HYDRODYNAMIC STUDIES IN FISCHER-TROPSCH DERIVED WAXES IN A BUBBLE COLUMN

Gas hold-up and Sauter mean bubble diameter measurements were made in a 0.051 m diameter by 3 m long glass bubble column in the system, nitrogen-molten wax, with three different waxes (paraffin wax FT-300, Sasol's Arge wax and Mobil's reactor wax). Paraffin wax has a tendency to foam and gas hold-up is a strong function of gas distributor type, temperature and start-up procedure, whereas the reactor waxes do not foam and are much less affected by these variables, In experiments at 265°C with a 1.85 mm single hole orifice plate distributor the gas hold-ups were nearly the same for all three waxes. However, significant differences in Sauter mean bubble diameters were obtained in experiments with different waxes; FT-300 wax produced the smallest Sauter mean bubble diameters whereas Mobil's reactor wax produced the largest bubbles. Addition of 1-octadecanol and octadecanoic acid (up to 10wt%) to the FT-300 paraffin wax caused an increase in gas hold-up and a delay in the foam break-up in runs at 265°C with the 1.85 mm orifice plate distributor.     

Hydrodynamic study in a slurry-bubble-column reactor

Local gas holdup, bubble diameter and bubble rise velocity in the nitrogen/Drakeol-10 oil system were measured at both laboratory (ambient temperature and pressure) and industrially relevant (high temperature and pressure) conditions using a dual conductivity probe in a slurry-bubble-column reactor. It was found that a constant superficial velocity, the Sauter mean bubble diameter decreases with increasing pressure and temperature. The bubble rise velocity significantly decreases as the pressure increases. Large bubbles rise faster than smaller bubbles. Akita and Yoshida's correlation [1] was utilized to compute the bubble size. Predicted values agree with the experimental data at high temperature.     

垂直筛板流化床中FCC催化剂的流化性能

以FCC催化剂颗粒研究垂直筛板流化床内构件对气固两相流化性能的影响,考察了板孔气速、颗粒循环量和帽罩开孔比等筛板结构对流化床压降和提升量强度的影响. 结果表明,气固两相总体逆流流动条件下,帽罩内气速达4 m/s,气固高速并流喷射无气泡,两相接触好、返混小,属快速流态化. 由于没有气泡,床层压力波动小,在塔板上颗粒返混小. 垂直筛板压降随板孔气速、帽罩底隙高度增大而增大,随帽罩开孔比、板孔径增大而减小,颗粒提升量大,床层压降大. 提升量强度随板孔气速、帽罩底隙高度、颗粒循环量增加而增大,随帽罩高度与塔节高度比增大而减少,随帽罩筛孔孔径变化存在最大值. 当帽罩开孔比为1.2~2.5、板孔面积与帽罩截面积比为0.42、帽罩底隙高与板孔孔径比为0.36~0.64时帽罩流化性能较好.     

Modeling of the evolution with length of bubble size distributions in bubble columns

Many of the existing methods, for the determination of the specific interfacial area in bubble columns, consider the column in a dynamic equilibrium between bubble coalescence and breaking-up. The aim of this work is to study if this consideration can be considered true for low superficial gas velocities. Two existing models have been chosen from literature in order to predict the break-up [Wang, T., Wang, J., Jin, Y., 2003. A novel theoretical breakup kernel function for bubbles/droplets in a turbulent flow. Chemical Engineering Science 58, 4629-4637] and the coalescence [Lehr, F., Millies, M., Mewes, D., 2002. Bubble size distributions and flow fields in bubble columns. A.I.Ch.E. Journal 48, 2426] rates. In order to confirm the validity of the models, predictions were compared with experimental results obtained by image analysis. Several simulations were performed for different superficial gas velocities and initial bubble size distributions. The column length needed to reach dynamic equilibrium was calculated for each simulation. The results show that the necessary length to reach the dynamic equilibrium does not depend on the shape of the initial distribution, but essentially on its Sauter mean diameter. The necessary length to reach the dynamic equilibrium is very important for low superficial gas velocities. The assumption that the entire column is in dynamic equilibrium is in general not valid. Therefore, the initial Sauter mean diameter and the total column length are important parameters for the determination of the specific interfacial area.     

