Removal of Carboxylic Acids from Fischer Tropsch Aqueous Product
Nuvaid Ahad and Arno de Klerk
Chemical and Materials Engineering, University of Alberta
Fischer–Tropsch is a process where in a gaseous mixture of carbon monoxide (CO) and hydrogen (H2), called syngas, is converted into hydrocarbons, oxygenates and water. It is often simplified as hydrocarbon synthesis: n CO + 2n H2 → (–CH2–)n + n H2O
After the Fisher–Tropsch reaction the reaction product is cooled down and one of the product phases is a liquid stream known as the aqueous product, or reaction water. The water also contains around 2 to 6% oxygenates, mainly light alcohols, carbonyls and carboxylic acids. Most of the dissolved alcohols and carbonyls can be recovered by distillation, resulting in a carboxylic acid containing bottom product, called acid water. It contains 1 to 1.5% v/v C2 – C4 organic acids of which ~85% is acetic acid. The acid water being acidic, due to presence of carboxylic acids, cannot be directly discharged into the water bodies or be used as process water, so it needs to be treated first.
Currently industries are treating the acid water biologically using aerobic or anaerobic systems. Since temperature has considerable effect on the efficiency of biological process, it is being used mainly in warmer climates. For places with colder climates, like Canada, heating of biological treatment systems is energy intensive. An alternative method that benefits from a colder climate is fractional freezing. Fractional freezing as acid water treatment strategy was investigated.
Thermodynamic data for fractional freezing was determined by decreasing the temperature of 1 wt% acetic acid, propionic acid and butyric acid aqueous solutions to their eutectic point. The decrease in temperature was done stepwise. This resulted in freezing of water and thus concentrating the acid solution. For each step change in temperature freezing was continued until solid-liquid equilibrium was attained. The experimentally determined solid-liquid equilibria matched literature data well and could be used with confidence in process design.
A conceptual design where fractional freezing of acid water was followed by distillation was developed. The concentrated acid solution from fractional freezing became feed for distillation. The ice from fractional freezing and the overhead product from distillation were combined to produce the water discharged from the process. The concentrated acid solution produced as by-product from cleaning had petrochemical value. Finally a cost evaluation of this alternative technique was performed and compared to the conventional biological methods currently used by industry. The processes were found to be of comparative economic value.