Showing papers in "Drying Technology in 1995"

Air-Drying Kinetics of Osmotically Dehydrated Fruits

Abstract: The air drying kinetics of fresh and osmotically dehydrated fruits (apples) was determined. Two sugars, glucose and sucrose, were used as osmotic dehydration agents. Three levels of sugar concentration (15%, 30% and 45%) and several times of immersion into the sugar solution were used. Following the osmotic preconcentration, the fruit samples were dried at 55°C and the weight of material was recorded. The effective water diffusivity of samples treated under various osmotic conditions was estimated and the results were related to the sugar content and the bulk porosity of the samples. The effective water diffusivity, resulting from the application of the diffusion equation to the drying kinetics of the apples was found to decrease significantly for the samples pretreated by a concentrated sugar solution (e.g. 45%), evidently due to the lower porosity and other physicochemical factors. The low diffusivity may be beneficial in the storage stability and utilization of dehydrated fruits.

An Analytical Model for Moisture Diffusion in Solid Objects During Drying

Abstract: An analytical model is proposed to determine the moisture diffusivities in geometrical solid objects (namely, infinite slab, infinite cylinder, sphere) subjected to drying applications. The analysis for moisture diffusion is carried out on the basis of two imponant criteria: 0.l⁢Bi⁢100 and Bi>100. New correlations for the mots of the characteristic equations corresponding these objects are developed. For a drying application, using the similarity between cooling and drying profiles, drying coefficient and lag factor are obtained. The model is then verified using available data from literature.Results show that the presented model can be used to determine the moisture diffusion coefficients in such solid objects in a simple and accurate manner in a variety of drying applications.

Freeze Concentratton by Layer Crystallization

Abstract: Layer crystallization in laminar falling films has been studied for freeze concentration applications. A binary mixture of water and sucrose has been tested in order to compare with literature results from suspension crystallizers. The concentration of sucrose in ice, which is a measure for loss of solute. were from 0.4 to 26.5% for bulk concenwlions of sucrose of 5 to 40%. respectively. The time averaged ice growth rates varied from 3. l0-7 l0 3-10-6 m/s. The results are related to expressions for maximum ice growth rate developed from the gradient criteria. Also an expression for maximum ice growth rate from irreversible thermodvnamics is oresented. Althoueh the exwrimental results show that one–step layercrystallization has amuch lower separation effect than suspension crystallizers with washcolumns, multistep layer crystallization could provide an economical favourable freeze concentration process. A multistep process that combines freeze concentration and reverse osmosis is proposed. A case ...

What Every Engineer Should Know About Finite Element Analysis

Abstract: History and development, G.E. Barron basic finite element concepts, John R. Brauer structural analysis, Vern D. Overbye thermal analysis, Vern D. Overbye electromagnetic analysis, John R. Brauer fluid analysis, Nancy J. Lambert design optimization, Robert S. Lahey interactive model building and results processing, Jeffrey J. Ruehl FEA - a guide to the future, Bruce E MacNeal.

Drying of Solids

Abstract: Often only small amounts of product are available for process development in the fine chemical and pharmaceutical industries. In such cases, a small apparatus to test the behavior of the product during drying is of great value. The Siemens drying laboratory has developed a unique agitated vacuum contact dryer that is especially suited to the online monitoring and recording of the entire drying process. This proprietary instrument offers the following key advantages:

Freeze-Drying of Pharmaceutical Crystalline and Amorphous Solutes in Vials: Dynamic Multi-Dimensional Models of the Primary and Secondary Drying Stages and Qualitative Features of the Moving Interface

Abstract: Dynamic and spatially multi-dimensional mathematical models of the primary and secondary drying stages of the freeze-drying of pharmaceutical crystalline and amorphous solutes in vials, are constructed and presented in this work. The models account for the removal of free and bound water and could also provide the geometric shape of the moving interface and its position. It is proved that the temperature of the moving interface can not be constant if the flux of heat flow to the sides of the vial is not zero. It is also proved that the slope of the free surface (moving interface) at the edge of the vial is always curved downward. The numerical solution of the nonlinear partial differential equations of the models would allow model simulations that could indicate design conditions, operating conditions, and control strategies that could provide high drying rates and could lead to a series of novel experiments in freeze-drying.

