Introduction of hydrogen sulfide gas
Hydrogen sulfide, which has the chemical formula of H2S occurs in crude petroleum, natural gas, volcanic emissions, and hot springs. In addition, it can also be a result from bacterial breakdown of organic matter, as well as human and animal wastes. In fact, bacteria found in a human mount and gastrointestinal tract produce hydrogen sulfide from decomposing vegetable or animal proteins. Hydrogen sulfide gas can also be produced in industrial activities, such as food processing, coke oven, paper mills, tanneries and petroleum refineries.
Hydrogen sulfide gas is both flammable, and colorless. It also has a characteristic odor, most often associated with rotten eggs. It is also known as hydrosulfuric acid, sewer gas, and stink damp. (ASTDR, 2009).
Hydrogen sulfide is usually released as a gas and spreads rapidly through the air. It can remain in the atmosphere for approximately eighteen hours. When it is released in its gaseous state, it will react with the atmosphere producing both sulfur dioxide and sulfuric acid.
Ninety percent of the sources that emit hydrogen sulfide into the environment are natural. In most cases, the release of hydrogen sulfide is a result of the natural decomposition of animal and plant material, especially in anaerobic environments, such as swamps. In addition, many instances of hydrogen sulfide releases are associated with volcanic activities.
However, there are a number of anthropogenic sources for hydrogen sulfide releases into the environment. These occur from industrial processes, mostly from the extraction and refining of oil and natural gas, or from paper and pulp manufacturing. (EPA, 1993). Hydrogen sulfide is naturally occurring both petroleum oil and natural gas, which are often trapped in sedimentary rocks. High sulfur kerogens, also associated with petroleum products through the decay of plant matter, release hydrogen sulfide during decomposition, and the resultant gas becomes trapped in the oil and gas deposits (Skrtic, 2006).
Operations related to oil and gas exploration and refinement can potentially emit hydrogen sulfide either routinely or accidentally into the environment. These releases can occur during nearly all parts of the process, including extraction, storage, transportation, or the processing stage. During the extraction stage, for example, hydrogen sulfide can potentially be released into the atmosphere at wellheads, pumps, along the pipeline, at various separation devices, or oil storage facilities. In addition, because it cannot sold, hydrogen sulfide is routinely disposed of through flare burns at the refinery stage. Because of the many areas where hydrogen sulfide can released into the environment, many safety and procedure cautions have been mandated to help reduce the amount of hydrogen sulfide released into the environment.
According to the EPA, well blowouts, line releases, extinguished flares, collection of hydrogen sulfide enriched gases in low lying areas, line leakage, and leakage of abandoned wells have all impacted the public health through documented accidental releases. Well blowouts are defined as uncontrolled releases from wells, and can occur during drilling, servicing or even production. The release from such an occurrence can last for an indeterminate length of time. In addition, if a well that has suffered a blowout is not properly sealed, hydrogen sulfide can continue to leak out of the well (Skrtic, 2006).
Hydrogen sulfide removal method
The use of hydrogen sulfide scavengers by the natural gas industry have grown removing low concentrations of hydrogen sulfide, usually up to and including 200 ppm, depending on the rate of gas flow. Increased concerns about the safety and environment associated with spent material disposal have prompted the introduction of new scavenger technologies. In addition, new, stricter environmental regulations have contributed to a stronger interest in identifying better, more environmentally sustainable hydrogen sulfide scavenging technologies (Foral, unknown)
One such hydrogen sulfide removal process that is still in wide use today is the use of an Iron Sponge. An early example of the use of an iron sponge occurred in the 1800s. In this case, the hydrogen sulfide was removed from the gas by passing it through beds containing hydrated ferric oxide supported by a media of wood chips. The ferric oxide reacted with the hydrogen sulfide to form ferric sulfide. These beds were then purged with air to oxidize the ferric sulfide into elemental sulfur and ferric oxide. The ferric oxide could again be utilized in the removal of more hydrogen sulfide, beginning the process again.
Although the basic method of utilizing an iron sponge is effective in the removal of hydrogen sulfide, it does have a number of significant drawbacks. The first is that it requires a relatively large area to operate in, and can be extremely labor intensive. In addition, when the ferric oxide becomes completely spent, it has a pyrophoric nature, making the disposal of the material problematic at best (Douglas, unknown).
