Countercurrent reverse osmosis

Ethanol-Water Separation by Countercurrent Reverse Osmosis... 

Figure 1. Diagram of the Countercurrent Reverse-Osmosis Process...

Figure 2. Comparison of Concentration Profiles Inside a Skinned Reverse-Osmosis Membrane Under Reverse-Osmosis and Countercurrent Reverse-Osmosis Conditions...

Countercurrent Reverse-Osmosis Tests. Flat-sheet 3N8 and TFC-801 membranes were characterized with and without permeate-side recirculation. Other flat-sheet membranes were not tested because of either their low ethanol-water selectivity or their tendency to degrade even in relatively dilute ethanol solutions. 

Table I. Calculated Countercurrent Reverse-Osmosis Performance of 3N8 Membranes at Different Operating Pressures to Enrich a 10-vOlZ Ethanol Feed to Produce a 1000 gal/hr Output of 50-vol2 Ethanol Concentrate...

We have demonstrated experimentally that the high osmotic pressure gradients associated with reverse-osmosis enrichment of ethanol can be reduced by adopting the process design of countercurrent reverse osmosis. However, the use of RO membranes in CCRO is limited at present by their inadequate selectivity and poor stability at high alcohol concentrations. 

Figure 19.5. The Permasep hollow fiber module for reverse osmosis, (a) Cutaway of a DuPont Permasep hollow fiber membrane module for reverse osmosis a unit 1 ft dia and 7 ft active length contains 15-30 million fibers with a surface area of 50,000-80,000 sqft fibers are 25-250 pm outside dia with wall thickness of 5-50pm (DuPont Co.), (b) The countercurrent flow pattern of a Permasep module.

Figure 5.27 Flow scheme showing the use of a reverse osmosis system to control nickel loss from rinse water produced in a countercurrent electroplating rinse tank...

F(=) Elutriation Countercurrent electrophoresis Filtration Ultrafiltration Reverse osmosis Pressure dialysis Zone melting Electrofiltration... 

The ionic concentration to be treated is an overriding consideration governing the cost of plant of a given design and therefore for very high ion concentrations it is foreseeable that membrane pretreatments such as reverse osmosis and electrodialysis will continue to fulfil an important role as might a more widespread revival of continuous countercurrent ion exchange. 

The product water from a reverse osmosis unit will have a low pH and most probably a high concentration of carbon dioxide. The carbon dioxide can be removed and the pH of the product increased by use of a decarbonator. A de-carbonator is a packed column in which product water is introduced at the top while either forced or induced air is introduced at the bottom. The air and water flow countercurrently over and around the column packing. The carbon dioxide is stripped from the water and exits from the decarbonator at the top in the air stream. In a well-designed decarbonator, the carbon dioxide content can be reduced to about 5 mg/C in the water effluent. 

The nickel plating industry is a typical candidate for the use of reverse osmosis in pollution control. Figure 4.17 shows a schematic of this industrial application. The workpiece travels from the plating bath with a concentration of 270,000 mg/E to the rinse tanks. There are three rinse tanks in series and rinse water flows countercurrent to the workpiece. The work piece drags out plating bath to the first rinse tank, first rinse tank solution to the second rinse tank and second rinse tank solution to the third rinse tank. Consequently, the first, second and third rinse tanks have concentrations of 3,000 mg/E, 333 mg/E and 37 mg/) , respectively. 

In the case of reverse osmosis, the relative flow configuration does not affect the performance to any large extent. As already indicated, the situation is quite different for gas permeation. While countercurrent flow improves the separation efficiency of hollow fiber modules in reverse osmosis only slightly, as far... 

The Monsanto hollow-fiber module for gas separation appears to be a compromise between the two preceding reverse osmosis module designs. As shown in Fig. 20.1-4, the hollow fibers are closed at one end with an epoxy plug or similar device, and feed enters at the side at one end. The feed then flows axially along the fibers, and the nonpermeant exits at the opposite end from which the feed enters. Permeant diflusing through the hollow-fiber wall into the bore moves countercurrently to the shell-side flow and ultimately is collected in the chamber where all the open ends of the fibers terminate. 

The cross-flow model for reverse osmosis is similar to that for gas separation by membranes discussed in Section 13.6. Because of the small solute concentration, the permeate side acts as if completely mixed. Hence, even if the module is designed for countercurrent or cocurrent flow, the cross-flow model is valid. This is discussed in detail elsewhere (HI). 

System type (4) Two miscible phases flow countercurrently in two regions of the device separated by a membrane. One of the phases may be generated from one feed phase by the application of pressure energy. Examples include reverse osmosis, ultrafiltration, microflltration, gas permeation, pervaporation. Examples where the other phase is introduced from outside are electrodialysis, dialysis, sweep vapor/ liquid based system. 

In the next step, the filtered fermentation broth is contacted with an extracting solvent in a mbter-setder type of device (Section 6.4.1.2) the solvent extracts the antibiotic from the broth, aiong with many reiated and nonrelated compounds. Countercurrent extracting cascades in the form of centrifugal extractors are employed (Section 8.1.4, Figure 8.1.35) to reduce the contact/residence time. This process is often called product isolation in that the product has been isoiated from the broth however, the solvent extraction process extracts other compounds as well fram the broth. Often, adsorption (Section 7.1.1) as well as membrane processes such as ultrafiltration and reverse osmosis may be used (Sections 7.2.1.3 and 7.2-1.2). Sometimes, such a step results in a significant increase in product concentration. 

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