A Bomb Calorimeter Practical Experiment Environmental Sciences Essay

A bomb calorimeter is an instrument used to determine the heat of combustion generated when a weighed amount of fuel is burned in a controlled environment. The colorimeter device allows for a controlled ignition of a multitude of fuels whilst constantly measuring the temperature of combustion. These temperature measurements are then converted to an energy output for the fuel [3]. This experiment entails the testing of five different fuels.

AIM OF THE EXPERIMENT

The aim of the bomb calorimeter experiment is to determine the heat capacity of a specific mass of a combustible fuel and from this determine the specific heat capacity of the fuel. The five fuels that will be tested are:

Petrol

Diesel

Methylated Spirits

Paraffin

Ethanol

APPARATUS

The following apparatus are used to perform the bomb calorimeter experiment:

Calorimeter

Filling Station (Oxygen)

Oxygen

Vessel

Preparation Stand

Sartorius Balance

Firing Cotton

Combustible Fuels – Petrol (0.5g)

Diesel (0.5g)

Methylated Spirits (0.5g)

Paraffin (0.5g)

Ethanol (0.5g)

Computer to Record Results

Figure 3 Bomb Calorimeter Apparatus [2]eco group.jpg

THEORETICAL BACKGROUND

In the following section the relevant literature that pertains to this experiment will be presented.

Bomb Calorimeters

The bomb calorimeter used in the practical experiment was the CAL2K-ECO bomb calorimeter. In the past calorimeters used a mass of water to insulate the combustion vessel, the temperature increase of the water surrounding the vessel was then used to calculate the energy increase due to combustion. The following figure depicts the general layout of a water insulated bomb calorimeter.

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Figure 4 Wet Bomb Calorimeter [4]

This large body of water surrounding the bomb calorimeter provides stability to the system. New advanced in calorimeter systems have lead to the development of dry isothermal calorimeter systems [3]. This new development allows for greater accuracy as changes in water volume affect the experimental results. The lack of water surrounding the colorimeter removes an experimental parameter and improves the results. The dry vessel used in this experiment utilises a conductive aluminium shell which is shrunk fit on to the vessel. Temperature sensors are then placed equally between the vessel and the conductive aluminium casing [3].

The vessel is purged and filled with oxygen at a pressure of 3 MPa, this provides an oxygen rich environment for the reaction to take place.

The calorimeter used in this experiment has the following specifications: [2]

Operating Temperature: 0 – 60ËšC

Temperature Resolution: 0.000001ËšC

Repeatability: 0-1%

Resolution: 0.001 MJ/Kg

Combustion Equations

Ethanol

When ethanol is combusted in an oxygen rich environment the following complete combustion equation is yielded:

Ethanol: C2H5OH

Oxygen: O2

Combustion Equation:

C2H5OH + 3O2 = 2CO2 + 3H2O + Heat

Diesel

When diesel is combusted in an oxygen rich environment the following complete combustion equation is yielded:

Regular Pure Diesel: C12H23

Oxygen: O2

Combustion Equation:

4C12H23+ 71O2 = 48CO2 + 46H2O + Heat

This equation represents the combustion of pure diesel which is not practical as diesel contains many additives and chemicals that will affect the combustion equation.

Petrol

When petrol is combusted in an oxygen rich environment the following complete combustion equation is yielded:

Regular Pure Petrol: C8H18

Oxygen: O2

Combustion Equation:

2C8H18+ 25O2 = 16CO2 + 18H2O + Heat

The above equation pertains to pure petrol. Petrol comes in many different forms with different octane ratings that would significantly change the chemical combustion equation.

Specific Heat Capacity

The specific heat capacity of a substance is the amount of energy that is required to raise a mass of 1 kilogram of the substance by 1ËšC or 1K. The heat capacity of a substance can be measured by the constant pressure specific heat (Cp) or the constant volume specific heat (Cv)

The constant pressure specific heat allows for the measuring of the change in enthalpy of a substance at a specific temperature change. [6]

The constant volume specific heat allows for the determination of internal energy at a constant volume over a change in temperature. [6]

For the purpose of this experiment the constant volume specific heat will be calculated although the constant pressure specific heat for uncompressible solids and liquids yields similar results. [6]

The amount of energy required to raise the temperature of a mass of a substance is calculated using the following formula:

Q = CvmΔT [6]

Where Q: Heat Added (MJ/Kg)

Cv: Constant Volume Specific Heat (KJ/Kg.K)

m: Mass of substance (Kg)

ΔT: Change in temperature (K)

Rearranging the equation for the purpose of this experiment yield the following equation with the specific heat as the subject of the formula:

METHOD

The following method was followed in order to perform the bomb calorimeter test:

The calorimeter crucible was cleaned and placed on the sartorius balance.

