Nitrogen Rejection and C02 Removal Made Easy

Molecular Gate™ Technology
for LNG Pretreatment

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Molecular Gate™ Technology
(Smaller Scale) LNG Pretreatment

Presented at the
2010 Gas Processors 89th Annual Convention
Austin, TX - March, 2010

Michael Mitariten, P.E., Guild Associates, Inc., Dublin, Ohio


The Molecular Gate adsorption process uses pressure swing adsorption (PSA) to remove nitrogen and carbon dioxide impurities from natural gas while using adsorbents with high selectivity between methane and these impurities. Two systems feeding smaller scale LNG plants are now operating where the methane rich feed is dehydrated and carbon dioxide is removed to 50 ppm or less. The Molecular Gate technology in LNG pretreatment is competitive with TSA based systems for lower carbon dioxide feed levels (such as for pipeline feed based peak shaver plants) and offers cost advantages and simplified operations as compared with amine plus TSA treatment for higher carbon dioxide concentrations. The technology removes water to <1 ppm and carbon dioxide to <50 ppm from any carbon dioxide feed level, with the treatment of carbon dioxide levels of up to 40% having
been demonstrated. The Molecular Gate technology is also widely used to remove nitrogen from natural gas and in this application any carbon dioxide in the feed is automatically co-removed to less than 50 ppm. This paper provides an overview of the technology, the basis of the operating plants and outlines the application of the technology to feed streams over a range of carbon dioxide concentrations.LNG is growing in applications for small scale stranded gas reserves, for peakshaver units for natural gas storage and for world scale production facilities. Prior to liquification CO2 and water must be removed to ppm levels and in some instances nitrogen removal is also desired.


The Molecular Gate process for removing nitrogen and/or carbon dioxide from contaminated natural gas streams continues to grow in popularity with over thirty systems provided to date. The system operates by adsorbing nitrogen and/or carbon dioxide at 100 - 500 psig (typically 100 psig) while delivering a methane product reduced in these impurities at 10 psi lower than the feed pressure. While most applications deliver pipeline gas, two systems are operating to deliver product gas streams
to small liquefiers (10,000 and 13,000 gallons per day).

While large LNG facilities receive the largest share of evaluation and study, small scale LNG facilities are becoming more popular, either for the purpose of distributed LNG or as peak shaver units. Indeed, natural gas storage through peak shavers is well established with well over 100 systems in the United States.

Historical Treatment for Peak Shaver Units Based on Pipeline Gas Feed

Where utilities seek to store natural gas during periods of slack demand for its subsequent use during high demand periods, peak shavers have commonly been used. In these systems, pipeline gas is treated, liquefied and stored by the utility and re-vaporized and injected into the pipeline distribution system as demand dictates. Upgrading pipeline gas as a feed to the liquefier is well established. Since the feed gas is pipeline quality it has already been treated for the removal of the bulk quantity of
carbon dioxide, such that the carbon dioxide is two percent or less (more commonly in the one percent range). The pipeline feed gas has also been bulk dehydrated and heavy hydrocarbons have been removed to meet typical pipeline specifications.

Because carbon dioxide freezes in a liquefier at levels above 50 ppm, the pipeline gas fed to a peak shaver unit requires pretreatment for the removal of the residual carbon dioxide in the pipeline gas. Further, the pipeline gas is typically dehydrated to levels of seven pounds per MM SCFD (approximately 150 ppmv) and since water will also freeze in a liquefier, the water level must be reduced to 1 ppm or less. Excess heavy hydrocarbons can form solids in the liquefier and their presence may need to be addressed in the design of the liquefier.

