Tubular membrane shapes

Figure 2.1 Polymeric membrane shapes and cross-sectional structures. Tubular membranes are similar to flat sheet membranes because they are cast on a macroporous tube as support. Capillary membranes are hollow fibers with larger diameter, that is, >0.5 mm.

Depending on the membrane shape (plate or tube) the reactor is different, but it is generally made of two chambers separated by the membrane. Figure 6 shows a reactor made of a tubular membrane and a conventional fixed-bed catalyst filling the inner part of the tube. In this example the reactant(s) is introduced into... 

Figure 11 Thin-walled tubular membrane catalyst in the shape of (a) a double-start flat spiral, (b) a side view, and (c) a top view of the spiral block.

The first commercial metal membranes for hydrogen separation and purification were made of palladium alloyed with 23-25 wt % silver. These membrane were of the unsupported type and tubular in shape. Nevertheless, the wall thickness was substantial by current standards—typically at least 100- an thick. Advances in drawing thin-walled metal tubes has allowed for palladium-silver tubular membranes to be made with much thinner walls, about 20- an thick. Composite membranes are also usually at least 25-/an thick. REB Research and Consulting (Oak Park, MI) provides tubular composite metal membranes consisting of a palladium coating over a tantalum base metal, although other group 4 or 5 base metals may be used. 

Reliable sealing technologies for use at temperatures up to 800°C are available for, e.g., alumina-based tubular membrane systems [11] but need further development for other shapes and materials. 

A significant recent effort in this area is a collaborative study by Amoco and the Argonne National Laboratory utilizing solid oxide type membranes [112-113]. The newly developed membranes show improved mechanical and thermal characteristics and are reported to remain stable for over 21 days at 900°C under CH4 partial oxidation conditions. The membrane used was tubular in shape. A CH4/Ar mixture was allowed to flow in the tubeside which was packed with a Rh based catalyst. Air was the source of oxygen on the outside... 

Fig. 11. Projection map at 20 A resolution of a negatively stained native tubular membrane of Rhodobacter sphaeroides. The basic unit, 198 A long and 112 A wide, contains an elongated S-shaped supercomplex composed of C-shaped structures facing each other. Figure source Jungas, Ranck, Rigaud, Joliot and Verm6glio (1999) Supramolecular organization ofthe photosynthetic apparatus of Rhodobacter sphaeroides. EMBO J 18 538.

Membrane materials are available in various shapes, such as flat sheets, tubular, hollow fiber, and monolithic. Flat sheets have typical dimensions of 1 m by 1 m by 200 pm thickness. Tubular membranes are typically 0.5 to 5.0 cm in diameter and up to 6 m in length. The thin, dense layer is on either the inside or the outside of the tube. Very small-diameter hollow fibers are typically 42 pm i.d. by 85 pm o.d. by 1.2 m long. They provide a very large surface area per unit volume. Honeycomb, monolithic elements of inorganic oxide membranes are available in hexagonal or circular cross section. The circular flow channels are typically 0.3 to 0.6 cm in diameter (Seader and Henley, 2006). 

The membrane shapes described are usually incorporated into compact commercial modules and cartridges. The four more common types of modules are (1) plate-and-frame, (2) spiral-wound, (3) tubular, and (4) hollow-fiber. Table 9.2 is a comparison of the characteristics of these four types of modules. The packing density refers to the surface area per unit volume of module, for which the hollow-fiber modules are clearly superior. However, hollow-fiber modules are highly susceptible to fouling and very difficult to clean. The spiral-wound module is very popular for most applications because of its low cost and reasonable resistance to fouling. 

Fig 5.2 is a schematic diagram of a tubular type gas-diffusion separator. The separator is column shaped, with two concentric channels. The tubular membrane forms the inner channel, usually reserved for the donor stream, and extends beyond the outer channel to be connected to the suppK and waste conduits. The terminals of the outer channel are extended to inlet and outlet ports on the column which are furnished with connectors for the acceptor stream conduits. 

