3laa Essam Thesis

Republic of Iraq

Ministery of Higher Education

and Sientific Reasearch

University of Technology

Chemical EngineeringDepartment

Heat Transfer in Bubble Columns Using Two Different Column Diameters

A THESIS

SUBMITTED TO THE CHEMICAL ENGINEERING DEPARTMENT

OF THE UNIVERSITY OF TECHNOLOGY IN PARTIAL FULFILMENT OF THE REQUIREMENTS

FOR THE DEGREE OF Master of Science in CHEMICAL ENGINEERING

By

Ola Issam Naji

B.Sc.in Chemical Engineering

Supervised by

Ass. Prof. Dr. Balasim Ahmed Abid

October 2010

ACKNOWLEDGEMENTS

Above all, my thanks go to God for His mercy and blessing.

I would like to express my deep gratitude to my supervisor,

Asst.Prof Dr. Balasim Ahmed Abid for suggesting the problem and

for his valuable support. I would like to thank his for the patience

that he always had with me.

I would like also to express my grateful admiration to Prof. Dr.

Mumtaz A.Zablouk haed of chemical engineering department.

My respectful regards goes to Dr.Khalid A. Suker, Mr.Bahha Shames Al.Deen Abid Alah for all the help, and also for Asst.Prof Dr. Mohammed Fadel.

I would like to express and thanks Dr. Jamal M. Ali for providing

facilities during the research.

My deepest thanks are given to my family for their constant support and encouragement.

Abstract

II

Abstract

Bubble columns are preferred in many chemical and biochemical processes for

gas-liquid contacting due to their simplicity in design, operation and maintenance. Solids, in some operations, are introduced into the bubble column as a third phase to enhance heat and mass transfer inside the bubble column.

The main object of this study is to investigate the effect of column diameter on the heat transfer coefficient, superficial gas velocity and gas holdup and bubble dynamics (bubble diameter and rise velocity).The heat transfer coefficient was measured for the air-water system in bubble columns of two different diameters, 0.15 and 0.3m .The superficial gas velocity Ug

From experimental data it was found that the heat transfer coefficient and gas holdup, increased with increasing superficial gas velocity. Also the bubble diameter and bubble rise velocity increased too. The experimental results emphasis significant influence of the column diameter on the hydrodynamics.

,was varied in the range from (0.002-0.012) m/s, for the 0.15m while at 0.3m (0.0005-0.0025) m/s and the velocity common between two column diameters are(0.002-0.0035) m/s for the homogeneous flow regime. The cause of this difference in velocities is the difference in the gas distributor for each column. The liquid phase level was kept constant at 1m for the two columns and gas holdup was measured also.

The experimental results of this research were compared with the Kast’s (1963) correlation which applied for column diameter equal 0.29 m, for low superficial gas velocity (0.002-0.06)m/s and the commn superficial gas velocity for two columns used in the experimental work is (0.002-0.0035)m/s which located between the superficial gas velocity of Kasts correlation:

22.022

1.0

−

=

l

pll

c

g

g

gcl

gpll KC

gDUUD

UCh µ

µρ

ρ

The heat transfer coefficient showed the same tendency of increase but there is a difference between the theoretical and experimental values due to the different system used.

Heat transfer coefficient, bubble diameter and bubble rise velocity showed an increase with increasing column diameter, while gas holdup decreased with increasing column diameter.

II

CONTENTS Subjects Page

Abstract I

Contents II

Nomenclature V

Chapter One: Introduction 1

1.1General 1

1.2 The Reasons For The Present Study 4

1.2.1 The Aims of The Present Study 4

Chapter Two: Literature Survey 5

2.1 Scope

5

2.2 Hydrodynamics Behaviour in Bubble columns 6

2.2.1 Flow Regime in Bubble Columns 7

2.2.2 Liquid Circulation in Bubble Columns

8

2.2.3 Bubble Rise Velocity

10

2.2.4 Bubble Coalescence and Break-up in Bubble Columns

11

2.2.5 Gas holdup 12

2.2.6 Gas sparger

12

2.2.7. Design and scale-up 13

2.3 Heat transfer in Bubble Columns

14

2.3.1 Heat Transfer Coefficient in Bubble Columns

16

2.3.2Axial/radial location of the heat transfer probe

28

2.4 Effect of Column Diameter

29

III

Chapter Three: Experimental Work 34

3.1 General Description 34

3.1.1 The Columns 34

3.1.2 Gas Distributors 37

3.1.3 Gas Supply System 38

3.1.4 Electric Power Measurement 38

3.1.5 Temperature Measurement System 38

3.1.6 The heater system 39

3.1.7 Measurement of the Gas hold up and Bubble Rise Velocity 40

3.1.8 Measurement of Bubble Diameter 40

3.2 Measurement of the Heat Transfer Coefficient 40

3.3 Experimental Procedure 41

CHAPTER FOUR RESULTS AND DISCUSSION 42

4.1 Gas Holdup 42

4.1.1 Effect of Superficial Gas Velocity and Column Diameter 42

4.2. Experimental Heat Transfer Study 43

4.2.1.1Effect of Superficial Gas Velocity 43

4.2.1.2 Effect of Column Diameter on the Heat Transfer

coefficients 48

4.2.3 Bubble Rise Velocity: 50

IV

4.3 Comparison of Experimental Data with Literature 51

Chapter Five: Conclusion and Recommendation 54

5.1Conclusion 54

5.2Recommendations 55

References 56

Appendices

Symbol Definition

A Column cross-sectional area (m2)

C Specific heat(J/Kg.K) p

Dc Column diameter (m)

d Bubble diameter (m) b

d Particle diameter (mm) p

d Outer diameter of heat transfer tube/probe (m) tube

Fl The liquid mass flow rates (kg/m2 s)

Fg The gas mass flow rates (kg/m2s)

Fr Froude number c

gg gD

UFr

2

=

g Gravitational acceleration (m/s2)

H Height of column (m)

H Initial height of the liquid in the column(m) o

h Heat transfer coefficient (W /m2.K)

h Heat transfer coefficient given in equation (2.4) (W /mc 2.K)

h Heat transfer coefficient in bulk in equation (2.4) (W /mb 2.K)

k Thermal conductivity (W /m.K)

q Heat flux (W /m2). Pr Prandtl number

l

pll

KCµ

=Pr

Re Reynolds numberl

cglg

DUµ

ρ=Re

St Stokes numberglpl UC

hStρ

=

T Temperature (K)

Tb bulk temperature (K)

Ts Surface temperature(K)

U Gas superficial velocity (m/s) g

U Liquid superficial velocity (m/s) l

Lc Characteristic vertical dimension of heater (m)

We Weber numberl

gclg

UDWe

σρ 2

=

Greek Letters Definition

ε Gas holdup ρg ,ρ Densities of gas and liquid, (kg/ml

3)

μg, μ Viscosities of gas and liquid, (Pa.s) l

Chapter One Introduction

1

CHAPTER ONE

INTRODUCTION 1.1 General

One of the major and modern equipment that play a vital role in chemical

industries for production is bubble column reactor. Bubble column are mostly used

in practice of gas-liquid contactor and liquid aeration. Vertical sparged reactors are

frequently used in various chemical processes including coal-liquefaction, Fischer

Tropsch synthesis and production of liquid fuels from biological materials.

The choice of bubble column has been motivated to decrease the sticky by

products that accumulate on the side walls, in the bubble column, gas bubbles flow

upward through a slower moving or stagnant liquid(Laari,2003).The heat transfer

rate in gas-liquid flow of bubble columns is generally 100 times larger than in

single phase flow (Deckwer 1980).

They are simple in construction and particularly suited for carrying out

relatively slow chemical reactions requiring large liquid hold-up in the

reactor(krishna, and Vanbaten.2002). The gas-liquid hydrodynamics in bubble

columns, however quite complex and influenced by several factors, such as:

1. Physical properties of gas and liquid phases

2. Operating pressure

3. Column diameter

4. Dispersion height… (Krishna.2000)

The industrial importance of bubble column remains undisputed mainly due to the

advantages (Degaleesan. 2001):

1. They need little maintenance due to simple construction and operation

and no problems with sealing due to the absence of moving parts.

2. High effective interfacial area.

3. Excellent temperature control.

4. High heat and mass transfer rates caused by strong gas-liquid interaction.


2

Bubble column belong to the general class of multiphase reactors which

consist of three main categories namely, the trickle bed reactor (fixed or packed

bed), fluidized bed reactor, and the bubble column reactor. A bubble column

reactor is basically a cylindrical vessel with a gas distributor at the bottom. The gas

is sparged in the form of bubbles into either a liquid phase or a liquid–solid

suspension. These reactors are generally referred to as slurry bubble column

reactors when a solid phase exists. Bubble columns are intensively utilized as

multiphase contactors and reactors in chemical, petrochemical, biochemical and

metallurgical industries.

Bubble columns frequently focuses on the following topics: gas holdup

studies bubble characteristics ,flow regime investigations and computational fluid

dynamics studies , local and average heat transfer measurements , and mass

transfer studies(Mohammed, 1997). Bubble column contactors with and without

suspended solids, are often used for chemical processes (Stegeman et al., 1996).

Slurry bubble column reactors provide benefits which have made them attractive

for a number of industrial processes in the areas of syngas conversion to fuel and

chemicals, heavy oil upgrading, environmental pollution control, and

biotechnology (Lie and Prakash, 1997). The advantages of these contactors include

the simplicity in their design, operation and maintenance, high heat and mass

transfer rates, isothermal conditions, plug-free operation, and on-line catalyst

addition and withdrawal.

High heat transfer rate is one of the most important characteristics in the

operation of bubble columns. This rate is influenced by a number of physical

parameters and operating conditions. Gas-holdup, superficial gas velocity,

circulation velocity and physical properties of liquid, all these factors are highly

interactive and control the bubble column performance.

Kollbel et al (1958) were the first who supposed that the enhancing effect

produced by the gas bubble on the heat transfer rate in bubble columns is related to

the removal of stagnant liquid portions from the transfer surface (that is, the

boundary layer).


3

Kast (1963) noted that the usual concept of the heat transfer through the

boundary layer does not apply to bubble column systems. It was pointed out that

the radial component of the liquid velocity; which is induced by the rising bubble,

is responsible for the high heat transfer coefficients in bubble columns.

Liquid circulation is one of the most important characteristics of bubble

columns, which represent the liquid flow induced by the rising bubbles and

governs the rate of heat and mass transfer.

This phenomenon is caused by the difference in buoyancy forces due to the

existence of bubble rich phase near the column axis (plume) and a relatively lean

phase in bubble near the wall. In the central plume the average axial velocity is

positive and in the annulus, it is negative (Joshi and Sharma 1979, and Yuanxin

and Al-Dahhan 2001). Figure (1-1) shows the velocity distribution in bubble

column due to the liquid circulation phenomena.

Figure (1-1) Velocity distribution in bubble column (Joshi and Shah 1981).


4

1.2 The Reasons for the Present Study:

The heat transfer coefficient was reported to be independent of the column

diameter in the majority of studies that investigated more than one column

diameter: e.g. Deckwer et al. (1980), Korte (1987), Fair et al. (1962). The value of

Dc at which this occurs may vary from 0.05 m to 0.19 m depending on the system

under study (Kim and Laurent, 1991; Saxena and Chen, 1994).The effect of

column diameter on heat transfer was investigated in detail by ( Saxena et al,1992).

The authors reported that heat transfer coefficients measured in a larger

diameter slurry bubble column (30.5 cm) was greater than in a smaller diameter

column (10.8 cm). They attributed this result to a higher mixing rate attained in the

larger diameter column. Wu et al. (2007) reported that the column diameter should

be greater than 0.15 m in order to avoid wall effects. The future experimental work

is recommended to be conducted in bubble columns at large diameter in order to

better understand the effect of column diameter on liquid circulation patterns. A

large diameter column would also permit experimentation using multiple internal

heat transfer tube configurations.

