Direct Contact Membrane Distillation Module Research Articles

A computational fluid dynamics study on TPMS-based spacers in direct contact membrane distillation modules

Feed spacers are incorporated in membrane modules to enhance fluid mixing, thereby enhancing membrane flux. Previous experimental work showed that triply periodic minimal surfaces (TPMS)-based spacers induce significant improvements in membrane distillation (MD) performance, which was attributed to the suppression of membrane boundary layer, reducing heat and mass transfer resistance. To corroborate those findings, in this study, we assess the performance of TPMS spacers in direct contact MD (DCMD) modules using computational fluid dynamics (CFD). Results showed that the Fischer-Koch S TPMS spacer topology achieved twice the flux of the empty channel case and exhibited a 13 % increment in flux compared to the net-type spacer, but this improvement was accompanied by a higher pressure drop in the feed channel. Therefore, a normalized power ratio (NPR) was used to evaluate the overall performance of spacers. For Reynold number (Re) ranging between 200 and 1200, Fischer-Koch S spacer had a 12 % improvement in NPR compared to the net-type spacer and resulted in higher shear stress near the membrane surface, indicating a promising mean for mitigating fouling in membrane distillation. This research highlights the importance of utilizing novel spacers in membrane distillation and showcases the power of CFD tools in optimizing the process.

3D-CFD simulation of hollow fiber direct contact membrane distillation module: Effect of module and fibers geometries on hydrodynamics, mass, and heat transfer

For achieving a successful membrane distillation operation, not only the properties of the membrane but also the module design must be optimized. Computational Fluid Dynamics (CFD) is a practical tool for investigating the transport phenomena in a membrane module. Thus this research aimed to investigate the momentum, heat, and mass transfer by means of a 3D-CFD simulation for a hollow fiber membrane module applied for direct contact membrane distillation (DCMD) of saline water. Momentum, heat, and mass transfer equations in three dimensions were coupled and solved using the finite volume method by the Comsol Multiphase® software, and the effects of feed and permeate flow rates, number of fibers, and fiber length on DCMD performance are examined. The results demonstrated that the number of fibers has a significant effect on the permeate flux through flow channeling. The impact of feed flowrate on the increase of permeate flux is much more considerable than the permeate side. The longer fibers resulted in lower average fluxes due to the development of a thermal boundary and concentration layers. The temperature profile shows abnormal deflections at feed entry and reject exit ports due to the fluid entrance effects. Finally, it could be concluded that enhancing the feed-side hydrodynamics is an easy and affordable technique for attaining considerable improvement in DCMD performance.

An innovative ocean thermal energy conversion system with zeotropic Rankine cycle and direct contact membrane distillation for enhanced efficiency and sustainability

This research introduces an integrated ocean thermal energy conversion water and power cogeneration system (OTEC-WPCS), combining a Zeotropic Rankine Cycle (ZRC) with Direct Contact Membrane Distillation (DCMD) for efficient power and water production. The hydrofluoroolefins (HFOs), chosen for their zero ozone depletion potential (ODP) and extremely low global warming potential (GWP), are used as working fluid components in the ZRC. Additionally, DCMD is implemented for freshwater generation. A mathematical model is established, incorporating both thermodynamic and CFD simulations. The goal is to identify optimal operating conditions that maximize the output of the system. By coupling DCMD and ZRC in series, the system's efficiency reaches 8.92 %, an improvement of 5.71 % compared to a standalone ZRC system. This configuration allows for more efficient use of surface seawater heat compared to an ammonia-based Organic Rankine Cycle (ORC)-DCMD system. The enhancements include an increase in power and water production per unit mass flow of surface seawater by 0.55 kW/(kg·s) and 30 %, respectively. Over 60 % of the system's exergy destruction occurs in the DCMD modules and evaporator, while nearly 80 % of the capital cost is associated with the evaporator, condenser, and DCMD heat exchanger. Enhancing the DCMD module structure and heat exchanger can improve both energy and exergy efficiency, and economic feasibility.

