Solids flow diagram of a CFB riser using Geldart B-type powders

Particuology 10 (2012) 51–61

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Solids flow diagram of a CFB riser using Geldart B-type powders Shiva Mahmoudi a , Chian Wen Chan a , Anke Brems b , Jonathan Seville c , Jan Baeyens c,∗ a

University of Warwick, School of Engineering, Coventry CV4 7AL, UK Katholieke Universiteit Leuven, Department of Chemical Engineering, 3001 Heverlee, Belgium c University of Surrey, Faculty of Engineering and Physical Sciences, Guildford GU2 7XH, UK b

a r t i c l e

i n f o

Article history: Received 26 March 2011 Received in revised form 24 June 2011 Accepted 22 September 2011 Keywords: Phase diagram Dilute riser flow Turbulent fluidized bottom bed Core-annulus flow Dense riser up-flow

a b s t r a c t Riser operating modes are vital to designing a circulating fluidized bed (CFB) reactor for a required process of either a gas–solid or a gas–catalytic nature. Different operating modes provide different solids’ residence times and mixing behaviors, which define the reactions’ efficiency and yield. The literature demonstrates distinct operating modes resulting from observed differences in slip factors and the range of particle velocities and their associated residence time distribution. The present research uses positron emission particle tracking (PEPT) in a riser of B-type bed material to determine the different operating modes by measuring (i) particle velocities and residence time distribution, (ii) population densities of these particles in the cross-sectional area of the riser, and (iii) solids flow pattern at the bottom of the riser. Data treatment defines four distinct solids hold-up regimes in the riser and proposes a “phase diagram” depicting the existence of the different operating modes (dilute, dense, core-annulus and combined) as a function of the superficial gas velocity and solids circulation flux in the riser. The delineated regimes have good agreement with available literature data and known industrial operations. Comparison with literature data for risers using A-type powders is also fair. The diagram enables CFB designers to better delineate operating characteristics. © 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

1. Introduction and objectives Circulating fluidized beds (CFBs) have widespread industrial applications, mostly in the petrochemical, energy and pollution control sectors. The high air velocity and solids’ circulation induce good mass and heat transfer in the riser where process reactions occur, enhancing process efficiency and yield. The CFB comprises a riser (the key-component), cyclone, downcomer, and solidsrecycle valve. The bed material can be sand, fly ash, catalyst or minerals depending on the specific application. Two types of applications generally use CFBs, i.e., gas–solid and gas–catalytic reactions (Mahmoudi, Seville, & Baeyens, 2010): the former mostly uses B-type bed material, and the latter mostly uses A-type catalysts (Kunii & Levenspiel, 1990; Squires, 1985). Tables 1a and 1b provide typical examples of current research and applications for gas–catalytic and gas–solid reactions, respectively (Van de Velden, Baeyens, & Smolders, 2007; Zhu & Bi, 1995). Although CFB history can be traced to the late 1930s (Squires, 1985) with subsequent widespread use, the CFB-riser’s solids flow mode is still not fully

∗ Corresponding author. Fax: +32 16 538729. E-mail address: [email protected] (J. Baeyens).

understood. The degree of interaction between the solid and gas phases can differ across operating modes. Several authors have reviewed the CFB hydrodynamic regimes and their specific operating conditions, and have presented results for both solids and gas flows in the riser. We critically assess these literature findings and our research results to present the reader with an overall picture of the different solids flow regimes. The results from our positron emission particle technique (PEPT) measurements provide on-line and real-time evidence of these operating regimes.

2. Literature findings on different riser regimes Gas velocity is the dominant operating parameter in fluidization. In circulating fluidized beds and pneumatic conveyors, the solids loading in general, and solids circulation rates in the CFB are also important. Avidan and Yerushalmi (1982) and Yerushalmi and Crankfurt (1979) presented the different operating regimes in terms of system voidage and slip velocity, respectively. Eq. (1) defines the slip velocity and relates particle velocity with interstitial gas velocity, U/ε. Uslip =

U − v¯ p . ε

(1)

1674-2001/$ – see front matter © 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

doi:10.1016/j.partic.2011.09.002

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Nomenclature Ar D dp F g G ReTR rv (rv )0.16 (rv )0.5 (rv )0.84 rt (rt )0.16 (rt )0.5 (rt )0.84 S(rt ) S(rv ) U Uc UCh Umf Usalt Uslip Ut UTR v¯ p ε εCh εmf εb g g p ϕ

