- Email: [email protected]
Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability
ARTICLE IN PRESS
JID: IJMF
[m5G;October 26, 2018;5:58]
International Journal of Multiphase Flow xxx (xxxx) xxx
Contents lists available at ScienceDirect
International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulflow
Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability Shaikh A. Razzak Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 13 April 2018 Revised 8 October 2018 Accepted 22 October 2018 Available online xxx Keywords: Gas-liquid-solid circulating fluidized bed Hydrodynamics Holdups Flow characteristics Flow regimes Superficial velocity
a b s t r a c t Gas-liquid-solid circulating fluidized bed (GLSCFB) is considered as superior technology over conventional Gas-liquid-solid fluidized beds. The GLSCFB consist of two interconnected riser and downer columns facilitating solid circulation between the columns by pressure balance. The overall pressure balance is an important parameter for stable operation of the system. The present communication reports an experimental investigation to correlate the pressure balance and hydrodynamics behavior and the stability of the GLSCFB. In this regard, two spherical shape glass bead particles (500 and 1200 μm in size), two irregular shape lava rock particles (500 and 920 μm in size) are used as solid phase, air as gas phase and saline-water as liquid phase. For flow characterization, superficial solids velocity (Us ), normalized superficial solids velocity ( UUs ), solids holdup (ɛs ) and gas holdups (ɛg ) are considered as measureable pal rameters. The experimental observations indicates that the solids holdup have significant impact of size, density and shape of the particles on hydrodynamics behavior. The drag effect on the particles and their terminal settling velocity are the main factors on solids holdup distribution. The average solids holdups decrease with the increase on net superficial liquid velocity (Ul − Ut ) and normalized superficial liquid velocity ( UUtl ). Effects of different parameters (Ua , Ul , Ug , UUtl , Ul − Ut ) are considered to study the hydrodynamics behavior on superficial solids velocity, solids and gas holdups. Two operating regimes are found under different operating conditions. In Region-I, superficial solids velocity increases with the increases of superficial, net superficial and normalized superficial liquid velocity whereas in Region-II solids velocities remain constant. For the case of cross-sectional phase holdups, both gas and solids holdups decrease with the increases of superficial liquid velocity. © 2018 Elsevier Ltd. All rights reserved.
1. Introduction Gas-liquid-solid circulating fluidized bed (GLSCFB) reactors have become more attractive due to their intrinsic features in chemical and biochemical applications (Atta et al., 2009). In GLSCFB, solids particles are circulated between riser and downer by proper pressure balance with higher liquid velocity compared to conventional fluidized beds. Circulating operations promotes good mixing, higher gas holdup, more uniform bubble sizes, better interphase contact (Razzak et al., 2009). In addition to good solid mixing, uniform mass and heat transfer capacity, lower particle backmixing, easy handling of particles, steady-state operation, promising product throughput per unit bed cross-section make GLSCFB a unique choice over conventional fluidized bed (no particle circulation) (Zhu et al., 20 0 0). Despite these advantages, limited investigations have been carried out in GLSCFB systems.
E-mail address: [email protected]
Flow characterization is very important to understand the overall flow behavior, system stability, phase holdups distribution in GLSCFB system. However, for the uniform, continuous, steady state operation under various operating conditions, system stability is necessary. By appropriate pressure balance between the riser and downer, system operational regime and system stability can be understood (Zheng and Zhu, 20 0 0). It also showed that the suitable operating conditions, required solids inventory height, solids circulation rate in the overall system. Flow behavior under the different operating conditions, effect of particles sizes and shape of the flow are also reported as important parameter for efficient system operations (Razzak, 2012). Phase holdup is the main parameter in the study of flow behavior in three-phase GLSCFB systems. The research in this area was limited because of the major challenge to distinguish the three phases and its quantitative measurement of phase holdups. Some articles are reported about the uses of optical fibre probe to distinguish gas holdup (Lee and de Lasa, 1987; de Lasa et al., 1984; Wang et al., 2001). Warsito and Fan (2003) used electrical and capacitance tomography (ECT) to distinguish gas bubble.
https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016 0301-9322/© 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
ARTICLE IN PRESS
JID: IJMF 2
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
2. Experimental set-up and methodology Notations Ar Ar Ad CD dP d∗ Gs H h r R Ret t Ua Ul UP Us Ug Ut Ul Ut
Ul − Ut U∗
Archimedes number Cross-sectional area of riser (m2 ) Cross-sectional area of downer (m2 ) Drag coefficient Particle diameter (m) D imensionless particle diameter Solid circulation rate (kg/m2 .s) Height of the riser (m) Height of accumulated particles (m) Radial position (m) Radius of the riser (m) Reynolds number at terminal settling velocity Time (s) Auxiliary liquid velocity (cm/s) Superficial liquid velocity (cm/s) Primary liquid velocity (cm/s) Superficial solid velocity (cm/s) Superficial gas velocity (cm/s) Terminal settling velocity (cm/s) Normalized liquid velocity Net superficial liquid velocity (cm/s) Dimensionless terminal settling velocity (cm/s)
Greek Letters ρs Density of solid particle (kg/m3 ) ρl Density of liquid (kg/m3 ) ɛs Solids holdups ɛg Gas holdups ɛl Liquid holdups ∅ Sphericity
All these study just simply distinguished gas phase qualitatively not have chance to measure quantitatively. Liang et al. (1995) for the first time quantitatively measured cross-sectional average gas holdup using conductive probe in GLSCFB system. On the other hand, Uchida et al. (1989) and Vatanakul et al. (2003) used ultrasound waves to measure solids holdup. For three phase distinguishing separately, George et al. (2001) developed a combined technique using electrical impedance tomography (EIT) and gamma-densitometry tomography (GDT) to measure phase distribution in a three phase flow system. Later, the present author employed electrical resistance tomography (ERT) and pressure transducers (PT) to distinguish three phases and quantitatively measured cross-sectional average solids, gas and liquid holdups in the bed (Razzak et al., 2007). The present investigation has been focused on system stability and flow characterization of the GLSCFB system under a wide range of operating conditions. The main operating parameters were used such as: (i) superficial liquid velocity (Ul ), (ii) auxiliary liquid velocity (Ua ), (iii) superficial solids velocity (Us ), (iv) superficial gas velocity (Ug ). An analysis of pressure balance in the whole loop was carried out for the GLSCFB system to understand the flow characteristics and system stability. The GLSCFB system was operated using two sizes of spherical glass bead particles and two irregularly shaped lava rock particles. This study also considered the effect of particle shape and size of the solid particle using two major parameters (v) normalized superficial liquid velocity (Ul /Ut ) and (vi) net superficial liquid velocity (Ul − Ut ). This article also discussed the flow region under a wide range of operating conditions.
Fig. 1 shows the schematic representation of the gas-liquidsolid circulating fluidized bed (GLSCFB) system employed for this study. It consists of two main sections, riser and downer, both made of Plexiglas. The riser is 5.97 m tall and 0.0762 m in diameter and the downer is 5.05 m tall and 0.2 m in diameter. Both riser and downer are interconnected by a solid feed pipe at the bottom and solids returning pipe at the top. Gas and liquid distributors are located at the bottom part and solid-liquid-gas separator at the top part of the riser. There are two liquid distributors at the bottom of the riser, the primary liquid distributor, made of seven stainless tubes occupying 19.5% of the total riser cross section and extending 0.2 m into the riser, and the auxiliary liquid distributor, a porous plate with 4.8% opening area at the base of the riser. The gas distributor is a tube of 19 mm in diameter and bent in a ring shape of approximately 0.0413 m in diameter, located at 0.34 m above the bottom of the riser and just above the tip of primary liquid distributor. There are 460 small holes of 0.5 mm in diameter on the ring, giving a total opening area of 361 mm2 , pointing upwards for gas flow. Liquid pumped from the reservoir and feed to the riser into two streams through the primary and the auxiliary liquid. Air used as gas phase, introduced through the gas distributor. The auxiliary liquid is employed to facilitate the flow of solid particles from the downer to the riser, with the main purpose of controlling the solids circulation rate and acting as a non-mechanical valve. Solid particles circulation is maintained by appropriate pressure balance between the riser and the downer. Solid particles entered into the riser at liquid distributor zone from the downer through the solids feed pipe. From the distributor zone, solid particles are carried up in the riser with the combined effects of primary (primary liquid velocity,Up ) and auxiliary (auxiliary liquid velocity, Ua ) flow. In addition, some degree of gas flow (superficial gas velocity,Ug ) have contribution to carry solid particles. Rising particles are entrained into the gas-liquid-solid separator located at the top of the riser. From the separator, air is vented, liquid is returned to the feed tank and separated solid particles are returned to the downer via conical flask through solids returning pipe. Solid particles farther start to settle down into the downer due to the gravity and passes through solids circulation rate measuring device. This measuring device located near the top of the downer and just below the solid returning pipe. Returned solid particles are accumulated at lower section of downer with slightly fluidized conditions called solids storage/inventory. This slight fluidization necessary to avoid chocking of particles at the bottom part of the downer. In addition, it allow smooth solids circulation and maintain necessary pressure balance. 2.1. Solids circulation rate measurement As mentioned above, solid particles return to the downer through a solid circulation rate measuring device. A vertical partition plate divides this device into two halves and have two half butterfly like flappable plates at both ends. By properly flipping these two half butterfly plates from one side to the other, returning solids can be collected on one side of the measuring section for a given time period. Using the height of the accumulated particles collected for a given period, the solids circulation rate (Solid flux) is calculated:
Gs =
hρb A2d t Ar
(1)
where Gs is the solids circulation rate (kg/m2 .s) h is the height of the accumulated particle (m), t is the accumulation time (s) and ρ b (kg/m3 ) is the bulk density, Ad cross-sectional area of the downer
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
ARTICLE IN PRESS
JID: IJMF
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
3
Solids Returning Liquid Recycle
Pipe
Liquid Recycle
Gas-Liquid-Solid Separator
Solids Circulation Measurement Device
Riser Downer
Solids Feed Pipe
Auxiliary Liquid
Air In Primary Liquid Fig. 1. The schematic diagram of the experimental set-up of the GLSCFB system.
and Ar cross-sectional area of the riser. Superficial solids velocity (Us ) was estimated by dividing the solids circulation rate with the density of the particles:
Us =
Gs
ρs
=
hρs (Ad /2 )εs h ( A d / 2 )ε s = t ρs A r t Ar
(2)
where Us is the superficial solids velocity (cm/s) and ρ s (kg/m3 ) is the solid particles density,
responding hydrostatic pressure data. Six pressure transducers (PT) installed on columns wall, connected with a personal computer via a 16-bits A/D converter for real-time data acquisition. For experimentations, signals of the differential pressure fluctuations collected at a frequency of 10 0 0 Hz and stored in the data acquisition system and computer hard disk. The total acquisition time was 20 s and thus the maximum length of the time series was 20,0 0 0 points. The locations of pressure taps changes depending on experiments requirements.