Although some authors reported that the heat transfer coefficient became

independent of the column diameter this is most likely attributable to the relatively

small column diameters studied. However, (Forret et al. 2006) reported that the

liquid axial velocity is a significant function of column diameter but the impact on

the heat transfer remains unknown. Therefore, there is a need for better

understanding of the role of the several column diameters on the hydrodynamic

and operating parameters and of the mechanisms of heat transfer in two-phase

(gas-liquid) bubble columns.

1.2.1The Aims of the Present Study Are:

To study experimentally the effect of gas superficial velocity, gas holdup and column

diameter on the heat transfer coefficient.

Chapter Two Literature Survey

5

CHAPTER TWO

LITERATURE SURVEY

2.1 Scope

Bubble column are used where higher interfacial areas between phases are

desirable. Bubble column reactors are used for reactions where the rate-limiting

step is the liquid phase or for slow reactions where contacting is not critical. These

seem to be exclusive choices wherever precise temperature control is required .

Bubble column reactors find applications in ethylene dimerisation and other

polymer reactions (Degaleesan 2001; Hikita 1981 ;Joshi and Sharma,1979). Works

are being carried out in hydrodynamic heat and mass transfer in bubble columns

for the past two decades, fractional gas hold up is a key parameter in bubble

column reactors. Superficial gas velocity, height to diameter ratio (H/D) and power

also play an indispensable role in determining dispersed phase hold up. (Kirk and

Othmer, 2004).

The bubble columns offer numerous advantages: good heat and mass transfer

characteristics, no moving parts, reduced wear and tear, higher catalyst durability,

and low operating maintenance costs. One of the main disadvantages of bubble

column reactors is significant bak-mixing, which can affect product conversion.

Excessive bak-mixing can be overcome by modifying the design of bubble

column reactors. Such modifications include the addition of internals, baffles or

sieve plates. Despite the wide use of bubble columns as gas-liquid contactors in

many applications (e.g. bio-reactors, blood oxygenators, and absorbers), their

design and scale up is still a difficult task, due to the generally complex structure

of the multiphase flow Encountered in this type of equipment.


6

Bubble size is an important design parameter, since it dictates the available

interfacial area for gas-liquid mass transfer. For example, in blood oxygenators

large bubbles favor CO2 removal, whereas small bubbles favor O2

2.2 Hydrodynamic Behaviour of Bubble Columns

transfer, but it

is more difficult to eliminate them in the debubbling section of the oxygenator.

Bubble size distribution depends extensively on column geometry, operating

conditions, physical properties of the two phases and type of gas sparger

(Mohammed, 1997).

The behavior of bubble columns is determined by their hydrodynamic

properties. The complex flow and mixing found in these units are often described

by means of the global parameters and phenomena including, gas holdup, liquid

circulation, flow regimes and bubble rise velocity and hence bubble size

distribution (Bennett et al. 1999, Kemoun et al., 2001, Yuanxin and Al-

Dahhan,2001).

Bubble columns are two –phase gas-liquid systems in which a gas is

dispersed through a sparger and bubbles through a liquid in vertical cylindrical

columns (Fig.(2.1)), with or without internals such as heat exchangers.

Figure (2.1) Schematic diagram of bubble column

(Dudukovic et al. 2002)


7

2.2.1 Flow Regime in Bubble Columns

Three different flow regimes have been identified to occur in bubble

columns, which are mainly determined by the gas superficial velocity. Forret et al.

(2006) described these three regimes as follows (for air-water system): -

1. Homogeneous regime or bubbly regime or dispersed regime, where the gas

holdup increases markedly with the superficial velocity. Bubble size is roughly

uniform (Krishna, 2000), and radial profile of gas holdup is nearly flat. In this

low gas velocity range, the distributor design affects gas holdup (Luo еt αl. ,

1999).

2. Transition regime where gas holdup may go through a maximum.

3. Heterogeneous regime or churn-turbulent regime or coalesced bubble regime,

where bubble coalescence and breakage is significant.

Roughly speaking, the breakage and coalescence mechanism is responsible

for two classes of bubbles:

1. Small bubble similar to those observed in the homogeneous regime, their

volume fraction is close to that observed at the beginning of the transition

regime.

2. Large bubbles that move quickly upwards as vapor bubbles in boiling liquid

The radial profile of gas holdup shows a maximum at the column centre-line,

and holdup is nearly zero at the wall (Krishna and Ellenberger, 1996; Krishna

еt αl.,1996).

For completeness, the slug regime can be found when the superficial gas

velocity is increased further. The slug regime is highly unstable. The gas passes

through the liquid in intermittent plugs while the liquid near the wall continuously


8

pulses up and down .This regime is generally limited to columns of small

diameter. A qualitative representation of the observed flow regime for the

experimental range of columns and superficial gas velocity is shown in (Figure

(2.2)).

Figure (2.2) Qualitative representation of the different flow regimes

(Urseanu,2000)

2.2.2 Liquid Circulation in Bubble Columns

In many multiphase (gas-liquid, gas-solid, liquid-liquid and gas-liquid-solid)

contactors, a lager degree of circulation of both discrete and continuous phases

occurs. This circulation causes a good degree of mixing and enhances heat and

mass transfer between fluid and walls (Joshi et al. 1980, Reilly et al. 1994, and

Gupta et al. 2001). The circulation of the liquid in the column is one of the major

observations, which should be taken into account when calculating mass or heat

transfer coefficients. This phenomenon is related to bubble size, bubble dynamic

and holdup. Therefore, these factors are very important in determining the

efficiency of contact in bubble columns (Whalley and Davidson 1974,

Viswanathan and Rao, 1983).


9

The main driving force, which induces the internal circulating flow of liquid,

is the difference in the apparent density of gas-liquid mixtures between the central

and peripheral regions of the column (annuals near the column wall). In the inner

region around the column axis, the so-called bubble street as given by Rietema and

Ottengraf (1970), liquid flows upwards with maximum velocity near the column

axis, whereas in the region near the reactor wall the liquid flows downwards

Between these two regions there is the shear zone, where the direction of flow

changes and velocity of the liquid becomes zero.

The radial position of this inversion of flow is dependent on the properties of

the gas/liquid system and on the operating conditions, and it can be used to

characterize liquid velocity profile (Walter and Blanch 1983). Joshi and Sharma

(1979) predicted the value of liquid circulation velocity and fractional gas holdup

for air-water system.

The authors analyzed the performance of bubble columns on the basis of

multiple circulation cells in the axial direction. The adjacent cells are normally

interacting and a considerable amount of inter-cell recirculation of the liquid

occurs. For the case of non-interacting liquid circulation cells, the authors have

shown, on the basis of minimum liquid vorticity, that the height of each circulation

cell equals the column diameter. Whalley and Davidson (1974) extended the work

given by Freedman and Davidson (1969) but, argued that an energy balance should

give better results than a pressure balance. They modified the liquid stream

function for the case of a three dimensional axi-symmetric cylindrical vortex for

low viscosity liquid. An energy balance was formed introducing energy dissipation

terms due to: -

1- Dissipation in the wake behind the bubbles.

2- Dissipation in the hydraulic jump, which arises at the surface of the liquid due

to the rising bubble street.


10

The sum of these dissipations was set equal to the energy input by the

introduction of bubbles at the base of the column. Many models have been

proposed to analyze and predict liquid circulation in bubble columns. Freedman

and Davidson (1969) developed a liquid circulation model exhibiting the so-call

“Gulf stream” effect. This consists of two vortex cells with upward flow in the

middle and downward flow near the wall .This model was analysed

mathematically by considering inviscid motion and sinusoidal distribution of

vorticity.

2.2.3 Bubble Rise Velocity

In all studies on bubble column technology there is a need to estimate the rise

velocity gas bubbles in liquids. The rise velocity of bubbles can be affected to a

significant extent by the dimensions of the bubble column (Krishna et al. 1999 b).

There is a large variety of experimental data on rise velocity available in the

literature for different bubble sizes and column diameters; it is difficult to compare

the results of one set of authors with those of others because of:

1- Difference in the physical properties of the liquids.

2- Presence of impurities in the liquid phase.

Each study is often restricted to a narrow bubble size range in a given

column diameter. It appears that it is necessary to know the bubble rise velocity if

the gas holdup needs to be calculated. The bubble rise velocity is mainly

dependent on bubble diameter (Wallis 1969, Heijnen and Riet 1984). Hills and

Darton (1976) studied experimentally the rise of single large air bubbles in a

uniform swarm of smaller bubbles in water, in two-dimensional and three-

dimensional columns. The authors pointed out that, a cap bubble rise is faster than

it would do in isolation.


11

They explained that the enhancement of velocity, is probably caused by

small-scale eddies in the liquid, produced by bubble swarm, which distort the

upper surface of the cap thereby changing the flow pattern around the bubble,

upon which its rising velocity depends. Krishna et al. (1999) developed a

procedure for the estimation of the rise velocity of a swarm of large gas bubbles in

a bubble column operating in the churn-turbulent flow regime. With the aid of an

extensive data set on the large bubble swarm velocity in columns of 0.051, 0.1,

0.174, 0.19, 0.38 and 0.63 m in diameter a correlation is developed for the

acceleration factor that accounts for the increase in the rise velocity of a bubble

because of its interaction with the wake of a bubble preceding it.

The authors found that, the large bubble swarm velocity is to be three to six

times higher than that of a single isolated bubble.

2.2.4 Bubble Coalescence and Break-up in Bubble Columns

Bubble coalescence and breakup are important factors that must be taken into

account in the design of bubble column. The diameter of the gas bubble of the

gas-distributor is not necessarily the same as the bubble diameter in the bulk of

column. The bubbles from the gas-distributor can undergo coalescence and/or a

(re) dispersion process. The coalescence rate is dependent on the liquid surface

properties, varying from coalescing (e.g. pure liquids) to non-coalescing (e.g.

water-salt systems). Therefore, the distinction between coalescing and non-

coalescing properties is very important in determining the performance of the

bubble column (Heijnen and Riet 1984).

Simon (2000) showed that, the coalescence of two bubbles is often assumed

to occur in three steps. First the bubbles collide trapping a small amount of liquid

between them. This liquid film then drains until the liquid film separating the

bubbles reaches a critical thickness. The film ruptures and the bubbles join

together.


12

2.2.5 Gas holdup:

Gas holdup is the amount of gas held by the liquid in reactor column, during

sparging of gas through liquid. Gas holdup is directly related to gas phase

residence time, and is also directly connected with the interfacial area. In order to

predict gas holdup values it is necessary to know the relationship between gas-

liquid slip velocity and gas holdup (Zhendong et al., 1986, Pino et al., 1990, and

Mohammed 1997). Daly et al., (1992) found that the holdup is independent of the

column height.

They obtained some differences in holdup with variation of the column

diameter. It was observed that the holdup in small column was slightly higher than

that in larger diameter columns. Gas holdup is one of the most important

parameters characterizing the hydrodynamics of bubble columns. It can be defined

as the percentage by volume of the gas in the two or three phase mixture in the

column, it has two fold applications. On one hand, gas holdup in two phase

systems gives the volume fraction of the phases present in the reactor and hence

their residence time.

On the other hand the gas holdup in conjunction with the knowledge of mean

bubble diameter allows the determination of interfacial area and thus leads to the

mass transfer rates between the gas and liquid phase. Gas holdup depends mainly

on the superficial gas velocity and often is very sensitive to the physical properties

of the liquid.

2.2.6. Gas sparger

Gas sparger type is an important parameter that can alter bubble characteristics

which in turn affects gas holdup values and thus many other parameters

characterizing bubble columns. The sparger used definitely determines the bubble

sizes observed in the column. Small orifice diameter plates enable the formation of

smaller sized bubbles. Some common gas sparger types that are used in literature


13

studies are perforated plate, porous plate, membrane, ring type distributors and arm

spargers.

Bouaifi et al. (2001) that, the smaller the bubbles, the greater the gas holdup

values. Thus, they concluded that with small orifice gas distributors their gas

holdup values were higher. In another study by Luo et al.(1999) , gas holdup was

found to be strongly affected by the type of gas distributor. The effect was more

pronounced especially for gas velocities below 6 cm/s. Schumpe and Grund

worked with perforated plate and ring type gas spargers. They concluded that with

ring type distributor, the total holdup was smaller. They also added that the small

bubble holdup showed a gradual increase with increasing superficial velocity with

ring type sparger.

