Reverse osmosis cross-flow filtration

Cross-Flow Filtration in Porous Pipes. Another way of limiting cake growth is to pump the slurry through porous pipes at high velocities of the order of thousands of times the filtration velocity through the walls of the pipes. This is ia direct analogy with the now weU-estabHshed process of ultrafiltration which itself borders on reverse osmosis at the molecular level. The three processes are closely related yet different ia many respects. 

Cross-flow filtration (CFF) also known as tangential flow filtration is not of recent origin. It began with the development of reverse osmosis (RO) more than three decades ago. Industrial RO processes include desalting of sea water and brackish water, and recovery and purification of some fermentation products. The cross-flow membrane filtration technique was next applied to the concentration and fractionation of macromolecules commonly recognized as ultrafiltration (UF) in the late 1960 s. Major UF applications include electrocoat paint recovery, enzyme and protein recovery and pyrogen removal. 

The major application today for cross-flow filtration is in the membrane filtration for bioprocessing or fine particle separations. Based on the size of the particles separated, membrane filtrations are categorized as microflltration (MF), ultraflltration (UF), nanoflltration (NF). and reverse osmosis (RO). The ratings of MF membranes are by micron ratings, just like other fabric filter media. In... 

Ultrafiltration and reverse osmosis have always used a fluid management technique known as "cross-flow filtration" to sweep away deposited particles from the membrane surface. "Cross-flow filtration" (CFF) is compared with "through-flow filtration" (TFF) (sometimes called "dead-ended filtration") in Figure 2.38. 

Membranes are being used increasingly for the removal of dissolved and colloidal contaminants in wastewater streams. Reverse osmosis (hyperfiltration) is well known for its ability to concentrate ionic species while ultrafiltration has found broad utility for the removal of dispersed colloidal oil, non-settlable suspended solids, and larger organic chemical molecules. One of the major problems these processes have faced is the fouling or blinding of the membranes after limited use. Various approaches have been developed in an effort to minimize this deterrent. Cross-flow filtration, where the contaminants are constantly flushed or washed from the membrane surface by the feedwater stream, is one of these approaches. The unit goes farther. Rather than a thin... 

This is a method of limiting cake growth by pumping the slurry through porous tubes at high velocities, so that the ratio of the axial flow velocity to filtration velocity through the tube walls is of the order of thousands. This is in direct analogy with the now well-established process of ultra-filtration applicable to much finer solids, which itself borders with reverse osmosis on the molecular level. It is therefore appropriate to review briefly the latter and then to follow on with ultra-filtration and, finally, with the relatively recently explored cross-flow filtration in porous tubes. 

There is no distinct limit between reverse osmosis and ultra-fdtration but the latter employs lower pressures of no more than 10 bar (seldom above 6 bar) and more open membranes for separation of large molecules and ultra-fine, sub-micron solids. Henry tried to make a clear distinction between ultra-filtration and cross-flow filtration by defining the former as retention of only dissolved species from solutions (as opposed to retention of particulate material from suspensions) but this has not caught on in practice, mainly because of the fact that dissolved and undissolved (ultra-fine) soUds are often separated together. Thus, ultra-filtration is used for example for the concentration of proteins from low-cost dairy byproducts, or in the separation of emulsified oil and suspended solids from waste waters. In such processes a cake or a layer of gel would form on the membrane and reduce the filtration rates if it were not for the cross-flow characteristics of most designs, in which the suspension flows at high speed across the membrane surface and prevents cake build-up. 

In order to desalinate seawater in a small-sized plarrt, it is envisaged to use a hollow-fiber modttle (Figure 14.10), the sketch of which has been formd in the Techniques de Tlngenietrr. Salt water flows inside drcular cylindrical hollow fibers of inner diameter 40 rm and outer diameter 80 jrm through which the permeate (salt free) is filtered by reverse osmosis as it passes from the inside to the outside of the fibers. In this cross-flow filtration device, a substarrtial fraction of the salt feed flow rate leaves without being filtered. 

Membrane separation utilizes cross-flow filtration in which feed water flows over the membrane surface, separating the feed water into two streams product water and concentrated water. The driving force for this filtration process is the pressure differential. Reverse osmosis membranes reject ionic species and operate at pressures of 700-4200 kPa (100-600 psig) for brackish water applications. Reverse osmosis is the process of forcing water through a semipermeable membrane against the natural osmotic gradient. When water is... 

The word membrane has stuck to a range of separation media that has expanded enormously from this early form, to embrace solid inflexible inorganic materials, especially ceramics, and an ever-increasing group of polymeric materials, and to applications that now extend through ultrafiltration into the miCTofiltration range. The existence of the membrane as a very effective filtration medium led to the development of the whole field of cross-flow filtration, which also now extends well beyond its reverse osmosis origins. 

A major development in filtration, which came with the first, reverse osmosis membrane process, was the fact that the flow of the fluid at the membrane snrface is tangential to it, rather than perpendicular to that surface. This has become known as cross-flow filtration (see Figure 2.21), and almost all membrane processes now operate in crossflow rather than through flow mode. Further scouring action is achieved by having the membrane medium move relative to the hquid flow, either rotating close to a stator, or vibrating. 

The separations feasible by filtration have expanded enormously over the last generation. The developments this symposium has commemorated, and the individuals it has honored, have been largely responsible. The removal of dissolved solutes or other low-molecular-weight substances from water by hyperfiltration or reverse osmosis, which the Loeb-Sourlrljan membrane made technically and economically feasible, has become an industrial-scale operation. Ultrafiltration of colloids and filtration of coarser materials from liquids have become much more efficient with the use of cross flow of liquid to slow the buildup of flltercake appreciation of the benefits from shear at the Interface has become much more general from the necessity of controlling concentration polarization and fouling in salt filtration. 

A membrane filtration plant suitable for process water from reactive dyeing and printing of cotton is a two step plant Pre-filtration by ultrafiltration and a final treatment by reverse osmosis. Pre-treatment technologies for RO spiral wound membrane filtration have focused on flat sheet polymer UF membranes in a high cross-flow filter. The quality of water produced by this plant will go beyond what most dyehouses use today and will be well suited for all processing, including reactive dyeing of cotton." ... 

The cross-flow principle (Figure 3.76) began with the hollow fibres used in reverse osmosis, and has expanded to become one of the most important components of the filtration industry. In order to keep the surface free of deposit, high-shear conditions are employed, and these can be created either by a high suspension velocity across the medium, or by some sort of movement (rotation, vibration, etc.) of the medium with respect to the liquid flow or a nearby non-porous surface. This latter group, of movement promoted filtration, is often termed dynamic cross-flow filter systems. 

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