Progress in Nuclear Energy 121 (2020) 103243
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Progress in Nuclear Energy
journal homepage: http://www.elsevier.com/locate/pnucene
Investigation of iodine removal efficiency in a venturi scrubber using mass
transfer model for CFD
Ammar Ahmed a, Ajmal Shah b, c, Kamran Qureshi a, Khalid Waheed b, Naseem Irfan b,
Waseem Siddique a, *, Masroor Ahmad b, Amjad Farooq b
a
b
c
Department of Mechanical Engineering, Pakistan Institute of Engineering and Applied Sciences, Pakistan
Department of Nuclear Engineering, Pakistan Institute of Engineering and Applied Sciences, Pakistan
Centre for Mathematical Sciences, Pakistan Institute of Engineering and Applied Sciences, Pakistan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Venturi scrubber
Mass transfer model
Iodine removal
Filtered containment venting system
Filtered containment venting system (FCVS) is important for the ultimate safety of a nuclear power plant owing
to its use in containment pressure reduction in case of severe accident. It vents a portion of containment air after
removal of harmful radioactive products including iodine through venturi scrubber. The purpose of this research
was to develop a mass transfer model for estimation of performance of a venturi scrubber for removal of
elemental iodine using Computational Fluid Dynamics. A mathematical model for transfer of iodine from air to
water was developed based on two film theory of mass transfer and experimentally validated. Simulations were
performed for self-priming non-submerged circular venturi scrubber using Fluent and the results were compared
with experimental ones. Experimental data was used for validation in which an aqueous solution of sodium
hydroxide and sodium thiosulfate was used as scrubbing solution for removal of iodine. For simulation, only
water was used instead of scrubbing solution for simplicity. Euler-Euler approach was used for two-phase
modeling and realizable k-epsilon model was used for turbulence modeling. The results obtained from simula­
tions had a good match with the experimental ones.
1. Introduction
Nuclear power plants provide clean and sustainable energy for
electricity generation and heat applications. Safety of nuclear power
plants along with their ability to handle severe accidents is the major
concern with this technology as was highlighted from the accidents at
Three Mile Island, Chernobyl and Fukushima Daiichi Nuclear Power
Plants. In these accidents, containment building designed to protect the
environment from harmful radioactive products was compromised. To
handle such severe accidents, filtered containment venting system
(FCVS) is designed which ensures containment integrity in case of core
meltdown and high-pressure steam buildup inside containment. This
system reduces containment pressure by venting some of the contain­
ment air into the atmosphere. Radioactive elemental iodine-131 is the
major product among others that needs to be removed from the venting
air. It has a half-life of around 8 years and is likely to cause thyroid
cancer if released into the atmosphere.
In FCVS, venturi scrubbers are widely used to remove the harmful
radioactive products from the venting air to prevent radioactivity
release outside the plant. Therefore, it is necessary to study the phe­
nomenon of iodine retention in venturi scrubber to effectively improve
its efficiency. Venturi scrubbers are converging diverging nozzles with
facility of scrubbing solution injection at throat. Harmful gases present
in air are absorbed in scrubbing solution while passing through the
venturi scrubber. Generally, based on the feed mechanism of injected
water, two types of venturi scrubbers are used: i.e. force-feed and selfpriming venturi scrubbers. Owing to the usage of venturi scrubber in
conventional plants and industry for removal of dust and gases like SO2,
some models are already available in literature for its effectiveness
analysis.
Uchida and Wen (1973) presented a unidimensional mass transfer
and fluid flow model using heat, mass and momentum balances and
formulating differential equations relating the velocity of liquid, gas
concentration and its partial pressure along axial direction. They used
Runge-Kutta-Gill method to numerically simulate the results of experi­
ments presented in three different researches and compared the overall
* Corresponding author.
E-mail addresses: ammarahmed@pieas.edu.pk (A. Ahmed), waseem@pieas.edu.pk (W. Siddique).
https://doi.org/10.1016/j.pnucene.2020.103243
Received 26 April 2019; Received in revised form 26 December 2019; Accepted 6 January 2020
Available online 15 January 2020
0149-1970/© 2020 Elsevier Ltd. All rights reserved.
A. Ahmed et al.
Progress in Nuclear Energy 121 (2020) 103243
efficiency for Sulfur Dioxide gas (SO2) removal. Ravindram and Pyla
(1988) proposed a theoretical model to predict the efficiency for
removal of gaseous pollutants from air using a venturi scrubber. Their
model was based on simultaneous diffusion and irreversible chemical
reaction of gases with alkaline solutions. For validation of their model,
they conducted experiments for gas absorption using sodium hydroxide
solution as scrubbing agent. The proposed model was found to be in
agreement with the experimental results for the cases of CO2 and SO2
removal. Talaie et al. (2012) proposed a three-dimensional mathemat­
ical model to evaluate the removal efficiency of SO2 in venturi scrubber
using alkaline solution and water. The mass transfer model presented in
their study was based on physical absorption by concentration differ­
ence of SO2 in liquid and gaseous phases. The authors used material
balance and dispersion equations to determine pollutant concentration
in gaseous phase. The model calculates overall removal efficiency of SO2
considering the difference in concentration at inlet and outlet of venturi
scrubber but there is no localized mass transfer phenomenon incorpo­
rated. Hills (1995) studied the performance of venturi scrubbers as
chemical reactors and modelled the absorption of a gas with chemical
reaction. In this study, Hills reported that the absorption of a gas in real
venturi scrubbers can be considered as a rapid reaction and controlled
by diffusion.
