HYDRODYNAMICS AND MASS TRANSFER IN A SPOUTED BED DRYER

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.     

Minimum spouting velocity of dense particles in shallow spouted beds

Quantitative method is used to experimentally measure the minimum spouting velocity in shallow conical spouted bed. And a new minimum spouting correlation for shallow conical spouted beds is developed. It is based on spherical ZrO₂ particles whose density is as high as 5890 kg/m³ while the other U_ms correlations published so far are mainly based on relatively deep conical beds composed of lower density particles with density around or lower than 3000 kg/m³. The new U_ms correlation can predict U_ms of heavy particles well within the range of the experimental matrix. © 2011 Canadian Society for Chemical Engineering     

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.     

Drying of industrial sludge waste in a conical spouted bed dryer. Effect of air temperature and air velocity

To determine the performance of a conical spouted bed dryer for the drying of sludge waste, an experimental study of drying in a spouted bed regime was performed under different experimental conditions. The drying performance was determined based on the time evolution of solid moisture content, and the influence of operating conditions (inlet air temperature, air flow rate, and bed mass) on the drying rate of sludge waste in spouted beds of a conical geometry was analyzed.     

DRYING OF PULP AND PAPER SLUDGE IN A PULSED FLUID BED DRYER

ABSTRACT

Hydrodynamics and drying kinetics for the pulp and paper primary sludge dried in a pulsed fluid bed dryer with relocated air stream are presented. Batch experiments have indicated that drying of disintegrated sludge to the required 12% moisture content takes place during the first drying period at practically constant material temperature close to the wet bulb temperature with respect to the inlet air conditions. Equations were developed for pressure drop, minimum pulsed-fluidization velocity, dynamic bed height, and volumetric mass transfer coefficient. Continuous experiments under drying conditions determined from the average residence time concept have confirmed that transportation of disintegrated sludge along the dryer follows the plug flow model.

     

Drying of Liquid Feedstocks in a Spout‐Fluid‐Bed with Draft‐Tube Submerged in Inert Solids: Hydrodynamics and Drying Performance

A spout‐fluid bed with draft tube submerged in a bed of polypropylene beads was used for drying maltodextrin solutions. The hydrodynamics of the dryer were studied by determining the annular air flow vertical profile at different spouting velocities, using an additional air flow rate through the annulus equivalent to 0.5 U_mf. The drying performance of the dryer was studied through the determination of several dryer response parameters (product moisture, evaporative capacity and volumetric evaporative capacity). These parameters were compared with those obtained in a conventional spouted bed with inert solids and a spray dryer.     

EXPERIMENTAL ANALYSIS OF A MECHANICALLY STIRRED SPOUTED BED DRYER

ABSTRACT

In order to increase the movement of the particles in the annular region of a conical spouted bed, a mechanical stirrer was incorporated. This led to a study of the fluid dynamics of the conical spouted bed of inert particles under different operating conditions: mass of inert particles, ratio of inert particles to suspension volumes, type of stirrer (2 designs) and stirrer speed.

The action of the stirrers resulted in increased movement of the particles, especially at low rpm values (≤ 90 rpm), with the loss of the annular region, and thus approaching the beds behavior to that of a fluidized bed. To characterize this novel type of spouted beds, the “minimum spouting flow” (Q_ms) parameter which is used usually was replaced by the “minimum pseudo-fluidizing flow” (Qmpr). The value of Q_mpf decreases when the agitation in the bed increases. A correlation for Q_mpf with the different operating conditions was developed.

Drying suspensions in the stirred spouted bed shows a considerable increase of drying capacity, over units without agitation.     

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.     

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

Abstract

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.     

EXPERIMENTAL RESEARCH ON MASS TRANSFER IN A CENTRIFUGAL FLUIDIZED BED DRYER

Abstract

The influences of bed thickness, particle diameter, rotating speed of the basket and operating gas velocity on the mass transfer between gas and particles in the centrifugal fluidized bed dryer (CFBD) were examined. The experimental correlation used to calculate the mass transfer coefficient in the centrifugal fluidized bed dryer was obtained.     

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.     

