Rotor-stator Mixer Research Articles

Organic salt-assisted liquid-phase shear exfoliation of expanded graphite into graphene nanosheets

How to improve the efficiency of liquid-phase shear exfoliation (LPSE) for mass production of large-size graphene nanosheets still remains an ongoing challenge. In this work, we have developed a LPSE method using a rotor-stator mixer. It is quite simple and efficient by exfoliation of expanded graphite (EG) in N-methyl-2-pyrrolidone (NMP) with the assistance of organic salts including sodium citrate, potassium citrate and sodium tartrate. The LPSE of EG in NMP can provide improved yields, up to 6 times as high as values from exfoliation of natural flake graphite (NFG). The additive of organic salts in NMP can make a further improvement in graphene yields, 1.5 times higher than that obtained only in NMP. Remarkably, the yields of the as-exfoliated graphene are as high as 10% under optimal conditions, and up to 50% after multiple-cycle exfoliation. Organic salts in LPSE act as analogue grinding aids enhancing the applied shear forces and thus contributing to the improved efficiency of LPSE, but they do not intercalate into the interplanar spaces of graphite. This facile LPSE method should have excellent potential in the large scale production of graphene nanosheets for numerous applications.

Residence Time Distribution and Micromixing Efficiency of a Dynamic Inline Rotor–Stator Mixer

Residence Time Distribution and Micromixing Efficiency of a Dynamic Inline Rotor–Stator Mixer

Formulation and Preparation of Water-In-Oil-In-Water Emulsions Loaded with a Phenolic-Rich Inner Aqueous Phase by Application of High Energy Emulsification Methods.

Currently, industry is requesting proven techniques that allow the use of encapsulated polyphenols, rather than free molecules, to improve their stability and bioavailability. Response surface methodology (RSM) was applied in this work to determine the optimal composition and operating conditions for preparation of water-in-oil-in-water (W/O/W) emulsions loaded with phenolic rich inner aqueous phase from olive mill wastewater. A rotor-stator mixer, an ultrasonic homogenizer and a microfluidizer processor were tested in this study as high-energy emulsification methods. Optimum results were obtained by means of microfluidizer with 148 MPa and seven cycles input levels yielding droplets of 105.3 ± 3.2 nm in average size and 0.233 ± 0.020 of polydispersity index. ζ-potential, chemical and physical stability of the optimal W/O/W emulsion were also evaluated after storage. No droplet size growth or changes in stability and ζ-potential were observed. Furthermore, a satisfactory level of phenolics retention (68.6%) and antioxidant activity (89.5%) after 35 days of storage at room temperature makes it suitable for application in the food industry.

Analysis of Power Input of an In-Line Rotor-Stator Mixer for Viscoplastic Fluids

In this work, the power draw and shear profile of a novel in-line rotor-stator mixer were studied experimentally and the laminar flow regime was simulated. The power draw of the rotor-stator mixer was investigated experimentally using viscoplastic shear-thinning fluid and the results of the obtained power consumptions were verified through simulations. The power draw constant and Otto-Metzner coefficient were determined from the result of experimental data and through simulations. A new method is suggested for the determination of the Otto-Metzner coefficient for the Herschel–Bulkley model and the term efficiency is introduced. It was shown that the proposed method can be applied successfully for the prediction of the Otto-Metzner coefficient for the mixing of viscoplastic shear-thinning fluids. The effect of geometry and rotor speed on power consumption and shear rate profile in the investigated mixer is discussed from the results of the simulations. It was found that numerical methods are a convenient tool and can predict the power draw of the in-line rotor-stator mixer successfully.

Scale-up of continuous microcapsule production

The objective of this work is to establish an experimental approach to scale-up the production of melamine formaldehyde (MF) microcapsules from a batch lab-scale to a pilot-scale inline rotor stator mixer (RSM). The inline RSM is operated in a continuous recycle mode, allowing multiple passes of the emulsion through the intensive mixing zone during continuous production. The liquid is continuously recirculated directly to the RSM, without a holding vessel, as is the case in conventional batch recirculation emulsification. The setup is operated at feed flow rates of 0.24 kg/h - 20 kg/h. Parameters including the Weber number, tip speed and energy density are investigated to correlate the mean capsule size of the batch and flow process. Rotational speeds range from 3000 to 26,000 rpm. The dimensionless maximum diameter correlated well with the Weber number to the power -0.4, for both devices operated at the same residence time. Volume and number based mean diameters showed little influence of the feed flow rate in the continuous recycle mode of operation. This inline RSM setup is therefore an ideal tool to optimize emulsification processes at lab scale and increase production by increasing the feed flow rate, while maintaining rotor-stator geometry.

