SIMULATION MODELS OF MULTI-CYLINDER PAPER DRYING

ABSTRACT

The literature on simulation models for multi-cylinder paper dryers is surveyed, 20 such models being reviewed. Models for intraweb heat and mass transfer are treated extensively, a separate chapter being devoted to them. In some of the models the specific physical transfer phenomena are modelled in detail, whereas in other models a more macroscopic approach is adopted. Normally the latter approach requires the incorporation of a large number of empirical constants into the model to describe the complicated heat and mass transfer processes involved and to obtain a reasonable agreement between the model and the measured data. In order to develop the simulation models further without use of an inordinately large number of empirical constants, the different physical phenomena which occur in the dryer must be modelled in greater detail, bnsed both on fundnmental theory and on empirical correlations.     

DESORPTION ISOTHERM MODELLING OF BLACK TEA USING ARTIFICIAL NEURAL NETWORKS

Neural network approach was selected for the modelling of desorption isotherms of black tea and a neural network model trained using Levenberg-Marquardt algorithm. The trained network was used for studying the influence of randomised training data of temperature in the range of 25-80°C and the water activity range of 0.10-0.90 and different number of hidden neurons. The obtained results, compared with the results already obtained with the well known phenomenological GAB model, already tested by the authors [9], have proven to be better.     

MODELLING OF DESORPTION ISOTHERMS OF CHESTNUT: INFLUENCE OF TEMPERATURE AND EVALUATION OF ISOSTERIC HEATS

The desorption isotherms of chestnuts (Castanea sativa Mill) were determined at 278, 293, 303, 313, and 323K by a gravimetric technique. The range of water activity was from 0.08 at 0.91. The sorption isotherms were of the type II according to Brunauer's classification. The effect of the temperature shows a cross over of the desorption isotherms at water activity 0.6. The experimental data are fitted with several models (Oswin, Smith, Henderson, Halsey, and G.A.B.). Oswin and G.A.B. models gave the best fittings. Particularly, with G.A.B. model parameters some interesting information as monolayer moisture content, heat of sorption of the first and nth layer adsorbed and net isosteric heat were obtained.     

DESORPTION ISOTHERMS AND DRYING CHARACTERISTICS OF SHRIMP IN SUPERHEATED STEAM AND HOT AIR

Desorption isotherms for shrimp were determined at the temperatures of 50, 60, 70 and 80°C. Amongst the moisture equilibrium predictions between the BET and GAB models, the latter has a better predictable capability. The GAB parameters are correlated with the temperatures by the Arrhenius expression. Drying characteristics of shrimp in drying media at the temperature range of 120-180°C for superheated steam and of 70-140°C for hot air have been examined. Drying rate and effective diffusion coefficient are used to quantify quantitatively the difference between the superheated steam and the hot air dryings. The temperature is more important effect on drying rate and effective diffusion coefficient in the superheated steam than in the hot air. Inversion temperature exists between 140 and 150°C. Comparing to the hot air, the shrimp dried by the superheated steam shows a lower degree of shrimp shrinkage. In addition, product colours are slightly different to those from the commercial sources.     

DESORPTION ISOTHERMS FOR JAPANESE NOODLE (UDON)

Desorption isotherms of Japanese noodle (udon) were obtained at 20, 30 and 40°C, and analyzed by several isotherm models. The desorption isotherm data satisfactorily correlated with the GAB equation and the Oswin and Smith equations. The binding energy of moisture was calculated by Clausius-Clapeyron equation and found to be negligibly small above the moisture content of 0.163 kg water/kg solid. The sorption enthalpy calculated from the Arrhenius plot of GAB parameter was similar to the binding energy from Clausius-Clapeyron equation at GAB monolayer moisture content.     

OPTIMIZING CONDITIONS FOR HEAT PUMP DEHYDRATION OF LACTIC ACID BACTERIA

ABSTRACT

Heat pump dryers were studied as an alternative to freeze drying for dehydration of lactic acid bacteria for starter cultures. Factors investigated were bacterial growth phase, addition of dehydro-protectants, drying conditions, drying media and storage life of the dried bacteria. The main criterion for evaluation was the activity of the dried product. Results indicated that single dehydro-protectants were not able to give high activities, but suitable combinations were found giving results at least equal to freeze-drying. Nitrogen purging was found to give a significant improvement in activity.     

