Original Methodology for the Screening of Catalysts Presenting High Reaction Rates at High Conversions

Introduction

The heterogeneous catalysts screening is the first and one of the most important steps in catalysts development. The main aim of screening is to select the catalyst that provides the best performance in terms of activity and selectivity. Thus, the experimental tests should be carried out without hydrodynamics and mass transfer limitations. The challenge appears in the case of industrial applications coupling very high reaction rates with high conversions, for which the influence of those phenomena cannot be neglected.

Objective and methodology

The aim of this work was to develop a global methodology allowing the screening of catalysts, presenting high reaction rates, at high conversion. This methodology includes both an experimental and a modeling approach, and it was validated by using the benzene hydrogenation as case study. The following figure illustrates the different steps of the methodology.

Figure 1 – Methodology description.

The first step of the methodology consist on determining the kinetics of the chemical system. Thus, experiments at different operating conditions must be carried out, and a simulation tool must be develop and coupled to an optimizer in order to regress the kinetics parameters. The simulation tool must take into account both hydrodynamics and mass transfer phenomena, and it must give access to the different species concentration along the reactor in each phase. The second step consist on analyzing the different concentration profiles in order to deduce the impact from both hydrodynamics and mass transfer phenomena on the results. If the impact is not negligible, modifications, based on numerical simulations, are made in the experimental protocol to reduce the impact from those phenomena. In case a good fit exists between model predictions and experiments, the modifications are validated and the catalyst screening can be carried out in good conditions. Otherwise, the model hypothesis must be reviewed and a new iteration must be done. This process is repeated until “convergence” achievement.

Results and discussion

The methodology described above was applied to the benzene hydrogenation reaction, which is interesting since the industrial process operates at very high conversions.

The initial experiments at different temperatures allowed the determination of an activation energy of about 6 kcal/mol for two different catalysts, which is quite low comparing to the values available in literature of about 12 kcal/mol. This may be a prove of existence of reactor limitations, internal diffusion limitations or both, even if a dilution factor of three (three volumes of inert per volume of catalyst) was used during those experiments.

A simulation tool coupling all the relevant phenomena within the reactor was developed. The kinetic law was extracted from literature and the kinetic parameters were regressed and compared with those determined in the literature. A very good agreement was found, which allows the simulation tool to be validated. The following figure presents the evolution of the different species concentration along the reactor in the gas and in the liquid phase for both catalysts.

Catalyst A	Catalyst B

Figure 2 – Species concentration evolution along the reactor in both the gas and liquid phases at 140 °C and 30 bar for a diluted catalyst bed. BZ – benzene; CH – cyclohexane. CL – liquid concentration; CI – liquid side interface concentration

The gas-liquid mass transfer limitation is negligible with the used experimental protocol, and the liquid-solid mass transfer is unlikely to be a limiting phenomenon since other experiments carried out at different liquid rates support this hypothesis. The simulation tool considers the diffusion within the catalyst, which allows the evolution of the different species concentration within the catalyst particle to be determined at different reactor heights. This is represented in figure 3. It is possible to verify that a strong internal limitation exists at these conditions, since the value for catalyst effectiveness is between 0.3 and 0.5. This strong limitation explains the low activation energy found for both catalysts, which is around a half from the one found in literature. This result consolidates the fact that a strong limitation coming from internal mass transfer exists for those experiments. This limitation is not critical since it can be transposed to the commercial scale. In fact, this is a limitation due to the catalyst itself, and as a result, it is independent from reactor features.

Catalyst A	Catalyst B

Figure 3 – Species concentration evolution within the catalyst at the reactor inlet, outlet and middle at 140 °C and 30 bar for a diluted catalyst bed. BZ – benzene; CH – cyclohexane.

The experimental protocol consist on testing those catalysts near industrial conditions in small fixed-bed reactors and by using a dilution factor of three. This allows a better performance from the thermal and from the mass transfer point of view, since the reactant converted quantity per unit of reactor length is lowered when comparing to the non-dilution case, while the transfer phenomena is kept constant. The simulation tool was used in order to generate results for the non-dilution case. The results are presented in the following figures.

Catalyst A	Catalyst B

Figure 4 – Species concentration evolution along the reactor in both the gas and liquid phases at 140 °C and 30 bar for a non-diluted catalyst bed. BZ – benzene; CH – cyclohexane. CL – liquid concentration; CI – liquid side interface concentration

Catalyst A	Catalyst B

Figure 5 – Species concentration evolution within the catalyst at the reactor inlet, outlet and middle at 140 °C and 30 bar for a non-diluted catalyst bed. BZ – benzene; CH – cyclohexane.

It is possible to verify that the dilution effect is very effective mainly for catalyst B, which is actually the one presenting the highest activity. In fact, an important gas-liquid mass transfer limitation exists for benzene and hydrogen. This result is very important, since, even if both catalyst can be differentiated under undiluted conditions, the performance of the best catalyst might not be sufficiently high to justify a further development if tested under undiluted conditions. Thus, it is important to couple both the experimental and the numerical approaches in order to perform the catalyst screening in an efficient way. 

The simulation tool was finally used to analyze the results at 90 °C. The results are shown in figures 6 and 7. At these conditions the gas-liquid mass transfer limitation is mainly observed for hydrogen. The results between the diluted and the non-diluted modes are quite similar, and are slightly better in the case of the undiluted bed. This result was expected since hydrogen is a reaction inhibitor under these conditions. Thus, a stronger gas-liquid mass transfer limitation on hydrogen allows the reaction to be accelerated, which was not observed at 140 °C because in this case, the benzene was affected by the gas-liquid limitation as well. The internal diffusion limitation is lower in this case, since the catalyst effectiveness is between 0.6 and 0.8. The diffusion model was validated by comparing the catalyst effectiveness of catalyst B to the experimental value of 0.85 at 80 °C. A simulated value of 0.86 was determined, which allows to consolidate the simulation tool validation.

Catalyst A	Catalyst B
Diluted	Diluted
Non-diluted	Non-diluted

Figure 6 – Species concentration evolution along the reactor in both the gas and liquid phases at 90 °C and 30 bar for both a diluted and a non-diluted catalyst bed. BZ – benzene; CH – cyclohexane. CL – liquid concentration; CI – liquid side interface concentration

Catalyst A	Catalyst B
Diluted	Diluted
Non-diluted	Non-diluted

Figure 7 – Species concentration evolution within the catalyst at the reactor inlet, outlet and middle at 90 °C and 30 bar for both a diluted and a non-diluted catalyst bed. BZ – benzene; CH – cyclohexane.

Conclusion

An original methodology allowing the screening of catalysts, presenting high reaction rates, at high conversion was developed and successfully applied to benzene hydrogenation. The results show that it is quite important to perform catalyst dilution at these conditions, even if special care must be taken in order to avoid some reactor by-passing and other hydrodynamics problems. Another important conclusion is the critical character of the numerical approach, that must be used in order to avoid high experimental costs or erroneous interpretations from experiments. In fact, the numerical approach permits a better understanding of what actually happens during the experiments. A good example is what happened when two different catalysts were numerically tested by using both the dilution and non-dilution experimental protocols at the same operating conditions. Even if the results are quite similar, it does not mean that gas-liquid mass transfer limitations are negligible in the non-diluted catalytic bed.