Archimedes number (–) diameter of riser (m) diameter of particle (m) cumulative distribution (–) gravitational acceleration (9.81 m/s2 ) solids circulation flux (kg/(m2 s)) Reynolds number at transport velocity (–) ratio of vp /¯vp (–) value of rv at F(rv ) = 0.16 (–) value of rv at F(rv ) = 0.50 (–) value of rv at F(rv ) = 0.84 (–) ratio of t/t¯ (–) value of rt at F(rt ) = 0.16 (–) value of rt at F(rt ) = 0.50 (–) value of rt at F(rt ) = 0.84 (–) Residence time span, ((rt )0.84 − (rt )0.16 )/(rt )0.50 (–) Velocity span, ((rv )0.84 − (rv )0.16 )/(rv )0.50 (–) superficial air velocity through the riser (m/s) transition velocity to turbulent fluidization (m/s) choking velocity (m/s) minimum fluidization velocity (m/s) saltation velocity (m/s) slip velocity (m/s) terminal velocity of particle (m/s) transport velocity (m/s) average net upward velocity of particle (m/s) voidage in riser (–) voidage at choking (–) voidage at minimum fluidization (–) voidage of the bottom bed (–) viscosity of air (kg/(m s)) density of air (kg/m3 ) density of solids (kg/m3 ) slip factor, ϕ = U/(εUS ) (–)

Fig. 1 combines and adapts previous presentations (Avidan & Yerushalmi, 1982; Yerushalmi & Crankfurt, 1979) with representative values of major characteristic parameters. Table 2 presents empirical equations as examples to predict the regime transition velocity, Utrans . For pneumatic conveying applications, additional critical velocities include the choking velocity (UCh ) for vertical transport and the saltation velocity (Usalt ) for horizontal transport. A CFB-riser operates at moderate to high slip velocities and ε  0.8. The process reactions occur in the CFB riser, where both the solid circulation flux, G (kg/m2 s), and superficial gas velocity in the riser, U (m/s), govern hydrodynamics. Gas and solids have close temperatures due to the high inter-phase heat transfer rates. The temperature profile along the riser length is uniform for low energy change reactions (Coudurier, Decottignies, Loukah, & Vedrine, 1993; Van de Velden, Baeyens, & Boukis, 2008), and the types and rates of chemical reactions thus occurring (which are temperature-dependent) can also be easily and uniformly controlled. Chang and Louge (1992) indicate that the flow in the riser is virtually independent from conditions in the feeder and solids return loop if G is controlled; the riser flow nature is thus isolated from other CFB parts and is a unique function of riser geometry and the common operating parameters (U, dp , p , g , D, g and g). The properties in the return loop (standpipe) influence the nature of flow in the riser if G is not properly controlled (Schnitzlein & Weinstein, 1988).

Table 1a Typical examples of CFB gas/catalytic reactors (7; Zhu & Bi, 1995). Reactions

References

Fluid catalytic cracking (FCC)

Squires (1985), Wu, Cheng, Ding, and Jin (2010), Jones and Pujado (2006), James and Glenn (2001), Sadeghbeigi (2000), Namkung and Kim (1998), Van Landeghem et al. (1996), King (1992), Avidan, Edwards, and Owen (1990), Almuttahar and Taghipour (2008) Lu and Lee (2007), Demirbas (2007), Steynberg, Dry, Davis, and Berman (2004, chap. 2), Tijmensen, Faaij, Hamelinck, and van Hardeveld (2002), Schönfelder, Hinderer, Werther, and Keil (1994), Bartholomew (1991), Shingles and McDonald (1988), Dry (1981)

Fisher–Tropsch

Paraffin oxidation Partial oxidation of n-butane to maleic anhydride

Oxidation of alkanes to alcohols and ketones Ammoxidation Benzonitrile from toluene Acrylonitrile

Allylic oxidation o-Xylene/naphthalene oxidation to phthalic anhydride Acrolein from propylene Dehydrogenation Oxidative dehydrogenation of butane to butadiene Ethylene from ethane Ethylene epoxidation

