Coke content, measurement

The only unknown parameter left in Eq. (o) is the tortuosity factor, r. This factor was determined from a comparison between the experimental rate at zero coke content, measured in a differential reactor and the surface fluxes. The latter were calculated using Pick s law and for a given t from the concentration profiles obtained by numerical integration of the system (Eqs. (g), (h), (i), and (j)). A value of T = 5 led to the best fit of all six experiments. This is the generally accepted value for the tortuosity factor in a catalyst of the type used in this work. It was also possible to calculate an effectiveness factor from these results. A value of 0.20 was obtained for a particle radius of 2.3 mm at 550°C. The Bischoff general modulus approach, presented in Chapter 3, leads to a value ofO.28. Finally, the heat transfer coefficients were calculated from the correlation of Handley and Heggs, mentioned in Chapter 3. 

The material balance is consistent with the results obtained by OSA (S2+S4 in g/100 g). For oil A, the coke zone is very narrow and the coke content is very low (Table III). On the contrary, for all the other oils, the coke content reaches higher values such as 4.3 g/ 100 g (oil B), 2.3 g/ioo g (oil C), 2.5 g/ioo g (oil D), 2.4/100 g (oil E). These organic residues have been studied by infrared spectroscopy and elemental analysis to compare their compositions. The areas of the bands characteristic of C-H bands (3000-2720 cm-1), C=C bands (1820-1500 cm j have been measured. Examples of results are given in Fig. 4 and 5 for oils A and B. An increase of the temperature in the porous medium induces a decrease in the atomic H/C ratio, which is always lower than 1.1, whatever the oil (Table III). Similar values have been obtained in pyrolysis studies (4) Simultaneously to the H/C ratio decrease, the bands characteristics of CH and CH- groups progressively disappear. The absorbance of the aromatic C-n bands also decreases. This reflects the transformation by pyrolysis of the heavy residue into an aromatic product which becomes more and more condensed. Depending on the oxygen consumption at the combustion front, the atomic 0/C ratio may be comprised between 0.1 and 0.3 ... 

There is a complex and little understood relationship between coke content, catalyst activity, and the chemical nature of the coke. For instance, the H/C ratio of coke depends on how the coke was formed its exact value will vary from system to system (Cumming and Wojciechowski, 1996). And it seems that catalyst decay is not related in any simple way to the hydrogen-to-carbon atomic ratio of the coke, or to the total coke content of the catalyst, or any simple measure of coke properties. Moreover, despite many and varied attempts, there is currently no consensus as to the detailed chemistry of coke formation. There is, however,... 

The coke content of the zeolite samples was measured on a Perkin-Elmer 240B CHN instrument which uses a combustion method to convert the sample elements to simple gases (C02, H20 and N2). The sample is first oxidized in a pure oxygen environment the resulting gases are then controlled to exact conditions of pressure, temperature and volume. Finally, the product gases are separated. Then, under steady-state conditions, the gases are measured as a function of thermal conductivity. The results are accurate to + 0.5%, absolute. 

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). 

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

A new reactor concept for the study of catalyst deactivation is presented, it consists of the combination of an electrobalance and a recycle reactor. With the electrobalance, the coke content on the catalyst is measured continuously. The recycle reactor operates gradientlessly at high conversion, with on-line gas chromatographic analysis of the effluent. Thus, the catalyst activity and product selectivities may be coupled directly with the coke content and the coking rate on the catalyst. 

In the electrobalance technique, the coke content on the catalyst is measured continuously. The combination with on-line gas chromatography couples the catalyst activity with its coke... 

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. 

The used catalysts were washed with toluene in a Soxhlet extractor, stored in purified toluene and dried before analysis. C, H, S and N elemental analysis were performed by combustion using a Carlo Erba apparatus. The coke content is therefore defined in this work as being the carbon content of a used catalyst washed by hot toluene. Metal contents were measured by X-ray fluorescence spectroscopy. The coke hydrogen content was determined by difference between the hydrogen content measured for the used catalyst and the hydrogen content measured for the fresh NiMo catalyst (0.6 wt %). 

Model compound testing was used to study the role of coke in the deactivation of hydrotreating catalysts. The approach used was to pick one of the compounds used in the initial adsorption experiments that gave a measurable increase in coke over a period of a few days and to study whether the rate of coking was affected by the presence of a sulfur or nitrogen compound that could be used to measure activity (HDS and HDNt respectively). If the rate of coking was unaffected, the activity was measured as a function of time, It would then be possible to relate activity to coke content. 

This can be realized by concentration- and temperatur-controlled experiments as described in section Alternatively the concentration vector of the reactants could be kept constant easily during time on stream is to feed the reactants at their thermodynamic equilibrium concentrations,so that any netto conversion of the reaction is avoided.This way the integral rise of coke content on the catalyst surface can be measured at wed defined reaction conditions. 

The coke content of the catalyst vs. time on streem measured this way is shown in Fig.7. 

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. 

In this project, the feasibility of catalyst regeneration by supercritical fluid extraction was studied. A spent catalyst from an industrial naphtha hydrotreater was extracted with tetrahydrofuran, pyridine, carbon dioxide, and sulfur dioxide under subcritical and supercritical conditions. The coke reduction and changes in the catalyst pore characteristics were measured and to a limited extent the catalyst activity was evaluated. It is shown that by supercritical extraction, the coke content of spent hydrotreating catalysts can be reduced and the catalyst pore volume and surface area can be increased. 

The catalyst was removed and analyzed for its coke content, pore size distribution, pore volume, and surface area. All the catalyst samples were analyzed after a Soxhlet extraction with tetrahydrofuran for 24 h and drying in oven at 100 C. The catalyst weight loss due to combustion at 550 C was considered as the coke on catalyst. The catalyst pore characteristics were measured by a Quantachrome Autoscan Porosimeter. 

