REFINING
PROCESSING
THERMAL METHOD
HISTORICAL DEVELOPMENT
1861: Cracking distillation (thermal decomposition with simultaneous removal
of distillate) was noticed by chance by a stillman. It was recognized as a means of
producing the valuable lighter product (kerosene) from heavier nonvolatile
materials avec une technique very simple (1870 to 1900)
As the need for gasoline arose in the early 1900s, the necessity of prolonging the
cracking process became apparent and pressure cracking evolved. The first
successful method of converting heavier oils into gasoline was developed by
Burton in 1912 which operated at about 400oC and 75 to 95psi. During the years
1914 to 1922, a number of successful continuous cracking processes were
developed by which gas oil was continuously pumped through a unit that heated
the gas oil to the required temperature, held it for a time under pressure, and then
discharged the cracked material into distillation equipment where it was separated
into gases, gasoline, gas oil, and tar.
As refining technology evolved throughout the 20th century, thermal processes,
such as vacuum flashing, visbreaking, and coking, came into wide usage by
refiners with the goal to achieve lighter products from the residuum or heavy
distillate from a distillation unit or upgrade the heavies oils.
Thermal Method
2
THEORETICAL BACKGROUND
Thermal processes are chemical transformations of pure hydrocarbons or petroleum
fractions under the influence of high temperatures. Most of the transformations are
cracking by a radicalic mechanism.
The thermal processes comprise the following types of industrial processes:
PYROLYSIS (STEAM CRACKING): used for production of ethene and propene
for the chemical industry. The pyrolysis of liquid feed stocks, leads also to
butadiene, isoprene, and C6-C8 aromatics. Characteristic for the pyrolysis process
are temperatures of about 900–950oC and low pressures (< 5 bar). At the present,
pyrolysis is the most important thermal process.
VISBREAKING: Used for producing fuel oils from heavy residues. The process
is characterized by relatively mild temperatures (around 500oC) and pressures of
15–20 bar. Recently, processes at much lower pressures, sometimes atmospheric,
were also developed
COKING: Used for producing petroleum coke from heavy residues. There are
two types of coking processes: the delayed coking realized at about 490oC, and a
pressure of 5–15 bar in coke drums, and fluid coking realized at about 570oC and
2–3 bar, in a fluidized bed. Of some importance is the production of needle coke,
which is used for the production of electrodes especially for electrometallurgy
processes (e.g. aluminum)
Thermal Method
3
THERMODYNAMICS
Thermodynamic calculations show that the thermal decomposition of alkanes of
higher molecular weight may take place with high conversions even at relatively
low temperatures. Thus, n-decane may convert to over 90% to form pentene and
pentane at 350oC and 1 atmospheric pressure.
The great number of parallel–successive reactions that may take place results in
the final product distribution being controlled by the relative rates of the reactions
that take place and not by the thermodynamic equilibrium.
The situation is different for the lower alkanes. Thus, in order to achieve a
conversion of 90% in the decomposition of butane to ethene and ethane at a
pressure of 2 bar, a temperature of near 500oC is required. In these conditions the
dehydrogenation reaction reaches a conversion at equilibrium of only about 15%
Thermal Method
4
The thermodynamic equilibrium for reactions
REACTION MECHANISMS
The initial chemical phenomenon, which take place at high temperatures,
is the breaking of the hydrocarbon molecule in two free radicals
The energy of the C–C bonds, for alkanes in general, is much lower than
the energy of the C–H bonds, the reaction of breaking a C–C bond will be
prevalent over breaking a C–H bond. Therefore, in the pyrolysis not only
for ethane but also of other alkanes, a C–C bond will be preferentially
broken. The cracking of the C–H bonds may be neglected, since their rate
is approximately two orders of magnitude lower than for C–C bonds
The energy of the C–C bonds of tertiary and especially quaternary carbon
atoms is lower than the others; thus they will be preferentially cracked.
