Mass Transfer Measurements Research Articles

Effects of baffle length on mass transfer in a parallel plate rectangular electrochemical cell

Global mass transfer measurements in unbaffled and baffled configurations using different baffle lengths and Reynolds numbers have been made in a parallel plate cell of rectangular geometry. The entry jet arrangement and the repeated 180° changes in direction of the flow, followed by the exit, produces extremely complex hydrodynamics in the cell. A plot of mass transfer coefficient against baffle length shows an increase in mass transfer with baffle length. Comparison of data for the present work with those of other workers for similar devices showed higher mass transfer due to the modifications incorporated in the present cell.

Hydrodynamics and Mass Transfer of Textile Vibrating-Valve Trays

New types of vibrating-valve trays with textile valves were investigated to determine their optimal construction. Hydrodynamic and mass transfer measurements were carried out in a pilot plant with different vibrating-valve and conventional valve trays under identical experimental conditions. The total tray pressure drop (Δ P ), column efficiency (η), volumetric mass transfer coefficient ( K G a ) and pressure drop per theoretical tray ( Δp/N th ) were measured and compared. The high mass transfer rate, the low investment and operating costs, the corrosion and heat resistance make the vibrating-valve trays attractive for gas absorption and air cleaning in environmental protection.

Fundamentals of hydrodynamics and mass transfer in a three-phase fluidized bed system

Particle dynamics of a three-phase fluidized bed are characterized using radioactive particle tracking (RPT). Solid–liquid mass transfer measurements were performed using a benzoic acid particle on a flexible tether. Columns with 12.7 cm inner diameter and 160–200 cm in length were used. The fluids used are water and air, and the solids are spherical polycarbonate particles 6.35 mm in diameter with a density of 1160 kg/m 3. All experiments are conducted at atmospheric pressure and room temperature. Analysis of RPT data, for free and tethered particles, shows that tethering does not affect particle velocities in the main bed region.

Heat flux measurement techniques

Heat flux measurement is used in the field of fluid mechanics and heat transfer to quantify the transfer of heat within systems. Several techniques are in common use, including: differential temperature sensors such as thermopile, layered resistance temperature devices or thermocouples and Gardon gauges; calorimetric methods involving a heat balance analysis and transient monitoring of a representative temperature, using, for example, thin-film temperature sensors or temperature sensitive liquid crystals; energy supply or removal methods using, for example, a heater to generate a thermal balance; and, finally, by measurement of mass transfer which can be linked to heat transfer using the analogy between the two. No one method is suitable to all applications because of the differing considerations of accuracy, sensitivity, size, cost and robustness. Recent developments including the widespread availability and application of thin-film deposition techniques for metals and ceramics, allied with advances in microtechnology, have expanded the range of devices available for heat flux measurement. This paper reviews the various types of heat flux sensors available, as well as unique designs for specific applications. Critical to the use of a heat flux measurement technique is accurate calibration. Use of unmatched materials disturbs the local heat flux and also the local convective boundary layer, producing a potential error that must be compensated for. The various techniques in common use for calibration are described. A guide to the appropriate selection of a heat flux measurement technique is provided according to the demands of response, sensitivity, temperature of operation, heat flux intensity, manufacturing constraints, commercial availability, cost, thermal disturbance and acceleration capability for vibrating, rotating and reciprocating applications.

Hydrodynamics and mass transfer in an airlift reactor

Hydrodynamics and mass transfer in internal and external flow airlift reactors have been studied in this work. Gas hold-ups and liquid velocities have been determined in a concentric tube airlift reactor, while oxygen mass transfer has been measured in an external loop airlift reactor. First, tracer measurements were used to verify a conventional axial dispersion model. Further, a model derived from multi-phase Navier–Stokes equations was verified with experimental data from the oxygen mass transfer measurements. Both models were in a good agreement with the measurements.

