When liquid nitrogen is drawn out frequently, how should the main tower be operated?
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Our newly installed three-stage oxygen production system requires liquid nitrogen. The usual process involves extracting only liquid oxygen from the main tower, with no liquid nitrogen being extracted. If liquid nitrogen is drawn off, I believe it will change the reflux ratio in the upper column, thereby affecting the operation of the main column. How can liquid nitrogen be drawn off without affecting the operation of the main tower?
Generally, liquid nitrogen is introduced into the small storage tank inside the tower through a throttle valve with a 40 MM diameter, which is placed behind the V2 valve. Once this small storage tank is full, the liquid nitrogen is transferred to an external storage tank. While ensuring the minimum reflux ratio in the distillation section of the tower as well as the appropriate reflux flow rate from the lower section, the 40 MM diameter valve for collecting liquid nitrogen is slightly opened, allowing the liquid nitrogen to be stored in the small tank inside the tower. Wait until there’s more liquid nitrogen in the small storage tank before draining it out
“While ensuring the minimum reflux ratio in the upper distillation section and the reflux liquid in the lower section, slightly open the 40 MM diameter nitrogen collection valve. The key is what measures and methods you will use to ensure that the minimum reflux ratio in the upper tower and the reflux liquid in the lower tower are not affected. Should it be to increase air flow or to increase expansion volume?
Whether to increase the air flow or the expansion volume – this issue should be related to your design. You mentioned that you need to extract liquid nitrogen frequently now – how much is extracted? As for the amount designed, sometimes it varies. If the amount drawn is roughly equal to the designed amount, then I believe the operation will not affect the reflux ratio. However, if the amount drawn is greater or smaller than the designed amount, then attention must be paid to the reflux ratio. With a distillation tower, even the slightest change can alter its operating conditions. On our end, there is 36,000 of cryogenic air; usually, it only flows into the liquid nitrogen storage tank, with a designed flow rate of 1,400 – the rest being nitrogen and contaminated nitrogen. (My personal opinion, for reference only)
There’s another issue: when the equipment is first started up, liquid nitrogen is certainly not drawn out yet; in other words, your liquid nitrogen outlet valve isn’t open. Wait until the operating conditions are established and normal before gradually opening it. So should we turn on the liquid nitrogen reflux at this point to assist?