HIGH-INTENSITY DRYING OF PAPER

High-intensity contact drying denotes drying under suf- ficiently intensive heating conditions that, following a brief warmup period, the mist paper web operates at internal tem- peratures in excess of the ambient boiling point. A simplified, two-zone analytical model is first presented. The paper is depicted as having a dry layer, of ever-increasing thickness, adjacent to the hot surface. Heat conduction through this layer (the rate-limiting step) causes evaporation at the interface with the “wet zone.” The vapor is then considered to flow through the wet zone into the ambient. Results of bench-scale experiments are discussed. Drying rates as much as twenty times conventional rates are indicated. The drying rate increases with hot surface/boiling point tem- perature difference and applied mechanical pressure. The instsn- taneous drying rate decreases continuously after a brief warmup period. The vapor pressure at the hot surfacelpaper interface rises quickly to a peak value, followed by a continuously- diminishing level. The heat flux shows a rapid rise to boiling- like conditions, followed by a drop to a range compatible with measured drying rates. The temperature of the open side of the sheet is constant during drying, after its initial rise to the boiling point. The experimental results are compatible with pre- dictions of the model.     

An approach for indirect monitoring of moisture content in rubberwood (Hevea brasiliensis) during hot air drying

Abstract

This study aimed to determine the equilibrium moisture content and to develop an indirect measurement technique for the moisture content (MC) by observing temperature and vapor pressure during hot air drying. The temperature and gas pressure were recorded during hot air drying at several points within wood sample specimens conforming to ASTM D 143 and AOAC, 1990. The moisture content was estimated from measured temperature and pressure. The accuracy of MC estimates was validated by oven-drying method. For validation, nine experiments at different temperatures of hot air drying were run and the indirect measurement was found to provide a good accuracy. The obtained statistics were R² = 82.5%, standard error (SE) ranging from 0.15 to 0.43, root mean square error (RMSE) ranging from 0.16 to 0.38 and mean absolute error (MAE) ranging from 0.4 to 1.1 respectively. It can be concluded that the air in wood was removed completely, especially when the MC was below the fiber saturation point. We have demonstrated an alternative moisture content monitoring method for potential adoption by the rubberwood industries.     

HIGH INTENSITY DRYING

A mathematical model simulating the heat and mass transfer process during high intensity drying of paper and board has been developed. The model is successful in predicting the vapor pressure developments, pressure driven bulk flow of liquid and vapor, and increased drying rates during high-intensity drying, closely matching the experimental determination.

The model predicts substantial amounts of water removal in the liquid form during high-intensity drying being pushed out of the web by pressurized vapor zone. Water removal by pressure flow of liquid could account for as much as one-third of the total water removed.

Similar to drying under conventional conditions, the existence of a dry zone, wet zone and an intermediate zone with accompanying advancing heat pipe has also been shown for drying under high intensity conditions.     

无恒速段干燥过程特征及实践

讨论了被干燥湿物料中含有在水中溶解度较大、且不挥发的第三组分对干燥过程的影响 ,运用化学热力学原理和干燥动力学理论对生产实践中的此类干燥问题进行分析 ,证明此类物系的干燥过程无恒速段 ,且溶剂饱和蒸气压下降较大 ,增加了干燥的难度。结论由实验得到验证。建议关注此类物料的干燥     

高初始含湿多孔介质喷雾干燥过程的热质耦合传递

多孔介质是大量干燥过程的主体,由于实际多孔介质干燥过程的复杂性,建立通用的干燥过程传热传质模型十分困难。通过分析喷雾干燥过程中高初始含湿多孔介质与干燥介质之间的传热传质机理以及各因素对传热传质的影响,根据马歇尔方程探讨了干燥介质与料雾之间的水蒸汽分压差在干燥过程中的变化情况,反映了多孔湿介质在喷雾干燥操作中的传热传质过程的几种特性,为确定实际生产中喷雾干燥器的操作条件指明了新的出路。     

DYNAMIC PRESSURE RISE IN THE DRYING CHAMBER AS A REMOTE SENSING METHOD FOR MONITORING THE TEMPERATURE OF THE PRODUCT DURING THE PRIMARY DRYING STAGE OF FREEZE DRYING

