Abstract:
An analyzer for continuously measuring the H 2 S content of a gas stream which utilizes a drying module for drying the gas, a compressor for compressing the gas, a means for diluting the compressed sample and an electrical chemical sensor. The invention is also a device for regulating the flow rate of air to a H 2 S oxidation system which utilizes the analyzer to control the ratio of air to H 2 S stream entering the process.

Description:
FIELD OF THE INVENTION 
     The invention relates to an analyser for continuously measuring the H 2 S contained in a gas. It also relates to a device including the said analyser for regulating the flow rate of air injected into a reactor for oxidizing H 2 S to sulphur. 
     BACKGROUND OF THE INVENTION 
     In order to recover the H 2 S present in low concentration, especially a concentration of less than 5% by volume, in gases of various origins, it is common practice to use processes involving oxidation, especially catalytic oxidation, of H 2 S to sulphur according to the reaction H 2 S+{fraction ( 1 / 2 )} O 2 →S+H 2 O. 
     In such oxidation processes, the gas to be treated containing H 2 S in the presence of a controlled amount of a gas containing free oxygen is made to come into contact with a catalyst for selective oxidation of H 2 S to sulphur, the said contact being achieved at temperatures either above the dew point of the sulphur formed, in which case the sulphur formed is present in the vapor state in the gaseous effluent resulting from the reaction, or at temperatures below the dew point of the sulphur formed, in which case the said sulphur is deposited on the catalyst, thereby requiring the sulphur-laden catalyst to be periodically regenerated by purging, by means of a non-oxidizing gas having a temperature of between 200° C. and 500° C. The gas containing free oxygen used for oxidizing the H 2 S to sulphur is usually air, but it may also consist of oxygen, oxygen-enriched air or else mixtures, in various proportions, of oxygen and an inert gas other than nitrogen. In the following, “air” is used to denote the said gas containing free oxygen. 
     The amount of air, with which the gas to be treated containing H 2 S is combined, is continuously adjusted in response to a parameter resulting from the superposition of a prediction parameter, representative of an air flow rate corresponding to an amount of oxygen proportional to the amount of H 2 S present in the gas to be treated and injected into the oxidation reactor, and of a correction parameter (a feedback parameter), representative of a corrective air flow rate for bringing the H 2 S content present in the gaseous effluent coming from the oxidation back to a set value. 
     The oxidation is carried out in a reactor having an upstream end and a downstream end which are advantageously separated by a bed of a catalyst for selective oxidation of H 2 S to sulphur, the said upstream end being equipped with a first line and a second line for the injection of the gas to be treated and of air into the reactor, respectively, and the said downstream end being equipped with an output line for the gases, in order to discharge the gaseous effluent resulting from the oxidation, and the flow rate of air injected into the oxidation reactor is adjusted with the aid of a regulating device combining (i) a prediction unit, which comprises a prediction computer receiving a signal from a flow meter and a signal delivered by a first H 2 S-content analyser, these being mounted in the first line at the upstream end of the oxidation reactor and generating, from the said signals, a signal representative of an air flow rate corresponding to an amount of oxygen proportional to the H 2 S content entering the oxidation reactor with (ii) a feedback unit, which comprises a correction computer receiving a signal delivered by a second H 2 S-content analyser, mounted in the output line of the oxidation reactor and generating, from the said signal, a signal representative of a corrective air flow rate in order to bring the H 2 S content present in the gaseous effluent passing through the said output line back to a given set value and with (iii) a flow regulator, which receives the signals generated by the prediction and correction computers and the signal delivered by a flow meter, mounted in the air injection line at the upstream end of the oxidation reactor and applying, to a valve with an adjustable opening, mounted in the said air injection line downstream of the flow meter, a control signal for adjusting the opening of the said valve, the said control signal being the resultant of the signals generated by the prediction and correction computers. 
     The analysers, which are mounted in the line for injecting the gas to be treated into the oxidation reactor and on the output line of the said reactor, respectively, may be, for example, gas chromatography analytical units (U.S. Pat. No. 3,026,184 and FR-A-2,118,365), differential spectrometry analytical units (FR-A-2,420,754) or infrared absorption analytical units, after selective transformation of the H 2 S into SO 2.    
