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Environment, Health, Safety & Security
June 1, 2019 | By Christopher Ristevski, Rosanna Kronfli, MacroTek
There are many industrial technologies for removing H 2 S from process gas, and each brings with it different benefits with regard to costs, efficiency and equipment layout
Hydrogen sulfide (H 2 S) is a toxic and corrosive gas that occurs naturally, but can also be produced through many industrial processes. In addition, H 2 S has an odor threshold of 0.01–0.15 parts per million (ppm), according to the Occupational Safety and Health Administration (OSHA; Washington, D.C.; www.osha.gov). Due to the very low odor threshold, in countries where nuisance odor is regulated, H 2 S removal from process gas or off-gas is required. Since H 2 S is produced through anaerobic digestion, it is prevalent where organic matter and sulfates are present. As a result, pipeline gas specifications exist to ensure gas quality. Furthermore, when natural gas, synthesis gas (syngas) or biogas is used in turbines or engines for power generation, H 2 S concentrations cannot exceed the engine manufacturer’s specifications, due to corrosion concerns. During combustion, H 2 S is oxidized to sulfur dioxide — a highly regulated air pollutant — which necessitates its removal before combustion. It is clear that H 2 S removal is important for the environment, industrial equipment integrity and human health.
H 2 S can be removed from process gas through various technologies, depending on the application, process conditions and removal requirements. This article describes selected proven technologies for controlling H 2 S in small- to medium-sized applications, including natural gas production, landfill gas recovery, waste-to-energy systems, biogas production and wastewater treatment plants.
Liquid redox refers to technologies that absorb H 2 S and oxidize it to sulfur in an aqueous system using a catalyst (most commonly chelated iron). The chelated iron converts H 2 S gas into solid sulfur, as shown in the below chemical reactions:
H 2 S + 2Fe 3+ → S + 2Fe 2+ + 2H + (Reduction of iron)
2Fe 2+ + ½O 2 + H 2 O → 2Fe 3+ + 2OH – (Oxidation of iron)
H 2 S + O 2 → S + H 2 O (Overall reaction)
As can be seen, only oxygen is consumed in the reaction. The chelating reagent is not consumed, because it is continually regenerated by forced oxidation using air. Although the reagent is not consumed in the process, a small amount of loss is typically experienced, mainly due to chelate degradation over time. Since solid sulfur is produced, filtration can be used to remove the sulfur and recycle the reagent back into the process. This eliminates or significantly reduces the production of wastewater and the associated costs. The sulfur that is produced can also potentially be sold as a product.
FIGURE 1. In a typical liquid redox system, a catalyst (typically chelated iron) converts H2S gas into sulfur in an aqueous system
The main vessels in a liquid redox system include an absorber and an oxidation vessel (Figure 1). In the absorber, H 2 S is absorbed into the liquid and converted into sulfur. The spent recirculating liquid is sent to the oxidation vessel where contact with air regenerates the solution into its active form. The regenerated solution is re-circulated back to the absorber to complete another reaction cycle. Some systems use a settling tank to concentrate the sulfur before sending it to a filtration system for removal from the process. Typically, 316L stainless steel is used as the material of construction for the process equipment.
Overall, liquid redox systems require minimal water and chemical addition and produce minimal waste. This results in very low operating costs compared to alternative technologies. The systems are also capable of handling large fluctuations in inlet composition and gas flow while maintaining high H 2 S removal efficiencies of greater than 99.9%. The systems are, however, higher in capital costs due to the increased control sophistication that is required. Although most liquid redox systems have very similar overall chemistry, different suppliers use different chelating agents to keep the iron in solution. Some of the chelating agents, such as nitrilotriacetic acid (NTA), are hazardous. Recent advancements in this technology have helped drive down the operating costs through the development of innovative chemical reagents that reduce chemical consumption rates. Similarly, the equipment and installation costs are being reduced due to modularization and skid-packaged offerings.
In a chemical oxidation scrubber, a base, typically sodium hydroxide (NaOH), is used to neutralize H 2 S gas after absorption into the scrubbing liquid. The absorbed H 2 S is then oxidized using a chemical oxidizing agent, typically hydrogen peroxide (H 2 O 2) or sodium hypochlorite (NaOCl), to form soluble sodium sulfate (Na 2 SO 4), as shown in the overall equation below. Na 2 SO 4 is removed from the system through a blowdown stream. This sulfate-containing wastewater (blowdown) must be treated or sent for disposal.
H 2 S + 2NaOH + 4H 2 O 2 → Na 2 SO 4 + 2H 2 O
(Overall reaction with NaOH and H 2 O 2)
FIGURE 2. A chemical oxidation system neutralizes H2S gas using a base, typically sodium hydroxide
Packed-bed scrubbers are the most common process equipment used for chemical oxidation (Figure 2). Vertical countercurrent packed-bed scrubbers are generally preferred due to their high efficiency. Compared to a spray tower, tower packing provides additional mass transfer and therefore smaller equipment size and pumping costs for the same performance.
