The 3 Phases of Large-Scale Methanol Production

Date Published:   01/06/2015

Understanding the molecular process of methanol applications has provided us with the opportunity to create practical solutions for its use. Beneficial products such as antifreeze, wastewater treatment, camping and boating stoves, de-staining products, and even acetic acid are all produced using methanol. Among its most prominent applications, methanol has been used to fuel ground transportation vehicles through its production of methyl tert-butyl ether (MTBE), which is most commonly applied as an octane booster in gasoline. Methanol can also be used to produce biodiesel as well as diesel and gasoline for high performance engines.

Looking to the future, researchers are studying methanol’s potential for sustainable energy use because of its short half life in groundwater. Whereas benzene can carry a half life of up to 730 days, methanol is considerably biodegradable with a half life of up to only 7 days. Some suggest that there may even be a use for methanol in powering electronic devices such as laptops and cell phones.

No matter the intended use of the methanol, there are three distinct stages of methanol production, including:

1. Autothermal Reforming of Natural Gas to Synthesis Gas

While methanol can be made from biomass, carbon dioxide plant emissions, and most fossil fuels, natural gas is the by far the most widely used feedstock for its convenience and efficient conversion. The first step in methanol production requires the formation of synthesis gas (“syngas” is a mixture of hydrogen and carbon monoxide gases) from natural gas.

Natural gas and steam in a 3:1 ratio are heated to 1,300 to 1,800 degrees Fahrenheit inside nickel catalyst-filled tubes. Methane, natural gas’s primary component, reacts with the water vapor to make syngas:

CH4 + H2O à3H2 + CO

The exact ratio of hydrogen and carbon monoxide in the product can be controlled through pressure, natural gas composition, steam-to-carbon ratio, and other reactor design and processing variables. The steam methane conversion equation above produces a 2:1 diatomic hydrogen to carbon monoxide ratio. This autothermal reforming of natural gas ratio is perfect for producing both fuel-grade and high-purity methanol outlined in step 2.

2. Methanol Synthesis

When exposed to a copper/zinc oxide, aluminum oxide catalyst, single carbon monoxide molecules combine with two diatomic hydrogens to synthesize methanol:

CO + 2H2 à CH3OH

Any carbon dioxide in the gas mix also reacts with hydrogen for additional methanol production:

CO2 + 3H2 à CH3OH + H2O

These reactions take place in a low-pressure synthesis loop reactor that allows for maximum recovery of the heat generated, as the three reactions are net exothermic. Use of isothermal reactors also allows for smaller sizes to keep investment and operating costs down. Composition of the catalyst is key, as high-performance and methanol synthesis selectivity are necessary for efficient conversion and long-term operation.

The two reactions are net exothermic, and the heat generated can be converted to high-pressure steam for use in other operations in the plant or to generate electricity. Many methanol plants have installed pre-reformers that optimize the natural gas feed for more complete syngas production. The adiabatic conversion transforms higher hydrocarbons in the feed into methane and other lighter compounds such as carbon oxides and hydrogen so the primary reformers can work more efficiently.

After treatment by the pre-reformers, the syngas enters a stand-alone, oxygen-fired autothermal reformer composed of a:

  • Burner
  • Combustion zone
  • Catalyst bed in a refractory lined pressure vessel

As the burner combusts, the feedstock in the oxygen-rich environment to complete the steam reforming and shift conversion reactions to equilibrium. The synthesis gas produced by ATR is rich in carbon monoxide, resulting in high reactivity of the gas. Removing carbon dioxide or recovering and recycling hydrogen makes it suitable for methanol production.

3. Methanol Purification and Distillation

Synthesized methanol and water travel to a two-stage separation and boiler tower for purification and distillation to concentrations of 99.85% by weight or even greater. The product flowing into the vaporizing units typically contains 18% water, as well as alkanes, alcohols and propanol that must be burned off to achieve the required purity (IMPCA or U.S. Grade AA).

  • Topping – First-stage purification is a topping process that removes “light end” impurities with relatively low boiling points. The water/crude methanol mixture is heating in the topping column to a temperature below methanol’s boiling point. “Light end” pounds boil off.
  • Refining – In the second stage, the methanol/water mixture enters a refining column consisting of series of boiling tanks. Methanol boils at a lower temperature than water, so as the first tank is boiled, methanol vapor rises (along with some water steam) into the next tank. The vapor is cooled, then boiled again in each tank. Every iteration of boiling/cooling process extracts the methanol and lesser and lesser quantities of water until the liquid methanol concentration approaches 100%.

Phoenix Equipment sells second-hand methanol synthesis loop equipment and turnkey plants immediately available for relocation. Buying used plants and components can save manufacturers significant capital and drastically shorten the time required to build or expand manufacturing operations. Contact us today so we can help you find the processing equipment or plant to suite your needs.

References
Plant Subcategory Methanol Plants
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