Ind. Eng. Chem. Res. 2009, 48, 253–261
253
Particle Motion in CFB Cyclones as Observed By Positron Emission Particle Tracking Chian W. Chan,‡ Jonathan P. K. Seville,‡ Xianfeng Fan,§ and Jan Baeyens*,† Department of Chemical Engineering, UniVersity of Birmingham, Birmingham B15 2TT, United Kingdom, School of Engineering, UniVersity of Warwick, CoVentry CV4 7AL, United Kingdom, and School of Physics and Astronomy, UniVersity of Birmingham, Birmingham B15 2TT, United Kingdom
Circulating fluidized bed (CFB) cyclones operate at high solids loadings. The paper presents, for the first time, particle trajectories within a cyclone obtained by positron emission particle tracking (PEPT), as a function of the solids loading (Cs). The pressure drop across a cyclone is a strong function of the solids loading. The objective of this work was to explain this behavior by direct observation of the particle movement. Cyclones normally operate in a stable particle movement mode, always with a spiral motion in the cylindrical part of the cyclone, followed by either a continued spiral in the cone (at low Cs) or by a much denser solids flow near the cone wall at higher Cs values. Data are used to obtain the tangential and axial velocity components of a tracer particle, the residence time of the particles in the cyclone, the thickness of the boundary layer in the cylindrical section of the cyclone, and the thickness of the dense wall layer in the conical section. This downward moving layer reduces the effective “free” cross section of the cyclone, thus increasing the air velocity and the pressure drop (∆P), especially for small cyclones, as in the present research. This effect will be negligible for larger cyclones, where the influence of solids film thickness is less important, when compared to cyclone diameter, and ∆P values are expected to remain almost constant with increases in the solids loading. 1. Introduction Cyclones are widely used in combination with bubbling fluidized beds (BFBs) and circulating fluidized beds (CFBs). The operating temperature and pressure vary within these applications. The applied particulate or solids loading (Cs) also varies, according to the application: this is expressed in terms of kilograms of solids per kilogram of gas and is determined as the ratio of delivered solids flow rate and delivered air flow rate, both given in units of kg/s. For common dust removal from BFB reactors, the value of Cs is generally well below 0.5. CFB cyclones operate at a very high solids loading, because of the applied solids circulation rate, with Cs close to medium (10-100) or dense phase (100-250) pneumatic conveying.1,2 The present paper considers the CFB cyclone to be responsible for efficiently separating and recycling the solids to the riser. Many previous studies have resulted in approaches to the prediction of the pressure drop (∆P) and separation efficiency of cyclones, mostly for low values of Cs. These studies have been reviewed and applied by Dewil et al.3 The present paper first presents some experimental results of ∆P measurements for a CFB cyclone over the range of solids loading, and thereafter presents views of the real-time particle motion within the cyclone by positron emission particle tracking (PEPT). Velocity and occupancy data indicate the presence of particle aggregates, moving in a dense flow near the cyclone wall, thus increasing the particle residence time in the cyclone. A cyclone works on the principle of centrifugal separation to achieve the separation of solids from the gas flow. Cyclones with axial inlet are usually called swirl tubes. There are three different types of commonly used cyclones with radial inlet configurations: the circular (or pipe) inlet, the slot (or tangential) inlet, and the “‘wrap-around” inlet. (See Figure 1.) * To whom correspondence should be addressed. E-mail address:
[email protected]. † Department of Chemical Engineering, University of Birmingham. ‡ School of Engineering, University of Warwick. § School of Physics and Astronomy, University of Birmingham.
The present research used two cyclones of different dimensions, both having slot inlets. The relevant dimensions of both cyclones are given in Table 1. 2. Fundamentals 2.1. Vortices. As gas enters tangentially into a cyclone, it creates an outer vortex (free vortex), where gas spirals downward. At the end of the outer vortex, the flow is reversed, forming the inner vortex (forced vortex), which flows upward to the vortex finder, as shown in Figure 2. The inlet and outlet of the tangential cyclone are perpendicular to each other. The flow in the tangential cyclone is highly turbulent, which, coupled with the vortex reversal in the cyclone, results in a relatively high pressure drop (∆P).3 Because of the effect of the vortices, heavy particles will strike the wall of the cyclone, losing their momentum and, subsequently, leaving the cyclone via the bottom opening (apex).
