Electrohydrodynamic Drying—A Concise Overview

Electrohydrodynamic (EHD) drying is a novel method of non-thermal processing. The drying can be carried out using either AC or DC high voltages. The thermodynamic considerations regarding the lowering of temperature under EHD drying include rapid rates of evaporation and exothermic interaction of the electric field with a dielectric material. Multi-point and plate electrode systems are efficient in accelerating drying of agricultural materials. Compared to hot air (convective) drying systems, EHD drying systems offer lower food production costs along with superior quality in terms of physiochemical properties such as color, shrinkage, flavor, and nutrient content. Compared to convective and freeze-drying, EHD drying systems, given their simpler design and lesser energy consumption, show great potential for bulk and industrial drying applications.     

Atmospheric Freeze Drying—A Review

This review article focuses on the development of atmospheric freeze drying (AFD): technological aspects, product possibilities, physical properties of products, drying kinetics, modeling, and simulation. The main motivation for developing atmospheric freeze drying as a new drying technology is the desire to reduce the energy consumption compared to vacuum freeze drying while maintaining a high product quality. One technical solution of atmospheric freeze drying is a combined atmospheric freeze drying and heat pump system with new environmentally friendly refrigerants. Temperature programs make it possible to customize products with desired qualities and properties, like retained color, instant properties, aroma, and nutritional value.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D_e) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X_o). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

ABSTRACT

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D _e ) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X _o ). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

HIGH ELECTRIC FIELD DRYING OF JAPANESE RADISH

Japanese radish (Raphanus sativus L.) slices were dried under an alternating current (AC) high electric field (HEF) of 430 kV/m using a multiple point electrode and a grounded copper mesh. HEF with multipoint to a mesh electrode system could evaporate 87.5% of the total moisture from fresh radish slices on 7 h exposure. Oven-drying at 60°C and ambient air at 25°C for 7 h could dry 86.9% and 26.5% of the total moisture from the fresh radish slices respectively. The average rate of HEF drying from second hour to sixth hour was 0.025 g/min which was 0.0012 g/min higher than the oven-drying. Quality factors for HEF-dried radish exhibited less shrinkage, high absorption of water, better rehydration, less solids loss and better color than the oven-drying.     

HIGH ELECTRIC FIELD DRYING OF JAPANESE RADISH

Japanese radish (Raphanus sativus L.) slices were dried under an alternating current (AC) high electric field (HEF) of 430 kV/m using a multiple point electrode and a grounded copper mesh. HEF with multipoint to a mesh electrode system could evaporate 87.5% of the total moisture from fresh radish slices on 7 h exposure. Oven-drying at 60°C and ambient air at 25°C for 7 h could dry 86.9% and 26.5% of the total moisture from the fresh radish slices respectively. The average rate of HEF drying from second hour to sixth hour was 0.025 g/min which was 0.0012 g/min higher than the oven-drying. Quality factors for HEF-dried radish exhibited less shrinkage, high absorption of water, better rehydration, less solids loss and better color than the oven-drying.     

Atmospheric Freeze Drying—Modeling and Simulation of a Tunnel Dryer

Drying is an important unit operation in processing of foods and other biotechnological products. Vacuum freeze drying is said to be the best drying technology regarding product quality of the end product, but the disadvantages are, among others, expensive operational costs and batch drying. Atmospheric freeze drying was introduced to lower the production costs of high-quality dried foods, and the need of simulation tools became important in estimations of the industrial drying processes.

A simplified mathematical model (AFDsim) is developed based on uniformly retreating ice front (URIF) considerations. The model is used to calculate theoretical drying curves of atmospheric freeze dried foods in a tunnel dryer. Studies of thermal and mass transfer properties during drying are essential for understanding the changes in product quality and for designing and dimensioning the drying process. The model can be used to simulate industrial atmospheric freeze drying of different foodstuff in a tunnel. The results from AFDsim modeling are in good accordance with the experimental results.     

