Hydrophobic mullite ceramic hollow fibre membrane (Hy-MHFM) for seawater desalination via direct contact membrane distillation (DCMD)

A low-cost hydrophobic mullite hollow fibre membrane (Hy-MHFM) fabricated via phase inversion/sintering technique followed by fluoroalkyl silane (FAS) grafting is presented in this study. The prepared CHFMs were characterized before and after the grafting step using different characterization techniques. The pore size of the CHFM surface was also determined using ImageJ software. The desalination performance of the grafted membrane was evaluated in direct contact membrane distillation (DCMD) using synthetic seawater of varying salt concentrations for 2 h at various feedwater temperatures. The outcome of the evaluations showed declines in the permeate flux of the membrane at increasing feed concentration, as well as increased flux with increased feed temperature. The long-term stability of the membrane was achieved at time 20 h, feed temperature 60 °C, and permeate temperature 10 °C, the membrane achieved a salt rejection performance of about 99.99 % and a water flux value of 22.51 kg/ m² h.     

Unsupported electrospun membrane for water desalination using direct contact membrane distillation

In this project, an unsupported electrospun poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) membrane was used for water desalination using direct contact membrane distillation (DCMD). The membrane was electrospun using a laboratory-scale machine with multiple nozzles that was developed in-house. Critical process parameters, including the applied voltage and polymer concentration, were optimized to obtain bead-free electrospun membranes with fiber diameters less than 300 nm. To improve the membrane thermal stability and performance, the selected electrospun membrane was heat-pressed at 160°C. The untreated and heat-pressed membranes were tested in a DCMD setup at different feed temperatures (60, 70, and 80°C) and feed flow rates (0.4, 0.6, and 0.8 L/min), while maintaining the permeate temperature and flow rate at 20°C and 0.2 L/min, respectively. The modified electrospun membrane exhibited a very high permeate flux (>37.5 kg/m²/h) and a salt rejection rate of 99.99% at a feed temperature of 70°C. The performance of the heat-pressed unsupported PVDF-HFP electrospun membrane was nearly identical to a commercially available polytetrafluoroethylene (PTFE) supported membrane. These promising results demonstrate that relatively low-cost electrospun membranes can be easily produced and successfully used in DCMD to minimize the capital cost and increase the energy efficiency of the process.     

直接接触式膜蒸馏过程的膜曝气强化

针对直接接触式膜蒸馏（direct contact membrane distillation,DCMD）过程存在的膜通量小及膜污染问题,设计了一种新型结构的膜蒸馏组件。以蔗糖溶液为处理液,考察了膜组件装填密度Φ、膜曝气量q、蔗糖浓度c与温度T₀对DCMD过程的影响。结果表明：随着Φ、q的增加,DCMD过程的膜通量先增大,后逐渐降低,Φ、q均存在最优值;随着c的增加,膜通量逐渐降低;随着T₀的增加,膜通量增大;对c为30%（mass）的蔗糖溶液进行DCMD法处理330 min时,膜曝气可使DCMD的初始膜通量J_initial提升24.7%、膜通量衰减率ΔJ降低55.0%,维持高膜通量的连续运行时间t₀延长4倍。主要原因是膜曝气强化了DCMD过程的传热传质,进而强化过程的分离性能;有效控制了DCMD过程的浓差极化,进而延缓过程的膜污染进程。研究结果有利于推进DCMD的工程化应用。     

Study on a new air-gap membrane distillation module for desalination

A new air-gap membrane distillation (AGMD) module for desalination with internal latent-heat-recovery which consisted of parallel hollow fiber membranes and heat exchange hollow fibers was successfully developed. The influences of feed flow rate, feed temperature and feed initial concentration on AGMD process were investigated. The vapor pressure polarization coefficient (η) was introduced to measure the reduction in the effective driving force for mass transfer with regard to the driving force imposed. Among all AGMD experiments, the maximum water vapor permeate flux (J_D) of 5.30 kg/m² h and the gained output ratio (GOR) of 5.70 were obtained. A theoretical model based on the mass and energy balances of the hot feed side was established to calculate the temperature and the local water vapor permeate flux distributions along the hollow fiber membrane, which showed that the temperature drop and local water vapor permeate flux drop were much larger at the upper part than those at the lower part of the membrane module in the hot feed side.     