Liquid oxidation catalyst(Lo-Cat ® )
The current generation of hydrogen sulfide removal processes began when it was discovered that certain organic compounds utilized in the tie dye industry were effective at oxidizing hydrogen sulfide to elemental sulfur. Eventually, these processes became further refined and known as liquid redox processes. Liquid redox processes employ successive cycles of oxidation of hydrogen sulfide by a catalyst containing an oxidizing agent. The catalyst is then re-oxidize in the air, producing element sulfur as a waste product. (Douglas, unknown).
One of the most common branded forms of this type of hydrogen sulfide removal is Lo-Cat ®. Lo-Cat ® is an oxidation process that uses iron as the catalyst agent. It is held in a chelating agent and has been shown as an effective way to oxidize hydrogen sulfide into elemental sulfur. In essence, a Lo-Cat ® removal process consists of three basic sections: an absorber, an oxidizer for catalyst regeneration, and a sulfur handling unit.
Gas Technology Products (GTP) and LGI are two of the manufacturers of catalytic sulfur removal technology such as Lo-Cat ®. GTP first commercialized the technology in 1978. Approximately three hundred units are currently in operation in various industries that require the removal of hydrogen sulfide, as well as the recovery of low levels of sulfur. These industries include natural gas sweetening, syngas operations, air purification, and refinery off-gas streams (NREL, 2006).
During the hydrogen sulfide removal process, the gas stream comes in contact with the Lo-Cat ® solution in the absorber, the hydrogen sulfide in the gas is converted to elemental sulfur. The spent iron catalyst, as well as the sulfur leave the absorber and then enter the oxidizer, where the catalyst is regenerated by contact with oxygen. The elemental sulfur is then concentration into sulfur slurry, which is then transported to sulfur handling unit.
Essentially, the Lo-Cat ® process removes hydrogen sulfide from gas streams by “scrubbing” the gas with an aqueous chelated iron solution. In the first portion of the process, the hydrogen sulfide gas is introduced to the iron solution, and the following chemical reaction takes place:
H2S(g) + 2Fe3+ → 2H+ S0 + 2Fe2+
In the second zone, or the downcomer, the spent catalyst material is sparged with oxygen, and the reduced iron chelate iron is oxidize into the original iron III by the following reaction:
2Fe2+ + H20 + ½ O2(g) → 2OH- + 2Fe3+
The Sulfur-Rite ® system is available in a broad range of sizes, and has been rated to handle flows up to 2500 scfm. They can operate in parallel, or they can be custom designed for higher flows and pressures. Systems, for example have been designed for high pressure applications, including natural gas processing.
Essentially this process converts hydrogen sulfide into iron pyrite, which is a stable non-hazardous substance. It does not require expensive equipment such as filters, additives, or flare gas makeup. There are no moving parts to maintain, no air emissions to control, and no liquid stream requiring additional disposal procedures.
A standard Sulfur Rite system unit is built with steel and corrosion resistant internal coating. It consists of both one or two vertical vessels and associated piping. In the dual vessel configuration, the vessels operate in a series, with the first vessel in the lead position, and the second in the lag position.
Comparison of Methods
In 1993, the Gas Research Institute (GRI) started a research program into hydrogen sulfide scavenging. They evaluated a number of systems, and attempted to quantify and qualify results in a number of different areas, including technical evaluation, laboratory testing and field evaluation.
The main purpose of the study was to evaluate the available commercial technologies, identify their respective process capabilities and limitations, and to establish relative investment and operating costs for each one. In order to accomplish task, a selection of well know hydrogen sulfide scavenger systems were designed using 1993 pricing for materials, disposal, man-hours, and maintenance for a mock trial. The mock trial consisted of the treatment of 2 MMSCFD, or million cubic feet per day of natural gas, containing 3, 60 and 100 lbs of sulfur per day in the form of hydrogen sulfide. The information obtained from these mock set ups were used to determine which were the most efficient method of hydrogen sulfide scavenging for the natural gas industry in 1993.
In the study conducted by GRI indicated that the maximum treatment amount of sulfur for iron sponge is approximately 100lbs per day. This corresponded roughly with the amounts for Sulfa-Scrub and Ecotreat, two additional brands that follow the same basic set up protocol for treatment (Foral, unknown). Results also indicated that in most cases Iron Sponge was most effective in areas where low amounts of hydrogen sulfide were encountered.