Modelling and Simulation of Batch and Continuous Fluidized Bed Dryers

Abstract: The performance of batch and continuous fluidized solids dryers has been modeled, with allowance for diffusional moisture transport in the dense phase particles and for interstitial gas-to-particle mass transfer within the dense phase, as well as for interphase exchange resistance between gas bubbles and the dense phase. Two types of boundary conditions are employed. Variations of the bed temperature and product moisture content in the bed with time are predicted numerically under various batch drying conditions. Exit product moisture contents, bed temperatures and outlet air humidities are also predicted for continuous drying at various mean residence times. The model can be used for homogeneous as well as bubbling fluidized bed drying. It can be used for a wide range of materials, including cereal grains and granular synthetic polymeric materials.

The Thin-Layer Drying Characterstics of White Onion Slices

Abstract: A batch–type experimental dryer with a computer–aided data acquisition system was designed and built for the thin layer drying studies of onion (Allium cew L., cv. Southport White Globe) slices. Twenty seven single–layer drying curves were established for a temperature range of 42.5–900C, an air velocity range of 0.6–1.4 m/s, an air humidity range of 0.0093–0.0442 kg of water/kg of dry air: and a slice thickness range of 0.002–0.005 m. The single–term exponential model adequately described the single–layer drying behaviour of the onion slices. l'he dependence of the drying rate constant on air temperature, absolute humidity, velocity and on slice thickness was best explained by an Arrhenius–type relationship. The drying rate constant in which moisture diffusion and shrinkage effects are lumped was greatly influenced by the sample thickness and drying air temperature, and to a lesser extent, by the air humidity and velocity.

Mathematical Modeling of Drying of Liquid/Solid Slurries in Steady State One-Dimensional Flow

Abstract: A mathematical model of simultaneous mass, heat and momentum transfer for two-phase flow of a gas and a solid/liquid slurry was developed. The model was applied to calculation of the drying process of coal-water slurry droplets in a gas medium in a steady one-dimensional flow. The model was based on the well-known two-stage drying process for slurry droplets. After the first period of drying, in which the evaporation rate is controlled by the gas phase resistance, the evaporating liquid diffuses through the porous shell (crust) and then, by convection, into the gas medium. Inside the dry external crust of the drop, a wet central core forms, which shrinks as evaporation proceeds. The temperature of the slurry droplet rises. The process ends when the temperature of the dry outer crust reaches the coal ignition temperature in the case of combustion or when the moisture of the particle reaches the final required moisture. The developed model was based on one-dimensional balance equations of mass, ene...

Microwave Drymg of Grapes in a Single Mode Cavity at 2450 Mhz - i: Drying Kinetics

Abstract: Grapes were dried into raisins using combined microwave and convective drying in a single mode cavity specially designed for the studies. Effect of processing parameters, viz.microwave power density (P) air temperature (T) and velocity (u) on drying kinetics was studied in an effort to optimize the operating conditions. The individual and interactive effects of these variables were srudied using three levels of P, four levels of T and two levels of u. Higher drying rates observed with the higher P and T were atuibuted to the increased product temperature and higher partial vapour pressure difference between the surface of the fruit and the air stream. Increase of air velocity increased the drying times. Low drying rate at higher air velocity was attributed to decreased fruit temperature due to rapid convective heat mnsfer at the surface.

Cross-Flow Fluidized Bed Paddy Dryer: Prototype and Commercialization

Abstract: The objective of this paper is to design and test a prototype, 0.82 ton/h capacity, fluidzed bed paddy dryer for high moisture paddy. Exhausted air is paritially recycled. Experimental results show...

