Spacer-filled Channels Research Articles

Experimental investigation of the effect of the spacer and operating conditions on mass transfer in direct contact membrane distillation

This study examines the effect of spacers and operating conditions (feed inlet temperature, velocity, and feed concentration) on mass transfer characteristics in direct contact membrane distillation (DCMD) systems. The experimental results showed that velocity of the feed and permeate influenced the mass transfer properties more significantly than the temperature and concentration factors. Among the five investigated spacers, the plastic spacer (PL4) had the most influence on mass transfer characteristics, which was proven through analysing the mass transfer coefficient (ks), boundary layer thickness (δ), shear stress (τ) as well as flow property in DCMD spacer-filled channels. Moreover, the mass transfer correlation for each spacer and a general mass transfer correlation for all spacers were determined. Through comparisons between theoretical and experimental Sherwood numbers, the mass transfer correlation developed by Grober for empty channels could be used to describe the mass transfer characteristic in spacer-filled channels in this study.

Computational Fluid Dynamics Modeling of the Resistivity and Power Density in Reverse Electrodialysis: A Parametric Study.

Electrodialysis (ED) and reverse electrodialysis (RED) are enabling technologies which can facilitate renewable energy generation, dynamic energy storage, and hydrogen production from low-grade waste heat. This paper presents a computational fluid dynamics (CFD) study for maximizing the net produced power density of RED by coupling the Navier–Stokes and Nernst–Planck equations, using the OpenFOAM software. The relative influences of several parameters, such as flow velocities, membrane topology (i.e., flat or spacer-filled channels with different surface corrugation geometries), and temperature, on the resistivity, electrical potential, and power density are addressed by applying a factorial design and a parametric study. The results demonstrate that temperature is the most influential parameter on the net produced power density, resulting in a 43% increase in the net peak power density compared to the base case, for cylindrical corrugated channels.

Towards improved hydrodynamics of the electrodialysis (ED) cell via computational fluid dynamics and cost estimation model: Effects of spacer parameters

The spacer geometries have a crucial effect on the hydrodynamics of spacer-filled channels in electrodialysis (ED) process, which are beneficial to mass transfer enhancement. Herein, a three-dimensional, isothermal, and steady flow model was developed by computational fluid dynamics (CFD) to illuminate the impacts of the spacer geometrical configurations on hydrodynamic behaviors in the spacer-filled channel for ED applications. Simultaneously, the finite element analysis method was employed to investigate the effects of spacer geometries on the velocity and pressure profiles. Furthermore, the normalized expenditure (NEXP) was introduced to acquire one comparable cost estimation index, which determined the optimal spacer geometries subserving the efficiency and economic competitiveness of ED process. Theoretical and techno-economic analysis validated that the optimal mesh parameters within the range of the test parameters were a cylindrical mesh, a filament diameter of 0.8 mm, a filament interval of 1.0 mm, a flow attack angle of 30°. Therefore, this work can provide a feasible strategy to visualize and quantitate the hydrodynamics in the spacer-filled channel and insight into the optimization of ED stack, potentially reducing the expenditure related to materials usage, energy consumption and test time.

Experimental investigation of two-side heat transfer in spacer-filled channels

In Membrane Distillation (MD), spacers support the membranes and promote mixing, thus reducing temperature polarization. Their efficient design requires a knowledge of the distribution of the local heat transfer coefficient h and of its dependence on Reynolds number, spacer geometry and flow-spacer relative orientation. In previous work, we applied Thermochromic Liquid Crystals (TLC) and digital image processing to the measurement of h distributions for different spacer configurations; data were used to validate CFD simulations and select turbulence models. For constructive reasons, the test section allowed only one-side heat transfer, while in most MD configurations (e.g. spiral-wound modules) heat transfer occurs from both sides of the feed water channels. Analytical and numerical solutions show that changing from one-side to two-side heat transfer deeply affects h values. This motivated the design and construction of an improved test section in which a hot channel is sandwiched between two cold channels, and twin cameras and lighting equipment allow the simultaneous acquisition of TLC images on both walls. This paper describes this new test section and the experimental technique, discusses measurement uncertainty, and presents preliminary results.

