Heat and mass transfer analysis in direct contact membrane distillation (original) (raw)
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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.
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.
Desalination and Water Treatment, 2021
This research aimed to examine the effect of the thermal conductivity model of hydrophobic membranes on performance modelling of direct contact membrane distillation systems. The parallel, series, and two types of Maxwell models were studied. Simultaneously, an iterative numerical model was developed to choose the most appropriate model by analysing the mass flux (Jw) and the heat transfer rate ( QP). Comparison with the experimental results, showing that Maxwell Type II was the most appropriate for modelling the thermal conductivity of the membrane. Also, based on the chosen model of membrane thermal conductivity, the direct contact membrane distillation performance (the global heat transfer coefficient, temperature polarization coefficient, energy efficiency, and gain output ratio) was studied. It was found that the membrane thermal conductivity model with a higher value of membrane thermal conductivity (km) resulted in an underestimation of the predicted mass flux, temperature ...
Journal of Membrane Science, 2012
A comprehensive analysis on the dominant effects for heat and mass transfer in the direct contact membrane distillation (DCMD) process has been performed with the aid of computational fluid dynamics (CFD) simulations for hollow fiber modules without and with annular baffles attached to the shell wall. Potential enhancement strategies under different circumstances have been investigated. Numerical simulations were carried out to investigate the effect of the MD intrinsic mass-transfer coefficient of the membrane (C) on the performance enhancement for both non-baffled and baffled modules. It was found that the temperature polarization coefficient (TPC) decreases significantly with increasing C value regardless of the existence of baffles, signifying a loss of overall driving force. However, the higher C compensated for this and the mass flux showed an increasing trend. A membrane with a lower C value was found to be less vulnerable to the TP effect. In this case, the introduction of turbulence aids such as baffles did not show substantial effect to improve system performance. In contrast, introducing baffles into the module can greatly enhance the mass flux and the TPC for a membrane with a high C value, where the main heat-transfer resistance is determined by the fluid side boundary layers. The effect of operating temperature on heat and mass transfer in the MD process was also studied with a membrane of a lower C value (2.0×10-7 kg•m-2 •s-1 •Pa-1). Although the TPC generally decreased with increasing operating temperatures, the mass flux N m increased significantly when operating temperature increased. A baffled module showed a more significant improvement than a non-baffle module at a higher temperature. Moreover, it was confirmed that higher operating temperatures are preferable for a substantial improvement in the heat/mass transfer as well as MD thermal efficiency, even with a relatively small transmembrane temperature difference of 10K.
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.
Energies, 2020
In this numerical study, a direct contact membrane distillation (DCMD) system has been modeled considering various angles for the membrane unit and the Reynolds number range of 500 to 2000. A two-dimensional model developed based on the Navier–Stokes, energy, and species transport equations were used. The governing equations were solved using the finite volume method (FVM). The results showed that with an increase in the Reynolds number of up to 1500, the heat transfer coefficient for all membrane angles increases, except for the inclination angle of 60°. Also, an increase in the membrane angle up to 90° causes the exit influence to diminish and the heat transfer to be augmented. Such findings revealed that the membrane inclination angle of 90° (referred to as the vertical membrane) with Reynolds number 2000 could potentially have the lowest temperature difference. Likewise, within the Reynolds numbers of 1000 and 2000, by changing the inclination angle of the membrane, temperature ...
Theoretical modeling and experimental analysis of direct contact membrane distillation
Journal of Membrane Science, 2009
A two-dimensional mathematical model was theoretically developed to predict the temperature polarization profile of direct contact membrane distillation (DCMD) processes. A concurrent flat-plate device was designed to verify the theoretical prediction of pure water productivity on saline water desalination. The numerical results from the temperature polarization profile were obtained using the finite difference technique to reduce the two-dimensional partial differential equations into an ordinary differential equations system. The resultant simultaneous linear equations system was solved with the fourth-order Runge-Kutta method. The results show theoretical prediction agreement with the measured values from the experimental runs. A combination of the Knudsen flow and Poiseuille flow models in the present mathematical formulation for membrane coefficient estimation was used to establish theoretical agreement. The influence of the inlet saline water temperature and volumetric flow rate on the pure water productivity as well as the hydraulic dissipated energy are also delineated.
Numerical study of the coupled heat and mass transfer in membrane distillation
2002
In order to deepen the understanding of the permeate characteristics of membrane distillation considering heat and mass transfers and concentration polarisation, a fully predictive mathematical model was developed. The first part that is devoted to the natural heat and mass transfer in the rectangular cavity, states the problem in Cartesian coordinates system, involves the use of a control-volume method and solves the full vorticity transport equation together with the stream function, mass and energy equations. The temperature fields and streamlines are plotted for various geometrical conditions to show some of the flow field characteristics. The second part was set up to simulate the phenomenon present in the membrane distillation process. The mass transfer was described with combined Knudsen and Maxwell-Stefan equations and coupled to the simultaneous heat transfer. The effect of parameters such as thickness of cavity, temperature and feed flow rate were investigated. The results were compared with the available data and the agreement is satisfactory.
Specific energy requirement of direct contact membrane distillation
Chemical Engineering Research and Design, 2017
The study aims to provide a clear picture of the thermal energy requirements of Direct Contact Membrane Distillation (DCMD) system as function of different variables influencing the specific energy consumption. This includes membrane properties, operating conditions, recovery factor and the option of heat recovery from the permeate and retentate streams. We simultaneously analyze the variation in specific energy demand and membrane surface area needed as a function of the membrane characteristics, operating conditions and recovery rate, taken as a design parameter. We observe that the specific energy demand of DCMD shows a relatively weak dependence on temperature polarization and membrane properties considered in the current study and a strong dependence on the recovery rate. The advantages of using a heat exchanger very much depends on the recovery rate of the process.