Abstract

A novel dynamic pressure rise method is developed as a remote sensing procedure for determining at different times during the primary drying stage of the freeze drying process (i) the temperature of the moving interface between the dried and frozen layers of the product, (ii) temperature close to the upper surface of the dried layer of the product, (iii) the temperature of the bottom surface of the frozen layer of the product, and (iv) the temperature profile of the frozen layer of the product. Furthermore, by knowing the temperature of the heating plate and determining the value of the temperature of the moving interface from the dynamic pressure rise method, the value of the position of the moving interface could be determined by an expression presented in this work     

ANALYSIS OF HEAT AND MASS TRANSFER DURING DRYING OF PAPER/BOARD

ABSTRACT

A mathematical model has been successfully developed to study the heat and mass transfer process during paper drying. This model takes into account the consective transfer of vapor and liquid apart from the known transport mechanisms of capillary flow of liquid, diffusion, vaporization-condensation, and heat conduction. The partial differential equations describing temperature, saturation and pressure change within the web during drying with associated boimdary conditions and initial conditions were solved using finite difference method. The model predictions show that during the drying process the web can be conveniently divided into three different zones, namely dry zone, wet zone and an intermediate zone. The movement of liquid and vapor in opposite directions in the intermediate zone is similar to the action of a heat pipe. Also, as drying proceeds the location of the intermediate zone and hence the heat pipe advances progressively through the thickness of the web.     

MVR空心桨叶干燥污泥的特性及动力学

基于空心桨叶干燥机建立了一套机械蒸汽再压缩式热泵干燥系统,将相同质量的污泥(100±0.1)kg,在真空压力约为95kPa、压缩机出口蒸汽温度为95~115℃进行恒温干燥实验,从而得到污泥干燥过程中含水率以及干燥速率等变化曲线,压缩机出口蒸汽温度为95~115℃,污泥临界湿含量从0.22增加到0.34。引入5种常用的污泥干燥模型,利用Origin软件对实验数据进行分段拟合分析,得到污泥加速阶段和降速阶段的干燥动力学模型MR=exp(-ktⁿ)。以分段函数形式表示的干燥方程分别为:MR=exp[-(2.78×10^-4T-0.01896)t^1.596],(加速阶段);MR=0.894-0.564(1.737×10^-4T-8.05×10^-3)t(恒速阶段);MR=exp[-(2.26×10^-4T-0.01365)t^1.984](降速阶段)。对实验得到的干燥模型进行验证,计算得到各干燥阶段污泥湿含量平均相对误差,并将实验所得干燥曲线和干燥速率曲线与模型计算值进行比较,可以看出分段处理能较好的描述污泥干燥规律。     

Solidification by Convective Drying of Particle Layer Wetted with Dilute Agar Gel

Abstract:

Solidified porous slab is formed through convective drying of glass particle layer wetted with aqueous dilute agar gel. Measured critical mean moisture content increases with increasing initial moisture or agar content. The agar gel moves in viscous flow caused by capillary pressure during drying. A new drying model based on the receding evaporation plane model is proposed. Drying period is divided into surface and internal evaporation periods. Wet slab consists of dried and wet zones during the internal evaporation period, while the wet slab consists of wet zone only during the surface evaporation period. In the new model, the evaporation rate from the wet zone in the internal drying period is estimated with the linear driving force (LDF) approximation in the field of adsorption engineering. Critical moisture content, that is, mean moisture content between the surface and internal periods, is estimated with a mass balance on the surface. Simulated results by the new drying model with reasonable fitting parameters agree very well with measured drying data.     

Solidification by Convective Drying of Particle Layer Wetted with Dilute Agar Gel

Solidified porous slab is formed through convective drying of glass particle layer wetted with aqueous dilute agar gel. Measured critical mean moisture content increases with increasing initial moisture or agar content. The agar gel moves in viscous flow caused by capillary pressure during drying. A new drying model based on the receding evaporation plane model is proposed. Drying period is divided into surface and internal evaporation periods. Wet slab consists of dried and wet zones during the internal evaporation period, while the wet slab consists of wet zone only during the surface evaporation period. In the new model, the evaporation rate from the wet zone in the internal drying period is estimated with the linear driving force (LDF) approximation in the field of adsorption engineering. Critical moisture content, that is, mean moisture content between the surface and internal periods, is estimated with a mass balance on the surface. Simulated results by the new drying model with reasonable fitting parameters agree very well with measured drying data.     

