Showing papers on "Countercurrent exchange published in 1996"
TL;DR: In this paper, a procedure for the optimal and robust design of the operating conditions of the unit is developed in the frame of equilibrium theory, where the adsorption equilibria are described through the constant selectivity nonstoichiometric Langmuir model, while mass-transfer resistances and axial dispersion are neglected.
Abstract: The separation of a multicomponent mixture in a countercurrent adsorptive separation unit is analyzed. A procedure for the optimal and robust design of the operating conditions of the unit is developed in the frame of equilibrium theory, where the adsorption equilibria are described through the constant selectivity nonstoichiometric Langmuir model, while mass-transfer resistances and axial dispersion are neglected. This allows for extending the results previously obtained for systems characterized by either the linear or stoichiometric Langmuir isotherm to cases where the adsorbates exhibit rather different loading capacities. Analogies and differences among these three equilibrium models with respect to the criteria for the choice of operating conditions of countercurrent separation units are discussed. A comparison between model predictions and experimental data assesses the reliability and accuracy of the theoretical results achieved in this work.
92 citations
TL;DR: In this article, a large-scale three-channel flowing-liquid-membrane module (selective membrane valve), designed for gas separation, is also described, which consists of PVTMS or PDMS/PPSQ non-porous membranes which acted as gas-permeable barriers.
78 citations
TL;DR: In this paper, a compressible countercurrent shear layer was investigated experimentally by establishing reverse flow around the perimeter of a supersonic jet and the results showed that spatial growth rates of the countercurrent layer significantly exceed those of the classical co-flowing layer at comparable density ratios and levels of compressibility.
Abstract: A compressible countercurrent shear layer was investigated experimentally by establishing reverse flow around the perimeter of a supersonic jet. Measurements demonstrate that spatial growth rates of the countercurrent shear layer significantly exceed those of the classical coflowing layer at comparable density ratios and levels of compressibility. Experiments also reveal the presence of coherent three-dimensional structures in the countercurrent shear layer at convective Mach numbers where similar structures are not present in coflowing layers. It is argued that these kinematic differences are responsible for the enhanced diffusion of the shear layer with counterflow. The spatio-temporal theory is used to examine the connection between the experimental observations and the existence of a transition from convective to absolute instability in high-speed shear layers.
73 citations
TL;DR: In this article, the CFDS-FLOW3D computational fluid dynamics code is used to determine the force exerted on a standing wave by gas flowing over it, and it is found that the drag on the wave is mainly due to the pressure variation around it and that it is a strong function of the channel dimensions.
62 citations
TL;DR: Simulation results show that this moderately high urea permeability in LDL is an order of magnitude too high for effective operation of the passive countercurrent multiplier, confirming the fundamental feasibility of the Passive hypothesis.
Abstract: The urine concentrating effect of the mammalian renal inner medulla has been attributed to countercurrent multiplication of a transepithelial osmotic difference arising from passive absorption of N...
59 citations
TL;DR: In this article, the experimental data of the countercurrent flow limitation (CCFL) for gas and liquid in a horizontal pipe with a bend are investigated. And the authors show that the onset of flooding appears simultaneously with the slugging of unstable waves that are formed at the crest of hydraulic jump.
56 citations
TL;DR: In this article, the transport mechanisms in a three-phase continuous, countercurrent sublation process are presented and two mathematical models, namely, the series CSTR model (SCM) and the two-phase axial dispersion model (ADM2), are developed.
Abstract: Solvent sublation is a nonfoaming wastewater treatment process that combines the benefits of bubble fractionation and liquid−liquid extraction in a way that does not require mixers, settlers, or subsequent downstream treatment. A review of past work on small lab-scale batch columns revealed that removal efficiencies of nonvolatile and volatile organic compounds are generally higher than those observed in bubble fractionation, air stripping, and conventional liquid−liquid extraction. In this work, the first of a three-part series, the transport mechanisms in a three-phase continuous, countercurrent sublation process are presented. Two mathematical models, namely, the series CSTR model (SCM) and the two-phase axial dispersion model (ADM2), are developed. It is shown that these two models are equivalent and can be used interchangeably with the aid of a simple expression. Nondimensional correlations, based upon simulated data obtained from the SCM, are generated to predict the steady-state fractional removal,...
36 citations
TL;DR: In this article, large-scale liquid and gas flow patterns were measured in a 0.5m diameter column dumped with 25-and 50-mm plastic Pall rings and 38-mm ceramic Intalox saddles.
