Capillary membrane reactors

Whole cells or enzymes immobilized in capillary membrane reactors... 

The basic hydrodynamic equations are the Navier-Stokes equations [51]. These equations are listed in their general form in Appendix C. The combination of these equations, for example, with Darcy s law, the fluid flow in crossflow filtration in tubular or capillary membranes can be described [52]. In most cases of enzyme or microbial membrane reactors where enzymes are immobilized within the membrane matrix or in a thin layer at the matrix/shell interface or the live cells are inoculated into the shell, a cake layer is not formed on the membrane surface. The concentration-polarization layer can exist but this layer does not alter the value of the convective velocity. Several studies have modeled the convective-flow profiles in a hollow-fiber and/or flat-sheet membranes [11, 35, 44, 53-56]. Bruining [44] gives a general description of flows and pressures for enzyme membrane reactor. Three main modes... 

When one of the components can condense within the pores, the capillary condensation mechanism is enabled (Fig. 9). The condensate fills the pores and then evaporates at the permeate side, where a low pressure is imposed [54]. Moreover, the transfer of rather big molecules is generally favored by this mechanism over rather small ones. Provided the pore dimension is small and homogeneous enough, and the pores themselves uniformly dispersed over the membrane, this mechanism allows for very high selectivities (separation factors between 80 and 10(X), as reported in [55]) limited only by the solubility of noncondensable molecules in the condensate. However, capillary forces are strong enough to promote this mechanism only with small pore sizes at relatively low temperatures. Hence, as for surface or multilayer diffusion, the practical chances of application appear poor in inorganic-membrane reactors. 

Existing ceramic, mesoporous membranes (with a 4 nm pore diameter) perform most gas separations according to Knudsen diffusion. The obtainable separation factors (Section 9.3.2.) are usually not economical for most gas separations and provide incremental but limited conversion enhancement in catalytic membrane reactor applications. Capillary condensation and preceding surface flow yield economically interesting separation factors but this mechanism is limited to easily condensable gases and is limited to rather low pressure drops due to stability problems (Sections 9.2.3. and 9.3.3.). 

Highly efficient enzyme membrane reactors can be also produced by immobilizing enzymes in membranes or in hollow fibers. For example, enzymes can be confined in the porous support matrix of an asymmetric capillary membrane, while substrate-containing solution flows through the fiber lumen. The dense skin layer at the lumen wall should be impermeable to the enzyme molecules. The latter diffuse through the inner wall of the fiber to the enzyme into the spongy part, where the conversion takes place. Applied transmembrane pressure and axial flow rate are parameters that contribute to control of the reactor performance. 

Figure 3. Flow sheet of laboratory experimental plant. REM, capillary enzyme membrane reactor SS,substrate reservoir, SP, permeate reservoir.

From the point of view of mechanical properties, performances of capillary membranes charged with cells are almost comparable to those of bacteria-free ones. The interesting conversions observed in lactose hydrolysis and the remarkable stability of immobilized bacterial fl-galac-tosidase encourage further studies for the development of an enzyme membrane reactor oriented to possible industrial applications. 

Finally, the dead volumes developed when capillary coimections and other junctions to the fluidic chip to provide off-chip processing, such as the interface to MS, can significantly degrade separation performance and introduce sample carryover or sample loss. For example, Gao and coworkers [124] estimated 4 pF for the dead volume associated with their miniaturized trypsin membrane reactor. As noted above, the typical peak volume associated with microchip electrophoresis is on the order of 10 pF, which demands that nearly all protein processing post-separation be done directly on-chip so as not to degrade separation... 

Overall enzyme amount in the reactor, M Number of capillary membranes in a bundle Product concentration, M L 3 Peclet number... 

Several authors have reported modelling of multi-phase membrane reactors and, in particular, of three-phase catalytic membrane reactors. Harold and Watson (1993) have considered the situation of a porous catalytic slab partially wetted by a liquid from one side and by a gas phase on the other side, and they have pointed out the complexity of the problem in presence of an exothermic reaction, capillary condensation and vaporization. 

Edwards et al. (1999a) reported that during removal of polyphenols in an enzyme-immobilized membrane reactor, in order to remove the colored quinone-type by-products, which passed through the membrane, a chitosan-packed post-adsorption column was required to be integrated into the system. In another study Edwards et al. (1999b) utilized chito-san gel as an immobilization matrix for polyphenol oxidase on polysul-fone capillary membranes. They reported that during the treatment of... 

