Doctor of Philosophy
Chemical and Biochemical Engineering
Dr. Jesse Zhu and Dr. Shahzad Barghi
A systematic and comprehensive study of hydrodynamics and reactor performance was conducted in a 76 mm i.d., 10 m high riser and a 76 mm i.d., 5.8 m high downer reactor under high density/flux operating conditions using fluid catalytic cracking (FCC) catalyst particles. An optical fiber probe was used to obtain a complete mapping of local solids holdup and particle velocity. Catalytic ozone decomposition reaction was employed to study the characteristics of reactor performance in the CFB riser and downer. The superficial gas velocity (Ug) and the solids circulation rate (Gs) were 3-9 m/s and 100-1000 kg/m2·s, respectively. Based on the spatial distributions of catalyst particles and gas reactant in the riser and the downer, hydrodynamics and reactor performance were fully characterized.
Solids suspension having a solids holdup of up to 0.2-0.3 could be maintained throughout the entire high flux/density riser. A homogenous axial flow structure was observed at Gs = 1000 kg/m2s. When Gs exceeded about 800 kg/m2s, the axial profile of the particle velocity became more uniform. The axial particle velocity was affected more significantly by high superficial gas velocity especially under high solids flux/density conditions. No net downward flow near the wall was one of the most important advantages of the high flux/density riser over the conventional low flux/density reactor, leading to a reduction of solids backmixing. Radial distributions of the solids holdup were nonuniform with a dilute region and a dense region. When Gs was higher than 700 kg/m2s, the dilute core region shrank to less than 20% of the cross-sectional area. Solids holdups thereafter increased monotonically towards the wall which could be up to 0.59. Moreover, solids holdup remained higher than 0.4 over a wide cross-sectional area (r/R = 0.7-1.0, about 60% of the cross-sectional area) even at the top section of the riser. Radial distribution of solids holdup in the downer was much more uniform than that in the riser. Radial profiles of solids holdup were characterized by a flat value covering a wide region of the cross section and a relatively high value near the wall in the fully developed section. The uniform distribution of solids flow provided a nearly plug flow condition in the downer reactor.
As to the ozone reaction in the CFB system, the axial and radial profiles of the ozone concentration were consistent with the corresponding profiles of the solids holdups which indicated that ozone reaction in the CFB reactors was controlled by the gas-solids flow structure. Strong interrelation was observed between the distributions of solids and reactant concentration. Higher solids holdups would give higher ozone conversions. Most conversion occurred in the entrance region, that is, the flow developing zone of the riser and downer reactors. Overall ozone conversions in CFB riser and downer deviated from plug-flow behavior indicating that hydrodynamics affected CFB reactor performance. The extent of the deviation of the conversion could be attributed to the different gas-solids contacting efficiency.
Wang, Chengxiu, "High density gas-solids circulating fluidized bed riser and downer reactors" (2013). Electronic Thesis and Dissertation Repository. 1572.