Bubble swarm behaviour and gas absorption in non-newtonian fluids in sparged columns

Mean relative gas hold up, slip velocity, bubble size distribution, and volumetric mass transfer coefficient of oxygen were measured in sparged columns of highly viscous non-Newtonian fluids (CMC solutions) as a function of the gas flow rate, and CMC concentration (fluid consistency index k, and flow behaviour index n).By comparison of the measured bubble swarm velocities with those calculated by relations for single bubbles the bubble swarm behaviour was investigated. It could be shown that small bubbles in swarm have higher rising velocities than single bubbles, expecially in highly viscous media. Large single bubbles rise with high velocity due to the change of their shape caused by the swarm of the smaller bubbles. No large bubbles with spherical cap shape could be observed. The volumetric mass transfer coefficient decreases rapidly with increasing CMC-concentration.A comparison of the volumetric mass transfer coefficients with those measured in mechanically agitated vessels indicates, that the performance of sparged columns is comparable with the one of agitated vessels. Because of their lower energy requirement sparged columns are more economical than mechanically agitated vessels. It is possible to improve the performance of sparged columns by the redispersion of large bubbles in a multistage equipment.     

Drop-breakage in agitated liquid—liquid dispersions

Experimental data from batch vessels on cumulative volumetric drop-size distributions at various times are shown to yield useful information on probabilities of droplet-breakup as a function of drop-size. Such information is sufficient for a priori prediction of drop-sizes in agitated dispersions in batch and continuous vessels. It may also be useful in predicting heat and/or mass transfer in liquid—liquid dispersions by accounting for the simultaneity of transport processes from individual drops and droplet breakage processes.     

SOLID-LIQUID MASS TRANSFER AT THE WALLS OF A RECTANGULAR AGITATED VESSEL

Rates of mass transfer between the walls of a rectangular agitated vessel and solution were studied by measuring the limiting current of the cathodic reduction of ferricyanide ion at the vessel wall. Variables studied were rotation speed of 45° pitched blade turbine impeller, physical properties of the solution and the presence of drag reducing polymer in lhe solution (Polyox WSR-301). The data were correlated for polymer free solution by the equation:

Sh= 1.925 Sc^0.33 Re^0.5

Comparison of the present data with previous heat and mass transfer studies conducted in cylindrical vessels has shown that the mass transfer behaviour of agitated rectangular vessels lie between baffled and unbaffled agitated cylindrical vessels. Polyox addition was found to reduce the mass transfer coefficient by an amount ranging from 5.6 to 37% depending on polymer concentration. Practical implications of using drag reducing polymers in agitated vessels were discussed.     

Cellular convection induced by absorption

The influence of cellular convection, induced by oxygen absorption in sodium sulphite solution, upon the mass transfer coefficient was studied experimentally both in a mechanically agitated non-aerated vessel (MANV), where absorption proceeds through the liquid surface, and in a mechanically agitated gas—liquid dispersion (MAD). It was found that the relation derived for correlation of the enhancement factor of the mass transfer coefficient for physical absorption as suggested in [1, 4] is not suitable for correlating the results obtained in the MANV. The probable mechanism of cellular convection occuring in this system is density driven, induced by concentration gradients. An empirical formula was obtained for the critical value of Rayleigh number. It was deduced from the formula that the onset of cellular convection does not depend on hydrodynamic conditions prevailing in the liquid phase within the whole range of Reynolds number used (3·4 × 10³ to 5·3× 10⁴). The enhancement factor R of the physical mass transfer coefficient caused by cellular convection is suitably described by the relation R = (Ra/Ra_cr)^u. The value of the exponent y is not constant.We present the correlation of experimentally determined values of y. For values of Reynolds number not higher than 3 × 10⁴ the value of y is constant and equal to 0·73. For higher values of Re the value of y is lower (y = 5·33 × 10²⁰Re^?4·66). It is shown that it would be more suitable to correlate the exponent y by the ratio of the time scale of initiation of cellular convection and thatof surface renewal.It was proved experimentally, that the absorption of oxygen into sodium sulphite solution in the MAD does not induce cellular convection in the studied range of oxygen absorption rated.     

