Oscillatory flow reactors

A careful analysis of the current portfolio of one major pharmaceutical company indicates that about 60% of the chemistry is suitable for continuous processing. About 50% of this chemistry is homogeneous and therefore readily transferable to existing continuous processing technology. The remaining 50% is heterogeneous and will therefore require implementation of some of the current advances in continuous flow equipment such as oscillatory flow reactors [13]. Technically, the transfer of these processes from batch to continuous could happen within... 

Issa, M., Wang, M., Vilar, G. and Williams, R. A., Measurements using high speed EIT on an oscillatory flow reactor 3rd International Tomography Workshop Japan, Tokyo, May... 

The effects of enhanced heat transfer in two reactor types, spinning discs and oscillatory flow reactors, are discussed based on two application examples. 

Basically the same conclusions are true for any intensified reactor. In case the intensified reactor allows a process to be heated with a small dT over the heat exchanger, but at the same time realises a substantial difference between solar supply and return temperature, there will be a high potential for heating the reactor with solar thermal energy with low-temperature collectors. This is further depicted in the next example, where an oscillatory flow reactor is analysed with gradual heating of the process medium. 

Technologies Reactors spinning disk reactor, static mixer reactor, microreactors, heat exchange reactors, monolithic reactors, oscillatory flow reactors, trickle bed reactors... 

The oscillatory baffle reactor/oscillatory flow reactor (OBR/OFR) types are seen as for niche applications, where one wants to convert a long residence time batch process to a continuous one. In the case of biodiesel, Dr Harvey indicates that a conversion could be carried out in 10 minntes, compared to 1-6 hours in continnons indnstrial processes. One variant is shown, by means of a flow diagram, in Figure 10.20, while Figure 10.21 shows components of the OFR. The aim is to make the plant portable so that it will fit into a standard shipping container. The unit could be sold worldwide to, for example, formers to produce their own fuel locally. 

Figure A3.14.4. P-T ignition limit diagram for H2 + O2 system showing first, second and third limits as appropriate to a closed reactor. The first and second limits have similar positions in a typical flow reactor, for which there is also a region of oscillatory ignition as indicated.

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... 

Figure A3.14.7. Example oscillatory time series for CO + O2 reaction in a flow reactor corresponding to different P-T locations in figure A3,14,6 (a) period-1 (b) period-2 (c) period-4 (d) aperiodic (chaotic) trace (e) period-5 (1) period-3.

In open, or flow, reactors chemical equilibrium need never be approached. The reaction is kept away from that state by the continuous inflow of fresh reactants and a matching outflow of product/reactant mixture. The reaction achieves a stationary state , where the rates at which all the participating species are being produced are exactly matched by their net inflow or outflow. This stationary-state composition will depend on the reaction rate constants, the inflow concentrations of all the species, and the average time a molecule spends in the reactor—the mean residence time or its inverse, the flow rate. Any oscillatory behaviour may now, under appropriate operating conditions, be sustained indefinitely, becoming a stable response even in the strictest mathematical sense. 

In chapters 2-5 two models of oscillatory reaction in closed vessels were considered one based on chemical feedback (autocatalysis), the other on thermal coupling under non-isothermal reaction conditions. To begin this chapter, we again return to non-isothermal systems, now in a well-stirred flow reactor (CSTR) such as that considered in chapter 6. 

Formulation of multi-component emulsions and mixtures are of interest in chemical and industrial processes (Vilar, 2008 Vilar et al., 2008). Standard stirred tank reactors (STR) and oscillatory baffled reactors (OBR) are traditional methods for the formulation of liquid-liquid mixtures and liquid-solid emulsions. Compared with STR, oscillatory baffled reactors provide more homogeneous conditions and uniform mixing with a relatively lower shear rate (Gaidhani et al., 2005 Harrison and Mackley, 1992 Ni et al., 2000). Figure 17 is a sketch of a typical oscillatory baffled reactor. It consists of the reactor vessel, orifice plate baffles, and an oscillatory movement part. The orifice plate baffles play an important role in the OBR for the vertex generation in the flow vessels as well as the radial velocities of the emulsions and mixtures. They are equally spaced in the vessel with a free area in the center of each baffle... 

Figure 17 Sketch of the oscillatory baffled reactor (top) and flow within baffled sections (bottom).

Obviously, to find a specific set of conditions for the onset of a concentration jump between stable states, or for the occurrence of oscillations, for that matter, would require the testing of a huge number of reaction mixtures under conditions maintained far from equilibrium. Since such an experiment is not easily accomplished in ordinary glassware, we conducted our studies of oscillatory reactions in continuous flow reactors patterned after those used at the Paul Pascal Research Center in Bordeaux, France35). 

Wen and Fan [6] have provided a comprehensive listing of various tracers and experimental techniques for determining the RTD in flow systems. Recent studies [10,11,12] have been performed employing an impulse tracer to determine the RTD in bubble columns and an oscillatory flow electrochemical reactor. The author [13,14] has employed both step-change and an impulse to determine the RTD of nozzle type reactors analysis of the RTD involves an atomic absorption spectrophotometer (AAS), a cine-projector, and a chart recorder. Figures 8-7 and 8-8 show the nozzle-type reactors and the AAS, respectively. Figure 8-9 gives a typical response curve from the AAS. 

If a chemical reaction is operated in a flow reactor under fixed external conditions (temperature, partial pressures, flow rate etc.), usually also a steady-state (i.e., time-independent) rate of reaction will result. Quite frequently, however, a different response may result The rate varies more or less periodically with time. Oscillatory kinetics have been reported for quite different types of reactions, such as with the famous Belousov-Zha-botinsky reaction in homogeneous solutions (/) or with a series of electrochemical reactions (2). In heterogeneous catalysis, phenomena of this type were observed for the first time about 20 years ago by Wicke and coworkers (3, 4) with the oxidation of carbon monoxide at supported platinum catalysts, and have since then been investigated quite extensively with various reactions and catalysts (5-7). Parallel to these experimental studies, a number of mathematical models were also developed these were intended to describe the kinetics of the underlying elementary processes and their solutions revealed indeed quite often oscillatory behavior. In view of the fact that these models usually consist of a set of coupled nonlinear differential equations, this result is, however, by no means surprising, as will become evident later, and in particular it cannot be considered as a proof for the assumed underlying reaction mechanism. 

Figure 4 shows a typical example of sustained kinetic oscillations occurring for particular conditions (pc0, p0r and T) during the catalytic CO oxidation on a Pt(llO) surface (40). The measurements were performed with an UHV system acting as flow reactor, where the C02 partial pressure is directly proportional to the rate. The simultaneously recorded CO pressure oscillates with the same period and with amplitudes of about 1%, whereby pco shows a minimum whenever the reaction rate is maximum. The work function A varies parallel to the rate R. This quantity is essentially determined by the oxygen coverage. Because under oscillatory conditions the rate is determined by oxygen adsorption (see above), it becomes plausible why A and R vary in phase. 

We, however, hope that our study will be considered asaflrst positive step towards this direction. We have Investigated the oxidation reaction of CO over Pt/y-Al203 type catalysts in.an all Pyrex glass flow reactor. We have carefully tried to eliminate all reactant impurities or possible interferences caused by the presence of temperature and flow controllers or high volume recycle streams. Furthermore, during our study, surface intermediates have been classified and monitored by the technique of IR transmission spectroscopy (IRTS) both under steady-state and oscillatory conditions. Our study is currently in progress. A few of our initial experimental observations are presented in this paper. Further details will be presented elsewhere [l6]. 

Fig. 5.32. The p-T regions for steady dark reaction, steady glow, oscillatory glow and oscillatory ignition, and the influence of added H2 for the CO + O2 reaction in a flow reactor with = 8 S , (a) no added H2 (b) 150 ppm added H2 (c) 1500 ppm added H2 (d) 7500 ppm added H2 (e) 10% H2 in final mixture. (Reprinted with permission from reference [64], Royal Society of London.)...

Exotic oscillatory and other types of non-linear behaviour are also features of most hydrocarbon oxidations [71-74]. The next chapter will provide a detailed mechanistic description of the basis for cool-flames etc., and their relevance in various situations. It is interesting, however, to apply the classification system developed in the previous sections to the global behaviour in these systems. We start with a description of the oxidation of acetaldehyde (ethanal) and again concentrate on modern studies in flow reactors where the effects of reactant consumption (which are much more significant in closed systems for these cases than for CO) are not a feature. 

In well-stirred flow reactors, oscillatory cool-flames are sustained indefinitely as long as the reactor temperature and flow control are held constant. 

A complementary ignition diagram to that for n-C4Hio in an unstirred closed vessel was obtained in a jet-stirred flow reactor by Proudler et al. [58]. The reactor was a 500 cm stainless steel cylinder, operated at a mean residence time of 9.4 0.4 s with a reactant mixture [n-C4Hio] [O2] = 1.13 1 (Fig. 6.16). Oscillatory cool-flames and ignitions were detected within narrow temperature ranges, but comparable with those of the closed... 

Fig. 6 16. p-Ta) ignition diagram obtained for the combustion of n-butane + oxygen in a well-stirred flow reactor at a mean residence time of 9.4 0.4 s. Different phenomena are identified as follows L, low-temperature stationary states H, high-temperature stationary states CF, oscillatory cool-flames I, oscillatory ignitions. A hysteresis of the boundaries was measured as a result of increasing (solid lines) and reducing (broken lines) the vessel temperature. (After Proudler et al. [58].)... 

Stonestreet, P. Van Der Veeken, P.M.J. The effect of oscillatory flow and bulk flow components on residence time distribution in baffled tube reactors. Chem. Eng. Res. Des. 1999, 77 (A8), 671-684. 

Figure 10.8 Oscillatory behavior of particle numberdensityfdy, > 0.3 rn) in a flow reactor study of coal gas containing NO at a concenuraiion of 11.6 ppm. Measurements of deposited particles were made with an optical counter. It is likely that many more particles were present in the ultraline range lip < 0.1 //m) and not counted. (After Badger and Drydcn. 1939.)...

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