In addition to certain purity requirements for medical oxygen (99.5% oxygen purity for the cryogenic process), the moisture content must not exceed a dew point temperature of -43°C. The volume fraction of carbon dioxide, as well as the contents of carbon monoxide, gaseous acids and alkalis, ozone, and other gaseous oxidants must also comply with the requirements of GB 8982-1998. Therefore, general industrial oxygen does not meet medical oxygen standards, meaning ordinary oxygen compressors and oxygen cylinders cannot be used to fill medical oxygen.
Using a diaphragm compressor is a measure to ensure oxygen quality. The liquid oxygen internal compression process is a safe and contamination-free process for producing medical oxygen. Specifically, liquid oxygen is extracted from the main condenser, subcooled, and then compressed to 16 MPa by a plunger-type liquid oxygen pump. It is then vaporized into high-pressure gaseous oxygen in an oxygen heat exchanger, recovering the cold energy. Many oxygen plants have implemented this modification for 50 m³/h and 150 m³/h oxygen generators. The main considerations are as follows:
- The oxygen heat exchanger operates under high pressure and must be structurally capable of withstanding high pressure. Additionally, due to the large cooling capacity of liquid oxygen, the amount of air cooled in the oxygen heat exchanger is approximately 1.7 times the oxygen quantity. The heat load on the main heat exchanger is reduced, and the insufficient reheating of the returning low-temperature gas expands the hot-end temperature difference, increasing cold loss;
- When adopting the internal compression process, the work consumed by the liquid oxygen pump becomes an additional cold loss for the unit. Meanwhile, the high-pressure oxygen leaves the unit under pressure, and the throttling effect refrigeration capacity is not utilized. To balance the cold loss, the refrigeration capacity of the expander must be increased. This requires raising the operating pressure of the air compressor, generally above 4.0 MPa;
If the unit needs to simultaneously produce medical oxygen and industrial oxygen, the sizes of the corresponding liquid oxygen subcooler, liquid oxygen heat exchanger, and liquid oxygen pump should be determined based on the required medical oxygen volume. In this case, the increase in cold loss is relatively small, and the operating pressure can also be reduced.
