Catalysts deactivation by coke deposits

Catalyst deactivation by coke deposition is a major concern in upgrading coal-derived oils. Coke forms as a results of a sequence of side reactions which may be simplified as follows ... 

Catalyst Deactivation by Coke Deposition. The catalyst employed in deep desulfurization of diesel fuel is deactivated by coke deposition onto the catalyst. Coke deposition affects not only the surface activity but also the diffusivity of the reactants, because the pore diameter is relatively small in this case. The catalyst deactivation data suggest that the effectiveness factor is smaller than 1.0. 

Figures lI.S.f-6, 11.5i"-7, and ll-SJ-S compare experimental data with results obtained from a simulation on the basis of the above equations and the best set of parameters. The agreement is quite satisfactory and the approach appears to be promising for the characterization of catalysts deactivated by coke deposition.

Another important process in which catalyst deactivation by coke deposits plays an important role is propane dehydrogenation, which can be performed with a variety of materials, including metal and metal oxide catalysts. Different in situ and operando spectroscopies have been applied to these catalysts, including UV-vis, Raman, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopies [4, 115, 140],... 

A series of experiments varying temperature, micro-sphere size and time on stream have been performed in a fixed fluidised bed microactivity reactor to study the role of intraparticle diffusion in commercial fluid catalytic cracking (FCC) catalysts, particularly on gasoline yield and catalyst deactivation by coke deposition, for the cracking of a vacuum gas oil. Additionally, a mechanistic model that describes interface and intrapartide mass transfer interactions with the cracking reactions, has been used to study the combined influence of pore size and intraparticle mass diffusion on the deactivation of FCC catalysts and the gasoline yield. 

Example 5.3.3.B Application to Industrial Processes Rigorous Kinetic Equations for Catalyst Deactivation by Coke Deposition in the Dehydrogenation of 1-Butene into Butadiene... 

RIGOROUS KINETIC EQUATIONS FOR CATALYST DEACTIVATION BY COKE DEPOSITION IN THE DEHYDROGENATION OF 1-BUTENE INTO BUTADIENE... 

It is a highly endothermic process and requires severe operational temperatures (800-1000°C) to reach high conversion levels in conventional fixed bed reactors. These severe operating conditions will result in catalyst deactivation by coke deposition due to deep cracking of methane, which is thermodynamically favored at high temperatures. In the MRs, it is possible to achieve either a higher conversion than the traditional process at a fixed temperature, or the same conversion but at lower temperatures. Catalyst deactivation can be suppressed by lowering the temperature, as coke deposition via methane decomposition is thermodynamically limited. 

Marin GB, Beeckman JW, Froment GF Rigorous kinetic-models for catalyst deactivation by coke deposition—apphcation to butene dehydrogenation, J Catal 97(2) 416—426, 1986. 

Figure 9.10. Scheme of an FCC Unit. Cracking ofthe heavy hydrocarbon feed occurs in an entrained bed, in which the catalyst spends only a few seconds and becomes largely deactivated by coke deposition. Coke combustion in the regenerator is an exothermic process that generates heat for the regeneration and for the endothermic cracking process. 

The problem associated with zeolites as nitration catalysts will be a reversible deactivation by coke deposition, and an irreversible deactivation by framework A1 removal (acid leaching). Optimization of zeolite activity, selectivity and life will be controlled by density of acid sites, crystalline size and hydrophobic/hydrophilic surface properties. 

The total acidity deterioration and the acidity strength distribution of a catalyst prepared from a H-ZSM-5 zeolite has been studied in the MTG process carried out in catalytic chamber and in an isothermal fixed bed integral reactor. The acidity deterioration has been related to coke deposition. The evolution of the acidic structure and of coke deposition has been analysed in situ, by diffuse reflectance FTIR in a catalytic chamber. The effect of operating conditions (time on stream and temperature) on acidity deterioration, coke deposition and coke nature has been studied from experiments in a fixed integral reactor. The technique for studying acidity yields a reproducible measurement of total acidity and acidity strength distribution of the catalyst deactivated by coke. The NH3 adsorption-desorption is measured by combination of scanning differential calorimetry and the FTIR analysis of the products desorbed. 

