(283f) Effect of Pressure, O/Ostoich Ratio and Temperature on the Partial Oxidation of Heavy Oil Model Compound Phenanthrene

An increasing world energy demand and the proportions of heavy oil reserves relative to lighter and more valuable feedstocks have enabled the development of new emerging research fields. A novel method to upgrade heavy oil through partial oxidation in supercritical water (SCW) is being developed in the research group. This method takes advantage of the properties of SCW (T=374˚C, P=220 bar and δ=0.32 g/ml and above), in which water changes from a good solvent for polar to non-polar species. At these conditions a change in the ion product, in the viscosity and a decrease in the dielectric constant are observed [1]. Studies with model compounds representing chemical structures found in heavy oils give relevant information about the reactivity in the medium and the reaction mechanisms occurring in the system. Reactions with polycyclic aromatic hydrocarbons have been studied in the presence and absence of an oxidizer [2, 3].

The aim of this work is to investigate and get an understanding about the  effect of process conditions pressure, temperature and O/O_stoich ratio in the yield and selectivity to different reaction products of the partial oxidation of heavy oil model compound phenanthrene at near and supercritical water conditions. 

 

Methodology:

Partial oxidation experiments were performed in a stainless steel micro-bomb reactor with a volume of 18 ml. The operation of the reactor has been described elsewhere [4]. The reactor was filled with 0.15g of phenanthrene 98% (Sigma-Aldrich), 1.2 ml of H₂O₂ 30% (VWR chemicals) and different volumes of deionized water to reach the desired operating pressure. The reactor was purged with helium in order to remove any air remaining in the system. Once purged the reactor is closed and immersed into a fluidized sand bath used to control the reaction temperature. Gas products formed in each reaction were analyzed in a Perkin Elmer Clarus GC with thermal conductivity detector. Liquid products were separated from the remaining water, extracted with a mixture of chloroform/methanol 4:1 and then filtered. Solids obtained after filtering were dried and analyzed in a Perkin Elmer TGA to determine yield to coke. The remaining liquids were analyzed in a Perkin Elmer Clarus GC with flame ionization detector and in a Varian Star 3400/Saturn 2000 GC/MS to identify the main products obtained.

 

Results and Discussion:

 Experiments to assess the effect of pressure in the system were performed at constant O/O_stoich ratio of 0.38, 60 min and reaction temperature of 450˚C. Conversions and yields to products were obtained at 210 bar, 230 bar, 250 bar and 275 bar. Results show that conversion of phenanthrene remained almost constant near or above the critical point of water. Interestingly, changes in pressure had a great impact on product distribution and yields. Yields to liquid products increased with an increase in pressure till operating pressures near the critical point of water. Further increase in pressure derived in an important decrease in the selectivity to liquid products. In addition, the increase in the system pressure led to a considerable increase in coke yield and a steady increase in the yields to gas. Moreover, analysis of the liquid fraction showed that the composition of the liquid products obtained was greatly affected by pressure. The widest range of liquid products was obtained near the critical pressure of water. At pressures below the critical point lower molecular weight and oxygenated products were obtained. This may be due to the occurrence of cracking and ring opening reactions, which also led to high gas productions. In contrast, higher molecular weight compounds as pyrene were obtained at pressures over the critical point. This may be due to ring annealing and polymerization reactions enhanced at higher pressures. At 230 bar, the highest product distribution was obtained and selectivity to oxygenated products with boiling points between 300 ˚C and 400 ˚C was higher.

Based on previous results, the effect of O/O_stoich ratio on the conversion of phenanthrene was studied. Reactions at different O/O_stoich ratio (0.1, 0.2, 0.38 and 0. 75) were performed at 450˚C, 230 bar and 60 min reaction time. It was observed that conversion of phenanthrene increases with an increase in the concentration of oxygen in the system. The yield to liquid products increased with the increase in O/O_stoich ratio. However, at higher concentrations of oxygen in the system the amount of gas produced increased considerably while the amount of coke produced remained almost constant. Therefore, it can be said that an increase in oxygen ratio make the liquid products obtained to continue to react towards undesirable gas. Based on observations made on this work an optimum O/O_stoich ratio to favor selectivity towards liquid products was found between 0.2 and 0.38.

Furthermore, experiments were performed to assess the effect of temperature in the system. For these set of experiments different temperatures (360˚C, 400˚C, 425˚C and 450˚C) were studied at a pressure of 230 bar and an O/O_stoich ratio of 0.2. It was observed that an increase in temperature increased the yield to all of the product fractions. However, it was noted that when temperature was raised from 425˚C to 450˚C, selectivity to liquid products decreased greatly mainly due to an important amount of gas produced. Results showed that when reactions took place in supercritical water, temperature has no major effect in the product distribution within the liquid fraction. However when temperature was below the critical point, two liquid products were mainly observed (fluorene and fluorenone) which suggest that the reaction occurs at a considerably slower rate than it occurs at supercritical conditions.

 Conclusions:

System pressure, O/O_stoich ratio and temperature have been proven to have a great influence in the partial oxidation of heavy oil model compound phenanthrene with water at near critical and supercritical conditions. It was found that the overall conversion of phenanthrene increased with an increase in O/O_stoich ratio and temperature and that these two variables greatly affect yields and selectivities to different reaction products. Conversion remained constant with an increase in pressure although, selectivity to liquid products reached a maximum at 230 bar near the critical pressure of water.  Yields to gas and coke also depended greatly on pressure. GC/MS analysis of the liquid product fractions showed that product distribution in the liquid fraction also depended on the system pressure and the O/O_stoich ratio. It has been observed that product distribution remain fairly constant with changes in temperature when operating above the critical point of water. These results are of great importance as they show that different target products can be obtained with slight variations of the main process conditions.

 References

1. H. Weingärtner, E.U. Franck, Angew. Chemie Int. Ed. 44 (2005), 2672–2692.
2. J.A. Onwudili, P.T. Williams, Int J Energ Res, 30 (2005), 523–33.
3. J.A. Onwudili, P.T. Williams, J Supercrit Fluid, 39 (2007), 399–408
4. J.L. Pinilla, P. Arcelus-Arrillaga, H. Puron, M. Millan, Appl. Catal. A 459 (2013) 17-25.