14.1 Introduction

In the above reactions, the first and third are exothermic reactions but the second reaction is endothermic. The reactions are carried out in vapour phase.
Catalyst: Silver or zinc oxide catalysts on wire gauge are used.
Operating temperature and pressure: Near about atmospheric pressure and 500 – 600°C

14.2.2 Process Technology (Figure 14.1):

• • Air is sent for pre-heating using reactor outlet product and heat integration concept.

• Eventually heated air and methanol are fed to a methanol evaporator unit which enables the evaporation of methanol as well as mixing with air.  The reactor inlet temperature is 54°C.

• The feed ratio is about 30 – 50 % for CH₃OH: O₂

• After reaction, the product is a vapour mixture with temperature 450 – 900°C

• After reaction, the product gas is cooled with the heat integration concept and then eventually fed to the absorption tower.

• The absorbent in the absorption tower is water as well as formaldehyde rich water.

• Since formaldehyde rich water is produced in the absorption, a portion of the rich water absorbent solution from the absorber is partially recycled at a specific section of the absorber.

• From the absorber, HCHO + methanol rich water stream is obtained as the bottom product.

• The stream is sent to a light end stripper eventually to remove any light end compounds that got absorbed in the stream.  The vapors from the light end unit consisting of light end compounds can be fed at the absorption unit at specific location that matches with the composition of the vapors in the absorption column.

• Eventually, the light end stripper bottom product is fed to a distillation tower that produces methanol vapour as the top product and the bottom formaldehyde + water product (37 % formaldehyde concentration).

Figure 14.1 Flow sheet of Formaldehyde production