Convective Mass Transfer Coefficients Research Articles

A novel, initial guess free optimization algorithm for estimating parameters of batch kinetics model used to simulate adsorption of pollutant molecules in aqueous streams

Adsorption kinetics involved in the removal of phenol molecules polluting waterbodies was scrutinized using the popular and rigorous Homogeneous Surface Diffusion Model (HSDM). Adsorption of phenol depends on its molecular properties, characteristics of the adsorbent and operating conditions (Liu et al., 2010). These affect the dynamic and equilibrium parameters of the HSDM. Due to a boundary condition that involves an integral and a nonlinear adsorption isotherm, an analytical solution is not possible. The parameters of the HSDM, viz. the convective mass transfer coefficient in the fluid phase and diffusion coefficient in the solid phase are difficult to estimate from experimental kinetics data, through conventional optimization algorithms. These algorithms are sensitive to the quality of initial guesses and may frequently stall if the error function surface has a nearly flat topology. A novel, initial guess free and robust algorithm, termed as Elephant Herd Optimization algorithm (EHO), was developed in-house for reliably estimating the HSDM parameters. EHO is based on the social behavior of a herd of elephants and their instinct to navigate to water sources based on their strong memory. The proposed algorithm was considerably improved from the well-known particle swarm optimization technique (Kennedy and Eberhart, 1995). Conventional methods such as Nelder-Mead, genetic algorithm, and simulated annealing were compared against EHO for the attainment of global minimum and computational efficiency. EHO was successful in identifying the parameters with the smallest attainable error sum of squares based on experimental and predicted concentration values, lesser computational effort, lesser number of function evaluations and greater precision. EHO typically reduced the time required for parameter estimations by about 30% on an average with 40% lesser number of iterations when compared to the conventional methods. Also, the standard deviation in the estimated parameters upon repeated application of the EHO was about 52% lower than the conventional methods on an average. The implications of inaccurate predictions of the transport parameters from conventional algorithms on the adsorption kinetics are discussed.

A Detailed Investigation on the Effect of the Sherwood and Nusselt Number Modelling for the Heating and Evaporation of a Single Suspended Water Droplet

The work described in this paper presents a comprehensive analysis of the convective heat and mass transfer coefficient modelling around a single water droplet using an in-house code. The most widely used approach is to rely on sub-models for the non-dimensional heat and mass transfer numbers (called hereafter the Nusselt, Nu, and the Sherwood, Sh, numbers) and which are shown to take the theoretical and functional form of and where Red is the droplet Reynolds number, Pr and Sc are the Prandtl and Schmidt numbers of the surrounding gas and K1 and K2 are constants. This formulation, which is generally referred to in the literature as the Ranz-Marshall model (with K1 = K2 = 0.6), is the most used approach in Computational Fluid Dynamics (CFD) codes for fire safety engineering. In this paper, we first assessed this formulation based on 36 experimental tests carried out in [Volkov and Strizhak, Applied Thermal Engineering (2017)] and where a single suspended water droplet of a diameter between 2.6 and 3.4 mm is heated up by a convective hot air flow with a velocity between 3 and 4.5 m/s and a temperature between 100 and 800°C. The results showed that the overall model uncertainty in the droplet lifetime prediction is about 34% with particularly poor results when the air temperature is 100 or 200°C. The droplet saturation temperatures were overestimated by around 20 to 30°C. After this initial assessment, we performed a sensitivity analysis and selected a combination of values for K1 and K2 that provided an overall simultaneous good agreement for both the droplet lifetimes (model uncertainty of 5%) and the droplet saturation temperatures (around 10°C). This analysis showed that, for high air temperatures (i.e., Ta ≥ 300°C), the value of K2 = 0.6 remains suitable. However, for these cases, the value of K1 needed to be increased (to 1.8 and up to 4) in order to promote the evaporation-induced cooling and improve the predictions in terms of droplet saturation temperatures. For Ta = 100°C, the ‘best’ combination was found to be K1 = K2 = 1.8. Such combination allowed to reduce the initially overestimated droplet lifetimes by promoting mass transfer (through an increased value of K1) without slowing down the heating process that generates water vapor at the droplet surface (explaining the equally increased value of K2). The case of Ta = 200°C appeared to be an intermediate case. The present results indicate that the ‘classical’ Ranz-Marshall approach with K1 = K2 = 0.6 is not optimal. A more thorough analysis, with eventually additional experimental data, is required.

