It is not unusual to discover natural gas deposits containing substantial levels of nitrogen. More often than not, these discoveries are not developed because the nitrogen levels exceed the four percent inert requirement for pipeline gas. A Nitrogen Rejection System may provide an economically practical approach to the development of these resources.

Cryogenic NRU designed to process 2-3 million cubic feet of natural gas per day with 8% Nitrogen.

Nitrogen rejection can be accomplished in several ways. Three of the most commonly used methodologies include CRYOGENIC systems, techniques that utilize a SOLVENT for hydrocarbon recovery, and the PRESSURE SWING ADSORPTION process. The technology you choose for removing nitrogen depends largely on a number of variables including the volume of gas to be processed, the quantity of natural gas liquids present in the methane mix, and the nitrogen level in the gas.

Cryogenic Processes: Cryogenic processes capitalize on the fact that methane and nitrogen change phase (from gas to liquid) at different temperatures. By manipulating and controlling the pressures and temperatures in the system, the methane is liquified and collected as it drops out of the gas. The gas is then re-vaporized, yielding a sales stream with nitrogen levels that conform to pipeline quality standards. The excess nitrogen is either flared or vented to the atmosphere. Because of the temperatures involved (-240 to -250 degrees), the long cool down time and extensive equipment required, cryogenic systems are most often used for large projects where processing volumes exceed five million cubic feet per day and in those instances where nitrogen levels range from 30% to 40% or higher.

Solvent Recovery Processes: In solvent recovery processes, the hydrocarbons in the feed stream are absorbed by a chemical solvent and the nitrogen that remains is dispersed by vent. The hydrocarbons are recovered from the solvent through a series of flash operations. Because the process requires that the gas stream be cooled to only -30 degrees, effective separation usually begins within 2 or 3 hours after start-up. In addition, solvent recovery processes provide the benefit of handling one of the problems commonly associated with nitrogen rejection: the presence of natural gas liquid elements in the feed stream. After nitrogen has been removed, the presence of these additional heavy hydrocarbons usually results in a stream with a hydrocarbon dew point that exceeds pipeline requirements. While other processes require an additional plant to remove the natural gas liquids, the solvent process conveniently handles both the separation of the nitrogen and the NGL's in a single plant. Of course, the recovered liquids can also be sold, enhancing the overall revenue stream of a project.

Pressure Swing Adsorption: The pressure swing adsorption process utilizes a carbon bed technology to separate the nitrogen from the hydrocarbons and can perform effectively even where there are large amounts of nitrogen in the feed stream. A particular advantage of the pressure swing adsorption process is its scaleability. It can be used in small plants designed to handle 100,000 cubic feet of gas per day as well as plants processing volumes in the millions of cubic feet.

Brake horsepower and compression requirements can be major issues in plant selections. Typically a PSA plant will only need about 72% of the horsepower required by a solvent plant. Where large quantities of liquids are present, a natural gas liquids plant may be needed to meet pipeline hydrocarbon dewpoint requirements. If other contaminants such as hydrogen sulfide and carbon dioxide are present, an amine plant may be necessary.

Tucker Gas Processing Equipment Corporation can help with your project and assist you in designing, building, and leasing or operating the project. We would be pleased to provide a process proposal utilizing the appropriate technologies to ensure that the best fit is made available for your gas volume and gas analysis.

TGPE can design nitrogen rejection systems for fields producing anywhere from 100,000 to more than ten million standard cubic feet per day. The systems can be engineered to handle natural gas streams with nitrogen levels ranging from five percent to more than fifty percent. By adding an amine unit, the process can be extended to provide for the removal of carbon dioxide and hydrogen sulfide. Raw helium can be recovered with the addition of a helium membrane.

Although we have designed and built cryogenic nitrogen rejection plants in the past, we believe that Pressure Swing Adsorption offers a more practical alternative, especially for most small applications. We can design and build PSA plants capable of handling volumes as small as 100 mscfd to easily within the 10 mmscfd range.

Pressure Swing Adsorption plants offer a number of advantages over cryogenic systems. The PSA process does not require a "cool down" period since no temperature differential is utilized. The plant operates on the basis of adsorbing the hydrocarbons into a carbon-based vessel. Through a series of depressurizations, the nitrogen is released while the hydrocarbons are trapped in the adsorption bed. Further depressurization frees the hydrocarbons from the bed so they can be removed from the vessel and repressurized for introduction into the pipeline.

These systems perform well at inlet pressures of approximately 50 psig, making them suitable for use following the gathering system. As indicated above, horsepower requirements are reasonable and maintenance and operator demands are low. The systems are skid-mounted and can be moved to a new location when a field has been depleted.

A gas analysis is needed to design a system that takes into account all the variables that affect the processing of gas. The analysis provides valuable information regarding the levels of heavier hydrocarbon liquids in the gas. This information is used to determine whether a Natural Gas Liquids Plant might be needed to remove the excess heavy liquids, bringing the gas into compliance with the hydrocarbon dewpoint requirements of the pipeline. The analysis may reveal the presence of carbon dioxide, hydrogen sulfide, and mercaptans that must be removed before the gas can enter the PSA system. If moisture appears to be a problem, dehydration may be included in the design.

We invite you to contact TGPE about your nitrogen problem. Let us propose a PSA-based solution.