Hutchenson, La Marca, Patience, Laviolette, and Bockrath (2010), Fernández, Vega, and Díez (2010), Shekari, Patience, and Bockrath (2010), Pérez-Moreno, Irusta, Soler, Herguido, and Menéndez (2009), Emig, May, and Scheidel (2002), Contractor (1999), Contractor et al. (1994), Pugsley, Patience, Berruti, and Chaouki (1992) Patience and Bockrath (2010), Subramani and Gangwal (2008), Pannek and Mleczko (1998), Lyons (1993) Gianetto, Pagliolico, Rovero, and Ruggeri (1990) Hu, Zhao, Wei, and Jin (2007), Fakeeha, Soliman, and Ibrahim (2000), Nakamura, Arai, Inaba, and Yamamoto, (1998), Beuther, Innes, and Swift, (1978), Lankhuyzen, Florack, and van der Bean (1976) Wainright and Hoffman (1974)

Patience and Mills (1994) Liu, Zhang, Luo, and Yang (1989) Coudurier et al. (1993) Park and Gau (1986)

For the operating gas velocity (U), a stable CFB-operation requires external solids circulation and is only possible at velocities exceeding transport velocity (UTR ). Van de Velden, Baeyens, Seville, and Fan (2008) demonstrated that Eq. (2), derived by Bi and Grace (1995), fairly predicts UTR for a wide variety of powders. Operating the riser at a velocity in excess of UTR is recommended, as empirical correlations are expected within 10% accuracy for different powder systems. ReTR = 1.53Ar 0.5 .

(2)

Previous studies observed a bed at the bottom and dilute phase at the top of the riser (Kato et al., 1989; Li, Tung, & Kwauk, 1988; Mori et al., 1992; Rhodes, Sollaart, & Wang, 1998; Van de Velden, Baeyens, Doughan, & McMurdo, 2007). The axial solids hold-up profile has an inflection point, and this profile is an S-profile. The bed progressively deepens under increasingly higher solid flux (Bai, Jin, Yu, & Zhu, 1992; Chan, Seville, Yang, & Baeyens, 2009; Karri & Knowlton, 2002; Malcus, Cruz, Rowe, & Pugsley, 2002; Manyele, Khayat, & Zhu, 2002). Other researchers, however, indicate that an exponential profile for solids hold-up exists without a dense bed but with an acceleration zone at the bottom of the riser, considered as characteristic of other regimes, such as core-annulus flow without bed, dilute transport and/or dense core flow (Bai et al.,

S. Mahmoudi et al. / Particuology 10 (2012) 51–61 Table 1b Typical examples of CFB gas/solids reactors (7; Gungor, 2008). Reactions

Table 3 Typical voidage (ε) for the distinctive hydrodynamic modes (Gungor, 2008). DRF

References

Combustion of coal, wood, biomass

Van de Velden, Baeyens, Doughan, et al., (2007), Krzywanski, Czakiert, Muskala, Sekret and Nowak (2010), Khan, de Jong, Jansens, and Spliethoff (2009), Koukouzas, Ward, Papanikolaou, Li, and Ketikidis (2009), Gungor (2008), Stamatelopoulos, Semedard, and Darling (2008), Nowak (2003), Smolders and Baeyens (2003), Anders, Beisswenger, and Plass (1991), Grace (1990), Dry and La Nauze (1990), Reh (1971) Boukis, Bezergianni, Grammelis, and Bridgwater (2007), Van de Velden et al. (2008), Van de Velden, Baeyens, Brems, Janssens, and Dewil (2010), Dai, Yin, Wu, Zhang, and Chen (2001), Bridgwater and Peacocke (2000) Bi and Liu (2010), Liao, Wu, and Yan ˜ Cabanillas, and (2007), García-Ibanez, Sánchez (2004), Raskin, Palonen, and Nieminen (2001) Reh (1971, 1995), Dai et al. (2001), Hirsch, Janssen, and Serbent (1986)

Pyrolysis of coal and biomass

Gasification of coal and biomass

Calcination of Al(OH)3 to alumina (Al2 O3 ), calcinations of clay Pre-calcining of cement raw material or calcination of limestone/dolomite Synthesis of AlF3 from Al(OH)3 and 98% HF Reduction of iron ore

