Sep-2023

Spent caustic treatment and reuse

Advances in environmentally friendly technologies, such as spent caustic treatment technologies, are key to the industry’s transition towards environmental sustainability.

James G Aiello
Merichem Company

Article Summary

Over the past 50 years, the oil industry has faced increasing reputational challenges across a wide range of stakeholders, including customers, employees, suppliers, shareholders, the communities in which they reside and serve, and the general public at large. With the heightened focus on a greener environment, the oil industry’s gifts to the world of ample, reliable, and affordable supply have come under question.

The processes and systems involved in oil production and distribution are highly complex. As the pressure to act builds, the technologies built around its provisions have become more innovative and imperative. In an industry that is not inherently sustainable, there is an impetus to change and reimagine its processes, including its waste streams.

Spent caustics
In refinery and midstream processes, caustic, an aqueous sodium hydroxide solution, removes impurities such as environmentally undesirable sulphur compounds and carbon dioxide (CO₂) from various liquid streams. The discharged spent caustic solutions, which generally include sodium or potassium hydroxide, water, and acidic contaminants, can contain contaminated waste products, in addition to sulphide minerals, carbonates, residual caustic, pH, salinity, and in many cases, heavy organic substances.

Mercaptans and H₂S naturally occur in crude oil and natural gas and are also produced by the microbial breakdown of organic materials. In midstream processing, the various gas and liquid hydrocarbons are separated and then washed with caustic as needed, which removes contaminants and improves the end products.

Spent caustics are one of the most problematic industrial wastes for disposal. When managed carelessly, spent caustic can be detrimental to health and the environment.

The serious consequences of mishandling spent caustics came glaringly into world view when, in 2006, an oil trading company produced spent caustic aboard a cargo ship when it refined coker naphtha to mix with gasoline to sell as fuel in the West African market. It generated more than 500 cubic meters 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 January 2020, the International Maritime Organization (IMO), an arm of the United Nations, established regulations that cap the sulphur content in marine fuels. Known as IMO 2020, it banned the use of fuels with a sulphur content greater than 0.5% globally with the aim of reducing sulphur emissions that are heavily produced by standard variations of marine fuel. Refiners around the world were then required to produce higher volumes of IMO-compliant low-sulphur fuel oil, as well as more valuable products like marine gas oil and diesel.

A domino effect of IMO 2020 was increased pressure on the handling and disposal of spent caustics. The cost of reclamation had begun to exceed the original cost of materials, so most reclamation facilities were reconfigured into disposal facilities. In the US, deepwell injection, which accounted for the disposal of more than 85% of toxic wastes from refineries, was grandfathered into environmental protection clauses.

Deepwell disposal was considered reasonably inexpensive and easy, but environmentalists soon criticised the practice when contaminants seeped into public places and water sources in South Florida, California, Oklahoma, and Louisiana. In 2012, there were more than 680,000 underground waste and injection wells nationwide, and federal regulators acknowledged they did not know how many of the sites leaked.

There became a fierce urgency to explore other ways of disposal that did not have detrimental effects on human health and the environment. Over the years, various research has been conducted on technologies for handling spent caustics, which includes those that reduce the consumption of caustic and the regeneration and reuse of the caustic solution.

On-site caustic treatment technologies
There are treatment technologies that can regenerate spent caustics for reuse depending on the contaminant. Regeneration is optimum for mercaptide-rich caustics and less so for sulphide-rich caustic streams. For mercaptide-rich caustic streams, regeneration allows returning a lean caustic stream for additional mercaptan removal. Gravity separation, solvent washing, gas stripping, and adsorption may be employed to maximise the removal of disulphide oil (DSO) from the regenerated caustic.

The removal of DSO, which results from  mercaptan oxidation, is required to meet low sulphur specifications in the treated LPG. This type of technology is ideal for high sulphur coker and FCC LPG streams and LPG products requiring less than 5.0 wppm total sulphur. The mercaptide-rich caustic is regenerated and returns a lean caustic with minimal DSO content to treat the LPG to the ultra-low sulphur levels required in the treated product. Ultimately, regeneration consumes far less fresh caustic, which results in far less spent caustic requiring disposal.

Other on-site treatment technologies can process spent caustics for BOD/COD reduction, odour control, and/or pH adjustment when regeneration is not viable. These employ either oxidation or acidification along with the option of final neutralisation to yield a neutral brine effluent stream acceptable for disposal in wastewater treatment facilities and evaporation ponds. These proprietary technologies are called Mericon I, II, and III, and the variations in the technologies are reflected in the advantages.

As shown in Figure 1, Mericon I is best suited for sulphidic spent caustics treatment with less demanding COD specifications. It oxidises sulphides to sulphate and thiosulphate and oxidises mercaptides to disulphide oils. The chemical oxygen demand (COD) is reduced by 75%, and because it operates under low pressure and temperature, stainless steel can be used, which reduces capital expenses. It also uses plant air, so no dedicated air compressors are required, thereby reducing operating expenses.

A non-catalytic wet air oxidation (WAO) process, Mericon II, treats all mixed refinery and ethylene caustics, including naphthenic and phenolic caustics, to low COD specifications. This technology oxidises spent caustics at high pressures (30-100 barg) and temperatures (200-300°C) to reduce COD by 98+%. It also oxidises all sulphides and thiosulphates to sulphates and destroys phenolics, naphthenics, and mercaptides to varying degrees, depending on oxidation temperature (Figure 2).