GB2052754A - Process and apparatus for the measurement of the concentration of odoriser in fuel gases - Google Patents

Abstract

The invention concerns a process and apparatus for measurement of the concentration of odorizer in fuel gases, particularly the mercaptan content therein. The measurement is effected in a galvanic cell (1) having silver measuring and reference electrodes, (2, 4). The galvanic cell (1) is made from an electrolyte-permeable material and formed as a cylindrical absorber (3). The fuel gas under test is brought into contact with strongly alkaline electrolyte saturated with Ag2O (5), that expediently contains 0.2 N KOH and 40 vol.% of an alcohol as well as 1 to 3 weight % KNO3. The electrolyte is kept in forced circulation by the gas and the gas is continuously absorbed. The Ag<+> concentration decreases adjacent the measuring electrode (2) and a galvanic current arises between the measuring electrode (2) and the reference electrode (4) that is in contact with the Ag2O. This current at constant gas flow rate is proportional to the odoriser concentration and is measured by a temperature- compensated method. <IMAGE>

Description

SPECIFICATION Process and apparatus for the measurement of the concentration of odoriser in fuel gases The invention concerns a process and apparatus for measuring the odoriser, mainly mercaptan, content of fuel gases, i.e. of the concentration of odorising agents in natural gas, propane-butane gas (hereafter: PB gas), town gas etc. wherein the energy liberated in the chemical reaction A+g+RSH=AYSR+H+ (1) taking place in a galvanic cell with silver electrodes provides the current which is an unambiguous function of the concentration to be measured under the circumstances of the measurement.

The odorising of energy-carrying gases (natural gas, PB gas, town gas) and the monitoring of the odorisation serve the fundamental safety interests of a large proportion of the population. In Hungary, exclusively ethyl mercaptan is used as an odoriser but elsewhere other mercaptans may be considered, e.g. tertiary butyl mercaptan and tetrahydro-thiophen.

A Hungarian Standard currently in force prescribes a test involving the human sensing organs for the measurement of the concentration of ethyl mercaptan. This test is cumbersome, is highly subjective and is subject to several disturbing effects. The laboratory method, utilising scientific instruments, is based on selective absorption, reaction with Ag+ or Hg++ ions and the electrochemical measurement of the residual ion concentrations. A known instrument called "Odotron" is based on gas-chromatographic separation and detection of electron capture but due to its weight and difficulty of operation it is not suitable for being an effective instrument for monitoring gas mains networks.

W.Breuer(VDI-Z. 107 (1965) Nr,30.1435) describes a galvanic cell for the measurement of the emissive concentration of hydrogen sulphide which in a few aspects resembles the solution according to the present invention. (A general description of galvanic cells may be found in a part of a book by P. Hersch entitled "ADVANCES IN ANALYTICAL CHEMISTRY AND INSTRUMENTATION" volume 3, John Wiley s Sons, London, New York, Sydney, Toronto).In this cell, a 0.01 N solution of Na2CO3 saturated with Agl is in contact at the surface of a diaphragm with the gas to be measured while a respective silver electrode is in contact with the solution saturated with Agl and with the Ag+ concentration which latter has been reduced under the effect of the hydrogen sulphide, as a consequence of which in the circuit connecting the two silver electrodes there is a flow of current which is a measure of the H2S concentration.

The Breuer method itself has numerous disadvantageous properties. Because of the low efficiency of material transfer related to hydrogen sulphide (and calculated on the ethyl mercaptan this value is smaller still!) the current intensity that can be calculated from Faraday's Law is only a small fraction of the calculated value as a consequence of which electronic amplification is required which, in turn, however, is disadvantageous from the point of view of protection against fire and explosion. The circulation of the electrolyte makes the measurement difficult and the sensitivity varies with changes in the rate of circulation.Since the material transfer is the process that determines the rate of velocity the sensitivity strongly varies with temperature (a few %/at). The detector is very sensitive to changes in the concentration of the I ion in the electrolyte and to its contamination since the ion product of Agl at 200C is 8.3 x 10-17, and hence an increase in the concentration of the I of 10-7 mol/l reduces the Ag + concentration to a tenth of its previous value).

