Monday, May 31, 2010



Pressure Swing Adsorption (PSA) is a process of separating molecules of gas from each other. It’s not a chemical process, such as “aBsorption”, rather it is physical process called “aDsorption. The first word is spelled using a “B” and the second is spelled using a “D”. This distinction helps you understand that a “pressurized” process, using carbon or zeolites, actually sifts molecules, which releases nitrogen from the air we breathe.

The air we breathe is 78 percent nitrogen. This process separates it and the only waste product from the process is oxygen, which is sent back into the air. By forcing air into a pressure vessel or sieve-bed, filled with sieve, some molecules are trapped and others released. The captured nitrogen molecules are stored in a tank ready for use in the desired application.

The  “membrane” process involves a series of hollow fibers that have holes in their walls. Compressed air is forced through the hollow fibers. The holes are small enough that nitrogen molecules are trapped. They are forced to escape. Heat is applied to excite the molecules in the hollow fiber to increase the chance they will permeate out through the holes.

PSA process better suits “critical” applications, while membrane is suitable for less critical requirements. When there is a need for high purity levels (up to 99.995) and low dewpoints, we often recommend a PSA system. The compressed air requirements are lower than the membrane systems and these systems are more tolerant to ambient temperature swings within a limited scope.

Between the PSA and membrane process for generating nitrogen, the answer is each meets special requirements, each has advantages and each has disadvantages. It takes a thorough understanding of the requirements to make the right choice. Some PSA systems may have a useful life of 20 years, while some membrane systems may fail within 7 or 8 years.

Membranes are approximately 30% more expensive than PSA systems.  They require more power than PSAs when operating at nitrogen purities of 98% and higher. PSAs are more robust and less sensitive to hot and cold ambient conditions. Membranes require 160 to 200 psig of compressed air for an efficient nitrogen separation to take place. On the other hand, PSAs require a much lower air pressure to achieve an efficient separation. Therefore, membranes will result in a higher outlet nitrogen pressure than PSA systems. PSAs generally operate longer with less maintenance costs.

Membranes have fewer moving parts, but membranes begin to age and degrade over a relatively short period of time. Replacement of membrane modules, represent a significant capital cost to the buyer. The biggest advantage of membrane systems is their inherent lightweight and small footprint.

The carbon molecular sieve material used in PSA units is extruded from carbonized coconut shells and represents a very hard and durable form of carbon with an exceptionally high propensity for separating compressed air into pure nitrogen product.

The CMS material literally has an indefinite life. And like membrane fibers, it too requires protection from compressor oil carry-over. The filtration system in both the PSA and membrane unit is designed to prevent hydrocarbons from coming in contact with the CMS material and membrane fibers

The PSA system utilizes a programmable logic controller (PLC)  to automatically control the operation of the Nitrogen Generator. This PLC consists of an 8k processor, power supply and I/O modules. The PLC performs all the controls and monitoring functions, including execution of the adsorption cycle. The PSA also has a built-in pressure switch and it monitors the nitrogen pressure. If no nitrogen is being used (or very little), the PSA will stop and go into standby mode. It will shut off the air inlet valve and nitrogen outlet valve so no air is used.

The air compressor also has a pressure switch or sensor that monitors downstream air pressure and it too, goes into a standby mode when the air pressure begins to rise. If the air compressor remains in this mode for more than three minutes, the main compressor motor will automatically shut down and wait until the air pressure is lowered before turning back on. This feature represents a tremendous power savings advantage.

The membrane system is totally reliant upon pressure switches to control its on and off operation. The pressure switch is installed in the nitrogen product line inside the control cabinet. The pressure switch can be set to turn the unit off and on at desired product pressure levels.

A PSA Nitrogen Generator is a two-bed absorption system and therefore, has an interrupted air demand to the system when air is diverted from one bed to another. An air surge tank located downstream of the air compressor and upstream of the PSA unit is required in order to smooth out surges in air demand. Membrane systems on the other hand, have a smooth constant demand for air and, otherwise do not necessarily require an air receiver..

Again, PSA systems require a nitrogen storage vessel in order to insure a smooth product delivery. Membrane systems do not require as large a nitrogen storage vessel, but is still recommended to prevent the system from starting and stopping frequently.

Evaporated liquid oxygen purity is 99.5% to 99.9% pure. Argon makes up about 4-5 percent of the oxygen product. After the compressed air leaves the air dryer, it is filtered in a series of filtration stages. The first and second stages consist of two acetate type filters with a pore space of 0.1 and 0.01 micron. These filters are designed primarily to remove water and particulates in the compressed air. The third and final stage is an activated carbon filter designed to eliminate any residual oil or hydrocarbon from the air before it passes into the adsorption vessels.

