Pulsed field gradient nuclear magnetic resonance （PFG NMR） has been performed to study the diffusion of organic solvents into semicrystalline polyethylene particles. Self-diffusion coefficients in different domains of the sample can be extracted through a bi- exponential fit to the echo intensity attenuation, which allows the precise determination of the tortuosity of the polyethylene particles. Further exploration comes from the measurements with branched polyethylene particles and it was found that the diffusion in polymer phase contributed significantly to the slow component of the exponential decay curve. 2007 Jing Dai Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Xiao Wei YanXiao Hong RenSiegfried StapJing Dai WangYong Rong Yang
Acoustic signals emitted from particles and bubbles in the gas-solid fluidized bed were collected by the transducer located under the distributor and the result showed that the energy and deviation of acoustic signals varied regularly with superficial velocity.Superficial velocity was the minimum fluidizing velocity or the minimum turbulent velocity, while energy or deviation changed suddenly.Then a criterion to determine the change of flow pattern was obtained that when the ratio of acoustic energy or deviation reached the maximum, the velocity was the minimum fluidizing velocity, while the ratio of acoustic energy or deviation reached the next maximum the corresponding velocity was the minimum turbulent velocity.Experiments were carried out in a fluidized bed with inner diameter of 250 mm and polyethylene resin particles.The minimum fluidizing and minimum turbulent velocity obtained from the new method agreed well with those deducted from classical equations and traditional methods, including pressure difference method and pressure fluctuation method.In conclusion, an easy, sensitive, exact and on-line way to detect fluidizing velocity is presented, and this method is applicable to industrial equipment.
The flow pattern of a fluidized bed is a key factor for heat transfer and new reactor design.The flow pattern of the gas-solid fluidized bed was experimentally measured by a novel acoustic emission(AE)technique.The acoustic energy generated by the collision of solids particles on the reactor wall or in the local space is a reflection of the speed and frequency of collision,and consequently the flow pattern of particles.The flow pattern of polyethylene particles with the average size 460 μm in a 150 mm fluidized bed in the range of superficial gas velocity from 0.3 m·s-1 to 0.7 m·s-1 was multi-circulation pattern with main-circulation zone,sub-circulation zone and stagnant zone.The multi-circulation flow pattern would transit to the single circulation flow pattern if superficial gas velocity exceeded 0.8 m·s-1 or average particle size decreased to below 365 μm.The height of stagnant zone remained unchanged with the increase of static bed height.It was also found that the type of the distributor had an evident effect on flow pattern.The single circulation flow pattern in the fluidized bed with a perforated stainless steel plate distributor would change to the multi-circulation flow pattern for the same particles and the same fluidized bed with a cone-shaped distributor.An empirical equation calculating the height of stagnant zone was presented and the experimental data could be correlated with the empirical equation with fairly good accuracy.