The balance was then zeroed in order to take into account the mass of the crucible.

Approximately 0.5g of the combustible fluid is then placed in the crucible.

The crucible with the combustible fluid is then weighed on the balance and the mass of the fluid recorded.

The mass of the fluid is then entered in to the calorimeter.

The crucible was then placed on the setup station where the crucible is fitted with firing cotton and the firing wire.

This setup is placed in the vessel where it is then filled with 3 MPa’s of pure oxygen.

The vessel was then placed in the calorimeter and the testing sequence was initiated.

After the test was completed the results were recorded into an excel file automatically by the calorimeter.

RESULTS

Petrol

On the 12th of April 2010 petrol of 0.58g was placed into the bomb calorimeter and the following results were produced:

Table Calorimeter Results for Petrol

Parameter

Value

Mass of Sample

0.58g

Minimum Temperature

22.3011ËšC

Maximum Temperature

38.98523ËšC

Change in Temperature (Max. Temp – Min. Temp)

16.68413ËšC

Specific Energy Released

37.0041 MJ/Kg

Figure 6 Graph Representing Sample Temperature vs. Time for Petrol

Using the results obtained from the bomb calorimeter a table was generated in order to calculate the heat capacity as well as the specific heat capacity of the fluid sample. The following table presents the results:

Table Theoretical Calculations for Petrol

Parameter

Formula

Value

Change in temperature (ΔT)

ΔT = Max. Temp – Min. Temp

16.68413ËšC

Specific Energy Released (U)

From Data

37.0041 MJ/Kg

Mass of sample (m)

Recorded in experiment

0.58g = 0.00058Kg

Specific Heat Capacity (Cv)

Cv = U/ΔT

2.217922 KJ/Kg.K

Heat Capacity (Cv.m)

Cv.m = (U.m)/ΔT

1.286395 J/K

From the table above the bomb calorimeter for 0.58 grams of petrol yielded the following results:

Specific Heat Capacity: 2.217922 KJ/Kg.K

Heat Capacity (0.58g Petrol): 1.286395 J/K

Diesel

On the 12th of April 2010 diesel of 0.5269g was placed into the bomb calorimeter and the following results were produced:

Table Calorimeter Results for Diesel

Parameter

Value

Mass of Sample

0.5269g

Minimum Temperature

25.84436ËšC

Maximum Temperature

44.15795ËšC

Change in Temperature (Max. Temp – Min. Temp)

18.31359ËšC

Specific Energy Released

43.4423 MJ/Kg

Figure 6Graph Representing Sample Temperature vs. Time for Diesel

Table Theoretical Calculations for Diesel

Parameter

Formula

Value

Change in temperature (ΔT)

ΔT = Max. Temp – Min. Temp

18.31359ËšC

Specific Energy Released (U)

From Data

43.4423 MJ/Kg

Mass of sample (m)

Recorded in experiment

0.5269g = 0.0005269Kg

Specific Heat Capacity (Cv)

Cv = U/ΔT

2.372135 KJ/Kg.K

Heat Capacity (Cv.m)

Cv.m = (U.m)/ΔT

1.249878 J/K

From the table above the bomb calorimeter for 0.5269 grams of diesel yielded the following results:

Specific Heat Capacity: 2.372135 KJ/Kg.K

Heat Capacity (0.5269g Diesel): 1.249878 J/K

Methylated Spirits

On the 15th of April 2010 methylated spirits of 0.5865g was placed into the bomb calorimeter and the following results were produced:

Table Calorimeter Results for Methylated Spirits

Parameter

Value

Mass of Sample

0.5865g

Minimum Temperature

23.59523ËšC

Maximum Temperature

36.40363ËšC

Change in Temperature (Max. Temp – Min. Temp)

12.8084ËšC

Specific Energy Released

27.8794 MJ/Kg

Figure 6 Graph Representing Sample Temperature vs. Time for Methylated Spirits

Table Theoretical Calculations for Methylated Spirits

Parameter

Formula

Value

Change in temperature (ΔT)

ΔT = Max. Temp – Min. Temp

12.8084ËšC

Specific Energy Released (U)

