【作者】 李良超；

【导师】 冯连芳； 李伯耿； 王嘉骏；

【作者基本信息】 浙江大学 ， 化学工程， 2010， 博士

【摘要】 气液和气液固反应器广泛应用于石油化工、生物化工和废水处理等工业过程中。气液反应过程的强化、反应装置的大型化趋势对于气液反应器的设计、优化提出了新的要求,高通气量操作条件下气液分散特性是目前研究工作的薄弱点,反应器内局部气液分散行为的深入解析可以为现有反应器的优化操作和新结构反应器的开发提供指导。本文采用实验测量和数值模拟相结合的方法对高通气量操作条件下气液反应器内的局部气液分散特性进行系统研究,取得了以下几个方面的研究结果。采用特制的双电导电极探针法对搅拌槽内气泡尺寸和局部气含率进行了测量,获得了中高通气量操作条件下搅拌槽内局部气液分散特性、以及操作条件对其的影响规律。结果表明：比之低通气量,在中高通气量下搅拌和通气对气液分散的控制能力逐渐减弱,全槽内气体分布的均匀性也变差；对于上层上翻斜叶桨和下层凹叶桨(PTU-CBT)双层组合式搅拌,通气量的增加导致下层搅拌桨向槽底部的气体循环能力减弱、下层搅拌桨局部区域气含率明显增加、上层搅拌桨区域气含率增加不明显；高通气量操作条件下宜采用靠近边壁通气的多管式气体分布器。在欧拉双流体模型的基础上耦合气泡数密度(BND)模型、引入气泡破碎和聚并函数,建立了大范围通气量下搅拌槽内气液分散的计算流体力学(CFD)数值计算方法。以PTU-CBT组合桨为例获得了气液搅拌槽内的气相和液相速度场、气泡大小与分布、局部气含率等重要信息,数值模拟与实验数据吻合良好,表明该方法能很好地对较高通气量搅拌槽内气液分散特性进行模拟计算并可视化。进一步的模拟研究表明,通气量的增加使气液相局部循环涡流和返混减弱、搅拌桨端区域的最大气液相速度明显减小；随通气量提高气泡尺寸明显增大并且分布均匀性变差,高通气量下液面附近气泡聚并增加易形成气泛；搅拌转速提高使平均气泡尺寸减小、搅拌桨作用区的局部气含率提高、全槽平均气含率也提高。基于传热和相变理论,采用UDF方法将流体相变模型加入到CFD中进行耦合计算,获得了不通气沸腾体系搅拌槽内流体力学特性。沸腾体系搅拌槽内汽含率分布与通气搅拌槽有很大的不同,沸腾体系搅拌槽内汽含率主要集中在靠近液百区域和上层桨叶片后部,槽体下部区域的汽含率几乎为零。另一方面,建立了预测搅拌槽内气体停留时间分布的拉格朗日-示踪粒子法和欧拉-示踪剂方法,对气(?)在搅拌槽内的返混和分散特性进行了研究。数值模拟结果表明：气泡尺寸、搅拌转速和通气量对搅拌槽内气体的返混和分散有明显的影响。对多种双层桨组合进行实验和CFD数值模拟研究发现,在较高通气量相同操作条件下,PCBDT-CBDT桨组合的气液分散特性最佳；桨叶片上开孔的桨组合比其相对应叶片未开孔桨组合气液分散效率更高。对多种形式气体分布器数值模拟研究发现,在较高通气量下,且下层桨为径向流桨时宜采用大直径的气体分布器。将气泡尺寸和局部气含率的实验测量与CFD数值模拟的研究扩展到鼓泡塔反应器。发现在鼓泡塔自下而上可分为鼓泡区、气泡过渡流域和充分发展流域,在过渡流域塔内气液分散特性随轴向高度变化敏感,在充分发展流域塔内气液分散基本不随轴向高度变化。根据塔内气液流型发展规律,提出了在鼓泡塔内设置多层气体分布器和增加导流筒内构件的创新构思,以调控塔内气液两相流。数值模拟研究表明：双层分布器相对单层分布器的鼓泡塔可改善在耗氧反应体系中氧浓度分布的均匀性,采用带导流筒的鼓泡塔可改善液体在塔内循环、使气液反应在全塔内更加均匀。更多还原

