Heat / Mass transfer intensification using helically coiled pipes: potentiality and comparison to alternative enhancement techniques

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Hydrodynamic-based techniques for improving transfer efficiency in membrane separation processes and heat exchangers

This chapter presents a state of the art of hydrodynamic-based solutions used to intensify separation processes and/or to improve heat transfer techniques. In the first part, the basic concepts of filtration, membrane separation and heat transfer processes are defined, and the main types of separation modules and heat exchangers are presented. Concentration and temperature polarizations are explained to be one of the main limitations for transfer efficiency. Then, a literature survey of seven hydrodynamic-based techniques for counteracting these effects is presented. A first technique is the use of turbulence promoters which are obstacles (parietal or placed in the bulk) that the fluid must circumvent and which produces recirculations favorable for mixing. Another enhancement solution is to vibrate the module wall, which limits the deposition of particles. Ultrasound technique is a very effective method to unclog filters. Gas (often air) may also be injected in the liquid: the passage of plugs (large gas bubbles) efficiently disrupts the liquid flow. Rotational cylindrical or disk systems take advantage of the hydrodynamic instabilities that appear in these geometries and induce recirculations with relatively low energy costs. Finally, the use of complex shapes devices is presented as a passive technique that induces Dean-type vortices or leads to chaotic flow behavior, which results in an extremely efficient mixing at a low energy consumption.

Filtration techniques and membrane separation

Filtration techniques and membrane separation

Tangential liquid filtration

Filtration is a unitary operation that consists in separating solids suspended in a fluid using a porous medium. Filtration techniques can be broadly classified into two main categories, frontal (or dead-end) and tangential (or crossflow) filtrations [1]. In frontal filtration, the flow of the suspension is orthogonal to the filtering wall as show in Figure 1.1a; all the carrying fluid passes through the porous wall while the solids are traped and retained by the filter (apart particles smaller than the characteristic threshold of the filter, which are not stopped) and accumulates in the form of a filter cake [1]. The clear liquid recovered is called filtrate. In tangential filtration, the flow of the suspension is parallel to the filtering wall. Such devices have one inlet and two outlets as show in Figure 1.1b. Part of the liquid, called filtrate or permeate, crosses the filtering wall while the other part, called concentrate or retentate, enriched in particles, is discharged downstream. The separation is not total in this case and it is refered to as clarification or concentration [1]. The concentrate stream is generally larger than the filtrate stream.
quired to obtain high shears, which is energy consuming and reduces the slurry residence time. The present PhD study mainly focuses on membrane separation and tangential filtration in the presence of Dean vortices, which constitutes an additional hydrodynamic-based enhancement of these processes efficiency. As will be discussed later, this technique is commonly used for improving the transfer rate in heat exchangers. Given the heat/mass transfer analogy, heat and mass transfer enhancement by Dean vortices will be simultaneously addressed in the following sections.

Membrane separation processes

Membrane contactors are membrane systems mainly used to contact two phases to promote the mass transfer between them. They are considered as one of the most promising intensification technologies for gas-liquid absorption processes in food, chemistry, energy and pharmaceutical industries. Mem-branes are thin, semi-permeable or permselective objects that allow the retention of solutes or particles contained in a fluid [4]. They can be porous or dense. Porous membranes contain a large number of small cavities called pores, with dimensions large compared to intermolecular distances in solids [5]. The fluid flow through the membrane is generally laminar, obeying Darcy law, or Ergun law for higher speeds [6].
In dense membranes, the medium is continuous. The crossing species dissolve in the membrane and diffuse according to Fick’s law [6]. Dense membranes have the major advantage in the sepa- ration of liquid or gaseous homogeneous mixtures. They are selected according to their selectivity and permeability. Because of a difference in affinity, they let through one of the constituents to be separated, but not the others. Membranes are used in separation processes such as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, dialysis and electrodialysis, gas permeation, pervapo-ration and membrane distillation [5], [7]. Microfiltration is a technique for filtering fine particles, between 0.1 and 10 µm in diameter. These particles may be inert solids, droplets or microorganisms (cells, bacteria). Microfiltration uses frontal filtration (depth filtration) or tangential filtration (flat, spiral, tubular modules). On the other hand, ultrafiltration aims at separating very fine particles or dissolved molecules or macromolecules, of size ranging between 10 and 100 nm [1], [5]. For this purpose, semi-permeable membranes are used in planar, spiral, tubular or hollow fiber modules and sometimes filter-press modules. The pumping pressure used is generally higher than in microfiltration and the transmembrane flow is less important. Nanofiltration is used for molecules smaller than 10 nm (Figure 1.2). Reverse osmosis consists in extracting water from a saline solution by applying enough pressure to the feed to reverse the osmotic flow. Selective membranes are used for this application, allowing water to pass through but not salt. The filter-press, spiral-plane and hollow fiber modules can be used to achieve reverse osmosis [5], [7].
Dialysis processes separate solutes (ionic or nonionic) dissolved in a solvent. The membranes used are permeable to the solute but impermeable to the solvent. This process is used to eliminate metabo-lite waste from the blood of people with renal failure (hemodialysis). In the case of electrodialysis, the solute to be separated is an ion that is transported through ionic membranes under the action of an electric field [5], [7]. Membranes are also used for gas separation and pervaporation. The separa-tion of a gaseous mixture is called gas permeation. In pervaporation, the mixture to be separated is initially liquid and one of the constituents undergoes evaporation before being transported through the membrane [5], [7]. Membrane distillation is used mainly for the desalination of seawater. In this process, water is transported through a hydrophobic porous membrane. On one side of the membrane, partially vaporized and salty water circulates at high temperature and on the other side circulates fresh water at a lower and imposed temperature. Because of the difference in temperature, a water partial pressure difference is established between the two sides which pushes the water vapor towards the cold side where it condenses [8].

Main geometries used

The classical tangential filtration and membrane separation modules are the planar modules, the spiral modules, the tubular modules and the hollow fiber modules [7], see Figure 1.3.
The planar modules are a kind of multi-layered membranes, alternating layers where the supply fluid circulates and layers in which the filtrate (or permeate) circulates, Figure 1.3a. The mixture to be treated enters the feed layers and enriches in particles/solute along its way as liquid flows through the membranes and exits through the permeate layers. Each layer contains internal objects called spacers that maintain a spacing between the membranes. The operation of the spiral modules is similar to that of planar modules, but the membranes are fixed to a collecting cylinder and wrapped around it, Figure 1.3b. The tubular modules consist of cylindrical filters of centimetral diameter, in closed with then cylindrical shell. The suspension circulates inside the tubes and is progressively concentrated in particles while the filtrate passes through the membrane, Figure 1.3c. The hollow fiber modules, which will be considered during this PhD, are composed of a large number (between 50 and 5000) of small pipes (the hollow fibers) disposed parallel in a housing, Figure 1.3d. The liquid to be treated can circulate inside the fibers (and the permeate outside) or outside, in the housing (and permeate inside).

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Heat exchanger processes

A heat exchanger is a device used to transfer heat between two or more fluids. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. These technologies can be used at an industrial or domestic scale and cover a large range of applications: heating and cooling in evaporators, refrigeration and air-conditioning, condensation in power plants, steam generation, waste heat recovery, fluid heating or cooling, radiators for space or terrestrial vehicles, etc. [10], [11]. There are a lot of types of heat exchangers in industrial appicaltion such as plate type heat exchanger , shell-and-tube heat exchanger, vertical mantle heat exchanger and a broad classification of heat exchangers based on their geometry structure shape outlined by Bhutta et al. (2012) [12] in the Figure 1.4. The design of these devices is complex, since it requires accurate estimations of the heat
Figure 1.4: Classification of heat exchangers on the basis of geometry structure shape, [12] transfer rate and pressure drop. Furthermore, engineers need to consider energy, material and cost saving aspects [13]. In fact, the major challenge in designing such facilities is to make the equipment compact and with a high heat transfer rate using minimum power to reduce the costs [11]. This approach can be referred to as Heat transfer Enhancement or Intensification [13]. To help engineers to decide between different methods and technologies, many performance criteria can be used to evaluate the performance of heat exchanger technologies. One of them is the thermal enhancement factor which is used to estimate the performance of different insert such as wire coil, twisted tape, etc [10]. There are active and passive methods to enhance the heat transfer in heat exchanger appilcations [14]. The active techniques are based on external forces to perform the augmentation [14]. The active technique is effective; however, it is not always easy to perform the compatible design with other component in a system. It also increases the total cost of the system. On the other hand, passive techniques employ special surface geometry. Using the geometry approach is easier, cheaper and does not interfere with other components in the system.

Concentration/ Temperature polarization

The membrane properties, trans-membrane pressure, liquid flow rate and bulk concentration/temperature affect the membrane efficiency. One of the basic phenomena limiting the separation efficiency in mem-branes is processes is a physical phenomenon that takes place near the membrane surface referred to as polarization. Depending on the applied driving force of the membrane system, the polarization phenomenon may designate either temperature polarization or concentration polarization, as shown in Figure 1.5 [15]. During separation, an over-concentration of the retained species appears in the vicinity of the membrane. This phenomenon is referred to as concentration polarization. It causes several problems for membrane transfer. First, the flow of species across the membrane is propor-tional sometimes to the concentration difference between the two membrane faces, which is reduced by polarization. In addition, molecules that accumulate may lead to fouling. Finally, above a con-centration threshold, the mixture becomes rheologically complex and may take the consistency of a gel that opposes the liquid and the species motion [16]. Similarly, when heat transfer is involved (in pure heat or combined mass and heat transfer processes), temperature polarization often occurs. In this phenomenon, temperature is higher (resp. lower) near the exchange surface than in the bulk flow, if the temperature on the other side of the membrane is high (resp. low). As a consequence, heat transfer, which is proportional to the temperature difference across the membrane, is limited. Concentration polarization takes place in both isothermal processes (e.g., reverse osmosis and forward osmosis) and in non-isothermal processes (e.g., membrane distillation) [15].

Table of contents :

Introduction
1 Synthesis and Objectives of the thesis
2 Organization of the Thesis
3 Publications
3.1 Journals
3.2 Conferences
3.3 Talks
Chapter 1 Hydrodynamic-based techniques for improving transfer efficiency in membrane separation processes and heat exchangers
1.1 Filtration techniques and membrane separation
1.1.1 Tangential liquid filtration
1.1.2 Membrane separation processes
1.1.3 Main geometries used
1.2 Heat exchanger processes
1.3 Concentration/ Temperature polarization
1.4 Hydrodynamic solutions to limit concentration/temperature polarization
1.4.1 Tangential flow
1.4.2 Turbulence promoters
1.4.3 Rough walls
1.4.4 Pulsate flow
1.4.5 Vibrating system in membrane separation
1.4.6 Ultrasound
1.4.7 Rotary systems
1.4.8 Gas sparging
1.4.9 Complex shapes
Chapter 2 Laminar flow friction factor in highly curved helical pipes: numerical investigation, predictive correlation and experimental validation using a 3D-printed model
2.1 Introduction
2.2 Materials and methods
2.2.1 Friction factor and dimensional analysis
2.2.2 CFD modeling and simulation
2.2.3 3D-printed helical pipe
2.2.4 Experimental setup for pressure drop measurements
2.3 Results and discussion
2.3.1 CFD results
2.3.2 Correlation development
2.3.3 Comparison with literature correlations
2.3.4 Correlation validation using experimental data from literature
2.3.5 Correlation validation using data acquired on the 3D-printed highly curved helix
2.4 Conclusion
Chapter 3 Optimal design of helical heat/mass exchangers under laminar flow: CFD investigation and correlations for maximal transfer efficiency and process intensification performances
3.1 Introduction
3.2 CFD computation of Nusselt (and Sherwood) number in helical pipe flows
3.2.1 Nusselt (and Sherwood) number in helical pipe flows
3.2.2 CFD modeling and simulation of heat transfer in helical pipes under laminar flow conditions
3.2.3 Heat and mass transfer analogy
3.3 Optimal packing density of helixes
3.4 Results and discussion
3.4.1 CFD results
3.4.2 Correlation for predicting Nusselt (and Sherwood) numbers in helical pipe laminar flows
3.4.3 Comparison between the current and literature correlations
3.4.4 Correlation and CFD data validation using experimental data from literature
3.4.5 Optimal packing density of helixes: results and correlation
3.4.6 Overall intensification factor and potentiality of highly curved helical pipes designs
3.5 Conclusion
Chapter 4 Transport phenomena in helical heat and mass exchangers under high Prandtl/Schmidt number conditions
4.1 Introduction
4.2 CFD computation of Nusselt (and Sherwood) number in helical pipe flows
4.2.1 Mesh-independence study
4.2.2 CFD modeling and governing equations
4.2.3 Thermally developing and hydrodynamically developed flow
4.3 Results and discussion
4.3.1 CFD Results
4.3.2 Non-periodic flow
4.3.3 Overall intensification factor and potentiality of highly curved helical pipes
4.4 Conclusion
Chapter 5 Heat / Mass transfer intensification using helically coiled pipes: potentiality and comparison to alternative enhancement techniques
5.1 Introduction
5.2 Transport phenomena in helical pipe flow
5.2.1 Helical pipes design, packing density and specific surface area
5.2.2 Hydrodynamics and heat/mass transfer in helical pipe flows
5.3 Alternative heat/mass transfer enhancement techniques
5.4 Results and discussion
5.4.1 Heat/Mass transfer enhancement per unit surface
5.4.2 Volumetric heat/mass transfer enhancement
5.4.3 Cost-effectiveness of heat/mass transfer enhancement per unit surface
5.4.4 Cost-effectiveness of volumetric heat/mass transfer enhancement
5.4.5 Cost-effectiveness of volumetric heat/mass transfer enhancement in ‘shell-andtube’ configurations
5.5 Conclusion
Chapter 6 Toward novel coiled heat/mass exchangers designs
6.1 Introduction of the Complex helical shapes
6.1.1 Wavy helical pipes
6.1.2 Double helical pipes
6.2 Conclusion

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