The tube-and-shell, or tubular, membrane module is easily adapted for use with drawn tubular membranes as well as membranes that are made by depositing a thin permselective metal layer onto a porous tube support. There are three significant variants to this module design. One is based on the membrane tubes fixed to a header at each end of the membrane tube. The second is similar in that both ends of the membrane tubes are fixed to a header, but to the same header. In the second design, the membrane tubes are bent into a U shape, which can be easily done with small diameter metal tubes. The third is based on a single header, to which open sides of closed-one-ended membrane tubes are fixed. The closed-ends of the membrane tubes are suspended freely. This latter design is more common for commercial applications, due to free thermal expansion and greater membrane durability (see above discussion), whereas laboratory test-and-evaluation practices favor the first variant for its ease of assembly. If the membrane is a drawn, thin-walled tube, the membrane tube will usually be brazed to the header. This is more difficult if the membrane tubes are to be fixed at both ends to head-... 

The next level up from the basic membrane architecture is the selection of membrane shape. The main two architectures studied have used tubular geometries and a modified planar geometry. Both have their advantages and disadvantages for oxygen separation. 

However, each aforementioned method presents benefits and drawbacks. For example, both CVD and ELP techniques are able to coat a complex-shaped component with a uniform thickness layer. Unfortunately, non desired compounds and impurities can be formed and incorporated in the palladium layer, reducing the flux of hydrogen through the film. Moreover, by ELP method, it is not easy to control the thickness of the film. On the contrary, an important benefit of the electroless coating is that it is well suited to applications on available commercial tubular membranes. CVD is not an economic process due to the strict conditions required for the process. 

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. 

In this type of module a number of membranes of tubular shape are encased in a container. For example, 18 tubes are connected in scries by headers at both ends of the Nitto NTR-1500-PI 8A module. Figures 7.9 and 7.10 show the structure of the module. Cellulose acetate membranes are formed in the internal wall of the support tube of 12-mm internal diameter The tubular membranes so prepared are inserted into plastic tubes with many holes, which are mounted in a module container. The feed liquid flows inside the tube, and the permeate flows from the inside to the outside of the membrane tube and is collected at the permeate outlet. There are also tubular modules in which the feed is supplied to the outside of the membrane tube. The main features of the tubular module are... 

Four configurations for membranes are plate, hoUow fine fiber, spiral wound, and tubular (32). With a variety of shapes, sizes, and materials many options exist for meeting the various needs in the dairy industry. 

Inorganic membranes employed in reaction/transport studies were either in tubular form (a single membrane tube incorporating an inner tube side and an outer shell side in double pipe configuration or as multichannel monolith) or plate-shaped disks as shown in Figure 7.1 (Shinji et al. 1982, Zaspalis et al. 1990, Cussler 1988). For increased mechanical resistance the thin porous (usually mesoporous) membrane layers are usually supported on top of macroporous supports (pores 1-lS /im), very often via an intermediate porous layer, with pore size 100-1500 nm, (Keizer and Burggraaf 1988). 

Tubular shaped membrane in double pipe configuration... 

Figure 7.1. Typical membrane reactor configurations (a) reactor with plate-shaped membranes, (b) tubular-shaped membrane in double pipe configuration and (c) multichannel monolith.

Some ISEs containing no inner reference solution, as well as tubular potentiometric sensors, has been used in conjunction with FI systems for the determination of vitamins B, and Bg in pharmaceutical preparations. The membranes used for this purpose were prepared from the vitamin tetra(2-chlorophenyl)borate dissolved in o-nitrophenyloctyl ether and immobilized in PVC. The intrinsic behaviour of the tubular electrodes was assessed by using a low-dispersion single-channel FI manifold and compared with those of conventionally-shaped electrodes using the same membrane the results provided by both were very similar [119]. 

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