Influence of Pore Structure on Membrane Wettability in Membrane Distillation (original) (raw)
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Chemical Engineering Science, 2018
Membrane distillation (MD) is considered as a key technology for desalination applications. It shows indeed numerous advantages compared to reverse osmosis and other desalination processes (e.g. thermal driving force, no osmotic pressure effect on membrane fluxes). Nevertheless, this technology still presents some issues, most notably due to pore wetting effects. This study proposes the use of a thin, self-standing dense membrane in place of microporous materials as a solution to avoid wetting in MD for water desalination. In a first step, a membrane contactor model is developed based on mass and energy balances and the simulations are validated using experimental results obtained on hollow fiber modules with microporous membranes. A comparative performance analysis is then achieved between a porous and a dense membrane module. A parametric study on the influence of dense membrane thickness and water permeability shows that a two fold increase in water flux, without significant impact on energy efficiency, is potentially achievable with thin and permeable dense materials compared to microporous membranes. Guidelines for the design of highperformance dense membrane modules for MD are finally proposed.
Journal of Membrane Science, 2002
Pore size distributions have been obtained for three hydrophobic porous membranes from air-liquid displacement measurements, assuming the common model of cylindrical capillaries for the membrane and using flux equations which includes both diffusive and viscous mechanisms for transport in the gas phase in pores. The pore size distribution so obtained and the same flux equations are used in order to predict the water vapour permeability through the membranes, characterised when employed in different membrane distillation configurations and operating conditions. In membrane distillation applications, where no viscous flux exists, the role of both Knudsen and molecular diffusion resistances is analysed. In membrane distillation applications which include diffusive and viscous transport, the contribution of both mechanisms is analysed.
Wetting phenomena in membrane distillation: Mechanisms, reversal, and prevention
Water research, 2018
Membrane distillation (MD) is a rapidly emerging water treatment technology; however, membrane pore wetting is a primary barrier to widespread industrial use of MD. The primary causes of membrane wetting are exceedance of liquid entry pressure and membrane fouling. Developments in membrane design and the use of pretreatment have provided significant advancement toward wetting prevention in membrane distillation, but further progress is needed. In this study, a broad review is carried out on wetting incidence in membrane distillation processes. Based on this perspective, the study describes the wetting mechanisms, wetting causes, and wetting detection methods, as well as hydrophobicity measurements of MD membranes. This review discusses current understanding and areas for future investigation on the influence of operating conditions, MD configuration, and membrane non-wettability characteristics on wetting phenomena. Additionally, the review highlights mathematical wetting models and...
2014
Membrane distillation (MD) is a thermally driven separation process that employs a hydrophobic membrane as a barrier for the liquid phase, allowing only vapor phase to pass through the membrane pores. Wetting of membrane pores by liquid streams (i.e. the loss of hydrophobic characteristics of membranes) is a crucial issue in MD treatment. This paper is organized into two parts. The first part provides an overview of the theoretical background of wetting phenomenon and guides the reader through the experimental techniques presented in the literature for determining liquid entry pressure (LEP) of MD membranes. In the second part, we provide experimentally measured data on LEP values of some commercially available hollow-fiber and flat-sheet membranes tested in our lab using different MD configurations. The LEP w value of the MD 020 CP 2N hollow-fiber membrane (Microdyn-Nadir GmbH, Wiesbaden, Germany) made of PP is found to be 0.97 bar using direct-contact membrane distillation (DCMD) configuration. The LEP w value of the Durapore TM GVPH flat sheet membrane (Merck Millipore Inc., Billerica, USA) made of PVDF is found to be 2.37±0.025 bar using static measurement technique and 1.90 bar using vacuum MD configuration. We also show that wetted membranes can be successfully regenerated by soaking them in ethanol and removing ethanol with evaporation at elevated temperatures. A novel concept of regeneration procedures applying vacuum have developed and have been proved to be effective for the tested flat sheet modules, however, failed on recovering the hydrophobic characteristics of the PP membrane in the hollow-fiber module.
Industrial & Engineering Chemistry Research, 2007
We report here direct contact membrane distillation results from modules having 0.28 m 2 of membrane surface area employing porous hydrophobic polypropylene hollow fibers of internal diameter (330 µm) and wall thickness (150 µm) with a porous fluorosilicone coating on the outside surface. The brine salt concentration and temperature and the distillate temperature and velocity were varied. Water vapor fluxes approach values obtained earlier in much smaller modules. As the brine temperature was increased from 40 to 92°C, water vapor flux increased almost exponentially. Increasing the distillate temperature to 60 from 32°C yielded reasonable fluxes. Salt concentration increases to 10% led to a small flux reduction. An extended 5-day run did not show any pore wetting. A model using the mass transfer coefficient k m as an adjustable parameter predicted the brine temperature drop, distillate temperature rise, and water vapor flux well for the large module and the smaller module of 119-cm 2 surface area.
