Flow reactor techniques

Graul S T and Squires R R 1988 Advances in flow reactor techniques for the study of gas-phase ion chemistry Mass Spectrom. Rev. 7 263-358... 

Unless carried out very carefully, data from flow reactors may be influenced by experimental uncertainties. Potential problems with the flow reactor technique include imperfect mixing of reactants, radial gradients of concentration and temperature, and catalytic effects on reactor walls. Uncertainties in induction times, introduced by finite rate mixing of reactants, presence of impurities, or catalytic effects, may require interpretation of the data in terms of concentration gradients, rather than just exhaust composition [442]. 

A combination of controlled atmosphere electron microscopy and flow reactor techniques have been used to investigate the influence of hydrogen sulfide on the catalytic activity of cobalt. Changes in the behavior of cobalt were monitored by the use of probe reactions which are sensitive to the chemical nature of the catalyst surface. These included graphite gasification in oxygen and hydrogen, and carbon deposition from decomposition of hydrocarbons. 

Brink, D.L., M.S. Massoudi, and R.F. Sawyer. A Flow Reactor Technique for the Study of Wood Pyrolysis, presented at the 1973 Fall Meeting of the Western States Section of the Combustion Institute in El Segundo, CA (October 1973). 

R. K. Jensen, S. Korcek, L. R. Mahoney, and M. Zinbo, Liquid-Phase Autoxidation of Organic Compounds at Elevated Temperatures. I. The Stirred Flow Reactor Technique and Analysis of Primary Products from n-Hexadecane Autoxidation at 120-180°C, Journal of the American Chemical Society, 79 1574—1579 (1979). 

The system can react to changes in the flow composition or temperature jump. A pulse method has also been introduced in heterogeneous catalysis and is used extensively. The transient flow reactor techniques that is frequently employed include the introduction of reactant as pulses, which provides unsteady state information, and step changes in reactant concentration, which provide information about the transient process from one steady state to another. The pulse method is generally less informative than the step change method in... 

The chief potential advantage of the stirred-flow reactor technique is in the study of complex reactions proceeding in several stages, since it avoids the use of cumbrous integrated rate equations, and facilitates the study by physical methods of transient reaction intermediates. 

Morita, A., Sugiyama, M., Koda, S. Gas-phase flow and diffusion analysis of the droplet train/ flow-reactor technique for the mass accommodation process. J. Phys. Chem. A107,1749-1759... 

Most ion-molecule techniques study reactivity at pressures below 1000 Pa however, several techniques now exist for studying reactions above this pressure range. These include time-resolved, atmospheric-pressure, mass spectrometry optical spectroscopy in a pulsed discharge ion-mobility spectrometry [108] and the turbulent flow reactor [109]. 

Another important reaction supporting nonlinear behaviour is the so-called FIS system, which involves a modification of the iodate-sulfite (Landolt) system by addition of ferrocyanide ion. The Landolt system alone supports bistability in a CSTR the addition of an extra feedback chaimel leads to an oscillatory system in a flow reactor. (This is a general and powerfiil technique, exploiting a feature known as the cross-shaped diagram , that has led to the design of the majority of known solution-phase oscillatory systems in flow... 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

Eisele and Tanner (146) have devised a similar scheme for the measurement of [HO ] via the chemical conversion of HO to H2 S04 by the addition of S02 to a flowing reactor followed by chemical ionization of gas-phase sulfuric acid to H S04 . The H 04 ion is uniquely identified and quantified in the flowing gas sample by a mass spectrometer. This technique is capable of sensitive, realtime measurement of [HO ], and although relatively new, appears to be perhaps the best overall technique devised to date. 

Chapter 2 developed a methodology for treating multiple and complex reactions in batch reactors. The methodology is now applied to piston flow reactors. Chapter 3 also generalizes the design equations for piston flow beyond the simple case of constant density and constant velocity. The key assumption of piston flow remains intact there must be complete mixing in the direction perpendicular to flow and no mixing in the direction of flow. The fluid density and reactor cross section are allowed to vary. The pressure drop in the reactor is calculated. Transpiration is briefly considered. Scaleup and scaledown techniques for tubular reactors are developed in some detail. 

Computational Scheme for Gas-Phase PFRs. A general procedure for solving the reactor design equations for a piston flow reactor using the marching-ahead technique (Euler s method) has seven steps ... 

The Knorr synthesis was chosen as an example to demonstrate that with an automated micro-reactor system a further step was made over existing parallel micro flow processing techniques, providing a 2 x 2 library in a chip micro reactor [20]. The new approach was designed for much higher diversity, aiming at a 7 x 32 library. 

From the time when it was shovm that micro flow reactors can provide valuable contributions to organic chemistry, it was obvious to develop them further and their workflow towards modern screening techniques [20]. It was especially the finding of high reaction rates, the capability to transport and transform minute sample volumes and the first integration of analytics that paved the way to a parallelization of micro flow processing. These benefits were combined with the ease of automation of a micro flow system. By this means, the potential of on-line analysis of the reactions can be fully exploited. 

For this last stage, the one-at-a-time procedure may be a very poor choice. At Union Carbide, use of the one-at-a-time method increased the yield in one plant from 80 to 83% in 3 years. When one of the techniques, to be discussed later, was used in just 15 runs the yield was increased to 94%. To see why this might happen, consider a plug flow reactor where the only variables that can be manipulated are temperature and pressure. A possible response surface for this reactor is given in Figure 14-1. The response is the yield, which is also the objective function. It is plotted as a function of the two independent variables, temperature and pressure. The designer does not know the response surface. Often all he knows is the yield at point A. He wants to determine the optimum yield. The only way he usually has to obtain more information is to pick some combinations of temperature and pressure and then have a laboratory or pilot plant experimentally determine the yields at those conditions. 

Except for the case of an ideal plug flow reactor, different fluid elements will take different lengths of time to flow through a chemical reactor. In order to be able to predict the behavior of a given piece of equipment as a chemical reactor, one must be able to determine how long different fluid elements remain in the reactor. One does this by measuring the response of the effluent stream to changes in the concentration of inert species in the feed stream—the so-called stimulus-response technique. In this section we will discuss the analytical form in which the distribution of residence times is cast, derive relationships of this type for various reactor models, and illustrate how experimental data are treated in order to determine the distribution function. 

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