Gas–liquid flows in medium and large vertical pipes

Gas–liquid bubbly flows with wide range of bubble sizes are commonly encountered in many industrial gas–liquid flow systems. To assess the performances of two population balance approaches – Average Bubble Number Density (ABND) and Inhomogeneous MUlti-SIze-Group (MUSIG) models – in tracking the changes of gas volume fraction and bubble size distribution under complex flow conditions, numerical studies have been performed to validate predictions from both models against experimental data of Lucas et al. (2005) and Prasser et al. (2007) measured in the Forschungszentrum Dresden-Rossendorf FZD facility. These experiments have been strategically chosen because of flow conditions yielding opposite trend of bubble size evolution, which provided the means of carrying out a thorough examination of existing bubble coalescence and break-up kernels. In general, predictions of both models were in good agreement with experimental data. The encouraging results demonstrated the capability of both models in capturing the dynamical changes of bubbles size due to bubble interactions and the transition from “wall peak” to “core peak” gas volume fraction profiles caused by the presence of small and large bubbles. Predictions of the inhomogeneous MUSIG model appeared marginally superior to those of ABND model. Nevertheless, through the comparison of axial gas volume fraction and Sauter mean bubble diameter profiles, ABND model may be considered an alternative approach for industrial applications of gas–liquid flow systems.     

精馏塔板上双液层三维模型的流体力学计算

在充分研究塔板上气液两相之间相互作用的基础上,运用双流体层三维模型,根据气体存在的形态将在泡沫工况下的液层在液体的厚度方向上分为喷射层和鼓泡层,将其特征与计算流体力学（CFD）结合起来,得到和实验观察比较一致的结果.     

Numerical simulation of multiphase flow in bubble column reactors. Influence of bubble coalescence and break-up

Population balance equations have been combined to a classical hydrodynamic Euler/Euler simulation to investigate the operation of a cylindrical bubble column. The MUSIG (mutiple-size-group) model implemented in the CFX 4.3 commercial software has been used. Hydrodynamic experimental variables, i.e. local axial liquid velocity and local gas hold-up, have been compared to the corresponding calculated values, showing a quite good agreement, except for the gas hold-up when the column is no more operating in the homogeneous regime. Bubble sizes have been investigated, showing that two domains of superficial gas velocities can be distinguished. In the first domain, coalescence occurs predominantly, Sauter diameter increases with the superficial gas velocity, bubble size distribution is narrow and Sauter diameter is continuously evolving along the column axis. In the second domain, break-up becomes more intensive and compensates coalescence, bubble size distribution becomes wider, since more small bubbles are formed, an equilibrium Sauter diameter appears when the superficial gas velocity increases. Furthermore an equilibrium Sauter diameter appears along the column axis, and it can be noticed that this phenomenon appears lower in the column when the gas flow rate is increased. In these two domains the characteristics of the bubbles are typical of those of the homogeneous and transition regimes.     

多孔挡板流化床气泡行为的研究

本文在内径为(?)120mm 的多孔挡板流化床中,用光导纤维法和电容法测定了 Al(OH)_3粉、铜粉和 FCC 三种不同物料体系的气泡频率和气泡速度,对操作条件和挡板参数对气泡行为的影响作了研究和分析。结果表明,在一定的气速下,挡板的开孔率、孔径和板间距(级间高径比小于3)对气泡频率和气泡速度的影响较小;对属 B 类的 Al(OH)_3粉和铜粉物料,气速对气泡频率的影响可以忽略,而对属 A 类的 FCC 物料,气泡频率随气速的增大而增大。     

装配水平筛板的气升式反应器中气液传质特性的数值模拟

利用Turbulent–Lehr组合模型对装配水平筛板的气升式反应器进行了计算流体力学(CFD)模拟,研究水平筛板对气含率、气泡直径、体积传质系数(kLa)和气液流速的影响。结果表明,筛板对气相的囤积作用和对液相的阻碍作用增加了反应器的整体气含率。筛板对气相的二次均布作用减弱了筛板和液面之间区域的气泡聚并过程,筛板筛孔对气泡的破碎作用产生了大量小于初始直径的气泡,增加了气泡比表面积(a);筛板对液相的阻碍作用提高了筛板附近的气–液相流动速度差,从而提高了该区域的液膜传质系数(kL),强化了反应器内的气液传质效果。     

EFFECT OF DISTRIBUTOR ON BUBBLE SIZE IN A CYLINDRICAL FLUIDIZED BED AT AN ELEVATED TEMPERATURE†

The effects of temperature and distributor on bubble diameter were investigated using a cylindrical fluidized bed of 147 mm in diameter. Three perforated distributors having different holes in diameter and the same ratio of holes to bed area were used. Eruption diameters of bubbles were measured using a high speed video-camera system under the following conditions: bed temperature = 300 and 600 K, bed particles = spherical glass beads of 272 μm in average size, excess gas velocity = 1-4 cm/s, and static bed height equals; 10-42 cm. The bubble diameter at 600 K was larger than that at 300 K. The difference became smaller with increasing the static bed height and with increasing the excess gas velocity. The distributor with larger holes gave larger bubbles. The effect of hole diameter of the distributor on the bubble diameter became insignificant with increasing the static bed height and with increasing the excess gas velocity.     

