Dispersion-Controlled Fast Reactions

As the final topic in this chapter, we consider the rates of chemical reaction in turbulent flow. Such flow produces rapid mixing, so that the fluid appears homogeneous. Such mixing turns out to be only macroscopic. In other words, if we take 10 samples, each of 1 cm, we find that the average concentrations of the samples differ by only a few tenths of a percent. However, if we take ten samples of 10 cm, we find that their concentrations vary widely. For example, if we are mixing acid and base, we might find that some samples contain 10 mol/1 H, and other have 10 mol/1 H.  

Such microscopic heterogeneity can sharply reduce the rate of a chemical reaction. Such reductions are not automatically bad. For example, in an automobile engine, we may wish to slow the combustion in order to reduce the maximum temperature and hence retard the formation of nitrogen oxide pollutants. It is important to know how this altered reaction rate can be estimated. Such estimates are the subject of this section. 

To begin these estimates, imagine that we continuously inject a small amount of a concentrated solution of dye into a rapidly flowing solvent. We measure the dye concentration downstream of this injection. Far downstream, the dye s concentration will be a uniform value of ci, as shown in Fig. 17.5-1. Closer to the injection point, the concentration Ci will fluctuate, both in position and time. The concentration fluctuations (ci — Cl) will sometimes be positive and sometimes negative their average over time will be zero. However, the squares of these fluctuations will always be positive. We can characterize the size of an average fluctuation as a root mean square 

Measurements of these fluctuations are reported as fluctuation sizes divided by the average concentration 

The relative fluctuation size 0 provides a measure of the effectiveness of the turbulent mixing in our particular experiment. 

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The thickness of the platelets can be adjusted by controlling the reaction conditions to produce interference colors. The pigment crystals are mechanically sensitive and show a fast sedimentation behavior because of the high density (6.4gcm 3). Agglomeration tendency and occupational health (toxicity) risks are the reason why Pb(OH)2 2PbC03 is not produced in powder form, but is flushed from the aqueous phase into suitable organic solvents or resins and handled as stabilized dispersions. Today, the application of basic lead carbonate is limited to artificial pearls, buttons, and bijouterie. 

Calorimetry (Fig. 4) proves that the key for thioresistance is the basicity of the surface which can be controlled by changing the CuO dispersion, showing a lower basicity (number and strength) when using a badly dispersed (highly loaded) CuO. Due to the fast reaction, the trap efficiency is mainly controlled by the size of the particles, so that the results can be largely improved. 

The first stage of a reaction involved the addition of sodium dispersed in toluene to a solution of adipic ester in toluene. The subsequent addition of iodomethane (b.p. 42°C) was too fast and vigorous boiling ejected some of the flask contents. Exposure of sodium particles to air caused ignition, and a violent toluene-air explosion followed [ 1 ]. When a reagent as volatile and reactive as iodomethane is added to a hot reaction mixture, controlled addition, and one or more wide-bore reflux condensers are essential. A similar incident involving benzene was also reported [2]. 

A feasible solution for this complex challenge is to implement at least two analytical methods with which the course of the reaction can be followed a fast first method that allows qualitative control of the status of the catalyst performance and a second accurate, and in most cases more time consuming, analysis method that will allow a detailed evaluation of catalyst performance. The two analysis methods can be run on one analytical unit, e.g. a gas chromatograph with two different analysis protocols, or separate analytical units such as a gas chromatograph for accurate performance evaluation in combination with a non-dispersive infrared unit for fast qualitative analysis. 

By reaction of ethyl acetoacetate with cyanoacetamides, pyridones are easily accessible. By selection of suitable substituents in the diazo components, shade and fastness properties and build up can be controlled (e.g., 39-42 and Disperse Yellow 211). 

Subsurface solute transport is affected by hydrodynamic dispersion and by chemical reactions with soil and rocks. The effects of hydrodynamic dispersion have been extensively studied 2y 3, ). Chemical reactions involving the solid phase affect subsurface solute transport in a way that depends on the reaction rates relative to the water flux. If the reaction rate is fast and the flow rate slow, then the local equilibrium assumption may be applicable. If the reaction rate is slow and the flux relatively high, then reaction kinetics controls the chemistry and one cannot assume local equilibrium. Theoretical treatments for transport of many kinds of reactive solutes are available for both situations (5-10). 

Multiphase reactions can be significantly affected by how well mixed the system is and how intimately dispersed the phases are. The reason for this is easy to explain, but more difficult to quantify although the course of any reaction is determined exclusively by the local concentrations of the reactants and the intrinsic reaction kinetic rates, in any real reactive system, the local reactant concentrations depend not only on how fast the reactants are depleted by the reaction, but also on how fast they are locally replenished from the bulk of the phases in which they initially reside. The latter phenomenon is directly related to the existence of a mass transfer step (in series with the reaction step), which determines the rate at which the reactants in different phases are brought in contact with each other. In many cases, especially if the rate of reaction is fast with respect to the mass transfer rate, the latter mechanism can become controlling over the former, and the overall reaction process is dominated by mass transfer and, hence, multiphase mixing. 

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