Iraqi Journal of Chemical and Petroleum Engineering 

 Vol.18 No.4 (December 2017) 35 - 45 

ISSN: 1997-4884 

 

 

 

 

 

Heat Transfer and Hydrodynamic in Internal Jacket Airlift 

Bioreactor with Microbubble Technology 
 

Mahmood K. H. Al-Mashhadani 

Department of Chemical Engineering, College of Engineering, University of Baghdad, 

Baghdad E-Mail: mkh_control@yahoo.com   

 

Abstract 

   Integration of laminar bubbling flow with heat transfer equations in a novel internal 

jacket airlift bioreactor using microbubbles technology was examined in the present 

study. The investigation was accomplished via Multiphysics modelling to calculate 

the gas holdup, velocity of liquid recirculation, mixing time and volume dead zone for 

hydrodynamic aspect. The temperature and internal energy were determined for heat 

transfer aspect.  

   The results showed that the concentration of microbubbles in the unsparged area is 

greater than the chance of large bubbles with no dead zones being observed in the 

proposed design.  In addition the pressure, due to the recirculation velocity of liquid 

around the draft tube, increased the retention time of microbubbles in the same area.    

   Thus it was expected that their effect on mass and heat transfer phenomena would 

be positive for biological applications. For example the gas fraction volume of 

microbubble in the downcomer region is 0.0063, while with fine bubble diameter of 1 

mm, this region was free from any bubble. Furthermore, the velocity of liquid in the 

center of ring diffuser would be 0.175 m/s, if the sparging system operated with 

bubble diameter of 100 micrometer, whilst would be 0.035 m/s with fine bubble 

diameter of 1 mm. 

   The study also proved the importance of bubble diameter on the heat transfer in gas 

and liquid phases in the proposed design. Microbubbles gave greater responsiveness 

to stability and homogeneity in all parts of the bioreactor. Finally, this study 

concluded the efficiency of the proposed design with the microbubbles technology 

thermally and hydrodynamically. 

 

Key words: Bioreactor, Microbubble, Airlift, Computational Model  

 

Introduction

   Climate change since the last century 

has become a matter of concern to the 

researchers [1]. Despite of confirmed 

evidences that the main reason for this 

change is the use of fossil fuels as a 

major energy source, still global 

demand for this type of sources is 

rising.  

   Carbon dioxide, as one of gases that 

produced from this source, is 

considered as a driving factor in that 

observed climatic change [2]. 

University of Baghdad 
College of Engineering 

Iraqi Journal of Chemical and 
Petroleum Engineering 

 

mailto:mkh_control@yahoo.com


Heat Transfer and Hydrodynamic in Internal Jacket Airlift Bioreactor with Microbubble Technology 

  

  

36                                  IJCPE Vol.18 No.4 (Dec. 2017)                -Available online at: www.iasj.net 
 

   There are many alternatives to 

address the problems of fossil fuels 

and environmental friendly as well 

[3,4]. However, and according to the 

available scientific possibilities, 

finding the main alternative to this 

source in the production of energy may 

be difficult and unreasonable [5]. In 

fact, the problem is not in the quality, 

but in their quantity and high cost of 

the production. 

   Fuel produced from the biomass via 

the microorganisms is a promising 

source to change this scenario[5], 

especially if it produced from food 

waste or inexpensive materials [6-8].   

   The operational conditions of these 

biological processes play an important 

role in improving the productivity such 

as the temperature       [9-12], since the 

metabolic activities of microorganisms 

increase with increasing the 

temperature [12,13]. Thus the 

temperature has an effective role in 

enhancing the consumption of organic 

matter and improving the production of 

biofuels. Production of clean energy 

that can be obtained from thermophilic 

processes is more than obtained from 

mesophilic process, such as anaerobic 

digestion process [14-16]. However, 

the energy required in such a process 

may be the main impediment to be at 

the forefront of the promising sources 

in the production of renewable energy.     

   In addition, the nature of the 

biological medium contributes in the 

complexity of variables during design 

of bioreactors, since these mediums 

usually contain solids, liquid and gas in 

the same time. Each of these phases 

requires a basic understanding of the 

mixing patterns to provide a suitable 

environment for living microorganisms 

as well as to get their food taking 

account the operational conditions 

such as temperature and prevent the 

problem of thermal stratification.  