The Effect of Spacer Orientations on Temperature Polarization in a Direct Contact Membrane Distillation Process Using 3-d CFD Modeling

Membrane distillation is an emerging technology that uses hydrophobic membranes to separate nonvolatile solids from liquids. The vapor pressure gradient between the feed and the permeate sides drives the process. Low-grade thermal energy is used to heat feed water and create a pressure gradient. A large vapor pressure gradient across the membrane surfaces results in high permeation rates. The feed spacer is an important element of the membrane module that forms channels for feed and permeate flow. A good feed spacer design helps improve permeation. In this paper, 3-d CFD simulations are carried out for spacer-filled channels, and the effect of inlet velocity, filament orientation and spacing on heat transfer is studied. Temperature polarization is used as the parameter for heat transfer performance evaluation. Shear stress and temperature polarization index have been calculated for different spacer orientations in a direct contact membrane distillation process. The results show a major influence of the studied parameters on temperature polarization and shear stress. A comparison of 2-d and 3-d analyses reveals that the average shear stress in the two approaches is nearly the same, but the standard deviation of shear stress is lower for the 2-d case. Similarly, the average value and the standard deviation of temperature polarization index are lower than those obtained in the 3-d analysis. The findings also show that for staggered axial filaments, the temperature polarization index distribution is more uniform suggesting that such orientations are more suitable for enhancing heat transfer in a membrane distillation process.

A 3D CFD comparative study between torsioned and non-torsioned net-type feed spacer in reverse osmosis

Spacer-filled channels are incorporated in membrane modules in reverse osmosis applications where the role of feed spacer is to promote turbulence, improve mass transport and mitigate concentration polarization. The objective of this study is to emulate a realistic spacer, due to manufacturing conditions, spacers tend to have a special complex geometry (deformations) which is usually not adopted in computational fluid dynamics (CFD) studies, this deformation tends to be of a torsioned shape. A total of 18 simulations were conducted for torsioned and non-torsioned spacers evaluating concentration polarization and mass transfer across membrane walls at different inlet salinities (co = 400, 500 and 600 mol/m3) and different Reynolds number (Re = 100, 192 and 384) based on the inlet velocity using fully-coupled three dimensional CFD, where the solution-diffusion model was incorporated for the calculations of water and solute fluxes across reverse osmosis membrane. The current comparison illustrates that the torsioned spacers have an overall enhanced performance compared to non-torsioned spacers. It is found that concentration polarization is mitigated in torsioned spacers which is the contrary with non-torsioned spacers having a tendency to concentration polarization, consequently, the water flux in torsioned spacers was found higher than those in non-torsioned spacers. Generally speaking, all membrane modules are found to have a higher performance at higher Reynolds number and at lower inlet salinity. This study proves that spacer geometric details are critical to predict accurately the RO membrane performance.

Turbulent heat transfer in spacer-filled channels: Experimental and computational study and selection of turbulence models

Heat transfer in spacer-filled channels of the kind used in Membrane Distillation was studied in the Reynolds number range 100–2000, encompassing both steady laminar and early-turbulent flow conditions. Experimental data, including distributions of the local heat transfer coefficient h, were obtained by Liquid Crystal Thermography and Digital Image Processing. Alternative turbulence models, both of first order (k-ε, RNG k-ε, k-ω, BSL k-ω, SST k-ω) and of second order (LRR RS, SSG RS, ω RS, BSL RS), were tested for their ability to predict measured distributions and mean values of h. The best agreement with the experimental results was provided by first-order ω-based models able to resolve the viscous/conductive sublayer, while all other models, and particularly ε-based models using wall functions, yielded disappointing predictions.

CFD Investigation of Spacer-Filled Channels for Membrane Distillation.

