pradeep sow paper.pdf

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Effect of asymmetric variation of operating parameters on EED

cell for HI concentration in IeS cycle for hydrogen production

Pradeep Kumar Sow, Anupam Shukla*

Department of Chemical Engineering, Indian Institute of Technology, Delhi 110016, India

a r t i c l e i n f o

Article history:Received 11 May 2012

Received in revised form

17 July 2012

Accepted 18 July 2012

Available online 11 August 2012

Keywords:

Electro-electrodialysis

IeS cycle

Open circuit voltage

a b s t r a c t

EED process for HI concentration was studied for the effect of individual operatingparameters such as I2/HI ratio, concentration of HIxHI=H2O, temperature and pressure.

Studies were conducted in an asymmetric system where the effects of operating param-

eters were varied for anolyte and the catholyte separately. Open circuit voltage (OCV) was

found to be a contributor toward the net potential drop across the EED cell. Ohmic resis-

tance was found to decrease with increase in I2/HI ratio in catholyte and was found to

increase with increase in I2/HI ratio in anolyte. Increase in xHI=H2 O decreased the resistance

for anolyte section whereas caused an increase in resistance for catholyte section. Increase

in temperature reduced the voltage drop and the resistance across the EED cell. A non-zero

differential pressure between the two compartments of the cell increased the resistance

across the cell without affecting the OCV value. Electrode potential studies at the graphite

electrodes showed an increase in the electro potential with increase in the iodine

concentration and decrease with the increase in the HI concentration. Energy required for

concentrating acid increased linearly with current density favoring operation at lowcurrent densities. Energy consumed in overcoming OCV contributed substantial fraction of

the total energy consumed in EED process at lower current densities.

Copyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

At present worlds energy architecture is heavily dependent

on the nonrenewable sources and mostly on the fossil fuel

reserves. Fossil fuels suffer from few other problems mostnotably carbon dioxide emissions [1]. To address the growing

energy need, search for alternative energy sources as

well as energy carriers has accelerated in past few decades

[2,3]. Other energy sources being explored such as wind,

hydro etc. still constitute a very limited fraction in the total

energy generated, which does not lead us to a large scale

energy solution [4,5]. One of the most widely researched

energy carriers is hydrogen owing to the higher energy density

per unit mass and environmentally benign combustion

product [5e8].

Most of the hydrogen requirement of the present time is

met by the steam reforming process that suffers from a major

drawback of CO2 as a byproduct [5,9]. Proposed alternativesinvolve hydrogen production from water by using a variety of

processes like water splitting process, fermentation of

biomass, from bio-ethanol etc. [6e12]. Large scale hydrogen

production by closed loop thermo-chemical cycles has

attracted a lot of attention owing to their lower heat demand

compared to direct thermal decomposition of water [13e15].

Thermo-chemical cycles essentially constitute the application

of both heat and chemicals for breaking down water into

* Corresponding author. Tel.: 91 11 26596290; fax: 91 11 26581120.E-mail address: [email protected] (A. Shukla).

Available online at www.sciencedirect.com

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / h e

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 3 9 5 8 e1 3 9 7 0

0360-3199/$ e see front matter Copyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.ijhydene.2012.07.068
mailto:[email protected]://www.sciencedirect.com/science/journal/03603199http://www.elsevier.com/locate/hehttp://dx.doi.org/10.1016/j.ijhydene.2012.07.068http://dx.doi.org/10.1016/j.ijhydene.2012.07.068http://dx.doi.org/10.1016/j.ijhydene.2012.07.068http://dx.doi.org/10.1016/j.ijhydene.2012.07.068http://dx.doi.org/10.1016/j.ijhydene.2012.07.068http://dx.doi.org/10.1016/j.ijhydene.2012.07.068http://www.elsevier.com/locate/hehttp://www.sciencedirect.com/science/journal/03603199mailto:[email protected]


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hydrogen and oxygen at temperatures much lower than the

direct decomposition of water. The high temperature needed

for the process can be supplied by nuclear energy from high

temperature gas cooled reactor. More than 200 thermo-

chemical processes have been proposed till date. Iodi-

neeSulfur (IeS) cycle, proposed by General Atomics (GA) is

found to be one of the most energy efficient [16]. IeS process

essentially consists of three major reactions which are the

Bunsen reaction, Hydroiodic acid decomposition and sulfuric

acid decomposition [13].

Bunsen reaction : I2 SO2g 2H2O!2HIaq: H2SO4aq:

(1)

Sulphuric acid decomposition : H2SO4aq:! H2Ol

SO2g

12

O2 (2)

Hydroiodic acid decomposition : 2HIaq:!H2 I2 (3)

In the above reaction scheme, iodine and SO2 act as recy-

cling agents and are recycled back from decomposition

sections to Bunsen reactor in a continuous operation. In thetraditional Bunsen reaction step of the IeS cycle, excess

amount of iodine and water is used for facilitating the phase

separation between the two acid phases and making the

reaction spontaneous, respectively [13,16]. The traditional IeS

cycle has lower efficiency compared to the theoretically

calculated value primarily due to downstream problems

created by these excess of reactants [17,18]. Although highest

fraction of total heat required is consumed in the sulfuric acid

decomposition section, it is generally believed that theres

little room for further optimization of that section [19]. A

significant amount of energy is also consumed in the

concentration of HIx solution coming out of Bunsen reactor.

The HIx from the Bunsen reaction stage is a pseudo-azeotropicmixture thereby increasing the heat requirement substan-

tially to affect the concentration process [17,20]. Corrosive

nature of the HIx solution further increases technical diffi-

culties especially material selection. One of the major tech-

nical challenges for efficient IeS process is an energy efficient

process for separation and concentration of hydroiodic acid

coming out of the Bunsen reaction.