Another disadvantage of the detector is that during operation polarisation may take place under the effect of the I ions. Furthermore, it is an unfavourable fact that the Breuer cell has a relatively high resistance which can change under the effect of components such as SO2 and CO2 which are present in the gas and which dissolve in the electrolyte; this is a very disturbing factor having regard to the fact that the current intensity is the parameter to be measured. A further disadvantageous phenomenon which leads to a change in the surface of the measuring electrode and thus ultimately to a change in sensitivity is that on the silver surface a Ag2S or silver mereaptide coating is formed, leading to an increase in cell resistance, known as "aging".

Significant difficulties arise also in the manufacture and replacement of these cells.

The present invention aims to eliminate or reduce the above-listed disadvantages and to develop a novel measurement cell operating with silver electrodes which fundamentally differs in its operation and construction from known silver cells.

As electrolyte a strongly alkaline medium (pH 13), expediently KOH of 0.2 N concentration with a significant salt content of 1 to 3 weight % of KNO3 is used, which significantly increases the conductivity of the cell. To ensure a constant Ag+ concentration, the solution is saturated with Ag2O: Ag2O+ H20a2AgOH (2) AgOHAg+ + OH-; K = 2.6 x 10-8 (3) The use of KOH has several advantages: it increases the conductivity; it increases the solubility and rate of solvation of the mercaptan; it sets or adjusts the necessary Ag+ concentration according to equation (3); it neutralises the acidity of the mercaptan (equation (1)) while at the same time its own concentration remains practically unchanged.

On analysing the formation of deposits on the silver measuring electrode of the cell, it has been found that this deposit is a side-reaction product wherein the mercaptide ion dissolved in the electrolyte does not react with the Ag+ ion but reacts instead with the material of the electrode itself, according to the reaction: Ag + RS- = AgSR +e- (4) which from the point of view of current generation is equivalent with the main process, but in its consequences, however, leads to an increase in the surface resistance of the electrode and ultimately to a decrease in cell sensitivity.

We have established that the deposit or surface coating formation on the electrode may be slowed down by: (a) increasing the Ag+ ion concentration which in practice in a given system means a decrease in the pld, but this in turn decreases the solubility of the mercaptan and also decreases the voltage of the ceil (because then the relative change in concentration of the Ag+ ions is also smaller) and all these factors ultimately lead to a decrease in sensitivity; the same result is obtained by decreasing the internal resistance of the cell and of the whole circuit, since the cell current itself also produces Ag+ ions; (b) decreasing the concentration of the mercaptide ions in the immediate vicinity of the measuring electrode.The impeding of the material transfer of the mercaptan by decreasing the pH and decreasing the contact surface area exerts its effect in this direction but it also decreases the sensitivity. Increasing the thickness of the liquid film separating the electrode and the gas phase by means of a thin filter material or filter paper coating on the measuring electrode also has this effect but increases the reaction time.

It is a disadvantageous property of galvanic cells that their sensitivity strongly varies with temperature and this increase may in certain cases exceed 10% per OC. This very high sensitivity variation has the consequence that the cell has to be brought to the appropriate temperature in the interests of measurement accuracy, but this renders the measurement and instrument intricate and expensive.

In this connection, we have established that the fundamental cause of the temperature dependence of sensitivity iies in the role played by the material transfer between the gas and liquid phases in determining the rate and sensitivity, and hence in order to reduce the temperature coefficient it is advantageous to provide conditions which render the absorption of the odoriser as complete as possible, that is to say the absorption of the odoriser should be effected: 1. in an alcohol solution, expediently in a 40 volume % isopropyl or n-propyl alcohol solution; 2. with a large absorbing surface: and 3. under turbulent flow conditions (high flow velocity).

