The distribution of argon in the upper column is not fixed. When the purity of oxygen or nitrogen changes-that is, when operating conditions shift even slightly-the distribution of argon within the column also changes accordingly. However, the extraction point of the argon fraction is fixed in position; therefore, the composition at the argon fraction extraction point will also vary. Experience has shown that a 0.1% change in oxygen purity will cause the oxygen content in the argon fraction to change by 0.8%–1%. The argon content in the argon fraction decreases as oxygen purity increases. Changes in the composition of the argon fraction directly affect the quantity of argon fraction entering the crude argon column. With a constant condensation rate in the crude argon column condenser, the higher the oxygen content in the argon fraction, the greater the quantity of argon fraction entering the crude argon column-and vice versa. At the same time, the liquid-to-gas ratio in the upper column also changes. Consequently, the operating conditions of the crude argon column become unstable, and it may even cease to function. The specific effects are as follows:
If the oxygen content in the argon fraction is too high, it will lead to increased oxygen content in the crude argon product, reduced argon yield, and lower argon recovery rate. It may also cause excessively high temperatures in the deoxidation furnace.
If the nitrogen content in the argon fraction is high, the temperature difference in the crude argon column condenser will decrease, even dropping to zero. This will reduce or halt the condensation of crude argon gas, rendering the crude argon column unable to operate normally. This, in turn, will reduce the argon fraction extraction rate, lower the upward gas velocity, and cause tray weeping. Moreover, as the argon fraction extraction rate decreases, the reflux ratio in the upper column will also decrease, oxygen purity will rise, and the nitrogen content in the argon fraction will correspondingly decrease. As a result, the temperature difference in the condenser-reboiler will expand again, the fraction extraction rate will automatically increase, and the nitrogen content in the argon fraction will rise again. This cyclical fluctuation prevents the crude argon column from operating normally. Therefore, the crude argon column can only be put into operation when the ASU operating conditions are particularly stable and both oxygen and nitrogen purities meet the required specifications.
When the argon fraction does not meet requirements and the nitrogen content is excessive, the oxygen delivery valve should be partially closed and the nitrogen vent valve opened wider. This will cause the argon-enriched zone in the stripping section to rise, reducing the nitrogen content in the argon fraction; at the same time, the oxygen content will increase and the argon content will decrease somewhat. When the argon content in the fraction is too low, the liquid nitrogen control valve should be partially closed to increase nitrogen vent purity, which will raise the argon content in the fraction. During operation, particular attention must be paid to fluctuations in the liquid oxygen level. Adjustments to oxygen and nitrogen production rates, as well as changes in air flow, must be carried out slowly, promptly, and appropriately, with every effort made to maintain a stable liquid oxygen level.
