Showing papers on "Countercurrent exchange published in 1997"

Collisional particle pressure measurements in solid liquid flows

Abstract: Experiments were conducted to measure the collisional particle pressure in both cocurrent and countercurrent flows of liquid-solid mixtures. The collisional particle pressure, or granular pressure, is the additional pressure exerted on the containing walls of a particulate system due to the particle collisions. The present experiments involve both a liquid-fluidized bed using glass, plastic or steel spheres and a vertical gravity-driven flow using glass spheres. The particle pressure was measured using a high-frequency-response flush-mounted pressure transducer. Detailed recordings were made of many different particle collisions with the active face of this transducer. The solids fraction of the flowing mixtures was measured using an impedance volume fraction meter. Results show that the magnitude of the measured particle pressure increases from low concentrations (>10% solid volume fraction), reaches a maximum for intermediate values of solid fraction (30-40%), and decreases again for more concentrated mixtures (>40%). The measured collisional particle pressure appears to scale with the particle dynamic pressure based on the particle density and terminal velocity. Results were obtained and compared for a range of particle sizes, as well as for two different test section diameters. In addition, a detailed analysis of the collisions was performed that included the probability density functions for the collisoin duration and collision impulse. Two distinct contributions to the collisional particle pressure were identified: one contribution from direct contact of particles with the pressure transducer, and the second one resulting from particle collisions in the bulk that are transmitted through the liquid to the pressure transducer.

Numerical Analysis of an Extremity in a Cold Environment Including Countercurrent Arterio-Venous Heat Exchange

Abstract: A model of the thermal behavior of an extremity, e.g., a finger, is presented. The model includes the effects of heat conduction, metabolic heat generation, heat transport by blood perfusion, heat exchange between the tissue and the large blood vessels, and arterio-venous heat exchange. Heat exchange with the environment through a layer of thermal insulation, depicting thermal handwear, is also considered. The tissue is subdivided into four concentric layers simulating, from the center outward, core, muscle, fat, and skin. Differential heat balance equations are formulated for the tissue and for the major artery and the major vein traversing the finger. These coupled equations are solved numerically by a finite-difference, alternating direction method employing a Thomas algorithm. The numerical scheme was extensively tested for its stability and convergence. This paper presents the model equations and results of the convergence tests, and shows plots of blood and tissue temperatures along the axis of the model for combinations of parameters including the effect of countercurrent heat exchange between the artery and the vein.

Vapor permeation system

Abstract: A gas-phase countercurrent sweep stream having a low partial pressure of a given vapor to be selectively removed from a mixture of condensable vapors is swept past the permeate side of a selectively permeable membrane, thereby rendering unnecessary the application of a vacuum to the permeate side of the membrane and at the same time maintaining a chemical potential gradient of the vapor to be removed across the membrane.

Lumen mass transfer in hollow‐fiber membrane processes with constant external resistances

Abstract: Membrane processes have recently become an accepted unit operation for a wide variety of separations in industry and in environmental applications. Hollow-fiber membrane processes with a constant external resistance having a constant or variable shell concentration resulting from an operational mode of cocurrent or countercurrent are studied. By solving numerically the continuity mass-conservation equation with the corresponding boundary conditions, the lumen laminar mass-transfer coefficients for both cases are correlated. The correlations greatly improve the calculating accuracy of the overall mass-transfer coefficient and can be used to obtain the lumen mixed-cup concentration by an algebraic equation substituting the partial differential equation. A separation factor m{prime} is introduced to characterize the effect of the operational mode. Calculation results demonstrate that the lumen mass-transfer coefficient is independent of the real lumen and shell concentrations, but it is greatly influenced by m{prime}. The countercurrent mode, compared to the cocurrent mode, provides not only a higher mean driving force, but a higher lumen mass-transfer coefficient. This conclusion is novel and valid for the tube-shell heat or mass-transfer processes and is supported by the experimental data in the literature and the authors` gas membrane separation experiments.

Comparison of hydrodynamic parameters for countercurrent and cocurrent flow through packed beds

Abstract: Hydrodynamic parameters have been determined in common equipment, i.e., same column and liquid and gas distributors, for cocurrent and countercurrent two-phase flow through fixed beds. The piston/dispersion exchange model (PDE) with usual Danckwerts' boundary conditions (closed/closed system) has been used to describe the liquid flow. A new imperfect pulse method has been used to estimate the PDE model parameters directly from the experimentally nonideal input and output response. The transition between trickle flow and pulse flow, for two-phase downflow, and the occurrence of flooding, for countercurrent flow, has been investigated using a macroscopic model for the two-phase flow.

