MODELING OF ROUGH RICE DRYING IN THIN VERTICAL COLUMNS

ABSTRACT

The solution of classical diffusion equation based on the assumption of average moisture diffusion coefficient did not adequately represent natural convection drying of rough rice in thin vertical columns exposed on both sides to hot air. Instantaneous moisture diffusivity coefficients determined from experimental drying curves decreased continuously with an increase in exposure duration and were linearly related to moisture ratio. The proponionality constant which was called apparent moisture diffusion coefficient was distinctly related to air temperature, relative humidity, and initial moisture content of rough rice. The modified moisture diffusion model using the instantaneous moisture diffusion coefficient was found to best represent the moisture removal from bulk rough rice.     

THIN LAYER AND DEEP BED DRYING OF LONG GRAIN ROUGH RICE

ABSTRACT

The paper presents new data for thin-layer drying characteristics of Thai long grain rough rice measured under various conditions of drying air temperature (35 to 60?°C), drying air relative humidity (30 to 70 % ) and the initial moisture content of rough rice (20 to 40 % dry basis). Empirical equations were developed using the instantaneous weight, the weight loss and drying time, with temperature, relative humidity and initial moisture content of rough rice as the independent variables. A computer program was developed to simulate the deep-bed drying process. The thin-layer drying equation developed before was used in the computer simulation. Experimental data from the fixed bed dryer were compared with the results from the calculation.     

THIN LAYER AND DEEP BED DRYING OF LONG GRAIN ROUGH RICE

The paper presents new data for thin-layer drying characteristics of Thai long grain rough rice measured under various conditions of drying air temperature (35 to 60 °C), drying air relative humidity (30 to 70 % ) and the initial moisture content of rough rice (20 to 40 % dry basis). Empirical equations were developed using the instantaneous weight, the weight loss and drying time, with temperature, relative humidity and initial moisture content of rough rice as the independent variables. A computer program was developed to simulate the deep-bed drying process. The thin-layer drying equation developed before was used in the computer simulation. Experimental data from the fixed bed dryer were compared with the results from the calculation.     

INTERMITTENT DRYING OF ROUGH RICE

Abstract

This work obtains thin-layer drying data for rough rice from 108 treatments. A thin-layer drying equation is also derived using these data with drying air absolute humidity, drying air temperature, tempering time interval and drying time interval as the independent variables. In addition, an intermittent drying equation is developed to predict the drying behavior of rough rice in a re-circulating type rice dryer.     

FINITE ELEMENT SIMULATION OF ROUGH RICE DRYING

Milled, brown and rough rice samples were dried in the laboratory with heated air and diffusivities of the endosperm, bran and husk were evaluated with a search technique using the finite element method. The endosperm had the highest diffusivity. The husk had a slightly higher diffusivity than the bran because the trapped air inside the husk was considered as an integral part of the husk. The finite element method predicted rough rice drying that was in good agreement with experimental results.     

COMPUTER SIMULATION ON INTERMITTENT DRYING OF ROUGH RICE

This study applied a partial differential equation model with newly-developed thin layer equations to simulate batch re-circulating dryers under different drying conditions, which are combinations of four parameters: drying air temperature, drying air absolute humidity, drying period duration, and tempering period duration. The moisture change and the drying rate, which were of particular concern with respect to the simulated data, were investigated. Validation drying tests were carried out in a lab scale re-circulating rice dryer. Two sets of experiment were performed involving different drying parameters to simulate re-circulating rice dryers which are extensively used in Asian countries. Comparing these two experimental data with two simulated drying curves respectively, it revealed they are quite consist with each other under the same drying conditions. Drying air temperature, drying air humidity, drying period duration and tempering period duration significantly influenced the drying rate. Under the same drying condition, the tempering period duration effect was insignificant to the drying rate in drying zone as the drying air humidity or temperature increased. And, a higher initial moisture content obtained higher time and energy efficiency for the re-circulating rice dryers.     

DEHUMIDIFICATION DRYING OF HIGH-MOISTURE ROUGH RICE IN A RADIAL-FLOW, CIRCULAR BIN

An experimental radial-flow, circular bin dryer with a dehumidifier was fabricated and tested for rough rice drying. Results showed that high-moisture paddy could be dried to 14% moisture content (wet basis) over a 24 h drying span at an average drying rate of 0.5%/h. The open-cycle system's specific moisture extraction rate and the dehumidifier's coefficient of performance were within the usual range of values cited for similar conventional systems or equipment. The quality of the dehumidifier-dried and naturally dried paddy was similar with respect to minimum grain Assuring, The system was found to be a technically feasible, more energy-efficient and safer alternative to electrical resistance heating in the conditioning of ambient air for drying under humid tropical conditions.     

