Table of Contents
Methanol Production Processes
Production of methanol from fossil fuels
The methanol production process is a low-pressure process. Methanol is produced from natural gas, collected gases from refining processes, gas synthesis, coal and coke, or even biomass.
Methanol is industrially synthesized from coal or natural gas. The reforming step in methanol production is done in different ways. Among the types of reforming used in this process are:
- Steam reforming
- Intensive reforming
- Autothermal reforming
- Combined reforming
- Hot gas reforming
Steps such as compression and synthesis of methanol are performed after the reforming operation. Raw methanol is separated from water, ethanol and other compounds in the separation unit. These impurities are separated in a system consisting of two distillation columns. In the first column, light impurities such as ethers, esters, acetone as well as insoluble gases are removed, and in the second column, heavier water and alcohols and similar heavy organic matter will be removed from the stream.
Today, with the exception of China, which uses coal to produce methanol, other parts of the world use cheap natural gas as feed to produce methanol. Economically, it is different to choose either synthesis gas or coal technology to produce methanol. In China, due to the huge resources of coal, building units based on this technique is a priority.
There is also a worldwide tendency for offshore units to use natural gas extraction facilities to produce methanol from natural gas due to access to cheaper gas reservoirs. There are different methods for preparing and synthesizing methanol with natural gas. These methods are:
- Steam reforming
- Partial oxidation
- Partial catalytic oxidation
- Photocatalytic method
Direct and indirect methods are used to convert natural gas to methanol. In the direct method, methane is converted directly to methanol at high temperature and pressure, which is limited in industry due to special operating conditions (high temperature and pressure).
The indirect method is done in three steps. In the first stage, methane first reacts with water to form carbon monoxide and hydrogen, which is reacted according to equation (1) and is called synthesis gas production. Due to the importance of this stage and the fact that it is extremely hot and requires a lot of energy, about 60% of the total cost of the process is related to this stage.
In the second stage, the synthetic gas produced in the first stage is converted to methanol:
After the production of methanol, in the third stage, the methanol is separated from the system and the uncharged gases return to the system in the return stream to reproduce the methanol.
Production of methanol by modern methods (Green Methanol)
Methanol is one of the most widely used and basic chemical products that can be collected and produced using carbon dioxide. Methanol production unit is one of the important petrochemical units. In Iran, due to its rich gas reserves, it is one of the main producers of methanol. So far, 5 methanol units with a production capacity of 5 million tons per year have been created and about 12 units with a capacity of 19 million tons per year are being studied and commissioned.
The main reason for paying attention to the production of methanol with recovered carbon dioxide is the production of methanol from any source of carbon dioxide. This advantage has made it possible to use any source of carbon dioxide produced and enter it into the recovery cycle to reduce carbon dioxide as a greenhouse gas.
In this cycle, carbon dioxide is collected from any source and converted to methanol in response to hydrogen and recovered. In this cycle, the required hydrogen is produced completely independent of fossil fuels and using electrolysis.
In this cycle, methanol is renewable, so that in other processes, methanol is used to produce petroleum compounds such as ethylene, propylene, gasoline and other products that are part of oil and gas derivatives, and then as soon as combustion. These products re-produce carbon dioxide and are released into the air, which is recovered and collected and sent to the methanol production cycle. The conversion of carbon dioxide to organic compounds and the preparation of fuels are the most useful ways to reduce carbon dioxide.
Hydrogen production process for methanol synthesis
Hydrogen does not exist in nature in its pure form, but it can be obtained from several other elements in several different ways. The process of producing hydrogen for the synthesis of methanol itself is one of the topics of interest and there are various methods for its production. For example, among the methods of hydrogen production, the following methods can be mentioned:
- Hydrogen production from fossil sources (non-renewable)
- Partial oxidation of heavy oil
- Natural gas converter with gas conversion process by steam
- Partial oxidation natural gas converter
- Hydrogen production from non-fossil sources (renewable)
- Thermolysis, radiolysis and electrodialysis of water
- Biomass materials
In general, hydrogen production methods are either carbon-free or obtained from fossil fuels. Non-carbon methods include the use of nuclear energy, wind energy and solar energy. In methods based on fossil fuels, synthetic gas is produced and hydrogen is produced through it.
In order to reduce the emission of hydrogen CO2 to produce methanol, it is better to produce it without a carbon method. For this purpose, hydrogen from electrolysis of water using a renewable electricity source can be considered as a carbon-free source. Biomass, solar radiation and wind are also the most common renewable sources proposed for power supply in water electrolysis, while wind is currently one of the most effective and rich sources of renewable energy.
Although various methods have been proposed today to produce hydrogen, about 98% of the total hydrogen produced in the world is now obtained from fossil fuels.
