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.     

市政污泥热力干化过程中黏滞特性研究进展

黏滞现象是城镇污泥脱水干化过程中面临的主要问题之一,污泥在干化设备中的黏附和结团,导致干化设备运行效率大幅降低,给污泥干化系统的经济性和安全性带来显著的负面影响。本文从污泥黏附力和结团力入手,简要介绍了污泥黏滞现象的形成机理以及低含水率条件下污泥黏滞性的主要测量方法,包括搅拌法、平板法、黏附-结团失效特性法、阻抗法、黏附量法和滑板黏度法等,并对比分析了各种分析方法的优缺点。综述了包括胞外聚合物（EPS）、温度和污泥含水率等主要因素对污泥黏滞特性的影响规律,并简要探讨了各因素的影响机理。概述了国内外污泥降黏方法的研究进展,讨论了各种降黏方法的工业应用前景,同时探讨了污泥黏滞特性研究领域未来值得深入研究的方向。     

Experimental Study on Thermal Hydrolysis and Dewatering Characteristics of Mechanically Dewatered Sewage Sludge

After mechanical dewatering, sewage sludge has a moisture content of around 80 wt% and further disposal is required. A new sewage sludge semi-drying (dewatering) process is proposed and verified. It combines thermal hydrolysis and subsequent mechanical dewatering, with less energy consumption than traditional thermal drying. Sludge treated using this new process satisfies further disposal requirements (e.g., landfill or autothermal incineration). In the present study, a high-pressure test reactor was used to study the thermal hydrolysis of dewatered sludge. Thermally hydrolyzed sludge was subsequently dewatered by centrifugal sedimentation or by pressure filtration. The amount of organic compounds returning to the water phase was also measured. According to the results from centrifugal settling tests, the optimal thermal hydrolysis treatment temperature was 180°C. The moisture content then dropped to 1.44 kg/kg dry solids (DS; 59 wt%) after dewatering under relative centrifugal force of 9,000 × g from 5.67 kg/kg DS (85 wt%). Pressure filtration further reduced the moisture content of filter cakes to only 0.5 kg/kg DS (33 wt%, hydrolysis temperature 180°C). After thermal hydrolysis, the heating value of sludge (moisture-free basis) was about 80% that of the untreated sludge.     

Direct thermal drying of sludge using flue gas and its environmental benefits

This paper reported a sludge disposal technology that uses 100–200°C flue gas to dry sludge through a systematic analysis of the relationship between sludge drying rate/temperature and moisture content. Using this direct drying technology, the sludge drying capacity for the dryer tested can reach 86?tons?d^?1 at 160°C. The experimental results show this technology can also preserve 95% calorific value in the sludge, and remove 16–42% PM_2.5, 26–55% PM₁₀, and 7–25% SO₂ from the flue gas. The exhaust gas from the sludge dryer consists mainly of chain alkanes while benzenes only 9.65% when dried at 100°C.     

NaHCO3-enhanced sewage sludge thin-layer drying: Drying characteristics and kinetics

Drying sewage sludge is a highly energy-extensive process. For this reason, this work seeks to identify a reagent that can enhance the effectiveness of the drying process. In this study, drying experiments of sewage sludge were conducted at drying temperatures ranging from 100 to 160°C. NaHCO₃ was selected as the drying reagent, which was added to the sludge before drying. The thin-layer drying characteristics of the sludge and sludge/NaHCO₃ mixtures were later investigated and compared. Various mathematical models were used to simulate the sludge drying curves. It was found that adding 2 and 6% (wet basis) of NaHCO₃ to the sludge was effective in improving the moisture diffusion during the drying process, whereas the drying rate of the sludge/NaHCO₃ mixtures decreased when the addition of NaHCO₃ was further increased to 10% (wet basis). When the addition ratio was 2%, the increase in the maximum drying rate was the largest. With coefficients of determination (R²) over 0.9999, the modified Midilli model proposed in this study was observed to be the most suitable model to describe thin-layer drying of sludge relative to the other models examined in terms of R², reduced χ², root mean square error, and residual sum of squares. The values of the diffusion coefficients at each temperature were obtained using Fick’s second law of diffusion, which varied from 3.700?×?10^?9 to 1.085?×?10^?8?m²/s over the temperature range (i.e., 100–160°C). The activation energy of moisture diffusion was determined to be 27.57?kJ/mol. Scanning electron microscope images of the dried sludge and sludge/NaHCO₃ mixtures indicated that the porosity of the sludge after drying increased with an increase in the NaHCO₃ addition ratio. Overall, the results suggested that NaHCO₃ is a suitable reagent to improve the drying efficiency of the sludge.     