DEHUMIDIFICATION DRYING OF HIGH-MOISTURE ROUGH RICE IN A RADIAL-FLOW,CIRCULAR BIN

Abstract

An experimental radial-flow, circular bin dryer with a dehumidifier was fabricated and tested for rough rice drying. Results showed that high-moisture paddy could be dried to 14% moisture content (wet basis) over a 24 h drying span at an average drying rate of 0.5%/h. The open-cycle system's specific moisture extraction rate and the dehumidifier's coefficient of performance were within the usual range of values cited for similar conventional systems or equipment. The quality of the dehumidifier-dried and naturally dried paddy was similar with respect to minimum grain Assuring, The system was found to be a technically feasible, more energy-efficient and safer alternative to electrical resistance heating in the conditioning of ambient air for drying under humid tropical conditions.     

THE STUDY OF INTERRUPTED DRYING TECHNIQUE FOR ROUGH RICE

Abstract

The purpose of this research was to study the optimum strategy of the interrupted drying technique for rough rice. The effects of interrupted drying on drying rates, cracking enhancement ratios, and the performance of a commercial circulating rice dryer were evaluated. The results indicated that the required interrupted period of grains with moisture of ranging from 18 to 20% was 2 around 3 hours. The interrupted drying technique can significantly increase the drying rate. The dryer performance e.g. fuel-consumption rate, drying rate, and drying time were all affected by interrupting the drying process, as expected.     

THE STUDY OF INTERRUPTED DRYING TECHNIQUE FOR ROUGH RICE

The purpose of this research was to study the optimum strategy of the interrupted drying technique for rough rice. The effects of interrupted drying on drying rates, cracking enhancement ratios, and the performance of a commercial circulating rice dryer were evaluated. The results indicated that the required interrupted period of grains with moisture of ranging from 18 to 20% was 2 around 3 hours. The interrupted drying technique can significantly increase the drying rate. The dryer performance e.g. fuel-consumption rate, drying rate, and drying time were all affected by interrupting the drying process, as expected.     

A COMPARTMENTAL MODEL OF THIN-LAYER DRYING KINETICS OF ROUGH RICE

Rough rice at about 21% (wet basis) was dried at various conditions of temperatures and evaporating capacities of air. The influence of both parameters on drying rate has been studied. At high temperatures, high drying rates can be achieved with low evaporating capacities. In addition, desorption isotherms of rough rice were measured at 35, 60 and 85°C and the experimental isotherms data were fitted using a modified Pfost equation.

A compartmental model was developed to simulate the grain moisture content. Heat and mass transfer coefficients were optimized using a Nelder & Mead method. Internal mass transfer coefficient was written as an exponential function of the average moisture content and temperature of the grain and the external mass transfer coefficient as a function of air temperature. The compartmental approach predicts very well the average moisture content with a mean error of about 5% in static and dynamic conditions.     

A COMPARTMENTAL MODEL OF THIN-LAYER DRYING KINETICS OF ROUGH RICE

ABSTRACT

Rough rice at about 21% (wet basis) was dried at various conditions of temperatures and evaporating capacities of air. The influence of both parameters on drying rate has been studied. At high temperatures, high drying rates can be achieved with low evaporating capacities. In addition, desorption isotherms of rough rice were measured at 35, 60 and 85°C and the experimental isotherms data were fitted using a modified Pfost equation.

A compartmental model was developed to simulate the grain moisture content. Heat and mass transfer coefficients were optimized using a Nelder & Mead method. Internal mass transfer coefficient was written as an exponential function of the average moisture content and temperature of the grain and the external mass transfer coefficient as a function of air temperature. The compartmental approach predicts very well the average moisture content with a mean error of about 5% in static and dynamic conditions.     

THIN-LAYER DRYING CHARACTERISTICS OF ROUGH RICE AT LOW AND HIGH TEMPERATURES

Thin-layer drying characteristics of rough rice were determined at temperature ranging from 11.8 to 51 °C and for relative humidities ranging from 37.1% to 91.3%, with initial moisture contents in the range of 24.7 to 41.6% dry basis. An oven, a self contain air conditioning unit, recently developed in Japan, was used for this experiments. With this apparatus, very smooth drying curves were obtained. The data of sample weight, and dry and wet bulb temperatures of the drying air were recorded continuously throughout the drying period for each test. The drying process was terminated when the moisture content change in 24 h was less than 0.2 % d.b. (weight change was less than 0.05 g). The final points were recorded as the dynamic equilibrium moisture contents.

The drying data were than fitted to the Page model. The model gave a very good fit for the moisture content with an average standard error of 0.294 % d.b¨ Both the drying parameters, K and N, are function of drying air temperature and relative humidity. The effect of variable initial moisture content was also described effectively by the empirical Page model. The drying time employed had a large effect on the K and N values. The results presented here, over typically 5 day drying, will be useful in the long term moisture transfer process occurring during ventilated storage.     

