Experimental Evaluation of a Portable Oxygen Concentrator Based on Pressure Swing Adsorption ^†

Abstract

Portable oxygen concentrators (POCs) can help reduce the load on hospitals and offer emergency care to patients in need of oxygen therapy. In this study, a POC was developed and tested at different pressures and cycling times. The device was made using low-cost materials to reduce the manufacturing cost. It was found that high air pressures resulted in an overall increase in oxygen concentration in the product air. Oxygen concentration was also found to increase as cycling time was extended. Pressurizing the air at 0.8 MPa for 15 s per cycle delivered 93% pure oxygen, which fulfills the medical need of intensive care unit (ICU) oxygen therapy.

1. Introduction

POCs have a history of development dating back to the 1970s when they were prototyped [1]. Over the past few decades, POCs have undergone significant improvements in terms of both cost and performance. Older POCs only provide 3–5 L per minute of oxygen [2]. The newer models can provide over four times that flow rate [3]. This increased efficiency has reduced the operational cost and reliability of POCs.

Pressure swing adsorption (PSA) is a common technique to concentrate oxygen. It is used to separate oxygen and nitrogen from ambient air, and it works on the principle of gas adsorption. PSA is a cyclical process that uses a bed of adsorbent material that can selectively absorb one or more gases from pressurized air. The adsorbed gas is then released by reducing the pressure. The process generates high-purity oxygen for medical applications, cylinder filling, oxyfuel cutting in metal fabrications, etc. [4].

Santos et al. presented a high-purity oxygen production process using pressure swing adsorption (PSA) with silver-exchanged zeolite, where oxygen/argon adsorption was conducted selectively [4]. The adsorption equilibrium isotherms of oxygen, nitrogen, and argon were determined in the study, and the PSA process was optimized using simulation techniques for experimental validation and testing.

Ahmad et al. introduced a flexible oxygen concentrator for medical applications based on a Skarstrom-type cycle consisting of four distinct steps [5]. Patel et al. explored a generic PSA technology for medical oxygen concentrators designed to reduce the adsorber size [6]. Similarly, Lee et al. developed a rotary valve multi-bed rapid cycle pressure swing adsorption (RCPSA) system to enhance the performance of a miniature oxygen concentrator [7].

While POCs based on PSA offer high concentrations of oxygen against lower power consumption compared to other processes, there remain significant challenges facing POCs. One of the most significant challenges is battery life. Consistent monitoring of oxygen concentration during operation cycles can reduce the device’s runtime. To address this issue, researchers have developed designs that optimize oxygen delivery and minimize battery usage [4].

In this study, we have developed a POC using the PSA process. The equipment was constructed with low-cost medically compliant materials to cut costs. This study explores the interdependent relationship between air pressure, pressure swing cycling durations, and oxygen concentration.

2. Materials and Methodology

2.1. Materials and Process

Table 1. Material specifications.

Item	Manufacturer	Model	Specifications	Dimensions (L × W × H)
Compressor	Ingersoll Rand	T30	30 CFM, 1 MPa	30” × 20” × 15”
Solenoid Valve	Parker Hannifin	5-2-W24V	2-way, 1/4” NPT	10” × 5” × 5”
Zeolite Columns	Zeochem	ZC-28-5	28” L, 5” D, 1/4” NPT	28” × 5”
Polyurethane Tubing	McMaster-Carr	5233K11	-	1 mL, 1/4” OD, 1/8” ID

shows the specifications of the equipment used to carry out the PSA-based oxygen saturation process. The air supply was provided by a compressor fitted with a built-in pressure regulator. The pressurized air from the compressor entered a cylindrical bed filled with blue silica beads to dehumidify it. This is necessary since compressed air is saturated with moisture, and humid air can limit the adsorption capacity of PSA [8]. Following dehumidification, the air was passed through a cylindrical bed containing a molecular sieve that can adsorb nitrogen from pressurized air and thereby produce oxygen-enriched gas [8]. The pressurized air was contained in the bed full of the molecular sieve for a few seconds to remove nitrogen from it and released with a solenoid valve to be supplied to the patient.

The device operates in two cycles, involving adsorption and desorption. During the adsorption cycle, one bed is connected to the compressor and receives pressurized air. The molecular sieve adsorbs nitrogen and releases oxygen-enriched gas. The other bed is isolated and undergoes desorption. In the desorption cycle, the isolated bed is vented to the atmosphere, releasing the adsorbed nitrogen. The pressure in the bed drops, and the molecular sieve regenerates for the next cycle. The valve system switches the roles of the two beds after each cycle. Figure 1 depicts a schematic of the process.

An anemometer and an oxygen sensor at the outlet of the solenoid valve were used to measure the oxygen concentration and the flow rate of the gas delivered to the patient, while a pressure transducer was used to monitor the pressure in the beds and control the compressor speed. This methodology is inspired by the project completed by ETH Zurich for the development of low-cost oxygen concentrators [9]. The design and parameters of the device were adapted from this project, which aimed to provide a solution for low-resource settings where oxygen supply is scarce and expensive. This study utilizes more tightly packed molecular sieve beds along with dehumidification chambers to attain higher oxygen concentrations at smaller cycling times.

2.2. Data Collection

The experiments were conducted in closed laboratory conditions during November and December 2023 in Islamabad, Pakistan. Each experiment was conducted between 11:00 AM and 2:00 PM, and the tests were repeated five (5) times until the air compressor heated up. Open-air ventilation was allowed through the lab to ensure that the ambient oxygen concentration was not altered during each experiment.

