Laboratory Dryer Research Articles

Effect of dipping treatment on air drying of plums

In this present study, samples of plums were pretreated in alkali solution containing ethyl oleate and dried in a laboratory dryer at 65 °C and 1.2 m/s air velocity. The effect of ethyl oleate solution on drying time of plum samples was investigated. Pretreatment of plums with ethyl oleate increased the rates of convective drying. Drying time of pretreated samples was 29.4% shorter than that of untreated samples. Diffusion coefficients of treated and untreated samples were estimated to lie between 2.171 × 10 −10 and 2.401 × 10 −10 m 2/s. Six semi-theoretical thin layer models were compared according to their coefficient of determination ( R 2), sum square error (SSE) and root mean square error (RMSE) to estimate air-drying curves. Comparing the R 2, SSE and RMSE values of all models, it was concluded that the two-term exponential model for untreated, Wang and Singh model for pretreated plums represents drying characteristics better than the other models.

Drying of eggplant and selection of a suitable thin layer drying model

The thin layer drying behaviour of eggplants in a laboratory dryer was examined. Drying characteristics of eggplants were determined using heated ambient air at temperatures from 30 to 70 °C and velocities from 0.5 to 2.0 m/s. Beside the effects of drying air temperature and velocity, the effects of pre-treatment and slice thickness on the drying characteristics, drying time and quality of dried product were also determined. Drying curves obtained from the experimental data were then fitted to the different semi-theoretical and/or empirical thin layer drying models. The effects of drying air temperature and velocity on the model constants and coefficients were evaluated by a multiple regression technique. All the models were compared according to three statistical parameter; i.e. root mean square error, chi-square and modeling efficiency. The results have shown that, increasing the drying air temperature and velocity causes shorter drying times. In an eggplant drying process, pre-treatment must definitely be applied to decrease drying time. A thinner slice also causes a shorter drying time. While the increasing drying air temperature decreased the colour lightness, increasing drying air velocity raised it. Slice thickness did not have any significant effect on the colour characteristics. Increasing drying air velocity raised the re-hydration ratio, and the highest value was obtained at a drying air temperature of 50 °C. The Midilli et al. model was found to be the best model for describing the drying curves of eggplants. The effects of drying air temperature and velocity on the drying constant and coefficient were also shown.

Dehydration kinetics of red pepper (Capsicum annuum L var Jaranda)

AbstractShredded and whole red pepper samples were dehydrated in a laboratory drier with a through‐flow air velocity of 0.5 m s−1 at 50, 55, 60 and 70 °C. Shredded peppers dried faster than whole peppers. The drying behaviour of whole samples was characterised by a constant‐ and a falling‐rate drying period, whilst that of shredded samples was characterised by a falling‐rate drying period only. The mass transfer coefficient for whole samples during the constant‐rate period was computed experimentally. The effect of temperature on the mass transfer coefficient was described by the Arrhenius model. The activation energy was 58 kJ mol−1. In the falling‐rate period the mass transfer was described by a diffusional model, and the effective diffusion coefficient at each temperature was determined. Diffusion coefficients were estimated to lie between 4.38 × 10−11 and 10.99 × 10−11 m2 s−1 for whole peppers and between 37.23 × 10−11 and 99.61 × 10−11 m2 s−1 for shredded peppers. The effect of temperature on the effective diffusion coefficient was described by the Arrhenius equation, with an activation energy of 44 kJ mol−1 for whole peppers and 56 kJ mol−1 for shredded peppers.© 2003 Society of Chemical Industry

Influence of processing parameters on the drying of spice paprika

Paprika is utilized for its strong colour and has very little pungency. The quality of ground paprika is mainly determined by its seasoning power i.e., capsaicin content its natural colour i.e., carotenoid content, its particle size and water content. Selection of proper drying conditions is necessary for minimizing thermal stress, over-drying and maintenance of relevant compounds which determine the quality of the product. Loss of red colour is caused by autoxidation of the carotenoids. It is reported that the stability of the quality of paprika, during storage is dependent on drying conditions and the degradation rate of quality increases as the drying temperature increases. In view of the importance of this spice product standardization of processing protocol is necessary. Transport properties in terms of interphase mass and heat transfer along with the diffusion coefficient were determined for paprika from drying experiments. The analysis includes the comprehensive process including the form of the product like whole fruit or pericarp, various preprocessing steps like cutting, separating, steam blanching and drying. Drying was carried out using both a conventional pilot scale convective dryer and a laboratory dryer with the additional facility of drying under inert atmosphere. For processing under inert gas, the pre- and post-drying steps of cutting, separating and grinding the product before storage were also carried out under inert conditions The influence of these pre-processing steps on the drying behaviour was characterized by determining the transport properties. Additionally, the product was also analyzed for its red colour intensity (ASTA) value, tocopherol, carotenoids and vitamin C content, to evaluate the best possible processing protocol to obtain a good quality product.

