THE EFFECT OF TEMPERATURE ON FLUIDIZED BED BEHAVIOUR

Most published correlations for the minimum fluidizing gas velocity have been derived from tests under ambient conditions and increasing discrepancy is found in their application over wider ranges of operating conditions. Up to 1000°C the Ergun equation is reliable but it requires a knowledge of the particle shape factor and bed voidage for its application. Bed voidage is found to vary with temperature for laminar gas flow conditions.

Paralleling changes in gas flow conditions with operating temperature are changes in bed-to-surface heat transfer coefficients. There is a distinct transition from the interphase gas convective to the particle convective component of heat transfer being the dominant mechanism as the operating temperature increases and Re_mf reduces through 12,5 at Ar ∼ 26000. This is thought to be a consequence of change in bed bubbling behavior.     

THE EFFECT OF TEMPERATURE ON FLUIDIZED BED BEHAVIOUR

Most published correlations for the minimum fluidizing gas velocity have been derived from tests under ambient conditions and increasing discrepancy is found in their application over wider ranges of operating conditions. Up to 1000°C the Ergun equation is reliable but it requires a knowledge of the particle shape factor and bed voidage for its application. Bed voidage is found to vary with temperature for laminar gas flow conditions.

Paralleling changes in gas flow conditions with operating temperature are changes in bed-to-surface heat transfer coefficients. There is a distinct transition from the interphase gas convective to the particle convective component of heat transfer being the dominant mechanism as the operating temperature increases and Re_mf reduces through 12,5 at Ar ～ 26000. This is thought to be a consequence of change in bed bubbling behavior.     

OPTIMAL DESIGN OF WELL-MLXED FLUIDIZED BED DRYERS

In a recent article, Baker described a novel technique for estimating the energy consumption of well-mixed fluidized bed dryers based on the use of experimental drying curves. An integrated approach to performing sizing and energy consumption calculations for such dryers using this technique is described in the present paper. A computer program, which includes not only the dryer simulator but also a heat recovery module and an exhaust air recycle option, is used to evaluate the viability of different energy-conservation strategies. The effects of operating parameters such as bed temperature, solids loading and air velocity on energy consumption and bed area are also explored. The observed drying kinetics are shown to have a major effect on the optimal design and operation of the dryer.     

OPTIMAL DESIGN OF WELL-MLXED FLUIDIZED BED DRYERS

ABSTRACT

In a recent article, Baker described a novel technique for estimating the energy consumption of well-mixed fluidized bed dryers based on the use of experimental drying curves. An integrated approach to performing sizing and energy consumption calculations for such dryers using this technique is described in the present paper. A computer program, which includes not only the dryer simulator but also a heat recovery module and an exhaust air recycle option, is used to evaluate the viability of different energy-conservation strategies. The effects of operating parameters such as bed temperature, solids loading and air velocity on energy consumption and bed area are also explored. The observed drying kinetics are shown to have a major effect on the optimal design and operation of the dryer.     

PRODUCT QUALITY MULTI-OBJECTIVE OPTIMIZATION OF FLUIDIZED BED DRYERS

Design of fluidized bed dryers constitutes a mathematical programming problem involving the evaluation of appropriate structural and operational process variables so that total annual plant cost involved is optimized. The increasing need for dehydrated products of the highest quality, imposes the development of new criteria that, together with cost, determine the design rules for drying processes. Quality of dehydrated products is a complex resultant of properties characterizing the final products, where the most important one is color. Color is determined as a three-parameter resultant, whose values for products undergone drying should deviate from the corresponding ones of natural products, as little as possible. In this case, product quality dryer design is a complex multi-objective optimization problem, involving the color deviation vector as an objective function and as constraints the ones deriving from the process mathematical model. The mathematical model of the dryer was developed and the fundamental color deterioration laws were determined for the drying process. Non-preference multi-criteria optimization methods were used and the Pareto-optimal set of efficient solutions was evaluated. An example covering the drying of sliced potato was included to demonstrate the performance of the design procedure, as well as the effectiveness of the proposed approach.     

