Catalyst stability testing

Two types of laboratory tests were conducted to evaluate contaminant tests, a catalyst stability test and a high-conversion bromine product test. For catalyst stability testing, only a small amount of catalyst was used (1.5 g) to ensure incomplete conversion of the HBr. If a feed contaminant causes catalyst deactivation, it is apparent as an immediate decrease in conversion. If an excess of catalyst was used instead, even if deactivation occurred at the inlet of the bed, it may not be detected until the region of deactivation moves considerably downstream. This could take many hours or days. 

Fig. 5. Catalyst stability testing at partial conversion using an HBr feed stream containing 5 % HCl.

The N4 complexes are rather stable in acidic solutions. However, sometimes the stability is not high enough, particularly so at higher temperatures. It was quite unexpected, therefore, to hud that after pyrolysis at temperatures of 600 to 800°C the catalytic activity of these compounds not only failed to decrease but in some cases even increased. The major result of pyrolysis is a drastic increase in catalyst stability. Tests have been reported where after pyrolysis such catalysts have worked for 4000 to 8000 h without activity loss. The reasons for the conservation of high activity after pyrolysis are not entirely clear. The activity evidently is associated with the central ion that has attained a favorable enviromnent of pyrolysis products. 

The catalyst stability test was performed for all the catalysts at 803 K for 4.5 hours at WHSV equal to 2.5 h. The n-butane conversion and aromatic selectivity over the catalysts were observed to be stable even after 4.5 hours. The coke formation over Ga-H-ZSM-5 and Zn-H-ZSM-5 catalysts was found to be smaller than over H-ZSM-5 catalysts, showing that Bronsted sites when combined with metal species are more resistant to coke formation. 

Figure 8.6 Catalyst stability testing (a) hydrotreating catalyst (b) isomerization catalyst.

One important aspect of catalyst stability testing is the selection of the operation mode (Marafi et al., 2008) fixed performance or fixed temperature. The former mode, in which reaction temperature is increased periodically to compensate for the activity loss, is more suitable for representing commercial operation. For this reason. 

FIGURE 8.7 Reaction temperature program and API Gravity of the upgraded oil during the catalyst stability test. (—Temperature, (o) API gravity. 

Process simulations with time-varying catalyst activity were performed based on a quasi-steady-state approximation (Lababidi et al., 1998). The underlying principle is that because catalyst aging is a relatively slow process in the operating cycle timescale, it can be assumed that the process is stable during short periods of time. In this case, it is considered that this time period is equal to the duration of the mass-balance runs during the catalyst stability tests (12 h). The simulation runs start at t=0 with the catalyst in its fresh state = 1.0 for the entire catalyst length). The concentration and temperature profiles are established from the steady-state solution of the heat and mass balances, as described previously. The next step is to estimate the local amount of MOC from the axial metal profiles in this period and after that to evaluate the deactivation functions for each reaction. The time step is increased and all the calculations are repeated. 

Table 8.4 contains the deactivation parameters estimated from the long-term catalyst stability test and the feedstock evaluation with HCO. d and d are the parameters of the hyperbolic function that describes the initial activity decay caused by coke formation, whereas 3 is the exponent of the power-type function that represents the slow deactivation process by metal deposition (see Equation 8.21). Each set of... 

FIGURE 8.9 Comparison between experimental deactivation data and model predictions, (a) Long-term catalyst stability test (b) Feedstock evaluations with HCO (—) 5%. 

From Figure 8.9 it can be observed that the model predicts reasonably well the catalyst deactivation curves obtained from the catalyst stability test and the evaluation with HCO. Most of the data randomly falls into the 5% interval, which confirms the validity of the proposed deactivation function. In the case of the test with HCO, it can be noticed how well the aging curves of each reaction (HDS, HDM, and HDAs) were fitted with a single set of parameters for the whole temperature range. 

S.6.2.2 Process Performance during the Catalyst Stability Test... 

In addition to actual synthesis tests, fresh and used catalysts were investigated extensively in order to determine the effect of steam on catalyst activity and catalyst stability. This was done by measurement of surface areas. Whereas the Brunauer-Emmett-Teller (BET) area (4) is a measure of the total surface area, the volume of chemisorbed hydrogen is a measure only of the exposed metallic nickel area and therefore should be a truer measure of the catalytically active area. The H2 chemisorption measurement data are summarized in Table III. For fresh reduced catalyst, activity was equivalent to 11.2 ml/g. When this reduced catalyst was treated with a mixture of hydrogen and steam, it lost 27% of its activity. This activity loss is definitely caused by steam since a... 

