Fluidised bed crystalliser and air gap membrane distillation as a solution to geothermal water desalination

Experimental study of air gap and direct contact membrane distillation configurations: application to geothermal and seawater desalination

Direct contact and air gap membrane distillation: Differences and similarities between lab and pilot scale

Experimental and simulation study of multichannel air gap membrane distillation process with two types of solar collectors

Membrane distillation (MD) is an evolving thermal separation technique most frequently aimed at water desalination, compatible with low-grade heat sources such as waste heat from thermal engines, solar collectors, and high-concentration photovoltaic panels. This study presents a comprehensive theoretical-experimental evaluation of three commercial membranes of different materials (PE, PVDF, and PTFE), tested for two distinct MD modules-a Direct Contact Membrane Distillation (DCMD) module and an Air Gap Membrane Distillation (AGMD) module-analyzing the impact of key operational parameters on the performance of the individual membranes in each configuration. The results showed that increasing the feed saline concentration from 7 g/L to 70 g/L led to distillate flux reductions of 12.2% in the DCMD module and 42.9% in the AGMD one, averaged over the whole set of experiments. The increase in feed temperature from 65 °C to 85 °C resulted in distillate fluxes up to 2.36 times higher in the DCMD module and 2.70 times higher in the AGMD one. The PE-made membrane demonstrated the highest distillate fluxes, while the PVDF and PTFE membranes exhibited superior performance under high-salinity conditions in the AGMD module. Membranes with high contact angles, such as PTFE with 143.4°, performed better under high salinity conditions. Variations in operational parameters, such as flow rate and temperature, markedly affect the temperature and concentration polarization effects. The analyses underscored the necessity of a careful selection of membrane type for each distillation configuration by the specific characteristics of the process and its operational conditions. In addition to experimental findings, the proposed heat and mass transfer-reduced model showed good agreement with experimental data, with deviations within ±15%, effectively capturing the influence of operational parameters. Theoretical predictions showed good agreement with experimental data, confirming the model's validity, which can be applied to optimization methodologies to improve the membrane distillation process.

Membrane distillation provides a feasible and optimal solution to potable water issues. The literature contains a number of studies and research studies that aim to understand the behavior of membrane distillation systems and to provide the best possible solutions under different conditions. The purpose of this article is to discuss the air gap membrane distillation (AGMD) specifically and its development to date. The areas for future research in the field of AGMD are suggested. Membranes used in AGMD were discussed, including nanocomposite membranes and graphene membranes. In addition, the long-term performance issues regarding membrane fouling and scaling and the ways to prevent and to reduce them were discussed. Performance parameters that have not been explored sufficiently, such as energy efficiency and performance ratio, are discussed. Evolution of new membrane distillation processes from AGMD, such as the material gap and permeate gap, and conductive gap membrane distillation, is discussed. A generalized theoretical model for heat and mass transfer is presented for air gap membrane distillation systems. Coupling AGMD to form a hybrid combination with renewable energy sources is considered as a good answer to energy specific issues. Hybrid renewable energy systems with AGMD are discussed in detail. Novel designs for coupling AGMD systems with different forms of renewable energies are suggested, which presents an excellent area to be considered for developing advanced hybrid AGMD systems. It is suggested that future research should include economic studies, long-run system performance, operational problems and maintainance requirements, and related issues for better understanding and better acceptance of AGMD systems for industrialization.

Membrane distillation (MD) has a great deal of potential and this is currently being explored by the scientific community. However, this technology has not yet been implemented by industry, and an estimation of final product costs is key to its commercial success. In this study a techno-economic assessment of air gap MD (AGMD) and water gap MD (WGMD) for seawater desalination under different capacities and heat source scenarios was developed. The simplified cost of water (SCOW) method, which estimates investment costs, fixed and variable costs, as well as amortization factors and price influence over time was applied. In addition, experimental data from a laboratory-scale MD desalination plant were also used. The results showed water costs in the range of 1.56 to 7.53 €/m3 for WGMD and 2.38 to 9.60 €/m3 for AGMD. Specifically, the most feasible scenario was obtained for WGMD with a capacity of 1000 m3 daily using waste and solar heat. Finally, the costs obtained for MD were similar to those of conventional desalination technologies at the same scale factor. Therefore, although large-scale pilot studies and optimization of manufacturing processes are needed, MD shows very promising results that should be considered further.

