Coking reactions

In conclusion, decrease in cyclohexanone oxime yield and caprolactam selectivity with time on stream is a major factor in the use of boria on alumina catalyst in the rearrangement reaction. Coke deposition and basic by-product adsorption have been suggested as a means of deactivation. In addition the conversion of water soluble boron, which is selective to lactam formation, to an amorphous water insoluble boron species is another factor that can account for the catalyst deactivation. 

In the case of gas-phase hydrocarbon reactions, coke retention occurs for two main reasons 1771 (1) the condensation under liquid or even solid state of coke molecules on the catalyst is generally observed at low temperature (<473 K) coke molecules are therefore not sufficiently mobile or volatile to be eliminated from the catalyst under operating conditions and (2) the steric blockage (trapping) within the pores that often occurs at high temperatures (>623 K), when the size of the product molecules formed within the pores becomes intermediate between the size of the cages or channels and that of the pore apertures. 

In theory, favorable conditions correspond to a pressure of 0.1.10 Pa and temperatures not exceeding 350 C However, cracking reactions (coke formadon) are excessive in this case, and the selecdvity of the operation is reduced. Hence the reactions producing aromatics must be activated selectively, and operations comlucted at a sufficiently hi partial pressure of hydrogen. 

Effects of diffusion on the reaction, coke formation, and deactivation (38,87-89). 

High sensitivity, fast response, and well-defined flow patterns make the TEOM an excellent tool for determining diffusivities of hydrocarbons in zeolites. Moreover, the TEOM has provided a unique capability for gaining knowledge about the effects of coke deposition on adsorption and diffusion under catalytic reaction conditions. An application of the TEOM in zeolite catalysis by combining several approaches mentioned above can lead to a much more detailed understanding of the catalytic processes, including the mechanisms of reaction, coke formation, and deactivation. 

Deactivation of light naphtha aromatization catalyst based on zeolite was studied, by kinetic analysis, micropore volume analysis and model reactions. Coke accumulates at the entrance of zeolite channel, blocks it and hinders reactant molecule to access active sites in zeolite channel. Our own stabilization technique passivates coke-forming sites at the external surface of the zeolite. This minimizes the coke formation at the entrance of zeolite channel and increases on-stream stability. The stabilized catalyst enabled us to develop a new light naphtha aromatization process using an idle heavy naphtha reformer that is replaced by CCR process. 

The experimental methods used to recover the compounds retained in the USHY zeolite after reaction ("coke") as well as to analyze them have already been described (ref. 9). It has been shown that the treatment of solubilization of the zeolite by hydrofluoric acid did not cause any transformation of the coke constituents (ref. 10). 

Hard" Delta Coke = Reaction Coke + Feed CCR Coke... 

In general the main emphasis and progress in the development of low-delta-coke Resid FCC catalysts has been in the reduction of reaction coke [4,5,10], Table 2 gives an impression of the improvements which have been obtained in recent years. 

A comprehensive model for the steam reformer should be developed. This model should take into consideration the characteristics of the combustion chamber as well as the details of the processes taking place in the catalyst tubes steam reforming reaction, coke formation, reoxidation of Ni and rereduction of NiO, mass and heat transfer between the catalyst pellets and the bulk gas (both external and intraparticle), heat transfer between the catalyst tubes and the combustion chamber. .. etc. 

Coke formation on the catalyst proceeds in same way as was shown in section 6.1 for thermal cracking. However, the presence of the catalyst changes the mechanism of the polycondesation reaction. Coke formation in all catalytic processes proceeds by the ion mechanism and not by the radical chain mechanism applicable for thermal processes. One example of a possible pathway for coke formation is shown in reaction (6.52). 

High hydrogen pressure can prevent coke formation, since hydrogen is a product of polycondensation reactions (coke formation). If alkali and alkaline-earth metals or their oxides are added to the catalyst, these promoters may increase the gasification of coke on the catalyst [19]. Less coke will be formed on a less acidic support [16]. 

The decrease in n-octane DHC activity is caused by coke deposition and proceeds steadily from the very beginning of the reaction. Coking decreases the pore volume and BEIT surface area more slowly than the DHC rate. The degree of deactivation is the same for the dehydrogenation of the small hydrogen and the large n-octane molecules. TTie results justify the assumption that coke reduces directly the activity and/or the number of the active sites responsible for the dehydrogenation and that catalyst deactivation is not caused by coke induced mass transport limitations. 

The deactivation of zeolites depends very much on their physicochemical characteristics and on the operating conditions. Although each reaction system (desired reaction + coking reaction) is a particular case in itself, some general rules can be proposed which will reduce coking and the deactivating effect of coke ... 

Then at 550-575°C in the Incoloy reactor and at 575-600°C in the 304 stainless steel reactor, significant reactions were noted. Products were obtained of what was apparently both gas-phase free-radical reactions and surface deconposition reactions coke and carbon oxides were both formed in significant amounts. The production of carbon oxides indicated that the propylene was reacting with surface oxides on the reactor surface. The level and the types of surface oxides were changing (e.g. Fe O was probably converted to Fe,0, or FeO), hence changing the sorface activ-... 

As a result of the cracking reactions, coke is deposited on the catalyst, consequently the catalyst is poisoned and has to be regenerated. This exothermic regeneration process is carried out by circulating it to a fluidized bed regenerator, where under excess oxygen, the coke is burned off the catalyst at a temperature and pressure of about 1272 T (690 °C) and 34 psia (2.3 bara) respectively. The process conditions should ensure that nearly all carbon monoxide produced in the bed is converted to carbon dioxide. The carbon monoxide concentration in the stack gas should meet the following constraint Xco < 10" mol/mol. 

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