Methanol to Gasoline MTG Process

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The MTG process was conceived and developed in response to the energy crisis of the 1970s. It was the first major new route to synthetic hydrocarbons since the introduction of the Fischer-Tropsch process before World War II, and provided an alternative pathway for the production of high-octane gasoline from coal or natural gas. Discovered by accident by a research team at Mobil, the MTG process was actually developed before the MTO process. In fact, the MTO process can be considered as a modified MTG process designed to produce mainly olefins instead of gasoline. For the MTG reaction, medium-pore zeolites with considerable acidity are the most suitable catalysts, with ZSM-5 recognized as the most selective and stable. Because of the defined structure and geometry of their pores, channels and cavities, they are shape-selective catalysts, able to control product selectivity depending on their molecular size and shape. Over this catalyst, methanol is first dehydrated to an equilibrium mixture of DME, methanol and water. This mixture is then converted to light olefins, primarily ethylene and propylene. Once these small olefins are formed, they can undergo further transformations to higher olefins, C3-C6 alkanes and C6-C10 aromatics.

Due to the shape selectivity of ZSM-5, heavier hydrocarbons containing more than 10 carbon atoms are practically not produced in this process. This is fortuitous, as C10 is also the usual limit for conventional gasoline. At the same time, this process produces aromatics (toluene, xylenes, trimethylbenzene, etc.), providing also a route to aromatic hydrocarbons. Depending on the catalyst and conditions used, the product distribution can be modified if desired. Using zeolites with larger pores, channels and cavities, such as ZSM-12 or Mordenite for example, leads to products with higher molecular weight.

Bi-functional acid-base catalysts such as tungsten oxide (WO3) supported on alumina active for the conversion of methanol to light olefins can also catalyze their further reaction to a mixture of higher hydrocarbons including alkanes, alkenes as well as aromatic compounds.

alkanes

CH3OCH3 "H2° > light olefins higher olefins aromatics

In 1979, the New Zealand government selected the MTG process developed by Mobil [233] for the conversion of natural gas from the large off-shore Maui field into gasoline. The New Zealand plant began operations in 1986, producing about 600000 tonnes of gasoline per year, supplying one-third of New Zealand's gasoline needs. Methanol was produced using the ICI low-pressure methanol process in two production units, each capable of producing 2200 tonnes of methanol per day. Crude methanol from these units was fed directly to the MTG section, where it was first converted over an alumina catalyst to an equilibrium mixture of methanol, DME and water. This mixture was then transferred to the gasoline synthesis reactor where it was reacted over ZSM-5 at 350-400 °C and 20 bar [232]. The crude gasoline was then treated to remove minimal amounts of heavier components. Without further distillation or refining necessary, the high-octane gasoline obtained can be blended directly with the general gasoline pool.

As mentioned earlier, the MTG process was developed in response to the energy crisis of the 1970s, which saw the price of oil and its products rise dramatically. However, as the oil price fell again during the 1980s, dipping briefly under $10 in 1986, the commercial interest for MTG dropped accordingly. Gasoline production at the New Zealand plant ceased because it was now cheaper to use low-cost gasoline derived from petroleum than to produce it from natural gas via methanol. Methanol production itself is, however, still in operation, providing methanol at a competitive cost. Increasing oil prices experienced during the past few years will most probably revive the interest in MTG.

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