Study On The Hydrodynamics Of Novel Multistage Airlift Loop Reactor

It has been well known that the conventional air-lift loop reactor is mainly composed of a column, a draft tube and a sparger. Though it possesses a very good hydrodynamic behavior, the problem of the conventional loop reactor has been encountered that in the downer region the local gas holdup is very low, and therefore the overall efficiency of the reactor is limited. In this dissertation a specially designed multi-stage loop reactor has been proposed. In this reactor the draft tube was made in several stages spaced by a number of holes drilled at different longitudinal positions on the wall of draft tube. In this way, the liquid flow possesses a special feature that the small local loop flows are formed around each individual stage and superimposed on the overall loop flow around the draft tube, leading to a novel multistage flow pattern and more uniform distribution of the local gas hold-up. The experimental part of this dissertation focuses on the hydrodynamic study on the multistage loop reactor including the measurements of the loop flow rates and the local gas holdup in different longitudinal location. The effects of the inner structure of the reactor, the physical characteristics of the fluid and the operation conditions on the hydrodynamic characteristics are also investigated. The results indicated that the hydrodynamic characteristics of the multistage loop reactor are influenced remarkably by the coalescence feature of the system, the bottom structure of the reactor, the height of the liquid hold up, the opening fraction and the position of the stages, etc. In addition, the mixing feature and the mass transfer rate are studied. The results show that the mixing time of the multistage airlift loop reactor is less than the conventional loop reactor, but longer than bubble column. The volumetric mass transfer rate in MSALR is found to be in the same range of the conventional loop reactor, and but is higher than that in bubble column. The bubble coalescence behavior in the loop reactor is also examined by using the advanced fibre probe technology. An experiential correlation is proposed to predict the coalescence feature of the system through the experimental data.It is seen that in the petrochemical industry and other industrial processes most of the multiphase reactions are conducted at high pressure and high temperature. Therefore the knowledge of the effects of pressure on the hydrodynamics characteristics is essential for both of process analyses and industrial reactor design. The experimental results show that the pressure has apparent affects on the overall gas holdup, but the liquid recirculation velocity is not affected significantly by pressure in the operating ranges. Additionally, the experimental data show that the mixing feature is good in airlift loop reactor. On the basis of experiments and theoretical analyses of the pressure profile along the draft tube, a detailed calculation and design approach for the multistage loop reactor is proposed. The two key parameters, namely the position of the first stage and the opening fraction of the holes, are well determined by this model. Besides, the other reactor parameters and operating properties can also be calculated. Based on this model, an industrial multistage loop reactor with capacity of 5,000 ton/year for hydrocracking of heavy oil is designed.