Microwave Drying of Grapes in a Single Mode Cavity at 2450 Mhz - 11: Quality and Energy Aspects

Abstract: Drying studies were conducted to optimize process parameters (air temperature T. microwave power density P and air velocity u) for quality and energy in microwave drying of grapes using a single mode cavity applicator at 2450 MHz. Thc quality was assessed by several attributes like colour. damage, darkness, crystallized sugar, stickiness and non-uniformity. Colour and damage attributes provided more preeise predietability when compared to the other quality attributes. Microwave dried raisins were lighter in colour and hence were superior to hot air dried samples. Quality attributes were sipnificantlv dependent on T. P and u. Increase in air velocitv resulted in better Quality raisins where as P and T had the opposite effect. Optimum selection of T, P and u was found to be critical to achieve an energy efficient process for a quality product. Response surface models in terms of temperature m d micowave power were obtained for each of the quality attributes. Process optimization of the operating va...

Mathematical Model for Exergy Analysis for Drying Plants

Abstract: A mathematical model is presented for exergy analysis of an industrial system for high-temperature forage drying. It allows qualitative analysis of the individual components as well as the entire system using a software package. This paper presents, as an example, the application of the model and the software, for forage drying, the changes of exergy and the basic elements and system operation quality indicators depending on the significant parameters.

Spray Drying: Retention of Volatile Compounds Revisited

Abstract: Retention or loss of trace volatile compounds during spray drying can be vital for product quality. Examples of cases where loss or retentions of volatile substances are important include retention of balanced flavor and aroma in food products, removal of odiferous substances, and control of the release of volatile organic compounds to the atmosphere. Factors and mechanisms determining losses of these substances are reviewed. Losses can occur during atomization, from undisturbed drops and as a result of morphological development. On the basis of this insight, several avenues for controlling the retention of volatile substances are identified and analyzed. These include control of atomizer pressure or rotation speed, choice of spray angle, configuration of air input, lteration of the air temperature profile, feed concentration, the presence of an oil phase and/or suspended solids, foaming of the feed, feed composition, and steam blanketing of the atomizer. The quality of a spray-dried product can ...

Test Model for Studying Sun Drying of Rough Rice Using Far-Infrared Radiation

Abstract: The effect of heat input, mixing at regular intervals, airflov over the surface and the relative humidity were investigated within a radiation chamber. Likening to sun drying as practiced in the tropics, the chamber was constructed to study the direct radiant heat drying of rough rice under various bed thicknesses. Graphic results showing the variations of rough rice surface temperature during the experiments are presented. Comparisons are also made between the relative contributions of bed depth and mixing on the drying rate. A non-mixed bed of 3 cm dried just as fast as a 6 cm mixed bed although moisture content was nonuniform in the former case. After drying, the grains were dehusked by hand and examined for physical defects like cracks. Possible constraints involved in drying thick beds of rough rice under direct radiation were investigated with the aim of defining suitable strategies for improving actual sun drying.

Freeze-Drying of the Black Currant Juice

Abstract: The effect of drying temperature of the black currant juice on the retention of vitamin C and anthocyanins content as well as drying kinetics in the freeze-dried process, has been studied. Non clarified, black currant juice was freered on plates in the layer of 15 mm thick until the temperature of -30 °C in the thermal center of sample has been reached. Plates with frozen layers of juice were subject of freeze-drying process at 5 different temperatures of the heating plate (20, 30. 40, 50 and 60 °C). Studies of the retention of vitamin C and anthocyanins after, drying were carried out. freeze-drying of black currant juice should be carried out at the temperature not higher than 40°C to preserve content of vitamin C and anthocyanins.

Combined Convective and Microwave Drying of Grapes

Abstract: (1995). Combined Convective and Microwave Drying of Grapes. Drying Technology: Vol. 13, No. 4, pp. 1029-1031.

A Method of Determination of Concentration Dependent Effective Moisture Diffusivity

Abstract: A method based on Fourier series solution to Fick's diffusion equation has been proposed to evaluate effective diffusivity (D) as a function of moisture content in agricultural materials undergoing shrinkage during drying process. The shrinkage kinetics of the particulate was used to correlate its instantaneous size (spherical equivalent diameter) as a function of material moisture content A computer program was used to evaluate D based on shrinkage kinetics and experimental drying data and relate it to moisture content. The method was used to obtain moisture diffusivity data for thin layer drying of grape and corn.