The temperature and humidity at the inlet and outlet of the POC were measured continuously. The temperature was measured using a Fluke 62 Max thermometer manufactured by Fluke Corporation and sourced from AliExpress, China. It was calibrated against the freezing point and boiling point of water in standard atmospheric pressure conditions. The humidity was measured using an XH-M452 hygrometer manufactured by Fafeicy and sourced from AliExpress, China. It was calibrated in a concentrated calcium chloride (CaCl₂) environment at 0% relative humidity (RH) and a fully humid environment at 100% RH. The oxygen concentration and flow rate of the oxygen were measured at the outlet of the POC.

The oxygen concentration levels were recorded using a KE-25 oxygen sensor manufactured by FIGARO and sourced from Instock.pk, Islamabad, Pakistan. It was calibrated against the ambient 21% oxygen concentration and a cylinder containing 95% oxygen. The flow rate was measured using a Metravi AVM-10 hot-wire anemometer manufactured by Metravi and sourced from AliExpress, China, which was factory-calibrated. The pressure in the POC was maintained using an Ingersoll Rand T30 compressor manufactured by Ingersoll Rand and sourced from Rawalpindi, Pakistan. The pressure was measured using a factory-calibrated QDW90A pressure gauge manufactured by Anhui Qidian Automation Technology Co., Ltd.

The pressurization and depressurization cycles were controlled with a manual Parker Hannifin 5 Way/2 Position solenoid valve manufactured by Parker Hannifin and sourced from Rawalpindi, Pakistan. The cycle times were measured using a smartphone stopwatch. The data was logged onto an STM32F103C8T6 microcontroller manufactured by STMicroelectronics and sourced from Islamabad, Pakistan.

The specifications of individual sensors are given in

Table 2. Sensor specifications.

Sensor	Manufacturer	Model	Range	Accuracy
Thermometer	Fluke	62 Max	−20 °C to 100 °C	±1.0 °C
Hygrometer	XH	XH-M452	0% to 100% RH	±1.0% RH
Pressure gauge	Anhui Qidian	QDW90-A	0 to 100 kPa	±1.0 kPa
Stopwatch	-	Cell phone watch	0 to 1000 s	±0.1 s
Anemometer	Metravi	AVM-10	0 to 30 m/s	±1.0 m/s
Weighing scale	iBELL	WHB28	0 to 200 g	±0.1 g
Oxygen sensor	Maxell	KE-25	0 to 100%	±1.0%

.

2.3. Uncertainty Analysis

Due to the least counts and accuracy errors associated with each instrument, uncertainties carry over into multiple measurements. The uncertainties were measured using Kline and McClintock’s method [10].

Table 3. Uncertainty values for all parameters.

Measurement	Unit	σR
Temperature	°C	±1.0
Time	s	±0.1
Relative Humidity	%	±1
Gas Pressure	kPa	±0.1
Gas Flow Speed	m/s	±0.5
Gas Flow Rate	Lpm	±0.5
Mass	g	±0.7
O2 Concentration	%	±1

shows the uncertainty values associated with each measurement.

$$\begin{array}{c}{\sigma }_R=\sqrt{{\left({\displaystyle \frac{\partial R}{\partial x_1}}{\sigma }_{x_1}\right)}^2+{\left({\displaystyle \frac{\partial R}{\partial x_2}}{\sigma }_{x_2}\right)}^2+\cdots +{\left({\displaystyle \frac{\partial R}{\partial x_n}}{\sigma }_{x_n}\right)}^2}\end{array}$$

(1)

3. Results

The variation in oxygen concentration against the compressor pressure at different pressurization and depressurization times is illustrated in Figure 2. It was observed that the oxygen concentration of the product air increased with air pressure in the molecular sieve chambers. This correlates to other studies in the literature [11]. This trend can be explained by the fact that molecular sieves can absorb more nitrogen from highly pressurized air, leaving the product air with a high concentration of oxygen [12].

Besides pressure, pressurization and purging time was also found to impact oxygen concentration. It was found that holding the air in a molecular sieve chamber for longer durations resulted in a greater oxygen concentration in the product air. This finding correlates with the literature [13]. It can be explained by the fact that longer pressurization cycles allow the molecular sieve to absorb a greater amount of nitrogen from the air, resulting in highly concentrated oxygen at the outlet of the molecular sieve chamber [14].

4. Conclusions

This research experimentally explored the impact of air pressure and cycling time on the oxygen concentration in POCs. The oxygen concentration in the product air supplied to the patient was found to increase with both air pressure and the duration for which the molecular sieves were allowed to hold pressurized air. The results were found to be congruent with the existing literature and demonstrated an improvement in oxygen concentration compared to other studies due to the use of a dehumidification chamber before the adsorption bed. Specifically, oxygen was concentrated from 21% to 93% in this study at a pressure of 1 MPa and a flow rate of 7.5 Lpm, matching the breathing requirements for most patients [5]. Comparable studies using similar cycling times have found oxygen concentrations between 88 and 92% but at much lower flow rates [8]. These results can be used to help with controlling the oxygen concentration according to the patient’s requirements measured in terms of blood oxygen level by adjusting the cycling time and air pressure.

Further studies can explore the use of other desiccant materials for dehumidification prior to adsorption; materials like lithium bromide (LiBr), lithium chloride (LiCl), and calcium chloride (CaCl₂) can be used to dry the air before it enters the adsorption chamber containing zeolite or the molecular sieve. More research can also be conducted to explore how temperature swing adsorption (TSA) can be used in parallel with pressure swing adsorption (PSA) to generate concentrated oxygen. The heat from the compressor motor can be used to heat the adsorption bed while PSA is conducted.