GB2150299A

GB2150299A - Oxygen cells

Info

Publication number: GB2150299A
Application number: GB08426863A
Authority: GB
Inventors: Michele Petrucci; Virgilio Cresti
Original assignee: Nuova Italsider SpA
Current assignee: Nuova Italsider SpA
Priority date: 1983-11-04
Filing date: 1984-10-24
Publication date: 1985-06-26
Also published as: BE900949A; DE3439286A1; IT1197738B; FR2554594A1; IT8349278A0; JPS60113144A; GB8426863D0; LU85609A1; NL8403355A; GB2150299B

Abstract

In a solid electrolyte oxygen cell in which the unknown oxygen partial pressure of a fluid medium is ascertained by measuring the emf between two electrodes 6 and 8 of the concen- cell, oxygen at a known partial pressure is produced as a reference by means of a substance 3 which reacts with the oxygen to produce compounds which are easily removed from the reaction area within a chamber 1. In this way, the problem of polarization of the cell is eliminated, for example, and so its useful life is considerably extended and the signal continues to remain extremely accurate over the long term. Substance 3 may be carbon or sulphur which produces gaseous products on reaction with the oxygen. Alternatively it may be lead or tin which produces products which slag easily and which substance is liquid at the temperature of the fluid medium. <IMAGE>

Description

SPECIFICATION Improvements in oxygen cells This invention relates to improvements in oxygen cells and particularly concerns the solution of the problem of long-term unreliability of measurements for determining the oxygen content in fluid media at high temperature.

The measurement of oxygen in fluid media at high temperature is finding increasing application for the monitoring and control of a large number of industrial processes, such as the production of metals and alloys in the molten state, the annealing and carburizing of metals, the sintering of ores, combustion, etc.

The oxygen cell is commonly used for this purpose. The cell consists of a chamber whose walls are made of suitably stabilized zirconium oxide which becomes pervious to oxygen ions above a given temperature, typically around 500 C.

By creating a known oxygen partial pressure in the chamber a concentration cell is obtained whose electromotive force (emf) is expressed by the Nernst equations:

where E is the emf of the cell, P02R is the known oxygen partial pressure and P 2x is the partial pressure it is desired to measure.

The problem is to obtain a PO2R which remains known over the long-term, so as to permit precise control at any instant.

There are two ways of obtaining a known oxygen partial pressure. The first way consists in passing into the chamber a continuous flow of gas of known composition. This means that the walls of the measuring cell and the gas supply line must be perfectly leak-proof or at any rate such that they do not permit any contaminants to enter the chamber which could alter the composition of the reference gas. Because of the thermal and mechanical shocks to which these cells are normally subjected, this condition is quite difficult to maintain, so that the cost is high. However, there still exists a doubt as to the effective long-term reliability of the measurements obtained.Furthermore, technical and economic requirements at the present time mean that the gas supply lines cannot exceed a certain length, typically around 70-100cm, thus considerably limiting the field of application of this kind of apparatus.

The second way consists in placing complex metal oxide mixtures in the chamber which, at equilibrium, ensure known oxygen partial pressures at various temperatures. The drawback with these devices is that they only function correctly for a limited period of time, since the oxygen which passes through the walls of the chamber reacts with the metal at the interface, causing polarization of the cell. Because of this the cell suffers from drift, the magnitude of which is unknown.

There is thus a problem of reliability of measurements over the long term, and hitherto this problem has been tackled by frequent replacement of the oxygen cell. However, this is not a satisfactory solution because of possible interruptions of the process under control and because of the intrinsic difficulties of the replacement operation, to say nothing of the cost of the apparatus which, although not enormous, is nevertheless appreciable, especially for the gas supply type of cell.

The object of the present invention is to resolve this problem by providing an improved type of oxygen cell which is completely reliable over the long term. The cell is also very sensitive so it can be placed in any position relative to the metering and signal-recording instruments, and moreover its cost is limited.

