comparison of novel and commercial static mixers – 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. Also, two novels and two commercial geometries were selected. Additionally, Velocity profiles, pressure, dispersive, and distributive mixing were analyzed. Finally, 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. In addition, these shapes are downstream of a woven mesh to improve its distributive mixing.

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

Graphical abstract

Download: Download high-res image (201KB)

Download: Download full-size image

Introduction

Mixing is an omnipresent, complex, operation in most process industries. Also, it aims at reducing concentration non-uniformities and/or helping enhance chemical reactions. Additionally, 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 safety. It will also have consequences on the 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]. Also, the interest in static mixers gained momentum due to their inherent safety, energy efficiency, durability, cleaner operation, and more compact footprint [5]. Furthermore, 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.

Performance

In the comparison of novel and commercial static mixers, researchers found that static mixers outperformed stirred tanks (and other conventional reactors). This is when dealing with fast competitive reactions and/or time-sensitive materials [9,10]. Furthermore, these mixers provide a quasi-uniform energy dissipation rate and are applicable in batch or continuous processes under varying conditions [2]. Additionally, this makes them suitable for different applications such as the mixing of miscible fluids and reacting systems [11], [12], [13], [14]. Finally, 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.

The comparison of novel and commercial static mixers and given that various geometries are available on the market. In addition, 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. Also, it has open designs with blades or vortex generators, corrugated plates, and multilayer designs. Finally, with 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].

Aerodynamcis

Additionally, researchers have employed screens for various purposes such as aerodynamic noise reduction, thickening, coalescing, and as Stirling engine regenerators among others [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.

High Dispersive Mixing Capabilities

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 reveals that only a few studies have compared the performance of various static mixers (excluding split-and-recombine, micro-, and milli-reactors), with most of them conducted 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 improving the mixing quality would be possible if they left a spacing between the elements. Regner et al. [6] characterized numerically the flow by evaluating the pressure drop, the helicity, and the rate of striation thickness.

Increased Flow Rates

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 terms of compactness, the authors discovered that the comparison of novel and commercial static mixers, or (n) mixers, outperformed all other designs in terms of energy efficiency. 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.

Simulations

Their CFD study was conducted in the laminar regime (Re_pipe = 10⁻⁴–100) and quantified both dispersive and distributive mixing. Researchers used CFD simulations 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.

Rotational Flow Sieve Tray

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 limitations included pressure drop, velocity, and RSM velocity measurements at the outlet relying on the LDA technique, and they evaluated mixing efficiency qualitatively using 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).

Flow Rate Ratio

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. Researchers developed pressure drop correlations based on the hydraulic diameter and the interstitial velocity.

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]. In addition, 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. Furthermore, the smallest residence time [41]. Meng et al. [43] compared the hydrodynamics, thermal, and mixing performance of static mixers. Finally, it was compared under turbulent flow conditions (Re_pipe = 4000–30,000).

Aspect Ratios

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 interaction between longitudinal and transversal vortices was attributed to the inherent geometry.

Energy Use

However, the improved efficiency was counterbalanced by an increased cost of energy, but they almost enhanced the heat transfer coefficient by 50%. Moreover, researchers found the pressure drop values 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. Also, this contrasts with the large number of mixers available on the market and/or proposed in the literature. In addition, the importance of these studies is that they allow comparisons under similar operating and design conditions. Furthermore, done by 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. Firstly, it proposes a novel design based on screen-type static mixers to enhance their distributive mixing behavior without excessive energy expenditures. Secondly, 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. Furthermore, to accomplish the aims of this study, we will present the mixer geometry. Finally, we will describe and analyze the velocity fields through the four different geometries. In addition, the pressure drops, and energy consumption of these mixers will be quantified. Once complete, we will conduct a comparison based on their dispersive and distributive mixing efficiencies.

Computational domain

In this investigation, researchers compare two newly proposed hybrid mixer geometries 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. Finally, we did not test larger diameters to avoid unnecessary computations.

Model Validation in the comparison of novel and commercial static mixers

To validate the current computational approach, researchers compared the pressure drop across the mixers against available literature data. However, researchers have never investigated novel geometries before. Finally, 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, researchers compared the performance of two novel mixer geometries based on screen-type static mixers with two widely used commercial designs, namely the standard mixers. The novel geometries consist of a combination of a woven mesh. Furthermore, researchers follow this with a pair of divergent inserts of either trapezoidal or rectangular shapes. Researchers compared both hydrodynamic and mixing efficiency perspectives in the comparison. 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.