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Make a process flow diagram FORMALIN PRODUCTION Formalin is a 3 7 wt % solution of formaldehyde in water. Formaldehyde and urea are used to

Make a process flow diagram
FORMALIN PRODUCTION
Formalin is a 37 wt% solution of formaldehyde in water. Formaldehyde and urea are used to make
urea-formaldehyde resins that subsequently are used as adhesives and binders for particle board
and plywood.
Process Description
Formalin (37 wt% formaldehyde in water) is produced from methanol using the silver catalyst
process. Air is compressed and preheated, fresh and recycled methanol is pumped and preheated,
and these two streams are mixed to provide reactor feed. The feed mixture is about 39 mol%
methanol in air, which is greater than the upper flammability limit for methanol. (For methanol, UFL
=36 mol%; LFL =6 mol%.) In the reactor, the following two reactions occur:
The reactor is a unique configuration, in which the silver catalyst is in the form of wire gauze,
suspended above a heat exchanger tube bank. Because the net reaction is very exothermic, the heat
generated in the adiabatic reactor section must be removed quickly, hence the proximity of the
heat-exchanger tubes. The heat exchanger resembles a pool boiler, with a pool of water on the shell
side. If the temperature of the effluent is too high, the set point on the steam pressure line is
lowered to increase the vaporization of boiler feed water (bfw). In general, the liquid-level controller
on the bfw is adjusted to keep the tube bundle fully immersed. The reactor effluent enters an
absorber in which most of the methanol and formaldehyde are absorbed into water, with most of
the remaining light gases purged into the off-gas stream. The methanol, formaldehyde, and water
enter a distillation column, in which the methanol overhead is recycled; the bottoms product is a
formaldehyde/water mixture that contains <=1 wt% methanol as an inhibitor. This mixture is cooled
and sent to a storage tank, which is sized at four days capacity. This storage tank is essential because
some of the downstream processes are batch. The composition in the storage tank exceeds 37 wt%
formaldehyde, so the appropriate amount of water is added when the downstream process draws
from the storage tank.
Storage of formaldehyde/water mixtures is tricky. At high temperatures, undesirable polymerization
of formaldehyde is inhibited, but formic acid formation is favored. At low temperatures, acid
formation is inhibited, but polymerization is favored. There are stabilizers that inhibit
polymerization, but they are incompatible with resin formation. Methanol, at concentrations
between 5 wt% and 15 wt%, can also inhibit polymerizaton, but no separation equipment for
methanol currently exists on site, and methanol greater than 1 wt% also causes defective resin
production. With <=1 wt% methanol, the storage tank contents must be maintained between 35\deg C
and 45\deg C.
Reaction Kinetics
Due to the very high temperature and large surface area of the wire gauze, the reaction
may be considered to be instantaneous.

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