The last two conditions may be assured by the use of a cylindrical absorber made from a filter material such as filter paper with a thickness of expediently less than 40 ,um in close contact with the measuring electrode.

We have also established that to compensate the change in sensitivity caused by the temperature coefficient, it is highly advantageous to use resistors, such as thermistors, which have a negative temperature coefficient. Their use may be achieved in two different ways, namely when using uA-meters of small internal resistance, these resistors can be used as shunts, while they may be used as a working resistor when using electronic amplification.

The cell and detector for measuring the odoriser concentration or mercaptan content of fuel gases is described in detail with reference to the accompanying purely exemplary drawings, wherein: Figure 1 illustrates an expedient embodiment of a galvanic cell; Figure 2 illustrates in diagrammatic elevation and plan the detector vessel, as well as certain of its components in elevation, Figure 3 is a block diagram of a portable device known as "Thiodet", which utilises the cell as a detector, and Figure 4-is a block diagram of an apparatus known as "Thiocontrol", which utilises the cell as a detector and which is a continuous measuring instrument that regulates in accordance with concentration.

Several constructional variants of a galvanic cell based on the operating principles described above have been tried in order to perfect a practical embodiment. The most advantageous embodiment has proven to be that shown in Figure 1. Here, the gas to be measured flows from below upwardly through a tube 1 made of a filter material, expediently of filter paper, and of 4 mm diameter and 110 mm length, and meets the electrolyte flowing radially into the tube under the effect of hydrostatic pressure. The outside of the paper tube is covered by a silver foil 2 wound thereon to serve as a measuring electrode, such that the measuring electrode expediently only covers the upper half of the tube.Filter paper layers 3 separate the measuring electrode from a reference electrode 4 which is also made from silver foil and covers the same portion of the tube as the measuring electrode. The reference electrode and the remaining surface of the tube are covered by an Ag2O precipitate layer 5 having a thickness of about 1 mm, the task of which is to prevent polarisation of the reference electrode and to adjust the equilibrium concentration of Ag+ in the electrolyte.

The reference electrode is covered by 2-4 layers of filter paper 6 having a thickness of 0.2-0.5 mm, held together by a wire braid (e.g.

Kanthal wire) which is inert to the electrolyte and which is used as mechanical stiffener. The lower end of the cell can be mounted on the inlet tube 1 5 for introducing fuel gas and can be resiliently and sealingly connected thereto (by means of rubber bands). The upper end of the cell can be fixed to a fuel gas conveying tube to assure the required mechanical rigidity for the cell.

Of course, in place of filter paper any other inert filter material is suitable provided that it retains the Ag2O precipitate, and is permeable both to the electrolyte and to the ions. According to our experience, the preferred thickness of the filter materials 3 and 6 is between 0.2-0.5 mm, that is to say 2 to 4 layers of ordinary commercial filter paper.

The silver electrodes 2 and 4 are expediently made from silver foil of 0.2-0.3 mm thickness, 2-3 mm width and 300 mm length to which silver wires are welded to provide circuit connections. The foils may be spirally wound on the appropriate cylindrical surfaces. Naturally, silver wire mesh or perforated sheets may also bye advantageously used.

The most expedient positioning for the silver measuring electrode 2 is on the upper half of the tube 1 since there it can come into contact with the electrolyte after the latter has had its Ag+ ion concentration strongly reduced as a consequence of the absorption of mercaptan. This determines also the position of the silver reference electrode 4 since the least internal resistance of the cell and thus its highest sensitivity is provided when the two electrodes are in geometrical overlap.

From the point of view of the sensitivity and life of the measuring electrode 2 it is advantageous if a silver layer of enhanced surface area, analogously to platinum black, is formed on its surface, by utilising a current density greater than that prescribed for the silvering process.

