CA1108698A - Device for monitoring a component in a fluid mixture - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An ion concentration analyser having a reference chamber and a sample chamber, maintained at a substantially common temperature, separated by an ion-conductive partition. A
known concentration of a reference material in the reference chamber is varied relative to the unknown concentration of a sample material in the sample chamber until the output across the ion-conductive partition reaches a set point from which the concentration of the sample material is calculated, for example as a function of the temperature of the ion-conductive partition.
Maintenance of the reference chamber and the sample chamber at a substantially common temperature significantly improves the resistance of the ion concentration analyser to thermal shock.

Description

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The present invention relates to a device and a method for monitoring a component in a fluid mixture and more particularly to such a device which utilizes a solid electrolyte.
~ork on the properties of solid electrolytes has been going on in many countries since the last century. Haber and St. Tolloczko were the first to study quantitatively the chemical changes of solid electrolytes and to find that they obeyed Faraday's law. Haber and others built high temperature c~ncentration cells using solid electrolytes which measured the e.m.f. produced by reactions between gases such as Co and 2 on one side of the cell keeping a constant 02 concentration on the other side. This enabled the calculation of thermodynamic data for the gas at different temperatures. It was proposed to use this method for power generation1 fuel cells, and for gas analysîs.
The validity of the Nernst equation:-e.m.f. = RT ln C2 ............... ~1) NF C
where:

R - Universal gas constant T - Absolute temperature F - Faraday constant ,~
N - Number of electrons transferred C~ - Concentration of a component on one side of electrolyte.
C2 - Concentration of same component on the other side of electrolyte.
(The e.m.f. is proportional to the chemical potential of the component under equilibrium conditions which in turn is related to its concentration or its partial pressure in gases).

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: :. -was the first demonstrated for solid electrolytes by Katayama who used amalgam concentration cells of the form (Hg + Pb) Cl ¦Solid electrolyte¦ (Hg + Pb) C2 lead Bromide By choosing an electrolyte system with the following features:
i) does not form deposits on the electrodes, ii) has the same basic reaction on either electrode, iii) has nearly pure ionic conductivity, and iv) has a conducting ion which is related to the component of interest, cells of the types mentioned can be utilized in a variety of modcs:-a) as concentration cells - if the concentration of a component on one side is known (reference) the output e.m.f.
will be related to the concentration of that component on the other side, concentration meter, thermodynamic data, etc.
b) as fuel cells for electric power generation, and c) as pumps - if an electronic current is passed through the electrolyte with the appropriate component having concentration Cl and C2 on either side a transfer of part of the component from one side to the other is effected, the extent of which is primarily determined by the amount of electric current passed.
For each of these modes, the cell design, the electrolyte material, and the electrode material is a matter of consideration.
Extensive use was made of solid electrolytes in the construction of galvanic cells to gather thermodynamic data and in the construction of fuel cells. Disc shaped solid electrolytes made of solid solutions such as CaO in ~rO2 and having oxygen ion vacancies in cells of the type A, A(O) /
solid electrylyte / B, B(O) 86~
where, A(0) - metal oxide of metal A, B(0) - metal oxide of metal B, have been used to determine the molar free energy of formation for a variety of oxides, sulfides and tellurides at elevates temperatures. Their work re-generated interesting the use of mixed crystals as solid electrolytes.
Now Nernst had observed, at the turn of the century, the evolution of oxygen at the anode whilst passing a ~.C. current in his "glow bar" element, which he usually made of mixed crystal solid solutions such as 0.85 ZrO2 0.15 Y2 03 . Wagner and Schotky related, thermo dynamically, the ionic defect concentrations of a compound (ionic or solid solution), and the deviation of the composition from exact stoichiometry, to the activity of the component in the surroundings. Wagner derived the expression:-e.m.f = -1 1 ~"02 (tion) d~02 ............. (2) ~F .
_ 11 1 0 2 where, tion - sum of ionic transference numbers of the electrolyte.
F - Faraday constant.
02 - Chemical potential of oxygen at cathode.
~1'02 - Chemical potential of oxygen at anode.
for the e.m.f. produced in a galvanic oxygen concentration cell involving a mixed conduction solid electrolyte.
Wagner a]so explained the electric conductivity of the Nernst "glow bar" as oxygen ion conduction resulting from large jl/~,~ -4-- .
- ~ : ~ - ;' 6v~3 concentration of mobile oxygen vacancies in the lattice. Hund who studied the density and x-ray diffractions of such mixed oxide solid electrolytes, found that a lower valent cation substituted in the lattice result in oxide vacancies providing thus a path for diffusing oxygen ions. Weininger and Zeemany were the first to demonstrate quantitatively that the oxygen ion is the carrier in solid electrolytes such as 0.85 Zr 02 0.15 Y203 by measuring the evolved oxygen and correlating it to the current passed through the electrolyte. Kingery and co-workers employed the stable isotope 018 and mass spectrometer analysis to determine the oxygen ion mohility in the cubic flourite-structure phase of the solid solution 0.85 ZrO2 0.15 CaO and found it to be near unity.
Interest in oxides such as zirconia, which early in the century was directed to its possible uses as a refractory material, had shifted by the fifties to its characteristic as a solid electrolyte and workers from many countries contributed to this knowledge. This enabled workers such as Peters and Mobuis to improve the design of the Haber basic gas concentration cell using mixed oxide solid electrolyte discs - ThO2, La203, ZrOz and Y203 and to use it to investigate the equilibria.
Co + 1/2 02 ~ Co2 and C + Co2 ~ 2Co at difEerent temperatures -1000 to 1600 K -. Peters and Mobuis disclose in German Democratic Republic Patent No. 21,673, granted August 7, 1961, a practical design of a gas analyse based on such a cell which operates at high temperatures and uses either a gas or a sealed metal/metal oxide mixture as a reference.
Ruka et al US Patent No. 3,400,054, issued September 3, 1968, also describes a cell construction capable of being used as a fuel cell, as an 2 from gas separator or as an 2 partial pressure sensor.

