Coarse argon column

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Analysis of the working principle and operation methods of the crude argon condenser. The crude argon condenser is one of the key components in argon extraction equipment; the proper operation of this device directly affects the yield and quality of crude argon. In severe cases, it can even affect the stability of the distillation operation in the air separation unit. In practical operation, it is common to employ the method of raising the liquid-air level in order to increase the heat load on the crude argon condenser, that is, to increase the resistance of the crude argon column. This method is effective when the liquid-air level in the crude argon condenser is low, but once it reaches a certain level, further increasing this liquid level not only fails to increase the resistance in the crude argon column; on the contrary, it reduces it. Why does this phenomenon occur? In fact, it is not difficult to understand why this phenomenon occurs if we analyze the working principle and structural features of the argon condenser. I. Working principle of the crude argon condenser: Compared with the main condensation evaporator, the crude argon condenser has structural similarities. It’s just that the medium on the crude argon condenser side is liquid air, while the medium on the condensation side is crude argon 1. The cooling capacity obtained from crude argon is used for condensation, while liquid air is evaporated. On the evaporation side, the liquid air flows within its channels at a certain circulation rate; that is, a large amount of liquid air circulates in these channels, while only a small portion of it is heated and vaporized. As shown in the figure. Under normal conditions, the liquid-to-air circulation rate is 7 to 10 times the vaporization rate. The higher the circulation ratio, the greater the heat transfer intensity, and thus the greater the thermal load on the crude argon condenser. This, in turn, leads to an increase in the resistance of the crude argon column. With a fixed condenser structure, the circulation ratio is primarily influenced by two factors: the temperature difference in the crude argon condenser and the liquid-air level. II. Effect of the temperature difference in the crude argon condenser on the circulation rate: The liquid air is heated by the crude argon gas on the condensation side within the evaporation channel; as a result, it gains heat, its temperature rises and it partially vaporizes, which reduces its specific gravity. In this way, the pressure inside the channel is lower than that at the inlet of the channel, causing the liquid-air mixture to flow upward within the channel. Therefore, the temperature difference in the industrial argon condenser is a prerequisite for generating the circulation rate; without this temperature difference, liquid air will not circulate automatically. The temperature difference in the crude argon condenser is primarily determined by the composition of the liquid air and the pressure at the evaporation point; of course, the liquid level of the liquid air also has an impact, which will be discussed later. The liquid-air composition on the evaporation side of the crude argon condenser ranges from 60 to 70% O2; at a constant pressure, different oxygen contents result in different evaporation temperatures. There are mainly two factors that affect the oxygen content in the liquid air: one is the oxygen content in the liquid air at the bottom of the tower. The higher the oxygen content in this liquid air, the higher it will be as well in the liquid air on the evaporation side, which in turn raises the evaporation temperature in the condenser and reduces the average temperature difference ； The other factor is the amount of liquid air returned; in product design, 2–10% of the total amount of liquid air taken is generally considered as the amount to be returned. Under normal circumstances, for every one percentage point increase in this ratio, the oxygen content in the liquid air decreases by 0.3 to 0.4 percentage points, which in turn lowers the temperature on the evaporation side, thereby increasing the temperature difference across the condenser. The effect of the liquid air evaporation side pressure on the condenser evaporation temperature is such that, generally, the lower the pressure on the evaporation side, the more favorable it is for increasing the temperature difference in the condenser. However, the pressure on the evaporation side cannot be reduced arbitrarily; it is constrained by the pressure of the liquid-air vapor from the crude argon condenser returning to the corresponding section of the upper column, that is, there is a one-to-one correspondence. However, it is acceptable to appropriately increase the pressure on the evaporation side within certain limits; that is, some users request that, when designing the crude argon column, a significant reduction in operating load be taken into account, and the pressure on the liquid-air evaporation side should only be increased in such cases to reduce the temperature difference in the condenser, thereby achieving the goal of reducing the load. In this case, it is necessary to install a control valve on the pipeline that carries the liquid air vapor back to the upper tower in order to achieve this. III. Influence of the liquid-air level on the circulation rate: In order to enable the liquid air to circulate at a certain rate within the liquid-air flow channels of the plate-fin heat exchanger, it is necessary to maintain a certain liquid-air level, while also ensuring a specific temperature difference with the condensing side of the condenser. Due to this liquid level, a certain pressure P2 is present at the bottom of the evaporation side, in order to overcome the resistance encountered by the liquid-air flow within the plate fins. Under normal conditions, as the liquid-air level rises, the pressure P2 at the bottom of the liquid level also increases gradually. The accelerated flow of liquid air leads to an increase in the circulation rate, thereby enhancing heat transfer in the condenser and facilitating an increase in the thermal load of the crude argon condenser. However, once the liquid level rises to a certain height, the increase in pressure P2 raises the saturation temperature at the bottom of the liquid-air mixture, which in turn increases the average temperature on the evaporation side and reduces the average temperature difference in the crude argon condenser. In this way, not only is the heat load on the crude argon condenser not increased, but it actually decreases; in severe cases, it can even disrupt the operation of the crude argon column. After understanding the working principle of the crude argon condenser and the influence of factors such as the liquid air level, the amount of reflux liquid air, and the oxygen content in the liquid air from the lower column, it becomes much easier to flexibly adjust the operating conditions of the crude argon condenser and the crude argon column in our actual operations. First of all, it is necessary to ensure that there is a certain temperature difference between the evaporation side and the condensation side of the crude argon condenser. The following measures can be taken: 1. Maintain a relatively stable operating condition for the upper tower, ensuring that as little nitrogen as possible is present in the crude argon gas, with the nitrogen content generally kept below 1.5% N2. 2. Adjust the operating conditions of the lower column so that the oxygen content in the liquid-air stream exiting the lower column is maintained within the range of 35.5–37.5% O2. 3. The oxygen content of the liquid air on the evaporation side is adjusted by opening or closing the liquid-air constant flow valve of the coarse argon condenser. Secondly, during debugging, tests can be conducted to adjust the liquid-air-liquid level height. That is, while ensuring that all other operating conditions remain normal, the liquid-air level is gradually increased from a low level to a high level. At first, as the level rises, the resistance in the crude argon column also increases accordingly; but once the level reaches a certain height, further increase in the level will result in a decrease in the resistance of the crude argon column. In such a situation, if one wishes to continue reducing the resistance in the crude argon column, then only the first to third methods mentioned earlier can be used for adjustment. Understanding this turning point allows us to avoid incorrect actions.