Forced Convection Drying Research Articles

Experimental study of drying techniques in retention of nutrients in aonla

This investigation deals with drying technique and proximate composition of aonla. The aonla was dried in natural and forced convection solar dryers and ground to obtain powder. Dried powder was analysed for its proximate composition and its nutritional characteristics on dry weight basis. It was found that crude fat, crude protein and crude fibre content were found to be highest in forced convection dryer (1.24 g, 1.74 g and 5.48 g). The ash content in natural convection dryer was found to be highest (1.95 g) in fresh aonla on dry weight basis. There was no significant difference (P > 0.05) in carbohydrate content in all three dried powders of aonla. The vitamin C content of natural convection dried aonla was 160.20 mg/100 g where as it was 175.60 mg/100 g for forced convection dried aonla powder and in fresh aonla vitamin C content was 195 mg/100 g. Thus, it can be inferred from the above results that forced convection solar drying is a superior technique of processing and for retaining maximum nutrients.

Thin layer drying kinetics of potato mash

The study was conducted to investigate the effect of temperature on hot-air drying kinetics of potato mash and to evaluate the best model predicting the drying kinetics along with the colour changes during mechanical tray drying. Commonly cultivated variety namely Kufri Jyoti was selected for the study. Potato mash was soaked in 0.2 percent KMS solution to reduce the reducing sugar followed by hot-air drying in single layer mash with thickness vary from 5–7 mm at different temperatures (50–70°C) in a forced convection dryer. In order to estimate and select the appropriate drying model, five different models which are semi theoretical and/or empirical were applied to the experimental data and compared. The goodness of fit was determined using the coefficient of determination (R2), reduced chi square (χ2), root mean square error (RMSE) and mean bias error (MBE). Among the models proposed, the semi-empirical logarithmic model was found to best explain thin layer drying behaviour of the potato mash as compared to the other models over the experimental temperature range. By increasing the drying air temperature, the effective moisture diffusivity values increased from from 1.43684 ×10−10 m2/s to 2.6582×10−10 m2/s as temperature increase from 50°C to 70°C. Samples dried at lower temperature had better lightness (higher L values) compared to those dried at higher temperature.

Drying and Quality Characteristics of Shredded Squid in an Infrared-Assisted Convective Dryer

Drying is one of the most common methods for processing and preserving squids. A novel forced convective dryer based on infrared heating was developed with an online temperature control. By setting the drying medium temperature of 50°C, we studied the effects of infrared wavelength and air velocity on drying characteristics of the shredded squid and qualities of dried squid products. We also compared it with the conventional hot-air drying (HAD) and advanced microwave vacuum drying (MVD). The infrared heating rate increase was faster than that of HAD. The heating and drying at the wavelength of 2.5–3.0 µm were more effective than those at the infrared wavelength of 5.0–6.0 µm. Specific energy consumption linearly increased with the air velocity. Microstructure observation showed that the infrared-dried rehydrated sample displayed a muscle fiber structure similar to the fresh sample. The infrared-dried squids had less drying shrinkage, brighter color, and better rehydration capacity than HAD products. Their sensory qualities were better than HAD and MVD products. Above all, infrared drying with wavelength of 2.5–3.0 µm and air velocity of 0.5 m/s was suggested as the best drying condition for squids in this study.

Effect of Thermophysical Properties on Coupled Heat and Mass Transfer in Porous Material during Forced Convective Drying

The convective drying kinetics of porous medium was investigated numerically. A mathematical model for forced convective drying was established to estimate the evolution of moisture content and temperature inside multilayered porous medium. The set of coupled partial differential equations with the specified boundary and initial conditions were solved numerically using a MATLAB code. An experimental setup of convective drying had been constructed and validated the theoretical model. The temperature and moisture content of the potato samples were dynamically measured and recorded during the drying process. Results indicate that thermal diffusion coefficient has significant positive impact on temperature distribution and mass diffusion coefficient might directly affect the moisture content distribution. Soret effect has a significant impact on heat flux and temperature distribution in the presence of large temperature gradient.

