Air Gap Membrane Distillation Research Articles

Integration of high-temperature PEMFC with air gap membrane distillation for simultaneous electricity generation and seawater desalination: A theoretical investigation

A novel hybrid system integrating high-temperature proton exchange membrane fuel cell (HT-PEMFC) with air gap membrane distillation (AGMD) is proposed to cogenerate electricity and freshwater. A mathematical model is established to evaluate the proposed system performance from both exergetic and energetic perspectives considering various irreversible effects, from which effectiveness and practicality of the proposed system are examined. Calculation examples reveal that maximum power density of the integrated system allows 83.4 % greater than that of the basic HT-PEMFC system, meanwhile, the according exergy destruction rate density is declined by 3.14 %. Furthermore, considerable sensitivity analyses indicate that the proposed system performance is benefited by elevating the feed water temperature while worsened by increasing the coolant water temperature, air gap width and thickness of hydrophobic membrane or feed water NaCl concentration. The feasibility of AGMD as an effective exhaust heat harvesting alternative is proved. The results acquired may be helpful in designing and optimizing such an actual hybrid system.

Investigations on the effect of spacer in direct contact and air gap membrane distillation using computational fluid dynamics

Direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD) have emerged as potential technologies for water and wastewater treatment. This investigation developed a comprehensive two-dimensional computational fluid dynamics model to describe and simulate heat transfer, mass transfer, and fluid flow in DCMD and AGMD processes. The effect of Reynolds number, inlet feed temperature, and distance between two successive spacer filaments on the performance parameters such as mass flux, temperature polarisation, concentration polarisation, and thermal efficiency was investigated in this work. The feed and permeate side heat transfer coefficient increased with Re in the DCMD and AGMD models. The thickness of the velocity boundary layers reduced upon introducing spacers in the feed and permeate channels. The mass flux on the introduction of spacers increased in DCMD and AGMD models because the boundary layer resistance was reduced. The models with spacer showed a lower oncentration polarisation. DCMD model demonstrated a loweroncentration polarisation than the AGMD model as the former has higher mass flux than the latter leading to a higher solute concentration on the membrane surface. The thermal boundary layer was smaller in the spacer-filled channel because of the fluid mixing. In the AGMD model, the permeate side thermal boundary layer was unaffected because of the poor conductivity of air than water.

Air gap membrane distillation applied to olive mill wastewater

In Mediterranean countries, the olive industry is a major economic sector. During the production of olive oil, a wastewater is generated namely, olive mill wastewater (OMW). This OMW needs to be treated otherwise it pollutes the environment. To lower the pollutants load air gap membrane distillation (AGMD) process is investigated. This study is focusing on investigating the effect of OMW feed temperature and flow rate in addition to membrane pore size on the performance of AGMD. Three different feed temperatures (50, 60, and 70 °C) along with three different feed flow rates (2, 3, and 4 ml/s) were employed. The utilized membranes were made of polytetrafluoroethylene (PTFE) having pore sizes of 0.2 and 0.45 µm. The major conclusion of this work is that all AGMD permeate samples have zero total phenols (TPh) and zero total solids (TS), however, their chemical oxygen demand (COD) values were around 17,700 and 20,400 mg/L for 0.2 and 0.45 µm membranes respectively. These results are considered low enough for the permeate to be allowed to be sent to centralized domestic wastewater treatment plants. In addition, a mathematical model was developed to calculate COD, TPh, TS and TSS (total suspended solids). The calculated values of these parameters were in good agreement with their measured counterparts. The activation energies of utilized membranes (0.2 and 0.45 µm) were calculated from experimental results and were found to be 50.7 and 49.1 kJ/mol respectively, whereas the mass transfer coefficients were 28.1 and 32.1 kg/m2.h.bar respectively.

Analysis and Experimental Study on Water Vapor Partial Pressure in the Membrane Distillation Process.

In membrane distillation, the vapor pressure difference is the driving force of mass transfer. The vapor pressure is generally assumed by the saturation pressure and calculated by the Antoine equation. However, in the actual operation process, the feed solutions usually flow in a non-equilibrium state, which does not meet the theoretical and measurement conditions of the vapor-liquid equilibrium (VLE) state. This study tested the actual water vapor pressure of the pure water, lithium bromide (LiBr) solution, lithium chloride (LiCl) solution, and calcium chloride (CaCl2) solution under different flow conditions. The results showed that the actual water vapor pressure was lower than the saturation pressure overall, and the difference increased with temperature but decreased with the mass concentration. Therefore, in vacuum membrane distillation (VMD), air gap membrane distillation (AGMD), and sweeping gas membrane distillation (SGMD), the membrane flux calculated by water vapor saturation pressure was higher than the actual membrane flux, and the relative difference decreased and was less than 10% after 60 °C. In direct contact membrane distillation (DCMD), the water vapor pressure difference on both membrane sides was almost the same by using the saturation vapor pressure or the tested data since the pressure errors were partially offset in parallel flow or counter-flow modes.

