Direct Contact Membrane Distillation Desalination Research Articles

Novel integration of thermoelectric distiller with direct contact membrane distillation

A thermoelectric distiller (TED) integrated with a direct contact membrane distillation (DCMD) unit was proposed and numerically investigated. The mathematical models of the hybrid TED--–DCMD system were developed and validated. In the proposed hybrid system, the TED provides the heating demand of the DCMD desalination unit in four novel configurations of TED-DCMD integration. The first configuration integrates the TED-DCMD by adding a DCMD heat exchanger coil inside the condensation tank of a TED, while the second configuration involves two TED condensation tanks with each tank containing a DCMD heat exchanger coil. The third configuration also consists of two TED tanks with an external heat exchanger where thermal energy from the TED produced water is transferred to the DCMD feed stream. The fourth configuration is a modification of the second configuration with a thermal recovery from TED produced water for inlet feed water preheating. To investigate the performance (distillate productivity, specific energy consumption, coefficient of performance, and ratio of TED productivity) of each integrated TED-DCMD configuration, the effects of different operating parameters such as DCMD feed flow rate, TEM current, number of TEMs, and TED produced water temperature were examined. Findings indicated that the third configuration is the most energy efficient, with specific energy consumption (SEC) reaching a minimum value of 217 kWh/m3 at low current operation of 2.1 A, and feed flow rate of 4.0 L/min for 8.0 TEMs in the TED. A maximum freshwater productivity and COPh of 2.74 kg/s and 3.24, are attained by the 4th and 3rd configurations, respectively. Results also indicated that elevating the current input encourages higher water productivity. Meanwhile, higher values of SEC and water productivity are attained when operating TED at saturation temperature.

Development of a high flux Janus PVDF membrane for oily saline water desalination by membrane distillation via PDA-TEOS-APTES surface modification

Membrane distillation (MD) is a promising approach for water desalination. However, in practice, this process is hindered by poor water flux and severe membrane fouling. In this study, a series of Janus membranes were designed for the desalination of oily saline water via direct contact membrane distillation (DCMD). Janus membranes were fabricated by a simple and facile method, direct deposition of dopamine (DA), tetraethylorthosilicate (TEOS), and aminopropyltriethoxysilane (APTES) on one side of the polyvinylidene fluoride (PVDF) membrane, which was modified by hydrophobic SiO2 nanoparticles. In brief, one side of the optimum Janus membrane was strongly hydrophobic (WCA: 149.7 ± 1°) while another side exhibited in air hydrophilicity (WA: 36.3 ± 1.2°) and underwater superoleophobicity. Additionally, the optimum Janus membrane experienced a remarkable enhancement in water flux, reaching an impressive value of 52.6 kg/m2h, (69.2 % increment compared to the pristine PVDF). Notably, the optimal Janus membrane demonstrated long-term stability and significant resistance toward oil fouling during the DCMD desalination of oily saline water while preserving its rejection and high vapor flux. The applied strategies may offer a fresh perspective on membrane design with efficient performance for MD by a facile method.

Electricity and water cogeneration using a small PWR of 75 MW(th) coupled to a DCMD desalination plant with heat recovery

The use of a small PWR of 75 MW(th) coupled to a Direct Contact Membrane Distillation (DCMD) desalination plant with heat recovery, for cogeneration of electricity and water, is investigated here. Part of the heat used for desalination is waste heat from the power plant, as the desalination plant collects warm seawater from the power plant condenser instead of taking water directly from the sea. Two distinct coupling strategies are considered. In the first one, the additional heating required for the desalination process is obtained from steam extractions, taken at the low pressure turbine, after the extracted steam has already done a substantial amount of mechanical work. The second strategy is based on splitting the steam produced in the steam generator into two parallel Rankine cycles. The condenser at one of those cycles is set to operate just below atmospheric pressure in order to provide extra heating, at a higher temperature, for the desalination plant. Both strategies analysed in this work, relying on the use of waste heat and employing a DCMD desalination plant with efficient heat recovery, are shown to be competitive with other well established thermal desalination technologies currently used in nuclear cogeneration plants, such as MED and MSF.