【Abstract】 Gas-liquid and gas-liquid-solid mechanically stirred reactors are widely applied in many process industries, for example, petrochemical, biochemical and sewage treatment process. With the intensification of gas-liquid reaction process and the scale-up of equipment, new requirement raised for optimizing the existing reactors or designing novel reactors with high performance. Investigation on the local gas dispersion in these reactors can provide guidance in optimization and design. However, due to the complexity of the gas-liquid system and the restriction of measurement techniques, the study on local gas dispersion under high superficial gas velocity is still weak in recent research work. In present study, the local gas dispersion in gas-liquid reactors was investigated in detail under medium and high superficial gas velocity by both experimental measurement and numerical simulation. The main contributions of the paper include:The local gas holdup and bubble size distribution were measured with double-tip conductivity probes in a dual-impeller agitated vessel under medium and high superficial gas velocity. The effects of stirring speed, gas inlet rate, impeller combinations on gas dispersion performance were investigated. It was found that the effects of stirring speed and gas inlet rate on gas dispersion decrease under medium and high superficial gas velocity compared with that under lower superficial gas velocity. Gas dispersion became worsen under medium and high superficial gas velocity. For the stirred vessel with impeller combination of pitch up turbine as upper impeller and concave blade turbine as lower impeller (PTU-CBT), the lower impeller discharge capacity of dispersing gas to the bottom region decreases with increasing of the gas inlet rate, local gas holdup increases greatly in lower impeller region while lightly in upper impeller region and upper circulation region. It is reasonable to mount the pipe gas distributors near wall region in stirred vessel under high superficial gas velocity condition.The gas dispersion under relatively high superficial gas velocity was simulated numerically with computation fluid dynamics (CFD). The Euler-Euler multiphase flow model, multi-reference frame method and a transport equation for the bubble number density (BND) function were used in the numerical simulation. The gas and liquid velocity, gas holdup and bubble size were achieved. The simulation results are in good agreement with the experimental value measured with double-tip conductivity probes, which indicated that the numerical method in our work could well predict the gas dispersion in stirred vessel under relatively high superficial gas velocity. The numerical simulation results show that, with the increasing of gas inlet rate, the eddy flow and back mixing of gas and liquid weaken, the maximum velocity of gas and liquid in impeller region decreases. Under high superficial gas velocity, the uniformity of bubble size distribution decrease, bubbles are inclined to coalesce in surface region. With increasing of the stirring speed, the global mean bubble size falls and the global gas holdup increases.The fluid flow in a boiling stirred vessel and gas residence time distribution in an aerated stirred vessel were numerically simulated, respectively. The phase change mode couple CFD method was used to predict the hydrodynamics in boiling stirred vessel. The numerical simulation results show that, gas holdup distribution in boiling stirred vessel is quite different from that in aerated stirred vessel. When heat was introduced from the bottom of the vessel, the boiling zone is mainly in surface region. The gas holdup in surface region is very high while is almost zero in bottom region. DPM method and tracer method were adopted to predict the gas residence time distribution. The numerical simulation results show that, bubble size, stirring speed and gas inlet rate have great effects on gas disperson and back mixing in aerated stirred vessel.Among the investigation of many dual-impeller combinations with experimental and CFD method, it was found that PCBDT-CBDT impeller combination is the best for gas dispersion under the same operating condition. The numerical simulation results also show that impeller combination with holes on the blades is more effective for gas dispersion than the corresponding impeller combination without holes on the blades. Gas distributor with large diameter is better for gas dispersion for agitated vessel mounted lower radial impeller under medium and high superficial gas velocity.The gas dispersion in bubble columns was experimental investigated and numerically simulated. Flow pattern in bubble column can be divided into bubbling region, transient flow region and fully developed flow region from lower side to upper side. Gas holdup and bubble size change sharply with the axial height in transient flow region while almost keeps unchanged in fully developed flow region. To improve the fluid flow, novel technique of mounting double stage gas distributor and flow draft tube in the bubble column was adopted. Bubble column with double stage gas distributors can improve the uniformity of oxygen distribution for oxidation reaction system compared with that with single stage gas distributor. Bubble column with flow draft tube can improve the liquid circulation and make gas liquid reaction evenly in the bubble column.更多还原