Study on Convective Drying Characteristics of Dredged Sludge from Dian Lake

In this work, the thin layer drying behavior of dredged sludge from Dian Lake by convective drying methods was investigated. The results showed that the Modified Page-I model was more suitable for thin-layer drying of dredged sludge. The values of the diffusion coefficients at each temperature were obtained using Fick’s second law of diffusion, and it was varied from 6.472×10^?9 to 1.143×10^?8 m²/s when the temperature was changed from 100 to 160°C for the dredged sludge of 10 mm. When the thickness was changed from 5 to 20 mm, the diffusion coefficients were varied from 4.036×10^?9 to 2.648×10^?8 m²/s at 140°C. The activation energy of moisture diffusion was 13.1 kJ/mol.     

Numerical Study on Moisture Transfer in Ultrasound-Assisted Convective Drying Process of Sludge

A coupled heat and moisture transfer model for ultrasound-assisted convective drying process of sludge was established. In this model, the permeable flow caused by acoustic pressure gradient in sludge was considered. The pore structure variety in sludge with ultrasonic irradiation was microscopically studied, and the pore size distribution of sludge was described by fractal geometry. Based on the fractal characterization, the physical properties of sludge including permeability, porosity, and tortuosity factor were determined, and the effective moisture diffusion coefficient of sludge under ultrasonic irradiation was also derived considering the effects of ultrasonic excitation energy and thermal effect on migration rate of water molecule. The effects of ultrasonic energy density and convective air temperature on convective drying process of sludge were numerically analyzed. The results showed that the ultrasonic irradiation changes the pore size distribution in sludge, the sludge flocs are dispersed, and the connectivity of pore structure is improved. Ultrasonic treatment is favorable to accelerating the moisture transport in the convective drying process of sludge, and the ultrasonic influence on moisture transport in sludge intensifies gradually with the increase of acoustic energy density from 0.2 to 0.6 W/ml. Furthermore, it can be also found that the enhancement effect of ultrasound on the average drying rate of sludge is more obvious at the connective air temperature of 65°C than that at 40°C under the uniform acoustic energy density and air velocity of 1.5 m/s.     

Experimental and Numerical Study of Flat Plate Sludge Drying at Low Temperature by Convection and Direct Conduction

This article deals with the drying of the waste by-product of an activated sludge manure treatment plant. The studies concern the low temperature drying of planar plates 3 cm thick. The upper face of the sludge is subjected to a tangential air flow controlled for speed, temperature, and humidity. The lower face can be subjected to contact heating. The maximum temperature for air and the bottom of the product is 60 °C. After characterizing the main thermophysical and hygric properties of the product, the laboratory set-up specifically designed for this study is described. Experiments are performed to determine temperature, mass loss, and deformation evolutions for different boundary conditions. These experimental data are used to build a numerical one-dimensional heat and mass transfer model to predict temperature and moisture content fields. Sludge is assimilated with a hygroscopic porous medium, and shrinkage is not taken into account. To validate this model, several comparisons between simulated and measured data are made for different drying methods (hot or fresh air convective drying, contact drying, and combined drying) and various drying conditions.     

Measurement of water content and moisture distribution in sludge by 1H nuclear magnetic resonance spectroscopy

Moisture distribution in sludge is essential for the examination of dewatering problems; however, sufficiently rapid and accurate methods of sludge moisture measurement are currently lacking. Hence, this study investigated a low-field ¹H nuclear magnetic resonance (NMR) method for measuring water content and moisture distribution in sludge. Moisture content measured by NMR was closely correlated with the thermal drying method (R² = 0.999). The loss of mechanical bound water from sludge was the primary cause of the decrease in water content from 96 to 37% during thermal drying at 40°C. NMR is more accurate, rapid, and nondestructive than other water distribution measurement methods.     

