Simplified treatment of mass transfer for gas-phase hydrogenation/dehydrogenation of heavy compounds

Using single catalyst pellets (5 mm) 15% Ptγ–Al₂O₃, we experimentally studied gas-phase benzene hydrogenation at normal pressure by thermocouple measurements of gas flow and the pellet center. Temperature of gas flow was varied in the range of 20 ‡C / 350 ‡C for three molar fractions of benzene vapor (0.1, 0.2, and 0.3) mixed with hydrogen. The ignition/extinction behavior of the flow-pellet temperature rise (maximum values up to 100 ‡C/ 200 dgC) is explained by internal-external mass transport limitations of the reaction rate and reaction reversibility at high pellet temperature. A simplified pseudobinary treatment of both multicomponent intrapellet mass transfer (in bimodal porous media) and multicomponent external mass transfer (under forced convection) is proposed on the basis of the analytical estimation. The validity of the suggested approach is confirmed by comparing the experimental data for benzene hydrogenation with rigorous (multicomponent) and approximated (pseudobinary) calculations obtained by using a mathematical model of a spherically symmetric pellet. The simplified approach appears to be quite accurate for reactions A+nH₂=B of hydrogenation (n>0) or dehydrogenation (n<0) of sufficiently heavy compounds, i.e. if D _AH ≈D _BH >>D _AB