There are a variety of strategies available that can alleviate the safety, environmental and operational problems caused by the storage and handling of problematic crude oils, refined products, and various other feedstocks and intermediates, such as condensates, naphthas, light distillates, gas oils, and residual materials.
In a perfect world, H2S could be managed by keeping storage temperatures and tank volumes low to minimize the concentration of H2S in the vapor phase. Tank-to-tank transfers, venting and air blowing are also methods that help dissipate H2S (with appropriate understanding of applicable environmental regulations). However, these practices are often impractical from cost, time and environmental perspectives, and can be unreliable as a means for reactive sulfur removal. Some of these practices may not be permitted by governmental regulations. Alternatively, there are treatment options using chemical scavengers to reduce H2S.
There are many chemicals that will readily react with H2S, including caustic, peroxides, formaldehyde, nitrites and many types of amines. All of these convert the H2S or mercaptans into other sulfur compounds.
Oxidizers, such as peroxide, convert H2S into elemental sulfur or sulfur oxides. However, oxidizers are not selective for sulfur compounds and will react with other components of the oil, resulting in high chemical consumption and potential degradation of the oil. Amine neutralizers react very quickly with H2S and are suitable for certain lower-temperature applications. However, the reaction products are not heat-stable and may regenerate H2S under certain conditions. The use of caustic (sodium hydroxide or a blend of sodium and potassium hydroxide) alone, or as a base treatment in combination with another scavenger, has been practiced. With sufficient agitation, caustic can be effective, but its use increases the sodium and/or potassium content of the treated oil and may lead to deposit formation and high-temperature corrosion in heaters, boilers and turbines. Additionally, caustic embrittlement of refinery rundown lines can occur if caustic is injected above 180°F (82°C).
The preferred method for scavenging H2S and mercaptans from hydrocarbon streams is to use a chemical conversion reagent. This type of additive reacts with H2S irreversibly to form thermally stable reaction products and does not revert back to H2S if exposed to higher temperatures farther downstream. Scavenger programs are selected depending on the nature of the H2S problem being addressed and the petroleum product being treated.
Water-soluble scavengers are among the most common scavengers and are often the product of choice for applications at temperatures below 200°F (93°C). Economical costs and fast reaction rates make them attractive options. Moreover, due to their water solubility, they add a minimum amount of nitrogen to the fuel. These are the preferred additives for use in flare gas, LPG, resids and crude oils. A common type of water-soluble H2S scavenger is a triazine-based chemistry. Triazine-based scavengers undergo the basic reaction with H2S outlined below.
Triazines made from monoethanolamine (MEA) or methylamine (MA) are the most common commercially available scavengers. MEA-based triazines react more efficiently in the liquid phase than other chemistries and are particularly useful if the immediate reduction of H2S is critical; as is the case when ships or barges are loaded with H2S-laden product and the safety of the crew is at risk. Other water-soluble H2S scavengers include polymeric, nitrogen-based products, as well as aldehyde-based, non-nitrogen chemistries. These scavengers are typically less likely to cause any fouling and corrosion problems in crude oil distillation units and refinery process equipment.
Oil-soluble scavengers are used in high-temperature applications or when water tolerance of the hydrocarbon is an issue. These products are typically amine-based and perform as effectively as water-soluble additives. These products react with H2S irreversibly to form a thermally stable, oil-soluble alkyl sulfide. They can be applied at a wide range of temperatures, from ambient up to 350°F (177°C), and are often the product of choice for viscous heavy oils and resids. They are also preferred by many users over water-soluble additives in finished fuels to avoid any possibility of haze formation.
Metal-based scavengers answer the specific needs of very high-temperature and high-H2S concentration applications. These additives can be used at temperatures in excess of 350°F (177°C) to form thermally stable products and are able to provide H2S reduction levels that other H2S scavengers cannot achieve. While these products are typically used to treat asphalt, other applications can be envisioned. These scavengers and reaction products remain in distillation column bottoms and could be used in refinery applications that are not sensitive to metal content.
