TOPIC 11 AGITATIONAND MIXING, Apuntes de Ingeniería Industrial

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TOPIC 11.

AGITATION AND MIXING

11.1. AGITATED VESSELS

11.2. VORTEX

11.3. POWER REQUIRED

TOPIC 11.

AGITATION AND MIXING

11. AGITATION AND MIXING

o Agitation consists of creating a turbulent and irregular movement of the fluid.

o Often times it is a secondary process, with the aim of making the heat or mass transfer easier, or avoiding the settling of the particles.

o There are four main reasons to carry out an agitation process:

o To increase the coefficients of heat or mass transfer. When gas-liquid or liquid-liquid processes are carried out, agitation causes the formation of bubbles and therefore, the area for heat or mass transfer increases in large quantities. o Mixing of soluble fluids. o Dispersion of an insoluble gas or liquid in another liquid. o To maintain the suspension of solid particles.

11.1 AGITATED VESSELS

o An agitator is composed of the following materials (Figure 11.1): the tank, the stirrer, and the complements.

o The tank is the place where the reaction or the process takes place.

o Many times the tank is cylindrical. And the bottom is often times round, not flat, to avoid dead zones.

o The impeller is mounted on an overhung shaft, a shaft supported from above, and is driven by a motor.

o Complements are: inlet and outlet pipes and connections, heat exchangers (cooling/heating coils or jackets), thermometers, wells to introduce other equipment to measure the temperature, etc.

o The upper side of the tank may be open or closed.

11.1 AGITATED VESSELS

o The pitch is an important characteristic of an impeller. This is the ratio between the displacement suffered by the fluid and the diameter of the tank. When the pitch is 1, it is referred to as squared.

o Viscosity is really important as well.

o Thus, in order to agitate fluids with low viscosity, propellers, turbines, and high efficiency impellers are used.

o Contrarily, in order to agitate fluids of high viscosity, special propellers are used.

11.1 AGITATED VESSELS

o Propellers:

o These axial impellers are used to make fluids of low viscosity flow, and their diameter is usually less than 50 cm. o They usually use squared pitches (Figure 11.2). o They usually turn at high speeds: small ones at 1100- rpm; big ones at 400-800 rpm. o The flow currents that come out of the agitator move in a certain direction until they are changed by the walls or the bottom of the tank. o The liquid in movement that leaves the propellers moves the liquid that is in its surroundings.

11.1 AGITATED VESSELS

o Turbines:

o These radial impellers are the most common ones in the industry. o 4 turbines are shown in Figure 11.2. A simple turbine, which rotates at around 20-150 rpm; a disc form turbine (adequate to disperse gases in liquids); a concave-blade turbine, and a pitched blade turbine. o Many parameters need to be established in order to design the process, such as: diameter of the tank, diameter of the impeller, placement of the impeller, etc. All those parameters fix the velocity, movement path, and power consumption of the process. o Because of that, the design needs to be carried out carefully.

11.1 AGITATED VESSELS

o Typical proportions are:

o Where Da , D (^) t, H, j, E, W, and L are the geometrical parameters of the tank and the impeller shown in Figure 11.3.

D

L

D

W

D

E

D

j

D

H

D

D

t a a

t t t

a

11.1 AGITATED VESSELS

o High efficiency impellers:

o These special impellers (Figure 11.4) are used to improve mixing and to reduce power consumption. o They are really efficient to propel low density liquids. o However, they are useless for agitating high viscosity liquids or dispersing gases.

Figure 11.4. High efficiency impellers.

11.1 AGITATED VESSELS

o Agitators for high viscosity liquids:

o If the viscosity of the liquid or the suspension is higher than 20 Pa·s, the impellers described up to now are not used. o Systems shown in Figure 11.5 are used instead. o The helical system is very useful, it may be used for fluids whose viscosity is 25000 Pa·s. o An anchor type may be used as well, mostly if we want to agitate the bottom of the tank, but the efficiency is lower. o If the fluid is very viscous and it has a suspension, instead of using an impeller, the whole tank is the one that rotates.

11.2 VORTEX

o The velocity of the fluid has 3 components inside a stirred tank: radial velocity (in the perpendicular way to the axis), longitudinal (parallel way to the axis), and tangential or rotational (rotating movement).

o In case the axis of the impeller is vertical, the tangential and radial components are in the horizontal plane, and the longitudinal on the vertical axis.

o The radial and longitudinal components create the necessary flow to mix the fluid. If the axis is vertical and is located in the centre of the tank, the tangential component of the velocity is not desirable.

o The tangential velocity follows a circular movement around the axis and generates a swirl on the surface of the liquid. This creates the stratification of the different fluid elements, which avoids mixing.

11.2 VORTEX

o The radial movement pulls the fluid against the wall and when the fluid touches the wall, the radial velocity becomes axial.

o As a consequence, the top surface of the liquid near the wall increases in height and the pressure of that column will be higher.

o A pressure gradient is formed, and that makes the fluid flow to a lower pressure area, towards the axis, and from there, it goes down the impeller.

o As the pressure is lower in the rotating element, that is a suction point and the height level of water in there is shorter, Figure 11.6.

o If a vortex appears, a larger volume is needed for the same amount of fluid.

11.2 VORTEX

o There are a few ways to avoid this situation:

o Working with closed tanks or vessels, with the volume of the tank being that of the liquid. The worst characteristic of this solution is that appearance of a vortex is not avoided. o Working at low velocities, the vortex will be small. A larger impeller is used to do so. It is not the best solution either. o Locating the impeller away from the center of the tank in small tanks; or placing it on one side in large tanks (Figure 11.7). o The use of baffles (Figure 11.8) is the preferred option, even though higher amounts of energy are needed.

11.2 VORTEX

Figure 11.7. Non-centered location of the impeller.

Figure 11.8. The use of baffles.