Mixing, mechanism

Pre-mixing of minor component with (part of) another component in an appropriate mixer 

Dust prilems can arise and the fluidized mixture may be difficult 

Is it possible to have the raw materials supplied at approximately the same particle size to minimis segregation  

This chapter will discuss the mechanisms of mixing fillers with gum rubbers. It is primarily concerned with the use of internal mixers. 

In addition, molecular diffusion always acts in such a way as to reduce inhomogeneities, but its effect is not significant until the fluid elements have been sufficiently reduced in size for their specific areas to become large. It must be recognised, however, that the ultimate homogenisation of miscible liquids 

mixing in liquids is achieved by several meehanisms which gradually reduce the size or scale of the fluid elements and then re-distribute them. If, for example, there are initial differences in concentration of a soluble material, imiformity is gradually achieved, and molecular diffusion becomes 

The description given here is a gross over-simplification, but it does give a qualitative representation of the salient features of turbulent mixing. This whole process is similar to that of the turbulent flow of a flmd close to a boimdaiy surface. Although some quantitative results for the scale size of eddies have been obtained and some workers [van der Molen and van Maanen, 1978 Tatterson, 1991] have reported experimental measurements on the structure of turbulence in mixing vessels, these studies have little relevanee to the mixing of non-Newtonian substances which are usually processed under laminar conditions. 

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A solution containing botli reactants and a catalyst may be mixed mechanically to bring tire constituents into efficient contact—otlierwise, tire rate of tire catalytic reaction would be affected by mass transport (e.g., diffusion)... 

Solids. For mixing of soflds the mixers can be categorized according to the mixing mechanism used. 

Mixing Mechanisms There are several basic mechanisms by which solid particles are mixed. These include small-scale random motion (diffusion), large-scale random motion (convec tion), and shear. 

Experimental confirmations of these mixing mechanisms are scarce. One study was with a 50-gal stirred tank reactor (Worrell and... 

The support materials for the stationary phase can be relatively inactive supports, e.g. glass beads, or adsorbents similar to those used in LSC. It is important, however, that the support surface should not interact with the solute, as this can result in a mixed mechanism (partition and adsorption) rather than true partition. This complicates the chromatographic process and may give non-reproducible separations. For this reason, high loadings of liquid phase are required to cover the active sites when using high surface area porous adsorbents. 

Substances with Unknown or Mixed Mechanism of Action... 

From a practical point of view, power consumption is perhaps the most important parameter in the design of stirred vessels. Because of the very different flow patterns and mixing mechanisms involved, it is convenient to consider power consumption in low and high viscosity systems separately. 

Finally, i should be noted that the calculation of the power requirement requires a knowledge of the impeller speed which is necessary to blend the contents of a tank in a given time, or of the impeller speed required to achieve a given mass transfer rate in a gas-liquid system. A full understanding of the mass transfer/mixing mechanism is not yet available, and therefore the selection of the optimum operating speed remains primarily a matter of experience. Before concluding this section, it is appropriate to indicate typical power consumptions in kW/m3 of liquid for various duties, and these are shown in Table 7.2. 

Figure 7.27. Twisted-blade type of static mixer operating in the laminar flow regime (a) Distributive mixing mechanism showing, in principle, the reduction in striation thickness produced (f>) Radial mixing contribution...

Reactor Design. The continuous polymerization reactions in this investigation were performed in a 50 ml pyrex glass reactor. The mixing mechanism utilized two mixing impellers and a Chemco magnet-drive mechanism. 

Confined flows typically exhibit laminar-flow regimes, i.e. rely on a diffusion mixing mechanism, and consequently are only slowly mixed when the diffusion distance is set too large. For this reason, in view of the potential of microfabrication, many authors pointed to the enhancement of mass transfer that can be achieved on further decreasing the diffusional length scales. By simple correlations based on Fick s law, it is evident that short liquid mixing times in the order of milliseconds should result on decreasing the diffusion distance to a few micrometers. 

