(267d) Influence Parameters and Modeling of Solids Circulation Rate in the High-Density Circulating Fluidized Beds

Circulating fluidized bed (CFB) has been utilized in many industrial processes. Two typical commercial applications are circulating fluidized bed combustor (CFBC) and the fluid catalytic cracking (FCC). The operating parameters are different in these two processes. For example, CFBC is usually operated with the solids circulation rate below 200 kg/m²s, while FCC process is operated with much higher solids circulation rate of 400-1200 kg/m²s. To distinguish these two processes, the circulating fluidized beds with high solids circulation of more than 200 kg/m²s along with high solids holdup of 10 % are defined as the high-density circulating fluidized beds (HDCFB). Those with solids circulation rate of lower than 200 kg/m²s are called low-density circulating fluidized beds (LDCFB). Right now, studies on CFB always focus on the LDCFB. The reason for less fundamental researches focusing on HDCFB is because it is hardly to obtain the high density operating condition in the laboratory-scale setup. The hydrodynamics of HDCFB are different from that of LDCFB. So it’s necessary to work on high-density circulating fluidized beds, especially on influences and modeling of high density operations.

In this work, experiments are mainly carried out in a riser with 80 mm diameters and 18 m height using the FCC particles with mean diameter of 85 μm and particle density of 1500 kg/m³. The experimental setup includes a downcomer with 450 mm diameters and a solids storage tank with 660 mm. The downcomer and storage tank can provide enough drive force to supply enough solids particle to the riser. Parameters on the solids circulation rate including superficial gas velocity, solids inventory height in the storage tank, the opening degree of the butterfly valve in the feeding pipe and the aeration air rate in the bottom of storage tank are comprehensively studied. The solids circulation rate can reach as high as 1300 kg/m²s at superficial gas velocity of 9 m/s with solids holdup nearly up to 20% in the riser indicating the high-density operating condition. The solids circulation rate increases linearly with the increase of superficial gas velocity from 5 to 9 m/s. Similar results are also found with increasing the solids inventory in the storage tank. The sensitivity of influence parameters is also studied in this work. Based on this work, an empirical model is established by fitting the experimental data which is shown as follows:

Gs = f(A, U_g, V_A)=(a*exp(A)*U_g²+b*V_A*U_g+c*A*V_A)*(A^d*V_A^e)

Where

G_s --- solids circulation rate, kg/m²s

A --- ratio of valve opening area, -

U_g --- superficial gas velocity, m/s

V_A --- aeration air rate in the bottom of the stored tank, m³/h

a, b, c, d, e --- the empirical coefficients, -

There are 3 independent variables and 5 empirical coefficients in the equation and those coefficients are shown in the Table 1. The solids circulation rate can be predicted by the empirical models according to the value of the solids inventory height in the storage tank, the opening degree of the butterfly valve, the aeration air in the bottom of the tank and the superficial gas velocity. The relative error between the predicting result and the experimental data is lower than 20%.

Table 1. Coefficients for the empirical predicting model

solids inventory height	a	b	c	d	e
4	-2.0035	88.4986	-109.8504	0.8906	-0.9896
6	-7.2611	158.2493	-282.6492	1.1061	-1.05
8	-13.2719	182.8993	-190.4830	0.8496	-1.0698
10	-9.2338	124.6352	158.7743	0.6943	-0.9279