Continuous Flow Regime Research Articles

Hydrodynamics of pulsing flow in three-phase chemical reactors

In the present study verification is carried out of mathematical criteria that define the boundary between two hydrodynamic regimes: the gas continuous flow (GCF) regime and the pulsing flow (PF) regime. Attention is focused on the criterion derived using the model of K. Grosser, R.G. Carbonell and S. Sundaresan, AIChE J., 34 (1988) 1850–1860, and the parametric sensitivity of this criterion is analysed. Next, based on the measured values of the gas pressure drop in a reactor the verification of some selected parameters is carried out by fitting the theoretical and experimental boundary lines separating the GCF and PF regimes. Experimental results are also presented concerning the determination of parameters characterising the pulsing flow through the bed, namely the frequency of pulsations and the pulse structure. These parameters were determined for the system nitrogen–water, and also for liquids differing from water in their physicochemical properties.

Competition of mixing and segregation in rotating cylinders

Using discrete element methods, we study numerically the dynamics of the size segregation process of binary particle mixtures in three-dimensional rotating drums, operated in the continuous flow regime. Particle rotations are included and we focus on different volume filling fractions of the drum to study the interplay between the competing phenomena of mixing and segregation. It is found that segregation is best for a more than half-filled drum due to the nonzero width of the fluidized layer. For different particle size ratios, it is found that radial segregation occurs for any arbitrary small particle size difference and the final amount of segregation shows a linear dependence on the size ratio of the two particle species. To quantify the interplay between segregation and mixing, we investigate the dynamics of the center of mass positions for each particle component. Starting with initially separated particle groups we find that no mixing of the component is necessary in order to obtain a radially segregated core.

Deformational controls on the dynamics of fluid flow in mesothermal gold systems

Abstract Coupling between deformation processes and rock permeability is a major factor influencing fluid migration and fluid-rock interaction during the formation of mesothermal lode gold systems in mid- to upper crustal regimes. The evolution of permeability during deformation is controlled by a dynamic competition between deformation-induced porosity creation processes and porosity destruction processes. Localization of deformation in faults and shear zones leads to flow localization, with large scale flow systems forming when active faults and shear zones link to create percolation networks. Broad regions of fluid focusing develop around the upstream segments of active shear networks and fluid discharge regions develop in the downstream parts of these systems. The architecture of flow within shear networks is influenced by the relative proportions of backbone, dangling and isolated structures within the network. Connectivity in the network may increase with growth of the shear network, but is expected to continually change as the locus and intensity of deformation changes. The typical distribution of mesothermal lode deposits within crustal scale shear networks indicates that mesothermal systems might develop most efficiently in networks that are close to the percolation threshold, i.e. with most flow occurring along a flow backbone forming only a small part of the total shear network. Dangling elements adjacent to the backbone, particularly in the downstream (discharge) parts of the system, provide some of the best potential for gold deposition by fluid-rock interaction processes. Contrasting styles of fluid flow are expected between the seismogenic and aseismic regimes of the shear- or fault-hosted hydrothermal systems associated with mesothermal lode gold formation. Below the seismic-aseismic transition, creep processes can lead to near-steady state permeabilities and produce continuous fluid flow regimes. In the seismogenic regime, large cyclic changes in fault permeability, fluid pressures and fluid flux occur during the seismic cycle, and lead to fault-valve behaviour and episodic fluid flow. The seismogenic regime allows for a richer variety of gold deposition processes than is generally available in the aseismic regions of hydrothermal systems associated with lode gold formation.

Poly(pyrogallol) film on glassy carbon electrode for selective preconcentration and stripping voltammetric determination of Sb(III)

Oxidative electropolymerization of pyrogallol at the glassy carbon electrode in phosphate buffer (pH 7.00) gave a stable, water-insoluble film, adherent on the electrode surface. This poly(pyrogallol) film modified glassy carbon electrode was exploited for the selective preconcentration of Sb(III) at an open circuit, followed by its determination by differential pulse anodic stripping voltammetry both in batch and continuous flow regimes. Factors affecting the accumulation, reduction, stripping and removal steps were investigated and an optimized procedure was then developed. Under optimized conditions, for batch determination, the calibration curve was linear in the range 5.00×10 −9 to 1.00×10 −7 M Sb(III) (10 min accumulation). A detection limit of 4.1×10 −10 M Sb(III) (0.050 ppb) ( S/ N=3) was found for a 10 min accumulation. For eight successive determinations of Sb(III) at concentrations of 1.00×10 −8 and 5.00×10 −9 M, relative standard deviations were 2.4% and 6.0%, respectively. Similar results were obtained for continuous flow determination. Interferences from selected metals and other inorganic ions and organic substances were examined. The developed method was applied to Sb determinations in human hair and seawater samples, both in batch and continuous flow systems.