Following process developments have been suggested in

literature to concentrate the HIx solution from the Bunsen

reaction:

a) Extractive distillation using phosphoric acid as the

extracting agent [13].

b) RheinischeWestfalische Technische Hochschule (RWTH),

suggested the feasibility of concentrating HIx solution

using reactive distillation under pressurized condition [18].

c) Electro-electrodialysis (EED) using ion exchange

membranes [21].

The EED process for concentration of HIx suggested by

Onuki et al. [21] was found effective in concentrating the HIxfor the HI decomposition step. The energy consumption was

found to be lower than the other alternatives described above.

EED process consists of a two-compartment cell separated by

an ion exchange membrane which performs the function of

a selective barrier between the HI-concentrated and the HI-

depleted electrolytes allowing only the selective passage of

hydronium ions. Reversible redox reaction at the anode and

cathode [Eqn (4)] involves conversion of iodine to iodide ion

and vice versa. Cation exchange membrane was preferred

over anion exchange membrane owing to lower resistance to

ion transport and lower energy consumption [21].

I2 2e%

cathode

anode2I (4)

Studies on EED cell also suggested that the resistance of theEED cell increases with increase in the iodine content of the

HIx feed solution in a symmetric system (identical anode and

cathode compartments and identical initial concentration of

anolyte and catholyte) [22]. Increase in the resistance was

ascribed to thereducedmobility of the iodide ion which forms

poly-iodide complex with iodine. Studies on the effect of

temperature as an independent operating variable on the EED

process were also reported in literature [23,24]. Increase in

temperature decreased both the transport number of the

protons through the membrane and the ohmic resistance of

the cell. The decrease in the resistance was due to the

increased thermal energy and hence increased the mobility of

the ions in the solution. Studies also suggested that themembrane potential drop decreases (w80%) as the operating

temperature is increased from 293 to 373 K [24]. A study was

carried out to determine the effect of graphite electrodes of

different BET surface area (1400 and 700 m2/gm) and different

I2/HI ratio [25]. It was found that the electrode with higher BET

surface area was more energy efficient in concentrating HIxsolution. Tanaka et al. studied the use of radiation grafted

membrane for the EED process based concentration of HIxsolution [26]. It was shown in the above mentioned studies

that the radiation grafted membrane offered lower resistance

as compared to Nafion 117 membrane.

Following information about the EED system is not avail-

able in the literature:

Nomenclature

a, b Tafel constants

Eeq/a equilibrium potentials at anode

Eeq/c equilibrium potentials at cathode

Ej potential drop across membrane

I2/HI ratio ratio of molarities of I2 to HI

Rsm combined solution and membrane resistance

OCV open circuit voltage

Vcell cell voltage drop

xHI=H2O mole fraction of HI on iodine free basis

ha anode reaction overpotential

hc cathode reaction overpotential

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 3 9 5 8 e1 3 9 7 0 13959
http://dx.doi.org/10.1016/j.ijhydene.2012.07.068http://dx.doi.org/10.1016/j.ijhydene.2012.07.068http://dx.doi.org/10.1016/j.ijhydene.2012.07.068http://dx.doi.org/10.1016/j.ijhydene.2012.07.068


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a) Studies were done in a system where the initial concen-

tration of anolyte and the catholyte sections was same. In

actual continuous process in conjugation with other IeS

cycle components, the steady state concentration of HI in

the catholyte section of EED would be higher

xHI=H2O > 0:157 than the anolyte section.

b) Studies on effect of I2/HI ratio on cell performance were

carried out by simultaneously varying the ratio in both theanolyte and catholyte sections. Thus, the effect of I2/HI

ratio of each of the anolyte and the catholyte section is not

known independently.

The presentwork focuses on the effects of the independent

variables on cell voltage where the physical parameters of

anolyte and catholyte were varied separately as shown in

Table 1. The concentration of HI in the catholyte chamber

xHI=H2 Ow0:185 was kept higher than the azeotropic concen-

tration xHI=H2Ow0:157 thus augmenting the information

available for the system in literature. In addition, effect of

pressure of the anolyte and the catholyte sections is also re-

ported. Effect of the electrolyte flow rate was studied sepa-rately for determining the mass transport resistance in terms

of cell voltage drop. Studies on the equilibrium potential at the

electrodes were done separately to determine the effect of

concentration of electroactive components on the OCV value

generated.

2. Theory

EED process for concentration of hydroiodic acid consumes

electrical energy. Optimizing electrical energy consumed in

EED is necessary to improve the overall performance of the

IeS process. Energy consumption depends upon the voltage

drop across the cell and the current efficiency. The cell voltage

drop (Vcell) is due to (a) the total ohmic resistance offered by

the solution and the membrane (Rohm), (b) overpotential at the

electrodes, and (c) the open circuit voltage (OCV) value and (d)

voltage drop required to overcome the mass transfer or the

diffusion resistance (Vmt) to the transport of the reactants

from the bulk to the electrode surface.

Vcell can be written as

Vcell ha hc Vohm Vmt OCV (5)

where ha and hc arethe overpotentialat the anode andcathode

respectively, Vohm is the ohmic potential drop, Vmt is the mass

transfer potential drop and OCV is the open circuit potential.

Vohm along with Vmt varies linearly with current [27].

Vohm Vmt iRohm Rmt iRsm (6)

The electrode overpotential at higher current density (such

that the reverse reaction at the electrode is negligible) can berepresented using Tafel equation [27]. Combined Tafel equa-

tion for both the electrodes can be expressed as

hnet hahc aaba lniacbc lniab lni (7)

where a is sum of ac and aa and b is sum of bc and ba. OCV

consists of three different potentials [27,28].

OCV Eeq=a Eeq=c Ej (8)

where Eeq/a and Eeq/c are the equilibrium potential at the

anode and cathode respectively and Ej represents potential

difference across the membrane. Studies conducted by

Tanaka and Onuki [28] on the equilibrium potential for the

EED cell suggested it to be independent of the current density.