Another conclusion about the type of spargers was that the contributions of

both small and large bubbles to gas velocity were lower with ring sparger as

compared to the perforated plate.

2.2.7 Design and scale-up

The design and scale-up of bubble columns have gained considerable

attention in recent years due to complex hydrodynamics and its influence on

transport characteristics. Although the construction of bubble columns is simple,

accurate and successful design and scale-up require an improved understanding

of multiphase fluid dynamics and its influences.

Industrial bubble columns usually operate with a length-to-diameter ratio, or

aspect ratio of at least 5 .In biochemical applications this value usually varies

between (2 and 5). The effects brought about by the selection of column

dimensions have found interest in bubble column reactor design. First, the use of

large diameter reactors is desired because large gas throughputs are involved.

Additionally large reactor heights are required to obtain large conversion

levels. However, there are also disadvantages brought about by the use of large


14

diameter and tall columns in terms of ease of operation. As a result it is necessary

to talk about an optimization process for best output.

2.3 Heat Transfer in Bubble Columns

The main heat transfer studies in bubble columns may be divided into two

groups concerning the wall –to-bed and inserted objects-to-bed. However most of

the previous studies on heat transfer rate in bubble columns are concerned with the

steady- state time-averaged heat transfer coefficient (AL-Dahan, 2001). Thermal

control in bubble columns is of importance since in many chemical and

biochemical processes, chemical reactions are usually accompanied by heat supply

(endothermic) or removal (exothermic) operation. Therefore, turbulent heat

transfer from the reactor wall and inserted coils to the liquid has been the topic of

many researches in the literature.

Bubble columns have been widely adopted in many industrial productions

and operations due to high heat transfer rates. The heat transfer rate in gas–liquid

bubble columns is reported to be generally 100 times greater than in single phase

flow .

Many hydrodynamic studies investigate the heat transfer between the heating

objectives and the system flow to understand the effects of hydrodynamic

structures on the heat transfer for improving the design and operation of bubble

column reactors Deckwer (1992). Chen et al., (2003) reported that the heat transfer

coefficient based on the measurements of energy input method using slow

assembly probe and the directed heat flux with aid of fast response advanced

probe.

Jamialahmadi et al. (2001) suggested a heat transfer mechanism in which the

heat transfer surface is divided into two zones. First the area, which is affected by

bubbles, Ab, in which heat is transferred into the fluid by transient heat conduction

from the heat transfer surface to the attached liquid.


15

The hot liquid film is transported into the liquid bulk and replaced by cooler

liquid. In the remaining heat transfer surface area, Ac

, heat is transferred to the

fluid by forced convection. Both mechanisms are assumed to occur in parallel in

separate zones of the heat transfer surface, as showing in Figure (2.3).

Figure (2.3) Area of the heat transfer surface affected by bubbles and by forced

convection (Jamialahmadi et al., 2001).

Flow patterns in two-phase gas-liquid bubble columns are investigated based

on local time-averaged heat transfer coefficients. The experiments are conducted

in a 0.28 m diameter Plexiglas column in air-water system over superficial gas

velocity range 0.05-0.3 m/s. The heat transfer measurements are made provided

local heat transfer coefficients. The measured heat transfer data are analyzed to

illustrate the effects of gas velocities on flow patterns in different regions of the

column.

The heat transfer measurements at different axial and radial positions

provided further insights into liquid circulation patterns in the column (Li and

Prakash 2002).Measurements of heat transfer coefficients in general require a heat

source and measurements of surface and bed temperatures. To estimate the local

instantaneous heat transfer coefficient h (w/m2.k) for heated object-to-bed system

for instance, the temperature difference between the wall surface and the bulk, and


16

the corresponding heat transfer flux, Q should be measured. The following relation

can be applied: 𝒉𝒉 = 𝑸𝑸∆𝑻𝑻

2.3.1 Heat Transfer Coefficient in Bubble Columns

Heat transfer coefficients have been reported by Fair (1962), Kast (1963),

Ruckenstein and Smigelschi (1965), Hart (1976), Mersimmn (1976), to be much

larger in bubble columns than for equivalent mass velocities in single-phase flow.

This phenomenon has been interpreted to be a result of increased liquid

circulation promoted by the gas phase. Fair et al. (1962) studied the gas holdup

and heat transfer coefficients in a commercial scale bubble column (18 inch and 42

inch diameter) by using air water system. The authors concluded that, the heat

transfer coefficients between the gas and the heating surface are high and vary

directly with superficial gas rate.

Also they found that the holdup and heat transfer data for bubble columns

are similar to those for vessels agitated by stirrers operating at moderate speeds.

On the other hand, they pointed that, neither vessel diameter nor type of heating

surface appears to influence heat transfer rates, and they submitted the following

correlation for heat transfer coefficient : -

U gh 22.08850= …………………………………….(2.1)

Where, h is the heat transfer coefficient

Ug

is the superficial gas velocity ( SI units ).

Hart (1976) studied heat transfer in bubble column (9.91 cm i.d. by 107 cm

high) using air and either water or ethylene glycol solutions. The column was

heated using an electric wall heater. Nine thermocouples measured the wall

temperature while 5 thermocouples – located within a copper tube submerged in

the column – measured the bulk temperature. Gas injection was via a single

nozzle (6.4 mm i.d.) at relatively low gas flow rates (0.0003 – 0.02 m/s).


17

The heat transfer coefficient was determined at the midpoint of the column.

It was determined that power dissipation per unit volume is a function of Ug

,

gravity, and ρl: similar dependencies were reported for h, with the addition of

liquid viscosity. The author proposed the following correlation for the wall heat

transfer coefficient:

25.036.0

125.0−

=

gU

KC

UCh

l

lg

l

pll

lgPl µρµ

ρ ……………… (2.2)

Nitrogen and water cooled by means of an external water jacket were used in

the wall heat transfer study by Holcombe et al. (1983) in a 7.8 cm i.d. and 1.8 m

high column. Six multiple thermocouple probes were evenly spaced every 0.305 m

along the column axis (see Figure 2.4).

The heat transfer coefficient was determined by measuring the bulk and

cooling water temperature profiles. The gas and liquid superficial velocities were

varied from 0 to 0.06 and 0.02 m/s, respectively, while three different pressures

(0.30, 0.51, and 0.71 MPa) were studied.

Constant radial temperature profiles indicated that there was good radial

mixing while the axial temperature profile was well described using the axial

thermal dispersion model as given by Equation (2.3): 46.03/426.1 gch UDD = …………………………..(2.3)

It was reported that pressure had no effect on hw for the system under study.

Based on their experimental finding, Holcombe et al. (1983) derived the following

correlation:

=

−

l

lc

l

lpl

b

g

g

gb

gpll

FDK

CgdUFd

UCh

µµ

µρ00024.0exp1.0

26.022

…… (2.4)

Where the units for μl and μg are kg/m.s and Fl and Fg are the liquid and gas mass

flow rates (kg/m2.s), respectively.


18

Figure (2.4) Experimental apparatus of Holcombe et al., 1983.

Heat transfer studies testing horizontal and vertical heat transfer tubes in a

vertical cylindrical column were completed by Kato et al. (1985; 1986). Two

different columns of 0.12 m i.d. (2.0 m high) and 0.19 m i.d. (2.5 m high) were

compared. Air served as the gas phase while water and carboxy-methyl cellulose

(CMC) served as the liquid phase.

The liquid viscosity was varied from 0.0011 to 0.017 Pa.s. The solid phase

consisted of glass beads (dp = 0.52 to 2.2 mm) or alumina (dp

Table (2.1) summarizes the probe dimensions. Several thermocouples

installed to measure heat transfer surface temperature. The temperature difference

was maintained at (2 – 3) K. Both studies reported that at low U

= 3.2 mm). The

column was equipped with a calming section beneath the perforated plate

distributor and was operated at ambient pressure and temperature. The heat

transfer surface consisted of a copper tube containing a sheathed heating wire (1.6

mm o.d.) surrounded with 5 to 30 μm copper particles and were installed 37 to 57

cm above the distributor.

l the heat transfer

coefficient increased with increasing Ul

; and that in the region of stable


19

fluidization h approached an asymptotic value. As well, h increased with Ug and

with decreasing μl and was found to be independent of Dc. The increase in h with

Ug was greater when dp

was relatively small.

Table (2.1) Probe dimensions used in the studies by Kato et al. (1985; 1986).

In the case of the horizontal tube, the following correlation was proposed

(Reported accurate to plus or minus 30 %) (Kato et al., 1986):

( ) ( )

33.05.0

212.065.05.0

3.11

02.012.0

1

+

−

+=

−

+

p

gFr

ll

pllpl

c

c

ll

lp

gdU

KdUC

LL

Khd h

ερ

εε

… (2.5)

Where in the case of the vertical tube, h appears to be independent of the heater

length once it exceeds 6 cm: the correlation for the vertical tube is given by

Equation (2.6) (Kato et al., 1985):

( ) ( )

5.075.0

022.03.2

1058.0

1

+

−

=−

tube

ll

pllpl

ll

lp dK

dUCK

hdε

ρεε

………… (2.6)

Although no direct comparison was made by the authors, the heat transfer

coefficient appears to be relatively larger for the horizontal tube than for the

vertical tube.

Verma (1989) studied the heat transfer in bubble column theoretically and

experimentally. And through his approach to heat transfer mechanism, which


20

depends on the conduction heat transfer, he showed that the heat transfer

coefficient over the entire surface is given by the following equation.

25.0

32/1 )()(

g ) 1( −−∝

µρεµ

ρgp

gp

Uk

CUC

h ………………………… (2.7)

This model is valid for air-water system. The proportionality constant from

the experimental data of the author was found to be 0.121. The values of the heat

transfer coefficient as estimated by this equation increase with superficial gas

velocity and become almost constant for Ug

Saxena (1989) investigated the hydrodynamic and heat transfer

characteristics of bubble columns in two-phase (air-water) systems operating in the

semi-batch and continuous modes. The average and local gas holdup and heat

transfer coefficient between an electrically heated cylindrical probe and air-water

dispersion were reported. Depending on his experimental results, the author

predicted the following correlation for the heat transfer coefficient, with good

agreement with the previous work in the literature.

> 0.1 m/s.

…… (2.8)

Saxena et al. (1989) compared the heat transfer and gas hold-up in the small

and large columns for both two- and three-phase systems (air-water and air-water-

glass beads, respectively). It was observed that εg was consistently higher when the

initial static bed height was set at 0.95 m compared to when it was set at 1.40 m.

The hydrodynamics of the two columns were reported as being similar except

when foaming was significant: with less foaming observed in the larger column.

Furthermore, the foaming increased with distance above the distributor plate

leading in turn to higher gas hold-up. Measurements also indicated that εg

increased monotonically with increasing Ug

308.0

3

4851.03/2 )()()(

271.0 σρ

µσµµ

ρgU

kC

UCh gp

gp

−

=

and decreased in the presence of

the solid particles – in agreement with studies on glass beads and red iron oxide


21

powders. The heat transfer coefficients for the three-phase system were reported

as being consistently higher than the two-phase system.

In all cases, the heat transfer coefficient was greater in the large column

(attributed to better mixing).The work by Westermeyer Benz (1992) may be seen

as an extension of the thesis by Korte (1987) but with the addition of a solid phase.

Different columns of 0.12 m i.d. (3.62 m high), 0.19 m i.d. (4.75 m high) 0.20 m

i.d (6.05 m high), 0.29 m i.d. (4.27 m high) and 0.45 m i.d (6.68 m high) were

compared.

Air served as the gas phase while pure water, saline water (10% NaCl),

ethylene-glycol or 1, 2- propylene-glycol served as the liquid phase. The liquid

viscosity was varied from 0.001 to 0.055 Pa.s. The solid phase consisted of glass

beads, plastic, or corundum powder; with Sauter mean diameter ranging from 60-

440 μm. The column internals consisted of 1 heat transfer tube (150 to 280 mm

long) and anywhere from 4 to 36 “dummy” tubes installed vertically above the

perforated plate distributor (see Figure 2.5).

