Comparison of hydrodynamic and mass transfer performances of an emulsion loop-venturi reactor in cocurrent downflow and upflow configurations

Hydrodynamic parameters (gas-induced flow rate and gas hold-up) and mass transfer characteristics (k_La, k_L and a) have been investigated in a gas–liquid reactor denoted “Emulsair” in which the distributor is an emulsion-venturi and the gas phase is self-aspired by action of the kinetic energy of the liquid phase at the venturi throat. Two configurations, respectively cocurrent downflow and cocurrent upflow were compared. A chemical method involving the dispersion of a CO₂–air mixture in a monoethanolamine (MEA) aqueous solution was used to measure mass transfer parameters. Experimental results showed that only the homogeneous bubbling regime prevailed in the upward configuration, while an annular regime could also be observed for cocurrent downflow at low liquid flow rate. Gas-induced flow rate and gas hold-up were usually smaller for cocurrent upflow, both at constant liquid flow rate and specific power input. The same stood for mass transfer properties. Conversely, specific power requirements were lower at constant liquid flow rate and mass transfer characteristics were enhanced at constant gas-induced flow rate for cocurrent upflow. A comparison with other gas–liquid contacting devices showed that the Emulsair reactor is a versatile tool avoiding the presence of mechanically moving parts when high and quickly adaptable dissolved gas supply is required. The cocurrent upflow configuration can be preferred when high gas flow rates are desired because the evolutions of gas-induced flow rate and mass transfer characteristics exhibit a stronger dependence on specific power input in the homogeneous bubbling regime for this configuration.     

Pressure Drop and Mass Transfer Study in Static Mixers with Gas Continuous Phase

The aim of this work was to characterize both the influence of the gas and the liquid flow rates on the pressure drop generated by static mixers type Sülzer, and on the mass transfer performances of these gas‐liquid contactors. The originality of this work rests on the use of the static mixers with a gas continuous phase. Several configurations were studied: vertical upward flow, vertical downward flow, and horizontal positions, with one to five mixing elements. It was concluded that the pressure drop is mainly generated by the gas phase, whatever the configuration chosen. Moreover, the volumetric mass transfer coefficients k_La found were lower than those obtained with the same static mixers used in classic conditions (that is with a liquid continuous phase), but greater than values obtained with classic reactors like bubble columns or packed columns. The efficiency of these gas‐liquid reactors was found high, as well as the energy dissipated, unfortunately.     

MASS TRANSFER IN GAS-SPARGED POROUS ELECTRODES

The electrochemical limiting current method was used to measure mass transfer coefficients with cocurrent upward gas-liquid flow in packed bed electrodes. Liquid-solid and overall gas-solid coefficients were determined from limiting current data in the presence of inert or reactive gas flows with the use of a plug flow model. The presence of inert gas flow increased mass transfer coefficients over those in single-phase flow by a factor of up to 1.7. As a result of boundary layer penetration by gas bubbles, sparging with reactive gas increased mass transfer rates by a factor of up to 3.5.     

MASS TRANSFER IN GAS-SPARGED POROUS ELECTRODES

The electrochemical limiting current method was used to measure mass transfer coefficients with cocurrent upward gas-liquid flow in packed bed electrodes. Liquid-solid and overall gas-solid coefficients were determined from limiting current data in the presence of inert or reactive gas flows with the use of a plug flow model. The presence of inert gas flow increased mass transfer coefficients over those in single-phase flow by a factor of up to 1.7. As a result of boundary layer penetration by gas bubbles, sparging with reactive gas increased mass transfer rates by a factor of up to 3.5.     

