Liste von Publikationen zur SOPAT Technologie:

Cocke, J. und Maaß, S., 2017
Cross Linking Between the Baffling Effect and Phase Inversion During Liquid–Liquid Monomer Mixing
Macromolecular Reaction Engineering, DOI: 10.1002 (link)
Recent studies have revealed unexpected developments in drop size by only small changes in the geometrical constraints of batch vessels used for suspension polymerization. The effect of the dispersed phase fraction, surfactant concentration, as well as baffle length on the evolving drop size distribution in different low viscous liquid–liquid systems is investigated. The analysis is focused on the drop-dominated systems by hindering the coalescence by polyvinyl alcohol (PVA) concentrations up to three times higher than the critical micelle concentration. The influence of PVA on drop size in breakage-dominated systems is well reproduced with population balance equation simulations. The Drops are measured using a photo-optical system with automated image analysis. The measured drop sizes increase with increasing dispersed phase fraction. As coalescence is completely hindered, all observed coalescence effects are connected to phase inversion, a catastrophic phenomenon during suspension polymerization for industrial production processes. Phase inversion can be reproduced for all studied solvents with and without the use of surfactants. In particular, the influence of the baffles can be reproduced. The system is adapted and trained to detect phase inversion as a warning system to make the suspension polymerization process more stable and robust.
Panckow, R. P., Reinecke, L., Cuellar, M. C. und Maaß, S., 2017
Photo-Optical In-Situ Measurement of Drop Size Distributions: Applications in Research and Industry
Oil Gas Sci. Technol. – Rev. IFP Energies nouvelles, 74 (3) (link)
The exact knowledge of Drop Size Distributions (DSD) plays a major role in various fields of applications to control and optimise processes as well as reduce waste. In the microbial production of advanced biofuels, oil droplets are produced under turbulent conditions in an aqueous medium containing many surface active components, which might hinder the recovery of the product. Knowledge of DSD is thus essential for process optimisation. This study demonstrates the capability of a photo-optical measurement method for DSD measurement in fermentation broth and in plate separators aimed at cost reduction in the microbial production of advanced biofuels. Measurements were made with model mixtures in a bioreactor, and at the inlet and outlet of a plate separator. In the bioreactor, the method was effective in detecting a broad range of droplet sizes and in differentiating other disperse components (e.g. microbial cells and gas bubbles). In the plate separator, the method was effective in determining the influence of the varied parameters on the separation Efficiency.
 Li, X., 2017
 Ionic liquids in separations: applications for pyrolysis oil and emulsion Systems
 University of Twente (link)
Solvent extraction is one of the main separation techniques and has been developed for a wide range of industrial applications. Ionic liquids (ILs) are often considered as environmentally friendly solvents and have been studied widely in various laboratory applications. Aiming to design effective extraction processes, in this work ILs have been employed for fractionation of pyrolysis oil, for demulsifying oil-in-water emulsions to recover surfactant, and for the design of a novel smart separation process type: fixed liquid extraction.
Pyrolysis oil, the liquid product of fast pyrolysis of lignocellulosic biomass, is a complex mixture containing hundreds of components. For the valorization of pyrolysis oil, value-added chemicals are required to be separated. A study on aromatics removal from pyrolytic sugar streams was described in chapter 2 and 3, and a study on fractionation of oxygenates (acetic acid, glycolaldehyde and acetol) from pyrolysis oil cuts in chapter 4.
In chemical enhanced oil recovery (CEOR), surfactant together with alkali and polymer are injected in the reservoir to enhance oil recovery yield. The surfactant is usually blended with water and oil in stable emulsions. Therefore, the first step for surfactant recovery is to destabilize these oil emulsions which is called demulsification. In chapter 5, in total 13 ILs were evaluated as demulsifiers for a model oil-in-water emulsion, and the efficiency and mechanism of demulsification were discussed.
