Accepted Manuscript Solids mixing in a shallow cross-flow bubbling fluidized bed Weibin Kong, Bin Wang, Jan Baeyens, Shuo Li, Hui Ke, Tianwei Tan, Huili Zhang PII: DOI: Reference:
S0009-2509(18)30289-6 https://doi.org/10.1016/j.ces.2018.04.073 CES 14205
To appear in:
Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
18 January 2018 17 April 2018 29 April 2018
Please cite this article as: W. Kong, B. Wang, J. Baeyens, S. Li, H. Ke, T. Tan, H. Zhang, Solids mixing in a shallow cross-flow bubbling fluidized bed, Chemical Engineering Science (2018), doi: https://doi.org/10.1016/j.ces. 2018.04.073
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Solids mixing in a shallow cross-flow bubbling fluidized bed Weibin Kong1, Bin Wang1,3, Jan Baeyens1, 2, Shuo Li1, Hui Ke1, Tianwei Tan1, Huili Zhang1,* 1
School of Life Science and Technology, Beijing University of Chemical Technology (BUCT),
Beijing, China 2
Beijing Advanced Centre of Soft Matter and Engineering, BUCT, Beijing, China
3
Qinhuangdao Bohai Biological Research Institute of BUCT, Hebei, China
*Corresponding authors: Huili Zhang, email:
[email protected];
Abstract Gas-solid, gas-catalytic and physical applications in a fluidized bed require a strict control of the solids residence time and a limited back-mixing for a required conversion. Since conversion proceeds with residence time, this residence time and its distribution (RTD) are essential design parameters in fluidized bed modeling. The experiments of the present research investigate the use of a shallow cross-flow bubbling fluidized bed as reactor. A tracer stimulus response technique was used to determine the RTD, in a fluidized bed with and without internals.
Experimental results were compared with fittings from several models. Although a cascade of perfectly mixed reactors or a plug flow with dispersion model can be applied, the latter is preferred, and the dispersion parameter, expressed as Peclet number, exceeds ~25.
The results are moreover used in a test case design of hexane devolatilization from rice bran cake. With the RTD model of the horizontal fluidized bed, and with batch kinetic experiments, the size of the desolventizer can be designed on the basis of the required residence time. The superficial fluidization velocity applied is normally 3 to 4 times Umf. A similar RTD-approach can be used for alternative physical processes (e.g. drying) and for chemical reactions (e.g. calcination of dolomite)
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provided kinetic data from batch experiments are known.
Keywords: shallow fluidized beds, cross-flow, thermal applications, group A powders, residence time distribution, modeling
Notations C(t)
Tracer concentration, g/g
dSV
Surface/volume particle diameter, m
E(t)
Residence time distribution of injection and riser system, dimensionless
F(t)
Cumulative residence time distribution, dimensionless
F
Feed rate of solids, kg/s
N
Number of perfectly mixed tank reactors, dimensionless
Pe
Peclet number based on U, dimensionless
T
Time, s Average residence time in the reactor, s
tav
Average residence time determined by Eq. (1), s
tm
Average residence time determined by Eq. (5), s
U
Superficial gas velocity, m/s
Umf
Minimum fluidization velocity, m/s
Umb
Minimum bubbling velocity, m/s
VBed
Bed volume, m3
Ρ
Density, kg/m3
τp
Plug flow fraction in the Fractional Tubularity (FT) model, dimensionless
Subscripts g,p
Gas and Particle, respectively
B
Bulk
2
1 1.1.
