A sulfur-iodine flowsheet using precipitation, electrodialysis, and membrane separation to produce hydrogen

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 6 6 0 4 e1 6 6 1 4

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A sulfur-iodine flowsheet using precipitation, electrodialysis, and membrane separation to produce hydrogen Youngjoon Shin a,*, Kiyoung Lee a, Yongwan Kim a, Jonghwa Chang a, Wonchul Cho b, Kikwang Bae b a b

Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yuseong-gu, Daejeon 305-600, Republic of Korea Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea

article info

abstract

Article history:

The preliminary flowsheet of an electrodialysis cell (EDC) and membrane reactor (MR)-

Received 4 November 2011

embedded SI cycle has been developed. The key components consisting of the preliminary

Received in revised form

flowsheet are as follows: a Bunsen reactor having a mutual separation function of sulfuric

15 February 2012

acid and hydriodic acid phases, a sulfuric acid refined column for the purification of the

Accepted 16 February 2012

sulfuric acid solution, a HIx-refined column for the purification of the hydriodic acid

Available online 16 March 2012

solution, an isothermal drum coupled to a multi-stage distillation column to concentrate the sulfuric acid solution, a sulfuric acid vaporizer, a sulfuric acid decomposer, a sulfur

Keywords:

trioxide decomposer, a sulfuric acid recombination reactor, a condensed sulfuric acid

Hydrogen production

solution and sulfur dioxide/oxygen gas mixture separator, a precipitator to recover excess

Sulfur-iodine flowsheet

iodine dissolved in the hydriodic acid solution, an electrodialysis cell to break through the

Precipitation

azeotrope of the HI/I2/H2O ternary solution, a multi-stage distillation column to generate

Electrodialysis

highly concentrated hydriodic acid vapor as a top product of the column, a membrane

Membrane separation

reactor to decompose hydrogen iodide and preferentially separate the hydrogen, and a hydrogen scrubber. The material and energy balance of each component was established based on a computer code simulation using Aspen Plus
. The thermal efficiency of the EDC and MR-embedded SI process has also been evaluated and predicted as 39.4%. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The SI cycle was initially introduced by the General Atomic (GA) Company [1] and has been extensively studied in Japan [2,3], France [4,5], and China [6]. Currently, the process is well defined as a whole according to the flowsheet studies performed by GA. As presented in Fig. 1, a combination of the following three chemical reactions leads to splitting water into oxygen and hydrogen without consuming other chemicals participating in

the process. Sulfur and iodine compounds are recycled throughout the process. - Bunsen reaction 2H2 O þ I2 þ SO2 ¼ H2 SO4 þ 2HI

(1)

- Sulfuric acid decomposition H2 SO4 ¼ H2 O þ SO2 þ 1=2O2

* Corresponding author. Tel.: þ82 42 868 2795; fax: þ82 42 868 4826. E-mail address: [email protected] (Y. Shin). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.02.082

(2)

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In the present study, a preliminary flowsheet is proposed for an electrodialysis cell (EDC) and membrane reactor (MR)embedded SI cycle. The material and energy balance in the EDC and MR-embedded SI cycle is established by a computer code simulation using Aspen Plus
, and the operation conditions of the process chemical reactors required in the EDC and MR-embedded SI cycle are introduced in order to achieve a higher thermal efficiency of the hydrogen production.

2. Review of key parameters for the EDC/MRembedded SI process

Fig. 1 e Schematic diagram of the SI cycle.

- Hydriodic acid decomposition 2HI ¼ I2 þ H2

(3)

The sulfur-iodine (SI) cycle can be divided into three sections based on the chemical reactions representing the characteristics of each section described in Fig. 1: a Bunsen reaction section (Section 1), a sulfuric acid concentration and decomposition section (Section 2), and a hydriodic acid concentration and decomposition section (Section 3). In Section 1, the two kinds of acids (H2SO4 and HI) are produced through the exothermic Bunsen reaction (Eq. (1)) with H2O, SO2 and I2. The reaction products of the Bunsen reaction are separated under excess iodine and water due to a difference in density and are sent to the decomposition sections for conversion into O2 and H2 at high temperatures, as expressed by Eqs. (2) and (3), respectively. The SI cycle requires high temperatures for the concentration of both acid streams and the decomposition of the two acids in Sections 2 and 3, which provides a high-efficiency conversion of the heat energy. The concentration of sulfuric acid should be higher than 90 wt% for the subsequent decomposition, and a distillation column or set of several flash drums is adopted to obtain the concentrated sulfuric acid from the dilute solution produced by the Bunsen reaction. The heat required in the SI cycle can be supplied through an intermediate heat exchanger (IHX) by the concentrated solar energy and the very high temperature gas-cooled reactor (VHTR). When using a high temperature energy source of 950  C generated from the VHTR, the VHTR-SI hydrogen production system consists of the nuclear reactor, primary and secondary helium cooling systems, and the SI process. The helium in the secondary cooling system acts as a medium when transferring high-temperature energy into the SI process. The temperatures of the endothermic reactions are well matched with the temperature of the helium gas coolant from the IHX, and the sensible heat of the coolant can provide the heat necessary for the whole SI cycle when the process parts are systematically integrated. Therefore, the SI cycle is suitable for a large-scale, centralized production of hydrogen, and produces no greenhouse gases from the feedstock or the energy source.

