EXPERIMENT # 11Date: 17-10-2013

OBJECTPerform the operation of mixing and show an analysis of effects of varying speed on mixing.

THEORYMixing is one of the common operations carried out in the chemical processing and allied industries. The term mixing is applied to the processes used to reduce the degree of non-uniformity, or gradient, a property in a system such as concentration, viscosity, temperature and so on. Mixing is achieved by moving materials from one region to another. It may be of interest simply as a means of achieving a desired degree of homogeneity but it may also be used to promote heat and mass transfer, often where a system is undergoing a chemical reaction.

APPARATUSMixing tank, variable speed motor, different size turbines, baffles, pH meter and small beakers.

PROCEDURE Add water so that the height becomes equal to the diameter of the tank. Take four baffles all should be of same size. Start the motor and add concentrated base. After every 15 seconds, take out a sample of about 30ml. After each sample, add 30ml water so that the level of water remains constant. Take about 20 readings. Change the water and repeat the experiment with 200, 250 and 300 Rpm. Draw graph for the change in pH for each speed. Determine the optimum speed. Scale up the results 8 times.

OBSERVATIONSTurbine size of 4cm with baffle size 2.2 cm and 200 Rpm

Turbine size of 4cm with baffle size 2.2 cm and 250 RpmTurbine size of 4cm with baffle size 2.2 cm and 300 Rpm

Time(sec)pH

08.314

1510.1

3010.96

4511.08

6011.13

7511.11

9011.13

10511.14

12011.14

13511.14

15011.14

16511.14

18011.14

Time(sec)pH

08.54

1510.21

3010.79

4510.9

6010.93

7510.94

9010.95

10510.91

12010.93

13510.94

15010.94

16510.94

18010.94

19510.94

21010.94

Time (sec)pH

08.51

1510.8

3010.9

4510.92

6010.94

7510.95

9010.95

10510.95

12010.96

13510.96

15010.96

16510.96

18010.97

19510.97

21010.97

22510.97

24010.97

25510.97

Turbine size of 4cm with baffle size 2.2 cm and 200 Rpm

Turbine size of 4cm with baffle size 2.2 cm and 250 Rpm

Turbine size of 4cm with baffle size 2.2 cm and 300 Rpm

Time (sec)O. speed (Rpm)

150200

75250

285300

RESULTThe optimum speed is found to 200 Rpm.DISCUSSIONAgitation is means whereby mixing phases can be accomplished and by which mass and heattransfer can be enhanced between phases or external surfaces. The operation of agitation, which includes mixing as a special case, is now well established as an important and in a wide variety of chemical processes.

Specifically, agitators are applied to three general classes of problems: To produce static or dynamic uniformity in multi component multiphase systems. To facilitate mass or energy transfer between the parts of a system not in equilibrium. To promote phase changes in multi component system with or without a change in compositions.Mixing in tanks is an important area when one considers the number of processes, which are accomplished in tanks. Essentially, any physical or transport process can occur during mixing in tanks. Qualitative and quantitative observations, experimental data, and flow regime identifications are needed and should be emphasized in any experimental pilot studies in mixing. A vortex is produced owing to centrifugal force acting on the rotating liquid. If vortex reaches the impeller severe air entrainment occurs. The depth and the shape of the vortex depend on impeller and vessel dimensions as well as on rotational speed.

Mixing efficiency in a stirred tank is affected by various numbers of parameters such as baffles, impeller speed, impeller type, clearance, tank geometry, solubility of substance, eccentricity of the impeller. Effects of speed of turbine and size of turbine and baffles on efficiency are discussed below:

BAFFLE SIZEIn the un-baffled vessel with the impeller rotating in the center, centrifugal force acting on the fluid raises the fluid level at the wall and lowers the level at the shaft. In baffled tanks, a better concentration distribution throughout the tank and therefore improvement in the mixing efficiency is achieved. Baffles are flat vertical strips set radially along the tank wall. Baffles avoid vortex formation. The larger the width of the baffles, the better is the mixing to some extent. So in above experiment as we increase the baffle sizes the efficiency of mixing increases.

SIZE OF IMPELLER/TURBINEFlow patterns can be changed according to the type of impellers, and fall into three categories: Axial, Radial and Tangential. In general, a mixer with a small diameter impeller type turning at a high speed, will result in the fluid seeing the applied power as mostly shear. Conversely, a low-speed mixer with a larger Hydrofoil diameter impeller will discharge a higher volume of Propeller fluid, resulting in high flow.In our experiment we observed that at constant speed increasing the size of impeller increases the discharge flow through turbine but decreases shear, and therefore mixing time increases.

SPEED OF TURBINEIn unsteady speed turbines the unsteady motion varied the location of the centers of the vortices in the vessel, non-mixing regions like doughnut rings disappeared and the mixing time drastically decreased. Unsteady speed mixing was found to be much more effective than steady speed mixing when a conventional impeller is used under laminar flow. Tip speed of turbines is usually in a range of 400 to 1500 FPM. Turbine should be operating below critical speed which is speed at which both frequencies of turbine (vibration and rotational) reinforce one another.

In our experiment it is observed that for same baffle size, the efficiency of mixing increases as the speed of turbine increases due to increase in shear.