WO2002066966A1

WO2002066966A1 - Method and apparatus for the determination of total sulfur in a gas

Info

Publication number: WO2002066966A1
Application number: PCT/US2002/000272
Authority: WO
Inventors: Loren T. Mathews; Radhakrishna M. Neti
Original assignee: California Analytical Instruments, Inc.
Priority date: 2001-01-08
Filing date: 2002-01-04
Publication date: 2002-08-29

Abstract

Method and apparatus for the determination of total sulfur is described. The method of the present invention for the determination of total sulfur comprising the steps of (1) diluting a sample of a gas in a ozone containing atmosphere to oxidize the sample to form sulfur dioxide from the oxidizable sulfur containing components thereof, (2) subjecting the oxidized sample to infrared radiation at a wavelength selected for the absorption thereof by sulfur dioxide and (3) determining the sulfur content as a function of the infrared absorption. The sulfur dioxide is determined indirectly utilizing methodologies such as nondispersive infrared, or Fourier transformation infrared technology may be utilized in determining the concentration of SO2 in the sample. The apparatus of the present invention includes a diluter (12) for the dilution of the sample gas with an oxygen containing gas and for the control of the flow rate of the diluted gas being monitored. The diluter communicates with a SO2 detection unit (24) for the determination and read out of the sulfur content of the monitored gas sample as the result of the absorption of infrared radiation by the sulfur dioxide of the sample gas.

Description

METHOD AND APPARATUS FOR THE DETERMINATION OF TOTAL SULFUR IN A GAS

Field of the Invention

This invention relates to the determination of total sulfur in a gas and more particularly to a method and apparatus for the determination of total sulfur dioxide in a gaseous sample employing infrared absorption techniques to measure the sulfur dioxide and to produce linear data output.

Background of the Invention There are several methods for the determination of different forms of sulfur in a gas. One such method involves exciting SO₂ to fluoresce and determining the sulfur content by ultraviolet emission fluorescence. Another method involves measuring hydrogen sulfide in the sample by contacting a lead acetate paper with the hydrogen sulfide containing sample to form lead sulfide on paper. The quantity of sulfur is then determined photometrically. These methods are specific for SO₂ and hydrogen sulfide respectively and are not quantifiable for mercaptans and disulfides. Accordingly they are not useful for the determination of total sulfur which includes all forms of sulfur.

Total sulfur can be determined by flame photometric methodology followed by quantitative determination by its ultraviolet absorption. The resulting concentration data is logarithmic and nonlinear. The above methods have several disadvantages. For example, as mentioned above, utilization of ultraviolet absorption produces nonlinear data that is only logarithmically related. For SO₂ determinations by fluorescence the samples must be carefully dried to avoid the presence of moisture which can mask or quench the sulfur output signal. In addition, the presence of interfering substances, such as nitrous oxide or polynuclear aromatics adversely effect the sensitivity and the minimum detection limits of the method employed.

Other methods involving the sampling of a gas after passing through a hydrogen flame followed by chemiluminescence determination produced by reacting the gas sample with ozone to oxidize the reduced S to SO₂ with the emission of light. Two such methods are described in U.S. PatentNo. 5,501,981 issued March 26, 1996toRayetal. andU.S.PatentNo.5,614,417 issued March 25, 1997 to Kubala et al.. Chemiluminescence methods are sensitive to the presence of moisture and hydrocarbon. These methods require that the chemiluminescence reaction be carried out at low pressure to avoid the condensation of moisture or, as described in Ray et al. and Kabula et al., the use of dual combustion zones.

Another method disclosed in U.S. patent 4,330,298, issued May 18, 1982 to Hawn et al. involves the reductive pyrolysis of organic sulfides in a sample to H₂S followed by electrochemical detection of the H₂S. The method for sulfur determination requires that the H₂S containing effluent gas from the pyrolysis be bubbled through a base electrolyte and thereafter the sulfide content is determined by plating an HgS layer on a mercury working electrode followed by scanning the working electrode potential the total H₂S derivatized sulfur content as a function of current or current derivative. This method requires a pyrolysis furnace with three separately controlled heating zones and a catalyst bed. The analysis cycle requires about 15 minutes and the ionic strength of the electrolyte solution should not exceed 1 and preferably should be about 0.5 so that the method is best suited for determining trace amounts of sulfur containing compounds. The method is best suited for the determination of trace amounts of sulfur in halogenated matrices.

