of conversion and selectivity levels achieved upon applying different samples of the same catalyst batch, indicated that flow maldistribution between the slits is not significantly affecting the performance of the IMPBRHE.

Through establishment of a systematic comparison strategy, the performance of the IMPBRHE was experimentally evaluated against a laboratory scale Fixed-Bed Reactor (FBR) with three dilution ratios. The IMPBRHE outperformed the undiluted FBR and gave CO conversions comparable to the diluted FBRs. The main difference seems to be the superior heat exchange properties of the IMPBRHE which can improve reactor performance for the exothermic methanol synthesis as compared with non-isothermal undiluted FBR. The results revealed the advantages of the IMPBRHE for robust scale up, not applicable for conventional lab-scale fixed-bed reactors due to different scaleup concept.

Experiments were carried out using two IMPBRHEs in series to investigate the effect of inter-stage condensation. Some 15 increase in carbon conversions was achieved when methanol and water were condensed, relative to the experiments done without inter-stage condensation.

A 2D-pseudo homogeneous mathematical model for a single slit packed bed microstructured reactor in the synthesis of methanol was developed in a 3D geometry using COMSOL Multiphysics software. The model is capable of predicting superior thermal behaviour and mass transfer properties as well as CO conversions in the IMPBRHE, all in a good agreement with the experimental data obtained. The verification of simulation results by experimental data shows promising features of the microstructured reactor for the compact methanol synthesis, as well as applicability of the developed model for further design and performance optimization of the reactor.