Determination of Critical Properties for Reactive Mixtures: Hydrodesulfurization of 4, 6-Dimethyldibenzothiophene
Advancing the chemical engineering fundamentals
Thermodynamics: General (T2-1e)
Keywords: Reactive residue curve, Critical point, Hydrodesulfurization
The mandatory reduction of sulfur content of diesel fuel agreeded by the European Union and USA (50 wppm since year 2005) is promoting changes in the oil refineries in terms of modifying the catalyst used and/or in the technology involved in the hydrodesulfurization (HDS) process. That is, higher activity of the commercial catalyst, and structural changes in the reactor configuration to increase the sulfur-compounds conversion, are needed. Specifically, the diesel produced in Mexican refineries contains around 500 wppm of sulfur and it is thought that the reduction to 50 wppm will require a very important economical investment. Therefore, the study and analysis of the different technological alternatives are considered to be high priority. An analysis of the operating conditions to obtain ultra-low sulfur diesel in a conventional HDS process suggests that reactive distillation could be an interesting technological alternative for deep HDS of diesel. The aplication of the reactive distillation concepts to the deep HDS of diesel requires a careful analysis of the reactive system in terms of at least three items: (i) thermodynamic behavior of the reactive mixture; (ii) reaction routes and kinetic expressions; and (iii) the heat and mass transport phenomena. In this work, only the first item is considered. The thermodynamic analysis plays a crucial role in the aplication of the reactive distillation technology. The thermodynamic properties of the reacting compounds and the appropriate determination of the phase behavior are needed to determine the feasibility of the separation. Up to now, only few papers have addressed the prediction of critical properties of reacting mixtures. Based on a thermodynamic analysis in terms of computation of the liquid-vapor phase equilibrium with chemical reaction in the liquid phase, a method to predict the critical properties of reactive mixture in the hydrodesulfurization of 4, 6-dimethyldibenzothiophene with tetralin has been developed. The thermodynamic analysis considers the simultaneous calculation of the reactive and non-reactive residue curve maps and the critical points from an equation of state, using the Heidemann and Khalil method (Heidemann and Khalil, 1981). The simulations were performed at different operating pressures (10, 15, 20, 25 and 30 atm). At P = 30 atm were identified several critical points for the reactive mixture. The visualization of the reactive residue curves and the critical points are posed in terms of elements (Pérez-Cisneros et al., 1997) and x-y-T phase diagrams. From the reactive x-y-T phase diagrams it is found that there is a temperature region for each pressure value where the elimination of the 4, 6-dimethyldibenzothiophene of the liquid phase is favored. Therefore, this temperature region should be carefully considered for the design of a reactive section in a reactive distillation column.
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Presented Tuesday 18, 16:20 to 16:40, in session Thermodynamics: General (T2-1e).
