Mechanistic Insight in Phase Transfer Agent Assisted
Scheme 4.2: Reaction mechanism for performic acid induced oxidative desulfurization
(5) The entropy change values (–ΔS) in different experimental categories are again a manifestations of the predominant chemical mechanism of oxidative desulfurization.
The highest (–ΔS) values are seen for category A.2, in which the chemical mechanism for oxidative desulfurization is purely radical–based and reactions occur almost instantly. The (–ΔS) values for mechanically stirred system increase with addition of PTA, indicating faster reactions. (–ΔS) values for category B.2 are intermediate between that of categories B.1 and A.2, indicating dual (ionic and radical induced) nature of the chemical mechanism of the PTA–assisted oxidative desulfurization. The dual nature of chemical mechanism in category B.2 also results in pseudo 1st order kinetic constants of intermediate value between the constants for categories A.2 and B.1.
(6) Despite significantly dissimilar chemical mechanisms that lead to dissimilar values of ΔH and (–ΔS), the Gibbs energy change (ΔG) is almost similar (±5%) for experimental categories B.1 and B.2, in which PTA was simultaneously applied with ultrasound. This essentially is an indication of the physical role played by ultrasound in PTA–assisted oxidative desulfurization. Moreover, lesser (ΔG) values for categories B.1 and B.2, as compared to categories A.1 and A.2, respectively, point out
supportive effect of the PTA in enhancement of the oxidative desulfurization by effective interphase transport of oxidant anion.
4.6 C
ONCLUSIONConcurrent analysis of extent of DBT oxidation and Arrhenius & thermodynamic parameters for different experimental conditions, and results of simulations of cavitation bubble dynamics presented in this study has revealed a cogent and coherent picture of the physical mechanism of the PTA–assisted ultrasonic oxidative desulfurization process. Essentially, this study has identified and established the links (or interactions) between the individual mechanisms of PTA and ultrasound/
cavitation in oxidative desulfurization process. Although ultrasonic oxidative desulfurization has the lowest activation energy and enthalpy change with the highest negative entropy change, the overall DBT oxidation achieved in this process is lesser due to low frequency factor, which is a consequence of high instability of the radicals generated by transient cavitation. Thus, contribution of chemical effect of transient cavitation to DBT oxidation is relatively small. The predominant mechanism of PTA–
assisted oxidative desulfurization is ionic, which results in relatively higher activation energy and enthalpy change as compared to ultrasonic oxidative desulfurization. The synergistic effect of fine emulsification generated due to physical effect of micro–
convection induced by transient cavitation and ultrasound, and PTA–assisted effective interphase transport of oxidant leads to almost complete conversion of DBT to DBT sulfone. It is thus established that prevalent role of ultrasound and cavitation in PTA–
assisted ultrasonic oxidative desulfurization process is of physical nature that helps in boosting the beneficial effect of PTA for enhancement of DBT oxidation.
R
EFERENCES1. Srivastava VC. An Evalution of desulfurization technologies for sulfur removal from liquid fuels. RSC Adv. 2012; 2: 759–783.
3. Doraiswamy LK. Organic synthesis engineering. New York; Oxford University Press:2001.
4. Mei H, Mei BW, Yen TF. A new method for obtaining ultra–low sulfur diesel fuel via ultrasound assisted oxidative desulfurization. Fuel. 2003;82:405–414.
5. Etemadi O, Yen TF. Selective adsorption in ultrasound–assisted oxidative desulfurization process for fuel cell reformer application. Energ. Fuel. 2007;21:2250–
2257.
6. Wan MW, Yen TF. Enhance efficiency of tetraoctyl ammonium fluoride applied to ultrasound assisted oxidative desulphurization (UAOD) process. Appl.
Catal. A. 2007;319:237–245.
7. Hagenson LC, Doraiswamy LK. Rate enhancement in a solid–liquid reaction using PTC, microphase, ultrasound and combination thereof. Chem. Eng. Sci.
1994;49:4787 – 4800
8. Hagenson, LC, Doraiswamy L.K. Comparison of the effects of ultrasound and mechanical agitation on a reacting solid–liquid system. Chem. Eng. Sci.
1998;53(1):131–148.
12. Zhao D, Ren H, Wang J, Yang Y, Zhao Y. Kinetics and mechanism of quaternary ammonium salts as phase–transfer catalysts in the liquid–liquid for oxidation of thiophene. Energ. Fuel. 2007;21:2543–2547.
13. Chakma S, Moholkar VS. Investigations in synergism of hybrid advanced oxidation processes with combinations of sonolysis + fenton process + UV for degradation of bisphenol–A. Ind. Eng. Chem. Res. 2014;53:6855–6865.
