It is not unusual to discover natural gas deposits containing substantial levels of Nitrogen. More often than not, these discoveries are ignored because the Nitrogen levels exceed the four percent inert requirement for pipeline gas. However, a Nitrogen Rejection System may provide an economically practical approach to the development of these resources.
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 the 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 capitalize on the fact that methane and nitrogen change phase from liquid to gas at different temperatures. By manipulating and controlling the pressures and temperatures in the system, the methane is liquefied 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.
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 beings 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.
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. However, when significant quantities of natural gas liquids are also present, it becomes troublesome to make separation without encountering difficulties in the process beds. Thus, the Pressure Swing Adsorption process is most useful on projects involving landfill gas, coal mine gas or the tail end of process plants where the natural gas liquids either are not present or have already been removed. One advantage of the Pressure Swing Adsorption process is its scale-ability. It can be used in small plants designed to handle 300,000 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 and where higher pressures are needed to remove other contaminants such as hydrogen sulfide and carbon dioxide, a natural gas liquids plant in conjunction with the PSA plant will provide for removal of larger quantities of liquids than can be handled by a solvent plant.
Tucker Gas Processing Equipment Inc. has sources to bring any of these technologies to your project and to assist you in designing, building, and leasing or operating the project. We would be pleased to provide a process proposal utilizing any one or more of these 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 500,000 to more than 10 million standard cubic feet per day. The systems can be engineered to handle natural gas streams with nitrogen levels ranging from 5% to more than 50%. By adding an amine unit, the process can be extended to probide for the removal of carbon dioxide and hudrogen sulfide. Raw helium can be recovered with the addition of a helium membrane.
Based on your actual gas analysis and the size of your reservior, Tucker Gas Processing Equipment can help determine whether or not Nitrogen Rejection might be a commercially viable alternative. TGPE can also work with yo in a variety of ways to make your project a financial reality. See Funding Alternatives.