- Email: [email protected]
Cl2 energy storage and power generation system using a CuCuCl catalyst
ht. .I. Hydrogen Energy, Vol. 22,No. 6, pp. 591-599,1997
Pergamon PII: S0360-3199(96)0018>8
CLOSED-CYCLE
AVINASH
0 1997InternationalAssociationfor HydrogenEnergy Elsevier Science Ltd All rightsreserved.Printedin GreatBritain 036&3199/97
$17.00+0.00
HCl/HJQ ENERGY STORAGE AND POWER GENERATION SYSTEM USING A Cu/CuCl CATALYST
K. GUPTA,*T
CHARLES F. SONA,* ROBIN Z. PARKER,* EDWARD J. BAIRS and ROBERT J. HANRAHANS *Solar Reactor Technologies, Inc. 3250 Mary Street, Miami, FL 33133, U.S.A. IDepartment of Chemistry, Indiana University, Bloomington, IN 47405, U.S.A. §Department of Chemistry, University of Florida, Gainesville, FL 32611, U.S.A. (Receiaedfor publication 8 July 1996)
energy storage system based on HCI-HZ-Cl2 chemistry is described. When dry HCl gas is passed over a copper surface, HZ is released and Cu is oxidized to CuCI. Subsequently, upon heating to high temperatures, the CuCl bed is decomposed back to Cu, releasing CIZ. The release of Hz from HCI on a Cu surface was studied as a function of temperature, flow rate, and specific surface area. The products H, and CuCl were measured between 400 and 600 K; temperature has only a small effect on the reaction rate, at most T”‘. The surface geometry and gas flow allowed several thousand gas-surface collisions per HCI molecule for complete conversion of HCI to HZ.The thickness of the CuCl layer is not the limiting factor in the reaction probability for layers lessthan a few thousand monolayers deep. Recovery of Cl, by the thermal decomposition of CuCl was examined as a function of temperature and pressure. Useful Cl, yields were found only for system pressuresbelow ca. 10 torr, at temperatures above 700 K, and using a flow of inert gas to sweep out the product. c; 1997International Association for Hydrogen Energy Abstract--An
NASA activities, would have been suitable for the housekeeping chores. It is evident that only space based reactors would possess the power capabilities required of burst mode activities. Only limited attention was given to proposed solar-dynamic solutions as described in this paper. However, it appears unlikely that the deployment of a substantial number of nuclear reactors in low space orbit (i.e., the nuclear reactor approach) would be accepted. On the other hand, solar dynamic solutions, which potentially could have been utilized for the SDIR’s requirements, continue to have potential applications at present and in the future. The energy storage methodology which we describe in this paper is based on the HCl-HZ-Cl, chemical system [l-4]. It is proposed that an inventory be maintained of molecular H, and Cl,. The former would preferably be stored as the hydride of a light metal, and the latter as liquified Cl,. For burst mode activity, the H, and Cl, would simply be combined in an appropriate burner, using the resulting heat to drive turbines for mechanical or electrical energy requirements. The product HCl would be temporarily trapped, in a large volume, low mass container such as the deployable PTFE bag. If the subsequent sequence of events so permitted, the HCl could be decomposed over a period of hours or days as necessary, utilizing a Cu based catalyst bed as described
INTRODUCTION In connection with the Strategic Defense Initiative Program, The Department of Defense established requirements for a self sustaining energy system which could be deployed on a space platform. The system was required to display both a “housekeeping mode” and “burst mode” of energy production. The former, which was required to be sustainable over several years, provided only a relatively low level of energy output sufficient for day to day energy needs of the installation. The latter mode, presumably to be used in connection with weapons capabilities, required tens of kW of power output. Although the national defense needs which formed the basis of the Strategic Defense Initiative program had largely disappeared by the early 1990’s, the related energy system concept may still be of interest, both for space and terrestrial applications. At the height of activity under the SD1 program (colloquially known as “Star Wars”), at least some of those responsible for the direction of the project seemed to prefer nuclear based energy systems. It is likely that radioisotope heat sources, which were used in the past in
t To whom correspondence should be addressed 591
A. K. GUPTA
592
later. The product H, and Cl, would be returned to inventory. The system efficiency could be increased somewhat in housekeeping mode. Under conditions of steady-state energy requirements, the Cl, gas would be passed through a chamber strongly illuminated by a solar collection device, prior to being admixed and burned with Hz. The resulting HCl would contain not only the heat released from the chemical reaction, but also additional caloric content resulting from the photolysis step [5]. The HCl would be back-decomposed over the Cu/CuCl catalyst bed, and the products, H, and Cl, returned to storage. Figure 1 shows a schematic block diagram of the overall system for energy storage and power generation. The present paper will be concerned specifically with the technology to back decompose HCl into Hz and Cl2 for return to inventory. The system is based on the concept that when dry HCl gas is passed over a Cu bed, Hz gas is released and the Cu is oxidized to CuCI. Subsequently, upon heating to high temperatures, the CuCl bed is decomposed back to Cu, releasing Cl, gas. The practicality of these two steps is examined in the experiments described below. Experiments were carried out to study the release of H, gas from HCl on the Cu metal surface as a function of temperature, flow rate, and specific surface area. The reaction products and their rate of formation was studied quantitatively. Also, the recovery of Cl, gas by thermal
PI ul
decomposition of CuCl was determined as a function of temperature and pressure. EXPERIMENTAL Experiments to study the decomposition of gaseous HCl were carried out by two separate methods. Method
1: (Preliminary
study)
One end of the reactor tube was connected to the hydrogen chloride and argon cylinders and the other end was bent downwards and immersed in a beaker containing NaOH. The reactor tube was a 30 cm long quartz tube with 2.4 cm ID, packed with silicon carbide support (16 mesh) and copper powder. The quartz reactor was placed in a tube furnace, with a temperature range from ambient to 1300 K. The furnace temperature was measured using a cromel-alumel thermo-couple. Two sampling ports were located upstream and downstream from the reactor for withdrawal of aliquots of gas to be injected into the gas chromatograph for analysis. A schematic diagram of the experimental setup for the formation of H, from HCI on a Cu bed is shown in Fig. 2. Analysis of products was carried out using a gas chromatograph equipped with a thermal conductivity detector. Both helium and argon were used as carrier gases. The column used was a 2 m long tube of l/8” diameter which was packed with 60/80 mesh celite support with N-decylpthalate as the stationary phase.
solar Radiation
-----b
HCI L F
Solar Pre-heat
Cl2 Storage
Regeneration System
Heat Exchanger
A
HCI Storage Fig. 1. Schematic block diagram of the overall system for energy storage and power generation
CLOSED CYCLE HCI-H&l,
ENERGY STORAGE SYSTEM
593
Tube Furnace
Hydrogen Chloride
NaOH Solution
Fig. 2. Experimental setup for the formation of hydrogen from hydrogen chloride using a copper bed.