Therefore, it would be of great advantage to have a straight forward method for the determination of total sulfur in a gas sample that is sensitive and that is capable of measuring low concentrations of sulfur.

Summary of the Invention Accordingly it is an obj ect of the invention to provide a method for the determination of total sulfur in a gas, which method can detect low concentrations of sulfur in the gas being monitored.

Another object of the invention is to provide a method for the determination of total sulfur which is specific for sulfur and not subject to interference by hydrocarbons or moisture.

Yet another object of the invention is to provide a method the determination of total sulfur that produces data that is linear in response.

Still another object of the invention is to provide apparatus for carrying out the method of the invention.

These objects and advantages are achieved by the method of the present invention for the determination of total sulfur comprising the steps of (1) diluting a sample of a gas in a ozone containing atmosphere to oxidize the sample to form sulfur dioxide from the oxidizable sulfur containing components thereof, (2) subjecting the oxidized sample to infrared radiation at a wavelength selected for the absorption thereof by sulfur dioxide and (3) determining the sulfur content as a function of the infrared absorption thereof by sulfur dioxide. The sulfur dioxide is determined indirectly by utilizing methodologies such as, nondispersive infrared or Fourier transformation infrared technology may be utilized in determining the concentration of SO₂ in the sample.

In a preferred embodiment of the invention, the SO₂ content of the oxidized sample is determined in an acoustic cell by subjecting the sample to pulses of infrared radiation at a wavelength most suitable for absorption by the SO₂. The infrared pulses are absorbed by the SO₂ which is heated thereby to produce temperature fluctuations in the sample that result in pressure fluctuations. The degree of sample heating is directly related to amount of infrared radiation absorbed which, in turn, is directly related to the concentration of SO₂ present in the sample. The pressure fluctuations are measured in the cell as acoustic signals. The acoustic signals are directly proportional to the SO₂ concentration in the sample that is directly correlated to the total sulfur content in the sample.

The apparatus of the present invention includes a diluter for the dilution of the sample gas with an oxygen containing gas and for the control of the flow rate of the diluted gas being monitored. The diluter communicates with a SO₂ detection unit for the determination and read out of the sulfur content of the monitored gas sample as the result of the absorption of infrared radiation by the sulfur dioxide of the sample gas.

In a preferred embodiment of the invention the apparatus includes a converter in which the dilution gas is subjected to ultraviolet radiation to convert at least a portion of the oxygen to ozone for the oxidation of the sulfur component of compounds in the sample gas to sulfur dioxide. The detection unit is preferably a photo-acoustic cell that is in optical communication with a source of infrared radiation. An optical filter carousel and mechanical chopper are disposed in the radiation path between the infrared source and the photo-acoustic cell to provide pulses of narrow band infrared radiation.

The above objects as well as other features and advantages of the invention will be more clearly understood from the following detailed description of the invention taken in conjunction with the sole figure.

Brief Description of the Drawings

The sole FIGURE is schematic flow diagram illustrating the apparatus and method of the present invention.

Description of the Preferred Embodiment of the Invention

The present invention relates to the determination of total sulfur in a gas by the determination of SO₂ formed by the oxidation of the sulfur of the components of the gas being monitored according to the following reaction: [RS] + O₃ → SO₂ + H₂0 where R is either hydrogen or an organic radical. The SO₂ thus formed is determined by infrared absorption utilizing one of several techniques and reported as total sulfur in the sample. Some samples may contain native SO₂ that is SO₂ that is initially present in the sample. The present method determines the native SO₂ and reports it as part of the total sulfur. The oxidation of the sulfur content of the sample gas to SO₂ and the use of infrared absorption provides a highly specific and sensitive method for the determination of total sulfur in the sample.

