Stirrer helical ribbon

A satisfactory correlation was also obtained for the combination turbine stirrer-helical ribbon stirrer in the case of non-Newtonian liquids, if Ne was plotted as a function of Re ff according to expression (1.47) [120] ... 

Laminar regime (Re < 102). For stirrers with very small wall clearances, such as the anchor or helical ribbon mixer, the laminar regime prevails for Rc < 100. In this regime, the viscous force dominates. The effect of inertial forces (density) is negligible, and thus the baffles are unnecessary. In this regime,... 

Whereas the pre-polymerization is carried out at temperatures of from 100 to 150 °C, the main polymerization is carried out at up to 180 °C. Its only aim is to increase the conversion and, thus, improve the economic efficiency of the processes. Target conversions are above 90 %. In order to be able to dissipate the heat of reaction from the solutions of exponentially increasing viscosity in a controlled manner, a number of reactors are generally connected in series. The designs vary considerably. For example, conical reactors with helical ribbon stirrers, horizontal tank reactors with paddle stirrers, reactor cascades and tower cascades have been proposed. 

The very slow-speed helical ribbon stirrer is a close-clearance stirrer [D/d 1.05) and is so operated that the liquid on the wall is transported downwards. Under these conditions this stirrer is the most suitable of all the stirrer types for homogenizing high viscosity liquids. 

The proportionality constant fe depends upon the stirrer type. For propeller stirrers fe = 10 ]68], for turbine stirrers fe = 11.5 and pitched-blade stirrers fe = 13 [104]. For blade stirrers k = 2.5, for cross-beam stirrer fe = 4.1 and for helical ribbon stirrer fe = 6.0 [411]. Calderbank [66] found that when turbine stirrers were used with Bingham and pseudoplastic fluids fe = 10 and when used with dilatant liquids fe = 12.8 (d/D) 5. Lower fe values were found as the viscoelasticity of fluids increased [104]. In the case of close-clearance anchor stirrer fe depended upon the wall clearance [24]. 

This graphical representation enables an interesting comparison to be made between the power characteristics of different stirrer types. In the first place it is evident that the Ne value for Re = 1, i.e. NeRe = const) increases with the surface area of the stirrer, it being with the anchor stirrer a factor of 10 greater and with the helical ribbon stirrer a factor of 25 greater than that of the propeller stirrer. The smaller the wall clearance of the stirrer, the longer laminar flow conditions remain. With gate and cross-beam stirrers these are maintained up to Re 10, with anchor stirrers and helical ribbon stirrers even up to Re 10. ... 

However, the flowed against front faces of the stirrers provide a far from proportional contribution to the stirrer power. Measurements on four helical ribbon stirrers with helices of different length confirm this (single and double helical ribbon stirrers with pitches of 0.5 and 1 helical ribbon area A ). The following relationship was found [611] ... 

However, it has to be pointed out that the high value of constj in equation (2.9) is taken - like Ne(Re) values for all the other stirrer types in Fig. 2.2 - from [611]. In this work the helical ribbons were mounted on a flat steel frame. Had they been directly connected with spokes to the stirrer shaft [262, 363], then const) in expression (2.7) would have been a factor of 3 lower NeRe = 340. 

In [120] power measurements are presented for tlie combination of turbine stirrer and helical ribbon stirrer in Newloniari liquids in the laminar range. It was found that the total Ne value with the combination was considerable larger than the sum of the individual Ne values NeRe - 70 for turbine stirrers, 150 for helical ribbon stirrers and 350 rather than 220 for the combination. This can be explained by the disturbance of the axial flow of the helical ribbon stirrer by the radial flow of the turbine stirrer in the lower vessel region. 

Measurements of power consumption of a I ARAVISC stirrer (combination of anchor and helical ribbon stiners) in dilatant (shear-thickening) fluids are presented in [708]. 

If the course of mixing with a helical ribbon stirrer in the represented laminar range is approximated with wfl = const, it can be said that the mixing time decreases directly proportionally with the stirrer speed. From about Re = 50 the forces of gravity begin to affect the flow pattern. Round the stirrer shaft a liquid cone remains under the surface, whose participation in the intimate mixing is retarded [611]. The helical ribbon stirrer is therefore only sensibly usable in the range Re < 100. 

