DIFFUSION OF MOISTURE IN DRYING OF SUGAR CANE FIBERS AND BUNDLES

Sugar cane fibers and arrangements of fibers in cylindrical bundles were dried in a thermoanalyzer and their diffusive coefficients were calculated using the slope method. The effect of temperature, moisture content as well as structural changes were analyzed. Diffusion coefficients changed nanlineariy with moisture content and followed an Arrhenius-like functionality with temperature. The analysis of these effects suggested a liquid diffusion transport mechanism of moisture transfer inside sugar cane fibers and bundles.     

DRYING OF SUGAR BEET PULP USING A LABORATORY AIR DRIER

ABSTRACT

Drying curves for sugar beet pulp were obtained in a laboratory-scale air drier. Dehydration conditions were: temperatures in the range of 35 to 90 ^°C, air flow velocities of 0.5 to 2.0 m/s, and air moisture content of 0.005 to 0.06 kg water/kg air. Using data from thin layer experiments, a mathematical model was derived to describe the dehydration behaviour under the described conditions. The model was able to predict data obtained from deep bed experiments. Results from this work could be used to develop a design methodology for dryers that operate at relatively low temperatures (<90^°C).     

EFFECTS OF GLYCEROL ON THE DRYING OF GELATIN AND SUGAR SOLUTIONS

ABSTRACT

The water diffusion coefficients D of gelatin, sucrose arid maltodextrin solutions were determined from the isothermal drying (desorption) rates as a function of water content u ( 0.1 < u < 1.0 ) and temperature (303, 323K) for various glycerol contents ( W_G = 0 – 0.43 kg-glycerol/kg-total solid). When W_G was above 0.2, D increased especially at low water contents (u < 0.25). The desorption isotherms were measured for the same systems. The equilibrium water content at a given water activity A_w decreased with an increase in W_G when A_w < 0.5. This indicates that adding glycerol weakens the water-solute interaction especially at low water contents. Glycerol also softened gelatin films and resulted in loss possibilities of surface cracks during drying.     

NONDESTRUCTIVE MEASUREMENT OF MOISTURE DIFFUSION COEFFICIENT IN WOOD DRYING

ABSTRACT

The computer tomography(CT image of wood was analyzed to estimate the moisture content from CT number. Comparing moisture contents, densities and CT numbers, moisture distributions were nondestructively determined by CT numbers detected with the scanner.

By periodically measuring CT images of the same cross section of specimens in drying, change of moisture distribution was computed. The coefficients of moisture movement at given positions inside the specimen were directly calculated from Fick's Law with known values of moisture content change, distance and drying time.

The diffiion coeficient was described as a function of various driving forces of moisture content, partial vapor pressure and chemical potential of moisture in wood. The results were comparable to data in the literature. Furthermore, it is shown that diffusion coefficients based on moisture content varies as a curve with the maximum value at about 15% m.c. during drying, and that the moisture content is actually adaptable to the driving force for moisture movement in wood because of capability over wide moisture content range.     

RELATIVE IMPORTANCE OF VAPOUR DIFFUSION AND CONVECTTVE FLOW IN MODELLING OF SOFTWOOD DRYING

ABSTRACT

The influence of vapour diffusion on the drying rate of a softwood board has been examined for drying temperatures varying from 60°C to 140°C. It is found that for very low temperature drying a model which considers both vapour convective and diffusion in wood predicts dry-rate curves matching the experimental data closely. For high temperature drying, both of the above drying model and a drying model which considers only vapour convective flow give predictions in agreement with the observed data. This illustrates that the diffusion of vapour and air is important in low temperature drying panicularly during the late stages of drying. However, for high temperature drying, the convective flow of moisture vapour is dominant and the diffusion component is negligible. The observation provides evidence for simplifying a drying model for high temperature drying without reducing its credibility in predicting drying rate curves.     

NONDESTRUCTIVE MEASUREMENT OF MOISTURE DIFFUSION COEFFICIENT IN WOOD DRYING

The computer tomography(CT image of wood was analyzed to estimate the moisture content from CT number. Comparing moisture contents, densities and CT numbers, moisture distributions were nondestructively determined by CT numbers detected with the scanner.