The earlier flowsheet introduced by GA in 1982 was designed to satisfy the mass flow of the HI decomposition process, which employs extractive distillation using phosphoric acid. In contrast with the GA flowsheet, the new EDC/MR-embedded SI cycle employing precipitation for the separation of excess iodine in this study requires modification not only in the stream line-up, but also in the mass and heat balance introduced by GA. Furthermore the SI cycle has been studied by several investigators [6e10] and while the process as a whole is well defined, there is some uncertainty about the best way of accomplishing the sulfuric acid and hydrogen iodide decomposition steps. Sections 2 and 3 in the SI cycle are the most crucial steps for the efficiency of the cycle as it presents the lower value. Four main difficulties in two sections must be overcome: the tube clogging problem due to the excess iodine that is absolutely required for the mutual separation of sulfuric acid phase (light phase) and hydrogen iodide phase (heavy phase) in Section 1, the presence of an azeotrope in the HIeI2eH2O mixture, which prevents efficient distillation of hydrogen iodide, the optimal operation condition at the most energy consuming parts of Sections 2 and 3, and an effective network of heat exchangers to recycle waste heat. Zhang et al. [6] mentioned that the tube clogging problem due to iodine solidification occurred at the closed cycle operation of the labscale SI process. In order to prevent this type of problem, the precipitation process of the excess iodine in the outlet from the Bunsen reactor should be effective just before introducing the hydriodic acid phase into Section 3. This means the solubility information of iodine in the pure water and HI/H2O binary mixtures is required as one of the key parameters for the successful operation of the SI process. The possibility of increasing the thermal efficiency of the SI process should also be analyzed in relation to the effect of sulfuric acid concentration on the heat demand at the sulfuric acid vaporizer and decomposer in Section 2, and also in relation to the effect of the operating pressure of the HIeI2eH2O multistage distillation column on the energy requirement at the reboiler and feeding pump in Section 3. A membrane reactor in Section 3 to decompose HI and separate H2 preferentially according to Le Chatelier’s principle enables the thermal decomposition yield of hydriodic acid to increase effectively.

2.1.

Solubility of iodine in HIeH2O binary mixtures

Although solubility information is important to prevent tube clogging due to iodine deposition inside the tube wall and to

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successfully transport HIx solution in the SI process, the experimental data of the iodine solubility in the HIeH2O binary system is scarce. Kracek in 1931 [11] summarized previous reliable values for the solubility of iodine in water in the temperature range of 0e60  C and reported his experimental data up to 200  C. Furthermore the iodine solubility in the HIeH2O binary system is absolutely required when the precipitation technology to recover excess iodine dissolved in the hydriodic acid solution is adopted in the SI process. Based on this necessity of solubility information, the Korea Atomic Energy Research Institute (KAERI) has carried out some experiments to obtain the iodine solubility data in the HIeH2O binary system [12]. The experimental variables were hydriodic acid concentrations in the binary mixture and dissolution temperature. The results are shown in Fig. 2, where I2 solubility is plotted as a function of temperature at the initial hydriodic acid concentration in HIeH2O mixture solvent. This diagram includes the solubility points measured by Kracek [11] and O’Keefe [13]. The solubility of iodine within the transportation temperature of HIx solution inside process tube or pipe is sensitive to temperature and hydriodic acid concentration. The solubility of iodine increases as the concentration of hydrogen iodide in HIeH2O mixture solvent is increased. This tendency of iodine solubility as a function of temperature supports the application of precipitation technology to recover excess iodine in the heavy phase outlet from the Bunsen reactor and the dissolution technology to recycle recovered excess iodine to the Bunsen reactor.