Valorization Study of Treated Deglet-nour Dates by Solar Drying Using Three Different Solar Driers

Deglet-nour variety is a specific date to the region of Maghreb countries and is a climacteric fruit with maturation spread out on the same palm. Dates fruits being harvested once a year; this situation makes the fruit post-harvest treatment necessary so as to minimize losses and avoid the eventual conservation and storage accidents particularly through rainy year or excessively hot climate. This paper presents an experimental investigation of Deglet-Nour treatment by hydration and solar drying process in order to recover and valorise this category. The first aim of this work is to study the effect of different temperature (20°C, 25°C 30°C, 35°C, 40°C, 45°C, 50°C and 55°C) of soaking in distilled water on the inversion reaction of sucrose and the second aim is to analyse the drying of hydrated Deglet-Nour dates by three different solar processes. Three drying kinetics have been established respectively in a direct natural convective solar drier, under indirect convective solar drier and in indirect natural solar drier. The experimental results show that the best case was obtained by soaking in water at T = 25°C to obtain hydrated dates with 0.5kg water/kg dry matter. Drying time to reach commercial water content (0.35kg water/kg dry matter) by the use of the three prototypes were respectively 22h, 11.5h and 2.5h. These tests also showed that the drying with indirect natural convective dryer provides a good product quality with acceptable duration, and it is satisfactory and competitive to a forced convective solar drying process. A good microbiological quality was observed on the final product obtained by each of the three different solar driers.

Forced Convection Drying of Khoa: A Heat Desiccated Milk Product

In this research paper, the convective heat transfer coefficients for khoa drying under indoor forced convection mode by hot air are delineated. The convective heat transfer coefficients have been determined for khoa sample of size 0.09 Χ 0.06 Χ 0.015 m 3 . The khoa sample has been dried as single thin layer for three consecutive days with the help of laboratory dryer until no variation in its mass is recorded. The experimental data have been used to determine the values of the experimental constants in the Nusselt number expression by simple linear regression analysis and then the convective heat transfer coefficients have been evaluated. The convective heat transfer coefficients have been observed to decrease with the drying day progression from the 1 st day to the 3 rd day. The average value of convective heat transfer coefficients have been found to decrease from 2.49 to 1.94 W/m 2 °C. The experimental error in terms of percent uncertainty has also been evaluated.

Understanding forced convective drying of apple tissue: Combining neutron radiography and numerical modelling

Abstract A multiphysics model for biological materials, coupling nonlinear viscoelastic deformation to water transport, was used to study forced convective drying of apple tissue samples (cv. Maigold). The accuracy of the model was verified with quantitative neutron radiography experiments, by comparing the total water loss, the transient water distribution profiles and the mechanical deformation. Both model simulations and experiments showed that the largest moisture gradients occurred at the air–tissue interface. The corresponding shrinkage behaviour was similar. Furthermore, the difference between simulation results from modelling the water exchange with the environment using a constant mass transfer coefficient or a spatially varying transfer coefficient from a flat-plate correlation was not significant, indicating that the drying kinetics were dominated by the water transport in the tissue rather than by the convective flow at air–tissue interface. The simulated results showed a satisfactory agreement with experimental observations. The validated model is clearly appropriate to be employed for optimization of convective drying processes of food. Industrial Relevance The paper adds a significant progress to the study the dynamics of drying (water transport and shrinkage) during dehydration of fruit tissue.

Integration of Photovoltaic Cells in Solar Drying Systems

Photovoltaic (PV) cells have not been sufficiently used in drying processes in the past, particularly for solar drying, due to their high price and low efficiency. This is now changing due to the important scientific and technological recent developments in the PV field. An increase in the number of published scientific works related to the integration of PV cells is clearly visible. It is revealed that PV cells are integrated in drying systems for two essential reasons. The first relates to their use as a part of the solar collector (generally solar air collector), which permits an improvement of their low efficiency. As a result, and in most of the studied cases, the total solar collector efficiency has reached 70%. The second reason relates to the recycling of the electrical energy consumed by other drying system components such as fans. The recycled electrical energy was directly used for instant energy consumption or stored in batteries. The main application of PV cells is their use in direct- and indirect-type forced convection dryers, generally for food and herb drying. In a study case, an economic analysis has shown that payback is dependent on the ratio of the PV cell surface to the total solar collector surface, with the possibility of an optimum payback in less than one year. In another study case, an additional heat pump significantly improved the performance of the photovoltaic–thermal (PV/T) solar dryer, reaching an efficiency of 70%. A proper design of solar drying plays an important role in attaining optimum results. In this case, particular care is given to the design of solar dryers with a detailed presentation of the influence of the different parameters such as the surface of the PV cell, geographical location, and materials used. For most of the presented systems, efficiency is calculated after application of the heat and mass balance for each solar dryer compound.