The double and triple bonds have bonding energies much higher that the
single C–C bond. The energy of C–C bonds in the position to a double
bond is higher than that of bonds in the position, and significantly lower
than the energy of the C–C bonds in alkanes
In all cases, the initial step in the thermal decomposition of hydrocarbons
is the breaking of a C–C bond. The cracking of various C–C bonds takes
place with comparable rates, with the exception of those in the position to
double bonds, to triple bonds, or to aromatic rings, the rate of which can
be neglected
Thermal Method
6
REACTION MECHANISMS
Types of transformations undergone by the formed radicals:
isomerization:
decomposition:
the reverse of which is the addition to the double bond:
substitution:
Thermal Method
7
KINETICS OF THERMAL PROCESSES
The kinetic equations can be deduced on the basis of the reaction mechanism. For
the thermal decomposition of a hydrocarbon, the chain mechanism may be
represented by the generalized scheme
Thermal Method
8
The kinetics of coke formation
The formation of coke during thermal
processes evidences two different aspects:
1. The formation of coke deposits inside the
pyrolysis tubes, where the feed and the
reaction products are all in vapor phase
2. The formation of coke as a consequence of the
reactions taking place in the liquid phase,
even if a vapor phase is also present - as in the
processes of visbreaking, delayed coking, and
in all classical processes of thermal cracking
Thermal Method
9
Coke formation in pyrolysis processes
Coke forms gradually on the inner wall of the furnace tubes during pyrolysis. The
rate of coke deposition increases as the molecular weight of the feed and its
aromatic character increase. However, coke formation is exhibited even by very
light raw materials such as ethane.
In the first stage, coke filaments are formed due to reactions on the metal surface
(iron and nickel). This catalytic effect is stronger at the beginning of the cycle
when the tubes are clean. Once the coke filaments have appeared, coke formation
is amplified in subsequent stages, by two mechanisms: a) very small droplets of
heavy liquid molecules resulting from reactions of dehydrogenation and
condensation accumulate in the filaments and b) the trapping of methyl, ethyl,
phenyl radicals and acetylene by free radicals existing on the coke surface.
The mechanism of coke formation during the pyrolysis of ethane and propane is
represented by the following sequence:
ethylene, propylene cyclic olefins benzene, alkyl-benzenes condensed
aromatics coke
Thermal Method
10
Coke formation during delayed coking and visbraking
In these processes, the initial stages of the thermal decomposition
reactions have a specific character since they take place in liquid phase: 1
liter of gas at 500oC and 1atm contains approx. 1022 molecules while 1
liter of liquid contains approx. 1024 molecules the concentrations in the
liquid phase are equivalent to the concentrations in the gas phase at
pressures of about 100 bar. Due to this effect, there will be an increase in
the liquid phase of bimolecular reactions such as polymerization,
condensation and the bimolecular interactions of thermal decomposition,
as compared to the gas phase.
As a result, the cracking reactions in the liquid phase convert the feed to
products of a higher average molecular mass such as asphaltenes, which
remain mostly in the liquid phase and undergo increasingly advanced
condensation and dehydrogenation, eventually becoming coke.
Thermal Method
11
Coke formation during delayed coking and visbraking
The following chain mechanism was suggested for the polycondensation reaction,
which transforms asphaltenes into coke precursors such as carboids
where A is a molecule of asphaltene and
M represents the light molecules that
go into the gas phase.
Reaction (a) is the chain initiation;
reactions (b),. . .,(d) are the chain
propagation and reaction (n) is the chain
termination.
is an inactive radical.
Thermal Method
12
INFLUENCE OF OPERATING CONDITIONS
temperature,
pressure,
feedstock properties,
steam introduced into the reactor
Thermal Method
13
Influence of temperature
The influence of temperature must be examined from the point of view of both the
modification of the thermodynamic equilibriums and of the relative rates of the
decomposition and other reactions that take place within the process
The increase in temperature has a favorable effect on the conversion to alkenes.
Therefore, considerable efforts are made to increase the coil outlet temperatures
(COT), in pyrolysis furnaces. Higher COT favor also the formation of acetylene
from ethene, of butene from butane and of butadiene from butene. However, it is
not possible to reach a maximum production of all these three hydrocarbons in the
same time. The reason is in the difference among the rates of the reactions that
produce them and in the fact that the desired products suffer further
decomposition.
