摘要:

AbstractThe influence of shear thinning and viscoelasticity on the power required for the mixing of viscous liquids using six different helical ribbon agitators has been investigated. Four Newtonian and 12 non‐Newtonian fluids prepared using several polymers dissolved in varying concentrations in different solvents cover a wide range of rheological properties. By a careful choice of test media, the specific and combined effects of shear thinning and viscoelasticity on the power requirement have been examined. Simple models are proposed to predict the effective shear rate in the tank from the knowledge of the torque or power number. The effective shear rate predictions compared with the effective shear rate estimated using the scheme of Metzner and Otto (1957) show that they slightly depend on the shear thinning properties. Fluid's elasticity increases appreciably the power requirement, and departures from the generalized Newtonian power curve in the laminar regime are observed at smaller Reynolds numbers for viscoelastic fluids. Bottom wall resistance of the mixing vessel makes a negligible contribution to the power consumptio

ISSN:0001-1541    

DOI:10.1002/aic.690390902

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA steady‐state method recently proposed for measuring two‐phase flow properties of a porous medium is supplemented by NMR imaging for measuring the saturation gradients which are allowed to persist in the specimen. The method relies on a strong capillary end effect which is assumed to be enforced by an outlet capillary pressure boundary condition, but direct verification of this has not hitherto been available. For constant flow rates, incremented stepwise, steady‐state profiles of hydrocarbon saturation Soare obtained which agree with predictions that they be monotonic, nonintersecting, and convex.Using D2O for the aqueous phase allows unambiguous detection of hydrocarbon by NMR, and imaging measurements of transverse relaxation time T2were included in the protocol. T2shows a dependence on Sowhich is mild but not negligible for quantitative NMR im

ISSN:0001-1541    

DOI:10.1002/aic.690390903

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe kinetics of hydrogen absorption and desorption in LaNi4.8Al0.2,LaNi4.9Al0.1and LaNi5, suspended in cyclohexane and LaNi5in ethanol have been investigated. The absorption process can be described in terms of mass transfer and reaction resistances in series. The rate‐limiting steps for this process are dissolution of hydrogen in the solvent and the reaction of hydrogen with the metal alloy. Over a broad range of hydrogen loadings, the reaction of hydrogen with the metal follows the rate equation:\documentclass{article}\pagestyle{empty}\begin{document}$$ R_{{\rm abs}} = k_a a_s \left({C_{{\rm H}_{2,S}} - C_{{\rm eq,}a}} \right) $$\end{document}This type of kinetic equation indicates that during hydride formation in a slurry a surface process is the rate‐determining step, which was confirmed by the observed influence of the solvent. Desorption of hydrogen can be described best with a shrinking core relation:\documentclass{article}\pagestyle{empty}\begin{document}$$ 1 - \left({1 - X} \right)^{1/3} = k_d ln\left({\frac{{P_{eq,d}}}{{P_{H_2}}}} \right)t $$\end{document}The phase transformation at the β/α boundary appears to be the rate‐determining step. Alloys with higher aluminum contents produce a lower reaction rate c

ISSN:0001-1541    

DOI:10.1002/aic.690390904

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA companion article (Chen and Hu, 1993a) discussed a statistical theory that for a system consisting of two closed subsystems, the residence time density (RTD) functions of the two subsystems will be statistically independent, if a two‐dimensional perfect mixing exists at the subsystem boundary. In this case, the overall RTD function is related to that for individual subsystems through the convolution integral. This theory has been validated experimentally using two die‐screw combinations.Based on this theory, a predictive RTD model for an intermeshing counterrotating twin‐screw extruder has been developed. The screw in the longitudincl direction has been treated as C‐chambers in series. The overall RTD of leakage flows has been calculated from the RTDs of these individual chambers and then converted into the RTD of the extruder. The predicted RTD has been tested against experimental results with

ISSN:0001-1541    

DOI:10.1002/aic.690390905

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe mixing and segregation behavior of liquid fluidized beds containing two different‐sized particle species are described mathematically. This model is based on a mass balance, rather than a volume balance, which has been used by most previous researchers. The model also includes the effects of density gradients that are present in the bed. Particle dispersion coefficients in a binary‐solid fluidized bed are evaluated using this model. Further, a correlation that allows the calculation of these particle dispersion coefficients is presented, which describes not only dispersion coefficients calculated using the above model, but also those presented by other researchers. It is applicable over a broad range of variables of practical interest and is based on relevant dimensionless parameters to account for the influence of important particle and fluid properties on the particle dispersion characterist