2.2. Pressure measurements 2.3. Phase holdups measurements Pressure gradients were measured using differential pressure transducers (OMEGA-PX61) along the fluidized beds. Transducers calibrated by changing different liquid levels and measured it cor-
One of the main challenges in the gas-liquid-solid three-phase system is distinguish phases and quantitative estimation of phase
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
ARTICLE IN PRESS
JID: IJMF 4
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
Cross-section of ERT
Riser Wall Electrode
ERT Sensor
Current Signal Voltage Signal
Data Acquisition System
Image Reconstruction System
Fig. 2. Schematic diagram of the measurement prinicple of ERT.
holdups. A combined approach of electrical resistance tomography (ERT) and pressure transducer (PT) were used to distinguished and quantitative measurement of cross-sectional average solid, gas and liquid holdups. ERT is a non-intrusive measurement technique by which phase holdup measures on the principle of conductivity. It can measure both qualitative and quantitative in any local points. A schematic diagram of ERT system has shown in Fig. 2, comprises an ERT sensor, a data acquisition system and a personal computer. A data acquisition system connected with the electrodes in ERT sensors is responsible for setting currents and reading voltages. The inner diameter of the ERT sensor is equal to the inner diameter of the riser. There are sixteen electrodes equally spaced are responsible to measure nonconductive phase holdups. The ERT system can provide phase distributions of a multiphase flow by measuring the peripheral resistance combinations, and reconstructing cross-sectional conductivity distributions for a given time. For a dispersed multiphase flow, ERT enable to convert the conductivity distributions to local phase holdups of the phases: electrically conductive phase and electrically non-conductive phase or phases. In this study, the former corresponds to saline water, i.e., the conductive liquid phase, while the latter to air and particles, i.e., the non-conductive gas and solid phases. Before the conversion, the local conductivity estimated as a non-dimensional parameters using the following equation:
σ=
σm − σ1 σ0 − σ1
(3)
where σ m denotes the estimated local conductivity, σ 1 denotes the local conductivity when the pipe is full of single liquid phase and σ 0 denotes the local conductivity when the pipe is full of gas or solid or both phases. The conductivity of the first phase (σ 1 ) can be found easily with available commercial conductivity meters, while the local estimated mixture conductivity (σ m ) is determined from the pixel conductivity of ERT image data.
The Maxwell relation is employed to convert the local conductivity to the local gas and solid hold-ups:
ε =1−
3σ ∗ 2 + σ∗
(4)
One disadvantage of the ERT is its inability of differentiating the three phases since it is based on conductivity. Therefore the gas and solid holdups are measured together as a single nonconductive phase. On the other hand, pressure transducers were used to measure the average solid holdup. Two pressure transducers located closest to the ERT measurement sensor were used to provide the local pressure data across the sensor section. Pressure drop in the riser is mainly due to liquid and solid static head, plus the friction at the wall. Since the fluidization velocity in GLSCFB is not very high compared to gas-solid fluidization, the wall friction is not significant. The measured pressure drop per unit length of the bed is therefore proportional to bed density,ρ bed , i.e.:
P = ρbed g = (εs ρs + εl ρl + εg ρg )g Z
(5)
where P is the pressure drop across the measured section of the bed and Z is the height of the measured section. Because ρ g is about two orders of magnitude smaller than either ρ l or ρ s , the gas effect is negligible and thus ignored. From ERT the average conductive liquid phase holdups can be obtained, which can be put into the above equation, to obtain the solids holdup.
εs
∼ =
P − ε l ρl / ρs Z ∗ g
(6)
ERT measured the non-conductive phase (εg + εs ). Combined approach of PT and ERT measured the gas holdups (ɛg ) quantitatively. Then the third phase, liquid phase holdups (ɛl ) also calculated by ERT as conductive phase. 2.4. Data preparation All experiments were conducted using glass bead and lava rock particles as solid phase, saline water as conductive liquid phases
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
ARTICLE IN PRESS
JID: IJMF
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
5
Table 1 Particle physical properties. Particle Denisty
Particles
Size, dp
GB-500 GB-1200 LR-500 LR-920
500 1200 500 920
(μm)
ρ S (kg/m3 )
Shape
Sphericity, ϕ
Terminal Settling Velocity, Ut (cm/s)
Reynold Number, Ret
Archimedes Number, Ar
2500 2500 2210 2210
Spherical Spherical Irregular Irregular
1.0 1.0 0.7 0.7
7.10 13.76 4.87 8.03
36.1 242 24.4 73.88
1920 31,588 1488 9243
and air as gas phase. The particle density of glass beads and lava rocks were 2500 kg/m3 and 2210 kg/m3 , respectively. Two different size and shapes are considered for this investigation to understand the effect of diameter and shape on flow behavior in GLSCFB system. The particle diameter of the lava rocks was determined by sieve analysis. Glass beads particles are regular spherical shape whereas lava rock particles are porous and irregular in shape. The details properties of four particles used in this investigation are provided in Table 1. The solid particles are referred as GB-500, GB1290, LR-500 and LR-920. Where, GB represents glass beads, LR represents lava rocks and the number represents their diameters in microns. For a moving solid particle within fluid the forces acting on the particle are drag force, buoyant force and gravity. The gas has very little impact on drag on solid particles thus the resultant force acting on the particle is mainly responsible by the liquid and particle movement in the fluid presented by the following equation:
FD + FB − FG = m
d us dt
1 2 FD = CD ρl Uslip A 2
(8)
The drag coefficient proposed by Wen and Yu (1966) on the basis of broad experimental conditions correlated with Reynolds number in liquid-solid systems as.
24
εl Ret
1 + 0.015(εl Ret )0.687
(9)
U
where, slip velocity Uslip = ( ε l − Uεss ) and for spherical shape glass l beads particles the terminal settling velocity, Ut (cm/s) can be calculated using by Clift equation (Clift et al., 1978):
log Ret = −1.814 + 1.347 log ND − 0.1243(log ND ) 3
+ 0.00634(log ND ) with, ND = CD Ret2 =
4ρl (ρl −ρs )gd3 3 μ2
2
(10) =
4 3 Ar
and where, CD is the
drag coefficient, Ret is the Reynolds number and Ar is the Archimedes number. For non-spherical lava rock particles the terminal settling velocity can be calculated using Haider and Levenspiel (1989) equation:
Ut = U∗
ρl2 gμ(ρl − ρs )
−1/3
(11)
where, U∗ is the dimensionless terminal settling velocity that can be determined by using Equation (Eq. (12)) for the range of sphericity 0.5–1.
U∗ =
Storage/ Inventory
Riser
(7)
where, FG is the gravity force (expressed by Newton’s law, FG =mg), FB is buoyant force (mass of the fluid displaced by the solids, (( ρm )ρl a) and FD the drag force due to the fluid friction on the S particle. When a fluid moves around the solid body, drag force exerted on the solid by the fluid is the combination of boundarylayer drag and form drag which can be expressed in terms of drag coefficient, CD . Drag force is estimated using the following equation:
CD =
GasLiquidSolid Separator
18 (2.3348 − 1.7439ϕ ) + d∗2 d∗0.5
−1
(12)
Fig. 3. Simplified diagram showing overall pressure balance of GLSCFB system.
where, d∗ = d p [
gρl (ρs −ρl ) 1/3 ]
μ2
The additional parameters such as auxiliary liquid velocity (Ua ), Primary Liquid Velocity (Up ), superficial liquid velocity (Ul ), superficial solids velocity (Us ), superficial gas velocity (Ug ). Since, the terminal settling velocity affects the distribution of solids holdups in a GLSCFB system, for better comparison the normalized superficial liquid velocity (Ul /Ut ) and net superficial liquid velocity (Ul − Ut ) are also introduced to analyse the radial non-uniformity of the solids holdups. 3. Pressure balance for system stability Solid particles are transported out from the top of the riser and move to the downer continuously during the operation. It is necessary to feed continuous particles at the inlet of the riser to maintain appropriate pressure balance to maintain steady solids circulation rate. A Simplified diagram showing overall pressure balance of GLSCFB system shown in Fig. 3. Auxiliary liquid flow is responsible to mobilize solid particles from the base of the riser to ensure the continuous solid particles feed from the downer to the riser and serve like a control device. Solid particles flow can be divided in two categories depends on the way to control, i.e. mechanical and non-mechanical. Mechanical valve usually control moving parts by opening or closing valve mechanically. GLSCFB system does not contain any moving parts but particles movement controlled by adjusting liquid flow rate in the riser and pressure drop around the solids entrance pipe. That is why control device applied in GLSCFB system is non-mechanical type. Mechanical valve are not commonly employed in the industrial high temperature and
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
ARTICLE IN PRESS
JID: IJMF 6
Fig. 4. Variation of superficial solids velocity of GB-50 0, GB-120 0, LR-50 0 and LR920 particles under different superficial liquid velocities at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s.