Activated carbon as a photothermal absorber in PVDF membranes for solar driven air-gap membrane distillation

Renewable energy-enabled desalination systems are crucial for solving the global water scarcity challenges in an environmentally friendly manner. In this work, we report an air gap membrane distillation (AGMD) process that utilizes polyvinylidene fluoride (PVDF) membranes blended with a photothermally active and relatively inexpensive activated carbon (AC). The composite membrane absorbed the solar radiation and generated local heat at the membrane surface, providing the driving force required in the AGMD process. This strategy overcame one of the key limitations of traditional MD—temperature polarization, and increased the energy efficiency significantly. We show that the blending of 5 to 9 %AC into the PVDF matrix can significantly boost the solar-energy-driven flux of AGMD by 281–1400 %, compared to the pristine membrane. PVDF membrane blended with 9 %AC exhibited an average AGMD permeate flux of 0.31 kg.m−2.h−1, with a GOR of 0.29 and a photothermal efficiency of 17.64 %. All PVDF-AC membranes showed excellent salt rejection, reaching 99.9 % for PVDF-9%AC. Finally, the AGMD performance of the fabricated membranes was compared by estimating their normalized MD coefficients (B/δ). PVDF-9%AC exhibited a B/δ value of 18.8E-5 kg s−1 m−3 Pa−1, as opposed to a meager 0.89E-5 kg s−1 m−3 Pa−1 exhibited by its pristine counterpart.

Effect of Configurations and Operating Parameters on the Desalination Performance of Membrane Distillation: A Review

A desalination is a promising approach to addressing the freshwater scarcity caused by limited freshwater resources and salt intrusion (pollution). Membrane distillation (MD) was proposed as a possible technology for desalination. This study review the efficiency of membrane distillation by comparing the permeate flux and thermal energy efficiency of the four configurations, namely, direct contact membrane distillation (DCMD), vacuum membrane distillation (VMD), air gap membrane distillation (AGMD) and sweeping gas membrane distillation (SGMD). It was observed that the sequence of permeate flux and thermal energy efficiency is VMD> DCMD> SGMD>AGMD and VMD> SGMD> AGMD> DCMD, respectively. The results show that the VMD provides the highest permeate flux at 15.2 kg/hm2 with 99.25% of salt rejection rate. Additionally, VMD possess good energy efficiency at 66% relative to other configuration at the recorded permeate flux. Subsequently, the feasibility of MD in desalination is studied using different case studies. Furthermore, the effect of operating parameters (feed temperature, feed concentration, feed flow rate, and long-term operation) on flux is discussed. The results suggested that the flux increases when feed temperature or feed flow is increased. At the same time, the flux will decrease when feed is in high concentration and long-term operation.

Performance and economic analysis of a solar membrane distillation pilot plant under various operating conditions

Although membrane distillation (MD) is a promising desalination process, its use is limited because it requires a large amount of thermal energy. To reduce the thermal energy consumption during MD, studies combining various renewable energy sources (e.g., solar, geothermal, and waste heat) are currently being conducted. Therefore, pilot plant experiments combining solar energy with an MD system were conducted. In particular, a module that could be changed into different MD configurations, including direct contact MD (DCMD) and air gap MD (AGMD), was reviewed. The performance of the pilot plant was analyzed for each configuration under various operating conditions and according to onsite weather conditions. Because DCMD can obtain a 43% higher water flux than can AGMD, a long-term DCMD test was conducted to achieve a high water flux. The specific energy consumption (SEC) and gained output ratio (GOR) were compared in the presence and absence of solar energy. A 30% decrease in the SEC and 17% increase in the GOR were observed for the sunny days compared with when no solar energy was used. These results indicate that combining the MD with solar energy can improve its performance during long-term operation. In addition, the cost per unit volume of product water was estimated based on the designed solar MD pilot plant.