By periodically measuring CT images of the same cross section of specimens in drying, change of moisture distribution was computed. The coefficients of moisture movement at given positions inside the specimen were directly calculated from Fick's Law with known values of moisture content change, distance and drying time.

The diffiion coeficient was described as a function of various driving forces of moisture content, partial vapor pressure and chemical potential of moisture in wood. The results were comparable to data in the literature. Furthermore, it is shown that diffusion coefficients based on moisture content varies as a curve with the maximum value at about 15% m.c. during drying, and that the moisture content is actually adaptable to the driving force for moisture movement in wood because of capability over wide moisture content range.     

DETERMINATION OF MOISTURE DIFFUSIVITY AND BEHAVIOR OF TOMATO CONCENTRATE DROPLETS DURING DRYING IN AIR

ABSTRACT

The drying mechanism and diffusion coefficient of water in spherical droplets (1.73 – 2.08 mm diameter) of tomato concentrates were successfully interpreted and modelled by using Fick's law. Solids content of the initial concentrate (5–15% w/w), and drying temperature (60^° – 100^° C) were varied but the drying air was kept at constant velocity and humidity.

The effective moisture diffusivity was estimated from the drying rate curves and expressed by an Arrhenius relation. Further, it was observed that case hardening has a large effect on the diffusion process causing the effective diffusional distance and the rate of moisture accumulation in the hardened crust to vary with the moisture content, according to a sorption controlled mechanism.     

PEAR DRYING PROCESS ANALYSIS: DRYING RATES AND EVOLUTION OF WATER AND SUGAR CONCENTRATIONS IN SPACE AND TIME

ABSTRACT

Dried pears are produced in Portugal by a traditional solar drying process and, because they represent unquestionable regional economic interest, some studies are being conducted in order to convert their production to industrial drying methods. In this work the main mass transfer phenomena occurring during drying of pears are investigated. Experiments were carried out to determine the time evolution of the radial profiles of water and sugar content. The drying rates were also studied at constant drying temperatures of 30, 40 and 50°C. The results enable to conclude that the concentration profiles of water and sugar, both in space and time, follow an expected pattern if one takes into account the rates of water removal and the diffusion mechanisms. These also allow to explain why the sugar concentration increases in the periphery of the fruit. The drying rate curves obtained are typical, with the drying process at 30°C being more gradual but also much slower than that at 50°C.     

PEAR DRYING PROCESS ANALYSIS: DRYING RATES AND EVOLUTION OF WATER AND SUGAR CONCENTRATIONS IN SPACE AND TIME

Dried pears are produced in Portugal by a traditional solar drying process and, because they represent unquestionable regional economic interest, some studies are being conducted in order to convert their production to industrial drying methods. In this work the main mass transfer phenomena occurring during drying of pears are investigated. Experiments were carried out to determine the time evolution of the radial profiles of water and sugar content. The drying rates were also studied at constant drying temperatures of 30, 40 and 50°C. The results enable to conclude that the concentration profiles of water and sugar, both in space and time, follow an expected pattern if one takes into account the rates of water removal and the diffusion mechanisms. These also allow to explain why the sugar concentration increases in the periphery of the fruit. The drying rate curves obtained are typical, with the drying process at 30°C being more gradual but also much slower than that at 50°C.     

CONVECTTVE HEAT AND MASS TRANSFER AND EVOLUTION OF THE MOISTURE DISTRIBUTION IN COMBINED CONVECTION AND RADIO FREQUENCY DRYING

ABSTRACT

In a previous study (Dostie and Navarri, 1994), experiments indicated that a non-uniform moisture distribution could develop in radio frequency drying depending on the applied power and initial conditions, making the design and scale-up of such a dryer a more difficult task. Consequently, a thorough study of the combined convection and RF drying process was undertaken. Experimental results have shown that the values of the neat and mass transfer coefficients decrease with an increase in evaporation rate caused by RF energy. This effect is adequately taken into account by the boundary layer theory. Furthermore, the usual analogy between heat and mass transfer has been verified to apply in RF drying. Experiments have also shown that a different mass transfer resistance on both sides of the product should not result in non-uniform drying. However, it appears that non-uniform drying is dependent upon the initial moisture distribution and the relative intensity of heal transfer by convection and RF- It was shown that the maximum drying rate occurs at a higher average water content and that the total drying time increases with non-uniformity of the initial moisture distribution.     

