Stirring propeller

This can be done in the old way with a stirring propeller entering through one of the necks of the flask, attached to a nonsparking motor. It is easier to sit the flask on a magnetic stirrer, and drop a magnetic stirring bar (preferably Teflon coated and egg shaped for round-bottom flask) in the solution. 

Pulverized cabbage was successfully liquidized using an autoclave with a stirring propeller by keeping the feedstock at 150°C for 1 hour. 

Continuous-Flow Stirred Tank Reactor (CSTR). In this flow reactor, a tank reactor is continuously fed with reactants that exist in a single fluid phase, which can be either a gas, liquid, or slurry (solids thickly suspended in liquids). The tank reactor may include a catalyst, and the reactants are mixed with a stirring propeller. Ideally, this complete mix creates the desired product, which is continuously removed from the tank. In practice, perfect mixing can be... 

Thick mixtures with viscosities greater than 10 Pa s are not readily mixed in conventional stirred pots with either propeller or turbine agitators. The high viscosity may be due to that of the matrix fluid itself, to a high slurry concentration, or to interactions between components. 

Batch Stirred Tanks Tanks agitated by coaxial impellers (turbines, paddles, or propellers) are commonly used for batch dissolution of solids in liquids and may be used for leaching fine solids. Insofar as the controlhng rate in the mass transfer is the rate of transfer of mate-... 

A basic stirred tank design is shown in Fig. 23-30. Height to diameter ratio is H/D = 2 to 3. Heat transfer may be provided through a jacket or internal coils. Baffles prevent movement of the mass as a whole. A draft tube enhances vertical circulation. The vapor space is about 20 percent of the total volume. A hollow shaft and impeller increase gas circulation (as in Fig. 23-31). A splasher can be attached to the shaft at the hquid surface to improve entrainment of gas. A variety of impellers is in use. The pitched propeller moves the liquid axially, the flat blade moves it radially, and inclined blades move it both axially and radially. The anchor and some other designs are suited to viscous hquids. 

FIG. 23-30 Basic stirred tank design and selected lands of impellers, (h) Propeller, (c) Turbine, (d) Hollow, (e) Anchor,... 

Zauner and Jones (2000a) describe an experimental set-up for determination of precipitation kinetics, as shown in Figure 6.19. Briefly, the jacket glass reactor (1) (300 ml, d = 65 mm) is equipped with a polyethylene draft tube and four baffles. The contents are stirred using a three-blade marine-type propeller (5) with motor (Haake), which pumps the suspension upwards in the annulus and downwards inside the draft tube. Measured power inputs ranged from 3.3 X 10- to 1.686 W/kg. 

The evolution of nitrogen aids in removing dissolved air. A salt bridge (4 mm tube) attached to the saturated calomel electrode is filled with 3 per cent agar gel saturated with potassium chloride and its tip is placed within 1 mm of the mercury cathode when the mercury is not being stirred this ensures that the tip trails in the mercury surface when the latter is stirred. It is essential that the mercury-solution interface (not merely the solution) be vigorously stirred, and for this purpose the propeller blades of the glass stirrer are partially immersed in the mercury. 

The kind of stirrer is not important. The submitters obtained similar results with a three-blade propeller -turning at 600 r.p.m. and a paddle stirrer turning at 78 r.p.m. They believe that a rocking autoclave could be substituted for a stirred one. 

This section is concerned with the UA xtiT — Text) term in the energy balance for a stirred tank. The usual and simplest case is heat transfer from a jacket. Then A xt refers to the inside surface area of the tank that is jacketed on the outside and in contact with the fluid on the inside. The temperature difference, T - Text, is between the bulk fluid in the tank and the heat transfer medium in the jacket. The overall heat transfer coefficient includes the usual contributions from wall resistance and jacket-side coefficient, but the inside coefficient is normally limiting. A correlation applicable to turbine, paddle, and propeller agitators is... 

