MD Process Research Articles

Nanoparticle Separation Using Direct Contact Membrane Distillation and Its Fouling Study

Direct contact membrane distillation (DCMD) which emerges as an alternative separation technology can effectively perform a colloidal separation process under thermal driven force. DCMD is capable of extracting pure water from aqueous solutions containing non-volatile nanoparticles through the hydrophobic microporous membrane when a vapour pressure difference was established across the membrane. This work aims to study the efficiency of the MD process in separating TiO2 nanoparticles. It was interesting to find out that below 1.0 g/L TiO2 concentration, no sign of flux reduction was noticed. It is indicated that the pore blocking phenomenon was not significant. However, as concentration exceeding 1.0 g/L, the flux started to decline due to the resistance of the gelation layer which impeded water from flowing through the membrane. The blocking law analysis showed that the cake layer was developed within 3 hours of operation. At higher feed velocity, the flux declination problem could be solved due to the surface scouring effect.

Membrane distillation pilot plant trials with pharmaceutical residues and energy demand analysis

In this study, an air gap membrane distillation (AGMD) system at pilot scale is applied for purification of effluent from a municipal wastewater treatment plant. A district heating network (DHN) is considered as a heat source for the membrane distillation system. Removal performance of pharmaceutical residuals, specific heat demand, and economic assessments were analyzed on the membrane distillation plant. Almost all targeted pharmaceutical compounds were removed to a very high degree, often below the method detection limit. The heat requirement for the MD process could be sufficiently supplied by the low-temperature district heating return line. Specific heat demands for the AGMD ranges from 692 to 875kWh/m3 without heat recovery and as low as 105kWh/m3 when heat recovery is possible. Different approaches to integrating the MD within the DHN system were analyzed; the advantages and shortcomings of each are discussed with emphasis on the MD system’s capacity requirement and annual heat demand. The thermoeconomic analyses from this study presented the potential for energy optimization regarding heat recovery and module design improvement of the current MD equipment.

Effect of pretreatment on fouling propensity of shale gas wastewater in membrane distillation process

AbstractMembrane distillation (MD) has potential to concentrate produced water from the hydraulic fracturing process, leading to reduced disposal cost for shale gas wastewater. However, it is anticipated that MD suffers from severe membrane fouling if it is directly applied to treat such wastewater. Accordingly, this study explored the techniques for pretreatment of shale gas wastewater to mitigated fouling in MD process. Microbubble treatment and filtration were considered in lab-scale experiments. A synthetic shale gas wastewater was prepared based on the compositions of the real produced water and flowback water.Experimental results showed that the microbubble pretreatment could be successfully applied for MD treatment of the synthetic shale gas wastewater. Without pretreatment, flux decline in MD system was found to be very high (>90% decrease within 600 min). After the pretreatment using microbubbles, the flux maintained constant even after 700 min. This was attributed to the removal of colloidal par...

Experimental Study and Numerical Simulation of the Air Gap Membrane Distillation (AGMD) Process

Abstract Some challenges, including inappropriate distribution of currents on the membrane surface, poor hydrodynamics and existing severe temperature polarization (TP) phenomenon in MD modules, impede industrialization of MD process. Computational fluid dynamics (CFD) method was used for numerical simulation of hydrodynamics in air gap membrane distillation modules. One of two simulated modules in this work is a novel developed one in which heat and mass transfer data was compared with available literature data. Moreover, the effect of using baffles in module was investigated. Comparison between the novel module and conventional module indicates higher trans-membrane mass flux and gained output ratio (GOR) coefficient by 7% and 15%, respectively. Moreover, the effects of different operating conditions including feed temperatures and feed flow rates on permeate flux were investigated.

