The effects of iCVD film thickness and conformality on the permeability and wetting of MD membranes

Membranes possessing high permeability to water vapor and high liquid entry pressure (LEP) are necessary for efficient membrane distillation (MD) desalination. A common technique to prepare specialized MD membranes consists of coating a hydrophilic or hydrophobic base membrane with a low surface-energy material. This increases its liquid entry pressure, making the membrane suitable for MD. However, in addition to increasing LEP, the surface-coating may also decrease permeability of the membrane by reducing its average pore size. In this study, we quantify the effects of initiated chemical vapor deposition (iCVD) polymer coatings on membrane permeability and LEP. We consider whether the iCVD films should have minimized thickness or maximized non-conformality, in order to maximize the permeability achieved for a given value of LEP. We determined theoretically that permeability of a single pore is maximized with a highly non-conformal iCVD coating. However, the overall permeability of a membrane consisting of many pores is maximized when iCVD film thickness is minimized. We applied the findings experimentally, preparing an iCVD-treated track-etched polycarbonate (PCTE) membrane and testing it in a permeate gap membrane distillation (PCMD) system. This study focuses on membranes with clearly defined, cylindrical pores. However, we believe that the principles we discuss will extend to membranes with more complex pore architectures. Overall, this work indicates that the focus of surface-coating development should be on minimizing film thickness, not on increasing their non-conformality.MIT & Masdar Institute Cooperative Program (02/MI/MI/CP/11/07633/GEN/G/00)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (W911NF-13-d-0001

Understanding wetting phenomena in membrane distillation and how operational parameters can affect it

Direct contact membrane distillation experiments were carried out under this work to study the influence of operational variables on membrane wetting. In the first part of this work, experiments were designed according to a Box-Behnken methodology and results were analyzed statistically using Pearson correlation coefficients, principal component/factor analysis and cluster analysis. The independent operational parameters were the temperatures of both the hot and cold streams (Tf, Tc) and their flow rates (Ff, Fc). The analyzed responses were the time and rate of wetting along with distillate flux. Statistical analysis showed strong evidence of a relationship between the selected variables and the wetting patterns. In general, parameters enhancing flux production led to suppression of wetting (both delayed wetting and reduced wetting rate). The second part of the work focused on reversing the wetting with minimal operation disruption by varying the operational parameters. The data generated helped in understanding the salt passage and wetting mechanisms. The wetting hypothesis developed herein is based on water bridging as a consequence of the weak hydrophobicity of the PVDF membrane and a net absolute transmembrane pressure. Data were analyzed through the Peclet number, the Poiseuille flow and a mass balance in order to understand the interplay between diffusion and convection/advection. High transmembrane temperature (¿T) (¿T=Tf-Tc) counteracts the build-up of a net absolute transmembrane pressure and reduces the viscous liquid flux. In this case, the diffusion of salt through the stagnant water layer in the membrane pores (a much slower mechanism) becomes more important and the wetting rate can be reduced and further reversed.Peer ReviewedPostprint (author's final draft

Scaling and fouling in membrane distillation for desalination applications: A review

Membrane distillation (MD) has become an area of rapidly increasing research and development since the 1990s, providing a potentially cost effective thermally-driven desalination technology when paired with waste heat, solar thermal or geothermal heat sources. One principal challenge for MD is scaling and fouling contamination of the membrane, which has gained growing attention in the literature recently as well. The present paper surveys the published literature on MD membrane fouling. The goal of this work is to synthesize the key fouling conditions, fouling types, harmful effects, and mitigation techniques to provide a basis for future technology development. The investigation includes physical, thermal and flow conditions that affect fouling, types of fouling, mechanisms of fouling, fouling differences by sources of water, system design, effects of operating parameters, prevention, cleaning, membrane damage, and future trends. Finally, numerical modeling of the heat and mass transfer processes has been used to calculate the saturation index at the MD membrane interface and is used to better understand and explain some of trends reported in literature.Masdar Institute of Science and Technology (Massachusetts Institute of Technology Cooperative Agreement 02/MI/MI/CP/11/07633/GEN/G/00

Ammonium recovery and concentration from synthetic wastewater using a poly(4-methyl-1-pentene) (PMP) liquid–liquid membrane contactor: Flux performance and mass transport characterization