TEMPERATURE AND MOISTURE CHANGES DURING MICROWAVE DRYING OF SLICED FOOD

ABSTRACT

Microwave drying characteristics of sliced foods were investigated using potatoes (Solarium tuberosum) as a test model. Sliced samples were dried to 7-10% moisture content at microwave power levels between 2.2 W/g and 3.6 W/g (raw material). Moisture and temperature changes during drying were monitored. Semi-empirical models were developed that followed temperature and moisture changes during microwave drying. Sliced potatoes experienced three distinct periods: a warming-up period with little removal of moisture; a constant temperature period in which most of the drying took place; and a heating up period in which the drying rate decreased and sample temperature increased rapidly, often causing partial charring. Product temperature during the second period of microwave drying increased with sample thickness and microwave power. Drying rates were not affected by slice thickness, but increased with the microwave power/mass ratio. Product charring towards the end of drying may be avoided by reducing microwave power and increasing ambient air velocity.     

超高分子量聚乙烯冻胶纤维萃取干燥工艺的研究

对超高分子量聚乙烯(UHMWPE)冻胶纤维进行萃取、干燥是改善其拉伸性能的主要技术关键.萃取、干燥的不同工艺条件都会较大地影响未拉伸干冻胶纤维的结构及其性能.其中主要有:萃取时间、萃取次数、干燥温度和干燥收缩率.本文借助电子强力试验仪、扫描电子显微镜(SEM)、X-射线衍射(WAXD、SAXS)和声速仪(SP)等对以三氯三氟乙烷(F113)、溶剂油120~#为萃取剂经不同萃取干燥工艺处理的冻胶纤维结构进行了测试,并对上述萃取、干燥处理过的冻胶纤维经超倍拉伸后所得的拉伸纤维性能进行了测试,得到了萃取、干燥的最佳工艺条件为:充分萃取、适温干燥、保持足够的干燥收缩率.     

MOISTURE DISTRIBUTION IN SPHERICAL FOODS IN MICROWAVE DRYING

Abstract

Moisture distribution in spheres of potato (Solanum tuberosum) and yam (Dioscorea japonica) tubers during microwave drying was investigated. Moisture variation at various locations within the samples was experimentally determined for different drying times. A numerical simulation model was developed based on a modified form of Lambert's law for microwave penetration in spheres and on moisture transfer in both liquid and vapor phases. This model was used to study various factors influencing moisture distribution. Moisture profile during microwave drying was strongly influenced by non-uniform microwave energy distribution within the foods. The core moisture contents were lower than other parts of potato and yam spheres of the various sizes tested in this study (2.6 cm to 6.0 cm diameter). The sphere radius to microwave penetration ratio (2αr_o) was an important parameter affecting the moisture profile in spherical foods. For 2αr_o of less than 2.0, the moisture content in the center of potato and yam spheres was markedly lower than the rest of the food, but for 2αr_o between 2.5 and 4, moisture contents at the center and close to the surface were lower than the average moisture content.     

INFRARED THERMOGRRPHY IN THE ANALYSIS OF MOISTURE FLUX FROM DRYING WOODEN SURFACES

ABSTRACT

Infrared thermography is used to visualize moisture evaporation from wooden surfaces. The initial moisture flux is shown to be dependent upon the growth ring structure as well as the heartwood and sapwood portions of a board.     

MATHEMATICAL SIMULATION OF TEMPERATURE AND MOISTURE FIELDS WITHIN A GRAIN KERNEL DURING DRYING

ABSTRACT

Non-linear partial differential equations are presented for two dimensional heat and mass transfer within a single grain kernel during drying. In this model, the moisture evaporation inside the kernel is considered. The moisture is assumed to diffuse to the outer boundary of the kernel in liquid form and evaporate on the surface of the kernel. The influence of temperature and moisture content on grain properties is also considered in the simulation. The Non-linear partial differential equations are solved using the finite element method and simulation data is verified on a thin layer dryer for wheat kernels. The comparison shows that the simulated results have a high accuracy with average relative error of about 5%. The results of the finite element analysis can be used for grain quality evaluation, drying simulation studies and stress analysis of grain kernel.     

