Other Views - click image for larger view

Discussion of Formaldehyde Production Processes

Date Published:   09/06/2018

Formaldehyde Production Processes

Formaldehyde is a main raw material for manufacturing value-added chemicals such as melamine, urea-formaldehyde and phenolic resins. The most common product is in a 37% aqueous solution, but concentration can be as high as 57%. Today, there are two main routes to produce formaldehyde in an industrial scale: oxidation-dehydrogenation using a silver catalyst, involving either the complete or incomplete conversion of methanol; and the direct oxidation of methanol to formaldehyde using metal oxide catalysts (Formox process).

Process Basis

The catalytic oxidation process is an optimized production method. It is a simple process as per the stoichiometric reaction steps shown below.

 

  1. Oxidation of methanol with oxygen present in the 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

 

The reaction is exothermic, i.e. it produces heat once started. This excess heat is removed from the reactor by means of cooling coils that are fitted to the reactor. Water is circulated through the coils and then into a heat exchanger. Excess heat (as steam) is then used to heat resin if it is available at the same site. Little or no extra heat needs to be added to enable resin manufacture. Standby boilers are used to supplement the steam requirement if the formaldehyde plant is not running. A standby generator is also installed and is used to ensure that the essential functions of the plant are maintained in the event of an electricity failure.

 

The unreacted methanol is stripped out from the formaldehyde solution and recycled to the process. Downstream from the methanol stripper requires a further purification column to remove water and increase the purity of formaldehyde, or adjust its concentration as per the customers’ order. However, by using a single vacuum distillation column, the separation can be achieved, eliminating the need of the two stripping columns.

 

Process Using Silver Catalyst

 

This process utilizes a silver catalyst and operates in an oxygen lean atmosphere. It involves passing a mixture of methanol vapor and air over a fixed bed catalyst at approximately atmospheric pressure, and absorbing the product gases in water. Silver-catalyzed processes for making formaldehyde from methanol can be characterized according to the number of catalytic stages used to affect the conversion. Single stage operation is widely used but suffers from the disadvantage that high amounts of unconverted methanol are contained in the product emerging from the catalyst bed. This phenomenon is customarily referred to as “methanol leakage”. Since, for many applications, methanol is an undesirable contaminant, it must be separated from the formaldehyde solution. This entails a substantial investment in distillation facilities and energy to carry out. It is usually necessary that the methanol content of the product be no greater than 2% by weight. One way of eliminating the need for facilities to distill off methanol is to use two catalytic stages with inter-stage cooling. Other two-stage processes use an adiabatic first stage reactor containing a silver catalyst and an isothermal second stage reactor containing a metal oxide catalyst.

 

There are two versions of the silver process used. One involves the complete conversion (97% - 98%) of methanol. In this process silver is used in the form of crystals and the reaction is carried out at 680°- 720°C (at atmospheric pressure). The feed is superheated and fed to the reactor where it passes through a bed of silver crystals 25 - 30 mm thick. The temperature is high enough to allow complete conversion (the rate and equilibrium of the endothermic dehydrogenation reaction increase with temperature). Gases are cooled when they leave the reactor (to prevent undesirable side reactions) and then fed to an absorption column where formaldehyde is eluted, giving a product that contains 40 wt% - 55 wt% formaldehyde, 1.3 wt% methanol and 0.01 wt% formic acid. The yield ranges between 89.5% and 90.5%. The largest known reactor for this process has a diameter of 3.2 m and an annual production of 72,000 tons, calculated as 100% formaldehyde.

 

The second version of the silver process involves incomplete conversion and distillative recovery of methanol. Superheated feed passes through a bed of silver crystals 1-5 cm thick or through layers of silver gauze. The reactor temperature lies in the range of 600°- 650°C. At these relatively low temperatures the undesirable secondary reactions are suppressed. The oxygen conversion is complete and the methanol conversion is between 77% and 87%. The gases are cooled after leaving the catalyst bed and fed to an absorption column. The product contains about 42 wt% formaldehyde and is sent to a distillation column to recover and recycle unreacted methanol. After leaving the distillation column, the formaldehyde solution is usually fed to an anion exchange unit to reduce the formic acid content to less than 50 mg/kg. The final product contains up to 55 wt% formaldehyde and less than 1% methanol. The overall yield is between 91% and 92%.

 

The tail gas for the silver process contains about 20% hydrogen and is burned to generate steam and eliminate emissions of carbon monoxide and other organics.

 

Process Using Metal Oxide Catalysts

 

The oxide process for formaldehyde production uses a metal oxide (modified iron-molybdenum-vanadium

oxide) catalyst. The feed mixture of steam, air and methanol contains relatively low amounts of methanol (to avoid the explosive range) and an almost complete conversion of methanol is obtained (98% - 99%). The reaction takes place at 250°- 400°C. All of the formaldehyde is made via reaction (the exothermic oxydehydrogenation of methanol). By-products are carbon monoxide, dimethyl ether, carbon dioxide and formic acid. Overall yields are in the range of 88% - 92 %.

 

The process begins with the vaporization of methanol. It is then mixed with air (and optional tail gas) and passed through catalyst-filled tubes in a heat exchanger reactor. A heat transfer fluid passes circulates outside the tubes and vaporizes, removing the heat of reaction. This fluid is then condensed to produce steam. The gases are cooled to 110°C in a heat exchange unit and then travels to the bottom of an absorber. Water is added to the top of the column, and the amount can be varied to control the product concentration. After leaving the column the product is fed through an anion exchange unit to reduce the formic acid content. The final product contains up to 55 wt% formaldehyde and 0.5 wt% - 1.5 wt% methanol.

