一种新型搅拌桨多相混合性能研究

提出了一种新构型的搅拌桨一错位桨,并以空气-水-石英砂三相体系为研究对象,与传统的径流桨(Rushton桨)和轴流桨(斜叶桨)在功率消耗、混合时间、气体循环方面进行了比较.结果表明,错位桨相对于传统Rushton桨,功率消耗降低.适应气速范围广,轴向混合能力明显提升;在同等条件下与斜叶浆相比,气体分散能力强,混合时间少.这种新型桨能克服径向流叶轮在轴向混合方面能力的缺陷,有较好的潜在工业应用价值.     

疏水Rushton搅拌桨的减阻性能

为了减小搅拌阻力与功耗,本研究提出了疏水叶片搅拌桨的设想。首先采用数值模拟的方法,对非疏水Rushton桨搅拌容器内的流场进行了模拟,通过与文献中实验结果的对比,验证了数值模型和模拟方法的可靠性。随后研究了湍流状态下疏水Rushton搅拌桨的流体动力学性能,分析了不同疏水状态下的流场结构、剪应力和压力分布以及减阻效果和搅拌功耗,并与非疏水桨进行了对比。结果表明,疏水处理后Rushton桨搅拌容器内的流场没有明显变化,但流体的轴向泵送能力有所增强,高速度区域略有扩大,超疏水时效果更明显。疏水处理可降低Rushton桨的剪应力和桨叶前后表面间的压差,具有减阻效果,超疏水时减阻幅度高达39.56%。另外,疏水Rushton桨的搅拌功耗有所降低,与非疏水桨相比,超疏水桨的功率准数降低了8.53%,具有显著的节能效应。     

固-液搅拌槽内桨型对颗粒悬浮特性影响的实验和模拟研究

以水为液相,玻璃珠为固相,在固-液搅拌槽内比较了传统径向流Rushton桨、轴向流下推式45°六斜叶桨以及新型半折叶搅拌桨的功耗、泵送能力和对固体颗粒的悬浮效果。并应用CFD （Computational fluid dynamics）方法研究了不同搅拌桨操作下颗粒的轴向速度分布。结果表明：在相同转速下,Rushton桨的功耗最大,新型半折叶桨与下推式45°六斜叶桨的功耗接近;新型半折叶桨的流量准数最大,泵送能力最强;在固-液体系中,新型半折叶桨与下推式45°六斜叶桨的流型类似,但3种桨中新型半折叶桨对固体颗粒的悬浮效果最好。     

基于正交法及Kriging模型的生物反应器搅拌桨优化设计

针对搅拌桨标准中设计参数区间大，生物反应器搅拌桨设计时参数难以确定、剪切力及搅拌功率缺少参考依据的问题。基于正交法及CFD对200 L生物反应器搅拌桨进行优化，由极差分析得到三斜叶桨不同搅拌桨直径、桨叶宽度、搅拌桨中心离底距离和转速对桨叶剪切速率和搅拌功率的影响规律，并结合流场分析选择有利于细胞培养的设计方案。根据正交优化方案确定多目标优化变量区间，以剪切力和搅拌功率为优化目标，基于Kriging模型和多目标遗传算法提出了有利于细胞培养的三斜叶桨设计变量区间，并优化得到桨叶平均剪切力为0.789 Pa、搅拌功率为0.395 W的搅拌桨最优参数组合，为生物反应器不同实际需求下的设计、优化提供了方法及数据参考。     

基于介尺度PBM模型的生物反应器放大模拟及实验研究

对于通气搅拌式工业生物反应器的放大设计而言,精确预测气泡尺寸和体积传质系数非常重要,因此需要建立合适的气泡聚并和破碎模型,以保证反应器的高效操作。以5 L通气搅拌式生物反应器为对象,以气泡尺寸和体积传质系数的实验数据为基准,模拟并考察了两种聚并模型和四种破碎模型对生物反应器内流体流动行为以及传质能力的影响。结果表明,基于介尺度理论的修正聚并模型与考虑黏流剪切的破碎模型组合,所得模拟结果与实验数据吻合最好,这为大型生物反应器的桨型优化提供了模型基础。因为工业化生物发酵通常是在大型生物反应器中进行,搅拌桨型对生物反应器效能至关重要,故本研究在选定最优气泡聚并破碎模型的基础上,通过叶轮末端剪切力相等的放大原则将5 L通气搅拌式工业生物反应器放大到400 m³,同时考察了六斜叶圆盘搅拌桨、非对称式抛物线搅拌桨、布鲁马金式搅拌桨以及六直叶圆盘搅拌桨等桨型组合对气泡破碎能力和气体分散效果的影响,并通过综合对比气含率、体积传质系数等参数,得到400 m³通气搅拌式生物反应器的最优桨型组合。     

Rushton搅拌釜内的气含率分布及其流动特性的模拟

利用雷诺应力模型结合多参考系法模拟了Rushton气液搅拌釜内的气液流动特性,得到了不同截面处气含率大小分布规律,并与γ射线CT实验所测得的结果做了比较,发现除排出流区CFD模拟值与实验值相差较大外,上循环区与下循环区都吻合得较好;结果表明排出流区的气含率最高,在搅拌桨高速旋转下,流体沿叶片端面急速排出,湍动也十分强烈...     