A typical experimental run took thirty minutes to an hour, once a set of experimental parameters (temperature, pressure, flow rates) had been established. Several samples were taken upstream and downstream from the reactor and injected into the gas chromatograph for analysis. When analyzing for Hz, argon was used as the carrier gas; for other pertinent gases helium was used as the carrier gas (when argon is used the GC detector shows little response to hydrogen chloride). Experiments were carried out over a range of temperatures. Also, the amount of Hz produced was measured at fixed temperatures by varying the number of moles of HCl. The apparatus to study the decomposition of CuCl consisted of a tank of helium connected through a needle valve to the quartz reactor (described above) packed with silicon carbide and CuCl. The quartz reactor tube was placed in a tube furnace. The other end of the reactor was connected to a cold trap and a mechanical pump. The experimental setup for the decomposition of CuCl is shown in Fig. 3. An experimental run began with the addition of a
known amount of CuCl to the reaction bed. The reactor was evacuated and the cold trap was filled with liquid nitrogen. A flow of He gas was started and the tube furnace was turned on. In separate experiments the pressure was lowered to ca. 10 torr. After the allowed time of the reaction the cold trap was valved off and removed from the system. The cold trap was then warmed and a small positive pressure (with respect to ambient air) of helium was leaked into the trap to bring the total pressure to one atmosphere. A gas chromatograph with He as the carrier gas was used to analyze Cl,. From the known volume of the cold trap, a knowledge of the mole fraction of Cl, in an aliquot taken from the trap yields the total number of the moles of Cl, gas produced from the reaction. The reaction time was varied from one to four hours. Method 2: (Detuiled study) The experimental apparatus consisted of the following components. A 14 liter Pyrex bulb stored the HCl or argon driver gas. The bulb (large enough to have nearly constant pressure during a run) was connected to the
Sampling Port
Helium
Fig. 3. Experimental setup for the decomposition of CuCl
A. K. GUPTA
594
quartz reactor placed in a heating assembly through a main manifold and a capillary of precision 0.025 mm fused silica tubing. The reactor outlet was connected to a titration cell with a magnetic stirrer containing standard base solution and indicator. A micrometer gas burette consisting of a 1” diameter ID precision bore Pyrex tube sealed by a teflon plunger was used to measure the volume of the gas which passes through the titration cell. The reactor pressure was maintained at a fixed value by changing the volume of the gas burette as the reaction progressed by adjusting the position of the plunger using a micrometer head having a sensitivity of 0.00645 cc/div. A baraton pressure transducer was used to monitor the pressure of the gas in the titration flask and the reactor. The complete experimental setup for the reaction between copper and hydrogen chloride is shown in Fig. 4. The reactor was a quartz tube 15 cm long and 2.2 cm ID. The central 7.5 cm of the inner wall was coated with copper by vacuum deposition before each experiment. A cross section of the metal vaporizer unit is shown in Fig. 5. It consisted of two aluminum end caps which sealed a clean quartz reactor tube to an oil diffusion pump. The reactor caps also acted as electrical terminals for the 0.030” tungsten wire along the axis of the reactor tube that heated the copper metal to the vaporization temperature, roughly 1100°C. Heavy steel spring in the aluminum cap exerted axial tension on the tungsten wire
Thermocouple Gauge
et al
to keep it from sagging when heated. The 0.20” copper wire was coiled tightly around the tungsten wire for about 1 inch, then cut into short sections and distributed along the tungsten wires. The reactor was then heated to 20& 225’C and evacuated to a pressure of less than lo-’ torr. Copper was evaporated on the hot quartz surface using about 50 amps from a 10 volt transformer operated from a variac. The heating assembly consisted of a stainless steel block 3” long by 3” diameter. Four-200 Watt cartridge heaters were encased in eight layers of cotronix ceramic paper. The temperature was measured by a chromelalumel thermocouple, and was controlled by an Omega Inc. microprocessor control, CN 9000 series. The aluminum end caps of the heater assembly were water cooled and held in good thermal contact with monel end caps of the reactor. A quartz insert which almost filled the quartz reactor was used to restrict gas flow to the annular space between the Cu surface and the insert. The surface to volume ratio was changed by using inserts of different diameter. The inserts were filled with 200 torr argon and sealed so that the pressure was roughly equal to the reactor pressure at the reaction temperature. The quartz reactor and the heating assembly are shown in Fig. 6. The flow rate was calculated using Poiseuille’s law and was experimentally verified by acid base titration of the HCl reagent. HCl gas was made to flow at various llow rates through the quartz reactor tube into the titration
Pressure Transducer
u
Gas Burette
Gas Titration Cell
Fig. 4. Complete experimental setup to study both the reaction between copper and the HCl and decomposition
of CuCI.
CLOSED CYCLE HCI-Hz-Cl,
Diffusion Pump L-.---d
ENERGY
STORAGE
Quartz Reactor
Tungsten heater with
Tube I
Copper Coils
595
SYSTEM
Tension Spring
I
?
Fluted Electrical Terminals
Fig. 5. Cross section schematic of the metal vaporizer unit. Quartz Reactor And Insert
Ceramic Paper Insulation
Heater Block Wii Cartridge Heaters
Water Cooled End Cap
Capillar
----Is
.
. * .
Fig. 6. Quartz reactor heating assembly. cell which contained 20 cc of 0.20 M standard NaOH solution with phenolphthalein indicator. At the equivalence point the color changed from pink to colorless. The flow rate of HCl was determined from the time required to neutralize a fixed number of moles of NaOH. The copper coated quartz reactor along with a quartz insert was then placed in the heating assembly. The system was evacuated and the heater was turned on to bring the quartz reactor to the desired operating temperature. The flow of HCI was then started and the insoluble H, gas was collected by changing the volume of the gas burette so as to keep the pressure constant. The time required for the indicator to change color was noted. From the volume of H2 collected at constant pressure and the time required to neutralize a known number of
moles of NaOH, the number of moles of Hz produced was calculated. Production of CIZ from the thermal decomposition of CuCl was measured by heating the reactor and sweeping the Cl, gas produced with argon gas. Measurements were made by flowing the stream through a standard KI solution with starch indicator in the titration cell.
RESULTS
AND
DISCUSSION
Thermodynamic calculations for the Cu/HCl/H,/C12 system are summarized in Table 1. Data for the computation was obtained from JANAF thermochemical tables [6].
