INFLUENCE OF DRYING CONDITIONS ON THE RHEOLOGICAL PROPERTIES OF PRUNES

Rheological properties of rehydrated prunes were obtained applying compression-relaxation tests by using a Texture Analyzer TAXT2i. A mathematical development was adopted to determine the stress and area, along the deformation. Experimental data of stress versus time was fitted by using three different rheological models: generalized Maxwell, Normand & Peleg and Maxwell. Results showed that generalized Maxwell model can be used to describe the viscoelastic behavior of the samples. The rheological parameters obtained indicated that prunes exhibited elastic behavior more pronounced at low moisture content and drying air temperature. At high moisture content and temperature the sample became a more viscous and less rigid.     

INFLUENCE OF DRYING CONDITIONS ON THE RHEOLOGICAL PROPERTIES OF PRUNES

ABSTRACT

Rheological properties of rehydrated prunes were obtained applying compression–relaxation tests by using a Texture Analyzer TAXT2i. A mathematical development was adopted to determine the stress and area, along the deformation. Experimental data of stress versus time was fitted by using three different rheological models: generalized Maxwell, Normand & Peleg and Maxwell. Results showed that generalized Maxwell model can be used to describe the viscoelastic behavior of the samples. The rheological parameters obtained indicated that prunes exhibited elastic behavior more pronounced at low moisture content and drying air temperature. At high moisture content and temperature the sample became a more viscous and less rigid.     

INFLUENCE OF LIQUID PROPERTIES ON THE DETERMINATION OF THE VOLUMETRIC MASS TRANSFER COEFFICIENT WITH THE HYDRAZINE METHOD

The oxidation of hydrazine in aqueous solution by atmospheric oxygen in presence of a homogeneous catalyst (copper tetrasulphophthalocyanine) is well suited for the determination of volumetric mass transfer coefficients. In contrast to other chemical methods the hydrazine oxidation permits to vary coalescence behaviour of the liquid phase by adding electrolytes or organic compounds, because without these solutes the reaction liquid does not inhibit bubble coalescence. With appropriate high molecular additives the hydrazine method can also be used for mass transfer measurements in liquids of high viscosity. Compared to the dynamic method the hydrazine method as a steady state method is far less influenced by systematic errors originating from the evaluation model. For this reason the application of the hydrazine method for testing semi-industrial and industrial gas-liquid contacting devices should be especially attractive.     

DRYING OF CLAY. II RHEOLOGICAL MODELISATION AND SIMULATION OF PHYSICAL PHENOMENA

The present study proposes the development of a complete mathematical modelling transfer phenomena involving at the same time heat, mass and momentum transfer during the drying process of clay. Clay is a generic example of colloid materials forming particulate gels. That can be considered as bi-constituent, homogeneous, isotropic, and highly deformable. The model was numerically solved by the finite difference method and validated by comparison of the numerical results with a previous set of experiments data. The simulation has allowed the determination of spatio-temporal evolution within the solid of different variables: temperature fields, moisture contents, displacement, deformation and stresses. The parametric sensibility has been analyzed in the case of thermophysical properties and the external heat transfer coefficient. Various values of external conditions have been analyzed.     

DRYING OF CLAY. II RHEOLOGICAL MODELISATION AND SIMULATION OF PHYSICAL PHENOMENA

ABSTRACT

The present study proposes the development of a complete mathematical modelling transfer phenomena involving at the same time heat, mass and momentum transfer during the drying process of clay. Clay is a generic example of colloid materials forming particulate gels. That can be considered as bi-constituent, homogeneous, isotropic, and highly deformable. The model was numerically solved by the finite difference method and validated by comparison of the numerical results with a previous set of experiments data. The simulation has allowed the determination of spatio-temporal evolution within the solid of different variables: temperature fields, moisture contents, displacement, deformation and stresses. The parametric sensibility has been analyzed in the case of thermophysical properties and the external heat transfer coefficient. Various values of external conditions have been analyzed.     

