No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Comparative characterization of three commercial spiral-wound membrane distillation modules
Abstract
At Plataforma Solar de Almería (PSA), different commercial spiral-wound MD modules were tested
coupled to a solar thermal field composed of stationary flat plate solar collectors. One of them is
the Solar Spring module with a permeate-gap membrane distillation (PGMD) configuration. Two
modules from Aquastill based on a configuration of air-gap membrane distillation (AGMD) were
also tested. A characterization of the modules is presented based on an extensive set of experiments
carried out using simulated seawater. The performance was evaluated by measuring the production
of distillate per unit surface of membrane and the heat efficiency, calculated through the thermal
energy consumption. Also, the quality of the product was evaluated by measuring the conductivity
of the distillate. The tests were performed changing the most significant operational parameters in
order to characterize their effect on the performance of the system. The feed flow rate was varied
between 400 and 600 l h–1 and the temperature of the hot feed from 60 to 80°C. The results show that
the internal design of the module is very important, and the differences in the channel length in
these modules have a stronger effect in their performance for seawater desalination than the configuration
of the gap.
... This plant was totally described in the work in Ref. [25] and it is composed of: a solar thermal field, with a nominal capacity of 7 kW th at 90 • C, a slope angle of 7 • and facing south (azimuthal angle of 0 • ); a storage tank, with a volume of 1.5 m 3 ; and a Membrane Distillation (MD) module. Although there are several MD modules available at PSA, in this work we use the Aquastill module which has a total membrane surface area of 24 m 2 providing a maximum distillate production of around 30 L/h under the best operating conditions [26]. • Nanofiltration plant: This facility is also based on a pilot plant located at PSA. ...
... Similarly, the equivalent circuit for photovoltaic cells provides the field's performance (η D,1 ) via another user-defined function [30]. The desalination and nanofiltration plants have been an object of analysis to determine their features as well [26,27] and only the storage systems are hypothetical elements, as justified in Ref. [31]. In respect to the resources considered as outputs (demands) of the energy hub, the electricity consumption of the pumping systems of the solar collectors (0.1943 kW), the MD plant (0.1720 kW), and the nanofiltration plant (0.2502 kW) were taken into account in the problem separately, as device-dependent outputs (O 2 , O 4 , and O 6 , respectively), allowing us to relate them to the on/off state of each pump. ...
... In respect to the resources considered as outputs (demands) of the energy hub, the electricity consumption of the pumping systems of the solar collectors (0.1943 kW), the MD plant (0.1720 kW), and the nanofiltration plant (0.2502 kW) were taken into account in the problem separately, as device-dependent outputs (O 2 , O 4 , and O 6 , respectively), allowing us to relate them to the on/off state of each pump. In addition, the value of κ in O 5 = κη D,3 P 13 is fixed at κ = 121.25 kW/m 3 , considering the experimental data and model provided in Ref. [26]. The rest of the demands (O 1 , O 3 , and O 7 ) were calculated by processing the real historical data measured by the sensor systems of each of the plants considered in the agro-industrial district (see Figure 4). ...
In this work, the optimal management of the water grid belonging to a pilot agro-industrial district, based on greenhouse cultivation, is analyzed. Different water supply plants are considered in the district, some of them using renewable energies as power sources, i.e., a solar thermal desalination plant and a nanofiltration facility powered up by a photovoltaic field. Moreover, the trade with the water public utility network is also taken into account. As demanding agents, a greenhouse and an office building are contemplated. Due to the different water necessities, demand profiles, and the heterogeneous nature of the different plants considered as supplier agents, the management of the whole plant is not trivial. In this way, an algorithm based on the energy hubs approach, which takes into account economic terms and the optimal use of the available resources in its formulation, is proposed for the pilot district with a cropping area of 616 m². Simulation results are provided in order to evidence the benefits of the proposed technique in two cases: Case 1 considers the flexible operation of the desalination plant, whereas in Case 2 the working conditions are forced to equal the plant’s maximum capacity (Case 2). A flexible operation results in a weekly improvement of 4.68% in profit, an optimized use of the desalination plant, and a reduction of the consumption of water from the public grid by 58.1%.
... Although PGMD has been studied by various researchers, most of them are based on flat sheet [7,[12][13][14][15] and spiral wound [10,11,16,17] membrane modules. In this research, a specifically designed PGMD module using hollow fiber membrane was manufactured and tested under various operating conditions, because hollow fiber membrane has a large specific area without any supporting structure [18]. ...
... These limitations should be considered during full scale hollow fiber PGMD module design. Some recent studies [16,17] further optimized spiral wound PGMD energy performance by using a solar system to supply the thermal energy, which could be taken into consideration during next step module optimization. ...
A hollow fiber Permeate Gap Membrane Distillation (PGMD) module was tested and compared with that operated in Direct Contact Membrane Distillation (DCMD) and Sweeping Gas Membrane Distillation (SGMD) modes. The results showed that the flux increased with increasing feed velocity and feed temperature. The global mass transfer coefficient of PGMD increased initially at low feed temperatures and decreased at high temperatures. Hollow fiber PGMD showed a lower STEC compared to DCMD and SGMD when flux was the same. It also achieved a better balance between flux and specific thermal energy consumption compared to spiral wound and flat sheet PGMD modules.
