Hollow dense membranes

A completely different approach was taken by Koresh and Soffer (1980, 1986, 1987). Their preparation procedure involves a polymeric system like polyacrylonitrile (PAN) in a certain configuration (e.g. hollow fiber). The system is then pyrolyzed in an inert atmosphere and a dense membrane is obtained. An oxidation treatment is then necessary to create an open pore structure. Depending on the oxidation treatment typical molecules can be adsorbed and transported through the system. 

Dual-layer polyethersulfone (PES)/BTDA-TD1/MD1 co-polyimide (P84) hollow fiber membranes with a submicron PES-zeolite beta mixed matrix dense-selective layer for... 

Hollosep High Rejection Type is characterized by Cellulose Tri Acetate (CTA) hollow fiber with dense membrane structure and high salt rejection, and also by the module configuration favorable for uniform flow of feed water through hollow fiber layers (5 ). These features suggest that Hollosep may be operated under the conditions of higher recovery ratio compared to conventional conditions. 

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. 

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... 

Gabelman A, Hwang S-T, and Krantz WB. Dense gas extraction using a hollow fiber membrane contactor Experimental results versus model predictions. J. Membr. Sci. 2005 257(1-2) 11-36. 

Some of the efforts, so far, to model such membrane bioreactors seem to not have considered the complications that may result from the presence of the biomass. Tharakan and Chau [5.101], for example, developed a model and carried out numerical simulations to describe a radial flow, hollow fiber membrane bioreactor, in which the biocatalyst consisted of a mammalian cell culture placed in the annular volume between the reactor cell and the hollow fibers. Their model utilizes the appropriate non-linear kinetics to describe the substrate consumption however, the flow patterns assumed for the model were based on those obtained with an empty reactor, and would probably be inappropriate, when the annular volume is substantially filled with microorganisms. A model to describe a hollow-fiber perfusion system utilizing mouse adrenal tumor cells as biocatalysts was developed by Cima et al [5.102]. In contrast, to the model of Tharakan and Chau [5.101], this model took into account the effect of the biomass, and the flow pattern distribution in the annular volume. These effects are of key importance for conditions encountered in long-term cell cultures, when the cell mass is very dense and small voids can completely distort the flow patterns. However, the model calculations of Cima et al. [5.102] did not take into account the dynamic evolution of the cell culture due to growth, and its influence on the permeate flow rate. Their model is appropriate for constant biocatalyst concentration. 

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. 

Most applications of GP use dense membranes of cellulose acetates and polysulfones. For high-temperature applications where polymers cannot be used, membranes of glass, carbon, and inorganic oxides are available, but they are limited in their selectivity. Almost all large-scale applications of GP use spiral-wound or hollow-fiber modules, because of their high packing density. 

The extraction of metals based on a membrane contactor system with conventional solvents is a process widely studied using different configurations, extractants, and extraction solvents. One of the upcoming applications of membrane contactors is supercritical extraction. This process is called porocritical extraction. Porocritical process or porocritical extraction is a commercial supercritical fluid extraction (SFE) technique that utilizes an hollow fiber membrane contactor (HFMC) to contact two phases for the purpose of separation. As an improvement, the extraction of Cu + from aqueous solutions by means of dense gas extraction was achieved by using a hollow fiber membrane contactor device [7]. The authors... 

Fujii et al. [13] studied morphological structures of the cross section of various hollow fibers and fiat sheet membranes by high-resolution field emission scanning electron microscopy. Figure 6.8 shows a cross-sectional structure of a flat sheet cellulose acetate RO membrane. The layer near the top surface is composed of a densely packed monolayer of polymeric spheres, which is supported by a layer formed with completely packed spheres. The contours of the spheres in the top layer can be observed. The middle layer is also composed of loosely packed and partly fused spheres, which are larger than the spheres in the surface layer. In the middle layer, there are many microvoids, the sizes of which are the same as the spheres. The layer near the bottom is denser than the middle layer, and the spheres are deformed and fused. Interstitial void spaces between the spheres, which may be called microvoids, are clearly observed. This structure seems common for the flat sheet as well as the hollow fiber membranes. For example. Fig. 6.9 shows a cross section of a hollow fiber made of PMMA B-2 (a copolymer containing methyl methacrylate and a small amount of sulfonate groups). The inside surface layer is composed of the dense structure of compactly packed fine polymeric particles. The particle structure of the middle layer... 

The macrostructure of the ceramic hollow fiber membranes can be controlled by modulating the suspension compositions as well as the spinning conditions, as shown in Figure 18.4." " When water is used as both internal and external coagulants, a sandwich-like structure, i.e. a central dense layer integrated with fingers on both sides is formed (Figure 18.4(a)). 

Membrane contactor (MC) is a phase-contacting device for use in gas absorption and stripping (degassing) processes as well as in biomedical gas transfer processes [44, 46]. The function of the membrane is to facilitate diflfusive mass transfer between contactir phases such as liquid-liquid, gas-liquid and gas-gas. The membrane phase contactor uses polyolefins, e.g., polypropylene (PP) microporous hollow fibres membranes, which are packed densely in a high surface area module. Since membranes are hydrophobic and have small pores (0.05—0.1 3m), water does not pass through the membrane pores easily. The pressure required to force water to enter the pore is called the breakthrough pressure, which for a PP membrane with a pore size of 0.05 pm is greater than 10 bar g. 

For gas separation applications, the feed stream is usually fed into the shell side of the module. This implies that the dense selective layer should be located on the outside of the polyaniline hollow fiber membrane. To achieve this desired morphology, the polyaniline hollow fiber was spun using an air gap between the spinneret and the coagulation bath. The residence time in the air gap influences the amount of solvent evaporation, which in turn governs the thickness of the dense separating layer on the outer surface of the hollow fiber. By adjusting the residence time from a few seconds to 30 s, the thickness of the dense separating layer on the outer surface of the hollow was successfully varied between 0.5 and 5 xm. 

Li, Y., Cao, C., Chung, T-S. and Pramoda, K.P. 2004. Fabrication of dual-layer polyether-sulfone (PES) hollow fiber membranes with an ultrathin dense-selective layer for gas separation. 245 53-60. 

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