The Gas-to-Particle Heat Transfer and Hydrodynamics in Spouted Bed Drying of Sawdust

This article presents experimental results for spouted bed drying of sawdust, carried out in a full-scale as well as in a laboratory-scale dryer using air as well as steam as drying media. The aim is to present design parameters for a spouted-bed sawdust dryer that can be used by the industry in designing full-scale dryers. A hydrodynamically stable spouted jet spouted bed was obtained. The heat transfer characteristics of the bed were represented in terms of a volumetric heat transfer coefficient (VHC). When sawdust is dried in a spouted bed, the mean VHC is increasing up to fiber saturation level (20-25% wb) from 40 to 110 W/m³ K. The VHC decreases with the residence time and with an increased static bed height. Gas temperature profiles are also presented for the bottom part of the drying chamber.     

A MATHEMATICAL MODEL FOR CONTINUOUS DRYING OF GRAINS IN A SPOUTED BED DRYER

ABSTRACT

A mathematical model for a continuous spouted bed dryer has been presented to predict moisture content, air and grain temperatures as well as energy consumption. To better understand the interactive influence of processes in each region of the spouted bed, solution schemes for the spout and downcomer were treated separately. The behavior of dryer was investigated experimentally and found that the dryer behaved differently from an ideal plug flow. The drying rate as simulated by the model is almost constant during grain movement in the dryer. Absence of airflow in the downcomer leads to a tempering process that takes place in the downcomer while intense heat and mass transfer occurs mainly in the spout due to the high airflow rate there. Furthermore, by considering the predicted grain temperature history as one of the indicators of product quality, one can, in principle, design appropriate successive processes in a continuous spouted bed dryer to minimize product damage.     

HEAT AND MASS TRANSFER DURING DRYING OF A LIQUID FILM FROM THE SURFACE OF A SINGLE INERT PARTICLE

ABSTRACT

This paper presents the results of theoretical and experimental studies on drying of aqueous suspensions of finely dispersed solids sprayed over the surface of an inert ceramic sphere. The effects of temperature and air velocity on kinetics of heat and mass transfer as well as peeling off the layer of a dry material from the sphere surface are described. The mathematical model of a drying process based on simplified ?gradientless? approach to transfer phenomena is proposed. The adequacy of the model developed for drying of the wet coat from a single sphere to the real drying process taking place in a bed of particulate carrier is confirmed by results of drying of several organic dyestuffs in an industrial spouted bed dryer with inert particles.     

使用导流管对连续喷动干燥器的影响

在欧洲市场,对使用诸如团块状的生物质燃料的关注日益增长。本文给出了使用带有不同类型导流管的连续喷动干燥锯末的实验数据。对不同进口和出口温度下湿锯末干燥进行了研究。在喷动干燥器中,干燥能力取决于热流量大小。实验结果表明,导流管能够提高干燥能力。此外,导流管的设计对热流量利用有影响。     

Drying Of Pastes In Spouted Beds Of Inert Particles: Design Criteria And Modeling

ABSTRACT

The drying of pastelike materials can be performed well in spouted beds (SB) of inert particles. In this work the drying performance of pastes in conical pastes in conical spouted beds is analyzed as a function of column dimensions, fluid flow characteristics and paste properties. imulated data on fluid flow together with the experimental results on drying of different pastelike materials are presented and discussed to provide criteria for the design of a conical spouted bed dryer for suspensions.     

Drying Process on Inert Particles in Mechanically Spouted Bed Dryer

ABSTRACT

Suspensions, slurries and paste-like materials can be dried in the Mechanically Spouted Bed ( MSB) dryer with men packing. The circulation of the men particles characteristic of classical spouted beds is provided with a houseless conveyor screw mounted in the vertical axis of the bed. Radioactive isotopic tracer technique was used for measurement of the cycle time distribution ( CTD) of the spherical inert particles as a function of the operational parameters of drying. The variances ( σ²) of the CTDs and the particle velocity m the various zones of the MSB dryer were calculated. The circulation of the mert particles can be characterised by nearly plug flow. According to the physical model of drying on inert packing the heat and mass transfer coefficients were calculated. Que to the relativeiy uniform film-like, wet coating formed on the surface of the spheical inert particles, the drying process may be characterised with the constant rate of drying. A method has been elaborated for calculation of drying time, hereby the partial processes of drying on inert particles can be synchronised.     

A MATHEMATICAL MODEL FOR CONTINUOUS DRYING OF GRAINS IN A SPOUTED BED DRYER

A mathematical model for a continuous spouted bed dryer has been presented to predict moisture content, air and grain temperatures as well as energy consumption. To better understand the interactive influence of processes in each region of the spouted bed, solution schemes for the spout and downcomer were treated separately. The behavior of dryer was investigated experimentally and found that the dryer behaved differently from an ideal plug flow. The drying rate as simulated by the model is almost constant during grain movement in the dryer. Absence of airflow in the downcomer leads to a tempering process that takes place in the downcomer while intense heat and mass transfer occurs mainly in the spout due to the high airflow rate there. Furthermore, by considering the predicted grain temperature history as one of the indicators of product quality, one can, in principle, design appropriate successive processes in a continuous spouted bed dryer to minimize product damage.     

