Reverse osmosis controls

In open fibers the fiber wall may be a permselective membrane, and uses include dialysis, ultrafiltration, reverse osmosis, Dorman exchange (dialysis), osmotic pumping, pervaporation, gaseous separation, and stream filtration. Alternatively, the fiber wall may act as a catalytic reactor and immobilization of catalyst and enzyme in the wall entity may occur. Loaded fibers are used as sorbents, and in ion exchange and controlled release. Special uses of hoUow fibers include tissue-culture growth, heat exchangers, and others. 

Fig. 23. Two types of hollow-fiber modules used for gas separation, reverse osmosis, and ultrafiltration applications, (a) Shell-side feed modules are generally used for high pressure appHcations up to - 7 MPa (1000 psig). Fouling on the feed side of the membrane can be a problem with this design, and pretreatment of the feed stream to remove particulates is required, (b) Bore-side feed modules are generally used for medium pressure feed streams up to - 1 MPa (150 psig), where good flow control to minimise fouling and concentration polarization on the feed side of the membrane is desired.

A second factor determining module selection is resistance to fouling. Membrane fouling is a particularly important problem in Hquid separations such as reverse osmosis and ultrafiltration. In gas separation appHcations, fouling is more easily controlled. Hollow-fine fibers are notoriously prone to fouling and can only be used in reverse osmosis appHcations if extensive, costiy feed-solution pretreatment is used to remove ah. particulates. These fibers caimot be used in ultrafiltration appHcations at ah. 

The success of a reverse osmosis process hinges direcdy on the pretreatment of the feed stream. If typical process streams, without pretreatment to remove partially some of the constituents Hsted, were contacted with membranes, membrane life and performance would be unacceptable. There is no single pretreatment for all types of foulants. Pretreatment methods range from pH control, adsorption (qv), to filtration (qv), depending on the chemistry of the particular foulant. Some of the pretreatment methods for each type of foulant are as foUow (43—45) ... 

Reverse Osmosis and Ultrafiltration. Reverse osmosis (qv) (or hyperfiltration) and ultrafilttation (qv) ate pressure driven membrane processes that have become well estabUshed ia pollution control (89—94). There is no sharp distinction between the two both processes remove solutes from solution. Whereas ultrafiltration usually implies the separation of macromolecules from relatively low molecular-weight solvent, reverse osmosis normally refers to the separation of the solute and solvent molecules within the same order of magnitude in molecular weight (95) (see also Membrane technology). 

Nitrate (N03)- Adds to solids content, but is not usually significant industrially useful for control of boiler metal embrittlement Demineralization, distillation, reverse osmosis, electrodialysis... 

Note 1 Older systems may have cation/anion deionizers in place of softeners/reverse osmosis. Note 2 Older systems may use chlorination for microbial control. 

Two other major factors determining module selection are concentration polarisation control and resistance to fouling. Concentration polarisation control is a particularly important issue in liquid separations such as reverse osmosis and ultrafiltration. Hollow-fine-fibre modules are notoriously prone to fouling and concentration polarisation and can be used in reverse osmosis applications only when extensive, costly feed solution pretreatment removes all particulates. These fibres cannot be used in ultrafiltration applications at all. 

Amjad, Zahid Zibrida, John F. Zuhl, Robert W. Silica Control Technology for Reverse Osmosis Systems. Ultrapure Water, Tall Oaks Publishing, Inc., USA, February 1999. 

Pet foods Ruminant feeds Feedback control, purpose of, 20 666 Feedback controllers, 20 666-667 tuning and stability of, 20 694 Feedback control systems, 20 691-695 Feedback loops, between science and technology, 21 615 Feed-back system, closed loop fuel metering system, 10 56 Feed characterization, in reverse osmosis, 21 666... 

RO (reverse osmosis) membranes, 25 890— 891. See also Reverse osmosis (RO) Roadbed stabilization/dust control... 

