Jul-2024

Sour gas and liquid treatments: Sulphur recovery and removal

Sulphur removal and sour gas processing ensure contaminants are removed from liquid and gas streams.

Cyndie Fredrick
Merichem Technologies

Article Summary

The oil and gas industry has always provided products and advantages to society and has come a long way in processing and manufacturing those benefits. However, in recent decades, there has been a greater understanding of the pollution caused by these processes and some of the solutions to prevent it. A specific focus has been on the removal of sulphur dioxide (SO₂) emissions, a contaminant that can affect health and the environment. SO₂ is formed by the combustion of any fuel containing sulphur. Short-term exposures to SO₂ can harm the respiratory system in humans and is particularly detrimental for children, the elderly, and those with asthma. Higher concentrations of SO₂ can generate reactions with other compounds in the atmosphere to form particulate matter pollution, which, in sufficient quantity, can penetrate deep into the lungs and contribute to more severe health problems.

In the environment, high concentrations of gaseous SO₂ and other sulphur oxides contribute to acid rain, which can create havoc on sensitive ecosystems. When acid rain is introduced into ecosystems and flows through the soil, acid rainwater can leach aluminum from soil clay particles. As the pH declines, acid-sensitive plants and animals that encounter it are compromised.

Sulphur products
Sulphur can be found in many parts of the oil and gas industry, including crude and continuing into finished products, both liquids and gases. Crude oils contain all kinds of sulphur compounds, both inorganic, as in the case of hydrogen sulphide (H₂S), and organic, as in the case of mercaptans and thiophenes. H₂S and mercaptans are highly odorous and volatile sulphur compounds. In the case of H₂S, its presence raises major safety considerations since it is a poison to animal life and humans. When crude oils are light (above 40° API gravity) and contain volatile sulphur in sufficient quantities, they require either treatment to remove the noxious sulphur to allow their storage and transportation or the use of more costly and sophisticated facilities along with special safety and environmental handling procedures.

There are two main reasons for removing acid gas and sulphur compounds from hydrocarbon liquids: to protect the environment by eliminating or reducing the amount of toxic H₂S and/or SO₂ released or formed during combustion and to protect process equipment that comes in contact with sour hydrocarbon liquid streams. In fact, these contaminants can cause corrosion of equipment unless the liquid is adequately dehydrated. H₂S is sufficiently corrosive at concentrations of more than 0.55 ppmw to cause liquefied petroleum gas (LPG) to fail a copper strip test, while 2 ppmw of elemental sulphur can cause failure of the copper strip.¹ If both H2S and elemental sulphur are present, the threshold numbers for copper strip failure go down significantly (ibid).

Mercaptans (RSH) are undesirable in hydrocarbon liquid products primarily because of odour but also contribute to corrosion. Up to 100 ppm mercaptans will not fail a copper strip and may even provide some inhibition to the H2S reaction with the copper strip. However, if elemental sulphur is also present, mercaptans will likely fail the copper strip.

Carbonyl sulphide (COS) and carbon disulphide (CS₂), although not corrosive in dry LPG, will hydrolyse slowly to H2S in the presence of free water, resulting in off-spec products.² If COS is not removed from the feed stream, it will concentrate in the propane stream of a fractionator. Finally, the presence of significant quantities of carbon dioxide (CO₂) can increase the vapour pressure and lower the heating value of the hydrocarbon liquids.³

The Clean Air Act, passed by Congress in 1970, set the National Ambient Air Quality Standards (NAAQS) for SO₂ and other harmful pollutants. NAAQS specifies maximum amounts of SO2 that can be present in outdoor environments. Under the Clean Air Act, the Environmental Protection Agency (EPA) has a mandate to protect society against the adverse effects of acid rain, including harmful ecological effects and consequences to vegetation and other sensitive ecosystems.

The standards set by the EPA are applied or implemented by controlling air pollution from emission sources and have significantly shifted the federal government’s role in air pollution control. However, the responsibility for controlling air pollution still rests with the states, which are required to develop a comprehensive plan to protect their jurisdiction. The plans must include components such as air quality monitoring and modelling, emission inventories and control strategies, and the policies and procedures for enacting them.

Elements of the Clean Air Act have been reviewed and updated over the years. In 2023, the National Environment Protection (Ambient Air Quality) Measure standards for SO2 were set at 0.10 parts per million (ppm) for one hour of exposure and 0.02 ppm for 24 hours of exposure. In April 2024, the EPA proposed replacing the current secondary SO2 standard with a new annual secondary standard of 10-15 parts per billion (ppb).

Technology solutions
Companies requiring sulphur removal solutions have a variety of technology solutions available that can support their goals whether this is for liquid or gas treating. Each type of treatment offers different benefits and options. Liquids treating is done predominantly to remove H₂S, RSH, and COS. Some options such as hydrotreating will remove additional contaminants.
Hydrotreating is a process for refining crude petroleum and producing transport fuels. In this process, a high volume of hydrogen gas is used to remove harmful impurities. As part of the hydrotreating process, sulphur is added to activate the catalyst, which then converts heavy organic sulphur in the oil into H₂S, ending up primarily in the gas phase. The H₂S is then removed by amine contacting and sent to a sulphur recovery unit (SRU). Although very effective at removing heavy organic sulphur, hydrotreating is energy intensive, requires a considerable capital investment, and generates higher operating costs than other treatment options.

Liquid treating
Removing sulphur is required for liquid-phase renewables and downstream, midstream, and petrochemical products. Sulphur species must be removed from a broad range of products before they can be considered saleable. LNG, natural gas liquids (NGL), gasoline, kerosene, and diesel are some of the streams that require treatment. Each of the fuels has differing specification requirements for treating to ensure products meet pipeline and product/sales specifications or other requirements, such as when the product is an intermediate feedstock and downstream catalysts must be protected.

Caustic treatment to remove impurities has been around for more than a century and has been used since the beginning of oil processing. Initial treatment involved heavy mixing of the stream to be treated with caustic, which supported the removal of the sulphur impurities. However, this method often caused significant caustic carryover into the product stream. Over time, technology solutions were developed to improve both the mixing of the streams and product specification attainment and minimise the carryover of the caustic, which can cause issues in downstream equipment or storage. These solutions have continued to be developed, and current versions involve trayed towers, settling vessels, fibre-packed contactors, and additional downstream treatment options, including water washes, sand filters, and other items to ensure the product has little to no caustic carryover.