For reactors with free turbulent flow without dominant boundary layer flows or gas/hquid interfaces (due to rising gas bubbles) such as stirred reactors with bafQes, all used model particle systems and also many biological systems produce similar results, and it may therefore be assumed that these results are also applicable to other particle systems. For stirred tanks in particular, the stress produced by impellers of various types can be predicted with the aid of a geometrical function (Eq. (20)) derived from the results of the measurements. Impellers with a large blade area in relation to the tank dimensions produce less shear, because of their uniform power input, in contrast to small and especially axial-flow impellers, such as propellers, and all kinds of inclined-blade impellers. 

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. 

The rheo cells can easily be replaced by various types of mixers, propellers or paddles (Figure 2.1.10). It is then possible to analyze the temporal evolution of chemical/physical reactions of mixing, demixing and sedimentation of materials in process engineering, e.g., during the mash process or fermentation [28, 29]. The stirring mechanics and speed can be optimized for various materials of different particle sizes and viscosity. 

The semi-crystalline mass exploded when stirred after standing overnight. The preparation was based on a published method used uneventfully for preparation of ammonium, dimethylammonium and piperidinium perchlorates [1], Its use in explosives and propellants has been surveyed [2],... 

The dominant fate process for chloroform in surface waters is volatilization. Chloroform present in surface water is expected to volatilize rapidly to the atmosphere. An experimental half-disappearance range of 18-25 minutes has been measured for volatilization of chloroform from a 1 ppm solution with a depth of 6.5 cm that was stirred with a shallow pitch propeller at 200 rpm at 25 °C under still air ( 0.2 mph air currents) (Dilling 1977 Dilling et al. 1975). Using the Henry s law constant, a half-life of 3.5 hours was calculated for volatilization from a model river 1 meter deep flowing at 1 meter/second, with a wind velocity of 3 m/second, and neglecting adsorption to sediment (Lyman et al. 1982). A half-life of 44 hours was estimated for volatilization from a model pond using EXAMS (1988). 

A glass stirred tank crystallizer with 350 ml volume was used, which had a marine type propeller located near the bottom. The rotational speed was 200 rpm throughout the experiments. The crystallizer had a filter at the center of the bottom to quickly separate the solution... 

A batch reactor is defined as a closed spatially uniform system which has concentration parameters that are specified at time zero. It might look as illustrated in Figure 2-4. This requires that the system either be stirred rapidly (the propeller in Fig. 24) or started out spatially uniform so that stirring is not necessary. Composition and temperature are therefore independent of position in the reactor, so that the number of moles of species in the system Nj is a function of time alone. Since the system is closed (no flow in or out), we can write simply that the change in the total number of moles of species j in the reactor is equal to the stoichiometric coefficient Vj multiplied by the rate multiphed by the volume of the reactor. 

In some multiphase reactors, stirring with an impeller or the flow pattern caused by gravity will control the interfacial area. By suitably designing and positioning propellers and reactant injection orifices or by using static mixers, it is possible to provide very efficient breakup of hquids into drops and bubbles. A factor of two decrease in drop or bubble size means a factor of four increase in interfacial area. 

The crowded conditions inside a reactor due to the presence of various probes and a phial-breaker usually limit the number and size of propeller blades which can be accommodated. However, these fittings also break up the laminar flow (which is inimical to efficient mixing) so that turbulence can be achieved at stirring speeds well below that which would be required for a cylindrical reactor free of solid obstructions. If there is sufficient space, mixing can be improved considerably by having two propeller blades, ca. 1.5 cm apart, on the same shaft, with opposite chiralities, so that the layer of fluid between them is subjected to an exceptional shear-rate. 

Stirring Device — The lid shall be equipped with a stirring device (Fig F13), mounted in the center of the lid and carrying two two-bladed metal propellers. A stirrer shaft may be coupled to the motor by a flexible shaft, or a suitable arrangement of pulleys... 

A Lightnin mixer (manufactured by the Mixing Equipment Company, Rochester, N. Y.) equipped with a propeller stirrer was used. If rapid stirring is not maintained, the reaction does not go to completion. 

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