Paraformaldehyde Production Technology
After many years development, formaldehyde industry in China has
made big progress in production scale, output, quality and
technology, and has approached or reached the international
advanced level in many aspects, however, the formaldehyde down
stream productsParaformaldehyde has more development spaces.
Section A. Paraformaldehyde Properties
Paraformaldehyde, short form PF is a white powder form or particle
form solid. It is an important linear polymer, molecular formula
(CH2O)n,; molecular weight （30）n; density 1.39 (water=1.0), melting
point: 120-170℃; flash point: 71℃; ignition temperature: 300-410℃;
heat of combustion :510 KJ/molheat of formation:177 KJ/molvapor
19Kpa/25℃low explosion limit of powder dust: 40g/m3
.Paraformaldehyde will be slowly decomposed into gas form
formaldehyde at ambient temperature and will form formaldehyde
water solution. Therefore, it has general formaldehyde properties.
Paraformaldehyde is a mixture of formaldehyde molecule and
Paraformaldehyde can be divided into tow types which is low
polymerization degree paraformaldehyde, and solid form
paraformaldehyde. Mainly use polymerization degreen( the molecules
number in the chain) for distinguishing, paraformaldehyde with low
polymerization degree, in which, n＜100, is easy dissolved in water;
The product is loose form with uniform grains, which is the ideal
raw material for formaldehyde, and is excellent replacement for
industrial formaldehyde. Therefore it has big development value and
Section B. Paraformaldehyde Application
Paraformaldehyde is mainly used in
Pesticides: synthesis of acetochlor, butachlor, and glyphosate.
Medicine and pharmaceutics: produce vitamin A.
Coatings: synthesis of high grade automobile paint.
Resins: applied in resin production such as ion exchanging resin
and melamine resin etc. Among them, 70-80% of paraformaldehyde are
used to produce synthetic glyphosate herbicide; in resin industry,
use paraformaldehyde to replace 37% of industrial formaldehyde to
produce phenolic resin; not only can produce high quality
thermoplastic and thermosetting phenolic resin, also can greatly
reduce waste water discharge and lower materials consumption more
Section C. Paraformaldehyde Production Process
So far, there are two paraformaldehyde production processes. One is
vacuum Rake type drying process, another is spray drying process.
1.Harrow type drying process
Raw material formaldehyde is firstly concentrated and dehydrated
until the formaldehyde concentration is increased from 37% to
65%then, it is sent to the vacuum Rake type dryer for further
dehydration and drying to make powders. The whole drying process is
operated under vacuum condition. The evaporation condensates
contain 10-15% of diluted formaldehyde, which returned to
formaldehyde shop to form 37% formaldehyde for sale or produce
methylal or hexamine. It is batch form operation. Each batch feed
for Rake type dryer is around 4-5m3drying time: 4-6 hours. This
process has long production period, low capacity and the products
are powder or block form.
2.Spray Drying Production Process
Formaldehyde concentration, through two stages of vacuum
concentration, reaches 85%, then, is pumped to spray dryer. The
concentrated formaldehyde is sprayed out to form particles of
paraformaldehyde. This is a continuous process, in which,
concentrated formaldehyde becomes to be solids within short time.
It is able to form short linear formaldehyde molecule chain; as a
result, product has good water solubility.
Section D. Low polymerization Degree Paraformaldehyde Production
Low polymerization degree paraformaldehyde reaction is as
nCH2（OH）2 (CH2O)n + (n-1)H2O
To produce low polymerization degree paraformaldehyde must take
care of the following issues:
Formaldehyde dehydration, polymerization, depolymerization must be
performed at the same time in the formaldehyde concentration
process. If process condition is not well controlled, it is easy to
cause large amount of formaldehyde loss or formaldehyde polymerized
to clog the system.
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.
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