Membrane Distillation: Recent Configurations, Membrane Surface Engineering, and Applications

Membrane distillation (MD) is a developing membrane separation technology for water treatment that involves a vapor transport driven by the vapor pressure gradient across the hydrophobic membrane. MD has gained wide attention in the last decade for various separation applications, including the separation of salts, toxic heavy metals, oil, and organic compounds from aqueous solutions. Compared with other conventional separation technologies such as reverse osmosis, nanofiltration, or thermal distillation, MD is very attractive due to mild operating conditions such as low temperature and atmospheric pressure, and 100% theoretical salt rejection. In this review, membrane distillation’s principles, recent MD configurations with their advantages and limitations, membrane materials, fabrication of membranes, and their surface engineering for enhanced hydrophobicity are reviewed. Moreover, different types of membrane fouling and their control methods are discussed. The various applications of standalone MD and hybrid MD configurations reported in the literature are detailed. Furthermore, studies on the MD-based pilot plants installed around the world are covered. The review also highlights challenges in MD performance and future directions.

The effect of minimal differences in the skin-to-air vapor pressure gradient at various dry-bulb temperatures on self-paced exercise performance

The effects of dry-bulb temperature on self-paced exercise performance, along with thermal, cardiovascular and perceptual responses, were investigated by minimizing differences in the skin-to-air vapor pressure gradient (Psk,sat-Pa) between temperatures. Fourteen trained male cyclists performed 30-km time trials in 13˚C and 44% relative humidity (RH), 20˚C and 70% RH, 28˚C and 78% RH, and 36˚C and 72% RH. Power output was similar in 13˚C (275±31 W; mean and SD) and 20˚C (272±28 W; P=1.00), lower in 36˚C (228±36 W) than 13˚C, 20˚C and 28˚C (262±27 W; P<0.001) and lower in 28˚C than 13˚C and 20˚C (P<0.001). Peak rectal temperature was higher in 36˚C (39.6±0.4˚C) than all conditions (P<0.001) and higher in 28˚C (39.1±0.4˚C) than 13˚C (38.7±0.3˚C; P<0.001) and 20˚C (38.8˚C±0.3˚C; P<0.01). Heart rate was higher in 36˚C (163±14 beats·min-1) than all conditions (P<0.001) and higher in 20˚C (156±11 beats·min-1; P=0.009) and 28˚C (159±11 beats·min-1; P<0.001) than 13˚C (153±11 beats·min-1). Cardiac output was lower in 36˚C (16.8±2.5 l·min-1) than all conditions (P<0.001) and lower in 28˚C (18.6±1.6 l·min-1) than 20˚C (19.4±2.0 l·min-1; P=0.004). Ratings of perceived exertion were higher in 36˚C than all conditions (P<0.001) and higher in 28˚C than 20˚C (P<0.04). Self-paced exercise performance was maintained in 13˚C and 20˚C at a matched evaporative potential, impaired in 28˚C and further compromised in 36˚C in association with a moderately lower evaporative potential and marked elevations in thermal, cardiovascular and perceptual strain.

