Laboratory-scale Dryer Research Articles

Evaluation of Thin-Layer Models for Mushroom (Agaricus bisporus) Drying

In the present research, seven well-known mathematical thin-layer drying models were fitted to mushroom (Agaricus bisporus) drying experimental data, implementing nonlinear regression analysis techniques. The experiments were conducted in two laboratory-scale dryers. A range of temperatures 50–65°C and air velocities 1.0–5.0 m/s were tested. The statistical analysis concluded that the best model in terms of fitting performance was the logarithmic model. Correlations expressing this model parameter dependence with the drying air coefficients are also reported.

Identification of the Limiting Mechanism in Contact Drying of Agitated Sewage Sludge

In spite of a great number of industrial applications, the thermal design of contact dryers for sewage sludge remains empirical. To improve the understanding of drying mechanisms, the penetration theory developed by Schlünder and coworkers for mono- and multidispersed packing is used to represent the experimental results from a laboratory-scale dryer. For granular packing, the only adjustment parameter of the model is the mixing number, which characterizes the dryer and its stirrer. For pasty-like materials, the pasty phase is assumed to be a saturated particulate phase. As the calculation of the effective properties calculation is cumbersome for a multi-granular packing, the particulate phase is considered as a monodispersed packing, whose dimension is unknown. To identify the two adjustment parameters, the mixing number was quantified from experiments performed on activated alumina balls, for which physical and thermal characteristics are known, and then the characteristic dimension of the sludge was determined by adjustment of experimental drying kinetics measured in a batch agitated dryer. According to this model, drying is exclusively controlled by the contact resistance between the wall and the biggest particles contained in the dewatered sludge. The model permits to find most of the tendencies experimentally observed for different operating conditions.

Studies on thin layer drying of bagasse

Thin layer drying experiments of bagasse were carried out to determine the drying kinetics and also to identify a suitable drying model. The drying experiments were conducted in a laboratory scale dryer for a wide range of air temperatures (80–120°C), velocities (0.5–2 m/s), humidity level of air (9–24 g water(kg of d.a)−1) and product thickness (20–60 mm). Air temperature, velocity, humidity and mass of product were recorded continuously during experimentation. The data were fitted to the different semi-theoretical or empirical models and compared based on their correlation coefficient (r), chi-square (χ2), root mean square error and mean bias error values. Among the models considered, the Page model gave accurate predictions with a correlation coefficient of 0.99627. It is found that the drying of bagasse takes place only in the falling rate period, even though the product has high initial moisture content. Copyright © 2006 John Wiley & Sons, Ltd.

Thin-layer drying behaviour of sludge of olive oil extraction

Drying characteristics of sludge were examined for average moisture content from 2 to approximately 0.7–0.08 kg water/kg dry matter using hot air of the range of 20–40–80 °C and simulating the interval that can be provided by an air solar heater and by air velocities of 1 m/s at a laboratory scale dryer. Different mathematical models were used for the simulation of the sludge drying curves, such as Lewis, Page, Modified Page, Henderson and Pabis, Wang and Singh, Logarithmic, Two term, Two Term exponential, Modified Henderson and Pabis, Midilli, Approximation of diffusion, Verma et al. and Simplified Fick’s diffusion models. The performance of these models was investigated by comparing four statistical parameters: the square of the coefficient of determination ( r 2), reduced chi-square ( χ 2), root mean square error (RMSE) and sum of residuals between the observed and predicted moisture ratio. The Midilli model represented the drying characteristics better than the others. The effective diffusivity coefficient of moisture transfer varied from 2.224 × 10 −10 to 6.993 × 10 −10 m 2/s over the temperature range. The temperature dependence of the effective diffusivity coefficient was expressed by Arrhenius type relationship. The activation energy for the moisture diffusion was found to be 15.77 kJ/mol.

Thin-layer drying of porous materials: Selection of the appropriate mathematical model and relationships between thin-layer models parameters

Experimental investigation of the drying kinetics of various types of materials was carried out in laboratory-scale dryers under different conditions of temperature, microwave heating power and pressure. Leather samples (mechanically and vacuum-dewatered bull napa and wet blue cutting), paperboards (grafopack, testliner), wood (alder, birch, willow) and two pharmaceutical powders (chlorpropamide and hydrochlorotiazide) were dried in a microwave dryer. Thin clay slabs, Al–Ni catalyst and chlorpropamide were dried in a convection dryer, while chlorpropamide and ketoprofen were dried in a vacuum dryer. In order to compare drying kinetics, experimentally obtained data, X = f( t), were correlated with the Lewis “thin-layer” equation, the modified Page equation and Fick's second law. The drying constant, effective diffusion coefficient, mass transfer coefficient and modified Page model parameters were estimated by fitting the selected mathematical models to experimental data. High levels of correlation between measured and calculated data were obtained for all materials and dryers using modified Page model. The application of the Lewis and Fick's equation is justified only for drying of clay, catalyst and leather. Mass transfer coefficient depends linearly on the drying constant. The relation between the modified Page model parameter and the drying constant can be represented by a unique power function.

