Download Determination of Heat of Combustion of Liquid

The Pennsylvania State University
College of Earth and Mineral Sciences
Department of Energy and Mineral Engineering
Petroleum and Natural Gas Engineering Program
PNG 482- Production Engineering Laboratory
Experiment Number 8- Determination of Heat of Combustion of
Liquid Hydrocarbons by Bomb Calorimeter
Instructor: Dr. Luis F. Ayala
Lab Assistant: Madhu Singh
Luke Spinuzza
Section 004
Group 10
Date Performed: March 6, 2015
Date Submitted: March 18, 2015
Results and Calculations
In this laboratory the lab group determined the heat of combustion of various liquid
hydrocarbons utilizing a bomb calorimeter. Additionally the lab group calibrated their results by
burning a benzoic acid tablet to ensure accurate results. The results of the experiment are
summarized below in table 1.
Table 1.
Chemical Name:
Initial Fuse Length (cm):
Final Fuse Length (cm):
Burned Fuse Length (cm):
Sample Weight (g):
Initial Temperature (Celsius):
Final Temperature (Celsius):
Heat Released (BTU/lb):
Benzoic Acid Tablet Crude Oil Ethanol
16
16
16
9.3
8
6.5
6.7
8
9.5
1.01
1.02 1.067
20.48
20.45 24.48
24.9
28.7 29.74
10853
20000 12968
Initial Burette Volume (cc):
Final Burette Volume (cc):
Volume used in titration (cc):
22
24.1
2.1
Time (min)
24.1
34.1
10
0.5
11.9
11.4
Temperature Change of Water (Celsius)
1
1.09
1.47
1.07
2
2.63
5.77
3.56
3
3.81
7.27
4.55
4
4.19
7.98
5.05
5
4.42
8.25
5.26
Net Corrected Temperature Rise:
4.42
8.25
5.26
As can be seen from table 1 the first calculation (determining the net temperature change) was
not necessary as the machine gave temperature change not final temperature when reading.
However, the final temperature was determined for the sake of being thorough through the
following equation:
𝑇𝑓 = 𝑇𝑖 + ∆𝑇
Where,
Tf = Final temperature (Celsius)
Ti= Initial temperature (Celsius)
∆𝑇= Temperature change (Celsius)
The values obtained for each reaction are already presented in the table above due to the ease of
the calculation.
The next step in correcting the machine-read values is to calculate thermos-chemical
corrections. There are three of these corrections and they are listed below.
𝑒1 = 𝑐1 ∗ 1
𝑐𝑎𝑙
𝑚𝑙
𝑒2 = 𝑐2 ∗ 𝑚 ∗ 14
𝑒3 = 𝑐3 ∗ 2.3
𝑐𝑎𝑙
𝑔
𝑐𝑎𝑙
𝑐𝑚
Where,
e1= correction for heat of formation of nitric acid
e2= correction for heat of formation of sulfuric acid
e3= correction for heat of combustion of fuse wire
and,
c1= ml of standard alkali solution used in titration
c2= percentage of sulfur in the sample (lab manual instructs to assume 0%)
c3= cm of fuse consumed in firing process
m= mass of the sample in grams
These calculations were performed for each individual chemical that was tested and the results
obtained are presented in table 2 on the next page.
Table 2.
Chemical Name: Benzoic Acid Tablet Crude Oil Ethanol
e_1 (cal)
2.1
10
11.4
e_2 (cal)
0
0
0
e_3 (cal)
15.41
18.4
21.85
Please note that the values of e2 are zero for each sample because the assumption was made that
there was no sulfur present.
The next step was to perform a benzoic acid standardization. This will give a value
which can be used to standardize the machines readings to the particular system. The equation
can be found reproduced below.
𝑊=
(𝐻 ∗ 𝑚) + 𝑒1 + 𝑒2
𝑇
Where,
W= energy equivalent of calorimeter in calories per degree Celsius
T= temperature change in degrees Celsius
H= heat of combustion of benzoic acid in cal/g
Before applying this equation however it is necessary to convert all of the heat of combustion
values read from the machine in BTU/lb to cal/g. This was done in the following manner:
1 BTU / pound =0.555927342 calories / gram
Utilizing this conversion this data is presented along with the value of W and percentage error in
table 3. The percentage error was calculated using the literature value of 2416 cal per degree
Celsius as provided by the lab manual.
Table 3.
Chemical Name:
Benzoic Acid Tablet Crude Oil Ethanol
Heat Released (cal/g):
6029.44 11111.11 7204.445
W (cal/deg_Celsius):
1381.73 N/A
N/A
Percentage Error:
42.81 N/A
N/A
As can be seen a fairly large error was experienced. This and possible contributing factors to this
error will be discussed in the discussion section of this lab report.
The final step in this laboratory is to calculate the gross heat of combustion which can be
done using equation 1 from the lab manual reproduced below.
