Mixing Kenics mixers

Static mixing of immiscible Hquids can provide exceUent enhancement of the interphase area for increasing mass-transfer rate. The drop size distribution is relatively narrow compared to agitated tanks. Three forces are known to influence the formation of drops in a static mixer shear stress, surface tension, and viscous stress in the dispersed phase. Dimensional analysis shows that the drop size of the dispersed phase is controUed by the Weber number. The average drop size, in a Kenics mixer is a function of Weber number We = df /a, and the ratio of dispersed to continuous-phase viscosities (Eig. 32). 

The Kenics mixer has been shown to provide thorough radial mixing. This results in a reduction in radial gradients in velocity, composition, and temperature. Because the unit possesses nearly plug flow characteristics, both temperature and product quality controls are achieved. 

To improve the mixing quality in the tubular reactor, Kenics type in-line static mixer reactor was employed. The in-line static mixers were designed to mix two or more fluids efficiently since an improved treinsport process such as flow division, radial eddying, flow constriction, and shear reversal eliminated the gradients in concentration, velocity and temperature. However, only 70 % conversion was achieved with one Kenics mixer unit. As shown in Table 2, five mixer units were required to achieve the maximum conversion. 

Avalosse, Th., and Crochet, M. J., Finite element simulation of mixing 2. Three-dimensional flow through a Kenics mixer. AlChE J. 43, 588-597 (1997b). 

Figure 8.31 Modern Kenics mixers with alternating twist directions a) cutaway view, and b) schematic with mixing striations (courtesy of Kevin G. Walsh of Chemineer, Incorporated)...

Figure 8.32 Kenics mixer and mixing quality as a function of the helix angle [58] (courtesy of Han Meijer of Technische Universiteit Eindhoven)...

The Kenics mixer, Figure 10.14(a), for example, consists of a succession of helical elements twisted alternately in opposite directions. In laminar flow for instance, the flow is split in two at each element so that after n elements the number of striations becomes 2". The effect of this geometrical progression is illustrated in Figure 10.14(b) and points out how effective the mixing becomes after only a few elements. The Reynolds number in a corresponding empty pipe is the major discriminant for the size of mixer, one manufacturer s recommendations being... 

For transitional flow, precise correlations are not available but for both SMV and Chemineer Kenics mixers, extra elements are required to achieve a certain degree of mixing. The SMV does not achieve significant mixing downstream of the mixer as in turbulent flow, so elements are not spaced out. The HEV is not recommended for transitional flow. 

Despite the wide use of the striation thickness concept in the early commercial literature, the CoV is now the most widely used mixing index. The following correlations are valid for viscosity ratios 0.01 < nBlnA < 100, feeding into the center of the pipe and with CoV measured 2dp downstream for Sulzer mixers, 3dp downstream for Kenics mixer. 

VL = 1 Wj), partial inversion. In the first case, N = 0 corresponds to a CSTR and N to a plug-flow reactor. It is shown that the best chemical conversion is obtained with complete flow inversion. The RTD in a Kenics mixer comprising 8 elements could be represented by this model with N = 3 and complete mixing. Static mixers could be used as chemical reactors for specific applications (reactants having large viscosity differences, polymerizations) but the published data are still very scarce and additional information is required for assessing these possibilities. 

A slit-type interdigital micromixer was used for fast mixing [30] and compared to a tubular reactor and five Kenics mixers connected in series. Details on further components of the micromixer rig were not given, but most likely a capillary reactor was added for efficient heat exchange. 

There is a need for improving the understanding of complex fluid and suspension flow behavior in mixing equipment. The concentration profiles obtained by MR imaging were used to monitor mixing with respect to axial position in a Kenics mixer, to which the two viscosity-matched test fluids were delivered at approximately the same volumetric flow rate. ... 

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. 

However, as laminar mixing is usually associated with fluids of high viscosity, it must also be expected that non-Newtonian fluid properties will be encountered in a significant number of cases. Wilkinson and Cliff report difficulties with the mixing of viscoelastic polyacrylamide in water solutions and Ottino has attempted to calculate the effect of non-Newtonian properties on static mixer performance. Studies have been made of the residence time distributions of Newtonian and non-Newtonian fluids in Kenics mixers -. ... 

The enhanced heat transfer coefncient is due to the increase in radial mixing compared with the empty tube and possibly a contribution due to the fin effect associated with the presence of the metal element— particularly if there is good contact between element and tube wall. It is difflcult to relate the heat transfer characteristics of static mixers to empty pipes but low pressure drop mixers (e.g. Kenics, SM X) give increases in heat transfer coefficient of the order of 300% for an increase of pressure drop of the order of 700%. Some simple correlations have been presented for the Kenics mixer ... 

In all of the work published to date the major concern has been with the description and evaluation of the blending characteristics of static mixers. In early discussions of mixing rate the concept of the number of sub-divisions of flow produced per static mixer element was used as an illustration of mixing rate. This was frequently interpreted as the hypothetical number of striations (r j) in a mixture pattern produced from equal volumes of segregated black and white materials. A relationship was proposed for the Kenics mixer. 

Four commercial mixers with quite different vane arrangements were included by Smith [2] with a summary of the Kenics mixer (Figure 11.1] regarded as the best known of this type for plastics extrusion. The example shown in Figure 11.2 is a plastic version used with a two component dispenser for mixing epoxy resin and hardener. 

The flow twisting arrangement of the Ross ISG mixer is completely different in that it uses two sets of crossing tubular ducts within each element to provide the redistribution, but this mechanism has a relatively high pressure drop compared with other mixers [1 ]. The Ross ISG and Kenics mixers and their mixing mechanisms have been described in some detail by Tadmor and Gogos [3]. 

Flow patterns in the Kenics static mixer are too complicated to determine the residence time distribntion analytically. Instead, experimental measurements were fit to a simple model. The model nsed for the Kenics mixer in Table 1-3 assumes regions of undisturbed laminar flow separated by planes of complete radial mixing, there being one mixing plane for every four Kenics elements. Simpler models are useful for systems in turbulent flow. 

The real power of using stretching computations to characterize chaotic flows lies in the fact that stretching is the link between the macro- and micromixing intensities in laminar mixing flows. In this section we describe the method for computing striation thickness distribution in our 3D example, the Kenics mixer. 

Figure 3-24 The distribution of intermaterial area density (p) in the standard Kenics mixer at four axial cross-sections at Re = 10(10). The probability density function of p/(p) reaches an invariant shape as the mixing process evolves.

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