Integrated forward osmosis-membrane distillation process for human urine treatment

This study demonstrated a forward osmosis-membrane distillation (FO-MD) hybrid system for real human urine treatment. A series of NaCl solutions at different concentrations were adopted for draw solutions in FO process, which were also the feed solutions of MD process. To establish a stable and continuous integrated FO-MD system, individual FO process with different NaCl concentrations and individual direct contact membrane distillation (DCMD) process with different feed temperatures were firstly investigated separately. Four stable equilibrium conditions were obtained from matching the water transfer rates of individual FO and MD processes. It was found that the integrated system is stable and sustainable when the water transfer rate of FO subsystem is equal to that of MD subsystem. The rejections to main contaminants in human urine were also investigated. Although individual FO process had relatively high rejection to Total Organic Carbon (TOC), Total Nitrogen (TN) and Ammonium Nitrogen (NH4+-N) in human urine, these contaminants could also accumulate in draw solution after long term performance. The MD process provided an effective rejection to contaminants in draw solution after FO process and the integrated system revealed nearly complete rejection to TOC, TN and NH4+-N. This work provided a potential treatment process for human urine in some fields such as water regeneration in space station and water or nutrient recovery from source-separated urine.

A novel microwave assisted photo-catalytic membrane distillation process for treating the organic wastewater containing inorganic ions

A novel microwave assisted photo-catalytic membrane distillation (MPMD) process was proposed in this study to treat organic wastewater containing inorganic ions. Humic acid (HA) solution containing Ca2+ was used as feed to investigate the effects of microwave irradiation and ultraviolet (UV) radiation on the MD process. The membrane surface and composition of the fouling layer was studied using scanning electron microscopy (SEM) coupled with energy dispersion spectrometry (EDS). Ca2+ affected the permeation flux by forming complex compounds with humic acids and resulted in coagulation on the membrane surface in a conventional MD process. Microwave irradiation effectively decreased the deposit on membrane surface by destabilizing complex compounds. Organic matter was photo-degraded by the microwave assisted photo-catalysis so that the membrane fouling was prevented. In the MPMD process, the flux remained nearly constant. After 45h of operation with the continuously concentrated solution, the concentration of the solution in photocatalytic reactor was about 2.8 times than the initial and J/J0 equaled to 94.5%. In the end of MPMD process, J/J0 equaled to 98.5%. The conductivity of the distillate over the operation time was less than 4μScm−1. MPMD was used to treat the coal gasification wastewater. The results clearly demonstrated that the MPMD process was sufficient to treat the coal gasification wastewater with a removal rate higher than 96% for CODcr and 98% for NH4+-N after an operation over 120h.

Electrospun Superhydrophobic Organic/Inorganic Composite Nanofibrous Membranes for Membrane Distillation.

Electrospun superhydrophobic organic/inorganic composite nanofibrous membranes exhibiting excellent direct contact membrane distillation (DCMD) performance were fabricated by a facile route combining the hydrophobization of silica nanoparticles (SiO2 NPs) and colloid electrospinning of the hydrophobic silica/poly(vinylidene fluoride) (PVDF) matrix. Benefiting from the utilization of SiO2 NPs with three different particle sizes, the electrospun nanofibrous membranes (ENMs) were endowed with three different delicate nanofiber morphologies and fiber diameter distribution, high porosity, and superhydrophobic property, which resulted in excellent waterproofing and breathability. Significantly, structural attributes analyses have indicated the major contributing role of fiber diameter distribution on determining the augment of permeate vapor flux through regulating mean flow pore size (MFP). Meanwhile, the extremely high liquid entry pressure of water (LEPw, 2.40 ± 0.10 bar), robust nanofiber morphology of PVDF immobilized SiO2 NPs, remarkable mechanical properties, thermal stability, and corrosion resistance endowed the as-prepared membranes with prominent desalination capability and stability for long-term MD process. The resultant choreographed PVDF/silica ENMs with optimized MFP presented an outstanding permeate vapor flux of 41.1 kg/(m(2)·h) and stable low permeate conductivity (∼2.45 μs/cm) (3.5 wt % NaCl salt feed; ΔT = 40 °C) over a DCMD test period of 24 h without membrane pores wetting detected. This result was better than those of typical commercial PVDF membranes and PVDF and modified PVDF ENMs reported so far, suggesting them as promising alternatives for MD applications.

