A methodology for determination of temperature dependent mass transfer coefficients from drying kinetics: Application to solar drying (original) (raw)
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Modeling of heat transfer and energy analysis of potato slices and cylinders during solar drying
Applied Thermal Engineering, 2009
In the present work, a method based on energy balance considering the effects of heat capacity of the food product, radiative heat transfer from food product to the drying chamber and solar radiation absorbed in the product during drying is proposed for determination of convective heat transfer coefficient, h c. A natural convection mixed-mode solar dryer is used for performing the experiments on potato cylinders and slices of same thickness of 0.01 m with respective length and diameter of 0.05 m. The present investigation indicates that the cylindrical samples exhibit higher values of h c and faster drying rate compared to those of slices, as expected. The h c values for each sample shape are correlated by an equation of the form Nu = C(Ra) n. Laplace transform is applied to solve the proposed heat transfer diffusion model considering the effect of moisture transfer rate to predict the transient sample temperature. The model is validated through a close agreement between calculated and experimental results of transient sample temperature. Results of energy analysis reveal that for both the sample geometries, decreasing product moisture content during drying resulted in significant reduction in specific energy consumption. For almost similar drying conditions, a considerable amount of reduction in specific energy consumption is achieved for cylinders, as expected.
In the present work, the thin layer drying kinetics of potato during natural convection solar drying was investigated experimentally. Cylindrical potato samples with length 50 mm and varying diameter of 8, 10 and 13 mm were dried in an in-house designed and fabricated laboratory scale mixed-mode solar dryer. Thirteen different thin-layer mathematical models were fitted to the experimental moisture ratio (MR) data. The obtained results indicated that the Modified Page model could satisfactorily describe the drying curve of potato cylinders with higher value of R 2 and lower values of RMSE and χ 2 . The shrinkage parameter is incorporated in the analytical diffusion model to study the moisture transfer mechanism of potato cylinders. It was observed that the values of effective diffusion coefficient (D eff ) and convective mass transfer coefficient (h m ) are overestimated in the range of 85. 02-90.27% and 39.11-45.11% for the range of sample diameter examined, without considering the shrinkage effect in the mass transfer analysis. A Multiphysics approach was adopted in this study to get insight into the drying behavior of potato cylinders in terms of food-moisture interactions during the solar drying process. The predicted results of MR are in close agreement with the experimental data. Moreover, the anisotropic behavior of shrinkage as well as the moisture distribution inside the potato cylinder was very well described by Multiphysics model.
ESTIMATION OF DRYING RATES USING LIQUID DIFFUSION MODEL - EFFECTS OF MULTIPLE MODES OF HEAT TRANSFER
Although a detailed mathematical model incorporating all physical mechanisms of moisture and heat transfer in the material would yield valuable design information, it is not feasible to do this on a routine basis for the design of dryers. A simple liquid diffusion model was developed in this report to quantitatively assess the influence of various operating parameters of engineering interest in drying of heat-sensitive materials. Heat of wetting, temperature and moisture dependent effective diffusivity and thermal conductivity, changes in product density and drying -induced ideal shrinkage of the product are considered in this model. The effects of combining convection with conduction, radiation and volumetric heating using a microwave field are simulated in view of the increasing interest in multimode heat input drying processes. Numerical results are reported on drying of potato slices to demonstrate how the moisture and temperature profiles as well as drying performance are affected by multi-mode heat input. Effects of stepwise change of drying air temperature, air relative humidity, flipping of product and performance of sun drying are computed and discussed. This model can be extended to two and three dimensional geometries including effects of drying on quality parameters.
Heat and mass transfer modeling in air drying of foods
Journal of Food Engineering, 1995
. aw a, : c A% ps' cpw AH,,, AH,, AH, D Do,% 4 eT NOTATION Air water activity Equilibrium air water activity Parameter of the GAB equation Empirical constants Heat of desorption (W/kg d.b.) Empirical constants Moisture diffusion coefficient (ml/s) Empirical constants Material temperature relative residuals 113 114 Z. B. Maroulis, C. T. Kiranoudis, D. Marinas-Kouris k Km km km kc, Li N ni ST sx t TA Ts u A XA 2 P Weighing factor Ps Apparent density ( kg/m3) Pso Empirical constant Material moisture content relative residuals Heat transfer coefficient (kJ/m2/s) Empirical constants Mass transfer coefficient (kg/m2/s) Empirical constants Material specific enthalpy (kJ/kg d.b.) Parameter of the GAB equation Material thermal conductivity (kW/m/k) Empirical constants Sample particle dimension (m) Number of experiments Number of measurements in experiment i Relative standard deviation between experimental and calculated values of material temperature (model accuracy to calculate the material temperature) Relative standard deviation between experimental and calculated values of material moisture content (model accuracy to calculate the material moisture content) Time (s) Air temperature ("C) Material temperature ("C) Air velocity (m/s) Air absolute humidity (kg/kg d.b.) Parameter of the GAB equation Material moisture content (kg/kg d.b.) Food Engng, 15,187-208.