 

   In summary, success of any 

biological process depends on the 

efficiency of the bioreactor in the 

hydrodynamic, mass transfer and heat 

transfer aspects. 

   As known, the airlift bioreactor has 

operational and economic advantages 

in biological applications over other 

conventional bioreactors, such as 

simple construction with low cost in 

manufacturing and maintenance, 

sufficient mixing and lower shear 

stress to microorganisms cells[17-19].    

   Although of many attempts in 

developing this type of bioreactors (as 

a design or enhancement of operational 

conditions) [20-22], there still remain a 

big challenge due to nature of 

biological mediums.  

   Thus, a deep knowledge of the fluid 

flow pattern and heat transfer inside 

the bioreactor is required for 

enhancement of metabolic activities of 

microorganisms or biomass 

productivity [23-26]. 

   This research is a serious attempt that 

aims to reduce the energy consumed in 

bioreactors through the integration of 

ring diffuser and internal jacket draft 

tube in the proposed design. This 

investigation is done by studying the 

momentum and heat transfer 

throughout the proposed bioreactor 

with microbubbles technology. 

 

Computational Model 

   The modelling of the process was 

carried out using COMSOL 

Multiphysics with two-dimension and 

axial symmetry. Fig.1 shows three-

dimension and two-dimension sketch 

of the suggested design of bioreactor 

used in the present study. 

   Laminar flow model was considered 

to simulate the airlift bioreactor using 

Navier-Stokes equation: 
 

      
                                                    … (1) 

 



Mahmood K. H. Al-Mashhadani  

 

-Available online at: www.iasj.net                      IJCPE Vol.18 No.4 (Dec. 2017)                        37 
 

   Where    ,    ,     ,  ,    ,    ,and    
are liquid volume fraction (m

3
/ m

3
), 

density of liquid,  velocity of liquid 

phase (m/s), time (sec), pressure (Pa), 

dynamic viscosity (Pa.s) and the 

gravity(m/s
2
) respectively. 

   The second model that was used in 

this current study was heat transfer 

model: 

 

 
                                                     … (2) 

 

 

 
Fig.1, Two and three-dimension sketch 

of bioreactor that suggested in this 

study 

Where    heat capacity [J/(mol K)],   
heat source/sink [W],     heat loss 
[W],   heat conductivity [W/(m K)], T 
temperature [K], and    pressure work 

[J]. 

 

   A time-dependent momentum 

balance model and heat transfer model 

were calculated in this model. For low 

gas concentration, the      was 
pointed to zero as considered in the 

present model. 

   The equation of gas phase was also 

used in present model with assumption 

that no mass transfer occurs between 

the two phases (gas and liquid). 

Therefore the equation was equalled to 

zero: 

 
     

  
   (      )           

                                                     … (3) 

 

Where    ,    , and     are the density 
of gas phase (kg/ m

3
), gas volume 

fraction (m
3
/ m

3
), and velocity of gas 

respectively.  

   The liquid volume fraction was 

calculated by          . The gas 

velocity can be determined as    

  +      , since       is relative 
velocity between two-phase fluid (gas 

and liquid). The       was calculated 
from pressure drag that is balanced 

with pressure forces according to the 

following equation [27]: 

 
   

   
 
 
|     |                       … (4) 

 

   No-slip boundary condition was used 

on the draft tube and ring diffuser, 

while slip boundary condition was 

considered at the top of the liquid. 

Extra fine mesh with 2141 elements 

was used in the present model. The 

general operational conditions that 

used in this research are illustrated 

throughout Table 1. 



Heat Transfer and Hydrodynamic in Internal Jacket Airlift Bioreactor with Microbubble Technology 

  

  

38                                  IJCPE Vol.18 No.4 (Dec. 2017)                -Available online at: www.iasj.net 
 

Table 1, operation conditions used in 

the computation model 

Operational Conditions for 

Computational Model 

Parameter Value Unit 

Temperature 

of Gas 
20 °C 

Temperature 

of Jacket 
50 °C 

Pressure of 

Process 
101 kpa 

Gas Mass 

Flux 
0.000754 Kg/m

2
.s 

 

   In fact, dealing with these kinds of 

models with time-dependent requires 

very high quality processor to 

complete the task of two dimensions 

model. Therefore, and to compute 

these complex models, the present 

study used an official software package 

of computational model with a special 

system and high quality that were 

provided by the University of 

Sheffield, UK. 

