Abstract:
The SO 3 , H 2  SO 4  content of a gas environment containing SO 2  and H 2  O is measured by cooling the gas to a temperature to convert SO 3  in the presence of H 2  O to H 2  SO 4  to effectively separate SO 3 , H 2  SO 4  from SO 2  to permit the individual measurements of SO x  (SO 2  +SO 3 ), SO 2  and SO 3 , H 2  SO 4 .

Description:
BACKGROUND OF THE INVENTION 
     Solid electrolyte detectors for the on-line monitoring of sulfur-bearing pollutants SO 2 , SO 3 , i.e. SO x , have recently been developed and are described in detail in the issued Canadian Pat. No. 1,040,264, entitled &#34;Solid State Sensor For Anhydrides&#34;, issued Oct. 10, 1978, which is assigned to the assignee of the present invention and incorporated herein by reference. This inventive concept is the subject matter of pending U.S. Patent application Ser. No. 718,511 now U.S. Pat. No. 4,282,078. The operation of the SO x  detector described in the above-referenced patent and patent application is based on potentiometric measurements across a solid electrolyte element of potassium sulfate (K 2  SO 4 ), wherein accurate measurements of sulfur-bearing pollutants over a concentration range of 0.1 parts per million to 10,000 parts per million can be realized. The sensor thus described is uniquely sensitive to SO x . The presence of other common pollutants such as CO 2 , CH 4  and NO x  does not interfere with the SO x  measurements. 
     SUMMARY OF THE INVENTION 
     The disclosed technique utilizes the condensation property of SO 3  as H 2  SO 4  (liquid) and uses this to quantitatively separate SO 3  and H 2  SO 4  from SO 2  by appropriate temperature control. In accordance with the disclosed technique, the above-referenced solid electrolyte electrochemical cell detector can be made to respond to total SO x , or separately to SO 2  or SO 3 , H 2  SO 4 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more readily apparent from the following exemplary description in connection with the accompanying drawings: 
     FIG. 1 is a graphical illustration of the conversion of SO 2  to SO 3  ; 
     FIG. 2 is a graphical illustration of the conversion of SO 3  to H 2  SO 4  ; 
     FIG. 3 is a graphical illustration of acid dew point as a function of H 2  SO 4  concentration; 
     FIG. 4A is a schematic illustration of an embodiment of the invention; 
     FIG. 4B is a graphical illustration of the operation of the embodiment of FIG. 4A; 
     FIG. 5A is a schematic illustration of an alternate embodiment of the invention; 
     FIG. 5B is a graphical illustration of the operating of the embodiment of FIG. 5A; 
     FIG. 6A is a second alternate embodiment of the invention; 
     FIG. 6B is a graphical illustration of the embodiment of FIG. 6A; and 
     FIG. 7 is a graphical illustration of the moisture content of a gas as a function of the acid dew point and the H 2  SO 4  concentration. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The SO x  solid electrolyte electrochemical cell sensor described in the above-referenced patent and patent application, and employed herein, consists of a solid electrolyte member having electrodes disposed on opposite surfaces thereof wherein the electrodes are typically platinum. The cell can be represented as follows: 
     
         SO.sub.3 (P.sub.1), O.sub.2 (P.sub.1 &#39;) Pt/K.sub.2 SO.sub.4 /Pt, O.sub.2 (P.sub.2 &#39;), SO.sub.3 (P.sub.2) 
    
     wherein the electrode reactions correspond to: 
     
         SO.sub.3 +1/2O.sub.2 +2e.sup.- ⃡SO.sub.4.sup.═ 
    
     The EMF of this solid electrolyte electrochemical concentration cell is defined by the Nerst equation: ##EQU1## where: 
     E=EMF; 
     R=gas constant; 
     T=temperature (°K.); and 
     F=Faraday. 
     Due to the following equilibrium relationship 
     
