The purification process of the molecular sieve is that before the compressed air enters into the cold box, it should first go through the molecular sieve purifier to remove the impurities such as moisture, carbon dioxide and so on in the air, and the air will not be separated and frozen again during the cooling process, it can ensure the normal operation of the air separation unit. Compared with the original purification process of switching heat exchanger, it has the following advantages:
1) while removing impurities such as water, carbon dioxide and so on, it adsorbs hydrocarbon such as acetylene. Generally, there is no need to set acetylene adsorber and corresponding liquid oxygen pump in the cold box, which greatly simplifies the process, the number of pipe valves and flanges can also be reduced;
2) replace the switching heat exchanger with the main heat exchanger which is purely heat exchange, omitting the switching valve which works frequently, reducing the failure rate of the equipment and reducing the switching noise. In addition, the heat exchanger channel is not subject to alternating stress, which can prolong the service life of the equipment;
3) simplify the equipment operation. Especially in the start-up stage, in order to pass through the phase of water and carbon dioxide precipitation safely, switching heat exchanger has strict requirements in operation and requires experienced operators to operate, in case of blocking of the expansion machine. While the process of molecular sieve purification does not need to worry about the freezing of moisture and carbon dioxide in the equipment, which greatly simplifies the start-up operation;
4) no special heating and thawing system is needed. Heating and drying can directly utilize the purified low-temperature raw material air, simplify the heating operation, reduce the equipment and reduce the heat effect brought by heating;
5) The return sewage nitrogen does not take away the frozen water and carbon dioxide from the heat exchanger channel, so there is no requirement for its quantity. Therefore, the output of pure nitrogen can be increased. The output ratio of oxygen to pure nitrogen in switching process is 1:1, while the ratio of molecular sieve purification process can reach 1: (2.5 ~ 3.5 );
6) because the switching time of switching heat exchanger is about 4 ~ 8 min, and the switching time of the adsorber can be extended to 2 ~ 4h
Therefore, the switching loss of air is greatly reduced, which can reduce energy consumption and improve the extraction rate of oxygen;
7) extend the operation cycle of the equipment. Under normal circumstances, the effect of molecular sieve purification is better than that of freezing self-cleaning, and the continuous operation cycle of the equipment can be extended from 1 year to 2 years.
Due to these advantages above, the new air separation unit adopts molecular sieve adsorption purification process.
