Vacuum Membrane Distillation Performance Research Articles

Study of membrane fouling in cross-flow vacuum membrane distillation

Membrane fouling is one of the main obstacles to membrane distillation (MD) applications in desalination and other fields. Membrane scaling influences the efficiency and performance of vacuum membrane distillation (VMD). In this study, the process of a periodic membrane scaling experiment was investigated in detail by using a cross-flow hollow fiber membrane module. In the experiment, water flux significantly decreases from 42.5kg/(m2h) to 4.1kg/(m2h) because of membrane scaling; by comparison, water flux decreases from 42.7kg/(m2h) to 30.4kg/(m2h) in the blank experiment which is only affected by concentration and polarization. The blank experiment is performed under the same operating conditions as the periodic membrane scaling experiments, in which pure NaCl solution is used as feed. Scaling deposits form on/in membrane pores. Gas permeability experiments indicate that the membranes exhibit poorer transport properties as a result of aggravation of fouling and have smaller yield of MD. The scaling layer negatively affects heat and mass transfer in the feed near the evaporation surface. Membrane scaling further enhances temperature and concentration polarization, thereby decreasing the permeate flux. Experimental results further indicate that the overall heat transfer coefficient is reduced from 4569.4W/(m2K) to 2346.6W/(m2K) and the mass transfer coefficient in the membrane is reduced from 3.88kg/(m2Pah) to 2.18kg/(m2Pah) as scaling increases. Scanning electron microscopy (SEM) coupled with energy dispersion spectrometry (EDS) was employed to analyze the morphologies and compositions of the deposits formed on/in the membrane. The major component of the deposits is Ca; small amounts of Mg and S are also detected. The different morphologies of the deposits are significantly determined by various feed compositions.Using Bohai seawater, we investigated membrane fouling on the basis of VMD. The results are important for VMD application in seawater desalination.

Effect of pretreatment on membrane fouling and VMD performance in the treatment of RO-concentrated wastewater

The polyvinylidene fluoride hollow fiber hydrophobic membrane was applied in vacuum membrane distillation (VMD) process for the treatment of reverse osmosis-concentrated wastewater from a steel plant. The effects of pretreatment methods on the CODcr removal, the performance and membrane fouling of VMD process were studied. The morphology and composition of the fouling layer were studied by scanning electron microscope and energy dispersive spectroscopy, respectively. The reason for the flux declination in VMD process was discussed. The results showed that the CODcr removal reached 40%, and the flux of VMD process was increased after the pretreatment of coagulation and ultrafiltration, when the concentration factor reached eight, the flux of VMD process using properly pretreated solution was 30% higher than that using un-pretreated solution as the feed. The membrane surface morphology and elemental analysis testified that the fouling of membrane surface was obviously alleviated by pretreatment.

Experimental and theoretical research on N-methyl-2-pyrrolidone concentration by vacuum membrane distillation using polypropylene hollow fiber membrane

In this work, N-methyl-2-pyrrolidone (NMP) aqueous solution was concentrated by means of vacuum membrane distillation (VMD) using polypropylene (PP) hollow fiber hydrophobic membrane. The effects of operating variables including feed concentration, feed temperature and vacuum degree on VMD performance were studied experimentally and theoretically. The results indicated that higher feed temperature and/or higher vacuum pressure could lead to an enhancement in H2O removal and no change in NMP rejection. The permeation flux of 9.5L/m2h and the rejection 98% can be achieved at the feed temperature of 80°C and vacuum degree of 0.09MPa. Higher feed concentration leads to a decrease in H2O flux and an increase in NMP flux and NMP molar fraction in permeate side. It proved that the Knudsen-viscous flow diffusion dominated the mass transform of H2O/NMP vapor in our experiment condition. Furthermore, “Dusty-Gas” model was used to predict membrane properties and Wilson equation method was used for predicting γ of the NMP aqueous solution. The theoretical data fitted well with the experimental data in our experiment conditions.