... scaling) leads also to lower permeate fluxes (Gryta, 2008a). Different MD configurations are employed for water recovery: direct contact (DCMD), air gap (AGMD), sweeping gas (SGMD), and vacuum (VMD), being the first one the most studied and the easiest to operate (El-Bourawi et al., 2006;Ruiz-Aguirre et al, 2017, 2018. ...
Management of in-land reverse osmosis (RO) desalination brines generated from surface brackish waters is a current challenge. Among the different near-Zero and Zero Liquid Discharge (ZLD) alternatives, Membrane Distillation (MD), in which the transport of water is thermally driven, appears as an attractive technology if a residual heat source is available. The aim of this study was to identify the limits of Direct Contact MD (DCMD) pre-treatments such as acidification and aeration, or the combination of both to quantify the scaling reduction potential when treating a RO brine from surface brackish water. Experimental data were used to evaluate the effectiveness of DCMD to achieve the highest concentration factors, depending on the chosen pre-treatment. Additionally, an economic analysis of the operational cost, taking as case study a site where the current management of the brine is the discharge to the sea, was also carried out. Results showed that pre-treatments enhanced MD performance by increasing the concentration factor achieved and highest volume reductions (about 3 times) were reached with the combination of acidification and aeration pre-treatments. Both processes reduced the precipitation potential of CaCO3(s) by reducing the total inorganic carbon (>90%); however, CaSO4·2H2O(s) precipitated. Results also indicated that even if a waste heat source is available, brine disposal into the sea is the cheapest option, while ZLD alternatives were not attractive in the current regulatory framework since their cost was higher than the discharge to the sea. Other options related to the Minimal Liquid Discharge may be more economically attractive.
... Fig. 13-b shows the corresponding velocity effect on the system performance. These predicted trends are in accordance with the results in (Aguirre et al., Jan. 2017) 4.2.1.3. Effect of channel height. ...
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.
... This is the reason that many current plants are powered by low-grade and/or renewable energy. As indicated in Table 1, twelve installations combine solar thermal and PV to supply the required energy [128,[132][133][134][138][139][140]142,[145][146][147][148]. Other plants use a combination of solar thermal and waste heat [133], electricity from the grid [144,150], geothermal energy [149] or a salinitygradient solar pond [141]. ...
Growing uncertainty in the future availability of freshwater sources has led to an increase in installations for
desalination of seawater. Reverse osmosis (RO), currently the most widely adopted technique, has caused
environmental concerns over the high associated greenhouse gas emissions and generation of large amounts of
chemicals-containing brine. Significant consumption of electricity for RO desalination is an additional challenge,
particularly in remote locations. In this review, forward osmosis (FO), membrane distillation (MD) and capacitive
deionisation (CDI) are assessed as potential substitute technologies and the major recent advancements in each
field are discussed. These emerging technologies offer significant advantages over RO, such as higher salt
rejection (CDI, MD), higher recovery of water (MD), fewer pre-treatment stages (MD, FO) and the ability to use
low-grade energy (MD, FO). In their current state, stand-alone technologies cannot compete with RO until certain
challenges are addressed, including pore-wetting (MD) and high energy consumption (MD, CDI, FO). Hybrid
systems that combine RO and emerging technologies may be useful for feed waters that cannot be treated by RO
alone and their benefits may be able to offset the increase in capital costs. These and other aspects, such as
operational stability should be considered in larger-scale, long-term studies.
... Therefore, MED cannot be scaled-down to less than several hundreds of m 3 /day, i.e. a scale much larger than the laboratory one [27]. However, the energy consumption of the MD process is higher than that of MED: in literature the reported specific thermal consumption is hardly below around 100-200 kWh/m 3 distillate for MD [28], while for MED it can reach values as low as 50-70 kWh/m 3 distillate [15]. Clearly, this difference would determine a decrease of the cycle efficiency and for this reason, MD turns out to be a competitive and cost-effective technology when low cost thermal energy is available [19,29], as in the case under investigation. ...
The coupling of Reverse Electrodialysis with Membrane Distillation is a promising option for the conversion of
waste heat into electricity. This study evaluates the performances of the integrated system under different operating conditions, employing validated model and correlations. This work provides a detailed description of the
behaviour of a real RED-MD heat engine and indicates the set of inlet concentrations, velocities and equipment
size which returns the highest cycle exergy efficiency. These operating conditions were selected for the pilot
plant developed within the EU-funded project RED Heat to Power. For the first time, a perspective analysis was
also included, considering highly performing RED membranes and future MD module. Relevant results indicate
that technological improvements may lead to interesting system performance enhancement, up to an exergy
efficiency of 16.5%, which is considerably higher than the values reported in literature so far.
