Reactors with hollow fiber catalysts

The design of stirred reactors with hollow fiber catalysts for Michaelis-Menten kinetics (with C. Georgakis and P.C.-H. Chan). Biotechnol. Bioeng. 57, 99-106 (1975). 

The main advantages of reactors with composite membrane catalysts arc the higher hydrogen permeability and smaller amount of precious metals in comparison with those presented in Section II. All constructions of the reactors with plane membrane catalyst may be used for composites of thin palladium alloy film and porous metal sheet The design of reactors with composite membranes on polymeric support may be the same as for diffusion apparatus with polymeric membranes (see, for example. Ref. 138). A very promising support for the composite membrane catalysts is hollow carbon fiber [139], once properly thermostable adhesives are found. 

Another favorable aspect of stirred batch reactors is the fact that they are compatible with most forms of a biocatalyst. The biocatalyst may be soluble, immobilized, or a whole-cell preparation in the latter case a bioconversion might be performed in the same vessel used to culture the organism. Recovery of the biocatalyst is sometimes possible, typically when the enzyme is immobilized or confined within a semi-permeable membrane. The latter configuration is often referred to as a membrane reactor. An example is the hollow fiber reactor where enzymes or whole cells are partitioned within permeable fibers that allow the passage of substrates and products but retain the catalyst. A hollow-fiber reactor can be operated in conjunction with the stirred tank and operated in batch or... 

Membranes that arc catalytically active or impregnated with catalyst do not suffer from any potential catalyst loss or attrition as much as other membrane reactor configurations. This and the above advantage have the implication that the former requires a lower catalyst concentration per unit volume than the latter. It should be mentioned that the catalyst concentration per unit volume can be further increased by selecting a high "packing density" (surface area per unit volume) membrane element such as a honeycomb monolith or hollow fiber shape. 

As mentioned earlier, one of the membrane element shapes with the highest packing density is hollow fibers. Typically several fibers are bundled to provide higher strength. In a packed-bed membrane reactor of this type, catalyst particles arc packed around the bundles. 

The hollow-fiber trickle-bed reactor, according to Yang and Cussler [59], is another variant of the hollow-tube theme. In this case the porous tube is not coated with a catalytic material. The outer shell surrounding the fibers is instead filled with catalyst pellets. The liquid is added to this outer shell, and the gas reactant is added to the inside of the fibers. Since no catalyst is present in the gas-liquid contact, this type of reactor functions merely as an effective gas-absorber. In comparison with the trickle bed, no flooding occurs with the hollow-fiber trickle-bed reactor at high liquid loads, which means a much higher reaction rate at high liquid flow rates than obtained with the traditional trickle bed. 

As one of the two common types of membrane modules, the hollow-fiber membrane module has shown excellent mass transfer performance due to its large surface area per unit volume (about 1000-3000ft2/ft3 for gas separation). In the modeling work, the WGS membrane reactor was configured to be a hollow-fiber membrane module with catalyst particles packed inside the fibers. 

The selectivity of metal catalysts improves in some reactions with alloying for example the alumina-supported Pd-Cu catalyst hydrogenates butadiene to 1-butene with 99% selectivity, i.e. the isomerization is less than 1%. The explanation is that hydrogen adsorption decreased on the Cu-containing catalysts" . Similarly, better selectivities were observed with a polymer anchored Pd, or a Pd-Co catalyst in the gas-phase hydrogenation of butadiene and cyclopentadiene in a hollow-fiber reactor" and in the liquid-phase hydrogenation of 1,5-hexadiene with Pd-Ag catalyst". ... 

Compared to batch processes, continuous processes often show a higher space-time yield. Reaction conditions may be kept within certain limits more easily. For easier scale-up of some enzyme-catalyzed reactions, the Enzyme Membrane Reactor (EMR) has been developed. The principle is shown in Fig. 7-26 A. The difference in size between a biocatalyst and the reactants enables continuous homogeneous catalysis to be achieved while retaining the catalyst in the vessel. For this purpose, commercially available ultrafiltration membranes are used. When continuously operated, the EMR behaves as a continuous stirred tank reactor (CSTR) with complete backmixing. For large-scale membrane reactors, hollow-fiber membranes or stacked flat membranes are used 129. To prevent concentration polarization on the membrane, the reaction mixture is circulated along the membrane surface by a low-shear recirculation pump (Fig. 7-26 B). 