Development of a Lycopodium powder-based superhydrophobic nanofiber membrane suitable for desalination

A biobased, green, inexpensive additive, Lycopodium particles, which are spores of the “Lycopodium clavatum” plant, were incorporated in the poly(vinylidene fluoride) PVDF electrospun nanofiber membranes (ENMs) for desalination by direct contact membrane distillation (DCMD). Superhydrophobic ENMs were prepared using this additive (PVDF-ENMs-Lyc). Thanks to their morphological structure and their prominent surface superhydrophobicity (anti-wetting) character, the resulting PVDF-ENMs-Lyc exhibited an improved liquid entry pressure (LEP), a high void volume fraction (greater than 87.2%), a good salt rejection factor (greater than 99.93%) and a reasonably high permeate flux (greater than 51.76 kg·m−2·h−1) at 80 °C, which are of great practical importance for water desalination by DCMD. The optimum membrane prepared with 3 wt% Lycopodium in the dope solution demonstrated a stable permeate flux of 52.4 ± 0.6 kg·m−2·h−1 with an electrical conductivity around 4.76 ± 0.46 μS/cm (NaCl rejection factor of 99.998 ± 0.036%) during 25 h DCMD desalination experiment using 35 g/L NaCl aqueous solution (similar to seawater concentration). The presented results pave the way for superhydrophobic nanofibrous membrane engineering suitable for membrane contactors by electrospinning in a single step without surfactants, organic additives, or chemical post-treatments, just by the incorporation of a green additive like Lycopodium powder.

Hydrophobic metal-organic framework@graphene oxide membrane with enhanced water transport for desalination

As a new generation of high-performance alternative, graphene-based membranes have shown great prospect in water purification. Among them, numerous efforts are concentrated on designing hydrophilic membranes while less attention is paid to hydrophobic membranes, which limit the potential application of graphene-based membranes, such as desalination via membrane distillation (MD) process. Anisotropic heat transfer and easy hydrophobic modification of graphene oxide (GO) make it suitable for MD membrane fabrication. Herein, we developed a novel porous and hydrophobic MOF-801@GO-PDMS membrane with enhanced water vapor transport for water desalination via direct contact membrane distillation (DCMD). MOF-801 nanoparticles were in-situ grown within GO interlayer followed with vacuum filtration to fabricate MOF-801@GO membrane which was subsequently modified with polydimethylsiloxane (PDMS) to fabricate the MOF-801@GO-PDMS membrane. The water-stable porous MOF-801 nanoparticle was introduced to increase the porosity of GO laminate and provide extra water vapor transport channels. PDMS was employed to ensure the membrane hydrophobic stability by utilizing its intrinsic hydrophobicity and segment entanglement. The optimum MOF-801@GO membrane exhibited water flux of 29.24 kg m−2 h−1 and NaCl rejection of over 99.99%, as well as robust stability during continuous DCMD desalination test. This work not only gives a promising candidate for water desalination, but also provides a method to design hydrophobic and porous graphene-based nanostructures for other potential applications.

Computational model and simulation of DCMD desalination systems with heat recovery

Desalination technologies can help mitigating the water scarcity problem facing humankind. Membrane distillation (MD) is a desalination technology attractive for its adequacy to use low-grade waste heat or renewable energy sources. This work addresses the modelling of a Direct Contact Membrane Distillation (DCMD) plant with heat recovery. The desalination unit comprises several DCMD modules, each of a shell and hollow fibre tube bundle type. A multiscale approach is needed to address the problem. It includes modelling of the vapour flux through the micro/nano porous membrane, models for the mass and energy conservation at the scale of a single DCMD module, and thermodynamic modelling at the scale of the desalination plant. Analytical solutions of the mass and heat transfer across the membranes are combined with finite volume discretised equations describing heat and mass conservation for the feed and permeate streams. An iterative scheme is devised to solve the model equations in order to determine the flow and temperature variables in the DCMD module. The computational model predictions show good agreement with experimental data available in the literature. Finally, the methodology presented herein is used to simulate the performance of a DCMD desalination plant with heat recovery.

