Continuous tubular membrane

Emulsification devices where the membrane is immersed in a stirred vessel containing the continuous phase, so as to obtain a batch emulsification device operating in deadend emulsification mode, have also been developed (Figure 21.13). Both flat-sheet and tubular membranes are used. In this membrane emulsification device, the continuous phase kept in motion creates the shear stress at the membrane surface that detaches the forming droplets. In a different operation mode, that is, when the continuous phase is not stirred, droplet formation in quiescent conditions is obtained. 

Rotating membrane emulsification is another type of batch emulsification. In this case a tubular membrane immersed in a continuous phase vessel is rotating itself and its angular velocity creates the shear stress at the membrane surface (Figure 21.14). 

The availability of and improvement in membranes has rekindled some interest in dialysis in aroma research. Benkler and Reineccius (19, 20) initially published studies on the use of Nafion (Dupont) membranes for the separation of fat from flavor isolates. This would permit solvent extraction to be used in the isolation of aroma compounds from fat containing foods. Chang and Reineccius (21) later used a continuous tubular counter current flow system to accomplish this fat/aroma separation more efficiently. These membranes can be obtained commercially and have been improved in terms of membrane thickness and purity. While the aroma isolate obtained using this membrane may not perfectly reproduce the aroma being studied, this is an alternate technique for aroma isolation. 

Fabrlcatlon/assembly of externally wound tubular membrane elements is accomplished by specially designed equipment that simultaneously and continuously winds, in helical fashion. Infinite lengths of membrane and backing material strips onto prefabricated tubular support structures. Membrane strip winding overlaps are solvent bonded during the winding process. 

Multiple tests of lab-scale tubular membranes and seals each operated continuously for over 6 months at 250 psig and 825°C with good performance stability... 

Tubular membranes and seal assemblies were tested in high-pressure, high-temperature lab-scale units under ITM S mgas and ITM H2 process conditions. In these tests, pre-reformed natural gas mixtures at process pressure were passed over the outer surface of the tubular membrane, while air at atmospheric pressure was fed to the inner surface of the tube. Multiple tests under ITM H2 conditions each operated continuously for over 6 months at 250 psig and 825°C with good performance stability. The results of one of these six-month continuous tests are shown in Figure 1. 

Dowding, P.J., Goodwin, J.W., and Vincent, B., Production of porous suspension polymer beads with a narrow size distribution using a cross-flow membrane and a continuous tubular reactor. Colloid Surface A, 180 (3), 301-309, 2001. 

In conventional membrane emulsification, droplets are formed at the membrane surface and detached from it by wall shear stress of the continuous phase (Figure 20.8, middle) [29,45,46]. In addition to tubular membranes made from ceramics such as aluminum oxide, special porous glasses such as SPG (Shiratsu Porous Class) membranes and polymers such as polypropylene (29, 47, 48], flat filter membranes made of PTFE [49, 50], nylon [51] and silicon (30, 51-55] have been used in emulsification. Silicon membranes are produced by microengineering techniques. This technology offers the possibility to influence precisely the structure of a membrane (arrangement of pores, pore shape, size and distance, porosity, surface characteristics, as shown in Figure 20.7). Very thin active layers reduce the pressure drop without losing mechanical stability. 

Belafi-Bako, K., Koutinas, A., Nemestothy, N., Gubicza, L., Webb, C. (2006). Continuous enzymatic cellulose hydrolysis in a tubular membrane bioreactor. Enzyme and Microbial Technology, 38, 155—161. 

A decade after Dr. Hassler s efforts, Sidney Loeb and Srinivasa Sourirajan at UCLA attempted an approach to osmosis and reverse osmosis that differed from that of Dr. Hassler. Iheir approach consisted of pressurizing a solution directly against a flat, plastic film. Their work led to the development of the first asymmetric cellulose acetate membrane in 1960 (see Chapter 4.2.1). This membrane made RO a commercial viability due to the significantly improved flux, which was 10 times that of other known membrane materials at the time (such as Reid and Breton s membranes). These membranes were first cast by hand as flat sheets. Continued development in this area led to casting of tubular membranes. Figure 1.3 is a schematic of the tubular casting equipment used by Loeb and Sourirajan. Figure 1.4 shows the capped, in-floor immersion well that was used by Loeb and students and is still located in Boelter Hall at UCLA. 

The volumetric productivity (g of lactic acid/l-h) is usually greater than ten-fold that of batch or continuous processes. Ceramic tubular membranes have been used which are both steam sterilizable and resistant to mechanical stress (Xavier et al. 1995). Fermentation production of lactic acid directly from starch was optimized in an MRB using I. amylovorus (O Table 1.12). No saccharification or preliquefaction of starch... 

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