The thermodynamic approach does not make explicit the effects of concentration at the membrane. A good deal of the analysis of concentration polarisation given for ultrafiltration also applies to reverse osmosis. The control of the boundary layer is just as important. The main effects of concentration polarisation in this case are, however, a reduced value of solvent permeation rate as a result of an increased osmotic pressure at the membrane surface given in equation 8.37, and a decrease in solute rejection given in equation 8.38. In many applications it is usual to pretreat feeds in order to remove colloidal material before reverse osmosis. The components which must then be retained by reverse osmosis have higher diffusion coefficients than those encountered in ultrafiltration. Hence, the polarisation modulus given in equation 8.14 is lower, and the concentration of solutes at the membrane seldom results in the formation of a gel. For the case of turbulent flow the Dittus-Boelter correlation may be used, as was the case for ultrafiltration giving a polarisation modulus of ... 

In temperate climate zones it may be more appropriate to install a nanofiltration process rather than reverse osmosis. Nanofiltration allows the production of drinking water from polluted rivers. As for reverse osmosis, pretreatment is important to control fouling of the membranes. One of the largest such plants produces 140,000 m3/day of water for the North Paris region(26). 

There are four basic methods by which to provide low solids water in the quantity and of the quality required for engineering control of x, (1) reverse osmosis (RO) (2) deionization... 

Agenson KO, Oh J-I, Urase T (2003) Retention of a wide variety of organic pollutants by different nanofiltration/reverse osmosis membranes controlling parameters of process. J Membr Sci 225 91-103... 

With particular reference to reverse osmosis systems involving cellulose acetate membranes and aqueous solutions, the membrane material has both polar and nonpolar character, and the solvent, of course, is polar. When these two components of the reverse osmosis system are kept constant, preferential sorption at the membrane-solution interface, and, in turn, solute separation in reverse osmosis, may be expected to be controlled by the chemical nature of the solute. If the latter can be expressed by appropriate quantitative physicochemical parameters representing polar-, steric-, nonpolar-, and/or ionic-character of the solutes, then one can expect unique correlations to exist between such parameters and reverse osmosis data on solute separations for each membrane. Experimental results confirm that such is indeed the case (18). 

Chlorine is the oldest and most widespread method of water disinfection. In reverse osmosis systems, chlorine may be added to feedwater for control of micro-organisms and, in addition, to prevent membrane fouling by microbiological growth. According to Vos et al. [i,2], chlorine will attack cellulose diacetate membranes at concentrations above 50 ppm. Membranes were found to show a sharp increase in salt permeability and a decrease in strength after one week of continuous exposure. Under milder conditions (10 ppm chlorine for 15 days) no detectable change in performance was observed. Spatz and Friedlander [3] have also found cellulose acetate membranes to be resistant to chlorine when exposed to 1.5 ppm for three weeks. 

Since Loeb and Sourlrajan s discovery (1 ) of a workable asymmetric reverse osmosis membrane, a lot of work has been done in order to elucidate and control the formation, structure and properties of a "skin", the surface layer of an asymmetric membrane. 

The economics of Reverse Osmosis Process will be highly favourable provided the desalination industry is taken up in a big way bringing down the capital investment. Water management and distribution particularly the water supply in the rural areas must be given top priority and should be under the direct control of central and federal government agencies and in this endeavour reverse osmosis has a potential... 

Oftentimes water is further purified using deionization or reverse osmosis to meet the requirements for chemical processing. There must be specifications for the quality of this purified water and periodic testing to demonstrate conformance. The water fed to these operations must also be of potable quality unless the purified water is used for noncontact purposes as discussed above. If the purified water is stored prior to use, it must be stored so as to prevent the build up of microbial organisms. There are many techniques suitable to control microbes such as treatment with ultraviolet light or ozone and circulation. Whatever control method is used, it should be demonstrated that it effectively prevents microbial build-up. 

Now that we have determined what processes the facility will be used for, we can finalize utility requirements. The following utilities are required for our solid-dose facility heating, ventilation, and air conditioning (HVAC), hot and cold water, steam, electrical service, compressed air, vacuum systems, dust collection, chillers, effluent stream, and purified water. For the more specialized processes or special material handling, we may need specialized gases and breathing air. Purified water is one of the more difficult utilities to maintain the quality of. From a source of potable water, a series of treatments must be performed to control microbiological quality. Typical treatment options include carbon filters, reverse osmosis, and UV radiation. 

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