Characteristics of gas back-mixing in micro fluidized bed
May 22-27, 2016
Micro fluidized bed (MFB) has been applied to isothermal differential analysis of gas-solid reactions (1), and the fluidized gas and gas product passing through the MFBR is expected to be plug flow. Literature shows that the gas flow is close to plug flow and in low axial gas back-mixing when the gas Peclet number (Pe) is over 50 (2). This work devoted to investigating the effects of inner bed diameter (D), superficial gas velocity (Ug) and static particle bed height (Hs) on axial gas back-mixing in MFB and to distinguishing the conditions for the MFB operation. The experiments of axial gas back-mixing testing are conducted by tracer-gas method in a fluidized bed using air as the fluidized gas and helium as the tracer gas. Fluid catalytic crack (FCC) catalyst particles (Geldart A particles) are selected as the fluidized agent. The mean residence time (), Pe and axial diffusion coefficient (Da,g) of gas are calculated to determine the state of gas flow in MFB. Pe generally decreases with the increase of D and the maximum Pe decreases from 200 to 40 when D increases from 5 to 50 mm as shown in Fig 1. When D of fluidized bed was below 15 mm (i.e. 5 and 10 mm), Pe increased observably from 20 to 200 with the increase of Ug from 10 to 65 times of the calculated minimum fluidization velocity (Umf), but Hs had little effect on it. When D was over 15 mm (15-50 mm), Pe first increased and then decreased to be constant as Ug increased. The higher Hs would lead to lower constant Pe. The suitable operating range and parameters leading to Pe above 50 can be obtained from three-dimensional diagram in which coordinate axis was non-dimensional shown in Fig.2. The empirical equations were further developed to predict the Pe in MFB from the major operating parameters.
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Jian Yu, Guangwen Xu, Sulong Geng, Dengguo Lai, and Junrong Yue, "Characteristics of gas back-mixing in micro fluidized bed" in "Fluidization XV", Jamal Chaouki, Ecole Polytechnique de Montreal, Canada Franco Berruti, Wewstern University, Canada Xiaotao Bi, UBC, Canada Ray Cocco, PSRI Inc. USA Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/fluidization_xv/11