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The Journal of Supercritical Fluids, 1995, 8, 263-270 263

Advanced Pd/Pt Relative Resistance Sensor the Continuous Monitoring of Dissolved

Hydrogen in Aqueous Systems at High Subcritical and Supercritical Temperatures

Chun Liu*t and Digby D. Macdonald

Center for Advanced Materials, The Pennsylvania State University, University Park, PA 16802

Received October 24, 1994; accepted in revised form March 3 1,1995

An advanced palladium/platinum resistance sensor has been developed to measure, in situ, dissolved hydrogen in high temperature aqueous systems. The measurement of hydrogen is based on the well- known fact that the resistance of palladium changes with the absorption of hydrogen into the lattice. Comparative electrical resistance measurements are made between a pair of palladium and platinum probes wound on an oxidized zirconium metal mandrel. Laboratory tests demonstrate that the sensor displays good sensitivity and responsiveness to changes of dissolved hydrogen concentration in an aqueous system at temperatures up to 410 “C and at pressures to 4000 psi (276 bar). The sensor is designed for use in water-cooled thermal power plant heat transport circuits and in super&&l-water reactor media for the re- ductive destruction of toxic waste, where continuous hydrogen monitoring may be necessary.

Keywords: supercritical water oxidation (SCWO), palladium, hydrogen, hydrogen sensor, high-temperature aqueous system

INTRODUCTON The efficient operation of water-cooled electrical en-

ergy generating systems (nuclear and fossil) requires the continuous monitoring of the chemical properties in the high-temperature aqueous heat-transport media. Such control is necessary, in order to suppress corrosion and the transport of corrosion products around the non-isothermal coolant systems. Mass transport can lead to fouling of heat-transfer surfaces, and to the dispersion of active nu- elides, such as 6oCo, into out-of core components in the case of nuclear generating systems. The corrosion product identity, solubility, and the rate of diss’olution are known to depend on the pH and on the redox properties of the systems;‘** therefore, both mass and activity transport phenomena might be minimized by careful control of the fluid chemistry,3*4 including the concentrations of redox species such as hydrogen.

In recent years, considerable interest has arisen in the use of supercritical water as a reaction media for the destruction of toxic waste.5-9 While most of the attention

t Current address: Polyvision Corporation, 866 North Main St., Wallingford, CT 06492.

has been focused on supercritical-water oxidation (SCWO), in which toxic organics are oxidized to CO*, H20, and HX (X = F, Cl, Br, . ..). interest also exists in the reduction of certain components to render them bio- logically or physiologically inactive, or as a means of producing a desired product (e.g., as in the liquefaction of coal and biomass).“‘-‘* In these cases, close control may be desired over the fugacity of hydrogen in the aqueous fluid, in which case a rugged, in situ, sensor is required.

Extensive effort has been deployed in this laboratory over the past several years to develop techniques for continuous monitoring of corrosion, mass transport, and water chemistry parameters in the heat transport circuits of water-cooled thermal power plants.13-17 These include techniques for measuring pH, redox potential, fluid conductivity, oxygen, and hydrogen concentrations, electrochemical noise induced by corrosion processes, and for monitoring the growth of cracks in components or in surveillance specimens in the heat transport circuits.

A potential application of in situ hydrogen monitoring of great current interest arises from the observation that austenitic stainless steels and high nickel alloys are susceptible to stress corrosion cracking (SCC) in high-

0896-8446/95/0803-0263$5.00/9 8 1995 PRA Press

264 Liu and Macdonald The Journal of Supercritical Fluids, Vol. 8, No. 3, 199.5

temperature aqueous systems at electrochemical corrosion potentials (ECPs) above a critical value (ECP,rit).‘8-22 In Boiling Water Reactors (BWRs), in which the heat transport fluid is oxidizing due to the formation of O2 and I-&& by the radiolysis of water, hydrogen is added to the feedwater to decrease the electrochemical corrosion potential and hence to prevent SCC in susceptible components. This process is referred to in the nuclear industry as Hydrogen Water Chemistry (HWC). The amount of hydrogen that is added to the feedwater is normally monitored at ambient temperature, and because H2 reacts with 4 and HZ% considerable uncertainty exists as to the actual concentration in the high temperature zones. This problem could be overcome by the development of techniques to monitor dissolved hydrogen, in situ, in the reactor water at various points around the coolant circuit.

