Chemical Engineering and Processing – Process Intensification

Volume 193, November 2023, 109559

Performance comparison between novel and commercial static mixers under turbulent conditions

Ranim Chakleh, Fouad Azizi

https://doi.org/10.1016/j.cep.2023.109559 Get rights and content

Highlights

Four different static mixers were compared under turbulent conditions.

Two novels and two commercial geometries were selected.

Velocity profiles, pressure, dispersive, and distributive mixing were analyzed.

The novel mixers reduced power consumption by a factor ranging between 2 and 10.

Abstract

This study compares the hydrodynamics and mixing performance of four different static mixers under turbulent flow conditions. Two novel static mixer geometries were compared numerically over a pipe Reynolds number ranging between 5000 and 30,000. The new geometries are based on the use of specially located divergent inserts of trapezoidal and rectangular shapes downstream of a woven mesh to improve its distributive mixing.

The performance of these mixers was compared based on the velocity fields, and pressure drop in addition to quantifying both the dispersive and distributive mixing efficiencies. The former was accomplished by employing extensional efficiency while the latter was based on the intensity of segregation (i.e., coefficient of variation). The results of this study show that the combination of screen-type static mixers with divergent inserts offers a good alternative for commercial designs where an improved distributive and dispersive behavior was obtained at reduced energy costs.

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Introduction

Mixing is an omnipresent, complex, operation in most process industries. It aims at reducing concentration non-uniformities and/or helping enhance chemical reactions, heat, and mass transfer operations. Many mixer designs exist to fulfill the diverse needs of the industry. Hence, the judicious selection of mixers is key to the success of these operations because inefficient mixing would have major consequences on the safety, efficiency, and feasibility of any process [1], [2], [3].

With the emergence of the concept of process intensification (PI), and the drive to integrate different functionalities in the same unit [4], the interest in static mixers gained momentum due to their inherent safety, energy efficiency, durability, cleaner operation, and more compact footprint [5]. Static mixers are motionless equipment of varying designs aiming to foster the chaotic behavior of the flow and enhance the distributive and dispersive mixing mechanisms [6,7]. Accordingly, near-plug flow conditions are achievable, and narrower residence time distributions can be obtained [8]. Thereby providing high mixing intensities in short residence times and at lower energy consumption rates.

Static mixers were found to outperform stirred tanks (and other conventional reactors) when dealing with fast competitive reactions and/or time-sensitive materials [9,10]. They provide a quasi-uniform energy dissipation rate and can be used in batch or continuous processes and under varying conditions [2]. This makes them suitable for different applications such as mixing of miscible fluids and reacting systems [11], [12], [13], [14], interface generation in multiphase systems [15,16], as well as heat transfer and thermal homogenization [13,17]. Moreover, they can handle fluids of different rheological properties [18] and they are appropriate for both laminar and turbulent flow regimes. Given that various geometries are available on the market, static mixers are usually classified based on their designs. Following the recent classification of Valdés et al. [3], these range from open designs with helices, open designs with blades or vortex generators, corrugated plates, multilayer designs, and closed designs with channels or holes, to screen-type designs.

The latter design employs woven wire screens, which have been recently investigated as static mixers in reactive systems and multiphase applications [19], [20], [21],15]. In it, grids are used as means for controlling turbulent flows whereby turbulence is produced or reduced and large-scale pressure and/or velocity non-uniformities are eliminated [22].

In addition, screens have also been employed as aerodynamic noise reducers, thickeners or coalescers, and Stirling engine regenerators among many other purposes [23]. Screen-type static mixers (STSM) thereby consist of a series of wire matrices whose function is to repeatedly create a tunable, radially uniform, highly turbulent, and plug flow field in pipes operating under high velocities [19,[24], [25], [26], [27]].

Various investigations focused on the effect of the geometric characteristics of screens on the mixer/reactor hydrodynamics and mixing efficiency under turbulent regimes [24,25,28]. Recently, Abou-Hweij and Azizi [29], [30], [31], fully characterized the three-dimensional bounded flow in STSM by investigating the hydrodynamics and mixing efficiency of these mixers under a wide range of operating conditions ranging between laminar and turbulent regimes.

These studies showed that STSMs possess high dispersive mixing capabilities, but their distributive mixing performance is weak. Based on this observation, Abou-Hweij [32] proposed the addition of divergent inserts downstream of the screens to improve distributive behavior by continuously redistributing the flow and creating complex patterns.

A search of the open literature shows that only a few studies compared the performance of various static mixers (excluding split-and-recombine, micro-, and milli-reactors), and most of them were performed under laminar flow conditions. For example, Rauline et al. [33] compared numerically the performance of six static mixers (i.e. by evaluating the extensional efficiency, stretching, mean shear rate, intensity of segregation, and pressure drop. The extensional efficiency showed that the flow is elongational at the edges and distributive within the mixer. Thus, the authors deduced that the mixing quality be improved if a spacing was left between the elements. Regner et al. [6] characterized numerically the flow by evaluating the pressure drop, the helicity, and the rate of striation thickness.

They showed that the mixing efficiency is better at low flow rates than at high flow rates. Meijer et al. [34] investigated the performance of industrially relevant static mixers in the Stokes regime for highly viscous flows. Using the mapping method and the flux-weighted area-averaged intensity of segregation they conducted their analysis using compactness and energy efficiency as two independent criteria. They concluded that open mixers exhibit high energy efficiency with minimal pressure drop, although at the cost of increased length.

In compactness, the authors found that the (n) mixers outperformed all other designs while energy efficiency was found to be more energy efficient. Meng et al. [35] characterized the flow in four different twisted tape inserts, namely, the standard helical type of mixer, the right-twist type RSM, the M-type, and the spiral type static mixers at Re_pipe = 0.1–100. By evaluating the Poincaré section, the stretching history, the extensional efficiency, and the CoV, they found that the standard performs better than its counterparts do. Soman and Madhuranthakam [36] investigated the effect of incorporating internal design modifications such as perforations of different sizes and/or serrations of varying shapes on the performance of the mixer.

Their CFD study was conducted in the laminar regime (Re_pipe = 10⁻⁴–100) and quantified both dispersive and distributive mixing. CFD simulations were used to compare the results of the modified geometries to the regular mixer. The authors concluded that circular serrations in the body rendered the least pressure drop and significantly improved mixing performance compared to the standard mixer. Haddadi et al. [8] also compared the performance, and a new model under laminar regimes (Re_pipe = 20–160) relying on the results of pressure drop, extensional efficiency, and CoV and reported that their proposed design is the most efficient.

Recently, Yang et al. [37] experimentally compared the mixing performance of a tridimensional rotational flow sieve tray for Reynolds numbers ranging between 197 and 987. The authors found that these types provided the best mixing efficiency, and then rendered a similar performance.

Under turbulent flow conditions, only a handful of comparative studies exist. Barrué et al. [38] compared the aerodynamic and mixing performance of a new gas-gas mixer, static mixers in turbulent regimes. Their study was limited to pressure drop, velocity, and RSM velocity measurements at the outlet relying on the LDA technique, and their mixing efficiency evaluation was based on a qualitative technique, namely the laser sheets visualization technique. Wadley and Dawson [39] relied on laser-induced fluorescence data to evaluate the mixing performance of SMV, standard, and mixers in turbulent and transitional regimes for Re_pipe ranging between 900 and 90,000. The performance of the various mixers was evaluated using only the concentration variance (i.e., CoV).

The effect of the flowrate ratio, the number of elements, and the initial injection position on the mixing efficiency were studied and their experimental results contradicted the correlations of the manufacturer. In another study, Theron et Le Sauze [7] also compared experimentally the performance of three designs in both single-phase and two-phase flow in turbulent regimes (Re_pipe ∼ 60–16,000). This comparison was based on the hydrodynamic and emulsification properties. Pressure drop correlations based on the hydraulic diameter and the interstitial velocity were developed. This study showed that improved versions are 50 % more energy-efficient than the design in single and two-phase flow. The analysis of the Sauter mean diameter distribution as a function of the mean energy dissipation rate per fluid mass showed that is the best compared to the two other types of mixers. Stec and Synowiec [40], [41], [42] carried out numerical and experimental tests to compare the performance under turbulent conditions (Re_pipe =1000–5000).

To accomplish this, they relied on the pressure drop data [40], RTD [41], and CoV values [42]. Their study revealed that 30 % is more energy-efficient, which was attributed to its compact geometry [40]. It was also found that presents lower values of CoV [42] but this has the narrowest RTD with the highest maximum, and the smallest residence time [41]. Meng et al. [43] compared the hydrodynamics, thermal, and mixing performance of static mixers under turbulent flow conditions (Re_pipe = 4000–30,000).

Their investigation highlighted that for aspect ratios greater than 1.5 the mixer is more energy-efficient than but its ability to enhance the dispersion mixing becomes less important. Recently, Meng et al. [44] also compared experimentally and numerically the hydrodynamics, mixing as well as thermal efficiencies of those of static mixers (Re_pipe = 2640–17,600). The pressure drops, Nusselt number, flow field, turbulent kinetic energy, and turbulent dissipation rate data showed that outperformed in terms of mixing and thermal efficiency. This was attributed to the inherent geometry of where longitudinal and transversal vortices interact.

However, the improved efficiency was counterbalanced by an increased cost of energy where the heat transfer coefficient was almost enhanced by 50 % while the pressure drop values were found to be 4.37 times higher.

It is therefore clear that the number of available numerical studies that compare the performance of static mixers under turbulent flow conditions is low. This is in contrast with the large number of mixers that are available on the market and/or being proposed in the literature. The importance of these studies is that they allow comparisons under similar operating and design conditions of both the flow field and mixing performance of these mixers.

Therefore, there is a need for systematic studies that undertake such tasks. Based on this, the current study serves two main objectives. First, it proposes a novel design based on screen-type static mixers to enhance their distributive mixing behavior without excessive energy expenditures. Second, it will numerically compare its hydrodynamics and mixing efficiency to the earlier design proposed by Abou-Hweij [32] and to two other commercially available static mixers.

To the authors’ knowledge, this will also be the first study that numerically compares the flow field and mixing performance of the various mixers under similar turbulent flow conditions. The simulations will be performed in the turbulent flow regime with Re_pipe ranging between 5000 and 30,000. To accomplish the aims of this study, mixer geometry will be presented, then the velocity fields through the four different geometries will be described and analyzed. In addition, the pressure drops, and energy consumption of these mixers will be quantified, and a comparison based on their dispersive and distributive mixing efficiencies will be conducted.

Computational domain

In this investigation, two newly proposed hybrid mixer geometries are compared to two commercial designs namely, the static mixers. Fig. 1 depicts the flow domain made of two representative units of each design.

The investigated computational domain of each mixer consists of a horizontal tube of internal diameter ��=12.7 mm equipped with 4 mixing elements of diameter equal to that of the pipe. Larger diameters were not tested to avoid unnecessary computational.

Model Validation

To validate the current computational approach, the pressure drop across the mixers was compared against available literature data. However, since the novel geometries have never been investigated before, a set of experiments to measure ΔP was carried out under the same operating conditions considered in the current work.

The pressure drop values across a mixer were expressed in terms of the Fanning friction factor, f, depicted in Eq. (7), and were compared against.

Conclusions and future work

In this study, the performance of two novel mixer geometries based on screen-type static mixers was compared to two widely used commercial designs, namely the standard mixers. The novel geometries consist of a combination of a woven mesh followed by a pair of divergent inserts of either trapezoidal or rectangular shapes. The comparison was undertaken from both hydrodynamic and mixing efficiency perspectives. For this purpose, three-dimensional CFD numerical

Credit authorship contribution statement.

Ranim Chakleh: Methodology, Formal analysis, Investigation, Visualization, Software, Writing – original draft, Data curation.

Fouad Azizi: Methodology, Investigation, Conceptualization, Validation, Writing – original draft, Writing – review & editing, Formal analysis, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.