Study On The Stability Of The Standpipe In A Circulating Fluidized Bed Reactor

Circulating fluidized bed reactors have been applied in many gas-solid heterogeneous processes,among which fluid catalytic cracking is the most representative.The processes prefer to be operated with high gas-solid contact efficiency,and large amounts of catalysts are needed in the circulation.Thus,the performance of the circulating solids would affect the stability of the reacting system directly.Stability of the standpipe which acts as the core part of the return system determines that of the whole circulating fluidized bed.However,a series of unpredicted and uncontrollable instabilities would occur in the running of the standpipe whose design and operation are usually achieved with rules of thumb.Therefore,the main research task of the dissertation was carried out in the lab-scale circulating fluidized bed cold model apparatus.The stability of standpipe was studied systematically according to flow characteristics inside,aiming to optimize the methods of standpipe design and operation,prevent the unstable phenomena and provide theoretical foundation and manipulating guidance for long-periodic smooth running of industrial units.Firstly,the unstable phenomena were deeply investigated and depicted in the aspects of vibration intensity,particles transport,pressure distribution and solids concentration distribution.Streaming downflow often occurs at the initial stage or when blocked seriously,so the solids circulating rates,solids concentration and pressure are all in a very low level,and no material seal is formed,resulting in severe gas reversal.Defluidization happens when the interstitial gas is compressed,followed with the non-fluidized packed flow or/and transitional packed flow.The strong particle-particle and particle-wall interactions consume much pressure energy and cause an abnormal reversal of pressure distribution,which reduces the impetus of solids circulation and brings out severe mechanical vibration.In the state of over-aerated flow,several flow regimes coexist in the standpipe and transformation between the regimes is repeated irregularly,leading to the oscillation of vibration intensity,solids flux,pressure profiles,and solids concentration.The aeration was introduced to realize the ideal flow of dense fluidized bed in the standpipe.The results show that aeration has the function of preventing defluidization,decreasing solids holdup and intensifying gas-solid reciprocity.Thus,the particle-particle and particle-wall interactions which result in pressure loss are weakened and solids circulating rates are increased.However,the influencing sphere of single aeration injection is limited,so multiple injections of aeration are needed to perform a synergistic effect to further reinforce the role of preventing defluidization and pressurizing.The oscillation intensity is therefore diminished under the comprehensive effects of aeration.As the results indicate,steady flow could be achieved in the standpipe and potential of conveying capacity could be taped further,thereby,the aerating method needs to be optimized.In the process of optimization,bigger promotion of standpipe performance is always obtained with more aeration injections and decreasing gas density from the lower to the upper.The ideal aerating method is that infinitely many injections of aeration are introduced into the standpipe in infinitesimal gas for each injection with gradually decreasing gas density from bottom to top.But in practical operation,it is acceptable when the aeration scheme satisfies the production requirements.In this experiment,the unit could reach the best condition when employing the scheme in which the aeration was introduced into the pipe in eight injections with increasing internals between adjacent gas taps towards the top,and the feasible range of aeration rate is 0.78-1.14 cm/s.Finally,a real-time monitoring and controlling method was put forward to eliminate the ambient interferences for the smooth running of the unit.