Drying of Inorganic Particulate Compounds

Inorganic particulates are usually dried in a fixed bed, fluidized bed, or spray dryers. These compounds are easy to dry, once their physical structure, with high porosity, allows moisture content removal with low resistances. For fluidized bed of alumina particle evaluations, a laboratory-scale drying unit was built. The drying experiments were carried out with alumina particles with different diameters to evaluate temperature and air flow rate effects on drying kinetics and bed height. In another case, the dehydration of a mixture of rare-earth chlorides in a fluidized bed was studied, aiming at the production of anhydrous rare-earth chlorides, used to obtain mischmetal by electrolytic and metallothermic processes. The spray drying experiments were carried out in a pilot plant. Spray drying is a technique largely applied in industrial processes to dry solutions or suspensions, converting their solid parts into a dried powder. A set of rare-earth drying experiments was carried out, aiming at the development of techniques to obtain a powder that could satisfy international morphological requirements. The results allowed evaluating the effects of air flow rate, feed concentration, atomizer model, rotation velocity, and atomization pressure on powder density and particle size distribution.     

流化床干化法在污水处理厂污泥处置中的应用

针对大伙房水库上游污水处理厂污泥处理过程中存在的问题,提出采用流化床污泥干化法对污水厂脱水后的污泥进一步处理,实现减量和无害化。文中给出了详细的工艺过程和参数,对污泥干化工程的实施有一定的参考价值。     

Spray Freeze Drying in a Fluidized Bed at Normal and Low Pressure

The aim of this study is to develop the spray freeze drying process and its hardware and to investigate its capabilities to dry thermosensible substances such as pharma-proteins at normal and low pressures. As the result, the spray freeze fluidized–bed dryer was constructed. During the study, the drying kinetic comparison between classical and spray freeze–drying technologies was done. Spray freeze drying has shown short process times and allows advanced control, product particle shape and size uniformity, and high solubility. This shows that the fluidized-bed freeze-drying process could be an alternative for classical freeze-drying processes. Identified problems are the low yield of the primary drying phase and the strong electrostatic effects during the secondary drying step. However, the innovative process has shown an excellent capability to dry and stabilize the thermosensitive substances, such as pharma-proteins.     

Spray Freeze Drying in a Fluidized Bed at Normal and Low Pressure

The aim of this study is to develop the spray freeze drying process and its hardware and to investigate its capabilities to dry thermosensible substances such as pharma-proteins at normal and low pressures. As the result, the spray freeze fluidized-bed dryer was constructed. During the study, the drying kinetic comparison between classical and spray freeze-drying technologies was done. Spray freeze drying has shown short process times and allows advanced control, product particle shape and size uniformity, and high solubility. This shows that the fluidized-bed freeze-drying process could be an alternative for classical freeze-drying processes. Identified problems are the low yield of the primary drying phase and the strong electrostatic effects during the secondary drying step. However, the innovative process has shown an excellent capability to dry and stabilize the thermosensitive substances, such as pharma-proteins.     

Process Design of a Multistage Drying Process via Flowsheet Simulation

Multistage drying processes including spray drying and fluidized bed unit operations are challenging to design due to the dependencies of the preceding process steps. Flowsheet simulation offers the possibility to simulate a complete process by the application of suitable short-cut models. In this contribution a novel model for a spray dryer based on a population balance approach is presented. Furthermore, a dynamic model for a fluidized bed dryer is introduced. Experimental data is used for validation and parameter optimization. The calibrated models are then used to design an industrial drying process of multiple drying stages.     

Combined Microwave/Fluidized Bed Drying of Fresh Peppercorns

Abstract

A fluidized bed dryer (FBD) and a combined microwave/fluidized bed dryer (CMFD) are used to dry the fresh ripe peppercorns. The average moisture content vs. elapsed drying time, and drying rate vs. average moisture content are experimentally investigated. It is found that the microwave field from the CMFD can increase the potential of the conventional fluidized bed drying. The drying rates of both dryers are dependent on the inlet air temperature and velocity. For the CMFD, the effects of the air velocity on the drying rate are found to be opposite to our previous results tested with white pepper seeds i.e., the drying rates of the fresh ripe peppercorns decreased with increasing air velocity. By using a CMFD, the drying time required to reach the desired moisture content can be reduced to 80–90% of the drying time required for a FBD at the same drying air temperature and velocity. The color of the product dried by a CMFD is also attractive: it becomes flaming yellow, instead of black as obtained from a FBD. The physical structure of the peppercorn, before and after the drying process is also investigated by a metallurgical macroscope and an image analyzer. Different from drying by a FBD, the external form and matter of the white pepper seed are still maintained, even after passing through the drying process.     

