Hydrodynamics and mass transfer study of aliphatic alcohols in airlift reactors

Gas holdup and gas–liquid mass transfer coefficient were considered in an external airlift reactor. Air was sparged through some aliphatic alcohols (methanol, ethanol, n-propanol, and n-butanol) with different concentrations (0–1%, v/v). It was observed that gas holdup and mass transfer coefficient increased with increasing the number of carbons in alcohols. Furthermore, an increment in alcohols concentration increased gas holdup and mass transfer coefficient. The same behavior was observed in external and internal loop airlift reactors although gas holdup and mass transfer coefficient values were less than those of internal airlift reactor. According to the experiments, two correlations for gas holdup and mass transfer were developed.     

Hydrodynamics of a slurry airlift reactor at high solid concentrations

The hydrodynamics of a slurry airlift reactor at high solid concentrations were experimentally studied. The influences of the average solid concentration, superficial gas velocity and particle size on the radial and axial profiles of the solid holdup, average gas holdup and liquid circulation velocity were investigated. The local solid holdup was measured with an electrical conductivity probe. At low solid concentrations or high superficial gas velocities, the radial profile of the solid holdup was uniform. At high solid concentrations, the radial profile of the solid holdup was nonuniform, with higher values near the wall. This radial nonuniformity increased with decreased superficial gas velocity or increased average solid concentration. The axial profile of the cross-sectional average solid holdup was uniform at all conditions in this work, even at high solid concentrations. The average gas holdup and liquid circulation velocity increased with the superficial gas velocity but decreased with the average solid concentration. A mathematical model based on the balance of the transverse lift force and turbulent dispersion force was proposed to predict the radial profile of the solid holdup. Reasonable predictions were obtained from this model with an adjustable model parameter.     

Simultaneous effect of particle size and solid concentration on the hydrodynamics of slurry bubble column reactors

The simultaneous effect of particle size and concentration on the total gas holdup of slurry bubble column reactors was investigated in this work. The total gas holdup was measured for air–water–glass beads systems. Three solid concentrations and three particle diameters were used. It was found that increasing particle size at high constant concentration decreases gas holdup. Moreover, increasing solid concentration decreases gas holdup and this decreasing effect is higher for larger particles. Also, solid particles have two effects on hydrodynamics, namely, changing the viscosity and density of the liquid phase as well as hindering the bubbles from rising within the column by the collision phenomenon. Therefore, a novel correcting factor was introduced to correct the gas holdup. The hindering factor considers both the collision efficiency affected by the particle size as well as the solid concentration. A novel correlation was developed to predict the experimental data of the three-phase gas holdup.     

剪切变稀体系同心双轴搅拌釜内的气液分散模拟

气液搅拌设备因其良好的适用性被广泛应用于过程工业中。为更好地比较不同工况下剪切变稀体系中的气液分散情况,通过实验研究整体气含率和相对功耗确定适宜的转动模式,进而模拟研究表观气速、体系黏度、搅拌转速对气含率和气泡尺寸的影响。结果表明,相同功率下内外双桨反向旋转模式在理想气液分散条件下,相较于单轴内桨和内外双桨同向旋转模式具有更高的气含率和更好的气体泵送能力;表观气速的增加有利于气泡的均匀分散,但气泡尺寸也会随之增大;有效黏度的增加使得搅拌桨的影响区域变小,不利于气泡的均匀分散,气泡尺寸也随之增大;搅拌转速的增加使得循环涡流的影响区域变大,高气含率区不断扩大。     

Effect of particles on hydrodynamics and mass transfer in a slurry bubble column: Correlation of experimental data

The effects of particle concentration and size on hydrodynamics and mass transport in an air–water slurry bubble column were experimentally studied. When the particle concentration α_s increased from 0% to 20%, the averaged gas holdup decreased by ~30%, gas holdup of small bubbles and gas–liquid volumetric mass transfer coefficient decreased by up to 50%, while the gas holdup of large bubbles increased slightly. The overall effect of particle size was insignificant. A liquid turbulence attenuation model which could quantitatively describe the effects of particle concentration and size was first proposed. Semi-empirical correlations were obtained based on extensive experimental data in a wide range of operating conditions and corrected liquid properties. The gas holdup and mass transfer coefficient calculated by the correlations agreed with the experimental data from both two-phase and three-phase bubble columns, with a maximum error <25%.     