A new enhancement technique on air gap membrane distillation

The most serious concern in the application of air gap membrane distillation (AGMD) configuration is the low permeate flux caused by an additional transport resistance owing to the air gap, by the temperature and concentration polarization and by the surface fouling. This paper presents an innovative design of a low-cost and high efficient membrane module with an advanced enhancement technique in an AGMD configuration, which not only yields a much improved permeate flux but also requires no additional facility for the enhancement. The new module design includes a tangent directional and rotational inlet turbulent flow of hot feed and a partial contact between the membrane and the cooling plate in a small air gap. The concrete structure of the module is introduced in detail in the paper. Using this new module the permeate flux can be obtained up to 119 kg/m²h for tap water when the temperature of the hot and cold water is 77 °C and 12 °C respectively, which is about a 2.5-fold improvement over the traditional AGMD technique at the almost the same conditions. Within the range of our experimental study, the optimum partial contact area ratio is about 75–80%. Mechanistically, the tangent and rotational inlet turbulent flow can accelerate the diffusive process of mass and heat, reduce the boundary layer thickness of temperature and concentration and wash the membrane surface so as to improve the temperature and concentration polarization near the membrane surface and to raise the efficiencies of mass and heat transfer. Because of the partial contact between the membrane and the cooling plate with a large area, the main heat transfer and permeate condensation in the gap both are carried out on the contact area, which is very different from either the common AGMD or DCMD (direct contact membrane distillation) so as to reduce the transport resistance in the gap and thus to raise the permeate flux significantly. The new enhancement technique is also applied for the desalination of 15 wt.% salt water, which shows the similar improvement in permeate flux.     

Energetic Performance and Permeate Flux Investigation of Direct-Contact Membrane Distillation for Seawater Desalination

The performance of the direct-contact membrane distillation (DCMD) process in desalting Mediterranean seawater was investigated. Theoretical and experimental optimization of various operating parameters was conducted. The effects of temperature differences, feed velocity, and membrane characteristics were studied. When using commercial polyvinylidene fluoride membranes, the vapor transfer throughout the membrane pores is dominated by the Knudsen-molecular diffusion model. Maximum permeate flux was obtained when increasing temperature, feed velocity, membrane pore size, and porosity and decreasing membrane tortuosity and thickness. Thermal efficiency, gained output ratio, and specific thermal energy consumption were improved when increasing feed temperature. By application of the DCMD process to Mediterranean seawater, a high water quality was obtained.     

Seawater desalination using PVDF-HFP membrane in DCMD process: assessment of operating condition by response surface method

Central composite design (CCD) was applied in this work to analyze the performance of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) flat sheet membrane in the seawater desalination using direct contact membrane distillation (DCMD) process. It is the most popular in response surface method (RSM). Development on Quadratic Regression model for membrane performance as a function of the operating conditions was studied. The ranges for each operating condition were selected as follows: feed temperature (T_f): 48–58?°C, feed flow rate (Q_f): 80–180?mL/min, permeate temperature (T_p):17–22?°C and permeate flow rate (Q_p): 80–180?mL/min. The model R-squared of 0.9759 (adjusted to degree-of-freedom), Lack-of-fit test (p?=?0.4764), predicted residual error sum of squared (PRESS) statistic of 10.3 suggest that the model is adequate to correlate the impact of operating conditions on permeates. ANOVA analysis showed that factors as feed flow rate, feed temperature, and permeate temperature have a valuable impact (p?≤?0.05) on the response variable. Additionally, the interaction among feed temperature-feed flow rate, feed flow rate-permeate flow rate, and the quadratic impact of feed temperature, permeate temperature, and permeate flow rate have shown an important impact (p?≤?0.05) on the permeate flux. Optimization of operating conditions to make the permeate flux and salt rejection high as possible was determined according to desirability function approach. A desirability of 0.969 was achieved at a feed temperature of 58?°C, feed flow rate of 180?mL/min, permeate temperature of 18.8?°C, and permeate flow rate of 145.3?mL/min in which a permeate flux of 12.56?kg/m²h and a salt rejection of 99.97% was obtained.     

