Study of Axial Solid Concentration Distribution in Slurry Bubble Columns

Study of Axial Solid Concentration Distribution in Slurry Bubble Columns

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Energy Procedia 157 Energy Procedia 00(2019) (2017)1537–1545 000–000 www.elsevier.com/locate/procedia

Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES18, Technologies and Materials for Renewable Energy, and Sustainability, TMREES18, 19–21 September 2018,Environment Athens, Greece 19–21 September 2018, Athens, Greece

Study of Axial Solid Concentration Distribution in The 15th International Symposium on District Heating and Cooling Study of Axial Solid Concentration Slurry Bubble ColumnsDistribution in Slurry Bubble Columns Assessing the feasibility the heat demand-outdoor Hiba of M. using Abdullah* Universityof Technology, Department, M.Engineering Abdullah* temperature function for Hiba a Chemical long-term districtBaghdad, heatIraqdemand forecast Universityof Technology, Chemical Engineering Department, Baghdad, Iraq

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*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre Abstract I. Andrić Abstract a IN+ Center for Innovation, Technology and Policy - Instituto Superior Técnico, Av.columns. Rovisco Pais 1, 1049-001 Portugal The axial distribution of solid concentration was Research studied in semibatch slurry bubble Tapered slurryLisbon, bubble column b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France (TSBC) bubble columnwas (CSBC) been usedslurry in this study.columns. Air, sodium chloride The axialand distribution of slurry solid concentration studiedhave in semibatch bubble Tapered slurry(NaCl) bubble aqueous column ccylindrical Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France solution 0.15 M, and quartz were have represented gas, in liquid solid phases, respectively. The superficial (TSBC) of andconcentration cylindrical slurry bubble columnsand, (CSBC) been used this and study. Air, sodium chloride (NaCl) aqueous velocity was varied0.15 fromM, 2.5and to 10 cm/s.sand, Quartz sands with a mean of 120 were used in the concentrations of solution of gas concentration quartz were represented gas,diameter liquid and solidµm phases, respectively. The superficial 3,5,10 &of20gas vol.was %.varied Influences solidsands loading longitudinal profile of µm solidwere concentration investigated. velocity fromof 2.5gas to velocity 10 cm/s.and Quartz withona mean diameter of 120 used in thewere concentrations of It was observed the longitudinal of solid solid loading concentration tends to be moreof uniform as superficialwere velocity of gas or 3,5,10 & 20 vol.that %. Influences of gasdistribution velocity and on longitudinal profile solid concentration investigated. Abstract solid increased. Under the same operation conditions, experimental showed thatasTSBC offered more of uniform It wasloading observed that the longitudinal distribution of solid concentration tends toresults be more uniform superficial velocity gas or distribution of solids along the column. The observed axial concentration distribution of solid particles in tapered slurry bubble solid loading increased. Under the same operation conditions, experimental results showed that TSBC offered more uniform District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the column wasofanalyzed on the basis precipitation-dispersion model. Thedistribution model successfully predicts thetapered axialthrough concentration distribution solids along the column. The observed axial systems concentration of solidwhich particles slurry bubble greenhouse gas emissions from theofbuilding sector. These require high investments are in returned the heat distribution systems TSBC. Empirical correlation for solidpredicts dispersion coefficient for solid column wasfor analyzed on therange basis of ofsolids precipitation-dispersion model. The model successfully axial could concentration sales. Due to narrow-sized the changed climate conditions andinbuilding renovation policies, heatthedemand in thethefuture decrease, particles in tapered slurry bubble column wassystems proposed. distribution for range of solids in TSBC. Empirical correlation for the solid dispersion coefficient for solid prolonging thenarrow-sized investment return period. particles in tapered bubble The main scope ofslurry this paper is column to assesswas the proposed. feasibility of using the heat demand – outdoor temperature function for heat demand ©forecast. 2018 TheThe Authors. Published by Elsevier Ltd. district of Alvalade, located in (Portugal), was used as a case study. The district is consisted of 665 © 2018 2019 The Authors. Published by Elsevier Elsevier Ltd. Lisbonlicense This is an open access article the CC period BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) © The Authors. Published by Ltd. buildings that vary in both under construction and typology. Three weather scenarios (low, medium, high) and three district This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the scientific Technologies andobtained Materials for Renewable Energy, This is an and open access article under the CC BY-NC-ND license committee (https://creativecommons.org/licenses/by-nc-nd/4.0/) renovation scenarios were developed (shallow, intermediate, deep). To of estimate the error, values were Selection and peer-review under responsibility of the scientific committee of Technologies and Materialsheat for demand Renewable Energy, Environment and Sustainability, TMREES18. Selection and peer-review under responsibility of the scientific committee of Technologies and Materials for Renewable Energy, compared with from a dynamic heat demand model, previously developed and validated by the authors. Environment andresults Sustainability, TMREES18. Environment and Sustainability, TMREES18. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications Keywords:Tapered slurrydemand bubble column, Axialthan solid 20% profile, coefficientconsidered). However, after introducing renovation (the error in annual was lower forSolid all dispersion weather scenarios Keywords:Tapered slurry bubble column, Axial solid profile, Solid dispersion scenarios, the error value increased up to 59.5% (depending on thecoefficient weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the * Corresponding author: Hiba M. Abdullah decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and E-mail address:[email protected] * Corresponding author: Hiba M. Abdullah renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the E-mail address:[email protected] coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102© 2018 The Authors. Published by Elsevier Ltd. Keywords: Heat demand; Forecast; Climate change license (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is an open access under the CC by BY-NC-ND 1876-6102© 2018 Thearticle Authors. Published Elsevier Ltd. Selection and peer-review under responsibility of the scientific of Technologies and Materials for Renewable Energy, Environment This is an open access article under the CC BY-NC-ND licensecommittee (https://creativecommons.org/licenses/by-nc-nd/4.0/) and Sustainability, TMREES18. Selection and peer-review under responsibility of the scientific committee of Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES18. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES18. 10.1016/j.egypro.2018.11.319