Experimental investigation of two novel arrangements of air gap membrane distillation module with heat recovery

Experimental investigations of an air gap membrane distillation (AGMD) unit were carried out to study the energy-saving by heat recovery. The spiral air gap membrane module in the water desalination unit was used. The unit was configured to operate in two novel arrangements and designed to minimize the loss of heat within the drains which maximize the unit productivity as well as the thermal efficiency. Water of salt concentrations 10,000 ppm and 20,000 ppm were used. Measurements of the heat source in the form of inlet and outlet temperatures, water productivity, and thermal efficiency are conducted for different flow rates of coolant and water feeds. The new arrangements are found to save much of the supplied heat, minimize the heat loss, and maximize the unit performance. An enhancement of 38.62% in the water productivity was achieved compared to the conventional AGMD unit. The considered heat recovery system showed water productivity of 155.92 m3/year for the proposed fresh potable water system with a cost of $12.85 per m3.

Performance of an air gap membrane distillation system and enhancement using a low-cost surface modification

ABSTRACT Water can be purified with a hydrophobic membrane in an air gap membrane distillation (AGMD) configuration. The present study presents experimental data for such a system and comparison with model predictions from simulations in a Matlab environment. The importance of the respective roles of feed water temperature and flow rate, cold water temperature and flow rate, and the extent of air gap on water production rate is studied, both from experiments and simulations. The comparison is found to be quite good, thus validating the choice of the mathematical model. In addition, possible improvements in the device performance are examined by using a surface modification of the hydrophobic membrane. In this respect, a layer of lead from a pencil is coated over the membrane. Evaporation rates through the pencil-coated membrane were measured in a solar simulator with 1 sun, 2 sun, and 3 sun settings. A distinct improvement in the evaporation rate was obtained when a pencil coated membrane was used. Following this observation, an augmented two-sided AGMD system is proposed, which carries a coated membrane exposed to solar radiation on one side and an uncoated membrane on the other.

Experimental characterization of the temperature gradient inside a membrane distillation module

One of the solutions to the current fresh water challenge is desalination using low-grade heat (solar radiation or waste heat), such as Air-gap Membrane Distillation (AGMD). Based on evaporation, induced by a temperature gradient across a hydrophobic membrane, this process has an improved energy efficiency thanks to the inclusion of the air-gap by limiting the heat losses, while hindering the mass transfer at the same time. Understanding of the temperature distribution in the module is crucial for working towards improved efficiency and freshwater output.This work describes a setup and optical measurement method dedicated to analysis of energy and mass transfer resistances through temperature gradient measurement in the bulk and observation of boundary layer at the membrane. In particular data acquisition, processing and interpretation framework is presented, and compared to existing literature. Initial results indicate that the method is successful, although a number of improvements are proposed to address the identified shortcomings.

Wave-Powered and Zero-Discharging Membrane-Distillation Desalination System: Conceptual Design and Analysis

There are many islands without full access to electricity around the world. These energy-poor regions generally have drinking water supply issues too. Renewable energy-powered desalination units can convert seawater to freshwater by using such as oceanic wave energy to mitigate the water limitation in small islands. A novel wave-powered floating desalination system (WavoWater) was proposed for easy on-site deployment and minimal environmental impact. WavoWater can produce freshwater using a vacuum-applied air-gap membrane distillation (AGMD) system, and the heat needed for the AGMD is provided through a heat pump powered by wave energy. Small-scale experiments were conducted to estimate the water generation rate of the vacuum-applied AGMD, and the WavoWater system modeling was developed based on the experimental results and wave data observed near the City of Newport, OR, USA. Fast Fourier transform was applied to estimate the wave energy spectrum in a random sea wave state. It was evaluated that 1 m-diameter WavoWater can produce 12.6 kg of fresh water per day with about 3.1 kWh of wave energy. With the performance evaluation, the aspects of zero discharging and minimal environmental impact were also highlighted for the stand-alone wave-powered desalination system.

Immobilized graphene oxide-based membranes for improved pore wetting resistance in membrane distillation

Membrane distillation (MD) is a useful method for the purification of difficult feedwaters but it cannot be applied in a range of industries due to pore wetting. In this work, graphene oxide (GO) laminate coatings are explored to overcome the pore wetting issues. Air gap MD (AGMD) configuration was considered, using a 35 g L−1 NaCl solution with 150 mg L−1 (150 ppm) of Triton X-100 surfactant as feed material. The GO is deposited as a laminate membrane on top of a commercial porous polyvinylidene fluoride (PVDF) support and good adhesion is achieved through the use of polydopamine (PDA) to form a hydrophilic tri-layer membrane. The small pore size achieved with the laminate GO led to increased pore wetting resistance for at least 90 h compared to 20 min with pristine commercial PVDF. Additionally, the extra layers of GO and PDA did not affect the membrane flux. Overall, a tri-layer immobilized GO membrane is synthesized with superior performance when compared to current commercial membranes, meaning that MD can be used for a new range of wastewater applications.

