Membrane lifetime

Fig. 28. Schematic representation of dead-end and cross-flow filtration with microfiltration membranes. The equipment used in dead-end filtration is simple, but retained particles plug the membranes rapidly. The equipment required for cross-flow filtration is more complex, but the membrane lifetime is...

Following successful testing of the bubble jet system [3] at pilot scale, the plant was scaled to full technical size (2.5 m2 elements) and successfully tested. The anolyte flow-out of the elements showed a completely pulsation-free operation with all benefits for the membrane lifetime. Despite the rather good results of this first run a design review was started to improve the electrolyser element design. 

Several test runs have been carried out using 1.5 dm2 laboratory cells with the F-8934 at 8kA m-2. The current efficiency at 8kA m-2 was about 1% lower than that at 5kAnT2, and no further decline was observed. F-8934 has also had a similar evaluation result in full-scale pilot cells at 8 kA m-2. AGC has been obtaining slightly lower current efficiencies than its desired target of 97% at the beginning of the membrane lifetime. This has led AGC to ensure that the other stepped-up design concept should be applied to the membrane for 8 kA m-2 operation. 

The APO system s internal microprocessor changes operating parameters for each pass of concentrate through the membrane. Pressures and process times are controlled in order to achieve long membrane lifetimes and very high concentrations of dilute metal salts. 

In the case of clarified feeds and low current densities, membrane lifetime can be as long as seven or 10 years for brackish water desalination or drinking water nitrate removal, respectively. However, if the feed solution is fouling or scaling or the ED separation plant has not been well designed or is not properly conducted, membrane lifetime is no longer than a year. 

Because of the challenging environment in which ultrafiltration membranes are operated and the regular cleaning cycles, membrane lifetime is significantly shorter than that of reverse osmosis membranes. Ultrafiltration module lifetimes are rarely more than 2-3 years, and modules may be replaced annually in cheese whey or electrocoat paint applications. In contrast, reverse osmosis membranes are normally not cleaned more than once or twice per year and can last 4-5 years. 

D.J. Edlund and J. McCarthy, The Relationship Between Intermetallic Diffusion and Flux Decline in Composite-metal Membranes Implications for Achieving Long Membrane Lifetime, 7. Membr. Sci. 107, 147 (1995). 

During reverse osmosis and ultrafiltration membrane concentration, polarization and fouling are the phenomena responsible for limiting the permeate flux during a cyclic operation (i.e., permeation followed by cleaning). That is, membrane lifetimes and permeate (i.e., pure water) fluxes are primarily affected by the phenomena of concentration polarization (i.e., solute build up) and fouling (e.g., microbial adhesion, gel layer formation, and solute adhesion) at the membrane surface [11]. 

Microfiltiation Reliability (fouling) Cost Selectivity Better fouling control could improve membrane lifetime significandy... 

Real membrane density (g/cm ) dynamic viscosity of the gas (Pa s) osmotic pressure at membrane surface (cm/s) fluid viscosity membrane lifetime (year)... 

Other remaining technical concerns with membrane cells relate to somewhat lower current efficiencies and to relatively short membrane lifetimes. At present, this is limited to 2-3 year of operation when coupled to much more careful brine pretreatment than is required for conventional asbestos diaphragm cells. A combination of mercury cell and membrane cell technologies has been recently tested for commercial feasibility [19]. The economics of the three primary chloralkali technologies have also been reviewed [20]. 

Ediund, D. J. McCarthy, J., The relationship between intermetallic diffusion and flux decline in composite-metal membranes implications for achieving long membrane lifetime. Journal of Membrane Science 1995, 107, (1-2), 147. 

To overcome the problems with SLM stability, the idea of their integration with other membrane processes was also investigated. Two approaches can be distinguished. Both could lead to significant increase ot the hquid membrane lifetime. One approach is to separate the hquid membrane from the feed and receiving phases. It can be achieved by placing hquid... 

Long membrane lifetime no fouling and blinding was observed the membranes are completely regenerable and used repeatedly... 

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