Reverse osmosis process considerations

To ensure the successful design of a reverse osmosis process, several factors should be considered. These considerations encompass the feed solution, the membrane module, and the use of other processes in the pre- and post-treatment processes. A thorough knowledge of the feed stream and its components is essential to the prevention of membrane damage and product impurities. Once the feed stream is characterized and the process objective is defined, design can be initiated. 

Albany International Research Co. has developed an advanced hollow fiber composite reverse osmosis membrane and module under the name of Quantro II . This composite membrane is comprised of a porous hollow fiber substrate on which has been deposited a rejection barrier capable of fluxes of commercial importance at high rejection of dissolved salts at elevated temperatures. Resistance to active chlorine has been demonstrated. Proprietary processes have been developed for spinning of the fiber, establishment of the rejection barrier and processing of the fiber to prepare modules of commercial size. Prototype modules are currently in field trials against brackish and seawater feed solutions. Applications under consideration for this membrane include brackish and seawater desalination as well as selected industrial concentration processes. 

A very common and useful approach to studying the plasma polymerization process is the careful characterization of the polymer films produced. A specific property of the films is then measured as a function of one or more of the plasma parameters and mechanistic explanations are then derived from such a study. Some of the properties of plasma-polymerized thin films which have been measured include electrical conductivity, tunneling phenomena and photoconductivity, capacitance, optical constants, structure (IR absorption and ESCA), surface tension, free radical density (ESR), surface topography and reverse osmosis characteristics. So far relatively few of these measurements were made with the objective of determining mechanisms of plasma polymerization. The motivation in most instances was a specific application of the thin films. Considerable emphasis on correlations between mass spectroscopy in polymerizing plasmas and ESCA on polymer films with plasma polymerization mechanisms will be given later in this chapter based on recent work done in this laboratory. 

Wild, P. M., G. W. Vickers, and N. Djilali (1997). Fundamental principles and design considerations for the implementation of centrifugal reverse osmosis. Proc. Inst. Mechanical Engineers, Part E J. Process Mechanical Eng. 211, E2, 67-81. MEP, London. 

The separation of suspensions is the selective removal of suspended solids, say, by the ordinary processes of filtration. Application can also made to the separation of colloidal suspensions of minute or microscopic solid particles, and even of emulsions, the suspension of minute immiscible liquid droplets within another liquid phase. A distinguishing feature of ordinary filtration is usually that the discharged liquid phase does not form a continuum on the downflow or reject side of the membrane, or filter, and more or less exists at atmospheric pressure. If otherwise, if a contiunuum is formed, the process is more that of reverse osmosis, also called hyperfiltration. In common use, notably for the upgrading or desalination of salt water or brackish water, reverse osmosis is a subject for special consideration. 

Membrane installations generate secondary wastes that have to be taken into consideration before plant design. Reverse osmosis produces permeate, which can be discharged after radioactivity control, and retentate that can undergo further processing. Usually the retentate is not suitable for solidification and further volume reduction is necessary. 

Reverse osmosis uses semipermeable membranes and high pressure to produce a clean permeate and a retentate solution containing salts and ions, including heavy metals. The technique is effective ifthe retentate solution can be reused in the process. The equipment tends to be expensive, and fouling of the membranes has been a common problem. Considerable research effort is being carried out on membrane processes, however, and they are likely to be more commonly applied in the future. Concentrations of dissolved components are usually about 34,000 ppm or less. 

The considerable energy expenditure in the dehydratation of ethanol, chiefly when it is produced by fermentation in concentrations often less than 10 per cent weight, encourages research into new processes for the separation of the watcr/alcohol mixture. Among those currently being investigated are the extraction of ethanol by C02 in the. supercritical state, solvent extraction, vacuum distillation, distillation with vapor recompression, adsorption on molecular sieves, low-temperature phase separation of mixtures with hydrocarbons, and reverse osmosis. 

Reverse osmosis is a cross-flow membrane separation process which separates a feed stream into a product stream and a reject stream. The recovery of a reverse osmosis plant is defined as a percentage of feedwater that is recovered as product water. As all of the feedwater must be pretreated and pressurized, it is economically prudent to maximize the recovery in order to minimize power consumption and the size of the pretreatment equipment. Since most of the salts remain in the reject stream, the concentration of salts increases in that stream with increased recovery. For instance, at 50% recovery, the salt concentration in the reject is about double that of the feed and at 90% recovery, the salt concentration in the reject is nearly 10 times that of the feed. In cases of sparingly soluble salts, such as calcium sulfate, the solubility limits may be exceeded at a high recovery. This could result in precipitation of the salt on the membrane surface resulting in decreased flux and/or increased salt passage. In addition, an increase in recovery will increase the average salt concentration in the feed/reject stream and this produces a product water with increased salt content. Consequently, the recovery of a reverse osmosis plant is established after careful consideration of the desired product quality, the solubility limits of the feed constituents, feedwater availability and reject disposal requirements. 

The first composite reverse osmosis membrane to be developed and described consisted of an ultrathin film of secondary cellulose acetate deposited onto a porous Loeb-Sourirajan membrane.3 The ultrathin film of cellulose acetate was fabricated by a water surface float-casting technique. This has been described to some extent in the published technical literature,4 5 and in considerable detail in several reports on government-funded research projects.3 6 Figure 5.2 illustrates this process schematically. 

It is understood that the economical success of any membrane process depends primarily on the quality of the membrane, specifically on flux, selectivity and service lifetime. Consideration of only the transport mechanisms in membranes, however, will in general, lead to an overestimation of the specific permeation rates in membrane processes. Formation of a concentration boundary layer in front of the membrane surface or within the porous support structure reduces the permeation rate and, in most cases, the product quality as well. For reverse osmosis. Figure 6.1 shows how a concentration boundary layer (concentration polarization) forms as a result of membrane selectivity. At steady state conditions, the retained components must be transported back into the bulk of the liquid. As laminar flow is present near the membrane surface, this backflow is of diffusive nature, i.e., is based on a concentration gradient. At steady state conditions, the concentration profile is calculated from a mass balance as... 

All six processes are easily operated once they have been started up and are under control. Time required for startup can vary from one hour or less for reverse osmosis units to four hours for multiple-effect evaporators. Ease of startup is not usually a major consideration because most desalination plants operate continuously with few shutdowns. 

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