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As operational experience was gathered from the use of such devices, many practical problems appeared and many improvements have been proposed. The most serious of these problems are:
a) Fragility and fracture due mainly to the use of large size ceramic units with low thermal conductivity, to the presence of temperature gradients across the ceramic, and to bad thermal matching of materials;
b) large errors due to active sensor areas being within a temperature gradient, to different temperatures at the sample and reference sides, to mounting the temperature sensor at a position different from the active sensor area, and to leakage across seals;
c) errors due to sample and reference components not reaching the equilibrium temperature;
d) complex designs leading to high manufacturing cost, and difficulty in manufacturing and servicing;
e) non-versatile design leading to specialized sensors.
In one particular aspect the present invention provides a concentration cell having an ion-conductive partition, first ion-supplying means arranged to apply ions to one side of said partition for conducting ions to the other side of said partition, second ion-supplying means arranged to apply ions to said other side of said partition for conducting ions to said one side of said partition, and regulating means for causing said first ion supplying means to supply ions to said one side of said partition in such concentration as to maintain the net conduction of ions across said partition at a predetermined fixed value, wherein said first ion-supplying means is a chamber having said one side of said J~ 6-' . . ;

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partition providing at least a portion of the inner surface of the wall of said chamber and also includes a chemical means for producing a concentration of gas inside said chamber in proportion to the temperature of said chemical means;
and heating means for heating the chemical means, said regulating means being responsive to the voltage across said partition and connected to said heating means for causing said heating means to vary the temperature of said chemical means such as to maintain the voltage across said partition at a fixed va]ue, said heating means arranged to keep the temperatures of ~he opposite sides of said partition substantially equal to each other.
In another particular aspect the present invention provides a method of measuring ion concentration which includes providing an ionic conductor, applying a known concentration of gas to a first portion of a surface of said conductor for conduction through said conductor of gas ions to a second portion of said surface, applying an unknown concentration of gas to said second portion, and varying sald known con-centration such as to malntain a fixed rate of conduction ofgas ions from one said portion to the other by exposing said one portion to oxygen gas evolving from a metal/metal oxide reference means, and varying the temperature of said refer-ence means such as to maintain said fixed rate, while maintaining said first portion and said second portion at substantially the same temperature.
A preferred embodiment of the invention provides a device comprising a small tube or cylinder having positioned across its center a small thin solid electrolyte disc having ionic ~., -6a-'. .- '-'~

