MATHEMATICAL MODELLING OF MDF FIBRE DRYING: DRYING OPTIMISATION

ABSTRACT

In the production of MDF, wet resinated fibre must be dried to its target moisture content, normally 9 to 11%, before compaction into a board by hot pressing. Fibre drying can be interpreted as an incorporated process involving gas-solid two phase-flow, inter-component transfer, and heat and mass transfer within the fibre. Based on these mechanisms, a mathematical model has been developed to simulate the MDF fibre drying process. From the model, fibre moisture content, air temperature and air humidity along the dryer length can be predicted and factors affecting the drying rate examined. The model can be employed to optimise drying conditions and to evaluate improvements in dryer design. A case study of drying improvement in reduction of dryer emissions and heat consumption is given to demonstrate the potential application of the developed dryer model.     

THE DEVELOPMENT AND VALIDATION OF A SYSTEM MODEL FOR A COUNTERCURRENT CASCADING ROTARY DRYER

ABSTRACT

An overall system model for a countercurrent rotary dryer has been developed with the ullimale aim of assessing controller pairings in these dryers. This model is based on heat and mass balances within dryer regions combined with two subsidiary models, one describing the equipment (which determines particle transport and heat transfer)and the other describing the behaviour of the material (the drying kinetics). Six partial differential equations have been set up to evaluate six state variables: solids moisture content, solids temperature, gas humidity, gas temperature, solids holdup and gas holdup as functions of time and rotary dryer length. A control-volume method has been used to reduce the six partial differential equations with respect to time and the length of the rotary dryer to six ordinary differential equations in time.

The drying model has been implemented in the SPEEDUP flowsheeting package (with FORTRAN subroutines) The model has been validated by fifteen experiments-in a pilot scale countercurrent-flow rotary dryer (0.2m in diameter and 2m in length)     

MODELING AND SIMULATION OF CROSSFLOW MOVING BED GRAIN DRYERS

ABSTRACT

The drying of grain in dryers of a crossflow moving bed type was theoretically and experimentally studied. Two different dryer configurations were analyzed, a dryer with central air distribution and another with multiple air duels. Experimental information was obtained in pilot-size dryers. A mathematical model to simulate the process was developed. Hindered drying was accounted for by using the concept of relative drying rate. An adjustable factor, specific to the dryers, was used to account for the uncertainties of the contact area and the transfer coefficients encountered in the literature. Agreement between experimental results and simulations was fairly good. Simulations showed that distance between inlet air and outlet devices, air to solid flow ratio and dryer height to cross section ratio have great influence on the process. The mathematical model may be a useful tool for process exploration and optimization of this type of dryers.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

ABSTRACT

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

Modeling of Muti-layer Far-Infrared Dryer

Abstract

The heat and mass transfer models applicable to biological products at each element in a far-infrared fixed-bed dryer were established. The model was validated by comparing the simulation results with experimental data of black mushrooms (Lentinus edodes) in terms of moisture content and drying rate. The very good agreement suggests that the models for heat transfer and drying rate of biological products in the far-infrared fixed-dryer can be obtained by establishing the equations of radiation and dehydration in interspace elements. An unevenness of change in moisture content and drying rate on the same horizontal layer in the dryer was found. Thus it is necessary to take some measures to solve this nonuniformity in drying if the proposed dryer is employed.     

Performance evaluation of parabolic greenhouse-type solar dryer used for drying of cayenne pepper

Abstract

This study examines the performance of a parabolic greenhouse-type solar dryer used for drying of cayenne pepper (Capsicum annuum) in Nan (northern Thailand). The dryer has a base area of 6.0?m × 8.2?m and a height of 3.25?m with the loading capacity of 100–200?kg for fruit or vegetables. It has a parabolic roof structure covered with polycarbonate sheets and is placed on a concrete floor. It is ventilated by three DC fans powered by a 50-Watt solar cell module. The produce is placed on trays with wire mesh base and located on steel supports. Among the advantages of this type of dryer is that the product is protected from rain, dust and insects. The pepper samples consisted of whole pods or cut pods. The drying experiments were carried out in the solar dryer and in an electrical (convective) tray dryer. The study was focusing on drying kinetics and on the effects of the drying treatments on one of the main quality attributes namely the capsaicin content in the dried products.     

SIMULATION OF FORESTRY BIOMASS DRYING IN A ROTARY DRYER

ABSTRACT

Drying of forestry biomass in a rotary dryer has been performed. The raw material used was Erica Arborea belonging to the ever-green, broad leaves ecosystem which covers Central Greece and other Mediterranean countries. The study was part of a project concerning a Greek biomass pyrolysis demonstration plant where drying of biomass is very important in the contribution to the global energy balance and product yields of pyrolysis.

