Natural gas processing is vital for a few reasons. First and foremost, raw natural gas is far from clean and needs impurities removed so as not to add harmful pollutants to the environment. Secondly, many of the components of raw gas are valuable fuels. These elements can be separated and sold individually to improve the overall market value of the gas resource. This article gives an overview of the various steps involved in processing natural gas, from the oil field to the point the gas is clean and ready to hit the natural gas market.
Oil and Gas Separator Process
The first step to attaining usable natural gas is the oil and gas separator process. In most cases, this is relatively simple. Pulling the oil and gas out of the ground reduces its pressure. This reduction of pressure causes the oil and gas to want to separate naturally. As naturalgas.org puts it, it’s “like opening a can of soda pop.” When you open the can, the pressure reduces, and the gas naturally comes out of the soda water. There are other less common cases where more processing is required to get the gas and oil to separate sufficiently. This is accomplished by adjusting temperature and pressure to force the separation.
Natural Gas Condensate and Water Removal
Just as most of the oil is easily removed at the oil well, the same holds true for most of the water. The water condensate naturally wants to separate from the gas. The rest of the water vapor is a little more challenging and can be removed at a processing plant using either absorption or adsorption. Simply put, absorption utilizes a dehydration agent (usually glycol) to absorb the water vapor, while adsorption condensates the water vapor out.
Adsorption is the most common and the most efficient method. It utilizes a solid-desiccant, like activated alumina or a granular silica gel. The gas passes through the desiccant in a tower. You will typically see two of these dehydration towers at a processing plant. These dehydration towers take their turns in the process. One will be in use, pulling out the water vapor as the wet gas flows through it. Meanwhile, the other is heated, evaporating and drying the desiccant so that it will be ready for its next shift.
Natural Gas Sweetening
Natural gas sweetening sounds strange to the common ear, but it is a crucial step in natural gas processing. Depending on the source, the raw gas may be rich in hydrogen sulfur. This sulfur-rich gas is known as sour gas and gives it the typical sulfur stench. But the problem isn’t the stench. Sour gas can be poisonous, even deadly when inhaled. On the other hand, once the sulfur is extracted, it becomes a valuable resource. According to naturalgas.org, about 15% of sulfur produced in the United States comes from processing natural gas.
To sweeten the gas, an amine process, or Girdler process is used. The sour gas runs through a tower containing the amines. As the gas runs through the amines, the amines absorb the sulfur, removing it from the gas, or sweetening the gas. Later, the sulfur can be removed from the amines and sold as sulfur powder.
Natural Gas Liquids Processing
There are two main-stream methods of natural gas liquids processing, absorption, and cryogenic processing. The absorption method is very similar to the absorption method of removing water but uses oil instead of glycol as the absorption agent. While the absorption method is excellent for extracting heavier NGLs, it’s not so good at separating the lighter NGLs. That’s where the second method, cryogenic gas processing, really shines.
With cryogenic processing, the wet gas is cooled to between -120 and -190 degrees Fahrenheit. At -120, 90% – 95% of the ethane is removed. At -190 degrees, practically all of the ethane is removed. The decision of how cold to chill the wet gas a simply a matter of the requirement due to the purity of the source raw gas.
At this temperature (-120 degrees to -190 degrees), the methane is still in a gaseous state, while nearly all of the other hydrocarbons become liquids. The gaseous methane is now pipeline quality natural gas, at 95% pure or better, and is ready to hit the midstream pipeline infrastructure.
NGL Fractionation Explained
The natural gas liquids are still a combination of several hydrocarbons, namely ethane, propane, butane, isobutane, and pentane. To separate out these gases, they undergo a process called fractionation. Each of these hydrocarbons has its own boiling point. This means that as the NGLs are slowly warmed up to ambient temperature, different gases will boil off at various stages of the warming process and can be separated out and sold individually.
A good way to understand this process is how alcohol stills works. While water boils at 212 degrees Fahrenheit, drinking alcohol or ethanol boils at 173 degrees. Hence, bringing the water/ethanol mixture to 174 degrees or better will boil out, or evaporate, the alcohol while leaving behind the water.
You may know that it’s over-simplifying to say that water boils at 212 degrees. Water boils at 212 degrees at sea level, but in the mountains where the air pressure is diminished, water can boil at 198 degrees. The same holds true for hydrocarbons. Cryo gas plants will use a combination of temperature and pressure to make the fractionation process as efficient as possible. But for this article, we are going to simplify things and talk about boiling points at ambient pressure.
NGL Fractionation Process Description
In the case of NGLs, they go through four fractionation processes to extract the individual fuels. As the names indicate, the deethanizer step separates out the ethane. As ethane’s boiling point is -128 degrees, it takes minimal effort to separate out the ethane from the chilled gas liquids.
Next, the liquids traverse to the depropanizer step. Propane evaporates at -44 degrees, so warming the NGLs up to above -44 degrees will evaporate out the propane so that it can be captured and sold separately.
This is followed by the debutanizer and butane splitter, which separates out the butane and isobutane. Butane evaporates at 30 degrees while isobutane evaporates at 11 degrees. After these four steps are completed what is left remaining are pentanes and heavy hydrocarbons that can also be sold as components to fuels and solvents.
Natural Gas Processing Plant Services
Milestone Companies is a natural gas construction company that builds gas processing plants and pipelines, as well as maintaining this vital part of our natural gas infrastructure. We invite you to check out our oil and gas construction services at https://milestoneco.us/oil-gas-construction-services.