W/O Pickering emulsion preparation using a batch rotor-stator mixer – Influence on rheology, drop size distribution and filtration behavior

HypothesisPickering emulsions (PE) are becoming of increasing interest for catalytic multiphase processes. Ultrafiltration of PE is a promising procedure for catalyst recovery to enable continuous processes. Dispersing conditions during production of PE are expected to significantly influence PE characteristics, and control of these properties is essential for robust process design. However, while the impact of PE composition has been studied before, knowledge on dispersing conditions is surprisingly scarce. ExperimentsThe influence of dispersing time, speed and emulsion volume during the preparation of PE with an UltraTurrax (2 dispersing tools) on the drop size distribution, rheology, stability and filtration was investigated. FindingsIn this first systematic study of PE preparation conditions, obtained Sauter mean diameters were correlated with energy density (R2 = 0.80), energy dissipation rate (R2 = 0.85) and tip speed (R2 = 0.86). All emulsions were stable for at least 10 weeks. With increasing tip speed (4–13 m/s), the dynamic viscosity first decreased, passed through a plateau value and then increased again. Filtration of concentrated PE was successful but strong membrane-particle-solvent interactions were revealed. This work contributes to a better understanding of PE properties that are essential for a sound application of PE in continuous multiphase catalysis.

Water in Oil W/O Pickering Emulsion Encapsulated in Oil in Water O/W Emulsion: A Double Emulsion for Poorly Soluble Drugs

There are two problems that hinder the use of double emulsions in pharmacies: large-scale equipment not available for one-step emulsification and obtaining a double emulsion and ingredients available to replace the tension-active agents as primary emulsifier because these surfactants are toxic. To overcome these difficulties, a two-stage emulsification strategy has been developed, first generating a water-in-oil Pickering emulsion stabilized by magnesium oxide particles and then the double W/O/W emulsion, thereby reducing significantly the amount of voltage-active. Pickering emulsions are surfactant-free emulsions, stabilized by colloidal particles. These systems are experiencing renewed interest on the one hand, because it is preferable to limit the use of synthetic surfactants for ecological reasons, and on the other hand, because the functionalization of particles has undergone recent advances. It is possible to make very simple calibrated emulsions of controlled size, exploiting a phenomenon called limited coalescence. The Bancroft rule served as a model for the formulation. The emulsification was carried out using a rotor stator mixer. The stability of these emulsions has been studied using several parameters (pH, conductivity, droplet size, dye test). The dye test and the conductivity measurement confirmed the W/O nature of the emulsion and W/O/W nature of the double emulsion. This study showed that we were able to develop a saturated double W/O/W emulsion.

PH-sensitive Pickering Emulsion Stabilized by Hydroxyapatite Nanoparticles: Stability and Controlled Release Study

This study investigates the formulation of surfactant-free Pickering emulsions that release a drug at a specific pH to improve its oral bioavailability. The stabilizing nanoparticles composed of hydroxyapatite were obtained by a process of nanoprecipitation. Pickering oil-in-water emulsions stabilized with hydroxyapatite nanoparticles and encapsulating a hydrophobic drug model (ibuprofen) were formulated using a high-energy process with rotor-stator turbo mixer (IKA® T25 digital ultra-Turrax). The experimental approach explored the impact of all formulation parameters, dispersed phase and amount of hydroxyapatite nanoparticles on the physicochemical properties of Pickering emulsions. The system was characterized by a methylene blue test, pH and conductivity measurements, and droplet size determination. In addition, Pickering emulsions stabilized by hydroxyapatite nanoparticles have the advantage of being destabilized in acidic medium leading to the release of the active principle via the droplets. The acidic medium release study (pH equal to 1.2) showed ibuprofen release as a function of initial droplet loading and saturation concentration. In the simulated intestinal medium at pH equal to 6.8, we found a better release of ibuprofen from emulsions that already had saturation in an acid medium. Thus, the interest of these Pickering emulsions lies on the fact that their non-toxicity and hydroxyapatite nanoparticles have advantage of being biocompatible because having the same mineral composition as bones and teeth. In addition, they allow destabilization of the emulsions and release of the drug. These emulsions not only protect patients from the side effects of acid-based drugs, but also contribute to increase the bioavailability of these acidic drugs.

