Introduction

The recovery of natural gas (primarily methane) from coalbed deposits is an established and growing source for the U.S and accounts for over 7% of its total production. The gas from on-purpose degassing is often pipeline quality and requires only compression and dehydration before being admitted to the pipeline grid. However, the raw gas is sometimes contaminated with nitrogen (N2) and/or carbon dioxide (CO2) that can make the gas inadmissible to the transportation pipeline. In general, the N2 must be 4% or less and the CO2 should be 2% or less.

In treating gas produced from virgin coal bed or from degassing operations where CO2 is present, amine systems are traditionally used for its removal. Engelhard’s Molecular Gate
adsorption system offers an attractive alternative due to its skid-mounted equipment, low pressure and unattended operation, cost and high onstream reliability.

Natural gas can also be sourced from the residual coal, after the coal has been mined. This source can be the GOB or gas remaining in abandoned coal mines. This source is almost universally contaminated, often with a combination of CO2 and N2. If the source is from air intrusion, the source can also be contaminated with O2. In treating CO2 / N2 contaminated feeds, the Molecular Gate adsorption system removes both of the impurities along with a level of O2 within a single processing step.

To date, ten Molecular Gate units are underway with three systems having been installed for upgrading gas from abandoned coal mines. The technology consists of specialty adsorbents and advanced PSA processes that are jointly offered as fabricated packaged equipment by Engelhard Corporation and Guild Associates, Inc.

Upgrading requirements, project challenges and project arrangements are discussed in this paper.

Molecular Gate Adsorbent

The Molecular Gate process for coal bed and coal mine methane takes advantage of a new type of molecular sieve that has the unique ability to adjust pore size openings within an accuracy of 0.1 angstrom. The pore size is precisely adjusted in the manufacturing process and allows the production of a molecular sieve with a pore size tailored to size-selective separations.

Nitrogen and methane molecular diameters are approximately 3.6 angstroms and 3.8 angstroms, respectively. In the Molecular Gate adsorption system for upgrading nitrogen-contaminated methane, a pore size of 3.7 angstroms is used. This adsorbent permits the nitrogen and carbon dioxide to enter the pore and be adsorbed while excluding the methane, which passes through the fixed bed of adsorbent at essentially the same pressure as the feed. This size separation is schematically illustrated in
Figure 1.

Figure 1.  Schematic view of the Molecular Gate pore size and relative molecule size

Carbon dioxide is an even smaller molecule than nitrogen at 3.3 angstroms and is easier to remove than nitrogen. One major advantage of the process in upgrading nitrogen contaminated feeds is that such feeds almost always have a level of CO2 contamination and, in the process; CO2 is completely removed in a single step with the nitrogen removed to pipeline specifications.

Since coal bed methane is produced at low pressure (vacuum is sometimes pulled to increase production) the possibility of introducing oxygen into the system exists. Oxygen is also a small molecule at about 3.5 angstroms and fits within the pore of the adsorbent and it is removed at about the same rate as nitrogen. Thus, if meeting pipeline specifications requires the removal of 70% of the nitrogen in the feed, the same percentage (70%) of the oxygen would also be removed (while carbon dioxide would be completely removed). Water is an even smaller molecule and can also be removed.

Process Description

The Molecular Gate adsorbent is applied in a pressure swing adsorption system (PSA), which operates by “swinging” the pressure from a high-pressure feed step that adsorbs the impurity to a low-pressure regeneration step to remove the previously adsorbed impurity. Since methane does not fit within the pore of the adsorbent, it passes through the bed at the feed pressure.

Figure 2. Generic isotherm for adsorption and PSA pressures

As can be seen in Figure 2, CO2 adsorbs at a higher capacity than N2 and thus is more easily removed. A system designed for N2 removal will require more adsorbent and, thus, have a higher cost than one for CO2-only removal.

PSA is widely used in light gas separations with hundreds of units in operation in the oil refining, petrochemical and air separation industries. The system is characterized by automatic and simple operation with high reliability.

Figure 3. Block flow diagram of Molecular Gate process

Figure 3 depicts the overall flow process for a typical Molecular Gate adsorption system applied to upgrading coal bed or coal mine methane. Feed gas from the wells is compressed from near atmospheric pressure to typically 100 psig where it is introduced into the Molecular Gate adsorption system. For coal bed and coal mine methane, a screw compressor to 100 psig is typically applied although operation from 80 – 800 psig is possible.

The process generates a low-pressure recycle stream that is rich in methane and recirculated back to the suction of the recycle compressor. By incorporating this recycle stream, the methane recovered as product sales gas is increased without adding additional compressors. The recycle rate is typically 10-15% of the raw feed rate.

To maximize the working capacity of the adsorbent to remove the N2, CO2 and/or other impurities, a single stage of vacuum is used to enhance the regeneration. The swing between the high adsorption pressure and regeneration at low pressure is completed in rapid cycles, on the order of a few minutes, to minimize the adsorbent inventory.

Typical methane recovery rates of 90-95% are achieved in the process. The rejected tail gas, containing the lost hydrocarbons, is often suitable as fuel to gas engines driving gensets or compressors and making use of the otherwise lost methane is a key part of the process optimization.

Field Experience

The introduction and proof of new technology is a challenge. To overcome the introduction hurdle, Engelhard / Guild built a small commercial unit to demonstrate Molecular Gate’s viability. This unit started operation in late 2000 at the Hamilton Creek site in SW Colorado and was operated for two years with excellent results. After the demonstration period, it was relocated to a commercial site in Ohio where it continues operation.

In Colorado, the unit was operated on a remote natural gas wellhead site where the feed contained 18% nitrogen and less than 1% carbon dioxide. The system operated to direct the product to one of two local pipelines that accepted either 3% or 6% nitrogen. This site was not easily accessed and lacked electric power, thus, a rental genset unit was used to provide power, and a packaged unit was used to provide instrument air. The feed capacity of the system was about 200 M SCFD with an operating pressure that was typically 400 psig.

The operation of the system at Hamilton Creek proved to be both effective and reliable. The pumper responsible for the wells operated the system and generally visited the site once per day for about a half hour to review a checklist for performance monitoring. The reliability of the system was very good and demonstrated a 99% availability factor.

In May of 2002, the first Molecular Gate adsorption system for the removal of carbon dioxide from associated natural gas was started up at a Tidelands Oil Production Company operated facility in California. A photo of the unit is shown in Figure 4. The system is skid-mounted with low capital and operating costs, and provides automatic and unattended operation with high on-stream factors. A dry product is produced with no need for further dehydration. Permitting and operability concerns were the main drivers for the selection of the Molecular Gate adsorption technology at this location.

The Hamilton Creek N2 rejection demonstration unit and the Tidelands CO2 removal unit established the experience base for the technology. Since the start-up of the Tidelands unit in 2002, eight additional units have been awarded and are operating or in fabrication. Two of these nine units are for CO2 removal and eight for N2 removal, including three units for CMM upgrading.

Commercial systems are designed to operate unattended and are being offered for flow rates as low as 0.5 MM SCFD. The systems are offered as a complete unit with maximum skid mounting of equipment for minimal installation cost. A single price and warranty is provided with the unit.

Figure 4. Tidelands Oil Production Company CO2 removal unit

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