Low-temperature methanol washing maintenance

It has better water solubility; ferrous sulfide should be in the high-temperature area

I think so too. Do you have any actual experience in disassembling and inspecting them?

The reboiler in our thermal regeneration tower has been washed with water; since ferrous sulfide was still on fire, though not intensely, just a small flame, we had water pipes ready, and the fire was extinguished as soon as water was applied. Another time, even though water washing was carried out, the wastewater treatment system could not handle such a large volume of water, which limited the amount of water that could be used for washing. Still, one maintenance worker experienced vomiting symptoms. As a solution, a shut-off valve was installed on the methanol side to facilitate handling of the waste fluid. If you are able to treat the alcohol-containing wash water after washing, then go ahead and use it; but if the alcohol content is too high and an explosion occurs, that would be serious

On May 9, 2017, we had just completed a major overhaul, and all the equipment used in the low-temperature methanol washing process needed to be inspected. Initially, we planned to use water for cleaning, but the amount of wastewater generated was too large; this not only led to waste of methanol but also resulted in a high water content in the system after it started operating. Therefore, this method was abandoned. Later, the methanol in all the systems’ equipment and pipelines was drained, and without shutting down the air separation unit, a large amount of nitrogen was used for displacement and purging over a period of 6 hours. We used nitrogen purging via gas lift, with a flow rate of 12,000 Nm3/h. After completion of the displacement process, the four manholes on the tower were opened, and fans were used to create convection; the sample analyses showed that the H2S and oxygen levels were within acceptable limits, so the gas could be used directly, though it had a slight odor. We carried out hot work in both the CO2 separation tower and the H2S concentration tower. However, due to the large amount of methanol solution, the methanol regeneration tower was used to store methanol, and it was not displaced. I believe the methanol scrubber is relatively dangerous, as lean methanol enters the upper section of this tower; if ferrous sulfide is attached to the tower walls and air is introduced after the manhole is opened, methanol could catch fire. Last year, the tower wasn’t replaced properly; when the manhole was opened to allow convection, black smoke came out of it, so the manhole was sealed immediately.

For the major overhaul in 2016, we used this same method for maintaining the stripping tower by installing blind flanges to isolate it from other towers. But this year, each of our towers has reached its pressure vessel inspection deadline, and the washing tower, methanol-water separation tower, and thermal regeneration tower have all experienced various issues to some extent. Now we have decided to carry out a thorough cleaning using water flushing and displacement.

Spend some money on chemical cleaning. The total amount of waste discharged is only a few hundred tons; with each tower and heat exchanger operating at full capacity, there is no odor at all, and anyone can enter the equipment freely. It took 6 days from the shutdown of my methanol washing unit to its handover; ferrous sulfide was present in areas with high temperatures, such as the thermal flash drum, especially in towers with a packed structure.