The aim of this paper is to determine the shape of a fixed-bed reactor which maxi- mizes the conversion rate under the constraints of process model equations (i.e. conti- nuity, Navier-Stokes, and mass balance equations), energy dissipation, iso-volume, and manufacturing. Incompressible fluid, laminar flow regime and steady-state conditions in the reactor are the main assumptions taken into account. The optimization method developed is based on the adjoint system method and OpenFOAM framework is used as CFD solver to compute the state vector and its adjoint variables introduced by the optimization approach. The algorithm developed is then tested on two different cases, a reactor where a first order homogeneous reaction takes place and another one involving a surface reaction. The optimization results show a significant reduction of the performance index by 10% in the first case, and by 20% in the second one. Finally, initial and optimal shapes are manufactured using a 3D printing technique.