Power consumption and mass transfer in agitated gas-liquid columns: A comparative study

Power consumption, gas holdup and oxygen mass transfer in agitated gas-liquid columns have been studied for an air-water system. Measurements have been carried out in a reciprocating plate reactor using five different types of perforated plates and in a stirred tank reactor with one, two and three Rushton turbines, a helical ribbon impeller with and without surface baffles. Each mixing vessel had an identical geometry with a working volume of 17 L. For reciprocating plate stacks, the gas holdup is a complex function of the perforation diameter, the frequency of agitation and the gas superficial velocity. For radial-type mixing devices, the gas holdup increases more rapidly with the speed of rotation for the helical ribbon. The power imparted to the fluid by the mixing device is independent of the gas superficial velocity for the plate stacks and the helical ribbon impeller for a given frequency or speed of agitation whereas it decreases for Rushton turbines. The correlation of the power consumption obtained for all mixing devices plotted against the reciprocating frequency or speed of rotation to the third power shows a linear fit. K_La values were correlated very well with the power input per unit volume and superficial gas velocity for all mixing devices. At lower power input per unit volume, K_La is a function of only the gas superficial velocity. At higher input power per unit volume, K_La increases rapidly with an increase in the intensity of agitation. Reciprocating plates with larger diameter perforations led to higher K_La values whereas the lowest K_La were obtained with the helical ribbon impeller. Correlations for one and three Rushton impeller assemblies were almost identical whereas measured K_La were much higher for the two-impeller assembly due to the presence of a highly mixed zone in the vicinity of the dissolved oxygen probe.     

Gas Sparger Orifice Sizes and Solid Particle Characteristics in a Bubble Column – Relative Effect on Hydrodynamics and Mass Transfer

The relative effects of the size of gas sparger orifices and properties of solid particles on gas‐liquid mass transfer are not yet fully understood. Here, the impact of sparger orifice sizes, solid particle shapes, and their loading amounts in a bubble column reactor on the absorption of oxygen in tap water was investigated. Their influence on the mass transfer coefficient and bubble hydrodynamic parameters was evaluated. The results show that the addition of solid particles can have both positive and negative effects on hydrodynamics and mass transfer, depending on the orifice size of the gas sparger. The introduction of ring‐shaped solid particles can improve the mass transfer rate by up to 28 % without requiring any significant additional power.     

A HUMBLE TRIBUTE TO CHURCHILL-SENSEI

The performance of pitched blade turbines in a gas-liquid dispersion has been studied. The two-phase hydrodynamics, gassed power consumption and mass transfer properties have been examined using six blade open turbines with blade angles from 30 to 60 degrees to the horizontal, mounted for down flow.

There are two distinct regimes by which gas leaving the sparger reaches the impeller: at low gas rates this is indirect via the recirculation loops, while at higher gas flow rates the flow is direct.

The transition between these regimes is reflected in power consumption and mass transfer characteristics and is related to the formation of large cavities behind the blades. It was also concluded that, with respect to mass transfer efficiency, a pitched blade turbine is at least as good as a Rushton turbine.     

Gas hold-up behavior of mechanically agitated gas-liquid reactors using pitched blade downflow turbines

Fractional gas holdup was measured in 0.57, 1.0, and 1.5 m i.d. vessels. Pitched blade downflow turbines (PTD) were used as the impeller. Design details of the impeller, such as the impeller diameter (0.22 T to 0.5 T) and blade width (0.25 D to 0.4 D), were studied. The effect of sparger type, geometry and size on fractional gas hold-up has been investigated in detail. Four different types of spargers (pipe, conical, ring and concentric ring spargers) were used. Sparger location was varied for all the types studied. Further, design details of the ring sparger, which gave the highest hold-up were then studied in detail. These included ring diameter, number of holes and hole size. All the reported correlations for fractional gas hold-up in mechanically agitated gas-liquid reactors were tested and compared. A better correlation has been developed for pitched blade turbines.     

