Intensified Catalytic Reactor Design for the Conversion of Volatile Fatty Acids Produced By Sewage Treatment Plant

Declining petroleum resources and increased demand for fuel are driving force for finding renewable sources of fuel particularly for transport as this sector consumes more than two third of energy produced. Volatile fatty acids (VFA) are produced as intermediates prior to methanogenesis during sewage treatment, and can be sustainably converted to a bio-diesel, since this bio-fuel will not interfere with the food production unlike other biomass based fuels. These VFAs, consisting mostly of acetic, propionic and butyric acids, can be separated and catalytically converted to a diesel-like fuel.

The reaction is exothermic, so heat management is a challenge, whilst efficient mixing of the reactants is critical in obtaining high conversion and selectivity.

An intensified catalytic reactor for the hydrogenation of VFAs has been designed and tested. A new mixing device for a better gas to liquid contact is also included in the design. Ni-Mo, Co-Mo and Pt/Al₂O₃ catalysts have been prepared and coated on metal foams. Both powder and coated catalysts can be tested using this simple reactor design. The Figure shows the reactor components with catalyst coated foams. The reactor is currently being tested at higher pressures upto 30 bar for VFAs conversion; typically reactions have been carried out at 350^oC and at ambient pressure. Gas Hourly Space Velocity of hydrogen and VFAs were maintained at 100 h^-1 and 1h^-1 respectively. The system was tested with a mixture of acetic acid, propionic acid and butyric acid in the ratio 40:40:20. A complete conversion of these mixed acids to hydrocarbons is achieved at the reaction temperature of 350^oC. Further studies for optimising conditions are in progress and will be reported in full with a detailed reactor design.