     The analysers of the aforementioned types, used for measuring the H 2 S content in gases containing this compound, do not always deliver continuous signals or do not always provide adequate sensitivity or adequate reliability, nor satisfactory operating simplicity. 
     SUMMARY OF THE INVENTION 
     The present invention provides an analyser for continuously measuring the H 2 S content of a gas containing it, which has a high sensitivity and the response of which shows no significant drift over time. 
     The analyser according to the invention, for continuously measuring the H 2 S content of a gas sample containing it, is characterized in that it comprises: 
     a dry-operating module for drying the gas sample, comprising an inlet, connected to a nozzle for taking and injecting the said sample, and an outlet for the dried sample; 
     a compressor module having a suction port, connected via a line to the outlet of the drying module, and a discharge port extended by a flow line for the compressed sample, the said line being equipped with an indicating and/or regulating primary flow meter; 
     a system for diluting the compressed sample, comprising an air intake line, which is mounted as a branch off the flow line for the compressed sample, downstream of the primary flow meter, and which is equipped with a regulating secondary flow meter adjusting the degree of opening of a valve having an adjustable opening, mounted in the air intake line downstream of the secondary flow meter, and a regulating module connected to each of the primary and secondary flow meters and slaving the secondary flow meter to the primary flow meter; and 
     an electrochemical sensor for measuring H 2 S, which is mounted in the flow line for the compressed sample, downstream of the air intake line, and delivers a signal proportional to the concentration of H 2 S in the said sample. 
     Advantageously, the nozzle for taking and injecting the gas sample, connected to the inlet of the dry-operating module for drying the gas sample, may be provided, at its remotest end from the said inlet, with a primary filter. Optionally, a finer filter may be provided at the other end of the said nozzle, located on the same side as the said module. If required, this nozzle may be surrounded by a jacket equipped with means for maintaining the temperature, for example by electrical heating or by the circulation of a heat-transfer fluid. 
     The dry-operating module for drying the gas sample may consist, in particular, of a dryer comprising permeation membranes such as the “SEC” dryer sold by Environnement SA. 
     The compressor module may be chosen from various miniaturized compressors having the required performance. Particularly suitable are diaphragm compressors. 
     The indicating and/or regulating primary flow meter mounted in the flow line for the compressed sample, as well as the regulating secondary flow meter mounted in the air intake line are, in particular, mass flow meters. In this case, the regulating module which is associated with them is a mass-regulating module. 
     The electrochemical sensor for measuring the H 2 S concentration is of the electrochemical transducer type for measuring the partial pressure of the compound measured. This sensor comprises a measurement cell, which contains a liquid electrolyte, in which a measurement electrode, a comparison electrode and a reference electrode are immersed, and which is separated, by a membrane, from the flow space for the gas sample on which the measurement is made. A constant electrical voltage is maintained between the measurement electrode and the reference electrode. The gas sample containing the compound to be measured, in the present case H 2 S, diffuses through the membrane into the liquid electrolyte. The aforementioned electrical voltage, the electrolyte and the material of the electrodes are chosen so that the compound, the concentration of which is to be determined, is transformed electrochemically at the measurement electrode and so that an electric current of intensity proportional to the concentration of the said compound passes through the measurement cell. An electrochemical reaction occurs at the same time, at the comparison electrode, with the oxygen of the dilution air. Such a sensor delivers an electrical signal of intensity proportional to the concentration of the compound to be measured, in this case H 2 S, present in the flowing gas sample in contact with the sensor. As an example of an electrochemical sensor that can be used in the analyser according to the invention, mention may be made of the sensor sold by Drager under the name “Polytron H 2 S”. 
     The analyser according to the invention, which allows the concentration of H 2 S contained, in relatively large amount, in a gas to be determined, is most particularly usable as an H 2 S analyser in an air-flow-regulating device with which a reactor for oxidizing H 2 S to sulphur is equipped. More especially, the analyser according to the invention can be used to form the analyser of a feedback unit and even the analyser of a prediction unit of the regulating device having the structure defined above, in order to regulate the flow rate of air injected into a reactor for oxidizing H 2 S to sulphur. 
