气升式环流反应器流动特性的数值模拟

应用Fluent计算流体力学软件,通过数值模拟的方法考察了气升式环流反应器内导流简直径、导流筒高度、导流筒安装高度以及液相进口速度对反应器内两相流动的影响。计算结果表明：导流简直径增大。液相循环量和上升段气含率均增大;导流筒高度增大,液相循环速度和循环量均增大,但是当导流筒的高径比增加到6．3后,液相循环速度和循环量反而减小;导流筒安装高度增加．液相循环速度及循环量均增大,升液管气含率降低;增加液相的进口速度,虽然上升段的液速有所增加,但是降液管循环液速及循环量基本不受影响。最后计算了反应器内液相速度随进口气速的变化情况,并与实际测量值进行了比较,模拟值和测氨值都表明随着进口气速的增加,液相循环速度随着进口气速的增加而增加．两者呈现良好的一致性。     

环隙气升式气固环流反应器内流体力学特性的理论分析

环流反应器的研究与应用一直局限于气液与气液固体系,将环流反应器移植到气固体系是一个具有独创性的探索。针对工业化中气固环流反应器的缺陷,提出了一种新型的环隙气升式气固环流反应器。并对床层空隙率、颗粒流动速度进行了实验研究与理论分析。建立了环隙区床层空隙率模型,发现环隙区床层空隙率随着环隙区表观气速的增加而增加;环隙区靠近导流筒外壁一侧颗粒流动速度明显大于靠近反应器内壁一侧,导流筒区颗粒流动速度沿径向的分布受气体分布器结构影响较大;环隙区颗粒流动速度基本不随轴向位置的变化而变化,导流筒区颗粒的流动属于密相输送,颗粒环流所受到的阻力主要集中在底部区域,其次为气固分离区,底部区域阻力大小由床层流化质量和导流筒下端距反应器底部的间隙所决定;建立了颗粒环流速度模型,发现环流速度随环隙区表观气速的增加而增加。     

中心气升式气-固环流反应器与自由流化床流动特性对比

对中心气升式气固环流反应器和自由床流化反应器的流动特性进行了对比. 结果表明,在同样的操作条件下,在轴向高度h=112~512 mm范围内,环流反应器导流筒区的径向不均匀指数RNI比自由床的RNI减小27%~36%;与自由床相比,环流反应器导流筒区的局部不均匀指数减小20%~59%,整体不均匀指数减小17%~43%,环流反应器的流化质量较高;环流反应器导流筒区和自由床内均存在气体返混现象,导流筒区各采样点示踪气体的浓度和轴向扩散系数Da,g比自由床均有所降低,在表观气速0.2~0.4 m/s范围内,导流筒区Peclet准数的绝对值比自由床增大12%~58%,说明导流筒区气体返混程度小于自由床,气-固两相流向上的宏观流动可有效抑制气体的轴向返混.     

气升式振荡环流反应器的数值模拟和结构优化

气升式振荡环流反应器（ARLR）作为一种新型的气升式环流反应器,能够有效地提高反应器的气含率和传质系数,并已得到生物发酵实验的验证。本文通过CFD的手段研究了反应器内的流动和传质状况,并利用CFD模拟和响应面分析相结合的方法,优化了反应器的结构参数,如高径比（H/D）、升液区降液区面积之比、导流筒高度等。经过实验测量,优化后的气升式振荡环流反应器与传统的气升式环流反应器相比,气含率提高了32%以上,传质系数提高了11%以上。结果表明,气升式振荡环流反应器作为生化反应器有着非常广阔的应用空间。     

中心气升式气固环流反应器中的能耗分布

基于能量平衡,推导出计算中心气升式气固环流反应器中不同区域内气固混合物流动能耗的理论模型,测量了不同区域内固相颗粒速度、气固混合物密度及床层压降,并根据实验数据确定出相应的模型参数,模型计算与实验结果吻合较好. 模型计算表明,中心气升式气固环流反应器内环隙区和气固分离区的能耗分别占反应器总能耗的近40%和30%,颗粒环流受到的阻力主要集中在这2个区域;而导流筒区及分布器影响区能耗较小,共占总能耗的近30%. 随导流筒区表观气速增加,环隙区能耗占总能耗的比重减小,气固分离区能耗所占比重增大,导流筒区和分布器影响区能耗基本保持不变. 气体分布器的安装位置对反应器内能量消耗的分布影响较大,中心气升式气固环流反应器内流动阻力更小.     

螺旋气升式内环流反应器流动特性

以水和空气两相系统为研究对象,建立螺旋气升式内环流反应器冷模实验装置,采用压差法测定导流筒升流区气含率,用电导法分别实测了升流区液相速度和反应器混合时间,研究螺旋气升式内环流反应器的流动特性,并与传统气升式内环流反应器的流动特性进行对比研究。结果表明,加螺环后反应器气含率明显增加,平均增幅为20%,升流区液相表观速度减小,混合时间增加。     

气升式三相内环流生物反应器内流体力学和传质性能的实验研究

本文通过实验研究了气升式三相内环流反应器在非牛顿流体中的流体动力学特性和传质性能.考察了表观气速、导流筒与反应器截面积之比、固体粒子加入量及CMC溶液浓度等操作条件对气升式三相内环流生物反应器中导流筒与环隙内液体循环速度、气含率、固含率及气-液体积传质系数等流动与传质特性参数的影响.     

