Membrane Distillation Unit Research Articles

Water–Energy Nexus in Membrane Distillation: Process Design for Enhanced Thermal Efficiency

A drawback of membrane distillation is the excessive use of heat, which is neither cost-effective nor environmentally effective considering the water–energy nexus. The present paper reports on the analysis and optimization of a bench-scale membrane distillation unit regarding thermal efficiency and transmembrane flux. The research work was developed using a phenomenological mathematical model which was validated against the experimental data. With the optimized process, the heat lost by conduction through the membrane from the retentate side is minimized by a proper design of the membrane properties. With the optimal set of membrane thickness, porosity, and thermal conductivity, the heat conduction across the membrane skeleton was null but retaining the heat transferred by water vapor flux (about 30%), which is intrinsically associated with the membrane distillation phenomenon. Additionally, the transmembrane flux increased by 2-fold using the optimal design for the cell, confirming that thermal efficiency is not in contradiction to water productivity.

Operation of Three-Stage Process of Lithium Recovery from Geothermal Brine: Simulation.

Lithium-rich geothermal waters are considered as an alternative source, and further concentration of lithium is required for its effective recovery. In this work, we have simulated a three-stage lithium recovery process including the brine softening by precipitation Ca2+/Mg2+ cations with sodium carbonate (calculated in PHREEQC), followed by an integrated system consisting of membrane distillation unit (water evaporation), crystallizer (NaCl precipitation), and membrane extraction (Li+ recovery), which was simulated in Simulink/MATLAB. It was shown that the deterioration of membrane performance in time due to scaling/fouling plays a critical role in the performance of the system resulting in the dramatic increase of the replaced membrane modules by a factor of 5. Low cost membranes are required. The process simulation based on the experimental and literature data on the high salinity solutions with the membrane distillation revealed that the specific productivity can be achieved in the range of 9.9–880 g (Li+) per square meter of membranes in the module used before its replacement. The increase of energy efficiency is needed. The mass-flow-rate of saline solution circulated to the crystallizer was set at its almost minimum value as 6.5 kg/min to enable its successful operation at the given parameters of the membrane distillation unit. In other words, the operation of the integrated system having 140 kg of saline solution in the loop and a membrane module of 2.5 m2 for concentration of lithium presence from 0.11 up to 2.3 g/kg would be associated with the circulation of about of 259 tons of saline solution per month between the distillation unit (60 °C) and the crystallizer (15 °C) to yield of up to 1.4 kg of lithium ions. The comprehensive summary and discussion are presented in the conclusions section.

Thermal analysis of a hybrid high concentrator photovoltaic/membrane distillation system for isolated coastal regions

The present study introduces an innovative method for fresh water and electricity generation in the isolated regions. This proposed system couples a concentrated photovoltaic (CPV) unit with a membrane distillation (MD) unit. The CPV unit converts solar energy into electrical energy with conversion efficiency of about 40%. The rest is converted to thermal energy, which may cause cells degradation if temperature exceeds manufacturer limits. An intermediate fluid is used as a coolant which transfers the excess energy to the feed of the MD unit through a heat exchanger. The generated thermal energy in the HCPV cells is used as the driving force for the distillation phenomena in the MD unit. Numerical models were built to simulate the hybrid system. It was found that, at a solar radiation concentration ratio of 1000 suns, the coolant flow rate should exceed 150 g/min for a maximum cell temperature less than 349 K. This arrangement should produce 177 W electric power, and 308 W thermal heat transferred to the coolant. At these conditions, the feed inlet temperature reaches about 323 K, at which, the MD unit produces about 5.88 kg/m2.h of pure water, thus allowing the system to simultaneously produce electricity and pure water for isolated coastal regions.

Simulation of Desalination of a Sodium Chloride Aqueous Solution by Membrane Distillation with a Porous Condenser

A scheme of a membrane distillation unit with the possibility of using solar energy collectors has been proposed for treatment of water–salt solutions. This scheme has been tested in the Simulink (MATLAB) simulation environment using a seawater desalination process as a model system. Based on solar radiation data received in Vietnam, a distillation process has been simulated using a solar collector and an electric heater. Experimental approbation of the model has been carried out by the desalination of a NaCl solution by membrane distillation with a porous condenser. The calculated productivity of the modeled system agrees with the obtained experimental data. Simulation showed the possibility of reducing energy consumption by 61% in the process of desalination of an aqueous solution of NaCl using low-grade heat.