As the population grows, one issue that is continually being addressed is the lack of clean water resources. In order to explore viable solutions, rapid experimentation and research has been underway to alleviate the water crisis. With the addition of new emerging technology, the development, improvement, and understanding of various techniques used to treat non-potable water has expanded. One subcategory of water filtration in particular that has seen rapid growth is Membrane Distillation (MD). MD is a filtration process that utilizes thermal energy to desalinate and decontaminate water. Compared to current industry leading techniques such as reverse osmosis, MD does not require such large operating pressures, leading to less power consumption. MD is accomplished primarily by flowing contaminated feed water at elevated temperatures across semi-permeable membranes. The membranes used are made to allow water vapors to penetrate through and separate from the contaminated liquid portion. By maintaining a temperature difference across the membrane, a pressure gradient is created, which drives the vapor of feed water through the pores in the membrane. Once the vapor passes through the membrane, it condenses through various methods and is collected. Air Gap Membrane Distillation (AGMD) has shown significant ability to desalinate water effectively in small scales. The air gap between the membrane and condensation plate minimizes heat loss through conduction, making AGMD a more attractive option for upscaling. In this project a laboratory-scale test cell was developed to test AGMD using different membranes, and operational parameters. In order to test such parameters, a unique design with baffled channels to induce turbulence was designed and manufactured. Feed water and coolant temperature differences, flow rates, membrane porosity, and air gap thickness are among the parameters that has been studied in this research. Temperatures of the hot feed were varied from 40°C to 80°C while the cold feed temperature was kept at a near constant temperature of 0°C. Flow rates of feed water and coolant water range from 1 to 3 L/Min. It was observed that the permeate flux is an increasing function of feed water temperature and membrane porosity. The air gap thickness plays a major role in permeate flux and energy consumption of the system.

This paper presents a comparative performance study of single-stage desalination processes with major configurations of membrane distillation (MD) modules. MD modules covered in this study are (a) direct contact MD (DCMD), (b) vacuum MD (VMD), (c) sweeping gas MD (SGMD), and (d) air gap MD (AGMD). MD-based desalination processes are simulated with rigorous theoretical MD models supported by molecular thermodynamic property models for the accurate calculation of performance metrics. The performance metrics considered in MD systems are permeate flux and energy efficiency, i.e., gained output ratio (GOR). A general criterion is established to determine the critical length of these four MDs (at fixed width) for the feasible operation of desalination in a wide range of feed salinities. The length of DCMD and VMD is restricted by the feed salinity and permeate flux, respectively, while relatively large AGMD and SGMD are allowed. The sensitivity of GOR flux with respect to permeate conditions is investigated for different MD configurations. AGMD outperforms other configurations in terms of energy efficiency, while VMD reveals the highest permeate production. With larger MD modules, utilization of thermal energy supplied by the hot feed for evaporation is in the order of VMD > AGMD > SGMD > DCMD. Simulation results highlight that energy efficiency of the overall desalination process relies on the efficient recovery of spent for evaporation, suggesting potential improvement in energy efficiency for VMD-based desalination.

Seawater membrane distillation desalination for potable water provision on remote islands − A case study in Vietnam

Statistical analysis of air-gap membrane desalination experimental data: Hypothesis testing

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.

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.

Membrane distillation (MD) is a thermally driven separation process. There are four types of MD: direct contact membrane distillation (DCMD), vacuum membrane distillation (VMD), air gap membrane distillation (AGMD) and sweep gas membrane distillation (SGMD). MD process has a number of potential advantages, namely, low operating temperature and hydraulic pressure, very high rejection of nonvolatile solutes, smaller footprint and potentially high water vapor flux for example in DCMD compared to conventional thermal distillation processes. For such reasons, MD has been considered as an emerging new technology in desalination and wastewater treatment. This chapter addresses a variety of applications of MD employing primarily the techniques of DCMD, VMD, and AGMD. State-of-the-art research results in different areas such as, desalination of seawater and brackish water, produced water treatment from oil exploration and coal seam gas production, high temperature DCMD, water treatment in bioreactors and oily wastewaters, treatment of processing streams from dairy, food, beverage industries and animal husbandry, concentration of acids, membrane distillation in biorefineries, mineral recovery and radioactive water treatment, are briefly presented and discussed in this chapter.

Membrane distillation (MD) is a hopeful desalination technique for brine (salty) water. In this research, Direct Contact Membrane Distillation (DCMD) and Air Gap Membrane Distillation (AGMD) will be used. The sample used is from Shat Al –Arab water (TDS=2430 mg/l). A polyvinylidene fluoride (PVDF) flat sheet membrane was used as a flat sheet form with a plate and frame cell. Several parameters were studied, such as; operation time, feed temperature, permeate temperature, feed flow rate. The results showed that with time, the flux decreases because of the accumulated fouling and scaling on the membrane surface. Feed temperature and feed flow rate had a positive effect on the permeate flux, while permeate temperature had a reverse effect on permeate flux. It is noticeable that the flux in DCMD is greater than AGMD, at the same conditions. The flux in DCMD is 10.95LMH, and that in AGMD is 7.14 LMH. In AGMD, the air gap layer made a high resistance. Here the temperature transport reduces in the permeate side of AGMD due to the air gap resistance. The heat needed for AGMD is lower than DCMD, this leads to low permeate flux because the temperature difference between the two sides is very small, so the driving force (vapor pressure) is low. 

Temporal performance indicators for an integrated pilot-scale membrane distillation-concentrated solar power/photovoltaic system