55225611
Flux Prediction in Direct Contact Membrane Distillation
Membrane distillation (MD) is a potential mean of water desalination. MD is a thermally driven desalination technology that has been employed in four basic configurations. One of these configuration is Direct Contact Membrane Distillation (DCMD). In DCMD, both hot and cold solution is maintained in direct contact with micro porous hydrophobic membrane material. Heat and mass transfer analysis was performed on DCMD. Based on Kinetic theory of gas, the performance of different models of membrane permeability (coefficient) was investigated under different DCMD operating parameters (feed temperature, coolant temperature and feed flow rate). Knudsen number provides the guideline in identifying the type of model of mass transfer to be considered under any given experimental conditions. Results revealed that for a given pore size under the same simulation and experimental conditions, Transition (Knudsen- Molecular diffusion) type of flow model predictions is in good agreement with the experimental results. Hence the best model to be consider for flux prediction in DCMD. The effect of membrane pore size was also studied. Results showed that permeate flux increases with increase in pore size up to the critical pore condition where the flux prediction remain constant (unchanged).
97585678
Membrane distillation - a theoretical study of evaporation through microporous membranes
Abstract Membrane distillation is a process in which, for example, water in a heated salt solution, evaporates through a porous membrane. The vapour condenses on a coolant surface on the other side of the membrane. The two liquid streams, the salt solution and the condensate, are separated by a highly porous hydrophobic membrane. Surface tension forces withhold liquids from the pores and prevent contact between the two liquids. The temperature difference, causing a corresponding vapour pressure difference across the membrane, provides the driving force of the membrane distillation process. Evaporation will occur at the solution surface if the vapour pressure on the solution side is greater than the vapour pressure at the condensate surface. Vapours then diffuse through the pores to the cooler surface, where they condense. The dependence of mass and heat transport upon different process and membrane parameters involved in membrane distillation has been investigated theoretically.
107392157
Membrane distillation : principles and applications
Modern membrane engineering is critical to the development of process-intensification strategies and to the stimulation of industrial growth. "Membrane Distillation (MD)" is a broad reference that covers specific information on membranes available and methods for MD membrane preparation and characterization. The book offers an introduction to the terminology and fundamental concepts as well as a historical review of MD development. Commercial membranes used in MD as well as laboratory-made membranes, including emerging membranes, are described in detail and illustrated by a number of clear and instructive schematic drawings and images. This is a comprehensive review on the development of MD membranes, MD modules, MD membrane characterization, MD configurations, applications in different areas and theoretical models. It offers an introduction to the terminology and fundamental concepts associated with MD as well as an historical review of MD development. It provides a description of commercial membranes used in MD as well as laboratory-made membranes, including emerging membranes.
13885422
Membrane-distillation desalination: Status and potential
Abstract 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.
96577334
Microporous polypropylene and polyethylene hollow fiber membranes. Part 3. Experimental studies on membrane distillation for desalination
Three polypropylene (PP) and one polyethylene (PE) microporous hollow-fiber membranes were used in direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) for the desalination of simulated seawater. The influence of feed temperature and feed flow on distillate pure water flux was investigated. The comparison of the PP and PE membranes in DCMD and VMD was carried out. It was found that the water flux increased with the feed temperature and feed flow in both DCMD and VMD. The data also showed that, compared with the PP membranes, higher water flux could be obtained by using PE membranes in both the DCMD and VMD processes.
101028899
Multiscale Modeling of Membrane Distillation: Some Theoretical Considerations
First, it is shown that the effective thickness of the membrane is the sum of the actual thickness, k0/UL, and λC/(UL) where k0 is the thermal conductivity of the membrane matrix, λ is the latent of vaporization of water, C is a parameter (defined as flux per unit thickness of membrane per unit of temperature driving force), and UL is a coefficient combining the heat-transfer coefficients on the feed side and the permeate side film. For typical conditions, the sum of the additional terms exceeds 100 μm, which clearly shows that the flux is not inversely proportional to membrane thickness. Also, to a first approximation, the thermal efficiency is independent of membrane thickness. This work and the development of an overall mass-transfer coefficient for direct contact membrane distillation build upon the pioneering work of Giulio Sarti. Second, a reassessment of the traditional method for combining the Knudsen diffusion coefficient and the molecular diffusion coefficient suggests that the traditional sum o...
14026609
Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement
Abstract New membrane distillation configurations and a new membrane module were investigated to improve water desalination. The performances of three hydrophobic microporous membranes were evaluated under vacuum enhanced direct contact membrane distillation (DCMD) with a turbulent flow regime and with a feed water temperature of only 40 °C. The new configurations provide reduced temperature polarization effects due to better mixing and increased mass transport of water due to higher permeability through the membrane and due to a total pressure gradient across the membrane. Comparison with previously reported results in the literature reveals that mass transport of water vapors is substantially improved with the new approach. The performance of the new configuration was investigated with both NaCl and synthetic sea salt feed solutions. Salt rejection was greater than 99.9% in almost all cases. Salt concentrations in the feed stream had only a minor effect on water flux. The economic aspects of the enhanced DCMD process are briefly discussed and comparisons are made with the reverse osmosis (RO) process for desalination.