Oxygen mass transfer coefficient in bubble column slurry reactor with ultrafine suspended particles and neural network prediction

The gas–liquid volumetric mass transfer coefficient was determined by the dynamic oxygen absorption technique using a polarographic dissolved oxygen probe and the gas–liquid interfacial area was measured using dual‐tip conductivity probes in a bubble column slurry reactor at ambient temperature and normal pressure. The solid particles used were ultrafine hollow glass microspheres with a mean diameter of 8.624 µm. The effects of various axial locations (height–diameter ratio = 1–12), superficial gas velocity (u_G = 0.011–0.085 m/s) and solid concentration (ε_S = 0–30 wt.%) on the gas–liquid volumetric mass transfer coefficient k_La_L and liquid‐side mass transfer coefficient k_L were discussed in detail in the range of operating variables investigated. Empirical correlations by dimensional analysis were obtained and feed‐forward back propagation neural network models were employed to predict the gas–liquid volumetric mass transfer coefficient and liquid‐side mass transfer coefficient for an air–water–hollow glass microspheres system in a commercial‐scale bubble column slurry reactor. © 2012 Canadian Society for Chemical Engineering     

GAS-LIQUID DISPERSION WITH PITCHED BLADE TURBINES

The performance of pitched blade turbines in a gas-liquid dispersion has been studied. The two-phase hydrodynamics, gassed power consumption and mass transfer properties have been examined using six blade open turbines with blade angles from 30 to 60 degrees to the horizontal, mounted for down flow.

There are two distinct regimes by which gas leaving the sparger reaches the impeller: at low gas rates this is indirect via the recirculation loops, while at higher gas flow rates the flow is direct.

The transition between these regimes is reflected in power consumption and mass transfer characteristics and is related to the formation of large cavities behind the blades. It was also concluded that, with respect to mass transfer efficiency, a pitched blade turbine is at least as good as a Rushton turbine.     

Gas hold‐up in stirred tank reactors

Based on a study of the gas hold‐up data for stirred tank reactor generated in the present work and the data available in the literature for large stirred tank reactors (T = 0.57 m to 2.7 m) equipped with disc turbines and pitched blade downflow turbines a correlation is presented which reliably predicts gas hold‐up data over wide range of system configurations and operating parameters. The parameter used, N/N_cd, relates gas hold‐up at impeller speed N with respect to the gas hold‐up at minimum impeller speed for complete dispersion of the gas, N_cd. It is shown that the gas hold‐up data of different workers when compared on the basis of N/N_cd, shows unanimity.     

Solid–liquid mass transfer characteristics of an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller

To develop an enhanced form of solid‐liquid apparatus, an unbaffled agitated vessel has been constructed, fitted with an agitation system using an impeller whose rotation alternates unsteadily in direction, i.e. a forward‐reverse rotating impeller. In this vessel, solid‐liquid mass transfer was studied using a disc turbine impeller with six flat blades. The effect of impeller rotation rate as an operating variable on the mass transfer coefficient was evaluated experimentally using various geometrical conditions of the apparatus, such as impeller diameter and height, in relation to the impeller power consumption. Mixing of gas above the free surface into the bulk liquid, i.e. surface aeration, which accompanied the solid‐liquid agitation, was also investigated. Comparison of the mass transfer characteristics between this type of vessel and a baffled vessel with a unidirectional rotating impeller underscored the sufficient solid‐liquid contact for prevention of gas mixing in the forward‐reverse rotation mode of the impeller. Copyright © 2008 Society of Chemical Industry     

Gas recirculation rate and its influences on mass transfer in multiple impeller systems with various impeller combinations

Both the numerical and experimental approaches were used to study the effects of the gas recirculation and non‐uniform gas loading on the mass transfer rate for each impeller in a multiple impeller system. By combining the calculated gas velocity and local gas holdup, the gas recirculation rate around each impeller was estimated. The local mass transfer coefficients for systems equipped with various combinations of the Rushton turbine impeller (R) and pitched blade impeller (P) were determined by using the dynamic gassing out method. It is found that the Rushton turbine impeller has to be served as the lowest impeller in order to have a better gas dispersion and to give a higher overall K_La for a multiple impeller gas‐liquid contactor. The upper pitched blade impeller always enforces the circulating flow around the Rushton turbine impeller just beneath it and gives a higher overall average mass transfer rate. However, the system equipped with only the pitched blade impellers results in a much lower mass transfer rate than the other systems owing to the poor gas dispersion performance of the pitched blade impeller.     

Gas‐Liquid Mass Transfer Characteristics in a Rotating‐Drum Bioreactor

The oxygen volumetric mass transfer coefficient is a key parameter to characterize the performance of aerobic bioreactors. A novel rotating‐drum bioreactor (RDB) fitted with a sparger as proposed in a previous work has demonstrated its excellent gas‐liquid mass transfer performance. To provide primary information on the design and scale‐up of the novel RDB, effects of reactor configuration including the number and width of lifters and operation conditions such as rotational speed, aeration rate, and solid volume fraction on mass transfer performance were systematically investigated in a new medium‐sized RDB. Compared with the stirred bioreactor and traditional RDBs, this new RDB exhibits better mass transfer performance. Taking both operational and reactor configuration parameters into consideration, an empirical correlation to predict the volumetric mass transfer coefficient in this type of RDBs was proposed which is valuable for its design and scale‐up.     

