Study On Flow Pattern In Gas Phase Ethylene Polymerization Fluidized Bed Reactor

Gas phase polymerization in fluidized bed reactor is used widely to produce polyolefin all over the world during decade years with significant commercial success. But the inherent disadvantages of gas phase process exist obviously due to weak ability of heat transfer in gas fluidized bed reactor. Generally there is a relationship between heat transfer and flow pattern within the reactor. So investigation of flow pattern or structure of fluidization has been a hot theme for a long time. However, there is not an effective apparatus to measure the flow pattern to this day. The changes of acoustic signal main-frequency in gas phase polymerization fluidized-bed reactor reflect the changes of acoustic energy in different segment. Based on this PSD measurement, a novel technique was used by means of an acoustic emission (AE) sensor to investigate flow pattern and the bed level in the fluidized bed in this work. Solid fluidization generally releases various AE signals comprised of particle-particle or particle-chamber collisions impact sound, particle-particle or particle-chamber friction sound, and air turbulence in fluidized bed. Therefore, measurement of AE energy and analysis could reflect the flow pattern and mixing of particles and bed level in fluidized bed. The innovative results can be summarized as following:(1) The flow pattern of a gas-solid fluidized bed was experimentally and industrially investigated by a novel Acoustic Emission technique. A multi-circulation flow pattern was found in the fluidization of polyethylene particles with the average size 511.6 μrn both in lab and industry, while a single circulation flow pattern was found in the fluidization of bimodal polyethylene particles with the average size 221.2 μm both in lab and industry. The multi-circulation flow pattern was also found in fluidization of LLDPE with the average size 540.0 μrn in condensation fluidized bed in industry;(2) The flow pattern of bimodal polyethylene particles in a gas-solid fluidized bed in malfunction was experimentally and industrially investigated by a novel Acoustic Emission technique. Single circulation was found when the agglomeration was small,only the energy changed in the agglomeration position. When the agglomeration was bigger enough, the multi-circulation flow pattern was found, and the variation of the particle was obviously found from the frequency figure;(3) The flow pattern of drafting A/C, B and D particles in Geldart particle classification was investigated for the first time. In the experiments, the flow patterns of the three kinds were similar when the static bed height was 0.05 m. When it rose to 0.15 m, multi-circulation was found in B particle, while single circulation was found in both drafting A/C and D particles, but the expandability and uniformity in drafting A/C particle were better than in D particle. When the static bed height reached 0.30 m, the flow patterns of three kinds were the same as those in 0.15 m height, and the expandability and uniformity of drafting A/C, B and D particle were all very good;(4) The transformation of flow pattern was investigated from drafting A/C particle to B particle for the first time. In the experiment, the flow pattern changed from single circulation to multi-circulation with the rising particle size. And the transformation was exactly the boundary of A and B particle, the formula is D*p≈296. It verified that different kinds of Geldart particles had different flow pattern. Also it is found high velocity was propitious to reaction and fluidization of particles, when the extents of the main circulation zone and stagnant zone were analyzed;(5) The granular temperature formula was presented based on analysis of the acoustic theory and granular temperature theory. And the formula was verified in the particles with the diameter of 0.640 mm. Based on the formula, granular temperature of particles with different size was investigated. The results indicated that when the operation velocity was higher than 5 times u_mf the particles would not stagnate in the stagnant zone of multi-circulation in fluidized bed.