Membrane Distillation Unit Research Articles

Membrane distillation hybridized with a thermoelectric heat pump for energy-efficient water treatment and space cooling

The current concept for cooling the indoors is far from ideal with respect to the total energy consumed and waste discharged. A novel concept for improving the energy efficiency is proposed via hybridizing the heat pump with a membrane distillation (MD) unit for simultaneous space cooling and water treatment. MD is well-acknowledged for utilizing low-quality waste heat for water treatment, which makes it feasible for coupling with a heat pump to make use of both the hot and cold reservoirs of the pump. Accordingly, the objective of the current effort was to investigate via experiments the efficacy of a thermoelectric heat pump coupled with a sweep-gas MD system (T-SGMD) by measuring the cooling capacity, condensate production and power consumption. The results from this study can be extended to other heat pumps. Three key highlights emanated from this study. Firstly, condensate production per unit energy consumed can be doubled with the T-SGMD system relative to thermoelectric dehumidification alone. Secondly, cool air recycle affected the condensate flux the most without a drastic loss of cooling compared to other tested parameters during the operation of the T-SGMD. Lastly, the T-SGMD system was able to provide an increase in condensate produced per unit energy without a loss in cooling capacity per unit energy input. These advantages of coupling heat pumps with MD, leveraging on the current advancements in MD, is promising for a hybridized system for decentralized water treatment, dehumidification and space cooling.

Prediction models to analyse the performance of a commercial-scale membrane distillation unit for desalting brines from RO plants

Desalting brines from Reverse Osmosis (RO) plants is one of the most promising applications of Membrane Distillation (MD) systems. The development of accurate models to predict MD system performances plays a significant role in the design of this kind of industrial applications. In this paper, a commercial-scale Permeate-Gap Membrane Distillation (PGMD) module was modelled by means of two different approaches: Response Surface Methodology (RSM) and Artificial Neural Networks (ANN). Condenser inlet temperature, evaporator inlet temperature, feed flow rate and feed water salt concentration were selected as inputs of the model, while permeate flux and Specific Thermal Energy Consumption (STEC) were chosen as responses. The prediction abilities of both RSM and ANN models were compared with further experimental data by using the Analysis of Variance (ANOVA) and the Root Mean Squared Error (RMSE). The results show that the ANN model is able to predict in a more precise way the behaviour of the module for the whole range of input variables. Thus, ANN model was used to find the optimal operating conditions, for the module operating at feed water salinity of 70 and 105 g/L, concentrations that can be reached when desalting RO brines.

Analysis of Absorption Cooling and MD Desalination Cogeneration System

An integrated system of solar absorption cooling and membrane distillation (MD) water desalination sub-systems is investigated to supply chilled water for space air-conditioning and desalination to provide freshwater for a typical family house. The system is based on single-effect LiBr– $$\hbox {H}_{\mathrm {2}}\hbox {O}$$ absorption refrigeration cycle where rejected heat is used to heat feed water in the MD system. Two different arrangements of the system are discussed, configurations A and B. Configuration (A) utilizes cooling seawater for the cold side of MD unit, while configuration (B) shares the chilled water produced from the absorption system with partial cooling load requirements. Results show that maximum cooling effect is produced by configuration (A) followed by configuration (B) with 25, 50 and 75% bypass percentages that produce 26, 19.5, 9.8 and 2.5 kW cooling effect, respectively. Meanwhile, configuration (B) with 75% bypass percentage has a better performance in terms of water productivity such that it produces up to 133 $$\hbox {kg/m}^{\mathrm {2}}$$ -hr of desalinated water compared to 125, 118 and 110 for the same configuration (B) at different bypass ratios of 50, 25% and configuration (A), respectively.