PSA generators These use carbon molecular sieve (CMS), which allows the smaller molecules in the input air (such as oxygen) to penetrate the CMS, whilst the larger molecules of nitrogen bypass the CMS to generate the output gas. A PSA generator consists of a pair of columns filled with CMS. PSA generators can achieve higher purity levels than membrane systems, up to 99.9995%

Membrane Generators  use a bundle of hollow fibres which separate gases by the principle of selective permeation. A membrane generator consists of these hollow fibres, encased within an outer shell of GRP. The faster permeating gases diffuse quickly through the membrane wall, leaving an output of nitrogen up to 99.9% pure. These systems are very compact and robust.
PSA generators are less sensitive than membrane generators. They are more robust, thus more suitable for potentially harsh environments.

Membrane generators seldom offer a purity higher than 99%. An optimal purity for membrane solutions is rarely higher than 94%. This is not the case with PSA solutions. The gas purity from PSA generators is often 99.5% as standard - and can easily reach 99.9%.

Even at high volumes, PSA generators consistently produce the same high amount of gas. By contrast, the capacity of membrane solutions tends to weaken over time

On average, maintenance costs for membrane solutions are 50% higher than for PSA solutions. This is due to the long lifespan of the pellets (Carbon Molecular Sieves) that form the hub of the PSA gas filtering process. The pellets can easily perform optimum separation for at least 10 years without being replaced. By contrast, membranes are more fragile and start deteriorating from the moment the on-button is pressed. And membranes are expensive to replace.



  1. Woooooooooooooooooooooooooooooow /Hey thanks man!! you are so good. I think this the perfect work.

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  2. your blogs r really helpful to the mariners 4 silors as well as those who r learning

  3. Thanking you Sir, The Information given in your blogs are very informative. Please continue the good work and help the juniors.

    4/E. Ponraj.

  4. hi ponraj,

    at sea knowledge is power.

    when you know your job, you are respected, and valued.

    this makes you happy--

    capt ajit vadakayil

  5. Thanks for every other informative site. The place else may just I am getting that type of info written in such a perfect approach? I have a undertaking that I am just now working on, and I have been at the look out for such information.AGS Filtration Pvt. Ltd is a Native Indian organization of Worldwide reputation in the field of multiple demand and program Filtration.Compressed Air Filte.

  6. dear capt saab thank you very much for sharing your knowledge for us(juniors)LIJO PAIPPATTU JOSEPH 4 E

  7. Thanks for this post.It is very useful for us.I am sharing a website that offers best Nitrogen generators in India . Nitrogen generators in India

  8. This comment has been removed by the author.

    1. hi s,

      the salary you gt in USA is NOT your salary - it is just a virtual imaginary figure.

      you salary is what you are able to put away in the bank ( savings ) , after paying US taxes, boarding / lodging , emergency expenses and travel. this is called perception.

      someone has to take care of your mother-- like when you get old , someone has to take care of you.

      india is rising, USA is falling.

      capt ajit vadakayil

  9. Dear Sri Vadakayil,

    One of my friend's mother is suffering from Sideroblastic Anaemia for an year. She is undergoing allopathy treatment and currently has to get blood transfusion/infused every two months.
    From what we know her spinal chord is unable to produce haemoglobin or key blood components.

    are there non-physical (mental/spirtual) causes for this? What non-allopathic approaches can we use to help her?

    From reading from your blog I understand that, suryanamaskar and pranayama is a basic need. Any other advice is appreciated.


    1. hi s,

      read my latest blog for the reply.

      capt ajit vadakayil

  10. Dear Sri Vadakayil

    Thank you very much. I read the post (and understood 10% of it) and spoke to my friend. The doctor has given pills to reduce iron levels (and not increase it). Upon more questioning I was informed that the person used to spend a lot of time working/replacing ink toners of old (10 to 20 years ago) xerox machines. Doctor has considered this as a potential for the problem. The family uses ramadev products and organic food as much as possible.

    The question is where do i go from here...

    I plan to research if these toner chemicals contained lead. Based on your previous post, I know taking lead out of the body is tough. may ask them to use (overnight) water from copper vessel with arecanut ...I am no doctor..but after reading your blogs I am able to understand things better and "trying" to solve some puzzles...

    thanks again. you helped a worthy lady (the reason she worked at the xerox equipment for long hours was to see her son progress)..a very well deserving mother...


  11. Thanks Ajit. it is useful information

  12. Dear Captain,

    I have been one of your silent readers who admires your work. When you asked to stop coming to your blogsite, I stopped for a bit. Owing to personal reasons I haven't been active in trying to post comments but I do spread your word orally and will do more when I can. Please don't harbor any ill will towards me since I am dealing with a lot in life already. I am not ungrateful nor do i come for anything selfish but for the same reasons that many or at least some of your readers do.
    If you do get my message, I want to thank you for everything you've done (even if I sometimes disagree with a few things). You're like a guru to all of us. Please don't be angry. I will do more when I am at a better place.
    Thank you again. Forgive me if I haven't been a good follower but I do try to incorporate ideas that I learn and agree with in life. I need your blessings and I honestly thank you. Please forgive me.