Formaldehyde is a main raw material for manufacturing value-added chemicals and adhesives used in wood products such as particleboard and plywood. Colorless but odiferous and flammable, formaldehyde is a key component in melamine, urea-formaldehyde, and phenolic resins that have strong bonding and heat resistance qualities. Formaldehyde also is a potent preservative with uses in sterilization and embalming.
Commercial producers typically use two methods to manufacture formaldehyde at scale: 1) Oxidation-dehydrogenation using a silver catalyst to achieve either the complete or partial conversion of methanol; and 2) the FORMOX process, which directly oxidizes methanol using metal oxide catalysts. In most cases, production targets a 37% aqueous solution (called formalin), but manufacturers may market concentrations as high as 57%. The solution is usually transported with methanol added to prevent polymer precipitation. Solutions that do not include methane stabilizers can avoid precipitation only when kept at 86 degrees Fahrenheit or higher. The advantage is lower shipping costs, as the higher concentrations can be diluted to the desired 37% saturation at the destination.
FORMOX: Formaldehyde Production Using Metal Oxide Catalysts
Conducted in a controlled reactor environment at around 500 to 900 degrees Fahrenheit, converts nearly all the methanol feedstock into formaldehyde and water:
2 CH3OH + O2 → 2 CH2O + 2 H2O
This high efficiency – 98% or higher conversion – reduces waste and requires less of the expensive starter material than the silver catalyst process to achieve the same volume of formaldehyde.
FORMOX introduces feedstock containing lower methane concentrations than the surrounding steam and the steam and air to ensure a stable reaction and eliminate the possibility of explosion. The process creates highly purified formaldehyde containing only trace amounts of carbon monoxide, dimethyl ether, carbon dioxide, and formic acid byproducts.
The reaction works by injecting vaporized methanol, air (and sometimes tail gases from previous cycles) over the iron oxide catalyst inside the reactor. As the methanol reacts with oxygen inside the tank, transfer fluid captures the heat generated and uses it to boil water and create steam for powering other industrial processes. Meanwhile the gases – mostly newly converted formaldehyde travel to an absorption chamber, where they condense and are absorbed into water fed into the chamber. An anion exchange reduces the formic acid content of the aqueous solution, the resulting product may contain up to 55% formaldehyde by weight, depending on the quantity of water introduced into the absorption chamber.
Formaldehyde Production Using Silver-catalyzed 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).
Requiring reactor temperatures of 1,100 degrees Fahrenheit and above, this process converts some of the methane in the same manner as the FORMOX process above, but also simply extracts hydrogen from the methane feedstock to product formaldehyde through a different, anaerobic reaction:
CH3OH → CH2O + H2
This direct dehydration is less efficient than FORMOX, so it demands more methane to be fed into the reactor in order to return similar concentrations and volumes of the finished formaldehyde.
So, why would manufactures use silver rather than the more efficient metal oxide catalyzation reaction? Despite higher methanol feedstock consumption, silver-catalyst formaldehyde production can deliver lower-cost products than iron oxide operations. This cost savings may offset efficiency losses, especially for smaller plants that can obtain methane cheaply to convert it in one of two subprocesses:
- (Nearly) Complete Conversion
With conversion as high as 98%, this process begins by heating the feedstock to approximately 1,200 degrees Fahrenheit to optimize the rate and equilibrium of the endothermic dehydrogenation reaction. Conversion occurs at a much lower temperature, however, and creates trace elements of carbon monoxide, carbon dioxide, hydrogen, and water along with the formaldehyde. Rapid cooling shuts off potential side reactions that can cause the formaldehyde to deteriorate. Absorbed into water, the formaldehyde creates a 40 to 55% aqueous solution, which also contains tiny amounts of methanol and formic acid, which are formed using the excess oxygen not used in the formaldehyde reaction.
- Incomplete Conversion and Distillate Recovery
The reactor heats the methane-containing feedstock to approximately 1,300 degrees Fahrenheit to forestall any unwanted secondary reactions. The reaction uses the oxygen in the chamber to convert 77 to 87% of the methane. Once cooled and absorbed into water, the solution contains about 43% formaldehyde, along with the 13 to 23% unconverted methane, formic acid, and other byproducts. Equipment removes the methanol and recirculates it into future conversion cycles. The next step is to further remain the solution to reduce the formic acid content, boosting its formaldehyde concentration to as much as 55% and its yield to around 90%.
Comparing and Contrasting Methods and Applications
As noted, manufacturers may choose FORMOX to capture formaldehyde reaction efficiencies or they may choose silver catalyzation to take advantage of lower operating costs. Plant operators should consider several other variables in deciding which process makes sense in their situation:
- Plant Capacity – For low-capacity facilities, producing up to 5,000 tons per year of formalin, the silver process is more economical. This process is less technical and requires fewer upfront costs, making it an attractive option for smaller operations. Larger plants producing up to 100,000 tons per year, however, likely should use the iron oxide catalyst process. Although this process requires higher capital expenditure on technology, the added capacity helps to offset these costs. The larger scale allows for greater economies of scale, improving the overall efficiency and profitability of the operation. Ultra-large operations may become inefficient due to excessively large gas conducts required for the metal oxide process. These producers should consider splitting production among medium-sized units that can capture all the metal oxide catalyzation efficiencies.
- Catalyst Cost: Silver catalysts must only last a few months, but because they can be fully regenerated, their overall cost is less than iron oxide catalysts that can last for over a year, but whose salvage value is limited to their molybdenum contents.
- Tail Gas Processing: The silver process generates tail gases containing approximately 20% combustion-facilitating hydrogen. Combustion creates steam and burns off carbon monoxide and other environmentally harmful organic compounds. Conversely, the tail gas produced in the iron oxide process is not flammable due to its low dimethyl ether, carbon monoxide, methanol, and formaldehyde content. Burning these gasses can only be accomplished by using a catalytic incinerator or adding fuel, making the process more complicated and costly.
- Steam Generation: The metal oxide process produces sufficient steam heat that manufacturers can export and use to power other processes, potentially providing energy efficiency and cost savings. Methanol rectification fully consumes the little steam generated in the silver-catalyst operation. Sometimes, the steam produced is insufficient and the reaction can only be completed by adding more.
- Product Purity: Metal oxide catalytic production yields formaldehyde with significantly less formic acid, heavy metals and unreacted methanol, and other impurities compared to the silver process. Industries increasingly require formaldehyde solutions with zero methanol and concentrated (up to a 4:1 formaldehyde to water ratio), urea-stabilized solutions, which can only be made in metal oxide-catalyst operations.
Phoenix Equipment stocks a full range of formaldehyde production equipment to help manufacturers start or scale operations. Whether you need reactors, absorption towers, boilers, evaporators, or a fully operational plant, we can help you get up and running in a fraction of the time and cost of buying new equipment. If you don’t see what you want on our website, contact our experts. We engage with suppliers and motivated sellers around the world and would be happy to put the word out.