利用Monte Carlo方法预测精馏塔板上气泡尺寸分布及相界面积

精馏塔板上气泡尺寸分布是计算精馏塔板效率以及设计和操作精馏塔的关键参数，但对它的预测受到理论不完善、实验测试手段落后及求解计算困难等方面的限制。本文以Kolmogoroff各向同性湍流理论为基础并结合格率理论，利用计算机图形处理技术对气泡的尺寸分布现象进行观察，分析气泡的聚并和破裂机理。通过已建立的气泡破裂和聚并速率模型，得到精馏塔板上气泡粒数衡算方程。针对气泡粒数衡算方程求解的复杂性，本文采用了Monte Carlo模拟技术来简化气泡粒数衡算方程的求解。求解结果与实验数据相吻合并显示气泡尺寸分布为对数正态分布，这与其他研究者的结论相一致。由所求得的气泡尺寸分布可以进一步计算出气泡的Sauter平均直径以及气液相界面积。     

TNAZ溶析结晶动力学研究

基于分级法[1]求解1,3,3-三硝基氮杂环丁烷(TNAZ)问歇溶析结晶过程的粒数衡算方程,通过激光粒度分布测量仪测量了结晶过程中粒度分布随温度、搅拌强度、溶析剂滴加速率的变化规律.利用已建立的TNAZ间歇溶析结晶过程的粒数衡算方程求解模型得到生长动力学参数,最后用最小二乘法模拟出成核速率和生长速率方程[2].成核速率...     

1,2,3-三硝基氮杂环丁烷溶析结晶动力学研究

基于分级法[1]求解了TNAZ间歇溶析结晶过程的粒数衡算方程,通过激光粒度分布测量仪测量了结晶过程中粒度分布随不同温度、搅拌强度、溶析剂滴加速率的变化规律。利用已建立的TNAZ间歇溶析结晶过程的粒数衡算方程求解模型得到了生长动力学参数,最后用最小二乘法模拟出了成核速率和生长速率方程[2]。成核速率和生长速率是结晶模拟过程中最核心的控制条件。     

基于气泡数密度模型的气体穿越液池过程气泡特性数值模拟

采用考虑了气泡破碎和聚并的平均气泡数密度(ABND)输运方程,并与计算流体力学(CFD)中的湍流双流体模型相结合,建立CFD-ABND耦合计算模型。运用该模型对含固体颗粒气体通过冷却管穿越液池过程中液池内的气泡尺寸和气液界面浓度进行数值模拟。定量获得了液池内气泡尺寸和气液界面浓度分布,并分析了气速变化对其影响规律。结果表明:所建立的CFD-ABND模型能够对液池中的气泡尺寸及气液界面浓度分布等气泡特性进行较好预测;在靠近冷却管外壁面附近区域形成较大尺寸气泡和较高气液界面浓度;液池内隔板的存在有助于气液扰动,使液池内总体的气泡尺寸得到有效降低及气液界面浓度得到提升。     

利用Monte Carlo方法求解粒数衡算方程分析影响气泡分布的各种因素

精馏塔板上气泡尺寸分布是计算精馏塔板效率以及设计和操作精馏塔的关键参数 ,为了对它进行更好的预测 ,文中以Kolmogoroff各向同性湍流理论为基础并结合概率理论 ,利用计算机图形处理技术对气泡的聚并和破裂现象进行观察研究 ,分析气泡的聚并和破裂机理 ,采用MonteCarlo模拟技术求解粒数衡算方程 ,对影响精馏塔板上气泡尺寸分布的各种因素进行研究分析。求解结果与实验数据相吻合并显示气泡尺寸分布为对数正态分布 ,这与其他研究者的结论相一致     

反应和沉淀中的质量传递

Shun Wachi 和 Alan G.Jones 模拟研究了气-液相反应结晶过程中的传质问题,他们的分析方法是把气-液反应传质膜理论与结晶过程的质量衡算及粒数衡算结合起来考虑,以文献报导的关于 Ca(OH)_2溶液吸收 CO_2过程为例进行了模拟,结果表明用可测的晶体粒粒度分布可表征由于传质阻力而引起的空间上过饱和度分布及成核粒子的非均匀性;在低传质     

苊冷却结晶动力学的间歇动态法研究

通过添加晶种的间歇冷却结晶实验研究了苊在乙醇中的结晶动力学。由苊晶体的粒数密度数据,通过矩量变换法按粒度无关生长模型求解粒数衡算方程,采用多元线性最小二乘法回归动力学数据,得到苊在乙醇中晶体生长及成核速率方程。对动力学方程的理论分析表明:搅拌速率对二次成核影响显著,随搅拌速率增加,晶体的成核速率明显增加。同时适宜的过饱和度及较低的悬浮密度,有利于苊晶体生长。该研究为苊冷却结晶特性的辨识、粒度分布的控制及工业放大提供了重要的理论指导。     

