Study On Reaction Engineering In Fixed Bed Synthesis Of Dimethyl Ether From Syngas

Dimethyl ether (DME) is a new chemical intermediate and clean fuel which can substitute diesel fuel with much lower NOX emission, and less engine noise. The one-step DME synthesis from synthesis gas (syngas) process is developed for the direct synthesis of DME from syngas in a single reactor over bi-functional catalysts composed of copper-based methanol synthesis catalysts and dehydration catalyst. The combination of methanol synthesis and dehydration reactions results in a synergistic effect relieving the unfavorable thermodynamic for methanol synthesis and increasing single conversion of CO. The direct DME synthesis process is one of the most important ways to solve our energy supply and energy security. The study on reaction kinetics and mathematical simulation of reactors guides the scale-up design and Optimal operation of DME synthesis reactors.An intrinsic kinetics model for the direct DME synthesis was established over the mixed catalyst. The diffusion-reaction model was established base on the kinetics model, and the effect of temperature and catalyst diameter on reactant concentration and temperature profile in catalyst was investigated. One-dimensional mathematic model for tube-shell type fixed bed reactor was established. For the reason to avoid too high temperature of hot spot, the methanol synthesis catalyst and mixed catalyst were graded packed in the reactor. The influence of different operation condition is simulated by the proposed reactor model.The intrinsic reaction kinetics of the dimethyl ether synthesis process over mixed catalyst was investigated using an isothermal integral reactor at 220-260℃,3-7 MPa,500-2000 mL/g-h and H2/CO was between 3 and 6. The mixed catalyst was prepared by physically mixing methanol synthetic catalyst (XNC-98) and dehydration catalyst (CNM-3) as mass ratio was 1:1. The L-H kinetics models were obtained. Methanol synthesis from CO, CO2 and methanol dehydration reaction were chosen as independent reactions. CO, CO2, and DME were chosen as the key components.The parameters of the kinetics models were determined by the arithmetic of universal global optimization integrated with calculus of least differences method. The residual error distribution and statistic test showed that the intrinsic kinetic models obtained were reliable and acceptable.The effect of operating condition on DME synthesis was investigated and the results showed that over the catalyst used in the experiment, the conversion of CO, total C and the selectivity of DME increased with the reaction temperature; the conversion of CO and total C increased, while the seclectivity of DME was increased slightly, with the pressure increased; higher space velocity caused lower conversion of CO, total C and selectivity of DME.The key components diffusion-reaction models of DME synthesis from syngas were developed and the internal diffusion effective factors of CO, CO2 and DME in the catalyst were calculated by the orthogonal collocation method. The global reaction rate data which were collected in the internal recycle gradientless reactor were used to examine the calculation results, the examination results showed that the absolute values of the relative error of CO, CO2 and DME were 8.42%,7.76% and 9.64%, respectively, and the models can be used for the internal diffusion effective factor calculation for DME synthesis from syngas.The diffusion-reaction model was used to calculate the concentration profile and the temperature profile in the catalyst particle. The range of the internal diffusion effective factor of CO, CO2 and DME were 0.26-0.38,0.13-0.64 and 0.34-0.51, respectively, which means that the reaction was influence by internal diffusion seriously. The internal temperature of the catalyst was higher than the temperature of the surface of catalyst, and the temperature difference between the inside and surface of the methanol synthesis catalyst was higher than that of the dehydration catalyst.The effects of catalyst diameter and temperature on internal diffusion factor were simulated. The simulation results showed thatζCO,ζCO2 andζDME increased with the diameter of the catalyst decreased;ζCO,ζCO2 andζDME decreased with the reaction temperature increased.The one-dimensional mathematical model of the tube-shell fixed-bed reactor was established base on the kinetics and reaction-diffusion models in this paper. The concentration of reactant gas and temperature profile can be hence calculated. The methanol synthesis catalyst and mixed catalyst were graded packed in the reactor to avoid the high temperature of hot spot, which would lead to catalyst deactivation.The simulation was made at the condition of tm= 220℃, Pin=5.1 MPa, tw=240℃, Pw= 3.35MPa, GHSV=10000h-1; and the length of bed was 6m, the diameter of tube wasφ40×3mm, the total volume of the catalyst bed was about 20m3. The component concentrations at the inlet of reactor were H20.7440, CO 0.1253, CO20.0306, N2 0.0554, CH4 0.0435, H2O 0.0002. The simulation result shows that the suitable packing volume of methanol synthesis catalyst should be 1:2 for each catalyst bed. The hotspot temperature was 258℃and CO conversion was 57.4%, while the yield of methanol and DME were 218.3t/d and 144.8t/d respectively.The effects of different operating conditions on the reactor performance were investigated. The inlet temperature was proved to have little effect on the reactor performance. Inversely, the space velocity increased leaded to a distinct increase of CO conversion and DME yield, while methanol yield was increased as well. The increase of inlet pressure, boiling water temperature and CO concentration in feed gas would increase the CO conversion and DME selectivity, while the catalyst bed temperature especially the hot spot temperature was increased too. Therefore, the CO content in the feed gas should not be too high.