带分段导流筒的气升式环流反应器的研究

带分段导流筒的气升式环流反应器是具有独特的两相流动和传质特性的一种多相反应器。本文用水－空气和亚硫酸钠（１０％－空气体系研究了分段导流筒环流反应器的气含率ε、氧消耗速率ｒｏ２、传质比表面ａ、以及体积传质系数ｋＬａ随表观气速变化规律，并与一段导流筒的环流反应器进行了比较。实验采用的表面气速下（０－６ｃｍ／ｓ），导流筒分段的环流反应的εｒｏ２，ａ和ｋＬａ有显著提高，平均比一段环流反应器高出１０％以上。     

气升式内循环反应器的数值模拟和结构参数

采用Euler-Euler双流体模型对内循环反应器(高1240 mm,直径165 mm,导流筒高590 mm)进行数值模拟,考察了表观气速、导流筒结构(导流筒内径比Dt/D,底隙高度)对反应器内上升区、下降区流体力学参数(气含率、液体速度)的影响. 结果显示,表观气速、导流筒内径、底隙高度对反应器气含率、液体速度有很大影响,随表观气速增加,反应器上升区、下降区气含率都增加,导流筒内径比为0.58时更易实现气液循环,底隙高度为30 mm时反应器内下降区气含率、气液速度都最小;气液分离器角度越大,进入下降区的气体越多,当气液分离器角度为45o时,能更好地实现气液循环.     

多相气升式环流反应器局部相含率和环流速度

气升式环流反应器流动特性研究大多都局限于流动参数的平均特性研究,涉及气-液-固系统流动特性的研究也较少。在200×2500mm气升式环流反应器内,实验研究了空气-水两相及空气-水-玻璃珠三相体系中局部气、固相含率以及浆(液)相环流特性等随操作条件以及液、固相物性的变化规律。结果表明:1两相体系导流筒中截面平均气含率轴向分布有局部极大值,而固体和乙醇的加入使得气含率轴向分布平缓;2两相体系循环液速在导流筒中呈径向抛物分布,在环隙分布较均匀,加入固体和乙醇后对循环速度分布和大小影响不大;3三相体系中导流筒内固含率沿轴向升高呈增大趋势,环隙内轴向分布均匀;导流筒内、外固含率径向分布均不均匀,以近壁处为大;加入乙醇后对固含率分布影响不大。     

Influences of top clearance and liquid throughput on the performances of an external loop airlift slurry reactor integrated mixing and separation

A new developed external loop airlift slurry reactor, which was integrated with gas–liquid–solid three-phase mixing, mass transfer, and liquid–solid separation simultaneously, was deemed to be a promising slurry reactor due to its prominent advantages such as achieving continuous separation of clear liquid from slurry and cyclic utilization of solid particles without any extra energy, energy-saving, and intrinsic safety design. The principal operating parameters, including gas separator volume, handling capacity, and superficial gas velocity, are systematically investigated here to promote the capabilities of mixing, mass transfer, and yield in the pilot external loop airlift slurry reactor. The influences of top clearance and throughput of the clear liquid on flow regime and gas holdup in the riser, liquid circulating velocity, and volumetric mass transfer coefficient with a typical high solid holdup and free of particles are examined experimentally. It was found that increasing the gas separator volume could promote the liquid circulating velocity by about 14.0% at most. Increasing the handling capacity of the clear liquid from 0.9 m~3·h~(-1) to 3.0 m~3·h~(-1) not only could increase the output without any adverse consequences, but also could enhance the liquid circulating velocity as much as 97.3%. Typical operating conditions investigated here can provide some necessary data and guidelines for this new external loop airlift slurry reactor to upgrade its performances.     

强制液体外循环气升式内环流反应器的流体力学特性

在有机玻璃制成的主体反应器(φ0.286 m×3.0 m)内,采用空气-水的气液两相体系考察了不同表观气速下带有中心下料管和环管式气体分布器的环流反应器内局部气含率和内环气泡上升速度的流体力学特性。结果表明：气含率均随表观气速的增大而增大;从外环流入内环的气泡数量比较少;在相同条件下外环的气含率远远低于内环的气含率;新型气体分布器的采用和液体外循环的引入可以增大床层气含率并使其分布趋于均匀,从而增大了内环中气液两相的接触面积和湍动强度,使传递过程得到强化。在此基础上采用商业软件ANSYS CFX10.0对该反应器进行了数值模拟研究,模拟结果与实验数据基本吻合,说明了基于实验验证的Euler-Euler法可以用于该反应器开发和放大研究。     

缩放型导流筒气升内环流反应器液体循环速度的研究

研究气升式内环流生物反应器液体循环速度,分别采用一种传统圆柱型导流筒和三种不同结构参数缩放型导流筒,试验条件分别为空气－水和空气－ＣＭＣ两相系统以及空气－水－树脂三相系统,试验结果表明,对于空气－水和空气－ＣＭＣ溶液两相系统以及空气－水－树脂三相系统,液体循环速度随能气速度提高而增大;     

