What is Natural Gas Processing and How is It Done?

Natural Gas and crude oil are found are both found under several layers of earth and are extracted by drilling , but they take very different routes from the wellhead to the consumer. Crude oil and refined oil products travel through pipelines, trucks, and tanker ships, while 99% of natural gas journeys through pipelines made of carbon steel. It is important to bring the recovered natural gas into the pipeline after it has been filtered of contaminants before entering the main interstate pipeline system.

The natural gas used to heat and cool homes and cook food is composed of clean-burning methane, which is the simplest form of hydrocarbon. However, natural gas from the ground is extracted with heavier hydrocarbons and impurities. Depending on the well’s location and the conditions that made the gas, contaminants like sulfur, water and natural gas liquids might be present. Gathering pipelines collect natural gas from wells and deliver it to local processing plants before it enters the interstate pipeline distribution system.

Natural gas comes out of the ground mixed with water vapor. This wet gas is separated using two main methods: glycol dehydration and solid-desiccant dehydration. With glycol dehydration, the wet gas goes through an inlet pipe into a contactor tank, and is then heated to vaporize the captured water. With solid-desiccant dehydration, a higher volume of natural gas is required under high pressure, making it more productive when processing large amounts than the first method.

Natural gas that contains sulfur is referred to as sour gas, primarily because of the rotten egg odor of sulfur. The process of removing sulfur from natural gas is thus called sweetening. The sulfur found in natural gas is hydrogen sulfide and has to be removed because it is very corrosive to pipeline walls and is toxic when inhaled. Also, when recovered in sufficient quantities, the sulfur can be neutralized to create pure and marketable sulfur. The most common sweetening process is similar to that of glycol dehydration.

The natural gas used in homes and transportation is methane and ethane, but natural gas from shale deposits often includes other hydrocarbons like pentane, butane and propane. These natural gas liquids come with their own unique properties that make them suited for different uses other than home heating and transportation. For instance, propane can also be used in space heaters for homes and businesses but different equipment is used to burn this fuel. Butane is primarily used as a key ingredient for synthetic rubber. Processing facilities remove natural gas liquids so they can be gathered and separated into cleaner hydrocarbons.

Most natural gas liquids can be removed with an absorption process which passes the mixed gas through gas-absorbing oils that allows methane gas to pass through while catching the heavier hydrocarbons. But light hydrocarbons like ethane will also pass through during this filtering process.  Ethane is separated from the methane chains by using a process known as cryogenic expansion.

A cryogenic process takes place at extremely low temperatures. With cryogenic expansion, cooled gas is blown into an expansion chamber which reduces the vapor pressure of the gas and reduces the gas temperature to about -100° Celsius. It is at this temperature that the gaseous methane can be separated from liquefied ethane.  More distillation may be required to separate methane from the heavier hydrocarbons not captured in the filtration process described above.

Since these distillation processes deal with extreme conditions of pressure and temperature, experts with cryogenic systems should be always on call to monitor for leaks, freezing of equipment and other challenges. The design of the piping systems will require cryogenic insulation experts to select the best solution for vacuum jacketed piping, high pressure valves and other cryogenic freezing equipment such as the cryogenic storage chambers. Hiring a field service engineer experienced with cryogenics is probably the best option for processing plants of natural gas.