Electrospun Nanostructured Membrane Engineering Using Reverse Osmosis Recycled Modules: Membrane Distillation Application.

As a consequence of the increase in reverse osmosis (RO) desalination plants, the number of discarded RO modules for 2020 was estimated to be 14.8 million annually. Currently, these discarded modules are disposed of in nearby landfills generating high volumes of waste. In order to extend their useful life, in this research study, we propose recycling and reusing the internal components of the discarded RO modules, membranes and spacers, in membrane engineering for membrane distillation (MD) technology. After passive cleaning with a sodium hypochlorite aqueous solution, these recycled components were reused as support for polyvinylidene fluoride nanofibrous membranes prepared by electrospinning technique. The prepared membranes were characterized by different techniques and, finally, tested in desalination of high saline solutions (brines) by direct contact membrane distillation (DCMD). The effect of the electrospinning time, which is the same as the thickness of the nanofibrous layer, was studied in order to optimize the permeate flux together with the salt rejection factor and to obtain robust membranes with stable DCMD desalination performance. When the recycled RO membrane or the permeate spacer were used as supports with 60 min electrospinning time, good permeate fluxes were achieved, 43.2 and 18.1 kg m−2 h−1, respectively; with very high salt rejection factors, greater than 99.99%. These results are reasonably competitive compared to other supported and unsupported MD nanofibrous membranes. In contrast, when using the feed spacer as support, inhomogeneous structures were observed on the electrospun nanofibrous layer due to the special characteristics of this spacer resulting in low salt rejection factors and mechanical properties of the electrospun nanofibrous membrane.

Improvement of nanostructured electrospun membranes for desalination by membrane distillation technology

A systematic study is carried out to determine the optimum electrospinning preparation condition to prepare an adequate electrospun nanofibrous membrane (ENM) for direct contact membrane distillation (DCMD). A structural properties investigation of ENM was carried out because of the significant impact of its architectural structure, nanofiber diameter, inter-fiber space and ENM thickness, on DCMD performance. The morphology, hydrophobicity, mechanical properties, crystallinity and DCMD desalination were investigated. A long-term DCMD experiment (100h) was carried out using 30 g/L NaCl aqueous solution, both in the feed and permeate side of the optimum ENM membrane to evaluate its potential to produce drinkable water in case of lack of distilled water, for instance in a remote area, emergency situation, and/or portable system. In this case, drinkable water could be produced after 28 h with a permeate flux of 57.5 kg/m2.h and a salt rejection factor greater than 99.9%. Another long-term DCMD experiment (65 h) was conducted using 30 g/L NaCl aquesous solution as feed but at a higher temperature and distilled water as permeate to evaluate the desalination stability, wettability and scaling of the optimum ENM. A permeate flux of 58.5 kg/m2.h was obtained with a salt rejection factor greater than 99.9%.

Flat sheet direct contact membrane distillation desalination system using temperature-dependent correlations: thermal efficiency via a multi-parameter sensitivity analysis based on Monte Carlo method

In this paper, a 1D model of direct contact membrane distillation is presented in which all fluid properties are temperature-dependent. In addition, a Nusselt number (Nu) relationship for developing flow in ducts (accounting for both thermal and hydrodynamic effects) is used to obtain the convective heat transfer coefficient at each side of the membrane. Simulated mass flux shows very good agreement with experimental measurements at various feed temperature, flow rate and concentration. A comprehensive sensitivity analysis of all operational and geometrical parameters, as well as Nu estimation parameters on water mass flux across the membrane $$(J_{\text{m}} )$$ and thermal efficiency, is also done. To determine the relative importance of each parameter, a multi-parameter sensitivity analysis (MPSA) based on the Monte Carlo method is applied, and the sensitivity index of each parameter at the defined range is computed. Results show that $$J_{\text{m}}$$ is highly sensitive to bulk feed inlet temperature ( $$T_{{{\text{in}},{\text{f}}}}$$ ) while both $$J_{\text{m}}$$ and thermal efficiency are highly sensitive to membrane porosity. Results show that among all parameters, just membrane porosity is highly sensitive which affects both mass flux and thermal efficiency especially at low $$T_{{{\text{in}}.{\text{f}}}}$$ .