Predicting the Quality of Dehydrated Foods and Biopolymers — Research Needs and Opportunities

Drying processes affect the quality of the final food biopolymer product. The texture, density, wetability, caking, rehydration capacity and mechanical properties of the dried food biopolymer depend on: the product properties, the drying process conditions and the product residence time distribution in the dryer. The interrelationship between dried product quality and drying process conditions is not sufficiently explored in literature because of limited understanding of the physico-chemical mechanisms affecting product quality, and of the effect of these mechanisms on transport properties. Recent ad- vances in material and polymer science research, however, provide the tools and methodologies needed to understand in depth the quality changes dur-ing drying. The current work reviews the approaches of conventional drying theories and presents recent advances in polymer science on modelling solvent absorption by glassy polymers based on the glass-transition theory. Oppor- tunities presented by the glass transition theory towards explaining quality changes during drying are identified and the framework of a recent rnathe- matical model that can be used to integrate the drying and polymer-science approaches is then presented.     

A Vacuum Drying System for Green Hardwood Parts

A vacuum drying system was designed and fabricated and that system was used to dry green rough hardwood dimension. The red oak samples, 76.2 (long) × 7.62 (wide) × 2.54 (thick) cm, were dried from green moisture content (MC) to 7% MC in this system. They were dried at a pressure of 12 mm Hg and temperatures ranging from 30 to 50°C within 25 to 70 h. Drying quality tested included warp, internal checking, and surface checking. Moisture gradients along the length and thickness were measured. The standard prong test was used to assess the drying stresses. Vacuum drying was fast and the drying rate increased as the temperature increased. It was found that the general drying quality was good with no color change. Drying stresses including longitudinal and transverse stresses were small. There were no internal checks.     

Mathematical modeling and simulation of shrunk cylindrical material's drying kinetics—Approximation and application to banana

In the present work, a diffusion-based model was adopted to represent the convective drying behavior of cylindrical banana samples, taking into consideration the shrinkage along drying. The developed model simulated a significant number of situations resulting from the variations of some operating conditions. The temperatures tested were 50, 60, 67 and 70 °C, the air velocities were 3, 4 and 4.5 m/s and the relative humidity range of the drying air was from 3.5 to 11.5%. The calculated drying curves were compared to the experimental ones in order to determine apparent moisture diffusivity. An empirical equation was suggested, describing the apparent moisture diffusivity within the banana versus product temperature and local moisture content. A good agreement was found between experimental and calculated drying kinetics.     

Enhancement of Drying Rate in Paper Machines with Multi-Functional Radial Jet Reattachment Blow Boxes — Theoretical Analysis

Multi-functional radial jet reattachment blow boxes deliver a dry air into the pockets of the dryer section of a paper machine by impinging directly onto the unsuppned sheet in the open diaw. The air flow also maintains or improves the sheet swbility.

The iadial jet reattachment blow box technology pmvides a significant enhancement in drying rates. The theoretical drying results are presented in terms of average drying rates, moisture and temperature pmfiles along the drying section with and without the presence of the multi-functional radial jet reaaachment blow boxes. The thcorclical results indicate that enhancements of drying rates within the range of 8% to 25% are feasible depending on ihe operating conditions and machine configuration.     

A NON DESTRUCTIVE METHOD FOR QUANTIFYING THE WOOD DRYING QUALITY USED TO DETERMINE THE INTRA AND INTER SPECIES VARIABILITY

ABSTRACT

A non destructive method for surface strain measurements is proposed to quantify the wood drying quality during convective drying. This method uses a visible laser scan micrometer intercepting needles maintained vertical at the board surface with a special device, and then gives shrinkage values at several surface points. The results analysis is related to heal and mass transfer phenomena.

Experiments were made on softwoods and hardwoods either with superheated steam or with moist air. Results show that stages of shrinkage agree with classical periods of transfer. Besides, shrinkage results from a compromise between global shrinkage of the board section and local effects bounded with drying conditions. We define two criteria for the drying quality. One compares experimental average shrinkage and free shrinkage, and the other quantifies the differences of shrinkage values between several surface points. Both criteria have to be associated in order to analyse the species behavior in terms of checking during the second drying period.     

