Coking and carbon formation

Fouling, coking, and carbon formation are used interchangeably and refer to the physical deposition of species from the fluid phase onto the catalyst surface, resulting in activity loss due to blockage of sites and/or pores. Coke-forming processes may also be accompanied by the chemisorption of condensed hydrocarbons which act as poisons. 

Effect of Aromatics on Formation of Coke and Carbon. Conventional liquid fuels contain widely differing levels of aromatics gasoline usually contains more than diesel. The studies show that these compounds cause more rapid deactivation than linear alkanes alone. A related result is that the steady state conversion of diesel fractions is also reduced in the presence of aromatics—i.e., these compounds act as kinetic inhibitors, limiting the production of H2. 

In another study by the same group, K, Ca and CaK promoters were added to Rh supported on MgAl204 spinel. Both modified and unmodified catalysts produced similar H2 and CO reformate concentrations of 23 and 25 vol%, respectively. The different modifiers did affect carbon production on the catalysts. The unpromoted Rh catalyst showed carbon levels of 0.03 wt% carbon, where the RhK catalyst had 0.02 wt% carbon, the RhCa 0.015 wt%, and the RhCaK only 0.01 wt%. Bimetallic Pt Rh with Li, Ba and Li-Ba modifiers supported on MgAl204 spinel were also examined for H2 and CO yields and carbon formation resistance, with similar results to previous work for the yields. The unpromoted Pt-Rh catalyst showed carbon levels of 0.01 wt% carbon, where the promoted Pt-Rh catalysts all showed reduced coking levels of 0.005 wt%. 

When alkanes are the sole products, Eqs. (3.46)-(3.49) represent the principal reactions with the formation of water, hydrogen, coke, and carbon oxides as byproducts eq. (3.49) describes the formation of aromatics ... 

Catalysts deactivate during use. The most common reason for deactivation is coke or carbon formation on and in the catalyst particles. The carbon can be burned off under carefully controlled conditions to regenerate the catalyst with activity and selectivity very nearly like that of a new catalyst. In the removal of carbon by oxidation, the quantity of oxygen and the temperature to which the catalyst particles are exposed must be limited. 

The capital cost of POX can be high because of the need for post treatment of the raw syngas to remove carbon and acid gases. There are also issues of coke and soot formation if the oxidation temperature becomes too low or the mixing of the feed components is incomplete. The addition of steam to the process allows for greater flame temperature control and suppression of carbon however, the hydrogen production efficiency is reduced due to more fuel being consumed in the combustion... 

The second broad grouping of papers (Chapters 13-17) describes useful design modifications for commercial plants. Surface reactions resulting in both the formation of coke and carbon oxides, and the destruction of olefins and other desired products are described. In quartz or Vycor glass reactors, however, such surface reactions are relatively unimportant and often have not been considered. Yet such reactions can be most significant in metal reactors when steam is used as a diluent of the entering feedstock. Data obtained in metal reactors are of value to the designer of plant equipment. 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

As the metal particle size decreases the filament diameter should also decrease. It has been shown that the surface energy of thirmer filaments is larger and hence the filaments are less stable (11,17-18). Also the proportion of the Ni(l 11) planes, which readily cause carbon formation, is lower in smaller Ni particles (19). Therefore, even though the reasons are diverse, in practice the carbon filament formation ceases with catalysts containing smaller Ni particles. Consequently, well dispersed Ni catalysts prepared by deposition precipitation of Ni (average metal particle size below 2-3 nm) were stable for 50 hours on stream and exhibited no filamentous coke [16]. 

We propose that the complicated dry oxidation of bitumen can be represented as the sum of contributions from two classes of oxidation reaction. One class of reactions is the partial oxidation that leads to deposition of coke and formation of "oxygenated bitumen", with very little production of carbon oxides and water. This class of reactions is concisely summarized by... 

Propylene cokage experiments followed by gravimetry have shown that higher is the 5A zeolite calcium content, higher are the cokage kinetics and carbon content inside the pores (Fig. 1). The total carbon contents retained in the porosity after desorption at 350°C of physisorbed propylene are 14.5% and 11% for 5A 86 and 5A 67 samples respectively. These carbon contents are relatively important and probably come from the formation of heavy carbonaceous molecules (coke) as it has been observed by several authors [1-2], The coke formation requires acid protonic sites which seems to be present in both samples but in more important quantity for the highly Ca-exchanged one (5A 86). 

Another cause of activity loss is carbon deposition, which can be avoided if a high steam to carbon (S/C) ratio is employed [45, 46], However, economic evaluations indicate that the optimum S/ C ratio tends to be low. The presence of tars in the reforming reactor enhances coking and it is the main cause of carbon formation in reforming a gas from biomass thermal conversion [29]. 

There are three major gas reformate requirements imposed by the various fuel cells that need addressing. These are sulfur tolerance, carbon monoxide tolerance, and carbon deposition. The activity of catalysts for steam reforming and autothermal reforming can also be affected by sulfur poisoning and coke formation. These requirements are applicable to most fuels used in fuel cell power units of present interest. There are other fuel constituents that can prove detrimental to various fuel cells. However, these appear in specific fuels and are considered beyond the scope of this general review. Examples of these are halides, hydrogen chloride, and ammonia. Finally, fuel cell power unit size is a characteristic that impacts fuel processor selection. 

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