AN ANALYSIS OF MASS TRANSFER IN AIR-DRYING OF FOODS

A theoretical model for the convective drying of food materials was developed. These materials shrink considerably, as moisture content decreases; for this reason, the proposed model takes shrinkage into account as a fundamental part of the drying operation. This is accomplished by modeling the movement of dry solids and the consequent movement of bound substance during drying. The resulting system of three non-linear partial differential equations is solved numerically and the predicted values of moisture content and temperature are compared with experimental data for the drying of Sultana grapes. The effective moisture diffusivity in the mass of grapes was described by an Arrhenius-type equation. A sensitivity analysis of the predicted values and the examination of the confidence regions of the estimated parameters showed that the activation energy for mass transfer was estimated with a high degree of precision. The agreement between the calculated results and the experimental measurements, using various criteria, was very satisfactory.     

AN ANALYSIS OF MASS TRANSFER IN AIR-DRYING OF FOODS

A theoretical model for the convective drying of food materials was developed. These materials shrink considerably, as moisture content decreases; for this reason, the proposed model takes shrinkage into account as a fundamental part of the drying operation. This is accomplished by modeling the movement of dry solids and the consequent movement of bound substance during drying. The resulting system of three non-linear partial differential equations is solved numerically and the predicted values of moisture content and temperature are compared with experimental data for the drying of Sultana grapes. The effective moisture diffusivity in the mass of grapes was described by an Arrhenius-type equation. A sensitivity analysis of the predicted values and the examination of the confidence regions of the estimated parameters showed that the activation energy for mass transfer was estimated with a high degree of precision. The agreement between the calculated results and the experimental measurements, using various criteria, was very satisfactory.     

ADIABATIC SORPTION OF BULK SINGLE ADSORBATE FROM AN INERT GAS-EFFECT OF GAS-SOLID MASS AND HEAT TRANSFER COEFFICIENTS

The effects of the gas-solid mass and heat transfer coefficients on the dynamics of adiabatic adsorption of a single adsorbate exhibiting Type I behavior were analyzed by numerically solving the conservation equations for a column. It was assumed that the adsorbate obeyed Langmuir equilibra and the local mass transfer rate could be described by the linear driving force model. The properties of the front zones were found to be controlled by mass transfer, while those of the rear zones were primarily determined by heat transfer. The separation between the two zones might vanish under certain conditions resulting in overlap of the two zones. The front zones were found to be constant pattern. The rear zones can be constant or proportionate pattern depending on the equilibrium properties and the feed condition of the adsorbate.     

NONUNIFORM STEADY STATES IN SYSTEMS OF INTERACTING CATALYST PARTICLES. THE CASE OF NEGLIGIBLE INTERPARTICLE MASS TRANSFER COEFFICIENT

The effect of micropore diffusion, macropore diffusion and adsorption on the transient uptake of porous solids having bimodal pore size distribution (for example, zeolites) is theoretically investigated in this paper. Two models commonly used in the literature—the pore diffusion model and the surface diffusion model— are considered and their equivalence is discussed. The bimodal diffusion model equations are solved analytically by using a generalized integral transform method. The solutions for the micropore, macropore concentrations and the fractional uptake are presented. Since two different processes control the transient uptake, the asymptotic solutions for micropore diffusion control case and macropore diffusion control case are obtained by using perturbation methods, so that these asymptotic solutions can be compared with that when these two processes equally contribute to the transient uptake.     

ADIABATIC SORPTION OF BULK SINGLE ADSORBATE FROM AN INERT GAS-EFFECT OF GAS-SOLID MASS AND HEAT TRANSFER COEFFICIENTS

The effects of the gas-solid mass and heat transfer coefficients on the dynamics of adiabatic adsorption of a single adsorbate exhibiting Type I behavior were analyzed by numerically solving the conservation equations for a column. It was assumed that the adsorbate obeyed Langmuir equilibra and the local mass transfer rate could be described by the linear driving force model. The properties of the front zones were found to be controlled by mass transfer, while those of the rear zones were primarily determined by heat transfer. The separation between the two zones might vanish under certain conditions resulting in overlap of the two zones. The front zones were found to be constant pattern. The rear zones can be constant or proportionate pattern depending on the equilibrium properties and the feed condition of the adsorbate.     

