NATURAL GAS AND IMPURITIES
The pure natural gas that we all use is, in San Antonio, almost 100% methane (CH4). It is important that the natural gas we burn is pure because it gives a correct and consistent amount of heat while minimizing the pollution produced.
Natural gas from the wellhead has impurities that cause problems when it is burned. Some of the impurities are heavier hydrocarbons such as ethane (C2H6), propane (C3H8), and heavier–longer chained hydrocarbons. These heavier hydrocarbons form natural gas liquids (NGL) when processed. These heavier hydrocarbons in the gas are stripped because they are sold as valuable liquids and, if left in the gas, greatly increase the heat of the burned gas.
Other common natural gas impurities, such as sulfur and benzene, cause pollution when burned. Another, water, also makes the gas more difficult to process. Of all these impurities listed, water must be removed first so that the remaining gas processing equipment can work properly. To remove the water from the gas stream, the gas is first sent to a dehydration unit (dehy). The dehydration unit commonly uses glycol to pull water from the natural gas, so it can also be called a glycol unit in that case.
WET GAS VS DRY GAS
In the reservoir, we consider a ‘wet gas’ to have a rich blend of liquid forming heavier hydrocarbons and a dry gas to have less liquids. However, in surface processing we will consider a ‘wet gas’ to have entrained water vapor regardless of the NGL content. To remove the water vapor, the wet gas stream is put into a separator featuring a glycol compound. Glycol compounds are liquids and have a strong affinity for water. The ‘high affinity’ means that when the water vapor comes into contact, the water prefers to be mixed with the glycol and it leaves the natural gas. Once the water vapor has left the natural gas and entered the liquid glycol, then gravity does the rest of the work. Natural gas is much lighter than liquid water and glycol so it rises to the top of the unit and the ‘dry gas’ is piped out while the liquids fall to the bottom.
The dry gas is sent to a gas plant where the components can now be separated by manipulating its temperature and pressure. The liquid water and glycol are recycled in a reconditioning process to be reused as fresh glycol in the dehydration separator. Glycols boil at a temperature higher than water. By heating boiling the water off of the glycol, the vapor evaporates away.
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