The droplet size plays an important role in mass transfer inside
venturi scrubber. Alonso et al. (2001) performed experiments for mea­
surement of diameter of water droplets inside cylindrical venturi
scrubber using laser diffraction technique by varying the gas velocities
and liquid to gas flow rate ratio. Sauter mean diameter values obtained
from Nukiyama and Tansawa (Nukiyama and Tanasawa, 1938) as well
as Boll et al. (1974) equations were compared with experimental results
and concluded that Sauter mean diameter is well predicted using Boll
et al. correlation. Also, velocity of gas has a major effect on the size of
droplet instead of liquid to gas flow ratio. Costa et al. (2004) also per­
formed the same experiments using a rectangular venturi scrubber
instead of a circular one and the results obtained agreed with the ones
presented by Alonso et al. Ahmadvand and Talaie (Talaie and Ahmad­
vand, 2010) developed a two-dimensional mathematical model to study
the droplet dispersion in cylindrical venturi scrubber using CFD.
Gamisans et al. (2004) experimentally studied the effect of diameter
and throat length on mass transfer in a jet-venturi scrubber for SO2
absorption in aqueous solutions. A model based on two film theory of
mass transfer was proposed by incorporating diffusion with irreversible
chemical reaction of SO2 with scrubbing solution. Concentration dif­
ference of SO2 was taken as the driving force for transfer of SO2 from air
to scrubbing solution. They mentioned that the scrubbing reaction is
very rapid and is diffusion controlled. Furthermore, the significance of
liquid film in mass transfer was proved experimentally. Comparison of
results was made for the cases considering the effect of liquid film on SO2
removal and neglecting its effect.
Pak and Chang (2006) developed a three-dimensional numerical
model for prediction of pressure drop as well as collection efficiency of a
circular venturi scrubber for dust particle removal. Inertial impaction
mechanism was considered for capturing of dust particles in water
droplets. Boll correlation was used for droplet size and
Eulerian-Lagrangian method for numerical simulation of dust-air-water
three phase mixture. In case of severe accident in a nuclear power plant,
containment is pressurized by high temperature steam mixed in air so
Ali et al. (2014) experimentally studied the removal efficiency of iodine
from steam in submerged venturi scrubber and concluded that removal
efficiency of iodine increases with increase in flow rate of steam. Also,
Ali et al. (2013) theoretically and experimentally studied the removal
efficiency of iodine in a self-priming venturi scrubber for the cases of
submerged and non-submerged conditions. Aqueous solution of 0.5%
sodium thiosulfate and 0.2% sodium hydroxide was used to absorb
iodine present in air inside venturi scrubber. A mass transfer model
based on two film theory proposed by Gamisans et al. (2004) was used.
Droplet diameter was calculated using Nukiyama and Tanasawa
equation whereas Steinberger and Treybal (1960) correlation was used
for mass transfer coefficient of gas phase. They reported that iodine
removal efficiency increases with increase in gas flow rate as well as
inlet iodine concentration. They also pointed out that the scrubbing
reaction is very rapid and the process is controlled by the diffusion at the
gas-liquid interface.
Ahmed et al. (Ahmedet al., 2018) investigated the removal efficiency
of self-priming venturi scrubber for dust particles removal using
computational fluid dynamics (CFD). They performed the analysis using
ANSYS CFX. 1-micron size Titanium oxide particles were used as dust.
They used cascade atomization and breakup (CAB) model for prediction
of deformation of water droplets and Eulerian-Lagrangian method for
multiphase analysis. They estimated the dust removal efficiencies for
different inlet air velocities and it was concluded that removal efficiency
increases with increasing air velocity.
Ashfaq et al. (Ashfaqet al., 2018) studied the effect of droplet
diameter of scrubbing solution on removal of elemental iodine from
containment air in a venturi scrubber using CFD. They used ANSYS
FLUENT to model the multiphase flow phenomenon and incorporated a
user defined function for mass transfer of iodine from air to water
(scrubbing). They done a tremendous job by introducing a
three-dimensional mathematical model for scrubbing iodine from air
and implementing the model in CFD software for the first time to the
best of my knowledge. However, there were some discrepancies in the
approach and mass transfer model they introduced.