Analysis of D2 law in case of Shengli lignite drying under inert and uninert environment

Shengli lignite coal, originated from inner Mongolia China, contains significantly high amount of moisture (more than 30%) which can cause spontaneous combustion or other application problems. Thus, it is of interest to understand the heat and mass transfer mechanism of the low-rank lignite drying under different drying environments such as N₂, CO₂, air, argon, and helium. In this study, fundamental drying experiments with different drying agents were performed on coal samples using thermogravimetric analysis (TGA) method. Lignites with size of 0.045–0.075?mm were heated up from ambient temperature to a target temperature of 175°C under different environments at heating rates of 5, 10, 20, 40, and 80 °C/min, respectively. It was found that thermal conductivity of drying media, heating rate, and initial moisture content are three most significant factors affecting lignite drying process. The highest moisture release rate and the lowest T_peak (when maximum moisture release rate occurred) were observed when drying with helium due to its highest evaporation constant (i.e., highest thermal conductivity). Moreover, higher heating rate and moisture content resulted in higher evaporation rate and T_peak. In the meantime, the classical D² law, which is used to simulate the liquid fuel droplet combustion, was further developed to describe the “group effect” of moisture evaporation process of solid fuel during drying. The D² law well explains the experiment results. Finally, the structures of the dried lignite samples under different drying mediums were investigated through scanning electron microscopy studies. It was found that lignite coals shrank and became more compact when dried out, especially with drying agent CO₂.     

Complex Dielectric Properties of the Sapwood of Aspen,White Birch,Yellow Birch,and Sugar Maple

The dielectric properties of four Canadian eastern wood species were measured as a function of moisture content and temperature, at frequencies between 0.4 and 2.47 GHz using the cavity perturbation technique. The temperature was varied from ? 20 to 58°C and moisture content from 40 to 136%. Dielectric properties varied with moisture content, wood temperature, and microwave frequency. However, the influence of moisture content and microwave frequency overshadowed the influence of wood species and wood temperature. The dependencies were established for the selected species and discrepancies are discussed, namely concerning their implication when used in wood industry (nondestructive testing, heating, and drying).     

A Small-Scale Pneumatic Conveying Dryer of Rough Rice

This article studies the possibility of reducing the high initial moisture content of wet rough rice using a small-scale low-cost pneumatic conveying dryer as a first stage dryer. The parameters investigated are final moisture content, surface temperature of rough rice, head rice yield, drying rate, power consumption per unit mass of evaporated water, and physical characteristics of rice. Parametric effects of the following variables are examined: velocity of drying air from 20 to 30 m/s, feed rate of rough rice from 150 to 350 kg/h, initial moisture content from 22 to 26% (wet basis), and drying air temperature from 35 to 70°C. From the experimental results, it is found that this drying method can be used for fresh rough rice with an initial moisture content of over 24% (wet basis). The drying process is able to lead to very rapid drying without any grain quality problems such as cracks in the rice kernel. The moisture content can be reduced to approximately 18% (wet basis) or about 5–6% of the initial moisture content within 3–4 s. The optimal drying air temperature is in the range of 50 to 60°C. A comparison of pneumatic conveying drying data obtained from the present study with fluidized bed drying data reported in the open literature is also discussed.     

Ultrasonic Convective Drying Kinetics of Clipfish During the Initial Drying Period

A hybrid drying system for high-intensity airborne ultrasound applied in convective drying was investigated for the drying of salted codfish (clipfish). Convective drying with ultrasonic assistance at 10, 20, and 30°C was compared to the same process without ultrasound. The Weibull model was used to model and investigate the drying behavior, and the effective diffusion in Fick's law was determined. The ultrasound decreased the drying time more at lower drying temperatures. The drying time was reduced by over 90% at a drying temperature of 10°C. For an industrial drying process at a temperature of 20°C, the drying time was reduced by 32.2%. The ultrasonic, convective drying of clipfish at a temperature of 20°C was faster than the same process without ultrasound at 30°C. The investigations showed a thermal effect for all products when ultrasound was applied. The specific moisture extraction ratio (SMER) in the investigated system was improved by 0.2 kg_water kWh^?1. The heat transfer coefficient in the system used was increased by 32.6% for a heating process in a separate investigation, whereas for a cooling process no increased heat transfer coefficient was determined. The thermal effect might (at least partially) explain the faster drying of ultrasonic-assisted convective drying. The results obtained demonstrate the potential of airborne ultrasound in convective drying with regard to drying time, energy consumption, and product quality. Documentation of the thermal effect should be included in future R&D on this topic.     

