Drying of a Porous Material in a Pulsed Fluid Bed Dryer: The Influences of Temperature,Frequency of Pulsation,and Air Flow Rate

In this work, a four-section pulsed fluid bed apparatus with a 0.18 m² cross-section area was used to investigate the influence of pulsed-fluidization variables on the drying process of molecular sieves, a test material that was chosen because it presents an initial constant drying rate period. A two-level factorial design was developed to evaluate the influences of the inlet gas temperature—40 and 70°C—the frequency of pulsation—250 and 900 rpm—and the air flow rate—500 and 600 m³(STP)/h—on the drying rate. In addition, a comparison was made between the drying rates achieved with conventional and pulsed fluidization. Results showed that all the investigated variables affect the drying rate. Moreover, drying rates with conventional fluidization are considerably higher, which shows that one must expect a lower drying rate when pulsation is used in a drying process controlled by the external evaporation. Concerning fluid dynamics, this work also analyzed the influence of the frequency of pulsation on the pressure drop across the bed. The higher the frequency, the higher the pressure drop. That result can be explained by the reduction of channeling.     

Drying Suspensions in a Pulsed Fluidized Bed of Inert Particles

The drying of carbohydrate suspensions on polypropylene particles in a pulsed fluidized bed was studied by means of a 2⁵ experimental design, to determine the effect of the air flow and temperature, suspension flow rate, and free section and rotating speed of the rotary plate on the Nusselt number, the moisture content of the product, and the percentage of solids retained inside the bed (which were minimized to 4.9 and 14.4%, respectively) with an air flow of 600 m³/h at 90°C and 720 mmHg, a suspension flow rate of 6 L/h, and a plate with 6% free section, rotating at 50 rpm.

Additionally, the effects of temperature, air flow, and suspension flow rate on the residence time distribution (RTD) were determined, using the stimulus-response methodology. The RTD was represented by 1.1 to 2 tanks in series, according to this model. The mean residence time of the dried carbohydrate particles was between 5.4 and 8.2 min.

Finally, an egg suspension could be dried at 4 L/h, with air at 90°C, with a mean residence time about 50% longer that that found for drying carbohydrate suspensions.     

DRYING OF CHOPPED SPRING ONION USING FLUIDIZATION TECHNIQUE

Abstract

This research investigates several parameters affecting the drying characteristics of chopped spring onion and develops an empirical model for predicting its drying behaviour in a fluidized bed dryer. Experimental results showed that at air temperature of 32 °C and relative humidity of 62 %, the minimum fluidization velocities were approximately 1.36, 1.20, 0.95 and 0.62 m/s at initial moisture contents of 95, 71, 56 and 5% w.b., respectively. Drying air temperature and specific air flow rate were parameters which affected drying rate. Page's model could predict the value closest to the experimental data. The air-product temperature should be kept lower than 53 °C to maintain the acceptable green color of the dried product.     

湿污泥颗粒的流化床干燥实验及模型

在鼓泡流化床内以河砂为干燥介质,对单颗粒湿污泥的流态化干燥特性进行实验研究,得到了流化床温度、污泥初始水分、污泥粒径及流化速度对干燥速率的影响规律：流化床温度及污泥粒径对干燥速率的影响都呈指数规律;污泥的水分越大,干燥速率越大;在鼓泡流化床流化速度达到2倍临界流化速度以上时,充分流化,流化速度再增大(2~5倍临界流化速度)对干燥速率没有明显影响. 在基本的扩散传质理论的基础上,利用实验数据回归得到湿污泥在鼓泡流化床内干燥的半经验模型,为流化床污泥干燥器的设计提供了基础数据和依据.     

