Thierry Lemenand a, Dominique Della Valle , Hassan Peerhossaini Show more add to MendeleyShareCite
https://doi.org/10.1016/j.cherd.2013.07.013Get rights and content
Highlights
- This review reports on the static mixing process in industrial applications.
- The first part of the study surveys the available commercial models.
- The second part addresses the fundamentals and mixing measurement techniques. Three mixing scales, macro-, meso-, and micromixing are defined.
Abstract
Static mixers and multifunctional heat exchangers/reactors (MHE/R) are qualified as efficient receptacles for processes including physical or chemical transformations accompanied by heat transfer due to their high productivity and reduced energy expenditures. The present work reviews recent conceptual and technological innovations in passive static mixers and continuous in-line reactors. Current industrial applications are discussed from a process intensification perspective, focusing on mixing and mass transfer performance.
Typical experimental techniques employed to characterize and quantify the mixing process are explored. The work is complemented by a review of mixing fundamentals, knowledge of which allows the development of theoretical models crucial for the analysis of experimental data like the chemical probe mixing assessment method. Considering the development of continuous flow equipment in numerous processes, advances in this field will certainly be of increasing interest to the scientific and industrial communities.
Introduction
Static mixers and multifunctional heat exchangers/reactors (MHE/R) are being increasingly incorporated in process industries for their mixing and heat transfer capabilities. Process intensification is a chemical engineering purpose that consists of seeking processes with higher productivity, safer operating conditions, reduced waste production, and lower energy consumption. New applications are being explored and new online exchanger/reactor designs are being developed offering several advantages compared to batch processing and mechanically stirred vessels.
The small space requirement, low equipment operation and maintenance costs, sharp residence time distribution, improved selectivity through intensified mixing and isothermal operation, byproduct reduction, and enhanced safety are the main features that have promoted the use of these devices in chemical, pharmaceutical, food processing, polymer synthesis, pulp and paper, paint and resin, water treatment, and petrochemical industries (Anxionnaz et al., 2008, Bayat et al., 2012, Ferrouillat et al., 2006a, Ferrouillat et al., 2006b, Shi et al., 2011, Thakur et al., 2003).
Characterizing mixing in industrial processes is an important issue for various economic and environmental considerations (Anxionnaz et al., 2008, Lobry et al., 2011, Stankiewicz and Moulijn, 2000) since it governs byproduct effluents and consequently process efficiency. In addition, due to the wide range of applications of mixers and micro-structured mixers, such as homogenization, chemical reaction, dispersion and emulsification, and heating or cooling processes, the mixing efficiency in these devices is a decisive criterion for overall process performance. Indeed, mixing affects various process parameters including heat and mass transfer rates, process operating time, cost and safety, as well as product quality.
Mixing Mechanism
To describe the mixing mechanism, Fournier et al. (1996a) and Baldyga and Bourne (1999) introduced macromixing, mesomixing, and micromixing as three parallel mixing stages of different scales. Macromixing concerns homogeneity at the reactor scale and is generally described by the residence-time-distribution (RTD) method (Castelain et al., 1997, Habchi et al., 2009a, Villermaux, 1986) as a signature of velocity field uniformity. At the intermediate scale, mesomixing reflects the coarse-scale turbulent exchange between the fresh feed and its surroundings governed by turbulent fluctuations, so it is characterized by the RMS of velocity fluctuations or the turbulent kinetic energy (TKE) (Habchi et al., 2010), and the length scale of these fluctuations.
When the fluid aggregates are reduced in size by the turbulent cascade to the Kolmogorov scale, micromixing starts by engulfment in the smallest vortices; it is then achieved in the viscous-convective subrange by laminar stretching and folding, associated with thickness reduction by striation, up to molecular diffusion at sub-Batchelor scales that rapidly dissipates the concentration variances (Batchelor, 1953).
The turbulence micro-scales are directly related to the turbulence energy dissipation rate ɛ (Hinze, 1955, Lemenand et al., 2005, Streiff et al., 1997). In this sense, the Kolmogorov scale is a key parameter for the selectivity of chemical reactions in the turbulent regime, since the limiting mechanism of the whole mixing process occurs at the smaller turbulence scale, hence governing species contact at the molecular scale, (Baldyga and Bourne, 1988, Baldyga and Bourne, 1989, Baldyga and Bourne, 1999, Baldyga and Pohorecki, 1995, Falk and Commenge, 2010, Guichardon and Falk, 2000, Komori et al., 1991, Villermaux, 1986).
