DETECTION AND DETERMINATION OF H 2 S AND TOTAL SULFUR IN NATURAL GAS Marshall T. Schreve Del Mar Scientific, LTD.

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1 DETECTION AND DETERMINATION OF H 2 S AND TOTAL SULFUR IN NATURAL GAS Marshall T. Schreve Del Mar Scientific, LTD Billy Mitchell, Addison Texas INTRODUCTION Hydrogen Sulfide is measured in the natural gas industry for several reasons: Personal Safety Corrosion Control Contractual Obligation H 2 S means that each molecule of gas is made up of two hydrogen atoms and one sulfur atom. Hydrogen Sulfide is formed naturally by the decomposition of organic (previously living) animal and/ or vegetable matter by bacterial action. It is, therefore, found in crude, natural gas, sewers, swamps, and is also produced as a product or by-product in a variety of industrial and biological processes. H 2 S DETECTION The most prevalent method of detection in the last thirty years has been Lead Acetate Tape Technology. A paper or cloth tape (white in color) is saturated with a lead acetate solution, dried, then spooled similar to a reel-toreel tape recorder. The gas sample is moistened as it flows through a humidifier, then passes over or through the tape in a small, confined area, typically 1 /8 in diameter. The gas sample is then vented out of the analyzer, usually to the atmosphere. The portion of the tape exposed to the sample gas stream will darken proportionally to the amount of H 2 S present, due to the formation of lead sulfide. Since the lead sulfide is colored, and the lead acetate and paper tape are not, the rate of formation of lead sulfide may be determined by measuring the rate of darkening of the tape. The degree of stain, or the color of the stain are not important; only the rate at which the stain occurs. TECHNOLOGICAL ADVANCES: 1970 S Detectors Analyzers during this period used two sets of detectors: Reference Detector: Consisting of an incandescent bulb for tape illumination, and an optical sensor which measured the degree of light reflection from the surface of the tape before exposure to the gas stream. Tape Analysis (read) Detector: An identical incandescent bulb and optical sensor which measured light reflection after the tape was exposed to the sample gas stream. Paper, or cloth tapes were a critical component, since the tape color, width and thickness could affect analyzer accuracy. Humidification Moisture was/is necessary to produce the chemical reaction of lead acetate with hydrogen sulfide to form lead sulfide. The gas stream was bubbled through a solution of acetic acid to pick up the required moisture for a chemical reaction. Note: 5% acetic acid is used in the bubbler since it has the same ph as hydrogen sulfide (2.6) and therefore does not act as a scrubber or filter to delete or add any components to the sample gas stream. Like all sulfur compounds, Hydrogen sulfide has an affinity for water. The water will act as a scrubber. Analysis The analyzer monitors the difference (imbalance) between the Reference Detector and the Read Detector, amplifies the imbalance and converts the resultant reading to ppm, grain or mole %. Applications Corrosion Control: H 2 S, in the presence of water, will react chemically with the iron content of metal. The resulting metal loss will weaken the structural capability of the metal to the point of potential failure. Embrittlement of Steel: Atomic Hydrogen can penetrate the lattice structure of steel, causing the metal to lose its ductility. Delayed Stress Failure: (Sulfide Stress Cracking) Atomic Hydrogen produced by the reaction of H 2 S with water can penetrate the lattice structure of steel and combine in a void or imperfection causing the creation of a hydrogen molecule. The creation of a hydrogen molecule, trapped in a void, will generate internal pressure. This internal pressure, in conjunction with the applied PAGE 116

2 stress can exceed the yield strength and cause sudden, or more often a delayed failure. Personal Safety: Since an individual must not be exposed to more than 10ppm H 2 S in a period of eight hours, a reliable method of detection is necessary. The reaction of H 2 S with the lead acetate impregnated tape produces a stain relative to the amount of H 2 S. The reaction is constant, and reliable. Whether the analyzer is fully calibrated, or not, the reaction takes place regardless. PROBLEMS WITH 1970 S TECHNOLOGY Accuracy of the analyzer was dependent upon keeping the reference detector and read detector balanced. Drift was a problem. Response time could be as long as minutes, which caused delays in identifying bad product (sour gas) or notifying personnel of hazardous conditions. Analyzers were flow and temperature sensitive since the reference and read functions were as much as minutes apart. Changes in flow and/or temperature would be averaged into the analysis for the length of the reference/read cycle. Calibration was very detailed and time consuming, requiring trained technicians to accurately identify system discrepancies, and perform the task of balancing the reference and read detectors. Tape consumption was 7-14 days per roll depending on the level of H 2 S present. Some analyzers from this period used tape drives that moved constantly, or advanced the tape continuously. Mercaptan Interference: The bubbler used to humidify the gas sample stream was capable of liquid carryover. The droplets of water containing mercaptans from the gas sample stream would decompose to H 2 S in the presence of the hot incandescent bulb used in the detector assembly. High H 2 S readings were displayed as a result. Spare Parts: A wide variety of spare parts were required to maintain the analyzer, including magnetic drives, bevel gears, pots, bulbs, optical sensors, circuit boards, etc S TECHNOLOGY Detectors A single optical sensor, or photo diode acts as the reference and read detector: At the beginning of each measurement cycle, the tape advances to a new, or unexposed portion of the tape. Instantly, as the tape stops, the optical sensor value is fed into a circuit driving a preamplifier into a A/D converter, and the resultant voltage level is stored in a microprocessor memory. A new optical sensor reading is compared to the stored information, At this point, the microprocessor determines the H 2 S level by comparing the two readings and then displays the results on a digital panel in ppm, grain or mole %. Simultaneously, the second reading is stored in the microprocessor for later comparison to a third, and a fourth reading. The cycle is repeated on an average of every second, until the tape is too dark for further analysis. At this point, the tape advances to a new (unexposed) spot and the process begins again. Effectively, a new zero or reference point is selected with each tape avance. Tape Due to the microprocessor based detector described above, the tape color and thickness are no longer critical. The lead acetate impregnated tape is 60% cotton and uniform in construction due to improvements in manufacturing techniques. Humidifier To eliminate the possibility of liquid carryover, as well as mercaptan interference, the gas sample stream is passed through a moist filter element instead of being bubbled through the acetic acid solution. The filter element, through capillary action, wicks the acetic acid solution from a reservoir to provide constant humidification. Response Time The microprocessor based analysis provides a 2-3 second initial response time for real-time analysis, and immediate response to gas quality control. Calibration Calibration normally takes less than fifteen minutes and can be achieved through an auto-calibration feature for critical applications such as EPA Fuel gas analysis. Accuracy Accuracy is normally 1% of full scale, with zero drift. (a ppm analyzer will be ±1 ppm accurate) Tape usage Since the tape does not move or advance until necessary, tape usage can be predicted at 3-4 weeks per roll depending on H 2 S levels. Technical Expertise Due to the simple modular design, and built-in diagnostics, end user personnel can usually perform any maintenance or repair required. Factory service calls are reduced as well as the associated costs. PAGE 117