Exergetic analysis of direct contact membrane distillation (DCMD) using PVDF hollow fiber membranes for the desalination brine treatment

The brines from desalination plants need to be disposed of due to their strong impact on the environment. Membrane operations, like direct contact membrane distillation (DCMD), provide a possible solution to reduce the amount of brine while producing further desalinated water. In this study, an exergy analysis of a laboratory membrane distillation unit working with brines from reverse osmosis (RO) is analyzed. Exergy analysis enables us to assess the energy lost in entropy generation; therefore, it commits us to identify the less efficient configuration of the DCMD module. Unlike other exergy analyses for distillation, in this study, only module inputs and outputs were incorporated. The exergy is calculated at different infeed temperatures, for both in-out and out-in feed configurations of hollow fiber membrane modules. Also, exergy difference, flux, and exergetic efficiency for both configurations are calculated. At high feed temperatures, there is an increase in both flux and exergy change, which increases water recovery and feed side exergetic efficiency. The highest flux that is obtained in the out-in configuration is 13.3 kg/h.m2 while it is only 6.23 kg/h.m2 for the in-out system of the module. Also, these exergy changes and feed efficiencies are higher in the out-in module configuration than in the in-out module configuration. Conversely, the exergetic efficiency of the permeate is higher at lower feed temperatures, due to the lower accumulation of concentration polarization along the membrane wall.

Effect of wavy corrugations on the performance enhancement of direct contact membrane distillation modules: A numerical study

This study employs three-dimensional (3D) computational fluid dynamics (CFD) simulations to investigate the influence of corrugations on direct contact membrane distillation (DCMD) modules. The accuracy of 3D CFD simulations is validated by the comparison of water flux and temperature polarization coefficient (TPC) against literature data for empty and spacer-filled DCMD modules. The dusty gas model is used to predict the permeate flux as a function of temperature and pressure over the surface of the membrane. We consider single and double sinusoidal corrugated membranes and examine the performance of DCMD membranes in terms of TPC, concentration polarization coefficient (CPC), power number (Pn), and Nusselt number (Nu) against various corrugation geometrical parameters. These parameters include the amplitude, streamwise, and spanwise frequency of corrugation waves. Our results show that corrugated membranes can successfully improve the membrane system by disrupting the boundary layer by an intense secondary flow. The mechanism works the best at high flow rates. Our results suggest that singly corrugated membranes improve the water flux and TPC by 32% and 25% at Re=1500. Also, doubly corrugated membranes provide 45% and 36% improvements in water flux and TPC at the same Reynolds number.

Performance enhancement of a nanofluid filtered solar membrane distillation system using heat pump for electricity/water cogeneration

Membrane distillation (MD) assisted by heat pump exhibits a favorable energy performance in water production, but it consumes huge precious electricity. In this work, a nanofluid filtered concentrating photovoltaic/thermal (CPV/T) technology is proposed to integrate with heat pump membrane distillation (HPMD) system for electricity/water cogeneration. Solar energy cascade utilization is considered in the system where electricity generated by CPV/T powers heat pump for MD heat recovery, and the collected thermal energy is used to heat seawater. A comprehensive study is carried out by establishing models of the receiver, direct contact membrane distillation (DCMD) and heat pump to investigate the performance of the nanofluid filtered solar membrane distillation system using heat pump. Results show that the seawater of 350.3 K is fed to the DCMD module while the electrical efficiency is above 12.2%. MD parametric analysis and equivalent specific thermal energy consumption (eSTEC) optimization are performed. We found that HPMD operates at the lowest eSTEC of 607.8 kWh·m−3 at heating temperature of 338 K and feed flow rate of 0.04 kg s−1. An overall system efficiency of 61% is obtained, and the comparison of permeate flux and energy consumption demonstrates advantages of the proposed system over other hybrid MD technologies.

Design of a multistage hybrid desalination process for brine management and maximum water recovery.