In accordance with the preferred embodiment of the invention the method for the determination of total sulfur comprises the steps of (1) diluting the test sample in an oxygen containing gas, (2) oxidizing the sulfur component of compounds in the monitored gas to SO₂ by ultraviolet radiation, and (3) determining the SO₂ concentration by infrared absorption. Moisture content is determined by infrared absorption at a wavelength specific for water and the moisture content of the sample is factored into the output at the sulfur wavelength to eliminate the effect of the moisture present in the sample.

The method and apparatus of the present invention are described herein in connection with the utilization of photo-acoustic technology for the determination of the SO₂ in a sample gas. The detection of components of gases by photo-acoustic techniques is disclosed generally in U.S. Pat No. 4,622,845 issued November 18, 1986 to Ryan andFeldman.

It will be understood, however, that the invention can be practiced using other technology for the SO₂ determination. For example, other methodologies such as, non-dispersive infrared or Fourier transformation infrared technology may be utilized in determining the concentration of SO₂ in the sample.

Referring to the FIGURE, the gas being monitored for total sulfur content is led into a diluter 12 through a line 14 and an oxygen containing diluent gas is led into the diluter through a line 16 that communicates with a source of diluent. Preferably, the flow rates of the air and of the monitored gas to the diluter 12 are closely monitored. The diluter 12 is of conventional design and is provided with flow restrictive devices, for example capillaries (not shown) and electronic flow control valves (not shown) for the precise control of the flow rates of the monitor gas and the diluent gas. Diluters capable of closely monitoring and controlling the flow rates of gases are well known in the art and do not per se form a part of this invention except as they are combined in the inventive apparatus described herein. For example, good results were achieved using a Model 101 diluter manufactured by California Analytical Instruments, Orange California.

The oxygen enriched diluted gas is introduced into an ultraviolet converter 18 through a line 20. hi the ultraviolet converter 18 the diluted monitored gas is subjected to ultraviolet radiation for converting some or all of the oxygen in the diluted monitored gas stream to ozone. The ozone reacts with the sulfur of the sulfur containing components of the monitored gas to oxidize it to SO₂. SO₂ itself may be initially present in the monitored gas and it would be expected that the SO₂ would for be oxidized to SO₃. Surprisingly, however, the SO₂ is not oxidized by the ozone and is thus determined and reported as part of the total sulfur in the sample. As will be discussed below, the diluent may be oxygen or ozone. In the case where the diluent gas is ozone, it will be apparent that it will be unnecessary to subject the diluted monitored gas to ultraviolet radiation since the purpose of exposure to ultraviolet radiation is to convert some or all of the oxygen in the diluted sample to ozone for reaction with the sulfur components in the sample to oxidize them to SO₂. A sample is taken from the ultraviolet converter 18 and is led through by a line 22 to an analysis cell 24 of a photo acoustic monitor and the sample is sealed in the cell. The analysis cell 24 is provided with a pair of microphones 26 for the detection of acoustic signals generated in the analysis cell. The acoustic signals detected by the microphones 26 are directed to a suitable data gathering and reporting unit 27. The data gathering and reporting unit 27 is preferably included as part of the acoustic analyzer unit to make it a stand alone unit although a separate computer processing unit can be employed with good results.

The analysis cell 24 is provided with an optical port 28 that is in optical alignment with an optical filter carousel 30 and a focused beam of infrared radiation emanating from an infrared source 32. A reflector 34 is provided for focusing and directing scattered infrared radiation to the optical port 28 of the analysis cell 24. An optical filter carousel 30 carrying several optical filters 31 is disposed between the infrared source 32 and the optical port 28. The optical filter carousel 30 is located so that at least one of its several optical filters 31 are in optical alignment with the beam of radiation from the infrared source 32. Interposed between the optical filter carousel 30 and the infrared source 32 is a mechanical chopper 36 that rotates to pulsate the beam of infrared radiation. The optical filters 31 are narrowband filters which transmit infrared radiation at a selected a relatively narrow range of wavelengths. At least one of the optical filters 31 is selected to transmit infrared radiation in a range of between about 1139 cm^"1 and about 1179 cm^"'. This range is preferred for SO₂ absorption. Water vapor is an interfering substance which, if present, contributes to the total acoustic signal in the analysis cell 24 and erroneously inflates the amount of total sulfur reported. For this reason a special optical filter 31 is installed in the optical filter carousel 30 to measure the contribution of water vapor during each measurement cycle. For example, a filter that transmits infrared at a wavelength of about 1020 cm^"1 is preferred, as water readily absorbs infrared at this wavelength. This measurement can be used to compensate for moisture interference in SO₂ measurements and thus water vapor is substantially eliminated as an interfering substance. The same technique can also be applied to compensate for other organic solvents such as, for example, alcohol.

