Paraformaldehyde production technology
After years of development, China's formaldehyde industry has made great progress in production scale, output, quality and technology, and has reached international advanced level in many aspects. As the downstream product of formaldehyde, paraformaldehyde has a broad space for development.

Introduction of paraformaldehyde properties

Paraformaldehyde referred to as PF, is a kind of white powder or granular solid. It is an important linear polymer in chemistry. The formula is HO·(CH2O)n·H, density 1.39 (water = 1), melting point 120-170 ℃, flash point 71 ℃, the ignition temperature 300-410 ℃, combustion heat 510 kj/mol, and generate heat 177 kj/mol, and the vapor pressure 0.19 kPa / 25 ℃, dust lower explosive limit is 40 g/m3. Paraformaldehyde will slowly decompose into formaldehyde gas at normal temperature, and form formaldehyde solution after depolymerization, so it has the general characteristics of formaldehyde. Paraformaldehyde is a mixture of 8-100 molecular units of formaldehyde - Polyoxymethylene glycol HO(CH2O)nH.

Paraformaldehyde has two types: low polymerized paraformaldehyde and solid paraformaldehyde, mainly by the degree of polymerization of formaldehyde - chain number of formaldehyde molecule n, low polymerized paraformaldehyde n < 12, water soluble, fully depolymerized, loose product with uniform particles, is the ideal raw material of pure formaldehyde as well as good substitute industrial formaldehyde, it has great potential in development.

Application of paraformaldehyde

Paraformaldehyde is mainly used in
( 1 )  Farm chemical: synthesis of ethyl oxalamide, butyl oxalamine and glyphosate
( 2 )  Medicine: produce vitamin A
( 3 )  Coating: synthetic high-grade automotive paint
( 4 )  Resin: used in synthetic resin (such as ion exchange resin, melamine resin, etc.), of which 70-80% is used in the production of pesticide synthetic glyphosate herbicide. In the resin industry, paraformaldehyde is used to replace 37% of industrial formaldehyde to produce phenolic resin, which can not only produce high-quality thermoplastic and thermosetting phenolic resin, but also greatly reduce the discharge of waste water and material consumption by more than 10%.

Paraformaldehyde production process

At present, there are two production processes of paraformaldehyde: one is vacuum rake dryer process; the other is spray dryer process.

Rake dryer process

The raw formaldehyde is firstly dehydrated for concentration to increase from 37% to 65%. Then it is sent to the rake dryer for further dehydration and drying until the powder is discharged. The whole dehydration process is operated under negative pressure. The evaporative condensate containing 10-15% dilute formaldehyde returns to formaldehyde section, which is configured as 37% formaldehyde for export or to produce methylal and hexamine. This process is a batch operation, each feed to rake dryer is 4-5m3, drying time is 4-6 hours, the processing cycle is long with low output, and product is powder block.

Spray dryer process

Formaldehyde is repeatedly concentrated under the vacuum conditions to increase the concentration up to 85%, and then pumped into spray dryer. Concentrated formaldehyde is ejected from the nozzle and condensed into granular paraformaldehyde solid. This process will produce low polymerized formaldehyde molecules chain with continuous production process, short time concentrated formaldehyde spray granulation and the product has good water-solubility.

Production technology of low polymerized paraformaldehyde

Low polymerized paraformaldehyde is carried out according to the following equation


In order to obtain paraformaldehyde with low polymerization, the following problems must be noted:

( 1 ) When formaldehyde concentration is processed, dehydration, polymerization and depolymerization are carried out at the same time. If the process condition is not well controlled, it is easy to cause big loss of formaldehyde or block polymerization system.
( 2 ) Formaldehyde content is low in the process of concentrated drying, water content is large, and the product is sticky and easy to agglomerate.
( 3 ) Due to the small amount of methanol, formic acid and other polar molecules in the raw formaldehyde, when these substances are concentrated and solidified, they will lead to the continuous polymerization of paraformaldehyde and the formation of insoluble polymers.

Formaldehyde concentration

Formaldehyde solution is a kind of copolymerized mixture: it is mainly composed of methylene glycol CH2(OH)2, polyoxymethenyl alcohol HO(CH2O)nH and hemiacetal HO(CH2O)n-1H. Monomeric formaldehyde is very rare and does not exceed 0.1% (wt) even in concentrated formaldehyde solutions. The water in formaldehyde solution is in two forms: free water and combined water. Combined water is the concentration of water in formaldehyde hydrate molecules, which is related to degree of polymerization and concentration of formaldehyde (refer to the following table).

Relation table of combining water and degree of polymerization  ( Unit wt% )

In the concentration process of formaldehyde solution, with the evaporation of free water, the condensation polymerization of methylene glycol is also in progress to generate a series of paraformaldehyde HO(CH2O)nH with different degrees of polymerization. However, in addition to the evaporation of free water during the concentration process, methylene glycol molecules themselves will polymerize to form high degree of polymerization of paraformaldehyde. The concentration of formaldehyde is related to the temperature of the polymer desorption from the solution. Therefore, it is very important to control the temperature to prevent the polymerization of formaldehyde. If formaldehyde reacts at over high temperature, cannizzaro reaction will occur, so the concentration of formaldehyde with vacuum dehydration is generally applied.

High formaldehyde concentration solidification
The relationship between the curing temperature of formaldehyde solution and the concentration of formaldehyde indicates that over high temperature will even lead to the simultaneous evaporation of formaldehyde and water followed with big loss of formaldehyde. If the temperature is too low, the dehydration rate of formaldehyde is slow, and it is easy to cause the deep polymerization of formaldehyde, and the water solubility of the product is poor, so the temperature control during polymerization drying is the key point in operation.

Equipment structure

When designing spray tower, what exerts great influence on product quality will be related to spray pressure, spray angle, spray tower diameter, height, material staying time in the tower, cooling medium temperature, flow rate and so on. For example, concentrated formaldehyde curing temperature closely related to the height of tower; Overheated spray temperature will lead to soft and sticky particles, easy scaling and affecting continuous operation.

 Paraformaldehyde flow chart

Main consumption index

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