Showing papers in "Chemical Engineering Communications in 1981"

Review of reverse osmosis membranes and transport models

Abstract: After a brief introduction to membrane processes in general, and the reverse osmosis process in particular, the structure and properties of membranes and membrane transport theory are described. The mechanism of salt rejection and transport properties of membranes are discussed in detail. Solubility, diffusivity, and permeability of membranes to solutes and solvents are reviewed critically and compared with each other. Special attention is given to two particular types of membranes, cellulose acetate (CA) and aromatic polyamide (AP) membranes, which are often used for water desalination. The major portion of this article is devoted to the review and discussion of membrane transport theory with application to the reverse osmosis and ultrafiltralion processes. It is shown that the solvent flux can be represented reasonably well by linear models such as the solution-diffusion model (Lonsdale, et al., 1965). The contribution of pore flow to the solvent flux is small. The solute flux, however, is not ...

Fully developed laminar convection from a helical coil

Abstract: A general correlating equation has been developed for all Prandtl and Dean numbers. This expression was constructed by joining the theoretical Nusselt number for a straight tube, a theoretical asymptote for the regime of creeping secondary flow, a semi-theoretical expression for the boundary layer regime and an asymptotic value of Nu for the intervening regime of flow.The arbitrary coefficients and exponents in the model were evaluated using experimental and numerically computed values. Slightly differing sets of coefficients are required for uniform wall temperature and longitudinally uniform heating with uniform peripheral wall temperature. All prior theoretical results were for toroidal flow (zero pitch). A numerical solution was developed for helical flow (finite pitch). These results confirm the validity of neglecting pitch for tightly wound coils but suggest a generalization of the correlating equation for large pitch.

Cavern sizes in agitated fluids with a yield stress

Abstract: Highly viscous, non-Newtonian Xanthan gum solutions and two transparent model fluids with similar Theological properties have been studied under aerated (up to 1 vvm) and unaerated conditions in a 0.29m diameter agitated vessel. Rushton disc turbines of size 1/3 and 1/2 of the tank diameter have been used alone and also in conjunction with 6-bladed, 45°-pitch axial flow turbines of the same size at speeds up lo 24 rev/s, enabling specific power inputs of up to 15 W/kg to be imparted. Flow patterns were studied by flow visualisation and hot film anemometry. When the fluids have a yield stress, the fluid divides into a turbulent well-mixed cavern which increases in size with increasing speed with the remainder stagnant. A model for the size of the cavern fits the experimental data well for both aerated and unaerated mixing. Large diameter combinations produce good mixing at about 1 to 2 W/kg which is about 1/3 to 1/4 of that required with small diameter combinations. Single disc turbine impellers a...

The spreading of aqueous surfactant solutions on glass

Abstract: An investigation of the spreading behavior of drops of a number of aqueous anionic, cationic and non-ionic surfactant solutions has demonstrated, for the first-time, spreading to a maximum solid-solution contact area, and then contraction to a smaller final size. The nature and kinetics of the spreading and contraction are shown to be dependent on the type and concentration of the surfactant. Generally, the maximum contact area decreases with increasing surfactant concentration, to a minimum (or no spreading) in the vicinity of the CMC. Above the CMC, anionic and nonionic surfactant solutions spread with peripheral fingering, followed by contraction, while cationic surfactant solutions do not spread at these concentrations. The results are explained in terms of the thin primary film spreading ahead of the drop.

Analysis of the multiplicity patterns of a cstr

Abstract: Continuous changes in the residence time of a cooled continuously stirred tank reactor, in which a single, exothermic, first-order reaction occurs, may lead to one of six different multiplicity patterns. A simple technique is developed for the exact prediction of the multiplicity pattern existing for any set of parameter values and of the influence of changes in the parameter values on the transition from one pattern to another.

Residence time distributions and micromixing

Abstract: This review is concerned primarily with the Lagrangian approach to mixing. First, the concepts of residence time and residence time distribution are introduced and defined. After a general treatment of this topic in the second chapter, the concept of micromixing is discussed in the third chapter with due emphasis on the application to chemical reactors. The final chapters extend the theory and applications to unsteady stale systems and to systems which are non-isothermal or non-homogeneous.