Application of ILs at industrial scale is restricted by their major drawbacks, such as high viscosity and corrosivity. IL-based emulsion systems that make use of stabilizers can potentially overcome these drawbacks. In chapter 6 the formation of several IL-based emulsions stabilized by microgel particles was studied, including the parametric influences on the drop size distributions. The influence of microgel particles on the extraction capacity and kinetics were investigated as well. In chapter 7, a novel separation process concept was explored for the processes, referred to as fixed liquid extraction, aiming for the design of separation process for extremely small S/F.
ILs can effectively remove aromatics from pyrolytic sugar streams. Value-added oxygenates can be fractionated by a two-step extraction process. The studied halogenide ILs and P666,14[N(CN)2] can demulsify heptane-in-water emulsions and the mechanism of demulsification is anion exchange. Several IL-based emulsions stabilized by microgel particles has been prepared, and the concept of fixed liquid extraction has been demonstrated for the first time.
Grafschafter, A. und Siebenhofer, M., 2017
Design Rules for the Taylor-Couette Disc Contactor
Chemie Ingenieur Technik, 89 (4): 409-415 (link)
The Taylor-Couette disc contactor (TCDC) is a stirred liquid-liquid phase contactor which is suitable for applications in bioseparations. For liquid-liquid reactor design, information about the specific mass transfer area is inevitable. Therefore, the drop size distribution and holdup in the TCDC were investigated under various operating conditions and appropriate correlations for the prediction of these parameters have been determined. Experimental data of drop size distributions were correlated with lognormal, Gaussian, and Weibull drop size distribution functions.
Skale, T., Hohl, L., Kraume, M. und Drews, A., 2017
Feasibility of w/o Pickering emulsion ultrafiltration
Journal of Membrane Science (link)
This study shows the possibility of separating w/o Pickering emulsions (PE) via ultrafiltration and consequently enables a continuous process concept for catalysed L/L reactions in Pickering emulsions which are currently receiving increased attention. In two types of filtration experiments, the stability of PE against coalescence caused by the applied filtration pressure and stirring was shown. The PE could be concentrated up to 80% water phase fraction. In pressure stepping experiments, permeabilities between 3 and 10 L/(m2 h bar) could be achieved depending on the drop size distribution of the PE. For the used 1 kDa PVDF membrane, an unexpected overproportional behaviour of the fluxes at higher pressures was observed. The presented results show that Pickering emulsions can be regarded as a promising alternative to conventional dispersions also in continuous L/L catalysis.
Marbà-Ardébol, A.-M., Emmerich, J., Neubauer, P. und Junne, S., 2017
Single-cell-based monitoring of fatty acid accumulation in Crypthecodinium cohnii with three-dimensional holographic and in situ microscopy
Process Biochemistry, 52: 223-232 (link)
To date, on line monitoring in bioprocesses is restricted to conventional parameters. Presently, advances in microscopy allow the monitoring of single-cell size distributions in a bypass or in situ. These data provide information regarding population heterogeneity, substrate conversion, or product synthesis as these parameters are related to the size of the cells. In this study, changes in the single-cell size distribution of the heterotrophic microalgae Crypthecodinium cohnii were tracked with holographic microscopy and a photo-optical microscopy probe, which is applicable in situ. This algae produces the polyunsaturated fatty acid docosahexaenoic acid (DHA). On the basis of the cell size and broadness of the size distribution, the applied methods enabled to distinguish between cells in the growth and production phase. Under conditions of low growth and high fatty acid accumulation, the cell size kept concomitantly changing. The correlation between cell size measurements and the intracellular DHA content was confirmed by regression analysis. The phase heterogeneity, which was measured by holographic microscopy, changed simultaneously with the DHA synthesis. The amount of information obtained by both digital holographic and in situ microscopy is similar to that obtained by flow cytometry but with reduced effort for a real-time Analysis.
