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Journal ArticleDOI

Passive Mixer cum Reactor Using Threaded Inserts: Investigations of Flow, Mixing, and Heat Transfer Characteristics

TL;DR: In this paper, the authors proposed a new desiggered tubular reactor for continuous manufacturing of fine and specialty chemicals, which can be used for continuous production of fine-and specialty chemicals.
Abstract: Significant efforts have been and are being spent on developing intensified tubular reactors for continuous manufacturing of fine and specialty chemicals. In this work, we have proposed a new desig...

Summary (4 min read)

1. Introduction

  • Several research institutes, universities and companies throughout the world have programs based on continuous flow synthesis and manufacturing at various levels.
  • Mixers/Reactors for continuous manufacturing of fine and specialty chemicals have been a hot topic not only in academics but have formed an integral part of variety of industrial processes.
  • 14,17,20 Many designs of passive mixers are studied over last two decades and are also reviewed to compare the designs, performance and flow patterns.

2. Design details and working principle of passive-mixer-cum-reactor with inserts

  • The schematic of a multiple threaded insert placed within a pipe has been shown in Figure 1a.
  • A detailed schematic of the configuration with two inserts having opposite direction threading is included as Figure S1 in the Supplementary information (SI).
  • The authors numerically investigated configurations with a different numbers of channels (from 1- channel to 9-channel design [an odd number of channels were considered]).
  • The direction of the threads formed over the insert surface is reversed, which will improve the interaction of fluid parcels between the channel and clearance in the region of reversal, consequently result in enhanced mixing.

3. Computational Model

  • The proposed passive mixer design with inserts has been computationally simulated for different insert designs.
  • Due to a large number of geometric parameters of the design, it is vital to know the effect of these parameters of flow dynamics for freezing a configuration providing required performance, and to be used for carrying out further experimental studies.
  • Five configurations viz., 5-channel, 7-channel, 9-channel, smooth surface (no threading) and smooth surface-extended rear end inserts have been studied here.
  • The numerical verification of the computational model and grid independence studies were carried out on one of the configurations.
  • Steady state Navier-Stokes equation was used for establishing the flow dynamics and heat transfer characteristics within the selected domain.

3.1 Model equations

  • The governing equations, viz., continuity, momentum and energy balance equations have been solved for simulating the flow, mixing and heat transfer in the selected configurations.
  • Multispecies transport equation has been used for carrying out multispecies flow in the passive mixer cum reactor.
  • The density and kinematic viscosity of the fluid and mass diffusivity of tracer in mixture were constant.

As discussed in design details section, there are a number of design parameters over which the performance

  • Two units of inserts of opposite threading direction have been placed at a defined distance concentrically within the tube.
  • Five different variants of designs have been selected based on modification in the insert surfaces, i.e., 5-channel, 7-channel, 9-channel, smooth surface (no threading) and smooth surface-extended rear end inserts .
  • These designs have been simulated for investigating the flow, mixing and heat transfer over the selected range of Reynolds number.
  • Simulation results for all the above cases have been explained in the results section.
  • The periodic inlet surface was split into two equal parts and defined as inlet velocity.

3.3 Quantitative parameters used in the study

  • The pressure drop across the considered periodic domain consisting of two threaded inserts for the selected configurations was calculated from these simulations.
  • The geometric details and nomenclature of the studied configurations have been discussed previously in Table 1.
  • The heat transfer enhancement ratio (𝜂) or Performance evaluation criteria (PEC) is used for comparing the performance of any passive mixer/reactor (insert design in their case) with the corresponding plain surface pipe.
  • Evaluation and quantification of different mixers using a very broad and universal set of criteria are impractical.

3.4 Computational details, Grid independency, Model verification, and Validation

  • The flow, mixing and heat transfer characteristics have been predicted by numerically solving the governing equations, i.e., mass, momentum and energy (Equations 1-4) using finite volume method using a commercial CFD code FLUENT (Ansys Inc., version 15.0).
  • Second order discretization scheme was used for the interpolation of pressure.
  • The high-quality surface mesh is later used to generate volume mesh in ANSYS Fluent.67 Tetra elements have been used for the volume mesh.
  • A comparison of the velocity profile has been made in the mixing region aft of the insert at x/L = 0.75.
  • Insignificant variation in the velocity magnitude is observed for considered grids of the highest number of elements .

4.1 Flow characterization

  • Laminar flow was simulated in a periodic domain of threaded mixer for Re ranging from 100 to 1600.
  • Negligible variation in the flow behavior was observed with and without studs, along with an insignificant rise in pressure drop (3 %) due to the presence of studs.
  • The cross-flow between the stream traces following the channel path and the stream traces flowing through the annulus generates strong shearing effects, which enhances mixing in the annulus region.
  • Since the passive mixer design consisting of the threaded insert have all the characteristics of the classes defined in Table 1 (i.e., by using bends: due to channels formed by threading, varying cross-sectional area, using inserts which allows splitrecombine of flow streams), higher pressure drop or Po are realized.
  • Therefore, the clearance between the insert and the pipe forms one of the major parameters for optimizing the flow through the annulus region and the threading/channels .

4.2 Heat transfer

  • 71 Number of passive heat transfer enhancement techniques has been developed with different insert designs which enhance convective heat transfer due to the generation of swirling effect or diffusion.
  • Nu has been normalized using theoretical values of straight pipe.70.
  • The threaded design is observed to perform better than other selected literature designs.
  • Therefore, fluid wall contact time increases with a decrease in the number of channels, thereby improving the heat transfer.
  • It is observed that significant enhancement in the heat transfer is achieved by using inserts compared to other conventional designs .

4.3 Mixing

  • Reliable characterization of mixing is crucial for an accurate prediction and designs of the reactors.
  • Such behavior of pathlines is observed only in the extension region of the mixer, whereas smooth deflection of the pathlines in the threading direction is realized within the annulus region.
  • Similar qualitative behavior of the pathlines and MF variation is observed for 7 and 9-channel design and has been shown in Figure S8 of the SI for reference.
  • Therefore, the mixing intensity, IM (Equation 15) is found to be increasing in the axial direction for the whole of the mixer/reactor .
  • The rate of increment of IM is higher in the annulus region due to the pinching effect caused by the nose of the insert and shearing due to the segregated stream traces.

4.4 Overcoming wake region or secondary vortices formation aft inserts

  • The presence of dead zones or secondary vortices within the reactor causes fluid parcels present in that region to remain unmixed or unreacted.
  • This may result in a significant loss in the throughput and the quality of the product obtained from the process.
  • The problem of secondary vortices /dead zones/backflow does exist in a number of well-established designs and has been discussed in Table 1.
  • As discussed in Section 4.3, the existence of a negative velocity region (wake) is the primary reason for the fall in mixing intensity.
  • The negative velocity region was found to be extending up to 1.3d for 5-channel design compared to the smooth surface (no threading) design .

4.5 Applications as intensified tubular reactor

  • There is an increasing trend to develop and use intensified tubular reactors for carrying out variety of reactions, particularly relevant to fine and specialty chemicals.
  • 5,79,80 For such reactor applications, it is essential to simultaneously realize enhanced mixing and heat transfer to intensify reactions.
  • The three channel configurations viz., 5-channel, 7-channel and 9-channel designs have been exhibited along with linear fit curves to indicate the corresponding Reynolds numbers.
  • 56; Theoretical (pipe) ─ Shah and London70 Note: some of the selected designs have IM values below 80% too.
  • Y-axis has been bounded to have a zoomed view.

5. Conclusions

  • A detailed computational study was carried out on screw inserts capable of enhancing shearing and diffusion by modifying the cross-sectional area and bends, and split-recombine of the flow streams.
  • Specific conclusions from this study may be summarized as: For threaded inserts, as the numbers of channels are increased, the deviation of the flow from the axial direction decreases, thereby resulting in less energy loss.
  • Cone angle of the ends of the insert (in degrees) Supporting information:.

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Passive Mixer cum Reactor Using Threaded Inserts: Investigations of
Flow, Mixing, and Heat Transfer Characteristics
Khalde, C. M., Ramanan, V., Sangwai, J. S., & Ranade, V. V. (2019). Passive Mixer cum Reactor Using
Threaded Inserts: Investigations of Flow, Mixing, and Heat Transfer Characteristics.
Industrial and Engineering
Chemistry Research
. https://doi.org/10.1021/acs.iecr.9b04606
Published in:
Industrial and Engineering Chemistry Research
Document Version:
Peer reviewed version
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Download date:10. Aug. 2022

1
Research Article
Passive Mixer cum Reactor using Threaded Inserts:
Investigations of flow, mixing and heat transfer characteristics
Chirag M. Khalde,
a,b
Vikram Ramanan,
c
Jitendra S. Sangwai,
a
and Vivek V. Ranade,
b
*
a
Gas Hydrate and Flow Assurance Laboratory, Petroleum Engineering Program,
Department of Ocean Engineering, Indian Institute of Technology Madras,
Chennai 600 036, India.
b
Multiphase Reactors and Intensification Group (mRING)
Bernal Institute, University of Limerick, Ireland and
School of Chemistry and Chemical Engineering, Queen's University Belfast, Belfast BT9 5AG
Northern Ireland, UK
c
National Centre for Combustion Research and Development,
Indian Institute of Technology Madras,
Chennai 600 036, India.
*Corresponding Author:
Vivek V. Ranade: V.Ranade@qub.ac.uk; Vivek.Ranade@ul.ie

2
Abstract
Significant efforts have been and are being spent on developing intensified tubular reactors for continuous
manufacturing of fine and specialty chemicals. In this work, we have proposed a new design of passive
mixer-cum-reactor for process intensification and development of continuous processes. The mixer/reactor
consists of threaded inserts with cone-shaped ends, placed concentrically in the tube such that fluid flows
through the annular region between the inserts and the tube. The proposed design is easy to fabricate,
maintain, and overcomes the limitations of scale up/scale down compared to most of the commercial
passive mixers. The split and recombine of flow around inserts, the swirling effect generated by threads,
change in the swirl direction due to change in the direction of screw threads, and pinching effect/expansion
at the cone-cone shaped ends realize desired enhancements in mixing and heat transfer. A detailed
computational study has been carried out on the mixer-cum-reactor to characterize flow, mixing and heat
transfer at different operating conditions using a verified and validated CFD model. Various designs and
configurations of threaded inserts were considered: 5-channel, 7-channel and 9-channel, smooth surface
(no threading) and smooth surface-extended rear end inserts. The flow, mixing and heat transfer were
characterized over the Reynolds number range of 100 to 1600. Structure of the generated swirling flow,
effect of pinching/expansion, direction reversal of flow, tracer fraction, temperature and path lines were
investigated systematically to gain new insights. Threaded inserts could achieve excellent mixing (>99 %
of mixing intensity) and heat transfer (7 times smooth inserts and 20 times without inserts). The presented
results will provide a sound basis for selecting appropriate threaded inserts for intensifying mixing and heat
transfer in tubular reactors. The work also provides a useful starting point for further work on multiphase
flows in a tubular reactor with threaded inserts.

3
1. Introduction
Several research institutes, universities and companies throughout the world have programs based on
continuous flow synthesis and manufacturing at various levels. Mixers/Reactors for continuous
manufacturing of fine and specialty chemicals have been a hot topic not only in academics but have formed
an integral part of variety of industrial processes. Enhanced mixing and heat transfer are primary
requirement in many different industrial applications such as: a) Chemical industry: reaction rates, heat
exchangers, etc. b) Petroleum industry: formation of homogenous drilling fluids which is a mixture of clay
and stabilized oil in water emulsion, production of crude oil, etc. c) Pharmaceutical industry: mixing of
different species of drugs, enzymes, etc. d) Paint industry: mixing of multispecies fluids like thinners,
blenders, etc. There has been a paradigm shift in approach for manufacturing chemicals by preferring
MAGIC (modular, agile, intensified, and continuous) processes over conventional ones. which has resulted
into the development of variety of mixers/reactors.
1,2
Mixers/reactors can be broadly grouped into two
classes: a) active: uses moving surfaces to enhance mixing (rotors, vibrations etc.) and b) passive: uses
geometrical configuration to enhance mixing and heat transfer.
3
Several designs and geometric
configurations involving folding structures, serpentine channels, splitting and recombining flows and
obstruction have been used in passive mixers.
4,5
Active mixers promote mixing by introducing mechanical energy (in the form of rotation, vibrations,
ultrasound, vapor pneumatic power), or dynamic differential pressure, ultrasonic or piezoelectric actuation,
magnetic actuation, light energy (through laser irradiations) or through electro-kinetic energy.
612
Although
active mixers are capable of providing intense mixing, they often require complicated device actuators and
fabrication protocols. Passive mixers are preferred over active devices for process intensification and
modular processes, owing to no moving parts, thereby less maintenance; ability to number up; large heat
transfer area; high mixing intensity; low pressure drop; cost-effective; compact, robust and simple for
manufacturing. Considering several advantages of passive mixers and microfluidic devices having such
features, they become the most obvious choice for mixing in modular processes for continuous flow
systems.
Several exhaustive reviews are available for classifying different types of passive mixers based on their
operating principles, commercialization and applications indicating their importance and implications.
3,5,13
19
The choice of mixer/reactor type is best determined by the specific application, available space for the
device and even the desired level of complexity of design with acceptable fabrication tolerance. The scales
at which they operate have been miniaturized for exploiting the increased surface to volume ratio. Many
investigators have studied flow and mixing in various components of micro-reactors.
14,17,20
Many designs
of passive mixers are studied over last two decades and are also reviewed to compare the designs,

4
performance and flow patterns.
21
Design of passive micromixers usually aims at increasing the tortuosity
along the channel length so as to increase the contact area between the fluid elements and enhance
diffusion.
2225
Another approach is to use hydrodynamic focusing to achieve better mixing. A third approach
is the flow separation and recombination approach that can be achieved through various complex multi-
layer channel structures, periodic or aperiodic alignment of various segments or objects in the channels,
etc.
10,2631
Such variations in the flow within the smaller length scales and low Reynolds number lead to
transverse flow or even tangential flow to generate chaotic advection. Depending upon the number of
channels the fluid stream splits and then combines (i.e., lamination), the mixing rates get enhanced
significantly.
Challenges in most of the passive mixer designs are that each of the designs has its operating range and
scale at which the best possible performance is achieved. An ideal passive mixer/reactor for realizing
modular, agile, intensified and continuous (MAGIC) process is expected to have following characteristics:
a) enhanced mixing; b) enhanced heat transfer; c) compact, robust, easy maintenance and high endurance;
and d) no dead zones or back mixing. A large number of passive mixers/reactors have been
developed/innovated to satisfy the above requirements. Unfortunately, no single mixer reactor design is
found to satisfy all of the above requirements for a wide range of operating conditions. Therefore, there is
a scope and need for developing novel designs that can overcome all the above limitations. We have
reviewed some of the recently developed passive mixers in Table 1. The passive mixers can be classified
into four categories based on their geometric configuration: a) Use bends with no change in cross-section
of channel/ tube; b) Use varying cross-section of channel/tube; c) Use channel branching for flow split and
recombine d) Use inserts to direct the flow in the channel.
In this work, we propose a new patented design of passive mixer which is based on threaded inserts (Figure
1a).
32
The working principle is discussed in Section 2. A computational study is carried out to understand
the flow physics, heat transfer and mixing characteristics of the proposed design. The purpose of this
exercise of carrying out single-phase flow simulations is to understand the influence of various geometric
and operating parameters on flow characteristics and use the results for designing appropriate experimental
setup. Therefore, we conclude that this numerical study is a pre-requisite for establishing the design of the
passive mixer/reactor. Preliminary experiments have been carried out for verifying the computational
model. The key results of flow, mixing and heat transfer for three different inserts are presented over the
Reynolds number up to 1600. The presented results will be useful for guiding the selection of threaded
inserts and will provide a basis for further work on extending applications of threaded inserts for multiphase
flow applications.

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Several designs of passive mixers are studied over last two decades and are also reviewed to compare the designs, performance and flow patterns this paper.