In carrying out the method of the present invention, the first step consists of diluting the monitored gas with an oxygen containing gas. Preferably the monitored gas is diluted with air that has been filtered to remove dust and oil that could contaminate the sample and result in erroneous results. Preferably equal parts of the air may be used as the diluent as this will provide excess oxygen for the formation of a sufficient amount of ozone for the oxidation of all the oxidizable sulfur in the sample. Oxygen, alone or admixed with other inert gases, can also be used as the diluting gas in which case the ratio is reduced to on the order of 25 parts of oxygen to 100 parts of the monitored gas. In carrying out the dilution it is highly preferred to precisely control the flow rate of the oxygen containing gas and the monitored gas. The dilution ratio is also closely monitored and controlled. Good results were achieved using a model 101 Diluter manufactured by California Analytical Instruments, Orange California.

Following dilution, the second step consists of reacting the oxidizable sulfur in the gas being monitored with ozone to form SO₂ as a reaction byproduct. In the preferred embodiment employing air or oxygen as the diluent gas, oxidation is preceded by the step of passing the diluted stream through ultraviolet radiation at a wavelength of between about 1849 angstroms and 4,000 angstroms. The ultraviolet radiation produces ozone from the oxygen present in the diluent gas which in turn oxidizes the sulfur containing compounds in the monitored gas to SO₂. As previously mentioned SO₂ which may be present in the monitored gas is not oxidized to SO3 and thus is determined along with the oxidizable sulfur and reported as part of the total sulfur in the sample.

It will be understood that if the diluent gas is ozone it will be unnecessary to subject the sample to ultraviolet radiation since there is no need to convert oxygen to ozone.

Following reaction with ozone, a sample of the ozone treated gas is sealed in the analysis cell 24 where it is subjected to infrared radiation at a wavelength that is that is selectively absorbed by the SO₂ component of the sample. The absorption of infrared radiation by the SO₂ produces an increase in the temperature of the sample which resultants in an increase in gas pressure. The beam from the infrared source 32 is passed through the mechanical chopper 36 so that the infrared radiation contacts the gas in the cell in pulses. In this manner the temperature of the gas is modulated and the resultant pressure modulations are sensed by the cell as acoustic signals. The acoustic signals sensed by the analysis cell 24 are directly proportional to the concentration of SO₂ in the sample.

Infrared absorption by SO₂ is highly selective and occurs in the wavelength range of between 1101 cm^'1 and 1291 cm^"1. Maximum sensitivity for sulfur determination occurs between about 1139 cm^"1 and about 1179 cm^"1.

A photo acoustic cell found to produce good results in the method of this invention is the photo acoustic cell manufactured by Innova Air Tech Instruments, Ballerup, Denmark. The acoustic cell 34 can be retrofitted on a conventional infrared analyzer. In addition to the photo acoustic cell, apparatus suitable for use in the present invention includes an infrared light source, a chopper to create the pulses of infrared radiation and an optical filter selected for its transmission of infrared light at a selected wavelength for SO₂.

The preferred method of the present invention is both highly selective and sensitive for the determination of total sulfur in a monitored gas. SO₂ can be determined in quantities as low as 0.1 pp . Once the monitored gas has been diluted, flow rate becomes less critical. Moisture quenching which has been a problem in the determination of total sulfur by prior art methods, has been substantially eliminated as an interfering substance.