conduction appropriate to the component to be monitored.
Symmetrical compartments on either side of the disc act as reference and sample chambers. A heater wire wound in grooves cut on the outside of the tube provide the disc with even heat resulting in equal temperature on either side of the disc.
Metallic particles define an electronic conducting area and hold a temperature sensing element in close contact with the disc on each side.

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8~ 8 The small si~e of the device and its symmetrical thermal design give it great resistance to thermal shock resulting in its ability to respond very quickly to changes in its required set temperature. This enables the device to be operated in a constant e.m.f. mode when a suitable reference is sealed in the reference chamber and a fixed e.m.f. operating point selected. The operating temperature is adjusted by an automatic electronic control system which alters it until the concentration of the reference component in relation to the sample component produces an output across the disc which is equal to the selected e.m.f.. The temperature of the disc is then related to the concentration of the sample component.
The preferred embodiment provides a simple design which is conductive to low cost and easy manufacturing and results in a very small and rugged sensor that can survive harsh environments, large temperature fluctuations, thermal cycling and vibration, while offering at the same time a more reliable and accurate measurement. Another feature of the design i9 its versatility, for example the reference and sample chambers are interchangeable.
Also, the ease of manufacturing a dlsc shaped electrolyte allows a wide choice of electrolytes to suit the application.
The feature of the embodiment, which is possible due to its fast thermal response time, is its operation in a constant e.m.f. mode when a suitabie reference is sealed in the reference compartment, and a fixed e.m.f. set or operating point selected. The operating temperature is adjusted by an automatic electronic control system which alters it until the concentration of the reference component in relation to the ~ 8-... . ..

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36~8 sample component produce an output across the disc which is equal to the selected e.m.f. set point. The temperature of the disc is then related to the concentration of the sample com-ponent.
These and other features and advantages of the present invention will become apparent from the following description of an embodiment thereof given by way of example when taken in conjunction with the accompanying drawings, in which:-Figure 1 shows a section through the centre of a device showing :its symmetry;
Figure 2 shows a way of making the disc and compartmentsin one piece;
Figure 3 shows a cross-sectional end view of the device shown in Figure l;
Figure 4 shows a block diagram of the electronic control system needed to operate the device in a constant e.m.f. mode;
Figure 5 shows a graph of the temperature distribution of the centre of the sensor along its length;
Figure 6 shows a graph of the thermal response of the sensor to a change in set temperature;
Figure 7 shows a graph of temperature difference across the disc due to variations in sample flow rate;
Figure 8 shows a graph of e.m.f. v temperature for a set of constant e.m.f. lines for a Pd/PdO sealed-in reference; and Figure 9 shows a graph of sensor temperature for a constant e.m.f. corresponding to 10% 02 set point.
The basic construction of the device is shown in Figure 1.
A small and thin solid electrolyte disc 1, having a high ionic conductivity appropriate to the measurement required and a low electronic conductivity, separates two chambers one being a reference chamber 2 and the other a sample chamber 3, the j 1/ _9_ 8~8 chambers being interchangeable due to the symmetry of the design.
A specific area 4 on each side of the disc is coated with a layer of porous electronically conducting powder, this can be applied by sputtering or by using a commercial paste. The choice of powder depends on whether an equilibrium state is desired, then a metal such as platinum would be suitable since platinum acts as a catalyst to speed up the operation, or a non equilibrium state is desired where a metal such as silver would be suitable.
The coating is used also to embed a thermocouple of very fine wire 5 into the centre of each face of the disc. In most cases it is found that the temperature of each face of the disc is nearly identical in which case one thermocouple suffices, and only an electrical conductor wire is needed on the opposite face. Each thermocouple provides an electric signal related to the temperature of the associated disc face.
The e m.f. across the disc can be measured using wires of the same material from each thermocouple. The chambers are defined by a thin walled ceramic cylinder manufactured out of an appropriate material having a zero porosity, thls could be of the same composition aæ the disc or a ceramic with matching thermal characteristics. The chambers and disc can be made in one part 6. Figure 2 if they have the same composition? or they can be made in separate cylindrical sections 7 as shown in Figure 1. When made in sections they can be joined to the disc with a metallic based, a glass based, or a ceramic based gas tight seal 8. The end sections 9 and 10 are also joined to the chambers and sealed gas tight. Means for admitting 11 and removing 12 a fluid into the chamber 2 are provided by pipes ., . ~ . .- . :