A. K. GUPTA et al.
596
Table 1. Reactions in the Cu/HCl,/H,/CI, system AG Favorable
Reactions
Unfavorable All temperatures
Cu(s) + 2HCl -+ CuCl,(s) + H,(g) Below 750 K
2’34s) + ZHCI(g) + 2CuCl(s) + H,(g) ZCuCl(s) + 2HCl(g) + 2CuCl,(s) + H,(g)
All temperatures All temperatures
2CuCl(s) + 2CuCl(v) 2CuCl(v) + 2cu + Cl,(g) 2CuCl(s) -+ Xu(s) + Cl*(g) 2CuCl,(s) + ZCuCl(s) + Cl,(g) CuCl,(s) + Cu(s)+ Cl,(g)
Below 1000K All temperatures Above 750 K High temperatures
(a) Reaction of HCl with Cu The reaction of Cu with gaseous HCI may yield CuCl, or CuCl along with H2. A series of thermodynamic computations indicate that the free energy for conversion of the reaction of Cu with HCI to form CuCl?, Reaction 1, is unfavorable at all temperatures considered, Table 1. Cu(s)+2HCl(g)
+ CuCl,(s) + H,(g).
(1)
By contrast, it is seen that the reaction between Cu and HCl to form CuCl, Reaction 2, is thermodynamically favorable from below room temperature up to about 700 K. 2Cu(s) + ZHCl(g) + 2CuCl(s) + H?(g).
(2)
It is possible that the CuCl(s) formed in Reaction 2 could further react with HCl to form CuCl,, Reaction 3. However, calculations indicate that this reaction is unfavorable and may yield no more than a trace amount of CuCl,(s). CuCl(s) + 2HCl(g) -+ 2CuCl,(s) + Hz(g).
(3)
Thus, from thermodynamic computations it seems the products of the reaction between Cu and HCl are CuCl(s) and H,. (b) Thermal decomposition of CuCl(s) Thermodynamic computations indicate that when CuCl(s) is heated to very high temperatures CuCl(v) is formed, Reaction 4. Free energy change for this reaction is not very favorable and the extent of vaporization is small. 2CuCl(s) + 2CuCl(v).
(4)
If CuCl(v) is produced it would spontaneously decompose to Cu and Cl, Reaction 5. 2CuCl(v) --t 2Cu(s) + Cl,(g).
(5)
Reaction 6, which is a sum of Reactions 4 and 5 shows the overall decomposition of CuCl(s) to Cu and Cl,. %CuCl(s) + 2Cu(s) + Cl*(g).
(6)
If any trace amount of CuCl, is formed by Reactions 1 and 3, it could be decomposed to CuCl(s) and Cl, at
moderate temperatures or to Cu and Cl, at high temperatures, Reactions 7 and 8. 2CuCl,(s) + 2CuCl(s) + Cl,(g)
(7)
CuCl,(s) + Cu(s) + Cl,(g).
(8)
Thus, in the overall reaction scheme for the formation of H, and Cl, from HCI, it is seen that (a) the reaction of Cu and HCl to form CuCl and H, is favorable, (b) CuCl(s) can be decomposed to Cu and Cl,, Reaction 6 (a combination of Reactions 4 and 5) provided that CuCl(s) is heated to high temperatures to form CuCl(v). Several runs were performed at temperatures above 700 K (713 K, 740 K, 770 K, 784 K, and 933 K) using Method 1 as described above. No significant amount of hydrogen evolution was observed, which tends to support the fact that CuCl and not CuC1, is the product of the reaction between Cu and HCl. The equilibrium constant calculated from the experimental data appears somewhat low, indicating a low residence time. Table 2 shows the partial pressures (HCl and HJ and equilibrium constants (theoretical and experimental) at various temperatures for the reaction between Cu and HCl. It is seen from Table 2 that the percent conversion of HCl to product increases with decrease in temperature and increase in HCl concentration (partial pressure). Using Method 2 the rate of H, production was measured by changing the surface to volume ratio and at various temperatures. Table 3 shows reaction of HCl with copper film under various conditions. The product formed is white in color indicating that the product is CuCl and not CuCl,. Detailed calculations are given in the report submitted to Wright Patterson Air Force Base
171. MODEL
FOR ANALYZING
DATA
To define conditions that lead to complete conversion of HCl it is necessary to distinguish the limitations that are caused by the surface geometry and flow rate at the copper surface from the conditions imposed by the reaction itself. The objective is to measure the extent of reaction using conditions that are sufficiently well defined that the results can be used to design a reactor in which the conversion of HCl to H, is virtually complete
CLOSED CYCLE HCI-HZ-Cl, ENERGY STORAGE SYSTEM
591
Table 2. Partial pressuresof HCI and H,, percent conversion and equilibrium constants at 626 K, 643 K, and 655 K for the reaction between Cu and HCI
T/K
P HC,,,“)
P HCl,o”t,
P”.
%Conv.
gperimental)
tieoretical)
626 626 626
0.80 0.54 0.27
0.30 0.25 0.15
0.15 0.13 0.07
62 53 44
1.29 1.44 1.76
1.91 1.91 1.91
643 643 643
0.68 0.62 0.34
0.40 0.35 0.19
0.18 0.18 0.08
41 43 42
1.06 1.21
1.51 1.51 1.51
655 655 655
0.85 0.67 0.34
0.38 0.32 0.20
0.22 0.16 0.07
59 52 41
Table 3. Reaction of HCl with copper film under various conditions Temperature, K Flow rate, pmoles/sec Surface/Volume ratio, cm-’ pmoles of Cu deposited pmoles of HCl reacted pmoles of H, formed pmoles Cl bound
500
500
600
400
500
11.8 11.8 11.8 11.8 11.8 5.88 11.8 11.8 11.8 11.8 198 266 592 743 787 58.0 69.3 80.7 80.7 69.3 27.5 33.0 41.4 36.2 37.5 59.2 73.5 78.8 78.8 64.8
The extent of the HCI reaction has a complex dependence on temperature, pressure, flow rate, surface area, and reactor volume. Temperature, for example affects both the mobility of the gas reactants and the probability of reaction per encounter between the reactants. The following model characterizes the components of reaction probability. The HCl flow through the reactor is resolved into radial and axial motions that are assumed to be independent. Axial flow is expressed as the average residence time of an HCl molecule passing through the reactor in the absence of reaction. The radial migration determines the probability that an HCl molecule will reach the surface during the residence time. The probability of reaction is the product of a sequence of probabilities, some features of which are distinguished by the present experiments. The probability that an HCl molecule will reach the copper surface is equal to the radial displacement (x) by random motions during its residence time in the reactor t, divided by the radial distance between successive encounters with the reactor surface. This probability can range from much greater than, to much less than 1, depending on the flow conditions and geometry. The solution of the l-dimensional random walk diffusion problem gives (x) in terms of the diffusion constant D. (x) = (2Dt,)“‘. The diffusion constant is calculated in terms of the average molecular speed (c) = (8RT/nM)“* and the hard sphere value for the mean free path 1 = RT/[2%P]
I .49
1.28 1.28 I .28
1.23
1.25 1.32
D = 1/.3(c)i. Using a hard sphere diameter of 3.0 A for HCl, the residence time is calculated from the reactor geometry and the mass flow rate of HCl,f(HCl).
w: - mLa,~ ” =
4f(HCl)RT
where d2 is the i.d. of the reactor, insert, and X,,,,, is the length of the probability, p(x, t,), is the ratio of distance to the width of the annular p(x, t,) = (x)/(d2
REACTION
d, is the o.d. of the copper deposit. The the total migration space.
