The impact of a low flow rate of high-pressure throttled liquid air entering the upper column on overall distillation
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Under full load, if the amount of high-pressure throttled liquid entering the upper column is 20% below the design value, what impact does this have on the distillation conditions in both the upper and lower columns? Will the purity of the oxygen-enriched liquid air at the lower tower be low? What is the impact of oxygen purity in the upper tower and the purity of the argon fraction?
The impact on distillation is mainly related to the reflux ratio in the upper column; an increased amount of expanded air fed into the air stream in the upper column affects this reflux ratio, which in turn results in a decrease in the reflux ratio and has an effect on the oxygen extraction rate.
I’ll take a certain set of equipment as an example: the designed temperature at the liquid expansion outlet (equivalent to the JT value) is -176.15°C, while the designed temperature of the liquid air at the bottom of the lower column is -173.82°C. If less liquid air is sent to the upper column due to high-pressure throttling (resulting in less liquid expansion), it will inevitably have to go to the lower column, causing the temperature of the liquid air to drop. This liquid air then reaches a position somewhere in the upper middle part of the upper column (above the location where the argon fraction is extracted). As a result, the reflux ratio in the stripping section increases, which makes it easier for the temperature in the upper middle part of the column to drop. The argon fraction thus contains more nitrogen, increasing the risk of nitrogen blockage and affecting the efficiency of argon and oxygen separation