Nitrogen Rejection and C02 Removal Made Easy

Molecular Gate®
     Adsorption Technology

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Production of Pipeline-Quality Natural Gas with
The Molecular Gate CO2 Removal Process

James Wills, P.E., SPE, Tidelands Oil Production Company; Mark Shemaria, SPE, Tidelands Oil Production Company;
Michael J. Mitariten, P.E., Guild Associates, Inc

Updated January 2009

Continued from Page 2 (page 3 of 3)

Oxygen Contaminated Feeds

Oxygen can be present in coalbed methane due to the low-pressure operation.  Even where not initially present, oxygen can be introduced when wells are put on vacuum or additional wells are added.

In a Molecular Gate system designed for the removal of carbon dioxide, oxygen will pass through the bed of adsorbent and be present in the methane product stream. Low quantities of oxygen are not generally a concern but the specification on oxygen should be clarified with the pipeline company. In our experience, we have seen a wide range of oxygen specifications from low ppm levels to a few thousand ppm.

In an amine system for coal bed methane oxygen can degrade the solvent and cause corrosion. In membrane systems the oxygen is not generally a concern but will split between the rejected CO2 and the pipeline sales gas product.

Note that Molecular Gate-Nitrogen Rejection systems designed for nitrogen removal will also remove part of the oxygen along with the nitrogen.

Applications - Natural Gas Upgrading

In most cases where the Molecular Gate system is applied for natural gas upgrading, feed is available at high pressure, and a slightly different flow scheme is applied as shown in Figure 5.

Block flow diagram for CO2 removal from natural gas Making use of the tail gas is the main consideration in applying the system to CO2 removal from natural gas.  For a coal bed methane application a use for the tail gas fuel exists in the fuel demand by the main feed compressor.  In natural gas applications the recycle compressor has a fuel demand but this is typically about 2% of the available feed, thus under typical recovery rates of 95% excess fuel exists.

This fuel balance is a site specific optimization but one that must be addressed for each project.  Where fuel demand is not available we have demonstrated over 99% methane recovery for high performance systems.

Unlike most coal bed methane feeds natural gas also contains at least some level of heavier hydrocarbons.  In treating natural gas feeds where heavy hydrocarbons are present, they are partly removed, with the carbon dioxide, through adsorption on the surface of the adsorbent.  Where justified by the quantity of the NGL components additional processing can be applied to recover them as a liquid. 

Comparison With Amine and Membrane Technology

Removal of carbon dioxide from natural gas feeds is widely practiced and a variety of amine solvents are commonly applied for its removal. Membrane systems are also attractive for certain applications and becoming more widely accepted by industry. To this range of applications, Molecular Gate fits within certain niches.

Coal bed methane is an ideal application for Molecular Gate systems, since they remove both carbon dioxide and water in a single step. Since feed compression is required there exists a recycle compressor and a home as fuel to the gas engine drive for the methane that is not recovered as sales gas.

The relative cost of the Molecular Gate system, amine system and membrane system are project specific and specific process requirements can be defined in which each of the technologies would be selected. As a general rule the overall costs are roughly similar and operational differences enter the evaluation. In general for low pressure applications the Molecular Gate system can have an advantage, especially where a use for the tail gas fuel exists while high-pressure applications can favor amine or membrane systems. A feed pressure of 100 - 400 psig is about optimum for the Molecular Gate system. In cases where the feed source is at low pressure but the pipeline is at high pressure it may be advantageous to operate the system at a compressor interstage (such as 100 ? 400 psig) and to compress the purified product gas.

A higher feed pressure would have a benefit for the amine unit due to higher loading of the carbon dioxide. High feed pressures can also make a membrane unit worthy of consideration. In the same manner as the Molecular Gate system the membrane unit would use the low-pressure carbon dioxide rich permeate stream as fuel to the main feed compressor.

NGL Recovery

In treating natural gas the removal of heavy hydrocarbons along with the carbon dioxide is an issue that needs to be addressed. These components can be used as fuel along with the rest of the tail gas but this may not always be possible or may result in excess available fuel. The co-removal of heavy hydrocarbons can have an advantage in meeting pipeline dew points but generally would be viewed as a disadvantage with the system penalized by the loss of the heavy components with the carbon dioxide impurity.

The recovery or use as fuel for NGL components is always a consideration for natural gas upgrading and processing for their removal from the feed or Molecular Gate tail gas can be economical.

Summary

In applications for carbon dioxide removal, Molecular Gate technology offers a new route for meeting the long established needs of the natural gas industry. The successful operation of the first system for carbon dioxide removal at Tidelands Oil Production facility in Long Beach, CA demonstrates the technology.

Subsequently 30 projects are underway with flows from 0.5 MM SCFD to 10 MM SCFD.

The Molecular Gate adsorbents and processes offer new technology for separations beyond those presented in this paper. Development of other separations using Molecular Gate technology is possible as the technology continues to evolve.

Acknowledgements

Grateful appreciation is given to the U.S. Department of Commerce, NIST, Advanced Technology Program for its support and funding of research on titanium silicate molecular sieves (ATP project # 335410).

 


SI Metric Conversion Factors

Btu ? 1.055 873
ft3 ? 2.831 685
?F (?F-32)/1.8
psi ? 6.894 757

E-01 = kJ
E-02 = m3
= ?C
E+00 = kPa

James H. Wills Jr. is currently a staff facilities engineer with Tidelands Oil Production Co. He is a licensed mechanical engineer with 25 years? experience in the oil and gas industry, performing supervision, engineering and design, and construction from concept to startup of multimillion-dollar projects. e-mail: jim.wills@tidelandsoil.com. His experience includes crude oil production, refined product, water and steam injection, and gas production and processing facilities. Mark Shemaria is the Manager of Environment, Health, and Safety with Tidelands Oil Production Co. and President of Tidelands? subsidiary Lomita Gasoline Co. Inc. e-mail: mark.shemaria@tidelandsoil.com. His professional career spans 22 years in the oil and gas production business in positions with increasing responsibilities at Thums Long Beach Co. and Tidelands Oil Production Co. His experience includes environment, health, and safety management; operations and construction supervision; facilities design; environmental engineering, including auditing, site assessments, and remediation; waste management; air-quality programs and permitting; water programs; emergency planning and response; and management systems. Shemaria holds a BS degree in engineering and industrial technology and an MBA, both from California State U., Long Beach, California. He recently was the Chairman of the SPE/EPA/DOE Environmental Conference and a Forum in 2003 and served on numerous Environment, Health and Safety Technical Program Committees for SPE?s ATC. He was the 2002 recipient of SPE?s 2002 Environment, Health, and Safety Award. Michael Mitariten was formerly the Business Manager for Engelhard?s Molecular Gate technology. He has 20 years? experience in gas separation with the technologies of adsorption, membranes, and cryogenic processing and with solvent-based acid-gas-removal systems. His focus has been on upgrading subquality natural gas and coalbed methane, the purification of hydrogen for refinery and chemical plant applications, and air separation. Mitariten holds a degree in chemical engineering from Manhattan College.

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