N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 4 2 4 e4 3 3

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Original Article

TWO-DIMENSIONAL SIMULATION OF HYDROGEN IODIDE
DECOMPOSITION REACTION USING FLUENT CODE FOR
HYDROGEN PRODUCTION USING NUCLEAR TECHNOLOGY
JUNG-SIK CHOI a, YOUNG-JOON SHIN b, KI-YOUNG LEE b, and JAE-HYUK CHOI c,*
a

The Institute of Machinery and Electronic Technology, Mokpo National Maritime University, 91 Haeyangdaehak-ro, Mokpo-si, Jeollanam-do,
South Korea
b
Korea Atomic Energy Research Institute, Daedeok-Daero 989-111, Yuseong-gu, Daejeon, South Korea
c
Division of Marine Engineering System, Korea Maritime and Ocean University, 727 Taejong-ro, Yeongdo-gu, Busan, South Korea

article info

abstract

Article history:

The operating characteristics of hydrogen iodide (HI) decomposition for hydrogen pro-

Received 5 October 2014

duction were investigated using the commercial computational fluid dynamics code, and

Received in revised form

various factors, such as hydrogen production, heat of reaction, and temperature distribu-

5 January 2015

tion, were studied to compare device performance with that expected for device devel-

Accepted 24 January 2015

opment. Hydrogen production increased with an increase of the surface-to-volume (STV)

Available online 27 March 2015

ratio. With an increase of hydrogen production, the reaction heat increased. The internal
pressure and velocity of the HI decomposer were estimated through pressure drop and

Keywords:

reducing velocity from the preheating zone. The mass of H2O was independent of the STV

Computational fluid dynamics
Fluent code
Hydrogen iodide decomposition
reaction
Hydrogen production
Sulfureiodine cycle


ratio, whereas that of HI decreased with increasing STV ratio.

1.

Copyright © 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society.

Introduction

Hydrogen not only has potential for use as an alternative to
fossil fuels, but also plays a key role in solving what is known
as a trilemma (economic growth, energy use, and environmental degradation) [1]. A number of thermochemical cycles

were first postulated by Funk and Reinstrom [2] as the most
efficient way to produce fuels (e.g., hydrogen, ammonia) from
stable and abundant species (e.g., water, nitrogen) using heat
sources.
 2H2O þ SO2 þ I2 / H2SO4 þ 2HI

* Corresponding author.
E-mail address: (J.-H. Choi).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://
creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
/>1738-5733/Copyright © 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society.


N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 4 2 4 e4 3 3

 2HI / H2 ỵ I2
 H2SO4 / H2O ỵ SO2 ỵ 1/2 O2
In a closed cycle system, various related processes have

been proposed and have received a great deal of attention. The
sulfureiodine (SI) process involving thermochemical
hydrogen production using nuclear energy was proposed by
General Atomics and this technology has been studied by
many researchers [3e5].
Extensive research on hydrogen iodide (HI) decomposition
for hydrogen production has been carried out for experimental
verification and measurement of several factors, such as conversion efficiency and kinetics [6]. The decomposition of HI is
the key to producing hydrogen in the SI cycle. The decomposition of HI in the absence of any catalysts is not efficient even
at 773.15 K; therefore, catalysts have been used to promote this
reaction [7]. Moreover, the rate of the homogeneous gas-phase
reaction is considerably low at temperatures <700 K. Thus, it is
desirable to use a catalyst to increase the reaction rate, which
has led to the study of platinum catalysts [8e10]. A decomposition test using 98% pure HI gas was carried out by Ilda [8] in
1978 using a platinum supported on polytetrafluoroethylene
catalyst. Moreover, Wang et al. [11] investigated the effects of
different supports (carbon nanotubes, active carbon, carbon
molecular sieve, graphite, and Al2O3), masses of catalyst, and
reaction temperatures on the decomposition of HI. Fresh and
used active carbon-supported platinum catalysts were also
characterized. Furthermore, different types of catalysts
including noble metals and support materials (active carbon
[7,12], Pt [13], Ni [14], and bimetallic catalysts [6]) have been
reported to catalyze HI decomposition.
To date, much progress has been made on the efficiency of
this process. However, insufficient research has been conducted on the role of water vapor in the HI decomposition
process. In addition, very few reports have been published on
the kinetic studies of HI decomposition in the presence of H2O.
Assessment of the real potential of the HI decomposition reaction requires a deep knowledge of the thermodynamic
behavior of hydrogen production systems. Improving the efficiency of hydrogen production and optimizing the reactor