COMPETING DRYING AND REACTION MECHANISMS IN THE FORMATION OF SOL-TO-GEL FILMS,FIBERS, AND SPHERES

ABSTRACT

Drying of films, fibers, and spheres undergoing sol-to-gel transformation is greatly affected by the strong dependence of the diffusion coefficients on composition, molecular weight, and temperature. This dependence is probed by solving the equations of mass and heat transfer by diffusion or conduction and associated convection, along with chemical reaction. A one-dimensional analysis is used that encompasses the planar, cylindrical, and spherical configurations. The solutions are obtained by Galerkin's method with finite element basis functions and entail large-scale computation.

Skinning and solidification phenomena in silica sol-to-gel systems are greatly affected not only by falling diffusion coefficients, but also by the competition between drying and reaction. The kinetics of silica reactions are modeled with the statistical scheme of Kay and Assink (1988). Gelation is predicted by the recursive technique of Bailey et al. (1990). Results show that at intermediate reaction rates, high molecular weight silica polymers are formed first near the evaporating free surface and gelation proceeds from that surface inwards, but at high or low reaction rates, gelation occurs before or after drying, respectively. These results help predict conditions in which uniform films can be cast, uniform fibers can be drawn, and uniform microspheres can be formed by spray drying.     

低温干燥对褐煤含氧基团及其吸水性能的影响

采用红外光谱法确定褐煤中含氧官能团的种类,利用化学法分析干燥前后褐煤表面含氧官能团的变化,吸水性能用最高内在水分来表征,研究低温干燥过程中温度、干燥程度和干燥气氛对褐煤含氧基团及其吸水性能的影响.结果表明,褐煤中羧基和酚羟基含量较高;惰性气氛低温干燥时,酚羟基含量基本不变,羧基发生少量分解;空气下干燥时,羧基含量先减小,之后在氧化作用下,羧基和酚羟基含量同时增大.随着干燥程度的加深,煤样最高内在水分先减小后增大.     

DRYING OF HIGH MOISTURE CONTENT PADDY IN A PILOT SCALE TRIANGULAR SPOUTED BED DRYER

From the successful drying of paddy in a lab-scale triangular spouted bed (TSB), a pilot scale unit with a capacity of 500kg/batch was constructed and used to dry paddy of high moisture contents. The drying tests showed that air temperatures of up to 160°C in the first stage, followed by a lower temperature of 80-100°C in the second stage, can be used to dry paddy from more than 25% down to around 15% (w.b.) without significantly changing grain quality in terms of head rice recovery. Moisture distribution in the pilot-scale TSB was observed to be satisfactorily uniform.     

THE INFLUENCE OF AIR TEMPERATURE ON EFFECTIVE DIFFUSION COEFFICIENT OF MOISTURE IN THE FALLING RATE PERIOD

ABSTRACT

The convection drying of spheres of Al-Ni catalyst and thin clay plate was investigated on a laboratory scale. The aim of the investigation was to determine the mechanism of moisture transfer within the firm sample skeleton.

By measuring the mass loss and the surface temperature of the samples during the drying process, basic dependencies necessary to describe the process kinetics was obtained. Found functional dependencies show high level of correlation with experimental data. Exponential model parameters are dependent of air temperature and velocity, and the type of the examined material.

The applicability of the chosen model to describe the drying kinetics is proven regardless of drying material origin and geometry, The comparison of different drying experiment and determination of the average diffusional coefficient at given experimental conditions is enabled.     

RELATIONSHIP OF PROCESS PARAMETERS AND STACK WIDTH IN TIMBER DRYING

ABSTRACT

Drying schedules applied in kiln drying, especially for hardwood species should be supplemented with values of air velocity. When dryers have no air velocity control their drying schedule can be suitably corrected. The correction should take into account factors related to dried material, i.e. wood moisture content, timber thickness, dried timber volume and primary width of a stack. Drying efficiency may be the criterion of modifications. The applied procedure of drying efficiency calculations lets to determine quantitative relationships between process and material factors.