Agitation final report in Lab 1

1. AGITATION
ELECCION, NICELY JANE R.
Department of Chemical Engineering
College of Engineering and Architecture
Cebu Institute of Technology – University
N. Bacalso Ave., Cebu City 6000
In Geankoplis (2009), agitation refers to forcing a fluid by mechanical means to flow in a
circulatory or other pattern inside a vessel. In this experiment the relationship between
the speed of rotation and impeller diameter on power requirement for baffled and
unbaffled tanks, as well as the relationship between the power number against the
Reynold's number for the baffled and unbaffled tanks are being determined. In this
experiment, the relationship between the power number against Reynold’s number for
baffled and unbaffled tanks can’t be achieved since only the baffled tank was used in the
experiment; however, in literature, power number in baffled tanks are greater than the
unbaffled tanks as the Reynold’s number increases. Greater impeller diameter would also
result to a greater power consumption in an agitator.
Engr. Jennifer M. Fernandez
March 6, 2018

2. 1. Introduction
Agitation is the process of providing bulk motion to a liquid, thus aiding mixing and
dispersion. Agitation of liquids is usually accomplished in a container equipped with
an impeller such as propeller, paddle, or turbine. The impeller is inserted into the liquid
and rotated in such a manner as to cause both bulk motion and fine – scale eddies in
the fluid. Mechanical energy is required to rotate the impeller, which in turn transmits
this energy to the fluid. Agitation in a process generally accomplishes physical
achanges, chamical changes, and/or increased rate of transport and may occur
simultaneously or singly.
Agitation equipment usually consists of a tank to hold the liquid, one or more
impellers to provide the shear flow, a motor or some other means to drive the impeller,
and usually wall baffles, where in the installation permits higher power input. Impeller
design as a strong impact on the agitation characteristics and the energy requirement
in an agitator.
This experiment seeks to understand the effect of speed of rotation on power
requirement for baffled and unbaffled tanks, specifically showing the relationship
between the power number against Reynold’s number and to know the effect of
Impeller Diameter on power requirement of Agitation on baffled and unbaffled tanks.

3. 2. Materials and Methods
2.1 Equipment and Materials
 Agitator – 2 – blade paddle or square – pitch propeller
 Distilled water (fluid)
 Tachometer
 Voltmeter
2.2 Methods
The agitator was opened by unscrewing its knobs and was filled with water in
such a way that the water level must not surpass 4 inches of the top of the baffles
to prevent the water to overflow. The valve was checked to prevent leakage of the
water. The voltmeter was connected with the power source with 12 amperes. The
agitator was closed and screwed shut. The power source and impeller was turned
on. The speed of the impeller was measured using the tachometer and was
recorded. The voltage of the agitator was also measured using the readings of the
voltmeter and was recorded as well. The procedure was repeated to the increase
of the impeller's speed.
3. Results
Table 3.1 Recorded Data of the Baffled Agitator
Trial Voltage (Volts) Power (Watts) Speed (rpm)
1 3.1 9.7 343.7
2 3.9 15.2 398.1
3 4.4 21 443.7

4. Table 3.2 Reynold’s Number and Power Number of Each Trial
Trial No. Reynold’s Number Power Number
1 63816.69 5.2
2 74080.91 5.2
3 82560.00 5.2
Note:
 Due to the unavailability of an ammeter, the current is assumed to be 12
amperes, on the basis of the series connection of the voltmeter and power
source, which has a current of 12 amperes.
 At room temperature, the viscosity and density of the water is assumed to
be 0.8937x10-3 Pa.s and 997.08 kg/m3 respectively.
4. Calculations
At V = 3.1 V
N = 343.7
𝑟𝑒𝑣
𝑚𝑖𝑛
(
1 𝑚𝑖𝑛
60 𝑠𝑒𝑐𝑠
) = 5.72
𝑟𝑒𝑣
𝑠𝑒𝑐𝑠
N’Re =
𝐷𝑜2
𝑁𝜌
𝜇
=
(0.1𝑚)2
(5.72
𝑟𝑒𝑣
𝑠𝑒𝑐𝑠
)(997.08
𝑘𝑔
𝑚3)
0.8937𝑥10−3 𝑃𝑎.𝑠
= 63816.69 or 6.4 x104
Note: Power Number, Np, can be read in Appendix 9.1 of this report.
Np = 5.2
P = NpρN3Do5 = (5.2) (997.08) (5.72)3 (0.1)5 = 9.7 W