... 12 A model optimization for seawater desalination showed that for very low circulation rate and feed temperature 70°C, GOR could be up to 6-7, although permeate flux was only 1 l/h m 2 . 34 Similar values of GOR and flux were obtained by Ruiz-Aguirre et al. 35 with a larger module (24 m 2 ) with longer channels (5 m long) using four times more circulation feed rate. This can be explained using the heat exchanger theory applied to MD by Swaminathan et al. 36 Reducing the feed flow rate is equivalent to lengthening the channel, increasing the number of heat transfer units (NTU) and improving heat recovery. ...
Abstract Membrane distillation is an attractive technology for solar-powered decentralized desalination that has not yet reached commercial breakthrough on a large scale. The main barriers are energy consumption and cost. Since the latter are mostly related to the former, thermal energy efficiency is key to assessing the potential of the different available membrane distillation systems at a commercial scale. As discussed here, existing membrane distillation technologies use mostly flat sheet membranes in plate and frame and spiral-wound modules. Modules based on hollow fibre membranes are also considered, as well as the concept of multi-effect vacuum membrane distillation for improved heat recovery. The heat efficiency of each system is analysed based on available experimental results. Better internal heat recovery and capacity for upscaling are found to be important elements of distinction which make multi-channelled spiral-wound modules working in air-gap configuration stand out currently, with the lowest heat consumption of all large scale modules. Potential for improvement of this and other technologies is also discussed, and an estimation based on the associated costs for solar energy is used for establishing boundary conditions towards the implementation of membrane distillation for solar desalination.
... These modules involved a plate and frame design with each featuring a 1 mm air gap, 2.3 m 2 membrane area, nine feeds and nine cooling channels (total stack thickness 17.5 cm). In another work, Aquastill spiral wound system based AGMD configuration was tested coupled to a solar thermal filed composed of stationary flat plate at PSA centre [248,249]. The performance plant was evaluated by measuring the flux and the heat efficiency. ...
Membrane distillation (MD) is a promising thermally driven membrane separation technique. In MD,
water vapor is being separated from the hot feed water solution using a microporous hydrophobic
membrane, due to the difference in temperature, and hence vapor pressure, across the membrane.
Air gap membrane distillation (AGMD) process is one of the common configurations of applying the
MD process for water desalination and other applications. In AGMD, a stagnant air gap is introduced
between the membrane and a condensation surface within the membrane module to reduce the conduction
heat loss through the membrane. In this review article, design characteristics and operating
conditions of AGMD and its modified designs to enhance the productivity and reduce the energy
consumption are surveyed and discussed. Previous work on pilot AGMD systems and multi-stage or
multi-effect systems with energy saving modules is highlighted. Membrane materials and developments
used with the AGMD modules are presented with discussion of membrane fouling and scaling
problems. In addition, modeling techniques based on the heat and mass transfer equations and simulation
approaches of the AGMD process are presented. The merits of operating the AGMD systems
with solar and other renewable energies are discussed along with the economic aspects. The future
research directions of AGMD are highlighted in this review. This will help researchers to direct their
research without repetition of previous known studies.
The use of vacuum enhancement in air gap membrane distillation (AGMD) has been shown to improve the permeate production and the thermal efficiency. This work presents the characterization of that improvement for different operating conditions (input temperatures and feed flow rate) in three commercial multi-envelope AGMD modules with different internal designs. Modules had different number and length of internal circulation channels, yielding different feed velocities and residence times, and thus allowing to investigate the role of these. Experiments were performed with simulated seawater in solar membrane distillation pilot plants, and the decrease of permeate flux (PFlux) and specific thermal energy consumption (STEC) were analysed for each operating condition. The greatest impact of vacuum enhancement in the performance improvement was observed in the modules with the longest residence time operated at low hot temperature. The lower the driving force, the more important the role of vacuum enhancement in the improvement of vapour diffusion was, independently of feed velocity. Increases of PFlux up to 88% and reductions of STEC down to 70% compared to AGMD operation were quantified. Moreover, vacuum enhancement led to the reduction of hydraulic pressure drop, decreasing 26% the specific electrical consumption.
The treatment of high salinity feeds, for example, brines from other desalination technologies such as reverse osmosis, is becoming one of the sought-after applications of solar membrane distillation processes. One of the most appropriate operating methodologies to carry out these treatments is a batch operation. Nevertheless, throughout this operation, the variability of the salinity in the feed water acts as a disturbance causing the optimal operating conditions required to reduce the process’ thermal energy consumption (expressed in kWh/m3 of freshwater production) to change. To face this issue, this study proposes two real-time optimization control approaches, with energy-saving achievements, based on the extremum-seeking control methodology. The first controller reduces the thermal energy consumption of the membrane distillation module by adapting the feed flow rate according to the level of salinity of the operation. The second one is based on the same idea but tries to search for a trade-off solution among energy consumption and water production. Both approaches are simulated under actual process circumstances using data and a validated model of a pilot plant located at Plataforma Solar de Almería (Spain). The results show how the proposed controllers improve the system’s daily operation concerning conventional operational procedures, reducing the system’s mean thermal energy consumption up to 35 % (from 993.81 kWh/m3 to 651.52 kWh/m3) in the best case. On the whole, the systematic implementation of the proposed control strategies allows enhancing the sustainability and thermal efficiency of the operation by minimizing unnecessary thermal energy usage. This fact can play a role in solar membrane distillation systems either to lengthen operations or reduce reliance on conventional backup devices in periods with inconstant solar irradiance.
