Coke and catalyst deactivation

In resid cracking the high feed metals and Conradson Carbon Residue (CCR) require careful consideration when assessing both catalyst design and performance evaluation. This paper addresses the issues of the latter with respect to coke, delta coke and catalyst deactivation. 

Catalysts for coal liquefaction require specific properties. Catalysts of higher hydrogenation activity, supported on nonpolar supports, such as tita-nia, carbon, and Ca-modified alumina, are reasonable for the second stage of upgrading, because crude coal liquids contain heavy polar and/or basic polyaromatics, which tend to adsorb strongly on the catalyst surface, leading to coke formation and catalyst deactivation. High dispersion of the catalytic species on the support is very essential in this instance. The catalyst/support interactions need to be better understood. It has been reported that such interactions lead to chemical activation of the substrate 127). This is discussed in more detail in Section XIII. 

One obvious method of cleaning the feed is to remove asphaltic material (asphaltenes plus resins) using a solvent such as propane in a deasphalting unit. The resulting deasphalted oil has less metals than the original feedstock but coke formation and catalyst deactivation are not completely eliminated. The byproduct stream is usually only acceptable as a raw material for asphalt manufacture. Even then, the asphaltic by-product may be unsuitable for a specification grade asphalt and require disposal by other means. 

The catalyst and oil are in plug flow and the contact time is short so that secondary reactions are avoided and catalyst deactivation by coke formation is properly simulated. The resulting product selectivity, then, is similar to commercial units. Experimental results from a laboratory scale unit can thus be translated to commercial units. 

It has been observed that, the indirect effect of delta coke on catalyst deactivation, and the direct effect of delta coke on the blocking of acid sites (early) in the riser seems to be a prime factor, which dominates conversion and selectivity effects in a resid type of operation. 

The present contribution is a description of the technique of INS spectroscopy of catalysts and a summary of some recent experimental results that illustrate the usefulness of neutron spectroscopy. These include the characterization of model systems, commercial catalysts, mechanisms of coke deposition and catalyst deactivation, and the identification of atomic hydrogen in the topmost atomic layers of... 

It has been proposed that, as an increase in the conversion of vacuum residue in the commercial fixed-bed reactors, a coke-controlled catalyst deactivation regime appears in the last bed, where coke blocks the active sites as well as decreases the diffusivity. The activity and diffusivity tests were conducted for aged and regenerated catalysts, which were used in the commercial reactors, to investigate mechanisms of the deactivation by coke and metal deposition. The effects of residue conversion, reactor position, and time on-stream on the deactivation were investigated, comparing the catalysts aged at different conditions. 

Coke Formation in Catalytic Processes Kinetics and Catalyst Deactivation... 

The present paper aims at reviewing progress in the modeling of coke formation and catalyst deactivation along the lines set in Table 1, although it stresses to a large extent the kinetic formulation, an area in which considerable progress has been achieved. 

The catalysts used in Fluid Catalytic Cracking (FCC) are reversibly deactivated by the deposition of coke. Results obtained in a laboratory scale entrained flow reactor with a hydrowax feedstock show that coke formation mainly takes place within a time frame of milliseconds. In the same time interval conversions of 30-50% are found. After this initial coke formation, only at higher catalyst-to-oil ratios some additional coke formation was observed. In order to model the whole process properly, the coke deposition and catalyst deactivation have to be divided in an initial process (typically within 0.15 s) and a process at a larger time scale. When the initial effects were excluded from the modeling, the measured data could be described satisfactory with a constant catalytic activity. 

For the models evaluated in this work, the best model to describe all experiments was the five lump model with a first order deactivation, although it did not describe the first part of the reactor correctly, obviously due to an incorrect description of the initial effects. When the initial effects were excluded, a model with a constant activity described the data satisfactory. Therefore, coke deposition and catalyst deactivation have to be divided in an initial process (<0.15 s) and a process on a longer time scale. 

Effective solutions to the problems of the vacuum residue hydrodesulfurization unit equipped with the fixed bed reactors, such as a hot spot, pressure-drop buildup, and catalyst deactivation by coke fouling, were discussed. Improving liquid distribution can prevent hot spot occurrence. Dispersing inorganic solids throughout the reactors can control a pressure-drop increase in the first bed. For a high conversion operation, controlling the conversion in each bed can minimize the coke deactivation in the fourth bed. 

Our results on the effects of coking on the catalyst and its performance are in agreement with the generally accepted view on the deactivation of residue HDS catalysts, in that the main cause of deactivation is coke and metal deposits near the pore mouths (21). These deposits lead to constricting, but not completely blocking the pores of the catalyst. However, the results of the present work highlight the importance of initial coking on catalyst deactivation. 

The oligomerization of olefins is an exothermic consecutive reaction, which benefits from the application of CD for enhanced selectivity to intermediate products. Catalytic distillation plays a particularly important role in enhancing the catalyst lifetime because in situ separation reduces the undesirable high-molecular-weight oligomers or polymers, which will form coke and deactivate the catalyst. The use of reaction heat for distillation also reduces the formation of hot spots and catalyst deactivation due to sintering. 

A review is given of the use of spectroscopic techniques to investigate the MTG process. Examples are quoted of catalyst characterization, investigation of the first carbon-carbon bond formation, alkene oligomerization and catalyst deactivation through coke formation. 

Shape selectivity and catalyst deactivation. A serious problem in catalytic cracking and other refinery operations is catalyst deactivation by coking. Coke forms on the catalyst from bulky molecules such as polyalkyl benzenes and polycyclic aromatics that are slow or unable to escape from the catalyst [57], These molecules, in turn are formed mainly from cracked olefins. Coking is severe in zeolites with window-and-supercage structure (chabazite, erionite, Linde A). Zeolites like ZMS-5, with straight channels and no supercages, are much less affected because the formation of bulky coke precursors is sterically inhibited [58]. 

Selective alkylation is therefore highly desired. Zeolites have proven to have excellent properties in this respect, and shape-selective reactions on these materials are well known [14]. Dow Chemical pioneered the shape-selective alkylation of polynuclear aromatics with olefins such as propylene, using as catalysts modified mordenite zeolites, which were not considered at the time to behave strictly as shape-selective catalysts [15,16]. Mordenite zeolites were not the catalyst of choice for such reactions because the alkylation of aromatics, and in particular of polynuclear aromatics, was recognized already as a first step in the reactions leading to coke formation and catalyst deactivation [17]. How was it possible to convert these unsuitable zeolites into stable and highly shape-selective catalysts for industrial applications The answer to this question will be used to illustrate the criteria and methods used to develop the so-called 3-DDM catalysts, or 3-dimensional dealuminated mordenites. 

Figure 5 shows the effect of the reduction temperature on the activity of the catalyst. It can be seen that an increase in the reduction temperature (i.e. an increase in the degree of reduction) produces a reduction in the coke formation due to the greater deactivation observed for the samples reduced at high temperature. The H2 production results confirm this fact, since the catalyst reduced at low temperature shows the lowest deactivation rate. An increase in the reduction temperature increases the size of the Ni crystallites (Fig. 1, curves b, d and e). This factor modifies the rates of nucleation, coke diffusion and catalyst deactivation [15,16]. 

There is a great similarity in the catalytic transformation of ethanol and of methanol on a HZSM-5 zeolite, either concerning the reaction mechanism [4], or in the spectra of products [5,6], Nevertheless, the high content of water in the feed in the BTG process markedly influences the distribution of products and catalyst deactivation [5-7], It has been proven that water attenuates deactivation by coke at moderate temperatures [6], but causes irreversible deactivation by dealumination of the zeolite at high temperatures. 

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