pressure conditions due to their associated sealing and mechanical problems. Since in GLSCFB system operated non-mechanically it can use at elevated temperatures and pressures. Driving force of the solid circulation rate is the appropriate pressure drop across the solids entrance pipe and overall pressure balance in whole system. Solids circulation rate mainly depends on the pressure drop (Pep ) across the solid entrance pipe. Solid pressure Pst at the entry region of the solid entrance pipe is the bottom pressure of the solid storage vessel (downer) and the exit region is Pr at the bottom region of the riser. Pressure drop in the riser mainly cause for liquid and solid static head, friction between liquid-solid-gas flows and friction at the wall. Since fluidization velocity in GLSCFB systems not very high like gas-solid fluidization so inter phase frictions and wall friction are not significant. Riser outlet exposed in the gas-liquid-solid separator and the level of water is not high enough so the outlet pressure is just atmospheric. Gas is vented to the atmosphere from the top of the separator. So pressure head at the bottom of the riser then calculated as just static head of the phases and can be written as,
Pr = ρs εs + ρl εl + ρg εg
(13)
where, εl + εg + εs = 1 and ρ indicates the density and ɛ is the holdup of the phases. Since the density of the gas is very low compare to the other phase can be neglected the effect of the static head of the gas. So the equation then simplified and reduces to,
1 − ε pst =
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
Fig. 5. variation of superficial solids velocity under different net superficial liquid velocities (Ul − Ut ) and (b) variation of normalized superficial solids velocity under different normalized superficial liquid velocities (Ul /Ut ) at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s.
1 − εse =
Gs
1 − εdst =
ρsUt
1 − εr p =
D2r D2se −D2r
Gs
Dr Dr p
2
ρsUt Gs
Dst
ρsUt
Particles are basically settled down due to the gravity in the gas-liquid-solid separator and then return back to the storage section through the solid returning pipe connected with the downer and bottom of the separator. Solid particles then return to the downer due to gravity and merge with storage, which act as slow moving packed bed. Since solid particles down flow due to gravity, it is assume that it down flow velocity is just the terminal settling velocity of the solid particles in the separator and the solids return pipe. So the liquid holdup can be calculated in the separator, solids return pipe, dilute phase section of the storage using mass balance.
(17)
where Gs is solid circulation rate and D is the diameter of the specific sections. Without introducing the auxiliary liquid velocity the solid particles could not start to move due to pressure drop does across the solid entrance pipe is not sufficiently high. When the auxiliary liquid flow introduce then the solids packed bed started to flow downward and solid particles started to upward through the riser. Another flow introduce to the downer to fluidize the packed region which reduce the internal friction of the particles and increase the flow ability of the particles. Since the solid particles started to move the actual packed bed height or solid inventory height in the storage vessel, L gets reduced toLpst . This reduce amount of the solids holdups in the riser, gas-liquid-solid separator, solid returning pipe and the dilute region above the reduced packed bed or inventory height. So the bed voidage in packed bed region can be calculated by,
(L − L pst )D2st
(14)
(16)
D r 2
H (1 − εl − εg )D2r + Lse (1 − εse ) D2se − D2r + Lr p (1 − εr p )D2r p + Ldst (1 − εdst )D2st
Pr = ρs εs + ρl εl
(15)
(18)
Pressure head at the bottom of the storage vessel or the entry region of the solid entrance pipe can be composed with four distinct pressures. This can be written as,
Pst = Pse + Pr p + Plst + Pdst + Ppst
(19)
where the pressure drops at the gas-liquid-solid separator (Pse ), solid returning pipe (Prp ), liquid disengaging section (Plst ), dilute bed section (Pdst ) and dense or packed bed section (Ppst ) in the storage vessel. Those can be calculated by,
Pse = [ρl εse + ρs (1 − εse )]Lse g
(20)
Pr p = [ρl εr p + ρs (1 − εr p )]Lr p g
(21)
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
JID: IJMF
ARTICLE IN PRESS
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
Plst = Llst ρl g
(22)
Pdst = [ρl εdst + ρs (1 − εdst )]Ldst g
(23)
Ppst = [ρl ε pst + ρs (1 − ε pst )]L pst g
(24)
7
To stable the system and steady state solids circulation dense pressure at the bottom of packed bed (storage/inventory) bed section (Pst ) and pressure at the solid returning pipe (Pr ) needs to maintain i.e. Pst − Pr = Pep . This pressure drop mainly controlled by the auxiliary and primary liquid flow and also partially with gas flow. In stable fluidization, at given operational parameters (such as auxiliary liquid, primary liquid flow, gas flow) the solids circulation rate remain constant. If any change of any parameter, system needs time to stable by adjusting overall pressure balance. When reach steady and system becomes stable no changes in successive solids circulation rate measurements observed. With the change of any operating parameters, system initially unstable then with time becomes stable and reached stead-state by adjusting overall pressure balance in the entire flow loop. 4. Flow characterizations 4.1. Solids circulation rate Solids circulation rate is the mass flux of solids circulated between the riser and the downer at steady operation. This circulation rate mainly controlled by the auxiliary liquid flow rate followed by primary liquid flow and some degree of gas flow. By increasing the auxiliary liquid flow rate, more particles are allowed to enter into the bottom of the riser and increase the solids circulation rate. Superficial solids velocity in the riser estimated from solids circulation rate (Eq. (2)) as an important influential parameter for flow characterization. Fig. 4 shows the variation of superficial solids velocity of GB50 0, GB-120 0, LR-50 0 and LR-920 particles under different superficial liquid velocities at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s. Under a constant auxiliary liquid velocity, the superficial solids velocities (Us ) initially increases with increasing of net superficial liquid velocities (Ul − Ut ) and then reaches constant values for all four particles. The increasing tendencies of superficial solids velocities are observed up to increasing 20 cm/s of net superficial liquid velocities. After then the superficial solids velocities no longer changes with the increase of net superficial velocities for all four particles. Similar results had been reported by Zheng et al. (1999) for the liquid-solid circulating fluidized bed. It is practically important to know the range of change of superficial solids velocity where particle holdups and retention time is important. It can be noted that the effect of size has important effect on superficial solids holdups. With increase of the size solids holdups decreases at any operating superficial liquid velocity conditions. The reason for this type of behavior can possibly for larger size particles generally heavy in weight. As a result, the gravitational force is more dominant and the contribution of drag is balanced by the gravitational component on the particles. However, effect of particle shapes are more intensive as shown in Fig. 4 for GB-500 and LR-500. At a given superficial liquid velocity, superficial solids velocities have a significant differences between both GB-500 and LR-500 particles and irregular shape LR-500 particles shows much lower value than that of GB-500 particles. It is happening because of the terminal settling velocity differs for the shape (sphericity). Since terminal settling velocity is an important parameters and have impact on flow characteristics, two important parameters are
Fig. 6. (a) The effect of particle diameter and (b) The effect of particle shape in variation of normalized superficial solids velocity under different normalized superficial liquid velocities at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s.
considered for farther analysis, i.e. net superficial liquid velocity (Ul − Ut ). Fig. 5 shows the variation of superficial solids velocity under different net superficial liquid velocities (Ul − Ut ) at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s. It can be noted that the onset velocity (the velocity where circulation of particles start) around 2 to 5 cm/s above the terminal settling velocity for the case of larger diameter particles (LR-920 and GB-1200). Whereas on set velocities found lower than the terminal settling velocities specially GB-500 and LR-500 particles. It is happening because local velocities varies from central radial position towards the wall. At central region, local fluid velocities much higher than that of terminal settling velocity thus the circulation still observed even at the position of net superficial velocity found zero. With the increase of net superficial liquid velocity, superficial solids velocities are increases. 4.2. Dimensionless analysis Farther studies carried out to investigate the variation of dimensionless normalized superficial solids velocity (Us /Ul ) with normalized superficial liquid velocity (Ul /Ut ). The effect of particle size and shapes on the flow behavior analyzed separately. Fig. 6(a) shows the effect of particle diameter in variation of normalized superficial solids velocity under different normalized super-
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
JID: IJMF 8
ARTICLE IN PRESS
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
Fig. 7. Flow regimes of GLSCFB for glass bead particles GB-500 under different auxiliary liquid velocities and superficial liquid velocities at superficial gas velocity Ug = 0 cm/s.
ficial liquid velocities at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s. An increase in the particle diameter, the normalized superficial solid holdups decreases under the same operating conditions. With the increase of normalized superficial liquid velocities, normalized superficial solids velocities increases initially and after some points it becomes constant. The operational range for increasing normalized superficial solids velocities for GB-500 are also higher than GB-1200. It is clear from the analysis that the solids circulation starts well ached for GB-500 particles whereas it starts when normalized superficial liquid velocity is 1.2 for GB1200. It indicates that terminal settling velocity is dominant factor on the size of the particle. It is also noticed that operational ranges in regards of normalized superficial liquid velocity for GB-1200 lies in between 1.2 and 3.8 whereas for GB-500 it is in between 0.5 and 6.3. On the other hand, GB-500 and LR-500 are same size particles but different in shape shows a wide range of normalized superficial liquid velocities under same operating conditions as shown in Fig. 6(b). The operational range of normalized superficial liquid velocities for LR-500 lies in between 1.8 and 6.5 which is almost similar for GB-500 which lies in between 0.5 and 6.3. The impact is the differences in terms of normalized superficial solids velocity. Which is more dominant for GB-500 and much higher than irregular shape LR-500 particles. For any normalized superficial liquid velocity conditions, LR-500 particles shows lower normalized superficial solids velocity than that of GB-500.
