摘要:

AbstractA model for a pneumatic‐conveying dryer is presented, with the focus on the superheated steam drying of wood chips, although it can also be used for other porous materials and drying media. It includes a comprehensive 2‐D model for the drying of single wood chips, which accounts for the main physical mechanisms occurring in wood during drying, including coupled transport of water, air, vapor and heat. This model allows for features such as initial condensation and flashing at the outlet, as well as the falling rate period when the drying is controlled by internal transport. External drying conditions in the dryer are calculated by applying mass, heat and momentum equations for each incremental step in dryer length. A plug‐flow assumption is made for the dryer model, and single‐particle and dryer models were solved iteratively. The irregular movement and nonspherical shape of wood chips are accounted for by measuring drag and heat‐transfer coefficients.Model calculations illustrate the complex interactions among steam, particles, and walls that occur in a flash dryer. The drying rate, the slip velocity, and temperature vary in a complex manner through the dryer, necessitating the use of a comprehensive single‐particle model, as in this case. Previous experimental data on the drying of bark chips in a pilot dryer was used to verify the model. The predicted temperature and pressure profiles, as well as the final moisture content of the material, agreed well with the measurements. Thus, the model provides a useful tool for the design and scale‐up of pneumatic‐conveying dryers. Effects of steam and material properties on the drying were investigated with different de

ISSN:0001-1541    

DOI:10.1002/aic.690431102

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractTo evaluate the effect of viscoelasticity on bubble formation, four rheological parameters of viscoelastic liquids were defined and measured by a rheometer. The effects of operating conditions and concentration of polyacrylamide aqueous solutions (PAA) as viscoelastic liquids on bubble volume and the growth curve were experimentally measured. A high‐speed video camera showed that the shapes of a bubble growing in vis‐coelastic liquids are not spherical. In this study, a nonspherical bubble formation model was proposed to theoretically estimate the volume, shape and growth curve of bubbles formed from an orifice submerged in viscoelastic liquids which are assumed to follow Maxwell's viscoelastic model. This experimental results in relatively low concentrations of PAA, as well as previous researchers' results, agreed relatively well with the calculated ones by this mo

ISSN:0001-1541    

DOI:10.1002/aic.690431103

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe metering operation in the transfer‐roll coating process is crucial for achieving a good uniformity and suitable thickness of the coating film. Experimental results carried out on a laboratory coater are reported. This coater mimics industrial flow conditions: it has a transfer roll and a metering‐rod diameter similar to those found on a pilot coater, it operates at commercial speed, and the cover of the transfer roll is deformable. The experimental apparatus is described first. Then, a video analysis of the rib pattern is presented. Some interesting features are shown, in particular the existence of an inertia regime at high speed. The nip flow hydrodynamics are investigated using measurements of the pressure profile. It is shown that the film stability is strongly linked to the flow at the exit of the nip. The analysis of the maximum pressure in the nip, for various fluid viscosities and operating parameters, is represented using a master cu

ISSN:0001-1541    

DOI:10.1002/aic.690431104

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe local structural function obtained by a microresistivity probe at different hydrodynamic regimes is examined. The structures, such as the vortex‐clinging structure, the appearance of one and two large cavities, small 3‐3 structure, large 3‐3 structure, and ragged cavities were recognized by frequency transformation of the time‐domain structural function. An optimized phase discrimination in signal processing was used. The distribution of the local void fraction (α) in a pilot‐size stirred tank was experimentally investigated, because almost no such data can be found in the literature. The two‐phase mixture was composed of air and water; α was measured at 190 nodes in the vertical half‐section plane of the vessel. Relative differences smaller than 9% between integrated values of α and measured gas holdups agreed reasonably well unde

ISSN:0001-1541    

DOI:10.1002/aic.690431105

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractInverse gas chromatography (IGC) was used to measure diffusion and thermodynamic parameters of volatile compounds in polymers. A comprehensive model for IGC is proposed, and a numerical solution for it is developed. Four capillary columns were prepared using PDMS and EPDM as the stationary phase. Toluene, ethanol, water, and some chlorinated compounds were the probes for the chromatographic experiments. Limitations of the IGC technique, although not reported in the literature, are explained. Nevertheless, the results for the systems chosen were satisfactory and agreed with the literature. The effect of adsorption on the column wall is important when water is used as a solute. The adsorption effect was incorporated into the model. The parameters obtained for water using the modified model became more reliable. The parameters obtained with IGC were used to get further understanding of the pervaporation process.

