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Файл:Chemical Engineering of Natural Fuels and Carbon Materials. Study Guide
.pdf
The chemical reaction is:
Substance
Standard heat of
formation,
kJ / mol
Standard heat of
combustion,
kJ / mol
Bond energies
Н
2
C2H4
C2H6
0
52.36
–84.79
–286.25
–1410.97
–1559.88
С−С 360
С=С 502
С−Н (ethane) 410
С−Н (ethylene) 444
Н – Н 435
H H
H
HCH
C
H
H H
H
C
C
H
+
H H
C2H6 ↔ C2H4 + Н2
Table 9.1 – Standard heat of formation and combustion for substances
Solution:
1)
2)
3)
Е ethane = 360 + 6 · 410 = 2820
Е ethylene = 502 + 444 · 4 = 227
11
;
;
.
, so the process is endothermic, for carrying out this process
heat should be expended (Q value in chemistry will be with the opposite
sign).
Example 1.2
Determine the amount of heat released at standard conditions in the
process of hydrogenation of 1000 kg of acetylene, if the final product
mixture contains 80% of ethylene, 2% of ethane and 18% of acetylene (%
wt.).
Solution:
СН≡СН + Н2 ↔ СН2=СН2 (I)
СН≡СН + 2Н2 ↔ СН
−СН
(II)
3
3
The values of heat of formation of the final products and of the
initial substances:
acetylene: 227.073 kJ/mol;
ethylene: 52.358 kJ/mol;
ethane: – 84.789 kJ/mol;
hydrogen: 0 kJ/mol.
The thermal effect of the reaction (I):
Н 1 = 52.358 – 227.073 = – 174.715 kJ/mol.
The thermal effect of the reaction (II):
Н 2 = – 84.789 – 227.073 = – 311.862 kJ/mol.
Amounts of substances formed as a result of the reactions:
The total thermal effect of the process of acetylene hydrogenation:
12
Н = Н
∙ 10
1
3
∙ n1 + Н
3
∙10
∙ n2 = – 5205.8∙103 kJ.
2
∆Н < 0, so the process is exothermic.
Task 1.1
In the process of catalytic reforming* 920 kg of toluene was
obtained, herein, 80 % of toluene was formed as a result of the reaction of
dehydrogenation of methylcyclohexane, and 20 % was formed as a result of
n-heptane dehydrocyclization reaction. Define the thermal effect of the
process if the values of the heat of formation of substances at the process
temperature are the following:
methylcyclohexane: – 182.09 kJ/mol;
toluene: + 33.23 kJ/mol;
n-heptane: – 217.13 kJ/mol.
*Catalytic reforming – is a chemical process used to convert
petroleum refinery naphthas, typically having low octane ratings, into high
octane liquid products called reformates which are components of high
octane gasoline (petrol).
Task 1.2
In the dehydrogenation of 2880 kg of isopentane 45 % of it has
transformed into 2-methyl-1-butene and 35 % was transformed into
isoprene. Find the standard thermal effect of the process.
Task 1.3
Calculate the thermal effect of the steam-cracking process* process
(under standard conditions) of 1500 kg of ethane and 1100 kg of propane,
if pyrogas (cracking gas) has the following composition in wt. % : Н2 – 8;
С2Н
– 17; СН4 – 5; С
6
– 19; С
3Н6
2Н4
– 45; С
3Н8
– 6.
*Steam-cracking process is thermal decomposition of
hydrocarbons for the purpose of obtaining olefin-containing gases, mostly
ethylene and propylene. It is a type of thermolysis.
Task 1.4
Calculate the thermal effect of a catalytic reforming process (under
standard conditions), if raw material (feedstock) contains:
n-hexane – 860 kg;
cyclohexane – 1680 kg;
and the composition of reaction products is the following:
13
benzene – 61.4% by weight.
cyclohexane – 16.5% by weight.
n-hexane – 16.9% by weight.
hydrogen – 5.2% by weight.
