Heating of air separation cryogenic tank
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During the maintenance of my unit, a heating process was carried out. The approximate timeline was as follows: on May 9 at 23:30, the expander was stopped and the liquid pump began to discharge the liquid; by 9:00 on May 10, all the liquid had been drained, and the temperature inside the cold box started to rise. At 20:00 on May 10, the temperature in the upper tower reached a minimum of -13 to -14°C, and gas was released and pressure was maintained in accordance with the company’s shutdown procedures. At 18:30 on May 15 (with the temperature inside the tower at around -46 to -49°C before gas introduction into the cold box), heating began as part of the gas introduction process. It took about 12 hours for the temperature inside the tower to reach 10°C; thereafter, it remained relatively stable at around 22°C. Since the dew point values were within acceptable limits, the expander was started up. Currently, the heat loss due to the high-pressure heat exchanger is quite high, with a temperature difference of around 9°C, which results in a significant increase in the load on the expander. The level of the main cooling liquid can just manage to be maintained. I would like to ask the experts: what are the criteria for thoroughly heating a cold box? (Such as temperature and time) How can we avoid a larger temperature difference across the heat exchanger and a worse heat exchange efficiency after heating?
Was effective positive pressure holding applied during that period? ~~Additionally, which heat exchange path is currently causing the increased temperature difference in the high-pressure plate? ～～
During the parking period, the pressure was not maintained; there was internal leakage in the valves. Before starting up, the valve had almost no pressure. The temperature difference of nitrogen in the lower tower was 15–16°C, while the differences for the other components were 8–9°C℃
A stream of gas can be introduced to maintain pressure~~ The backup liquid nitrogen will vaporize~~
Yes, maintaining pressure while parking the system is particularly important. I think the high-pressure oxygen channel isn’t properly heated; previously, a high-pressure air throttling purge valve was used in conjunction with a high-pressure oxygen purge valve to achieve heating and purging, but this time that wasn’t done – instead, only the inlet and outlet valves of the oxygen pump were opened to utilize the heat from the upper tower for heating.
Before starting the expander, the temperatures of the forward and reverse flow gases at the high plate had already reached 21°C – is the equipment still cold?
This post was last edited by hait3202843 on 2016-6-6 23:06: open the inlet and outlet valves of the oxygen pump to use heating from the upper tower. ----That’s how we do it too.
Oh, I made my judgment based on our other air separation heating method. I’m quite confused about this heating process now; I don’t know what’s causing the cooling issue – is it because the heating time is too short?
It has nothing to do with temperature; the key is that the dew point must be within acceptable limits. It could be a problem with pressure retention, which causes moist air to be drawn back in, or certain areas may not have been properly ventilated during heating. It’s also possible that the heating gas itself is of poor quality. Additionally, there’s a possibility that pearlescent sand has gotten into the plate structure. Analyze the situation based on your specific conditions~~
If normal pressure holding is maintained and the low-pressure gas source is heated, there should be no major problems~~ But if water gets in accidentally or something, that could be a problem~~