Combined Microwave/Fluidized Bed Drying of Fresh Peppercorns

A fluidized bed dryer (FBD) and a combined microwave/fluidized bed dryer (CMFD) are used to dry the fresh ripe peppercorns. The average moisture content vs. elapsed drying time, and drying rate vs. average moisture content are experimentally investigated. It is found that the microwave field from the CMFD can increase the potential of the conventional fluidized bed drying. The drying rates of both dryers are dependent on the inlet air temperature and velocity. For the CMFD, the effects of the air velocity on the drying rate are found to be opposite to our previous results tested with white pepper seeds i.e., the drying rates of the fresh ripe peppercorns decreased with increasing air velocity. By using a CMFD, the drying time required to reach the desired moisture content can be reduced to 80-90% of the drying time required for a FBD at the same drying air temperature and velocity. The color of the product dried by a CMFD is also attractive: it becomes flaming yellow, instead of black as obtained from a FBD. The physical structure of the peppercorn, before and after the drying process is also investigated by a metallurgical macroscope and an image analyzer. Different from drying by a FBD, the external form and matter of the white pepper seed are still maintained, even after passing through the drying process.     

Drying Characteristics of Coriander Seed Particles in a Reduced Pressure Superheated Steam Fluidized Bed

Drying characteristics of coriander seed particles were experimentally analyzed in a reduced pressure superheated steam fluidized bed. The typical moisture gain, reported in some other studies during the warm-up period of the process, was reduced in most of the cases by supplying additional heat into the column. The experimental results demonstrated that the drying rate increases and the equilibrium moisture content decreases by increasing the operating temperature. However, variation of the operating pressure (40–67 kPa) and the superficial steam velocity (2.3–4.0 m/s) did not present significant effects on the moisture contents. The degree of superheating was found to be the most important parameter for the process. The experiments also showed that the equilibrium moisture content decreases upon increasing the degree of superheating. Finally, employing a reduced pressure superheated steam fluidized bed appears as an option to carry out drying processes at relatively lower temperatures.     

Drying of Pepper Seed Particles in a Superheated Steam Fluidized Bed Operating at Reduced Pressure

A series of drying experiments was performed in a reduced-pressure superheated steam fluidized bed, employing pepper seed particles and some novel data were obtained. Experiments were carried out using different chamber pressures (40–67 kPa), temperatures (90–122°C), steam velocities (2.35–4.10 m/s), and mass flow rates (0.0049–0.0134 kg/s). In the majority of the experiments, the moisture gain observed in some other studies in the warm-up period of the process was prevented through some supplementary heat provided to the column. The drying rate was found to be increasing by operating temperature; however, it was not affected much by the superficial gas velocity and the operating pressure. Nevertheless, the reduced pressure operation increases the degree of superheating that appears as the most important parameter of the process. The experimental results showed that the equilibrium moisture content decreases by the increasing degree of superheating. On the other hand, the critical moisture content assumes higher values for the greater degrees of superheating. It was concluded that a relatively lower temperature process can be achieved through a reduced-pressure superheated steam fluidized bed.     

Simulation and Validation of Microwave-Assisted Fluidized Bed Drying of Soybeans

A general mathematical model of heat and mass transfer was developed to simulate the microwave-assisted fluidized bed drying of bulk grain. The model was solved using the well-known Runge-Kutta-Gill method. The model is capable of predicting the moisture content of soybean as well as the drying air parameters (i.e., drying air temperature and moisture content) during drying. The values of mean relative deviation (MRD) were less than 8 and 10% for prediction of grain moisture content and outlet air parameters, respectively, which reflects an acceptable accuracy. In comparison with conventional fluidized bed drying of soybean, microwave-assisted fluidized bed drying led to 83.39–98.07% savings in drying time and 82.07–95.22% savings in specific energy consumption when reducing soybean moisture content from 18.32 to 12% (db).     