Two extraction runs, runs 1 and 2, were conducted with tetrahydrofuran at a reduced pressure of 1.05 and a reduced temperature of about 1.35 for 5 and 11 h respectively. The reduction of the coke content of catalyst was insignificant. The catalyst activities before and after extraction were measured at 355 C. The hydrogenation activity of the catalyst did not change, whereas, the quinoline HDN activity of the catalyst was decreased from 50% to about 20% when the catalyst was extracted for only 5 h. This initial low activity gradually increased to 50% in about 4 h. However, the catalyst that was extracted for 11 h, after stabilization showed an HDN activity of about 80%. These changes in the catalyst activity are not related to the coke. They are rather attributed to the interaction of tetrahydrofuran with the adsorbed species on the catalytic sites responsible for HDN reactions. 

The kinetics of reactions in zeolites is conventionally related to the reactant concentrations in the gas phase. Reaction within the pores of zeolites, however, involves adsorption, diffusion of reactants into the pores, reactions of adsorbed species inside the pores, desorption of products, and diffusion of products out of the pores (92). Therefore, intrinsic kinetics based on the concentration of species adsorbed inside the pores is expected to be very useful for catalyst development. TEOM is an excellent technique for measurement of adsorption of reactants under reaction conditions as well as measurement of this adsorption as a function of the coke content (3,88). This technique makes it possible to obtain intrinsic activity of each acidic active site directly and to understand deactivation mechanisms in detail. 

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. 

An obvious choice for a measure of the deactivation of a catalyst is the comparison, for a given gas phase composition, of the intrinsic rate of a reaction, at timej t, or coke content,... 

Coronene adsorbs on catalyst sites present on both the alumina support and on the active NiMo sulfide phase 3). It has been found that adsorption decreases with coke content to a very Jow value at high coke levels (4), Therefore, it appears that coronene adsorption on the coke is nil. On this basis, it is assiimmed that the loss in adsorption with increasing coke is proportional to the loss in pore surface area due to coverage by coke. The results of coronene adsorption measurements on the VGO-coked catalysts show an initial drop for the 2% C sample, but little change thereafter for higher coked catalysts (Fig. lA) This implies that the coke Is concentrated near the mouth of the pores, On the other hand, catalyst dlffusivity measurements show a continual and sig-... 

An exponential function of the coke content has been derived for a variety of processes [De Pauw Froment, 1975 Dumez Froment, 1976 Hatcher, 1985 Beimaert et al, 1994], The approach symbolized by (1) is easier since it does not require measuring the coke content of the catalyst, as (2) does. However, measurement of coke content is no longer a problem adequate equipment has been developed that permits the simultaneous study of the main reaction(s) and coke... 

When the coke content of the catalyst is measured a distinction can be made between the deactivation function for the main reaction, (p, and that for the coking reaction tpc defined by ... 

Clearly the rate equation for the cracking of methane, i.e. for the coke formation is not fundamentally different from that of one of the main reactions (12). What remains to be done is to link the coke content of the catalyst to the rate of the main reactions. Thereby a specific aspect of coke formation on Ni/alumina catalysts has to be accounted for, namely whisker formation. The rate equation (16) is not directly applicable because it contains the concentration of coke adsorbed on the Ni-surface, which is not accessible, just like Cc 4., C. i,... The latter are eliminated through adsorption-isotherms in favor of the measurable gas phase partial pressures PcH4. Ph2> but this is not possible for coke. What is done in the derivation of (5) and (6), where the same problem is already encountered, is to manipulate the expressions so as to factor out the Cq, thus yielding the... 

Although the catalysts showed high initial activity, rapid deactivation was also observed. For example, when using a Pt/t -alumina catalyst at 250 C, essentially complete TCA conversion was observed initially however, after 15 h TCA conversion had declined to < 25 percent. To understand the deactivation process, surface acidity and basicity, coke content, chlorine content, and platinum content were measured for both the fresh and the used catalysts. These measurements showed that up to 40 wt% coke formed on the supported platinum catalyst and that the acidity changed significantly during the reaction at 350°C. 

Coke combustion was studied by differential scanning calorimetry (Setaram DSC 111) at constant temperature 500, 525, 540 and 550°C. Coke combustion was followed by on-line FTIR analysis of the gaseous products (CO, CO2 and H2O) in a Nicolet 740 FTIR spectrophotometer. The evolution of coke H/C ratio throughout combustion and the value of this ratio for the initial coke were calculated from the measurement of the combustion products. After a period of kinetic study at each one of the previously indicated temperatures, additional combustion at 600°C was carried out to total combustion. In this way, coke conversion and coke H/C ratio are referenced to total coke content. 

The catalyst in each reactor section can be unloaded without mixing and its coke content determined by a highly sensitive TPO technique [4], using a modified Altamira temperature-programmed unit (Model AMI-1). In this modification, the gas exiting the reaction cell enters a methanator where CO2 and CO are converted to methane over a Ru catalyst with a constant supply of hydrogen. The methane formation rate is measured by an FID detector. 

A NH4Y zeolite (LZ 54 from UOP) was used. The zeolite was compressed, at a pressure of 1.5 tomie/cm in the form of a cylinder 12 mm long and 6 mm in diameter. This pellet was heated under nitrogen flow up to 720 K, then coked by cracking n-heptane at the same temperature for periods between 2 and 10 hours to obtain various coke contents 2.5, 7.5 and 10 % w/w. The coke content is measured by weighting the sample before and after coking. 

Measurement of T2 from the slope of the linear transform of equation 4, shows that it is very short (225 coke content the carbonaceous residues are essentially polyaromatic (graphitic) and that it is in the most coked zones that the coke is the most graphitic (T2 shortest). 

---