Pumping capacity of stirrers

Under the term pumping capacity q of the stirrer is meant the liquid flow (throughput), which is conveyed through the surface swept by the stirrer. This surface is given by 7rci /4 for axially working stirrers and by ndb for radially working 

Continuously operated stirred tanks can only attain homogenization if q q (where q is the liquid throughput through the vessel). The mixing time is then 0 oc V/q (where V is the liquid volume of the vessel). Knowledge of q, in the case of propeller stirrers, enables the flow velocity along the heat exchanging surfaces of one or more concentric cylindrical coils round the stirrer (and consequently the heat transfer rates) to be calculated. Such an installation may be indispensable in case of extremely exothermic reactions. 

In the turbulent flow range (Re 10 ) the following process characteristics apply for individual stirrer types in baffled vessels with H/D = 1  

Propeller stirrers with pitch s — 1 (where s is the tangent of the pitching angle of the vane) and distance h from the bottom from h/d = 1, according to [137]  

Pitched-blade stirrer with 6 paddles (a = 45°, b/d = 0.2) according to [136]  

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For an open system, the homogenization can only be achieved if the pumping capacity of the stirrer is greater than the liquid flow rate through the vessel. This pumping capacity expressed in a dimensionless form Q = q /Nd3 is a function of the Reynolds number and the vessel geometrical parameters (d-f/dy) and HJdt. In turbulent flow (Re > 103), Q is independent of the Reynolds number. The nature of the relationship also depends upon the nature of the stirrer and the agitated vessel. 

The pumping capacity of the stirrer or the liquid throughput of the stirrer is the liquid flow rate that is displaced through the area swept by the stirrer. For axial flow stirrers, q is determined from the circulation time of particles with Ap = 0. For radial flow stirrers, it is obtained from the integral value of the measured velocity distribution of the flow. 

The flow condition was measured in a vessel with dished bottom and turbine stirrer or two different axially working stirrers with a two-component LDA [359], which was operated in back-scattering mode, such that the flow field could be averaged over a long period. The measurements were limited to a vertical plane in the middle between two baffles. The liquid viscosity was varied, so that the flow conditions could be measured both in the virtually laminar zone and in the turbulent zone. The internal liquid circulation and the pumping capacity of the stirrers was determined from the velocity measurements. An increase in the viscosity significantly reduced the rate of liquid circulation, as expected. 

A certain flow rate of liquid Fl or gas Fq is recirculated between various modules by the action of the impeller (pumping capacity of the stirrer Fp). 

Cooper and Wolf (1968) examined the velocity profile and pumping capacity for turbine stirrers with six paddles (Fig. 21g). They concluded that the normalized radial velocity profiles of the turbine jet are independent of turbine speed and independent of turbine diameter for geometrically similar turbines. The radial velocity profile is parabolic but becomes increasingly plug in shape with greater radial distance. The tangential profile is somewhat flat in nature. The pumping capacity can be expressed as... 

Figure 8 gives details of the piston injectors and vapour pressure cell. Only one of the injectors is shown. The piston injector is a commercial 100 cm capacity liquid metering pump capable of operating under vacuum conditions. The displacement volumes can be read to 0.01 cm . The vapour-pressure cell consists of a 100 cm glass vessel connected to a metal lid E by an O-ring seal. The lid supports a magnetic stirrer, a set of baffles, and an evacuation port H. 

Place 80 g, of hydroxylamine sulphate (or 68-5 g. of the hydrochloride), 25 g. of hydrated sodium acetate, and 100 ml. of water in a 500 ml. flask fitted with a stirrer and a reflux water-condenser, and heat the stirred solution to 55-60°. Run in 35 g (42 nil,) of -hexyl methyl ketone, and continue the heating and vigorous stirring for ij hours. (The mixture can conveniently be set aside overnight after this stage.) Extract the oily oxime from the cold mixture twice with ether. Wash the united ethereal extract once with a small quantity of water, and dry it with sodium sulphate. Then distil off the ether from the filtered extract, preferably using a distillation flask of type shown in Fig. 41 (p. 65) and of ca, 50 ml, capacity, the extract being run in as fast as the ether distils, and then fractionally distil the oxime at water-pump pressure. Collect the liquid ketoxime, b.p. 110-111713 mm. Yield, 30-32 g. 

Dimethylaniline (40 g. 0 33 mole) is dissolved in 250 c.c. of approximately 5 N-hydrochloric acid (one part of concentrated acid and one part of water) in a filter jar (capacity 1 1.) The jar is immersed in ice 200 g. of ice are dropped in, and then with good stirring—preferably with a mechanical stirrer—a cold solution of 25 g. of sodium nitrite in 100 c.c. of water is run in gradually from a dropping funnel (Fig. 51, p. 146). The temperature should not rise above 5° and no nitrous gases should be evolved. After the mixture has stood for one hour the orange-yellow hydrochloride is filtered thoroughly at the pump and washed several times with dilute hydrochloric acid (say 2 N). The salt is sufficiently pure for subsequent... 

The suspension of cut nitrocellulose fibres in water is transferred with the help of a pump into a boiler (poacher). This is a vertical (Fig. 160) or horizontal (Fig. 161) cylindrical vessel of 10-20 m3 capacity, constructed of sheet-metal, equipped with a stirrer and heated by direct steam injection. 

The bituminous binder is kept in insulated and heated storage tanks of appropriate capacity. The tanks should be equipped with either agitators (stirrers) or circulation pumps, or both. Agitators are preferred when modified binders are to be used. 

A 1.2 Oil Bath of sufficient depth to allow submersion of the evaporation cell to the proper level and capable of being controlled at the desired test temperature within 0.S C ( rF), with a maximum variation throughout the bath of 0.S C ( 1 F). Circulation of the oil heating medium by a pump or stirrer is recommended. Sufficient heat capacity shall be provided to return the bath to the required temper-... 

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