Water desalination using membrane distillation with acidic stabilization of scaling layer thickness

Membrane scaling during water desalination by membrane distillation has been studied. A tap water was applied as a feed and diluted HCl solutions were used for periodical cleaning of module. The membrane morphology and the composition of deposits were determined using scanning electron microscopy coupled with energy dispersion spectrometry. The CaCO3 deposit covered not only the membrane surfaces but was also formed in the pores' interior. During the membrane rinsing an HCl solution dissolved the deposit inside the pores, which is synonymous with flooding of these pores by liquid. Due to the membrane wetting, a significant flux decline was observed. It was confirmed that the rate of membrane wettability might be limited by shorting a period between the cleaning operations. In this work the membranes were cleaned every 1–5h, and as a result, the penetration of deposit into the pores was restricted up to the depth of 30μm during 200h of MD process. A periodical rinsing of membranes by HCl solutions did not have a negative influence on the membrane performance, and the MD module efficiency was stabilized during this period.

Interaction of humic substances on fouling in membrane distillation for seawater desalination

The interaction of humic substances on organic and biofouling during the treatment of seawater using direct contact membrane distillation (DCMD) was studied. Organic fouling was investigated at different feed temperatures in terms of distillate flux pattern and detailed organic characterization using liquid chromatography-organic carbon detection. The penetration of organics through the membrane pores was observed with SEM cross-section analysis. Also, chemical cleaning analysis showed seawater organic fouling was irreversible in DCMD. Humic substances and low molecular weight (LMW) organics are dominant organic contents in seawater. In the tested DCMD system, humic substances were thermally disaggregated to LMW–humic organics. This phenomenon was more significant in the presence of salt (NaCl), while inorganic scalant (CaSO4) reduced the disaggregation of humic substances due to the binding effect. In this study, the assimilable organic carbon (AOC) test was used to estimate the biological fouling potential during the MD process. AOC concentration is closely associated with biofilm growth in water and on the membrane (biofouling). The AOC concentration increased as the concentration of LMW–humic organics increased. This indicated the possibility of biofouling increasing in the feed and on the membrane in the MD process.

Evaluation of MD process performance: Effect of backing structures and membrane properties under different operating conditions

The paper presents investigations on the process performance of different membranes with and without backing structure in direct contact membrane distillation. Influences of backing structures and their orientation are identified. An integrated membrane and backing model was developed. Laminates with different backing designs were examined over a wide range of operating conditions and compared to results for the same membranes without backing. Experimental results and model predictions for flux and thermal efficiency are compared. The model predictions agree well with the experimental results. An influence on the effective area for diffusion, an increase in effective diffusion path length in the membrane and the formation of a complex network of thermal resistances are considered to be the main effects, leading to a significant reduction in process performance due to backing structures. The backing was identified to be one of the key components for further improvement in MD applications. An integrated assessment of membrane properties in combination with backing structures was carried out by analysing membranes with different thicknesses and different pore sizes. The experimental observations show good agreement with theoretical considerations. A combined analysis of specific flux and thermal efficiency leads to comprehensive understanding of the process and its potential for optimisation.

셰일가스 플랜트 용수 처리를 위한 직접 접촉 막 증발법 적용 가능성 연구

Non-conventional oil resources such as shale gas are becoming increasingly important and have drawn the attention of several major oil companies all over the world. Nevertheless, the market-changing growth of shale gas production in recent years has resulted in the emergence of environmental and water management challenges. This is because the water used in the hydraulic fracturing process contains large amount of pollutants including ions, organics, and particles. Accordingly, the treatment of this flowback water from shale gas plant is regarded as one of the key technologies.<BR> In this study, we examined the feasibility of membrane distillation as a treatment technology for the water from shale gas plants. Direct contact membrane distillation (DCMD) is a thermally-driven process based on a vaper pressure gradient across a hydrophobic membrane, allowing the treatment of feed waters containing high concentration of ions. Experiments were carried out put in the lab-scale under various conditions such as membrane types, temperature difference, flow rate and so on. Synthetic feed water was prepared and used based on the data from literature. The results indicated that DCMD is suitable for treating not only low-range flowback water but also high-range flowback water. Based on the theoretical calculation, DCMD could have over 80% of recovery. Nevertheless, organic pollutants such as oil and surfactant were identified as serious barriers for the application of MD. Further works will be required to develop the optimum pretreatment for this MD process.