Hydrodynamics and mass transfer issues in a countercurrent gas-liquid internally finned monolith reactor

The hydrodynamic behavior of countercurrent gas–liquid flow and the gas–liquid mass transfer characteristics have been investigated in an internally finned monolith. Special attention has been paid to the flooding limits, the pressure drop, and the liquid hold-up. Gas–liquid mass transfer has been investigated experimentally and the influence of the liquid and gas flow rate has been determined. From the flooding experiments it can be concluded that countercurrent flow is possible at industrially relevant flow rates and that the limitations are mainly engendered by the liquid outlet. The flooding correlation of Wallis can be used to describe the flooding data. In the wavy annular flow regime, below the flooding limits, both pressure drop and liquid hold-up can be predicted by a model that is based on a theoretical analysis of the momentum transfer. It was found that the deviations between the model and the experimental results are to a large extent induced by inlet effects and only moderately determined by interfacial waves. The mass transfer measurements showed that k GL a GL , which ranged from 0.03 to 0.16 s -1, was much higher in the upper part of the monolith. The influence of the liquid flow rate becomes significant at higher liquid flow rates while the gas flow rate only has a moderate effect. Developing waves probably cause some extra mixing in the liquid film resulting in improved mass transfer.

Mass transfer and influence of the local catalyst activity on the conversion in a riser reactor

Gas–solids contacting in risers has been studied based on measurements of mass transfer controlled CO oxidation over a Pt/γ-alumina catalyst, and on experimental results published by Ouyang et al. (1995) for the kinetically controlled ozone decomposition. In the present experiments, the catalyst activity was varied by mixing the active catalyst particles with similar, but inert γ-alumina particles (in ratios from 150 to 2500 m3inert/m3cat), whereas Ouyang and co-workers varied the operating temperature ( de 300 à to 500 K). Mass transfer controlled CO oxidation occurs at temperatures above 750 K. A negative square root dependency has been observed for the relationship between the Sherwood number and the solid hold-up. Increasing the gas velocity always improves the gas–solids contacting. The local catalyst activity appears to be an important parameter. As an important conclusion of the present work, it can be stated that at a high local activity, the conversion rate per unit volume of catalyst decreases significantly due to local depletion of reactant. On a étudié le contact gaz–solides dans des colonnes montantes à partir de mesures d'oxydation du CO contrôlée par le transfert de matière sur un catalyseur de Pt/γ-alumine et à partir dés résultats expérimentaux de Ouyang et al. (1 995) pour la décomposition contrôlée cinétiquement de I'ozone. Dans les présentes expériences, on a fait varier I'activité du catalyseur en mélangeant les particules de catalyseur actives avec des particules de γ-alumine semblables mais inertes (dans des rapports de 150 à 2500 m3inert/m), tandis qu'Ouyang et ses collaborateurs faisaient varier la température de fonctionnement (de 300 à 500 K). L'oxydation du CO contrôlée par le transfert de matière survient à des températures supérieures à 750 K. Une dépendance en racines carrées négatives a été observée pour la relation entre le nombre de Sherwood et la rétention des solides. Le fait d'accroître la vitesse du gaz améliore toujours le contact gaz–solides. L'activité du catalyseur locale semble être un paramètre important. Comme conclusion importante du présent travail, on peut dire qu'avec un activité locale élevée, le taux de conversion par unité de volume de catalyseur diminue de maniére significative en raison de I'épuisement local du réactif.

99/01467 Measurement of mass transfer between the bubble and dense phases in a fluidized bed combustor : Campos, J. B. L. M. et al. Combust. Flame, 1999, 116, (1/2), 105–119

99/01467 Measurement of mass transfer between the bubble and dense phases in a fluidized bed combustor : Campos, J. B. L. M. et al. Combust. Flame, 1999, 116, (1/2), 105–119

Energetic Correlation of Local Mass Transfer in Swirling Pipe Flow

Mass transfer measurements in a decaying swirl flow in a pipe were carried out, using the electrochemical limiting diffusion current technique. The measurements were taken from local electrodes of 1 mm diameter without a developing concentration boundary layer around. Swirl was generated using short helical guide-vanes with vane angles between 15 and 60° at the outer edge of the vanes to the duct axis. The pressure drop measurements were also taken to perform an energy analysis. For a Reynolds number range of 1700−10000, the overall energy correlation for a representative length of 36.5 diameter were evaluated to be Shs(Sc)-0.33 = 0.3705(1 + tan θo)-0.120 X0.236, where θo is the vane angle at the outer edge of the helical swirler and X is the energy dissipation parameter.

