Coke deposition catalyst deactivation

Figure 5 shows that coke deposition is very fast at the beginning of the run. As catalyst deactivates, coke deposition rate decreases. N-heptane conversion decreases from the initial 30 wt% to 17 wt%. Changes in selectivity show the different toxicity of the coke for the... 

In the course of the reaction, all the catalysts deactivated by deposition of coke. Several deactivation laws were fitted with the experimental data. The Voohries law, r = r t n, often claimed to represent ageing of acid catalysts did not apply. The best fit for the rate law of the deactivation process was obtained with -dr/dt = kd>r°<, with o(= 1+0.2 depending on the catalyst. Therefore a first order deactivation rate applies which takes the integral form r = rQ expt-k. t). In most cases, correlation coefficients better than 0.9 were obtained when determining kd (refs.5,15). 

In spite of the intense effort carried out in the past, the deactivation of catalysts by coke deposition, and its subsequent regeneration, still poses one of the most important problems in industrial catalytic processes. 

DEACTIVATION MECHANISMS OF A CHROMIA-ALUMINA CATALYST BY COKE DEPOSITION... 

The reaction occurs over a relatively small zone in the catalyst bed. As the reaction moves down the catalyst bed, coke deposits deactivate the front part of the bed. The reaction continues down the bed until a substantial part of the catalyst is deactivated and unconverted methanol "breakthrough" is detected in the reactor effluent stream. Use of sufficient catalyst permits reactor onstream periods, or cycles, sufficiently long to avoid excessive regenerations. To enable this to be done onstream, multiple reactors are provided and operated in parallel on a cyclic mode. The New Zealand plant is designed to operate with four reactors onstream, with a fifth reactor in regeneration. 

As shown in figure 1 for ZF 520, the activity of the catalysts seems to decrease significantly after about 5 hours of reaction. This can be due either to the deactivation of the catalyst by "coke" deposition and/or to the inhibition of the reaction by the products. 

First studies on the influence of intraparticle diffusional mass transfer on catalytic reactions, and about deactivation of cracking catalysts by coke deposition, started with Thiele [1] and Voorhies [2], respectively. To-date, Thiele s analysis remains valid, however the approach followed by Voorhies in which coke formation is expressed as a function of time on stream (fos), although still used in many studies [3], is not adequate. It has been stated that deactivation, due to the coverage of active sites by coke deposition and to pore blockage by coke growth [3, 4], should be directly related to coke itself emd not to tos [5]. In this way, coke formation is linked to the operating conditions, the nature of the feedstock and the type of catalyst. 

Since the deposition of metal compounds determines catalyst life we developed a model in which we used the deposition of metals as the principal parameter to describe the deactivation phenomena for equilibrium-coked catalyst. When coke deposition becomes over-... 

This was a Hquid-phase process which used what was described as siUceous zeoUtic catalysts. Hydrogen was not required in the process. Reactor pressure was 4.5 MPa and WHSV of 0.68 kg oil/h kg catalyst. The initial reactor temperature was 127°C and was raised as the catalyst deactivated to maintain toluene conversion. The catalyst was regenerated after the temperature reached about 315°C. Regeneration consisted of conventional controlled burning of the coke deposit. The catalyst life was reported to be at least 1.5 yr. 

The appHcations of supported metal sulfides are unique with respect to catalyst deactivation phenomena. The catalysts used for processing of petroleum residua accumulate massive amounts of deposits consisting of sulfides formed from the organometaHic constituents of the oil, principally nickel and vanadium (102). These, with coke, cover the catalyst surface and plug the pores. The catalysts are unusual in that they can function with masses of these deposits that are sometimes even more than the mass of the original fresh catalyst. Mass transport is important, as the deposits are typically formed... 

Catalysts in this service can deactivate by several different mechanisms, but deactivation is ordinarily and primarily the result of deposition of carbonaceous materials onto the catalyst surface during hydrocarbon charge-stock processing at elevated temperature. This deposit of highly dehydrogenated polymers or polynuclear-condensed ring aromatics is called coke. The deposition of coke on the catalyst results in substantial deterioration in catalyst performance. The catalyst activity, or its abiUty to convert reactants, is adversely affected by this coke deposition, and the catalyst is referred to as spent. The coke deposits on spent reforming catalyst may exceed 20 wt %. 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

These metals, when deposited on the E-cat catalyst, increase coke and gas-making tendencies of the catalyst. They cause dehydrogenation reactions, which increase hydrogen production and decrease gasoline yields. Vanadium can also destroy the zeolite activity and thus lead to lower conversion. The deleterious effects of these metals also depend on the regenerator temperature the rate of deactivation of a metal-laden catalyst increases as the regenerator temperature increases. 

Figure 9.10. Scheme of an FCC Unit. Cracking ofthe heavy hydrocarbon feed occurs in an entrained bed, in which the catalyst spends only a few seconds and becomes largely deactivated by coke deposition. Coke combustion in the regenerator is an exothermic process that generates heat for the regeneration and for the endothermic cracking process. 

Coke formation on these catalysts occurs mainly via methane decomposition. Deactivation as a function of coke content (see Fig. 3 for Pt/ y-AljO,) seems to involve two processes, i e, a slow initial one caused by coke formed from methane on Pt that is non reactive towards CO2 (see Table 3) In parallel, carbon also accumulates on the support and given the ratio between the support surface and metal surface area at a certain level begins to physically block Pt deactivating the catalyst rapidly. The coke deposited on the support very close to the Pt- support interface could be playing an important role in this process. 

J. Wood, L. F. Gladden 2003, (Effect of coke deposition upon pore structure and self-diffusion in deactivated industrial hydroprocessing catalysts), Appl.Cat. A General, 249, 241. 

The two limiting cases for the distribution of deactivated catalyst sites are representative of some of the situations that can be encountered in industrial practice. The formation of coke deposits on some relatively inactive cracking catalysts would be expected to occur uniformly throughout the catalyst pore structure. In other situations the coke may deposit as a peripheral shell that thickens with time on-stream. Poisoning by trace constituents of the feed stream often falls in the pore-mouth category. 

Coke builds up on the catalyst since the start up of operation. In the first weeks of operation, an amount between 5% and 8% of coke accumulates on the catalyst. The rate of deposition decreases with time on stream, a careful monitoring of temperature and of feed/H2 ratio is the basis for controlling deposition. Coke deposition primarily affects the hydrogenation reactions (and so denitrogenation), but the deposition rate determines the catalyst life. As mentioned above, deactivation is compensated by an increase in temperature (and some times in pressure, when denitrogenation has to be adjusted, as well). However, increasing severity, increases coke deposition and shorten catalyst life. 

---