Characteristics of Three-Phase (Gas–Liquid–Solid

740 Journal of Chemical Engineering of Japan Copyright © 2018 The Society of Chemical Engineers, Japan

Journal of Chemical Engineering of Japan, Vol. 51, No. 9, pp. 740–761, 2018

Characteristics of Three-Phase (Gas–Liquid–Solid) Circulating Fluidized Beds

Yong Kang1, Min Kon Kim2, Si Woo Yang1 and Sang Dong Kim3

1 Department of Chemical Engineering, Chungnam National University, Daejeon 34134, Korea (Rep. of)2 Chemie und Bioingenieurwesen, Freidrich Alexander Universität, Schloßplatz 4, 91054 Erlangen, Germany

3 Chemical and Biomolecular Engineering. KAIST, Daejeon 34141, Korea (Rep. of)

Keywords: Three-phase (G/L/S), Circulating Fluidized Beds, Hydrodynamics, Heat and Mass Transfer

The characteristics of three-phase (gas–liquid–solid) circulating �uidized beds (TPCFBs) are discussed in order to visualize the unique features and advantages thereof due to the intrinsic �ow and contacting behaviors of multi-phases, providing a better understanding of the state of art of TPCFBs and their feasible applications as multi-phase reactors and contac-tors. The hydrodynamics such as individual phase holdups, bubbling �ow behaviors, bubble properties and liquid phase dispersions, and heat and mass transfer characteristics in the riser were examined based on the previous investigations. Although the information in the riser of various kinds of TPCFBs deviated from each other depending on the solid circula-tion modes and experimental conditions, they were summarized and consociated for the elucidation of the present states and views of the investigations. Rational guides to predict the hydrodynamics and heat and mass transfer phenomena in the riser of the TPCFB were possible by analyzing and synthesizing the results reported in the literatures presently avail-able. Especially, the e�ects of operating variables including gas (UG) and liquid (UL) velocities, properties of �uidized solid particles and continuous liquid media, and solid circulation rate (Gs) on the hydrodynamic parameters and the heat and mass transfer characteristics such as heat transfer coe�cient (h) and resistance, volumetric mass transfer coe�cient (kLa), gas–liquid interfacial area (a) and liquid side mass transfer coe�cient (kL) were determined. The information on the heat and mass transfer, however, were extremely limited comparing with those of hydrodynamics. Some correlations were suggested to predict the hydrodynamic parameters, and the values of h, kLa, a, and kL in the riser of the TPCFBs to provide insights for the present and future studies. The mechanism of heat and mass transfer and their modeling should be con-ducted for the better prediction of the performance of the TPCFB-reactors and contactors in the future.

Introduction

A three-phase circulating fluidized bed (TPCFB), of which a typical schematic diagram can be seen in Figure 1, usually consists of three main sections including a riser column, gas–liquid–solid separator and solid recycle device. The characteristics of TPCFB have been mainly studied for the riser, since the riser could have a great potential for ap-plication in various fields of multi-phase reacting and con-tacting processes. The riser of TPCFB constitutes upstream of solid particles which are fluidized and entrained by means of the gas and liquid phases flowing upward continuously. The entrained particles are separated from gas and liquid phases by means of a separator which is located at the top of the riser column. The separated solid particles flow down to the particle storage system and are re-circulated into the bottom of the riser through the solid recycle device (Zhu et al., 2000; Kim and Kang, 2006; Atta et al., 2009).

Taking advantage of the solid circulation mode with high

velocity between the riser and the downer (downcomer), the liquid–solid circulating fluidized bed (LSCFB) was de-veloped in order to utilize the particles as bio-solid, ion-exchanger, absorbent, adsorbent, catalyst or catalyst media effectively (Jin et al., 1994; Zhu et al., 2000; Lan et al., 2002a; Zheng et al., 2002; Cho et al., 2005; Atta et al., 2009; Kalaga et al., 2012; Jin et al., 2013).

The TPCFB, which was proposed by introducing the gas phase into the LSCFB, has been developed due to the great demands of potential applications in the diverse fields of biochemical, petrochemical, food, energy, environmental, metallurgical and medical engineering due to their highly unique features and advantages (Zhu et al., 2000; Kim and Kang, 2006; Atta et al., 2009). It can overcome the restric-tions and drawbacks of the conventional three-phase fluid-ized bed (TPFB) owing to the various kinds of properties of particles and continuous liquid media, since the convention-al bed should be operated within the normal range of UL be-tween the minimum fluidization and terminal velocities of solid particles in the liquid media. In addition, the TPCFB could realize special functions as a reactor or a contactor with highly effective contacting among different gas, liquid and solid phases, high heat and mass transfer rates, continu-ous processing and regeneration of the contaminated media or deactivated catalysts, effective removal or supply of heat as required, high fractional conversion, minimal dead zone,

Received on June 19, 2017; accepted on September 25, 2017DOI: 10.1252/jcej.17we205Correspondence concerning this article should be addressed to Y. Kang (E-mail address: [email protected]).Presented at The 5th ASCON-IEEChE 2016 at Yokohama, November 2016

Journal Review

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and the high production efficiency per unit cross sectional area (Liang et al., 1995a, 1995b, 1995c; Zhu et al., 2000; Kim and Kang, 2006; Atta et al., 2009).

In the riser of TPCFB, the flow modes of gas, liquid and solid particles are co-current in the upward direction with rapid liquid velocity (UL) higher than the terminal velocity of the solid particles. Moreover, the injection of gas phase into the flow field of liquid–solid riser of LSCFB could rise to the unpredicted bubbling phenomena. Those flow modes of TPCFB can lead to quite different hydrodynamics, heat and mass transfer phenomena from those of conventional TPFB and LSCFB. Several attempts, therefore, have been made to investigate and analyze the phenomena due to the inherent unique features of TPCFB. The investigations are including the individual phase holdups, liquid dispersion and mixing, flow regime, bubble properties, hydrodynamic-modeling, simulation and similarity, heat transfer, and gas–liquid mass transfer.

Although several studies have been conducted on the hy-drodynamics, the information on the heat and mass transfer in TPCFBs have been extremely scarce until now as de-scribed in Tables 1–3. In various investigations, the veloci-ties of gas (UG) and liquid (UL) phases, properties of liquid phases such as viscosity (μL) and surface tension (σL), prop-erties of solid particles such as size (dP) and density (ρP), and solid circulation rate (GS) have been important parameters

and variables.However, there have been little valuable and realistic

information to predict and understand the hydrodynamic characteristics and heat and mass transfer phenomena in TPCFBs until now (Zhu et al., 2000; Kim and Kang, 2006; Atta et al., 2009). Thus, in the present review, the important and unique features in the riser of the TPCFB were summa-rized and discussed in view of rational design, operation and application in possible industries. Comparing to TPCFB, the characteristics of the LSCFB have been more interesting and analyzed up to now for effective applications. Since analysis on the relatively simpler LSCFB could help the understand-ing of the TPCFB, the characteristics of LSCFBs were also surveyed and examined in this review.

1.　Phase Holdup and Flow Behavior in the Riser

In analyzing the dynamic multi-phase flow systems such as circulating fluidized beds, the basic attempt was to con-sider the hydrodynamic phenomena by measuring the in-dividual phase holdups and related flow behaviors. In the riser of both LSCFBs and TPCFBs, the solid particles are flowing as a discrete phase in the continuous liquid media, thus the axial and radial distributions of solid particles could be vital roles in elucidating the characteristics of circulating fluidized beds. It has been noted that the axial and radial distributions of εS are relatively uniform in TPCFBs as well as LSCFBs at fully developed flow conditions, as shown in Figure 2, comparing with those of conventional fluidized beds. This could be due to the rapid liquid velocity which is higher than the terminal velocity of fluidized solid par-ticles (Cho et al., 2001a; Zheng et al., 2002; Liu et al., 2003; Vatanakull et al., 2003; Cao et al., 2009). Effects of operating variables on the values of εS and εG in TPCFBs and LSCFBs are summarized in Figures 3–5. The individual effects of each variable are discussed in the following sections.

1.1　Liquid–solid circulating �uidized beds (LSCFBs)The holdups of solid and liquid (void fraction) phases

were determined by measuring the pressure drop (∆P) in a given section. The axial and radial distributions and profiles of εS in the riser could be the resultant phenomena of liquid and solid flow behaviors; therefore, the distributions and profiles were used to analyze the hydrodynamics of LSCFBs. The axial distribution of εS was almost uniform (Liang et al., 1997b; Zheng et al., 1999; Rao et al., 2007; Chavan et al., 2009; Gnanasundaram et al., 2014a) under the fully devel-oped flow conditions, at which the flow behavior of a liquid and solid mixture could attain the steady state condition. The flow regime and liquid velocity should be adjusted to let the system attain the steady state flow condition in the riser depending on the terminal velocity of particles (Natarajan et al., 2008b, 2009, 2011).