Heeres, A. S., Heijnen, J., van der Wielen, L. A. M. und Cuellar, M. C., 2016
Gas bubble induced oil recovery from emulsions stabilised by yeast components
Chemical Engineering Science, 145: 31-44 (link)
In the search for advanced biofuels, microorganisms have been developed that make and secrete long chain hydrocarbons, resulting in a four phase fermentation mixture (cells, aqueous liquid, organic product, and (produced) gas). The product immiscibility offers the potential for a straightforward recovery, but surface active components in the fermentation broth emulsify the product droplets. In the current process, multiple centrifugation steps with chemical de-emulsifiers are used for product recovery, posing economic and environmental burdens on the process. In this paper, an alternative separation method is presented, in which gas bubbles induce coalescence of the emulsified oil droplets, obtaining a continuous oil layer. The oil layer formation was influenced by the gas flow rate, nozzle diameter, column geometry, and emulsion properties, offering the possibility for process optimisation. The developed gas bubble induced oil recovery method does not require chemical additives, uses mild process conditions, and can potentially be integrated with the fermentation, giving a low cost alternative for the conventional recovery method. Using this technology, another step can be made towards economically feasible production of advanced biofuels.
Hohl, L., Schulz, J., Paul, N. und Kraume, M., 2016
Analysis of physical properties, dispersion conditions and drop size distributions in complex liquid/liquid systems
Chemical Engineering Research and Design, 108: 210-216 (link)
Tunable multiphase systems can be applied for the hydroformylation of long-chained olefins. These reaction systems enable highest yields and fastest separation processes under three phase operating conditions. However, little is known about the characteristic properties of these complex dispersions. In this study, a simplified model system consisting of water, 1-dodecene and a solvo-surfactant was analysed. The minimum aggregation concentration, interfacial tension, density and viscosity were determined. Additionally, the phase behaviour was investigated using conductivity measurements. The phase volume fractions under three phase conditions were analysed. An endoscope measurement technique was applied to determine the dispersion conditions and drop size distributions in a stirred tank as a function of composition and stirrer speed.
Fries, T., Dittler, I., Blaschczok, K. , Löffelholz, C., Dornfeld, W., Schön, R., Drwes, A. und Eibl, D., 2016
Quantifizierung der hydromechanischen Beanspruchung von Pumpen auf tierische Zellen mittels des nicht-biologischen Modellsystems Emulsion
Chemie Ingenieur Technik, 88(1-2): 177-182 (link)
Zur Quantifizierung der Beanspruchung durch die Zentrifugalpumpe PuraLev® 200SU wurde der mittlere Sauter-Durchmesser des Modellsystems Emulsion in Abhängigkeit des massenspezifischen Leistungseintrages ermittelt. Anschließend wurde mit ausgewählten Parametern die Absterberate von CHO-Zellen untersucht und mit den Ergebnissen der Emulsionsversuche verglichen. Es konnte gezeigt werden, dass für das biologische System neben dem Leistungseintrag weitere Parameter wie die Impellerumfangsgeschwindigkeit Einfluss auf die Partikelbeanspruchung haben.
Amokrane, A., Maaß, S., Lamadie, F., Puel, F. und Charton, S., 2016
On droplets size distribution in a pulsed column. Part I: In-situ measurements and corresponding CFD–PBE simulations
Chemical Engineering Journal, 296: 366–376 (link)
The pulsed column is one of the most used contactors in solvent extraction processes, especially in the nuclear fuel treatment industry and for hydrometallurgical applications. The optimisation of solvent extraction operations needs a thorough understanding of the diphasic flow’s properties inside the apparatus, especially that of the dispersed phase. For the first time, in-situ measurements of the drop size distribution (DSD) were achieved in a small diameter pulsed column (lab scale), by using the SOPAT-VF light reflectance video probe on two water-in-oil (W/O) systems exhibiting low and high viscosities of the oil phase. Separate effects of the pulse amplitude and frequency on the DSD and the phase hold-up were evidenced for both O/W systems, despite their different viscosity levels. This technique allowed very precise measurements of droplet-diameters in quite a confined space, three orders of magnitude, which is remarkable. Experimental findings allowed the validation of a computational fluid dynamics (CFD)–population balance equation (PBE) coupled model and the improvement of breakage and coalescence kernel parameters. Rupture and coalescence mechanisms were correctly captured. The dependence of emulsion’s properties on the column hydrodynamics could then be illustrated over one period of the pulsed flow. Incidence of higher levels of pulsation intensity, hardly achievable experimentally, and separate investigation of the effects of viscosity and surface tension are presented.
