Analysis of mass transfer behavior in membrane distillation: Mathematical modeling under various conditions (original) (raw)
Related papers
Journal of Membrane Science, 2003
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.
Desalination
In order to improve water production of membrane distillation (MD), the development of high performance membrane having better mass transfer and enhancement of convection heat transfer in MD module have been continuously investigated. This paper presents the relationship between the heat and mass transfer resistance across the membrane and the performance improvement. Various ranges of mass transfer coefficient (MTC) from normal (0.3×10-6 to 2.1×10-6 kg/m 2 sPa: currently available membranes) to high (>2.1×10-6 kg/m 2 sPa: membranes under development) were simulated using an experimentally validated model at different ranges of convection heat transfer by varying the inlet flow rates and spacer enhancement factor. The effect of mass transfer and convection heat transfer on the MD performance parameters including temperature polarization coefficient (TPC), mean permeate flux, and specific energy consumption were investigated in a direct contact MD (DCMD) configuration. Results showed that improving the MTC at the low ranges is more important than that at the high ranges where the heat transfer resistance becomes dominant and hence the convection heat transfer coefficient must be increased. Therefore, an effort on designing MD modules using feed and permeate spacers and controlling the membrane surface roughness to increase the convection heat transfer and TPC in the channel aiming to enhance the flux is required because the currently developed mass transfer has almost reached the critical point.
Separation and Purification Technology, 2017
The application of vacuum to direct contact membrane distillation (vacuum enhanced direct contact membrane distillation, V-DCMD) removed condensable gasses and reduced partial pressure in the membrane pores, achieving 37.6% higher flux than DCMD at the same feed temperature. Transfer mechanism and temperature distribution profile in V-DCMD were studied. The empirical flux decline (EFD) model represented fouling profiles of V-DCMD. In a continuous V-DCMD operation with moderate temperature (55 ºC) and permeate pressure (300 mbar) for treating wastewater ROC, a flux of 16.0±0.3 L/m 2 h and high quality distillate were achieved with water flushing, showing the suitability of V-DCMD for ROC treatment. A Thermal conductivity (kW/m•K) P Poiseuille flow diffusion L Membrane module length (m) K Knudsen diffusion ρ Fluid density (kg/m 3) m Membrane surface Cp Specific heat capacity of fluid x Transversal (x-direction) λ Latent heat of water z Axial (z-direction) Rm Membrane resistance (Pa.m 2 .h/L) t Time (s)
Heat and mass transfer analysis in direct contact membrane distillation
Desalination, 2008
This work aims to provide a detailed analysis of the heat transfer in direct contact membrane distillation (DCMD). The influence of mass transfer on heat transfer flux was identified in the feed thermal boundary layer, across the membrane and through the permeate thermal boundary layer. A mathematical model was proposed to evaluate the experimental values of the thermal boundary layers' heat transfer coefficients, the membrane/liquid interface temperatures, the temperature polarization coefficient, the membrane mass transfer coefficient and the evaporation efficiency. This model was solved numerically using MATLAB ® and it is based on direct experimental observations of the feed solution inlet/outlet temperatures and the mass fluxes. The obtained results showed that the mass transfer contribution to the overall heat transferred was significant only in the membrane region while it was negligible in both feed and permeate boundary layers. The membrane conductive heat transfer coefficient was estimated using trial and error procedure simultaneously with the developed model equations solution. The obtained values of the conductive heat transfer coefficient showed an agreement with experimental values reported in literature. The effect of feed temperature variation on the boundary layers' heat transfer coefficients, the temperature polarization coefficient, the evaporation efficiency, the membrane conductive heat transfer coefficient, the membrane mass transfer coefficient and the permeate flux was shown and fairly discussed.
Heat and Mass Transport in Modeling Membrane Distillation Configurations: A Review
Frontiers in Energy Research, 2018
Identification and mitigation of challenges associated with membrane distillation (MD) modeling are very crucial to the applicability of MD technology in the industry. Several research studies have been carried out on direct contact membrane distillation (DCMD) modeling because of its simplicity, while other MD configurations have gained little attention. Most studies conducted on MD modeling were achieved based on uniform membrane pore size and pore size distribution assumption. This study exploits the homogeneity of these assumptions to conduct a modeling review for temperature polarization (TP) and concentration polarization (CP), as they apply to MD configurations. TP and CP phenomena have been identified as two of the main challenges to advance MD modeling for further development of MD technology. Their impact are detailed in the heat and mass transfer mechanisms discussed. Thermal conductivity of common hydrophobic commercial membrane materials at different temperatures are presented in this study. The use of optimal operating flow rates, suitable membranes, and proper module design are recommended as viable solutions to reduce the effect of TP and CP on permeate flux decay.
