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

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.     

INSTRUCTIONS FOR PREPARATION OF MANUSCRIPTS FOR DIRECT REPRODUCTION (REVISED, 1992)

ABSTRACT

A software for simulating the drying operation of diluted suspensions in conical spouted beds of inert particles has been developed. This computer program combines the fluid flow and the solids circulation models with the drying kinetic equations. The simulated results for drying animal blood in a spouted bed of inert particles compare with data reported in the literature.     

Spouted Beds of Inert Particles for Drying Suspension

ABSTRACT

A brief review of Brazilian research on drying process in spouted beds shows its relevance and evolution trends. Needs for applying this technique to dry pasty materials are discussed, focusing on a new model approach in order to overcome operational problems with particle agglomeration and with thermal efficiency. Based on this approach, two cases, drying of black liquor in a conical spouted bed of inert polypropylene particles under intermittent operation and drying of egg emulsion in a conical spouted bed of inert glass beads under continuous operation, are presented and analyzed.     

Simulation Of Drying Suspension In A Conical Spouted Bed

A software for simulating the drying operation of diluted suspensions in conical spouted beds of inert particles has been developed. This computer program combines the fluid flow and the solids circulation models with the drying kinetic equations. The simulated results for drying animal blood in a spouted bed of inert particles compare with data reported in the literature.     

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 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.     

Spouted Beds of Inert Particles for Drying Suspension

A brief review of Brazilian research on drying process in spouted beds shows its relevance and evolution trends. Needs for applying this technique to dry pasty materials are discussed, focusing on a new model approach in order to overcome operational problems with particle agglomeration and with thermal efficiency. Based on this approach, two cases, drying of black liquor in a conical spouted bed of inert polypropylene particles under intermittent operation and drying of egg emulsion in a conical spouted bed of inert glass beads under continuous operation, are presented and analyzed.     

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.     

SIMULATION OF DRYING SUSPENSIONS IN SPOUT-FLUID BEDS OF INERT PARTICLES

In spouted and spout fluid bed dryers, the suspension is spread into the bed of inert particles, covering these particles with a thin layer. As the inert particles circulate, this suspension layer is dried and must become brittle enough to break off by the particle attrition. The powder produced is then carried out by air. Problems with the spout stability, particle agglomeration and powder deposits inside the column should be overcome by controlling the drying operation. The present work is aimed at modeling and simulating the drying of suspensions, such as organic and biological pastes, in conical spout-fluid beds of inert particles. A computer program has been developed combining the air flow and particle circulation models with the mass and energy balances and the drying kinetic equations in order to describe this drying process. The effect of cohesive forces is also incorporated to the fluid flow model. Simulation results are analyzed and compared with experimental data reported in the literature. Implication of these results in drying suspensions is also discussed.     

Drying of Pharmaceuticals: The Applicability of Spouted Beds

ABSTRACT

This study evaluates the performance of conventional spouted beds (CSB) with inert bodies in the drying of pharmaceutical pastes, based on the dry product quality. The powder obtained by drying annatto extracts shows properties comparable to one of the commercial products, indicating that the spouting is competitive with other industrial and lab-scale processes. The contents of bixin and caffeine in annatto and guarana dry powders demonstrated a strong dependence on the inlet air temperature. Also, the properties of indomethacin-containing microcapsules dried in CSB and spray dryers revealed that spouting is a promising technique to obtain pharmaceutical forms with unique drug release qualities.     

RADIO FREQUENCY ENHANCED DRAUGHT TUBE SPOUTED BED DRYER

ABSTRACT

A study of the operation of a draught tube spouted bed in conjunction with dielectric heat addition was carried out. The addition of radio frequency allowed a greater amount of energy to be input into the particulate material in any given time, increasing the drying rate: however, the drying pattern was unaffected by the ratio of radio frequency to convective heat addition. The rate of moisture loss was proportional only to the total energy input. The synergy which often occurs with low levels of radio frequency addition does not occur with rf enhanced spouted beds.

The properties of spouted beds seem to be improved only for moisture contents at which packed bed drying can perform effectively.

Although the properties of spouted beds can be improved by radio frequency heat addition, their qualities although useful for specific operations, are unsuitable for more general use.     

Moisture Prediction During Paste Drying in a Spouted Bed

The objective of this work was to derive and experimentally verify a hybrid CST/neural network model to determine the moisture content of the powders produced during paste drying in a spouted bed and describe the highly coupled heat and the mass transfer. The model was derived from overall energy and mass balances with effective drying kinetics given by a neural network. Simulations were performed in MatLab and drying experiments for model verification were carried out for different pastes in a conical, semi-pilot-scale spouted bed.     

