Anchor stirrers

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,... 

Values of the heat transfer coefficient in the reactor side were determined. Stirring with an anchor stirrer at 100 RPM, the value obtained for bromine at - 5°C is 115 W/m2-K and for BrCl at - 10°C 190 W/m2-K. 

Model predictions are caipared with experimental data In the case of the ternary system acrylonitrlle-styrene-methyl methacrylate. Ihe experimental runs have been performed with the same recipe, but monomer feed composition. A glass, thermostat ted, well mixed reactor, equipped with an anchor stirrer and four baffles, has been used. The reactor operates under nitrogen atmosphere and a standard degassing procedure is performed Just before each reaction. The same operating conditions have been maintained in all runs tenperature = 50°C, pressure = 1 atm, stirring speed = 500 rpm, initiator (KgSgOg) 0. 395 gr, enulsifier (SLS) r 2.0 gr, deionized water = 600 gr, total amount of monomers = 100 gr. 

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. 

Church27 suggest using a paddle or anchor type stirrer of medium speed at 20-60 rpm. Since styrene is only slightly soluble in water and all the other substances are present in very small quantities, it is assumed that the properties of water, the continuous medium, can be used in the design of the agitator. The size of the motor will then be doubled to correct for any errors and then doubled again to obtain the necessary power for startup. 

The mixture was stirred with a mechanical anchor stirrer at 50 rpm at 60 °C until completely dissolved. A stoichiometric amount of adipic acid (69.92 g, 0.48 mol) was... 

A 2.5 m3 stainless steel stirred tank reactor is to be used for a reaction with a batch volume of 2 m3 performed at 65 °C. The heat transfer coefficient of the reaction mass is determined in a reaction calorimeter by the Wilson plot as y = 1600Wnr2KA The reactor is equipped with an anchor stirrer operated at 45 rpm. Water, used as a coolant, enters the jacket at 13 °C. With a contents volume of 2 m3, the heat exchange area is 4.6 m2. The internal diameter of the reactor is 1.6 m. The stirrer diameter is 1.53 m. A cooling experiment was carried out in the temperature range around 70 °C, with the vessel containing 2000 kg water. The results are represented in Figure 9.16. 

For laboratory-size experimentation, the most suited vessel is a widenecked flask with a clamp-on lid. This style has several advantages, one being that fitting an anchor stirrer is much easier. The discharging of the final product is relatively quick with this type of vessel. When a batch has been prepared, all parts that have been in contact with the polyurethane must be thoroughly cleaned and dried. Polyurethanes are a very effective adhesive and can be formed by the reaction between the resin produced and the moisture in the air. 

Most polyurethane systems tend to cling to the glass surface, so an anchor-style stirrer is the best to use. This provides a sweeping motion that helps prevent a very viscous layer from forming on the surface of the vessel. 

Fig. 33 Work-sheet for determining optimal operating conditions for heat removal in a vessel with an anchor stirrer with two different wall clearances (D/d <= 1.00 - without wiper blades -and D/d = 1.10) in the laminar flow range (Re < 100) from [59]...

If more than the determined amount Rmax has to be removed, an anchor stirrer with D/d = 1.1 can be replaced with one with D/d = 1.0 (no wiper blades). Consequently, at a slightly lower optimum stirrer speed (n = 17 min-1), the removal of Rmax 60 kW is possible. 

The reactor, a 750-ml. conical glass flask provided with a stainless steel anchor-type stirrer, which contained the initial charge, was then heated to 80°C., and the monomer feed emulsion was added gradually with stirring in about 2 hours. The temperature was kept at 80 °C. After addition of the emulsion, stirring was continued for another 2 hours, the temperature being kept at 80°C. The latex then was cooled with stirring to room temperature. Monomer conversion was 99.8%, the pH 4-4.5. 

According to the reported literature, the exponent of the viscosity, m, for both heating and cooling varies between 0.14 and 0.24. For an anchor stirrer in the laminar flow regime, Uhl and Voznick (1960) report m = 0.18. The value of the constant a is generally for turbulent flow. For laminar flow, Uhl and Voznick (1960) report a — 0.43. 

The heat transfer is particularly important for the laminar flow regime, which generally prevails for viscous (e.g., polymeric) systems. Zlokarnik (1969) showed that for a vessel with a low-clearance anchor stirrer, the effects of Re and Pr on Nu decrease steadily as the Reynolds number decreases. He suggested a two-parameter equation of the type... 

The mixing time for viscous liquids was examined by Hoogendoorn and Den Hartog (1967). The types of mixers examined in this study are illustrated in Fig. 23. The mixing time was measured by a decoloration and a thermal response technique (see Section IX). In truly viscous flow, the mixing time was inversely proportional to the stirrer speed. The performance of the various mixers were compared using the two dimensionless correlations 02P/(dfp) and pdf/(fid). The turbine and anchor mixers were found to be unsatisfactory for viscous mixing. 

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,... 

Conventional stirred-tank polymeric reactors normally use turbine, propeller, blade, or anchor stirrers. Power consumption for a power-law fluid in such reactors can be expressed in a dimensionless form, Ne = Reynolds number based on the consistency coefficient for the power-law fluid. Various forms for the function f(m) in terms of the power-law index have been proposed. Unlike that for Newtonian fluid, the shear rate in the case of power-law fluid depends on the ratio dT/dt and the stirrer speed N. For anchor stirrers, the functionality g developed by Beckner and Smith (1962) is recommended. For aerated non-Newtonian fluids, the study of Hocker and Langer (1962) for turbine stirrers is recommended. For viscoelastic fluids, the works of Reher (1969) and Schummer (1970) should be useful. The mixing time for power-law fluids can also be correlated by the dimensionless relation NO = /(Reeff = Ndfpjpti ) (Tebel et aL 1986). 

In polymerization reactions viscosity builds up as the molecular weight increases. For such highly viscous media, helical stirrers that bring about axial motion or anchor stirrers are used. The latter also brings a scrapping motion, which helps in cleaning the reactor walls. 

A propeller-type stirrer is of value for stirring up mixtures containing heavy solids such as iron powder or zinc dust. Such stirrers, an example of the container and should be large enough to nearly cover the bottom, of which is shown in Figure 17 (page 94), should reach to the bottom. Anchor- or paddle-type stirrers are also useful under these conditions. 

Closed reaction kettles, as a rule, have their stirrers built in, usually of the anchor-type which reach nearly to the bottom and sides of the kettle. 

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