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 (page 1 of 3)

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

Tidelands System

Abstract

In May of 2002, the first Molecular GateCarbon Dioxide Removal system for the removal of carbon dioxide and water was started-up at the Tidelands Oil Production Company operated facility in Long Beach, California and continues operation today.  The feed source for the unit is hydrocarbon rich, water-saturated, associated gas from water flood enhanced oil recovery operations.  The feed CO2 concentration varies widely and is typically over 30% while the unit reduces the carbon dioxide level to less than 2%.  The unit removes the carbon dioxide, heavy hydrocarbons and water producing pipeline specification gas for sale to the local natural gas utility company. 

Introduction

The first Molecular Gate-Nitrogen Rejection system operated for over two years removing 18% nitrogen from glycol dehydrated wellhead gas while producing pipeline specification product at an unattended site at Hamilton Creek in Southwest Colorado prior to the commercial system for Tidelands.

The Molecular Gate adsorbent used at Hamilton Creek facility is from a new family of titanium silicate molecular sieves with the unique ability to be manufactured with a desired pore size, in the case of Nitrogen Rejection with a 3.7-Angstrom pore. This pore size permits nitrogen (3.6 angstrom) to enter the pore while the larger methane molecule (3.8 angstrom) does not fit within the pore and passes through the fixed bed of adsorbent at high pressure. In this manner the system is similar to fixed bed dryers where water is adsorbed from the natural gas feed with the dry product produced at high pressure.

The adsorbent is utilized in a pressure swing adsorption system consisting of carbon steel adsorber vessels and a valve and piping skid network alongside the skid to control the feed, product and tail gas flows between the adsorber vessels. Adsorption occurs at high pressure, typically at 100 psig, and in the case of the Hamilton Creek unit at 400 psig, with the adsorbed nitrogen removed through a single stage vacuum pump and discharged at low pressure.

The Hamilton Creek system is operated by the pumper responsible for the gas wells through a daily visit to the site. Under these conditions it has achieved an excellent level of reliability and has been available for 99% of the time. No major issues have been identified with the system and trips are mostly attributed to valve and instrumentation drift or failure. The pumper can normally repair and restart the system within 15 minutes of his arrival to the site.

Since the start-up of the Hamilton Creek system thirty projects are underway for nitrogen rejection and carbon dioxide removal or both.


Carbon Dioxide CO2 Removal

One of the initial advantages recognized for the nitrogen rejection technology is that carbon dioxide (3.4 Angstroms) is a smaller molecule than nitrogen and can easily be removed when present in a system designed for nitrogen removal. This co-removal of carbon dioxide is attractive to project economics and operation since it eliminates the need for a separate amine treating unit.

Pressure swing adsorption systems have, in a few instances, been used for the bulk removal of carbon dioxide from methane, such as through the use of activated carbon adsorbent. The advantages of PSA are in its simplicity but the technology is limited by a relatively low selectivity between methane and carbon dioxide using conventional adsorbents. This means that a large amount of methane is co-adsorbed along with the carbon dioxide leading to high losses of methane into the tail gas and larger adsorbent inventories.

The low selectivity of carbon dioxide over methane has been addressed in the Molecular Gate – Carbon Dioxide Removal system through the tailoring of the pore sizes of the adsorbent and designing for a low methane adsorption level on the adsorbent. Using a single stage vacuum pump for regeneration as well as a recycle to feed of a methane rich stream further enhances this inherent adsorbent selectivity to provide high methane recovery rates.

The carbon dioxide removal application was not initially targeted for Molecular Gate technology, but in mid-2001 Tidelands requested the removal of carbon dioxide from a heavy hydrocarbon rich associated gas stream at their Long Beach, California facility. In response to this request (and a few earlier requests) pilot plant studies were performed and a preliminary design prepared. The system was awarded in late 2001 and delivered to the site in about 16 weeks and continues to operate with no loss in performance to date.

The system offers a new route for the removal of bulk levels of carbon dioxide using proprietary adsorbent that has a high affinity for carbon dioxide while having a low capacity for methane. The system has advantages over the traditional amine and membrane processes for certain applications. It is ideally suited for coal bed methane, landfill gases and biogas and is appropriate for a wide range of conditions.

Tidelands Long Beach, California Facility

The majority of the hydrocarbon resources in the southern portion of the Wilmington oil field are owned by the State of California and the City of Long Beach, California. Production facilities are operated by Tidelands Oil Production Company. The giant Wilmington Oil Field has been a prolific producer of oil since its discovery in the early 1930’s. Today the field’s production is maintained through a water flood EOR operation. The location is challenging for oil and gas production with a need to address environmental and operational concerns in an urban location. Maintaining ground surface levels and oil production requires the removal and subsequent rejection of 200,000 b/d of total liquids of which 6500 b/d is oil. Along with the oil 1.5 MM SCFD of associated natural gas is produced.