Improving the Distillate Prediction of a Membrane Distillation Unit in a Trigeneration Scheme by Using Artificial Neural Networks

An Artificial Neural Network (ANN) has been developed to predict the distillate produced in a permeate gap membrane distillation (PGMD) module with process operating conditions (temperatures at the condenser and evaporator inlets, and feed seawater flow). Real data obtained from experimental tests were used for the ANN training and further validation and testing. This PGMD module constitutes part of an isolated trigeneration pilot unit fully supplied by solar and wind energy, which also provides power and sanitary hot water (SHW) for a typical single family home. PGMD production was previously estimated with published data from the MD module manufacturer by means of a new type in the framework of Trnsys® simulation within the design of the complete trigeneration scheme. The performance of the ANN model was studied and improved through a parametric study varying the number of neurons in the hidden layer, the number of experimental datasets and by using different activation functions. The ANN obtained can be easily exported to be used in simulation, control or process analysis and optimization. Here, the ANN was finally used to implement a new type to estimate the PGMD production of the unit by using the inlet parameters obtained by the complete simulation model of the trigeneration unit based on Renewable Energy Sources (RES).

An anaerobic membrane bioreactor – membrane distillation hybrid system for energy recovery and water reuse: Removal performance of organic carbon, nutrients, and trace organic contaminants

In this study, a direct contact membrane distillation (MD) unit was integrated with an anaerobic membrane bioreactor (AnMBR) to simultaneously recover energy and produce high quality water for reuse from wastewater. Results show that AnMBR could produce 0.3–0.5L/g CODadded biogas with a stable methane content of approximately 65%. By integrating MD with AnMBR, bulk organic matter and phosphate were almost completely removed. The removal of the 26 selected trace organic contaminants by AnMBR was compound specific, but the MD process could complement AnMBR removal, leading to an overall efficiency from 76% to complete removal by the integrated system. The results also show that, due to complete retention, organic matter (such as humic-like and protein-like substances) and inorganic salts accumulated in the MD feed solution and therefore resulted in significant fouling of the MD unit. As a result, the water flux of the MD process decreased continuously. Nevertheless, membrane pore wetting was not observed throughout the operation.

Innovative Design of Solar-Powered Desalination (SPD) System using Vacuum-Multi Effect Membrane Distillation (V-MEMD) Process

This research focused on the development of an innovative design of solar-powered desalination (SPD) system which was expected to solve the water and energy problem simultaneously. We have developed a portable and hybrid solar-powered desalination (SPD) system for producing potable water from saline water. It is a self-contained and integrated system which combines solar-thermal collector and solar-photovoltaic for its operation, and thus the system can operate to produce water by only using solar energy. Therefore, the system is highly suitable to be implemented in remote arid and coastal areas without infrastructures or connection to the grid (water and power), but blessed with abundant solar irradiation, like in Saudi Arabia. A Memsys Vacuum Multi-Effect Membrane Distillation (V-MEMD) unit was used as the core of the SPD system. A heat pump was also integrated into the SPD system for energy recovery and to improve the performance of the system. The system could be considered as sustainable and “green” desalination technology, which will be very useful for the Kingdom of Saudi Arabia. To study the performance of the system, small-scale tests have been carried out at the Engineering College - King Saud University, Saudi Arabia. Based on the experimental results, the system has run successfully by only utilizing solar energy.

Modelling and optimization of a utility system considering different water desalination technologies

Water and energy are essential for human life and inextricably linked to each other. Water is used in the processes of energy generation and transportation and energy is also used in the processes of water exploitation and transportation. In a utility plant, water is used as working fluid for power generation and heat carrier. Large amount of fresh water is required in the utility system to compensate for the water loss due to boiler blowdown, cooling tower blowdown, cooling tower water evaporation, and process hot utility utilization. Since the requirement of boiler feed water is strict, the water treatment in the industrial utility plant is quite energy intensive. Thus, the simultaneous design and optimization of water desalination process and energy generation process is significant. In this paper, a novel superstructure of water desalination and energy generation system is proposed. Thermal membrane distillation unit is applied for water desalination together with reverse osmosis unit. A mathematical model is formulated for the thermodynamic analysis and optimization of the novel system. Two objective functions are used to evaluate the design scheme. A case study is elaborated to test the proposed system and developed model.