4.3. Flow regime In the visual observation during experimentation, at zero auxiliary liquid velocity, no particles in the riser are fluidized and bed is acted as packed bed. With the introducing of auxiliary liquid flow and increase of superficial liquid velocity above the minimum fluidized bed velocity, the bed start to expand and fluidization achieved. This velocity called minimum fluidization velocity. Above this velocity, smooth expansion of bed observed. With gradual increasing of superficial liquid velocities leads to stage of particle entrainment. It is indicating that the bed is operating in the conventional fluidized bed regime where no entrainment of particles achieved. At the onset velocity, the first particle entrained and start to circulate from riser to downer. When the liquid velocity is sufficiently high, a large quantity of solid particles are transport from the riser to the downer and create circulating fluidization regime. As superficial liquid velocity increases, solids circulation rate increases thus the superficial solids velocity.
Fig. 8. Flow regimes of GLSCFB for glass bead particles GB-500 under different normalized superficial solids velocity vs normalized superficial liquid velocities (a) effect of auxiliary liquid velocity (b) effect of superficial gas velocities.
Fig. 7 shows the flow regimes of GLSCFB for glass bead particles GB-500 under different auxiliary liquid velocities and superficial liquid velocities at superficial gas velocity Ug = 0 cm/s. It could be observed that there are two region exists, Region I: the initial zone in which superficial solids velocities increases rapidly with the increase of the superficial liquid velocities up to 32 cm/s. and Region II: the fully developed zone where the superficial solids velocities becomes typically constant with increase of superficial liquid velocity from 32 cm/s. It also can be noted that with the increases of auxiliary liquid velocities superficial velocities increases at any operating superficial liquid velocity. Dimensionless analysis for flow regimes of GLSCFB for glass bead particles GB-500 are carried out to see the effect of both auxiliary liquid velocity and superficial gas velocity. Fig. 8(a) shows the effect of auxiliary liquid velocity and Fig. 8(b) the effect of superficial gas velocities on normalized superficial solids velocity under different normalized superficial liquid velocities. For both conditions shows two flow regimes under wide ranges of operations. Two clear flow regimes also exist in both cases. Normalized superficial solids velocities increase with the increase of normalized liquid velocities from 0.4 to 4.3 in Region I and after then it is no longer changes in region II. It can be noticed that with the increases of auxiliary velocity have great impact on increases of normalized superficial solids velocity but the case of gas holdups not significantly increases. When introducing gas, slightly increase of
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
JID: IJMF
ARTICLE IN PRESS
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
9
cantly increase. It is confirmed that in the bed with increase of gas holdups, solids holdups not significantly changed and indicates the liquid holdups decreased in the bed. 5. Conclusions System stability and flow characterization of the three-phase GLSCFB under a wide range of operating conditions have been studied. Two regular spherical shape glass bead particles (500 and 1200 μm in size), two irregular shape lava rock particles (500 and 920 μm in size) were used as solid phase, air as gas phase and saline-water as liquid phase. For flow characterization, superficial solids velocity (Us ), normalized superficial solids velocity ( UUs ), l
solids holdup (ɛs ) and gas holdups (ɛg ) were considered measureable parameters. Superficial liquid velocity (Ul ), auxiliary liquid velocity (Ua ), superficial gas velocity (Ug ), normalized superficial liquid velocity (Ul /Ut ) and net superficial liquid velocity (Ul − Ut ) were used as main operating parameters. The followings are the conclusion of this study:
Fig. 9. Effect of superficial liquid velocity on cross-sectional average (a) solids holdups and (b) gas holdups under different superficial gas velocities at axial location H = 3 m in GLSCFB riser.
superficial solids velocity observed but farther increases of superficial gas velocities does not increase that much. It means that there is very little impact on the drag force from the gas phase. 4.4. Phase holdups
1. With the change of any operating parameters, system became initially unstable then gradually returned to stable. The desired level stead-state can be reached by adjusting overall pressure balance in the entire flow loop. 2. Under a constant auxiliary liquid velocity, it was found that the superficial solids velocity initially increased with increasing superficial liquid velocities followed by reached a constant values for all four particles. 3. Normalized superficial solids velocity for GB-500 particle was higher than that of GB-1200 particles. On the other hand, GB500 and LR-500 (same size but different in shape) showed a wide range of normalized superficial liquid velocities under same operating conditions. 4. An increase in the particle diameter, the normalized superficial solids holdups decreased under the same operating conditions. For any normalized superficial liquid velocity conditions, irregular shape LR-500 particles had lower normalized superficial solids velocities than GB-500. 5. Two flow regimes found in circulating fluidized bed conditions. Region I: the initial zone in which superficial solids velocities increased rapidly with the increase of the superficial liquid velocities and Region II: the fully developed zone where the superficial solids velocities became typically constant with increase of superficial liquid velocity. 6. In the case of phase holdups, at a given superficial liquid velocity, with the increase of superficial gas velocities solids holdups not significantly increased or changed whereas gas holdups significantly increased. Acknowledgments
Phase holdups is the important parameters that indicates the amount of solids, gas and liquid phases are present in per crosssectional area in the bed. The effect of superficial liquid velocities on cross-sectional average solids holdups and gas holdups under different superficial gas velocities at axial location H = 3 m in GLSCFB riser shows in Fig. 9. The dotted lines show the trends of the data points. Fig. 9(a) shows that cross-sectional average solids holdups decrease sharply from 5 to 12 cm/s when operated in different superficial gas velocities. After then the decreasing rate slower from 12 cm/s to 35 cm/s. It also noticed that at any superficial liquid velocity condition, solids holdups not change that much with the increasing of superficial gas velocities. Fig. 9(b) shows that cross-sectional average gas holdups decrease sharply from 5 to 23 cm/s when operated in different superficial gas velocities and after then decreasing rate get slower. But for a given superficial liquid velocity, with the increase of gas velocity, solids holdups not significantly increased or changed whereas gas holdups signifi-
The author would like to gratefully acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. NSTIP # 13-WAT96-04 as part of the National Science, Technology and Innovation Plan. The author would like to acknowledge to Powder Technology Research Center (PTRC) and Professor Jesse Zhu at the University of Western Ontario for experimental support. The authors would also like to thank M. M. Hossain for his assistance in the editing of this manuscript. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ijmultiphaseflow.2018. 10.016.
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
JID: IJMF 10
ARTICLE IN PRESS
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
References Atta, A., Razzak, S.A., Nigam, K.D.P, Zhu, J-X., 2009. (Gas)-liquid-solid circulating fluidized bed reactors: characteristics and applications. Ind. Eng. Chem. Res. 48 (17), 7876–7892. Clift, R., Grace, J.R., Weber, M.E., 1978. Bubbles, drops, and Particles. Academic Press, New York. de Lasa, H.I., Lee, S.L.P., Bergougnou, M.A., 1984. Bubble measurement in three-phase fluidized beds using a U-shaped optical fiber. Can. J. Chem. Eng. 62 (2), 165–176. George, D.L., shollenberger, K.A., Torczynski, J.R., O’ Hern, T.J., Ceccio, S.L., 2001. Three-phase material distribution measurement in a vertical flow using gamma-densiometry tomography and electrical-impedance tomography. Int. J. Multiphase Flow 27, 1903–1930. Haider, A., Levenspiel, O., 1989. Drag coefficient and terminal velocity of spherical and nonspherical particles. Powder Technol. 58, 63–70. Lee, S.L.P., de Lasa, H.I., 1987. Phase holdups in three-phase fluidized beds. AIChE J. 8 (33), 1359–1370. Liang, W.-G., Wu, Q.W., Yu, Z.Q., Jin, Y., Bi, H.T., 1995. Flow regimes of the three-phase circulating fluidized bed. AIChE J. 41, 267–271. Razzak, S.A., 2012. Hydrodynamics modeling of an LSCFB riser using ANFIS methodology: Effects of particle shape and size. Chem. Eng. J. 195, 49–61. Razzak, S.A., Barghi, S., Zhu, J-X., 2007. Electrical resistance tomography for flow characterization of a gas-liquid-solid three-phase circulating fluidized bed. Chem. Eng. Sci. 62, 7253–7263.
Razzak, S.A., Zhu, J.-X., Barghi, S., 2009. Radial distributions of phase holdups and phase propagation velocities in a three-phase gas-liquid-solid fluidized bed (GLSCFB) riser. Ind. Eng. Chem. Res. 48 (1), 281–289. Uchida, S., Okamura, S., Katsumata, T., 1989. Measurement of longitudinal distribution of solids holdup in a three-phase fluidized bed but ultrasonic technique. Can. J. Chem. Eng. 67, 166–176. Vatanakul, M., Zheng, Y., Couturier, M., 2003. Flow Characterization of a three-phase circulating fluidized bed using ultrasonic technique, 81, 1121–1129 Wang, T., Wang, J., Weiguo, Y., Yong, J., 2001. Experimental study on gas-holdup and gas-liquid interfacial area in TPCFBs. Chem. Eng. Commun. 187, 251–263. Warsito, W., Fan, L.-S., 2003. ECT imaging of three-phase fluidized bed based on three-phase capacitance model. Chem. Eng. Sci. 58, 823–832. Wen, C.Y., Yu, Y.H, 1966. A generalized method for predicting the minimum fluidization velocity. AIChE J. 12 (3), 610–612. Zheng, Y., Zhu, J., Wen, J., Martin, S.A., Bassi, A., Margaritis, A., 1999. The axial hydrodynamic behavior in a liquid-solid circulating fluidized bed. Can. J. Chem. Eng. 77 (2), 284–290. Zheng, Y., Zhu, J-X., 20 0 0. Overall pressure balance and system stability in a liquid— solid circulating fluidized bed. Chem. Eng. J. 79, 145–153. Zhu, J., Zheng, Y., Karmanev, D., Bassi, A., 20 0 0. (Gas)–liquid–solid circulating fluidized beds and their potential applications to bioreactor engineering. Can. J. Chem. Eng. 78, 82–94.
