Formaldehyde process: steam production 4

[roker]

Hello,

In Formaldehyde plants (with metal oxide catalyst) steam is produced in a heat exchanger by condensing vapors of a thermal fluid which is circulated in the Formaldehyde reactor jacket.

Can someone clarify why the water is not circulated directly in reactor jacket?

regards,
roker

 

[katmar]

If water were used directly it would have to kept under pressure to avoid having it boil in the reactor. I forget the actual operating temperature of the reactor, but it is well above 100 C. Formaldehyde reactors are large and to make this as a pressure vessel would be very expensive. It is more economic to use a heat transfer fluid and then produce steam in a smaller unit.

There may also be statutory factors. Here in South Africa a boiler is legally defined as "a device generating vapours from a source other than steam". This would make the reactor a boiler, with all the rules and regulations that go with it. It is easier to satisfy the regulations (at least here in SA) in a separate vessel.

 

[cmark]

It comes down to the selective properties of the catalyst. Normal operating range for the mixed oxide catalyst is 257 to 308 C. Diphenyl Oxide/Biphenyl Blend with an atmospheric boiling point of 257 C is used as the thermal fluid.

HCHO plants use shell and tube reactors with anywhere from 10,000 to 18,000 tubes. Typically these are 5.0 ft x 1.0 inch OD, 13ga and set at a pitch of about 1.25 inch. Max operating pressure using the oil is about 2.6 bar, with water you would require in excess of 85 bar!