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Continuous In Situ Method for the Measurement of Dissolved Hydrogen in High-Temperature Aqueous Systems Digby D. Macdonald,' Michael C. H. McKubre, Arthur C. Scott, and Paul R. Wentrcek Materials Research Labomby, SRI International, Menb Park, Callfornla 94025
A temperature-compensated palladium resistance probe has been developed for in situ monitoring of the concentratlon of hydrogen in aqueous systems at elevated temperatures. The performance of the probe has been investigated in 0.1 m B(OH)3at 275 "C and for hydrogen Concentrations ranging from 0 to 1.5 ppm. In the absence of a high concentration of oxygen, the probe exhibits a fast response for hydrogen absorption but a slow response for desorption. Prior exposure of the probe to oxygenated water, however, reverses this relationship in that absorption tends to be slow or exhibits an indwAon period and desorption is rapid. The effect of oxygen is dlscwsed in terms of a model that assumes a surface process to be rate-controlling for the absorptlon/desorption behavlor of hydrogen in palladium.
Introduction The efficient operation of water-cooled electrical energy generating systems (nuclear and fossil) requires continuous methods for monitoring the chemical properties of the high-temperature aqueous heat transport media. Such control is necessary in order to suppress corrosion and the transport of corrosion products around the nonisothermal systems. This latter phenomenon can lead to the fouling of heat transfer surfaces, and in the case of nuclear generating systems, to a dispersion of active nuclides such as ' W o into out-of-core components. Because corrosion product identity, solubility, and rate of dissolution are known to depend on the pH and the redox properties of the system (Sweeton and Baes, 1970;Macdonald et al., 1972,1974;Macdonald, 1972,1975,1976; Macdonald and Rummery, 1973,1974;Rummery and Macdonald 19731, it is expected that both mass and activity transport phenomena (Burrill, 1977,1979; Lister et al., 1971;Lister, 1979) can be minimized by careful control of the fluid chemistry. In two previous publications from this laboratory (Macdonald et al., 1980,1981) we have described techniques for the continuous monitoring of the pH and redox potential of high-temperature aqueous systems under simulated power plant conditions. In this paper we describe a method for the on-line measurement of dissolved hydrogen in high-temperature aqueous systems. This device makes use of the well-known variation in the electrical resistance of palladium with the content of hydrogen in the lattice (Lewis, 1967). Wright and Stiteler (1958) have previously described an instrument based on this phenomenon for the measurement of hydrogen in primary circuits of pressurized water reactors (PWRa). In this case, relatively high levels of hydrogen are present in the fluid, and the work of Wright and Stiteler considered hydrogen concentrations ranging from 0 to -85 cm3/kg of H20 (0-7.6 ppm). However, the secondary coolant circuit of a PWR, and the coolant circuit of a BWR (boiling water reactor), operate at very much lower hydrogen levels (" 0.4
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of the redox potential vs. time curve can be accounted for by mixed potential theory (Macdonald et al., 1980),including the gradual change and subsequent abrupt drop after the start of hydrogen injection. It is important to note that the sudden drop in the redox potential is predicted to occur even though the hydrogen content of the solution changes continuously with time. Since the palladium probe resistance is a monotonic function of hydrogen concentration (see Figure 8), it appears that after the second hydrogen injection the response of the resistance probe is characterized by an induction time that is
considerably larger than can be accounted for by the mass transfer characteristics of the loop. Also, the resistance data plotted in Figure 11suggest that the appearance of an induction time in the hydrogen monitor response requires a certain minimum concentration of oxygen in the system, because no induction period was observed after the first injection of 100% hydrogen. In this case,the prior oxygen concentration was far lower than that before the second hydrogen injection (17% vs. 50% O2 in H2). Absorption/Desorption Mechanisms. The data presented above indicate that the rate of absorption and
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fnd. Eng. Chem. Fundam., Vol. 20, No. 3, 1981 22 HOUF 100‘0 t i 2 c _
