Nitrogen Rejection Unit
Encyclopedia
A nitrogen rejection unit (NRU) selectively removes nitrogen
from a gas. The name can be applied any system that removes nitrogen from natural gas
.
For high flow-rate applications, cryogenic processing is the norm . This is a distillation
process which utilizes the different volatilities of methane
(boiling point
of -161.6°C) and nitrogen
(boiling point of -195.69°C) to achieve separation. In this process, a system of compression and distillation columns drastically reduces the temperature of the gas mixture to a point where methane is liquified and the nitrogen is not. For smaller applications, a series of heat exchangers may be used as an alternative to distillation columns.
For smaller volumes of gas, a system utilizing Pressure Swing Adsorption
(PSA) is a more typical method of separation. In PSA, methane and nitrogen can be separated by using an adsorbent with an aperture size very close to the molecular diameter of the larger species, in this case methane (3.8 angstroms). This means nitrogen is able to diffuse through the adsorbent, filling adsorption sites, whilst methane is not. This results in a purified natural gas stream that fits pipeline specifications. The adsorbent can then be regenerated, leaving a highly pure nitrogen stream. PSA is a flexible method for nitrogen rejection, being applied to both small and large flow rates.
It should be noted that the operating conditions of various PSA units are quite variable. Depending on the vendor, high degrees of pretreatment of the gas stream (removal of water vapor and heavy hydrocarbons) may be necessary for the system to operate optimally and without damage to the adsorbent material. Moreover, the degree of hydrocarbon recoveries (75% vs 95%) and purities can vary considerably. The economic viability of any PSA unit will be highly dependent on such factors.
An estimated 25% of the US natural gas reserves contain unacceptably large quantities of nitrogen. Nitrogen is inert and lowers the energy value per volume of natural gas. It also takes up capacity in pipelines that could be used for valuable methane.
Pipeline specifications for nitrogen are extremely variable, though "no more than 4% nitrogen" seems to be the most typical specification .
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
from a gas. The name can be applied any system that removes nitrogen from natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
.
For high flow-rate applications, cryogenic processing is the norm . This is a distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
process which utilizes the different volatilities of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
(boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of -161.6°C) and nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
(boiling point of -195.69°C) to achieve separation. In this process, a system of compression and distillation columns drastically reduces the temperature of the gas mixture to a point where methane is liquified and the nitrogen is not. For smaller applications, a series of heat exchangers may be used as an alternative to distillation columns.
For smaller volumes of gas, a system utilizing Pressure Swing Adsorption
Pressure swing adsorption
Pressure swing adsorption is a technology used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperatures and so differs from cryogenic distillation...
(PSA) is a more typical method of separation. In PSA, methane and nitrogen can be separated by using an adsorbent with an aperture size very close to the molecular diameter of the larger species, in this case methane (3.8 angstroms). This means nitrogen is able to diffuse through the adsorbent, filling adsorption sites, whilst methane is not. This results in a purified natural gas stream that fits pipeline specifications. The adsorbent can then be regenerated, leaving a highly pure nitrogen stream. PSA is a flexible method for nitrogen rejection, being applied to both small and large flow rates.
It should be noted that the operating conditions of various PSA units are quite variable. Depending on the vendor, high degrees of pretreatment of the gas stream (removal of water vapor and heavy hydrocarbons) may be necessary for the system to operate optimally and without damage to the adsorbent material. Moreover, the degree of hydrocarbon recoveries (75% vs 95%) and purities can vary considerably. The economic viability of any PSA unit will be highly dependent on such factors.
An estimated 25% of the US natural gas reserves contain unacceptably large quantities of nitrogen. Nitrogen is inert and lowers the energy value per volume of natural gas. It also takes up capacity in pipelines that could be used for valuable methane.
Pipeline specifications for nitrogen are extremely variable, though "no more than 4% nitrogen" seems to be the most typical specification .