Invited review hydrodynamic and mixing models for bubble column reactors

Abstract: The knowledge of the flow patterns of each phase of bubble column reactors is of considerable importance for the rational design and scale-up The hydrodynamic models for the liquid phase have been reviewed The models have been based on some form of pressure balance or energy balance These two approaches have been compared and recommendations have been made regarding the range of applicability of the individual models A comparison between the predicted and the experimental liquid velocities has been presented whenever possible The range of variables has been pointed out for which further investigations are needed The empirical and theoretical models for the mixing behavior of the different phases of bubble column reactors have also been reviewed

Application of turbulence fundamentals to reactor modelling and scaleup

Abstract: Applications of the fundamentals of turbulent mixing become clear once those fundamentals are understood. The first article in this series presented those fundamentals, in order to show how to apply turbulent mixing fundamentals modelling and scaleup, this article covers the following topics: 1. reaction types and their interaction with mixing; 2. closure of the Reynolds equations for mixing and reactions; 3. application to complex geometries; 4. random coalescence-dispersion modelling; 5. application to complex chemistry. The most difficult problem in applying our knowledge of turbulence to mixer modelling and scaleup is the choice of model complexity. The levels of model complexity available and how to apply them to various problems are presented following the introduction.

Newtonian fluid sphere with rigid or mobile interface in a shear-thinning liquid: drag and mass transfer

Abstract: The drag force and the mass transfer rate of a Newtonian fluid sphere, having mobile or rigid interface, moving in a power law fluid, are obtained by an approximate solution of equations of motion in the creeping flow regime. It is shown that both the drag and mass transfer increase as the flow index of the external fluid decreases. The increase of drag due to the pseudoplastic anomaly is more significant at large viscosity ratio parameter. The results obtained are in good agreement with available experimental data and with those analyses based on variational principle when the non-Newtonian flow behavior is not very pronounced. Also, the predicted mass transfer rates are in good agreement with the trends presented in the literature. Unlike in the case of drag force, the effect of the pseudoplastic anomaly on mass transfer rate is more pronounced for low values of the viscosity ratio parameter. The analysis was extended to include the case when the surface of the sphere was immobilized by surface...

The effect of fin density on the heat transfer and pressure drop performance of low-finned tube banks

Abstract: Heat transfer and pressure drop data are presented for equilateral-triangular pitch tube bundles containing low-finned tubes with fin counts between 0.4 and 1.06 fins per mm. Two types of finned tubes were tested. The first was a 19-mm O.D. integral finned tube with a 1.65-mm fin height and with 0.75 and 1.06 fins per mm. The second was a 31.75-mm O.D. plain tube wound with 3.18-mm soldered fin stock and containing 0.4, 0.71, and 0.98 fins per mm. A new low-fin heat transfer correlation was developed which predicted these new data and additional data from five other sources within an error range from minus 20 percent to plus 31 percent. A necessary dimensionless group required to obtain this accuracy was the fin diameter/fin spacing ratio. Existing friction factor correlations were not successful in predicting these new pressure drop results. A new low-fin friction factor correlation was developed which predicted these data and additional published data within an error range from minus 19 percent to plus ...

Chemical reactor modelling for purposes of controller design

Abstract: The relation between modelling and design of control systems for chemical reactors is discussed, using several practical examples. Criteria for controlability and satisfactory response are proposed. The dependence of the controller structure on the properties of the reactor model is investigated, and it is shown that the most important decisions in controller design occur during process design. The information required for controller design is different from the information required for scaleup, and depends on the design approach. Examples chosen are control of a fluid catalytic cracker, a hydrocracker, and a crystallizer. Control and reactor modelling are not only often separate activities by themselves, but they are done in separate groups and current practice is to call the control engineering in after the design is finished or the plant either operates or is scheduled to start operation. If we want to make significant advances, we have to realize that some of the most important decisions that...

Protein precipitation-analysis of particle size distribution and kinetics

Abstract: (1981). PROTEIN PRECIPITATION-ANALYSIS OF PARTICLE SIZE DISTRIBUTION AND KINETICS. Chemical Engineering Communications: Vol. 12, No. 1-3, pp. 203-219.