The study includes two parts. First, the experimental part concerns the influence of air flowrate, temperature, rotation speed and inclination of a laboratory rotary dryer to biomass residence time and biomass outlet moisture content. The second part concerns the development of a mathematical model for biomass drying in a rotary dryer. Experimental measurements in a rotary dryer were compared to the data from the model, in order to check the validity of the model.     

The Development of Ginger Drying Using Tray Drying,Heat Pump–Dehumidified Drying,and Mixed-Mode Solar Drying

Mature ginger was pretreated by soaking in citric acid prior to drying in a single layer in a tray and heat pump dehumidified dryer at three temperatures of 40, 50, and 60°C and in a mixed-mode solar dryer at 62.82°C and a radiation intensity of 678 W/m². The drying data were applied to the modified Page model. Diffusivities were also determined using the drying data. Quality evaluation by color values, reabsorption, and 6-gingerol content showed best quality for ginger with no predrying treatment and dried at 40°C in a heat pump–dehumidified dryer. At drying temperature of 60 to 62.82°C, no pretreated dried ginger from mixed-mode solar dryer provided the shortest drying time and retained 6-gingerol as high as heat pump–dehumidified dryer.     

SCALE-UP OF LAYER DRYERS: A UNIFIED APPROACH

Abstract

The concept of Specific Drying Rate is introduced for analysis of experimental drying data for layer dryers and as a convenient scale-up factor. In conjunction with data obtained from a batch laboratory simulation the method is applicable to most types of continuous layer dryers. Three illustrations of the use of this approach for scale-up are presented including a continuous convection heated rotary shelf dryer, an indirectly heated paddle dryer, and an indirectly heated plate dryer with heating zones.     

Dynamic Analysis of Drying Energy Consumption

Abstract

A concept of instantaneous drying and energy efficiencies has been applied to analyze energy consumption in a through-circulation conveyor dryer and a batch fluid bed dryer for synthetic rubber. It is shown that the energy performance of the conveyor dryer can be improved by leveling of the moisture content distribution across the material layer by mechanical agitation, and sectioning of the air plenum in order to reduce inlet air temperature in the last two sections of the dryer. It is also shown that drying of a synthetic rubber in the fluid bed dryer is more energy-efficient than in the conveyor dryer, especially in the constant-rate period. Thus, the largest energy savings could be obtained in a two-stage dryer comprised of the plug-flow fluid bed dryer, and the belt conveyor dryer for removal of the bulk and residual water, respectively.     

Modelling Diffusion-Limited Drying Behaviour in a Batch Fluidized Bed Dryer

ABSTRACT

Fluidized bed dryers are often used to extract water from granular materials. When the drying process is mainly limited by the resistance against water transport inside the particle the drying behaviour is said to be diffusion-limited. In the literature there are several models that predict this drying process with very diverging results. In this study a model is set up to arrive at a better prediction for this drying process. The heat and mass transfer in the granular material and the drying air is described. The resulting equations are solved numerically. The model must be extended to incorporate the heat capacity of the dryer.     

PHOTOVOLTAIC DRYER WITH DUAL PACKED BEDS FOR DRYING MEDICAL HERB

Abstract

This work presents design and optimization of a cylindrical photovoltaic dryer with dual packed beds thermal energy storage for drying medical herb. The dryer is provided with electrical heater where the electrical energy is generated by using photovoltaic system. The electrical heater is designed and sized to realize continuouse drying (day and night) to minimize the drying time. Two packed beds are used to fix the drying temperature in dryer during day and night. The main packed bed thermal energy storage is charged during the sun-hours directly, to realize continued drying after sunset. An efficient PV dryer is devised to work under forced air created by air blower and heated by the electrical coils.     

Comparison Between Drying of Timber in a Solar Dryer and in an Electrically-Heated Kiln

ABSTRACT

Solar dryers have been considered for timber drying in a number of countries because of the expected savings in drying costs. From a review of past works on solar, natural, and conventional drying it was observed that while solar dryers were able to dry timber faster compared to natural drying, the difference was only marginal in some instances. The drying rates are expected to be dependent upon ambient conditions in which the dryera are operated. Solar dryers would operate more efficiently in countries with low humidity than in tropical regions. Thus the thermal performance and also the economics of solar dryer is country dependent. In the present paper, a comparison of the drying rates obtained with a solar dryer is made with that obtained with an electrically operated drying kiln.     

