Weekly Topic 3.28, Issue 20: Investigation on the Non-condensable Gas Scrubber in Methanol Distillation
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As the title suggests, I would like everyone to share information on the operation of methanol distillation non-condensable gas scrubbers, as well as details regarding the design of such equipment!
Three-tower process! The temperature of the non-condensable gas is 42°C; the wash liquid at the bottom of the tower can be sent to the resolver, or to the pre-cooling reflux tank, or even to an underground tank! Choose according to the actual situation!
Distillation capacity: 450,000 tons per year. The material of the wash towers is carbon steel; after about half a year of operation, corrosion and perforations occurred in the tower walls, and there were many such instances. The cause was attributed to an excessive level of CO2. Use clamps for temporary leak sealing; replace the equipment after major repairs!
This post was last edited by wu0406 on 2011-3-28 at 17:21. We keep the temperature around 40 degrees; the washing liquid can be fed into the analyzer, or into the pre-treatment reflux tank, or into the underground tank. The gas can be used as fuel or sent for compression
Due to the low pressure in the pre-tower, at around 40 degrees, the partial pressure of methanol is high. Especially in distillation systems under high load, the amount of methanol recovered is considerable. The common practice is to wash it with water; the high presence of CO2 can cause corrosion of the equipment. If an alkaline solution is added to the washing water, the methanol recovered in this way can easily cause the acid value of the product to exceed the specified limit. In winter, it is particularly important to maintain the temperature of the washing water. (Some methanol plants have very strict requirements, which need to be taken into account.)
Our control is around 40, though it’s usually around 30 degrees; however, it seems that either a higher or lower value doesn’t have much impact on the distillation process. The product obtained is sent partly to the fusel oil tank and partly back to the reflux tank ~
Here, we have designed a liquid inlet to the pre-coating return tank, or an underground tank； The gas was directly vented to a higher point
Generally, non-condensable gases are sent to heaters or other locations as fuel; releasing them into the atmosphere is quite wasteful! However, non-condensable gases tend to cool down during transportation, which causes some of them to liquefy; when these liquefied gases enter the heating furnace to burn, it results in a fire storm. We have experienced this phenomenon twice here!
Ours is quite wasteful; it should be part of an earlier process. The vehicle was put into operation in 2005, and during the first cooling stage, the temperature is maintained at 53–58 degrees before the fluid enters the return tank. During the second cooling stage, the temperature is kept as low as possible. After condensation, the liquid goes into the fusel oil tank, while the non-condensable gases are sent to the water seal tank for absorption, thereby preventing air from entering.
It’s of course possible to burn this mixture since there is condensate present, but a separator needs to be added afterward; otherwise, the liquid content will be too high. In my opinion, it’s better to send the liquid at the bottom of the tower to the fusel oil tank, as the components in that liquid aren’t clearly identified, and they could cause the product to fail quality standards, which would be problematic
The non-condensable gas from our pre-distillation column first passes through a non-condensable gas cooler; the cooled liquid is then returned directly to the pre-reflux tank, while the uncooled gas goes through an additional non-condensable gas superheater to ensure that it does not condense during transportation. The heat source for this process comes from the condensate of the pre-column reboiler. Non-condensable gas is sent to the fuel gas network as fuel for the heating furnace.