Radial segregation of granular mixtures in rotating cylinders

Simultaneous mixing and segregation of granular materials is of considerable practical importance; the interplay among both processes is, however, poorly understood from a fundamental viewpoint. The focus of this work is radial segregation—core formation—due to density in a rotating cylinder. The flow regime considered is the cascading or continuous flow regime where a thin layer of solids flows along a nearly flat free surface, while the remaining particles rotate as a fixed bed along with the cylinder. The essence of the formation of a central segregated core of the more dense particles lies in the flow, mixing, and segregation in the cascading layer. The work involves experiments and analysis. A constitutive model for the segregation flux in cascading layers is proposed and validated by particle dynamics and Monte Carlo simulations for steady flow down an inclined plane. The model contains a single parameter, the dimensionless segregation velocity (β), which is treated as a fitting parameter here. Experimental results for the equilibrium segregation of steel balls and glass beads are presented for different fractions and different extents of filling. There is a good match between theoretical predictions and all experimental results when the value of dimensionless segregation velocity is taken to be β=2. The extent of segregation is found to increase with increase in the dimensionless segregation velocity and dimensionless diffusivity but is independent of the level of filling. Lagrangian simulations based on the theory and experiments demonstrate the competition between segregation and mixing. In the case of slow mixing, the intensity of segregation monotonically decreases to an equilibrium value; for fast mixing, however, there exists an optimal mixing time at which the best mixing is obtained.

Study into nucleation of steam during expansion through a nozzle

The development of nucleation theory makes possible to predict droplet formation in expanding steam. The present study is conducted to investigate the nucleation process occurring during the flow through convergent-divergent nozzles. A kinetic theory approach for determining the rate of droplet growth has been developed successfully. The governing flow equations are developed for the free and continuous flow regimes. A numerical scheme is introduced to solve the resulting equations. To validate the predictions, an experiment is carried out to measure the droplet size and thermodynamic properties of steam expanding through a convergent-divergent nozzle. It is found that the amount of supersaturation is limited with pressure increase and the limiting degrees of supercooling and the resulting droplet sizes depend on the Wilson pressure and the expansion rate. The theoretical predictions are in good agreement with the experimental measurements and the process of nucleation in high pressure steam can, possibly, be described satisfactorily by the model developed in the present study.

Influence of surfactants on absorption of CO2 in a stirred tank with and without bubbling

We determined volumetric mass transfer coefficients k(L)a for the absorption of bubbled and unbubbled CO2 in a stirred tank, with and without surfactants in the absorbent liquid. In the absence of surfactants, results obtained under continuous liquid flow regimes agreed closely with batch results; accordingly, for convenience, subsequent experiments with surfactant additives were performed only with continuous liquid flow. The results of experiments to determine the dependence of k(L)a on stirring rate and the kind of bubbling device were fitted to within a 4% error by expressions of the form k(L)a = K(P(e)/V)0.4 where P(e) is the effective power supplied to the tank, V is the volume of liquid in the tank and K depends on surfactant concentration and the bubbling device and increases linearly with the liquid flow rate. Surfactant reduced k(L)a by an amount that, to within a 3% error, was proportional to both the corresponding value of k(L)a in the absence of surfactant and the surface concentration of surfactant.

Vibration of a Tube Bundle in Two-Phase Freon Cross-Flow

Abstract Two-phase cross-flow exists in many shell-and-tube heat exchangers. The U-bend region of nuclear steam generators is a prime example. Testing in two-phase flow simulated by air-water provides useful results inexpensively. However, two-phase flow parameters, in particular surface tension and density ratio, are considerably different in air-water than in steam-water. A reasonable compromise is testing in liquid-vapor Freon, which is much closer to steam-water while much simpler experimentally. This paper presents the first results of a series of tests on the vibration behavior of tube bundles subjected to two-phase Freon cross-flow. A rotated triangular tube bundle of tube-to-diameter ratio of 1.5 was tested over a broad range of void fractions and mass fluxes. Fluidelastic instability, random turbulence excitation, and damping were investigated. Well-defined fluidelastic instabilities were observed in continuous two-phase flow regimes. However, intermittent two-phase flow regimes had a dramatic effect on fluidelastic instability. Generally, random turbulence excitation forces are much lower in Freon than in air-water. Damping is very dependent on void fraction, as expected.