5. At V = 3.9 V
N = 398.1
𝑟𝑒𝑣
𝑚𝑖𝑛
(
1 𝑚𝑖𝑛
60 𝑠𝑒𝑐𝑠
) = 6.64
𝑟𝑒𝑣
𝑠𝑒𝑐𝑠
N’Re =
𝐷𝑜2
𝑁𝜌
𝜇
=
(0.1𝑚)2
(6.64
𝑟𝑒𝑣
𝑠𝑒𝑐𝑠
)(997.08
𝑘𝑔
𝑚3)
0.8937𝑥10−3 𝑃𝑎.𝑠
= 74080.91 or 7.4 x104
Note: Power Number, Np, can be read in Appendix 9.1 of this report.
Np = 5.2
P = NpρN3Do5 = (5.2) (997.08) (6.64)3 (0.1)5 = 15.2 W
At V = 4.4 V
N = 443.7
𝑟𝑒𝑣
𝑚𝑖𝑛
(
1 𝑚𝑖𝑛
60 𝑠𝑒𝑐𝑠
) = 7.40
𝑟𝑒𝑣
𝑠𝑒𝑐𝑠
N’Re =
𝐷𝑜2
𝑁𝜌
𝜇
=
(0.1𝑚)2
(7.40
𝑟𝑒𝑣
𝑠𝑒𝑐𝑠
)(997.08
𝑘𝑔
𝑚3)
0.8937𝑥10−3 𝑃𝑎.𝑠
= 82560.00 or 8.3 x104
Note: Power Number, Np, can be read in Appendix 9.1 of this report.
Np = 5.2
P = NpρN3Do5 = (5.2) (997.08) (7.40)3 (0.1)5 = 21 W

6. 5. Sketch

7. 6. Discussion
It has been observed that as the speed of the impeller was increased, the power
requirement increases as well. This explains that increasing the impeller size will also
increase the cost of operation to higher operating costs; however, an impeller
operating at high speed does not automatically mean that it is operating efficiently as
it can lead to a "vortex" (a whirling mass of water that will drag everything to its center).
Furthermore, the Reynold's number in each trial results to a high value, and since it's
greater than 4000, it can be concluded that the flow of water is turbulent. As the
Reynold's number increases, the power number doesn’t show evident changes since
the Reynold’s number ranges from 10^4 and as shown in Figure 9.1 where in a
theoretical graph on the power correlations for various impellers and baffles is given,
at that range, power number is constant and/or is close to the value 5.2. The power
number is also said to be analogous to a friction factor or a drag coefficient. This
means that as the water becomes more turbulent (which is relevant on the Reynold's
Number) the friction of the impeller to the fluid decreases or is constant at a certain
level, making it ineffective in mixing the fluid.
The figure below shows the relationship between the power number against
Reynold’s number for baffled and unbaffled tanks in literature. It is shown that as the
Reynold’s number increases, the baffled tanks’ power number is evidently greater
compared to the unbaffled tanks. Unbaffled tanks also have a constant power number
as its Reynold’s number touches 1000 compared to the fully baffled tanks wherein it
slowly increases as the Reynold’s number increases.

8. 7. Conclusion
In this experiment, the relationship between the power number against Reynold’s
number for baffled and unbaffled tanks is asked and based on the experiment, it can’t
be achieved since only the baffled tank was used in the execution of the experiment;
however, in literature, power number in baffled tanks are greater than the unbaffled
tanks as the Reynold’s number increases as discussed in the previous section.
Greater impeller diameter would also result to a greater power consumption in an
agitator based on the calculations presented in this report.
8. Recommendation
In this experiment, it is best to use the both the baffled and unbaffled agitator
in order to know the relationship of the power number agains the Reynold’s number
as asked in the objectives. It is also recommended that a functioning, high-quality
apparatus/equipment must be used to have proper execution of the experiment by the
students and setting the experiment in the best atmosphere where there are no
distractions and the like that may alter results in order to achieve accurate data
especially in getting the speed of the impeller through the tachometer.
9. Appendix
Figure 9.1. Power correlations for various impellers and baffles

9. 10.References
[1] Geankoplis, C.J. (2009) Principles of Transport Processes and Separation
Processes. 1st edition. Pearson Education South Asia PTE. LTD.
11.Web References
[1] Chapter 6: Agitator. (2005). Retrieved March 4, 2018, from
https://classes.engineering.wustl.edu/eece503/Lecture_Notes/ChE_512_Chapter_6.
pdf
[2] Chapter 9: Agitation. (1998). Retrieved March 4, 2018, from
https://classes.engineering.wustl.edu/eece503/Lecture_Notes/Agitation%20%28STR
%29.pdf