Effect of cohesive forces on fluidized and spouted beds of wet particles

Fluid beds are now being used for processing pasty materials including production of fine powders through drying suspensions in beds of inert particles; coating of tablets or pellets; granulation, etc. In such processes, the fluid bed operation becomes more complex due to the development of cohesive forces resulting from liquid bridges between particles. Such forces can affect gas and solids flow leading to uncontrollable particle agglomeration and to poor gas–solid contact. This work is aimed at analyzing and quantifying the differences of flow behavior in fluidized and spouted beds of wet and dry particles. Experimentally, surface stickiness is induced by application of metered amounts of glycerol. Based on pressure drop vs. fluid flow rate curves, solids circulation rates and bed porosity variations, two types of particle–particle interaction forces are identified and their effect on air–solid flow is quantified as a function of glycerol concentration. Implications of these results in coating, granulation and drying of suspensions in these beds are also discussed.     

Comparison of numerical simulations and experiments in conical gas-solid spouted bed

Flowbehavior of gas and particles in conical spouted beds is experimentally studied and simulated using the twofluid gas-solid model with the kinetic theory of granular flow. The bed pressure drop and fountain height are measured in a conical spouted bed of 100mmI.D. at different gas velocities. The simulation results are compared with measurements of bed pressure drop and fountain height. The comparison shows that the drag coefficient model used in cylindrical beds under-predicted bed pressure drop and fountain height in conical spouted beds due to the partial weight of particles supported by the inclined side walls. It is found that the numerical results using the drag coefficient model proposed based on the conical spouted bed in this study are in good agreement with experimental data. The present study provides a useful basis for further works on the CFD simulation of conical spouted bed.     

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).     

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.     

BATCH DRYING OF SHELLED CORN IN TWO-DIMENSIONAL SPOUTED BEDS WITH DRAFT PLATES

ABSTRACT

Geometrically similar spouted beds with draft plates were used to obtain the drying characteristics of freshly harvested shelled corn with 0.28 to 0.31 kg/kg initial moisture content at different air inlet temperatures and bed heights. Thermal equilibrium between air and grains was achieved at minimum spouting conditions. The drying kinetics of shelled corn in a drafted two-dimensional spouted bed was found to be of the "thin layer" type. Expressions for the model parameters in Page's equation accounting for the bed geometry, grain moisture content, and drying conditions were developed.     

SLURRY DRYING IN GAS-PARTICLE CONTACTORS: FLUID-DYNAMICS AND CAPACITY ANALYSIS

The fluid-dynamic characteristics and evaporation capacities of several substrates (pure water, a mixture of water and carbohydrates, and apple pulps with maltodextrines) for conical spouted bed, fluidized bed and pneumatic dryers was compared.

For fluidized and spouted beds, the presence of fluids increased the fluid-dynamics parameters over the values when only inert particles were present in the beds. In both cases it was proposed correlations to predict those effects. In the spouted bed drier, increasing the load of inert particles resulted in a slight increase of the water evaporation capacity, while in the fluidized bed drier the effect was more marked. For fluidized and spouted beds, under similar operation conditions, the maximums of the suspension drying capacities are lower than the values of water evaporation, but for pneumatic drying this effect was the opposite. In general terms, the maximal capacities of fluidized bed dryers are higher than those observed in the spouted bed dryers.     

Simulation of the Hydrodynamics and Drying in a Spouted Bed Dryer

Gas-particle flow behavior in a spouted bed of spherical particles was simulated using the Eulerian-Eulerian two-fluid modeling approach, incorporating a kinetic-frictional constitutive model for dense assemblies of the particulate solid. The interaction between gas and particles was modeled using the Gidaspow drag model and the predicted hydrodynamics is compared with published experimental data. To investigate drying characteristics of particulate solids in axisymmetric spouted beds, a heat and mass transfer model was developed and incorporated into the commercial computational fluid dynamics (CFD) code FLUENT 6.2. The kinetics of drying was described using the classical and diffusional models for surface drying and internal moisture drying, respectively. The overall flow patterns within the spouted bed were predicted well by the model; i.e., a stable spout region, a fountain region, and an annular downcomer region were obtained. Calculated particle velocities and concentrations in the axisymmetric spouted bed were in reasonable agreement with the experimental data of He et al. (Can. J. Chem. Eng. 1994a, 72:229; 1994b, 72:561). Such predictions can provide important information on the flow field, temperature, and species distributions inside the spouted bed for process design and scale-up.     

Analysis of conical spouted bed fluid dynamics using carton mixtures

The pyrolysis has risen as an important alternative technology for generating value from waste. Among the modern solid wastes, the post-consumer carton packaging highlights due to the high value-added of the primary products obtained from pyrolysis. In an attempt to use conical spouted beds (CSBs) as a pyrolysis reactor for processing cartons, this present research aims at analyzing experimentally the air–carton mixtures flow dynamics in CSBs and stating comparisons with characteristic fluid dynamics obtained by using CFD technique. The flow behavior of air–carton disk is experimentally investigated by analyzing data of bed pressure drop, air velocity and fountain height. For the carton disk and polyethylene mixtures up to 50% cartons (in mass), and carton disks and sand mixtures comprising 5 and 10% cartons (in mass), the analysis of the experimental data shows that the stable spouted regimes are achieved. Furthermore, the simulated results demonstrate that the Eulerian approach using the Syamlal drag model is able to predict qualitatively the flow behavior in conical spouted beds comprising non-spherical particle mixtures.