Determination of Minimum Spouting Velocities in Conical Spouted Beds

Minimum spouting velocities in conical spouted beds have been obtained from pressure drops versus the superficial gas velocity curves, based on both increasing and decreasing the superficial gas velocity. It has been shown that the minimum spouting velocity from decreasing the superficial gas velocity is lower than from increasing the superficial gas velocity in most cases. This phenomenon is similar to that in conventional spouted beds and different from the early works. The experimental results also showed that there isn't significant difference in the pressure drop and U_ms under identical operating conditions between semi‐circular and circular conical spouted beds, and the same U_ms can be obtained from absolute pressure drops at any position above the gas inlet. The U_ms is found to increase with increasing the cone angle and static bed height, as well as the gas inlet diameter to a less extent.     

Modeling of Moisture and Temperature Changes of Wheat during Drying in a Triangular Spouted Bed Dryer

A mathematical model of temperature and wheat moisture content distribution inside a triangular spouted bed dryer was developed. The model is based on analysis of heat and mass transfer inside the dryer. In addition to that, an empirical bulk density model has been developed for wheat and included in the drying simulation. A laboratory-scale triangular spouted bed (TSB) dryer was used to dry wheat grain to validate the model. The dryer was divided into three sections, namely spouting, downcomer, and fountain. A series of drying runs were conducted to record moisture and temperature profile. There were two distinct regions observed during wheat drying. A constant rate period was observed during the initial drying stage and the falling rate period took place at the later drying stage. Initial moisture content and operating drying temperature governed the timing of transition from constant rate period to falling rate period. The model can be used to accurately predict the moisture content of wheat during drying. The temperature prediction inside the TSB dryer was less accurate, especially at high temperatures due to heat losses in the experimental dryer. Further studies are needed to improve the accuracy of this model, especially with regard to the temperature prediction.     

Modeling of Moisture and Temperature Changes of Wheat during Drying in a Triangular Spouted Bed Dryer

A mathematical model of temperature and wheat moisture content distribution inside a triangular spouted bed dryer was developed. The model is based on analysis of heat and mass transfer inside the dryer. In addition to that, an empirical bulk density model has been developed for wheat and included in the drying simulation. A laboratory-scale triangular spouted bed (TSB) dryer was used to dry wheat grain to validate the model. The dryer was divided into three sections, namely spouting, downcomer, and fountain. A series of drying runs were conducted to record moisture and temperature profile. There were two distinct regions observed during wheat drying. A constant rate period was observed during the initial drying stage and the falling rate period took place at the later drying stage. Initial moisture content and operating drying temperature governed the timing of transition from constant rate period to falling rate period. The model can be used to accurately predict the moisture content of wheat during drying. The temperature prediction inside the TSB dryer was less accurate, especially at high temperatures due to heat losses in the experimental dryer. Further studies are needed to improve the accuracy of this model, especially with regard to the temperature prediction.     

A TWO-REGION MATHEMATICAL MODEL FOR BATCH DRYING OF GRAINS IN A TWO-DIMENSIONAL SPOUTED BED

A two-region non-equilibrium model of a two-dimensional spouted bed dryer has been developed. The model, based on heat and mass transfer interactions between the spout and downcomer regions, predicts the air and grain temperatures and moisture content throughout each elementary thin layer of grain in the spouted bed. The model was validated with the experimental results on batch drying of corn. Drying and heating processes are shown to occur in both the spout and downcomer regions. Overall, the agreement between experimental and simulated results is satisfactory.     

Mass transfer in liquid-fluidized and spouted beds of ion exchange resin at low Reynolds number

Rates of mass transfer from the liquid phase to small ion exchange resin particles (0.78 mm in mean diameter) in fluidized and spouted beds were studied experimentally. Dilute aqueous solution of sodium hydroxide was fed into the beds of strong cation exchange resin and the exit concentration of the solution was determined by conductivity measurement. In spouted beds, the initial conversion and K_l increased with bed height, but decreased with fluid flowrate. The model, applying material balance of the reactant and axisymmetric flow of fluid in the annulus of a spouted bed, predictions of the initial conversion in spouted beds are satisfactory. In fluidized beds, the obtained mass transfer coefficients were correlated and compared with other works.     