Abstract: Large-scale liquid and gas flow patterns were measured in a 0.5-m diameter column dumped with 25- and 50-mm plastic Pall rings and 38-mm ceramic Intalox saddles. Liquid gas velocity and profiles measured for these packings were shown as a function of the initial distribution of the liquid and gas, packed height and liquid and gas flow rates. There was an emphasis in the experiments to study the development of liquid wall flow as well as the interaction between countercurrent liquid and gas. In addition, the effect of maldistribution was studied by means of water cooling experiments. It was of a particular interest to see the extent to which liquid temperature profiles, or analogously concentration profiles, were affected by known velocity profiles.
36 citations
TL;DR: In this paper, an ultrathin skinned hollow fiber membrane module coupled with a liquid absorbent was investigated experimentally for the removal of CO 2 from a gas mixture, and the overall mass transfer coefficients of carbon dioxide were obtained in the gas phase.
Abstract: An ultrathin skinned hollow fibre membrane module coupled with a liquid absorbent was investigated experimentally for the removal of CO 2 from a gas mixture. The module consists of a bundle of hollow fibres having a dense skin layer at the outer edge of the fibre. The gas mixture containing 4% CO 2 was introduced into the hollow fibre lumen and was in countercurrent contact with the liquid (either water or NaOH solution) fed into the module shell. The overall mass transfer coefficients of carbon dioxide were obtained in the gas phase. A study ofmass transfer in the membrane module indicates that the overall mass transfer coefficients, K AG, are controlled by both the liquid film and membrane resistances. It was also shown experimentally that the use of the ultrathin skinned hollow fibres module for CO 2 absorption has two advantages. Firstly, the dense skin layer of the hollow fibre membrane eliminates the wetting problem commonly encountered in microporous membranes. As a result, the mass transfer operations are stable with long term exposure of the membrane to the liquid absorbent. Secondly, operations of the feed pressure are flexible. The feed gas pressure of 200 k P a higher than the liquid pressure was maintained without any noticeable bubble formation in the liquid phase. The higher operating pressure in the gas phase suggests that the reduction of the mass transfer rate due to the higher membrane resistance could be compensated by an elevation of the feed gas pressure, i.e. increase of the driving force.
26 citations
TL;DR: In this paper, the authors compare the results of the four-section and three-section countercurrent models and conclude that the equivalent true countercurrent model is fully adapted to represent the 4-section scheme, but a major discrepancy arises with the 3-section model: the raffinate concentration is not correctly predicted.
Abstract: This article is intended as a complement to a recent article in this journal, Comparative Study of Flow Schemes for a Simulated Countercurrent Adsorption Separation Process by Ching et al. (1992). These authors carried out experiments on continuous separation of three carbohydrate mixtures with both the usual four-section scheme and a three-section scheme: fructose-dextran (MW≅9400), raffinose-dextran (MW=6000), and fructose-raffinose. These two schemes were modeled as equivalent true countercurrent systems. From the experiments and model results they conclude that the equivalent true countercurrent model is fully adapted to represent the four-section scheme, but a major discrepancy arises with the three-section scheme: the raffinate concentration is not correctly predicted. From these results it may be concluded that when a physically meaningful modeling of the equivalent true countercurrent is realized, the apparent discrepancy disappears. From another standpoint, such a mismatch between the experiments and model predictions would not have been observed with a model that considers the periodic port motion. Such a model foresees a cyclic steady state where extract and raffinate compositions vary throughout the period T; they can be considered as constant only when averaged over the period. It is worth comparing the results of these two models, especially when the equivalent countercurrent representation is not very close to the physical phenomenon it should represent. We agree with their major finding that a three-zone scheme represents necessarily a higher desorbent demand than a four-zone scheme.
25 citations
TL;DR: Using mathematical models, the effects of blood flow, permeability, and anatomical arrangement of microvessels on the steady‐state distribution of solute in the blood and the interstitial fluid (ISF) are explored.
Abstract: Objective: As a first step in modeling microvascular exchange in the renal medulla, we developed mathematical models to explore the effects of blood flow, permeability, and anatomical arrangement of microvessels on the steady-state distribution of solute in the blood and the interstitial fluid (ISF).Methods: Single capillaries and countercurrent capillary loops were used to model microvessels that were surrounded by a secretory epithelium over either the whole or part of the capillary length. Solute concentration in the vessels and the ISF were derived analytically. We also derived approximate solutions that ignored axial diffusion of solute.Results: The full and approximate solutions were in good agreement with data based on measurements in the renal medulla. Model results revealed that concentration in the ISF falls rapidly with distance beyond the region of solute secretion and equilibrates with the concentration in capillaries, even with countercurrent exchange between the two limbs of the capillary l...