Preparation and properties of PVDF composite hollow fiber membranes for desalination through direct contact membrane distillation

► PVDF composite hydrophobic hollow fibers were prepared for membrane distillation. ► Influence of nano-particles on the membranes characteristics was investigated. ► The mechanical properties of the PVDF composite membranes were enhanced obviously. ► The spun fiber presented excellent performance stability and hydrophobicity. The polyvinylidene fluoride (PVDF) composite hydrophobic hollow fiber membranes were fabricated for membrane distillation through non-solvent induced phase inversion by dispersing hydrophobic modified calcium carbonate nano-particles in the PVDF casting solution and using the mixture of LiCl and polyethylene glycol (PEG) as non-solvent additive. The influence of nano-particles on the spun hollow fiber membrane characteristics was investigated. The addition of hydrophobic nano-particles could optimize the sandwich-like morphology, narrow the pore size distribution, improve the membrane porosity and increase the membrane surface roughness and contact angle to some extent. The nano-particles also enhanced the crystallinity degree and thermal stability of the hollow fiber. In addition, the composite membranes presented stronger mechanical properties compared to the fiber without particles. During the desalination process of 3.5 wt% sodium chloride solution, the maximum transmembrane permeate flux about 46.3 kg/m 2 h with a lower thermal loss was obtained from the composite fiber when the hot feed inlet temperature and the cold distillate inlet temperature were at 80.5 °C and 20.0 °C, respectively. Furthermore, the composite membrane exhibited satisfying performance stability compared with the pure PVDF membrane in the 30 days continuous desalination experiments, indicating that the as-spun composite fiber may be of great potential to be utilized in MD process for desalination.

Analysis of heat and mass transfer by CFD for performance enhancement in direct contact membrane distillation

A comprehensive analysis on the dominant effects for heat and mass transfer in the direct contact membrane distillation (DCMD) process has been performed with the aid of computational fluid dynamics (CFD) simulations for hollow fiber modules without and with annular baffles attached to the shell wall. Potential enhancement strategies under different circumstances have been investigated. Numerical simulations were carried out to investigate the effect of the MD intrinsic mass-transfer coefficient of the membrane (C) on the performance enhancement for both non-baffled and baffled modules. It was found that the temperature polarization coefficient (TPC) decreases significantly with increasing C value regardless of the existence of baffles, signifying a loss of overall driving force. However, the higher C compensated for this and the mass flux showed an increasing trend. A membrane with a lower C value was found to be less vulnerable to the TP effect. In this case, the introduction of turbulence aids such as baffles did not show substantial effect to improve system performance. In contrast, introducing baffles into the module can greatly enhance the mass flux and the TPC for a membrane with a high C value, where the main heat-transfer resistance is determined by the fluid side boundary layers. The effect of operating temperature on heat and mass transfer in the MD process was also studied with a membrane of a lower C value (2.0 × 10−7 kg m−2 s−1 Pa−1). Although the TPC generally decreased with increasing operating temperatures, the mass flux Nm increased significantly when operating temperature increased. A baffled module showed a more significant improvement than a non-baffle module at a higher temperature. Moreover, it was confirmed that higher operating temperatures are preferable for a substantial improvement in the heat/mass transfer as well as MD thermal efficiency, even with a relatively small transmembrane temperature difference of 10 K.

Application of membrane distillation process for tap water purification

Membrane distillation process was used for purification of pre-treated natural water (tap water). The rejection of inorganic and organic compounds in this process was investigated. The obtained rejection of inorganic solutes was closed to 100%, but the volatile organic compounds (VOCs) diffused through the membrane together with water vapour. The content of trihalomethanes (THMs) in the obtained distillate was two-three fold higher than that in the feed, therefore, the rejection of the total organic compounds present in the tap water was reduced to a level of 98%. The intensive membranes scaling was observed during the water separation. The morphology and composition of the fouling layer was studied using scanning electron microscopy coupled with energy dispersion spectrometry. The influence of thermal water pre-treatment performed in a heat exchanger followed by filtration on the MD process effectiveness was evaluated. This procedure caused that significantly smaller amounts of <TEX>$CaCO_3$</TEX> crystallites were deposited on the membrane surface, and a high permeate flux was maintained over a period of 160 h.

