This article analyses how the configuration of an industrial fixed-bed reactor affects the cycle length of a heavy oil hydroprocessing unit. It is well-known that during the hydroprocessing of heavy feeds, catalyst aging is counterbalanced by continuously increasing reaction temperature. In addition, the exothermality of the reaction provokes a huge temperature rise along the reactor, which is why quenching is necessary. Thus, there is an increasing temperature profile that evolves with time until a maximum allowable temperature is reached and then the operation is shut down. For this reason, there is an optimum reactor configuration (i.e. number of quenches and their positions) that must be established when designing new processes in order to maximize unit run length. To evaluate this problem, a reactor performance model with time varying catalyst activity was constructed. Kinetic and catalyst aging data were obtained from bench-scale tests. The model showed to reproduce sufficiently well the experimental data set. The analysis of various reactor designs indicated that for this process the use of single-bed or double-bed reactors is unpractical in terms of cycle length. A more complex configuration consisting of multiple beds of increasing lengths is necessary to delay shut down. (C) 2011 Elsevier Ltd. All rights reserved.