Nonporous dense membranes

Nonporous Dense Membranes. Nonporous, dense membranes consist of a dense film through which permeants are transported by diffusion under the driving force of a pressure, concentration, or electrical potential gradient. The separation of various components of a solution is related directiy to their relative transport rate within the membrane, which is determined by their diffusivity and solubiUty ia the membrane material. An important property of nonporous, dense membranes is that even permeants of similar size may be separated when their concentration ia the membrane material (ie, their solubiUty) differs significantly. Most gas separation, pervaporation, and reverse osmosis membranes use dense membranes to perform the separation. However, these membranes usually have an asymmetric stmcture to improve the flux. 

In Figure 10.10a, it can be seen that for porous membranes, the partial pressure and concentration profiles vary continuously from the bulk feed to the bulk permeate. This is not the case with nonporous dense membranes, as illustrated in Figure 10.10b. Partial pressure or concentration of the feed liquid just adjacent to the upstream membrane interface is higher than the partial pressure or concentration at the upstream interface. Also, the partial pressure or concentration is higher just downstream of the membrane interface than in the permeate at the interface. The concentrations at the membrane interface and just adjacent to the membrane interface can be related according to an equilibrium partition coefficient KM i. This can be defined as (see Figure 10.10b) ... 

Nonplant cost, 9 527 Nonpoint contamination source, 13 310 Nonpolar adsorbents, 1 674 for gas adsorption, 1 632 Nonpolar solvents, VDC polymer degradation in, 25 717-718 Nonporous dense membranes, 15 799 Nonporous silicone tubing, flow through, 15 722, 723... 

According to Cuperus and Nijhuis [5], the mechanisms in which NF membrane works are not completely clear. Possibly both size exclusion and solution-diffusion mechanisms play a role. This is in agreement with the work of Subramanian et al. [29]. These authors observed that solution-diffusion is the predontinant mechanism of the transport of vegetable oil constituents through nonporous (dense) membranes. The effect of viscosity (temperature) on permeation suggests that transport by convective flow exists in these membranes but the extent observed is not significant. 

Nonporous, Dense Membranes. Nonporous, dense membranes consist of a dense film through which permeants are transported by difftision imder the... 

Isotropic mioroporous membrane Nonporous dense membrane Electrically charged... 

Isotropic microporous membrane Nonporous dense membrane... 

Nonporous, dense membranes consist of a dense film through which per-meants are transported by diffusion under the driving force of a pressure, concentration, or electrical potential gradient. The separation of various components of a mixture is related directly to their relative transport rate within the membrane, which is determined by their dififusivity and... 

Dense nonporous isotropic membranes are rarely used in membrane separation processes because the transmembrane flux through these relatively thick membranes is too low for practical separation processes. However, they are widely used in laboratory work to characterize membrane properties. In the laboratory, isotropic (dense) membranes are prepared by solution casting or thermal meltpressing. The same techniques can be used on a larger scale to produce, for example, packaging material. 

A modified extraction cell containing a bag-shaped membrane made of LDPE, instead of an FS membrane, was designed to contain the extraction solvent for the extraction of polycyclic musk compounds and pharmaceuticals in wastewater.60 The extraction cell was further developed in terms of membrane design and material. A dense nonporous PP membrane was preferably chosen as a membrane bag in the extraction cell, which was incorporated into a fully automated MASI device that is now commercially available from Gerstel (MUlheim an der Ruhr, Germany). 

The methods for preparation of nonporous composite membrane catalyst are discussed in Ref. 10. The porous stainless steel sheets were covered with a dense palladium alloy film by magnetron sputtering [113] or by corolling of palladium alloy foil and porous steel sheet. The electroless plating of palladium or palladium alloy on stainless steel [114] or on porous alumina ceramic [115,116] gives the composite membranes with an ultrathin, dense palladium top layer. 