Experimental Study of the Heat and Mass Transfer During Drying in a Fluidized Bed Dryer

ABSTRACT

Drying curves obtained in a pilot-scale fluidized bed dryer using biological source solids (sawdust, soya and fish meal) were used to estimate the parameters involved in heat and mass transfer phenomenas: heat transfer coefficient and moisture diffusivity coefficient. Parameters involved in mass transfer were estimated from drying models based on diffusional mechanisms and others that in addition consider internal and external resistance to the mass transfer. The estimate ef ective diffusivity coefficient was between 2x10-11 to lx10 (m2/s) for the considered products. Heat transfer coefficient was estimated from drying data points in the constant drying rate period when the external resistance to the mass transfer controls the process.     

MODELING AND SIMULATION OF AIRFLOW IN SPOUTED BED DRYERS

ABSTRACT

This paper presents an axisymmetric three-dimensional semi-empirical model that describes the airflow in both the spout and annular regions. Both the pressure and air velocity distributions can be calculated at any point inside the dryer. Thus, the effect of the fluid dynamics on the heat and mass transfer coefficients and, consequently, on the drying process, can be evaluated. The finite element method was used to solve the model. Pressure distributions were measured for several conditions in which stable spouted bed flow regime was attained. Simulated pressure distributions agreed well with experimental data.     

HYDRODYNAMICS AND MASS TRANSFER IN A SPOUTED BED DRYER

ABSTRACT

The disposal of sludge generated by water treatment plants poses major financial and environmental problems. Drying of the sludge is an essential pan of any disposal process which may include incinerating, landfilling or upgrading. In the present study, experiments were carried out to investigate the drying of sludge in a spouted bed and to characterize the hydrodynamics and mass transfer mechanisms. The effect of bed moisture content on the minimum spouting velocity (U_ms) was examined for sludge granules. U_ms was found to increase with increasing the moisture content of the sludge particles. The gas phase mass transfer coefficient in the dryer was determined using porous calcined alumina particles and the results were compared with predictions by correlations available in the literature. The existing correlations gave poor predictions for the mass transfer coefficient. A new correlation for the mass transfer coefficient in a spouted bed dryer was developed based on the experimental results.     

SELF-TUNING CONTROL OF EGG DRYING IN SPOUTED BED USING THE GPC ALGORITHM

ABSTRACT

A spouted bed is used to dry many and different materials ranging from granules to pastes and suspensions. This work presents an implementation and tests of an advanced control strategy on a spouted bed dryer. Water was used as the ideal paste for tests. An adaptive control algorithm GPC (Generalized Predictive Control) was implemented. A microcomputer was used to maintain humidity and temperature set points of air in the dryer by manipulating electric power of heat exchanger and paste feed flow rate. Instrumentation was set-up with different sensors, interface and final control elements for operation. Stability and performance analysis of the control strategy was accomplished. A robust stable controller was obtained which had also encouraged us to extend the investigation of this control strategy to the drying of other pastes.     

SCALE-UP OF SPOUTED BED DRYERS: CRITERIA AND APPLICATIONS

Abstract

The criteria required for reliable scale-up of the aerodynamics of spouted bed dryers of grains are stated and discussed. The dryer modeling role for effective scale-up, design and analyses of such dryers will be outlined. Suggestions are made on how small scale drying and aerodynamic data may be scaled up for spouted bed dryers. Appropriate empirical and semi-empirical correlations are listed. A flowchart is provided for design and analysis of a conventional and a two-dimensional spouted bed dryers.     

Fundamental Research on Drying Process in Institute of Engineering Thermophysics,CAS

Abstract

Some results of our fundamental research on drying processes are summed up in this article. It consists of three parts: (1) Multistage fluidized bed drying, including particle flowing characteristics, heat and mass transfer between particles and drying medium, drying characteristics of drying materials; (2) Impinging stream drying, the flowing and drying characteristics of a vertical impinging stream dryer, one-stage and multistage semi-circular impinging stream dryer and combined vertical and semi-circular impinging stream dryer are discussed; (3) The effects of rapid transient heat and mass transfer on drying processes, such as time and space scales for nonFourier or nonFickian and influence of extreme heat and mass transfer are also discussed.     

An Experimental Study on the Influence of Using a Draft Tube in a Continuous Spouted Bed Dryer

Further increasing the production of processed biofuel also increases the demands on drying capacity. With the aim of increasing the heat capacity flow, experimental tests have been performed on the process of drying sawdust in a continuous spouted bed dryer with nine different draft tube designs. The results showed that a draft tube with an increased length and an increased disengagement height decreased the dry substances’ flow rate throughout the dryer. The results also showed that the mass of the material in the dryer was approximately the same in all the tests. This means that the draft tubes, no matter their size, do not influence the amount of material in the dryer.     