-d,).
PROBABILITY PER SURFACE ENCOUNTER
The observed reaction probability p(obs) is the ratio of the mass flow rate of H, to that of HCl. The mass flow rate of HCl is measured in a preliminary experiment by timing the HCl flow required to neutralize a quantity of standard base. The mass flow rate of Hz is determined from the rate of change of gas volume measured in the gas burette. dabs) = 2ff(Hd/-if(HCU The observed reaction probability is the product of the gas diffusion probability and the probabilities that make up the reaction probability per encounter of an HCl molecule with the copper surface. p(react) = P(obs)/P(x, t,) Results in Table 4 show that p(react) is independent of Table 4. Components of the observed reaction probability T(K)
P (torr) d2-d,
500 400 500 500 600
600 600 600 600 600
t, (set)
0.311 12.37 0.170 8.75 0.170 7.00 0.170 7.00 0.170 5.83
Pb. fr)
p (obs) p (react)
19.9 25.9 27.4 27.4 28.7
0.0137 0.0174 0.0189 0.0192 0.0222
0.00069 0.00067 0.00069 0.00070 0.00077
598
A. K. GUPTA et al.
surface/volume ratio and nearly independent perature, T”* at most.
of tem-
DEPTH
Time, hours
where Nay is Avagadro’s number. By definition the number of moles of copper in a monolayer is = 1.67 x 10e7moles
~d~x,,,,,dx,d~~~)/M(~"~
and the depth of penetration monolayer is m = 2n(H,)/n,
of HCl into the copper
= 390 monolayers/cc H,.
To examine the linearity of H2 production over an extended period of time an experiment was performed in which the H, was measured in a series of stopped-flow steps. The HCl was stopped for 8 min, then allowed to flow normally while the accumulated hydrogen was measured. The total amount of hydrogen corresponds to a significant fraction of the weight of the copper film. The amount of hydrogen accumulated in each step was constant, as shown in Fig. 7. Examination of the film at the end of the experiment
6L 2,000
5-
i
-I
84
1,500
5
Temperature, K
1
2
3
4
640 685 816 987
0 10 66 82
0 15 68 88
0 17 12 88
0 19 83 89
OF PENETRATION
From the observed rate of formation of H, further information about the reaction probability is obtained. Penetration of HCl into the solid Cu film in terms of monolayers is calculated from the density of the copper, d(Cu) = 8.92 g/cc. The thickness of the monolayer is
n, =
Table 5. Percent decomposition of CuCl at various temperatures
-
F x3I -
1,000
I 2 0 d 2
Conditions: Constant flow of argon gas at ca. 10 torr through 30 cm long tube. CuCl coated on silicon carbide. Effluent trapped at liquid nitrogen temperature and subsequently analyzed by gas chromatography. showed that parts of the copper film were completely penetrated by the reaction. The nonlinearity observed may be due to a decrease in the reactive surface area. For film depths of about l&1000 monolayers there was no indication that the thickness of the CuCl film had a significant affect on the reaction rate. Experiments carried out at 400-650 K to study the decomposition of CuCl by Method 2 using KI and starch indicator solution show that Cl, in the flow stream is negligible at this temperature. Qualitative examination of the films at the end of the experiment show that there is a significant migration of CuCl across the annular space to the insert. The coating on the insert is more or less uniform. In view of extremely small vapor pressures at these temperatures, the results are encouraging. According to Method I which involves lowering of pressure, useful chlorine yields were found for system pressures below 10 torr and above 700 K using a flow of inert gas to sweep out the chlorine. It is seen from Table 5 that the percent decomposition of CuCl increases with temperature. Results of the experiments indicate that the decomposition of HCl can be carried out by passing dry HCl gas over a Cu bed to release H, and form CuCI. Subsequently, upon heating, CuCl can be decomposed back to Cu releasing Cl, gas, provided the pressure is below 10 torr and the product gas is swept out using an inert gas. Other metal/halides such as Ag/HCl or Ag/HBr might prove superior in this application. Also, for the release of halogens, metal halides like AgCl and AgBr would be ideal, since photo-decomposition of these halides is known to occur.
f
CONCLUSIONS
22-
Reaction of HCI with CM
-I 500
:t/?: 0
20
40
Time,
60
loo0
mid&
Fig. 7. Volume of hydrogen produced and the corresponding monolayers reacted versus reaction time.
(1) Cu and HCl(g) form CuCl(s) and Hz in the temperature range 40&600 K. (2) Percent conversion to product increases with HCl pressure. (3) Temperature has only a small effect on the reaction rate, at most T”‘. (4) The surface geometry and gas flow must allow at least a few thousand gas-surface collisions per HCl molecule for complete conversion of HCl to H,.
CLOSED CYCLE HCl-Hz-Cl,
(5) Thickness of the CuCl layer is not a limiting factor in the reaction probability for layers less than a few thousand monolayers deep. Production of Cl, by thermal decomposition of CuCl (1) No Cl, was detected in a stream of argon flowing over a CuCl film at 650 K and 1 atmosphere. (2) CuCl is more mobile at temperatures in the 400-600 K range than might be expected from its equilibrium thermodynamic properties. (3) Cl: is produced at pressures below ca. 10 torr at temperatures above 700 K. (4) Other metal/halide combinations such as Ag/HCl or Ag/HBr might prove superior to Cu/HCl in this application since photo-decomposition of AgCl and AgBr is known to occur. AcknoM,ledgemeni-This work was sponsored by Wright Patterson Air Force Base under contract #F33615-86-C-2716.
ENERGY
STORAGE
SYSTEM
599
REFERENCES 1, Hanrahan, R. J. and Gupta, A. K., US Patent No. 4,848,087, 1989. 2. Williams, L. O., Hydrogen Power-An Introduction to Hydrogen Energy and its Applications, 1980, Pergamon Press, NY.
3. Nishimoto, Y., Mizumoto, Y., Mitsuoka, S. and Hasegawa, 4.
5.
6. I.
S., (Mitsubishi Heavy Industries, Ltd) Japan, Kokai 76/548892 (Cl COlBjO3) (14 May 1976) Appl. 74/128,658 (8 November 1974). Mellor, J. W., A Comprehensive Treatise on Inorganic Chamistry and Theoretical Chemistry, Vol. II (1922), and Supplement II, Part I (1956). Parker, R. Z., Hanrahan, R. J. and Cox, J. D., US Patent No. 4,848,087, 1989. JANAF Thermochemical Tables, Second Edition, National Standard Reference Data Series, National Bureau Standard (U.S.) (June 1971). Solar Augmented Fluids Technologies-A Report from Solar Reactor Technologies to Wright Patterson Air Force Base, OH 45433-6563. Report No. WRDC-TR-89-2036.
