C2 hydrogenation startup
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Before starting up, hydrogenation is carried out first; when starting up the carbon dioxide hydrogenation process, low-pressure nitrogen is used to maintain pressure initially, followed by the introduction of CO, then high-pressure nitrogen, followed by ethylene gas, and finally, the process is started up by filling it with cracking gas. Is this the correct sequence? What are the standards for introducing CO? And is this order of pressure application correct?
Specific data should be obtained by consulting the catalyst manufacturer; contact them in advance before starting up to obtain technical support.
Low-pressure nitrogen, CO, ethylene, high-pressure nitrogen; finally, gas charging is carried out via the bypass of the main line interlock valve. Regarding the injection criteria for CO, different catalyst manufacturers provide varying recommendations for the amount to be injected; it is not a fixed value.
Hello, I’ve seen many documents suggesting the sequence of low-pressure nitrogen filling → CO passivation → high-pressure nitrogen filling → pyrolysis gas filling; ethylene gas is not used in this process. Could you explain the role of ethylene gas during operation, as well as the advantages and disadvantages of not using it? I would like to ask: after the carbon dioxide hydrogenation process, low-pressure nitrogen is used for automatic cooling; why is it necessary to wait until the temperature drops to 80° before starting the cooling of propylene? What are the consequences of starting the cooling at a higher temperature?
A safer approach is to abandon the small bypass for the pyrolysis gas punching. After blowing with low-pressure nitrogen, increase the pressure of ethylene to 2.5 and that of nitrogen to 3.5. Open the four interlock valves in the sequence of inlet 1 then outlet 1, inlet 2 then outlet 2, and quickly retract the bypass. The first time, we used ethylene to reach a pressure of 2.5, high-pressure nitrogen to reach 3.2, and the pyrolysis gas to reach 3.5. Due to the interlock valves for inlet and outlet and untimely adjustment of the bypass, the pyrolysis gas entering through the small bypass remained in Bed A; the space velocity was too low, which caused the temperature to soar immediately. As soon as the main line was started and the large bypass was reactivated, the temperature rose again. One can only release it again and start over; on the second attempt, the small bypass was discarded, allowing for a smooth air intake. There are also disadvantages to this approach; too much nitrogen flow places a strain on the high-pressure tower and even on the downstream separation processes. Quenched propylene was added as soon as the bed temperature dropped to 100 degrees. The reason it couldn’t be added directly at the beginning was, in my opinion, the risk of cold extraction in the equipment or thermal degradation of the catalyst.
Ethylene gas is not necessary. We started operations from the beginning without using ethylene gas, and everything ran smoothly. Ethylene gas was used only later when flying and restarting. I think that if cooling by stamping alone is used, low-pressure nitrogen and high-pressure nitrogen are sufficient. It just takes a little longer; it will definitely cool down as well, and the pressure can also be reduced. Acrylic acid is added for rapid cooling, while high-pressure ethylene is used for compression; the purpose of this is to ensure that the subsequent processing steps proceed smoothly as quickly as possible, thereby avoiding an increased burden on the separation process. Acrylic acid returns to the second-stage tank, and ethylene, being the target product, can be recovered. Why use ethylene gas? I need to learn more about it*.
After pressurizing with pure ethylene gas and balancing it with the discharge pressure at the fifth stage, the reactor is reset and put back into operation. The catalyst addition process does not tend to cause excessive temperature rises. Pressurizing with nitrogen also presents issues regarding system purging, whereas pressurizing with pure ethylene gas solves this problem much better
My understanding is that ethylene is pressurized in the initial stage; at this time, the carbon dioxide hydrogenation process operates in a flare mode. It takes about an hour to adjust the carbon dioxide hydrogenation unit to operate properly, and during that time all the substances present are flared off, so there is no way to recover the ethylene that was pressurized. . There’s another issue: during the pressurization of the pyrolysis gas compression line, the flight temperature rises due to the low space velocity. The temperature of bed A is around 40° during pressurization; I remember that the initial reaction temperature should be 50°. Does the reaction take place at 40°?
This is when the torch is introduced, right? There shouldn’t be any need to send it to the separation unit; by the time the carbon dioxide hydrogenation process is complete, the nitrogen would have already been released. I’m not sure if my understanding is correct
Also, after the temperature rises, many experiences suggest that the bed temperature should be below 80° before propylene can be introduced. Introducing propylene ahead of time has two consequences: 1) it causes cooling of the catalyst, and 2) propylene undergoes hydrogenation and releases heat. It’s not clear which of these views is correct
More towards cold extraction. Due to the differences among catalyst manufacturers, as well as the low selectivity of carbon dioxide hydrogenation catalysts for propylene, cold extraction is a more reasonable approach.