Fermentative energy conversion: Renewable carbon source to biofuels (ethanol) using Saccharomyces cerevisiae and downstream purification through solar driven membrane distillation and nanofiltration

Experimental studies on fermentative production of ethanol from renewable sugarcane juice as carbon source were carried out in a bench scale fermenter integrated with microfiltration, nanofiltration and membrane distillation units (MF-NF-MD) for downstream purification and concentration enrichment of ethanol. This new design is flexible, compact, eco-friendly, and modular and, operated in a cross flow module that ensures almost fouling-free membrane separation while overcoming the product inhibition of traditional batch fermenters. The system is blessed with steady and continuous ethanol production from sugarcane juice with comparatively high productivity 9.2gL−1h−1, yield 0.47gg−1 and concentration 54.3gL−1 using Saccharomyces cerevisiae (NCIM 3205) vis-à-vis the existing technologies. The new system enables continuous product withdrawal, separation and recycle of the biomass as well as residual carbon source back to the fermenter under high cell density without product inhibition. Membrane distillation in the final stage, increases product purity and results in significant concentration enrichment. The study culminates in development of a green and sustainable process for a biofuel production.

Assessing the Thermal Efficiency of Brackish Water Desalination by Membrane Distillation Using Exergy Analysis

Membrane distillation (MD) is a newly emerging technology that receives growing attention in the water desalination and purification industries. A mathematical model for MD unit is validated using experimental data. The mass and energy balances were simultaneously calibrated against the collected distillate production rate and measured temperature of the reject permeate stream. The validated model is utilized to evaluate the performance of the process using energy and exergy analysis. This analysis tool will help in process design and operating condition selection that saves energy and improve performance. It is revealed that the exiting warm permeate is responsible for great amount of exergy losses if disposed to the environment. Repeated recycling the outlet permeate to successive MD units arranged in parallel improves the thermal energy effectiveness and increases the production rate. In fact, recovery ratio is increased from 6 to 25% and the exergy lost from the system with exiting permeate is reduced from 1.2 kW to almost null.

Total water production capacity inversion phenomenon in multi-stage direct contact membrane distillation: A theoretical study

The low thermal efficiency and low water production are among the major challenges that prevent membrane distillation (MD) process from being commercialized. In an effort to design an efficient multi-stage direct contact MD (DCMD) unit through mathematical simulation, a new phenomenon that we refer to as total water production capacity inversion (WPI) has been detected. It is represented by a decrease in the total water production beyond a number of stages or a certain module length. WPI phenomenon, which was confirmed by using two different mathematical models validated experimentally, was found to take place due to the decrease in water vapor flux across the membrane as well as the increase in heat loss by conduction as the membrane length increases. Therefore, WPI should be considered as a critical MD design-criterion, especially for large scale units. Investigations conducted for a simulated multi-stage DCMD process showed that inlet feed and permeate temperatures difference, feed and permeate flow rates, and feed salinity have different effects on WPI. The number of stages (or module length at constant width) that leads to a maximum water production has been determined for different operating parameters. Decreasing inlet feed and permeate temperatures difference, or inlet feed and permeate flow rates and increasing inlet feed temperature at constant temperature difference or inlet feed salinity cause the WPI to take place at lower number of stages. Even though the feed salinity affects negligibly the mean permeate flux, it was clearly shown that it can affect WPI. The results presented herein unveil a hidden phenomenon that is likely to occur during process scale-up procedures and should be considered by process engineers for a proper choice of system design and operating conditions.

