Heat and mass transfer analysis in direct contact membrane distillation

Vapor transfer is an important phenomenon in various thermally driven membrane processes such as membrane distillation (MD), membrane evaporation and membrane condensation. In this study, we explore the mass transfer phenomena in sweeping gas membrane distillation (SGMD) by systematically investigating the effects of operational parameters on the process performance. It is found that mass transfer in SGMD is principally determined by both the evaporation temperature and the sweeping gas flow rate, and is significantly influenced by the operational parameters (i.e. fluid velocities) through multiple effects, including the boundary layer effect on both sides of the membrane, and the temperature polarization effect. We also prove that at low gas flow rates (i.e. insufficient gas vapor-holding capacity) the sweeping gas becomes saturated and water vapor forms droplets due to condensation on the gas side of the membrane. To the best of our knowledge, this is the first study reporting the interesting mass transfer phenomena in terms of vapor condensation and droplet re-evaporation in SGMD, which are of great significance for the heat and mass transfer in many thermally driven membrane processes using stripping gas.

Section: Mass Transfer Mechanisms Through a Porous Membranementioning

confidence: 99%

“…Heat and mass transfer in MD, particularly in DCMD, have been intensively studied since the 1980s [1,4,5,[9][10][11][12][13][14][15][16]. However, less than 5% of MD publications deal with SGMD [5].…”

Section: Introductionmentioning

confidence: 99%

Condensation studies in membrane evaporation and sweeping gas membrane distillation

Zhao

Feron

Xie

et al. 2014

“…The vapour pressure difference across the membrane depends on the temperature T 1 and T 2 , and the driving force is therefore P T1 -P T2 where P T1 and P T2 are the vapour pressures at T 1 and T 2 , respectively. The feed concentration C f increases across the feed boundary layer to C 1 [8,14]. …”

Section: Heat Transfer and Mass Transfer In Dcmdmentioning

confidence: 99%

Identification of material and physical features of membrane distillation membranes for high performance desalination

Zhang

Dow

Duke

et al. 2010

254

156

In this paper, the performance of various membranes were assessed in direct contact membrane distillation (DCMD) under different feed velocities and inlet temperatures. The membranes studied included a polyvinylidenefluoride (PVDF) microfiltration membrane with a non-woven support layer, a polytetrafluoroethylene (PTFE) microfiltration membrane with a non-woven support layer, and three MD membranes made from PTFE of different pore size and all with a structured scrim support layer. The results showed that distillation using PTFE membranes produced much higher flux than that of the PVDF microfiltration membrane at the same operational conditions, and the support layer affected not only the flux, but also the energy efficiency (0.51 -0.24). The results also show that increasing the velocity of the feed and its inlet temperature increased the flux, but the rate of flux increase diminishes at high velocities. The mass transfer coefficient improved for thinner support and active layer membranes, leading to fluxes as high as 46 L.m -2 h -1 at 80˚C. The heat transfer characteristics were found to be superior for the open scrim backed membranes compared to the non-woven support membranes, resulting in significantly greater thermal efficiency for the scrim backed membranes.

“…This has proved to be helpful in analyzing the heat-transfer process and calculating the local heat-transfer coefficients and membrane surface temperatures and hence determining the temperature-polarization coefficient [1,13,[18][19][20]. Unfortunately, these empirical equations were developed based on non-porous and rigid shell and tube heat exchangers that are not coupled with mass transfer and phase changes [14].…”

Section: Introductionmentioning

confidence: 99%

“…On the other hand, much effort has been devoted to the MD process in order to mitigate the TP effect and further increase permeation flux [5][6][7][8][9][10]. These studies have examined the permeation flux and flux decay in terms of operating conditions, long term performance, membrane properties, module design and thermal energy consumption, etc [11][12][13]. It is well-recognized that the feed and permeate temperatures, the vapor pressure difference and the feed and permeate flow velocities play important roles in MD performance [14].…”

Section: Introductionmentioning

confidence: 99%

Analysis of heat and mass transfer by CFD for performance enhancement in direct contact membrane distillation

Yu¹,

Yang²,

Wang³

et al. 2012

136

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.