- Manufacture of Formaldehyde from Methanol
Formaldehyde(HCHO) can be manufactured from methanol via two different reaction routes:
1. DeHydrogenation or oxidative dehydrogenation in the presence of Ag or Cu catalysts.
2. Oxidation in the presence of Fe-containing MoO, catalysts.
The oxidative dehydrogenation process with Ag or Cu metal is operated above the explosion limit with a small quantity of air.
In the oxidation process, a small amount of methanol is reacted below the explosion limit with a large excess of air. The thermal energy balance of the reaction is essential for process design in this manufacturing method.
Ag catalysts are preferred for the dehydrogenation and oxidative dehydrogenation of methanol. In the BASF, Bayer, Borden, Celanese, Degussa, Du Pont, ICl, Mitsubishi Gas, Mitsui Toatsu, and Monsanto processes the catalyst (silver crystals or gauze) is arranged as a few centimeter thick layer in the reactor. In the CdF Chimie process, silver is supported on carborundum. In the initial step methanol is dehydrogenated:
Hydrogen can be combusted exothermically on addition of air, resulting in the following formal equation for the oxidative dehydrogenation:
A less-than-stoichiometric amount of air is employed in the industrial process. The air is fed so that the reaction temperature remains constant (± 5 °C) in the range 600-720 °C. At temperatures of about 600-650°C, conversion of methanol is incomplete, and a methanol recycle is necessary. At higher temperatures of 680-720 °C and with the addition of H2O, there is almost complete conversion of the methanol in a single pass. The water has another favorable effect on the life of the Ag catalyst in that the steam delays the deactivation caused by sintering of the thin layer of red-hot silver crystals. The catalyst has a lifetime of 2-4 months and the spent catalyst can be easily regenerated electrolytically without loss of Ag. The catalyst is sensitive to traces of other metals as well as halogens and sulfur.
The hot gases from the reaction are very quickly cooled to ca. 150 °C and washed in a counterflow with H2O in several absorption stages. The solution is stabilized towards polymerization with a residual amount of methanol (1-2 wt%). A distillation can follow to produce concentrated formaldehyde solutions (37 to 42 wt%). The yield of formaldehyde exceeds 92% with a selectivity of over 98%.
The byproducts are CO and CO2. There is virtually no formic acid present.
In the oxidation process, the formaldehyde formation occurs practically as a pure methanol oxidation. A mixture of 18-19 wt% Fe2O3 and 81-82 wt% MoO3 is employed as catalyst. Under carefully controlled conditions, it is onverted into the catalytically active iron(Ⅲ)-molybdate.
Excess MoO3 is frequently added in order to make up for losses resulting from formation of molybdenum blue. This compound goes to the cooler end of the catalyst bed where it lowers both the catalytic activity and selectivity. Cr and Co oxide can be used as promoters.
In the industrial process, methanol vapor together with a large excess of air is passed over the catalyst in a tubular reactor at 350-450 °C. The heat of reaction is removed by a liquid which surrounds the tubes. After cooling to 100 °C the gases from the reaction are scrubbed with H2O in a bubble column. By adjusting the amount of water, the concentration of the formaldehyde solution can be varied between 37 and 50 wt%, or, with a new development from Nippon Kasei, up to a maximum of 55 wt%.
An aqueous urea solution can also be used in the column to absorb the formaldehyde and produce urea-formaldehyde precondensates, which can be converted to thermosetting resins. The methanol conversion is roughly 95-99% and the formaldehyde selectivity reaches 91-94%.
The byproducts are CO, CO2 and formic acid. The formic acid can be removed in a coupled ion exchanger.
The lifetime of the catalyst is roughly two years.
The Perstorp-Reichhold ('Formox'), Hiag-Lurgi, Montecatini, SBA, IFP-CdF Haldor Topsoe, Nippon Kasei, and Lummus processes were developed in accordance with this principle.
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