Regarding the radial distribution, the local value of εS increased with increasing radial coordinate to the wall of the riser (Liang et al., 1996; Chavan et al., 2009), as shown in Figure 2. The radial profile of εS could be due to the resis-

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1. Riser 7. Compressor 13. A/D convertor2. Downer 8. Control valve 14. Computer3. Hoppor 9. Flowmeter 15. G/L distributor4. L/S separator 10. Resistivity probe 16. Liquid reservoir5. Pressure tap 11. Amplifier 17. Pump6. Butterfly valve 12. Low-pass filter

Fig. 1　Schematic diagram of a three-phase circulating fluidized bed

742 Journal of Chemical Engineering of Japan

Table 1　Summary of Hydrodynamics Studies in LSCFBs (LS) and TPCFBs (TP)

Researchers System mode Riser

Solid Liquid

RemarksKind dP×103 m ρP

[kg/m3] Kind μL×103 Pa·s

σL×103 N/m

Liang et al. (1995a, 1995b, 1996)

TP 0.140 m (ID) Glass beads 0.405 2460 Water 1.0 72.75 ·Phase holdups (εG, εS), flow regimeLS 3.0 m (H) ·Radial distribution

Liang and Zhu (1997a), Liang et al. (1997b)

LS 0.140 m (ID) Glass beads 0.40 2460 Water 1.0 72.75 ·Flow regime and εS

3.0 m (H) Silica gel beads 0.385 1360 ·Core-annulus modelHan et al. (1998) TP 0.140 m (ID) Glass beads 0.405 2460 Water 1.0 72.75 ·Da and Dr (infinite space model)

3.0 m (H) ·Effects of UG, UL, GS

Kim et al. (1999) TP 0.102 m (ID) Glass beads 2.1 2500 Water 1.0 72.75 ·Phase holdups (εG, εL, εS)3.5 m (H) ·Effects of UG, UL, GS

Yang et al. (1999a) TP 0.140 m (ID) Glass beads 0.40 2460 Water 1.0 72.75 ·Radial distribution of ULr3.0 m (H) ·Effects of UG, UL on Da and Dr

Cho et al. (2001a, 2001b) TP 0.102 m (ID) Glass beads 2.1 2500 Water 1.0 72.75 ·Radial εG and axial εS

3.5 m (H) ·Bubble properties (UB, LB, FB)Roy and Dudukovic (2001a);

Roy et al. (2001b, 2005, 2014)

LS 0.152 m (ID) Glass beads 2.5 2500 Water 1.0 72.75 ·GS in a closed loop3.35 m (H) ·Flow mapping and modeling

Shin et al. (2003) TP 0.102 m (ID) Glass beads 2.1 2500 Water 1.0 72.75 ·Liquid radial dispersion coefficient3.5 m (H)

Wang et al. (2001a, 2001b) TP 0.140 m (ID) Glass beads 0.4 2460 Water 1.0 72.75 ·εG and radial distribution3.0 m (H) ·Radial εG and UB distribution

Kang et al. (2003) TP 0.102 m (ID) Glass beads 2.1 2500 Water 1.0 72.75 ·Radial liquid dispersion3.5 m (H) ·Bubble properties and distribution

Lee et al. (2003) LS 0.102 m (ID) Glass beads 1.0, 1.7, 2500 Water ·εS and hydrodynamics3.5 m (H) 2.1, 3.0 CMC solution 1.0–38.0 ·Effects of UG, UL, dp, GS and μL

Liu et al. (2003) TP 0.076 m (ID) Glass beads 1.3 2500 Water 1.0 72.25 ·Radial εG and εS distribution2.0 m (H) ·Effects of UG, UL and GS

Vatanakull et al. (2003) TP 0.076 m (ID) Glass beads 0.4, 1.3 2500 Water 1.0 72.75 ·εG, εS and distribution2.0 m (H) ·Effects of UG, UL, dp, GS

Wang et al. (2003) TP 0.140 m (ID) Glass beads 0.4 2500 Water 1.0 72.75 ·UB and radial distribution3.0 m (H)

Yu et al. (2004) TP 0.102 m (ID) Glass beads 2.1 2500 Water 1.0 72.75 ·Axial liquid dispersion3.5 m (H)

Cho et al. (2005) LS 0.102 m (ID) Glass beads 1.0, 1.7, 2500 Water 1.0–38.0 ·Effects of P-fluctuation on Dr

3.5 m (H) 2.1, 2.0 CMC solution ·Effects of UL, dp, GS and μL

Rao et al. (2007) LS 0.076 m (ID) Plastic beads 0.5 1080 Water 1.0 72.75 ·Solid holdup3.0 m (H) Glass beads 0.6 2500 ·Solid circulation rate

Ceramic beads 0.23 1850Granite stone 0.31 2560

Son et al. (2007a) TP 0.102 m (ID) Glass beads 1.0 2500 Water 0.96–38.0 72.9–73.6 ·Stochastic analysis of P-fluctuations3.5 m (H) CMC solution ·Effects of UG, UL, μL and P on LB

Natarajan et al. (2008a, 2008b, 2009, 2011, 2014)

LS Bluestone 0.33 2850 Water 1.0 ·εS, drift flux modelSand 0.55 2775 ·Regime transition, stability

Silica gel 0.55 1060 ·Slip velocityCation resin 0.46, 0.55, 0.65 1325

Chavan et al. (2009) LS 0.05 m (ID) Ion exchange resin

0.36, 0.61, 0.72 1325 Water 1.0 72.75 ·εS, Up and their distribution

2.0 m (H) ·Effects of UL, dp

Shilapuram et al. (2008, 2009, 2011)

LS Glass beads 1.36 2468 Water ·εS, μL effectsGlycerol solution 0.892–1.36 ·Macroscopic properties

Shilapuram and Sai (2012)Kalaga et al. (2012) LS 0.05 m (ID) Resin particles 0.36–0.72 1100 Water 1.0 72.75 ·Axial liquid dispersion

2.0 m (H) Glass beads 0.1–0.7 2500 ·RTD measurementJin et al. (2013) LS 0.102 m (ID) Glass beads 0.5, 1.0 2500 Water 0.96 55.7–65.2 ·εS

2.5 m (H) 2.0, 3.0 Ethanol+Water 0.94–0.95 ·Effects of UL, dp and σS

Gnanasundaram et al. (2014a, 2014b)

LS 0.1 m (OD) Sand 0.5 2400 Tap water 1.0 72.75 ·GS, US, axial εS

2.4 m (H) Glass beads 2.0 2460 Aqueous glycerol 1.08–1.36 ·Effects of UL, dp, ρS and μL

Lim et al. (2014) LS 0.102 m (ID) Glass bead 0.5, 1.0,2.0, 3.0 2500 Water 1.0 72.75 ·εS and flow behavior3.5 m (H) ·Energy dissipation rate

Kim et al. (2015) TP 0.102 m (ID) Glass beads 0.5, 1.0, 2.0, 3.0

2500 Water 1.0 72.75 ·Solid circulation rate3.5 m (H) ·Effects of UG, UL, dp, L

Hong et al. (2016) LS 0.102 m (ID) Glass beads 0.5, 1.0, 2.0, 3.0

2500 Water 1.0–50.0 72.90–73.6 ·Solid circulation rate3.5 m (H) CMC solution ·Effects of UL, dp, μL, L

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tance to solid up-flow increasing with approaching towards the wall of the riser. Actually, the upward velocity of fluid-ized particles decreased with increasing radial coordinate in the riser (Chavan et al., 2009). The radial profile of up-flow particle velocity became more uniform with decreasing dP due to the decrease of dP/D ratio.

For the estimation of GS in LSCFBs, the method could be divided into two groups: one is based on the information of the velocity and volume fraction of particles in the riser (Roy and Dudukovic, 2001a; Roy et al., 2014), and the other is the estimation by measuring the amount of solid particles piled up in the solid recycle device (Kang et al., 2002a; Shin et al., 2005; Kalaga et al., 2012; Jin et al., 2013; Gnanasunda-ram et al., 2014b). From the results of several investigators, it could be stated that the value of GS increased with increas-ing UL (increasing UL1 and UL2), L or μL but decreased with increasing dP (Gnanasundaram et al., 2014b; Hong et al., 2016). The values of GS could be predicted from the Eq. (1) (Hong et al., 2016).

−= 0.553 0.635 0.392 0.101 0.303S L1 L2 P L31.016G U U d μ L (1)

Since the solid circulation is a unique feature in LSCFBs, the value of GS had been considered as a scaling parameter in a form of GS/ρS UL, of which the physical meaning is the ratio of particle velocity (US) and liquid velocity (UL). In

addition, the value of GS could be correlated in terms of di-mensionless groups as Eq. (2) (Hong et al., 2016).

= =

0.112 0.289L S L P L L P

S S L119.82U ρ U d U ρ d

G U μ L (2)

Herein, L is height of solid particles piled up in the solid recycle device.

Regarding the mean solid particle holdup (εS), it de-creased with increasing UL or μL but increased with dP, GS or σL in LSCFBs as shown in Figures 3 and 4 (Zheng et al., 1999; Lee et al., 2003; Vatanakull et al., 2003; Razzak et al., 2007; Atta et al., 2009; Shilapuram et al., 2011; Jin et al., 2013; Lim et al., 2014; Gnanasundaram et al., 2014a). The decrease of εS with UL or μL could be due to the decrease of residence time of particles in the riser, since the drag force acting on the particles by continuous upward liquid flow could increase with increasing UL or μL. That is, the fluidized particles could easily be entrained by an increase of upward drag force from liquid flow.

