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 $35\backslash$deg C

and $45\backslash$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.