   The dead zones in airlift bioreactor 

have investigated by Al-mashhadani 

[27]. According to their results, 

decreasing the bubble size decreases 

the dead regions. However, reducing 

the bubble size, in spite of its 

advantages, may be undesirable in 

some biological applications because it 

causing separation of microorganisms 

or their nutrients by floatation process. 

Then this will lead to failure of the 

process. Therefore, 100 microns were 

considered in the present study. While 

the bubbles diameter of 1 mm and 0 

micrometer were used for 

compression. 

 

Results and Discussion 

Hydrodynamic Study 

   In the present study, the suggested 

design was examined through two 

aspects; hydrodynamic and heat 

transfer aspects. Gas fractions, velocity 

of re-circulation liquid and volume of 

dead regions are main objective 

variables that were investigated for 

hydrodynamic study. While the 

temperature of the liquid, internal heat 

flux were addressed for heat transfer 

section.Fig. 2 shows the gas-holdup in 

the downcomer region of the reactor at 

the bubble diameter of 100 micron and 

1 mm. It can be seen that the existence 

of bubbles of 100 microns is more than 

that with 1 mm. For example the gas 

volume fraction of microbubble in the 

downcomer region is 0.0063, while 

with fine bubble diameter is 6.89e-12.    

   Thus, the retention time of small 

bubbles increases in this unsparged 

region, allowing greater exchange 

between the gas and liquid phases 

across the interfacial layers depending 

on the known principles of material-

heat transfer. 

   Fig. 3 shows the snapshots of gas 

volume fraction at different bubble size 

after 12 min (i.e after steady state).     

   This figure demonstrated that bubble 

size is critical factor in the design of 

bioreactor. In fact, the weak buoyancy 

force of small bubble size (i.e 100 

microns) is sufficient to present such 

bubbles in downcomer region.   

   However, the velocity of liquid 

circulation around the draft tube of 

reactor plays an important role in 

determining the amount of gas in this 

unsparged region as a result of the 

competition between the flow force of 

the liquid to that area and buoyancy of 

bubbles. 

 
Fig. 2, Gas volume fraction in the 

down-comer region 



Mahmood K. H. Al-Mashhadani  

 

-Available online at: www.iasj.net                      IJCPE Vol.18 No.4 (Dec. 2017)                        39 
 

   Therefore, it is possible to observe 

that the biggest amount of bubbles 

present in the down-comer region was 

in the first 3 minutes, due to the 

primary velocity of the circulation 

liquid in the riser region of the reactor 

and in this time as can be shown in 

Fig. 4. Then the steady state of gas 

holdup and velocity of liquid begin. 

 

 
Fig. 3, gas volume fraction at different 

bubble size A) at 100 micron, B) at 

1mm 

 

 
Fig. 4, Liquid velocity in the riser 

region of bioreactor 

 

   Fig. 5 illustrates the fluid flow 

pattern at 0.1, 2, and 5 min when the 

bioreactor is sparged with 100 

micrometer.  

   The figure indicates that the change 

was generated at the first moments of 

operation, after which, the liquid flow 

appeared to be stabilized. This stability 

resulted in homogeneity of the fluid in 

all parts of the bioreactor, especially 

after the second minute of simulation 

time. 

   In general, the biological processes 

use domestic and industrial wastes as 

main biological media. These wastes 

usually are heavy because they 

containing the suspended solids. In 

addition, the biological reactions, 

naturally, are very slow, thus, 

sedimentation of these wastes at the 

bottom of traditional bioreactors is 

possible. Even if sediments are already 

not present in the biological medium, 

the biomass generated by the 

biological reactions will form a 

sediment component during 

cultivation, as in algal cultivation. This 

will cause inhomogeneity in the 

temperature and pH as well as the 

famine in those layers of sediments 

due to separation from the suspended 

nutrients. 

 

 
Fig. 5, snapshot of liquid velocity at 

0.1, 2, and 5 min 

 

   The best solution may be to place the 

diffuser at the bottom of the reactor, 

thus preventing the suspended material 

from precipitation.  