H2 to dram Themmcouole

Test Cell

Pt hydrogen

To Resislometer

Figure 1. Schematic of the test loop.

f-- 254.Omm A

In this paper, we describe an advanced sensor for monitoring hydrogen in aqueous environments over wide ranges of temperature and pressure. The technique is based on the well-known fact that the electrical resistance of palladium varies with the amount of hydrogen absorbed into the lattice.23 Because of their ability to reversibly absorb hydrogen and their relatively large coefficients of electrical resistivity with respect to hydrogen concentration in the lattice, palladium and palladium alloys have been previously used in many types of hydrogen sensors, and related devices, that make use of this property.24-2g However, the harshness of high temperature aqueous environments renders many of these sensors unsuitable for the present application. Furthermore, while hydrogen sensors for application in high temperature aqueous environments have been fabricated previously,27Js they were limited to subcritical temperatures, because of the use of organic ma- terial (e.g., PTFE) for electrical insulation. These materials cannot withstand harsh aqueous environments that exists at temperatures in excess of 320 0C.28 The advanced hydrogen sensor developed in this work uses a zirconium mandrel, whose surface has been oxidized to form a thin and continuous zirconia layer, as the support for the Pd and Pt sensing elements. The sensor can be used at supercritical temperatures (as demonstrated in this work), because zirconia is particularly immune to corrosion in high temperature aqueous solutions.29 The sensor has been fabricated using a palladium wire for sensing hydrogen, a platinum wire for temperature compensation, and a high precision resistorneter to measure and compare the differential resistance between the palladium and platinum probes. TO our knowledge, this is the first time that dissolved hydrogen has been measured, in situ,.in aqueous environments above the critical point of water.

Double-threads nickel Wring ($=063mm) Double-threads plakmr~ wiring (@.2Omm)

6.35mm Zr tubbmg covered with ZrO2

Double threads Pd and pr wiring (~=O.lOmm)

A

660 4mm

Single-threads nickel wing (@.63mm)

6.35mm Zr tubing Autoclave Engineers hning

Conax fimng

6.35mmType 316 SS rubmg

Figure 2. Schematic of the hydrogen sensor assembly.

similar to those used in previous studies, and details of the design and construction can be found in the literature.15,16 The high-temperature measurement cell was fabricated from Type 3 16 SS, with an outer diameter of 19.05 mm and an inner diameter of 13.51 mm. Figure 2 shows schematically the design of the hydrogen sensor that we have developed in this study. The mandrel is a 6.35-mm (o.d.) zirconium tube, which was covered with a thin, continuous zirconia film after threads were machined down part of its length to accommodate the palladium and platinum wires. The thin zirconia layer on the zirconium mandrel was formed by high temperature oxidation, in such a way that only the end exposed to the high- temperature solution was fully covered with oxide.

EXPERIMENTAL APPARATUS A once through/recirculating high-temperature,

high-pressure loop, capable of operating at temperatures to 500 “C and pressures to 310 bar (4500 psi), was developed to test the hydrogen sensor (Figure 1). This loop is

A four-wire configuration was used to ensure high precision resistance measurements (Figure 3). Pure nickel wire of 0.63-mm diameter was chosen as the leads, because of its resistance to corrosion in this type of application. The leads are arranged inside the zirconium tube and are guided by a four-channel alumina tube in order to avoid mutual electrical contact and contact with the zirconium tube. The outer diameter of the alumina tube is 4.0

The Journal of Supercritical Fluids, Vol. 8, NO. 3, 1995 Advanced Pd/Pt Relative Resistance Sensor 265

palladium wire platinum wrire

b b

I v Common v I

Figure 3. Simplified electrical connection schematic of the hydrogen sensor.

mm, and each channel has a diameter of 0.7 mm. The palladium and platinum sensing elements are connected together at one end (common) and are spotwelded to the zirconium mandrel. The graded profile of the zirconium oxide ensures that the zirconium tube is available for a metal-to-metal seal using an Autoclave Engineers high- pressure fitting (AE 6MX94K2 316 SS). The zirconium tube was then connected through a Swagelok fitting to a 6.35 mm Type 316 SS tube, which was then sealed, with the multi-channel alumina tube, by a Conax fitting with PTFE sealant (Gland MHC2-020-B4). The tube is bent to a 90” angle to avoid extrusion of the leads under pressure, and to fit the entire sensor into a confined space in field applications. The body of the loop is used as the common lead, and the electrical continuity is assured by the spot-weld of the common lead to the zirconium tube and by the metal-to-metal fittings.

The sensing elements are O.lO-mm diameter palladium and platinum wires, with the later being used to compensate for the effect of temperature fluctuations. Short lengths of pure nickel wire, with a diameter equal to 0.5 mm, were connected to both ends of the sensing elements, via an intermediate platinum wire (0.20-mm diameter), to enhance the ruggedness of the sensor.

