Parallel-plate reactor processes

Catalyst Monolith. The previous discussion in this chapter focused primarily on chemical reactions taking place in packed-bed reactors. However, when a gaseous feedstream contains significant amoimts of particulate matter, dust tends to clog the catalyst bed. To process feedstreams of this type, parallel-plate reactors (monoliths) are commonly used. Figure 11-11 shows a schematic diagram of a monolith reactor. The reacting gas mixture flows between the parallel plates, and the reaction takes place on the smface of the plates. 

Nevertheless, despite the fact that no quantitative predictions can yet be made with regard to reactor scale-up, it is of interest to consider some possible implications in the simplest case of a parallel plate reactor in which the field is considered to be perfectly uniform. Furthermore, if it is assumed that solid surfaces play no part in the chemical processes and that the reaction rates are only dependent upon the reduced field (E/p), then for a reactor having an electrode spacing d and operating at a voltage V and pressure p,... 

Many such processes continue to use parallel plate reactors, a number of which are now available for purchase (e.g. the SU cell[37], the dished electrode cell[38] and the FM21 cell[22]) but many syntheses fail to be applied because economic assessments are based on unnecessarily complex designs and expensive cell components developed for other application e.g. membranes, Ti cell bodies, electrode materials. Hence it is particularly good to see the development and application of very cheap configurations, for example the new Monsanto process[24] or the Swiss Roll cell[35j. It is to be hoped that more companies give consideration to cell systems specifically designed for particular processes. 

Plasmas can be used in CVD reactors to activate and partially decompose the precursor species and perhaps form new chemical species. This allows deposition at a temperature lower than thermal CVD. The process is called plasma-enhanced CVD (PECVD) (12). The plasmas are generated by direct-current, radio-frequency (r-f), or electron-cyclotron-resonance (ECR) techniques. Eigure 15 shows a parallel-plate CVD reactor that uses r-f power to generate the plasma. This type of PECVD reactor is in common use in the semiconductor industry to deposit siUcon nitride, Si N and glass (PSG) encapsulating layers a few micrometers-thick at deposition rates of 5—100 nm /min. 

The process of "characterizing" a reactor can be illustrated for a parallel-plate cold-wall reactor operated at 50 kHz.8 System power was kept at 500 W, pressure at 200 mTorr, and wafer temperature of 240°C. Wafers are placed on a circular electrode which is rotated to promote uniformity of deposition. Therefore, we are only interested in the radial variation of deposition rate. Reactive gases enter at the center and flow out at the periphery. 

Based on these fundamentals, many systems and apparatus have been built, being operative on an industrial scale. Different types of reactors have also been designed. The electrodes may be parallel plates [162, 163] or sacrificial Al pellets as anode [164,165]. The feeding of pressurized air has been implemented in many electrocoagulation-electroflotation systems [159,166-168]. Some plants have a press to remove water from the sludge [169,170] and a processing tank with a closed S-shaped one-way flow path [171]. 

In building up multiple units (Sections 5.1.2 and 5.1.3), alternative systems of electrical and hydraulic connections are possible, each of which have advantages. The choice often depends on the scale of the process and whether batch or continuous operation is to be used. In batch processes with flow electrolysis, the conversions per cell pass are usually small and the type of hydraulic connection will depend on mechanical aspects of reactor design and operation. With parallel plate cells, parallel electrolyte flow is certainly the most common option. In continuous processes the type of hydraulic connections—series or parallel—may affect the production capacity of a unit. Further information can be found in Picket s book" on reactor design. 

The parallel-plate geometry is a popular and convenient choice of reactor for a diverse range of processes. Reasons for this include ... 

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