Добавил:
ivanov666
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Chemical Engineering of Natural Fuels and Carbon Materials. Study Guide
.pdf
Task 5.1.
Determine the reaction activation energy at a temperature 427 °C if
temperature gradient of the reaction is α = 8 °C.
Task 5.2.
The ethane dehydrogenation reaction into ethylene (first order
reaction) at 820 °C proceeds up to 75 percent in 1.1 seconds. How long will
it take for this reaction to reach the same degree of conversion at 860 °C, if
the activation energy of the reaction is Ea = 68.1 kJ / mol?
Task 5.3.
Determine the activation energy of the reaction of n-pentane
isomerization into iso-pentane at 227 °C, if the temperature gradient of this
reaction is α = 12 °. What is the temperature coefficient of the reaction?
Task 5.4.
During hydrotreating process, the hydrogenation reaction of ethyl
mercaptan, which is the second order reaction, at 500 °C proceeds up to
65 % in 1 minute. How long will it take for the reaction to reach the same
depth at 520 °C? The initial concentrations of the substances are equal, and
the activation energy of this reaction is Ea = 32.7 kJ / mol.
Task 5.5.
For the reaction of ethane dehydrogenation into ethylene (the first
order reaction) half-life time at 860 °C is equal to 0.5 seconds. Determine
the reaction rate constant under these conditions. Determine the necessary
time for the reaction to proceed up to 80 percent under these conditions?
31
6. BOND STRENGTH IN THE MOLECULES
OF HYDROCARBONS.
REACTIONS OF THE RADICALS. THERMAL CONVERSION
OF HYDROCARBONS
Data on the bond strength in chemical compounds is used to predict
the probability of product formation in thermodynamics.
According to thermal stability hydrocarbons can be arranged in the
following increasing order: paraffins (alkanes) < mono-olefin hydrocarbons
(alkenes) < diolefin hydrocarbons (dienes) < naphthenes (cyclanes,
cycloalkanes) < aromatic benzene series (arenes) < condensed
hydrocarbons.
The ratio of radical reactions rates
Since the radical can undergo several different reactions, the
preferred direction of its conversion can be determined by the possible ratio
of reaction rates:
where е is the exponent, w is the reaction rate, K is the reaction rate
constant, A is the pre-exponential factor, E is activation energy, R is the gas
constant.
The activation energy of radical reactions depends on their thermal
effect Q and can be described with sufficient accuracy by Polanyi Semenov rule:
E = 48.2 – 0.25 ∙ Q – the expression for exothermic reactions;
E = 48.2 + 0.75 ∙ Q – the expression for endothermic reactions.
Bond strength in the molecules of hydrocarbons
Double (olefinic bond) and triple (acetylene bond) bonds are
stronger than single bonds (ordinary bond). The bonds in beta-position
relative to multiple bonds in a molecule are weaker than others.
C-C bond (carbon-carbon bond) in the aromatic rings is stronger
32
than C-C bonds in paraffins. C-C bond in the lateral substituents in betaposition relative to the aromatic ring is weakened.
The strength of C-C bond in molecules of undivided paraffins
decreases from the ends towards the center of the molecule.
The bond between the primary carbon atoms is stronger than the
bond in combinations with secondary and especially with tertiary carbon
atoms.
The C-H binding energy (C-H bond energy) decreases nearer the
center of the molecule.
The hydrogen bond with the secondary and tertiary carbon atoms is
weaker than the hydrogen bond with a primary carbon atom.
C-H bonds in beta-position relative to multiple bonds are weaker
than others.
C-H bond energy in aromatic rings is comparable with the strength
of C-H bond in methane. C-H bond in beta-position relative to an aromatic
ring is weakened approximately to the same extent as in the beta position to
the double bond in the olefin molecules.