The actual detector is formed by the cell and a detector vessel containing it as well as electrolyte, see Figure 2. This vessel 8 is a cylindrical glass vessel, expediently of polished glass, and provided with a lid or stopper 7, expediently also polished, the vessel volume being approximately 300 ml. At the top of the polished stopper there are two polished openings 9, 10. The opening 9 may receive a polished capillary member 11 through which the gas to be measured exits from the detector while the opening 10 receives a body for immersing a sealed thermistor 1 6 in the electrolyte, the thermistor being connected as a shunt. Two platinum wires 12 are sealed in the top of the stopper to mechanically connect the electrical leads of the cell.A tubular fitting 13 serves to introduce the gas to be measured and this fitting is continued as a multiply-bent tube 14; the long length of the tube 14 assures that the gas to be measured takes up the actual temperature of the electrolyte. The tube 14 ends in the already mentioned upwardly bent fitting 1 5 to which the above-described galvanic cell is sealingly connected. The detector vessel is filled with electrolyte to such a depth that the upper end of the cell projects by a few mm, expediently 2-3 mm, from the electrolyte.

The operation of the detector may be readily ascertained from Figures 1 and 2. In the absence of gas flow, the interior of the cell is filled with electrolyte of a composition practically identical with that of the electrolyte in the detector vessel.

At this stage, the cell has no current output. On starting up the flow of gas the electrolyte is expelled from the interior of the cell except for an amount of electrolyte that flows radially into the interior of the tube through the filter layer due to the hydrostatic pressure drop across it. In the interior of the tube the electrolyte comes into intensive contact with the gas to be measured as a consequence of which the Ag+ concentration of the electrolyte is reduced. The measuring electrode of the cell detects the reduced Ag+ concentration under the effect of which the cell produces a current causing Ag+ ions to dissolve at the measuring electrode to compensate the reduction of the Ag+ ions caused by reaction (1).

At the same time, silver is deposited on the reference electrode. For a given temperature and a given flow rate, the current output of the cell is a direct function of the ethyl mercaptan concentration.

Any hydrogen sulphide present would disturb the operation of the galvanic cell; put another way, the cell may also be used for measuring the concentration of hydrogen sulphide, moreover, in practice with a doubled sensitivity. However, to eliminate the disturbing effect of hydrogen sulphide, a hydrogen sulphide absorber may be used, which may be an acidic (pH 2), buffered, aqueous solution of CdSO4, or possibly it may be a CdSO4 layer formed with an aniline + HCI buffer on a solid carrier of thermolite.

It has been found that this galvanic cell may be used also for the measurement of tetrahydrothiophen odoriser if the tetrahydrothiophen content is first transformed in a microreactor containing an incandescent platinum wire to hydrogen sulphide and then the hydrogen sulphide is used in the measurement.

The invention will be further described with reference to non-limiting Examples given below: EXAMPLE 1 A portable instrument under the name of "Thiodet" utilising the above-described cell as a detector is shown in block diagram form in Figure 3. The gas to be measured is conveyed in a linenbacked rubber tube via a needle valve, a hydrogen sulphide absorber 18 and a bubbling manostat 1 9 to the detector 20, the construction of which has been described above with reference to Figures 1 and 2. The cell current is measured and indicated by a micro-ammeter 21 which is calibrated directly in ethyl mercaptan concentration. A potentiometer 22 is provided for calibration purposes and a switch 23 enables the cell to be short-circuited to promote its regeneration.A resistor 24 and the thermistor 1 6 are provided for automatic temperature compensation.

