Continuously Stirred-Tank Reactors Agitated with Ultrasound

Continuously stirred tank reactors (CSTR) are widely applied for various chemical reactions including catalysis, emulsion chemistry, polymerization, synthesis, extraction and crystallization. Slow reaction kinetics is a common problem in CSTR, which can easily be overcome by the application of power-ultrasonication. The intense mixing, agitation and the sonochemical effects of power-ultrasound accelerate reaction kinetics and improve the conversion rate significantly. Ultrasonicators can be easily integrated into CSTRs of any volume.

Why Applying Power-Ultrasound to a Continuously Stirred Tank Reactor?

A Continuously Stirred Tank Reactor (CSTR, or simply stirred tank reactor (STR)) is in its principal characteristics quite similar to the batch reactor. The major important difference is that, for the continuous stirred tank reactor (CSTR) setup the feed of material must be provided in continuous flow into and out of the reactor. Feeding the reactor can be achieved by gravity flow or forced-circulation flow using a pump. The CSTR is sometimes called a back-mixed flow reactor (BMR).
CSTRs are commonly used when agitation of two or more liquids is required. CSTRs can be used as single reactor or be installed as a series of configurations for different concentration streams and reaction steps. Besides the use of a single tank reactor, the serial installation of various tanks (one after each other) or the cascade setup are commonly used.
Why Ultrasonication? Ultrasonic mixing and agitation as well as the sonochemical effects of power ultrasound are well known to contribute to the efficiency of chemical reactions. The improved mixing and particle size reduction due to ultrasonic vibrations and cavitation provide a significantly accelerated kinetics and enhanced conversion rate. Sonochemical effects can deliver the necessary energy to initiate chemical reactions, switch chemical pathways, and give higher yields due to a more complete reaction.

Ultrasonically-intensified CSTR can be used for applications such as:

• Heterogeneous liquid-liquid reactions
• Heterogeneous solid-liquid reactions
• Homogeneous liquid-phase reactions
• Heterogeneous gas-liquid reactions
• Heterogeneous gas-solid-liquid reactions

Ultrasonication as High-Speed Synthetic Chemical System

High-speed synthetic chemistry is a novel reaction technique used to initiate and intensify chemical synthesis. In comparison to traditional reaction pathways, which need several hours or days under reflux, ultrasonically-promoted synthesis reactors can minimize reaction duration to a few minutes resulting in a significant accelerated synthesis reaction. Ultrasonic synthesis intensification is based on the working principle of acoustic cavitation and its related forces including locally confined superheating. Learn more about ultrasound, acoustic cavitation and sonochemistry in the next section.

Ultrasonic Cavitation and its Sonochemical Effects

Ultrasonic (or acoustic) cavitation occurs when power ultrasound is coupled into liquids or slurries. Cavitation is the transition from a liquid phase into a vapour phase, which occurs due to a pressure drop down to the level of the vapour tension of the fluid.
Ultrasonic cavitation creates very high shear forces and liquid jets with up to 1000m/s. These liquid jets accelerate particle and cause inter-particle collisions thereby reducing the particle size of solids and droplets. Additionally – localized within and in close proximity of the imploding cavitation bubble – extremely high pressures on the order of hundreds of atmospheres and temperatures on the order of thousands of degrees Kelvin are generated.
Although ultrasonication is a purely mechanical processing method, it can produce a locally confined extreme temperature rise. This is due to the intense forces generated within and in close proximity to the collapsing cavitation bubbles, where easily temperatures of several thousands of degrees Celsius can be reached. In the bulk solution, the temperature increase resulting from a single bubble implosion is almost negligible, but the heat dissipation from numerous cavitation bubbles as observed in cavitation hot-spots (as generated by sonication with high-power ultrasound) can finally cause a measurable temperature increases in the bulk temperature. The advantage of ultrasonication and sonochemistry lies in the controllable temperature effects during processing: Temperature control of the bulk solution can be achieved by using tanks with cooling jackets as well as pulsed sonication. Hielscher Ultrasonics’ sophisticated ultrasonicators can pause the the ultrasound when an upper temperature limit is reached and continue with the ultrasonication as soon as the lower value of a set ∆T is reached. This is especially important when heat-sensitive reactants are used.

