(215c) Elucidating Chromium Contaminates Distribution in a Circulating Fluidized Bed for Light Alkane Dehydrogenation

Propane dehydrogenation (PDH) becomes more and more attractive to produce propylene. Commercial PDH plants employ either fixed beds (e.g. Catofin process) or moving beds (e.g. Oleflex process). However, both technologies are facing the problems of frequent regeneration of catalysts and/or reaction heat supply due to the highly endothermic nature of the dehydrogenation reaction and the fast deactivation of the catalysts. This has triggered the utilization of the fluidized bed technology for PDH to help alleviate those shortcomings, where the chromium-based catalyst is commonly used. Although it is well-known that Cr⁶⁺ is quite harmful to the environment and human health, its distribution in a Circulating Fluidized Bed (CFB) setup is still unclear.

Methodology

The distributions of chromium with various valences (i.e., Cr⁶⁺, Cr³⁺, and Cr²⁺) on the dehydrogenation catalysts, the detailed fluid flow, heat and mass transfers, and coupled chemical conversions in a whole CFB unit need to be precisely quantified. Fortunately, the Multi-Phase Particle-In-Cell (MP-PIC) methodology, has offered real possibilities to vary many physical parameters linked to CFBs and study their effects on the process performance at large scales. Thus, MP-PIC simulations on the whole CFB unit were carried out in this work.

Selected Results

With the MP-PIC methodology, the predicted products yield and temperature distributions are in a very good agreement with the experimental data, with all relative errors below 3.31%. Figure 1a presents the distributions of Cr⁶⁺, Cr³⁺, and Cr²⁺ on catalysts. As it can be seen from Figure 1a, while Cr³⁺ and Cr²⁺ are mainly distributed in the PDH fluidized bed reactor, Cr⁶⁺ is distributed in the whole CFB setup. Figure 1b shows the flow diagrams of original and updated processes. Numerical results indicate that compared to the original PDH process, the distribution of Cr⁶⁺ in the whole setup can be inhibited if a pre-reduction reactor is added into the process.

Key Conclusions

Numerical predictions with MP-PIC methodology show a very good agreement with experimental data. According to the full-loop MP-PIC simulations, catalysts covered with Cr⁶⁺ are distributed in the whole CFB unit. Therefore, effective measures must be taken to avoid chromium contaminates. For instance, a reactor for Cr⁶⁺ reduction can be set before the PDH reactor in the continuous process.

Acknowledgements

Financial supports from the National Natural Science Foundation of China (21908186) and the Natural Science Foundation of Shandong Province (ZR2023MB109) are acknowledged.