Asymmetric membranes with porous

Figure 1.1 Schematic diagrams of the principal types of membranes (a) porous symmetric membrane (b) non-porous/dense symmetric membrane (c) liquid immobilized membrane (d) asymmetric membrane with porous separation layer (e) asymmetric membrane with dense separation layer.

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. 

There are several processes for the separation of liquid mixtures using porous membranes or asymmetric polymer membranes. With porous membranes, separation may depend just on differences in diffusivity, as is the case with dialysis, where aqueous solutions at atmospheric pressure are on both sides of the membrane. For liquid-liquid extraction using porous membranes, the immiscible raffinate and extract phases are separated by the membrane, and differences in the equilibrium solute distribution as well as differences in diffusivity determine the extract composition. 

ILMs can be fabricated with any any porous matrix in which the liquid phase wets the matrix material and the pores are sufficiently small. Since commerciidly available asymmetric membranes with thin (0.1 - 2pm) p us skins supported by a macroporous backing exist, it should be possible to fabricate liquid membrane versions of the supported ultra-thin polymer membranes figure 9). 

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. 

Osmosis and Its Applications, Fig. 1 Forward osmosis profiles of the solution concentration and the osmotic pressure under the effects of membrane stmetures and orientations, (a) A symmetric dense semipermeable membrane with only ECP effects, i.e., concentrative ECP and dilutive ECP. (b) An asymmetric membrane with the dense-selective layer (DSL) facing against draw solution (normal mode) with the profile of the solution concentration illustrating concentrative ICP and dilutive ECP. (c) An asymmetric membrane with the porous support layer (PSL) facing against draw solution (reverse mode) with the profile of the solution concentration illustrating dilutive ICP and concentrative ECP. The key parameters Ch... 

Let us take polysulfone as an example. This is a polymer which is frequently used as a membrane material, both for microfiltration/ultrafiltration as well as a sublayer in composite membranes. These applications require an open porous structure, but in addition also asymmetric membranes with a dense nonporous top layer can also be obtained which are useful for pervaporation or gas separation applications. Some examples are given in table ni.S which clearly demonstrate the influence of various parameters on the membrane structure when the same system, DMAc/polysulfone(PSf), is employed in each case. How is it possible to obtain such different structures with one and the same system To understand this it is necessary to consider how each of the variables affects the phase inversion process. The ultimate structure arises through two mechanisms i) diffusion... 

To quote an example. A polysulfone/DMAc system can be immersed in either water or i-propanol. Since the miscibility of DMAc with water is much better than with i-propanol, instantaneous demixing consequently occurs in water resulting in a porous rnembrane with ultrafiJtration properties. With i-propanol as the nonsolvent delayed demixing occurs, which results in an asymmetric membrane with a dense nonporous top layer with pervaporation or gas separation properties. The cross-sections of these membranes are shown in figure HI - 45. 

A verj large number of combinations of solvent and nonsolvent are possible all with their own specific thermodynamic behaviour. Table III.8 shows a very general classification of various solvent/nonsolvent pairs. Where a high mutual affinity exists a porous membrane is obtained, whereas in the case of low mutual affinity a nonporous membrane (or better an asymmetric membrane with a dense nonporous top layer) is obtained. It should be noticed that this holds for ternary systems. In case of multicomponent systems with additives the thermodynaics and kinetics change, as do the membrane properties. 

Membrane pressure-driven processes, namely, MF, UF, NF, and RO are normally carried out in the liquid phase. Although water permeates through the membrane, other species are partially or completely rejected. According to Fane et al.. The MF-UF range can be considered as a continuum [11] both processes involve porous membranes. MF is carried out using symmetric membranes, with pore size ranging from 0.05 to 10 pm. UF, instead, requires asymmetric membranes, with pore size from 1 to 100 nm. The NF and RO spectrum is also considered as a continuum [11]. NF/RO membranes are usually thin-film composite (TFC) structures with nonpo-rous skin. The most important features of pressure-driven membrane processes are resumed in Table 1.3. 

Terms such as symmetric and asymmetric, as well as microporous, meso-porous and macroporous materials will be introduced. Symmetric membranes are systems with a homogeneous structure throughout the membrane. Examples can be found in capillary glass membranes or anodized alumina membranes. Asymmetric membranes have a gradual change in structure throughout the membrane. In most cases these are composite membranes... 

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

Internal concentration polarization occurs as a result of salt accumulation in the porous substrate of asymmetric membranes, and is unaffected by stirring. Internal concentration polarization can only be reduced to an acceptable level by using membranes with an open substra,te. Without due regard for internal concentration polarization, it is unsafe to project PRO performance from RO performance. 

Two Asymmetric Membrane Sheets, Sandwiching A Porous "Spacer", Are Adhesively Bonded At Edges And Spirally Wrapped About Permeate "Core", To Which "Spacer" Empties Through Penetrations. These Spiral Wraps Are Separated From One Another By An Open-Lattice Feed-Channelling Material, Concurrently Wrapped With The Membrane "Sandwich". The Resultant Permeator Element Is Then Over-Wrapped With Fiberglass Lay-Up, And Provided Necessary Seals And Fittings. 

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

An important point to consider about hollow fiber membranes is their morphology. Hollow fiber membranes can be either symmetric or asymmetric.16 Symmetric membranes have continuous pore structure throughout. Asymmetric membranes have a dense upper layer or skin layer that is then supported with a sublayer that is significantly more porous. Figure 6.2 shows SEM images of... 

Both microdialysis and ultrafiltration collection obtain analytes from a sample in the reverse direction regardless of how a normal hemodialysis membrane is used. In hemodialysis, the blood is passed through the inner fiber lumen and filtrate is then collected on the outside of the hollow fiber. When these fibers are used as microdialysis or ultrafiltration devices for collection of samples, the outside of the fiber is interfaced with the sample and the analyte is collected into the inner fiber lumen of the hollow fiber. This is important particularly for the asymmetric membranes that have their large porous support layer on the outside facing the tissue sample. 

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