requires an understanding of the thermodynamics of the
process. The thermodynamic model for the HIx system used
by Roth and Knoche was proposed by Neumann [15] in 1987,
and was based mostly on total pressure measurements per€ lische Technische Hochschule
formed at Rheinisch-Westfa
(RWTH) Aachen [16] and on liquideliquid equilibrium data. In
the simulation studies, hydrogen pressures higher than those
expected for homogeneous gas-phase HI decomposition in the
direct decomposition of HIeH2OeI2 solution were achieved
€ user et al. [18] compared the calculated homo[17]. Berndha
geneous gas-phase reaction results with those of direct HI
decomposition from HIeH2OeI2 solutions, and found that the
rate of direct HI decomposition was several orders of magnitude higher than that of the gas-phase reaction. Furthermore,
Lanchi et al. [19], Murphy and O'Connell [20], and Hadj-Kali
et al. [21], reviewed the phase equilibrium properties of the
HIeI2eH2O (HIx) system and proposed new thermodynamic
models for describing the thermodynamic properties of this
system. Many researchers have attempted to develop appropriate models for the HI decomposition reaction in the SI

425

process [22,23]. Commercial process simulators such as Aspen
Plus (Aspen Technology, Inc., Massachusetts, USA) [24e26]
and ESP (simulator program, Microsoft, USA) [27e29] have
been used to develop these thermodynamic models.
In addition, in recent years, interest in and research on nuclear hydrogen technology, which involves direct decomposition of water to produce a large amount of hydrogen at high
temperatures in a nuclear reactor as the heat source of the
thermochemical cycle, has increased [30,31]. In particular,
following the successful continuous operation of a bench-scale
closed cycle gas turbine by the Japanese Atomic Energy Agency,

SI thermochemical hydrogen production technology has taken
center stage as one of the high-practical-potential hydrogen
production technologies that could be coupled with the very
high temperature reactor (VHTR) [32e36]. However, transferring the high heat output of the gas coolant for VHTR to the SI
thermochemical cycle involves the great challenge of developing a heat-exchanger material capable of withstanding high
temperatures (1173 K) and pressures (approximately 50e70 kg/
cm2). In the HI decomposition reaction, which requires a lot of
heat in the SI cycle, if the decomposition efficiency of HI can be
maintained at high pressures, the design and development of
the heat exchanger and reactor can be flexible.
In this study, an optimum decomposition reactor in the
thermochemical VHTReSI cycle operating characteristics of
the HI decomposition reaction were investigated using the
code. Several factors, such as hydrogen production, heat of
the reaction, and temperature distribution, were studied to
compare the device performance with that expected for device development.

2.

Materials and methods

2.1.

Basic equation for the HI decomposition reaction

The decomposition reaction of HI is as follows:

2HI / I2 ỵ H2

(1)


In this study, the simulation was performed using the
LangmuireHinshelwood-type rate equation on the catalytic
decomposition of HI in the presence of an active carbon
catalyst proposed by Oosawa et al. [13]
rHI ¼ ÀkHI P RHI

RHI



p
K PFe
xH2 xI2 1 ỵ I22
xHI


ẳ

1 ỵ KI2 PxI2
Kp 1 þ KI2 PxI2

(2)

(3)

where


À8; 250 cal$molÀ1

KHI ¼ 0:158 exp
RT

(4)



16; 480:1 cal$molÀ1
KI2 ¼ 5:086 Â 10À11 exp
RT

(5)

where Fe is the equilibrium conversion, rHI (mol/m3/s) is the
reaction rate, kHI (mol/m3/Pa/s) is the reaction rate coefficient,


