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 water treatment process sludge in a fluidized bed dryer

The drying characteristics of water treatment process (WTP) sludge were investigated with a fluidized bed. The equilibrium moisture ratio of WTP sludge increased with relative humidity and decreased with temperature of drying air. However, equilibrium moisture ratio of WTP sludge was more sensitively dependent on relative humidity than temperature of drying air. When the sludge was dried in a batch fluidized bed, the drying rate of sludge decreased as the moisture ratio of sludge in the bed decreased. The periods of constant drying rates were apparently not observed on the drying rate curves. In addition, the maximum drying rates were increased with bed temperature and superficial air velocity. As the fluidized bed was operated continuously, the degree of drying of WTP sludge increased with bed temperature but was weakly dependent on superficial air velocity. However, the drying efficiency was decreased with bed temperature and relatively insensitive to superficial air velocity and increased with feed rate of sludge.     

The air temperature modulation impact on the drying of soybeans in a fixed bed

This work aimed to explore an alternative to reduce costs in the soybean drying process using a periodic drying operation, which can enhance mass and energy transfer, leading to lower energy consumption in comparison with the conventional operation carried out with entrance drying air in steady-state conditions. A fixed bed dryer for laboratory scale was used to dry soybean grains and experiments were carried out, applying the periodic and conventional operations with the same drying time and energy consumption. Results indicated with 5% of significance that the experiments conducted with entrance air temperature modulation achieved higher levels of the percentage of evaporated water improving the dryer performance. These results were in agreement with predicted data obtained with heterogeneous models and they indicated that the periodic drying operation can demand less energy to achieve a specific safe level of soybean moisture content for storage.     

Experimental and Theoretical Study of Agitated Contact Drying of Sewage Sludge under Partial Vacuum Conditions

Agitated contact drying experiments were carried out in a cylindrical lab-scale paddle dryer to study the drying kinetics of sewage sludge under partial vacuum conditions. To explore the drying mechanisms, the penetration model for vacuum contact drying of agitated particulate materials, developed by Schlünder and Mollekopf, was applied to simulate the experimental results. The influence of the drying parameters (system pressure, heating temperature, stirrer speed, and dryer load) on the drying kinetics was studied both experimentally and theoretically. The results indicate that pressure and temperature significantly influence the drying rates of sewage sludge, whereas stirring speed and dryer load have no obvious effects on drying rates in the examined range. The experimental and simulated drying rates were in acceptable agreement as long as the temperature difference between dryer and sludge was small. A crust is likely to form on the dryer surface, however, when the temperature difference becomes large; this crust leads to markedly lower drying rates than the calculated ones. Furthermore, a comparison between partial vacuum contact drying and atmospheric contact drying is presented. The results indicate that the mass transfer resistance increases with rising drying potential in the examined experimental range.     

Effects of Air Velocity,Air Temperature,and Berry Diameter on Wild Blueberry Drying

Abstract:

In order to design, manufacture, and commission a commercial dryer to dry individually quick frozen (IQF) wild blueberries (Vaccinium angustifolium), The Nova Scotian Fruit Company completed a series of experiments to characterize the effect of air velocity, air temperature, and packed bed depth on drying. Based on previous experience with forced air packed bed drying systems at air temperatures up to 65°C, the experiments focused on measuring the effect of air temperature and velocity during the first few hours of drying. The data collected suggest that drying occurs solely in the falling rate period. These data were used to successfully design, build, and commission a commercial dryer with a tenfold increase in production capacity over previous equipment.     

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.     

ANALYSIS OF THE DRYING OF RESIDUAL SLUDGE: FROM THE EXPERIMENT TO THE SIMULATION OF A BELT DRYER

The drying of residual sludge is a current environmental problem not sufficiently described in the literature, hence research investigations on this dewatering process are required. This paper proposes a modelling of a conveyor dryer belt. However the study of residual sludges dewatering in order to design or simulate drying equipment requires some experimental investigations. An analysis in terms of drying kinetics is proposed in this paper. Specific experiments were also needed in order to better describe some particular aspects such as crusting, shrinkage, and particle size impact. According to these data some analytical expressions have been derived and integrated in a belt dryer model. The set of air operating conditions (T, RH, v) was reduced to a single parameter, the drying potential. The results of the simulations show that a conveyor belt dryer is not well adapted to activated sludge. This is essentially due to a crust phenomenon. For a PVC industrial sludge this kind of dryer is more efficient and allows drying with reasonable residence times. Several simulations were run in order to point out the relevant parameters of the process.     