Sonochemistry Improves Reaction Kinetics

Since sonication generates intense vibrations and cavitation, chemical kinetics are affected. The kinetics of a chemical system correlates closely with the cavitation bubble expansion and implosion, whereby impacting the dynamics of bubble motion significantly. Dissolved gases in the chemical reaction solution affect the characteristics of a sonochemical reaction via both, thermal effects and chemical effects. The thermal effects influence the peak temperatures which are reached during bubble collapse within the cavitation void; the chemical effects modifies the effects of gases, which are directly involved in a reaction.
Heterogeneous and homogeneous reactions with slow reaction kinetics including Suzuki coupling reactions, precipitation, crystallization and emulsion chemistry are predestined to be initiated and promoted through power-ultrasound and its sonochemical effects.
For instance, for the synthesis of ferulic acid, low-frequency (20kHz) sonication at a power of 180 W gave a 94% ferulic acid yield at 60°C in 3 h. These results by Truong et al. (2018) demonstrate that the use of low frequency (horn type and high-power irradiation) improved the conversion rate significantly giving yields higher than 90%.

Ultrasonically Intensified Emulsion Chemistry

Heterogeneous reactions such as emulsion chemistry benefits significantly from the application of power ultrasound. Ultrasonic cavitation diminished and distributed the droplets of each phase homogeneously within each other creating a sub-micron or nano-emulsion. Since the nano-sized droplets offer a drastically increased surface area to interact with different droplets, mass transfer and reaction rate are significantly improved. Under sonication, reactions known for their typically slow kinetics show dramatically improved conversion rates, higher yields, less by-products or waste and better overall efficiency. Ultrasonically improved emulsion chemistry is often applied for emulsions polymerization, e.g., to produce polymer blends, water-borne adhesives and specialty polymers.

10 Things You Should Know, Before You Buy A Chemical Reactor

When you choose a chemical reactor for a chemical process there are many factors to that influence the optimum chemical reactor design. If your chemical process involves multi-phase, heterogeneous chemical reactions and has slow reaction kinetics, reactor agitation and process activation are essential influencing factors for successful chemical conversion and for economical (operational) costs of the chemical reactor.
Ultrasonication improves the reaction kinetics of liquid-liquid and liquid-solid chemical reactions in chemical batch reactors and inline reaction vessels significantly. Hence, the integration of ultrasonic probes in a chemical reactor can reduce reactor costs and improve overall efficiency and the quality of the final product.
Very often, chemical reactor engineering lacks the knowledge about ultrasonically-assisted process enhancement. Without profound knowledge about the influence of power ultrasound, ultrasonic agitation, acoustic cavitation and sonochemical effects on chemical reactor performance, chemical reactor analysis and conventional design fundamentals can produce only inferior results. Below, you will get an overview over the fundamental benefits of ultrasonics for chemical reactor design and optimization.

The Advantages of Ultrasonically Intensified Continuous Stirred Tank Reactor (CSTR)

• • Ultrasonically enhanced reactors for lab and production:
    Easy scalability: Ultrasonic processors are readily available for lab size, pilot and large-scale production
    Reproducible / repeatable results due to precisely controllable ultrasonic parameters
    Capacity and reaction speed: ultrasonically intensified reactions are faster and thereby more economical (lower costs)
  • Sonochemistry is applicable for general as well as special purposes

– adaptability & versatility, e.g., flexible installation and setup options and interdisciplinary use

• Ultrasonication can be used in explosive environments
  – purging (e.g., nitrogen blanket)
  – no open surface
• Simple cleaning: self cleaning (CIP – clean-in-place)
• Choose your preferred materials of construction
  – glass, stainless steel, titanium
  – no rotary seals
  – wide choice of sealants
• Ultrasonicators can be used in a wide range of temperatures
• Ultrasonicators can be used at a wide range of pressures
• Synergistic effect with other technologies, e.g., electrochemistry (sono-electrochemistry), catalysis (sono-catalysis), crystallization (sono-crystallization) etc.
• Sonication is ideal to enhance bioreactors, e.g., fermentation.
• Dissolution / Dissolving: In dissolution processes, particles pass from one phase to the other, e.g. when solid particles dissolve in a liquid. It is found that the degree of agitation influences the speed of the process. Many small crystals dissolve much faster under ultrasonic cavitation than one in conventionally stirred batch reactors. Here, too, the reason for different speeds lies in the different mass transfer rates at particle surfaces. For instance, ultrasonication is successfully applied to create supersaturated solutions, e.g., in crystallization processes (sono-crystallization).
• Ultrasonically-promoted chemical Extraction:
  – Liquid-Solid, e.g. botanical extraction, chemical extraction
  – Liquid-Liquid: When ultrasound is applied to a liquid–liquid extraction system, an emulsion of one of the phases in the other is created. This formation of emulsion leads to increased interfacial areas between the two immiscible phases resulting in an enhanced mass transfer flux between the phases.