The work presented here is an extension of the study performed by
Ashfaq et al. (Ashfaqet al., 2018). In this work an effort has been made to
remove the discrepancies in the approach and CFD model for scrubbing
of iodine from air. Ashfaq and his coworkers performed the simulations
to study the effect of droplet diameter on the process of scrubbing. They
did this by selecting various droplet diameters and running the simu­
lations for each case. However, the droplet diameter is a function of the
air flow rate and varies as the flow rate is changed. In the present work,
the droplet diameter has been calculated using the correlation of Boll
et al. (1974) to overcome the discrepancy of the Ashfaq et al.
(Ashfaqet al., 2018) model. Ashfaq et al. (Ashfaqet al., 2018) assumed
Iodine distribution parameter equal to unity as did by Crank (1975).
However, Crank (1975) has also mentioned that distribution parameter
is unity only when the diffusion process is very rapid. Further, Gamisans
et al. (2004) and Ali et al. (2013) have mentioned that, in the scrubbing
process, reaction rate is rapid as compared to diffusion so the process is
diffusion controlled. Therefore, in the present study the distribution
parameter has not been taken as unity, rather it has been calculated by
using the scrubbing efficiencies obtained by experiments and CFD. In
this study a range of distribution parameter has been calculated and
finally an average value of the distribution parameter has been pre­
sented. The value of the distribution parameter, reported in this work for
the first time to the best of our knowledge, reflects the process of iodine
diffusion from air to liquid, thus giving an insight of the governing
process. Ashfaq et al. (Ashfaqet al., 2018) has not validated the scrub­
bing model against experimental results, therefore, in the present study
the CFD results have been validated with experimental results. So, the
present study presents a much-improved mathematical model of the
iodine scrubbing process, which has not only been validated by exper­
imental results but also reflects the dynamics of the actual process by
calculating the value of the distribution parameter.
2. Material and procedure
In this study, Computational Fluid Dynamics analysis was performed
for simulation of mass transfer phenomenon in a non-submerged cir­
cular venturi scrubber using Fluent module of ANSYS. A mathematical
model was developed for evaluating the mass transfer of iodine from gas
to liquid phase and was incorporated with the software using a user
defined function (UDF). The efficiency of venturi scrubber for removal
of iodine from air was obtained from the difference of concentration of
2
A. Ahmed et al.
Progress in Nuclear Energy 121 (2020) 103243
Fig. 1. Experimental setup (Nawaz, 2017).
Fig. 2. Geometry of venturi scrubber (Nawaz, 2017).
iodine in air at inlet and outlet of venturi scrubber.
�
2.1. Multiphase model and governing equations
�
m_ qp þ Sq
þ
n �
X
! _
R pq þ mpq !
v pq
m_ qp !
v qp
qp
(1)
(3)
q
lift;q
wl;q
vm;q
td;q
�
�
∂
∂pq
α ρ h þ r: αq ρq hq !
u q ¼ αq
þ τq
∂t q q q
∂t
: r!
uq
r:!
q q þ Sq þ
n
X
Qpq þ m_ pq hpq
m_ qp hqp
�
(4)
p¼1
Where hq , !
q q and Sq represent specific enthalpy, heat flux and source
term of qth phase respectively. Qpq and hpq show intensity of heat ex­
change between pth and qth phases and interphase enthalpy
respectively.
Iodine was treated as a chemical specie in air and scrubbing solution
phases. It was modelled by specie transport model with following con­
servation equation for iodine in any phase q,
αq rp þ r:τq þ αq ρq !
g
�
�
2
v qI
μq r:!
3
represent external body force, lift force, wall lubrication force, virtual
mass force and turbulent dispersion force respectively. Interaction force
!
between phases is shown by R pq and pressure shared by all phases
represented by p.
The conservation of energy equation in Eulerian model is written as
Where mass transfer from pth to qth phase is shown by m_ pq and from qth
to pth phase by m_ qp .
The momentum balance equation for a phase q is given by
�
�
∂
αρ!
v þ r: αq ρq !
v q!
vq ¼
∂t q q q
T
Where μq and λq represent shear and bulk viscosity of phase q. !
v pq and
! !
!
!
!
!
v
show interphase velocities. F ; F
; F
; F
and F
Eulerian-Eulerian approach was used for solution of complex twophase flow consisting of air-water-iodine mixture. The Eulerian model
solves a set of continuity and momentum equations for each phase.
In this model, continuity equation for a generic phase q flowing with
velocity !
v q is
n
�
� X
∂
m_ pq
αq ρq þ r: αq ρq !
vq ¼
∂t
p¼1
�
�
v q þ r!
τq ¼ αq μq r!
v q þ αq λ q
(2)
p¼1
! !
!
!
! �
þ F q þ F lift;q þ F wl;q þ F vm;q þ F td;q
Where τq represents stress-strain tensor of qth phase given by
3
A. Ahmed et al.
Progress in Nuclear Energy 121 (2020) 103243
Table 1
Experimental data reported by Nawaz (2017) and used for benchmarking.
Sr.