Stanmore and Gilot (2005), Hartman, Trnka, and Svoboda (2000) Reh (1971) Suzuki, Kunitomo, Hayashi, Egashira, and Yamamoto (1990), Yu (1994) Saxena and Jotshi (1996), Hallström and Karlsson (1991), Rickman, Holder and Young (1985) Leuschke, Bleckwehl, Ratschow, and Werther (2008), Yi, Sauer, Leuschke, and Baege (2005), Sauer and Baege (1998), Hansen, Toher, Lanois, and Sauer (1991), Porter (1994), Graf (1986)

Incineration

Desulphurization

ε=

Table 2 Prediction of transition velocities (100 ␮m particles, p : 2600 kg/m3 ). Equations

Umf

1.07 2 Ar = 1823Remf + 21.27Remf

Uc UTR

Rec = 1.24Ar0.45 , for 2 < Ar < 108 ReTR = 1.53Ar0.5

Value (m/s)

UCh

2gD(ε−4.7 −1) Ch (UCh −Ut )2

Usalt

Usalt = 4.43

= 0.00872g0.77

 p −g g

dp

Wu and Baeyens (1991) Bi and Grace (1995) Bi and Grace (1995)

0.009 1.33 2.05

Punwani, Modi, and Tarman (1976)

a

Brook (1985)

1.9

a

UCh is a function of the variable solids loading, so a specific UCh value could not be calculated.

1992; Bi & Grace, 1995; Hartge, Rensner, & Werther, 1988; Kato et al., 1989; Rhodes & Geldart, 1989). From the extensive literature survey and our experiments, four distinct solids hydrodynamic modes emerge. We hereafter use common terms and their associated acronyms to avoid confusion; some previously used terms, such as dilute pneumatic transport or dense core flow, are used interchangeably between CFB risers and pneumatic transport lines while the basis of their operation could be completely different. The four distinct regimes are depicted as follows. Dilute riser flow (DRF): In DRF, the solids predominantly move upwards with negligible downward flow. The axial voidage profile is typically exponential with one acceleration zone. The Geldart

CAF U U+(G/p )

ε=

U U+(2G/p )

CAF with TFBB

DRU

U CAF: ε = U+(2G/ p) TFBB: εb = U+1 U+2

ε=

U U+(G/p )

equation (Geldart, 1986) can predict solids velocity using the terminal particle velocity as slip velocity:

v¯ p =

U − Ut . ε

(3)

More generally, an alternate Eq. (4) has expressed the particle velocity, introducing the slip factor, ϕ, providing:

v¯ p =

U . εϕ

(4)

In DRF-regime, the slip factor, ϕ, has a previously reported values between 1 (Hartge et al., 1988) and 1.2 (Chan, Seville, Parker, & Baeyens, 2010). Core-annulus flow (CAF): The solids motion in CAF is an upward core flow and downward flow in the annulus. Voidage is typically exponential along the axial direction and Eq. (4) can predict the solids velocity, with ϕ values close to 2 (Matsen, 1976; Ouyang & Potter, 1993). Core-annulus flow (CAF) with turbulent fluidized bed at the bottom (TFBB): Here, the axial voidage profile has a typical S-nature, because a turbulent fluidized bed at the bottom (TFBB) exists. The residence time for CAF with TFBB is significantly longer than CAF and DRF due to the fully mixed TFBB. Chan et al. (2009) demonstrated that the residence time for solids in TFBB alone ranges from 10 to 20 s. The TFBB voidage ranges from 0.7 to 0.9, and the empirical equation below can predict it (King, 1989): εb =

Utrans

53

U+1 . U+2

(5)

The characteristics of the CAF region above the TFBB are similar to those of a sole CAF flow, as described above. Dense riser upflow (DRU): Chan, Seville, et al. (2010) found that the DRU regime has similar characteristics as DRF, mainly differing in fractionally higher ϕ values, ranging between 1.2 and 1.6, with an average of 1.3 in DRU. Fig. 2 schematically represents these various hydrodynamics regimes, where the evolution of bed voidage with height in the riser is characteristic of operating modes, with voidages ranging from approximately 0.98 in a dilute flow (DRF), 0.7–0.9 in a bottomfluidized bed (TFBB), 0.95–0.98 in core-annulus mode (CAF), to ∼0.9 in a dense riser upflow DRU (Brems, Chan, Seville, Parker, & Baeyens, 2011; Smolders & Baeyens, 2001a, 2001b). Each regime shows similar behavior in the acceleration zone (AZ), except CAF with TFBB mode, where two acceleration zones exist, including an initial acceleration zone (IAZ) at the base of the TFBB. The equations in Table 3 can predict typical voidage values (Gungor, 2008). Recent experiments, reported in Chan, Brems, et al. (2010) examined the riser exit effect on the solids flow mode in the riser. When using an abrupt riser exit, densification of the flow occurs near the exit. The enhanced particle reflux near the wall is then responsible for the co-existence of downward flow in the riser. A further qualitative and quantitative distinction of the hydrodynamic regimes as a function of the dominant parameters, U and G, can be obtained from PEPT results of the velocity distribution, residence time distribution (RTD), acceleration zones, slip velocities and slip factors (ϕ). We describe and discuss this below.