Dissertation•
01 Aug 1996
Patent•
03 Apr 1996
TL;DR: In this article, a fluidized bed reactor operating under countercurrent gas-solid flow conditions was designed to maintain gas and solids flows of at least 70% of flooding conditions.
Abstract: A fluidized bed reactor operating under countercurrent gas-solid flow conditions wherein the reactor is supplied with horizontal baffles having separate passages through which the gas and the solid flow, such that the ascending gas leaves the baffle in a point higher than the point in which the descending solid leaves the same baffle, and the use of the reactor to perform reactions under countercurrent flow conditions while maintaining gas and solids flows of at least 70% of flooding conditions.
TL;DR: In this paper, a closed mathematical model of the liquid friction factor for flow occurring in triangular grooves is presented, which considers the interfacial shear stresses due to liquid-vapor frictional interactions for countercurrent flow.
TL;DR: In this article, a countercurrent shear layer with a nominal convective Mach number (M c ) of 2 was generated using a unique arrangement, and the growth rate was found to be twice that of conventional, co-flowing shear layers at comparable M c.
Abstract: A countercurrent shear layer with a nominal convective Mach number (M c ) of 2 is generated using a unique arrangement. The shear layer growth rates were found to be twice that of conventional, coflowing shear layers at comparable M c . It is hypothesized that the countercurrent shear layer becomes self-excited, leading to a modification of the turbulent properties. Instantaneous flow visualization images clearly reveal the presence of large, highly convoluted, turbulent structures believed to be responsible for the enhanced growth rates. The flowfield was also examined for the presence of eddy shocklets that have been observed in numerical simulations under similar compressible flow conditions. Even though the countercurrent flowfield appears to satisfy the criteria for the creation of shocklets, a careful and thorough search revealed no evidence of their presence.
TL;DR: In this article, the critical heat flux of a two-phase thermosyphon was analyzed in a countercurrent annular flow, where a liquid film and a vapor flow exist in the countercurrent flow.
Abstract: An analytical study has been done on the critical heat flux of a two-phase thermosyphon, in which a liquid film and a vapor flow exist in a countercurrent annular flow. The CHF point on the thermosyphon is proved to correspond to a maximum falling liquid rate fed to the thermosyphon, which can be determined from three equations of momentum, its partial derivative with void fraction, and mass balance in the thermosyphon. This maximum point, furthermore, becomes identical to the point at which an envelope line generated from the momentum equation and its partial derivative intersects the mass balance line. The CHF calculated from the maximum liquid rate is found to be in fairly good agreement with the existing CHF data.
TL;DR: In this paper, an investigation was carried out to clarify the mechanism of countercurrent flow limitation or flooding, that is, limitations in the falling water mass flux in countercurrent two-phase flow in vertical channels, and to identify the effects of predominant parameters regarding CCFL, adopting the criterion that the CCFL condition be given by an envelope of momentum equation applied for the entire length of the channel with respect to any void fraction.
Abstract: In this study, an investigation was carried out to clarify the mechanism of countercurrent flow limitation (CCFL) or flooding, that is, limitations in the falling water mass flux in countercurrent two-phase flow in vertical channels, and to identify the effects of predominant parameters regarding CCFL, adopting the criterion that the CCFL condition be given by an envelope of momentum equation applied for the entire length of the channel with respect to any void fraction. As a result, it was found that the analytical model proposed could adequately predict all existing experimental results investigated in this study. In the channel configuration, circular, rectangular, and annular or planar channels, channel dimensions of diameter, gap size, width or circumference, and length, interfacial and wall friction, water injection mode, and inlet water subcooling were dominant parameters. Therefore, both the mechanism and the quantitative effects of CCFL have been identified.
Patent•
01 May 1996TL;DR: In this paper, a regeneration phase consisting of a co-current depressurization step, in order to participate in the repressurization of another adsorber, and then a countercurrent elution step at the low pressure of the cycle with oxygen taken off from production.