Pergamon PII: S0360-3199(96)0018>8
CLOSED-CYCLE
AVINASH
0 1997InternationalAssociationfor HydrogenEnergy Elsevier Science Ltd All rightsreserved.Printedin GreatBritain 036&3199/97
$17.00+0.00
HCl/HJQ ENERGY STORAGE AND POWER GENERATION SYSTEM USING A Cu/CuCl CATALYST
K. GUPTA,*T
CHARLES F. SONA,* ROBIN Z. PARKER,* EDWARD J. BAIRS and ROBERT J. HANRAHANS *Solar Reactor Technologies, Inc. 3250 Mary Street, Miami, FL 33133, U.S.A. IDepartment of Chemistry, Indiana University, Bloomington, IN 47405, U.S.A. §Department of Chemistry, University of Florida, Gainesville, FL 32611, U.S.A. (Receiaedfor publication 8 July 1996)
energy storage system based on HCI-HZ-Cl2 chemistry is described. When dry HCl gas is passed over a copper surface, HZ is released and Cu is oxidized to CuCI. Subsequently, upon heating to high temperatures, the CuCl bed is decomposed back to Cu, releasing CIZ. The release of Hz from HCI on a Cu surface was studied as a function of temperature, flow rate, and specific surface area. The products H, and CuCl were measured between 400 and 600 K; temperature has only a small effect on the reaction rate, at most T”‘. The surface geometry and gas flow allowed several thousand gas-surface collisions per HCI molecule for complete conversion of HCI to HZ.The thickness of the CuCl layer is not the limiting factor in the reaction probability for layers lessthan a few thousand monolayers deep. Recovery of Cl, by the thermal decomposition of CuCl was examined as a function of temperature and pressure. Useful Cl, yields were found only for system pressuresbelow ca. 10 torr, at temperatures above 700 K, and using a flow of inert gas to sweep out the product. c; 1997International Association for Hydrogen Energy Abstract--An
NASA activities, would have been suitable for the housekeeping chores. It is evident that only space based reactors would possess the power capabilities required of burst mode activities. Only limited attention was given to proposed solar-dynamic solutions as described in this paper. However, it appears unlikely that the deployment of a substantial number of nuclear reactors in low space orbit (i.e., the nuclear reactor approach) would be accepted. On the other hand, solar dynamic solutions, which potentially could have been utilized for the SDIR’s requirements, continue to have potential applications at present and in the future. The energy storage methodology which we describe in this paper is based on the HCl-HZ-Cl, chemical system [l-4]. It is proposed that an inventory be maintained of molecular H, and Cl,. The former would preferably be stored as the hydride of a light metal, and the latter as liquified Cl,. For burst mode activity, the H, and Cl, would simply be combined in an appropriate burner, using the resulting heat to drive turbines for mechanical or electrical energy requirements. The product HCl would be temporarily trapped, in a large volume, low mass container such as the deployable PTFE bag. If the subsequent sequence of events so permitted, the HCl could be decomposed over a period of hours or days as necessary, utilizing a Cu based catalyst bed as described
INTRODUCTION In connection with the Strategic Defense Initiative Program, The Department of Defense established requirements for a self sustaining energy system which could be deployed on a space platform. The system was required to display both a “housekeeping mode” and “burst mode” of energy production. The former, which was required to be sustainable over several years, provided only a relatively low level of energy output sufficient for day to day energy needs of the installation. The latter mode, presumably to be used in connection with weapons capabilities, required tens of kW of power output. Although the national defense needs which formed the basis of the Strategic Defense Initiative program had largely disappeared by the early 1990’s, the related energy system concept may still be of interest, both for space and terrestrial applications. At the height of activity under the SD1 program (colloquially known as “Star Wars”), at least some of those responsible for the direction of the project seemed to prefer nuclear based energy systems. It is likely that radioisotope heat sources, which were used in the past in
t To whom correspondence should be addressed 591
A. K. GUPTA
592
later. The product H, and Cl, would be returned to inventory. The system efficiency could be increased somewhat in housekeeping mode. Under conditions of steady-state energy requirements, the Cl, gas would be passed through a chamber strongly illuminated by a solar collection device, prior to being admixed and burned with Hz. The resulting HCl would contain not only the heat released from the chemical reaction, but also additional caloric content resulting from the photolysis step [5]. The HCl would be back-decomposed over the Cu/CuCl catalyst bed, and the products, H, and Cl, returned to storage. Figure 1 shows a schematic block diagram of the overall system for energy storage and power generation. The present paper will be concerned specifically with the technology to back decompose HCl into Hz and Cl2 for return to inventory. The system is based on the concept that when dry HCl gas is passed over a Cu bed, Hz gas is released and the Cu is oxidized to CuCI. Subsequently, upon heating to high temperatures, the CuCl bed is decomposed back to Cu, releasing Cl, gas. The practicality of these two steps is examined in the experiments described below. Experiments were carried out to study the release of H, gas from HCl on the Cu metal surface as a function of temperature, flow rate, and specific surface area. The reaction products and their rate of formation was studied quantitatively. Also, the recovery of Cl, gas by thermal
PI ul
decomposition of CuCl was determined as a function of temperature and pressure. EXPERIMENTAL Experiments to study the decomposition of gaseous HCl were carried out by two separate methods. Method
1: (Preliminary
study)
One end of the reactor tube was connected to the hydrogen chloride and argon cylinders and the other end was bent downwards and immersed in a beaker containing NaOH. The reactor tube was a 30 cm long quartz tube with 2.4 cm ID, packed with silicon carbide support (16 mesh) and copper powder. The quartz reactor was placed in a tube furnace, with a temperature range from ambient to 1300 K. The furnace temperature was measured using a cromel-alumel thermo-couple. Two sampling ports were located upstream and downstream from the reactor for withdrawal of aliquots of gas to be injected into the gas chromatograph for analysis. A schematic diagram of the experimental setup for the formation of H, from HCI on a Cu bed is shown in Fig. 2. Analysis of products was carried out using a gas chromatograph equipped with a thermal conductivity detector. Both helium and argon were used as carrier gases. The column used was a 2 m long tube of l/8” diameter which was packed with 60/80 mesh celite support with N-decylpthalate as the stationary phase.
solar Radiation
-----b
HCI L F
Solar Pre-heat
Cl2 Storage
Regeneration System
Heat Exchanger
A
HCI Storage Fig. 1. Schematic block diagram of the overall system for energy storage and power generation
CLOSED CYCLE HCI-H&l,
ENERGY STORAGE SYSTEM
593
Tube Furnace
Hydrogen Chloride
NaOH Solution
Fig. 2. Experimental setup for the formation of hydrogen from hydrogen chloride using a copper bed.
