STEAM DRYLNG OF WOOD FUELS

ABSTRACT

The objective of this work is to investigate some of the important aspects in the design of a steam band dryer for wood fuels. For this purpose the drying of the material in a bench-scale fixed bed dryer has been studied.

Drying times and thermal efficiencies for experiments conducted under different conditions are compared. The investigated materials are soft-and hardwood chips and softwood bark.

The thermal efficiency, the part of the sensible heat which is used during one passage of the steam through the bed, increases with increasing mass load (mass of dry matter per unit area) and with decreasing steam mass flux. At a mass load of 30 kg/m the thermal efficiency is about 0.85 even at steam mass fluxes as high as 0.6 kg/m² s (1.2 m/s). The thermal efficiency proves to be almost independent of pressure and temperature of the steam.

Due to the very inhomogeneous materials the steam mass flow distribution was uneven. This causes a decreased thermal efficiency. When bark     

DRYING OF PADDY IN A BED DRYER

The experimental results of a bed dryer (at Ludhiana) have been analyzed. The experimental results are well predicted by the small scale laboratory experiments on a spouted bed dryer.     

Investigation of Temperature Distribution and Heat Transfer in Fluidized Bed Using a Combined CFD-DEM Model

Fluidized beds are widely used in many industries because they are effective for both mixing and drying. The distinct element method (DEM) has recently received more attention for investigating the phenomena of multiphase flow because the technique is effective in gathering detailed information on complex phenomena without physically disturbing the flows. However, most studies have focused on the aerodynamics of the particles. In this study, a combined computational fluid dynamics (CFD)-DEM model, which allows prediction of gas and particle temperature profiles and heat transfer coefficients in a two-dimensional fluidized bed, was developed. The predicted results were compared with the experimental results at the superficial gas velocities of 2.04, 2.22, and 2.41 m/s and at the controlled inlet temperature of 343 K. Based on the comparison between the predicted and experimental results, it was found that the developed model performed adequately in predicting the gas temperature profiles, and the predicted particle temperature profiles were higher than the experimental data. The predicted heat transfer coefficient was slightly higher than the experimental data. However, the predicted and experimental results had a similar trend in which the heat transfer coefficient increased as a function of an increase in superficial gas velocity. In addition, the minimum fluidization velocity predicted by the developed model agreed well with the experimental data. Such predictions can provide essential information on temperature and heat transfer coefficients inside the fluidized bed for design and scale-up.     

A New Model and Quality of Unfrozen and Frozen Cooked Rice Dried in a Microwave Vibro-Fluidized Bed Dryer

This study aimed to develop a suitable drying model for microwave vibro-fluidized bed drying in a single-mode applicator (MVFB-SMA drying) of cooked rice with and without prefreezing treatment and to investigate the effects of prefreezing treatment and drying temperature (110–185°C) on quality of dried cooked rice. During the process of drying cooked rice from 60 to 10% (wet basis), results indicated that drying rate increased, whereas drying time decreased with prefreezing treatment and increased drying temperature. The drying rate and drying time of unfrozen and frozen cooked rice ranged from 0.196 to 0.497 g water/g dry matter/min and 0.228 to 0.554 g water/g dry matter/min; and from 7 to 2.5 min and 5.5 to 2 min, respectively. A new model was proposed in this study (MR = exp(?k t ⁿ ) + bt + c) to compare with 11 commonly used drying models. The new model describes the MVFB-SMA drying data most satisfactorily. The values of effective diffusivity were between 1.70 × 10^?7 and 5.72 × 10^?7 m²/s for the unfrozen sample and between 1.99 × 10^?7 and 5.86 × 10^?7 m²/s for the frozen sample. Their activation energy values were 23.66 and 21.19 kJ/mol, respectively. Prefreezing treatment provided a whiter product with a less uniform porous structure and higher bulk density. Slower ability to rehydrate was also observed for the frozen cooked rice dried at 160 and 185°C. An increase in drying temperature resulted in changes in whiteness, microstructure, bulk density, and rehydration capability. No prefreezing treatment and drying at 160°C seemed to be the optimal process condition for cooked rice, ensuring whiteness, a porous structure, low bulk density, and high rehydration capability.     