Impact of test parameters on the water vapor permeability of textiles

PurposeUnlike other materials, textiles associate with aesthetic and mechanical properties such as flexibility and removability that allow them to be deployed or folded as required and which make them good candidates for clothing and furnishing but also, eventually, for other applications such as building. Actually, the clothing should ensure appropriate heat and mass transfers between the human body and its environment in order to maintain the thermo-physiological comfort. For that, it is important to determine water vapor permeability (WVP) of textile. Several normalized procedures with variants depending on the nature of the tested material exist to measure the WVP. One of the methods used is the “dish method” described by the British Standard (BS 7209). The purpose of this paper is to determine the influence of the test parameters on the WVP measurements.Design/methodology/approachConsequently, WVP of different textiles was measured while varying several parameters like: nature of fabrics, air layer thickness, vapor pressure gradient and air velocity.FindingsA decrease in the WVP values was observed with an increase in the air layer thickness and the number of textile layers. On the other side, an increase in the water pressure gradient induces an increase in the WVP value. Finally, it was also observed that air velocity has an impact on the WVP measurements.Originality/valueIn addition to intrinsic properties of fabrics, i.e., nature of fiber, woven structure, the influence of the several extrinsic properties, i.e., the influence of the air layer thickness, the number of textile layers, the vapor pressure gradient and the air velocity, on the WVP were investigated. Some researchers have already investigated the impact of these parameters on the WVP measurement separately. However, this study presents a difference from other studies that it takes into account the influence of the both intrinsic and extrinsic properties on WVP. In addition to these, this work combine several extrinsic properties which are presented separately during other studies. The first time, in this study the influence of the air velocity on WVP was investigated. Results on both hydrophilic and hydrophobic fabrics showed a great variation in the results when varying the location of the cups inside the climatic chamber. This is the reason why future studies look at studying more deeply the effect of air velocity on the WVP properties on different types of fabrics by connecting WVP values with air velocity values. It is also planned to make tests with the rotation device and by fixing the value of the temperature and RH. The objective will be to obtain reliable values that do not take into account the effect of air velocity.

Effects of Atmospheric Surface Layer Stability on Turbulent Fluxes of Heat and Water Vapor across the Water–Atmosphere Interface

Widely used numerical models to estimate turbulent exchange of latent heat flux (LE) and sensible heat flux H across the water–atmosphere interface are based on the bulk transfer relations linked indirectly to atmospheric stability, even though the accurate prediction of the influence of stability on fluxes is uncertain. Here eddy covariance data collected over the water surface of Ross Barnett Reservoir, Mississippi, was analyzed to study how atmospheric stability and other variables (wind speed, vapor pressure gradient, and temperature gradient) in the atmospheric surface layer (ASL) modulated LE and H variations in different stability ranges. LE and H showed right-skewed, bell-shaped distributions as the ASL stability shifted from very unstable to near neutral and then stable conditions. The results demonstrate that the maximum (minimum) LE and H did not necessarily occur under the most unstable (stable) conditions, but rather in the intermediate stability ranges. No individual variables were able to explain the dependence of LE and H variations on stability. The coupling effects of stability, wind speed, and vapor pressure gradient (temperature gradient) on LE (H) primarily caused the observed variations in LE and H in different stability ranges. These results have important implications for improving parameterization schemes to estimate fluxes over water surfaces in numerical models.

Modeling of Moisture Distribution Evolution in High-Performance Concrete under Fire Exposure

High-performance concrete (HPC) will undergo severe damage under fire conditions. It is well known that vapor pressure induced by high temperatures plays an important role in the damaging process. Therefore, the determination of the moisture distribution evolution in concrete is essential to the damage analysis of heated HPC. This paper presents a numerical method for the prediction of the moisture distribution evolution in HPC under fire conditions. In the method, the vapor pressure and the moisture transport induced by the vapor pressure gradient are analyzed. The effect of the thermal decomposition on the moisture distribution and the effects of the slippage flow and the water saturation degree on the permeability are considered. The proposed method is applied to the moisture distribution analysis of a concrete cube with 90% initial moisture content under fire conditions and can be further used for the analysis of the thermal damage of heated HPC.

Study on Water Transport Phenomena Through Micro-Porous Layers in PEFC

Micro-porous layers (MPLs) play an important role in the water management of polymer electrolyte fuel cells (PEFCs), but details of the mechanism that works to suppress water flooding has not been fully understood. In this study, the authors investigated water distribution at the interface between the MPL and the catalyst layer (CL) at the cathode side to clarify the effect of the MPL on the discharge of produced water. A freezing method was applied to observe the distribution of the condensed water, and the ice distribution on the CL surface was quantified by image processing. The effects of operating conditions on the water distribution were examined at normal temperature conditions. The distribution of ice formed on the surface of the CL with and without MPL after −10°C cold start operation was also established. Water transport rate in the vapor phase was analyzed based on temperature and vapor pressure gradient considerations. The experiments and the analysis showed that the MPL functions to prevent accumulation of water on the surface of the CL, resulting in less water flooding.