Application of response surface methodology for modeling and optimization of membrane distillation desalination process

In this work, response surface methodology (RSM) was applied for modeling and optimization of operating parameters for water desalination by direct contact membrane distillation (DCMD) process using polypropylene membrane (PP) with low pore size. Operating parameters including vapor pressure difference, feed flow rate, permeate flow rate and feed ionic strength were selected and the optimum parameters were determined for DCMD permeate flux. The developed model for permeate flux response was statistically validated by analysis of variance (ANOVA) which showed a high value coefficient of determination value (R² = 0.989). The obtained optimum operating parameters were found to be 0.355 × 10⁵ Pa of vapor pressure difference, feed flow rate of 73.6 L/h, and permeate flow rate of 17.1 L/h and feed ionic strength of 309 mM. Under these conditions, the permeate flux was 4.191 L/(m² h). Compared to a predicted value, the deviation was 3.9%, which confirms the validity of the model for the DCMD process desalination optimization. In terms of product water quality, the DCMD process using hydrophobic PP membrane can produce high quality of water with low electrical conductivity for all experimental runs.     

Evaluation of commercial PTFE membranes in desalination by direct contact membrane distillation

In this study, nine flat-sheet commercially available hydrophobic PTFE membranes were used in desalination by direct contact membrane distillation and their characteristics were investigated under different operating conditions including feed temperature, feed flow rate, cold stream flow rate, and feed concentration. Membrane properties, i.e. pore size, thickness, support layer, and salt rejection were also studied. Moreover, membrane module designs including flow arrangements (co-current, counter-current and tangential) for process liquid and depth both on hot and cold sides were tested experimentally. Finally, the long-term performance of the selected membranes for direct contact membrane distillation as a stand-alone desalination process was investigated. The results indicated that increasing feed temperature, hot feed flow rate, and module depth on the cold side led to increase permeate flux. On the other hand, increasing membrane thickness and module depth on the hot side (at constant flow rate) had negative effects on the flux. The highest permeation flux and salt rejection was achieved when the membranes with a pore size of 0.22 μm were used in the cross-current follow arrangement of hot and cold streams. In addition, the requirements for support layer for a successful DCMD process has been extensively discussed.     

Effects of embedding functionalized multi-walled carbon nanotubes and alumina on the direct contact poly(vinylidene fluoride-co-hexafluoropropylene) membrane distillation performance

Flat-sheet membranes were fabricated by incorporating alumina (Al₂O₃) and functionalized multiwalled carbon nanotubes (MWCNTs; MWCNTs-COOH) in PVDF-co-HFP membrane via the phase-inversion method for application in membrane distillation (MD) application. Scanning electron microscopy and atomic force microscopy were performed on the resulting membranes to investigate the effects of functionalized MWCNTs. The results revealed that the embedding of functionalized MWCNTs led to a significant modification of the membrane characteristics, including the structural morphology, thickness, roughness, porosity, pore size, and pore size distribution. The effects of operational parameters such as the hot feed solution temperature (47–67?°C), feed flow rate (0.35–0.55?L/min), and feed concentration (0–100?g/L) on the performance of the fabricated membrane were tested using the DCMD system. The experimental results demonstrated that the permeate flux was enhanced by approximately 32.43% by using functionalized MWCNTs, reaching a value of 16.35?kg/m² h at 35?g/L feed concentration, 67?°C hot feed temperature, and 0.55?L/min feed flow rate, at the constant temperature of 20?°C and 0.35?L/min flow rate. The functionalized MWCNTs embedded within the membrane successfully modified the interactions between water and the membrane to improve the water vapor transport while inhibiting salt penetration into the pores.     

Permeate-Flux Declination for Ultrafiltration along Membrane Tubes

The correlation equations for predicting local permeate fluxes in tubular-membrane ultrafilters were derived from mass and momentum balances by the modified resistance-in-series model with the considerations of the increment of concentration polarization and the declines of transmembrane pressure and flow rate, along the membrane tube. Ultrafiltration of dextran T500 aqueous solution in a tubular microporous ceramic module has been carried out under various feed concentrations, transmembrane pressures, and feed flow rates, and many experimental data of ten-point local permeate fluxes along the tube were obtained to confirm the correlation predictions. The increment of concentration polarization, as well as the decline of permeate flux, along the tube was also discussed.     

