As the air of the air separation plant of the molecular sieve purification process removes impurities such as moisture and carbon dioxide through the adsorption of the molecular sieve, compared with switching the purification process, the start-up operation is simple and easy to control. It doesn't need to consider the complex influences such as self-cleaning of water and carbon dioxide, precipitation of water and carbon dioxide in the expansion machine, and the attention of starting process mainly focuses on giving full play to the refrigeration capacity of the expansion machine, reasonably allocate cooling capacity and fully cool the equipment. It can be divided into three stages: cooling equipment, accumulation of liquid and adjustment of rectification working condition. Compared with the start-up mode of switching process, it can be called "comprehensive cooling method" or "one-time cooling method ".
The following points should be paid attention to when starting the operation:
1) the molecular sieve used for the first time needs one-time activation and regeneration, with the purpose of removing the water and carbon dioxide adsorbed during the process of transportation and filling. The activation temperature should generally be higher than 200℃ and lower than 250℃. When the outlet temperature reaches 80℃, it can be blown cold. The activation time is no less than two switching cycles;
2) when the molecular sieve adsorber is started, the process of air supply and pressure rise should be slow, and when the vent valve is closed, be cautious to prevent the molecular sieve in the bed from being damaged due to pressure fluctuation;
3) when two expansion machines need to be started, the outlet backflow valve of the booster machine should be fully opened to lower the pressure of the first running expansion machine, and then load the two expansion machines at the same time, prevent surge of the back-starting booster;
4) pay attention to the temperature control of the middle part of the main heat exchanger:
① The temperature difference in the middle of the control unit is generally not more than 3 ~ 5℃;
② The temperature of the middle part should not be too low, and the cooling amount should not be concentrated on the main heat exchanger too much, which will cause the temperature difference of the hot end to increase. After the cold end temperature reaches the air liquefaction temperature, the cooling volume should be transferred to the rectification system to make the rectification system cool down fully, accumulate liquid as soon as possible and establish rectification;
5) pay attention to the work of the air cooling tower to ensure that the air temperature after precooling meets the design requirements. To prevent pressure and water level from fluctuating, in case that the air carries water and affects the performance of the molecular sieve.
595. Can the molecular sieve adsorption purification clean out the hydrocarbon such as Block B, in order to ensure the safe operation of the unit, what problems should be paid attention to in the operation?
Answer: The molecular sieve adsorber can adsorb moisture and carbon dioxide at normal temperature, and at the same time, it can adsorb hydrocarbon such as acetylene. According to the data provided by Linde company, the formula and experimental data of the adsorption efficiency of molecular sieve removing acetylene and other hydrocarbon are as follows:
ηx=(1-yci/yji)×100%
In the formula, ε x -- the average adsorption efficiency of a certain hydrocarbon CnHm;
Yci -- the concentration of a certain hydrocarbon CnHm at the outlet of the adsorber;
Yji -- the concentration of a certain hydrocarbon CnHm injected into El By The adsorber.
Thereinto, the adsorption efficiency of 96.h6 and C2H6 is almost zero; that of C3H8 is 89%; that of C2H4 is 97%; that of C3H6, C2H2 and C4H10 is almost 100%. It can be seen from the above data that molecular sieve purification can be used to clear acetylene, propylene and so on, but it is ineffective to methane and acetane.
In order to ensure the safe operation of the device, the following problems should be paid attention to during the operation:
1) the molecular sieve adsorber should be in good working condition with complete desorption and regeneration, full and smooth bed and no short-circuit airflow;
2) keep the main cold liquid oxygen with a certain emission (generally not less than 1% of the product oxygen );
3) keep the stability of the main cold liquid level. For the main cooling of full immersion operation, the liquid level should not drop below the full immersion position;
4) If there is liquid oxygen safety adsorber, the normal operation and regeneration of the adsorber should be kept;
5) pay attention to monitoring the content of hydrocarbon in the main cold liquid oxygen, and take corresponding measures when exceeding the provisions.