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desorption of hydrogen in palladium is controlled by a surface process. This hypothesis is in agreement with the findings of Auer and Grabke (1974), who found that the rate-controlling step for the absorption of hydrogen in palladium and palladium-silver alloys from the gas phase is the dissociation of molecular hydrogen to form adsorbed atoms. We assume that this same reaction controls the rate of absorption and desorption of hydrogen in palladium in aqueous systems. Additionally, however, we have found that the desorption reaction is catalyzed by oxygen, but for the reverse process the presence of high prior levels of oxygen give rise to an induction time in the response of the probe to hydrogen absorption. The kinetic data summarized above can be rationalized in terms of the mechanisms in Scheme I, in which the recombination reaction is assumed to be rate determining. Thus, catalysis by oxygen is due to the fast reaction between adsorbed oxygen atoms and hydrogen atoms on the palladium surface which occurs in parallel with the slow recombination reaction for the formation of molecular hydrogen. The induction time for the entry of hydrogen into the lattice arises because adsorbed oxygen competes with dissociating hydrogen for surface sites. Only after most of the adsorbed oxygen has reacted to form water are
sufficient adjacent sites available for disssociation of molecular hydrogen to form adsorbed hydrogen atoms which can then enter the paliadium lattice. In other words, the induction time for absorption corresponds to the time required to react most of the adsorbed oxygen atoms at the metal surface with hydrogen to form water. Acknowledgment The authors wish to thank the Electric Power Research Institute, Palo Alto, Calif., Dr. T. 0. Passell, Project Manager, for financial support of this work under Contract NO.RP 1168-1. Literature Cited Auer. W.; Grabke, H. J. Ber. Bunsenges. phys. Chem. 1974, 78, 58. Burrill, K. A. Corrosbn 1970, 35, 84. Bwiii, K. A. Can. J. Chem. fng. 1977, 55, 54. Handbook of Chemlstw and Physics”, 56th ed.; CRC Press: Cleveland. Ohio, 1976; p F-166.~ Lewis, F. A. ”The Palladium-Hydrogen System”, Academic Press: London, 1967.
Listei,-D. H. Corrosbn 1079, 35, 89. Lister, D. H.;Charlesworth, D. H.; Bowen, B. D. Conosbn 1971, 27, 261. Macdonakl, D. D. “The Thermodynamics of MetacWater System at Elevated Temperatures. IV. The NMeCWater System”, AECi-4139, 1972. Macdonald, D. D. ”The Electrochemistry of Metals in Aqueous System8 at Elevated Temperatues”, In “Modem Aspects of Electrochemistry", Bockris. M. O’M.; Conway, B. E., Ed.; Vol. 11, Plenum: New York, 1975; Chapter 4, p 141. Macdonald, D. D. Corras. Scl. 1076, 16, 461. Macdonald, D. D.; Rummery, T. E. “The Thermodynamicsof Metal Oxides In Water Cooled Nuclear Reactors”, AECL Report-4140, 1973. Macdonald D. D.; Rwnmery, T. E.; Tomlkrson, M. “Stability and SOkrblRty of Metal Oxldea In High Temperature Water”; Presented to the 4th IAEA Symposium on the Thenrodynemlcs of Nuclsar Materials, Vienna, Oct, 1974. “Thennodynamlcsof Nuclear Materials”, 1974, Vd. 2, p 123, Published by the Internatbnsl Atomlc Enerey Agency, Vienna. Macdonald, D. D.; Scott, A. C.; Wentrcek, P. J . Elechochem. SOC. 1981, 128, 250. Macdonald, D. D.; Scott, A. C.; Wentrcek, P.; McKubre, M. C. H. “Monltahg Techniques for pH, Hydogen, and Redox PotenUal In Nudear Reactor Circuits”; Final Report to the Electric Power Research Institute, PaQ Alto, Calif., Contract No. RP 1166-1, 1979. Macdarald, D. D.; Shlennan, G.; Butler, P. ”Thermodynamics of Metal-Water Systems at Elevated Temperatures. I. The Water and Copper-Water Systems“; AECL-4138, 1972.
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Macdonald, D. D.; Shlerman, Q.; Butler, P. "The Thermodynamlcs of MetalWater Systems at Elevated Temperatures. 11. The Iron-Water System"; AECL-4137, 1972. Maecdonald, D. D.; Shlerman, G.; Butler, P. "The Thermodynamlcs of MetalWater Systems at Elevated Temperatures. 111. The Cobalt Water System"; AECL-4138, 1972. Macdoneld, D. D.; Wentrcek, P.; Scott, A. C. J. Eh9ctmhem. Soc. 1980, 127, 1745. " O v , Q. B.; Ryshenko. B. N.; Khodakovsky, 1. L. "Handbook of Thermodynamic Data", USQS Trend. USGSWRD-74-001, 1974. R u m " y , 1.W.; Macdoneld D. D. "The Thermodynamics of Selected Tran-
skion Metal Ferrttes In Hlgh Temperature Aqueous Systems"; AECL Report-4577, 1973. Rummery, 1. W.; Macdonald, D. D. J. Nucl. Meter. 1875, 55, 23. Sweeton, F. H.; Baes, C. F. J . Chem. Thermodyn. 1970, 2, 249. Wdght, J. M.; Stlteler, D. J. The Bettls Technlcal Review, WAPD-BT-7, AvaU. Office of Technical Service, Department of Commerce, Waahlngton, DC, 1958.
