Methanol to Olefins: State of the Art and Perspectives

摘要

Natural gas is an attractive feedstock on account of its vast reserves worldwide. World gas reserves have continued to increase as a result of the discovery of new gas fields and increasing conservation methods used in the recovery of associated gas from crude oil production. While processes are not yet available for the direct conversion of natural gas to olefins, its conversion to methanol is well established, with the trend being toward the construction of ever larger methanol plants, presently being built for up to 5,000 metric tons per day (MTD) and soon possibly for up to 10,000 MTD. The availability of low cost methanol provides a very attractive economic incentive to selectively convert it to ethene and propene and further to downstream derivatives such as polyethene, polypropene and PVC through the Gas-To-Olefins (GTO) and Gas-To-Polymers (GTP) concepts. The UOP/Hydro MTO process has been jointly developed by UOP and Norsk Hydro for the selective production of ethene and propenefrom either crude or refined methanol. The catalyst used in the process is based on a silicoaluminophosphate, SAPO-34. The technology has been extensively demonstrated in a demo plant by Norsk Hydro. The MTO process converts methanol to ethene andpropene at close to 80% carbon selectivity in a fluidized bed reactor. The carbon selectivity approaches 90% if butenes are also accounted for as part of the product slate. There is considerable flexibility in the operation of an MTO unit. Typically, the C_2~=/C_3~= ratio can be modified within a range from about 0.75 to 1.5 by adjusting the operating severity, with higher temperatures leading to higher C_2~=/C_3~= ratios. The UOP/Hydro MTO process is a major step forward compared to earlier MTO technologies. For gas monetization GTO is more economical than GTL (fuels) and LNG, and MTO competes well with naphtha crackers for light olefin production. GTO/MTO introduces through the use of cheap natural gas a new feedstock which is less sensitive to oil price and offers an ethene/propene flexibility that can meet the increased future need for propene.