Qualitative Investigation
Qualitative investigation of the mixing process using optical techniques can give valuable information on the flow hydrodynamics. However, understanding and quantifying the mixing mechanism is essential in designing industrial processes involving fast reactions that can present characteristic reaction times smaller than the characteristic mixing time. This fundamental property of the turbulent field (Wallace, 2009) can be determined by classical velocimetry methods such as laser Doppler anemometry, particle image velocimetry, or hot-wire anemometry, all of which give access, in three-dimensional space, to the contributions of the turbulent energy dissipation rate.
Alternative methods to characterize mixing based on observations of a chemical system have been recently developed, especially by Baldyga and Bourne (1990), Bourne et al., 1992a, Bourne et al., 1992b, and Fournier et al. (1996a): Villermaux–Dushman reactions or the iodide/iodate method (Baldyga and Bourne, 1989, Durandal et al., 2006, Dushman, 1904, Guichardon and Falk, 2000, Guichardon et al., 2000, Mohand Kaci, 2007, Oates and Harvey, 2006, Wheat and Posner, 2009).
Techniques
These techniques, called “chemical probe methods”, are based on the competition between mixing and well-known chemical kinetics by the straightforward observation of reaction selectivity through monitoring the secondary product concentrations. Under optimal conditions, the slowest reaction time is equal to the mixing time. From the knowledge of the mechanism, kinetics, and stoichiometry of the chemical reaction, the local turbulent energy dissipation rate can readily be derived from the measured selectivity by using phenomenological mixing models (Bourne et al., 1992a, Fournier et al., 1996a, Guichardon and Falk, 2000).
The following sections present an overview of static mixers and multifunctional heat exchangers/reactors, their applications, and mixing capabilities. Then mixing fundamentals and experimental techniques developed for its assessment are reviewed. The iodide/iodate method based on the concept presented above is then detailed and the adaptive procedure and mixing models are discussed. The final section includes concluding remarks on static mixers, their present state, and future opportunities, with comments on the mixing characterization techniques presented.
Distributive mixing in static mixers
A static mixer can be a hollow tube or channel with a specific geometrical construction that influences the flow structure in a manner to promote secondary transverse flows that enhance mass and heat transfer in the cross-section. Another type of static mixer concept is the insert-type configuration in which the typical design is a series of identical, stationary inserts, called elements, that can be installed in pipes, channels, or ducts. The purpose of the elements is to redistribute the
Mixing fundamentals
Mixing is a central issue in process engineering and many industrial fields. How reagents are mixed, whether at the reactor scale, affected by the flow structures, or at molecular scales, influences the selectivity and hence the productivity of reactions.
Methods to characterize mixing in continuous flows
Understanding and describing the degree of mixing is a crucial issue. In addition, process control and optimization sometimes necessitate parameterization, measurement, and quantification of mixing. This urge has led to the development of several qualitative and quantitative measurement techniques that are sometimes case-specific; each of which is adapted to a certain type of flow; some of them have been the subject of extensive research, and the list of references cited hereafter is not exhaustive.
Iodide/iodate chemical probe method
This chemical probe method, developed by Fournier et al., 1996a, and Fournier et al., 1996b to study partial segregation in stirred tanks, implements a system of parallel competing reactions producing iodine. The coupling of the borate neutralization and the Dushman reaction in this system allows the measurement of mixing efficiency in reactors by monitoring the amount of iodine produced. As mentioned before, this technique has been extensively used in different types of reactors. A novel adaptive
Conclusions
This review reports on the static mixing process in industrial applications—the mechanisms, the devices, the characterization methodologies—in an attempt to summarize the basic information needed to handle a mixing application on an industrial scale.
The first part of the study surveys the available commercial models of static mixers and heat exchangers/reactors and their fields of application. All have the common characteristics of safe operating conditions, compactness, good reaction control,
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