Mixers flow coefficient

Heat transfer in static mixers is intensified by turbulence causing inserts. For the Kenics mixer, the heat-transfer coefficient b is two to three times greater, whereas for Sulzer mixers it is five times greater, and for polymer appHcations it is 15 times greater than the coefficient for low viscosity flow in an open pipe. The heat-transfer coefficient is expressed in the form of Nusselt number Nu = hD /k as a function of system properties and flow conditions. 

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. 

This chapter reviews the various types of impellers, die flow patterns generated by diese agitators, correlation of die dimensionless parameters (i.e., Reynolds number, Froude number, and Power number), scale-up of mixers, heat transfer coefficients of jacketed agitated vessels, and die time required for heating or cooling diese vessels. 

Laboratory Extractors. Pilot-Scale Testing, and Scale-Up. Several laboratory units arc useful in analysis, process control, and process studies. The AKUFVE contactor incorporates a separate mixer and centrifugal separator. It is an efficient instrument for rapid and accurate measurement of partition coefficients, as well as for obtaining reaction kinetic data. Miniature mixer-settler assemblies set up as continuous, bench-scale, multistage, countercurrent, liquid-liquid contactors are particularly useful Tor the preliminary laboratory work associated with flow-sheet development and optimization because these give a known number of theoretical stages. 

The time resolution of the measurement also depends on several important mixer parameters the diffusive mixing time (determined by the jet width and the diffusion coefficient of the ions (Knight et al., 1998)), the beam size, uncertainty in the mixing time resulting from the hydrodynamic focusing process, and depth-dependent variations in the flow speed of the jet. Each of these issues has been addressed independently, but will be briefly reviewed here for completeness. 

Mass-Transfer Models Because the mass-transfer coefficient and interfacial area for mass transfer of solute are complex functions of fluid properties and the operational and geometric variables of a stirred-tank extractor or mixer, the approach to design normally involves scale-up of miniplant data. The mass-transfer coefficient and interfacial area are influenced by numerous factors that are difficult to precisely quantify. These include drop coalescence and breakage rates as well as complex flow patterns that exist within the vessel (a function of impeller type, vessel geometry, and power input). Nevertheless, it is instructive to review available mass-transfer coefficient and interfacial area models for the insights they can offer. 

Other important features in assessing industrial extractions are whether one or more extraction stages are to be used. This will depend in part on the partition coefficient of the system of interest, and on the flow pattern of the solvent and extracted phase (the raffinate) to be used, if more than one extraction stage is employed. Single-stage extraction requires a mixer to bring about intimate contact between the two phases, followed by a settler which allows phase separation and a means for independent removal of the two phases (Fig. 10.7). 

In Fig. 8.6, the variance coefficient of these mixers is plotted as a function of the I/D ratio for laminar flow and in Fig. 8.7 for turbulent flow. (This measure is invariant with respect to the sample size [101]). The numerical value behind the mixer type in these figures denotes the CfRe constant in expression (8.9). This representation shows, how the mixing length can be shortened by increasing the pressure drop Ap/y for a given degree of mixing. For details over the variance... 

Fig. 8.6 Variance coefficient

In [132] the second moment of the variance coefficient M = (a/c) was calculated for a single phase (air) turbulent flow in a pipe with a Tee mixer with the CFD code (Phoenix, version 1.6) for two distances x/D = 3 and 5 from the addition point and different values of momentum length/pipe diameter ud/ yD The K-e model utilized contained two additional laws of conservation for the mean kinetic energy K and the dissipation s. The parameter range extended over 0.026 model used reproduces well the existing experimental data and only a few adjustments are necessary to the already existing process relationship, whose constant is ca. 70% lower than the newly acquired one. 

In [211] the flow conditions in a Kenics mixer consisting of six mixing elements were mathematically investigated with a commercially available software packet, in which the path of the mixing elements was followed through the flow field. In this way the residence time distribution, the layer formation and the variance coefficient were determined as a function of the number of mixing elements. The results obtained agreed very well with the published experimental data. 

Co and Cj are the feed and desired effluent concentration, respectively, N is the number of stages, is the water phase flow rate, I) is the permeation rate constant, is the holdup ratio (volume ratio of emulsion to external phase in the mixer). While this approach is simple, there are nevertheless certain drawbacks to its use. The permeation coefficient D or equivalently the membrane film thickness varies depending on conditions. Hatton and... 

---