However, the residence time of particles in the riser could increase with increasing dP in a given operating condition due to the decrease in the upward velocity of particles, since the weight of particles could increase with dP. In addition, the increase of GS could lead to the increase of amount of solid particles in the riser. Thus, the value of εS could in-


Keys ■ ● ▲ ▼ ▽ ◁ ▷ ◆ ◇ ★ ☆ □ ○ △

Mode LS TP TP TPUG×102 m/s — 0.329–1.645 0–0.367 1–5UL×102 m/s 10–42 15.22 9.7 2US×102 m/s 0.2 — — —dP×103 m 508 1.3 0.433 1.45GS [g/m2 s] 14.2 14.3 6H [m] 0.3–1.7 1.2 — 0.6

Authors Zheng et al. (2002) Liu et al. (2003) Vatanakull et al. (2003) Cao et al. (2009)

Fig. 2 Radial profiles of εS in LSCFBs and TPCFBs

Fig. 2　Radial profiles of εS in LSCFBs and TPCFBs


crease with increasing dP or GS (Lee et al., 2003; Kim and Kang, 2006; Atta et al., 2009; Chavan et al., 2009; Jin et al., 2013; Lim et al., 2014; Gnanasundaram et al., 2014a). Those trends could be closely related to the hydrodynamic energy dissipation rate and turbulence intensity increasing with in-

creasing dP or GS (Lim et al., 2014). Effects of σL on the solid holdup were studied by Jin et al. (2013), who noted that the εS increased gradually with increasing σL in LSCFBs.

To predict and describe the overall and resultant flow behavior of solid particles in LSCFBs, Roy and Dudukovic

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Keys □ ○ △ ▽ ◇ ■ ● ▲ ▼ A B C D E ◆

Mode TP TP TP TP TP LSUG×102 m/s 1.83–6.0 1–7 0.33–1.645 0–0.303 0–4.88 —UL×102 m/s 6.0–12.0 30 15.22 9.7–20.8 11.2–26.63 10–42US×102 m/s 0.1–0.3 — — — — —dP×103 m 0.405 2.1 1.3 0.433 1.3 0.5μL×103 Pa·s 0.96 0.96 0.96 0.96 0.96 0.96GS [kg/m2 s] — 2 14.2 4.5–14.3 8–60 10σL×103 N/m 72.75 72.75 72.75 72.75 72.75 72.75

Authors Liang et al. (1995d) Cho et al. (2001a) Liu et al. (2003) Vatanakull et al. (2003) Razzak et al. (2007) Atta et al. (2009)

Keys ★ ☆ ◐ ◑ F G H I J K L M P Q R S T U

Mode TP LS LSUG×102 m/s 0–6.1 — —UL×102 m/s 5.6–35.0 15.5–36.5 22–44(UL−Ut )×102 m/s — — 1–7dP×103 m 0.25 0.5–3.0 1.0–3.0μL×103 Pa·s 0.96 0.96 24GS (kg/m2 s) — 2.5–6.0 2.0σL×103 N/m 72.75 72.75 72.75

Authors Razzak et al. (2009) Lim et al. (2014) Lee et al. (2003)

Keys V W X Y

Mode LS LSUG×102 m/s — —UL×102 m/s 7.4–34.7 2.1–9.2US×102 m/s 0.2 —dP×103 m 0.508 0.5μL×103 Pa·s 0.96 0.892–1.36GS [kg/m2 s] — —σL×103 N/m 72.75 —

Authors Zheng et al. (1999) Gnanasundaram et al. (2014a)

Fig. 3 Effects of UG, UL and dP on the solid holdup in LSCFBs and TPCFBs

Fig. 3　Effects of UG, UL and dP on the solid holdup in LSCFBs and TPCFBs

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(2001a) employed a two-fluid CFD model based on the kinetic theory of granular flow. Considering the effects of operating variables such as dP, ρS and size of equipment on the hydrodynamic model of LSCFBs, Cheng and Zhu (2005, 2008) examined the two-phase flow structures by means of CFD modeling. Shilapuram et al. (2009, 2011), Shilapuram and Sai (2012) examined the macroscopic flow properties in LSCFBs by considering the effects of different operation methods, liquid viscosities and liquid distributors. By means of numerical modeling and investigation, the effects of other operating parameters such as GS and UL including primary and auxiliary liquid velocities were discussed by Razzak et al. (2008, 2010) using CFD modeling. Roy et al. (2014) proposed a modeling framework for the simulation of the hydrodynamic characteristics of LSCFBs by using CFD modeling and a cluster structure-dependent drag model was examined by Liu et al. (2015). In addition, the hydrody-namic similarity was examined to obtain the new similarity

parameter for scale-up (Natarajan et al., 2014) and a full fac-torial design approach was discussed to develop a model for the prediction of hydrodynamics (Palkar and Shilapauram, 2015). Those works could contribute to the feasible design, operation and scale-up of LSCFBs.

1.2　Three-phase circulating �uidized beds (TPCFBs)Because the TPCFB is composed of fluidized particles

and concurrent upward flows of gas and liquid phases, the flow behavior in the riser can be highly complicated and ir-regular. The complicated flow behaviors were analyzed with adopting the stochastic methods, by choosing the pressure as an independent variable. The pressure fluctuations were analyzed by means of phase space portraits and Kolmogorov entropy (Cho et al., 2001a; Son et al., 2007b) to predict the overall and resultant flow behaviors in the riser. The phase space portraits of pressure fluctuation time series could visualize the complex flow behaviors of multiphase in the


Keys ■ ● ▲ ▼ ◆ □ ○ △ ▽ ★ ☆ ◇

Mode LS LS TP TP LS TPUG×102 m/s — — 0 1.46 — 0.083UL×102 m/s 10 21.5–33.5 11.2 20.75 24–40 15.2dP×103 m 0.5 0.5–3.0 1.3 1.3 1.7–3.0 0.433μL×103 Pa·s 0.96 0.96 0.96 0.96 0.96–38 0.96GS (kg/m2 s) 5–15 2.5–8.0 28–39 10–20 — 9.8–19.6σL×103 N/m 72.75 72.75 72.75 72.75 72.75 72.75

Authors Atta et al. (2009) Lim et al. (2014) Razzak et al. (2007) Liu et al. (2003) Lee et al. (2003) Vatanakull et al. (2003)

Keys ◐ ◑ A B C D E F G H I

Mode LS LS LSUG×102 m/s — — —UL×102 m/s 19 23.5dP×103 m 0.5 1.36 1.0μL×103 Pa·s 0.892–1.36 0.9–3.22 0.94–0.96GS (kg/m2 s) — 2.5–8.0σL×103 N/m 72.75 55.73–72.75

Authors Gnanasundaram et al. (2014b) Vidyasagar et al. (2011) Jin et al. (2013)

Fig. 4 Effects of GS, μL and σL on the solid holdup in LSCFBs and TPCFBs

Fig. 4　Effects of GS, μL and σL on the solid holdup in LSCFBs and TPCFBs


riser. That is, the attractor became more scattered and com-plex with an increase in UG, μL, or GS; however, it tended to be less complicated with increasing UL at the higher range, which could be corresponding to the complexity of the mul-tiphase flow in the riser.

The scale of time dependent multiphase flow in the riser was expressed more easily by using the Kolmogorov entropy of pressure fluctuations, since it could be a measure of the generation rate of new information in the system. The varia-tions of Kolmogorov entropy of pressure fluctuations in the riser agreed well with the shapes of attractors, implying that the turbulence and aggressive contacting among multiphase in the riser would increase with increasing UG or GS (Cho et al., 2001a). Son et al. (2007a) used the correlation dimen-sion of pressure fluctuations to consider the quantitative measure of the resultant bubbling phenomena in the riser due to the flow and contacting of three phases. The value of correlation dimension of pressure fluctuations increased with increasing UG or μL, indicating that the flow could be more complicated with increasing UG or μL.

The axial and radial profiles and distributions of individu-al phase holdups could also be resultant expressions of mul-tiphase flow in the riser of the TPCFB; therefore, the trends of distributions were used to analyze the flow characteristics in the riser. As in the cases of LSCFBs, the axial distribution of εS in the riser of the TPCFB was found to be almost uni-form (Cho et al., 2001a; Vatanakull et al., 2003), due to the relatively high UL.

Considering the effects of operating variables on εS, it was

found that the value εS increased with increasing UG, dP or GS but decreased with increasing UL or μL in TPCFBs as shown in Figures 3 and 4 (Cho et al., 2001a; Liu et al., 2003; Vatanakull et al., 2003; Razzak et al., 2007, 2009). Regard-ing the GS in TPCFBs, Kim et al. (2015) suggested Eq. (3) to obtain the value of GS in terms of dimensionless groups of operating variables from the experimental results.

+=

0.266p L1 L2 L pS

G L L

ρ(0.00837

)d U U dGU ρ μ L (3)

In TPCFBs, the gas holdup (bubble holdup), εG, is one of the inherent important features comprising the reaction and contacting modes in the systems. Regarding the gas holdup, it was noted that the local εG tended to increase near the center of the riser, showing a parabolic form (Jin et al., 1997; Cho et al., 2001a; Wang et al., 2001a). The difference of local εG with radial direction increased with increasing UG but decreased with an increase in UL, due to the more even distribution with increasing UL. (Cho et al., 2001a; Wang et al., 2001a); however, the local εG showed a local maximum sometimes with increasing UL (Cao et al., 2009). The aver-aged value of εG was found to increase with increasing UG (Wang et al., 2001a; Liu et al., 2003; Son et al., 2007a, 2007b, 2009) or dP (Son et al., 2009) but decrease with increasing μL; however, it decreases or does not change considerably with increasing UL or GS (Wang et al., 2001a; Liu et al., 2003; Son et al., 2007a, 2007b, 2009), as shown in Figure 5.