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Fig. 1. Characteristics of various fluidization regimes.

3. Detailed investigation by PEPT 3.1. Experimental set-up The experimental set-up for the PEPT study should based on a complete view to properly define the hydrodynamic phase diagram of a CFB riser, as shown in Figs. 3(a) and (b), differing in ␥-ray

camera positions. The setup shown in Fig. 3(a) has been used to show the fully developed solids hydrodynamics zone, as presented by Van de Velden et al. (2007), Van de Velden, Baeyens, Seville, et al. (2008) and Chan, Seville, et al. (2010), and further examined in the present paper. The set-up of Fig. 3(b) studies the bottom part of the riser to view the acceleration zone, and determines the conditions whereby a bottom bed starts to appear and disappear. This

Fig. 2. Characteristic hydrodynamic regimes in a riser, with zone-numbers corresponding to the description in the text (zone II: dilute riser flow DRF combined with bottom acceleration zone AZ; zone III: core-annulus flow CAF with bottom acceleration zone AZ; zone IV: initial acceleration zone IAZ, followed by a turbulent fluidized bottom bed TFBB, and a second acceleration zone prior to the CAF zone; zone V: bottom acceleration zone followed by dense riser upflow DRU).

S. Mahmoudi et al. / Particuology 10 (2012) 51–61

55

of the riser was 4 m. The L-valve re-entry was situated 5 cm above the distributor plate. For PEPT imaging of the developed riser flow, the ␥-ray camera was located between 1.5 and 2.1 m above the distributor. For the bottom imaging, the camera was located from the distributor plate to ∼0.6 m higher up the riser. The bulk bed material is a typical Geldart B-type material: a rounded sand with a mean diameter of 120 ␮m and a particle density of 2260 kg/m3 , typical for CFB gas–solid operations. A single particle of size ∼120 ␮m was labeled with radioactive 18 F using anion-substitution surface adsorption (Fan, Parker, & Smith, 2006). The positrons emitted by the tracer particle annihilate with electrons to produce 2 co-linear ␥-rays, detected by the pair of ␥-ray cameras, having surface area 0.59 m × 0.47 m each. The particle positions were determined in real-time (∼1 location every 4–10 m s). A list of consecutive locations in three dimensions (X, Y and Z coordinates) was obtained. The operating values of U and G were varied between 1 and 10 m/s and between 25 and 622 kg/(m2 s) respectively. Geiger counters checked whether the particles circulated and estimated the solid circulating rate from the tracer velocity in the downcomer to adjust G to the desired value. 3.2. PEPT results

Fig. 3. Experimental set-up, with (a) and (b) differing in ␥-ray camera positions.

zone extends to a height of 0.2–0.4 m above the solids circulation re-entry point only and can be predicted from the balance of forces on the particles within this zone (Chan et al., 2009). Different risers were used, all with sharp (abrupt) exit configuration and dimensions of 0.046, 0.09 and 0.16 m I.D. The total height

3.2.1. Illustration of PEPT imaging Van de Velden et al. (2007) and Van de Velden, Baeyens, Seville, et al. (2008) observed that, under a constant G but with increasing U, the riser hydrodynamics gradually shift from one operating mode to another, as shown in Fig. 4. With increasing U values but at a constant G, the annulus progressively disappears, and the core flow gradually occupies the entire riser cross-section at U = 5.1 m/s. Fig. 5 shows additional observations, where U is maintained constant while G is gradually increased. At low G-values (<∼10 to 20 kg/(m2 s)), the solids descend below the solids entry point, i.e., the L-valve, then they are accelerated and conveyed up the riser in a dilute transport mode. At intermediate values of G and/or

Fig. 4. Cross-sectional view of the riser with downwards moving particles (top) and upwards moving particles (bottom), at a solids circulation rate of 260 kg/(m2 s) (Van de Velden et al., 2007; Van de Velden, Baeyens, Seville, et al., 2008).

