Vacuum Pressure Swing Adsorption vs. Pressure Swing Adsorption

First is to compare the differences between absorption and adsorption. While they do sound similar, they are two different processes. Absorption is the process where molecules enter a bulk phase, either in liquid or solid state. The best way to demonstrate this process would be to soak water into a sponge. The molecular structure of both the sponge and the water remain separate but combine into one solid object. In the absorption process, the objects can easily be separated. The water can easily be separated from the sponge by squeezing the water out.

Adsorption focuses on a molecular level rather than a matter’s state of being. The idea of adsorption is binding or attracting certain molecules to gather and remain on a solid surface. The molecules will stick to the surface of the solid, but they will not combine with the surface. Once the molecules are caught, they can be removed and filtered out. A good way to think of this is with a face mask. When wearing a mask, you want to filter out the harmful chemicals in the air. When breathing in, a filter will adsorb those harmful chemicals while allowing clean air to pass through. The undesired molecules will stick to the filter, so you don’t breathe them in. Once you’re done with the mask, the chemicals can be cleaned or treated to remove those chemicals.

The Adsorption process come in handy when it comes to thinking about concentrating Oxygen remotely. To make concentrated Oxygen, you’d need to be separate it from the air we breathe, which is 70% Nitrogen, 20% Oxygen, and the rest is other trace elements such as Carbon Dioxide. Two issues occur here: First, you’d need an effective and adsorbent surface to capture the Nitrogen to produce a purer Oxygen. Second, consider that as a state of matter, gas molecules tend to move fast and randomly. As such, you’d need a highly effective material to catch the desired molecules. In Nitrogen/Oxygen separation, a material called Zeolite, which is highly attracted to nitrogen gas, is used. By capturing the Nitrogen, the result is higher concentration of Oxygen gas.

Now with the right materials, all that’s needed is the process. There are two industry processes commonly used to accomplish oxygen concentration. The first is called Pressure Swing Adsorption (PSA). The idea behind it is that if you put gas molecules under enough pressure and force them to stay together, the gas molecules will be attracted to solid surfaces. The higher the pressure, the more molecules you can force to adsorb onto the solid surface. Once the desired gas is filtered out, depressurizing the concentrator will release the molecules stuck to the adsorbable solid surface. Basically, you’re using pressure to attract molecules and releasing pressure to clean off the solid plate they were stuck to.

In Nitrogen/Oxygen applications, natural air is sucked into the air compressor. From there, the air is pressurized to around 4–8 barg. To put it into perspective, 4–8 barg is 400 kPa–800 kPa. The air pressure at sea level is 101 kPa. That’s 4 times normal air pressure we experience on land. To get to 400 kPa naturally, you would have to swim around 40 meters under the surface of the ocean. Under pressure, the Zeolite attracts Nitrogen to its surface and allows the Oxygen and Argon to pass through. Once the Oxygen had been separated from the air, the pressure is released to normal atmospheric levels, and the Nitrogen detaches from the Zeolite. The amount of pressure placed on the air in the compressor determines the purity of the Oxygen produced. As such, the pressuring process is both the means to force the Nitrogen to stick to the Zeolite plate and the means to clean off the plate once the air is separated.

While an effective process, the trouble behind using PSA is that it consistently needs a high amount of energy to compress and decompress the gas, which can be costly depending on the amount and purity of Oxygen required. High energy usage means high energy costs. Also, consider that constantly subjecting Zeolite plates to high amounts of pressure means that the equipment will need higher amounts of maintenance and regular replacements. All of which costs money.

The second industry process is Vacuum swing adsorption (VPSA). While, like PSA, it uses Zeolite, it is more cost effective in comparison as it uses less energy and requires less maintenance to produce an equal oxygen output. The main difference between the processes is that while PSA uses pressure to clean off the Zeolite plate, VPSA uses a vacuum process.

Remember that gases like to move and occupy space whenever possible. Think of your favourite space movie. When a hole is blown into the side of a space craft, everyone and everything in that room will be sucked into space. Why? Space is just as it is named—Space. Space does not have any gases floating around like on Earth. Spaceships are concentrated little containers full of compressed trapped gas. When a hole is made in that container, that gas now has a lot of space to expand into. In this case, Space is a vacuum and sucks out the Oxygen from the spacecraft. It is this same vacuum process that VPSA uses to clean off the Zeolite plate.   

VPSA uses a blower instead of an air compressor. An air compressor forces concentrated air at high speeds through a hose. A blower works by blowing forced air at a high velocity. Its job is to make a positive pressure above the vacuum or the atmospheric pressure. The big difference between the two is that air compressors operate at high pressure ratios while blowers operate at low pressure ratios. As such, the need for all the equipment such as a dryer, a filter, and a buffer tank in PSA is eliminated, and less energy is needed as a result. The air is forced into a slightly higher than normal pressure state of 1.25–1.5 barg (125–150 kPa). The Nitrogen will adhere to the Zeolite and the Oxygen will continue to pass through at this pressure. Once the Oxygen has passed, a vacuum state will be created. The vacuum state will release all the Nitrogen off the Zeolite plate and reset the concentrator to restart the process. What’s great about this process is it eliminates the need to keep a constant high/low pressure to produce the same quality of oxygen, which lowers overall operational and maintenance costs.

The VPSA process is made viable by using Lithium-exchanged Zeolites. This means that there is further protection for the Zeolite which means it has a higher nitrogen capacity at lower pressure. While Lithium-exchanged Zeolite is more expensive than the Zeolite in a pressure-swing system, the Lithium-exchanged Zeolite does not need to be replaced as frequently in part due to its durability and due to its lack of subjection to pressure change, saving money on maintenance costs.

If visualized on a scale, Zeolite is kept at a state of 101 kPa, which is the natural atmospheric pressure. In PSA, the Zeolite is forced to the 400–800 kPa levels, keeping it under pressure and cleaning the plate off in the process. VPSA works the other way. It keeps the Zeolite at natural atmospheric pressure, and then will create a vacuum state of 20 kPa–0 kPa to clean off the plate. Creating a vacuum environment is less harmful on a Zeolite plate, meaning there won’t be a worry of needing to maintain or replace the plates as frequently.