The electrical resistances of the palladium and platinum sensing elements were measured using a high precision resistometer (Automatic Systems Laboratories Inc., Model F250). The resistometer is essentially a 4-wire, autobalancing, AC resistance ratio bridge, which uses a l- mA square wave current excitation at 375 Hz to make comparative resistance measurements. The resolution of the resistometer is 0.001 R. The resistometer was monitored by an IBM/PC compatible 486 computer using an IEEE488.2 interface. Compared to a DC carrier, an AC carrier presents the advantage that the sensing elements will not be polarized by the carrier current. Note that the inductance of the palladium and the platinum resistance due to the configuration is extremely low, so that the AC part of resistance (i.e., oL) is negligible. However, due to the smaller metal/solution impedance for an AC carrier, the parallel paths through the solution may decrease the precision of measurements, as discussed later in this paper.

The solution was prepared in the loop reservoir by saturation with selected pressures of pure hydrogen gas. In the dehydrogenating process, hydrogen-free solution was obtained by purging with nitrogen gas. Finally, solutions containing sub-ppm levels of hydrogen were obtained by purging the reservoir with mixtures of nitrogen containing 1 and 10% hydrogen.

RESULTS AND DISCUSSION Practical Considerations and Theory of

Measurement. The operation of this hydrogen sensor makes use of comparative resistance measurements of a hydrogen-sensitive (palladium) probe with respect to a hydrogen-insensitive probe (platinum, in our case). We have chosen platinum, because this metal exhibits excel- lent corrosion resistance and possesses an electrical resistivity which is similar to that of palladium.30 Because the resistometer has limited precision (0.001 Q), it is advantageous that the sensing elements have an electrical resistance that is as high as possible. This consideration implies that the palladium and platinum wires should be small in diameter and long in length. In addition, the thinner the palladium wire, the more rapidly it should re- spond to variations in hydrogen concentration. However, in our case, the physical dimensions of the test cell con- strains the length and diameter of the mandrel that is used for winding the platinum and palladium wires, and wires of too small diameter are susceptible to fracture. On con- sidering the literature on hydrogen absorption rate as a function of the diameter of palladium wire,31 we have found that palladium and platinum wires of 0. lo-mm diameter, double wound on a 40 threads-per-inch mandrel, provided the best compromise.

Assume that the resistance of the palladium wire is a function of temperature, T, and hydrogen concentration, C. Thus,

dRPd =(+)dT+(%)dC

for a finite change in T and C, we get,

6R,, = (%)ST+(++-C

where

6R,, = RPd - R;,,

&=T-To

6C= c-c,

and RF,, , Co, and To refer to a reference state. Thus,

(2)

266 Liu and hlacdonald The Journal of Supercritical Fluids, Vol. 8, No. 3, 199.5

RPd =@d+(+)(T-7,)+(%)(C-CO). (3)

Defining

$=aln(RPd)- 1 aRPd -LaRPd ~- -

dT Rpd aT RFd JT

and

y;d=+(RPd) _ 1 aRPd _ 1%

ac Rpd dC - $$ ac

gives

RPd=$?d[~+j’;d(~-~&~~d(C-CO)]

Likewise for platinum,

Rp, = RFt[l+ $(T-TO)].

(4)

(5)

(6)

(7)

If no hydrogen . is contained in the palladium, we may de- fine a differmtial resistance as,

A$’ =R;d+@?d$d(T-To) (8)

where RFh is the resistance of a palladium at temperature Tp w1th no hydrogen absorbed. The difference between eq 8 and eq 7 gives,

ARnh = R;d ( -R;~)+[$d’;d - R;$](T- TO). (9)

This is a constant for any given sensor. Consequently,

AR = R,, -RPt =@h+R;dj’;d(C-Co). (10)

Remanging eq 10 gives,

C=(AR-AR”h)/R;dy;d (11)

because the reference state is chosen with Co = 0. Seneitivity. Let AR - AP = DR, as being the

exper1mentally measured quantity. However, the sensitivity also depend s on the resistance of the palladium wire Rjd ) thusy frOIll eq 11, we have,

or,

s-~~R -‘R;.

