NUMERICAL SIMULATION OF INTERMITTENT AND CONTINUOUS DEEP-BED DRYING OF BIOLOGICAL MATERIALS

This paper presents a simulation study of dryers. A two-dimensional deep-bed drying model, which incorporates modifications in existing one-dimensional models, has been developed and has the capability of handling both non-uniform velocity profiles and material properties dependence on temperature and moisture content. A corn drying system was simulated in order to analyze its temporal and spatial drying behavior. Both intermittent and continuous drying were simulated. The results showed that continuous drying is more efficient than intermittent drying with respect to drying time. However, intermittent drying led to more uniform temperature and moisture distributions, which improve grain quality. The model can be easily extended to three-dimensional drying analysis.     

OPTIMUM AIR DRYING STRATEGY FOR DEHUMIDIFIER TIMBER DRYERS

This paper presents an economic model for analyzing the cost behavior of dehumidifier timber drying process. Various costs, including those of air drying, are identified and quantified. Reasonable assumptions are made to simplify the calculation process. For species of both softwoods and hardwoods, the optimum air drying strategies are investigated based on two scenarios of existing commercial dehumifier dryers and well-run high performance dehumidifier dryers. This paper finds that when the timber maximum air drying degrade rate is less than 5%, air drying adds significant value to the operation of dehumidifier dryers. This benefit accelerates with the extent of air drying. With the use of high performance dehumidifier dryers, the importance of air drying tends to decrease. The cost of air drying is very sensitive to both the assumed interest rate and timber maximum air drying degrade rate.     

OPTIMUM AIR DRYING STRATEGY FOR DEHUMIDIFIER TIMBER DRYERS

This paper presents an economic model for analyzing the cost behavior of dehumidifier timber drying process. Various costs, including those of air drying, are identified and quantified. Reasonable assumptions are made to simplify the calculation process. For species of both softwoods and hardwoods, the optimum air drying strategies are investigated based on two scenarios of existing commercial dehumifier dryers and well-run high performance dehumidifier dryers. This paper finds that when the timber maximum air drying degrade rate is less than 5%, air drying adds significant value to the operation of dehumidifier dryers. This benefit accelerates with the extent of air drying. With the use of high performance dehumidifier dryers, the importance of air drying tends to decrease. The cost of air drying is very sensitive to both the assumed interest rate and timber maximum air drying degrade rate.     

盘式连续干燥器与回转窑干燥机干燥镍精矿的比较

介绍了盘式连续干燥器与回转窑干燥机的工作原理和干燥效果,通过对盘式连续干燥器的实验数据和回转窑的实际生产数据的分析比较,证明盘式连续干燥器干燥镍精矿的优越性及节能环保效果。     

CONVENTIONAL BASIC DESIGN FOR CONVECTION OR CONDUCTION DRYERS

The items to be considered prior to selection of dryers are explained, and a simple method for a rough estimation of dryer sizes was proposed based on data obtained from operating industrial dryers.

The equations of basic design for batch or continuous type dryers were derived. The heat was supplied to materials by convection and/or conduction. The equations were simplified to the case when the falling rate of drying is proportional to the moisture content of materials under the constant drying conditions. The heat transfer coefficient used in the equations can be determined based on the calculations or the data obtained from the experimental or industrial dryers. The equations are useful for estimating the scale-up effect of dryers.     

Mathematical Modeling of Rotary Drying of Woody Biomass

Woody biomass in the form of forestry industry residues has been recognized as a promising resource for renewable energy and liquid fuels. Drying of the woody biomass is one of the key operations in development of the energy conversion technologies. Rotary drying is an effective method due to the enhanced contact between the solids and the drying medium (hot air). In this work, a mathematical model was developed to simulate the drying of the woody biomass as chips in a rotary dryer, based on energy and mass balance and transfer, experimental drying kinetics of the wood chips, and using literature correlations for the residence time. A new correlation between the theoretical maximum drying rate and the actual constant drying rate for the wood chips was obtained from the drying experiments, which was incorporated in the drying model. The model was applied both for cocurrent and countercurrent rotary dryers, and the simulation results are consistent with the observed trend. However, the accuracy of the model needs to be further investigated through experimental validation of the residence time correlation.     

Modeling of heat pump tumble dryer energy consumption and drying time

The use of heat pump tumble dryers is nowadays more common because they offer huge energy savings compared to conventional tumble dryers. Earlier studies made on conventional tumble dryers have shown that parameters such as heater power, fan speed, drum speed, weight and initial moisture content of textiles and air leakage have a huge impact on the energy efficiency and drying time. In the present study, a modified commercial heat pump tumble dryer was evaluated for energy consumption and drying time by changing operating parameters including fan speed, drum speed, and mass load. The total energy consumption and drying time were measured and corrected for the initial and final moisture content in the textiles. The experimental results based on 27 drying tests were evaluated to develop linear regression models for energy consumption and drying time, which show a good agreement with the experimental data. The results show that a large mass load, a high drum speed, and a low fan speed give the highest energy efficiency, i.e. the lowest energy consumption per kg of drying load. Larger loads extend the length of the drying cycle while higher fan and drum speeds result in shorter drying time.     

