Processing Natural Gas: 8 Steps to Know
Natural gas is valuable both as a clean source of energy and as a chemical feedstock in the manufacture of plastics. Before it reaches the consumer, it has to pass through several steps known as processing natural gas. These steps are necessary for the recovery of valuable components contained in the gas, as well as the efficient transport of gas over long distances through the pipeline.
What is natural gas?
Processing natural gas is a complex industrial process used to purify natural gas that is extracted from underground gas fields. This process separates impurities and other various non-methane fluids and hydrocarbons to produce pure methane (CH4), also known as dry pipeline quality natural gas. Industrial, commercial, and even residential consumers use this processed natural gas as fuel.
Natural gas in its final state used by the consumer is very different then the type of gas that is found underneath the earth. Raw natural gas is full of impurities, while the final product after processing is almost pure methane. These impurities include nitrogen, oil, hydrates, water vapor, carbon dioxide, hydrogen sulfide, and heavy hydrocarbons that consist primarily of pentane, butane, propane, and ethane.
While some of this processing is completed at the well site, the process is completed at the processing plant. Before natural gas enters the pipeline, removal of most or all impurities is required. Processing natural gas as whole requires several separate steps, however there are four primary processes: oil, water and condensate removal, hydrogen and carbon dioxide removal, dehydration, and the removal of NGL.
Processing Natural Gas
In order for consumers to benefit from natural gas, we must start with processing natural gas so that the consumer can use it. There are 8 steps when processing natural gas. These steps illustrate what needs to happen for natural gas to be delivered to the end consumer.
1. Large-Particle Impurities
When the gas leaves the well, the first step for processing natural gas is to remove all of the water and condensates. Generally, this step is performed at the site of the well. First, scrubbers and heaters are implemented to reduce temperature drops in the gas and then to remove large-particle types of impurities.
2. Separating Oil from Gas
Next, a conventional separator is used to separate the oil from the gas. The separator is comprised of a closed tank that uses the force of gravity to separate solids and liquids. When this process is not accomplished with gravity alone, separators use high-pressure to cool the gas, which then is transferred through a high pressure liquid to remove a quantity of the water and any remaining oil.
When water, condensates, and oil have been removed from the natural gas, any hydrogen sulfide and carbon dioxide must also be removed. This step in the process is also known as “sweetening” the gas. This is because of sulfur’s strong or “sour” scent. This step is critical because hydrogen sulfide in itself is corrosive and even lethal.
While product specifications and regulations to prevent sulfurs effects are put forth by the National Agency of Corrosion Engineers, also known as NACE International, the Occupational Safety and Health Administration, OSHA, requires these additional preventative measures.
Once the carbon dioxide and hydrogen sulfide have been removed, next comes the dehydration of the natural gas. This step is essential to remove any excess water that can create freezing, corrosion, and hydrates issues, which do not meet pipeline standards. Water vapor is removed through the process of adsorption or absorption. Adsorption is the condensing and collection of water vapor on the surface, or a passive dehydration system. Absorption is the removal of water vapor using a dehydrating agent.
5. Mercury Removal
This step is not always expedient, but high mercury levels cause environmental pollution and aluminum heat exchanger corrosion. When deemed necessary, there are two forms of mercury removal that take place in regenerative and nonregenerative processes. The first uses alumina or sulfur-activated carbon, while the second used silver located on a molecular sieve.
6. Nitrogen Rejection
Nitrogen, a non-flammable inert gas lowers natural gas’s overall temperature. The gross heating value of natural gas must maintain a level of 900 to 1200 btu (British thermal unites). Purified natural gas maintains a levels of 1010 btu. In most cases, the heating value is too low to meet pipeline standards where there is the presence of nitrogen. The removal of nitrogen from natural gas is known as nitrogen rejection.
7. NGL Recovery
Natural gas liquids, or NGLs, are heavy in hydrocarbons including natural gasoline, iso-butane, butane, propane, and ethane. Although natural gas liquids hold a high btu, they are not of pipeline quality. However, these products are valuable when they are sold separately. Therefore, natural gas liquids must be removed in a process known as NGL recovery.
The first step of this recovery process is to remove all present NGLs from the natural gas. This is accomplished through the cooling of gas temperature. This cooling transforms vapor into liquid, making the case for easy removal. The process also removes hydrates and reduces the gross heating value.
Once the natural gas liquids are extracted they are divided into their individual end products. This step is known as fractionation. Natural gas liquids that have been separated have high selling values and are beneficial in a variety of use cases. The separating of pentanes, butanes, propane, and ethane by volatility is accomplished with a piece of equipment known as the fractionator train.
Importance of Natural Gas
As the world strives to diversity its energy mix into a single source, natural gas and the natural gas process is becoming increasingly important.
Not only is natural gas viewed as “lower carbon” compared to other fossil fuels, the construction of natural gas process plants is considered to be relatively quick (2 years). The International Energy Agency (IEA) forecasts the share of natural gas in the global energy mix the to increase up to 2% annually until 2020.
With this growth has raised natural gas dependency in a variety of uses. Natural gas is not only used in the power sector, but also in industrial heat generation, the creation of ammonia in fertilization, and in the transportation division as LNG for long haul trucks or CNG (compressed natural gas).
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