Answer
The production of high-purity Nitrogen (N2) on board gas carriers relies on separating nitrogen from ambient air. The two primary technologies used on ships are Hollow-Fiber Membrane Separation and Pressure Swing Adsorption (PSA).
Both systems operate on the fundamental principle that atmospheric air is composed of approximately 78% Nitrogen, 21% Oxygen, and 1% Argon/other trace gases, and by utilizing physical properties, oxygen molecules can be trapped or filtered out to leave high-purity nitrogen.
1. General Working Principle of a Marine N2 Generator System
Regardless of whether the ship uses a Membrane or PSA system, the overall machinery train follows a standardized process to prepare and process the air:
Air Compression: Ambient air is drawn in and compressed by a dedicated, high-capacity feed air compressor (usually a screw-type compressor) to a pressure between 7 bar and 13 bar.
Air Cleaning & Filtering (Crucial Phase): The compressed air is highly contaminated with moisture, oil aerosols from the compressor, and atmospheric dust. It passes through:
An Air Cooler and Water Separator to drop the temperature and knock out bulk condensed water.
A series of Coalescing Filters (Pre-filter, Fine-filter, and Micro-filter) to remove solid particulates down to 0.01 micron.
An Activated Carbon Tower (Oil Vapor Adsorber) to eliminate any trace oil vapors. Any oil carryover will permanently ruin and "poison" the downstream PSA carbon sieves or membranes.
Air Heating: The clean, dry air is slightly reheated to a stable operating temperature (around 40∘C to 50∘C) to optimize the separation efficiency.
Separation: The treated air enters the separation module (PSA towers or Membrane bundles) where Oxygen is vented to the atmosphere and pure Nitrogen is directed to the deck receiver.
2. The PSA (Pressure Swing Adsorption) Principle
The PSA system uses a physical filtration process at the molecular level, leveraging the difference in molecular sizes and adsorption rates between Oxygen and Nitrogen.
The Core Medium: Carbon Molecular Sieve (CMS)
The heart of a PSA generator consists of two pressure vessels filled with a porous material called Carbon Molecular Sieve (CMS).
An Oxygen molecule (O2) has a molecular diameter of approximately 3.46 A˚ (Angstroms ) or 3.46 x 10-10 mtr.
A Nitrogen molecule (N2) has a molecular diameter of approximately 3.64 A˚.
Because the oxygen molecule is slightly smaller and has a higher diffusion velocity, it penetrates the microscopic pores of the CMS much faster than nitrogen and becomes trapped (adsorbed) inside the sieve. The larger nitrogen molecules pass straight through the gaps between the CMS pellets.
The "Swing" Process (The Two-Vessel Cycle)
The system is called "Pressure Swing" because it constantly shifts (swings) between High Pressure (Adsorption) and Low Pressure (Desorption/Regeneration) across two identical towers (Tower A and Tower B) via automated, cyclic valve operations:
Step 1: Adsorption under High Pressure (Tower A Active, Tower B Regenerating) Compressed, clean air enters the bottom of Tower A at high pressure (≈7−9 bar). The CMS adsorbs the oxygen molecules. High-purity Nitrogen gas exits from the top of Tower A into the ship's cargo tank distribution manifold. Simultaneously, Tower B is depressurized down to atmospheric pressure, causing its saturated CMS to release (desorb) the trapped oxygen molecules, venting them out to the deck atmosphere.
Step 2: Pressure Equalization Just before Tower A becomes completely saturated with oxygen, the feed air inlet is isolated. A valve opens connecting the tops of both towers. High-pressure nitrogen gas flows from Tower A to Tower B to equalize their pressures. This saves energy and pre-pressurizes Tower B.
Step 3: Adsorption Shifts (Tower B Active, Tower A Regenerating) The feed air is now directed into the bottom of Tower B at high pressure to produce nitrogen. Tower A is opened to the atmosphere; its internal pressure drops rapidly, forcing the trapped oxygen to detach from the CMS pores and exhaust out.
This continuous alternating loop (typically cycling every 60 to 90 seconds) ensures an uninterrupted, non-stop flow of high-purity nitrogen to the deck.
⚓ MEO Class 1 Oral Exam Quick-Reference Checkpoints
When explaining the operation and maintenance of a PSA system to an MMD surveyor, be sure to highlight these technical operational factors:
Purity vs. Throughput Control: Emphasize that nitrogen purity (≥99.5%) is directly proportional to the residence time inside the CMS. If you open the deck delivery valve too much (increasing flow rate), the gas passes through the CMS too fast, causing the oxygen content to rise. Purity is regulated using an automated Flow Control Valve linked to an Oxygen Analyzer.
The Danger of Oil Contamination: If the compressor's oil separators or activated carbon filters fail, oil carryover coats the surface of the CMS. This permanently blocks the micro-pores, destroying its adsorption capacity and requiring a highly expensive complete replacement of the CMS charge.
Air Consumption Ratio: Note that a PSA system requires roughly 2.5 to 3 volumes of compressed air to produce 1 volume of high-purity nitrogen. The remaining volume (enriched oxygen air) is safely vented to an open deck area away from any ignition sources.