Reactive flow near surfaces

Correct modeling of the flow near the front of a stream requires a rigorous solution of the hydrodynamic problem with rather complicated boundary conditions at the free surface. In computer modeling of the flow, the method of markers or cells can be used 124 however this method leads to considerable complication the model and a great expenditure of computer time. The model corresponds to the experimental data with acceptable accuracy if the front of the streamis assumed to be flat and the velocity distribution corresponds to fountain flow.125,126 The fountain effect greatly influences the distribution of residence times in a channel and consequently the properties of the reactive medium entering the mold. 

Problem 11-6. Mass and Heat Transfer for Chemical Vapor Deposition. Consider the following model for chemical vapor deposition (CVD) on a surface. A reactive species is transported toward the surface by a 2D flow near a stagnation point, as illustrated in the figure. Far away from the surface the flow is given by... 

The VOF technique thus works best analyzing flow situations where the macroscopic interface motion may be nearly independent on the microscopic interface physics. For example, many analyzes on stratified or free-surface flows may be performed successfully by such a model. It is also noted that the VOF model concept is not primarily intended for multicomponent reactive flow analyzes, so no interfacial heat- and mass transfer fluxes or any variations in the surface tension are usually considered. 

A sequel to this study was presented later by the same group (Freund et al., 2003), this time for the simple first-order reaction A->B in a cylindrical bed of spheres with N — 5. The reaction was again taken to be mass-transfer limited and to occur on the surfaces of the catalyst particles, but at a very low flow rate at Re — 6.5. It was found that concentration peaks occurred near the wall at values close to the inlet value of species A, indicating that channeling was taking place. There were also local peaks of product concentration that indicated areas of high reactivity that could give rise to hotspots in practice. 

UV exposure (at k < 300 nm) of the AZ resist prior to plasma etching causes polymer cross-linking (167, 168) or decomposition (169) of the resist photosensitizer near the surface. Thus, a hardened shell or case is formed that permits a higher bake temperature without resist flow and also reduces the etch rate due to plasma exposure. Exposure to inert plasma (e.g., N2) causes similar effects (170), possibly because of ion and electron, as well as UV, bombardment of the resist surface. When F-containing discharges are used, fluorination of the resist surface occurs that strengthens the resist (because of the formation of C-F bonds) and minimizes reactivity (171). 

In extraction columns, it is possible to find droplet swarms where the local velocities near the droplet surface are higher, this being due to the lower free area available for the countercurrent flowing continuous phase. Wake and Marangoni influences make the prediction of a physical mass transfer coefficients difficult. With reactive extraction the influence of interfacial kinetics on overall mass transfer is generally not negligible. In any case, a combination of reactive kinetics with any eddy mass transfer model is recommended, whereas the latter could rely on correlations derived for specific column geometries. 

When plasma polymerization is carried out in a flow system, in which glow covers the entire cross-section of reactor with respect to the direction of monomer flow, the monomer molecules coming into the reactor first encounter the luminous gas phase. It is very unlikely that the molecules pass through the luminous gas phase without interacting with it and reach the relatively narrow zone in which IG or DG, located near the electrode surface, occurs. Therefore, the mode of activation that occurs in LPCAT without the influence of ionization is important in terms of the creation of chemically reactive species in LCVD. The creation of reactive species by the luminous gas is the mechanism considered here. 

Another NG-fired OTM-based oxy-fuel plant is the zero emission ion transport membrane oxygen power (ZEITMOP) cycle (Fig. 10.11) proposed by Yantovski and co-authors. The cycle is based on a supercritical CO2 cycle, where CO2 is compressed in an intercooled compressor to over 200 bar, heated in a recuperative heat exchanger and expanded in a high pressure turbine to 15 bar. It is then used as sweep gas in an OTM, where it is enriched with O2 separated from a stream of compressed air. The CO2/O2 flow is used as oxidant in a NG combustor which produces high temperature oxidized gas to be expanded to nearly ambient temperature in a low pressure turbine. Efficiencies of 50.4-52.0% with virtually zero CO2 emissions are reported. " A reactive membrane configuration can also be adopted to reduce the required membrane surface area. In this case, cycle efficiency... 

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