Browsing by Person "Kohlus, Reinhard"

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    Beschreibung und Optimierung der Vorgänge der dynamischen Gefriertrocknung
    (2018) Pliske, Roland; Kohlus, Reinhard
    Freeze-drying is a gentle but also time-consuming drying method. One reason for the longer drying times is the formation of a dry layer during drying, which is a heat and mass transfer resistance. One approach for reducing the drying time is removing these resistances. The detail of an approach to remove the dry layer within a special powder mixer has been investigated. The process of freeze-drying while agitating has been termed ‘dynamic freeze-drying’. The used mixer was a plow-share type, in which the dry layer is actively rubbed-off permanently during the drying process. In this process the drying always takes place on the moister particle surface. This corresponds to the characteristics of a constant drying rate period, which can be considered confirmed by independent dynamic freeze-drying experiments. Freeze-drying process typically do not show a constant drying rate period. The drying front retreats immediately at the start of drying into inside of the particle. Therefore, drying rate of dynamic-freeze drying could be increased. The drying rate can be furthermore increased applying higher heating temperature in the case of dynamic freeze-drying compared to static freeze-drying. The danger of a collapse is prevented by abrasion of the dry layer during dynamic freeze drying. It has also been shown that under identical drying conditions, dynamic freeze-drying has an up to tenfold faster drying rate compared to conventional, static freeze-drying. One reason for this is a higher conductive heat flux into the bed. Another reason is the conversion of the kinetic energy into heat energy during the mixing of the bed, which is additionally used for the sublimation. Since the dry layer is removed during dynamic freeze-drying, the advantage should lie by larger initial diameters, because there are greater heat and mass transfer resistances compared to smaller initial particle diameters. This effect is overcompensated by the number of particles that are present if the same initial mass will be used for creation smaller particles than bigger particles. The contact number of particles to mixer wall determines the heat transfer by conduction and particle to particle determines the heat transfer by friction. For this reason, the drying time of the dynamic freeze-drying of smaller diameter beds is always lower. All results indicate that the number of contact points of particles to the mixer wall and other particles is relevant for the energy transfer to the bed during dynamic freeze-drying. As the particles become smaller during the drying process, however their number remains constant, and so is the effective heat transfer coefficient. A positive effect on drying rate was determined for the dried powder, which is within the mixer during the drying process. While drying with low rotational frequency less dried powder was discharged from the mixer and the experimental drying times always were lower than the modeled ones. The powder is heated at the mixer wall and is then afterwards reintroduced into the bed. At high rotational frequencies the powder is fluidized up more intensively and discharged with the water vapor from the mixer. During the drying process the water vapor leaves the mixer and partially the dried final product, too, and the load decreases and the energy input as well. Freeze-drying covers a large part of microorganism conservation so called starter culture conservation. First trials in using dynamic freeze-drying for this application have been conducted. Dynamic freeze-drying has been used in the drying of microorganisms in order to compare the viable count and the activity of the dried microorganisms with those from static freeze-drying. The presented results show that the viable count of the dynamic freeze-dried microorganisms is reduced. The activity however is partly higher than that of static freeze-dried microorganisms, which indicates a stress activation. These results were found using starter cultures that were frozen without adding "protective medium". Whether trials using protective medium will show similar results is currently unclear. The phenomenon of stress activation has to be confirmed using a large variety of lactic acid bacteria.
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    Characterization of the rehydration behavior of food powders
    (2019) Wangler, Julia; Kohlus, Reinhard
    The rehydration behavior of food powders is of high importance in terms of powder processing and product quality. Rehydration of powders mainly depends on the physical powder characteristics particle size, porosity and wettability, the latter being expressed by the contact angle between solid and rehydrating liquid. With focus on food powders, it could be shown that the rehydration behavior is strongly influenced by dynamic changes of these physical characteristics. This includes the initiation of dissolution and swelling directly after powder-liquid contact. Especially in case of biopolymers, which were investigated in detail by the example of xanthan gum, guar gum and alginate, these processes are important to describe their rehydration behavior. Due to the special characteristics of these biopolymers dissolution and swelling result in an increase of viscosity as well as in a decrease of bulk porosity. The kinetics and interactions of these processes significantly affect the individual steps of rehydration and have to be considered in describing the process of food powder rehydration. For inert powder-liquid systems capillary liquid uptake into a powder bulk can be described by the Washburn equation which equates the capillary pressure and the hydrodynamic flow resistance. This approach was used as basic equation to describe capillary liquid uptake of food powders. The validity of the original approach is restricted to the case of constant powder and liquid properties. With regard to food powders, changes within the powder-liquid system were considered by a stepwise adaption of the variables of the Washburn equation. Thus, the first part of this thesis focused on establishing and defining methods to characterize the dynamics of the physical properties particle size, bulk porosity, viscosity and contact angle. This enabled a more detailed characterization of the interactions between food powder and liquid during rehydration. Wettability of food powders in contact with dist. water was assessed by contact angle measurements. Contact angles were 52° for alginate, 58.1° for xanthan gum and 70° for guar gum which confirmed their hydrophilic character. To describe the change of the bulk porosity a rheological measurement set-up was constructed to quantify the swelling behavior. Influence of viscosity on rehydration was determined by measuring the concentration dependent viscosity increase and the rate of viscosity increase over time. The change of viscosity as a consequence of dissolution allowed conclusions about the dissolution rate of biopolymers in highly concentrated situations. These results indicated that rehydration of guar gum is mainly influenced by viscosity effects whereas swelling has the highest impact on the rehydration behavior of xanthan gum and alginate. Further methods such as Nuclear Magnetic Resonance analysis enabled a more detailed characterization concerning the dynamics of powder-liquid interactions and the strength of water binding within these biopolymer gels. The strength of water binding was found to correlate with the stability of highly concentrated biopolymer aggregates. The aggregate stability was determined by rheological analyses and is of importance, particularly with regard to powder dispersability. To predict food powder rehydration, a model was established using a VoF approach. To simulate capillary liquid rise based on physical characteristics, dynamic changes were resolved both spatial and temporally. To describe particle and liquid properties more precisely, a model system consisting of biopolymer coated glass beads was developed by fluid bed technology. By the variation of the coating layer thickness and the coating material, dynamic changes within the system could be controlled which enabled a more differentiated description. A parameter variation study was conducted to simulate the influence and interaction of dynamic processes on capillary liquid uptake into such powder systems. Capillary liquid uptake into the coated glass beads was investigated experimentally. It could be shown that even with coating layers of 0.5 µm dynamic effects are sufficiently strong to cause a stop of capillary liquid uptake. It has been shown that viscosity development dominates guar gum rehydration whereas swelling is the prevalent mechanism in xanthan gum and alginate rehydration. Simulation of capillary liquid rise demonstrated that the influence of the coating layer thickness is not significant. This result could be explained by the slow dissolution rates of the biopolymer samples. Calculations indicated that even a coating layer of 0.5 µm could only be dissolved partially after a dissolution time of 250 s. This explains the little impact of coating layer thickness on viscosity development and thus on capillary liquid uptake. Further explanations focus on biopolymer swelling. Simulation showed that coating layers of 0.5 µm are sufficient to cause swelling-induced pore-blocking conditions.
