NOVELTY - A reactor comprises a monolith having primary (1706, 1716) and secondary channels. The primary channel is parallel and connected to each of at least three other primary channels by the secondary channels. The axis of the primary channel is at an angle with respect to a portion of the reactor wall of more than 0 to less than 90degrees.    USE - The reactor, e.g. catalytic reactor, is used for carrying out reactions of fluid (claimed). The fluid includes exhaust gas from internal combustion engine, or blends of natural gas and water used to produce hydrogen by steam reforming.    ADVANTAGE - The reactor ensures a uniform distribution of residence time of the fluid in the reactor, thus increasing the reactor efficiency. The pressure drop of the flow through the reactor is kept to a minimum.    DETAILED DESCRIPTION - A reactor comprises an inlet (1750); an outlet (1752); a reactor wall (1708); and a monolith (1760). The monolith comprises primary and secondary channels. At least one of the primary channels is parallel to each of at least three other primary channels. The primary channel is connected to each of the other of the three primary channels by the secondary channels. The axis of the primary channel is at an angle with respect to a portion of the reactor wall of more than 0 to less than 90degrees. At least one of the primary channels is open at both of its ends. The monolith is placed within the reactor such that at least a portion of a fluid entering the inlet of the reactor will subsequently pass through the monolith and then exit the reactor through the outlet. An INDEPENDENT CLAIM is also included for reacting a fluid with a reactor comprising:    (A) causing the fluid to enter the reactor; and    (B) allowing the fluid to exit the reactor.    DESCRIPTION OF DRAWING(S) - The figure is a partial perspective cut away view of a reactor comprising a monolith of frustoconical corrugated layers.    Primary channels (1706, 1716)    Reactor wall (1708)    Inlet (1750)    Outlet (1752)    Monolith (1760)