Optimization of operational parameters in air-gap membrane distillation using central composite design applied in recovery of dye manufacturing wastewaters

ABSTRACT The challenge for water recovery from dye manufacturing wastewaters that contain high salinity and high dye concentration is recovery efficiency and water quality. The permeate flux and water quality mainly reflect the membrane distillation performance in treatments of these wastewaters. This study investigated the effects of Air Gap Membrane Distillation’s (AGMD) operational parameters and its interactions on permeate fluxes and recovered water quality in treatments of real dye manufacturing wastewaters using design of experiments (DOE). The DOE method used the response surface methodology (RSM) of central composite design (CCD) to predict and optimize the permeate flux and recovered water quality. The results showed a reliable predictive model of actual permeate flux (15.5 kg.m−2.h−1) with the adjusted R2 adj of 0.995 and only 34.4%–73.8% of the theoretical permeate flux because of salt contents’ main effect. The rejection rate of chloride ion (Cl−), sulfate ion (SO4 2-), and ADMI (American Dye Manufacturers’ Institute) color index from real dye manufacturing wastewaters with high salinity was more than 99,9%. The COD removal rate of raw wastewater (C1) from dye manufacturing was 99,4%. This study help in understanding theoretical and actual membrane flux and the scale-up of a membrane treatment plant.

Boron removal by using vacuum assisted air gap membrane distillation (VAGMD)

Vacuum-assisted air gap membrane distillation was used to remove boron from different waters in this study. Four different vacuum pressures (0.02 bar, 0.04 bar, 0.06 bar, and 0.08 bar) were applied and air gap membrane distillation (AGMD) configuration was also used to compare the effect of vacuum pressures on the process. Six commercial hydrophobic membranes which have the 110° contact angle value at least were used. Membranes with a 44.18 cm 2 effective area were tested with saline water, synthetic boron solution, and real geothermal water as the feed solutions which is the only study with VAGMD in the literature. Salt removal efficiencies of all experiments were higher than 98.2%. Permeate boron concentrations of synthetic boron solution and real geothermal water were lower than the 0.5 mg/L limit value for drinking water set by WHO and in terms of removal efficiency it means higher than 99% removal efficiency. Vacuum pressures enhanced the water fluxes as expected and also decreased the specific energy consumption. • The use of a supplementary vacuum pump on the AGMD system improved the flux. • Increasing pressures increase flux and GOR without any considerable rejection loss. • Higher than 99% boron removal efficiencies were achieved. • All permeate boron concentrations were obtained below 0.5 mg/L. • Specific energy consumption values did not increase although using a vacuum pump.

Membrane Distillation Technology Using for Desalination of Associated Oil Water Production and Study Efficiency of Feed Water Operation Parameter on Air Gap Membrane Distillation Process Production

In this study; membrane distillation technology using to associated oil water production desalination by air gap membrane distillation unit to removed salt from the water after separating oil product, via PTEF commercial hydrophobic membrane distillation with 0.22 µm porous and 0.011 m contact surface area, evaluated the desalination system and operation parameter for feed water effected on process production by employment the distillation under boiling point temperature that getting commercial increments and study the energy gain by calculating the GOR of the desalination process.
 The study focusing on the air gap membrane distillation desalination process by a range of temperature after primary simple sedimentation and filtration it's obtained salt rejection up to 98.9% that proves process separation efficiency. Evaluated the mean operation parameters of feed water affected on permeate water production when selected the feed temperature and flow rate with fixed coolant temperature, coolant flow rate, and air gap width to get an optimum range for feed water operation parameter to obtain optimum value to permeate water production and saving energy.

Intermittent direct joule heating of membrane surface for seawater desalination by air gap membrane distillation