PRODUCT QUALITY MULTI-OBJECTIVE OPTIMIZATION OF FLUIDIZED BED DRYERS

ABSTRACT

Design of fluidized bed dryers constitutes a mathematical programming problem involving the evaluation of appropriate structural and operational process variables so that total annual plant cost involved is optimized. The increasing need for dehydrated products of the highest quality, imposes the development of new criteria that, together with cost, determine the design rules for drying processes. Quality of dehydrated products is a complex resultant of properties characterizing the final products, where the most important one is color. Color is determined as a three-parameter resultant, whose values for products undergone drying should deviate from the corresponding ones of natural products, as little as possible. In this case, product quality dryer design is a complex multi-objective optimization problem, involving the color deviation vector as an objective function and as constraints the ones deriving from the process mathematical model. The mathematical model of the dryer was developed and the fundamental color deterioration laws were determined for the drying process. Non-preference multi-criteria optimization methods were used and the Pareto-optimal set of efficient solutions was evaluated. An example covering the drying of sliced potato was included to demonstrate the performance of the design procedure, as well as the effectiveness of the proposed approach.     

EFFECT OF PARTICLE SIZE DISTRIBUTION ON LOCAL VOIDAGE IN A BENCH-SCALE CONICAL FLUIDIZED BED DRYER

ABSTRACT

The effect of particle size distribution (PSD) on local voidage has been investigated in a conical fluidized bed containing dried placebo pharmaceutical granule. For each of the five PSDs examined, the static bed height was varied between 0.12 and 0.17 m and the superficial gas velocity was varied between 0.05 and 0.75 m/s. The local voidage was measured using a twin-plane electrical capacitance tomography (ECT) system. A wide PSD containing 12 wt% solids with a diameter of 2 mm or larger resulted in two different types of gas flow: an annular gas flow up to a gas velocity of 0.50 m/s and a centrally concentrated gas flow above 0.50 m/s. The mixtures containing less coarse material exhibited a centrally concentrated gas flow surrounded by a dense phase at the walls of the bed over the entire range of gas velocities and bed heights examined. Consideration of previous work by other researchers suggests that the behavior of the wide PSD mixture is due to segregation at the lower velocities. The local voidage was sensitive to small changes in static bed height. For the wide PSD mixture at a fixed gas velocity, the gas tended to spread more uniformly over the bed cross-section as static bed height increased. The opposite was true of the other mixtures, i.e., the gas flow became more centralized with increasing bed height.     

EFFECT OF PARTICLE SIZE DISTRIBUTION ON LOCAL VOIDAGE IN A BENCH-SCALE CONICAL FLUIDIZED BED DRYER

The effect of particle size distribution (PSD) on local voidage has been investigated in a conical fluidized bed containing dried placebo pharmaceutical granule. For each of the five PSDs examined, the static bed height was varied between 0.12 and 0.17 m and the superficial gas velocity was varied between 0.05 and 0.75 m/s. The local voidage was measured using a twin-plane electrical capacitance tomography (ECT) system. A wide PSD containing 12 wt% solids with a diameter of 2 mm or larger resulted in two different types of gas flow: an annular gas flow up to a gas velocity of 0.50 m/s and a centrally concentrated gas flow above 0.50 m/s. The mixtures containing less coarse material exhibited a centrally concentrated gas flow surrounded by a dense phase at the walls of the bed over the entire range of gas velocities and bed heights examined. Consideration of previous work by other researchers suggests that the behavior of the wide PSD mixture is due to segregation at the lower velocities. The local voidage was sensitive to small changes in static bed height. For the wide PSD mixture at a fixed gas velocity, the gas tended to spread more uniformly over the bed cross-section as static bed height increased. The opposite was true of the other mixtures, i.e., the gas flow became more centralized with increasing bed height.     

ADVANCED CONTROL STRATEGIES FOR FLUIDIZED BED DRYERS

ABSTRACT

Generating the best possible control strategy comprises a necessity for industrial processes, by virtue of product quality, cost reduction and design simplicity. Three different control approaches, namely an Input-Output linearizing, a fuzzy logic and a PID controller, are evaluated for the control of a fluidized bed dryer, a typical non-linear drying process of wide applicability. Based on several closed loop characteristics such as settling times, maximum overshoots and dynamic performance criteria such as IAE, ISE and ITAE, it is shown that the Input-Ouput linearizing and the fuzzy logic controller exhibit a better performance compared to the PID controller tuned optimally with respect to the IAE, for a wide range of disturbances; yet, the relevant advantage of the fuzzy logic over the conventional nonlinear controller issues upon its design simplicity. Typical load rejection and set-point tracking examples are given to illustrate the effectiveness of the proposed approach.     

