Experimental Study On Forces Acting On Baffles Immersed In Gas-solid Fluidized Beds

Internals are widely used in industrial fluidized bed reactors,which is one of the most effective ways to improve their reaction efficiency and product selectivity.However,while improving gas-solids flow behavior,these internals also suffer from forces induced by violent gas-solids movements in bed.Moreover,industrial fluidized reactors often have very strict requirements on equipment reliability.An indicator is that industrial fluidized reactors usually long repair periods,at least one year or even several years.The continuous acting of various forces on internals is likely to result in excessive overload or fatigue,leading permanent damage of these internals.Therefore,how to ensure the long-period reliability of internals is another important issue concerned strongly by the industrial community.To keep the long-period reliability of internals,we need to scientifically design their structure and strength.These designs must be based on the knowledge of forces on the internals in different operating states of fluidized bed reactors.For this reason,we built a large cold model fluidized bed apparatus to investigate the forces acting on internals under different operating states(i.e.defluidized,start-up,fluidized state)systematically.In this study,the baffle made of multiple slats is chosen as the research object for their wide applications in industry.The forces acting on the baffle are measured by adhering strain gauges on its surface.Meanwhile,several pressure taps are arranged along the bed height to know the macro gas-solids flow behavior in the bed and then to analyze the relationship between forces acting on the baffle and the gas-solids behaviors in the bed.In steady fluidized state,the experimental results show that the measured force signals on a single slat immersed in the dense bed are made of random pulses.The effective frequency of the force signals ranges from 0 and 10 Hz.The main frequency is usually lower than 5 Hz.Moreover,the effective frequency is close to that of the pressure signals synchronously measured in the bed.In the bubbling flow regime,the vertical RMS forces on the immersed slat increase linearly with increasing superficial gas velocity.The slat is subject to higher RMS forces in a deeper fluidized bed.The RMS forces on the slat are approximately proportional to the initial bed height.The RMS forces on the slat decrease significantly with increasing inclination angle due to the reduction of interaction area between the slat and rising bubbles.On the other hand,force signals are quite different from those measured in the fluidized beds of different groups of solids.It is found that the RMS forces on the slat immersed in Group B particles are about 2?3 times larger than that measured in fine Group A particles.Under the same operating conditions,the standard deviation of pressure signals measured in a bed of Group B particles is much larger than that measured in a bed of Group A particles,which further indicates that there is a close relationship between forces on the slat and bubbles motion in the beds.In addition,for an array made of multiple inclined slats,the slat located at the opposite of oriented direction will encounter stronger forces.With decreasing space between adjacent pair of slats,the measured RMS forces on the slat in a slat array is significantly reduced compared with a single slat immersed in the bed due to an "air cushion" layer appearing below the slat array.During starting up a fluidized bed,there is a high force impulse acting on the internals immersed in the dense bed.The duration of the force impulse is about 0.5?2 s and its peak is about 2?5 times of the average amplitudes of the measured force signals in steady fluidized state.Furthermore,by systematically investigating the influence of different parameters,we find that the peak forces on the slat increase with increasing increment of superficial gas velocity and the initial bed height,but they decrease with increasing inclination angle of the slat.Moreover,once the bed has been transformed into a fluidized state,there is no a similar high force impulse appeared in the measured force signals,even if the superficial gas velocity has a large increment.Therefore,when starting up a fluidized bed,it is necessary to make the bed transform from a fixed-bed state into a fluidized state at a low increment of superficial gas velocity,and then adjust the gas velocity to the required operating conditions to avoid damage of the immersed internals.On the other hand,for a slat array,the slats located near the column wall will subject to higher force impulse.Besides,for a slat array with small space between adjacent pair of slats,the peak force impulses on a slat in the slat array are much smaller than that on a single slat immersed in the bed.During starting up a fluidized bed with a layer of mesh gird,it is found that the peak forces on the test slat located above a layer of mesh grid are about 2/3 lower than that on a single test slat immersed in the bed.The lower mesh grid plays a protecting role on the upper test slat or other internals during the start-up stage.