In the vertical countercurrent design, gas flows upward while scrubbing liquid flows down through spray nozzles or a liquid distributor above the random packing section. The absorption of gases occurs in the packing section. A mist eliminator inside the vessel removes entrained liquids from the gas before exiting the scrubber. A regulated amount of the reagents is added to the re-circulating liquid to maintain setpoint pH and oxidation-reduction potential (ORP) levels. Fiber-reinforced plastic (FRP) is typically the material of choice for the process vessels, particularly if sodium hypochlorite is used as the oxidizing agent.
One of the disadvantages of chemical oxidation is that caustic is not selective to H 2 S in the presence of other contaminants, such as carbon dioxide (CO 2). Not only does CO 2 increase chemical consumption, but the reaction products can cause scaling in the packing. Care must be taken during the equipment design and selection of process setpoints to ensure CO 2 interference is minimized.
Chemical oxidation scrubbers are compact and low in cost. They are also capable of handling large fluctuations in inlet composition and gas flow while maintaining high H 2 S removal efficiencies of greater than 99.9%. The chemicals that are used are commodity chemicals, and therefore, availability is rarely a concern. The rate of chemical consumption can be high relative to the amount of H 2 S that is treated. For example, for every 1 mole of H 2 S, 2 moles of NaOH and 4 moles of H 2 O 2 are required, as shown in the overall reaction. Furthermore, handling the sulfate-containing wastewater stream adds significantly to the operating cost of the scrubber.
FIGURE 3. Biotrickling processes use autotrophic microbes to consume and transform H2S
There are several types of H 2 S removal technologies that use biological or biochemical processes. This article focuses on biotrickling filters, which use autotrophic microbes to consume H 2 S and convert it into sulfuric acid. Biotrickling filters consist of a vessel with a packed or porous media section (Figure 3). The media provides a large surface area for the microbes to grow on. Makeup water is required to maintain the pH, and a blowdown stream of dilute acid is removed from the system. NPK (nitrogen, phosphorus and potassium) fertilizers or other mixtures (such as unchlorinated raw water) are used to provide the nutrients that are required by the microbes. Nutrient addition can be done manually or can be mixed at a fixed ratio with the makeup water. When a biotrickling filter is installed, it may take several days for the biofilm to develop on the media, and during this phase, the removal efficiency will be low. Concrete structures are often used for large gas-flow systems, while FRP can be used for smaller units.
The bacteria are sensitive to humidity, temperature and fluctuating H 2 S inlet loading. Recirculating liquid supplied by a pump, or once-through water, is required to keep the packing or media wet. If the media is allowed to dry, the microbes will become inactive. Furthermore, biotrickling filters require long residence times, resulting in large vessels. Little operator intervention or maintenance is required other than daily checks of pH and media pressure drop.
Scavengers can either be liquid- or solid-phase chemicals that react with H 2 S. Typically, these reactions are irreversible (non-regenerative), resulting in the need to periodically replace and dispose of the scavenger. Depending on the type of scavenger used, byproducts can be hazardous, making disposal costly. The most common liquid scavengers are triazines, which react with H 2 S to form water-soluble sulfur compounds. Common solid scavengers include metal oxides, particularly iron, which react with H 2 S to form sulfides. Regenerative liquid and solid scavengers (such as amines, molecular sieves and so on) are not discussed here, as they do not eliminate the H 2 S. Instead, a waste gas with a high H 2 S concentration is produced during the scavenger-regeneration process, which must be treated or sent for disposal.
Like other H 2 S-removal technologies, most scavengers require dedicated process equipment (Figure 4). In some cases, direct inline injection is possible with liquid scavengers. The process equipment consists of vertical towers where gas flows up through the liquid or media. For liquid systems, the gas is usually bubbled through a liquid-filled absorber vessel. For solid systems, the gas flows up or down through a fixed bed.
FIGURE 4. A scavenger system involves the injection of a liquid or solid chemical to irreversibly react with H2S
Furthermore, since the scavenger reagent is consumed by the reaction, two vessels in parallel are often installed such that one vessel can be taken offline in order to replace the scavenger. Scavenger systems use simple process equipment and require only basic controls, resulting in a relatively low capital cost. On the other hand, operating costs can be high due to high chemical consumption and treatment of waste streams. Carbon steel is often used as the material of construction for the process equipment.
The advantage of scavengers over other discussed technologies is their selectivity of H 2 S over CO 2. However, the disadvantage is that most scavengers can be sensitive to high temperature and require gas with high humidity.
