About Us | Nitrogen Rejection & Removal | Carbon Dioxide CO2 Removal | N2 Rejection & CO2 Removal SPEC Plants
Molecular Gate Adsorption Systems Technology | Biogas & Digester Gas Purification | Pretreatment for LNG Facilities
Natural Gas Vacuum Pumps & Blowers | Heavy Hydrocarbon Removal | Dehydration Systems | Inquiry | News | Home
Nitrogen Rejection and C02 Removal Made Easy
Molecular Gate® |
 |
Production of Pipeline-Quality Natural Gas with
The Molecular Gate CO2 Removal Process (page 1 of 3)
James Wills, SPE, and Mark Shemaria, SPE, Tidelands Oil Production Co., and
Michael J. Mitariten, Engelhard Corp. 
Summary
In May 2002, the first Molecular Gate* Carbon Dioxide Removal system for the removal of carbon dioxide and water was started at the Tidelands Oil Production Co. facility in Long Beach, California. The feed source for the unit is hydrocarbon-rich, watersaturated, associated gas from waterflood enhanced-oil-recovery (EOR) operations. The feed CO2 concentration varies widely and is typically more than 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 system operated for more than 2 years, removing 18% nitrogen from glycol-dehydrated wellhead gas while producing pipeline-specification product at an unattended site at Hamilton Creek in southwest Colorado.The adsorbent used at the 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, as in the case of nitrogen rejection with a 3.7-Angstrom (au) pore. This pore size permits nitrogen (3.6 au) to enter the pore, while the larger methane molecule (3.8 au) does not fit and passes through the fixed bed of adsorbent at high pressure. In this manner, the system is similar to fixed-bed dryers in which water is adsorbed from the natural gas feed with the dry product.
The adsorbent is used in a pressure-swing-adsorption (PSA) 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 vessels. Adsorption occurs at high pressure; in the case of the Hamilton Creek unit, it was 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 through a daily visit to the site by the pumper responsible for the gas wells. Under these conditions, it has achieved an excellent level of reliability and has been available 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 on site.
Since startup of the Hamilton Creek system, a second nitrogenrejection unit is operating, and three are in the fabrication stage, along with a second CO2 removal system.
CO2 Removal
One of the initial advantages recognized for nitrogen rejection
technology is that carbon dioxide (3.4 au) is a smaller molecule
than nitrogen and can easily be removed when present in a system
designed for nitrogen removal. The coremoval of carbon dioxide is
attractive to project economics and operation because it eliminates
the need for a separate amine treating unit.PSA systems have been used in a few instances for bulk removal of carbon dioxide from methane, such as through the use of activated carbon adsorbent. The advantage of PSA is its simplicity, but the technology is limited by a relatively low selectivity between methane and carbon dioxide with conventional adsorbents. This means that a large amount of methane is coadsorbed 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 vs. methane has been addressed in the system by tailoring the pore sizes of the adsorbent and designing for a low methane adsorption level. Using a singlestage vacuum pump for regeneration as well as recycling a methane- rich stream back to the feed further enhances this inherent adsorbent selectivity to provide high methane recovery rates.
The carbon dioxide removal application was not initially targeted for this technology, but in mid-2001, Tidelands requested the removal of carbon dioxide from a heavy hydrocarbon-rich associated gas stream at its Long Beach, California, facility. In response to this request (and a few earlier ones), pilot plant studies were performed, and a preliminary design was prepared. The system was awarded in late 2001 and was delivered to the site in approximately 16 weeks.
The system offers a new route for the removal of bulk levels of carbon dioxide with a proprietary adsorbent that has a high affinity for carbon dioxide and a low capacity for methane. The system has advantages vs. the traditional amine and membrane processes for certain applications. It is ideally suited for coalbed methane and is appropriate for a wide range of natural gas 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 Co. The giant Wilmington oil field has been a prolific producer since its discovery in the early 1930s. Today, the field’s production is maintained through a waterflood 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 total liquids, of which 6,500 B/D is oil. Along with the oil, 1.5 MM scf/D of associated natural gas is produced.The associated gas is contaminated with carbon dioxide (more than 30%) and also includes a smaller level of nitrogen and a large quantity of heavy hydrocarbons. It is produced at approximately 20 psig. In the facility, a portion of the associated gas is used as fuel to operate internal combustion (IC) engines and other facility equipment. The local fuel consumption leaves more than 0.5 MM scf/D of excess fuel that previously had been flared.
The Tidelands staff of more than 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 unit at Tidelands is a relatively small system that treats approximately 1.0 MM scf/D of the associated gas. Even at this small size, the elimination of flaring and the 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. The system is not designed to remove nitrogen; because the gas utility company originally imposed a total inert specification of 4%, excess nitrogen in the feed could lead to the system being noncompliant with the pipeline specification, even if carbon dioxide was completely removed. This concern was addressed by reducing nitrogen sources.
The design and typical operation are shown in Table 1. The contaminated associated gas is split into a compressed stream for feed to the Molecular Gate unit and a bypassed stream. In the unit, CO2, water, and heavy hydrocarbons are removed, and pipeline quality gas is produced, metered, and sold.
The system’s adsorbents allow modification of one or more properties. These can include the pore sizes, cations exchanged into the adsorbent, or the binder amount. 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 adsorbent’s surface, when a compound bed of adsorbents is used, or when a weak adsorbent is used to protect from water or the heaviest hydrocarbons. The adsorbent also can 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 to be used as fuel.
The adsorbent is robust and does not degrade during normal operation. The main requirement is to protect the adsorbent from deactivation, preventing liquid carryover onto the adsorbent, generally through feed-stream filters and automatic shutdown on high liquid levels.
The low-pressure tail gas from the unit contains CO2, water, and heavy hydrocarbons and is blended with the portion of the contaminated associated gas that bypassed 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 Fig. 1.
The startup and operation of the Tidelands unit resulted in a few unexpected developments. The feed-stream CO2 level typically operates at approximately twice that of the design rate (37 vs.
18%). While the unit is still able to operate at full capacity, some portion of it was gained by relaxing the product specification such that up to 2% CO2 is permitted into the product stream, as compared to the design level of less than 2,000 ppm.In other respects, the startup was uneventful, and the time required from feed-in to normal, unattended operation was a few days. It is desirable to operate the unit continuously, and this has generally been the case since startup, with on-stream factors of more than 99%.
Operating difficulties have been fairly minor. One modification made to the unit shortly after startup placed 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 unit, they were causing oscillating loads on the feed compressor.
The unit required reactivation of an old pipeline with scale that caused filter blockage, which was cleared up with further use.
The rotary-vane 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 because downstream consumers (IC engines and/or flares) were running at greater than the design pressure. Tidelands’ strategy on startup of the unit was to get it running, then adjust downstream operations to optimize performance. In some cases, this meant removing duplicate pressure regulators on IC engines so they could operate on a lower supply pressure; in other cases, it meant lowering the regulated pressure. In addition, some units required tuning for the different fuel-gas qualities. Within a few days of startup, the discharge pressure was lowered, and the suction to vacuum compressor was also subsequently lowered.
 |
 |
Download Article
| Further Information: If you would like an evaluation of how the Molecular Gate technology can solve your gas treatment needs simply complete and fax back the Estimate Request Form or contact Michael Mitariten at 908-752-6420 or by email mike@moleculargate.com. |