According to the invention, there is provided a device for measuring oxygen concentration in a fluid medium at high temperature, comprising an oxygen concentration cell in which, in use, an electromotive force is generated between two electrodes one of which in contact with the fluid medium at high temperature having an unknown oxygen concentration and the other of which is in an environment having a known oxygen concentration which is produced by reaction between the oxygen and a reactive substance in a chamber having walls which are pervious to oxygen ions, wherein the substance which reacts with oxygen forms products which are easily removed from the reaction area within the chamber.

This substance preferably produces gaseous products on reaction with oxygen, or alternatively is a liquid at the temperature of the fluid medium and, as a result of reaction with oxygen, produces reaction products which can be solid, but which float on the liquid.

Examples of these substances are all those elements capable of reacting with oxygen to give volatile products or which, while giving solid reaction products, having a melting point lower than the temperature at which the operations are performed, such as, for instance, aluminium, antimony, bismuth, carbon, lead, alkaline-earth metals, phosphorus, sulphur, tin, etc. Of course, in practice, the choice is reduced for various reasons such as excessive reactivity of the element, its contaminating effect or the hazards caused by it or its products of reaction.

In order that the invention may be more fully understood, a preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which the single figure shows a vertical section through a reaction vessel.

Referring to the drawing, a chamber with a zirconium oxide wall 1 having a cover 2 is filled with the reactive substance 3, as described above.

Electrodes 8 and 6 with their insulated conductors 7 and 5 are mounted on the outer and inner sides respectively of the wall 1. The internal conductor 5 leaves the chamber through a small hole 4 (which serves also for extraction of the gaseous products of reaction) in the cover 2. The cover 2 also preferably has a second hole ii through which pass the wires 9 and 10 of a thermocouple the junction 12 of which is set as close as possible to electrode 6 and wall 1.

This arrangement of the thermocouple is useful for ensuring the best possible measurement of the temperature of the oxidation reaction which occurs in the vicinity of the electrodes. For each reaction temperature there is a precise oxygen partial pressure which is used as the reference value P02R in the Nernst equation referred to earlier.

In operation an emf is established between electrodes 6 and 8 due to the difference between P02R and P 2x, the latter being derived by measuring the emf at the terminals of conductors 5 and 7 at a known temperature.

Various prototype oxygen cells in accordance with the above description have been built and tested under industrial conditions. Tests have been run so farforthree successive months without interruption.

The signal emitted by the cell during use has been found to be completely independent of the age of the cell, and it has also been observed that there are no special constraints regarding the respective positions of the cell(s) and the recording and metering instruments.

Of the numerous elements which have been employed for generating the reference oxygen partial pressure, lead, tin, carbon and sulphur have given the best results. However to date the greatest reliability, with complete absence of problems of any kind and the highest sensitivity, has been achieved with tin and carbon, obtaining 1 x1018 atmosphere as the iower iimit of measured concentration as ascertained by the mass spectrometer and with H2/H2O gas mixtures of known oxygen content.

Claims

1. A device for measuring the concentration of oxygen in a fluid medium at high temperature, comprising an oxygen concentration cell in which, in use, an electromotive force is generated between two electrodes one of which in contact with the fluid medium at high temperature having an unknown oxygen concentration and the other of which is in an environment having a known oxygen concentration which is produced by reaction between the oxygen and a reactive substance in a chamber having walls which are pervious to oxygen ions, wherein the substance which reacts with oxygen forms products which are easily removed from the reaction area within the chamber.

2. A device according to claim 1, wherein said substance produces gaseous products on reaction with oxygen.

3. A device according to claim 1, wherein said substance is a liquid at the temperature of the fluid medium and, as a result of reaction with oxygen, produces solid products which slag easily.

4. A device according to claim 2, wherein said substance is carbon or sulphur.

5. A device according to claim 3, wherein said substance is lead or tin.

6. A device for measuring the concentration of oxygen in a fluid medium at high temperature, substantially as hereinbefore described with reference to the accompanying drawing.