Reverse osmosis membranes effects

Szabo G.T., More Gy. Ramadan Y. (1996), Filtration of organic solutes on reverse osmosis membrane. Effect of counter-ions. Journal of Membrane Science, 118, 295-302. 

Some data iEustrating the effect of pressure on the water and salt fluxes and the salt rejection of a good quaUty reverse osmosis membrane are shown ia Figure 34 (76). 

The Effect of Halogens on the Performance and Durability of Reverse-Osmosis Membranes... 

The Effect of Phosphoric Acid as a Casting Dope Ingredient on Reverse-Osmosis Membrane Properties... 

Size ranges for membrane processing by reverse osmosis, ultrafiltration and microfiltration are shown in Figure 19.1. Reverse osmosis is effective in removing solvents away from dissolved molecules. Because of limitations in crushing strengths of membranes, pressures are limited to maxima of about 1000 psi... 

In coupled transport and solvent dehydration by pervaporation, concentration polarization effects are generally modest and controllable, with a concentration polarization modulus of 1.5 or less. In reverse osmosis, the Peclet number of 0.3-0.5 was calculated on the basis of typical fluxes of current reverse osmosis membrane modules, which are 30- to 50-gal/ft2 day. Concentration polarization modulus values in this range are between 1.0 and 1.5. 

Many nanofiltration membranes follow these rules, but oftentimes the behavior is more complex. Nanofiltration membranes frequently combine both size and Donnan exclusion effects to minimize the rejection of all salts and solutes. These so-called low-pressure reverse osmosis membranes have very high rejections and high permeances of salt at low salt concentrations, but lose their selectivity at salt concentrations above 1000 or 2000 ppm salt in the feed water. The membranes are therefore used to remove low levels of salt from already relatively clean water. The membranes are usually operated at very low pressures of 50-200 psig. 

It is necessary to minimize the y intercept ( ) = RTKilPiVi = a KilPi), thereby maximizing the diffusive water permeability and minimizing the hydraulic water permeability. Reverse osmosis membranes can be characterized simply by observing salt rejection as a function of effective pressure. Figure 34.14 is a... 

Sahachaiyunta et al. [38] conducted dynamic tests to investigate the effect of silica fouling of reverse osmosis membranes in the presence of minute amounts of various inorganic cations such as iron, manganese, nickel, and barium, which are present in industrial and mineral processing wastewaters. Experimental results showed that the presence of iron greatly affected the scale structure on the membrane surface when compared to the other metal species. 

Nakase Y. Irradiation effects on properties of reverse osmosis membrane based on crosslinked aromatic polyamide. J Nucl Sci Technol 1994 31 1214 1221. 

Summary of Physicochemical Parameters. In the previous section steric parameters 4 . and were introduced to describe the effective size of solution components and the average size of the transport corridor, respectively. A variety of quantities that could be used to represent

dense membranes and the skin layer of reverse osmosis membranes in which the transport corridors are beyond the resolution capabilities of modern instruments and may be dynamic in nature. Therefore, any discussion of membrane material selection based on steric considerations must be qualitative. 

An effect not considered in the above models is the added resistance, caused by fouling, to solute back-diffusion from the boundary layer. Fouling thus increases concentration polarization effects and raises the osmotic pressure of the feed adjacent to the membrane surface, so reducing the driving force for permeation. This factor was explored experimentally by Sheppard and Thomas (31) by covering reverse osmosis membranes with uniform, permeable plastic films. These authors also developed a predictive model to correlate their results. Carter et al. (32) have studied the concentration polarization caused by the build-up of rust fouling layers on reverse osmosis membranes but assumed (and confirmed by experiment) that the rust layer had negligible hydraulic resistance. 

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