Hydrodynamics and mass transfer in a three-phase fixed-bed reactor with cocurrent gas—liquid downflow

The paper presents experimental results concerning dynamic liquid holdup, wetting efficiency and local mass transfer coefficients between the liquid and solid surface in a fixed-bed three-phase reactor, in which both the gas and liquid flow cocurrently downwards. The experiments were conducted for two basic regimes of operation of trickle-bed reactors, namely the gas continuous flow regime and the pulsing flow regime under atmospheric pressure. The measurements of dynamic liquid holdup have been performed for a wide range of gas and liquid flow rates, three different packing diameters, and for two systems of working media of different physical properties. The common correlation developed determines with good accuracy the holdup values in both regimes as well as in the transition region between the two hydrodynamic modes. In experiments concerning the wetting efficiency a dynamic tracer method has been employed, for which an original mathematical model has been formulated. The results are presented in the form of diagrams and appropriate correlating formulae. The experiments concerning local solid—liquid mass transfer coefficients were carried out for two diameters of spherical particles, with the flow rates of both phases and physicochemical properties of the liquid varied over a wide range. The experimental results are correlated and compared with the appropriate literature data. A mathematical model describing the time-varying solid—liquid mass transfer process is formulated for the pulsed flow regime. The time-averaged mass transfer coefficients calculated on the basis of the model developed are compared with the experimental values. A good agreement between these two sets of values fully confirms the assumptions of the model elaborated.

Mean-field theory of sandpile avalanches: From the intermittent- to the continuous-flow regime.

We model the dynamics of avalanches in granular assemblies in partly filled rotating cylinders using a mean-field approach. We show that, upon varying the cylinder angular velocity \ensuremath{\omega}, the system undergoes a hysteresis cycle between an intermittent- and a continuous-flow regime. In the intermittent-flow regime, and approaching the transition, the avalanche duration exhibits critical slowing down with a temporal power-law divergence. Upon adding a white-noise term, and close to the transition, the distribution of avalanche durations is also a power law. The hysteresis, as well as the statistics of avalanche durations, are in good qualitative agreement with recent experiments in partly filled rotating cylinders.

Ohmic heating of complex fluids

This paper describes the basic principles and physical modelling of systems based on the ohmic heating of liquids by passage of mains frequency electric current through the liquid itself, in order to heat it on a continuous flow regime. Preliminary numerical results for non-Newtonian fluids of Herschel-Bulckley type (Carbopol 940) are presented and the effect of natural convection, and shear and temperature dependence are emphasized.

Characterization of bismuth-manganese oxide catalysts for methane oxidative coupling

This paper reports a finding, namely a cooperation between separate phases, which may help understand the mechanism of oxidative coupling of methane. The activity of co-precipitated bismuth-manganese oxides in the oxidative coupling of methane, measured in the continuous flow regime, is maximum for Bi : Mn = 1:2. The catalysts have been investigated by X-ray diffraction. X-ray photoelectron spectroscopy, transmission electron microscopy and microanalysis; catalytic pulse experiments were performed. The Bi2Mn4O10 phase is present in these catalysts. But, surprisingly, although it corresponds to the Bi : Mn ratio of 1:2 giving maximum activity, it decomposes to α-Bi2O3 and α-Mn2O3 in a few hours during the catalytic test. The higher catalytic activity observed at Bi:Mn = 1:2 thus does not seem due to the Bi2Mn4O10 phase. The high activity catalyst is actually composed of Bi2O3 in contact with an other phase. This might be a Bi-depleted Bi2Mn4O10 like phase; it is speculated that this phase could donate oxygen to Bi2O3 and make it more active. This would suggest that oxygen mobility or spill-over between phases, observed to be beneficial in other reactions occurring at 350–450° C, also promotes the selective oxidative coupling of methane above 700° C.

Flow-reversal flow-injection analysis Enhancement of flow-injection titrations

Most FIA applications involve simple unidirectional continuous-flow regimes. This paper continues the development of the flow-reversal (frFIA) approach and its applications. Flow-reversal flow programming achieves flexibility in operation without requiring manifolds to be refitted. The sensitivity of FIA titrations can be increased by use of frFIA.

