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0360-3199189 $3.00 + 0.00 Maxwell Pergamon Macmillan plc. International Association for Hydrogen Energy.
Int. J. ttvdrogen Energy, Vol, 14, No. 8, pp. 545~549, 1989. Printed in Great Britain.
THERMOCHEMICAL WATER SPLIT-FING THROUGH DIRECT HI-DECOMPOSITION FROM HzO/HI/I2 SOLUTIONS M. ROTH and K. F. KNOCHE Institute for "Technische Thermodynamik', RWTH Aachen, Schinkelstr. 8, 5100 Aachen, F.R.G.
(Received forpublication 16 November 1988) Abstract--Sulfur/iodine cycles have a considerable potential for thermochemical hydrogen production. GA Inc, have investigated these cycles for many years. They have been able to demonstrate the feasibilityof such cycles and evaluated first chemical engineering flowsheets. The HI concentration and decomposition seems to be the most expensive and energy consuming step. Therefore, an alternative has been developed at the Institute for Technical Thermodynamics in Aachen, in which HI is decomposed directly from liquid H20/HI/I2 solution under high pressure and temperature. After thermodynamic correlations for the phase equilibrium in the quaternary mixture of H20/HI/IJH2 have been established, a separation column could be designed and theoretically evaluated.
T H E R M O D Y N A M I C D A T A O F T H E SYSTEM H20/HI/I2/H2 The sulfur/iodine process according to General Atomic To develop an alternative for the Sections III and IV [1] can be characterized with the following reaction of the original G. A. flowsheet thermodynamic data of scheme: the quaternary system H20/HI/IE/H2 must be known. (1) The vapor pressure of this system with HI2 H20 + 12 + 5 0 2 + 2 HI + H 2 SO4 concentrations up to 17 mol% in the liquid phase and (2) temperatures up to 580 K is measured by Engels et al. H2504 ---+H 2 0 + 502 + 1/2 0 2 [3]. It was found that the relative vapor pressure minima (3) of this quaternary system shows a strong temperature 2 HI --~ H2 + 12. dependence for low iodine contents in the liquid. The use of excess water and iodine in the Bunsen Remarkable hydrogen pressures were only found in the reaction (1) leads to sufficient reaction rates and a equilibrium vapor of solutions with HI-contents higher separation of the reaction products into two liquid than the pseudo-azeotropic compositions. These results phases (Section I in the G . A . proposal). The upper lead us to develop a model to investigate the direct phase contains H2SO4 and H20, the lower phase HI, 12 dissociation of theoretical HI. Therefore a thermodynaand water. The HzSO4-phase is decomposed under mic model was developed to describe the phase equilibconsumption of high temperature heat according to rium of this quaternary system. A method to describe reaction (2) (Section II of the G . A . proposal). The lower electrolytic mixtures developed by Engels [4] was used phase of the Bunsen reaction is treated with highly for the calculation of all thermodynamic data necessary concentrated phosphoric acid as an extraction agent for the mass, energy and exergy balances. (Section III of the G . A . proposal). So HI can be gained as a pure liquid. The dissociation of the pure hydrogen iodine according to reaction (3) is carried out at low THE DISTILLATION COLUMN temperature either in the liquid or in the gas phase. With With the measurements and the correlation menan exergy, or second law, analysis Funk and Knoche [2] have shown that one weak point of the G.A.-flowsheet is tioned above we are able to design a column for a direct the reconcentration of the phosphoric acid. To get a dissociation of HI in the liquid phase. In this proposal theHzOIHI/I2 stream coming from the good heat recovery a high amount of electrical energy is Bunsen reaction (1) (Section I) is the feed stream of the required. The flowsheet for the original G . A . process plant column. We get the temperature and pressure of this includes five sections as shows in Fig. 1, which also shows feed stream from Fig. 2 which shows the phase equilibthe actual molar flows of the components. There is a rium of H20/HI/I2 mixtures with a constant 12 mol large recirculation of water between Sections I and III, fraction of 39%. This is the I2 mol fraction coming from and an even greater recirculation of water and iodine Section I. In Fig. 2 we find a small temperature between Section I and III. Section V is not part of the dependence of the azeotropic point. As mentioned chemical process. It includes heat transfer and power above we know that remarkable hydrogen pressures are generation equipment and accomplishes energy transfer only found in the vapor over solutions with HI content higher than the azeotropic tool fraction. That deteramong the various sections of the chemical plant. INTRODUCTION
545
546
M. ROTH AND K. F. KNOCHE
½~=
H,O
J
HzS& I~!
SectionI
mines the minimum temperature and pressure of the feed stream. We find a pressure of 22 bars and a temperature of 262°C. The stream coming from Section 1 is pumped to 22 bars and preheated to the boiling temperature of 262°C and then enters the distillation column. The product from the bottom consists of mostly iodine and small parts of water and HI. To close the water balance, a side stream must be taken out of the column. This stream consists of water, HI and a small amount of iodine. The ratio of HI/H20 mol fractions of the side stream must be lower than the ratio of the feed stream. The bigger the difference between the ratios, the smaller is the feed stream. But on the other hand, the HI mol fraction of the side stream must be high enough to get an equilibrium vapor with a sufficient HI pressure. These two conditions are the reason for the high mol numbers of the feed stream. The product from the top of the column consists of hydrogen and HI. The water and iodine content is negligible. Figure 3 shows the phase equilibrium of the quaternary system H20/HI/I2/H2 at a pressure of 22 bars, because that is the pressure of the column. We see a strong dependence of the azeotropic HI tool fraction for different iodine mol fractions in the solution. Figure 3 shows the phase equilibrium of the HI mol fraction in the liquid and the vapor and supplementary the hydrogen mol fraction in the vapor. We see that we get high hydrogen mol fractions with low iodine content and
SectionII
12HzO+SO22 *IH - ~SC+ I2H~ INv_. T~HS 'O _H -'O ' ÷SU O~ :" +30, 1
'" 2HI 1OH,Ofk\ 5H,O IOH,OI 81~/ ,,70, 8Izl l \x
;
!
SectionV
i
I
"d
(
/
/
/
/
/
Secto i nIV
Primory Energy [
2HI-- H.~+I2
Source [
H~ ....... Energy Tronspor t Mclteri(IITransport
-
Fig. l. Flowsheet of the G.A. process.
Pha,eEquLL.:H20/HI/12 (x-J2-0.39)
I
-Zo. R~ @
o (_ Q.
--
J
180 C
J
i
/
|quid
J
ca o 0.0
0.1
0.2
0.3
x-HI,
9-HI
Fig. 2. Phase equilibrium for le = 39%.
0.4
0.S
547
THERMOCHEM1CAL WATER SPLITTING
Phase g q u L L . :
H20/HI/I2/H2
p - 22 b a r
o tn
o t~
x-HJ
I
¢,1 cD --
10.7)
o
~ { 0 . 3 9 1 o
4ol
i
{0.1)
o
~-H2
ol o
0.0
o.~
0.1
o.~
d.~
-~.~
x-HI, 9-HI, ~-H2
Fig. 3. Phase equilibrium at column pressure.
ConcentrotLon Proftte8
Concentrotton ProfLtes H2 c~
o
HI
H20
I o
o
H20
L ~:-
C
~o 0 ~o
12 {3
o
c~
o o
0.0
0.2
0.t vapor
0.6 moL
OL8
fractLon
Fig. 4. Concentration profiles of the vapor.
I I. 0
0.0
0.2
0.4
0.6
0.8
LLquLd moL fractLon
Fig. 5. Concentration profiles of the liquid.
1.0
548
M. ROTH AND K. F. KNOCHE
temperatures about 220°C or lower. The profiles of the concentrations and the temperature along the column stages are shown in this diagram. We find the feed stream in Fig. 3 at the azeotropic point for the iodine mol fraction of 0.39. Down to the bottom of the column the distillation follows the azeotrope. Up to the top we get over azeotropic concentrations. If we follow the concentration profiles from the feed to the top, we see that we get a remarkable hydrogen content only for very little iodine in the liquid. That means the HI dissociation takes place only in the upper part of the column. But we can see in this diagram that the dissociation will increase very rapidly when it starts. Figure 4 shows the concentration profiles in the vapor at every stage of the column. We see that the main hydrogen production takes place at the last three stages. Also we see very complicated behaviour of the water and HI profiles. Additional to the last diagram Fig. 5 shows the concentration profiles in the liquid phase. If we compare Fig. 4 and Fig. 5 we find only for negligible iodine content an increase of the hydrogen production.
t n H20 HI H2
= = -
As mentioned above we get large mol flows which we see in Fig. 6. Per 1 kmol hydrogen which is produced we have a feed stream of 125.8 kmol. We need this large feed stream because we take, with the side stream, a large amount of HI out of the column. This side stream makes about 60% of the inlet mol flow.
THE F L O W S H E E T The calculation of this column leads to a new flowsheet (Fig. 7). This flowsheet should only be connected to Section I of the G.A. process for the material flows. But we need a large amount of heat at the bottom for a temperature of 320°C. The preheating of the feed stream may be done by cooling the side stream and the bottom stream. There is a lot of heat to transport in heat exchangers but we must remember that we have only liquid streams. Table 1 lists a summary of the energy which is required per 1 kmol of hydrogen. The main advantage of the present flowsheet compared with the G.A. proposal is that there is only a little
221 C 3.32 kmol 54 16 % 30 t n H2 HI
t n t n H20 HI I2
= = -
262 C 125.8 51% 10 % 39 %
H20 HI I2
kmol
-+ t n H20
= : -
310 C 41.7 kmol 4 %
-
95
HI I2
1 %
Fig. 6. The distillation column.
= =
= --
251 76.4
-
80. 1% 12.5 % 7.4 %
25 C 1 kmo] 99.7 0.3 %
C kmol
THERMOCHEMICAL WATER SPLITI'ING
549
tIE 6 H20 SEE. I =~,9
)o
H~O
. S H~O secl
I I
I
lIE 6
)1
Fig. 7. The flowsheet.
Table l. Energy summary (according to Fig. 7) energy (kmol H2) Pump HE 1 HE 2 HE 3 HE 4 HE 5 HE 6 HE 7 HE 8
5,8 608.5 405.2 207.7 -78.0 -8.8 237.0 -39.6 -26.6
MJ MJ MJ MJ MJ MJ MJ MJ MJ
electrical energy needed to drive the pumps. But there is also an important disadvantage. That is the very large feed stream and following there is a large a m o u n t of heat required at a t e m p e r a t u r e of 320°C (see H E 6 of Fig. 7).
CONCLUSION D u e to the availability of sufficient t h e r m o d y n a m i c data concerning the quaternary system H20/HI/I2/H2 it was possible to develop a new proposal for the H I decomposition of the General A t o m i c process. This proposal needs no extraction agent and a minimum of electrical energy.
internal heat exchange internal heat exchange internal heat exchange
REFERENCES 1. J. H. Norman, G. E. Besenbruch and D. R. O'Keefe, Thermochemical Water-Splitting for Hydrogen Production/Final Report, General Atomic Company, 10955 John Jay Hopkins Drive, San Diego, California (1981). 2. J. E. Funk and K. F. Knoche, Hydrogen by Thermochemicl Water Splitting, Proceedings of the Tenth Technical Workshop, Tokyo, Japan (27-29 July 1987). 3. H. Engels, K. F. Knoche and D. H6ckelmann, Vapor Pressure, Pseudo Aceotrop and Miscibility Gap of the System HI~H20~12, Proceedings of the l.E.A.-Workshop, Honolulu (1985), 4. H. Engels, Anwendung des Modells der lokalcn Zusammensetzung auf Elektrlytl6sungen. Thesis RWTH Aachen, FRG (1985).
Int. J. ttvdrogen Energy, Vol, 14, No. 8, pp. 545~549, 1989. Printed in Great Britain.
THERMOCHEMICAL WATER SPLIT-FING THROUGH DIRECT HI-DECOMPOSITION FROM HzO/HI/I2 SOLUTIONS M. ROTH and K. F. KNOCHE Institute for "Technische Thermodynamik', RWTH Aachen, Schinkelstr. 8, 5100 Aachen, F.R.G.
(Received forpublication 16 November 1988) Abstract--Sulfur/iodine cycles have a considerable potential for thermochemical hydrogen production. GA Inc, have investigated these cycles for many years. They have been able to demonstrate the feasibilityof such cycles and evaluated first chemical engineering flowsheets. The HI concentration and decomposition seems to be the most expensive and energy consuming step. Therefore, an alternative has been developed at the Institute for Technical Thermodynamics in Aachen, in which HI is decomposed directly from liquid H20/HI/I2 solution under high pressure and temperature. After thermodynamic correlations for the phase equilibrium in the quaternary mixture of H20/HI/IJH2 have been established, a separation column could be designed and theoretically evaluated.
T H E R M O D Y N A M I C D A T A O F T H E SYSTEM H20/HI/I2/H2 The sulfur/iodine process according to General Atomic To develop an alternative for the Sections III and IV [1] can be characterized with the following reaction of the original G. A. flowsheet thermodynamic data of scheme: the quaternary system H20/HI/IE/H2 must be known. (1) The vapor pressure of this system with HI2 H20 + 12 + 5 0 2 + 2 HI + H 2 SO4 concentrations up to 17 mol% in the liquid phase and (2) temperatures up to 580 K is measured by Engels et al. H2504 ---+H 2 0 + 502 + 1/2 0 2 [3]. It was found that the relative vapor pressure minima (3) of this quaternary system shows a strong temperature 2 HI --~ H2 + 12. dependence for low iodine contents in the liquid. The use of excess water and iodine in the Bunsen Remarkable hydrogen pressures were only found in the reaction (1) leads to sufficient reaction rates and a equilibrium vapor of solutions with HI-contents higher separation of the reaction products into two liquid than the pseudo-azeotropic compositions. These results phases (Section I in the G . A . proposal). The upper lead us to develop a model to investigate the direct phase contains H2SO4 and H20, the lower phase HI, 12 dissociation of theoretical HI. Therefore a thermodynaand water. The HzSO4-phase is decomposed under mic model was developed to describe the phase equilibconsumption of high temperature heat according to rium of this quaternary system. A method to describe reaction (2) (Section II of the G . A . proposal). The lower electrolytic mixtures developed by Engels [4] was used phase of the Bunsen reaction is treated with highly for the calculation of all thermodynamic data necessary concentrated phosphoric acid as an extraction agent for the mass, energy and exergy balances. (Section III of the G . A . proposal). So HI can be gained as a pure liquid. The dissociation of the pure hydrogen iodine according to reaction (3) is carried out at low THE DISTILLATION COLUMN temperature either in the liquid or in the gas phase. With With the measurements and the correlation menan exergy, or second law, analysis Funk and Knoche [2] have shown that one weak point of the G.A.-flowsheet is tioned above we are able to design a column for a direct the reconcentration of the phosphoric acid. To get a dissociation of HI in the liquid phase. In this proposal theHzOIHI/I2 stream coming from the good heat recovery a high amount of electrical energy is Bunsen reaction (1) (Section I) is the feed stream of the required. The flowsheet for the original G . A . process plant column. We get the temperature and pressure of this includes five sections as shows in Fig. 1, which also shows feed stream from Fig. 2 which shows the phase equilibthe actual molar flows of the components. There is a rium of H20/HI/I2 mixtures with a constant 12 mol large recirculation of water between Sections I and III, fraction of 39%. This is the I2 mol fraction coming from and an even greater recirculation of water and iodine Section I. In Fig. 2 we find a small temperature between Section I and III. Section V is not part of the dependence of the azeotropic point. As mentioned chemical process. It includes heat transfer and power above we know that remarkable hydrogen pressures are generation equipment and accomplishes energy transfer only found in the vapor over solutions with HI content higher than the azeotropic tool fraction. That deteramong the various sections of the chemical plant. INTRODUCTION
545
546
M. ROTH AND K. F. KNOCHE
½~=
H,O
J
HzS& I~!
SectionI
mines the minimum temperature and pressure of the feed stream. We find a pressure of 22 bars and a temperature of 262°C. The stream coming from Section 1 is pumped to 22 bars and preheated to the boiling temperature of 262°C and then enters the distillation column. The product from the bottom consists of mostly iodine and small parts of water and HI. To close the water balance, a side stream must be taken out of the column. This stream consists of water, HI and a small amount of iodine. The ratio of HI/H20 mol fractions of the side stream must be lower than the ratio of the feed stream. The bigger the difference between the ratios, the smaller is the feed stream. But on the other hand, the HI mol fraction of the side stream must be high enough to get an equilibrium vapor with a sufficient HI pressure. These two conditions are the reason for the high mol numbers of the feed stream. The product from the top of the column consists of hydrogen and HI. The water and iodine content is negligible. Figure 3 shows the phase equilibrium of the quaternary system H20/HI/I2/H2 at a pressure of 22 bars, because that is the pressure of the column. We see a strong dependence of the azeotropic HI tool fraction for different iodine mol fractions in the solution. Figure 3 shows the phase equilibrium of the HI mol fraction in the liquid and the vapor and supplementary the hydrogen mol fraction in the vapor. We see that we get high hydrogen mol fractions with low iodine content and
SectionII
12HzO+SO22 *IH - ~SC+ I2H~ INv_. T~HS 'O _H -'O ' ÷SU O~ :" +30, 1
'" 2HI 1OH,Ofk\ 5H,O IOH,OI 81~/ ,,70, 8Izl l \x
;
!
SectionV
i
I
"d
(
/
/
/
/
/
Secto i nIV
Primory Energy [
2HI-- H.~+I2
Source [
H~ ....... Energy Tronspor t Mclteri(IITransport
-
Fig. l. Flowsheet of the G.A. process.
Pha,eEquLL.:H20/HI/12 (x-J2-0.39)
I
-Zo. R~ @
o (_ Q.
--
J
180 C
J
i
/
|quid
J
ca o 0.0
0.1
0.2
0.3
x-HI,
9-HI
Fig. 2. Phase equilibrium for le = 39%.
0.4
0.S
547
THERMOCHEM1CAL WATER SPLITTING
Phase g q u L L . :
H20/HI/I2/H2
p - 22 b a r
o tn
o t~
x-HJ
I
¢,1 cD --
10.7)
o
~ { 0 . 3 9 1 o
4ol
i
{0.1)
o
~-H2
ol o
0.0
o.~
0.1
o.~
d.~
-~.~
x-HI, 9-HI, ~-H2
Fig. 3. Phase equilibrium at column pressure.
ConcentrotLon Proftte8
Concentrotton ProfLtes H2 c~
o
HI
H20
I o
o
H20
L ~:-
C
~o 0 ~o
12 {3
o
c~
o o
0.0
0.2
0.t vapor
0.6 moL
OL8
fractLon
Fig. 4. Concentration profiles of the vapor.
I I. 0
0.0
0.2
0.4
0.6
0.8
LLquLd moL fractLon
Fig. 5. Concentration profiles of the liquid.
1.0
548
M. ROTH AND K. F. KNOCHE
temperatures about 220°C or lower. The profiles of the concentrations and the temperature along the column stages are shown in this diagram. We find the feed stream in Fig. 3 at the azeotropic point for the iodine mol fraction of 0.39. Down to the bottom of the column the distillation follows the azeotrope. Up to the top we get over azeotropic concentrations. If we follow the concentration profiles from the feed to the top, we see that we get a remarkable hydrogen content only for very little iodine in the liquid. That means the HI dissociation takes place only in the upper part of the column. But we can see in this diagram that the dissociation will increase very rapidly when it starts. Figure 4 shows the concentration profiles in the vapor at every stage of the column. We see that the main hydrogen production takes place at the last three stages. Also we see very complicated behaviour of the water and HI profiles. Additional to the last diagram Fig. 5 shows the concentration profiles in the liquid phase. If we compare Fig. 4 and Fig. 5 we find only for negligible iodine content an increase of the hydrogen production.
t n H20 HI H2
= = -
As mentioned above we get large mol flows which we see in Fig. 6. Per 1 kmol hydrogen which is produced we have a feed stream of 125.8 kmol. We need this large feed stream because we take, with the side stream, a large amount of HI out of the column. This side stream makes about 60% of the inlet mol flow.
THE F L O W S H E E T The calculation of this column leads to a new flowsheet (Fig. 7). This flowsheet should only be connected to Section I of the G.A. process for the material flows. But we need a large amount of heat at the bottom for a temperature of 320°C. The preheating of the feed stream may be done by cooling the side stream and the bottom stream. There is a lot of heat to transport in heat exchangers but we must remember that we have only liquid streams. Table 1 lists a summary of the energy which is required per 1 kmol of hydrogen. The main advantage of the present flowsheet compared with the G.A. proposal is that there is only a little
221 C 3.32 kmol 54 16 % 30 t n H2 HI
t n t n H20 HI I2
= = -
262 C 125.8 51% 10 % 39 %
H20 HI I2
kmol
-+ t n H20
= : -
310 C 41.7 kmol 4 %
-
95
HI I2
1 %
Fig. 6. The distillation column.
= =
= --
251 76.4
-
80. 1% 12.5 % 7.4 %
25 C 1 kmo] 99.7 0.3 %
C kmol
THERMOCHEMICAL WATER SPLITI'ING
549
tIE 6 H20 SEE. I =~,9
)o
H~O
. S H~O secl
I I
I
lIE 6
)1
Fig. 7. The flowsheet.
Table l. Energy summary (according to Fig. 7) energy (kmol H2) Pump HE 1 HE 2 HE 3 HE 4 HE 5 HE 6 HE 7 HE 8
5,8 608.5 405.2 207.7 -78.0 -8.8 237.0 -39.6 -26.6
MJ MJ MJ MJ MJ MJ MJ MJ MJ
electrical energy needed to drive the pumps. But there is also an important disadvantage. That is the very large feed stream and following there is a large a m o u n t of heat required at a t e m p e r a t u r e of 320°C (see H E 6 of Fig. 7).
CONCLUSION D u e to the availability of sufficient t h e r m o d y n a m i c data concerning the quaternary system H20/HI/I2/H2 it was possible to develop a new proposal for the H I decomposition of the General A t o m i c process. This proposal needs no extraction agent and a minimum of electrical energy.
internal heat exchange internal heat exchange internal heat exchange
REFERENCES 1. J. H. Norman, G. E. Besenbruch and D. R. O'Keefe, Thermochemical Water-Splitting for Hydrogen Production/Final Report, General Atomic Company, 10955 John Jay Hopkins Drive, San Diego, California (1981). 2. J. E. Funk and K. F. Knoche, Hydrogen by Thermochemicl Water Splitting, Proceedings of the Tenth Technical Workshop, Tokyo, Japan (27-29 July 1987). 3. H. Engels, K. F. Knoche and D. H6ckelmann, Vapor Pressure, Pseudo Aceotrop and Miscibility Gap of the System HI~H20~12, Proceedings of the l.E.A.-Workshop, Honolulu (1985), 4. H. Engels, Anwendung des Modells der lokalcn Zusammensetzung auf Elektrlytl6sungen. Thesis RWTH Aachen, FRG (1985).