DRYING KINETICS AND CHARACTERISTIC DRYING CURVE OF LIGHTLY SALTED SARDINE (SARDINELLA AURITA)

Drying of lightly salted sardine (Sardinella aurita) was accomplished using three air temperatures (35°C, 40°C, 50°C) and three air velocities (0.5 m/s, 1.5 m/s, 2 m/s); the effects of drying conditions on drying kinetics were studied. As for all biological products, air temperature is the main factor influencing the drying kinetics. However, over a given temperature which seems to correspond to protein modification (50°C), and at a high air flow rate (2 m/s and 2.5 m/s) a crust formation on the surface of the fish, due to the combined effect of heat and salt was observed. This phenomenon inhibited the drying rate. From the drying curves, two falling rate periods were observed. The dimensionless drying rate versus a dimensionless moisture content data were regressed by the Marquardt Levenberg non-linear optimization method to obtain an empirical equation describing the salted sardine characteristic drying curve.     

Air drying characteristics and moisture diffusivity of carrots

The effects of air temperature on drying kinetics of carrot cubes were investigated. Convective drying characteristics of carrot cubes in a spout-fluidized bed were evaluated through the effect of air temperature on drying kinetics. Drying was carried out at 60, 70, 80 and 90 °C and the falling drying rate data were used to calculate the effective diffusion coefficients from the Fick's equation. Four mathematical models available in the literature were fitted to the experimental data. The Two-term model is given better prediction than the Henderson and Pabis, Page and Lewis model and satisfactorily described drying characteristics of carrot cubes.     

Effect of Segment Scale in a Pore Network of Porous Materials on Drying Periods

Drying periods of porous glass disks of nonhygroscopic MPG glass and hygroscopic Vycor glass were investigated. A simple model of dimensionless segment sizes is proposed where a dimensionless segment size is equal to the value of the tortuosity factor of gas diffusion. This model was applied to the estimation of the segment size and the drying periods of a porous Vycor glass disk. The drying rate curve of the Vycor glass disk showed that the first falling rate period appeared to be included in the constant rate period and that the second falling rate period started right after the constant rate period ended, whereas the drying rate curve of the MPG glass disk estimated from previous models showed that the first falling rate period and the second falling rate period were clearly distinguished. The difference in drying periods between two porous glass disks was caused by a large difference in the respective segment sizes.     

OPTIMIZATION OF CONVECTIVE DRYING PROCESS BASED ON ECONOMIC INDICATORS

ABSTRACT

Drying of lightly salted sardine (Sardinella aurita) was accomplished using three air temperatures (35°C, 40°C, 50°C) and three air velocities (0.5 m/s, 1.5 m/s, 2 m/s); the effects of drying conditions on drying kinetics were studied. As for all biological products, air temperature is the main factor influencing the drying kinetics. However, over a given temperature which seems to correspond to protein modification (50°C), and at a high air flow rate (2 m/s and 2.5 m/s) a crust formation on the surface of the fish, due to the combined effect of heat and salt was observed. This phenomenon inhibited the drying rate. From the drying curves, two falling rate periods were observed. The dimensionless drying rate versus a dimensionless moisture content data were regressed by the Marquardt Levenberg non-linear optimization method to obtain an empirical equation describing the salted sardine characteristic drying curve.     

DRYINC RATES DURING HIGH TEMPERATURE DRYING OF YELLOW POPLAR

Experimental drying rates during high temperature drying of Yellow Poplar is analyzed to determine types of drying mechanism. Drying rate during the constant rate period and effective diffusivity during the falling rate periods were obtained at dryer temperatures ranging from 100°C to 150°C.

The drying curve shows three drying regimes: a constant-rate period, and two falling rate periods. The constant rate period, where the drying rate is a linear function of time, is controlled by the rate of heat transfer to the wood surface. Both falling rate periods show a drying rate which is a linear function of the square root of time. Floisture migration during the first falling rate is by simultaneous capillary action and liquid diffusion. while moisture'migration takes place by vapor diffusion during the second falling rate period. This occurs when the moisture content of the wood falls below the fiber saturation point. The effective diffusion coefficient during both periods is found to be constant.     

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.     

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

ABSTRACT

The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.     

Kinetics of Drying of Fibre Materials and Fibre-Forming Polymers with Warming

Drying with heating of fibres, fibre materials, and fibre-forming polymers takes place with low-intensity energy feed, with an increase in the thickness of the material dried, or with a high moisture content of the drying agent. Heating of the material in the initial drying period is analyzed and the corresponding equation for the drying kinetics is obtained. The dependences for describing the kinetics of drying with heating are obtained for different materials and conditions. The general kinetic curve describes periods of heating and a constant and diminishing drying rate. Drying with first and second periods (instantaneous heating of the material) is obtained from the overall dependence as a special case. The dependences obtained are compared with the experimental data for different types of drying of fibres, fabrics, and fibre-forming polymers. The calculated drying parameters are found. The calculated curves are in good agreement with the experimental data in the working range. A method for calculating drying using the dependences obtained is described.     