Co-axially electrospun superhydrophobic nanofiber membranes with 3D-hierarchically structured surface for desalination by long-term membrane distillation

Electrospun nanofiber membranes (ENMs) have gained increasing interest in membrane distillation (MD) applications due to their high surface area, high hydrophobicity and porosity, and controllable pore size and membrane thickness. However, despite these advantages, ENMs still suffer from wetting issues in MD. Co-axial electrospinning is an attractive strategy for the one-step fabrication of non-woven membranes with core-sheath structures and improved wetting resistance for MD application. In the present study, we investigated poly (vinylidene fluoride-co-hexafluoropropylene) (PH) as the core and PH/silica aerogel (SiA) as the sheath to obtain superhydrophobic co-axial composite ENMs. The surface characterization results indicated that the active layer (i.e., PH) of the co-axial ENMs was rough, highly porous (>80%), and superhydrophobic (contact angle >160°). Further, the co-axial ENMs possessed small pore sizes (<0.39 μm) and a suitable liquid entry pressure (>1.72 bar). Upon the application in long-term (one month) direct contact MD testing using a 3.5 wt% NaCl solution as the feed, high water vapor flux and salt rejection of 14.5 L/m2h and 99.99% were achieved, respectively, when applying the optimal 4 wt% SiA solution loading at the sheath. The ENMs fabricated using the versatile co-axial electrospinning showed great potential for long-term applications in direct contact MD desalination.

Efecto de antiincrustantes y variables de operación en la eficiencia de desalinización por destilación por membranas por contacto directo (DMCD)

The effect of hydrodynamic conditions (feed and permeate flow rates) and the action of six different types of antiscalants on desalination of seawater from the Gulf of Mexico by direct contact membrane distillation (DCMD) was studied. The effect of feed temperature during the DCMD process was also evaluated on performance of two microfiltration membranes (0.5 µm) having different thickness (300 and 250 µm). Results showed more than 100 % increments on water vapor flux by raising feed temperature from 50 to 70 oC and feed flow rates up to 7 L·min-1. No significant effect on process performance was determined by raising permeate flow rates in the range tested. Choosing the type and concentration of antiscalants is critical during seawater desalination by DCMD. Performance of this process increased by up to 49.2 % during desalination of real seawater by adding the KMRO S-516 antiscalant designed to disperse iron, silica and calcium carbonate salts. During DCMD no important water vapor flux decay and no increase on conductivity of the distillate was observed by antiscalants’ dosage. In addition, scanning electron microscopy (SEM) from the membrane surface after DCMD desalination of seawater treated with the KMRO S-516 antiscalant confirmed the absence of scaling.

Crosslinked PVDF based hydrophilic-hydrophobic dual-layer hollow fiber membranes for direct contact membrane distillation desalination: from the seawater to oilfield produced water

Crosslinked polyvinylidene fluoride (PVDF) based hydrophilic-hydrophobic dual-layer hollow fiber membranes were fabricated for direct contact membrane distillation (DCMD). PVDF/ammonia/water dope solution was formulated with a spinning process delay of 9 days for the formation of a mechanically robust and hydrophobic crosslinked PVDF outer layer, and polyethylene glycol 6000 (PEG-6000) was introduced to create a thick hydrophilic PVDF/PEG-6000 inner layer. The influences of ammonia/water contents and the spinning process delay on the membrane morphology were studied. The DCMD performances were investigated using both simulated seawater and actual oilfield produced water as the feed solutions. The incorporation of ammonia/water mixture allowed a slow PVDF crystallization in the dope solution during the 9 days of spinning process delay. The obtained membrane DN2H2-9 showed contact angles of 133.6° and 47.1°on the surfaces of the crosslinked PVDF outer layer and PVDF/PEG-6000 inner layer, respectively. The membrane DN2H2-9 showed a superior permeate water flux of 97.6 kg m−2 h−1 and an energy efficiency of 92.8% to desalinate a 3.5% NaCl solution. The result from a 200 h continuous DCMD operation revealed a stable permeate water flux and more than 99.9% of salt rejection, which was attributed to a relatively high liquid entry pressure (LEP) of 1.95 bar and the simultaneously enhanced mechanical strength of the membrane. The membrane DN2H2-9 also demonstrated promising DCMD performance in the desalination of the real oilfield produced water with a high total dissolved solids (TDS), including almost 100% of permeate water recovery and more than 99.9% of salt rejection in 72 h of operation.