 

The tail gas from the oxide process does not burn by itself as the combustible content (dimethyl ether, carbon monoxide, methanol and formaldehyde) is only a few percent. It can be burnt in a catalytic incinerator or by adding fuel.

 

The metal oxide catalyst charge lasts from 12 to 18 months, but when exhausted, it cannot be regenerated: only the main metal (molybdenum) can be recovered to produce new catalyst.

 

Typical performance figures of Metal-Oxide Process (based on 37 wt-% concentration)

 

Per metric ton

Methanol

421-426

 

Electricity (for blower)

30-70 kWh

Cooling water

35 m3

Catalyst

0.03- 0.5 kg

Steam produced (4)

780 kg

Steam pressure

12-22 bar g

 

 

Comparing the Silver Catalyst and Metal Oxide Processes

 

Apart from the catalyst used and the reaction mechanisms, some important differences between the silver process and the metal oxide process are:

 

  1. With the silver catalyst process, catalyst and energy costs are less than with the metal oxide process, but with lower yields than the metal oxide process, which will produce more formaldehyde per unit of methanol. Therefore, when methanol prices are low, the silver catalyst process tends to be more economical, while when methanol prices are high – the metal oxide process has a cost advantage.  

 

 

  1. The level of impurities in the formaldehyde solution (formic acid, methanol, heavy metals) tends to be considerably lower in the metal oxide process than in the silver catalyst process.

 

 

  1. Even when recycled gas is used (to reduce the oxygen concentration of the feed and therefore the amount of air needed to avoid the flammable range), the total volume of gas passing through the oxide process is 3 - 3.5 times that of the silver process. This means that the equipment used for the oxide process must have a higher capacity. The absorption column in particular is much larger.  This tends to translate into higher capital costs with the metal oxide process.

 

  1. The metal oxide process produces a surplus of steam that can be used for other processes, while the silver catalyst process does not and at times needs steam added to the process.

 

  1. A 5,000 TPY  37% solution plant is more economical as a silver catalyst plant than a metal oxide plant, as the metal oxide plant’s technology costs tend to exceed even the equipment cost at this scale; while at the medium size scale – metal oxide plants tend to be less expensive as a total capital outlay. And at the very large scale (>100,000 TPY 37% solution), the metal oxide process plants have to be split into separate units because of the excessive size of the gas conduct as a single unit.

 

 

  1. In the silver catalyst process, there is only a partial conversion of methanol in the reactor.  Subsequently, there is an added step where the unconverted methanol has to be absorbed from the solution and recycled in a catalytic reactor; while in the metal oxide process the conversion is near complete and the product is obtained by a single-step absorption.

 

 

  1. Another important distinction in the two processes are the reaction temperatures.  The metal oxide reaction takes place at 250°- 400°C and between 600°- 720°C for the silver catalyst process.  This adds another layer of complexity to the silver catalyst process versus the metal oxide process.

 

 

  1. The metal oxide process has a major advantage in making a urea stabilized formaldehyde solution, as there is no methanol in the absorption column, and the solution can be acquired with little additional expense.  This cannot be done in the silver catalyst process.  In addition, the metal oxide’s concentrated solution of a 4:1 formaldehyde/water ratio has significant merits versus the maximum 1.5:1 ratio of the silver catalyst made solution, when manufacturing ureic resins or glue, as water distillation is expensive, and is not necessary with the 4:1 concentrated solution from the metal oxide process.  This is why most all companies producing or considering to produce urea glue choose the metal oxide process over the silver catalyst process.

 

 

  1. The tail gas from the oxide process is noncombustible, while the tail gas from the silver process can be burned. The tail gas from the silver process contains hydrogen which can be used for other processes.

 

  1. The reactor is run adiabatically for the silver process, while a heat exchanger reactor is used for the metal oxide process. Proper temperature control is needed for the metal oxide process to achieve 99% conversion. If the temperature is allowed to rise above 470°C, the side reaction involving the formation of carbon monoxide and water from formaldehyde and oxygen increases considerably.

 

  1. The silver process does not require ion exchange to remove formic acid. The metal oxide process produces more formic acid, requiring an extra step to remove it.

 

 

Technology Providers:

 

Metal Oxide

Silver Catalyst

  • Alder
  • ATEC
  • Haldor Topsoe
  • Dynea
  • Johnson Matthey
  • Poerner
 
   
   

 

 

Phoenix Equipment buys and sells second hand formaldehyde plants that are immediately available for purchase and relocation. Buying a used plant can save you significant capital and drastically shorten the time required to build a plant.  Here are some formaldehyde plants we currently have for sale:

 

  • Formaldehyde Plant – 60,000 TPY # 442

(https://www.phxequip.com/plant.2306/formaldehyde-plant-60-000-tpy.aspx)

 

  • Formaldehyde Plant – 25,000 TPY to 35,000 TPY #395

(https://www.phxequip.com/plant.2233/formaldehyde-plants-25-000-35-000-tpy.aspx)

 

If you are interested in learning about any of the above plants, please contact Edward Zhang (Tel: 732-520-2187; email: edz@phxequip.com).

References
Stock# 442
Search Keyword Formaldehyde
Plant Subcategory Formaldehyde Plants
Other Articles Customers Viewed