Son et al. (2009) tried to enhance the εG in the riser by

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Keys ◇ ★ ☆ ■ ● ▲ ▼ ◀ □ ○ △ ▽ ◆ A B C D

UG×102 m/s 1.8–5.4 0.3 1–7 0.329–1.645 0–0.303UL×102 m/s 0.63–12.72 0.054 1.0–3.0 38–44 15–22 9.7dP×103 m 0.405 0.4 0.4 1.0–3.0 1.3 0.433μL×103 Pa·s 0.96 0.96 1.0 0.96 0.96GS kg/m2 s — — 2.0–8.0 2.0–8.0 9.84–19.67 4.5–14.1σL×103 N/m 72.75 72.75 72.75 72.75 72.75 72.75

Authors Liang et al. (1995c) Wang et al. (2001a) Son et al. (2007a, 2007b) Son et al. (2009) Liu et al. (2003) Vatanakul et al. (2003)

Fig. 5 Effects of operating variables on the bubble (Gas) holdup in TPCFBs

Fig. 5　Effects of operating variables on the bubble (Gas) holdup in TPCFBs

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injecting the auxiliary liquid by means of swirling flow. The ratio of swirling liquid injected into the riser tangentially was in the range of 0–0.5. They could increase the εG up to 25–30% by increasing the residence time of gas (bubble) phase by adjusting the liquid swirling in the riser.

2.　Bubble Properties in the Riser

The frequency shape, size and rising velocity of bubbles have to change inevitably during the flow in the riser, due to coalescence and disintegration. It has been understood that the bubbles could contact not only other bubbles but also fluidized solid particle in the fields of turbulent liquid flow, which results in the coalescence and disintegration of bubbles in the riser of TPCFBs. Since the density of the bubble phase is quite different from that of solid particles and continuous liquid media, the bubble properties could affect the flow behavior of mixtures of gas, liquid and solid phases, and thus hydrodynamics and heat and mass transfer in the riser. By considering the effects of operating variables on the bubble properties, the unique bubbling phenomena could be easily elucidated. The bubble properties such as size (DB or LB), rising velocity (UB) and frequency (FB) were measured by using an electric resistivity probe (Kang et al., 2002a, 2002b, 2003), fiber optic probe (Wang et al., 2001a, 2001b, 2003) or electrical resistance tomography (Razzak et al., 2009).

The bubble size in TPCFBs was not uniform in radial

direction. It was reported that the bubble size in the riser of the TPCFB generally decreased with increasing the radial distance from the center, due to the tendency of bubbles to move to the center avoiding contact with the wall of the riser (Wang et al., 2001b; Kang et al., 2003; Liu et al., 2017). The effects of UG on the bubble size could depend on the experimental conditions and systems. That is, in the bed of a riser with 2.1 mm glass beads, the bubble size (in terms of LB) increased with increasing UG (Cho et al., 2001b; Kang et al., 2003; Son et al., 2007b). However, in the bed of a riser with 0.4 mm glass beads, the bubble size decreased slightly with increasing UG (Wang et al., 2001b), as shown Figure 6. The different trend could be due to the different dP, εS level, UL range and solid circulating device. The bubble size de-creased with increasing UL or GS from the measurement of the former group but it did not change considerably with UL and increased slightly with GS from the latter group.

The value of UB increased with increasing UG in both beds with relatively large (dP=2.1 mm) and small (dP=0.4 mm) particles (Cho et al., 2001b; Wang et al., 2001b; Kang et al., 2003). In addition, it increased with increasing UL but de-creased slightly with increasing GS in the beds with relatively large particles (Cho et al., 2001b; Kang et al., 2003; Son et al., 2007b), while it did not change with UL but increased slightly with increasing GS in the beds of the riser with relatively small particles (Wang et al., 2003). The difference trend of bubble properties such as LB (or DB) and UB with varying operating variables could be due to the different


Keys □ ○ △ ▽ ◆ ◇ ★ ☆ ◑

UG×102 m/s 1.0–7.0 1.0–7.0 0.02–0.06UL×102 m/s) 25–31 17–23 0.065–0.12dP×103 m 2.1 2.1 0.4μL×103 Pa·s 1.0 0.96–38 0.96GS [kg/m2 s] 2.0–8.0 2.0–8.0 0.02–0.14σL×103 N/m 72.86 72.9–73.6 72.75

Authors Cho et al. (2001a, 2001b) Son et al. (2007a, 2007b) Wang et al. (2001b)

Fig. 6 Effects of operating variables on bubble properties in TPCFBs

Fig. 6　Effects of operating variables on bubble properties in TPCFBs


bubbling phenomena in the different bubble flow regimes. That is, in the bed of the riser with relatively large particles (dp=2.1 mm) and relatively high UL range over Ut of par-ticles, the level of εS in the riser was relatively low level, which resulted in the formation of a bubble disintegration regime in the riser. Therefore, the DB (or LB) increased with increasing UG but decreased with increasing UL or GS, since the turbulence intensity for bubble breaking could increase with increasing UL (0.19–0.32 m/s) or GS (2.0–8.0 kg/m2s) in the riser. However, in the bed of relatively small par-ticles (dp=0.4 mm) operated with relatively low range of UL (0.065–0.12 m/s) and GS (0.02–0.14 kg/m2s), the εS level could be relatively high, and thus bubbles could coalesce constituting a bubble coalescence regime, where the DB did not change considerably with UL and increase slightly with increasing GS. It was reported that the bubble frequency (FB) increased with increasing UG, UL or GS (Cho et al., 2001b; Kang et al., 2003; Ojima et al., 2015; Sasaki et al., 2015).

Son et al. (2007a, 2007b) measured the pressure fluctua-tions in a viscous TPCFB bioreactor and analyzed them by adopting the concept of phase space portraits and correla-tion dimension, in order to consider the relation between the P-fluctuations and the bubble size. They reported that the bubble size in terms of LB increased with increasing UG or μL but decreased with increasing UL, indicating the linear relationship between the bubble size and the correlation di-mension of P-fluctuations. It was understood that the corre-lation dimension of P-fluctuations could be the quantitative measurement of bubbling behavior in the riser. The infor-mation on the bubble properties in TPCFB has been highly limited until now; therefore, more systematic investigations are required in the future.

3.　Liquid Dispersion in the Riser

3.1　Liquid–solid circulating �uidized beds (LSCFBs)Dispersion and mixing of liquid phase in the riser of

LSCFBs were analyzed in the axial and radial directions by employing the axial dispersion model and infinite space model, respectively. The axial dispersion coefficient of liquid phase (Da,LS) in LSCFBs was found to increase with increas-ing UL but decrease with increasing dP or ρS by Kalaga et al. (2012), who determined the value of Da,LS by means of a dis-persed plug flow model from the information on the RTD of tracer. However, the value of Da,LS had been reported in very limited literature.

The radial dispersion coefficient of the liquid phase (Dr,LS) was examined by Cho et al. (2005), who estimated the value of Dr,LS from the parabolic concentration of tracer in the ra-dial direction of the riser, based on the infinite space model. They reported that the value of Dr,LS increased with increas-ing dP or GS but decreased with increasing UL. In addition, they analyzed the liquid radial dispersion by means of pres-sure fluctuations arising from the liquid–solid contacting and flow behavior in the riser. They expressed the resultant or overall liquid–solid flow behavior by means of the cor-relation dimension of pressure fluctuations. Generally, the

variations of the pressure fluctuations with respect to the operating variables coincided with those of the Dr,LS values, implying that the liquid radial dispersion in LSCFBs could be due to the liquid–solid contacting and flow behavior (Cho et al., 2005).

Effects of μL on the liquid radial dispersion coefficient (Dr,LS) was analyzed by Cho et al. (2005). They measured the pressure fluctuations and obtained the dominant frequency by means of the autocorrelation functions and power spec-tral density functions. The relation between the pressure fluctuations and the liquid radial dispersion was found as that the value of liquid radial dispersion coefficient (Dr,LS) increased with increasing the dominant frequency of pres-sure fluctuations, since the flow behavior of solid particles and their clusters (groups) could be expressed by means of pressure fluctuations. That is, the behavior of solid particles could be restricted gradually with decreasing the value of dominant frequency of pressure fluctuations in the riser (Cho et al., 2005). Values of Dr,LS are closely related to the values of εS, since both of them increased with increasing dP or GS, but decreased with increasing UL or μL. In addition, the turbulence due to the contacting between the particles and the fluid elements of liquid media could also increase with increasing εS. Those phenomena could explain the ef-fects of operating variables on the value of Dr,LS. The mixing or dispersion of liquid phase in the axial and radial direc-tion could also be one of the factors to analyze and predict the complex multiphase flow in the riser; therefore, those aspects should be investigated in some detail in the future.

3.2　Three-phase circulating �uidized beds (TPCFBs)The dispersion characteristics of continuous liquid media

in the axial and radial directions of TPCFBs were studied by very few investigators (Han et al., 1998; Yang et al., 1999a; Kang et al., 2003; Yu et al., 2004), although the information on the flow of continuous liquid media could affect the vol-ume and production efficiency of the systems and reactors in considering the design and their performance.