Heat and Mass Transfer at the Wall of a Square Mechanically Stirred Gas-Liquid-Solid Catalytic Reactor

Wall mass and heat transfer rates in a square gas-sparged, mechanically stirred reactor were measured by the electrochemical technique under the effect of various geometrical and hydrodynamic variables. For the 45° impeller, the mass transfer data fit the equation Sh = 0.7Sc^0.33Re^0.2Re_g^0.5 with an average deviation of ±6.9 %. For the 90° impeller, the data fit the equation Sh = 0.95Sc^0.33Re^0.14Re_g^0.53 with an average deviation of ±7.5 %. Gas sparging enhanced the wall mass transfer rates by factors of up to 2.61 and 3 for the 90° and 45° impellers, respectively, with a significant decrease in the total power consumption. The contribution of the present results to the operation of multiphase mechanically agitated vessels in different aspects is outlined.     

A physical interpretation of drop sizes in homogenizers and agitated tanks,including the dispersion of viscous oils

Theoretical studies include firstly, the applicability of the Kolmogoroff-Hinze theory of droplet break-up in a turbulent liquid, testing it over a range of local power dissipations per unit mass of liquid from 5 to 10⁹ W kg⁻¹. Secondly, the turbulent fluctuation velocity is related to the size and rotational speed of the impeller blades in agitated vessels. Thirdly, the influence on droplet size of the viscosity of the dispersed phase is included in a new, simple basic theory, which is tested using data on agitated tanks, static mixers and homogenizers.     

Power demand and mass transfer capability of mechanically agitated gas-liquid contactors and their relationship to air-lift fermenters

Analysis of established correlations for power uptake, gassed power uptake and overall volumetric mass transfer coefficient (k_La) shows that when operating at constant impeller speed small increases in gas superficial velocity will result in both an increase in k_La) and a decrease in the total power input to the vessel. This is confirmed experimentally. At or near flooding, increasing the superficial velocity will continue to increase k_La still further, but with no further decrease in the agitator power uptake, total power input will increase.The transition region tends to lie on the corresponding k_La vs power input relation for an air-sparged situation (i.e. no mechanical agitation).A given mass transfer capability (k_La) value will be achieved at the same power/unit volume with both air sparged vessels and mechanically agitated vessels providing the gas superficial velocity is greater than 25 mm/sec.     

超临界条件下甲醇合成的气液传质系数测定

以液体石蜡为惰性液相载体,正己烷为超临界介质,合成气制甲醇为研究体系,测定了超临界条件下三相浆态床中甲醇合成的气液传质系数。在反应温度238℃、合成气分压3.7 MPa、气体空速2744 h^-1条件下,通过不断增加催化剂浓度提高气液传质阻力和反应阻力的相对大小,采用外推法获得完全处于气液传质控制下的气液传质系数。计算结果表明：催化剂浓度对CO的气液传质系数的影响较大,而对CO₂的气液传质系数的影响较小;液相条件下CO、CO₂的气液传质系数分别是0.161、0.03 s^-1,而超临界三相甲醇合成中CO、CO₂的气液传质系数分别是0.199、0.042 s^-1,说明三相浆态床甲醇合成中引入超临界流体利于气液传质,验证了超临界介质中三相甲醇合成的优越性。     

A new method for the investigation of fluid/fluid mass transfer—I: Mass transfer in gas/liquid systems

A new dynamic electrochemical method is introduced to investigate gas/liquid mass transfer in mechanically agitated contactors. Current transients were recorded and evaluated with a linear regression analysis in two contactors at different rotation rates. The volumetric mass trasnfer coefficient a₁k₁ and Henry's constant can be calculated from these experiments. Reliable results are obtained over a wide range of hydrodynamic conditions.     