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
JID: IJMF
[m5G;October 26, 2018;5:58]
International Journal of Multiphase Flow xxx (xxxx) xxx
Contents lists available at ScienceDirect
International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulflow
Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability Shaikh A. Razzak Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 13 April 2018 Revised 8 October 2018 Accepted 22 October 2018 Available online xxx Keywords: Gas-liquid-solid circulating fluidized bed Hydrodynamics Holdups Flow characteristics Flow regimes Superficial velocity
a b s t r a c t Gas-liquid-solid circulating fluidized bed (GLSCFB) is considered as superior technology over conventional Gas-liquid-solid fluidized beds. The GLSCFB consist of two interconnected riser and downer columns facilitating solid circulation between the columns by pressure balance. The overall pressure balance is an important parameter for stable operation of the system. The present communication reports an experimental investigation to correlate the pressure balance and hydrodynamics behavior and the stability of the GLSCFB. In this regard, two spherical shape glass bead particles (500 and 1200 μm in size), two irregular shape lava rock particles (500 and 920 μm in size) are used as solid phase, air as gas phase and saline-water as liquid phase. For flow characterization, superficial solids velocity (Us ), normalized superficial solids velocity ( UUs ), solids holdup (ɛs ) and gas holdups (ɛg ) are considered as measureable pal rameters. The experimental observations indicates that the solids holdup have significant impact of size, density and shape of the particles on hydrodynamics behavior. The drag effect on the particles and their terminal settling velocity are the main factors on solids holdup distribution. The average solids holdups decrease with the increase on net superficial liquid velocity (Ul − Ut ) and normalized superficial liquid velocity ( UUtl ). Effects of different parameters (Ua , Ul , Ug , UUtl , Ul − Ut ) are considered to study the hydrodynamics behavior on superficial solids velocity, solids and gas holdups. Two operating regimes are found under different operating conditions. In Region-I, superficial solids velocity increases with the increases of superficial, net superficial and normalized superficial liquid velocity whereas in Region-II solids velocities remain constant. For the case of cross-sectional phase holdups, both gas and solids holdups decrease with the increases of superficial liquid velocity. © 2018 Elsevier Ltd. All rights reserved.
1. Introduction Gas-liquid-solid circulating fluidized bed (GLSCFB) reactors have become more attractive due to their intrinsic features in chemical and biochemical applications (Atta et al., 2009). In GLSCFB, solids particles are circulated between riser and downer by proper pressure balance with higher liquid velocity compared to conventional fluidized beds. Circulating operations promotes good mixing, higher gas holdup, more uniform bubble sizes, better interphase contact (Razzak et al., 2009). In addition to good solid mixing, uniform mass and heat transfer capacity, lower particle backmixing, easy handling of particles, steady-state operation, promising product throughput per unit bed cross-section make GLSCFB a unique choice over conventional fluidized bed (no particle circulation) (Zhu et al., 20 0 0). Despite these advantages, limited investigations have been carried out in GLSCFB systems.
E-mail address: [email protected]
Flow characterization is very important to understand the overall flow behavior, system stability, phase holdups distribution in GLSCFB system. However, for the uniform, continuous, steady state operation under various operating conditions, system stability is necessary. By appropriate pressure balance between the riser and downer, system operational regime and system stability can be understood (Zheng and Zhu, 20 0 0). It also showed that the suitable operating conditions, required solids inventory height, solids circulation rate in the overall system. Flow behavior under the different operating conditions, effect of particles sizes and shape of the flow are also reported as important parameter for efficient system operations (Razzak, 2012). Phase holdup is the main parameter in the study of flow behavior in three-phase GLSCFB systems. The research in this area was limited because of the major challenge to distinguish the three phases and its quantitative measurement of phase holdups. Some articles are reported about the uses of optical fibre probe to distinguish gas holdup (Lee and de Lasa, 1987; de Lasa et al., 1984; Wang et al., 2001). Warsito and Fan (2003) used electrical and capacitance tomography (ECT) to distinguish gas bubble.
https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016 0301-9322/© 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
ARTICLE IN PRESS
JID: IJMF 2
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
2. Experimental set-up and methodology Notations Ar Ar Ad CD dP d∗ Gs H h r R Ret t Ua Ul UP Us Ug Ut Ul Ut
Ul − Ut U∗
Archimedes number Cross-sectional area of riser (m2 ) Cross-sectional area of downer (m2 ) Drag coefficient Particle diameter (m) D imensionless particle diameter Solid circulation rate (kg/m2 .s) Height of the riser (m) Height of accumulated particles (m) Radial position (m) Radius of the riser (m) Reynolds number at terminal settling velocity Time (s) Auxiliary liquid velocity (cm/s) Superficial liquid velocity (cm/s) Primary liquid velocity (cm/s) Superficial solid velocity (cm/s) Superficial gas velocity (cm/s) Terminal settling velocity (cm/s) Normalized liquid velocity Net superficial liquid velocity (cm/s) Dimensionless terminal settling velocity (cm/s)
Greek Letters ρs Density of solid particle (kg/m3 ) ρl Density of liquid (kg/m3 ) ɛs Solids holdups ɛg Gas holdups ɛl Liquid holdups ∅ Sphericity
All these study just simply distinguished gas phase qualitatively not have chance to measure quantitatively. Liang et al. (1995) for the first time quantitatively measured cross-sectional average gas holdup using conductive probe in GLSCFB system. On the other hand, Uchida et al. (1989) and Vatanakul et al. (2003) used ultrasound waves to measure solids holdup. For three phase distinguishing separately, George et al. (2001) developed a combined technique using electrical impedance tomography (EIT) and gamma-densitometry tomography (GDT) to measure phase distribution in a three phase flow system. Later, the present author employed electrical resistance tomography (ERT) and pressure transducers (PT) to distinguish three phases and quantitatively measured cross-sectional average solids, gas and liquid holdups in the bed (Razzak et al., 2007). The present investigation has been focused on system stability and flow characterization of the GLSCFB system under a wide range of operating conditions. The main operating parameters were used such as: (i) superficial liquid velocity (Ul ), (ii) auxiliary liquid velocity (Ua ), (iii) superficial solids velocity (Us ), (iv) superficial gas velocity (Ug ). An analysis of pressure balance in the whole loop was carried out for the GLSCFB system to understand the flow characteristics and system stability. The GLSCFB system was operated using two sizes of spherical glass bead particles and two irregularly shaped lava rock particles. This study also considered the effect of particle shape and size of the solid particle using two major parameters (v) normalized superficial liquid velocity (Ul /Ut ) and (vi) net superficial liquid velocity (Ul − Ut ). This article also discussed the flow region under a wide range of operating conditions.
Fig. 1 shows the schematic representation of the gas-liquidsolid circulating fluidized bed (GLSCFB) system employed for this study. It consists of two main sections, riser and downer, both made of Plexiglas. The riser is 5.97 m tall and 0.0762 m in diameter and the downer is 5.05 m tall and 0.2 m in diameter. Both riser and downer are interconnected by a solid feed pipe at the bottom and solids returning pipe at the top. Gas and liquid distributors are located at the bottom part and solid-liquid-gas separator at the top part of the riser. There are two liquid distributors at the bottom of the riser, the primary liquid distributor, made of seven stainless tubes occupying 19.5% of the total riser cross section and extending 0.2 m into the riser, and the auxiliary liquid distributor, a porous plate with 4.8% opening area at the base of the riser. The gas distributor is a tube of 19 mm in diameter and bent in a ring shape of approximately 0.0413 m in diameter, located at 0.34 m above the bottom of the riser and just above the tip of primary liquid distributor. There are 460 small holes of 0.5 mm in diameter on the ring, giving a total opening area of 361 mm2 , pointing upwards for gas flow. Liquid pumped from the reservoir and feed to the riser into two streams through the primary and the auxiliary liquid. Air used as gas phase, introduced through the gas distributor. The auxiliary liquid is employed to facilitate the flow of solid particles from the downer to the riser, with the main purpose of controlling the solids circulation rate and acting as a non-mechanical valve. Solid particles circulation is maintained by appropriate pressure balance between the riser and the downer. Solid particles entered into the riser at liquid distributor zone from the downer through the solids feed pipe. From the distributor zone, solid particles are carried up in the riser with the combined effects of primary (primary liquid velocity,Up ) and auxiliary (auxiliary liquid velocity, Ua ) flow. In addition, some degree of gas flow (superficial gas velocity,Ug ) have contribution to carry solid particles. Rising particles are entrained into the gas-liquid-solid separator located at the top of the riser. From the separator, air is vented, liquid is returned to the feed tank and separated solid particles are returned to the downer via conical flask through solids returning pipe. Solid particles farther start to settle down into the downer due to the gravity and passes through solids circulation rate measuring device. This measuring device located near the top of the downer and just below the solid returning pipe. Returned solid particles are accumulated at lower section of downer with slightly fluidized conditions called solids storage/inventory. This slight fluidization necessary to avoid chocking of particles at the bottom part of the downer. In addition, it allow smooth solids circulation and maintain necessary pressure balance. 2.1. Solids circulation rate measurement As mentioned above, solid particles return to the downer through a solid circulation rate measuring device. A vertical partition plate divides this device into two halves and have two half butterfly like flappable plates at both ends. By properly flipping these two half butterfly plates from one side to the other, returning solids can be collected on one side of the measuring section for a given time period. Using the height of the accumulated particles collected for a given period, the solids circulation rate (Solid flux) is calculated:
Gs =
hρb A2d t Ar
(1)
where Gs is the solids circulation rate (kg/m2 .s) h is the height of the accumulated particle (m), t is the accumulation time (s) and ρ b (kg/m3 ) is the bulk density, Ad cross-sectional area of the downer
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
ARTICLE IN PRESS
JID: IJMF
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
3
Solids Returning Liquid Recycle
Pipe
Liquid Recycle
Gas-Liquid-Solid Separator
Solids Circulation Measurement Device
Riser Downer
Solids Feed Pipe
Auxiliary Liquid
Air In Primary Liquid Fig. 1. The schematic diagram of the experimental set-up of the GLSCFB system.
and Ar cross-sectional area of the riser. Superficial solids velocity (Us ) was estimated by dividing the solids circulation rate with the density of the particles:
Us =
Gs
ρs
=
hρs (Ad /2 )εs h ( A d / 2 )ε s = t ρs A r t Ar
(2)
where Us is the superficial solids velocity (cm/s) and ρ s (kg/m3 ) is the solid particles density,
responding hydrostatic pressure data. Six pressure transducers (PT) installed on columns wall, connected with a personal computer via a 16-bits A/D converter for real-time data acquisition. For experimentations, signals of the differential pressure fluctuations collected at a frequency of 10 0 0 Hz and stored in the data acquisition system and computer hard disk. The total acquisition time was 20 s and thus the maximum length of the time series was 20,0 0 0 points. The locations of pressure taps changes depending on experiments requirements.
