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Introduction
The objective of the experiment conducted was to determine the energy
performance of the reboiler in the given distillation column system. A distillation column
is used for separation processes when a mixture is used and a pure substance is required
and/or desired. Water was used as the liquid for the experimental system in the lab. The
experiment ran smoothly with no detected problems and continuously with no need for
stoppage. All experiments were conducted in EMCS 120.
1
Theory
dT
 Qin  Qloss
Figures 1 anddt
2 describe what is taking place in the following equations.
M cw * C p *




dT
dt is calculated data
wise and graphically
Cp is looked up in a
handbook
Mcw and Qin are
measured
Now we can determine
Qloss
U 
Qloss
Mcw = 6.5 kg
Qloss
Qin = 2750
W
Figure
A * (Treboiler
1Troom )
TRoom
Qin
Qloss
Treboiler
Area = 0.44 m2
Figure 2
The calculations for this experiment were based on two equations. The second is
dependent on the first. The main equation for unsteady state energy performance is (See
Figure 1):
M cw * C p *
dT
 Qin  Qloss
dt
2
Where Mcw is the mass of cold water in kilograms (kg), Cp is the heat capacity of water
in Joules (J) per kg-oC, dT per dt is the change in temperature (oC) per change in time
(minutes), Qin is the heat input in Watts (W), and Qloss is the heat loss in Watts (W). Qloss
is solved for and used in the next equation. The equation for heat loss is as follows (See
Figure 2):
Qloss  U * A * (Treboiler  Troom)
Where A is the surface area of the reboiler in m2, T is the temperature in oC, and U is the
heat transfer coefficient in W per m2-K. The heat transfer coefficient is what is solved for
from this equation. The units for temperature, in Celsius or Kelvin, do not matter in this
case because the change in temperature is the same for either.
3
Equipment
Figure 3 is a schematic drawing of a reboiler. Heat is input through the calrod
heaters. The reboiler heats the liquid until it reaches its boiling point. The vapor leaves
the reboiler makes its way through the distillation column. Figure 4 is a picture of the
actual reboiler from the distillation column located in EMCS 120 that was used for the
experiment.
Figure 3
Figure 4
4
Procedure
The experiment began by filling the feed tank with water. The feed pump was
then turned on and set at a desired rate to pump water to the reboiler. Once the reboiler
reached the correct level, the feed was turned off and heat was added to start the
separation process. Temperature measurements were taken from the reboiler from the
beginning (room temp) all the way to steady state (boiling point of water) in increments
of one minute. All measurements were taken by the computer system connected the
distillation column. The only physical measurement was the measure of the mass of water
in the reboiler. The reboiler was drained, and the volume of water collected was
measured. To get the mass, the volume was multiplied by the density.
5
Results
Heat loss was calculated from a known heat input, looking up heat capacity in a
handbook, measuring the mass of water physically, and calculating the change in
temperature with the respect to time graphically from data collected. Heat loss, surface
area of the reboiler, and the difference in temperature between the reboiler and the room
was used in calculations to find the heat transfer coefficients.
Figure 5 is a graph of temperatures during the time interval the experiment was
conducted. The transient portion of the graph is where
dT
is found.
dt
Reboiler Temperature Rate
Transient Qin = 2750 W
Temp (deg C)
120
100
80
Qin = 0
60
40
Steady State dT/dt = 0
20
0
10
20
30
time (min)
Figure 5
6
40
50
60
Figure 6 describes the change in temperature with the respect to time through this
temperature range. Data from the transient portion of was plotted versus the temperature
at which the change in temperature with respect to time occurred.
Temp Rate vs Temp
dT/dt (oC/min)
5
4
3
2
1
60
65
70
75
80
Temp (oC)
7
85
90
95
100
Figure 7 shows the heat loss during the range of temperatures as the system
approaches steady state. The data from this graph is used to calculate U.
Qloss vs Temp Reboiler
2500
Qloss (Watts)
2000
1500
1000
500
Figure
0 6 shows U, the heat transfer coefficient, during the temperature range
60
65
70
75
80
85
90
95
100
Figure 6
Temp Reboiler (oC)
Figure 7
8
We were only concerned with normal operating conditions of the reboiler (65 oC
– 97oC). Figure 8 shows U, the heat transfer coefficient, during the temperature range.
Heat Transfer Coeffiient vs Temp Boiler
U (W/sq m-K)
65
60
55
50
45
40
60
70
80
90
100
Te mp Boile r (oC)
Figure 8
The average heat transfer coefficient, U, was calculated to be 51 W per m2-K.
Table 1 is a list of variables, known and/or measured, used for calculations
Table 1
A (m2)
0.44
Mw (kg)
6.5
Qin (W)
2750
9
Cp
(J/kgo
C)
4184
Troom
(oC)
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Discussion of Results
The change in temperature with respect to time decreases as time and/or
temperature increases. This occurs with the reason being… as the system approaches
steady state, the change in temperature with respect to time approaches zero. Steady state
is defined as no change (zero) in temperature with respect to time. Heat loss increases as
the temperature of the reboiler increases. Heat is transferred out of the reboiler and to
other parts of the distillation column. The average U for this experiment was 51 W per
m2-K.From page 8 of Fundamentals of Heat and Mass Transfer, 4th Edition by Frank P.
Incropera and David P. DeWitt, the typical values of U, the heat transfer coefficient, are
50-1000 W per m2-K.
Conclusions
Transient analysis of the reboiler energy performance was conducted. The
average heat transfer coefficient, U, was calculated to be 51 W per m2-K. It has a
standard deviation of ± 8 with a confidence level of 95%. That means that 95% of values
for U will be between 43 and 59 W per m2-K.
10
Appendix
Sample Calculation
M cw * C p *
dT
 Qin  Qloss
dt
rearranging gives
Qloss  Qin  M cw * C p *
dT
dt
Qloss = (2750 W)-(6.5 kg)*(4184 J/kg-oC)*(4.11 oC/min)*(min/60 sec)
Qloss = 885 W
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