According to related theories, almost all formaldehyde in formaldehyde aqueous solution exists in the form of monohydrate and polymeric hydrate. In the past, it was once believ
ed that most formaldehyde and water formed methylene glycol and existed in the aqueous solution, which provided some guidance for the formaldehyde concentration mechanism. Under the condition of 60 ℃, the total free formaldehyde of the formaldehyde aqueous solution accounts for 0.1 wt% of the total; under the condition of 100 ℃, the total free formaldehyde of the formaldehyde aqueous solution accounts for 1 wt% of the total, and the change in the content of free formaldehyde is consistent with the reaction mechanism of formaldehyde with water. of. In order to obtain a higher concentration of formaldehyde, it is necessary to achieve better separation of formaldehyde and water. At present, industrial production mainly uses two methods to achieve this goal:
Utilize the reaction of formaldehyde and alcohols
Known to be put into industrial applications are the process of DuPont of the United States and the process of Asahi Kasei of Japan. The DuPont process uses 50% formaldehyde solution to react with isooctyl alcohol to generate ethylhexyl semi-aqueous formaldehyde solution, which is dehydrated and thermally cracked to obtain high-concentration formaldehyde
Utilize the azeotrope of formaldehyde and water
At 100 ℃ and 1 atmosphere, the azeotropic composition of formaldehyde and water is 21%:79%, and the azeotropic temperature is about 96 ℃. The azeotropic composition increases with increasing temperature and pressure. When the temperature decreases and the pressure decreases, the formaldehyde content in the azeotrope decreases. When the pressure is lower than 26.6 KPa(A) and the temperature is about 65 ℃, the azeotrope phenomenon disappears.
Pressure concentration
Theoretically, pressurization can be used to increase the composition of formaldehyde in the distillate, so as to achieve the concentration of formaldehyde. However, when the concentration of formaldehyde is more than 45%, the steam that needs to be consumed and the severe corrosion of high-temperature and high-concentration formic acid gas caused by this Such problems have severely restricted the further increase of the concentration of formaldehyde. Therefore, the current industry mainly uses the pressure distillation process to recover the low-concentration formaldehyde solution as a raw material to prepare a formaldehyde solution with a concentration of about 45%.
Vacuum flash
Vacuum flash evaporation is currently the mainstream method of producing high-concentration formaldehyde solutions. It is operated in azeotropic area, and higher vacuum and lower temperature should be used as much as possible. In addition to azeotropic conditions, the storage and transportation of concentrated formaldehyde is also restricted. It depends on the concentration of the concentrated formaldehyde solution produced. At present, it is known that the highest concentration of formaldehyde obtained by vacuum concentration is 74%. Through industrial production practice, industrial production devices with 65% to 70% concentrated formaldehyde can already achieve long-term stable operation.
In addition, the process of using other azeotropic agents to preferentially azeotropically remove them with water, and then remove the azeotropic agent from the high-concentration formaldehyde solution to obtain a high-concentration formaldehyde solution has also achieved a certain scale of industrial production.