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What is the pH value of the wastewater at the bottom of the methanol-water separation tower? If it is acidic, is it necessary to add alkali? Should this be done intermittently or continuously?
Alkali should be added to prevent acidic corrosion of the tower bottom and the pipelines carrying the residues from there. The acidity of the methanol-water solution varies from one plant to another; therefore, the amount of alkali required also differs. It is sufficient to maintain a pH level of around 8. If the feed volume to the distillation tower is high, continuous addition of alkali can be used, while if the feed volume is low, intermittent addition is appropriate – the choice depends on the actual conditions. The above guidelines apply to the methanol distillation used in dimethyl ether production. In the case of methanol production from coal, the pH level at the tower bottom is generally kept around 6, as it needs to remain acidic; otherwise, the acid value of the product might exceed acceptable limits.