2.2. Pressure measurements 2.3. Phase holdups measurements Pressure gradients were measured using differential pressure transducers (OMEGA-PX61) along the fluidized beds. Transducers calibrated by changing different liquid levels and measured it cor-
One of the main challenges in the gas-liquid-solid three-phase system is distinguish phases and quantitative estimation of phase
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
ARTICLE IN PRESS
JID: IJMF 4
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
Cross-section of ERT
Riser Wall Electrode
ERT Sensor
Current Signal Voltage Signal
Data Acquisition System
Image Reconstruction System
Fig. 2. Schematic diagram of the measurement prinicple of ERT.
holdups. A combined approach of electrical resistance tomography (ERT) and pressure transducer (PT) were used to distinguished and quantitative measurement of cross-sectional average solid, gas and liquid holdups. ERT is a non-intrusive measurement technique by which phase holdup measures on the principle of conductivity. It can measure both qualitative and quantitative in any local points. A schematic diagram of ERT system has shown in Fig. 2, comprises an ERT sensor, a data acquisition system and a personal computer. A data acquisition system connected with the electrodes in ERT sensors is responsible for setting currents and reading voltages. The inner diameter of the ERT sensor is equal to the inner diameter of the riser. There are sixteen electrodes equally spaced are responsible to measure nonconductive phase holdups. The ERT system can provide phase distributions of a multiphase flow by measuring the peripheral resistance combinations, and reconstructing cross-sectional conductivity distributions for a given time. For a dispersed multiphase flow, ERT enable to convert the conductivity distributions to local phase holdups of the phases: electrically conductive phase and electrically non-conductive phase or phases. In this study, the former corresponds to saline water, i.e., the conductive liquid phase, while the latter to air and particles, i.e., the non-conductive gas and solid phases. Before the conversion, the local conductivity estimated as a non-dimensional parameters using the following equation:
σ=
σm − σ1 σ0 − σ1
(3)
where σ m denotes the estimated local conductivity, σ 1 denotes the local conductivity when the pipe is full of single liquid phase and σ 0 denotes the local conductivity when the pipe is full of gas or solid or both phases. The conductivity of the first phase (σ 1 ) can be found easily with available commercial conductivity meters, while the local estimated mixture conductivity (σ m ) is determined from the pixel conductivity of ERT image data.
The Maxwell relation is employed to convert the local conductivity to the local gas and solid hold-ups:
ε =1−
3σ ∗ 2 + σ∗
(4)
One disadvantage of the ERT is its inability of differentiating the three phases since it is based on conductivity. Therefore the gas and solid holdups are measured together as a single nonconductive phase. On the other hand, pressure transducers were used to measure the average solid holdup. Two pressure transducers located closest to the ERT measurement sensor were used to provide the local pressure data across the sensor section. Pressure drop in the riser is mainly due to liquid and solid static head, plus the friction at the wall. Since the fluidization velocity in GLSCFB is not very high compared to gas-solid fluidization, the wall friction is not significant. The measured pressure drop per unit length of the bed is therefore proportional to bed density,ρ bed , i.e.:
P = ρbed g = (εs ρs + εl ρl + εg ρg )g Z
(5)
where P is the pressure drop across the measured section of the bed and Z is the height of the measured section. Because ρ g is about two orders of magnitude smaller than either ρ l or ρ s , the gas effect is negligible and thus ignored. From ERT the average conductive liquid phase holdups can be obtained, which can be put into the above equation, to obtain the solids holdup.
εs
∼ =
P − ε l ρl / ρs Z ∗ g
(6)
ERT measured the non-conductive phase (εg + εs ). Combined approach of PT and ERT measured the gas holdups (ɛg ) quantitatively. Then the third phase, liquid phase holdups (ɛl ) also calculated by ERT as conductive phase. 2.4. Data preparation All experiments were conducted using glass bead and lava rock particles as solid phase, saline water as conductive liquid phases
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
ARTICLE IN PRESS
JID: IJMF
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
5
Table 1 Particle physical properties. Particle Denisty
Particles
Size, dp
GB-500 GB-1200 LR-500 LR-920
500 1200 500 920
(μm)
ρ S (kg/m3 )
Shape
Sphericity, ϕ
Terminal Settling Velocity, Ut (cm/s)
Reynold Number, Ret
Archimedes Number, Ar
2500 2500 2210 2210
Spherical Spherical Irregular Irregular
1.0 1.0 0.7 0.7
7.10 13.76 4.87 8.03
36.1 242 24.4 73.88
1920 31,588 1488 9243
and air as gas phase. The particle density of glass beads and lava rocks were 2500 kg/m3 and 2210 kg/m3 , respectively. Two different size and shapes are considered for this investigation to understand the effect of diameter and shape on flow behavior in GLSCFB system. The particle diameter of the lava rocks was determined by sieve analysis. Glass beads particles are regular spherical shape whereas lava rock particles are porous and irregular in shape. The details properties of four particles used in this investigation are provided in Table 1. The solid particles are referred as GB-500, GB1290, LR-500 and LR-920. Where, GB represents glass beads, LR represents lava rocks and the number represents their diameters in microns. For a moving solid particle within fluid the forces acting on the particle are drag force, buoyant force and gravity. The gas has very little impact on drag on solid particles thus the resultant force acting on the particle is mainly responsible by the liquid and particle movement in the fluid presented by the following equation:
FD + FB − FG = m
d us dt
1 2 FD = CD ρl Uslip A 2
(8)
The drag coefficient proposed by Wen and Yu (1966) on the basis of broad experimental conditions correlated with Reynolds number in liquid-solid systems as.
24
εl Ret
1 + 0.015(εl Ret )0.687
(9)
U
where, slip velocity Uslip = ( ε l − Uεss ) and for spherical shape glass l beads particles the terminal settling velocity, Ut (cm/s) can be calculated using by Clift equation (Clift et al., 1978):
log Ret = −1.814 + 1.347 log ND − 0.1243(log ND ) 3
+ 0.00634(log ND ) with, ND = CD Ret2 =
4ρl (ρl −ρs )gd3 3 μ2
2
(10) =
4 3 Ar
and where, CD is the
drag coefficient, Ret is the Reynolds number and Ar is the Archimedes number. For non-spherical lava rock particles the terminal settling velocity can be calculated using Haider and Levenspiel (1989) equation:
Ut = U∗
ρl2 gμ(ρl − ρs )
−1/3
(11)
where, U∗ is the dimensionless terminal settling velocity that can be determined by using Equation (Eq. (12)) for the range of sphericity 0.5–1.
U∗ =
Storage/ Inventory
Riser
(7)
where, FG is the gravity force (expressed by Newton’s law, FG =mg), FB is buoyant force (mass of the fluid displaced by the solids, (( ρm )ρl a) and FD the drag force due to the fluid friction on the S particle. When a fluid moves around the solid body, drag force exerted on the solid by the fluid is the combination of boundarylayer drag and form drag which can be expressed in terms of drag coefficient, CD . Drag force is estimated using the following equation:
CD =
GasLiquidSolid Separator
18 (2.3348 − 1.7439ϕ ) + d∗2 d∗0.5
−1
(12)
Fig. 3. Simplified diagram showing overall pressure balance of GLSCFB system.
where, d∗ = d p [
gρl (ρs −ρl ) 1/3 ]
μ2
The additional parameters such as auxiliary liquid velocity (Ua ), Primary Liquid Velocity (Up ), superficial liquid velocity (Ul ), superficial solids velocity (Us ), superficial gas velocity (Ug ). Since, the terminal settling velocity affects the distribution of solids holdups in a GLSCFB system, for better comparison the normalized superficial liquid velocity (Ul /Ut ) and net superficial liquid velocity (Ul − Ut ) are also introduced to analyse the radial non-uniformity of the solids holdups. 3. Pressure balance for system stability Solid particles are transported out from the top of the riser and move to the downer continuously during the operation. It is necessary to feed continuous particles at the inlet of the riser to maintain appropriate pressure balance to maintain steady solids circulation rate. A Simplified diagram showing overall pressure balance of GLSCFB system shown in Fig. 3. Auxiliary liquid flow is responsible to mobilize solid particles from the base of the riser to ensure the continuous solid particles feed from the downer to the riser and serve like a control device. Solid particles flow can be divided in two categories depends on the way to control, i.e. mechanical and non-mechanical. Mechanical valve usually control moving parts by opening or closing valve mechanically. GLSCFB system does not contain any moving parts but particles movement controlled by adjusting liquid flow rate in the riser and pressure drop around the solids entrance pipe. That is why control device applied in GLSCFB system is non-mechanical type. Mechanical valve are not commonly employed in the industrial high temperature and
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
ARTICLE IN PRESS
JID: IJMF 6
Fig. 4. Variation of superficial solids velocity of GB-50 0, GB-120 0, LR-50 0 and LR920 particles under different superficial liquid velocities at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s.
pressure conditions due to their associated sealing and mechanical problems. Since in GLSCFB system operated non-mechanically it can use at elevated temperatures and pressures. Driving force of the solid circulation rate is the appropriate pressure drop across the solids entrance pipe and overall pressure balance in whole system. Solids circulation rate mainly depends on the pressure drop (Pep ) across the solid entrance pipe. Solid pressure Pst at the entry region of the solid entrance pipe is the bottom pressure of the solid storage vessel (downer) and the exit region is Pr at the bottom region of the riser. Pressure drop in the riser mainly cause for liquid and solid static head, friction between liquid-solid-gas flows and friction at the wall. Since fluidization velocity in GLSCFB systems not very high like gas-solid fluidization so inter phase frictions and wall friction are not significant. Riser outlet exposed in the gas-liquid-solid separator and the level of water is not high enough so the outlet pressure is just atmospheric. Gas is vented to the atmosphere from the top of the separator. So pressure head at the bottom of the riser then calculated as just static head of the phases and can be written as,
Pr = ρs εs + ρl εl + ρg εg
(13)
where, εl + εg + εs = 1 and ρ indicates the density and ɛ is the holdup of the phases. Since the density of the gas is very low compare to the other phase can be neglected the effect of the static head of the gas. So the equation then simplified and reduces to,
1 − ε pst =
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
Fig. 5. variation of superficial solids velocity under different net superficial liquid velocities (Ul − Ut ) and (b) variation of normalized superficial solids velocity under different normalized superficial liquid velocities (Ul /Ut ) at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s.