2.2.

Heat demand at H2SO4 vaporizer and decomposer

The effect of sulfuric acid concentration on the heat demand of the vaporizer and decomposer was analyzed. The heat demand to produce 300 mol/s of sulfur dioxide in Section 2 was calculated under an operation pressure of 7 bars as a function of the mole fraction of sulfuric acid. The concentration of sulfuric acid depends entirely on how to operate a multi-stage distillation column for the concentration of sulfuric acid solution. Even if the reboiler of the multi-stage distillation column requires thermal energy to boil the sulfuric acid solution, it is assumed that such thermal energy can be

Fig. 2 e Solubility of iodine as a function of temperature.

supplied by the sensible and condensing heat of the outlet gas and vapor stream from the sulfur trioxide decomposer keeping the highest temperature of 850  C in the SI process. In this case, the high temperature outlet stream of the sulfur trioxide composer, which is composed of SO2, O2, H2O, and residual SO3 and H2SO4, is introduced into the tube side of the reboiler, and then discharged from the reboiler at 375.9  C. The temperature of a 98 wt% sulfuric acid solution inside the reboiler is 233.8  C. In order to minimize the exergy loss of the discharged process stream, the discharged process stream is passed through the wall jacket of the isothermal flash drum to concentrate primarily a sulfuric acid solution, and then finally put into the phase separator for the separation of condensed sulfuric acid liquid phase and SO2/O2 mixed gas phase at 180  C. Fig. 3 shows the total heat demand and that of each component as a function of sulfuric acid concentration. The total heat demand is decreased from 165,000 kWth to 140,676 kWth as the sulfuric acid concentration increases until reaching a mole fraction of sulfuric acid of 0.9, which is near the azeotropic concentration of a H2SO4/H2O binary system. The difference of the total heat demand is equivalent to a 4% change of an overall hydrogen production thermal efficiency. This figure gives information on a feasible concentration at the 0.9 mol fraction of sulfuric acid.

2.3. Energy demand at HIx distillation column and feeding pump On the other hand, the effect of operating pressure of the HIx distillation column for the obtained HI vapor in Section 3 on the energy demand at the HIx distillation column and feeding pump was also analyzed. The heat demand to produce 1000 mol/s of HI vapor at the top partial condenser of the HIx distillation column in Section 3 was calculated at a total feeding rate of 27,676 mol/s, which had a mole fraction of HI/ H2O/I2 ¼ 0.14/0.63/0.23 as a function of the operating pressure of the HIx distillation column using Aspen Plus
. The HIx distillation column has 7 equilibrium stages with a partialvapor condenser and a reboiler. This production rate of HI vapor is equivalent to a hydrogen production rate of 300 mol/s Fig. 4 shows the total energy demand and that of each

Fig. 3 e Heat demand in Section 2 vs. mole fraction of sulfuric acid.

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iED ¼

Fig. 4 e Energy demand in HIx distillation system vs. operating pressure.

component as a function of the operating pressure. The heat demand at the reboiler of the HIx distillation column for the HI vapor production rate of 1000 mol/s is decreased as the operating pressure is increased. However, the energy demand of the feeding pump is increased as the operating pressure is increased. The minimum total energy demand is revealed at an operating pressure of 40 bars. This figure gives information on the feasible operating pressure at 40 bars.

2.4. Modeling of electrodialysis cell and membrane reactor One of crucial points in the SI cycle is how to break through an azeotrope in the HIeI2eH2O mixture discharged from the Bunsen reactor. Several methods to solve this problem were reported. The Japan Atomic Energy Agency (JAEA) proposed a concept of elctrodialysis for the HIx concentration [2] and the Korea Institute of Energy Research (KIER) extensively studied it [14]. On the other hand, the use of permselective membranes was considered by GA [15] and the University of Sheffield carried out the feasibility study and experimental study on membrane separations in the HIx processing section of the SI cycle [16,17]. Since an electrodialysis cell (EDC) and a membrane reactor (MR) are not standard ASPEN Plus
components, custom component models are required for inclusion in the overall process simulation of Section 3. The Aspen component separation block was applied to model the EDC cell, which has a function in which the outlet concentration can be simply identified by multiplying the inlet concentration with a given fraction value. The outlet stream is always set to 12.5 mol HI/kg H2O. The energy demand in the EDC ðHED Þ can be calculated by Eq. (4). HED

iED ,VED ¼ he ,n

DHIcathode  F tþ

(5)

