PTQ Q3 2023 Issue

Steam, o-gas, nitrogen, air, water Caustic Sulphuric acid Brine Acid oils

PDC

O-gas

Acid oils

Nitrogen

Scrubbing phase separation

Neutralisation

Process water

Oxidation

Cooling water

Alkaline solution

Oxidation air

Fresh caustic

Cooling water

MP steam

Neutralised brine

Cooling water

Spent caustic

Sulphuric acid

Figure 1 Mericon I is best suited for sulphidic spent caustics treatment with less demanding COD specs

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 mercap- tide-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 technol- ogies are reflected in the advantages. As shown in Figure 1 , Mericon I is best suited for sulphidic spent caustics treatment with less demanding COD speci- fications. 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, includ - ing naphthenic and phenolic caustics, to low COD speci- fications. 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, naph- thenics, and mercaptides to varying degrees, depending on oxidation temperature ( Figure 2 ). Mericon II effectively treats sulphidic caustics at low

pressures and temperatures and treats mixed refinery caustics, including naphthenic caustics, at higher pressures and temperatures, which leaves minimum contaminants in the final brine. Even when all COD is not removed, any toxic organics are usually degraded sufficiently to be no longer be toxic to downstream bacterial treatment. As shown in Figure 3 , Mericon III treats unregenerated sulphidic, mercaptide, and naphthenic spent caustics most appropriately. Instead of destroying contaminants via oxidation, it reforms the original impurities by acidifying the spent caustics at low pressures and temperatures. It releases most high COD impurities from caustic, thereby reducing COD by 90-95%. The technology can recapture organic compounds, naphthenics, and phenolics if required. The flexibility of changing the acidification pH during oper - ation can quickly and responsibly manage varying com- positions and quantities of spent caustic, especially the removal of naphthenates and phenolics in caustic streams. Beneficial reuse Increasingly stringent environmental regulations have stim- ulated innovative management strategies for spent caustics with a focus on reuse. Spent caustics can be beneficially reused, without reclamation or manifestation as hazardous waste, in industrial processes when reused as feedstock substitutes for commercial products. Beneficial reuse has earned its place at the top of the waste management hierarchy and has become a prior- ity method of handling caustic materials. Under the US Environmental Protection Agency’s (EPA) beneficial reuse regulations, a waste material can be reused as feedstock for new products or as a substitute for commercially available products if certain regulatory requirements are met. When such conditions are satisfied, beneficially reused spent caustic becomes a valuable commodity excluded from the EPA’s solid waste definition. Not only does it reduce the

PTQ Q3 2023