Distillate Flux Research Articles

Fabrication of PVDF hollow fiber membranes via integrated phase separation for membrane distillation

Abstract In this study, polyvinylidene fluoride (PVDF) hollow fibers with interpenetrating network morphologies were fabricated via complex thermally induced phase separation (c-TIPS) by integration of non-solvent induced phase separation (NIPS) and thermally induced phase separation (TIPS) at 80 °C and then tested in membrane distillation (MD). The effects of solvents, additive and bore fluid on the membrane morphology, pore structure and MD performance were investigated. Direct-contact membrane distillation (DCMD) for desalination was carried out to evaluate membrane permeability and salt rejection. Using a weak solvent like triethylphosphate (TEP) and weak bore fluid like polyethylene glycol (PEG-200) in the c-TIPS favors the formation of an interpenetrating network morphology. In addition, PEG-200 increased the roughness and water contact angle of the inner skin. The hollow fibers fabricated via the c-TIPS at low temperature presented high permeability, mechanical strength and long-term stability. In desalination of formulated seawater, a distillate flux of 61.6 kg/m2h with NaCl rejection of 99.99% was achieved at feed temperature of 71 °C.

Enhancement of energy utilization using nanofluid in solar powered membrane distillation

Nanofluids have excellent solar energy utilization efficiency due to the localized surface plasmon resonance phenomenon. In this study, photothermal nanofluids were employed as the feed solution for energy harvesting in solar powered membrane distillation. Ten different nanofluids were compared and TiN (titanium nitride) was chosen following UV-Vis-NIR-waveband (ultraviolet–visible–near-infrared) optical absorption analysis, zeta potential measurement, and membrane distillation flux testing. Desalination experiments were conducted using a range of TiN concentrations and solar radiation powers. The results showed that water flux and solar energy utilization efficiency increased with increasing TiN content. Compared to the base fluid (35 g/L NaCl aqueous solution), flux increased from 0.47 to 0.74 kg/(m2∙h), while energy utilization efficiency improved from 32.1% to 50.5% for 100 mg/L TiN nanofluid. Flux also increased with the increasing of solar radiation power markedly. With 5 kW/m2 solar radiation power, the flux reached 2.77 kg/(m2∙h). Furthermore, the permeate water produced was of excellent quality contained less than 10 mg/L salinity when using 35 g/L NaCl feed solution. And no nanoparticles were detected transport through the membrane during the process. The nanofluid enhanced solar powered membrane distillation represents a promising perspective for better solar energy utilization.

Silica fouling during direct contact membrane distillation of coal seam gas brine with high sodium bicarbonate and low hardness

The presence of silica in coal seam gas (CSG) reverse osmosis (RO) brine complicates the membrane distillation (MD) process due to silica polymerization and metal silicate formation at high water recovery. This study reported that direct contact MD (DCMD) of CSG RO brine at electrical conductivity (EC) of 17.8 mS/cm achieved 90% brine recovery (overall recovery of 95%) with distillate flux decline of 16%, and distillate EC of 15 μS/cm over 150 h of operation. A series of experiments were conducted using synthetic solutions to evaluate the effect of CSG RO brine water quality on silica fouling in DCMD process. Results showed that CaCO3 formation was fast and did not co-precipitate with dissolved silica, while magnesium and silica were co-precipitated as magnesium silicate compounds. Results also revealed that polymerized silica formed a non-porous layer on the membrane surface. This fouling layer not only significantly reduced distillate flux but also led to a dramatic increase of distillate EC. Further study on silica fouling mitigation suggested that rapid cooling and filtration/clarification could be adopted for removal of silica from MD brines containing high silica concentration. It also showed that disodium ethylenediaminetetraacetic acid (Na2EDTA) was effective to hinder the formation of CaCO3 and magnesium silicate in MD.