The membrane distillation (MD) process for water desalination is affected by temperature polarization, which reduces the driving force and the efficiency of the process. To counteract this phenomenon, spacer-filled channels are used, which enhance mixing and heat transfer but also cause higher pressure drops. Therefore, in the design of MD modules, the choice of the spacer is crucial for process efficiency. In the present work, different overlapped spacers are investigated by computational fluid dynamics (CFD) and results are compared with experiments carried out with thermochromic liquid crystals (TLC). Results are reported for different flow attack angles and for Reynolds numbers (Re) ranging from ~200 to ~800. A good qualitative agreement between simulations and experiments can be observed for the areal distribution of the normalized heat transfer coefficient. Trends of the average heat transfer coefficient are reported as functions of Re for the geometries investigated, thus providing the basis for CFD-based correlations to be used in higher-scale process models.

Investigation on hydrodynamics and mass transfer in a feed channel of a spiral-wound membrane element using response surface methodology

Using the response surface methodology (RSM) coupled with computational fluid dynamics, the hydrodynamic and mass transfer have been investigated in a spacer-filled channel. Geometry of an industrial reverse osmosis spiral-wound membrane module has been set to optimize the pressure drop, water permeation flux and the spacer configuration efficacy with respect to the average inlet velocity, the spacer attack angle and the spacer mesh angle. Several tools have been used to implement the response surface methodology: The Latin Hypercube Sampling Design method for design points generation, the Kriging model for interpolation and regression, and the genetic algorithm for optimization. It was found that the optimum configuration taking all the output operational parameters into account was stood up at the attack angle of 75°, the mesh angle of 85.9°, and the average inlet velocity of 0.0428m/s. Also the global sensitivity analysis has been performed, which the average inlet velocity and the attack angle were obtained the most and the least effective parameters on optimum conditions, respectively. Due to the change in the velocity along the RO module with several SWM elements, the optimum performance was achieved by adjusting the geometric structure of the spacers with velocity variations in each RO element.

Determining effects of spacer orientations on channel hydraulic conditions using PIV

Feed spacer orientation affects the velocity pattern and pressure drop of spacer-filled channels such as those encountered in Spiral-wound Membrane (SWM) modules of Reverse Osmosis (RO). However, there are only limited numbers of experimental studies on this topic. This study sets out to reveal more detailed information on the pressure drop and velocity patterns of spacer-filled channels. Particle Image Velocimetry (PIV) is used to provide high-resolution velocity maps for three commercial feed spacers of different thicknesses at a flow attack angle of 45° and 90°. The pressure drop is measured for the applied operational conditions (Re < 250). Results showed higher pressure losses, a better mixing of flow, a lower variation of temporal velocity, and a smaller variation of velocity over the channel height in the orientation with a flow attack angle of 45° as compared to 90°. The results presented here can be used to validate numerical studies, determine the fouling-sensitive regions in a spacer-filled channel and consequently, design the optimal spacer with respect to its orientation and thickness.

Prediction of spatial-temporal evolution of membrane scaling in spiral wound desalination modules by an advanced simulator

There is no method available at present to determine the spatial-temporal evolution of scale throughout the membranes in desalination plants, although it would be most useful for desalination plant design and monitoring. For such tasks, various approaches are followed to estimate the scaling propensity of feed-water or concentrate, including the use of inadequate single-value indices (e.g. LSI, S&DSI). A novel method is presented herein for simulating spatial-temporal scale evolution. It is based on a comprehensive theoretical model, describing transport phenomena in the spacer-filled channel flow of spiral-wound membrane (SWM) modules, and an experimentally determined correlation (akin to a constitutive relation) of two key parameters of the scaling process; i.e. the depositing salt supersaturation ratio at the membrane surface Sw and the rate of scale-mass deposited per unit membrane area, F g/(m2·h), which represent the driving force and effect of scaling, respectively. Very good agreement is obtained by comparing experimental data from the literature on spatial scale distribution with new computational results. The challenges and priorities are discussed regarding further development of the proposed method as well as applications related to important tasks, such as the design and operation of scale-control and monitoring systems of RO/NF desalination plants.