Task 1.5
Determine the thermal effect of delayed coking* process of 1000 kg
of oil tar, if product yield is the following in wt. %:
gas – 6.2;
gasoline (in British English – petrol) – 12.1;
light gas oil – 39.4;
heavy gas oil – 22.7;
coke – 19.6.
Standard heat of combustion of components in kilocalories per kg
are following: gas – 12668; gasoline – 11260; light gas oil – 10720; oil tar –
10400; heavy gas oil – 10620; coke – 8470.
*Delayed coking is a type of thermal processes of heating of
petroleum residues up to their thermal cracking temperature in
a furnace with the purpose of obtaining petroleum coke.
14
2. THERMAL PROPERTIES OF PETROLEUM PRODUCTS.
М
С
C
sp
12
tt
q
C
a
15
15
),0034.0762.0(
1
15
15
.ТCa
),1.2()273(00371.0444.1
15
15
aa
ТC
TEMPERATURE DEPENDENCE
OF THERMAL EFFECT OF REACTION
Heat capacity and enthalpy (heat content) are among the main
thermal properties of petroleum products.
The molar heat capacity is the heat capacity per one mole of a pure
substance. The specific heat capacity, often simply called specific heat, is
the heat capacity per one mass unit of a material.
Specific heat of the substance is the amount of energy required for
heating 1 kg of a substance by 1 degree. In other words, it is the ratio of
the heat added to or subtracted from an object to the
resulting temperature change.
Its system unit of measure is J/kg·K (joule per kg ∙ K).
.
Average heat capacity is the amount of heat (q), required for heating
1 kg of a substance from the temperature t1 to t2:
The average heat capacity of liquid petroleum products boiling up to
200 °C and having the relative density
can be calculated according to
Craig formula:
where T is the temperature of a petroleum product, K (Kelvin scale).
The average heat capacity can also be calculated according to the
equation of Fortch and Whitman:
15
where Ta is the average temperature of boiling range of petroleum
,
32
0
ТТТQQ
Т
fraction, K.
There are special graphs which reflect the dependence of heat
capacity of liquid petroleum fractions and oil vapors on their density and
temperature.
Enthalpy (or heat content) of liquid petroleum product
(petrochemical) is the amount of heat (in joules) required for heating 1 kg of
a substance from 0 °C to target temperature (enthalpy at 0 °C
conventionally is considered to be equal to zero).
Enthalpy of vapor at some set temperature is the amount of heat
required for heating 1 kg of a substance from 0 °C to this set temperature
taking into account the heat of evaporation at the same temperature and the
heat of superheating the vapor.
There are empirical formulas, tables, and graphs for liquid
petroleum products and oil vapor enthalpies determination.
The thermal effect dependence on temperature
This dependence is expressed by the Nernst equation:
where Qo is a conditional thermal effect at zero degrees Kelvin; QT is the
thermal effect at the determined temperature, Kelvin scale; α, β, γ are the
coefficients that can be calculated from the temperature dependence of the
specific heat of substances taking into account their stoichiometric
coefficients in the chemical reaction equation:
а
Temperature dependence of the specific heat of the substances can
be expressed by the following equations:
16
с = а + b ∙ T;
),(
..293 initialprodT
QQ
),293()293()293(
3322
293
ТТТQQ
Т
4
5
3623
1046.2
1021.31011.1332.16
Т
ТТТQQ
о
c = a + b ∙ T + d ∙ T2;
c = a + b ∙ T + d / Т2.
The change (delta) of the specific heat of the substances in a
chemical reaction can be calculated according to the formula:
∆ ср = ∑с
р (products)
– ∑с
р (initial substances).
The thermal effect of the reaction can also be calculated using the
enthalpies of the substances, participating in this reaction:
where ∑I
initial.
and ∑I
are the sums of the enthalpies of the initial
prod.
substances and reaction products in kJ/mol.
