Membranes integrally skinned asymmetric

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

One setback for the production of thin-film composite membranes and integrally skinned asymmetric membranes with separating layer thickness of less than 0.2 pm is the defects. A thin coating of a highly permeable polymer can help eliminate the defects. Surface coatings are also applicable in improving the fouling resistance of membranes for UF or NF applications (44). 

An integrally skinned asymmetric membrane with a porous skin layer (hereafter called substrate membrane) is prepared from a polymer solution by applying the dry-wet phase inversion method and dried according to the method described later, before being dipped into a bath containing a dilute solution of another polymer. When the membrane is taken out of the bath, a thin layer of coating solution is deposited on top of the substrate membrane. The solvent is then removed by evaporation, leaving a thin layer of the latter polymer on top of the substrate membrane. 

Integrally skinned asymmetric asymmetric membranes used for gas and liquid separations consist of a thin skin layer supported by a porous substructure. The skin layer determines the permeability and selectivity of the membrane, whereas the porous substructure functions primarily as a physical support for the skin. Both layers are composed of the same material and are integrally bonded. The skin layer usually has a thickness on the order of several hundred to several thousand angstroms. 

H.-L. Wang, J. Gao, J.-M. Sansinena, and P. McCarthy, Fabrication and characterization of polyaniline monolithic actuators based on a novel configuration integrally skinned asymmetric membrane, Chem Mater., 14 (6), 2546-2552, (2002). 

Integrally Skinned Asymmetric Membrane and Thin-Film-Composite Membrane.36... 

The membrane can be a solid, a liquid, or a gel, and the bulk phases can be liquid, gas, or vapor. Membranes can be classified according to their structures. Homogeneous or symmetric membranes each have a structure that is the same across the thickness of the membrane. These membranes can be porous or have a rather dense uniform structure. Heterogeneous or asymmetric membranes can be categorized into three basic structures (1) integrally skinned asymmetric membrane with a porous skin layer, (2) integrally skinned asymmetric membrane with a dense skin layer, and (3) thin film composite membranes [13]. Porous asymmetric membranes are made by the phase inversion process [14,15] and are applied in dialysis, ultrafiltration, and microfiltration, whereas integrally skinned asymmetric membranes with a dense skin layer are applied in reverse osmosis and gas separation applications. 

Integrally skinned asymmetric reverse osmosis and nanofiltration membranes also feature small roughness parameters, ranging from 0.84 to 5.14 nm. 

Fig. 6.2. FE-SEM image of integrally skinned asymmetric polysulfone membrane dried under T2E (ethanol). Reprinted from [2], Copyright 1999, with kind permission from the American Chemical Society...

Norris, I.D., A.G. Fadeev, J. Pellegrino, and B.R. Mattes. 2005. Development of integrally skinned asymmetric polyanUine hollow fibers for membrane applications. Synth Met 153 57. 

Kim, I., H. Yoon, K. M. Lee, Formation of Integrally Skinned Asymmetric Poly-etherimide Nanofiltration Membranes by Phase Inversion Process. Journal of applied polymer science, 2002, 84(6), 1300-1307. 

Solvent-resistant NF (SRNF) represents a fairly new and interesting application of NF in different industrial fields (e.g., food, chemical, and pharmaceutical) for purification, recovery, or recycling of oligomers, catalysts, and solvents. This process requires membranes to be able to withstand aggressive environments, with high chemical resistance, coupled with desired permeability and selectivity. Not only TFC but also integrally skinned asymmetric membranes can be used for SRNF. The most widely used polymers for the preparation of SRNF membranes are polyimide, PAN, polyelectrolyte complex membranes (PECMs), and polydimethylsiloxane (PDMS). 

As pointed out by Nunes and Peinemann [108], inorganic membranes are usually preferred because many processes at the industrial level are carried out at high temperature. However, polymeric membranes can be used for H2/hydrocarbon separation in the platformer off gases from refineries and for CO2 separation in coal plants. Polymeric manbranes for GS can be symmetric or asymmetric, but should have a dense selective layer. Three types of membrane structures can be employed (1) homogeneous dense manbranes (symmetric) (2) integrally skinned asymmetric membranes and (3) composite membranes. 

Brunetti et al. [107] produced integrally skinned asymmetric membranes from PEEK-WC and studied the influence of different preparation parameters, such as the composition and temperature of the coagulation bath and casting knife gap set, on the membrane morphology and transport properties, permeance, and selectivity. Membranes were prepared by immersion precipitation, using THE as solvent. Three different coagulation baths were tested a mixture of methanol/water 70/30, pure methanol (MeOH), or pure IPA. 

It should be however kept in mind that the membrane preparation procedure could influence its structure and gas transport properties. Thus, casting of the integrally skinned asymmetric membranes from p-DMePO solution using different nonsolvent additives produced the nodule structures in the surface skin layer of the membranes, which affected the permeance ratios for O2/N2 and CO2/CH4 [72]. In the homogeneous films of polyphenylene oxides considered in this chapter such structures apparently do not exist. 

This chapter will focus mainly on the development of chemically modified poly (2, 6 - dimethyl - 1, 4 - phenylene oxide) (PPO) membranes for gas separation applications. Gas separation characteristics of dense films from PPO and modified PPO will be described. Formation of integrally skinned asymmetric membranes as well as thin film composite (TFC) membranes from PPO and modified PPO along with gas transport through these membranes will also be discussed. A brief discussion on the effect of the molecular weight of PPO on the mechanical and gas transport properties of the membranes prepared from PPO and modified PPO will also be included. 

Fluxes through integrally skinned asymmetric membranes and TFC membranes are expressed as permeances that are reported in gas permeation units, GPU, where ... 

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