PTQ Q1 2025 Issue

ILs tend to have high viscosities at ambient tempera- tures, which can limit mass transfer rates, although the high removal efficiencies result from a large mass transfer driving force. The adsorption of organics from spent caustic solutions on bleaching earth, alumina, and petroleum coke has been reported under alkaline conditions.3 Bleaching earth could reduce COD values by 60%, prob- ably by ion exchange and/or intercalation of the organic ani- ons between layered structures. However, alumina-based adsorbent reacted chemically with alkali, resulting in adsor- bent bed collapse in the column. With petroleum coke, there was no removal of COD com- ponents from the spent caustic. Instead, the spent caustic desorbed other organic compounds, increasing the COD values. It is not surprising due to the ionic nature of organic compounds under strongly alkaline conditions. The result- ing contribution to the COD does not allow their adsorption on coke. When pH is reduced and phenolic compounds are converted into a molecular form in the present studies, their adsorption on activated carbon improves significantly. Conclusion The treatment of highly alkaline waste streams using CO 2 provides a sustainable solution. The phenolic compounds are removed by bringing the pH of the solution below pKas of phenol(s) and efficiently separating the oil phase. (In molecular form, phenol and cresylic acids have poor solu- bility in water). The phase separation needs to be improved by a solvent extraction step, using an organic solvent that can dissolve the separated phenolic oil. However, neutralisation by CO 2 is restricted to free alkali and alkali salts of phenols. Thiophenols and all naphthenic acid(s) remain in the solu- tion because of their stronger acidic nature. The limitation of slower solubilisation of CO 2 due to its poor solubility in water can be overcome using a suitable unit to increase mass transfer area between gas and liq- uid phases, allowing maximum CO₂ use in the process. It is also possible to use gaseous effluent from thermal energy plants or refineries, having CO₂, nitrogen (N₂), nitrogen oxide (NOx) and sulphur dioxide (SOx), depending on fuel type. However, if SOx and NOx are present, they will react faster than CO₂ and further reduce the pH. The risk of H2S evolution remains in such a case if pH drops to an acidic

level, demanding a secondary treatment unit for the cap- ture of H 2S. In refineries, the spent caustic stream is mixed with other aqueous solutions, diluting the spent caustic effluent. If the spent caustic from the kerosene treatment unit, with its remarkably high load of phenolics, is treated directly with CO 2 , the process may provide an economic incentive to recover phenols as a valuable product. We believe that using a CO 2 waste stream to treat another waste solution is a sustainable solution. Such positive solu - tions can have a positive and profitable effect across the entire spectrum of the oil industry. References 1 Pino-Cortés, E., Montalvo, S., Huiliñir, C., Cubillos, F., Gacitúa, J., (2020) Characteristics and Treatment of Wastewater from the Mercaptan Oxidation Process: A Comprehensive Review, Processes , 8, 425; doi:10.3390/pr8040425. 2 Rita, A.I., Rodrigues, C.S.D. Santos, M. Sanches, S. Madeira, L.M., (2020) Comparison of different strategies to treat challenging refin - ery spent caustic effluents, Sepn. Purifn. Technol., 253, 117482-97, https://doi.org/10.1016/j.seppur.2020.117482. 3 Czimer, B., Kovács, A., Petró, J., (2015) Treatment of Spent Merox Caustic Waste in Industrial Ecology Frames, Period. Polytech. Chem. Eng. , 59(3), 215-220, DOI: 10.3311/PPch.7601). 4 Roudsari, M. H., Soltani, M., Seyedin, S. H., Chen, P., (2017) Investigation on New Method of Spent Caustic Treatment, J. Multidisc. Eng. Sci. Technol. , 4(6), 7459-7464. 5 Kumfer, B.J., Felch, C.L., Brandenburg, B., Ishmann, R.R., (2014), US patent, 8,734,648 B2, Siemens Energy, Inc., Orlando, FL (US). 6 Doble, M., Kruthiventi, A. K., Gaikar, V.G., (2004) Biotransformation’s and bioprocesses, Marcel Dekker, New York. 7 Sabri, M.A., Ibrahim, T.H., Khamis, M.I., Nancarrow, P., Hassan, M.F., (2018) Spent caustic treatment using hydrophobic room temperatures ionic liquids, J. Ind. Eng. Chem ., 65, 325–333, https://doi.org/10.1016/j. jiec.2018.05.002.

Dr Vilas G Gaikar is Bharat Petroleum Chair Professor. Email: vg.gaikar@ictmumbai.edu.in

K V Seshadri is Adjunct Professor in the Department of Chemical Engineering at the Institute of Chemical Technology, Matunga, Mumbai-400019, India. Vaibhav B Kamble is a post-graduate student in the Department of Chemical Engineering at the Institute of Chemical Technology, Matunga, Mumbai-400019, India.

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PTQ Q1 2025

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