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.     

甘油水溶液中气液传质过程的研究

对二氧化碳气泡群在不同质量分数的甘油水溶液中的气液传质过程进行了研究。利用二氧化碳探针测定了不同操作条件下二氧化碳的体积传质系数,通过图像处理技术得到了气泡群的界面面积,考察了液相质量分数,气体流量对体积传质系数、界面面积和传质系数的影响。结果表明:体积传质系数均随气体流量的增大而增大,随溶液质量分数的增大而减小。气体流量通过影响界面面积对传质速率产生作用,而甘油水溶液的质量分数对传质速率的影响则归因于黏度对传质系数的影响。     

Gas/liquid mass transfer in a slurry bubble column

Gas holdups and volumetric mass transfer coefficients (k_La) for oxygen were measured in suspensions of kieselguhr (7 μm), aluminium oxide (8 μm) and activated carbon (5 μm) in water or 0.8 molar sodium sulfate solution. By comparison to previously reported specific interfacial areas (a) determined by the sulfite oxidation technique, the liquid side mass transfer coefficients (k_L) can be evaluated. Most data are well correlated as a decreasing function of the effective viscosity. However, higher k_L values are determined at low concentrations of solids with a high density.     

Particle-liquid mass transfer in three phase sparged reactor

Particle-liquid mass transfer in a three phase sparged reactor has been studied over a wide range of particle sizes, for the chum-turbulent regime. The particle-liquid Sherwood number has been correlated in the usual form with the Reynolds and Schmidt numbers. Use of the hindered particle settling velocity in the Reynolds number yields good agreement (±20%) with the present as well as most of the literature data. The proposed correlation also holds for power law non-Newtonian liquids when the effective viscosity is used.     

Note on mass transfer at gas sparged electrodes

Mass transfer at vertical plates with gas sparging obeys formally the same correlation as mass transfer in turbulent natural convenction due to concentration or temperature differences. A model is presented which allows a theoretical interpretation of this result.     

Gas/liquid mass transfer processes in a carbon dioxide/alkane system

A gas/liquid mass transfer process has been studied using carbon dioxide/alkane systems in a stirred vessel. Four linear alkanes (n‐heptane, n‐octane, n‐decane and n‐dodecane) have been used as the liquid phase in the present paper, and they have been employed to study the influence of the carbon length upon the mass transfer velocity. The mass transfer along the liquid phase has been studied using the mass transfer coefficient of the liquid phase, k_L. Pure carbon dioxide has been employed as the phase in all cases for this reason. The effects of the power supplied to the liquid phase and the gas flow rate upon the absorption process have also been analysed. Finally, the equations that allow calculation of the mass transfer coefficients have been applied for these systems, with acceptable results. Copyright © 2005 Society of Chemical Industry     

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.     

三相机械搅拌反应器气液传质

由空气-水、液体石蜡-细颗粒黄沙、石英砂、催化剂构成三相体系,常压下在表观气速0.10×10^-2～1.5×10^-2cm•s^-1 、固体浓度为0～0.34 g•ml^-1溶剂、搅拌转速450～1500 r•min^-1的实验条件下,采用溶氧仪研究了三相机械搅拌反应器的气液传质特性,考察了操作参数和液体性质、颗粒粒径及密度等物性因素对液相容积传质系数k_La的影响,用改进的高斯-牛顿法进行参数估值,得到k_La与上述因素的量纲1关联式,统计检验表明,所得量纲1关联式与实验值拟合情况良好,可为后续搅拌反应釜中三相淤浆床甲醇合成过程分析与模拟提供传质基础数据.     

Empirical approach to solid-liquid mass transfer in a three-phase sparged reactor

Solid-liquid mass transfer coefficients were determined in three-phase sparged reactors (TPSRs) using benzoic acid dissolution. Experiments were performed in three acrylic column reactors of internal diameter 0.1, 0.2 and 0.3 m respectively. The superficial gas velocities were varied up to 0.35 m s⁻¹. Using experimental data generated in this work and data reported in the literature for a 0.4-m diameter reactor, the effect of the reactor diameter on the solid-liquid mass transfer coefficient, k_SL, was investigated. It is demonstrated that an empirical approach can be used to determine k_SL from an appropriate mass transfer correlation useful for the design of TPSRs.     

Particle suspension and mass transfer rates in agitated vessels

This paper reviews and presents new information on three aspects of the behaviour of suspended solid particles in agitated tanks. The first is the minimum rate of stirring N required to keep the particles just suspended (JS) against their tendency to settle to the bottom of the tank. The second is the rate of mass transfer from suspended particles. The third is the relation between these two rates, that is, between N_JS and the corresponding mass transfer coefficient k_LJS.Theoretical and practical aspects of these three problems are discussed here, and it is shown that, starting from basic turbulence theory, one can predict N_JS as a function of particle concentration c.The reported constancy of the experimental k_LJS when the tank geometry is widely varied follows from the theoretical equations of the present paper.     

