Formaldehyde is a main constituent for manufacturing valuable chemical like melamine, urea and phenolic resins. It is available in 37% aqueous solution. It is identified by some of it properties like molecular weight about 30.3, boiling point -118 oC, melting point at -19 oC and density of 0.815 at -20 oC.
Formaldehyde Manufacturing Technologies:
Out of two important methods of manufacture formaldehyde in large scale, methanol process route is optimized and established for industrial production.
- Non catalytic oxidation of methane/ butane or propane.
- Dehydrogenation of methanol by catalytic oxidation.
- Silver catalyst process
- Metal oxide catalyst process
Catalytic oxidation process is a optimized production method. Let us discuss about the technology in this process. It is a simple process as per the stoichiometric reaction steps shown below.
1. Oxidation of methanol with oxygen present in air
- CH3OH + ½ O2 → HCHO + H20 ∆H = -37 Kcal
2. Pyrolysis endothermic reaction
- CH3OH → HCHO + H2 ∆H = +20 Kcal
3. Side reaction – complete combustion producing heat energy
- CH3OH + 3/2 O2 → 2H2O +CO2 ∆H = -162 Kcal
Brief process description of methanol dehydration for formaldehyde production:
Silver Catalyst Method: Initially Air compressed to pressure 0.2 atm by an air compressor and fed to the bottom of methanol vaporizer. The ratio of methanol and air maintained about 35-45%. This mixture heated to the reaction temperature 550-600 oC by series of preheater before entering into the silver catalyst reactor. The catalytic reactor is fixed bed type filled with silver catalyst used for converting methanol to formaldehyde. The fraction of conversion depends on the catalyst type and the temperature of the reactor at fixed operating pressure. Advance reactor controls both reactions, exothermic combustion reaction and endothermic dehydration reaction simultaneously in single step.
The product stream sent to purification and recovery section. Unreacted methanol fed back to the process at methanol vaporizer. Recycle stream contain 15% unreacted methanol of feed to vaporizer. Formaldehyde obtained as heavy end of alcohol stripper column. It is in the form of aqueous containing 63% water. Overall process has yield ranging from 85-90% on weight basis.
Technology challenges on formaldehyde catalytic oxidation are selection of catalyst and air to methanol ratio. When metal oxides are mixed, and then there are chances of obtaining greater than 90% overall yield of desired product. Reducing the concentrations of CO + CO2 in the streams is one of the demanding tasks for a process engineer.
Methanol stripper: In above flow diagram the unreacted methanol is stripped out from formaldehyde solution and recycled to the process. The down stream from methanol stripper requires further purification column to remove water and increase the purity of formaldehyde or adjust it concentration as per costumer order. However by using a single vacuum distillation column the separation can be achieved eliminating the two stripping columns.
Metal oxide catalyst process: Iron–molybdenum oxide catalyst replaced the costly silver improving the methanol conversion to 99% it surface is very much suitable for formation of H2CO. The reactions in the shell and tube reactor are completely exothermic. 250-345oC temperature is maintained in the reactor by removing the excess energy from reactor tube using dowtherm oil.