LIQUID-SOLID MASS TRANSFER AT HORIZONTAL WOVEN SCREENS WITH UPWARD COCURRENT GAS-LIQUID FLOW

Rates of liquid-solid mass transfer at horizontal single screens and an array of horizontal parallel separated screens were studied under upward cocurrent gas (N₂)-liquid bubbly flow using the electrochemical technique. Variables studied were gas and liquid flow rates, and screen characteristics (e.g., mesh number and wire diameter)

Under the present conditions where relatively low solution flow rates were used the rate of mass transfer was found to be mainly determined by the gas flow rate. For a given gas flow rate, the mass transfer coefficient decreased with increasing solution flow rate. The data for single screen were correlated with a dimensionless equation. Rates of mass transfer at an array of separated horizontal screens were lower than those at the single screen by an amount ranging from 3 to 45% depending on screen mesh number and flow conditions. The importance of the present study for building continuous high space time yield catalytic, and electrochemical reactors suitable for electrochemical air pollution control is highlighted.     

LIQUID-SOLID MASS TRANSFER AT HORIZONTAL WOVEN SCREENS WITH UPWARD COCURRENT GAS-LIQUID FLOW

Rates of liquid-solid mass transfer at horizontal single screens and an array of horizontal parallel separated screens were studied under upward cocurrent gas (N₂)-liquid bubbly flow using the electrochemical technique. Variables studied were gas and liquid flow rates, and screen characteristics (e.g., mesh number and wire diameter)

Under the present conditions where relatively low solution flow rates were used the rate of mass transfer was found to be mainly determined by the gas flow rate. For a given gas flow rate, the mass transfer coefficient decreased with increasing solution flow rate. The data for single screen were correlated with a dimensionless equation. Rates of mass transfer at an array of separated horizontal screens were lower than those at the single screen by an amount ranging from 3 to 45% depending on screen mesh number and flow conditions. The importance of the present study for building continuous high space time yield catalytic, and electrochemical reactors suitable for electrochemical air pollution control is highlighted.     

Mass transfer and hydrodynamic characteristics in a co‐current downflow contacting column

Hydrodynamic and mass transfer characteristics of water–air system in a co‐current downflow contacting column (CDCC) were studied for various nozzle diameters at different superficial gas velocities and liquid re‐circulation rates. Gas hold‐up and liquid‐side mass transfer coefficient increased with increasing superficial gas velocity and liquid flow rate but decreased with increasing nozzle diameter. It is shown that correlations developed, which are based on liquid kinetic power per liquid volume present in the column, and superficial gas velocity explains gas hold‐up and the mass transfer coefficient within an error 20% for all gas and liquid flow rates and nozzle diameters used. The constants of correlations for gas hold‐up and mass transfer coefficient were found to be considerably different from other gas–liquid contacting systems. © 2003 Society of Chemical Industry     

Electrohydrodynamic Spraying (EHDS) of Conductive Liquids and Mass Transfer in EHDS Distillation Columns

A physicomathematical model for electrohydrodynamic spraying (EHDS) of a conductive liquid and the mass transfer in a parallel plate electrodes system of an EHDS distillation column is presented. The influence of the electric field on the formation and general characteristics of the finely dispersed aerosol flow of the conductive liquid in EHDS distillation columns has been determined from the model. It allows to explain in detail the mechanism of the influence of the electric field on the mass transfer in the gas‐liquid system. In addition, the maximum possible liquid flow rate in the mass transfer section of the EHDS distillation column has been estimated. The role of process parameters is discussed in detail.     

PERFORMANCE OF A PACKED-BED REACTOR

We examined the conversion rates in a packed bed catalytic reactor with a two phase upward flow in a wide range of operating conditions. The oxidation of ethanol to acetic acid in the liquid phase on a Pd-Alumina catalyst was chosen as the test reaction.

Global reaction rates were measured by changing gas velocities, temperature, and feed concentrations of ethanol in the liquid phase. The observed rates were compared with those calculated using two models, assuming a total external wetting of the catalyst. In the first model, a “kinetic” conversion rate was calculated by neglecting any interphase mass transfer resistance. In the second model the interphase mass transfer resistance was considered and expressed by an overall coefficient evaluated from published correlations. The results show that there is an hydrodynamic influence, probably due to the mass transfer and/or to the partial effective wetting of the catalyst. Mass transfer, on the other hand, is better than that observed in other cases. A comparison with the performances of a downflow trickle-bed reactor operating at the same tested conditions showed a much smaller influence of mass transfer and hydrodynamics on the overall conversion rate for the upflow reactor.     