ISSN:0001-1541    

DOI:10.1002/aic.690390906

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe growth kinetics and contact nucleation of calcite crystals were studied in a pH‐stat crystallizer. Large seed crystals up to 230 μm were successfully prepared by the gel growth technique. The crystal growth data over a wide range of crystal sizes were evaluated from titration curves by an initial slope method and then analyzed by the two‐step model. The mass‐transfer and surface integration rates were found to be functions of crystal size with the exception of mass‐transfer rates of small crystals less than 10 μm. The controlling step of crystal growth process was tested using the effectiveness factor concept. Large seed crystals of calcite induced contact nucleation when a stainless‐steel impeller was used for agitation. The important features of contact nucleation for calcite crystals were explored and compared with those of soluble

ISSN:0001-1541    

DOI:10.1002/aic.690390907

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractSeparation of a binary mixture by a three‐step one‐column pressure swing adsorption process in isothermal and adiabatic cases has been studied. The separation performance of the process is characterized by three parameters: enrichment, recovery and productivity. It has been compared at various operating conditions. The evolution of the bed performance from cycle to cycle and the bed dynamics at cyclic steady state have also been addres

ISSN:0001-1541    

DOI:10.1002/aic.690390908

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractSpatiotemporal temperature patterns on a polycrystalline nickel disk were recorded using infrared video imaging during atmospheric hydrogen oxidation and characterized by the proper orthogonal decomposition pattern analysis technique. The system was studied at two different residence times, 3.2 s and 6.4 s. At moderate feed temperatures, steady‐state multiplicity and rate oscillations were found. Oscillations at a residence time of 6.4 s were periodic and essentially spatially uniform. At a residence time of 3.2 s, however, the surface temperature became nonuniform, and rate oscillations occurred via traveling temperature waves which emanated from “pacemakers” (locally active regions) on the edge of the catalyst. During periodic oscillations, the waves were emitted synchronously from the pacemakers, while during chaotic oscillations, the pacemakers were desynchronized and emitted waves independently of each other. Nonlocal gas‐phase coupling between distance surface elements caused spatial desynchronization during rate oscil

ISSN:0001-1541    

DOI:10.1002/aic.690390909

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe study of diffusion and permeation of methane, nitrogen, and helium in miscible blends of poly(styrene‐stat‐acrylonitrile) (PSAN) and poly (methyl methacrylate) (PMMA) at 35–140°C shows that permeation coefficients (P) of helium followed the linear logarithmic mixing rule. Positive deviations from the linear logarithmic mixing rule were observed for permeation and diffusion coefficients (D) of methane and nitrogen below the glass transition temperature (Tg). The deviations decreased with increasing temperature and disappeared above Tg. The experimental results were analyzed by free volume and activated state theories. The Arrnenius plots of log D or log P vs. the reciprocal of temperature exhibited discontinuities in the glass transition region for all gases and blend compositions. The discontinuities are caused by large thermal expansion coefficient differences between the rubbery and glassy states of PSAN and PMMA. The sorption of methane in 50/50 PSAN/PMMA has dual‐mode characteristics

ISSN:0001-1541    

DOI:10.1002/aic.690390910

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe linear chemical‐physical theory (LCPT) model for liquid metal solution thermodynamics has been extended to the determination of the liquidus curves for binary intermetallic compound‐forming systems. The equations developed include corrections to the observed melting point temperature and heat of fusion for compounds that dissociate partially on melting. The primary advantages of the LCPT model for solid‐liquid equilibria are the small number of physically realistic parameters required, ease of implementation, and wide applicability. In addition, the model also permits the incorporation of compounds in modeling the liquidus curves that are not necessary for representing the liquid‐phase thermodynamic properties. For the seven systems studied, the agreement between calculated and experimentally measured liquidus curves is qui

ISSN:0001-1541    

DOI:10.1002/aic.690390911

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY