Heterogeneous process reactor surface material

On the other hand, studying the influence of the surface material on the DMTM process in one [140] of a series of works, where a very high selectivity of methanol formation (P = 30 atm, T = 350 °C, tj- > 100 s) was observed, revealed no significant differences in the selectivity and yield of methanol in reactors with different surfaces, such as Pyrex, Teflon, stainless steel, silver, and copper. In these experiments, the selectivity of methanol formation on all surfaces reached values close to 90% or more, whereas the methanol yield was as high as 10.7%. However, the authors do not exclude the possibility of influence of the surface material on the temperature and reaction time. In particular, it was found that the maximum selectivity in reactors with metal surfaces was achieved at temperatures higher by nearly 50 °C. This is probably due to the fact that, at lower temperatures in the presence of a metal surface, the oxidation occurs mostly in the heterogeneous mode with the formation of mainly deep-oxidation products. 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

For chemical processes requiring heterogeneous catalysts a high density of surface exposed active sites in a given reactor volume is desirable. This can be achieved by using porous materials, either as catalysts or as supports for small clusters of a catalytic material. For catalytic processes with large molecules, the pore dimensions are of interest in particular, for liquid phase catalysis multidimensional pore with diameters exceeding 2.0nm may be required. 

We have already considered steady-state one-dimensional diffusion in the introductory sections 1.4.1 and 1.4.2. Chemical reactions were excluded from these discussions. We now want to consider the effect of chemical reactions, firstly the reactions that occur in a catalytic reactor. These are heterogeneous reactions, which we understand to be reactions at the contact area between a reacting medium and the catalyst. It takes place at the surface, and can therefore be formulated as a boundary condition for a mass transfer problem. In contrast homogeneous reactions take place inside the medium. Inside each volume element, depending on the temperature, composition and pressure, new chemical compounds are generated from those already present. Each volume element can therefore be seen to be a source for the production of material, corresponding to a heat source in heat conduction processes. 

---