Liquid dispersion mechanisms in agitated tanks: part i. pitched blade turbine

Abstract: An oil into water dispersion, created by a pitched blade turbine, was observed using high speed, stereoscopic motion pictures. Two different dispersion mechanisms were responsible for the break-up of the oil drops, even though both mechanisms occurred in the vortex system trailing from the impeller blade tips. The first mechanism could be described as ligament stretching, since large oil drops were stretched by fluid shear to form elongated ligaments, which subsequently ruptured into small drops. The second mechanism was turbulent fragmentation, where large oil drops were shattered into large droplet clouds the instant they entered the trailing vortex system. Observations of the oil drops undergoing ligament stretching also indicated that velocities in the trailing vortex system were proportional to impeller tip speed.

Fundamentals of turbulent motion, mixing and kinetics∗

Abstract: The principles of mixing on a macroscopic level have been known and used for a number of years for the scale-up of mixing processes. However, these scale-up methods have been empirical in nature due to the lack of information about mixing theory on a microscopic level. The complexity of problems involving turbulence, such as mixing, has made a quantitative description difficult. However, the modern turbulence approach has led to a much deeper understanding of the field of mixing and of related turbulent scalar transport processes. The analysis allows us to define measurable mixing criteria. A knowledge of the parameters of the turbulence provides the information necessary to estimate the degree of mixing. The parameters can often be estimated from the geometry of the flow system and from simple empirical relationships. With a knowledge of the mixing, the problem of chemical kinetics, when mixing cannot be ignored, can be studied. A theoretical approach is possible that with limited assumptions al...

Fluid bed heat exchangers- a new model for particle convection energy transfer

Abstract: A new model is presented for the wall-to-bed heat transfer from solid surfaces immersed in fluidized beds. This model makes use of some basic ideas adopted from the kinetic theory of gases in order to describe the mechanism of energy transfer through the moving particles (“particle convection”). The parallel—and in most cases less important—gas convective and radiative contributions to the total heat transfer coefficient are calculated from well known equations from the literature. The predictions of the new model are in good agreement with many experimental observations, in particular as to the non-monotoneous variation of heat transfer coefficients with gas velocity and particle diameter. Literature data for particle diameters from 0.01 to more than 10 mm, various particle materials, some different gases, various pressures, and temperatures up to more than IOOO°C are compared with the model calculations.

Motion of and mass transfer from an assemblage of solid spheres moving in a non-newtonian fluid at high reynolds numbers

Abstract: An approximate solution for the motion of an assemblage of solid spheres moving in a power-law fluid in the high Reynolds number region is obtained using a combination of Happel's free-surface cell model and the boundary layer theory. It is theoretically predicted that the drag coefficient will decrease with the increase of the shear-thinning anomaly. The results of the present analysis are in reasonably good agreement with the available experimental data for fixed and fluidized beds. The influence of the non-Newtonian behavior on the mass transfer rate from an assemblage of solid spheres is also discussed.

The effect of gravity and friction on the stability of boiling flow in a channel

Abstract: The linear stability or boiling How in heated 'parallel channels is considered. A nondimensional lumped parameter model is derived from the Conservation equations of homogeneous two-phase Row. The marginal stability surfaces are constructed in a systematic way using the D-partition method. It is found that frictional and gravitational effects can lead to islands of instability within the stable region, which can produce higher order instabilities. Moreover, the model yields a unified treatment of Ledine[ggrave]g¨ and density-wave instability modes.

Invited review microbial growth kinetics

Abstract: (1981). INVITED REVIEW MICROBIAL GROWTH KINETICS. Chemical Engineering Communications: Vol. 8, No. 4-6, pp. 181-211.

Sedimentation of particles in non-newtonian fluids

Abstract: The influence of non-Newtonian flow behavior on sedimentation velocity of particles is investigated using an approximate solution for the motion of an assemblage of solid spheres presented previously by the authors. It is theoretically predicted that the pseudoplaslicity decreases the sedimentation velocity and its reduction is pronounced at large voidage. The present theory is discussed using the available empirical correlations.

Secondary nucleation: an analysis

Abstract: (1981). SECONDARY NUCLEATION: AN ANALYSIS. Chemical Engineering Communications: Vol. 12, No. 1-3, pp. 161-169.

