Catalyst coke

A completely new approach for BTX production has emerged in recent years. It converts to paraffins into aromatics using a modified ZSM-5 zeoHte catalyst which contains gallium (19). An example of this approach, the Cyclar process, has been in commercial operation by British Petroleum at Grangemouth, Scotiand since August 1990 (20). It uses C —feed and employs UOP s CCR technology to compensate for rapid catalyst coking. 

The presence of contaminant metals on the equiUbrium catalyst can significantly increase the catalyst coking tendency, which in turn results in an increase in regenerator temperature if all other factors remain unchanged. As one example, if the metals on an FCCU equiUbrium catalyst increased from an equivalent-nickel value of 2000 wt ppm to 3500 wt ppm, the catalyst coke factor would increase 30—50%. If all controllable parameters remained constant, the regenerator temperature would be expected to increase 35—50°C and conversion would drop. 

A large quantity of hydrogen-rich separator gas is normally recycled with the feed stream. Recycle rates may vary from 2,000 to 10,000 MSCF/B. The recycle gas serves to suppress catalyst coke make but normally has relatively little direct effect on gasoline yields or catalyst requirement. However, at lower recycle levels, where an increase in recycle rate may significantly increase reactor hydrogen partial pressure, the effect is similar to a small increase in total... 

The design frequency of regeneration is normally from three to six months for semi-regenerative units, and one reactor every 24 hours in cyclic units. For either case, an increase in regeneration frequency would result in a reduction in average catalyst coke level. Thus, gasoline yields would increase and catalyst requirements decrease. 

Dumez, F.J. and G.F. Froment, "Dehydrogenation of 1-Butene into Butadiene. Kinetics, Catalyst Coking, and Reactor Design", Ind Eng. Chem. Proc. Des. Devt., 15,291-301 (1976). 

Lebreton, R. Brunet, S. Perot, G., et al., Deactivation and characterization of hydrotreating NiMo/AL203 catalyst coked by anthracene. Studies in Surface Science and Catalysis, 1999. 126 p. 195. 

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

Catalyst coking studies were carried out on the protonated and modified ZSM-5 zeolites. Approximately 0.8 g of zeolite was heated to 823 K followed by exposure to 2-butene at 100 ml/min for a specific time to obtain samples with varying degrees of coking, as shown in Table I. To achieve a similar wt % coke, the modified Na, H-ZSM-5 zeolite required a 33% longer exposure time to 2-butene. 

On active Ni-based catalysts, coke formation is apt to occur. The primary site of carbon formation is the acidic metal-promoted supporting oxide [7], This catalytically active oxide is however necessary for the majority of catalytic reactions and is essential for high steam... 

The deactivation of a lanthanum exchanged zeolite Y catalyst for isopropyl benzene (cumene) cracking was studied using a thermobalance. The kinetics of the main reaction and the coking reaction were determined. The effects of catalyst coke content and poisoning by nitrogen compounds, quinoline, pyridine, and aniline, were evaluated. The Froment-Bischoff approach to modeling catalyst deactivation was used. 

A trickle-bed reactor was used to study catalyst deactivation during hydrotreatment of a mixture of 30 wt% SRC and process solvent. The catalyst was Shell 324, NiMo/Al having monodispersed, medium pore diameters. The catalyst zones of the reactors were separated into five sections, and analyzed for pore sizes and coke content. A parallel fouling model is developed to represent the experimental observations. Both model predictions and experimental results consistently show that 1) the coking reactions are parallel to the main reactions, 2) hydrogenation and hydrodenitrogenation activities can be related to catalyst coke content with both time and space, and 3) the coke severely reduces the pore size and restricts the catalyst efficiency. The model is significant because it incorporates a variable diffusi-vity as a function of coke deposition, both time and space profiles for coke are predicted within pellet and reactor, activity is related to coke content, and the model is supported by experimental data. 

Figure 4. Most frequent pore diameter vs. catalyst coke content. Key model prediction A, experimental data.

Catalyst coke content is a good measure of activity. Both hydrogenation and hydrodenitrogenation can be related to coke content. 

The present study has shown that during the initial deactivation of a residue hydrodemetallization catalyst, coke formation is very rapid but is far less poisoning than small amount of vanadium well dispersed in the grain volume. This suggests that metal deactivation predominates over coke deactivation at the beginning of a run of a resid hydrotreater. 

In this study we address hydrocracking of VGO at moderate hydrogen pressures (30 bar) and elevated temperatures (450°C) using catalysts with little or no acidity [1]. The moderate pressures are attractive from a capital investment point of view. A potential drawback could be that the severe conditions lead to considerable coke deposition on the catalyst. In order to control the level of catalyst coking a careful balance of catalyst and process parameters is a prerequisite. 

Several factors determine the deactivation of Ga/H-MFI(Si,Al) catalysts. Coke deposition is the major issue. 

Since DBT does not affect the coking rate, it is possible to measure HDS activity while coking the catalyst with pyrene. Results are shown in Fig. 7 for three repeat tests of HDS activity as a function of run length. The three tests were operated for different periods of time 40 hours, 65 hours and 110 hours. The resultant levels of carbon for the samples aged 40 and 110 run hours fit (9.5% wt and 13.5% wt, respectively) the data in Fig. 6 perfectly. However, the carbon level found for the 65 run hour aged sample was somewhat larger than expected, 15.7% wt vs. the expected 12.5% wt. The reason for deposition of the additional coke is unresolved. The data in Fig. 7 (solid boxes) show an unexpected activity drop between run hour 40 and 50. This activity drop is most likely caused by coke deposition on the catalyst (coke is the only source of deactivation during these runs ). We... 

This demonstrates the importance of catalyst coking in the deactivation process and also that it is more the amount of coke rather than the nature of coke that determines the extent of coke deactivation. 

The deactivation of cracking catalysts by coking with vacuum gas oils (VGO) is studied in relation to the chemical deactivation due to site coverage, and with the increase of diffusional limitations. These two phenomena are taken into account by a simple deactivation function versus catalyst coke content. The parameters of this function arc discussed in relation to feedstock analysis and change of effective diffiisivity with catalyst coke content. 

Accelerated tests of catalyst coking What do they tell us ... 

Table 1 shows the catalyst compositions. I and II are Pt-Re/Al203 commercial catalysts coked by their use in a commercial naphtha reforming unit operated at 1.5 MPa. I was sampled at the end of the operation cycle (7 months), and II at the middle of the cycle. Ill is a fresh sample of the same catalyst coked in the laboratory at 0.1 MPa. The coked catalysts were ground and the 35-80 mesh fraction was used. 

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