A double‐effect membrane distillation (DE-AGMD) process was developed by equipping air gap membrane distillation (AGMD) with internal heat recovery. In this arrangement, the latent heat of the first effect will be recycled as a source of heat for the second effect. More produced water without adding energy. This configuration improves the efficiency of the process, and SEC (specific energy consumption) decreases. The potentialities of DE-AGMD to desalt seawater were investigated. Response surface methodology (RSM) is used to model and optimize the operating parameters for water desalination by DE-AGMD. Permeate flux (Jp) and salt rejection (R %) were the main performance responses to optimize. The temperature difference (ΔT), width of air gap (δ), feed flow rate (Qf) and cooling solution flow rate (Qc) were the operating parameters investigated to optimize the performance of DE-AGMD. The obtained quadratic regression model is validated by analysis of variance (ANOVA). The high values of R2 = 99.86 and R2 adj = 99.7 confirmed that the quadratic equation properly fitted the experimental data. The optimal operation parameters are found to be 52.28 °C for the temperature difference, 25.62 L h−1 for the feed flow rate, 23.87 L h−1 for the permeate flow rate and 3 mm for the air gap width. Under these conditions, the permeate flux could reach 4.69 L m−2 h−1. The experimental and predicted results show a good agreement confirming the fit of the models by 97.78%. The modeling of salt rejection is found to be difficult due to the steadiness of response for all experimental runs (> 97%).
Membrane distillation using multi-envelope modules in vacuum enhanced air gap (V-AGMD) operation has been demonstrated as the best alternative to the current thermal desalination technologies. This work presents for the first time the use of response surface methodology (RSM) to model and validate the performance of V–AGMD operation for seawater desalination in three multi-envelope modules with different internal designs. Permeate productivity (PFlux) and condenser outlet temperature (TCO) were modelled and validated from results obtained in steady-state experiments, considering the evaporator inlet temperature, the condenser inlet temperature, and the feed flow rate as inputs. To gain accuracy in the estimations of thermal efficiency regarding previous works, a novel strategy was applied, consisting in calculating the specific thermal energy consumption (STEC) using the models of PFlux and TCO, developed from direct measurements. The models of the three modules were used to search the optimal operating conditions that minimize the levelized cost of water (LCOW) in a large-scale MD plant under different scenarios. Different costs of thermal and electrical energy were considered and their influence on the number of modules required in the facility was also discussed.
The use of thermal evaporation processes, such as multieffect distillation (MED) and membrane distillation (MD), is presented as a very interesting option to regenerate salt solutions when a closed-loop system is considered in a salinity gradient power heat engine (SGP-HE). It is due to the possible increase in the operating temperature of the evaporation processes by the use of working fluids preventing ions from scaling formation. This chapter shows the potential of both thermal evaporation processes in terms of thermal energy consumption to regenerate the salt solutions used in an SGP-HE. Several case studies are presented for MED and MD, using three different salt solutions: sodium chloride (NaCl), lithium chloride (LiCl), and potassium acetate (KAc) as working fluids. Results prove the high potential of both thermal evaporation processes to regenerate these salts, achieving minimum values of specific thermal energy consumption of 21.3 kWh/m³ in MED for concentrating a solution of NaCl from 1 to 5 M and 302 kWhth/m³ from 1.8 to 3.6 M for MD.
The water–energy–food nexus has captured the attention of many researchers and policy makers for the potential synergies between those sectors, including the development of self-sustainable solutions for agriculture systems. This paper poses a novel design approach aimed at balancing the trade-off between the computational burden and accuracy of the results. The method is based on the combination of static energy hub models of the system components and rule-based control to simulate the operational costs over a one-year period as well as a global optimization algorithm that provides, from those results, a design that maximizes the solar energy contribution. The presented real-world case study is based on an isolated greenhouse, whose water needs are met due to a desalination facility, both acting as heat consumers, as well as a solar thermal field and a biomass boiler that cover the demand. Considering the Almerian climate and 1 ha of tomato crops with two growing seasons, the optimal design parameters were determined to be (with a solar fraction of 16% and a biomass fraction of 84%): 266 m2 for the incident area of the solar field, 425 kWh for the thermal storage system, and 4234 kW for the biomass-generated power. The Levelized Cost of Heat (LCOH) values obtained for the solar field and biomass boiler were 0.035 and 0.078 €/kWh, respectively, and the discounted payback period also confirmed the profitability of the plant for fuel prices over 0.05 €/kWh. Thus, the proposed algorithm is useful as an innovative decision-making tool for farmers, for whom the burden of transitioning to sustainable farming systems might increase in the near future.
