Catalyst pellets coke deposition

There are several factors that may be invoked to explain the discrepancy between predicted and measured results, but the discrepancy highlights the necessity for good pilot plant scale data to properly design these types of reactors. Obviously, the reaction does not involve simple first-order kinetics or equimolal counterdiffusion. The fact that the catalyst activity varies significantly with time on-stream and some carbon deposition is observed indicates that perhaps the coke residues within the catalyst may have effects like those to be discussed in Section 12.3.3. Consult the original article for further discussion of the nonisothermal catalyst pellet problem. 

The catalysts used for processing heavy feedstocks inevitably deactivate with time due to accumulation of nickel and vanadium sulfides and carbonaceous residues (coke) on the catalyst. The former deposits have been studied intensely, in part because the metal deposits tend to accumulate near the surface of catalyst pellets, rendering the interior ineffective. Both metal sulfides and coke may contribute to loss of activity. 

A trickle-bed reactor was used to study catalyst deactivation during hydrotreatment of a mixture of 30 wt% SRC and process solvent. The catalyst was Shell 324, NiMo/Al having monodispersed, medium pore diameters. The catalyst zones of the reactors were separated into five sections, and analyzed for pore sizes and coke content. A parallel fouling model is developed to represent the experimental observations. Both model predictions and experimental results consistently show that 1) the coking reactions are parallel to the main reactions, 2) hydrogenation and hydrodenitrogenation activities can be related to catalyst coke content with both time and space, and 3) the coke severely reduces the pore size and restricts the catalyst efficiency. The model is significant because it incorporates a variable diffusi-vity as a function of coke deposition, both time and space profiles for coke are predicted within pellet and reactor, activity is related to coke content, and the model is supported by experimental data. 

Avoiding carbon deposition on the catalyst is a major challenge [2, 3]. Carbon can be present as graphite-like coke and in the form of whiskers, or carbon nanofibers. The latter lead to detachment of the nickel crystallites from the support and breaking of the catalyst pellets. This may cause blockage of the reformer reactor tubes and the formation of hot spots. Higher hydrocarbons exhibit a larger tendency to form... 

Coke on the catalyst is, thus, largely responsible for catalyst deactivation by loss of surface area, and this could be minimized by increasing the hydrogen pressure. However, increasing pressure has been reported to increase vanadium deposition more near the exterior surface of the catalyst pellet (13,14). In essence, an increase in the hydrogen pressure has a beneficial effect in suppressing coke formation, but can lead to shorter catalyst life due to rapid accumulation of vanadium at pore mouths. 

In a fixed-bed reactor, if coke is formed primarily from the reactants, there will be a gradient in catalyst activity, with the highest coke content and the lowest activity in the first part of the bed. If a reaction product is the main coke precursor, the coke level will be highest and the activity low toward the end of the bed. Additional complexity arises if the coke-deposition reaction is diffusion limited, which leads to a gradient of coke content within the catalyst pellet. Many theoretical studies have been made showing the shape of profiles of coke content and catalytic activity in catalyst beds subject to fouling, but there are few comparisons of theory and experiment and no way to predict the fouling rate for a new system. 

Pyrolytic coke R49 Encapsulation of catalyst pellet, deposits on tube wall high temperature, long residence time, presence of olefins, sulphur poisoning... 

It has been shown [440] [518] for coke deposits on cracking catalysts that the reactivity after oxidation depends mainly on the surface area of the coke and that the rate quickly becomes diffusion limited with a risk of overheating the catalyst pellet. In practice, the bum-off of coke can easily be performed by adding a few percent of air to the steam flow at temperatures above approximately 450°C as illustrated in Figure 5.38 [388] [389]. 

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