Liquid oxidation catalyst(Lo-Cat ® )
In various case studies conducted by both the manufacturer and a few select customers that utilize Lo-Cat ® technology, the efficiency of removal ranged between 92% and 99% hydrogen sulfide removal (GTP, 2008, ARTC, 2008). For example, in 1999, PT Pertamina, Indonesia’s state run oil company chose to install a Lo-Cat ® hydrogen sulfide oxidation process system for the Mudi field in East Java, Indonesia. The oil production at that field was approximately 11 MMSCFD of associated sour gas, which was then slated to be used to strip additional hydrogen sulfide from the sour crude, and to generate power for three gas fired turbines.
In order for the Lo-Cat ® system to be effective, it had to meet a number of criteria. The first was that both the gas capacity turndown and the hydrogen sulfide concentration turndown both had to have a relative ratio of 3:1. Second, the hydrogen sulfide gas in the effluent had to be less than or equal to 10 ppmv, the system had to operate successfully with minimal operator oversight, and the produced sulfur product had to be suitable for agricultural use. In order to meet these demands, PT Pertamina had a customized Lo-Cat ® unit built. The system was designed to remove 15 long tons per day (tpd) of sulfur by treating 11 MMSCFD of the sour associated gas at 60 psig, containing approximately 3.62% by volume of hydrogen sulfide. The end result was sulfur cakes containing approximately 60% sulfur by weight, an effluent “significantly below the required outlet level of 10ppmv”, and an overall operator attention of approximately 2 hours per day (ARTC, 2008).
In most case studies presented, the overall removal efficiency of a Sulfur Rite system varied between 89% and 95% when operated at an optimal level, with the majority of instances above 90%. However, According to the Alaska Department of Environmental Conservation, Sulfur-Rite ® is only designed to handle hydrogen sulfide streams of less that 10 ppmv (Eastern Research Group, 2008).
Over time, regulations have become more restrictive in the methods of classification and disposal of spent Iron Sponge. There is a potential of exposure to the toxic off gas and spontaneous combustion of the spent material when exposed to air. In addition there is also a potential of having sodium hydroxide, sodium carbonate, sodium bicarbonate, and sodium sulfide, especially if caustic soda is added for pH control. In addition, the components in both the fresh and spent scavenger material can potentially be listed under one or more regulations including RCRA (40 CFR 261.33), CERCLA 1980 (40 CFR Part 302 Table 302.4) and the Department of Transportation (DOT) from 49 CFR 172.102. (Foral, unknown).
Liquid oxidation catalyst (Lo-Cat ® )
In general, there are two main areas of environmental impact seen with the use of a Lo-Cat ® system. The first is the creation of sulfur waste that may require specialized disposal in a landfill or by alternative means depending on the concentrations, conditions of the sulfur, and what impurities might be found in the waste material. In addition, normal operation of a Lo-Cat ® system will often utilize small amounts of caustic solution to regulate the pH in the reaction vessel. Depending on the amount of material, safe handling and disposal of various effluents and waste material potentially could include hazardous material considerations (Alaska Department of Environmental Conservation 2008)
In the case of Sulfur-Rite ® the resultant product is an insoluble iron pyrite. The spent product is reported to by non-pyrophoric, and landfillable, making the disposal of the product somewhat easier than that of both iron sponge and Lo-Cat ® . (Feldmeier).
There are a number of factors that contribute to the overall cost of the removal of hydrogen sulfide. One of the major cost considerations is the cost of the disposal of hazardous material. In general terms, the disposal cost of hazardous material is greater than for a non-hazardous material. For example, if the spent scavenger material contains a high water and hydrocarbon content, a standard recycler will charge approximately $200 per ton to dispose of the material. However, if the spent material is in cake form, and have a minimum amount of water or hydrocarbon, the cost of disposal could be significantly lower (Foral, unknown).
In the study previously detailed and conducted by GFI, the operating cost of a hydrogen sulfide scavenger system was shown to include the following basic parameters for the iron sponge system. It included the cost of the bed change out, basic labor costs, and the cost of the actual scavenger material. The cost of transportation, loading, and unloading spent material was not included in the study, since these costs are often negotiated on a case by case basis. The typical cost for the change out labor, materials, and set up for an average Iron sponge system was approximately $10,875 per install, according to 1993 pricing. In addition, disposal costs for the spent material averaged approximately $5,438 per unit. This would bring the estimated total cost of operation of an Iron Sponge unit on average between $15,000 and $20,000 dollars depending on system requirements. Again, this total does not include the transportation of materials, and is based on 1993 cost estimates. In addition is should be noted that these estimates are essentially for one round of use for the Iron Sponge. Depending on the project specifications, multiple uses of the iron sponge might be required.