For helical ribbon stirrers not only the power characteristic but also the mixing time characteristic is clearly dependent upon the size of the helical surface (single or double helix pitch 0.5 or 1.0). For the range Re < 100 it was found that ... 

Of the many publications, which have concerned themselves with the homogenization of liquids, only a few of the most comprehensive will be initially mentioned here [190, 220, 552, 611] and then those which deal with particular stirrer types helical ribbon stirrers [261, 363, 651, 694] enamaled stirrer types [159] turbine and paddle stirrers [457] PARAVISC stirrer (combination of anchor and helical ribbon stirrers) [658]. Mixing times in gassed liquids have been dealt with in [524]. 

Finally mixing time characteristics were mentioned [535], which were determined with seven different designs of helical ribbon stirrers, in which the gap... 

This relationship is reproduced in Fig. 3.12a with the test results from Hoogen-dom and den Hartog [220]. It shows that FIs = const applied in the laminar range for several axially conveying stirrer types (helical ribbon stirrers propeller stirrers in a draught tube), i.e. the stirrer power per unit volume, which is required for a particular homogenization time, is here proportional to the viscosity n of the liquid and independent of the tank diameter. FIj and therefore P/D P/V in this range is actually a scale-up criterion for these stirrers. 

In Fig. 3.13 below, the authors [363] plotted their own measurements for the helical ribbon stirrer for different wall clearances d/D. It shows (see the table below right), that the stirrer power of the helical ribbon stirrer decreased rapidly with increasing wall clearance and that, to a smaller extent, the nd values also decreased. This stirrer type, which up to now has proved to be the most suitable for homogenization in the laminar flow range, should if at all possible be utilized with d/D = 0.9. 

The helical ribbon stirrer is particularly favorable for homogenizing in the laminar flow range, but has two serious drawbacks, namely, the high cost of manufacture and the complicated introduction and installation in the tank. These disadvantages... 

Tecante and Choplin [542] investigated a stirrer arrangement consisting of an propeller mixer and a helical ribbon stirrer in a laboratory tank (D = 210 mm d = H = 185 mm D/d = 1.14). Aqueous solutions of CMC, xanthan and polyacrylamide were used as material systems. The evaluation proceeded according to equation (4.32). The following sorption characteristics were obtained ... 

In this study [614] the comparison between the anchor stirrer and a double helix ribbon stirrer with a pitch of 0.5 for the same d/D = 0.98 established, that the helical ribbon stirrer for Re l provided a Nm value which was approximately double that with the anchor stirrer, but that this superiority had already been lost at Re 100. Apparently, here the radial flow component is entirely sufficient for heat transfer. 

A considerable removal of the boundary layer from the inside wall of the tank is only possible with stirrers with wiper blades. (For the numerical analysis of the three-dimensional flow and thermal behaviour in a scraped-surface heat exchanger see [647].) A first review over such stirrers in [443] brought little of substance. Nagata [0.6] provided the following relationship for helical ribbon stirrers with wipers ... 

Judat [250] investigated the heat exchange behavior in a stirred tank (D = 444 mm, H = 1080 mm H/D = 2.43) with helical ribbon stirrers (d = 340 mm pitch 1), on which one, two or four wiper pairs (number of wipers i = 2 4 8) were so mounted on the shafts, that they pressed firmly on the inside wall of the tank upon rotation, as a result of hydraulic forces. 

Fig. 7.2 Heat transfer characteristic of a helical ribbon stirrer with hydraulic pressing wiper blades (2 wiper blade pairs,... 

Fig. 7.3 Comparison of heat transfer characteristics of an anchor stirrer with d/D — 0.98 from [614] and the helical ribbon stirrer with hydraulic pressing wiper blades (2 wiper blade pairs, i — 4) after [250]...

G., Power requirement when mixing a shear-thickeningfiuid with a helical ribbon stirrer, Chem. Eng. Technol. 23 (2000) 4, p. 329-336... 

A distinction is usually made between close clearance/large diameter and small diameter/high speed impellers. The first class is used for viscous reaction mixtures. It includes anchor stirrers and helical ribbons. The diameter of these agitators typically is between 70 and 100 % of the tank diameter. 

On the right side a vessel with a helical ribbon stirrer with d/D = 0.9 used for high viscous liquids... 

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