PH—Postharvest Technology: Modelling High Temperature, Thin Layer, Drying Kinetics of Olive Bagasse

The thin layer drying kinetics of olive bagasse at relatively high temperatures is characterized. The influences of product granularity, gas velocity and temperature on the kinetics have been experimentally analysed. A laboratory dryer and its data acquisition system were designed and constructed to enable a wide range of operating velocities and temperatures. The drying conditions, investigated in this study, included combustion products of air and propane with dry-bulb temperatures ranging from 125 to 250°C, relative humidity lower than 1% and gas velocities ranging from 0·5 to 2·0 ms−1. The results of the drying kinetics experiments were fitted to a model based on Fick's law, which allowed for the determination of the effective moisture diffusivity as a function of temperature. Accurate agreement between experimental results and predicted curves was found for gas velocities above a critical velocity, quantified as 1·5 ms−1.

DETERMINATION OF PARAMETERS AND PRETREATMENT SOLUTION FOR GRAPE DRYING

The drying of Iranian seedless white grapes (sultana) was investigated in a batch operation in a laboratory dryer. The effect of pretreatment and temperature on the drying rate of grapes at constant air velocity were studied. Pretreatment solutions contained different alkaline materials in different concentrations and temperatures. Dipping grapes in an alkaline solution increased the drying rate substantially. Increasing the air temperature in the dryer from 60 to 70°C accelerated the drying rate (for a decrease in drying time) of the pretreated grape. Grapes dried in 450–900 min depending on pretreatment and air temperature. The shorter drying time and best quality dried product were obtained with grapes dipped in a solution of potassium carbonate of 5% at 42°C. The mean apparent diffusivity of moisture for K2CO3 pretreatment was calculated at 50, 60, and 70°C, and the values were fitted to an Arrhenius type model. Moisture sorption isotherms of currants were determined at 20, 30, and 40°C using static gravimetric methods. A modified Halsay equation was used to predict experimental data for water activity in a range of 0.1–0.85.

A SIMPLIFIED MODEL FOR DETERMINATION OF MOISTURE DIFFUSIVITY OF DATE FROM EXPERIMENTAL DRYING CURVES

ABSTRACT Experimental drying curves for Tunisia Deglet Nour dates were obtained in a laboratory dryer under different drying conditions The air temperature was varied from 30 to 69°C, relative humidity from 11.6 to 47.1 % and air velocity from 0.9 to 2.7 m/s. A numerical method to obtain a solution of a diffusion equation in which the diffusivity depends upon temperature and moisture content has been proposed to investigate the moisture movement in a date by assuming the sample to be a homogenous infinite cylinder. To rind the fitting moisture and temperature dependent diffusivity, the calculated drying curves are compared with the observed drying curves and an empirical equation for the moisture diffusivity of the date has presented as a function of temperature and moisture. It has been shown that the moisture distribution in the date during drying can be obtained by using the empirical equation presented.

EXPERIMENTAL STUDY OF MATTER TRANSFER IN COCOA BEANS DURING FERMENTATION AND DRYING

A study was carried out of the changes in moisture and acetic acid contents during the fermentation and convection drying of cocoa beans. A laboratory drier operating by convection or conduction and a series of three volatile acidity extractors based on the Mothe-Efter method were used. An original cutting system made it possible lo analyse five inner layers of cocoa bean from the centre to the outside edge. Processing of the results enabled the determination of the local and overall changes in the moisture and acetic acid contents and showed the influence of air parameters on drying in thin layers.

Color and density of apple cubes dried in air and modified atmosphere

The color, density and porosity of apple cubes dried in air and modified atmospheres were measured to identify the potential of modified atmosphere drying for foods. The modified atmospheres were achieved by flushing nitrogen gas through a specially designed laboratory dryer. The drying temperature was varied from 17 to 75°C. The results indicated that dried apple cubes with no browning could be produced under low oxygen atmospheres. Thus, chemical additives, such as sulfur dioxide can be completely eliminated from the process. The temperature and oxygen levels in the drying atmosphere were important factors in achieving the best quality in terms of color. Apple cubes dried in low oxygen atmospheres gave more porous products than those dried in air. Moreover, apple cubes dried in a nitrogen atmosphere had the greater number of open pore structures connected to the the exterior surface of the cubes.

