Effective Gasoline Production Strategies for the Catalytic Cracking of a Rapeseed Vegetable Oil Under Realistic FCC Conditions

Our heavily dependence on the finite fossil fuel resources forced many counties to look at the possibilities for other alternatives, especially renewable energy sources, to meet the sustainable future energy and transportation fuel supply and demand. Of the many available sources, converting the raw materials from the biomass (e.g., vegetable oils) in the existing equipments (e.g., fluid catalytic cracking (FCC) process) into valuable transportation fuels and light olefins would be a very attractive option.

            Most of the studies available on the catalytic cracking of vegetable oils have been carried out in a fixed bed micro-activity (MAT) reactor, which does not have the hydrodynamics of a commercial unit. Dupain et al. [1] have, however, studied the thermal and catalytic cracking of a rapeseed oil in an isothermal plug flow microriser reactor, which mimics the actual riser unit very well and in addition to that it has the ability to operate at various contact times and catalyst-to-oil (CTO) ratios. Despite the differences in the reactors, the catalytic cracking of vegetable oils resulted in very large amounts of aromatics in the liquid fuel product in both the gasoline and diesel product slate. The aromatic formation depends, however, on the degree of unsaturation (presence of double bonds) of vegetable oils; the higher the unsaturation, the higher the aromatic content in the fuel. Furthermore, the LCO (diesel blending component) and gasoline yields were also found to depend on the degree of unsaturation: saturated vegetable oils result in more gasoline selectivity and yield, while unsaturated ones give more LCO selectivity and yield [1]. The thermodynamic is favorable for aromatization under the FCC operation conditions (high temperature and low pressures). Additionally, the oxygen of the triglycerides is removed mainly in the form of water [1], though carbon oxides (CO_x) formation is also possible, but is observed to minor extent [1]. The above observations clearly emphasize that there is a need to improve the process in terms of reducing the aromatic content of the liquid fuel product to the gasoline product distribution and also removing the oxygen at the same time. Thus, the present study aims to design a process that can overcome the above problems associated with vegetable oils, especially, unsaturated oils.

            The main strategy to address the above issue is considering an option of co-processing H₂ into the system that may help removing water and/or saturating the double bonds present in the system. However, H₂ alone is not expected to serve the purpose under realistic FCC conditions (near atmospheric pressure). This means we need to have a catalyst that can make H₂ to function in an efficient way under such conditions. To achieve this, the catalysts that can have a control over the rate of dehydrogenation activity, would be ideal because this would consequently reduce the aromatic formation as well. Thus, in this study, we have chosen some metals such as nickel (Ni) and platinum (Pt) along with H₂ co-feeding into the system to understand the their presence on the aromatic formation and also on the liquid product yields.

            For this study, both the same commercial equilibrium catalyst (Ecat) and vegetable oil were used [1]. Initially, the incorporation of Ni and Pt onto the commercial equilibrium catalyst (Ecat) is achieved by the incipient wetness impregnation method and calcined at 600 °C. The catalysts are not reduced in H₂ atmosphere before testing. Therefore, the Ni and Pt are essentially in their oxide form since this same oxidation state as the FCC catalyst enters the riser reactor from the regenerator. The Ni and Pt loadings used are 1 and 0.25 wt%, respectively. The reactions are performed in the microriser reactor at a temperature of 525 °C and CTO ratio of ~5. The details of the microriser are given elsewhere [1]. The base Ecat and other metal-containing Ecats are tested for the cracking of a rapeseed oil with and without co-feeding H₂ into the system. The liquids products are analyzed by SIMDIS gas chromatograph (GC), while the gas by the standard GC. The coke on the spent catalysts is measured by LECO analyzer. The aromatic content was determined by standard HPLC analysis.

             The results clearly demonstrated that the H₂ presence indeed helps in improving the gasoline yield on both the E-cat and Ni-Ecat systems. This means H₂ alone (no metal on Ecat) some beneficial effects on the product yields. Similarly, Ni alone (no H₂ co-feeding) also resulted in improved gasoline yields. On the other hand, the Ni-Ecat system in the presence of H₂ showed a remarkable activity for improving both the gasoline yield (by 10 wt%) and reducing the aromatic content of the gasoline fraction by 15 % and in fact the total aromatics by 17 %. On the other hand, Pt has not improved the process, and rather reduced the liquid product yield compared to that of the base Ecat. The coke yield for the Pt-Ecat is much higher than that for the Ecat and Ni-Ecat.

            In summary, the enhancement of gasoline yield and the reduction of aromatic content of the liquid fuel products by the cracking of (unsaturated) vegetable oils, a renewable feedstock can be improved substantially by co-feeding H₂ along with the incorporation of metal functionality. These results certainly would  trigger many people interests in the refining catalysis field to focus more on the rational catalyst design to improve the catalytic cracking performance of vegetable oils.

Reference:  

1. X. Dupain, D.J. Costa, C.J. Schaverein, M. Makkee, J.A. Moulijn, Appl. Catal. B: Env. 72 (2007) 44.