Drying characteristics and modeling of yam slices under different relative humidity conditions

The drying characteristics of yam slices under different constant relative humidity (RH) and step-down RH levels were studied. A mass transfer model was developed based on Bi-Di correlations containing a drying coefficient and a lag factor to describe the drying process. It was validated using experimental data. Results showed that the drying air with constant RH levels of 20, 30, and 40%, temperature of 60°C, and air velocity of 1.5 m/s had an insignificant effect on drying time. This phenomenon was likely attributed to the fact that higher RH led to a rapid increase in sample’s temperature. The higher sample temperature could provide an additional driving force to water diffusion and thereby promote the moisture movement, which could minimize the negative effect of lower the drying rate in the initial drying stage. Applying air with 40% RH for 15 min in the initial stage achieved the desired color and reduced the drying time by 25% compared to the drying time under continuous dehumidification from an initial RH of 40%. Using the developed Bi-Di correlation, the estimated Biot number, effective moisture diffusivity, and mass transfer coefficient ranged from 0.1024 to 0.1182, 1.1133 × 10^?10 to 8.8144 × 10^?9 m²/s, and 1.8992 × 10^?9 to 1.7364 × 10^?7 m/s, respectively. A rather high correlation coefficient of determination (R² between 0.9871 and 0.9971) was determined between the experimental and predicted moisture contents. The present findings contribute to a better understanding of the effect of relative humidity on drying characteristics. The developed Bi-Di correlation provided a new method to determine the effective diffusivity of moisture in drying.     

Drying of Sewage Sludge During Composting in an Updraft Dryer with the Concept of Using Self-Energy Generated Within the System

An innovative technology for sewage treatment systems is proposed. A mixture of sewage sludge and char particle is subjected to drying in the progress of composting in an updraft column. Exothermic heat generated during composting is utilized for energy of sludge drying. The char particle in the mixture contributes to enhancement of composting and drying rate. This study presents drying and composting behaviors of sludge to examine the effect of char addition. A fundamental drying experiment was carried out by hot-air heating of the sample in a small vessel. The drying characteristic curve for a mixture of sludge and char was greater than the sample of only sludge over a whole period if the drying rates were compared with the same moisture content. The drying behavior could be analyzed with a reasonable agreement by the water front receding model. The enhancement effect of drying and composting of sludge by char addition was confirmed by examination using a pilot scale of the updraft column whose volume was 50 m³. Sludge with 400% dry base (80% wet base) in moisture content could be dried successfully to lower moisture content than 67% dry base (40% wet base) only by exothermic heat during composting without supplying any auxiliary energy from the outside, and the treatment time was reduced by mixing char in sludge. The behavior was analyzed with a satisfactory agreement by a theoretical model employing moisture diffusion parameters determined from the drying characteristic curve.     

Heat and Mass Transfer During Fry-Drying of Sewage Sludge

Deep-frying, which consists of immersing a wet material in a large volume of hot oil, presents a process easily adaptable to dry rather than cook materials. A suitable material for drying is sewage sludge, which may be dried using recycled cooking oils (RCO) as frying oil. One advantage is that this prepares both materials for convenient disposal by incineration.

This study examines fry-drying of municipal sewage sludge using recycled cooking oil. The transport processes occurring during fry-drying were monitored through sample weight, temperature, and image analysis. Due to the thicker and wetter samples than the common fried foods, high residual moisture is observed in the sludge when the boiling front has reached the geometric center of the sample, suggesting that the operation is heat transfer controlled only during the first half of the process followed by the addition of other mechanisms that allow complete drying of the sample. A series of mechanisms comprising four stages (i.e., initial heating accompanied by a surface boiling onset, film vapor regime, transitional nucleate boiling, and bound water removal) is proposed. In order to study the effect of the operating conditions on the fry-drying kinetics, different oil temperatures (from 120 to 180°C), diameter (D = 15 to 25 mm), and initial moisture content of the sample (4.8 and 5.6 kg water·kg^?1 total dry solids) were investigated.     