A concept for a methanol (or ethanol) fuel processor based upon steam reforming and membrane separation was presented by Gepert et td. [400]. As shown in Figure 5.33, the alcohol/water mixture was evaporated and converted by steam reforming in a fixed-bed catalyst, into which palladium capillary membranes were inserted. The retenate then entered the combustion zone, which was positioned concentrically around the reformer bed at the reactor wall. Air was fed into the combustion zone and residual hydrogen, carbon monoxide and unconverted methanol combusted therein. The sealing of the membranes at the reactor top was an issue solved by air-cooled elastomers. 

Fig. 15. Standing Turing patterns observed in the capillary tube reactor (inner diameter 0.2 mm, length 3.0 mm). Reactor respectively filled with (a) polyacrylamide gel (5% dry material) loaded with Thiodene (130 g/1 of gel) [65]. (b) agarose gel (1.2% dry material) loaded with Thiodene (80 g/1 of gel), (c) An aqueous solution of Thiodene 100 g/1. The starch is maintained in the tube by dialysis membranes at each end (see text). Experimental conditions [KI] = 2.5 x 10" M, [NaOH] = 2 x [CH2(COOH)2] = 8.15 x 10 M,...

Hicke, HG, Becker, M, Paulke BR and Ulbricht, M (2006), Covalently coupled nanoparticles in capillary pores as enzyme carrier and as turbulence promoter to facilitate enzymatic polymerizations in flow-through enzyme-membrane reactors , / Membr Sci, 282,413-422. 

The main building block of a membrane reactor system is the module, which as a unit operation bears more resemblance to a membrane unit than to a traditional reactor. For commercial apphcations, it is desirable to fabricate membranes into a modular form that maximizes both productivity and selectivity. The most common commercial membrane geometries are flat sheet and tubular, and currently there are five module types namely, plate-and-frame and spiral-wound modules, based on flat membranes, and tubular, capillary and hollow-fibre modules, based on tubular membrane geometries. The module types of most relevance to siUca membrane modules and membrane reactors are the plate-and-frame and tubular types. The inflexibility of silica membranes prevents them from being used in a spiral-wound set-up and the technology to produce capillary and hoUow-fibre membrane geometries has not been adequately explored for sihca membranes. Plate-and-frame modules are easily constructed by... 

Hollow fiber reactors [7] and dialysis reactors [8] avoid shear stress by separating cells and flowing media. In both reactors nutrient supply takes place by diffusion through the capillary wall or the dialysis membrane. 

The reactors are cylindrical in shape and can carry up to 30 mg of resin. Polymer sieves at the top and bottom of the cylinders serve for liquid feed and withdrawal. The array of reactors is attached to a capillary system allowing feed to either columns or rows. This distribution system is said to provide uniform charges to the various reactors. A specific detail of the reaction system is that mixing is achieved by pneumatic actuation using a fluoropolymer membrane (Figure 4.36). 

HOLLOW-FIBER MEMBRANES. A hollow-fiher membrane is a capillary having an inside diameter of - inn and an outside diameter < I mm and whose wall functions as a semipermeahlc membrane. The fibers can he employed singly or grouped into a bundle which may contain tens of thousands of fibers and up to several million libers as in reverse osmosis (Fig. 11. In most eases, hollow fibers are used as cylindrical membranes that permit selective exchange of materials across (heir walls. However, they can also he used as containers to effect the controlled release of a specific material, or as reactors to chemically modify a permeate as il diffuses through a chemically activated hollow-liher wall. e g., loaded with immobilized enzyme. 

Figure 8 (Top) Electrochemical flow cell for the oxidation of phenol and aniline (a) Pb anode feeder (b) packed bed of 1-mm lead pellets (c) stainless steel cathode plate (d) Nation membrane (e) stainless steel screen (f) Luggin capillary (g) glass beads (h) gasket (i) reactor inlet (j) reactor outlet. (Bottom) Schematic of apparatus (a) electrochemical reactor (b) peristaltic pump (c) water bath (d) heater (e) anolyte reservoir (t) gas sparging tube (g) C02 adsorbers. (From Ref. 39.)... 

Finally, a nonpermselective membrane can be used in multiphase applications. The original idea belongs to Harold and Cini [137]. Their reactor consisted of a supported catalytically active tubular membrane (Pd/y-A Oi on a two-layer a-Al203 porous support) separating the two reactants the gas flows at the tube side (membrane side), the liquid at the shell side (support side). Capillary forces let the liquid penetrate the pores of the... 

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