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S. Mahmoudi et al. / Particuology 10 (2012) 51–61

Fig. 5. PEPT views of the riser bottom, at U − UTR = 2.1 m/s, for G = (a) 5.5, (b) 20.1, (c) 55.5 and (d) 210 kg/(m2 s) (Chan et al., 2009).

U, a TFBB forms. At very high G-values, no TFBB is seen and the tracer immediately assumes a core flow mode, previously referred to as a DRU mode. A TFBB can therefore only exist in a specific range of combined U and G operating values (Chan et al., 2009). Van de Velden et al. (2007) and Van de Velden, Baeyens, Seville, et al. (2008) recommended that DRU operation occurs at U > (UTR + 1) m/s and G ≥ 200 kg/(m2 s). The present paper further refines these findings and presents a PEPT-based phase diagram, identifying four regimes in a CFB riser: DRF, CAF, CAF with TFBB and DRU. 3.2.2. Assessment and discussion of PEPT results 3.2.2.1. Acceleration zone. The acceleration length and time are nearly constant at approximately 0.26 m and 0.21 s regardless of U and G and are independent from solids hydrodynamic modes, as previously demonstrated (Chan et al., 2009). For industrial risers with heights of 10–20 m, the acceleration zone has limited influence on the overall solids hydrodynamics. 3.2.2.2. Particle velocity and residence time distribution. Recent experiments have determined average particle velocity and their distribution (Chan, Seville, et al., 2010; Mahmoudi, Baeyens, & Seville, 2011). With known particle viewing length, i.e., the PEPT camera height, velocities were respectively transformed to residence time distributions. Results were expressed in cumulative

distribution functions of velocities, F(rv ), and residence times, F(rt ), with respective spans S(rt ) and S(rv ). Detailed analyses of these findings occur in previous studies (Chan, Seville, et al., 2010; Mahmoudi et al., 2011). For velocity distribution, S(rv ) is defined as: S(rv ) =

(rv )0.84 − (rv )0.16 . (rv )0.50

(6)

For residence time distribution, S(rt ) is defined as: S(rt ) =

(rt )0.84 − (rt )0.16 . (rt )0.50

(7)

The spans of the velocity distribution curve, S(rv ), and the residence time distribution curve, S(rt ), define the degree of real solids flow deviating from a plug flow, understood as the constant velocity of all particles throughout the riser cross-section and height. The F-values at 84% and 16% represent the standard deviation in a Gaussian distribution. For plug flow, S would be 0, as (rv )0.84 = (rv )0.16 = (rv )0.50 and (rt )0.84 = (rt )0.16 = (rt )0.50 . Larger values of S indicate more mixing, deviating from plug flow illustrated in cited references (Chan, Brems, et al., 2010; Fan et al., 2006) and not included in the present behavior. The evolution of velocity and residence time distributions with U and G are extensively paper.

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Fig. 6. S(rt ) vs. U − UTR at various G ranges.

Fig. 6 depicts all PEPT residence time distribution findings, with their respective S(rt ) as a function of U − UTR within specific G value ranges. The S(rt ) dependency illustrates that operating modes differ as a function of U − UTR and/or G. A low value of S(rt ) represents operating in a dilute mode or DRF (low G, <30 kg/(m2 s), irrespective of U − UTR ), closely resembling solids plug flow. Intermediate values of G show high values of S(rt ), especially at lower ranges of U − UTR . This behavior involves significant solids back-mixing and is representative of the CAF flow mode. At higher U − UTR values and intermediate G-values, the S-values diminish with increasing U − UTR , indicating that the operation moves towards a more pronounced plug flow and less pronounced back mixing mode. At high values of G (>80 kg/(m2 s)), S-values are close to 1 at low values of U − UTR , but decrease steadily with further increasing U − UTR to approach the condition representing solids plug flow. This operating mode corresponds to DRU. 3.2.3. Slip factor Slip factor is also expected a function of hydrodynamic regimes. This has been previously examined in detail (Fan et al., 2006) and is summarized below, directly related to the operating modes, as viewed by PEPT occupancy plots (Fig. 4). PEPT particle velocity measurements determined the slip factor ϕ, represented in Fig. 7. Fig. 7(a) presents results for the dilute and dense riser flows. Provided U − UTR exceeds about 2 m/s, the slip factor ϕ has fair agreement with the previous findings of ϕ ∼ 1 in DRF (dilute) and ϕ ∼ 1.3 in DRU (dense upflow). Only at low values of U − UTR does ϕ deviate from these average values. This deviation occurs when, at these low values of U − UTR (<1.5 m/s), the dilute or dense flow regimes are not fully established and a core-annulus regime better represents the operation. This leads to significant back mixing in the annulus region and a net upward velocity below the predicted value, due to the internal annulus downward circulation flux. This finding corresponds with earlier results in Patience, Chaouki, Berruti, & Wong, 1992), where the slip factor at U ≤ 6 m/s (or U − UTR < 3.8 m/s in our experiments) exceeds the theoretical value and can be predicted by the following: ϕ =1+