Modeling and simulation of moisture transfer during solar drying of carrot slices

AbstractCarrot slices of 0.03 m diameter and 0.005 m thickness were dried in direct, indirect and mixed mode type natural convection solar dryers. Thin layer drying behavior of carrot slices was studied using 10 empirical and semi‐empirical mathematical models. Wang and Singh model described the drying behavior of carrot accurately with maximum values of coefficient of determination (R2) and minimum reduced chi‐square (χ2), and root mean square error (RMSE). The effective diffusivity (Deff) and convective mass transfer coefficient (hm) of carrot slices ranged from 2.59 × 10−8−6.36 × 10−8 m2/s and 3.15 × 10−7 to 4.28 × 10−7 m/s, respectively, with maximum values obtained for mixed mode solar dried samples followed by direct and indirect solar dried samples. The diffusivity of carrot slices dried in direct, indirect and mixed‐mode solar dryer were expressed as a function of temperature using Arrhenius relation. Finite element model was developed using COMSOL Multiphysics software to simulate moisture ratio during drying. The developed model predicted the transient moisture migration phenomenon inside the samples precisely during the drying process.Practical applicationsThe ever increasing worriment on climate change and growing demand for enactment of clean energy technologies in food industries is the motivation for investigating the solar drying behavior of carrot slices. It is well known that the physico‐chemical and sensorial changes during drying are very much dependent on the moisture content of food product. In this regard, proper understanding and controlling of the solar drying process by accurately predicting moisture distribution inside the product will result in superior quality dried products. Moreover, mathematical models will assist in predicting the drying behavior of food product by analyzing the influence of process parameters and will be helpful to minimize the cost of drying process and deterioration of quality attributes of the product. The use of solar dryers is a proven technology and the food industry should promote its practical applications in food processing sector.

Effect of convective, infrared and microwave heating on drying rates, mass transfer characteristics, milling quality and microstructure of steam gelatinized Paddy

AbstractPaddy (var. Sworn Masuri) was dried by hot air (50 °C), infrared hot air (4,185 W m−2 radiation intensity and 40 °C) and microwave rotary drum (0.37, 0.78, and 1.23 kW kg−1 power density) to evaluate the most suitable method of drying. Before drying, the paddy grains were soaked in water (60 °C, 6 h) followed by steaming (200 kPa, 10 min) to gelatinize the starch. The drying rate constants, apparent moisture diffusivity (Da), convective mass transfer coefficient (hm), head rice yield (HRY) and the broken percentage were estimated. The X‐ray diffraction analysis and scanning electron microscopy (SEM) of starch granules and outer husk of the dried paddy were carried out. Microwave rotary drum drying (MWD) was found to be faster as compared to hot air drying (HAD) and infrared hot air drying (IRHD). The hm in MWD was found to be 90.76–97.01% higher than HAD and 33.33–78.44% higher than IRHD. The HRY and specific energy consumption in MWD were, respectively, 5.87% and 89.16% lower than HAD. The microwave dried rice showed the lowest crystallinity percentage. The SEM of rice starch obtained by MWD showed cracks and fissures whereas IRHD induced microstructural changes only in paddy husk.Practical applicationsRice is a staple food of 90% Asian population. The parboiled paddy is dried in the open sun (2–3 days) and by hot air (6–9 h) on farms and in the rice industry. These are highly laborious and time‐consuming methods. Therefore, the study was undertaken from industry suggestion to develop a cost‐effective microwave‐assisted drying method for parboiled paddy. The microwave rotary drum drying was found to give similar quality rice compared to hot air with a 90% reduction in specific energy consumption. Therefore, traditional open sun and hot air drying can be replaced by microwave rotary drum to save expenses towards the capital costs, energy, labor, and grain handling.