Hohl, L., Paul, N., und Kraume, M., 2016
Dispersion conditions and drop size distributions in stirred micellar multiphase systems
Chemical Engineering and Processing, 99: 149–154 (link)
Micellar multiphase systems can be applied to enable reactions like the hydroformylation of long-chained olefins. These liquid/liquid systems combine advantages of homogenous catalysis like high and specific yield or mild reaction conditions with a fast phase separation process. In previous studies highest yields were observed in systems under three phase operation conditions whereby the reaction rate was a function of stirrer speed. Hence, dispersion conditions and drop size distributions need to be taken into consideration. In this study, micellar three phase systems were analysed using an endoscope measurement technique and image analysis in a stirred tank. A methodical approach to identify the respective phases and to clarify the dispersion conditions was found. The mean Sauter diameters were quantified as a function of the system composition. By applying abrupt changes of the stirrer frequency, the dynamic behaviour and coalescence effects were investigated.
Aksamija, E., Weinländer, C., Sarzio, R. und Siebenhofer, M., 2015
The Taylor-Couette Disc Contactor: A Novel Apparatus for Liquid/Liquid Extraction
Separation Science and Technology 50(18):2844-2852 (link)
In the present project, the design of the Rotating Disc Contactor (RDC) has been fundamentally simplified and optimized in terms of separation efficiency, load capacity, and suitability for downstream processing of bio-based materials. Optimized performance was obtained when the stator discs were abandoned and their function was adopted by enlarged rotor discs. Mass transfer experiments showed improved separation efficiency in the novel apparatus, while the operating range was simultaneously increased. Due to recurring drop formation and re-coalescence in every compartment, large droplets were not observed, resulting in a narrower drop size distribution compared to the RDC.
Panckow, R., Comandè, G., Maaß, S. und Kraume, M., 2015
Determination of Particle Size Distributions in Multiphase Systems Containing Nonspherical Fluid Particles
Chemical Engineering & Technology, 38(11): 2011-2016 (link)
The presented results are obtained by means of an automated image analysis software capable of measuring particle diameters in photographs taken in situ by a 2D photo-optical measurement device. Initially, the applicability of this measurement method must be proven for fluid particle systems on the basis of spherical-shaped particles. As a first approach in targeting the issue of irregularly shaped fluid particles, a pure gas-liquid system is analyzed to investigate the influence of the variation of operating parameters. In the second step, the spherical shape is changed by reducing the surface-active auxiliaries. The usage of shape factors helps to interpret the appearance of the 2D projected objects. By examining the capabilities and limitations, it is demonstrated that the size of fluid particles can be detected online and in situ in order to analyze and control the particular chemical process.
Dittler, I., Dornfeld, W., Schöb, R., Cocke, J., Rojahn, J., Kraume, M. und Eibl, D., 2015
A Cost-effective and Reliable Method to Predict Mechanical Stress in Single-use and Standard Pumps
Journal of Visualized Experiments, (102): 10.3791/53052 (link)
Pumps are mainly used when transferring sterile culture broths in biopharmaceutical and biotechnological production processes. However, during the pumping process shear forces occur which can lead to qualitative and/or quantitative product loss. To calculate the mechanical stress with limited experimental expense, an oil-water emulsion system was used, whose suitability was demonstrated for drop size detections in bioreactors1. As drop breakup of the oil-water emulsion system is a function of mechanical stress, drop sizes need to be counted over the experimental time of shear stress investigations. In previous studies, the inline endoscopy has been shown to be an accurate and reliable measurement technique for drop size detections in liquid/liquid dispersions. The aim of this protocol is to show the suitability of the inline endoscopy technique for drop size measurements in pumping processes. In order to express the drop size, the Sauter mean diameter d32 was used as the representative diameter of drops in the oil-water emulsion. The results showed low variation in the Sauter mean diameters, which were quantified by standard deviations of below 15%, indicating the reliability of the measurement technique.