Numerical simulation and theoretical study on simultaneous effects of operating parameters in vacuum membrane distillation

The Persian Gulf and its coastal areas are the world's largest single source of crude oil, and related industries dominate the region. Also the waste heat from refineries and petrochemical industries, which are located close to the Persian Gulf, can be used according to the simple feed inlet condition in vacuum membrane distillation (VMD). Hence, in order to enhance the performance of VMD in desalination of Persian Gulf water and to get more flux, a simultaneous heat and mass transfer model in VMD system has been developed and validated with experimental data. The model has been solved numerically using MATLAB. The influence of some operating conditions such as feed temperature, vacuum on permeate side, feed concentration, heat transfer coefficient, feed velocity and some membrane characteristics such as thickness, porosity, pore size distribution and tortuosity on permeate flux has been considered. The effects of feed temperature, feed velocity and membrane thickness on temperature polarization have also been taken into account. The results showed that the permeate flux improved with increase of feed temperature, velocity, heat transfer coefficient, porosity and pore size distribution, and decreased with enhancement of vacuum pressure, feed concentration, thickness and tortuosity of membrane.

Permeate Flux Curve Characteristics Analysis of Cross-Flow Vacuum Membrane Distillation

In this Article, the heat and mass transfer processes were analyzed to build a mathematic model of cross-flow vacuum membrane distillation (VMD) for deriving the theoretical relationship of permeate flux with operation conditions and better understanding of the novel VMD performance. The common characteristics of permeate flux curves were studied experimentally under different operation conditions in cross-flow VMD. Three main operation conditions (feed inlet temperature, vacuum level, and feed velocity) affect the permeate flux curves with different mechanisms, which form different types of permeate flux curves. Feed inlet temperature mainly affects saturated vapor pressure of feed at membrane surface, and the characteristic of the curves of permeate flux versus feed inlet temperature is opening up incremental parabola. Vacuum level mainly affects the vapor pressure of permeate side, and the characteristic of the curves of permeate flux versus vacuum level of permeate side is the linear growth line. Feed velocity mainly affects residence time of feed in membrane module and convective heat transfer coefficient of boundary layer in feed side, and the characteristic of the curves of permeate flux versus feed velocity is right opening growth parabola. Temperature polarization on the feed side intensifies with the increase of permeate flux under any operation condition, and even with the increases of feed inlet temperature, vacuum level of permeate side, and feed velocity.

Performance evaluation of vacuum membrane distillation for desalination by using a flat sheet membrane

Vacuum membrane distillation (VMD) process received a great deal of attention by many investigators because of its promising applications in several separation areas. It is a rising technology for seawater or brine desalination process. The process simply consists of a flat sheet hydrophobic microporous PTFE membrane and diaphragm vacuum pump without a condenser for the water recovery or trap. In this work, VMD performance was investigated for aqueous NaCl solution. In order to enhance the performance of the VMD process in desalination, that is, to get more flux, it is necessary to study the effect of operating parameters on the yield of distillate water. The influence of operational parameters such as feed flow rate, feed temperature, feed salt concentration and permeate pressure on the membrane distillation (MD) permeation flux have been investigated. The VMD performance showed that this device could reach a desalting degree of 99.99% which was not affected by feed concentration. The membrane distillati...

Preparation and morphological characterization of narrow pore size distributed polypropylene hydrophobic membranes for vacuum membrane distillation via thermally induced phase separation

Isotactic polypropylene (iPP) hydrophobic microporous flat sheet membranes were developed via thermally induced phase separation for vacuum membrane distillation (VMD). The synthesized membranes were characterized by contact angle measurements, poremeter and scanning electron microscopy. The membranes had an asymmetric cross-sectional structure with cellular pores on the skin layer and dendritic pores on the non-skin layers. Pure water and 0.5 mol L − 1 aqueous NaCl solution VMD performance was developed with the resultant membranes. The factors of iPP concentration, iPP melt index and nucleating agent concentration affect pore morphology and performance of the membranes significantly. The formation of the membrane is not very sensitive to quench bath temperature, although temperature has a small effect on porosity. It was found that the diluent agent has a strong effect on the morphology and performance of the membranes; soybean oil is a good diluent agent for preparation of narrow pore size iPP hydrophobic membranes with pore size distribution of about 0.02–0.2 μm.

