Purity Dependant Switching (PDS) has been developed by Wirac Automation for its Noblegen range of Industrial Nitrogen Gas Generators.

PSA EXPLAINED

PSA Nitrogen Gas Generators involves a continuous time cycle to operate valves to control the generation and re-generation cycle of a Pressure Swing Adsorption system (PSA).

As shown here, the ‘GREEN’ CMS1 vessel is generating nitrogen while the ‘RED’ CMS2 vessel is re-generating which is set using a time cycle via a timer or PLC. 

At a pre-determined time before the end of the exhausting cycle the vent closes which allows good quality nitrogen from the CMS1 to purge CMS2 prior to the change from one side to the other.

PSA CONSIDERATIONS

As a fixed time cycle is used to control the generating and re-generating modes there are often considerations to be made when sizing and specifying generators such as:-

• Carbon Molecular Sieve batch and performance changes
• Filling the vessels evenly and ensuring settlement

 These lead to uncertainty of overall generator performance resulting in often having to be conservative with the output flow against purity. This could result in having to offer a larger system than necessary to ensure performance which means an increased air demand, more expense, higher running costs and reduced efficiency.

PDS EXPLAINED

With fixed time cycles the compressed air and energy required to drive the nitrogen generator is fairly constant regardless of the outlet flow, and performance is based around the generator output being at 100%.

In reality the generator output is rarely operating at full capacity, often with some over capacity built in to the sizing along with periods of reduced flow. Perhaps during shift work, overnight or over weekends and holidays where the flow is greatly reduced.

Where fixed time cycles are used generators are set up for maximum efficiency at 100% output.

Reduced output flow from a PSA Nitrogen generator controlled with fixed time cycles results in a higher purity of gas, which is not required and wastes energy from running compressors, dryers and the generator itself.

PDS is a more scientific and energy efficient approach to PSA control,  by measuring the purity swing at the top of the CMS column to determine the generation and re-generation cycle. This means that no 2 cycles are the same and the valves switch according to the conditions to maintain a constant purity of nitrogen gas regardless of output flow.

PDS BENEFITS

A specially selected oxygen sensor and sample valve allow the oxygen reading of each column to be measured and controlled using our HMI touch-screen PLC.

As the output flow decreases, the purity in the column improves and the PSA cycle automatically extends to regulate the output oxygen levels to the desired purity rather than maintaining a very high purity which wastes energy.

The new AIR:N2 ratios shown above are based on 100% output flow @ 20-25°c ambient air temperature in PDS Energy Management mode.

The table shows the typical energy savings though using PDS technology.

AIR:N2 ratios decrease but the fact the generator output is dropping using the same amount of carbon molecular sieve must be considered, this has been accounted for in the table.

Where generators operate at 70-90% capacity is where the largest savings in air and energy can be made.

Savings are made through the following:-

• As the nitrogen demand decreases, cycle times increase and the AIR:N2 reduces, so a variable speed drive compressor output and energy consumption will reduce.
• As generation and re-generation cycles change in PDS mode the solenoid valve operations are less frequent and the overall power consumption of the generator reduces.
• With less solenoid valve operations, gives longer maintenance intervals, longer life and lower running costs.
• When using a desiccant dryer prior to the generator in DDS (dew-point dependant switching) mode, the lower air demand from the generator will save dryer purge air and thus reducing compressor power.
• As the cycles extend more purge gas is saved in the other column, removing the need for equalisation at very low flows.