Coke content effect

The specification requirements for electrode binder pitch, eg, high C/H ratio, high coking value, and high P-resin content, effectively ruled out pitches from gasworks or low temperature tars. The cmde tar is distilled to a medium-soft pitch residue and then hardened by heating for several hours at 385—400°C. This treatment increases the toluene-insoluble content and produces only a slight increase in the quinoline-insoluble (Ql) material, the latter by the formation of mesophase. 

Chen and co-workers have studied the role of coke deposition in the conversion of methanol to olefins over SAPO-34 [111]. They found that the coke formed from oxygenates promoted olefin formation while the coke formed from olefins had only a deactivating effect The yield of olefins during the MTO reaction was found to go through a maximum as a function of both time and amount of coke. Coke was found to reduce the DME dilfusivity, which enhances the formation of olefins, particularly ethylene. The ethylene to propylene ratio increased with intracrystal-line coke content, regardless of the nature of the coke. 

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. 

Figure 4. The linear relationship of poisoning might be due to uniform poisoning, i.e., sites of equal activity were deactivated at zero coke content. Figure 4 shows that pyridine and quinoline are more poisonous than aniline. It shows that the higher basicity compounds have greater effectiveness as poisons. Quinoline which has a higher molecular weight and lower basicity than pyridine showed a slightly lower effectiveness than pyridine.

The effectiveness factors and n, defined as the ratios of the actual reaction rates at time 0 to the maximum reaction rates on a clean catalyst, are obtained nEmerically from equations [4] -[9]. An explicit finite difference method was used to solve the partial differential equations without further simplifications. Densities, porosities and clean catalyst pore diameters were measured experimentally. The maximum coke content is assumed to be that which fills the pore completely. The tortuosity is taken as 2.3, as discussed by Satterfield et al. (14). 

Information concerning the limitation or the blockage of the access of the reactant n-heptane to the micropores caused by coke can be obtained from adsorption experiments. Two adsorbates were chosen n-hexane with the same kinetic diameter as the reactant and nitrogen, a less bulky molecule. Indeed at high reaction temperature (450°C) the effect of coke is probably less pronounced that at the temperature chosen for n-hexane adsorption (0°C). The pore volume accessible to the adsorbates Va was compared over a large range of coke contents to the volume really occupied by coke VR (estimated from coke composition) (Figures 6),... 

The first mode of deactivation is clearly shown with HZSM5, At low coke content, Vr/Va is close to 1 4 coke molecules are needed to deactivate one acid site, This weak deactivating effect can be explained by a competition for adsorption on the acid sites between the reactant and the coke molecules which are too weakly basic to be "irreversibly adsorbed at the reaction temperature. However limitations in the rate of diffusion of the reactant can also be responsible for deactivation. The size and the basicity of the coke molecules increase with the coke content, which causes an increase in the deactivating effect of the coke molecules. Beyond a certain size of the coke molecules the channel intersection is completely inaccessible to the reactant and to the adsorbates and Vr/Va can be lower than T This first mode of deactivation occurs also with USHY. However the deactivating effect of coke molecules is initially very high because coke molecules are formed on the strongest (hence the most active) acid sites. 

During the first minutes of the experiment, measurement of the coke content is disturbed by several transient effects adsorption of hydrocarbons, pressure stabilization and gradual displacement of the pretreatment gas by the feed. To determine the total amount of coke deposited on the catalyst, the coked catalyst is stabilised at the end of the experiment in the pretreatment gas. The weight difference between the uncoked catalyst and the coked catalyst gives the total amount of non-desorbable products. 

Another way to distinguish between the effects of coke and conversion is to perform a series of experiments at a constant inlet hydrocarbon pressure, but different space times. In these experiments a given conversion corresponds with different coke levels. Interpolation at a given conversion and at a selected coke content permits to unravel the effects of both variables on the reaction kinetics. 