A Quasi-stationary Approach to Drying Kinetics of Fluidized Particulate Materials

A modified quasi-stationary method has been proposed to describe drying kinetics of particulate materials dried in convective dryers with active hydrodynamic regimes. Both our own results and literature data were used. These include sliced celery, cranberry, diced carrot, wheat and polystyrene granules dried in four types of dryers: pulsed fluid bed dryer, fluid bed dryer with a mixer, spouted bed dryer with a draft tube, and vortex dryer. The method was validated by comparing experimental data with results of modeling in terms of a reduced moisture content and material temperature. A new form of the generalized drying curve has been proposed with the reduced time and the index of hydrodynamic intensity as parameters. The equations developed can be used to calculate the total drying time and determine the temporal variation of the moisture content and material temperature.     

A Comparative Study Between Spray-Drying and Fluidized Bed Coating Processes for the Preparation of Pramipexole Controlled-Release Microparticles for Orally Disintegrating Tablets

In the present study, controlled-release microparticles for orally disintegrating tablets (ODT) were prepared using two different processes, spray drying and fluidized bed coating processes. Pramipexole dihydrochloride monohydrate (PRM), an anti-Parkinson's disease agent, was selected as a model drug. The in vitro release rate and morphology of microparticles were evaluated and compared. The size of microparticles prepared by spray drying (SD microparticles) and fluidized bed coating (FC microparticles) was around 10 and 200 µm, respectively. The latter size was defined by the size of an inert core bead. The release behavior of SD microparticles was characterized by a large initial burst release prior to slow release. In the case of FC microparticles, the initial burst release was smaller than that of SD microparticles and the compression process damaged the release-controlling layer, which led to a change in release rate. The results indicated the importance of carefully considering the manufacturing process for microparticles during the design of controlled-release ODT.     

Numerical Spray Model of the Fluidized Bed Coating Process

In this study, a new model for the batch top-spray fluidized bed coating process is presented. The model is based on the one-dimensional (axial) discretization of the bed volume into different control volumes, in which the dynamic heat and mass balances for air, water vapor, droplets, core particles, and coating material were established. The coupling of the droplet phase's mass and heat transfer terms with the gas and solid phases was established by means of a droplet submodel in which droplet trajectories were individually simulated.

The model calculation method combines a Monte Carlo technique for the simulation of the particle exchange with the first-order Euler's method for solving the heat and mass balances, enabling the prediction of both the dynamic coating mass distribution and the one-dimensional (axial) thermodynamic behavior of the fluidized bed during batch operation. The simulation results were validated using experimental two-dimensional spatial air temperature and air humidity distributions, which were measured in a fluidized bed pilot reactor using a scanning probe.

Sensitivity analysis was carried out to study the effect of controllable process variables, such as fluidization air and atomization air properties, as well as the properties of the spraying liquid upon the simulated dynamic temperature and humidity distributions. Also, the effects of relevant process variables on growth rate uniformity and process yield were studied. Based on these sensitivity studies it was concluded that nozzle parameters, such as air pressure and positioning with respect to the bed, are as important as the fluidization air properties (humidity, temperature, and flow rate) for the coating growth rate uniformity and process yield.     

Formulation and Cost-Effective Drying of Probiotic Yeast

Saccharomyces boulardii yeast is considered as a probiotic according to the World Health Organization (WHO). Like any other probiotic, Saccharomyces boulardii is available as a freeze-dried formulation. Although freeze drying is the most preferred method of preserving the microorganisms, the process is very expensive. The cost of capsules containing freeze-dried probiotic yeast is certainly out of reach of the underprivileged population of the world. Hence, the present work focuses on developing a new formulation and cost-effective drying process for probiotic yeast. Freeze drying was compared with fluidized bed drying (FBD) and heat pump–assisted fluidized bed drying (HP-FBD) techniques for drying of Saccharomyces boulardii formulation. The process involves addition of lactose and microcrystalline cellulose to the biomass obtained by centrifugation and then making the dough. This blend is then extruded and spheronized to form granules. Heat pump fluidized bed drying of the formulation resulted in higher activity retention compared to atmospheric fluid bed drying. This encouraged further experimentation using HP-FBD to study the effect of various operating variables such as air temperature, air humidity, and velocity on the drying kinetics and the deactivation kinetics. It was observed that the temperature and humidity play a major role in the deactivation and drying, whereas the change in air velocity has comparatively less effect on both parameters. The effective diffusivity and the deactivation rate constants were correlated to the temperature using an Arrhenius-type equation.     

