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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: ππ = ππ + βπ Where, Tf = Final temperature (Celsius) Ti= Initial temperature (Celsius) βπ= 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. π1 = π1 β 1 πππ ππ π2 = π2 β π β 14 π3 = π3 β 2.3 πππ π πππ ππ 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. π= (π» β π) + π1 + π2 π 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. π»π = (π β π) β π1 β π2 β π3 π 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 groupβs 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: ββ¦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.β 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