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sealed gas tight to the end section 9. The choice of pipe material is dependant on the intended operating temperature range and on the degree ofthe corrosion potential of the fluid. The thin electric wires are brought outside the chamber, through holes 13 which are sealed gas tight, and then fused to thicker wires 14 of the same composition which provide the electrical terminations through the end section 9. ;;
A heater wire 15 is tightly wound round the ceramic cylinder in grooves cut on its outside and covered with a ceramic cement 16. The end connections of the heater winding are also brought out through the end section 9.
A cylinder of insulating material such as an appropriate ceramic 17 is cemented with a refractory cement to the end sections and the space between it and the heater can be filled with a high temperature insulation fibrous material 18. A thin stainless steel outer cover 19 cemented to the end sections acts as an overall sheath. Reference material is sealed in the reference chamber 2 obviating the need for the inlet and outlet pipes.
In another embodiment, when the fluid to be tested is in an open environment into which the sensor is inserted, the fluid could diffuse into the test chamber through a filter disc or a flame trap if necessary. The outer sheath 19 then to be replaced by an extended sheath having an opening at the trap or filter disc.
When the device has a sealed in reference, the device is operated in a constant e.m.f. mode as outlined in the block diagram shown in Figure 4. The desired set point or constant e.m.f. is selected from voltage divider 22. An electronic 8~
comparator 23 compares the set e.m.f. with the output voltage of the decive and produces a signal, when they differ, of polarity indicating the sense of the difference which is fed to the temperature controller 2~l. This either increases or decreases the electrical energy reaching the heater 15 as needed to decrease the difference. The device settles down to a temperature at which the sealed-in reference produces a concentration of the component related to the sample component so that the output voltage of the device is identical to the set point voltage.
The temperature of the device as read by the thermocouple 5 and after suitable conditioning 25 can be used to drive a display 26. The most common use of solid electrolyte devices at present is for the measurement of the partial pressure of oxygen in a gas mixture, so this application will be chosen for the following example although by proper choice of the materials other components can be measured. For example, if it were desired to measure chlorine, a chlorine ion conductive solid electrolyte would be chosen and a reference of metal/metal chloride would be used.
Example 1 This is an example of selecting an appropriate sealed reference for oxygen measurement in the constant e.m.f. mode.
The reference is required to have an oxygen partial pressure, which is a function of temperature, that is very stable and repeatable. A system of metal/metal oxide has this feature, and the second requirement would be to find partial pressures corresponding to the range of 2 measurement and in a temperature range that is within the working range of the ~ 12-,:
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, .-: . : . .~, ` ~rf~ ~6$8 solid electrolyte used. The range of oxygen is chosen to be within 1-100% and one of the common oxygen type ion conductor electrolytes that operate between 800 - 1300 K is used.
An oxygen ion conducting material was chosen for the disc which was 5 mm diameter and 0.5 mm thick and an active area of about 2 mm diameter. Pt/13% Rh 0.001 diameter wire was used for the thermocouple which were cemented using Johnson and Matt'ney N 758 Pt. paste. The disc was fused to the two ~ylindrical halves using thin copper foil and heated to over 1000C. The heater winding was 0.002 Pt wire covered with sauereisen no. 8 cement, this was also used for all other seals, but other suitable cements may be used. The temperature distribution of the centre of the sensor along its length with 20 W input power is shown in Figure 5. The measured temperature difference between the faces of the disc was less than 1C. The response of the sensor to a chang'e in set temperature, in this case switching on 20 W from cold, is shown in Figure 6 where a time constant of about 15 seconds is evident.
Repeated thermal cycling and shock showed no sign of fracture in the sensor. By appropriately positioning the inlet pipe the-effect of the fluld cooling the disc can be minimised as shown in Figure 7.
Now for a metal/metal oxide system the oxygen partial pressure C2 at a given temperature is given by:-ln C2 = A + BT ................................... (3) where A and B are constants dependant on the oxide system used.
Using this with equation (1), n = 4 for oxygen.
e.m.f. = A + BT ~ 4F ln Cl ............................ (4) An oxide system that satisfies our requirement is aPd/Pd 0 system and using the data given by Fouletier J. Appl.