Attrition of Victorian brown coal during drying in a fluidized bed

Analyzing the attrition of Victorian brown coal during air and steam fluidized bed drying, the change in particle size distribution over a range of initial moisture contents (60% to 0%) and residence times (0 to 60 minutes) was determined. Dried at a temperature of 130°C with a fluidization velocity 0.55 m/s and an initial particle size of 0.5–1.2 mm, both fluidization mediums show a shift in the particle size distribution between three and four minutes of fluidization, with a decrease in mean particle size from 665 µm to around 560 µm. Using differential scanning calorimetry (DSC), the change in particle size has been attributed to the transition between bulk and non-freezable water (approximately 55% moisture loss) and can be linked to the removal of adhesion water, but not to fluidization effects. This is proved through the comparison of air fluidized bed drying, steam fluidized bed drying, and fixed bed drying—the fixed bed drying is being used to determine the particle size distribution as a function of drying. The results show the three drying methods produce similar particle size distributions, indicating that both fluidization and fluidization medium have no impact upon the particle size distribution at short residence times around ten minutes. The cumulative particle size distribution for air and steam fluidized bed dried coal has been modeled using the equation P_d = A₂ + (A₁ ? A₂)/(1 + (d/x₀)^p), with the resultant equations predicting the effects of moisture content on the particle size distribution. Analyzing the effect of longer residence times of 30 and 60 minutes, the particle size distribution for steam fluidized bed dried coal remains the same, while air fluidized bed dried coal has a greater proportion of smaller particles.     

Drying of Citrus sinensis Peels in an Inert Fluidized Bed: Kinetics,Microbiological Activity,Vitamin C,and Limonene Determination

Citrus sinensis peel drying kinetics in a fluidized bed with inert material were investigated. Drying impact on microbiological activity as well as limonene and vitamin C content was also studied. Drying parameters studied were as follows: temperatures of 40°C, 50°C, and 60°C; air velocities of 0.73 m/s and 0.85 m/s; and orange peel:sand weight ratios of 1:0, 1:1, and 1:2. High temperatures, high air velocity, and the presence of inert material increased the drying rate. Nine thin-layer drying models were fitted to the experimental data of Citrus sinensis peels. The Midilli et al. model was found to be the most suitable model for describing the drying kinetics of Citrus sinensis peels. Vitamin C and limonene content were higher in the product dried using a fluidized bed than in the sun-dried product. Drying of Citrus sinensis peel in a fluidized bed also may reduce microorganism growth, increasing storage life.     

Prediction of Energy and Exergy of Carrot Cubes in a Fluidized Bed Dryer by Artificial Neural Networks

In this study both static and recurrent artificial neural networks (ANNs) were used to predict the energy and exergy of carrot cubes during fluidized bed drying. Drying experiments were conducted at air temperatures of 50, 60, and 70°C; bed depths of 3, 6, and 9 cm; and square-cubed carrot dimensions of 4, 7, and 10 mm. Five hundred eighteen patterns, obtained from experiments, were used to develop the ANN models. Initially, a static ANN was applied to correlate the outputs (energy and exergy of carrot cubes) to the four exogenous inputs (drying time, drying air temperature, carrot cube size, and bed depth). In the recurrent ANNs, in addition to the four exogenous inputs, two state inputs and outputs (energy and exergy of carrot cubes) were used. To find optimum ANN models, various numbers of hidden neurons were investigated. The energy and exergy of carrot cubes were predicted with R ² values of greater than 0.95 and 0.97 using static and recurrent ANNs, respectively. Accordingly, the optimal recurrent model could be utilized for determining the appropriate drying conditions of carrot cubes to reach the optimal energy efficiency in fluidized bed drying.     

SIMULATION OF FLUIDIZED-BED DRYING OF CARROT WITH MICROWAVE HEATING

ABSTRACT

A mathematical model of coupled heat and mass transfer was applied to batch fluidized-bed drying with microwave heating of a heat sensitive material—carrot. Four kinds of microwave heating with intermittent variation were examined. The numerical results show that different microwave heating patterns can affect the fluidized bed drying significantly. Changing the microwave input pattern from uniform to intermittent mode can prevent material from overheating under the same power density. Supplying more microwave energy at the beginning of drying can increase the utilization of microwave energy while keeping temperature low within the particle. For a particle diameter of 4 mm, fluidization velocity of 2 m/s, inlet airflow temperature of 70°C and the bed area factor of 80, the drying time are 750 and 1000 s, respectively, for the two good operating conditions with on/off periods of 125/375 s and 375/375 s. The cumulative microwave energy absorbed by particles at the end of drying is 1415 and 2300 kJ/kg (dry basis), respectively.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

ABSTRACT

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D _e ) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X _o ). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

Development of Cross-Flow Fluidized Bed Paddy Dryer

ABSTRACT

A cross-flow fluidized bed paddy dryer with a capacily of 200 kgh was designed, fabricated and tested. Experimental results showed that final moisture content of paddy should not be lower than 23 % dry-basis if quality is to be maintained. Drying air temperature was keot constant at 115°C according to the recommendation of previous work. Results obtained from the mathematical model developed in this study indicatedthat optimum operating parameters should be as follows : air speed of 2.3 m/s, bed thickness of 10 cm and fraction of air recycled of 80 %. At this condition, energy consumption was close to the minimum while drying capacity was near the maximum. A prototype fluidized bed dryer with a capacity of 1 t/h was designed, fabricated and installed with the collaboration of a private company. The unit has been used for almost the whole past harvesting season in 1994 at a paddy merchant sile with preference compared to conventional column continuous dryers. More than 300 tons of paddy were dried without any problems.     