The scale-up of aerated mixing vessels for specified oxygen dissolution rates

Values of the mass transfer product, k_La, have been determined using unsteady state measurements of oxygen dissolution rates in water, in 0.60 m³ and 0.043 m³ baffled mixing vessels fitted with geometrically similar flat-bladed turbines and spargers. It was seen that the assumption of quasi steady-state in the gas phase, as often practised hitherto, can lead to large errors in k_La. A correlation for the scale-up of aerated mixing vessels is proposed.     

Mechanism of gas absorption enhancement in presence of fine solid particles in mechanically agitated gas-liquid dispersion. Effect of molecular diffusivity

The effect of fine particle addition in physical gas desorption and absorption with fast reaction (sulphite oxidation in the presence of a cobalt catalyst) has been studied in a stirred cell with a flat gas-liquid interface and mechanically agitated gas-liquid bubble dispersion in a wide range of stirring speeds. Activated carbon and TiO₂ were used at low loadings . The desorption was used to avoid supersaturation effect which was observed during oxygen and hydrogen absorption into liquid saturated with nitrogen. Using two gases with sufficiently different diffusivity (O₂, H₂), the effect of molecular diffusivity on the mass transfer coefficient was estimated in the form k_L∼Dⁿ, with the exponent n indicating the surface mobility accompanying the effect of the particles. The value n=2/3 indicates a fully rigid and the value 1/2 a fully mobile mass transfer interface.Chemisorption experiments confirmed that the particles do not affect mass transfer area of the agitated dispersion. After addition of particles, k_L for physical desorption from bubbles in dispersion was increased by 10-30% in water and by 20-60% in sulphate solution with decreasing agitation rate. In the stirred cell, the increases were much higher reaching 200% and 230% for water and sulphate solution, respectively. The exponent n exhibited a significant decrease in the presence of particles ranging from 10% (for dispersion in water) to 33% (for stirred cell in sulphate solution). The decrease in n encountered in dispersion indicated the transition from a partially mobile to a fully mobile surface. The reduction of k_L was interpreted through the physicochemical effect of surfactants removal from the gas-liquid interface by activated carbon particles.The results have confirmed that the mechanism of mass transfer enhancement in the presence of fine particles, based on the removal of surface contaminants from the liquid by adsorption onto the hydrophobic surface of the particles, as suggested by Kaya and Schumpe [2005. Surfactant adsorption rather than “shuttle effect”? Chemical Engineering Science 60, 6504-6510] for stirred cell, is valid also for the enhancement of absorption rate from bubbles in mechanically agitated dispersion.     

Solid‐liquid mass transfer in a rotary drum

The solid‐liquid mass transfer coefficient for slurries agitated in rotating drums with baffles was measured using the dissolution of β‐naphthol as a tracer in silica sand slurries. The effects of drum rotation rate (0.33 to 15 min^?1), slurry holdup (4.4% to 17%) and solids volume fraction (0 to 0.62) on the solid‐liquid mass transfer were investigated with a slurry particle diameter of 4.6 × 10^?4 m. Mass transfer coefficients were corrected for the effect of particle shrinkage due to dissolution of the tracer. The mass transfer coefficient increased with increasing rotational speed of the drum from 0.005 to 0.045 mm.^5‐1. Two types of slurry motion were observed; well mixed slurry flowing over the baffles and segregation of solids on the baffles. The mass transfer coefficients from the present study, along with literature data, correlated with the Froude number to the 0.36 power, for Froude numbers from 10^?6 to 10^?1.     