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1. Introduction Slurry bubble columns (SBCs) are extensively utilized in the chemical, petrochemical, and biochemical industries for carrying out a variety of chemical reactions, e.g., hydrogenation, oxidation, chlorination, alkylation, bioremediation and polymerization. These three-phase reactors have many merits, like uniform temperature, large contacting area, minimal cost of operation, lower loss in pressure, good productivity, and ability of addition or withdrawal of catalyst without intermittent of process. In spite of their advantages, SBCs have some drawbacks such as considerable back-mixing, difficulty in solid separation from liquid phase, and uncertainty in scale-up procedure. Because their merits often overcome the drawbacks, SBCs are the desired type of reactors for numerous processes, particularly highly exothermic ones, such as Ficher-Tropscher synthesis and liquid-phase methanol synthesis [5, 18, 24]. Nomenclature Cs concentration of solids in withdrawn sample of slurry C�� average solid concentration in slurry Cv volumetric solid concentration, vol.% D diameter of column Db diameter of column at the bottom Di diameter of column at height z dp diameter of a solid particle Es dispersion coefficient of solid particles g gravitational acceleration Sb area ratio= Di2/D2b t time Ug superficial velocity of gas UP hindered settling velocity of solid particles USL superficial velocity of slurry z axial position in the column Greek letters ɛg gas holdup µL liquid viscosity ρL liquid density ρS solid density φL volume fraction of liquid in slurry

[_] [Pa.s] [kg.m-3] [kg.m-3] [_]

Abbreviation TSBC tapered slurry bubble column, CSBC cylindrical slurry bubble column Dimensionless numbers �� ��

��

���������� ������ �

������ ������ � ������ ������ �

��

,

���

U� z E�

�� � ������ �� ��� �� �

[kg.m-3] [kg.m-3] [_] [m] [m] [m] [m] [m2.s-1] [m.s-2] [_] [s] [m.s-1] [m.s-1] [m.s-1] [m]

