Static mixer, simulation

Wetox uses a single-reactor vessel that is baffled to simulate multiple stages. The design allows for higher destmction efficiency at lower power input and reduced temperature. Its commercial use has been limited to one faciHty in Canada for treatment of a complex industrial waste stream. Kenox Corp. (North York, Ontario, Canada) has developed a wet oxidation reactor design (28). The system operates at 4.1—4.7 MPa (600 to 680 psi) with air, using a static mixer to achieve good dispersion of Hquid and air bubbles. 

Lang. E.. Drtina, P.. Streiff, F. and Fleischu, M. Chem. Eng. Sci. 38 (1995) 2239. Numerical simulation of the fluid flow and mixing process in a static mixer. 

Another comparison is due to Van Wageningen et al. (2004) who performed a similar study (in terms of the numerical scheme used) on unsteady laminar flow in a Kenics static mixer. They found that the LB code was 500-600 times faster than FLUENT in terms of simulation time per grid node per time step and that FLUENT used about 5 times more memory than LB. 

The number of cross bars, parallel bars, and elements will affect the performance of an SMX static mixer. Singh at al. [62] performed simulations of the two-component flow through an SMX mixer. The number of cross bars and the number of elements were varied, and the results are shown in Fig. 8.35. Contrary to intuition, which would tend toward the most bars to get optimum mixing, the optimum was shown to be six cross bars in the channel. Moreover, after only four elements the number of striations and thus the interfacial surface area was extremely high such that the black and white patterns are becoming difficult to see. 

Tubular reactors are also used to carry out some multiphase reactions. Wamecke et al. (1999) reported use of a computational flow model to simulate an industrial tubular reactor carrying out a gas-liquid reaction (propylene oxide manufacturing process). In this process, liquid is a dispersed phase and gas is a continuous phase. The two-fluid model discussed earlier may be used to carry out simulations of gas-liquid flow through a tubular reactor. Warnecke et al. (1999) applied such a model to evaluate the influence of bends etc. on flow distribution and reactor performance. The model may be used to evolve better reactor configurations. In many tubular reactors, static mixers are employed to enhance mixing and other transport processes. Computational flow models can also make significant contributions to understanding the role of static mixers and for their optimization. Visser et al. (1999) reported CFD... 

Visser, J.E., Rozendal, P.F., Hoogstraten, H.W. and Beenackers, A.A.C.M. (1999), Three-dimensional numerical simulation of flow and heat transfer in the Sulzer SMX static mixer, Chem. Eng. Sci., 54, 2491-2500. 

Craig [312] reports on an investigation of the performance of a large diameter static mixer used as a continuous reactor for styrene polymerization. It was shown the mixer behaved adiabaticaliy. This was confirmed by computer simulation using models that had been verified experimentally. Better results were obtained with a static mixer that carries a heat transfer fluid supplied via manifold connections from external headers. A comparison of different static mixers with respect to thermal homogenization, pressure drop, and mixing efficiency was published by Mueller [313]. 

Fig. 12. Simulation of the mixing pattern for two identical shear-thinning inelastic liquids in a static mixer after passing six blades oriented at 140°. Reproduced from Ref. 44. Similar images for Newtonian liquids may be found in Ref. 45.

This section applies conservation equations, explained in Section 6.2, to three real-world physical systems. It shows a step-by-step computational modeling approach of three conventional chemical multiphysics systems. The first case study is a 3D computational model of a laminar static mixer with twisted blades, the second case study is a 3D computational model of a porous reactor with injection needle, and the last case study is a 3D model of an isothermal heat exchanger. All three case studies are computationally modeled and simulated with one of the commercial software tools COMSOL Multiphysics , the CFD and multiphysics modeling software tool developed by COMSOL AB from Sweden. 

Figure 14.2. Simulation of the mixing pattern for two identical Carreau-Yasuda fluids with n = 0.1 after passing through a static mixer with six blades with a twist angle of 180°. Reprinted with permission from Galaktionov et al., Int. Polym. Proc., XVIII, 138 (2003).

Clearly, the helicity has a value of zero in 2D simulations. In 3D simulations, it gives an indication of how well the local rotation of a fluid element is aligned with the velocity of the element. It is useful for illustrating longitudinal vortices, or spiral motion, as is often found in vortex cores. In Figure 5-21, isosurfaces of helicity are used to depict the longitudinal vortices generated in the Kenics static mixer described in Section 5-7.10. 

Data for a Kenics twisted-ribbon static mixer geometry obtained by Baldyga et al. (1997) is shown in Figure 13-31. In this case only final yields for a complex reaction were measured. The static mixer used by Baldyga et al. (1997) was 0.04 m in diameter. The method developed above was used to simulate the reactions in the static mixer. Even though it is not true in individual elements of the static mixer, plug flow overall was assumed. Also, in contrast with the... 

The mixing and flow patterns of gravitational dry particulate flows in continuous mixer tubes with helical, Kenics-type [1] static mixer elements have been simulated by the distinct element method (DEM) under steady state conditions. In the particular system the subsequent mixer elements were twisted in opposite direction a mixing element twisted clockwise is followed by an element twisted counter-clockwise and so on. A state diagram that gives a general relationship between the mass flow rate and the solids volume fraction in the mixer tube was determined for various construction parameters. 

The flow and mixing characteristics of dry particulate flows were studied in continuous mixer tubes with helical static mixer elements. In accordance with the visual observations and the actual experimental results, DEM simulation confirmed the presence of three different flow regimes. 

Fig. 20.20 Mixing of two model liquids to simulate the mixing of a separate feed of initiator and monomer solution within the nozzle measured by an optical measurement device two results presenting the mixing with the use of a static mixer and without...

Rahmani RK, Keith TG, Ayasoufi A, 2006. Numerical simulation and mixing study of pseudoplastic fluids in an industrial helical static mixer, J. Fluids Eng. 128 467. 

M 31] [P 28] The time evolution of the flow patterns in the cross-shaped micro mixer with two static mixing elements was monitored by simulation at time intervals of 50,150, 500 ps and 1 ms after application of pressure [71]. In addition to seeing the evolution of the swirling patterns, it was concluded from this analysis that at 500 ps a nearly homogeneous distribution of the mass fractions is given and at 1 ms this is indeed completed. Hence the theoretical mixing time of the mixer may be below 1 ms. 

M 31] [P 28] The flow patterns simulated by the ConventorWare and FLUENT 5 software concerning the flow in the cross-shaped micro mixer with two static mixing elements the same hence the predictability of the ConventorWare software was demonstrated [71]. 

Table 1.5 Measures for mixing efficiency calculated from mass contour plots yielded by CFD simulation - benchmarking cross-shaped mixers with and without static mixing elements (SME) [71].

Qualitative, numerical simulations were performed with the commercial tool FLUENT-5 to evaluate mixing efficiency [2], The simulations were oriented on concepts employed for conventional 3-D static mixing. The micro-mixer geometries were laid out using the GAMBIT predecessor as well as the meshing of surfaces and volumes and the specification of boundary conditions. Entrance and exit sections were also simulated. 

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