Study On Global Kinetics Of Direct Dimethyl Ether Synthesis From Syngas And Mathematical Simulation For Bubble Column Slurry Reactor

Dimethyl ether (DME) is a new chemical intermediate and ideal clean fuel which can substitute diesel fuel and liquefied petroleum gas for for its high cetane number and environmentally benign properties. From thermodynamical point of view, it is favorable to the direct DME synthesis from syngas process which breaks through the thermodynamics equilibrium limitation of methanol synthesis. In the result, a higher CO single conversion and a higher DME selectivity can be obtained. For its higher heat transfer coefficient for the existing of liquid inert media, bubble column slurry reactor has received growing attention in direct DME synthesis from syngas and easy to achieve constant temperature operation in the case of a strongly exothermic reaction which can cause temperature runaway in traditional tubular fixed-bed reactor and alleviate greatly carbon deposition. Commercial bi-functional catalysts were dispersed in liquid medium after smashing and grinding, and lead easily to irreversible deactivation for the existence of wateradhering to the surface of catalysts. Cu-Zn-Al-Zr slurry catalysts used in this study were prepared by a novel complete liquid-phase technology from a solution to the slurry, and always dispersed in the organic medium, which has the advantages of small and uniform granularity, high dispersive, higher activity stability, well rheological property, stable surface structure and a good stability during the reaction of 440 h without significant catalyst deactivated. Study on global kinetics for the direct DME synthesis from syngas and mathematical simulation in BCSR over the Cu-Zn-Al-Zr slurry catalysts has important guiding significance to the industrial design and scale up and optimal operation of DME synthesis process.A global kinetics for DME direct synthesis has been proposed based on Langmuir-Hinshelwood mechanism in continuous stirred tank reactor over Cu-Zn-Al-Zr slurry catalyst. Bubble characteristic parameters in air/water/hollow glass beans system are measured experimentally using dual-tip conductivity probes, and gas-liquid volumetric mass transfer coefficient was determined experimentally by the means of a dynamic oxygen absorption technique in bubble column slurry reactor at ambient temperature and normal pressure. Empirical correlations by dimensional analysis and feed-forward back propagation neural network models were obtained to predict bubble characteristic parameters and gas-liquid volumetric mass transfer coefficient. It has been further developed a steady state one- dimensional mathematical model for an industrial scale bubble column slurry reactor with annual output of 100,000 tons dimethyl ether based on the global kinetic model for direct DME synthesis and and sedimentation-dispersion model of catalyst grains.Directy Dimethyl ether synthesis process from syngas was studied in 500mL stirred autoclave over the novel Cu-Zn-Al-Zr slurry catalyst in wide range of experimental operation conditions:pressure with 3-7MPa, temperature with 220-260℃, WHSV with 0.6-1.2L/(gcat-h) and the composition of syngas with yH2 0.65-0.75, yCO 0.14-0.20,yCO2 0.04-0.08 in the experiments. The influences of pressure, temperature and WHSV on the once-through conversion of CO, selectivity of DME were studied in detail:The conversion of CO and selectivity of DME increase gradually, yet selectivity of methanol has different changes with increasing pressure and increasing temperature, but both conversion of CO and selectivity of DME decrease gradually with the increasing WHSV.A global kinetics model for liquid phase DME direct synthesis based on Langmuir-Hinshelwood mechanism has been proposed by choosing methanol synthesis from CO hydrogenation and CO2 hydrogenation and methanol dehydration as independent reactions. The global kinetics model is suitably fitting experimental data, and its reliability was verified by statistical test and residual error analysis. Bubble characteristic parameters(d32,εG, aL) in air/water/hollow glass beans systems are measured experimentally using dual-tip conductivity probes, and mass transfer coefficient (kLaL,,kL) was determined by the means of a dynamic oxygen physical absorption technique with a polargrafic dissolved oxygen probe in bubble column slurry reactor at ambient temperatures and normal pressures. Empirical correlations by dimensional analysis were obtained to predict bubble characteristic parameters and gas-liquid mass transfer coefficient for air/water/hollow glass beans system. Feed-forward back propagation neural network models with the same 3-layer [3,15-12,1] topology are employed to predict these parameters. The increasing of axial height in the column has little or no influence on local gas holdup and gas-liquid interfacial area and slightly enhanced the Sauter-mean bubble diameter; Local gas holdup, gas-liquid interfacial area and Sauter-mean bubble diameter all increased with the superficial gas velocity increasing, and the Sauter-mean bubble diameter reached gradually stable value in coalesced bubble regime. Gas-liquid interfacial area decreased gradually and Sauter-mean bubble diameter increased gradually with the increasing solid holdup. Local gas holdup decreased with the increasing solid holdup in heterogeneous flow regime and then almost not changed in homogeneous flow regime; With axial height of the column increasing, volumetric mass transfer coefficient has remained approximately constant, while liquid-side mass transfer coefficient was fluctuated for bubble recirculation in bulk slurry phase; Volumetric mass transfer coefficient kLaL increased and liquid-side mass transfer coefficient kL decreased with the increase of superficial gas velocity. kLaL decreased generally with the increasing solid holdup, but have a slight increase in low solid holdup less than<5% which can enhance gas-liquid mass transfer characteristics, and kL has the corresponding changing tendency of the increasing firstly and and then decreasing with the increasing temperature.Based on the global kinetics of direct dimethyl ether synthesis and sedimentation-dispersion model of catalyst grains, a steady-state one-dimensional mathematical model for three-phase bubble column slurry reactor with annual output of 100,000 tons dimethyl ether has been established. The influence of operation conditions and reactor structure on catalyst axial concentration distribution with simulation results was discussed. It turns out that the particle diameter and reactor diameter are the main factors influencing concentration distribution uniformity. For ultrafine slurry catalyst, the influence of catalyst content on the catalyst axial concentration distribution has been not obvious. it can be carried out under the slurry catalyst with a higher mass content to achieve higher methanol/DME yield.The simulation calculation of bubble column slurry reactor design was carried out for 100,000 t/a dimethyl ether in the typical industrial operating conditions with the composition of coal-based syngas:yH20.70, yN20.10,yCO0.15, yCO20.05, Solid constent 30wt.%, reactor diameter 2.50m, temperature 240℃, pressure 5MPa. The simulation results were listed:static bed height 18.23 m, operating bed height 22.83 m, average gas holdup 0.20, superficial space velocity 0.260m·s-1, mass of paraffin 52.40t, mass of catalyst 22.46t, gas composition in the outlet:yH2 0.6021, yCO 0.0068, yCO2 0.0819, yH2O 0.0703, yM 0.0181, yDME 0.0837, yN2 0.1371, total carbon conversion 0.6765, DME seliectivity 0.8419, methanol yield 16254.65 t/a, DME yield 108351.84 t/a.The influence of temperature, pressure and reactor diameter on total carbon conversion, DME seliectivity, products output and bed height were simulated and analyzed.Total carbon conversion, selectivity of DME and yield of methanol/DME are all increased as pressure increases. Operating bed height is decreased greatly, while static bed height decreases slightly. Total carbon conversion, selectivity and yield of DME decrease slightly with increasing temperature. Static bed height and operating bed height drop suddenly as the temperature increases. Total carbon conversion, selectivity of DME, and yield of methanol/DME keep almost constant with reactor diameter. But both operating bed height and static bed height are increased obviously because average gas holdup and superficial space velocity are increased as reactor diameter decreases. The optimal operation conditions in BCSR were proposed: temperature at 240℃, pressure at 5MPa and reactor diameter of 2.5m.