High-salinity water desalination using VMD

Abstract This research focuses on vacuum membrane distillation (VMD) using high-concentration NaCl aqueous solutions as feed. A new membrane module was investigated to improve water desalination and experiments were carried out using a commercial polypropylene (PP) membrane with a pore size of 0.2 μm. In order to enhance performance of VMD in desalination and to get more flux, effects of operating parameters on the performance were studied. Water fluxes were measured at different feed temperatures, feed concentrations, vacuum pressures and flow rates. The new configuration provides better mixing and this increases heat and mass transfer coefficients, and as a result, reduces temperature and concentration polarization effects.

Hydrophobic modification of poly(phthalazinone ether sulfone ketone) hollow fiber membrane for vacuum membrane distillation

New hydrophobic poly(phthalazinone ether sulfone ketone) (PPESK) hollow fiber composite membranes coated with silicone rubber and with sol–gel polytrifluoropropylsiloxane were obtained by surface-coated modification method. The effects of coating time, coating temperature and the concentration of silicone rubber solution on the vacuum membrane distillation (VMD) properties of silicone rubber coated membranes were investigated. It was found that high water permeate flux could be gotten in low temperature and low concentration of silicone rubber solution. When the coating temperature is 60 °C, the coating time is 9 h and the concentration of silicone rubber solution is 5 g L −1 the water permeate flux of the silicone rubber coated membrane is 3.5 L m −2 h −1. The prepolymerization time influence the performance of polytrifluoropropylsiloxane coated membranes, and higher prepolymerization time decrease the water permeate flux of the membrane. The water permeate flux and the salt rejection was 3.7 L m −2 h −1 and 94.6%, respectively in 30 min prepolymerization period. The VMD performances of two composite membranes during long-term operation were studied, and the results indicated that the VMD performances of two composite membranes are quite stable. The salt rejection of silicone rubber coated membrane decreased from 99 to 95% and the water permeate flux fluctuated between 2.0 and 2.5 L m −2 h −1. The salt rejection of polytrifluoropropylsiloxane coated membrane decreased from 98 to 94% and the water permeate flux fluctuated in 1 L m −2 h −1 range.

Novel membrane and device for vacuum membrane distillation-based desalination process

In vacuum membrane distillation (VMD) process for desalination, water vapor flux is strongly affected by the hot brine heat transfer coefficient and membrane mass transfer coefficient. The VMD process has been studied here using porous hydrophobic polypropylene hollow fibers having three different dimensions, two different wall thicknesses and pore sizes. The outside surface of the hollow fibers has been coated with a variety of porous/microporous plasmapolymerized silicone fluoropolymer coatings. A large number of rectangular modules having the hot brine in cross flow over the outside of the fibers and vacuum on the fiber bore side have been investigated for their VMD performances to hot brine (1% NaCl) over a brine temperature range of 60–90 °C. Studies were carried out with hot water as well; further hot water flow in the tube side and vacuum in the shell side were also implemented. The shell-side Reynolds number was varied between 9 and 95. Module MXFR #3 containing larger size fibers having a much more open coating and larger pore size yielded at a high Reynolds number a water vapor flux as high as 71 kg/m 2 h from a 85 °C hot feed in cross flow. The temperature polarization was reduced considerably with the temperature polarization coefficient varying between 0.93 and 0.99. Zukauskas equation for heat transfer coefficient in cross flow over a tube bank allows one to predict the water vapor flux behavior over a wide range of Reynolds number of the shell-side brine provided an empirically determined membrane mass transfer coefficient is employed.

Membrane distillation. I. Module design and performance evaluation using vacuum membrane distillation

Pure water vacuum membrane distillation (VMD) experiments were performed to evaluate the heat and mass transfer boundary layer resistances in a new laboratory-scale membrane module. The membrane module designed for this work is unique in that it can use flat-sheet membranes without a support. Additionally, the membrane module and associated apparatus were designed to achieve relatively high feed and permeate Reynolds numbers within the module. These two factors led to a dramatic reduction in boundary layer resistances, which resulted in improved VMD fluxes. This paper also examines a new complete VMD model based on the dusty-gas model, which accounts for both Knudsen and viscous mass transport across the membrane. The new model was used to predict the performance of VMD with pure water and ethanol-water solutions.