Drying Characteristics In Superheated Steam Drying at Reduced Pressure

The superheated steam drying at reduced pressure is performed, and the effects of operational conditions such as drying pressure and temperature on the drying characteristics are examined. In order to obtain the basic guideline for the design of the superheated steam dryer at reduced pressure, the heat flux to sample was calculated and the optimal conditions were estimated.

After the sample temperature reached at the boiling point, the temperature was maintained at the boiling point and the drying rate became almost constant. Once the sample was dried out, the temperature suddenly increased up to the drying gas temperature. From the calculation of combined heat flux, the followings were found. The contribution of radiative heat transfer to the combined heat flux became larger as the drying pressure was lower. The combined heat flux had a maximum value against the drying pressure. The optimum drying pressure, which gave the maximum heat flux, became lower as the drying gas temperature decreased. It was found that reduction in the drying pressure is effective for the enhancement in drying performance.     

颗粒表面料层干燥机理

颗粒表面料层干燥是喷雾流化干燥技术的基本环节. 为了探讨喷雾流化干燥机理,着重探讨了物料表面蒸汽分压与物料湿含量的关系,首次建立了单颗粒表面料层干燥的物理模型,并进行了数值计算,计算结果与实验结果吻合较好.分析了惰性载体的密度、比热容、直径及热导率对干燥时间的影响,对喷雾流化干燥的深入研究以及放大、优化设计等具有重要的指导意义.     

棒状MgCO3·3H2O的干燥动力学研究

以六水氯化镁为原料,碳酸氢铵为沉淀剂,采用直接沉淀法,制备出长约为80μm,直径为5～10μm的棒状三水碳酸镁;通过不同热空气温度及不同湿物料层厚度下的干燥动力学实验,获得棒状三水碳酸镁的干燥曲线和干燥速率曲线。采用相变动力学理论,对干燥动力学实验数据进行拟合分析,得到棒状三水碳酸镁的干燥方程、干燥速率方程及干燥动力学参数。     

Drying Characteristics In Superheated Steam Drying at Reduced Pressure

Abstract

The superheated steam drying at reduced pressure is performed, and the effects of operational conditions such as drying pressure and temperature on the drying characteristics are examined. In order to obtain the basic guideline for the design of the superheated steam dryer at reduced pressure, the heat flux to sample was calculated and the optimal conditions were estimated.

After the sample temperature reached at the boiling point, the temperature was maintained at the boiling point and the drying rate became almost constant. Once the sample was dried out, the temperature suddenly increased up to the drying gas temperature. From the calculation of combined heat flux, the followings were found. The contribution of radiative heat transfer to the combined heat flux became larger as the drying pressure was lower. The combined heat flux had a maximum value against the drying pressure. The optimum drying pressure, which gave the maximum heat flux, became lower as the drying gas temperature decreased. It was found that reduction in the drying pressure is effective for the enhancement in drying performance.     

喷雾干燥粘壁的原因与解决途径

对喷雾干燥粘壁的类型和原因进行了分析。喷雾干燥粘壁主要有半湿物料粘壁、低熔点物料的热熔性粘壁和干粉表面粘附三种类型。半湿物料粘壁主要与喷雾干燥塔结构、雾化器结构、安装、操作以及热风在塔内的运动状态有关。物料熔点低于干燥温度时容易产生热熔性粘壁。干粉表面粘附与塔壁的几何形状、粗糙程度、空气流速、静电力等有关。提出了解决各种类型粘壁的技术措施。     

HYBRID DRYING OF GRANULAR MATERIALS BY COMBINED RADIATIVE AND CONVECTIVE HEATING

ABSTRACT

As one of the basic steps of the R&D on hybrid drying by combined radiative and convective heating, drying experiments of a stagnant horizontal wet granular layer heated simultaneously by infrared lamps and hot air were conducted throughout a whole period of the drying. An opaque model, in which the incident radiation is assumed to be absorbed only on the surface of the layer, was introduced to explain experimental trends. Then, the effects of combined heating on drying characteristics of three kinds of samples, a silica sand, a brick and a brown coal were discussed by comparing the experimental data with the calculated results.

