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Low Cost Nitrogen Natural Gas Removal Equipment

Local natural gas pipeline specifications in the United States vary, but nearly all companies purchase natural gas with the condition that its heat content is at least 950 BTU/cu ft. As a result, as much as 17% of natural gas reserves in the United States are labeled as sub standard natural gas. Nitrogen is hence the major target for removal in upgrading significant Natural Gas volumes of otherwise unsalable pipeline quality gas. A major portion of the United States nitrogen rich low-BTU natural gas is trapped in modest to small fields, these fields often owned by stripper operators.

These smaller fields are not agreeable to Natural Gas upgrading technologies such as large conventional pressure swing adsorption (PSA) systems and cryogenic separation because these fields cannot deliver a large enough feed volume for profitable operations utilizing more traditional technologies.

The advantages of using a micro nitrogen rejection system are that they are inexpensive and require only the use of activated carbon as the adsorbent. They may be skid mounted and require only a small footprint. NRU units have limited moving parts and require only very little maintenance throughout the year. Natural gas rejection systems can be configured to be used in remote or isolated areas. Other advantages are; the systems can be upgraded to larger mcf/d with minimal costs. The systems do not emit volatile organic compounds (VOC’s) and majority of states require no permitting or special requirements.

Twin activated carbon beds are required and switch between adsorbing and desorbing of the natural gas flow. The average nitrogen rejection system (twin towers) uses approximately 6400 pounds activated carbon. Additional equipment is a 50 to 70 horse natural gas powered engine or electrical motor for the compressor which is designed to create a vacuum. With lower BTU feed gas a vacuum up 30” Hg is necessary. Condensate removal tower catches any liquid vapors and an optional surge tank to blend the natural gas vapors for a more consistent blend of natural gases.

There are only a few requirements to use the micro natural gas nitrogen rejection system. They are as follows; that the field gas have a minimum of 630 BTU’s per cubic foot, that the inlet field gas pressure able to pressurize the rejection tanks between 20 to 70 psi (adjusted based on (BTU content); and the system be able to vent the nitrogen vapors from the adsorption tank between 2 and 13 psi.

The micro nitrogen rejection system efficiency upgrades averages 54% to 58% by volume to the minimal natural gas requirement of 950 BTU. The feed gas simply requires minimal heavy hydrocarbon with a fraction of 3.8%.The BTU recovery efficiency increases to around the 75% with improved feed having a 715 BTU per cubic foot or above.

The micro plant nitrogen rejection system is capable of adsorbing and desorbing 98% of the heavy hydrocarbons captured from the feed stock of low BTU natural gas. Heavy hydrocarbons are considered C2H6+.

The nitrogen rejection system works very well on a minimal hydrocarbon content of 63% (CH4+ % mole) which the Nitrogen Rejection Unit will upgrade to saleable natural gas to around 85%, this nitrogen scrubbing system results in a 73% hydrocarbon recovery with a 75% BTU recovery. The vented off gas will contain about 63% Nitrogen (% mole) which results in an average nitrogen rejection efficiency of 76%

Conclusions

·         Pressure Swing Adsorption Plants upgrade low-BTU gas (as low as 630 BTU/cu ft) to pipeline quality (> 950 BTU/cu ft) using a low maintenance, cost-effective micro-scale nitrogen rejection unit (NRU).

 

·         Approximate plant construction costs of $120,000 and assuming gas prices at $4/mcf and a feed of 200 mcf/d, the ROI is estimated at 5 to 6 months for 615 BTU/cu ft feed. Higher BTU/cu ft. streams increase ROI rates.

 

·         The towers have to be evacuated (desorbed) from vent pressure (around 2 psi) to maximum vacuum (≈25 to 28” Hg) to maximize heavy hydrocarbon recovery and to lower cycle time, which is inversely related to plant throughput.

 

·         Both nitrogen content and the fraction of heavy hydrocarbons in the feed control the optimum NRU plant settings and determine the systems efficiency.

 

·         NRU Plant settings, namely tower charge pressure and vent pressure, will have to be adjusted if feed composition (BTU and C2H6+/CH4+ ratio) changes over 10%; this may be controlled by use of a programmable logic controller. Greater amounts of heavy hydrocarbons in feed results in higher sales/feed ratio and thus better plant operating economics.

 

·         Low BTU Reference data:

ü  In general, the shallower zones tend to produce low-BTU gas.

ü  Hydrocarbon-wetness increases with age and depth of the producing zone.

ü  Nitrogen-to-helium ratios are unaffected by the age of the pay zone.

ü  Given the limited data set available, the deeper formations appear to display

greater compositional ranges for hydrocarbon and non-hydrocarbon gases.

 

For more information about partnerships, joint ventures or purchasing a Nitrogen Rejection System (Pressure Swing Adsorption Plant) visit our website: www.american-environmental.us  or call today +1 918 708-1253.

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