Numerical Spray Model of the Fluidized Bed Coating Process

In this study, a new model for the batch top-spray fluidized bed coating process is presented. The model is based on the one-dimensional (axial) discretization of the bed volume into different control volumes, in which the dynamic heat and mass balances for air, water vapor, droplets, core particles, and coating material were established. The coupling of the droplet phase's mass and heat transfer terms with the gas and solid phases was established by means of a droplet submodel in which droplet trajectories were individually simulated.

The model calculation method combines a Monte Carlo technique for the simulation of the particle exchange with the first-order Euler's method for solving the heat and mass balances, enabling the prediction of both the dynamic coating mass distribution and the one-dimensional (axial) thermodynamic behavior of the fluidized bed during batch operation. The simulation results were validated using experimental two-dimensional spatial air temperature and air humidity distributions, which were measured in a fluidized bed pilot reactor using a scanning probe.

Sensitivity analysis was carried out to study the effect of controllable process variables, such as fluidization air and atomization air properties, as well as the properties of the spraying liquid upon the simulated dynamic temperature and humidity distributions. Also, the effects of relevant process variables on growth rate uniformity and process yield were studied. Based on these sensitivity studies it was concluded that nozzle parameters, such as air pressure and positioning with respect to the bed, are as important as the fluidization air properties (humidity, temperature, and flow rate) for the coating growth rate uniformity and process yield.     

Sugarcane Bagasse Drying in a Cyclone: Influence of Device Geometry and Operational Parameters

Sugarcane bagasse is becoming more and more commonly used in generating electrical energy, steam, and bioethanol. Drying is important in sugarcane and other types of biomass because it can be used to improve the calorific value and overall energetic use. In this work, sugarcane bagasse was treated by drying in a cyclonic dryer. The influence of the geometry of the device (the conical part of the cyclone) and process parameters (bagasse mass flow rate and temperature) were tested. The modification on the conical part was related to two different angles and with two different inferior outlets (B). Experimental design was carried out for each geometry. The independent variables were the drying agent temperature (35 to 275°C) and the bagasse mass flow rate (0.1 × 10^?2 to 2.9 × 10^?2 kg s^?1). The air flow rate was kept constant at 7.5 × 10^?2 kg s^?1. The dependent variables were moisture reduction (MR) and average particle residence time (t_res) in the cyclonic dryer. For both cyclonic geometries, it was observed that MR was directly proportional to the temperature and inversely proportional to the bagasse mass flow rate. t_res was also inversely proportional to the bagasse mass flow rate. Decreasing B tended to increase tres and MR.     

Effect of Drying Methods on Crystal Transformation of Trehalose

Dihydrate trehalose was transformed into anhydrous crystals by dehydration using fluidized bed drying, indirect heat drying, and vacuum drying, and characteristics of the reaction of crystal transformation were compared. In any drying method, the dihydrate trehalose was transformed into the stable β-form anhydrous trehalose crystal. However, the surface structure and drying characteristics were different. The moisture change was correlated by the Avrami equation, which can be generally applied to the crystal transformation.     

Effect of Drying Methods on Crystal Transformation of Trehalose

Dihydrate trehalose was transformed into anhydrous crystals by dehydration using fluidized bed drying, indirect heat drying, and vacuum drying, and characteristics of the reaction of crystal transformation were compared. In any drying method, the dihydrate trehalose was transformed into the stable β-form anhydrous trehalose crystal. However, the surface structure and drying characteristics were different. The moisture change was correlated by the Avrami equation, which can be generally applied to the crystal transformation.     

Drying of Suspensions in a Fluidized Bed of Inert Particles Under Reduced Pressure

A drying method in which a heat-sensitive material is immersed in a fluidized bed under reduced pressure was used to continuously obtain dispersed, dry, fine powders of that material from a dilute suspension or solution at a low temperature with a high drying rate. The mass velocities of the drying gas, sample flow rate, and chamber pressure were varied, and the effects of these variations on the corresponding drying characteristics were examined.

The fluidization state of a fluidized bed of inert particles strongly affects the drying rate in the bed and has a greater effect than the chamber pressure on the corresponding drying characteristics. In other words, it is important to maintain a vigorous fluidization state to achieve a high drying rate. Although the maximum drying rate is independent of the chamber pressure, it can be achieved at a low mass velocity of the drying gas under reduced pressure. That is, at a low chamber pressure a high drying rate can be achieved, even at a relatively low mass velocity of the drying gas. The bed temperature at the time of drying is strongly influenced by the drying rate and decreases linearly with an increase in the maximum drying rate when the sample flow rate is equal to the drying rate.