An experimental heat transfer investigation of using spacer in direct contact membrane distillation

Membrane distillation (MD) is a thermally-driven separation technology that can produce highly pure water. Direct contact membrane distillation (DCMD), where both hot and cold solution is directly in contact with the hydrophobic membrane is the most studied system due to its condensation method without an external condenser as well as its overall structural simplicity. In this paper, heat transfer analysis was carried out in DCMD experimentally to investigate how the presence of a spacer can effect on heat transfer coefficient. Results revealed that the heat transfer coefficient for spacer-filled channels (plastic and metallic material) is approximately from 3 to 3.5 times higher than that for empty channels. Additionally, from specific pumping power (SPP) for 1 cubic meter of freshwater production, it is predicted that DCMD with spacer-filled channels can consume far less electrical or mechanical energy than that with empty channels.

Impact of Modified Spacer on Flow Pattern in Narrow Spacer-Filled Channels for Spiral-Wound Membrane Modules

A modified spacer, which was constructed with arched filaments and zigzag filaments, was designed to improve vortex shedding and generate a directional change in flow patterns of membrane modules, especially in the vicinity of the feed spacer filament, which is most affected by fouling. A unit cell was investigated by using a three-dimensional computational fluid dynamics (CFD) model for hydrodynamic simulation. The results of CFD simulations were carried out for the fluid flow in order to understand the effect of the modified spacer on vortices to the performance of arched filaments at different distances. From 2D velocity vectors and shear stress contour mixing, the flow pattern and dead zone flushing were depicted. The ratio of low shear stress area to the total area increased with the inlet velocity closed to 20%. The energy consumption with respect to flow direction for the arched filament was 80% lower than that in the zigzag filament. Compared with previous commercial spacers’ simulation, the friction factor was lower when the main flow was normal to the arched filament and the modified friction factor was close to the commercial spacers. The homogenization was realized through the flow pattern created by the modified spacer.

Temperature and Velocity Effects on Mass and Momentum Transport in Spacer-Filled Channels for Reverse Electrodialysis: A Numerical Study

Concentration polarization is one of the main challenges of membrane-based processes such as power generation by reverse electrodialysis. Spacers in the compartments can enhance mass transfer by reducing concentration polarization. Active spacers increase the available membrane surface area, thus avoiding the shadow effect introduced by inactive spacers. Optimizing the spacer-filled channels is crucial for improving mass transfer while maintaining reasonable pressure losses. The main objective of this work was to develop a numerical model based upon the Navier–Stokes and Nernst–Planck equations in OpenFOAM, for detailed investigation of mass transfer efficiency and pressure drop. The model is utilized in different spacer-filled geometries for varying Reynolds numbers, spacer conductivity and fluid temperature. Triangular corrugations are found to be the optimum geometry, particularly at low flow velocities. Cylindrical corrugations are better at high flow velocities due to lower pressure drop. Enhanced mass transfer and lower pressure drop by elevating temperature is demonstrated.

CFD Modelling of an Electrodialysis Cell

Electrodialysis (ED) cells are based on the selective transport of ions through membranes which ideally exclude the co-ions, so that when a potential difference is applied to the cell all ions are transported in solution but only the counter ions can cross the membrane. Thus, concentration polarization occurs in solutions adjacent to membrane surfaces due to the different transport number between the membrane and the solution. The concentration profiles formed in the solution near the membrane depends on the mass transport fluxes and hydrodynamic pattern in the channel; operating conditions and geometrical configuration of the cell affect greatly the concentration polarization [1]. Therefore, the performance of ED cells depends not only on the membrane properties but also on the electrolyte hydrodynamics being important to provide an adequate flow patter avoiding channeling, stagnant zones, jet formation, among others flow deviations. Keeping a homogeneous flow patter minimizes variations of mass transfer fluxes and current density over the membrane surface preventing high polarization zones that could give rise to water dissociation, low efficiency and premature membrane failure. The examination of relevant mass transport mechanisms by modelling can help to evaluate the effect of design characteristics and operating conditions on the performance of an electrodialysis cell through the analysis of concentration, potential and current density distributions. In this task, CFD has proven to be a valuable tool for the analysis of flow and mass transfer in membrane separation systems [2]. Therefore, this work presents the modelling of a laboratory ED cell coupling hydrodynamics and mass transport, taking into consideration changes of Donnan potential. The flow pattern inside channels of ED cell were obtained by 3D CFD modeling and the results were used to calculate the residence time distribution (RTD) to compare with experimental RTD. The hydrodynamic behavior was used then to solve the mass transport model to obtain a description of the concentration distribution of each of the ionic species in the cell, as well as of the amounts derived as fluxes of each of the components, current density and Donnan potentials at every point of the membrane.[1] Gurreri L., Tamburini A., Cipollina A., Micale G., Ciofalo M., CFD prediction of concentration polarization phenomena in spacer-filled channels for reverse electrodialysis J. Membr. Sci. 468, 133–148 (2014).[2] Fimbres-Weihs G.A., Wiley D.E., Review of 3D CFD modeling of flow and mass transfer in narrow spacer-filled channels in membrane modules, Chem. Eng. Process. 49 759–781 (2010).

Effect of non-woven net spacer on a direct contact membrane distillation performance: Experimental and theoretical studies

This study provides a comprehensive and systematic overview of the fundamental characteristics of heat and mass transfer in the direct contact membrane distillation (DCMD) process that employs different types of spacers on one or both surfaces of the membrane. Detailed theoretical investigations were carried out to demonstrate the effects of spacers adjacent to the membrane surface on heat and mass transfer enhancement in the DCMD with a PTFE/PP composite membrane, complemented with experimental data for model validation. Thus, this work aimed to propose and demonstrate the heat transfer correlation for spacer-filled channels to reliably predict the heat and mass transfer improvement by non-woven net spacers in the DCMD process. The results showed that the permeate flux enhancement by the spacers ranged between 7% and 19% only for the spacer-filled permeate channels and between 21% and 33% only for the spacer-filled feed channels even at higher flow rates, thus indicating lower flux enhancements in the spacer-filled permeate channels. This was because the influence of spacers on flux improvement became more evident at higher temperatures owing to higher temperature polarization. In this study, the maximum flux enhancement of approximately 43% over the empty channels was achieved using the thinnest and densest spacer with a hydrodynamic angle of 90°, adjacent to both membrane surfaces.

New Insights into Computational Fluid Dynamic Modeling of the Resistivity and Overpotential in Reverse Electrodialysis

The resulting electrical potential of a reverse electrodialysis is reduced both due to ohmic and non-ohmic resistances. The non-ohmic resistance is mainly controlled by concentration polarization which is a considerable challenge in a membrane based processes and is a result of accumulation or depletion of specific ions adjacent to the ionic exchange membranes compared to the bulk solution. This phenomenon effectively reduces the driving force across the membrane, hence affects the performance of the process. The present work aims to present a numerical model based on coupled Navier-Stokes and Nernst-Planck equations to predict flow and pressure drop as well as concentration and electrical potential for optimizing the performance of the system, using OpenFOAM. The model is demonstrated in a flat and spacer-filled channel for different Reynolds number. The results reveal that reducing the Reynolds number and introducing flow promoters such as cylindrical corrugations in a dilute solution channel reduces the resistivity of a RED unit cell, hence increasing the produced electrical potential. However, introducing cylindrical corrugations in a concentrated solution channel has an adverse effect on the resistivity, leading to an unfavorable resistivity increment.

New Insights into Computational Fluid Dynamic (CFD) Modeling of Concentration Overpotential

Salinity gradient power (SGP) is a promising source of sustainable energy production involving technologies such as pressure retarded osmosis (PRO), reverse electrodialysis (RED) and capacitive mixing (CAPMIX). Coupling these technologies to their counterparts used for desalination, salinity gradient batteries can in principle be developed [1], as recently demonstrated for RED/ED [2]. Electrode and membrane processes typically involve an accumulation or depletion of specific ions compared the bulk solution, resulting in so called concentration polarization. Concentration polarization is a considerable challenge in RED/ED, effectively reducing the driving force across the membrane, hence reducing the performance of the process [3]. In addition to concentration polarization, the presence of concentration gradients will result in both Ohmic and non-Ohmic contributions to the electrical potential [3], further influencing the characteristics of the system. Different strategies have been proposed in order to mitigate the influence of concentration gradients, specifically increasing the flow velocity and introducing flow promoters – typically by optimizing the spacer geometry [3, 4] – in order to alter momentum and concentration boundary layers. The interaction between ionic species and fluid flow can be achieved by solving the coupled Navier-Stokes (NS) and Nernst-Planck (NP) equations [5,6], allowing for simultaneous prediction of flow and pressure fields, concentration and electrical potential, all of which of interest for the overall performance of the system. The current work describes a simulation framework for simulation of the NS-NP system based on the open source CFD platform OpenFOAM [7], aiming to predict the influence of flow conditions and geometry on concentration and electrical potential. Figure 1 (left) shows constant, linear and parabolic concentration profiles, corresponding to the extreme conditions of plug and Poiseuille flow in a flat channel, with the corresponding results for electrical potential in Figure 1 (right), clearly demonstrating the strong coupling between the two. The suggested framework is applied to spacer filled channels aiming to describe pros and cons of different configurations such as height and pitch of spacers. The presented approach sheds light on future concentration polarization modeling, providing better descriptions of the mass transfer and electric potential and highlighting the role of spacer-filled channel in membrane based processes.

Fluid Dynamics and Mass Transfer in Spacer-Filled Membrane Channels: Effect of Uniform Channel-Gap Reduction Due to Fouling

The time-varying flow field in spacer-filled channels of spiral-wound membrane (SWM) modules is mainly due to the development of fouling layers on the membranes that modify the channel geometry. The present study is part of an approach to tackling this extremely difficult dynamic problem at a small spatial scale, by uncoupling the fluid dynamics and mass transfer from the fouling-layer growth process. Therefore, fluid dynamics and mass transfer are studied for a spacer-filled channel whose geometry is altered by a uniform deposit thickness h. For this purpose, 3D direct numerical simulations are performed employing the “unit cell” approach with periodic boundary conditions. Specific thickness values are considered in the range 2.5–10% of the spacer-filament diameter D as well as other conditions of practical significance. The qualitative characteristics of the altered flow field are found to be very similar to those of the reference geometry with no gap reduction. For a given flow rate, the pressure drop, time-average wall-shear stresses and mass-transfer coefficients significantly increase with increasing thickness h due to reduced channel-gap, as expected. Correlations are obtained, applicable at the “unit cell” scale, of the friction factor f and Sherwood number Sh, which exhibit similar functional dependence of f and Sh on the Reynolds and Schmidt numbers as in the reference no-fouling case. In these correlations the effect of channel-gap reduction is incorporated, permitting predictions in the studied range of fouling-layer thickness (h/D) = 0–0.10. The usefulness of the new results and correlations is discussed in the context of ongoing research toward improved modeling and dynamic simulation of SWM-module operation.

An experimental study for the characterization of fluid dynamics and heat transport within the spacer-filled channels of membrane distillation modules

The thermo-fluid dynamic behavior of spacer-filled channels for membrane distillation was investigated experimentally. Several different geometry were investigated thanks to customized reference spacers manufactured using a 3D printer. In particular, two sets of experiments were conducted: in the first set, cylindrical filaments were orthogonally arranged and the flow attack angle was made to vary from 0o to 90o; in the second set, the flow attack angle was kept symmetrical and the filament angle was made to vary from 30° to 150°. Each spacer was tested for Reynolds numbers between 200 and 900 in the hot channel, while maintaining a constant temperature difference of 13°C between the hot and the cold channels. Thermochromic Liquid Crystal (TLC) sheets were used for surface temperature measurements. Results showed that, for all spacers, the heat transfer coefficient increases with hot feed flow rate and that the combination of attack angle 45° and filament angle 90° gave the highest heat transfer. Pressure drop measurements showed that spacers could be categorized into two groups relevant to the values of the Darcy friction coefficients (f). One group, associated with spacer designs with filament angles greater than 90°, led to high f values, while the other, associated with filament angles less than 90°, led to moderate f values.