A TWO-REGION MATHEMATICAL MODEL FOR BATCH DRYING OF GRAINS IN A TWO-DIMENSIONAL SPOUTED BED

A two-region non-equilibrium model of a two-dimensional spouted bed dryer has been developed. The model, based on heat and mass transfer interactions between the spout and downcomer regions, predicts the air and grain temperatures and moisture content throughout each elementary thin layer of grain in the spouted bed. The model was validated with the experimental results on batch drying of corn. Drying and heating processes are shown to occur in both the spout and downcomer regions. Overall, the agreement between experimental and simulated results is satisfactory.     

DRYING OF PULP AND PAPER SLUDGE IN A PULSED FLUID BED DRYER

Hydrodynamics and drying kinetics for the pulp and paper primary sludge dried in a pulsed fluid bed dryer with relocated air stream are presented. Batch experiments have indicated that drying of disintegrated sludge to the required 12% moisture content takes place during the first drying period at practically constant material temperature close to the wet bulb temperature with respect to the inlet air conditions. Equations were developed for pressure drop, minimum pulsed-fluidization velocity, dynamic bed height, and volumetric mass transfer coefficient. Continuous experiments under drying conditions determined from the average residence time concept have confirmed that transportation of disintegrated sludge along the dryer follows the plug flow model.     

Evaluation Of Fluidized Versus Spouted Bed Drying Of Baker'S Yeast

ABSTRACT

Drying of baker's yeast (Saccharomyces cerevisiae) was examined experimentally in laboratory scale fluidized bed and spouted bed dryers. Pressedyeast samples (70% moisture content, wb) were prepared in granular form (0.8–1 mm diameter) and dried first in a fluidized bed to characterize their fluidizationand drying properties.

It was observed that high moisture yeast particles (up to 35% wb) belong to group C and D of Geldart's classification, especially for higher (>200 mm) bed heights making their fluidization very difficult. On the other hand, fluidization of pre-dried yeast particles with moisture content under 35%, wb (group B) was very easy. This characterization suggests a new concept for drying of baker's yeast i.e. spouted bed drying in the first stage and fluidized bed in the second stage or spouted bed drying for the entire process. The quality parameter examined during the drying process was viability of yeast cells at a standard moisture content (6–8%, wb).     

Industrial Trials of Paste Drying in Spouted Beds Under QDMC

Abstract

Spouted bed has been extensively investigated by many authors for years, but the issue of controlling its operation still remains in basic developing stages. The present work consists of the application of an advanced control strategy (QDMC—Quadratic Dynamic Matrix Control) to automate the operation of a large-scale spouted bed dryer. The work on control strategies for spouted bed was motivated by the lack of references on this subject and the encouraging results obtained by Corrêa et al. (Corrêa, N.A.; Freire, J.T.; Corrêa, R.G. Improving operability of spouted beds using a simple optimizing control structure. Brazilian Journal of Chemical Engineering 1999 16 (4), 359–368; Corrêa N.A. Corrêa, R.G.; Freire, J.T. Adaptive control of paste drying in spouted bed using the GPC algorithm. Brazilian Journal of Chemical Engineering 2000 17 (4–7), 639–648; Corrêa, N.A.; Corrêa, R.G.; Freire, J.T. Self-tuning control of egg drying in spouted bed using the GPC algorithm. Drying Technology 2002 20 (4), 813–828.) for a laboratory-scale apparatus. The industrial-scale spouted bed dryer has a height of 4 m and a diameter of 66 cm. The same configuration of data acquisition unit, sensors, and interface developed for the laboratory-scale unit was employed. The dryer is capable of processing up to 20 L/h of pasty material. The powder moisture content (Ys), inferred from measurements of temperature at the exhaustion of the bed bulk, and the powder production rate (Pr), measured on-line by an electronic balance, are the controlled variables. The manipulated variables are: the paste inflow rate (W), regulated by an automatic pump, and the electrical power supply for air heating (P), regulated by a thyristor. The bed pressure drop, the airflow rate, and the information from the environment are monitored by means of an interfaced microcomputer. Experimental runs of drying up to 400 kg/day of an Al₂O₃ 10% aqueous suspension were carried out. The results showed that the QDMC control is robust with respect to the main variables involved and is efficient in maintaining the operation of the bed under the constraints of the system. The major aim concerning the scale-up of spouted bed, which is the determination of the minimum amount of energy required to ensure spout stability, was accomplished by the constrained control strategy.