Adsorption of H 2 S is a physical process where H 2 S is captured onto the surface of activated carbon. The media has a large specific surface area due to its inner pore structure, which provides a large adsorptive capacity. Carbon beds are particularly suitable when very low outlet concentrations are required. In addition to the physical process, some activated carbons have catalytic properties that oxidize H 2 S into water-soluble sulfur compounds. This allows the carbon to be washed for regeneration until spent, at which point, disposal is required.
Since the process does not occur in the liquid phase, liquid recirculation, chemical addition and the associated controls are not required, resulting in relatively simple process equipment and operation. A carbon-bed vessel is typically sized based on the carbon usage rate and superficial gas velocity. The vessel configuration is typically vertical, with gas flowing up through the fixed-carbon bed (Figure 5). When the activated carbon is spent, the media must be replaced. Some plants will install two units in parallel for continuous operation. Since the superficial gas velocity is very low through carbon beds (higher superficial gas velocities result in excessive pressure drops), large-diameter vessels are required even for low flowrates. Therefore, carbon beds are often better suited for polishing applications or for low H 2 S loadings, and as such, some plants do not include an installed spare vessel, but instead elect to exceed their H 2 S emissions during the time that they are replacing the carbon.
FIGURE 5. Adsorption of H2S using a fixed-bed activated-carbon process is appropriate when a very low outlet concentration of H2S is required
Activated carbon is sensitive to humidity and temperature, among other parameters, such as particulate matter loading. Water blocks the adsorption sites and, as a result, decreases its effectiveness. Similarly, increased temperature has negative effects on capacity. These systems also work best under positive pressure, because increasing pressure increases the amount of H 2 S adsorbed. The most common materials of construction for carbon beds include coated carbon steel, FRP and other plastics.
Selection of the appropriate H 2 S-removal technology is mainly governed by cost and technical suitability. Table 1 provides a summary of the process features offered by each technology. Generally, for high H 2 S loadings, regenerative or biological technologies are more economically feasible than non-regenerative types due to their lower operating cost per unit of H 2 S removed. This is mainly because of the lower consumption of chemicals or media. Furthermore, non-regenerative technologies are less complex than regenerative systems and the capital cost is consequently lower. Generally, regenerative systems result in short payback periods for applications where the inlet H 2 S loading is more than 50 kg/d. However, the exact breakeven point will vary for each application. For reference, a gas flow of 1,000 std. ft 3 /min containing 850 ppm of H 2 S corresponds to a loading of 50 kg/d of H 2 S.
Most technologies are suitable for applications where high process variability or high turndowns are required, with the exception of biotrickling filters, which require a constant amount of H 2 S in order to maintain the biofilm. This makes biotrickling filters difficult to use in applications with frequent shutdowns or variable inlet conditions unless periodic H 2 S excursions are acceptable. This is why biotrickling filters excel in wastewater treatment plants, but are rarely utilized in other industrial processes.
The choice between the three non-regenerative technologies discussed here often comes down to plant preference and ability to manage the chemical inputs and waste streams. Chemical oxidation scrubbers have the smallest footprint and are continuous processes, while scavenger units and activated carbon beds are larger and must be shut down to replenish the spent media or chemical unless a standby unit is installed. Solid media is also less desirable to handle on a regular basis. Therefore, solid scavengers and carbon beds are typically used for low-H 2 S-loading applications or as a polishing stage to avoid frequent change-out.
In the past, liquid redox systems were best suited for applications with very high H 2 S loadings, particularly for natural gas and landfill gas processing. The high capital cost of a liquid redox system resulted in a technology that was not economically feasible for lower H 2 S loadings. As a result, scavenger systems and chemical oxidation systems became the technology of choice, even though the operating costs are greater. However, emerging technology advancements have resulted in lower capital costs. This has allowed liquid redox systems to become feasible and attractive alternatives in small- to medium-sized application ranges.
Edited by Mary Page Bailey
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Christopher Ristevski currently leads the Process Engineering team at Macrotek Inc. (421 Bentley Street, Unit 1, Markham, Ontario, L3R 9T2; Email: cristevski@macrotek.com). With 10 years of experience, he has a wide range of expertise in air-pollution-control systems, including system integration, process design, process modeling and equipment and controls selection. He has also led the development of innovative new air-pollution-control technologies, such as the SULFCAT process, and is now responsible for the implementation of these systems worldwide. He holds a degree in chemical engineering from the University of Toronto.
Rosanna Kronfli is an applications engineer at Macrotek Inc. (Same address as above; Email: rkronfli@macrotek.com). She joined Macrotek in 2015 and has a wide range of experience in air-pollution-control equipment and process design. She holds bachelor of applied science and master of applied science degrees in chemical engineering, both from the University of Toronto, and is a licensed Professional Engineer with Professional Engineers Ontario.
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