251Process Intensification.© 2012 Elsevier Ltd. All rights reserved.2013

Intensified Mixing 7CHAPTER

OBJECTIVES IN THIS CHAPTER

There are several types of mixer, some giving greater intensification than others. As with separations, it is often the active enhancement methods that can produce the greatest intensification of mixing, but passive methods currently dominate. Both types are described here and recent developments directed at intensify mixing are also introduced.

7.1 IntroductionMixing is one of the less exciting areas of process engineering, but its importance in a range of sectors cannot be underestimated. Without effective mixing, many food products would not be possible and chemical processes would, at best, con-sume much additional time and energy. Some years ago it was observed by John Middleton, then BHR Solutions mixing consultant (see Appendix 4), that attention to the mixing process typically yielded increases in plant productivity of 10–20%, in some cases reaching 40%.

The stirred tank, of course, uses the stirrer for mixing, but the performance, even with modern designs of paddles, does little to ensure highly uniform reactions – the main point of stirring (or mixing) the fluid(s) within the pot. The most common types of mixer are in-line units and rotor stator mixers. Other mixer types consid-ered below include variants based upon ejectors, fluidics, types using venturi aera-tion and ideas based upon spinning discs.

Mixing is frequently linked to reactions, and in earlier chapters, as well as in the discussion of PI applications, the reactor (or heat exchanger reactor) is designed, ideally, to have good mixing characteristics. The static tube insert introduced in Chapter 3 is an example where mixing can be used to intensify heat transfer and thus reduce the size of a heat exchanger for a given duty. Good mixing is neces-sary for uniform and efficient reactions. Often mixing and reaction take place in the same location or the mixer may be located upstream of the reactor. The terminol-ogy can be confusing, with some mixers actually being mixer reactors, and some being heat transfer enhancement devices. Mixers are essential components of some tubular reactors (see Section 2.10). The PDX unit – discussed in Chapter 10 (see Section 8.3.8) on the food industry – is commonly described as a reactor, but can be simply a mixer, albeit an intense one!

252 CHAPTER 7 Intensified Mixing

7.2 Inline mixersIn-line mixers are used for continuous mixing in a fluid stream – tending to operate with much smaller continuous inventories than batch or semi-batch stirred tanks. The uniformly distributed turbulent flow in in-line mixing units helps to ensure that the bubbles or drops generated within them tend to have a controllable size distribu-tion within a narrow range. Most mixers are located in pipe work or tubes.

7.2.1 Static mixersThere are many types of static mixer available commercially, all with similar operating principles. Most units have a number of mixing elements, the combina-tion of element configurations depending upon the mixer type. These are inserted into the pipework at the point where mixing is needed. Their main advantage is the lack of moving parts, keeping operating and capital costs relatively low. Cleaning is, of course, required where product or feedstock changes may be involved.

Static mixers are used to achieve good homogeneity between two or more streams, which can help in achieving good conditions upstream of, for example, a catalyst where reactions between the two are taking place. The intensification pro-cess using these mixers allows the components of the stream to be highly mixed within a distance of a few pipe diameters, with no external energy input apart from the modest additional pressure drop.

Andrew Green of BHR Group (Green et al., 2001), a leading UK laboratory on mixing and reactor technologies, compared the static mixer, where 100 s of W/kg in mixing energy could be delivered, with a stirred tank, where 1–2 W/kg was more typical. The up to 100-fold higher mass transfer rates in static mixers (compared to stirred tanks) is characterised by uniform energy dissipation, plug flow and good radial mixing.

Static mixers are perhaps the simplest and most versatile of process intensifica-tion equipment, with application in reactions where at least one phase is a liquid. They are tube inserts that use the pumping energy/pressure drop to induce mixing and can be roughly divided into three categories:

1. Turbulent flow mixers that rely on the vortices shed from tabs positioned on the walls of the device. They promote mixing in an axial direction and so approxi-mate well, to plug-flow devices.

2. Laminar flow mixers that physically redistribute, stretch and fold the fluid.3. Those used for both regimes.

Such mixers are marketed as heat transfer enhancement devices by several com-panies, including Cal-Gavin, whose product is called HiTran (see Chapter 3 and Appendix 3). The in-line static mixer has also proved to be effective in gas–liquid contacting (a phenomenon explained fully in Chapter 6). Al Taweel et al. (2005) used a specific design of static mixer to generate narrow-sized liquid–liquid disper-sions and succeeded in generating contact areas of the order of 2,200 m2/m3 in the

2537.2 Inline Mixers

region close to the woven wire-screen mixer elements. The performance of the sys-tem with regard to industrially relevant streams such as those containing surfactants had yet to be characterised.

Hessel and colleagues at IMM Mainz (2005) have investigated mixers at the micro-scale. At the top of Figure 7.1 is a slit type micro-mixer made in glass, with flow rate capabilities in the range 10–1,000 ml/h. The lower units are stainless steel mixers for pilot and production scale uses, with capabilities an order of magni-tude greater than the micro unit illustrated.

Flow distribution zone

Mixing channel forgeneration oflamellae

500 µm

FIGURE 7.1

Micro-mixers in glass and stainless steel (Hessel et al., 2005).

254 CHAPTER 7 Intensified Mixing

At the lower end of the mixing scale we are within the micro-fluidics regime as discussed in Chapters 3 and 10. The application of active enhancement methods to increase or improve mixing at these scales can, of course, be fruitful. These may include ultrasound and electrokinetic forces. As with any flows at the small scale, particularly if multiple phases are involved, the possible adverse effect of thermo-phoresis and electrophoresis may work against mixing.

7.2.1.1 Mixing in the context of micro-fluidicsIt is interesting to note that mixing is moving into the micro-fluidics regime. Naturally enough, such static mixers are classed as passive, as opposed to active, and the problem with mixing at such a small-scale is the absence of turbulence. Thus the mixing relies upon diffusion, which can of course be slow if the diffusivity of the fluid being mixed is low. Recent research in Germany and The Netherlands on passive chaotic micro-mixers (Sarkar et al., 2011), selecting specifically the staggered herringbone micro-mixer (SHM) as a case to examine – a type that has proved to be successful at this scale. In this type of mixer, ‘herringbones’ are placed inside micro-channels. In a typical channel size examined by the research team – 96 microns × 192 microns × 1536 microns in length – the lattice Boltzmann method was used to describe the fluid flow, which is likened to chaotic advection. This is the enhancement process brought about by the herringbone structures. It was found that the mixing length decreased by over 30% using the herringbone structure to enhance the mixing above that of simple diffusion.

7.2.1.2 Example of a mixer heat exchangerIn many conventional types of heat exchangers, the cooling or heating of laminar-flowing, viscous fluids can be difficult. This is due to the formation of boundary layers which can sharply inhibit thermal exchange. For applications of the above kind, there has been a notable increase in the use of static mixer heat exchangers which provide continuous radial mixing, and prevent, or at least strongly reduce, the build-up of boundary layers. A further extremely positive feature is the narrow residence time spectrum, which can be a decisive factor in the processing of tem-perature-sensitive viscous media, e.g. polymers.

KOCH mixer heat exchangers are available either in monotube or multitube ver-sions. These static mixers increase the heat transfer coefficient by a factor of 6 to 8 compared with empty pipe configurations. They are used not only for rapid heating up applications, (for example, the heating of polymer solutions during manufacture of low density polyethylene and similar types of plastics) but also for the efficient cooling of viscous media.

The KOCH mixer reactor type SMR is a special type of mixer heat exchanger. Its mixing elements are formed of a series of hollow tubes through which the heat transfer medium flows. The arrangement of the tubes in the apparatus is such that they induce strong radial mixing. The SMR is used mainly either as a polymerisa-tion reactor or a cooler. In the SMR, the fluid being treated under plug flow condi-tions with a narrow residence time spectrum is continuously subjected to mixing

255

across the entire cross-section of the flow, and at the same time heat is either being introduced or removed. Besides purely cooling tasks, the SMR reactor is employed everywhere where a reaction has to be carried out under defined time/temperature conditions.