Forced Convective Drying of Wastewater Sludge with the Presentation of Exergy Analysis of the Dryer

The main objective is studying the fundamental aspect, by means of drying kinetics and the application of forced convective drying of wastewater sludge with the determination of the optimum drying conditions. The drying system is composed of two units; small samples of 2.5 g are dried in the first unit and a bed of sludge weighing 250 g is dried in the second unit. The experiments are performed under air temperatures varying between 80°C and 200°C. The range of the air velocity and its humidity is 1–2 m/s and 0.005–0.05 kg_water/kg_{dry air}, respectively. The experiments are performed for two different sludges: activated sludge (AS) and thermalized and digested sludge (TDS). Usually, three main drying phases are observed during drying of bed of sludge. These phases are reduced to only two for small samples. Determination of the influent parameters shows that the temperature of the drying air and sludge origin can profoundly influence the drying kinetic of the sludge. The exergy analysis of the two units of the drying system allows selecting 140°C, 2 m/s, and 0.05 kg_water/kg_{dry air} as optimum drying conditions with an exergy efficiency reaching 90%.     

ANALYSIS OF THE DRYING OF RESIDUAL SLUDGE: FROM THE EXPERIMENT TO THE SIMULATION OF A BELT DRYER

ABSTRACT

The drying of residual sludge is a current environmental problem not sufficiently described in the literature, hence research investigations on this dewatering process are required. This paper proposes a modelling of a conveyor dryer belt. However the study of residual sludges dewatering in order to design or simulate drying equipment requires some experimental investigations. An analysis in terms of drying kinetics is proposed in this paper. Specific experiments were also needed in order to better describe some particular aspects such as crusting, shrinkage, and particle size impact. According to these data some analytical expressions have been derived and integrated in a belt dryer model. The set of air operating conditions (T, RH, v) was reduced to a single parameter, the drying potential. The results of the simulations show that a conveyor belt dryer is not well adapted to activated sludge. This is essentially due to a crust phenomenon. For a PVC industrial sludge this kind of dryer is more efficient and allows drying with reasonable residence times. Several simulations were run in order to point out the relevant parameters of the process.     

MOBILE FLUIDIZED BED PADDY DRYER

The objectives of this research are to design, construct and test a mobile fluidized bed paddy dryer with a drying capacity of 2.5-4.0 t/h. Suitable drying conditions are recommended as follows : drying capacity 3.8 t/h, bed velocity 2.8 m/s, average drying air temperature 144 °C, bed height 13.5 cm, fraction of air recycled 0.8. Residence time of paddy was approximately 1.3 minutes. Test results showed that moisture content of paddy was reduced from 32.6 % dry-basis to 25.8 % dry-basis. Consumption of electrical power and diesel fuel was 12.9 kW and 21.71 1/h respectively. Primary energy consumption was 910.9 MJ/h. The dryer could evaporate water 218.8 kg/h. Specific primary energy consumption was 4.2 MJ/kg-water evaporated. Cost of paddy drying was 1.48 baht/kg-water evaporated of which 0.53 was fixed cost and 0.95 was energy cost (US$1 =34baht).     

