Stirrers, high-speed

A mathematical analysis of the action in Kady and other colloid mills checks well with experimental performance [Turner and McCarthy, Am. Inst. Chem. Eng. J., 12(4), 784 (1966)], Various models of the Kady mill have been described, and capacities and costs given by Zimmerman and Lavine [Co.st Eng., 12(1), 4-8 (1967)]. Energy requirements differ so much with the materials involved that other devices are often used to obtain the same end. These include high-speed stirrers, turbine mixers, bead mills, and vibratoiy mills. In some cases, sonic devices are effec tive. 

The methylene blue test can also be used to determine cation exchange capacity of clays and shales. In the test a weighed amount of clay is dispersed into water by a high-speed stirrer. Titration is carried out as for drilling muds, except that hydrogen peroxide is not added. The cation exchange capacity of clays is expressed as milliequivalents of methylene blue per 100 g of clay. 

A significant advance was made in this field by Watarai and Freiser [58], who developed a high-speed automatic system for solvent extraction kinetic studies. The extraction vessel was a 200 mL Morton flask fitted with a high speed stirrer (0-20,000 rpm) and a teflon phase separator. The mass transport rates generated with this approach were considered to be sufficiently high to effectively outrun the kinetics of the chemical processes of interest. With the aid of the separator, the bulk organic phase was cleanly separated from a fine dispersion of the two phases in the flask, circulated through a spectrophotometric flow cell, and returned to the reaction vessel. 

Mass Transport. Cavitation improves mixing but, on a macroscopic scale, it is probably less effective than a high speed stirrer. On a microscopic scale, however, mass transport is improved at solid surfaces in motion as a result of sound energy absorption. This effect is called acoustic streaming and contributes to increasing reaction rates. 

A 1-1. three-necked flask equipped with a nitrogen inlet, a high-speed stirrer, and a Dewar condenser having a gas outlet... 

The overall yield of cyclodecanone is comparable to the overall yield obtained by conversion of dimethyl sebacate to sebacoin 11 and subsequent reduction to cyclodecanone.6 In addition, the present procedure does not require the use of a high-speed stirrer, the rigorous exclusion of air, and the high dilution that are necessary in preparing sebacoin. 

It is worth noting that the use of a very high-speed stirrer e. g. an Ultra-Turax can produce similar effects to sonication in heterogeneous systems. This may well be a case of chemistry induced by hydrodynamic rather than acoustic cavitation [32, 33]. 

Fig. 5.10 Computer-assisted extraction kinetics-measuring apparatus for highly stirred phases (A) high-speed stirrer (B) stirrer shaft (C) sample inlet (D) Teflon stirring har (E) Teflon phase separator (F) water hath (G) flow-cell (H) spectrophotometer (I) peristaltic pump (J) chart recorder (K) A/D converter (L) clock (M) minicomputer (N) dual-floppy disk drive (O) printer, (P) plotter. (From Ref. 16.)...

The amine solution (4) is now immediately run in dropwise over a period of 30 s and the stirrer speed reduced to 2000 rpm After 2-3 min the high-speed stirrer is replaced by a normal paddle stirrer and the dispersion stirred (500 rpm) for a further 30 min at room temperature to complete the interfacial polycondensation between terephthaloyl dichloride and diethylenetriamine. 

In a 4-1. beaker, equipped with a high-speed stirrer (Note 3), are placed 20 g. (0.185 mole) of quinone (Note 4), 34 g. (0.4 mole) of sodium bicarbonate, 50 g. of chopped ice, and 500 ml. of water. About 10 ml. of the above diazonium salt solution is added (Note 5). After the frothing has subsided (Note 6), the diazonium salt solution is added in 10- to 20-ml. portions over a period of about an hour (Note 7). The temperature of the reaction mixture is kept below 15° during this period by the addition of ice. After the diazonium salt solution has been added, the mixture is allowed to warm up to room temperature, and the stirring is continued for an additional hour. The precipitate of 2-/>-acetyl-phenylquinone is collected on a Buchner funnel and washed thoroughly with approximately 11. of water. The yield of crude yellow-brown solid is 40-41 g. (96-98%). The melting point ranges from 125-135° to 134-136° (Note 8). 