Effect of Pretreatments on Drying and Rehydration Kinetics and Color of Sweet Potato Slices

The effect of air temperature and pretreatments (KMS: citric acid) on drying kinetics of sweet potato slices was investigated. Drying experiments were performed in a tray dryer. In falling rate period, moisture transfer from sweet potato slices was described by applying the unsteady-state Fickian diffusion model, and the rate constant (k) were calculated. The effect of temperature on k could be interpreted according to Arrhenius law. Drying rate and therefore k values were found to be affected by pretreatments. Rehydration rates of dried sweet potato slices at 25, 40, 80°C were also determined and found to be independent of drying conditions and rehydration temperature. The ΔE value was found to be the highest for slices treated at 50°C with 0.5:1.0% KMS and citric acid.     

Water migration in clay slabs during drying: A three-dimensional numerical approach

Clay products for construction offer durability, thermal and acoustic comfort as well as low cost. Drying is an important stage of the production process of these products, and it was studied herein. Experiments on convective drying of clay slabs with initial moisture content of 0.23 (db) were performed at 50, 60, 70, 80 and 90 °C. Drying occurred in two distinct periods: constant and falling rate. In the first period, the process was described by an empirical equation and, in the second one, by the diffusion equation with boundary condition of the third kind. To eliminate some restrictions found in the literature, a three-dimensional numerical approach was used to describe the process. Equations were obtained to express the process parameters as a function of the drying air temperature and local moisture content. A simulation using the values obtained from these equations for the process parameters is coherent with the experimental data.     

A New Variable Diffusion Drying Model for the Second Falling Rate Period of Paddy Dried in a Rapid Bin Dryer

The objectives of this article is to propose a new drying model for the second falling rate period known as the variable diffusion controlled period that follows after the first falling rate period and to propose a new method to determine the second critical moisture content that separates these two periods. Experimental work on paddy drying at minimum fluidization velocity was carried out in a rapid bin dryer. The effects of operating temperatures (60-120°C) and bed depths (2-6 cm) on the paddy drying characteristics were investigated. It was found that the normalized drying rate of paddy was proportional to the normalized moisture content in the first falling rate period but in the second falling rate period, the normalized drying rate of the material varies exponentially with the normalized moisture content. The different relationship between the normalized drying rate and the normalized moisture content in the first and second falling rate periods indicate that two different mechanism of moisture transport are at work. The new exponential model of the second falling rate period and the linear model of the first falling rate period were found to fit the experimental data very well. Derivation from variable diffusion equation shows that the linear model is the result of constant diffusion coefficient whereas the new exponential model is the result of linear diffusion coefficient. This also implies that the first falling rate period is a constant diffusion controlled period and the second falling rate period is a variable diffusion controlled period. In addition, drying kinetics data of a drying process that fits the exponential model over a very slow drying period will show that the drying process is under the effect of a linear diffusion coefficient. It was also found that the proposed new method to determine the second critical moisture content that distinguishes between the first and second falling rate periods by using a sudden change in the value of the drying rate gradient to a much lower value at that point is more rigorous and yet simpler than the method of determining the specific location of the receding drying boundary since it is based on the behavior of the actual drying kinetic data.     

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.     

Model-Based Estimate for Time-Dependent Apparent Diffusivity

This article presents a method allowing the effective diffusion coefficient to be determined from experimental drying kinetics as a time-dependent parameter. The method combines an analytical solution of Fick's equation in which the Fourier number is raised to the empirical power (a), with a semi-theoretical equation derived for quasi-stationary conditions. The method has been validated by applying the resulting equation to literature data on drying kinetics for parboiled rice and cut tobacco leaves as they approach a sphere and a slab, considered to be standard geometries in diffusion problems.     

Microwave-Assisted Thin Layer Drying of Wheat

Drying characteristics of Canada Western Red Spring (CWRS) wheat were studied using a domestic microwave convective oven. The effects of microwave power level, grain bed thickness, and initial grain moisture on the drying kinetics were investigated. Wheat samples with initial moisture levels of 0.18 to 0.29 kg water/kg of dry matter were dried for different drying periods of 180 to 360 s. The moisture loss data were recorded at regular short intervals. Then moisture loss data were fitted to various models (Page equation, modified drying equation, and Midilli equation) to study the drying kinetics of wheat. The results showed that wheat moisture loss increased with increasing microwave power level. A mathematical model was developed by coupling mass and energy balances, resulting in a system of non-linear equations. The predicted moisture loss data from the developed model were compared by fitting to experimental microwave data that were in good agreement.     

Comparative study of droplet drying models for CFD modelling

CFD simulation is used to study wall deposition and agglomeration phenomena commonly encountered in industrial spray dryers. This paper initially provides a comparison of two drying kinetics models: Characteristic Drying Curve (CDC) and Reaction Engineering Approach (REA). Comparisons are made with experimental data with application to carbohydrate droplet drying obtained from past workers. These models were then extrapolated to actual drying conditions to assess their performance. The REA model predicts the progressive reduction in drying rate better than the CDC model for the carbohydrate droplets. A modified CDC model incorporating a convex falling rate produced better agreement than the conventional linear falling rate model. Further analysis showed that the REA model can be extended to simulate the particle surface moisture which may affect the agglomeration process. The proposed concept was compared with reported simulation results from a diffusion model which showed reasonable fit with data.     

