Membrane Distillation Configurations Research Articles

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.

Performance of sweeping gas membrane distillation for treating produced water: Modeling and experiments

Membrane distillation (MD) has shown promise for desalination of produced water (PW), but there are very few studies reported on the performance of MD with real PW and the characteristics of the fouling and scaling in MD. Among different MD configurations, sweeping gas MD (SGMD) is the least studied configuration. Here we report the application of SGMD for treating high-salinity PW (123 g L−1). The SGMD operating conditions were first carefully optimized through pure water MD experiments by varying operating conditions. The flux evaluation was accompanied by a detailed Aspen Custom Modeler simulation based on the electrolyte-NRTL thermodynamic model to better understand the heat and mass transfer in the feed and permeate channel. Supported by experiments, modeling results confirmed the partial condensation of the vapor in the permeate channel. Additionally, it was concluded that the saturation of the sweeping gas is controlling the overall mass transfer through the membrane. Selected membranes and optimized conditions were then used to conduct short-term and long-term performance tests with PW. In long-term MD test, the concentration of the PW increased by a factor of two (50% water recovery), while the membrane exhibited small flux decline. Elemental analysis showed that strontium and sodium were the major scalant compounds.

A Facile Approach of Thin Film Coating Consisted of Hydrophobic Titanium Dioxide over Polypropylene Membrane for Membrane Distillation

In this work, the hydrophobic modification of TiO2 nanoparticles (HTiO2) was carried out by reacting with dodecylphosphonic acid (DDPA) and hexylamine solution. A facile approach of the self-assembly technique was used for the coating of hydrophobic HTiO2 layer over the microporous polypropylene (PP) membrane. The self-assembled layer was formed between the interface of trimesoyl chloride (TMC) (in hexane) and trimethylamine (in water) solutions. The high porosity for the coated membranes ascribed to the selfassembled trimesic acid (TMA) layer and its potential to generate open and loosely packed, two-dimensional hydrogen-bond networks on the membrane surface. The dispersion of HTiO2 was accomplished in the TMC in hexane leading hydrophobic and porous surfaces than the neat PP membrane. The initial average pore size of the PP membrane was reduced from 0.4 μm to 0.2 μm with a coating of 2.0 wt% of HTiO2. The new membranes showed high reliability, high rejection, and water flux during the real seawater desalination tested in direct contact membrane distillation (DCMD) configuration. The maximum transmembrane permeate flux of 45.4 kg/m2h with >98% salt rejection was obtained for the coating layer with 2.0 wt% HTiO2 at 80oC demonstrating the future potential application towards seawater desalination.

Economic performance of membrane distillation configurations in optimal solar thermal desalination systems

In this study we provide a comprehensive evaluation of the economic performance and viability of solar membrane distillation (MD). To achieve this goal, process models based on mass and energy balances were used to find the minimum cost of water in MD systems. Three MD configurations: direct contact, sweeping gas, and vacuum MD, were compared in terms of economic cost and energy requirements in optimized, solar-driven desalination systems constrained to produce 10 m3 d−1 of distillate from 3.5% or 15% salinity water. Simulation results were used to calculate the water production cost as a function of 13 decision variables, including equipment size and operational variables. Non-linear optimization was performed using the particle swarm algorithm to minimize water production costs and identify optimal values for all decision variables. Results indicate that vacuum MD outperforms alternative MD configurations both economically and energetically, desalting water at a cost of less than $15 per cubic meter of product water (both initial salt levels). The highest fraction of total cost for all configurations at each salinity level was attributed to the solar thermal collectors—approximately 25% of the total present value cost. Storing energy in any form was economically unfavorable; the optimization scheme selected the smallest battery and hot water tank size allowed. Direct contact MD consumed significantly more energy (primarily thermal) than other MD forms, leading to higher system economic costs as well.

Electrothermally Driven Membrane Distillation for Low-Energy Consumption and Wetting Mitigation.

Membrane distillation (MD) is a promising alternative approach for desalination, especially for high-salinity brines. Its application has been limited by its high operational cost because of the energy consumption required for hydraulic circulation and heating the entire circulating feed. Localized heating of the feed by Joule heating diminishes energy consumption, but the potential charging on the electrothermal material surface causes water splitting and membrane degradation in high-salinity environments. Herein, a novel reverse Joule-heating air gap MD method was designed in which an electrothermal material was placed at the air gap, isolating itself from saline water. Even though the Joule-heating layer was at the air gap side, 90.56% of heat flowed into the saline water for heating the feed. The opposite temperature gradient in the membrane matrix as opposed to conventional MD-mitigated membrane wetting was caused by capillary condensation. This novel electrothermal-driven MD configuration is worthy to be introduced into applications.

Mass transfer during membrane distillation treatment of wastewater from hot-dip galvanization

Hot-dip galvanized products are widely applied in many fields in China. The treatment of the wastewater containing acid and many metal ions from the hot-dip galvanizing industry is an important environmental issue. In this paper, we experimentally investigated the membrane distillation technique to treat the wastewater from the hot-dip galvanizing industry, and determined the factors that affect the membrane distillation performance. The transport behavior of multi-components (i.e. HCl and H2O) through the porous membranes was studied in detail. The results indicate that it is promising to use the air gap membrane distillation configuration to treat the wastewater and recover hydrochloric acid. The increase of the feed temperature leads to the increase of the permeate flux in general. The presence of FeCl2 and the acidity of the feed affects the acidity of the permeate. FeCl2 in the feed can enhance the acidity of the permeate significantly, especially when the acidity of the feed is low. The adding of 9.5 wt% FeCl2 in the feed can increase the acidity of the permeate by about 10 times when the acidity of the feed is 6 wt%. The overall metal ion (Fe2+) rejection ratio is higher than 99.99% by membrane distillation technique.

Optimization of module length and number of stages in a multistage membrane distillation configuration

ABSTRACT Multistage membrane distillation units connected in series for water desalination is considered. The influence of the number of stages and the module length per stage on the overall economics is investigated via simulations using a validated model. Depending on the degree of feed salinity, a suboptimal condition occurs at a structure size of 40 ~ 35 stages with an average module length of 15 ~ 12 m. The corresponding specific operating and fixed costs are $8 ~ 12.5/m3 and $0.97~−.8/m3, respectively. For a high salinity feed, the optimum specific operating cost increases due to reduced mass production. Membrane porosity and thickness showed a pronounced effect on cascade structure economics. The specific operating cost can be further reduced by an average of 60% using permeate heat recovery systems.

Evaluation of Permeate Quality in Pilot Scale Membrane Distillation Systems.

In this work, the salinity of permeate obtained with membrane distillation (MD) in pilot scale systems was analyzed. Experiments were performed with three different spiral-wound commercial modules, one from Solar Spring with 10 m2 surface membrane area and two from Aquastill with 7.2 and 24 m2. Intermittent operation meant that high permeate conductivity was measured in the beginning of each experiment, which was gradually decreasing until reaching a constant value (3–143 µS·cm−1 for seawater feed). The final quality reached did not depend on operating conditions, only the time it took to reach it. This can be because the permeate flux dilutes the minimal feed leak taking place through pinholes in the membranes. Larger feed leak through the membrane was observed when operating in vacuum-enhanced air-gap MD configuration (V-AGMD), which is compatible with this explanation. However, for the increase of feed leak with salinity (up to 1.8 M), a conclusive explanation cannot be given. Pore wetting due to crystallization is discarded because the high permeate quality was recovered after washing with distilled water. More studies at higher salinities and also at membrane level are required to investigate this.

Effect of Adding Functionalized Graphene on the Performance of PVDF Membrane in Direct Contact Membrane Distillation

In this paper, the effect of adding modified graphene nanoplatelets (MGNPs) as a filler on the characteristics and performance of Polyvinylidene fluoride (PVDF) membrane in a direct contact membrane distillation (DCMD) configuration was investigated. Both pure PVDF and PVDF/MGNPs composite (2%) membranes were fabricated by using electrospinning technique. The fabricated membranes were characterized using different analyses techniques such as SEM imaging, XRD analysis, static water contact angle as well as membrane porosity and liquid enter pressure measurements. Also, the average fiber diameter and the average membrane pore diameter were estimated using ImageJ software. The prepared PVDF/MGNPs composite membrane exhibited lower fiber diameter by about 5.7%, whereas the contact angle increased by 10 and liquid entry pressure increased by 11.7%. The membrane also showed an enhanced flux that reached about 19.8 kg/m2∙h at feed inlet temperature of 65°C, feed flow rate of 30 l/h. and feed inlet concentration of 10000 ppm. This represents about 13.46% improvement over the pure PVDF membrane at the same conditions. The produced membrane presents a viable alternative to commercial MD membranes.

Open-source industrial-scale module simulation: Paving the way towards the right configuration choice for membrane distillation

Understanding the impact of configuration selection of industrial-scale membrane distillation (MD) is critical for a productive and energy-efficient operation of this emerging process for desalination. However, it is rarely considered even as new high flux membranes and wider applications are developed. In this context, three open-source simulators were developed on the Matlab GUI platform for the performance prediction of industrial-size direct contact membrane distillation (DCMD), submerged vacuum membrane distillation (S-VMD), and cross-flow vacuum membrane distillation (X-VMD). Using laboratory-scale experimental results as simulation inputs, the developed simulators were able to predict large-scale MD performance. Furthermore, design considerations on appropriate module scale-up for all three configurations were demonstrated. More importantly, the configuration that shows the optimal mass and heat transfer behaviour was revealed through the performance comparisons across different full-size MD configurations. In addition, these simulators are open-source allowing researchers to use these tools for the development of specific scale-up strategies of their own MD membranes — a critical step towards commercialisation.

Development of a novel perfluoropolyether (PFPE) hydrophobic/hydrophilic coated membranes for water treatment

A novel method to prepare hydrophilic/hydrophobic membranes via dip-coating and UV in situ polymerization was presented. Fluorolink® MD700, a UV-curable perfluoropolyether (PFPE) hydrophobic compound, was employed as surface modifier via coating technique on two different commercial hydrophilic membranes with different pore size (0.1-0.2-0.45 μm): polyamide (PA), and polyethersulfone (PES). The influence of starting membrane material (PA-PES) was studied and all coated membranes were characterized in terms of surface morphology, water and oil contact angle, porosity, pore size, liquid entry pressure and mechanical tests. The coating resistance and stability in time with salty solution NaCl 0.6 M and chemicals cleaning agents, such as KMnO4 0.1 wt%, Hypochlorite NaClO pH 4.25, HCl pH 2.5, NaOH pH 11.5, were evaluated. Membranes performance was tested in direct contact membrane distillation (DCMD) configuration, using deionized water and salty solution 0.6 M (NaCl) as feed. The effect of the starting material on the permeate fluxes was investigated. The measured permeate water fluxes ranged between 3 and 22 kg/m2h and 2.9–19.5 kg/m2 h for PA and PES respectively, at different feed temperatures (40–60 °C). Both flux (∼8.3 kg/m2 h) and salt rejection (∼99.95%) were kept constant during a prolonged test (158 h) carried out at 50 °C on the coated PA 0.45 μm membrane.

Influence of Hydraulic Pressure on Performance Deterioration of Direct Contact Membrane Distillation (DCMD) Process.

Direct contact membrane distillation (DCMD) is a membrane distillation (MD) configuration where feed and distillate directly contact with a hydrophobic membrane. Depending on its operating conditions, the hydraulic pressures of the feed and distillate may be different, leading to adverse effects on the performance of the DCMD process. Nevertheless, little information is available on how hydraulic pressure affects the efficiency of DCMD. Accordingly, this paper investigates the effect of external hydraulic pressure on the process efficiency of DCMD. Gas permeabilities of MD membranes were measured to analyze the effect of membrane compaction by external pressure. Mass transfer coefficients were calculated using experimental data to quantitatively explain the pressure effect. Experiments were also carried out using a laboratory-scale DCMD set-up. After applying the pressure, the cross-sections and surfaces of the membranes were examined using a scanning electron microscope (SEM). Results showed that the membrane structural parameters such as porosity and thickness were changed under relatively high pressure conditions (>30 kPa), leading to reduction in flux. The mass transfer coefficients were also significantly influenced by the hydraulic pressure. Moreover, local wetting of the membranes were observed even below the liquid entry pressure (LEP), which decreased the rejection of salts. These results suggest that the control of hydraulic pressure is important for efficient operation of DCMD process.

Heat and Mass Transport in Modeling Membrane Distillation Configurations: A Review

Identification and mitigation of challenges associated with membrane distillation (MD) modeling are very crucial to the applicability of MD technology in the industry. Several research studies have been carried out on direct contact membrane distillation (DCMD) modeling because of its simplicity, while other MD configurations have gained little attention. Most studies conducted on MD modeling were achieved based on uniform membrane pore size and pore size distribution assumption. This study exploits the homogeneity of these assumptions to conduct a modeling review for temperature polarization (TP) and concentration polarization (CP), as they apply to MD configurations. TP and CP phenomena have been identified as two of the main challenges to advance MD modeling for further development of MD technology. Their impact is detailed in the heat and mass transfer mechanisms discussed. Thermal conductivity of common hydrophobic commercial membrane materials at different temperatures are presented in this study. The use of optimal operating flow rate, suitable membranes and proper module design are recommended as viable solutions to reduce the effects of TP and CP on permeate flux decay.

How to select the optimal membrane distillation system for industrial applications

Despite increasing academic interest in membrane distillation systems, industry adoption of the technology remains low. We propose a simple yet comprehensive method for selecting the optimal membrane distillation design for any industrial process. This flexible, system level analysis procedure yields a holistic view of the technology, which could help identify promising industries for commercial MD systems. The method consists of comparing membrane distillation designs on the basis of their total water production cost. Membrane distillation configuration, module type, heat exchange arrangement, operating conditions, and membrane properties all influence the total cost of the system. To illustrate our methodology, we apply the analysis procedure to a case study, optimizing the MD system design for an MD unit coupled to a condenser of a steam power plant. The total water production cost for the optimized system is $2.11 per cubic meter of permeate with current commercial membranes or $1.58/m3 with improved membrane material.

Analysis of process intensification and performance assessment for fermentative continuous production of bioethanol in a multi-staged membrane-integrated bioreactor system

The work analyses process intensification in production of fermentative biofuel (bioethanol) in a multi-staged membrane integrated bioreactor system from sugarcane juice (SCJ) using Saccharomyces cerevisiae (NCIM 3205). Membrane based clarification, sterilization and concentration of sugar cane juice could replace several energy-intensive and polluting steps of conventional production schemes. Integration of traditional bioreactor with membrane-based devices for downstream purification of bioethanol enabled the system for recycling of cells and residual sugars to the fermentation unit through largely fouling free cross flow microfiltration and nanofiltration membrane modules. The new process design is characterized by high yield (0.48 g/g), productivity (14.6 g/L/h) and concentration capability (97.6 g/L) using concentrated SCJ of 22 wt% under high cell density in a flexible, compact, eco-friendly and modular plant. The final membrane distillation stage concentrated ∼98% pure ethanol in a solar-driven direct contact membrane distillation configuration using PTFE/PET hydrophobic membrane with enhanced flux with vis-à-vis the existing processes. A rectangular flat sheet cross flow membrane module in counter-current flow mode of hot and cold streams was employed. The membrane-based system achieves quite high levels of process intensification essential to sustainable operation at industrial scale. The achieved process intensification in the new system has been analysed vis-a-vis the existing conventional systems in terms of eco-friendliness, flexibility, cost of equipment and production, consumption of energy and material, profitability, E-factor, atom efficiency and business sustainability.

An industrial perspective on membrane distillation processes

AbstractProcess intensification has led to significant developments in both distillation and membrane technology. Membrane distillation (MD) is an emerging technology for fluid separations that are typically performed by conventional separation processes, such as distillation or reverse osmosis (e.g. water desalination, or water removal). Compared with other membrane technologies, the driving force in MD is the difference in vapor pressure across the hydrophobic membrane, rather than the total pressure. MD can be a cost effective separation process, especially when more sustainable alternative sources of energy (e.g. geothermal and solar) or waste heat sources are used. Many review papers on MD are available in the open literature, but most of them focus on the membrane characteristics (e.g. material aspects).This industrial perspective paper assesses the MD technology and reports on relevant issues by offering a concise overview of MD technology, addressing different MD configurations, current major applications, operating parameters and their effect on the MD process, commercially available membranes, as well as cost estimations to determine the feasibility of MD processes. While successfully applied in desalination and a few other niche applications, MD has failed to make a strong industrial impact in other areas still dominated by distillation. A key message is that membrane distillation is still a growing technology for separation and purification processes, but it needs further exploration and optimization to become a mature technology applicable to more industrial processes in the chemical process industry. © 2018 Society of Chemical Industry

Experimental and numerical evaluation of membrane distillation module for oxygen-18 separation

A novel module design for membrane distillation is attempted in this work. For this purpose, three-dimensional numerical simulations were carried out to investigate the effect of baffle configuration on the performance of three modules for liquid gap membrane distillation. Pure water was considered as feed and the simulations were performed based on dusty gas model, which describes the diffusion of vapor through the air entrapped in the membrane pores. The simulation predicted 15–25% improvement for vapor flux by utilizing triangular baffles in the module compared to a module without baffle. In particular, the newly designed triangular baffle provided uniform temperature distribution over the membrane surface, thus contributing to the improvement of vapor flux. The simulated module was constructed in order to conduct 18O isotope water separation from pure water. It is worth noting that the novel module design can be used in any membrane distillation configurations.

Economic modelling and model-based process optimization of membrane distillation

Membrane distillation (MD) is a thermally driven separation process, operated at moderate temperature, allowing for the use of waste heat as driving force. While the literature is saturated with lab-scale models, almost none exist for designing a complete MD system. Based on previously published and thoroughly validated models, this work demonstrates a graphical user interface tool, capable of designing a complete membrane distillation system, including all of the supporting equipment and able to predict the price of the obtained distillate for the most commonly studied and used membrane distillation configurations. The user can also optimize the module geometry based on specific requirements. Four different case studies are discussed, ranging from 2 to 1000 m3 of distillate per day, with final brine salinity up to 20 wt%, feed temperature up to 80 ° C. The optimal system design for each case is demonstrated. The distillate price varied from 25 €/m3 of distillate for the smallest scale to 2.1 €/m3 for the largest scale. Finally, the reader is presented with a simplified cost model that can be used to quickly estimate the price of the produced distillate at different production scales and concentration factors.

Testing of three different PVDF membranes in membrane assisted-crystallization process: Influence of membrane structural-properties on process performance

Throughout the world an intensified use of desalination for reducing current and future water scarcity is observed. Despite the enormous benefits of desalination processes, improvements are still required in terms of lower desalted water cost, higher productivity, better water quality and enhanced eco-sustainability of the desalination process. Environmentally sensitive concentrate management is today recognized as a significant hurdle to extensive implementation of desalination technologies. Membrane assisted crystallization (MCr) offers a suitable solution for redesigning desalination process through the treatment of brine streams for the production of salts and water. In this work, three different Hyflon/polyvinylidene fluoride (PVDF) composite membranes were tested in MCr operations. The coating of PVDF membranes with Hyflon allowed increasing membrane surface hydrophobicity. The latter is the membrane essential characteristic when MCr is implemented according direct contact membrane distillation configuration like in the present work. The aim was to analyze the MCr performance in terms of membrane chemical–physical properties and process operating conditions.

Comparative studies of different membrane distillation configurations and membranes for potential use on board cruise vessels

Cruise tourism industry is expanding worldwide. An average cruise ship produces over 1000m3 per day of fresh water. To date, multi-stage flash distillation (MSF) and seawater reverse osmosis (SWRO) are the most used desalination technologies on board. However, their application has some drawbacks: a great volume needed on board (MSF), increases of fuel consumptions (SWRO), carbon dioxide emissions and total costs. In this way, membrane distillation (MD) is proposed to overcome limitations of existing conventional technologies. Different configurations (DCMD, AGMD and WGMD) and two membranes (0.45 and 0.20μm) were tested and compared with each other in order to study potential use of MD on board cruise ships, taking advantage of waste heat generated in the engine cooling system. Operational conditions were established based on an approximation of real conditions encountered in a cruise vessel. Results obtained for simulated and real seawater tests showed advantages for WGMD configuration using PTFE 0.45μm pore size membrane, reaching a high permeate quality (99.99% of salt rejection) and average fluxes around 13Lm−2h−1, being these similar to those produced by SWRO. In conclusion, use of MD, in the cruise industry, can be a sustainable water production choice that should be considered.