Flow reactor techniques applications

The above experiments are generally difficult to perform and the interpretation of the results may not necessarily be straightforward. The low abundance of the neutral products collected and the likelihood of mass spectral interference between reagents and products make these techniques applicable only to special cases. An independent approach to this problem has been proposed by Marinelli and Morton (1978) who have used an electron-bombardment flow reactor allowing in principle for larger collection of neutral products followed by glc and mass spectral analysis. 

Many investigators use pulse techniques in which a catalyst reacts with hydrocarbons, oxygen etc. separately in time. This can provide an insight into the nature and significance of the individual reaction and sorption steps, but it should be emphasized that selectivities and other data may be unrepresentative for conditions in a flow reactor. In particular, selectivities may be considerably lower under steady state conditions. If the selectivity differences between pulse and flow experiments are very large, a cyclic mode of operation may be attractive for the practical application of the catalyst concerned. Oxidation and reduction are then separated. 

The three kinds of reactors already described in this section are all traditional cross-flow reactors with permeable plates or membranes. The electrochemical filter-press cell reactors used, e.g., for electrosynthesis, are equipped with cation-selective membranes to prevent mixing of the anolyte and the catholyte. These cell reactors are therefore good examples of the extended type of cross-flow reactors according to the definition transferred from the filtration field. The application of the electrochemical filter-press cell reactor technique... 

This method of obtaining spectra is truly in situ spectroscopy. It is superior to the previously described high pressure method in that the reaction mixture need not be transferred to a separate cell, thus allowing cooling and undesirable reactions to occur. However, the external sampling technique offered by the flow reactor described above may be more applicable to large scale industrial use. 

Chapter 8 ignored axial diffusion, and this approach would predict reactor performance like a PFR so that conversions would be generally better than in a laminar flow reactor without diffusion. However, in microscale devices, axial diffusion becomes important and must be retained in the convective diffusions equations. The method of lines ceases to be a good solution technique, and the method of false transients is preferred. Application of the false-transient technique to PDFs, both convective diffusion equations and hydrodynamic equations, is an important topic of this chapter. 

The phenomenon of continuous oscillation has been attributed to the intermittent particle generation coupled with a slow washout, as described previously. The application of advanced control techniques to stabilize these oscillations has met with limited success [17,41,42] and it has been shown that the use of a plug flow reactor (PFR) as the first reactor will stabilize the system. All particle nucleation takes place in the PFR. Subsequent growth of the particles takes place in the downstream CSTRs. The segregation of particle nucleation and growth prevents the onset of oscillation. 

The study of reaction kinetics in flow reactors to derive microkinetic expressions also rehes on an adequate description of the flow field and well-defined inlet and boundary conditions. The stagnation flow on a catalytic plate represents such a simple flow system, in which the catalytic surface is zero dimensional and the species and temperature profiles of the estabhshed boundary layer depend only on the distance from the catalytic plate. This configuration consequently allows the application of simple measurement and modehng approaches (Sidwell et al., 2002 Wamatz et al., 1994a). SFRs are also of significant technical importance because they have extensively been used for CVD to produce homogeneous deposits. In this deposition technique, the disk is often additionally forced to spin to achieve a thick and uniform deposition across the substrate (Houtman et al., 1986a Oh et al., 1991). 

Immobilized enzymes and whole cells have found well-documented applications in industry, medicine, and analytical chemistry. Theoretically, it should be possible to carry out any enzymatic reaction with the help of the respective immobilized enzyme or whole cell containing the enzyme. The technique of using an immobilized enzyme for a chemical transformation is not basically different from using the soluble enzymes. In commercial applications, the immobilized enzymes can be used in a continuous-flow reactor. However, the optimum conditions for a specific reaction will have to be redetermined before maximum turn-over can be achieved. Thus, proteolytic enzymes such as trypsin, when immobilized on an anionic matrix such as cofpolyethylene-maleic anhydride), require a much lower pH for reaction than in solution. Some typical applications of immobilized enzymes that are currently being made, or are in the process of development, are mentioned in Table 15-1. 

Catalysis For technical reasons it can be necessary to have a catalytic active sohd material supported to improve, e.g., its mechanical stability and reduce its flow resistance when used in a flow reactor. The sol-gel fluorination synthesis provides a convenient way for depositing high surface area metal fluorides on supports. For example, HS-AIF3, which as fine powder makes problems when used as catalyst in flow systems, could be supported by Y-AI2O3 whereby its Lewis acidity and consequently its catalytic activity remains almost unchanged [64]. For other catalytic applications, like micro-reactor techniques, deposition of catalytically active thin layers of metal fluorides is also of interest. 

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