140171275
Advances in Membrane Distillation for Water Desalination and Purification Applications
Membrane distillation is a process that utilizes differences in vapor pressure to permeate water through a macro-porous membrane and reject other non-volatile constituents present in the influent water. This review considers the fundamental heat and mass transfer processes in membrane distillation, recent advances in membrane technology, module configurations, and the applications and economics of membrane distillation, and identifies areas that may lead to technological improvements in membrane distillation as well as the application characteristics required for commercial deployment.
97698271
Heat and mass transfer in membrane distillation
Abstract Equations for heat and mass transfer in membrane distillation (MD) have been developed and tested experimentally. The concept of temperature polarisation is introduced and shown to be important in the interpretation of experimental results. Vapour transport through the membranes tested is reasonably described by combined Knudsen and molecular diffusion. The significance of temperature polarisation in the design and operation of large-scale MD modules is discussed, and hollow fibre and tubular systems shown to be potentially the most effective.
53362941
Identification of material and physical features of membrane distillation membranes for high performance desalination
In this paper, the performances 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.
95094723
Study of Asymmetric Polarization in Direct Contact Membrane Distillation
Abstract The objective of this study was to analyze the polarization phenomena in each side of a microporous hydrophobic membrane using a direct contact membrane distillation process. Experiments were conducted with distilled water and sodium chloride aqueous solutions as feeds. Different flow rates and temperatures at both membrane sides were employed. Thefeed and permeate temperature polarization coefficients as well as thefeed and permeate vapor pressure polarization coefficients were defined and evaluated. Two methods: a velocity extrapolation method and a semiempirical method that considers the heat and mass transfer empirical correlations, were used. It was proved that there is an asymmetric polarization in direct contact membrane distillation.
96101722
Factors affecting flux in membrane distillation
Abstract This paper summarises a model of the Direct Contact Membrane Distillation (MD) process. The factors affecting flux are shown to be film heat transfer, membrane ‘thermal’ permeability, the partial pressure of air within the pores and the presence of solute in the feed. Solute effects mainly influence vapour pressure and film transfer coefficients. Deaeration significantly increases gas phase permeability, but the effect on flux is modest due to increased temperature polarisation. MD of salt and sucrose solutions at concentrations up to 25 and 30% respectively give fluxes of 60 to 70% of that for a pure water feed. The flux reduction for salt is largely due to vapour pressure reduction, and for sucrose is due to increased viscosity.
98143734
Gas and vapour transport through microporous membranes. II. Membrane distillation
Abstract Experimental membrane distillation (MD) fluxes were increased by up to 50% by deaeration. A new theoretical model shows that under these conditions, membrane permeability increased by around seven-fold, but temperature polarisation decreased the thermal driving force by five-fold. Greater flux increases are predicted using higher film heat transfer coefficients. Heat loss by conduction across the membrane was shown to decrease with deaeration. Results indicate that the membrane is rarely the controlling resistance and that future developments should be towards improved MD modules rather than specialised MD membranes.
98623351
Membrane distillation: A comprehensive review
article i nfo Membrane Distillation (MD) is a thermally-driven separation process, in which only vapour molecules trans- fer through a microporous hydrophobic membrane. The driving force in the MD process is the vapour pres- sure 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 pre- sented and discussed.
5382602
Modelling mass transport through a porous partition: Effect of pore size distribution
Abstract Direct contact membrane distillation process has been studied using microporous polytetrafluoroethylene and polyvinylidene fluoride membranes. The membranes were characterized in terms of their non-wettability, pore size distribution and porosity. The mean pore sizes and pore size distributions were obtained by means of wet/dry flow method. The mean pore size and the effective porosity of the membranes were also determined from the gas permeation test. A theoretical model that considers the pore size distribution together with the gas transport mechanisms through the membrane pores was developed for this process. The contribution of each mass transport mechanism was analyzed. It was found that both membranes have pore size distributions in the Knudsen region and in the transition between Knudsen and ordinary diffusion region. The transition region was the major contribution to mass transport. The predicted water vapor permeability of the membranes were compared with the experimental ones. The effect of considering pore size distribution instead of mean pore size to predict the water vapor permeability of the membranes was investigated.
97506196
Effect of pore size distribution and air flux on mass transport in direct contact membrane distillation
Abstract The concept of mass transfer regions within the membranes was introduced to study the mass transport in membrane distillation processes. Mass transfer model for direct contact membrane distillation (DCMD) was derived to examine the influence of pore size distribution and air fluxes on water vapor fluxes across the membranes. The pore size distributions of the membranes were determined by field emission scanning electron microscopy (FESEM) and the image analysis program. DCMD experiments with pure water were carried out under laminar and turbulent flow conditions so as to compare the experimental results with the predictions. The calculation results showed that Knudsen and transition regions were found in the membranes studied, while the transition region was the major contribution to mass transport. The model including the effect of pore size distribution and air fluxes predicted water fluxes with the average discrepancy 5% of the experimental results. The mass transfer analysis indicated that the influence of pore size distribution and air fluxes on water fluxes was insignificant. Therefore, the mass transfer model with the assumptions of air trapped in membrane pores and single pore size is adequate to describe mass transport in DCMD. The concept of mass transfer regions was also applied to analyze the effect of pore size distribution on flux in vacuum membrane distillation and gas permeation.