A thermophilic bacterium (growth temperature 50°C), Thermomonospora curvata JTS 321 was identified as a catalyst for the same reaction96-97. Three different reactor systems have been compared, namely packed-bed and fluidized-bed reactors with immobilized cells (polyacrylamide- hydrazide gels) as well as a hollow-fiber reactor96. The highest productivity (up to 1400mgh 1 L-197 was observed with the latter. 

We have developed a one-dimensional non-isothermal model for the countercurrent WGS membrane reactor with a C02-selective membrane in the hollow-fiber configuration using air as the sweep gas. Figure 1 shows the schematic of each hollow-fiber membrane with catalyst particles in the reactor. The modeling study of the membrane reactor is based on (1) the CO2 / H2 selectivity and CO2 permeance reported by Ho [1, 2] and (2) low-temperature WGS reaction kinetics for the commercial catalyst copper oxide, zinc oxide, aluminum oxide (CuO/ZnO/ AI2O3) reported by Moe [3] and others [4]. In this modeling study, the model that we have developed has taken into account critical system parameters including temperature, pressure, feed gas flow rate, sweep gas (air) flow rate, CO2 permeance, CO2 /H2 selectivity, CO concentration, CO conversion, H2 purity, H2 recovery, CO2 concentration, membrane area, water (H20)/C0 ratio, and reaction equilibrium. 

Figure 3 Some examples of the principle of coupling a membrane technique with an HCR. (a) Hollow-fiber membrane module (b) reactor. Case 1, separation of a soluble catalyst by NF case 2, MR contactor in two-phase reaction (only phase 1 and 2) case 3, MR contactor in three-phase reaction (phases 1, 2, and 3).

A useful variation of this design, shown in Figure 24.2b, consists of three concentric tubes (Oertel et al., 1987). The inner of the two annular spaces formed is filled with the catalyst, and selective permeation of products to the central (product) tube is achieved by placing a number of tubular membranes inside this packed volume. Therefore, it is called the packed-bed inert selective multimembrane reactor (IMMR-P). In yet another version of an IMR-P, the membrane is supported on the inner surface of a hollow fiber membrane tube and the catalyst is loaded around the hollow fiber (Figure 24.2c). 

Figure 16.12 In situ separation of oxygen from ambient air for the CPOM Performance of a BCFZ hollow fiber membrane reactor in the CPOM as a function of temperature with CH4 conversion (O), CO selectivity ( ), CO2 selectivity (O) and H2/CO ratio (A)7 Experimental details Flow rate on the core side of the BCFZ hollow fiber membrane = 150 mL min air, flow rate on the shell side = 20 mL min (10 mL min CH4 + 10 mL min He). 0.88 cm effective membrane area, 0.8 g Ni/Al203 steam reforming catalyst as packed bed on the shell side.

Figure 16.14 Schematic drawing of the reactor set-up and an incorporated BCFZ hollow fiber for OCM. At both ends the 30 long fiber was coated with gold to obtain a 3 cm long gold-free isothermal oxygen permeation zone. The active surface area for the oxygen permeating BCFZ hollow fiber is 0.78 cm. The OCM catalyst was dipersed between the outer dense alumina tube and the fiber, which was inserted into a porous alumina tube.

The status and fiitine trends in the development and application of novel reactors equipped with solid electrolyte based membrane materials was analyzed in a review article [105]. BaCo Fe Zr Oj perovskite hollow-fiber membrane reactor was used with permeable and passivated surface segments to enable controlled oxygen insertion into the reactor. The reduced oxygen concentration offered higher ethene selectivity. At low and moderate ethane conversion, the performance of the membrane reactor was on par with the best catalysts used in cofeed mode. The ethene yield was comparable with that of the industrial steam cracking process. However, the reaction temperature was 100°C lower [106],... 

In 2000, Itoh and Haraya constructed the first CMR and experimentally examined the performance of a dehydrogenation reaction. Asymmetric polyimide hollow fibers were pyrolyzed in a vacuum oven at 1023 K in order to obtain hollow fiber carbon membranes. Their CMR consisted of SS in which 20 carbonized hollow fibers (0.295 mm diameter and 128 mm long) and catalyst pellets (0.5 wt% Pt/Al203) were allocated. The reactor, used for cyclohexane dehydrogenation to benzene at 468 K, showed a fair improvement over equilibrium conversions. In detail, the temperature dependency of the permeation rates showed that the carbon membrane had micropores with an average diameter close to those of the gas molecules and therefore the permeation process was molecular-sieving controlled. The ideal H2/Ar... 

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