Heat Transfer and Hydrodynamic in Internal Jacket Airlift Bioreactor with Microbubble Technology 

  

  

40                                  IJCPE Vol.18 No.4 (Dec. 2017)                -Available online at: www.iasj.net 
 

   But if the waste contains large 

particles, with time they will cause 

blockage in the diffuser. Al-

Mashhadani [27] has noticed this 

problem by putting the diffuser at a 

height of 5 cm from the bottom to 

ensure that this problem does not 

occur. However, they reported that the 

volume dead zoon may be generated at 

the bottom of the reactor. 

   The study takes into account the 

most likely problems by placing the 

diffuser 5 cm from the bottom of the 

reactor, but using a circular diffuser 

instead of disk diffuser and with a 

specific design as can be shown in Fig. 

6 .This construction allows the fluid to 

flow vigorously in that area to prevent 

formation of dead areas. 

   In addition, and although, the present 

study suggested to use the 

microbubbles to increase momentum, 

heat, and mass transfer rate, the 

potential for foaming and stabilized 

emulsions in the reactor is a significant 

negative potential and it is one of the 

challenges facing the production of 

microbubbles as well as if is 

considered undesired property in many 

applications. Previous studies have 

addressed this problem, and showed 

that decrease the bubbles size increases 

the height of foam [28]. 

 

 
Fig.6, schematic of integration of ring 

diffuser and internal Jacket draft tube 

   The current study also took into 

account these problems and it sought 

to get an appropriate design to help 

using advantage of the microbubbles 

and reduce the problems resulting from 

the use of such technology.   

   Effect of foam, for example, can be 

reduced through increasing the 

circulation of the liquid around the 

draft tube in the bioreactor. Although, 

the velocity of the liquid may be 

somewhat slow in the upper part of the 

downcomer of reactor it is unable to 

address the problem as can be seen in 

Fig.7. This velocity  increases as the 

liquid moves down the reactor. In fact, 

the proposed geometry of current 

reactor has greatly contributed in 

improving the liquid speed at the 

bottom. This Figure shows the velocity 

of the liquid at different areas of the 

reactor. It can be observed that there is 

a significant increase in fluid velocity.   

   This result will greatly contribute to 

move the biological sediments formed 

at the bottom of the reactor with great 

uniformity in temperature and pH.   

   Therefore, and according to these 

figures, the dead areas will be very few 

if not vanished in this suggested 

design. 

 
Fig. 7, liquid velocity at different 

positions of reactor, a) in the 

downcomer (0.08, 0.14), b) in the 

bottom of reactor (0.04, 0.03), in the 

center ring of diffuser (0, 0.06) 



Mahmood K. H. Al-Mashhadani  

 

-Available online at: www.iasj.net                      IJCPE Vol.18 No.4 (Dec. 2017)                        41 
 

   However, the microbubbles size can 

play an important role in determining 

liquid velocity,  the increase in the 

central velocity of liquid during the 

reactor’s length is observed in Fig. 8.     

   This behaviour is due to the high 

momentum of the microbubbles to 

push the liquid up or down causing a 

decrease in the pressure and density of 

the mixture, which helps in generating 

high flux of the fluid due to 

momentum transfer between gas and 

liquid. 

 

 
Fig. 8, liquid velocity along the bio-

reactor 

 

Heat Transfer Study 

   The effect of bubble size on the 

suggested design in heat transfer aspect 

was also investigated in this section.     

   This factor represents the 

fundamental step in increasing the 

metabolic processes of 

microorganisms. Traditional heating 

methods for biological processes have 

some disadvantage such as formation 

different thermal layers in the same 

reactor, and thus will affect the 

microorganisms. The current design is 

radically different from what was done 

in these attempts through Fig. 6, which 

shows schematic diagram of suggested 

part in bioreactor by integrating the 

ring diffuser with internal draft jacket.   

    

 

 

   From this integration, the moving of 

the gas and liquid upwards generates 

driving force in inner side of jacket 

draft tube. This force has sufficient 

power to remove the thermal thin 

layers, which are generated on the 

internal jacket during heat transfer 

process, thus, reduces the formation of 

the thermal resistance in this part of 

reactor. Removing this layer creates an 

environment suitable for more heat 

transfer rate. In other words, 

decreasing the thermal layer thickness 

increases the heat transfer rate if the 

process was under sparging system.   