ISSN:0001-1541    

DOI:10.1002/aic.690431106

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractNew correlations, based on the rough hard‐sphere theory and an extensive literature database, were developed for the determination of the binary diffusion coefficients of liquid and solid solutes in supercritical solvents. The correlations were tested to predict 107 solute‐solvent systems with good results. The input data required for the correlations are the solute molecular weight, temperature, density and the solvent basic properties (molecular weight, critical volume, and critical pressu

ISSN:0001-1541    

DOI:10.1002/aic.690431107

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractTransport models of diffusion‐induced bubble growth and collapse in viscous liquids are developed and evaluated. A rigorous model for these important phenomena is formulated taking into account convective and diffusive mass transport, surface tension, and viscous and inertial effects. Predictions for bubble growth dynamics demonstrate the importance of viscous effects in the liquid surrounding the bubble for a wide range of conditions having relevance to polymer processing operations. It is also shown how previous models employing various approximations can be derived from the rigorous model by using different mathematical methods and dimensional analysis. Predicted bubble growth dynamics from the rigorous and approximate models are compared to establish the ranges of validity for two commonly used approximations. These comparisons indicate that models using a thin boundary‐layer approximation have a rather limited range of applicability. A new approximate model, based on a previously published result, gives reasonably accurate bubble growth rate predictions with a significant reduction in computational eff

ISSN:0001-1541    

DOI:10.1002/aic.690431108

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe algebraic solution for linear, ideal simulated moving‐bed (SMB) chromatography derived previously revealed important new properties of the SMB process. This solution was validated by comparing with experimental data and with the results of numerical solutions of the equilibrium–dispersive and the lumped kinetic model. This earlier solution, however, was limited to the simple case in which the safety factor was assumed to be the same for the flow rates in all four sections. An extension of this solution to the case in which the safety factors differ and allow an independent selection of the four different flow rates (within limits) is derived. This solution accounts well for most practical situations. It permits the discussion of the influence of different flow rates on the performance of an SMB unit, an issue of critical importance for the optimization of the design and operation of these separat

ISSN:0001-1541    

DOI:10.1002/aic.690431109

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractA Sorbex‐type SMB separation instrument was operated for the separation of mixtures of 2‐phenyl ethanol and 3‐phenyl‐l‐propanol on a series of eight columns packed with Zorbax C18 bonded silica, using a 60:40 (u/v) solution of methanol and water as the mobile phase. The experiments were carried out with low‐concentration mixtures under linear conditions. The elution profiles of both compounds between any two successive columns of the set under steady‐state conditions were recorded while compositions of the raffinate and extract were measured. The four safety margin factors, βj, were divided into two groups: β1and β4; β2and β3with different influence. Within the framework of the linear, ideal model of chromatography, a range of flow rates enables an SMB to operate successfully by completely separating the feed into its two components. The influence of different flow rates on the performance of the separator was studied by calculations and experiments with excellent agreement between both sets. With a simple optimization strategy, the four flow rates are selected to maximize the production rate or the concentration of extract and raffinate products, or to minimize the desorbent consumption. These optimum conditions are obtained by controlling the locations of concentration profiles inside the SMB. A reliable model for SMB operation allows a rapid and easy selection of the optimum exper

ISSN:0001-1541    

DOI:10.1002/aic.690431110

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractOptimal thermodynamic feed conditions for distillation of ideal binary mixtures are identified through thermodynamic efficiency analysis of distillation columns. Due to simplifying assumptions, temperatures do not appear explicitly in the final thermodynamic efficiency equations. These equations are found to be functions of only feed composition, the thermodynamic state of the feed, and the relative volatility of the more volatile component A with respect to component B! When a single one‐phase feed, AB, is used, the preferred state of the feed is all liquid when the amount of heavy component B is greater than 50% and is all uapor when the amount of volatile component A is greater than 50%. For a single two‐phase feed, a simple heuristic is presented to adjust the liquid fraction of a given feed to maximize the thermodynamic efficiency of the distillation column. For most binary mixtures, two feeds with the same composition, but one saturated vapor and the other saturated liquid, can improve a distillation column's efficiency substantially. The maximum benefit is obtained for a 50–50 mixture. This two‐feed case is less efficient than the single two‐phase feed case when the relative volatilities exceed certain calculab

ISSN:0001-1541    

DOI:10.1002/aic.690431111

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY