Paste Residence Time in a Spouted Bed Dryer. III: Effect of Paste Properties and Quality Interactions

The aim of this study was to evaluate the effects of paste properties on residence time during drying in a spouted bed dryer with inert bodies. The effect of paste solids content, surface tension, and viscosity on the residence time distribution and the mean residence times were studied using factorial experimental designs. The inert bodies used were glass and polyethylene beads. The mean residence times varied from 13.6 to 16.3 and 12.2 to 17.7 min for drying on glass and polyethylene beads, respectively. The analysis of variance showed that mean residence times significantly depended on solids content and surface tension for glass beads and also on viscosity for polyethylene beads. The residence time distributions for all conditions studied fitted well to the perfect mixing cell when applying the continuous stirred vessels in series model analysis. The powder density, flowability, and particle size depended on paste properties and inert type.     

Paste Residence Time in a Spouted Bed Dryer. II: Effect of Spout Operational Conditions

Residence time distributions for an aqueous solution of 10% sodium chloride in a spouted bed dryer of inert particles were determined using the stimulus-response technique. Glass and polyethylene beads with diameters 2.6 and 3.4 mm were used as inert bodies in a cylindrical column of 14.0 cm diameter and 60° conical base. The effects of inert bodies load, air, and paste flow rates on the mean residence times and RTD were determined following 2³ factorial designs. The RTD could be correlated to the perfect mixing cell model with R² varying from 0.8684 to 0.9815. The mean residence times in CSBD varied from 10.8 to 13.9 and 10.7 to 13.3 min for glass and polyethylene beads, respectively. For both inert particles, mean residence times increased with bed height and decreased with paste feed rates. Also, terms of interaction among the factors were significant in some cases, showing a complex behavior of paste residence times. Equations obtained by response surface regression could predict mean residence times on glass and polyethylene beads with deviations lower than ±10%.     

Paste Residence Time in a Spouted Bed Dryer. II: Effect of Spout Operational Conditions

Residence time distributions for an aqueous solution of 10% sodium chloride in a spouted bed dryer of inert particles were determined using the stimulus-response technique. Glass and polyethylene beads with diameters 2.6 and 3.4 mm were used as inert bodies in a cylindrical column of 14.0 cm diameter and 60° conical base. The effects of inert bodies load, air, and paste flow rates on the mean residence times and RTD were determined following 2³ factorial designs. The RTD could be correlated to the perfect mixing cell model with R² varying from 0.8684 to 0.9815. The mean residence times in CSBD varied from 10.8 to 13.9 and 10.7 to 13.3 min for glass and polyethylene beads, respectively. For both inert particles, mean residence times increased with bed height and decreased with paste feed rates. Also, terms of interaction among the factors were significant in some cases, showing a complex behavior of paste residence times. Equations obtained by response surface regression could predict mean residence times on glass and polyethylene beads with deviations lower than ±10%.     

Paste Residence Time in a Spouted Bed Dryer. IV: Effect of the Inert Particle Size Distribution

The residence time distribution and mean residence time of a 10% sodium bicarbonate solution that is dried in a conventional spouted bed with inert bodies were measured with the stimulus-response method. Methylene blue was used as a chemical tracer, and the effects of the paste feed mode, size distribution of the inert bodies, and mean particle size on the residence times and dried powder properties were investigated. The results showed that the residence time distributions could be best reproduced with the perfect mixing cell model or N = 1 for the continuous stirred tank reactor in a series model. The mean residence times ranged from 6.04 to 12.90 min and were significantly affected by the factors studied. Analysis of variance on the experimental data showed that mean residence times were affected by the mean diameter of the inert bodies at a significance level of 1% and by the size distribution at a level of 5%. Moreover, altering the paste feed from dripping to pneumatic atomization affected mean residence time at a 5% significance level. The dried powder characteristics proved to be adequate for further industrial manipulation, as demonstrated by the low moisture content, narrow range of particle size, and good flow properties. The results of this research are significant in the study of the drying of heat-sensitive materials because it shows that by simultaneously changing the size distribution and average size of the inert bodies, the mean residence times of a paste can be reduced by half, thus decreasing losses due to degradation.     