where DHIcathode is the difference in the inlet and outlet HI mole flow rates (mol/sec) in the cathode side, and F is the Faraday constant (C/mol). The dimensionless transportation number of protons (tþ) is assumed to be 1 according to the experimental data [14]. Hwang et al. in 2003 [18] measured experimentally the dimensionless transportation number and electroosmosis coefficient of the cation exchange membrane used in the ECD. The MR model using a stoichiometric reactor block, a separation block, and a valve block was prepared for the simulation. The stoichiometric reactor is used to set the HI decomposition. The separation block is used to specify the separated materials expected by the membrane. The valve block is used to set the product-side pressure of the membrane reactor. This MR model in the Aspen Plus
code requires only the separation factor and decomposition yield of hydriodic acid. These parameters are given by experimental values. The applied values in the simulation are 60% of the decomposition yield and 99.99% of the separation factor. In order to design the MR and propose its permeance characteristics to satisfy the Aspen Plus
simulation results of the MR model, the membrane reactor was simulated using the plug flow model and the inlet and outlet boundary conditions obtained from the Aspen Plus
simulation. Fig. 5 shows a schematic diagram of a catalytic membrane reactor model. The reactor consists of a reaction zone and a permeation zone, and HI over the catalyst converts into I2 and H2 in the reaction zone. The decomposition rate of HI(A) per volume, rA, is given by rA ¼ kPr RHI

(6)

where k is the rate constant, Pr is the pressure in the reaction zone and, RHI represents the arbitrary rate expression determined experimentally. Hwang et al. in 2001 reported the following equations [19]. 
1 k ¼ 0:158exp  34; 310 Jmol =RT RHI


 1 KD ¼ 5:086  1011 exp 86; 660 Jmol =RT

VED is the potential of the EDC (V), he is the heat-to-electricity conversion efficiency and represents 0.48 at the Brayton cycle, and n is the alternating current to direct current converting efficiency, and is usually given by 95%. The electric current of the electrodialysis cell (iED ) can be calculated by Eq. (5).

(8) (9)

where 42 is the equilibrium conversion value, KD is the equilibrium constant of the HI decomposition, Pe,i is the Fi = Pei(PrxA – PpyA)/tM

Permeation zone

Pp N i,L+dL

N i,L (4)

(7)

n ¼ XA =ð1 þ KD Pr XA Þ  ðXC þ XD Þ1=2 ð1 þ KD Pr 4e =2Þ= KP ð1 o þ KD Pr XD Þ2

Pr

Reaction zone

L

dL

Fig. 5 e Schematic diagram of the catalytic membrane reactor.

permeability constant of component i per membrane surface area, and xi is the mole fraction of component i in reaction zone. The production rates of H2(C) and I2(D) are given stoichiometrically by the following: rC ¼ 0:5kPr RHI

(10)

rD ¼ 0:5kPr RHI

(11)

The permeation rate of each component through the membrane is assumed to obey Fick’s law. Afterward:
 Fi ¼ Pei Pr xi  Pp yi tM

(12)

where Pp is the total pressure in the permeation zone, yi is the mole fraction of component i in the permeation zone, and tM is the thickness of the membrane. The components flow through the reaction and permeation zone in co-current flow mode. Assuming an isothermal operation, plug flow, no pressure drop, constant permselectivity, and no reaction in the permeation zone of the reactor, the basic material balance for each component by referring to the elementary section of the reactor as shown in Fig. 5, can be expressed as simultaneous ordinary differential equations. Ni and Fi are the mole flow rates of each component in the reaction and permeation zones, respectively. For HIðAÞ :
 dNA ¼ Sr  kPr RHI  Cr  PeA Pr xA  Pp yA dL

(13)


 dFA ¼ Cr  PeA Pr xA  Pp yA dL

(14)

For H2 ðCÞ :
 dNC 1 ¼ Sr  kPr RHI  Cr  Pec Pr xc  Pp yc dL 2

(15)


 dFc ¼ Cr  Pec Pr xc  Pp yc dL

(16)

Mole flow rate in permeation zone [mol/sec]

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2500

2000

H 2O

1500

1000

500

H2 0 0

1

2

3

4

5

6

7

8

9

Tube Length [m]

Fig. 6 e Mole flow rates of hydrogen and steam in the permeation zone along the column length.

