Browsing by Subject "Trocknung"

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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.
Development and optimisation of a low-temperature drying schedule for Eucalyptus grandis (Hill) ex Maiden in a solar-assisted timber dryer
(2006) Bauer, Konrad; Mühlbauer, Werner
The Brazilian furniture industry consumes about 45 million m³ of sawnwood per year which is mainly supplied by deforestation of the tropical rainforest. At the same time, fast growing eucalypt species are produced on almost 3 million ha for the production of wood pulp and charcoal. Meanwhile, several Brazilian companies try to substitute the expensive natural woods by hardwood from eucalypt trees for the production of high quality sawnwood. However, eucalypt wood has to be dried very carefully under controlled conditions to prevent drying defects. Ambient air drying is not suitable since missing control causes high losses and long drying times. Beside this, the low wood moisture content required in the furniture industry cannot be achieved. Artificial drying technologies reduce the drying time, the timber can be dried to a low wood moisture content and the quality can be improved. However, sophisticated high-temperature dryers cause high investments. Locally manufactured timber dryers do not allow an adequate control of the drying process. Furthermore, the required slow drying process is increasing the thermal and electrical energy consumption causing high drying costs. To overcome the existing problems, the Institute of Agricultural Engineering in the Tropics and Subtropics of the University of Hohenheim (ATS) developed in close co-operation with the German company THERMO-SYSTEM Industrie- & Trocknungstechnik Ltd (THS), Alfdorf and the Brazilian forest company CAF Santa Barbara Ltda (CAF) a solar-assisted dryer for sawnwood with integrated solar collector and biomass backup heating system. Aim of this research work was to analyse the newly developed solar dryer and to develop a suitable drying schedule which allows the economical production of high quality sawnwood for the furniture industry. Therefore, a prototype of the greenhouse type dryer was installed under subtropical climate in Brazil. Due to the low thermal insulation of the solar dryer and missing experience with the extremely sensitive Brazilian varieties of Eucalyptus grandis, a new type of drying schedule had to be developed. The new schedule considers not only the general drying demands of eucalypt sawnwood but also the system immanent characteristics of the solar dryer and the ambient air conditions. An oscillation of the drying air temperature according to the ambient air allowed to speed up the drying process and reduces the condensation of water on the cover without a negative impact on the timber quality. In more than 80 drying tests with 16 000 m³ of eucalypt sawnwood the schedule was tested and improved. Based on experiments and information from literature, the course of the temperature, relative humidity and velocity of the drying air and the mode of remoistening was systematically optimised. Thereby, the influence of the changing drying conditions on the drying time, the timber quality, the energy consumption and the drying costs were analysed. With the final version of the drying schedule, 27 mm thick boards could be dried in the solar dryer from a medium wood moisture content of 60 to 12 % d.b. in 27 days. This drying time was about 20 % higher than in a high temperature dryer. However, a drying time of at least 60 days was required to reduce the moisture content to about 20 % d.b. at ambient air drying. The electric energy consumption in the solar dryer was reduced to about 20 kWh per m³ dried eucalypt sawnwood. This is only 20 % of the energy usually consumed in a high temperature dryer. The thermal energy consumption was 1.2 GJ per m³ which is about 60 % less than the energy required in conventional high temperature dryers. The low thermal and electrical energy consumption combined with the considerable lower investment costs for the solar-assisted timber resulted in average drying costs of 7.90 Euro per m³. This is only half of the costs caused by drying 27 mm thick eucalypt hardwood in a high temperature dryer. For an economic evaluation, a sensitivity analysis was done for the most important cost parameters. The electrical energy costs, the currency exchange rate and the interest rate for credits were found to be the main influencing parameters on the Brazilian market conditions. However, solar drying was generally more cost efficient than conventional high temperature drying. In the framework of this research work, it was proved that Brazilian eucalypt timber can be dried economically to a low moisture content of 10 to 12 % at a high quality level by applying the developed drying schedule in the optimised solar-assisted dryer. Meanwhile, approximately 35 000 m³ of eucalypt hardwood is dried annually in two solar-assisted drying plants contributing significantly to the protection of the natural rain forests.
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.
Modelling of air resistance during drying of wood-chips
(2011) Müller, Joachim; Karaj, S.; Barfuss, Isabel; Schalk, J.; Reisinger, G.; Pude, R.
The objective of this study was to investigate the parameters that affect the drying process of wood chips at low air flow conditions. This objective was determined by measuring the air pressure resistance being produced by wood chips by examining different variables such as: air flow rate, air velocity, wood chip size, bulk density, bulk height and porosity. The air flow resistance was measured inside a 3 meter high cylindrical air duct constructed at University of Hohenheim. Physical properties of two different Spruce wood chip fractions were analyzed and their characteristics were considered on fitting the model expression. The analysed model expresses the physical behaviour of air flow resistance. Statistical analyses show high correlation of air speed versus air flow resistance. The model could be used for determination of drying conditions with low air mass flow. The height of bulk density according air mass flow generated or the necessary air mass flow needed for transporting air through the bulk height.
Monitoring quality change of fruit during drying by application of laser light in the red spectrum
(2011) Müller, Joachim; Nagle, Marcus; Romano, Guiseppe
The main task of this research is to apply laser backscattering technology to simultaneously predict variations in moisture content and hardness of apples during drying. The backscattering area in pixel numbers, representing the illuminated area after laser light injection, and light luminescence measured by grey values were used for estimating changes in internal quality parameters during drying. Laser light measurement at 635 nm was found to be adequate for predicting changes in moisture content and SSC of apple during drying over different stages. On the contrary, photon scattering at 635 nm is not recommended as estimator of change in hardness during apple drying, based on the results.
Untersuchungen und Verbesserungen der Querstromtrocknung von Getreide
(1980) Kuppinger, Heinz; Kutzbach, Heinz Dieter
Das Querstromtrocknungsverfahren findet weit verbreitete Anwendung zur Getreidetrocknung. Allerdings hat dieses Trocknungsverfahren den Nachteil der geringen Trocknungsleistung, des hohen Wärmebedarfs und der ungleichmäßigen Trocknung der Körnerschüttung.. Ziel der vorliegenden Arbeit war es daher, zu untersuchen, inwieweit durch eine Variation der das Trocknungsverhalten beeinflussenden Trocknungsparameter und durch eine Modifikation des Querstromverfahrens Verbesserungen der Querstromtrocknung zu erreichen sind.