GAS-LIQUID MASS TRANSFER CHARACTERISTICS OF LARGE-SCALE IMPELLERS: EMPIRICAL CORRELATIONS OF GAS HOLDUPS AND VOLUMETRIC MASS TRANSFER COEFFICIENTS IN STIRRED TANKS

Gas dispersion with large-scale impellers consisting of modified large paddle impellers in stirred tanks, with rather large ratios of both impeller diameter and impeller height to tank diameter, was experimentally examined in transition and turbulent mixing ranges. Gas holdups and volumetric gas-liquid mass transfer coefficients with large-scale impellers, i.e., Maxblend and Fullzone impellers, were measured in 0.31 and 0.6 m I.D. stirred tanks, and the gas dispersion performance of large-scale impellers was compared with that of double conventional small-scale high-speed impeller systems, i.e., double four-flat blade disk turbine impellers and double four-flat paddle impellers.

The gas holdups of the large-scale impellers were comparable with those of the small-scale impeller systems at a given rotational speed. The volumetric gas-liquid mass transfer coefficients for large-scale impellers were also similar to those of the small-scale impeller systems. It was found that the large-scale impellers are not more energy efficient than the small-scale impellers in obtaining good gas dispersion.

Empirical correlations for gas holdups and volumetric gas-liquid mass transfer coefficients were developed. They fit the experimental data in transition and turbulent mixing ranges reasonably well, with correlation factors greater than 0.84.     

INFLUENCE OF MIXING INTENSITY ON THE MASS TRANSFER COEFFICIENT ACROSS LIQUID-LIQUID INTERFACES

The influence of mixing intensity on the liquid-liquid mass transfer have been investigated using a binary system (water/n-butanol) in a modified stirred cell type contactor which overcomes the shortcomings associated with the previous contactors. It is observed that the mass transfer coefficient is affected by the degree of mixing intensity in both phases. The dependence of the aqueous and organic mass transfer coefficients on the mixing rate is about the same when both phases are agitated at the same speed and these coefficients become asymptotic values for the coefficients obtained at non identical agitation rate in both phases.     

RHEOLOGICAL PROPERTIES OF SUSPENSIONS OF SOLID SPHERES IN NON-NEWTONIAN FLUIDS

A theoretical expression based on the cell model has been developed to predict the dependence of the flow properties of a suspension in a wide range of solids concentration when the dispersing medium is a non-Newtonian liquid. Theoretical predictions show that the increase of the relative consistency coefficient of a suspension with increasing particle concentration is less dramatic when the flow index of the suspending medium is lower. Comparison with published data shows that the predictions of the present model for the relative consistency coefficient of a suspension in a non-Newtonian fluid are fairly accurate.     

CONJUGATE HEAT AND MASS TRANSFER IN CONVECTIVE DRYING OF MULTIPARTICLE SYSTEMS PART II: SOYBEAN DRYING

This is the second of two papers concerning conjugate transport in convective drying of multiparticle systems. In the first paper (part 1), a solution methodology for conjugate transport problems was porposed and successfully tested for assemblages of spheres. This paper deals with the application of the proposed methodology to a soybean drying problem. The analysis in this study allowed certain phenomena, that are usually not present in single-kernel and deep-bed drying analyses, to be captured by the drying rate curves. The results presented here reinforce the need to take into account particle interactions when studying the drying mechanisms of multiparticle systems.     

CONJUGATE HEAT AND MASS TRANSFER IN CONVECTIVE DRYING OF MULTIPARTICLE SYSTEMS PART II: SOYBEAN DRYING

Abstract

This is the second of two papers concerning conjugate transport in convective drying of multiparticle systems. In the first paper (part 1), a solution methodology for conjugate transport problems was porposed and successfully tested for assemblages of spheres. This paper deals with the application of the proposed methodology to a soybean drying problem. The analysis in this study allowed certain phenomena, that are usually not present in single-kernel and deep-bed drying analyses, to be captured by the drying rate curves. The results presented here reinforce the need to take into account particle interactions when studying the drying mechanisms of multiparticle systems.     