Paste Residence Time in a Spouted Bed Dryer. I: The Stimulus-Response Methodology

The stimulus response methodology was evaluated in an experimental study conducted to measure NaCl solution residence time distributions in a spouted bed dryer of inert particles. The influence of tracer volume, concentration, and injection mode upon the measured mean residence time and residence time distribution was determined following experimental design techniques. The injection mode showed no significant effect on residence times, whereas tracer volume and concentration had significant effects at the levels of 1 and 5%, respectively, under the conditions chosen initially. The steepest ascent and factorial design methods were applied to determine the range in parameters for optimum stimulus response. The procedure repeated for a polymeric paste showed similar results. However, the results also showed that care must be exercised when using the stimulus response technique and the optimum injection conditions must be determined. The procedure presented in this study may be adopted in future investigations of paste residence time distributions in spouted bed dryers of inert bodies.     

Paste Residence Time in a Spouted Bed Dryer. I: The Stimulus-Response Methodology

The stimulus response methodology was evaluated in an experimental study conducted to measure NaCl solution residence time distributions in a spouted bed dryer of inert particles. The influence of tracer volume, concentration, and injection mode upon the measured mean residence time and residence time distribution was determined following experimental design techniques. The injection mode showed no significant effect on residence times, whereas tracer volume and concentration had significant effects at the levels of 1 and 5%, respectively, under the conditions chosen initially. The steepest ascent and factorial design methods were applied to determine the range in parameters for optimum stimulus response. The procedure repeated for a polymeric paste showed similar results. However, the results also showed that care must be exercised when using the stimulus response technique and the optimum injection conditions must be determined. The procedure presented in this study may be adopted in future investigations of paste residence time distributions in spouted bed dryers of inert bodies.     

INFLUENCE OF SOLIDS MOISTURE CONTENT ON THE AVERAGE RESIDENCE TIME IN A ROTARY DRYER

The solids mean residence time in a rotary dryer is influenced by several variables such as dryer dimensions and solids characteristics. One of these characteristics, usually not taken into account in correlations proposed to estimate the mean residence time, is the solids feed moisture content. Although it is well known that the solids moisture content has a major impact on the ability of the solids to move along the rotary dryer, it does not enter as a parameter in available correlations. In this investigation, numerous experiments were performed in a pilot-scale rotary dryer to study the influence of solids moisture content and drying gas temperature on the mean residence time. Sand employed in cement makeup was used to perform these experiments. Results show that the mean residence time for a moisture content in the range of 8% to 12% is four times higher than for dry solids. The moisture content and the drying gas temperature influence significantly the shape of the residence time distribution curve.     

阳离子易染共聚酯的热性能研究

在半连续大装置上合成了含第三单体间笨二甲酸乙二脂-5-磺酸钠(SIPE),第四单体聚乙二醇(PEG)的阳离子易染共聚酯(ECDP);采用热失重分析、差示扫描量热法对ECDP的热性能进行了研究,并对ECDP切片干燥和熔融过程的特性粘数降进行了测试.结果表明:随行PEG含量的增加,ECDP的热分解温度下降,玻璃化转变温度相...     

内热式惰性粒子流化床中膏状物料干燥模型

通过对惰性粒子流化床中膏状物料干燥机理的分析,得到了干燥时间及单位面积床层水分汽化量的数学计算式,可对干燥器的性能进行预测。采用带浸没加热管的惰性粒子流化床对膏状钛白物料进行干燥中试研究,采用气流式喷嘴将膏状物料分散成200～400μm的小液滴喷洒在惰性粒子表面进行干燥,探索了适宜的干燥条件,测定和确定了最佳的干燥工艺参数、操作参数和设备参数。结果表明:该干燥工艺能强化床内传热传质,促进高黏性膏糊状物料很好地分散,床层温度分布均匀,干燥器的操作弹性大,热量消耗低,干燥强度高,传热系数可达300W·m-2·K-1以上。     

氯乙烯-醋酸乙烯酯共聚糊树脂次级粒子特性研究

讨论氯乙烯-醋酸乙烯酯共聚乳液后处理工艺与树脂次级粒子特性间的关系。根据氯醋糊树脂性能特点,调整喷雾干燥器进出口温度,研究干燥工艺条件与树脂次级粒子结构及树脂增塑糊粘度间的关系,乳胶固含量对树脂次级粒子及糊粘度的影响规律,确定合适干燥工艺及乳液含固量,降低树脂糊粘度。对优化条件下制备的氯醋糊树脂颗粒特性、流变性能进行分析和对比,获得良好结果。     

