1 TECH-LAB: EXPERIMENT 31-ME. DETERMINATION OF HEAT OF COMBUSTION OF BIOFUELS. Instructors dr hab. Grzegorz Litwinienko (room 132, consultations: Thursdays 13 00 -14 00 ) (some parts of the text were translated into English by mgr Agnieszka Krogul) LITERATURE: 1. R. T. Morrison, R. N. Boyd: Chemia organiczna, tom 2 (rozdz. 33: Tłuszcze) PWN, Warszawa 1985. (in Polish) or R. T. Morrison, R. N. Boyd: Organic Chemistry, vol. 2.(in English). 2. S. Bredsznajder, W. Kawecki, J. Leyko, R. Marcinkowski: Podstawy ogólne technologii chemicznej, PWN Warszawa 1973. (in Polish) 3. S. E. Manahan: Environmental Chemistry, Brooks/ Cole Publishing Company, 1984. 4. H. Koneczny: Podstawy technologii chemicznej, (rozdz. V: Paliwa i ich przerób), PWN, Warszawa 1973. (in Polish) 5. R. Bogoczek, E. Kociołek-Balawejder: Technologia chemiczna organiczna, rozdz. 2, WAE, Wrocław 1992. (in Polish) 6. Spalanie i paliwa praca zbiorowa pod red. J. Kordylewskiego, Oficyna Wydawnicza Politechniki Wrocławskiej, Wrocław 2005. (in Polish) 7. J. Ciborowski: Inżynieria chemiczna. Inżynieria procesowa, WNT, 1973. (in Polish) ATTENTION: Student are obliged to search information on the presented subjects from additional sources (physical chemistry and thermochemistry textbooks from the library, internet sources etc.) Requirements Definitions: energy and conversion of energy, fuels, biofuels (bioethanol, biodiesel), heat / thermal capacity, calorimetric conctant, combustion, diffusion, fats and vegetable oils, fatty acids and their esters, triglycerides (triacylglycerols), FAME (fatty acid methyl esters), physical and chemical properties of fats. 1. Combustible materials (flammable substances). Fuels – definition and classification. Combustion processes, chain reactions, heat of combustion and calorific value , explosions, explosion limit, spark- ignition engine, compression-ignition engine (Diesel engine, diesel). Cetane Number, Octane Number. 2. Basic concepts and definitions of thermodynamics and thermochemistry: functions of state, thermodynamic functions, heat, work, laws of thermodynamic and thermochemistry (Hess’s law, Kirchoff’s law). 3. Heat transfer and mechanisms of heat exchange (heat conduction, convection, thermal radiation). 4. Calorimeters: definition, design, the calorimeter constant (heat capacity of calorimeter), typical calorimetric plot of temperature versus time. 5. Knowledge on the procedures of analysis of biofuels and analysis of combustion products, described in this manual.
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1
TECH-LAB: EXPERIMENT 31-ME.
DETERMINATION OF HEAT OF COMBUSTION OF BIOFUELS.
Instructors
dr hab. Grzegorz Litwinienko (room 132, consultations: Thursdays 1300
-1400
)
(some parts of the text were translated into English by mgr Agnieszka Krogul)
LITERATURE:
1. R. T. Morrison, R. N. Boyd: Chemia organiczna, tom 2 (rozdz. 33: Tłuszcze) PWN, Warszawa
1985. (in Polish) or R. T. Morrison, R. N. Boyd: Organic Chemistry, vol. 2.(in English).
2. S. Bredsznajder, W. Kawecki, J. Leyko, R. Marcinkowski: Podstawy ogólne technologii
chemicznej, PWN Warszawa 1973. (in Polish)
3. S. E. Manahan: Environmental Chemistry, Brooks/ Cole Publishing Company, 1984.
4. H. Koneczny: Podstawy technologii chemicznej, (rozdz. V: Paliwa i ich przerób), PWN,
Warszawa 1973. (in Polish)
5. R. Bogoczek, E. Kociołek-Balawejder: Technologia chemiczna organiczna, rozdz. 2, WAE,
Wrocław 1992. (in Polish)
6. Spalanie i paliwa praca zbiorowa pod red. J. Kordylewskiego, Oficyna Wydawnicza
Politechniki Wrocławskiej, Wrocław 2005. (in Polish)
7. J. Ciborowski: Inżynieria chemiczna. Inżynieria procesowa, WNT, 1973. (in Polish)
ATTENTION: Student are obliged to search information on the presented subjects from additional
sources (physical chemistry and thermochemistry textbooks from the library, internet sources etc.)
Requirements
Definitions: energy and conversion of energy, fuels, biofuels (bioethanol, biodiesel), heat / thermal
capacity, calorimetric conctant, combustion, diffusion, fats and vegetable oils, fatty acids and their
esters, triglycerides (triacylglycerols), FAME (fatty acid methyl esters), physical and chemical
* Values of lower (LEL) and upper (UEL) explosion limits.
When the combustion is performed in order to obtain heat - a process is carried out in
furnaces or boilers. Then, the thermal energy can be converted into mechanical work e.g. in steam
turbines. A more direct and more effective way to convert chemical energy into mechanical work is
the use of internal combustion engines (more specifically, heat engines with internal combustion). In
8
reciprocating engines5 the energy is converted into the work of piston or pistons moving in a cylinder
(cylinders). This movement is converted into the torque of the crankshaft.
In spark-ignition engines (Figure 3) combustion is initiated by a spark of the spark plug. Fuels
used in this type of engines may be natural gas, light fractions of petroleum (liquid gas, petrol,
leaded petrol) as well as flammable and volatile organic substances (e.g. methanol, ethanol). Spark
engines have light constructions and simple design and they easily receive high speed, high power
gain, and they are easy to start.
Figure 3 Scheme of four-stroke engine (four-cycle engine) with spark ignition.
Figure 4. Spark-ignition engine cycle (according to Combustion and fuels edited by J. Kordylewskiego, Publishing House of Wroclaw University of Technology, Wroclaw, 2005).
*************************************************** ******************* optional material
5 A reciprocating engine (or a piston engine) is a heat engine using one or more reciprocating pistons to convert
pressure into a rotating motion.
9
Motor efficiency, η, refers to the amount of useful work dW that we can get from a definite quantity of supplied heat, dQh. Since the useful work (dW = - pdv) is the difference between the heat absorbed (dQh <0) and heat discharged (dQc >0), we have:
h
c
h
c
h
ch
h dQ
dQ
dQ
dQ
dQ
dQdQ
dQdW +=
−−=
−−−=
−−= 11η
The efficiency of reciprocating combustion spark-ignition engine is given by: κεη −−= 11
where ε is the compression ratio (the ratio of cylinder volume to the harmful volume V1/V2, see
Figure 4, position 1 and 2) and κ = Cp / Cv. Compression ratio in combustion engines is in the 8:1 to
11:1 range, but above 8.5:1 the combustion with engine knocking may occur (on the course of
explosive), leading to the destruction of the piston, rings and bearings.
(from: Ćwiczenia Laboratoryjne z chemii fizycznej translation from
German, PWN Warszawa 1975.])
Connection to power causes the glowing of ignition wire and ignition of cotton thread, thus, the
flame is transferred to the sample.
An example of calorimetric plot resulting from combustion experiment is presented in
Figure 4. The measurement can be divided into three periods shown as three segments /sectors on
the calorimetric plot. After the calorimetric vessel is charged with a sample and placed within
calorimeter bath a few minutes is needed to thermally equilibrate the system. Then, the initial
period of measurement starts and the changes of temperature are recorded to monitor the rate of
heat exchange between calorimetric system and the surrounding. Usually this period takes five
minutes and 5-10 values of temperature are recorded (30 or 60 second intervals). On Figure 4 this
initial period is presented as segment from T1to T3. In the main period a combustion of the sample
occurs (at T3) and the energy is transferred to the whole calorimetric system (an increase of
temperature can be seen on Fig. 4). Temperature increases until point T4 is reached and the final
period starts. During that period a temperature is still monitored and recorded in the same way as in
the initial period. Duration of the final period is the same as duration of initial period – in both these
time segments a rate of heat exchange between calorimeter and surrounding is monitored and
recorded.