Determining Cyclone Particle Holdup by Pressure Drop for a CFB Boiler

An experimental study was conducted to assess the possibility of determining particle holdup by measuring the pressure drop of a conventional cyclone used in a circulating fluidized bed (CFB) boiler. It was found that within a wide range of inlet solid concentrations, i.e., 0.54–4.42 kg/kg‐gas, the cyclone pressure drop increased linearly with inlet solid concentration at a given gas velocity, while the pressure drop between the dust exit and the vortex finder of the cyclone remained almost constant. Since particle holdup increases virtually linearly with solid flow rate, the particle holdup in the cyclone can be derived from the cyclone pressure drop, and therefore, an equation set was proposed to calculate the particle holdup from the cyclone pressure drop.     

Hydrodynamics of an annulus airlift reactor

The hydrodynamics of an annulus airlift reactor (AALR) was studied and compared with that of a slurry bubble column reactor (SBCR) with silica sands of 75-125 μm in size as solids, city tapping water as liquid phase, and air as gas phase in the present investigation. The effects of superficial gas velocity and solids concentration on gas holdup and solids distributions were investigated. The results showed that the local average gas holdup decreased along the column height, and the average gas holdup decreased with the increasing solids concentration, but this tendency became less at higher solids concentrations. It was found that the effect of superficial gas velocity on axial solids distribution was negligible over the gas velocity range investigated, as long as the solids in the column could be suspended. Increasing solids concentration led to flatter axial solids holdup profiles. The axial distributions of solids concentration and gas holdup in the AALR were much more uniform than those in the SBCR, and slurry circulation in the AALR damped the effects of increasing solids concentration on the hydrodynamics. These advantages of an AALR over a SBCR are especially important for some catalytic reaction processes in three-phase systems such as Fischer-Tropsch synthesis and methanol synthesis.     

Liquid Holdup in a Pilot‐Scale Turbulent Contact Absorber – An Experimental and Comparative Study

Liquid holdup in a turbulent contact absorber was determined experimentally. Experiments were performed in a 44.7 cm diameter Perspex column. Hollow spherical high‐density polyethylene balls were used as packing. The effect of liquid and gas velocities, static bed height, diameter and density of packing on liquid holdup was investigated for the range of gas velocities greater than minimum fluidization velocities. Also, the effect of gas and liquid distributors on liquid holdup was studied. Correlations for liquid holdup were developed and compared with those in the literature. It was observed that liquid holdup increased with the increase in liquid velocity, packing density, and the decrease in static bed height. Liquid holdup also increased with gas velocity when the gas distributor section was included, while no effect was observed for the bed. Lack of information on the contribution of liquid and gas distributors seems to be the logical explanation for the wide variation in data reported in the literature.     

Experimental analysis and development of correlations for gas holdup in high pressure slurry co-current bubble columns

The effect of liquid and gas velocities, solid concentrations, and operating pressure has been studied experimentally in a 15 cm diameter air-water-glass beads bubble column. The superficial gas and liquid velocities varied from 1.0 to 40.00 cm/s and 0 to 16.04 cm/s, respectively, while the solid loading varied from 1 to 9%. The gas holdup in the column was reduced sharply as we switched from batch to co-current mode of operation. At low gas velocity, the effect of liquid velocity was insignificant; while at high gas velocity, increasing liquid velocity decreased the gas holdup. Drift flux approach was applied to quantify the combined effect of liquid and gas velocities over gas holdup. For co-current three phase flows, the gas holdup decreased with increase in solid loading for all pressures. But for batch operations, when solid loading was 5% or more, settling started leading to higher gas holdup. Increasing pressure from atmospheric conditions increased the gas holdup significantly, flattening asymptotically.     