Korea Institute of Energy Research (KIER) team, who is KAERI’s partner to develop the SI thermochemical hydrogen production process, studied the manufacturing of ceramic membrane tube with the tube diameter of 10 mm to apply to the HI thermal decomposition part in 2009. From the model calculation results of Figs. 6 and 7, it is determined that 2000 membrane reactor tubes with a length of 8.36 m is theoretically required to achieve a hydrogen production rate of 300 mol/s. Figs. 6 and 7 shows the mole flow rate of each component along the tube length. At the same time, a diminishing rate of HI was found in this zone. As simulation results, it was found that the permeances (mol Pa1 m2 s1) of I2, H2O, and H2 based on the permeance of HI (2.0  1010) were 3.0  1010, 6.0  106, and 4.0  103, respectively. These values can be used as the manufacturing specification of the ceramic membrane for the MR.

3.

Process simulation and results

The preliminary process flowsheet for a hydrogen production rate of 300 mol/s was woven through a Aspen Plus
code simulation. As a result of the simulation, the preliminary SI

For I2 ðDÞ :
 dND 1 ¼ Sr  kPr RHI  Cr  PeD Pr xD  Pp yD dL 2

(17)


 dFD ¼ Cr  PeD Pr xD  Pp yD dL

(18)

2500

For H2 OðIÞ :
 dNI ¼ Cr  PeI Pr xI  Pp yI dL

(19)


 dFI ¼ Cr  PeI Pr xI  Pp yI dL

(20)

Sr is the cross-section area in the reaction zone, and Cr is the surface area of the membrane. When it was assumed that the diameter of the reactor tube was 10 mm, the number of tubes and tube length were determined by the model calculation using Eqs. (13)e(20).

Mole flow rate in reaction zone [mol/sec]

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2000

H 2O

1500

1000

HI

500

I2

0 0

1

2

3

4

5

6

7

8

Tube Length [m]

Fig. 7 e Mole flow rate of each component in the reaction zone.

9

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Fig. 8 e Preliminary flowsheet of the EDC/MR-embedded SI process.

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Table 1 e Specific parameters of the EDC/MR-embedded SI process. Hydrogen production rate Bunsen reactor outlet temperature SO3 decomposer outlet temperature HI decomposer outlet temperature Operating pressure of Bunsen reactor Operating pressure of sulfuric acid concentration system Operating pressure of sulfuric acid decomposition system Operating pressure of EDC Operating pressure of HI distillation column and decomposer Power conversion system efficiency (he)

300 mol H2/s 120  C 850  C 450  C 3.0 bar 0.1 bar 7.0 bar 1.0 bar 40.0 bar 0.48

flowsheet using precipitation, electrodialysis, and membrane separation has been developed and presented in Fig. 8. There are four chemical reactors in Section 1. The role of a Bunsen reactor is not only the Bunsen reaction of Eq. (1) but also a mutual separation of the immiscible sulfuric acid and hydrogen iodide phases generated from the Bunsen reaction. The HIx-refined column removes the residual sulfuric acid from the separated hydrogen iodide phase. Finally two oxygen scrubbers recover the sulfur dioxide from the oxygen/sulfur dioxide mixture before venting oxygen. These reactors do not need to obtain thermal energy from an external thermal source. Section 2 consists of a sulfuric acid refined column, an isothermal flash drum, a sulfuric acid distillation column, a sulfuric acid vaporizer, a sulfuric acid decomposer, a sulfur trioxide decomposer, and a phase separator for the separation of condensed sulfuric acid liquid phase and SO2/O2 mixed gas phase. Most of the chemical reactors in this section need

a heat supply from an external heat source. When an array of these reactors is determined by the operational temperature order, the high-temperature helium use to heat these reactors has to go through the sulfur trioxide and sulfuric acid decomposers, and the vaporizer in the series. The cooled helium vented from the vaporizer is recycled to the intermediate heat exchanger to receive thermal energy from an external heat source, such as solar or nuclear energy. The sulfuric acid distillation column and isothermal flash drum that operate at a lower temperature can be heated by the sensible and latent heat of the process gas, as shown in Fig. 8. Section 3 has a HIx solution distillation column for an additional concentration of the HIx solution, discharged from the electrodialysis equipment, which preliminarily concentrates the HIx solution by using the cationic membrane in the electric field. The outlet hydriodic acid solution from the Bunsen reactor has to be introduced into an iodine precipitator to recover excess iodine, and then puts it into the cathode chamber of the electrodialysis cell. Two iodine precipitators are installed in parallel in Section 3 for the continuous operation of the SI process. In this case, one is operated for the removal of excess iodine in the HIx solution discharged from the HIx-refined column. The other is simultaneously operated for the dissolution of the precipitated iodine particles with the anolyte discharged from the anode chamber of the EDC, and the I2-dissolved anolyte is then recycled to the Bunsen reactor. These functions of the two precipitators are regularly interchanged with each other. A membrane reactor, which is one of the key pieces of equipment in Section 3, has two functions: the catalysis decomposition of HI and the preferential separation of hydrogen from the decomposed gas mixture of H2/I2/HI/H2O. The membrane reactor is heated by the circulated helium discharged from the sulfuric acid decomposer. The HIx distillation column is heated by the sensible and latent heat of

