Overcoming the complexities of spent caustic treating
Case studies focus on sustainable solutions for treating highly alkaline spent caustic refinery waste with widely available carbon dioxide
Vilas G Gaikar, K V Seshadri and Vaibhav B Kamble Institute of Chemical Technology
S ulphur compounds such as mercaptans, thiols, and thiophenols are usually first removed in the sweet - ening process from petroleum products by extraction using aqueous alkaline solutions, which generates caustic waste. The sulphidic spent caustic comes from scrubbing of liquefied petroleum gas (LPG) and pentane from a fluid catalytic cracker (FCC) and crude distillation unit (CDU). Naphthenic spent caustic comes from the treatment of kerosene, diesel, and jet fuels, while cresylic spent caus - tic comes from the treatment of vis-breaker gasoline. The major volume of the spent caustic is produced in the treat - ment of kerosene. The multiple spent caustic streams from different units finally add up to a mixed spent caustic that holds sulphidic, naphthenic, and cresylic compounds at significantly high concentrations. The undissolved organics and/or grease may also appear in the spent caustic because of incomplete phase separation in the earlier processes. Catalytic mercaptan Merox oxidation technology, used globally, is based on a special UOP catalyst to accelerate the oxidation of mercaptans to disulphides at optimum operating conditions in an alkaline environment. The disul - phides usually form a separate oil phase from the remain- ing aqueous alkaline solution, and the separated caustic is recycled to the reactive extraction operation. Between the extraction and oxidative regeneration cycles, the concentration of sodium hydroxide (NaOH) depletes as it reacts while components with stubborn resistance to oxi - dation, such as phenols and cresols (as sodium salts), emul - sified naphthenates, and catalyst residuals, accumulate along with inorganic sulphides, thiosulphates, carbonates, and Fe+2 precipitates, needing frequent purge. 1 These spent caustic waste solutions usually have a large free alkalinity (caustic concentration in the range of 2-15% with pH~13) with appreciable concentrations of sulphides, phenols, mercaptans, amines, naphthenic acids, and/or other acidic organic compounds. The chemical oxygen demand (COD) values of the spent caustic effluent exceed 100,000, particularly those with cresylic and naphthenic acids. In a few cases, as much as 3% phenolic content has been seen in the spent caustic. 2,3 Spent caustic treating The complexities of processes involved in treating the spent
caustic are challenging. It is one of the most problematic industrial wastes in terms of disposal because it is difficult to deal with by biological treatment and wet air oxidation. Phenols, beyond specific concentrations, are highly inhibit - ing compounds in the metabolism of micro-organisms. The naphthenate salts may result in excessive foaming and/or emulsification of a substantial amount of the oil phase. The physical separation of disulphides from the alkaline aqueous stream is poor because of an exceedingly small difference in the densities of the two phases. For example, di-methyl-disulphide, di-ethyl-disulphide, and ethyl-propyl-disulphide have specific gravities of 1.057, 0.992, and 0.964, respectively. 3 Thus, aqueous solutions tend to carry some colloidal oil phase (1-2%) in a finely dispersed form, contributing sub - stantially to higher values of COD of the waste stream. The toxic components of the spent caustic must be reduced before the effluent can be subjected to conventional treat - ment facilities, and the cost of the pretreatment depends on the nature of the impurities. If these compounds are recov - ered by some physical means with/without chemical treat - ment, it would bring down the impact of these compounds on the environment and ease the treatment of residual COD in conventional biological waste treatment plants. The neutralisation of free alkali by a mineral acid to drop the pH of the spent caustic is a straightforward choice. However, deep neutralisation to pH level <3 generates a large volume of mal-odoriferous gases containing hydro - gen sulphide (H2S) and volatile mercaptans, which must be treated in an auxiliary unit that generates a secondary aqueous waste. Selection of the appropriate metallurgy is also required to protect the equipment from severe corrosion at low pH con - ditions. Restricting the pH above 8.0 can avoid the forma - tion of acidic gases. Waste incineration is rarely employed because of its high energy consumption and the release of hazardous compounds such as furans and dioxins. Wet air oxidation (WAO) of phenols requires a minimum tempera - ture of 200ºC. High-temperature WAO at 240-260°C can oxidise all phenols and reduce the COD, producing a biodegradable effluent. To reduce the formation of copious amounts of ferrous iron sludge in the Fenton oxidation process, new
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PTQ Q1 2025
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