Assessment of a pilot system for seawater desalination based on vacuum multi-effect membrane distillation with enhanced heat recovery

This work presents the evaluation of an innovative system based on vacuum multi-effect membrane distillation modules (V-MEMD) for seawater desalination at pilot scale. This four-effect unit introduces a remarkable modification from previous V-MEMD systems, consisting of the use of the seawater feed flow as cooling in the condenser, rather than a separate circuit. Preheating the feed in the condenser improved heat efficiency (maximum gained output ratio obtained for seawater was 3.2). Maximum distillate fluxes reached 8.5 l h−1 m−2 for hot feed temperature 75 °C and feed flow rate 150 l h−1. Increasing both parameters to raise the productivity was hindered by the inability of the condenser to cope with all the steam generated in previous effects, resulting in overheating and overpressure. Furthermore, a loss of 40% of distillate production was measured due to the increase of seawater cooling temperature by 8 °C along the year. Finally, it was observed that scaling reduced distillate production up to 50%. Acid cleaning successfully removed scaling and restored the performance. Subsequently, the use of an antiscalant as a pre-treatment was sufficient to prevent it.

Pore channel surface modification for enhancing anti-fouling membrane distillation

Membrane surface modification by forming a functional layer is an effective way to improve the anti-fouling properties of membranes; however, the additional layer and the potential blockage of bulk pores may increase the mass transfer resistance and reduce the permeability. In this study, we applied a novel method of preparing anti-fouling membranes for membrane distillation by dispersing graphene oxide (GO) on the channel surface of polyvinylidene fluoride membranes. The surface morphology and properties were characterized by scanning electron microscopy, atomic force microscope, and Fourier transform infrared spectrometry. Compared to the membrane surface modification by nanoparticles (e.g. SiO2), GO was mainly located on the pore surface of the membrane bulk, rather than being formed as an individual layer onto the membrane surface. The performance was evaluated via a direct-contact membrane distillation process with anionic and cationic surfactants as the foulants, separately. Compared to the pristine PVDF membrane, the anti-fouling behavior and distillate flux of the GO-modified membranes were improved, especially when using the anionic surfactant as the foulant. The enhanced anti-fouling performance can be attributed to the oxygen containing functional groups in GO and the healing of the membrane pore defects. This method may provide an effective route to manipulate membrane pore surface properties for anti-fouling separation without increasing mass transfer resistance.

Hierarchical Composite Membranes with Robust Omniphobic Surface Using Layer-By-Layer Assembly Technique

In this study, composite membranes were fabricated via layer-by-layer (LBL) assembly of negatively charged silica aerogel (SiA) and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FTCS) on a polyvinylidene fluoride phase inversion membrane and interconnecting them with positively charged poly(diallyldimethylammonium chloride) (PDDA) via electrostatic interaction. The results showed that the PDDA-SiA-FTCS coated membrane had significantly enhanced the membrane structure and properties. New trifluoromethyl and tetrafluoroethylene bonds appeared at the surface of the coated membrane, which led to lower surface free energy of the composite membrane. Additionally, the LBL membrane showed increased surface roughness. The improved structure and property gave the LBL membrane an omniphobic property, as indicated by its good wetting resistance. The membrane performed a stable air gap membrane distillation (AGMD) flux of 11.22 L/m2 h with very high salt rejection using reverse osmosis brine from coal seam gas produced water as feed with the addition of up to 0.5 mM SDS solution. This performance was much better compared to those of the neat membrane. The present study suggests that the enhanced membrane properties with good omniphobicity via LBL assembly make the porous membranes suitable for long-term AGMD operation with stable permeation flux when treating challenging saline wastewater containing low surface tension organic contaminants.

The unidirectional regulatory role of coagulation bath temperature on cross-section radius of the PVDF hollow-fiber membrane

