What is psva?

The post answers how a typical PSVA operates from the beginning to end with schematics

In the present situation in India when COVID is at its peak and no one knows when his turn comes. For people with poor autonomous respiration ability, like chronic obstructive pulmonary disease (COPD) patients there is a growing need to address their health and quality of life through a lightweight and portable oxygen concentrator with a medical-grade oxygen supply (O2 concentration: ~88–92 vol %).

Background reading

Currently, oxygen concentration techniques are based on air separation processes such as cryogenic technique, membrane separation, and pressure swing adsorption (PSA) that can effectively produce enriched oxygen using ambient air as the source. Although cryogenic distillation acts as the leading process for air separation in an industrial oxygen plant, the pressure swing adsorption (PSA) technique has the advantage of reducing energy consumption for small production scales. Compared to the membrane separation technique, which requires higher pressure and expensive selective permeable materials, pressure swing adsorption using nano-size zeolite adsorbents is a more feasible solution for portable oxygen concentrator development. Due to the large microporous surface area inside the zeolite adsorbents, the portable oxygen concentrator using the PSA technique can adsorb nitrogen from air and output an enriched oxygen stream under high-pressure conditions. The concentrator can be regenerated by decreasing the pressure to release the adsorbed nitrogen. In this way, a continuous oxygen stream supply can be generated to assist an individual and increase the fraction of inspired oxygen. The oxygen concentration system is usually coupled with a filtration system to remove PM 2.5 and improve the quality of the product gas stream.

What is a portable oxygen generator?

Schematic of Oxygen concentrator

Portable pressure swing adsorption oxygen concentrators have been designed and developed in the past two decades due to the synthesis of more effective zeolites with higher gas adsorption capacity and selectivity. The PSA oxygen concentrator is able to provide a continuous oxygen flow using the ambient air as the gas source. Two-bed or multi-bed PSA systems are utilized alternating adsorption and desorption operations of the zeolite columns to achieve continuous oxygen separation.

For on-site medical grade oxygen production, a cylinder gas source or several powerful air compressors are usually installed for the rapid pressure swing adsorption process to maintain the productivity of a high purity oxygen stream. Considering the mobility of small-scale concentrators, a variety of simplified designs have been added to the pressure swing adsorption process for system miniaturization.

Flow directions Schematics of the normally open and distributor three-way valve under pressure and vacuum conditions.

In a portable oxygen concentrator, the size of the adsorption column could be as small as 3 cm in diameter and 20 cm in length. A portable concentrator design uses the pressure/vacuum swing adsorption (PVSA) cycle with a deep evacuation step (−0.82 barg) instead of desorption with purge flow to simplify the oxygen production process. The output of the oxygen concentrator is a ~90 vol % enriched oxygen stream in a continuous adsorption and desorption cycle (cycle time ~90 s).

The concentrator consists of two adsorption zeolite columns, a dual-head air compressor, five three-way solenoid valves, a back-pressure regulator, an oxygen sensor, an Oxygen analyzer, a surge tank, a pressure sensor, and a cooling fan to avoid the temperature rise of the air compressor The concentrator operation consists of four different steps. In step 1, column 1 is pressurized by the compressor while column 2 is connected to ambient air to decrease the pressure. In step 2, the pressure inside column 1 reaches the working pressure (1.79 barg) with enriched oxygen flowing out through the backpressure regulator (set at 1.79 barg) to the surge tank. Meanwhile, the pressure in column 2 decreases to vacuum (set at −0.82 barg) via the air compressor to regenerate the zeolite. In steps 3 and 4, the adsorption and desorption positions of columns 1 and 2 are reversed to regenerate the saturated column 1 and pressurize the regenerated column 2. By switching the valve sequence (step 1 → 4 → 1), a continuous enriched oxygen stream can be obtained from the surge tank. The flow directions of the two different types of valves (normally open and distributor three-way solenoid valves) are shown above in the above image.

In the above image, with two different roles in the pressure swing operations. Valves 1 and 2 are normally open 3-way valves directing the pressurizing flow and counter-current blowdown flow while valves 3 and 4 are distributor 3-way valves for pressurizing and depressurizing port control at the column end.

Adsorption equilibrium isotherms for nitrogen and oxygen onto zeolite LiX at 20 °C. Reproduced with permission from, Industrial & engineering chemistry research, 2008.

X-axis Pressure Barg

According to the adsorption isotherm of zeolite LiX shown in the image above (Santos et al., 2008), zeolite capacity and selectivity of nitrogen to oxygen is higher under pressure vacuum swing range compared to conventional pressure swing.]. In order to reduce the compressor size, a dual-head air compressor is considered to perform pressurization and vacuum operation simultaneously.

The valve operation sequence for the PVSA cycle.

P1, P2 represents valve positions 1 and 2.

Portable pressure swing adsorption oxygen concentrators have been designed and developed in the past two decades due to the synthesis of more effective zeolites with higher gas adsorption capacity and selectivity.