Spiral-wound asymmetric

Spiral-wound asymmetric polyethersulfone membrane (molecular weight cutoff of 3 kDa)... 

Commercial laccase was immobilized onto a spiral-wound asymmetric polyethersulfone membrane. The laccase membrane reactor was applied to the biodegradation of a model phenol solution. The feasibility of using a hollow-fiber membrane reactor for the Upase-catalyzed interesterification reaction of triglycerides and fatty acids in a micro aqueous n-hexane system was developed by Basheer et al. In this case they use a stirred-tank reactor as well as a hollow-fiber membrane reactor system. Moreover, in 2004 a new immobilization of lipase into microporous... 

Reverse Osmosis. This was the first membrane-based separation process to be commercialized on a significant scale. The breakthrough discovery that made reverse osmosis (qv) possible was the development of the Loeb-Sourirajan asymmetric cellulose acetate membrane. This membrane made desalination by reverse osmosis practical within a few years commercial plants were installed. The total worldwide market for reverse osmosis membrane modules is about 200 million /yr, spHt approximately between 25% hoUow-ftber and 75% spiral-wound modules. The general trend of the industry is toward spiral-wound modules for this appHcation, and the market share of the hoUow-ftber products is gradually falling (72). 

The geometries for asymmetric mixed-matrix membranes include flat sheets, hollow fibers and thin-fihn composites. The flat sheet asymmetric mixed-matrix membranes are formed into spirally wound modules and the hollow fiber asymmetric mixed-matrix membranes are formed into hollow fiber modules. The thin-film composite mixed-matrix membranes can be fabricated into either spirally wound or hollow fiber modules. The thin-film composite geometry of mixed-matrix membranes enables selection of different membrane materials for the support layer and low-cost production of asymmetric mixed-matrix membranes utilizing a relatively high-cost zeolite/polymer separating layer on the support layer. 

In 1968 we started investigations of RO applications for desalting brackish water. In the course of the investigations, we have found the spirally wound module of asymmetric cellulose acetate RO membrane shows excellent durabilities against fouling materials and free chlorine. 

Hollow-tiber membranes are subjected lo extensile studies lor gaseous separation (e.g.. CO-. 11-. CL. Ny. 1LS. CO. CH4). where the capillary configuration has an advantage over the spiral-wound fiat Hint and plate und-lramc devices. Another significant area of development and commercialization is pervaporation. These membranes are dense, rather than porous. structures. Generally asymmetric composite constructions arc employed with the ulirathin membranes on an open support. 

Salts rejected by the membrane stay in the concentrating stream but are continuously disposed from the membrane module by fresh feed to maintain the separation. Continuous removal of the permeate product enables the production of freshwater. RO membrane-building materials are usually polymers, such as cellulose acetates, polyamides or polyimides. The membranes are semipermeable, made of thin 30-200 nanometer thick layers adhering to a thicker porous support layer. Several types exist, such as symmetric, asymmetric, and thin-film composite membranes, depending on the membrane structure. They are usually built as envelopes made of pairs of long sheets separated by spacers, and are spirally wound around the product tube. In some cases, tubular, capillary, and even hollow-fiber membranes are used. 

The spiral wound membranes tested for extraction of impurity-free NaSCN from aqueous process solution were polyamide (PA-300), CTA-700, PERMA-400, and PERMA-250. PA-300 was prepared by interfacial polymerization technique, while the PERMA membranes were prepared by coating a novel proprietary copolymer onto a microporous polysulfone substrate followed by cross-linking of the top layer. Thus, the morphology of these membranes was TFC. CTA-700 was asymmetric in nature and was prepared by solution casting and phase inversion method. 

Dorr-Oliver began to search for other polymers suitable for casting asymmetric UF membranes. By 1965, the first laboratory-scale UF membranes and cells appeared on the market. The ten-year period between 1965 and 1975 was a period of intense development where chemically and thermally resistant membranes were made from polymers like polysulfone (PS) and even polyvinylidene difluoride (PVDF) in molecular weight cut-offs (MWCO) from 500 to 1,000,000. Hollow fibers were also developed during this decade and a whole host of module configurations. Tubes, plate and frame units, and spiral-wound modules became available. 

Figure 6.3927 shows a two-stage cascade for the purification and dehydration of sour gases, mainly removing C02 and H2S. Again, spiral wound modules with asymmetric cellulose acetate membranes are employed. It should be noted, that in this case as in all other cases discussed here, no compressors had to be installed. This is the main reason why these applications show excellent payback times. 

Most commercial membrane separations use natural or synthetic, glassy or rubbery polymers. To achieve high permeability and selectivity, nonporous materials are preferred, with thicknesses ranging from 0.1 to 1.0 micron, either as a surface layer or film onto or as part of much thicker asymmetric or composite membrane materials, which are fabricated primarily into spiral-wound and hollow-fiber-type modules to achieve a high ratio of membrane surface area to module volume. 

PV combines the evaporation of volatile components with their permeation through a membrane. It involves the use of a Hquid feed to produce a vapour permeate and a hquid reject. The feed is usually suppHed at above 100°C and at a pressure sHghdy above 1 bar. The permeate pressure is maintained at vacuum well below atmospheric. Vapourisation occurs as the permeating species pass through the membranes. Mosdy asymmetric composite hydrophihc membranes such as composite poly vinyl alcohol (PVA)/PS (or polyacrylonitrile) are used [43,44]. Membranes made from a thin layer of PDMS cast on PAN have been found to be usefiil in separating polar compounds from non-polar ones in deal-coholisation of Hquors. Flat sheet and spiral wound modules are commonly used. 

Figure 7.2.1. (a) Crossflow membrane module for gas permeation (b) model for crossflow gas permeation in an asymmetric or composite membrane (c) tube-side feed crossflow hollow fiber module (d) shell-side feed hollow fiber module (e) spiral-wound module. 

The majority of gas separation membranes currently used in industry arc polymeric. Polymer membranes arc easily processed into a variety of forms such as asymmetric spiral-wound or hollow fibers while maintaining their separation efficiency however, the innate separation performance of solution-processable polymers is limited. Robeson (1991) discussed the separation performance of a variety of polymeric membranes for the O2/N2 and CO2/CH4 separations and formulated the semiempirical upper-bound trade-off line between the permeability and selectivity for solution-processable polymers see Figure 23.1. Bums and Kotos (2003) have shown similar results for the CaHe/CaHg separation. Freeman (1999) has also addressed the limitations of polymers in terms of the trade-off line. The logical extension of these earlier analyses is that new membrane materials that exceed this trade-off limit must be developed. 

Diffusion across thin membranes can sometimes produce chemical and physical separations at low cost. These low costs have spurred rapid development of membrane separations, especially during the last 20 years. This rapid development has sought both high fluxes and high selectivities. It has included the separation of gases, of sea water, and of azeotropic mixtures. It has used hollow fibers and spiral wound modules it has centered on asymmetric membranes with selective layers as thin as 10 nm. This rapid development is a sharp contrast to other diffusion-based separations like absorption, where the basic ideas have been well established for 50 years. 

In this section, we want to describe the fundamentals of diffusion across membranes and the actual physical construction of the membrane. We will extend these basic ideas to specific types of separations in latter sections. The fundamentals of diffusion across membranes include the effects of partition coefficients, concentration units, and resistances in series. The physical construction of membranes includes both the membranes themselves and the modules in which the membranes are used. The membranes themselves may be symmetric or asymmetric the modules include hollow fibers, spiral-wound elements, and plate-and-frame assemblies. 

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