MECHANISMS OF COKE BUILDUP AND GROWTH ON QUARTZ, VYCOR, AND STAINLESS STEEL SURFACES VIA THERMAL AND METAL-CATALYZED DECOMPOSITIONS OF TOLUENE AND ACETYLENE (PYROLYSIS, CRACKING, CARBON)

This investigation substantially clarifies coke formation on solid surfaces. In laboratory experiments, toluene and acetylene were pyrolyzed in a Vycor glass reactor at 780 - 1080(DEGREES)C and at partial pressures of from 7 to 38 torr; coke deposited on quartz and various stainless steel coupons. The morphology and metal composition of coke which formed over a wide range of operating conditions were determined using a scanning electron microscope equipped with EDAX. The results indicate for the first time the complicated interrelationships that exist between three coking mechanisms: Mechanism 1 involves metal-catalyzed reactions to produce filamentous and other coke. In mechanism 2, globular coke forms on solid surfaces by deposition of tar droplets and/or coke particles. In mechanism 3, coke grows when gaseous microspecies adsorb and react on the coke surface. A model based on the gas-phase coking mechanisms was developed to correlate the coke buildup on quartz surfaces as a function of temperature and residence time. A comprehensive coking mechanism is proposed that incorporates the three coking steps. This mechanism explains the character of coke produced in the coils, transfer lines, and transfer line exchangers of large ethylene plants, starting with clean, coke-free metal surfaces through well-coked surfaces. The mechanism also explains the complicated differences in coke buildup that were found in the laboratory unit as a function of temperature, residence time, type of solid surface, and feedstock. In industrial units, coke deposits are formed by a dynamic growth process and metal transfer is a key factor affecting the character of the coke. First, filamentous and other metal-catalyzed coke form. Metal-catalyzed coke is predominant near the metal/coke interface, and the concentration of metal in the coke at this interface is often as high as 1 - 2%. At distances greater than about 0.1 mm from the metal/coke interface, the metal content is about 0.1 - 0.3% and the type of coke depends less on the metal-catalyzed mechanism. Next, a fill-in operation occurs in which mechanisms 2 and 3 predominate. The filamentous and globular cokes present on the surface thicken and grow. Hence, the coke deposits are frequently non-porous over a considerable portion of their thickness