The effect of HCl on catalyst stability was tested using an aqueous HBr stream containing 5 % HCl. No decrease in conversion occurred during 24 hours on stream with a 300°C inlet temperature (Fig. 5). 

Fig.3. Stability test of 12.8CaO-6.4MnO/CeC>2 catalyst in CO2 OCM reaction 4. CONCLUSIONS...

Stability tests of catalyst. All catalysts deactivate during their life by various causes (see Chapter 3). The aim of stability tests is to examine the cause and rate of deactivation. These experiments are usually performed at conditions similar to those planned for the commercial unit. In some cases, accelerated tests are carried out using a feedstock with an elevated level of impurities or at a temperature significantly higher than that anticipated for the full-scale reactor. A laboratory reactor used for such tests is usually a down-scaled reactor or a part of the full-scale-reactor. Standard analytical equipment is used. 

Stability tests of the FePcClie-S catalyst confirms that this material is stable under the reaction conditions, as it was previously reported (17). Neither leaching of the complex nor significant changes in the UV-vis spectra of the catalyst after reaction (Figure 49.3) were detected. The catalyst can be reused at least three times without a significant activity loss. 

The Ni/Re on carbon catalyst was also evaluated in a 1700 hour continuous reactor test to determine the stability of the catalyst. This test was performed with a different model compound than xylitol. Shown in Figure 5, the results from the lifetime test of the Ni/Re catalyst operated at constant process conditions sampled intermittently for 1700 hours. This shows that for a similar aqueous hydrogenation reaction deliberately operated to near completion, the catalyst retained its activity and product selectivity even in the face of multiple feed and H2 interruptions. We feel that this data readily suggests that the Ni/Re catalyst will retain its activity for xylitol hydrogenolysis. 

The few examples where SILP catalysis has been tested so far showed highly encouraging results. It is very likely that other applications where ionic catalyst solutions were tested in liquid-liquid biphasic reactions could be reinvestigated under SILP conditions. If very high catalyst stability over time can be realised or simple catalyst regeneration protocols can be developed than SILP catalysis can be expected to make its way into industrial processes. 

Commercial MgO was impregnated with Ni acetate in toluene solution. Addition of 1-3 wt% K suppresses the Ni sintering and extends the catalyst stability. Catalysts were evaluated for 500 h endurance test. Process reported to be suitable for producing H2 for MCFC... 

The authors339 found that the best catalysts were those whereby the ceria was impregnated prior to calcination of the hydrotalcite-based Cu-Zn precursors. The addition of ceria led to an important improvement in catalyst stability. For example, during stability tests (0.7 nl/min N2, 0.3 nl/min CO, 0.3 nl/min H20, T = 300 °C, P = 3 atm), the undoped catalyst calcined at 400 °C dropped to 25% of its initial activity within the first couple of hours on-stream. In contrast, the ceria-doped catalyst slowly decreased to 25% of its initial activity in about 50 hours. The activity was linked to the metallic Cu surface. In either case, the deactivation is considered to be very rapid. 

Carbides have been studied for ORR in acidic media.219,220 Tungsten carbide was shown to be promising for ORR in acidic media,219 though WC has a corrosion problem in acidic systems.221 To increase the stability of the catalyst in PEM fuel cell conditions, tantalum was added to tungsten carbide.220 The Ta-WC catalyst was tested under fuel cell conditions and compared to WC. The corrosion resistance was markedly improved as well as the activity for ORR. It is thought that a Ta-W alloy acted as a stabilizer for the catalyst while WC remained the active site for ORR.220... 

The practical application of a catalyst not only depends on its catalytic activity but also on its stability. Therefore, it was of interest to study the stability of the three catalysts during three successive acetophenone hydrogenation reactions. Tests carried out for this purpose consisted in hydrogenating acetophenone until reaching 100% conversion. The catalyst was then washed with isopropyl alcohol and allowed to act again, so that catalysts were tested in a series of three hydrogenation cycles. 

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