MOBILE FLUIDIZED BED PADDY DRYER

ABSTRACT

The objectives of this research are to design, construct and test a mobile fluidized bed paddy dryer with a drying capacity of 2.5-4.0 t/h. Suitable drying conditions are recommended as follows : drying capacity 3.8 t/h, bed velocity 2.8 m/s, average drying air temperature 144 °C, bed height 13.5 cm, fraction of air recycled 0.8. Residence time of paddy was approximately 1.3 minutes. Test results showed that moisture content of paddy was reduced from 32.6 % dry-basis to 25.8 % dry-basis. Consumption of electrical power and diesel fuel was 12.9 kW and 21.71 1/h respectively. Primary energy consumption was 910.9 MJ/h. The dryer could evaporate water 218.8 kg/h. Specific primary energy consumption was 4.2 MJ/kg-water evaporated. Cost of paddy drying was 1.48 baht/kg-water evaporated of which 0.53 was fixed cost and 0.95 was energy cost (US$1 =34baht).     

Analysis of the drying process of a biopolymer (poly-hydroxybutyrate) in rotating-pulsed fluidized bed

A rotating-pulsed fluidized bed (RPFB) dryer was employed to conduct the drying of poly-hydroxybutyrate (PHB) cohesive granules. Along the experiments, it was possible to identify, visually, 3 different dynamic regimes that were related with the temperature profile, the drying kinetics and the fluid dynamic behavior. The drying kinetics of PHB showed a short constant drying rate period followed by a decreasing drying rate period. The constant drying rate (N_c) and final moisture content (dry basis) were related to the rotation frequency (responsible for the pulsation effect), temperature and velocity of the inlet air. Furthermore, measurements of molecular mass (gel permeation chromatography analysis) and Carr Index (flowability test) on PHB samples were done before and after the drying. The RPFB dryer showed to be appropriate to dry the PHB granules, resulting in an excellent fluid dynamic behavior that provided uniform drying of the solid. The best conditions of drying were identified at 7 Hz of rotation frequency, 90 °C and 0.55 m/s of inlet air temperature and velocity. At these conditions the dried PHB reached final moisture content of 0.56% (wet basis) after 2 h of drying.     

Drying characteristics of millet in a continuous multistage fluidized bed

The effects of gas velocity, inlet gas temperature and the solid feed rate on the drying efficiency, the outlet solid moisture content, bed temperature in each stage, the outlet gas humidity and temperature in a rectangular acryl multistage fluidized bed (0.172 m×0.192 m×1.5 m-high) with a downcomer (0.04 m-I.D.) were investigated. The experiments were performed by using 1.9 mm millet particles. The final moisture contents of the solids increased with increasing the solid feed rate. The drying efficiency increased with increasing the wetted solid feed rate but decreased with increasing the inlet gas temperature. The drying performance of the multistage fluidized bed was compared with the single-stage fluidized bed and found to be superior under identical operation conditions. The model predicted values were well matched with the experimental data in the multistage fluidized bed dryer. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

Drying Behavior of Carrots Dried in a Spout–Fluidized Bed Dryer

The effect of water blanching treatment and the inlet air temperature on drying kinetics as well as the quality attributes of carrot cubes dried in a spout–fluidized bed dryer at 60, 70, 80, and 90°C were analyzed. The material shrinkage and the rehydration potential were calculated to assess the changes in quality of dried carrots. It was found that the value of the air velocity during the drying of carrot cubes in a spout–fluidized bed dryer should be related to the moisture content of the carrot particles. A high value of air velocity at the beginning of the drying cycle and a lower value for the later stages were also required. The linear equation was correlated to the data of shrinkage of raw and blanched carrots. Blanching significantly influenced the coefficients in the shrinkage model derived for drying of carrot cubes in a spout–fluidized bed dryer, while drying temperature did not influence the shrinkage of carrot particles. The intensity of heat and mass transfer during spout–fluidized drying of carrot cubes was dependent on the drying temperature. A correlation was developed to calculate the values of effective moisture diffusivity of dried carrot cubes as a function of the moisture content and temperature of the material. It was observed that for any given time of rehydration, both the moisture content and the rehydration ratio calculated for samples dried at 60°C were higher than for samples dried at temperatures of 60, 70, 80, and 90°C.     

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.     

