Gas‐liquid mass transfer in low‐ and medium‐consistency pulp suspensions

The volumetric gas—liquid mass transfer coefficient (k_La) was measured for low‐ and medium‐consistency pulp suspensions using the cobalt‐catalyzed sulfite oxidation technique. Mass transfer rates were measured in a high‐shear mixer for a range of operating parameters, including the rotor speed (N = 10 to 50 rev/s), gas void fraction (X_g = 0.10 to 0.40) and fibre mass concentration (C_m = 0.0 to 0.10). k_La measurements were compared with the macroscale flow regime in the vessel (characterized using photographic techniques) and correlated with energy dissipation, gas void fraction and suspension mass concentration in the mixer. We found that gas‐liquid mass transfer was significantly reduced in pulp suspensions, even for low suspension concentrations. Part of this reduction was associated with dissolved components leached from the fibres into the liquid phase. This could account for reductions in k_La of up to 30% when compared with distilled water. The fibres further reduced k_La, with the magnitude of the decrease depending on the fibre mass concentration. Correlations were developed for k_La and compared with results available in the literature.     

Hydrodynamics and mass transfer of gas–liquid flow in a falling film microreactor

In this article, flow pattern of liquid film and flooding phenomena of a falling film microreactor (FFMR) were investigated using high‐speed CCD camera. Three flow regimes were identified as “corner rivulet flow,” “falling film flow with dry patches,” and “complete falling film flow” when liquid flow rate increased gradually. Besides liquid film flow in microchannels, a flooding presented as the flow of liquid along the side wall of gas chamber in FFMR was found at high liquid flow rate. Moreover, the flooding could be initiated at lower flow rate with the reduction of the depth of the gas chamber. CO₂ absorption was then investigated under the complete falling flow regime in FFMR, where the effects of liquid viscosity and surface tension on mass transfer were demonstrated. The experimental results indicate that k_L is in the range of 5.83 to 13.4 × 10^?5 m s^?1 and an empirical correlation was proposed to predict k_L in FFMR. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Gas holdup for high gas velocities in a gas–liquid cocurrent upward‐flow system

Gas holdup has been measured in an 83‐mm diameter, 2.2‐m high column at high gas superficial velocities — 0.22 to 2.7 m/s — and at liquid (water) superficial velocities of 0 to 0.47 m/s, by means of a differential pressure transducer. The equation of Hills (1976) based on the slip velocity gives good predictions of the gas holdup for 0.1 ≤ E_g ≤ 0.4. However, the holdups predicted by this approach are considerably higher than the experimental values at gas velocities high enough that E_g > 0.4. Other equations from the literature are also shown to be inadequate. The new data and earlier data at high gas velocities are therefore correlated with a new dimensional equation for U_l ≤ 0.23 m/s.     

Gas—liquid mass transfer in pulp suspension mixing operations

The rate of mass transfer from the gas to water phases was measured in a commercial, high-shear, laboratory mixer under conditions typical of medium-consistency bleaching. The gas—liquid volumetric mass transfer coefficient, k_La, was measured using the cobalt-catalyzed sulfite oxidation technique. Suspensions of fully-bleached kraft pulp and synthetic nylon fibres were used, with mass transfer rates measured over a range of suspension compositions and mixer operating conditions. In the presence of pulp fibre, mass transfer rates were significantly reduced over the comparable water cases. The same dramatic decrease in mass transfer was not observed for the nylon suspensions, although k_La did decrease with increasing suspension concentration. Comparison of this data with that obtained from ozone bleaching experiments confirmed that at medium-consistency gas—liquid mass transfer controls ozone bleaching.     

Mass transfer at the confining wall of helically coiled circular tubes with gas–liquid flow and fluidized beds

Experiments were conducted to investigate the effect of various dynamic and geometric parameters on mass transfer coefficients in two-phase helically coiled flow systems. Computation of mass transfer coefficients was facilitated by the measurement of limiting current at the electrodes fixed flush with the inner surface of the tube wall. Two flow systems were chosen: a two-phase liquid solid fluidized bed and a two-phase gas–liquid up flow. An equimolar potassium ferrocyanide and potassium ferricyanide solution in the presence of sodium hydroxide was used as the liquid phase. In the fluidized bed, glass spheres and sand of different sizes were employed as fluidizing solids. In two-phase flow system nitrogen was employed as inert gas. The pressure drop in the presence of fluidizing solids in helical coils was found to increase with increase in the pitch of the coil and was maximum for straight tube. The mass transfer coefficients were found to increase with increase in liquid velocity. The mass transfer coefficients in case of gas–liquid flow were found to be independent of liquid velocity and the pitch of the coil, and were largely influenced by gas velocity only. The data were correlated using j_D factor, Helical number, Froude number and Stanton number.     