Role of sparger design on gas dispersion in mechanically agitated gas-liquid contactors

Critical impeller speeds for gas dispersion and gas recirculation were measured in 0.57, 1.0 and 1.5 m i.d. vessels, using visual observations, measurements of power consumption and liquid-phase mixing time. A pitched blade down-flow turbine impeller (PTD) was used. Design parameters of the PTD impeller such as diameter (0.22Tto 0.57T), blade width (0.25D to 0.4D) and blade thickness (2.8, 4.3 and 6.4 mm) were studied. The effect of sparger type and geometry on N_CD has been investigated. For this purpose, pipe, ring, conical and concentric ring spargers were employed. Design details of the ring sparger such as ring diameter, number of holes and hole size were also studied in detail. The sparger location with respect to the impeller was found to be the most important parameter. Correlations have been developed for N_CD and N_CR.     

Influence of impeller type on hydrodynamics and gas‐liquid mass‐transfer in stirred airlift bioreactor

The influence of impeller type in a mechanically stirred airlift bioreactor was analyzed in relation to the non‐Newtonian viscous fluids. The agitation was carried out through a marine impeller (axial impeller) and a paddle impeller (radial impeller) located along with the gas sparger in the region comprised by the riser. The bioreactor was sparged with air under different velocities (0.036–0.060 m s^?1). Carboxymethylcellulose 1.94% and xanthan 1.80% were used as a fluid model. The gas holdup and volumetric mass‐transfer coefficient increased in up to five and three times, respectively, when compared to a conventional airlift bioreactor; however, better results were obtained when the straight paddle impeller type was used. The results suggest that the studied bioreactor can be used successfully in viscous fluid, and it can be more efficient than conventional airlift bioreactors. The results obtained suggest the use of radial impellers. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3159–3171, 2015     

搅拌槽内粘稠物系中气液相间氧传递

以发酵罐中气液相间氧传递为背景，考察了搅拌槽内搅拌器形式、物系流变性质、通气搅拌操作条件等对假塑性粘稠物系中氧传递过程的影响。结果表明，这些因素主要通过改变气体分散状态和相间传质面积来影响氧传递速率。根据气泡在搅拌槽内不均匀分布现象，多层搅拌下气液相间传质过程可以用气泡运动分区分布模型来描述。它说明了采用轴向流桨和涡轮桨组合的搅拌形式在氧传递方面的优越性，为强化发酵罐中供氧指明一条有效途径     

Liquid‐Solid Mass Transfer Behavior of a New Stirred‐Tank Reactor with a Packed Bed Fixed to its Wall

Rates of liquid‐solid mass transfer at a packed bed of Raschig rings fixed to the wall of a stirred tank were measured by a technique which involves the diffusion‐controlled dissolution of copper in acidified dichromate. Variables studied were impeller rotation speed, impeller geometry, Raschig ring diameter, bed thickness, presence of baffles, physical properties of the solution, and effect of superimposed flow. Mass transfer data for the batch reactor were correlated by a dimensionless equation. For a given set of conditions, the radial‐flow impeller was found to produce higher rates of mass transfer than the axial‐flow impeller. The presence of baffles increased the rate of mass transfer inside the bed. Applications of the suggested reactor in conducting different diffusion‐controlled liquid‐solid reactions were evaluated.     

外循环气升式膜反应器中的体积传质系数

传质系数是决定气升式反应器传质特性的主要参数之一,文中介绍了一种新型的将陶瓷膜曝气系统与外环流反应器耦合的外环流膜反应器,考察了不同曝气气量,曝气压力,曝气方式以及曝气位置条件下反应器中体积传质系数的变化情况。结果表明,曝气气量,曝气压力,曝气孔孔径都能影响体积传质系数,孔径的影响尤为显著,采用膜微孔曝气,体积传质系数相比曝气头曝气提高近2倍。将膜管曝气和曝气头曝气耦合起来,可在获得良好气液循环的条件下有较大的气液接触面,从而优化气液传质。     

Role of sparger design in mechanically agitated gas-liquid reactors. Part II: Liquid phase mixing

