Reverse osmosis solute retention

Retention Rejection and Reflection Retention and rejection are used almost interchangeably. A third term, reflection, includes a measure of solute-solvent coupling, and is the term used in irreversible thermodynamic descriptions of membrane separations. It is important in only a few practical cases. Rejection is the term of trade in reverse osmosis (RO) and NF, and retention is usually used in UF and MF. 

Equations (22-86) and (22-89) are the turbulent- and laminar-flow flux equations for the pressure-independent portion of the ultrafiltra-tion operating curve. They assume complete retention of solute. Appropriate values of diffusivity and kinematic viscosity are rarely known, so an a priori solution of the equations isn t usually possible. Interpolation, extrapolation, even precuction of an operating cui ve may be done from limited data. For turbulent flow over an unfouled membrane of a solution containing no particulates, the exponent on Q is usually 0.8. Fouhng reduces the exponent and particulates can increase the exponent to a value as high as 2. These equations also apply to some cases of reverse osmosis and microfiltration. In the former, the constancy of may not be assumed, and in the latter, D is usually enhanced very significantly by the action of materials not in true solution. 

The relevance of LSC data to reverse osmosis stems from the physicochemical basis (adsorption equilibrium considerations) of liquid-solid chromatography (52), and the principle that the solute-solvent-membrane material (column material) Interactions governing the relative retention times of solutes in LSC are analogous to the interactions prevailing at the membrane-solution Interface under reverse osmosis conditions. The work already reported in several papers on the subject (53-58) indicate that the foregoing principle is valid, and hence LSC data offer an appropriate means of characterizing interfacial properties of membrane materials, and understanding solute separations in reverse osmosis. 

The temperature of the water used to precipitate the casting solution is important this temperature is controlled in commercial membrane plants. Generally low-temperature precipitation produces lower flux, more retentive membranes. For this reason chilled water is frequently used to prepare cellulose acetate reverse osmosis membranes. 

Microporous membranes can be used in a manner similar to reverse osmosis to selectively allow small solute molecules and/or solvents to pass through the membrane and to prevent large dissolved molecules and suspended solids from passing through. Microfiltration refers to the retention of molecules typically in the size range from 0.05 to 10 pm. Ultrafiltration refers to the range from 1 to 100 nm. To retain even smaller molecules, reverse osmosis, sometimes called hyperfiltration, can be used down to less than 2 nm. 

Whereas the nature of membrane retention of particles in UF is molecular screening, the nature of membrane retention in MF is that of molecular-aggregate screening. On the other hand, comparing RO and UF, RO presents a diffusive transport barrier. Diffusive transport refers to the diffusion of solute across the membrane. Due to the nature of its membrane, RO creates a barrier to this diffusion. Figures 8.2 through 8.4 present example installations of reverse osmosis units. 

Membranes for Reverse Osmosis. The first commercially successful membrane was the anisotropic or asymmetric structure invented by Loeb and Sourirajan (1960 cited by Sourirajan, 1970). It is made of cellulose acetate and consists of a dense layer 0.2-0.5 jjim diameter. The thin film has the desired solute retention property while offering little resistance to flow, and the porous substructure offers little resistance to flow but provides support for the skin. The characteristics of available membranes for reverse osmosis and ultrafiltration are listed in Tables 19.2 through 19.4. 

Plants are cleaned, sanitized, and rinsed immediately after processing, and right before processing to ensure satisfactory initial process conditions from microbiological standpoint [3]. Because chlorine is freely permeable to most membranes that it is able to sanitize the permeate side of the system as well as the retentate side, using solutions of sodium hypochlorite containing 100-200 ppm of active chlorine is a common sanitation technique for many membranes, except cellulose acetate reverse osmosis membranes, which can only tolerate brief exposure to chlorine at 10-50 ppm level [3]. 

The influence of different cross-linking reagents on the properties of the membranes was Investigated by reverse osmosis experiments. A procedure for preparing the membranes was devised that yielded membranes of medium retention of phenol against an aqueous phenol solution of 2 g/litre at pH 13. 

In biotechnology, the products concerned are removed from aqueous solution by extraction with methylacetate, butylacetate, isobutyl methyl ketone etc. The remaining aqueous substrate is saturated with the extraction solvents. Sometimes this causes problems with regard to environmental regulations. Table V shows that the solvents can be remored almost entirely by reverse osmosis. The concentrate consists of two phases, namely, the solvent saturated with water and the water saturated with solvent. These can be separated by means of a settler. The water phase is recirculated to the reverse osmosis. The saturated solubility in Water at room temperature is 19 OOO mg/litre for isobutyl methyl ketone, 3300 mg/litre for butyl acetate and 9 500 mg/litre for methyl acetate. As the results in table V show, the retention for isobutyl-methyl ketone increases with increasing concentration. This result is remarkable, as generally a decrease in retention is observed with increasing concentration. 

This extends the previous work (I ) In which the Lennard-Jones type surface potential function and the frictional function representing the Interfaclal forces working on the solute molecule from the membrane pore wall were combined with solute and solvent transport through a pore to calculate data on membrane performance such as those on solute separation and the ratio of product rate to pure water permeation rate in reverse osmosis. In the previous work (1 ) parameters Involved in the Lennard-Jones type and frictional functions were determined by a trial and error method so that the solutions in terms of solute separation and (product rate/pure water permeation rate) ratio fit the experimental data. In this paper the potential function is generated by using the experimental high performance liquid chromatography (HPLC) data in which the retention time represents the adsorption and desorption equilibrium of the solute at the solvent-polymer interface. 

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. 

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