Why is the refined methanol product alkaline?

It may be caused by a change in the quality of crude methanol.

Because when crude methanol is processed through distillation to produce pure methanol, it is necessary to make the methanol basic in order to remove organic acids, which facilitates their removal and thus allows the production of pure methanol; therefore, pure methanol is basic in nature

Do you produce dihydroxyalkanes? If so, it’s possible that ammonia was introduced into the synthesis tower; methanol is basic, so that’s likely what happened.

Methylamine, ethylamine, and the like in the crude alcohol have not been completely removed； Control of alkali addition amount

It’s probably because too much alkali was added during the previous dosing process. . . Check whether there is any change in the dosage of the chemical added, or in the concentration of the base being used. .

It is caused by the formation of methylamine, particularly dimethylamine and trimethylamine, from ammonia and methanol in the feed gas. When methylamine is introduced into pure methanol, it becomes alkaline. Ammonia in syngas may originate from carbonization processes. In the carbonization process, the ammonia washing in the recovery and cleaning tower is not thorough; this may also be due to the use of fresh gas from the ammonia synthesis process as the cooling gas in the diol tower, which often carries ammonia released from the copper solution. The fundamental measure is to prevent ammonia from entering the urea synthesis system. In many factories, the recovery and cleaning towers used in carbonization processes suffer from unsatisfactory ammonia removal efficiency due to poor tray design. A packed tower or bubble column with good gas-liquid contact should be used, and the tray plates must be regularly inspected and replaced to improve the ammonia removal efficiency. High-pressure hydrogen or nitrogen should not be used as the cooling gas in the syngas synthesis tower, as this leads to energy waste and may also introduce ammonia; instead, the gas entering the tower after syngas production should be used. Therefore, the cooling pipelines need to be carefully designed, with resistance levels that are not too high.

It should be due to an excess of NaOH added or changes in the product components

pH value is also one of the important indicators for assessing the quality grade of pure methanol products. The substances that affect the acidity and alkalinity of pure methanol mainly come from the synthesis process of crude alcohol. During the methanol synthesis process, when the moisture content of the syngas is high, the following reaction occurs: CO + H2O → HCOOH. Other side reactions may also take place during this synthesis: 2CO + 2H2 → CH3COOH, and CH3OH + CO → CH3COOH. In the production of dimethyl ether, factors such as the recycling of raw gas can lead to the presence of amine impurities in the crude methanol, particularly monomethylamine, dimethylamine, and trimethylamine. These substances are slightly basic, and their basicity increases as the number of hydrogen atoms in the ammonia molecule is replaced by alkyl groups. If such impurities are not removed thoroughly during the distillation process, it will result in a high alkalinity value of the purified methanol product, affecting its quality grade. Controlling the pH of pure methanol is mainly achieved through the operation of the distillation process. High acidity or alkalinity not only leads to substandard product quality but also easily corrodes equipment. Some manufacturers add an alkaline solution of NaOH with a concentration of 2% at the top of the pre-treatment tower; the amount added is generally 0.05% to 0.1% of the feed volume of crude alcohol in that tower, and it is adjusted according to the process requirement that the pH value of the crude alcohol should be maintained between 7 and 9. This is because adding an alkaline solution can not only neutralize the organic acids in the alcohol, but also promote the decomposition of organic amines and hydroxyl compounds, thereby allowing them to be removed further during distillation.

The refined methanol product has a high alkalinity – how can this be avoided? 1) Main reasons: a) The crude methanol feed liquid contains impurities such as methylamine (especially dimethylamine and trimethylamine); b) The extraction distillation in the pre-tower is not sufficient, and the condensation temperature at the top of the pre-tower is high. 2) Measures to avoid this issue: a) Improve the purification of the gas used in crude methanol synthesis to prevent ammonia from entering the methanol synthesis system, thereby avoiding the formation of methylamine; b) Control the extraction distillation process in the pre-tower as well as the condensation temperature at its top in accordance with the specified requirements; c) Increase the temperature of the air discharged from the main tower and reduce the height at which the refined methanol is extracted.