CONVENTIONAL BASIC DESIGN FOR CONVECTION OR CONDUCTION DRYERS

Abstract

The items to be considered prior to selection of dryers are explained, and a simple method for a rough estimation of dryer sizes was proposed based on data obtained from operating industrial dryers.

The equations of basic design for batch or continuous type dryers were derived. The heat was supplied to materials by convection and/or conduction. The equations were simplified to the case when the falling rate of drying is proportional to the moisture content of materials under the constant drying conditions. The heat transfer coefficient used in the equations can be determined based on the calculations or the data obtained from the experimental or industrial dryers. The equations are useful for estimating the scale-up effect of dryers.     

Application of Markov Chain Analysis and Tanks-in-Series Model in Mathematical Modeling of Impinging Stream Dryers

In spite of the fact that the principles of impinging stream reactors have been developed for more than half a century, the performance analysis of such devices, from a viewpoint of the mathematical modeling, has not been investigated extensively. In this study two models are proposed to describe the drying performance of particulate materials in two-impinging stream dryers. The models are developed based on the Markov chain analysis and the tanks-in-series model. The required parameters for each model are determined by using RTD data obtained in a two-impinging stream dryer and the governing equations are solved numerically. Comparison of the results of the models with available experimental data shows that the stirred tanks-in-series model successfully explains the drying behavior in impinging stream dryers. Nevertheless, the results of the model that is developed based on the Markov chain analysis are not in exact agreement with corresponding experimental data because of the extremely short residence time of the particles inside the dryer. Also, the effects of some operating parameters on the performance of such dryers are investigated. The results indicate that the drying efficiency of the dryer increases when solid-to-gas flow rate ratio, initial moisture content, and diameter of the particles decrease and when the temperature of the carrier gas increases.     

AUTOMATIC CONTROL OF COMMERCIAL CROSSFLOW GRAIN DRYERS

An unsteady state model of crossflow grain drying has been developed. The model has been used to develop several empirical models as process models for a feedforward dryer control system.

The control system consists of an empirical drying model, an on-line moisture meter, a tachometer, data acquisition software, a microcomputer, and the feedforward/feedback control software. It has been tested on several commercial crossflow maize (corn) dryers. With the inlet grain moisture content varying between 16.1% to 34.3% (w.b.), the new control system controlled the outlet grain moisture content to + 0.6% of the set point.     

AUTOMATIC CONTROL OF COMMERCIAL CROSSFLOW GRAIN DRYERS

ABSTRACT

An unsteady state model of crossflow grain drying has been developed. The model has been used to develop several empirical models as process models for a feedforward dryer control system.

The control system consists of an empirical drying model, an on-line moisture meter, a tachometer, data acquisition software, a microcomputer, and the feedforward/feedback control software. It has been tested on several commercial crossflow maize (corn) dryers. With the inlet grain moisture content varying between 16.1% to 34.3% (w.b.), the new control system controlled the outlet grain moisture content to + 0.6% of the set point.     

MODELING AND SIMULATION OF CROSSFLOW MOVING BED GRAIN DRYERS

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.     

Mathematical Model Based on DSMC Method for Particulate Drying in a Coaxial Impinging Stream Dryer

An impinging stream dryer (ISD) belongs to a unique class of dryers that has proved to be an excellent alternative to flash dryers for removing surface moisture of particulate materials due to the collision of streams and particles in the dryer. However, the performance analysis of such devices, from a viewpoint of mathematical modeling, has not been investigated extensively. In this study, a mathematical model based on the direct simulation Monte Carlo (DSMC) method is proposed to describe the drying process of particulate materials in a coaxial ISD. The collisions between particles and the heat exchange between impacting particles are included in the present mathematical model. The predicted results were in good agreement with the experimental data, which indicates the validity of the present model. The drying process and the effects of various parameters, including the feeding mode and impinging distance, on the drying performance of the dryer were then numerically investigated and discussed.     