Empirical relationship developed is given by

OCV 4:7 106Texp

1:6 103=T ln

0BBBBB@

xHI;AxHI;C

2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

xI2 ;AxI2;C

s1CCCCCA (9)

Using Eqns. (6) and (7), Eqn. (5) can be written as:

Vcell Da Dblni i Rsm OCV (10)

3. Experimental

Two different sets of experiments were done. The first set

involved experiments on an EED cell while the second set of

experiments were done to determine behavior of electrodes in

contact with their respective electrolytes.

3.1. Materials and instrumentation

Electrolyte solutions were prepared using 55 wt% hydroiodic

acid (AR grade) supplied by CDH Pvt. Ltd., and iodine

Table 1 e Different independent operating variables varied in the EED experiments. Table showing the values of theoperating variables under three headers of low mid and high. Values of other parameters were: I2/HI ratio anolyte: 0.73, I2/HI ratio catholyte: 0.43, xHI=H2O anolyte: 0.112, xHI=H2O, catholyte: 0.183, temperature:293 K, anolyte pressure:1 bar, catholytepressure: 1 bar, differential pressure: 0 bar.

Levels Independent operating variable varied

I2/HI ratioanolyte

I2/HI ratiocatholyte

xHI=H2Oanolyte

xHI=H2Ocatholyte

Temperature(K)

Anolyte pressure(bar)/Differential

pressure (Pano Pcatho) (bar)

Catholyte pressure(bar)/Differential pressure

(Pano Pcatho) (bar)

Low 0.56 0.26 0.08 0.145 293 1 (0) 1 (0)

Mid 0.73 0.33 0.093 0.16 308 1.5 (0.5) 1.5 (0.5)

323

High 0.92 0.43 0.112 0.183 341 2 (1) 2 (1)



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(AR grade) supplied by Fischer Chemicals Ltd. without any

further purification. Nafion 117 (supplied by Electrochem Inc.,

USA) was used in the EED cell as a separator between the

compartments. Flexible graphite sheets supplied by ONS

Engineers & Consultants India Ltd., Mumbai were used as the

electrodes. Sodium thiosulphate and sodium hydroxide

(AR grade) procured from Fischer Chemicals Ltd. were used

without further purification for determination of hydroiodicacid and iodine concentration. An autotitrator (make: Mettler

Toledo, model: DL-15) was used for the estimation of hydro-

iodic acid concentration and iodometric titration was used for

the determination of the iodine content in the sample. A

potentiostat (make: Gamry model: reference 600) was used for

the measurement of electrode potentials. A d.c. power supply

(Instek GPS-2303) was used for the EED experiments.

3.2. Electro-electrodialysis experiment

Experimental setup shown in Fig. 1 can be divided into three

major sections, namely the anolyte loop, the catholyte loop

and the two-compartment EED cell. Two leak-free (Durion

pressure plus) bottles were used as reservoirs for the anolyte

and thecatholyte. Thereservoirs were mounted on a hot plate

magnetic stirrer for providing necessary heat to increase the

reservoir temperature and mixing. Electrolyte solutions from

the reservoir were pumped to the EED cell through silicon

tubing using variable flow peristaltic pumps (Miclins PP 30 EX).

Nitrogen was used for pressurizing the anolyte and the cath-

olyte sections by monitoring pressure with the help of pres-

sure gauges connected to each reservoir. Digital temperature

sensors were used to continuously monitor the electrolytes

temperature.

The two-compartment EEDcell wasfabricated using Teflon

and the compartments were separated by Nafion 117

membrane. The membrane was supported with two neoprene

rubber gaskets to prevent the electrolyte leakage from the EED

cell under pressure. Each of the Teflon compartment housed

a graphite electrode. The graphite electrodes were ultra-

sonicated and dried in air oven for 10 h before use. Baffles

made of Teflon were placed inside both compartments of thecell to improve the flow of the electrolyte through the cell and

avoid channeling or dead zones. Active area of the cell was

10 cm2.

Before the start of the experiment, the anolyte and the

catholyte reservoirs were filled with aqueous HIx solution and

all the tubings connecting reservoir to the cell were purged

with nitrogen for 10 min. After that the purge lines were

closed and the pressures of the reservoirs were adjusted to the

required values. The heaters and stirrers were switched for

raising the electrolyte solution to desired temperature. Both

the electrolytes were circulated continuously in their respec-

tive loops (reservoir to their compartment of the cell and back

to reservoir)using peristaltic pumps. After the system reacheda steady state (constant temperature), the voltage drop across

the cell was noted as the open circuit voltage (OCV). There-

after, electro-electrodialysis was started by applying

a constant current to the EED cell. Current density was varied

from 0.002 to 0.275 A/cm2 and the corresponding steady state

cell voltage was recorded to obtain the currentevoltage (IeV)

curve. Each experiment was repeated thrice and the average

values of the resulting data have been reported.

Table 1 shows the values of the various parameters that

were used for the EED experiments. xHI=H2O in catholyte was

kept higher at 0.183 while for anolyte it was 0.112. I2/HI ratio

was kept 0.43 for the catholyte whereas it was kept higher for

the anolyte at 0.73. The operating temperature was kept at293 K with both the anolyte and catholyte under atmospheric

pressure (differential pressure 0 bar).

First set of experimentswere done at different flow ratesof

anolyte and catholyte. Two types of flow rate studies on the

cell voltage drop were conducted. In the first type, experi-

ments were done with identical flow rates for both anolyte

and catholyte. The linear flow velocity was varied from 0.95 to

3.53 cm/s. In the second type, flow rate was varied in one of

the sections (in the linear velocity range of 0.95e3.53 cm/s)

keeping the flow rate in the other section constant (linear

velocity 3.53 cm/s).

Further experiments were done with anolyte and catholyte

linear velocity of 3.53 cm/s. Experiments were done by varyingthe xHI=H2O of anolyte and catholyte separately keeping the I2/

HI ratio constant at 0.73 for the anolyte and 0.43 for the

catholyte section. The anolyte xHI=H2 O was varied in the range

of 0.08e0.113 whereas the catholyte xHI=H2 O was varied

between 0.145 and 0.183.

For experiments to determine the effect of I2/HI ratio, the

xHI=H2O values for both the anolyte and the catholyte were held

constant at 0.112 and 0.183 respectively. The I2/HI in the

anolyte was varied in the range of 0.56e0.92 whereas its value

for the catholyte section was varied from 0.26 to 0.43.

EED experiments were also done by varying pressures of

anolyte and catholyte separately using nitrogen gas. In these

experiments, the xHI=H2 O values for anolyte and catholyte were

1: Anolyte reservoir

2: Catholyte reservoir

3: EED cell

4: Heating plate and stirrer

5: Nitrogen cylinder

6: Pressure Gauge

7: Digital Temperature Indicator8: Peristaltic Pump

9: Valve

13

4 4

5

5

6 6

77

8 8

99

2

Fig. 1 e A schematic diagram of the EED experimental

setup used for concentration of hydroiodic acid.



5/13

fixed at 0.112 and 0.183 respectively and the I2/HI ratio in the

anolyte and catholyte were kept at 0.73 and 0.43 respectively.

Experiments were also done at four different temperatures

between 293 and 341 K. Magnetic stirrer was used to keep the

temperature as well as the concentration uniform inside the

reservoirs.

The values of the different operating variables in the

different EED experiments are summarized in Table 1.

3.3. Electrode potential studies

Equilibrium potential of graphite electrode in different

concentration HIx solutions was measured in a three electrode

cell using a potentiostat/galvanostat (make: Gamry, model:

Reference 600) Ag/AgCl/KCl (3 M) electrode was used as the

reference electrode and Pt wire as counter electrode. Graphite

strip of 2 cm2 area was used as the working electrode. The

electrodes were first washed in distilled water then ultra-

sonicated for 10 min to remove all the adsorbed impurities.

The cleaned electrode was then dried in an air oven for 10 h

before use in experiments.In this case, two different sets of experiments were done.

First set consisted of measurement of electrode potential on

HIx solutions with constant iodine concentration of 0.63M and

varying HI concentration between 2.2 and 5.5 M. The second

set usedsolutions with constant HI concentration (2.75 M) and

varying iodine concentration between 0.32 and 1.9 M.

4. Results and discussion

In a continuous IeS cycle, EED operates along with the other

units of the IeS process and the concentration of a stream at

any point of flowsheet remains constant at the steady state.Thus, in the EED operation, various points (along length of the

stack) will have different concentrations of anolyte and

catholyte. The outlet concentration of HIx solution from

Bunsen reaction that goes to EED unit is nearly azeotropic

xHI=H2 Ow0:155. Different flowsheets have been proposed in

literature where the anolyte stream of EED is obtained by

mixing a part of Bunsen outlet HIx stream with liquid stream

coming out of flash/distillation or from HI decomposer (after

H2 removal). These streams are rich in iodine and also contain

water. Thus the EED anolyte inlet xHI=H2O is expected to be

lower than 0.15. Also, the anolyte xHI=H2O decreases along the

length of EED unit. In this work therefore xHI=H2O in the anolyte

section was taken as 0.08, 0.093 and 0.112. xHI=H2 O for catholytewas taken as 0.145, 0.160 and 0.183 since the HI concentration

of the catholyte increases along the length of EED unit. Also,

optimal value for EED exit concentration was reported to be

between 0.175 and 0.197 [29]. xHI=H2O ratio of 0.183 represents

above-azeotropic concentration and the vapor in equilibrium

with this solution has high HI mole fraction (>60 mol%) [30].

Therefore, this solution can be further concentrated using

distillation.

A lower value of I2/HI ratio was used as compared to the

value in the outflow stream of traditional Bunsen reaction

(w4). This was done because newer schemes of carrying out

Bunsen reaction like membrane electrolysis are being

proposed in literature and in these schemes a lower value of

I2/HI molar ratio (w0.5) is used [31]. Also even for traditional

scheme of Bunsen reaction (where I2/HI ratio is more), sepa-

ration schemes are reported where HI rich phase is subjected

to flash separation first. During flashing a large amount of

iodine is separated from the HIx solution. The HIx solution

from the flash column is further concentrated before being

sent for decomposition. Thus even with traditional scheme of

Bunsen reaction, the EED operation can be used for concen-trating HIx solution after flash operation [14,32]. Concentra-

tion of anolyte iodine increases along the length of EED stack

and hence I2/HI molar ratios of 0.56, 0.73 and 0.92 were used

in this work. On the other hand, concentration of catholyte

iodine decreases along the length of EED stack. Therefore

0.26, 0.33 and 0.43 were selected as catholyte I2/HI molar

ratios.

The effect of different concentrations of both HI and iodine

on Vcell is required to calculate energy required for EED oper-

ation. Therefore in this work, experiments were performed

with higher concentration of HI in catholyte and lower

concentration in anolyte. Effect of different operating

parameters was obtained using anolyte and catholyte flowrates at which mass transfer effects were minimal. One of the

desired targets is to operate EED at conditions where the mass

transfer resistance is reduced to a minimum. Mass transfer

resistance causes a non-linear increase in the cell voltage

(with current density). Mass transfer effects can be reduced by

changing the flow rates of anolyte and catholyte.

4.1. Effect of catholyte and anolyte flow rate

Fig. 2 shows variation of Vcell with (identical) flow rates of

electrolytes in the anolyte as well as the catholyte at two

different current densities (0.01 A/cm2 and 0.175 A/cm2). Vcell

decreased as the linear velocity of electrolytes was increasedfrom 0.95 to 3.53 cm/s. This was due to the decrease in mass

transfer resistance with increased convection. In another set

of experiments, flow rates of anolyte and catholyte were

varied separately. The studies were done at four different

linear velocities (0.95, 1.89, 2.78 and 3.53 cm/s). While the flow

rate ofone side of the cell was varied theflow rate of other side

was kept constant at the highest flow rate (3.53 cm/s). Fig. 3

shows the Vcell variation with anolyte and catholyte flow

rates respectively. Vcell decreased as the linear velocity of the

catholyte increased from 0.95 to 3.53 cm/s. On the other hand,

change in Vcell with increase in anolyte flow rate was insig-

nificant. This suggested that the mass transfer resistance of

the cell was dominated by the cathode side. Cathode sidemass transfer resistance was dominant because of the

concentration and nature of the reacting species. Therefore,

drop in Vcell with increase in anolyte flow rate (reduced

thickness of diffusion boundary layer) was insignificant.

Decrease in Vcell was however small (w30 mV) for 0.175 A/cm2

and insignificant (w8 mV)for low current density (0.01 A/cm2).

Thus no significant advantage could be gained by operating at

higher flow rates at low to moderate current densities. Further

experiments were therefore performed at anolyte and cath-

olyte flow rates of 3.53 cm/s. Also, flow rates of both anolyte

and catholyte were kept same, to ensure that no local pressure

gradient across the membrane are generated at any point

inside the cell.



6/13

4.2. Effect of variation ofxHI=H2O

The effect of HI concentration was considered in form of

xHI=H2O because the azeotropic concentration of HIx solution is

conveniently expressed using this ratio. Vcell variation withchange in HI concentration was measured by independently

changing the xHI=H2O of the anolyte and the catholyte. Even

though the anode and cathode reaction were reverse of each

other, OCV value was non-zero due to unequal concentrations

of anolyte and catholyte. OCV value reduced by about 19%

(Fig. 4(b)) as the xHI=H2O of catholyte was reduced from 0.183 to

0.145. Fig. 4(a) shows the measured Vcell at current densities

between 0.001 A/cm2 and 0.275 A/cm2 for xHI=H2O values of

0.183, 0.16 and 0.145 (anolyte xHI=H2O was kept constant at

0.112). Measured Vcell values were fitted using Eqn. (9) and the

values of parameters of the equation are listed in Table 2. The

value of the Tafel parameters Db (of order 104) and Da

(of order 103) was negligible. Using these parameter values

the calculated electrode overpotential values were very small

fraction of the Vcell (e.g., 0.2% ofVcell at 0.275 A/cm2). Ie

Vdataof the cell, therefore, can be approximated with a linear

response as given below:

Vcell i Rnet OCV (11)

Total resistance equivalent of the cell (Rnet) and the

regressed value of open circuit voltage (V0) are also given in

Table 2. Rnet increased (Fig. 4(b)) from 0.145 to 0.183 U/cm2 as

the catholyte xHI=H2O increased from 0.145 to 0.183. OCV was

significant proportion of the Vcell ranging from 12 to 14% at

higher current densities to w75% at lower current density of

(0.02 A/cm2).

Variation in Vcell with change was measured at three

different anolyte xHI=H2O (0.112, 0.093 and 0.08). IeV data

0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.176

0.178

0.180

0.182

0.184

0.186

Voltage(V

)

Linear flow velocity (cm/sec)

Catholyte

Anolyte

0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.720

0.725

0.730

Voltage(V)


Catholyte

Anolyte

a

b

Fig. 3 e Effect of independent variation in flow rate of

anolyte and catholyte on cell potential drop at current

density of (a) 0.01 A/cm2, and (b) 0.175 A/cm2. Linear

velocity of one of the compartments was fixed at 3.53 cm/s

when that of the other was varied.

0.5 1.0 1.5 2.0 2.5 3.0 3.50.178

0.180

0.182

0.184

0.186

0.188

0.190

Voltage(V)


0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.720

0.725

0.730

0.735

0.740

0.745

0.750

0.755

Voltage(V)


a

b

Fig. 2 e Variation of cell potential drop with simultaneous

change in linear flow velocity of anolyte and catholyte at

current density of (a) 0.01 A/cm2, and (b) 0.175 A/cm2.



7/13

showed linear trend indicating the combined electrodes

overpotential were insignificant fraction of total Vcell(Fig. 5(a)). In contrast to the trend observed in the catholyte

section, Vcell was higher at lower xHI=H2 O. OCV value increased

(Fig. 5(b)) considerably (w55.5%) on reducing xHI=H2 O from

0.112 to 0.08. This can be attributed to a sharp change in thechemical potential of the anolyte. This is important in light of

the fact that OCV value constituted a major fraction ofVcell at

lower current densities (w70% at 0.02 A/cm2) and remained

non-negligible even at higher current densities (w15% at

0.275 A/cm2). Vcell data at different current density was fitted

using Eqn. (11) to obtain the Rnet value. Fig. 5(b) shows the Rnetvalues for different values of anolyte xHI=H2O. Rnet decreased

by w10% as the xHI=H2 O was reduced from 0.112 to 0.08.

Increase in the cell voltage with time of operation has been

reported in literature [21e25]. The above reported results

suggested that increase in cell potential was due to devel-

opment of differential concentration across the membrane of

the EED.

0.00 0.05 0.10 0.15 0.20 0.25 0.30

0.0

0.2

0.4

0.6

0.8

1.0

Voltage(V

)

Current density (A/cm2)

xHI/H O

: 0.183

xHI/H O

: 0.160

xHI/H O

: 0.145

0.14 0.16 0.18

0.10

0.12

0.14

0.14 0.16 0.18

OCV

Rnet

xHI/HO

(catholyte)

OpenCircuitVoltage(OCV)

(V)

3.00

3.05

3.10

3.15

3.20

3.25

ResistanceR

net(-cm2)

a

b

Fig. 4 e Effect ofxHI=H2O in catholyte on (a) cell voltage drop,

and (b) OCV and calculated value of cells ohmic resistance

(Rnet).

Table 2 e Table shows the simulated values of thederived parameters using the Tafel equation and thelinear fitting for varying xHI=H2O in the catholyte section.

xHI=H2O(catholyte)

Parameters Tafel equation Linear fit

0.183 OCV (V) 0.137

Rnet (U

/cm

2

) 3.103 3.09962Da (V) 0.0019 0.0008

Db 0.0002263 e

0.16 OCV 0.117

Rnet 3.045 3.05279

Da 0.0012 0.00064

Db 0.0002208 e

0.145 OCV 0.111

Rnet 3.001 3.008

Da 0.0013 0.0002

Db 0.0002194 e

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage(V)


xHI/H O

: 0.112

xHI/H O

: 0.093

xHI/H O : 0.080

0.08 0.10 0.12

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.240.08 0.10 0.12

OCV

R

xHI/HO

(anolyte)

OpenCircuitVoltage(O

CV)

(V)

3.05

3.10

3.15

3.20

3.25

3.30

3.35

3.40

3.45

ResistanceR

net(-cm2)

a

b

Fig. 5 e Effect ofxHI=H2O in anolyte on (a) cell voltage drop,

and (b) OCV and calculated value of cells ohmic resistance

(Rnet).



8/13

4.3. Effect of the variation of I2/HI ratio

Fig. 6 shows OCV and currentevoltage data respectively, at

three different I2/HI ratios (0.26, 0.33 and 0.43) in the catholyte.

OCV values were significant part of the Vcell and contributed

up to 60% to Vcell at lower current densities (up to 0.02 A/cm2)

andw15% at higher current densities. However, OCV did not

change appreciably (decreased by w2%) with increase in I2/HIratio (Fig. 6(b)). Vcell varied linearly with current density and

the data was regressed linearly. Rnet for different I2/HI ratio

(Fig. 6(b)) decreased with increase in iodine concentration.

Fig. 7 shows OCV and currentevoltage data respectively, at

three different I2/HI ratios (0.56, 0.73 and 0.92) in the anolyte.

OCV increased by 40% as the I2/HI ratio was increased from

0.56 to 0.92. Difference in iodine concentration of the two

sections became large as the anolyte iodine concentrationwas

increased. This may be the reason for significant increase in

OCV on changing anolyte iodine concentration while an

insignificant increase with change of catholyte iodine

concentration. Vcell varied linearly with current density and

the Rnet values obtained at different I2/HI ratio are shown in

Fig. 7(b). In contrast to trend for iodine increase in catholyte,

Rnet value increased with increase in iodine concentration of

the anolyte.

Increase in the iodine concentration affected the ohmicresistance of the cell as well as the OCV. Anolyte iodine

concentration affected Vcell more strongly as compared to the

catholyte iodine concentration. EED cell resistance was re-

ported to increase with increase in iodine concentration [22].

Separate increase in iodine concentration of anolyte and

cathode done in this work, suggested that cell resistance

increased more by increase in iodine concentration of anolyte

than the catholyte.

4.4. Effect of pressure on the cell voltage

Effect of pressure on cell voltage has been analyzed by

defining differential pressure as:

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage(V)


I2/HI : 0.26

I2/HI : 0.33I2/HI : 0.43

0.25 0.30 0.35 0.40 0.450.142

0.143

0.144

0.145

0.146

0.147

0.148

0.149

0.150

0.151

0.1520.25 0.30 0.35 0.40 0.45

OCV

R

I2/HI ratio (Catholyte)


(V)

3.10

3.15

3.20

3.25

3.30

R

esistanceR

net(-cm2)

a

b

Fig. 6 e Effect of I2/HI ratio in catholyte on (a) cell voltage

drop, and (b) OCV and calculated value of cells ohmic

resistance (Rnet).

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage(V

)


I2/HI : 0.56

I2/HI : 0.73

I2/HI : 0.92

0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95

0.09

0.10

0.11

0.12

0.13

0.14

0.15

0.16

0.17

0.18

0.19 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95

OCV

R

I2/HI ratio (Anolyte)


(V)

3.0

3.1

3.2

3.3

3.4

3.5

ResistanceR

net(-cm2)

a

b

Fig. 7 e Effect of I2/HI ratio in anolyte on (a) cell voltage


resistance (Rnet).



9/13

PDiff Panolyte Pcatholyte (12)

Differential pressure was created by pressurizing either the

anolyte or the catholyte and keeping the other section at

atmospheric pressure (1 bar). Experiments were done at five

differential pressures between 1 bar and 1 bar. In addition

the currentevoltage data was also measured at zero differ-

ential pressure but with both the anolyte and catholyte at2 bar pressure and theresponse was found to be similar to that

with that of the base case. OCV was unaffected by differential

pressure changes. Fig. 8(a) shows the currentevoltage data at

different differential-pressures. Vcell varied linearly with

current density for all the differential pressures. Rnet value

obtained from linear regression of current voltage data was

least for zero differential pressure and increased for both

positive and negative values of differential pressure (Fig. 8(b)).

Increase in Rnet was more for positive differential pressure

(6.2% for 1 bar compared to 4% for 1 bar at 0.275 A/cm2). Rnetvalue remained unchanged at zero differential pressure for

both 1 bar and 2 bar pressure of the system. A possible

explanation for increase in Rnet at non-zero differential pres-

sures can be sticking of iodine to the membrane. A differential

pressure caused iodine to deposit on membrane at the higher

pressure side and the amount deposited increased with

increase in differential pressure. Presence of iodine on

membrane surface increased the resistance of the membrane.

No specific experiments were conducted to determine the

effect of iodine sticking. However iodine was found on themembrane as indicated the change in color of the membrane

to light brown after the experiment.

