PSA Nitrogen Generator

Pressure Swing Adsorption (PSA) systems are one of the most common systems to produce high purity nitrogen (99.9995%). This system relies on the concept of selective adsorption, which is a gas’ tendency to stick to a solid surface under high pressure. The adsorber is the solid surface that adsorbs the gas. In the case of PSA nitrogen generators, carbon molecular sieves are used to adsorb oxygen from dry air, leaving high purity nitrogen.

PSA systems operate at ambient conditions and therefore do not require the air to be heated up. Air enters one of the pressure vessels at high pressure, where gas separation takes place. Oxygen is adsorbed onto the surface of the vessel while high purity nitrogen exits for use. When the adsorbent becomes saturated and not able to adsorb any more oxygen, the process “swings” to the other vessel and is blocked off from the inlet air entrance. The saturated vessel is depressurized to vent the oxygen out to the atmosphere while inlet air now enters the other vessel for separation. This system swings back and forth to continuously produce Nitrogen, with one vessel depressurizing and the other adsorbing.

What is a Membrane?

A membrane is a permeable barrier that selectively permits entities to pass through it. Gas Land utilizes membrane technology to separate air (composed of 20.8% Oxygen , 78% Nitrogen, and 1.2% other gases) to produce a Nitrogen rich gas product.  Gas Land's membrane separator modules consist of a cylindrical housing with densely packed hollow fibers. Pressurized, dried, and filtered air enters the membrane module and fast gases such as Oxygen permeate through the fibers leaving behind  Nitrogen rich gas. The separation process depends on several factors such as the size of the molecules, speed of the molecules, and chemical interactions with the fibers. Typical Gas Land Nitrogen Generation Systems are designed to deliver between 96% and 98.5% Nitrogen purity depending on the customer’s needs. The primary advantage of membrane gas separation is that the process does not require any moving parts. This reduces maintenance and repair costs and ensures the system continues to perform for years to come.

Designing a Nitrogen Generator

There are many factors that one must consider while designing a Nitrogen Generation System. 

1) Nitrogen Flow Required

2) Purity of Nitrogen

3) Inlet Air Pressure

4) Inlet Air Availability 

5) Utilities Available

6) Nitrogen Discharge Temperature

7) Turndown Capability

These parameters form the basis of the design for the System. Higher inlet air pressure results in better separation of Nitrogen from Air. Additionally, one must also ensure that the compressors are sized sufficiently; so, they can provide the necessary feed air required for the Nitrogen Generator. Electrical utilities play a role in sizing of the Heater and related controls. Some applications may have a constraint on the discharge temperature of Nitrogen, which is a key parameter in establishing the process temperature for the separation to take place. It is also important to understand the turndown capability of the System as during startup downstream requirement may be much lesser than the intended design

Typical Nitrogen Generation Plant

Nitrogen forms a critical component of the utility system as it is used for various applications in a LNG Plant. Nitrogen is mainly used for purging, blanketing, and as a seal gas for various compressors and pumps. The above figure depicts a typical Nitrogen Plant set up upstream and downstream of the Nitrogen Generator. The Nitrogen is distributed to various trains, or users via the Nitrogen Distribution Header. A secondary Nitrogen Generation system such as the Liquid Nitrogen Storage & Vaporization package or another Membrane Nitrogen Generator is generally used as a backup system.