Seawater membrane distillation (SWMD) is a promising separation technology due to its ability to operate as a stand-alone desalination unit operation. This paper reviews approaches to improve laboratory-to-pilot-scale MD performance, which comprise operational strategies, module design, and specifically tailored membranes. A detailed comparison of SWMD and sea water reverse osmosis is presented to further analyze the critical shortcomings of SWMD. The unique features of SWMD, namely the ability to operate with extremely high salt rejection and at extreme feed concentration, highlight the SWMD potential to be operated under zero liquid discharge (ZLD) conditions, which results in the production of high-purity water and simultaneous salt recovery, as well as the elimination of the brine disposal cost. However, technical challenges, such as thermal energy requirements, inefficient heat transfer and integration, low water recovery factors, and lack of studies on real-case valuable-salt recovery, are impeding the commercialization of ZLD SWMD. This review highlights the possibility of applying selected strategies to push forward ZLD SWMD commercialization. Suggestions are projected to include intermittent removal of valuable salts, in-depth study on the robustness of novel membranes, module and configuration, utilization of a low-cost heat exchanger, and capital cost reduction in a renewable-energy-integrated SWMD plant.
This paper presents the first experimental evaluation at pilot scale of the operation of vacuum-enhanced air-gap membrane distillation (V-AGMD) using two commercial spiral-wound modules at Plataforma Solar de Almería's solar desalination test facilities. The main difference between the modules was the channel length (1.5 and 2.7 m) as a result of having different membrane surface area (7.2 m² and 25.9 m² respectively) and different number of envelopes. Suction of air from the gap improved the vapour transfer through the membrane pores and the performance of the modules was significantly increased in relation to common air-gap (AGMD) operational mode, especially in the treatment of high salinity feeds. Increases of up to 234% in permeate flux and decreases of 68% in specific thermal energy consumption were measured. Depending on the channel length of the modules, the effect of vacuum led to extreme permeate productivity (8.7 l h⁻¹ m⁻²) in the shortest, or to extreme energy efficiency (49 kWhth m⁻³, equivalent to a GOR of 13.5) in the longest. These are the best experimental performances obtained so far with pilot scale modules in 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.
This work presents the evaluation of an innovative system based on vacuum multi-effect membrane distillation modules (V-MEMD) for seawater desalination at pilot scale. This four-effect unit introduces a remarkable modification from previous V-MEMD systems, consisting of the use of the seawater feed flow as cooling in the condenser, rather than a separate circuit. Preheating the feed in the condenser improved heat efficiency (maximum gained output ratio obtained for seawater was 3.2). Maximum distillate fluxes reached 8.5 l h⁻¹ m⁻² for hot feed temperature 75 °C and feed flow rate 150 l h⁻¹. Increasing both parameters to raise the productivity was hindered by the inability of the condenser to cope with all the steam generated in previous effects, resulting in overheating and overpressure. Furthermore, a loss of 40% of distillate production was measured due to the increase of seawater cooling temperature by 8 °C along the year. Finally, it was observed that scaling reduced distillate production up to 50%. Acid cleaning successfully removed scaling and restored the performance. Subsequently, the use of an antiscalant as a pre-treatment was sufficient to prevent it.
An experimental characterization of the performance of two multi-channel spiral-wound air-gap membrane distillation modules at commercial-scale was carried out for seawater desalination. The only difference between them was the membrane surface areas: 7.2 m² and 24 m² and thus the length of the channels (1.5 and 5 m respectively). The influence of the main operating parameters (evaporator inlet temperature, condenser inlet temperature and feed flow rate) was quantified. It was observed that the trade-off between productivity and energy efficiency was mostly related to the heat recovery inside the module. This was affected by the operation and also by the design of the modules: the longer the channel, the better the thermal efficiency but the worse the productivity. Empirical models were derived for each module. Optimum values of the three operating parameters to simultaneously maximize permeate flux and minimize specific heat consumption were obtained using surface response methodology (RSM). In the longest module it was possible to find operating conditions that maximize the productivity without losing thermal efficiency, but in the shortest module a compromise had to be found between both.
Solar Membrane Distillation (SMD) is an under-investigation desalination process suitable for developing self-sufficient small scale applications. The use of solar energy considerably reduces the operating costs, however, its intermittent nature requires a non-stationary optimal operation that can be achieved by means of advanced control strategies. In this paper, a hierarchical control system composed by two layers is used for optimizing the operation of a SMD pilot plant, in terms of thermal efficiency, distillate production and cost savings. The upper layer is formed by a Nonlinear Model Predictive Control (NMPC) scheme that allows us to obtain the optimal operation by optimizing the solar energy use. The lower layer includes a direct control system, in charge of attaining the variable references provided by the upper layer. Simulation and experimental tests are included and commented in order to demonstrate the benefits of the developed control system.
In this paper, a commercial spiral wound PGMD module was modeled and optimized for seawater desalination using Response Surface Methodology (RSM). Permeate flux (Pflux) and specific thermal energy consumption (STEC) were the main performance parameters to optimize, while evaporator inlet temperature (Tevap), condenser inlet temperature (Tcond) and feed flow rate (F) were the three operating parameters chosen. Analysis of variance (ANOVA) was used to evaluate statistically the response surface models. According to the study, Tevap had the strongest effect on Pflux and STEC, increasing the former and decreasing the latter, F increased both responses, and Tcond had a weak effect on Pflux and practically none on STEC. The models were validated with further experimental data and a good correlation between experimental and predicted values of the responses was obtained for Pflux and STEC respectively. An optimization was performed to determine the operating conditions that produce a maximum value of Pflux and a minimum value of STEC simultaneously. The result of the multiple responses optimization using desirability function was a maximum Pflux of 2.66 l/h·m² and a minimum STEC of 255.8 kWh/m³.
