Deactivating catalysts rate from experiment

In the experiments presented in Fig. 4, the catalyst rnass was approximately six times larger than for those shown in Fig. 5- XF5 (x-ray fluoresence spectroscopy) artalysis of the used catalyst shows that this results in a distinct axial Mo-profile, as shown in Fig. 6 for similar experiments. In this case the degree of fcductioo does not have the major influence on the activity. Instead, the rate of the loss of catalyst mass becomes determining. The long term deactivation of the catalyst was found to occur concurrently with the loss of Mo from the catalyst (ref. 7,S). This has been determined using a modified integral reactor (multibasket reactor) where, after the deactivation experiments the baskets can be separated and analyzed separately. Fig. 6 shows the Mo/P profile measured at different times on stream. Depicted there is the molar Mo/P ratio of the deactivated catalyst from different axial positions in the multibasket reactor, normalized with the Mo/P ratio of the fresh catalyst It is dearly seen that Mo is lost mainly in the inlet section, while no Mo loss was found in the outlet section of the reactor. 

Pre-reduced catalysts. The previous experiments were repeated again but catalysts were reduced before poisoning. Reduction of the film at various temperatures before sulfur deposition decreased very dramatically the rate of reaction compared to fresh unreduced films. Total deactivation is attained at much lower levels of surface sulfur poisoning than in the case of the unreduced catalysts. For a S/Pd ratio of 0.18, conversion decrease from 60 (fresh unreduced) to 7% when the reduction temperature is as low as 200°C. For a reduction temperature of 300°C only a 1% conversion is measured, and no conversion is detected when the reduction temperature is 400°C. The AFM images of these catalysts show that the surface breaks up in islands of varying sizes. As the reduction temperature increases, the sizes of these islands decrease, but their heights increase. 

In eqn. 2, e a(T)t j a factor that expresses the extent of catalyst deactivation with time, and may be regarded as a time constant for catalyst activity loss. This form of catalyst deactivation factor derives from the assumption that catalyst activity decays exponentially with time, at constant temperature. A and E are, respectively, the preexponential factor and the activation energy for the rate constant. E can be determined from an Arrhenius plot for k1 under conditions where the quantity e a(T)t is essentially constant. Fig. 4 shows the results of experiments that were conducted at the end of the catalyst life tests at 498°K and 538eK shown in Fig. 3. The greater extent of deactivation at 538°K is clearly evident in Fig. 4, since the data at 538°K fall well below those at 498°K. The activation energies derived from the slopes of the two lines on Fig. 4 average t7.9 kcal/mole and agree to within 0.8 kcal/mole. 

Our study also showed that the catalyst deactivates with time-on-stream even at low conversions. The activity dropped 30% from its initial value over a few hours. The present work further investigates this deactivation phenomenon in order to evaluate more thoroughly the potential application of copper oxide catalysts for OMR. Experiments were conducted to determine the cause of deactivation and the effect of the support on deactivation rate. Zirconia has been explored as an alternative support to ZnO and/or alumina. Reaction and deactivation rate data for 18-hour OMR reactions are reported for these catalysts. 

The average catalyst deactivation rate over the entire experiment was 0.0291%/mscf/lb. The rate of deactivation during the initial 462 hrs of operation at a fresh feed space velocity of about 2090/hr (216 scfh) was very low, 0.0017%/mscf/lb from 500 hrs to 841 hrs with 2990/hr space velocity, the deactivation rate increased to 0.040% /mscf/lb. Catalyst deactivation rates were calculated (Table IX) for various operating periods and fresh feed space velocities. 

As noted for experiment HGR-13, the deactivation rate increased significantly when the fresh feed space velocity was increased from 2000 to 3000/hr. During the period of 841-1058 hrs, the fresh feed space velocity was returned to 2000/hr and the CGR ratio was increased from — 3 1 to 9 1 to give a low deactivation rate of 0.0027%/mscf/lb. When the CGR ratio was returned to 3 1 at 1058-1760 hrs, the rate of catalyst deactivation increased to 0.0187%/mscf/lb. After 1760 hrs, the unit was shut down and put in standby condition under hydrogen. After the unit was restarted, the deactivation rate had increased greatly to 0.0821%/ mscf/lb which indicates that the increase in deactivation rate was associated with this particular shutdown. Since this experiment had previously undergone three unscheduled shutdowns at 215, 798, and 894 hrs with no adverse effect on performance, some unknown factor unique to the shutdown at 1760 hrs was responsible for the subsequent rapid decline in activity. 

