Réacteurs à lit fixe intensifiés par ultrasons

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.
• Amélioration du transfert de chaleur : 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.

Catalyseurs à lit fixe

Les lits fixes (parfois également appelés lits garnis) sont généralement chargés de pastilles de catalyseur, qui sont généralement des granulés d'un diamètre de 1 à 5 mm. Ils peuvent être chargés dans le réacteur sous la forme d'un lit unique, de coquilles séparées ou de tubes. Les catalyseurs sont principalement basés sur des métaux tels que le nickel, le cuivre, l'osmium, le platine et le rhodium.
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.

Intensification par ultrasons des réactions catalytiques

Le mélange et l'agitation par ultrasons améliorent le contact entre les particules de réactifs et de catalyseurs, créent des surfaces hautement réactives et initient et/ou renforcent la réaction chimique.
La préparation ultrasonique des catalyseurs peut entraîner des changements dans le comportement de cristallisation, la dispersion/désagglomération et les propriétés de surface. En outre, les caractéristiques des catalyseurs préformés peuvent être influencées par l'élimination des couches de surface passivantes, une meilleure dispersion et l'augmentation du transfert de masse.

Examples of Ultrasonically-Improved Reactions

• Prétraitement par ultrasons du catalyseur Ni pour les réactions d'hydrogénation
• Un catalyseur Raney Ni sonifié avec de l'acide tartrique permet d'obtenir une très haute énantiosélectivité
• Ultrasonic synthesized Fischer-Tropsch catalysts
• Catalyseurs en poudre amorphe traités par voie sonochimique pour une réactivité accrue
• Sono-synthèse de poudres métalliques amorphes

Récupération du catalyseur par ultrasons

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 propose différents processeurs ultrasoniques et variantes pour l'intégration des ultrasons de puissance dans les réacteurs à lit fixe. Différents systèmes à ultrasons sont disponibles pour être installés dans les réacteurs à lit fixe. Pour les types de réacteurs plus complexes, nous proposons ultrasons personnalisés solutions.
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!
Prenez contact avec nous dès aujourd'hui ! Nous nous ferons un plaisir de discuter avec vous de l'intensification par ultrasons de votre processus chimique !
Le tableau ci-dessous vous donne une indication de la capacité de traitement approximative des sonicateurs Hielscher :