Deactivation of Catalysts and Reaction Kinetics for Upgrading of Renewable Oils

Doktorsavhandling, 2019

The transport sector is one of the main contributors of greenhouse gas emissions in the world. Advanced biofuels from renewable oils can play a decisive role in reducing carbon emissions from the transport sector. Advanced biofuels from waste streams like tall oil, used cooking oil etc. can lower the CO2 emissions in a range of up to 90% making our future and society more sustainable. Catalytic hydrodeoxygenation (HDO) is a process in which oxygen is selectively removed from renewable oils to produce advanced biofuels. These biofuels are drop-in hydrocarbons which can substitute fossil-based fuels without infrastructure or vehicle changes. This thesis focuses on aspects of catalyst deactivation and reaction kinetics during the production of such biofuels via HDO reactions.
Renewable oils can be sourced from varied streams like tall oil (paper industry residue), animal fats, used cooking oil etc. due to which their composition and innate contaminants can vary significantly. Phosphorus, alkali metals like potassium or sodium, iron, silicon, chlorides etc. are some of the common poisons present in renewable feedstocks which can cause catalyst deactivation during the upgrading process. In the first section of this thesis, the influence of iron (Fe), phosphorus (from phospholipid) and potassium (K) as poisons during HDO of fatty acids over molybdenum based sulfided catalysts was investigated. A range of concentration of poisons was evaluated to show that these poisons severely impacted the activity of catalysts. A change in selectivity was also seen, which is an important parameter to consider during the industrial production of biofuels. Different characterization techniques were employed to study the poison distribution on catalyst samples from lab experiments as well as from a refinery. It was suggested that Fe deposits preferentially near Ni-rich sites which deteriorated the ability of these catalysts to create active sites i.e. via sulfur vacancies. However, phosphorus resulted in irreversible phase transformation of the support to aluminum phosphate (AlPO4) which resulted in catalyst deactivation via pore blockage. In the comparative experiments, with spherical catalyst particles (1.8 mm), the Fe caused the strongest deactivation among P and K, based on the quantity added to feed oil. Although, considering the decrease in surface area per unit of deposited element after the experiment, then P caused the most deactivation. It was concluded that Fe deposited mostly near to the outer surface irrespective of concentration while P and K penetrated deeper in catalyst particles such that the distribution profile was dependent on the concentration.
Reaction kinetics of HDO of fatty acids provides critical knowledge which could be applied at the refining scale in process design and optimization. The activity and selectivity of NiMo catalyst during HDO of stearic acid was studied by varying reaction conditions like temperature, pressure, feed concentration and batch-reactor stirring rate and using intermediates like octadecanal and octadecanol. A deeper understanding of the reaction scheme and selectivities was developed based on the experimental results. A Langmuir–Hinshelwood-type mechanism was used to develop a kinetic model which well-predicted the changes in selectivities at varying reaction conditions.