鼓泡破泡一体化高效精馏塔盘流动特性与CFD模拟

就痕量精馏中塔板传质效率低、需强化气液传质的问题,研究者提出了新型鼓泡破泡一体化高效精馏塔盘,通过在筛板上泡沫层高度范围内设置一层破泡装置,打破大气泡,减小气泡体积,强制界面进行更新,从而提高传质效率。采用双欧拉模型分别对鼓泡破泡一体化塔盘和筛板进行了气液流场的数值模拟,并对模型进行了验证。对比两种塔板的计算结果可以看出：在相同操作条件下,破泡装置将大气泡破裂成无数小气泡,使高气含率区域面积较普通筛板进一步增大,且气含率梯度变化更均匀;增加破泡装置后,在相同气速条件下气泡上升速度下降,气体在液层中的滞留时间延长,使鼓泡层高度增加,可显著提高传质效率,且降低了气体雾沫夹带量;破泡装置还明显改善了气相的纵向分布,气含率由塔板底部向上逐渐增大且存在明显分界;破泡装置附近湍动较剧烈,气泡破碎喷出的气体会进一步撕裂液膜,气体破碎作用会抑制气泡聚并,促进界面的快速更新更有利于传质过程的进行。研究结果可对工业塔板设计和优化提供指导。     

Effect of bubble contamination on rise velocity and mass transfer

An apparatus where individual bubbles are kept stationary in a downward liquid flow was adapted to simultaneously (i) follow mass transfer to/from a single bubble as it inevitably gets contaminated; (ii) follow its shape; and (iii) periodically measure its terminal velocity. This apparatus allows bubbles to be monitored for much longer periods of time than does the monitoring of rising bubbles. Thus, the effect of trace contaminants on bubbles of low solubility gases, like air, may be studied.Experiments were done with air bubbles of 1-5 mm initial equivalent diameter in a water stream. The partial pressure of air in the liquid could be manipulated, allowing bubbles to be either dissolving or kept at an approximately constant diameter.Both drag coefficient and gas-liquid mass transfer results were interpreted in terms of bubble contamination kinetics using a simplified stagnant cap model. Drag coefficient was calculated from stagnant cap size using an adaptation of Sadhal and Johnson's model (J. Fluid Mech. 126 (1983) 237).Gas-liquid mass transfer modelling assumed two mass transfer coefficients, one for the clean front of the bubble, the other for the stagnant cap. Adjusted values of these coefficients are consistent with theoretical predictions from Higbie's and Frössling's equations, respectively.     

Small bubble formation via a coalescence dependent break-up mechanism

A mechanism has been elucidated for the coalescence-mediated break-up of bubbles in gas-liquid systems. Images taken from dynamic systems (a coalescence cell and laboratory-scale bubble columns) show that in some instances the coalescence of two bubbles is accompanied by the formation of a much smaller daughter bubble. Following the coalescence process an annular wave is formed due to the very rapid expansion of the hole following the instant of film rupture. As the wave moves along the length of the bubble, away from the point of rupture it causes a rippling effect which distorts the newly coalesced bubble and may result in the formation of an unstable extension. Instabilities due to the annular wave pinch off a portion of this extension, resulting in the generation of a small daughter bubble. In coalescence dominated systems the process results in the generation of significant numbers of bubbles much smaller (100- diameter) than the Sauter mean diameter (3-).     

Numerical simulation of the gas–liquid flow in a laboratory scale bubble column: Influence of bubble size distribution and non-drag forces

In the present work, a computational model based on an Eulerian–Eulerian approach was used for the simulation of the transient two-phase flow in a rectangular partially aerated bubble column. Superficial gas velocities (U_G) ranging from 0.24 to 2.30 cm/s were used throughout both the experiments and the simulations. The calculated results were verified by comparing them with experimental data including measurements of gas hold-up, plume oscillation period (POP) and Sauter mean bubble diameter. The study shows the effect of mesh refinement, time-step and physical model selection, the latter regarding the role of bubble size distribution and non-drag forces, on the computational results. According to the results presented here, the representation of bubble populations using multiple size groups (MUSIG model) instead of a single group improves the prediction of the experimental parameters under study. Additionally, the results obtained after including the virtual mass force term do not differ considerably from those obtained including only the drag force. On the contrary, as a consequence of introducing the lift force term into the model, the gas hold-up is overestimated and a non-symmetric bubble plume oscillation appears, a fact that is not experimentally observed.     