While the invention has been described in connection with the preferred photo acoustic technology, it will be understood by those skilled in the art that other methods for the determination of SO₂ may be employed. For example, conventional non-dispersive infrared analysis techniques or

Fourrier transformation infrared methodology (FDIR) can also be employed. Although not preferred, ultraviolet absorption can also be employed for the determination of SO₂.

As will be understood by those skilled in the art, various arrangements which lie within the spirit and scope of the invention other than those described in detail in the specification will occur to those persons skilled in the art. It is therefor to be understood that the invention is to be limited only by the claims appended hereto.

Having described the invention we claim:

Claims

1. A method for the determination of total sulfur in a gas comprising the steps of: a. diluting a sample of a gas in an ozone containing atmosphere to oxidize the sample to form sulfur dioxide from the oxidizable sulfur containing components thereof; b. subjecting the oxidized sample to infrared radiation at a wavelength selected for the absorption thereof by sulfur dioxide; and c. deteπriining the sulfur content as a function of absorption of the infrared radiation by the sulfur dioxide of the sample.

2. The method of claim 1 further including the step of first diluting the sample with an oxygen containing gas to enrich the oxygen content thereof and subjecting the oxygen enriched sample to ultraviolet radiation to convert at least a portion of the oxygen content thereof to ozone thereby to form the ozone containing atmosphere in which the sample is diluted.

3. The method of claim 2 wherein the sample is diluted with about 25 to 100 parts of oxygen containing gas to about 100 parts of the sample gas.

4. The method of claim 2 wherein the sample gas is diluted with equal parts of air.

5. The method of claim 2 wherein the sample gas is diluted with oxygen in a ratio of about 25 parts of oxygen to about 100 parts of the sample gas.

6. The method of claim 1 wherein the oxidized sample is placed in a photo-acoustic cell, the photo-acoustic cell being in optical communication with a source of pulsed infrared radiation, an optical filter carousel being disposed between the source of infrared radiation and the photo-acoustic cell, the carousel carrying at least one filter for transmitting the infrared radiation in a narrow wavelength selected for the adsorption thereof by sulfur dioxide, radiating the oxidized sample in the chamber with pulsed infrared energy at a wavelength adsorbed by sulfur dioxide to modulate the temperature of the sample as a result of the adsorption of infrared radiation by the sulfur dioxide and to generate pressure modulations as a result thereof, detecting the pressure modulations as acoustic signals that are proportional to the concentration of the sulfur dioxide present in the sample.

7. The method of claim 6 wherein the oxidized sample is subjected to infrared radiation at a wavelength of between about 1139 cm^"1 and 1179 cm^"1.

8. The method of claim 6 wherein the optical filter carousel further includes a filter for transmitting the infrared radiation in a narrow wavelength selected for the adsorption thereof by moisture, detecting the resultant acoustic signals and utilizing the acoustic signals to compensate for moisture in the sample.

9. Apparatus for the detection of sulfur dioxide in a sample gas comprising: a. a diluter for the dilution of the sample gas with an oxygen containing dilution gas; b. an infrared source for producing a beam of infrared radiation; c. a chopper for intermittently interrupting the beam to produce pulses of infrared radiation; d. a photo-acoustic monitor disposed in the path of the beam of infrared radiation and in communication with the diluter for receiving an aliquot of the diluted sample gas, the monitor including an analysis cell having an optical port aligned with the radiation beam, and acoustic detectors for the generation of signals in response to pressure waves produced by the absorption of infrared radiation by the sulfur dioxide content of the sample gas and the temperature modulation of the sample resulting therefrom; e. a data gathering and reporting unit.

10. The apparatus of claim 9 further including a converter unit wherein the diluted sample gas is subjected to ultraviolet radiation to convert at least a portion of the oxygen of the diluted sample gas to ozone for the production of sulfur dioxide from sulfur components in the sample gas.

11. The apparatus of claim 9 further including an optical filter carousel disposed in the path of the infrared beam, the carousel carrying at least an optical filter for the transmission of infrared radiation at a selected wavelength specific for absorption by sulfur dioxide.