   Fig. 9 shows simple schematic 

diagram of removal and generation of 

thermal layer during sparging system. 

At time equal zero (no bubbling 

system), the thermal layer is already 

generated. When the sparging system 

starts, the created layer will be 

removed. Thus more heat will transfer 

due to decreasing the thermal 

resistance. 

 

 

Fig.9, schematic diagram of removal 

the thermal layer by bubbling system 
 

   The influence of the bubble sizes on 

the heat transfer rate in the current 

design was also considered. Fig. 10 

displays the temperature response in 

the two reactor areas (downcomer and 

riser region) for 12 min until the steady 

state is achieved.  



Heat Transfer and Hydrodynamic in Internal Jacket Airlift Bioreactor with Microbubble Technology 

  

  

42                                  IJCPE Vol.18 No.4 (Dec. 2017)                -Available online at: www.iasj.net 
 

   It is noted that the temperature in the 

first zone is higher than that in the 

second zone as well as the fastest up to 

the steady state. The main reason for 

this change is the amount of internal 

heat transferred from the riser region to 

downcomer region as a result of the 

momentum transfer generated by 

sparging process. This behaviour 

demonstrates a real integration 

between air bubbles and the liquid that 

work together in heat transfer process.   

   Therefore it is a significant delay in 

the temperature response in the 

downcomer region due to delay the 

recirculation of the liquid and gas 

between the two areas. Figure 11 

shows snapshot of transfer of internal 

energy from the riser region to 

downcomer region at 0.1 min, 5 min 

and 8 min. 

   However, determining the 

temperature difference between these 

two regions depends largely on the 

diameter of the micro bubbles. Figure 

12 shows that the temperature and 

speed of access to the steady state is 

almost very simple change between the 

two regions, while with no bubbling 

process, there was no observed change 

in temperature at any region. 
 

 

Fig.10, temperature profile in riser and 

downcomer region at 100 micron 

bubble size 
 

 

 

Fig. 11, internal energy in the 

suggested bioreactor at different 

simulation time; 0.1, 5, and 8 min. the 

last number was taken as value at 

steady state situation 

 

 

Fig. 12, temperature profile in riser and 

down-comer region at 1mm bubble 

size 

 

   Figure 13 illustrates temperature 

profile in unsparged area (downcomer 

region) at different bubble size; 1mm, 

100 and 0 micrometer. It can be seen 

from this figure that the flow, whatever 

its velocity is better than the flow 

without a bubbles, due to improved 

temperature, speed and homogeneity.    

   However, microbubbles give great 

heat efficiency compared to large 

bubbles, not by the high speed of heat 

transfer alone but with improving the 

features of the hydrodynamic. 

 



Mahmood K. H. Al-Mashhadani  

 

-Available online at: www.iasj.net                      IJCPE Vol.18 No.4 (Dec. 2017)                        43 
 

 

Fig. 13, temperature profile in 

downcomer region at 1 mm, 100, and 0 

micron bubble size 

 

Conclusions 

   Current research addressed the 

hydrodynamic and heat transfer study 

of the proposed reactor design using 

laminar bubbling flow model and heat 

transfer model in internal Jacket airlift 

bioreactor. Through the integration of 

these two models, the results indicated 

the importance effect of microbubbles 

on the main variables in biological 

processes such as retention time and 

mixing system. This will harmonize 

with the operating conditions in the 

process to get the equitable distribution 

of nutrients and prevent the formation 

of dead areas. In terms of the study of 

heat transfer, bubbles are important 

factor in the efficiency of heat transfer.    

   However, reducing the bubble 

diameter increases the frequency of the 

temperature response and reaching the 

steady state quickly. 

 

Acknowledgment 

   The author would like to thank Prof. 

William B. Zimmerman from the 

University of Sheffield, United 

Kingdom and Prof. Steven Wilkinson 

from Chester University, United 

Kingdom for their support for 

providing the official software package 

and information.  

   He would like also to acknowledge 

the support of Dr. Hemaka 

Bandalusena (Loughborough 

University, United Kingdom). The 

Author would like to thank the 

University of Baghdad-Department of 

Chemical Engineering. 
 

 
 

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