FORMALDEHYDE PRODUCTION PROCESSES
Formaldehyde is produced from methanol substantially by means of two types of processes: the oldest one is based on silver catalyst and is working at a methanol concentration above the explosion limit, the most modern one is based on metal oxides catalyst and is working at a methanol concentration under the explosion limit.
The main features of the two types of processes (the former called “S” and the latter called “MO”) are compared hereunder.
Of course the comparison has to be rather general, as also not all the various “S” processes available on the market are exactly the same, and similarly there are many differences between the various “MO” processes offered by the various know-how suppliers.
The formaldehyde production cost is generally made for over the 80% by the raw material consumption, and therefore the methanol yield is the most important item to be considered.
In the “S” process the methanol losses (there is the amount which is not converted in formaldehyde, as it is remaining unconverted plus the amount which is being oxidized to other substances) are usually the 10 to 12% of the feed.
In the “MO” process the methanol losses are usually the 7 to 8% of the feed, and with the most modern catalysts even the 6% of the feed.
As in the “MO” process the methanol concentration in the gas fed to the reaction is low, large amounts of gas have to be compressed, so that the energy specific consumption is usually higher in this process as in the “S” process.
The silver catalyst charge has to be replaced every 3 to 4 months and can be regenerated.
The metal oxide catalyst charge lasts from 12 to 18 months, but when exhausted cannot be regenerated: only the main metal (molybdenum) can be recovered to produce new catalyst.
Taking into account these different characteristics and the time involved in the charge replacement, in the two types of processes the catalyst cost influences in the formaldehyde production cost have similar values.
In the “MO” process steam is produced and exported from the production unit.
In the “S” process in the best situations the produced steam is totally used for the methanol rectification, and sometimes steam has also to be fed from outside.
Very small plants (up to 5000 t/y 37% solution) can be economically built only according to the “S” process (whose technology is widely known) but not according to the “MO” process (when the technology cost may be larger as the equipments cost).
Medium sized plants are usually less expensive when built with “MO” process than with “S” process.
Very large plants (over 100,000 t/y 37% solution) with the “MO” process are unpractical due to the too large gas conducts, and over this size can be better split in many separate units.
In the “MO” process the conversion of methanol in the reactor is practically complete and the product is obtained by means of a simple absorption.
In the “S” process the conversion of methanol in the reactor is usually only partial, so that the unconverted methanol has to be first absorbed and then rectified from the solution in order to be recycled to the catalytical reactor.
The reaction temperature is of 250° – 350°C in the “MO” process and of 550° – 700°C in the “S” process.
For both factors the former process is simpler than the latter.
Both process operates with a gas mixture kept outside the explosion limits, and very often reducing the oxygen amount with inert gas dilution for additional safety.
But in case of leak and contact of the reacting gas with the outside air, no harm derives in the “MO” process, but a dangerous explosive mixture is formed in the “S” process.
Also the total amount of methanol present in the plant is much lower in the “MO” process than in the “S” process, so that the intrinsic safety against fire and explosion of the former is higher than the safety of the latter.
Generally the amount of impurities (formic acid, methanol, formals, heavy metals) in the formaldehyde solution is much lower in the “MO” process than in the “S” process, and only in the “MO” process the widely required methanol-free formaldehyde can be obtained.
In the special cases when a methanol stabilized solution is desired, this solution may be better obtained in the “S” process (where in any case the methanol is contained in the formaldehyde solution and has to be extracted from it by means of rectification).
Only in the “MO” process, where no methanol is present in the absorption column, a concentrated urea stabilized solution can be directly obtained in the formaldehyde plant, practically without any extra cost. Instead such a solution cannot be directly obtained in the “S” process.
This concentrated solution, with a formaldehyde/water ratio up to 4:1 (instead of the maximum obtainable 1.5:1 of the “S” process) is used with many advantages when producing ureic resins or glue, as no water has to be expensively distilled from the product.
Such characteristic of the “MO” process is economically so important, that in the recent times almost every company in the world interested in urea glues production has chosen for its new units the “MO” process, also when having already available the know-how of the “S” process.