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Transcript of Agitation Experiment
What will you hear today?
What is Agitation?
It is the induced motion of a material in a specific way, usually in a circulatory pattern inside some sort of container.
Why study Agitation?
Occurs in almost every process plant and works hand in hand with mixing
Example: reactors, separators, etc.
Equipment and Procedure
Results and Discussion
To determine the torque and power consumption as a function on agitator speed.
To find the effects of baffles and shape factors .
Compare results to literature values.
To determine the efficiency of mixing using a tracer and fit data to a two parameter model.
Baffles or No Baffles
The Mixing Model
Torque/moment of a force is the tendency of a force to rotate an object about an axis.
Power is the work done per time
In rotational systems Power is related to Torque(τ) and angular velocity(ω)
Baffles are flow directing vanes or panels found in vessels.
No baffles causes vortex formation
Ideally a tank with baffles can take more agitation as the flow is "controlled".
Impeller Vs Propeller
Comparison of experimental and predictive
Graph of res Time vs RPS
Scale up Graph
Graph of Torque vs RPS
Various types of shaft serve different purposes.
1.Make sure the drain valve is closed
2. Turn on the LabVIEW Program, RPM meter, and the voltage meter
for calibration and data acquisition.
3. Calibrate the torque meter. This is done by taking a zero reading with no
weight and taking a reading while balancing a 40.5 oz-in weight on the tip
of the torque meter. A value of 1.140 mv means the torque meter is properly calibrated
9. Based on minimum and maximum RPM range, determine 6 to 8 RPMs within the range and obtain the average torque at each RPM.
10. Make sure to record the average torque (oz-in) value from the LabVIEW program when all three lights turn green.
11. Repeat step 5 to 9 (In the large tank, standard three-bladed propeller with and without baffles as well as six-bladed impeller with and without baffles. In the small tank, six-bladed impeller with and without baffles.)
6. Measure the diameter of the agitator. Connect the
agitator (impeller or propeller) with or without the baffles and lock it into the
motor shaft and lower it down to where the diameter of the impeller is equal
to the distance from the agitator to the bottom of the tank.
7. Make sure the tank is placed in the center.
8. Turn the system on and determine the minimum and maximum RPM for the unit for each set up. Minimum RPM can be found based on the lowest RPM that gives us a first average torque reading. For the maximum RPM, if there is vortex formation, then the RPM can be increased by the increments of 20 RPM until the height of the water level reaches to the top of the tank due to the vortex formed.Typically, indication of the maximum RPM is as follows: When a tank does not have baffles, the maximum RPM can be found when the tank starts to wobble too much. With the presence of baffles, the maximum RPM is reached when system
1). Fill the system with distilled water.
2). Make sure to keep record of the volume added or measure the height of the water in the tank and the diameter of the tank to calculate the volume of the water later for calculation purposes.
3). Connect an agitator (impeller or propeller) with or without the baffles and lock it into the motor shaft and lower it down to where the diameter of the impeller is equal to the distance from the agitator to the bottom of the tank.
4). Measure 10 g KCl and add 500mL of water to prepare the solution to inject into the tank.
5. Turn on the conductivity LabVIEW program
6). Insert the conductivity probe into the tank and turn on the conductivity meter.
7). For each set of the run, turn the motor on (Use the same RPMs obtained from the first part of the experiment). Once the impeller or the propeller starts rotating, inject about 15mL-20mL of KCl solution through the funnel and start the LabVIEW program at the same time.
8). Prior to starting the experiment, save each file as #####.prn (For instance, MJINB70.prn indicating my initial, impeller without baffles at 70 RPM.)
9). Set the time delay appropriate for each set of run On the LabVIEW program.
10). Adjust the initial conductivity.
11). Start the LabVIEW program and run the program until it stops.
12). Repeat step 8 to 11 for each set at each RPM. Increase the RPM accordingly and then save the data in separate files as #####.prn.
Make sure to increase the number of sampling accordingly as the RPM increases.
13). After completing the 4 sets of run, obtain the fraction of total volume at the core (b1) and volumetric flow rate (b2) at each RPM that can be found via curve-fitting the data using ‘QuickBASIC’.
Safety & Precaution
1. Safety glasses should be worn throughout experiment to for safety and good lab practice..
2. Wear jeans or slacks, a long sleeved shirt, and sturdy shoes that give good traction on possibly wet floors.
3. Make sure to wear gloves when measuring the KCl.
Effects of Baffles
1. Power and torque do increase with RPM
2. The presence of baffles increase torque and power consumption
3. Mixing is improved and requires less time with the presence of baffles
4. To scale up keep shape factors constant.
They are various linear measurements converted to dimensionless ratios by dividing each one by an arbitrary chosen basis.
The diameter of the impeller Da and the tank diameter Dt were chosen for the base measurement.
These help to estimate the power required to rotate an impeller from empirical correlations of power or power number with other variables. (graphs)
Shape factors are denoted by S1, S2, S3, …, Sn, which can be expressed as follows.
S1=Da/Dt , S2=E/Da , S3=L/Da , S4=W/Da , S5=J/Dt , S6=H/Dt
Assuming the liquid is Newtonian (fluids such as water and most gases which have a constant viscosity), power correlations for power number is expressed as
Da is the impeller diameter, and µ is the viscosity of the fluid
six blade impeller
3 bladed propeller
Describes a mathematical model of the flow pattern in the vessel that can be used to provide an objective measure of mixing time
Mixing Time (Residence time) = V/b2
P = Power in N-m/s
n = rotational speed in rps
τ = observed torque in N-m
Calculations are based of empirical correlations. (Rushton Graphs or MaCabe, Smith and Harriot correlation graphs)
μ= viscosity of water
Da= agitator diameter
n= rotational speed
ρ=density of water
gc = 1
Step by step
Calculate the Reynolds number
Calculate the Froude Number
From the appropriate Rushton graph, Find φ
From φ calculate Np
Find Predictive Power and
six blade impeller
3 bladed propeller
There are multiple purposes of agitation, depending on the objectives of the processing step.
1. Suspending solid particles.
2. Blending miscible liquids, for example, methyl alcohol and water.
3. Dispersing a gas through the liquid in the form of small bubbles.
4. Dispersing a second liquid, immiscible with the first, to form an emulsion or a suspension of fine drops.
5. Promoting heat transfer between the liquid and a coil or jacket.
The significance of the experiment is to understand the basic principle and the factors to consider when designing an agitator vessel.