THIN-LAYER DRYING CHARACTERISTICS OF ROUGH RICE AT LOW AND HIGH TEMPERATURES

Abstract

Thin-layer drying characteristics of rough rice were determined at temperature ranging from 11.8 to 51 °C and for relative humidities ranging from 37.1% to 91.3%, with initial moisture contents in the range of 24.7 to 41.6% dry basis. An oven, a self contain air conditioning unit, recently developed in Japan, was used for this experiments. With this apparatus, very smooth drying curves were obtained. The data of sample weight, and dry and wet bulb temperatures of the drying air were recorded continuously throughout the drying period for each test. The drying process was terminated when the moisture content change in 24 h was less than 0.2 % d.b. (weight change was less than 0.05 g). The final points were recorded as the dynamic equilibrium moisture contents.

The drying data were than fitted to the Page model. The model gave a very good fit for the moisture content with an average standard error of 0.294 % d.b? Both the drying parameters, K and N, are function of drying air temperature and relative humidity. The effect of variable initial moisture content was also described effectively by the empirical Page model. The drying time employed had a large effect on the K and N values. The results presented here, over typically 5 day drying, will be useful in the long term moisture transfer process occurring during ventilated storage.     

STUDY ON ROUGH RICE FISSURING DURING INTERMITTENT DRYING

The Assuring of rough rice during intermittent drying was studied through experimental and numerical methods. The moisture distribution inside the rice kernel during drying and tempering was obtained by diffusion model. The moisture gradients were used to analyze the hydro stresses in the rice kernel during intermittent drying.

Discontinuing the drying process with tempering can decrease the hydro stresses in the rice kernel. Decreased unit drying time or increased intermittent ratio caused decrease of the stresses in the rice kernel during intermittent drying. Less fissured rice was also observed by discontinuing the drying process in the experiments. Higher intermittent ratio or lower unit drying time caused lower percentage of fissured rice in the experiments.     

STUDY ON ROUGH RICE FISSURING DURING INTERMITTENT DRYING

Abstract

The Assuring of rough rice during intermittent drying was studied through experimental and numerical methods. The moisture distribution inside the rice kernel during drying and tempering was obtained by diffusion model. The moisture gradients were used to analyze the hydro stresses in the rice kernel during intermittent drying.

Discontinuing the drying process with tempering can decrease the hydro stresses in the rice kernel. Decreased unit drying time or increased intermittent ratio caused decrease of the stresses in the rice kernel during intermittent drying. Less fissured rice was also observed by discontinuing the drying process in the experiments. Higher intermittent ratio or lower unit drying time caused lower percentage of fissured rice in the experiments.     

DRYING OF AIR IN SILICA GEL PACKED COLUMNS

Silica gels that were made humidity indicating by impregnation of CoCl₂ were used for air drying in isothermal packed column. The effects of type of silica gel, packing height, particle size and air flow rate on breakthrough curves were studied. The breakthrough curves predicted by Rasmuson and Neretniek's analytical solution were in good agreement with experimental data.     

DRYING OF AIR IN SILICA GEL PACKED COLUMNS

Silica gels that were made humidity indicating by impregnation of CoCl₂ were used for air drying in isothermal packed column. The effects of type of silica gel, packing height, particle size and air flow rate on breakthrough curves were studied. The breakthrough curves predicted by Rasmuson and Neretniek's analytical solution were in good agreement with experimental data.     

STOCHASTIC MODELING OF GRAIN TEMPERATURE IN NEAR-AMBENT DRYING

Stochastic analytical model was developed to describe the heat transfer in a thin layer of grain ventilated with ambient air. As an excitation to the drying system, a stochastic analytical model of ambient air temperature variations during the period of drying, based on a sine function, was used. Mean, correlation function, variance and the standard deviation were determined for air and grain temperature, both treated as random processes. The stochastic model of air temperature variation was fitted to the observed historical data. The effect of the standard deviation of air temperature of the three different climates (Canadian in Winnipeg, Polish in Poman and English in Weddington) on the standard deviation of grain temperature is shown.     

SOLAR DRYING SIMULATION OF PRUNES ARRANGED IN THIN LAYERS

Solar drying of prunes arranged in thin layers in a multi-shelf type drier is simulated. The weather conditions are those found during harvest in humid pampa region (hot-temperate and humid climate). A mathematical model based on Luikov's theory of drying and including an equation that describes the material shrinkage is employed. The airstream humidification is taken into account in order to analyse the influences of shelves spacing and drier length in the final product characteristics. It is found that the relative moisture content heterogeneity is a useful design criterion. The influence of weather conditions in the drying time and the drying capacity are evaluated.