搅拌槽中酯化反应热失控的CFD模拟

热失控是化工过程中常见的安全风险之一。在间歇釜式反应器中,桨叶的机械转动可以增强流体的循环流动、湍流强度、混合程度以及传热,进而有效防范热失控。防控效果与反应器结构和搅拌桨型密切相关。针对丙酸异丙酯酯化反应,采用计算流体力学模拟研究了桨型(Rushton桨、30o PBT桨及60o PBT桨)、转动方向和挡板对釜式反应器内温度演化的影响,从流动结构方面分析了原因。基于散度的失控判据比较了三种搅拌桨抑制热失控的能力,抑制能力为Rushton桨＞30° PBTD桨＞60° PBTD桨。本研究可为搅拌反应器热失控的优化设计提供一定的理论依据。     

基于CFD的羰基合成反应器放大及桨型优化

对带三层组合搅拌桨的羰基合成反应器进行计算流体力学(CFD)模拟,对比分析了反应器放大前后及底层桨优化前后反应器内流场、气含率分布和搅拌功率等参数的变化规律,研究反应器的放大及桨型优化效果。模拟结果表明,当釜径从4 600mm放大到5 200mm时,反应器内流场分布和气含率分布与放大前相似,具有相近的混合效果,说明放大后的搅拌桨能达到预定的效果,所选用的放大准则是合理的。采用新弯叶径向流搅拌桨替换底层直叶径向流桨后,发现该搅拌桨不仅具有更好的搅拌混合效果,而且搅拌功率较直叶桨搅拌功率降低了约51%。     

生物搅拌反应器内混合情况的CFD模拟及在发酵中的应用

用计算流体力学(CFD)方法模拟了50 L生物反应器中不同的,搅拌桨组合对搅拌流场、混合时间的影响,并从流体力学角度对生物反应器搅拌桨组合进行了优化.将模拟优化结果用于重组大肠杆菌发酵过程,结果表明,优化后的搅拌桨组合可以改善发酵罐内部的流场和气体分布,能够明显降低获得相同溶氧所需的转速和最大通气量,而菌体生物量和产酶水平也略有提高.     

多层桨搅拌槽内的液-液分散特性

在直径为0．476m的椭圆底圆柱形搅拌槽内,以水／航空煤油及水／环己烷为实验体系,研究了Rushton涡轮式搅拌桨(RT-6)、六半椭圆管涡轮式搅拌桨(HEDT)、折叶轴流式搅拌桨及翼形轴流式搅拌桨(CBY)的6种不同组合桨的液-液分散特性,用取样法测定了分散相体积分数的轴向分布及体系澄清时间。结果表明,组合桨中的底桨在液-液分散中发挥了主要作用。单位体积输入功率相同时,底桨为CBY的组合桨进行液-液分散后,液滴的平均滴径最小,体系澄清时间较长;底桨为HEDT的组合桨的分散效果次之;底桨为RT-6的组合桨因滴径分布较宽,虽然平均滴径最大,但澄清时间也较长。     

A CFD model for predicting the flow patterns of viscous fluids in a bioreactor under various operating conditions

Computational fluid dynamics simulation is becoming an increasingly useful tool in the analysis and design of simultaneous saccharification fermentation (SSF) and saccharification followed by fermentation process (SFF). To understand and improve mixing and mass transfer in a highly viscous non-Newtonian system, it was necessary to simulate the flow behavior in this bench scale bioreactor (BioFlo 3000). This study focused on designing a high concentration medium agitation system for such a process using the commercial computational fluid dynamics package Fluent (V. 6.2.20) and its preprocessor Mixsim (V. 2.1.10). The objective of this study is to compare performance of various designs of a bioreactor and identify the flow pattern and related phenomena in the bench scale tank. The configuration of the physical model for simulating a mixing tank with a Rushton impeller consists of an ellipsoidal cylindrical tank with four equally spaced wall mounted baffles extending the vessel bottom to the free surface, stirred by a centrally located six-blade Rushton turbine impeller. Simulations were performed with the original and a modified design in which the lower bottom shaft mounted a Lightnin A200 impeller. The results suggest that there is a potential for slow or stagnant flow between top impellers and bottom of the tank region, which could result in poor nitrogen and heat transfer for highly viscous fermentations. The results also show that the axial velocity was significantly improved for the modified geometry in the bottom of the tank.     