The thermal effect of the reaction can also be calculated using the
thermal effect of the reaction at 20 °С (293 degrees Kelvin):
where Т is temperature, Kelvin scale.
Example 2.1
Determine the thermal effect of acetylene hydrogenation into
ethylene at temperature 177 ºC, if the temperature dependence of this
reaction is the following:
, cal/mole
Solution:
С2Н
+ Н2 ↔ С
2
2Н4
17
Kelvin:
3
1011.13
4
5
298
1046.2
3
1011.13
4
5
450
1046.2
ТC
Н
р
0016.041.28
2
2
0000482.01362.061.5
42
ТТC
НС
р
2
0000687.0142.039.9
62
ТТС
НС
р
The thermal effect of the reaction at 298 º K:
∆Н
= 12496 – 54194 = – 41698 cal/mole.
298
The conventional thermal effect of the reaction at zero degrees
∆H
= – 41698 + 16.32 ∙ 298 –
о
∙298
2
+ 3.21 ∙ 10
-6
∙ 2983 +
+
= – 37914 cal/mole.
∆H
= – 37914 – 16.32 ∙ 450 +
450
∙ 4502 – 3.21 ∙ 10
–
= – 42895 cal/mole.
∆H
= – 42895 ∙ 4.19 = – 179730 J/mol = – 179.73 kJ/mol.
450
Task 2.1.
For the reaction of ethylene hydrogenation into ethane, determine
the expression for the temperature dependence of thermal effect and
calculate the thermal effect of this reaction at 727 ºC (1000 K), if the
temperature dependences of the specific heat are following:
, J/mol·K;
, J/mol·K;
, J/mol·K.
Task 2.2.
Determine the thermal effect of the dehydrogenation reaction of 300
kg of cyclohexane into benzene at 427 ºC if the temperature dependence of
the heat capacities of the reactants are the following:
18
-6
∙ 4503 –
С6Н
ТC
НС
р
143.036.12
126
ТC
НС
р
061.02.14
66
ТC
Н
р
0004.078.6
2
26
107.681362.061.5
42
ТТC
НС
р
25
1073.160475.0243.21
126
ТТC
НС
р
↔ С
12
6Н6
+ 3Н2
, J/mol·K;
, J/mol·K;
, J/mol·K.
Task 2.3.
Determine the thermal effect of the ethylene trimerization reaction
at 327ºC if temperature dependences of the heat capacities of the reactants
are the following:
, J/mol·K;
, J/mol·K.
19
3. GIBBS FREE ENERGY AS A MEASURE
OF THE THERMODYNAMIC FEASIBILITY
OF A CHEMICAL REACTION
The condition of chemical process feasibility in the forward
direction can be expressed by the following inequality:
∆G < 0,
where ∆G is Gibbs free energy or isobaric-isothermal potential (IUPAC
recommended term is Gibbs energy or Gibbs function).
When ∆G° is negative (∆G° < 0) the condition for reaction
to proceed is favorable.
When ∆G° is positive (∆G° > 0) the condition for reaction to
proceed is unfavorable.
Gibbs free energy equation can be used to determine whether the
reaction can proceed or not. Or we can say the change in Gibbs free
energy associated with a chemical reaction is a useful indicator of the
reaction proceeding or not.
The more negative the value ΔG is, the more thermodynamically
probable the reaction is.
If ΔG > 0, the reaction from a thermodynamic point of view does
not take place at all.
Many chemical reactions, including oil refining, under the received
process conditions, can not occur at all. However, in the presence of
catalysts, they may be implemented.
Gibbs energy of a substance is related to its enthalpy and entropy
with following equation:
G = Н – T∙S
Accordingly, Gibbs energy change of a chemical reaction can be
determined by the equation:
∆G°Т = ∆Н°Т – T · ∆S°Т,
where ∆Н
and ∆SТ are the changes of enthalpy and entropy of the reaction
Т
products and reactants.
Let's review the following hypothetic reaction as an example:
20
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