Equipment Membranes

R. G. Semerad, "Sanitary Considerations Involved with Membrane Equipment," Proceedings of the Whej Product Conference Adantic City, N.J., 1976. 

FIG. 22-55 Typical capital-cost schematic for membrane equipment showing trade-off for membrane area and mechanical equipment. Lines shown are from families for parallel hues showing hmiting costs for membrane and for ancillary equipment. Abscissa Relative membrane area installed in a typical membrane process. Minimum capital cost is at 1.0. Ordinate Relative cost. Line with positive slope is total membrane cost. Line with negative slope is total ancillary equipment cost. Curve is total capital cost. Minimum cost is at 1.0. 

Because membrane equipment, capital costs, and operating costs increase with the membrane area required, it is highly desirable to maximize membrane flux. 

Gas separation through membranes achieved commercialization after the introduction of the Prism process by Monsanto a decade ago. Originated for hydrogen recovery, high area membrane equipment is now used for other gases, notably C02 [1]. Hydrogen, carbon dioxide, and other components are now being removed from mixtures on an industrial scale [2, 3],... 

Membrane equipment for industrial scale operation of microfiltration, ultrafiltration and reverse osmosis is supplied in the form of modules. The area of membrane contained in these basic modules is in the range 1-20 m2. The modules may be connected together in series or in parallel to form a plant of the required performance. The four most common types of membrane modules are tubular, flat sheet, spiral wound and hollow fibre, as shown in Figures 8.9-8.12. 

World capacity of commercial electric membrane equipment, as detailed above, currently stands at a little under 600,000 gallons per day. New orders received or in prospect lead to the prediction that world capacity of commercial electric membrane plants will be at least doubled by early 1961, reaching 1,000,000 to 1,500,000 gallons per day or more. 

There are a number of critical variables in the design of successful electric membrane equipment. Among these are the properties and cost of the membranes, the design and cost of the spacers, the size, cost, and construction of the alternating membrane-spacer-assembly—called the membrane stack—and the current density at which the unit is operated. 

The commercial availability of two-stage continuous units with high rates of demineralization per square foot of membrane substantially decreases the cost of demineralization for supplies in the neighborhood of a few thousand to a few tens of thousands of gallons per day. It is expected that the increased simplicity, economy, and reliability of this equipment will considerably broaden the use of electric membrane equipment in a number of fields and contribute to continued technical advancement in the field. 

My introduction to membranes was as a graduate student in 1963. At that time membrane permeation was a sub-study of materials science. What is now called membrane technology did not exist, nor did any large industrial applications of membranes. Since then, sales of membranes and membrane equipment have increased more than 100-fold and several tens of millions of square meters of membrane are produced each year—a membrane industry has been created. 

The development and application of membrane separation processes is one of the most significant advances in chemical and biological process engineering in recent years. Membrane processes are advanced filtration processes which utilise the separation properties of finely porous polymeric or inorganic films [1,2]. Membrane separations are used in a wide range of industrial processes to separate biological macromolecules, colloids, ions, solvents and gases. They also have important medical uses, especially in renal dialysis. The world-wide annual sales of membranes and membrane equipment are worth in excess of 1 billion. 

Krack, R., Chemical cleaning agents and costs in cleaning and disinfection of membrane equipment, in Fouling and Cleaning in Pressure Driven Membrane Processes, Special Issue 9504, International Dairy Federation, FIL-IDF, Brussels, 1995, p. 151. 

A variety of reverse osmosis membrane systems based on cellulose acetate, aromatic polyamides, and other polymers have been tested for their potential applications. Reverse osmosis membrane equipment is available for large-scale operation since the process is widely used for the production of potable water from sea or brackish waters and upstream of ion exchange in the preparation of ultrapure water for steam-generating boilers. In these applications, the feed concentrations may vary from 500 to 40,000 mg/L of dissolved solids. The RO technique can be used at pH values between 3 and 12 and up to 45°C. 

The field of reaction enhancement is more complex and developments more difficult to predict. The ability of ceramic membranes to run at higher temperatures greatly increases the number of reactions, which can be the considered as enhancement candidates. These reactors can be further enhanced, for example, by using the ceramic tube to support a catalyst as well as a membrane. Equipment of this type is already under development. 

Most of ceramic membrane equipment suppliers have entered the beer and wine filtration market with membrane products adapted to the specificity of these products [76-79]. The pore diameter of available membranes ranges between 0.1 and 1.5 pm, but the choice of pore size cannot be systematically categorized for each family of products, in particular for wines. In fact, filtration requirements are not the same for red wines and white wines. Even, in each of these categories the quality properties of the final product are not exactly predictable and render the choice of the membrane... 

The membranes and module sales in 1998 were estimated at more than US 4.4 billion worldwide [1], shared by different applications (Fig. 1.1). If equipment and total membrane systems are also considered, the estimate would be double. At least 40% of the market is in the United States [2, 3], 29% of the market is shared by Europe and the Middle East. The markets in Asia and South America are growing fast A more recently published study [3] estimates the combined market for membranes used in separation and nonseparation applications to be worth 5 billion only in the US, with an aimual growth rate of 6.6%. According to another recent study [4], the demand for pure water will drive the market for crossflow membrane equipment and membranes worldwide from 6.8 billion in 2005 to 9 biUion in 2008. 

HWS is commonly used to kill microorganisms. RO membranes reject bacteria and other microbes. However, due to defects in the membrane surface, the rejection is not 100% or absolute so that microorganisms can pass through the membrane into the permeate side of the membrane where they can multiply. Hence, sanitisation of a membrane system is essential for the production of US Pharmacopeia (USP) water and water for injection to meet the very low bacterial limits [57-59]. HWS of RO membrane (and EDI) systems is a relatively recent development made possible by the development of TFC membranes and membrane equipment capable of handling hot water at 85° C for a short time membrane elements are manufactured with special adhesives, permeate tubes and connectors to withstand elevated temperatures. These membranes also make it possible to sanitise and protect the RO membranes from bio-fouling especially when the use ofbiocides is not acceptable. Typical membrane manufacturers specifications of hot water sanitisable TFC membranes are given in Table 2.14. 

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