Dissolution mass transfer in a turbine agitated baffled vessel

Mass transfer coefficients were measured for the dissolution of crystalline electrolytes suspended in a baffled vessel by means of disc turbines. A 3-fold range of density differences, a 5-fold range of diffusivities and a 50-fold range of particle size were covered and the effect of impeller speed, size and clearance from the base was studied. The increase in coefficent with speed is both a function of the clearance and solid density and size and is difficult to quantify. However, in the most economic configuration of small clearance and large impeller, this is not of great importance since the coefficient increases but slowly with speed and the minimum suspended coefficient can be calculated. This is done by means of a slip velocity theory which has been extended to include larger particles, which allows for the effect of concentration dependent physical properties and which is based on the impeller speed at which particle suspension occurs.     

Initial response analysis of mass transfer in a gas sparged stirred vessel

A method for evaluating the mass transfer coefficient, k_La, in a gas sparged stirred vessel from the initial portion of the liquid dynamic response curve is described. The method assumes perfect liquid mixing but is independent of the gas mixing characteristics.Results obtained from two geometrically similar air-water systems (volume ratio 8:1) correlate well over a wide range of operating conditions and are in good agreement with evaluations made on the smaller system using an established and more involved experimental technique.     

Bubble size and mass transfer characteristics of sparged downwards two-phase flow

When gas is continuously fed through a sparger into a downflowing liquid in a pipe a ventilated cavity is often formed. The cavity remains attached to the sparger even in the presence of high liquid flow rates that would wash away a free slug bubble. Small bubbles are shed from the base of this cavity by the falling liquid film at the wall of the pipe and these bubbles are swept downwards forming a bubbly flow that is highly effective for mass transfer. The ventilated cavity is undesirable since it reduces the driving force for liquid circulation when the pipe is the downcomer of an external air loop fermenter or analogous gas/liquid reactors. The cavity also reduces the available interfacial area for mass transfer. It has been shown [Thorpe et al., 1997. Proceedings of the Fourth International Conference on Bioreactor and Bioprocess Fluid Dynamics; Lee, 1998. Ph.D Thesis, University of Cambridge, UK], that the length of the cavity can be reduced by replacing the common industrial design of a horizontal sparger (HS) with two novel spargers; a peripheral sparger (PS) and a plunging jet sparger (PJS) (Fig. 3). In this paper we investigate the effect of PS and PJS on mass transfer and the resulting bubble size.Experiments were carried out with air and water in a large circulating rig with a 0.105 m diameter test section. The local average bubble size in the bulk two-phase flow region below the ventilated cavity was determined using photography for three combinations of liquid and gas volumetric flow rates. The average bubble size was essentially the same (differences within 10%) for the PS, central spranger (CS) and HS. The PS created the largest bubble in all cases examined. The PJS created smaller bubbles than all the other spargers and did not allow the formation of cavities, which suggests that it has the superior performance. The estimated increase in k_La due to the smaller bubble size for the PJS was by a factor of 1.3.In order to check this result, the effects of sparger type on the volumetric mass transfer coefficient (k_La) were also measured. The k_La was determined with a dynamic method, by using unsteady state absorption of oxygen. The results confirmed the apparent superiority of PJS over the other spargers. An average increase of 19% in the k_La was observed when the PJS was used instead of the industrial design (HS). The CS and PS showed similar k_La values again within 10% of the HS.However the power consumption is larger when the PJS is used instead of the industrial design HS. Hence an attempt was made to adjust the bubble size and mass transfer coefficients of the PJS to account for the differences in energy consumption. When this is done the PJS and HS produce roughly the same bubble size and have the same mass transfer performance. Still the PJS had the important operational advantages of producing shorter cavities and having the greater resistance to stall at low liquid flow rates.     

微孔膜气体分布器气液传质比表面积的研究

研究了以微孔膜作为气体分布器的反应器的气液传质比表面积,主要考察了气体流量、液面高度、表面张力等操作因素对气液传质比表面积的影响。结果表明,气液传质比表面积随着气体流量的增大先呈增大的趋势,到一定值时,开始呈减小趋势;随着表面张力的增大而减小;随着液面高度的增大缓慢减小。与传统塔设备比较,微孔膜气体分布器的气液传质比表面积要高,分布器强化了气液传质过程。最后,运用量纲分析方法对试验数据进行非线性回归得到关于气液传质比表面积的关联式,此式较好地吻合了试验数据。     

Bubble size and mass transfer coefficients in dual-impeller agitated reactors

The study relates to the mass transfer and the bubble size in a non standard vessel equipped with various dual-impeller combinations. The effects of the rotational speed, gas flow rate, impeller type and diameter are investigated. The volumetric mass transfer coefficient k_La and the bubble size d_bs were studied. The liquid side mass transfer coefficient k_L and the volumetric interfacial area a were estimated separately. A comparison has been made with some existing correlations.     

Gas-liquid and solid-liquid mass transfer in three-phase sparged reactors with and without ultrasound

Gas-liquid and solid-liquid mass transfer in three-phase sparged reactor has been studied with and without super-imposed ultrasound. The results show marginal enhancement in mass transfer coefficient due to ultrasound. Thus it has been concluded that the reported enhancement in the rates of heterogeneous reactions due to ultrasound is likely due to other, e.g., chemical, effects of ultrasound.     

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