Key points for starting small-scale air separation equipment
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The startup of low-pressure small air separation units takes place in the early stages of the cooling phase; ensuring that the temperature difference on the hot side remains relatively constant, air enters the expander through the middle valve or the bottom valve of the main heat exchanger. Which option is more effective in reducing the startup time? Also, is local cooling better or overall cooling? Our workshop has debated this issue for a long time; alas, it’s because we lack solid evidence to support our arguments. Moreover, there aren’t many opportunities to carry out experiments when starting up air separation units. I hope experts can provide compelling evidence – I’m new to this industry.
First of all, this information is included in the operating instructions. If it is in forward flow, when operating the system one should first open the bottom valve, as the temperature there is lower; this results in a much larger amount of expansion within the expander, leading to more cooling effect, and thus faster operation. As for overall cooling and local cooling, I personally think overall cooling is better, as it helps to extend the lifespan of the equipment.
Finally, someone has replied to my post :) I’m starting to think that operating with intermediate extraction is a good approach. This approach is based on the principle of high temperature and large enthalpy drop in the expander – the higher the temperature of the air entering the expander, the greater the temperature difference between its inlet and outlet, and thus the greater the amount of cooling produced. When an air separation unit is first started up, the temperature is relatively high, and using intermediate extraction helps to accelerate the rate of temperature drop. From another perspective, if we consider the entire tower as being in an ideal state with no heat exchange with the outside world, that is, a state where there is no temperature difference at the hot end, then the initial cooling capacity depends entirely on the unit cooling efficiency of the expander and the volume of expansion that occurs. As suggested by Teacher LS, the issue of expansion volume wasn’t taken into account; indeed, when the temperature is high, the expansion volume isn’t as large as it would be with bottom extraction. It’s difficult to determine which factor is more important, and calculations may be needed to compare them. Regarding local cooling versus overall cooling, I also believe that overall cooling is better, as it helps to keep the temperature at the expander’s inlet at a higher level, ensuring efficient operation of the expander. With local cooling, the process of liquid accumulation would be much slower.
I hope to learn more about this area from everyone*.
The principle behind high temperature and high enthalpy drop in expanders is correct; I have calculated this, and the effect of the amount of expanded gas on the amount of cooling produced is greater than the effect of the temperature difference. Cooling capacity generated by the expansion machine = Volume of expanded gas * Actual enthalpy drop. Actual enthalpy drop = Theoretical enthalpy drop * Efficiency of the expansion machine
Oh, I see. Next time I get the chance to drive, I’ll give it a try and see how much the effect of pulling the throttle mid-drive or at the end of the stroke is on the level of expansion. Thank you to the teacher who replied above
Thank you for the explanation; now I understand
There is really a need for knowledge in this area; I hope to see more of it