Solar Drying Kinetics of Cassava Slices in a Mixed Flow Dryer
please write the names of authors as follows: first names and then surnames. As an example to illustrate: Katarina KOLLAROVA (that means, Katarina is the first name and KOLLAROVA is the surname), please also write the complete addressthe print version of the journal will be sent to you to that address ABSTRACT Drying characteristics of cassava slices was investigated in a mixed mode natural convection solar dryer to obtain a suitable mathematical model describing the drying. The average drying chamber temperature was between 34 ± 2 °C and 50 ± 1.8 °C, while 10 commonly used thin layer drying models were used for drying curve modelling. Coefficient of determination (R 2 ) and root mean square error (RMSE) were used to determine the models performances. The drying curve of cassava slices showed a reduction of moisture content with increased drying time in the solar dryer, and the variation of moisture ratio exponentially decreased with increased drying time. The Midilli and Logarithmic models showed better fit to the experimental drying data of cassava slices. As compared with other models tested, there were no significant differences (p >0.05) in the R 2 values obtained for the Midilli and Logarithmic models; hence, the Logarithmic model was preferable because of the lower RMSE. The diffusion mechanism could be used to describe the drying of cassava slices that was found to be in the falling rate period. A diffusion coefficient (Deff) of 1.22 × 10 -8 m 2 s -1 was obtained, which was within the established standard for food products.
Analysis of Moisture Transport in the Solar Drying of Food Items
2008
In this study, the principle of moisture transport in porous materials was used to analyze the rate of moisture removal from food items during the solar drying process. A cabinet solar dryer was designed, fabricated, and tested to evaluate moisture flux from samples of food items. The dryer exhibited sufficient ability to dry the samples reasonably to a safe moisture level. The results obtained show that at lower liquid concentrations the moisture flux increases with increase in the concentration, while the moisture flux was constant at liquid concentrations above 70 kg m–3 for shelled corn and above 150 kg m–3 for yam chips. The drying temperature and the intensity of solar radiation were the main factors that influenced the rate of moisture removal from the food items.
International Journal of Energy Engineering, 2014
In this study, convective drying kinetics of tomato slices was conducted in order to identify the characteristic parameters of such high moisture content product and to establish mathematical models of the drying kinetic for various range of temperatures typically encountered in industrial dryers. The results allow the calculation of the effective diffusivity and the activation energy. Drying experiments were conducted in Thermal Process Laboratory of the Research and Technology Centre of Energy. The drying characteristics of tomato were studied at 38°C, 44°C, 52°C, 57°C and 64°C with three air flow rates of 1ms -1 , 2ms -1 and 3ms -1 . The drying continues until samples' mass reaches a final moisture content of 11%. Some different mathematical models available in the literature (Newton, Page, Henderson and Pabis, Two-term, modified Wang and Singh, Logarithmic ...) were used and compared to estimate drying curves of tomatoes. The results indicated that to reach a final moisture content of 11% at 38°C, 44°C, 50°C, 57°C and 64°C, the drying time for 1 m/s, respectively, was 22.4h, 18.5h, 16.7h, 13.5h and 9.9h. For an air flow rate of 1ms -1 , 2ms -1 and 3ms -1 , the drying time of tomato slices at 50°C were 16.7h, 14.2h and 11.7h, respectively. The effective diffusivity and activation energy varied from in the temperature range (38°C -64°C). Out of the considered mathematical models, the drying curves were fitted to Logarithmic and Two-Term equation.
Intensification of the Plant Products Drying Process by Improving Solar Dryer Design
Journal of Engineering Thermophysics, 2018
The article presents the rationale for production of dried fruits and vegetables using a solar drying unit. To intensify the drying process, convection of drying agent flow in the proposed drying chamber is studied using Navier–Stokes equations. Numerical methods are used for solving equations describing the process of convective heat transfer. As a result, graphical interpretations of isolines of drying agent flow are obtained and location of passive zones in the dryer chamber are identified. Uniformity of the temperature zones in the chamber is ensured by supplying additional drying agent into the passive zones. Temperature values at various levels of the drying chamber are experimentally obtained. Results for drying cut-up mass of vegetables and fruits are presented.
Heat and mass transfer modeling for fruit drying: a review
MOJ Food Processing & Technology
In this review, coupled heat and mass transfer phenomena (drying) is discussed .Drying is an effective method for fruit storage/preservation. Drying could retain quality end products, which is challenging, because all fruits are variable in structure, so, heat and mass transfer modeling (operating parameters) is a useful technique to deal with it. This can only be done by selecting the right type of drying equipment and understanding the science behind the drying process including thermal properties of fruit. Drying process have many effects on different heat sensitive fruits components and equipment (sensors etc.) as well which result into increase in maintenance cost, diffusion rate goes to critical limits etc. Because, selection of an appropriate drying method and equipment is most important regarding product quality and its economic value. Modeling of a drying process considering different drying parameters and their effects on final quality of products and economic importance are also discussed here. We should have knowledge about the drying mechanics. So, that knowledge of heat and mass transfer process for fruit drying helps to identify best operating conditions and saves the maximum amount of energy.