The increased reactivity with H2S gives these products a favorable cost-performance ratio relative to other H2S scavengers. Scavenger system controls H2S in flare gas. A refinery involved Baker Petrolite in a custom H2S scavenging program to be used when the H2S content of flare gas threatened to exceed acceptable levels; reduction rates for SOX were set to less than 200 pounds a day and H2S levels to less than 4,000 ppm. After a thorough survey, a four-nozzle injection system was used in existing flare gas process lines. Field trials were conducted to verify that all customer requirements could be met. SOX emissions were reduced from over 500 to less than 70 pounds a day. H2S levels were reduced from over 18,000 to 900 ppm (Fig. 4). This program will allow the refinery to maintain compliance with government regulations and avoid possible fines or penalties. The refinery was also able to avoid installation of a multi-million dollar processing unit. Finally, the refinery will not have to decrease throughput to remain in compliance. H2S scavengers help control tank roof corrosion In a study conducted by Baker Petrolite, the corrosion rate of the roof of a storage tank located in a coastal region was monitored for one year using metal coupons and electrical resistance probes attached to the tank roof. This particular tank contained a residual fuel with liquid phase H2S levels of 15 to 25 ppm. The H2S content of the vapor phase ranged from 100 to 600 ppm during the study period. When the H2S content of the vapor space increased, a corresponding increase in the rate of corrosion was recorded (Fig. 5). In this location, corrosion rates had been high enough to reduce the lifetime of storage tank roofs to less than five years. Implementation of an H2S scavenger program helped to control tank roof corrosion. By reducing the concentration of H2S in storage tank headspace, use of an H2S scavenger reduced corrosion rates measured on the tank roofs from nearly 14 to < 1 mils per year (0.36 to 0.0254 mm per year). Off-spec propane treated in cavern A products pipeline system that uses underground salt caverns to store propane prior to shipment had several caverns containing propane off-specification on the ASTM D1838 Copper Strip Corrosion Test. While H2S was the suspected culprit, the pipeline company needed assistance in identifying and resolving the problem. The Baker Petrolite PREPARED TO RESPOND™ Services program offered prompt and effective remediation.
After the pipeline consolidated 42,000 barrels of the off-specification propane into a 100,000-barrel capacity cavern, Baker Petrolite applied SULFIX™ 9272 Scavenger over a three-hour period via an inlet to a 4-in. steel pipe that serves to circulate the cavern contents. The propane in the cavern was circulated for 12 hours and allowed to settle for an additional 10 hours. Adequate settling time is necessary to allow the water-soluble H2S scavenger to separate from the hydrocarbon phase. The treated propane passed the copper strip corrosion test with a “1” rating; furthermore, no H2S was detected after treatment. The treatment and its responsiveness enabled the client to minimize the amount of money lost as a result of missed shipments and lack of storage capacity.
In many cases, tank farm or terminal operators have very little time in which to correct H2S-related problems. These could impact operations (personnel safety issues) or profitability (product transport/transfer issues) in a time-critical manner. Baker Petrolite offers unique PREPARED TO RESPOND™ Services that are focused on quickly remediating distressed feedstocks. The services include product selection, mobilization of chemicals, injection equipment and experienced personnel that can all arrive on site within 24 hours in most major refining and fuel transportation centers worldwide to safely and effectively bring cargos back on specification, thereby increasing safety and profitability. Effective removal yields clear benefitsH2S scavengers are industry-proven methods to remediate dangerous conditions that present safety and regulatory issues. Additional issues caused by H2S contamination include corrosion problems, odor concerns and product degradation. Experience demonstrates that proper selection and application of these proven classes of chemicals are critical to successful H2S-abatement programs. The combination of effective products and application expertise results in a successful program that can maximize safety, minimize or eliminate corrosion, and improve profit margins.
The primary content of these blog posts is a Baker Hughes white paper available here.