The associated gas is contaminated with carbon dioxide at over 30% and also includes a smaller level of nitrogen and a large quantity of heavy hydrocarbons. It is produced at about 20 psig. In the facility a portion of the associated gas is used as fuel to operate internal combustion engines and other facility equipment. The local fuel consumption leaves over 0.5 MM SCFD of excess fuel that has previously been flared.

Tidelands staff of over 100 individuals is focused on the environment and safe operations. Upgrading the contaminated associated gas to pipeline quality was a highly desirable goal, however, distracting the ongoing operations with a complex facility, or one that could compromise the environment, was not acceptable.

System Design

The Molecular Gate Unit at Tidelands is a relatively small system that treats about 1.0 MM SCFD of the associated gas.

Even at this small size the elimination of flaring and revenue generated through the sale of the pipeline quality gas to the local gas utility company allowed an acceptable return for the project.

One challenge in the design was the level of nitrogen contained in the feed stream. Since the gas utility company originally imposed a total inert specification of 4% and the system is not designed to remove nitrogen, excess nitrogen in the feed could lead to the system being non-compliant with the pipeline specification even if carbon dioxide was completely removed. This concern was addressed by reducing nitrogen sources.

The contaminated associated gas is split into a compressed stream for feed to the Molecular Gate unit and a bypassed stream. In the Molecular Gate unit CO2, water and heavy hydrocarbons are removed and pipeline quality gas is produced. This pipeline quality gas is metered and sold.

The Molecular Gate adsorbents allow modification of one or more properties.  These can include the pore sizes, cations exchanged into the adsorbent or the amount of binder.  Such modifications change the adsorption of the targeted molecules or other feed components.  In feeds containing heavier hydrocarbons the adsorbents can remove heavy components, mostly by adsorption onto the surface of the adsorbent or where a compound bed of adsorbents is used where a weak adsorbent is used to protect from water or the heaviest hydrocarbons.  The adsorbent can also be designed to pass a level of the C2 and C3 into the product stream.  The compound bed of adsorbents used at Tidelands adsorbs the water and heavier hydrocarbon components and removes them to the tail gas since there is a use for the tail gas as fuel.

The design and typical operation is shown in Table 1.

Tidelands Design and Actual Performance Comparison


The low-pressure tail gas from the Molecular Gate unit contains the CO2, water and heavy hydrocarbons and is blended with the portion of the contaminated associated gas bypassed around the unit.  This combined stream provides fuel for gas engines driving pumps that reinject water into the formation. A schematic of the flow balance is shown in Figure 1.

Tidelands' Process Flow SchematicThe operation and start-up of the Tidelands unit resulted in a few unexpected developments. The feed stream CO2 level typically operates at about twice that of the design rate (37% versus 18%). While the unit is still able to operate at full capacity, some portion of this capacity was gained by the relaxation of the product specification such that up to 2% CO2 is permitted into the product stream as compared to the design level of less than 2000 ppm.

In other respects the start-up was uneventful and from feed-in to normal, unattended operation, the time required was a few days. It is desirable to operate the unit continuously and this has generally been the case since start-up with on-stream factors over 99%.

Operating difficulties have been fairly minor. One modification made to the unit shortly after startup was to place additional pressure control between the feed compressor and the unit to minimize swings in the discharge pressure of the feed compressor. Although such pressure swings are not problematic for the Molecular Gate Unit, they were causing oscillating loads on the feed compressor.

The unit required the reactivation of an old pipeline, whose scale caused the blockage of filters and which was cleared up through further use.

The tail gas vacuum compressor has provided good operation. Vacuum compressor suction pressure was raised at startup to allow for higher discharge pressures at the vacuum compressor outlet. This was required as downstream consumers (I.C. engines and/or flares) were running at higher than design pressure. Tidelands strategy on startup of the Molecular Gate unit was to get the unit running then adjust downstream operations to optimize performance of the Molecular Gate Unit. In some cases this meant removing duplicate pressure regulators on IC engines so they could operate on a lower supply pressure, and in other cases it meant lowering the regulated pressure. In addition some units required tuning for the different quality fuel gas. Within a few days from startup the discharge pressure was lowered and the suction to vacuum compressor was also subsequently lowered.

The unit has operated for over six years and continues to operate with no changes in performance noted. Recently an automatic product purity control has been added to maintain the product at 2% carbon dioxide.

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