When changing the freezing purification process into the molecular sieve purification process, what changes should be made to the air precooling system correspondingly,

The adoption of molecular sieve purification system greatly simplifies the operation and improves the safety and reliability of the equipment. Therefore, many units of low-pressure air separation plant that used the freezing method to remove moisture and carbon dioxide in the air have also considered reforming the original equipment in succession. In order to ensure the purification effect of the molecular sieve adsorber, the air needs to be precooled to about 8℃, so the original air cooling tower can not meet the requirements any more. In order to save the cost of transformation, a precooling system is usually added after the original precooling system to cool the air to the required temperature.
The precooling system firstly needs to provide a cold source whose temperature is lower than 8℃ in order to cool the air; the second is to add a heat exchanger to realize the heat exchange between the air and the cold source; thirdly, separate the moisture eluated from the air temperature drop in the separator in advance, and then enter the adsorber.
According to different cold source or cooling methods, the following four types have been adopted:
1) ammonia evaporation cooling system. For chemical enterprises with ammonia stations, rich liquid ammonia is used as cold source. After throttling, the high pressure liquid ammonia is partially gasification, and the temperature drops to the saturation temperature corresponding to the pressure after throttling. After going through the ammonia liquid separator, the low-temperature saturated ammonia liquid enters the air cooler and cools the air to 7℃, then the ammonia liquid evaporates and gasification, and then returns to the ammonia station. Ammonia evaporator is Aluminum Plate-fin Heat Exchanger;
2) refrigerating fluid cooling air system. Take the evaporator of working medium (R22, etc.) in the refrigerating unit as the air cooler. The air is cooled to 7℃ by the evaporation of working medium. The refrigeration unit can be made up of several smaller refrigerators in parallel to increase the flexibility of adjustment. The cooling effect of this system is relatively good, but the evaporator needs to be specially designed and manufactured;
3) low temperature water indirect cooling system. Use the low temperature water about 5℃ provided by the air-conditioning refrigeration unit as the cold source of the air cooler to cool the air to 7℃. The water chilling unit of this system can be matched from the ready-made product series, and the cooling capacity can be adjusted flexibly;
4) direct cooling system of low temperature water. Pump the low temperature water provided by the water chilling unit to the top of the air cooling tower for spraying, and conduct contact heat exchange with air. The air cooling tower is similar to the original one. Its cooling effect is good, but attention should be paid to prevent accidents with water.
Each unit can determine the best scheme through technical and economic comparison according to the actual situation.