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About half of the product from iron-based high-temperature Fischer–Tropsch synthesis is an aqueous product containing dissolved oxygenates. Volatile oxygenates can be recovered by distillation, but the bulk of the carboxylic acids remain in the water, which is called acid water. Fractional freezing was explored as a process for producing a more concentrated carboxylic acid solution from which the carboxylic acids could be recovered as petrochemical products, while concomitantly producing a cleaner wastewater. Solid–liquid equilibrium data were collected for aqueous solutions of acetic acid, propionic acid, and butyric acid. A synthetic Fischer–Tropsch acid water mixture (0.70 wt% acetic acid, 0.15 wt% propionic acid, and 0.15 wt% butyric acid) was prepared and the liquid phase concentrations of the acid species at solid–liquid equilibrium were determined. Control experiments with material balance closure on each of the carboxylic acid species were performed at selected conditions. Having more than one carboxylic acid species present in the mixture meaningfully changed the solid–liquid equilibrium versus temperature of the system. The carboxylic acids partitioned between the solid phase and the liquid phase and a practical design would require multiple duty-controlled solid–liquid equilibrium stages, with most of the separation taking place in the temperature range 0 to − 5 °C.

Fischer–Tropsch synthesis produces roughly equal amounts of water and oil as products. The water product is acidic due to dissolved carboxylic acids. Treating this acid water in small-scale facilities designed for remote locations is a challenge, because conventional biological treatment might be impractical. Kolbe electrolysis was investigated as potential process for acid water treatment in such small-scale facilities. The Kolbe electrolysis of acetic acid in dilute aqueous solution produced methane, ethane and CO2 as expected. However, current efficiencies in the operating range 10–30 °C were less than 3% and the energy required for acetic acid conversion was >2 GJ/kg acid. Hydrogen was co-produced by water electrolysis and the overall current efficiency of acetic acid and water electrolysis was of the order 20%. Despite being technically viable, it appeared unpromising compared to alternative electrolysis processes for use in small-scale remote Fischer–Tropsch facilities.