The column was operated at ambient pressure while the temperature was

varied from 20 to 60 °C. Three different tube diameters (15, 25, 63 mm) and tube

pitch were studied (40 to 159 mm). A conductivity probe was installed midway up

the 0.19 m i.d. column to measure the radial solid phase hold-up. Westermeyer

(1992) concluded that the effect of the superficial gas velocity on the heat transfer

coefficient was pronounced when Ug < 20 cm/s: h increased with increasing Ug

Furthermore, h increased with decreasing μ

,

then in the region of stable fluidization h approached an asymptotic value.

l

( )[ ] [ ] 6/148/536/113/12 RePrRe115.0 −

∫

−= AWeFrFrSt ggggg

and was determined to be

independent of Dc. The experimental results were summarized by Equation (2.9).

…………. (2.9 )


22

Figure ( 2.5) Tube arrangements studied by (Westermeyer, 1992).

Where Af

l

gclg

l

pll

c

gg

l

cglg

glpl

UDWe

KC

gDU

FrDU

UChSt

σρµ

µρ

ρ

22

;Pr;;Re; =====

is the free cross-sectional area of the column (i.e. the area not occupied by

the tubes) and the dimensionless groups are defined below:

A wide range of gas velocity, column diameter, together with different gas-

liquid and gas-liquid-solid system has been studied in published literature. A

summary of these studies are given in table (2.2).A summary for the previous

experimental studies for columns with and without internals are given below in

(Appendix A).


23

Table 2.2 summary heat transfer coefficient correlations Reference Correlation Note (s) / Comment (s) (Konsetov,1966) 14.03/13/1

2

323/125.0

=

w

l

l

pll

l

clg

l

cw

KCDg

KDh

µµµ

µρε

(96) 1) Theoretical treatment only

14.03/13/13/1

2

323/118.0

=

w

l

l

pll

tube

c

l

tubelg

l

tube

KC

dDdg

Khd

µµµ

µρ

ε (97)

(Suh and Deckwer.1989)

( )( )[ ]

g

ssl

l

llggllssgl

lslplll

and

gUUU

whereCKh

εε

βββη

ερερερε

µηρ

ρν

ν

−=

−=

−+++=Ρ

Ρ=

1;

64/3915.2

1.0

2/12/1

(98)

1) For both wall and internal heat transfer 2) r2 = 0.98 and maximum deviation of ±15% 3) Theoretical treatment only

(Kawase and Moo- Young, 1987)

4/34/123/1

134.0

=

−

KUD

gDU

KC

KDh gcl

c

g

l

pll

l

c ρµ (99)

1) For Newtonian fluids 2) K = power-law consistency index (Pa.sn) 3) Theoretical treatment only

(Joshi et al., 1980)

( ) 14.033.066.033.033.033.1

48.0

−= ∞

w

l

l

lpl

l

bggcl

l

c

KCVUgD

KDh

µµµ

µερ (100)

3) Theoretical treatment only

(Kast, 1962; Kast,1963)

22.022

1.0

−

=

l

pll

c

g

g

gcl

gpll KC

gDUUD

UCh µ

µρ

ρ (101)

1) Dc = 0.29 m i.d.; Hc = 4 m 2) Air/water/isopropanol (45 wt. %) 3) Ug = 0.0025 to 0.06 m/s


24

Table 2.2cont. summary heat transfer correlations

Reference Correlation Note (s) / Comment (s)

(Burkel, 1972)

22.048.22

11.0

−

=

l

pll

c

g

l

cgl

gpll KC

gDUDU

UCh µ

µρ

ρ (102)

1) Immersed coil in air-water system 2) Dc = 0.19 m 3) Ug < 0.5 m/s – h constant for Ug > 0.1 m/s 4) Correlation cited from (Schluter et al,1995)

(Bieszk, 1986; Bieszk and Hammer, 1988) 69.025.02

15.0−−

=

l

pll

b

g

l

bll

gpll KC

gdUdU

UCh µ

µρ

ρ (103)

1) Dc = 15 cm i.d.; Hc = 1.85 m 2) Single heat probe 3) Ug = 2 to 10 cm/s 4) Solids: 0.46 mm glass beads (Θs = 5 wt. %) 5) Water and glycerine solutions: μl = 0.89 to 9.35 MPa.s; ρl = 998 to 1155

(Pauli, 1988) 78.03/122

097.0

−

=

l

pll

c

g

l

tubegl

pllg KC

gDUdU

CUh µ

µρ

ρ (104)

1. industrial chlorine liquefaction plant 2. gas and liquid phase: chlorine 3. tube bank: 100 mm o.d. 2 m high

(Mersmann, 1976; Mersmann, 1977)

226.0

8107.0

=

l

l

lll a

vavgKh

(105)

( ) 2/13/16/12

12.0 pllll

gl

l

CKvgh ρ

ρρρ

−

= (106)

1. Equation (105) for 0.03 < Pr < 100 2. Equation (106) for 1 < Pr < 100 3. Dc = 200 mm, H0 = 900 mm 4. Probe: 39.5 mm o.d.; 111.4 mm long 5. liquid-liquid column: toluene in water, or water in tetrachlorethylene 6. dispersed phase velocity = 0.002 to 0.15 m/s 7. 6.9 < db < 15.1

(Zaidi et al., 1987)

29.02249.0

076.0

−−

=

c

g

l

lcg

l

pll

gpll gDUDU

KC

UCh

µρµ

ρ (107)

1. Dc = 0.10 m and Hc = 1.6 m 2. Ug = 0.01 to 0.10 m/s 3. biomass reactor 4. air/Xanthan solutions of varying concentrations and flow indices between 0.18 and 0.70 5. inserted heating elements (64 cm2 total surface area) 6. cross-flow


25

Table 2.2cont. summary heat transfer correlations

Reference Correlation Note (s) / Comment (s)

(Napp and Hammer, 1983

( ) 78.026.0 PrRe25.0 −−= FrUC

h

gpllρ (108)

1. Dc = 15 cm; Hc = 125 cm 2. Ug = 0.005 to 0.11 m/s; Ul = 0 to 0.01 3. water/PE-glycol/KCl/benzoic acid 4. glass beads: 0.095, 0.145, 0.275. 0.460 mm 5. μl = 0.92 to 20 mPa.s 6. no general correlation for taking into account solid suspension effect

(Zehner, 1982; Zehner, 1986a; Zehner, 1986b)

( )

( )3

3

3

22

5.2

6

118.0

l

gcglf

gbb

lb

flplllg

UgDU

dl

where

lU

CKh

ρρρ

επ

µρ

ρε

−=

=

−=

(109)

1. Dc = 0.139 m and Hc = 0.298 m 2. lb is mean distance between bubbles and Uf is the eddy velocity 3. liquid circulation model based on transversely layered cylindrical eddies 4. compared to data obtained using sucrose, ethanol, spindle oil, water, glycerine, and glycol solutions 5. liquid velocity was measured using a fanwheel anemometer

(Fazeli et al., 2008)

gppgps

ppsgpgp

ppgspsps

gpps

URHUHRHURUH

RHURHURHh

2.240576.0

0287.05.2138.3

20.7140.0158.00027.0

91.226.5155.00276.003.2

+Θ

−Θ+−

+−Θ+Θ−Θ

++++Θ+=

1. See Table 11 for experimental details.

Lin and Fan (1999) studied the heat transfer and bubble characteristics in

high pressure bubble columns. The experiments are conducted at pressures up to

15.2 MPa and at a temperature of 27°C. In the bubbling regime, the heat transfer

coefficient has a significant increase with increasing pressure and nozzle gas

velocity. At high pressure, the authors noted that, the rate of heat transfer

coefficient and the nozzle gas velocity are largely increased.

In the jetting regime, however, the influence of pressure on the heat transfer

coefficient is small. Yang et al (2000) have studied the effect of pressure and


26

temperature on heat transfer characteristics in bubble columns. The effect of

pressure and temperature on heat transfer was included through their effects on gas

holdup .

Jamialahmadi et al. (2001) investigated the effects of heat addition on the

performance of bubble column reactors, using distilled water, isopropanol and

sodium sulfate solutions as liquid phase and air as gas phase. They studied the

effect of operating parameters on the heat transfer coefficients from a submerged

heat transfer surface to the fluid and of fluid temperature on the gas holdup.

Significant improvement in heat transfer coefficient was observed due to

presence of air bubbles in the column. In the bubbly flow regime, the heat transfer

coefficient increases with gas velocity and when the flow regime changed to

churn-turbulent, no further significant improvement in the heat transfer coefficient

was observed.

The authors proposed a model for prediction of heat transfer coefficients in

bubble column as follows:-

h = hc + 4 ε (1- ε) 2.39 (hb – hc

Where:-

) ………………………… (2.10)

hc

h

= the heat transfer coefficient produced by forced convection.

b

ε = gas hold up

= the heat transfer coefficient in the bulk.

The proposed model predicts all experimental data with good accuracy. A

comparison between the heat transfer coefficients from several correlations is

shown in Figure (2.6). All correlations shown in Figure (2.6) predict a considerable

increase in the heat transfer coefficient with increasing gas velocity.


27

Figure (2.6) Comparison of measured and predicted heat transfer coefficients

from various correlations (Jamialahmadi et al., 2001)

In the most recent study from Prakash and coworkers they studied the effect of high pressure on heat transfer coefficients in a gas-liquid bubble column using air and water (Wu et al., 2007). In this study a perforated plate distributor was used and the column was constructed of stainless steel. Before each experiment the static liquid height was varied in order to maintain a constant dynamic bed height.

It was reported that pressure has significant effect on column

hydrodynamics. As the pressure increases, ρg increases which caused the initial bubble size (db

From this it would appear that bubble size is a dominant factor in determining the heat transfer rate and that the positive effect due to increasing bubble number is not as strong as the negative effect of decreasing bubble diameter (Wu et al., 2007). The decrease in h due to the increase in pressure was more pronounced at low U

) to decrease and increases the rate of bubble break-up which decreases the overall bubble size and bubble size distribution. Increasing the pressure (0.1 – 1 MPa) resulted in a decrease in the measured heat transfer coefficient at both the wall and in the centre of the column.

g

Abdulmohisin R.S (2008) studied the heat transfer and bubble characteristics in a large –scale bubble column. The experiments are conducted at time-averaged local heat transfer coefficient profiles in a 0.45 m bubble column diameter using air-water system. The effects of the superficial gas velocity, axial and radial locations on the heat transfer coefficient were investigated in bubble column. By increasing the superficial gas velocity from 0.05-0.45 m/s the heat transfer coefficient increased and the values in the center of the column were 9-13٪ greater than those near the wall region. The properties of bulk flow region are large variation in radial direction and little in axial direction for the values of heat transfer coefficients.


28

. Finally, the radial profile of h was observed to flatten as P increased.

2.3.2Axial/radial location of the heat transfer probe

The position of the heat transfer probe in the column was also reported to

alter the values of the heat transfer coefficient. Thus, several studies were

performed by locating the heat transfer probe at various axial/radial locations in the

column and determining the corresponding values of the heat transfer coefficients

at those locations. In fact, the axial heat transfer measurement differences in the

column stem from measurement of the distance to the gas distributor and radial

differences from the bubble populations. Saxena et al (1990). Compared the heat

transfer coefficients at two different axial locations. The probes were at 2.9m and

0.52 m from the distributor. Their results indicated that the heat transfer

coefficients at 2.9 m were systematically higher than at the 0.52 m. This was

attributed to the influence of the distributor region.

The height of 0.52 m from bottom was less than two times the column

diameter (0.305 m) corresponding to the developing region for bubble growth and

liquid phase flow pattern. The influence of the distributor region is reported usually

to extend up to three or four times the column diameter. In the distributor region

the bubble sizes are definitely smaller than the ones in the bulk region. This is due

to the fact that the external pressure around the bubble decreases as the bubble

rises up in the column. Thus, large bubbles would be more dominant away from

the distributor. Since faster moving large bubbles would be more effective on heat

transfer as compared to small bubbles, higher heat transfer coefficient values could

be observed at the top sections of the column, i.e. away from the distributor as

compared to the distributor region.