Investigation of Nucleation under High-Shear Conditions for a Pharmaceutical Compound in an Unseeded System

The effect of in situ high energy mixing on primary nucleation of an active pharmaceutical compound (ABT-089) has been investigated by measuring the induction time for 1, 5, and 140 mL scales via different shear-inducing mechanisms, namely, magnetic stirrer, Couette device, and high-shear rotor–stator mixer known as wet-mill in pharmaceutical industry. Experiments performed on previously untested scale and equipment show that the onset time of nucleation decreases with an increase in average energy input per unit volume of the solution (i.e., specific energy input) as previously reported (Liu, J.; Svard, M.; Rasmuson, Å.C. Influence of Agitation on Primary Nucleation in Stirred Tank Crystallizers, Cryst. Growth Des. 2015, 15, 4177–4184). The variability in the induction time is found to decrease with an increase in volume and specific energy input. Studies at different scales are compared using the Rasmuson (Liu, J.; Svard, M.; Rasmuson, ÅC. Influence of Agitation on Primary Nucleation in Stirred Tank Crystallizers, Cryst. Growth Des. 2015, 15, 4177–4184) approach based on the classical nucleation theory. It is shown that the effect of specific energy input can be adequately described by the kinetic term “A” or pre-exponential factor within the classical nucleation theory framework because the critical free-energy term appears to be constant for the same scale. A semiempirical correlation specific to ABT-089 is proposed between the energy input to a system and the average onset of primary nucleation for each of the means of shear induction. The generic approach presented in this work can be used as part of the overall development strategy to scale-up and scale-down crystallization experiments by estimating the specific energy input or the corresponding tip speed requirement to induce primary nucleation within a specified time range. However, given the practical limitations on the amount of data that can be generated and used to develop such correlations, appropriate caution is necessary in defining the scope of their applicability.

Linking power and flow in rotor-stator mixers

The relationship between power and flow characteristics of batch rotor-stator mixers has been studied using CFD simulations with experimental power validation. The mixer studied was the Silverson L5M batch mixer with the standard emulsor head. The size of the holes in the screen and the constriction of the base hole were changed in small increments. The MRF technique was used to model rotor rotation. A model is developed in this study which links the power and flow numbers of the mixer. Since power is easy to measure experimentally, one can use this model to predict the flow number by measuring torque. A second model is also developed which allows one to predict the flow number using solely the geometry of the mixing head. This study greatly enhances our understanding of the relationship between power, flow and mixer geometry in rotor-stator mixers.

Numerical and experimental evaluation of droplet breakage of O/W emulsions in rotor-stator mixers

Emulsification of a high viscosity oil in water mixture was studied in a rotor-stator mixer. Experimental characterization of drop sizes and high-speed visualizations of drop breakup phenomena were performed, combined with computational fluid dynamics analysis of the flow field and population balance modeling. Single-phase flow simulations were carried out with a sliding mesh CFD model and combined with analysis of streamlines, turbulent eddy dissipation fields and high-speed visualizations of drop breakage. From frames taken over few seconds, with the oil-water system initially fully segregated, three drop breakage mechanisms were identified, supported by the CFD analysis: (i) a combination of the large scale circular motion in the tank and the fluid jet emanating from the stator holes; (ii) droplets that are pulled into the rotor – stator system from the bottom and flow through the gap between the stator and rotor, undergoing extremely high shear stresses, being expelled through the stator slots in several fragments, and iii) droplets that remain caught at the recirculation zones in the stator holes, increasing exposure time to high shear during the rotor blade passage. Finally, an optimization algorithm was coupled to a population balance model and the measured drop size distributions were reproduced in good agreement with experimental data, including the bimodal behavior attributed to the generation of satellite droplets.

