(713d) Controlling Foaming during Thermal Cracking - a Non-Silicon Based Antifoaming Agent

Foaming in hydrocarbon systems is a dynamic process that occurs when gas bubbles travel to the surface of a liquid without breaking or coalescing. The life of foam is related to its stability. The stability of foam in turn is related to forces such as surface tension, gravity, viscosity and capillary forces acting on the liquid film around the bubble. During thermal cracking of heavy oils, the presence of cracked vapors traveling through a liquid phase can cause foaming. The foam produced during this process can be stabilized by natural surfactants present in heavy oils such as asphaltenes and resins. Foaming can pose serious issues in the refinery if it overflows from the reacting vessel such as in a delayed coke drum. Silicon-based antifoams (AF) are commonly used to mitigate foaming. These antifoams contribute significantly to operating costs and can result in contamination of catalysts downstream of the delayed coking unit and in some cases; it leads to undesirable coke morphology that can pose risk to operators when decoking the reactor. An antifoaming agent that effectively controls foam without contaminating the liquid stream with silicon is desirable.

This paper shows how a non-silicon based antifoam is used to control foaming during thermal cracking in a pilot unit delayed coker drum. A vacuum resid with known properties was fed to the 3-inch diameter coker equipped with a gamma densitometer that allows for a transient analysis on foam growth and collapse during the experiment. This non-silicon antifoam; Tire oil, is an aromatic compound that does not contain any contaminants which can affect the downstream units or catalyst. Two modes of AF injection are tested; intermittent and continuous injections (at 0.36 cc/min and 0.18 cc/min after initial injection). Both modes were found to be effective at controlling the foam in the coke drum. The continuous foaming case resulted in a morphology change and liquid yield increased. Two AF concentrations are also tested (a neat injection and a mix with a hydrocarbon carrier) using the intermittent injection mode. Both concentrations were effective at controlling the foam.

The processing capacity of an industrial coke drum utilizing this non-silicon AF may be increased by decreasing the zone used to impede foamovers as a result of the eliminated risk of catalyst poisoning typically caused by foamover of silicon containing liquid. This increases the profitability of the delayed coker process.