Multilayer coking

Equation (5) is an example of the Poisson process in probability theory [9]. S decays slowly with t, so that enough sites survive even at long times to continue forming new coke. This simple model lends itself naturally to the distinction of coke depositing on clean sites, monolayer coke concentration C , and coke depositing on already-coked sites, multilayer coke concentration Cm- That is, C (/) = CJf) + CM t) at all times. A simple site balance yields... 

Note that C (/) is concave, while C t) is convex for small t and concave beyond the inflection point at / = / = l/(oro), the characteristic time scale of coke buildup. Initially, there is no monolayer coke and thus no multilayer coke. As the monolayer coke begins to build up, multilayer coke formation accelerates. But after a significant portion of the active sites are coked, the rate of multilayer coke buildup slows down. It bears emphasizing that both the rates of monolayer and multilayer coke buildup depend on active sites (more on this later). 

Figure 5. Monolayer and multilayer coke vs. time on Pt-Re/Al203...

Often when organic compounds react on catalyst surfaces with adsorbed molecules undergoing and leading to species which are retained on the catalyst, these species stay on the surface and build up several carbonaceous layers. The model for multilayer coking was proposed by N. Ostrovskii (Figure 8.38) and in the treatment below we will follow his considerations. 

This equation includes linear and exponential dependences as special cases. When multilayer coking is not profound, then (p = k /km > 0 and Cc / Cm = (1 - a). When the... 

Consider coking taking place on an energetically uniform catalyst surface. Suppose that a coke precursor, P, can form coke upon deposition onto sites whether the sites are vacant or occupied by coke. This multilayer coking can be represented by the following irreversible elementary surface reactions ... 

The coke content due to multilayer coke or coke growth is contained in the second term in the bracket of Eq. 5.93 since Cc/Q = y when only monolayer coke is involved. For strictly monolayer coking, Kp approaches infinity, kp approaches zero, and Eq. 5.93 reduces to that for monolayer coking. For structure-insensitive reactions, the fractional catalytic activity remaining after deactivation, A. is equal to (1 — y). This can be used in Eq. 5.93 to obtain the following relationship ... 

Lobera et al. (2008) found that the content of coke formed on a Pt-Sn-K/ AI2O3 catalyst increased sharply at the initial state, followed by a more moderate linear increase. This phenomenon could be illustrated and described by using monolayer and multilayer coke growth model. For the coking reaction... 

---