A comparison of power and flow characteristics between batch and in-line rotor-stator mixers

Rotor-stator mixers are used in many industries to perform emulsification and de-agglomeration processes. Despite previous research, different modes of operation have not been compared in terms of flow and power characteristics. The aim of this study was to use CFD to investigate power and flow characteristics of a Silverson L5M mixer operating in batch and in-line mode. MRF was used along with the standard k-∊ turbulence model for the simulations. Results suggest that batch mixers can be characterised in the same was as in-line mixers, and for a given mixing head design, the characterisation is independent of the mode of operation. A new way of the calculating flow number is proposed, which explains some discrepant results from previous studies. Overall, this work enhances our understanding of rotor-stator mixers and allows for better design choices of mixer.

Emulsion Formation by Homogenization: Current Understanding and Future Perspectives

Emulsion formation by homogenization is commonly used in food production and research to increase product stability and to design colloidal structures. High-energy methods such as high-pressure homogenizers and rotor-stator mixers are the two most common techniques. However, to what extent does the research community understand the emulsion formation taking place in these devices? This contribution attempts to answer this question through critically reviewing the scientific literature, starting with the hydrodynamics of homogenizers and continuing by reviewing drop breakup and coalescence. It is concluded that although research in this field has been ongoing for a century and has provided a substantial amount of empirical correlations and scaling laws, the fundamental understanding is still limited, especially in the case of emulsions with a high-volume fraction of the disperse phase, as seen in many food applications. These limitations in the current understanding are also used to provide future perspectives and suggest directions for further investigation.

Reducing the Induction Time Using Ultrasound and High-Shear Mixing in a Continuous Crystallization Process

Continuous crystallization in tubular crystallizers is of particular interest to the pharmaceutical industry to accurately control average particle size, particle size distribution, and (polymorphic) shape. However, these types of crystallizers require fast nucleation, and thus, short induction times at the beginning of the flow process, which is challenging for larger and complex organic molecules. High shear and/or the presence of bubbles were identified to influence the nucleation behavior. This work investigates the effects of both high-shear mixing and ultrasound on the anti-solvent crystallization of paracetamol in acetone–water. Both devices generate intense amounts of shear and gas bubbles. Generally, the results show that increasing input power decreases the induction time significantly for both the rotor–stator mixer and ultrasound probe. However, the induction time is almost independent of the supersaturation for the ultrasound probe, while the induction time significantly increases with decreasing supersaturation for the rotor–stator mixer. In contrast, the particle size distribution for the rotor–stator mixer is independent of the supersaturation, while increasing supersaturation decreases the particle size for the ultrasound probe.

Exfoliation of Graphite into Graphene by a Rotor–Stator in Supercritical CO2: Experiment and Simulation

High-shear-induced exfoliation of graphite into graphene using a rotor–stator mixer in supercritical CO2 is a promising approach to massively produce high-quality graphene. The exfoliation mechanism of the rotor–stator and its geometry influence on the exfoliation yield were investigated in this work. The results showed that the active region of peeling off graphite to graphene was located between the rotor (including the rotating fluid) and the stator, in which the velocity gradient was the highest. The exfoliation time was valid only when the graphite particles fell in the active region. The volume of the effective area and the active exfoliation time affected the graphene yield significantly. The optimal ratio of the wall area of the stator is about 80%. Both the lengthened rotor–stator and the multiwall stators increased the yield by 40%. Also, the similar results were obtained in other solvents like water and NMP by the optimal structure of the rotor–stator in terms of exfoliation efficiency. The fin...

Flow and power characteristics of an axial discharge rotor-stator mixer

Flow and power characteristics of a two-stage inline axial flow rotor-stator mixer have been investigated via CFD simulation of the RANS equations. The realizable k–ε turbulence model with enhanced wall treatment was used to predict the mean velocity field, turbulent kinetic energy, and energy dissipation rate, as well as flow and power number. The results from different computational meshes are compared. The periodic behavior of throughput and torque is attributed to rotor position and shown to be related to number of blades and stator slots. The flow, power number, and energy dissipated in different fluid regions are reported for various backpressure operating conditions.