426

N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 4 2 4 e4 3 3

P (Pa) is the system pressure, xHI, xI2, and xH2 are the mole
fractions of HI, I2, and H2, respectively, KI2 is the absorption
equilibrium constant of iodine (1/Pa), and Kp is the equilibrium
constant for the decomposition of HI. Further, Kp can be obtained from HSC 5.1 (chemistry-software, USA) [37] as follows:

Kp ¼


0:5  0:5

CeH2
CeI2

Ce
F ¼ 1 À HI ¼ 1 À
CHI;in
e

(6)

CeHI
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
À e ÁÀ e Áffi
CH2 CI2
Kp CHI;in

(7)

Kp ¼ 4:515 Â 10À3 À 4:519 Â 105 T ỵ 1:437 107 T2
8:662 1011 T3 ỵ 1:828 1014 T4

(8)

where CeH2 (mol/m3), CeI2 (mol/m3), and CeHI (mol/m3) are the
equilibrium concentrations of H2, I2, and HI, respectively, CHI,in
(mol/m3) is the inlet concentration of HI, and T (K) is the reaction temperature.

2.2.

Model design


The reactive distillation flow sheet developed by GA Company
(General Atomics, San Diego, CA, USA) [38] is shown in Fig. 1.
Fig. 1A illustrates the HI decomposition section. The dashed

line in Fig. 1B shows the multistage distillation column for the
periodic acid solution and the HI decomposer.
The HIeI2eH2O mixture gas was discharged from the
condenser part of the multistage distillation column for the HI
reaction, and flowed into the HI decomposer. Then, the H2eI2
gas was generated after decomposition of the mixture gas. In
this study, the characteristics of the thermal decomposition
device (HI decomposer, Fig. 2A) were investigated using the
CFD simulator model (Fig. 2B).
Fig. 2A shows the HI decomposer model and the heating
device consisting of a sealed three-stage electric heater. A
Hastelloy HC276 tube was inserted, and its outer diameter,
inner diameter, and height were 60.5 mm, 52.7 mm, and
1500 mm, respectively. This tube was filled with active carbon
catalyst and Al2O3 Raschig ring. Each of the three electric
heaters had built-in heating elements (i.e., 3.65-kW Kanthal A1) and the HC276 tube was surrounded by a low-density
ceramic fiber board for insulation. Moreover, a proportionalintegral-derivative controller was used to maintain the temperature of the three electric heaters at a set value. The
HIeI2eH2O mixture gas discharged from the condenser of the
multistage distillation flowed into the bottom of the inlet of
the HI decomposer at an absolute pressure of 5 kgf/cm2
(506,625 Pa). The temperature gradually increased to reach the
set temperature of the electric heater of the HI decomposer
and was maintained at this value. Meanwhile, helium (He) gas
or N2 gas was supplied to the HI decomposer to maintain the


Fig. 1 e Reactive distillation flow sheet in the SI cycle. (A) HI decomposition section. (B) Multistage distillation column and HI
decomposer. DI, deionized; HI, hydrogen iodide; SI, sulfureiodine.


N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 4 2 4 e4 3 3

427

Fig. 2 e Designed HI decomposer for the SI cycle and the CFD model. (A) Designed HI decomposer. (B) CFD domain.
CFD, computational fluid dynamics; HI, hydrogen iodide; ID, internal diameter; OD, outer diameter; SI, sulfureiodine.

pressure at 506,625 Pa, and thermal equilibrium was achieved
using an external heating chamber of the HI decomposer. The
mixture gas was supplied to the inlet under the conditions
that the temperature reached the set point and the pressure
was reliably maintained at 5 kgf/cm2.

size, and amount of catalyst. The effect of the surface-tovolume (STV) ratio on hydrogen production was also considered. When the reactor was developed, the STV ratio, defined
as the specific surface of the catalyst, was approximately 0.4 in
the preliminary simulation. The effects of setting the STV
ratio at 0.3, 0.4, and 0.5 on hydrogen production and reaction
heat were investigated.