MIXING CHARACTERISTICS IN SLURRY STIRRED TANK REACTORS WITH MULTIPLE IMPELLERS

Hydrodynamic parameters such as power consumption, gas holdup, critical impeller speed for solid suspension and mixing time were measured in slurry stirred tank reactors with multiple impellers. The experiments were mainly conducted in a stirred tank of 0.2mi.d. with baffles. It contained two four-pitched blade downflow turbines for gas dispersion and one Pfaudler type impeller for solid suspension. As a part of scaling studies, additional experiments were also carried out in a larger stirred tank reactor (0,8m i.d.) geometrically similar to the smaller one. Glass beads and polymeric particles were used as a solid phase. Solid concentration was in the range of 0-20% (K/K). Tap water and methanol were used as a liquid phase

The power consumption decreased due to an introduction of gas and the presence of solids caused a decrease in the extent of reduction in power consumption. A correlation for power consumption in aerated slurry systems was proposed, It was found that the presence of solids is responsible for a decrease in gas holdup. A new correlation for gas holdup in gas-liquid-solid three-phase stirred tank reactors was developed. It fit the present experimental data reasonably. The critical impeller speed for solid suspensions increased with increasing gas flow rate. However, its increase was rather smaller as compared with the predictions of the correlations available in the literature. We proposed a correlation of the critical impeller speed for solid suspension in the presence of gas. The mixing time complicatedly increased or decreased depending on gas flow rate, impeller speed, solids type and concentration.     

喷射环流膜生物反应器水力特性的研究

通过清水试验,确定了喷射环流膜生物反应器的最大吸气量,探讨吸气量、循环水量等因素对气含率、液相循环速度的影响,测定膜组件加入前后的气含率、液相循环速度大小,并与已有的生化反应器进行了比较.结果表明,装置的最大吸气量可达到0.67 m~3·h~(-1)吸气量是影响气含率的主要因素,并且随着吸气量的增加,气含率也随着增加;循环水量是影响液相循环速度的主要因素,随着循环水量的升高循环液速升高;膜组件的加入对气舍率、液相循环速度的影响非常小,与已有的生化反应器比较,JLMBR在水力特性方面具有明显的优势.所得结果可以为该类反应器的放大设计提供借鉴.     

Dynamic and local gas holdup studies in external loop recirculating airlift reactor with two rolls of fiberglass packing using electrical resistance tomography

BACKGROUND: Airlift bioreactors have been used extensively in biotechnology industries in recent years in a variety of arrangements and applications. The insertion of packing inside the bioreactors has the potential to provide high productivity within a compact size through utilizing immobilized species. RESULTS: A novel recirculating external loop airlift bioreactor that has two rolls of fiberglass packing and a gas distributor in between was designed and built. Electrical resistance tomography (ERT) images showed that the gas holdup increased after installing the packing and the gas distributor. Gas holdup in the riser increased with decreasing static liquid height in the bioreactor. This decreased the liquid superficial velocity, which contributed to a higher gas holdup in the bioreactor. Results also showed that riser gas holdup varied slightly with different sparger configurations. Higher gas holdup increases the oxygen mass transfer rate by increasing the residence time and interfacial mass transfer area. CONCLUSION: ERT results showed that fiberglass packing with an installed gas distributor in bioreactors can achieve higher gas holdup at higher superficial gas velocity. This can contribute to improved conversion in bioreactors with packing through utilizing higher biomass concentrations and higher oxygen concentration. © 2012 Society of Chemical Industry     

典型内循环生物流化床的水力及充氧特性研究

以挂膜后的活性碳为载体，在不同投加率下研究了典型内循环生物流化床的水力特性及充氧特性。实验结果表明，流体循环时间及混合时间均随气速及栽体投加量增加而缩短；循环时间随液速增大而缓慢减小；混合时间随液速变化规律较复杂；各区气相含率随气速及载体含率的增大而升高；且受气速影响显著；KLa随气速、栽体投加率增加总趋势为增大，但幅度较小。     

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。     

Gas holdup in a two‐phase reversed flow jet loop reactor

Experimental investigations have been carried out in Reversed Flow Jet Loop Reactor (RFJLR) to study the influence of liquid flow rate, gas flow rate, immersion height of two‐fluid nozzle in reactor and nozzle diameter on gas holdup without circulation, that is, gas–liquid mixture in draft tube only (E_gd) and gas holdup with circulation loop (E_g). Also critical liquid flow rate required for transition from draft tube to circulation loop has been determined. Gas holdup was measured by isolation valve technique. Gas holdup in draft tube and circulation loop increased with increase in liquid flow rate and gas flow rate. It is observed that the increased flow rate is required for achieving a particular value of gas holdup with larger nozzle diameter. Nozzle at the top edge of draft tube have higher gas holdup as compared to other positions. It has been noted that, no significant recirculation of gas bubbles into the top of draft tube from annulus section has been observed till a particular liquid flow rate is reached. A plot of gas holdup with no circulation and with circulation mode determines minimum liquid flow rate required to achieve complete circulation loop. Critical liquid flow rate required to achieve complete circulation loop increases with increase in gas flow rate and is minimum at lowest immersion height of two‐fluid nozzle.     

