A process includes preparing a diluted catalyst slurry in a catalyst slurry preparation system. The diluted catalyst slurry contains solid particulate catalyst and liquid hydrocarbon diluent. The catalyst slurry preparation system includes a mixing vessel with a top part, a bottom part and a rotatable impeller system which is actuated by a motor. The rotatable impeller system has a magnetic actuated agitator shaft which is positioned along a longitudinal axis of the mixing vessel and extends through the top part of the mixing vessel. At least two double-bladed hubs are fixed to the magnetic actuated agitator shaft.