Cracked Naphtha Reactivity: Effect of Free Radical Reactions

The problems faced when dealing with thermally cracked naphtha are vast, from gum formation in storage units to fouling and deactivation of catalysts. Our interest was to gain a better understanding of the initiation of cracked naphtha fouling. The thermal reactivity of cracked naphtha from an industrial bitumen upgrading facility was investigated. The naphtha was exposed to temperatures of 200, 250, and 300 °C for a period of 1 h. No free radicals were detected in the naphtha feed using electron spin resonance spectrometry, yet, after heating, evidence of free radical reactions was found. The influence of heating was investigated for both the naphtha on its own and the naphtha in combination with α-methylstyrene (AMS). AMS was employed as a probe molecule that facilitated detection of free radical transfer reactions. The potential contribution of oxygen as free radical initiator was also investigated by heating naphtha at different oxygen partial pressures. Heating of naphtha and AMS mixtures resulted in the formation of compounds linked to free radical termination. Self-initiation by AMS was ruled out with control experiments using a mixture of AMS in n -pentane. The possible contribution of oxygen could also be ruled out in reactions performed under inert atmosphere. Reactions conducted at different partial pressures of oxygen resulted in a different product distribution than found in reactions conducted in an inert atmosphere. It appeared that the thermally cracked naphtha contained molecules capable of initiating free radical reactions at the lowest temperature studied in the absence of oxygen. An initiation pathway by molecule induced homolysis was discussed as a possible explanation of the observations.