Setting of the main cryogenic liquid nitrogen outlet to the lower reflux valve
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In our plant’s old units, a nitrogen circulation process is used: expanded nitrogen is fed into the lower tower. There is a liquid nitrogen level gauge in the main cooler, and under normal operation the system operates automatically, with the reflux valve at the top of the lower tower being controlled accordingly. However, the new 40000 unit uses an air expansion process for feeding material into the upper tower; there is no liquid nitrogen level gauge in the main cooler, and the reflux valve leading to the lower tower has been replaced by a manual butterfly valve. In the material balance diagram, there is no reflux under normal conditions – all the liquid nitrogen is removed as product or through the reflux at the top of the upper tower. Are there any plants that operate using such a process? Is the knowledge regarding this manual butterfly valve only applicable before and after starting up the system to allow fluid accumulation? Is it adjustable during normal operation? This post was last edited by lxy6240@tom.com on 2009-4-15 12:02.]
The process you mentioned is now very common; this manual butterfly valve is used only before and after starting up the system to allow liquid to accumulate, and it is not activated during normal operation once the purity level has been adjusted.
The “U”-shape bend is widely used as a liquid seal to maintain the lowest liquid level on the main cryogenic nitrogen side, thereby maximizing the heat exchange area. As for the setting of the butterfly valve, it is closed only when the equipment is dry, during startup, and during the liquid accumulation process; it remains fully open under normal conditions. There are also those designed to be used in conjunction with tower throttling valves; there are differing opinions, but I advocate for operating this valve at full open. (For reference)
In the air expansion process, liquid nitrogen should be returned to the lower column; otherwise, there will be no reflux liquid at the top of the column (unlike the nitrogen expansion process, where external liquid nitrogen is used for reflux). The reflux butterfly valve is rarely adjusted during normal operation; generally, adjustments can be made by controlling the liquid nitrogen valve leading to the tower.