SPRAY DRYING DYNAMIC MODELING WITH A MECHANISTIC MODEL

In this work we suggest the dynamic modeling of a spray dryer considered as a series of well-stirred dryers. That is, a series of dryers in which the output variables are equal to the state variables. The state equations were obtained from the heat and water mass balances in product and air. Additionally, heat and water mass balances in interface jointly with water equilibrium relation between product and air were considered. A pilot spray dryer was modeled assuming one, two, five and 20 well stirred steps. Low-fat milk with 10-20% of solids was dried at different inlet air temperatures (120-160°C), air flow rate of 0.19 kg dry air s^-1 and different feed rates (1.4 - 4.2 × 10^-4 kg dry solids s^-1). Stationary result showed that the model predicts the experimental air outlet temperature, at different inlet conditions with a maximum deviation of 6°C. The dynamic simulation reproduce the experimental one with moderate accuracy. Experimental dynamic showed that the pilot plant spray dryer has a well-stirred process behavior. The model represents a method for estimate outlet product moisture as function of the outlet air temperature. This has application for automatic control because there is not an easy way to measure on-line measure the outlet product moisture content.     

循环逆流式枸杞二次烘干机的设计

针对目前枸杞没有专用的二次烘干设备的现状,设计了循环逆流式枸杞二次烘干机。本设计根据枸杞干燥特性,以干燥计算理论为基础,参照带式干燥器原理,通过热气流和物料相互对流,并经过多层反复使气流与物料接触时间加长,对流强度增大,在输送带运转下达到连续干燥效果,从而提高干燥效率和生产效率。     

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.     

SPRAY DRYING DYNAMIC MODELING WITH A MECHANISTIC MODEL

ABSTRACT

In this work we suggest the dynamic modeling of a spray dryer considered as a series of well-stirred dryers. That is, a series of dryers in which the output variables are equal to the state variables. The state equations were obtained from the heat and water mass balances in product and air. Additionally, heat and water mass balances in interface jointly with water equilibrium relation between product and air were considered. A pilot spray dryer was modeled assuming one, two, five and 20 well stirred steps. Low-fat milk with 10–20% of solids was dried at different inlet air temperatures (120–160°C), air flow rate of 0.19 kg dry air s^?1 and different feed rates (1.4 ? 4.2 × 10^?4 kg dry solids s^?1). Stationary result showed that the model predicts the experimental air outlet temperature, at different inlet conditions with a maximum deviation of 6°C. The dynamic simulation reproduce the experimental one with moderate accuracy. Experimental dynamic showed that the pilot plant spray dryer has a well-stirred process behavior. The model represents a method for estimate outlet product moisture as function of the outlet air temperature. This has application for automatic control because there is not an easy way to measure on-line measure the outlet product moisture content.     

HYDRODYNAMICS OF VIBRO-FLUIDIZED BEDS

Vibro-fluidized bed dryers are being extensively used in the industry to dry granular particles of wide particle size distribution. For drying applications of limited air inlet temperature and hard to dry granules--high diffusion resistance of moisture inside the qranules--baffles are normally provided in these dryers to increase the residence time of particles. The residence time distribution of granules in vibro-fluidized bed dryers is of high importance to successfully model such dryers. There has been some studies reported in the literature for average residence time of granules in vibro-fluidized beds without baffles, but there has been no work reported for average residence time in vibro-fluidized beds with baffles and residence time distribution for beds with or without baffles. Experimental runs were carried out in this study in the 0.93 m2 (10 ft2 ) pilot plant vibro-fluidized bed dryer with baffles to determine the residence time distribution of dry granules and granule height profile through the length of the dryer.

Among all the dryer operating variables considered in this study, vibration amplitude and baffle spacing were identified as most significant. Values of the average residence time and particle diffusivity were obtained for the entire range of the expected operation of the pilot-scale or commercial scale dryers. These values of average residence time and particle diffusivity, along with the appropriate drying rate (kinetic) data, enable full modeling of vibro-fluidized bed dryers.     

HYDRODYNAMICS OF VIBRO-FLUIDIZED BEDS

Vibro-fluidized bed dryers are being extensively used in the industry to dry granular particles of wide particle size distribution. For drying applications of limited air inlet temperature and hard to dry granules--high diffusion resistance of moisture inside the qranules--baffles are normally provided in these dryers to increase the residence time of particles. The residence time distribution of granules in vibro-fluidized bed dryers is of high importance to successfully model such dryers. There has been some studies reported in the literature for average residence time of granules in vibro-fluidized beds without baffles, but there has been no work reported for average residence time in vibro-fluidized beds with baffles and residence time distribution for beds with or without baffles. Experimental runs were carried out in this study in the 0.93 m2 (10 ft2 ) pilot plant vibro-fluidized bed dryer with baffles to determine the residence time distribution of dry granules and granule height profile through the length of the dryer.

Among all the dryer operating variables considered in this study, vibration amplitude and baffle spacing were identified as most significant. Values of the average residence time and particle diffusivity were obtained for the entire range of the expected operation of the pilot-scale or commercial scale dryers. These values of average residence time and particle diffusivity, along with the appropriate drying rate (kinetic) data, enable full modeling of vibro-fluidized bed dryers.