How Does Sonication Improve Chemical Reactions in Stirred Tank Reactors?

• Larger Contact Surface Area: In reactions between reactants in heterogeneous phases, only the particles that collide with each other at the interface can react. The larger the interface, the more collisions can occur. As a liquid or solid portion of a substance is broken into smaller droplets or solid particles suspended in a continuous phase liquid, the surface area of this substance increases. Furthermore, as a result of the size reduction, the number of particles increases and therefore the average distance between these particles decreases. This improves the exposure of the continuous phase to the dispersed phase. Therefore, the reaction rate increases with the degree of fragmentation of the disperse phase. Many chemical reactions in dispersions or emulsions show drastic improvements in reaction speed as a result of ultrasonic particle size reduction.
• Catalysis (Activation Energy): Catalysts are of great importance in many chemical reactions, in lab development and in industrial production. Often catalysts are in solid or liquid phase and immiscible with one reactant or all reactants. Hence, more often than not, catalysis is a heterogeneous chemical reaction. In the production of the most important basic chemicals such as sulfuric acid, ammonia, nitric acid, ethene and methanol, catalysts play an important role. Large areas of environmental technology are based on catalytic processes. A collision of particles leads to a chemical reaction, i.e. a regrouping of atoms, only if the particles collide with sufficient kinetic energy. Ultrasonication is a highly efficient means to increase the kinetics in chemical reactors. In a heterogeneous catalysis process, the addition of ultrasonics to a chemical reactor design can lower the requirement for a catalyst. This can result in the use of less catalyst or inferior, less noble catalysts.
• Higher frequency of contact / Improved mass transfer: Ultrasonic mixing and agitation is a highly efficacious method to generate minute droplets and particles (i.e., sub-micron and nano-particles), which offer a higher active surface for reactions. Under the additional intense agitation and micro-movement caused by power-ultrasound, the frequency of inter-particle contact is drastically increased resulting in a significantly improved conversion rate.
• Compressed plasma: For many reactions, a 10 Kelvin increase in reactor temperature causes the reaction rate to roughly double. Ultrasonic cavitation produces localized highly reactive hotspots of up to 5000K within the liquid, without substantial heating of the overall liquid volume in the chemical reactor.
• Thermal energy: Any ultrasonic energy that you add to a chemical reactor design, will finally be converted into thermal energy. Therefore, you can reuse the energy for the chemical process. Instead of a thermal energy input by heating elements or steam, ultrasonication introduces a process activating mechanical energy by means of high-frequency vibrations. In the chemical reactor, this produces ultrasonic cavitation that activated the chemical process on multiple levels. Finally the immense ultrasonic shearing of the chemicals results the conversion to thermal energy, i.e. heat. You can use jacketed batch reactors or inline reactors for cooling in order to maintain a constant process temperature for your chemical reaction.

High-Performance Ultrasonicators for Improved Chemical Reactions in CSTR

Hielscher Ultrasonics designs, manufactures and distributes high-performance ultrasonic homogenizers and dispersers for the integration into continuous stirred tank reactors (CSTR). Hielscher ultrasonicators are used world-wide to promote, intensify, accelerate and improve chemical reactions.
Hielscher Ultrasonics’ ultrasonic processors are available at any size from small lab devices to large industrial processors for flow chemistry applications. Precise adjustment of the ultrasonic amplitude (which is the most important parameter) allows to operate Hielscher ultrasonicators at low to very high amplitudes and to fine-tune the amplitude exactly to the required ultrasonic process conditions of the specific chemical reaction system.
Hielscher’s ultrasonic generator feature a smart software with automatic data protocolling. All important processing parameters such as ultrasonic energy, temperature, pressure and time are automatically stored onto a built-in SD-card as soon as the device is switched on.
Process monitoring and data recording are important for continuous process standardization and product quality. By accessing the automatically recorded process data, you can revise previous sonication runs and evaluate the outcome.
Another user-friendly feature is the browser remote control of our digital ultrasonic systems. Via remote browser control you can start, stop, adjust and monitor your ultrasonic processor remotely from anywhere.
Contact us now to learn more about our high-performance ultrasonic homogenizers can improve your continuously stirred tank reactor (CSTR)!
The table below gives you an indication of the approximate processing capacity of our ultrasonicators: