May-2024

Sulphur reduction, sulphur removal, and spent caustic reuse

Demand for flexible sulphur recovery solutions is increasing, including wet air oxidation, deep neutralisation, incineration, and processes that need sustainable strategies.

Richard Stambaugh
Merichem Technologies

Article Summary

More than a hundred different crude oils are traded on the market today. The most desirable crudes are light and sweet, containing less than 0.5% (by weight) sulphur. They are highly efficient, leave minimal residue in the refinement process, and the low density makes them easy to distil and transport. Demand for light, sweet crude grades rose exponentially across Europe, Africa, and the US as refineries adjusted their slates ahead of the International Maritime Organization (IMO) 2020 lower sulphur cap.

In contrast, heavy, sour crudes contain more than 0.5% sulphur. They are generally extracted from the Gulf of Mexico, Mexico, South America, Canada, and the 24 OPEC Member Nations. Sour crudes are more complicated to refine into lower-sulphur fuels, and the refiners who can process them have a distinct advantage over those who cannot, particularly considering IMO 2020. The impetus has been on refineries to determine the best methods of separating sulphur compounds from sour crude. In doing so, US refineries can take in almost any crude oil and turn it into clean-burning fuel.

The problem
Sitting midway to 2030 net zero goals, it has become clear to scientists and environmentalists that incremental and fragmented change is insufficient to achieve the Sustainable Development Goals (SDGs) in the seven years remaining. Meeting (net zero goals) requires transformative and accelerated actions, outlined in the 2023 Global Sustainable Development Report (GSDR), a new phase of accelerated progress by UN member states towards the Sustainable Development Goals leading up to 2030. It outlines the shifts required to transform the energy sector, and the increasingly stringent fuel specifications have made crude oil desulphurisation obligatory.

The sulphur-containing compounds in sour crudes are among the most undesirable. Hydrogen sulphide (H2S) is toxic to humans and particularly dangerous because, at sufficiently high concentrations, it paralyses olfactory nerves even when exposed to it for only a few minutes. According to the National Institute for Occupational Safety and Health (NIOSH), H2S environmental concentrations of 100 ppm are immediately dangerous to life or health. Concentrations greater than 500 ppm can cause a person to collapse within five minutes, and concentrations exceeding 700 ppm can cause immediate collapse and death within just one or two breaths.

Spent caustics
When the malodorous and potentially dangerous sulphides and mercaptans are expended from sweetened hydrocarbons, the residual caustics are considered spent. The complexities and numerous processes involved in treating spent caustic are challenging. It is one of the most problematic industrial wastes in terms of disposal. When managed poorly, spent caustic can cause significant harm to health and the environment. Since 1997, the Environmental Protection Agency (EPA), under the Resource Conservation and Recovery Act (RCRA), has classified spent caustics as hazardous waste, with strict handling and disposal requirements because of the high levels of contaminants.

Spent caustics typically have a high pH value (>12) and high sulphide concentrations (2-3% w/w), not accounting for the phenols, mercaptans, amines, or other organic compounds that are emulsified in the caustic. Spent caustic is classified as an EPA Hazardous Waste Number of DOO3, hazardous waste under the US Resource Conservation and Recovery Act (RCRA), and a Class 8 (corrosive) hazard throughout

United Nations (UN) participating countries use the UN Globally Harmonized System of Classification and Labeling of Chemicals (GHS), which is defined as substances that can cause significant harm to living tissue and/or corrode steel and aluminium upon contact. Spent caustic is also on the Right to Know Hazardous Substance List cited by the Occupational Safety and Health Administration (OSHA), American Conference of Governmental Industrial Hygienists (ACGIH), Department of Transportation (DOT), National Institute for Occupational Safety and. Health (NIOSH), The National Fire Protection Association (NFPA), and Environmental Protection Agency (EPA) – and is listed on the Special Health Hazard Substance List.

Until the 1960s, most hazardous waste was dumped in waterways, which caused pollution perilous to ecologies and drinking water. Oil companies rectified the problem by developing deep well injection sites to safely dispose of liquids deep underground beneath aquifers to trap the liquid waste under multiple impermeable layers of rock.

The severity of the effects of mismanaging spent caustics is well documented.
In 2006, an oil trading company produced spent caustic aboard a cargo ship that refined coker naphtha to mix with gasoline to sell as fuel in the West African market. It generated more than 500 cubic metres of toxic waste in the ship’s waste tanks. After being turned away by several countries, the company had the waste illegally dumped in Côte d’Ivoire. Shortly afterwards, tens of thousands of Ivorians suffered extreme health issues, and authorities recorded 15 deaths – all of which caught the attention of Amnesty International and Greenpeace.

In 2007, a spent caustic tank exploded in a major Midwestern US refinery. The explosion occurred while the operators were transferring spent caustic into the atmospheric-relief cone roof storage tank that had been in service since 1956. There were no injuries or fatalities, but property damage was extensive enough to interrupt the spent caustic disposal process. Unit operation was eventually restored by commissioning an alternative, inherently safe, spent caustic storage system.

Public policy solution
The OSHA, a department of the US Department of Labor, requires chemical manufacturers, distributors, or importers to provide applicable Safety Data Sheets to communicate information and risks on hazardous materials. In 1974, US refineries and petrochemical complexes were mandated to treat spent caustic to prevent adverse environmental impacts and meet environmental regulations.

The European Union (EU) developed a chemicals policy in the late 1960s that was regulated at the organisational level rather than within its 27 individual countries. The policy was overhauled in 2007 with the launch of the Registration, Evaluation, and Authorization of Chemicals (REACH), which united chemicals regulation in Europe into a single system.

Canadian provincial governments play a role in the country’s environmental regulation, with the province of Ontario leading in toxics reduction legislation. Its Toxics Reduction Strategy helps protect the health and environment by reducing toxic substances in air, land, water, and consumer products. In 2009, the Ontario Legislature passed the Toxics Reduction Act, the cornerstone of the Toxics Reduction Strategy, which requires regulated facilities to track and quantify toxic substances they produce and develop plans to reduce their use and creation.