Utilization of solar energy for direct contact membrane distillation process: An experimental study for desalination of real seawater

Membrane distillation (MD), a non-isothermal membrane separation process, is based on the phenomenon that pure water in its vapor state can be extracted from aqueous solutions by passing vapor through a hydrophobic microporous membrane when a temperature difference is established across it. We used three commercially available hydrophobic microporous membranes (C02, C07 and C12; based on the pore size 0.2, 0.7 and 1.2 μm respectively) for desalination via direct contact MD (DCMD). The effects of operating parameters on permeation flux were studied. In addition, the desalination of seawater by solar assisted DCMD process was experimentally investigated. First, using solar power only short-term (one day), successful desalination of real seawater was achieved without temperature control under the following conditions: feed inlet temperature 65.0 °C, permeate inlet temperature 25.0 °C, and a flow rate of 2.5 L/min. The developed system also worked well in the long-term (150 days) for seawater desalination using both solar and electric power. Long-term test flux was reduced from 28.48 to only 26.50 L/m²hr, indicating system feasibility.     

气液两相流强化卷式纳滤膜分离硫酸镁水溶液

气液两相流强化卷式纳滤膜分离实验是针对DK2540卷式纳滤膜,采用气液两相流强化分离技术,对硫酸镁溶液进行研究,较系统地研究了温度、料液浓度、过膜压力、料液流速、气体流速等因素在分离硫酸镁溶液时,对膜通量、截留率和膜通量增加率的影响,并总结了气液两相流强化效果。结果表明,气液两相流强化卷式纳滤膜分离有明显的效果。温度宜在30~40℃。料液浓度越大、过膜压力越小、气液比越大,气液两相流强化效果越明显。     

Optimization and modeling of the performance of polydimethylsiloxane for pervaporation of ethanol−water mixture

Response surface methodology was used to optimize the performance of pervaporation of ethanol aqueous solution using polydimethylsiloxane hollow-fiber membrane. The effects of four operating conditions, that is, the feed temperature (30–50°C), the feed flow rate (10–50 L/h), ethanol concentration (5–20 wt%), and the vacuum pressure (10–50 KPa) on the membrane selectivity and the total flux of permeation were investigated with response surface methodology. The results showed that a quadratic model was suggested for both selectivity and total flux showing a high accuracy with R² = 0.9999 and 0.9995, respectively. The developed models indicated a significant effect of the four studied factors on both selectivity and total flux with some significant interactions between these factors. The optimum selectivity was 15.56, achieved for a feed temperature of 30°C, feed flow rate of 10 L/h, ethanol concentration of 15 wt%, and a permeate pressure of 10.74 KPa whereas the optimum total flux was 1833.66 g/m².h was observed for at a feed temperature of 50°C, a feed flow rate of 50 L/h, ethanol concentration of 15 wt%, and a permeate pressure of 49.38 KPa.     

Development of a Simulation Model Predicting Performance of Reverse Osmosis Batch Systems

Abstract

A model describing concentration profiles and performance relationships for the operation of a reverse osmosis system with a spiral-wound membrane module has been developed. The model is an enhancement of a previously proposed treatment and encompasses a discussion of mass transfer relationships and overall and component mass balances on a system operating in a closed-loop concentrating or recycling pattern. The presentation of mass transfer within a spiral wound membrane module is treated with an overall module approach with solution-diffusion mass transfer parameters empirically determined. A comparison with various methods to represent membrane feed side concentration and their effect on membrane performance is presented. A perspective is made on representing concentration polarization in this type of membrane configuration. The simulation model is verified with experimental data on simple aqueous salt systems. The simulations are excellent in predicting feed concentration profiles. Permeate flux deviates moderately and the permeate concentration only deviates significantly at high recoveries. A more exact depiction of the feed-side concentration improves the correlation to experimental data, but a more simplistic treatment may suffice under certain process conditions. The concentration polarization coefficient utilized was found to depend more heavily on the increase in flux due to a decrease in feed-side concentration rather than in the direct increase in feed concentration.     