Enhancement of Flux Performance in PTFE Membranes for Direct Contact Membrane Distillation.

This work focused on enhancing the flux on hydrophobic polymeric membranes aimed for direct contact membrane distillation desalination (DCMD) process without compromising salt rejection efficiency. Successful coating of commercial porous poly-tetrafluoroethylene membranes with poly(vinyl alcohol) (PVA) was achieved by solution dipping followed by a cross-linking step. The modified membranes were evaluated for their performance in DCMD, in terms of water flux and salt rejection. A series of different PVA concentration dipping solutions were used, and the results indicated that there was an optimum concentration after which the membranes became hydrophilic and unsuitable for use in membrane distillation. Best performing membranes were achieved under the specific experimental conditions, water flux 12.2 L·m-2·h-1 [LMH] with a salt rejection of 99.9%. Compared to the pristine membrane, the flux was enhanced by a factor of 2.7. The results seemed to indicate that introducing hydrophilic characteristics in a certain amount to a hydrophobic membrane could significantly enhance the membrane distillation (MD) performance without compromising salt rejection.

Innovative swirling flow-type microbubble generator for multi-stage DCMD desalination system: Focus on the two-phase flow pattern, bubble size distribution, and its effect on MD performance

In this study, we proposed a novel air-assisted swirling flow-type microbubble generator (MBG) to improve the downstream transmembrane flux of a multistage direct contact membrane distillation (MDCMD) based seawater desalination system, and experimentally demonstrated its performance under various operating conditions. To simulate the downstream operating conditions of the MDCMD system, low temperature and high concentration conditions were implemented on the feed side in the experiments. It was observed that the formation of a gas–liquid two-phase flow in the feed stream demonstrated a positive influence on the performance of the direct contact membrane distillation (DCMD) process; particularly, microbubbles less than 100 μm performed an important role in improving the heat and mass transfer. The DCMD performance using MBG was demonstrated to be more pronounced at lower feed temperatures and higher feed flow rates, where the enhancement in permeation flux was as high as 37%. The optimal air flow rate to achieve maximum DCMD performance was determined to be 50 cc/min at a given feed flow rate, while a further increase in air flow rate incurred a slug flow in the feed channel, which resulted in a decrease in the number of microbubbles and hence a decrease in the DCMD performance. In addition, the process performance was evaluated using highly concentrated brine as the feed solution, and the results demonstrated the feasibility of applying MBG to practical desalination processes.

Activated carbon enhanced hydrophobic/hydrophilic dual-layer nanofiber composite membranes for high-performance direct contact membrane distillation

In this study, activated carbon (AC) enhanced hydrophobic/hydrophilic dual-layer nanofiber composite membranes were prepared for direct contact membrane distillation (DCMD) by incorporating different concentrations (0–3.0 wt%) of AC nanoparticles into the polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) hydrophobic active layer. The membranes were composed of two layers, a thin hydrophobic PVDF-HFP/AC electrospun active layer and a thick hydrophilic support layer. Results indicated that the incorporation of AC nanoparticles have greatly increased the membrane properties and performance. The membrane containing 1.5 wt% AC nanoparticles (M3) showed the best flux performance (45.6 L/m2 h) compared to other membranes fabricated in this study and a commercial PTFE benchmark membrane (41.8 L/m2 h) without compromising the salt rejection. A two-dimensional model was developed to investigate the flux profile of the PVDF-HFP/AC membranes. The present study suggests that the incorporation of AC nanoparticles into PVDF-HFP active layer can impart additional functionalities to the composite membranes that would enhance the membranes DCMD performance. The fabricated PVDF-HFP/AC composite membranes present great potential in the application of DCMD desalination.