4.5. Effect of temperature

Fig. 9(a) shows the currentevoltage data at four different

temperatures (293 K, 308 K, 323 K and 341 K). OCV value of the

EED cell did not change with increase in the temperature

(Fig. 9(b)). This suggested that the change in chemical poten-

tial of both the anolyte and catholyte with temperature was

nearly same. Vcell decreased with increase in temperature and

the difference was higher at higher current densities (w11% at

0.275 A/cm2). Insignificant change of OCV suggested that drop

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.2

0.4

0.6

0.8

1.0

1.2

V

oltage(V)


PDIff

: 0 Bar

PDIff

: + 0.5 Bar

PDIff : + 1 BarP

DIff: - 0.5 Bar

PDIff

: - 1 Bar

-2 -1 0 1 2

0.14

0.16

-2 -1 0 1 2

OCV

R

Differential Pressure (bar)


(V)

3.15

3.30

3.45

R

esistanceR

net(-cm2)

a

b

Fig. 8 e Effect of differential pressure on (a) cell voltage


resistance (Rnet).

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.2

0.4

0.6

0.8

1.0

T : 293 K

T : 308 KT : 323 K

T : 341 K


Voltage(V)

290 300 310 320 330 3400.10

0.11

0.12

0.13

0.14

0.15290 300 310 320 330 340

OCV

R

Temperature (K)

OpenCircuitVoltage(O

CV)

(V)

2.7

2.8

2.9

3.0

3.1

3.2

ResistanceR

net(-cm2)

a

b

Fig. 9 e Effect of cell operating temperature on (a) cell

voltage drop, and (b) OCV and calculated value of cells

ohmic resistance (Rnet).



10/13

in Vcell with temperature was entirely due to the decrease in

the ohmic resistance of the cell. Fig. 9(b) shows the Rnet value

of the cell at different temperatures obtained by linear

regression of the currentevoltage data. Rnet decreased by 14%

for a temperature difference of 48 K. Similar trends have been

reported in literature [23,24].

4.6. Equilibrium potential studies on graphite electrode

OCV contributed a substantial fraction to the Vcell, thus it is

important to understand how OCV is affected by different

operating variables. As discussed above, OCV was not affected

by pressure and temperature was changed significantly with

the I2/HI ratio as well as the xHI=H2O. As discussed earlier [Eqn.

(8)] the net OCV generated is function of the electrode equi-

librium potential along with the membrane potential. Exper-

iments were done in a three electrode setup to determine how

electrode potential (against Ag/AgCl reference electrode)

varied with concentration of HI and iodine. The experimen-

tation was done in two sets, one with constant HI concen-

tration and the other with constant iodine concentration.

Fig. 10 shows the electrode potential at iodine concentrations

between 0.32 M and 1.9 M (HI concentration was 2.75 M).

Maximum increase in the electrode potential (Eeq) in the

experimental concentration range was found to bew13%. The

rate of increase however reduced at higher iodine concen-

tration. The average rate of increase per unit change in iodine

molarity was found to be around 19:6 mV=MI2 . Fig. 11 shows

electrode potentials at HI concentrations between 2.2 and

5.5 M (at constant iodine concentration of 0.63 M). The elec-

trode potential showed a progressive increase (by about 53%)

with the increase in HI concentration in the experimental

range along with average rate of decrease was about 42 mV/

MHI. Thus, electrode potential increased with increase in

iodine concentration and decreased with increase in the HI

concentration. Also, the electrode potential was affected more

strongly by the HI concentration. This explains smaller

change in OCVwith change I2/HI ratio compared to the xHI=H2O.

4.7. Optimal operating parameters

EED consumes electric energy which itself is obtained at 40%

efficiency from heat source. Hence energy consumption in

EED will significantly affect the overall efficiency of the IeS

process. Recent simulation studies reported in literature also

suggested that the reduction in potential drop required in EED

cell results in major increase in theoverall IeS cycle efficiency

[33]. For comparison of the EED process, energy required is

expressed in terms of the heat equivalent the total electricenergy required to concentrate enough acid that on decom-

position gives one mole of H2. Since one mole of HI on

decomposition gives half mole of H2, heat equivalent of energy

required can be written as:

UH2 2FVcellhIhH

(13)

where hI is the current efficiency and hH is the efficiency of

conversion of heat to electricity (taken as 0.45). Reported

current efficiency values varies in the range of 0.8e0.95

[21e24]. Presentstudies showed that OCV and ohmic potential

drop constitutedmajor fraction of the total potential drop. Cell

potential drop reduced on loweringxHI=H2O of the catholyte, sothe exit catholyte xHI=H2 O of EED should be kept at the

minimum value after which simple distillation can be used

effectively for further concentration of HI. Optimal catholyte

exit xHI=H2 O value can only be found by simultaneous energy

calculation over EED and distillation of HIx solution. Optimal

value for EED exit concentration was reported to be between

0.175 and 0.197 [29]. The catholyte exit xHI=H2 O value of around

0.18 is in the range of optimal values. EED cell potential drop

increased with decrease in anolyte xHI=H2 O. So the inlet anolyte

xHI=H2 O must be kept at the highest possible value which is the

exit concentration of Bunsen reaction i:e:; xHI=H2Ow0:15. The

change in HI concentration of catholyte and anolyte of EED is

related more or less in stoichiometric way. Thus, an exit

0.0 0.5 1.0 1.5 2.0220

230

240

250

260

HI - 2.75 MEeq

(mVvsAg/

AgClref.)

Iodine Concentration (M)

Fig. 10 e Equilibrium potential of the graphite electrode in

HIx solutions of different iodine concentration and a fixed

HI concentration of 2.75 M. The potential reported is wrt

Ag/AgCl reference electrode.

2 3 4 5 6

120

160

200

240

280

I2

- 0.63 M

Eeq

(mVvsAg/AgClref.)

HI Concentration (M)

Fig. 11 e Equilibrium potential of the graphite electrode in

HIx

solutions of different HI concentration and a fixed

iodine concentration of 0.63 M. The potential reported is

wrt Ag/AgCl reference electrode.