间歇结晶过程中几种可能的粒度分布

对间歇溶液结晶过程,依据粒数衡算方程,在忽略二次过程如聚集、破碎和老化等且混合良好情况的假设下,采用分离变量方法对粒度相关生长或生长分散的粒度分布进行理论分析,推导出间歇结晶过程中的几个粒数密度与结晶时间和晶体粒度的关系式.这些关系式可以用于间歇结晶过程中,选择合适的晶体生长机理与测定线性晶体生长速率,预测粒度分布与优化结晶过程,以及回归粒度分布.结果表明,本方法在溶液结晶过程中具有良好的应用价值.     

十水硫酸钠冷却结晶动力学的研究

在混合悬浮混合排料(MSMPR)结晶器中利用连续稳态法研究了十水硫酸钠冷却结晶动力学.以粒数衡算方程为基础,利用非线性最优化方法直接拟和由实验所得的数据,确定了十水硫酸钠冷却结晶的成核和生长速率方程.结果表明,十水硫酸钠晶体生长速率与粒度相关,而且实验数据符合MJ-2模型,即G(L)=3.96×10-4exp(-2.20×104/RT)ΔC0.13[1-exp(-1.53×104L)].     

吹气发泡法制备闭孔泡沫铝发泡过程的三维数值模拟

吹气发泡法制备闭孔泡沫铝的过程实质上是搅拌流场中复杂的两相流动过程,应用计算流体力学方法分析由倾斜轴倾斜叶片引起的发泡熔池内气液两相强旋湍流流动过程. 在双流体模型基础上引入多重参考系法描述搅拌两相流场,通过分析相间相互作用及湍流模型进行封闭. 解气泡数密度函数的输运方程来分析气泡聚合和破碎引起的气泡尺寸变化. 应用体积积分的方法,计算平均及局部气含率及气泡直径. 考察了桨叶转速及气体流率对气泡直径及其分布的影响. 结果显示,气含率随桨叶转速和气体流率增加而增大;气泡直径随气体流率增加而增大,随桨叶转速增大而减小.     

Simulation of oscillatory baffled column: CFD and population balance

In the present work, an attempt has been made to combine population balance and a CFD approach for simulating the flow in oscillatory baffled column (OBC). Three-dimensional Euler-Euler two-fluid simulations are carried out for the experimental data of Oliveira and Ni [2001. Gas hold-up and bubble diameter in a gassed oscillatory baffled column. Chemical Engineering Science 56, 6143-6148]. The experimental data include the average hold-up profile and bubble size distribution in the OBC. All the non-drag forces (turbulent dispersion force, lift force) and the drag force are incorporated in the model. The coalescence and breakage effects of the gas bubbles are modeled according to the coalescence by the random collision driven by turbulence and wake entrainment while for bubble breakage by the impact of turbulent eddies. Predicted liquid velocity and averaged gas hold-up are compared with the experimental data. The profile of the mean bubble diameter in the column and its variation with the superficial gas velocity is studied. Bubble size distribution obtained by the model is compared with the experimental data.     

Experimental and modelling study of gas dispersion in a double turbine stirred tank

Gas dispersion in a double turbine stirred tank is experimentally characterised by measuring local gas holdups and local bubble size distributions throughout the tank, for three liquid media: tap water, aqueous sulphate solution and aqueous sulphate solution with PEG. For all these media, bubble coalescence generally prevails over breakage. Where average bubble size decreases, this can be attributed to the difference in slip velocity between different sized bubbles. Most of the coalescence takes place in the turbine discharge stream.A compartment model that takes into account the combined effect of bubble coalescence and breakage is used to simulate gas dispersion. The model predicts spatial distribution of gas holdup and of average bubble size, with average bubble size at the turbines as an input. Reasonable agreement between experiment and simulation is achieved with optimisation of two parameters, one affecting mainly the slip velocity, the other related mainly to the bubble coalescence/breakage balance. Different sets of parameters are required for each of the three liquid systems under study, but are independent of stirring/aeration conditions. The model only fails to simulate the smaller average bubble diameters at the bottom of the tank.     