14. Gogate PR, Tatake PA, Kanthale PM, Pandit AB. Mapping of sonochemical reactors: review, analysis, and experimental verification. AIChE J. 2002;48:1542–
1560.
15. Naik SD, Doraiswamy LK. Phase Transfer Catalysis: Chemistry and Engineering. AIChE J.1998;44:612–646.
16. Wang M–L, Yang H–M. Kinetic study of synthesizing 2,4,6–tribromophenol allyl ether by phase–transfer catalytic reaction. Ind. Eng. Chem. Res. 1990;29:522–
526.
17. Wang M–L, Yang H–M. Simplified dynamic model for the allylation of 2,4,6–
tribromophenol by phase transfer catalyst. Ind. Eng. Chem. Res. 1991;30:631–635.
18. Wang M–L, Yang H–M. Dynamic of phase transfer catalyzed reaction for the allylation of 2,4,6–tribromophenol. Chem. Eng. Sci.1991;46:619–627.
19. Bhattacharya A. General Kinetic Model for Liquid–Liquid Phase–Transfer Catalyzed Reactions. Ind. Eng. Chem. Res.1996;35:645–652.
20. Wang M–L, Chen W–H. Kinetic study of synthesizing dimethoxydiphenylmethane under phase–transfer catalysis and ultrasonic irradiation.
Ind. Eng. Chem. Res. 2009;48:1376–1383.
21. Toegel R; Gompf B; Pecha R; Lohse D. Does Water Vapor Prevent Upscaling Sonoluminescence? Phys. Rev. Lett. 2000;85:3165–3168.
22. Storey BD, Szeri AJ. Water Vapor, Sonoluminescence and Sonochemistry.
Proc. R. Soc. London, Ser. A.2000;456:1685–1709.
23. Sivasankar T, Paunikar AW, Moholkar VS. Mechanistic approach to enhancement of the yield of a sonochemical reaction. AIChE J. 2007;53:1132–1143.
24. Moholkar VS, Sable SP, Pandit AB. Mapping the cavitation intensity in an ultrasonic bath using the acoustic emission. AIChE J. 2000;46:684–694.
25. Krishnan SJ, Dwivedi P, Moholkar VS. Numerical investigation into the chemistry induced by hydrodynamic cavitation. Ind. Eng. Chem. Res. 2006;45:1493–
1504.
26. Kumar P, Khanna S, Moholkar VS. Flow regime maps and optimization thereby of hydrodynamic cavitation reactors. AIChE J. 2012;58:3858–3866.
27. van Wijngaarden L. On the equations of motion for mixtures of liquid and gas bubbles. J Fluid Mech. 1968;33:465–474.
28. Capocelli M, Musmarra D, Prisciandaro M, Lancia A. Chemical effect of hydrodynamic cavitation: Simulation and experimental comparison. AIChE J.
2014;60:2566–2572.
29. Reid RC, Prausnitz JM, Poling BE. Properties of gases and liquids. New York:
McGraw−Hill, 1987.
30. Hirschfelder JO, Curtiss CF, Bird RB. Molecular theory of gases and liquids.
New York:Wiley, 1954.
31. Condon EU, Odishaw H. Handbook of physics. New York: McGraw− Hill, 1958.
32. Zhou J, Lu X, Wang Y, Shi J. Molecular dynamics investigation on the finite dilute diffusion coefficients of organic compounds in supercritical carbon dioxide.
Fluid Phase Equilib. 2000;172:279–291.
33. Zhu Y, Lu X, Zhou J, Wang Y, Shi J. Prediction of diffusion coefficients for gas, liquid and supercritical fluid: Application to pure real fluids and infinite dilute binary solutions based on the simulations of Lennards–Jones fluid. Fluid Phase Equilib. 2000;194−197:1141–1159.
34. Press WH, Teukolsky SA; Flannery BP; Vetterling, WT. Numerical recipes.
New York; Cambridge University Press:1992.
35. Keller JB, Miksis MJ. Bubble oscillations of large amplitude. J. Acoust. Soc.
Am. 1980; 68: 628–633.
36. Brenner M, Hilgenfeldt S, Lohse D. Single–bubble sonoluminescence. Rev.
Mod. Phys. 2002; 74: 425–484.
37. Chemical Equilibrium Calculation (Bioanalytical Microfluidics Program, Colorado State University), (accessed January 2013).
38. Leighton TG. The acoustic bubble. San Diego: Academic Press, 1994.
29. Grossmann S, Hilgenfeldt S, Zomack M, Lohse D. Sound radiation of 3 MHz driven gas bubbles. J Acoust Soc Am. 1997;102:1223–1227.
40. Moholkar VS, Warmoeskerken MMCG. Integrated approach to optimization of an ultrasonic processor. AIChE J. 2003;49:2918–2932.