As with much of the intensification work associated with mixing, the end use is a chemical reactor. Work in France on the intensification of heat transfer and mix-ing using heat exchangers incorporating vortex generators (Ferrouillat et al., 2006) revealed that such devices – which could also be used as intensified reactors – were very efficient in terms of heat transfer enhancement and macro-mixing, but micro-mixing was less effective. (Vortex generators are static enhancement devices and are used in plate-fin heat exchangers, amongst others, to improve mixing and heat transfer.) Figure 7.2 shows the streamlines in the wake of two vortex generators. Re is 4600.

7.2.2 EjectorsEjectors, for gas–liquid contacting, consist of four main sections. A rotating distrib-utor takes the pumped liquid and orientates and stabilises its flow before it passes through a nozzle that provides a high velocity jet of fluid to create suction in the gas chamber and entrain gas into the ejector. In the following mixing tube the liquid jet attaches itself to the tube wall resulting in a rapid dissipation of kinetic energy. This creates an intensive mixing zone or shock where the high turbulence produces a fine dispersion of bubbles with a large interfacial area for mass transfer. High-energy dissipation associated with this fast process gives excellent mixing, which is carried out for a period downstream in the diffuser.

Z

FIGURE 7.2

Streamlines downstream of a vortex generator.

7.2 Inline Mixers

256 CHAPTER 7 Intensified Mixing

Static mixers and gas–liquid ejectors are plug-flow devices and this can improve selectivity for consecutive reaction schemes. Due to their very high-energy dissipation rates they can increase the mass-transfer coefficient by an order of magnitude compared to stirred tanks. The large levels of turbulent energy dissipation produced in these high intensity mixers acts to reduce the dispersion size, dramatically increasing the interfacial area – typical gas bubble sizes range from 0.5 to 2.0 mm, compared to 1.0 to 5.0 mm in stirred tanks and bubble columns. These factors combined give such devices the very major benefit of enhanced mass transfer rates of typically 10–100 times those of an STR. A significant benefit of high-intensity in-line mixers is that they have no moving parts, therefore sealing, high pressure and hazardous materials are less of a problem.

Wu et al., in Australia (2007), used a form of ejector nozzle developed at CSIRO to intensify the mixing of oxygen into slurry, and this has been successfully applied in a leaching tank at a gold refinery.

A form of injector rather than ejector is the Pursuit Dynamics PDX mixer/reactor. This is described in Chapter 10 (Section 10.3.7) and is successfully used in areas such as mashing systems in breweries. Based on PDX reactor technology, the PDX mashing system uses direct steam injection to heat and mix the mash. According to the company The efficient and effective transfer of heat eliminates for-mation of burn-on in the mash tun and mixes the mash to a uniform homogeneity unrivalled by current conventional mash agitation technology. The PDX mashing system converts standard boiler steam to food-grade via the steam conditioning unit and the production of the beer by direct steam injection conforms to the German Purity Law (Deutsches Reinheitsgebot).

7.2.3 Rotor stator mixersA rotor stator mixer has, as its name implies, a rotor component that moves relative to a fixed stator. The gap of a few mm between the two allows very high shear rates to be obtained, implying high energy dissipation. The rotor stator mixer is able to perform reactions in very short times, sometimes less than 1 second. Its high energy dissipation rates make it suitable for disintegration, homogenisation, solubilisation, emulsification, blending and dispersion duties. Units can be applied in either batch processes for recirculation or continuous reaction processes where short residence times are needed. They can help reduce mixing times for liquids up to 1,000,000 cP. In liquid–liquid applications they can reduce droplet sizes below 1 micrometer – significantly increasing mass transfer rates.

Recent research has been directed at studying the micro-mixing characteristics of these mixers, (Jasinska et al., 2012), where tests were carried out on the Silverson 150/250 rotor stator mixer, a double-screen type. Encouraging results were obtained using CFD modelling, giving qualitative agreement with experimental data. This allows one to make a proper choice of the concentrations of the reactant and flows. The work was in collaboration with Unilever Research and Development.

