Desalination by membrane distillation

This work advances the state of the art by presenting a transport analysis of air gap membrane distillation (AGMD) and of direct contact membrane distillation (DCMD), based on a two-dimensional conjugate model in which, the temperature and concentration of the hot and cold solutions both normal to the membrane and along it are solved, so that the sensitivity of the permeate flux to the major system parameters could be better evaluated. Employment of spacers in the flow channels for improving the process by reducing the convective resistance is also investigated. Significantly, this is the first comprehensive analysis and exposition of all resistance to heat and mass transfer in the process. The solutions were validated in comparison with available experimental results. The modeling and sensitivity analysis provide useful basic detailed information about the nature of the process, and are helpful for process improvement and optimization. Some of the principal conclusions are: (1) the air/vapor gap has the major role in reducing the parasitic heat loss in the process, (2) the gap width has an important effect: decreasing it 5-fold increases the permeate flux 2.6-fold, but the thermal efficiency improves only slightly because the conductive heat loss increases too, (3) increasing the inlet temperature of the hot solution has a major effect on the permeate flux and also increase the thermal efficiency, while decreasing the coolant temperature has a lesser effect on the flux increase, and even slightly reduces the efficiency, (4) the feedwater salt concentration has a very small effect on the permeate flux and thermal efficiency, (5) the inlet velocities of the hot and cold solutions have a relatively small effect, (6) reducing the thermal conductivity of the membrane material improves the process thermal efficiency somewhat, (7) the permeate flux of DCMD is higher than that of AGMD by about 2.3-fold at Thi = 80°C and becomes even higher for low inlet saline feedwater temperatures (at Thi = 40°C, JDCMD/JAGMD = 4.8), (8) the sensitivity of DCMD to the main process parameters is more noticeable than that in AGMD, (9) for MD it appears that the central type is the most effective one, and can improve the flux by about 33% over the empty channel. (Abstract shortened by UMI.

Desalination by membrane distillation

This work advances the state of the art by presenting a transport analysis of air gap membrane distillation (AGMD) and of direct contact membrane distillation (DCMD), based on a two-dimensional conjugate model in which, the temperature and concentration of the hot and cold solutions both normal to the membrane and along it are solved, so that the sensitivity of the permeate flux to the major system parameters could be better evaluated. Employment of spacers in the flow channels for improving the process by reducing the convective resistance is also investigated. Significantly, this is the first comprehensive analysis and exposition of all resistance to heat and mass transfer in the process. The solutions were validated in comparison with available experimental results. The modeling and sensitivity analysis provide useful basic detailed information about the nature of the process, and are helpful for process improvement and optimization. Some of the principal conclusions are: (1) the air/vapor gap has the major role in reducing the parasitic heat loss in the process, (2) the gap width has an important effect: decreasing it 5-fold increases the permeate flux 2.6-fold, but the thermal efficiency improves only slightly because the conductive heat loss increases too, (3) increasing the inlet temperature of the hot solution has a major effect on the permeate flux and also increase the thermal efficiency, while decreasing the coolant temperature has a lesser effect on the flux increase, and even slightly reduces the efficiency, (4) the feedwater salt concentration has a very small effect on the permeate flux and thermal efficiency, (5) the inlet velocities of the hot and cold solutions have a relatively small effect, (6) reducing the thermal conductivity of the membrane material improves the process thermal efficiency somewhat, (7) the permeate flux of DCMD is higher than that of AGMD by about 2.3-fold at Thi = 80°C and becomes even higher for low inlet saline feedwater temperatures (at Thi = 40°C, JDCMD/JAGMD = 4.8), (8) the sensitivity of DCMD to the main process parameters is more noticeable than that in AGMD, (9) for MD it appears that the central type is the most effective one, and can improve the flux by about 33% over the empty channel. (Abstract shortened by UMI.

Performance of asymmetric hollow fibre membranes in membrane distillation under various configurations and vacuum enhancement

Hollow fibre membrane distillation (MD) modules have a more compact structure than flat sheet membrane modules, providing potentially greater advantage in commercial applications. In this paper, a high-flux asymmetrically structured hollow fibre MD module was tested under various conditions. The results show that increasing velocity and temperature are positive for flux, and salt rejection was more than 99% over the entire experimental range. The hollow fibre module also showed great variation in flux when altering the hot feed flow from the lumen side to the shell side of the fibre, and this phenomenon was analysed based on the characterisation of the asymmetric structure of the hollow fibre. The largest mass transfer resistance was determined to be in the small pore size skin layer on the outer surface of the membrane, and having the hot feed closest to this surface provided the greatest vapour pressure difference across this high resistance mass transfer layer. The results also show that placing the suction pump on the permeate outlet increased the flux by lowering the pressure within the pore and hence increased the rate of vapour mass diffusion. A maximum flux of 19 L m−2 h−1 was obtained at 85 °C when hot feed was entering the shell side, and the mass transfer coefficient was relatively constant across the entire temperature range when operated at high velocities. These outcomes suggest that asymmetric hollow fibre MD modules should be operated with hot brine feed closest to the high resistant skin layer, and that vacuum enhanced MD further increases vapour transport and flux

Cattaneo–Christov Heat Flux Model for Three-Dimensional Rotating Flow of SWCNT and MWCNT Nanofluid with Darcy–Forchheimer Porous Medium Induced by a Linearly Stretchable Surface

In this paper we investigated the 3-D Magnetohydrodynamic (MHD) rotational nanofluid flow through a stretching surface. Carbon nanotubes (SWCNTs and MWCNTs) were used as nano-sized constituents, and water was used as a base fluid. The Cattaneo–Christov heat flux model was used for heat transport phenomenon. This arrangement had remarkable visual and electronic properties, such as strong elasticity, high updraft stability, and natural durability. The heat interchanging phenomenon was affected by updraft emission. The effects of nanoparticles such as Brownian motion and thermophoresis were also included in the study. By considering the conservation of mass, motion quantity, heat transfer, and nanoparticles concentration the whole phenomenon was modeled. The modeled equations were highly non-linear and were solved using homotopy analysis method (HAM). The effects of different parameters are described in tables and their impact on different state variables are displayed in graphs. Physical quantities like Sherwood number, Nusselt number, and skin friction are presented through tables with the variations of different physical parameters

Modelling heat and mass transfers in DCMD using compressible membranes

A model for predicting the flux and evaporation ratio in direct contact membrane distillation (DCMD) using a compressible membrane is presented. Polytetrafluoroethylene (PTFE) membranes, one of the most common types of membranes employed in MD, are characterised with high porosity (∼90%) and high hydrophobicity, and therefore have high water vapour permeability and high wetting resistance. However, the PTFE membrane is compressible due to its structure. Compression of the membrane will cause a change of its physical structure, such as porosity, thickness, and pore size. As a result, the thermal conductivity and vapour permeability of the membrane will be altered, causing a change in flux and energy efficiency. Such effects need to be accounted for when scaling up from laboratory data to full scale design, because there may be significant differences in the applied pressure. Therefore, in this paper, the influence of pressure on the flux of the compressible PTFE membrane was modelled. This paper also provides a mathematical method to correlate the applied pressures with physical structure changes based on the assumption of constant tortuosity. The modelling results were compared with experimental results over a range of variable process parameters, i.e., temperatures, velocities, membrane lengths, and pressure applied to the membrane. The errors between the model predictions and experimental results were less than 10% within the operating range used in this investigation