Managing mercaptans

Numerous sulfur-containing compounds are present in oil and gas streams. Besides the most common and problematic contaminant, hydrogen sulfide (H2S), mercaptans (also known as thiols) are a significant issue in the oil and gas industry due to their toxicity and foul odour. These compounds cause hazards for workers and environmental pollution, as well as severe corrosion in pipelines and storage facilities. Mercaptans pose a particular risk in refineries, as they poison and deactivate the catalysts, which leads to significantly higher replacement and maintenance costs.

Regulatory restrictions have been imposed on oil producers to decrease mercaptan emissions and allay air quality and the public health concerns. Mercaptans levels are closely monitored along the oil value chain process: with new climate disclosure regulations taking effect in various regions, the industry has been compelled to improve its emissions management strategies. This requires the adoption of advanced monitoring technologies and practices to minimise mercaptan emissions.

What are mercaptans?

Mercaptans are forms of hydrogen sulfide where one hydrogen atom is replaced by a hydrocarbon group, resulting in the general formula RSH. Their properties are defined by the length of the hydrocarbon chain, R. Similar to H2S, mercaptans exhibit acidic properties, but the presence of the hydrocarbon group makes them much weaker acids. As the length of the hydrocarbon chain increases, mercaptans exhibit properties closer to its hydrocarbon chain, R, than to those of acids. In the workplace, mercaptans expose a severe health risk: they can be rapidly absorbed through inhalation but to a lesser extent through skin and eye contact. Low-level exposure can lead to irritation of the eye, skin, and upper respiratory tract, as well as symptoms like headaches, vomiting, and dizziness can occur. Higher levels of exposure can rapidly result in more severe respiratory paralysis. From an engineering point of view, mercaptans pose a severe corrosion threat: these chemicals interact with metals and form metal sulfides that lead to corrosion, especially in the presence of moisture. The consequences of corrosion can include damage to production equipment, leaks, spills, pump blockages and loss of productivity. If not properly controlled, mercaptans can cause severe economic problems in many operations in the oil and gas industry.

General properties of mercaptans

Mercaptans display different properties, which are dependent on the chain length of the hydrocarbon. They are known for a really strong unpleasant smell of rotten eggs. They are highly reactive due to the -SH group and form disulfides on oxidation and react with metals to form mercaptides. Boiling points of the mercaptans are very low, particularly the short-chain methyl mercaptan (CH3SH) displays a very low boiling point of 6°C, which results in serious health, safety and environmental problems. Short-chain mercaptans are more soluble in water than longer-chain mercaptans.

Current treatment technologies

The choice of treatment technology depends on various factors, including the concentration of mercaptans, the nature of the hydrocarbon stream, economic considerations, and environmental regulations. Often, a combination of methods is employed to achieve the desired level of mercaptan removal. This article focuses on organic chemical scavengers. It discusses both the advantages of this treatment technique and its limitations. The use of a well-known H2S scavenger for mercaptan reduction is analysed and substantiated by various laboratory tests and field trials.

Organic scavengers – limitations and benefits in scavenging mercaptans

Organic mercaptan scavengers are used in natural gas and oil processing and wastewater systems, for example, to meet pipeline specifications, reduce unwanted odours and avoid corrosion. They are also used in refining to protect catalyst and ensure good fuel quality. In petrochemical production, they are applied to improve the quality of raw materials and end products. Organic scavengers are chemicals that are used to remove mercaptans from environments or processes. With specific technologies, scavengers react irreversibly with the mercaptans to form less harmful or more manageable products. The advantages and limitations of using organic scavengers to remove mercaptans are explained below. Common organic mercaptan scavengers are triazines and oxazolidines, which are often used in the oil and gas industry for gas sweetening and the removal of H2S and mercaptans.

Other scavengers like amines react with mercaptans to form reversible salts and formaldehyde-based scavengers that react with mercaptans to form thioacetals. These types of scavengers show several advantages over other treatment technologies: they can effectively remove low concentrations of mercaptans from gas and liquid streams, resulting in improved product quality and safety. They are designed to react selectively with mercaptans, minimising interference with other compounds in the process stream. Depending on application type and mercaptans to be treated, scavenger can be a viable and relatively inexpensive alternative to mechanical processes.

However, organic scavengers do have their limitations. The success of mercaptan removal can vary significantly depending on the concentration, the carbon chain length, the type of hydrocarbon medium and ultimately, kinetics. Light ends (C1-C4) are known to be easier to treat than heavier chains but there is difficulty in the success of scavengers in particular crudes across the globe even when mercaptan levels are known. In some cases, multiple treatments or higher doses of chemical than necessary may be required to meet customer specs, which can be uneconomical. With large-scale applications, costs of treatment can easily inflate: this includes the cost of the scavenger itself and the associated operating expenses, like transport, handling and storage. Also, formed reaction by-products may require further treatment or disposal, making the process even more complex. Additionally, most organic scavengers can be hazardous, requiring careful handling, storage, and disposal. This can increase operational complexity and costs.

The use of organic scavengers for mercaptan removal is an effective method with numerous advantages, such as a generally simple and quick-to-implement treatment/injection with low capital investment costs. However, there are also limitations such as chemical cost, potentially complicated handling and environmental impact factors. Careful selection and handling of scavengers is crucial to maximise their benefits while minimising the drawbacks.

Stabicor® S 100 (MBO) is an oxazolidine-based scavenger that consists of nearly 100% active ingredient. It contains no water or solvent. It is completely soluble in both water and oil and offers high chemical capacity and fast reaction kinetics. The excellent low-temperature properties of MBO ensure that it remains pumpable even at very low temperatures without the need for external heating. In addition, it is thermally stable up to 160°C and readily biodegradable, resulting in a favourable environmental profile. MBO does not need to be used in high dosages and excess: this significantly reduces storage and transport costs and this is particularly important in environments where space is limited, such as oil platforms. However, MBO is an alkaline product and can be incompatible with certain brines leading to the precipitation of inorganic salts. MBO does introduce nitrogen into the system, but at a significantly smaller amount than the industry standard MEA triazine.

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