A typical experimental run took thirty minutes to an hour, once a set of experimental parameters (temperature, pressure, flow rates) had been established. Several samples were taken upstream and downstream from the reactor and injected into the gas chromatograph for analysis. When analyzing for Hz, argon was used as the carrier gas; for other pertinent gases helium was used as the carrier gas (when argon is used the GC detector shows little response to hydrogen chloride). Experiments were carried out over a range of temperatures. Also, the amount of Hz produced was measured at fixed temperatures by varying the number of moles of HCl. The apparatus to study the decomposition of CuCl consisted of a tank of helium connected through a needle valve to the quartz reactor (described above) packed with silicon carbide and CuCl. The quartz reactor tube was placed in a tube furnace. The other end of the reactor was connected to a cold trap and a mechanical pump. The experimental setup for the decomposition of CuCl is shown in Fig. 3. An experimental run began with the addition of a
known amount of CuCl to the reaction bed. The reactor was evacuated and the cold trap was filled with liquid nitrogen. A flow of He gas was started and the tube furnace was turned on. In separate experiments the pressure was lowered to ca. 10 torr. After the allowed time of the reaction the cold trap was valved off and removed from the system. The cold trap was then warmed and a small positive pressure (with respect to ambient air) of helium was leaked into the trap to bring the total pressure to one atmosphere. A gas chromatograph with He as the carrier gas was used to analyze Cl,. From the known volume of the cold trap, a knowledge of the mole fraction of Cl, in an aliquot taken from the trap yields the total number of the moles of Cl, gas produced from the reaction. The reaction time was varied from one to four hours. Method 2: (Detuiled study) The experimental apparatus consisted of the following components. A 14 liter Pyrex bulb stored the HCl or argon driver gas. The bulb (large enough to have nearly constant pressure during a run) was connected to the
Sampling Port
Helium
Fig. 3. Experimental setup for the decomposition of CuCl
A. K. GUPTA
594
quartz reactor placed in a heating assembly through a main manifold and a capillary of precision 0.025 mm fused silica tubing. The reactor outlet was connected to a titration cell with a magnetic stirrer containing standard base solution and indicator. A micrometer gas burette consisting of a 1” diameter ID precision bore Pyrex tube sealed by a teflon plunger was used to measure the volume of the gas which passes through the titration cell. The reactor pressure was maintained at a fixed value by changing the volume of the gas burette as the reaction progressed by adjusting the position of the plunger using a micrometer head having a sensitivity of 0.00645 cc/div. A baraton pressure transducer was used to monitor the pressure of the gas in the titration flask and the reactor. The complete experimental setup for the reaction between copper and hydrogen chloride is shown in Fig. 4. The reactor was a quartz tube 15 cm long and 2.2 cm ID. The central 7.5 cm of the inner wall was coated with copper by vacuum deposition before each experiment. A cross section of the metal vaporizer unit is shown in Fig. 5. It consisted of two aluminum end caps which sealed a clean quartz reactor tube to an oil diffusion pump. The reactor caps also acted as electrical terminals for the 0.030” tungsten wire along the axis of the reactor tube that heated the copper metal to the vaporization temperature, roughly 1100°C. Heavy steel spring in the aluminum cap exerted axial tension on the tungsten wire
Thermocouple Gauge
et al
to keep it from sagging when heated. The 0.20” copper wire was coiled tightly around the tungsten wire for about 1 inch, then cut into short sections and distributed along the tungsten wires. The reactor was then heated to 20& 225’C and evacuated to a pressure of less than lo-’ torr. Copper was evaporated on the hot quartz surface using about 50 amps from a 10 volt transformer operated from a variac. The heating assembly consisted of a stainless steel block 3” long by 3” diameter. Four-200 Watt cartridge heaters were encased in eight layers of cotronix ceramic paper. The temperature was measured by a chromelalumel thermocouple, and was controlled by an Omega Inc. microprocessor control, CN 9000 series. The aluminum end caps of the heater assembly were water cooled and held in good thermal contact with monel end caps of the reactor. A quartz insert which almost filled the quartz reactor was used to restrict gas flow to the annular space between the Cu surface and the insert. The surface to volume ratio was changed by using inserts of different diameter. The inserts were filled with 200 torr argon and sealed so that the pressure was roughly equal to the reactor pressure at the reaction temperature. The quartz reactor and the heating assembly are shown in Fig. 6. The flow rate was calculated using Poiseuille’s law and was experimentally verified by acid base titration of the HCl reagent. HCl gas was made to flow at various llow rates through the quartz reactor tube into the titration
Pressure Transducer
u
Gas Burette
Gas Titration Cell
Fig. 4. Complete experimental setup to study both the reaction between copper and the HCl and decomposition
of CuCI.
CLOSED CYCLE HCI-Hz-Cl,
Diffusion Pump L-.---d
ENERGY
STORAGE
Quartz Reactor
Tungsten heater with
Tube I
Copper Coils
595
SYSTEM
Tension Spring
I
?
Fluted Electrical Terminals
Fig. 5. Cross section schematic of the metal vaporizer unit. Quartz Reactor And Insert
Ceramic Paper Insulation
Heater Block Wii Cartridge Heaters
Water Cooled End Cap
Capillar
----Is
.
. * .
Fig. 6. Quartz reactor heating assembly. cell which contained 20 cc of 0.20 M standard NaOH solution with phenolphthalein indicator. At the equivalence point the color changed from pink to colorless. The flow rate of HCl was determined from the time required to neutralize a fixed number of moles of NaOH. The copper coated quartz reactor along with a quartz insert was then placed in the heating assembly. The system was evacuated and the heater was turned on to bring the quartz reactor to the desired operating temperature. The flow of HCI was then started and the insoluble H, gas was collected by changing the volume of the gas burette so as to keep the pressure constant. The time required for the indicator to change color was noted. From the volume of H2 collected at constant pressure and the time required to neutralize a known number of
moles of NaOH, the number of moles of Hz produced was calculated. Production of CIZ from the thermal decomposition of CuCl was measured by heating the reactor and sweeping the Cl, gas produced with argon gas. Measurements were made by flowing the stream through a standard KI solution with starch indicator in the titration cell.
RESULTS
AND
DISCUSSION
Thermodynamic calculations for the Cu/HCl/H,/C12 system are summarized in Table 1. Data for the computation was obtained from JANAF thermochemical tables [6].
A. K. GUPTA et al.
596
Table 1. Reactions in the Cu/HCl,/H,/CI, system AG Favorable
Reactions
Unfavorable All temperatures
Cu(s) + 2HCl -+ CuCl,(s) + H,(g) Below 750 K
2’34s) + ZHCI(g) + 2CuCl(s) + H,(g) ZCuCl(s) + 2HCl(g) + 2CuCl,(s) + H,(g)
All temperatures All temperatures
2CuCl(s) + 2CuCl(v) 2CuCl(v) + 2cu + Cl,(g) 2CuCl(s) -+ Xu(s) + Cl*(g) 2CuCl,(s) + ZCuCl(s) + Cl,(g) CuCl,(s) + Cu(s)+ Cl,(g)
Below 1000K All temperatures Above 750 K High temperatures
(a) Reaction of HCl with Cu The reaction of Cu with gaseous HCI may yield CuCl, or CuCl along with H2. A series of thermodynamic computations indicate that the free energy for conversion of the reaction of Cu with HCI to form CuCl?, Reaction 1, is unfavorable at all temperatures considered, Table 1. Cu(s)+2HCl(g)
+ CuCl,(s) + H,(g).
