Deactivation front

Do not infer from the above discussion that all the catalyst in a fixed bed ages at the same rate. This is not usually true. Instead, the time-dependent effectiveness factor will vary from point to point in the reactor. The deactivation rate constant kj) will be a function of temperature. It is usually fit to an Arrhenius temperature dependence. For chemical deactivation by chemisorption or coking, deactivation will normally be much higher at the inlet to the bed. In extreme cases, a sharp deactivation front will travel down the bed. Behind the front, the catalyst is deactivated so that there is little or no conversion. At the front, the conversion rises sharply and becomes nearly complete over a short distance. The catalyst ahead of the front does nothing, but remains active, until the front advances to it. When the front reaches the end of the bed, the entire catalyst charge is regenerated or replaced. 

The ammonia oxidation reaction proceeds in the first part of the catalyst bed [Fig. 16(a)]. This part is subsequently deactivated, mainly by nitrogen species. The high activity of the catalyst is maintained due to the movement of the reaction front to the next positions in the catalyst bed. When [ Nj-NH3 is injected at the moment that the reaction was already 20 seconds on-stream, labelled N species adsorb further on in the catalyst bed. Thus, in time to come, the deactivation front moves to the end of the catalyst bed. When this front reaches the end of the bed, the catalyst is covered with reaction species and the deactivation is observed in the concentration of the products. An experiment with half an amount of the catalyst also supports this reaction front movement. This experiment showed the formation and concentration of the products in the same manner, however, the catalyst remained active for half the time of the normally applied catalyst bed. Thus, below 413 K, the catalyst remains initially active because the reaction zone moves to the next bed positions, after the previous positions became fully covered with the adsorbed reaction species. Injection of a [ N]-NH3 or [ 0]-02 pulse after the initial deactivation, confirmed that the platinum surface is fully covered and that conversion of ammonia and oxygen is low. No significant amount of nitrogen or oxygen species remains adsorbed at the catalyst surface. 

Industrially relevant consecutive-competitive reaction schemes on metal catalysts were considered hydrogenation of citral, xylose and lactose. The first case study is relevant for perfumery industry, while the latter ones are used for the production of sweeteners. The catalysts deactivate during the process. The yields of the desired products are steered by mass transfer conditions and the concentration fronts move inside the particles due to catalyst deactivation. The reaction-deactivation-diffusion model was solved and the model was used to predict the behaviours of semi-batch reactors. Depending on the hydrogen concentration level on the catalyst surface, the product distribution can be steered towards isomerization or hydrogenation products. The tool developed in this work can be used for simulation and optimization of stirred tanks in laboratory and industrial scale. 

Some simulation results for trilobic particles (citral hydrogenation) are provided by Fig. 2. As the figure reveals, the process is heavily diffusion-limited, not only by hydrogen diffusion but also that of the organic educts and products. The effectiviness factor is typically within the range 0.03-1. In case of lower stirrer rates, the role of external diffusion limitation becomes more profound. Furthermore, the quasi-stationary concentration fronts move inside the catalyst pellet, as the catalyst deactivation proceeds. 

For a high functional stability an excess of the analyte converting enzyme is generally fixed in front of the sensor. In this way, in spite of the time-dependent deactivation of the immobilized biocatalyst, complete analyte conversion is maintained over a long operation time. Under these conditions the sensitivity and the response time are determined by the rate of mass transfer both outside and inside the enzyme layer. 

Band-Aging - Especially with fresh catalysts, the reaction occurs over a relatively small zone in a fixed bed. This reaction front marches down the catalyst bed as the coke deposits first deactivate the front part of the bed (Figure 4). Use of a sufficient catalyst volume permits a fixed-bed design in which on-stream periods are long enough to avoid overly frequent regeneration cycles. 

The deactivation takes place in the front part while reaction occurs in the rear section. Evidently, the speed of the travelling wave is given by the rate of deactivation. From Figure 9, we can infer that an envelope of temperature profiles exist. This envelope is determined by a completely deactivated bed. In the part of the reactor specified by the envelope, the fresh feed is... 

Effect of Poison Concentration. The effect of poison concentration can be anticipated according to the type of deactivation. For the travelling wave deactivation, the higher concentration of the poison in the feed will increase the speed of the front. Evidently, the velocity of the travelling wave is given by the rate of deactivation. For the case of low E and r r, the speed of the wave is proportional to the poison concentration in the feed, as shown in Figure 14. 

Sulfur, usually present as H2S, has to be below 0.1 ppm, but even with such low concentrations, the catalyst is slowly poisoned. The ZnO adsorbs the sulfur and it finally transforms into bulk ZnS. When the ZnO is exhausted in a given layer of the catalyst, the H2S causes deactivation of the copper by sintering. The poisoning process moves through the catalyst as a relatively sharp front and can be seen in the change of the catalyst temperature profile over time [619], [620] (Figure 64). 

We can model the FCC catalyst system as a combination of a shrinking core of sites not yet deactivated by coke and a progressing shell of large hydrocarbon molecules and metal contaminants, penetrating into the catalyst particle.The relative velocities of these fronts will be of great importance and will be strongly determined by the accessibility of the various functional sites of the catalyst [40]. 

Empirical Methods. The grcphical deactivation plot is a very useful empirical method for prediction of the catalyst performance and for estimation of catalyst lifetime (18,19). The deactivation plot shows the length of the reaction front as a function of time. This illustrates the movement of the temperature profile caused by the progressive deactivation of the catalyst. The method is illustrated in Figure 3. The temperature increase over the catalyst bed is calculated as AT = Texii - Twet and a certain percentage hereof, e.g. 90% (AT90) is calculated. The axial distance in the... 

Fig. 5. Energy dissipation (dashed arrow) by vibrational deactivation and internal conversion (reproduced with permission front Barrow (47)].

Such an approach was used in a previous study by Duvenhage, et al. in 1994 [5]. These authors performed a series of deactivation studies on catalysts which were first used in an industrial fixed bed reactor at SASOL, then removed, under nitrogen, from sections along the bed, and transferred to microreactors for further study. Their results [5] showed that the activity lost at the front of a fixed bed is caused by S poisoning. However, downstream into the reactor, they observed a clear correlation between the loss in activity and an increase in magnetite concentration. Further, they observed that in the region of highest activity, the catalyst contained... 

As expected, the behavior of Catalyst System-C was similar to that of Catalyst System-B in view of similar HDM catalyst guard at the front end. However, the reactors pressure drop was much higher from the SOR itself, enforcing a lower Gas/Oil ratio that resulted in enhanced catalyst fouling and deactivation. The higher pressure drop was related to the characteristics (i.e. size and shape) of the catalyst. The products yield was, as expected, similar to that of Catalyst System-B. 

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