THE DEPENDENCE OF PAPER MACHINE DRYING DRYING CHARACTERISTIC ON THE MACHINE GEOMETRY

ABSTRACT

The effect of the dryer cylinder diameter on the dryer specific evaporation rate is examined theoretically. It is observed that, in the range of cylinder diameters applicable in practice, the dryer specific evaporation rate is nearly independent of the selected cylinder diameter when the dryer geometry remains unchanged. In practice, however, the dryer geometries have changed slightly according to the cylinder diameter.

TO clarify the effects of the various component factors pertaining to the dryer geometry, this paper presents a drying characteristic, i.e.. a curve showing the dependence of the drying rate on the condensing steam temperature. The drying characteristic is given in the form of a nomogram to facilitate the evaluation of the effect of the ratio between the free draw length and the cylinder diameter, and the wrapping angle, to the dryer specific evaporation rate. Further, by means of an example, the specific evaporation rates obtainable for machine geometries currently used by various machine manufacturers are compared.     

Two-Fluid,Two-Dimensional Model for Pneumatic Drying

Abstract

Pneumatic drying is a widely used process in the chemical industries and includes simultaneous conveying and heat and mass transfer between the particles and the heat gas. The increase in the use of this unit operation requires reliable mathematical models to predict processes in the industrial facilities. In the present study a Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a two-dimensional steady-state condition and considering axial and radial profiles for the flow variables. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle is assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The numerical procedure includes discretization of calculation domain into torus-shaped final volumes, solving conservation equations by implementation of the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm and controls over coupling of phases by IPSA (Interphase Slip Algorithm). The developed model was applied to simulate a drying process of wet PVC particles in a large-scale pneumatic dryer and to a drying process of wet sand in a laboratory-scale pneumatic dryer. The numerical solutions are compared successfully with the results of independent numerical and experimental investigations. Following the model validation, the two-dimensional distributions of the flow characteristics were examined.     

Heat and Moisture Transfer Model for Onion Drying

Abstract

Onion slices of 3 mm thick were dried in an atmospheric batch tray dryer in order to investigate the influence of air temperature and drying time on parameters such as sample moisture content and drying rate. A model is proposed which takes into account both moisture and temperature distributions in the sample and is in a fair agreement with the experimental data. The models suggested so far by other workers take only the moisture distributions into account for onion drying.     

Drying of a Root Crop in Vibro-Fluidized Beds

ABSTRACT

Drying of sliced corm of Moyu which represents a group of hard-to-dry herbaceous plants was carried out in a batch laboratory vibrated fluid bed to determine drying kinetics and identify operating parameters for required product quality. Results from laboratory studies were verified in a pilot-scale continuous VFB dryer. Field trials performed over the Moyu harvest season confirmed the resulls extrapolated from laboratory data.     

PSEUDO TWO-DIMENSIONAL MODEL FOR A PNEUMATIC DRYER

ABSTRACT

Pneumatic drying of chemical products has been frequently used in chemical industries. The increase in the use of this unit operation requires the knowledge of the dynamic of the gas-solid flow in tubes. The mathematical models of vertical pneumatic conveying found in the literature mostly consider the flow steady and one dimensional. However, experimental evidences suggest that radial profiles of the basic variables of the flow exist. In this work a model is proposed for vertical pneumatic conveying considering axial and radial profiles for gas and solids velocities, porosity and pressure. The conservation equations for energy and mass of water were written to extend the model to a pneumatic dryer. The equations of the model were solved using finite difference method and the results show the axial and radial variations of gas and solid temperatures, gas humidity and particle moisture content in the dryer.     

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.     

ON THE STUDY OF TIME-VARYING TEMPERATURE DRYING—EFFECT ON DRYING KINETICS AND PRODUCT QUALITY

ABSTRACT

The ability of heat pump dryer to produce controlled transient drying conditions, in terms of temperature, humidity and air velocity, has given it an edge over other drying systems. Exploiting this characteristic, we studied and compared the effect of different temperature-time profiles on the quality of agricultural products in a tunnel heat pump dryer capable of providing up to 14.6 kW of cooling capacity. The product quality refers to the color change of the products. Samples of banana and guava were dried in batches in a two-stage heat pump dryer. The effects of the starting temperature of a selected profile and the cycle time on both drying kinetics and product quality were studied. It was observed that by employing a step change in drying air temperature with the appropriate starting temperature and cycle time, it was possible to reduce significantly the drying time to reach the desired moisture content with improved product color.