Radial Profiles of Gas Bubble Behavior in a Gas‐Liquid Bubble Column Reactor under Elevated Pressures

The effects of operating conditions on radial variation of gas holdups, bubble swarm rising velocity, and Sauter mean diameter were investigated in a bubble column reactor under elevated pressures using a conductivity probe method. Air served as gas phase and tap water as liquid phase with varying gas velocity and pressure. All three parameters increased constantly with higher superficial gas velocity. Maximum holdup, velocity, and Sauter mean diameter were found at the center of the cross section. Two different cases for Sauter mean diameter distribution were observed. The gas holdups increase continuously with higher system pressure, but decrease for bubble swarm rising velocity and Sauter mean diameter. According to experimental results, an empirical correlation of the gas holdup profiles is presented.     

Inviscid bubble formation on porous plates and sieve plates

We experimentally and theoretically investigate the formation of bubbles on wetted and non-wetted sieves and porous plates, which are submerged in an inviscid liquid. It is assumed that the process of bubble formation occurs as if each bubble were alone on the plate. The interaction between orifices is modeled by estimating the gas flow field caused by a growing bubble. Depending on the material below the plate, a growing bubble influences the formation of bubbles at other orifices either in its vicinity or over the whole plate, and bubbling sites are regularly or irregularly distributed over the plate. Values for the volume fluxes at which the bubbling behavior changes are given, e.g., the minimum gas volume flux at which a disperser becomes effective or the volume flux at which growing bubbles densely populate the plates.For an example system of air in water we discuss the four cases of wetted sieve, non-wetted sieve, wetted porous plate and non-wetted porous plate. We present data for a non-wetted porous plate and find fair agreement between theoretical predictions and experiment.     

Bubble mechanisms and characteristics at pore scale in a packed-bed reactor

An image processing technique was used to study dominant bubble mechanisms in a two-dimensional packed-bed at pore level under the bubbly flow regime. Bubble breakup and coalescence were identified as dominant mechanisms using a large number of image samples. Two types of coalescence mechanisms were identified that occur due to compression and deceleration associated with the bubbles and three breakup mechanisms were identified that are result of liquid shear force, bubble acceleration, and bubble impact. Data on various two-phase parameters, such as local void fraction, bubble velocity, size, number, and shape were obtained from the images. Results indicated that when a flow regime changed from bubbly to either trickling or pulsing flow, the number of average sized bubbles significantly decreased and the shape of the majority of the bubbles was no longer spherical. Although a mean bubble velocity of all sized bubbles was uniform for given gas and liquid superficial velocities, individual bubble velocities were quite different depending on the bubble location in the pore. The present bubble size distributions were compared with previous studies and the results on bubble size are in general agreement.     

S型垂直筛板流化床的性能

在S型垂直筛板流化床中,以FCC催化剂和空气作为流化介质,对垂直筛板流化床的流化性能进行冷模实验研究,考察了板孔气速、固体循环量和帽罩结构对床层压降和帽罩提升量的影响,用提升量收集器测量帽罩提升量。实验结果表明,床层压降随板孔气速和帽罩底隙高度的增加而增加,而随塔板开孔直径、帽罩开孔直径和开孔率的增加而下降。帽罩提升量则随板孔气速、固体循环量、塔板开孔直径及帽罩底隙高度的增加而增加;随帽罩开孔直径的增加,提升量先增加后下降。     

Analysis of bubble behaviors in bubble columns using electrical resistance tomography

The distribution of gas holdup, the rise velocity of gas bubble swarm and the Sauter mean bubble size are estimated with a small diameter laboratory scale bubble column using electrical resistance tomography (ERT). The theory of gas disengagement based on ERT methods has been developed for estimations of bubble size and bubble rise velocity. The gas holdups of large bubble swarm and small bubble swarm, the distribution of both bubble size are derived through the analysis of gas disengagement based on the differences of the rise velocity of bubble swarm at the cross-section imaged by electrical resistance tomography. Experimental results are in very good agreement with correlations and conventional estimation obtained using pressure transmitter methods. The proposed methodology can be also used as an analysis tool for quantifying and optimizing the performance of other types of complex reaction systems.