Reverse osmosis problems

For small distances from the channel entrance x/h—>0), the concentration layer is developing, and, as with the reverse osmosis problem, the solution is self-similar. We therefore choose the same similarity variable as defined by Eq. (4.4.18), namely. 

The analysis for the ion concentration distribution now follows exactly as in Section 4.4. A function f T]) is defined as in Eq. (4.4.17), and the resulting ordinary differential equation is solved exactly as for the reverse osmosis problem. The result for the wall concentration from Eq. (4.4.28) is... 

Eq. (7.37) with the conditions (7.35), (7.39), and (7.40) is the same as in the problem of concentration profile development in reverse osmosis (see Section 6.4). A comparison with this problem shows that the condition (7.39) takes place in the reverse osmosis problem if we replace i/2z+F by VwC . The density of current is i = Fz+j+, where j+ is the flux of ions. It is now possible to use the solution derived in Section 6.4. At small distances from the entrance (x/h 0), diffusion layer thickness is insignificant, and it is possible to introduce the selfsimilar coordinate (6.76)... 

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. 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

In most industrial applications, it is rare that a single RO module can be used to address the separation task. Instead, a reverse-osmosis network (RON) is employed. A RON is composed of multiple RO modules, pumps and turbines, llie following sections describe the problem of synthesizing a system of RO modules and a systematic procedure for designing an optimal RON. Once a RON is synthesized, it can be incorporated with a mass integration framework (see Problem 11.6). 

Electro-osmosis has been defined in the literature in many indirect ways, but the simplest definition comes from the Oxford English Dictionary, which defines it as the effect of an external electric held on a system undergoing osmosis or reverse osmosis. Electro-osmosis is not a well-understood phenomenon, and this especially apphes to polar non-ionic solutions. Recent hterature and many standard text and reference books present a rather confused picture, and some imply directly or indirectly that it cannot take place in uniform electric fields [31-35]. This assumption is perhaps based on the fact that the interaction of an external electric held on a polar molecule can produce only a net torque, but no net force. This therefore appears to be an ideal problem for molecular simulation to address, and we will describe here how molecular simulation has helped to understand this phenomenon [26]. Electro-osmosis has many important applications in both the hfe and physical sciences, including processes as diverse as water desahnation, soil purification, and drug delivery. 

Hyperfiltration (Reverse Osmosis) is a form of membrane distillation or desalination (desalting) operating with membrane pore sizes of perhaps 1 to 10 Angstrom units. The various individual RO component technologies have improved tremendously over the last 20 to 25 years, and resistance to fouling and permeate output rates have benefited. Nevertheless, all RO plants remain susceptible to the risk of fouling, and adequate pretreatment and operation is essential to minimize this problem. 

This method is used mainly to remove high-molecular-weight materials such as proteins, colloids, viruses, and bacteria. The same types of problem encountered with the use of reverse osmosis membranes are encountered here, and the proposed solutions are the same. 

Membrane processes such as ultrahltrahon or reverse osmosis have been proposed as oil removal processes. Laboratory tests have indicated favorable oil removal, although relatively low flux rates, membrane fouling, and membrane life problems have presented concerns for the practical applicahon of membrane processes to oil removal. 

The aim of this study is to investigate the ability of ortho-phosphoric acid to function as the pore-producing additive for the preparation of CA reverse osmosis membranes. Ortho-phosphoric acid (PA) is known to be a strongly hydrogen bonded liquid, and it has been claimed ( ) to be a promising additive for the asymmetric CA membrane formation. To our knowledge, there has not been a thorough study on the problem. 

However, few successful reverse osmosis membranes have been made by these techniques. One problem has been the limited solvent resistance of polysulfone support film. Only water, lower alcohols and aliphatic hydrocarbon solvents can be used as solvents. 

Reverse osmosis procedures concentrate over 90% of the total organic material present in water into an aqueous brine (29). A problem has been the efficient transfer of the organic components to a solvent suitable for the bioassays (30). Another problem is the loss of chemicals having molecular weights below 200-400. 

The first level of treatment, with sand filters and chlorination to remove suspended matters and disinfection of pathogens, may be good enough for the low-cost water. The removal of discoloration and bad smell is accomplished by activated charcoal absorption. Ozone and ultraviolet treatments are much more expensive for the removal of microbes and organic matter, and should be considered only when necessary to solve a technical problem, or to satisfy an advertisement need. Reverse osmosis is the most effective method used to recover clean water from brackish water, and to remove inorganic minerals such as sodium, copper, iron, and zinc. The removal of calcium and magnesium ions can be accomplished by the method of ion exchange with sodium, which would also increase the sodium concentration, and could cause objections. Different levels of treatment require a variety of costs, and can produce different levels of customer satisfaction. 

Filtration can remove fine suspended solids and microorganisms, and microfiltration membranes of cellulose acetate or polyamides are available that have pores 0.1-20 /xm in diameter. Clogging of such fine filters is an ever-present problem, and it is usual to pass the water through a coarser conventional filter first. Ultrafiltration with membranes having pores smaller than 0.1 fim requires application of pressures of a few bars to keep the membrane surface free of deposits, water flows parallel to the membrane surfaces, with only a small fraction passing through the membrane. The membranes typically consist of bundles of hollow cellulose acetate or polyamide fibers set in a plastic matrix. Ultrafiltration bears some resemblance to reverse osmosis technology, described in Section 14.4, with the major difference that reverse osmosis can remove dissolved matter, whereas ultrafiltration cannot. 

It is the rate of separation rather than the efficiency of salt retention that is the primary practical issue in the development of reverse osmosis desalination. In addition to a variety of other factors, the rate of reverse osmotic flow depends on the excess pressure across the membrane. Therefore the problem of rapid flow is tied into the technology of developing membranes capable of withstanding high pressures. The osmotic pressure of sea water at 25 °C is about 25 atm. This means that no reverse osmosis will occur until the applied pressure exceeds this value. This corresponds to a water column about 840-ft high at this temperature. 

Reverse osmosis for concentrating trace organic contaminants in aqueous systems by using cellulose acetate and Film Tec FT-30 commercial membrane systems was evaluated for the recovery of 19 trace organics representing 10 chemical classes. Mass balance analysis required determination of solute rejection, adsorption within the system, and leachates. The rejections with the cellulose acetate membrane ranged from a negative value to 97%, whereas the FT-30 membrane exhibited 46-99% rejection. Adsorption was a major problem some model solutes showed up to 70% losses. These losses can be minimized by the mode of operation in the field. Leachables were not a major problem. 

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