Nitrogen Rejection and C02 Removal Made Easy

Molecular Gate™ Technology
for LNG Pretreatment

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Molecular Gate™ Technology for (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

Continued from Page 2 (page 3 of 3)

Heavy Hydrocarbon Removal

An advantage that the Molecular Gate Technology brings to LNG facilities is the ability to remove heavy hydrocarbons along with the carbon dioxide and water vapor. Though heavy hydrocarbons do not fit with the pore they adsorb on the surface of the Molecular Gate adsorbent. The Molecular Gate technology can target the relative removal quantity of these heavy hydrocarbons which can be adjusted to that desired for the liquefier. In our experience, we have seen requests for C6+ removal to levels as low as 10 ppm.

In removing heavy hydrocarbons, the system is flexible and can be designed to target the removal of the C5 and C6+ fractions while leaving behind the bulk of the ethane, propane and a portion of the butane in the product to the liquefier. Since heavy hydrocarbons can form solids in the LNG plant and since controlling the heating value of the LNG product can be desirable, this flexibility can be an attractive benefit.

The Molecular Gate system can also be designed to remove mid-range hydrocarbons to match the desired product LNG heating value. Systems have been provided to treat feeds with high levels of ethane and propane to remove a portion of these components (along with essentially all the C6+) to meet CARB standards for CNG product quality.

Table 2 - Typical Higher Carbon Dioxide/Rich Gas Treatment

The adsorbent also has a high affinity for hydrogen sulfide and when carbon dioxide is removed below 50 ppm, any hydrogen sulfide in the feed is removed from the methane product. To date, feed streams with up to 6000 ppmv of hydrogen sulfide have been treated.

Mercaptan and Mercury Removal

Where the feed contains mercaptans they are adsorbed and removed from the product to the liquefier. At the current time, data on mercury remo6val is lacking, however, where mercury is present in the feed stream it is expected to be removed (at least in part).

Nitrogen plus Carbon Dioxide Rejection

Of the thirty Molecular Gate Systems in operation, more than half have been designed for the removal of nitrogen (1) to typical pipeline standards of three to four percent. For all the nitrogen rejection units, the feed stream contains a level of carbon dioxide ranging from a few thousand ppm to several percent. Because carbon dioxide adsorbs more strongly than nitrogen in the Molecular Gate process, inherently a feed stock containing mixed nitrogen plus carbon dioxide will contain a product with less than 50 ppm carbon dioxide when the nitrogen is removed to typical pipeline specifications. Thus, the Molecular Gate Unit removes both carbon dioxide and nitrogen in a single step.

Nitrogen in the feed stream to a liquefier is undesirable because nitrogen is soluble in the LNG and thus increases the refrigeration duty or lowers the heating value of the LNG product. While nitrogen removal by flash from the LNG is possible (and studies using the Molecular Gate process to purify the flash gas or boil-off gas have been conducted) the Molecular Gate process allows removal of nitrogen before it reaches the liquefier which can be attractive in the liquefier design.

In our experience, small scale LNG plants target nitrogen levels in the three to four percent range and, thus, the typical product nitrogen levels from Molecular Gate units match that required by LNG facilities. To date, there are two operating Molecular Gate units that feed small scale liquefiers with both of these in landfill gas service.

In upgrading the landfill gas to feed the liquefiers, the gas is available near atmospheric pressure and contains about 50% methane along with water vapor, a wide variety of VOC's, hydrogen sulfide, ~40% carbon dioxide, ~1% oxygen and nitrogen in the 6% to 15% range. The typical process steps are (1) compression (2) pre-treatment to remove the VOC's and hydrogen sulfide (3) treatment to remove the bulk of the carbon dioxide (using either membranes or a first stage Molecular Gate system) and (4) nitrogen rejection. In the feed to the Molecular Gate unit removing the nitrogen and carbon dioxide the resultant gas after steps 1 to 3 contains a few percent of carbon dioxide and 10 to 25 percent nitrogen (dependent on the raw feed nitrogen concentration). In the two existing Molecular Gate Units, the nitrogen is removed to three to four percent and the carbon dioxide is essentially completely removed. In both applications, the Molecular Gate Unit is the final treatment system before feeding the liquefier. In this demonstrated service, the first unit treating a contaminated feed of approximately 1.2 MM SCFD has been in operation in the UK for over two years while a second unit treating a contaminated feed of 1.8 MM SCFD has been operating in California since last year.


Peak shavers and small scale LNG units have been a common and growing process for the natural gas industry. The Molecular Gate technology offers a new route for feed gas pre-treating. The technology brings the advantage of removing water vapor and any level of carbon dioxide while offering a flexible design that can be targeted to remove a desired amount of heavy hydrocarbons while also removing other impurities. Where the feed gas contains nitrogen, Molecular Gate Systems can remove nitrogen and deliver methane product to the LNG facility at a typical three to four percent nitrogen concentration, while inherently removing carbon dioxide in the feed to the ppm levels required for the liquefier.

1. Mitariten, M.J., "Nitrogen Removal from Natural Gas with the Molecular Gate Adsorption Process", 88th GPA Proceedings,
San Antonio, TX, 2009.

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is solely responsible for all representations regarding the technology made herein.

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