Abstract: The process comprises a regeneration phase including a co-current depressurization step, in order to participate in the repressurization of another adsorber, a countercurrent depressurization down to the low pressure of the cycle without gas intake, and then a countercurrent elution step at the low pressure of the cycle with oxygen taken off from production. The cycle time does not exceed 20 seconds. The process is particularly application to the production of oxygen having a purity greater than 90% with flow rates of 2 to 6 liters/minute for oxygenotherapy.
TL;DR: In this article, the authors investigated the initiation conditions of liquid ascent in a countercurrent two-phase flow of air and water using vertical pipes of 10-26 mm in diameter.
TL;DR: In this article, a countercurrent gaseous diffusion model was developed to describe graphite oxidized through porous coatings and scales, where graphite was oxidized by a porous alu...
Abstract: A countercurrent gaseous diffusion model was developed to describe oxidation through porous coatings and scales. The specific system modeled involved graphite oxidized through a porous alu...
TL;DR: In this article, the distribution of fluid temperatures in a direct contact heat exchanger is analyzed analytically by integration of the one-dimensional energy equations, and the influences of decisive parameters on the temperature distributions along the column are examined.
TL;DR: In this paper, mass transfer characteristics were determined for countercurrent extraction in a spray column with supercritical carbon dioxide solvent, where feeds were 5 and 10 vol% aqueous ethanol and iso-propanol at 25-65°C and 6.89-15.51 MPa.
Abstract: Mass transfer characteristics were determined for countercurrent extraction in a spray column with supercritical carbon dioxide solvent. Feeds were 5 and 10 vol% aqueous ethanol and iso-propanol at 25–65°C and 6.89–15.51 MPa. Results obtained in a 22 mm inside diameter column showed enhanced extraction efficiencies over comparable liquid-liquid systems. The models based on conventional liquid-liquid extraction were modified to correlate overall mass transfer coefficients for supercritical conditions. The constant value in the model of measuring individual mass transfer coefficient was influenced by the density of carbon dioxide and the system used.
TL;DR: In this article, the absorption of NOx into nitric acid solutions in a countercurrent flow packed column has been modified by taking into account separate energy balances for the gas and liquid phases and variation of the molar flow rates of water and Nitric acid in the liquid phase.
TL;DR: In this paper, a numerical approach based on the two-dimensional boundary layer equations is presented and validated with experiments available in literature to examine the coupled heat and mass transfer between two separated cocurrently flowing phases in vertical pipes.
TL;DR: In this paper, a countercurrent Trickle Flow Reactor for acidifying 6-Aminopenicillanic acid from Penicillin G is described. But the complexity of the countercurrent trickle flow Reactor requires a mathematical model, incorporating the effects of hydrodynamics, heterogeneous biocatalysis and multicomponent ion exchange.
TL;DR: Based on the two-phase separated model, a detailed one-dimensional model for binary mixtures had been proposed to study the in-tube evaporation in a horizontal tube.
TL;DR: Simulated countercurrent chromatography is a continuous flow method for separation of binary mixtures or for separating of multicomponent mixtures into two fractions as discussed by the authors, where the number of columns and the location of the inlet and outlet ports were varied to investigate product purity and productivity.
Abstract: Simulated countercurrent chromatography is a continuous flow method for separation of binary mixtures or for separation of multicomponent mixtures into two fractions. Countercurrent flow is simulated by moving the feed point past several fixed adsorbent beds at a rate between the single column breakthrough time of each component. The number of columns and the location of the inlet and outlet port were varied to investigate product purity and productivity. Concentration profiles of the effluent product streams were measured. The maximum product concentration exceeds the feed concentration during part of the feed-switching cycle and drops to zero during part of the cycle. Three configurations were tested with the total number of columns varying between three and eight. For the test separation chosen, gaseous propylene and dimethyl ether on Chromosorb 101, high purity (> 99%) product streams were obtained with an optimal four-column configuration using three desorbent streams.
TL;DR: In this paper, an experimental test facility was constructed to determine the vapor flow influence on the friction factor of countercurrent liquid flowing in small channels, and the amount of this increase was shown to depend on the ratio of the average vapor velocity to the average liquid flow velocity.
TL;DR: In this paper, the operation of a continuous countercurrent adaption system is modelled and optimized, and an optimal choice of operating variables corresponding to a compromise between recovery and purity is also suggested.
TL;DR: In this article, heat transfer correlations are given in terms of Nusselt, Reynolds and Rossby numbers, the flow mode (parallel or countercurrent) and the inner rotating-pipe surface area.