Hypersaline brine production from desalination plants causes huge environmental stress due to the untenable conventional discharge strategies. Particularly, brine production is expected to drastically increase in the coming few decades due to the increasing desalination capacity in attempts of forestalling water scarcity. Thereby, zero liquid discharge (ZLD) is a worth-considering solution for strategic brine management. ZLD or minimal liquid discharge (MLD) systems provide maximum water recovery with least or zero liquid waste generation and valuable salt production. In this work, a theoretical design of ZLD/MLD system is proposed for reverse osmosis (RO) brine management. Different scenarios are investigated utilizing multistage freeze desalination (FD) and its hybridization with multistage direct contact membrane distillation (DCMD), and eutectic freeze crystallization (EFC) technologies. The design is based on the experimental assessment of the indirect FD process at different feed salinities, i.e., 2g/L to 155g/L. FD experiments showed that ice quality is reduced at greater crystallinity levels and initial concentration. Moreover, a computational fluid dynamics (CFD) model is utilized to assess the performance of DCMD. A single DCMD module could produce 53kg/(m2.h) of pure water operating with 69% thermal efficiency. Eventually, water recovery, water quality, as well as specific energy consumption (SEC) are evaluated for the whole system. Based on different configurations of the hybrid ZLD system, the proposed design can achieve water recovery between 40 and 93% with SEC range of 28-114 kWh/m3. Results also showed that the produced water quality exceeds drinkable water standards ( 500mg/L). This work has provided great evidence in the practicality of ZLD/MLD systems for sustainable brine management.

Comprehensive experimental and theoretical investigations on the effect of microbubble two-phase flow on the performance of direct-contact membrane distillation

This study provides a comprehensive and systematic overview of the application of gas-liquid two-phase flow with microbubbles in the feed stream to improve heat and mass transfer in direct-contact membrane distillation (DCMD) processes for seawater desalination. A swirl-flow-type microbubble generator (MBG) was installed at the feed-side inlet of the DCMD module to investigate its effect on transmembrane flux. The maximum improvement in the MBG-assisted DCMD permeation flux was found to be approximately 18% at a lower feed temperature (40 °C) and optimal air flow rate (50 cc/min), and an optimal MBG geometry comprising a swirler, a nozzle tip of diameter 2 mm, and a diffuser at an angle of 30°. The results were observed to be related to the number density of microbubbles less than 100 µm in size, which plays an important role in improving heat and mass transfer in two-phase flow. In addition, the simulation results based on conventional heat transfer correlations of bubbly flow underestimated the experimental results. Therefore, this study also aims to propose and verify a new two-phase flow heat transfer correlation. The proposed correlation considers the effects of bubble size distribution to accurately predict the performance of MBG-assisted DCMD processes.

Hybrid nanofluid filtered concentrating photovoltaic/thermal-direct contact membrane distillation system for co-production of electricity and freshwater

The membrane distillation (MD) systems integrated with photovoltaic/thermal (PV/T) collectors can hardly obtain large permeate flux and favorable electrical performance simultaneously. Therefore, an innovative hybrid nanofluid filtered concentrating PV/T (CPV/T)-direct contact membrane distillation (DCMD) system is proposed. The novelty is that Ag/saline water nanofluid acts as a multifunctional fluid capable of absorbing solar spectrum, which cannot be efficiently utilized by solar cells, and converting it into heat to directly produce water in DCMD. Mathematical models of both nanofluid filtered CPV/T and DCMD are established to evaluate the system performance. Results indicated that nearly 70% of solar radiation is converted into useful energy by the nanofluid filtered CPV/T device. Besides, detailed parametric studies are performed to explore how the system performance is affected by the operation and design parameters. It is found that Ag/saline water nanofluid can be heated to 353 K for desalination, at which, the DCMD module can produce 26.4 kg m−2 h−1 of fresh water. Meanwhile, the electrical efficiency of the system is 12.2% and the equivalent system thermal efficiency can reach a maximum of 71.3%. Furthermore, we demonstrate that the use of Ag/saline water nanofluid as feed stream can mitigate the temperature polarization and improve DCMD performance.

Solar Energy Driven Membrane Desalination: Experimental Heat Transfer Analysis

In the direct contact membrane distillation (DCMD) system, the temperature polarization due to boundary layer formation limits the system performance. This study presents the experimental results and heat transfer analysis of a DCMD module coupled with a salinity gradient solar pond (SGSP) under three different flow channel configurations. In the first case, the feed and permeate channels were both empty, while in the next two cases, the feed and permeate channels were filled with a porous spacer material. Two different spacer geometries are examined: 1.5 mm thick with a filament angle of 65°, and 2 mm thick with a filament angle of 90°. The study considers only the heat transfer due to conduction by replacing the hydrophobic membrane normally used in a DCMD module with a thin polypropylene sheet so that no mass transfer can occur between the feed and permeate channels. The Reynolds number for all three configurations was found to be between 1000 and 2000, indicating the flow regime was laminar. The flow rate through both the feed and permeate sides was the same, and experiments were conducted for flow rates of 5 L/min and 3 L/min. It has been found that the highest overall heat transfer coefficient was obtained with the spacer of 2 mm thickness and filament angle of 90°.

Computational fluid dynamics simulations of solar-assisted, spacer-filled direct contact membrane distillation: Seeking performance improvement

Significant downstream performance reduction and concentration polarisation reduce direct contact membrane distillation (DCMD) efficiency. These challenges are not well researched since they are difficult to implement experimentally and numerically. Hence, this study examined the impact of solar absorbers and different spacer filaments upon DCMD performance. A 2D computational fluid dynamics model that considered simultaneous mass and heat transfer across the membrane and throughout the channels was developed to simulate water flux in DCMD modules under the use of solar absorbers and spacer filaments of various designs. The simulation outcomes were in excellent agreement with experimental results provided by two different studies, with the assisted solar absorber module and the spacer-filled module deviating <5 % and 3 % from the experimental results, respectively. A module equipped with the solar absorber membrane enhanced the DCMD performance better than a module with a solar absorber plate. The results also illustrated that a module with cylindrical detached spacer filaments improved DCMD performance more than a module with rectangular and semicircular attached spacers. Finally, it was shown that a module equipped with an integrated developed spacer and solar absorber membrane significantly enhanced water flux and both concentration and temperature polarisations, specifically when the inlet velocity was increased. Water flux and the temperature polarisation coefficient increased by 220 % in the integrated module, and the concentration polarisation coefficient decreased by 10 %. Moreover, the developed module resulted in a substantial increase in downstream performance.

Numerical and experimental investigation of thermoelectric materials in direct contact membrane distillation

The high energy penalty associated with the direct contact membrane distillation (DCMD) technology is a major challenge precluding its commercial deployment. Although DCMD requires less aggressive operating conditions than reverse osmosis (∼1 bar vs 55 bar), its throughput is way lower (∼10 L/m2-h vs 70 L/m2-h) and its energy consumption is more (∼1700 kWh/m3 vs 7 kWh/m3). One approach to improve these metrics is the use of direct thermal energy sources inside the membrane module. A Peltier device installed inside the DCMD module to provide the needed thermal energy can reduce heat exchanger losses to near zero as the Peltier device is sandwiched between the hot feed and cold permeate in a thermoelectric-DCMD system. Previous experimental results have indicated gains in performance metrics using this approach. Given these results, robust high-fidelity computational fluid dynamics (CFD) simulations validated with experiments are developed to predict the performance of the thermoelectric-DCMD module whilst conducting a parametric analysis of the operational conditions and geometrical configurations. The Peltier device’s heat transfer rates are predicted from a simple analysis using the manufacturer’s performance datasheet and used as boundary conditions for the CFD simulations.

Effect of membrane surface wetting on the performance of direct contact membrane distillation for seawater desalination

Membrane distillation (MD) is a standalone process to generate fresh water and is especially attractive when low-grade waste heat or renewable thermal energy is available. Surface wetting hinders the commercialization of Membrane distillation (MD) technology by deteriorating the permeate quality, thermal efficiency, and transmembrane flux. There is still a lack of understanding on how and to what extent the partially wetted membranes affect the performance of direct contact MD (DCMD) systems. It is of great importance to optimize the operating conditions under such conditions. The DCMD performance was addressed in the literature by considering non-wetted or fully wetted membranes. This study for the first time proposes a computational model to investigate the effect of membrane surface wetting ratio (R = L wet /L total ) on the transmembrane (J) and thermal efficiency (η) of a DCMD module. Parametric and sensitivity analyses were performed to display the effect of system parameters (feed temperature, feed velocity, permeate temperature, permeate velocity, membrane thickness, and membrane surface wetting ratio) on the Key Performance Indicators (KPIs) of the DCMD module. The obtained results indicate that the permeate side temperature has more effect (more than twice) on KPIs in wetted membranes (∼30% and ∼15% rise in J and η) compared to the non-wetted ones (∼15% and ∼5% enhancement in J and η), and the negative effect of membrane surface wetting could be minimized by adjusting the permeate side operational conditions. The effect of membrane wetting ratio on the performance of the DCMD module in thin membranes (≤0.2 mm) and thick membranes (≥0.25 mm) strongly depends on the permeate and feed temperatures. The parametric and sensitivity analysis performed in this study will be beneficial to optimizing the operational conditions of MD systems for maximizing their performance and could serve as valuable guidelines in the development of efficient water desalination systems. • A numerical model was developed to investigate the performance of the partially wetted DCMD module. • The effect of surface wetting ratio (R = L wet /L total ) was investigated using a parametric study. • A sensitivity analysis was performed to quantify the relative impact of operating conditions. • The effect of permeate properties is more dominant for surface wetted membranes. • Permeate and feed temperatures dramatically alter the R ratio effect in thin and thick membranes, respectively.

Sonicated direct contact membrane distillation: Influence of sonication parameters

The effect of sonication on the performance of the direct contact membrane distillation (DCMD) is studied in this work. A sonicated DCMD module equipped with a commercial PTFE membrane is built and tested in an immersed temperature-controlled water bath. High fidelity transient and thermally conjugated CFD model is also developed for the DCMD unit accounting for the sonication. Flow metrics, including temperature polarization coefficient (TPC), heat- and mass- flux under different sonication frequencies and amplitudes were evaluated. Parametric study showed a clear improvement in the DCMD system metrics and more pronounced at higher temperature. It was observed that the application of sonication impedes the development of both kinetic and thermal boundary layers due to ultrasonic-induced mixing. The results showed that the application of sonication to DCMD at the intensity of 4525 W/m2 resulted in increase of mass flux from 4.25 L/m2.h to 8.1 L/m2.h corresponding to 90% increase. It is economically unviable due to electrical power demand, i.e., ~ 35 kW.h/m3 (~$5/m3), but can be feasible below 474 W/m2 demanding 3 kW.h/m3 (nearly $0.42/m3). It was also observed that an interesting equivalence exists between the effect of acoustic amplitude and acoustic frequency on the performance of DCMD. For example, sonication at 1 kHz with 0.5 μm amplitude produced similar results, in terms of mass flux and TPC, as sonication at 5 kHz with 0.1 μm amplitude.

Computational model and simulation of DCMD desalination systems with heat recovery

Desalination technologies can help mitigating the water scarcity problem facing humankind. Membrane distillation (MD) is a desalination technology attractive for its adequacy to use low-grade waste heat or renewable energy sources. This work addresses the modelling of a Direct Contact Membrane Distillation (DCMD) plant with heat recovery. The desalination unit comprises several DCMD modules, each of a shell and hollow fibre tube bundle type. A multiscale approach is needed to address the problem. It includes modelling of the vapour flux through the micro/nano porous membrane, models for the mass and energy conservation at the scale of a single DCMD module, and thermodynamic modelling at the scale of the desalination plant. Analytical solutions of the mass and heat transfer across the membranes are combined with finite volume discretised equations describing heat and mass conservation for the feed and permeate streams. An iterative scheme is devised to solve the model equations in order to determine the flow and temperature variables in the DCMD module. The computational model predictions show good agreement with experimental data available in the literature. Finally, the methodology presented herein is used to simulate the performance of a DCMD desalination plant with heat recovery.

Heat-Integration of Solar-Heated Membrane Distillation and Fuel Cell for Desalination System Based on the Dynamic Optimization Approach

The heat integration feasibility of the proton exchange membrane fuel cell (PEMFC) coupled with the solar-heated direct contact membrane distillation (DCMD) module is evaluated in this study. The additional waste heat from the PEMFC increases the DCMD system’s ability to produce fresh water and electricity. Two systems units to be assessed mainly include a flat plate solar collector, a heat storage tank with an internal heat exchanger, and the DCMD module with and without the PEMFC module. The importance of daily operation continuity is emphasized through a preliminary dynamic simulation and proper sizing of the solar-heated DCMD distillation. Sensitivity analysis is implemented to analyze the relationship between the essential variables and the daily freshwater production. The design variables of both configurations are rigorously optimized in terms of minimum unit production cost (UPC). The proposed heat integration feasibility is evaluated to obtain critical insights on the design strategy of the hybrid systems.

Study of spacer structure on the enhancement of heat and mass transfer in direct contact membrane distillation modules

Membrane distillation technology, which combines the membrane process with evaporation, has many benefits, including operation at atmospheric pressure, simple configuration, usage of low-grade heat sources, etc. In the current work, a new type of curved spacers was invented for the direction contact membrane distillation. A three dimensional numerical method was established, which coupled the transmembrane mass transfer with fluid flow in the channel by adding user-defined functions. Different distances between the spacer and the top wall, defined as d, were studied. 4 straight spacers (Sd0, Sd1.5, Sd3,Sd4.5) and 4 curved spacer (Cd0, Cd1.5,Cd3,Cd4.5) were investigated in detail. The curved spacers performed better in mixing and enhancing permeate flux than the straight spacers. The averaged performance enhancement factor (αc) was introduced to evaluate the performance enhancement of the curved spacers compared to the straight spacers. When d was 1.5 mm, αc was 1.33, which was the maximal among the studied cases. The curved spacer performed better in terms of heat transfer, but at a considerable cost in terms of pressure drop. The comprehensive evaluation index (Pec) was proposed to evaluate the overall performance of heat transfer and pressure drop. Pec varied with Reynolds number (Re) and spacer configurations. When Re was in 200 - 550, the maximal value of Pec was achieved by Cd1.5 spacer. When Re was in 500 - 920, the maximal value of Pec was achieved by Cd3 spacer.

Development of machine learning and stepwise mechanistic models for performance prediction of direct contact membrane distillation module- A comparative study

The development of accurate and fast modeling tools to predict the performance of direct contact membrane distillation (DCMD) modules can result in their performance improvement. Conventional models ignoring the variations of operational parameters along the membrane's length may cause inaccuracy. Moreover, models considering these variations are often complex and computationally expensive. To propose an accurate and fast modeling tool, the possibility of using machine learning models for the performance prediction of the DCMD module has been studied for the first time in this study. The robustness of three machine learning models (ANN, SVR, and RF) has been thoroughly compared with the stepwise mechanistic modeling approach in terms of models’ accuracy, trend predictability, and computational time. The results show that ANN and SVR models exhibit an enhanced performance over the mechanistic model, possessing a MAPEtest of 3.46% and 4.78% as compared to the mechanistic model with a MAPEtest of 7.31%. Further, compared to the mechanistic model, the machine learning models have the privilege of simplicity, enhanced accuracy, and significantly lowered computational time. The feature importance analysis also showed that the feed flow temperature is the most influencing parameter on permeate flux in the DCMD system.

Thermo-economical and environmental analyses of a Direct Contact Membrane Distillation (DCMD) performance

As a promising desalination technology, membrane distillation is expected to be developed continuously due to its wide application and low sensitivity to salt concentration in the feed stream. The performance of a direct contact membrane distillation (DCMD) has been investigated based on energy, exergy, economic and environmental analyses in the current study. Performance evaluation of a DCMD unit is comprehensively carried out in terms of the operating parameters, including gained output ratio (GOR), specific energy consumption (SEC), concentration polarization coefficient, temperature polarization coefficient, recovery ratio, exergy efficiency, specific exergy consumption, carbon cost, and cost of water. This study presents one of the most comprehensive investigations on DCMD behavior performed in the available literature by developing a robust computational code in MATLAB software. In addition to pervasive energy and exergy analyses, a complete economic study for freshwater production plants with different capacities based on heat extracted through geothermal energy and wasted heat, and investigation of the environmental effects of using heat recovery MD cycle to reduce greenhouse gas emissions, are presented. The results showed that the utilization of heat recovery heat exchangers significantly improves the system's operating conditions. The exergy efficiency of the DCMD module with and without heat recovery is 38.23% and 56.71%, respectively, indicating a 48.34% improvement in exergy efficiency. Finally, the economic and environmental analyses showed that for a 15000 m3/day desalination unit, recovering the wasted heat could reduce the distillate water production cost down to 1.31 $/m3 while CO2 emissions and energy consumption also decreased by 59%.

Numerical Study of the Effect of Wavy Corrugations in Direct Contact Membrane Distillation Modules

Numerical Study of the Effect of Wavy Corrugations in Direct Contact Membrane Distillation Modules