1 − εse =
Gs
1 − εdst =
ρsUt
1 − εr p =
D2r D2se −D2r
Gs
Dr Dr p
2
ρsUt Gs
Dst
ρsUt
Particles are basically settled down due to the gravity in the gas-liquid-solid separator and then return back to the storage section through the solid returning pipe connected with the downer and bottom of the separator. Solid particles then return to the downer due to gravity and merge with storage, which act as slow moving packed bed. Since solid particles down flow due to gravity, it is assume that it down flow velocity is just the terminal settling velocity of the solid particles in the separator and the solids return pipe. So the liquid holdup can be calculated in the separator, solids return pipe, dilute phase section of the storage using mass balance.
(17)
where Gs is solid circulation rate and D is the diameter of the specific sections. Without introducing the auxiliary liquid velocity the solid particles could not start to move due to pressure drop does across the solid entrance pipe is not sufficiently high. When the auxiliary liquid flow introduce then the solids packed bed started to flow downward and solid particles started to upward through the riser. Another flow introduce to the downer to fluidize the packed region which reduce the internal friction of the particles and increase the flow ability of the particles. Since the solid particles started to move the actual packed bed height or solid inventory height in the storage vessel, L gets reduced toLpst . This reduce amount of the solids holdups in the riser, gas-liquid-solid separator, solid returning pipe and the dilute region above the reduced packed bed or inventory height. So the bed voidage in packed bed region can be calculated by,
(L − L pst )D2st
(14)
(16)
D r 2
H (1 − εl − εg )D2r + Lse (1 − εse ) D2se − D2r + Lr p (1 − εr p )D2r p + Ldst (1 − εdst )D2st
Pr = ρs εs + ρl εl
(15)
(18)
Pressure head at the bottom of the storage vessel or the entry region of the solid entrance pipe can be composed with four distinct pressures. This can be written as,
Pst = Pse + Pr p + Plst + Pdst + Ppst
(19)
where the pressure drops at the gas-liquid-solid separator (Pse ), solid returning pipe (Prp ), liquid disengaging section (Plst ), dilute bed section (Pdst ) and dense or packed bed section (Ppst ) in the storage vessel. Those can be calculated by,
Pse = [ρl εse + ρs (1 − εse )]Lse g
(20)
Pr p = [ρl εr p + ρs (1 − εr p )]Lr p g
(21)
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
JID: IJMF
ARTICLE IN PRESS
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
Plst = Llst ρl g
(22)
Pdst = [ρl εdst + ρs (1 − εdst )]Ldst g
(23)
Ppst = [ρl ε pst + ρs (1 − ε pst )]L pst g
(24)
7
To stable the system and steady state solids circulation dense pressure at the bottom of packed bed (storage/inventory) bed section (Pst ) and pressure at the solid returning pipe (Pr ) needs to maintain i.e. Pst − Pr = Pep . This pressure drop mainly controlled by the auxiliary and primary liquid flow and also partially with gas flow. In stable fluidization, at given operational parameters (such as auxiliary liquid, primary liquid flow, gas flow) the solids circulation rate remain constant. If any change of any parameter, system needs time to stable by adjusting overall pressure balance. When reach steady and system becomes stable no changes in successive solids circulation rate measurements observed. With the change of any operating parameters, system initially unstable then with time becomes stable and reached stead-state by adjusting overall pressure balance in the entire flow loop. 4. Flow characterizations 4.1. Solids circulation rate Solids circulation rate is the mass flux of solids circulated between the riser and the downer at steady operation. This circulation rate mainly controlled by the auxiliary liquid flow rate followed by primary liquid flow and some degree of gas flow. By increasing the auxiliary liquid flow rate, more particles are allowed to enter into the bottom of the riser and increase the solids circulation rate. Superficial solids velocity in the riser estimated from solids circulation rate (Eq. (2)) as an important influential parameter for flow characterization. Fig. 4 shows the variation of superficial solids velocity of GB50 0, GB-120 0, LR-50 0 and LR-920 particles under different superficial liquid velocities at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s. Under a constant auxiliary liquid velocity, the superficial solids velocities (Us ) initially increases with increasing of net superficial liquid velocities (Ul − Ut ) and then reaches constant values for all four particles. The increasing tendencies of superficial solids velocities are observed up to increasing 20 cm/s of net superficial liquid velocities. After then the superficial solids velocities no longer changes with the increase of net superficial velocities for all four particles. Similar results had been reported by Zheng et al. (1999) for the liquid-solid circulating fluidized bed. It is practically important to know the range of change of superficial solids velocity where particle holdups and retention time is important. It can be noted that the effect of size has important effect on superficial solids holdups. With increase of the size solids holdups decreases at any operating superficial liquid velocity conditions. The reason for this type of behavior can possibly for larger size particles generally heavy in weight. As a result, the gravitational force is more dominant and the contribution of drag is balanced by the gravitational component on the particles. However, effect of particle shapes are more intensive as shown in Fig. 4 for GB-500 and LR-500. At a given superficial liquid velocity, superficial solids velocities have a significant differences between both GB-500 and LR-500 particles and irregular shape LR-500 particles shows much lower value than that of GB-500 particles. It is happening because of the terminal settling velocity differs for the shape (sphericity). Since terminal settling velocity is an important parameters and have impact on flow characteristics, two important parameters are
Fig. 6. (a) The effect of particle diameter and (b) The effect of particle shape in variation of normalized superficial solids velocity under different normalized superficial liquid velocities at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s.
considered for farther analysis, i.e. net superficial liquid velocity (Ul − Ut ). Fig. 5 shows the variation of superficial solids velocity under different net superficial liquid velocities (Ul − Ut ) at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s. It can be noted that the onset velocity (the velocity where circulation of particles start) around 2 to 5 cm/s above the terminal settling velocity for the case of larger diameter particles (LR-920 and GB-1200). Whereas on set velocities found lower than the terminal settling velocities specially GB-500 and LR-500 particles. It is happening because local velocities varies from central radial position towards the wall. At central region, local fluid velocities much higher than that of terminal settling velocity thus the circulation still observed even at the position of net superficial velocity found zero. With the increase of net superficial liquid velocity, superficial solids velocities are increases. 4.2. Dimensionless analysis Farther studies carried out to investigate the variation of dimensionless normalized superficial solids velocity (Us /Ul ) with normalized superficial liquid velocity (Ul /Ut ). The effect of particle size and shapes on the flow behavior analyzed separately. Fig. 6(a) shows the effect of particle diameter in variation of normalized superficial solids velocity under different normalized super-
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
JID: IJMF 8
ARTICLE IN PRESS
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
Fig. 7. Flow regimes of GLSCFB for glass bead particles GB-500 under different auxiliary liquid velocities and superficial liquid velocities at superficial gas velocity Ug = 0 cm/s.
ficial liquid velocities at Ua = 1.4 cm/s and superficial gas velocity Ug = 0 cm/s. An increase in the particle diameter, the normalized superficial solid holdups decreases under the same operating conditions. With the increase of normalized superficial liquid velocities, normalized superficial solids velocities increases initially and after some points it becomes constant. The operational range for increasing normalized superficial solids velocities for GB-500 are also higher than GB-1200. It is clear from the analysis that the solids circulation starts well ached for GB-500 particles whereas it starts when normalized superficial liquid velocity is 1.2 for GB1200. It indicates that terminal settling velocity is dominant factor on the size of the particle. It is also noticed that operational ranges in regards of normalized superficial liquid velocity for GB-1200 lies in between 1.2 and 3.8 whereas for GB-500 it is in between 0.5 and 6.3. On the other hand, GB-500 and LR-500 are same size particles but different in shape shows a wide range of normalized superficial liquid velocities under same operating conditions as shown in Fig. 6(b). The operational range of normalized superficial liquid velocities for LR-500 lies in between 1.8 and 6.5 which is almost similar for GB-500 which lies in between 0.5 and 6.3. The impact is the differences in terms of normalized superficial solids velocity. Which is more dominant for GB-500 and much higher than irregular shape LR-500 particles. For any normalized superficial liquid velocity conditions, LR-500 particles shows lower normalized superficial solids velocity than that of GB-500.
