Catalytic Upgrading of Waste Oils to Advanced Biofuels – Deactivation and Kinetic Modelling Study

Licentiatavhandling, 2018

The demand for liquid hydrocarbons as transportation fuels is enormous and ever growing.

Advanced biofuels is one of the promising solutions to keep pace with the global transition

to cleaner energy by reducing greenhouse gas emissions from the transport sector. It is

possible to selectively remove oxygen from waste oils like tall oil, used cooking oil etc. via

a catalytic hydrodeoxygenation (HDO) process to produce advanced biofuels. These

biofuels have similar molecules as in the traditional fossil-based fuels and exhibit improved

performance. This thesis focuses on aspects of catalyst deactivation and kinetic modelling

of HDO reactions.

 

In the first study, the influence of iron (Fe) as a poison during HDO of a model compound

for renewable feeds (Oleic acid) over molybdenum based sulfided catalysts was

investigated. Fe is a potential contaminant in renewable feeds due to corrosion during

transportation and storage in iron vessels. A series of experiments with varying Fe-oleate

concentration in the feed over MoS2/Al2O3 and NiMoS/Al2O3 catalysts. There was a salient

drop in the activity of the catalysts. At higher Fe concentration, for the NiMoS catalyst, the

selectivity for the direct hydrodeoxygenation product (C18 hydrocarbons) increased.

However, it was opposite for the MoS2 catalyst. There was a decrease in the yield of direct

hydrodeoxygenation products and an increase in yield of decarbonated products. It was

proposed that Fe interacted with these two catalyst systems differently. Fe influenced the

critical step of creation of sulfur vacancies in a negative way which resulted in lower

activity. Microscopic analysis indicated that Fe was preferentially deposited close or

around the nickel promoted phase, which explained why the role of Ni as a promoter for

the decarbonation route was subdued for the NiMoS catalyst.

 

In the second study, the kinetics during HDO of stearic acid (SA) over a sulfided

NiMo/Al2O3 catalyst were explored to investigate the reaction scheme. Model compounds

like octadecanal (C18=O) and octadecanol (C18-OH) were employed to understand the

reaction steps and quantify the selectivity. A Langmuir–Hinshelwood-type kinetic model

was used to investigate the kinetics. The results from the proposed kinetic model were

found to be in good agreement with experimental results. In addition, the model could

effectively reproduce the observed experimental profiles of different intermediates like

C18=O and C18-OH and illustrate phenomena like inhibiting effects of the fatty acid.