(1)
By contrast, it is seen that the reaction between Cu and HCl to form CuCl, Reaction 2, is thermodynamically favorable from below room temperature up to about 700 K. 2Cu(s) + ZHCl(g) + 2CuCl(s) + H?(g).
(2)
It is possible that the CuCl(s) formed in Reaction 2 could further react with HCl to form CuCl,, Reaction 3. However, calculations indicate that this reaction is unfavorable and may yield no more than a trace amount of CuCl,(s). CuCl(s) + 2HCl(g) -+ 2CuCl,(s) + Hz(g).
(3)
Thus, from thermodynamic computations it seems the products of the reaction between Cu and HCl are CuCl(s) and H,. (b) Thermal decomposition of CuCl(s) Thermodynamic computations indicate that when CuCl(s) is heated to very high temperatures CuCl(v) is formed, Reaction 4. Free energy change for this reaction is not very favorable and the extent of vaporization is small. 2CuCl(s) + 2CuCl(v).
(4)
If CuCl(v) is produced it would spontaneously decompose to Cu and Cl, Reaction 5. 2CuCl(v) --t 2Cu(s) + Cl,(g).
(5)
Reaction 6, which is a sum of Reactions 4 and 5 shows the overall decomposition of CuCl(s) to Cu and Cl,. %CuCl(s) + 2Cu(s) + Cl*(g).
(6)
If any trace amount of CuCl, is formed by Reactions 1 and 3, it could be decomposed to CuCl(s) and Cl, at
moderate temperatures or to Cu and Cl, at high temperatures, Reactions 7 and 8. 2CuCl,(s) + 2CuCl(s) + Cl,(g)
(7)
CuCl,(s) + Cu(s) + Cl,(g).
(8)
Thus, in the overall reaction scheme for the formation of H, and Cl, from HCI, it is seen that (a) the reaction of Cu and HCl to form CuCl and H, is favorable, (b) CuCl(s) can be decomposed to Cu and Cl,, Reaction 6 (a combination of Reactions 4 and 5) provided that CuCl(s) is heated to high temperatures to form CuCl(v). Several runs were performed at temperatures above 700 K (713 K, 740 K, 770 K, 784 K, and 933 K) using Method 1 as described above. No significant amount of hydrogen evolution was observed, which tends to support the fact that CuCl and not CuC1, is the product of the reaction between Cu and HCl. The equilibrium constant calculated from the experimental data appears somewhat low, indicating a low residence time. Table 2 shows the partial pressures (HCl and HJ and equilibrium constants (theoretical and experimental) at various temperatures for the reaction between Cu and HCl. It is seen from Table 2 that the percent conversion of HCl to product increases with decrease in temperature and increase in HCl concentration (partial pressure). Using Method 2 the rate of H, production was measured by changing the surface to volume ratio and at various temperatures. Table 3 shows reaction of HCl with copper film under various conditions. The product formed is white in color indicating that the product is CuCl and not CuCl,. Detailed calculations are given in the report submitted to Wright Patterson Air Force Base
171. MODEL
FOR ANALYZING
DATA
To define conditions that lead to complete conversion of HCl it is necessary to distinguish the limitations that are caused by the surface geometry and flow rate at the copper surface from the conditions imposed by the reaction itself. The objective is to measure the extent of reaction using conditions that are sufficiently well defined that the results can be used to design a reactor in which the conversion of HCl to H, is virtually complete
CLOSED CYCLE HCI-HZ-Cl, ENERGY STORAGE SYSTEM
591
Table 2. Partial pressuresof HCI and H,, percent conversion and equilibrium constants at 626 K, 643 K, and 655 K for the reaction between Cu and HCI
T/K
P HC,,,“)
P HCl,o”t,
P”.
%Conv.
gperimental)
tieoretical)
626 626 626
0.80 0.54 0.27
0.30 0.25 0.15
0.15 0.13 0.07
62 53 44
1.29 1.44 1.76
1.91 1.91 1.91
643 643 643
0.68 0.62 0.34
0.40 0.35 0.19
0.18 0.18 0.08
41 43 42
1.06 1.21
1.51 1.51 1.51
655 655 655
0.85 0.67 0.34
0.38 0.32 0.20
0.22 0.16 0.07
59 52 41
Table 3. Reaction of HCl with copper film under various conditions Temperature, K Flow rate, pmoles/sec Surface/Volume ratio, cm-’ pmoles of Cu deposited pmoles of HCl reacted pmoles of H, formed pmoles Cl bound
500
500
600
400
500
11.8 11.8 11.8 11.8 11.8 5.88 11.8 11.8 11.8 11.8 198 266 592 743 787 58.0 69.3 80.7 80.7 69.3 27.5 33.0 41.4 36.2 37.5 59.2 73.5 78.8 78.8 64.8
The extent of the HCI reaction has a complex dependence on temperature, pressure, flow rate, surface area, and reactor volume. Temperature, for example affects both the mobility of the gas reactants and the probability of reaction per encounter between the reactants. The following model characterizes the components of reaction probability. The HCl flow through the reactor is resolved into radial and axial motions that are assumed to be independent. Axial flow is expressed as the average residence time of an HCl molecule passing through the reactor in the absence of reaction. The radial migration determines the probability that an HCl molecule will reach the surface during the residence time. The probability of reaction is the product of a sequence of probabilities, some features of which are distinguished by the present experiments. The probability that an HCl molecule will reach the copper surface is equal to the radial displacement (x) by random motions during its residence time in the reactor t, divided by the radial distance between successive encounters with the reactor surface. This probability can range from much greater than, to much less than 1, depending on the flow conditions and geometry. The solution of the l-dimensional random walk diffusion problem gives (x) in terms of the diffusion constant D. (x) = (2Dt,)“‘. The diffusion constant is calculated in terms of the average molecular speed (c) = (8RT/nM)“* and the hard sphere value for the mean free path 1 = RT/[2%P]
I .49
1.28 1.28 I .28
1.23
1.25 1.32
D = 1/.3(c)i. Using a hard sphere diameter of 3.0 A for HCl, the residence time is calculated from the reactor geometry and the mass flow rate of HCl,f(HCl).
w: - mLa,~ ” =
4f(HCl)RT
where d2 is the i.d. of the reactor, insert, and X,,,,, is the length of the probability, p(x, t,), is the ratio of distance to the width of the annular p(x, t,) = (x)/(d2
REACTION
d, is the o.d. of the copper deposit. The the total migration space.
