The issue of adding alkaline compounds in low-temperature methanol washing
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Linde Corporation suggests that \"to prevent corrosion of carbon steel equipment, alkaline compounds (sodium hydroxide, ammonia) can be added.\" (Amines, etc.)” Please share whether your respective facilities have adopted this measure If it is adopted, what will be the effects of its use, and what are the substances added? Join location? Amount to add?
What are the consequences of adding alkaline compounds to low-temperature methanol washing, along with carbon dioxide and hydrogen sulfide, in terms of chemical reactions?
Some devices available in China have addition and subtraction functions, while others do not. I think a plus/minus adjustment device should still be installed; our device doesn’t have one. The pH of the wastewater at the bottom of the dehydration tower is low, sometimes only 4–5, which still poses a certain degree of corrosion risk. Alkaline solutions are generally added directly to the dehydration tower, usually using NaOH.
Are there actually any devices that use alkalis to control pH in use these days?
Some units have it, but our unit does not; the inlet for adding alkali is usually located in the reboiler section of the methanol-water tower, that is, at the bottom of the methanol-water tower. This is used to keep the system slightly alkaline, 1. to prevent corrosion, and 2. some units state that maintaining a slightly alkaline environment helps prevent organic substances from undergoing polymerization reactions, which could otherwise cause blockages in the trays or heat exchangers. Personally, I lean more towards the first viewpoint
Generally, a dilute sodium hydroxide solution is used! There are few companies that add ammonia! Moreover, sodium hydroxide is usually added at the bottom of the methanol-water separation tower; the bottom of the tower does not contain hydrogen sulfide or carbon dioxide, so no reactions will occur, so you need not worry!
From the design of many units, it can be seen that most are equipped with alkali addition pipelines, but it is understood that many of them are not in use; and even when they are used, the alkali addition is intermittent rather than continuous! The specific reason is not very clear; please engage in active discussions! @
Alkali should be added to adjust the pH value; it is generally added in the methanol-water separation tower. In our design, ammonia is used for this purpose (added intermittently), as the higher the water content, the more severe the corrosion becomes!
I’ve heard that some organizations and companies suggest that keeping the ammonia content in recycled methanol at 30–60 mmol/L can effectively prevent corrosion in the system. In practice, the pH value can be increased by reducing the amount of water sprayed at the top of the ammonia scrubber, thereby allowing a small amount of NH3 from the raw gas to enter the methanol washing system. Is this method widely used? Friends who have read this post might as well share their thoughts openly!
The units in use don’t involve the use of alkali solutions; it’s the first time I’ve heard of that (due to my lack of knowledge, I still need to learn more*). But what properties does methanol possess, exactly?
Hehe, in methanol distillation, alkali solution is even added directly to the methanol!