#
Mass Flow
Rate of Air
(m3/s)
Inlet I2
Concentration
(ppm)
Outlet I2
Concentration
(ppm)
Removal
Efficiency (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1.389 �
10 3
50.1
34.6
27.9
42.0
48.6
69.7
44.0
39.5
34.0
37.6
32.7
35.6
49.4
45.1
41.4
37.2
4.9
5.2
5.0
5.1
4.8
5.6
4.9
4.7
2.5
2.5
2.4
2.5
2.5
2.5
2.5
2.4
90.2
85.0
82.1
87.9
90.1
92.0
88.9
88.1
92.6
93.4
92.7
93.0
94.9
94.5
94.0
93.5
1.528 �
10 3
1.667 �
10 3
1.806 �
10 3
∂ q q q
q
v Y qÞ ¼
ðρ α Y Þ þ r:ðρq αq !
∂t
n
X
!q
m_ pq
r:αq J þ αq Rq þ αq Sq þ
m_ qp
�
p¼1
þR
(5)
2.2. Mass transfer model
The mathematical model developed for mass transfer of iodine from
air to droplets of water is based on two film theory. The driving force for
mass transfer is the iodine’s concentration difference (diffusion) and
takes place at interface of air and water films. It is assumed that the
reaction is kinetically rapid and hence, rate of mass transfer is gas-film
controlled (Gamisans et al., 2004). Furthermore, all droplets of water
are spherical and represented by a Sauter mean diameter. Mass transfer
rate of iodine in a single droplet of water is given by the following
relation (Gamisans et al., 2004),
Nl ¼ 4πr2 kg mðCin
Ci Þ
(6)
The distribution factor (m) in Equation (5) was assumed by Crank
(1975) and Ashfaq (Ashfaqet al., 2018) as unity considering the same
concentration of solute present in film as well as in solution. However, it
was mentioned that in reality, the concentration of solute in liquid film
is m times the concentration of solute in the gas phase (Crank, 1975) i.e.
m¼
Equilibrium ​ Concentration ​ of ​ iodine ​ in ​ liquid ​ phase
Equilibrium ​ Concentration ​ of ​ iodine ​ in ​ gaseous ​ phase
Fig. 3. Meshed Geometry (a) Complete (b) Outlet region (c) Throat region (d)
Inlet region.
Since the scrubbing process is diffusion controlled (Gamisans et al.,
2004; Ali et al., 2014), the value of distribution parameter cannot be
assumed unity as mentioned by Crank (1975). Therefore, the distribu­
tion parameter becomes very important to understand the process of
diffusion in iodine scrubbing. To determine the mass transfer rate at
local computational cell, it is required to find the number of droplets (n)
in the cell, which is calculated by considering volume of water and
volume of spherical droplets in the computational cell.
n¼
3αl Vc
4π r 3
3αl kg mCin
M_ ¼
r
2.3. The mass transfer coefficient is calculated using sherwood number
formula
(7)
Sh ¼
Rate of mass transfer in a unit cell of domain in Equation (8) can be
obtained by the product of Equations (6) and (7).
m_ ¼
3αl kg mðCin
r
Ci Þ Vc
(9)
kg dd
Dg
(10)
Where Sherwood number is calculated using Steinberger and Treybal
correlation (Steinberger and Treybal, 1960)
�0:62
Sh ¼ 2 þ 0:347 Re Sc0:5
(11)
(8)
And Reynolds and Schmidt number are given by Equations (12) and
(13).
With the assumption of reaction being gas film controlled, Ci ¼ 0 for this
case (Ali et al., 2013). On per unit volume of cell basis, the rate of mass
transfer is given by Equation (9).
4
A. Ahmed et al.
Progress in Nuclear Energy 121 (2020) 103243
Fig. 6. Distribution parameter values.
Fig. 4. Mesh independence study.
Table 2
CFD models and boundary conditions used in simulation.
Multiphase model
Turbulence model
Specie transport
Phase-1 (Gas phase)
Phase-2 (Liquid
phase)
Mass Transfer
Venturi scrubber
inlet
Orifice inlets at
throat
Venturi scrubber
outlet
Wall
Eulerian-Eulerian
Realizable k-epsilon
Iodine taken as specie in gas and liquid phase
Air-Iodine mixture
Water-Iodine mixture
From phase-1 iodine to phase-2 iodine at a user defined rate
using UDF
Boundary Conditions
Mass Flow Inlet of Phase-1 with certain concentration of I2
Pressure Inlet of Phase-2 with no concentration of I2
Pressure Outlet at atmospheric pressure
No slip conditions at wall
Fig. 7. Comparison of simulation and experimental removal efficiency
of iodine.
