Modelling the simultaneous heat and mass transfer of direct contact membrane distillation in hollow fibre modules

Effect of operational parameters on distillate flux in direct contact membrane distillation (DCMD): Comparison between experimental and model predicted performance

Direct contact membrane distillation (DCMD) offers an attractive operation for the separation of mixtures at atmospheric pressure with reasonable energy requirement. A new simultaneous heat and mass transfer model in DCMD in a hollow fiber configuration is presented. Flow regime in feed and permeate side, the variations of mean temperature and concentration along the membrane module, the length of the membrane, and various properties of membrane characteristics are taken into account in the present model. A system of nonlinear equations describing the DCMD process is solved numerically for each cell using the FSOLVE coding, which is a built‐in function in MATLAB® to find the influence of the temperature and velocity of the feed and permeate streams, and the salt concentration of the feed along the module on the permeate flux. The predicted results by the new model show a good accord with a wide range of various experimental results available in the literature. © 2012 American Institute of Chemical Engineers AIChE J, 59: 589–603, 2013

The path for water molecules transported across a membrane in real porous membranes has been considered to be a constant factor in the membrane distillation (MD) process (i.e., constant tortuosity); as such, its effect on membrane performance at various operating conditions has been ignored by researchers. Therefore, a simultaneous heat and mass transfer model throughout the direct contact membrane distillation (DCMD) module was developed in this study by taking into account the hypothetical path across the membrane as a variable factor within the operating conditions because it exhibits the changes to the mass transfer resistance across the membrane under the DCMD run. The DCMD process was described by the developed model using a system of nonlinear equations and solved numerically by MATLAB software. The performance of the poly-tetra-fluoroethylene (PTFE) membrane was examined to treat 200 g/L NaCl saline at various operating conditions. The simulation results in the present work showed that the hypothetical proposed path across the membrane has a variable value and was affected by changing the feed temperature and feed concentration. The results estimated by the developed model showed an excellent conformity with the experimental results. The salt rejection remained high (greater than 99.9%) in all cases. The temperature polarization coefficient for the DCMD ranged between 0.88 and 0.967, and the gain output ratio (GOR) was 0.893. The maximum thermal efficiency of the system was 84.5%.

Principles of unit operations: by A. S. Foust, L. A. Wenzel, C. W. Clump, L. Maus and L. B. Andersen. 578 pages, diagrams, [formula omitted] New York, John Wiley & Sons, Inc., 1960. Price, $15.00

Simultaneous heat and mass transfer to flat plate in humid air stream under frosting conditions

Heat and mass transfer in hollow-fiber modules for direct contact membrane distillation: Integral transforms solution and parametric analysis

Simultaneous heat and mass transfer in free convection

In pure heat transfer, specifications of effectiveness, fluid properties, and flows enable calculation of the heat exchanger area. In the case of falling film absorption, a simultaneous heat and mass transfer governs the performance of the absorber. The exchange of mass across the liquid-vapor interface involves the generation of heat. The heat effects associated with the mass exchange increase the temperature, which affects the equilibrium state of the pressure and composition and in turn affects the mass. The falling film flow rate coupled to the physical properties of kinematic viscosity and surface tension govern the flow regime of a vertical falling film. Wavy-laminar, roll-wave laminar, and turbulent flows will develop convective contributions that can enhance the transfer of mass into the film. The combined interaction of all these factors makes the absorption process very difficult to analyze and predict. A study of simultaneous heat and mass transfer was therefore conducted on a vertical falling film absorber to better understand the mechanisms driving the heat and mass transfer processes. Falling films are characteristically unstable, and a wavy-laminar flow was observed during the experimental study. The wavy flow further complicates the problem; therefore, only limited information is known about the temperature and concentration profiles along the length of the absorber that describe the local heat and mass transfer rates. Hence, this study presents much-needed experimental data on the heat and mass transfer processes in the absence of heat and mass transfer additive. Absorption experiments were conducted in a mini-absorber test stand at various falling film flow rates, at various absorber pressures, and with various compositions of the binary salt solution. Thermographic phosphors were successfully used to measure the temperature profile along the length of the absorber test tube. These measures of the local variations in temperature enabled calculation of the bulk concentration along the length of the absorber. The bulk concentration varied linearly, from which one may infer that the concentration gradient in the direction of flow is approximately constant. The implication is that the mass flux, and therefore the absorber load, can be solved for by using a constant flux approximation.

Investigations on the effect of spacer in direct contact and air gap membrane distillation using computational fluid dynamics

Simultaneous mass and heat transfer with reactions in a multicomponent, laminar, falling liquid film

Numerical simulation and theoretical study on simultaneously effects of operating parameters in direct contact membrane distillation

14 - Green Body Drying

Effect of non-woven net spacer on a direct contact membrane distillation performance: Experimental and theoretical studies

Direct contact membrane distillation (DCMD) process using polyvinylidene fluoride (PVDF) membrane was used for fluoride removal from aqueous solution. This study has been carried out on heat and mass transfer analyses in DCMD. The dusty-gas model was used to analyze the mass transfer mechanism and to calculate the permeate flux. The heat transfer is analyzed based on energy balance, and the different layers are considered as a series of thermal resistances. Mass transfer analysis showed that the transition Knudsen-molecular diffusion is the dominant mechanism to describe the transport of water vapor through the pores of the PVDF membrane. The most significant operating parameter is the feed temperature. The permeate increases sensitively with feed temperature and velocity, and it shows insignificant change with feed salts concentration. Heat transfer analysis showed the conduction through the matrix of the membrane presents the major part of available energy. The increasing feed temperature leads to increase thermal efficiency (TE) and decrease temperature polarization coefficient (TPC). The experimental results are in good agreement with theoretical values. Therefore, it is suggested to work at high feed temperature, which will benefit both the thermal efficiency and permeate flux. The experimental results proved that DCMD process is able to produce almost fluoride-free water suitable for many beneficial uses.

A simultaneous heat and mass transfer due to microrotating Darcy-Forchheimer flow of hybrid nanofluid over a moving thin needle is investigated. Darcy-Forchheimer medium accommodating hybrid nanofluid flow yields greater heat transfer rate, thereby leading to greater mass transfer rate over thin needle in industrial applications such as blood flow problems, aerodynamics, transportation, coating of wires, lubrication, and geothermal power generation. The thermophoresis and Brownian motion phenomena are introduced to enrich thermal treatment. Heat and mass transfer are accompanied by Cattaneo-Christov heat and mass flux. The hybrid nanofluid is radiative and dissipative in nature. Arrhenius pre-exponential factor law is introduced. Entropy generation analysis is carried out. The 4th order Runge-Kutta method along with shooting technique is devised to get requisite numerical solution of the transformed non-dimensional system of equations. Darcy-Forchheimer effect to simultaneous heat and mass transfer of microrotating hybrid nanofluid flow over thin needle subject to non-linear slip is the novelty of present study which is beyond of previous investigations. Rise in Forchheimer number (strengthening Darcy Forchheimer medium) leads to surface viscous drag decreases by 11.11% for hybrid nanofluid and 10.78% for pure nanofluid indicating the control of momentum transfer, thereby regulating heat transfer rate effectively.