Novel Application of Neutron Radiography to Forced Convective Drying of Fruit Tissue

Neutron imaging is a promising technique to study drying processes in food engineering as it is a non-intrusive, non-destructive technique, which provides quasi-real-time quantitative information of the water loss during drying and of the internal water distribution, at a high spatial and dynamic resolution. Particularly, the high sensitivity to water is its main advantage for drying studies, despite the limited accessibility to reactor facilities, which produce neutrons. This technique was used to investigate forced convective drying of fruit tissue (pear and apple), placed in a small wind tunnel. Water loss, water distribution in the sample and sample shrinkage were evaluated as a function of time. The water loss, determined quantitatively from neutron radiographs, was underestimated slightly compared to gravimetrical measurements. The overall drying behaviour agreed well with control measurements performed in a climatic chamber and was very similar for both fruit tissues. The corresponding shrinkage behaviour of both tissues was also similar. The large shrinkage, which is characteristic for soft biological materials such as food products, however, hindered post-processing to some extent. From the internal water distribution, the water gradients within the sample, induced by drying, were visualised and were found to predominantly occur at the air–material interface, indicating that the water transport inside the tissue dominated the water loss, instead of the convective exchange with the air flow. Neutron imaging was shown to exhibit unique benefits for studying drying processes of food.

Effect of Air Velocity and Temperature on the Drying Kinetics of Drumstick Leaves (Moringa Oleifera)

Abstract The present study examines the effect of air velocity on drying kinetics of the drumstick leaves in a forced convective dryer. The drumstick leaves were dried in the temperature range of 50–800 C, at different air velocity (Dv) of 0.5 and 1.3 m/s. The results indicated that drying temperature and air velocity are the factors in controlling the drying rate. Experimental data obtained for the samples for color, drying rate and drying time proved that air velocity of 1.3 m/s yielded the product superior in terms of both quality and energy efficiency as compared to the samples at 0.5 m/s. Activation energy for drumstick leaves dried with air velocity, 0.5 and 1.3 m/s was 12.50 and 32.74 kJ/mol respectively. The activation energy relates similarly with the effective moisture diffusivity which also increased with increase in air velocity and temperature.

Development of convective heat transfer correlations for common designs of solar dryer

The knowledge of convective heat transfer coefficient hcpf (absorber plate to flowing air) is necessary to predict or evaluate thermal performance of any solar dryer. In order to determine hcpf, laboratory models of direct (cabinet), indirect and mixed mode solar dryer are designed and constructed to perform no-load steady state experiments for natural and forced air circulation. The dryers are operated under indoor simulation conditions for absorbed thermal energy and air flow rate for the range of 300–800W/m2 and 1–3m/s, respectively. Separate methods depending on mode of heat utilisation are proposed for determination of hcpf for different dryers. Correlations of hcpf in terms of dimensionless numbers are developed for each dryer operating under natural and forced convection. Levenberg–Marquardt algorithm is used to develop temperature dependent correlations. A close agreement between experimental and predicted hcpf values obtained from proposed correlations for natural convection dryers demonstrates their reliability. However, for forced convection dryers, there is a need to use temperature dependent Nu–Re correlation for more accurate results. The low uncertainty ranging from 0.3% to 0.8% in the determination of hcpf confirms the accuracy of experimental data obtained for various dryer designs operated under different conditions.

Drying of garlic slices: Kinetics and nonlinearity measures for selecting the best equilibrium moisture content equation

The objective of this work was to analyze the drying process of garlic slices based on an experimental study involving its equilibrium moisture content and drying kinetics. The equilibrium data were obtained by the static method using saturated salt solutions. The main equilibrium equations for biological materials were discriminated using some measures of nonlinearity. The drying kinetics was studied in a forced convection dryer, and the influence of three types of slices was analyzed. The results showed that the modified Halsey equation was the most suitable to represent the equilibrium data for garlic slices. The Page equation adequately represented the drying kinetics data. The highest drying rates of sliced garlic were obtained with crosswise cut.