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    Erklärung der Strukturbildung und Trocknungskinetik von Einzeltropfen im Ultraschall-Levitator
    (2022) Hülsmann, Ramona; Kohlus, Reinhard
    In recent years, acoustic levitation has been increasingly used to investigate individual particles during the drying process. In this process, a droplet is positioned in a standing ultrasonic field in such a way that non-contact observation can take place. Within the scope of this work, an experimental setup for a levitator was designed and the drying and levitation behavior of various substances was investigated.
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    Investigation of fluidised bed coating : measurement, optimisation and statistical modelling of coating layers
    (2017) van Kampen, Andreas; Kohlus, Reinhard
    Fluidised bed coating describes a process to encapsulate particles. The coating layer is applied in order to protect the core material from chemical reactions with the environment, to control the release of drugs or to mask bad taste. Depending on the application, the coating layer must fulfil various quality requirements, such as completeness, homogeneity and minimum layer thickness. The measurement of the coating layer thickness is therefore necessary in order to determine appropriate parameters for an optimal coating process. This, however, is difficult in the investigated core particle size range of 100 to 500 μm with a coating layer thickness of around 10 μm. Fluorescent imaging of sliced particles or imaging of optical slices using confocal laser scanning microscopy are possible ways to make the coating layer visible and to measure the coating layer thickness using image analysis techniques. This leads to detailed images of the coating layer and an accurate description of the coating layer thickness distribution, but is rather time consuming due to tedious sample preparation and long image acquisition times. Consequently only relatively few particles are measured and used to draw conclusions on the population. Other methods like measurement of the change of particle size using laser diffraction or assessment of the volume ratio of coating to core material usually only deliver the mean thickness and no information on completeness and homogeneity of the coating. In the first part of this thesis a quick method for coating thickness measurement was developed based on a dissolution test. Sodium chloride was used as a core material and maltodextrin DE21 was used as a coating material. When dissolved in deionised water, sodium chloride raises the conductivity in contrast to maltodextrin. Therefore, the measurement of conductivity can be used to assess the dissolution curve of the core material. The coating layer delays the dissolution of the core and by comparison with the dissolution curve of pure sodium chloride the coating thickness distribution can be assessed by deconvolution. It was shown that this method is well reproducible and delivers reliable results comparable to other methods. The method is fast, which enables the measurement of many samples with replicates and using appropriate sample division should provide a good representation of the population. The shape of the thickness distribution allows the quantification of the three aforementioned quality parameters. The method was therefore used in the second part of this thesis in order to investigate the coating process using design of experiments. The four factors spray rate, air temperature, air velocity and concentration of the coating solution were investigated using a central composite design of experiments. The dissolution method was used to assess the coating quality. The particle size distribution was measured in order to quantify the agglomeration rate and the mass of deposited coating material was assessed by quantifying a tracer colour in order to assess the efficiency of the process. Significant quadratic models were fitted to all response variables. These were successfully used to find a local optimum within the investigated parameter space which allowed the formation of an optimal coating layer within a short time frame. The results of the previous investigations showed that the thickness distribution can be well described by a Weibull distribution. Furthermore, it was possible to confirm effects that were previously described in the literature, i.e. that a low concentration of the coating solution leads to more homogeneous coating layers. In order to give a general description of the coating layer, a statistical model of the coating thickness distribution was developed in the third part of this thesis and verified by a Monte-Carlo simulation. The model reproduces the experimentally determined effect of the concentration of the coating solution qualitatively and is able to calculate the mean thickness distribution with given concentration, contact angle, sprayed mass and core particle and droplet size. Appropriate adjustments of these parameters lead to a good agreement between the model and measured thickness distributions of real experiments. It was concluded that predominant spray drying of small droplets and an increase of concentration of the remaining droplets due to pre drying negatively affects the homogeneity of the coating layer. It was further confirmed that the Weibull distribution can be used to describe the coating layer thickness in the investigated thickness range. The thickness distribution transitions from the Weibull distribution to a normal distribution as the coating becomes thicker. Thin coatings with defects can be described by a clinched Weibull distribution containing the uncoated area fraction as an offset.