898.    FERRIS, S. W.  Limiting Compositions in Hydrocarbon Conversion.  Ind. Eng. Chem., vol. 33, 1941, pp. 752-759; Chem. Abs., vol. 35, 1941, p. 7163.

      Several investigators have attempted to show that the process of cracking hydrocarbons tends to approach an equilibrium.  This term is not used in the present paper.  Instead, attention is called to the tendency for a synthetic crude to be formed in the cracking reactions.  The synthetic crude oil includes all the liquid material resulting from a reaction, under such conditions of condensation that vapor pressure approximates 10-15 lb./sq. in.  From very different stocks the curves of distillation of the products of cracking are seen to approach a limiting band.  The same tendency can be traced from the literature in catalytic as in thermal cracking, in destructive hydrogenation, alkylation, the Fischer-Tropsch synthesis from CO and H₂, catalytic reforming, and even in polymerization.  The effect was similar when a naphthenic petroleum and successive synthetic crude oils from it were subjected to thermal cracking.  The 3 passes after the 1^st made little differences in the approach of the synthetic crude oils to the limiting band.  It was found that if the distillation curve of the original stock lay to the right of the limiting band, the synthetic crude oil could not be forced to the left of it.  Petroleums of this character were probably residual crude oils from which the light fractions had escaped in the course of geologic change.  The petroleums whose distillation curves started at the left of the limiting band were regarded as distillate crude oils.  If pyrolysis is pushed far enough, a 2d limiting composition can be detected in which benzene and naphthalene predominate.  This is a conclusion based on the data reported by Rittman, Dutton, and Dean.  The conditions for the production of the limiting band depend upon an original specific gravity of 0.800-0.930.  At higher specific gravities the available supply of H₂ is so limited that the tendency is toward benzene-naphthalene limiting composition.