Removal of Carboxylic Acids from Fischer-Tropsch Aqueous Product

Nuvaid Ahad
Fischer-Tropsch is a process whereby a gaseous mixture of carbon monoxide (CO) and hydrogen (H2), called syngas, is converted into hydrocarbons, oxygenates and water. 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
more » ... onyls can be recovered by distillation, resulting in a carboxylic acid containing bottom product, called acid water. The acid water contains around ~1 % carboxylic acids of which ~85% is acetic acid, with lower amounts of propionic and butyric acids. Due to the presence of carboxylic acids, the pH is low and the chemical oxygen demand of the water is high. It cannot be directly discharged into water bodies, or used as process water, so it needs to be treated first. Currently the industry is treating the acid water biologically using aerobic or anaerobic processes, but in all cases the facilities are located in warm climates. The overall objective of the research was to find an alternative technique for treating Fischer-Tropsch acid water that is suitable for application in Canada, which has a cold climate. The alternative techniques for treating the acid water had to be both technically feasible and economically viable. Three processes were identified as potential alternatives for treating Fischer-Tropsch acid water: (1) Fractional freezing, (2) Electrolysis, and (3) Catalytic decomposition. For each process aqueous product effluent used for experiments was close to industrial acid water. Each process was evaluated experimentally to demonstrate proof of concept and generate engineering data. The experimental work was followed by a conceptual design to demonstrate technical feasibility for meeting Canadian effluent standards, and determine the cost. This was also done for biological treatment processes, which served as base case for an economic evaluation on the same design basis. It was found that all three of the alternative processes were technically feasible, but that fractional freezing was the most cost competitive with biological treatment. iii
doi:10.7939/r3x34n090 fatcat:noaxvqclkbfmtoaradv6lyclz4