Air gap membrane distillation (AGMD) is a low-temperature separation process applied commonly to desalination of (hyper)saline feeds. The low energy efficiency of heating the bulk feed solution to drive vapor transport in MD must be addressed through novel materials and techniques. In this work, we introduce the concept of intermittent electrical heating at the membrane surface using electrically conductive carbon nanotube structure (CNS) films pressed on commercial polypropylene (PP) membranes. First, the Joule heating effect was optimized by varying the mass loading of the CNS layer between 1.4 and 4.2 mg cm −2 . The effect of mass loading on porosity, electrical properties and cross-sectional morphology of the CNS layer was also investigated. Electrical conductivity of CNS films increases with increasing CNS mass loading. A CNS loading of 1.4 mg cm −2 yielded the highest electrical resistance, which translated into higher Joule heat generation, and highest steady-state temperature in response to applied current. When an alternating current of 2 A was passed through the CNS films in air, the maximum surface temperature achieved was 184 °C, 126 °C and 100 °C at CNS loadings of 1.4 mg cm −2 , 2.8 mg cm −2 and 4.2 mg cm −2 , respectively. Maximum surface temperature and spatial temperature distribution of CNS films were verified using a steady-state thermal-electric simulation. CNS films exhibited rapid thermal cycling between on and off states, due to their low thermal mass, with low thermal time constants for both heating and cooling. CNS film with a loading of 1.4 mg cm −2 was then pressed onto a commercial PP membrane and applied to AGMD for seawater desalination, with the conductive layer in direct contact with the feed solution. Intermittent heating was carried out by passing a current of 2 A for 10 min at 20 min intervals, for 10 cycles. Feed outlet temperature increased by > 9 °C during each cycle, which improved average permeate flux by 78%. While specific thermal energy consumption increased by 34%, a 25% enhancement was found in GOR, indicative of higher thermal energy efficiency. We have demonstrated the effectiveness of intermittent membrane surface heating for simultaneous performance enhancement and improved energy efficiency for seawater desalination using MD. • Direct surface heating applied to lab-scale air gap membrane distillation system for seawater desalination. • Carbon nanotube nanostructures coated onto hydrophobic membrane to enable Joule heating. • Intermittent application of electrical field improves GOR by 25% and productivity by 78%. • Long-term application of high-grade electrical energy not required.

Solar chimney power plant combined with membrane distillation (SCPPMD), part I: Principle and operation characteristics

In this paper, a novel solar chimney power plant combined with membrane distillation (SCPPMD) was proposed. SCPPMD could not only achieve power output but also product more freshwater, by installing a novel air gap membrane distillation module vertically in the disc distiller. A transient heat and mass model was established to predict the system performance. Then, the operation laws of SCPPMD under unsteady weather conditions and the effect of seawater thickness were explored. The results show that the peak power generation is about 4p.m. and the peak water yield lags two hours. The hourly water yield of the membrane is much higher than that of the disc distiller and its valley value reaches 11.55 t/h, 14.26 times the valley value of the disc distiller and 0.875 times its peak value. Membrane distillation still has considerable freshwater output even when the water production of disc distiller is weak. As the seawater layer thickens, the peak-valley difference of power generation is reduced and the daily power generation drops; The peak hourly water yield of disc distiller drops, the valley value increases and its daily water yield drops; Both the peak and valley value of membrane hourly water yield increase, and its daily water yield rises; The change of the system water yield is basically the same as that of membrane.

Statistical modeling and optimization of helical air gap membrane distillation desalination system

Statistical modeling and optimization of helical air gap membrane distillation desalination system

Scale-up of membrane distillation systems using bench-scale data

A procedure to design full-scale air gap membrane distillation (AGMD) processes is presented. A mathematical model was then developed for both direct contact membrane distillation (DCMD) and AGMD. The model is centered on solving local mass and energy balances using a finite difference approach. The full-scale model was calibrated by utilizing the membrane distillation coefficient (MDC) determined by DCMD bench-scale experiments, as the sole adjustable parameter. The MDC was then used to model the water production and energy efficiency of a spiral-wound AGMD full-scale element. The model yields accurate representation of full-scale AGMD elements using polytetrafluoroethylene (PTFE) and polyethylene (PE) membranes. Full-scale experimental results obtained over a wide range of feed flow rates (2 to 4.5 L/min), temperatures (40 to 80 °C), and salinities (0 to 200 g/L NaCl) confirmed that the developed procedure can be applied to model and design large-scale AGMD elements. Furthermore, the model guides the selection of specific temperature and flow conditions at a given salinity and element geometry to maximize water production and energy efficiency. This methodology is suitable for rapid evaluation of novel MD membranes performance in field AGMD applications.

Performance improvement of an air gap membrane distillation process with rotating fan

The current paper presents an experimental study on the performance of a novel air gap membrane distillation (AGMD) desalination system where a fan blade is installed within the air gap to improve the system’s productivity and energy utilization. The installed blade creates suction inside the air gap behind the membrane and increases the air turbulence, thereby reducing the vapor mass transfer resistance across the membrane and improving the productivity and energy efficiency of the system. Results revealed that the proposed design and process improve the vapor permeate flux by about 250% when compared to the conventional AGMD module. Furthermore, the average improvement in the gained output ratio of the system is about 67%, with a corresponding 77% reduction in the specific thermal energy consumption. Influences of the feed temperature and the motor speed are dominating the performance of the new system.