The natural gas in underground reservoirs is not quite the same as the finished product sold on the market. Consumers have no use for gas in its raw state, so it has to undergo several phases of treating and processing before it can be purchased and used. This article is a great outline for those of us interested in how gas gets from the ground into the hands of consumers.
From Gas In The Ground To Gas In The Grid
Gas in the ground is naturally saturated with water vapor, and it can also be contaminated with heavy hydrocarbons or other impurities. As a natural gas producer, we must remove these impurities to create a gas that will meet consumer specifications, such as gas of a consistent quality. Additionally, consumers often require us to supply gas at a certain rate, and problems can ensue we don’t meet this expectation.
Several steps are necessary to prevent problems with pipelines that deliver gas to consumers. The goal is to create gas that can be transported long distances at ground temperature and high pressure without forming liquids, since liquids can cause corrosion or blockage, decrease pipeline efficiency and damage downstream equipment. This process includes somewhat obvious standards, like making sure there is no dust, and treating chemicals or other contaminants in the gas. However, customer requirements are often a lot more detailed.
A Four-Step Guide To Gas Sale Specifications
While companies and customers have their own particular requirements, specifications often include maximum CO2, H2S, organic sulfur and residual water vapor content. Non-chemical variables are specified as well, such as maximum hydrocarbon dewpoint temperature and minimum heating value. There’s often a maximum allowable temperature when gas leaves the plant, as well as a minimum allowable pressure when it enters the transmission grid.
Removing Hydrogen Sulfide
Sour gas is one term all gas processors ought to know. Gas is considered sour if its hydrogen sulfide (H2S) content is above the maximum limit specified by the consumer; if our gas is sour, we must initiate a chemical reaction to decrease the amount of H2S present — a process called sweetening. Sweetening involves a solvent capable of removing H2S as well as any other sulfur compounds (mercaptans like ethyl and propyl are common in sour gas) that are above the customer’s prescribed limit.
Removing Carbon Dioxide
It’s important to note that the sweetening process will also remove carbon dioxide (CO2) from the gas. However, even if it’s not necessary to sweeten the gas and remove sulfides, we still need to remove the CO2 if the amount is over the customer’s requisite — and there are specific processes available to remove just this component.
Removing Water Vapor
Finally, water vapor must also be removed from the gas until it no longer exceeds the maximum limit for residual water content, a process called dehydration. There are several ways to complete dehydration, including absorption with glycol or a deliquescent salt, and adsorption with a dry desiccant.
Hydrocarbon Recovery - Dew Point Control
Recovery of natural gas liquids (NGL) from natural gas is of great economic importance as well as a common occurrence in natural gas processing. There are reasons to select liquid-recovery process:
- To produce transportable gas
- To meet sales-gas specifications
- To maximize liquid recovery
If condensation is in the transportable gas there are two drawbacks:
- Two phase flow requires larger pipe diameter than single phase flow for the same pressure drop.
- When a two phase flow arrives at a destination, slug catchers may be required to protect the downstream equipment
Tips To Make The Most Of Treating And Processing
Optimum pressure and temperature for gas transmission is variable and dependent on several criteria:
- Specifications of our customers
- Required flow rate of sold gas
- Composition of raw gas and condensate
- Keep the pressure under 1,000 psig to avoid formation of liquids in a pipeline where gas is being transmitted a long distance.
- Extract liquefiable hydrocarbon components, like ethane or propane. Their value on the market, once extracted, is often much greater than their heating value if they’re left in the gas. However, be aware these components have their own requirements regarding purity before they can be sold.
Treating and processing gas is necessary and simple. To treat our customers well, we must treat our gas well. To get the most out of your gas treating and process, give us a call!