SPOUTED AND SPOUT-FLUIDIZED BEDS FOR GRAM DRYING

The objectives of this work are to analyze the drying performance of conical-cylindrical spouted bed (CSB) dryers for three different grains (rice, corn and wheat), and to compare the drying efficiency of CSB dryers with that of spout-fluid bed (SFB) dryers. A PC-program was developed for: (I) -optimization of the CSB dryer dimensions; (2) -simulation of drying grains in the optimized CSB dryer (including start-up period); and, (3) -analysis of the drying performance in a similar SFB dryer.

The liquid diffusion model is used to describe the falling rate drying period. Semi-empirical correlations available in the literature as well as information obtained in the authors' laboratory for spouted and spout-fluidized beds of grains are used to describe the aerodynamic parameters.

The results are presented in terms of the size of the dryer, energy consumption, air handling requirement, drying characteristics etc for different drying conditions. The drying effeciency in a CSB is compared with that in a similar SFB for different grain feed rates and drying temperatures.     

BEAN DRYING IN FIXED,SPOUTED AND SPOUT-FLUID BEDS: A COMPARISON AND EMPIRICAL MODELING

ABSTRACT

In this work the drying of IAC-Carioca bean was analyzed experimentally in fixed, spouted and spout-fluid beds. A laboratory scale, batch operated apparatus was used. To obtain experimental data, a factorial design technique was used and the influences of the following variables on the process were evaluated: the solids toad, initial grain moisture and drying air temperature. This analysis was done for each dryer by means of drying curves, evolution of grain temperature over time, drying rate and statistical results obtained from factorial design. An energy evaluation was also done and used as a means of comparison between the dryers studied. The three dryers operating under the same conditions showed very similar behavior and the fixed bed dryer showed a performance slightly better. With this work the potential of using spouted bed grain dryerswas confirmed and the possibility of using the spout-fluid bed as a dryer was verified. Two empirical models were proposed and their parameters were correlated with experimental variables. A sensory evaluation was also performed for the purpose of evaluating the effects that the high drying temperatures had on the grain's potential value as food.     

Drying characteristics of a draft tube spouted bed

Using the results of earlier work on the flow pattern of gas and solids in a draft-tube spouted bed, the drying characteristics of such a spouted bed are discussed. Paddy (unpeeled rice) was dried in a 30 cm bed with a 5.5 cm draft tube and temperatures and moisture content of solid and gas phase were measured as a function of drying time at various points in the system. It appears that the (constant) drying rate is determined by the heat transfer in the recirculation zone (just above the air inlet) and in the draft tube. After the short and rapid heating in the tube, the temperature and moisture gradients in the particle equilibrate in the annulus. It is concluded that a draft-tube spouted bed is a promising dryer design for heat-sensitive particles with slow intra-particle mass transfer coefficients.     

Drying Sawdust in a Pulsed Fluidized Bed

The objective of this work was the experimental and theoretical study of sawdust drying, in batch and continuous experiences, using a pulsed fluidized bed dryer.

In the batch experiences, a 2³ factorial design was used to determine the kinetics of drying, the critical moisture content, and the effective coefficients of both diffusivity and heat transfer, all of them as a function of the velocity and temperature of the air, the speed of turning of the slotted plate that generates the air pulses in the dryer, using sawdust with 65% moisture in each run.

In the continuous operation, a 2³ factorial design was used to study the effect of the solid flow and the velocity and temperature of the air on both the product moisture and the distribution of residence times. In order to determine these last ones, digital image processing was used, utilizing sawdust colored by a solution of methylene blue as tracer.

The statistically significant factors were the velocity and the temperature of the heating air, for both the continuous and batch operations. Although the speed of turn of the slotted plate was not significant, it was observed that the air pulses increased the movement of particles, facilitating its fluidization, especially at the beginning of drying.

The heat transfer coefficients were adjusted according to the equation Nu = 0.0014 Re _p ^1.52, whose standard deviation of fit is 0.145.

The period of decreasing rate was adjusted to several diffusivity models, giving the best fit the simplified variable diffusivity model (SVDM). The curve of distribution of residence times was adjusted using the model of tanks in series, with values between 2.6 and 5 tanks.     

BATCH VIBRATING FLUID BED DRYER FOR SAWDUST PARTICLES: EXPERIMENTAL RESULTS

ABSTRACT

In a batch experimental equipment, the behavior of a sawdust dryer in a vibrating fluidized bed is analyzed. Empirical data concerning fluidization velocities, pressure drops and drying kinetics was obtained, and advantages of using vibration in the drying chamber, relative to a conventional fluidized bed, are shown. This technique is presented as an alternative to solve problems of solid agglomeration and bed defluidization. Results show that it is possible to dry sawdust with more than 2 kg/kg moisture, in a vibrating bed keeping a high degree of bed homogeneity and high quality of fluidized state.     

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

ABSTRACT

The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.