Methanol to Olefin MTO Process

The methanol to olefin technology, or MTO, was developed as a two-step process, which first converts natural gas via syn-gas to methanol, followed by its transformation to light olefins. The driving force for the development of this technology was to utilize natural gas sources far from major consumer centers. The conversion of methanol to olefins proceed through the pathway:

The initial step is the dehydration of methanol to dimethyl ether (DME), which then reacts further to form ethylene and propylene. In the process, small amounts of butenes, higher olefins, alkanes and some aromatics are also produced.

Besides the synthetic aluminosilicate zeolite (ZSM-5) catalysts, numerous other catalysts were also studied. UOP developed silicoaluminophosphate (SAPO) molecular sieves such as SAPO-34 and SAPO-17, which have demonstrated high activity and selectivity for the MTO process. Both type of zeolites have defined three-dimensional crystalline structures. They are microporous solids permeated with channels and cages of very specific size. The many different zeolite catalysts differ by their chemical composition and the size and structure of these channels and cages, which have molecular dimensions ranging from 3 to 13 A. The catalytic sites responsible for the catalyst's activity are placed in the pores and channels of these catalysts. Access to these sites will therefore be limited to chemical reagents small enough to enter the zeolites pores and channels. At the same time, the size of the reaction products is also governed by space constraints imposed by the catalysts structure. As a result, zeolites can be highly shape-selective catalysts. ZSM-5, for example has pore openings of some 5.5 A, allowing much faster diffusion of para-xylene than meta- or ortho-xylene with a larger molecular size. With a pore size of only 3.8 A, SAPO-34 used in the MTO process, allows effective control of the size of the olefins that emerge from the catalyst. Larger olefins diffuse out at a lower rate, making smaller olefins such as ethylene and propylene the predominant products.

Independent from zeolites, bi-functional supported acid-base catalysts, such as tungsten oxide over alumina (WO3/Al2O3) were found by Olah and coworkers during the 1980s also to be active for the conversion of methanol to ethylene and propylene at temperatures between 250 and 350 °C [234,235], and subsequently to hydrocarbons. These heterogeneous bi-functional catalysts catalyze the reaction despite the fact that they lack the well-defined three-dimensional structure of shape-selective zeolite catalysts.

Based on SAPO-34, an MTO process has been developed jointly by UOP and Norsk Hydro [236, 237]. This process converts methanol in more than 80% selectivity to ethylene and propylene. In addition, about 10% is converted to butenes, which are also valuable starting materials for a variety of products. Depending on the operating conditions, the propylene to ethylene weight ratio can be modified from 0.77 to 1.33. This allows considerable flexibility and adaptation to changing market conditions. The technology has been extensively tested in a demonstration plant in Norway, and more than ten years of development have now been completed. Currently, the UOP/Hydro MTO process is commercialized in Nigeria and units are being considered in other locations. A methanol plant, built close to the capital city of Lagos and with a capacity of 2.5 million tons per year will be the largest in the world. The methanol produced will then be converted to ethy-lene and propylene to produce 400 000 tonnes per year of polyethylene and 400 000 tonnes per year of polypropylene. This will be the first commercial large-scale demonstration of the MTO technology.

Lurgi has also developed an MTO process [238] which, unlike the UOP/Hydro technology, is designed to yield mostly propylene. It is thus described as a methanol to propylene (MTP) process. In a first step, methanol is dehydrated over a slightly acidic catalyst to produce DME. The DME is then reacted over a ZSM-5-based catalyst under moderate pressure (1.3-1.6 bar) and temperatures between 420 °C and 490 °C to form light olefins. The achievable overall yield of propylene is above 70%. The process has been demonstrated at Statoil's Tjeldbergodden methanol plant in Norway, and is now ready for commercialization. Propylene obtained from this unit with a purity of 99.7% has also been successfully polymerized to polypropylene, showing the feasibility of producing polypropylene directly from methanol obtained from natural gas.

Mobil (now ExxonMobil), which pioneered the MTO technology using the company's ZSM-5 catalyst (ZSM meaning literally Zeolite Synthesized by Mobil) also demonstrated this technology on a 100 barrel-per-day scale in Wesseling, Germany. In addition, Mobil also developed the subsequent Olefins to Gasoline and Distillate (MOGD) process. In this process, which was originally developed as a refinery process, the olefins from the MTO unit are oligomerized over a ZSM-5 catalyst to yield, with selectivity greater than 95%, hydrocarbons in the gasoline and/or distillate range. Depending on the reaction conditions, the ratio between gasoline and distillate can be varied considerably, allowing a significant flexibility in production. When operated at relatively low temperatures and high pressures (200-300 °C, 20-105 bar), called the distillate mode, the products are higher molecular weight olefins which, after being subjected to hydrogenation, produce fuels including diesel and premium quality jet fuels. Changing the operating conditions to higher temperatures and lower pressures leads to the formation of products of lower molecular weight but with higher aromatic content (i.e., high-octane gasoline). Besides being a good catalyst for the MTO and MOGD processes, ZSM-5 was also found to catalyze the direct conversion of methanol to hydrocarbons in the gasoline range. This led to the development of the Methanol to Gasoline process (MTG).

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Responses

  • alfrida
    Are olefins more valuable than methanol?
    3 months ago

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