| One of the utilizations of COG is for the synthesis of methanol. The industrial operation practice of the methanol synthesis units that use COG as the feedstock demonstrates that the methanol synthesis catalyst deactivates faster and its life cannot reach the expected time. It is speculated that one of the reasons for the faster deactivation of the methanol synthesis catalyst is likely that the sulfur content in COG is beyond the required level. In the industrial plants,.the sulfur compounds in COG are generally removed via a two stage hydrodesulfurization (HDS) process. Although this technology has been applied for more than one decade in China, the operation of the setups relies mainly on experience, since no detailed research report concerning the conversion of the COG components as well as the HDS of the sulfur compounds on the catalysts is published in the open literature. Based on this fact, in the paper the conversion of the components in COG, especially the HDS of the sulfur compounds, was studied and the operation of the industrial reactor is simulated. The results obtained in this study provide the basis for the improvement and optimization of the COG purification process and is important for improving the profit of the methanol plants. With successful industrial application of the large-scale coal gasification technology, the production capacity of methanol in China has significantly increased in recent years. Production of the high volume chemicals such as olefins and gasoline for methanol is important for the sustainable and stable development of methanol industry. The conversion of methanol to oil (MTO) is catalyzed mainly by HZSM-5 zeolite. The acidic properties and pore structure of HZSM-5 zeolite show a significant influence on its performance. Hydrothermal treatment of the ZSM-5 zeolite is an important technique for tailoring the surface acidity and pore structure. Properly hydrothermal treatment is able to improve the activity, selectivity as well stability of the ZSM-5 zeolite. In this paper, the influence of hydrothermal treatment of the ZSM-5 zeolite on the catalytic performance in the MTO was studied. The obtained results provide an important reference for improving the catalytic properties of ZSM-5 zeolite used in MTO via hydrothermal treatment method.The main research works and results are summarized as followings:1. Based on the fact that the current analysis method of COG is complicated, time-consuming and poor accurate, a new analysis method was proposed that uses methane as a cross-linked component and correlated area normalization to quantify the amount of permanent gas and hydrocarbons containing in COG. The new method is not only time-saving but also more accurate as compared with the current method. It provides a reliable analysis method for studying the conversion of COG, HDS kinetics and for the optimization of the industrial COG purification process.2. Under the conditions close to the industrial HDS reactors, the conversion and reaction kinetics of the main components in COG on the two commercial Fe-Mo/Al2O3 catalysts (T-202 and JT-8) were investigated. On the T-202 catalyst, the hydrogenation of CO and CO2 contained in COG hardly occurred. Complete conversion of diolefins and alkynes could be reached even under mild conditions (T=240?, P=1.6MPa). With increasing in temperature and pressure, the olefins could be progressively hydrogenated, and the conversion degree of olefins depends on their molecular structure. The sulfur containing compound CS2 can be converted to H2S with a fast rate and complete conversion. Though HDS of COS is also easy, it complete conversion is impossible due to the thermal dynamic limitation and the presence of relatively large amount of CO in COG. The produced H2S from the HDS of CS2 and COS could react with unsaturated hydrocarbons in COG forming ethyl mercaptan and thiophene at mild conditions, which then underwent HDS progressively with increasing temperature and pressure. On the JT-8 catalysts, hydrogenation of CO2 to CO could occur to a small degree. The hydrocarbons could undergo also a small degree hydro-cracking reaction, resulting a slight increase in the content of the methane. Ethylene, propylene and isobutylene could be completely hydrogenated under the conditions used in this study while the saturation of straight chain butene isomers could not be realized. With increasing temperature and pressure, complete HDS of ethyl mercaptan and thiophene could be reached, while complete HDS of COS was impassible due to the above mentioned reasons. Based on the experimental results obtained on the T-202 and JT-8 catalysts, kinetic models for the HDS of sulfur compounds was proposed and the parameters of these models were estimated via the regression of the experimental results.3. Simulation of the two stage industrial HDS reactors for the purification of COG was performed using the pseudo homogeneous one dimension model and the obtained HDS kinetic models. In the first reactor, the complete HDS of ethyl mercaptan can be realized easily, while the complete HDS of COS and thiophene is impossible. As a consequence, it is necessary to use the second stage HDS reactor. In the second stage reactor, complete HDS of ethyl mercaptan and thiophene can be realized, while the complete HDS of COS is still impossible even under severe conditions. The amount of COS in the COG is always higher than 1.6ppm. Since the COS cannot be complete removed by the following ZnO absorbent, therefore, the COG purified by the two stage HDS process may contain more than 0.1 ppm sulfur required by the methanol synthesis catalyst.4. The hydrothermally treated and the hydrothermally treated followed by citric acid washing HZSM-5 zeolite (SiO2/Al2O3=38) samples were characterized in terms of XRD,27AlMAS NMR, pyridine-IR adsorption and N2 adsorption-desorption methods. The catalyst performance of these samples in the MTO reaction was evaluated in a continuous flow fixed-bed tubular reactor. The results indicated that hydrothermal treatment at 550,650 and 750? for 240min resulted the dealumination of the NaZSM-5 zeolite, however, no framework collapse occurred. The zeolite samples still remain good crystalline structures. After converted to HSZM-5, it was found that the hydrothermal treatment results in a significant reduction in the amount of the surface acidic sites, especially the B sites. The treatment also leads to a reduction in the specific BET surface area, an increase in the specific surface area of the external framework, the pore volume as well as the average pore diameter. Upon the treatment, mesopores were generated in the HZSM-5 samples and their pore structure became complicated. The citric acid washing of the hydrothermal treated samples could remove a part of the non-framework aluminum species produced in the hydrothermal treatment process, making the blocked channels reopening. The hydrothermal treatment as well as the hydrothermal treatment followed by citric acid washing could prolong the running time of the HZSM-5 catalysts in the MTO reaction. This can be attributed, on one hand, to the reduced amount of surface acidic sites which show a lower coking rate, on the other hand, to the complicated pore structure which fevers the diffusion of the reactant and products and has a higher capacity to accommodating coke deposition. The hydrothermal treatment of the ZSM-5 zeolites did not influence on the conversion of the methanol to dimethyl ether, but did result in a reduction in the conversion of dimethyl ether to the hydrocarbons and a reduction in the selectivity of aromatics. This means that the conversion of the dimethyl ether to hydrocarbons and the formation of aromatics on the HZSM-5 zeolite is associated to its acidic properties. The HZSM-5 sample which has a higher amount of surface acidic sites shows a higher ability to convert dimethyl ether to hydrocarbons and a higher selectivity to aromatics. In the HZSM-5 catalyzed MTO process, the reaction product distribution varies with time on stream. |
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