Effects of High-Temperature Drying on Physicochemical Properties of Various Cultivars of Rice

Abstract

Three varieties of paddy rice, namely Langi and Amaroo from Australia and Chainart I from Thailand, were dried from high initial moisture content of about 27% down to 13–14% wet basis using a two-stage drying system. A fluidized bed dryer reduced the moisture content down to 18%. Drying experiments were carried out at 100, 125, and 150°C. Further moisture content reduction down to 14% was achieved by shade drying. As a result of these treatments, head rice yield increased proportionally with the drying temperature. In contrast to that, the yellowness, measured by colorimeter in terms of b value, showed an opposite trend. Starch characteristics were studied by Rapid Visco Analyser (RVA), x-ray diffraction, and differential scanning calorimetry (DSC). Pasting properties were affected by the drying temperature. The peak viscosity and break down were decreasing with the increase of drying temperature in all varieties while the setback values were increasing in Langi and Amaroo only. All starch samples displayed the typical A type x-ray diffraction pattern. The apparent crystallinity determined by x-ray diffraction was reduced with increasing drying temperature. The gelatinization peak shifted to higher temperature while the endothermic enthalpy of gelatinization decreased with increasing drying temperature.     

Design and Modeling of Plug Flow Fluid Bed Dryers

Plug flow fluid bed dryers (PFFBD) have been used for drying of particulate solids such as salts, ion exchange resins, grains, and a variety of other products. The present article describes the use of a mathematical model for the scale-up of lab-scale batch fluidization data to design an industrial-scale PFFBD. Axial dispersion theory was used in conjunction with the tanks-in-series model to describe the non-ideal flow. The model was implemented in Matlab 6.5 and it can be used for easily fluidizing particulate materials. The proposed model is capable of analyzing both the exponential falling rate and constant rate drying periods. The model predicts the required dryer dimensions for a given throughput and desired final moisture content. The model can also be used to study the effect of different process parameters such as solids feed rate, inlet air velocity, and temperature on the required dryer dimensions and it can also be used to predict the moisture and temperature profiles along the length of the PFFBD.     

HOW TO USE FLUIDIZATION TO OBTAIN DRYING KINETICS COUPLED WITH QUALITY EVOLUTION

Abstract

This paper presents the main activities of the Gas Fluidization and Drying Team of the Chemical Engineering Laboratory at Toulouse. The emphasis has been put on the modeling of a fluidized bed dryer and its derivates such as a flotation fluidized bed, by the analysis of heat and mass transfer together with the physico-chemical transformations of the product during drying. Several studies have been performed on different solids (cereal and chemical products) to illustrate the effect of the operating conditions on dehydration and degradation kinetics The knowledge of heat and mass transfer coefficients in flotation fluidized bed allows to determine intrinsec drying and degradation kinetics. These kinetics can be used in any conductive or convective drier model.     

INTERMITTENT THIN LAYER CORN DRYING

ABSTRACT

In this study a block oriented modelling technique was introduced in order to simulate the intermittent drying technology process. First, the relevant blocks of the parameters concerning to drying air and the properties of material to be dried were developed. These blocks were then used to set up the full modell of a fix bed dryer including the temperature and moisture distribution of drying air and The material along the depth of the dryer. The simulation results were compared to Bakker-Arkema model with a good agreement. Finally, several measurements were performed for the case of thin layer of com to prove the advantage of the intermittent drying technology. It has been founded that in both energy saving and quality of dried product the intermittent technology has a significant contribution.     

DRYING RESEARCH AND DEVELOPMENT IN CZECHOSLOVAKIA

This paper provides general information on drying R 6D activities in Czechoslovakia. This survey report lists the main topics in drying according to individual industrial sectors in which drying plays an important role. Drying R 6D carried out in Czechoslovak Universities is also cited. The paper includes an extensive list of references, mainly proceedings of national drying conferences, symposia, and meetings     

Influence of Drying Techniques on the Characteristics of Chitosan and the Quality of Biopolymer Films

In this work, spouted bed and tray-drying techniques were employed at different drying air temperatures to produce dried chitosan, and the chitosan powder was used to produce biofilms. The products obtained from each drying technique were compared in relation to quality aspects (molecular weight, lightness, and hue angle). The results found for chitosan in spouted bed drying (90°C) showed lower alteration and best quality aspects in relation to the chitosan powder. However, in tray drying under the best condition (60°C) the chitosan molecular weight increased about 50% in relation to the initial value and browning was observed. The biofilms produced from chitosan dried in the spouted bed showed the best mechanical properties (tensile strength of 42 MPa and elongation of 29%) and lower water vapor permeability (3.95 g mm m^?2 kPa^?1 day^?1).