Rapid mixer

The rapid mixer is composed of layers that are fabricated separately and then assembled together. The main four channel device, represented by the cartoon of Fig. 12.2, is etched through a 1-mm-thick silicon wafer using an anisotropic Bosch process RIE (Unaxis 770, Unaxis). The depth of this etch requires a thick mask. We use a 7 pm layer of PECVD silicon dioxide (GCI PECVD Group Sciences Incorporated, San Jose, CA). This mixer is sandwiched between two 100 pm thick poly(dimethylsiloxane) (PDMS) layers (Duffy et al, 1998), which contain channels in a T configuration. 

Finally, fluidized-bed mixers are rapid mixers, but cannot, of course, deal with cohesive powders or a very wide distribution of particulate sizes neither are they suitable for powder mixtures with pronounced size, density, and shape variations because of segregation problems. They generate small static electric charges. 

Figure 5.6 This home-made rapid mixer houses two 10 ml gas-tight Hamilton syringes that are encased in a block of plexiglass equipped for a temperature-controlled water bath. Three-way taps allow for solution uptake into the syringes and then equal volume delivery into a small mixing chamber before delivery into the cuvette.

G. Imanidis, S. Winzap, and H. Leuenberger, Agglomeration kinetics and process monitoring in rapid mixers , in Agglomeration 85 (ed. C. E. Capes), Proceedings of Fourth International Symposium on Agglomeration, Toronto, Canada, The Iron and Steel Society, Inc., Warrendale, Pa., USA, 1985, pp. 489-96. 

There are a variety of other spectroscopy techniques available for investigating reaction kinetics. However, MALDI-TOF-MS allows for the determination of rate constants regardless of the incorporation of a chromophore, and at a wide variety of buffer concentrations. Both of these advantages are useful if one wishes to study enzymatic reactions under naturally occurring conditions. The advantages of MALDI-TOF-MS can be compared to spectrophotometric methods, which require a chromophore, and electrospray ionization mass spectrometry, which is sensitive to buffer concentrations. As MALDI-TOF-MS is an off-line technique requiring quenching for the analysis we performed, a rapid mixer must be incorporated into the system. 

The design and operation of the custom-built rapid mixer are described elsewhere (3,4). An effective rapid-mix step rendered a flocculation step unnecessary and enabled the use of Fe(III) doses as high as 20.6 mg/L (60 mg/L as FeClj) without any fouling. Moreover, backwash intervals as high as 29 min were... 

The ideal design arises from measurements of chemical kinetics unhindered by mixing or mass transfer effects. This is possible for reaction times greater than about 10 milliseconds for some liquid systems, using rapid mixers or by mixing at low temperature then suddenly increasing the temperature to cause reaction. This is the temperature jump technique . Such instruments are available commercially. 

Most practical reactions are more complicated than has been inferred here, but with the aid of careful experiments this can often be simplified to one controlling step. The rapid mixer plus plug flow reactor is useful for unravelling the details. If such a simplification cannot be made, the results of the kinetics experiments will have to be fed into a computer reactor model (see section 15.10.6) for interpretation and the model used for scale-up. 

Semicontinuous crystallization processes often eombine the best features of both batch and continuous operation. For example, a rapid mixer with a short residence time (possibly an on-line deviee, e.g. Figure 9.14) can discharge its product slurry into an agitated residenee tank. In many eases, a series of tanks may have to be installed, whieh can then be operated as individual units or arranged in cascade (section 9.1.2). 

Turbo rapid mixer, compactor, mill + stock surcharge... 

Various possibilities are available for homogenising the powder and binder. For example, the material can be mixed with a twin screw extruder, a shear roller extruder, or a rapid mixer. 

In a recent development by Cabot [2] a rubber latex and a carbon black slurry are cocoagulated in a continuous, rapid mixer with vigorous agitation. The process is particularly suited to NR latex, which has a delicate balance of stability, although in principle, it is applicable to any rubber latex. The advantages over dry mixing are demonstrated in several ways ... 

Park, N S, Park, H and Kim, J S (2003), Examining the effect of hydraulic turbulence in a rapid mixer on tmbidity removal with CFD simulation and PIV analysis . Journal of Water Supply Research and technology-AQUA, 52(2) 95-108. 

Figure C3.1.1. The basic elements of a time-resolved spectral measurement. A pump source perturbs tlie sample and initiates changes to be studied. Lasers, capacitive-discharge Joule heaters and rapid reagent mixers are some examples of pump sources. The probe and detector monitor spectroscopic changes associated with absorjDtion, fluorescence, Raman scattering or any otlier spectral approach tliat can distinguish the initial, intennediate and final...

In the adiabatic process, the reactants are mixed rapidly ia a smaH-volume, high throughput static mixer without cooling (65). The hot product is used direcdy. A 250-mL reactor of this type can produce 8 t/d of peroxomonosulfuric acid. 

For suspension of free-settling particles, circulation of pseudoplastic slurries, and heat transfer or mixing of miscible liqiiids to obtain uniformity, a speed of 3.50 or 420 r/min should be stipulated. For dispersion of dry particles in hquids or for rapid initial mixing of hquid reactants in a vessel, an 11.50- or 1750- r/min propeller should be used at a distance Df/4 above the vessel bottom. A second propeller can be added to the shaft at a depth below the hquid surface if the submergence of floating hquids or particiilate solids is other wise inadequate. Such propeller mixers are readily available up to 2.2 kW (3 hp) for off-center sloped-shaft mounting. 

High-Intensity Mixer. Mixers such as that shown in Fig. 18-44 combine a high shear zone with a fluidized vortex mixing action. Blades at the bottom of the vessel scoop the batch upward at peripheral speeds of about 40 m/s (130 ft/s). The high shear stress (to 20,000 s" ) and blade impact easily reduce agglomerates and aid intimate dispersion. Since the energy input is high [200 kW/m (about 8 hp/fE)h even powdery material is heated rapidly. 

These mixers are particularly suited for rapid mixing of powders and granules with liquids, for dissolving resins or solids in liquids, or for removal of volatiles from pastes under vacuum. Scale-up is usually on the basis of maintaining constant peripheral velocity of the impeller. 

In most mixers, the metal wall has a negligible thermal resistance. The paste film, however, usually has high resistance. It is important, therefore, while minimiziug the resistance of the heating or coohng medium, to move the paste up to and away from the smooth wall surface as steadily and rapidly as possible. This is best achieved by having the paste flow so as to follow a close-fitting scraper which wipes the film from the wall with each rotation. Typical overall heat-transfer coefficients are between 25 and 200 J/(m -s-K) [4 to 35 Btu/(h-fF-°F)j. 

Frictional heat develops rapidly in some units such as a Baubuiy mixer. The first temperature rise may be beneficial in softening the materials and accelerating chemical reactions. High temperatures detrimental to the product may easily be reached, however, and pro-... 

For most rapid mixing, in addition to diffusive (fine-scale) mixing, there should be a means by which large groups of particles are intermixed. This can be accomphshed by either the convective or the shear mechanism. A ribbon mixer illustrates the former, whereas a plain tumbler gives the latter. 

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