The use of solar energy to feed the MD desalination process is being evaluated at Plataforma Solar de Almería, the largest European facility for solar energy research, located in SE Spain. A test bed for the evaluation of membrane distillation modules is under operation coupled to a field of static solar collectors. Different commercial modules and real-scale prototypes are tested in continuous operation and coupled with a solar thermal source, in order to obtain data in conditions closer to real applications than the tests performed at laboratory scale. This particular study shows an evaluation of two different modules using spiral-wound membranes, one with a liquid-gap configuration (built by Solar Spring) and the other with an air-gap configuration (built by Aquastill). An assessment of the influence of the operational parameters in the performance was done within the allowed ranges of operation of each prototype, with special attention to the temperatures and the feed flow rate. Also, the influence of the salinity was investigated using feed water with salts at different values of conductivity. The characterization of the systems is done based on the quality of the distillate, as well as the measured values of distillate production and thermal performance, choosing the specific distillate flux obtained and the gain output ratio (GOR) as performance indicators. The main results of the analysis are summarized and compared, discussing the particular operational experiences in each case.
Membrane Distillation (MD) is a thermally-driven separation process, in which only vapour molecules transfer through a microporous hydrophobic membrane. The driving force in the MD process is the vapour pressure difference induced by the temperature difference across the hydrophobic membrane. This process has various applications, such as desalination, wastewater treatment and in the food industry.This review addresses membrane characteristics, membrane-related heat and mass transfer concepts, fouling and the effects of operating condition. State of the art research results in these different areas will be presented and discussed.
In many places world wide drinkable water is already a scarce good and its lack will rise dramatically in the future. Missing energy sources and no grid connections complicates the use of standard desalination techniques in these places. Fraunhofer ISE develops solar thermally driven compact desalination systems based on membrane distillation (MD) for capacity range between 100 and 500 L/day and larger systems for the capacity range up to 10 m(3)/day. All systems can be operated energy self sufficient and almost maintenance free. Membrane distillation is a technique which is operated with thermal energy but also uses a membrane for the separation of pure water from the concentrated solution. With regard to the implementation in solar driven stand-alone desalination systems it holds important advantages. Altogether eight pilot plants were installed in five different countries.
The development of small to medium size, autonomous and robust desalination units is needed to establish an independent water supply in remote areas. This is the motivation for research on alternative desalination processes. Membrane distillation (MD) seems to meet the specific requirements very well. This work is focused on experimental studies on full scale demonstration systems, utilizing a parallel multi MD-module setup. Three different plant concepts are introduced, one of them is waste heat driven and two of them are powered by solar thermal collectors. Design parameters and system design are presented. After the analysis of plant operation a comparison among the plants as well as a comparison with laboratory experiments is carried out and discussed. Impact of different feed flow rates, salinities, operating hours and process temperatures are taken into consideration and put into relation. GOR values and specific thermal heat demand are derived and compared. Energy balances of all three plants are given, uncovering heat losses and identifying room for improvement.
Brine management is a major bottleneck for coal seam gas (CSG) production in Australia. This study investigated the concentration of CSG reverse osmosis (RO) brine using a pilot membrane distillation (MD). The system was equipped with a novel spiral-wound air gap membrane distillation (AGMD) module. By operating the pilot MD system at low feed temperature and a small temperature gradient, a stable distillate production rate could be maintained. The resulting low permeate flux can be offset by a high packing density of the spiral-wound membrane module. Here, using a module with diameter, height, and total membrane surface area of 0.4 m, 0.5 m, and 7.2 m2, respectively, the pilot MD system sustainably achieved 80% water recovery and produced 10 L/h of distillate from CSG RO brine. Overall, 95% water recovery could be obtained from CSG produced water for beneficial uses by a combination of RO and AGMD without any observable membrane scaling. A preliminary thermal energy demand analysis suggests that if installed in New South Wales (Australia), 1 ha of flat-plate solar thermal collector arrays could provide sufficient thermal energy to treat 472 m3/day (2970 bbl/day) of CSG produced water using the proposed RO/AGMD treatment train.
The paper presents investigations on the process performance of different membranes with and without backing structure in direct contact membrane distillation. Influences of backing structures and their orientation are identified. An integrated membrane and backing model was developed. Laminates with different backing designs were examined over a wide range of operating conditions and compared to results for the same membranes without backing. Experimental results and model predictions for flux and thermal efficiency are compared. The model predictions agree well with the experimental results. An influence on the effective area for diffusion, an increase in effective diffusion path length in the membrane and the formation of a complex network of thermal resistances are considered to be the main effects, leading to a significant reduction in process performance due to backing structures. The backing was identified to be one of the key components for further improvement in MD applications. An integrated assessment of membrane properties in combination with backing structures was carried out by analysing membranes with different thicknesses and different pore sizes. The experimental observations show good agreement with theoretical considerations. A combined analysis of specific flux and thermal efficiency leads to comprehensive understanding of the process and its potential for optimisation.