Heeres, A., Schroën, K., Heijnen, J., van der Wielen, L. und Cuellar, M., 2015
Fermentation broth components influence droplet coalescence and hinder advanced biofuel recovery during fermentation
Biotechnology Journal, 10(8): 1206-1215 (link)
Developments in synthetic biology enabled the microbial production of long chain hydrocarbons, which can be used as advanced biofuels in aviation or transportation. Currently, these fuels are not economically competitive due to their production costs. The current process offers room for improvement: by utilizing lignocellulosic feedstock, increasing microbial yields, and using cheaper process technology. Gravity separation is an example of the latter, for which droplet growth by coalescence is crucial. The aim of this study was to study the effect of fermentation broth components on droplet coalescence. Droplet coalescence was measured using two setups: a microfluidic chip and regular laboratory scale stirred vessel (2 L). Some fermentation broth components had a large impact on droplet coalescence. Especially components present in hydrolysed cellulosic biomass and mannoproteins from the yeast cell wall retard coalescence. To achieve a technically feasible gravity separation that can be integrated with the fermentation, the negative effects of these components on coalescence should be minimized. This could be achieved by redesign of the fermentation medium or adjusting the fermentation conditions, aiming to minimize the release of surface active components by the microorganisms. This way, another step can be made towards economically feasible advanced biofuel production.
Dittler, I., Kaiser, S., Blaschczok, K., Löffelholz, C., Bösch, P., Dornfeld, W., Schöb, R., Rojahn, J., Kraume, M. und Eibl, D., 2014
A cost-effective and reliable method to predict mechanical stress in single-use and standard pumps
Engineering in Life Sciences, 14(3): 311-317 (link)
The suitability of oil–water emulsions to predict shear forces in stirred bioreactors under cost-effective and time-saving conditions has been demonstrated several times, but no application to pumps has been described so far. In this report, the drop sizes in a model oil–water system were determined for the Levitronix PuraLev® multiuse (MU) series (PuraLev® 200MU and PuraLev® 600MU), a peristaltic pump (Masterflex® I/P Easy Load), and 4-piston diaphragm pump (Quattroflow 1200-SU, where SU is single-use) using inline endoscopy. It was determined that the Sauter mean diameter could be used as a comparison criterion to estimate mechanical stress in pumps. The investigation showed that PuraLev® MU pumps are characterized by up to 59% larger Sauter mean diameters than their counterparts at comparable operational conditions. This indicates lower hydrodynamic stress in the PuraLev® MU pumps. Using computational fluid dynamics, a well-streamlined fluid flow and low turbulent energy dissipation rates were found in the PuraLev® MU pumps, which correlated well with experimental results. A calculation model was used to predict the Sauter mean diameter by combining both experimental and computational fluid dynamics data. Good agreement with deviations below 13% was determined between model predictions and experimental data.
Ngo, T. H., 2013
Gas absorption into emulsions
Dissertation | Technische Universität Carolo-Wilhelmina zu Braunschweig (link)
Gas-liquid-liquid systems, in which a gas is absorbed into an oil-water emulsion with or without chemical reaction, are of scientific, technical and commercial importance due to their wide industrial application.

Through the development of the aqueous biphasic technique in homogeneous catalysis, gasliquid-liquid systems have gained increasing attention in various chemical processes, e.g. hydroformylation, carbonylation, hydrogenation and oligomerization (Cornils, 1999). This aqueous biphasic technique involves using water-soluble ligands to solubilize the metal complex catalyst in an aqueous phase, which is easily separated from reactants and reaction products by decantation. Hydroformylation of propene to n-butyraldehyde is a typical example for important industrial applications of gas-liquid-liquid reaction systems. In this process, the reactive gas mixture of carbon monoxide, hydrogen and propylene is converted to n-butyraldehyde in an aqueous solution of rhodium catalyst (Cents et al., 2004).  Three different phases therefore exist in the reactor: the gas phase containing CO, H2, and propylene; the aqueous catalyst phase (water phase); and the dispersed organic butyraldehyde phase (oil phase).