Investigation of the Effects of Temperature on the Microstructure of PTFE Microfiltration Membranes Under Membrane Distillation Conditions

Polytetrafluoroethylene (PTFE) microfiltration membranes are commonly used in Membrane Distillation (MD) systems, and parameters such as the pore size and porosity have significant influence on their performance. The operating temperature of a membrane distillation unit is typically 60-80˚C, and while PTFE is considered to be thermally stable it does expand with increasing temperature. When dealing with a porous microstructure this expansion becomes significant. It was found that increasing the membrane temperature resulted in an expansion of the fibrous PTFE material and subsequently an increase in pore size. The membrane structure was observed over a period of 80 minutes, this time was deemed necessary given that PTFE has low thermal conductivity and therefore would heat up slowly. Pore size increased by 32% in the first 60 minutes, when the sample was heated to 80˚C. A lumped system analysis of the heat transfer inside the SEM chamber was used to determine a heat transfer coefficient of 0.72 W/m2K. The temperature dependence of pore size will result in fluctuations in performance when the membrane is used intermittently and therefore heated and cooled periodically.

Integration of multi effect evaporation and membrane distillation desalination processes for enhanced performance and recovery ratios

This work focuses on the integration of multiple effect evaporation (MEE) and membrane distillation (MD) processes. The MEE brine is treated by the MD allowing at the same time a preheating of the MEE feed water. Different structures of integration are assessed theoretically. Simulation models based on the heat and mass transfer balances are developed for the parallel feed (PF), parallel cross (PCF), forward feed (FF)-MEE, and direct contact MD (DCMD) configurations. The main performance parameters such as the performance ratio, recovery ratio, and specific energy consumption are investigated via simulation of various MEE-MD configurations and operating parameters. The results show that hybrid MEE and MD structures outperform the standalone MEE systems. The performance ratio increased by more than 25% when hybrid structure is used as compared to standalone MEE. The results revealed also that the hybrid parallel PF-MEE-MD configuration yields the best performance compared to other hybrid configurations. It also outperforms the standalone PCF-MEE which is widely adopted and used in practice. Besides, the performance of the hybrid configurations is especially sensitive to the feed temperature's variation in both MEE and MD units. Increasing MD feed temperatures has a positive impact on the recovery ratios of the combined systems.

Secondary effluent purification towards reclaimed water production through the hybrid post-coagulation and membrane distillation technology: A preliminary test

Shortage of water resource makes the production of high-grade reclaimed water become an important topic. This study proposed the post-coagulation and membrane distillation hybrid technology for further treatment of the secondary effluent to produce the reclaimed water with high quality. The emerging high-efficient polytitanium coagulant was utilized in post-coagulation unit, with the results suggesting its superior coagulation performance than the conventional polyaluminum chloride in terms of both particulates and organic matter removal. The titanium coagulated effluent then flowed to the subsequent direct contact membrane distillation unit for further purification. Permeated flux of the direct contact membrane distillation reached stable value of around 5.1 L/m2·h, which was accompanied by the production of the filtrate with high quality (turbidity < 0.4 NTU and dissolved organic carbon < 1.0 mg/L). Post-coagulation could not only withdraw the foulants from raw secondary effluent, but also improve the subsequent permeate flux and mitigate membrane fouling during membrane distillation process. The foulants attached on membrane surface was the main cause of membrane fouling during membrane distillation procedure. Both economical and practical prospect are included in this study, with the results demonstrated that post-coagulation of the secondary effluent followed by the subsequent membrane distillation procedure was a feasible and high-efficient strategy towards reclaimed water production.

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

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

Developing and validating a dynamic model of water production by direct-contact membrane distillation.

We consider the development and fitting of a dynamic model for desalinated water production by a direct-contact membrane distillation (DCMD) unit. Two types of dynamic-model structures, namely, lumped parameter and spatial, were evaluated. Both the models were validated using experimental response data generated by step testing the inlet hot stream temperature of a DCMD pilot plant. Both the model structures failed to follow the dynamic response adequately. However, a modification of the model by adding a heat loss term resulted in enhanced predictions for both model structures. The overall relative error in the model-plant mismatch was approximately 3%. This is reasonable considering the random uncertainties associated with the plant operation. This observation also improves our understanding of the importance of using better correlations for heat-transfer coefficients, to develop a more reliable and accurate predictive model for a wide range of operating conditions.