Effect of tube size on liquid side mass transfer in co-current gas-liquid upward flow

Liquid side mass transfer in cocurrent upward gas-liquid flow in empty tubes of three different sizes, viz. 10, 15 and 20 mm has been studied by absorbing carbon dioxide from a mixture with air into a buffer solution of sodium carbonate and sodium-bicarbonate. A method for predicting the partial liquid side mass transfer coefficients, based on the Levich model of damped turbulence at the gas liquid interface, has been proposed for the case of annular flow.     

Mass transfer performance of structured packings in a CO2 absorption tower

This paper studies the mass transfer performance of structured packings in the absorption of CO2 from air with aqueous NaOH solution. The Eight structured packings tested are sheet metal ones with corrugations of different geometry parameters. Effective mass transfer area and overall gas phase mass transfer coefficient have been measured in an absorption column of 200 mm diameter under the conditions of gas F-factor in 0.38–1.52 Pa0.5 and aqueous NaOH solution concentration of 0.10–0.15 kmol·m?3. The effects of gas/liquid phase flow rates and packing geometry parameters are also investigated. The results show that the effective mass transfer area changes not only with packing geometry parameters and liquid load, but also with gas F-factor. A new effective mass transfer area correlation on the gas F-factor and the liquid load was proposed, which is found to fit experiment data very well.     

Hydrodynamics and Mass Transfer in Gas‐Liquid‐Solid Circulating Fluidized Beds

Although extensive work has been performed on the hydrodynamics and gas‐liquid mass transfer in conventional three‐phase fluidized beds, relevant documented reports on gas‐liquid‐solid circulating fluidized beds (GLSCFBs) are scarce. In this work, the radial distribution of gas and solid holdups were investigated at two axial positions in a GLSCFB. The results show that gas bubbles and solid particles distribute uniformly in the axial direction but non‐uniformly in the radial direction. The radial non‐uniformity demonstrates a strong factor on the gas‐liquid mass transfer coefficients. A local mass transfer model is proposed to describe the gas‐liquid mass transfer at various radial positions. The local mass transfer coefficients appear to be symmetric about the central line of the riser with a lower value in the wall region. The effects of gas flow rates, particle circulating rates and liquid velocities on gas‐liquid mass transfer have also been investigated.     

Experimental investigation of fast-mode liquid modulation in a trickle-bed reactor

Unsteady-state operation of trickle-bed reactors (TBRs) is a promising technique to improve reactor performances especially when mass transfer phenomena are rate controlling. Among the different techniques, fast-mode modulation of the liquid flow rate seems to be one of the most successful. In fact cycling the liquid flow rate at very low frequencies can induce the reactor to work at the high-interaction regime where mass and heat transfer phenomena are strongly enhanced. Fast-mode periodic operation, then, can be considered an extension of the natural high-interaction regime at a mean range of gas and liquid flow rate normally associated with trickling regime in steady-state conditions.Experimental tests have been performed in a TBR employing α-methyl styrene hydrogenation on Pd/C catalyst in unsteady-state conditions by “on-off” fast-mode liquid modulation. Results have been compared with the steady-state experiments at the corresponding average liquid flow rate, revealing a conversion rate improvement up to 60%. All experiments have been performed in isothermal conditions, so conversion improvement can be ascribed only to mass transfer increase and not to thermal effects. The variation of gas and liquid flow rates and liquid cycle parameters presented several important implications about the optimal working conditions.     

