Natural gas is the source for a high concentration of methane in the earth crust. Natural gas contains compounds from ethane to hexane along with nitrogen, hydrogen sulphide and carbon dioxide. Components other than methane present in lesser percentage on average maybe around 15%. Methane molecule made up of one carbon and four hydrogen atoms and it belongs to C1 or alkane group. It can be easily split and can be converted to other organic chemical products, so natural gas is purified and separations are conducted to remove other than methane compounds. Methane concentration varies from 70 to 98 % based on the location of the gas reserves. Oil well reserve also has a natural gas fraction with crude oil. Natural gas is used directly as fuel for heating applications in process and power industries.
Processing natural gas by dehydration methods (initial step)
The necessity to remove the moisture content present natural gas by pre-treatment operation is essential to prevent formation plugging compounds due to the reaction of hygroscopic materials.
Three main effects caused by moisture present in natural gas:
- Plugging of the pipeline due to hydrates formation
- Water accumulation at bends and joints
- Corrosion of pipe at traps and drains points by condensed water
Process plants wildly use two kinds of separation operation to remove water vapour from natural gas by cooling the gas to water dew point, by which water vapour condenses and separated by:
- Gas-liquid separator
- Separation by desiccant (liquid type or solid bed like molecular sieves)
Factors affect the separation operation
- Water vapour concentration in natural gas
- Gas flow rate inlet and outlet of the separation unit
- Pressure drop in the equipment
Processing natural gas by chemical method (second step):
The dehydrated natural gas contains H2S and CO2 the acid gas constitutes. In the chemical process industry, acid gas components are removed by absorption and stripping technology. N-methyl-diethanolamine (MDEA) solution is used as an absorbing agent in absorber that operates at low temperature and high pressure. H2S and CO2 are scrubbed out from natural gas and absorbed into the MDEA solution. The rich methyl diethanolamine solution is sent to stripping column to strip out the acid gas components. The regenerated methyl diethanolamine (MDEA) again sent to the top section of the absorber.
Process flowsheet of processing natural gas by absorption and stripping system using MDEA solution:
Petrochemical industries use methane-rich natural gas for producing other chemical products. Based on the structure and chemistry of methane number of specified chemical process routes developed. They are classified as direct process and indirect process. A particular chemical product is produced from methane with one step of reaction comes under the section of the direct process. A vast gap in research can be seen in this area. Conversion of methane and catalyst selection is the important parameters for the researcher. The direct process handles the chemical reaction like:
- 2CH4+ O2 → C2H4 + 2H2O
- 2CH2 + ½ O2 → C2H6 + H2O
C2 compounds are created from C1 compound (methane). As for now, this conversion achieved the yields of 20 to 22%.
Indirect process section covers those processes that involve more than one-step chemical reaction processing. For example one of the processes which was adopted in most chemical industries all over the world that natural gas is changed into a gas mixture called synthesis gas having a major component of H2 and CO. This so produced synthesis gas used to make a selected compound with help of a catalyst. Natural gas is processed different kinds of technologies like reforming, auto-thermal reforming, partial oxidation and steam reforming.
Let see what products are formed by direct and indirect processing natural gas:
Direct process: Natural gas with high methane is reacted with oxygen to produce formaldehyde, hydrogen, C2H6, C2H4, (CO, H2) mixture with the ratio of 1:2 respectively. When treated with steam it gives CO, H2 mixture with 1:3. When processed with CO2 the same mixture with 1: 1 produced. The mixture converted to syngas. Finally, it is the main source of producing heat energy in chemical industries.
Indirect process: Products like dimethyl ether, ammonia, methanol, acetic acid, phosgene, oxo-alcohols, metal carbonyls and hydrocarbons are produced as the second step in sequence from the first step of syngas production. Now scientist are developing a process that used to produce any chemical product we want from natural gas in this sector
As the environment concern the carbon dioxide produced in the above process technologies, converted to a useful product with modern catalyst technologies. To name one, carbon dioxide reformed to methane again by water shift gas, which is the old form, but now carbon dioxide converted to dimethyl ether, which is the famous replacement for diesel fuel.
Primary fuel and chemical product from natural gas:
By autothermal reforming, it is an easy way to produce hydrogen from natural gas rather generating it from water using electrolysis method. Hydrogen production from natural gas is the simplest process than all other conversion processes. It generates energy by the reaction and consumes less energy for running the process plant. The reactor’s best optimum condition requires less operating pressure normally slightly above atmospheric pressure is selected. However, the reactor temperature would be around 500oC. In the case of the autothermal reactor, the water to carbon ratio maintained near 4 and oxygen to carbon ratio nearly 0.9.