明胶热风干燥特性的实验研究

采用静态法研究了明胶的平衡含湿质量分数,得到了20℃下吸湿和解吸等温线,结果表明,当空气相对湿度在16%—39%,存在吸湿滞后现象。在对流干燥实验台上进行了明胶干燥特性的实验,以不同厚度的明胶块为实验对象研究了热风温度、风速、湿度对干燥过程的影响。实验结果表明:明胶的干燥过程只有降速阶段,提高热风温度、加大风速均可以在前期提高干燥速率,但在干燥后期干燥速率反而降低;明胶块中心温度受胶块厚度、热风温度影响较大,而在实验范围内空气相对湿度的变化对明胶中心温度影响甚微;明胶的相对含湿质量分数随时间呈指数规律下降,提高风温、加大风速后明胶含湿质量分数在开始阶段下降较快,但最终含湿质量分数反而偏高。     

不同加热功率下含油污泥的干燥

辽河油田的含油污泥含有大量的水分,为了更有效地进行油泥热解和减少热解过程中的能耗,需运用干燥技术处理油泥。实验考察了不同干燥功率对油泥干燥速率、湿分比的影响。实验结果表明:油泥干燥可以分成3个阶段,即恒速干燥阶段、第1降速干燥阶段和第2降速干燥阶段;加热功率越大,达到平衡含水质量分数所用的时间越短,恒速干燥速率越大。然后运用干燥方程求得干燥时间。最后运用热分析动力学对实验数据进行处理,得到干燥动力学的数学模型。模拟结果表明修正Page模型符合油泥干燥过程。     

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。     

生物质型煤干燥能耗的瞬态分析及预测模型

通过在不同干燥温度(140,160,180℃)、不同风速(0.4,0.8,1.2 m/s)下对生物型煤进行了干燥特性实验,并对其瞬时单位能耗进行了计算和分析,结果表明:生物质型煤干燥过程中,单位能耗曲线分为3个阶段:下降阶段、恒定阶段和上升阶段;当干燥速率处在升速阶段时,单位能耗随干燥温度和风速的提高下降迅速;当干燥速率处在恒速阶段时,单位能耗随干燥温度和风速的提高而降低;当干燥速率处在降速阶段时,单位能耗随干燥温度和风速的提高而快速上升。基于干燥特性数学模型——Sabbet方程,得到了生物质型煤干燥时瞬时单位能耗的预测模型,其可以有效地反映出生物质型煤在干燥过程中单位能耗瞬态变化,为生产和工艺改进提供指导。     

含油污泥为黏结剂制型煤干燥过程研究

为提高干燥效率、优化干燥效果,采用以含油污泥为黏结剂,与粉煤掺混干燥制备型煤的方式对含油污泥进行资源化处理。研究了掺煤量、干燥风速、干燥温度和型煤粒径对干燥过程的影响,并得到含水率和干燥速率随时间变化曲线。结果表明,以含油污泥为黏结剂制型煤的干燥过程可分为三个阶段:升速阶段、等速阶段和降速阶段。掺煤量主要影响降速阶段,风速主要影响升速阶段,温度主要影响升速阶段和等速阶段,粒径主要影响降速阶段。掺煤量越大、风速越大、温度越高、粒径越小,则干燥速率就越大。试验筛选出最佳干燥条件:掺煤量为65%、风速为2.75m/s、温度为105℃、粒径为10mm。对最佳干燥条件下的干燥过程进行干燥动力学模型拟合,求解干燥方程,得出Page(Ⅱ)模型相关系数平方为0.991时,拟合效果较好,能够反应其干燥特性。     