         SO.sub.3 ⃡SO.sub.2 +1/2O.sub.2 
    
     the cell will respond to either SO 2  or SO 3  and the species involved in the cell reaction will depend on the temperature of the cell. The temperature dependence of the SO 2  -SO 3  equilibrium is shown in FIG. 1. The equilibrium in the vicinity of the platinum electrode, considering platinum to be catalytic, is reached very rapidly. 
     The operation of the cell in accordance with the Nernst equation requires the establishment of a stable SO x  reference at a reference electrode of the cell such that at a given cell temperature the change in EMF produced by the cell is a function of the SO x  of the monitored gas mixture at the opposite, or sensing, electrode. A change in the EMF is indicative of a change in the partial pressure of SO x  at the sensing electrode. The Nernst equation for monitoring the partial pressure of SO 2  in a monitored gas mixture present at the sensing electrode of the cell is represented as follows: ##EQU2## The operating temperature for the cell is typically between 600° and 1000° C. 
     In accordance with the above mode of operation of the previously disclosed SO x  responsive solid electrolyte cell it is not possible to discriminate between SO 3  and SO 2  and in fact the measurement provided is an indication of the total SO x  concentration. Furthermore, as is evident from the above representation of the Nernst equation, the oxygen content of the monitored gas must be known or the oxygen pressures pO 2  and pO 2  &#39; maintained equal. 
     It has been determined that SO 3  reacts spontaneously and reversibly with water vapor to form H 2  SO 4 . The thermodynamic equilibrium for this gas phase reaction as a function of temperature is shown in FIG. 2. 
     Although the equilibrium is a function of the water vapor content of the gas, it has been determined that for typical ambient or stack humidities, the equilibrium will be entirely in the direction of H 2  SO 4  at temperatures below 200° C. Below this temperature H 2  SO 4  will condense. The condensation temperature, i.e. acid dew point, is dependent on the concentrations of H 2  SO 4  and H 2  O in the gas mixture. The relationship between the dew point temperature and H 2  SO 4  concentration for several water vapor concentrations is shown in FIG. 3. It is seen that if the gas mixture is cooled to approximately 100° F., virtually all of the H 2  SO 4  will be condensed even at water vapor concentrations typical of the ambient air (approximately 1.5 vol. %). SO 2  will not condense at this temperature and thus an effective means of separating SO 2  from SO 3  and H 2  SO 4  is provided. This principle forms the basis for the use of the Goksoyr-Ross coil method for determining the SO 3 , H 2  SO 4  concentration in stack gases. The information illustrated in FIG. 4 can be verified in an article appearing in Chemical Engineering Progress, Vol. 71 (1974) by Verhoff and Banchero. The above-referenced Goksoyr-Ross coil method is the subject of an article appearing in the Journal of Fuel Instrumentation, Vol. 177 (1962). 
     It should be noted however, that the monitored gas mixture cannot be cooled below the water dew point inasmuch as condensation of water in the condenser will result in conversion of SO 2  to SO 3 . The novel technique disclosed herein involves the combination of the above-disclosed technique for the controlled cooling of the sample of monitored gas and the use of a solid electrolyte SO x  detector to produce a device for real time monitoring of the presence of SO 3 , H 2  SO 4  in a monitored stack gas. In a stack gas SO 3  and H 2  SO 4  are closely associated and distinct from SO 2 . It will be apparent from the following discussion that numerous variations of the disclosed technique can be employed to adapt the device for use with gases of widely varying SO 3  content. 
     The embodiments of FIGS. 4A, 5A and 6A are those of sampling systems wherein a sample of the monitoring gas, i.e. ambient air, exhaust gas, etc., is drawn into a measuring apparatus. 
     The embodiment of FIG. 4A illustrates an SO 3 , H 2  SO 4  monitoring system suitable for use in monitoring stack gases with relatively high SO 3  content, i.e. greater than 10 parts per million. The detector system 10 of FIG. 5A includes a heated filter 20, a hot/cold leg gas bypass apparatus 30 and an SO x  detector 40. A sample of the monitored gas environment G is initially passed through a prefilter 20 heated to a temperature of about 300° C. to remove particulates from the gas sample before the gas sample reaches the detector 40. By switching the sample of monitored gas G between the hot leg 32 at a temperature of about 300° C., and the cold leg 34 at a temperature of about 40° C., of the gas bypass apparatus 30, total SO x  and SO 2  can be separately and alternately measured by the detector 40. The difference between the measured concentration of SO x  and SO 2  is a measurement of the SO 3 , H 2  SO 4  concentration in the monitored gas G. The SO 3  content is quantitatively condensed as H 2  SO 4  in the cold leg 34. The operation of the system 10 on the basis of 5 minute cycle times between the sample gas flow through the hot leg 32 and the cold leg 34 is graphically illustrated in FIG. 4B. The EMF output signals sequentially generated by the detector 40 in response to the alternate supply of the monitored gas through the hot and cold legs 32 and 34 is transmitted to a conventional subtracting circuit 50. The circuit 50 subtracts the two signals and generates an output indicative of the SO 3 , H 2  SO 4  concentration of the monitored gas G. 