2.3.
CFD analysis methods and boundary conditions of
the CFD domain
STV ratio ¼ A/V
The CFD simulation model of the HI decomposer is presented
in Fig. 2B. Size, properties, and boundary conditions identical
to those of the actual operating system were applied, and then

the CFD analysis was conducted using the Fluent program.
Moreover, to describe the actual operating system, a userdefined function was applied. At t ¼ 0 and 464.85 K
(191.7  C), the HIeI2eH2O mixture gas with flow rates of
0.008 mol/s HI, 0.0002 mol/s I2, and 0.020 mol/s H2O was
injected into the HI decomposer at an absolute pressure of
5 kgf/cm2. Maintaining a fixed porosity of 0.4, the temperatures of the electric heaters were set at 773.15 K (500 C),
873.15 K (600 C), and 973.15 K (700 C). The porosity value was
approximately predicted by considering several parameters,
such as the reactor size, volume, catalyst density, catalyst

where A is the surface area of the pore walls and V is the
unit volume.
For efficient CFD analysis, the following criteria were
assumed:
 The temperature of the heater was always maintained at
the set temperature.
 N2 gas was fully filled inside the HI decomposer, as the
initial condition of the simulation.
 The backward (reverse) reaction was not considered.
 The absolute pressure and temperature were set at 5 kgf/
cm2 (506,625 Pa) and 673.15 K (400 C), respectively, and
then the mixture gas was injected.


428

N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 4 2 4 e4 3 3

Fig. 3 e Estimates of hydrogen production according to changes in heater temperature. (A) Heater set value ¼ 773.15 K. (B)
Heater set value ¼ 873.15 K. (C) Heater set value ¼ 973.15 K. STV, surface-to-volume ratio.


3.

Results and discussion

3.1.
Effect of the heater temperature on hydrogen
production
To investigate the effects of the heater temperature on
hydrogen production, hydrogen production at HI decomposer
temperatures of 773.15 K, 873.15 K, and 973.15 K were estimated at STV ratios of 0.3, 0.4, and 0.5, as shown in Fig. 3.
Under each condition, the amount of hydrogen produced was
monitored at 10-minute intervals and the total operation time
was 180 minutes.
Fig. 3A shows the hydrogen production measured at
773.15 K (heater temperature). Up to 20 minutes after the
operation of the HI decomposer (i.e., after the start of the
decomposition process), no hydrogen production occurred
regardless of the STV ratio. From 20 minutes to 120 minutes,
the amount of hydrogen production varied according to the
STV ratio: the hydrogen production was 1.11e1.12 mol/h,
1.98e1.99 mol/h, and 3.09e3.10 mol/h at the STV ratios of 0.3,
0.4, and 0.5, respectively. As presented in Fig. 3B, at 873.15 K,
the amount of hydrogen produced was very low for 20 minutes, but after 120 minutes, it increased according to the STV
ratio. After 120 minutes, hydrogen production was
1.11e1.12 mol/h, 1.98e1.99 mol/h, and 3.09e3.10 mol/h at STV
ratios of 0.3, 0.4, and 0.5, respectively. Fig. 3C shows hydrogen
production at 973.15 K. Similar to the reaction at 773.15 K and
873.15 K, the chemical reaction occurred, and then, hydrogen


was produced at 1.11e1.12 mol/h, 1.98e1.99 mol/h, and
3.09e3.10 mol/h at STV ratios of 0.3, 0.4, and 0.5, respectively.
As shown in Fig. 3AeC, hydrogen production was more
strongly affected by the STV ratio than by the heat supplied by
the heater. The inner temperature of the HI decomposer was
higher than the temperature for catalyst activation
(T > 623.15 K) initially, and the inlet mass flow rate was the
same regardless of the heater temperature.

3.2.