MATHEMATICAL MODELING OF THROUGH-HOLE-PLATING IN ELECTROCHEMICAL MICROFABRICATION II. AXISYMMETRIC TWO-DIMENSIONAL CONVECTIVE MASS TRANSFER

Axisymmetrical, two dimensional mass transfer in a tube and a through-hole was studied. The elliptic convective-diffusion equation was solved numerically by taking into account both the axial and radial diffusion in the entrance region of an infinite tube. The whole mass-transfer region in a tube of finite length was also studied by dividing the domain into several regions according to different mass-transfer mechanisms. A similar analysis is then conducted for a through-hole geometry by assuming a modified Hagen-Poiseuille fluid pattern. Results show that mass-transfer rate in a through-hole is larger than that in a tube by approximately 10%. This is due primarily to faster fluid flow within the concentration boundary layer in a through-hole.     

真空辅助成型工艺环氧树脂体系化学流变特性研究

通过实验研究了环氧树脂的化学流变特性,分别考察了双阿累尼乌斯黏度模型、工程黏度模型及联合黏度模型在真空辅助成型工艺专用环氧树脂(2511-1A)应用的有效性。通过比较分析发现,联合黏度模型更加准确地反映了该树脂的时间-温度-黏度的变化情况,进而预测了树脂的工艺时间窗口。根据时间-温度的变化关系,建立了非恒温下2511-1A环氧树脂体系的联合黏度模型,预测了在不同升温速率下黏度的变化,预测结果与实验结果较吻合,可进一步应用于热压工艺技术参数的选择。     

POLAROGRAPHIC DETERMINATION OF MASS TRANSFER IN A STAGNATION FLOW

An electrochemical technique, oxygen polarography, was used to characterize the mass transfer coefficient for diffusion of oxygen to the surface of a circular cathode in a stagnation flow; because the electrical current output depended upon the magnitude of the impinging velocity, this method could form the basis of an electrochemical velocity probe. Four cathode assemblies consisted of 1.28 cm diameter/2.54 cm long polymethylmethacrylate rods with a circular cathode of diameter 1.28 cm, 0.64 cm, 0.32 cm or 0.13 cm vacuum deposited on their flat surface. Each cathode assembly was placed in a 2.54 cm diameter flow loop such that a fully-developed flow (8000 ≥ Reynolds number ≥ 200) of aerated saline solution impinged upon the cathode surface

The effect of radial position on electrical current output was studied by placing the cathode assemblies at the center of the tube, halfway between the centerline and the wall, and at the tube wall. For the two largest cathodes, the output currents decreased as the cathode assembly was moved to the wall. The smallest cathode exhibited the opposite trend. The output correct of the cathode of intermediate diameter, 0.16 cm, was virtually unaffected by the radial position. Mass transfer coefficient (K _L) values were computed at the centerline position and a power law relationship with mean axial tube velocity (ū) was postulated: k _L = γūⁿ, where γ and n are positive parameters. The regressed values of γ and n were 1.11 × 10³ (cm7sol;s)^0.5 and 0.502, respectively, during laminar flow and 1.14 × 10^?3 (cm/s)^0.6 and 0.391, respectively, for the turbulent regime. The final regressed value of n in the laminar regime was essentially the same as the theoretical value of 0.5.     

COMPUTER SIMULATION OF MASS TRANSFER IN AGGREGATIVE FLUIDIZED BEDS

A mathematical model for simulation of unsteady-state mass transfer in an aggregative fluidized bed of porous particles is proposed. A quasi-steady state approach is being used to solve the model. A computer program is written for solving the model and the rate of drying of air in a fluidized bed of silica-gel particles is calculated as an example. To evaluate the accuracy of the proposed model, a pilot-scale fluidized bed is erected and the experimental results are compared to the calculated values. A good agreement is observed between them.