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Transcript of 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.
Chapter One Introduction
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).
Chapter One Introduction
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).
Chapter One Introduction
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.
Chapter Two Literature Survey
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)
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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).
Chapter Two Literature Survey
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.
Chapter Two Literature Survey
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.
Chapter Two Literature Survey
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.
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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.
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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).
Chapter Two Literature Survey
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.
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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 )
Chapter Two Literature Survey
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).
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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.
Chapter Two Literature Survey
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.
Chapter Two Literature Survey
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.
Chapter Two Literature Survey
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
Chapter Two Literature Survey
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).
Chapter Two Literature Survey
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).
Chapter Two Literature Survey
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.
Chapter Two Literature Survey
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.
Chapter Three Experimental Work
35
Figure (3.1) A view of the experimental apparatus
Chapter Three Experimental Work
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
Chapter Three Experimental Work
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
Chapter Three Experimental Work
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).
Chapter Three Experimental Work
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
Chapter Three Experimental Work
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
Chapter Three Experimental Work
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]
Chapter Four Results and Discussion
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.
Chapter Four Results and Discussion
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.
Chapter Four Results and Discussion
45
Figure (4.4) Effect of superficial gas velocity on heat
transfer coefficients at different heat flux, q=375W, for
bubble column diameter equal 15cm.
Figure (4.5) Effect of superficial gas velocity on heat transfer coefficients at different heat flux, q=525W, for bubble column
diameter equal 15cm.
Chapter Four Results and Discussion
46
Figure (4.6) Effect of superficial gas velocity on heat
transfer coefficients at different heat flux, q=150 W for
bubble column diameter equal 30cm.
Figure (4.7) Effect of superficial gas velocity on heat transfer
coefficients at different heat flux, q=250 W for bubble column
diameter equal 30cm.
Chapter Four Results and Discussion
47
Figure (4.8) Effect of superficial gas velocity on heat transfer
coefficients at different heat flux, q=375 W for bubble
column diameter equal 30cm.
Figure (4.9) Effect of superficial gas velocity on heat transfer
coefficients at different heat flux, q=525 W for bubble
column diameter equal 30cm.
Chapter Four Results and Discussion
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.
Figure (4.10) Effect of superficial gas velocity on heat transfer
coefficients at different column diameter D=30, 15 cm for
q =150W.
Chapter Four Results and Discussion
49
Figure (4.11) Effect of superficial gas velocity on heat
transfer coefficients at different column diameter D=30, 15
cm for q =250W.
Figure (4.12) Effect of superficial gas velocity on heat transfer
coefficients at different column diameter D=30, 15 cm for
q =375W.
Chapter Four Results and Discussion
50
Figure (4.13) Effect of superficial gas velocity on heat transfer
coefficients at different column diameter D=30, 15 cm for
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.
Chapter Four Results and Discussion
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 :
Chapter Four Results and Discussion
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°
Chapter Four Results and Discussion
53
Figure (4.16) Comparison of measured and predicted heat transfer coefficients from Kast (1963) correlations for D =30 cm at Tave
.=31C°
Figure (4.17) Comparison of measured and predicted heat transfer coefficients from Kast (1963) correlations for D =30 cm at Tave
.=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
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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
For the column diameter (15 cm)
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)
For the column diameter (15 cm)
N=84 hole
For the column diameter (30 cm)
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
Table ( C-10) Comparison of measured and predicted heat transfer coefficients from Kast (1963) correlations for D =30 cm at Tave
.=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
Table ( C-11) Comparison of measured and predicted heat transfer coefficients from Kast (1963) correlations for D =30 cm at Tave
.=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