Mathematical modeling on hot air drying of thin layer apple pomace

The hot air convective drying characteristics of thin layer apple pomace were evaluated in a laboratory scale dryer. The drying experiments were carried out at 75, 85, 95 and 105 °C and at the air velocity of 1.20 ± 0.03 m/s. Different mathematical models were tested with the drying behavior of apple pomace in the dryer. The results indicated that the Logarithmic model can present better predictions for the moisture transfer than others; the drying process took place in two falling rate periods, the effective diffusivities in the second period were about six times greater than that in the first period; the general relationship of moisture ratio against drying duration in the thin-layer convective drying of apple pomace were found and can be used in design and operation of the convective drying, suggested by 0.9923 of square correlation coefficient and 0.00001 of chi square.

Final Drying of Whole Milk Powder in Vibrated-Fluidized Beds

Vibro-fluidized beds are widely used in drying sticky powders and agglomerated materials as milk powder. Using a vibro-fluidized laboratory scale dryer, this work is aiming at analyzing preliminarily the effect of its operational variables on the drying kinetics and characteristics of whole milk powder. The full-factorial design technique with three replications at the central point has been employed to generate data and correlations to quantify the effect of inlet air temperature, air flow rate, and vibration bed amplitude on the drying curves and the milk powder properties related to its faster reconstitution in water (as tapped density, Hausner cohesion ratio, agglomerate size distribution, and internal pore concentration). Results obtained are analyzed and discussed to identify the adequate operation condition for final drying whole milk powder in vibro-fluidized beds, assuring a high product quality.

The Gas-to-Particle Heat Transfer and Hydrodynamics in Spouted Bed Drying of Sawdust

This article presents experimental results for spouted bed drying of sawdust, carried out in a full-scale as well as in a laboratory-scale dryer using air as well as steam as drying media. The aim is to present design parameters for a spouted-bed sawdust dryer that can be used by the industry in designing full-scale dryers. A hydrodynamically stable spouted jet spouted bed was obtained. The heat transfer characteristics of the bed were represented in terms of a volumetric heat transfer coefficient (VHC). When sawdust is dried in a spouted bed, the mean VHC is increasing up to fiber saturation level (20–25% wb) from 40 to 110 W/m3 K. The VHC decreases with the residence time and with an increased static bed height. Gas temperature profiles are also presented for the bottom part of the drying chamber.

Mathematical modelling of drying of bay leaves

This paper presents a study on the mathematical modelling of bay (Laurus nobilis L.) leaves. Drying experiments were performed in a laboratory scale dryer constructed in the Department of Agricultural Machinery, Faculty of Agriculture, Ege University, Izmir, Turkey. The leaves were 90–100 mm long and 30–40 mm wide, and ones with no blemish were selected and used for the drying tests. The tests were performed with various relative humidities (5%, 15% and 25%) and temperatures (40, 50 and 60 °C) at a constant air velocity of 1.5 m/s. Fifteen different mathematical drying models were compared based on their correlation coefficient, root mean square error, mean bias error, reduced chi-square and t-statistic method to estimate the drying curves. It may be concluded that the Page model could sufficiently describe drying of bay leaves.

Modelling of olive cake thin-layer drying process

Olive cake is a sub-product of the mechanical olive oil extraction industry, which consists of pit and pulp of the olive fruit, olive oil and vegetable water. It has been using for direct combustion in bakeries and olive oil mills due to its energy content. However, the initial moisture content of olive cake is approximately 44.78% ± 0.5 (wet basis), and this has to be reduced to about 5% to recover its energy content. Therefore, the characterization of the thin layer drying kinetics was investigated at a wide range of drying temperatures (50–110 °C), a constant sample thickness and air velocity of 1.2 ± 0.03 m/s at a laboratory scale dryer. Various mathematical models simulated drying curves of olive cake. The logarithmic model was found to give better predictions than the others. In addition, the temperature dependence of the effective diffusivity coefficient was expressed by an Arrhenius type relationship. The effective diffusivity varied between 0.3 and 1.1 × 10 −8 m 2/s with an activation energy of 17.97 kJ/mol.