The demand of freshwater has surpassed the renewable limit and new water sources are associated with an intensive use of energy. Coincidence between scarcity of water and availability of solar radiation makes solar energy the most suitable option to mitigate the water deficit. This paper analyzes the use of energy for decentralized water production using membrane desalination systems fed with solar energy. An analysis is performed based on experimental results from the most advanced commercial prototypes of different technologies of membrane distillation using various configurations, i.e., air-gap, permeate-gap and vacuum; with flat-plate and spiral-wound membranes. The systems operate with thermal energy, although there is some electrical consumption for pumping and in some cases for sustaining vacuum. The thermal energy requirements per unit volume of water produced are assessed in each case, considering the effect of different operational conditions like the temperature regime and the salinity of the input water.
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 difference across the membrane (ΔT). Water vapor flux of 88.8 kg/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 the 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.
Membrane distillation (MD) is considered a promising technology for desalination applications. In this paper, the main transport phenomena in membrane distillation are analyzed theoretically. One of the main limiting factors is the molecular diffusion resistance caused by stagnant air in the membrane pore volume. A great potential for flux enhancement has been identified to be the removal of air in the membrane pores. Preliminary experiments on water deaeration using a commercial membrane contactor were carried out. The dynamics of membrane and air gap deaeration using deaerated feed water were studied experimentally. A module test facility was equipped with a feed water deaeration setup. Full-scale spiral-wound modules with a membrane area of 5 and 10m2 have been used for comprehensive experimental studies on the use of deaerated feed water. The impact of deaeration at different pressures, feed flow rates, feed water salinities and temperature levels is quantified and discussed. A comparison of module operation with and without preceding feed water deaeration is given. The experimental results include module output rate, thermal energy demand, thermal efficiency and an estimation of the electrical energy demand. In all experiments, the beneficial effect of feed water deaeration can clearly be shown for total flux and thermal energy demand simultaneously. For selected operation points, the relative effect was identified to be more than 50%50% and the specific thermal energy demand dropped to values of less than 100kWhthm−3.
This paper presents the analysis of the performance of two different pre-commercial MD desalination modules developed by the Singaporean enterprise Keppel Seghers tested under real conditions intermittently for at least 2400 h during two years, directly coupled with a static collector's solar field. Both modules are based on the air gap membrane distillation (AGMD) configuration with a total membrane surface of 9 m(2) each. One of the modules is a compact single design while the other one consists of three stages connected in series. The performance of both modules (i.e. distillate production and quality, thermal efficiency and recovery ratio) has been characterized as a function of operational parameters. Especial attention was paid to thermal efficiency issues. Aqueous NaCl solutions of 1 and 35 g L-1 concentration were used as feed. Minimum specific thermal energy consumption was in the range of 1805 kWh(t)m(-3) for the compact prototype and around 294 kWh(t) m(-3) for the multi-stage one. Distillate quality was excellent (in the range of 2-5 mu S cm(-1)) and practically not affected by feed flow rate, hot inlet temperature or feed salt concentration. The paper shows the production and distillate's quality as well as the thermal efficiency results as a function of the operational parameters.
Multivariate statistical techniques, such as cluster analysis (CA), factor analysis (FA), principal component analysis (PCA) and discriminant analysis (DA) were applied to the data set on water quality of the Gomti river (India), generated during three years (1999–2001) monitoring at eight different sites for 34 parameters (9792 observations). This study presents usefulness of multivariate statistical techniques for evaluation and interpretation of large complex water quality data sets and apportionment of pollution sources/factors with a view to get better information about the water quality and design of monitoring network for effective management of water resources. Three significant groups, upper catchments (UC), middle catchments (MC) and lower catchments (LC) of sampling sites were obtained through CA on the basis of similarity between them. FA/PCA applied to the data sets pertaining to three catchments regions of the river resulted in seven, seven and six latent factors, respectively responsible for the data structure, explaining 74.3, 73.6 and 81.4% of the total variance of the respective data sets. These included the trace metals group (leaching from soil and industrial waste disposal sites), organic pollution group (municipal and industrial effluents), nutrients group (agricultural runoff), alkalinity, hardness, EC and solids (soil leaching and runoff process). DA showed the best results for data reduction and pattern recognition during both temporal and spatial analysis. It rendered five parameters (temperature, total alkalinity, Cl, Na and K) affording more than 94% right assignations in temporal analysis, while 10 parameters (river discharge, pH, BOD, Cl, F, PO4, NH4–N, NO3–N, TKN and Zn) to afford 97% right assignations in spatial analysis of three different regions in the basin. Thus, DA allowed reduction in dimensionality of the large data set, delineating a few indicator parameters responsible for large variations in water quality. Further, receptor modeling through multi-linear regression of the absolute principal component scores (APCS-MLR) provided apportionment of various sources/factors in respective regions contributing to the river pollution. It revealed that soil weathering, leaching and runoff; municipal and industrial wastewater; waste disposal sites leaching were among the major sources/factors responsible for river quality deterioration.
