Continuous Desalination Research Articles

Effects of calcium carbonate nano-particles on the properties of PVDF/nonwoven fabric flat-sheet composite membranes for direct contact membrane distillation

The polyvinylidene fluoride (PVDF) flat-sheet composite membranes were prepared for membrane distillation using hydrophilic polyester nonwoven fabric as a support and the mixture of LiCl and PEG as non-solvent additive through coating and wet phase inversion process. The effects of hydrophobic calcium carbonate nano-particles on membrane morphology, porosity, pore size, pore size distribution, hydrophobicity, thermal properties and permeability were investigated. The nano-particles optimized membrane structure, enlarged pore diameter, narrowed pore size distribution and improved membrane porosity to some extent. In addition, the nano-particles could also enhance membrane surface roughness and contact angle while increasing the crystallinity degree of the prepared membranes. Nonetheless, the excess nano-particles addition would deteriorate membrane structure, reduce membrane porosity and decrease pore size. During the desalination process of 35g/L sodium chloride solution, the maximum transmembrane permeate flux about 49.37kg/m2h was obtained with the feed solution at 83ºC and the cold distillate water at 20.0ºC. Furthermore, the flat-sheet composite membrane exhibited satisfying performance stability compared with the membrane without nano-particles addition in the 300h continuous desalination experiments, indicating that the prepared PVDF/nonwoven fabric flat-sheet composite membrane may be of great potential to be utilized in membrane distillation process for desalination.

Mathematical Modeling and Parametric Study of a Continuous Solar Desalination System

A mathematical model is presented to analyze the performance of a counter-current solar desalination system. Besides, the heat transfer equations, the mass transfer is also considered to improve the model precision. A new approach is used for analyzing the radiative heat transfer in these systems. A low enough value for feed flow rate, a moderate value for glass temperature, taking the advantage of high flux at the appropriate time and appropriate insulation of the floor could effectively increase productivity. The system length is also a significant parameter. The model can be sufficiently extended for other continuous solar desalination systems.

Circulatory osmotic desalination driven by a mild temperature gradient based on lower critical solution temperature (LCST) phase transition materials

Abrupt changes in effective concentration and osmotic pressure of lower critical solution temperature (LCST) mixtures facilitate the design of a continuous desalination method driven by a mild temperature gradient. We propose a prototype desalination system by circulating LCST mixtures between low and high temperature (low T and high T) units. Water molecules could be drawn from a high-salt solution to the LCST mixture through a semipermeable membrane at a temperature lower than the phase transition temperature, at which the effective osmotic pressure of the LCST mixture is higher than the high-salt solution. After transfer of water to the high T unit where the LCST mixture is phase-separated, the water-rich phase could release the drawn water into a well-diluted solution through the second membrane due to the significant decrease in effective concentration. The solute-rich phase could be recovered in the low T unit via a circulation process. The molar mass, phase transition temperature, and aqueous solubility of the LCST solute could be tuneable for the circulatory osmotic desalination system in which drawing, transfer, release of water, and the separation and recovery of the solutes could proceed simultaneously. Development of a practical desalination system that draws water molecules directly from seawater and produces low-salt water with high purity by mild temperature gradients, possibly induced by sunlight or waste heat, could be attainable by a careful design of the molecular structure and combination of the circulatory desalination systems based on low- and high-molar-mass LCST draw solutes.

Preparation and characterization of PVDF/nonwoven fabric flat-sheet composite membranes for desalination through direct contact membrane distillation

The polyvinylidene fluoride (PVDF) flat-sheet composite membranes were prepared for membrane distillation using hydrophilic polyester nonwoven fabric as a support through coating and wet phase inversion process. The influence of non-solvent additive on membrane morphology, porosity, pore size and pore size distribution, hydrophobicity and permeability were evaluated. It was found that the membrane with a mixture of acetone and H3PO4 as non-solvent additive had a thinner top skin and a more porous network sponge-like layer from the morphology study. This membrane also exhibited a narrow pore size distribution and its mean pore size was about 0.22μm in diameter. The rough membrane surface with smaller nodules produced a higher contact angle of 82.6±0.7° with water according to atomic force microscope images analysis. During the direct contact membrane distillation (DCMD) process of 35g/L sodium chloride solution, the maximum transmembrane permeate flux about 47.6kg/m2h was obtained with the feed solution at 80.5°C and the cold distillate water at 20.0°C. Furthermore, the membrane showed satisfying performance stability in the 240h continuous desalination experiment, indicating that the prepared PVDF/nonwoven fabric flat-sheet composite membrane may be of great potential to be utilized in DCMD process for desalination.

Preparation and properties of PVDF composite hollow fiber membranes for desalination through direct contact membrane distillation