Table 2 e Calculated stream conditions in Section 1. Stream no.

101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120

Molar flow rate (mol/s) H2SO4

HI

I2

H2O

SO2

O2

N2

Total

309.85 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 9.79 9.79 9.83 0.00 309.85 0.00 0.00 0.00 0.00 0.00

0.00 3737.37 3737.70 0.00 0.00 3737.70 0.00 0.15 0.00 0.00 0.00 0.15 0.15 3137.22 0.00 0.00 3137.22 0.15 0.00 0.00

5.47 14,204.52 14,203.26 1.09 1.09 14,203.26 0.76 0.78 0.12 0.00 0.12 0.90 0.90 14,508.74 5.47 5.47 14,503.28 0.15 0.00 5.47

1252.75 19,550.27 19,548.17 2.38 63.61 19,548.17 3.41 302.71 0.56 0.00 969.61 1272.32 1781.65 19,561.16 1252.75 13.14 19,548.02 299.87 0.00 13.14

14.38 14.38 0.00 14.38 314.19 0.00 14.38 12.00 2.38 0.00 3.56 15.56 15.75 13.20 14.38 13.20 0.00 0.00 0.00 13.20

4.69 0.00 0.00 0.00 150.00 0.00 150.00 0.00 150.00 150.00 0.00 0.00 0.00 4.69 4.69 4.69 0.00 0.00 0.00 4.69

0.00 0.00 0.00 10.00 10.00 0.00 10.00 0.00 10.00 10.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 10.00 0.00

1587.13 37,506.53 37,489.13 27.85 538.90 37,489.13 178.54 315.64 163.06 160.00 983.08 1298.72 1808.28 37,225.01 1587.13 36.49 37,188.51 300.16 10.00 36.49

Pressure (bar)

Temp ( C)

3 3 3 3 3 1.0 3 3 3 3 3 3 3 3 4.3 4.3 3 3 3 3

120 120 120 120 120 119.9 120 120 120 120 120 120 120 120 120 170 120 25.4 120 150

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Table 3 e Calculated stream conditions in Section 2. Stream no.

201 202 203 204 205 206 207 208 209 210 211 212 213 214 215

Molar flow rate (mol/s) H2O

H2SO4

SO3

SO2

O2

Total

1239.61 270.56 563.49 54.16 54.16 341.24 502.93 504.78 354.16 292.93 61.23 292.93 969.05 509.33 969.05

309.85 300.06 451.35 451.31 451.31 164.22 2.53 0.68 151.30 151.29 0.01 151.29 9.79 0.04 9.79

0.00 0.00 0.00 0.00 0.00 287.09 239.58 150.62 0.00 0.00 0.00 0.00 0.00 0.00 0.00

1.18 0.00 0.19 0.00 0.00 0.00 209.19 300.00 300.00 0.19 299.81 0.19 1.18 0.19 1.18

0.00 0.00 0.00 0.00 0.00 0.00 104.60 150.00 150.00 0.00 150.00 0.00 0.00 0.00 0.00

1550.64 570.61 1015.02 505.46 505.46 792.55 1058.83 1106.08 955.46 444.41 511.05 444.41 980.02 509.56 980.02

the hydrogen-eliminated gas mixture discharged from the membrane reactor. In this step, a heat pump must be used to maximize the heat utilities. Table 1 shows the specific parameters used to evaluate the EDC/MR-embedded SI process capable of producing 300 mol$H2/s. Based on these specific parameters, the calculated stream conditions at different points in Fig. 8 are shown in Tables 2, 3 and 4 in each section. In order to maximize the thermal efficiency of the hydrogen production process, analyses in a variety of a network of heat exchangers were performed by the Aspen Plus
code simulation using these stream conditions. The energy balance of this preliminary SI process flowsheet for the hydrogen production rate of 300 mol/s was