A Method to Measure Emissions from Dryers with Diffuse Leakages 2—Sensitivity Studies

Measurements of VOC emissions from dryers have several known difficulties. An earlier published method to measure emissions of VOC from dryers deals with problems associated with diffuse emissions and high moisture content of the drying medium.^[1] In this article, a different calculation path are used and the influence of the main parameters are studied in more depth. For one experiment, confidence intervals for all of the parameters are calculated. Experimentally induced uncertainties are found to be more important than a theoretically important parameter. Suggestions are made on how to increase accuracy.     

Development of SAPGD—A Simulation Software Regarding Grain Drying

Properties of air are interrelated in a drying process. Equations of psychrometrics and fluid dynamics developed by previous researchers enable the development of simulation models, which become an indispensable tool for the study of drying technology. A Windows based software, entitled SAPGD (Simulation of Air Properties and Grain Drying), written in MATLAB, was developed to simulate the equations/models related to the following four aspects:

1. The equilibrium moisture content of grains under the diverse conditions of atmosphere.
2. The pressure drop to airflow of various grains.
3. The relationship between drying time and moisture content under the model of thin layer drying of grains.
4. The relationships among drying time, moisture content, grain depth, and grain temperature under the logarithmic model of deep bed drying of grains.

     

A Simple Data Processing Approach for Drying Kinetics Experiments

A simple mathematical approach is proposed to be applied to drying kinetics raw data processing. The data collected in a drying experiment of powder cork under constant air drying conditions served as case study to present the methodology. Two functions (linear and third-degree polynomial) were used to fit solid moisture content in the constant drying rate and the falling rate periods. The drying rate curve was obtained by differentiation and the time at which the drying rate period's transition occurs was determined iteratively until virtually continuous functions were achieved. The critical moisture content was easily identified and two falling drying rate periods were detected.

The powder cork moisture decrease was also used to test several semiempirical models available in the literature. The Logarithmic, Midilli, and Page Modified I models were the ones that revealed the best correlations performance. When the methodology proposed was applied using these models, the critical moisture content was underpredicted.

The effective moisture diffusivity and the activation energy were also obtained for powder cork after the proposed mathematical approach has been applied on the raw data obtained in experiments performed at different air drying temperatures.     

A SHORT-CUT METHOD FOR DETERMINING CONCENTRATION DEPENDENT DIFFUSIVITY IN LIQUID FOODS AND POLYMER SOLUTIONS FROM REGULAR REGIME DRYING CURVES

A simple method for determining concentration (water or solvent content) dependent diffusivity in liquid foods and polymer solutions from the regular regime drying curve was developed on the basis of the power-law diffusion model. The method was first tested to the drying data generated from numerical solutions of the diffusion equation with various types of concentration dependent diffusivities. The present method was found to be very simple yet accurate compared with the previous methods. Then, the drying rates of various sugar solutions experimentally obtained were analyzed on the basis of the present method to determine the moisture content dependent diffusivities.     

A SHORT-CUT METHOD FOR DETERMINING CONCENTRATION DEPENDENT DIFFUSIVITY IN LIQUID FOODS AND POLYMER SOLUTIONS FROM REGULAR REGIME DRYING CURVES

A simple method for determining concentration (water or solvent content) dependent diffusivity in liquid foods and polymer solutions from the regular regime drying curve was developed on the basis of the power-law diffusion model. The method was first tested to the drying data generated from numerical solutions of the diffusion equation with various types of concentration dependent diffusivities. The present method was found to be very simple yet accurate compared with the previous methods. Then, the drying rates of various sugar solutions experimentally obtained were analyzed on the basis of the present method to determine the moisture content dependent diffusivities.     

A rate—controlling step in langmuir—hinshelwood kinetic models

Langmuir‐Hinshelwood models with and without rate controlling step assumptions have been compared. Hydrogenations of toluene, cinnamaldehyde and mixture of methylesters of fatty acids were used as model systems. Integral data were obtained in semibatch reactors. The proposed models have been simplified by neglecting statistically insignificant parameters. The predicted mixture composition and surface coverages have been compared and the reliability of parameter estimates tested.