A solar desalination system based on membrane distillation (MD) is presented and evaluated. In the context of a European project, the MEDESOL project, a pilot plant was built to evaluate the system, which consists of three commercial MD modules coupled with a static solar collector's field. The MD modules employed have been developed and manufactured by the Swedish company Scarab AB. They have a flat sheet air gap membrane distillation (AGMD) configuration with a total membrane surface area per module of 2.8m2. The MD system is intended to be technically simple to operate, robust and able to cover water demands of small settlements. It also contemplates the use of a multi-stage layout to minimize energy consumption.Experiments were run during solar hours (the layout didn’t include heat storage) and addressed to characterize the performance of the system (i.e. distillate production and quality, thermal efficiency and recovery ratio) as a function of operation variables and salt concentration, as well as to identify the operating capacities and the potential improvements of the MD technology. Aqueous NaCl solutions of 1 and 35g/l concentration were used as feed. Temperatures up to 85°C in the feed and up to 75°C in the refrigeration were employed. Maximum specific distillate flux values registered were in the range of 7l/hm2. Multi-stage layouts were tested in order to evaluate the improvement of the system's thermal efficiency and recovery ratio. The MD technology assessed proved to be suitable for coupling with transient solar thermal energy but inefficiencies inherent to scaling-up compared to laboratory experiences reported in literature were also identified, namely affecting specific distillate production and thermal consumption. The results of the characterization, performance assessment and operational issues description of the pilot plant are shown.
Membrane distillation (MD) is considered a promising technology for desalination applications. Usually, the investigations stated in MD literature cover lab-scale experiments to analyse the technology's potential. This work is focussed on experimental studies on full scale spiral wound MD-modules with a membrane surface area of 5–14m2. Module technology and module fabrication are introduced, as well as the fully automated performance test facility and the characterisation procedures. Statements regarding module fabrication quality are given by a comparison of a four module production charge. The experimental results consider module output rate and specific energy consumption for a broad variety of module operation points. The interaction of feed flow rate, temperature levels, feed water salinity and geometrical module design parameters are quantified and discussed. Special attention is given to the influence of salinity on module performance. The salt rejection rate throughout all experiments is very high and almost independent from any studied operational parameters. Finally, a preliminary approach shows the feasibility of high purity distillate production with the spiral wound modules. In a emulated double stage process the distillate conductivity was dropped down to 0.19μS/cm.
This paper presents an assessment of membrane distillation (MD) based on the available state of the art and on ourpreliminary analysis. The process has many desirable properties such as low energy consumption, ability to use low temperature heat, compactness, and perceivably more immunity to fouling than other membrane processes. Within the tested range, the operating parameters of conventional MD configurations have the following effects:(1) the permeate fluxes can significantly be improved by increasing the hot feed temperature (increasing the temperature from 50 to 70°C increases the flux by more than three-fold), and by reducing the vapor/air gap (reducing the vapor air gap thickness from 5 to 1 mm increase the flux 2.3-fold); (2) the mass flow rate of the feed solution has a smaller effect: increasing it three-fold increases the flux by about 1.3-fold; (3) the concentration of the solute has slight effect: increasing the concentration by more than five-fold decreases the flux by just 1.15-fold; (4) the cold side conditions have a lower effect (about half) on the flux than the hot side; (5) the coolant mass flow rate has a negligible effect; (6) the coolant temperature has a lower effect than the mass flow rate of the hot solution. Fouling effects, membranes used, energy consumption, system applications and configurations, and very approximate cost estimates are presented. The permeate fluxes obtained by the different researchers seem to disagree by an order of magnitude, and better experimental work is needed.
Reverse osmosis membrane technology has developed over the past 40 years to a 44% share in world desalting production capacity, and an 80% share in the total number of desalination plants installed worldwide. The use of membrane desalination has increased as materials have improved and costs have decreased. Today, reverse osmosis membranes are the leading technology for new desalination installations, and they are applied to a variety of salt water resources using tailored pretreatment and membrane system design. Two distinct branches of reverse osmosis desalination have emerged: seawater reverse osmosis and brackish water reverse osmosis. Differences between the two water sources, including foulants, salinity, waste brine (concentrate) disposal options, and plant location, have created significant differences in process development, implementation, and key technical problems. Pretreatment options are similar for both types of reverse osmosis and depend on the specific components of the water source. Both brackish water and seawater reverse osmosis (RO) will continue to be used worldwide; new technology in energy recovery and renewable energy, as well as innovative plant design, will allow greater use of desalination for inland and rural communities, while providing more affordable water for large coastal cities. A wide variety of research and general information on RO desalination is available; however, a direct comparison of seawater and brackish water RO systems is necessary to highlight similarities and differences in process development. This article brings to light key parameters of an RO process and process modifications due to feed water characteristics.