Liquid phase mixing time was measured in 0.57, 1.0 and 1.5 m i.d. mechanically agitated gas-liquid reactors. Transient conductivity technique was used for the mixing time measurement. Pitched blade downflow turbine was employed. The design details of PTD impellers such as diameter (0.22 T to 0.5 T) and blade width (0.25 D to 0.35 D) were studied. The influence of sparger types and their design on mixing time has been investigated. For this purpose, pipe, ring, conical, and concentric ring spargers were employed. The design details of the ring sparger, i.e. ring diameter, number of holes and hole size were also studied in depth. Sparger location with respect to the impeller was found to be the most important variable and, therefore, it was varied for practically all the spargers studied in this work. It was found that the liquid phase mixing time depends on the impeller design, sparger design, sparger location, impeller speed and superficial gas velocity. Correlations have been developed for the dimensionless mixing time.     

Bubble size in a forced circulation loop reactor

BACKGROUND: The bubble size distribution in gas‐liquid reactors influences gas holdup, residence time distribution, and gas‐liquid interfacial area for mass transfer. This work reports on the effects of independently varied gas and liquid flow rates on steady‐state bubble size distributions in a new design of forced circulation loop reactor operated with an air–water system. The reactor consisted of a cylindrical vessel (～26 L nominal volume, gas‐free aspect ratio ≈ 6, downcomer‐to‐riser cross‐sectional area ratio of 0.493) with a concentric draft tube and an annular riser zone. Both gas and liquid were in forced flow through a sparger that had been designed for minimizing the bubble size. RESULTS: Photographically measured bubble size distributions in the riser zone could be approximated as normal distributions for the combinations of gas and liquid flow rates used. This contrasted with other kinds of size distributions (e.g. bimodal, Gaussian) that have been reported for other types of gas‐liquid reactors. Most of the bubbles were in the 3 to 5 mm diameter range. At any fixed low value of aeration rate (≤1.8 × 10^?4 m³s^?1), increase in the liquid flow rate caused earlier detachment of bubbles from the sparger holes to reduce the Sauter mean bubble size in the riser region. CONCLUSION: Unlike in conventional bubble columns where bimodal and Gaussian bubble size distributions have been reported, a normal bubble size distribution is attained in forced circulation loop reactors with an air–water system over the entire range of operation. Copyright © 2007 Society of Chemical Industry     

Experimental determination of gas holdup and volumetric mass transfer coefficient in a jet bubbling reactor

The hydrodynamics and mass transfer characteristics of a lab-scale jet bubbling reactor(JBR) including the gas holdup, volumetric mass transfer coefficient and specific interfacial area were assessed experimentally investigating the influence of temperature, p H and superficial gas velocity. The reactor diameter and height were 11 and 30 cm,respectively. It was equipped with a single sparger, operating at atmospheric pressure, 20 and 40℃, and two p H values of 3 and 6. The height of the liquid was 23 cm, while the superficial gas velocity changed within 0.010–0.040 m·s~(-1) range. Experiments were conducted with pure oxygen as the gas phase and saturated lime solution as the liquid phase. The liquid-side volumetric mass transfer coefficient was determined under unsteady-state oxygen absorption in a saturated lime solution. The gas holdup was calculated based on the liquid height change, while the specific interfacial area was obtained by a physical method based on the bubble size distribution(BSD) in different superficial gas velocities. The results indicated that at the same temperature but different p H, the gas holdup variation was negligible, while the liquid-side volumetric mass transfer coefficient at the p H value of 6 was higher than that at the p H = 3. At a constant p H but different temperatures, the gas holdup and the liquid-side volumetric mass transfer coefficients at 40℃ were higher than that of the same at 20℃. A reasonable and appropriate estimation of the liquid-side volumetric mass transfer coefficient(kla) in a pilot-scale JBR was provided which can be applied to the design and scale-up of JBRs.     

CFD在自吸反应器气液流动和传质特性研究中的应用

针对配置气体分布器的六叶轮自吸反应器建立了欧拉气液两相流三维瞬态模型,耦合Higbie气液传质模型,采用CFX软件对其气液混合过程的流场、气含率、吸气速率及溶氧传递过程进行数值模拟,获得了反应器的流动特性、气液分散性能、吸气特性及气液传质特性.分析了反应器内水平及竖直位置上的流型特征及溶氧传递性能,结合实验数据及经验关联式对比分析了对气含率及吸气速率的预测作用.结果表明,六叶轮转子及其配置的气体分布器可以获得较均匀的气液混合,气含率及吸气速率的预测与实验值偏差分别为5.2% 和17.6%,模拟发现在反应器底部近壁处溶氧及混合效果不佳.