Progress in the Field of Converting Methanol into Light Olefins over the ZSM-5 Zeolite Catalyst

ReviewsProgress in the Field of Converting Methanolinto Light Olefins over the ZSM-5Zeolite CatalystLiu Yue' ; Guo Qi2(1.SINOPEC Corp, Beijing 100029; 2. Tianjin Branch Company, SINOPEC, Tianjin 300271)Abstract: Technical progress in the field of conversion of methanol into ethylene and propylene over theZSM-5 catalyst was summarized. The economical analysis of the technology, the mechanism of chemicalreaction and reaction kinetics were introduced. The factors including the effect of the operating conditions,the influence of catalyst preparation conditions and modification of ZSM-5 zeolite on the reaction and cokeformation were also discussed.Key Words: methanol; olefin; catalyst; technical progress1 IntroductionMethane + Steam-一+ CO+H, 一Methanol一→Low-carbon olefinsLow-carbon olefins- in particular ethylene and propylene asthe basic organic chemical feedstocks are playing a pivotal The MTO technology is soon to be commercialized. Egypt isrole in modern petroleum and chemical industry. There are to install in Suez a commercial integrated unit, which adoptstwo routes for manufacture of low-carbon olefins: one is thethe fluidized MTO process licensed by UOP/HYDRO, andpetroleum-based route and the other is the non-petroleum the unit is expected to be in operation in 2004. The Innerbased route. Till now most countries in the world predomi- Mongolia Autonomous Region of China will construct a com-nantly adopt traditional method of naphtha cracking to manu- mercial unit adopting the fixed-bed MTO process licensed byfacture ethylene, propylene and other low-carbon olefinsl2. Lurgi GmbH, and this unit will be put on stream in 2005.Owing to limited oil production in China and low naphthacontent in crude oils designated for manufacture of low-car-2 Techno-Economic A ssessment of thebon olefins, it is necessary to pay attention on development MTO Processof technology for manufacture of light olefins from non-pe-troleum feedstock in compliance with the situation in China UOP of USA has analyzed the economic aspects of the MTOalong with the improvement of traditional methods of steam technology and ethylene manufacture through traditionalcracking and catalytic cracking to solve the gap between the naphtha cracking!3: At a similar ethylene production scale ofsupply and demand for ethylene and propylene.500 kt/a the investment in MTO based on natural gas is 43%higher than that of naphtha cracking (excluding the invest-As of the end of 2002 the estimated proven reserves of natu- ment in construction of refinery for providing naphtha), butral gas in China reached 1.51 tillion cubic meters, and the its production cost is only 20% of that of naphtha cracking.potential of gas resources greatly exceeds that of oil. The Furthermore, the investment in the former case is simple andnatural gas can be used as the feedstock to manufacture the rate of return is by 3% higher.methanol, which is then converted into olefins through theMTO (methanol to olefins) process that can reduce the con- UOP considers that the MTO process can be applied in threesumption of petroleum for olefin production. The process different scenarios: firstly, the GTO combination unit can beroute is as follows:setupontheg中国煤化I>d of gas field;fYHCNMH G7.China Petroleum Processing and Petrochemical TechnologyNo.2, June 2004secondly, the methanol produced can be routed into a region to further formation of the first C- e bond.or regions that are close to the end users; and thirdly, a MTOreactor interface can be added to the existing naphtha or LPG3.2 The carbene mechanisml16,17]pyrolysis unit. Since the total olefins output can be enhancedafter increase of a slight load in the product recovery section，Under the synergistic action of the acid and basic sites metha-the ROI can exceed 36% as compared to a ROI of27.5% for nol is deprived of water to give carbene (:CH), which is thenthe previous two cases.subjected to polymerization or can be inserted into methanolor dimethyl ether molecules to form olefins:3 Reaction Mechanism for MTOH- CH, OH-H,O + :CH,The MTO reaction process can be divided into three steps: n:CH,一 → (CH)n, n=2,3,4,5formation of methoxy radicals on the zeolite surface, forma-tion of first C- E bond, and formation of C, and C。3.3 The free radical mechanism[18]The methoxy radical on the surface is an important intermedi- The reactions involving conversion of methanol into olefinsate during the catalytic reaction in the presence of may be comprised of free radical reactions.zeoltel-9. At a relatively high temperature (400C) themethoxy radical on the catalyst surface is highly active andis CH,OH* CH,●+OH.apt to participate in the reactionl8. During the MTO reaction 2CH,. .dimethyl ether (DME) is formed after dehydration of one mole CH, ●+CH。一+ CH,+CH,CH,●of water from methanol, and an equilibium mixture between CH, ●+CH,CH.一→ CHmethanol and DME is quickly establishedIol. Methanol/DMEmolecules can enter into reaction with the acid sites on the 3.4 The carbon pool mechanism19 22]1zeolite to form methoxy radicals, possibly two kinds ofmethoxy radical1.I21. The first is formed after reaction of Some researchers think that in the course of methanol reac-methanol/DME molecules with the Bronsted acid sites, andtion there exist transition species such as (CH)n in whichthis kind of methoxy species can play a critical role in the 0