Understanding the impact of membrane properties and transport phenomena on the energetic performance of membrane distillation desalination

Direct contact membrane distillation (DCMD) is a thermal desalination process that is capable of treating high salinity waters using low-grade heat. As a water treatment process, DCMD has several advantages, including the utilization of waste heat (below 100℃), perfect rejection of nonvolatile solutes, low areal footprint, and high scalability. However, the energy efficiency of DCMD is relatively low compared to other work-based and thermal desalination processes. In this study, we aim to quantify how membrane properties and process conditions affect the exergy or second-law efficiency (ηII) of a DCMD desalination system with external heat recovery. In particular, we analyze how the membrane permeability coefficient (B) and thermal conduction coefficient (K̅) impact MD performance. We show that increasing the B value of a membrane by reducing its thickness, initially leads to an increase in ηII before conductive heat loss through the membrane causes ηII to fall. For a typical MD membrane with a porosity of 0.90, material thermal conductivity of 0.20Wm−1K−1, and a nominal pore diameter of 0.6μm, we find that the optimal permeability coefficient is 1.59×10−6kgm−2s−1Pa−1 (572kgm−2h−1bar−1). This value corresponds to an optimal membrane thickness of around 95μm. Our analysis stresses the importance of effective heat recovery in DCMD. We show that an external heat exchanger with a minimum approach temperature of 5℃ reduces energy consumption by 72%. Finally, we demonstrate that increasing the ratio B/K̅, rather than just the B value, is key to increasing the exergy efficiency of DCMD desalination. For example, increasing membrane porosity from 0.70 to 0.90, which yields a 160% increase in B/K̅, leads to a 42% increase in ηII from 5.3% to 7.6%. The advantages of reducing the bulk pressure (P) in the membrane pores are also explored. For a typical membrane, halving P from 1.0bar to 0.5⁢bar, results in a 21% increase in ηII from 7.0% to 9.2%. We conclude by identifying that the maximum exergy efficiency achievable as membrane porosity tends to unity is 10% for a bulk membrane pressure of 1.0bar and 12% for a bulk membrane pressure of 0.5bar, given perfect heat recovery.

Superhydrophobic nanofiber membrane containing carbon nanotubes for high-performance direct contact membrane distillation

Tailoring the membrane to have superhydrophobicity, coupled with high porosity, adequate pore sizes and narrow pore size distribution, and thin thickness could find potential application for high-performing direct contact membrane distillation (DCMD) process. Electrospinning is an excellent approach in fabricating nanofiber membranes with adequate properties required of an MD membrane. In this study, superhydrophobic, robust, mixed matrix polyvinylidene fluoride-co-hexafluoropropylene (PcH) nanofiber membranes were fabricated incorporating different concentrations (1–5wt%) of carbon nanotubes (CNTs) as nanofillers to impart additional mechanical and hydrophobic properties. The electrospun membrane has been designed to have two cohesive layers, a thin CNT/PcH top layer and a thick neat PcH bottom layer. Through different characterization techniques, CNTs were found to be widely distributed on/in the nanofibers, where more beads-on-string were formed at higher CNT content. However, the beads-on-string did not significantly affect the membrane porosity and pore size, as well as did not degrade the MD performance. Highly-porous structure was observed for all membranes and the nanofiber membrane showed comparable pore sizes with a commercial flat-sheet PVDF membrane but at a much higher porosity (>85%). The contact angle increased to superhydrophobic at 158.5° upon the incorporation of 5wt% CNTs in the nanofiber due to increased roughness and added effect of hydrophobic CNTs. The liquid entry pressure also increased when 5wt% CNT was added compared to the neat PcH nanofiber membrane. The resulting flux of the 5wt% CNT-incorporated nanofiber membrane (24–29.5L/m2h) was consistently higher than the commercial PVDF membrane (18–18.5L/m2h), with an average increase of 33–59% depending on the feed water type (35 or 70g/L NaCl solution) without compromising the salt rejection (>99.99%). The present nanofiber membranes containing CNTs with one-step electrospinning fabrication show high potential for DCMD desalination application.

