Membrane loop reactor

As is the case with pure bubble columns and gas-operated loop reactors, most bioreactors in technical use are aerated with oxygen or air. Reactors with pure surface aeration, such as roller bottles, shake flasks and small stirred reactors or special reactors with membrane aeration, are exceptions. The latter are used for the cultivation of cells and organisms which are particularly sensitive to shearing (see e. g. [28 - 29]). The influence of gas bubbles in increasing stress has been described in many publications (see e.g. [4, 27, 29, 30]). In principle it can be caused by the following processes ... 

Tubular loop reactors, 25 710 Tubular membrane modules, 25 821 Tubular pervaporation modules,... 

In the literature many examples of more or less exotic bioreactors can be found. Few actually are applied, outside the laboratory. Here two novel designs, the membrane and the liquid-impelled loop reactor, are discussed briefly. These two reactors are simple to use and, to a certain extent, liable to scale-up and both integrate the actual biocatalysis with part of the down-stream processing. 

Fig. 3 Schematic representation of batch-wise passive membrane dialysis (A) and continuous membrane filtration dead-end-filtration (B) and loop reactor (C)...

In general, two types of CFMRs are applied in homogeneous catalysis the dead-end-filtration reactor (Fig. 3B) and the loop reactor (Fig. 3C) [19]. In the dead-end-filtration reactor the nanosized catalyst is compartmentalized in the reactor and is retained by nanofiltration membranes. Reactants are continuously pumped into the reactor, whereas small molecules (products and substrates) cross the perpendicularly positioned membrane due to the pressure exerted. Unreacted materials can be processed by adding them back into the reactor in this set-up. Concentration polarization of the catalyst near to the membrane surface can occur using this technique. In contrast, when a loop reactor is used, such behavior is prevented, since the solution is continuously circulated through the reactor and no pressure is exerted in the direction of the parallel-positioned membrane, so small particles cross the membrane laterally. 

The substrate specificity of the amino acylase from Aspergillus oryzae is very broad, and a wide range of proteinogenic and non-proteinogenic N-acetyl and N-chloroacetyl amino acids are transformed in the presence of the L-amino acylase. The enzyme membrane reactor (Fig. 5) is operated continuously as a loop reactor, and the enzyme is retained by an ultrafiltration hollow-fiber membrane (molecular weight cut off 10000 Dalton). 

The recovery of homogeneous catalysts can be performed batchwise, subjecting the catalyst alternately to a catalytic reaction in a batch reactor and to a batch ultrafiltration (e.g., using an apparatus as depicted in Figure 3 for the filtration), or it can be carried out in a continuous fashion. Continuous filtration units comprise continuously operated membrane reactors, or loop reactors (Figure 8). 

With respect to catalytic membrane reactors, processes can be divided in homogeneously and heterogeneously catalyzed reactions (see Fig. 5.2). In homogeneously catalyzed processes, the membrane modules can be used in loop reactors. For heterogeneously catalyzed reactions several configurations are possible ... 

Membrane bio-reactor (MBR) A system for simultaneous reaction and separation with a membrane. Could be conducted in the same physical enclosure or in separate enclosiures if the reaction vessel and membrane unit are coupled in a semi-closed-loop configuration. 

Bioreactors. Bioreactors that utilize hving cells are typically called fermenters. There are several different types of bioreactors mechanically stirred or agitated tanks bubble columns (cylindrical tanks that are not stirred but through which gas is bubbled) loop reactors, which have forced circulation packed-bed reactors membrane reactors microreactors and a variety of different types of reactors that are not easily classified (such as gas-Uquid reactors and rotating-disk reactors). Biochemical engineers must choose the best bioreactor type for the desired purpose and outfit it with the right instrumentation and other features. 

Figure 3.30 Ignition/extinction loops for ammonia oxidation over platinum performed in micro reactors with different membranes [19],...

Membrane reactors allow a different option for the separation of biocatalysts from substrates and products and for retention in the reactor. Size-specific pores allow the substrate and product molecules, but not the enzyme molecules, to pass the membrane. Membrane reactors can be operated as CSTRs with dead-end filtration (Figure 5.5e) or as loop or recycle reactors (Figure 5.5f) with tangential (crossflow) filtration. 

Similarly, enantiomerically pure hydrophobic (S)-l-phenyl-2-propanol, (S)-4-phe-nyl-2-butanol, and (S)-6-methylhept-5-en-2-ol (sulcatol) were obtained with high purities in a dual-loop enzyme membrane reactor unit with separate membrane extraction unit (Kruse, 1996). Whereas substrate concentrations were low at 9-12 mM, space-time yields higher than 100 g (L d) 1 as well as concentrated product solutions were obtained. 

The obtained results have shown that the configuration where the recirculation tank was irradiated and the catalyst was used in suspension appeared to be the most interesting for industrial applications [73]. Moreover, it was observed that the degradation rate was higher when an immersed lamp was used compared to a system with an external lamp [81]. Therefore, actually the studies in progress are realized in the system described elsewhere [39] consisting of a Pyrex annular photoreactor with a 125-W medium-pressure Hg lamp axially positioned inside the reactor. The separation module containing the flat-sheet membrane was connected to the photoreactor in a recirculation loop. 

Until now, bioreactors of various types have been developed. These include loop-fluidized bed [14], spin filter, continuously stirred turbine, hollow fiber, stirred tank, airlift, rotating drum, and photo bioreactors [1]. Bioreactor modifications include the substitution of a marine impeller in place of a flat-bladed turbine, and the use of a single, large, flat paddle or blade, and a newly designed membrane stirrer for bubble-free aeration [13, 15-18]. Kim et al. [19] developed a hybrid reactor with a cell-lift impeller and a sintered stainless steel sparger for Thalictrum rugosum cell cultures, and cell densities of up to 31 g L1 were obtained by perfusion without any problems with mixing or loss of cell viability the specific berberine productivity was comparable to that in shake flasks. Su and Humphrey [20] conducted a perfusion cultivation in a stirred tank bio-... 

Reactions were carried out in the 140 ml reaction vessel. ( )-menthol and 200 mg of the enzyme peparation were placed in the reactor and the reactor connected to the system. The whole system was flushed with CO2 after which pressure and temperature were adjusted to 100 bar and 50°C. Water activity was set to the desired value by adding portions of water via the HPLC valve. The reaction was started by the addition of 1 ml of isopropenyl acetate once again via the HPLC valve. Final substrate concentrations were 20 mM menthol and 54 mM isopropenyl acetate. Stirring of the enzyme reactor was accomplished by a magnetic stirrer. In addition the reaction medium was pumped in a circle using a gear pump. The enzyme was retained in the enzyme reactor by a nylon membrane. Samples were taken via the HPLC-valve with a 500 pi sample loop, the content of which was expanded into hexane, and analyzed on a HP 5890 Series II gas chromatograph. 

The most suitable driving force in BI is the reduction of the diffusion path that already operates in transport processes across biological bilayers. Consequently, biocatalyst membranes and specially designed bioreactors, such as jet loop and membrane reactors, are available to intensify biochemical reactions. " " Supported biocatalysts are often employed to enhance catalytic activity and stability and to protect enzymes/ microorganisms from mechanical degradation and deactivation.f Immobilization of the cells is one of the techniques employed to improve the productivity of bioreactors. 

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