Replacement waste heat boilers in plants for the production of nitric acid, formaldehyde and caprolactam

Nitric acid is produced by means of catalytic oxidation of ammonia to nitric oxides. The catalyst used is a finely woven platinum gauze with approximately 15 % rhodium. The oxidation is an exothermic process in which the gas is heated to approx. 900 °C. The process operates under a pressure of about 5 bar (500 kPa). The released thermal energy is recovered by cooling the gas to 300 °C, which in turn generates and superheats steam. Superheated steam temperatures of up to 500 °C are possible. The usual steam pressure levels are between 40 bar (4,000 kPa) and 80 bar (8,000 kPa).

The integrated waste heat boiler is of a forced circulation design to ensure the necessary compactness. The heating surfaces are either flat spiral elements or meander coil elements in a square section gas duct. Usually evaporator, superheater and economizer are arranged in one shell. In larger plants, however, the feed water preheating is sometimes outside the waste heat boiler. A different design comprises a spirally coiled superheater in front of an evaporator as a two-pass fire-tube boiler

Among others, formaldehyde is produced by the „Silver Contact Process“. The feedstock for the reaction is a gas mixture of methanol, water and air. The reaction type is a catalytic oxy-dehydration, which heats the product gas to 600 to 700 °C.

The reactors used to produce the formaldehyde have an integrated waste heat boiler to recover the heat reaction from the gas. Saturated steam is produced when the gas cools down to 150 °C.

The waste heat boiler is designed as a fired-tube boiler. The produced steam will be delivered to the main process. The feedstock enters the reactor in the upper section with the catalyst, the product gas is cooled down in the subsequent tube bundle. Usual steam parameters for pressure and temperature are 20 bar (2,000 kPa) and 200 °C.