Fluid mixing in stirred tank involve in wide variety of operations. The operation parameters and type of impellers and the presence of baffle introduce to different process efficiency. Liquids are agitated in tank where the bottom of tank is rounded. Small scale tank which falls below 10litres are constructed using Pyrex glass. An impeller is mounted on an overhung shaft which is the motor driven that creates a flow pattern. The three patterns are axial, radial and tangential flow. Impellers can be classified into two main groups.
Impellers with small blade area will rotate at high speeds. These include turbine and marine propellers
Impellers with large blade area will rotate at low speeds. These include anchors, paddles.
Type of impeller that been used in this experiments is flat blade paddle and turbine impeller. Flat paddle acts as horizontal circular flow in low speed and radial flow in high speed. This impeller often used at low speeds, between 20 and 200 rpm. The most common used flat paddles are two-bladed and four-bladed flat paddle.
Turbine impeller is a multi-bladed paddle with short blades. The diameter of impeller is between 30 to 50% of vessel diameterCITATION DrM l 1033 (AzmierAhmad, 2016). This type of impeller is suitable for wide range of viscosity. In a low viscosity it generates strong currents which continue throughout the vessel. The principal currents produces are radial and tangential. Shown in Figure 1 is a type of impellers.
Figure SEQ Figure * ARABIC s 1 1 Type of various impeller
This experiment is also look into a presence of baffles with the type of impellers used. Baffle is a vertical plate that is perpendicular to the tank wall used to optimize and stabilize the mixing flow pattern and minimize variation in agitator power draw. Baffles are suitable for large tanks with vertical agitators. With the use of paddle and turbine impellers, it gives a good radial flow in the plane of the impeller with the flow dividing at the wall to form two separate circulation patterns. One portion will flows downward along the wall and back to the center and the other portion flows upward toward the surface and back to the impeller from above. Most common baffles are straight flat plates of metal. Most vessels will have at least three baffles but the most common is four and is often referred as “fully baffled”. Proper baffle implementation dramatically impacts process results. Agitation in vessel without baffle however leads to swirling flow with vortex formation and poor distribution. As shown in figure 2 is a different between having a baffle and not having a baffle.
Figure SEQ figure * ARABIC 2- Flow pattern with and without baffleCITATION yAn14 l 1033 (Anchor Institute _ Dharamsinh Desai University, 2014)The power consumed can be calculated from these equations;
Angular speed (?) = rpm x (2? / 60)
Torque (T) = Force x radius
Force (F) = Torque / radius
Power (W) = Torque (T) x Angular Speed ? (rad/s)
APPARATUS AND MATERIAL
•Fluid Mixing Apparatus
The objective in this experiment has been achieved. It is true that different impellers create a different type of flow patterns in the tank. From the experiment we know that Flat Blade impeller produce radial flow type and suitable for mixing low viscosity liquids at low speeds. On the other hand turbine impeller also create tangential and radial flow and suitable at moderate speeds. Other than that, baffles are one of the most beneficial addition that can be equipped in the mixer. These components allow improve the flow pattern in a mixer in a way that does not put a great deal of strain in the impeller shaft. Industrial mixing is a very important process because the quality of the final product will be derived by that quality of the mix. If fluid mixing is not going to be done well, it will lead to a non-homogenous products which lacks consistency, therefore when it comes to industrial phase, process of mixing need to be design and selected carefully so that there are effective and efficient mixing.
Anchor Institute _ Dharamsinh Desai University. (2014). agitator flow pattern. Course Material Process Engineering: Agitation & Mixing, 15.
AzmierAhmad, D. M. (2016). Fluid Flow for Chemical Egineer. Retrieved from Slideplayer: https://slideplayer.com/slide/10756399/