Evaluation of heat utilization in membrane distillation desalination system integrated with heat recovery

Aiming to optimize the system-level heat utilization, a pilot-scale direct contact membrane distillation desalination system integrated with heat recovery (DCMD–HX) was studied using Aspen Plus. An implicit expression of gain output ratio (GOR) was derived to reveal the interplay of heat utilization and process parameters including operating conditions, module specifications as well as membrane properties in the DCMD–HX desalination system. Compared to operating temperatures, the feed/permeate recirculating flowrates were identified as the most influential operational factors affecting the GOR. In the current settings, the maximal GOR of 6.0 was observed at low and equivalent feed- and permeate-side flowrates regardless of module specifications. Low flowrates, however, resulted in undesirable low water productivity, which was consistent with the trade-off relationship observed between the heat utilization efficiency and water recovery rate in MD. Employing membranes with high heat-transfer resistance (low conductivity and thicker membrane wall) helped to improve the GOR up to 32%. Simulated results also showed that the GOR value increased by 1.3-fold with the preheater parameter ΔTHX varying from 5 to 0°C. The non-linear scale-up relationship existed between the membrane area and heat utilization (i.e., GOR) was also observed, indicating the possible uncertainty in accurately predicting the GOR value for industrial-scale desalination systems based on lab-scale module testing.

A predictive model for the assessment of the temperature polarization effect in direct contact membrane distillation desalination of high salinity feed

Temperature polarization is one of the major sources responsible for flux drop in membrane distillation systems due to the reduction in the driving force across the membrane. The present study offers a predictive model developed for the estimation of the temperature polarization coefficient across the membrane taking into consideration the simultaneous heat and mass transfer phenomena. The uniqueness of the developed model is its ability to predict the intermediate temperatures (temperatures along the flow path of the membrane sheet) which can be used to estimate the local flux and local temperature polarization coefficients as opposed to the methods used by others which estimate the TPC, using the average bulk temperatures, resulting in a tool that enables the estimation of the temperature polarization coefficient (TPC) at different operating conditions. It was found that higher feed temperatures result in higher temperature polarization effect and hence a lower TPC. It was also observed that TPC increases with feed flow rate. The highest TPC value of 0.82 was achieved for a flow rate of 3L/min and a feed–permeate temperature system of 60–20. The use of flow promoters further enhances the performance of the DCMD system and was reflected on increasing the TPC values (0.66 for a spacer filled channel compared to 0.47 for a spacer free operation) at 1.5L/min flow condition with 70–30 temperature system. The axially integrated local flux values predicted by the model were in good agreement with the experimentally measured fluxes.

Performance evaluation of the DCMD desalination process under bench scale and large scale module operating conditions

The flux performance of different hydrophobic microporous flat sheet commercial membranes made of poly tetrafluoroethylene (PTFE) and poly propylene (PP) was tested for Red Sea water desalination using the direct contact membrane distillation (DCMD) process, under bench scale (high ΔT) and large scale module (low ΔT) operating conditions. Membranes were characterized for their surface morphology, water contact angle, thickness, porosity, pore size and pore size distribution. The DCMD process performance was optimized using a locally designed and fabricated module aiming to maximize the flux at different levels of operating parameters, mainly feed water and coolant inlet temperatures at different temperature differences across the membrane (ΔT). Water vapor flux of 88.8kg/m2h was obtained using a PTFE membrane at high ΔT (60°C). In addition, the flux performance was compared to the first generation of a new locally synthesized and fabricated membrane made of a different class of polymer under the same conditions. A total salt rejection of 99.99% and boron rejection of 99.41% were achieved under extreme operating conditions. On the other hand, a detailed water characterization revealed that low molecular weight non-ionic molecules (ppb level) were transported with the water vapor molecules through the membrane structure. The membrane which provided the highest flux was then tested under large scale module operating conditions. The average flux of the latter study (low ΔT) was found to be eight times lower than that of the bench scale (high ΔT) operating conditions.