Design and thermodynamic assessment of a biomass gasification plant integrated with Brayton cycle and solid oxide steam electrolyzer for compressed hydrogen production

In this paper, a comprehensive thermodynamic evaluation of an integrated plant with biomass is investigated, according to thermodynamic laws. The modeled multi-generation plant works with biogas produced from demolition wood biomass. The plant mainly consists of a biomass gasifier cycle, clean water production system, hydrogen production, hydrogen compression, gas turbine sub-plant, and Rankine cycle. The useful outputs of this plant are hydrogen, electricity, heating and clean water. The hydrogen generation is obtained from high-temperature steam electrolyzer sub-plant. Moreover, the membrane distillation unit is used for freshwater production, and also, the hydrogen compression unit with two compressors is used for compressed hydrogen storage. On the other hand, energy and exergy analyses, as well as irreversibilities, are examined according to various factors for examining the efficiency of the examined integrated plant and sub-plants. The results demonstrate that the total energy and exergy efficiencies of the designed plant are determined as 52.84% and 46.59%. Furthermore, the whole irreversibility rate of the designed cycle is to be 37,743 kW, and the highest irreversibility rate is determined in the biomass gasification unit with 12,685 kW.

Performance evaluation of the different nano-enhanced polysulfone membranes via membrane distillation for produced water desalination in Sert Basin-Libya

A Polysulfone-Polyethylene glycol (PS/PEG) flat sheet membrane was prepared by phase inversion technique. Dimethyl Formamide (DMF) was utilized as a solvent and deionized water was utilized as the coagulant. Polyethylene glycol (PEG) of a various dose of PEG 2000 was utilized as the polymeric improvers and as a pore-forming agent in the casting mixture. The single-walled carbon nanotube (SWCNTs), multi-walled carbon nanotube (MWCNTs), aluminum oxide (Al2O3) and copper oxide (CuO) nanoparticles (NPs) were utilized to improve the PS/PEG membrane performances. The characterizations of the neat PS, PS/PEG, PS/PEG/Al2O3 (M1) PS-PEG/CuO (M2), PS-PEG/SWCNTs (M3) and PS/PEG/MWCNTs (M14) nanocomposite (NC) modified membranes were acquired via Fourier-transform infrared analysis (FTIR), water contact angle estimation (WCA), scanning electron microscope (SEM), dynamic mechanical analyzer (DMA) and thermogravimetric analysis (TGA). Enhanced Direct contact membrane distillation (EDCMD) unit was used for estimating the efficiency of the performance of the synthesized NC membranes via 60 °C feed synthetic water and/or saline oil field produced water samples containing salinities 123,14 mg/L. Adjusting the operational procedures and water characteristics confirmed a high salt rejection of 99.99% by the synthesized NC membranes. The maximum permeate flux achieved in the order of SWCNTs (20.91) > Al2O3 (19.92) > CuO (18.92) > MWCNT (18.20) (L/m2.h) with adjusted concentration of 0.5, 0.75, 0.75, 0.1 wt% compared with PS weight, i.e. 16%. The optimum operational circumstances comprised feed and permeate temperatures 60 °C and 20 °C, respectively. The achieved flux was 5.97 L/m2.h, using brine oil field produced water, via PS/PEG/SWCNTs membrane with 0.5 wt% of SWCNTs. Moreover, the membrane indicated sustaining performance stability in the 480 min continuous desalination testing, showing that the synthesized PS/PEG/SWCNTs NC modified membrane may be of magnificent potential to be activated in EDCMD procedure for water desalination.

Hybrid Membrane Distillation and Wet Scrubber for Simultaneous Recovery of Heat and Water from Flue Gas.