Pressure (bar)

Temp ( C)

4.3 0.1 0.1 0.1 7.0 7.0 7.0 7.0 7.0 7.0 7.0 0.1 0.1 0.1 0.1

170.0 170.0 158.1 233.4 233.8 467.0 750.0 850.0 180.0 180.0 180.0 179.9 170.0 158.1 27.9

calculated through a Aspen Plus
code simulation and an additional series of simulations was carried out to investigate the maximum possible efficiency when the optimized heat network was employed. The sulfuric acid vaporizer (SA Evaporator), sulfuric acid decomposer (SA Decomposer-1), and sulfur trioxide decomposer (SA Decomposer-2) in Section 2, and the heat exchanger (HE301) for preheating the inlet stream of the HI membrane reactor and HI membrane reactor (MR) in Section 3, need a heat supply from an external heat source such as the secondary helium in the VHTR system. When an array of these reactors is determined by the operational temperature order, the high temperature helium to heat these reactors has to go through SA Decomposer-2 and SA Decomposer-1 in series, and the helium flow is then

Table 4 e Calculated stream conditions in Section 3. Stream no.

301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 315A 316 317 318 319

Molar flow rate (mol/s) H2O

I2

HI

H2

Total

16870.34 17569.66 17569.66 17569.66 16275.60 16275.60 1294.06 1294.06 1293.91 1293.91 17569.51 17569.51 16870.19 19548.02 0.15 0.15 300.00 300.00 0.28 299.87

6577.27 6227.61 6227.61 6227.61 5992.70 5992.70 234.91 234.91 534.93 534.93 6527.62 6527.62 6877.28 14503.28 0.15 0.15 0 0 0.00 0.15

3179.19 3878.51 3878.51 3878.51 2900.51 2900.51 978.00 978.00 377.52 377.52 3278.03 3278.03 2578.71 3137.22 0.15 0.15 0 0 0.00 0.15

0 0 0 0 0 0 0 0 0 0 0 0 0 0 300.17 300.17 0 0 300.01 0.16

26626.80 27675.77 27675.77 27675.77 25168.81 25168.81 2506.97 2506.97 2206.36 2206.36 27375.16 27375.16 26326.18 37188.52 300.62 300.62 300.00 300.00 300.29 300.33

Pressure (bar)

Temp ( C)

1.0 1.0 40.0 40.0 40.0 40.0 40.0 40.0 39.5 39.5 39.5 1.0 1.0 1.0 38.0 38.0 1.0 40.0 38.0 38.0

25.0 100.0 103.4 274.4 285.3 108.2 274.4 450.0 295.3 180.0 115.5 114.1 100.0 100.0 25.0 450.1 25.0 25.4 25.5 25.2

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Table 5 e Summary of energy requirements. Unit no. HE301 MR R301 HE302

Heat pump P301 P302 T301 EDC P201 Diffuser HEATER-1 S201 R201 SA evaporator SA decomposer-1 SA decomposer-2 HE202 P101 P102 T101 T102 T103 Total energy requirements

Description Heat exchanger for preheating the inlet stream of MR HI membrane reactor HI distillation column reboiler primary tube side: HE302 secondary heat transfer from HE302 using heat pump Primary shell side: R301 Secondary heat transfer to R301 using heat pump Tertiary shell side: HEATER-1 Fourth shell side: S201 For heat transfer from the outlet stream of MR to R301 Pump for feeding a HIx solution to HI distillation column Pump for feeding water to H2 scrubber Turbine Electrodialysis cell at 0.112 V and electroosmosis coefficient of 1. Pump for feeding a 98 wt% H2SO4 solution to SA evaporator Turbine Sulfuric acid refined column tube side: HE302 Isothermal flash for H2SO4 concentration primary tube side: HE302 secondary tube side: HE202 H2SO4 distillation column reboiler tube side: HE202 Sulfuric acid vaporizer H2SO4 thermal decomposer SO3 thermal decomposer Primary shell side: R201 Secondary shell side: S201 Pump for feeding a H2SO4 solution to H2SO4 refined column Pump for recycling a HIx solution to Bunsen reactor Turbine Turbine Turbine

divided into two ways. One is for heating the SA Evaporator and the other is for heating MR and HE301 in series. The cooled helium vented from the SA Evaporator and HE301 is recycled to the intermediate heat exchanger to receive thermal energy from the VHTR. On the other hand, the sulfuric acid distillation column reboiler (R201) and isothermal flash for H2SO4 concentration (S201) in Section 2 can be heated by the sensible and latent heats of the outlet stream of SA Decomposer-2. Section 3 has an HI distillation column for an additional concentration of the HIx solution discharged from the electrodialysis cell (EDC), which is used to preliminarily concentrate the HIx solution using the cationic membrane in the electric field. The HI distillation

h¼

Thermal energy (kWt) 16,749.00 3790.10 152,937.61 39,465.60 113,472.01 15,709.80 5387.10