The axial dispersion coefficient of the liquid phase in the riser of a TPCFB (Da,TP) was studied by adopting the axial dispersion model based on the interstitial velocity of the liquid phase in the riser. It was revealed that the value of Da,TP increased with increasing UG or GS, as shown in Figure 7 (Han et al., 1998; Yang et al., 1999a; Yu et al., 2004). However, the value of Da,TP decreased slightly (Yu et al., 2004) or did not change considerably (Han et al., 1998; Yang et al., 1999a), with increasing UL. These differ-ent phenomena could be ascribed to the different changes in holdups of gas and solid particles with increasing UL, which promote the mixing of multiphase, and thus in the liquid phase. In addition, the increase of UL resulted in the decrease of residence time of liquid phase in the riser. This could lead to the liquid mixing time being too short to be mixed and dispersed. However, the increase of UL could lead to the increase of turbulence, which could enhance the axial dispersion of liquid phase. Due to these competing factors, the value of Da,TP decreased only slightly or did not change

Vol. 51 No. 9 2018 749

considerably with increasing UL.The liquid radial dispersion coefficient in TPCFBs (Dr,TP)

was studied by employing the infinite space model (Han et al., 1998; Yang et al., 1999a; Kang et al., 2003). Generally, the value of Dr,TP increased with increasing UG, dp or GS but decreased with increasing UL and μL, as shown in Figure 8 (Han et al., 1998; Yang et al., 1999a; Kang et al., 2003). It was noticed that the liquid radial dispersion could be enhanced by means of rising bubbles, since the turbulent wakes which are trailing behind the bubbles could be formed and separat-ed during their rise. In addition, the non-uniform velocity and size distribution of bubbles in the radial direction could let the liquid element flow in the radial direction. Thus, the value of Dr,TP increased with increasing UG. However, the radial flow of liquid element would decrease with increasing axial bulk flow of continuous liquid phase, since the increase

of UL could decrease the velocity gradient of rising bubbles in the radial direction. Thus, the value of Dr,TP decreased with increasing UL.

The reason why the value of Dr,TP increased with increas-ing GS could be due to the increase of εS with increasing GS (Figure 4). It has been understood that the turbulence intensity in the bed could increase with increasing εS in the riser, which could result in the decrease of bubble size and increase of uniformity of bubble distribution in the radial direction. In addition, the contacting frequency of concur-rent rising bubbles and liquid element with fluidized solid particles could increase with increasing GS, which could in-duce the radial distributions of bubbles and liquid elements. Therefore, the value of Dr,TP increased with increasing GS (Kang et al., 2003). Based on this purported deduction, the effects of μL on the value of Dr,TP could be explained, since

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Keys ■ ● ▲ □ ○ △ ▽ ◇ ▼ ◆ ★ ☆ ◑

Mode TP TP TPUG×102 m/s 1.08–2.71 1–7 1.8–3.6UL×102 m/s 6.67–9.38 25–33 6.49–9.92dP×103 m 0.405 2.1 0.4μL×103 Pa·s 0.96 0.96 0.96ρS [kg/m3] 2460 2460GS [kg/m2 s] 1.08–3.1 4 —σL×103 N/m 72.75 72.75 72.75

Authors Han et al. (1998) Yu et al. (2004) Yang et al. (1999a)

Keys A B C D E F

Mode TP LSUG×102 m/s 1–7 —UL×102 m/s 25–33 1.3–3.0 1.7–3.4dP×103 m 2.1 0.36–0.72 0.1–0.2μL×103 Pa·s 0.96 1100 2500ρS [kg/m3] 2500 1.0 1.0GS [kg/m2 s] 2.0–8.0 — —σL×103 N/m 72.75 72.75 72.75

Authors Yu et al. (2004) Kalaga et al. (2012)

Fig. 7 Effects of operating values on Da in LSCFBs and TPCFBs

Fig. 7　Effects of operating values on Da in LSCFBs and TPCFBs


the value of εS decreased with increasing μL (Figure 4).Since the liquid is a continuous phase, the local or radial

velocity of liquid phase (ULi) could affect the Dr,TP as well as local phase holdups in the riser of TPCFB. The value of ULi was higher in the center than that in the near wall re-gion, indicating the non-uniform radial profile (Cao et al., 2009). In addition, the value and radial non-uniformity of ULi increased with increasing UG, or μL but decreased with increasing GS. The simulation to predict the value and radial profile of ULi could support the experimental measurement.

4.　Heat Transfer in the Riser

The heat flow as well as temperature fluctuations and dis-tributions in LSCFBs and TPCFBs could not be maintained uniformly owing to the complicated mixing and contacting among gas, liquid and fluidized solid particles. Thus, the study on the heat transfer in those highly complicated sys-tems could be indispensable for the reasonable design and operation of reactors and contactors employing LSCFBs and TPCFBs. However, the heat transfer studies in these systems have been very limited until now, as described in Table 2.

4.1　Liquid–solid circulating �uidized beds (LSCFBs)For LSCFBs, most of the heat transfer studies have been

conducted in the immersed heater-to-bed heat transfer sys-tems in the riser. The effects of liquid velocity (UL), fluidized particle size (dP), solid circulation rate (GS) and properties of continuous liquid media on the overall heat transfer coeffi-

cient (hLS) in the riser have been studied, to analyze the heat transfer phenomena in the LSCFB. Shin et al. (2003) pointed out that the complexity of temperature fluctuations in the riser increased with increasing dP or GS but did not change considerably with increasing UL from the stochastic analysis of temperature fluctuations. They predicted the heat transfer coefficient in the riser from the correlation dimension of temperature fluctuations.

The variation trends of correlation dimension of T-fluc-tuations from Shin et al. (2003) generally agreed with those of heat transfer fluctuations obtained by Kuramoto et al. (1999), who studied heat transfer phenomena of LSCFBs with 0.049 m (ID). However, the values of hLS obtained by Kuramoto et al. were extremely high (9000–15000 W/m2K) comparing with other data owing to the very small diameter of the riser (Kuramoto et al., 1999; Hashizume and Kimura, 2008).

Generally, the value of hLS between the immersed heater and the bed of riser increased with increasing dP or GS in LSCFB, as shown in Figure 9, due to the increase of turbu-lence for heat transfer. The increase of dP or GS could lead to the increase of εS in the riser, which resulted in the promo-tion of turbulent flow and effective contacting between the particles and the heater surface in the riser (Lee et al., 2003; Jin et al., 2013). However, the increase of UL resulted in the decrease of εS although it could lead to the increase of mo-mentum of fluidized particles; therefore, the value of hLS did not change considerably with varying UL (Lee et al., 2003; Jin et al., 2013).


Keys ■ ● ▲ ▼ □ ○ △ ▽ ◆ ◇ ★ ☆ A B C D E

Mode LS TP TP TPUG×102 m/s — 1.08–2.71 1.8–3.6 1–7UL×102 m/s 16–38 6.67–9.38 6.79–9.15 25–33dP×103 m 1.0–3.0 0.405 0.4 2.1μL [mPa·s] 0.96–38.0 0.96 0.96 0.96GS [kg/m2 s] 2.0–8.0 1.08–3.1 2.0–8.0σL×103 [N/m] 72.75 72.75 72.75 72.75

Authors Cho et al. (2005) Han et al. (1998) Yang et al. (1999a) Kang et al. (2003)

Fig. 8 Effects of operating values on Dr in LSCFBs and TPCFBs

Fig. 8　Effects of operating values on Dr in LSCFBs and TPCFBs

Vol. 51 No. 9 2018 751

The value of hLS in the riser decreased with increasing μL. This can be ascribed to the turbulence for heat transfer decreasing owing to the decrease of holdup and flow mo-mentum of particles, with an increase in μL (Lee et al., 2003). The effects of liquid surface tension (σL) on the hLS in the riser were reported by Jin et al. (2013), who found that the value of hLS increased only slightly with increasing σL. They

explained that the increase of σL could lead to the increase in εS, and thus increase in the turbulence in the riser; however, the increase of σL resulted in the decrease in the thermal conductivity of liquid phase (kth). Due to these compet-ing factors for heat transfer, the value of hLS increased only slightly with increasing σL.

The heat transfer phenomena were analyzed by means of

Vol. 51 No. 9 2018 9

Keys ■ ● ▲ ▼ □ ○ △ ▽ ◆ ◇ ★ ☆

UL×102 m/s 20.5–30 40 14–44dP×103 m 0.5–3.0 3.0 1–3μL×103 Pa·s — 0.96–38.0 0.96–38GS kg/m2 s 2–8 2–8 2–8Voidage [—] — — —σL×103 N/m 55.73–72.75 72.9–73.6 72.9–73.6

Authors Jin et al. (2013) Shin et al. (2005) Lee et al. (2003)

Fig. 9 Effects of operating variables on hLS in LSCFBs

Fig. 9　Effects of operating variables on hLS in LSCFBs

Table 2　Summary of Heat Transfer Studies in LSCFBs and TPCFBs

Researchers System mode Riser

Solid Liquid



σL×103 N/m

Kuramoto et al. (1999) LS 0.049 m (ID) Glass ballotini 0.182 2580 Water 1.0 72.75 ·T-fluctuations4.0 m (H) ·Effects of UL, εS on hLS

Cho et al. (2001b) TP 0.102 m (ID) Glass beads 2.1 2500 Water 1.0 72.75 ·Effects of εG, UG, UL, GS on hTP

