Feb-2025

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

Article Summary

Sulphur compounds such as mercaptans, thiols, and thiophenols are usually first removed in the sweetening 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 caustic comes from the treatment of vis-breaker gasoline. The major volume of the spent caustic is produced in the treatment 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 disulphides usually form a separate oil phase from the remaining 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 oxidation, such as phenols and cresols (as sodium salts), emulsified 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 inhibiting 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 substantially 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 treatment facilities, and the cost of the pretreatment depends on the nature of the impurities. If these compounds are recovered by some physical means with/without chemical treatment, 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 hydrogen 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 conditions. Restricting the pH above 8.0 can avoid the formation 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 temperature 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 homogeneous and heterogeneous catalysts have been recommended. However, the process works to its best under acidic conditions, and neutralising caustic still becomes a prerequisite.⁴

Treating oxidation-resistant compounds
In our quest for developing sustainable solutions to environmental issues of petroleum refining, in the current work, as a sustainable approach for treating spent caustic, the use of carbon dioxide (CO2) is proposed if the major components of the spent caustic are oxidation-resistant phenolic or cresylic compounds.

CO2 is widely available as a waste gaseous stream in manufacturing industries and petroleum refining. CO2 reacts with the excess alkali of the spent caustic effluent in an acid-base reaction, forming sodium carbonate (Na2CO3)and reducing the pH below 9. Being more acidic than phenolic compounds, CO2 reacts further with the alkali metal salts of phenols and/or thiols. Thiophenol (pKa~6.7) salts may not react completely, while naphthenate salts will not react at all with CO2.

The phenol(s) and thiols, generated in the neutralisation reaction, separate as an oil phase from the spent caustic. Surprisingly, no neutralisation of alkaline streams with CO2 is practised in refineries, although the use of CO2 for the pH adjustment of spent caustic before WAO has been suggested.⁵ The pH adjustment was limited to 10, which was not enough to separate phenolic compounds from the spent caustic.

For recovery of phenols, the pH must be brought at least one unit below the pKas of cresols/phenols. The treatment of Merox unit spent caustic with CO2 gave a 65% COD reduction when the pH decreased below 9.0, while the second stage of solvent extraction gave an overall 90+% COD reduction. The process is controlled by the kinetics of mass transfer of CO2 into aqueous solution.

Figure 1 shows a schematic of a laboratory setup consisting of a stirred vessel equipped with a six-blade Rushton turbine agitator running at 1,500 rpm and a flow meter to measure the amount of CO2 sparged just below the impeller. The exit gaseous stream from the stirred vessel was connected to a packed column with a provision for spraying an alkali solution (1% w/v) to chemically absorb any acidic gases that might be liberated in the reaction or stripped off from the solution by CO2. Additionally, residual CO2 in the exit gas reacts with alkali in this column. For comparison, the spent caustic waste was also neutralised using mineral acids with continuous pH monitoring, initially to a neutral pH and then to a pH below 3.