3 Typical Specification: Range: 0-100% Repeatability: +/- 1% Response Time: <1 minute to 90% Zero Drift: None Temperature Range: Classification: Typical Options: 0-50 C General Purpose Class I Division I Class I Division II Power Supply: Outputs/Alarms Diagnostics Data Logging: Pressure Regulator Cabinet Heater Auto-Calibration: Communication: 12,24 VDC 110,220 VAC 4-20Ma Digital Outputs Alarm Output 32 Meg Gas or liquid Electric or Gas Field Selectable Analog and Digital FIGURE 1. Hydrogen Sulfide Analyzer Lead Acetate Tape Technology Basic Design Drawing Capabilities The microprocessor design provides for: Auto-Calibration: Easily cycled according to customer or contract requirements. Multistream Capability: Due to the speed of the response and recovery times, a single analyzer can effectively monitor two or more streams. Control/Communication: The ability to control and communicate with the analyzer from a distant location has created many new applications. Applications Corrosion Control: (as mentioned above) Embrittlement: (as mentioned above) Delayed Stress: (as mentioned above) Personal Safety: (as mentioned above) Gas Loss to Flare: Remote facilities do not always have the capability to recycle sour gas, therefore bad product must be flared once it leaves the process area. The 2-3 second response time of modern analyzers prevents the sour gas from leaving the process area untreated, and subsequently, eliminates the loss of product to the atmosphere. Reduction in Treating Cost: Sour Gas Treaters, using products such as Alkanolamines (Amine Solution) can be operated with closer (tighter) tolerances resulting in reduced consumption of amine solution. It is common practice to use higher concentrations of amine solution than necessary to guarantee an end product within contractual specifications. The speed and accuracy of modern analyzers can eliminate the cost or need for over-dosing. Note: Although over-dosing of amine solutions will usually provide a margin of safety, corrosion rates are excellerated through this practice, resulting in reduced service life for process equipment. Reduced Fuel Consumption: Accurate monitoring of the condition of inlet (untreated) gas will result in being able to optimize the operation of process equipment. Over-dosing, and elevated operating temperatures are eliminated. PAGE 118

4 Typical specification: Range: 10ppb to % level Repeatability: +/- 1% Response Time: <1 minute to 90% Zero Drift: None Temperature Range: 0-50 C Classification: General Purpose Class I Division I Class I Division II FIGURE 2. Total Sulfur Analysis Blending Possibilities: Real time analyzers can be used to blend small sour gas streams into larger sweet gas streams through stream switching. ADVANCES FOR THE YEAR 2002 Many advances are being introduced with the latest versions of Lead Acetate Technology: Programmable Range: Customer selectable, changeable ranges to eliminate field service calls when an analyzer is required to monitor a sample gas stream outside the original range set at the factory. Reduced Need for Dilution: Most analyzers require a dilution system for H 2 S levels above 20 ppm. The valves, solenoids and piping needed for dilution control increase the cost of the analyzer, require more frequent maintenance, and create a more complex system. Analyzers are now available that can operate at levels as high as 300 ppm without a dilution system. Long-Term Operation: Today s analyzers can operate from days without tape changes or humidifier servicing. Modular Design: Alarms, contacts, and control features can be changed in the field through software upgrades or laptop interface. Self-Calibration: analyzers are able to calibrate themselves every few minutes, and report any defaults. Communication: Dual RS-232 capacity will ensure communication at critical locations and during system upsets. Data Logging: Data storage of up to ten years at the analyzer, with uploading and downloading through laptop interfacing. Simultaneous Dual Stream Capability: A single analyzer will be capable of monitoring two gas sample streams simultaneously. PAGE 119

5 TotalSulfur Analysis through the use of a conversion furnace: All sulfur species can be converted to H 2 S, and then monitored by a typical H 2 S analyzer. The following conditions must be met to convert the sulfur species to H 2 S: The gas sample stream must be blended with an adequate amount of sulfur-free hydrogen. A hydrogen rich environment will convert all of the sulfur compounds, while an insufficient amount of hydrogen will allow some of the sulfurs to remain unchanged. The gas sample stream must flow through a conversion furnace at or above 1000 Degrees C Once the sulfur compounds are reduced to H 2 S the analyzer is capable of detecting the total sulfur concentration down to less than 10 ppb. Marshall Schreve PAGE 120