3.5 m (H) ·T-fluctuations on hTP

Kang et al. (2002a) TP 0.102 m (ID) Anion polymer resin

0.4×0.4 1130 Synthetic wastewater

·T-fluctuations in bioreactors1.0 m (H) ·hTP in bioreactors

·Effects of εS on hTP

Shin et al. (2005) LS 0.102 m (ID) Glass beads 1.0, 1.7, 2.1, 3.0

2500 Water 0.96–38.0 72.9–73.6 ·Heat transfer model3.5 m (H) CMC solution ·Mechanism of heat transfer

CMC solution ·Effects of UL, dP, GS, μL

Kang et al. (2007) TP 0.102 m (ID) 2.1 2500 Water 1.0 72.75 ·Heat transfer model & resistances3.5 m (H) ·Effects of UG, UL, GS

Hashizume and Kimura (2008)

LS 0.024 m (ID) Glass beads 2.19–4.95 2530 Water 1.0 72.75 ·Dimensionless groups correlation0.012 m (ID) Ceramics 2.1–3.15 3700

Son et al. (2008a) TP 0.102 m (ID) Glass beads 1.0, 1.7, 2.1, 3.0

2500 Water 0.96–38.0 72.9–73.6 ·Heat transfer coefficient (hTP)2.5 m (H) CMC solution ·Effects of UG, UL, dP, GS, μL

Lim et al. (2011) TP 0.102 m (ID) Glass beads 0.5, 1.0, 2.0, 3.0

2500 Water 0.92–0.96 47.53–72.75 ·T-fluctuations & stability2.5 m (H) Ethanol solution ·Effects of UG, UL, dP, GS, σL

Jin et al. (2013) LS 0.102 m (ID) Glass beads 0.5, 1.0, 2.0, 3.0

2500 Water 0.960.94–0.95

72.7555.7–65.2

·Heat transfer coefficient (hLS)2.5 m(H) Ethanol solution ·Effects of UL, dP, GS, σL


two resistances-in-series model (Shin et al., 2005). Based on the model, the overall heat transfer resistance (1/hLS) could be composed of the resistances in the heater surface region (1/hh,LS) and in the bed proper (1/hb,LS), as expressed in Eq. (4) (Shin et al., 2005).

= +LS h,LS b,LS

1 1 1 h h h (4)

They considered the thickness of the thermal boundary layer (δ) at the heater surface from the radial temperature profile in the riser and determined the two resistances con-stituting the overall heat transfer resistance. They noticed that the value of δ did not change considerably with varying UL but decreased with increasing dP or GS and increased with increasing μL. In addition, the heat transfer resistance in the bed proper (1/hb,LS) increased with increasing UL or μL but decreased with GS, while the heat transfer resistance in the region adjacent to the heater surface was dominant in determining the overall resistance. The value of hLS reported in the literature could be correlated in terms of dimension-less groups as Eq. (5).

−= 0.224H M0.0549J Re (5)

Herein, JH=(h⁄(ρLCpLUL))εLPr2⁄3 and ReM=dPULρL/μL,app(1−εL).

4.2　Three-phase circulating �uidized beds (TPCFB)The overall heat transfer coefficients in the riser of

TPCFBs (hTP) were determined by measuring the time se-ries of temperature difference (∆T) between the immersed

vertical heater surface and the riser proper. Since the tem-perature fluctuations can be directly related to the heat transfer phenomena, the ∆T-fluctuations were analyzed by the Shannon entropy (HS), which could be used to express the degree of uncertainty involved in predicting the output of a probabilistic event (Cho et al., 2001b). They showed the close relationship between the values of hTP and HS.

From the experimental results of previous investigators, it had been noted that the value of hTP increased with in-creasing UG, dP or GS; however, it did not change consider-ably with varying UL. The increasing trend of hTP with UG, dP or GS could be due to the increase in the turbulence in the region adjacent to the heater surface and bed proper in the riser (Cho et al., 2001a; Kang et al., 2007; Lim et al., 2011; Mo et al., 2015), as shown in Figure 10. Son et al. (2008b) examined the effects of μL on the value of hTP. They considered the μL in the range of 0.96–38.0×10−3 Pa·s and reported that the value of hTP increased with increasing UG, UL, dP and GS in the riser with viscous liquid media, while it would decrease with increasing μL. They explained that the effects of μL on the hTP were mainly due to the restricted mo-tions and flow behaviors of bubbles and particles, since the drag force exerted on the bubbles and particles increase in the viscous liquid media.

The effects of the σL on the hTP were considered by Lim et al. (2011), who used a mixture of water and ethanol to change σL. From them, the value of hTP decreased with in-creasing σL due to the decrease of bubbling phenomena and bubble holdup with increasing σL. It seemed that the stable condition of the heat transfer system could be examined by means of the temperature difference (∆T) fluctuations


Keys ■ ● ▲ ▼ □ ○ △ ▽ ◆ ◇ ★ ☆ C ◐ ◑ A B

UG×102 m/s 4–12 0–0.8 1–7 1–7UL×102 m/s 36.6–50.3 0–6 23–33 25–33dP×103 m 2.0–6.5 4 2.1 2.1μL×103 Pa·s 0.92–0.96 0.96–38 0.96 0.96GS [kg/m2 s] 2–6.5 2.0–8.0 2.0–8.0σL×103 N/m 47.53–72.75 72.9–73.6 72.75 72.75

Authors Lim et al. (2011) Son et al. (2007a, 2007b) Cho et al. (2001a, 2001b) Kang et al. (2007)

Fig. 10 Effects of operating values on hTP inTPCFBs

Fig. 10　Effects of operating values on hTP in TPCFBs

Vol. 51 No. 9 2018 753

between the heater surface and the riser proper. Lim et al. (2011) pointed out the elapsed time during which the value of ∆T-fluctuations could become a constant value. That is, from that point onward the heat transfer process could be a quasi-steady state condition, and thus stable. The elapsed time increased with increasing σL, indicating that it would take more time to attain the steady and stable heat transfer condition with increasing σL. This could be because the heat transfer would be less effective with increasing σL. It is interesting to note that the elapsed time during which the value of ∆T could be constant decreased with increasing dP, indicating that the value of hTP increased with increasing dP (Lim et al., 2011).

In a TPCFB bioreactor for treating wastewater, Kang et al. (2002a, 2002b) measured the hTP value and reported that the value of hTP increased with increasing UG or holdup of bio-film media (anion polymer resin) but increased only slightly with increasing UL. Although the operating conditions were quite different from other investigations (the range of UG and UL were very small), the results agreed well with others (Cho et al., 2001a; Son et al., 2008a, 2008b).

The heat transfer mechanism was analyzed by means of heat transfer resistances based on the two resistances-in-series model (Kang et al., 2007), as in the case of LSCFBs. The model was considered with the notion that the heat transfer process could be divided into two steps; the heat transfer in the region adjacent to the heater surface, and that in the riser proper. In the former step, the heat could be transferred by means of conduction through a very thin liquid film formed around the heater surface, while in the latter step, the heat could be transferred by means of convec-tion. They reported that the heat transfer resistance in the region adjacent to the heater surface, which was determined by measuring the thickness of the thermal boundary layer at the heater surface, was the dominant resistance for the total heat transfer. From the experimental results determining the effects of operating variables such as UG, UL and GS on the

overall as well as individual heat transfer resistances, they reported that both resistances decreased upon increasing UG and GS; however, the heat transfer resistance in the region adjacent to the heater surface decreased while the resistance in the bed proper increased slightly, upon increasing UL.

The values of hTP reported in the literature (Cho et al., 2001b; Kang et al., 2002a, 2002b, 2007; Son et al., 2008a, 2008b; Lim et al., 2011) could be predicted from the follow-ing correlations in terms of operating variables (Eq. (6)) and dimensionless groups (Eq. (7)), respectively.

−= 0.21 0.03 0.14 0.05 0.08TP G L S P L2252h U U G d σ (6)

− −= 2.52 1.46 0.400.14 Nu Pr Re We (7)

5.　Mass Transfer in the Riser

It has been understood that the information on the gas–liquid mass transfer could help to predict the performance of the heterogeneous reactors and contactors, since the mass transfer phenomena could act as one of the rate determining steps for controlling the conversion level of the reaction and contacting efficiency among multiphase systems (Fan, 1989; Kim and Kang, 1996, 1997; Atta et al., 2009). However, there has been little information on the gas–liquid mass transfer in TPCFBs, as described in Table 3. Most of the studies to investigate the mass transfer in TPCFBs had considered the effects of operating variables such as UG, UL, dp, GS and μL on the values of volumetric mass transfer coefficient (kLa), gas–liquid interfacial area (a) and liquid-side mass transfer coefficient (kL), as shown in Figures 11 and 12.