,



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According to the reactor shape, it can be classified into cylindrical column and tapered column [28].Cylindrical column is the main kind industrial reactors. On the other hand, tapered columns are utilized extensively in numerous processes such as treatment of waste water, exothermic reactions, biofilm reactions [9], nuclear fuel particles coating, coal gasification, sulfide ores roasting[16] and food processing [4]. In tapered columns, a velocity gradient exists in the axial direction of the column leading to unique dynamic characteristic of the column. Due to this characteristic, heavy particles are suspended at the bottom of column where the superficial gas velocity is high while light particles entrainment is prevented at the top of column where the superficial gas velocity is low. Therefore, tapered columns are very useful for the operation with a wide particle size distribution [26] and also for extensive particle mixing [19].Webster and Perona, [25] observed a downward flow of particles at walls of the tapered bed while a chaotic motion of particles was observed at walls of the cylindrical bed. Huang et al., [6] reported that tapered reactors have unrivaled features over conventional ones especially in biochemical processes. For instance, the bioparticles in a tapered bioreactor do not entrained from the reactor due to lower superficial velocity of the liquid at the top section of column. This makes tapered bioreactors can be operated at a broader range of flow rate. Despite of the widespread application of tapered columns, the majority of researches concerned with tapered columns have been dedicated on the gas-solid and liquid-solid systems [1, 7, 12]. In the field of gas-liquid-solid system, most of studies have been done in the cylindrical columns [27, 28].In the three-phase bubble column reactor, it was observed that as the catalyst loading increased the surface area of reaction zone increased while the residence time of liquid decreased correspondingly. The mixing characteristics of catalyst particles have a pronounced influence on the selectivity and distribution of products [20, 23].The authors reported that the dispersion of solid particles have strong effect on the conversion of reactants. Nedeltchev and Schump, [14] reported that the key process parameters that effect the performance of a three-phase bubble column reactor are the superficial velocity of gas and liquid, catalyst loading, diameter of solid particles, and the aspect ratio of reactor. In order to increase the productivity of a three-phase bubble column reactor, Krishna et al., [11] suggested that the operation with a high catalyst loading. Several works have been published on the axial solid concentration profile in cylindrical slurry bubble columns. Knesebeckiand Guardani, [10] studied particle concentration profile of different sizes in a threephase column using low ranges of superficial velocities of gas and liquid phases. They found that particles of different sizes have considerably different longitudinal distribution throughout the column. The analysis of quantitative results for axial profile of solid concentration in cylindrical slurry bubble column is based on the precipitation-dispersion model [2, 3, 20, 22]. Two parameters (i.e., longitudinal dispersion coefficient of solid and particle falling velocity) are included in this model. Many empirical-based correlations have been suggested to estimate these parameters. However, little studies have been done on three-phase tapered columns [26]. In present study axial distribution of solid concentration was studied in tapered slurry bubble column (TSBC) and cylindrical slurry bubble column (CSBC). Precipitation–dispersion model was adopted to predict the axial profile of solid concentration in the tapered slurry bubble column. Furthermore, empirical correlation of solid dispersion coefficient for tapered slurry bubble column was presented. 2. Experimental section 2.1. Experimental apparatus and procedure Fig. 1 demonstrates a schematic diagram of the experimental apparatus. Experiments were conducted by using two types of slurry bubble columns (i.e., TSBC and CSBC) which were made of Perspex. Height of TSBC was 150 cm. It was composed of two sections. The main section of column was conical. Its height was 120 cm and the tapered angle was 3º. Size of the conical section was increased from 12.7 cm internal diameter at the bottom to 25.4 cm internal diameter at the top. The other section of TSBC was cylindrical of 30 cm height with 25.4 cm internal diameter was utilized to avoid overflowing of slurry. The cylindrical column was 12.7 cm in diameter and 150 cm in height. Four sampling taps, 0.5 cm in diameter, were connected vertically to each column with equally-spaced distance. A porous ceramic plate was used as the gas distributer. The gas, liquid, and solid phases were air, sodium chloride (NaCl) aqueous solution, and quartz sand, respectively. The physical properties of these materials used in the present work were presented in Table 1.The column was operated in a semibatch manner. All experimental runs

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were conducted at 1 atm and (25±2) °C. Air from a compressor was inserted via a rotameter into the bottom of each column and then through a porous distributer. Samples were withdrawn from the taps after the steady state operation was attained. The slurry samples were weighed then put into an oven. The dried samples then weighted to measure the local concentration of solid at each position of the column. Table 1 The physical properties of the materials utilized in the present work

Material Air Sodium Chloride solution (0.15 M) Quartz sand

Density (kg/m3) 1.2 1005 2630

Viscosity (Pa.s) 1.7*10-5 1*10-3 -

2.2. Experimental conditions The operating parameters of the present study are shown in Table 2. Table 2 operating parameters

Operating condition Superficial gas velocity (cm/s) Particle size (μm) Solid concentration % (v/v) Static slurry height (cm) Gas Distributer Pore size (μm)

Range 2.5-10 100-140 3, 5, 10, 20 85 240

1. Compressor; 2. Rotameter; 3.Valve; 4.Gas distributer; 5.TSBC; 6.CSBC; 7. Sampling tap

Fig. 1 Schematic of experimental setup

3. Analysis of solid particles behavior Axial solid distribution in the tapered slurry bubble column was predicted based on precipitation - dispersion model which has been suggested by Cova [3] and Suganuma and Yamanishi [22] for cylindrical slurry bubble columns. The flux of solid phase in TSBC can be described as follows:



Hiba M. Abdullah / Energy Procedia 157 (2019) 1537–1545 Hiba M. Abdullah/ EnergyProcedia 00 (2018) 000–000

��� ��

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(1)