ESTABLISHMENT OF A NEW MASS (HEAT) TRANSFER MEASUREMENT TECHNIQUE AND ITS VALIDATION

A convective mass (heat) transfer measurement technique based on the reflective properties of drying surfaces has been established and validated. The convection surface is covered with an absorbent paper and wetted with a volatile liquid. The drying rate is determined from measurements of changes in the intensity of reflected or transmitted portion of a light beam using a light emitter detector system. The paper needs to be calibrated before using it in mass transfer measurements. Various convective flow systems have been tested for calibration of the paper. In the view of the present findings and the analysis of results of previous studies using this technique, a better way of carrying out the technique has been proposed. The technique has been validated by circumferential mass transfer measurements around circular cylinders in a uniform crossflow. The close agreement of the present data with those from the literature prove that the present technique could offer an alternative to existing heat and mass transfer measurement methods.

Overall mass transfer in the swirling flow induced by a tangential inlet between coaxial cones

An electrochemical method is used to measure mass transfer coefficients between an electrolytic solution and the inner core of a system formed by stationary coaxial cones of the same apex angle. A swirling decaying flow is induced by means of a tangential inlet at the system base. The average mass transfer coefficients are measured at three axial positions from the tangential inlet for both laminar and turbulent flow regimes. Pressure drops between the inlet and the outlet of the experimental device are also investigated. Flow visualization revealed the existence of axially fixed toroidal vortices. The overall mass transfer coefficients along the conical gap are found to be greater than those measured in annular swirling decaying flow for the same values of the annular gap thickness, of the tangential inlet diameter and of the Reynolds number based on the mean axial velocity at the bottom of the conical gap. The enhancement in mass transfer, up to 50% compared with that measured in a cylindrical arrangement, is not counter-balanced by an increase in pressure drop, which remains of the same order of magnitude as that measured in a classical annular configuration.

Measurement of mass transfer between the bubble and dense phases in a fluidized bed combustor

An experimental study is described on mass transfer between the bubble and dense phases in a fluidized bed, used as a coke combustor. The experimental technique allowed quantification of the mass transfer rate during bubble formation and during a bubble’s rise through the bed. The combustion experiments were performed at 1 atm and 1223 K, in a fluidized bed (i.d. 120 mm) of sand (average diam. 325 μm) with static heights of 0.10–0.21 m. The bubbling flow rate ranged from 2.5 to 5.0 times that at incipient fluidization. The coke particles were 3.0 or 3.5 mm in diameter. Results indicate that the equivalent bed height, L eq (the height a bubble must rise to transfer to the dense phase the same quantity of oxygen as during its formation) is independent of the bubbling air flow rate. The mean value L eq = 50 mm suggests that for shallow beds the mass transferred during bubble formation is a significant part of the total mass transferred. The measured mass transfer factor between phases during a bubble’s rise ( x′ = X/ L mf ) is independent of the bubbling air flow rate and substantially lower than the theoretical predictions of Kunii and Levenspiel [1]. This disagreement is explained by the fact that the theoretical model is for an isolated bubble and does not account for the strong interaction between consecutive bubbles; this increases a bubble’s velocity and induces their coalescence, leading to a decrease in mass transferred between phases.

