Nitrogen Rejection and C02 Removal Made Easy

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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 1 (page 2 of 3)

Tidelands SystemTidelands continues to optimize the facility and recently installed a new gas engine driving a water injection pump which has reduced the excess fuel routed to the Molecular Gate unit and decreased the quantity of gas sold to the pipeline. Taking advantage of the unit’s current excess capacity may allow the opening of currently shut-in high CO2 wells.

The carbon dioxide removal requirements at Tidelands are somewhat unique.  The main difference compared to typical field operations is a relatively low operating pressure and the local use for a large quantity of fuel.  This fuel demand provides a convenient outlet for the tail gas from the Molecular Gate Unit but also means that the system need not achieve high methane recovery rates.  At the Tidelands site the methane recovery rate is about 75%; acceptable for Tidelands but insufficient for most field operations where recovery rates in the 95% range are a more typical target.  Recovery rates in the mid-90’s are achieved through the addition of a methane vent recycle that recovers methane otherwise lost into the tail gas.

Applications - Coal Bed Methane

Methane from coalbeds is a rapidly growing source of natural gas and now accounts for about 7% of USA production. The gas is normally produced from shallow wells and is quite lean, rarely containing substantial quantities of hydrocarbons heavier then methane. The gas as produced is water saturated and commonly contaminated with carbon dioxide. The level of carbon dioxide varies widely with the source and location. We have commonly seen levels on the 4-5% range in the Powder River Basin, about 12% in the San Juan Basin and much higher levels in deeper (and heavy hydrocarbon containing) formations.

Tidelands System Gas VolumeThe gas produced at the wellhead is at low pressures and typically routed through a screw compressor to boost the pressure to about 80-120 psig. The gas is then further compressed to high-pressure and processed to pipeline specifications.

Because the pipeline requirement is at high pressure any system to remove the CO2 will operate at elevated pressure. Both amine and membrane systems prefer high pressures of 800+ psi. The Molecular Gate operating system pressure is flexible (80 600 psig) but in most cases prefers a lower pressure at the discharge of a booster screw machine running at 100-200 psig. The choice of the design pressure is project specific for any CO2 removal technology.






A typical arrangement for the system for coalbed methane is shown in Fig. 4.

Typical Coalbed Methane Process Flow Schematic It is common for coal bed methane to contain carbon dioxide and many amine-based systems are used for the removal of this impurity.  In addition to the usual operating challenges of these amine systems, coal bed methane is generally H2S free and corrosion concerns can require consideration.

A glycol dehydration unit to remove water and meet the pipeline specification typically follows the amine system (while not being required by the Molecular Gate system).

In the Molecular Gate system carbon steel construction is used and, since it is a dry system, corrosion is not a concern. Since the tail gas from the vacuum pump contains water and carbon dioxide, attention to corrosion is required there and prevention of liquid water carryover into the gas engine fuel is required.

In addition to the removal of carbon dioxide the system also dehydrates the feed stream and a water free pipeline gas product is produced in a single step.

Carbon dioxide is removed by the system to low levels, typically 2 percent, and lower levels are achievable. Most market interest is to meet the pipeline specification of 1.5 3.0% and maximizing the level of carbon dioxide in the product provides the lowest cost and highest methane recovery design. The product purity is flexible and lower levels can be achieved through a simple change in the adsorption time. These low levels include the ppm levels required for LNG pretreatment.

To maximize the methane recovery rate a low-pressure recycle stream is extracted during the process cycle and recycled back to the main feed compressor. This process step requires an incrementally larger feed compressor. It permits high methane recovery rates, typically ninety-five percent, to be achieved by the Molecular Gate system.

Process Optimization

Because the system does not recover all the methane and loses a portion into the tail gas, use of the tail gas is a process optimization for each project. Where the coal bed methane feed gas contains less than about 8 percent carbon dioxide, the tail gas from the system has a sufficient heating value to provide fuel to the feed compressor. This is an important consideration. Since the main feed compressor will typically consume about five percent of the available feed, a Molecular Gate PSA unit operating at ninety-five percent recovery of methane allows the resulting tail gas to be in balance with the fuel demands of the main feed compressor. In this manner, there is essentially no loss of methane from the system. The design of the fuel supply to the gas-engine is an important consideration and is evaluated on a case-by-case basis with gas engine suppliers.

Where the coal bed methane contains higher levels of carbon dioxide, the tail gas fuel is of low heating value. In this application the system can be configured to split this tail gas into a higher heating value portion suitable for driving the gas engine and a lower heating value component. This design aims to minimize the loss of methane into the low heating value portion of the tail gas. While this solves the need to make use of the tail gas it adds cost. Alternately, depending on the overall fuel demand, a portion of the feed stream (or recycle stream) can be blended with the tail gas to spike the heating value such that it can be used to drive the gas engine.

The level of carbon dioxide in the feed can also vary over time since as the pressure of the wells decreases the carbon dioxide level can increase. Design for a flexible system is required and some increase in capacity is possible through cycling the adsorbent beds more rapidly. If substantial flow rate increases are anticipated the system can be sized to allow the addition of adsorber vessels/adsorbent in the future for debottlenecking purposes. In general such expansions can be accomplished for a fraction of the cost of grass root facilities.

Example - Coalbed Methane

In the example in Table 2, Coal Bed Methane with about 6% carbon dioxide requires removal to 2%.  In this design the raw feed is 5 MM SCFD and the recycle rate back to the feed compressor is about 0.60 MM SCFD (12%).

Example for Coalbed Methane Material Balance

Fuel Evaluation:

Fuel consumption
Wellhead pressure
Feed pressure
Feed compression
Fuel required for compressor

17 psia
315 psia
~1,000 horsepower
~10 MM Btu/hr
Fuel availability
Tail gas heating value:
Tail gas contained heating value:


~500 Btu/ft3
7.9 MM Btu/hr

In the above example the fuel demand and contained heat in the tail gas are in near balance. This balance is site dependent, especially with respect to the feed pressure to the CO2 removal system.

Some gas engine manufacturers commonly address operating gas engines on low quality fuel. In general a dual fuel carburation system or a fast acting air/fuel ratio controller such that the engine can operate with the normal tail gas as fuel but also operate with the raw feed (or pipeline gas) for start-up or upset conditions.

The vacuum pump on the Molecular Gate tail gas is a single stage system and is chosen between positive displacement blowers, rotary vane machines and liquid ring machines. Such machines are commonly used in coal bed methane production to enhance the gas flow rate from the coal bed and its operation does not present any special challenges. The system is fitted with oxygen sensors and automatic shutdown due to the possibility of air (oxygen) ingress under vacuum conditions.

The vacuum pump is normally electrically driven, though gas drives can also be utilized.


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