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)
Water Flux Prediction in Direct Contact Membrane Distillation Subject to Inorganic Fouling
Membranes, 2022
Freshwater is a limited resource, which has driven the development of new purification and water-reuse technologies. One promising technology for water treatment is membrane distillation (MD). One of the main problems of MD, and of many desalination technologies, is membrane fouling, which reduces the performance of the membrane. This work presents a mathematical model that aims to predict distillate fluxes in direct-contact MD when fouling occurs as salts are deposited onto the membrane surface, forming an inorganic fouling layer. The mathematical model uses a heat- and mass-transfer formulation for prediction of the distillate flux under steady state conditions, and it is combined with the cake-filtration theory to represent the distillate fluxes after the onset of membrane fouling. Model results agree well with experimental observation of distillate fluxes, both before (~12–14 kg m−2 h−1) and after the onset of membrane fouling, with root-mean-square errors smaller than 1.4 kg m−...
Chemosphere, 2018
Substantial amounts of trace hazardous elements have been detected in industrial wastewater (e.g fluoride > 900 mg/L). Feed water characteristics, operational parameters, and membrane properties are major factors affecting flux and rejection of the MD process. Membrane parameters such as membrane material type and pore size have been investigated. Fluoride ion rejection was selected to setup amethodology to remove trace elements from wastewater by adjusting the membrane parameters in DCMD. Study of the fouling thickness of the MD membrane using pH and feed water composition revealed that a PVDF membrane with a smooth surface holds a thicker fouling layer, which enhances fluoride rejection while reducing the permeate flux. On the other hand, PTFE and PP membranes showed higher mass transfer and higher wetting performance, respectively. Therefore,a PVDF membrane with low organic feed water at higher alkaline pH can be utilized to obtain high-quality permeate, while PTFE can provide the highest flux with acceptable permeate water quality. Therefore, this methodology can be applied toidentify the optimum membrane to fit the required
Heat transport and membrane distillation coefficients in direct contact membrane distillation
Journal of Membrane Science, 2003
This work aims to provide detailed understanding of heat transport in direct contact membrane distillation (DCMD). The influence of mass transfer on heat transfer rates and on the heat transfer coefficient was identified, and the relative significance of each heat transfer mechanism was evaluated. The role of spacers in heat transfer improvement was analyzed. Alternative methods to evaluate the membrane thermal conductivity were also proposed. The heat transfer analysis of the experimental results showed that the effects of mass transfer on the heat transfer rates and on the film heat transfer coefficients were negligible. The heat transfer due to the vapor flow (q v) in the membrane was equal to or greater than the heat conduction (q c) for the membranes studied and increased with the feed temperature. When the feed temperature was lower than 323 K, the heat loss due to heat conduction across the membrane was the major contribution of the total heat transfer in the membrane. In addition, the temperature distributions in the membranes were closely linear. The membrane distillation (MD) coefficients for each membrane were constant over the flow rates and temperatures studied. The flow pattern in the spacer-filled channel was probably transition flow rather than turbulent flow. The alternative models for calculating the membrane thermal conductivity showed better agreement than the commonly used model.
Thermodynamic design and fouling of membrane distillation systems
arXiv: Applied Physics, 2015
As water shortages intensify globally under the stresses of increasing demand, aquifer depletion, and climate change, the market for efficient desalination technologies has grown rapidly to fill the void. One such developing technology, membrane distillation (MD), has experienced keen academic interest and an increase in start-up businesses in the past decade. MD has expanded into a niche of small scale thermal desalination using solar and waste heat resources, due to its fouling resistance, scalability, and acceptable efficiency. Recent studies indicate that MD could attain the efficiencies of state-of-the-art mature thermal desalination technologies, although additional engineering and scientific challenges must first be overcome. The aim of this research is to better understand and provide solutions for two major challenge areas for MD: efficiency and membrane fouling. Studies on improving MD efficiency included examining the effects of tilt angle on MD performance using numerica...