Gas absorption into a turbulent liquid

Mass transfer coefficients were measured for the liquid phase for gas absorption into a turbulent liquid flowing down a long wetted-wall column. Helium, hydrogen, oxygen and carbon dioxide were absorbed into distilled water over a Reynolds number range of 1300–8300. The results indicate that the liquid phase mass transfer coefficient is proportional to the molecular diffusion coefficient raised to the 0·54 power. The results are interpreted in terms of an eddy diffusivity model and indicate that the eddy diffusivity increases as the square of the distance from the interface.     

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.     

Oxygen absorption in a baffled tank agitated by delta paddle impeller

The power consumption and the volume-based mass transfer coefficient (k_la) for oxygen uptake from air into water have been measured for delta paddle impellers (10 cm and 20 cm diameter) in water in a 30 cm square sectioned baffled tank. Power consumption under ungassed conditions was measured in viscous mineral oil. For the 20 cm impeller, the effects of minor design modifications, such as sweeping the blade forward or back, have also been examined. The power numbers are approximately 2.0 in the absence of sparged gas, but are reduced by over 50% when gas is supplied beneath the impeller. Mass transfer data can be correlated with the power input per unit volume and the superficial gas velocity, showing that the performance with aeration of water is comparable to that reported by previous workers for the Rushton turbine impeller.     

Gas absorption in the plunging liquid jet reactor

A pseudo first-order gas absorption survey has been made of the contacting system formed when a coherent liquid jet plunges through an ambient reactive gaseous atmosphere into a bath of jet liquid. Using the hypochlorite ion catalysed reaction between pure carbon dioxide and a carbonate ion-bicarbonate ion buffer solution, the reactor has been found to be analogous to a gas sparged stirred tank contactor with the plunging jet acting as both the reactor agitator and gas bubble generator. Resolution of the gas—liquid interfacial area and rate of surface renewal absorption parameters was thus made possible. Specific interfacial areas in the range 20–110 m^?1 and rates of surface renewal in the range 40–160 sec^?1 for the subsurface reactor have been related to the plunging jet surface roughness and velocity or the entrainment rate of the plunging jet.     

Gas self-ignition in a plane vortex chamber

This paper describes the numerical modeling of gas flow in a plane vortex chamber by using the Navier–Stokes equations. The model is based on the laws of conservation of mass, momentum, and energy for nonstationary two-dimensional compressible gas flow in the case of axial symmetry with a tangential component of the gas velocity. The processes of viscosity, thermal conductivity, and turbulence are accounted for. It is shown that the transition of the kinetic energy of gas into thermal energy as a result of transfer processes leads to the formation of hot spots in the boundary layers near the walls of the chamber. The gas temperature at these hot spots can exceed the gas combustion temperature, while the gas remains rather cold in the neighboring regions. This could be the reason for the cold gas self-ignition observed in the experiments. The turbulence of the flow and the processes of mixing and diffusion of the components make a significant contribution to the capacity of gas self-ignition.     

Liquid—liquid mass transfer in systems agitated by submerged jets

Mass transfer across a plane interface has been investigated for liquid—liquid systems, with the liquid phases agitated by vertically opposed, submerged jets, one in each phase. Mass transfer coefficients were determined for the systems ethyl acetate—water and butanol—water, and for the transfer of dissolved helium between water and toluene, and toluene between toluene and water. The latter two solutes provided a wide range of molecular diffusion coefficients. When turbulent eddies penetrated to the interface the mass transfer coefficients were found to be proportional to D^0.5, and could be represented by the Levich-Davies mass transfer model for mass transfer between turbulent liquids. The characteristic turbulence velocities in this model were related to the velocities of the liquids from the jet nozzles, and to the equipment dimensions (e.g. the distances of the jets from the interface, the radius of the vessel and the diameters of the jet nozzles) by an expression based on the hydrodynamic behaviour of jets.For the low interfacial tension system butanol—water, a flat disc had to be placed in the interface at the region of jet impingement, to prevent disruption of the interface, but, even so, only a limited range of jet flow rates could be used. Only at the highest flow rates were turbulent conditions obtained, and most of the experimental mass transfer coefficients for this system were between the values predicted by the Levich-Davies model and the Levich “three-zone” model for boundary layer flow.     

Gas absorption of carbon dioxide in a hollow fiber contained liquid membrane absorber

Experiments on the absorption of CO₂ into a hollow fiber contained liquid membrane absorber were performed. The feed gas was a mixture of CO₂ and N₂, absorbent liquid was 2-amino-2-methyl-l-propanol and the hollow fiber was a microporous hydrophobic polytetrafluoroethylene membrane. Outlet concentration of CO₂ from the absorber decreased as absorbent concentration increased, gas flow rate increased and were held constant for speed of agitation, but had a maximum value in the range of inlet concentration of CO₂ from 5 to 40 mole%. The reaction rate constant obtained for CO₂-amine system was 231 I/mol · s at 25 °C using a flat stirred vessel, and the membrane-side-mass-transfer coefficient was 1.217 × 10⁻⁵ mol/cm² · s · atm in CO2/N2-amine system. A diffusion model based on mass transfer with fast-reaction was proposed to predict the performance of the absorber.     

