After the feed gas for low-temperature methanol washing is cut off, cold air leaks downward
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Currently, to reduce the cost of installing such units, the upper part of the absorption towers used in low-temperature methanol washing processes is typically made of low-temperature steel (LTS), while the lower part is made of low-temperature carbon steel (LTCS). However, once the feed gas supply is cut off, the low-temperature methanol in the upper part of the tower transfers its cold energy to the lower part. The low-temperature tolerance of LTS is generally around -80°C, whereas that of LTCS is around -45°C; as a result, the lower part of the tower ends up being exposed to temperatures beyond its acceptable range. I would like to ask those with more experience how this issue can be addressed Thank you.
You’ve heard that it’s a slow process; the cooling unit also reduces its load, and the nitrogen used for gas lift decreases as well. As a result, the temperature of the lean methanol doesn’t drop that low.
Reduce the amount of circulating methanol to lower the load on the chiller, decrease the nitrogen used for stripping, and adjust the amount of methanol fed into the upper column.
Our factory has encountered this situation before as well; once the gas was cut off, it immediately cooled down. The pipeline for extracting carbon-rich methanol from the middle section of the tower is equipped with a -35°C interlock to spray methanol from the top. Shutdown of low-temperature methanol washing. Later, due to a shortage of staff, the management decided to keep the methanol circulation running despite the difficulties, and thus adjusted the interlock setting to -45 degrees. After the gas cut, the methanol circulation rate dropped to 100 T/h. The liquid level valves of each cryocooler are closed, and the system is filled with nitrogen. Maintain liquid level balance in all towers and tanks.
Won’t reducing the nitrogen used for stripping cause overpressure in the thermal regeneration tower?
Normal shutdown is slow, but what if it’s necessary to cut off the feed gas urgently?
How is that possible? By reducing the amount of nitrogen, the amount of carbon dioxide absorbed by the methanol during the shutdown process also decreases. So there won’t be overpressure, but you also need to reduce it slowly; you can’t cut it down all at once.
May I ask again, what if the raw gas is urgently removed?
With emergency shutdown, thermal regeneration won’t cause overpressure either.
Perhaps I didn’t explain it clearly enough. Under normal production conditions, if the feed gas supply is interrupted due to issues upstream or for other reasons, the circulation rate of methanol needs to be reduced. However, if the amount of nitrogen used for stripping is decreased, components such as CO2/H2/CO that are dissolved in the H2S concentration tower cannot be completely stripped away; they end up in the thermal regeneration tower, where they are released from methanol under the heating effect of the reboiler at the bottom of the tower. This results in an increase in the volume of gas at the top of the tower, causing overpressure in the thermal regeneration tower. Therefore, I don’t understand why the amount of nitrogen used for stripping should be reduced. I hope this explanation is clear now.