Introduction Novel applications of upflow bubbling fluidized beds in solar receivers
The Upflow Bubbling Fluidized Bed (UBFB) has recently been promoted for its specific application of solar energy capture and storage [1–4], with experiments carried out in short vertical tubes of small internal diameter (I.D.), and using group-A powders to limit sensible heat losses associated with the fluidizing air flow [5,6]. A further scale-up to multi-megawatt capacity is however less obvious since it will involve the use of the parallel tube-concept but needing tubes of a significantly taller height to efficiently make use of the concentrated heliostat beams and provide sufficient heat capture surface area. In molten salt applications, receiver heights of 10 to 18 m are now common practice. This will hardly be possible using the UBFB since its gas-solid hydrodynamics depend strongly on the geometry of the tubes, with special emphasis on their I.D. and height. It has indeed recently been shown that even in group-A powders, common unrestrained bubbling can be transformed into slugging when the bubble sizes approach the size of the tube, and both wall slugs above ~ 0.5 m bed height [7,8] and axi-symmetric slugs in beds exceeding a 1.5 m depth seriously hamper the solids mixing and considerably reduce the tube wall-to-suspension heat transfer [5]. Applicable solids circulation rates, expressed as solid flux (kg/m2s), are moreover limited to avoid choking [9]. It was therefore deemed important to look for alternative fluidized bed layouts, possibly used in cascade, to overcome the drawbacks of deep-bed slugging, choking and reduced heat transfer.
1.2.
The use of a shallow cross-flow fluidized bed
Industrial fluidized beds are mostly of the “well-mixed” type, and generally executed as deep beds of circular cross section. In their “well-mixed” performance, the residence time distribution of the particles is represented by the perfect mixing law particles with residence times between (t - dt),
with E(t)dt the fraction of the mean residence time. Due to the near
perfect mixing, the bed has close to a uniform composition and temperature, equal to the outlet stream in a continuous operation. Fluidization gas also exits the bed at bed temperature. Despite the good mixing of feed particles within the bed, the main disadvantage is the residence time
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distribution leading to a distribution in product characteristics since about 40% of the particles stay in the bed for less than half of , and about 10% for less than one-tenth of .
For specific applications, where strict product properties are required, where a temperature gradient from feed to discharge is desired, or where the pressure drop across the bed needs to be limited, the use of continuous cross-flow fluidized beds can be applied. Particles flow along a channel of high length/width radio, and the objective is to approach plug flow of the particles with a near-equal residence time. Straight channel designs are often provided with baffles normal to the direction of particle flow to further enhance plug flow. In addition to having a narrow spread on residence time and product properties, a plug flow bed will normally require a smaller bed volume than a well-mixed bed to achieve the same performance. Shallow beds with low bed pressure drop are hence commonly applied. Thermal gradients will exist along the particle path, and the thermal efficiency of a simple plug flow bed is lower than for a well-mixed bed. This is even more so if several well-mixed beds are used in a countercurrent superposed contacting mode [10,11], however with disadvantages of a higher total pressure drop, difficulties to maintain a stable down flow among the counter currently staged beds, and a more difficult design in general. A single stage cross-flow fluidized bed is tentatively presented in Figure 1. As in the UBFB case, parasitic losses (pressure drop, amount of sensible heat loss with fluidizing gas), the required high flow rate of fluidization gas, and large size of equipment will be limited by using Geldart group A-powders, both as inert bed material (SiC, cristobalite, …), or as feedstock to be treated in the thermal reactor (e.g. dolomite, cement raw meal, milled phosphate rock, …) or in physical applications (fluidized bed heat exchanger, drying, desolventizing,...).
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b)
Figure 1. Illustration of a cross-flow bubbling fluidized bed: (a) in a solar thermal application [12] with ①
feed of particles, ② horizontal fluidized bed, ③
discharge of particles, ④
multi-orifice air distributors, ⑤ exhaust air; and (b) as air pre-heater in a solar air-Brayton turbine cycle [13].
The aeratable group A powders have attracted considerable interest, since fluidizing at low excess velocities (< 1 cm/s), while easy to circulate around fluidized and pneumatic conveying loops and characterized by excellent mixing and heat transfer [14].Their bubble size is only slightly affected by the average particle size, and the rise velocities of small bubbles (
