Liquefied air when China was in a factory in Shandong province, the factory reported that for a long time, the 21000Nm 3/h air separation plant and the 15000Nm 3/h air separation plant both had the problem of difficult start-up of crude argon tower. In this regard, the author had an in-depth discussion with the staff of the factory, analyzed the reasons why it was difficult to start the crude argon Tower, put forward the solution to this problem, and put it into practice, which achieved good results.
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The description of the problem of difficulty in starting the crude argon Tower:
At the initial stage of starting the crude argon Tower, the non-condensable air emission valve of the crude argon Tower maintains a relatively large opening, and the liquefied air vapor of the crude argon tower goes back to the upper tower valve, when the liquefied air starts to be imported into the crude argon condenser, the liquid level of the crude argon condenser rises to 600 ~ 700mm, the pressure of non-condensable gas emission pipeline of crude argon condenser decreases from 35KPa to 5 ~ 6KPa, the outlet is discharged from the initial liquid discharge to no gas discharge, and the rectification load of the crude argon tower starts to decrease until it completely stops working, and the start of the crude argon Tower fails.
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Cause Analysis of difficulty in starting crude argon Tower:
From the initial stage of starting the crude argon Tower, the phenomenon that the pipeline can discharge liquid from the air to almost no gas can be inferred as follows.
A) there is no mechanical blockage in the discharge pipeline.
B) the reason for the poor discharge of non-condensing gas is that after the crude argon tower is put into use, the discharge pipeline is blocked by foreign matter, and the source of this foreign matter may be:
L The argon of the crude argon condenser solidifies, resulting in the pressure drop at the discharge pipeline. However, since the crude argon tower is put into use with oxygen-enriched fraction above 90% in the initial stage, the condensate of the crude argon condenser is mainly liquid oxygen, therefore, the possibility of argon curing should be excluded.
L There is a U-shape bend in the discharge pipeline of the crude argon condenser. After the crude argon condenser is put into operation, the opening degree of the discharge valve is too large, and the flow rate of the discharge pipeline is too high, the condensate of the crude argon condenser will be clamped into the U-shape bend and stored, and finally fill the bottom of the whole U-shape bend, blocking the emission of non-condensing nitrogen. Nitrogen is enriched in the crude argon condenser, cause nitrogen plug and lead to the failure of the crude argon tower. The following figure is the pipeline diagram of the discharge pipe of the crude argon condenser:
C) at the initial stage of starting the crude argon Tower, the condensing gas of the crude argon condenser is over 90% oxygen-enriched gas. If the operating pressure of the crude argon tower is too low, oxygen-enriched gas may be too cold after liquefaction, and it will be more difficult to vaporize and discharge after being clamped into the bottom of U-shaped bend. Therefore, in order to reduce the accumulation of oxygen-enriched liquid in U-shaped bend and increase the evaporation pressure on the side of liquefied air, reducing the heat transfer temperature difference of the crude argon condenser is an effective method to improve the liquid storage at the U-shaped bend, which is clearly proved by the following operation trend chart of the crude argon tower.
D) since there is no liquid discharge pipeline in the low part of U-shape bending of the crude argon condenser discharge pipeline, there is no chance to install an discharge pipeline in the low part of U-shape bending under the condition that the Pearl sand is not pulled out from the cold box. Therefore, the new direction to solve this problem is to reduce or avoid the liquid clamps of the non-condensing gas pipeline when starting the crude argon tower.
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New plan for starting crude argon Tower
A) when the main cold liquid level of the main tower has been fully immersed, the purity of the main cold oxygen nitrogen is basically qualified, and the product starts to be delivered, the crude argon tower has the conditions to start.
B) under the condition of keeping the main cold and full immersion, slowly import the liquefied air to the crude argon condenser, improve the liquid level of the crude argon condenser, and observe the side pressure of the liquefied air of the crude argon condenser at the same time, pass hallway small liquefied air steam back tower valve the pressure maintained at 70 ~ 80KPa (the high operating pressure is because the original design is non-full immersion operation, and the liquefied air is transformed into full immersion operation ).
C) under the condition that the purity of oxygen in the main tower is qualified, the liquid level of liquefied air in the crude argon condenser reaches about 94%, and the oxygen content in the fraction is about, gradually open the non-condensable gas discharge valve of the crude argon condenser (this valve must be closed when it stops in ASU to prevent a large amount of nitrogen from entering the crude argon Tower after the air separation stops to increase the nitrogen discharge time when the crude argon Tower starts, delay the start of argon Tower ). The opening degree of the discharge valve should not be too large to prevent the liquid ribbon from entering U-shaped bend due to the too large opening degree. The judgement basis for judging the aggravation of the liquid ribbon is the continuous slow decline of the pressure at the discharge pipeline.
D) after the load of the crude argon tower is added up, the slightly higher oxygen content in the fraction must be maintained to avoid the nitrogen plug in the starting process, and at the same time, the opening degree of the non-condensing gas emission valve must be reduced gradually, in order to minimize the loss of argon and accelerate the start-up speed of argon system.
E) after the start of the crude argon Tower, the liquid at the bottom of the crude argon tower can be imported into the upper tower. Be careful not to affect the purity of oxygen.
4
The effect of starting the new plan of crude argon Tower:
This new start-up scheme was immediately put into practice on the 21000Nm 3/h air separation plant, which smoothly realized the start-up of the crude argon tower and solved the previous problem of difficult start-up, the new operation method also avoids the possible transformation of the discharge pipeline of the crude argon Tower, reduces the maintenance time and cost, and also reduces the gas supply loss during the period of parking and maintenance, it improves the operation efficiency of the factory.
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Summary:
In the air separation plant, there will always be some situations that the operation parameters of the unit are abnormal due to the design and manufacturing reasons, which will affect the normal judgment of the failure causes by the process personnel. As long as we collect wisdom and do not miss any link, we can study and find out the reason why the operation parameters are abnormal and work out the most economical and effective solution. In order to maximize the operation benefits of the factory.
