Paraformaldehyde plant , Paraformaldehyde equipment ,
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 products ——Paraformaldehyde 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/mol,heat of formation:177 KJ/mol,vapor
pressure:0.19Kpa/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
HO(CH2O)nH etc. Paraformaldehyde can be divided into tow types
which is low polymerization degree paraformaldehyde, and solid form
paraformaldehyde. Mainly use polymerization degree ——n( the
molecules number in the chain) for distinguishing, paraformaldehyde
with low polymerization degree, in which, n＜100, is easy dissolved
in water; depolymerization thoroughly. 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
⑴ Pesticides: synthesis of acetochlor, butachlor, and glyphosate.
⑵ Medicine and pharmaceutics: produce vitamin A.
⑶ Coatings: synthesis of high grade automobile paint.
⑷ 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.Rake 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 4-5m3,drying 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:
⑴Formaldehyde 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.
⑵In 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.
⑶Because 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 Unit:
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
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.
Section E. Main Consumption
Rake Type Dryer
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; and the single plant capacity can reach to
30000tons/year. Kai Mao technology has been included in the
national environmental friendly directory; at present, it is the
sole supplier of paraformaldehyde technology in domestic China, who
owns one-stop independent intellectual property rights including 1