Flue gas contains high amount of low-grade heat and water vapor that are attractive for recovery. This study assesses performance of a hybrid of water scrubber and membrane distillation (MD) to recover both heat and water from a simulated flue gas. The former help to condense the water vapor to form a hot liquid flow which later used as the feed for the MD unit. The system simultaneously recovers water and heat through the MD permeate. Results show that the system performance is dictated by the MD performance since most heat and water can be recovered by the scrubber unit. The scrubber achieved nearly complete water and heat recovery because the flue gas flows were supersaturated with steam condensed in the water scrubber unit. The recovered water and heat in the scrubber contains in the hot liquid used as the feed for the MD unit. The MD performance is affected by both the temperature and the flow rate of the flue gas. The MD fluxes increases at higher flue gas temperatures and higher flow rates because of higher enthalpy of the flue gas inputs. The maximum obtained water and heat fluxes of 12 kg m−2 h−1 and 2505 kJm−2 h−1 respectively, obtained at flue gas temperature of 99 °C and at flow rate of 5.56 L min−1. The MD flux was also found stable over the testing period at this optimum condition. Further study on assessing a more realistic flue gas composition is required to capture complexity of the process, particularly to address the impacts of particulates and acid gases.

Thermodynamic investigation of a concentrating solar collector based combined plant for poly-generation

The detailed thermodynamic evaluation for combined system assisted on solar energy for poly-generation are studied in this paper. This poly-generation cycle is operated by the concentrating solar radiation by using the parabolic dish solar collector series. The beneficial exits of this integrated plant are the electricity, fresh-water, hot-water, heating-cooling, and hydrogen while there are different heat energy recovery processes within the plant for development performance. A Rankine cycle with three turbines is employed for electricity production. In addition to that, the desalination aim is performed by utilizing the waste heat of electricity production cycle in a membrane distillation unit for fresh-water generation. Also, a PEM electrolyzer sub-component is utilized for hydrogen generation aim in the case of excess power generation. Finally, the hot-water production cycle is performed via the exiting working fluid from the very high-temperature generator of the cooling cycle. Moreover, based on the thermodynamic assessment outputs, the whole energy and exergy efficiencies of 58.43% and 54.18% are computed for the investigated solar plant, respectively.

Application of solar energy to seawater desalination in a pilot system based on vacuum multi-effect membrane distillation

The evaluation of a novel solar seawater desalination system implemented at the University of Almeria (Spain) is presented. It integrates a solar thermal field based on static collectors and a thermal desalination system based on the vacuum multi-effect membrane distillation technology. The distillation unit has a particular innovation to increase its thermal performance, using a seawater flow to condense the steam and preheat the feed. Experiments were made under different environmental conditions to assess the role of the thermal storage system for minimizing the effect of disturbances in solar radiation. Thermal energy could be delivered at a stable temperature to the distillation module, even with variable solar radiation. A simulation analysis based on a quasi-dynamic model was also performed to evaluate the distillate production profile and the operating time during a typical year considering different temperature setpoints at the inlet of the membrane module (60, 70, and 80 °C). The simulated volume of distilled water generated annually ranged from 41.7 to 70.5 m3, depending on the setpoint. The membrane distillation unit produced water almost uniformly along the year, with an average flux of (5.5 ± 1) l h−1 m−2 at the maximum setpoint, which was proved the most favourable.

Optimization of module length and number of stages in a multistage membrane distillation configuration

ABSTRACT Multistage membrane distillation units connected in series for water desalination is considered. The influence of the number of stages and the module length per stage on the overall economics is investigated via simulations using a validated model. Depending on the degree of feed salinity, a suboptimal condition occurs at a structure size of 40 ~ 35 stages with an average module length of 15 ~ 12 m. The corresponding specific operating and fixed costs are $8 ~ 12.5/m3 and $0.97~−.8/m3, respectively. For a high salinity feed, the optimum specific operating cost increases due to reduced mass production. Membrane porosity and thickness showed a pronounced effect on cascade structure economics. The specific operating cost can be further reduced by an average of 60% using permeate heat recovery systems.

Membrane distillation of power plant cooling tower blowdown water

The objective of this study was to examine the recovery of the power plant cooling tower blowdown water (CTBD) by membrane distillation. The experiments were carried out using a flat plate poly vinylidene fluoride (PVDF) membrane with a pore diameter of 0.22 um by a direct contact membrane distillation unit (DCMD). The effects of operating parameters such as transmembrane temperature difference (delta-T), circulation rate and operating time on permeate flux and membrane fouling have been investigated. The results indicated that permeate flux increased with increasing delta-T and circulation rate. Whereas maximum permeate flux was determined as 47.4 L/m2.h at delta-T of 50 degree Celcius for all short term experiments, minimum permeate flux was determined as 7.7 L/m2.h at delta-T of 20 degree Celcius. While 40 degree Celcius was determined as the optimum delta-T in long term experiments. Inorganic and non-volatile substances caused fouling in the membranes.