𝐻𝑔 =
(𝑇 ∗ 𝑊) − 𝑒1 − 𝑒2 − 𝑒3
𝑚
Where,
Hg= gross heat of combustion in cal/g
This equation was applied to the current data set and the results are provided in table 4 below.
Table 4.
Chemical Name:
Benzoic Acid Tablet Crude Oil Ethanol
Gross Heat of Combustion (cal/g)
6029.44 11147.92 6780.37
Gross Heat of Combustion (BTU/lb)
10853.00 20066.25 12204.66
These results show what was expected from the crude oil and the ethanol. The lab group had
expected the energy of the crude oil to be greater because of the larger hydrocarbons present.
Discussion
In this laboratory the lab group successfully used a bomb calorimeter to determine the
heat of combustion of two different hydrocarbons and calculated the gross heat of combustion
based on benzoic acid standardization. The group had anticipated prior to beginning the
experiment that the crude oil would have a greater heat of combustion than the ethanol sample.
This is due to the fact that crude oil contains many larger hydrocarbon components which
translates to a greater energy potential. Additionally, it can be noted that hydrocarbons such as
ethanol can be extracted from crude oil at refineries through distillation and various other
processes. This is also how other pure hydrocarbons like butane are produced. Knowing this it
is easy to see why the crude would be expected to generate more heat energy.
This laboratory was not completed without issues however, and this can be seen simply
from the percentage error found in the benzoic acid standardization process. There are several
potential reasons behind this large error all of which should be considered cumulatively. The
first and most apparent error is that at the time of our lab the o-ring on the calorimeter would
leak randomly. It did not do this for every run but once the lid was closed on the machine we
would not know if ignition could possibly initiate another leak. This could cause error because
the calorimeter would contain less oxygen for the reaction to go to completion.
The remaining sources of error are inherent to this particular type of experiment and not
specific to our lab group’s experience. The accuracy of a bomb calorimeter relies on the reaction
taking place within a closed system, in other words this intended to be an adiabatic reaction.
However, in order to start the reaction we must initiate it with a fuse which we account for in the
correction calculations. This has inherent error as the very thin wire can be difficult to measure
accurately especially when it breaks into multiple small pieces. Additionally, there is no way to
perfectly insulate the chamber to prevent any heat loss to the rest of the equipment or
atmosphere. Further the accuracy of the temperature sensor must be taken into account as all
equipment has limitations. Finally, it is very possible that the water in the chamber was not in
full equilibrium before firing occurred. To achieve full equilibrium could take a long time, so in
the interest of limited lab time when the group did not notice a change in temperature the test
was began. This could have been remedied by waiting much longer.
Conclusions
This laboratory involved testing the heat of combustion of both ethanol and crude oil.
This was accomplished through the use of a bomb calorimeter and the results were calibrated for
multiple factors. The values were corrected for the factor of the fuse wire burning in the bomb
along with the formation of nitric acid. Sulfur content was assumed to be zero. Then these
corrected values were adjusted once more utilizing benzoic acid standardization. This
standardization did have a large error but the sources of that error were covered in the discussion
portion of the lab report.
After completion of the experiment and calculations it became apparent that the crude oil
had a much larger heat of combustion (almost double) than that of ethanol. This is what the lab
group had predicted would be the result but that does not diminish the findings. Larger chain
hydrocarbons contain more energy than smaller chain hydrocarbons. Additionally crude oil
contains both long and short chain hydrocarbons which can be separated and sold as pure
substances or as mixtures.
The purpose stated in the pre lab portion of this lab report was: “…to become familiar
with the constant volume bomb calorimeter and to test the heat of combustion of various organic
substances relative to a benzoic acid standard.” This purpose has been accomplished and more.
In addition to accomplishing the previously stated goals the lab group was also forced to become
familiar with the limitations and potential sources of error of a bomb calorimeter due to the high
deviation from literature data. Errors in experimentation are not always for naught and many
times more can be learned from an experiment that was not entirely performed correctly than one
that had no error.
Post-lab Questionnaire
Responding to Question 2
Heat of combustion is not a very specific term and must be made more specific in order
to truly convey the information accurately. The gross heat of combustion was determined in this
laboratory experiment and is essentially the heat released by combustion of a unit mass of fuel in
a constant volume bomb after the water vapor produced has condensed back to the liquid phase.
The constant volume constraint means that pressure is not constant and neither is temperature
making this an adiabatic process.
The other type of heat of combustion is net heat of combustion. This is similar to gross
heat of combustion but occurs at constant pressure rather than constant volume. The net heat of
combustion is the heat released by combustion of one unit mass of fuel at one atmosphere
pressure and the water remaining in the vapor state. This would be the heat you could expect to
utilize if that fuel was used for something such as cooking. For example a can of some sort of
camping cooking fuel relies on net heat of combustion because the situation is not constant
volume and any water vapor produced likely escapes around the cookware.
References
PNG 480 Lab Manual
Adiabatic Bomb Calorimeters
http://www.cal2k.com/index.php/adiabatic-bomb-calorimeters
Last accessed: March 18, 2015