Membranes flat-sheet preparation

Superiority of PI membranes subjected to PV over other polymeric membranes has been highlighted by Jiang et al. (2009). Membranes prepared from soluble Pis derived from 4,4 -diamino-3,3 -dimethyldiphenylmethane was used as a PV membrane for EtOH-water mixture (Wang et al.2006). The average value of the separation factor and total permeation flux were 46-108 and 660-1380 g/m h, respectively. Yanagishita et al. (1995) used asymmetric PI (aromatic) membranes (flat sheet) prepared by a phase inversion process, for the separation of alcohol solution by PV. The membrane exhibited separation factors of afHjO/EtOH) = 900 and a(H20/2-Pr0H) = 11,000 with a flux of 0.45 kg/m h for 95 vol% alcohol aqueous solution at 60°C. The membrane showed stable performance for 3 months. 

Kulprathipanja and coworkers reported the preparation of integrally skinned siUcaUte-1/cellulose acetate flat sheet asymmetric mixed-matrix membranes via phase inversion technique in 1992 [73]. The O2/N2 separation performance of these membranes was investigated. It was demonstrated that the separation factor of... 

Polymer precipitahon by cooling to produce microporous membranes was hrst developed and commercialized by Akzo [33,37], which continues to market microhltration polypropylene and poly(vinylidene fluoride) membranes produced by this technique under the trade name Accurel . Flat sheet and hollow fiber membranes are made. Polypropylene membranes are prepared from a solution of polypropylene in N, A-bis(2-liydroxyethyl)tal lowamine. The amine... 

The first reported zeolite-based membranes were composed of zeolite-filled polymers [3-9]. The incorporation of zeolite crystals into these polymers resulted in a change of both permeation behavior and selectivity, due to the alteration of the affinity of the membrane for the components studied. Up to now, most known inorganic, zeolitic membranes have consisted of supported or unsupported ZSM-5 or silicalite [10-27]. Other reported membranes are prepared from zeolite-X [21], zeolite-A [21,28], or AIPO4-5 [29]. The materials used as support arc metals, glass, or alumina. The membrane configurations employed are flat sheet modules and annular tubes. 

Polymeric flat sheet membranes are easy to prepare, handle, and mount. For gas separation, the flat sheet membranes are composites with a selective polymer coated on a support. A commercial configuration that has been quite successful for hydrocarbon vapor recovery is the Borsig envelope type module (see Figure 4.19) [107]. Packing densities for flat sheet membranes may be in the range of 100 00 m /m [1]. 

Classically, flat-sheet porous PTFE or polypropylene membranes are used as support for the membrane liquid and mounted in holders (cells, contactors) permitting one flow channel on each side of the membrane [1,3,6,8,25]. See Figure 12.1. Such membrane units are typically operated in flow systems and in principle apphcable to aU versions of membrane extraction for analytical sample preparation or sampling. Such a setup can be easily interfaced with different analytical instmments, such as HPLC and various spectrometric instmments, and thereby provides good possibdities for automated operation. Drawbacks of this type of devices are relatively large costs and limited availability, as well as some carryover and memory problems as the membrane units are utilized many times, necessitating cleaning between each extraction. 

The term membrane element refers to the basic form in which a membrane is prepared. There are three types of membrane elements flat sheets, hollow hbers, and tubular membranes. The device within which the membrane element is housed is referred to as the membrane module. The design of the membrane module largely depends on the type of membrane element, as well as on additional requirements such as the need for cleaning and disassembling, the required transmembrane pressure (TMP), and the required hydrodynamic conditions. Some of the different modules types are (see Figures 18.3 through 18.7) ... 

However, ELMs are quite difficult to prepare and after transport, the oil droplets have to be separated and broken up to recover the receiving phase. Compared to the ELM, the BLMs are easier to operate. The supported liquid membranes (SLM) are categorized into two types of supports, namely, a flat-sheet supported liquid membrane (FSSLM) or a hollow fiber supported liquid membrane (HFSLM). Here a polymeric filter with its pores filled with the organic phase acts as membrane. The three different types of liquid membranes have already been schematically represented in Chapter 29. A schematic representation of a hollow fiber semp is shown in Figure 31.2. 

When discussing membrane preparation, not only must the physical structure be considered, but one must also consider the membrane form or shape. In an effort to combat concentration polarization and membrane fouling and to maximize the membrane surface area per unit module volume, membranes are produced in the form of flat sheets (used either In plate-and-frame or spiral wound modules), supported and unsupported tubes, and hollou fibers. Although much of the technology associated with membrane development and membrane production Is closely guarded as proprietary Information, some of the details are beginning to appear in the literature (6,9-13,16-20). 

Shape Bead, flat sheet or hollow fiber membrane, amorphous aggregate Crystal Ease of filtration, Control of diffusion path length and flow properties Simple preparation... 

While the previously described three membrane modules required flat sheet membrane material for their preparation, special membrane configurations are needed for the preparation of the tubular, capillary, and hollow fiber modules. The tubular membrane module consists of membrane tubes placed into porous stainless steel or fiber glass reinforced plastic pipes. The pressurized feed solution flows down the tube bore and the permeate is collected on the outer side of the porous support pipe, as indicated in Figure 1.33 (d). The diameters of tubular membranes are typically between 1-2.5 cm. In some modules, the membranes are cast directly on the porous pipes and in others they are prepared separately as tubes and then installed into the support pipes. 

Figure 1.35 Schematic diagram indicating the function of a casting machine used for the preparation of supported flat sheet membranes.

Asymmetric membranes are made from solution in the form of a hollow fiber, but the process used to form a thin, pore free dense layer on these hollow fibers is not disclosed.45 46 However, US patent 4,440,64312 describes a unique process for producing pore-free polyimide composite membranes. An asymmetric polyimide porous substrate is prepared from solution. When fully imi-dized, the substrate is insoluble. The substrate can now be coated with a poly-amic acid from dilute solution (— 1 %). When fully imidized, the resultant polyimide coating becomes the separating layer. This process allows use of the same or different polyimides for the substrate and the separating membrane. While the examples in the reference describe preparation of flat sheet membranes, this process could be used to prepare hollow fiber membranes. 

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