Hollow fiber membrane contactors are a promising technology for the removal and recovery of ammonia from liquid effluents. However, a better understanding of the process engineering (e.g. mass transport of ammonia over water) and performance optimization is required. In this study, the performance of a hollow fibre liquid–liquid membrane contactor (HF-LLMC), incorporating a new polymer chemistry (i.e., poly (4-methyl-1-pentene (PMP) and an asymmetric fibre structure, for the recovery and concentration of ammonium from synthetic aqueous solutions, was investigated. The influence of the feed and acid flow rates was evaluated experimentally by determining the overall mass transfer coefficient (), the ammonium recovery as a function of time and the acid consumption. In addition, the experimental results were fitted to a mathematical model to determine the membrane permeabilities to ammonia () and water () and identify the mass transfer resistance regime. The highest values experimentally obtained were in the range of 3·10–3 to 3.51·10–3 m/h with corresponding ammonia recovery rates of 94 and 96.2% after 10 h, operating at a feed and acid flow rates of 180 L h-1 and 500 L h-1, respectively, which are in the upper range of the HF-LLMC literature. The overall results of this study were not only the upper range of the HF-LLMC ammonia recovery literature but a much-lower water transport was confirmed indirectly by the concentration factor (CF) values obtained experimentally. The remarkable selectivity of the membrane towards ammonia over water (i.e., = 87–180 L m–2h-1 bar-1 and = 1.2–1.4·10–3 L m-2 h-1 bar-1 at NTP conditions) is attributed to the asymmetrical membrane structure and the polymer chemistry (i.e., PMP). The proven high ammonia selectivity of the HF-LLMC makes it a promising technology for the recovery and concentration of ammonium from urban (e.g. wastewaters) and industrial (e.g. soda ash and fertilizers production) diluted streams.Peer ReviewedPostprint (author's final draft

Decarbonization of industrial processes: technologies, applications and perspectives of low-temperature solar heat (80-150°C)

Low-temperature (80-150°C) solar collectors guarantee a very high efficiency (up to 60%) in the conversion of solar radiation into useful thermal energy. Moreover, solar thermal technologies are already reliable solutions, relatively cheap and widely available in the market. For that reason, solar collectors operating at low temperatures are among the most important sustainable technologies that can reduce the fossil fuel consumption of industrial processes and their corresponding carbon footprint. Unfortunately, Solar Heat for Industrial Processes (SHIP) is still mostly unused for several reasons, e.g., not easy identification of the appropriate applications (e.g., cleaning processes, drying, desalination) or lack of knowledge of the potential environmental and economic benefit of the use of SHIP technologies. For that reason, this work includes i) an overview of solar technologies for low/medium -temperature SHIP (80-150°C) ii) results obtained on the innovative design of the mirrors used in evacuated receiver tube by means of a variation in the shape of its internal reflector iii) estimation of CO2 saving using a solar field based on evacuated tube collector (ETC). The work also includes a comparison of the standard ETC solar plant with an ETC solar plant embedded with reflectors with innovative shape

Scaling and fouling in membrane distillation for desalination applications: A review

Membrane distillation (MD) has become an area of rapidly increasing research and development since the 1990s, providing a potentially cost effective thermally-driven desalination technology when paired with waste heat, solar thermal or geothermal heat sources. One principal challenge for MD is scaling and fouling contamination of the membrane, which has gained growing attention in the literature recently as well. The present paper surveys the published literature on MD membrane fouling. The goal of this work is to synthesize the key fouling conditions, fouling types, harmful effects, and mitigation techniques to provide a basis for future technology development. The investigation includes physical, thermal and flow conditions that affect fouling, types of fouling, mechanisms of fouling, fouling differences by sources of water, system design, effects of operating parameters, prevention, cleaning, membrane damage, and future trends. Finally, numerical modeling of the heat and mass transfer processes has been used to calculate the saturation index at the MD membrane interface and is used to better understand and explain some of trends reported in literature.Masdar Institute of Science and Technology (Massachusetts Institute of Technology Cooperative Agreement 02/MI/MI/CP/11/07633/GEN/G/00