11/13

catholyte xHI=H2 O of about 0.185 would correspond to an exit

anolyte xHI=H2 O of about 0.11e0.12. EED potential drop

increased with increase in anolyte I2/HI ratio and therefore it

must be kept at the minimum possible value. Mixing of recycle

streams from flash or HI decomposer with inlet stream of EED

will increase the I2/HI ratio of the anolyte and should be

avoided. Mass balance calculation reveals that for inlet xHI=H2 O

and I2/HI ratio of 0.155 and 0.5 respectively and outlet xHI=H2O of0.183, the outlet I2/HI molar ratio will be 0.26. Similarly, with

identical inlet for anolyte, the outlet I2/HI molar ratio will be

0.78. Thus for a Bunsen outlet stream (after partial iodine

removal in case of direct contact mode of Bunsen reaction) of

composition xHI=H2 Ow0:155 and I2/HI molar ratio of 0.5, the

minimum energy demand is expected for identical inlet ano-

lyte and catholyte streams and exit concentration of catholyte

ofxHI=H2 O of 0.185, I2/HI molar ratio of 0.26. The corresponding

anolyte exit concentration would be xHI=H2O of 0.115 and I2/HI

molar ratio of 0.78.

Cellpotentialdecreased with increase in the catholyte I2/HI

ratio from 0.26 to 0.43. The iodine concentration in the inlet

catholyte stream can be increased by mixing of recycle streamfrom the HI decomposer which is rich in iodine and has

comparatively lower water content. However, energy required

in flash operation (immediately after EED unit) increases with

increase in iodine content of the EED product stream. Thus

inlet iodine of catholyte stream cannot be increased indefi-

nitely to lower energy consumption of HI decomposition

section. Data from the I2/HI ratio patterns suggest that for

a value of 0.56 the EED potential drop shows the minimum for

all the variation. Energy calculations were therefore per-

formed for two different inlet catholyte stream concentra-

tions (as shown in Table 3). First concentration corresponds to

out stream from Bunsen reactor and the second inlet cath-

olyte stream composition is based on the mixing of recyclestream from HI decomposer. Transport number and electro-

osmotic flow are important parameters which determine

performance and energy consumption of EED cell. Their

measurements require long duration EED operations where

inlet and outlet EED streams concentration are sufficiently

different. For this reason, average values of these parameters

are reported in literature. Since the focus of this work is on

determination of variation of EED cell performance with

asymmetric variation of independent variables, point deter-

mination of transport number and electro-osmotic coefficient

could not be determined. Energy calculation was therefore

done using average value of transport number and current

efficiency value reported in the literature.

Potential drop varied almost linearly with current density;

hence the energy consumption increased linearly with

current density. The energy consumption based on the initial

voltage value can be further subdivided into the energy

consumed in overcoming the OCV, energy consumed in

concentration process (electrode reactions) and the energy

consumed in overcoming the ohmic resistance of the cell.

Fig. 12 shows bar chart of the energy consumed at threedifferent current densities of 0.01, 0.05 and 0.275 A/cm2 for

two different inlet catholyte stream compositions. The

UeOCV corresponds to the energy consumed in overcoming

the OCV. U-Act represents the cumulative sum of energy

consumed in concentration process and overcoming the

ohmic drop. Energy consumption with addition of recycle

stream (109 kJ/mol-H2 at a current density of 0.01 A/cm2) was

higher than the case where the recycle stream (91 kJ/mol-H2 at

a current density of 0.01 A/cm2) was added. The energy

consumed in overcoming the OCV is independent of the

applied current density. UeOCV was significant contributor to

the total energy consumption at low current density while the

U-Act dominated the energy consumption at higher currentdensity.

The above stated energy consumption values exclude the

pumping costs. Typical heat demand for the IeS cycle is re-

ported in the range of 600e650 kJ/mol-H2 [29]. Thus EED

operation would be feasible only at lower current densities.

OCV was a major component of EED potential drop at low

current densities and was affected strongly by the electrolyte

concentrations viz. I2/HI ratio and xHI=H2O while other oper-

ating parameters such as temperature and pressure had little

effect on OCV. It was found that the increase in the difference

in concentrations of the electroactive components on two

compartments of the cell resulted in higher OCV. This sug-

gested network streams and their relative mass flow in HIdecomposition of the cycle should be designed such that

difference in concentration of electrolytes in two compart-

ments of the EED cell be minimized. Increasing the tempera-

ture however reduced the ohmic resistance of the cell.

Table 3 e I2/HI ratio of the inlet catholyte and anolytestreams of EED with and without addition of recyclestream from HI decomposer. The other variablesremained arexHI=H2O (catholyte)e 0.183, xHI=H2O (anolyte)e0.112, DP e 0 bar, Te 293 K.

I2/HI

Anolyte Catholyte

Solution 1 0.73 0.26

Solution 2 0.73 0.43

0

100

200

300

400

500

600

700

S-

2

S-

2

S-

1

S-

1

S-

2

S-

1

EnergyRequirement

(kJ/mole-H2

)

0.01 A/cm2

0.05 A/cm2

0.275 A/cm2

U-OCV

U-Act

Fig. 12 e Bar chart representation of the energy consumed

in EED cell for solution 1 (S-1), and solution 2 (S-2) at three

different current densities 0.01, 0.05 and 0.275 A/cm2.



12/13

5. Conclusion

Effects of different operating parameters on the EED cell

potential drop were investigated. Studies on the EED cell

with asymmetric variation of electrolyte concentration

added to the information available in literature on effect of

operating variables on performance of EED cell. OCV wasidentified as a major potential drop component in the EED

cell operation. It was also found that the equilibrium elec-

trode potential changes rapidly with concentration of redox

species and thus difference in concentration of these species

in the two compartments of the cell rapidly increased the

OCV of the cell. Increase in I2/HI ratio in catholyte from 0.25

to 0.45 resulted in slight lowering of the energy consump-

tion. In contrast, increase in I2/HI ratio in anolyte resulted in

increase in the energy consumption. The energy calculation

revealed that the operation at lower current density is

preferred and at lower current densities OCV form a signifi-

cant part of the EED cell potential and hence energy

consumption.

Acknowledgment

Authors acknowledge the financial support from ONGC

Energy Centre for carrying out this work.

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