EXAMPLE 2 Figure 4 illustrates in block diagram form an instrument known as Thiocontrol and utilising the above-described galvanic cell as a detector. This instrument is a continuous measurement instrument that regulates in accordance with concentration. Through a three-way (threeposition) cock 26, the odoriser containing gas to be measured, unodorised gas and calibration gas can be connected to the instrument in turn. The gas passes via a needle valve 27, a hydrogen sulphide absorber 28 and a flow meter 29 into the detector 30 the electrolyte level of which is kept at a constant value by means of electrolyte storage vessel 31. An automatic temperature compensator including a thermistor 32 ensures that the voltage drop across it should be independent of temperature.An amplifier 33 is provided to amplify the current of the cell to the value required to serve as an input to the recording/registering instrument 34; at the same time, by virtue of having a variable gain the calibration can also be made use of here.

The amplifier can be set by means of a zeroiser or null adjuster 35 when unodorised gas is passed through the system. A comparator circuit 36 as a difference forming unit compares the basic signal of the actual concentration and the control signal of the concentration to be maintained, in the event of impermissible limits being exceeded, it operates an alarm 37 while at the same time issues a command signal to the unit 38 connected to an intervention unit 39 to provide an output to change the flow rate of the odoriser in the sense and extent required by the comparator 36.

EXAMPLE 3 In the Thiodet instrument described above, an electrolyte of the following composition was used: To an aqueous solution of 1 litre, in which 0.2 mol, 11.2 g KOH and 50 g KNO3 have been dissolved and approximately 2 g Ag2O have been suspended, 0.667 litre isopropyl-alcohol has been mixed; and at various temperatures a micro ammeter of low internal resistance (353 ohm) has been used to measure the current outputs provided by the cell (at a constant 50 I/h gas flow rate) on passing PB gas with a concentration of 40 mg ethyl mercaptan per cubic metre.The values are shown in the Table below: TABLE 0C ,uA OC ,uA 6.7 79.4 18.8 116 12.0 95.0 20.1 120 17.2 111.5 25.3 148.8 30.0 171.4 The logarithm of the current values has been illustrated as a function of the temperature and the straight line has been obtained the tangent to which is 0.0143/0C from which the temperature dependence of the sensitivity is 0.0143 x 2.303 x 100 per OC = 3.3% per OC.

This value can be simply and automatically compensated by a resistor or thermistor of negative temperature coefficient as a consequence of which the measured current strength will depend only on the concentration.

EXAMPLE 4 By utilising a hydrogen sulphide absorber the galvanic cell will only be sensitive to mercaptan.

Tetrahydro-thiophen utilised as odoriser may also be used in the galvanic cell if a micro-reactor provided with a platinum wire is interposed between the hydrogen sulphide absorber but before the cell or detector. The chief characteristics of the micro-reactor are as follows: Glass tube Inner diameter 8 mm Length 80 mm Incandescent platinum wire Diameter 0.07 mm Resistance at 200C 7.2 ohm Along the longitudinal axis of the glass tube, the platinum wire is wound helically on a mica carrier of 5 mm diameter along a length of 40 mm. At a flow rate of 2.5 I/h methane it is heated at 1 7-20 W whereby in the micro-reactor the tetrahydro-thiophen is converted to hydrogen sulphide with an efficiency of practically 100% and is measured in the galvanic cell as such.

Claims (23)

1. A process for the measurement in a galvanic cell provided with a silver measuring electrode and a silver reference electrode of the concentration of odoriser material in a fuel gas wherein said galvanic cell is made of an electrolyte-permeable material, and the fuel gas is brought into contact with a strongly alkaline electrolyte saturated with; Ag2O so as to cause the gas to be continuously absorbed as a consequence of which the Ag+ ion concentration is reduced in the vicinity of the silver measuring electrode, and thus a current is generated between the measuring electrode and the reference electrode which latter is arranged to be in contact with the Ag2O, and this galvanic current, which is proportional to the concentration of the odoriser material in the gas, is measured at steady gas flow conditions by a temperaturecompensated method.

