results and discussions conclusions me73

7/31/2019 Results and Discussions Conclusions ME73

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Results and Discussions

The heating value of fuels is one of the properties used to compare characteristics of one

sample from another. The total heating value of the sample fuels were determined by burning the fuel

in a bomb calorimeter and measuring the amount of energy released. In the experiment conducted,

diesel fuel and an unknown sample fuel have been tested. The parts of the experiment were conducted.First, the unknown sample is used and on the second, the diesel sample is used. Three trials were done

for each fuel sample to compare. In the experiment, 2L of water has been used. Bomb calorimeter at

constant volume is a process used to determine the energy of combustion reactions. The constant

volume of the process restricts the energy produced in the reaction to be turned entirely into heat

energy. After the motor stirrer has been turned on and the set up was allowed to stand for 2 minutes,

the temperature at one-minute interval was obtained, this is the pre-period. This is the time were the

fuel hasnt been burned yet, but has already been exposed to the oxygen and water. After 7 minutes of

temperature measurement, the bomb calorimeter has been ignited.

The energy after the ignition is fast and violent. The steel container of the bomb calorimeter

holds in the explosion. In air, not all the samples will burn, but this is not a problem in the bomb. Pure

oxygen promotes combustion, and high-pressure oxygen greatly enhances combustion. The burned fuel

in the calorimeter has evaporated almost completely, leaving fragments of the fine iron wire used. The

calorimeter turns the energy stored in these fuel samples into heat flow. The temperature increase of

the water and metal of the calorimeter reveals the heating values of the samples used.

At this point, there has been a drastic rise in temperature observed. The temperatures were also

obtained every 12 seconds for 3 minutes. This is the rise period. Then for the next 14 minutes, the

temperatures were observed at a one-minute interval. This is the period where the rate of the

temperature change has become constant. This is the post period.

(a.)

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0 1 2 3 4 5 6 7 8 9 1011121314151617181920212223242526

Temperature(C)

Time (s)

Sample 1a (Temperature Vs.

Time)


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(b.)

(c.)

Figure 1. Time versus Temperature plot of the unknown sample in (a.) trial 1, (b.) trial 2, and (c.) trial 3.

(a.)

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Temperature(C)

Time (s)

Sample 1b (Temperature Vs.

Time)

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0 5 10 15 20 25 30

Temperature(C)

Time (s)

Sample 1c (Temperature Vs.

Time)

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Temperature(C)

Time (s)

Sample 2a (Temperature Vs.

Time)


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(b.)

(c.)

Figure 2. Time versus Temperature plot of the diesel sample in (a.) trial 1, (b.) trial 2, and (c.) trial 3.

The time and temperature at firing and the maximum temperature with its corresponding time

were obtained from the graphs. With these gathered data, the temperature at which the reaction

reaches 60% of the total increase was computed using the formula:

The corresponding time b of the temperature at point b is being interpolated within the range it

is closest in the given data. This is assuming that between the two points where temperature b is

located, the relationship between time and temperature is approximately linear. Given that points a, b

and c of the graph is located, the corrected ignition temperature and the corrected final temperature

has been computed using the following equations:

[ ]

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Temperature(C)

Time (s)

Sample 2b (Temperature Vs.

Time)

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Temperature(C)

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Sample 2c (Temperature Vs.

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C The corrected temperature rise is computed by subtracting the corrected ignition Temperature

from the corrected final temperature. From this temperature rise, the heating value of the sample can

be computed using the formula:

( ) Where, mcal,H2O = 2000g, mequiv,cal = 432g, CH2O = 1cal/g

oC.

The tables below show the computed values for b, temperature b, corrected ignition

temperature, corrected final temperature, corrected temperature rise and the heating value.

Table 1. Computed Values for Unknown Sample

Sample1a Sample1b Sample1c

a 7 min 7 min 7 min

Ta 28.299 C 30.29 C 29.771 C

b 8.223804878 min 8.025 min 8.159186047 min

Tb 31.1478 C 32.69 C 32.0978 C

c 13 min 14 min 13 min

Tc 33.047 C 34.29 C 33.649 C

Corrected ignition temperature 28.29253132 C 30.27682143 C 29.75907694 C

Corrected final temperature 33.11734033 C 34.403525 C 33.72425265 C

Corrected temperature rise 4.824809011 C 4.126703571 C 3.96517571 C

Heating Value 11030.24693 cal 11209.65091 cal 11015.42296 cal

Table 1. Computed Values for Unknown Sample.

Table 2. Computed Values for Diesel Sample

Sample2a Sample2b Sample2c

a 7 min 7 min 7 min

Ta 29.314 C 29.01 C 28.789 C

b 8.125856698 min 8.146 min 7.888502994 min

Tb 31.393 C 31.392 C 31.1188 C

c 13 min 14 min 14 min

Tc 32.779 C 32.98 C 32.672 C

Corrected ignition temperature 29.30547566 C 29.00181429 C 28.84510262 C

Corrected final temperature 32.8463518 C 33.073664 C 32.75633866 C

Corrected temperature rise 3.540876142 C 4.071849714 C 3.911236041 C

Heating Value 11019.93945 cal 11194.76496 cal 9153.491517 cal

Table 2. Computed Values for Diesel Sample.


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From the tables, the average heating values for the two samples were obtained. The unknown

sample has an average heating value of 11085.11cal/goC. The sample diesel has an average heating

value of 10456.07cal/goC. To compare the two samples, the percent difference between the heating

values with respect to the diesel fuel which is known.

There is only a 6.02% deviation from the unknown sample compared to the diesel fuel. This

shows that the properties of a diesel fuel are near to the properties of the unknown sample.

Possible source of error that might cause deviation of the experimented data is the problem in

determining the starting temperature of the combustible product and which data point are going to be

the a, b, and c in the temperature versus time graph. Another possible source of error is the reaction

taken place when the bomb is being pressurized at a distance from the laboratory.

Conclusions

It can be concluded that in determining the heating value of a sample fuel with a bomb

calorimeter, the reaction takes place in constant volume. The sample is confined in oxygen atmosphere

and ignited with the thin hot wire. The heat generated during the combustion of the sample raises the

temperature of the water that surrounded the bomb. And since it is known that the reaction takes place

in constant volume, the heat combustion that can be calculated is the internal energy.

The average heating values of the two samples were obtained from the graphs and tables. It is

calculated that the average heating values of the unknown sample and the diesel fuel are

11085.11cal/goC and 10456.07cal/g

oC, respectively. This is computed to have a 6.02% deviation with

respect to the known fuel, diesel. It is also concluded that the properties of both sample are similar due

to the small % deviation.

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