Application of supercritical fluid technology to citrus oil processing

Abstract: Two continuous processes for the fractionation of citrus oil by supercritical carbon dioxide (SCCO2) have been developed to obtain the highly concentrated product of flavor; (1) countercurrent extraction process, (2) pressure swing adsorption process. In the countercurrent process, effects of pressure and solvent-to-feed ratio (S/F ratio) on the separation selectivity were investigated at a temperature of 333 K. The selectivity was improved with the increase in the S/F ratio. The HETS was calculated from the phase equilibria estimated by PR equation of state for a model mixture and the measured HETS decreased from 4 to 0.2 m with increase in the S/F ratio. In the pressure swing adsorption process, a cyclic adsorption and desorption process in SCCO2 was developed with silica gel adsorbent at a temperature of 313 K. Highly concentrated fraction of flavor was continuously obtained at the desorption step and blowdown step.

Turbulence structure and prediction of interfacial heat and mass transfer in wavy‐stratified flow

Abstract: The turbulence structure of the liquid phase near a wavy gas-liquid interface in stratified flow was experimentally investigated in a 50 mm (H) × 100 mm (W) rectangular duct. The characteristic parameters of the organized motion under the liquid waves such as the frequency of appearance and length scale were estimated by using the variable-interval time-averaging and the photochromic dye activation techniques. These characteristics were used in a hybrid surface renewal-eddy cell model to predict the interfacial heat- and mass-transfer coefficients in stratified two-phase flow. The predictions for cocurrent and countercurrent flows agreed reasonably well with experimental data available in the literature.

Hydroprocessing in a countercurrent reaction vessel

Abstract: Liquid petroleum or chemical streams are upgraded by passing the stream countercurrent to the flow of a treat gas such as a hydrogen-containing gas in at least one reaction zone (r1, r2, r3). The reaction vessel (R) used in the practice of the present invention contains vapor passageway means (VB1, VB2, VB3, VB4, VB5) and optionally liquid passageway means (LD) to bypass one or more catalyst beds. This permits more stable and efficient reaction vessel operation.

Countercurrent reaction vessel

Abstract: A reaction vessel for processing liquid petroleum or chemical streams wherein the stream flows countercurrent to the flow of a treat gas, such as a hydrogen-containing gas, in at least one interaction zone (r1, r2, or r3). The reaction vessel contains vapor, optionally liquid, passageways (VB1-5) to bypass one or more packed beds, preferably catalyst beds (r1-3). This permits more stable and efficient vessel operation.

Countercurrent extraction of sparingly soluble gases for membrane introduction mass spectrometry

Abstract: Membrane introduction mass spectrometry has been applied to inert gas measurements in blood and tissue, but gases with low blood solubility are associated with reduced sensitivity. Countercurrent extraction of inert gases from a blood sample into a water carrier phase has the potential to extract most of the gas sample while avoiding dependence of signal on blood solubility. We present the design of a membrane countercurrent exchange (CCE) device coupled with a conventional direct insertion membrane probe to measure partial pressure of low solubility inert gases in aqueous samples. A mathematical model of steady-state membrane CCE predicts that countercurrent extraction with appropriate selection of carrier and sample flow rates can provide a mass spectrometer signal nearly independent of variations in solubility over a specified range, while retaining a linear response to changes in gas partial pressure over several orders of magnitude. Experimental data are presented for sulfur hexafluoride and krypton in water samples. Optimal performance is dependent on adequate equilibration between the sample and carrier streams, and the large resistance to diffusion in the aqueous phase for insoluble gases presents a substantial challenge to the application of this principle.

Impact of Model Error on the Measurement of Flow Properties Needed to Describe Flow Through Porous Media

Abstract: Indirect methods are commonly employed to determine the fundamental flow properties needed to describe flow through porous media Consequently, if one or more of the postulates underlying the mathematical description of such indirect methods is invalid, significant model error can be introduced into the measured value of the flow property In particular, this study shows that effective mobility curves that include the effect of viscous coupling between fluid phases differ significantly from those that exclude such coupling Moreover, it is shown that the conventional effective mobilities that pertain to steady-state, cocurrent flow, steady-state, countercurrent flow and pure countercurrent imbibition differ significantly Thus, it appears that traditional effective mobilities are not true parameters; rather, they are infinitely nonunique In addition, it is shown that, while neglect of hydrodynamic forces introduces a small amount of model error into the pressure difference curve for cocurrent flow in unconsolidated porous media, such neglect introduces a large amount of model error into the pressure difference curve for countercurrent flow in such porous media Moreover, such neglect makes it difficult to explain why the pressure gradients that pertain to steady-state, countercurrent flow are opposite in sign It is shown also that improper handling of the inlet boundary condition can introduce significant model error into the analysis This is because, if a short core is used with one of the unsteady-state methods for determining effective mobility, it may take many pore volumes of injection before the inlet saturation rises to its maximal value, which is in contradiction with the usual assumption that the inlet saturation rises immediately to its maximal value Finally, it is pointed out that, because of differences in flow regime and scale, the effective mobilities measured in the laboratory may not be appropriate for inclusion in the data base for a reservoir-scale simulation