Observations of solid–liquid systems in anchor agitated vessels

Although process development is often done in well agitated vessels (e.g. with a retreat curve, pitched blade turbine etc.), there are a sizeable number of contract manufacturers’ still deploying a significant number of anchor agitated process units. For the purpose of observing the Zwietering constant value ‘S’ and few industrially important solid–liquid systems, we conducted extensive suspension experiments with anchor agitated vessels for varying D/T ratios (0.74 and 0.73). In this study, Zwietering's N_js (just suspension speed) and the corresponding ‘S’ factor were obtained for each system over a range of solid loadings. We found that the Zwietering constant was strongly dependent on the nature of the solid–liquid system; i.e. different systems had different ‘S’ values for the same geometrical configuration.     

Gas/liquid mass transfer in sparged agitated slurries

Volume referred mass transfer coefficients k_La were determined for six slurry systems in an agitated vessel of standard configuration. The measurements were carried out under variation of power input, gas flow rate and solids concentration. The k_La data can be fitted well by correlations proposed for agitated gas/liquid tanks. Preference is given to the concept introduced by Zlokarnik (Adv. Biochem. Engng8, 133, 1979) as by means of this the experimental Stanton numbers can be described excellently as function of only one dimensionless group which involves total power input and slurry phase properties.     

On the effect of phase fraction on drop size distribution of liquid–liquid dispersions in agitated vessels

The theory of Kolmogorov–Hinze is the base for many studies that have been done on mean drop size and drop size distribution of liquid–liquid dispersions in agitated vessels. Although this theory has been used extensively in the literature, but it does not always give a satisfactory result in the studies and therefore needs to be modified. This paper addresses the effect of phase fraction on drop size distribution in agitated vessels and on the proportionality coefficient and Weber number exponent in the relation d₃₂/D ∝ We^m. The experimental data that were taken from Pacek et al. (1998) and Desnoyer et al. (2003) have been applied to this relation to investigate the effect of phase ratio. It is shown that even at low phase fractions, the Kolmogorov–Hinze theory necessarily does not give the best result with the −0.6 exponent for the Weber number. Furthermore, for the non-coalescing system, a range of exponent for the Weber number typically from −0.6 to −0.43 can be considered where the system may be approximated as a pseudo-coalescing system at Φ = 0.4 in which the obtained results are in good agreement with the results of Pacek et al. (1998).     

Solid-liquid mass transfer coefficients in gas-solid-liquid agitated vessels

The research on mass transfer coefficients in solid-liquid agitated systems has received substantial attention in the past, due both to the interest in fundamental aspects of mass transfer between particles and turbulent fluids and to the importance of practical applications. In contrast, little information is available on solid-liquid mass transfer when a third gaseous phase is also dispersed into the system, in spite of the importance of the applications of gas-solid-liquid agitated systems. In this work a suitable dissolution technique was used to measure the solid-liquid mass transfer coefficient in gas-solid-liquid vessels stirred by either radial or axial impellers. The mechanical power dissipated by the stirrers at various agitation speeds and gas flow rates was also measured by means of a new technique. The mass transfer data obtained were found to be well correlated to the 0.25 power of the specific power dissipation, indicating that the Kolmogorov's theory of mass transfer applies to these systems, while no clear influence of the gas hold-up was ascertained.     

Local Turbulent Energy Dissipation Rate in an Agitated Vessel: Experimental and Turbulence Scaling

The hydrodynamics and the flow field in an agitated vessel were measured using 2-D time resolved particle image velocimetry (2-D TR PIV). The experiments were carried out in fully baffled cylindrical flat bottom vessels 300 and 400 mm in inner diameter. The 300 mm inner diameter tank was agitated by a Rushton turbine 100 mm in diameter, and the 400 mm inner diameter tank was agitated by a Rushton turbine 133 mm in diameter. Three liquids of different viscosities were used as the agitated liquid: (i) distilled water (ν = 9.35 × 10^–7 m²/s), (ii) a 28 vol % aqueous solution of glycol (ν = 2 × 10^–6 m²/s), and (iii) a 43 vol % aqueous solution of glycol (ν = 3 × 10^–6 m²/s). The velocity fields were measured at an impeller rotation speed in the range from 300 to 850 rpm, which covers the Reynolds number range from 50000 to 189000. This means that fullydeveloped turbulent flow was reached. The experiments were performed to investigate the applicability of the following relations: ε* = ε/(u⁴/ν) = const, vK/u = const, Λ/ηK = const, τ_Λ/τ_K = const, ε* = ε/((Nd)4/ν) = const, Λ/d ∝ Re^–1, ηK/d ∝ Re^–1, vK/(Nd) = const, Nτ_Λ ∝ R^–1, Nτ_K ∝ Re^–1, and ε/(Nq) ∝ Re. These formulas were theoretically derived in our previous work, using turbulence theory, in particular, using turbulence spectrum analysis. The correctness of the proposed relations is investigated by statistical hypothesis testing.     