Received for review June 9,1980 Accepted May 6,1981
COMMUNICATIONS Jet Penetration in a Pressurized Fluidized Bed A correlation has been developed which, with engineering accuracy, correlates the jet Penetration data available in the literature obtained from fluidized beds of sand (pP= 2629 kg/m3), FMC char (pP= 1158 kg/m3), and siderite (pp = 3988 kg/m3) at pressures ranglng up to 5300 kPa (-53 atm). The limiting case of the developed correlatkm at 1 atm pressure (101 kPa) reduces to a form similar to that developed earlier using jet penetration data cdlected in our own laboratory at ambient pressure, though the numerical coefficients are slightly different. A new method was also introduced to estimate the complete fluidization velocity for a material of wide size distribution at elevated pressures.
Introduction With the increase in application of fluidized-bed reactors for fossil fuel processing, the studies on critical phenomena occurring in the fluidized-bed reactors are also receiving increased attention. One of these critical areas is the jetting phenomenon. The jetting phenomenon is an important consideration in designing fluidized-bed reactors because the fluidizing gas introduced through distributors and the solids fed through the pneumatic transport systems appear as gas or gas-solid jets in the reactors. For chemical reactors where the reaction rate is fast, much of the conversion may occur in the jetting region. The studies on different aspects of the jetting phenomenon in the fluidized beds available in the literature were reviewed recently by Yang and Keairns (1979). The present paper details the development of a correlation which correlates, with engineering accuracy, the jet penetration data available in the literature obtained from fluidized beds of sand (p, = 2629 kg/m3), FMC char (p, = 1158kg/m3), and siderite bP= 3988 kg/m3) at pressurea ranging up to 5300 kPa (-53 atm). The limiting case of the developed correlation at 1 atm pressure (101 kPa) reduces to a form similar to that developed earlier using jet penetration data collected in our own laboratory at ambient pressure, though the numerical coefficients are slightly different (Yang and Keairns, 1978). Jet Penetration in a Pressurized Fluidized Bed All of the jet penetration studies reported in the literature so far are limited to a pressure at or around 1atm pressure. A recent study by Knowlton and Hirsan (1980) reported the effect of pressure on jet penetration in a semicylindrical fluidized bed up to a pressure of 5300 kPa (-53 atm) using three materials of widely different densities and a relatively wide particle size distribution. The physical characteristics of the solids are summarized in Table I. 0190-43 13/61 i 1020-0297$01.25/0
Table I. physical Characteristics of Bed Materials particle size average particle distribution, particle density, bed materials mesh size,pm" kg/m3 Ottawasand -20+60 438 2629 FMC char -2Ot60 419 1158 siderite -20+60 421 3988 Determined by
a, = l/cXi/dpj
The experiments were performed in a semicircular bed 30 cm in diameter with a jet nozzle of 2.5 cm in diameter using nitrogen as the fluid medium. In addition to the flow through the jet nozzle, the bed was also fluidized at a velocity of lUd, 2Ud, or 3U& Knowlton and Hirsan (1980) compared their experimental data with the existing correlations developed based on data obtained at atmospheric pressure and found that all of the correlations underpredicted the effect of pressure. They defined the jet penetration in three different ways as shown in Figure 1. Among them, L , is the most distinct and physically the most significant parameter. The penetration length L , was also the parameter correlated earlier using a two-phase Froude number defined as pfUo2/(p,- &do (Yang and Keairns, 1978). Using the same set of data, a correlation was developed which correlated L , to within f40% for operating pressures up to 5300 kPa. The dimensionless quantities of the data L,/do were first plotted against the two-phase Froude number in logarithmic scale as suggested earlier. Trends were discernible. Straight lines of similar dopes could usually be obtained for data obtained at the same operating pressure. The intercepts, however, were larger for data obtained at higher pressures. This indicated that, employing the twephase koude number alone, the earlier correlation was indeed Tderpredicting the effect of pressure on jet penetration. It was found experimentally that the voidage at 0 1981 American Chemlcal Societv
