Research and Optimization of Multiphase Flow and Mixing Inside a Overflow Stirred Tank Reactor
|Keywords||stirred tank slurry polymerization multi-layer agitator slurry externalloop slurry entrainment load variation reactor scale-up acoustic emission (AE) computational fluid dynamics (CFD) Hilbert-Huang transform (HHT)|
Chemical industry is one of the most important pillar industries in national economy. Stirred tanks are of great importance due to their comprehensive application in chemical industry. What is more important, the mixing performance of stirred tanks usually has huge impact on product quality, process economy, environment and some other problems during industrial production. Research on stirred tank began early, but people have not received deep understanding of it because of its extreme complexity. Especially, the lack of knowledge upon the commonly used three-phase stirred systems, in which single-phase mixing, gas-liquid dispersion, and solid-liquid suspension interact and interconnect with each other, has seriously constrained the optimization of stirred tank design and operation.A representative example is the scale-up of gas-liquid-solid stirred tanks used in slurry polymerization process of polyethylene (CX technology or CX process for short). Due to the extremely complex polymerization process and multiphase flow inside the reactors, research on CX technology have encountered four great puzzles: inefficient axial mixing performance of impellers, lack of basic research on slurry external loop, over-abundant slurry entrainment at the top of reactors, and insufficient understanding upon heat and mass transfer and operation characteristics of the polymerization reactors.In this thesis, we carried out elaborate research on the multi-layer stirred tank used in CX technology. We relied on acoustic emission (AE) technique, Hilbert-Huang transform (HHT) analysis, and computational fluid dynamics (CFD) simulation method, expecting to acquire theoretical basis of the four great puzzles in deep. Thus, we could bring forward relevant engineering solutions. In particular, we conducted deep theoretical study on flow field of the stirred tank, multiphase mixing performance of external loop, mechanism of slurry entrainment, heat and mass transfer inside the reactor and some other aspects. Apart from that, we proposed a new method, AE technique combined with HHT analysis, for measuring important process parameters and setting up relevant prediction models. In addition, we put forward several useful optimization suggestions of the polymerization device. The results we obtained were of great significance to the design and operation of industrial reactors.The main achievements are as follows.1. Single-phase flow field inside the multi-layer stirred tank was studied by means of Fluent software. Advantages and disadvantages of the original impellers were found by studying the functions of each impeller and related internals. In addition to that, a new combined multi-impeller agitator was designed aimed by CFD simulation, expecting to reduce the axial mixing inefficiency of the original impellers.1) CFD simulation was used to study the flow field inside the stirred tank used in CX technology. It was found that the original impellers had deficiencies of poor axial mixing performance, serious subregion phenomena, and poor mixing on top of the plate. It was reasonable to try to give optimization or transformation.2) CFD was used to optimize the impellers. The plate diameter of the upper impeller was enlarged, and enlarging twice as long as the vane was found the best choice. After the optimization of the upper impeller, axial mixing under the plate got stronger and gas phase residence time became longer.3) The type of the middle impeller was changed to45°pitched blade disc turbine, and up flow type was chosen the best. After the optimization of the middle impeller, axial mixing in the middle of the tank got stronger and subregion phenomena were impaired apparently.4) A45°pitched blade turbine, which had the same length as the middle impeller, was added to the top of the optimized agitator. Consequently, mixing performance in the top area inside the stirred tank became better. Meanwhile, the flow pattern was optimized so as to control the slurry entrainment generated on the level.2. The impact of external loop on single-phase mixing, gas-liquid dispersion, and solid-liquid suspension inside multiphase stirred tank was studied by cold mode experiments, revealing the fluid mixing performance of external loop. Moreover, criteria for external loop design were proposed. Therefore, it could be easy to optimize the external loop parameters with the help of CFD simulation.1) In the whole tank mixing performance was influenced by the flow rate of external loop. On one hand, a certain degree of external loop would intensify axial flow in the tank, so mixing time could be reduced. On the other hand, too large flow rate of external loop would destroy the original multi-loop flow pattern inside the tank, so mixing could be impaired. In the local area, where the external loop entered into the tank, the velocity of external loop was more remarkable. Destruction to the local flow field would be more significant under larger velocity of external loop, even influencing the whole flow pattern inside the tank.2) AE technique, based on HHT analysis, was used to measure the critical dispersion speed under different flow rates of external loop. From the results, it could be found that the critical dispersion speed, which represented for the gas-liquid dispersion performance of a stirred tank, became larger while the flow rate of external loop increased. The reason was found that external loop could fairly reduce the radial flow in the tank, which was important for gas dispersion. Therefore, external loop was detrimental to gas-liquid dispersion.3) AE