Methanol-to-Gasoline Process
Published on 8 May 2023
The Methanol-to-Gasoline (MTG) process is a method to produce synthetic gasoline developed by Mobil in the 1970s. It has already been used at industrial scale, either in New Zealand, exploiting a cheap source of natural gas, or in China to convert coal into liquid fuel. In any case, the carbon source is converted into syngas, then converted into methanol, itself transformed into gasoline with the MTG process. Following the same approach, hydrogen from electrolysis and captured CO2 could be combined into syngas, then converted into eGasoline.
Reactions and reactors
The first step of methanol (CH3OH) to gasoline conversion is the dehydration of methanol over an aluminium-based catalyst to produce dimethyl ether (CH3OCH3). The reaction occurs in slightly pressurised reactors (< 15 bar) and releases about 1750 kJ/kg CH3OH. As it is an exothermic reaction, it is favoured by low temperature (300-320°C) and active reactor cooling is required.
Dimethyl ether is then fed to a second reactor over a ZSM-5 zeolite catalyst to produce a mix of hydrocarbons of size C12 or lower. The conversion into hydrocarbons includes 2 sub-steps: conversion of dimethyl ether into light C2-C4 olefins, and then conversion of light olefins into C4+ paraffins, aromatics, larger olefins and polycyclic aromatics. More recent developments have shown that ZSM-5 catalyst is preferable when targeting gasoline-like component while SAPO-34 provides a higher olefinic yield and is therefore preferred when looking for olefins (feed for non-transportation renewable petrochemicals).
Total conversion of CH3OCH3 into hydrocarbons is obtained, with 25% being light gaseous hydrocarbons and 75% being heavier liquid hydrocarbons. The reactor operates at higher temperature (400-420°C) but reaction is still exothermic and active reactor cooling is required.
The C2-C12 range of produced hydrocarbons is perfect for synthetic gasoline. The lighter gaseous component are generally recycled by converting them back into syngas, while the liquid hydrocarbons can be distilled and upgraded following standard refinery process, and exploiting heat produced in the first 2 reactors.
Mass and energy balance
The MTG process is a licensed product from ExxonMobil with no other source of information than corporate presentations.
Because methanol has a far larger content in hydrogen and oxygen than gasoline, such atoms need extraction during the process, mostly in the form of water molecules. Starting from 1000 kg of methanol, about 560 kg of water and 440 kg of hydrocarbons are produced. Out of the 440 kg of hydrocarbons, 85 to 90% are compounds of the synthetic gasoline, while the remainder is Liquefied Petroleum Gas (LPG) compounds. The weight of produced gasoline is therefore only about 40% of the weight of incoming methanol. Yet, this is not totally surprising as methanol has a far lower energy content per unit of mass than gasoline
From an energetic point of view, the balance is more advantageous with the produced gasoline containing about 95% of the energy that was initially in methanol. As the methanol to dimethyl-ether and dimethyl-ether to hydrocarbon steps both release excess heat, most auxiliaries in the subsequent gasoline refining process are self-propelled, so that even after accounting for energy consumption in the auxiliaries, the energetic efficiency lies between 90 and 93%.
Quality of produced gasoline
The MTG process is capable of producing synthetic gasoline of various qualities. The industrial scale unit in New Zealand in the 1980s was producing RON 92 gasoline (enough to then satisfy the local market), while the recent Porsche/ExxonMobil collaboration demonstrated high gasoline quality with RON up to 102.
The process initially converts dimethyl-ether into light olefins that are subsequently converted into selected aromatics and paraffins. Because the gasoline composition is essentially engineered, the synthetic gasoline can have a very similar composition to crude-based fuels (mainly paraffins with contained fraction of aromatics and a low content of olefins) and be compatible with existing regulations like EN228 gasoline definition in Europe, making it a drop-in fuel for existing cars.
Because it is a synthetic fuel, gasoline is sulfur-free and benzene content can be kept extremely low. Yet, the MTG process tends to produce durene, a heavily-branched large aromatics with high RON number but a freezing temperature of 79°C. Such species needs either to be removed as it would create solid deposits in fuel tanks or the process needs to be tuned to restrict production. Durene production is sensitive to pressure and temperature with less durene produced at low pressure and high temperature.