On the fundamental heat and mass transfer analysis of the counter-flow dew point evaporative cooler

The performance of the dew point evaporative cooling (DPEC) is dominated by its convective heat and mass transfer mechanism. Existing mathematical models are mainly developed for the thermodynamic analysis of DPEC under various operating and geometric conditions. The convective heat and mass transfer coefficients are estimated using the Nusselt number and Sherwood number at constant surface conditions. However, as the channel surface is subjected to a naturally-formed boundary condition, the cooler’s actual heat and mass transfer performance remains unclear and has never been investigated. Therefore, we propose an experimental and numerical study, to examine at the fundamental level, the convective heat and mass transfer process of the DPEC. The temperature and humidity distributions of a counter-flow dew point evaporative cooler are measured under different test conditions. The magnitude of the convective heat and mass transfer coefficients are determined using the log mean temperature/humidity difference method. Concurrently, a 2-D mathematical model has been formulated to simulate the heat and mass transfer performance of the cooler. The model agrees well with the acquired experimental data with a maximum discrepancy of ±7.0%. The product air temperature, convective heat and mass transfer coefficients and the Nusselt number and Sherwood number, are further examined under different conditions. Key findings emerged from this study reveal that ReD,r,HL,δH and π are the dominant factors related to the heat and mass transfer performance. The average convective heat and mass transfer coefficients are found to be 26.8–29.9 W/(m2·K) and 0.025–0.027 m/s. The corresponding Nu‾D,d,Nu‾D,w and Sh‾D,w span 8.67–9.95, 8.68–9.21 and 8.17–8.67, respectively, under varying dimensionless numbers/groups.

Vacuum drying of apples (cv. Golden Delicious): drying characteristics, thermodynamic properties, and mass transfer parameters

In this work, apples of cv. Golden Delicious were cut into slices that were 5 and 7 mm thick and then vacuum dried at 50, 60 and 70 °C and pressure of 0.02 bar. The thin layer model drying kinetics was studied, and mass transfer properties, specifically effective moisture diffusivity and convective mass transfer coefficient, were evaluated using the Fick’s equation of diffusion. Also, thermodynamic parameters of the process, i.e. enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG), were determined. Colour properties were evaluated as one of the important indicators of food quality and marketability. Determination of mass transfer parameters and thermodynamic properties of vacuum dried apple slices has not been discussed much in the literature. In conclusion, the Nadi’s model fitted best the observed data that represent the drying process. Thermodynamic properties were determined based on the dependence of the drying constant of the Henderson and Pabis model on temperature, and it was concluded that the variation in drying kinetics depends on the energy contribution of the surrounding environment. The enthalpy and entropy diminished, while the Gibbs free energy increased with the increase of the temperature of drying; therefore, it was possible to verify that variation in the diffusion process in the apple during drying depends on energetic contributions of the environment. The obtained results showed that diffusivity increased for 69%, while the mass transfer coefficient increase was even higher, 75%, at the variation of temperature of 20 °C. The increase in the dimensionless Biot number was 20%.

Comparison heat and mass transfer coefficients in the shell side of the hollow fiber membrane module

The article discusses the theory associated with the determination of total heat and mass transfer coefficients in a membrane module. The equations for determining Nusselt and Sherwood numbers on the shell side of the module are presented. The design of the membrane module and the test bench are also discussed. The convective coefficients of heat and mass transfer on the shell side have been determined on the basis of the measurements. Calculation results obtained from correlation equations were then compared with the results of experimental investigations of convective heat and mass transfer coefficients in the air-water system. The equation is indicated appropriate for use in the case analysed.

Effect of Pre-Drying and Hydrocolloids Batter Coating during Heat and Moisture Transfer of Coated Fried Potato Chips

Fat uptake is considered the major nutritional critical point of deep frying because of the obesity and negative effect of excess oil consumption on human health. Hence, this study investigated the effect of pre-drying and hydrocolloids batter coating during heat and moisture transfer of coated fried potato chips. The potato slices were pre-dried coated and deep fried at 180°C for 5 min. Chemical composition, color profile; thermo-physical properties, dimensionless numbers associated with convective heat and mass transfer coefficient and sensory properties were evaluated. The result revealed that chemical composition and color profile were significantly (p ≤ 0.05) affected by pre-drying and hydrocolloids batter coating. The density, viscosity, thermal conductivity, specific heat capacity and thermal diffusivity ranged from 333.3-444.0 kg/m3, 0.5281-0.5377 pas, 22.686-31.403 W/m °C, 198.686-258.383 KJ/kgK and 2.730 × 10-4-3.826 × 10-4 m2/s, respectively. The estimated Grashof, Prandtl, Schmidt, Nusselt and Sherwood numbers ranged between 0.024-0.032, 4.14-4.71, 2.053 × 10-5 and 2.739 × 10-5, 0.0167-0.0209, 9.70 × 10-8 and 1.02 × 10-7, respectively. The heat and mass transfer coefficient values ranged from 25.26 to 42.98 W/m2 °C and 3.80 × 10-12 to 4.01 × 10-12 m/s, respectively. Hydrocolloid type distinctly dictated considerable quality attributes and apparently potato chips pre-dried, coated with egg albumin and carboxyl methylcellulose were gum tragacanth demonstrates great reduction in the fat content of the chips which will in turn decline the tendency of obesity and equally reduce cholesterol level.

