An important ingredient in the chemicals used to produce resins, adhesives, plywood and particleboard, insulation, and lubricants, formaldehyde is produced commercially by the catalytic oxygenation of methanol. The production of formaldehyde from methanol using a silver catalyst is performed at high temperatures and yields formalin, a 37% solution of formaldehyde in water. In a formaldehyde production process, a catalyst of iron oxide is used with molybdenum or vanadium, also often yielding formalin, but it also can be optimized to produce concentrations up to 57%. The higher concentration can reduce transport and storage costs and can be later diluted to the desired concentration. A formaldehyde production process that uses metal oxide catalysts is considered more advanced and is common in high volume, high technology manufacturing facilities. One common metal oxide catalyst formaldehyde production method is the The Formox® process.
Production of Formaldehyde Using Metal Oxide Catalysts
This process, performed at a reactor temperature of 250 to 500 degrees Celsius, oxidizes methanol efficiently, converting 98 percent or more:
2 CH3OH + O2 → 2 CH2O + 2 H2O
Therefore, less methanol is required to achieve better results to those in the silver catalyst process. This is advantageous because feed stock is a major input cost in formaldehyde production. In this formaldehyde production process, the feed mixture contains low quantities of methanol compared to the steam and air, keeping it below the explosive range. As the vast majority is converted to formaldehyde, the resulting solution remains relatively pure, with only minimal amounts of carbon monoxide, dimethyl ether, carbon dioxide, and formic acid byproducts.
Vaporized methanol, air (and sometimes tail gases from previous cycles) is passed over the iron oxide catalyst inside the reactor. Transfer fluid captures the heat emitted during the conversion of the oxygen and methanol into formaldehyde. The heat is converted to steam, which can be used for other plant purposes, such as the heating of resin if it is produced at the same location. The gases – including formaldehyde travel to an absorption chamber, where it condenses to an aqueous solution in the water that is fed into the chamber. After anion exchange reduces the formic acid content of the solution, the resulting product may contain up to 55% formaldehyde by weight, depending on the quantity of water introduced into the absorption chamber.
Production of Formaldehyde from Methanol using Silver Catalyst
Performed at a reactor temperature of 600 degrees Celsius and above, the silver catalyst reaction is less efficient that the iron oxide process. That is because, while iron oxide converts nearly all the methanol according to the stoichiometry noted above (2 CH3OH + O2 → 2 CH2O + 2 H2O) the silver process converts some methanol through a dehydrogenation reaction:
CH3OH → CH2O + H2
In contrast with the formaldehyde production process that is designed to convert nearly all the methanol, the silver process is conducted with a higher concentration of methanol, which consumes nearly all the oxygen. Manufacturing plants using silver catalysts use one of two sub-processes:
(Nearly) Complete Conversion of Methanol
Heating the feed stock to approximately 700 degrees Celsius increases the rate and equilibrium of the endothermic dehydrogenation reaction sufficiently to convert 97% to 98% of the methanol. After cooling to shut down undesirable side reactions, the vaporous mixture enters an absorption column, which elutes the formaldehyde to 40-55% formaldehyde by weight, with small amounts of aqueous methanol and formic acid (created from excess oxygen present during formaldehyde production).
Incomplete Conversion and Distillate Recovery
Feed stock is heated to around 630 degrees Celsius to minimize the undesirable secondary reactions. Formaldehyde production consumes all the oxygen, but the methanol is converted at only 77-87%. Again, the gases are cooled and sent to an absorption column. The aqueous product contains about 42 percent formaldehyde by weight. Because the solution may contain 20 percent or more unconverted methanol, it is moved to a distillation column to strip out this unreacted gas for recycling into the process. The stripped solution can be further processed to reduce the formic acid content and increase the concentration of formaldehyde up to 55% by weight and a yield of more than 90%. Some manufacturers install a second catalytic to convert more methanol, avoiding leakage and eliminating the need for distillation and recycling.
Comparing and Contrasting the Methods
We have noted that the iron oxide formaldehyde production process is the more efficient method for producing formaldehyde from methanol. However, plants using the silver catalyst process may present lower operating costs, apart from feedstock expenses. Therefore, if a cheap source of methanol can be obtained, silver may be the better option, especially for smaller operations. There are several other variables involved in deciding which formaldehyde production process to employ:
- Plant Capacity – Low-capacity formaldehyde manufacturing facilities – up to 5,000 tons per year production of formalin probably will be more economical using the less technical silver process. Larger plants up to 100,000 tons per year are best suited for the metal oxide catalysts formaldehyde production process, where added capacity can help recoup the higher capital expenditure on technology. Note that the excessively large gas conducts that would be required make facilities over 100,000 tons per year inefficient. It is recommended that large operations split production among medium-sized metal oxide processing units.
- Catalyst Cost – Though they must be replaced a few times a year, silver catalysts cost less overall than iron oxide catalysts that last for a year or more. That’s because silver catalysts can be fully regenerated while only the molybdenum can be recovered from iron oxide catalysts.
- Tail Gas Processing – Tail gases in the silver process contains about 20% hydrogen, which allows it to be burned, creating steam and eliminating carbon monoxide and other environmentally harmful organic compounds. Tail gas produced in the iron oxide process is not flammable, as the dimethyl ether, carbon monoxide, methanol and formaldehyde content is too low. Burning requires a catalytic incinerator or the addition of fuel.
- Steam Production –The metal oxide process produces sufficient steam heat to be exported from the production unit to power other processes. The relatively lower steam production in the silver process is fully consumed in methanol rectification – sometimes even this is insufficient, and steam must be fed from outside. There is never excess steam for export.
- Product Purity – Metal oxide catalytic production generally produces formaldehyde containing significantly fewer impurities such as formic acid, heavy metals, and unreacted methanol than the silver process. Industries increasingly require formaldehyde with zero methanol in the solution and concentrated (up to 4:1 formaldehyde to water ratio), urea-stabilized solutions. These products are only possible using metal oxide catalysts in formaldehyde production.
Metal Oxide and Silver Catalyst Technology Suppliers
- Haldor Topsoe
- Johnson Matthey
No matter what formaldehyde production process you choose, Phoenix Equipment is here to help you find the exact formaldehyde production plant to suit your needs. Buying a used plant can save you significant capital and drastically shorten the time required to build or expand a manufacturing facility. Even if we do not have a plant listed that matches what you are looking for, Phoenix has built an expansive network within the chemical and processing industry and we are confident we can help you find exactly what you are looking for. Contact us today to speak to one of our experts. We look forward to serving you.