Heat transfer measurements at different radial locations were carried out by

(Li and Prakash et al.2002) It was reported that the column centre heat transfer

coefficients were higher than the near wall heat transfer coefficients, due to the fact

that large bubbles collect more dominantly at the centre. In addition to that,

obviously there existed more turbulence in the centre as compared to near wall,

due to possible wall effects.


29

2.4 Effect of Column Diameter

The heat transfer coefficient was reported to be independent of the column

diameter in the majority of studies that investigated more than one column

diameter: e.g. Deckwer et al. (1980), Korte (1987), Fair et al. (1962). The value of

Dc at which this occurs may vary from 0.05 m to 0.19 m depending on the system

under study (Kim and Laurent, 1991; Saxena and Chen, 1994).

Saxena et al. (1989) compared the heat transfer and gas hold-up in the small

and large column for both two- and three-phase systems (air-water and air-water-

glass beads, respectively). It was observed that εg

was consistently higher when the

initial static bed height was set at 0.95 m compared to when it was set at 1.40 m.

The hydrodynamics of the two columns was reported as being similar except

when foaming was significant: with less foaming observed in the larger column.

Furthermore, the foaming increased with distance above the distributor plate

leading in turn to higher gas hold-up.

Measurements also indicated that εg increased monotonically with increasing

Ug

Saxena and Patel (1991) studied the gas holdup and heat transfer coefficients

in small column diameter (9.91cm) and in high (107cm) by using the three

different heat transfer probe diameters (19, 31.8, 50.8 mm). Three different sizes of

glass beads (50.119, 143 µm) were used at concentrations of 0 and 10 wt٪, while

the gas and liquid phases consisted of air and water. The gas holdup and heat

transfer coefficient were both reported to be independent of the particle and solid

concentration. The gas holdup was also reported to be independent of the tube

probe diameter, however, the heat transfer coefficient decreased with increasing


30

d

and decreased in the presence of the Solid particles – in agreement with studies

on glass beads and red iron oxide powders. The heat transfer coefficients for the

three-phase system were reported as being consistently higher than the two-phase

system. In all cases, the heat transfer coefficient was greater in the large column

(attributed to better mixing).

tube

(due to the poorer mixing resulting from the decrease in the spacing between

the tube and outer wall). The following correlation incorporates the hydraulic

diameter of the column;

−=

c

probecg D

DDUh 21.01483 ……………………………. (2.11)

Where :-

h = heat transfer coefficient, Dc = column diameter, Dprobe =

Saxena and Chen (1994) stressed the importance of the presence of a suitably

designed gas distributor in order to ensure an initially uniform distribution of

bubbles. The gas hold-up is not dependent on Dc (or column height) if the column

diameter is larger than 0.10 m (Kantarci et al., 2005; Saxena and Chen, 1994). It

has been observed that as the column diameter increases there is a significant

increase in the axial liquid circulation velocity while the radial liquid circulation

velocity decreases (Forret et al., 2006) and in one study Saxena et al (1990)

reported an increase in the heat transfer rate with increasing Dc (10.8 and 30.5 cm

i.d. columns).

heat transfer probe

diameter

Krishna and van Baten, 2001, studied experimentally the hydrodynamics of

bubble columns in 0.051 and 0.1 m diameter bubble columns with air-water system

and found that the bubble rises faster in the wider column. The reason for this is

the restraining effect of the walls .For operation with superficial gas velocity in the

range (0-0.04 m/s), (Krishna et al 2001), found that the total gas holdup decreases

with increasing column diameter. The reason for this scale dependency is because

the strength of the liquid circulations increasing with increasing scale. Such

circulations accelerate the bubble traveling upwards in the central core in bubble

column diameter (0.38m) as shown in figure (2.7).


31

Figure(2.7) Gas holdup as a function of superficial gas velocity Ug

Table (2.3) presents a summary of the general consensus as to the effect of

key process parameters on the heat transfer coefficient. Where the consensus is not

clear, the varying conclusions are presented along with a reference to the relevant

sources.

for column

diameter Dc =0.38m (Krishna et al 2001).


32

Chen et al.(2003) studied the instantaneous local heat transfer were measured

by using a hot-wire probe in three bubble columns of different diameters of(0.2,0.4

and 0.8 m i.d by 3m high). This study concentrated on the time-dependent local

hydrodynamics on the assumption that the use of average heat transfer caused the

loss of information regarding the effect of instantaneous bubble dynamics on heat

transfer.

It is for this reason, that the authors surmise that most correlations do not

remain valid over a wide range of gas flow rates. The local maximum

instantaneous h was associated with the passage of the bubble wake. The data

examined using a rescaled range analysis and chaos analysis (including evaluation

of the correlation dimension and Hurst exponent), which indicated the behavior in

the bubble column was highly nonlinear and differed with scale.

The authors claim that the artificial ANN showed good scale-up potential.

The artificial neural network (ANN) was applied to correlate instantaneous local

heat transfer with dynamic motions of bubble and liquid. The (ANN) was

optimized and trained by only using the experimental data measured at one

location of 20cm column. The trained (ANN) model shows good performance for

the generalized use to predict the dynamic heat transfer rate in three columns

(20,40,80cm) over whole experimental conditions studied ,indicating the ANN is

capable of capturing the universal relation between instantaneous heat transfer and

local bubble dynamics. After well trained, the prediction performance of the ANN

model was examined using the never-seen-before data of test data set.

A typical comparison of the predicted and measured heat transfer coefficients

is shown in figure (2.8). Note that the predicted values for liquid phase are outputs

of the ANN model while for gas phase the average value of measurement results is

simply plotted as the predicted value. From this figure, it can be seen that the

predicted heat transfer coefficients approach well to the measured once. Because

the heat transfer behavior is predominantly determined by the local bubble

dynamics, the instantaneous heat transfer rate measured reflects the nature of

nonlinear hydrodynamics in bubble columns.


33

Figure (2.8) The comparison of measured and predicted heat transfer

coefficients using the ANN model in 20cm column, Ug

It was demonstrated that the ANN-based model only trained with the

dynamic data obtained from one specified location in 20cm column is capable of

predicting the instantaneous local heat transfer rates of different locations in bubble

columns with diameter up to 80cm.

=45mm/s,axial

position=900mm, radial position, r/R =0 (Wei Chen et al. 2003).

Wu et al. (2007) reported that the column diameter should be greater than

0.15 m in order to avoid wall effects.

Chapter Three Experimental Work

34

CHAPTER THREE

EXPERIMENTAL WORK

3.1 General Description

Experiments were carried out in two bubble columns of 0.15 and 0.3 m in

diameter used for heat transfer study. Air was used as the gas phase. The liquid

phase used in the experiments was water. Each column had a gas distributor plate

with perforated holes of (1mm, 2mm) diameter respectively. The rate of air flow

into all columns was regulated by the use of rotameters.

3.1.1 The Columns

Two columns were made of Plexiglass, the inside diameter of these two is

15, 30 cm and the same height of 150 cm. The top of the columns is opened to the

atmosphere. The gas distributor plate, at the bottom of each column, a conical

shape reducer was installed for the purpose of minimizing the fluctuation of the gas

phase.

A view of the experimental apparatus is shown in Figure (3.1) and Figure

(3.2) shows a Schematic diagram of the experimental system.


35

Figure (3.1) A view of the experimental apparatus


36

Figure (3.2) Schematic diagram of the experimental apparatus.

1 Compressor 8 Gas distributor

2 Pressure gauge 9 Heater

3 Air rotameters. 10 Graduated ruler

4 Valve 11 Cylindrical paking

5 Needle valve 12 Variac

6 Non-retarn valve 13 Digital thermometer

7 Conical section 14 15cm diameter column

15 30cm diameter column


37

3.1.2 Gas Distributors

The distributor plate was designed directly depending on the procedure that

was suggested by Ruff and Pilhofer (1978). Appendix (B) shows the complete

procedure of the gas distributor design. For the 15 cm diameter column, air was

introduced into the system through perforated stainless-steel plate of 3mm

thickness .The plate contained 84 holes, with a diameter of 1 mm. For the 30 cm

diameter, air was introduced into the system through perforated plastic plate of 6

mm thickness .The plate contained 218 holes, with a diameter of 2 mm. The

distributor was placed firmly between the cylindrical section of the column and the

conical section using two flanges equipped with gaskets.

A) The gas distributor of 84 holes, with whole diameter of 1mm.

B) The gas distributor of 218 holes, with whole diameter of 2mm.

Figure (3.3) Dimensions of the gas distributor used in the two bubble columns


38

3.1.3 Gas Supply System

Air is used as the gas phase and it was supplied by means of an air-

compressor. The air metered by rotameter before being continuously introduced

into the system at ambient temperature of 30 C˚. Air flow rate was maintained at

the desired values with the aid of needle valves and rotameters.

3.1.4 Electric Power Measurement

The power consumed by the heater was controlled by means of variac

transformer to give 150,250,375 and 525W for different surface heater and bulk

temperatures. A clamp meter was used also to measure the power directly for more

accurate results. The electrical circuit is shown in Fig (3.4).

Figure (3.4) The Electrical Circuit of Heat Transfer Element

3.1.5 Temperature Measurement System

Infra Red was used to measure the surface heater and bulk temperatures by

using a digital instrument named (Lutron, High temp., IR) thermometer to have

more accurate results because of the fact that this instrument takes (0.95sec.) for

each measuring period and addition to that had been calibration the instrument and

add it to Appendix B. The temperature measuring system is shown in figure (3.5).


39

Figure (3.5) Temperature Measurement System

3.1.6 The Heater System

The heater assembly is shown in Figure (3.6), which was installed vertically at 0.5 m

above the distributor as a heat source in the immersed heater-to-bed system with

0.012 m diameter by 0.5 m length, 1000W/220V electric heating U shaped elements.

Figure (3.6) Schematic diagram

of The Heater


40

3.1.7 Measurement of the Gas hold up and Bubble Rise Velocity

In determining the gas holdup, the gas flow rate was adjusted using one

rotameter at a time. Sufficient time was given for steady state to be reached in the

column after which the increase in dispersion height was recorded. The total gas

holdup εg

is defined as:

ΗΗ−Η

=ο

ε g ……………………………………………………….. (3.1)

Where H◦ ungassed column height and H is the dispersion height due to the

pressure of gas bubble.

The bubble rise velocity ub was related to superficial gas velocity Ug and gas holdup εg

g

gb

Uu

ε=

by Taitel et al. (1980) equations:

……………………………………….(3.2)

3.1.8 Measurement of Bubble Diameter

The following correlation were recommended (Heijnen and van’t Riet, 1984)

for perforated plate to determine bubble diameter db

3/1])(

[7.1g

dd

gl

lb ρρ

σ−

= °

is …………………………… (3.3)

Where ρg ,ρl (1.2,1000) densities of gas and liquid, (kg/m3lσ), =0.072 (pa.m)

surface tension of water, g gravitational acceleration (m/s2°d), diameter of hole.

3.2 Measurement of the Heat Transfer Coefficient

The heater was located at 0.5m above the air distributor. The liquid level was

kept constant for all experiments at 100 cm for the two bubble columns that were

used in the present experimental work. The heat flux given by the electrical heating

element was calculated using the voltage across the heater and the current passed

through it. The columns were allowed to run at a given set of conditions at


41

approximately 15 minutes. This time was greater than that required to reach steady

state. During this period the three surface heater and bulk temperatures were

continuously recorded.

3.3 Experimental Procedure

The liquid level was kept constant for all experiments at 100 cm for the two bubble columns that were used in the experimental work and the gas phase was gradually allowed to flow via the gas distributor through the column.

