摘要:

AbstractWe investigate the utility of digital particle imaging velocimetry (DPIV) for performing kinematic measurements in non‐Newtonian flows. With the advances in numerical techniques for simulation of viscoelastic flows, acquisition of spatially dense 2‐D kinematic data in steady and time‐dependent deformations can be useful in verifying predictions of the corresponding computational studies. Furthermore, kinematic measurements of the velocity field and rate of deformation in prototypical industrial processes can significantly enhance the rational design and optimization of polymer processing unit operations. Application of a high seeding density DPIV technique in viscoelastic media is discussed, and quantitative data are obtained in a number of industrially relevant flow geometries. The issues of velocity‐position assignment and the effects of a velocity gradient across DPIV correlation regions are discussed. A simple yet effective averaging technique preserves the order of accuracy and assigns the velocity vectors to their appropriate positions using an overlapping discretization scheme. The examples studied experimentally include steady flow in circular pipes, flow past obstructions, flow in a lid‐driven cavity, and time‐dependent free‐surface extensional flows in a liquid filament. With the exception of the first example, these flow geometries constitute an important collection of configurations in which quantitative experimental data for non‐Newtonian fluids are scarce

ISSN:0001-1541    

DOI:10.1002/aic.690430202

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe convective diffusion equations were used to obtain mean concentrations and concentration fluctuation covariances during turbulent reactive mixing of fast reversible reactions. The mean concentrations are directly applicable to reactions that are much faster than the mixing, while the covariances can serve as a first approximation closure for slower reactions. The covariances should also be useful for testing more general closure models. Numerical examples are presented for the simple mass‐action rate law with stoichiometric coefficients of unit

ISSN:0001-1541    

DOI:10.1002/aic.690430203

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractTo study the influence of particle concentration on the hydrodynamics of bubble‐column slurry reactors operating in the heterogeneous flow regime, experiments were carried out in 0.10, 0.19, and 0.38‐m‐dia. columns using paraffinic oil as the liquid phase and slurry concentrations of up to 36 vol. %. To interpret experimental results a generalization of the “two‐phase” model for gas–solid fluid beds was used to describe bubble hydrodynamics. The two phase identified are: a dilute phase consisting of fast‐rising large bubbles that traverse the column virtually in plug flow and a dense phase that is identified with the liquid phase along with solid particles and entrained small bubbles. The dense phase suffers backmixing considerably. Dynamic gas disengagement was experimented in the heterogeneous flow regime to determine the gas voidage in dilute and dense phases. Experimental data show that increasing the solid concentration decreases the total gas holdup significantly, but the influence on the dilute‐phase gas holdup is small. The dense‐phase gas voidage significantly decreases gas holdup due to enhanced coalescene of small bubbles resulting from introduction of particles. The dense‐phase gas voidage is practically independent of the column diameter. The dilute‐phase gas holdup, on the other hand, decreases with increasing column diameter, and this dependence could be described adequately with a slight modification of the correlation of Krishna and Ellenberger developed

ISSN:0001-1541    

DOI:10.1002/aic.690430204

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractA mechanistic model was developed to predict pressure drop and flooding in packed columns equipped with corrugated packing of the regular type. It was developed after considerating the interaction of falling liquid film with the gas phase, based on mass‐and momentum‐conservation equations. Among the most common structured packings, the behavior of the Mellapak and BX types was analyzed. The aim of this work is to demonstrate how mechanistic models, developed for simple geometry, can also be used to compute pressure drops in cases where the geometry is more complex, as with a structured packing. This approach, based on the geometric characteristics of the packing and measurable parameters such as liquid holdup, enables the development of a basic model by limiting the number of adjustable parameters, which are numerous in all the available models. Because of its nature, this model is extremely easy to extend to different types of structured packi

ISSN:0001-1541    

DOI:10.1002/aic.690430205

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractPast research shows that for the same power per unit volume, flocculation performance varies with tank size and impeller type. This study was performed to characterize effects of scale and impeller design on turbulence produced in the flocculation process. The study was performed with a Rushton turbine and an A310 foil impeller in three square tanks of 5‐, 28‐ and 560‐L volume. Fluid velocities were measured using a dual‐channel laser Doppler velocimeter with an enhanced burst spectrum analyzer. Flocculation tanks were operated at a constant average unit‐mass energy‐dissipation rate of 0.0016 m2/s3. The results show that the turbulence intensity and local turbulent energy‐dissipation rate were higher for the Rushton turbine than for the A310 impeller. The turbulence intensity was found to increase with increasing tank size regardless of impeller type. The local turbulent energy dissipation rate decreased for the Rushton turbine and remained constant for the A310 impeller with increas