A combined approach to optimize the drying process of flavonoid-rich leaves ( Inga edulis) using experimental design and mathematical modelling

The process of drying of the leaves of Inga edulis, a flavonoid-rich plant from Amazonia, was optimised in a forced convection dryer by combining experimental design and mathematical modelling. First, the monolayer moisture content was determined by hygroscopic studies at 25 °C and the value of this parameter (5.86 g/100 g dry matter) was fixed as a minimum to be achieved. Next, a 3 2 full-factorial design was applied, with temperature (40, 55 and 70 °C) and airflow velocity (0.6, 1.2 and 1.8 m/s) as the independent variables, the dependent variable being the drying time required to reduce the moisture content until the level determined in the monolayer was achieved. Nine kinetic models, with one to four parameters, were fitted to the experimental data. The fitting performance of these models was assessed using statistical criteria and, from these results, the Henderson and Pabis model was found to be sufficient to predict the monolayer drying times. Subsequently, a multiple linear regression was carried out by applying a second-order model to describe the relationship between the independent and dependent variables, which established the adequacy of this model ( R 2 > 0.97) and, thus, the applicability of the combined statistical and mathematical approaches. Finally, an experimental domain that could minimise the drying times ( time < 40 min) was determined. An experimental validation of the model was accomplished in a point of this domain and good agreement was found between the experimental and predicted values. Moreover, no flavonol degradation was observed, whereas a loss of 19.4% for the total flavanols was found.

PRODUCTION OF DRIED STARCH-ALBUMEN POWDER: EFFECT OF TEMPERATURE AND STARCH ON SOME FUNCTIONAL PROPERTIES

The effect of cassava starch inclusion (10–30%), as a preconcentration step, on the functional properties of starch-albumen powder (SAP) was studied. A natural convective hot air drying method at 40–60C was used. Drying time at loading density of 0.5 kg/m2 ranged from 7.4 to 8.0 h (about 22–32% reduction in time for drying pure liquid albumen). The products had safe moisture levels ranging from 6.0 to 8.0% (dry basis). Generally, the functional properties of SAP were not significantly affected by the drying temperature when compared with pure albumen powder. With increasing starch content, the foaming capacity, foam stability, oil absorption capacity and emulsion stability were enhanced (P < 0.05), whereas the emulsion capacity of SAP was significantly reduced (P < 0.05). The study highlighted prospective applications of the new product and the need to design a more efficient forced air convective drying facility to enhance throughput and reduce energy cost. PRACTICAL APPLICATIONS The study has shown the possibility of preserving egg white in powdered form using a simpler and low-cost convective air drying technology compared with the conventional spray drying technology. The product's characteristics indicate that it may be used as breading material in fried food, as basal weaning food and as ingredient in some baked products.

Theoretical model of a drying system including turbulence aspects

Biological products contain high moisture content in the harvest period. Such moisture could cause their deterioration in storage. Drying is an interesting solution to keep the quality of these products. Generally, drying process is based on contact of a dry air phase with a wet solid. A theoretical model was developed to describe the dynamic behavior of a forced convection drying process. This model takes into account the entire dynamic phenomenon including turbulence, diffusivity and tortuosity effects. Only the mass transfer of water molecules during vaporization was referred to as an empirical expression. The model thus strongly build, was compared to Thompson’s empirical model. Corn grain was used in the validating process. Transient temperature and water content results are presented.

Secagem de fatias de abóboras (Cucurbita moschata, L.) por convecção natural e forçada

A abóbora (Cucurbita moschata, L.) é uma importante fonte de provitamina A, de baixo custo, e sob a forma desidratada oferece diferentes opções de utilização e consumo. Secagens por convecção natural e forçada foram comparadas quanto ao grau de secagem atingido e encolhimento, em função da temperatura, velocidade de ar e dimensões do produto. Os resultados obtidos mostraram que o uso de fatias com volume de 6,25 cm³ em secador por convecção forçada a 50 °C e à velocidade de 5,5 x 10-4 ms-1 resultaram em produtos de menor encolhimento, sendo recomendadas estas condições.