Figure 1. Schematic diagrams of (a) circular (or “pipe”) inlet, (b) tangential (or “slot”) inlet, and (c) “wrap-around” inlet. Table 1. Dimensions of the Cyclones Value characteristic dimension
Cyclone 1
Cyclone 2
diameter, Dc length of cylindrical section, Ley length of conical section, Lco height of inlet, hi width of inlet, wi diameter of apex, Da diameter of vortex finder, Dvf penetration depth of vortex finder
95 mm 100 mm 260 mm 70 mm 30 mm 35 mm 45 mm 70 mm
200 mm 400 mm 400 mm 120 mm 55 mm 60 mm 90 mm 110 mm
10.1021/ie800213g CCC: $40.75 2009 American Chemical Society Published on Web 08/08/2008
254 Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009
Figure 3. Radial distribution of the tangential air velocity (Vt) of gas in the cross-sectional area of the cyclone, as proposed by Corte´s and Gil.11
the tangential velocity profile across the radius of the cyclone is shown in Figure 3. 2.2. Pressure Drop and Cyclone Separation Efficiency. The pressure drop across a cyclone consists of a combination of local inertia-related losses and a frictional loss. The local losses include an expansion loss at the cyclone inlet and a contraction loss at the entrance of the vortex finder. The frictional loss includes a swirling loss due to the friction between the gas flow and the cyclone wall, and a friction loss of the gas flow in the outlet. In most cases, the contraction loss at the entrance of the vortex finder and the friction loss associated with the swirling motion of vortices are the major factors.6 The static pressure drop (∆P) between the inlet and outlet of a cyclone is proportional to the square of the flow rate (F), with a proportionality resistance coefficient defined in the early years of cyclone design based on the inlet velocity (Vi ) F/(wihi)); this latter term is referenced as ξc.
Figure 2. Inner and outer vortices in a typical cyclone.
Light and/or small particles will exit through the vortex finder of the cyclone, together with the gas stream.4,5 The flow areas of the upward vortex and downward vortex flow are roughly one-third and two-thirds of the cross-sectional area of the cyclone.6 The tangential velocity of gas in the outer vortices can be calculated using the equations of Table 2, while
Table 2. Equations Used To Predict the Tangential Gas Velocity in the Outer Vortex reference Meissner and Lo¨ffler7
equation
Vi / Vθw
Alexander8
Barth9
Muschelknautz and Kambrock10
()
(1)
( )
(2)
wi + 0.889 R
) -0.204
Vtw Ai ) 2.15 Vi DcDvf
()
ViRin wi ) 1 - 0.4 VtR R
Vt )
ViRin RR
R)
1 1ξ
ξ)
(3)
(4)
{
wi R
0.5
-ξ ) [( 2ξ ) - 2ξ ]1 - (1 - ξ1 )(2ξ +c
1+4
2
2
2
o
}
(5)
(6)
Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 255
Figure 4. Compression of inlet gas flow.
Figure 6. Schematics of the experimental setup. Legend: (1) riser, (2) cyclone inlet, (3) cyclone, (4) downcomer and L-valve, (5) tracer, (6) γ-ray camera, (7) cyclone outlet to bag filter, and (8) compressed air. Figure 5. “Short-circuiting” of inlet air flow.
Svarovsky12,13 related the pressure drop to the gas velocity in the cyclone body, i.e., based on the cyclone diameter (Vc ) F/(πDc2/4)) and to a friction (resistance) coefficient; this latter term is referenced as the dimensionless Euler number (Eu). The pressure drop ∆P is then written as ∆P ) ξc or
( ) FgVi2 2
(7)
( )
FgVc2 (8) 2 The use of Vi as a characteristic velocity is not recommended, because the essential characteristic dimension (Dc) is not taken into account: cyclones of equal dimension Dc but with different inlet (or outlet) sections of relative size can produce an equal pressure drop ∆P, despite operating at a different value of Vi.12,13 Their Euler numbers will be equal, but ξc will differ for these cyclones. Hence, it is recommended to use Dc and Eu as fundamental design parameters.13 These cyclones have more or less fixed geometric equivalence ratios; therefore, both of the previous equations can be converted, because ∆P ) Eu
Eu ) ξc
( ) π2Dc4
16wi2hi2
(9)
The literature still mostly quotes ξc, and the prevailing equations are listed by Dewil et al.3 However, these equations are known to apply only at low dust loadings. For operations at low to moderate dust loadings, a correction coefficient is proposed. Svarovsky13 quotes Eu values for different cyclones based on the cyclone cross-sectional velocity, whereas Dewil et al.3 related Eu to the cyclone dimensions. The separation efficiency of cyclones is dependent on the particle size and is often referred to as “grade” efficiency. It increases from zero for micrometer-sized particles to 100% for coarse particles. The particle size equivalent to 50% separation is called the cut size (or d50). The grade efficiency curve
Figure 7. Measured ∆P values at various Vc and Cs values for both cyclones tested.
represents the percentage of the different particle sizes separated: coarse particles (those with a size greater than d50) will be removed at >50% and smaller particles at