Heat transfer in isotropic turbulence

Abstract: The direct interaction approximation (Kraichnan 1959) was used to calculate thermal eddy diffusivities, intensities or temperature fluctuations, and various correlation functions for heat transfer in an isotropic turbulent flow with a uniform mean temperature gradient The calculated results compare reasonably well with values from experiments in a heated wind tunnel and may be of value in the analysis of more complex heat transfer processes

Analysis of simultaneous radiative and conductive heat transfer in fluidized beds

Abstract: In the bubbling regime of operation for fluidized beds, the major mechanism for heat transfer is transient conduction to periodic packets of densely packed particles at the heat transfer surface. The well known Mickley and Fairbanks model, with various subsequently proposed modifications, adequately describes this transient conduction mechanism. However, no adequate theory exists for heat transfer in high-temperature fluidized beds where radiative contribution becomes significant. Analysis of the radiative contribution is complicated by the nonlinear interaction of radiation with conduction/convection. This paper describes a differential formulation of the combined radiative/ conductive heat transfer process. The discrete flux method used by Churchill et al. for radiative transport in heterogeneous media is applied here to the problem of transient heat transfer to packets in fluidized beds. Packets are modeled as radiatively participating media with absorption, scattering, and emission of radiati...

Liquid dispersion mechanisms in agitated tanks: part ii. straight blade and disc style turbines

Abstract: The dispersion of oil in water in an agitated vessel was studied for two types of radial discharge impellers, straight blade and disc style turbines. Two different dispersion mechanisms, ligament stretching and turbulent fragmentation, were observed to occur in the vortex systems of the impeller discharge. Although these two dispersion mechanisms were similar to pitched blade turbine performance, differences in the velocity magnitudes and vortex interactions were observed with the radial flow impellers. The ligament stretching mechanism was observed between the vortex formation regime and the transition to the fragmentation regime. The turbulent fragmentation mechanism was observed only in highly turbulent flow. Blade thickness was found to influence the ligament stretching mechanism. A thin blade on the straight blade turbine created higher vortex velocities and smaller drop sizes than a thick blade for the same tip speed and processing time. The consequences of this blade thickness effect could be signi...

Application of a stochastic network pore model to a catalyst pellet

Abstract: A previously developed stochastic two-dimensional network pore structure model has been applied directly to a nickel/alumina commercial catalyst pellet. A technique has been devised to determine the pore diameter distribution function of cylindrical pore segments for any prescribed network dimension which will exactly replicate the experimentally observed mercury penetration curve for the pellet in the method of mercury porosimetry. This stochastic network representation for the catalyst pellet pore structure is superior to the classical parallel bundle model since it incorporates the element of pore interconnectivity. The model has been extended to investigate the phenomena occurring during the accumulation of a foulant deposit (e.g. coke in hydrocarbon catalysis) for the pore structure determined for a 10 × 10 set of network elements for the pellet under study. The analysis shows how the build-up of a deposit causes physical blocking of the smaller pores at quite low accumulations of foulant wh...

A simulation model of particle deposition on single collectors

Abstract: A simulation model based on the concepts developed previously (Tien el. al. 1977, Wang el. al. 1977) for the deposition of particles from suspensions flowing past single spherical and cylindrical collectors is presented. The model is capable of providing detailed information about the formation and growth of particle deposits at the surface of the collector. Reasonably good agreement with experimental data was observed.

Atomization in a venturi scrubber

Abstract: High speed motion pictures were made or the breakup of a single jet of water in the throat of a venturi scrubber for various air and water jet velocities and different nozzle diameters. Atomization seemed to occur as the result of several different mechanisms, depending on the flow conditions, the most common mechanisms being breakup due to either capillary or acceleration waves and also breakup by “steady shear,” all of which have been observed by other investigators. The so-called “cloud-type” atomization proposed by Hesketh (1970) was not observed despite several attempts to achieve it. Measured values of pressure drop across the test section of the scrubber compare well with previously published results.

Heat transfer between packed, agitated and fluidized beds and submerged surfaces

Abstract: Heat transfer between packed, agitated and fluidized beds and submerged surfaces is treated by a common theoretical concept. The thermal properties appearing in this concept may be predicted a priori. The remaining problem is how to describe the particle motion. It is shown that the penetration model, which is well known from gas liquid systems, also applies to gas-solid systems. The results of the theoretical approach are compared with numerous experimental data obtained from literature.