PREDICTING DRYING BEHAVIOR OF SPAGHETTI IN A CONTINUOUS INDUSTRIAL DRYER WITH THE MODELS DETERMINED IN A LAB SCALE BATCH SYSTEM

In the first stage of this study experimental data were obtained by using a lab scale batch type dryer, operating under various constant temperature/ constant relative humidity combinations; then a mathematical model was determined for the effective diffusivity of moisture in spaghetti. In the second stage of the study the model obtained in the lab scale batch type dryer was used to calculate the moisture content of the spaghetti leaving continuous industrial scale dryer, and its success was tested by comparing with the experimentally determined moisture content of the product. The model was not found successful and corrected. Mathematical models based on experimental data obtained in a bench-top laboratory dryer underestimated the apparent equilibrium moisture content and over estimated the effective diffusivity of water in spaghetti in industrial scale continuous drying equipment. The structural differences in the product, extrapolation of the data beyond the range of the original experiments, variations in mode (batch versus continuous), surface-to-volume ratios, flow patterns, and geometry associated with each drying system were anticipated as the cause of the scale up problem.

DRYING KINETICS OF SOME FRUITS

ABSTRACT Drying kinetics of four Fruits (namely, apple, pear, kiwi and banana) were studied by introducing an one-parameter empirical mass transfer model, involving a characteristic parameter (drying constant), as a function of process variables. The model was tested with data produced in a laboratory dryer, using non-linear regression analysis. The investigation involved a wide range of characteristic dimensions of samples and air conditions (temperature, humidity, and superficial velocity). The parameters of the model considered were found to be greatly affected by sample characteristic dimension and air temperature. The model also involved the equilibrium material moisture content which was formulated in the form of the GAB equation. Its parameters were evaluated by means of non-linear regression analysis to independent experiments covering two process variables (temperature and water activity of the surrounding air).

Étude expérimentale du séchage infrarouge et convectif de peintures automobiles modèle sur supports plastiques ou métalliques

This work aims to study car paint drying and to better understand the complex physico-chemical transformations which occur during this process. Two different model painting systems were studied: • - a water-based epoxy-amine paint applied on steel support; • - a solvent based polyurethane paint applied either on sheet molding compound support (SMC) or on steel support. In these two cases, drying is an important step and determines significantly the final paint film quality. Indeed, water is a solvent which is difficult to evaporate and SMC supports are plastic and porous materials which can induce the apparition of bubbles on the dry paint film. After the description of the two paint systems and of the near infrared and convective laboratory drier, we will sum up the different results obtained for different experimental conditions, in terms of drying kinetics and dry paint film quality. These experiments show the influence of the physical process parameters (air temperature and velocity), of the infrared flux density, of the paint system (film thickness, chemical properties...) and of the supports properties.

A New Laboratory Dryer for Analysing the Deterioration Kinetics of Biomaterials

Convective drying is a cost saving presewtion technique for the production of bulk products. However, heat senstive bioprodusts may be damaged by convective drying. This paper presents the conception of a convective laboratory dryer with particular features for a simple and rapid examnination of deterioration processes during the drying of biomaterials. The drayer is described and results are given concerning the preservation of microorganisms for environmental applications. The results show that deterioration of the gram-negative bacteria Pseodomonas putida during drying is mainly caused by dehydration. Membrane damage is one of the dehydration deterioration mechanisms. Drying temperatures below 40°C ensure that thermal death of P. putida is neglegible.

DETERMINATION of HEAT TRANSFER COEFFICIENTS DURING DRYING of FOODSTUFFS

Heat transfer coefficients during drying of foods were evaluated from drying data and heat and mass balances. Experimental drying curves for slices and cylinders of potato, apple and carrot were obtained in a laboratory dryer under different drying conditions. the specific heat of the foodstuffs was measured as a function of water content and modelled through a linear relationship. Experimental heat transfer coefficients were correlated by the dimensionless expression NuD= 0.249 ReD0.64 and compared with predictions of different equations.

