WO1998013687A1

WO1998013687A1 - Passive cell protection for solid electrolyte gas analyzer

Info

Publication number: WO1998013687A1
Application number: PCT/US1997/017066
Authority: WO
Inventors: Boris Y. Farber; Joanne M. Starling; Mark Little
Original assignee: Rosemount Analytical Inc
Current assignee: Rosemount Inc
Priority date: 1996-09-24
Filing date: 1997-09-22
Publication date: 1998-04-02
Anticipated expiration: 1999-03-24
Also published as: JP2000501512A; DE19781049T1; GB2321314A; GB9810139D0

Abstract

A solid electrolyte gas analyzer (40) includes a probe (42) for immersion in the gas of interest. The probe has a passageway (57) connecting the gas of interest to a heated solid electrolyte cell (60). A collection of beads (56A), each having an outer surface comprising a catalytic material such as platinum, is disposed in the passageway (57). The platinum surfaces of the beads (56A) act as a catalyst to protect the cell (60) from sulfur and other corrosive elements originating from the gas of interest. In a preferred embodiment the beads (56A) are disposed proximate the cell (60) and a heater (66) surrounds both the cell (60) and the beads (56A) to heat them to a predetermined temperature.

Description

PASSIVE CELL PROTECTION FOR SOLID

ELECTROLYTE GAS ANALYZER

BACKGROUND OF THE INVENTION The present invention relates generally to the segment of the field of gas analysis instrumentation that involves the use of solid electrolyte cells to measure gaseous species. More specifically, the invention relates to techniques for protecting solid electrolyte cells from corrosive components in the measured gas. As used herein, the term solid electrolyte cell means a quantity of the solid electrolyte, for example zirconia-yttria ( (ZrO_z) ₍₁._x) (Y₂0₃)_x), and porous electrical contacts or electrodes connected thereto, usually made of platinum or other suitable materials.

Gas analyzers having a probe with a solid electrolyte cell to measure gaseous oxygen are well known. See, for example, U.S. Pat. Nos . 3,400,054 and 3,928,161, incorporated herein by reference. Another example is the World Class 3000 Oxygen Analyzer sold by Rosemount Analytical Inc. of Orrville, Ohio, available with replacement cells under part no. 4847B61G01/02/03. A common application for such analyzers is the measurement of gaseous oxygen in a flue or duct such as a smokestack. At a given elevated temperature, the solid electrolyte cell generates an EMF V_celi as a function of the concentration of oxygen (or other gaseous specie) exposed to the cell. The probe is configured with a heater and thermocouple, controlled by circuitry in a separate housing, to maintain the solid electrolyte cell at a constant elevated temperature (e.g. , 750°C) . It is also known that solid electrolyte cells can corrode from exposure to certain gas constituents. One known solution is to encapsulate an exposed electrode of the cell in a refractory overlayer. See, e.g., U.S. Pat. No. 3,645,875. Another known solution is to pack platinum fibers in a passageway between the cell and the measured gas. Corrosive constituents in the measured gas chemically react with the fibers before they have an opportunity to contact the cell. An object of the invention is to reduce exposure of the cell to corrosive constituents in the gas of interest in a relatively low-cost manner and without having to alter the design of the cell e.g. by applying protective coatings to the cell . Other objects of the invention will become apparent from the detailed description of the invention and the accompanying drawings .

SUMMARY OF THE INVENTION A solid electrolyte gas analyzer has a probe with a distal end adapted for immersion in the gas of interest. A passageway in the probe connects the gas of interest to a solid electrolyte cell providing an output as a function of a constituent of the gas of interest. The analyzer further includes a catalyst disposed in the passageway, the catalyst including a collection of beads each having an outer surface comprising a catalytic material such as platinum. In a preferred embodiment the beads are sandwiched between a first and second screen, the screens having matching concave shapes. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of a preferred gas analyzer.

Figure 2 is a sectional view of the distal end of an analyzer probe according to the invention. For convenience, items in the figures having the same reference symbol are the same or serve the same or a similar function.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred solid electrolyte analyzer 40 is shown in Figure 1. Analyzer 40 comprises an analyzer probe 42 having a distal end 42A, a proximal end 42B, and a mounting flange 44 affixed therebetween to a tubular member 46. Flange 44 mounts to a wall of a flue or duct such that distal end 42A projects into the flue and proximal end 42B remains outside the flue. An analyzer circuit, housed in a can 48, measures the cell output V_cell and the cell temperature, controls the cell heater, and calculates the oxygen content of the gas of interest. Can 48 preferably mounts to the probe 42 at the proximal end 42B through a standoff 52, resulting in a combined analyzer/probe having reduced installation costs compared to present day analyzers which require a separate mounting site for the analyzer circuit and an interconnecting cable. Further, corruption of the outputs from the cell and thermocouple (both mounted in distal end 42A, see FIG. 2) is minimized by keeping the length of the interconnecting wires short and shielding them with the analyzer housing comprising the tubular member 46, can 48, and standoff 52, all of which are preferably metallic.

