12. Diffusion and Reaction in Porous Catalysts

The hydrogenation of aromatics in a napthenic lube oil distillate takes place in a trickle bed reactor. The bed is 1 m in diameter and 6 m deep. The solubility of hydrogen in the oil is 0.0068 mol/dm ³atm. The concentration of aromatics in the feed is 0.07 mol/ dm ³. The superficial liquid flow rate is 8 kg/m ²s, while the hydrogen flow rate is 0.2 kg/m ²s. The reaction is first-order in hydrogen and in aromatics on the

-in. catalyst pellet. The true specific reaction rate is 2 x 10 ^-5 m ⁶ /kmolkgs. Nitrogen enters with the hydrogen at a rate of 0.05 kg/m ²s. The reaction occurs at 100°C and 50 atm.

(a) Make a plot of conversion of aromatics as a function of bed depth.
(b) Calculate the percent resistance of each transport step.
(c) How would the exit conversion change if the gas flow rate were reduced by a factor of 4?
(d) How could the conversion be increased?

Additional information:

Effective liquid diffusivity of H ₂ in oil: 8 x 10 ^-9 m>² /s
Effective liquid diffusivity of aromatics in oil: 6 x 10 ^-11 m ² /s
Viscosity of oil: 3.0 cP
Bed porosity: 0.35
Pressure gradient for flow of 0.25 kg gas/m ²s: 30 kPa/m
Density of oil: 900 kg/m ³
Pellet Density: 1500 kg/m ³
Tortuosity: 3.8
Porosity: 0.5
Construction: 0.8