In this paper, the low-temperature thermally induced phase separation (L-TIPS) method was adopted to prepare a high-performance polyvinylidene fluoride (PVDF) hydrophobic membrane for membrane distillation (MD) with non-solvent polyethylene glycol-400 (PEG400) and solvent triethyl phosphate as a fully water-soluble mixed diluent. In the process of L-TIPS membrane formation, the unidirectional regulatory role of coagulation bath temperature on cross-section radius of the PVDF hollow-fiber membrane was analyzed. The results show that the tensile strength of the membrane increases with the increase of the external coagulation bath water temperature, which is the opposite of the general rule. PEG400 is employed to regulate the membrane structure, and more specifically to increase the cellular structure (a positive impact on tensile strength) and reduce the sphere-packed aggregation structure (a negative impact on tensile strength). When the coagulation bath temperature is 60°C, the tensile strength of the membrane reaches 6.21MPa (more than 2–4 times higher than the hydrophobic membranes reported). Meanwhile, the PVDF hollow-fiber hydrophobic membrane also has a high flux, the vacuum membrane distillation flux reaches 30.6kgm−2h−1, which is 14.6 times higher than that at the coagulation bath temperature of 15°C.

Distillate flux enhancement of the concentric circular direct contact membrane distillation module with spiral wired flow channel

Abstract The modeling equations for predicting distillate flux in the direct contact membrane distillation (DCMD) module with embedded spiral wire of various spiral wire pitches in concentric circular module were developed theoretically and verified experimentally. The temperature distributions, distillate flux and energy consumption of both concurrent- and countercurrent-flow operations were presented with the volumetric flow rate, inlet saline temperature, the spiral wire as parameters, and the experimental data was incorporated into the modeling equations to validate the theoretical predictions. A description of the average Nusselt number was made to evolve a correlating equation of heat transfer coefficients incorporating distillate flux and Reynolds numbers with the use of experimental data. The effects of operating and design parameters such as flow rate, inlet saline temperature and spiral wire pitch on the pure water productivity and power consumption under both concurrent- and countercurrent-flow operations were also analyzed.

Distillate flux enhancement in the air gap membrane distillation with inserting carbon-fiber spacers

ABSTRACTA new design of air gap membrane distillation (AGMD) with inserting carbon-fiber spacers with various hydrodynamic angles in flow channels for eddy promoting under concurrent-flow operations was developed theoretically and experimentally. Attempts to enlarge eddy flow in aiming to reduce the temperature polarization were achieved with the inserted carbon-fiber spacers that enhance the heat and mass transfer in the AGMD system. A mathematical model considering heat and mass transfer mechanisms has been developed, and the Nusselt number was correlated with the experimental data. The effects of various operation parameters on the distillate flux enhancement were studied as compared to the modules without inserting carbon-fiber spacers (empty channels).

A theoretical approach of a vacuum multi-effect membrane distillation system

A detailed mathematical model, based on mass and energy balances, was developed in order to provide guide lines for the optimal design and operation of a vacuum-multi-effect membrane distillation system. Since there is no well-established solution for the V-MEMD system, two case studies were used for the feed channel. The comparison between experimental and theoretical values of distillate flux showed low deviations (1.9%–11.1%) for tap water as regards the second case study. However, the deviations were higher, in the range of 2%–23%, when saline water was used as feed solution. The model was also utilized to investigate the effect of the operating conditions, membrane structural properties and design parameters on the performance indicators. High values of the hot water inlet temperature and feed inlet temperature lead to increase of water productivity and recovery ratio. Membranes with high porosity, large pore size and low thickness are the best candidates for enhancing the water productivity. Introduction of spacer improves the mass and heat transfer in the channels of the system. Also, the increment of the number of effects from 2 to 4 significantly lowers the specific thermal energy consumption.

Experimental and theoretical study of a lab scale permeate gap membrane distillation setup for desalination

Abstract Membrane distillation (MD) as a novel thermally-driven process with moderate operating temperatures is a known effective technology for salt-water desalination. In this research, a lab scale plate-and-frame permeate gap membrane distillation (PGMD) module with internal heat recovery characteristic is designed. The developed PGMD module performance is experimentally investigated for fresh and saline water feed in terms of permeate water flux rate, specific thermal energy consumption (STEC) and gained output ratio (GOR). The experimental results show that for a feed sample with 130 (g/kg: ppt) concentration (nearly four times seawater salinity), increasing the feed flow rate from approximately 0.4 to 1 L/min, led to increasing the distillate flux from 3 to 5 kg/m 2 h. However, increasing the feed flow rate in this range also led to approximately 40% increase in the STEC of the system. Furthermore, a single node theoretical model based on the PGMD module configuration is developed and the modelling results validated with experimental values at different feed water flow rate and salinity. The comparison shows a good agreement between the developed model results and experimental outcomes. It is also concluded, optimization of the MD module performance to improve internal heat recovery and produce higher fresh water rate would be achievable by adjusting the effective membrane surface area and feed flow rate.

