Stirrer hollow

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. 

The catalysts were obtained from their respective commercial sources and for some of these tests they were subjected to RPT by heating them to 400°C for 2 hours under a flow of hydrogen. The enantioselective ketone hydrogenations were carried out in a 50 ml stainless steel autoclave stirred with a magnetic stirring bar with 10 to 30 mg of catalyst, 10 to 30 mg of MeOHCd, 5 ml substrate and 20 ml AcOH at 60 bar and 25°C for 30 minutes. The crotonic acid hydrogenations were carried out in an ethanolic solution at atmospheric pressure and room temperature stirred at 2000 rpm with a hollow shaft bubbling stirrer. 

The preparation is electrolytic. The anode consists of a few inches of thin platinum rod, to one end of which a 1 X f in. piece of platinum foil is attached. This is surrounded by a hollow cylinder of platinum gauze of diameter 1 in. and length 1 - in. The other end of the platinum rod is attached to an electric stirrer. ... 

The core of double membrane stirrer perfusion bioreactors is a stirrer on which two microporous hollow fiber membranes are mounted, one of them being hydrophobic and used for bubble-free aeration, the second of them being hydrophilic and used for cell-free medium exchange [15]. This system has been reported to provide viable cell densities of 20 million cells per miUiliter for more than two months [106]. Although Lehmann et al. [15] have described the scale-up of this system to the 20-L and 150-L scale, it has been most commonly employed at the bench-scale. 

Figure 11. Electrodes in microcalorimetric vessels. A Schematic diagram of a section through a titration-perfusion microcalorimetric vessel equipped with a polarographic oxygen electrode and a pH electrode, a, sample compartment, volume 3 ml b, hollow stirrer shaft c, steel tube d, turbine stirrer e, O-rings f, combination pH electrode protected by a steel tube g, polarographic oxygen sensor (Clark electrode). B Record from a growth experiment with T-lymphoma cells. The vessel was completely filled with medium. Once the baseline had been established, the experiment was started (as indicated by the arrow) by the injection of 100 pi concentrated cell suspension.

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-... 

Example 22 Process characteristics of a self-aspirating hollow stirrer and the determination of its optimum process conditions... 

As a result of their form, hollow stirrers utilize the suction generated behind their edges (Bernoulli effect) to suck in gas from the head space above the liquid. As rotating ejectors , they are stirrers and gas pumps in one and are therefore particularly suitable for laboratory use (especially in high pressure autoclaves) because they achieve intensive gas/liquid contacting via internal gas recycling without a separate gas pump [58/1]. A particularly effective type of this stirrer - the pipe stirrer - is depicted in Fig. 28. 

In operating a hollow stirrer, both target quantities - gas throughput q and the stirrer power P -adapt themselves simultaneously. Both target quantities depend on the following parameters ... 

III is named the gas throughput number Q, Yl2 is the inverse Reynolds number and 113 is the inverse Froude number. The gas throughput characteristics of a hollow stirrer then reads ... 

Model experiments with another type of hollow stirrer (3-edged stirrer, see sketch in Fig. 30) were performed in water/air under defined experimental conditions and the scale factor p = dT/dM was changed in the range ofp=l 2 3 4 5. The results... 

These results impressively demonstrate to which extent information can be compressed by dimensional analysis. - These process characteristics present a reliable basis for the scale-up of this hollow stirrer under the given geometric conditions. But they also allow a further optimization of this process as will be demonstrated by... 

In this range, the hollow stirrer is still less effective, because the power per unit gas throughput increases with the square of the scale (d2). 

Under these circumstances, which can be described by small is beautiful , it can be clearly shown that hollow stirrers are not suitable for sucking in large amounts of gas on a full-scale. In this case, it is advisable to decouple gas throughput and the power consumption by using a high speed stirrer (e.g. turbine stirrer) and supply it with gas from underneath it via a blower. 

However, there are many chemical reactions in the G/L system in which the gas throughput plays no role because micro-kinetics is rate determining. In such cases, the hollow stirrers, due to their dual role as stirrers and gas conveyers, play a prime role, particularly in high pressure chemical engineering. 

Self aspirating devices (hollow stirrers, ejectors, funnel-shaped nozzles) have to overcome the hydrostatic pressure Aphydr = pg H. This is taken into account by the combination Fr = Fr (d/H ). By the way, this extended Froude number, Fr, represents the reciprocal value of the Euler number, Eu ... 

Fig. 68 Comparison of the heat transfer behaviour of a mixing vessel with a self-aspirating hollow stirrer [58/2] and a bubble column [90].

In contrast, Fig. 72 shows the results of mass transfer in the system aqueous 1-n sodium sulphite solution/air. These measurements were carried out under steady-state conditions in vessels with hollow stirrers on the scale p = 1 5 [58/1, 92]. In this material system, the high salt concentration (70 g/1) fully suppresses bubble coalescence. In the case of the self-aspirating hollow stirrer (see Fig. 28), the stirrer power and gas throughput were coupled via the stirrer speed and were therefore dependent on each other. Consequently, v does not occur explicitly in the representation in Fig. 72, because it is a function of (P/V)". 

Fig. 72 Sorption characteristic of a mixing vessel with a selfaspirating hollow stirrer in a material system (70 g Na2S03/l) with fully suppressed coalescence taken from [58/2, 92],...

Particularly in co-current down-flow operation, very high gas-slurry interfacial areas per cubic meter of reactor volume can be realized [6]. To improve the solids suspension and/or to improve mass or heat transfer, in many cases a stirrer is added to the system. Particularly where pure gases have to be absorbed in the slurry and no gases are produced, the gas may be sparged into the liquid via a hollow shaft stirrer, sucking the gas from the free board above the slurry. 

Fig. 6. Hollow stirrer (tube type). (Reprinted with permission from the publisher, VCH Publishers, Inc., after Zlokarnik and Judat, 1988.)...

Because the gas is aspirated through a hollow stirrer in a gas-inducing impeller, the gas must overcome the hydrostatic pressure of the liquid above the stirrer, H pg. The liquid head over the stirrer, H = (Hh — HJ, therefore has a significant influence on the gas flowrate q. A uniform aeration of... 

The surface tension a and the dynamic viscosity of the liquid /i (as long as it is low) do affect gas flowrate. For viscosities greater than 0.5 PaS, however, the use of hollow stirrers for aeration becomes meaningless. Since aeration deals with a material system with large density differences, the operation is strongly influenced by the parameter gAp/p g at normal pressure, and therefore by the Froude number. 

The gas throughput characteristic of a hollow stirrer generally has the form /VA = /(Fr dy H, Ga, dr/dt, HJdf), where NA = qt/(Ndf) is the dimensionless flowrate number, Fr s Ndjg the Froude number, and Ga = dfg/v2 the Galileo number. For liquids with viscosities close to that of water and for HJd = 1, the gas throughput characteristics for the tube stirrer shown in Fig. 9 are as follows ... 

For a 0.4 m contactor containing a triangular hollow gas-inducing stirrer, Zlokarnik (1966) proposed the following correlation for the heat-transfer coefficient to a heating coil ... 

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