ADVANCED CONTROL STRATEGIES FOR FLUIDIZED BED DRYERS

Generating the best possible control strategy comprises a necessity for industrial processes, by virtue of product quality, cost reduction and design simplicity. Three different control approaches, namely an Input-Output linearizing, a fuzzy logic and a PID controller, are evaluated for the control of a fluidized bed dryer, a typical non-linear drying process of wide applicability. Based on several closed loop characteristics such as settling times, maximum overshoots and dynamic performance criteria such as IAE, ISE and ITAE, it is shown that the Input-Ouput linearizing and the fuzzy logic controller exhibit a better performance compared to the PID controller tuned optimally with respect to the IAE, for a wide range of disturbances; yet, the relevant advantage of the fuzzy logic over the conventional nonlinear controller issues upon its design simplicity. Typical load rejection and set-point tracking examples are given to illustrate the effectiveness of the proposed approach.     

DYNAMIC ANALYSIS OF CONTINUOUS WELL-MIXED FLUIDIZED/SPOUTED BED DRYERS

A diffusion-based mathematical model is presented for the prediction of the dynamics of drying in continuous well-mixed fluidized/spouted beds. Numerical techniques are used to solve the model equations. The outlet solids moisture content, the outlet air humidity and solids temperature are predicted as a function of time for the falling rate drying period. The model is helpful in describing the drying process during the startup periods and in studying open loop behavior of drying process. The model is also useful in designing control system for fluidized/spouted bed dryers.     

MULTIVARIABLE PROCESS IDENTIFICATION AND CONTROL OF CONTINUOUS FLUIDIZED BED DRYERS

ABSTRACT

Dynamic models that rigorously describe fluidized bed dryers based on the fundamental principles of the process are usually so complex to be employed in control system design. To obtain simple reduced-order models for such systems, a sequence of step changes in the manipulated and load variables is introduced into the rigorous model. The obtained input–output dynamic response data are used for off-line model identification. Different types of linear models are generated, which are shown to be adequately representing the fluidized bed drying dynamics. The derived models are useful to develop model-based control algorithms such as Internal Model Control (IMC) and Model Predictive Control (MPC). Performance and robustness properties of these controllers are analyzed. Simulation results demonstrate a good performance in terms of tracking and load rejection capabilities.     

MULTIVARIABLE PROCESS IDENTIFICATION AND CONTROL OF CONTINUOUS FLUIDIZED BED DRYERS

Dynamic models that rigorously describe fluidized bed dryers based on the fundamental principles of the process are usually so complex to be employed in control system design. To obtain simple reduced-order models for such systems, a sequence of step changes in the manipulated and load variables is introduced into the rigorous model. The obtained input-output dynamic response data are used for off-line model identification. Different types of linear models are generated, which are shown to be adequately representing the fluidized bed drying dynamics. The derived models are useful to develop model-based control algorithms such as Internal Model Control (IMC) and Model Predictive Control (MPC). Performance and robustness properties of these controllers are analyzed. Simulation results demonstrate a good performance in terms of tracking and load rejection capabilities.     

EFFECT OF FLUIDIZED BED DRYING ON STABILIZATION OF RICE BRAN

ABSTRACT

Rice bran was treated in a batch fluidized bed at temperatures starting from 90°C up to 130°C in steps of 10°C for different treatment periods. Drying characteristics of rice bran so obtained were used in a mathematical model to predict the maximum possible storage period of rice bran treated under fluidizing conditions. It was observed that the fluidized bed treatment of rice bran requires much lower treatment times for stabilization of rice bran compared with packed bed processes and therefore could be very attractive in commercial applications.     

EFFECT OF FLUIDIZED BED DRYING ON STABILIZATION OF RICE BRAN

Rice bran was treated in a batch fluidized bed at temperatures starting from 90°C up to 130°C in steps of 10°C for different treatment periods. Drying characteristics of rice bran so obtained were used in a mathematical model to predict the maximum possible storage period of rice bran treated under fluidizing conditions. It was observed that the fluidized bed treatment of rice bran requires much lower treatment times for stabilization of rice bran compared with packed bed processes and therefore could be very attractive in commercial applications.     