Vibration of Tube Bundles in Two-Phase Cross-Flow: Part 2—Fluid-Elastic Instability

Abstract An extensive experimental program was carried out to study the vibration behavior of tube bundles subjected to two-phase cross-flow. Fluid-elastic instability is discussed in Part 2 of this series of three papers. Four tube bundle configurations were subjected to increasing flow up to the onset of fluid-elastic instability. The tests were done on bundles with all-flexible tubes and on bundles with one flexible tube surrounded by rigid tubes. Fluid-elastic instabilities have been observed for all tube bundles and all flow conditions. The critical flow velocity for fluid-elastic instability is significantly lower for the all-flexible tube bundles. The fluid-elastic instability behavior is different for intermittent flows than for continuous flow regimes such as bubbly or froth flows. For continuous flows, the observed instabilities satisfy the relationship V/fd = K(2πζm/ρd2)0.5 in which the minimum instability factor K was found to be around 4 for bundles of p/d = 1.47 and significantly less for p/d = 1.32. Design guidelines are recommended to avoid fluid-elastic instabilities in two-phase cross-flows.

A New Method of Contaminant Plume Analysis

ABSTRACTThis paper develops an analytical expression for contaminant transport from a finite source in a continuous flow regime. The model requires some numerical integration and its degree of accuracy for near‐field problems depends on discretization procedures applied to the source boundary. A second model for a continuous source is developed by extending a well‐known pulse model. This second model is particularly useful in that it permits the determination of several potential unknowns directly from a concentration distribution. These include the source concentration, source dimensions, the position of the center of mass which is the product of the seepage velocity and the time since the contaminant first entered the ground water, and up to three dispersivities for a three‐dimensional problem. As a demonstration of its utility, this second model is applied with reasonable success to a well‐defined field condition. A comparison of the two models indicates that, except for minor differences in the very near field, the results from each are virtually identical.

An immobilized cell reactor with simultaneous product separation. II. Experimental reactor performance

The simultaneous separation of volatile fermentation products from product-inhibited fermentations can greatly increase the productivity of a bioreactor by reducing the product concentration in the bioreactor, as well as concentrating the product in an output stream free of cells, substrate, or other feed impurities. The Immobilized Cell Reactor-Separator (ICRS) consists of two column reactors: a cocurrent gas-liquid "enricher" followed by a countercurrent "stripper" The columns are four-phase tubular reactors consisting of (1) an inert gas phase, (2) the liquid fermentation broth, (3) the solid column internal packing, and (4) the immobilized biological catalyst or cells. The application of the ICRS to the ethanol-from-whey-lactose fermentation system has been investigated. Operation in the liquid continuous or bubble flow regime allows a high liquid holdup in the reactor and consequent long and controllable liquid residence time but results in a high gas phase pressure drop over the length of the reactor and low gas flow rates. Operation in the gas continuous regime gives high gas flow rates and low pressure drop but also results in short liquid residence time and incomplete column wetting at low liquid loading rates using conventional gas-liquid column packings. Using cells absorbed to conventional ceramic column packing (0.25-in. Intalox saddles), it was found that a good reaction could be obtained in the liquid continuous mode, but little separation, while in the gas continuous mode there was little reaction but good separation. Using cells sorbed to an absorbant matrix allowed operation in the gas continuous regime with a liquid holdup of up to 30% of the total reactor volume. Good reaction rates and product separation were obtained using this matrix. High reaction rates were obtained due to high density cell loading in the reactor. A dry cell density of up to 92 g/L reactor was obtained in the enricher. The enricher ethanol productivity ranged from 50 to 160 g/L h while the stripper productivity varied from 0 to 32 g/L h at different feed rates and concentrations. A separation efficiency of as high as 98% was obtained from the system.

Chronic Effects of Coal-Liquid Dispersions on Fathead Minnows and Rainbow Trout

Abstract Partial-life-cycle bioassays were conducted on fathead minnows Pimephales promelas and rainbow trout Salmo gairdneri, under continuous-flow regimes, with water-soluble fractions (WSFs) derived from a coal liquid. Phenols constituted 95% of the organic carbon in stock WSFs. Growth of larval fathead minnows was significantly reduced at 0.25 mg/liter total phenols as determined by dye photometry. Spawning of adult fathead minnows exposed to coal-liquid WSFs was inhibited at 1.27 mg/liter total phenols and was significantly reduced at 0.62 mg/liter total phenols. Spawning inhibition was not permanent at concentrations tested; after 21 days of exposure, fathead minnow pairs resumed spawning upon transfer to control water. The minimal concentration of WSF that resulted in significant mortality of rainbow trout embryos was time-dependent. No rainbow trout embryos survived 14 days of exposure to ≥2.98 mg/liter total phenols because of egg mortality or premature hatching. Swim-up rainbow trout suffered ra...