Re ¼
ρg vg dd
μg
(12)
Sc ¼
μg
ρg D g
(13)
The diameter of water droplet is calculated using correlation of Boll
et al. (1974)
�
�1:932
l
4:22 � 10 2 þ 5:77 � 10 3 1000Q
Qg
dd ¼
(14)
v1:602
r
2.4. Experimentation
The validation of a computational model generally requires experi­
mental results for comparison. As part of this ongoing project Nawaz
(2017) performed some experimental work, which has been used as
validation data in this current study. The details of Nawaz (2017)
experimental work are mentioned below.
Fig. 5. Variation in distribution parameter values.
5
A. Ahmed et al.
Progress in Nuclear Energy 121 (2020) 103243
form of a spectrum which gives the concentration of material present in
the sample. Calibration curve was drawn for absorbance of light versus
known value of concentration and later used for determination of con­
centration of iodine in samples. The removal efficiency was calculated
using the difference in concentration of iodine at inlet and outlet of
scrubbing column.
Table 1 shows part of the experimental data reported by Nawaz
(2017) and used as benchmark for simulating the scrubbing of iodine
from containment air through venturi scrubber. Mass flow rate of air was
mentioned in units of cubic meters per hour, concentration of iodine in
air at inlet and outlet of venturi scrubber was mentioned in parts per
million. Removal efficiency was calculated using the difference in con­
centration of I2 in air at inlet and outlet. Further details of the experi­
mental work are given in Nawaz (2017).
2.5. CFD simulation
CFD simulations were performed using ANSYS Fluent version 14.0.
The venturi scrubber geometry was obtained from the experiments
performed by Nawaz (2017). The geometry was made in ANSYS Design
Modular and meshed in ANSYS Meshing module. The geometry with
dimensions is shown in Fig. 2.
The venturi scrubber used in this study is of self-priming type; In
order to apply pressure boundary conditions at orifices, orifices were
extended 5 mm from throat. The meshed geometry with tetrahedral
mesh at orifices and hexahedral elsewhere is shown in Fig. 3.
The mesh independent study was performed. Variation of iodine
concentration in air was obtained on axis of venturi scrubber by grad­
ually increasing the number of mesh elements. The concentration (mass
fraction) of iodine in air became independent of mesh size at around
112875 elements as shown in Fig. 4.
The Eulerian-Eulerian model was used to obtain in-depth two-phase
analysis, constant droplet diameter was used by calculation using Boll
correlation, realizable k-epsilon model was used for turbulence and mass
transfer model was incorporated using user defined function in ANSYS
Fluent. The boundary conditions used were mass flow inlet at air inlet,
pressure inlet at orifices and pressure outlet at outlet of venturi scrubber.
The boundary conditions were used according to the data obtained from
Fig. 8. Variation of liquid droplet diameter with flow rate of air.
The experiments were performed on a circular self-priming sub­
merged venturi scrubber made of brass with dimensions shown in Fig. 2.
The schematic of experimental setup is shown in Fig. 1. The filtered air
from compressor was passed through iodine sublimation chamber. A
certain amount of solid iodine was loaded in iodine chamber before each
experiment. The sublimated iodine vapors get mixed with air in iodine
chamber. The amount concentration of iodine mixing in air was deter­
mined using iodine traps before inlet of venturi scrubber. The air-iodine
mixture was passed through the venturi scrubber submerged in scrub­
bing solution (0.5% NaOH and 0.2% Na2S2O3). Iodine concentration in
air at inlet as well as outlet of venturi scrubber was measured at 5-min
intervals using traps as shown in Fig. 1. Air streams containing iodine
vapors from inlet and outlet were bubbled through traps containing 0.1
M KOH solution and analyzed on UV–visible spectrophotometer that
works on absorption spectroscopy principle. The amount of light
absorbed as a function of wavelength is measured and plotted in the
Fig. 9. vol fraction of (a) gas phase on axial plane (b) gas phase on cross-sections along length (c) liquid phase on axial plane (d) liquid phase on cross-sections
along length.
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A. Ahmed et al.
Progress in Nuclear Energy 121 (2020) 103243
Fig. 10. Zoomed contour of volume fraction of (a) Phase-1 (b) Phase-2.
the experimentation (Nawaz, 2017). Because of unavailability of iodine
in the Fluent library, properties of iodine were manually entered in the
software. Because of very low concentration of NaOH and Na2S2O3 in
the solution, its flow was simulated using water whereas the iodine
absorption of the solution was simulated by mass transfer model. The
convergence criterion used for the simulation was 10 4. The CFD set­
tings have been summarized in Table 2.
diameter, using Boll et al. (1974) correlation. Further the experimental
results of scrubbing efficiency were used to calculate the iodine distri­
bution parameter which was assumed unity previously by Ashfaq et al.
(Ashfaqet al., 2018). In the following topics the distribution parameter,
droplet diameter, validation of mass transfer model and contours of
volume fraction, mass transfer, pressure and velocity have been dis­
cussed in detail to throw light on the scrubbing process of iodine from
containment air using submerged venturi scrubber.