Panckow, R., Maaß, S., Emmerich, J. und Kraume, M., 2013
Automatisierte Quantifizierung von Blasengrößenverteilungen in einem gerührten Luft/Wasser-System
Chemie Ingenieur Technik, 85(7): 1036-1045 (link)
Die exakte Bestimmung der Phasengrenzfläche in Gas/flüssig-Systemen ist unabdingbar für die optimierte Prozessführung verfahrenstechnischer Anlagen. Die experimentelle Studie zeigt Untersuchungen an einem Luft/Wasser-System mittels dreier Messtechniken hinsichtlich der Bestimmung der Veränderung der Blasengröße in Abhängigkeit von der Rührerdrehfrequenz. Die äußerst präzise und selbstreferenzielle Methode der foto-optischen Analyse wird bzgl. der Bildverarbeitung näher betrachtet. Es wird gezeigt, dass die neuesten Entwicklungen im Bereich der Foto-Optik und der Bildverarbeitung wertvolle Grundlagen zur Etablierung einer präzisen Echtzeitgrößenmesstechnik geliefert haben. Dies ist notwendig, da laserbasierte Methoden nur unzureichende Prozessdaten liefern.
Schilder, L., Maaß, S. und Jess, A., 2013
Effective and Intrinsic Kinetics of Liquid-Phase Isobutane/2-Butene Alkylation Catalyzed by Chloroaluminate Ionic Liquids
Industrial & Engineering Chemistry Research, 52(5): 1877-1885 (link)
The kinetics of the IL-catalyzed reaction of isobutane with 2-butene and the size distribution of the dispersed IL droplets were investigated in a stirred tank reactor. The results were used to calculate the intrinsic reaction rate constant with a model accounting for the interplay of external and internal mass transport and chemical reaction. The intrinsic reaction rate was found to be very high, leading to a low effectiveness factor of the IL catalyst. The used fraction of the IL droplets with a diameter of about 400 μm was very small, and the actual reaction took place only in a thin spherical shell with a thickness of about 5 μm.
Ngo, T. H. und Schumpe, A., 2012
Oxygen Absorption into Stirred Emulsions of n-Alkanes
International Journal of Chemical Engineering, 2012 (link)
Absorption of pure oxygen into aqueous emulsions of n-heptane, n-dodecane, and n-hexadecane, respectively, has been studied at 0 to 100% oil volume fraction in a stirred tank at the stirring speed of 1000 min−1. The volumetric mass transfer coefficient, , was evaluated from the pressure decrease under isochoric and isothermal (298.2 K) conditions. The O/W emulsions of both n-dodecane and n-hexadecane show a maximum at 1-2% oil fraction as reported in several previous studies. Much stronger effects never reported before were observed at high oil fractions. Particularly, all n-heptane emulsions showed higher mass-transfer coefficients than both of the pure phases. The increase is by upto a factor of 38 as compared to pure water at 50% n-heptane. The effect is tentatively interpreted by oil spreading on the bubble surface enabled by a high spreading coefficient. In W/O emulsions of n-heptane and n-dodecane increases with the dispersed water volume fraction; the reason for this surprising trend is not clear.
Maaß, S., Rojahn, J., Hänsch, R. und Kraume, M., 2012
Automated drop detection using image analysis for online particle size monitoring in multiphase systems
Computers and Chemical Engineering, 45: 27-37 (link)
Image analysis has become a powerful tool for the work with particulate systems, occurring in chemical engineering. A major challenge is still the excessive manual work load which comes with such applications. Additionally manual quantification also generates bias by different observers, as shown in this study. Therefore a full automation of those systems is desirable. A MATLAB® based image recognition algorithm has been implemented to automatically count and measure particles in multiphase systems.

A given image series is pre-filtered to minimize misleading information. The subsequent particle recognition consists of three steps: pattern recognition by correlating the pre-filtered images with search patterns, pre-selection of plausible drops and the classification of these plausible drops by examining corresponding edges individually. The software employs a normalized cross correlation procedure algorithm. The program has reached hit rates of 95% with an error quotient under 1% and a detection rate of 250 particles per Minute depending on the System.

