Membrane unit operations

Various membrane operations are available today for a wide spectrum of industrial applications. Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), gas and vapor separation (GS, VS), pervaporation (PV), dialysis (D), electrodialysis (ED) and membrane contactors (MCs) are only some of the best-known membrane unit operations. 

Catalytic reactions can be combined in membrane-assisted integrated catalytic processes with practically all the membrane unit operations available today. Many examples of integration of membrane contactors, pervaporation, gas separation, nanofiltration, microfiltration, and ultrafiltration operations together with catalytic reactions, have been proposed in the literature. 

Membrane unit operations are today largely used in many different applications for their higher efficiency in comparison with traditional separation systems. Moreover the integration of different membrane operations in the same unit, or in combination with conventional ones, offers important benefits in terms of product quality, plant compactness, environmental impact, and energetic aspects. 

Davis, R.A. (2002) Simple gas permeation and pervaporation membrane unit operation models for process simulators. Chemical Engineering Technology, 25 (7), 717-722. 

Block diagram of a plant redesigned with the integration of some membrane unit operations. Reprinted from P. Bernardo, G. Barbieri, E. Drioli, An exergetic analysis of membrane unit operations integrated in the ethylene production cycle, Chem. Eng. Res. Des, 2006,84,405 11, with permission of Elsevier. " ... 

The goal of the demonstration project was to study the effectiveness of the formed-in-place hyperfiltration membrane unit, operating in a cross-flow mode, for the concentration of PAHs (and PCP) from contaminated groundwater found at a wood treating site. The original plans called for a companion study of a proprietary biodegradation process for the concentrate however, a decision was made during this study not to carry out that portion of the study. 

In this chapter the possibility of integrating different membrane unit operations in the same industrial cycle or in combination with conventional separation systems is analysed and discussed. Many original solutions in water desalination, agro-food productions and wastewater treatments are reviewed highlighting the advantages achievable in terms of product quality, compactness, rationalization and optimization of productive cycles, reduction of environmental impact and energy saving. 

Silicones find practical application in different membrane unit operations for treating gaseous and liquid mixtures. This is due to their solubility controlled transport, which allows the selective separation of organics from air or from water. Polymer blending, polymer grafting, addition of different solid fillers or ionic Hquids, are the most effective strategies for improving the stabihty as well as the selective transport of silicones. The industrial applications of silicone-based membrane systems present environmental benefits such as reduced waste and recovered/recycled valuable raw materials that are currently lost to fuel or to the flares. 

This section describes the use of separation processes which utilize membranes. Placement in this chapter is in recognition of the recent ascendency of industrial-scale rnernbrane-based separations, but it also reflects the iew that within a decade, many of these separation processes will be mainstream unit operations. Some approach that status already. Figure 22-46 shows the relath e size of things important in membrane separations. 

Processes and/or unit operations that fall under this classification include adsorption, ion exchange, stripping, chemical oxidation, and membrane separations. All of these are more expensive than biological treatment but are used for removal of pollutants that are not easily removed by biomass. Often these are utilized in series with biologic treatment but sometimes they are used as stand-alone processes. 

Filtration is a fundamental unit operation that, within the context of this volume, separates suspended particle matter from water. Although industrial applications of this operation vary significantly, all filtration equipment operate by passing the solution or suspension through a porous membrane or medium, upon which the solid particles are retained on the medium s surface or within the pores of the medium, while the fluid, referred to as the filtrate, passes through. 

The authors provide selection criteria, by which the suitability of a process for a distributed production can be assessed [139]. These are explicitly given for the categories of feedstock, processes, customer products, and waste products. This is completed by a list of suitable device types for distributed production such as plate heat exchangers, pressure and temperature swing units, electrostatic dispersers, and membrane units. The various operations often rely on the use of electricity and therefore are said to be particularly suited for operation at the mini scale. 

As mentioned earlier, a major cause of high costs in fine chemicals manufacturing is the complexity of the processes. Hence, the key to more economical processes is reduction of the number of unit operations by judicious process integration. This pertains to the successful integration of, for example, chemical and biocatalytic steps, or of reaction steps with (catalyst) separations. A recurring problem in the batch-wise production of fine chemicals is the (perceived) necessity for solvent switches from one reaction step to another or from the reaction to the product separation. Process simplification, e.g. by integration of reaction and separation steps into a single unit operation, will provide obvious economic and environmental benefits. Examples include catalytic distillation, and the use of (catalytic) membranes to facilitate separation of products from catalysts. 

Applications Membranes create a boundary between different bulk gas or hquid mixtures. Different solutes and solvents flow through membranes at different rates. This enables the use of membranes in separation processes. Membrane processes can be operated at moderate temperatures for sensitive components (e.g., food, pharmaceuticals). Membrane processes also tend to have low relative capital and energy costs. Their modular format permits rehable scale-up and operation. This unit operation has seen widespread commercial adoption since the 1960s for component enrichment, depletion, or equilibration. Estimates of annual membrane module sales in 2005 are shown in Table 20-16. Applications of membranes for diagnostic and bench-scale use are not included. Natural biological systems widely employ membranes to isolate cells, organs, and nuclei. 

Membrane Ultrafiltration Membrane ultrafiltration is often one of the favored unit operations used for the isolation and concentration of biomolecules because they can be easily scaled up to process large feed volumes at low costs. Toward the end of an ultrafiltration operation, additional water or buffer is added to facilitate the passage of... 

The unit operation demonstrates that membrane life over two years has been demonstrated and that the selected materials of construction are correct. The recovery of sulphate-lean brine is 85-90% during normal operation. In a trial run under extreme conditions, with the unit modified to operate in recycle mode, the concentration of sodium sulphate in the reject stream was increased to 190g l-1 a 90% sulphate rejection rate was achieved during this trial. The sodium chloride concentration decreased on the concentrate side of the membrane and increased in the... 

A membrane reactor offers the possibility of combining two individual processes in the same unit operation. (1) Selective permeation (thus separation) can be coupled directly with the reaction by means of either a catalytically active membrane or of a passive membrane placed next to the... 

While the plant is in operation, but not producing water that is of the required quality, two additional discharge streams can be produced (Mauguin and Corsin 2005). This usually occurs during the startup of the plant. The first stream is the pretreated water which has not reached an acceptable quality to pass through the membrane unit. Its composition is largely similar to that of the feed, and if membrane processes are not used as pretreatment, its salinity should be the same as feed. The second stream is the permeate which has not yet reached the desired quality of the final water product. This stream will have a lower salinity than the feed, and should not contain great quantities of harmful chemicals. Both these streams should be safe for disposal in the concentrate stream. 

The preparation of food for consumption, as well as manufacturing of various products from different raw materials, usually involves the application of several discrete unit operations and processes. Many operations, such as washing, trimming, milling, leaching, disintegrating, mechanical separation, and use of membrane techniques, may decrease the natural toxicity of some raw materials by eliminating specific undesirable components. Examples include the removal of most of the fluorine compounds from Antarctic krill... 

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