超重力旋转床中水脱氧过程的模型化研究

超重力旋转床是一种高效的强化传质和混合的新型设备。今提出了超重力旋转床中的水脱氧过程的传质模型,分别采用欧拉方法和拉格朗日方法对超重力旋转床中的气相和液滴的运动行为进行了数值模拟;在此基础上计算了液滴的传质系数,计算结果和实验结果符合较好,平均误差为7．9％。当超重力旋转床中液体存在的主体形式更接近于液滴时,模型计算结果误差减小。进一步讨论分析了液体和气体流量、转速以及填料内径的变化对于超重力旋转床体积传质系数的影响,分析表明旋转填料对液体剧烈地剪切破碎分散作用是强化传质过程的主要原因。     

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.     

外环流气升式反应器中催化丙烯环氧化反应

本论文研究了外环流气升式反应器中TS-1催化丙烯环氧化反应,以单位催化剂空时收率为目标,进行了反应工艺条件的优化,考察了结构参数,上升管与下降管横截面积之比Ar Ad对反应的影响。     

Gas holdup and overall volumetric mass transfer coefficient in a modified reversed flow jet loop reactor

Experiments were conducted in a modified reversed flow jet loop reactor having the liquid outlet at the top of the reactor to determine the gas holdup and overall volumetric mass transfer coefficient in the air-water system. The influence of gas and liquid flow rates, and the draft tube to reactor diameter ratio were studied. It was observed that both gas holdup and volumetric mass transfer coefficient increased with increased gas and liquid flow rates and were found to be significantly higher in the modified reactor compared to the conventional one. The optimum draft tube to reactor diameter ratio was found to be in the range of 0.4 to 0.5. Empirical correlations are presented to predict gas holdup and overall volumetric mass transfer coefficient in terms of operational and geometrical variables.     

Mass transfer in cocurrent gas-liquid flow: Gas side mass transfer coefficients in upflow,interfacial areas and mass transfer coefficient in gas and liquid in downflow

A comparison of the values of interfacial area in cocurrent downward flow of gas and liquid with those obtained in upward flow revealed very little difference.Volumetric as well as true liquid side mass transfer coefficients in downflow were found to be several times lower than in upflow. Only in the smallest 10 mm tube a dependence of the mass transfer coefficient on the gas flow rate could be detected, no effect of the gas velocity was observed in the 15 and 20 mm tubes. A correlation for the liquid side mass transfer coefficient was obtained in terms of the Reynolds number of the film.The volumetric gas side mass transfer coefficient in upflow was generally independent on the liquid flow rate, except at high gas velocities in the two smaller tubes. Correlations were obtained for the volumetric mass transfer coefficient in terms of the specific rate of energy dissipation, and of the true mass transfer coefficient in terms of dimensionless groups. Much lower values were obtained for the gas side mass transfer coefficient in downflow. Only the data for the 10 mm tube could be correlated with some success by the formulas proposed for upflow.     

新型气液逆流撞击洗涤喷嘴的优化

针对一种新型气液逆流撞击式洗涤喷嘴,通过冷模实验,采用溶氧法考察了不同结构喷嘴的气液两相传质性能。结合解析率及流型变化,考察了喷嘴出口直径、切向进液口倾角、旋流室收敛段锥角、切向进液口直径、喷口长度5个参数对传质的影响,确定了优选喷嘴的结构尺寸,分析了该优选喷嘴在不同操作条件下(气速、表观液气比和轴切比)的传质效果。结果表明,优选喷嘴在轴切比为0.4~0.6且气速较高时传质效果较好。     

Effect of Tween 80 on Bubble Size and Mass Transfer in a Bubble Contactor

Gas absorption in aqueous solutions with Tween 80 and absorption processes based on hydrodynamics and mass transfer is determined. The impact of surfactant concentration on gas holdup and gas‐liquid interfacial area is analyzed, observing an increase of these parameters with surfactant concentration. The influence of liquid‐phase contamination on the absorption process is investigated on the basis of the liquid‐film mass transfer coefficient, removing the effect caused by the presence of a surfactant and the gas flow rate on the interfacial area and, thereby, on the volumetric mass transfer coefficient. The opposite effect on the mass transfer coefficient can be observed which decreases in the presence of the surfactant.