Influence of drying conditions on the moisture diffusion and fluidization quality during multi-stage fluidized bed drying of bovine intestine for pet food

In order to establish the influence of the drying air characteristics on the drying performance and fluidization quality of bovine intestine for pet food, several drying tests have been carried out in a laboratory scale heat pump assisted fluid bed dryer. Bovine intestine samples were heat pump fluidized bed dried at atmospheric pressure and at temperatures below and above the materials freezing points, equipped with a continuous monitoring system. The investigation of the drying characteristics have been conducted in the temperature range ?10 to 25 °C and the airflow in the range 1.5–2.5 m/s. Some experiments were conducted as single temperature drying experiments and others as two stage drying experiments employing two temperatures. An Arrhenius-type equation was used to interpret the influence of the drying air temperature on the effective diffusivity, calculated with the method of slopes in terms of energy activation, and this was found to be sensitive to the temperature. The effective diffusion coefficient of moisture transfer was determined by the Fickian method using uni-dimensional moisture movement in both moisture, removal by evaporation and combined sublimation and evaporation. Correlations expressing the effective moisture diffusivity and drying temperature are reported.Bovine particles were characterized according to the Geldart classification and the minimum fluidization velocity was calculated using the Ergun Equation and generalized equation for all drying conditions at the beginning and end of the trials. Walli's model was used to categorize stability of the fluidization at the beginning and end of the drying for each trial. The determined Walli's values were positive at the beginning and end of all trials indicating stable fluidization at the beginning and end for each drying condition.     

Vibrofluidized Bed Drying of Adipic Acid

ABSTRACT

Adipic acid is a white crystalline solid and is the most industrially important of all aliphatic dicarboxylic acids. Drying is the principal unit operation after its crystallization. The acid has a tendency to agglomerate when moistened, due to its high cohesiveness. The fluidization regime does not occur and the particles cannot be adequately treated in a conventional fluidized bed. This article deals with the drying of adipic acid in a vibrofluidized bed. The equipment used in the drying tests was specially designed and constructed to maintain the fluidized regime of fine particles of moistened adipic acid. The drying curves presented the constant and the falling rate periods, yielding final moisture contents that depend mainly on the drying temperature. The quality of the dried acid was also verified by the crystals' granulometric analysis, which showed breakage of crystals for drying at higher air temperatures. Results showed to be satisfactory for drying adipic acid with moisture content up to 5.1% w.b. in the vibrofluidized bed, keeping a high degree of bed homogeneity and quality of the fluidized state.     

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 OF POWDERY MATERIALS IN A PULSED FLUID BED DRYER

ABSTRACT

Drying of recycled polypropylene powder was studied experimentally in a laboratory pulsed fluid bed dryer (PFB) with relocated air stream. It was proved that fluidization of fine particles having a large specific surface area and a broad size distribution is technically feasible when using the composite supporting grid. Drying and hydrodynamic characteristics for a pulsed fluid bed of fine particles are found to be similar to the ones for coarse particles. Equations for minimum pulsed fluidization velocity, pressure drop, and surface and volumetric heat transfer coefficients are given.     

OSMO-CONVECTIVE DRYING OF GRAPES

ABSTRACT

In order to reduce browning of grapes during drying, a special drying method was developed and evaluated using a laboratory scale fluidized bed dryer. Fresh Thompson seedless grapes were initially dried by immersion in a fluidized bed of sugar. The mass ratio of grapes to sugar was 1:1. The flow rate of hot air (at 45 and 60^°C) was sufficient to fluidize the sugar bed, while grapes placed on the screen, 3 cm above the drying air distributor, remained generally stationary.

