【Weekly Topic】Week 25, 2011——Short shutdown of air separation unit: What actions should be taken before shutting down production?

The liquid level inside the tower should be increased as much as possible to ensure an adequate supply of cooling capacity for use during startup.

This post was last edited by you*n*nyou on 2011-6-21 at 19:03. 1. First, cut off the connection between oxygen and the downstream system; then separateively cut off the connections for nitrogen and pressurized nitrogen. This prevents any impact on the operation of the downstream system due to oxygen separation unit operations. 2. After all connections to the downstream system have been severed, the oxygen production can be reduced. The method involves gradually shutting down the frequency converter of the liquid oxygen pump while also stopping the oxygen pump’s pressure relief function. 3. At the same time, use the high-pressure throttle valve to control the temperature in front of the expansion machine as well as its speed. Pay attention to the pressures in the second and third stages of the compressor, as well as the exhaust pressure of the air compressor. 4. Once the frequency converter of the liquid oxygen pump has been turned off to its initial setting, shut off the oxygen pump’s pressure relief function. The expander should also have reached its critical speed by now, so it can be stopped. The principle is to operate slowly while ensuring that the booster does not experience overpressure. 5. Transfer all the liquid oxygen from the lower tower to the upper tower; this can reduce the startup time of the air separation system. 6. Once the air separation unit has almost no load left, it’s possible to prepare to stop the compressor system. The specific procedures aren’t detailed here; only a general overview is given, as there are many additional details that won’t be covered one by one. In short, during short interruptions in air separation, it is necessary to ensure that overpressure does not occur and that the normal operation of subsequent processes is not affected. This is my personal opinion; if there are any mistakes, please feel free to point them out

If the goal is to drive faster next time, then it’s important to maintain a high speed during the gas release process; of course, this must be done while ensuring that the unit can adjust properly, and avoiding the situation where all the liquid in the main cooler and the upper tower is used up during this process； The valve leading to the storage tank was closed immediately. The liquid level in the argon tower can be set a bit higher; this way, the purity of argon will reach the required standard more quickly during the next activation of the argon system ； Try to send the liquid from the lower tower to the upper tower; this is safe and can also speed up the next startup time.

The liquid level of the main cooler can be lowered in advance to leave some space for the liquid coming from the upper tower packing and the lower tower.

During a short-term shutdown of the air separation unit, first close the liquid oxygen and liquid nitrogen valves; release the high-pressure oxygen first, then gradually reduce the oxygen production volume. At the same time, lower the flow rate at the second stage of the compressor and reduce the expansion amount of the expander, as well as narrow the valve that allows nitrogen from the upper tower to flow into the water cooling tower. During operation, it is necessary to pay attention to the outlet pressure of the air compressor, the inlet temperature of the expander, the outlet pressures of the first and second stages of the booster pump, as well as the temperature difference at the hot side of the high-pressure plate heat exchanger. Once the outlet pressure of the high-pressure oxygen pump drops to around 30 BAR, the expander should be stopped; at the same time, the valve for liquid nitrogen return to the lower tower should be closed, and the valve connecting the lower tower to the upper tower should be opened. The valve for pressurized nitrogen gas leaving the cold box should also be closed. After the expander stops operating, the booster pump is unloaded, and the valves connecting the upper and lower towers are adjusted (the valve for liquid nitrogen in the lower tower can be closed once there is no more liquid in it). After the valve for liquid nitrogen return to the lower tower is closed, the valve for waste nitrogen gas and the valve for pressurized nitrogen gas are also closed, thereby allowing the gas inside the tower to be released. Finally, the valve for low-pressure air entering the tower should be closed. In the control room, it is important to monitor the outlet pressure of the air compressor. After that, the molecular sieve and the pre-cooling system can be shut down. As for the speed of the next cold-start operation, as long as there is sufficient cold capacity inside the tower – this is achieved by first removing oxygen and then stopping the expander – it will be possible to maintain enough cold capacity in the tower. Secondly, it depends on the operating procedures.

Try to maintain cold conditions, but be aware that liquid evaporation can cause pressure to rise