21
Figure 4. Plot of temperature (T) of calorimeter system versus time (t) of calorimetric experiment.
Letters A, B, C denote periods: initial, main, and final, respectively.
A corrected increase of temperature, ∆Tcal , can be calculated from the equation:
∆Tcal = Tn-To+c (x12) where:
Tn – last value of temperature in the main period,
To - last value of temperature in the initial period
c –correction for heat exchange between calorimeter and surrounding.
Correcting factor c is calculated by Bunte’s formula:
c = nt [(∆i+ ∆f) + r ∆f] (x13) where: nt is a number of temperature points recorded per minute, ∆i is a mean value of temperature
increase (in K or °C) per one minute during the initial period, ∆f is a mean value of temperature
decrease (in K or °C) per one minute during the final period, r is number of temperature points
recorded during the main period. Value of ∆Tcal is usually less than 3 degrees (precision of
temperature measurement is usually 0.0001°C or less).
For analyzed process if ∆T is known, the amount of heat can be calculated from the heat
balance and it is a sum of heat consumed (absorbed) by calorimeter and the heat exchanged with the
surrounding. Therefore, the amount of heat measured with calorimeter is given by the equation:
��������� = −� × ∆��� (x14)
where K – calorimetric constant (heat capacity of the calorimeter), ∆Tcal – change (increase
/decrease) of temperature calculated with corrections (see eq. x13). Since combustion is exothermal
process, the results will have negative sign, thus, in the formula x14 a minus is added before the
K∆Tcal. expression. If the mass of the analyzed sample is known, the heat can be expressed in cal/g or
J/g, however, the heat measured during experiment is a sum of heat from combustion of the sample
and some extraneous energy generated during combustion of other materials present in the system
(for example, combustion aids, igniters, sulfuric acid and nitric acid formed from molecular nitrogen).
The heat of combustion of the cotton thread that ignites the sample and the electric energy needed
for the ignition would result in distorted values of the measurement. This effect is taken into
consideration in the calculation with a correction value. Almost all substances to be analyzed contain
sulfur and nitrogen. Under the conditions that prevail during calorimetric measurements, sulfur and
nitrogen undergo combustion and form SO2, SO3 and NOx. 9 Sulfuric and nitric acid arise in
combination with the water resulting from combustion and humidity. To obtain the standard gross
calorific value, the effect of the heat of solution on the gross calorific value is corrected.
The calorimetric constant (heat capacity of the calorimeter) is calculated from equation:
� =������ �� �� �
∆���� (x15)
where:
Qst is a heat of combustion of the standard substance used for callibration[cal/g or J/g],
mst is a mass of the standard substance [g],
∆Tcal – change of temperature monitored during experiment [°C or K],
c1- correction for heat released during combustion of steel wire (c1 should be included when older
type of calorimetric bomb is used, for platinum wire c1=0),
cb - correction for heat released during combustion of cotton thread,
cN- correction for heat released during combustion of nitrogen to form HNO3.
Corrections cb i cN are calculated in the following ways:
9 Nitrogen oxides formed during ignition and combustion can catalyze the oxidation of SO2 to SO3.
22
cb = 4200 [cal/g] × mass of cotton thread [g], (in the experiment the heat of combustion of cotton
thread: 50 J = 11.94 cal has been determined by producer),
cN=1.43 × vNaOH
where 1,43 is a heat of formation of 1.00 mL of 0,1M nitric acid (in cal/mL) and vNaOH is the volume
(in milliliters) of 0.1 M NaOH used for titration of nitric acid formed during combustion.
ATTENTION: If the calculations are to be proformed in joules, the 1.43 factor has to be recalculated
into joules.
23
TECH-LAB: EXPERIMENT 31-ME.
HEAT OF COMBUSTION OF BIOFUELS -
- INSTRUCTION MANUAL
The purpose of this experiment is to determine heat of combustion and total calorific value of solid
and liquid organic compounds (including fuels and biofuels). During the experiment, students will
carry out a series of measurements in order to do:
a) calibration of the calorimeter (determination of calorimetric constant, K),
b) determination of the heat of combustion of biofuel,
c) determination of the heat of combustion of other organic substance,
d) titration of combustion products – to calculate corrections for heats of combustion.
