| Lurgi multistage fixed bed reactor MTP process can realize the stable operation of the plant with an annual output of 1 million tons of propylene,but the process has high energy consumption,low yield and poor economy.Although the development of new catalysts can improve the catalyst lifetime,activity,product selectivity and process economy,it requires a lot of time and capital investment,and has relatively large risk.In order to solve this problem,this thesis believes that we can make full use of the existing equipment,from the perspective of raw material selection,process optimization and the improvement of the existing catalyst efficiency,to achieve the quality and efficiency of the production process.Therefore,an improved MTP process with crude methanol as raw material was proposed.By studying the influence mechanism of impurities on MTP reaction,the process was adjusted,and the long-period pilot study,simulation and industrial verification were carried out to determine the optimal process of crude methanol feed,which significantly improved the economy.Specific contents and conclusions are as follows:1.Through the characteristic analysis of crude methanol,the contents of the impurities were identified,which were mainly alkali metal ions,miscellaneous alcohols,water,and small amount of other substances like esters and ketones.The influence of representative impurities on MTP reaction was studied respectively.At the initial stage of MTP reaction,a small amount of alkali metal ions in crude methanol has no obvious effect on the methanol conversion,which can reach 99%.However,after a period of time,the methanol conversion gradually decreases,and the higher the content of alkali metal ions,the faster the catalyst deactivated.Compared with the refined methanol feed,the selectivity of ethylene and propylene is lower,and with the increase of alkali metal concentration,the selectivity of ethylene and propylene is decreased,the selectivity of C5+ is increased,and the selectivity of CH4 is slightly increased.The presence of alkali metal can lead to the increase of the proportion of heavy components in the product,which increases the yield of wax and coke to some extent.At the same time,the selectivity of ethylene and propylene is still significantly reduced after the calcination regeneration of the catalyst,the selectivity of C5+ is significantly increased,and the selectivity of CH4 is also increased,indicating that the toxification effect of the alkali metal to the catalyst is irreversible,and should be removed before raw methanol feeding.2.Taking ethanol,the main component of miscellaneous alcohols,as the representative,the influence of ethanol on MTP reaction and catalyst performance was investigated through reaction performance evaluation,physicochemical performance analysis before and after catalyst reaction,and in-situ characterization.The results show that methanol and ethanol can react with MTP catalyst to produce methane,ethylene,propylene,C4 mixed hydrocarbon,C5+ mixed hydrocarbon and other organic compounds.The inclusion of ethanol in the methanol feed can significantly improve ethylene selectivity and reduce the liquid hydrocarbon yield.After the reaction,the crystal structure of the catalyst remained basically unchanged,and the addition of ethanol was more conducive to maintaining the framework of ZSM-5 molecular sieve during the reaction process,but the total acid content,especially the acid content of strong acid,significantly decreased,the average carbon deposition rate of the catalyst increased,promoting the formation of carbon deposition species of tetramethylbenzene,ethanol molecules were more easily adsorbed on the B acid site of the catalyst,and accelerated the formation of aromatic hydrocarbons.3.The introduction of water impurities in methanol can prolong the catalytic lifetime,and at the same time,the low carbon olefins in the product,especially propylene,increases to a certain extent.Since water reduces the acidity of the catalyst,it can significantly affect the performance of the initial stage of MTP reaction.By activating the catalyst with steam,the acidity of the catalyst can be reduced,and the lifetime and selectivity of the catalyst can be improved obviously.For raw methanol feed,the current hydrothermal treatment conditions used in industrial plants at 480 ℃are suitable,the overall acid content of catalyst is reduced slightly,and the requirements of raw methanol feed reaction can be met without changing the hydrothermal treatment conditions.4.The lumped effect of crude methanol containing various impurities on the MTP reaction after the removal of alkali metal ions showed that crude methanol as a lumped feed had little effect on the MTP reaction after the removal of alkali metal ions.The initial propylene selectivities of the two feedstocks were almost the same,and the feeding method had no significant effect on the induction period of the reaction.The variation trend of ethylene and propylene was similar.The selectivity of propylene increased gradually with the reaction time,while that of ethylene decreased gradually.The selectivity of C3H8,C4,C5+C6 is similar.The catalyst lifetime of crude methanol feed is slightly longer than that of refined methanol feed.5.The study found that the selectivity of methane of crude methanol feeding showed a unique rule,and the selectivity of methane of the two feeding methods showed different trends.At the initial stage of the reaction,the methane selectivity of crude methanol is 3～4% higher than that of refined methanol feed by 2.5%.As the reaction goes on,the methane formation rate of crude methanol feed decreases,while the methane selectivity of refined methanol feed increases gradually and then stabilizes between 3.5% and 4%.According to the mechanism of the formation of methane from dimethyl ether via free radical,the influence mechanism of raw methanol feeding on the reaction was deeply analyzed,which provided scientific guidance for the separation and regulation of by-products in industrial applications.6.The multi-cycle industrial verification of crude methanol feed found that with the increase of reaction time,the catalyst skeleton aluminum shed and the acid density decreased,resulting in the transformation of polymethyl benzene to insoluble carbon deposition.The MTP reaction mechanism under the influence of carbon deposition was analyzed,and the catalyst regeneration strategy of temperature matching carbon deposition species was proposed to guide the industrial production.7.The purification(rectification)process of crude methanol and the MTP process of crude methanol in the 500,000 tons/year coal-to-propylene plant were simulated.The results showed that when the water content of crude methanol increased,the equilibrium conversion and content of methanol decreased linearly.Under the same propylene yield,crude methanol had little effect on the operation of MTP reactor,and the conversion of methanol and dimethyl ether was almost constant.The latent heat of water vaporization causes the significant increase of heat load of DME reaction feed heater,which restricts the application of crude methanol in the existing MTP process.Based on the heat balance theory,the strategy of increasing heat exchange is proposed to obtain the relationship between water content and heat supplement in crude methanol.8.The industrial experiment results show that the production capacity and product quality of the MTP unit are basically unchanged,the energy consumption of the methanol refining unit is greatly reduced,and the economic benefits are significant after the existing refined methanol feed is replaced by crude methanol.The thesis contains 91 pictures,41 tables,and 210 cited references. |
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