2.+Formation

__Methods of Formation﻿ __
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Due to the nature of fouling on catalysts, there are several different ways in which molecules can build-up on catalysts. Since there are many fouling molecules and different catalysts being used, the main focus of this section will be on coke formation.This section focuses on the effects of different operating conditions, metal catalysts and metal oxide catalysts. In general though, here are some examples of what fouling looks like with a before and after on a metal catalyst:

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=Operation State =

Plants and facilities can only operate under two different modes of operation: transient (during start-up and shut-down) and steady state. For the transient state, we can see fouling occurring mainly through phase change. Due to the nature of plant operation, the activation/shut down of different parts often is a very complicated and difficult procedure and this leads to temperature fluctuations within the reaction chamber causing some of the gas molecules to condense onto the catalyst. During steady state operation we see many different pathways which fouling can occur but the root of all these mechanisms is from the high temperature of operation. At higher temperatures we see many more radicals and smaller by-products form which normally aren’t feasible at lower temperatures. At this higher temperature all the molecules have more energy (therefore more motion) and in general, smaller particles can diffuse through Brownian motion into crevasses and pores in catalysts more frequently while larger molecules have the capability of forming into complex aromatics and adsorbing to catalysts(Fu et al. 1998).

=Coke Formation =

One of the most prominent types of fouling is coke formation; it appears much more than any other form of fouling in catalysts mainly because the applications where it occurs are very common(ie hydrocrackers and catalytic converters). It has been noted that there are two important types of coke formations, coke sensitive and coke insensitive. Coke insensitive formations are usually formed by carbon compounds which are generally unreactive and cannot be readily removed while the coke insensitive is usually more reactive carbon compounds (such as those with double bonds or aromatics) which can be removed with hydrogen or other gasifying agents. While this definition is useful for generalizing reactions, most catalytic fouling is based on the catalyst itself and whether it’s a pure metal or uses addition non-metals to bolster selectivity(Bartholomew 2000).

=Metal Catalysts =

With both fixed-support and un-supported metal catalysts, the carbon formation is heavily dependent on the temperature and feed, but there are three general ways that carbon compounds form. In the first mechanism, carbon compounds strongly chemisorb to the surface of the metal compound or adsorb to form multiple layers on the catalyst. In another mechanism, the carbon molecules adsorb and slowly attach to each other to envelope the catalyst. The third mechanism is where the carbon compounds get stuck in the pores and slowly clogs the catalyst and builds up from the inside out. In certain cases when the carbon compounds enter the pores, they can cause an unnatural stress increase within the pore. This stress can evertually fracture the catalyst, completely destroying the pore site or even the entire catalyst in rare circumstances. In general with metal catalysts due to their stability and relative unreactive pores sites, the carbon compounds end up adsorbing to the surface and either enveloping the carbon form the inside to the out, the outside to the inside, or forming multiple layers over the catalyst (Bartholomew 2000).

=Metal Oxide/Sulfide Catalysts =

 Metal oxide catalysts differ from plain metal catalysts because they usually contain acidic pore spaces which affect the formation of coke. These acidic pores aid in the alteration of basic carbon compounds into cyclic and aromatic chains which bond with carbon radicals to form high molecular weight compounds which stick to catalysts. The formation of these compounds is increased when high molecular weight carbon compounds are entered into the feed stream. Comparable to the metal compounds, it can be seen that at different temperatures different coke forms and also with different catalysts (ie zeolites) the pore size can effect what coke is formed. One of the main problems with the fouling of metal oxides is that the coke can strongly adsorb onto the catalyst surface making it difficult to remove; this also allows it to create several layers and attract more carbon compounds.