� �� m���

Where m� is the mass flux which obtained from Fick's law in order to elucidate the influence of gravitational force on the solid particles [3]. As first step for solving the equation above, the solid concentration is assumed to vary only axially in the column. By using this assumption, the following equation can be obtained: �

�� �� �� �

𝐸𝐸� �

��� ��



���

����

���

� �� �=

��

(2)

The following step is to assume that the operation of slurry bubble column at steady state. Therefore, Eq. (2) is integrated to obtain: � 𝐸𝐸�

��� ��� ��

� 𝐶𝐶� �𝑧𝑧� �

���

����

� �� � � �

(3)

In a batch operation, the superficial velocity of slurry, USL, becomes zero. For model simplification, additional assumptions are considered as follows [20]: The slurry phase (i.e., solid and liquid) behaves as a pseudo-homogeneous phase. All particles have the same terminal falling velocity in the slurry. Gas hold-up, ɛg, fraction of liquid in slurry ψL, are independent of the axial position, z. By integration Eq. (3), the following formulation can be obtained: C� �𝑧𝑧� � 𝐶𝐶� � 𝐶𝐶� exp ��

�� � ��

� � 𝐶𝐶� � 𝐶𝐶� exp����� �

(4)

Where Pez is Peclet number for the solid particle and z is the longitudinal position in the column. The following boundary conditions are proposed to solve the constants C1 and C2 in equation (4):

CS= CS1 CS=0

at z=z1 at z= ∞

Utilizing the boundary conditions into Eq. (4) to obtain: 𝐶𝐶� �𝑧𝑧� � 𝐶𝐶��

�������� �

��������� �

(5)

The quantity UP/ES can be acquired by using regression analysis on the solid concentration profile data against longitudinal position. 4. Results and discussion 4.1. Influence of gas velocity

Figure 2 (a-d) shows the influence of gas velocity on the axial profile of solid concentration. As can be seen from this figure, as gas velocity increased the axial distribution of solid becomes more uniform for 10 vol. %(C�� �263 kg/m3) and 20 vol.% (C�� =526 kg/m3). This behaviour is attributed to the dependence of solid mixing on the turbulent dispersion induced by rising gas bubbles and entrainment due to bubble wake. The higher gas velocity supplies more kinetic energy to the system and makes the movement of solid particles easier in the column. The up flow of bubbles tends to apply more shearing force upon the swarm of particles and as a result, the non-uniform distribution is reduced. Furthermore, the higher gas velocity means more energetic bubbles transport particles in

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their wakes. This result is supported by previous researchers [10, 17]. For 3 vol. % (C�� =78.95 kg/m3) and 5 vol. % (C�� �131.5 kg/m3), the increasing in gas velocity has a negligible effect on the axial profile distribution. Such behaviour has been previously seen by Kato et al., [8]. (a)

(b)

120

200

110

Ug= 0.025 m/s Ug= 0.100 m/s

100

160

90

140

C s (k g/m 3 )

C s (k g/m 3 )

80 70 60 50 40

120 100 80

40

20

20

10 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.0

0.8

0.1

0.2

0.3

0.4

z (m) (c)

0.6

0.7

0.8

(d) 800

Ug= 0.025 m/s Ug= 0.1 m/s

550 500

Ug= Ug= Ug= Ug=

700

450

600

C s (k g/m 3 )

400

C s (k g/m 3 )

0.5

z (m)

600

350 300 250 200 150

0.025 m/s 0.050 m/s 0.083 m/s 0.100 m/s

500 400 300 200

100

100

50 0 0.0

0.025 m/s 0.050 m/s 0.083 m/s 0.100 m/s

60

30

0 0.0

Ug= Ug= Ug= Ug=

180

0.1

0.2

0.3

0.4

0.5

z(m)

0.6

0.7

0.8

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

z (m)

Fig.2 Influence of gas velocity on axial solid distribution for (a) 3 vol. % (b) 5% vol. % (c) 10 vol. % (d) 20 vol. %

4.2. Influence of slurry concentration Figure 3 (a-b) illustrates the influence of solid loading on axial profile of solid particles. As shown, the axial solid profile becomes more uniform with an increasing in solid loading. This positive dependence of axial solid distribution on solid loading is attributed to the viscosity of slurry which increases as the solid concentration increased. The increasing of viscosity means higher drag force exerted on the solid particles which in turn reduces the hindered settling velocity. In addition high particle concentrations reduce the flow area and increase the linear velocity of the fluid and as a result stronger degree of solid mixing is attained. Similar observations were reported by [15, 26].