5.6 + 0.47Fr 0.47 . Fr

(8)

In the experimental rig (0.09 m I.D.), Fr ∼ 1 at low velocities and ϕ can reach values of ≥6, as confirmed in Fig. 7(a). In the intermediate regime of U and/or G, corresponding with operations in core-annulus flow or core-annulus flow with

Fig. 7. Slip factor, ϕ, versus U − UTR for various ranges of G: (a) in dilute (DRF) and dense (DRU) riser flows regimes and (b) in the sole CAF riser flow regime.

bottom turbulent fluidized bed, a distinction between these two flow regimes must be made since the total residence time of particles in the TFBB should be deducted from their total residence time in the riser, if the sole contribution of the CAF regime should be considered (Chan et al., 2009). Assuming that this TFBB residence time is between 5 and 10 s as a function of reducing U and/or increasing G (Van de Velden et al., 2007), the corrected residence time in the sole CAF regime was used to calculate ϕ, as presented in Fig. 7(b). In the separate CAF flow regime, ϕ exceeds the value of 2 at lower U − UTR values. This finding again partly corresponds with earlier results in Patience et al. (1992), showing that a slip factor of 2 is only valid provided U > 6 m/s. At operating velocity conditions of U ≤ 6 m/s, ϕ exceeds 2 and can be predicted by Eq. (8). Smolders and Baeyens (2001a, 2001b) further demonstrated that the slip factor in the CAF-region is a function of height in the riser. Because Fr is a function of U only, Eq. (8) does not cover the measured ϕdependency of G. The use of ϕ = 2 is thus not recommended for the CAF regime. Clearly, ϕ is on average close to 1 in the DRF regime and ∼1.3 in the DRU regime for U − UTR > 2 m/s. For both the DRF and DRU regimes, ϕ increases at lower values of U − UTR . ϕ ∼ 2 in the core-annulus (CAF) regime, that is, for G-values between 40 and 100 kg/(m2 s) and U − UTR > 2 m/s. For lower values of U − UTR , predictions by Eq. (8) have fair agreement with the experimental data. To obtain more complete insight into the CAF (and TFBB) regime, a fundamental model must be established to account for the complex flow mode in the core-annulus regime, that is, to couple a core of dominant upflow and an annulus of dominant downflow, with particles transferring in between. Such a model is currently under development.

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Fig. 8. Distribution of PEPT data for different hydrodynamic flow modes.

3.2.4. Combination of the previous findings with respect to the parameter influences The general diagram of Fig. 8 summarized all findings of crosssectional occupancy plots, axial velocity profiles, particle velocities and their distributions, as well as the regime-dependent ϕ factors. The data corresponding to Fig. 5 are given as separate symbols, (a)–(d). The diagram and its tentative separation curves illustrate the dependence of operating modes on U and/or G. The demarcation lines between the regimes in Fig. 8 separate the different experimentally observed regimes and can be fitted by the appropriate equations: II/III :

G = 10 + (U − UTR )1.8 , 2

(9)

III/IV :

G = 20 + (U − UTR ) ,

(10)

IV/V :

G = 60 + 15(U − UTR )0.5 .