Concentration of apple juice using direct contact membrane distillation

The direct contact membrane distillation process (DCMD) has been used to investigate the concentration of apple juice. Results show that at a constant temperature of juice in the hot cell, an increase in the flux permeates of DCMD resulted in reducing the temperature of cooling water in the cold cell. Increasing the temperature of juice in the hot cell reduces influence of the cooling water temperature in the cold cell on the flux permeate of DCMD. The influence of temperature polarization on the effectiveness of DCMD in apple juice concentration has also been detected. The dependence of flux permeates on operating temperature. The concentration of soluble substances in concentrate and hydrodynamic conditions in the experimental equipment has also been studied. In the concentration of apple juice, 50% of solids content was obtained when the permeate flux reached about 9 l/m2·h. Further concentration of juice to 60–65% solids resulted in reduced productivity (3.8–3.0 l/m²·h) and therefore a decrease in the biological value of the concentrate.     

Performance characterization and steady-state modelling of spinning basket membrane module

Spinning basket membrane module is a special type of shear-enhanced device with inbuilt cleaning facility. In this article, the performance of this module has been reported in the ultrafiltration of bovine serum albumin. High permeate flux (=8.7 × 10^–5 m³.m^–2.s^–1) under moderate condition of transmembrane pressure (TMP) (=588.4 kPa) clearly indicates the efficacy of the design in reducing concentration polarization. A two-parameter, semi-empirical, steady-state model has also been proposed and validated under varied parametric conditions of TMP, feed concentration and rotational speed. Maximum absolute deviation between the model prediction and the experimental data was restricted only within 6%.     

Pervaporation of flavors with hydrophobic membrane

Using a pervaporation process, a surface-modified hydrophobic membrane was used for recovery of esters which are volatile organic flavor compounds; ethyl acetate (EA), propyl acetate (PA), and butyl acetate (BA). A surface-modified tube-type membrane was used to evaluate the effects of the feed concentration (0.15–0.60 wt%) and feed temperature (30–50 °C) on the separation of EA, PA, and BA from dilute aqueous solutions. The permeation flux increased with the increasing feed ester concentration and operating temperature. EA, PA, and BA in the permeate were concentrated up to 9.13–32.26, 11.44–34.95, and 14.96–36.37 wt%, respectively. The enrichment factors for the 0.15–0.60 wt% feed solution of EA and BA were in the range of 48.5-62.8 and 97.7-101.5, respectively. Phase separation occurred in the permeate stream because the ester concentration in the permeate was above the saturation limit. This meant that selectivity of the membrane was high enough for the recovery of esters from dilute aqueous solution, even though the enrichment factor of the membrane was lower than that of non-porous PDMS membrane. The fluxes of EA, PA, and BA at 0.60 wt% (6,000 ppm) feed concentration and 40 °C were 254, 296, and 318 g/m².hr, which are much higher than those obtained with polymer membranes. In the case of non-porous PDMS at feed concentrations of 90-4,800 ppm and at 45 °C, it was reported that the permeate flux of EA was 1.1–5.8 g/m^2.h. Compared to non-porous PDMS, the surface-modified membrane investigated in this study showed a much higher flux and enough selectivity of esters.     

Application of membrane distillation technology in the treatment of table olive wastewaters for phenolic compounds concentration and high quality water production

Direct contact membrane distillation process (DCMD) is proposed for the treatment of table olive wastewaters (TOW) for high quality water production and concentration of their phenolic compounds. The main objective was to investigate the effectiveness of DCMD process to concentrate phenolic compounds from TOW that can be reused as a potential source for powerful natural antioxidants. The performance of three commercial membranes, made from polytetrafluoroethylene supported by polypropylene net (TF200, TF450 and TF1000), were tested. The permeate fluxes and polyphenols concentration in both the permeate and retentate have been monitored under different DCMD operating temperatures. It was found that the three membranes exhibit an excellent separation coefficient (greater than 99.5%) even after 4 h of DCMD operation with TOW. High concentration factors were obtained with the membrane TF450 at 70 °C, while the membrane TF200 having the lowest pore size was found to be more resistant to fouling phenomenon compared to the other membranes since the reduction of its water permeate flux after TOW treatment did not exceed 2.9%. High quality of the permeate was obtained with phenolic concentration lower than 16 mg of TYE/L. In addition, the values of electrical conductivity of the permeate were lower than 193 μS/cm for the membranes TF450 and TF200, and lower than 355 μs/cm for the membrane TF1000. Consequently, DCMD proved to be an effective process for the treatment of TOW for high quality water production and a phenolic-rich concentrate.