Gas treatment
Before natural gas can be fed into the supply grid, it has to be treated. The composition of the natural gas supplied is dependent on the type and depth of the reservoir. The gas will certainly be mixed with water in the form of reservoir brine and condensation. In 1,000 cubic metres of gas there can be anything between 10 and over 100 liters of water. To prevent any corrosion or impermeation through gas hydrate formation in the supply pipelines, this damp gas has to be dried. Since it is mainly water that is separated off, one normally refers to the process as gas dehydration.
However, other undesirable components of natural gas are highercarbon chain hydrocarbons and traces of mercury. These substances are also removed in the treatment process. The facilities where this process is carried out are known as gas dehydration plants. Every well site has a gas dehydration plant so the gas does not have to pumped over any great distance before treatment.
Gas treatment by the low temperature separation (LTS) method
Like all other gases, natural gas cools down when pressure is reduced. This is known as the Joule-Thomson effect. It is this characteristic of gas that RWE Dea experts utilize in treating gas by the LTS method. By gradually reducing the pressure, the temperature of the gas is lowered to -32° C. As a result, the water and the higher hydrocarbons and mercury condense, leaving the natural gas ready for use.
Gas treatment (simplified presentation)
1. Wells
The natural gas comes up from the reservoir via the production string and reaches the surface at pressures of up to several hundred bars. From the well head the gas is pumped along a high-pressure pipeline into the first section of the gas dehydration plant.
2. Free water knockout
The first step in the treatment process takes place in the so-called free water knockout unit. The tiny drops of water carried in the gas stream are segregated.
3. Preheating and pressure reduction
In order to utilize the Joule-Thomson effect, the gas has to be at a specific temperature at the beginning of the process. First a preheater heats up the cold gas to the required temperature. Later the temperature is reduced in a cooling unit. Reducing valves lower the pressure of the gas in two stages to 115 - 125 bars with the result that the water vapor condenses and is collected in the pre-separator. In the subsequent cold separator the remaining water, mercury and higher hydrocarbons are also abstracted. The hydrocarbons are transported to a refinery for further processing.
4. Glycol absorption
The final drying process to remove any remaining moisture from the gas involves spraying glycol into the gas stream. Glycol's hygroscopic qualities enable it to absorb any remaining water vapor in the gas. The saturated glycol is separated, the water removed and the purified glycol returned to the dehydration plant.
After dehydration and pressure reduction the natural gas is ready for use and can be fed into the supply pipelines at pressures of around 80 bars.