OPTIMIZING CONDITIONS FOR HEAT PUMP DEHYDRATION OF LACTIC ACID BACTERIA

Heat pump dryers were studied as an alternative to freeze drying for dehydration of lactic acid bacteria for starter cultures. Factors investigated were bacterial growth phase, addition of dehydro-protectants, drying conditions, drying media and storage life of the dried bacteria. The main criterion for evaluation was the activity of the dried product. Results indicated that single dehydro-protectants were not able to give high activities, but suitable combinations were found giving results at least equal to freeze-drying. Nitrogen purging was found to give a significant improvement in activity.     

DESORPTION ISOTHERMS FOR JAPANESE NOODLE (UDON)

Desorption isotherms of Japanese noodle (udon) were obtained at 20, 30 and 40°C, and analyzed by several isotherm models. The desorption isotherm data satisfactorily correlated with the GAB equation and the Oswin and Smith equations. The binding energy of moisture was calculated by Clausius–Clapeyron equation and found to be negligibly small above the moisture content of 0.163 kg water/kg solid. The sorption enthalpy calculated from the Arrhenius plot of GAB parameter was similar to the binding energy from Clausius–Clapeyron equation at GAB monolayer moisture content.     

A CALCULATION METHOD OF OBTAINING DESORPTION ISOTHERMS OF BEECH (FAGUSMOESL4CA) WOOD

ABSTRACT

This paper presents the specific methods to obtain sorption isotherms. Sorption isotherms were first obtained experimentally for 20°C and 100°C and then, by using the presented method, isotherms were also obtained for 50°C and 80°C. The same treatment can be applied in finding any isotherm sorption in the temperature range between 20°Cand 100°C.

The experiments were carried out on beech (Fagus moesiaca) and sample dimensions were 3×3×3cm. Based on the results of the experiment, by regression analysis, the most suitable analytic formula was proposed. It relsites temperature and air humidity with the equilibrium moisture content in wood. Experimental results were compared with psychrometric tables and Bramhall's formula for desorption curves with good agreement. This proves that the method presented in this paper is precise and efficient to decrease the lime needed to obtain sorption isotherms experimentally.     

APPLICATION OF PELEG'S EQUATION IN DESORPTION STUDIES OF FOOD SYSTEMS: A CASE STUDY WITH SAGO METROXYLON SAGU ROTTB STARCH

ABSTRACT

Moisture desorption is pertinent to the modelling of dehydration of food systems. The variation of moisture content with time during desorption of sago starch from six towns in Papua New Guinea was analysed using Peleg's equation. Three temperatures (30° 407deg; and 45° C) and four aw ranging from 0.103 - 0.923 were studied. The equation gave a highly significant fit to the experimental data ( r2 - 0.998 - 1.000), the two constants (Ki and K2) were obtained and both constants varied significantly (p< 0.05) with the source of the starch, temperature and aw. Kz was used to calculate the (true) equilibrium moisture content and this was unaffected by the length of the curve used in the computation. The true equilibrium moisture content approximated the moisture content that would have been obtained when weight changes were fairly constant (pseudo-equilibrium moisture content) The true equilibrium moisture content was shown to be more reliable and consequently recommended for sorption studies.     

DESORPTION ISOTHERMS OF WATER IN TRIFOLIATE YAM )Dioscorea dumetorum Pax.(

Abstract

Desorption isotherms of water in trifoliate yam (Dioscorea dumetorum Pax) were determined using a dynamic gravimetric method at temperatures of 20°, 34°, 46° and 62°C, in the relative humidity range from 11 to 86% A nonlinear least squares regression program was used to fit the measured data to the Modified Henderson, the Modified Halsey, the Chung-Pfost, and the GAB sorption models. The GAB model gave the best fit. The net isosteric heat of sorption, estimated using the Clasius-Clapeyron equation, ranged from 1412 kJ/kg at a moisture content of 0.05 kg/kg, dry basis, to 325 kJ/kg at 0.20 kg/kg. Results obtained in this study should be important in controlling the water content of yam during processing, handling, packaging and storage.     