The method of preparation also influences the properties of the film. Cast films of varying properties can be prepared by variation inter alia of the solvent power of the casting solution containing the polymer, although the complex processes involved in film formation are not yet fully understood. It is clear, however, that the conformation of the polymer chains in concentrated solution just prior to solvent evaporation will determine the density of the film, and the number and size of pores and voids. Dmg flux through dense (nonporous) polymer membranes is by diffusion flux through porous membranes will be by diffusion and by transport in solvent through pores in the film. With... 

POLYMER MEMBRANES. The transport of gases through dense (nonporous) polymer membranes occurs by a solution-diffusion mechanism. The gas dissolves in the polymer at the high-pressure side of the membranes, diffuses through the polymer phase, and desorbs or evaporates at the low-pressure side. The rate of mass transfer depends on the concentration gradient in the membrane, which is proportional to the pressure gradient across the membrane if the solubility is proportional to the pressure. Typical gradients for a binary mixture are shown in Fig. 26.2. Henry s law is assumed to apply for each gas, and equilibrium is assumed... 

Thin, dense, nonporous polymeric membranes are widely used to separate gas and liquid mixtures. Diffusion of solutes through certain types of polymeric solids is more like diffusion through liquid solutions than any of the other solid-diffusion phenomena, at least for the permanent gases as solutes. Consider, for example, two portions of a gas at different pressures separated by a polymeric membrane. The gas dissolves in the solid at the faces exposed to the gas to an extent usually directly proportional to the pressure. The dissolved gas then diffuses from the high- to the low-pressure side in a manner describable by Fick s first law. 

Membranes can be macroporous, microporous, or dense (nonporous). A microporous membrane contains interconnected pores that are small (on the order of 10 to 100,000 A), but large in comparison to the size of the molecules to be transferred. Only micoporous and dense membranes are permselective. However, macro-porous membranes are widely used to support thin microporous or dense membranes when significant pressure differences across the membrane are necessary to achieve a reasonable flux. 

Norby, T, Haugsrud, R. Dense ceramic membranes for hydrogen separation. In SammeUs, A. F, Mundschau, M. V, editors. Nonporous inorganic membranes. Weinheim Germany Wiley-VCH 2006. pp. 1-48. 

Based on pore size, membranes are described qualitatively as being nonporous, dense or homogeneous (with pore sizes around 1 nm) microporous (with pore sizes 0.001-20 pm) and macroporous (with pore sizes usually 100-500 pm). 

Schauer et al. (2003) used poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) membranes to separate water-EtOH mixtures by PV. Asymmetric membranes were prepared from solutions containing chloroform as a solvent and 1-butanol as a nonsolvent via the phase inversion technique. Dense membranes were prepared from chloroform solution by evaporation. Nonporous membranes (membranes precipitated from solutions with a small amount of the nonsolvent or prepared by evaporation) were preferentially permeable to water. Microporous membranes (prepared from solutions with a large amount of the nonsolvent) were preferentially permeable to EtOH, provided the membrane was not wetted by the feed solution. 

A very large body of data on the gas permeability of many rubbery and glassy polymers has been published in the literature. These data were obtained with homopolymers as well as with copolymers and polymer blends in the form of nonporous dense (homogeneous) membranes and, to a much lesser extent, with asymmetric or composite membranes. The results of gas permeability measurements are commonly reported for dense membranes as permeability coefficients, and for asymmetric or composite membranes as permeances (permeability coefficients not normalized for the effective membrane thickness). Most permeability data have been obtained with pure gases, but information on the permeability of polymer membranes to a variety of gas mixtures has also become available in recent years. Many of the earlier gas permeability measurements were made at ambient temperature and at atmospheric pressure. In recent years, however, permeability coefficients as well as solubility and diffusion coefficients for many gas/polymer systems have been determined also at different temperatures and at elevated pressures. Values of permeability coefficients for selected gases and polymers, usually at a single temperature and pressure, have been published in a number of compilations and review articles [27—35]. 

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