technique, based on HHT analysis, was used to measure the critical suspension speed under different flow rate of external loop. From the results, it could be found that the critical suspension speed, which represented for the solid-liquid suspension performance of a stirred tank, became smaller while the flow rate of external loop increased. The reason was found that external loop could fairly enhance the axial flow in the tank, which was important for solid suspension. Therefore, external loop was favorable to solid-liquid suspension.4) External loop parameters, such as velocity, flow rate, incident height, and incident angle, were optimized. Relevant criteria were proposed. To specify, the flow rate of external loop should comply with two conditions, impact on gas dispersion should be less than20%and increase of heat removing be more than20%. The suggested flow rate was from400m3·h-1to600m3·h-1, especially580m3·h-1. The velocity of external loop should comply with two conditions, nonoccurrence of solid deposition and least possible destruction to the flow field. The suggested velocity was from4m·s-1to9m·s-1, especially6m·s-1. The incident height of external loop should comply with three conditions, least axial flow, largest tangential flow, and radial flow directing to the shaft. The suggested incident height was from1.9m to2.3m, especially2.05m. The incident angle of external loop should comply with the condition that most possible to change the horizontal flow to vertical flow. The suggested incident angle was from45°to60°, especially60°.3. Slurry entrainment in the reactors used in CX technology was deeply studied. On one hand, AE technique, based on HHT analysis, was used to propose a model to predict entrainment amount quantitatively. This technique could be applied to industrial devices, and it would be technical assurance for reactor control optimization. On the other hand, theoretical research of the mechanism of slurry entrainment in the reactor was done, and the theoretical model was found. Based on the theoretical model, it was sensible to propose suggestions for equipment optimization. Thus, slurry entrainment could be controlled radically to ensure long-time operation.1) AE Technique, based on HHT analysis, was used to measure the droplet or slurry entrainment in cold mode apparatus and industrial reactor, expecting to set up predictive model. Firstly, droplet entrainment in horizontal pipe gas-liquid mist flow was quantitatively measured, and the predictive model based on AE technique was acquired, giving an average absolute relative deviation of10.99%. The possibility of measuring entrainment quantity was preliminarily proved. Secondly, the relation between entrainment quantity and gas flow rate was obtained with the help of cold mode apparatus. The possibility of measuring entrainment quantity was further proved. Finally, AE technique was applied to industrial devices, giving desired results.2) Pseudo-liquid method was introduced to deal with the slurry in the polymerization reactor. Then a theoretical model of slurry entrainment was found, based on gas-liquid two phase flow dynamics and gas-liquid mixing principles. The acquired theoretical model was constituted of entrainment mechanism on the level and above the level.3) Based on the theoretical model of slurry entrainment acquired, two optimization methods were proposed to reduce slurry entrainment. One is using new-design combined impellers (impeller type optimization), and the other is using reactor with an expanding section (reactor type optimization). 4. Operation characteristics during the load variation process were studied. It contained three aspects, important parameters analysis, heat balance analysis of reactor, and gas-liquid dispersion and mass transfer performance analysis.1) By elaborately analysis of load increasing process from6.5t·h-1to7.5t·h-1. it was found that some key parameters varied remarkably. Slurry concentration had a huge increase, and it even exceeded the design value under the load of7.5t·h-1. Meanwhile, the residence time of catalyst dropped apparently, resulting in lower catalyst yield and more ash in the product. Apart from that, load increasing brought about a significant expansion of recycle gas flow rate. As a result, top heat exchanger would be hard to sustain, and slurry concentration would be heavier. These all affected the long-time operation of industrial equipment.2) The result of heat balance analysis indicated that total heat removing ability of the apparatus was insufficient after the load increasing. The main reason was that the top heat exchanger had reached the limit.3) The result of gas-liquid mass transfer analysis indicated that gas-liquid mass transfer performance was deteriorated after the load increasing. That could be found in the increase of critical dispersion speed, mean bubble diameter and the decrease of specific interfacial area, mass transfer coefficient.5.300,000tons per year HDPE polymerization reactor was design by both geometrically similar scale-up and dissimilar scale-up.1) A stirred tank with a diameter of5.2m, a height of7.96m, and a volume of150m3was designed by geometrically similar scale-up. The construction was the same as the original reactor.2) A reactor with an expanding section was designed by dissimilar scale-up. The scale-up criteria were the same superficial gas velocity in the entrainment region and the height diameter ratio adjusted to1.3. As a result, the diameter of entrainment region was5.2m. The diameter of slurry region was4.48m. Total height of reactor was8.45m, and the total volume127m3. Compared with the reactor acquired by geometrically similar scale-up, new design reactor could save space and cost for manufacturing and transporting. In addition to that, it could have better mixing performance, better heat removing ability, and better ability to control slurry entrainment.