酒精污泥在内热式振动流化床中的干燥特性研究

以干污泥颗粒为惰性粒子,采用内热式振动流化床对酒精污泥进行了干燥实验,考察了流化气速、进气温度、振动频率和内加热功率对污泥干燥特性的影响,分析了污泥湿含量和干燥速率的变化规律。结果表明,干燥速率随着流化气速、进气温度和内加热功率的增加而增加,随着振动频率增加呈先增长后降低的趋势。适度提高流化气速、进气温度、振动频率和内加热功率可以降低最终湿含量。将干燥过程分为降速I段和降速Ⅱ段,根据扩散传质理论,建立了内热式振动流化床中污泥干燥的数学模型,模型计算值和实测值的误差在20%以内,符合较好。实验结果为酒精污泥干燥过程的工业设计和操作提供了实验依据。     

HEAT,MOMENTUM, AND MASS TRANSFER MEASUREMENTS IN INDIRECT AGITATED SLUDGE DRYER

ABSTRACT

Whereas indirect agitated drying has been extensively studied for granular materials, little is known in the case of pasty products. We describe an experimental set up specially designed for the investigation of drying kinetics, of heat transfer coefficient evolution, and of the mechanical torque necessary for stirring. This device was applied to municipal sewage sludge. Preliminary experiments were performed to investigate the influence of ageing of sludge on the drying kinetics. It appears that ageing does have no effect except for the first two days. The influences of the wall temperature, the stirrer speed, the dryer load and the location of the stirrer against the heated wall were studied. Three different rheological behaviors were observed during sludge drying. In particular, the sludge goes through a “glue” phase, and high levels of mechanical strain are recorded when the compact mass begins to break up. A critical stirrer speed is found within the range 40–60 rpm. To shorten the drying time, it seems better to adjust the wall temperature in accordance with the moisture content of the sludge.     

Design and Performance Evaluation of a Solar-Assisted Biomass Drying System with Thermal Storage

A solar biomass hybrid air heating system that does not require a conventional auxiliary heater but can still provide a daily load fraction exceeding 90% and supply hot air at a steady temperature and flow rate continuously for 24 h a day has been developed. The system, which combines an unglazed transpired solar collector, rock bed, and a biomass gasifier stove with heat exchanger, was evaluated by drying chilli using air at 60°C and 90 m³/h. The chilli was dried from 76.7% moisture (w.b.) to 8.4% over 32.5 h of continuous drying. The dryer reduced the drying time by 66% compared to open sun drying and provided 91.6% load fraction during the 24-h operation. The temperature of hot air supplied was stable at 60±3°C for about 21 h during the entire drying duration.     

Drying Efficiency of Indonesian Lignite in a Batch-Circulating Fluidized Bed Dryer

This research explores the production of low-moisture, high-rank coal using a batch-type, laboratory-scale, circulating fluidized bed to dry low-rank Indonesian coal with a high moisture content of 35 wt%. The operation was performed using air as a fluidization gas in a riser (a 4-m-tall pipe with an inner diameter of 0.04 m) at a gas velocity ranging from 2.0 to 2.7 m/s and a riser temperature of 80 to 150°C. The electric heaters were installed in the upper part of a downcomer to prevent the condensation of the evaporated moist- ure. The drying rate of the coal was investigated in terms of the inlet gas temperature, the gas velocity, and the drying time in order to determine the optimum operating conditions. Changes in the moist- ure content of the coal, before and after the experiments, were char- acterized by a proximate analysis, an ultimate analysis, the higher heating value (HHV), the lower heating value (LHV), a particle size analysis, and by the equilibrium moisture content. The results show that 70 to 80 wt% (wet basis, wb) of the total moisture can be reduced when the gas velocity of the riser is 2.0 m/s and the gas temperature is 150°C. In experiments, a simple mathematical model based on the heat and mass balances and a thin-layer drying model were simul- taneously used to predict the drying behavior of coal under the given operating conditions. The results of the model are similar to those of the experiment.