Studies on the nature of gum formed in cracked naphthas

Cracked naphthas from fluid catalytic cracking (FCC) operations are generally the major blending components in gasoline. Naphthas from thermal cracking operations also find their way in gasoline pool in some refineries. Composition of these two generic classes of naphthas, their gum forming tendencies and nature of gum formed have been studied. The soluble and insoluble gum have been characterized for functional groups by infrared (IR) spectroscopy and the molecular weight profile, using gel permeation chromatography. The nature of the gum is correlated with the composition of the naphthas. Two FCC naphthas, two visbreaking naphthas and one coker naphtha were taken for the study. The FCC naphthas contain relatively higher levels of mono-olefins and conjugated diolefins with high branching. The thermal cracking naphthas have higher levels of alpha-olefins and are bound in di-, tri- and cyclo-olefins. The infrared spectra of gum produced under identical accelerated ageing conditions show that the hydroxyl functionalities in the gum from FCC and thermal cracking naphtha are of the same nature but hydrogen bonding in gums from thermal cracking naphthas is stronger. Carbonyl functionalities indicate formation of different types of esters in gum formed in naphthas from two different routes. Molecular weight of both the soluble and insoluble gums are distributed from 100 to around 2000 in both types of naphthas. However, the distribution shows that the insoluble gums contain higher amount of high molecular weight polymers as compared to the soluble ones.