| One step dimethyl ether (DME) synthesis from syngas has attracted more and more attention at present. Compared with the methonal synthesis process, the CO conversion is raised substantially in one step DME synthesis because of breaking of the thermodynamic limits. This paper mainly foucused on the process of one step DME synthesis from syngas. The optimal hybrid catalyst and the optimal operation conditions were selected and the stability of the catalyst was studied. The mechanism of the methanol synthesis reaction, methanol dehydration reaction and water-gas-shift reaction was elucidated. The kinetic equation of the reaction was established, and the kinetic parameters were calculated according to the experiment data.The main contents of the paper are as follows.Firstly, the catalysts of one step DME synthesis were studied. Different catalysts were prepared by physical mixing method and co-precipitation method, respectively. The reaction test is carried out in a fixed bed reactor. By analyzing the test data, XNC-98is chosen as the methanol synthesis component and HZSM-5whose Si/Al=40as methanol dehydration component for the hybrid catalyst. The optimal operation temperature is240℃, and the optimal ratio of two components is XNC-98/HZSM-5=4:1. Kaolin as methanol dehydration component is also studied. It was found that after dealt with the sulfuric acid, Kaolin can be used as the methanol dehydration component part of the hybrid catalyst. Silica sol is added as binder for the purpose of enhancing the attrition resitance of the catalyst. The spherical of the catalyst particles is improved by spray drying. Attrition test shows that the silica sol containing40-50nm colloidal particls is adapt to the binder.Secondly, stability of the hybrid catalyst is evaluated. Stability test is carried out at3MPa and240℃in a fixed bed reactor. Different conditions such as the reduction procedure, the ratio of H2and CO in raw gas, the synergistic effect between the methanol synthesis catalyst and methanol dehydration catalyst and space velocity are dealt with. The catalysts before and after deactivation are analyzed by XRD,N2-adsorption and TGA. Results show that the deactivation of the DME synthesis catalyst is mainly caused by the ageing of the methanol synthesis catalyst. Sintering of Cu particles is the main reason leads the catalyst to deactivate, and the process is deeply affected by reduction process and the synergistic effect. At the same time, coking may be another reason that has a little effect on the catalyst stability.Thirdly, mechanism of one step DME synthesis reaction was investigated. Results of XPS and XAES showed that after reduction at220℃,the copper on the catalyst surface is turned to Cu0instead of Cu+,so it can be deduced that the active center of the catalyst is Cu0. The reaction route of methanol synthesis and water-gas-shift is studied by in-situ FTIR,and the results are elaborated as follows when used CO2free syngas.For methanol synthesis reaction,the mechanism is given by:(1)H2+2s(?)2Hs (2) Hs+ZnO(?)Zn-OH+s (3) CO+s(?)COs (4) COs+Zn-OH(?)HCOO-Zn+s (5) COs+Os(?)CO2.s+s (6) CO2.s+Os(?)CO3.s+s (7) CO3.s+Hs(?)HCOO s+Os (8) HCOOs+ZnO(?)HCOO-Zn+Os (9) HCOO-Zn+2Hs(?)CH3O-Zn+Os+s (10) CH3O-Zn+Zn-OH+Os(?)CH3OH·ZnO+ZnO+s (11) CH3OH·ZnO(?)CH3OH+ZnO (12) Os+2Hs(?)H2Os+2s (13) H2Os(?)H2O+sFor water-gas-shift reaction,the mechanism is given as follows:(1) CO+s(?)COs (2) H2O+s(?)H2Os (3) H2Os+2s(?)2Hs+Os (4)2Hs(?)H2+2s (5) COs+Os(?)CO2.s+s (6) CO2.s(?)CO2+sFinally, intrinsic kinetic model is built. Studied on the methanol synthesis, methanol dehydration and water-gas-shift reaction, the intrinsic kinetic equations are achieved. The kinetic data are colected in a U type fixed bed reactor and the simplex method was used to scan the models and correlate the kinetic parameters. The equations are showed as follows.Methanol synthesis reactionMethanol dehydration reactionWater-gas-shift reaction k1=7.938×107exp(-6.2744×104/RT) k2=4.734×1012exp(-7.2983×104/RT) k3=1.210×1014exp(-8.5197×104/RT)... |
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