碳化硅超细粉球团热风干燥特性及干燥模型的研究

利用热风干燥实验装置并采用130℃、150℃、170℃的风温及2m/s、3m/s的风速对碳化硅超细粉球团的干燥特性及干燥模型进行研究。结果表明:碳化硅超细粉球团的干燥过程分为三个阶段,即升速、恒速和降速阶段,其中升速阶段的干燥时间较短约为20～30min,恒速阶段的干燥时间约为50～80min,降速阶段的干燥时间约为180～210min;在碳化硅超细粉球团干燥的恒速阶段,干燥速率由高到低的顺序为:170℃150℃130℃,在降速阶段,风温170℃时干燥速率下降最快;当风速高于2m/s时,碳化硅超细粉球团的干燥速率变化不大。通过对在不同温度和不同风速条件下的碳化硅超细粉球团干燥的实验数据与8个常用的干燥模型进行拟合分析,发现修正Page模型(Ⅱ)干燥模型拟合度最好,可以较好地反映出碳化硅超细粉球团在不同温度和不同风速下的干燥特性。     

过热蒸汽间歇干燥酒精糟研究

以高湿物料酒精糟为研究对象,进行了过热蒸汽间歇干燥实验研究。利用自行设计的间歇干燥实验台,酒精糟的初始含水质量分数为66.7%,研究了不同实验条件对干燥速率的影响。结果表明:随着干燥介质温度的升高,干燥速率明显加快;被烘物料质量越少,烘干时间越短;随着物料颗粒直径的减小,恒速段的干燥速率基本不变,而降速段的干燥速率明显增加;过热蒸汽质量流量越大,干燥速率越高。因此随着蒸汽过热度的升高、质量流量的增大,过热蒸汽干燥速率加快,干燥效率越高;随着进料质量的增加、颗粒直径的增大,过热蒸汽干燥速率却有所降低。     

氢氧化镁纳米棒的干燥速率曲线

以碱式氯化镁纳米棒为原料,采用沉淀转化法制备出氢氧化镁纳米棒滤饼;在不同干燥介质温度和不同物料床层厚度下,通过干燥动力学实验,得到了干燥速率曲线和干燥温度曲线。研究结果表明:在恒定干燥条件下,随着干燥介质温度的提高(或物料床层厚度的降低),氢氧化镁纳米棒的干燥速率加快,干燥时间缩短。当干燥介质温度较低(或物料床层厚度较大)时,对于某一干燥介质温度(或物料床层厚度)下的一条干燥速率曲线,氢氧化镁纳米棒的干燥过程可以分为升速、恒速、第一降速和第二降速四个干燥阶段。随着干燥介质温度的提高(或物料床层厚度的降低),干燥速率曲线中的恒速干燥阶段范围逐渐变小,直至消失。整个干燥速率曲线图可以分成为升速干燥区、恒速干燥区、第一降速干燥区和第二降速干燥区。     

基于气固流态化原理的油页岩干燥动力学

为了考察气固流化床干燥器能否使油页岩含水质量分数达到要求,以柳树河油页岩颗粒为原料,研究进口气体温度和颗粒直径对油页岩干燥性能的影响,采用薄层干燥模型,对油页岩干燥实验数据进行模拟,确定油页岩干燥方程和干燥速率方程,建立油页岩干燥速率特征常数和有效扩散系数之间的关联式。研究结果表明:薄层干燥模型中修正Page模型Ⅰ适合描述油页岩的干燥过程;油页岩在流化床内干燥过程主要发生在降速干燥阶段,进口气体温度越高,油页岩颗粒直径越小,所需干燥时间越短,进口气体温度为350℃时,使2.4 mm油页岩含水质量分数低于5%,所需干燥时间为2.5 min。     

胡萝卜对流干燥过程理论分析——临界水分质量比

收缩和非收缩条件下,对胡萝卜对流干燥特征进行了理论分析。发现临界水分质量比是试样几何形状、尺寸大小以及热空气温度、速度、相对湿度的函数。在收缩条件下,只有薄片状试样对流干燥过程包含恒速干燥阶段和降速干燥阶段;而圆柱状和球状试样干燥速率从干燥起始就随试样水分质量比的下降降低。数值实验结果表明,根据物料临界水分质量比很难确定对流干燥过程处于外部干燥条件控制的第1干燥阶段还是物料内部条件因素控制的第2干燥阶段。临界水分质量比不是表明物料干燥特性的特征参数。     

