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Paraformaldehyde Production Process
There are two kind of process for producing paraformaldehyde：one is vacuum rake drying production process,the other Is spray drying production process.
1.Vacuum rake type drying production process
Raw material FMD is concentrating firstly,and the concentration is increased from 37%to 65%.After that it is sent to rake dryer to secondary stage concentrating and drying until become powder form for discharging.The whole thickening process is under negative pressure, evaporative condensing liquid containing 10-15% dilute FMD go back to process section of FMD and then it is collocated to 37%FMD for selling or made into methylal,hexamine.This process is batch opreration,rake dryer dosing is 4-5m3 at each time,drying time is 4-6 hours,production cycle takes very long,production capacity is lower and also the products present in powder.
2.Spary drying production process
Formaldehyde goes through secondry concentration under vacuum,which concentration is increased to 85%,and then it is sent to sprary dryer by pump.Concentrated formaldehyde sprays out through nozzle and become grainy solid paraformaldehyde.This process is a continous production process and take very short time to concentrate papaformaldehyde to short liner paraformladehyde molecular chain,therefore it have good water-solubility.
Low Polymerization Paraformaldehyde Production Process
Low Polymerization Paraformaldehyde reaction as below：
To produce Low Polymerization Paraformaldehyde,we must pay attention to the following aspects：
(1)When FMD concentrated,the dehydration，Polymerization and depolymerization all happen at the same time.If the process conditions is not well controled, it is easy to cause large amount of formaldehyde loss or formaldehyde polymerized to clog the system.
(2)In the concentrating and drying process, if the formaldehyde concentration is not high enough, with relatively high water content; the product will be sticky and easy to be agglomerated.
(3)Because the raw material formaldehyde contains small amount of methanol and formic acid polar molecules, in the concentration process, these materials will lead to further paraformaldehyde polymerization and form insoluble polymers.
Formaldehyde water solution is a co-polymerization mixing solution; mainly contains methylene glycol CH2(OH)2; HO(CH2O)nH and methylal HO(CH2O)n-1H. No more than 0.1%（wt）formaldehyde monomers exist, even in the concentrated formaldehyde solution. There are free water composition and hydration water composition two types of formaldehyde water solution to be existed. Hydration water is namely the water concentration in formaldehyde hydration molecules, which is related to polymerization degree and formaldehyde concentration. (see the following table)
|Hydration water and polymerization degree relationship table
|water concentration in hydration composition
|Formaldehyde concentration in hydration composition
In the formaldehyde solution concentration process, along with the vaporization of free water, methylene glycol condensation polymerization reaction also takes place at the same time to form a series of different polymerization degree paraformaldehyde HO(CH2O)nH, but, besides free water vaporization, in the concentration process, methylene glycol itself also will occur self-polymerization to form high polymerization paraformaldehyde. The concentration of formaldehyde is related to the temperature, at which ,the polymer crystallized from the solution. So, it is important to control the temperature for preventing formaldehyde from polymerization. Formaldehyde will have Cannizzaro reaction at over high temperature, therefore, formaldehyde concentration usually use vacuum dehydration.
2.Concentrated formaldehyde solidification
The relationship between formaldehyde solution cooling solidification temperature and formaldehyde concentration indicates that over high temperature will cause formaldehyde and water evaporation at the same time, big formaldehyde loss will occur. Temperature too low also will cause formaldehyde dehydration process too slow and cause formaldehyde deep polymerization. The product will have poor water solubility, so, it is crucial to control polymerization and drying temperature.
When design the dryer, spraying pressure, spray angle, and the diameter, height of sprayer chamber, material retaining time in the tower, and cooling medium temperature, flow etc. all have big impact to the product quality. For example, the concentrated formaldehyde solidification temperature will affect the height of dryer; if spraying temperature too high it will cause the grain soft and sticky, easy scaling and affect continuous operation. Therefore, spray drying design is a systematic engineering work; copy normal dryer is not practical.
Flow chart of Paraformaldehyde production