Direct synthesis of dimethyl ether in multi-tubular fixed-bed reactors: 2D multi-scale modelling and optimum design

The scope of this work is to explore the viability of the direct synthesis of dimethyl ether (DME) over bifunctional catalysts, such as mixtures of CuO/ZnO/Al2O3 and gamma-Al2O3 at industrial scale. To accomplish this purpose, the process is simulated using a phenomenological mathematical model considering momentum, mass and energy balances, applied to both the catalyst particles and reactor bed, which is solved in 2D axisymmetric coordinates. This constitutes a step beyond most of the available studies for the modelling of the DME synthesis reaction, based on simple 1D isothermal models. The use of this detailed model revealed the importance of intraparticle mass and heat transfer, with effectiveness factors within the range 0.5-1.1. At the reactor scale, radial phenomena were found to be relevant. A design-sensitivity analysis of mass flux, catalyst fraction, pressure, feed temperature, cooling potential and tube diameter on the reactor performance was carried out. An optimized reactor design that provides 80% CO conversion operating at inlet temperature and pressure 245 degrees C and 40 bar, corresponds to 0.02 m diameter, 8.50 m length and 3600 h(-1) gas-hourly space velocity with a yield of dimethyl ether of 0.53.