Influence of Pressure on the Hydrodynamics and Mass Transfer Parameters of an Agitated Bubble Reactor

The present study deals with the pressure effects on the hydrodynamic flow and mass transfer within an agitated bubble reactor operated at pressures between 10⁵ and 100 × 10⁵ Pa. In order to clarify the flow behavior within the reactor, liquid phase residence time distributions (RTD) for different operating pressures and gas velocities ranging between 0.005 m/s and 0.03 m/s are determined experimentally by the tracer method for which a KCl solution is used as a tracer. The result of the analysis of the liquid‐phase RTD curves justifies the tank‐in‐series model flow for the operating pressure range. Good agreement is obtained between theoretical and experimental results assuming the reactor is operating as perfectly mixed. Two parameters characterizing the mass transfer are identified and investigated in respect to pressure: the gas‐liquid interfacial area and volumetric liquid‐side mass transfer coefficient. The chemical absorption method is used. For a given gas mass flow rate, the interfacial area as well as the volumetric liquid mass transfer coefficient decrease with increasing operating pressure. However, for a given pressure, a and k_La increase with increasing gas mass flow rates. The mass transfer coefficient k_L is independent of pressure.     

Mass Transfer Coefficients in Mechanically Agitated Gas‐Liquid Contactors

There are a large number of correlations given in literature for the prediction of volume‐related liquid‐side mass transfer coefficients in mechanically agitated gas‐liquid contactors. Significant disagreement can be observed concerning the proposed correlations, so that no single correlation exists representing all of the mass transfer data given in the literature. The observed differences can mainly be ascribed to the differences in the geometry of the system, the range of operational conditions and the measurement method used. On the basis of a comparative study of mass transfer phenomena in agitated Newtonian and non‐Newtonian aerated liquids, a critical discussion of the literature results is presented in this review article, so that final conclusions can be drawn for the k_Lα values in the different single‐ and multiple‐impeller agitated systems studied in the literature.     

Kinetic study of hydrogen sulfide absorption in aqueous chlorine solution

Hydrogen sulfide (H₂S) is currently removed from gaseous effluents by chemical scrubbing using water. Chlorine is a top-grade oxidant, reacting with H₂S with a fast kinetic rate and enhancing its mass transfer rate. To design, optimize and scale-up scrubbers, knowledge of the reaction kinetics and mechanism is requested. This study investigates the H₂S oxidation rate by reactive absorption in a mechanically agitated gas–liquid reactor. Mass transfer (gas and liquid sides mass transfer coefficients) and hydrodynamic (interfacial area) performances of the gas–liquid reactor were measured using appropriated physical or chemical absorption methods. The accuracy of these parameters was checked by modeling the H₂S absorption in water without oxidant. A sensitivity analysis confirmed the robustness of the model. Finally, reactive absorption of H₂S in chlorine solution for acidic or circumneutral pH allowed to investigate the kinetics of reaction. The overall oxidation mechanism could be described assuming that H₂S is oxidized irreversibly by both hypochlorite anion ClO⁻ (k = 6.75 × 10⁶ L mol⁻¹ s⁻¹) and hypochlorous acid ClOH (k = 1.62 × 10⁵ L mol⁻¹ s⁻¹).