The experimental steps for each one of the two columns were as follows:

1. The airflow was turned on, and the globe valves were adjusted to provide

the desired superficial gas velocity. The range of the superficial gas velocity

for:-

a- 15 cm [(2-12)*10-3

b- 30 cm [(5-25)*10

m/s]

-3

c- The velocity common between two diameters [(2-35)*10

m/s]

-3

2. When the system reached a stable gas velocity, the amount of heat was

obtained by the electrical circuit switched on. The quantity of heat was

controlled by variac transformer; this amount of heat was checked by the

use of clamp meter connected directly to the wear that sources the heater

with electricity.

m/s]

3. When the surface temperature of the heater became constant, a digital

thermometer was used for recording bulk temperature with a time interval

of (15 min) between each reading.

4. The total heat transfer coefficient was calculated from the following

equation, ( )TbTsAqh−

= ………………….(3.4), Where, q=VI, Ts; surface

temperature, Tb; the bulk temperature

5. The above procedure was repeated two times for each reading

Chapter Four Results and Discussion

42

CHAPTER FOUR

RESULTS AND DISCUSSION

This work has been developed to study the effect of column

diameter and superficial gas velocity on the gas holdup and the heat

transfer coefficient.Tables in appendix (C) show all the experimental

results.

4.1 Gas Holdup

The gas velocity-holdup relationship is the most important

design parameter in gas-liquid bubble column reactors. Providing the

basis for the prediction of heat transfer coefficients and information

on the hydrodynamic conditions.

4.1.1 Effect of Superficial Gas Velocity and Column Diameter

The gas holdup is found to decrease slightly with increasing

column diameter see Figure (4.1). This decrease in gas holdup is

evident in the homogeneous flow regimes and it is due to increased

liquid recirculation with increasing column diameter, due to this

strong circulation, the bubbles will be accelerated. This acceleration

effect causes a significant reduction in gas holdup with increasing

diameter. Also it has been found that with increasing superficial gas

velocity the gas holdup increases. This behavior is due to the increase

in the accumulation of gas through the liquid phase.

This result is in agreement with the observation of many

investigators [Al-Banna, 2005.Hana 2000, Krishna et al 1999, and

Krishna and Van baten 2002]


43

Figure (4.1) Effect of superficial gas velocity on

gas holdup at two different bubble column

diameters, D=15,30cm.

4.2. Experimental Heat Transfer Study:

4.2.1Effect of Superficial Gas Velocity:

The effect of superficial gas velocity were investigated for two

different column diameters at (15, 30) cm respectively shown in

Figures (4.2) to (4.9) for different heat fluxes. This show that

increasing in the superficial gas velocity causes increase in the heat

transfer coefficients. This must be related to the fact that at low

superficial gas velocity the small bubble sizes are formed in bubbly

flow regime, in addition when the superficial gas velocity increased

continued the magnitude of the increase in the heat transfer

coefficients, since faster bubbles coalescence and breakup come to

balance at a certain velocity.


44

Figure (4.2) Effect of superficial gas velocity on heat

transfer coefficients at different heat flux, q=150w, for

bubble column diameter equal 15cm.

Figure (4.3) Effect of superficial gas velocity on heat transfer coefficients at different heat flux, q=250W, for bubble column

diameter equal 15cm.


45


transfer coefficients at different heat flux, q=375W, for


Figure (4.5) Effect of superficial gas velocity on heat transfer coefficients at different heat flux, q=525W, for bubble column



46


transfer coefficients at different heat flux, q=150 W for


Figure (4.7) Effect of superficial gas velocity on heat transfer

coefficients at different heat flux, q=250 W for bubble column



47


coefficients at different heat flux, q=375 W for bubble

column diameter equal 30cm.


coefficients at different heat flux, q=525 W for bubble

column diameter equal 30cm.


48

4.2.2 Effect of Column Diameter on the Heat Transfer coefficients:

The effect of column diameter on the heat transfer coefficients

is shown in Figures (4.10) to (4.13). These figures show that

increasing the column diameter causes an increase in the heat transfer

coefficients at different values of heat flux.

It can also be seen that the heat transfer coefficients in the 15,

30 cm column diameter, increase with increasing gas velocity over the

range (0.002-0.0035) m/s, and heat flux (150-525) w.


coefficients at different column diameter D=30, 15 cm for

q =150W.


49


transfer coefficients at different column diameter D=30, 15

cm for q =250W.



q =375W.


50



q =525W.

4.2.3 Bubble Rise Velocity:

The bubble rise velocity bu was calculated using the correlations

of Taitel et al. (1980). As shown in equation (3.2) the bubble rise

velocity was related to superficial gas velocity. Figure (4.14) show the

effect of superficial gas velocity on bubble rise velocity. It can be seen

that the rise velocity of bubbles slightly increased with increasing gas

flow rate because of increased bubble diameter and bubble rises faster

in the wider column. The reason for this is the restraining effect of the

walls. This result is in agreement with the were recommended

(Heijnen and van’t Riet, 1984) for perforated plate to determine

bubble diameter.


51

Figure (4.14) Effect of superficial gas velocity on bubble rise velocity

at different column diameter D=30, 15 cm. (Taitel et al. 1980).

4.3 Comparison of Experimental Data with Literature

A number of studies have been proposed in the literature to

represent the heat transfer coefficient in bubble columns (Forret 2006,

Saxena and Chen, 1994, Kantarci 2005, Krishna and van Baten 2001,

Wu 2007,Co-Workers 1994,Verma 1989, Kast 1963). It was observed

in the present hydrodynamic study, that the effect of, superficial gas

velocity, axial and radial liquid circulation velocity, column diameter

and gas holdup on the resulting heat transfer coefficient.

The results of this present were compared with the Kast’s (1963) correlation which applied for column diameter equal 0.29 m , air-water- isopropanol (45 wt. %) system and with low superficial gas velocity (0.002-0.06)m/s and the commn superficial gas velocity for both columns used in the experimental work is (0.002-0.0035)m/s which located between the superficial gas velocity of Kasts correlation :


52

22.022

1.0

−

=

l

pll

c

g

g

gcl

gpll KC

gDUUD

UCh µ

µρ

ρ……………..(4.1)

The average wall temperature for the column which equal 0.3 m is used to calculate the dimensionless groups were applied in Kast’s (1963) correlation to have the theoretical heat transfer coefficient. The heat transfer coefficient shows the same tendency of increasing when both temperature and superficial gas velocity increased as that of the praetical heat transfer coefficient. All of these relation shown in figures (4.15) to (4.17).

Figure (4.15) Comparison of measured and predicted heat transfer coefficients from Kast (1963) correlations for D =30 cm at Tave

.=29C°


53


.=31C°


.=35C°

Chapter Five Conclusions and Recommenations

54

CHAPTER FIVE

CONCLUSIONS AND RECOMMENDATIONS

5.1 CONCLUSIONS

From the present study, the following conclusions are drawn:

1. Increasing column diameter was found to cause a decrease in the gas

holdup; this behavior is due to the increase in the accumulation of gas

through the liquid phase.

2. An increase in the superficial gas velocity in each of the two columns leads

to an increase in the bubble diameter.

3. The rise velocity of the bubbles increases slightly with increasing gas flow

rate .The bubble rises faster in the wider column, the reason for this is the

restraining effect of the walls.

4. Increasing column diameter from 15 to 30 cm caused an increase in the heat

transfer coefficient, by (16) %. This percentage was based on calculation of

heat transfer coefficient for all values taken at two points on two different

columns.

Chapter Five Conclusions and Recommenations

55

5.2 RECOMMENDATIONS

1. The effect of fluid properties on the heat transfer coefficient should be

investigated.

2.The heat transfer phenomena in bubble columns of commercial scale for

both low and high viscosity liquids (petroleum products) are needed (scale

up problem).

3. The effect of solid loading and different particle diameter on the heat

transfer in different bubble columns.

4. The effect of liquid surface tension on the heat transfer in bubble

columns is too importance.

5. Other types of gas distributor, like porous and spider gas distributors

should be used to investigate their effect on the bubble columns.

References

56

REFERENCES

Abdulmohsin, R. S “Heat transfer in bubble column operating in churn-

turbulent flow regime”, Ph.D. Thesis, University of Technology (2008).

Al – Banna B.F.M. “Hydrodynamics of bubble column operating with NaCl

solution”, M.Sc. Thesis, University of Technology (2005).

Bennett, M. A., Luke, S. P., Jia, X., West, R. M., and Williams, R. A.,

“Analysis and Flow Regime Identification of Bubble Column Dynamics”, 1st

Bouaifi M, Hebrard G, Bastoul D, Roustan M. “A comparative study of gas holdup, bubble size, interfacial area and mass transfer coefficients in stirred gas –liquid reactors and bubble columns”. (2001). (Cited in Review Bubble column reactors. Nigar Kantarcia, Fahir Borakb, Kutlu O. Ulgena) (2004)

Wold Congress on Industrial Process Tomography, Buxton, Great Manchester,

April, p-54 (1999).

Chen W, Hasegawa T, Tsutsumi A, Otawara K, Shigaki Y. “Generalized dynamic modeling of local heat transfer in bubble columns”. (2003). (Cited in Review Bubble column reactors Nigar Kantarcia, Fahir Borakb, Kutlu O. Ulgena) (2004).

Daly JG, Patel JG, Bukur DB. Measurement of gas holdups and sauter mean

bubble diameters in bubble column reactors by dynamic gas disengagement

method. Chem Eng Sci 1992;47:3647–54.

Deckwer, D. W., “On the Mechanism of Heat Transfer in Bubble Column

Reactors”, (1980).(Cited in International Journal of Chemical Reactor

Engineering Vol. 7 [2009], Review R1.

Deckwer, W.D. Bubble Column Reactors. New York: Wiley (1992).

Degaleesan S, Dudukovic M, Pan Y. Experimental study of gas induced liquid-flow structures in bubble columns”.(Cited in Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008, 113-118).

References

57

Dudukovic, M.P., Larachi, F., and Mills, P.L. "Multiphase catalytic reactors:

a perspective on current knowledge and future trends." (Cited in International

Journal of Chemical Reactor Engineering Vol. 7 [2009], Review R1.

Fair, J. R., Lambright, A. J., and Andersen, J. W., “Heat Transfer and Gas

Holdup in a Sparged Contactor”, (1962).p.50. (Cited in International Journal of

Chemical Reactor Engineering Vol. 7 [2009], Review R1.

Forret, A., Schweitzer, J.M., Gauthier, T., Krishna, R., and Schweich, D.

"Scale up of slurry bubble reactors." Oil & Gas Science and Technology 61

(2006): 443 - 458. 84

Forret, A., Schweitzer, J.M., Gauthier, T., Krishna, R., and Schweich, D.

“Influence of Scale on the Hydrodynamics of Bubble Column Reactors:An

Experimental study in Columns 0.1,0.4,1 m Diameters” ”, Chem. Eng.(2003)

Sci.,58 , 719-724.

Gupta, P., Ong, B., Al-Dahhan, M. H., Dudukovic, M. B., and Toseland, B.

A., “Hydrodynamics of Churn Turbulent Bubble Columns: Gas-Liquid

Recirculation and Mechanistic Modeling”, Catalysis Today, Vol. 64, p-253,

(2001).

Hart, W. F., “Heat Transfer in Bubble-Agitated Systems. A general correlation”,

(1976).p.12. (Cited in International Journal of Chemical Reactor Engineering

Vol. 7 [2009], Review R1.

Hanna F.Z. “An Experimental study of gas holdup and mass transfer

coefficient in two-phase bubble column with ethanol-water mixture” M.Sc.

Thesis, University of Technology (2000).

Holcombe, N. T., Smith, D. N., Knickle, H. N. and Willam, O., “Thermal

Dispersion and Heat Transfer in Non-isothermal Bubble Columns”,

References

58

(1983).(Cited in International Journal of Chemical Reactor Engineering Vol. 7

[2009], Review R1.

Heijnen, J. J., and Riet, K. V., “Mass Transfer and Heat Transfer Phenomena

in Low Viscosity Bubble Column Reactors”, Chem. Eng. J., Vol. 28, p. B21-

B42, (1984).