Experimental Study on Microwave Vacuum Drying of Two-Porosity Level Media

Microwave vacuum drying experiments were performed on a laboratory scale dryer with a two-level porosity material: a packed bed of porous alumina beads. The incident microwave power and the vacuum pressure level were fixed, the main varying parameters being the beads diameter and porosity, and the mean pore diameter. The drying kinetics and the evolution of the product temperature are presented. The drying kinetics can be divided into two main periods. The first one corresponds to the drying from the bed voids according to evaporation mechanism that we describe with a stagnant film law. The second one corresponds to the drying from the particle pores and we divide it into two parts: we suggest that the former is dominated by capillarity driven moisture transport, and the latter is limited by the desorption kinetics of the few water layers left.

Microwave vacuum drying of porous media: experimental study and qualitative considerations of internal transfers

This paper deals with experimental results obtained on a laboratory scale dryer. Two porous media were dried, a packing of initially water saturated glass beads and a packing of initially unsaturated pharmaceutical granules. For the first one, the main parameter is permeability of the bed varying the beads diameter and for the second one it is the porosity of the bed varying the amount of product. These experiments were carried out at fixed incident power and pressure level. In the first period of the drying, it was observed that permeability was a limiting parameter to internal mass transfers when free liquid water is expulsed, revealing high gas pressure gradients inside the product. They occur when we have a massive vaporisation with ebullition. As for the porosity of the granules bed, it has no effect on mass transfers. In the second period, we have no influence of both parameters on the drying kinetics. In the last period, so in the hygroscopic field or below the irreducible saturation, the internal mass transfers are governed by the electromagnetic energy absorption.

MICROWAVE VACUUM DRYING OF POROUS MEDIA: VERIFICATION OF A SEMI-EMPIRICAL FORMULATION OF THE TOTAL ABSORBED POWER

Vacuum drying experiments were performed on a laboratory scale dryer with two porous materials: a packing of initially saturated glass beads and of unsaturated pharmaceutical granules. Several incident powers and two vacuum pressure levels were tested in order to demonstrate different drying mechanisms. The drying kinetics, temperature of the product and the absorbed power are presented and the coupling between the drying rate and the absorbed power is shown. A semi-empirical formulation of the total absorbed power is proposed taking into account the water content, the temperature and the dielectric properties of each phase. This formulation is based on a quasi-static assumption which allows the local electric field inside the material to be expressed with an analytical equation.

EFFECT OF DRYING AIR TEMPERATURE AND GRAIN TEMPERATURE OF DIFFERENT TYPES OF DRYER AND OPERATION ON RICE QUALITY

Quality of aromatic rice (KDML-105) and non-aromatic rice (Suphanburi I) was studied using different methods and scales of drying. At the lab scale, rough rice at high moisture (21±1% w.b.) was subjected to shade drying, sun drying, oven drying at 45 °C and 60 °C and fluidized bed drying (for different time periods) at 120 °C, with tempering for 2hrs. On a commercial scale, single stage drying (sun drying, column drying) and multi stage drying in a fluidized bed dryer coupled with column and shade drying were investigated. Head rice yield, color, hardness, stickiness and hardness ratio, water absorption, and structure of milled rice were examined for each method of drying. The results revealed that water absorption and ratio of stickiness and hardness decreased but the b-value and hardness increased with increase in dry grain temperature, longer exposure/drying time, longer tempering time, and rapid rate of water removal. Head rice yield of samples dried at high temperature (>60 °C) followed by tempering and of those dried at low temperature (<45 °C) in case of lab scale drying was increased and no significant difference (p<0.05) was seen relative control samples. For single stage drying, at low temperature, and multi stage drying in the fluidized bed drier (interment tempering), at high temperature, head rice yield was decreased and exhibited significant difference (p<0.05) relative to control sample. The yield however increased for a sample subjected to a 12hr tempering period and then shade drying stage subsequent to drying in a fluidized bed drier. Drying of rough rice at low temperature (<45 °C) did not affect cooking and eating qualities of rice which were affected by drying at high temperature (>60 °C). Cooking and eating qualities however changed at both single and multi stage drying on a commercial scale.

VACUUM DRYING OF DISPERSED MATERIALS FROM MULTICOMPONENT LIQUID SYSTEMS: VERIFICATION OF MODEL

ABSTRACT Experimental data on vacuum drying of chemical and microbial synthesis products under laboratory and industrial conditions are presented. Kinetic relations of vacuum drying of dispersed materials with the removal of a multicomponent liquid system are verified on both laboratory and industrial scale dryers under different operating conditions. The results of the research were generalised and a number of empirical relations needed for drying process calculation are given. The relation between heat transfer coefficient and heat flux and motive force for the vacuum drying process is established.