Eulerian–Lagrangian simulation of bubble coalescence in bubbly flow using the spring-dashpot model

The Eulerian–Lagrangian simulation of bubbly flow has the advantage of tracking the motion of bubbles in continuous fluid, and hence the position and velocity of each bubble could be accurately acquired. Previous simulation usually used the hard-sphere model for bubble–bubble interactions, assuming that bubbles are rigid spheres and the collisions between bubbles are instantaneous. The bubble contact time during collision processes is not directly taken into account in the collision model. However, the contact time is physically a prerequisite for bubbles to coalesce, and should be long enough for liquid film drainage. In this work we applied the spring-dashpot model to model the bubble collisions and the bubble contact time, and then integrated the spring-dashpot model with the film drainage model for coalescence and a bubble breakage model. The bubble contact time is therefore accurately recorded during the collisions. We investigated the performance of the spring-dashpot model and the effect of the normal stiffness coefficient on bubble coalescence in the simulation.The results indicate that the spring-dashpot model together with the bubble coalescence and breakage model could reasonably reproduce the two-phase flow field, bubble coalescence and bubble size distribution. The influence of normal stiffness coefficient on simulation is also discussed.     

精馏塔板上气液相界面积的测量与预测

利用现代的电子光学测试仪器和计算机图像处理技术较准确地测量了精馏塔板上鼓泡液体中气泡的粒径分布和相界面积.同时，还从理论上分析了湍流液体中气泡变形与破碎的机理，提出了预测气液相界面积的多相湍流动力学模型.模拟计算结果与实验测量数据的比较证明，此模型有较高的准确性，而且形式简单，应用方便.     

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。     

Theoretical prediction of flow regime transition in bubble columns by the population balance model

Theoretical prediction of flow regime transition in bubble columns was studied based on the bubble size distribution by the population balance model (PBM). Models for bubble coalescence and breakup due to different mechanisms, including coalescence due to turbulent eddies, coalescence due to different bubble rise velocities, coalescence due to bubble wake entrainment, breakup due to eddy collision and breakup due to large bubble instability, were proposed. Simulation results showed that at relatively low superficial gas velocities, bubble coalescence and breakup were relatively weak and the bubble size was small and had a narrow distribution; with an increase in the superficial gas velocity, large bubbles began to form due to bubble coalescence, resulting in a much wider bubble size distribution. The regime transition was predicted to occur when the volume fraction of small bubbles sharply decreased. The predicted transition superficial gas velocity was about 4 cm/s for the air-water system, in accordance with the values obtained from experimental approaches.     

网格和数值格式对TBPBM-CFD模型模拟鼓泡塔的影响

基于气泡动力学属性的现有认识,把气泡分成大、小气泡,首次建立了完整的双气泡相-群平衡模型(TBPBM)以预测气泡尺寸.通过编写用户自定义程序实现了TBPBM模型、Luo破碎模型以及Prince 聚并模型,并耦合TBPBM与CFD双流体模型对直径440 mm鼓泡塔进行数值模拟,详细考察了网格与数值格式对TBPBM-CFD模型模拟结果的影响.结果表明,网格与数值格式对各物理变量的模拟结果影响非常大,特别是网格和体积分数方程对流项离散格式的影响最为显著.随着计算精度的提高,湍流耗散率和整体气含率分布梯度增大,气泡平均直径减少,大气泡所占气相比率降低,液相速度及气含率径向分布与实验值更趋吻合.     

DYNAMICS OF BUBBLE SIZE DISTRIBUTION IN TURBULENT GAS-LIQUID DISPERSIONS

The population balance equation coupled with the proposed breakage kernel and the previously developed breakage model is applied to the analysts of bubble size distribution for non-coalescing systems in a bench-scale airlift column. Good agreement obtained between the theoretical results and the experimental data is encouraging and indicates that the model is suitable for predicting dispersion properties such as bubble size and interfacial area in turbulent gas-liquid dispersions.     

加温加压下CFD-PBM耦合模型空气-水两相流数值模拟研究

计算流体力学与群体平衡模型（CFD-PBM）结合可有效地模拟鼓泡塔内流体行为,较准确地预测流场、相含率以及局部气泡尺寸分布。以直径100 mm、高1.3 m的加温加压鼓泡塔为模拟对象,在系统压力为1 MPa、表观气速为0.08~0.24 m/s、温度为30~160℃条件下系统地考察了空气-水体系的表观气速、温度以及固含率对平均气含率、大小气泡气含率、气泡直径和气泡尺寸分布等参数的影响。结果表明,平均气含率的模拟结果和实验值在10%的误差范围内吻合较好;温度的变化主要影响了塔内气泡的聚并和破碎,并用聚并破碎的机理解释了温度对其流体行为的影响。