Migration of antioxidants from polylactic acid films, a parameter estimation approach: Part I – A model including convective mass transfer coefficient

A two-step solution based on the boundary conditions of Crank's equations for mass transfer in a film was developed. Three driving factors, the diffusion (D), partition (Kp,f) and convective mass transfer coefficients (h), govern the sorption and/or desorption kinetics of migrants from polymer films. These three parameters were simultaneously estimated. They provide in-depth insight into the physics of a migration process. The first step was used to find the combination of D, Kp,f and h that minimized the sums of squared errors (SSE) between the predicted and actual results. In step 2, an ordinary least square (OLS) estimation was performed by using the proposed analytical solution containing D, Kp,f and h. Three selected migration studies of PLA/antioxidant-based films were used to demonstrate the use of this two-step solution. Additional parameter estimation approaches such as sequential and bootstrap were also performed to acquire a better knowledge about the kinetics of migration. The proposed model successfully provided the initial guesses for D, Kp,f and h. The h value was determined without performing a specific experiment for it. By determining h together with D, under or overestimation issues pertaining to a migration process can be avoided since these two parameters are correlated.

A mathematical model for heating and evaporation of a multi-component liquid film

A new model for heating and evaporation of a multi-component liquid film, based on the analytical solutions to the heat transfer and species diffusion equations inside the film, is suggested. The Dirichlet boundary condition is used at the wall and the Robin boundary condition is used at the film surface for the heat transfer equation. For the species diffusion equations, the Neumann boundary conditions are used at the wall, and Robin boundary conditions are used at the film surface. The convective heat transfer coefficient is assumed to be constant and the convective mass transfer coefficient is inferred from the Chilton-Colburn analogy. The model is validated using the previously published experimental data for heating and evaporation of a film composed of mixtures of isooctane/3-methylpentane (3MP). Also, it is applied to the analysis of heating and evaporation of a film composed of a 50%/50% mixture of heptane and hexadecane in Diesel engine-like conditions.

Historical Trends and Recent Developments in Solar Greenhouse Dryer Operated Under Active Mode: A Review

Background/Objectives: Today the greenhouse drying technology has reached at a remarkable stage due to its tremendous contribution towards saving the agricultural products from spoilage. This has helped the whole world to progress at a fast rate as they are able to save a lot of conventional energy requirements which was being wasted earlier in food. The main objective of this paper is to provide a general idea of progress that has occurred in the field of active greenhouse solar dryer time to time. Methods: This paper focuses on some historical work in the field of active greenhouse drying and recent development in this field to improve the performance in terms of temperature and quality of the agricultural products. Findings: This review paper provides detailed and systematic information of research carried out in the field of solar greenhouse drying operated under forced convection mode. Applications: This review work not only provides the general reference to the researchers working in the same field but also helps to farmers for the possible modifications in the method of drying and further improvements in future. Keywords: Convective Mass Transfer Coefficient, Forced Convection, Performance, Solar Greenhouse Dryer

Influence of airborne particles on convective mass transfer of SVOCs on flat surfaces: Novel insight and estimation formula