Paste extrusion of polytetrafluoroethylene (PTFE): Surface tension and viscosity effects

The effects of the lubricant physical properties on the processing of polytetrafluoroethylene (PTFE) fine powder resins are studied. Lubricants having different surface tension and viscosity were used; the two properties changed independently. These effects were studied by using dies of various contraction angle and reduction ratio for resins having a variety of molecular architecture. It was found that the wettability (surface tension) of the lubricant strongly affects the pressure needed to extrude the PTFE pastes. The viscosity of the lubricant was also found to play a significant role in the process since a lubricant with a low viscosity causes the paste to be extruded at a lower pressure. These effects of the physical properties on the extrusion pressure influence significantly the mechanical properties of the final extrudates. The latter are functions of the degree of fibrillation, which is significantly influenced by the wettability and viscosity of lubricants. Finally, the effects of die geometry on extrusion pressure and mechanical properties of extrudates were also assessed in order to determine the geometrical characteristics and operation conditions for the optimization of the process.     

Coating and Drying in Spouted Bed: Influence of the Liquid-Particle Work of Adhesion

The aim of this work is to analyze the resulting process: coating or drying, and the respective fluid dynamic behavior, by bottom-spraying polymeric suspensions on a spouted bed of inert particles. Glass beads, ABS®, polypropylene (PP), and polystyrene (PS) were chosen as inert particles and were analyzed and characterized by their physicochemical properties. The polymeric suspensions were characterized by density, surface tension, rheology, and wettability. The fluid dynamic behavior of the bed was correlated with the particles and suspensions characteristics for each process: coating and drying. Each process performance was correlated with the polymeric suspension-particle work of adhesion.     

Coating and Drying in Spouted Bed: Influence of the Liquid-Particle Work of Adhesion

The aim of this work is to analyze the resulting process: coating or drying, and the respective fluid dynamic behavior, by bottom-spraying polymeric suspensions on a spouted bed of inert particles. Glass beads, ABS®, polypropylene (PP), and polystyrene (PS) were chosen as inert particles and were analyzed and characterized by their physicochemical properties. The polymeric suspensions were characterized by density, surface tension, rheology, and wettability. The fluid dynamic behavior of the bed was correlated with the particles and suspensions characteristics for each process: coating and drying. Each process performance was correlated with the polymeric suspension-particle work of adhesion.     

内置换热管振动流化床传热特性实验研究

在内置换热管的振动流化床中以玻璃珠为惰性粒子实验物料,测定惰性粒子振动流化床与加热管之间的传热系数,研究了惰性粒子处于流化状态时的传热特性,分析了操作气速、振动频率、惰性粒子直径等因素对传热过程的影响,建立了传热系数与各影响因素之间的关联式。研究结果可为干燥膏状物料时确定适宜的操作参数提供参考。     

超细粉体对水泥净浆流变行为影响研究

采用截锥圆模法测定不同掺量超细粉体水泥复合浆液的流动度.采用ZNN-D6B型旋转黏度计研究超细玻璃粉和偏高岭土两种超细粉体对水泥净浆流变行为的影响,得到了剪切速率-剪切应力(γ-τ)曲线和剪切速率-表观粘度(γ-μa)曲线,并分别采用宾汉姆模型和赫-巴模型对γ-τ流变曲线进行拟合,得到不同掺量超细玻璃粉-水泥(GP-C)复合浆液和偏高岭土-水泥(MK-C)复合浆液的动切力、塑性粘度、稠度系数和流性指数等流变参数.结果表明:超细粉体的加入降低了复合浆液的流动度.随着掺量的增加,两种复合浆液的宾汉动切力τ0、塑性粘度η、赫-巴动切力τy均逐渐增大,MK-C复合浆液的稠度系数K和流性指数n逐渐减小,GP-C复合浆液的稠度系数K呈现增大-减小-增大的趋势,而流性指数n呈现减小-增大-减小的趋势.所有样本表观粘度μa都随着剪切速率的增大而减小,呈现剪切稀释现象.     

