Stirrer turbine

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. 

Figure 5.4-5. Streamlines for a radial-flow stirrer (turbine).

Extractor 10 Back pressure regulator 15 Stirrer (Turbine type impeller)... 

Close-Clearance Stirrers For some pseiidoplastic fluid systems stagnant fluid may be found next to the -essel walls in parts remote from propeller or turbine impellers. In such cases, an anchor impeller maybe used (Fig, 18-6), The fluid flow is principally circular or helical (see Fig, 18-7) in the direction of rotation of the anchor. Whether substantial axial or radial fluid motion also occurs depends on the fluid iscosity and the design of the upper blade-supporting spokes. Anchor agitators are used particularly to obtain irnpro ed heat transfer in high-consistency fluids,... 

Fit up the apparatus shown in Fig. 64. It consists of a large wide-necked bottle, in which the ammonia solution is placed The solution is stirred by a mechanical stirrer, rotated by means of a water-turbine. The solution of the chloracetic acid in 50 c.c. water, is dropped in from a tap-funnel. After standing 24 hours the liquid is poured into a flask, and the C lcess of ammonia is removed by passing in a current of steam, and evaporating at the same time on the water-bath until the last traces of ammonia disappear. The solution now contains gly-... 

Compare the capital and operating costs of a three-bladed propeller with those of a constant speed six-bladed turbine, both constructed from mild steel. The impeller diameters are 0.3 and 0.45 m respectively and both stirrers ate driven by a 1 kW motor. What is the recommended speed of rotation in each case Assume operation for 8000 hr/year, power at 0.01/kWh and interest and depreciation at 15%/year. 

A typical stirred-tank reactor is shown in Fig. 5.4-3. It is a cylindrical vessel with elliptical or torospherical bottom and cover. It is equipped with an axially mounted stirrer rotating with a speed from 25 rpm (large scale) to 2000 rpm (laboratory). Fig. 5.4-4 shows the stirrers that are mostly used in fine chemicals manufacture, viz. the marine propeller, turbine, flat- or pitched-blade agitator, and anchor. Agitators move the fluid into axial and radial direction. Marine propellers and pitched-blade stirrers predominantly impose axial motion. 

In contrast, flat-blade stirrers and turbines mainly cause radial movement (see Figs. 5.4-5 and 5.4-6). An anchor is used to enhance heat transfer between the reaction mixture and the cooling medium. 

Figure 5.4-52. A system with an axial-flow turbine (upper stirrer) and a four-bladed flat turbine (lower stirrer) (adapted from Fasano and Penney, 1991b).

The presence of a gas in the suspension results in an increase of the stirrer speed required to establish the state of complete suspension. The propeller usually requires a higher speed than the turbine. Furthermore, a critical volume gas flow exists above which drastic sedimentation of particles occurs. Hence, homogenisation of the suspension requires an increase of the rotational speed and/or a decrease of the gas flow rate. The hydrodynamics of suspensions with a solid fraction exceeding 0.25-0.3 becomes very complex because such suspensions behave like non-Newtonian liquids. This produces problems in the scale-up of operations. Hydrodynamics, gas hold-up, mass-transfer coefficients, etc. have been widely studied and many correlations can be found in literature (see e.g. Shah, 1991). 

CTSR 2, Interphase gas/liquid 3, Self-rotating floating baffle 4, Annulus for position limiting 5, Rushton disk turbine 6, Interphase liquid/liquid 7, Pitched blade turbine upward (mixer/stirrer) 8, Aqueous-... 

The FTS experiments were conducted in a 1 L CSTR equipped with a magnetically driven stirrer with turbine impeller, a gas inlet line, and a vapor outlet line with an stainless steel (SS) fritted filter (7.0 microns) placed external to the reactor. A tube fitted with an SS fritted filter (2.0 micron opening) extends below the liquid level of the reactor for withdrawing reactor wax to maintain a nearly constant liquid level in the reactor. Another SS dip tube (1/8 inch OD) extends to... 

In the case where no correlations are available (i.e., the application involves an exotic fluid, a non-traditional stirrer or a very small reactor), experimental measurements of kLa must be performed to afford power law correlations valid for very similar reactor, turbines and fluids. Several techniques for kLa determination have been published [56]. 

Chemically pure reagents were used. Cadmium was added as its sulfate salt in concentrations of about 50 ppm. Lanthanides were added as nitrates. For the experiments with other metal ions so-called "black acid from a Nissan-H process was used. In this acid a large number of metal ions were present. To achieve calcium sulfate precipitation two solutions, one consisting of calcium phosphate in phosphoric acid and the other of a phosphoric acid/sulfuric acid mixture, were fed simultaneously in the 1 liter MSMPR crystallizer. The power input by the turbine stirrer was 1 kW/m. The solid content was about 10%. Each experiment was conducted for at least 8 residence times to obtain a steady state. During the experiments lic iid and solid samples were taken for analysis by ICP (Inductively Coupled Plasma spectrometry, based on atomic emission) and/or INAA (Instrumental Neutron Activation Analysis). The solid samples were washed with saturated gypsum solution (3x) and with acetone (3x), and subsequently dried at 30 C. The details of the continuous crystallization experiments are given in ref. [5]. 

At present our 6-m tank reactor gives 75% conversion for the first order reaction A R. However, since the reactor is stirred with an underpowered paddle turbine, we suspect incomplete mixing and poor flow patterns in the vessel. A pulse tracer shows that this is so and gives the flow model sketched in Fig. E12.2. What conversion can we expect if we replace the stirrer with one powerful enough to ensure mixed flow ... 

Figure 11.3 shows a number of stirrers that are used. The turbine stirrer, being easy to construct and having a high power number, is the most widely used. The other types are less intensively applied. A detailed description of all types of stirrers can be found in Zlokamik (1972). 

We examine the power consumption of a turbine stirrer, the so-called Rushton turbine (inset in Fig. 3, p.l2) installed in a baffled vessel and supplied by gas from below. 

Let us assume that we have a geometrically similar laboratory device of D = 0.4m (F 0.050 m ) with a turbine stirrer of rotational speed of the stirrer is n = 750/min. Which must the gas throughput be to obtain Q = idem in the laboratory device The answer is... 

From Ne = 1.75 found in laboratory measurement, the power P of the industrial turbine stirrer of J=0.8m and a rotational speed of n = 200/min is calculated as follows ... 

Figure 11 Power characteristics of a Rushton turbine stirrer under given geometric conditions, measured in two differently scaled vessels (scale 1 2) and fitting the flow behavior of the viscoelastic fluid [polyacrylamide (PAA) solution] by changing its viscosity. Source From Ref 13.

Figure 11 depicts the power characteristics of a Rushton turbine stirrer in geometrically similar cylindrical vessels (///D = l D/J=2) without baffles. To keep the Hedstrom number constant at different scales, viscosity of the PAA solutions had to be fitted as discussed above. 

Dimensionless numbers, such as Reynolds and Froude numbers, are frequently used to describe mixing processes. Chemical engineers are routinely concerned with problems of water-air or fluid mixing in vessels equipped with turbine stirrers where scale-up factors can be up to 1 70 (3). This approach has been applied to pharmaceutical granulation since the early work of Hans Leuenberger in 1982 (4). 

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