Challenging the oxygen infrastructure

Abstract

Medical oxygen is crucial for effective treatment of patients, yet many low- and middle-income countries (LMICs) without a good healthcare infrastructure have been unable to provide sufficient oxygen therapy for years now. The COVID-19 pandemic highlighted the critical need for more oxygen, leading to supply shortages and rising prices even in developed countries. Currently, most medical oxygen is produced by a few large companies using centralized Cryogenic Air Separation Units (CAS). Pressure Swing Adsorption (PSA) offers a promising alternative, enabling local production of medical oxygen without reliance on suppliers. PSA leverages the pressure dependency of adsorption isotherms to selectively adsorb nitrogen, concentrating the oxygen.

This research has investigated the possibilities of designing a system that not only meets the specific oxygen demands of a Dutch hospital but also complies with the Dutch legal frameworks and forms an economically feasible case. A case study at Reinier de Graaf hospital (RdGG) in Delft used real oxygen consumption data. The applicable regulatory frameworks have been obtained by research in literature and with help of a representative at RdGG. For the technical design and optimisation, a numerical model of the PSA-cycle has been developed. This model is based on the Ideal Adsorption Solution Theory (IAST) and makes use of the Linear Driving Force model for adsorption kinetics. The model behaviour has been validated by experiments within a self-built, small-scale PSA-unit in the Process & Energy (P&E) lab. After this, the setup was scaled to hospital size within the model. A schematic PSA-plant has been designed based on the regulatory frameworks and system requirements which came from the numerical modelling. The economic viability has been addressed based on quotes, consumption data and electricity prices which were provided by RdGG.

Dutch regulations require hospitals to have three oxygen sources, each capable of independently providing the full design flow, with PSA units obligated to produce 93% oxygen concentration (± 3%) and meet strict impurity limits. To achieve the required oxygen purity, a vacuum pressure of 0.05 bar and an adsorption pressure of 6 bar with Oxysiv MDX zeolite are optimal. Depressurization should occur close to atmospheric pressure (1.05 bar) for efficiency. The adsorption column is most effective with a diameter-to-length ratio of 0.1 and a dimensionless time, tc, of 9.56 seconds for maximum oxygen production. The dimensionless time represents the time it takes for the gas to pass through the length of the column based on its inlet velocity.
The PSA unit at RdGG must handle a normal demand of 10 Nm³/h and a peak demand of 80 Nm³/h for three consecutive days. The second source can handle peak demand for three days, while the backup source can manage only 9 hours (tvital). If the PSA-storage has been depleted and the second source fails, tvital is the maximum refill time of the PSA-storage. This requires a production rate of 0.40 mol/s and two packages of storage cylinders (1.6 m³) which store the oxygen at 200 bar. This setup necessitates two 336L columns (3.5m height, 0.35m diameter, 0.36 m/s inlet velocity) and an air inlet flow of 126 m³/hr, supported by a vacuum pump and a compressor. The cycle steps for the adsorption, depressurization and vacuum (tAD = 79 s, tDP = 67 s, tVC = 112.1 s) total 258.7 s, allowing two columns to run in parallel. A 3 m³ air pressure vessel at 10 bar maintains PSA operation for three minutes during compressor disturbances. This setup achieves 93.61% oxygen purity with 41% recovery for 93% oxygen concentration, increasing to 58% recovery for 90% oxygen concentration. Each column requires 127 kg of zeolite, producing 5.75 mol O₂ kg⁻¹h⁻¹ (0.66 m³ O₂ kg⁻¹h⁻¹ at adsorption pressure), with an energy consumption of 29.38 kJ per mole of oxygen, totalling 255 kWh/tO₂.

The total investment for a PSA plant is estimated at 215,190 EUR, with fixed equipment costs of 176,800 EUR. Using straight-line depreciation over 20 years results in annual depreciation of 9,890 EUR. Annual costs also include energy and maintenance.
For an oxygen demand of 234 tons in 2023, the annual cost would be 25,590 EUR using PSA, significantly less than the current 60,840 EUR with the Liquefied Oxygen (LOX) installation. The levelized cost of oxygen over a period of 20 years for PSA is 128.61 EUR/ton, which is 2.1 times lower than the LCOO of LOX which is 270.52 EUR/ton.

The implementation of PSA plants for localized oxygen production in the Netherlands is both technically and economically feasible. A case study at Reinier de Graaf hospital shows significant cost savings compared to current liquid oxygen setups, with potential annual savings of 35,250 EUR. Required equipment is available and the setup size allows it to be installed inside or next to the hospital. Regulatory frameworks are already present and define clear requirements for using a PSA system as primary source. When implementing the right flow and pressure mechanisms the PSA plant can be connected to the current second and backup source, minimizing system adjustments for a smooth integration.

Future research should focus on several key areas to enhance the feasibility of using PSA in Dutch hospitals. Regulatory frameworks need approval or revision by experts in medical oxygen production. Analysing zeolite degradation and validating its performance are crucial for simulating the PSA unit's lifecycle. Improved modelling techniques, considering temperature variations, research on the validity of current assumptions and preventing backflow in the experimental setup will yield more accurate results. Additionally, industry practices should be reviewed to refine the proposed setup and obtaining precise cost estimates from quotes or hospital bills will improve the economic analysis.