As a result, students will be able to use the calorimeter apparatus (calorimetric bomb), they will
calculate the heat of combustion and calorific value of organic materials as well as they will be able
to compare the calorific values of fossil fuels with biofuels (biodiesel, FAME). After the experiment the
students gain the ability to perform qualitative and quantitative description of the fuels and biofuels
and ability to present resulting data and to critically assess the quality and applicability of (bio)fuels.
Equipment
lab glass and small equipment safety equipment
Calorimeter IKA C2000
decomposition vessel (bomb)
stainless steel crucible
thermostatic bath
can with compressed O2
pressure regulator
laboratory press
analytical balance
computer + printer
burette 1 item
conical flasks 4 items
squeeze bottle 2 items
tweezers 1 item
pipette 10 mL 1 item
disposal pipettes
disposal syringes
latex gloves
lab glasses
Figure 5. Scheme of IKA C2000 calorimetric
system.
1. Control panel, 2. Keyboard, 3. Display, 4.
Electronics unit, 5. Measuring cell. 6.
Temperature sensor. 7. Oxygen filling device. 8.
Decomposition Lessel. 9. Measuring cell cover.
Figure copied from original operating
instruction for IKA C2000 basic calorimeter
system.
24
Figure 6.
Individual parts of the
decomposition vessel.
1. Cap skrew.
2. Oxygen valve.
3. Cover.
4. Crucible.
5. Electrical ignition contact.
6. Ignition wire.
7. Crucible holder.
Figure 7.
Fastening a cotton thread (1) onto
the ignition wire.
Figure 8.
Pressure reducer valve with
manometers, mounted to pressure
gas can with O2.
Chemicals
a) benzoic acid is used as thermochemical standard during the experiment – its heat of combustion
is 6324 cal/g (if weight is measured in air at temp. 20°C) or 6319 cal/g (if weight is measured in
vacuum). The standard should be of analytical grade characterized by the following parameters:
melting point 122°C
assay 99,90 %
ashes no more than 0,02 %
chlorides no more than 0,0005 %
sulfates no more than 0,005 %
heavy metal ions none
water no more than 0,03%
b) cotton thread (heat of combustion 50 J )
c) 0.1 M solution of NaOH
d) 0.1 M solution of HCl
e) phenolophtalein, 1% in ethanol
f) biofuel (FAME)
25
Performing the measurement
1. Check the connection of the gas can with oxygen, turn on the main valve and check the oxygen
pressure on the manometers.
2. Turn the chiller (thermostate) on and set up the working temperature as 20°C.
3. Turn the calorimeter on.
4. Turn on the computer and printer (chose CALWIN icon, option: isoperibol, „new measurement”)
Determination of calorimeter constant
Heat capacity of the calorimeter is determined by the means od standard material (benzoic acid,
described in the previous section).
5. Preparation of the standard (reference) material: using laboratory (“two decimal place”) balance
weigth about 0.6-0.9 g of benzoic acid (the amount of reference has to be adjusted to obtain 2-3°C
increase of temperature during calorimetric measurement) and make tablet by the means of the
laboratory press. All parts of pellet press and plates have to be clean (before use clean them with
acetone or ethanol and make sure the parts are dry).
6. Using analytical balance weight clean and dry crucible from calorimetric vessel, record its weight
with accuracy 0,0001 g (0,1 mg), remove it from the balance, place the tablet of standard compound
(from point 5) inside the crucible and weigh the total mass of the crucible with content. Calculate
the weight of standard, mst.
7. Using pipette add 10,0 cm3 of distilled water to the calorimetric vessel.
8. Fasten a cotton thread around ignition wire (see Figure 7). Crucible with tablet within the crucible
holder (Fig. 6) and align the cotton thread with a pair of tweezers so that it hangs down into the
crucible and is immersed in the sample. This ensures that during the ignition process the
burning thread will ignite the sample.
9. Close the decomposition vessel.10
Always hold the decomposition vessel securely on the top by
the skrew cap.Keep the vessel always in vertical position.
10. Guide the decomposition vessel into the filler head of the open measuring cell cover until it
catches in place.