Numerical modeling and economic evaluation of two multi-effect vacuum membrane distillation (ME-VMD) processes

A hollow fiber membrane module cascaded with a heat exchanger to recover the latent heat of vapor is a one-stage vacuum membrane distillation (VMD) unit. In this work, two multi-effect VMD (ME-VMD) systems, first-stage heating and inter-stage heating processes with n one-stage-VMD units cascaded together, were firstly considered and compared in terms of efficiency using computational fluid dynamics (CFD). The inter-stage heating ME-VMD system had a relatively high water recovery, however, its gained output ratio (GOR) was low and the system complexity and operating cost increased due to the presence of the heaters in each stage. Thus, we focused on the first-stage heating ME-VMD system. An ME-VMD system containing 30 one-stage VMD units with first-stage heating was then evaluated coupling with the mathematic model of single module via CFD, and an economic evaluation model was established. The effects of feed inlet temperature and stage number on the distillate flux, water recovery and GOR were comprehensively discussed. The highest GOR could reach to 12.1. The water production cost mainly depended on the heat exhaustion and the lowest cost was estimated at $0.59/t for an optimal four-stage ME-VMD system with first-stage heating, which can be competitive to the reverse osmosis technique.

Fabrication of a novel nanofibers-covered hollow fiber membrane via continuous electrospinning with non-rotational collectors

A novel nanofibers-covered hollow fiber membrane (N-HFM) with high porosity (83.5%) was prepared via employing electrospinning to deposit nanofibers onto a porous supporting tube. Electrospun nanofibers covered on a porous supporting tube serve as the outer layer of the N-HFM. The supporting tube fixed on the soft electrode serves as the collector as well as the inner layer of the N-HFM. When the feeding rate was 1mlL−1, it was possible to continuously fabricate N-HFM with a non-rotational collector. The prepared N-HFM was used for desalination by direct contact membrane distillation (DCMD), and a high distillate flux of 17kgm−2h−1 was obtained while the feed and permeate temperatures were fixed at 333K and 293K, respectively. It is expected that the novel hollow fiber membranes with high porosity fabricated by this continuous electrospinning technique may be attractive in various separation and purification processes.

Characteristics of membrane foulants at different degrees of SWRO brine concentration by membrane distillation

Fouling development at different degrees of SWRO brine concentration by a DCMD system was investigated in this study. The experiments were done with a 60°C feed inlet temperature and 20°C permeate inlet temperature to investigate the fouling dynamic based on the distillate flux and detailed organic and inorganic characterizations. The fouling test was classified into 4 phases based on the flux decline percentage. The initial distillate flux (40.1±1.0kg/m2h) was reduced by 96.6%, when the feed concentration factor reaches 3.0 of uninterrupted operation. The fouling mass increased nearly 378 times from VCF 1.1 (1.3g/m2) to VCF 3.0 (490.9g/m2). The deposit morphology and the composition of the foulants at each phase have been determined using SEM-EDS, XRD, ICP-OES, ATR-FTIR and LC-OCD. The fouling data revealed that membrane fouling is dominated by deposited inorganic foulants in combination with organic foulants. The accumulation of small mass of organic foulants (0.3‰) promoted fouling development because of their adhesion to the membrane and salt bridging. In addition, some organic materials penetrated the membrane despite a lack of apparent membrane wetting.

Vacuum membrane distillation for deep seawater: experiments and theory

This paper investigates the feasibility and potential of vacuum membrane distillation (VMD) for concentrating deep seawater (DSW) of 34 g/kg and its reverse osmosis (RO) brine of 62 g/kg. In this work, a small pilot-scale VMD system was developed and distillate fluxes were measured for feed solution...