Liquid‐bridge flow in the channel of helical string and its application to gas–liquid contacting process

To solve the problems of the traditional packings, such as high pressure drop, mal‐distribution and short liquid residence time, a helical flow structured packings was proposed. Two different flow patterns, liquid‐bridge flow and liquid‐drop flow were identified when the width of the channel of the helical string was adjusted. Moreover, the characteristics of the helical liquid‐bridge flow including maximum liquid loading, mean thickness of liquid film, mean residence time and effective specific surface area, were examined. And the separation efficiency was studied by the lab‐scale distillation column. In comparison, the effective specific surface area of the helical flow type packings is almost as large as the traditional B1‐350Y structured packings, but with thinner liquid film, longer liquid residence time and finally higher separation efficiency. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3360–3368, 2018     

Influence of the gas—liquid separator design on hydrodynamic and mass transfer performance of split-channel airlift reactors

Three geometric configurations of gas–liquid separators were used in split-channel airlift reactors (0·1 m³ liquid volume; riser-to-downcomer cross-sectional area ratio = 0·7; aspect ratio = 3.6) to test the effect of geometry on hydrodynamic performance and oxygen transfer behaviour. For otherwise fixed conditions, the design of gas–liquid separators affected the induced liquid circulation rate, the depth of penetration of the bubble layer in the downcomer, the gas holdup in the downcomer, the mixing time and the overall volumetric gas–liquid oxygen transfer coefficient. The gas holdup in the riser was only marginally affected by the design of the separator. The impact of the various separator designs on hydrodynamic behaviour could be explained as emanating from a combination of the gas–liquid separating ability of the design and its hydraulic resistance.     

Investigating the liquid film characteristics of gas–liquid swirling flow using ultrasound doppler velocimetry

A novel gas–liquid two‐phase flow metering method was proposed. A spiral vane mounted in the inner pipe was used to transform inlet flow patterns into gas–liquid swirling annular flow. The thickness and velocity profile of liquid film were measured by ultrasound Doppler velocimetry. The liquid flow rates were obtained by integrating of velocity profile during the liquid film zone. Experiments were carried out in an air–water two‐phase flow loop and an ultrasonic transducer was installed under the bottom of the test section with the Doppler angle of 70°. The flow patterns included stratified wavy, annular, and slug flows. Compared with non‐swirling flow, the liquid film thickness at the bottom reduces greatly. The measurement accuracy of liquid flow rate was independent of inlet flow patterns, gas and liquid velocities. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2348–2357, 2017     

A multi‐scale theoretical model for gas–Liquid interface mass transfer based on the wide spectrum eddy contact concept

On the basis of the wide spectrum eddy contact concept and the isotropic turbulence theory, a multi‐scale theoretical model for the prediction of liquid‐side mass transfer coefficient in gas–liquid system was developed. The model was derived from an unsteady‐state convection and diffusion equation and considered the contributions of eddies with different sizes to the overall mass transfer coefficient. The proper contact time distribution at the surface is need to be determined to obtain satisfactory results with this model. Moreover, a simplified model was also proposed based on the assumption of steady‐state mass transfer mechanism for single eddy. The results predicted by this model showed a very good agreement with the available experimental data in a comparatively wide range of turbulence intensities. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Chemical hydrodynamics of a downward microbubble flow for intensification of gas‐fed bioreactors

Bioreactors are of interest for value‐upgrading of stranded or waste industrial gases. Reactor intensification requires development of low cost bioreactors with fast gas–liquid mass transfer rate. Here we assess published reactor technology in comparison with a novel downward bubble flow created by a micro‐jet array. Compared to known technology, the advanced design achieves higher volumetric gas transfer efficiency (k_La per power density) and can operate at higher k_La. We measure the effect of four reactor heights (height‐to‐diameter ratios of 12, 9, 6, and 3) on the gas transfer coefficient k_L, total interfacial area a, liquid residence time distribution, energy consumption, and turbulent hydrodynamics. Leading models for predicting k_L and a are appraised with experimental data. The results show k_L is governed by “entrance effects” due to Higbie penetration dominate at short distances below the micro‐jet array, while turbulence dominates at intermediate distances, and finally terminal rise velocity dominates at large distances. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1399–1411, 2018     

Motion of pulp fibre suspensions in rotary devices

Suspensions of semi-bleached kraft pulp at low and medium mass concentrations were sheared in a rotary shear tester at angular velocities up to 524 rad/s (5000 rpm). The use of baffled housings, with rotors having lugs, imposed shear within the body of the suspensions and prevented slip at the vessel walls. The torque was measured as a function of angular velocity, and high speed photographs were taken through a transparent plate to aid in interpretation of the findings. The torque vs. angular velocity curves and flow phenomena were found to depend on the mass concentration of the suspension, the gap width between the rotor and housing, and the air content of the suspension. With little gas present, the flow initially involved tangential flow in an active cavity whose radial extent increased with increasing angular velocity. When the flow reached the outer baffles, a marked flow transition occurred, probably explaining what has been called “fluidization” by Gullichsen and Harkonen (1981). As the gas content of the suspension increased, the gas congregated around the rotor, impeding the transfer of momentum to the suspension. The apparent viscosity of a 10% by mass suspension was of the order of 16 Pa · s under the test conditions.     

