Technology Comparison

PSA vs Membrane Application

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Holtec Gas Systems manufactures nitrogen generators based on both Pressure-Swing Adsorption and Membrane separation technologies. But which technology is right for you? While cost is a significant driver of this decision, there are many other factors which you should consider before choosing the best technology for your application.

Pressure-Swing Absorption (PSA) - Process Description

CMS DiagramPSA technology works on the concept of adsorption - the process by which molecules temporarily adhere to the surface of materials they are in contact with. A PSA nitrogen generator consist of two or more adsorbers filled with separation material called carbon-molecular sieve (CMS). CMS is produced specifically to have pore sizes that correspond to the relatively small size of an oxygen molecule (compared to the relatiely larger nitrogen molecule.

In a PSA nitrogen generator, cormpressed air is drawn from the atmosphere and directed into one adsorber. Under pressure, oxygen in the compressed air gets "stuck" in the pores of the CMS, allowing the remaining constituents (mostly nitrogen) to pass through the system unhindered.

Sounds simple, doesn't it? There is only one problem. Eventually, oxygen molecules separation will no longer occur. This is called saturation, when oxygen molecules completely cover the surface of the CMS and there is no room for more to be adsorbed.

Nitrogen Generation Illustration Rev 2.1 Final AI6Fortunately, the adsorption process is reversed by depressurizing the adsorber. This is why PSA technology uses two or more adsorbers. At ambient pressure, oxygen is released from the surface of the CMS and is returned to the atmosphere as waste gas, thus regenerating the adsorber for use in the next cycle. While the first adsorber is regenerating, the second adsorber is actively producing nitrogen. At the end of the cycle, the first adsorber is once again ready to produce nitrogen and the second adsorber regenerates - and so on.

However, each time an adsorber cycles back on-line, there is a brief period during pressurization that no nitrogen is being produced. This is one reason why every PSA nitrogen generator requires a process nitrogen receiver tank.

Additionally, compressed air usage during the pressurization step is greater than the average flow. In order to ensure there is always enough clean, dry, oil-free compressed air to supply the system, a process air receiver is also usually needed.

PSA Process Steps:

Purification
1. Pressurized air in
2. Absorption of oxygen
3. High purity nitrogen out

Regeneration
4. Equalization of pressure (not depicted)
5. Regenerating bed depressurizes completely, allowing oxygen enriched gas to desorb
6. Waste gas exits through silencer

Typical PSA Process Flow

 PSA Flow

  1. Compressor - Compressed air is required with all PSA nitrogen generators. If your site has compressed air available, typically between 100-125 psig (6.9-8.6 barg), this air can be used to feed a new PSA. Depending on what type of compressor and air treatment already in place, the diagram above may change. If a new dedicated compressor is needed, a lubricated screw compressor is the most common choice for cost and maintenance reasons. The equipment recommendations above assume this type of compressor.
  2. Liquid separator - Used to remove bulk water and oil from the compressed air.
  3. Dryer - Used to condense and remove more water and oil from the feed air. If the system is installed indoors where temperatures will always be above freezing, a refrigerated dryer can be used to reduce the feed air dewpoint below 38 °F / 3 °C. If the system is outdoors in a climate where freezing temperatures are possible, a desiccant dryer will be needed.
  4. 0.01 Micron coalescing filter - Used to reduce oil aerosols to less than 0.0008 ppm.
  5. Activated Carbon Bed - Even after a high quality coalescing filter, there will still be oil vapor in the compressed air whenever a lubricated compressor is used. Holtec therefore always recommends the use of a large bed of activated carbon pellets in order to adsorb any remaining hydrocarbons, making the compressed air virtually oil-free. Cartridge style activated carbon filters are too small, meaning residence time will not be long enough to adsorb all oil and therefore this style of filter is not recommended.
  6. Process Air Receiver - Compressed air requirements calculated are always an average. But the PSA process requires regular pressurization and depressurization which means that the instantaneous air flow is always changing throughout the PSA cycle. It may be much lower than average or several times higher. An air receiver is therefore required to ensure optimum feed pressure is always available to the nitrogen generator and to make sure filters are not overflowed.
  7. PSA Nitrogen Generator - We hope you will choose a high quality, reliable, Holtec PSA nitrogen generator for this piece of equipment!
  8. Process Nitrogen Receiver - Just as compressed air flow changes throughout the cycle, so will the nitrogen production rate. Product purity will also vary, and there will also be several seconds each cycle where nitrogen is not being produced at all. A process nitrogen receiver is therefore required with every PSA system in order to provide your process with a constant, stable flow of nitrogen at the required pressure and purity.

Through rigorous research and testing, Holtec has designed a proprietary simulation program in order to ensure that all filters, tanks, valves, and line sizes are properly sized for optimal operation, guaranteeing the least amount of customer problems. Maintenance is typically limited to standard compressor and dryer PM and regularly changing filters and activated carbon, which facilities utilizing compressed air are already familiar with.

Membrane Separation - Process Description

Membrane DiagramMembrane nitrogen generators separate nitrogen from atmosphere by passing compressed air through a permeable membrane. The process relies on the principle of selective gas permeation. Each gas has a different permeation rate, so as the compressed feed air stream passes across the membrane, gases with faster permeation rates like 02 and CO2 are released back into the atmosphere as waste gas (along with some N2)membranefibers

The product nitrogen, now stripped of most of the oxygen and carbon dioxide, passes out the other end of the separator at a slightly lower pressure for collection or directly into your application. Unlike a PSA system, a nitrogen receiver tank is not necessarily needed.

Typical Membrane Process Flow

Membrane Process Flow

  1. Compressor - Compressed air is required with all Membrane nitrogen generators. If your site has compressed air available, typically between this air can be used to feed a membrane system. Depending on what type of compressor and air treatment already in place, the diagram above may change. Membrane productivity and efficiency is significantly improved by higher pressures, and they are typically operated between 100-350 psig (6.9-24.1 barg).
  2. Liquid separator - Used to remove bulk water and oil from the compressed air.
  3. Dryer - Depending on the application, existing equipment, and the type of membrane selected, a dryer may or may not be required.
  4. 0.01 Micron coalescing filter - Used to reduce oil aerosols to less than 0.0008 ppm.
  5. Activated Carbon Bed - Even after a high quality coalescing filter, there will still be oil vapor in the compressed air whenever a lubricated compressor is used. Holtec therefore always recommends the use of a large bed of activated carbon pellets in order to adsorb any remaining hydrocarbons, making the compressed air virtually oil-free. Cartridge style activated carbon filters are too small, meaning residence time will not be long enough to adsorb all oil and therefore this style of filter is not recommended. They can, however, be utilized when oil-free compressors are used in order to eliminate other contaminants.
  6. Dust Filter - Dust from activated carbon would collect in the membrane separator and reduce performance, so we include a filter to remove.
  7. Process Air Receiver - Depending on the application and existing equipment, an air receiver may be necessary. The best location in process flow may change depending on the application.
  8. Membrane Nitrogen Generator - We hope you will choose a high quality, reliable, Holtec membrane nitrogen generator for this piece of equipment!
  9. Process Nitrogen Receiver - Unlike a PSA system, membrane nitrogen production is a steady state process where nitrogen is being produced all the time at a stable rate. A nitrogen receiver is therefore not a necessity, but one can be included when the customer wants to have stored nitrogen as well.

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