Adjustment of the operating conditions of air separation units

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The main production cost of air separation units during normal operation is electricity consumption; reducing reactive power generation and lowering the oxygen release rate are important measures to save electricity. When there is a need to reduce the production of products such as oxygen, it is necessary to operate under varying conditions, and this can be achieved in several ways: by switching from producing mainly gaseous products to producing more liquid products such as liquid oxygen and liquid nitrogen ； Or reduce the load and decrease the production volume of gaseous products, etc. 1. Increasing or decreasing the variable-load operation of gas products: As the demand from users for products such as oxygen decreases, the pressure in the oxygen distribution network rises. In order to reduce waste gas emissions and save energy, it is often necessary to decrease the production volume of oxygen. The specific steps are as follows: 1) First, reduce the output volume of oxygen products, while simultaneously reducing the volume of nitrogen products in proportion. Under normal circumstances, the amount of contaminated nitrogen gas remains under automatic control based on the pressure in the main pipe leading out of the cold box. 2) Based on the reduced amount of oxygen, reduce the air volume entering the cold box by a factor of 5 or more, that is, decrease the opening degree of the air compressor vanes. 3) Adjust (slightly close) the pure liquid nitrogen reflux valve at the lower column to maintain a basically constant pressure at the bottom of the lower column. 4) Adjust the valve for pure liquid nitrogen feed to the upper column in order to maintain a constant oxygen content in the liquid-air mixture in the lower column. 5) The amount of expanded air can be adjusted according to the liquid oxygen level in the main coolant and the required quantity of liquid oxygen product. If the level of liquid oxygen in the main cooling system is too high and there is no particular need for liquid oxygen product, the amount of expanded air should be reduced accordingly. 6) Appropriately reduce the liquid-air level in the crude argon condenser, lower the load on the crude argon condenser, and at the same time decrease the output volume of crude argon product. 2. Procedures to increase oxygen production: The procedures for increasing oxygen production are essentially the reverse of reducing load; therefore, the steps are as follows: 1) Slightly increase the guide vanes of the air compressor to boost the amount of air supplied. 2) The pressure at the bottom of the lower column is controlled by adjusting the pure liquid nitrogen reflux valve in that column, so as to keep its pressure constant. 3) Adjust the liquid nitrogen inlet valve of the upper column, paying attention to the oxygen content in the liquid air from the lower column. 4) Gradually increase oxygen production. 5) Increase nitrogen production in the same proportion. 6) The amount of expanded air can be appropriately increased based on the liquid oxygen level in the main coolant. 3. Variable-condition operation to increase the volume of liquid products: When the demand for oxygen decreases, more liquid oxygen can be produced by increasing the amount of air used for expansion. The steps for this operation are as follows: 1) Reduce the oxygen output valve to decrease the amount of oxygen produced. 2) Gradually increase the amount of air being expanded; if necessary, another expander can be added. However, when adding another expander, it is necessary to first reduce the load on the existing expander, after which both expanders can have their loads increased gradually. The increase in the amount of expanded air must be gradual, with the additional expanded air being bypassed through a bypass valve. 3) Slowly close the liquid nitrogen reflux valve at the bottom of the column to maintain a constant pressure at the bottom of the column. 4) Adjust the liquid nitrogen inlet valve of the upper column to keep the purity of the liquid oxygen in the lower column constant. 5) Slightly reduce the liquid-air level in the crude argon condenser to lower the load on the crude argon column, and at the same time reduce the output volume of crude argon product. 4. Points to note during manual load variation operations: When using the manual load variation method, it is necessary to have a thorough understanding of the performance characteristics of the entire air separation unit. The operator must be well aware of the range of variation for all relevant parameters, the potential errors that may occur, and the proportional relationships between them. They should be able to operate the equipment skillfully and handle various abnormal situations and faults effectively. In addition, they need to understand the basic principles, operating methods, procedures, and steps involved in load variation before proceeding with such operations. The following key points should be noted during variable load operations: 1) When performing manual variable load operations, it is necessary to follow the principle of \"seeking stability while making changes, and seeking stability within those changes.\" We know that the air separation process itself is a process of gas inflow and product outflow. The amounts of gas and liquid flow are controlled by adjusting the opening degrees of various valves, thereby regulating the instantaneous flow rate of fluids in different pipelines and achieving control over this constantly changing process. Therefore, during variable load operation, it is very important to operate the equipment and adjust various controlled parameters in accordance with the aforementioned principles. During manual load variation operations, it is also important to note that when increasing the load, one should start by increasing the amount of processing air, and gradually adjust the setpoints of the relevant parameters in sequence. Each change in magnitude should not exceed 0.5% of the oxygen production. Under normal circumstances, the cycle for changing the set point from start to finish is 10 minutes. Conversely, when reducing the load, start by decreasing the oxygen flow rate to the cold box, and then adjust each set point in sequence in the opposite direction; the extent to which oxygen production is reduced should be greater than the extent to which air volume is reduced. The operating conditions of the air separation unit are stable, and the values of the various key parameters displayed are accurate; they are not fake values, and they generally conform to the principles of material balance and heat balance. 2) Technical limitations of the variable load range. In practical applications, we have found that during variable-load operations, the production in the crude argon column is the most affected. Crude argon is extracted using the argon fraction gas drawn from the lower part of the upper column; the quality of the distillation process in the upper column directly influences that of the crude argon column. In particular, even if the purity of the oxygen and nitrogen products remains unchanged and the distillation process in the upper column operates normally, the distillation process in the crude argon column can still be disrupted. After analysis, we found that this is caused by a change in the concentration gradient distribution of the rising steam in the upper tower and the downward-flowing liquid. For example, a change in the concentration gradient of the rising steam in the upper column can affect the purity of the argon fraction. In particular, when the nitrogen content in the argon fraction increases, non-condensable gases (nitrogen) accumulate suddenly in the condenser of the crude argon column, resulting in a decrease in the heat transfer temperature difference. This reduces the efficiency of heat transfer in the crude argon condenser, causing a sudden reduction in the reflux flow rate in the crude argon column. As a consequence, the oxygen content in the crude argon increases, with both oxygen and nitrogen levels rising simultaneously. If the oxygen content in the argon fraction is adjusted or reduced solely based on the high oxygen content in the crude argon, the situation will worsen. Therefore, it is relatively simple to adjust the load on air separation units without argon; it is sufficient to consider only the purity of nitrogen at the top of the upper column and the purity of oxygen at its bottom. Moreover, the purity of the oxygen and nitrogen products can be easily maintained during the load adjustment process. However, in air separation units operated with argon, it is also necessary to consider the distribution of the concentration gradient in each section of the column; otherwise, the normal operation of the argon system cannot be ensured. We know that it is very difficult to maintain a constant concentration gradient in the upper tower during load changes. Under low-load operation, the distillation efficiency of each tray decreases, and the concentration gradient inevitably changes, which prevents the normal operation of the argon system. Therefore, the range of variable load is limited, provided that argon production is not affected. In production practice, it has been found that when the load on the air separation unit is below 80%, the operation of the argon system is sometimes disrupted, whereas the purity and output of oxygen and nitrogen products remain unaffected. 3) When performing manual load variation operations, efforts should be made to avoid the influence of other external factors; for example, no load variation operations should be carried out within 10 minutes before pressure equalization of the molecular sieve and 10 minutes after it is completed. :):):):):):):):):):):):)