Rotor-Stator Mixers: From Batch to Continuous Mode of Operation—A Review

Although continuous production processes are often desired, many processing industries still work in batch mode due to technical limitations. Transitioning to continuous production requires an in-depth understanding of how each unit operation is affected by the shift. This contribution reviews the scientific understanding of similarities and differences between emulsification in turbulent rotor-stator mixers (also known as high-speed mixers) operated in batch and continuous mode. Rotor-stator mixers are found in many chemical processing industries, and are considered the standard tool for mixing and emulsification of high viscosity products. Since the same rotor-stator heads are often used in both modes of operation, it is sometimes assumed that transitioning from batch to continuous rotor-stator mixers is straight-forward. However, this is not always the case, as has been shown in comparative experimental studies. This review summarizes and critically compares the current understanding of differences between these two operating modes, focusing on shaft power draw, pumping power, efficiency in producing a narrow region of high intensity turbulence, and implications for product quality differences when transitioning from batch to continuous rotor-stator mixers.

Tramadol Encapsulation in Aqueous Phase of Water/Oil Pickering Emulsion Stabilized by Magnesium Oxide Particles

Pickering emulsions are systems without surfactants, stabilized by solid particles. These emulsions are experiencing a renewed interest, on the one hand, because it is preferable to limit the use of synthetic surfactants for toxicological and environmental reasons and, on the other hand, the need to make new formulations in order to control the drug release patterns by encapsulation or controlled release. Thus, we were interested in the formulation and evaluation of W / O Pickering emulsions stabilized by particles of magnesium oxide with tramadol hydrochloride in the internal phase. The Bancroft rule served as a model for the formulation. The emulsification was carried out by progressively adding an aqueous phase dispersed in an oil-dispersing phase using a turbo rotor stator mixer. The stability of these emulsions was studied using several parameters (droplet size, pH, viscosity, conductivity...) and the qualitative and quantitative analysis of the active ingredient by UV-visible spectrophotometry. The results obtained showed that the dye test and the conductivity measurement confirmed the W / O nature of these emulsions. Some parameters such as droplet size, pH and viscosity were strongly influenced by the amounts of Magnesium oxide particles and the active ingredient. The qualitative and quantitative analysis of the active ingredient confirmed the presence of tramadol in the internal aqueous phase. Thus, we have succeeded in developing a stable W / O Pickering emulsion with magnesium oxide particles. In addition, we were able to encapsulate tramadol in the dispersed aqueous phase.

Pumping capacity of inline dynamic mixers and its effect on process flow control

A semi-empirical (dimensional) model has been developed to predict the performance of an inline dynamic mixer within a specific application to ensure effective process design and operation. Inline dynamic mixers (or rotor-stator mixers) are known to behave like centrifugal pumps, particularly when operating at high rotor speeds. A mismatch between the process flowrate and the mixer’s inherent pumping action can result in either high or low pressure at the inlet of the rotor-stator mixer, which can influence process control. Experiments were conducted at pilot scale using three models of inline dynamic mixers from Silverson Machines Ltd., with water as the process fluid. A wide range of flowrates and rotor speeds were covered where the pressures at the inlet and outlet of the Silverson were measured in each condition. A good correlation (R2=0.9998) between Silverson inlet pressure, process flowrate and rotor tip speed has been established but the expression requires knowledge of the nominal flowrate of the mixer model at a benchmark rotor tip speed. This reported model can serve as design criteria when selecting mixer models even at production scale.

An energy transport based evolving rheology in high-shear rotor–stator mixers

Rotor–stator mixers such as the inline Silverson are widely used by the process industry. Existing literature on experimental and computational investigations of these devices focus on characterising the power draw and turbulent mixing of Newtonian fluids and non-Newtonian fluids such as emulsions. The current knowledge on the performance of these mixers in blending and mixing fluids with an underlying complex structure is limited. Modelling and simulation of such structured liquids has traditionally been challenging due to the complexity of the constitutive governing equations which are to be solved for the prediction of rheology. In this paper a novel approach to model evolving rheology is proposed. This approach incorporates important physical phenomenon such as the strain rate history effects in the generalised Newtonian fluid model. The new approach is used to model mixing in a pilot scale inline Silverson mixer via Computational Fluid Dynamics (CFD) simulations. A sliding mesh algorithm coupled to eddy viscosity turbulence closure is used. Experiments have been performed with the inline Silverson mixer placed in a recirculation loop for two different rotor speeds, and rheological measurements have been performed on the samples taken at the outlet of the mixer. Computational results are compared with the viscosity measurements and it is found that the model predictions for the evolution of viscosity are in reasonable agreement with the experimental data.