THE DESIGN AND PERFORMANCE OF CONTINUOUS WELL-MIXED FLUIDIZED BED DRYERS - AN ANALYTICAL APPROACH

Abstract

In a previous study, a unified approach to the calculation of the size and energy consumption of well-mixed fluidized bed dryers was developed. A computer code was written and a parametric study undertaken to determine the effects of the principal operating variables. The results were shown to depend on the form of the drying kinetics. The present paper describes the formulation of an analytical model of a well-mixed fluidized bed dryer, which applies for materials that exhibit linear falling-rate drying. These materials are typified by small hygroscopic particles, which are frequently dried in such equipment. Equations are presented that enable the drying time, specific energy consumption, heater duty and bed area to be calculated. The predictions of these equations agreed well with previously published experimental findings and with the results of computer simulations for a typical solid (ion exchange resin) exhibiting the required drying characteristics.     

PREDICTING THE ENERGY CONSUMPTION OF CONTINUOUS WELL-MIXED FLUIDIZED BED DRYERS FROM DRYING KINETIC DATA

ABSTRACT

Previous work has shown that it is possible to predict the size of a continuous welt-mixed fluidized bed dryer from batch drying curve measurements. This approach has been extended in the present study to include energy consumption calculations. A computer code was written to simulate the performance of the dryer and to determine its specific energy consumption E_s. Starting in this case with an isothermal bed batch drying curve, the program first calculates the mean solids residence time required under specified operating conditions. Mass and energy balances are then used to calculate the heat duty and E_s. The bed temperature was found to have a significant effect on specific energy consumption in all cases. However, the influences of air flowrate and humidity, and of solids loading, were shown to depend on the solids drying characteristics.     

EFFECT OF DISTRIBUTOR ON BUBBLE SIZE IN A CYLINDRICAL FLUIDIZED BED AT AN ELEVATED TEMPERATURE†

The effects of temperature and distributor on bubble diameter were investigated using a cylindrical fluidized bed of 147 mm in diameter. Three perforated distributors having different holes in diameter and the same ratio of holes to bed area were used. Eruption diameters of bubbles were measured using a high speed video-camera system under the following conditions: bed temperature = 300 and 600 K, bed particles = spherical glass beads of 272 μm in average size, excess gas velocity = 1-4 cm/s, and static bed height equals; 10-42 cm. The bubble diameter at 600 K was larger than that at 300 K. The difference became smaller with increasing the static bed height and with increasing the excess gas velocity. The distributor with larger holes gave larger bubbles. The effect of hole diameter of the distributor on the bubble diameter became insignificant with increasing the static bed height and with increasing the excess gas velocity.     

THE DESIGN AND PERFORMANCE OF CONTINUOUS WELL-MIXED FLUIDIZED BED DRYERS - AN ANALYTICAL APPROACH

In a previous study, a unified approach to the calculation of the size and energy consumption of well-mixed fluidized bed dryers was developed. A computer code was written and a parametric study undertaken to determine the effects of the principal operating variables. The results were shown to depend on the form of the drying kinetics. The present paper describes the formulation of an analytical model of a well-mixed fluidized bed dryer, which applies for materials that exhibit linear falling-rate drying. These materials are typified by small hygroscopic particles, which are frequently dried in such equipment. Equations are presented that enable the drying time, specific energy consumption, heater duty and bed area to be calculated. The predictions of these equations agreed well with previously published experimental findings and with the results of computer simulations for a typical solid (ion exchange resin) exhibiting the required drying characteristics.     

ＣＦＢ稀相段直长对颗粒内循环流动规律的影响

采用光纤探头测定Ｄ１m射流循环流化床（半圆形）稀相区颗粒速度、颗粒浓度和颗粒流通量的径向分布规律,并考察了操作条件及其轴向的影响,实验结果表明,在循环流化床（ＣＦＢ）中,颗粒流动在床层径向有较大的不均匀性,并呈明显的内循环流动结构,进而考察了影响其流动规律的,以期进一步理解循环流化床颗料流动机理,强化和改善反应器的设计操作。