Direct contact membrane distillation: effect of mass transfer on heat transfer

Journal of Membrane Science

Duke

et al. 2013

Six modules with different packing densities and lengths were fabricated for conducting vacuum membrane distillation (VMD) experiments. The performances of the modules with different packing densities showed similar results for flux at different temperature and feed velocities. However, shorter modules show both higher flux and global mass transfer coefficient than that of longer modules. It was also found that the increased flux at higher velocity was largely due to the increased average feed temperature rather than reduced temperature polarisation. The flux decay in this study was considered to be caused by gradual compression of the membrane which was confirmed by a compression test. This phenomenon should be noted in the study of VMD, since the pressure difference across the membrane is in general more than 100 kPa and the porosity of the membrane is in general high which results in reduced mechanical strength.

Section: Influence Of Module Design On Vmd Performancesupporting

confidence: 81%

Influence of module design and membrane compressibility on VMD performance

Journal of Membrane Science

Duke

et al. 2013

“…This is the simplest configuration capable of producing reasonably high flux. It is best suited for applications such as desalination and concentration of aqueous solutions (e.g., juice concentrates) [1,[15][16][17][18][19].…”

Section: Configurations Of Membrane Distillationmentioning

confidence: 99%

“…As shown in Equation (17), reducing the heat transfer coefficient h m of the membrane material will directly impact the global heat transfer coefficient and reduce process heat losses [226,227]. As h m is directly proportional to the thermal conductivity, an accurate measurement of the thermal conductivity or of the thermal diffusivity can therefore lead to a better understanding of the heat transfers in DCMD.…”

Section: Heat Transfer In MDmentioning

confidence: 99%

Advances in Membrane Distillation for Water Desalination and Purification Applications

Camacho

Dumée

et al. 2013

Water

642

342

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. Keywordsa function of temperature, vapor pressure, and of the gas molecular mass K 0 membrane characteristic defined by Equation (9) Kn Knudsen number K(T) a function of temperature and molecular weight of the gas l mean free path of the molecules l m distance between parallel spacer fibres (m) LEP Limit Entry Pressure (kPa) M molecular mass (g/mol) M w molecular weights of water (g/mol) M a molecular weights of air (g/mol) n number of CNTs per unit cross section in bucky-paper P pressure in the air gap (kPa) half time to reach the maximum intensity-laser flash technique (s) t proportion of conductive heat (balance due to evaporative heat) loss through the membrane T mean temperature in the pores (K)

“…This is consistent with the boundary layer thickness being less sensitive to flow velocity at high velocities [23]. 12 13 The fluxes as a function of hot side inlet temperatures (29-90°C) for Setup I and II are shown in flattened part of the curves in Figure 9 to reduce the effect of the boundary layer on flux and remained constant at 1.7 m/s b vap across the entire temperature range, and the high S The global mass transfer coefficient shown in Figure 10 decreased slightly when the temperature rose, suggesting increased polarisation at higher temperatures [24].…”

Section: Fluxes and Global Mass Transfer Coefficients At Different Vementioning

confidence: 99%

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

Journal of Membrane Science

Duke

et al. 2010

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 Lm -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.