Studies on the Interaction of Arabic (Acacia senegal) and Mesquite (Prosopis juliflora) Cum as Emulsion Stabilizing Agents for Spray-Dried Encapsulated Orange Peel Oil

Abstract: The encapsulation properties of pure mesquite and arabic gum powders and mixtures thereof with orange peel oil were measured The highest rate of encapsulated oil was found to be 935 % for a ratio of 6040% of arabic to mesquite gums Pure arabic gum preparations were able to encapsulate similar amounts of the oil Sensory evaluvation of the microencapsulates showed no significant differences (a≤05) in flavor intensity beween orange peel oil encapsulated with pure arabic gum and mixtures containing mesquite gum

Control Strategies for Corn Mixed-Flow Dryers

Abstract: This paper relates work which has been conducted in order to obtain a control strategy for French mixed-flow dryers. Special emphasis is given to the system analysis ( static and dynamic). Potential difficulties for the dryer control are: non-linearities, long delay, high order, high disturbances. Special emphasis is also made on the importance of signal pre-processing. Then, different control slrategies are compared on a simulation basis : classical Pl. pole-placement and non-linear Pl controllers. The conclusion is experimentally based.

Measurements of particle residence time distributions in a co-current spray dryer

Abstract: Measurements of particle residence time distributions by means of tracer analysis were done in a co-current pilot plant spray dryer operated with a pressure–nozzle. A system is described for injecting tracer into the feedstream just before the nozzle. Tracer concentrations were measured in two product streams: the tower product stream (mean particle size 134 microns) and the cyclone product stream (mean particle size 67 microns). The measurements show a very wide range of residence times: some particles have residence times shorter than 3 seconds, others have residence times longer than 10 minutes. The median of the distribution is 58·5 s for the tower product stream and 42.2 s for the cyclone product stream.

The Application of Mathematical Models to the Commercial High-Temperature Drying of Softwood Lumber

Abstract: High temperature drying of softwood is used because it provides much faster drying rate than is possible at lower temperatures. However, the occurrence of some drying defects limits its use where the quality is critical. In order to understand the drying phenomena and to describe the drying processes, numcrous mathematical models have been developed in the past two decades. The diffusion model is the earliest attempt to describe wood drying processes and is relatively simple in form, so it is often used for stress analysis. However.further substantial work is still required before it is possible to apply the stress model to kiln control. Recently. transpon-based mathematical models have been receiving attention in modelling studies. This review discusses one of these models, a physiological-transport-based model, which has been further applied to the drying of mixed sap/heartwood boards and the drying of a kiln-wide stack. The mixed boards with a thin heanwood layer parallel to the flat surface a...

Introduction to Surface Chemistry and Catalysis

Abstract: (1995). Introduction to Surface Chemistry and Catalysis. Drying Technology: Vol. 13, No. 1-2, pp. 507-508.

Deformation Behavior of Ceramic Slabs by Nonuniform Drying

Abstract: The presenr paper is to study the transient deformation characteristics of molded ceramics undergoing nonunifom drying The linear viscoelasticity was assumed for the strain-stress analysis Three dimensional equations of the strain-stress as well as heat and moisture transfer werc solved numerically by the Galerkln finlle element melhod using isoparametric parameters of twenty-nodes so to allow easily the application to any feature of ceramics Both the analysis and experiments were carried out to be compared on the behavior of drying and deformation for a slab clay in preheating and constant drying rare periods When the bottom surface of the slab uras shielded to model the nonuniform drying a warp phenomrnon to the opposite side of the shielded surface was observed experimentally as drying by hot air heating proceeds The behavior could be successively simulaled by the present theoretical model The degree of warp became rcmarkabel with a rise in air temperature or nonuniformity of drying Th