If the effect of the coke content on the rate of one of the reactions of the network has to be expressed, the partial pressures of the related reactants have to be kept constant. For any of the primary reactions of n-hexane cracking, this is ensured by keeping the hexane conversion constant, but not for the other reactions, however. For secondary reactions, Table 3a does not only reflect the effect of coke, but also of concentration. 

To quantify the deactivation effect of coke on the various reaction rates, a deactivation function (Cc) was defined as the ratio of the reaction rate at a given coke content to the... 

The effect of the H2/oil ratio on the coke content of a NtV/SiOj catalyst (low HDS activity, thermal coke predominates) is shown in Figure 6. The distinct maximum of the coke deposited with the H2/oil ratio is apparent from both experiment and theory. Detailed analysis of the model output indicates that at low gas rates the VGO feedstock is mainly in the liquid phase throughout the reactor, whilst at the highest gas rates the reactor is operated in the gas phase already at the reactor inlet. In both limits the amount of coke deposited is modest. Intermediate gas rates (1000 Nl/kg), however, lead to much higher rates of coke... 

Figure 6. Effect of H2foil ratio on the coke content of the catalyst. Experiment Feedstock Kuwait VGO, catalyst NiV/SiOj, temperature 450 C1 pressure 30 bar, WHSV 2.2 kg/(kg-h), run length 200 h.

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. 

Figure 2. Chemical deactivation versus initial Figure 3 Effectiveness factor ratio versus coke content (legends in fig, L) initial coke content (legends in fig. 1)...

The substitution of equations (9,11) into equation (6) produces an asymptotic relation between the effective diffusivity and the coke content (figure 5). 

Figure 5. Effective diffusivity versus initial coke content when x ...

An industrially spent hydrotreating catalyst from naphtha service was extracted with tetrahydrofuran, carbon dioxide, pyridine and sulfur dioxide under subcritical and supercritical conditions. After extraction, the catalyst activity, coke content, and pore characteristics were measured. Tetrahydrofuran was not effective in the removal of coke from catalyst, but the other three solvents could remove from 18% to 54% of the coke from catalyst. 

Beyond providing a tool for investigation of deactivation and adsorption/diffu-sion effects as a function of the coke content, TEOM makes possible the determination of the catalytic reaction kinetics directly as a function of the concentration of reactants inside the catalyst pores (and not just in the gas phase). 

On the basis of these observations, the coke formed in MTO and DTO is classified into two categories unreactive coke, formed from adsorbed olefins having a deactivating effect on DTO and MTO and reactive coke, formed from oxygenates, having a promoting effect on DTO and MTO. The activities of the catalyst for the MTO and DTO reactions at various coke contents depend on the nature of the coke, in particular on the ratio of the reactive to the unreactive coke (7). 

The following section illustrates that TEOM provides a unique opportunity for direct measurements of diffusivity and investigation of changes in diffusion rates and the effects of diffusion on the MTO reaction as it depends on the coke content. Diffusion of methanol has been investigated by uptake measurements under conditions of no catalytic reaction at low temperatures as well as by measurements of the kinetics of MTO on SAPO-34 crystals of various sizes. 

The classical method of investigation of effects of diffusion on reactions is typically to run a reaction with catalyst particles of various sizes. For zeolites, the resistance of intracrystalline diffusion is normally much larger than that characteristic of molecular diffusion or Knudsen diffusion that could occur in the spaces between the zeolite crystals in a catalyst particle. Thus, the crystal size of the zeolite has to be varied instead of the particle size to determine the effects of diffusion on zeolite-catalyzed reactions. Kinetics of the MTO reaction has been measured with SAPO-34 crystals with identical compositions and sizes of 0.25 and 2.5 pm 89). The methanol conversion was measured as a function of the coke content of the two SAPO-34 crystals in the TEOM reactor. 

Fig. 15. Changes in effective diffusivity of methanol in SAPO-34 and intrinsic reaction rate constants for methanol conversion on SAPO-34 as a function of the coke content in crystals with average sizes of 0.25 and 2.5 m (89).

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