     The detector 40 consists of a solid electrolyte electrochemical cell 42 comprising a solid electrolyte member 43 having a sensing electrode 44 disposed on one surface and a reference electrode 45 disposed on the opposite surface. The solid electroltye cell 42, which is constructed as described above in accordance with the teachings of the above-referenced patent, is positioned within a housing 48 so as to isolate the monitored sample gas G which contacts the sensing electrode 44 from the reference electrode 45. An SO x  stable reference is provided in contact with the reference electrode such that the operation of the cell 42 in accordance with the above Nernst equation. The temperature of the detector 40 is maintained constant by the heater 47. The monitored sample gas introduced to the sensing electrode 44 via the inlet tube 35 is removed from the housing via a pump system 60. 
     A technique for monitoring H 2  SO 4  in atmospheric gas where little, if any, SO 3  is present is illustrated in FIGS. 5A and 5B. The gas G is initially cooled by a cooler 73 to separate H 2  SO 4  from SO 2 . The H 2  SO 4  is collected by a filter on trap 70. The remaining gas volume is removed by the pump 60 through a gas meter 61 which measures the volume of gas sampled. This information, in accordance with FIG. 5B, is used in determining the SO 3  concentration of the gas sampled during a predetermined period of time. Following a collection period of several hours or days, the filter 70 is heated through the operation of a temperature controller 71 and an over member 72 to volatilize the trapped H 2  SO 4  and produce H 2  SO 4 . The H 2  SO 4  is then transmitted to the SO x  detector 80 consistingof the solid electrolyte electrochemical cell 82 positioned within housing 84 and an EMF signal is generated by the cell 82 which is indicative of the H 2  SO 4  concentration collected over a period of time. Referring to the graphical illustration of FIG. 5B, the area under the H 2  SO 4  response curve is indicative of the amount of acid collected from a known volume of ambient gas. As indicated with respect to the embodiment of FIG. 5A, a heated prefilter 75 heated to a temperature of about 150° C. or higher can be included upstream of the filter 70 to retain solid particulate matter. 
     Yet another embodiment of the SO 3 , H 2  SO 4  detection technique is illustrated in FIGS. 6A and 6B. In this implementation the inlet tube 90 to the SO x  detector 98 includes a heated prefilter 89 and a variable temperature trap 91 consisting of an oven 92 and a temperature control 93. The prefilter 89 is heated to a temperature of about 150° C. or higher. At temperatures of approximately 150° F., both SO 2  and SO 3 , H 2  SO 4  through the trap 91 and are detected by the sensor 98 to provide a total SO x  indication. As the temperature of the trap 91 is lowered via the temperature controller 93, a temperature will be reached where SO 3  begins to be converted to H 2  SO 4  and condensing begins in the trap 91. This temperature is the H 2  SO 4  acid dew point. If cooling is continued, a temperature is reached where all of the SO 3  is converted to H 2  SO 4  and is condensed out of the sample gas stream G entering inlet tube 90. The total change in the EMF signal generated by the detector 98 is equivalent to the SO 3 , concentration of the sample gas G. This information regarding the acid dew point temperature and SO 3 , H 2  SO 4  concentration of the monitored gas G can be used to determine the moisture content of the monitored gas G in accordance with the graphical illustration of FIG. 7. 
     It is apparent from the above that a single apparatus can be employed to measure four important parameters of an exhaust gas, i.e. SO 2  concentration, SO 3  concentration, H 2  O concentration and H 2  SO 4  acid dew point temperature. 
     In each of the monitoring systems described above, the choice of materials should eliminate those which would act as catalysts for the conversion of SO 2  to SO 3 . While most metals promote oxidation of SO 2 , quartz and porcelain can be considered as inert materials for use as monitored gas input lines, variable temperature coils and traps, etc. A variety of filter materials including quartz fiber, nucleopore, and Teflon have been found to be inert, efficient collectors for H 2  SO 4  in the presence of SO 2 .