It should be noted that the hydrogen produced in the industry is considered as a chemical product and the commercial sale of hydrogen is less than 10% of its production in the world, which means that 90% of the hydrogen produced is consumed at the production site.
The following is a brief description of some of the methods of hydrogen production:
Production of hydrogen using the conversion of natural gas to steam
Conversion of natural gas by steam is one of the most common methods of hydrogen production. Methane (the main element of natural gas) participates in the equilibrium reaction with steam and the product of the reaction is mainly hydrogen and carbon monoxide during reaction (3):
In addition to methane, other hydrocarbons can also produce hydrogen in response to water vapor conversion. Hence the general form of the water vapor conversion reaction can be shown as (4):
The main role of steam in steam conversion reactions is to transfer the reaction to the production of products, i.e. the production of hydrogen and carbon monoxide, and due to the equilibrium of the reaction, to produce more hydrogen and carbon monoxide, the reaction must be done at high temperature and low pressure. This reaction usually takes place at a constant pressure, so it moves towards hydrogen production in order to raise the temperature. Figure (1) shows a simple diagram of the hydrogen production process in this way.
Fig (1): A simple diagram of the process of hydrogen production using the conversion of natural gas to steam
Production of hydrogen gas using partial oxidation of natural gas
In cases where the use of natural gas is not economical or heavy oil is available cheaply, partial oxidation is used to produce hydrogen. Partial oxidation reactions take place in this process, which are as follows:
Partial oxidation is an exothermic reaction and takes place at high temperatures in the range of 1500-1200 C without a catalyst. The advantage of using this method over catalytic processes is that there is no need to remove sulfur compounds from the feed. Also, the high temperature in partial oxidation makes it possible to use heavier oil slices that could not be used in catalytic processes and converted to hydrogen in this process.
This process may also be performed in the presence of platinum and nickel catalysts. The efficiency of the partial oxidation process is lower than that of the water vapor conversion process, and less hydrogen is produced per molecule of methane. A partial oxidation converter is used to prepare fuel cells. In this converter, by changing the ratio of air to fuel, the reaction temperature and consequently the reactor temperature is controlled and no other converter is needed. Figure (2) shows a simple diagram of hydrogen production using a partial oxidation process.
Fig (2): Hydrogen production using the partial oxidation process
Hydrogen generation using Auto Thermal Process
In the Auto Thermal process, water vapor is another process used to produce hydrogen. In this oxidizing process, which is usually oxygen or air, it enters the burner part of the reactor and then passes over the surface of the catalyst at high temperature.
Reactions in this process are a mixture of partial oxidation exothermic reactions and alternative heat exchangers with water vapor, and in fact the energy required for the water vapor conversion reaction is provided by the partial oxidation reaction. Therefore, the reaction temperature and consequently the reactor temperature can be controlled by changing the air to fuel ratio. The general form of the reaction performed in this process is presented in reaction (8).
That is why this type of converter is called Auto thermal. The advantage of this process over conventional water vapor systems is the low steam to heat supply as well as the supply of heat required by the combustion of part of the fuel.
Hydrogen conversion using pyrolysis process
Another process that exists to produce hydrogen from hydrocarbons is the heating of hydrocarbons in the absence of air, during which the hydrocarbons are broken down and decomposed into hydrogen and carbon. The most important advantage of the thermal failure process is the production of high purity hydrogen. By adding air to the hot reactor, carbon is released from the system as carbon dioxide. In some cases, the use of propane thermal refraction process to provide hydrogen for use in polymer fuel cell systems has been suggested.
Hydrogen production using coal gasification
To produce hydrogen or hydrogen-rich gas, coal is gaseous using pure oxygen with a high percentage of purity at high temperatures and pressures, and hydrogen is produced according to the following reactions:
Figure (3) also shows a simple diagram of the process of hydrogen gas production by the process of gasification of coal.
Fig (3): A simple diagram of the process of hydrogen gas production by the process of coal gasification
Hydrogen production using the iron vapor process
One of the oldest processes for producing hydrogen is the use of the iron vapor process. This process is based on coal and the gas from coal is used to reduce iron oxide and convert it to iron. In this process, in the first stage, coal under the influence of steam and air is used to produce hydrogen and carbon monoxide reducing gases during the following reaction:
In the second stage, these gases react with iron oxides and produce reduced iron oxides according to the following reactions:
In the third stage of the reaction, the reduced components are introduced into the steam-iron reactor in the presence of oxidized water and iron, and finally Fe3O4 and hydrogen-rich gas are produced. Figure (4) also shows a simple diagram of the hydrogen production process using the iron vapor process.
Fig (4): A simple diagram of the hydrogen production process using the iron vapor process