Liquid oxidation catalyst(Lo-Cat ® )
Generally speaking, the cost of using a Lo-Cat ® hydrogen sulfide removal system will depend on a number of factors, including cost of set up, ongoing operational cost, cost of waste disposal, and cost of regulatory compliance measures. On average, the control cost for this method is approximately $1,089 per ton of hydrogen sulfide, according to the manufacturer. This control cost is only an average estimation, and may significantly fluctuate based on project parameters.
However, when the operational costs are considered, the budget for operating a Lo-Cat ® system can vary widely depending on the scope and project duration. Estimates for overall operational costs have varied between $25,000 and $73,000 for normal operation of approximately 48 hours per a seven day week period.
In comparison, the average annual cost of a Sulfur-Rite ® system depends mainly on the cost of initial set up, routine maintenance, and the cost of disposal. There are few moving parts in the system, and therefore operation oversight is not as much of a concern. Depending on the required performance levels, the annual cost of operating a Sulfur-Rite ® system can vary between $5,000 and $24,000.
In the case of the iron sponge process, the basic design consists of the uniform distribution of an iron oxide hydrate within a supporting material such as wood shavings or wood chips. In essence, the sour hydrogen sulfide gas is passed through the medium, combines briefly with the water and ferric oxide of the material to form an iron sulfide, and water. The spent material may then be regenerated by introducing oxygen into the systems, which convert the sulfides back into the original iron oxide form and elemental sulfur. This process is repeated until the iron sponge material cannot convert the hydrogen sulfide gas into iron sulfides.
The development and refinement of this process that led to the introduction of a wood chip media offer a number of advantages. The first advantage was that the increased porosity led to a reduction of the potential for pressure drop difficulties. In addition, it was discovered through experimentation that the most effective crystalline structures included the alpha and gamma structures. In addition, a number of manufacturing techniques were developed to bind the iron oxide hydrate to the substrate media. This provided a relatively high quality, uniform iron sponge media, which allowed the focus to shift towards the optimization of the system equipment.
Liquid oxidation catalyst (Lo-Cat ® )
In the case of the Lo-Cat ® system design, there are essentially four main types. These include an aerobic design, an anaerobic design, an auto circulation design, and one designated as a mini-cat design. Each follow essentially the same basic set up, but there are some notable differences.
In the aerobic system design, the hydrogen sulfide gas enters the absorber vessel, where it makes contact with the Lo-Cat ® catalyst solution. The resultant chemical reaction produces elemental sulfur, which is then filtered out. Oxygen is continuously introduced into the system, allowing the catalyst material to regenerate and be used again. Once the hydrogen sulfide is removed from the influent air stream, the resultant effluent is released into the atmosphere.
In contrast, a Lo-Cat ® anaerobic design system separates the absorber and oxidizer vessels, making the product possible. The hydrogen sulfide that is removed in much the same process, but the spent material is then circulated into the oxidizer tank and regenerated by contact with air.
The auto circulation system, however was developed to take the advantages of both the aerobic and anaerobic systems and combine them. In these systems, the oxidizing and absorption tanks are combined, and have been shown to be a cost effective treatment for anaerobic, non-explosive gas streams. Once the hydrogen sulfide is removed, both the oxidizing stream and the resultant sweet gas are discharged into the atmosphere. Because the chemical reactions take place in a single vessel, there is no need for catalyst circulation pumps, and the need for high concentration of catalyst material is lessened.
The most recent process design development in the Lo-Cat ® series is the development of the Mini-Cat ™. It was designed to treat smaller sulfur loads of equal to or less than 200kg/day. .According to information provided by the manufacturer, this latest installment in the series will provide a reduction in chemical cost of approximately 80%, will continuously remove hydrogen sulfide from the gas stream without unexpected “breakthroughs” and will eliminate the need for reactor change outs.
The Sulfur-Rite ® process design is based on a lead-lag configuration. The product is an iron-oxide media impregnated onto a ceramic base, with supplemental chemicals added to aid in the reaction process. It is a dry, free flowing granular material that is non-hazardous in both its fresh and spent forms, exhibits a relatively low pressure drop during operation, and also requires a minimal amount of operational oversight.
