DETERMINATION OF CALORIMETRIC VALUE OF COIL SAMPECalorimetry.ppt

Calorimetry

1. Fu Qin, Principal Chemist, Ph.D.
Sherritt Technologies

2. Calorimetry is the science of measuring
quantities of heat through combustion.
The instruments used for such
measurements are known as calorimeters.
Oxygen bomb calorimeters are often used
for measuring calorific values of solid and
liquid combustible samples.
2

3. Heat of combustion.
Defined as the heat liberated by a certain amount
of sample when burned with oxygen in an
enclosure of constant volume.
Expressed as:
j/g or cal/g or Btu/lb
3

4. 1 cal/g = 4.1868 j/g
1 Btu/lb= 2.326 j/g
1 cal/g= 1.8 Btu/lb
j = joule
cal = calorie
Btu = British thermal unit
4

5. 5

6. 6

7. 7

8. Weigh about 1 g of sample into a sample holder.
Place the sample in the bomb.
Connect a measured piece of ignition fuse.
Close the bomb and sink it into water bucket.
Apply a 25 to 35 bar pressure from an oxygen tank.
Start the combustion and observe the temperature change in
the water bucket.
The temperature change is usually from 3 to 5
o
C and with a
precision from 0.01 (using a mercuric thermometer) to 0.0001
o
C (using a thermister).
8

9. 9

10. Pre-period
To gain temperature baseline. The rise in
temperature is due to the heat generated by
stirring. This will be corrected by software.
Rise period
Combustion heat drives the temperature of
the whole system up.
Post period
Temperature drops due to heat loss to the
environment. This will be corrected.
10

11. C.V., (gross) = ((tE) - e1 - e2 - e3 - e4)/m
Where,
E = Heat capacity of the calorimeter (j/g)
t= Corrected temperature rise (
o
C)
e1= Acid correction (j)
e2= Fuse correction (j)
e3= Sulphur correction (j)
e4= Combustion aid correction (j)
m = Mass of the sample (g)
11

12. The temperature rise is usually between 3 to
5
o
C with precision of 0.0001
o
C at the best.
The heat loss between the water bucket and
the jacket is corrected by the instrument
software in a modern instrument.
12

13. Follow the sample determination procedure with ONE
benzoic acid tablet as samples.
E = ((Hc  m) + e1 + e2)/t
Hc= 26,435 (j/g) for benzoic acid
m = Mass of benzoic acid (g)
e1= 43 j, acid correction
e2= 55 j, fuse correction (done by instrument)
t= Temperature rise (
o
C)
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14. Heat capacity is vessel specific (since every vessel is slightly different),
therefore the Heat Capacity is different.
Each vessel has its own Heat Capacity.
The calibrated bombs are labeled by black strips.
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16. Nitrogen trapped in the bomb is oxidized and
combined with water vapor to form nitric acid.
Under a normal combustion, nitrogen does not
form nitric acid at all.
N
2
(g) + 5/4O
2
(g) + H
2
O (l) = HNO
3
e1 = V (j)
where,V = mL of 3.76 g/L Na
2CO
3 consumed
Sulphur is combustible but does not form SO
3
under normal conditions.
SO
2
+ 1/2O
2
(g) = SO
3
SO
3
+ H
2
O = H
2
SO
4
Sulphuric acid formed from sulphur is titrated
by Na
2CO
3 and also be partially corrected here.
16

17. Fuse used to ignite the sample contributes
to the total heat generated, therefore,
must be subtracted from the total heat.
For a modern calorimeter, fuse correction
is just a constant entered into the system.
The system will do the correction
automatically.
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18. The residual difference of formation heat
between HNO
3 and H
2SO
4 is corrected here.
e3 = 55.45 j/g  m  S%
where,
m= Mass of sample
S%= Percent sulphur in sample
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19. Some samples burns slowly, therefore a
complete combustion is very hard to achieve.
Combustion aid is needed to speed up the
combustion.
The heat generated from the combustion aid
should be subtracted from the total heat.
Common combustion aids are:
benzoic acid, ethylene glycol, butyl alcohol or
decalin.
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20. 20
0.26%0.17%0.00%
31.35%
68.22%
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
Heat%

21. Coal is an ideal material for the
calorific value determination. It does
not need any combustion aid at all.
Only two corrections are need for the
corrected C.V. calculation:
Acid correction
Sulphur correction
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22. Rinse the bomb after the combustion is
finished.
Collect all the rinses into a Mettler titration cup.
Titrate this solution using 1 N NaOH.
The autotitrator will print the equivalent volume
of 3.76 g/L Na
2CO
3 in mL.
1 mL of 1 N NaOH is 14.1 mL of 3.76 g/L
Na
2
CO
3
equivalent.
22

23. Sulphur% is determined by using Leco
C/S analyzer.
This determination does not need any
combustion aid.
Sulphur correction term:
= 55.2 J/g  S%  Sample Weight (g)
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24. If acid correction value (V, mL) and sulphur
correction value (S%) are not entered in the
calorimeter, the machine reported calorific
value needs off-line correction.
C.V., corrected = (C.V., printed - e1 - e3)/m
where,e1= mL of 3.76 g/L Na
2CO
3
e3=55.2 J/g  S%  m
24

25. In the lab operation, one just needs
to enter the volume of 3.76 g/L
Na
2CO
3 and S% into an Excel spread
sheet. The corrected C.V. will be
generated automatically.
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26. 26
Semi-Automated

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Weigh not over 1 g of
coal sample.
The instrument will
pass current to the
ignition wire to made
it red hot.
The red hot ignition
wire ignites the
cotton thread.
 The cotton thread
pass the small fire to
the sample.

30. 30
The bomb pressure release valve needs to be
cleaned every month.
The Measurement Cell pressure release
needle mechanism needs to be cleaned every
3 months.
The water batch needs to be cleaned every 12
months.