Enhanced Freshwater Production Using Finned-Plate Air Gap Membrane Distillation (AGMD)

Air Gap membrane distillation (AGMD), a special type of energy efficient membrane distillation process, is a technology for producing freshwater from waste water. Having some benefits over other traditional processes, this method has been able to draw attention of researchers working in the field of freshwater production technologies. In this study, a basic AGMD system with flat coolant plate has been modified using a specially designed channelled coolant plate of portable size to observe its effect over the production rate and performance of the system. Attempt has been made to increase the amount of distillate flux by using the “fin effect” of the channelled coolant plate. A finned plate have been used instead of a flat coolant plate and experiments were conducted to compare the effect. Coolant temperature and feed temperature of the system have been varied from 10°C to 25°C and 40°C to 70°C respectively. Comparing the data, around 50% to 58% distillate enhancement has been observed for channelled coolant plate. Also, it was seen that the enhancement was higher for higher feed temperatures and coolant temperatures. With these findings, a better performing AGMD module has been introduced to mitigate the scarcity of freshwater.

Photothermal nanocomposite membranes for direct solar membrane distillation

Photothermal membranes that convert light to heat locally on the membrane surface and significantly improve membrane distillation flux and energy efficiency have been reported.

Variation of distillate flux in direct contact membrane distillation for water desalination

Variation of distillate flux in direct contact membrane distillation for water desalination

Effects of membrane structure and operational variables on membrane distillation performance

A bench-scale, sweeping gas, flat-sheet Membrane Distillation (MD) unit was used to assess the importance of membrane architecture and operational variables to distillate production rate. Sweeping gas membrane distillation (SGMD) was simulated for various membrane characteristics (material, pore size, porosity and thickness), spacer dimensions and operating conditions (influent brine temperature, sweep gas flow rate and brine flow rate) based on coupled mass and energy balances. Model calibration was carried out using four membranes that differed in terms of material selection, effective pore size, thickness and porosity. Membrane tortuosity was the lone fitting parameter. Distillate fluxes and temperature profiles from experiments matched simulations over a wide range of operating conditions. Limitations to distillate production were then investigated via simulations, noting implications for MD design and operation. Under the majority of conditions investigated, membrane resistance to mass transport provided the primary limitation to water purification rate. The nominal or effective membrane pore size and the lumped parameter ε/δτ (porosity divided by the product of membrane tortuosity and thickness) were primary determinants of membrane resistance to mass transport. Resistance to Knudsen diffusion dominated membrane resistance at pore diameters <0.3µm. At larger pore sizes, a combination of resistances to intra-pore molecular diffusion and convection across the gas-phase boundary layer determined mass transport resistance. Findings are restricted to the module design flow regimes considered in the modeling effort. Nevertheless, the value of performance simulation to membrane distillation design and operation is well illustrated.

Application of Salt Additives and Response Surface Methodology for Optimization of PVDF Hollow Fiber Membrane in DCMD and AGMD Processes

In this study, the influence of the salts as an additive on the performance of the membrane was investigated and an extensive work was performed to optimize PVDF hollow fiber membranes through a response surface methodology (RSM). The prepared membranes were characterized by SEM, contact angle and LEP measurement. Then, the RSM was used for the optimization of surface pore size, porosity and hydrophobicity of the synthesized hollow fiber at different conditions (polymer concentration, salt concentrations, and air gap). Under MD conditions (feed concentration, 100 mg/l; feed temperature 80 °C, and cooling temperature 15 °C), the optimum membrane was compared with the virgin one in the same condition. In addition, the influence of distillate flux at different feed concentrations and temperatures was evaluated. The results show that the optimum hollow fiber membrane was fabricated in the polymer concentration of 22 %w/w, BaCl2 concentration of 2.9 %w/w and an air gap of 34.5 cm. Consequently, the optimum fiber was examined for the desalination of water with 35, 50 and 70 g/l salt concentration by DC and AG membrane distillation. Our findings show that the distillate flux with the salt rejection of 99.9% was increased to 46% and 31% for DCMD and AGMD, respectively.