Mass transfer from rotating circular cylinders in a submerged slot jet of air

Isothermal convective mass transfer behaviour of a horizontal rotating circular cylinder exposed to a two dimensional slot jet of air was determined as a function of the flow and the geometrical parameters using a photo-evaporative mass transfer measurement technique. The rotational Reynolds number ( Re w) varied from 0–8.0×10 4, the jet exit Reynolds number ( Re j) from 4.6×10 4–2.70×10 5, the ratio of the cylinder diameter to the jet slot height ( d\\ D) from 3.333–15, the ratio of the distance between the jet exit and the front of the cylinder to the slot height ( L\\ D) from 1–16, the ratio of the off-set between the axis of the cylinder and the symmetry plane of the jet to the slot height ( E\\ D) from −0.555–2.133 and the jet impingement angle ( φ) from 20°–90°. Some correlations of the mean Sherwood number ( Sh ) have been produced by using the experimental data. Comparison of the mass transfer performance of the present configuration with those of a stationary cylinder in the same jet flow and in a uniform crossflow of the similar velocities indicated that the rotation had a deleterious effect on the mean mass transfer.

Platinum catalyzed oxidation of carbon monoxide as a model reaction in mass transfer measurements

The oxidation of CO with oxygen over a Pt/γ-alumina catalyst is proposed as a model reaction to be used for the determination of mass transfer coefficients in packed and fluidized beds. It is applicable at relatively low temperatures (<800 K) and for very small particles (<100 μm). In the present work, the kinetics of this reaction have been verified in a small fixed bed facility (average particle diameter 54 μm), for various reactant concentrations, temperatures and superficial gas velocities. As a result, Langmuir–Hinshelwood kinetics ( E a =75.4 kJ mol -1) appeared to describe the experimental results better than a power law expression ( E a =90.6 kJ mol -1). Three temperature regimes can be identified upon interpretation of experimental results with a suitable single particle model: a reaction rate controlled regime (I) at relatively low temperatures, characterized by a reaction order for oxygen of plus one and a carbon monoxide reaction order of minus one, an intermediate temperature interval (regime II) for which the reactions rate is influenced by both mass transfer and kinetics, and where the apparent reaction order in CO and O 2 change to values lower than minus one and higher than one, respectively, and the high temperature regime (III) where mass transfer resistances are dominant, and the apparent reaction orders in O 2 and CO are changed to values of zero and plus one, respectively. In case of carbon monoxide oxidation over a platinum catalyst, the observed orders in O 2 and CO provide an extra instrument to recognize the prevailing conversion rate controlling phenomenon, apart from known indicators like the observed activation energy value or the influence of hydrodynamic conditions. As a consequence, the reliability of mass transfer measurements is significantly improved. This has been verified by the application in mass transfer measurements for a packed bed and for a riser system.

Measurements of friction, heat transfer, and mass transfer in a severely outgassing tube bank

The influence of outgassing on the transport coefficients for flow through a staggered row tube bank was measured. Friction factors and heat and mass transfer coefficients were measured for 8000 < Re < 16000 and blowing rates up to v 0/ U x = 5×10 −3. Because of outgassing, heat and mass transfer coefficients reduced by as much as 70%. New blowing factors and correction functions were obtained that quantify the effect of outgassing on the transport coefficients.

Flow and mass transfer measurements for a flat plate of finite thickness in pulsating flow

Laboratory measurements were made of flow and mass transfer over a blunt flat plate of finite thickness, which is placed in a pulsating free stream, U x = U 0 (1+ A 0 cos 2 πƒ p t). Low turbulence-intensity wind tunnel experiments were conducted for small and moderate Reynolds numbers, 770 ⩽ Re H ⩽ 8000. Pulsation was generated by means of an acoustic speaker. The majority of experiments were carried out in the ranges of ƒ p = 20.0–80.0 Hz and A 0 ⩽ 0.15. Flow properties were measured by I-type and split-film probes. Mass transfer rates were measured by employing the naphthalene sublimation technique. The present results for non-pulsation flows ( A 0 = 0.0) were shown to be consistent with the published data. For pulsating approach flows, results are provided for the distributions of the wall static pressure, the longitudinal mean velocity and turbulent intensity, and the Sherwood number, Sh, as a function of the stream-wise distance x * measured from the leading-edge separation point. As A 0 or ƒ p increases, the time-mean reattachment length is reduced significantly. This implies that the height and length of the separation bubble shrink simultaneously: the position where C p is recovered moves upstream, and the minimum value of C p decreases; the reverse flow is intensified: and a substantial augmentation of turbulent energy is discernible. In the separation bubble, the effect of pulsation on Sh is conspicuous. Sh decreases monotonically from the separation point to the minimum value near the secondary separation point; and Sh increases appreciably with increasing x *, after passing the secondary separation point to the maximum value at the reattachment point; and afterward, Sh decreases. The secondary separation point and the position where Sh has a maximum move further upstream, as A 0 or ƒ p increases. At large Re H, the relative influence of pulsation on Sh weakens.