Sensor‐based flow pattern detection—gas–liquid–liquid upflow through a vertical pipe

Flow distribution during gas–liquid–liquid upflow through a vertical pipe is investigated. The optical probe technique has been adopted for an objective identification of flow patterns. The probability density function (PDF) analysis of the probe signals has been used to identify the range of existence of the different patterns. Dispersed and slug flow have been identified from the nature of the PDF, which is bimodal for slug flow and unimodal for dispersed flow. The water continuous, oil continuous, and emulsion type flow distributions are distinguished on the basis of the PDF moments. The method is particularly useful at high flow rates where visualization techniques fail. Based on this, a flow pattern detection algorithm has been presented. Two different representations of flow pattern maps have been suggested for gas–liquid–liquid three phase flow. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3362–3375, 2014     

Dispersed phase holdup and bubble size distributions in gas–liquid cocurrent upflow and countercurrent flow in reciprocating plate column

Dispersed phase holdup and the bubble size distribution were measured in a reciprocating plate column under cocurrent upflow and countercurrent flow of gas and liquid phases. The response of the system to a variation in design and operating conditions was found similar to that for liquid–liquid contacting; the magnitude of response, however, differed significantly between them. Taking into consideration the dominant forces encountered in gas–liquid dispersions, the experimental data are satis–factorily correlated in terms of Froude, Weber and Gallileo numbers.     

Mean residence time of gas in mechanically agitated gas–liquid contactors

In the present study of gas–liquid contactors, mean residence/contact time was calculated from knowledge of superficial velocity and the gas phase hold-up, for various gas rates and impeller geometry and speeds, and compared with values obtained from RTD measurements. A new correlation, involving Flow Number, Froude Number, system geometry and the physical properties, is proposed. This uses the authors data and those available in literature.     

Gas–liquid mass transfer studies for biomass support materials in a bioreactor

BACKGROUND: Oxygen mass transfer can be described and analyzed by means of the mass transfer coefficient k_La, which is the most important parameter involved in the design and operation of mixing–sparging equipment for bioreactors. In the present study, the effect of biomass support materials on the gas–liquid mass transfer coefficient was studied in a bioreactor under variable process conditions. The biomass support materials used were activated carbon, pumice and loofa sponge. RESULTS: Compared with the case with distilled water only, the presence of the biomass support materials negatively influenced mass transfer. On the other hand, the mass transfer coefficient increased with increased impeller speed, air flow rate and temperature; and decreased with the increase of liquid viscosity and biomass support material concentration for all cases. CONCLUSIONS: Evaluation of the experimental data showed that k_La values were affected by process variables. Besides the major exponential correlations used in the literature, satisfactory linear correlations for the relationship between the k_La and process variables were obtained. Copyright © 2010 Society of Chemical Industry     

Dispersed phase hold-up in mechanically agitated gas–liquid contactors

In the present study, in mechanically agitated gas–liquid contactors (MAC), a generalized correlation for the direct estimation of dispersed gas phase hold-up is proposed. This present proposed new correlation in terms of fundamental operating variables, involving gas velocity, impeller speed in terms of Froude number, vessel geometry and the physical properties, is more accurate than those previously reported and also simpler to use. © 1998 SCI     

Gas–liquid mass transfer of aqueous Taylor flow in monoliths

The gas–liquid mass transfer of a monolith operating in the Taylor flow regime is presented. Mass transfer measurements are compared with a literature model derived for single capillaries. The comparison resulted in a prediction of the unit cell length (gasbubble+liquidslug). Independent measurements of the liquid slug length showed that the predicted unit cell length is close to the measured ones. This leads to the conclusion that mass transfer models for single capillaries may indeed be used for monoliths. Additionally, it is shown that the liquid slug length may also be estimated from pressure drop measurements.     

The mechanism model of gas–liquid mass transfer enhancement by fine catalyst particles

In order to present the enhancement of gas–liquid mass transfer by heterogeneous chemical reaction near interface, the mechanism model has been proposed to describe the mass transfer rate for a gas–liquid–solid system containing fine catalyst particles. The composite grid technique has been used to solve the model equations. With this model the effect of particle size, first-order reaction rate constant, distance of particle to gas–liquid interface and residence time of particle near gas–liquid interface on the mass transfer enhancement have been discussed. The particle–particle interaction and slurry apparent viscosity can be considered in the model. The experimental data have been used to verify the model, and the agreement has been found to be satisfied.