Stages of the radical-chain mechanism of thermal conversion
1. Initiation of the chain (usually by monomolecular decomposition)
is due to C-C bond decay. In these reactions, a single molecule breaks apart
into two free radicals. Only a small fraction of the feed molecules actually
undergoes initiation, but these reactions are necessary to produce the free
radicals that promote the rest of the reactions. In steam cracking, initiation
usually involves breaking a chemical bond between two carbon atoms,
rather than the bond between a carbon and a hydrogen atom.
2. Chain transfer. At this stage radicals, formed at the stage of chain
initiation, interact with the initial molecule forming new radicals. Hydrogen
abstraction takes place. In these reactions a free radical removes a hydrogen
atom from another molecule, turning the second molecule into a free
radical.
3. The growth of chain (continuation of chain) stage includes
various transformations of radicals. According to the process conditions,
radical decomposition or radical addition reactions occur. In radical
decomposition reactions, a free radical breaks apart into two molecules, one
being a molecule of alkene, the other being a free radical. This is the process
that results in alkene products. In radical addition reactions, the reverse of
radical decomposition reactions, a radical reacts with an alkene to form a
33
larger single, free radical. These processes are involved in forming the
3362
НСНСНC
524623
НСНСНСНC
2232
CHCHHCHHC
52262
НСННСH
42252
НCHHCH
6252
НCHCH
83523
НCHCНС
42625252
НCНCHCНС
aromatic products that are formed when heavier feedstocks are used.
4. Chain termination accompanied by recombination and
disproportionation reactions, which lead to the disappearance of free
radicals. In these reactions, two free radicals react with each other to
produce products that are not free radicals. Two common forms of
termination are recombination, where the two radicals combine to form one
larger molecule, and disproportionation, where one radical transfers a
hydrogen atom to the other, resulting in an alkene and an alkane.
A large number of chemical reactions take place during the thermal
cracking process, most of them are based on free radicals and include
hundreds or even thousands of reactions. Radical-chain mechanism of
ethane thermal conversion can be expressed by the following reactions:
1.
2.
3.
4.
Thermal processes of hydrocarbon processing
Processes of hydrocarbon transformations, including decay and seal
reactions, conducted under high temperatures without using catalysts, are
called thermal processes.
Until the mid of XX century, the main purpose of thermal processes
was to obtain the additional amount of gasoline out of heavy oil residues.
34
With the introduction and development of more effective catalytic
processes, such as catalytic cracking, catalytic reforming, alkylation and
others, thermal cracking as gasoline producing process has lost its industrial
importance. However thermal cracking remains important in producing
naphtha, gas oil, and coke and more sophisticated forms of thermal cracking
have been developed for various purposes. These include visbreaking, steam
cracking, and coking.
In a modern refinery, the following types of thermal processes are
found:
Thermal cracking of high boiling distillate or residues is conducted
at high pressures (2–4 MPa) and temperatures of 500–540 °C to obtain gas
and liquid products.
Visbreaking is a light thermal non-catalytic cracking process with
limited depth of thermolysis conducted at low pressures (1.5–3 MPa) and
temperatures, which reduces or "breaks" the viscosity of oil residues. The
purpose of a visbreaking process is to reduce the quantity of residues
produced in the distillation of crude oil and to increase the yield of more
valuable middle distillates. During the
process large hydrocarbon molecules are thermally cracked by heating in
a furnace to reduce their viscosity and to produce small quantities of light
hydrocarbons (LPG, liquefied petroleum gas and gasoline).
Coking is a long thermolysis process of heavy residues or
aromatized high boiling distillates, conducted at low pressures and at
temperatures of 470–540 °C. The primary purpose of coke process is the
production of petroleum coke. Under the right conditions delayed coking
can produce valuable needle coke, highly crystalline petroleum coke used in
the production of electrodes for steel and aluminium industries.
Steam cracking is high temperature thermolysis (750–800 °C) of
gases, a light or medium distillate hydrocarbon feedstock, conducted at low
pressure and extremely short reaction time. The main purpose of steam
cracking is the production of olefin containing gases. The actual reaction is
known as homolytic fission and produces alkenes, which are the basis for
the economically important production of polymers and other feedstocks for
the petrochemical industry.