Visualization and determination of local heat transfer coefficients in shell-and-tube heat exchangers for staggered tube arrangement by mass transfer measurements

Local heat transfer coefficients on the outer surface of tubes in shell-and-tube heat exchangers with staggered tube arrangement are visualized and determined from mass transfer measurements. The mass transfer experiments are carried out using a technique based on absorption, chemical and coupled colour reaction. Local mass transfer coefficients are measured for fully developed flow conditions on each tube surface. These coefficients are transformed to heat transfer coefficients by employing the analogy between heat and mass transfer. The averaged heat transfer coefficients and the pressure drop are compared with the predictions from the literature.

Heat Transfer in a Two-Pass Internally Ribbed Turbine Blade Coolant Channel With Cylindrical Vortex Generators

The effect of vortex generators on the mass (heat) transfer from the ribbed passage of a two-pass turbine blade coolant channel is investigated with the intent of optimizing the vortex generator geometry so that significant enhancements in mass/heat transfer can be achieved. In the experimental configuration considered, ribs are mounted on two opposite walls; all four walls along each pass are active and have mass transfer from their surfaces but the ribs are nonparticipating. Mass transfer measurements, in the form of Sherwood number ratios, are made along the centerline and in selected interrib modules. Results are presented for Reynolds number in the range of 5000 to 40,000, pitch to rib height ratios of 10.5 and 21, and vortex generator-rib spacing to rib height ratios of 0.55 and 1.5. Centerline and spanwise-averaged Sherwood number ratios are presented along with contours of the Sherwood number ratios. Results indicate that the vortex generators lead to substantial increases in the local mass transfer rates, particularly along the side walls, and modest increases in the average mass transfer rates. The vortex generators have the effect of making the interrib profiles along the ribbed walls more uniform. Along the side walls, vortices that characterize the vortex generator wake are associated with significant mass transfer enhancements. The wake effects and the levels of enhancement decrease somewhat with increasing Reynolds number and decreasing pitch.

Triphase catalysis: a new rotating disk contactor for measuring mass transfer coefficients

Triphase catalysis (TPC) is a practically attractive way of carrying out phase transfer catalyzed reactions. The analysis of triphase catalytic systems requires a knowledge of mass transfer coefficients, but no correlations for estimating these are available. Therefore, a new rotating disk contactor (RDC) for measuring the external mass transfer coefficients of TPC systems has been designed and operated. Details of the design and construction of the contactor are discussed. In order to test the apparatus, experiments were carried out on a test system: synthesis of benzyl acetate from benzyl chloride and sodium acetate with polymer supported tributylmethylammonium chloride as the immobilized phase transfer catalyst. This reaction was found to be mass transfer controlled at low agitation speeds, and it was therefore possible to determine the external mass transfer coefficient as a function of the bulk agitation speed. The values ranged from 1.4×10 -3 to 2.0×10 -3 cm/min.

Effect of baffle spacing on pressure drop and local heat transfer in shell-and-tube heat exchangers for staggered tube arrangement

Local heat transfer and pressure drop on the shell side of shell-and-tube heat exchangers with segmental baffles were investigated for different baffle spacings. The distributions of the local heat transfer coefficients on each tube surface within a fully developed baffle compartment were determined and visualized by means of mass transfer measurements. Per-tube, per-row and per-compartment average heat transfer coefficients were drawn from the local values. The local pressure measurements allow the determination of the shell-side flow distributions. For same Reynolds number, the pressure drop and average heat transfer are increased by an increased baffle spacing due to a reduced leakage through the baffle-shell clearance. The experimental results were compared with literature values.