Nitrogen Rejection System - Pressure Swing Adsorption
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Nitrogen Rejection System (Pressure Swing Adsorption) - Local pipeline specifications vary, but nearly all
companies buy natural gas with the requirement 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. N2 is hence
the major target for removal in upgrading significant NG volumes of otherwise unsalable pipeline quality gas. A
major portion of the United States N2-rich low-BTU gas is trapped in moderate to small fields owned by stripper
operators.

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

Advantages of the N3 Rejection System
    • Uses inexpensive activated charcoal as the adsorbent bed.
    Modular skid mounted units which are scalable as per changing NG feed volumes.
    • Has a small real estate foot print starting at (400 sq. ft).
    • Does not emit any volatile organic compounds (VOCs).
    • Has few moving parts (other than the blower and compressor) to reduce labor
     and maintenance costs.
    Operates in remote locations by solar panels and low-BTU feed gas.
    Can economically upgrade low-volume (30 mcf/d to 1.5mmcf/d) and low-pressure (<100 psi) feed gas.

Equipment
    • Carbon Bed Towers from 36" to 76" x 10’)
    Solenoid Valves
    • Activated Carbon from 1100 to 5200lbs. Per Tower
    50-200 HP NG gas-fired engine, operating on the low-BTU feed gas
    Compressor unit or vacuum pump
    Condensate removal tower
    Surge Tank (Optional)
    Connection pipe to pipeline.

Operating Parameters
    • Field Gas above 630 BTU/cub. ft.
    • Inlet pressure between 20 to 100 psi depending on BTU content.
    • Venting N2 from Adsorption Tank

Conversion Ratios
    • 54% to 58% of the feed gas by volume is upgraded from 630 BTU/cub. ft. to 950+ BTU/cub. ft. with
    minimal heavy hydrocarbon fraction of 3.8%. BTU-recovery efficiency decreases to around 59% as
    compared to 75% obtained with a superior feed having an average of 715 BTU/cu ft.

    • Effective in adsorbing and then desorbing 98% of the entrained heavy hydrocarbons (C2H6+) from a
    feed stream of low-BTU gas

    • Granulated Carbon Swing Bed Nitrogen Rejection Plant works well on a average hydrocarbon content of
    63% (CH4+ % mole) which upgrades to a saleable stream containing around 84% of CH4+ (% mole),
    resulting in 73.2% of hydrocarbon recovery and 75.7 % BTU recovery. The vented gas contains about
    63.1% N2 (% mole) resulting in an average N2 rejection efficiency of 76.7%.























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 small plant construction costs of $120,000 for lower psi vacuum systems. Assuming natural
    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 2 psi to maximum vacuum 60 psi 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 its 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:
  • In general, the shallower zones tend to produce low-BTU gas.
  • Hydrocarbon-wetness increases with age and depth of the producing zone.
  • Nitrogen-to-helium ratios are unaffected by the age of the pay zone.
  • 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, please call
or email:
sales@american-environmental.us
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