Bond cleavage and reactive radical intermediates in heavy tar thermal cracking

Thermal cracking is an important step of heavy tar processing, in which covalent bonds are initially cleaved to form free radical fragments. The free radical fragments, namely the reactive intermediates, react with each other to form lighter products and coke. Quantity and activity of these reactive radicals determine the products distribution and process operation. Since few studies can be found about the reactive radicals from thermal cracking of heavy tar, we quantify their generation and analyze their activity using two tars provided by ExxonMobil Chemical Company. The tars are heavy fractions of steam cracking of naphtha and heavier hydrocarbon streams. It is found that these heavy tars crack significantly at 350 °C. The reactive radicals generated in 10 min for Tar-1 at 350 and 400 °C are 2.62 and 4.73 mmol/g-tar, respectively, and 1.27 and 3.88 mmol/g-tar, respectively, for Tar-2. The bonds cleaved in 10 min at 400 °C are about 1.50% and 1.30% of total covalent bonds in Tar-1 and Tar-2, respectively, and the bond cleavage activation energies are 31.00 and 38.80 kJ/mol, respectively. The differences in bond cleavage were analyzed from the viewpoint of the composition and structural differences between the two heavy tars, in terms of asphaltene content, aromaticity, and substitutive degree of aromatic ring, for example. The reactive radicals were further categorized into low activity ones and high activity ones and were correlated with stable radicals which have survived the cracking.