Transient behavior of pulsed particulate fluidized beds

Abstract: Transient phenomena in solid–liquid fluidized-bed systems are important in designing pulsed, countercurrent (multistage) fluidized-bed contactors of the Cloete-Streat type at high-solids flow rate Of particular interest are the residence times or corresponding velocities of porosity gradients in the bed and the excess or overshoot height of the bed after refluidization Theory assuming local equilibrium between holdup and velocity of the phases (local-equilibrium model) for stepwise perturbations in the liquid flow is readily available It is investigated whether the local-equilibrium theory can be used for more complex perturbations and whether inertia effects, such as are encountered in countercurrent multistage fluidized-bed systems, can be ignored Therefore, the detailed particle-bed model of Foscolo and Gibilaro, which incorporates inertia effects, was applied to investigate the transient behavior of fluidized-bed systems Transient fluidization experiments were performed with a broad range of water-fluidized particles in a laboratory-scale multistage fluidized-bed contactor The operating conditions corresponded to those for countercurrent contact Numerical simulations with the particle-bed model predict satisfactory experimental results The “overshoot” heights of the fluidized bed were estimated correctly by the particle-bed model, whereas the local-equilibrium model only provides a conservative estimate However, the local-equilibrium model allows an analytical solution that is more interesting for design, as it avoids tedious calculations The residence time of the last perturbation before the fluidized bed relaxes to steady state was estimated with similar accuracy by both models

Measurement System of Bubbly Flow Using Ultrasonic Velocity Profile Monitor and Video Data Processing Unit, (II) Flow Characteristics of Bubbly Countercurrent Flow

Abstract: The authors have developed a measurement system which is composed of an ultrasonic velocity profile monitor and a video data processing unit in order to clarify its multi-dimensional flow characteristics in bubbly flows and to offer a data base to validate numerical codes for multi-dimensional two-phase flow. In this paper, the measurement system was applied for bubbly countercurrent flows in a vertical rectangular channel. At first, both bubble and water velocity profiles and void fraction profiles in the channel were investigated statistically. Next, turbulence intensity in a continuous liquid phase was defined as a standard deviation of velocity fluctuation, and the two-phase multiplier profile of turbulence intensity in the channel was clarified as a ratio of the standard deviation of flow fluctuation in a bubbly countercurrent flow to that in a water single phase flow. Finally, the distribution parameter and drift velocity used in the drift flux model for bubbly countercurrent flows were calculated from the obtained velocity profiles of both phases and void fraction profile, and were compared with the correlation proposed for bubbly countercurrent flows.

Flowing solids dynamic holdup in the countercurrent gas-flowing solids-fixed bed reactor

Abstract: The effect of packing properties on the flowing solids dynamic holdup in the countercurrent gas-flowing solids-fixed bed reactor was investigated. The correlations for the prediction of flowing solids dynamic holdup in preloading and loading regimes were proposed.

Mass transfer in a countercurrent, supercritical extraction system for solutes in moist solids

Abstract: Solutions of partial differential equations (PDE) that describe mass transfer in conventional continuous, countercurrent solid-liquid extraction systems also successfully describe mass transfer in nearly continuous, contercurrent extraction systems where supercritical fluid (SF) is used to extract solutes from moist solids. These solutions can also be used to describe mass-transfer behavior in absorbers where solutes are transferred from SF to showers of liquid drops if information about drop diameters and velocities, effects of circulation in drops and, most important, axial dispersion is available. The variables involved include: dimensionless concentrations, Ficks number, F = Dst/a2 , the stripping factor, a the Peclet number, UL/Da and the mass-transfer Biot number, Bi. In extractors, a slightly greater than 1.0 are needed to provided efficient extraction without excessive circulation of solvent; in absorbers, α should be < 1.0. α, Pe and Bi are used in PDE solutions to determine F and extrac...

Steady‐state multiplicity in cocurrently cooled autothermal reactors

Abstract: The steady-state simulation of a cocurrently cooled autothermal fixed-bed reactor was carried out using a 2-D heterogeneous mathematical model. The ammonia synthesis was chosen as a case study. Unlike the not-autothermal cocurrent reactor, which is unconditionally stable, the autothermal cocurrent reactor shows multiple steady states within a broad range of operating conditions. This finding, not reported in the literature, is explained through the mass transport from the bottom to the top of the reactor, associated with a feedback of energy. The feedback of heat, which is inherent to autothermal reactors, leads to an ignition-extinction phenomenon similar to that found in the countercurrent configuration. The influence of different parameters on the stability of the autothermal cocurrent reactor was analyzed. The regions where steady-state multiplicity occurs were compared with those presented by the autothermal countercurrent reactor. The influence of an additional heat exchanger on the reactor stability was considered.