SELECTION OF EMC/ERH ISOTHERM EQUATIONS FOR SHELLED CORN BASED ON FITTING TO AVAILABLE DATA

Abstract

Five hundred and ninety-one experimental EMC/ERH data points for shelled corn are extracted from twenty publications identified in literature. These data are grouped into adsorption, desorption, average, yellow dent, dent and yellow flint data sets according to varieties and sorption processes, Thirteen out of the twenty publications contain data over a wide range of temperatures, thus producing nineteen data sub-sets suitable for comparing EMC/ERH isotherm equations. Three commonly cited shelled com EMC/ERH equations (Modified-Henderson, Modified-Chung-Pfost and Modified-Oswin equations) are evaluated for their ability to fit these nineteen data sub-sets. The evaluation, based on the residual sum of squares (RSS), standard error of estimate (SEE) and mean relative deviation (MRD), shows that the Modified-Oswin and Modified-Chung-Pfost equations have a better ability to describe the EMC/ERH isotherms of shelled com, with the Modified-Oswin equation slightly better than the Modified-Chung-Pfost equation. Both equations are then used to fit the adsorption, desorption, average, yellow dent, dent and yellow flint data sets and the best fitted coefficients are given. These fitted equations are based on a large data set and therefore provide a sound basis for future work on drying and storage of shelled com.     

MODELING VACUUM-CONTACT DRYING OF WOOD: THE WATER POTENTIAL APPROACH

ABSTRACT

A two-dimensional mathematical model for vacuum-contact drying of wood is presented. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady state conservation equation of dry air. Most of the model parameters were determined during independent experiments. The set of equations is then solved in a coupled form using the finite element method. The validation of the model is performed using experimental results obtained during vacuum-contact drying of sugar maple sapwood. The experimental and calculated data are in good agreement. Nevertheless, some discrepancies are observed which can be attributed to the boundary conditions used and to the fact that heat transfer by convection was neglected.     

DRYING CHARACTERISTICS OF THE HAZELNUT

Abstract

Equilibrium moisture content isotherms for Spanish hazelnut (Corylus avellana L.) at different temperatures (30°C-80°C) were determined using static gravimetric method. Thin layer drying experiments were done with forced air circulation and were conducted with different operating conditions to determine the drying characteristics of hazelnuts. The effect of air temperature (30°C-70°C), air velocity (0.5 m/s - 2 m/s) and drying bed loading density (50 kg/m² - 150 kg/m²) on drying of unshelled and shelled hazelnuts was studied. Six mathematical models were used to fit the experimental equilibrium moisture content data, from which the G.A.B. model was found to give the best fit. Diffusion coefficients were determined by fitting experimental thin-layer drying curves to the Fick's diffusion model. Variation of the effective diffusion coefficient with temperature was of the Arrhenius type. The Page equation was found to describe adequately the thin layer drying of hazelnut. Page equation drying parameters k and n were correlated with air temperature and relative humidity.     

Modeling the Rehydration Process of Dried Tomato

Rehydration of air-dried tomatoes was investigated at four temperatures (25, 40, 60, and 80°C). To describe the rehydration kinetics, two empirical models, Peleg and Weibull, were considered. The empirical models described the rehydration process properly. The equilibrium moisture content decreased as rehydration temperature increased, whereas the kinetic constants of the Peleg and Weibull models, k ₁ and β, respectively, increased with increasing temperature. This influence was described in terms of an Arrhenius relationship. The Peleg constant k ₂ was also found to be a function of temperature, increasing as temperature increased, whereas the Weibull shape parameter α decreased with increasing temperature. In addition, a mathematical model based on one-dimensional, steady-state, fully developed capillary flow of water with negligible inertia effects within the air-dried tomato was developed. This model was found to adequately describe the rehydration behavior of dried tomatoes.     