DRYING OF OILSEEDS

Drying of oilseeds is a post harvest operation required for safe storage of harvested seeds. Oilseeds have gained importance in India over the last few years. Fluidized bed (FB) drying and Spouted Bed (SB) drying of oilseeds were studied as potential dryers for these seeds. Experimental pilot fluidized bed dryers were developed and a 2 TPH capacity (mustard seed) FB dryer plant was designed and installed in the lab. Experimental units and a pilot spouted bed dryer were also developed. The dryer units and the plant have been extensively tested and satisfactory drying performance has been achieved. Drying data and drying characteristics have been generated for different oilseeds such as mustard, sunflower, soybean and groundnut etc. The drying rates in different regimes of drying characterized by constant rate and falling rate periods have been evaluated. The salient results of the work are briefly outlined.     

Drying rates of polyamic acid solution film under (i) natural convection and (ii) induced draft drying conditions

Drying rate studies were carried out with polyamic acid solution synthesized in the laboratory in dimethyl acetamide solvent, with a view to determine the process design parameters of a continous polyimide film casting machine. Studies reveal that drying under natural convection condition takes an unduly long time, whereas drying under induced draft/recirculation with partial recycle condition takes only ? the time that of natural convection drying. Another important observation made was that under natural convection drying condition the initial adjustment period was too long and that the drying process was evenly spread over the initial adjustment, constant, and falling rates of the drying periods, whereas under induced draft condition drying mostly took place during the falling rate of drying period. The drying rate data obtained was used for generating the necessary process design, and operating parameters for a continuous polyimide film casting machine.     

Relationship of Product Structure,Sorption Characteristics,and Freezing Point of Atmospheric Freeze-Dried Foods

Drying is an important unit operation in processing of foods with a long shelf life. The drying process influences product properties and quality; the products may shrink, break, or undergo rheological, physical, and biochemical changes. Important parameters responsible for product quality changes during drying are temperature, relative humidity, and residence time. Studies of thermal and mass transfer properties during drying are essential for understanding the changes in product quality and for designing and dimensionalizing the drying process.

Drying kinetics, sorption properties, shrinkage, and freezing point depression were determined during atmospheric freeze drying (AFD) of pieces of apple, turnip cabbage, and cod. Adsorption rate and sorption isotherms were determined in the end product. The drying temperature affected the physical properties. Drying at ?5°C resulted in a larger shrinkage than drying at ?11°C. GAB modeling was used to characterize the sorption properties of the products. No typical sigmoidal shape was found of the moisture sorption isotherms of the products, which is in accordance with the Guggenheim constant found from the same results. Experimental data on freezing point depression were used to find product constants E and b in the Schwartzberg equation for the freezing point depression. Freezing point depression, as a function of the dry matter content, was determined using Schwartzberg's equation and a component composition model (CCM). The result indicates an influence of structural effects on freezing point depression.     

Microwave Drying of Multi-component Powder Systems

The rate of uptake of microwave technology within the pharmaceutical sector has been slow due to stringent regulatory procedures and unsuccessful microwave trials. However, microwave systems offer significant energy and time savings compared with conventional dryers. Common pharmaceutical excipients, lactose, stearic acid, and active ingredients, Aspirin, Paracetamol, were wetted with water and dried within an experimental atmospheric microwave drying system (2.45 GHz, 90 W). The drying curves obtained showed a constant drying rate, followed by two falling rate periods. Two- and three-component mixtures were dried under the same conditions. The drying characteristics were found to lie between those of the pure components. Drying kinetics of samples wetted with a range of solvents (methanol, ethanol, acetone), and two-component solvent mixtures, were examined. The relative evaporation rates of the solvents showed some dependence on their dielectric properties; latent heat of vaporisation and boiling point having greater influence on drying characteristics. Clearly this research provides information vital to achieve increased acceptance of this technology within the pharmaceutical industry.     

DRYING OF GELATINIZED WHOLE WHEAT

Whole grains of gelatinized durum and soft wheat were dried by forced and natural convection at 40, 60, 80, and 100°C. Magnetic resonance images taken periodically during drying indicated that Fick's diffusion is not applicable to describe the moisture transfer during drying of the gelatinized wheat grains. A simple mathematical model based on overall moisture balance fitted the experimental data very well. The drying took place in the falling rate period, which was approximated by two regions - first and second falling rate periods (FFRP and SFRP). The internal drying coefficient linearly increased with increasing drying temperature, and was almost an order of magnitude (from 10⁴ to 10⁵ s^-1) higher during FFRP than SFRP. The soft wheat dried faster than the durum wheat. The effect of forced convection was more pronounced during FFRP than SFRP.