Enhanced mixing

Flares. Flares are used for the combustion of waste hydrocarbon gases in which the rates may vary over a wide range and for emergency releases. Steam injection is usually used to enhance mixing and the formation of a clean flame. 

For the Michael addition of 2,4-pentanedione enolate to ethyl propiolate, improvements in conversion were determined. This example serves also to demonstrate that proper process conditions are mandatory to have success with micro-reactor processing. A conversion of only 56% was achieved when using electroosmotically driven flow with a two-fold injection, the first for forming the enolate and the second for its addition to the triple bond (batch synthesis 89%) [151]. Using instead a stopped-flow technique to enhance mixing, a conversion of 95% was determined. 

The conversions observed followed the sequence of reactivity known from batch experiments carried out in advance. For example, only 15% conversion was found for the less reactive reagent benzoylacetone in the micro reactor experiment, while 56% was determined when using the more reactive 2,4-pentanedione (batch syntheses 78% and 89%, respectively) [8]. Using the stopped-flow technique (2.5 s with field applied 5.0 s with field turned off) to enhance mixing, the conversions for both syntheses were increased to 34 and 95%, respectively. Using a further improved stopped-flow technique (5.0 s with field applied 10.0 s with field turned off), the conversion could be further enhanced to 100% for the benzoylacetone case. For the other two substrates, diethyl malonate and methyl vinyl ketone, similar trends were observed. 

When deciding on the type of the reactor required for a particular chemical or physical transformation, the first question that needs to be addresses is whether the cavitation enhancement is the result of an improved mechanical process (due to enhanced mixing). If this is the case, then cavitation pretreatment of a slurry may be all that is required before the system is subjected to conventional type transformation scheme and the scale up of the pretreatment vessel would be a relatively simpler task. 

Acrylonitrile alone has little tendency to produce duodenal ulcers in animals, but pretreatment with phenobarbital or Aroclor results in a marked increase in the incidence of such ulcers (Szabo et al. 1983, 1984). Although the mechanism of the ulcerogenic effect is not obvious, these data indicate that agents which enhanced mixed-function oxidase activity may also increase the toxicity of acrylonitrile. 

Peristalsis is the intestinal pump that maintains a suitable velocity and enhances mixing. We describe this reactor as nearly plug flow, although the backmixing of bacteria is essential in keeping them in the reactor. 

Dispersion is the enhanced mixing of material through spatial variations in velocity. When it is of interest (when we are not keeping track of the three-dimensional mixing), dispersion is typically one or two orders of magnitude greater than turbulent diffusion. The process of dispersion is associated with a spatial mean velocity. The means used in association with diffusion, turbulent diffusion, and dispersion are identified in Table 6.2. 

Brone D, Muzzio F. Enhanced mixing in double-cone blenders. Powder Technol 2000 110(3) 179M89. 

In reality, of course, mixing cannot be perfectly instantaneous. In practice, CSTR reactors are designed to create high-intensity turbulence that enhances mixing. Also very low-pressure systems can often be considered as stirred reactors. At low pressure (e.g., below 1 Torr), the molecular-diffusion rate is very high owing to long mean-free paths. 

When there are many (quasi)bound states, healing can occur in an off-diagonal manner in that the (quasibound) state that they come back to need not be the same as the (quasibound) state that they dissociated from. Healing thus provides for an enhanced mixing of the bound phase space. 

M 11] [P 9] Increasing the peak-to-peak amplitude further enhances mixing. By multiple-bubble microstreaming mixing at 5 kHz and 40 V, the dye completely fills the micro chamber within about 6 s (see Figure 1.29) [24]. Mixing based on pure diffusion in the same volume would have taken about 8 h. 

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