Nitrogen Generators Using Compressed Air

Air is 78% nitrogen, 21% oxygen. Nitrogen generator's job is removing oxygen, keeping nitrogen.

How to separate? Industry commonly uses two paths: PSA pressure swing adsorption, membrane separation.

PSA Principle and Membrane Separation Principle

PSA relies on carbon molecular sieve. This stuff's surface is full of micropores, oxygen molecules are small, get in fast, nitrogen molecules are big, get in slow. Pressure comes up, oxygen gets held by micropores, nitrogen slips by. Pressure releases, oxygen comes back out, molecular sieve can keep working.

Membrane separation is more direct. Hollow fiber membrane has selectivity for gases, oxygen permeates faster than nitrogen. Compressed air pumped into membrane tube, oxygen passes through the wall and leaves, nitrogen goes along the tube and comes out. No adsorption-regeneration cycle, no valves frequently actuating, structurally much simpler.

PSA Method

Dual Tower System

Membrane

Hollow Fiber Module

Controls

System Monitoring

Both methods have requirements for feed gas, PSA is pickier. Carbon molecular sieve fears water and oil, water molecules will occupy micropore positions, oil will clog micropores. Clogging is irreversible, molecular sieve is ruined. So PSA must have adsorption dryer upstream, push dew point down to minus forty degrees. Membrane separation has looser dryness requirements, sometimes refrigerated dryer processed air can go directly.

PSA can push purity to 99.999%, membrane separation generally 99.5% is the limit. Go higher and membrane separation efficiency drops badly, producing same amount of nitrogen burns more power, eats more air, not economical.

That Gas Treatment Equipment Upstream

Air straight from compressor can't go directly into nitrogen generator.

Temperature is high, moisture content is high, also carrying oil mist and dust. Air receiver catches it first, lets airflow settle. Filters catch particles and oil droplets. Refrigerated dryer drops temperature, condenses out water vapor to drain. Precision filter does another pass, catches stuff down to hundredths of a micron. Activated carbon absorbs oil vapor.

This equipment can't be skipped. Skip it and nitrogen generator main unit life shortens, molecular sieve or membrane modules fail early, costs more money.

How Big a Compressor to Match

Nitrogen generator consumes compressed air. Consumption ratio relates to nitrogen generation method and purity requirements.

PSA doing 99.5% purity, producing one cubic meter nitrogen takes about two to three cubic meters air. Membrane separation doing 99% purity, ratio is lower. These numbers are experience ranges from equipment manufacturers, specifics depend on model.

When calculating compressor, first figure out how many cubic meters air per minute needed, then multiply by margin factor, typically 1.1 to 1.2. Select discharge volume based on this number, pressure select 7 bar and up.

Which to Choose

Look at purity requirements. Below 99% either works, membrane separation has lower equipment investment. Above 99%, PSA is more suitable. Above 99.5%, basically only PSA.

Look at volume. High volume choose PSA, lower operating energy consumption, wins in long-term accounting. Low volume choose membrane separation, simpler equipment, less maintenance.

Look at usage pattern. Intermittent use, frequent starts and stops, membrane separation responds fast. Continuous use, stable load, PSA is more efficient.

Make Your Own or Buy Outside

Buying liquid nitrogen or bottled nitrogen doesn't need equipment investment, buy as much as you use. Unit price is expensive, small quantities doesn't matter.

Volume goes up and you need to do the math. Tens of cubic meters per hour, running every day, building your own station makes sense. Equipment investment pays back in one to two years, savings after that is all profit.