ISSN:0001-1541    

DOI:10.1002/aic.690430206

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractAn experimental investigation was conducted to determine the boiling heat‐transfer characteristics in a thermosyphon condenser‐reboiler of air‐separation plants. The test section was a plate‐fin heat exchanger of practical size of 0.106 m width and 2.56 m length. The testing fluid was nitrogen. Experiments were carried out within a range of pressure from 150 to 300 kPa, submergence from 1.95 to 2.80 m, and heat flux up to 11 kW/m2under constant wall‐temperature condition. The liquid superheat at the onset of boiling was rather significant. The boiling heat‐transfer coefficients obtained agreed well with Chen's correlation modified by Colburn's J factor within a mean deviat

ISSN:0001-1541    

DOI:10.1002/aic.690430207

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractPressure fluctuations in gas–solid fluidized beds have long been recognized as an important index for the quality of fluidization. Due to their highly random nature, many researchers have resorted to stochastic signal‐processing methods to extract useful information. However, methods such as power‐spectral density are based on a priori hypothesis of the stationarity of the signals, whose validity has not been investigated. In this article, using the Wigner distribution (WD), the signal's strong nonstationary feature is demonstrated and signal properties are addressed. Local frequency information is extracted from the defined parameter—the local peak weighted average (LPWA), which can be regarded as a generalized concept of the major frequency of power‐spectral density in the nonstationary case. Experimental results indicate that LPWA is stochastically much more reproducible than the major frequency of power‐spectral density. The change in LPWA under different operating conditions may reflect the corresponding change in the bubble phase. This information is useful in characterizing the quality of f

ISSN:0001-1541    

DOI:10.1002/aic.690430208

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractWhen a fluidizing gas is switched from a low‐density gas to a high‐density gas, particles in the emulsion phase agglomerate and bubbles disappear. At the same time, channeling occurs and a decrease in the pressure drop over the bed is observed. The disturbance of fluidization is temporary, and normal fluidization is restored after several minutes. The study of mechanisms of the transient phenomena shows that the difference in diffusion rates of the two gases in the emulsion phase caused the agglomeration of particles. Because the mobility of particles was reduced, channels were formed and the fluidizing gas bypassed through them. Effects of the properties of the fluidizing gases and particles on the intensity of the transient phenomena were also studied. The intensity increased with increasing difference in the densities of the two gases. The transient phenomena were considerable for small‐ and light‐particle

ISSN:0001-1541    

DOI:10.1002/aic.690430209

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractBatchwise fluidized‐bed calcination and sulfation of a limestone were done to investigate particle comminution phenomena and their relation with parallel occurrence of reactions. Operating conditions of the bed were those typical of atmospheric bubbling fluidized‐bed combustors. A general framework of comminution phenomena is outlined, which includes different types of fragmentations as well as attrition by abrasion. Comminution processes were characterized by following the modifications of bed sorbent particle‐size distribution and the elutriation rates of fines throughout conversion. Mutual interactions between comminution processes and the progress of chemical reactions are ass

ISSN:0001-1541    

DOI:10.1002/aic.690430210

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY

摘要:

AbstractThe attainable region approach for reaction – mixing systems is combined with geometric methods for the feasibility of separations. The result is a systematic approach to identify the feasible compositions that can be achieved in processes combining simultaneous reaction, mixing, and separation. An activity‐based formulation for both nonideal VLE and reaction‐rate expressions is applied to develop hybrid reactor–separator models for the multiphase CSTR and PFR with simultaneous vapor removal. A reaction–separation vector is defined that satisfies the same geometric properties as the reaction vector. Therefore, the attainable region can be constructed following the existing procedure for reaction–mixing systems. This approach provides a method to generate feasible process alternatives. The technique is demonstrated on two nonideal ternary mixtures: the production of dimethyl ether by dehydration of methanol, and the production of methyl tert‐butyl ether from isobutene and methanol. It is shown that for hybrid reaction–separation devices the entire composition space is not always attainable. In such cases, combining a hybrid device with traditional nonreactive separation is required to attain c

ISSN:0001-1541    

DOI:10.1002/aic.690430211

出版商:American Institute of Chemical Engineers    

年代:1997

数据来源: WILEY