Frequently Asked Questions:
1. What is PSA generation? (FAQ’s section)
PSA stands for pressure swing adsorption. Absorption is actually a process which involves a chemical change. Generally accepted use of the term "absorb" implies a sponging effect. You can see the term has been misused for a long time. Since nothing we do is chemical, only physically separating molecules, our process is correctly called adsorption. It’s the process of separating molecules of gas from each other. The process is not chemical, produces no waste, and has no hazardous materials or effects. We use two sieve beds in all of our generators to achieve a constant flow of the Nitrogen or Oxygen delivery. The compressed feed air is fed into one sieve bed, then the feed swings to pressurize the other sieve bed hence the name Pressure Swing Adsorption.
2. How does PSA work?
The holes of the CMS are so small that the oxygen molecules are able to go through and get captured inside the particles, while the larger nitrogen molecules remain outside. After allowing this separation process to go on for sometime, the free floating nitrogen molecules are released out of the vessel at low pressure and captured in a storage tank. When the nitrogen has been drawn out of the tank (or sieve bed), the pressure in it is released quickly, allowing the oxygen to escape into the air, cleansing the CMS for the next cycle. The Sieve bed is then re-flooded with pressurized air and the cycle begins again.
To gather oxygen, the Zeolite has larger porous surfaces, which attract the nitrogen molecules. As both oxygen and nitrogen flow through the Zeolite, the oxygen moves freely past, while the nitrogen is captured. This is because the Zeolite has an affinity for the nitrogen. The same process then takes place, the oxygen molecules are free floating and are drawn off into a storage tank. When these have been drawn off, the tanks are then purged ready for the next cycle.
3. What purity can they achieve?
Our generators come as standard with an inbuilt oxygen analyzer so your purity is measured and displayed on the touch panel in real time. For our nitrogen systems, we have a patented Purity Exchange Valve which lets you choose three different purities with the one system – al controlled via the touch screen control panel.
99%USP (US Pharmacopiea) grade Oxygen
FDA cleared medical Oxygen
4. How long do they last?
We expect about 20-25 years life from a PSA Nitrogen system and we know we get 15–18 years in a PSA Oxygen system. Membranes are a different story. Since the membranes start degrading immediately and level off after one year, a unit will over-perform at first, however, if you are putting high-pressure air through the membrane and/or pushing high volumes of air through the unit to increase flow, the unit will need replacement in about 5 – 7 years. If you think of the logic, it’s obvious: membranes degrade because molecular particles which get past filtration plug the holes through which the O2, CO, CO2 molecules ferment. The more air at higher pressure (more excited molecules bouncing around, moving, giving a better chance for impurities to plug holes) the more quickly the membrane deteriorates. Since the membrane is 85% of the cost of a membrane system, things go downhill pretty fast. Here is what happens: as membrane purity goes down, customers must increase flow of input air to ensure purity. The more air, the faster the degradation of the membrane until you are not producing Nitrogen any longer.
5. Are they reliable?
Our technology is designed around using the simplest and most direct path to achieve the goal. We seek reliability through simplicity. We use all off-the-shelf parts so that our systems can be serviced by any mechanically inclined person, anywhere in the world. We have worked with and tested all technologies involving PSA gas production and continue to find that our time-proven basic technology, constantly tweaked and upgraded with innovation, makes the most reliable system in the world. The true genius is to make a complex process simple and understandable; then keep it that way.
However, saying that, just ask us and we will be glad to put you in touch with any of our long standing customers. Most of the time the answer you will hear is that they forget the system is even there. Don’t just believe what we say, talk to people who use the system and let them tell you the real story.
6. The truth about gas delivery
What are you really paying for with bulk liquid?
Let’s follow the process of filling a liquid N2 tank. There are no flow meters for liquid N2. There are no tank gauges like you have on your car for bulk liquid gases. A bulk liquid tanker truck is filled at the cryogenic air separation plant and weighed. The weight of the truck determines the number of gallons of liquid in the truck. You are charged based on this figure. Since the DOT has strict guidelines about the pressure allowed in the truck, it starts releasing N2 into the air as soon as it’s filled. Estimates vary between 3 and 5% of the truck’s volume is released during the road trip to your plant. If the truck stops at another customer’s location before yours, a best-guess estimate based on tank inches is made as to how much goes to one customer and how much goes to the next and the next, etc. In fact, the best-case scenario is that you gets about 90-95% of what you are charged for into their bulk tank. Then the tank evaporation begins.
The truth about bulk liquid tanks
Since Nitrogen boils at 320 degrees Fahrenheit below zero, the N2 in a bulk liquid tank is always boiling. The standard bulk tank has a relief valve like a pressure cooker, which releases evaporated (boiled off) liquid N2 when the pressure builds to a certain point. This means that eventually, all of the liquid N2 will boil, vaporize and release into the atmosphere. The pressure builds up from the surface of the liquid N2. The speed with which the customer uses N2 has very little effect on the amount of boil-off. Picture a pressure cooker in which we draw off water from the bottom. Evaporation and pressure build up comes from the surface and fills the space above the liquid. Since pressurization is fairly rapid and vaporous, if the cooker is nearly full or nearly empty, it has little effect on the speed of build up of vapor above the surface of the liquid. Further, since the N2 is boiling at such a low temperature, it has very little effect on the process if the tank is located in Queensland or is Tasmania, i.e. ambient temperature. The reality, and this figure comes directly from a bulk gas specialist at AirGas in the United States, is that a bulk cryogenic tank boils off at a rate of 2% of the volume of the tank (not the volume of liquid in the tank) per day. This means that if a tank is a 1,500 gallon tank, 30 gallons per day or 3,000 SCF will vent into the atmosphere every 24 hours. This figure will be the same if the tank is full or nearly empty.
A great deal for the gas company
Bulk tanks can cost the gas company $50,000 or higher. The company receiving the tank has to put in a pad 7-9 feet thick of concrete depending on the state (US) to support the heavy weight of the tank and the liquid N2. Very few customers own their own bulk tanks. To that end, the gas company charges the customer a rental charge for the tank as well as a delivery charge for the liquid. In some states there are environmental charges as well for traveling over the road with liquid N2. When you compute your potential savings with liquid, always be sure to figure in tank rental, fuel surcharges and environmental charges.
Gas company contracts
The contract that you or one of your predecessors signed with the Gas Company is a document that makes most attorneys shiver. It reads like the Trump prenuptial agreement and you’re the bride. If it’s like most gas company contracts, it goes for 7 years before it automatically renews unless you notify them at least on year in advance of the end of the contract. It probably says that if you change your gas needs in a way that requires a hardware change on their part, the contract automatically starts again for another 7 years. It probably says that, if the gas company decides to raise your cost per liter of liquid or m3 of gas, you have the right to go out for bid. However, if you get a better price, the gas company has the option to meet that price. Did I mention that the contract then automatically begins again for another 7 years? You are driven to sign such a contract because every gas company forces it in one form or another. If you do not sign, you pay an outrageous cost per m3. It appears that it gives you control of your cost but it only indentures you to the gas company. This contract walks a fine line of legality and, at best, forces you to deal with one gas company almost for life. Owning or leasing your own system can free you from the contract. If you wish to have a liquid backup system, we can help you buy a good used or new cryo tank. Once you have your own tank, you can call all the gas companies and let them compete for your business on a daily basis instead of once every seven years.
*Information taken from our supplier - On Site Gas Systems Inc
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