Integration of photocatalysis and membrane distillation for removal of mono- and poly-azo dyes from water

The paper reports investigations on the application of anatase-phase TiO 2 for the removal of azo dyes in a hybrid system coupling photocatalysis with direct contact membrane distillation (DCMD, MD). The process was conducted in a laboratory-scale installation equipped with a PP capillary module. The influence of reaction temperature and initial concentration of azo dyes on the effectiveness of their photodegradation was especially investigated. Two mono-azo dyes: Acid Red 18 (AR18) and Acid Yellow 36 (AY36) and one poly-azo dye, Direct Green 99 (DG99) were applied as model compounds. The increase of the reaction temperature from 313 to 333 K resulted in an improvement of the efficiency of photodecomposition of the dyes, as was found on the basis of changes of their masses in the feed solution. The comparison of the results obtained during photocatalysis alone and hybrid photocatalysis–MD process revealed that the reduction of feed volume in MD did not affect the photodegradation rate of the azo dyes. An improvement of the effectiveness of the degradation of dyes was obtained by an application of solutions with lower initial concentration (10 instead of 30 mg/dm 3). Regardless of the process parameters applied, the product (distillate) was almost pure water with conductivity lower than 0.3 mS/m and pH above 5.2.

Evolutionary artificial neural networks by multi-dimensional particle swarm optimization

In this paper, we propose a novel technique for the automatic design of Artificial Neural Networks (ANNs) by evolving to the optimal network configuration(s) within an architecture space. It is entirely based on a multi-dimensional Particle Swarm Optimization (MD PSO) technique, which re-forms the native structure of swarm particles in such a way that they can make inter-dimensional passes with a dedicated dimensional PSO process. Therefore, in a multidimensional search space where the optimum dimension is unknown, swarm particles can seek both positional and dimensional optima. This eventually removes the necessity of setting a fixed dimension a priori, which is a common drawback for the family of swarm optimizers. With the proper encoding of the network configurations and parameters into particles, MD PSO can then seek the positional optimum in the error space and the dimensional optimum in the architecture space. The optimum dimension converged at the end of a MD PSO process corresponds to a unique ANN configuration where the network parameters (connections, weights and biases) can then be resolved from the positional optimum reached on that dimension. In addition to this, the proposed technique generates a ranked list of network configurations, from the best to the worst. This is indeed a crucial piece of information, indicating what potential configurations can be alternatives to the best one, and which configurations should not be used at all for a particular problem. In this study, the architecture space is defined over feed-forward, fully-connected ANNs so as to use the conventional techniques such as back-propagation and some other evolutionary methods in this field. The proposed technique is applied over the most challenging synthetic problems to test its optimality on evolving networks and over the benchmark problems to test its generalization capability as well as to make comparative evaluations with the several competing techniques. The experimental results show that the MD PSO evolves to optimum or near-optimum networks in general and has a superior generalization capability. Furthermore, the MD PSO naturally favors a low-dimension solution when it exhibits a competitive performance with a high dimension counterpart and such a native tendency eventually yields the evolution process to the compact network configurations in the architecture space rather than the complex ones, as long as the optimality prevails.

Membrane processes used for separation of effluents from wire productions

AbstractMetal wires are produced from different metals using drawing methods. The metal used influences both the technology applied and the composition of effluents generated during wires production. Ultrafiltration and nanofiltration are used for the separation of waste emulsions from cable factories. Membrane distillation was proposed for the treatment of acidic saline wastewater generated during steel wire manufacturing (etching). The possibility of the previously mentioned processes application for water reuse is presented. The application of poly(vinylidene fluoride) (PVDF) membranes (FP 100) with the molecular weight cut-off (MWCO) of 100 kDa in the ultrafiltration process resulted in the reduction of 99 % of oil and lubricants in the treated emulsions and allowed complete removal of suspended solids and colloidal substances. Such pre-treated emulsion was subsequently purified by nanofiltration (NF-90-2540) and a 98 % rejection of copper ions was achieved, resulting in a decrease of the permeate electrical conductivity from 3200 μS cm−1 to 260 μS cm−1. The obtained permeate was suitable for preparation of fresh oil emulsion utilized for lubrication in the wire drawing process. The spent etching baths (from steel wire production), which mainly contained FeSO4 and about 1 mass % of sulfuric acid, were separated by membrane distillation. The obtained permeates were: clean water with electrical conductivity at a level of 3–5 μS cm−1. Concentrates (190–200 g of Fe per L) from the MD process were cooled to 295 K, which enabled the FeSO4 crystallization. Application of the above-mentioned membrane processes allows producing high quality product water, over 90 % of water was recovered from the treated wastewaters.