From Data

27.8794 MJ/Kg

Mass of sample (m)

Recorded in experiment

0.5865g = 0.0005865Kg

Specific Heat Capacity (Cv)

Cv = U/ΔT

2.17665 KJ/Kg.K

Heat Capacity (Cv.m)

Cv.m = (U.m)/ΔT

1.276605 J/K

From the table above the bomb calorimeter for 0.5865 grams of methylated spirits yielded the following results:

Specific Heat Capacity: 2.17665 KJ/Kg.K

Heat Capacity (0.5865g Methylated Spirits): 1.276605 J/K

Paraffin

On the 16th of April 2010 paraffin of 0.5262g was placed into the bomb calorimeter and the following results were produced:

Table Calorimeter Results for Paraffin

Parameter

Value

Mass of Sample

0.5262g

Minimum Temperature

23.47467ËšC

Maximum Temperature

42.94966ËšC

Change in Temperature (Max. Temp – Min. Temp)

19.47499ËšC

Specific Energy Released

46.2688 MJ/Kg

Figure 6 Graph Representing Sample Temperature vs. Time for Paraffin

Table Theoretical Calculations for Paraffin

Parameter

Formula

Value

Change in temperature (ΔT)

ΔT = Max. Temp – Min. Temp

19.47499ËšC

Specific Energy Released (U)

From Data

46.2688 MJ/Kg

Mass of sample (m)

Recorded in experiment

0.5262g = 0.0005262Kg

Specific Heat Capacity (Cv)

Cv = U/ΔT

2.375806 KJ/Kg.K

Heat Capacity (Cv.m)

Cv.m = (U.m)/ΔT

1.250149 J/K

From the table above the bomb calorimeter for 0.5262 grams of paraffin yielded the following results:

Specific Heat Capacity: 2.375806 KJ/Kg.K

Heat Capacity (0.5262g Paraffin): 1.250149 J/K

Ethanol

On the 19th of April 2010 ethanol of 0.5204g was placed into the bomb calorimeter and the following results were produced:

Specific Heat Capacity: 2.376786 KJ/Kg.K

Heat Capacity (0.5204g Ethanol): 1.236879 J/K

DISCUSSION OF RESULTS

The results from the bomb calorimeter were collated into a table and compared to theoretical results. The calorimeter results and the theoretical results were then compared and a percentage error calculated.

Table Comparisons of Calorimeter Results and Theoretical Results [1]

Fuel

Specific Heat Capacity (Calorimeter)

Specific Heat Capacity (Theory)

Percentage Difference (%)

Petrol

2.217922 KJ/Kg.K

2.22 KJ/Kg.K

0.093604

Diesel

2.372135 KJ/Kg.K

2.01 KJ/Kg.K

18.0167

Methylated Spirits

2.176650 KJ/Kg.K

2.40 KJ/Kg.K

9.30625

Paraffin

2.375806 KJ/Kg.K

2.13 KJ/Kg.K

11.5402

Ethanol

2.376786 KJ/Kg.K

2.30 KJ/Kg.K

3.33852

The specific heat calculated for petrol using the bomb calorimeter yielded a resultant specific heat of 2.217922KJ/Kg.K, this was compared to the theoretical value of 2.22KJ/Kg.K. The percentage difference between theoretical and practical results was only 0.094% and the result can therefore be regarded as relatively accurate.

The percentage difference between theoretical values and practical values for ethanol yielded a difference of 3.34%. This difference in calculated specific heat is possibly due to the theoretical value being taken at an ambient temperature of 20ËšC whilst the calorimeter value was taken at 21.69ËšC. The error calculated for ethanol is in the acceptable range.

The specific heats calculated for the other three samples (Diesel, Methylated Spirits, Paraffin) all yielded large experimental errors. The large errors produced may be due to the mass of sample used and measured. Once the samples were decanted into the crucible they began evaporating. Once the mass of the sample was measured it still needed a fuse added to the sample and the sample needed to be placed into the vessel. This reduced the mass of the sample and may have given an inaccurate result.

CONCLUSION

From the results obtained from the bomb calorimeter practical the results were comparable to those from other literature sources. The percentage error for two of the five tests yielded accurate results where as the other three test showed an error ranging from 9 – 18%. This error was justified by experimental errors from the mass entered into the bomb calorimeter as well as differences in atmospheric temperature between the practical results and the results in the relevant literature researched.

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