There is one further point of comparison. Interpretation of results from a stirred-tank reactor depends on the assumption that the contents of the tank are well mixed. Interpretation of results from a tubular reactor rests on the assumption of plug flow unless the flow is laminar and is treated as such. Which of these two assumptions can be met most satisfactorily in practical experiments Unless the viscosity of the reaction mixture is high or the reaction extremely fast, a high speed stirrer is very effective in maintaining the contents of a stirred tank uniform. On the other hand, a tubular reactor may have to be very carefully designed if back-mixing is to be completely eliminated, and in most practical situations there is an element of uncertainty about whether the plug flow assumption is valid. 

Twin-screw extruders or high-speed stirrers may be used to obtain good dispersions in the thermoset precursors. The main drawback is the possibility of producing agglomerates of particles during storage or processing. 

Sodium dispersion is prepared by melting sodium in toluene at just over 100°C in a reactor equipped with a high-speed stirrer engineered to maximize shear of the liquid sodium. On cooling, stable particle sizes of 5-10 p can be achieved. The stirrer design may be any of the types illustrated in Figure 16. 

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. 

Lithium Benzenetellurolate3 In a 250 ml, three-necked flask fitted with a high speed stirrer and a nitrogen inlet tube is placed a solution of 4.1 g (0.01 mol) of diphenyl ditellurium in 100 w/of tetrahydrofuran and the flask is flushed with nitrogen. 0.14 g (0.02 mol) of freshly cut, small pieces of lithium are added through the third neck which is then stoppered. A positive nitrogen pressure is maintained and the mixture is stirred at 20 for 6 h. Any unreactcd lithium is then removed the yellow solution of lithium benzenetellurolate must be kept under nitrogen to prevent oxidation to diphenyl ditellurium. 

Fig. 5-14. High-speed stirrer. (Courtesy of Dr. A. Morton and Analytical Chemistry.)...

A device which has much the same effect as a high-speed stirrer is a vibrating stirrer known as a Vibromischer (Fig. 5-15). The stirring shaft is attached to the flask by means of a flexible rubber sheet and vibrates up and down at 60 cps. The shaft may be fitted with various... 

Sodium powder is prepared by melting S g of carefully cleaned cut sodium metal in 250 mL of boiling xylenes. After removal of the heating bath a slow stream of nitrogen is passed over the solvent, and a high speed stirrer (20,000 rpm, e.g., Ultra-Turrax, model T 18/10) is introduced and run for 15 s. The finely divided metal is filtered and washed (three l5-mL portions of pentane) under inert gas and sucked dry. It can be stored in a dry box for months without loss of activity. ... 

For example, n-pentylsodium may be prepared in 80-90% yield by reaction between Na sand (25 jxra particle size) and 1-chloropentane in n-pentane at — 10°C in a creased flask equipped with a high-speed stirrer. Comparable 3delds of n-pentylsodium may be obtained in n-heptane, in which the yield of organoalkali decreases only from 85 to 75% after storage at RT for 24 days. In contrast, in n-BujO the yield of n-pentylsodium is 63%, but after 10 days most of the reagent has decomposed by reaction with the solvent. In 1,2-dimethoxyethane (DME) and 12 the yields of n-pentylsodium are < 15 %. Even under inert solvents samples of n-pentylsodium must be prepared and stored at RT or lower because pyrolysis is appreciable at 50°C and rapid at 100°C. 

In acetic acid there was no precipitate, but the product from the addition of two equivalents of ozone was colored and contained both the sulfoxide and the sulfone. In a reactor in which the ozone was dispersed by a high-speed stirrer, the precipitate formed in chloroform did not interfere with the addition of ozone, but the results were the same as they were in acetic acid. Attempts to oxidize diphenyl sulfide and dibenzothiophene cleanly to the sulfones using several solvents and ozone or hydrogen peroxide as the oxidizing agents gave the same results. More work on this phase is clearly indicated. 

Of the stirrer types which set the liquid in a radial motion - or into a tangential flow in the case of high viscosities - only the turbine stirrer ) (so-called Rushton turbine , a disk 2d/3 in diameter supporting 6 blades each d/5 high and d/4 wide [474]) belongs to the high speed stirrers. It can be sensibly utilized only with low viscosity liquids and in baffled tanks. Its diameter ratio D/d is 3-5. The turbine stirrer causes high levels of shear and hence is well suited for dispersion processes. 

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