Superheated Steam Drying of a Single Particle in an Impinging Stream Dryer

Abstract: Impinging stream contactors provide a novel configuration for drying and/or chemical reactions of particulates, pastes or suspensions which can be dispersed in a flowing gas stream. Essentially they consist of one or more highly turbulent “impingement” zones formed by collision of two opposing jets (OJ) in a confied channel or duct. The objective of this paper is to present computational fluid dynamic predictions for two-dimensional turbulent opposing jets over a range of nozzle-to-nozzle separations and jet Reynolds numbers for the simulation of single particle drying in these systems using superheated steam. A number of different turbulence models were tested ( e.g. high Reynolds, Lam-Bremhorst, Launder and Sharma models etc.). Predictions are performed in two distinct parts. In the first part a power law, finite volume method based on the “SIMPLEC” algorithm is used to solve the momentum and energy conservation equations for air in OJ systems in order to gain insight into their

Electrochemical Aspects of Electroosmotic Dewatering of Clay Suspensions

Abstract: An attempt is made to elucidate the electrochemical aspects of the electroosmotic dewatering (EOD) of clays as reported in some recent work, especially that on interrupted DC power electroosmotic dewatering published by Rabie, Mujumdar and Weber (2). These authors showed that the dewatering by EOD stops after the DC power has been on for several minutes or hours; on interruption of their power and on short-circuiting of the electrodes, conditions can be created again for some further dewatering by DC power EOD. This discovery, of Rabie et al. is interpreted as a fuel cell effect and it is shown that it affords clues to several other electrochemical strategies for the possible enhancement of the efficiency of the EOD by DC power. Further, the open circuit potentials observed by Rabie et al. (2) on the interruption of DC power are given an electrochemical interpretation which leads to quantitative estimates in agreement with the experimental values.

Flow, Heat Transfer and Particle Drying Characteristics in Confined Opposing Turbulent Jets: a Numerical Study

Abstract: Impinging stream contactors provide a novel configuration for drying and/or gas/solid chemical reactions involving particulates. pastes or suspensions which can be dispersed in a flowing gas stream. Essentially they consist of one or more highly turbulent “ impingement ” zones formed by normal collision of two confined opposing jets in a channel or duct. Little information exists in the current literature on the flow and thermal characteristics of such flows. The objective of this paper is to present computational fluid dynamic predictions for confined two-dimensional opposing turbulent jets over a range of nozzle-to-nozzle separations and jet Reynolds numbers and superheated steam drying of panicles in such contactors. The standard k-E model is used to close the governing conservation equations. Some results on the prediction of single particle trajectories and drying in such flows are presented. Predictions are performed in two distinct pans. In the first part a hybrid. finite volume method bas...

A Control-Volume procedure compared with the Finite-Element method for calculating Stress and Strain during Wood Drying

Abstract: This paper proposes a suitable method for calculating drying stresses. This is an application of the finite element method (FE) and the so-called control volume (CV) method. The latter has been chosen because most of coupled heat and mass transfer codes are based on CV. A parallel between the resulting formulation and a particular case of FE method (quadrilateral four-nodes element and Galerkin's method) has been established Both analytical expressions and numerical results were compared. Furthermore, it is shown how to choose interpolation coefficients in CV procedure in order to get exactly the results obtained with FE. Such a stress calculation has been added to the code TRANSPORE in order to obtain a complete drying code which, in addition to heat and mass transfers, solves stress and strain due to shrinkage, Interesting and promising simulations of non-symmetric convective drying are presented. Indeed, for this drying configuration, the stresses induce a global curvature of the section.

Steam Drying — History and Future

Abstract: The concept of steam drying originates from the mid of the last century. However, a broad industrial acceptance of the technique has so far not taken place. The paper deals with applications of steam drying within certain industrial sectors where the technique has been deemed to have special opportunities. The applications discussed involve drying of fuels with high moisture contents, cattle feed exemplified by sugar beet pulp, lumber, paper pulp, paper and sludges. Steam drying is compared to flue gas drying of biofuels prior to combustion in a boiler. With reference to a current installation in Sweden, the energy losscs, as manifested by loss of co-generation capacity, are discussed. The energy saving potential when using steam drying of sugar beet pulp as compared to other possible plant configurations is demonstrated. Mechanical vapour recompression applied to steam drying is analysed with reference to reported data from industrial plants. Finally, environmental advantages when using steam dr...