Q&A with Clariant Catalysts on optimising the formaldehyde production

Could you explain the formaldehyde production process, with details regarding the reaction conditions?

Production of formaldehyde from methanol using a iron-molybdenum oxide catalyst:

Methanol is vaporised in process air and fed together with a recycle airflow to the tubular reactor. At a reactor temperature of around 260 to 320°C, methanol is oxidised over the Fe/Mo catalyst: 2 CH₃OH + O₂ → 2 CH₂O + 2 H₂O

Depending on the process design, the methanol concentration varies between 6 vol.% to 11 vol.% in the feed as stream.

A second parameter which differentiates the process design is the linear velocity. The linear velocity is a measure of contact time of the gas and the catalyst, calculated from total feed gas stream vs cross-section of the reactor tubes into which the catalyst is loaded. The linear velocity may range from approximately 1.2 up to 2.5 Nm/s.

This wide range of industrially applied process conditions, the methanol concentration and linear velocity, are one key element that requires different catalyst configurations for achieving optimum performance. Clariant addresses this need by offering different catalyst types and proprietary, layered catalyst designs.

On the shell side of the reactor tubes, a molten salt mixture or more commonly a diathermic oil is used as heat transfer fluid (also called cooling medium), which absorbs the heat produced during the conversion of the oxygen and methanol into formaldehyde. The temperature of the cooling medium in the reactor is controlled by converting the absorbed heat to steam, which can be used within the formaldehyde plant and excess stream can be exported or converted to electricity.

The gas exiting from the reactor contains formaldehyde and with it a small fraction of unconverted methanol and by-products from the reaction such as CO, CO₂ and dimethyl ether (DME). This gas is cooled and fed into an absorption tower, where the formaldehyde is absorbed by water to form an aqueous formaldehyde solution. Depending on the downstream use of the formaldehyde, the concentration of formaldehyde in the solution typically ranges from 37% to more than 50% by weight.

The exit gas from the absorber must be treated remove the remaining organic components and for converting CO in CO₂. Technologies for off-gas treatment include catalytic oxidation, for which Clariant offers the EnviCat^® series of catalysts, or thermal oxidation.

What is pressure drop, and how does preventing it affect production?

Feedstock is introduced into the reactor at elevated pressure. As the gas passes through the reactor and over the catalyst and also through the downstream process it loses pressure. The reactor inlet pressure is directly linked to the pressure drop over the process and the catalyst. A lower pressure drop leads to lower inlet pressure, for which less energy is consumed to feed the same airflow to the reactor. Thus producers look for minimal pressure drop in order to operate their process in the most energy efficient way.

For some customers the design of the air blower is limiting their ability to feed process gas to the reactor and thus reduces plant capacity.

FAMAX^® DS is a specially shaped catalyst for methanol to formaldehyde oxidation: engravings, on the top and bottom surfaces of the catalyst ring, generate a free path for gas flow and a larger surface area of the catalyst. Thus, the pressure drop is reduced significantly. This allows the producers, based on their specific need, to operate with:

• Increased gas throughput at the same pressure drop compared to conventional ring-shaped catalysts.
• Benefit from reduced energy consumption at the same gas throughput.

Both operating scenarios bring clear advantages for process efficiency and economics.

“…allows operation at reduced cooling medium temperatures” Could you please clarify what cooling medium temperatures are?

The cooling medium temperature is the temperature that is applied on the shell side of the reactor, which is an important parameter to control the reaction (oxidation of methanol to formaldehyde). The cooling medium in the reactors of the formaldehyde process can be molten salts or more commonly a diathermic oil. A typical temperature range is 260 to 320°C.

The cooling medium temperature is increased during the lifetime of the catalyst for compensating it’s deactivation.

Are the FAMAX catalyst and multilayer bed system used in conjunction by the formaldehyde production technology?

Yes, FAMAX catalysts are offered in a multi-layer design: different versions of the catalyst are arranged in up to five layers in the reactor tube. Through the resulting better distribution and tailoring of the catalyst activity along the tube height, high methanol conversion and formaldehyde yield can be achieved. It also maximises the lifetime of the catalyst.

How long does catalyst replacement typically take, and how long does it take with Clariant’s specialised automated machines?

Clariant has created a mechanised loading machine to rapidly load thousands of tubes into the tubular reactor to reduce loss of production days during catalyst replacement. With these machines, loading the catalyst usually takes 12 - 48 hours, depending on the reactor size, instead of several days which would be needed for hand loading (without the use of automated machines).

Could you please tell me which other companies have contracts with Clariant for this technology?

While we cannot disclose individual names, we can say that more than 80 formaldehyde producers currently use FAMAX catalysts from all regions globally (Asia, China, India, Europe, Russia, North America, South America, the Middle East, Africa).

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/12082019/qa-with-clariant-catalysts/