Maaß, S. und Kraume, M., 2012
Prozesse auf Basis der Partikelgröße steuern
CIT Plus, 6: 31-33 (link)
CITplus – Das Sopat-System (Smart On-line Particle Analysis Technology) ist die Kombination einer photo-optischen Messtechnik und einer Bildverarbeitungs-Software. Diese analysiert im Gegensatz zu herkömmlichen inline-Sonden auch in hochkonzentrierten Dispersionen Größe und Konzentration von festen oder fluiden Partikeln in Echtzeit. Dies findet – anders als bei offline Methoden – direkt im Reaktor statt, wodurch erstmals eine Prozesssteuerung auf Basis der Partikelgröße möglich wird.

Maaß, S., Paul, N. und Kraume, M., 2012
Influence of the dispersed phase fraction on experimental and predicted drop size distributions in breakage dominated stirred systems
Chemical Engineering Science, 76: 140-153 (link)
The effect of the dispersed phase fraction on the evolving drop size distribution in different low viscous agitated liquid/liquid systems was investigated. The analysis focused on the drop breakage phenomena by hindering the coalescence completely. Therefore, polyvinyl alcohol concentrations were used around three times higher than the critical micelle concentration.

The measured drop sizes were increasing with increasing dispersed phase fraction. As coalescence was completely hindered and also the measured dispersion viscosity showed no influence on the dispersed phase hold-up, the size increase is proposed to be a result of turbulence hindering.

The influence of the dispersed phase fraction on the drop sizes in breakage dominated systems was well reproduced with population balance equation (PBE) simulations. The used breakage models require a turbulence damping factor (1+φd), which is used in most of the common models. Summarizing the various PBE simulations we can conclude that drop sizes in systems with different dispersed phase fractions can be easily predicted, if the model parameters are fitted to one set of experiments studying the same physical system. The change of the solvent was successfully simulated with outstanding results for two of the three further investigated organics.

The used Weber correlations were also able to reproduce the linear interdependency between the drop size and the dispersed phase fraction. Unfortunately, every change in the dispersed phase needed new parameter estimation. As at least three out of four different liquid/liquid systems were predicted with excellent results, the PBE is proposed as a more robust tool which gives additionally information about the transient behavior of the system. Therefore, PBE should be used rather than the classical correlations widely used in academics and industries.

Maaß, S. und Kraume, M., 2012
Determination of breakage rates using single drop experiments
Chemical Engineering Science, 70: 146-164 (link)
An evaluation of several breakage rates from the literature based on single drop experiments was carried out. This data was collected in a single drop breakage cell under turbulent conditions, comparable to those in a stirred tank. For a constant initial diameter and flow velocity at least 750 single drops have been investigated to measure the breakage time and probability, using high-speed imaging. These results were used for the determination of breakage rates by the product of the inverse of breakage time and the breakage probability. The same subdivision was carried out for the literature models. These differentiations in the analysis showed that published models for the breakage probability are more or less similar and in good agreements with the experimental results. Proposed approaches for the breakage time are contrary. The experiments support the assumption of some researchers that the breakage time rise with increasing drop diameter. The magnitude of the predicted values of the breakage time for all kind of models is one or more magnitudes higher than experimental results in this study and from literature. Furthermore the influence of the physical properties, like viscosity or interfacial tension, is only poorly reflected in the available models. These analysis results lead to an improved breakage time model, which takes into account different breakage mechanisms and the influence of viscosity and interfacial tension. Combined with a breakage probability from literature, this new model leads to an excellent prediction of the breakage rate for the investigated single drops.

