Liquid-Phase Thermal Treatment of Tall Oil Soap into Hydrocarbon Fuels

Abstract

Liquid-phase thermochemical processing of tall oil soap has been experimentally investigated. The organic matter of tall oil soap, which is a by-product of kraft pulping, originates mainly from wood extractives. Conventional processing of tall oil soap involves acidulation to yield crude tall oil and subsequent distillation of the oil at centralised refineries. Because tall oil originating from birch wood is far less valuable than that from pine, there is an economic incentive in the Nordic countries to develop alternative conversion processes for the tall oil soap produced at pulp mills where birch is widely used as feedstock.

Both catalytic hydrotreatment and liquid-phase thermal treatment of a mixed pine/ birch soap have been investigated in the laboratory. Liquid and gaseous hydrocarbons were the principal products formed in both types of treatment. Although the liquid product formed under hydrotreatment conditions contained a higher proportion of aliphatic compounds, it was clear that the thermal treatment process would be, economically, the more favourable option. In the thermal process, a predominantly hydrocarbon oil product was obtained at 450 °C using a residence time of at least 60 min. About 50% of the energy content of the tall oil soap was recovered as oil product, about 30% as gases, and about 20% as organic material contained in an aqueous slurry phase. When the oil product was carefully separated from the aqueous slurry phase, only trace amounts of impurities, such as water, sodium and organic acid salts were present in the product. A techno-economic evaluation of the process concept has been performed. The economics of the thermal treatment process appear to be favourable in comparison with those of the conventional acidulation process.