Reactores de leito fixo intensificados por ultra-sons

Sonication can improve catalytic reactions in fixed-bed reactors mainly by intensifying mass transfer around and inside the packed catalyst bed. Additionally, sonication removes passivation and fouling layers from the catalyst surface thereby continuously regenerating the catalyst.

How Sonication Improves Fixed-Bed Catalysis

In a fixed-bed reactor, the catalyst particles remain stationary while liquid, gas, or multiphase reactants flow through the bed. Reaction performance is often limited by external mass transfer, pore diffusion, channeling, fouling, and heat-transfer gradients. Ultrasound can reduce several of these limitations by generating acoustic cavitation, microstreaming, shear forces, and pressure oscillations.

Key Effects of Ultrasonically-Intensified Fixed Bed Reactions

• Improved external mass transfer: Ultrasonic microstreaming reduces the stagnant boundary layer around catalyst particles, allowing reactants to reach active sites more efficiently.
• Enhanced pore accessibility: Cavitation-induced pressure fluctuations and liquid movement can improve penetration of reactants into catalyst pores and removal of products from pores.
• Reduction of fouling and passivation: Sonication can help remove deposits, polymer films, coke precursors, or other passivating layers from catalyst surfaces, maintaining catalytic activity for longer.

Improved liquid-solid contact: Ultrasound promotes better wetting of catalyst particles, which is especially useful in trickle-bed, slurry-fed, or liquid-phase fixed-bed systems.

• Reduced channeling in packed beds: In micropacked-bed studies, ultrasound has been shown to modify flow behavior and reduce dispersion, helping the reactor approach more ideal plug-flow behavior.
• Transferência de calor melhorada: Acoustic streaming and turbulence improve local heat dissipation, reducing hot spots or cold zones in the catalyst bed.
• Higher conversion and yield: By improving mass transfer and catalyst accessibility, sonication can increase reaction rate, conversion, and product yield, especially when the reaction is transport-limited rather than purely kinetically limited.

How does Sonication Improve Fixed Bed Catalysis?

The main mechanism is acoustic cavitation: ultrasonic waves create microscopic bubbles that grow and collapse violently. Their collapse generates local shear, microjets, shockwaves, and intense mixing. Near catalyst surfaces, these effects can clean, activate, and refresh the solid-liquid interface. Reviews of sonocatalysis describe this as a synergy between ultrasound and solid catalysts, involving improved heat transfer, mass transfer, and localized effects at catalytic surfaces.

Sonication is most beneficial when the fixed-bed reaction suffers from:

• slow diffusion into catalyst pores,
• poor wetting of catalyst particles,
• product accumulation inside pores,
• fouling or surface passivation,
• mass-transfer-limited kinetics,
• multiphase flow maldistribution,
• channeling through the packed bed.

Catalisadores de leito fixo

Os leitos fixos (por vezes também designados por leitos empacotados) são normalmente carregados com granulados de catalisador, que são normalmente grânulos com diâmetros de 1-5 mm. Podem ser carregados no reator sob a forma de um único leito, de conchas separadas ou de tubos. Os catalisadores são maioritariamente baseados em metais como o níquel, o cobre, o ósmio, a platina e o ródio.
The effects of power ultrasound on heterogeneous chemical reactions are well known and widely used for industrial catalytic processes. Catalytic reactions in a fixed bed reactor benefit from sonication treatment, too. Ultrasonic irradiation of the fixed bed catalyst generates highly reactive surfaces, increases the mass transport between liquid phase (reactants) and catalyst, and removes passivating coatings (e.g. oxide layers) from the surface.

Intensificação ultra-sónica de reacções catalíticas

A mistura e agitação ultra-sónicas melhoram o contacto entre o reagente e as partículas do catalisador, criam superfícies altamente reactivas e iniciam e/ou melhoram a reação química.
A preparação de catalisadores por ultra-sons pode causar alterações no comportamento de cristalização, dispersão / desaglomeração e propriedades de superfície. Além disso, as caraterísticas dos catalisadores pré-formados podem ser influenciadas pela remoção de camadas superficiais passivantes, melhor dispersão, aumento da transferência de massa.

Examples of Ultrasonically-Improved Reactions

• Pré-tratamento ultrassónico do catalisador de Ni para reacções de hidrogenação
• O catalisador Raney Ni sonicado com ácido tartárico resulta numa enantioselectividade muito elevada
• Ultrasonic synthesized Fischer-Tropsch catalysts
• Catalisadores em pó amorfo tratados sonoquimicamente para aumentar a reatividade
• Sono-síntese de pós metálicos amorfos

Recuperação do catalisador por ultra-sons

Solid catalysts in fixed-bed reactors are commonly used in the form of spherical beads, pellets, extrudates, or cylindrical particles. During chemical reactions, the catalyst surface can become passivated by a fouling layer, resulting in a gradual loss of catalytic activity and/or selectivity over time.
The timescale of catalyst deactivation varies considerably. For example, the deactivation of a cracking catalyst may occur within seconds, whereas an iron catalyst used in ammonia synthesis may remain active for 5–10 years. Nevertheless, catalyst deactivation is observed in virtually all catalytic processes. Although different deactivation mechanisms can occur – including chemical, mechanical, and thermal degradation – fouling is one of the most common causes of catalyst decay.
Fouling refers to the physical deposition of species from the fluid phase onto the catalyst surface and within its pores. These deposits block reactive sites, restrict pore accessibility, and reduce contact between reactants and the active catalyst surface. Catalyst fouling by coke or carbonaceous deposits is often a rapid process; however, in many cases it can be partially or fully reversed by ultrasonic regeneration.

Ultrasonic cavitation is an effective method for removing passivating fouling layers from catalyst surfaces. During sonication, high-intensity ultrasound generates cavitation bubbles in a liquid medium. Their collapse produces localized shear forces, microjets, shock waves, and intense micro-mixing. These effects help detach fouling residues from the catalyst surface, reopen blocked pores, and restore access to active sites.
Ultrasonic catalyst recovery is typically carried out by dispersing the catalyst particles in a liquid, such as deionized water or a suitable solvent, and exposing the suspension to controlled ultrasonic treatment. This process can remove fouling residues from various catalyst materials, including platinum/silica fibre catalysts, nickel catalysts, and other supported metal catalysts. As a result, sonication can contribute to catalyst regeneration, extended catalyst lifetime, and improved process sustainability.

Click here to learn more about the ultrasonic regeneration of spent catalysts!

Sonicators for the Integration into Chemical Reactors

Hielscher Ultrasonics oferece vários processadores ultra-sônicos e variações para a integração de ultrassom de potência em reatores de leito fixo. Vários sistemas de ultra-sons estão disponíveis para serem instalados em reactores de leito fixo. Para tipos de reactores mais complexos, oferecemos ultra-sons personalizados soluções.
Learn how sonication improves chemical reactions in various reactor designs!
To test the effects of sonication on your chemical reaction, you are welcome to visit our ultrasonic process lab and technical center in Teltow!
Contacte-nos hoje mesmo! Teremos todo o prazer em discutir consigo a intensificação ultra-sónica do seu processo químico!
O quadro seguinte dá-lhe uma indicação da capacidade aproximada de processamento dos sonicadores Hielscher: