Distillate Flux Research Articles

Innovative hydrophobic/hydrophilic perfluoropolyether (PFPE)/polyvinylidene fluoride (PVDF) composite membrane for vacuum membrane distillation

Though membrane distillation (MD) has gained more and more attention in the field of desalination, the wetting phenomenon was still a non-negligible problem. In this work, a method combined dip-coating and UV in situ polymerization for preparing hydrophobic/hydrophilic perfluoropolyether (PFPE)/polyvinylidene fluoride composite membranes. This composite membrane consisted of a top thin hydrophobic coating layer and hydrophilic substrate membrane. In terms of anti-wetting properties, contact angle and liquid entry pressure of all composite membranes (except for those based on 0.45 μm) exceeded 160° and 0.3 MPa, respectively. In particular, the desalination performance was tested in vacuum membrane distillation tests by feeding 3.5% (mass) saline solution (NaCl) at 60 °C. The composite membranes with larger support pore size and lower PFPE content had higher membrane distillation flux. And for stability tests (testing the 0.22 μm membrane coated by 5% (mass) PFPE), the highest MD flux 29.08 kg·m−2·h−1 and stable salt rejection (over 99.99%) during the period. Except that, the effects of coating material concentration and pore sizes of substrate membrane were also investigated for surface morphology and topography, porosity, mechanical strength and pore size characteristics. This work provided a simple and effective alternative to prepare excellent hydrophobic composite membranes for MD applications.

Boron rejection from aqueous solution and wastewater by direct contact membrane distillation

Boron is widely used in various areas of modern technology. Due to the environmental problems arising during the production and use, the studies on the removal and recovery of boron from wastewater have been increased recently. Membrane distillation (MD) system is smaller in size with respect to other common distillation systems and needs lower operating temperatures. In addition, the equipment costs are reduced and the safety of the process increases since it operates at lower pressures. Moreover, the membrane distillation process can remove pollutants from water without using chemicals. In this study, boron rejection from aqueous solutions and wastewater was investigated by using direct contact membrane distillation (DCMD) system where both surfaces of a porous hydrophobic membrane were in contact with liquid streams. The effects of various parameters (pH, feed concentration, feed temperature, etc.) on boron rejection were investigated and the highest boron rejection was found to be 50 % when pH=10 at 50 °C and with feeding by a pump of 54 rpm. According to the test results of wastewater from Kırka Borax treatment plants, the mean distillate fluxes were found as 13, 16 and 14 L m-2 h-1 at the feed temperatures of 30, 40 and 50 °C, respectively. The boron removal percentages were found to be 47, 64 and 48 % at 30, 40 and 50 °C, respectively. It was observed in the XRD spectra that the crystals in wastewater mainly consist of Na2B(OH)4Cl and Mg2B2O5 structures.

Effective treatment of leachate concentrate using membrane distillation coupled with electrochemical oxidation

The performance of sequential electrochemical oxidation (EO) and membrane distillation (MD) process was investigated for treating leachate concentrate with the characteristics of salinity and biorefractory organics. EO was achieved by direct and indirect oxidation on the Ti/PbO2 or Ti/Ru-Ir anode, by comparing the performance under different variations. Results showed that at an applied current density of 100 mA/cm2, operation time of 5 h and the choice of the Ti/PbO2 anode, the EO process could minimize the organic fouling by decomposing the larger molecular humic-acid that the removal efficiency was 83.4% for TOC. MD was used for treating the EO-pretreated concentrate due to its high rejection of salt and non-volatile components when treating salinity water. Flux results showed that the sequential system made the process resistant toward the sharp flux decline due to organics fouling, leading to a steady distillate flux in the earlier stage, and antiscalant was further added to retard the scaling in the long-time treatment. A strong interaction occurred between the foulants and antiscalant in the feed solution, and thus slowed down the scaling on the membrane which was reflected by the membrane flux, morphology and layer thickness. Results further showed that the ratio of actual and theoretical soluble Ca2+ was up to 61.6% and the energy barrier increased by 427.0% under optimum 20 mg/L PBTCA dosage. This study suggests that the sequential EO-DCMD system with antiscalant dosage can provide a potential alternative for treating the leachate concentrate.

Robust dual-layered omniphobic electrospun membrane with anti-wetting and anti-scaling functionalised for membrane distillation application

A robust dual-layer superhydrophobic and omniphobic electrospun nanofiber membrane was developed for membrane distillation application, especially with feed that contains surfactant and dissolved organic compounds. 1H, 1H, 2H, 2H-Perfluorooctyltriethoxysilane (FAS) functionalising of the electrospun polyvinylidene fluoride-co-hexafluoropropylene (E-PVDF-HFP) membrane, followed by surface grafting with FAS-functionalised zinc oxide (ZnO) nanoparticles, achieved low membrane water sliding angle with re-entrant morphologies on the membrane surface. Field-emission scanning electron microscopy with energy-dispersive X-ray images revealed the regular and uniform formation of re-entrant structures due to the ZnO nanoparticles. The optimised 25% w/w ZnO (ePFP-25Z) membrane achieved the highest contact angles for water (>161°), oil (131.5° ± 1.8°) and ethanol (131° ± 2.9°), had the lowest surface energy (0.75 ± 0.43 mN m−1) and high surface roughness (3.26 μm). The ePFP-25Z membrane maintained a stable membrane distillate flux and salt rejection (>99.9%) even after 80 h operation with 0.1 mM sodium dodecyl sulphate in super-saline solution (1 M NaCl). Anti-scaling and anti-wetting properties were illustrated in the presence of the dissolved organic compounds humic acid and alginate in super-saline feed.

A sustainable solution for fresh-water demand in mining sectors: Process water reclamation from POX effluent by membrane distillation

Direct contact membrane distillation (DCMD) was applied to the oxidation under pressure (POX) effluent treatment at different feed temperatures (50–70 °C) and recirculation flowrates (0.3–1.5 L/min). Two different commercial membranes made of poly(tetrafluoroethylene) (PTFE) and poly(vinylidenefluoride) (PVDF) were compared, including the contribution of an additional spacer, in terms of distillate recovery rate and retention efficiency. Lower wetting rates were obtained for lower feed temperatures and recirculation flowrate, which allowed for a greater permeate recovery rate. In contrast, an increase in both feed temperature and recirculation flowrate led to lower resistances. Among them, the resistance imposed by the membrane had the highest contribution to mass transfer resistance, whereas fouling resistance had negligible effects. Due to its lower hydrophobicity, PVDF membranes were more subjected to wetting if compared to PTFE membranes. Under the best conditions (PTFE with spacer, feed temperature of 60 °C and 0.3 L/min) the process had an average distillate flux of 6.82 L/m2 h and a permeate recovery rate of 33.91%, without compromising the permeate physicochemical quality. That corresponds to high values of metals and acidy retention, >95.7% and >99.9%, respectively, generating a permeate that can be used directly as reuse water in the mining plant. Regardless of that, both membranes maintained its rejection efficiency and distillate flux after 240 days exposed to the effluent under similar experimental conditions. Moreover, no significant changes were observed in their morphology or structure. Overall, DCMD was considered as a promising alternative for process water reclamation since the final temperature of this effluent can reach up to 80 °C, which corroborates the use of its residual heat as a driving force of DCMD, thus reducing the energy demand for the processing of this effluent.

Ultrafiltration pretreatment enhances membrane distillation flux, resilience and permeate quality during water recovery from concentrated blackwater (urine/faeces)

In this study, the pretreatment of concentrated blackwater using ultrafiltration (UF) was shown to improve the permeability, selectivity and robustness of membrane distillation (MD) for application to wastewater treatment. Concentrated blackwater comprises urine and faeces, with minimal flushwater added. The faecal contribution increased the soluble organic fraction and introduced coarse and colloidal particles into the urine, which increased resistance to filtration during dead-end UF. Ultrafiltration removed the particulate and colloidal fractions (MW > 500 kDa) from the blackwater, which permitted similar permeability and robustness for MD to that observed with urine (29.9 vs 25.9 kg m−2h−1), which comprises a lower colloidal organic concentration. Without UF pretreatment, a higher density organic layer formed on the MD surface (197 vs 70 gCODm−2) which reduced mass transfer, and transformed the contact angle from hydrophobic to hydrophilic (144.9°to49.8°), leading to pore wetting and a dissipation in product water quality due to breakthrough. In comparison, with UF pretreatment, MD delivered permeate water quality to standards satisfactory for discharge or reuse. This is particularly timely as the ISO standard for non-sewered sanitation has been adopted by several countries at a national level, and to date there are relatively few technologies to achieve the treatment standard. Membrane distillation provides a robust means for concentrated blackwater treatment, and since the energy required for separation is primarily heat, this advanced treatment can be delivered into areas with more fragile power networks.

Membrane Distillation Desalination System With Gap Circulation and Cooling Using a Built-in Heat Exchanger

Abstract A comprehensive experimental investigation is conducted to evaluate the performance of a new flux-enhanced compact water gap membrane distillation (WGMD) module design with gap circulation and cooling for water desalination. The new design uses a separate circulation loop to circulate the gap water, and a built-in heat exchanger coil implanted inside the coolant stream channel for cooling the circulated gap water. The WGMD modules with circulation and with circulation and cooling are compared with conventional WGMD without circulation. Variations of distillate flux, temperatures, and energy consumption are presented at different design operating conditions. Circulation and cooling of the gap water greatly enhance the output flux due to gap water motion and increase the temperature difference between membrane surfaces. However, the enhancement in flux was achieved at the expense of energy consumption. Circulation and cooling of gap water are more effective with bigger gap widths. Feed flowrate showed significant effects with gap water circulation and cooling. The electrical specific energy consumption (SEC) showed the best value of 7.9 and 8.8 kWh/m3 at a feed temperature of 70 °C for both conventional WGMD and WGMD with circulation modules, while the best value of SEC for the WGMD module with gap circulation and cooling was 9.4 kWh/m3 at a feed temperature of 80 °C.

Experimental and numerical evaluation of the energy requirement of multi-stage vacuum membrane distillation designs

Although vacuum membrane distillation (VMD) has a high distillate flux, its potential is limited by its high specific heat consumption (SHC). Flat sheet VMD systems recover the brine energy by employing a cascade system to improve the SHC. Hollow fiber VMD systems, however, have not yet been optimized for energy recovery. To overcome this knowledge gap, four multi-stage configurations (with and without brine energy recovery) were investigated via experimentation and through numerical simulations. The results show that having permeate flux promoting conditions, such as high feed temperature or lower vacuum pressure, improves the SHC for VMD configurations without brine recirculation, but have little impact for configurations with brine energy recovery. A Pareto multi-objective optimization showed that the optimized first-stage heating MS-VMD without brine recirculation has the highest SHC (908 kWh/m3) but one of the lowest LCOW (2.37 USD/m3). In contrast, the configuration with the first-stage heating with brine recirculation provided the lowest SHC (585–629 kWh/m3) and an acceptable LCOW (2.27–8.30 USD/m3). This study reveals that the development of MS-VMD represents a promising direction for thermal desalination technologies, particularly for high saline water applications.

Enhanced performance of membrane distillation using radio-frequency induction heated thermally conducting feed spacers

In the last decade, there is a growing interest in membrane distillation (MD) technology for treating hypersaline water and wastewater. However, MD commercialization is still limited by technical challenges such as temperature polarization, high energy required for heating the bulk feed solution, and heat losses. We demonstrate a proof-of-concept approach to overcome these limitations using stainless steel thermally conducting feed spacers. Feed spacers were directly heated by induction heating, a contactless and instant heating method, using radio frequency (RF) altering electromagnetic fields. Results show that heat was efficiently transferred to the water-membrane interface by conduction and advection, thus, eliminating the need to continuously heat the bulk feed solution. Furthermore, the RF heated spacers were shown (experimentally and by numerical simulations) to reduce temperature polarization. The influence of operating conditions (i.e., flow velocity, vacuum pressure, and power) and the spacer’s material and geometry were evaluated and results were compared to a conventional MD process using a polymeric spacer. Overall, results show that higher spacer mass and larger porosity led to an increase in distillate flux and that the use of thermally conducting spacers heated by RF significantly enhance the distillate flux while reducing the specific heating energy.

Modelling of the coupling of desalination plants with the thermal solar energy system

Abstract This work aims to develop models able to describe and predict the thermal solar collector coupled with vacuum membrane distillation (VMD) unit to determine the growth evolution of the temperature in the solar collector and the membrane module of the VMD. These models will be used to evaluate the distillate flow rates during the year. The global solar radiation data recorded on an inclined surface as well as ambient temperature data can be included in a program. First, the models were validated with some experimental data studies from appropriate literature and then they were used to predict the unit's performance under different operating conditions in the City of Bouzaréah (Algiers). The results obtained from the proposed models showed that the distillate flux water reached 10.5 kg/h·m2 in September and around 5.6 kg/h·m2 in December during midday.

Scaling mitigation in radio-frequency induction heated membrane distillation

Calcium sulfate (CaSO4) scaling is a limiting factor in membrane distillation as a result of concentration polarization and elevated temperatures. We assessed the influence of radio-frequency magnetic fields, used as a method of heating the membrane's surface, as part of a membrane distillation (RF-MD) process. The influence of RF heating on CaSO4 scaling is addressed in terms of distillate flux and crystal formation, and the results are compared to conventional MD systems. The RF-MD system included a thermally conducting dual-layer membrane containing a magnetic hydrophilic layer based on iron oxide-carbon nanotubes coated on a hydrophobic membrane. Heating the solution was mainly done directly at the membrane-water interface, and scaling was assessed at different feed flow velocities using feed solutions containing elevated concentrations of CaSO4 and NaCl and at ionic composition simulating RO brine. Results show only sporadic small CaSO4 crystals on the membrane surface following RF-MD, while high concentrations of small crystals were detected at the concentrate stream exiting the filtration module. Furthermore, the RF-MD system was able to produce enhanced distillate flux. The scaling mitigation mechanism is discussed in detail and hypothesized to be a result of the high-frequency movement and collision of the ions in the solution.

Electrochemical Oxidation-Membrane Distillation Hybrid Process: Utilizing Electric Resistance Heating for Distillation and Membrane Defouling through Thermal Activation of Anodically Formed Persulfate.

This study reports distillation-based salt removal by Ohmic heating in a hybrid process, in which electrochemical oxidation (EO) and direct contact membrane distillation (DCMD) are performed sequentially. In addition to anodically destructing the organics, the hybrid process also separated the sulfate-based electrolytes from treated water through distillation, without consuming external energy, owing to the temperature of the aqueous sulfate solution being elevated to 70 °C via resistive heating. The hybrid process treated organic compounds in a nonselective fashion, whereas DCMD alone did not completely reject (semi)volatile organics. Integrating EO with DCMD made the hybrid process resistant toward the wetting phenomenon; the process exhibited a steady distillate flux and salt rejection as the initial loading of amphiphilic sodium dodecyl sulfate was increased to 0.3 mM. Anodic persulfate formation from the sulfate and Ohmic heating caused an in situ yield of the sulfate radical in the feed solution; this eliminated membrane fouling, according to the observation that the water flux, which was drastically reduced upon adding alginate, was recovered immediately after an electric current was applied. The hybrid process concurrently decomposed spiked organics and removed naturally present inorganic ions in actual flue gas desulfurization wastewater, without an external supply of electrolyte and heat energy.

Comparison of different mass transfer models for direct contact membrane distillation flux evaluation

Comparison of different mass transfer models for direct contact membrane distillation flux evaluation

Porous heat exchange tube with ultra-thin dense skin layer via NIPS for AGMD process

In this paper, a kind of poly(vinylidene fluoride) (PVDF) porous heat exchange tube (HET) was prepared via non-solvent induced phase separation (NIPS) method. This porous HET possessed an ultra-thin dense skin layer of 2 μm. Since the thermal conductivity of water is higher than that of PVDF, filling the pores with water can significantly improve the heat performance of the porous HET. A full plastic double-pipe air gap membrane distillation (AGMD) module was fabricated with the porous HETs and PVDF hollow fiber membranes. In the AGMD process, the wetting treatment can make the membrane distillation flux (J) and the gained output ratio (GOR) increased by 30%. During the long-term stability test, the maximum conductivity of distillate water was only 17.2 μS/cm. The experimental results demonstrated that the porous HET can be applied for the AGMD process to treat high saline brine, and the ultra-thin dense layer can prevent the leakage of saltwater.

Analysis of an air-cooled air gap membrane distillation module

An air-cooled design for an air gap membrane distillation (AGMD) process offers potential for significantly lowering energy requirements and reducing cost for desalination by eliminating or reducing the balance of plant components associated with the coolant flow system. Experiments were conducted on an air-cooled AGMD module to study the effects of air gap, support mesh conductivity and hydrophobicity, and the condensing surface hydrophobicity. A novel modular design was developed in which modules could be arranged in a series configuration to increase the distillate flux. The output from the series configuration was found to yield about three times the distillate water compared to a single pass water-cooled system with the same temperature difference (ΔT). The results also indicate that the mesh conductivity has a favorable effect on the flux value whereas the hydrophobicity of the mesh has no significant effect. The hydrophobicity of the condensing surface is favorable on two accounts: first, it increases the distillate flux at temperatures below 60 °C and secondly, the temperature difference of the saline feed when it enters and leaves the module is lower, which can lead to energy savings and higher yield when used in a series configuration.

Novel hydrophobic PVDF membranes prepared by nonsolvent induced phase separation for membrane distillation

Polyvinylidene fluoride (PVDF) was used for membrane preparation using the nonsolvent induced phase separation (NIPS). In this paper, the effect of the polymer concentration in the dope solution and the nonsolvent bath composition is widely investigated. In particular, the effect of increasing concentrations of ethanol in the coagulation bath on the morphology and characteristics of the membranes, as well as on their performance in membrane distillation of pure water and a brine was evaluated. FESEM analysis, coupled with contact angle determination highlighted different coagulation paths that allowed to obtain superhydrophobic membranes at higher ethanol concentrations. Moreover, the polymer concentration had major effects on the overall porosity and pore size of the membrane. By tuning the membrane preparation conditions, it was possible to obtain membranes that showed great distillate flux as well as a complete salt rejection during the vacuum membrane distillation tests.

Ethanol recovery from a model broth by the fermentation-extraction-distillation coupling process

The fermentation-extraction-distillation coupling (FEDC) process for ethanol recovery from a simulated model broth using 1-dodecanol as an extractant was studied by varying distillation temperature, inlet gas flow rate, extract height and feed concentration. Results showed that the temperature field was strongly impacted by the inlet gas flow rate and the distillation temperature, but the concentration field was almost not affected by these two factors. The distillate flux was greatly influenced by the distillation temperature, the inlet gas flow rate and the feed concentration. The ethanol concentration in the distillate was increased with the increasing of all the four factors. The distillate flux and the separation factor were respectively up to about 720 g/(m2·h) and 20.4, which were respectively comparable to the permeation flux and the separation factor of pervaporation.

Portable Solar-powered Membrane Distillation System to Solve Water and Energy Problems Simultaneously

With increasing human population, the demand for water as well as energy also increasing. This will lead to global water scarcity and energy crisis, due to limited freshwater resources and decreasing fossil fuel resources. The most potential solution to solve this water-energy problem is by using desalination technology powered by renewable energy like solar energy. This paper reports the design and development of a portable and stand-alone solar-driven desalination system. Due to portability, this system is expected to be transported to anywhere like remote areas. The system consists of three major parts, which are solar-thermal collector, solar photovoltaic (PV) panel, and Vacuum Multi-Effect Membrane Distillation (V-MEMD) unit as the main part of the system. In this paper, a small-scale operation test of the system was carried out and analyzed. The system run successfully for about 7 hours, which started at 09:00 AM. The feedwater used was brackish-water with conductivity of about 2500 μS/cm and the flowrate was maintained at 69 L/h. The system produced distillate/freshwater at average rate of 5.98 L/h. Whereas, the total distillate produced was approximately 34.8 L with conductivity of about Total Dissolved Solid (TDS) of 2.67 mg/L. The distillate flux of the current system was in the range of 3.2 – 5.2 L/m2.h.

A three-dimensional plasmonic spacer enables highly efficient solar-enhanced membrane distillation of seawater

A 3D plasmonic spacer for membrane distillation: rational-design of a 3D spacer with nano-gaps in plasmonic hybrids for localized heating could enhance the distillate flux and reduce energy consumption of membrane distillation.

Metallic spacers to enhance membrane distillation

Despite membrane distillation (MD) being well acknowledged as a green technology due to the capability to treat water using waste heat, the efficient use of this heat remains one of the biggest challenge for MD to be cost-effective. In this study, we harnessed the high thermal conductivity of metallic spacers to improve the energy efficiency in DCMD, with the performance studied via both simulations and experiments. The frequently used polypropylene spacer with each mesh dimension of 3 mm was used as a benchmark for comparison. Two metals were investigated, (namely, nickel and copper), along with three spacer densities (namely, 3 mm mesh, 1.5 mm mesh and foam). The noteworthy results include: (i) the metallic spacers placed on the feed side of the membrane gave higher temperatures that were more uniform along the membrane surface; (ii) whereas the surface-averaged membrane temperature for the polypropylene material decreased with spacer density, that for the metallic materials decreased then increased with spacer density; (iii) although the experimental distillate fluxes were similar, the heater input energy per unit volume distillate was lower for the metallic foam spacers by up to 16%; and (iv) platinum-coated Ni foam gave a remarkable reduction in energy input of 28% under light irradiation. This study demonstrated the feasibility of using metallic spacers with inherent high thermal conductivity to improve energy efficiency, providing a new platform for further improvements through other commercially available metallic foams or spacer modifications.