During steam cracking, saturated hydrocarbons are broken down
into smaller, often unsaturated, hydrocarbons. It is the principal industrial
method for producing the lighter alkenes (or commonly olefins),
including ethene (or ethylene), and propene (or propylene). Steam cracker
units are facilities in which feedstock such as naphtha, liquefied petroleum
35
gas (LPG), ethane, propane or butane is thermally cracked through the use
of steam in furnaces without the presence of oxygen to produce lighter
hydrocarbons. The products obtained depend on the composition of the
feed, the hydrocarbon-to-steam ratio, and on the cracking temperature and
furnace residence time. In modern cracking furnaces, the residence time is
reduced to milliseconds to improve yield. After the cracking temperature
has been reached, the gas is quickly quenched to stop the reaction in a
transfer line heat exchanger.
Also during the steam cracking, highly aromatic liquid products are
formed. The process also results in the slow deposition of coke, a form
of carbon, on the reactor walls. This degrades the efficiency of the reactor,
so reaction conditions are designed to minimize this. Nonetheless, a steam
cracking furnace can usually run only for a few months at the time between
de-cokings. Decoking requires the furnace to be isolated from the process
and then a flow of steam is passed through the furnace coils. This converts
the hard solid carbon layer to carbon monoxide and carbon dioxide. Once
this reaction is complete, the furnace can be returned to service.
The production of carbon carbon black is high temperature (above
1200 °C) thermolysis of heavy aromatized distillates, conducted at low
pressure and low process duration.
Bitumen production is a continuous process of oxidative
dehydration (carbonization) of heavy oil residues (tars, asphalts of
deasphalting process), conducted at moderate temperatures of 250–300 °C
and atmospheric pressure.
Task 6.1.
Select the most and the least strong C-C bond (carbon-carbon bond)
in molecules of the following hydrocarbons:
1. nonene-4-yne-8 (or octene -3 -yne-7);
2. n-butylbenzene;
3. hexene-2;
4. n-hexane.
Task 6.2.
Arrange the following hydrocarbons in order of increasing their
thermal stability: n-octane, meta-xylene, iso-octane, ethyl cyclohexane,
octene-1.
36
Task 6.3.
In each of the given hydrocarbon molecules specify the weakest
C-H bond:
1. n-pentane;
2. iso-butane;
3. butene-1;
4. n-butyl-benzene.
Task 6.4.
Define the activation energy values and the ratio of reaction rates of
radical decomposition at 1000 K:
Thermal effects of these reactions are 126, 138, 188 kJ/mol,
respectively.
Task 6.5.
In substitution reactions, the interaction of methyl radical with a
molecule of propylene can lead to the formation of three unsaturated
radicals:
The thermal effects of these reactions are 109, 4,2, 4,2 kJ/mol,
respectively.
Find the difference in the rates of these reactions at 727 °C.
Task 6.6.
Write a mechanism of ethane pyrolysis by stages.
Task 6.7.
Write a mechanism for propane thermal conversion by stages.
Note. The formation of isopropyl or n-propyl radicals depends on
37
conditions. Hydrogen bond with a primary carbon atom in the propane
molecular is higher than hydrogen bond with a secondary carbon atom. At
lower temperatures, mainly isopropyl radical is formed. At higher
temperatures, the difference in C-H bond energies of the primary and
secondary carbon atoms becomes insignificant and, as the amount of
primary hydrogen atoms is 3 times more than that of secondary hydrogen
atoms in propane, the probability of hydrogen abstraction from the primary
carbon atoms becomes higher, and under such conditions, n-propyl radical
is formed.
Therefore, at the higher temperature (880 °C) during the pyrolysis
of propane ethylene yield will be higher, and at relatively low temperatures
(780 °C), the propylene yield will be higher.
Task 6.8.
Define the initial products of the thermal decay of n-hexane and its
isomers.
Note. Isoparaffinic hydrocarbons give less ethylene yield. Methane
yield during pyrolysis of isoparaffins is higher. Not only alpha-olefins, but
olefins with the other double bond location are formed.