无霜空气源热泵系统冬季运行性能实验

为了解决传统空气源热泵系统冬季运行室外换热器结霜温度,提出了一种溶液除湿型无霜空气源热泵空调系统。该热泵系统在冬季可以无霜高效运行的同时夏季性能也有所提高。通过搭建该系统实验平台,研究了室外空气干球温度、湿度、供热水温度、供热水流量、溶液流量、溶液质量分数、室外空气流量等对冬季工况系统供热性能的影响,还研究了溶液流量、溶液温度、室外空气流量等对冬季工况系统再生性能的影响,得出了室外空气湿度、溶液质量分数对系统的供热性能影响较小,随着室外空气干球温度、供热水流量、溶液流量、室外空气流量等参数的升高和供热水温度的减小,系统供热性能逐渐增大最高可达3.11,而随着溶液流量、空气流量等参数的升高和溶液温度的减小,系统再生性能逐渐增大最高可达4.03,系统供热综合COP在实验工况相较逆循环除霜系统有所提升,实验验证了该系统适用于低温高湿地区。     

THE INFLUENCE OF THE DRYING MEDIUM ON HIGH TEMPERATURE CONVECTIVE DRYING OF SINGLE WOOD CHIPS

High temperature convective drying of single wood chips with air and superheated steam respectively is studied theoretically. The two-dimensional model presented describes the coupled transport of water, vapour, air and heat. Transport mechanisms included are the convection of gas and liquid, intergas as well as bound water diffusion. In the initial part of the drying process, moisture is transported to the surface mainly due to capillary forces in the transversal direction where evaporation occurs, As the surface becomes dry, the drying front moves towards the centre of the particle and an overpressure is simultaneously built up which affects the drying process

The differences between drying in air and steam respectively can be assigned to the physical properties of the drying medium. The period of constant drying rate which does not exist (or is very short) in air drying becomes more significant with decreasing amounts of air in the drying medium and is clearly visible in Dure superheated steam drying. The maximal drying rate is larger in air drying, and shorter drying times are obtained since the heat flux to the wood chip particle increases with increasing amounts of air in the drying medium. The period of falling drying rate can be divided into two parts: in the first, the drying rate is dependent upon the humidity of the drying medium whereas in the second, there is no such correlation.     

THE INFLUENCE OF THE DRYING MEDIUM ON HIGH TEMPERATURE CONVECTIVE DRYING OF SINGLE WOOD CHIPS

ABSTRACT

High temperature convective drying of single wood chips with air and superheated steam respectively is studied theoretically. The two-dimensional model presented describes the coupled transport of water, vapour, air and heat. Transport mechanisms included are the convection of gas and liquid, intergas as well as bound water diffusion. In the initial part of the drying process, moisture is transported to the surface mainly due to capillary forces in the transversal direction where evaporation occurs, As the surface becomes dry, the drying front moves towards the centre of the particle and an overpressure is simultaneously built up which affects the drying process

The differences between drying in air and steam respectively can be assigned to the physical properties of the drying medium. The period of constant drying rate which does not exist (or is very short) in air drying becomes more significant with decreasing amounts of air in the drying medium and is clearly visible in Dure superheated steam drying. The maximal drying rate is larger in air drying, and shorter drying times are obtained since the heat flux to the wood chip particle increases with increasing amounts of air in the drying medium. The period of falling drying rate can be divided into two parts: in the first, the drying rate is dependent upon the humidity of the drying medium whereas in the second, there is no such correlation.     

