DRYING OF BIOLOGICAL PRODUCTS WITH SIGNIFICANT VOLUME VARIATION. EXPERIMENTAL AND MODELING RESULTS FOR POTATO DRYING.

ABSTRACT

This work is devoted to the experimental investigation and theoretical modeling of biological products with high initial moisture content which implies significant volume contraction. Firstly, experimental equipment which was designed to continuously determine variations in the global moisture content and in the temperature of the sample is introduced. Furthermore, the equipment is controlled by a PC which records experimental information in real time. In particular, measurement of the radial and temporal distributions of moisture content and temperature during the drying process of the homogeneous porous media potato are given. The mean moisture content and temperature curves are also given.     

DRYING OF BIOLOGICAL PRODUCTS WITH HIGH WATER CONTENT IN SOLAR REGIME. APPLICATION TO THE POTATO DRYING.

The experimental results on the drying of biological porous media (potato) are modeled using the Luikov equations, under simulate insolation conditions. The temperature of the environment was regulated, within the typical range corresponding to hot-temperature and rather wet climate (Argentina's Humid Pampa region).

The comparison between measurements and calculations gave good agreement. The present study confirms independently the identification of several characteristic parameters of the product obtained under constant conditions in a previous work [1].     

A DIFFUSIONAL MODEL FOR DRYING WITH VOLUME CHANGE

A diffusional model to study the drying process when volume change is significant is proposed. The effect of the foodstuff shape is taken into account. An empirical relation between moisture content and volume is presented. The resulting differential equations are numerically solved using a moving boundary finite differences technique. A good agreement between numerical results and experimental data is obtained.     

A DIFFUSIONAL MODEL FOR DRYING WITH VOLUME CHANGE

A diffusional model to study the drying process when volume change is significant is proposed. The effect of the foodstuff shape is taken into account. An empirical relation between moisture content and volume is presented. The resulting differential equations are numerically solved using a moving boundary finite differences technique. A good agreement between numerical results and experimental data is obtained.     

COMPARISON OF EXPERIMENTAL RESULTS WITH ANALYTICAL SOLUTIONS AND TWO DIHENSIONAL MODEL OF OAK DRYING IN AN EVACUATED KILN

ABSTRACT

In continuation of a series of tests, the original results of oak drying in an evacuated kiln are presented here for different plate temperatures and for various pressures in the kiln. These results include more particularly the drying curves, the evolution of temperature, of moisture and of pressure in and on the wood. They evidence the pressure and the levels of temperature occurring in the wood during the drying period.

These results also allow the development of two types of drying models a simple monodimensional model of drying curves from the analytical solutions of the equations of water diffusion in the wood and, moreover, a model, in two dimensions, of temperature, moisture and pressure fields in the wood by applying the finite element method. The boundary conditions of the second model can be fixed with precision thanks to the results of the first model. In both cases, the proposed solutions are justified by experimental results.     

COMPARISON OF EXPERIMENTAL RESULTS WITH ANALYTICAL SOLUTIONS AND TWO DIHENSIONAL MODEL OF OAK DRYING IN AN EVACUATED KILN

In continuation of a series of tests, the original results of oak drying in an evacuated kiln are presented here for different plate temperatures and for various pressures in the kiln. These results include more particularly the drying curves, the evolution of temperature, of moisture and of pressure in and on the wood. They evidence the pressure and the levels of temperature occurring in the wood during the drying period.

These results also allow the development of two types of drying models a simple monodimensional model of drying curves from the analytical solutions of the equations of water diffusion in the wood and, moreover, a model, in two dimensions, of temperature, moisture and pressure fields in the wood by applying the finite element method. The boundary conditions of the second model can be fixed with precision thanks to the results of the first model. In both cases, the proposed solutions are justified by experimental results.     