The calculated results were found to be in a close agreement with the experimental data obtained on the time behaviors of the drying rate and the temperature distribution in the- layer during not only the preheating and the constant rate periods but also the falling rate period. In the case of the coal layer, however, the falling drying rate different from that in ordinary convective drying had to be considered since it appeared that the radiative heating had an effect to enhance the falling drying rate of the coal owing to some interactions of the coal and water.     

二氧化硅气凝胶常压干燥工艺的研究进展

二氧化硅气凝胶因具有低密度、高比表面积、稳定的物理化学性质等特性在吸附分离、隔热保温等领域表现出巨大的应用潜力。但长耗时、高成本的制备工艺限制了它的发展,尤其是湿凝胶向气凝胶转变的干燥工艺。本文介绍了二氧化硅气凝胶在常压干燥的过程中面临的主要难点及解决方法,虽然常压干燥方法工艺简单、过程安全、对设备要求低且可连续制备,成为近年来的研究热点,但也存在制备周期长、体积收缩大、需要消耗大量有机溶剂和改性剂等不足。文中从凝胶基体增强与优化、降低毛细管力与减少不可逆收缩两种角度,介绍了二氧化硅气凝胶常压干燥的改进方法及其发展现状,分析归纳了不同改进方法的优缺点,总结了二氧化硅气凝胶常压干燥目前面临的技术挑战。并且,立足于目前二氧化硅气凝胶基体增强和表面改性技术发展的趋势,对今后二氧化硅气凝胶常压干燥过程中结构可控、成本降低以及产品多功能化的发展路线进行了展望。     

控制分压的过热水蒸气定湿度干燥分析

引出定湿度干燥这一概念,对干燥流程进行简略的介绍。具体分析了水蒸气分压与混合气体湿度,混合气体各参数与物料目标含湿量之间的关系。对器壁的绝热性做了简略的阐述,分析了干燥室的工作模式,提出了片层化模型分析法,并在物料的干燥质量和所需能量上验证了模型的合理性。依照模型设定理论干燥时间,并详述设定时间需考虑的因素,及实际干燥时间的测定原理和方法。     

HYBRID DRYING OF GRANULAR MATERIALS BY COMBINED RADIATIVE AND CONVECTIVE HEATING

As one of the basic steps of the R&D on hybrid drying by combined radiative and convective heating, drying experiments of a stagnant horizontal wet granular layer heated simultaneously by infrared lamps and hot air were conducted throughout a whole period of the drying. An opaque model, in which the incident radiation is assumed to be absorbed only on the surface of the layer, was introduced to explain experimental trends. Then, the effects of combined heating on drying characteristics of three kinds of samples, a silica sand, a brick and a brown coal were discussed by comparing the experimental data with the calculated results.

The calculated results were found to be in a close agreement with the experimental data obtained on the time behaviors of the drying rate and the temperature distribution in the- layer during not only the preheating and the constant rate periods but also the falling rate period. In the case of the coal layer, however, the falling drying rate different from that in ordinary convective drying had to be considered since it appeared that the radiative heating had an effect to enhance the falling drying rate of the coal owing to some interactions of the coal and water.     

不挥发溶质对混合溶剂沸点及蒸气压的影响

测定了若干不挥发溶质对醇 (甲醇、乙醇、正丙醇 ) -水双液系恒压 (100.3kPa)汽液平衡的影响 ,其中包括 16种不挥发溶质对正丙醇 -水恒沸混合物沸点的影响 .通过理论分析 ,得到少量不挥发溶质对具有固定组成的二元混合溶剂沸点及蒸气压影响的普遍性结论 :不挥发溶质使难挥发组分的气相组成增加是引起溶液蒸气压降低或沸点升高的充分条件 ;在恒沸混合物中无论不挥发溶质使气相组成发生何种变化都会引起溶液蒸气压降低或沸点升高 .经检验 ,该结论与实验结果及大量文献数据完全相符 .