Journal of Membrane Science, 2010
In this paper, the performance of various membranes were assessed in direct contact membrane distillation (DCMD) under different feed velocities and inlet temperatures. The membranes studied included a polyvinylidenefluoride (PVDF) microfiltration membrane with a non-woven support layer, a polytetrafluoroethylene (PTFE) microfiltration membrane with a non-woven support layer, and three MD membranes made from PTFE of different pore size and all with a structured scrim support layer. The results showed that distillation using PTFE membranes produced much higher flux than that of the PVDF microfiltration membrane at the same operational conditions, and the support layer affected not only the flux, but also the energy efficiency (0.51-0.24). The results also show that increasing the velocity of the feed and its inlet temperature increased the flux, but the rate of flux increase diminishes at high velocities. The mass transfer coefficient improved for thinner support and active layer membranes, leading to fluxes as high as 46 L.m-2 h-1 at 80˚C. The heat transfer characteristics were found to be superior for the open scrim backed membranes compared to the non-woven support membranes, resulting in significantly greater thermal efficiency for the scrim backed membranes.
Membrane Distillation for Desalination and Current Advances in MD Membranes
Journal of Applied Membrane Science & Technology, 2023
Desalination is a great technique to address the growing demand for water because it is essential for humans. Water treatment and desalination are two common uses for the membrane-based, non-isothermal MD (Membrane Distillation) process. It works at low pressure and temperature, and heat from waste and solar energy can meet the process's heat requirements. In MD, dissolved salts and nonvolatile contaminants are rejected as the vapors go through the membrane's pores and start condensing at the permeate side. However, because to the lack of a suitable and adaptable membrane, biofouling, wetting and water efficacy are the main problems for MD. Many researchers have recently worked on membrane composites and attempted to create effective, appealing, and unique membranes for membrane distillation. This review article talks about water shortages in the 21st century, the rise of desalination, the use of membrane distillation (MD), recent developments in membrane distillations, developments in pilot scale MD technologies, New developments in membrane fabrication and modification, the desired properties of membranes, and desalination membranes.
Membrane distillation process: Fundamentals, applications, and challenges
IntechOpen eBooks, 2024
Traditional thermal-based processes such as multistage flash and multi-effect distillation have been used for thousands of years to obtain freshwater from saline water. Recently, with the development of membrane-based technology, membrane distillation (MD) as a thermally driven membrane process has received significant attention. The driving force in MD is the vapor pressure gradient induced by temperature difference through hydrophobic microporous membrane pores. The membrane used for MD should be hydrophobic and microporous. In MD, the mechanism of transport involves simultaneously heat and mass transfers, which moves from the hot feed side to the cold permeate side. The performance of MD is evaluated based on various performance metrics including permeate flux, recovery ratio, thermal efficiency, gained output ratio, and specific thermal energy consumption. It has good ability for various industrial uses due to its moderate applied temperature and pressure, high rejection rate, less membrane fouling tendency and its ability to treat high-saline water. The water production cost still remains high compared to conventional processes. Therefore, MD can be cost-effectively when integrated with solar energy, geothermal energy and waste heat. Nevertheless, MD process requires focused research to improve its efficiency to become more mature and economically competitive at large scale.
Evolution of Membrane Surface Properties for Membrane Distillation: A Mini Review
Journal of Applied Membrane Science & Technology
To date, the membrane development for membrane distillation (MD) application is growing in line with the increasing volume of various types of wastewaters discharged into environment. MD is a liquid-vapor separation process and a hydrophobic membrane is used to retain the liquid. Theoretically, the hydrophobic membrane can achieve 100% rejection of non-volatile components that dissolved in feed liquids. As a result, MD has received significant attention in water recovery from saline water as well as wastewaters. Nevertheless, in addition to the scaling problem due to salts, the hydrophobicity property of membrane becomes a concern when dealing with challenging wastewaters which contain various types of low surface tension components such as oils, grease, alcohols, organics and surfactants. The membrane pore wetting due to salts deposition fouling and low surface tension components subsequently reduces the flux and even fails the liquid-vapor separation process. This article briefly ...
Applied Thermal Engineering, 2021
A theoretical analysis of the energy efficiency of a direct contact membrane distillation (DCMD) module with external heat recovery intended for desalination applications is carried out. A porous media model is proposed and validated against previously reported numerical and experimental results. A 2 n planning scheme is employed to determine the most decisive membrane properties for maximizing the energy efficiency of the desalination system. The interaction between membrane parameters are found to be weak, allowing for the selection of manufacturing processes for emphasis in certain parameters. The porosity is shown to be a dominant factor, responsible for at least 40% of the variation of the energy efficiency metric, while the relative importance of other parameters is dependent on the heat recovery system effectiveness. An optimum membrane thickness is identified and observed to become smaller for better heat recovery systems, improving the energy efficiency of the process. The results obtained offer guidance to future membrane development efforts as to what should be emphasized to maximize the amount of distilled water produced for a given heat input. In particular, harnessing the interaction between the membranes parameters and the heat recovery system is essential to leverage the energy efficiency of the desalination system.