Abatement of hydrogen sulphide in asphalt
Risks of exposure at asphalt loading platforms require reliable monitoring of H2S levels and a programme of abatement using effective chemical scavengers
Jennifer Draper and Joseph Stark
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Hydrogen sulphide (H2S) is a naturally occurring gas contained in many of the world’s crude oils. It is also formed by the degradation of sulphur compounds in the oil when it is exposed to high temperatures or catalysts in the refining process. Since the sulphur content of crude oils is gradually increasing (see Figure 1), H2S issues are expected to remain a challenge for all aspects of the refining industry, especially when handling heavy oils.
Vacuum tower bottoms (VTBs), the primary blending component for asphalt production, have particularly high concentrations of H2S because they do not undergo additional processing to remove H2S through distillation, stripping and sweetening processes as is done with more refined fuels such as gasoline, diesel fuel and home heating oils. VTBs are among the heaviest of the products coming out of the refinery and typically the product in which sulphur compounds concentrate. Due to the viscosity of asphalt, it is stored at high temperatures (300–400°F; 149–204°C). These temperatures are high enough to promote further thermal cracking of sulphur-containing compounds and the formation of additional H2S. The amount of cracking and H2S generation is dependent on the structure of the sulphur compounds present in the oil and on the temperatures encountered during processing. Figure 2 shows a typical relationship between storage temperature and H2S formation.
Since H2S is a gas at typical storage temperatures, it has a tendency to migrate from the liquid phase into the vapour phase above the oil. Certain tank conditions, such as increased liquid volume, agitation and high temperatures, can exacerbate this already hazardous situation by changing the partition coefficient (the ratio of H2S in the liquid and vapour phases of the crude oil) of the H2S in favour of the vapour space. Asphalt is especially problematic because more H2S partitions into the vapour phase relative to other petroleum products (see Table 1). Typically, 1 ppm of H2S in the liquid phase of asphalt correlates to 400 ppm in the vapour phase. Therefore, asphalt can contain extremely high levels of H2S in the vapour phase, even exceeding 3% (30 000 ppm), which can cause a variety of problems and risks.
Concerns with asphalt H2S
The foremost consideration when dealing with hydrocarbons containing large amounts of H2S is safety of personnel involved in its storage, handling and transportation, and of the community. Exposure to very low levels of H2S can result in significant health ramifications. H2S is especially insidious because it deadens the sense of smell at concentrations as low as 30 ppm, and death can occur within a few breaths at concentrations of 700 ppm.
Due to the toxicity of H2S, there are regulatory limits at federal, state and local levels, as well as limitations placed by refineries and terminals:
• The American Conference of Governmental Industrial Hygienists (ACGIH) recently lowered the recommended threshold limit value (TLV) for H2S from 10 to 1 ppm, and the short-term exposure limit (STEL) from 15 to 5 ppm
• The Occupational Safety and Health Administration (OSHA) limits H2S exposure to 20 ppm (ceiling) and 50 ppm (peak, not to exceed 10 minutes)
• The National Institute for Occupational Safety and Health recommended exposure limit is 10 ppm maximum
• Typical best practices limit the levels of H2S to 100 ppm at barges and 10–15 ppm at truck loading racks.
Other regulatory limits are focused on ambient air quality. The air quality of communities located around facilities that may emit H2S, and the chronic health effects of these emissions on nearby residents, is a concern. Health effects from chronic exposure to low levels of H2S include headaches, insomnia, nausea, shortness of breath and eye irritation. Many states and municipalities regulate the amount of H2S permissible in the ambient air, most of these as low as a few parts per billion (ppb) of H2S. This can limit asphalt production and the filling of asphalt tanks due to emissions during filling operations.
In addition to the inherent dangers of chronic exposure to low levels of H2S, nuisance odours can also be a problem if the refinery or storage facility is located close to residential neighbourhoods. Wind speed, direction and the topography of the neighbouring areas can either exacerbate or mitigate the magnitude of the problem. H2S is heavier than air and will tend to settle in any low spots around the storage facility, making these prime areas for odour complaints, particularly if the wind direction moves the H2S towards the low spot.
Methods of analysis
Reliably determining the concentration of H2S in asphalt is difficult. In addition to the inherent danger of working with a product that contains high levels of H2S, there are added complications relative to the temperature of the material, its viscosity and its tendency to quickly lose H2S during testing. Testing is most appropriately done on-site with fresh asphalt sampled directly from the unit into the sampling container. Figure 3 shows the rapid loss of H2S from sealed sampling containers as a function of time and loss of temperature. In this example, three untreated asphalt samples were taken consecutively from the unit, sealed and tested at five-minute intervals. After 10 minutes, a greater than 50% loss of H2S was observed.
Baker Hughes field can test method
Since the methodology for measuring the performance of scavengers is inherently variable, field trials are the most reliable way to determine performance. Laboratory results can be useful in predicting trends and relative performance, but dosage optimisation may only be achieved through a field trial. Baker Hughes has developed a method to measure the H2S content of asphalt safely and to allow for a more reproducible means of establishing the vapour phase in the sample container. This method employs quart-sized metal cans that are half-filled with asphalt, sealed and shaken using a specially designed container that allows adequate agitation of the sample while protecting the operator from accidental leakage of hot asphalt (see Figure 4). Untreated samples are tested immediately using gas detection tubes. For the purposes of screening additive performance, the scavenger should be added to the sample container prior to introducing the asphalt, then transported to an oven in an insulated container, where it is stored at temperatures and time periods consistent with those experienced in the field.
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