Measurement of constant drying rate of wet material placed in a fluidized bed of inert particles under reduced pressure

The objective of this study is to estimate the drying characteristics of a relatively large material immersed in a fluidized bed under reduced pressure by measuring the constant drying rate. The constant drying-rate period in a fluidized bed under reduced pressure is difficult to measure because it is extremely short. To maintain the constant drying-rate period, distilled water is directly supplied to the drying material. Through our experiment, the heat transfer coefficient of the material surface was also determined. The results were compared with data on hot air drying. The constant drying rate is higher for fluidized bed drying than for hot air drying. It suggests that the heat transfer coefficient on the surface of the drying material is much larger for fluidized bed drying than for hot air drying. For fluidized bed drying, the effect of pressure in the drying chamber on the heat transfer coefficient is slight at the same normalized mass velocity of dry air (G/G_mf). The temperature difference between the inside of the drying chamber and the drying material is much smaller for fluidized bed drying than for hot air drying. The constant drying rate increases as the pressure in the drying chamber decreases.     

Study of dynamic multi-dimensional temperature and concentration distributions in liquid-sprayed fluidized beds

A physically based model was developed for heat and mass transfer processes in liquid-sprayed fluidized beds. Such fluidized beds are used for granulation, coating and agglomeration. The complex correlations of a number of microprocesses, spraying, wetting, drop deposition, heat transfer, drying and mass transfer were studied, and transient three-dimensional distributions of air humidity, air temperature, particle wetting efficiency, liquid film temperature, particle temperature, local liquid loading and local evaporation rate were calculated. For the evaluation of the model, the stationary spatial air temperature distributions were measured at a fluidized bed pilot plant of the institute. The fluidized bed of monodisperse wood- or glass beads was sprayed with clear water. Conclusions are drawn on the relevance of particle dispersion, spraying and drying to simulating temperature and concentrations distributions.     

Exergetic Analysis of Textile Convective Drying with Stenters by Subsystem Models: Part 2—Parametric Study on Exergy Analysis

In this study, the effects of exhaust air humidity ratio, the residual moisture content of fabric outlet, and the temperature of the drying air on the exergy destruction and efficiency of stenters were investigated. The exergy efficiencies of the direct gas heated stenter (DGHS) and hot oil heated stenter (HOHS) were calculated to be varying from 8.5 to 17.5% and from 6.8 to 14.0%, depending on the exhaust air humidity ratio, respectively. The increase in the drying air temperature led to an increase in the exergy efficiency, especially in the constant rate and second rate period of the drying. On the other hand, the application of the gradual temperature method caused the highest total exergy efficiency due to the highest drying rates in the first chambers where considerably high air temperatures were set. Overdrying resulted in the higher irreversibility due to the increase in the fuel consumption in the falling rate period of drying. Thus, the exergy efficiency decreased drastically.     

Kinetics of drying inorganic spheres: Simultaneous modeling of moisture and temperature during the constant and falling rate periods

The kinetics of the dehydration of four sizes of spherical inorganic materials was studied experimentally with warm air using a fixed-bed dryer at laboratory scale. Following the evaluation of the effect of bed thickness, air mass rate, and air temperature on drying kinetics, a kinetic model consisting of two equations was proposed. The main novelty of this study resides in the possibility of utilizing a new model for simultaneous heat and mass transfer in an efficient manner for practical applications and appropriate system optimization, especially during the falling rate period.     

SUPERHEATED STEAM DRYING: A BIBLIOGRAPHY

An experimental setup was developed to study the through air–drying characteristics of permeable grades such as tissue and towel under commercially relevant conditions of basis weight, airflow rate, temperature, and humidity conditions. The experimental setup is capable of evaluating the transient fluid flow, heat, and mass transfer characteristics of relatively larger samples (TAPPI standard hand sheets; 0.1524 m) and is capable of studying the effect of local heterogeneity and structure on convective heat and mass transfer. The system is capable of airflow rates of 0.5–10 m/s with corresponding high-speed data collection and acquisition for measuring important variables such as exhaust air humidity. To study the effect of nonuniformity, local temperature and velocity profiles can also be measured using grid of thermocouples and hot wire anemometers. The instantaneous drying rate and airflow characteristics during through air drying was measured and dry permeability, wet permeability, and convective heat and mass transfer characteristics were then calculated. The experimental results were verified by comparing with the results from literature. Typical experimental results were presented to show the effect of sheet basis weight, initial moisture content, and airflow rates on the drying characteristics for two different types of paper samples.