-d,).
PROBABILITY PER SURFACE ENCOUNTER
The observed reaction probability p(obs) is the ratio of the mass flow rate of H, to that of HCl. The mass flow rate of HCl is measured in a preliminary experiment by timing the HCl flow required to neutralize a quantity of standard base. The mass flow rate of Hz is determined from the rate of change of gas volume measured in the gas burette. dabs) = 2ff(Hd/-if(HCU The observed reaction probability is the product of the gas diffusion probability and the probabilities that make up the reaction probability per encounter of an HCl molecule with the copper surface. p(react) = P(obs)/P(x, t,) Results in Table 4 show that p(react) is independent of Table 4. Components of the observed reaction probability T(K)
P (torr) d2-d,
500 400 500 500 600
600 600 600 600 600
t, (set)
0.311 12.37 0.170 8.75 0.170 7.00 0.170 7.00 0.170 5.83
Pb. fr)
p (obs) p (react)
19.9 25.9 27.4 27.4 28.7
0.0137 0.0174 0.0189 0.0192 0.0222
0.00069 0.00067 0.00069 0.00070 0.00077
598
A. K. GUPTA et al.
surface/volume ratio and nearly independent perature, T”* at most.
of tem-
DEPTH
Time, hours
where Nay is Avagadro’s number. By definition the number of moles of copper in a monolayer is = 1.67 x 10e7moles
~d~x,,,,,dx,d~~~)/M(~"~
and the depth of penetration monolayer is m = 2n(H,)/n,
of HCl into the copper
= 390 monolayers/cc H,.
To examine the linearity of H2 production over an extended period of time an experiment was performed in which the H, was measured in a series of stopped-flow steps. The HCl was stopped for 8 min, then allowed to flow normally while the accumulated hydrogen was measured. The total amount of hydrogen corresponds to a significant fraction of the weight of the copper film. The amount of hydrogen accumulated in each step was constant, as shown in Fig. 7. Examination of the film at the end of the experiment
6L 2,000
5-
i
-I
84
1,500
5
Temperature, K
1
2
3
4
640 685 816 987
0 10 66 82
0 15 68 88
0 17 12 88
0 19 83 89
OF PENETRATION
From the observed rate of formation of H, further information about the reaction probability is obtained. Penetration of HCl into the solid Cu film in terms of monolayers is calculated from the density of the copper, d(Cu) = 8.92 g/cc. The thickness of the monolayer is
n, =
Table 5. Percent decomposition of CuCl at various temperatures
-
F x3I -
1,000
I 2 0 d 2
Conditions: Constant flow of argon gas at ca. 10 torr through 30 cm long tube. CuCl coated on silicon carbide. Effluent trapped at liquid nitrogen temperature and subsequently analyzed by gas chromatography. showed that parts of the copper film were completely penetrated by the reaction. The nonlinearity observed may be due to a decrease in the reactive surface area. For film depths of about l&1000 monolayers there was no indication that the thickness of the CuCl film had a significant affect on the reaction rate. Experiments carried out at 400-650 K to study the decomposition of CuCl by Method 2 using KI and starch indicator solution show that Cl, in the flow stream is negligible at this temperature. Qualitative examination of the films at the end of the experiment show that there is a significant migration of CuCl across the annular space to the insert. The coating on the insert is more or less uniform. In view of extremely small vapor pressures at these temperatures, the results are encouraging. According to Method I which involves lowering of pressure, useful chlorine yields were found for system pressures below 10 torr and above 700 K using a flow of inert gas to sweep out the chlorine. It is seen from Table 5 that the percent decomposition of CuCl increases with temperature. Results of the experiments indicate that the decomposition of HCl can be carried out by passing dry HCl gas over a Cu bed to release H, and form CuCI. Subsequently, upon heating, CuCl can be decomposed back to Cu releasing Cl, gas, provided the pressure is below 10 torr and the product gas is swept out using an inert gas. Other metal/halides such as Ag/HCl or Ag/HBr might prove superior in this application. Also, for the release of halogens, metal halides like AgCl and AgBr would be ideal, since photo-decomposition of these halides is known to occur.
f
CONCLUSIONS
22-
Reaction of HCI with CM
-I 500
:t/?: 0
20
40
Time,
60
loo0
mid&
Fig. 7. Volume of hydrogen produced and the corresponding monolayers reacted versus reaction time.
(1) Cu and HCl(g) form CuCl(s) and Hz in the temperature range 40&600 K. (2) Percent conversion to product increases with HCl pressure. (3) Temperature has only a small effect on the reaction rate, at most T”‘. (4) The surface geometry and gas flow must allow at least a few thousand gas-surface collisions per HCl molecule for complete conversion of HCl to H,.
CLOSED CYCLE HCl-Hz-Cl,
(5) Thickness of the CuCl layer is not a limiting factor in the reaction probability for layers less than a few thousand monolayers deep. Production of Cl, by thermal decomposition of CuCl (1) No Cl, was detected in a stream of argon flowing over a CuCl film at 650 K and 1 atmosphere. (2) CuCl is more mobile at temperatures in the 400-600 K range than might be expected from its equilibrium thermodynamic properties. (3) Cl: is produced at pressures below ca. 10 torr at temperatures above 700 K. (4) Other metal/halide combinations such as Ag/HCl or Ag/HBr might prove superior to Cu/HCl in this application since photo-decomposition of AgCl and AgBr is known to occur. AcknoM,ledgemeni-This work was sponsored by Wright Patterson Air Force Base under contract #F33615-86-C-2716.
ENERGY
STORAGE
SYSTEM
599
REFERENCES 1, Hanrahan, R. J. and Gupta, A. K., US Patent No. 4,848,087, 1989. 2. Williams, L. O., Hydrogen Power-An Introduction to Hydrogen Energy and its Applications, 1980, Pergamon Press, NY.
3. Nishimoto, Y., Mizumoto, Y., Mitsuoka, S. and Hasegawa, 4.
5.
6. I.
S., (Mitsubishi Heavy Industries, Ltd) Japan, Kokai 76/548892 (Cl COlBjO3) (14 May 1976) Appl. 74/128,658 (8 November 1974). Mellor, J. W., A Comprehensive Treatise on Inorganic Chamistry and Theoretical Chemistry, Vol. II (1922), and Supplement II, Part I (1956). Parker, R. Z., Hanrahan, R. J. and Cox, J. D., US Patent No. 4,848,087, 1989. JANAF Thermochemical Tables, Second Edition, National Standard Reference Data Series, National Bureau Standard (U.S.) (June 1971). Solar Augmented Fluids Technologies-A Report from Solar Reactor Technologies to Wright Patterson Air Force Base, OH 45433-6563. Report No. WRDC-TR-89-2036.