     The invention will be more clearly understood on reading the description given below of one of its embodiments, made with reference to the appended drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 gives a schematic representation of an analyser according to the invention, and 
     FIG. 2 shows schematically a reactor for oxidizing H 2 S to sulphur, equipped with a device for regulating the flow rate of air injected into the reactor, the said device including an analyser as illustrated in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the analyser  1  illustrated comprises a permeation-membrane dryer  2  forming a dry-operating module for drying the gas sample, the said dryer having an inlet  3  and an outlet  4 . The inlet  3  of the dryer is connected to a nozzle  5  for taking and injecting a gas sample. The end of the said nozzle, which is furthermost from the inlet of the dryer, is provided with a primary filter  6 , namely a filter consisting of sintered material, and especially of ceramic, which retains the solid particles having a size greater than, for example, 20 μm, and penetrates the line  7  through which the gas to be sampled for analysis flows. At its end connected to the inlet of the dryer  2 , the nozzle  5  is provided with a fine filter (not shown) which retains the solid particles having a size greater than 0.3 μm, for example. 
     The outlet  4  of the dryer  2  is connected via a line  8  to the suction port  9  of a diaphragm compressor  10  forming a compression module. The said compressor has a discharge port  11  which is extended by a gas flow line  12  on which an indicating and/or regulating primary mass flowmeter  13  is mounted. An air intake line  14  is mounted as a branch off the flow line  12  at a point  15  on the latter, located downstream of the flow meter  13 , the said line  14  being provided with a valve  16 , having an adjustable degree of opening, and with a regulating secondary mass flow meter  17  located upstream of the valve  16  and used for adjusting the degree of opening of the latter. A mass-regulating module  18  receives, via an electrical linkage  19 , the signal delivered by the primary flow meter  13  and sends, via an electrical linkage  20 , a signal to the said secondary flow meter  17  in order to slave the secondary flow meter  17  to the primary flow meter  13 . The unit formed by the air intake line  14  with its valve  16  and the secondary flow meter  17  and by the mass-regulating module  18  constitutes a dilution system for the contents of the line  12 . 
     An electrochemical sensor  21  for measuring the H 2 S concentration is mounted in the flow line  12 , downstream of the point  15  where the air intake line  14  joins the flow line  12 , the said sensor delivering a signal proportional to the measured H 2 S concentration via an electrical conductor  22 . 
     That part of the nozzle  5  provided with the filter  6  and that part  23  of the line  12  located downstream of the sensor  21  form, respectively, the inlet of the analyser for the gas sample to be analysed and the outlet of the analyser for the said gas sample, while the conductor  22  constitutes the measurement output of the analyser. 
     Referring to FIG. 2, a reactor  30  for oxidizing H 2 S to sulphur has an upstream end  31  and a downstream end  32  which are separated by a bed  33  of a catalyst for selective oxidation of H 2 S to sulphur, the said upstream end being equipped with a first line  34  and with a second line  35  for the injection of a gas to be treated containing H 2 S and of air into the reactor, respectively, and the said downstream end being equipped with an output line  36  for the gases, in order to discharge the gaseous effluent resulting from the oxidation. 
     The oxidation reactor is equipped with a device for regulating the flow rate of air injected into the oxidation reactor, the said regulating device consisting of the combination of a prediction unit, a feedback unit and an air flow regulator. 
     The prediction unit comprises a prediction computer  37  which receives a signal  38  from a flow meter  39  and a signal  40  delivered by a first H 2 S-content analyser  41 , these being mounted in the first line  34  located at the upstream end of the oxidation reactor  30 , and which generates, from the said signals, a signal  42  representative of an air flow rate corresponding to an amount of oxygen proportional to the amount of H 2 S entering the oxidation reactor. 
     The feedback unit comprises a correction computer  43  which receives a signal  22  delivered by a second H 2 S-content analyser  1 , mounted in the output line  36  of the oxidation reactor, and which generates, from the said signal  22 , a signal  44  representative of a corrective air flow rate in order to bring the content of the H 2 S present in the gaseous effluent flowing through the said output line back to a given set value. 
     The flow regulator  45  receives the signals  42  and  44 , generated by the prediction computer  37  and the correction computer  43  respectively, and the signal  46  delivered by a flow meter  47 , mounted in the air injection line at the upstream end of the oxidation reactor, and applies, to a valve  48  having an adjustable opening, mounted in the said air injection line downstream of the flow meter  47 , a control signal  49  for adjusting the opening of the said valve, the said control signal being the resultant of the signals  42  and  44  generated by the prediction computer  37  and the correction computer  43 , respectively. 