Reaction heat prediction with different STV ratios

Fig. 4AeC presents the heat of the reaction at STV ratios 0.3,
0.4, and 0.5. The reaction was endothermic. As shown in
Fig. 4A, at an STV ratio of 0.3, the reaction heat was measured
at different heater temperatures. A similar consumption of
reaction heat was observed regardless of the heater temperature. After 60 minutes, the reaction heat required to produce
hydrogen was 0.0827e0.0831 kW/h, whereas after 120 minutes, when the hydrogen production was constant, it was
0.1116e0.117 kW/h. Therefore, the reaction heat increased
with the increase of hydrogen production regardless of the
temperature of the heater. Fig. 4B, C shows similar results at
STV ratios of 0.4 and 0.5, respectively. At an STV ratio of 0.4,
the reaction heat was 0e0.1604 kW/h regardless of the heater
temperature; however, it increased with increasing hydrogen
production. At an STV ratio of 0.5, the reaction heat was
0e0.2147 kW/h and showed a similar correlation with
hydrogen production as that observed at the 0.4 STV ratio.

Fig. 4 e Reaction heat according to different STV ratios. (A) STV ratio 0.3. (B) STV ratio 0.4. (C) STV ratio 0.5. STV, surface-tovolume ratio.



N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 4 2 4 e4 3 3

429

Fig. 5 e Inner temperature of HI decomposer according to temperature changes of heater. (A) Heater set value ¼ 773.15 K
with different STV ratios at 160 minutes. (B) STV ratio of 0.3 with different temperatures at 160 minutes. (C) Heater set
value ¼ 773.15 K with different STV ratios at 160 minutes. (D) STV ratio of 0.3 with different temperatures at 160 minutes.
HI, hydrogen iodide; ITC, inner thermocouple; STV, surface-to-volume ratio.


430

N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 4 2 4 e4 3 3

Fig. 5 e (continued).

As shown in Fig. 4AeC, the reaction heat used in the HI
decomposition reaction for hydrogen production was correlated to the specific surface area of the catalyst and it had a
direct relation with the production of hydrogen. However, the
temperature changes of the HI decomposer had no direct effect. The mixture gas was introduced into the HI decomposer
when the heat supplied from the heater passed through the
decomposer completely and the temperature was higher than
the activation temperature (i.e., 673.15 K). Moreover, the
temperature of the mixture gas from the preheating zone was
increased up to the temperature needed for the catalyst
decomposition reaction. Therefore, to enhance thermal efficiency, lowering the temperature of the heater was more advantageous at the STV ratio.

3.3.

Dependence of the HI decomposer inner temperature
distribution on the heater temperature
The inner temperature distribution of the HI decomposer was
monitored at different temperatures (773.15 K, 873.15 K, and
973.15 K) at various STV ratios (0.3, 0.4, and 0.5) during an
operation time of 160 minutes (Fig. 5AeC). Moreover, the
temperatures are presented in the form of area-weighted averages at inner thermocouple (ITC) 1e5. The temperature at
the center of the catalyst zone was lower than that at the
preheating zone for all STV ratios (Fig. 5A). The heat was used
to catalyze the decomposition reaction. The temperatures of
the outlet part, which discharged the mixture gas, and the
temperature of the zone above the catalyst zone were relatively higher than that of the catalyst zone. The temperature
distribution was as follows: 778e801 K at ITC 1, 779e792 K at

ITC 2, 777e794 K at ITC 3, 790e805 K at ITC 4, and 789e820 K at
ITC 5, regardless of the STV ratio.
Especially, at an STV ratio of 0.3, the temperatures of ITC
1, ITC 2, and ITC 3 at 973.13 K were higher than the temperatures at 773.15 K and 873.15 K for the same STV ratio
(Fig. 5B). Because the decomposition is an endothermic reaction, it seems that less reaction heat was supplied, and
therefore the inner temperature was higher at 973.15 K than
in the other cases. Using different STV ratios, there was no
significant difference in all of the temperatures. However,
different trends were exhibited at various temperatures
even if the differences were small. At 973.15 K, the inner
temperature was found to be higher than in the other cases.
The main reaction takes place well with the reaction temperature or more operation conditions, so it is not necessary
to supply the over temperature.