; -13-Electrochem. Vol. 5, III (1975) in conjunction with equation (4), a family of curves can be drawn each corresponding to a constant e.m.f. and relating the partial pressure of oxygen with the temperature Figure 8. ~y examining this we can select a constant e.m.f. line which is suited for the o~ygen range and temperature range of the sensor. Assume that the zero e.m.f. line is selected, this gives us the partial pressure of the oxygen at the sample side log Cl = 10.3 - 1175~ ............................... (5) Thls shows that the working temperature of the device ls related to the sample concentration when it is operated in a constant e.m.f. mode and the variation in temperature corresponding to an oxygen sample range of 15 - 5~ 02 for a zero e.m.f. is shown in Figure 9.
The sensor is connected in a circuit as shown in Figure 4 whlch controls the temperature of the sensor and hence the refe~ence material in the reference chamber 2 until the concentration of the reference component equals the concentration in the sample component and hence no e.m.f. is generated across the disc. The temperature of the disc is thus related to the concentration of the sample component and the temperature indication can be used as an indication of concentration.

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A concentration cell having an ion-conductive partition, first ion-supplying means arranged to apply ions to one side of said partition for conducting ions to the other side of said partition, second ion-supplying means arranged to apply ions to said other side of said partition for conducting ions to said one side of said partition, and regulating means for causing said first ion supplying means to supply ions to said one side of said partition in such concentration as to maintain the net conduction of ions across said partition at a predetemined fixed value, wherein said first ion-supplying means is a chamber having said one side of said partition providing at least a portion of the inner surface of the wall of said chamber and also includes a chemical means for producing a concentration of gas inside said chamber in proportion to the temperature of said chemical means; and heating means for heating the chemical means, said regulating means being responsive to the voltage across said partition and connected to said heating means for causing said heating means to vary the temperature of said chemical means such as to maintain the voltage across said partition at a fixed value, said heating means arranged to keep the temperatures of the opposite sides of said partition substantially equal to each other.

2. A concentration cell according to Claim 1, wherein said partition is provided mid way along the length of a bore of a tube so as to form symmetrical chambers, the heating means being arranged to heat said chambers equally.

3. A concentration cell according to Claim 2, wherein the partition is integral with the tube.

4. A concentration cell according to Claim 2, wherein the partition is sealed to the bore of the tube using a cement.

5. A concentration cell according to Claim 2, wherein the heating means comprises a coil of wire wound around the exterior of the tube.

6. A method of measuring ion concentration which includes providing an ionic conductor, applying a known concentration of gas to a first portion of a surface of said conductor for conduction through said conductor of gas ions to a second portion of said surface, applying an unknown concentration of gas to said second portion, and varying said known concentration such as to maintain a fixed rate of conduction of gas ions from one said portion to the other by exposing said one portion to oxygen gas evolving from a metal/metal oxide reference means, and varying the temperature of said reference means such as to maintain said fixed rate, while maintaining said first portion and said second portion at substantially the same temperature.

7. A method according to Claim 6, wherein the metal/metal oxide reference means is a Pd/Pd O system.

8. A concentration cell according to Claim 1, said heating means being arranged to heat said chambers equally.