Figure 4. Ethane mole fraction contour plots in the catalytic reactor with an integral heat exchange structure in an array of longitudinally disposed, adjacent reaction passageways or channels which are either catalyst-coated or catalyst-free.
The velocity contour plots of flow in the catalytic reactor are illustrated in Figure 5 with an integral heat exchange structure in an array of longitudinally disposed, adjacent reaction passageways or channels which are either catalyst-coated or catalyst-free. A catalyst structure comprising a heat resistant support material composed of a plurality of common walls which form a multitude of adjacently disposed longitudinal channels for passage of a gaseous reaction mixture wherein at least a part of the interior surface of at least a portion of the channels is coated with a catalyst and the interior surface of the remaining channels is not coated with any catalyst such that the interior surface of the catalyst-coated channels are in heat exchange relationship with the interior surface of adjacent catalyst-free channels. The preferred catalyst structure can be readily modified to increase the number and tortuosity of the catalytic channels by inserting additional corrugated sheets having a herringbone corrugation pattern between the two flat sheets with sharp peaked corrugations. If additional corrugated sheets are inserted in the repeat unit, they can be coated on one side of the other or remain uncoated depending on the catalyst structure desired. Catalytic channels and non-catalytic channels are formed by selectively coating one side of the two flat sheets and one side of one of the corrugated sheets with a catalyst. The reactor can incorporate streamwise variation of the fluid velocities. Specifically, the velocity of the fluid in the exothermic catalytic reaction channel can be decreased after the fluid enters the channel by streamwise geometric changes in the reactor wherein the channel's entrance flow area is less than the channel's cross-stream flow area at some streamwise location where the catalyst is deposited. The channel's entrance flow area is defined as the channel's cross-stream flow area immediately downstream of the most downstream conduit inlet. If the conduit wall thicknesses are approximately constant, streamwise changes in flow velocity can be produced by providing, for instance, either contracted sections of the conduits, expanded sections of the casing, or a combination of both. In a reactor employing expanded-end conduits, the cross-stream area of the conduits decreases just after the cooling flow enters the cooling passages, where the expanded conduit sections taper down to the nominal conduit size in the central portion of the reactor. As a result, the cooling flow velocity is increased to a value greater than its entrance velocity. Conversely, because the casing is of constant cross-sectional size, the cross-stream flow area of the exothermic catalytic reaction channel increases just after the fuel-oxidant mixture enters. As a result, the flow velocity in the exothermic catalytic reaction channel is decreased to a value less than its entrance velocity. The gas flow velocity entering the exothermic catalytic reaction channel should exceed the minimum required to prevent flashback into the fuel-oxidant stream upstream of the reactor if the fuel-oxidant mixture entering the exothermic catalytic reaction channel is within the limits of flammability. The laminar flame propagation velocity is typically less than 0.8 meters per second for hydrocarbon fuels in air, but the turbulent flame propagation velocity may exceed 8 meters per second and may approach 28 meters per second for highly turbulent flow. To prevent flashback, the gas flow velocity should exceed 8 to 28 meters per second at gas turbine engine conditions, or more if a safety margin is allowed. Because catalyst light-off becomes increasingly difficult with increasing velocity, it is desirable to reduce the velocity of the fuel-air stream once it has entered the exothermic catalytic reaction channel, by a streamwise variation of cross-stream area. The flow velocity of the fuel-air stream over the exothermic reaction surface in the exothermic catalytic reaction channel is nominally 28 meters per second or less. This reduction in velocity is achieved by a streamwise increase in the cross-stream area for flow over the exothermic reaction surface. Streamwise changes in cross-stream area are fixed by the geometry of the reactor, and do not change in time. The velocity of the cooling stream should exceed the maximum flame propagation velocity at the exit of the cooling conduits, if the cooling fluid exiting the cooling passages is within the limits of flammability, to prevent flashback from a downstream combustion chamber. If expanded conduit outlets are employed, it is also very important that the downstream increase in cross-stream area of the cooling conduits is sufficiently gradual that recirculation of the cooling flow does not occur, so that there is no possibility of flashback or flame holding in the downstream expanded conduit section. Typically, the cone angle for an axis-symmetric diffuser section should not exceed approximately 6 to 8 degrees for good pressure recovery and minimal recirculation. If continued backside cooling is a consideration, the angle should be especially shallow to ensure that there is no local separation of the cooling flow and concurrent loss of cooling effectiveness.