The Taylor-Couette reactor (see Chapter 5) is also a rotor stator mixer, but is dis-cussed separately. Units such as the Marbond HEX-reactor demonstrate mixing plus reactions plus heat transfer in one unit, and these are also discussed in Chapter 5.

257References

7.3 Mixing on a spinning discRecent experiments on rotating discs have shown that the apparatus can be used to produce crystals of a specific size where mixing is a critical stage in the operation. The intensity of mixing plays a fundamental role in determining the crystal size dis-tribution of the final product. In order to obtain very small crystals in a narrow size range, a homogeneous nucleation is required, that is, the occurrence of a very high nucleation rate in the absence of crystals. Further data on spinning discs as crys-tallisers are given in Chapter 6 and 8. For measurements of the behaviour of thin liquid films on a rotating disc, see Ghiasy et al. (2012), which deals primarily with the heat transfer aspects. Although these, of course, are critical to mixing in that the viscosity of the fluid on the disc is a function of temperature.

7.4 Induction-heated mixerThe mixer used, for example, in a stirred tank, can be given a boost in performance by generating heat in the blades of the mixer itself. Conventionally, in a stirred ves-sel, the heat input is via a jacket, with conduction of heat through the vessel wall and some forced convection thereafter in the ‘mixed’ product. Using electrical induction heating via a coil, the paddles themselves can be heated, allowing local reductions in viscosity.

7.5 SummaryThe chapter on mixers is comparatively short – not because mixers are unimpor-tant, but because mixing is such an integral part of other intensive processes (heat exchange, reactions and crystallisation/precipitation for example that the topic is addressed continuously through other equipment and application chapters. The point is well made by Wu et al. (2007), who stress in their review that PI needs to be implemented via increased mixing, as well as heat and mass transfer – hence the success in this area of the spinning disc and the oscillatory baffle reactors.

ReferencesAl Taweel, A.M., Yan, J., Azizi, F., Odebra, D., Gomaa, H.G., 2005. Using in-line static mix-

ers to intensify gas–liquid mass transfer processes. Chem. Eng. Sci. 60, 6378–6390.de Weerd, K., 2001. Intensification of gas-liquid reactions. NPT Procestechnologie (2), 38–41.

March–April.Ferrouillat, S., Tochon, P., Garnier, C., Peeerhossaini, H., 2006. Intensification of heat trans-

fer and mixing in multifunctional heat exchangers by artificially generated streamwise vorticity. Appl. Therm. Eng. 26, 1820–1829.

Ghiasy, D., Boodhoo, K.V.K., Tham, M.T., 2012. Thermographic analysis of thin liquid films on a rotating disc: approach and challenges. Appl. Therm. Eng. 44, 39–49.

258 CHAPTER 7 Intensified Mixing

Green, A., Johnson, B., Westall, S., Bunegar, M. Symonds, K., 2001. Combined chemical reactor/heat exchangers: validation and application in industrial processes. Proceedings of the Fourth International Conference on Process Intensification in the Chemical Industry, 215–225, BHR Group, Cranfield.

Hessel, V., Lowe, H., Schoenfeld, F., 2005. Micromixers: a review on passive and active mix-ing principles. Chem. Eng. Sci. 60 (8–9), 2479–2501.

Jasinska, M., Baldyga, J., Cooke, M., Kowalski, A., 2012. Application of test reactions to study micromixing in the rotor-stator mixer. Appl. Therm. Eng. <http://dx.doi.org/10.1016/j.applthermaleng.2012.06.036> (in press).

Sarkar, A., Narvaez, A., Hating, J., 2011. Numerical optimisation of passive chaotic micro-mixers. Proceedings of the Third Micro and Nano Flows Conference, Thessaloniki, Greece, 22–24 August, 2011.

Wu, J., Graham, L.J., Noui-Mehidi, N., 2007. Intensification of mixing. J. Chem. Eng. Japan 40 (11), 890–895.