3. Results and discussion
3.1. Distribution parameter and validation of mass transfer model
In this study, an improved computational mass transfer model of
iodine scrubbing from containment air has been presented and validated
through experimental results. As mentioned earlier, this model was
applied for the first time by Ashfaq et al. (Ashfaqet al., 2018) for their
computational study of iodine scrubbing and the present study is an
extension of the work of Ashfaq et al. (Ashfaqet al., 2018). In this work,
the scrubbing model has been modified by calculating the droplet
The distribution parameter is defined as the ratio of equilibrium
concentration of specie in liquid phase to its equilibrium concentration
in gas phase. It is an important parameter in flows where the mass
transfer is diffusion controlled as is the case in iodine scrubbing from
containment air. It can be considered as unity if the diffusion process is
very fast as mentioned by Crank (1975). Ashfaq et al. (Ashfaqet al.,
2018) assumed the value of distribution parameter as unity in the mass
transfer model of iodine scrubbing, however, in the present study a more
realistic approached was used to calculate its value using the experi­
mental value of scrubbing efficiency. In this study, a series of simula­
tions were performed by keeping the experimental value of scrubbing
efficiency as the bench mark to ascertain a range of distribution
parameter under different operating conditions. The distribution
parameter values as a function of concentration of iodine in air and air
flow rate are shown in Fig. 5. The values of distribution parameter vary
from 0.00144 to 0.00205 for an air flow rate of 1.389 � 10 3 m3/s to
1.806 � 10 3 m3/s and iodine concentration of 24 ppm–70 ppm in air.
These results indicate that the distribution parameter is not a strong
function of air flow rate and iodine concentration in air. However, these
values indicate that the process of iodine scrubbing from containment
air is strongly diffusion controlled as the values of distribution param­
eter are very small as compared to unity. These values, therefore, vali­
dates the assumption, employed in the mass transfer model, that the
process of iodine scrubbing is diffusion controlled rightly mentioned by
Gamisans et al. (2004) and Ali et al. (2013) too.
The narrow band of the values of distribution parameter, under
different operating conditions, as shown in Fig. 6 provoked us to search
for a single value of the distribution parameter valid for the simulation
of all the cases with reasonable error. So, a mean value of the distribu­
tion parameter was obtained from the data of Fig. 6 and it was 0.00185.
Fig. 11. The mass fraction of iodine in Phase-1 and Phase-2 on axial centerline
of venturi scrubber.
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A. Ahmed et al.
Progress in Nuclear Energy 121 (2020) 103243
Fig. 12. Mass fraction of iodine in (a) gas phase on axial plane (b) gas phase on cross-sections along length (c) liquid phase on axial plane (d) liquid phase on crosssections along length.
All the simulations were performed again using this single value of the
distribution parameter and the iodine removal efficiency was calculated
and compared with the experimental values as shown in Fig. 7. It was
found that the maximum error in the iodine removal efficiency is 6.1%,
which looks acceptable.
entering the venturi scrubber. Smaller the diameter of these droplets,
more will be the mass transfer because of an increased surface area as
depicted by Equation (9). Ashfaq et al. (Ashfaqet al., 2018) used a
constant mass flow rate of 0.09 kg/s and considered different diameters
of water droplets for the same flow rate to study the effect of changing
diameter on different parameters. However, droplet diameters are rep­
resented by a Sauter mean diameter which is a function of flow rates of
gas and liquid (Alonso et al., 2001). In this improved model, the Boll
et al. correlation was introduced to calculate the droplet diameter. With
the edition of this correlation the model is now able to calculate the
droplet diameter according to the existing flow conditions. Fig. 8 in­
dicates the droplet diameter variation with the air flow rate obtained
through modified model. This Figure indicates that the diameter of
liquid droplet depends on the air mass flow rate and decreases as the air
mass flow is increased. So, the water atomizes into finer droplets as the
air flow increases. The variation in droplet diameter is from 32 μm to 47
3.2. Droplet diameter
Droplet diameter is an important parameter in scrubbing process of
iodine from containment air because it defines the surface area offered
by liquid phase to the iodine present in air. If larger surface area is
offered by liquid the scrubbing will be efficient and vice versa. The
droplet diameter decreases with increase in gas flow rate as reported by
Alonso et al. and Costa et al. (Talaie et al., 2012; Nukiyama and Tana­
sawa, 1938). The liquid phase enters the venturi scrubber by pressure
difference through tiny holes present at throat of venturi scrubber as
shown in Fig. 3. The liquid atomizes into very small droplets after
Fig. 14. Mass transfer rate contour on (a) axial cross-section (b) radial crosssections along length (c) enlarged view near orifices of axial cross-section.
Fig. 13. Enlarged view of contour of mass fraction of I2 in Phase-1 near orifices.
8
A. Ahmed et al.
Progress in Nuclear Energy 121 (2020) 103243
Fig. 15. Axial profiles of (a) pressure and (b) velocity at different flow rates of gas.