Due to the simultaneous osmotic and convection drying effects, the drying time was reduced by factor ～1.5 as compared to drying under the similar conditions without added sugar. A special pre-treatment of dipping of grapes in ethyl oleate (2% solution in 0.5% sodium hydroxide) at 80?deg; C for 30 s further reduced the drying time by factor 2 in both cases. The color of osmo-convective dried grapes were comparable to that of sulfur dioxide treated grapes. The texture of osmo-convective dried grapes was more pliable (softer) than convective dried samples. The major problem associated with die osmo-convective drying of grapes on a sugar bed was the stickiness, caused by sugar, on the fruit surface. This was reduced by partially substituting sugar with semolina (maintaining a 1:1 ratio) to create fluidized bed.     

Transient Thermal Behavior and Stress Development within a Cylindrical Wooden Disk during Drying in a Fluidized Bed Dryer

Thermal behavior and stresses developed in a wood particle are predicted using a mathematical model formulated for drying in fluidized bed. Drying experiments have been carried out on 10 mm long and 30 mm diameter cylindrical disks at a bed temperature of 403 K (130°C). Drying time, mass reduction, and shrinkage during drying have been measured and used to validate the thermal model. The stress model is validated with an analytical solution available in literature. The drying time is experimentally found to be 630 s. The wood particle shrinks by 4.6% in the radial direction and by 1.0% in the longitudinal direction. The model indicates splitting failure at the circumference, which was also noticed during experiments.     

Vibrofluidized Bed Drying of Adipic Acid

Adipic acid is a white crystalline solid and is the most industrially important of all aliphatic dicarboxylic acids. Drying is the principal unit operation after its crystallization. The acid has a tendency to agglomerate when moistened, due to its high cohesiveness. The fluidization regime does not occur and the particles cannot be adequately treated in a conventional fluidized bed. This article deals with the drying of adipic acid in a vibrofluidized bed. The equipment used in the drying tests was specially designed and constructed to maintain the fluidized regime of fine particles of moistened adipic acid. The drying curves presented the constant and the falling rate periods, yielding final moisture contents that depend mainly on the drying temperature. The quality of the dried acid was also verified by the crystals' granulometric analysis, which showed breakage of crystals for drying at higher air temperatures. Results showed to be satisfactory for drying adipic acid with moisture content up to 5.1% w.b. in the vibrofluidized bed, keeping a high degree of bed homogeneity and quality of the fluidized state.     

A Review of: “FLUIDIZATION VI” Editors: J.R. Grace,L.W. Shemilt and M.A. Bergougnou Publisher: AIChE (for Engineering Foundation) 345 East 47th St., New York,N.Y. 10017, 1989, 750 p.

ABSTRACT

Heat pumps in combination with fluidized bed driers have been investigated extensively at The Norwegian Institute of Technology. A laboratory plant was constructed and a series of experiments on different types of temperature sensitive products were executed. By using heat pump, good temperature and humidity regulation is achieved. Drying temperatures can be regulated from ?20°C to 50°C. This gives the possibility of freeze-drying at atmospheric pressure, and drying at temperatures above 0°C in the same plant. By using a temperature program, including an initial period of freeze drying, it is possible to regulate the products' physical properties. Examples are: the sinking velocity of fish fry feed, rehydration ability, colour and taste of dried fish, meat and vegetable products. Further, biotechnological products like bacterial cells and biomolecules/enzymes were dried with improved biological activity of the products compared to traditional drying methods like vacuum freeze drying or spray drying.     

Drying Kinetics of Oil Palm Frond Particles in an Agitated Fluidized Bed Dryer

The drying kinetics of oil palm frond particles in a laboratory-scale agitated fluidized bed dryer were investigated under various operating conditions: inlet air temperature (50–80°C), superficial air velocity (0.6–1.0 m/s), bed load (200–300 g), and agitation speed (300–500 rpm). To study the effects of these variables on the drying time and drying rate, an experimental design using Taguchi orthogonal array was employed. Based on analysis of variance (ANOVA), the results indicated that inlet air temperature greatly affected the drying rate, followed by superficial air velocity and bed load. The effect of agitation speed on the drying rate was found to be small. The experimental drying kinetics data were compared with the values obtained from three different models, namely, the Page model, modified quasi-stationary method (MQSM), and a new composite model. It was found that the proposed new model could satisfactorily predict the complete drying rate curve for the drying of oil palm fronds.