Hydrodynamics in rectangular air-lift reactors: Scale-up and the scale-up and the influence of gas-liquid separator design

Experiments were conducted on air-lift reactors with different liquid volumes, aspect ratios, and gas-liquid separator configurations, with constant downcomer to riser cross-sectional area ratio and riser/downcomer height. Reactor performance was highly dependent on the gas-liquid separator design. By changing the latter, the mode of operation could be changed from that of a concentric tube reactor (with gas recirculation) to an external loop reactor (without gas recirculation). Liquid velocity and overall gas hold-up data were correlated to the power per liquid volume and “Disengagement Ratio”, a geometric parameter describing the separator's configuration. During scale-up, similar gas hold-ups and liquid velocities were found in laboratory and pilot reactors. A modified air-lift reactor with an enlarged channel for a gas-liquid separator also was studied.     

Modeling of modified airlift loop reactor with a concentric double-draft tube

In this work, mathematical modeling based on the tanks-in-series model was employed to characterize the performance of a novel modified airlift loop reactor. Liquid mixing characteristics, i.e., mixing time and circulation time, were employed to describe the performance of the proposed reactor. These values were determined by using the classical tracer response technique in a column 18.5 cm in diameter and 90 cm in height, and two concentric draft tubes 14 and 8.5 cm in diameter and 80 cm in height. Matlab 7.1 software was used to solve the model equations in the Laplace domain and determine the model parameter. A comparison between the numerical solution and experimental data showed that the applied model can accurately describe the behavior of a modified airlift reactor. Experimental results in the homogeneous bubble flow regime showed that the proposed airlift reactor configuration with a double-draft tube significantly improves the liquid mixing quality compared to a conventional concentric-tube airlift reactor with an identical operating volume. By using this high-performance reactor, the mixing and circulation times can be reduced by up to 48.3% and 35.5%, respectively.     

The gas–liquid contacting effect on the operation of small scale upflow hydrotreaters

The effect of reactor geometry and bed dilution on the extent of gas oil hydrodesulfurization was tested by conducting hydrodesulfurization experiments in two laboratory reactors of different scale with non-diluted and diluted beds in ascending flow. The superficial gas and liquid velocities and the catalyst bed height were kept constant while the main difference between the two reactor scales was the reactor diameter. The diluted bed of the mini-reactor showed the best performance and its results were identical in upflow and downflow mode. The differences between the performance of the mini- and the bench-scale reactor operating in upflow mode have been investigated. Reactor performance simulation was attempted by a mathematical model that takes into account axial dispersion of the liquid phase and gas–liquid mass transfer. Bench-scale reactor operation was characterized by lower mass transfer rates than the corresponding mini-scale one. Combining model predictions and mock up operation it is concluded that the stronger mass transfer resistances calculated for the bench-scale reactor are associated with poorer gas distribution through the catalyst bed. Reduction of the bed diameter results in better gas–liquid contact by forcing the gas bubbles to distribute more effectively into the liquid phase.     

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.     

EFFECTS OF AERATED LIQUID LEVEL ABOVE DOWNCOMER ON HYDRODYNAMIC CHARACTERISTICS OF AN EXTERNAL-LOOP AIRLIFT REACTOR

Effects of the aerated liquid level above the downcomer on riser superficial liquid velocity, gas flow rates, and gas holdups in individual sections were investigated in an external-loop airlift reactor. The aerated liquid level is an important operating parameter, and the valve connected to the extension tube could adjust it. As the aerated liquid level was increased up to 0.158 m, which was slightly larger than the diameter of the riser, both the liquid circulation velocity and the gas flow rate in the extension tube increased, whereas the riser gas holdup, the downcomer gas holdup, and the gas flow rate in other individual sections except the extension tube decreased. A combination of the gas-liquid separation ability and hydraulic resistance of the head region and the surface aeration could explain the impact of the aerated liquid level on the hydrodynamic characteristics. However, the effect was negligible when the aerated liquid level was higher than 0.158 m.     

EFFECTS OF AERATED LIQUID LEVEL ABOVE DOWNCOMER ON HYDRODYNAMIC CHARACTERISTICS OF AN EXTERNAL-LOOP AIRLIFT REACTOR

Effects of the aerated liquid level above the downcomer on riser superficial liquid velocity, gas flow rates, and gas holdups in individual sections were investigated in an external-loop airlift reactor. The aerated liquid level is an important operating parameter, and the valve connected to the extension tube could adjust it. As the aerated liquid level was increased up to 0.158 m, which was slightly larger than the diameter of the riser, both the liquid circulation velocity and the gas flow rate in the extension tube increased, whereas the riser gas holdup, the downcomer gas holdup, and the gas flow rate in other individual sections except the extension tube decreased. A combination of the gas-liquid separation ability and hydraulic resistance of the head region and the surface aeration could explain the impact of the aerated liquid level on the hydrodynamic characteristics. However, the effect was negligible when the aerated liquid level was higher than 0.158 m.