Transient gas-solid reactions in a fixed-bed reactor have been modeled and applied to hydrodesulfarization with the Langmuir-Hinshelwood mechanism. Three levels of process space are used: the bed, pellet, and grain in meter, millimeter, and micron sizes, respectively. The resulting nonlinear partial differential equations were solved via finite differencing and alternating-direction-iterative methods. The model was validated through comparison of experimental and theoretical results. Theoretical conversion was 99.98% compared to an experimental value of 99.90% for thiophene at steady state. Transient temperature profiles and the mass balance approach were used for verification. Steady-state temperature was predicted within 0.5%.