     The H 2 S-content analyser  1 , mounted in the output line  36  of the oxidation reactor, is an analyser having the structure of the analyser described above with reference to FIG.  1 . The H 2 S-content analyser  41 , mounted in the first line  34  located at the upstream end of the oxidation reactor  30 , may be an analyser similar to the analyser  1  or may consist of an analyser of another type, for example an infrared-absorption analyser, after the H 2 S has been selectively transformed into SO 2 , a gas-chromatography analyser or a differential-spectrometry analyser. 
     The analyser according to the invention and the regulating device including it, which are described above, operate as indicated below. 
     A sample of the H 2 S-containing gas to be analysed is taken from the output line  36  of the oxidation reactor  30 , which corresponds to the line  7  shown in FIG. 1, via the nozzle  5 , through the filter  6  and the said sample is injected, via the said nozzle maintained by electrical heating at a temperature of approximately 135° C., into the permeation-membrane dryer  2  in which the water vapor contained in the sample is almost completely removed. The dry gas sample is drawn into and compressed in the diaphragm compressor  10  and then sent into the flow line  12  in which, after it has passed through the primary mass flow meter  13 , it is diluted by the injection of air entering via the line  14  with a mass flow rate which is slaved, by the action of the regulating module  18  acting on the regulating secondary flow meter  17  adjusting the degree of opening of the valve  16 , to the mass flow rate of the sample measured by the flow meter  13 . The flow rate of dilution air is chosen so that the H 2 S content of the diluted sample lies within the allowed concentration range for the sensor. Next, the diluted gas sample flows in contact with the electrochemical sensor  21  for measuring the H 2 S concentration of the said sample, after which the diluted sample is sent to a flare (not shown) in order to be burnt. The sensor  21  delivers an electrical signal  22  proportional to the H 2 S content of the sample analysed. 
     The prediction computer  37  receives, from the flow meter  39 , a signal  38  representative of the flow rate of H 2 S-containing gas injected into the oxidation reactor  30  and, from the analyser  41 , a signal  40  representative of the H 2 S content of the said gas and it generates, from these signals, a signal  42  representative of an air flow rate corresponding to an amount of oxygen proportional to the amount of H 2 S entering the oxidation reactor. The coefficient of proportionality corresponds, in particular, to the molar ratio O 2 /H 2 S chosen for carrying out the H 2 S oxidation, the said ratio possibly ranging, for example, from 0.5 to 10 and more particularly from 0.5 to 4. Advantageously, the coefficient of proportionality may be gradually increased during the oxidation step, for example from a value of 0.5 to a value of 4, in order to prevent gradual deactivation of the catalyst during the said step. 
     The correction computer  43  receives, from the analyser  1 , a signal  22  representative of the H 2 S content of the gaseous effluent leaving the oxidation reactor  30  via the output line  36  and it generates, from the said signal, a signal representative of a corrective air flow rate in order to bring the content of the H 2 S present in the gaseous effluent flowing through the said output line back to a given set value. 
     The flow regulator  45  receives the signals  42  and  44  generated by the prediction computer  37  and the correction computer  43 , respectively, and the signal  46  delivered by the flow meter  47 , mounted in the air injection line  35  at the upstream end of the oxidation reactor  30 , and it generates, from the said signals, a control signal  49  which it applies to the valve  48  with an adjustable opening, which is mounted in the said air injection line  35  downstream of the flow meter  47 , in order to adjust the opening of the said valve, the said control signal  49  being the resultant of the signals  42  and  44  generated by the prediction computer  37  and the correction computer  43 , respectively. 
     In an alternative embodiment, the line  34  supplying the H 2 S-containing gas to the oxidation reactor is the output line of a hydrogenation and/or hydrolysis reactor in which a sulphur plant residue gas is treated in order to convert all the sulphur compounds that it contains into H 2 S. In this case, instead of being placed in the said line  34 , the flow meter  39  may be mounted in the line for feeding the sulphur plant residue gas into the hydrogenation and/or hydrolysis reactor.