133 Other Reactors

One textbook reactor which also has several industrial applications, is a tubular reactor. For example, polymerization of ethylene is carried out in a tubular reactor. Although, a lot of information is available on global fluid dynamics of tubular reactors, when reactions and fluid dynamics are intimately connected via mixing, it is necessary to develop a comprehensive computational flow model including chemical reactions. Recently Kolhapure and Fox (1999) used such a computational flow model (based on a multi-environment reactive mixing model) to understand the effect of micromixing on LDPE (low density polyethylene) polymerization in a plant-scale tubular reactor. The relevant model equations and necessary numerical methods are already discussed in earlier chapters. The study by Kolhapure and Fox (1999) shows that imperfect mixing of species reduces monomer conversion, increases the polydispersity index and may cause local hot spots (with possible degradation and adverse product quality). Apart from giving important physical insights, such a computational flow model can also be used to carry out 'virtual experiments' to evaluate various design alternatives to minimize imperfect mixing in industrial LDPE reactors.

Another major application of computational flow modeling is for engineering chemical vapor deposition (CVD) reactors. Various types of CVD reactor are used in the microelectronics industry (Kleijn, 1991). In CVD reactors, fluid dynamics, transport processes and chemical reactions are again strongly interrelated and computational fluid dynamics based models can make substantial contributions to the design of industrial-scale CVD reactors. Recently, Komiyama et al. (1999) reported the application of computational flow models to simulate a tubular CVD reactor. The model was used to simulate growth rates and composition on a 5-inch wafer placed in a vertical, axis-symmetric, cold wall reactor. No fitting parameter was used for these simulations. Their predicted results are shown in Fig. 13.11a and 13.11b. It can be seen that CFD captured the growth rate and composition quite adequately. The simulation results indicate that decreasing mass transfer resistance near the wall led

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