VOLUME REDUCTION AND BIOLOGICAL STABILIZATION OF SLUDGE IN SMALL SEWAGE PLANTS BY SOLAR DRYING

ABSTRACT

High investments, high dumping costs and high electrical energy consumption turn waste water treatment in small sewage plants an expensive process. Therefore, the University of Hohenheim and the company Thermo-System have developed a fully automatic low temperature solar drying plant allowing further stabilization and volume reduction of sewage sludge by continuous mixing and aerating. To test the potential of the technology, the drying process was investigated in a commercially operated municipal sewage treatment plant with aerobic sludge digestion. During experiments, the changes of the dry solids concentration and the volatile solids, the climatic conditions and the energy consumption were constantly measured and documented. It was found, that the sludge could be dried from a dry solids concentration of 3 to 93% w/w in 64 (flocculated) or 83 days (non-flocculated). At the same time, the volatile solids content was reduced from 65 to 34% w/w, respectively. The electrical energy consumption was only 22 to 28 kWh per ton of evaporated water, compared to 70 to 110 kWh required for conventional drying processes. The thermal energy for evaporation was fully covered by solar energy.     

VOLUME REDUCTION AND BIOLOGICAL STABILIZATION OF SLUDGE IN SMALL SEWAGE PLANTS BY SOLAR DRYING

High investments, high dumping costs and high electrical energy consumption turn waste water treatment in small sewage plants an expensive process. Therefore, the University of Hohenheim and the company Thermo-System have developed a fully automatic low temperature solar drying plant allowing further stabilization and volume reduction of sewage sludge by continuous mixing and aerating. To test the potential of the technology, the drying process was investigated in a commercially operated municipal sewage treatment plant with aerobic sludge digestion. During experiments, the changes of the dry solids concentration and the volatile solids, the climatic conditions and the energy consumption were constantly measured and documented. It was found, that the sludge could be dried from a dry solids concentration of 3 to 93% w/w in 64 (flocculated) or 83 days (non-flocculated). At the same time, the volatile solids content was reduced from 65 to 34% w/w, respectively. The electrical energy consumption was only 22 to 28 kWh per ton of evaporated water, compared to 70 to 110 kWh required for conventional drying processes. The thermal energy for evaporation was fully covered by solar energy.     

EXPERIMENTAL INVESTIGATION AND MODELING OF THE INFLUENCE OF INDIRECT HEATING ON FLUIDIZED BED DRYING

Experimental results of fluidized bed drying with additional energy input by indirect heating are presented. A clear influence of moisture on wall-to-bed heat transfer coefficients is observed for particle diameters of 250 μm and 50 μm. Two different reasons for this influence may be distinguished in both, the measurements and theoretical considerations: 1 The change of bed and particle properties and 2 the increase of the local wall-to-particle heat transfer.     

EXPERIMENTAL INVESTIGATION AND MODELING OF THE INFLUENCE OF INDIRECT HEATING ON FLUIDIZED BED DRYING

Experimental results of fluidized bed drying with additional energy input by indirect heating are presented. A clear influence of moisture on wall-to-bed heat transfer coefficients is observed for particle diameters of 250 μm and 50 μm. Two different reasons for this influence may be distinguished in both, the measurements and theoretical considerations: 1 The change of bed and particle properties and 2 the increase of the local wall-to-particle heat transfer.     

POLARIZATION BEHAVIOR OF PACKED-BED ELECTRODES: A COMPARATIVE STUDY OF EXPERIMENTAL AND MODELING RESULTS

The current distribution and overall polarization behavior of electrodeposition at a flow-through (packed bed or fluidized bed) electrode are modeled by means of a one-dimensional model involving a primary reacting species and a simultaneous side reaction. The model equations are solved by orthogonal collocation; the time and storage requirements compare favorably to those of finite-difference methods. Experimental data obtained using a packed-bed electrode are compared with the model predictions and various methods of fitting the data are compared.     

POLARIZATION BEHAVIOR OF PACKED-BED ELECTRODES: A COMPARATIVE STUDY OF EXPERIMENTAL AND MODELING RESULTS

The current distribution and overall polarization behavior of electrodeposition at a flow-through (packed bed or fluidized bed) electrode are modeled by means of a one-dimensional model involving a primary reacting species and a simultaneous side reaction. The model equations are solved by orthogonal collocation; the time and storage requirements compare favorably to those of finite-difference methods. Experimental data obtained using a packed-bed electrode are compared with the model predictions and various methods of fitting the data are compared.     