Gas holdup in pulp fibre suspensions: Gas voidage profiles in a batch-operated sparged tower

Gas holdup in a semi-batch operated slurry (pulp fibre suspension) bubble column was investigated for two pulp types (softwood and hardwood kraft pulps) over a range of suspension mass concentrations (C_m=0–9% by mass) and superficial gas velocities (U_g=0.0027–0.027 m/s). Three techniques were used: height difference between gassed and ungassed operation; pressure difference as a function column height; and electrical resistance tomography (ERT). Depending on the technique used the average, axial and radial holdup profiles could be determined. In the pulp suspensions, the ERT determined gas holdups correlated well with those determined using the differential height method. In water, the ERT determined gas holdups were significantly lower, but the agreement was significantly improved by increasing the background conductivity by adding 1 g/L salt to the water. This, however, reduced the overall gas holdup due to the effect of the electrolyte on bubble coalescence. Other differences between the three measurement techniques were attributed to limitations in the detection methods and the averaging procedures used to compare results.The presence of pulp fibres reduced gas-holdup at all gas flow rates and suspension concentrations studied and is attributed to increased bubble coalescence which increases bubble size and consequently bubble rise velocity through the suspension. Gas holdup (as determined by ERT) increased with column height. The radial gas profiles were non-uniform and more peaked than the corresponding water profiles. At low suspension concentrations this was attributed to asymmetric suspension recirculation within the column. As suspension concentration increased, channels formed in the suspension with the average void fraction leveling off to a plateau.     

气液向上或向下并流固定床动持液量测定

引　言固定床也可用作气 -液 -固三相反应器 ,其中气体和液体可并流向下或并流向上通过催化剂颗粒床层而发生化学反应 .它广泛应用于各种化工过程[1].床层中的动持液量是一个估算液体平均停留时间和液膜厚度必不可少的参数 ,也是反应器内液 -固接触效率的一个量度 ,它直接影响催化剂的润湿程度和反应器的性能 ,因此持液量常是关联固定床 (或滴流床 )反应器内传质系数和传热系数的变量之一 .随着化工工业的发展 ,往往要进行原料脱杂质处理过程 (一般是加氢过程 ) ,需要足够的催化剂床层持液量 .工业上采取催化剂颗粒小型化、异形化和脉动操作…     

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.     

Local hydrodynamic properties of gas phase in an internal-loop airlift reactor

The local hydrodynamic properties of the gas phase in an internal-loop airlift reactor were investigated in this study. The hydrodynamic properties including gas holdup, bubble velocity and bubble chord length were measured by dual electrical resistivity probes. The chord length distribution was then transformed to the bubble size distribution by modeling the bubbles as ellipsoids. It was found that the gas holdup increased with decreasing bubble velocity. In addition, most bubbles tended to rise along the riser central axis. Thus, the gas holdup in the axis was higher. The bubble size, bubble velocity and gas holdup were relatively constant in the axial direction of the riser except in the zones near the gas sparger and the gas–liquid separator. The bubble velocity became slower when the bubbles approached the gas–liquid separator. Moreover, the bubble size and bubble velocity for the three-phase system were relatively insensitive to the radial direction compared to those for the two-phase system. It was also found in this study that the bubble rise velocity and bubble size for the three-phase system were lower than that for the two-phase system. However, the gas holdup for the three-phase system were higher than that for the two-phase system due to bubble breakage caused by the solid particles.     

Bubble and gas holdup characteristics in a bubble column of CMC solution

The local bubble behavior such as holdup, bubble frequency, bubble size and rising velocity in a bubble column of CMC solution was measured using the eleclroresislivity probe technique, and the effects of gas velocity and CMC concentration on the behavior were investigated. Also, the total gas holdup was measured from the liquid level in the column, and its relation with gas velocity and CMC concentration was studied. Two correlations of mean bubble size and total gas holdup with dimensionless groups, composed of gas velocity and physical properties of gas and liquid, were obtained from the experimental results.