Loop impeller

If the application requires high interfacial area (i.e., small drop diameters), a high-shear impeller, such as the Rushton or radial disk turbine (RDT), is a good choice (Fig. 1). Acceptable substitutes include the Scaba and Chemineer s BT-6 and CD-6 impellers, commonly used for gas-liquid dispersion. If moderate, yet gentle shear is required, such as for emulsion polymerization, the retreat-curve impeller is commonly chosen. When larger drops of a narrow size distribution are required, the loop impeller is a reasonable choice. Broad-blade paddles are also used. 

A loop impeller consists of rectangular-shaped arms made from pipe. The diameter is usually D T = 0.67, and the height is nearly equal to the depth of liquid in the vessel. The number of arms is commonly two, but four arms have been used successfully. The ratio of Vtan/ tip at the tip of the impeller determines the value of k, where Vtm is the tangential velocity component at the impeller tip. 

The system consisted of free radical initialed styrene-divinylbenzene monomers dispersed in water containing 0.2% dissolved polyvinyl alcohol. The dispersed phase was 50 vol %. The process was to be carried out in a vessel containing a loop impeller (see Figure 12-28) operating at low-shear conditions. Bench scale studies showed important variables to be speed, impeller diameter, baffling, selection of the suspending agent, and continuous phase viscosity. Polymerization reactions were completed and bead size distributions were determined by sieve analysis. 

Leng and Quarderer (1982) reasoned that dispersion occurred in the boundary layer adjacent to the loop impeller surfaces and that the impeller vertical elements could be approximated by cylinders moving through the suspension at the relative impeller tip speed. When laminar shear forces predominated, it was shown that... 

Another version of a fluidized-bed reactor has been introduced by Vogelbusch (Austria). This reactor has an internal recirculation loop set up by means of a high flow impeller. The system utilises porous glass beads for immobilizing the cells. However, glass beads may not work with all types of cells. 

Mixing by agitation of liquids normally involves the transfer of momentum from an impeller to the liquid. In some cases, mixing is achieved by gas injection or circulation via a pump loop. An impeller, which is mounted on a shaft driven by an electric motor, is divided into two operation categories ... 

Figure 2.5 Possible technological solutions to bioprocess problems a) Fed-batch culture b) Continuous product removal (eg dialysis, vacuum fermentation, solvent extraction, ion exchange etc) c) Two-phase system combined with extractive fermentation (liquid-impelled loop reactor) d) Continuous culture, internal multi-stage reactor e) Continuous culture, dual-stream multi-stage reactor f) Continuous culture with biomass feedback (cell recycling). (See text for further details).

A gas-inducing agitator system is an alternative to a multistirrer system. It contains a hollow shaft with orifices above the liquid level and a hollow impeller. A typical hollow impeller consists of a tube that is, at the centre, connected to the hollow shaft. Both ends of the impeller are cut at 45 so that, at rotation, the open portions of the tube are at the near side of the stirrer. There are several modifications of this design. Obviously, there is a minimum impeller speed at which the onset of gas induction occurs. Loop reactors are also successfully used. 

Liquid-impelled loop reactor Production of anthraquinones Plant cell culture 124... 

Phase-averaged values of 4 in a plane midway between two baffles of a stirred tank have been plotted in Fig. 1 (from Hartmann et al., 2004a) for two different SGS models (Smagorinsky and Voke, respectively) in LES carried out in a LB approach. The highest values, i.e., the strongest deviations from isotropy, occur in the impeller zone, in the boundary layers along wall and bottom of the tank, and at the separation points at the vessel wall from which the anisotropy is advected into the bulk flow. In the recirculation loops, the turbulent flow is more or less isotropic. 

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. 

A novel bioreactor, especially designed to work with two liquid phases, is the liquid-impelled loop reactor (Figme 11.10), in which the advantages of air lifts and... 

Figure 11.10 Schematic representation of a liquid-impelled loop reactor.

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-... 

Flower [25], Panda et al. [26], Doran [27]. and Payne et al. [28]. Several kinds of bioreactors, such as the stirred tank bioreactor with hollow paddle and flat blade impellers, the bubble column, the airlift bioreactor with internal and external loops, the rotating drum bioreactor, the stirred-tank with a draft tube, and the mist bioreactor have been attempted for plant cell, tissue and organ cultures (Fig. 1). 

The impeller within a large sodium chloride brine pump in a closed loop system of a chlorine plant needed to be replaced. The job was not assigned to the regular maintenance crew, but... 

Mixing time is directly proportional to the longest loop length, which depends on the type of the impeller and the position. 

A common form of stirred-tank reactor has an impeller and a draft tube, which together cause the contents to circulate in a toroidal loop without much mixing except in the region of the feed. This is a simple case of a more general description suggested by van de Vusse (2). Such a reactor can be represented as a loop, described by the differential equation... 

Figure 5 Phillips Particle-forming process a) catalyst hopper b) double loop reactor c) flash tank d) purge drier e) powder-fed extruder f) impeller g) sedimentation leg.

To overcome the pressure drop (5-20 bar) in the synthesis loop re-compression of the recycle gas is required. In practically all modern ammonia plants, the shaft of the final casing also bears the impeller for the compression of the recycle gas. Depending on synthesis configuration, mixing of make-up gas and recycle can be performed inside the casing or outside (three or four-nozzle arrangement Fig. 77). 

Bubble columns Loop reactors Stirred tanks Hydrocyclones Reasonable Reasonable Reasonable Reasonable Modeling of chum-turbulent flow regime Modeling of chum-turbulent flow regime Improved geometrical representation of impeller and baffles Improved geometrical representation of system... 

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