PTQ Q3 2024 Issue

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

T he oil and gas industry has always provided prod- ucts and advantages to society and has come a long way in processing and manufacturing those bene- fits. 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 sys - tem 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 indus- try, including crude and continuing into finished products, both liquids and gases. Crude oils contain all kinds of sul - phur 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 suffi - cient 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 con - taminants can cause corrosion of equipment unless the liq - uid is adequately dehydrated. H₂S is sufficiently corrosive at concentrations of more than 0.55 ppmw to cause lique - fied 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 H2 S and elemental sulphur are present, the threshold numbers for copper strip failure go down signif- icantly (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 H 2 S reac- tion 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 H 2 S 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 SO 2 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 pol - lution 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 mon - itoring and modelling, emission inventories and control strat- egies, and the policies and procedures for enacting them. Elements of the Clean Air Act have been reviewed and

PTQ Q3 2024