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

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

Experimental parametric study of membrane distillation unit using solar energy

Solar distillation is a promising method for the supply of freshwater to rural communities because water is a vital factor for the survival of humans. Therefore potable water demand is increasing continuously because of the industrial, population and, fast agricultural applications. Nowadays, different techniques for water purification have been created to provide freshwater from salty and polluted water such as solar distillation method to distill brackish/saline water. This work presents an experimental study of the solar membrane distillation unit which is coupled with direct contact membrane (DCMD) which located at Kairouan University. The system is installed as part of a cooperation project research and development between Tunisian Electromechanical Systems Laboratory and German Institute for Solar Energy Systems entitled: Solar driven membrane distillation for resource efficient desalination in remote areas. For the heating of the hot feed water stream, a heat exchanger is included which can be operated with any external heat supply between 60 and 80 °C. For the cooling of the cold feed water stream, no heat exchanger is included. The cooling can be provided over the intake of cold feed water. The effect of solar irradiation and all temperature on the journal productivity of the unit has been presented.

The effect of ‘High-pH pretreatment’ on RO concentrate minimization in a groundwater desalination facility using batch air gap membrane distillation

This study compared the efficacy of two different scale control strategies: (i) ‘High-pH pretreatment’; and (ii) antiscalant (AS) addition for reverse osmosis (RO) concentrate minimization, in a lab-scale air gap membrane distillation (AGMD) unit. In contrast with previous studies that used a direct contact membrane distillation set-up, we systematically investigated the performance of batch AGMD configuration with and without the preliminary reduction of scale-forming constituents, using high salinity, high alkalinity, high SiO2 and high magnesium hardness RO concentrate. Results indicated that ‘High-pH pretreatment’ by means of the cold lime-soda ash process was more productive than only AS addition to the RO concentrate, resulting in significant salt precipitation when the concentration factor (CF) in the AGMD system increased above 1.3. The High-pH precipitation process provided significant concentration reduction of SiO2 (96%), magnesium (96%) and calcium (86%). Following ‘High-pH pretreatment’, pH re-adjustment and final AS addition, the use of AGMD allowed us to minimize the existing RO concentrate with an initial total dissolved solids (TDS) level of 10.8 g/L by a CF of 3.2. In addition, this chemical demineralization process enabled operation of the AGMD unit at a higher temperature, thus increasing permeate flux.

Analysis of a rich vapor compression method for an ammonia-based CO2 capture process and freshwater production using membrane distillation technology

Post-combustion capturing of CO2 through chemical solvent absorption is a promising technique for reducing the CO2 emissions from fossil fuel power plants. However, the energy penalty associated with the absorbent regeneration continues to be a critical challenge in the chemical solvent absorption process. In this study, the operating parameters of ammonia-based CO2 capture were optimized to reduce the energy penalty. This optimized process was considered a base process to which process modifications were added, with the goal of further reducing the energy consumption. These process modifications included absorber inter-cooling and rich vapor compression (RVC) combined with cold solvent split (CSS) processes. The combined RVC and CSS process was compared with the base process and advanced NH3-based CO2 capture processes, such as the rich split process and the inter-heating process. Compared to the base process, the combined process reduced the energy requirements by 20.2%, which was higher than the 11.6% and 8.26% energy reductions obtained via the rich split and inter-heating processes, respectively. The combined process was also compared with MEA-based process modifications. The energy savings from the combined process were higher than those of the MEA-based process modifications. To estimate the trade-offs between the energy savings resulting from the combined process vs. the capital cost of the additional equipment required, the Aspen Capital Cost Estimator (ACCE) was used. The results showed that the combined process saved $0.707 million per year. Furthermore, a membrane distillation (MD) technology was integrated with the CO2 capture unit to produce freshwater. This additional process produced freshwater at a rate of 719.240 m3/day at a feed stream temperature to the MD unit of 35.66 ℃.