3.4.
Inner velocity and pressure of HI decomposer

according to temperature change of heater
In Fig. 6AeC, the static pressure and velocity magnitude at
STV ratios of 0.3e0.5 after 160 minutes of operation are presented at three different heating temperatures. The static
pressure and velocity magnitude are area weighted and
averaged. As shown for positions AeE, the static pressure was
2.71 Pa at position A (0 mm), 26.95 Pa at position B (10 mm),
18.66 Pa at position C (510 mm), 2.41 Pa at position D
(1490 mm), and 2.25 Pa at position E (1500 mm). The mixture
gas flowed to the preheating zone filled with the Al2O3 Raschig
ring and it collided with the solid Al2O3 Raschig ring, thus
generating high pressure. Therefore, a pressure drop was


N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 4 2 4 e4 3 3

431

Fig. 6 e Pressure and velocity of HI decomposer according to temperature changes of the heater. (A) STV ratio 0.3, (B) STV
ratio 0.4. (C) STV ratio 0.5. Heater set at 773.15 K, 873.15 K, and 973.15 K. HI, hydrogen iodide; STV, surface-to-volume ratio.
evident from the increase of the pressure value. When the
mixture gas from the preheating zone flowed to the catalyst
zone, a pressure drop was generated but the magnitude was
smaller than in the previous case.

As shown for AeE, the velocity magnitude also had no correlation with the heater temperature and STV ratio. The velocity was 0.016 m/s at position A (0 mm), 0.04 m/s at position B
(10 mm), 0.016 m/s at position C (510 mm), 0.016 m/s at position


432


N u c l E n g T e c h n o l 4 7 ( 2 0 1 5 ) 4 2 4 e4 3 3

D (1490 mm), and 0.14 m/s at position E (1500 mm). At the outlet
at position E, the velocity of the mixture gas was highest
(0.14 m/s). In the empty space (i.e., AeB), the velocity of the
mixture gas was 0.016e0.04 m/s, but it slowed down to 0.016 m/
s when the gas flowed to the BeD region that was filled with the
solid. The gas collided with the solid upon decreasing the flow
rate, and then the resulting mixture gas was well dispersed.
As shown in Fig. 6AeC, the static pressure and velocity
magnitude were independent of the heater temperature and
STV ratio, and this could be attributed to the fact that the fluid
porosity was 0.4 in all cases. Therefore, the pressure drop of the
flowing mixture gas and velocity drop were affected not by the
thermal heat and surface area involved in the catalyst reaction,
but were rather affected by fluid porosity (solid factor).
In conclusion, the CFD analysis for HI decomposition
simulation was performed by applying the actual operation
conditions and HI decomposer design. Several factors, such as
hydrogen production, reaction heat, inner temperature,
pressure, velocity, and mass distribution of the HI decomposer, were investigated.
 The hydrogen production depended on the STV ratio. In
this study, the predicted hydrogen production was
1.12 mol/h, 1.99 mol/h, and 3.10 mol/h for STV ratios of 0.3,
0.4, and 0.5, respectively. The hydrogen production at an
STV ratio of 0.5 was higher than that at 0.3 by 2 mol/h.
 For the three different STV ratios, the heat required for the HI
decomposition reaction was investigated. With an increase
in the amount of hydrogen production, the reaction heat
increased, and its value was 0.1117 kW/h, 0.1604 kW/h, and

0.2147 kW/h at STV ratios of 0.3, 0.4, and 0.5, respectively.
 The inner temperature of the HI decomposer was studied
in the form of contour images at ITC 1e5. A temperature
higher than 750 K was required to generate the reaction
stably. In addition, at ITC 2 and ITC 3 in the center of the
catalyst zone, the temperature was 773e801 K, which was
lower than that at the preheating region.
 The inner pressure and velocity of the HI decomposer were
investigated through pressure drop and reducing velocity
starting from the preheating zone. At this point, the pressure drop was approximately 24 Pa and the reducing velocity was 0.024 m/s.
For future research, the effect of fluid porosity and STV
ratio on hydrogen production/reaction heat/hydraulic thermal analysis of the HI decomposer will be studied. The findings of this research can be used as a basis for the optimal
design of the HI decomposer.

Conflicts of interest
All contributing authors declare no conflicts of interest.

Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government
(MSIP; Grant No. 53154-14).

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