Figure 5. Sensible enthalpy contour map in the reactor wherein the leading edge of the subsurface layer has a tapered shape that corresponds to the angle of cross-bars in the top-most layer so that fluids from the bulk flow path are not trapped beneath the top-most layer.
The enthalpy contour map in the reactor is plotted in Figure 6 wherein the leading edge of the subsurface layer has a tapered shape that corresponds to the angle of cross-bars in the top-most layer so that fluids from the bulk flow path are not trapped beneath the top-most layer. Smaller values of the temperature difference range have single phase heat transfer and low heat fluxes. There is a threshold temperature difference where nucleate boiling starts and increasing the difference slightly can result in larger heat fluxes, as nucleate boiling starts to occur. Critical heat flux occurs when the temperature difference reaches a point where the heat transfer rate changes from nucleate and bubbly flow to local dry out and gas phase resistance starts to dominate heat transfer. Critical heat flux can occur before dry-out. Critical heat flux results in larger hydraulic diameters are fairly well characterized. Channel size and vapor quality are related to average wall heat flux in saturated boiling [61, 62]. Consequently, higher process heat flux quickly approaches local critical heat flux via higher vapor generation rate and accumulated vapor amount. The difference between the wall temperature and the saturation temperature is defined as the overage temperature. For a matrix of aligned microchannels where the local heat flux varies from channel to channel the difficulties described above become more challenging. Potential unit operations that would have a varying heat flux profile over a matrix of connecting channels include but aren't limited to the following factors: Exothermic chemical reactions, catalytic or homogeneous, distillation tower heat removal, desorption stage in an absorption or adsorption system, exothermic mixing processes, and the like. This can occur when the microchannels are aligned cross-flow to the direction of the other unit operation's channels. For the varying channel flux situation there may be need for more flow in channels with the higher heat fluxes and less flow to channels with less heat fluxes to sustain convective boiling. It should be noted that from a heat transfer performance standpoint, isolated bubbles are most desirable. The catalyst may be supported on a porous support structure such as a foam, felt, wad or a combination thereof. The term foam is used herein to refer to a structure with continuous walls defining pores throughout the structure. The term felt is used herein to refer to a structure of fibers with interstitial spaces therebetween. The term wad is used herein to refer to a structure of tangled strands, like steel wool. The catalyst may be supported on a honeycomb structure. The catalyst may be supported on a flow-by support structure such as a felt with an adjacent gap, a foam with an adjacent gap, a fin structure with gaps, a wash-coat on any inserted substrate, or a gauze that is parallel to the flow direction with a corresponding gap for flow.