Drying Rates of 25 Argentinian Varieties of Soybean: A Comparative Study

Argentinian soybean production reached almost 11 million tonnes in 1991, after a growth of 660% in 14 years. Soybean drying became a major post-harvest activity involving long drying times and high fuel consumption. Thus, production of varieties with high drying rates should be encouraged. In this work, the drying rates of 25 varieties of soybean were determined at 60°C in a laboratory dryer. Drying times required to reduce the dimensionless moisture from 1 to 0.5 were 97.3 ± 11.5 min. An analysis was then performed to explain differences in drying times. Sizes of grains, (radius squared) was a significant cause of the differences in drying time. Effective diffusion coefficients were also determined for the 25 varieties. Values found were 7.62 × 10-11 m2/s with a variation coefficient of ±9.1%. The combination of size and diffusion coefficient accounted for the 93.8% of the variations in drying times. No practical effect of oil and protein content on the drying rate was encountered. The diffusion coefficients tend to be greater for larger grains, probably due to the smaller contribution of the tegument to the grain specific resistance to water movement.

SCALE-UP OF CONTACT DRYERS

ABSTRACT The scale-up of contact dryers is still based on experimental drying curves. In order to keep the effort to a minimum the drying curve is determined using a small laboratory or pilot dryer of similar geometry to the production dryer. This paper introduces a new scale -up method for contact dryers. The new scale-up method is based on the assumption that heat transfer is the controlling mechanism. The scale-up method is derived from the material balance, the energy balance, the kinetic equation of heat transfer and thermodynamic equilibrium. The scale up method can be used to convert the drying time required to achieve a certain residual moisture content from the laboratory or pilot dryer to the production dryer and/or different drying conditions. The scale-up method was verified by drying test with four different products in conical mixer dryers of 1, 60, 250, 1000 I volume. Two products were free flowing and two products were non free flowing in the wet state. The products can be considered non-h...

Drying Parboiled Rough Rice in Stationary, Semi-fluidized, and Fluidized Conditions

Fluidization in drying a moist product has been one unit operation attributed to acceleration of the drying process and improvement of quality of the dried product. A sample laboratory dryer capable of having stationary, semi-fluidized, and fluidized drying conditions in a single unit was designed and fabricated. Drying experiments were conducted for parboiled rough rice with 10, 15, and 20 cm initial bed thicknesses at 40 to 80 C of drying air for each of the three drying conditions. It was observed that parboiled rough rice could be dried under semi-fluidized conditions without any significant milling loss and that drying time would also be largely reduced as compared to stationary-bed conditions. An extensive loss of head rice yield in milling and higher energy consumption took place in case of drying in fluidized bed. Recirculation of drying air at various proportions with fresh air was tried in semi-fluidized bed showing an improved drying efficiency and a higher milling yield.

STATE ESTIMATION IN THE BATCH DRYING OF FOODS

ABSTRACT Measurement of material moisture content is necessary for the control of product quality in batch drying. However, this variable cannot be measured on-line, and state estimation techniques are proposed. A non-linear dynamic model is developed for batch drying of foods. Process disturbances and measurement errors are modeled as stochastic processes and a hybrid extended Kalman filter is employed for state estimation. This filter is based on the local linearization of the process model around the suboptimal filter estimates. The moisture estimation approach was applied to experimental points obtained in a laboratory dryer with quite satisfactory results.

Modelling and adaptive control of concurrent-flow driers

Exhaust-air sensing and control is a well-established principle used to control grain driers. It is often preferred to direct measurement and control of the moisture content because of its cheapness and simplicity. The process dynamics of a drier varies with grain and drier characteristics. This motivates the use of an adaptive controller, since this controller is able to continuously track the dynamics and tune itself in an optimal way. An adaptive controller was developed and tested on a laboratory concurrent-flow drier. The controlled drier variables were either the final grain temperature or the approximate maximum grain temperature. The controller is based on linear discrete-time models. In order to investigate whether the drier could be well described by such models, process-identification experiments were made with data from the laboratory drier. In the experiments two linearizing principles were used. The control input to the process was defined to be inversely proportional to the discharge rate, and sampling of the process variables was performed with respect to the grain's vertical displacement in the drier. It was found that the two temperatures were then modelled quite well and the orders and delays of these models were used in synthesis of the adaptive controller. Control experiments were made where either the final grain temperature or the approximate maximum grain temperature were used as the controlled variable. In the experiments very accurate control of the temperatures was obtained but the ability to maintain an even outlet moisture content needs to be further evaluated.

Modelling and adaptive control of continuous grain driers

A model for cross-flow driers was developed for use in the development of adaptive control systems. An adaptive controller based on this model was derived. This controller is able to tune itself in order to maintain good control despite changes in grain or drier characteristics. Control experiments with a small scale laboratory drier were carried out. They showed that the exhaust air temperature at the bottom of the drying section can be accurately controlled with the adaptive controller. They also showed that accurate control of this temperature led to good control of the outlet moisture content.