Mass transfer from suspended solids to a liquid in agitated vessels

A new fundamental approach to the formulation of mass transfer from suspended solids to a liquid in agitated vessels is presented. Influence of various dimensionless groups on mass transfer is studied using the water-banzoic acid system for experimental work.Results are compared with previous works in agitated vessels and a correlation is proposed:Sh = 0.046Re^0.283Ga^0.173U^?0.011(T/d)^0.019Sc^O.461     

An experimental and computational study of the vortex shape in a partially baffled agitated vessel

The vortex shape in a non-standard partially baffled agitated vessel in the form of a glass-lined, under-baffled stirred vessel has been investigated using both experimental and numerical approaches for an air/water system for different rotation speeds of the agitator. A simple and flexible experimental strategy was developed for determination of the time-averaged location of the unstable free surface using a process involving superimposition of images. CFD simulations were made to predict the vortex shape by using an Eulerian-Eulerian multiphase model coupled with a homogenous turbulence model. The simplifying assumptions of a constant bubble size, a constant drag coefficient and use of the k-ε turbulence model were made. An assessment of the capability of the numerical method to predict the vortex shape was carried out through comparison between experimental data and numerical results. Considering for comparison purposes a water isosurface volume fraction equal to 0.9, to account for the existence of air/water mixture present at the interface in the experiments, instead of the classical value of 0.5, gave very good agreement with the experimental data.     

The absorption of carbon dioxide by a quiescent liquid

An interferometric technique is described by which the absorption of carbon dioxide into quiescent water has been examined. There is no measurable resistance at the interface to solution of the gas into pure water, the rate of absorption into the liquid being governed by molecular diffusion. A measurable interfacial resistance is found when carbon dioxide is absorbed by solutions of surface-active agents.     

Effect of surface resistance on gas absorption accompanied by a chemical reaction in a gas—liquid contactor

An analysis of gas absorption accompanied by chemical reaction in the presence of interfacial resistance is presented. The analysis indicates that the effect of interfacial resistance on interphase mass transfer is significantly higher in presence of a reaction compared to the pure absorption case. For fixed values of surface resistance and contact time, the difference between the amount of gas transferred across the interface with and without surface resistance increases as the value of reaction velocity increases. For ranges of contact time and surface resistance of practical relevance, the influence of surface resistance is too high to be neglected while designing gas-liquid contactors.     

Gas absorption with exothermic bimolecular reaction in a thin liquid film: Fast reactions

A problem of gas absorption with a fast nonisothermal bimolecular reaction in a thin liquid film has been analysed. Approximate analytical solutions for the interfacial temperature rise and enhancement factor have been derived incorporating the influence of depletion of liquid reactant concentration at the interface. The effects of heat generation, heat dissipation and reaction rate parameters on enhancement factor and interfacial temperature rise have been discussed. For a certain range of parameters, multiple steady state solutions of enhancement factor and interfacial temperature rise are observed. Another interesting observation is that as the value of a dimensionless parameter ? increases, the enhancement factor approaches its instantaneous asymptote which is the same as that for the isothermal case in an instantaneous regime. The analysis presented here is simple and avoids complex numerical computations and it is likely to be useful in design applications.     

Enhancement of absorption caused by density driven convection at gas—liquid interface induced by absorption with chemical reaction

The superimposition of mechanical agitation and density-driven cellular convection has been studied. The experiments were carried out using a sodium sulfite solution into which oxygen was being absorbed across the liquid surface. Experimental results suggest that, with respect to the convective pattern, two regions exist in the neighbourhood of the liquid surface: (i) at low agitation rates the cellular convection is stable and no bulk liquid penetrates into the interface, (ii) at high agitation rates both the cellular convection and the surface renewal proceed simultaneously. For the second case, a mathematical model based on the integral balance method was derived and good agreement between the enhancement factors estimated from the model and those found experimentally was attained.     

Spectroelectrochemistry at the liquid|liquid interface: Parallel beam UV-vis absorption

We present two novel UV-vis absorption spectroelectrochemical cells for the study of electrochemical processes at polarized liquid|liquid interfaces based on the parallel incidence of the light beam with respect to the interface. The first one is a crude design (Cell 1) based on an existing cylindrical cell for liquid|liquid electrochemistry. The second design (Cell 2) is superior and based on the assembly of the four-electrode set-up inside a rectangular quartz spectrophotometric cuvette. In both cases it is demonstrated that absorption spectroelectrochemistry in parallel beam configuration can be a sensitive probe of processes such as ion transfer at the polarized liquid|liquid interface, if the contribution from the bulk of the sampled phase is removed. In particular, the electrochemical and spectroscopic performance of both cell designs was evaluated using the well-known transfer of Ru(bpy)₃²⁺ across the water|1,2-DCE interface as a model reaction.     

浆料反应器中吸附剂粒子对CO2气体的吸收强化

研究了吸附剂粒子对气体吸收的增强作用。在一个带有搅拌的间歇反应釜中,分别测定了CO2在活性炭/水、S iO2/水浆液以及在活性炭/环己烷、S iO2/环己烷浆液中的吸收速率和增强因子。实验温度为298.15 K,初始压力为0.1 MPa,浆液中粒子的固含率为0—2 kg/m3,搅拌速度为1 s-1和4 s-1。结果表明,活性炭/水、S iO2/环己烷体系对CO2的吸收具有明显的增强作用,而S iO2/水、活性炭/环己烷体系则没有发现增强作用。因此,只有具有界面亲合能力且对溶质有吸附作用的颗粒才能对气体吸收产生显著的强化作用。将界面划为粒子覆盖和未覆盖区,提出了一个预测增强因子的一维双区模型,并进行了求解,模型预测值与实验结果吻合良好。