3
to 1.806 � 10 3
m3/s. The modified model is now capable of calculating the Sauter mean
diameter according to the flow conditions and draw back present in
Ashfaq et al. (Ashfaqet al., 2018) model was removed.
μm as the air flow rate is varied from 1.389 � 10
liquid (phase-2) phases have been shown in Fig. 9 (a and c) and on
different cross sections along the length of the venture Fig. 9 (b and d),
respectively. The boundary condition of mass flow inlet for gaseous
phase was applied at inlet of venturi scrubber. The pressure inlet con­
dition (corresponding to the hydrostatic head for a column height of
0.762 m mentioned in experiments (Nawaz, 2017)) for liquid phase was
used at eight orifices present at throat of venturi scrubber. The liquid
enters from the orifices present at throat of venturi scrubber because of
applied pressure. The volume fraction contours shown in Fig. 8 show the
behavior of water entering through orifices at inlet air flow rate of 1.389
� 10 3 m3/s.
At the inner walls of venturi scrubber downstream of orifices, the
3.3. Contours of volume fraction
The interaction between the liquid and gas phases results in mass
transfer of iodine between them. The localized mass transfer rate at any
local position inside venturi scrubber is also dependent on volume
fraction of liquid present there as evident from Equation (8). The con­
tours of volume fraction on an axial plane of gaseous (phase-1) and
Fig. 16. Contour of (a) pressure on axial plane (b) pressure on cross-sections along length (c) air velocity on axial plane (d) air velocity on cross-sections along length.
9
A. Ahmed et al.
Progress in Nuclear Energy 121 (2020) 103243
solution entering from orifices and hence, transfer of iodine from air to
scrubbing solution. On the contrary, the concentration of iodine present
in liquid phase (Fig. 12 c and d) increases gradually downstream of
orifices which is a proof of transfer of iodine specie from gaseous phase
to liquid phase by virtue of the mass transfer model added. This behavior
was also reported by Ashfaq et al. (Ashfaqet al., 2018).
There is peculiar behavior of iodine concentration in air at inner wall
of venturi scrubber just downstream of orifices. The iodine concentra­
tion decreases and then increases near the wall. The behavior has been
highlighted by Fig. 13 which shows an enlarged view of iodine mass
fraction in phase-1 contour near the orifice region. The reason for this
behavior may be the presence of liquid film at walls as observed in
Fig. 10 resulting in a large concentration of liquid phase. This means a
very small amount of gas-phase is present in that region. So, concen­
tration of iodine drops too low in that region. It increases downstream
where the film vanishes and the air from center expands to move to walls
in diverging section. This behavior has been observed for the first time to
the best of our knowledge.
Fig. 14 shows the contours of mass transfer rate of iodine from gasphase to liquid-phase. Mass transfer rate is maximum at throat region
near the orifices because concentration of iodine is maximum in this
region as seen in Fig. 12 and also because volume fraction of water is
highest as seen in Fig. 9. As the iodine gets transferred continuously from
gas phase to liquid phase as gas moves downstream of orifices, its con­
centration reduces and as a result, mass transfer rate decreases as shown
in Fig. 11. Ashfaq et al. (Ashfaqet al., 2018) mentioned the same effect.
Fig. 14 (c) shows the enlarged view mass transfer rate contour near
orifices. It has been observed that maximum mass transfer occurs at the
region near the wall where the film of liquid phase is formed as observed
in Fig. 10. The reason for this might be the presence of negligible amount
of phase-1 at the region and a large volume fraction of phase-2.
Fig. 17. Variation of removal efficiency of iodine with throat gas velocity.
volume fraction contours show almost zero volume fraction of gas
phase. The reason is the film formation of liquid phase along walls of
venturi scrubber after entry and before droplet formation. Also, the
momentum of air forces the incoming water to stay at the walls before
droplet formation and dispersion in the whole venturi scrubber. Fig. 10
shows an enlarged view of the contour of volume fraction near orifices
on an axial plane. The film formation of liquid phase can be seen clearly
in Fig. 10, a phenomenon captured by CFD for the first time to the best of
our knowledge. It was observed that the film of liquid phase was present
in a small region just downstream of orifice and it vanishes when moving
further away into diverging section. It can be concluded from the afore
discussion that the film might be trapped in this localized region.
3.5. Contours of pressure and velocity
The behavior related to flow physics of phases inside venturi
scrubber was obtained as an additional information apart from the mass
transfer. The pressure and velocity variation at an axial centerline of
venturi scrubber is shown in Fig. 15 (a) and (b) respectively. The in­
crease in velocity and reduction in pressure downstream of inlet may be
attributed to the converging section of venturi scrubber where a gradual
decrease in area of flow causes such a behavior. The reduction in slope of
both profiles is because of straight throat region after converging sec­
tion. A sharp peak in velocity profile and the corresponding valley in
pressure profile may be because of water injection at orifices which
reduces the area of flow for air. Downstream the orifices, is the diverging
section of venturi scrubber and hence the velocity decrease and pressure
recover because of increasing area of flow. With increase in flow rate the
maximum pressure at inlet increases but the minimum pressure at throat
further decrease because of the corresponding increase in velocity in
that region. Also, the velocity change with changing flow rate of air is
significant at throat but in other regions, it is little affected by it.