LUMPED‐PARAMETER APPROACH FOR PREDICTION OF DRYING KINETICS IN FOODS

ABSTRACTDrying kinetics of the product are the most important data required for the design and simulation of air dryers. This essential information can be obtained through simple lab‐scale experiments. Nevertheless, due to the uncertainty of the process, specially when dealing with biological materials, all the necessary data at different operating conditions are not usually available. On the other hand, drying kinetic models are either too simple to expect reasonably good predictions, or too complicated to be applied easily.The key assumptions of a previously developed lumped‐parameter model for air drying kinetics were revised to examine their application to biological materials. A short cut method has been developed to simulate kinetic curves from a minimum number of experimental data. A simple parameter study was carried out to detect the most important variables that affect the mass transfer Biot number in order to predict kinetics data safely. Experimental (hot air) drying curves for bananas, grapes and mushrooms were obtained in a lab scale dryer, at different air temperatures (50 to 70C) and air velocities (1 to 3 m/s). A comparison between experimental and predicted data was done for different operating conditions.

Two-Dimensional Simulation Model of Steady-state Mixed-flow Grain Drying. Part 2: Experimental Validation

Abstract A model of a mixed-flow grain dryer at steady state, described in a previous paper, was used to simulate a laboratory-scale dryer in which all input conditions and the grain moisture, temperature and viability loss had been recorded at the discharge and, for most runs, at points in the grain bed. The model was used to calculate grain flow rate for the observed moisture reduction, and the air and grain conditions throughout the grain bed for 11 drying experiments. A unique feature of the model allows it to calculate two drying regimes, one for the grain which passes near the inlet ducts, the other for the grain flowing near the exhaust ducts. Although grain input was overestimated in all cases by the model, by a mean of 10·9%, the loss of viability was predicted with an absolute average error of only 3·1% points of germination. The model underpredicted the viability loss where it was small, but predicted the loss at 125°C very accurately. The moisture difference between the two drying regimes was predicted well in one run and underpredicted in another, and the magnitude of the observed moisture difference between inlet side and outlet side, 0·08 dry basis, showed the importance of the phenomenon in mixed-flow dryers. Predicted air exhaust temperatures were higher than measured, but balances for energy and mass performed on the dryer suggest that heat losses were partly responsible. The model predicted that cross-flow elements of the bed resulted in rapid heating of the grain, showing that this aspect of mixed-flow dryers is important in achieving accurate calculation of viability loss. A simpler model of mixed-flow drying predicted the grain input within 6·1% but because it did not represent the intense heating near inlet ducts, its predictions of viability loss were very poor.

Evaluation of Textile Bobbins Drying Processes: Experimental and Modelling Studies

ABSTRACT Drying of textile bobbins was carried out on two kinds of laboratory-scale dryers: in a cross circulation drying rig, atmospheric pressure heating air is used, while in the through circulation drying apparatus, the heating air is pressurized before the drying process takes place. In both cases, a bobbin was fitted with seven thermocouples, placed in a plane normal to the bobbin axis but at different radial distances, in order to provide temperature data during the drying process. In cross circulation drying, plots of temperature against time indicated the presence of two moving fronts, one starting from inside the bobbin, another from the outside, while in through circulation drying, similar plots showed the presence of an evaporation front, moving from inside the bobbin to the outside. In both canes, numerical models were developed to simulate the drying processes.

High-intensity infrared drying study: Part I. case of capillary-porous material

Experiment involving the drying of a thin horizontal unconsolidated wet sand layer under intensive infrared radiation (IR) have been performed in a laboratory-scale dryer. The drying rates and solid temperature data were much higher than in conventional convective processes. Coupled heat and mass-transfer phenomena have also been analyzed. It was observed that medium-infrared radiation (medium-IR) was more efficient than near-infrared radiation (near-IR) and that the analogy between heat and mass transfer is still valid in the case of high mass fluxes. A simple receding model based on the assumption that IR radiation is absorbed at the surface of the product could predict the drying rates and the temperature profiles up to the zero moisture content. The experimentally determined parameters of the main model were the mass transfer and infrared absorption coefficients.

The effect of temperature and particle size on the fluid bed drying of Northern Ireland lignite

This paper investigates fluid-bed drying of lignite, in a batch-wise laboratory scale dryer. Based on experimental data, the rate controlling step was partly characterized by determining the observed activation energy for the process. The effects of temperature and particle size on drying rate were studied to obtain information relating to optimum operating conditions. The boundary layer resistance was also shown to affect the rate of evaporation of moisture from the particulate solid.