The majority of safety concerns that stem from hydrogen sulfide relate to its effect on the physical body through inhalation, contact with the eyes, and skin. The majority of organ systems are susceptible to the effects of hydrogen sulfide, and it is therefore considered to be a broad spectrum toxicant. For example, organs such as eyes and the nose, which have exposed mucous membranes or those with a high demand for oxygen, such as the lungs or the brain are the main targets of hydrogen sulfide gas. Its effect on the body is very similar to hydrogen cyanide, essentially interfering with the cytochrome oxidase as well as the aerobic metabolism. Hydrogen sulfide essentially blocks respiration on the cellular level, leading to depravation of oxygen and eventual death on the cellular level. In general, the human body can detoxify itself by oxidizing the hydrogen sulfide into sulfate or thiosulfate either by the use of hemoglobin or liver enzymes, but there is a limited amount of hydrogen sulfide concentration that the body can handle before becoming overwhelmed. In most cases, an exposure to 100 ppm of hydrogen sulfide gas is considered to be immediately dangerous to life or health by NIOSH standards. In most cases, the characteristic odor can be detected at 5ppm, and physical effects of the hydrogen sulfide gas can be observed at concentrations as low as 9 ppm. Since the physical effects can include loss of eyesight, dizziness, nausea, loss of consciousness and even death, maintaining adequate protection against over exposure of hydrogen sulfide gas is essential.
While using an iron sponge unit for the removal of hydrogen sulfide gas can be an effective method, there are some signficant safety concerns inherent in the process. The first is that the iron sponge material is highly flamable and proned to combustion, especially in it’s spent form. It has the potential to ignite on contact with moisture, and can exhibit a flare buring effect. In addition, it has been known to decompose explosively when heated or involved with a fire.
If the iron sponge material were to ignite, the resultant fire would also have a number of safety and health concerns associated with it. The first would be the production of irritating, corrosive or toxic gases. In addtion, the decompostion produces, when inhaled can lead to serious injury or death. Dermal contact with the iron sponge material has also been shown to cause severe burns.
Liquid oxidation catalyst (Lo-Cat ® )
In most cases, the safety concerns that stem from the utilization of a Lo-Cat ® hydrogen sulfide removal system relate to the disposal of spent materials. In most cases, the resultant elemental sulfur can be shown to be inert and to have little effect on the safety of the workers or environment. However, because the use of oxygen in the regeneration process is often called for, careful monitoring of the oxygen content would be warranted. As with any use of oxygen, an oxygen enriched atmosphere can increase the hazard of explosion.
Many of the safety concerns illustrated by iron sponge are reflected in the Sulfur Rite process. Since both systems make use of iron oxide in their mediums, the probability of flammability and combustibility is somewhat increased. In addition the resultant fire would again have the potential to produce substances hazardous to human health.
Based on the available literature, the most cost effective and safest way to remove hydrogen sulfide from a petroleum development or refinement system various greatly on the scope of the project, the amount of hydrogen sulfide, the regulatory restraints, and the overall time frame in question. The three methods of removal discussed here, namely the iron sponge method, the Lo-Cat ®, and the Sulfur-Rite ® all show significant advantages, and disadvantages depending on the nature of the project. Overall annual operational costs vary significantly between roughly $15,000 and $80,000. In addition, the costs of permits, disposal, and transportation of material should be factored into the overall cost of each program.
Agency for Toxic Substances and Disease Registry (2006) “Hydrogen Sulfide CAS# 7783-06-4 Fact Sheet”
ARTC (2008) “Merichem Gets the Sulfur Out in Indonesia” ARTC 11th Annual Meeting March 2008
Douglas (unknown) “Hydrogen Sulfide Oxidation by Napthoquinone Complexes the Hiperion Process” Ultrasystems Engineers and Constructors, Incorporated Irving, California.
Eastern Research Group (2008) “Technical Analysis Report for Air Quality Control Construction Permit No. AQ0181CPT06 BP Exploration (Alaska), Inc. (BPXA) Endicott Production Facility, Liberty Development Project” Alaska Department of Environmental Conservation Air Permits Program
EPA (1993) “Report to Congress on Hydrogen Sulfide Air Emissions Associated with the Extraction of Oil and Natural Gas.” EPA-453/R-93-045, October 1993. ” p.III-4.
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Matthiasdottir, K. (2006) Removal of Hydrogen Sulfide from Non-Condensable Geothermal Gas at Nesjavellir Power Plant” Department of Chemical Engineering, Lund Instituted of Technology, Lund, Sweden, June 2006
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Skrtic, Lana (2006) “Hydrogen Sulfide, Oil and Gas and People’s Health” Submitted in partial fulfillment of Master’s of Science Degree Environmental Resources Group University of California, Berkley
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