96671859
Direct contact membrane distillation mechanism for high concentration NaCl solutions
Abstract Using high concentration NaCl aqueous solutions, direct contact membrane distillation (DCMD) experiments are implemented in this work where water fluxes are measured at different feed/permeation temperatures, feed concentrations and flow rates. The results under the experimental conditions show that (1) water fluxes decrease as concentrations of NaCl solutions increase; (2) when concentrations of NaCl solutions are above 25%, water fluxes decrease sharply; (3) when concentrations of NaCl solutions are up to saturation, water fluxes gradually approach steady levels. Variations of membrane-fouling resistance are found to result in the above observations. Analysis also shows that concentrations of NaCl solutions, operation temperatures and flow rates can affect the speed of membrane fouling. Among them, the concentration is the most important factor. In this work, a new model is also proposed which takes membrane resistance, concentration polarization resistance and membrane fouling resistance all into consideration. Model predictions show good agreement with the experimental results.
98777750
Mass transfer mechanisms and transport resistances in direct contact membrane distillation process
Abstract The objective of the present work was to investigate the mass transport and fouling mechanism of direct contact membrane distillation (DCMD) process. The experiments were performed on a flat sheet module using pure water and humic acid solution as feeds. The membrane employed was hydrophobic PVDF of pore size 0.22 μm. The mass transfer models based on Dusty gas model were applied to fit the flux data and the pressure blocking filtration laws were employed to explain the membrane fouling. The results showed that molecular diffusion was the most suitable model for predicting fluxes of both laminar and turbulent flow. Fouling of the membrane by humic acid aggregates can be described by cake filtration model. The transport resistance of the feed boundary layer was higher than other resistances, and fouling resistance increased significantly with time.
119735792
An experimentally optimized model for heat and mass transfer in direct contact membrane distillation
Membrane distillation (MD), a thermally driven process involving hydrophobic micro-porous membranes has gained widespread interest in academic research and is set to become an alternative solution to other membrane separation processes such as reverse osmosis (RO). Although extensive experimental studies have been carried out since the 1980s [1,2], clear understanding of the heat and mass transport phenomena has yet to be established. This manuscript presents experimental results of direct contact membrane distillation (DCMD) with de-ionized water and aqueous salt solutions of NaCl with concentration levels of up to 15 ppt as feed together with an experimentally optimized and validated model for the prediction of the permeate flux in DCMD for GE Aspire Membrane QL 833 (GE Energy). Different heat transfer prediction methods in combination with the three different forms of the Dusty Gas model for mass transport were used in the comparison of our experimental data in the laminar and turbulent flow regimes under steady-state conditions. The comparison between experimental and predicted results confirmed our expectation that the Knudsen-molecular diffusion transition model yielded the best prediction. We have also identified, based on the comparison of the data, the most accurate heat transfer correlation for the laminar and turbulent flow regimes, taking into account the experimental and permeate prediction uncertainties to optimally address the heat and mass transport equations used in DCMD studies. Hence, it is highly recommended that these heat transfer correlations and the Knudsen-molecular mass transport equation be used in the prediction of heat and mass transfer for flat sheet DCMD experiments.
95161087
Temperature and concentration polarization in membrane distillation of aqueous salt solutions
Abstract In this paper we have studied water transport in membrane distillation using a flat PTFE membrane. Experiments have been carried out with water and aqueous solutions of NaCl as feed. The effects of temperature and concentration polarization on the reduction of vapour pressure differences across the membrane with regard to the vapour pressure differences corresponding to the bulk phases which are separated by the membrane, are evaluated. A coefficient which measures this reduction has been introduced. This coefficient and the temperature polarization coefficient coincide when water is used as feed, but they are more and more different when the salt concentration of feed increases. The measured flux results and the calculated polarization results are discussed for different temperatures, recirculation rates and solution concentrations.
94991309
Transport analysis of an Air Gap Membrane Distillation (AGMD) process
Abstract Membrane distillation (MD) desalination is an emerging technology for fresh water production. This process incorporates phase change and transport of vapour through a hydrophobic membrane caused by vapour pressure across the membrane. The results from experimental studies and the one-dimensional transport analyses of the heat and mass transfer processes on an air gap MD (AGMD) unit are presented in this paper. The effects of different operating variables including feed and coolant temperatures, air gap, membrane support mesh size, feed concentration and feed and coolant flow rates were investigated. Mass transport through membrane, membrane support, air gap and condensation on the coolant plate has been analysed and expression for global mass transfer coefficient has been derived. The maximum distillate flux achieved was 5.11 kg m−2 h−1 at a feed temperature of 60°C, coolant temperature of 10°C and an air gap of 2.5 mm. Feed temperature and air gap width were found to have significant influence o...