Pure water permeability

Geong and coworkers reported a new concept for the formation of zeolite/ polymer mixed-matrix reverse osmosis (RO) membranes by interfacial polymerization of mixed-matrix thin films in situ on porous polysulfone (PSF) supports [83]. The mixed-matrix films comprise NaA zeoHte nanoparticles dispersed within 50-200 nm polyamide films. It was found that the surface of the mixed-matrix films was smoother, more hydrophilic and more negatively charged than the surface of the neat polyamide RO membranes. These NaA/polyamide mixed-matrix membranes were tested for a water desalination application. It was demonstrated that the pure water permeability of the mixed-matrix membranes at the highest nanoparticle loadings was nearly doubled over that of the polyamide membranes with equivalent solute rejections. The authors also proved that the micropores of the NaA zeolites played an active role in water permeation and solute rejection. 

All symbols are defined at the end of the paper. Equation 10 defines the pure water permeability constant A for the membrane which is a measure of its overall porosity eq 12 defines the solute transport parameter D /K6 for the membrane, which is also a measure of the average pore size on the membrane surface on a relative scale. The Important feature of the above set of equations is that neither any one equation in the set of equations 10 to 13, nor any part of this set of equations is adequate representation of reverse osmosis transport the latter is governed simultaneously by the entire set of eq 10 to 13. Further, under steady state operating conditions, a single set of experimental data on (PWP), (PR), and f enables one to calculate the quantities A, Xy 2> point... 

Membrane Specifications. At a specified operating temperature and pressure, a cellulose acetate membrane is completely specified in terms of its pure water permeability constant A and solute transport parameter D /k6 for a convenient reference solute such as sodium chloride. A single set of experimental data on (PWP), (PR), and f at known operating conditions is enough to obtain data on the specifying parameters A and (DAM/X6)jjg(. 2 at any given temperature and pressure. 

Plotting the reciprocals of membrane constant(pure water permeability) against time elasped as shown in Fig. 3, a much more straight line can be obtained for each operating pressure 2000, 1500,1000 and 600 psi. But during the initial period of the operation of less than seven to three hours, the reciprocal values increase steeply untill they reach the straight lines. 

Figure 12. An increase of the solute and pure water permeabilities for homogeneous membranes due to a decrease of acetyl content and f,(CA) and fi(CA) vi. acetyl content...

Figure 16. Decrease of separation (or increase of solute permeabilities) of seawater reverse osmosis desalination at several concentration levels of NaOCl. Initial membrane constants pure water permeability constant = 97.0 nmol m Pa s and the solute permeability constant for NaCl = 0.9 X 10 cm s . Operational conditions k = 7.(97 X 10 cm s Ap = 6.0 MPa, and T = 25°C.

Reverse-Osmosis Experiments. All reverse-osmosis experiments were performed with continuous-flow cells. Each membrane was subjected to an initial pure water pressure of 2068 kPag (300 psig) for 2 h pure water was used as feed to minimize the compaction effect. The specifications of all the membranes in terms of the solute transport parameter [(Dam/ 6)Naci]> the pure water permeability constant (A), the separation, and the product rate (PR) are given in Table I. These were determined by Kimura-Sourirajan analysis (7) of experimental reverse-osmosis data with sodium chloride solution at a feed concentration of 0.06 m unless otherwise stated. All other reverse-osmosis experiments were carried out at laboratory temperature (23-25 °C), an operating pressure of 1724 kPag (250 psig), a feed concentration of 100 ppm, and a feed flow rate >400 cmVmin. The fraction solute separation (/) is defined as follows ... 

In these equations, the membrane characteristics are described by three parameters, pure water permeability Lp, solute permeability P and reflection coefficient a. The determination of these parameters is very important. The term C in Eq. (10) shows the average concentration on both sides of a membrane and the logarithmic mean is usually used. 

Pure water permeability Lp was obtained from the experiment of pure water permeation using Eq. (9), in which the osmotic pressure difference All is zero. 

The Preferential Sorption/Capillary Flow Model (Sourirajan and Matsuufa (1985)) is based on the assumption that a layer of water sorbs at the membrane surface, creating a deficit of solute at the surface. The membrane is viewed as a microporous medium, and transport is controlled by the surface chemistry of the membrane and water transport through the membrane. Ions with large hydrated radii are retained better, since they also have to overcome more energy to strip off the water. Ions diffuse through the laj et of strucmred water at the membrane surface and through water cluster channels in the membrane (Staude (1992)), where B is the pure water permeability of the membrane. 

Table 4.1. Pure water permeability of NE-1812 and the retention rate of monosaccharide and sodium chloride by NE-1812 membrane under different operating pressures [12].

Pressure (MPa) Pure water permeability Lp(10 m.s -MPa ) Robs (glucose) Robs (maltose)... 

Membrane name (type) Flux decline J/Jo (%) Salt (NaCl) rejection (%) Pure water permeability (10 mPas ) Average roughness, Ra (nm)... 

The measured average pure water permeabilities (P ) for the unmodified and the modified membranes are summarized in Table 5.1. It can be observed that all the modified membranes showed a lower water permeability than the unmodified membrane (NFPESIO) with a water permeability as low as 0.58 l/m h bar. 

NF of UV-grafted manbranes was carried out with distilled water feed and the pure water permeability was measured. The results are shown in Table 5.2. The... 

FIGURE 5.16 The effects of UV-irradiation time on pure water permeability (P ) at different AA concentrations. (From Journal of Membrane Science, 355, Abu Seman, M.N., Khayet, M., Bin Ali, Z.I., and Hilal, N., Reduction of nanofiltration membrane fouling by UV-initiated graft polymerization technique, 133-141, Copyright (2010), with permission from Elsevier.)... 

Pure water permeability SPEEK coated nanofiltration membrane Lm-2 h- MPa- 6000 500... 

The low-fouling property of membranes is evaluated with a nonionic surfactant aqueous solution. Test result shows that, in operation, low-fouling RO membrane has a relatively small permeability declaration ratio of 27%, compared with initial pure water permeability and shows stable operation. On the other hand, conventional fully aromatic polyamide membranes, SU-700 and SUL-G, show 36-47% declaration ratio, even if they show stable operation. And concerning the chemical cleaning properties, low-fouling RO membrane shows better recovery of permeability after chemical cleaning. 

In the literature, different modifications of the spinneret design were proposed as follows (1) spinnerets with different flow angles, to tailor pore size distribution and control pure water permeability [46] and (2) spinnerets with microstructured annulus or needle, to increase the active surface area [47,48],... 

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