Impact of airborne particles on convective mass transfer coefficient (hm) of semivolatile organic compounds (SVOCs) at flat surfaces has been investigated by assuming gas-particle equilibrium partition. In realistic indoor environments, however, this equilibrium partition might not be attained because the time required to reach equilibrium could be longer than residence time of airborne particles. No quantitative result of influence of particles on hm under a non-equilibrium state has been given yet. A mass transfer model was used in this study to quantify this influence for the laminar air flow. The influence, calculated as ratio of hm with particles present to hm with particles absent, ε, was found to significantly depend on three dimensionless parameters: Bip, R, and Cp∗(∞). Bip represents the ratio of mass transfer rate between gas- and particle-phases SVOCs to mass transfer rate of gas-phase SVOCs transporting across the concentration boundary layer, R is the ratio of SVOC mass in particle-phase to that in gas-phase at equilibrium state, and Cp∗(∞) characterizes non-equilibrium degree of particle-phase SVOCs in the bulk air (higher non-equilibrium degree as it deviates further away from one, as one corresponds to equilibrium). At equilibrium state (Cp∗(∞)=1), the presence of particles enhances hm (ε>1) to the same extent for source surfaces and sink ones. At non-equilibrium state, ε varies linearly as Cp∗(∞) changes (Cp∗(∞)>1 for source surface, and Cp∗(∞)<1 for sink surface). The presence of particles always enhances hm for SVOC source surfaces. In contrast, it can either enhance (ε>1) or weaken (0<ε<1) hm for sink surfaces, depending on non-equilibrium degree between gas-phase SVOCs and particles (characterized by Cp∗(∞)). An empirical formula relating ε with these three parameters was obtained. Good agreement between predictions by the formula and experimental results in the literature moderately supported its reliability. Results of this study could be useful in better estimating indoor SVOC concentrations, as well as assessing and controlling human exposure to indoor SVOCs.

Spectral method for simulating 3D heat and mass transfer during drying of apple slices

In the present study, a numerical method is proposed for simulating the coupled three dimensional heat and mass transfer processes during convective drying of apple slices. Spectral collocation method (pseudospectral method) is applied for discretizing both the space and time variables based on the Jacobi Gauss Lobatto (JGL) interpolation points. Also operational matrices of differentiation are implemented for approximating the derivative of the spatial and temporal variables. The external flow and temperature fields were simulated through the Fluent CFD package. The convective heat transfer coefficient is calculated from the lumped system analysis and convective mass transfer coefficient is computed through the analogy between the thermal and concentration boundary layers. The model is validated against experimental data in a range of air temperatures from 60 °C up to 90 °C. The results illustrate a remarkable agreement between the numerical predictions and experimental results, which confirm robustness, computationally efficient and high accuracy of the proposed approach for predicting the simultaneous heat and mass transfer in apple slices.

Determination of the convective mass transfer coefficient in the dissolution of NaCl particles in non-Newtonian fluids

The main objective of this work was to study the dissolution of salt (NaCl) particles in water and in non-Newtonian fluids that contain xanthan gum. In an agitated tank, experimental profiles of salt concentration as a function of time in the liquid phase were obtained. The experimental data were used to evaluate the mass transfer coefficient in different operational conditions and to validate the mathematical model. It was found that the dissolution kinetics of the salt particles was affected by both the fluid's xanthan gum concentration and initial salt concentration. Adding xanthan gum to the fluid decreased the salt dissolution kinetics. The mass transfer coefficient for a system with xanthan gum was smaller than the one for pure water. The values of the mass transfer coefficient for pure water base fluids varied from (0.836 ± 0.003) × 10−4 to (1.110 ± 0.003) × 10−4 m/s, and for xanthan gum solutions from (0.526 ± 0.003) × 10−4 to (0.865 ± 0.003) × 10−4 m/s. The mathematical model based on the mass conservation equations for the liquid and solid phases predicts the salt concentration profiles simulating the dissolution process. In addition, it was proposed an equation to model the decrease in the diameter of the particles due to dissolution. The mathematical model fitted the experimental data with an average relative error of 2.2%.

Modeling of the Garification Process of Fermented Cassava Mash

This study was carried out to analyze the garification process using Artificial Neural Network (ANN) based model of steady state simultaneous heat and mass transfer. Convective heat and mass transfer coefficients were obtained during garification process of fermented mash from cassava ages of different maturity. Empirical equations developed for heat, (hc) and mass, (hm) transfer coefficients [hc=0.017t2-0.388t+3.039, hm=0.042t2-0.914t+5.481]; with (R2>0.9) were best described by polynomial relationships. The optimum ANN model that produced convective heat and mass transfer coefficients for the garification process consisted of two hidden layers and twenty-five neurons in each hidden layer, with mean square error, mean absolute error, sum square error and R2 of 0.000015, 0.0030, 0.0082% and 0.995, respectively. The developed ANN model can be useful in the determination of heat and mass transfer rate for garification process and wide range of physical conditions. These results are equally important considerations for obtaining quality gari for commercial production.

