Experimental techniques to understand catalyst performance evolution .1 Accelerated decay tests .1 Accelerated decay tests

Accelerated decay tests are commonly used in industry as a pragmatic method to examine long term stability and performance of catalyst formulations. Levenspiel (1972) divides the concept of catalytic performance evolution into five key categories, the effects of which can be promoted by accelerated ageing:

 Parallel: Depends on concentrations of reactants, Creac (such as poisoning or fouling due to deposition of side-products from main reaction).

 Series: Depends on concentrations of reaction products Cprod (such as poisoning, fouling or structural changes caused by decomposition of main reaction products).

 Side-by-side: Depends on concentrations of other components, Cpois that are not involved in the main reaction (such as a feedstock impurity).

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 Independent: Driven by structural transformations occurring at reaction temperature.

These are independent of fluid composition.

 Multi-component: Catalytic performance evolution due to more than one of the above effects.

These mechanisms are probed by harsher conditions than would be experienced in actual plant operation (e.g. higher temperatures, greater concentration of feed poisons). These tests are most effective when one particular mode of decay dominates, however, care must be taken when designing these tests to ensure that operational conditions do not stray too far from typical process settings (Birtill, 2004a). For example, in an investigation of poison effects, a large increase in reaction temperature may induce sintering effects whilst reducing the effect of the poison (e.g. promoting desorption). In short, these tests are useful but require clear initial thinking before they are carried out.

Such catalyst performance evolution processes can be further demonstrated by an examination of selectivity evolution across the catalyst beds (Birtill, 2003). This is tested by modifying the flow rate over the largest catalyst bed so that a reference initial reactant conversion is restored. All smaller catalyst beds are then exposed to same flow rate and a shift in product selectivity can be compared. Often selectivity evolution is linked directly to overall catalyst activity: active sites do not fundamentally change but are simply lost or gained over time. In these situations intrinsic selectivity of the catalyst is constant but observed selectivity may still change, as a function of conversion.

In some systems, intrinsic selectivity may change, with no simple direct link to overall catalyst activity. In these situations, modelling activity evolution will be more complex; multiple mechanisms may be prevalent and relative quantities of active sites may shift.

2.1.2.2 Parallel difference tests

Parallel difference testing of catalyst formulations using multi-tube, fixed-bed reactor units is a strong candidate for analysing catalyst performance evolution in detail. Parallel difference testing of a single catalyst provides the capability to estimate performance of discrete segments of a catalyst bed (see Figure 2.3). Multiple reactors are operated using identical feedstock concentrations, flow rates and temperature, but with different catalyst

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masses. The approach is data rich and effectively tracks kinetic profiles of an integral bed with time on stream, which is excellent for kinetic modelling purposes.

Figure 2.3: Example of a parallel difference test setup using 4 micro-reactors in parallel filled with catalyst and inert silicon carbide packing

Example data (Figure 2.4, overleaf) shows a test whereby three catalyst beds are utilised and performance over time is calculated for each segment. It is essential to carry out repeats for this kind of experiment, as error can be propagated in the species concentration changes between segments. This is due to subtraction of one value from another, both of which will have a degree of uncertainty, dependent of the quality of experimental measurements.

Parallel difference testing can provide clues as to the type of evolution mechanism observed over time on stream. The graphs in Figure 2.4 provide simple examples of the four mechanisms previously discussed. Upon restoration of initial reactant conversion across the 100% length bed the following typical behaviours can be observed:

 Parallel: Large changes are seen in performance at the front of bed resulting from high reactant concentrations at this point. Subsequent parts of the catalyst bed accommodate for these changes over time. A return to initial conversion reveals a large performance evolution at the front of the catalyst bed.

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 Series: Front of catalyst bed slowly changes in activity however subsequent catalyst bed sectors do not compromise this change due to the effect of product concentrations.

 Side-by-side: Performance profiles are often similar to those seen for a parallel mechanism. Further testing using high purity feedstock may be required to fully decouple these mechanisms.

 Independent: Changes in segment performance are often similar to those for a series mechanism, however a return to initial overall conversion sees each bed segment return to its initial conversion also. Catalyst performance evolution is uniform along the catalyst bed.

Figure 2.4: Characteristic catalyst performance evolution plots for parallel difference testing. In each case all beds receive identical feeds for the entire test except for the final

data points at time = 40 h. At time = 40 h, the feed-rates to the 100% length beds are reduced so as to restore conversion to the initial value of 98% and the feed-rates to the shorter beds are reduced to the same extent (taken from Birtill, 2004a). Source does not

describe the origin of the data presented. 0.0 – 0.3, 0.3 – 0.7, 0.7 – 1.0 denotes the fractional sections of the catalyst bed.

Parallel Decay

66 2.1.2.3 Transient response tests

Transient response tests, such as those which incorporate step changes in feed composition over an operational catalyst, initially at steady state, can extract considerable mechanistic understanding of catalyst behaviour by probing individual steps of catalytic reaction sequences (Weller, 1992; Berger et al., 2008).

As discussed in Chapter 1, the nature of these experiments can take multiple forms (e.g. choice of flow-through reactor, nature of perturbation) however in this thesis study, application will focus around flow-through, gas phase fixed beds. Examples of such operation are shown in Figure 2.5. In the example from Lietti et al., (1997), catalyst formulation comparison is demonstrated. These data can be applied to non-steady state adsorption-desorption-reaction kinetic models in order to elucidate key differences in catalyst functionality and link to formulation. The background to this modelling process is discussed in Sections 2.3.1 and 2.5.1.

Figure 2.5: Examples of step-change transient response tests: NO and NH₃

concentration response over two different catalyst formulations when a 700 ppm NO/1%

O₂/He feed is doped with 700 ppm NH₃ (at t = 0 s) which is subsequently removed at t = 1000 s at 553 K (Lietti et al., 1997)

67 2.2 Catalyst characterisation

The support of experimental catalyst performance evolution data with real physical and chemical observations on the catalyst may be used to strengthen the basis of mechanistic models generated in this study. Whilst the individual results chapters of this thesis delve into specific techniques appropriate to the catalyst system in question, this section summarises ex situ, in situ and operando options in this area.