Drying and Heat Transfer Characteristics for a Novel Fluidized Bed Dryer

Abstract

The use of a fluidized bed dryer with a lateral air flow and mechanical agitation to the drying of sludge from a wastewater treatment plant was investigated. Experimental curves of moisture content vs. drying time, as well as heat transfer coefficients and the size characteristics of the products, were determined at temperatures between 80°C and 110°C, a stirring rate of 55 rpm and air velocity of 0.9 m/s for 3 kg sludge batches with initial moisture contents of 0.55 and 0.65 (d.b.). Experimental drying kinetics were compared with values derived from three models based on Fick's second law, namely: the constant diffusivity model, the simplified variable diffusivity model, and the modified quasi-stationary model.     

An Experimental Study and Mathematical Simulation of Wheat Drying

A fixed-bed dryer was designed and constructed and drying experiments with fixed beds of wheat were carried out under various conditions of drying air with wheat of several initial moisture contents. The air temperature and moisture content of wheat at various levels within the beds were measured periodically. A computer program based on energy and mass balances was developed to simulate the deep bed drying process. Experimental data from the dryer were compared with the results from this program. The results showed that there was good agreement between the simulated drying rates between the layers and those experimentally observed. In addition, there was a good agreement with respect to the shapes of the drying air temperature profiles.     

Experimental and Theoretical Investigation of Drying of Carrots in a Fluidized Bed with Energy Carrier

A pilot scale fluidized bed dryer with an inert energy carrier (steel, glass beads ranging from 2.7 to 6.5 mm) was used to investigate the drying of carrots. The effects of sample diameter, inert material type, inert material diameter, amount of inert material, air velocity, and temperature on the rate of drying were studied. A mathematical model was proposed for predicting the drying rate and temperature of drying material. It was found that presence of inert particles enhance the rate of drying. The results of this study also revealed that, although the rate of drying increases with decreasing sample diameter, increasing the inert material thermal conductivity, and increasing air temperature, but the inert material diameter and air velocity have no significant effects on the rate of drying. The independence of rate of drying on air velocity especially in well-fluidized systems indicates that external diffusion is not a controlling step in this process. Also the presence of inert materials causes the drying material to reach more rapidly to its final internal temperature.     

The drying kinetics of seeded grape in solar dryer with PCM-based solar integrated collector

This paper presents a novel type of dryer for experimentally evaluating the drying kinetics of seeded grapes. In the developed drying system, it has been particularly included an expanded-surface solar air collector, a solar air collector with phase-change material (PCM) and drying room with swirl element. An expanded-surface solar air collector has been used to achieve high heat transfer and turbulence effect whiles a solar air collector with PCM has been used to perform the drying process even after the sunset. On the other hand, the swirl elements have been located to give the swirl effect to air flow in drying room. These advantages make the proposed novel system a promising dryer in that lower moisture value and less drying time. The drying experiments have been carried out simultaneously both under natural conditions and by the dryer with swirl flow and without swirl flow at three different air velocities. The obtained moisture ratio values have been applied to six different moisture ratio models in the literature. The model having the highest correlation coefficient (R) and the lowest Chi-square (χ²) value has been determined as the most relevant one for each seeded grape drying status.     

Partial Crystallization Behavior during Spray Drying: Simulations and Experiments

Simulations of crystallization behavior in a spray dryer have been performed using modifications to a model developed by previous workers and applied to a Buchi-290 laboratory-scale dryer. The potential crystallization behavior has been modeled using Williams-Landel-Ferry kinetics. Explorations using the model have suggested that the air inlet temperature is an important variable affecting crystallization in the dryer. The explorations suggested drying conditions that permitted reasonable drying while controlling the degree of crystallization. These conditions were examined and tested experimentally, also showing that the apparent degree of crystallinity was affected by the inlet air temperature over the range of inlet temperatures from 134°C (∼55% crystalline) to 210°C (∼76% crystalline). The simulation also predicted the trends in the experimental results from previous workers where their experimental results are considered in terms of the measurement techniques used in each case, suggesting that the simulation is a reasonable tool for developing operating conditions for drying equipment to give low or high degrees of crystallinity in the products.