Increasing The Density Of Circulating Fluidized Bed Reactors — Hydrodynamics In High-density Riser And Downer

Fluid catalytic cracking(FCC) plays an important role in petroleum refining industry, which is one of the most important processes to convert the heavy oil to light products. In view of the trend of crude oil becoming heavy and in consideration of the demand of environmentally friendly fuels and light olefins, it is better to develop a novel FCC reactor so that it can be operated under high severity of operating conditions such as high temperature, high catalyst to oil ratio and appropriate residence time to product light olefins and environmentally friendly gasoline. Based on the fundamental researches, it is found that high density circulating fluidized beds(HDCFBs) have so many advantages such as high gas and solids throughput, high solids concentration, relatively uniform radial distribution of gas-solids suspension, less axial backmixing that they are suitable for FCC for the purpose of intermediate products. According to their unique characteristics of the HDCFBs, a CFB cold model was modified so that it could be operated at high density conditions. In the modified CFB system, hydrodynamics in HDCFB riser and downer were comprehensively investigated. With the systematic experiments and theoretical analysis, the following conclusions were obtained:The solids circulation rate reaches as high as 1000 kg/m2 s which has never been achieved before in an academic setting. At high density operating conditions, the axial flow structure becomes uniform and the cross-sectional mean solids holdup reaches 0.22 to 0.32 throughout the entire riser. Compared to a typical core-annulus structure, the radial distributions of the solids holdup becomes much less uniform with a shrinking core and transits to a monotonic increasing profile towards the wall. Speed of flow development differs at various radial positions with almost instant development in the center even at the highest solids flux of 1000 kg/m2 s and then becoming slower towards the wall. Fluctuations in high density circulating fluidized beds are significantly greater than those in low density ones, leading to more vigorous interactions between gas and solids phases. As a result, better gas-solids contacting and mixing, plus the uniform axial profiles of solids holdup, provide better reactor performance for the high solids flux/density risers than low flux/density ones.The axial distribution of particle velocity becomes more uniform in such extremely high solids flux conditions than cases with lower solids flux. Radial profiles of particle velocity and solids flux become increasingly steep with increasing solids circulation rate. No net downward flow near the wall is detected, which is considered an important advantage of the high density riser leading to reduced solids backmixing. Correlations of particle velocity against solids holdup are stronger for low density conditions than for high density cases, suggesting that gas-particle interaction dominates in low density risers, while particle-particle interaction plays a key role for the motion of particles in the extremely high density ones.Axial distributions of particle velocity and solids holdup in the downer are significantly affected by the operating conditions. Superficial gas velocity is the predominance for particle velocity distribution while solids circulation rate plays a key role in solids holdup distribution. Development of solids flow can be accelerated by increasing superficial gas velocity and/or decreasing solids circulation rate. High solids holdup, higher than 0.06, can be achieved in the entire downer with relatively uniform axial profiles at solids circulation rate of 700 kg/m2 s indicating a preferential flow with reduced backmixing in the downer.Although the radial distribution of solids holdup is somewhat less uniform under very high flux conditions, it is still much more uniform compared to riser reactors. Radial profiles of solids holdup, particle velocity and solids flux are significantly affected by the operating conditions. It is also found that relationships between local solids holdup and particle velocity are totally different in the downers from that in the risers due to their different flow characteristics. Compared with the riser reactor, downer has a self-adjusting mechanism to perform with the nearly ’’ideal’’ plug flow nature. Therefore, high density downer reactors exhibit a prospect of much wider applications requiring high solids/gas feed ratios and high solids concentration.