Higher operating temperatures also favor the equilibrium for the formation of
aromatic hydrocarbons from alkylcyclohexanes
With increasing temperature the average molecular mass of the formed products
decreases
In the cracking processes of crude oil fractions, the reactions of the intermediary
products resulting from the decomposition, have higher activation energies than
the reactions of the feed. Higher operating temperatures will reduce the ratio of
the rate constants and, also reduce the maximum concentration of the intermediary
product. The time for obtaining the maximum will also be shorter
Thermal Method
14
Rate constants for the conversion
of asphaltenes to coke:
1-asphaltenes precipitation;
2-coke formation
Thermal Method
15
Influence of pressure
The reduction of pressure favors the increase in conversion of alkane (ethane to
ethene, propane to propene, 1-butane to butadiene) and in the dehydrogenations of the
6-carbon atoms ring cyclo-alkanes to aromatic hydrocarbons
The polymerization of the alkenes, produced during the processes of cracking under
high pressure, is favored by the increase in pressure
The effect of pressure upon the kinetics of the radical chain decomposition may be
analyzed also by examining its influence on the mechanisms of the following
reactions:
cleavage of free radicals: The decomposition of the radicals is a succession of cracking
steps that occur in position, at time intervals corresponding to the medium life duration of
the radicals. Since the reaction kinetics is of first order, the time during which the
transformation took place to some extent does not depend on the concentration
substitution reactions, i.e. the reactions of radicals with feed molecules: The substitution
relations are of second order. Therefore, the time to reach a certain degree of transformation
is inversely proportional to the concentration
The increase of pressure, causes an increase of the concentration. The result is that the
frequency of the substitution reactions is higher than that of the decomposition
reactions. This means that the substitution reactions may take place before all the
cleavages in position took place. For this reason the average molecular mass
resulting from the decomposition is higher at higher pressure.
Thermal Method
16
Influence of pressure
The pressure also influences the precipitation of asphaltenes, which is a determining
stage in the formation of coke at the temperatures of thermal cracking. By increasing
the pressure, the liquid phase will become enriched in lighter hydrocarbons, in which
asphaltenes are less soluble. This facilitates their precipitation and increases the rate of
coke formation. This effect is the usual cause of coking in thermal cracking.
However, if the system pressure exceeds the critical pressure of the hydrocarbons
mixture in the vapor phase, an opposite effect may come into play. In supercritical
conditions, the solubility of the heavy hydrocarbons is increased and their precipitation
as well as coking will be retarded. As an end effect, the formation of coke becomes
disfavored by the increase of pressure.
With respect to coke formation, the processes of cracking at high pressure requires a
thorough analysis in order to take into account the nature of the hydrocarbons present
in the system and the pressure in the reactor.
In the presence of other favorable process parameters such as high temperatures, a
reduced pressure in the process may lead to a strong increase of the aromatization
reactions especially by the dehydrogenation of cycloalkanes
Thermal Method
17
Influence of pressure and temperature
The mentioned effects of the temperature and pressure on the thermal processes,
resulted in the selection of two extreme regimes, determined by the objective of the
process:
very high temperatures (limited only by the possibilities of technical implementation) and
partial pressures as close as possible to the atmospheric, for pyrolysis processes. Here, the
main objective is the production of ethene and propene.
moderate temperatures (as low as possible, but sufficient for achieving reaction rates
conducive to reasonable reactor sizes) and high pressures (limited by economic criteria),
when the production of liquid fractions is targeted.
The consequences of these two extreme regimes on the yields and the chemical
character of the products, may be illustrated by the following data:
In the pyrolysis of liquid fractions, the gases represent approximately 73% for the pyrolysis
of gasoline and about 50% for the pyrolysis of gas oil, whereas for the cracking at higher
pressures, the maximal yield is about 12%.
In the gases from pyrolysis (the fraction C1 –C4), ethene represents 30–35% by weight,
propene 14–18%, butadiene about 5%, and unsaturated hydrocarbons represent around 75%.
In the gases produced in the processes operating at high pressures, the alkenes generally
represent about 25%.
Thermal Method
18
Influence of Feed composion
The molecular mass and the chemical composition of the feedstock determine the
conversion as well as the distribution of the products obtained from thermal
processes. This connection between the characteristics of the feedstock and the
result of the process is expressed quantitatively by the kinetic equations and the
rate constants.
Because the thermal cracking and the pyrolysis processes are strongly influenced
by temperature, the overall conversion may be modified without difficulties by a
small change in the temperature at the reactor exit. It must be taken into account
that, in visbreaking and delayed coking, which are performed at temperatures
around 5008C, the reaction rate doubles for a temperature increase of 12–158C. In
pyrolysis, the doubling of the rate requires an increase of about 258C. A more
important change of the temperature may be limited by the highest value tolerated
by the tubes of the furnace. This is, among others, the reason for the special care
paid to the selection of the value of the kinetic constants used in the design of the
pyrolysis furnaces. This is of lesser importance for processes that use lower
temperatures (thermal cracking, visbreaking, delayed coking, etc.).
Thermal Method
19
Influence of Feed composion
The variation of the conversion to GO for two feeds having different chemical character:
1-paraffinic, low in resins and asphaltenes;
2-naphthenic—aromatic, with more resins and asphaltenes
Thermal Method
20
- Xem thêm -