5.1　Volumetric Gas–liquid mass transfer coe�cients (kLa) in TPCFBs

The volumetric mass transfer coefficients (kLa) in the riser of TPCFB were obtained mainly by means of two methods. One is a method using the steady-state axial dispersion

Table 3　Summary of Gas–Liquid Mass Transfer Studies in TPCFBs

Researchers Riser

Solid Liquid



σL×103 N/m

Yang et al. (1999b, 2001) 0.140 m (ID) 0.4 2460 Water 1.0 72.75 ·kLa from axial dispersion model3.0 m (H) ·kL and a from εG and DB

Kim et al. (2001) 0.102 m (ID) Glass beads 1.0, 1.7, 2.1, 3.0

2500 Water 1.0 72.75 ·kLa from axial dispersion model3.5 m (H) ·Effects of UG, UL, dP and GS

Wang et al. (2001a, 2001b) 0.140 m (ID) Glass beads 0.4 2460 Water 1.0 72.75 ·a from εG and DB

3.0 m (H) ·Effects of UG, UL, GS

Liu et al. (2003) 0.076 m (ID) Glass beads 1.3 2500 Water 1.0 72.75 ·Radial kLa distribution2.0 m (H) ·Axial dispersion model

Son et al. (2007b) 0.102 m (ID) Anion Polymer resin

0.4 1130 Synthetic waste water 1.0 72.75 ·εG and kLa in a bioreactor1.0 m (H) ·Axial dispersion model

Son et al. (2008a, 2008b, 2009) 0.102 m (ID) Glass beads 1.0, 1.7, 2.1, 3.0

2500 Water 0.96 72.75 ·εG and kLa enhancement3.5 m (H) CMC solution 0.96–38.0 73.2–73.6 ·Effects of UG, UL, dP, GS, μL

Lee et al. (2013) 0.102 m (ID) Glass beads 0.5, 1.0, 2.0, 3.0

2500 Tap water 1.0 72.75 ·kLa, a and kL from chemical reactionSodium carbonate-

bicarbonate solution1.0 ·Effects of UG, UL, dP, GS

3.5 m (H) ·Simultaneous desorption & absorption


model, from the gas (oxygen) balance in the liquid phase around a differential volumetric section of the riser (Method 1) (Fan, 1989; Kang et al., 1991, 2002a, 2002b; Deckwer, 1992; Yang et al., 1999b; Kim et al., 2001; Wang et al., 2001b; Liu et al., 2003; Kim and Kang, 2006; Son et al., 2007a, 2007b, 2008a, 2008b, 2009). The other is a method using the simultaneous physical desorption and chemical absorption of gas phase into the liquid phase (Method 2) (Dhanuka and Stepanek, 1980; Nguyen-Tien et al., 1985; Chang et al., 1986; Lee et al., 2013). The axial dispersion model, which is based on the gas concentration in the axial direction in the riser, was generally adopted to determine the value of kLa. Fortunately, the calculated axial distribution of dissolved gas concentration in the continuous liquid medium based on the axial dispersion model has been reasonably well fitted to the experimentally measured one (Yang et al., 1999a, 1999b; Son et al., 2007a, 2007b, 2008a, 2008b, 2009). In employing

Method 2, the gas–liquid interfacial area (a) and the true liquid-side mass transfer coefficient (kL) were measured si-multaneously, in addition to obtaining the value of kLa (Lee et al., 2013).

In TPCFBs, the gas holdup has been found to be non-uni-form in the radial direction, which could influence the local gas–liquid mass transfer. The radial distribution of kLa in the riser was reported by Liu et al. (2003) who determined the kLa value by fitting the measured oxygen concentration pro-file at the axial position of the riser (Method 1). The value of kLa tended to decrease with increasing the radial position from the center of the riser, with an increase of UG. This can be ascribed to the gas holdup being greater in the cen-ter than that near the wall. In addition, they reported that the value of kLa decreased with increasing GS in the beds with relatively small particles (dP=1.3 mm), constituting the bubble coalescence regime. It can be noted that the increase

Vol. 51 No. 9 2018 11

Keys ■ ● ▲ □ ○ △ ▽ ▼ ◆ ◇ ★ ☆ ◐ ◑ A B C D E F

UG×102 m/s 0–6.8 1–7 0.329–2.485 1–8 1–8UL×102 m/s 1–7 6.4–12.0 17–23 18.0–22.0 38–44 24–32dP×103 m 1–3 1.7–4.0 1 1.3 3.0 0.5–3.0μL×103 Pa·s 0.96 0.96 0.96–38 0.96GS [kg/m2 s] 2–8 0 2 9.84–14.2 2–8 2–8σL×103 N/m 72.75 72.75 72.9–73.6 72.75

Authors Son et al. (2007a, 2007b) Chang et al. (1986) Kim et al. (2001) Liu et al. (2003) Son et al. (2008a, 2008b) Lee et al. (2013)

Keys G H I J K L

UG×102 (m/s) 0.4–3.6 5.0–8.0 3.0–5.2UL×102 (m/s) 6.2–10.0 6.1–8.8 6.5–11.2dP×103 (m) 0.4 5.0–6.0 1.98μL×103 (Pa·s) 0.96GS (kg/m2 s) 7.2σL×103 (N/m) 72.75

Authors Wang et al. (2001a) Yang et al. (2001) Nguyen-Tien et al. (1985) Dhamka and Stepanek (1980)

Fig. 11 Effects of UG and UL on kL α, α and kL in TPCFBs

Fig. 11　Effects of UG and UL on kLa, a and kL in TPCFBs

Vol. 51 No. 9 2018 755

of GS could lead to the promotion of bubble coalescence in the bed with relatively high εS level of small particles (Liu et al., 2003).

The value of kLa increased with increasing UG or UL from most investigations as shown in Figure 11, although the effects of UG were dominant (Yang et al., 1999b; Kim et al., 2001; Liu et al., 2003; Son et al., 2007a, 2007b, 2008a, 2008b; Lee et al., 2013; Shibata et al., 2016a, 2016b). As shown in Figure 12, the kLa increased with increasing dP (Kim et al., 2001; Son et al., 2007a, 2007b, 2008a, 2008b; Lee et al., 2013). It has been understood that the larger particles could have more inertia force to generate the turbulence required for the gas–liquid mass transfer at the interface by break-ing the bubbles and contacting with the liquid fluid element flowing upwardly in the riser (Son et al., 2008a, 2008b; Lee et al., 2013).

The kLa increased with increasing GS from the data ob-

tained by means both of methods (Methods 1 and 2) (Kim et al., 2001; Son et al., 2007a, 2007b, 2008a, 2008b, 2009; Lee et al., 2013), as shown in Figure 12. As expected, the turbu-lence intensity which affects the contacting efficiency at the gas–liquid interface could increase with increasing GS, since the εS increased with increasing GS. It has been understood that the contacting frequency and intensity among gas, liquid and solid phases could increase with increasing εS. Therefore, the value of kLa increased with increasing GS (Son et al., 2007a, 2007b, 2008a, 2008b, 2009; Lee et al., 2013). However, the kLa showed a local maximum (Yang et al. 1999b) or decreased (Liu et al., 2003), with increasing GS. The different trends of kLa with GS from Yang et al. (1999b) and Liu et al., (2003) could be due to the small particle size and relatively high solid loading in the riser, which could result in the severe decrease in the gas holdup owing to the high holdup of solid particles.


Keys ■ ● ▲ ▼ ◆ ◇ ★ ☆ ◑ ◐ A B C D □ ○ △ ▽

UG×102 m/s 2.0–8.0 1.8–5.0 1.0–7.0 1.0–7.0 3–5UL×102 m/s 26–32 8.0 21.0 28.0 24.0–40.0(UL−Ut)×102 m/s — 1.0–7.0 1.0–7.0 3.0dP×103 m 0–4.0 0–4.0 1.0–3.0 1–3 1.7–3.0μL×103 Pa·s 0.96 0.96–38.0 0.96GS [kg/m2 s] 2.0–8.0 0–8.0 2.0–8.0 2.0–8.0σL×103 N/m 72.75 72.9–73.6 72.75

Authors Lee et al. (2013) Chang et al. (1986) Kim et al. (2001) Son et al. (2008a, 2008b) Son et al. (2009)

Keys E F G H

UG×102 m/s 0.786 5UL×102 m/s 20.75 28(UL−Ut)×102 m/s 2.1dP×103 m 1.3μL×103 Pa·s 0.96 2.0–8.0GS [kg/m2 s] 0–19.67σL×103 N/m 72.75

Authors Wang et al. (2001b) Liu et al. (2003) Son et al. (2007a, 2007b) Wang et al. (2001a)

Fig. 12 Effects of dP, GS and μL on kL a, a and kL in TPCFBs

Fig. 12　Effects of dP, GS and μL on kLa, (a) and kL in TPCFBs


The effects of μL on the value of kLa were considered by Son et al. (2008a, 2008b), who reported that the value of kLa decreased with increasing μL in the apparent range of 0.96– 38.0×10−3 Pa·s. They explained that the turbulence intensity and surface renewal rate for mass transfer of the gas phase at the bubble–liquid interface could decrease with increasing μL, owing to the decrease of holdups of bubbles and micro eddies with high energy in addition to the decrease of mo-bility of particles with increasing μL.

In order to enhance the value of kLa in the bioreactors employing a TPCFB, Son et al. (2009), suggested the swirl-ing flow of liquid medium in the riser. They adjusted the ratio of swirling liquid, which was in the range of 0–0.5, by controlling the volume ratio of swirling liquid based on the total volume flow introduced into the riser. As a result, the values kLa could be increased up to 20–25% by means of liquid swirling flow in the riser. It can be noted that, as men-tioned in the earlier section (Holdup and Flow Behavior), the increase of εG and turbulence intensity with increasing the ratio of liquid swirling could be important factors for the increase of kLa value. The values of kLa in the literature (Wang et al., 2001a, 2001b; Yang et al., 2001; Liu et al., 2003; Lee et al., 2013) were correlated in terms of operating vari-ables as Eq. (8).