Comparative performance evaluations of nanomaterials mixed polysulfone: A scale-up approach through vacuum enhanced direct contact membrane distillation for water desalination

Doping of multi-walled carbon nanotube (MWCNT), silicon dioxide (SiO2), titanium dioxide (TiO2) and zinc oxide (ZnO) into polysulfone (PSf) flat sheet membranes was prepared by phase inversion process. The characterizations of the PSf and PSf-MWCNT, PSf-SiO2, PSf-TiO2 and PSf-ZnO membranes were achieved using Fourier transform infrared spectroscope, contact angle measurement, dynamic mechanical analyzer, thermo-gravimetric analysis and scanning electron microscope. Vacuum enhanced direct contact membrane distillation unit was used for evaluating the efficacy of prepared membranes in water desalination. Optimizing the operational procedures and water characteristics ensured a high slat rejection of 99.99% using the prepared membranes. The highest permeate flux obtained in the order of MWCNT (41.58)>SiO2 (38.84)>TiO2(35.6)>ZnO (34.42L/m2·h) with optimized concentration of 1.0, 0.5, 0.75, 0.5wt% relative to PSf weight, i.e. 15%. The optimum operational conditions included feed and permeate temperatures 60°C and 20°C, respectively, synthetic NaCl feed water with salinity was 10,000ppm.

Design study of the coupling of an air gap membrane distillation unit to an air conditioner

This work presents the design of a membrane distillation (MD) unit coupled to a refrigeration device. The operation of an MD pilot plant is first tested then simulated using EES solver. The range of hot water feed temperature provided by the condenser of the refrigeration device may vary from 25 to 65°C as needed by the MD unit. The results of the experimental study enable to validate a 1D theoretical model with a maximum deviation of ±6%. This model is combined to a heat pump model to simulate the operation of a coupled machine.The system performance is studied following two scenarios. In Scenario 1 (continuous mode), the system is connected to a seawater network. In Scenario 2 (batch mode), the seawater reservoir is filled up manually every day. The simulation study shows that the increase of the feed water temperature enhances significantly the produced permeate flux but decreases the performance of the heat pump. This system exhibits high exergy efficiency and GOR for a high feed water temperature. Exergy efficiency and gained output ratio (GOR) decrease as the water flow increases and the hot water temperature decreases. Conversely, the coefficient of performance (COP) of the refrigerator increases.

Methodical design and operation of membrane distillation plants for desalination

The operation of pre-industrial membrane distillation (MD) units is of great importance in order to implement this new technology. The systematic development from module to plant design for MD is conducted in an industrial research-cooperation. The consequent up-scaling from a lab plant at Hamburg University of Technology (TUHH) for MD module evaluation to a pre-industrial plant is presented. Permeate gap MD (PGMD) and Direct Contact MD (DCMD) spiral wound modules with different channel lengths have been used in this study to evaluate the most suitable module configuration and design. It has been decided to go along with DCMD modules with a relatively short channel length. A pilot plant for on land testing has been built and operated for 9 months before constructing the pre-industrial unit. In the pre-industrial plant waste heat was used and a heat recovery and recycle concept was implemented and tested successfully. It has been proven that long-term operation with real seawater is possible without significant performance decline. Besides operational data also MD plant design specifics will be discussed. The importance of ambient pressure conditions for the MD modules was determined. In addition, the possibility of piping system deaeration is discussed.

Energy efficient configuration of Membrane Distillation units for brackish water desalination using exergy analysis

Membrane Distillation (MD) became a promising technology for water desalination. MD is energy-driven process and with the limitation and cost of energy resources it is imperative to examine improving the energy effectiveness of such units. For this purpose, Exergy analysis is applied to a validated model of typical MD module to identify the sources of inefficiencies. Reject brine is found to be an origin for exergy losses when the energy of the geothermal feed is not fully utilized. In this case, feeding the reject brine to sequential MD units connected in series can capitalize the production rate and energy utilization. In fact, the exergy losses are reduced from 70% to 30%. The enhanced energy efficiency made the exiting warm permeate responsible for another exergy losses if disposed to the environment. Repeated recycling of the outlet permeates to successive MD vessels connected in parallel improves the process performance. This cascaded structure of the desalination process converts most of the available exergy in the geothermal source into work to produce additional fresh water and dispose the salty brine and warm permeate at very low exergy levels.

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

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

Recovery of ethanol from scrubber-water by district heat-driven membrane distillation: Industrial-scale technoeconomic study

Membrane distillation (MD) integrated with a district heating network (DHN) to supply the heat demand has been investigated in a bioethanol production plant (BP). The specific application considered here is ethanol recovery from fermentation off-gas (CO2) scrubber water, where the main objective of this study was to develop an air gap MD system that is driven by heat from DHN, thereby offloading the steam-driven distillation column. Experiments conducted on an MD laboratory facility combined with data from the bioethanol industry were used to assess the technological and economic feasibility of this integrated MD-DHN-BP system. Comparisons were also made between the distillation column and MD units regarding heat demand and economic savings. Results of the study showed that MD could be a competitive technology for ethanol recovery given that low-grade heat such as from district heating network or waste heat is accessible.

Improving Liquid Entry Pressure of Polyvinylidene Fluoride (PVDF) Membranes by Exploiting the Role of Fabrication Parameters in Vapor-Induced Phase Separation VIPS and Non-Solvent-Induced Phase Separation (NIPS) Processes

Polyvinylidene fluoride (PVDF) is a popular polymer material for making membranes for several applications, including membrane distillation (MD), via the phase inversion process. Non-solvent-induced phase separation (NIPS) and vapor-induced phase separation (VIPS) are applied to achieve a porous PVDF membrane with low mass-transfer resistance and high contact angle (hydrophobicity). In this work, firstly, the impacts of several preparation parameters on membrane properties using VIPS and NIPS were studied. Then, the performance of the selected membrane was assessed in a lab-scale direct-contact MD (DCMD) unit. The parametric study shows that decreasing PVDF concentration while increasing both relative humidity (RH) and exposure time increased the contact angle and bubble-point pore size (BP). Those trends were investigated further by varying the casting thickness. At higher casting thicknesses and longer exposure time (up to 7.5 min), contact angle (CA) increased but BP significantly decreased. The latter showed a dominant trend leading to liquid entry pressure (LEP) increase with thickness.

Polyurethane Nanofiber Membranes for Waste Water Treatment by Membrane Distillation

Self-sustained electrospun polyurethane nanofiber membranes were manufactured and tested on a direct-contact membrane distillation unit in an effort to find the optimum membrane thickness to maximize flux rate and minimize heat losses across the membrane. Also salt retention and flux at high salinities up to 100 g kg−1 were evaluated. Even though the complex structure of nanofiber layers has extreme specific surface and porosity, membrane performance was surprisingly predictable; the highest flux was achieved with the thinnest membranes and the best energy efficiency was achieved with the thickest membranes. All membranes had salt retention above 99%. Nanotechnology offers the potential to find modern solutions for desalination of waste waters, by introducing new materials with revolutionary properties, but new membranes must be developed according to the target application.

Exergy analysis of a solar-powered vacuum membrane distillation unit using two models

A detailed exergy analysis of a solar powered VMD unit was performed using two models: the ideal mixture model and the model using the thermodynamics properties of seawater. The exergy flow rates of process steam, given by the two models differed of about 18%, on average. Despite these differences, the two models agree that during the step of condensation, the most important fraction of exergy was destroyed. Moreover, in this work, two forms of exergy efficiency are calculated. The overall exergy efficiency of the unit with reference to the exergy collected by the solar collector was 3.25% and 2.30% according to Cerci and Sharqawy models, respectively. But, it was 0.182% and 0.128%, when referenced to the exergy of solar radiation, according to Cerci and Sharqawy models, respectively. Besides, the utilitarian exergy efficiency was 9.96%. Since the heat exchanger, the hollow-fiber module and the condenser have a very high exergy performance, then it can be concluded that the enhancement or reduction of exergy losses will be mainly by recovering heat lost in brine discharges and in the rejection of the cooling water. In addition, the influence of the rejection rate on exergy efficiencies was studied.

Effects of membrane structure and operational variables on membrane distillation performance

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