Hills, J. H., and Darton, R. C., “ The Rise Velocity of a Large Bubble in a

Bubble Column Swarm”, Trans. Inst. Chem. Eng., Vol. 54, p-258 (1976).

Hikita, H., Asai, S., Kikukawa, H.Zaike, T.and Ohue, M. �”Gas holdup in bubble columns”�. (1980). (Cited in Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008, 113-118). Jamialahmadi, M., Sarrafi, A., Muller-Steinhagen, H., and Smith, J., “Gas

Holdup and Heat Transfer in Bubble Column Reactors”, Int. J. Transport

Phenomena, Vol. 3, p.85 (2001).

Joshi, J. B., “A Axial Mixing in Multiphase Contactors”, Trans. Inst. Chem.

Eng., Vol. 58, p-155 (1980).

Joshi, J. B., and Sharma, M. M., “A Circulation Cell Model for Bubble

Columns”, Trans. Inst. Chem. Eng., Vol. 57, p-244 (1979).

Joshi,J.D,and Pandit,”Some Aspect in the Design of Bubble Column”,(1983).(Cited in Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008, 113-118).

Kast, W, Chem. Eng. Tech., Vol. 35, p-785 (1963).

Kantraci N.et al “Review:Bubble Column Reactors”Process bio chemistry

40,(2005) P(2263-2283).

References

59

Kemoum, A., Rados, N., Al-Dahhan, M. H., Dudukovic, M. P., Mills, P. L.,

Leib, T. M., and Lerou, J. J., “Gas Holdup in a Trayed Cold Flow Bubble

Column”, Chem. Eng. Sci, Vol. 56, p-1197 (2001).

Kato, Y., Uchida, K., Kago, T., and Morooka, S. "Liquid holdup and heat

transfer coefficient between bed and wall in liquid-solid ang gas-liquid-solid

fluidized beds." (1981.(Cited in International Journal of Chemical Reactor


Korte, H.J. "Wärmeübergang in blasensäulen mit und ohne einbauten."

(1987).p.51. (Cited in International Journal of Chemical Reactor Engineering


Kollbel, H., Siemes, W., Maas, R., and Müller, K. "Heat transfer in

bubblecolumns." Chemie Ingenieur Technik 20.6 (1958): 400.

Kim, S.D., and Laurent, A. "The state of knowledge on heat transfer in three

phase fluidized beds." (1991).p.42. (Cited in International Journal of Chemical

Reactor Engineering Vol. 7 [2009], Review R1.

Kirk and Othmer, Encyclopedia of Chemical Technology ,Wiley Inter science.(Cited in Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008, 113-118).

Krishna, R., Urseanu, M. I., Baten, J. M., and Ellenberger, J., “Rise Velocity of a

Swarm of Large Gas Bubbles in Liquids”, Chem. Eng. Sci., Vol. 54, p-171

(1999).

Krishna, R., Urseanu, M. I., Baten, J. M., and Ellenberger, J., “Wall Effect on the Rise

of Single Gas Bubbles in Liquids”, Int. Comum. Heat Mass Transfer, Vol. 26,

No. 6, p-781, (1999 ).

Krishna, R. and Ellenberger, J. (1996)” Gas Holdup in Bubble Column Reactors

Operating in the Churn-Turbulent Regime”. AIChE J., 42, 2627-2634.

References

60

Krishna R. and Sie. S.T. (2000) Selection,” Design and Scale-UpAspects of

Fischer-Tropsch Reactors”. Fuel Processing Technology, 64, 73-105.

Krishna R, and J.M.Vanbaten.”Scaling up Bubble Column Reactors with AID

of CFD). (2001).(Cited in International Journal of Chemical Reactor


Laari A., Ikka Turunen “Experimental determination of bubble Coalescence and Break –up rates in a bubble column”. (Cited in Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008, 113-118).

Li, H., and Prakash, A. "Analysis of flow patterns in bubble and slurry bubble

columns based on local heat transfer measurements." Chemical Engineering

Journal 86 (2002): 269 - 276.

Li H, Prakash A. Heat transfer and hydrodynamics in a three-phase slurry bubble column”. (1997). (Cited in Review Bubble column reactors. Nigar Kantarcia, Fahir Borakb, Kutlu O. Ulgena) (2004)

Lin, T. J., and Fan L. S., “Heat transfer and bubble characteristics from a nozzle in

high-pressure bubble columns”, Chem. Eng. Sci., Vol. 54, p-4853 (1999).

Luo, X., Lee, D.J., Lau, R., Yang, G.Q. and Fan, L.S. (1999) “Maximum Stable Bubble Size and Gas Holdup in High-pressure Slurry Bubble”. (Cited in Review Bubble column reactors. Nigar Kantarcia, Fahir Borakb, and Columns Kutlu O. Ulgena) (2004)

Lapin A, Paaschen T, Junghans K, Lu¨bbert A. Bubble column fluid dynamics,

flow structures in slender columns with large-diameter ring-spargers. Chem.

Eng Sci 2002;57:1419–24.

Mohammed, Th. J., “Hydrodynamic Interaction in Bubbly Two-Phase Flow”,

Ph.D Thesis, University of Technology, Baghdad (1997).

References

61

Pino, L. Z., Yepez, M. M., Saez, A. E., and Drago, G. D., “An Experimental

Study of Gas Holdup in Two-Phase Bubble Columns With Foaming Liquids”,

Chem. Eng. Commun., Vol. 89, p-155 (1990).

Perry, R. H., and Chilton, C. H., “Chemical Engineering Handbook”, McGraw-Hill, seventh Edition (1997), P(18-68). Ruff, K., Pilhofer, T., and Mersmann, A., “Ensuring Flow Through all the Openings

of Perforated Plates for Fluid Dispersion”, Int. Chem. Eng., Vol. 18, No.3, p-

395 (1978).

Reilly, L. G., Scott, D. S., De Bruijn, T. J., and MacIntyre, D., “The Role of

Gas Phase Momentum in Determining Gas Holdup and Hydrodynamic Flow

Regimes in Bubble Column Operations”, Can. J. Chem. Eng., Vol. 72,

February, p-3 (1994).

Rietema, K., and Ottengraf, S., Trans. Inst. Chem. Eng., Vol. 48, T-54

(1970), (Cited in Rietema, K., 1982).

Saxena, S., “Heat Transfer From a Cylindrical Probe Immersed in a Bubble

Column”, Chem. Eng. J., Vol. 41, p.25 (1989).

Saxena, S., Vadivel, R., and Saxena, A. C., “Gas Holdup and Heat Transfer From

Immersed Surfaces in Two and Three Phase in Bubble Columns”, Chem. Eng.

Commun., Vol. 85, p-63 (1989).

Saxena, S.C., and Chen, Z.D. "Hydrodynamics and heat transfer of baffled

and unbaffled slurry bubble columns." (1994).(Cited in International Journal of


Saxena, S.C., and Patel, B.B. "Heat transfer and hydrodynamic

investigations in a baffled bubble column: air-water-glass bead system."

(1990).p.58 (Cited in International Journal of Chemical Reactor Engineering


References

62

Saxena, S.C., and Patel, B.B. "Heat transfer investigations in a bubble column

with immersed probes of different diameters." (1991).p.58 (Cited in nternational

Journal of Chemical Reactor Engineering Vol. 7 [2009], Review R1.

Saxena SC, Rao NS, Saxena AC.” Heat-transfer and gas-holdup studies in a bubble column: air–water–glass bead system”. (1990).(Cited in Review Bubble column reactors Nigar Kantarcia, Fahir Borakb, Kutlu O. Ulgena) (2004)

Shaikh, A.,and Al-Dahhan, M. H.”A review on flow regime transition in

bubble columns”.Intern, Jour. Of Chem .Eng. Vol,5 ,RI (2007).

Simon, L., “Application of Population Balance to CFD Modeling of Gas

Liquid Reactors”, The conference of Trends in Numerical and Physical

Modelling for Industrial Multiphase Flows”, UK, September (2000)

Thorat .ND. Joshi JB” Hydrodynamic Characteristics of Bubble Column”,(1998), Cited in Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008, 113-118). Taitel, Y.; Bornea, Dvora; Dukler, A. E., (1980). ‘Modeling flow pattern

transitions for steady upward gas-liquid flow in vertical tubes”. (Cited in

International Journal of Chemical Reactor Engineering Vol. 5 [2007], Review

R1.

Verma, A.K. "Heat transfer mechanism in bubble columns."

(1989):p.24.(Cited in International Journal of Chemical Reactor Engineering


Viswanathan, K., and Rao, D., “Circulation in Bubble Columns”, Chem. Eng.

Sci. Vol. 38, p.474 (1983).

References

63

Walter, J. F., and Blanch, H. w., “Liquid Circulation Patterns and Their Effect

on Gas Holdup and Axial Mixing in Bubble Column”, Chem. Eng. Common.

Vol. 19, p-243 (1983).

Wallis, W. G., “One Dimensional Two Phase Flow”, McGraw-Hill, New York

(1969).

Westermeyer Benz, H. Wärmeübergang und Gasgehalt in zwei- und dreiphasig

betriebenen Blasensäulenreaktoren mit längseingebauten Rohren. Düsseldorf:

VDI-Verlag, (1992).p.65 (Cited in International Journal of Chemical Reactor


Whalley, P. B., and Davidson, J. F., “Liquid Circulation in Bubble Columns”,

Multi- Phase Flow Systems Symposium, Vol. 11, University of Strathelyde,

Glasgow, Scotland, (1974).

Wu, C., Al-Dahhan, M.H., and Prakash, A. "Heat transfer coefficients in a

highpressurebubble column." (2007). (Cited in International Journal of


Yang, G.Q., Luo, X., and Fan, L.S. "Heat-transfer characteristics in slurry

bubble columns at elevated pressures and temperatures." Industrial Engineering

Chemistry and Research 39 (2000): 2568 - 2577.

Yuanxin, W., and Al-Dahhan, M. H., “Prediction of Axial Liquid Velocity

Profile in Bubble Columns”, Chem. Eng. Sci., Vol. 56, p-1127 (2001).

Zhendong, Y., Ulrich, R., Rainer, B., and Ulfert, O., “Profile of Liquid Flow

Bubble Columns”, Chem. Eng. Commun., Vol. 49, p-51 (1986).

Appendices

b-1

APPENDIX (B)

GAS DISTRBUTOR DESIGN

The design of gas distributor in bubble column is one of the most important

steps in determining the hydrodynamic behaviour of such units. Many authors

pointed to the importance of the design of gas distributors and their direct effect on

the fluid dynamic and heat and mass transfer in bubble columns (Ruff et al., 1978,

Deckwer etal., 1982, and Chen etal., 1999). Therefore, the proper design of the gas

distributor is the key to successful operation of bubble column. This appendix

contains the design procedure of the perforated plate gas distributor that is used in the

two bubble columns of the present study. The design procedure was based on the

procedure that given by Ruff et al. (1978). The minimum gas velocity in the

perforations must be calculated in order to ensure that flow occurs through all the

perforations, or that weeping does not occur. Therefore, the following steps are to

find out the number of holes required to ensure flow through all the perforations and

to prevent weeping of the continuous phase.

Calculation of the diameter do

8/52/1

gg

g 32.2

−

= ρσ

ρρσ

liqod

g

liq

according to equation(b-1):

………………………………… (b-1)

For the column diameter (15 cm)

do=

For the column diameter (30cm)

1.12 mm ≈ 1mm

do= 2.73

mm

Appendices

b-2

Then, depending on the physical properties of the system, Ruff et al. (1978) pointed

to that, the Weber number must be of a constant value and equal to 2, then the

following relation was used,

2 2

==liq

gg UdWe

σρ …………………………………………………(b-2).

where, Ug


is the superficial gas velocity

Ug = 0.64 m/s, and by applying a safety factor of 40 % gives value of Ug

For the column diameter (30cm)

= 0.9 m/s

Ug = 7.7695 m/s, and by applying a safety factor of 40 % gives value of Ug

The number of holes in the gas distributor, then can be calculated depending on the

following formula:-

=

10.8773 m/s

π24

og dUQN = ……………………………………..(b-3)


N=84 hole


N=218 hole

Appendices

b-3

Free area = ……………(b-4)

1- For 15 Cm Free Area 2- For 30 Cm Free Area

All these values are between the range (0.1-5)%.