= 0.49 0.09 0.37 0.29L G L P L S0.0069k a U U d Gμ (8)

5.2　Gas–liquid interfacial area (a)The value of gas–liquid interfacial area (a) was obtained

simply by the expression of a=6εG/Db by measuring the bubble size (Db) and εG (Fan, 1989; Deckwer, 1992; Wang et al., 2001a, 2001b; Yang et al., 2001), with the assumption of a bubble shape factor of 0.8 (Wang et al., 2001b). Based on this scheme, the value of a increased with increasing UG or UL but decreased slightly with increasing GS. Recently, Lee et al. (2013) determined the value of a from the chemi-cal reaction and reported that the value of a increased with increasing UG or UL. They explained that the increasing trend of a with UG can be ascribed to the increase of gas holdup with increasing UG.

The effects of UL on the value of a can be explained by the following. The increase in turbulence in the riser due to the increase UL can bring gas and liquid phases into vigor-ous contact. Actually, the bubble size has been reported to decrease with increasing UL (Figure 6). These could act as positive factors in determining the value of a. However, the residence time of turbulent eddies decreases with increas-ing UL, which could be a negative factor on the increase of a with UL. Thus, the value of a increases only slightly with increasing UL.

From the simultaneous measurement of kLa and a by means of chemical reaction, it was reported that the value of a increased with increasing dP or GS in the riser, as can be seen in Figure 12 (Lee et al., 2013). They considered the dP from 0.5 to 3.0×10−3 m and explained the effects of dP and GS on the value of a as follows. The increase of dP could lead to the increase in the bubble breaking potential in the

riser. Further, the hindrance effects of solid particles on the gas–liquid flowing in the upward direction could increase with increasing dP. Therefore, the value of a increased with increasing dP.

It has been understood that the turbulence intensity gen-erated by fluidized solid particles could increase with in-creasing GS, which consequently results in the bubble break-ing, and thus, the value of a increased with increasing GS. The effects of GS on the value of a could also be related to the level of εS in the riser, because the level of εS could affect the bubble coalescence in the relatively low range of UL in the beds of relatively small particles. That is, in the beds of a riser with relatively small particles, the low range of UL could lead to the relatively high level of εS. In this case, the increase of GS results in the further increase of εS in the riser, which consequently results in the increase of the system apparent viscosity and the bubble coalescence (Wang et al., 2001a, 2001b; Yang et al., 2001). Therefore, the value of a de-creased slightly with increasing GS in these cases. However, in the relatively high range of UL, the apparent viscosity of the gas–liquid–solid mixture could be maintained within the lower level and the probability of bubble coalescence disappeared, due to the relatively low level of εS in the riser. In this case, the breaking rather than the coalescence of bubbles could occur by means of turbulence generated from the complicated and rapid flow of multiphase. Therefore, the increase of dP or GS could increase in the turbulence among multiphase flow for the bubble breaking in the riser, which consequently could lead to the increase of a (Lee et al., 2013). In the conventional three-phase fluidized beds, it has been understood that the bubble coalescence could occur upon the occurrence of bed contraction in the beds of rela-tively small particles (Fan, 1989; Kim and Kang, 1997).

To predict the value of a, Lee et al. (2013) suggested Eq. (9) from the correlation of previous information on the value of a.

= 1.67 1.28 0.65 0.99G L P S0.010a U U d G (9)

5.3　Liquid-side mass transfer coe�cient (kL)The value of kL based on the liquid phase (liquid-side

mass transfer coefficient) was generally obtained from the data on kLa and a. As shown in Figure 11, the value of kL in-creased only slightly with increasing UG or UL (Chang et al., 1986; Yang et al., 2001; Lee et al., 2013). This could be due to the competitive effects of turbulence intensity and residence time of the turbulent eddies in the riser in determining the value of kL, with varying UG or UL. That is, based on the surface renewal theory (Deckwer, 1992), the surface renewal rate at the gas–liquid interface increases with increasing UG or UL; however, the contact time between the bubble and liquid medium decreases with increasing UG or UL, which results in the slight increase in the value of kL with UG or UL (Lee et al., 2013).

The variations of kL with dP and GS can be seen in Figure 12, where the value of kL increased with increasing dP and GS. It has been understood that the particles could contrib-

Vol. 51 No. 9 2018 757

ute to the effective contacting for gas–liquid mass transfer by reducing the resistance at the gas–liquid interface; there-fore, the surface renewal rate and the value of kL could in-crease with increasing dP and GS (Lee et al., 2013).

From the previous investigations in the literature, Lee et al. (2013) proposed the following correlation to obtain the value of kL in TPCFBs.

= 0.11 0.16 0.14 0.16L G L P S41k U U d G (10)

In addition, they expressed the kL data in terms of dimen-sionless groups based on the concept of the local isotropic turbulence theory (Fan, 1989; Deckwer, 1992; Kim and Kang, 1997) as Eq. (11).

0.01G0.37 0.51

PL

0.845Sc USh Re U (11)

6.　Possible Applications

By utilizing the different unique features of the riser and downer (downcomer) simultaneously, the mode of circulat-ing fluidized bed has been extended in its highly potential applications in several fields, including wastewater treat-ment, protein separation and recovery, enzymatic polymer-izations, lactic acid production, etc. (Atta et al., 2009). That is, by taking advantages of LSCFBs as a reactor, alkylation of alkylbenzene (Liang et al., 1995c; Liang and Zhu 1997c), polymerization of phenol (Trivedi et al., 2006) and extrac-tive fermentation of lactic acid (Patel et al., 2008) were conducted effectively. In treating the wastewater, the LSCFB was used effectively as a bio-reactor for the biological nutri-ent removal from municipal and high strength wastewater (Cui et al., 2004; Patel et al., 2005a, 2005b, 2006; Chowdhury et al., 2008, 2009; Andalib et al., 2012). By adopting the ion-exchange extraction scheme, the LSCFB was also employed to conduct the continuous protein recovery (Lan et al., 2000, 2002a, 2002b), cesium removal (Feng et al., 2003) and adsorption of nickel ion effectively by regenerating the resin (Gaikwad et al., 2008). With the aid of modeling and simulation of a LSCFB ion exchange system for continuous protein recovery (Mazumder et al., 2009a, 2009b), it was employed to recover soy protein using a solvent-free process (Prince et al., 2012). However, the application of TPCFB has been still scarce and extremely limited, although it has many advantages for potential applications.

Kang et al. (2002b) employed the TPCFB as a bioreac-tor for the removal of nitrogenous component (NH4

+) and COD from the wastewater. The removal efficiency could be enhanced considerably by using the substrate particles in order to let microorganism adhere at the surface of the porous substrate. The optimum holdup or amount of sub-strate was dependent upon the initial concentration of NH4

+ in the wastewater. Son et al. (2007b) adopted a TPCFB as a bioreactor to treat the ammonium component in the viscous wastewater from food processing industries by using micro-organism. Recently, some attempts have been made to de-velop a circulating fluidized bed evaporator by adopting the

TPCFB by Li et al. (2016, 2017). They considered the effects of dP on the particle distribution of a two-pass evaporator with a baffle.

Conclusions

An overview was conducted by surveying the information in the literature available at present in order to organize and analyze the characteristics of TPCFBs and LSCFBs, for bet-ter understanding and applications. The effects of operating variables on the hydrodynamics such as phase holdup, flow behavior, bubble properties and liquid dispersion in the riser were determined. Special attention was given to the char-acteristics of heat and mass transfer in the riser of TPCFBs and LSCFBs to elucidate the strengths and advantages of them for the feasible applications in the extended fields of emerging industries. The observations and deductions for the hydrodynamics and heat and mass transfer phenomena were examined and consociated for the consolidated analy-ses and explanations. This review should be supplemented by the unique features and notions of TPCFBs and LSCFBs in the near future. For better understanding and utilization, information on the heat and mass transfer characteristics should be investigated in the future. The unified correlations and plausible mechanisms for the hydrodynamic, heat and mass transfer parameters should be proposed in TPCFBs and LSCFBs, for the scale-up and commercial utilization in the future.

Nomenclature

a = gas–liquid interfacial area [1/m]D = molecular diffusivity [m2/s]DaL = liquid axial dispersion coefficient [m2/s]DB = bubble size [m]dP = particle diameter [m]DrL = liquid radial dispersion coefficient [m2/s]FB = bubble frequency [1/s]GS = solid circulation rate [kg/m2s]h = heat transfer coefficient [W/m2K]H = height of the riser [m]JH = modified Colburn j-factor, (h⁄ρLCpLUL)εLPr2⁄3 [−]kL = liquid-side mass transfer coefficient [m/s]kLa = volumetric gas–liquid mass transfer coefficient [1/s]kth = thermal conductivity of liquid phase [W/mK]L = Height of particles piled up in the solid recycle device [m]LB = bubble chord length [m]Pr = Prandtl number, CpLμL,app/kL [−]ReM = modified Reynolds number, dP UL ρL/μL,app (1−εL) [−]ReP = particle Reynolds number, GSdP/μL [−]Sc = Schmidt number, μL/ρLD [−]Sh = Sherwood number, kLdP/D [−]UB = bubble rising velocity [m/s]UG = superficial gas velocity [m/s]UL = superficial liquid velocity [m/s]ULi = initial liquid velocity in the riser [m/s]US = Particle velocity [m/s]We = Weber number, dPρL (UG+UL)2/σL [−]


‹Greek letters›ε = phase holdup [−]ρ = density [kg/m3]μ = viscosity [Pa·s]μL,app = apparent liquid viscosity [Pa·s]σ = surface tension [N/m]

‹Subscript›B = bubble phaseG = gas phaseL = liquid phaseLS = liquid–solid circulating fluidized bedS = solid phaseTP = three-phase circulating fluidized bed

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