Reducing the working pressure of the air separation equipment can reduce the unit energy consumption of the product. The working pressure of the full low-pressure air separation equipment is close to the working pressure of the lower tower, while the working pressure of the small air separation equipment is much higher than the pressure of the lower tower. The working pressure is low, and the unit cooling capacity generated by expansion is also small. In order to maintain the balance of cooling capacity, it is first required that the unit cooling loss is also small. For large-scale air separation equipment, the unit cooling loss decreases as the unit capacity increases. At the same time, a smaller hot end temperature difference is also selected during design. The unit cooling loss due to incomplete heat exchange is relatively small, which is a good way to reduce the working pressure. Favorable conditions were created.

In addition, the low working pressure requires the expander to have high efficiency so that it can produce a larger cooling capacity under the same pressure difference. As the capacity of the turboexpander increases, the optimal rotation speed decreases and the efficiency increases. Therefore, it is most suitable for large-scale air separation equipment, making it possible to reduce working pressure.

For small air separation equipment, the cooling loss is relatively large. Even if a turbine expander is used, the speed is as high as 105r/min or more, the efficiency is low, and the maintenance and management requirements are very high. In addition, for large-scale air separation equipment, the amount of expansion is small relative to the amount of process air. The expanded and refrigerated air can still participate in distillation and extract oxygen from it. If a small air separation equipment adopts a low-pressure process, because the amount of expansion gas required to generate refrigeration capacity is large and cannot be fully involved in the distillation, the oxygen extraction rate will be very low, and the energy consumption per unit product will still be high. Therefore, the full low-pressure process is most suitable for large and medium-sized air separation plants.

At present, with the adoption of molecular sieve adsorption purification and booster turbine processes, as well as the advancement of plate-fin heat exchanger technology, the minimum capacity of low-pressure air separation equipment has been designed to 340m3/h oxygen production and 800m3/h nitrogen production (KDON -340/800), the exhaust pressure of the air compressor is 0.59MPa.

What is the pressure swing adsorption molecular sieve purification process, and what are its characteristics compared with the temperature swing adsorption purification process?

Molecular sieves have the function of selective adsorption of mixed gases, and their adsorption capacity changes with changes in temperature and pressure. Its adsorption capacity increases at low temperature and high pressure, and decreases at high temperature and low pressure. Temperature swing adsorption (TSA) is based on the principle that molecular sieves adsorb at room temperature and desorb at high temperatures, while pressure swing adsorption (PSA) is based on the principle that molecular sieves adsorb at high pressure and desorb at low pressure.

The so-called pressure swing adsorption molecular sieve purification process is to use the molecular sieve pressure swing adsorption process to remove water, carbon dioxide, and hydrocarbons in the air, eliminating the need for air pre-cooling systems and regeneration heaters, as shown in Figure 15.

Use 1% to 1.5% purified air to regenerate molecular sieves. The general switching cycle is 9 to 14 minutes. There are 2 to 6 adsorbers according to the specifications of the air separation unit. The adsorbent capacity is 4 times that of the TSA adsorbent of similar air separation units. Each adsorbent is The container is equipped with 6 switching valves.

Compared with TSA, the advantages of PSA are:

     1) Simplify the process and eliminate the need for equipment such as air cooling towers, evaporative cooling towers, low-temperature water pumps, and regenerative heaters;

     2) There is no steam consumption required for TSA heating regeneration. For a 60,000m3/h air separation unit, 1,800kg/h of steam (approximately 1,000kW·h/h) can be saved.

Its disadvantages are:

     1) Air switching loss is 1% to 1.5% and consumes 400kW more power;

     2) The switching cycle is short and the switching valve is prone to failure;

     3) It is difficult to completely desorb the regenerated molecular sieve at room temperature, which will affect the adsorption performance of the molecular sieve and bring some harmful gases into the air separation unit, which will have a certain impact on the safety of large-scale air separation units;

     4) The investment is slightly larger.