DR RPd (14)

Because SRFt = 0 for any given sensor, therefore, the sensitivity of the hydrogen sensor is expressed as,

~2%. DR

(15)

The hydrogen concentration in the solution is calcu- lated from the saturation pressure in the reservoir using Henry’s law,32

1321 P&J = - T -+10.703-0.01468T (16)

and the concentration of dissolved hydrogen in solution is given by,

where T is temperature in Kelvin, pB,p * is -log Ku, Ku is Henry’s constant for hydrogen dissolution, fu, is the hydrogen fugacity, and C is the molal hydrogen concentration in mol kg-‘. Because the hydrogen concentration was established in the solution reservoir at room temperature, the same molal concentration exists at the elevated temperature, assuming a single phase system and no losses through the loop walls. Accordingly, the fugacity of hydrogen in the test zone (temperature 7’) is given as,

fT =(KH,To 6T)fTo (18)

where To is the temperature of the reservoir. For practical purpose, fT, = Pro, where PT, is the partial pressure of hydrogen in the reservoir.

Influence of Solution Conductivity. The solution conductivity gives rise to a major concern in the design of the hydrogen sensor, in that the sensing elements are exposed to an electrically conductive environment. The contribution of the solution to the electrical conductance can be assessed by reference to the electrical analog shown in Figure 4. Assume that the contribution of the solution conductance along the circuit, excluding the actual palladium and platinum wires, is negligible compared to the contribution from the conductance of the

and hence,

The Journal of Supercritical Fluids, Vol. 8, No. 3, 1995

solution Rpg so, Solution Rpt, sol

Common

Figure 4. Simplified equivalent electrical connection schematic of the hydrogen sensor in taking into account of solution conductivity.

solution in contact with the palladium and platinum wires. We consider this assumption to be reasonable, because the resistance of the sensor is dominated by the palladium and platinum sensing elements. Thus, the resistance, Rlsol can be considered as infinite, Rlsol = 00 (Figure 4), and the measured resistance, taking into account the solution conductivity, can be expressed as,

1

(19)

and

(20)

where RpMd and RE are the electrical resistances as measured by resistometer with an AC carrier, and R$ and

-3.5

-3.7

-3.9

-4.1

-4.3

-4.5

Advanced Pd/Pt Relative Resistance Sensor 267

RR’ are the equivalent resistances from solution to the palladium and platinum circuits, respectively. Note that because of the geometrical similarity of the palladium and platinum circuit, we expect that RG’ = R$. By differentiating the left-hand side of eq 20, we have,

(21)

Differentiating the right-hand side of eq 20 we obtain,

(22)

Comparing eqs 21 and 22, we get,

where K = g - . KPd

Equation 23 indicates that the major influence of so-

(23)

lution conductivity is to decrease the sensitivity of the measurement by a factor of R$/RPd. From experiment, this factor can easily be determined by measuring the resistance of the palladium probe using an AC carrier, which yields RpMd ; and a DC carrier current, which yields RPd. This is attributed to a low voltage DC current not passing through the solution due to the double layer ca- pacitance that exists at the metal-solution interface and due to the development of concentration overpotential. However, for a given solution that has a constant compo- sition, the solution conductivity will not impact the measurement other than through the factor K mentioned

change of hydrogen concentration

[H21=5.79ppm

4 6

Time (h)

Figure 5. The differential resistance as a function of time at 360 “C for a constant hydrogen concentration.

The Journal of Super-critical Fluids, Vol. 8, No. 3, 1995

t El 360°C

3800 psi

t (5.79ppm) (5.79rw)

B

I.5 j- II J N c

0

49 50

0 (ODDIn) . .(OPTpl) . I . . mL?Rm~ . ’ .-.- ...a. ’ . . ’ . ’

0 25 50 75 100

Figure 6 Time(h)

function 0f.t. ‘he normalized differential resistance (with respect to the differential resistance at 360 “C with no hydrogen) as a ‘% at 360 “C during hyd r og en cycling (hydrogen concentrations in ppm are indicated in parenthesis).

above. ln 360 “c, we Oar 49.069 Q ha,

exPerience, involving deaerated water at e measured for the palladium probe, RpMd =

&t$~~;M$Qwi~h $zI~K = 0.95. illustrates . in terms of the Stability of the

Figure 5

th hydrog en sensor, at 360 ‘C,

time for a

shw that for e differential resistance, as a function of

ConStant hydrog en concentration. The data SenSor yields

The Continuous monitoring of up to 10 h, the

. . rtc0rMant and stable output. r%l rise

factor In Industrial monitoring applications. We have ob- time of a sensor is a very important

ge in the differential resistance to 90% a change in the partial pres-

occurs in about 30 min the real response of the sens-

’ 1Qt a pumping rate of 4.0 mL min-‘, Sed in the experiment, the time required to re-

Ucted at ambient temperature, by intro-

e in hydrogen concentration Literature data indicate that the rate

exppessed in terns of (dR/Rdt)(h-I), increases

increases by a factor of six, in the and by a factor of four, in

the case of hydrogen desorption.33 Because of this in- crease in diffusivity of hydrogen in palladium with in- creasing temperature, we believe that the actual response time of the sensor at 360 “C is, at the most, of the order of a few tens of seconds.