Moisture transfer modeling during solar drying of potato cylinders considering shrinkage

ABSTRACTIn 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 R2 and lower values of RMSE and χ2. 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 (Deff) and convective mass transfer coefficient (hm) 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.

Infrared thin layer drying of saffron (Crocus sativus L.) stigmas: Mass transfer parameters and quality assessment

Saffron is the most precious and expensive agricultural product. A dehydration treatment is necessary to convert Crocus sativus L. stigmas into saffron spice. To the best of our knowledge, no information on mass transfer parameters of saffron stigmas is available in the literature. This study aimed at investigating the moisture transfer parameters and quality attributes of saffron stigmas under infrared treatment at different temperatures (60, 70, …, 110°C). It was observed that the dehydration process of the samples occurred in a short accelerating rate period at the start followed by a falling rate period. The effective moisture diffusivity and convective mass transfer coefficient were determined by using the Dincer and Dost model. The diffusivity values varied from 1.1103×10−10m2·s−1 to 4.1397×10−10m2·s−1 and mass transfer coefficient varied in the range of 2.6433×10−7–8.7203×10−7m·s−1. The activation energy was obtained to be 27.86kJ·mol−1. The quality assessment results showed that the total crocin content increased, when the temperature increased up to 90°C but, in higher temperatures, the amount of crocin decreased slightly. The total safranal content of the samples decreased slightly when drying temperature increased from 60°C to 70°C and then continuously increased up to 110°C. Also, the amount of picrocrocin increased from 83.1 to 93.3 as the drying temperature increased from 60°C to 100°C.

Mathematical Modelling and Determination of Mass Transfer Characteristics of Celeriac Slices under Vacuum Drying

This study aimed at mathematical modelling and determination of mass transfer parameters of celeriac slices during vacuum drying at different temperatures and vacuum pressures. The usefulness of eight mathematical thin layer models to simulate the drying kinetics was examined and the Midilli-Kucuk model best described the drying curves with the minimum values of root mean square error and some of squares error. The effective diffusivity of the samples was obtained in the range of 2.1908 × 10-10 − 8.9304 × 10-10 (m2/s). Diffusivity increased with increasing drying temperature and decreasing vacuum pressure. The obtained results showed that the convective mass transfer coefficient had ascendant trend during the drying process. Any increment in the drying temperature and vacuum pressure led to an increment and decrement in the convective mass transfer coefficient, respectively.

Mass transfer parameters of celeriac during vacuum drying

An accurate prediction of moisture transfer parameters is very important for efficient mass transfer analysis, accurate modelling of drying process, and better designing of new dryers and optimization of existing drying process. The present study aimed to investigate the influence of temperature (e.g., 55, 65 and 75 °C) and chamber pressure (e.g., 0.1, 3, 7, 10, 13 and 17 kPa) on effective diffusivity and convective mass transfer coefficient of celeriac slices during vacuum drying. The obtained Biot number indicated that the moisture transfer in the celeriac slices was controlled by both internal and external resistance. The effective diffusivity obtained to be in the ranges of 7.5231 × 10−10–3.8015 × 10−9 m2 s−1. The results showed that the diffusivity increased with increasing temperature and decreasing pressure. The mass transfer coefficient values varied from 4.6789 × 10−7 to 1.0059 × 10−6 m s−1, and any increment in drying temperature and pressure caused an increment in the coefficient.

Dehydration Characteristics of Whole Lemons in a Convective Hot Air Dryer

In this study, whole lemons were dried using a laboratory convective hot air dryer and the effects of drying temperature on dehydration behaviour and mass transfer characteristics of the lemons were investigated. The drying experiments were conducted using air temperatures of 50, 60 and 75 °C and air velocity of 1 m/s. It was observed that the drying temperature affected the drying time and the drying rate significantly. Drying rate represented no constant period and the entire dehydration process took place in the falling rate period. The usefulness of five mathematical models to simulate the drying kinetics was evaluated and Midilli and logarithmic models were found as the best models describing the drying curves. The effective moisture diffusivity values were obtained to be 1.15×10-10, 2.29×10-10, and 7.63×10-10 m2/s for the applied temperatures, respectively. The activation energy was also determined to be 71.32 kJ/mol. The convective mass transfer coefficient values were calculated by the analytical model and obtained to be 4.078×10-7, 1.023×10-7 and 4.346×10-8 m/s for drying temperatures of 75, 60 and 50 °C, respectively.