ADN, 19.4% CL, 1.3% ACAM, 2.2% CVAM and 30.9% others and was hydrogenated with Raney Co 2724 under identical conditions to the above. The reaction showed an initial hydrogen uptake rate of 7.8 psi (53.8 kPa)/minute. After 240 minutes, the reaction had consumed 525 psig (3.72 MPa) a sample was removed from the reactor for analysis. It comprised 39% HMD, 18% CL, and by-products. The reaction showed no evidence of catalyst deactivation. While the rate of hydrogen consumption was detectably larger in this experiment than in the Raney Co experiment with C02 and NH3, the differences are not sufficiently large to infer a mechanistic difference. 

Specimens of catalysts (0.125 gram) were deactivated at 360° C for desorption experiments by using continuous (rather than pulsed) operation. Purified liquid benzene or cumene was pumped to the injection port of the microreactor system with a syringe pump at the rate of 0.00241 moles/hour. Propylene was fed from a gas lecture bottle through a rotameter at a rate of 0.00245 moles/hour. Parent H-mordenite catalyst samples were de-... 

In some experiments we have deliberately poisoned acidic sites on the catalyst by adding pyridine to the gas phase. In table 3, two experiments are compared which were perfonned under identical conditions, apart from the presence of pyridine. As can be seen from the results, the presence of pyridine does significantly slow down the rate of deactivation. 

The experiments were carried out at ambient pressure. All hydrocarbons were tested at a S/C ratio of three and all alcohols at a corresponding oxygen to carbon ratio. Decreasing conversion was found for the various fuels with increasing feed rates except for methanol owing to the very high reaction temperature of 725 °C. Table 2.9 summarizes some of the results presented for the various fuels. The proprietary catalyst showed only minor deactivation after 70 h TOS. It was deactivated reversibly by sulfur. Load changes of the liquid input from 100 to 10% resulted in a system response after 5-10 s. 

A theory has been developed which translates observed coke-conversion selectivity, or dynamic activity, from widely-used MAT or fixed fluidized bed laboratory catalyst characterization tests to steady state risers. The analysis accounts for nonsteady state reactor operation and poor gas-phase hydrodynamics typical of small fluid bed reactors as well as the nonisothermal nature of the MAT test. Variations in catalyst type (e.g. REY versus USY) are accounted for by postulating different coke deactivation rates, activation energies and heats of reaction. For accurate translation, these parameters must be determined from independent experiments. 

A problem with monofunctional reactions, e.g., cracking, alkylation, etc. is that they have a tendency to quickly deactivate because of coke deposition. This problem is usually not of concern with bifunctional reactions, e.g., those that employ a metal function in addition to the acid sites. However, we avoided the use of metal function because of the possible unknown modifications that could be introduced to a given sample by the metal deposition procedure. This is especially important when dealing with samples like VPI-5. Thus, to minimize the rate of deactivation, the alkylation experiments were conducted at 463 K. This low temperature introduces another problem, namely, the adsorption of reactants and products. At the experimental conditions employed here, the catalyst bed becomes saturated at time of 10 minutes or less (depending on sample). From this point onward, deactivation is clearly observable via the decrease in conversion with time. The data reported here were obtained at 11-13 minutes on-line. Since meta-diisopropylbenzene proceeds through several reaction pathways that lead to a number of products, it is most appropriate to compare the catalytic data at the constant level of conversion. Here we report selectivities at approximately 25 % conversion. For each catalyst, the results near 25 % conversion were repeated three times to ensure reproducibility. 

Abstract Alkylation of benzene with ethylene over Y-type zeolite has been carried out under supercritical conditions. Two aspects of the reaction have been paid attention to slowing down the deactivation rate and decreasing the by product selectivity. Experiments have revealed the existence of some coke precursors that are partly removed from the catalyst surface. By product xylenes are decreased and are explained due to high diffiisivity in the supercritical fluid. 

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