Fig. 4.5.16 Schematic drawing of a boundary layer mixing mechanism. It is proposed that a thin layer with thickness 8 has a linear velocity profile with average velocity V/2. Material with bulk droplet volume fraction ( >in is drawn into the creamed layer (area Ac) and material with average creamed layer volume fraction (j)ou, is swept out. The remainder of the emulsion (inside the dashed circle) is stagnant.

Thus Saltiel has concluded that the small increase in [T]S/[C], in going from benzene to acetone indicates that a mixed mechanism is operative for acetone-sensitized isomerization, that is, both triplet energy transfer and, to a minor extent, Schenck intermediates are involved. When acetophenone or benzophenone is used as a sensitizer the pss is close to the thermodynamic... 

The tailing is probably caused by a mixed mechanism, for instance adsorption on active silica sites that are not end-capped. To reduce this, we can try adding a salt to the water. To get better resolution we need to change the selectivity, a, which means changing the chemistry of the mobile phase, or increasing the plate number of the column, or both. 

The separation is not totally orthogonal, as shown in Fig. 18.1, and is typical of most 2DLC separations (Kilz et al., 1995). Low molecular weight polymers that can diffuse into the packing pores exhibit both hydrophobic and size exclusion mechanisms in RPLC, and this mixed mechanism is shown by the Brij 70 series of peaks, dl through d3. The lower molecular weight material (dl) is more retained on the RPLC column since it can further diffuse into the pores. 

A batch of black powder, modified by addition of potassium chlorate, was being mixed mechanically with added water. A friction-sensitive crust appears to have formed, leading to initiation of the explosion which followed. 

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

PdxZS catalysts. Appropriate amounts of HZSM-5 or NaZSM-5 and Pd(acac)2 are mixed mechanically for 15 min in a mortar. The resulting solid is then treated in flowing helium or flowing oxygen (25 ml/min) at 2°C/min to 500°C and kept at this... 

The mixing mechanism of this mixer revolves around two axes. The mixing blades revolve around a common axis and also their own axis of rotation. This mixing action is particularly effective especially for less viscous solutions. 

In practical applications, dispersion calculations need to be performed in topographically complex environment. Thus, solid obstacles intervening in the area dispersion should be counting in the computations. In previous validation works, it has been proved that CFD codes constitute powerful tools for complex terrain dispersion simulation providing high accuracy results with excellent visualization capabilities, which can be helpful in quantitative risk analysis applications [55]. The dominating mixing mechanism between... 

Considerations of mixed mechanisms and mixed reactive states might apply to these cases also, but there is no direct evidence that cyclo-hexenone itself reacts from other than the lowest zn-n state. Both cis and trans products are produced, except possibly in the acrylonitrile reaction 97>, and cis and trans-2-butene give the same mixture of products. A predominant biradical mechanism would seem the logical choice. 

Put in ordinary terms, the more successful we are in causing a separation, the more propensities there are for a re-mixing of the components. There are many ways this can occur but there are a fewer number of important routes to mixing. It seems reasonable that we examine these before we consider all the possible ways in which thermodynamics can be controlled in general terms. In almost all equilibrium separation systems, the separation can occur either in a packed bed of particles or fibers or in an open channel or tube. The stationary phase is either coated on the walls of the channel or on the particles/fibers of the packed bed. If there were no mixing mechanisms an infinitely narrow packet containing the components would become a series of infinitely narrow packets of pure components moving at different velocities toward the end of the packed bed or tube. 

If flow occurred in an open channel with no particles or fibers and if there were no other mixing mechanisms, all particles would transit the same distance from beginning to end. In a packed bed, each time a molecule or atom encounters a particle or fiber it must go around it to continue on. It is analogous to encounter a tree in a field—one either walks around it to the right or the left and that is equivalent to flipping a coin. Some molecules will encounter more particles than others as illustrated in the following scheme where each encounter causes a chance in direction and the path of a hypothetical molecule is traced by a line (Scheme 1). 

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