(31d) Catalytic Upgrading of Maya Oil Vacuum Residue through Partial Oxidation in Supercritical Water

Background

An increasing world energy demand combined with the gradual decline in light oil reserves have made heavy oil production and upgrading crucial for the future of the global energy market. A raise in world’s heavy oil production from 5.2 mbpd in 2012 to 13 mbpd in 2035 is expected (1). Heavy oil feedstocks are difficult to process due to their low API gravity, high viscosity and composition. In addition, heavy oil feedstocks have a high concentration of asphaltenes, heteroatoms and metals which cause severe problems in refining stages, low yields to high value fractions and catalyst deactivation (2). In order to address these challenges, technologies to process heavy oil fractions in a more efficient and environmentally friendly way have to be developed. An interesting approach is the use of water at near critical or supercritical conditions as a medium to upgrade heavy oil. Water at temperatures above 374 ˚C and pressures of 221 bar becomes a supercritical fluid and changes its properties from a good solvent for polar species to a solvent for non-polar species. Moreover, an increase in ion product, changes in viscosity and a decrease in the dielectric constant are observed (3). In addition, it has been reported that water near or above its critical point can play the role of solvent, reactant and catalyst in the same process (4). Potential application of near critical (NCW) and supercritical water (SCW) has been found for oil upgrading processes as it decreases the viscosity of heavy oil and inhibits the formation of coke (5). Furthermore, it has been found that SCW alone has the potential of removing sulfur from heavy oil feedstocks and that desulphurization can be improved with the addition of a catalyst in the absence of molecular hydrogen (6). In this work, a novel method to upgrade heavy oil feedstocks through partial oxidation in SCW has been developed. The effect of the addition of a Mn-Y-Zeolite as catalyst in the upgrading of heavy oil was also studied.

Methodology

Experiments were performed in a stainless steel tee type microbomb reactor with a volume of 18 mL. Details on the reactor configuration and operation have been extensively described elsewhere (7). Experiments were conducted with an initial 0.5g loading of Maya crude vacuum residue (VR) with 33% asphaltene, 7% sulphur and 0.7% nitrogen content as well as 211 ppm of vanadium and 207 ppm of nickel. All reactions were carried out in supercritical water conditions at 230 bar, 60 min, an oxygen/VR ratio of 0.15 and a catalyst/VR ratio of 0.25 when catalyst was used. The Mn-Y-zeolite catalyst used was synthesized following a traditional ion exchange method as reported in literature (8). Experiments were performed at different temperatures, namely 400°C, 425°C and 450°C. Products of reaction were recovered with a 4:1 chloroform-methanol mixture and filtered. Solids recovered were dried and analyzed in a thermogravimetric analyzer to determine coke yields. Liquid products were dried at room temperature under N₂ flow and afterwards were fractionated into maltene and asphaltene fractions.  The maltene fraction was further analyzed in a Perkin Elmer Clarus 500 Chromatographer fitted with flame ionization detector using the ASTM 2887 method to determine yields to maltenes with boiling points (bp) below 450°C (light maltenes). Both maltene and asphaltene fractions were analyzed to determine molecular weight distributions through size exclusion chromatography (SEC) with a Mixed D packed column with polystyrene/polydivinylbenzene from polymer labs. Gas yield were calculated through mass balance. Nitrogen and sulphur concentrations were determined through combustion and elemental microanalysis and metal concentrations through ICP technique.

 Results and Discussion

Experimental results show that VR conversion and yields to products are greatly affected by changes in temperature. At low temperature (400°C), a conversion of 35% of the organic material with bp > 450°C was achieved with high yields to light liquid products (bp < 450°C) above 20%, keeping the yields to gas and coke low at values around 10% and 5%,respectively. Further increments in temperature to 425°C and 450°C resulted in a considerable increase in VR conversion to 58% and 80%, respectively. However, the opposite effect was observed in the yields to light liquid products with bp < 450°C which decreased with an increase in temperature. This also resulted in an important increase in the yields to gas and coke at 450°C to reach values above 25% and 40%, respectively. These results suggest that working at lower temperature maximized the selectivity to light liquid fractions and that increasing the temperature resulted in an increase in VR conversion mainly due to an important increase in the yields to coke and gas. The addition of Mn-Y-Zeolite catalyst resulted in an increase of 8% in VR conversion and an increase in the yields to light liquid fractions of 6% in experiments performed at 400°C with no impact in gas and coke yields. At higher temperatures the effect of the catalyst addition decreases to a slight increase in conversion of 2%. A similar trend was observed in the yields of light liquids obtained which were slightly higher than the ones obtained without a catalyst just 3% and 2% higher at 425°C and 450°C. A marginal increase in the yields to gas and coke was registered in the presence of a catalyst at higher temperatures. The previous suggest that the use of catalyst increases VR conversion and improves selectivity to light liquid products at low temperatures, but the catalytic effect is not observed at higher temperatures as the reaction is mainly thermally driven. Besides this, XRD analysis performed over the spent catalysts revealed that the faujasite-type structure of the zeolite collapsed at 425 and 450 ºC, therefore explaining the reduced catalytic effect at these conditions. SEC results showed that when Mn-Y-Zeolite was added to the system, lighter maltene and asphaltene fractions with a narrower molecular weight distribution were obtained at all temperatures studied. Results on heteroatom removal showed that S and N removal increased from 19% to 22% and 5% to 12%, respectively, with the use of a catalyst at 400°C. It was observed that an increase in temperature up to 450 ºC improved the S removal capacity of the catalytic system up to 15% higher removal capacity than in the absence of a catalyst. On the contrary, the presence of Mn-Y-Zeolite catalyst seems to make no difference in N removal capacity of the system at high temperatures. The presence of the catalyst in reactions at 400°C resulted in an increase in V removal from 6% to 24% than in reactions without a catalyst. In a similar trend than in the case of nitrogen, small differences in the percentage of vanadium removed between experiments with or without a catalyst at higher temperatures were observed. As previously mentioned, this observation may be related to the collapse of the faujasite structure in the zeolite, and as result the access of the VR molecules to the actives sites is hindered.

References

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