Task 6.9.
Write a decay scheme of neopentane.
Note. The reaction products are isobutene and methane formed in
equal amounts.
Task 6.10.
Write thermal decomposition reactions of cyclohexane and
cyclopentane.
Note. Naphthenic hydrocarbons (cycloalkanes) in a thermal
cracking can be converted in three ways:
a) dehydrogenation of a naphthenic ring with the formation of
aromatic hydrocarbons;
b) avulsion of the side chain of the ring;
c) ring opening with the formation of straight-chain hydrocarbon.
The primary decay of six-membered naphthenes by C - C bond in
the ring leads to the formation of a biradical. Biradical then decays into
stable molecules. The decomposition of naphthenes leads to the
accumulation of olefins in the reaction mixture, and a process starts to
38
develop by a radical chain mechanism due to the decay of unsaturated
hydrocarbons. Formed radicals interact with an initial naphthenic
hydrocarbon molecule, taking away a hydrogen atom from it and becoming
a stable molecule.
At high temperatures, the dehydrogenation of cycloolefin radical
does not proceed according to the chain mechanism and leads to the
accumulation of cycloolefin and aromatic hydrocarbons in cracking
products.
Task 6.11.
Write a thermal decomposition reaction of decalin
(decahydronaphthalene, perhydronaphthalene)
Note. Bicyclic naphthenes at a temperature above 600°C can
undergo decyclization, dealkylation, and dehydrogenation. Bicyclic
cycloalkanes (in the reaction) can lead to the formation of hydrocarbons
tetralin and naphthalene series.
Task 6.12.
Write the most probable way of n-decyl cyclohexane thermal
cracking.
Note. When cracking cycloalkanes with a long side chain, which are
also unstable at high temperatures, as well as corresponding alkanes, the
side chain splitting off primarily occurs. Wherein the side chain is shortened
and low molecular weight alkane and cycloalkane with an alkenyl
substituent or alkene and cycloalkane with an alkyl substituent can be
formed.
Task 6.13.
Write the mechanism of 2-hexene thermal decomposition.
Note. The mechanism of primary reactions of thermal
decomposition of olefins, as of paraffins, is a radical chain mechanism. The
primary decay of olefin takes place at the weakest C-C bond in β-position in
relation to a double bond. Then, formed radicals will react with the initial
molecule of alkene, taking a hydrogen atom in β- C-H bond from it.
Task 6.14.
What products can be formed during the thermal decomposition of
propylene?
Note. At a high temperature and low pressure, propylene
39
decomposes with the formation of hydrogen, methane, ethylene, and allene.
116
HC
116
HC
At the relatively low temperature and atmospheric pressure, the main
reaction of the allyl radical is an addition reaction to the double bond of
initial molecule. Thus, the dimerization of propylene takes place to form
radical. Reactions of
radicals give a complex mixture of
products.
Task 6.15.
Write the reactions of the thermal decomposition of cyclohexene.
Note. Cyclic olefins are more stable than the corresponding openchain olefins and decay by a non-chain mechanism through the formation of
biradicals.
Task 6.16.
Write a molecular reaction of two molecules of butadiene in thermal
processes and the reaction of their condensation with benzene.
Note. Dienes and acetylenic hydrocarbons tend to seal by
polymerization reactions and condensation with each other and with alkenes
to form cyclic hydrocarbons, which are easily dehydrogenated to arenes.
Task 6.17.
Write the chain mechanism of acetylene decomposition.
Note. As a result of these reactions, highly unsaturated compounds
and higher molecular weight aromatic hydrocarbons are formed.
Task 6.18.
Write the mechanism of n-butyl-benzene thermal decomposition.
Task 6.19.
Write the reactions of the thermal decomposition of iso-propylbenzene and iso- butyl-benzene.
Task 6.20.
Write the mechanism of the thermal decomposition of toluene.
40
Соседние файлы в предмете [НЕСОРТИРОВАННОЕ]