Abstract
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.
Highlights
Fischer–Tropsch synthesis is an industrially proven pathway for converting synthesis gas, i.e., H 2 and CO, to synthetic oil [11]
As was explained in the Experimental section, a fresh solution was prepared with 2.6 wt% acetic acid, which was cooled down to − 5.00 ± 0.02 °C to obtain the solid–liquid equilibrium measurement point
The results indicated that acid partitioning between the liquid and solid phases was such that fractional freezing had poor selectivity for concentrating a mixed acid solution at − 25 °C
Summary
Fischer–Tropsch synthesis is an industrially proven pathway for converting synthesis gas, i.e., H 2 and CO, to synthetic oil [11]. The composition of the oil can be manipulated through the Fischer–Tropsch synthesis. This provides flexibility in application, making Fischer–Tropsch synthesis a useful process for both transport fuel and petrochemical production [5, 6]. When Fischer–Tropsch synthesis is applied to petrochemical production, the emphasis is on olefins and oxygenates, since these classes contain major commodity petrochemicals, ethylene, propylene, methanol, ethanol and acetic acid, among others. These petrochemicals are most abundant in the product from iron-catalyzed high-temperature Fischer–Tropsch synthesis.
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