Membrane Type Nitrogen Generation System Working Principle

In this article, we will see the Membrane Type Nitrogen Generation System Working Principle.

Working Principle of Membrane

The membranes are produced in the form of hollow fibres. Gas separation membrane work on the principle of selective permeation through a membrane surface. The difference between partial pressure of the gas on the internal side of the membrane and external side. If the greater this difference, the greater the permeation of gas through the membrane.

The permeation rate of each gas depends on its diffusion rate in the membrane. Gases with a higher diffusion rate and lower molecular size penetrate the membrane faster than large, lesser diffusion gases.

Each membrane module contains millions of hollow fibres. The high density of the fibre bundle provides a vast membrane surface area through which the air components permeate with different velocities. Nitrogen has a lower permeation velocity than oxygen, CO2 and water vapour. Hence these gases pass through the membrane and vented to atmosphere at the safe location (particular attention shall be given to the Vented gas since it contains gas enriched with oxygen). Only nitrogen come out of the membrane. Refer to the above figure.

Membrane Type Nitrogen Generation Working

The figure shows the simplified schematic of a membrane-type nitrogen generation. The main components of nitrogen generations are

• Air Compressor
• Dryer Unit
• Inlet Filters
• Air Heater (Optional)
• Membrane
• Nitrogen Storage Reservoir

Normally, the air feed from a compressor passes through a dryer unit where the moisture from the air as removed.

Then the dry air passes through the series of filters. The filters are used to remove the contamination present in the air. Usually, three types of filters are used in the nitrogen generation system. The first filter used to remove the particles up to 1 micron. The second filter used to filter the particles up to 0.01 micron. The third filter, an activated carbon filter used to remove hydrocarbons up to 0.004 ppm.

Generally, the performance of the membrane directly related to the operating temperature of the air. If the operating temperature of the air is below 45^oC, then the membrane performance will reduce drastically. Hence a heater is introduced into the nitrogen generation system to maintain the temperature of the air around 55~60^oC. If the feed air temperature near to 55^oC, then the heater not required.

Then the air enters into the membrane bundle. Each membrane module contains millions of hollow fibres. The high density of the fibre bundle provides a vast membrane surface area through which the air components permeate with different velocities. Nitrogen has a lower permeation velocity than oxygen, CO₂ and water vapour. Hence these gases pass through the membrane and vented to the atmosphere. Only nitrogen come out of the membrane and stored in the nitrogen storage vessel (nitrogen receiver). Then the nitrogen sent to the end-users (ie., compressor, pumps, tanks etc.,) through nitrogen distributor networks.

Points to taken care while selecting Membrane type nitrogen generator

1. Membranes work better at a constant temperature. Usually, the membrane’s efficiency gets better at a higher temperature. Therefore, the electric heater has provided before the nitrogen generator unit to raise the inlet air temperature to 50~60°C.
2. The maximum operating temperature of the membrane is around 65~75°C. If the operating temperature of air reaches around 65^oC, the nitrogen generation unit will trip.
3. An Inlet filter installed before the membrane-type generator package in order to reduce membranes plugging.
4. An oxygen analyzer installed at the outlet of the nitrogen generation system to indicates performance and controls nitrogen purity to be within the requirement.

Advantages:

• Proven technology also for off-shore applications
• Simple and very reliable operation
• Lower weight, footprint and noise compared to other technologies
• Easy to operate and maintain
• Future scale-up of membranes capacity is easy, only additional membrane banks are needed

Disadvantage:

• Higher feed air consumption compared to other technologies
• Higher electrical consumption compared with pressure Swing Adsorption for inlet air heating
• Very sensitive to water, oil droplets and impurities
• Irreversible fouling and easy breakage are the main problems concerning hollow fibre filtration.

The membranes are more likely to break when under high strain compared to other methods of nitrogen generation