Maaß, S., Buscher, S., Hermann, S. und Kraume, M., 2011
Analysis of particle strain in stirred bioreactors by drop breakage investigations
Biotechnology Journal, 6(8): 979-992 (link)
Understanding of particle strain and drop breakage is relevant for various technical applications. To analyze it, single drop experiments in a breakage cell and evolving drop size distributions in an agitated system are studied. The mechanisms for particle strain and drop breakage are assumed to be comparable for the investigated turbulent flow regime. The agitation process is simulated using a population balance model. This model provides transient prediction capacities at different scales and can be used for scale-up/down projects. The number and the size distributions of daughter fragments for single drops have been studied. The results clearly support the assumption of binary breakage. The most common assumption of a Gaussian distribution for the daughter drop size distribution could not be supported. The evolution of a breakage-dominated toluene/water system was then simulated using different daughter drop size distributions from literature. The computational results were compared with experimental values. All simulations were able to predict the transient Sauter mean diameter excellently but varied strongly in the results on the shape of the distribution. In agreement with the experimental single drop results, the use of a bimodal or a very broad bell-shaped distribution of the daughter drops is proposed for the simulations. Although these results were obtained in a particular vessel for a specific phase system, it can be applied to simulate transient multiphase systems at different scales. We would expect that the general trends observed in this study are comparable to various applications in multiphase bioreactors.

Maaß, S., Rehm, T. und Kraume, M., 2011
Prediction of drop sizes for liquid-liquid systems in stirred slim reactors – Part II: Multi stage impellers
Chemical Engineering Journal, 168(2): 827–838 (link)
Based on the successful predictions of transient drop sizes in the first part of this research (Maaß et al., Prediction of drop sizes for liquid–liquid systems in stirred slim reactors—part I: single stage impellers. Chem. Eng. J., 162 (2010) 792–801), this part is a straight continuation and extension of the earlier work. The predictive capabilities of the used population balance equation model are increased for single stage impellers and transferred to scale-up procedures of such applications. Therefore different scale-up rules for liquid–liquid systems are tested experimentally and by simulations in two different sized, geometrically similar vessels. The multi stage impellers are tested against comparable single stage impellers in terms of power consumption, mixing time and minimum impeller speed. Especially for high aspect ratios (larger than three), multi stage impellers successfully compete with the single stage ones. The measured drop size distributions in slim reactors with multi stage impellers showed no dependency on the local position, although the dispersion process is tedious due to the compartmentalization. The simulations are not able to reflect this initial phase of the dispersion process, but are in close agreement with the experiments after complete dispersion is fulfilled. Based on these experiences the aspect ratio is increased up to five and the resulting drop size can be predicted with reasonable deviations (lower than 10%). The results of the scale-up of this multi stage impeller liquid–liquid system do not lead to a clear conclusion. Although the simulations recommend the use of constant power input, the experiments could not support this. None of the other traditional scale-up rule are supported by the experiments. Overall, the results of power consumption, mixing time and dispersion behavior show the great potential of multi stage impellers for process optimization and intensification in slim reactors.

Maaß, S., Wollny, S., Voigt, A. und Kraume, M., 2011
Experimental comparison of measurement techniques for drop size distributions in liquid/liquid dispersions
Experiments in Fluids, 50(2): 259-269 (link)
An online measurement technique for drop size distribution in stirred tank reactors is needed but has not yet been developed. Different approaches and different techniques have been published as the new standard during the last decade. Three of them (focus beam reflectance measurement, two-dimensional optical reflectance measurement techniques and a fiber optical FBR sensor) are tested, and their results are compared with trustful image analysis results from an in situ microscope. The measurement of drop sizes in liquid/liquid distribution is a major challenge for all tested measurement probes, and none provides exact results for the tested system of pure toluene/water compared to an endoscope. Not only the size analysis but also the change of the size over time gives unreasonable results. The influence of the power input on the drop size distribution was the only reasonable observation in this study. The FBR sensor was not applicable at all to the used system. While all three probes are based on laser back scattering, the general question of the usability of this principle for measuring evolving drop size distributions in liquid/liquid system is asked. The exterior smooth surface of droplets in such systems is leading to strong errors in the measurement of the size of the drops. That leads to widely divergent results. A different measurement principle should be used for online measurements of drop size distributions than laser back scattering.


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