A STOCHASTIC APPROACH TO THE DRYING OF SOLIDS IN A MULTI-STAGE FLUIDIZED BED

The residence times of solids flowing through a fluidized bed dryer exhibit dispersion about the mean. In this paper, expressions for the probability density functions of solids moisture content in the various stages of a multi-stage dryer are derived. A simple recurrence relationship for the moments of the distribution is also presented. The analysis is applied to the drying of cereal grains, and it is shown that the degree of drying increases with the number of stages in the dryer. Probability density functions of the moisture content are presented.     

A STOCHASTIC APPROACH TO THE DRYING OF SOLIDS IN A MULTI-STAGE FLUIDIZED BED

The residence times of solids flowing through a fluidized bed dryer exhibit dispersion about the mean. In this paper, expressions for the probability density functions of solids moisture content in the various stages of a multi-stage dryer are derived. A simple recurrence relationship for the moments of the distribution is also presented. The analysis is applied to the drying of cereal grains, and it is shown that the degree of drying increases with the number of stages in the dryer. Probability density functions of the moisture content are presented.     

Solids flow pattern in gas-flowing solids-fixed bed contactors. Part I: Experimental

An experimental investigation of the solids flow pattern in gas-flowing solids-fixed bed contactors is presented. The apparatus and procedures for determining the dynamic and static solids holdups, solids residence time distribution and the extent and rate of the exchange between particles in the static and dynamic solids holdup are described in detail.Experiments were performed in a bench scale system, containing a column (diameter ) packed with glass beads of 16 mm in diameter packed up to the height of 0.8 m. Tracer experiments with a step input in flowing solids phase were used for determining the residence time distribution and exchange between particles. Fine solids (spheres with mean diameter of ) of two different colors (all other properties being the same) were used in the tracer experiments to determine the residence time distribution and the exchange between static and dynamic solids holdup. In both types of experiments, the response curves have been obtained via color analysis of digital photos. All experiments have been repeated at different operating conditions, with a broad variation of solids mass flux and gas velocity, and reproducibility at set conditions was checked.The obtained experimental results are discussed and the observed important characteristics of the solids flow pattern are outlined. The effects of the solids flux and gas velocity on the solids flow pattern are presented and analyzed.     

Evaporation of Distilled Water in a Fluidized Bed of Various Types of Fluidizing Particles Under Reduced Pressure

An objective of this study is to continuously obtain dispersed, dry, fine powders from a dilute suspension of heat-sensitive materials at a low temperature and high drying rate. A fluidized bed under reduced pressure was used in this study and, as a first step, only distilled water (without solid powders) was used as a sample. Drying characteristics were examined for various types of fluidizing particles. The diameter of fluidizing particle varied for inert particles (glass beads). Three kinds of hygroscopic porous particles (silica gel beads; 3A, 4B, and 5D) were also used as fluidizing particles. Under reduced pressures, the maximum drying rate was found to be higher than that at atmospheric pressure, while the bed temperature became lower with an increase in the maximum drying rate (i.e., drying at lower temperature with a higher drying rate is possible under reduced pressures. As diameter of the fluidizing particle was increased, the maximum drying rate became higher, although the amount of gas required for fluidization also increased. The maximum drying rates for silica gel beads were found to be almost equal to those of glass beads, with the exception of 3A silica gel beads (having a smaller pore diameter). The bed temperature was lower for silica gel beads compared to glass beads at the same maximum drying rate (i.e., silica gel beads (hygroscopic porous particles) are superior to glass beads (inert particles) with regard to drying at low temperatures at a high drying rate).     

Effect of aggregates and microcracks on the drying rate of cementitious composites

We investigated the effect of aggregate concentration on the drying rate of cementitious composites with glass beads, sand grains, or expanded polystyrene (EPS) beads as aggregates. The drying rate of composites with non-diffusive aggregates (glass beads and sand grains) decreased with aggregate concentration. Composites with 60% glass beads dried a factor 1.3 slower than plain cement paste with the same w/c-ratio. Composites with EPS-beads showed the opposite trend: an increased drying rate for composites with higher aggregate concentrations. However, the effective diffusion coefficients of the EPS-composites decreased with increasing aggregate concentration. A higher aggregate concentration means that less water needs to diffuse out of the material to reach a specific degree of drying, and this effect mainly determined the drying rate of the composites with EPS-beads. The development of drying shrinkage microcracks had a small effect on the drying rate of the studied composites.