The pressure and velocity contours are shown in Fig. 16. The con­
tours were plotted at an axial cross section passing through two out of
eight orifices. The same behavior of velocity and pressure can be seen in
these contours as mentioned above. Negative pressure present at throat
of venturi scrubber causes the water to enter through orifices by pressure
difference. Pressure inlet boundary condition was used at orifices so
pressure decreases towards center of venturi scrubber where negative
pressure maintains. Similar pressure and velocity contours were re­
ported by Ashfaq et al. (Ashfaqet al., 2018).
The variation of removal efficiency of iodine with throat gas velocity
is shown in Fig. 17. The smaller droplet diameter at higher gas velocities
results in increase of mass transfer rate. Higher mass transfer rate of
iodine from gas to liquid phase means higher removal efficiency.
3.4. Contours of mass transfer
Since the process is assumed to be diffusion controlled, the concen­
tration of iodine present in both phases play a vital role in determining
the mass transfer rate. Higher the concentration of iodine present in
gaseous phase in any localized region of venturi scrubber, the more will
be the mass transfer rate in that area, provided the scrubbing solution is
present there. The mass transfer of iodine from gas phase to liquid phase
is governed by Equation (8). According to Equation (9), the increase in
mass flow rate of air decreases the droplet diameter of water and hence
increases the mass transfer rate. In Fig. 10 the mass fraction of iodine in
air as well as water has been plotted along the axis of the venture for
different flow rates of the air stream. In Fig. 11 it has been observed that
the mass fraction of iodine in air decreases after the throat section of
venturi scrubber (where water is entered to the venturi scrubber) due to
mass transfer from air to water. At the same time the mass fraction of
iodine is increased in the water as a result of scrubbing process. It has
also been observed that the process of scrubbing of iodine from air oc­
curs at a higher rate as the air flow rate is increased and vice versa. The
reason is that at higher flow rate of air the water droplet size decreases as
can be seen from Fig. 8, which results in increased mass transfer from air
to water as can be seen from Equation (9) as well.
The contour of mass fraction of iodine present in air and at axial
cross-sectional plane are shown in Fig. 12 (a and c), respectively. Fig. 12
(b and d) represent the iodine mass fraction in air and water at many
cross-sections downstream of orifices along the length of venturi
scrubber, respectively. The iodine concentration in air (Fig. 12 a and b)
was constant upstream of orifices because of no liquid present as
mentioned in section 3.4 and hence no mass transfer. The mass fraction
of iodine downstream of orifices gradually reduces because of scrubbing
10
Progress in Nuclear Energy 121 (2020) 103243
A. Ahmed et al.
4. Conclusion
� It was observed that the removal efficiency of venturi scrubber in­
creases with an increase in mass flow rate of gas as well as the
concentration of iodine in air at inlet
� A new model has been proposed to simulate the mass transfer phe­
nomenon of a gas inside venturi scrubber using computational fluid
dynamics
� An averaged value of distribution parameter has been proposed for
which a maximum error of 6.1% was observed between simulated
and experimental removal efficiency
� The model was used to simulate the mass transfer phenomenon
under various conditions and the behavior of trends of volume
fractions, mass transfer rate, velocity and pressure inside venturi
scrubber were reported
� There is a decrease in mean diameter of liquid droplets with increase
in flow rate of gas through venturi scrubber which results in an in­
crease in removal efficiency
CRediT authorship contribution statement
Ammar Ahmed: Formal analysis, Writing - original draft. Ajmal
Shah: Conceptualization, Supervision, Writing - original draft. Kamran
Qureshi: Conceptualization, Supervision, Writing - original draft.
Khalid Waheed: Conceptualization, Writing - review & editing.
Naseem Irfan: Funding acquisition. Waseem Siddique: Project
administration. Masroor Ahmad: Funding acquisition. Amjad Farooq:
Funding acquisition.
Nomenclature
Q
mf
Mass transfer rate in one droplet (kg s 1)
Radius of droplet
Mass transfer coefficient
Distribution parameter
Concentration (kg m 3)
Number of droplets
Volume
Volume fraction
Mass transfer rate in a unit cell (kg s 1)
Mass transfer rate in a unit cell (kg m 3 s 1)
Sherwood number
Reynolds number
Schmidt number
Diameter
Diffusion coefficient
Density
Velocity
Dynamic viscosity
Volume flow rate
Mass Fraction
g
in
i
w
c
l
d
r
Gas phase
Inlet
Interface
Water
Cell
Liquid phase
Droplet
Relative
N
r
k
m
C
n
V
α
m_
M_
Sh
Re
Sc
d
D
ρ
v
μ
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.pnucene.2020.103243.
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