刚柔组合搅拌桨与刚性桨调控流场结构的对比

高黏度流体处于层流状态时,普遍存在的混合隔离区,降低了流体的混合效率。减小或消除隔离区,是实现流体高效混合的基本途径。采用实验研究与数值模拟相结合的方法,对刚性六直叶涡轮桨（刚性桨）和刚柔组合六直叶涡轮桨（组合桨）的流场结构进行研究,对比分析了两种桨叶在相同功耗（3 kW·m^-3）时的轴向、径向和切向的速度矢量图、速度云图以及速度分布散点图。结果表明,刚性桨的能量集中在桨叶尖端部分,远离桨叶区域的流体速度很小甚至为0 m·s^-1;而组合桨可将能量从桨叶尖端扩散至全槽,使槽内流体均具有一定的流速,提高了混合效率,且显色实验与数值模拟结果一致,组合桨体系的混合隔离区在短时间内就可消除,混合良好,而刚性桨体系的混合隔离区始终存在,混合效果不佳。     

Measurement of gas and liquid flows in stirred tank reactors with multiple agitators

The gas flow in a 3:1 aspect ratio vessel agitated by triple Rushton turbines has been measured by an ultrasound Doppler probe and by means of residence time studies. Strong recirculation around each impeller is found which fits in well with the compartmentalisation found in earlier liquid mixing studies. Surprisingly, when two axial A315 impellers above a Rushton turbine were used, gas recirculation around each impeller was still found. Study of the liquid phase mixing by a decolourisation technique confirmed that the gas flow essentially destroyed the strong axial liquid flow expected. Indeed, even under unaerated conditions, compartmentalisation was found between each impeller.     

Gas-Liquid Mixing in a Grid-Disc Impeller Stirred Tank

To enhance the gas-liquid mixing performance in stirred tanks, the grid-disc impeller was designed by replacing the solid disc of the standard Rushton impeller with a grid disc. Gas-liquid hydrodynamics of the new impeller was studied by employing the Eulerian-Eulerian two-phase model coupled with the dispersed k-ε turbulence model. Rotation of the impeller was simulated with the multiple reference frame method. Flow field, gas holdup, and power consumption were investigated and compared with the standard Rushton impeller. The numerical method was validated by comparing the gas holdup with literature. The grid-disc impeller performed better than the Rushton impeller in terms of gas dispersion performance, axial pumping capacity, and energy requirement, indicating its potential for gas-liquid mixing applications.     

Effect of shaft eccentricity on the laminar mixing performance of a radial impeller

The effects of impeller eccentricity and Reynolds number on the mixing performance were studied for Rushton turbine stirred tank systems operated in the laminar regime (Re < 10). A digital image analysis of an acid–base decolourization reaction was used to characterize the mixing efficiency. Results show that both parameters have remarkable effects on the destruction of the toroidal segregated regions surrounding the Rushton turbine in laminar flow. Criteria are given to prevent the formation of these segregated regions in a tank. It is also shown that shaft eccentricity does not impact on the impeller power consumption.     

Characterization on the hydrodynamics of a covering-plate Rushton impeller

A modified Rushton impeller with two circular covering-plates mounted on the upper and lower sides of the blades was designed. There are gaps between the plates and the blades. The turbulent hydrodynamics was analyzed by the computational fluid dynamics (CFD) method. Firstly, the reliability of the numerical model and simulation method was verified by comparing with the experimental results from literature. Subsequently, the power consumption, flow pattern, mean velocity and mixing time of the covering-plate Rushton impeller (RT-C) were studied and compared with the standard Rushton impeller (RT) operated under the same conditions. Results show that the power consumption can be decreased about 18%. Compared with the almost unchanged flow field in the lower stirred tank, the mean velocity was increased at the upper half of the stirred tank. And in the impeller region, the mean axial and radial velocities were increased, the mean tangential velocity was decreased. In addition, the average mixing time of RT-C was shortened about 4.14% than the counterpart of RT. The conclusions obtained here indicated that RT-C has a more effective mixing performance and it can be used as an alternative of RT in the process industries.     

Application of KHX Impeller in a Low-shear Stirred Bioreactor☆

In our previous work, a low-shear stirred bioreactor was explored. With a pitched blade turbine impeller downflow (PBTD) used, the shear stress generated is high compared with that in some low shear ax...     

Gas-liquid hydrodynamics in a vessel stirred by dual dislocated-blade Rushton impellers☆

Towards the objective of improving the gas dispersion performance, the dislocated-blade Rushton impeller was applied to the gas–liquid mixing in a baffled stirred vessel. The flow field, gas hold-up, dissolved oxygen, power consumption before and after gassing were studied using the computational fluid dynamics (CFD) technique. Dispersion of gas in the liquid was modelled using the Eulerian–Eulerian approach along with the dispersed k–εturbulent model. Rotation of the impeller was simulated with the multiple reference frame method. A modified drag coefficient which includes the effect of turbulence was used to account for the momentum exchange. The predictions were compared with their counterparts of the standard Rushton impeller and were validated with the experimental results. It is concluded that the dislocated-blade Rushton impeller is superior to the standard Rushton impeller in the gas–liquid mixing operation, and the findings obtained here lay the basis of its application in process industries.