(11)

The velocity range close to UTR is not indicated, as some data points are in a riser flow mode, while others are still in a turbulent fluidized bed mode. Respecting a velocity margin of about 0.2 m/s above UTR is therefore recommended. 3.2.5. Comparison to literature data Each main regime has its own distinct limits of U and G in which it can exist. Literature data for risers operating with B-powders are limited to the lower (U − UTR ) range and for G-values between 30 and 70 to 90 kg/(m2 s) only. These data points are all within the CAF operating mode, with or without a TFBB. From the published data, distinguishing CAF without TFBB to CAF with TFBB regime is difficult, due to the lack of detailed axial voidage and/or axial pressure drop gradient profiles. A wider range of (U − UTR ) and/or G has been studied and more fully reported for A-type powders, generally with specification of the observed riser flow mode. Literature data for B-type powders are represented in Fig. 9(a), together with the above defined demarcation lines. Fig. 9(b) presents a similar figure for A-type powders. In general, the operating mode demarcations, as predicted by the appropriate equations, correspond fairly well with the

Fig. 9. The different operating regimes compared to literature data for (a) group B and (b) group A powders.

literature data, though the region of DRF extends to higher G-values at low U − UTR values for A-type powders. The CAF-regime disappears at velocities exceeding U − UTR ∼ 6 m/s, after which operations only in DRU and DRF have been reported. Within the same CAF-regime, the literature findings do not distinguish between CAF and CAF with TFBB sub-regimes. Considering particle entrainment can, however, distinguish between the CAF and CAF with TFBB sub-regimes. A TFBB forms when the flux of the recycled solids, expressed by G, exceeds the carry-over flux. Smolders and Baeyens (2001b) have reviewed equations to determine this carry-over flux, preferring the Geldart equation (Smolders & Baeyens, 2001a), which slightly overestimates the carry-over flux at high operating velocities.

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that includes all known operating regimes has been presented. To operate in a DRU mode, as preferred for gas–catalytic reactions requiring strict residence time control, G should exceed approximately 100 kg/(m2 s). To operate in a CAF mode typical for applications such as calciners, gasifiers and combustors, possibly with a turbulent fluidized bed at the bottom (TFBB) of the riser, lower G-values must be applied, as illustrated in Fig. 10. The proposed CFB phase diagram enables CFB designers to better delineate the operating regimes and improve the predictability of CFB behavior. References

Fig. 10. Operating modes of a CFB riser, expressed as G vs. U − UTR depending on the different hydrodynamic flow modes.

Applying these findings to a CFB system with Ut = 0.7 m/s and UTR = 2.2 m/s reveals that the carry-over flux is limited to ∼20 kg/(m2 s) at U = 2.3 m/s (U − UTR = 0.1 m/s) or ∼75 kg/(m2 s) at U = 5.2 m/s (U − UTR = 3 m/s). At higher velocities (U − UTR > 6 m/s), the system moves into DRU mode. The CAF regime clearly should be divided into 2 sub-modes according to the limiting carry-over criterion. This theoretical sub-division of the CAF regime, calculated by the illustration above, has fair agreement with Eq. (10) for the III/IV transition. 3.2.6. Comparison with commercial riser operation Mahmoudi et al. (2010) and Van de Velden et al. (2007) and Van de Velden, Baeyens, Seville, et al. (2008) reviewed the range of operating U and G for known commercial risers, with reported CAF operation in the range of U − UTR = 0.5–5 m/s and G-values of 10 to about 80 kg/(m2 s). CFB operations in DRU mode are reported for U − UTR values exceeding about 8 m/s and G-values between 150 and 1200 kg/(m2 s). These reported modes have good agreement with the proposed operation diagram in Fig. 9. 3.2.7. Overall solids flow diagram Fig. 10 overall represents the operating modes of CFB riser by combining PEPT observations and literature data for the various regimes, without the data points. Transitions between the DRF, CAF, CAF with TFBB and DRU modes are indicated according to the respective equations. This diagram enables CFB designers to better delineate operating modes using combined (U, G)-values. At very high U values, only DRF and DRU modes are maintained. The above reported modes for commercial risers have good agreement with the operation diagram proposed in Fig. 10. 4. Conclusions The present paper used particle emission positron tracking (PEPT) to study the movement of a tracer particle through (i) the solids velocities; (ii) the population densities of these solids in the cross-sectional riser area; (iii) the solids flow patterns near the distributor and L-valve; (iv) the existence of four different operating solids hold-up regimes in the riser. A phase diagram

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