Evaporation from Snow in the Central Sierra Nevada of California

Evaporation from snow was calculated with the mean-profile method using single-level meteorological data at eight locations representing a variety of alpine and sub-alpine terrain in the central Sierra Nevada, California. Four to six years of data were analyzed at most of the sites. Evaporation from snow was highest in mid-winter, when the vapor pressure gradient between the snow surface and the air was at its maximum, and declined throughout the snowmelt season. Near the end of snowmelt, condensation of water vapor on the snow surface often matched or exceeded evaporative loss. Annual evaporation from the snowpack varied from 12 to 156 mm. We estimate mean annual regional evaporation in the sub-alpine and alpine zones (excluding evaporation from tree-captured snow and during wind re-deposition) as 80 to 100 mm, approximately 7 per cent of the maximum accumulation during an average snow year. Evaporation during snowmelt contributed only minor amounts to the total seasonal loss, typically around 25 mm, or about 2% of the maximum average accumulation.

Dew-point hygrometry system for measurement of evaporative water loss in infants

Ariagno, Ronald L., Steven F. Glotzbach, Roger B. Baldwin, David M. Rector, Susan M. Bowley, and Robert J. Moffat.Dew-point hygrometry system for measurement of evaporative water loss in infants. J. Appl. Physiol.82(3): 1008–1017, 1997.—Evaporation of water from the skin is an important mechanism in thermal homeostasis. Resistance hygrometry, in which the water vapor pressure gradient above the skin surface is calculated, has been the measurement method of choice in the majority of pediatric investigations. However, resistance hygrometry is influenced by changes in ambient conditions such as relative humidity, surface temperature, and convection currents. We have developed a ventilated capsule method that minimized these potential sources of measurement error and that allowed second-by-second, long-term, continuous measurements of evaporative water loss in sleeping infants. Air with a controlled reference humidity (dew-point temperature = 0°C) is delivered to a small, lightweight skin capsule and mixed with the vapor on the surface of the skin. The dew point of the resulting mixture is measured by using a chilled mirror dew-point hygrometer. The system indicates leaks, is mobile, and is accurate within 2%, as determined by gravimetric calibration. Examples from a recording of a 13-wk-old full-term infant obtained by using the system give evaporative water loss rates of ∼0.02 mgH2O ⋅ cm−2 ⋅ min−1for normothermic baseline conditions and values up to 0.4 mgH2O ⋅ cm−2 ⋅ min−1 when the subject was being warmed. The system is effective for clinical investigations that require dynamic measurements of water loss.

197 ENERGY BALANCE OF URBAN SURFACES AFFECTS GAS EXCHANGE OF CRABAPPLE AND NORWAY MAPLE

We investigated gas-exchange response of norway maple and crabapple to the energy balance of turf, bark-mulch, and asphalt surfaces. In each surface stomatal conductance, leaf temperature (T1), and photosynthesis, were measured during two dawn-to-dusk studies concurrent with soil (To), top surface (Ta), and air temperature (Ta) measurements. Different properties affected the energy balance of each surface. Turf transpiration moderated To and Ts while low thermal conductivity of the mulch resulted in To similar to turf but Ts23C higher. Higher thermal conductivity of the asphalt resulted in higher To but Ts intermediate to mulch and turf surfaces. We did not detect differences in Ta, probably due to close proximity to one another that allowed substantial air mixing between treatments. Higher Ts increased longwave radiation flux that raised midday T1 of trees in the mulch and asphalt 3 to 8C higher than trees in the turf. Differences in T1 between the asphalt and mulch were minimal. Stomatal conductance declined with increasing leaf-to-air vapor pressure gradient in all trees, and was consistently lower for trees in the mulch and asphalt through the day due to larger gradients induced by higher T1. Midday photosynthesis was highest for trees in the turf and lowest for those in the mulch. Foliar interception of higher energy fluxes from mulch and asphalt surfaces apparently limited gas exchange in both species due to over-optimal leaf temperatures as compared to trees in the turf