Area of holes

Area of distributor 2

2

4

4

c

o

D

Nd

π

π=

( )( )

%373333.0

%10015.0

84001.02

2

=

××

=

( )( )

%98888.0

%1003.0

218002.02

2

=

××

=

Appendices

b-4

Thermometer Calibration

Calibration of the thermometers prior to use by using the specifications of the manufacture of the equipment or the following procedures will be implemented. Calibration in ice water: 1. Add crushed ice and distilled water to a clean container to form a watery slush. 2. Place thermometer probe into slush for at least one minute, taking care to not let the probe contact the container. 3. The thermometer reading is recorded. This was (+.2) degrees C. Calibration in hot water: 1. Heat a clean container of water. After the water in the container has reached a complete boiling”. 2. Place the thermometer probe into the hot water, for at least one minute, taking care not to let the probe contact the container. 3. The thermometer reading is recorded. . This was adjust to 99.5 degrees C.

Infrared Thermometer Calibration

Thermometers in use will be checked against a certified thermometer during calibration, if available. Otherwise, all thermometers will be calibrated either against each other, or against a thermometer that is used only during calibration. The following procedure will describe how to check the accuracy of the infrared thermometer used in experimental work (TM-949) compared with already calibrated thermometer : 1. Tap water has been added to a clean container at room temperature. A calibrated thermometer immersed in the container immediately. 2. After one minute the temperature recorded from the immersed thermometer and the infrared thermometer used to record the temperature simultaneously.

Appendices

b-5

3. Then the water is heated during this, the temperature recorded every one minute by using both thermometers. Until it reaches the boiling point. 4. After results comparison, the accuracy between the tow thermometers was -/+ 3%.

عايرة جهاز معايرة جهاز قياس درجات الحرارة كل دقيقة م

قياس درجات الحرارة كل دقيقة(InfraRed

thermometer)C˚ Thermometer CP

˚

28 28

39 39

51 53

62 64

72 75

83 83

87 89

93 95

98 100

معايرة جهاز قياس درجات الحرارة كل دقيقة

0

20

40

60

80

100

120

28 39 51 62 72 83 87 93 98

(IR)

Ther

mo

C-1

EXPERIMENTAL RESULTS

Table (C-1) Average heat transfer coefficient for different bulk and surface temperatures as a function of Ug for ,[ Dc=15cm]

Run

U (m/sec)

g Average heat transfer coefficient, h (W/m2 K) Tb1

oC Tb2oC Tb3

oC TbavoC Tsav

oC h(w/m2.k) 1

0.00188 41 42 42 41.6 45 2495.6 0.00377 43 44 44 43.6 46 3535.5 0.00566 45 45 45 45 47 4242.5 0.00754 45 47 47 46.3 48 4991.2

0.010 47 48 48 47.6 49 6060.8

2

0.00188 49 50 50 49.6 52 5892.44 0.00377 52 52 53 52.3 54 8318.74 0.00566 55 56 56 55.6 57 10101.33

0.00754 57 57 57 57 58 14141.87

0.010 58 58 58 58 59 14141.87

3

0.00188 52 52 53 52.3 55 7856.59 0.00377 57 56 57 56.6 59 8838.66 0.00566 58 59 59 58.6 60 15152.00 0.00754 59 60 60 59.6 61 15152.00

0.010 61 62 63 62 63 21212.80

4

0.00188 56 56 56 56 58 14848.96 0.00377 58 59 59 58.6 60 21212.80 0.00566 60 61 61 60.6 62 21212.80 0.00754 63 63 63 63 64 296970.90

0.010 66 66 66 66 67 296970.90

C-2

Table (C-2) Average heat transfer coefficient for different bulk and surface temperatures as a function of Ug for ,[ Dc=30cm]

Run

U (m/sec)

g Average heat transfer coefficient, h (W/m2 K) Tb1

oC Tb2oC Tb3

oC TbavoC Tsav

oC h(w/m2.k) 1

0.00045 27 28 27 27.3 29 1250 0.00094 28 28 28 28 30 1063.8 0.00142 28 29 29 28.6 30 1666.7 0.00188 29 29 29 29 30 2122.2 0.00235 30 30 31 30.3 31 3061.2

2

0.00045 29 30 29 29.3 31 2083.3 0.00094 30 31 30 30.3 32 2210.7 0.00142 31 31 31 31 32 3537.1

0.00188 33 32 32 32.3 33 5052.9

0.00235 34 33 33 33.3 34 5052.9

3

0.00045 30 31 31 30.6 33 2210.8 0.00094 32 32 33 32.3 34 3106.8 0.00142 34 34 34 34 35 5305.6 0.00188 34 35 35 34.6 35 13264.00 0.00235 35 36 36 35.6 37 13264.00

4

0.00045 32 31 32 31.6 34 3094.90 0.00094 34 34 34 34 36 3713.90 0.00142 35 36 36 35.6 37 5713.70 0.00188 36 37 37 36.6 37 18569.60 0.00235 37 38 38 37.6 38 18569.60

C-3

Table (C-3) Effect of superficial gas velocity on heat transfer coefficients at different heat flux , a:q=150, b:q=250, c:375, d:q=525w for bubble

column equal.

A) 15cm in diameter

U(m/sec)

g h(w/m2.k) h(w/m2.k) h(w/m2.k) h(w/m2.k)

0.00188 2495.6

5892.44

7856.59

14848.96

0.00377 3535.5

8318.74

8838.66

21212.8

0.00566 4242.5

10101.33

15152

21212.8

0.00754 4990

14141.87

15152

29697.9

0.010 6061

14141.87

21212.8

29697.9

B) 30cm in diameter

U(m/sec)

g h(w/m2.k) h(w/m2.k) h(w/m2.k) h(w/m2.k)

0.00045 1250

2083.3

2210.7

3094.9

0.00094 1063.8

2200.6

3106.8

3713.9

0.00142 1666.7

3500

5305.6

5713.7

0.00188 2122

5000

13264

18569.6

0.00235 3061 5052.9 13264 18569.6

C-4

Table (C-4) Effect of heat flux on heat transfer coefficients at different bubble column diameters for Dc=15,30cm, at constant range of

superficial gas velocity

A) 15 cm

q(watt) h(w/m2

.k) h(w/m2

.k) h(w/m2

.k) h(w/m2

.k)

150 2000

2200 2900

3000

250 3700

5000 5200

5500

375 6600

7900 8000

8100

525 7500

15000 16300

21000

B) 30 cm

q(watt) h(w/m2

.k) h(w/m2

.k) h(w/m2

.k) h(w/m2

.k)

150 2500

3100

3500

4000

250 5100

6000 6300

7100

375 7300

13100

13200

13200

525 14000 18500 19000

21000

C-5

Table (C-5) Effect of superficial gas velocity on heat transfer coefficients at different bubble column diameters for Dc=15,30cm , at constant range

of superficial gas velocity

Run Ug (m/s)*10^-3

h(w/m2

Dc=30cm .k) h(w/m2

Dc=15cm .k)

1 2 2500

2000

2.5 3100

2200

3 3500

2900

3.5 4000

3000

2 2 5100

3700

2.5 6000

5000

3 6300

5200

3.5 7100

5500

3

2 7300

6600

C-6

3 2.5 13100

7900

3 13200

8000

3.5 13200

8100

4 2 14000

7500

2.5 18500

15000

3 19000

16300

3.5 21200

21000

Table (C-6) to (C-7) Effect of superficial gas velocity on gas holdup and bubble rise velocity at different bubble column diameters for

Dc=15,30cm, at constant range of superficial gas velocity

Table (C-6) Effect of superficial gas velocity on gas holdup:

U(m/sec)

g εDc=15cm

g εDc=30cm

g

0.002 0.02 0.019 0.0025 0.022 0.02 0.003 0.025 0.23 0.0035 0.027 0.024

C-7

Table (C-7) Effect of superficial gas velocity on bubble rise velocity

U(m/sec)

g ubDc=15cm

(m/sec) ubDc=30cm

(m/sec)

0.002 0.1 0.105 0.0025 0.11 0.125 0.003 0.12 0.130 0.0035 0.13 0. 145

Table (C-8) ) Effect of bubble diameter at different bubble column diameters for Dc=15,30cm.

db (m) Dc (cm) 0.144 15 0.198 30

Table ( C-9) Comparison of measured and predicted heat transfer coefficients from Kast (1963) correlations for D =30 cm at Tave

.=29C°

Ug

(m/s) Kast (1963) h(W/m2

.K)

D =30 cm h(W/m2.K)

0.002 780 2500 0.0025 850 3100 0.003 900 3500 0.0035 950 4000

C-8


.=31C°

Ug

(m/s) Kast (1963) h(W/m2.K)

D =30 cm h(W/m2

.K)

0.002 810 5100 0.0025 950 6000 0.003 1080 6300 0.0035 1200 7100


.=35C°

Ug

(m/s) Kast (1963) h(W/m2.K)

D =30 cm h(W/m2.K)

0.002 840 14000 0.0025 900 18500 0.003 960 19000 0.0035 1230 21200

1

الخالصة

حظت االعمدة الفقاعية باهتمام كبير حيث أنها تستخدم كثيرا في الصناعات الكيمياوية و

وهي توفر عدة )صلب سائل – –غاز(الكيمياوية الحيوية وذلك لضمان التالمس بين االطوار

. فوائد من حيث التصميم والتشغيل و الصيانة

أن الهدف االساسي من هذا البحث هو دراسه تأثير قطر العمود على قيمة معامل انتقال

. )قطر الفقاعة وسرعتها(وعلى ديناميكيه الفقاعة الحرارة وايضا على قيمة الغاز المحتجز

و 0.15ماء في اعمدة فقاعية من قطرين مختلفين تم قياس معامل انتقال الحرارة لنظام هواء –

مختلفه حيث كانت العمل ضمن مدى سرع غازذلك بوغطت الدراسة نظام التدفق الفقاعي .م 0.3

تراوح م ت 0.3بينما لعمود الفقاعة ذات ,ثا| م) 0.012-0.002(م تتراوح من 0.15لعمود الفقاعة ذات

ثا ومدى السرعة المشتركة بين العمودين الفقاعيين كانت تتراوح من |م) 0.0025 -0.0005(من

. ثا|م) 0.002-0.0035(

من البيانات التجريبية وجد ان معامل انتقال الحرارة و الغاز المحتجز يزدادان بزيادة سرعة

حيث اظهر معامل .الغاز السطحية وتؤكد النتائج التأثير الهام لقطر العمود على الهيدروديناميكيا

قلت بزيادة ز زيادة بزيادة قطر العمود بينما قيمة الغاز المحتجوقطرالفقاعة وسرعة انتقال الحرارة

والتي تسخدم لعمود فقاعة ) Kast 1963(قارنة النتائج بأستخدام عالقة تم م .قطر العمود

ثا |م) 0.06-0.002(م ولمدى سرع غاز قليلة 0.29بقطر

22.022

1.0

−

=

l

pll

c

g

g

gcl

gpll KC

gDUUD

UCh µ

µρ

ρ

رة بزيادة سرعة الغاز السطحية في اظهرت النتائج النظرية زيادة في معامل انتقال الحرا .مع اختالف القيم وذلك الختالف الظروف والنظام المستخدمين م 0.3عمود الفقاعة

وزارة التعليم العالي والبحث العلمي

الجـامعة الـتكنولوجـية

قســم الهنـدسة الكيمياويـة

انتقال الحرارة في االعمدة الفقاعية بأستخدام عمودين فقاعيين مختلفين في االقطار

الجامعة التكنولوجية كجزء من رسالة مقدمة الى قسم الهندسة الكيمياوية –متطلبات الدراسة لنيل درجة ماجستير علوم في الهندسة الكيمياوية

اعــــــداد

عال عصام ناجي . م

باشراف االستاذ الدكتور

بالسم احمد عبد

2010

Documents

3laa Essam Thesis