4.3. Flow regime In the visual observation during experimentation, at zero auxiliary liquid velocity, no particles in the riser are fluidized and bed is acted as packed bed. With the introducing of auxiliary liquid flow and increase of superficial liquid velocity above the minimum fluidized bed velocity, the bed start to expand and fluidization achieved. This velocity called minimum fluidization velocity. Above this velocity, smooth expansion of bed observed. With gradual increasing of superficial liquid velocities leads to stage of particle entrainment. It is indicating that the bed is operating in the conventional fluidized bed regime where no entrainment of particles achieved. At the onset velocity, the first particle entrained and start to circulate from riser to downer. When the liquid velocity is sufficiently high, a large quantity of solid particles are transport from the riser to the downer and create circulating fluidization regime. As superficial liquid velocity increases, solids circulation rate increases thus the superficial solids velocity.
Fig. 8. Flow regimes of GLSCFB for glass bead particles GB-500 under different normalized superficial solids velocity vs normalized superficial liquid velocities (a) effect of auxiliary liquid velocity (b) effect of superficial gas velocities.
Fig. 7 shows the flow regimes of GLSCFB for glass bead particles GB-500 under different auxiliary liquid velocities and superficial liquid velocities at superficial gas velocity Ug = 0 cm/s. It could be observed that there are two region exists, Region I: the initial zone in which superficial solids velocities increases rapidly with the increase of the superficial liquid velocities up to 32 cm/s. and Region II: the fully developed zone where the superficial solids velocities becomes typically constant with increase of superficial liquid velocity from 32 cm/s. It also can be noted that with the increases of auxiliary liquid velocities superficial velocities increases at any operating superficial liquid velocity. Dimensionless analysis for flow regimes of GLSCFB for glass bead particles GB-500 are carried out to see the effect of both auxiliary liquid velocity and superficial gas velocity. Fig. 8(a) shows the effect of auxiliary liquid velocity and Fig. 8(b) the effect of superficial gas velocities on normalized superficial solids velocity under different normalized superficial liquid velocities. For both conditions shows two flow regimes under wide ranges of operations. Two clear flow regimes also exist in both cases. Normalized superficial solids velocities increase with the increase of normalized liquid velocities from 0.4 to 4.3 in Region I and after then it is no longer changes in region II. It can be noticed that with the increases of auxiliary velocity have great impact on increases of normalized superficial solids velocity but the case of gas holdups not significantly increases. When introducing gas, slightly increase of
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
JID: IJMF
ARTICLE IN PRESS
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
9
cantly increase. It is confirmed that in the bed with increase of gas holdups, solids holdups not significantly changed and indicates the liquid holdups decreased in the bed. 5. Conclusions System stability and flow characterization of the three-phase GLSCFB under a wide range of operating conditions have been studied. Two regular spherical shape glass bead particles (500 and 1200 μm in size), two irregular shape lava rock particles (500 and 920 μm in size) were used as solid phase, air as gas phase and saline-water as liquid phase. For flow characterization, superficial solids velocity (Us ), normalized superficial solids velocity ( UUs ), l
solids holdup (ɛs ) and gas holdups (ɛg ) were considered measureable parameters. Superficial liquid velocity (Ul ), auxiliary liquid velocity (Ua ), superficial gas velocity (Ug ), normalized superficial liquid velocity (Ul /Ut ) and net superficial liquid velocity (Ul − Ut ) were used as main operating parameters. The followings are the conclusion of this study:
Fig. 9. Effect of superficial liquid velocity on cross-sectional average (a) solids holdups and (b) gas holdups under different superficial gas velocities at axial location H = 3 m in GLSCFB riser.
superficial solids velocity observed but farther increases of superficial gas velocities does not increase that much. It means that there is very little impact on the drag force from the gas phase. 4.4. Phase holdups
1. With the change of any operating parameters, system became initially unstable then gradually returned to stable. The desired level stead-state can be reached by adjusting overall pressure balance in the entire flow loop. 2. Under a constant auxiliary liquid velocity, it was found that the superficial solids velocity initially increased with increasing superficial liquid velocities followed by reached a constant values for all four particles. 3. Normalized superficial solids velocity for GB-500 particle was higher than that of GB-1200 particles. On the other hand, GB500 and LR-500 (same size but different in shape) showed a wide range of normalized superficial liquid velocities under same operating conditions. 4. An increase in the particle diameter, the normalized superficial solids holdups decreased under the same operating conditions. For any normalized superficial liquid velocity conditions, irregular shape LR-500 particles had lower normalized superficial solids velocities than GB-500. 5. Two flow regimes found in circulating fluidized bed conditions. Region I: the initial zone in which superficial solids velocities increased rapidly with the increase of the superficial liquid velocities and Region II: the fully developed zone where the superficial solids velocities became typically constant with increase of superficial liquid velocity. 6. In the case of phase holdups, at a given superficial liquid velocity, with the increase of superficial gas velocities solids holdups not significantly increased or changed whereas gas holdups significantly increased. Acknowledgments
Phase holdups is the important parameters that indicates the amount of solids, gas and liquid phases are present in per crosssectional area in the bed. The effect of superficial liquid velocities on cross-sectional average solids holdups and gas holdups under different superficial gas velocities at axial location H = 3 m in GLSCFB riser shows in Fig. 9. The dotted lines show the trends of the data points. Fig. 9(a) shows that cross-sectional average solids holdups decrease sharply from 5 to 12 cm/s when operated in different superficial gas velocities. After then the decreasing rate slower from 12 cm/s to 35 cm/s. It also noticed that at any superficial liquid velocity condition, solids holdups not change that much with the increasing of superficial gas velocities. Fig. 9(b) shows that cross-sectional average gas holdups decrease sharply from 5 to 23 cm/s when operated in different superficial gas velocities and after then decreasing rate get slower. But for a given superficial liquid velocity, with the increase of gas velocity, solids holdups not significantly increased or changed whereas gas holdups signifi-
The author would like to gratefully acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. NSTIP # 13-WAT96-04 as part of the National Science, Technology and Innovation Plan. The author would like to acknowledge to Powder Technology Research Center (PTRC) and Professor Jesse Zhu at the University of Western Ontario for experimental support. The authors would also like to thank M. M. Hossain for his assistance in the editing of this manuscript. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ijmultiphaseflow.2018. 10.016.
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016
JID: IJMF 10
ARTICLE IN PRESS
[m5G;October 26, 2018;5:58]
S.A. Razzak / International Journal of Multiphase Flow 000 (2018) 1–10
References Atta, A., Razzak, S.A., Nigam, K.D.P, Zhu, J-X., 2009. (Gas)-liquid-solid circulating fluidized bed reactors: characteristics and applications. Ind. Eng. Chem. Res. 48 (17), 7876–7892. Clift, R., Grace, J.R., Weber, M.E., 1978. Bubbles, drops, and Particles. Academic Press, New York. de Lasa, H.I., Lee, S.L.P., Bergougnou, M.A., 1984. Bubble measurement in three-phase fluidized beds using a U-shaped optical fiber. Can. J. Chem. Eng. 62 (2), 165–176. George, D.L., shollenberger, K.A., Torczynski, J.R., O’ Hern, T.J., Ceccio, S.L., 2001. Three-phase material distribution measurement in a vertical flow using gamma-densiometry tomography and electrical-impedance tomography. Int. J. Multiphase Flow 27, 1903–1930. Haider, A., Levenspiel, O., 1989. Drag coefficient and terminal velocity of spherical and nonspherical particles. Powder Technol. 58, 63–70. Lee, S.L.P., de Lasa, H.I., 1987. Phase holdups in three-phase fluidized beds. AIChE J. 8 (33), 1359–1370. Liang, W.-G., Wu, Q.W., Yu, Z.Q., Jin, Y., Bi, H.T., 1995. Flow regimes of the three-phase circulating fluidized bed. AIChE J. 41, 267–271. Razzak, S.A., 2012. Hydrodynamics modeling of an LSCFB riser using ANFIS methodology: Effects of particle shape and size. Chem. Eng. J. 195, 49–61. Razzak, S.A., Barghi, S., Zhu, J-X., 2007. Electrical resistance tomography for flow characterization of a gas-liquid-solid three-phase circulating fluidized bed. Chem. Eng. Sci. 62, 7253–7263.
Razzak, S.A., Zhu, J.-X., Barghi, S., 2009. Radial distributions of phase holdups and phase propagation velocities in a three-phase gas-liquid-solid fluidized bed (GLSCFB) riser. Ind. Eng. Chem. Res. 48 (1), 281–289. Uchida, S., Okamura, S., Katsumata, T., 1989. Measurement of longitudinal distribution of solids holdup in a three-phase fluidized bed but ultrasonic technique. Can. J. Chem. Eng. 67, 166–176. Vatanakul, M., Zheng, Y., Couturier, M., 2003. Flow Characterization of a three-phase circulating fluidized bed using ultrasonic technique, 81, 1121–1129 Wang, T., Wang, J., Weiguo, Y., Yong, J., 2001. Experimental study on gas-holdup and gas-liquid interfacial area in TPCFBs. Chem. Eng. Commun. 187, 251–263. Warsito, W., Fan, L.-S., 2003. ECT imaging of three-phase fluidized bed based on three-phase capacitance model. Chem. Eng. Sci. 58, 823–832. Wen, C.Y., Yu, Y.H, 1966. A generalized method for predicting the minimum fluidization velocity. AIChE J. 12 (3), 610–612. Zheng, Y., Zhu, J., Wen, J., Martin, S.A., Bassi, A., Margaritis, A., 1999. The axial hydrodynamic behavior in a liquid-solid circulating fluidized bed. Can. J. Chem. Eng. 77 (2), 284–290. Zheng, Y., Zhu, J-X., 20 0 0. Overall pressure balance and system stability in a liquid— solid circulating fluidized bed. Chem. Eng. J. 79, 145–153. Zhu, J., Zheng, Y., Karmanev, D., Bassi, A., 20 0 0. (Gas)–liquid–solid circulating fluidized beds and their potential applications to bioreactor engineering. Can. J. Chem. Eng. 78, 82–94.
Please cite this article as: S.A. Razzak, Flow characteristic studies on the gas-liquid-solid circulating fluidized bed based on system stability, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.10.016