Effects of Surface Concentration on Drying Behavior of Carbohydrate Solutions

As the surface properties of the drying materials are very important not only for the drying rate but also for the quality change during drying, the effects of surface concentration on the drying behavior of liquid foods (sugar solutions) were investigated by isothermal drying experiments and by numerical calculation experiments. The isothermal drying experiments with gelled sugar solution systems (sucrose and maltodextrin) were carried out at various relative humidity (RH) values (RH = 0 to 84%). Separate experiments were carried out for determination of the desorption isotherms.

The isothermal drying curves of sugar solutions at RH = 0 to 51% were very similar. Numerical simulations also showed that the drying curves of these sugars at the surface concentration = 0 and 0.1 are almost the same, although the concentration distributions are different.

When a small amount of gelatin was added to sugar solutions, the drying rate decreased remarkably as the gelatin might form a thin film (skin) near the surface, and consequently the retention of ethanol increased.     

Effects of Surface Concentration on Drying Behavior of Carbohydrate Solutions

ABSTRACT

As the surface properties of the drying materials are very important not only for the drying rate but also for the quality change during drying, the effects of surface concentration on the drying behavior of liquid foods (sugar solutions) were investigated by isothermal drying experiments and by numerical calculation experiments. The isothermal drying experiments with gelled sugar solution systems (sucrose and maltodextrin) were carried out at various relative humidity (RH) values (RH = 0 to 84%). Separate experiments were carried out for determination of the desorption isotherms.

The isothermal drying curves of sugar solutions at RH = 0 to 51% were very similar. Numerical simulations also showed that the drying curves of these sugars at the surface concentration = 0 and 0.1 are almost the same, although the concentration distributions are different.

When a small amount of gelatin was added to sugar solutions, the drying rate decreased remarkably as the gelatin might form a thin film (skin) near the surface, and consequently the retention of ethanol increased.     

Drying of sardine muscles: Experimental and mathematical investigations

The aim of this work was to study the effect of air drying process on the dehydration kinetics of sardine muscles (Sardina pilchardus). Experimental drying kinetics were measured at five air temperatures (40, 50, 60, 70 and 80 °C), two relative humidity and at a constant air velocity of 1.5 m/s. The sardine drying kinetics were accelerated by increasing air temperature and were showed down when increasing air humidity. Moisture desorption isotherms of sardine muscles were determined at three temperatures (40, 50 and 70 °C) by using the static gravimetric method. The equilibrium moisture contents of sardine muscles were used to treat mathematically the experimental drying kinetics. Experimental drying kinetics and desorption isotherms of sardine muscles were described by using empiric models available in the literature. Eight models (GAB, BET, Henderson–Thompson, Modified Chung & Pfost, Modified Halsey, Oswin, Peleg and Adam & Shove models) were compared in order to describe the desorption isotherms. The Peleg model showed the best fitting of experimental data. For the drying kinetics, the Page model allowed a better fitting than the Newton and the Henderson and Pabis models. The Page model was thus used for simulating the drying kinetics of sardine muscles between 40 and 80 °C.     

PREDICTING THE SORPTION BEHAVIOUR OF GARLIC SLICES

ABSTRACT

Moisture desorption properties of fresh garlic (cv. Early Californian) were investigated at 20 °C to 70°C and relative humidities ranging from 11% to 85% using the gravimetric static method. The slices were allowed to equilibrate in a constant humidity environment (hygrostats) maintained by salt solutions. The effect of temperature and relative humidity on the equilibrium moisture content was highly significant (P<0.0001). Four models available in the literature, namely the Chung-Pfost, modified Halsey, modified Henderson and the Guggenheim-Anderson-de Boer (GAB) were evaluated (the correlation coefficient, F-ratio and plots of residuals) to determine the best fit for the experimental data. On the basis of the F-test, The modified Henderson model was found adequate and the GAB model superior for characterising the sorption behaviour of garlic slices in the temperature and water activity range investigated.