Historically, the treatment for such LNG facilities uses a thermally regenerated molecular sieve adsorption system (TSA) consisting of two or more adsorbent beds. While one bed removes the water and carbon dioxide, a second bed is regenerated by being heated and subsequently cooled before being placed back on the feed step. Because the amount of adsorbent required is large relative to the feed rate, three bed TSA units as shown in Figure 1, are commonly used to allow relatively fast cycle times
to minimize the quantity of the regeneration gas and the heating duty. In the TSA system, the amount of regeneration gas required is a function of the amount of adsorbent required. For this reason, feed streams with high carbon dioxide levels require large
adsorbent quantities and high regeneration gas flow rates. Where the carbon dioxide exceeds about two percent, the need to heat and cool the large beds results in using a large portion of the product gas for regeneration, leaving minimal product gas as liquefier feed. For this reason carbon dioxide levels above 2% are not typically treated in TSA systems.

Figure 1 - Three Bed TSA Process

The molecular sieve beds in this process operate at moderate pressure levels as dictated by the design of the liquefier and/or the available pipeline pressure. While the pressure can range widely, 400 psig is a typical operating pressure for historical peak shaver liquefiers, with recent small liquefiers operating at lower pressures. The molecular sieves used in this application are chosen to maximize the capacity for carbon dioxide. Unlike molecular sieves targeted at dehydration alone, a relatively large
amount of adsorbent is required to remove the carbon dioxide. This is due to the relatively high carbon dioxide feed concentration and the lower adsorbent capacity for carbon dioxide as compared to the capacity for water vapor. Due to the relatively low capacity of molecular sieves for carbon dioxide in a TSA based system, it is desirable to have the feed gas temperature as low as possible to improve the molecular sieves carbon dioxide capacity. Since pipeline gas is generally at temperatures below summer ambient conditions, there is a temperature advantage in helping to minimize the required adsorbent quantity for pipeline feed streams.

The regeneration of the adsorbent bed takes place at high temperatures, typically in the range of 500 degrees F, and results in a rejected stream containing the previously adsorbed carbon dioxide and water, which is in turn either utilized as local fuel or returned to the pipeline. The treated gas product is commonly used as the regeneration gas, though using the LNG boil-off gas for regeneration is practiced. As with all adsorption systems, finding a home for the regeneration stream is a process optimization and a critical site-specific optimization.

Feed Gases Containing High Carbon Dioxide Levels

Because pipeline gas is already treated for bulk carbon dioxide and water vapor removal the amount of processing to meet the needs for the liquefier is minimal. However, LNG systems are also applied where the feed gas is from wellhead gas. In these applications, the level of carbon dioxide, heavy hydrocarbons and water vapor is higher than that of a pipeline. Pretreatment of such wellhead sites would commonly use amine based systems carbon dioxide removal and since the amine systems are aqueous, the amine system product effluent is saturated in water.

In a wellhead application, the dedicated amine system can be designed to remove the carbon dioxide to levels lower than that required for pipeline specifications. For example, the carbon dioxide can be removed to <50 ppm followed by TSA drying. This low carbon dioxide slip requires a larger amine plant and consideration for specialty amines in the amine plant design should be provided. The carbon dioxide slip into the product can also be balanced against the removal of carbon dioxide in the
amine plant versus removal of the carbon dioxide in the TSA dryer. A simplified process flow scheme for an amine/TSA dryer is shown in Figure 2.

Since the product from the amine plant is water saturated, it is possible to install a glycol unit for bulk dehydration prior to the TSA system to limit the size of the molecular sieve TSA system and minimize the quantity of the regeneration stream. In general, membranes in place of the amine system would not be considered since the removal of the carbon dioxide to very low levels is required.

Regardless of the route chosen for carbon dioxide and water removal, the heavy hydrocarbons are not removed and remain with the treated gas. Since heavy hydrocarbons can form solids in the liquefier they require that the liquefier design address the potential impact of heavy hydrocarbons. Since smaller liquefiers do not have the luxury of the economy of scale found with world-scale plants, the additional processing to separately remove heavy hydrocarbons is generally undesirable.

Figure 2 - Amine Plus TSA Treatment

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