OPTIMISATION OF HARDWOOD DRYING SCHEDULES ALLOWING FOR BIOLOGICAL VARIABILITY

ABSTRACT

The amount of biological variability in timber creates considerable problems in producing timber of adequate and reproducible quality with a predictable amount of variability in final moisture contents. The development of optimised drying schedules for addressing these problems is therefore desirable. Previous methods (largely of a stochastic type) are reviewed in this work and their limitations assessed. The physical parameters which have the greatest impact on the stress levels (and hence quality) of the timber have been assessed using a diffusion model for the drying of Australian hardwood timber. This deterministic model is then used, together with statistical methods for quantifying the confidence regions for the variability in the physical parameters with the greatest impact, in a systematic technique to develop a new optimised schedule for grey ironbark timber (Eucalyptus paniculaia). This new schedule is then compared with a previous optimised schedule, which did not take this variability into account. The productivity (amount of good quality timber divided by drying time) appears to be maximised when the schedule is such that 90% of the timber produced is good quality.     

OPTIMISATION OF HARDWOOD DRYING SCHEDULES ALLOWING FOR BIOLOGICAL VARIABILITY

The amount of biological variability in timber creates considerable problems in producing timber of adequate and reproducible quality with a predictable amount of variability in final moisture contents. The development of optimised drying schedules for addressing these problems is therefore desirable. Previous methods (largely of a stochastic type) are reviewed in this work and their limitations assessed. The physical parameters which have the greatest impact on the stress levels (and hence quality) of the timber have been assessed using a diffusion model for the drying of Australian hardwood timber. This deterministic model is then used, together with statistical methods for quantifying the confidence regions for the variability in the physical parameters with the greatest impact, in a systematic technique to develop a new optimised schedule for grey ironbark timber (Eucalyptus paniculaia). This new schedule is then compared with a previous optimised schedule, which did not take this variability into account. The productivity (amount of good quality timber divided by drying time) appears to be maximised when the schedule is such that 90% of the timber produced is good quality.     

COMPARISON OF EXPERIMENTAL AND MODELLING STUDIES FOR THE MICROWAVE DRYING OF IRONBARK TIMBER

ABSTRACT

Temperature profiles through boards during the microwave-assisted drying of Australian Ironbark timber have been investigated in this work in order to explore evidence for timber degrade via “charring” at internal temperatures below those required for pyrolysis (Brooke et al., 1998). A previously published model (Turner and Jolly, 1990a) describing one-dimensional microwave power absorption, based on the solution of Maxwell's equations, has been reviewed and significant limitations identified and overcome. Improvements included the use of a linear-mixing approach for the dielectric constants, the incorporation of temperature and moisture dependencies for these dielectric properties and the inclusion of diffusion within an overall system model. A control-volume technique has been used for predicting both moisture and temperature distributions within the timber, with a second-order finite-difference method being used to solve Maxwell's equations. The final model showed     

CONVECTIVE DRYING OF AGRICULTURAL PRODUCTS. EFFECT OF CONTINUOUS AND STEPWISE CHANGE IN DRYING AIR TEMPERATURE

Samples of banana were dried in a two-stage heat pump dryer capable of producing stepwise control of the inlet drying air temperature while keeping absolute humidity constant. Two stepwise air temperature profiles were tested. The incremental temperature step change in temperature of the drying air about the mean air temperature of 30 °C was 5 °C. The total drying time for each temperature-time profile was about 300 minutes. The drying kinetics and color change of the products dried under these stepwise variation of the inlet air temperature were measured and compared with constant air temperature drying. The stepwise air temperature variation was found to yield better quality product in terms of color of the dried product. Further, it was found that by employing a step-down temperature profile, it was possible to reduce the drying time to reach the desired moisture content.     

EXPERIMENTAL RESULTS OF A PULL-OUT TEST PERFORMED WITH SINGLE- AND MULTI-FIBER SAMPLES

Widely-used methods for characterising the fibre/matrix interface in polymeric composites are the fragmentation test and the droplet test as a special kind of the single-fibre pull-out test. A severe disadvantage of these tests is that non-realistic model samples are investigated which contain only one fibre in the matrix. In order to obtain data about the effect of the different residual stress situations for fibres in such samples and in composites, pull-out tests of E-glass fibres in polystyrene and polycarbonate are performed using samples, where the investigated fibre is surrounded by 0 to 3 other near fibres. Neighbouring fibres can increase the pull-out forces by a factor of three and the interfacial toughness by a factor of four. This has to be taken into account, if the tests are performed not only for comparison reasons but for measuring interface properties.