Developments in H2S scavenging
Advances in the application of triazine scavengers have significantly improved process efficiencies.
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The easy to extract sweet oil and gas deposits of the past have evolved into technically advanced new plays requiring extensive horizontal fracturing. New fields and existing fields
with different zones have been exploited all over North America, and much of the resulting new production contains hydrogen sulphide (H2S).
A strict regulatory environment, including regulations for pipeline specifications, flaring, transportation safety, corrosion, venting and other emissions, has necessitated more innovative and exceptional H2S treatment methods. In turn, stronger demands on H2S removal has ensured that suppliers of traditional triazine scavengers develop more competitive pricing and improve chemistry formulations, making many more applications cost effective.
H2S is associated with methane and other hydrocarbons in all phases of production and all over the world, including land based production, transportation, storage and processing and offshore production and storage facilities. With much of current global production having a level of H2S as part of its composition, producers, midstream companies and facility operators require improved chemistry formulations and more cost effective H2S treatments.
This article discusses how inexpensive triazine based liquid scavengers, coupled with more efficient equipment options, provide a more economical and comprehensive solution to H2S problems in every phase of production.
H2S scavenger overview
H2S is a light, volatile compound that must be eliminated from hydrocarbon gases and liquids in order to produce consumable products. Historically, H2S scavenging has been used at pipelines and well sites to remove H2S during the gas phase of production, but today’s scavenging chemistries and processes can effectively remove this poisonous and corrosive compound from both hydrocarbon gas and liquids throughout production and processing stages.
Lower hydrocarbon pricing mandates either less expensive production costs or shutting in production in regions entirely. There are both economic and technical limitations to removing H2S, but with regulations and emission limits, as well as ever present safety requirements, H2S treatments are increasingly required and often mandated. Governmental bodies, environmental bodies and the more intricate midstream and custody transfer network tend to require H2S treatments earlier in the production process and in many more application types.
Globally, triazine is the most common chemical to be used for H2S removal. More than 400 million kg of this alkanolamine/aldehyde condensate are consumed annually (315 million kg in North America), and increased consumption from North America’s shale boom has eroded triazine pricing to commodity levels – a benefit to producers needing cost effective H2S removal to compete in a market with marginal oil and gas pricing. In addition, evolved liquid scavenger processes more efficiently treat a wider array of operating conditions, sources and compositions.
Used with current liquid scavenging technologies, triazine can efficiently and effectively reduce H2S concentrations to as low as 0 ppm and partially remove some light mercaptans (methyl, ethyl, and propyl) as well. Most inorganic sulphur compounds and other heavier sulphur compounds are not reactive with the current triazine chemistry.
Different triazine formulations contain additives to help enhance cold temperature operations, to reduce scale formation, to provide varying mass transfer characteristics and to maintain different capacities of reaction. Strengths of the chemical and the economics of storage and transportation vary with the application, and formulations differ by supplier. MEA triazine is the most common chemical used with current liquid methodologies.
The triazine reaction is a non- reversible chemical substitution reaction with limited uptake capacity. The capacity varies with each chemical strength, but most commercially available formulations have a stoichiometric uptake capacity of approximately 1.0-1.2 lb H2S per US gallon (0.15 kg/l). The maximum practical capacity limit of triazine is approximately 80% of the stoichiometric value and can routinely be attained in practice in the field with the current process technologies. Other commercially available chemical formulations have different uptake capacities, and each formula should be evaluated independently.
The principal H2S reaction with MEA triazine is shown in Figure 1.
The efficiency of a process is defined as the percentage of the potential reaction that has taken place during the process compared to the actual reaction completion. For example, if a specific chemical can remove 1 lb H2S/gal, but the process actually removes 0.8 lb/gal, then we would consider this to be 80% efficient. The triazine reaction with H2S is strongly kinetically favoured and therefore not meaningfully affected by pressure. Temperature is best in the 80-120°F (27-49°C) range, but practical applications have stretched the range from 50-160°F (10-70°C) with some losses of efficiency.
Controlled contact between triazine and H2S is critical, as either excessive contact, even with low H2S concentrations, or lesser contact but with high concentrations of H2S can over-react the triazine and lead to polymerisation and precipitation of the chemical. Figure 2 shows over- reacted solid material that occurred with a 35 ppm concentration in the gas, but in a pipeline with velocity too low to carry reacted triazine through. The over-reacted product then built up in the low points of the pipeline.
While certain operating conditions may not be conducive to achieving the desired efficiency, more specialised technologies are now able to reach practical efficiency limits and minimise the risk of chemical overspending and solid material precipitation in the processes and downstream equipment. Additionally, processes developed to treat unusual operating conditions, wide swing variations in operating parameters, exceptionally high H2S concentrations, large gas flows and hydrocarbon liquids are available.
The spent chemical in all the processes is a liquid waste product that is water dispersible and requires proper disposal. To date, there is no secondary use for the spent chemical, and it is most often disposed of in salt water disposal wells with produced water generated at the facility.
Chemical suppliers have discussed alternatives to triazine and conducted many proprietary trials, but the H2S uptake capacity, speed of reaction, corrosivity or cost of treatment have limited the use of these chemicals to very specialised applications or trials. Chemical trials include proprietary triazine formulations, formaldehyde, caustic solution, glyoxal and sodium nitrite, but none have become a competitive product in the scavenging market. So for the time being, triazine is the most cost effective and widespread chemistry for low level H2S removal.
Historic H2S removal
Because H2S primarily resides in the gas phase, H2S treatments have long been linked with natural gas production. Applications have evolved over time and include:
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