Methanol adsorption and decomposition on noble metals have been the subject of many surface-analytical investigations (e.g., References 94,171,320,350,378, 478 94). CH3OH dehydrogenation on palladium catalysts could be a valuable source of synthesis gas or hydrogen, but unfortunately catalyst deactivation by carbon deposits (coking) seriously limits this process (495-498). In this respect, the probability of O H vs. C O bond scission is important, the first path resulting in CO and H2, and the second in carbon or carbonaceous species (CH x = 0-3), CH4, and H2O (see scheme in Fig. 49 details are discussed below). 

The Mizushima Oil Refinery of Japan Energy Corporation first implemented a high conversion operation of vacuum residue, versus a constant desulfurization operation, in the commercial residue hydrodesulfurization unit equipped with fixed-bed reactors, to produce more middle distillates as well as fuel oil with lower viscosity. The catalysts will be replaced when the sulfur content in the product oil reaches the allowable limit. Since we have believed that an increase in the residue conversion decreases the catalyst activity by coke deposition, we have been interested in controlling the coke deactivation to maximize the residue conversion during a scheduled operating period. 

Catalyst A with a small pore size rapidly lost its activity due to plugging of the catalyst pores from SOR to EOR. This type of catalyst is not strongly deactivated by coke deposition, because asphaltenic compounds, which easily make coke on the catalyst surface, are not allowed to diffuse into the catalyst pores. However, this catalyst is easily deactivated through pore plugging by metal deposition. 

The Likun Process (China) uses a two-stage cracking process under normal pressures where the waste plastics are first pyrolyzed at 350-400°C in the pyrolysis reactor and then the hot pyrolytic gases flow to a catalyst tower where they undergo catalytic reforming over zeolite at 300-380°C. By having the catalyst in the second stage this overcomes the problems of rapid catalyst deactivation from coke deposits on the surface of the catalyst. 

In relation to catalyst deactivation by cuke deposits, two questions on the coke topology are of major Importance, viz,... 

The bimetallic Pt-Re/Al203-Cl catalyst is the most widely used in naptha reforming. The addition of Re strongly improves the stability of the traditional monometallic Pt catalyst. Such improvement is explained by a double effect of rhenium stabilization of the metallic phase on the support and higher resistance to deactivation by coke deposition [1-8]. Nevertheless the role and the nature of the interaction between Pt and Re are the subject of many controversies [9-14]. 

Coke deactivation on Pt/Al203 catalysts have been studied intensively in the literature. Previous works have focused on the kinetics of catalyst deactivation [7] the influence of additives on coke formation [8] the coke deposition on different morphologic surfaces [9] the structure [10] and chemical composition of coke [11]. Deactivation by coke deposition on niobia supported catalysts, or even on other reducible supports which promote SMSI effect has not been studied. 

In similar studies, the extent of formation of coke deposits during the initial stages of reaction has been found to decrease with the acidity of a range of catalyst [28]. Once deposited, the coke was found to reduce the activity of catalysts for acid-catalysed reactions such as isomerisation [24] or hydrocracking [29]. The inference is clear - initial coking involves acidic sites on the catalyst which are deactivated by coke deposition. 

Deactivation of catalysts, particularly by coke deposition (the main means of reversible FCC catalyst deactivation) has been the subject ofintensive study over the past 50 years (2-4). Initially, the loss of activity was correlated with the time on stream, but it is now generally accepted that a more appropriate approach to understanding the effect of deactivation by coke is to relate deactivation to the deposited coke concentration (5). Furthermore, few studies on the effect of catalyst formulation on both the product distribution and coke formation have appeared in the open literature. 

The dehydrogenation of butene to butadiene over a 20% chrome on alumina catalyst is an important industrial process. This catalytic reaction, that is deactivated by coke deposition, has been extensively studied by Froment et al. [10,11]. Faccio et al. [7] have simulated this process using a site-bond-site model on a Bethe network. 

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