► PVDF composite hydrophobic hollow fibers were prepared for membrane distillation. ► Influence of nano-particles on the membranes characteristics was investigated. ► The mechanical properties of the PVDF composite membranes were enhanced obviously. ► The spun fiber presented excellent performance stability and hydrophobicity. The polyvinylidene fluoride (PVDF) composite hydrophobic hollow fiber membranes were fabricated for membrane distillation through non-solvent induced phase inversion by dispersing hydrophobic modified calcium carbonate nano-particles in the PVDF casting solution and using the mixture of LiCl and polyethylene glycol (PEG) as non-solvent additive. The influence of nano-particles on the spun hollow fiber membrane characteristics was investigated. The addition of hydrophobic nano-particles could optimize the sandwich-like morphology, narrow the pore size distribution, improve the membrane porosity and increase the membrane surface roughness and contact angle to some extent. The nano-particles also enhanced the crystallinity degree and thermal stability of the hollow fiber. In addition, the composite membranes presented stronger mechanical properties compared to the fiber without particles. During the desalination process of 3.5 wt% sodium chloride solution, the maximum transmembrane permeate flux about 46.3 kg/m 2 h with a lower thermal loss was obtained from the composite fiber when the hot feed inlet temperature and the cold distillate inlet temperature were at 80.5 °C and 20.0 °C, respectively. Furthermore, the composite membrane exhibited satisfying performance stability compared with the pure PVDF membrane in the 30 days continuous desalination experiments, indicating that the as-spun composite fiber may be of great potential to be utilized in MD process for desalination.

Identification, Modelling, and Control of Continuous Reverse Osmosis Desalination System: A Review

Desalination is a separation process used to reduce the amount of dissolved salts in seawater or brackish water to a usable or potable level by distillation, multiple effect vapor compression, evaporation, or by membrane processes such as electro-dialysis reversal, nano-filtration, and reverse osmosis (RO). RO is the most widely used desalination process. Recent advances in RO technology have led to more efficient separation and now is the most cost-effective process to operate. The performance of the Reverse Osmosis process is dependent on the concentration of dissolved solids in the feed-water, feed-water pressure, and the membrane strength to withstand system pressure, membrane solute rejection, membrane fouling characteristics, and the required permeate solute concentration. RO is a promising tool that uses cellulose acetate (or) polyamide membrane and is widely chosen as the cost of production is reduced by the use of energy-efficient and process-control techniques. This article presents a review of literature survey of identification of parameters, dynamic modelling, and control of desalination system in the past twenty years by collecting more than 65 literatures.

Desalination of brackish groundwater by direct contact membrane distillation

The direct contact membrane distillation (DCMD) applied for desalination of brackish groundwater with self-made polyvinylidene fluoride (PVDF) membranes was presented in the paper. The PVDF membrane exhibited high rejection of non-volatile inorganic salt solutes and a maximum permeate flux 24.5 kg m(-2) h(-1) was obtained with feed temperature at 70 degrees C. The DCMD experimental results indicated that the feed concentration had no significant influence on the permeate flux and the rejection of solute. When natural groundwater was used directly as the feed, the precipitation of CaCO(3) would be formed and clog the hollow fibre inlets with gradual concentration of the feed, which resulted in a rapid decline of the module efficiency. The negative influence of scaling could be eliminated by acidification of the feed. Finally, a 250 h DCMD continuous desalination experiment of acidified groundwater with the concentration factor at constant 4.0 was carried out. The permeate flux kept stable and the permeate conductivity was less than 7.0 microS cm(-1) during this process. Furthermore, there was no deposit observed on the membrane surface. All of these demonstrated that DCMD could be efficiently used for production of high-quality potable water from brackish groundwater with water recovery as high as 75%.

Direct seawater desalination by ion concentration polarization

A shortage of fresh water is one of the acute challenges facing the world today. An energy-efficient approach to converting sea water into fresh water could be of substantial benefit, but current desalination methods require high power consumption and operating costs or large-scale infrastructures, which make them difficult to implement in resource-limited settings or in disaster scenarios. Here, we report a process for converting sea water (salinity approximately 500 mM or approximately 30,000 mg l(-1)) to fresh water (salinity <10 mM or <600 mg l(-1)) in which a continuous stream of sea water is divided into desalted and concentrated streams by ion concentration polarization, a phenomenon that occurs when an ion current is passed through ion-selective membranes. During operation, both salts and larger particles (cells, viruses and microorganisms) are pushed away from the membrane (a nanochannel or nanoporous membrane), which significantly reduces the possibility of membrane fouling and salt accumulation, thus avoiding two problems that plague other membrane filtration methods. To implement this approach, a simple microfluidic device was fabricated and shown to be capable of continuous desalination of sea water (approximately 99% salt rejection at 50% recovery rate) at a power consumption of less than 3.5 Wh l(-1), which is comparable to current state-of-the-art systems. Rather than competing with larger desalination plants, the method could be used to make small- or medium-scale systems, with the possibility of battery-powered operation.

Continuous desalination using cyclic mass transfer on bifunctional resins

Continuous desalination using cyclic mass transfer on bifunctional resins

Piezodialysis

In this paper a previously developed model for a piezodialysis membrane is summarized and extended. Relationships between transport properties of resins and transport properties of a composite membrane are derived. These relationships provide considerable insight into the piezodialysis process and some useful direction in the search for better piezodialysis membranes. A relationship between geometric factors of the membrane and test apparatus and the effective circulating current path length is presented. Performance of a module which demonstrates continuous desalination by piezodialysis is described.

Perturbation analysis of a continuous system for desalination by reverse osmosis

Blasius perturbation solutions are developed for the incompressible laminar boundary layer at high Schmidt numbers. The eigenvalues have been found to be multiples of 3/2 and the eigenfunctions can be expressed as confluent hypergeometric functions. The eigenfunctions are employed in analyzing a continuous desalination system. The reverse-osmosis boundary condition leads to an integral equation which is, in turn, solved by iterative techniques. The first-order calculations show that the results are most accurate for large ratios of osmotic to applied pressures. For medium ratios, the results are reasonably good.