17,454.96 4096.15 28.17 2594.00 7954.83 109.64 28.36 15,709.80 47,992.20 37,299.90 63,181.50 43,725.50 33,769.00 37,299.90 42,605.10

161,215.10 kWt

7.13 363.98 193.26 27.32 13.30 27,158.62 kWe

utilities. The outlet stream, which donates its sensible heat for heating R301, still has enough thermal energy to heat the sulfuric acid refined column (HEATER-1) and to additionally heat S201. A summary of the energy requirements in this flowsheet are shown in Table 5. In Table 5, the positive sign indicates energy received from the surroundings or electricity consumption, while the negative sign represents energy donated to the surroundings or electricity generation. The thermal efficiency of hydrogen production (h) for this network of heat exchangers, based on the high heating value (HHV) of the produced hydrogen, can be calculated by Eq. (21), and the efficiency value of 39.4% is anticipated in the EDC and MR-embedded SI cycle.

HHV  H2 Production Rate Thermal Energy Requirement þ Electric Energy Requirement=he

column reboiler (R301) is heated by the sensible heat of the outlet stream of the MR. R301 has a two-stage heating method. The primary heating is carried out using a Kettletype floating-head reboiler, while the secondary heating adopts the application of a heat pump to maximize the heat

Electric energy (kWe)

(21)

Figs. 9 and 10 show the effect of the EDC operating potential and H2SO4 vaporizer feed concentration on the thermal efficiency of hydrogen production, respectively. From these results, it is anticipated that the EDC operating potential has crucial influence on the thermal efficiency of hydrogen production.

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by the sensible and latent heat of the SI process stream to achieve the highest thermal efficiency. Additional studies are being planned to investigate how to effectively start up the process, taking into account the thermal energy and material pathways.

Acknowledgment This study was performed under the mid- and long-term Nuclear R&D Projects supported by the Ministry of Education, Science and Technology, Republic of Korea.

Nomenclature Fig. 9 e Effect of electrodialysis cell potential on hydrogen production thermal efficiency.

Cr HED DHI F tþ iED KD Pe,i Pp Pr R Sr T tM VED xi yi

Fig. 10 e Effect of H2SO4 vaporizer feed concentration on hydrogen production thermal efficiency.

4.

Conclusions

The preliminary flowsheet of an electrodialysis cell(EDC)/ membrane reactor(MR)-embedded SI cycle was developed. The material and energy balance in the EDC and MRembedded SI cycle was calculated by a computer code simulation using Aspen Plus
. The primary conclusions from this study are as follows: - The thermal efficiency of hydrogen production of up to 39.4% can be expected based on the EDC and MR-embedded SI process. - The secondary helium from the IHX flows through the SO3 and H2SO4 decomposers, and is then split in two ways, to the H2SO4 vaporizer and to the HI decomposer. - The thermal energy required at the HI distillation column, the sulfuric acid refined column, the isothermal flash drum, and the sulfuric acid distillation column has to be supplied

surface area of membrane, m2 energy demand in EDC, W difference in inlet and outlet HI mole flow rates of EDC cathode chamber, mol/sec Faraday constant, 96,500 C/mol dimensionless transportation number of protons electric current of EDC, A equilibrium constant of HI decomposition permeability constant of component i, mol/(m2 Pa s) total pressure in permeation zone of MR, Pa total pressure in reaction zone of MR, Pa Gas constant, 8.314 J/(mol K) cross-section area in reaction zone of MR, m2 temperature, K thickness of membrane, m electric potential of EDC, V mole fraction of component i in reaction zone of MR mole fraction of component i in permeation zone of MR

Greek letters h hydrogen production thermal efficiency power conversion system efficiency he n alternating current to direct current converting efficiency equilibrium conversion value 4e Abbreviations EDC electrodialysis cell HHV high heating value IHX intermediate heat exchanger MR HI membrane reactor SI sulfur-iodine VHTR very high temperature gas-cooled reactor

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