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 1 (page 2 of 3)

Molecular Gate Technology for Carbon Dioxide Removal

Figure 3 - InstallaionThe Molecular Gate Technology for carbon dioxide removal uses a highly selective adsorbent with a high affinity for carbon dioxide over that of methane which is applied in a pressure swing adsorption system. Feed gas is introduced into the Molecular Gate Unit at feed pressure, which is typically 100 psig but can range as high as 500 psig or more, and the product methane stream is
delivered at near feed pressure. In the Molecular Gate System, water vapor and carbon dioxide is adsorbed in much the same way as in a TSA dryer and a product of less than one ppm water vapor and less than 50 ppm carbon dioxide is produced. The Molecular Gate System regenerates using pressure reduction rather than thermal regeneration and the feed stream is split into a product stream at near feed pressure and a reject regeneration stream that is delivered after a vacuum pump at a few psi above
atmospheric pressure. A photo of a typical Molecular Gate system is shown in Figure 3.

Because the Molecular Gate system operates with rapid cycles, on the order of a few minutes, and the feed/regeneration pressures can be changed rapidly, the technology allows both bulk and trace removal of impurities. The attributes of the pressure swing adsorption technology allows the amount of carbon dioxide that can be removed in a single step to be much higher than that achieved by the TSA system. This is compared to the hours-long cycle times of thermally regenerated systems which are more appropriate for the removal of low quantities of impurities.

Historically, conventional adsorbents in PSA service have high methane adsorption capacities which upon regeneration result in high methane losses. However, the very high selectivity between methane and carbon dioxide of the Molecular Gate adsorbent allows the delivery of a product that contains ppm levels of carbon dioxide while also achieving high methane recovery rates. The inherent higher adsorption for carbon dioxide over methane of all adsorbents is enhanced in the Molecular Gate system due to the pore size control which limits adsorption of methane. This size selective effect is shown in Figure 4.

Figure 4 - Molecular Gate Schematic

In the Molecular Gate process, nearly any level of carbon dioxide can be removed to 50 ppm or less in a single step. As a high carbon dioxide feed example the removal of 40% carbon dioxide, such as from a digester gas, to less than 50 ppm of carbon dioxide has been demonstrated.

In a single step Molecular Gate Unit, the methane recovery to the LNG facility is, in part, a function of the amount of carbon dioxide in the feed stream. For example, feed stocks containing one to two percent carbon dioxide such as pipeline gas can achieve methane recovery rates of 95%, while higher levels of carbon dioxide such as five to ten percent would have recovery rates in the 90% range. In some cases where the tail gas is to be returned to the pipeline, high recovery is not desired since the
returned tail gas must be diluted to meet the pipeline carbon dioxide limits. Thus, the Molecular Gate design and hydrocarbon recovery is a site-specific optimization depending largely upon the uses of the regeneration stream. A single stage flow sheet is shown in Figure 5.

The feed pressure used is commonly 100 psig, though higher pressure can also be accommodated. In general, higher pressure operations would include a recycle compressor to allow high methane recovery and minimize hydrocarbon losses.

Table 1

Figure 5 - Typical Molecular Gate Processing

As with most separation processes, there are considerable flexibilities in the design performance and the use of a recycle stream or two-stage processing allows the adjustment of the hydrocarbon recovery rates to be higher or lower as needed to optimize the process for a particular site.

In treating streams with high carbon dioxide or where the highest methane recovery is desired,
two stage processing can be considered. Such a process is shown in Figure 6 where a first stage unit removes the bulk of the carbon dioxide while the second stage polishes the carbon dioxide to ppm levels. The opportunity to recycle the tail gas from the second stage allows for minimizing the hydrocarbon losses.

Figure 6

Figure 7 shows an alternate two stage approach targeted at moderate feed levels of carbon
dioxide but where high recovery rates are desired. In such a design, recovery rates of 98% or more can
be achieved. In this configuration, the first stage unit produces the treated product while the second
stage treats the compressed tail gas from the first stage for bulk carbon dioxide removal with
hydrocarbon recycle to the first stage to allow additional methane recovery.

Figure 7 - Two Stage Processing Schematic #2

In the examples above, a two stage Molecular Gate process is considered, however, one of the
stages can be amine, membrane or TSA based. The use of two-stage processing adds cost and the
overall cost must be balanced against the value of the recovered hydrocarbons as LNG and the sitespecific
use of the regeneration stream.

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Further Information:

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Guild is a licensee of Engelhard's Molecular Gate® Adsorbent Technology and
is solely responsible for all representations regarding the technology made herein.

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