Tube-wall reactor parallel reaction

It is evident that in many situations the reaction rate will be directly proportional to the surface area between phases whenever mass transfer hmits reaction rates. In some situations we provide a fixed area by using solid particles of a given size or by membrane reactors in which a fixed wall separates phases Ifom each other. Here we distinguish planar walls and parallel sheets of sohd membranes, tubes and tube bundles, and spherical solid or liquid membranes. These are three-, two-, and one-dimensional phase boundaries, respectively. 

In tubular reactors, heat is removed either by providing cooling tubes running parallel inside the reactor, or through external heat exchangers. Radial temperature gradients are normally observed in tubular reactors. In the case of exothermic reactions the temperature is maximum at the center of the tube and minimum at the tube wall. Similarly, in the case of endothermic reactions the temperature is minimum at the center of the tube and maximum at the tube wall. For highly exothermic reactions, packed bed reactors are usually avoided. 

We shall develop next a single-channel model that captures the key features of a catalytic combustor. The catalytic materials are deposited on the walls of a monolithic structure comprising a bundle of identical parallel tubes. The combustor includes a fuel distributor providing a uniform fuel/air composition and temperature over the cross section of the combustor. Natural gas, typically >98% methane, is the fuel of choice for gas turbines. Therefore, we will neglect reactions of minor components and treat the system as a methane combustion reactor. The fuel/air mixture is lean, typically 1/25 molar, which corresponds to an adiabatic temperature rise of about 950°C and to a maximum outlet temperature of 1300°C for typical compressor discharge temperatures ( 350°C). Oxygen is present in large stoichiometric excess and thus only methane mass balances are needed to solve this problem. 

Figure 12.11 A schematic diagram of two configurations for photo-assisted CVD. CVD reactors have windows inserted into the reactor tube (vacuum vessel) wall through which light supporting the reaction enters. A detector in parallel incidence can be included to measure absorption in the gas to determine the gas composition and density.

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