Standoff 52 helps the analyzer circuit survive the high temperatures near the flue by distancing electronics can 48 from the flue. Standoff 52 also functions as a heat sink between tubular member 46 and can 48 by virtue of cooling fins or ridges 52A on its outer surfaces to dissipate heat to surrounding ambient air. The wall thickness of tubular member 46 is reduced between flange 44 and standoff 52 to reduce heat conduction along the tubular member. The wall thickness can be reduced by removing material from the outside diameter of tube 46 or from the inside diameter or both. Standoff 52 has a face 52B that carries pneumatic fittings 54A,54B,54C used for the introduction of a reference gas, of a calibration gas, and for a vent. By placing fittings 54A,54B,54C on standoff 52, the pneumatic and electrical connections are divided between standoff 52 and electrical can 48. This separation simplifies operation and serviceability of analyzer 40 by permitting pneumatic connections to be serviced independent of the electrical connections and vice versa. Further details of the housing and analyzer circuit can be found in the co-pending applications referenced above.

A closeup of probe distal end 42A is shown in FIG. 4, further including a dome-shaped ceramic filter 26. Oxygen gas in the flue migrates through filter 26 and through a catalyst 56 before contacting a side 60A of a solid electrolyte cell 60. During a calibration procedure calibration gas having a known oxygen content is provided via a pneumatic tube 58 and routed through a flange 62, displacing the flue gas and thus contacting side 60A of cell 60. The opposite side 60B of cell 60 is continuously exposed to reference gas emitted from a pneumatic tube 64 and having an oxygen partial pressure P(°₂)_REF- Cell 60 is disposed inside a cell heater 66 which heats cell 60 to a substantially constant cell temperature T. Heater 66 is preferably of conventional design, formed of a length of nichrome wire helically wrapped around a quartz support cylinder.

The EMF voltage generated by cell 60 can be approximated by the Nernst equation: e_en = C + S^«log{P(0₂)/P(0₂)_REF}, where C is a cell constant, S is a cell slope which is a function of T, and P(0₂) and P(0₂)_REF are the oxygen partial pressure at sides 60A,60B respectively of cell 60. Actual solid electrolyte cells deviate from the Nernst equation to some extent. A thermocouple 68 disposed proximate heater 66 and cell 60 is used for temperature control. Wires 70,72,74 extend through tubular member 46 and standoff 52 to connect cell 60, heater 66, and thermocouple 68 respectively to the analyzer circuit housed inside can 48.

Catalyst 56 operates to shield cell 60 from corrosive gaseous components such as gaseous sulfur and oxides of sulfur (S0_X) in the flue gas by catalyzing such components before they migrate to side 60A. Catalyst 56 comprises a quantity of beads 56A sandwiched between stainless steel screens 56B,56C. Screens 56B,56C are preferably formed into matching concave shapes as shown to increase surface area while providing a uniform axial depth of beads 56A. The screens are held in place inside a tube 57, that also holds cell 60, by spot welding or brazing. The screens are preferably made of stainless steel because of its resistance to corrosion but can alternately be made of other corrosion-resistant materials. Beads 56A are all preferably about 2mm diameter alumina spheres, such as part No. C5035 available from Engelhard Corp. of Seneca, South Carolina, but other suitable sizes, shapes, and compositions can also be used. Equally sized spheres are preferred because of their uniform packing characteristics, promoting a spatially uniform migration of gas throughout the volume of beads . Each bead has a thin coating of catalytic material, preferably platinum. Other catalytic materials similar to platinum, such as palladium, can be used instead of platinum. Catalyst 56 is held inside cell heater 66 and maintained at the cell temperature T.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention as defined by the claims appended hereto. The invention relates to solid electrolyte gas analyzers but does not encompass the solid electrolyte cell itself, which can have various configurations and compositions.

Claims

WHAT IS CLAIMED IS:

1. In a gas analyzer of the type having a probe carrying a solid electrolyte cell that provides a cell EMF as a function of a constituent of a gas of interest, the probe being adapted for immersion in the gas of interest and having a passageway connecting the solid electrolyte cell to the gas of interest, a cell protection apparatus comprising: a catalyst disposed in the passageway, the catalyst including a collection of beads each having an outer surface comprising a catalytic material.

2. The cell protection apparatus of claim 1, wherein the catalytic material is selected from the group consisting of platinum and palladium.

3. The cell protection apparatus of claim 1, wherein the gas analyzer also has a heater operable to heat the cell to an elevated temperature and the probe has a distal end, wherein the cell and catalyst are disposed in the distal end of the probe proximate the heater.

4. The cell protection apparatus of claim 3, wherein the heater surrounds both the eel] and the catalyst .

5. The cell protection apparatus of claim 1, further comprising a first and second screen mounted in the passageway such that the collection of beads is sandwiched between the first and second screen.

6. The cell protection apparatus of claim 5, wherein the first and second screens have a concave shape .

7. The cell protection apparatus of claim 6, wherein the first and second screens have substantially matching concave shapes and are oriented in the same direction.

8. The cell protection apparatus of claim 1, wherein each bead comprises a body of inert material covered with a film of the catalytic material.

9. The cell protection apparatus of claim 8, wherein the inert material comprises alumina.