11. Input to the software (CALWIN) all information on the sample (weght, name of a file). Make
certain that the desired operating mode is set. Press “start” button.
12. Filling with high pressure oxygen, stabilization of the calorimeter and measurements of
temperature are carried out automatically. After the experiment is complete, the measuring cell
cover opens. As soon as the message BOMB↑ appears onthe small display, you can remove the
decomposition vessel. Decompress the vessel with safe way using the venting button.
13. Open the vessel and check whether the sample underwent a complete combustion. If you notice
a presence of unreacted compound -the determination should be calcelled and its result has to be
rejected.
14. Thansfer the water from the decomposition vessel to conical flask, use three or four additional
portions of distilled water (from a squeeze bottle) to completely rinse out all the products generated
during combustion and absorbed in water.
15. Clean the decomposition vessel with cover and crucible - they should be clean and dry before
the next measurement.
16. Place the conical flask containing solution (from point 14) in water bath (95°C) for 5 minutes,
then remove and cool it, add 10,00 cm3 of 0.1000 M NaOH, add two drops of 1% alcoholic solution
of phenolophtalein and titrate excess of unreacted NaOH with 0.1000 M HCl.
10
You do not need to remove air from decomposition vessel. A presence of nitrogen is sometimes desired
(when?) and the results are corrected on the basis of analysis of the amount of nitric acid generated during
combustion.
26
Calculation of the calorimetric constant
17. Determine the increase of temperature ∆Tcal (using eq. X12) including correction factor according
to eguation X13. (an example of calculation of ∆Tkal: see example #1 in section entitled: Examples of calculations).
Calculate the calorimeter constant on the basis of equation X15, taking the weight of the sample,
correction for heat released by the cotton thread and correction for nitrogen combustion. (an example of calculation of K l: see example #2 in section entitled: Examples of calculations).
18. Repeat points 5-17 twice. Each time calculate the K. Take arithmetic mean as the final result.
Determination of heat of combustion of biodiesel (FAME)
19. On analytical balance weight clean and dry crucible from calorimetric vessel, record its weight
with accuracy 0,0001 g (0,1 mg), remove it from the balance, pour inside the crucible 0.5-0.8 cm3 of
biofuel (Fatty Acid Methyl Esters), and weigh the total mass of the crucible with content. From the
difference calculate the mass of the sample (ms)
20. Determine the heat of combustion of the studied sample in the same way as for standard
material (as described in points 7-16). Each time calculate ∆Tcal as it was described in point 17.
21. The heat of combustion of bio(fuel) Qfuel can be calculated from modified equation X14 with
appropriate corrections:
���� =�×∆����� �� �
��
[cal g⁄ ] (x16)
where: K- calorimetric constant, ∆Tcal – temperature increase determined according to point # 20,
ms-mass of the sample, cb-correction for heat released during cotton thread combustion,
cN- correction for heat released during combustion of nitrogen to form HNO3 (including molecular
nitrogen from air and nitrogen from the analyzed compound).
22. Repeat activities described in the points 19-21. Each time calculate Qfuel. Take an arithmetic
mean as the final result.
23. Calculate the error of determination.
27
Examples of calculations
Example 1 – Calculation of ∆Tcal
measured values:
initial period: main period: final period:
time / min. temp. °C time / min. temp. / °C time / min. temp. /°C
0
1
2
3
4
24,3527
24,3561
24,3599
24,3640
24,3681
0
1
2
3
4
5
6
7
26,5350
27,0851
27,2273
27,2737
27,2885
27,2919
27,2909
27,2881
0
1
2
3
4
27,2845
27,2807
27,2769
27,2732
27,2694
The increase of temperature ∆Tcal can be calculated from eqns. X12 and X13.
Tn = 27.2881°C,
To = 24.3681°C,
nt = 1 value per minute,
∆i = (24.3681°C-24.3527°C) / 4 min = 0.0154°C/min
∆f = (27.2694°C-27.2845°C) / 4 min = 0.0151°C/min
r= 8
c = nt [(∆i+ ∆f) + r ∆f]= 1/min × [0.0154°C/min +0.0151°C/min+8 × 0.0151°C/min] = 0.1513°C