Effect of Hydrogen Cycling. The variation of the differential resistance as a function of time during hydrogen cycling is shown in Figure 6. Data obtained at 360 “C indicate that the output correlates uniquely with the hydrogen concentration, in that the same output is ob- served for the same hydrogen concentration after hydrogen cycling. Because of the different mechanisms in hydrogen absorption and desorption, which involve different reac- tions, the absorption rate is different from that of desorption. 34 Hence, a hysteresis phenomenon may develop when the sensor is used to monitor variation of hydrogen concentration in a de-hydrogenating process. However, the literature data indicate that the difference in absorption and desorption rates tends to diminish as the temperature increases. Thus, it was found that, at room temperature, a factor of 3.6 exists between desorption and absorption rates for hydrogen in palladium, while at 180 ‘C, this factor becomes 2.3.34 This indicates that at higher temperatures (such as in supercritical aqueous environments), the hydrogen sensor described in this paper should become equally responsive to decreases and increases in the hydrogen concentration in the immediate environment. Finally, the data that we have obtained in this study indicate that the sensor is reversible to the hydrogen concentration.

Effect of Hydrogen Concentration. The variation of the differential resistance with respect to hydrogen concentration and temperature is presented in

The Journal of Supercritical Fluids, Vol. 8, No. 3, 1995 Advanced Pd/Pt Relative Resistance Sensor 269

0.08

0.06

0.04

0.02

0 I.T... I...,. .I.. I

0 I 2 3 4

.jiGi(rm)

Figure 7. The normalized differential resistance (with respect to the average resistance of the palladium and platinum wires at room temperature) as a function of hydrogen concentration and temperature.

Figure 7. The data indicate that, within the temperature range investigated (from 250 to 410 “C), the output increases linearly with C”.s. This is an important feature for on-line monitoring, in that once the sensor is cali- brated for a specific set of conditions (temperature, solution, pressure, range of hydrogen concentration, etc.), it can be used to monitor dissolved hydrogen accurately under other conditions (but at the same temperature).

As stated previously, the resistometer has a limited precision, which, in our case, is 0.001 iz. Therefore, it is always advantageous that the sensing elements have as high an electrical resistance as possible so as to produce large change in the absolute value of the resistance for a given hydrogen concentration. In our case, physical and mechanical ruggedness considerations limit the maximum electrical resistance that can be achieved. The resolution for this sensor can be readily estimated from Figure 7, by calculating the equivalent hydrogen concentration corre- sponding to the precision of the resistometer. This calcu- lation yields a resolution of 4.3 ppb (which corresponds to a resistance change of 0.001 Q) at 410 “C and 7.4 ppb at 360 “C. However, we see no reason why the resolution cannot be reduced to the sub-ppb level by appropriate changes in the design. Furthermore, taking into account the amount of hydrogen injected in a typical BWR cool- ing system to be in the order of a few hundreds ppb to a few ppm,35-37 we consider that the hydrogen sensor developed in this study has adequate sensitivity for this type of application.

SUMMARY AND CONCLUSIONS A sensor has been developed to measure in situ dis-

solved hydrogen in high temperature aqueous systems. The sensor is based on the principle that the electrical resistance of palladium varies with the amount of hydrogen absorbed into the lattice. Our experimental data show that the sensor has adequate sensitivity and responsiveness to changes in dissolved hydrogen in aqueous system at tem-

peratures up to 410 “C and at total pressures to 4000 psi (276 bar). The response time is estimated to be a few tens of seconds, at most, when used at 360 “C. The sensitivity is such that hydrogen concentrations of less than 10 ppb can be monitored in supercritical aqueous systems. The simplicity and ruggedness of this hydrogen sensor make it a promising technique for industrial applications where on line, continuous in situ hydrogen monitoring is required.

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of

this work by Idaho National Engineering Laboratory (INEL) under contract No. C88-101857, and by the Army Research Office (ARO) under Grant No. DAAL03-92-G- 0397. We also would like to thank Dr. Leo B. Kriksunov for many helpful discussion and assistance in fabricating the sensor.

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an advanced pd/pt relative resistance sensor for the continuous monitoring of dissolved hydrogen in...

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