Variation of the Physical Properties of Sheanut (Vitellaria Paradoxa Gaertn.) Kernels during Convective Drying

The effect of moisture content and drying temperature of Vitellaria paradoxa Gaertn kernels on some of its physical properties was investigated. The kernels which were harvested at a moisture content of about 60% (wet basis) and hence prone to high post harvest losses from two ecological zones of Cameroon (Bangoua in West province and Tchabal in Adamawa province) were dried in a forced convection dryer at 40oC, for 6, 20, 48, 72 and 96 hours to give moisture contents ranging from 10 to 60% wet basis. Ten trees from each of the zones were carefully selected to serve as sources for the ripe kernel bearing shea fruits that were used as samples for this study. For each parameter studied, a sample population of 30 kernels selected at random per tree was used. The results obtained revealed that there was a significant difference in the physical properties of the kernels from different trees irrespective of the locality. The bulk density, true density, sphericity and porosity varied non-linearly with the moisture content. Kernels with larger masses showed a different variation pattern of bulk density and porosity with moisture content compared to the lighter kernels. The variation of the bulk density, sphericity, porosity of sheanut kernels with moisture content and temperature was satisfactorily modelled with empirical equations. The samples underwent considerable shrinkage (up to 35%) during the drying process. Three empirical models were used to describe the shrinkage behaviour of the kernels and it is proposed that these models could be incorporated in drying models.

Mathematical modeling of handmade recycled paper drying kinetics and sorption isotherms

The objective of this work is to analyze and compare the natural and forced convective drying of handmade recycled paper. Drying of recycled cellulose pulp was carried out under laboratory environment conditions and in a convective dryer with forced air circulation and controlled conditions of air temperature and velocity. The tests were conducted following a two-factor central composed factorial design of experiments, with six runs at the central point. The drying results were analyzed and fitted to mathematical models of Fick, Henderson and Pabis (Fick’s modified equation), Page and He (considering the nonlinear Fick effect). The model of Page represented best the experimental data and the one of Henderson and Pabis resulted in an adequate fit for the paper drying kinetics. Sorption isotherms were determined for the dried paper and the models of GAB (Guggenheim-Anderson-de Boer) and GDW (Generalised D’Arcy and Watt) resulted in excellent fits of the experimental data. The water sorption mechanism was suggested by the analysis of the calculated parameters of the GDW model.

A generalized conjugate model for forced convection drying based on an evaporative kinetics

A model describing the heat and mass transfer involved in food drying is presented. The aim is to determine the effect of air temperature on the performance of the drying process applied to fresh-cut vegetable slices, but other effects can be easily incorporated in the model. The model allows to disregard one of the most limiting parameters in such modeling, i.e. the average heat and mass transfer coefficients at the food/drying substrate interface, which are generally taken from the literature. Such assumptions are limiting in the sense that they are referred to average transfer conditions and general geometries. The presented model relies upon a finite-element solution of time-dependent differential equations for simultaneous and conjugate heat and moisture transfer in a two-dimensional domain, without any inference in such empiricism. A special formulation for drying kinetic of the substrate is also exploited, and a treatment of the dependence of the properties upon the residual moisture content is included. After proper validation with the available experimental measurements, the numerical solution is discussed by presenting each involved field variables, emphasizing on the conjugate nature of the drying process. Due to its flexibility and generality, the model can be used in common industrial driers’ optimization, even in the assumption of a laminar flow field.

Experimental and mathematical investigations of convective solar drying of four varieties of olive leaves

The aim of this work is to study the drying kinetics of four varieties of Tunisian olive leaves ( Chemlali, Chemchali, Zarrazi and Chetoui) by using an indirect forced convective solar dryer consisting of a solar collector, an auxiliary heater, a circulation fan and a drying cabinet used for drying experiments. Experiments are conducted at three temperatures (40, 50 and 60 °C), ambient relative humidity (29–32%) and at a drying air flow rate of 0.0556 m 3/s. Three models available in the literature were used to describe experimental kinetics. The experimental drying curves show only the falling rate period. Air temperature has a significant effect on drying kinetics. The statistical parameters (correlation coefficients and standard errors) show that the Page model could be used to determine the curve drying equations from the experimental drying curves. The characteristic drying curves of olive leaves are determined empirically by using the corresponding curve drying equations. Apparent moisture diffusivities of olive leaves were determined from the analytical solution of Fick's equation. Values vary from 2.95 × 10 −10 to 3.60 × 10 −9 m 2/s. They depend on the olive leaves variety and on temperature.