4.+Removal

__Coke Removal __
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The loss of catalytic activity in most processes is inevitable. Thus when the activity has declined to a critical level, one must decide on four alternatives in order to continue the process:

1. Restore the activity of the catalyst. 2. Use the catalyst for another application. 3. Reclaim and recycle the important and/or expensive catalytic components. 4. Discard the catalyst.

The first alternative (regeneration and reuse) is almost always preferred; catalyst disposal is usually the last resort especially in view of environmental considerations. Fortunately due to the reversibility of carbon and coke formation which deactivates the catalyst by fouling, restoration of the fouled catalysts is easily done.(Bartholomew 2001)

=General Mechanism of Coke Removal =

﻿﻿Coke and other carbonaceous materials can be removed by gasiﬁcation with oxygen, hydrogen, water and carbon dioxide, as described by the following generalized reactions (Trimm 2001):



Specifically the coke undergoes several different processes during removal:

1. Pyrolysis – occurs as the coke heats up, volatiles are released and lighter carbon compounds are produced. 2. Combustion – some of the lighter carbon compounds and the volatiles that were released react with oxygen to form carbon dioxide and carbon monoxide, providing heat for the subsequent gasification reactions 3. Gasification – the lighter carbon compounds react with carbon dioxide and steam to produce carbon monoxide and hydrogen (this process becomes more favorable in the presence of a catalyst) 4. In addition, the reversible gas phase water gas shift reaction reaches equilibrium very fast with increasing temperature. This balances the concentrations of carbon monoxide, steam, carbon dioxide and hydrogen.

The following equation describes the reversible gas phase water gas shift reaction:

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">This resulting gas mixture from the third process is a useful product that can be used as a fuel or reactant in other processes. Problems can arise due to the inconsistent composition of the coke and the sensitivity of the catalyst to high temperatures. Therefore an important consideration when executing this process is the temperature required to gasify these deposits at a reasonable rate without causing high temperatures to further deactivate the catalyst. This is especially true when a fouled catalyst is regenerated by heating the catalyst in the presence of air, since it is a very exothermic process ( Trimm 2001 ).

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">Air regeneration is commonly used to remove coke from catalysts in: catalytic cracking, hydrotreating processes, and catalytic reforming. The use of air over any other gasification reagent is because of the relative rates of gasification that can be generalized to be in the following order:

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;"> <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">It was (Walker 1959) reported that the specific relative rates of uncatalysed gasification at 800◦C were:

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;"> <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">But this is a generalization and subject to correction based on the catalysts present and other pertinent reaction conditions. Furthermore, the rates of gasification of coke are accelerated by the same metal or metal oxide catalyst that had been fouled by the coke.

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">The combustion process is typically controlled by initially feeding low concentrations of air and by increasing oxygen concentration with increasing carbon conversion; nitrogen gas can be used as a diluent in laboratory-scale tests while steam is used as a diluent in full-scale plant operations. <span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;"> =<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 80%; line-height: 115%;">Coke Removal from Fouled Zeolite Catalysts =

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">The process for removal of coke from zeolite catalysts is similar for other catalysts. But due to the complexity of zeolite structures, the choice of operating conditions, especially temperature is important for limiting the detrimental effect the water produced by coke oxidation has on the zeolite catalyst: including processes which alter the structure of the zeolite.(Gulsnet et al. 1997)

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">A simple solution for limiting the degradation of the zeolite is to operate the removal of coke in two stages: In the first stage the majority of the hydrogen atoms of coke molecules are oxidized at sufficiently low temperature, so as minimize the degradation of the catalyst by avoiding its contact with steam at a high temperature. The second stage occurs at a higher temperature to remove the remaining carbon in the form of carbon monoxide and carbon dioxide. To illustrate the temperatures at which various gasification products are producedthe following figure shows the evolution of water, carbon monoxide and carbon dioxide as function of oxidation temperature for CBV 500 ( 8.5 wt.-% C).(K. Moljord et al. 1995)



<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">The pore structure of the zeolite and the coke content has only a limited effect on the rate of coke oxidation. However the composition of the zeolite seems to have a significant effect. For example the main parameter that contributes to the rate of oxidation of coke on acid zeolites is the density of the acid sites. This was explained by the fact that radical cations formed through reaction of molecular oxygen on coke molecules adsorbed on protonic sites were proposed as intermediates in coke oxidation (K. Moljord et al. 1995).

<span style="font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">In addition, to facilitate coke removal combustion accelerators (such as platinum or palladium) can be introduced in low amounts in the zeolite catalysts in order to obtain an easier elimination of coke and to convert carbon monoxide into carbon dioxide. The advantages of using the accelerators include a greater activity of the regenerated zeolite, a shorter period of regeneration, and no need for a carbon monoxide afterburner.