Photocatalytic membrane reactor (PMR) coupling photocatalysis and membrane distillation—Effectiveness of removal of three azo dyes from water

The results of the investigation on the possibility of coupling photocatalysis and direct contact membrane distillation (DCMD, MD) for degradation of azo dyes (Acid Red 18, Acid Yellow 36 and Direct Green 99) in aqueous solution are described. TiO 2 Aeroxide ® P25 (Degussa, Germany) was applied as a photocatalyst. It was found that the addition of TiO 2 P25 to the feed did not affect the distillate flux, regardless of the catalyst concentration applied. The MD process is very effective in separation of photocatalyst particles. After 5 h of the experiment, the turbidity of distillate was similar to that measured for ultrapure water, regardless of the TiO 2 loading used. The highest effectiveness of photodecomposition was obtained in case of Acid Red 18, and the most difficult degraded was Acid Yellow 36, having the lowest molecular weight from all the dyes used. The composition of the feed solution slightly influenced the quality of distillate. Among the volatile compounds that passed through the membrane some organic compounds were present as was indicated by the measurements of TOC concentration. After 5 h of the process the TOC concentration, conductivity and TDS content in distillate did not exceed the value of 1.2 mg C/dm 3, 2.2 μS/cm and 1.4 mg/dm 3, respectively, being the highest in case of Direct Green 99.

Water demineralisation by NF/MD integrated processes

The production of demineralised water from drinking water by nanofiltration/membrane distillation system (NF/MD) has been investigated. The precipitation of CaCO 3 on the membrane surface and a rapid flux decline was observed when tap water was used directly as a feed. The problem of scaling was eliminated by the acidification of the feed to pH = 4 and stable module efficiency was observed. The performance of the MD module was significantly improved when tap water was subjected to softening in NF process. The precipitation of the silica compounds during the operation of MD process with the NF permeate was found. These compounds caused clogging of the capillary membrane inlets, resulting in a gradual decline of the module efficiency. The application of filtration net assembled at the module inlet prevented blocking of the capillary membrane inlets. The MD process can be run without a flux decline, associated with the scaling, when the pH of NF permeate was adjusted at a level of 4. A partial wettability of the membrane resulting in a flux decline by 30% was observed after 1100 h of process duration. The quality of obtained distillate was stable and practically independent of the feed concentration. The produced distillate has the electrical conductivity in the range of 1.5–2.5 μS cm −1 and contained 0.5 ppm of inorganic carbon and below 1 ppm TOC.

Analysis of a solar-powered membrane distillation system

Nowadays, in dry and rural areas, solar-powered membrane distillation (SPMD) technology is considered a feasiblemeans for the production of pure water from brackish water. Prior to the design and construction of a SPMD pilot plant, there is a need to predict its performance theoretically by means of a computational simulation program. Unlike previous approaches followed by other investigators to develop a mathematical model that can describe the components of a SPMD pilot plant, the developed mathematical model in this study is based on the fact that the SPMD process by nature is unsteady. The performance of a proposed SPMD pilot plant is then obtained by means of a numerical solution of the model with the aid of a simulation computer program. The results reveal that the proposed SPMD pilot plant has some unique features, which differ from a similar MD process operated at steady-state conditions in a laboratory. The analysis of the system has shown that heat recovery via an external heat exchanger is not only possible, but even effective, and an economical way to intensify the SPMD process. The plant productivity can be improved by increasing the heat-exchanger capacity (KA), decreasing the flow rates of both feed and permeate or otherwise by increasing the effective surface area of the membrane. The achieved enhancements in the SPMD pilot plant productivity are directly related to an improved heat recovery rate in the heat exchanger. However, further analysis reported in this paper shows that the increase in KA and membrane area should be optimized for any planned capacity in the design of a SPMD pilot plant.