7

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Hiba M. Abdullah / Energy Procedia 157 (2019) 1537–1545 (b)

(a) 500

80 75

Cv= 3% Ug= 0.025 m/s Cv= 5% Ug= 0.025 m/s

70

400

60

350

55 50

C s (k g/m 3 )

C s (k g/m 3 )

Cv= 10% Ug= 0.1 m/s Cv= 20% Ug= 0.1 m/s

450

65

45 40 35 30 25

300 250 200 150

20

100

15 10

50

5 0 0.0

1543

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.0

0.8

0.1

0.2

0.3

0.4

z (m)

0.5

0.6

0.7

0.8

z (m)

Fig.3 Influence of solid loading on axial solid distribution

4.3. Comparison of axial profile of solid phase in TSBC and CSBC Figure 4(a-b) shows the axial distributions of solid phase in TSBC and CSBC at identical operation conditions. As can be seen from this figure, tapered column offers more uniform axial profile of solids than cylindrical one. This is due to the increasing of mixing rate and the decreasing of hindered settling velocity in TSBC and as a result the ratio UP/Es is decreased. This ratio reflects the uniformity of longitudinal solid distribution in the column. A lower UP/Es means a more uniform solid distribution. This reduction in UP/Es is related to the increasing of surface area of the tapered column which increases the retarding resistance of the fluid in the column, thus decreasing the hindered settling velocity and consequently lowering the ratio UP/Es. Similar findings were reported by [26, 28]. (a)

(b)

240

500

TSBC C v = 3% U g = 0.025 m/s CSBC C v = 3% U g = 0.025 m/s TSBC C v = 5% U g = 0.083 m/s CSBC C v = 5% U g = 0.083 m/s

220 200 180

400

C s (k g/m 3 )

C s (k g/m 3 )

C v =10% U g = 0.05 m/s C v = 10% U g = 0.05 m/s C v =20% U g = 0.10 m/s C v = 20% U g = 0.10 m/s

350

160 140 120 100 80

300 250 200 150

60

100

40

50

20 0 0.0

TSBC CSBC TSBC CSBC

450

0.1

0.2

0.3

0.4

z (m)

0.5

0.6

0.7

0.8

0 0.0

0.1

0.2

0.3

0.4

z (m)

Fig. 4 Comparison of axial profile of solid in TSBC and CSBC

0.5

0.6

0.7

0.8

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4.4. Model confirmation The use of equation (5) from the model to predict axial concentration of solid particles in the TSBC is illustrated in Fig. 5 (a-b). This figure shows that the precipitation - dispersion model fits the experimental values very well. (a) C v= 3% C v= 5% C v= 10% C v= 20% ___ Predicted

600

Cv = 3%

450

Cv = 5% Cv = 10%

400

C v = 20% ___ P redicted

350

Cs (kg/m3 )

500

Cs (k g/m 3 )

(b)

500

700

400 300

300 250 200 150

200

100

100

50

0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

z (m)

0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

z (m)

Fig.5 Predicted and measured longitudinal solid concentration vs. axial position for (a)Ug= 0.025 m/s and (b) Ug= 0.1 m/s

4.5. Empirical correlation for solid dispersion coefficient Axial solid dispersion is a characteristic of the slurry bubble columns. Based on solid Peclet number, the dispersion coefficient of solid is described quantitatively [15, 20, 21]. The relevant parameters to quantify the dispersion coefficient of solid in TSBC are: -Properties of liquid, such as viscosity (µL) and density (ρL). -Properties of Particle such as density (ρS) and particle size (dP). -Superficial velocity of gas, (Ug). -The volume fraction of liquid in slurry (φL). -Taper angle which is function of (Di) and (Db) where Di is the diameter of column at each axial position and Db is the diameter of column at the bottom. A regression analysis of the solid dispersion coefficient in terms of solid Peclet number has been used to correlate the dependence of the solid dispersion coefficient on these parameters. This analysis gives expression for the solid Peclet number for TSBC as a function of following dimensionless numbers as follows: Pe= 0.0136 Fr-0.677Ar0.438φL3.621Sb2.014

(6)

Correlation coefficient (R2) = 0.93 Conclusions The present work was devoted to study the axial profile of solids in both tapered and cylindrical slurry bubble columns. Experimental results revealed that TSBC can offer more uniform axial profile of solid phase than the CSBC. In TSBC, results show that the more gas velocity and solid concentration the more uniformity in solid profile was obtained. This uniformity in concentration profile could be useful in many bioreaction and environmental applications. It was shown that the precipitation-dispersion model, utilized in present work, successfully accounts for the axial profile of solid phase in TSBC. An empirical correlation for solid dispersion coefficient in terms of Peclet number was obtained with correlation coefficient (R2) = 0.93.



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