2. A process according to claim 1 wherein the electrolyte contains 0.2 N KOH.

3. A process according to claim 1 or 2, wherein the electrolyte contains 40 vol.% isopropyl alcohol or n-propyl alcohol.

4. A process according to any preceding claim, wherein the electrolyte contains 2-3 weight % KNO3.

5. A process according to any preceding claim, wherein a thermistor is employed in said temperature compensated method.

6. A process according to any preceding claim wherein said odoriser is a mercaptan.

7. A process according to any preceding claim wherein said odoriser is tetrahydro-thiophen.

8. A process for the measurement of the concentration of odoriser material in a fuel gas substantially as herein described with reference to and as shown in Figures 1,2 and 3 or Figures 1,2 and 4 of the accompanying drawings.

9. Apparatus for the measurement of the concentration of odoriser material in a fuel gas, comprising a galvanic cell made of electrolytepermeable material and including a silver measuring electrode and a silver reference electrode in contact with a source of silver ions, means for causing an odoriser-containing fuel gas to pass through the cell, means for causing a strongly alkaline electrolyte to pass into the cell and for intensively mixing with said fuel gas, and circuit means effective to measure in a temperature-compensated manner the galvanic current generated between said electrodes as a consequence of the depletion of silver ions in the vicinity of the measuring electrode due to absorption of odoriser in the electrolyte.

10. Apparatus according to claim 9, further including means for setting a constant gas flow rate and a detector vessel containing said electrolyte and said galvanic cell immersed in the electrolyte to function as a detector.

11. Apparatus according to claim 9 to 10, wherein said silver electrodes are arranged in a concentric configuration.

12. Apparatus according to any of claims 9 to 11, wherein said cell is a tube made of filter material, preferably filter paper.

13. Apparatus according to claim 12, wherein said tube has an internal diameter of 4 mm and a length of 110 mm.

14. Apparatus according to claim 12 or claim 13, wherein said filter material is thinner than 40 lem.

1 5. Apparatus according to any of claims 12 to 14, wherein the upper half of the tube is externally tightly wound with an electrolyte-permeable spiral silver layer foil to form said measuring electrode and is provided with welded silver wires for external connection, the measuring electrode being separated from a fully identically dimensioned constructed and geometrically overlappingly positioned reference electrode separated from the measuring electrode by filter material.

16. Apparatus according to claim 15, wherein the measuring electrode foil is of 0.2 to 0.3 mm thickness 2-3 mm width and 300 mm length.

1 7. Apparatus according to claim 1 5 or 16, wherein the filter material is of 0.2 to 0.5 mm thickness.

18. Apparatus according to any of claims 1 5 to 17, wherein the filter material is made of 2-4 layers of filter paper.

1 9. Apparatus according to any of claims 1 5 to 18, wherein the reference electrode and the filter material are covered along their full length by Ag2O precipitate in a thickness of 1 mm.

20. Apparatus according to any of claims 1 5 to 19, wherein the galvanic cell is covered on its outside along its full length by a filter material of thickness of 0.2 to 0.5 mm made up of two to four filter paper layers, and this filter material is held together in the system by a chemically inert but electrolyte-permeable mechanical construction.

21. Apparatus according to claim 2G, wherein said mechanical construction is wire braid wound around the cell.

22. Apparatus according to any of claims 10 to 21, wherein the detector vessel is cylindrical, is of glass, preferably polished glass, has a stopper or lid provided with bungs and is preferably also of polished glass, the vessel being filled with electrolyte to a level of 2-3 mm below the upper end of the galvanic cell, the lid or stopper having three openings of which one opening has a capillary for discharging the gas to be measured from the detector, a second opening for passing therethrough a thermistor or equivalent immersed in the electrolyte to provide for temperature compensation while the third opening serves for introducing gas to be measured through a multiply-bent tube and an upwardly directed outlet portion secured to the galvanic cell by way of a seal, and electrode outlets for the cell connected to platinum wires fitted mechanically in the lid.

23. Apparatus according to claim 9, substantially as herein described with reference to and as shown in Figures 1,2 and 3 or Figures 1,2 and 4 of the accompanying drawings.