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Файл:Chemical Engineering of Natural Fuels and Carbon Materials. Study Guide
.pdf
Task 7.16.
Write all possible schemes of n-heptane dehydrocyclisation.
Task 7.17.
Write the mechanism of meta-xylene conversion in catalytic
reforming.
51
8. THE CHEMISTRY OF HYDROCATALYTIC PROCESSES
Hydrogenation processes include:
- Hydrotreating of fuel and petroleum oil fractions;
- Hydrodesulfurization and hydrodemetallization of heavy oil
residues;
- Hydrocracking.
Hydrotreating is the process of petroleum products (petrochemicals)
refinement by removing heteroatom containing compounds. Sulfur-,
nitrogen- and oxygen- containing compounds are hydrogenated, olefinic
hydrocarbons and polycyclic aromatic hydrocarbons are also saturated with
hydrogen. Also, there is a removal of organometallic compounds.
In these reactions, the weakest C-heteroatom bond breaking takes
place. The strength of this bond is less than C-C bond.
The stability towards hydrogenation increases in the following
range of compounds:
Sulfur-containing < oxygen-containing < nitrogen-containing.
Sulfur is found in petroleum and petrochemicals in the form of
elemental sulfur, hydrogen sulfide, mercaptans, aliphatic and aromatic
sulfides, disulfides, cyclic sulfides (thiophane) thiophenes, and
benzothiophene.
Mercaptans are hydrogenated with forming hydrogen sulfide and
corresponding hydrocarbons.
The stability of sulfur compounds increases in the following order:
mercaptans < disulfides < sulfides < thiophanes < thiophenes.
Hydrogenation reaction of nitrogen-containing compounds is
similar to the hydrogenation reaction of sulfur-containing compounds and is
accompanied by the release of ammonia.
Nitrogen-containing compounds may be represented by pyridine,
piperidine, quinoline, pyrrole, indole.
Oxygen-containing compounds in petroleum fractions can be
represented by alcohols, ethers, phenols and naphthenic acids.
Oxygenates are easily hydrogenated to form the corresponding
hydrocarbon and water.
Hydrocracking is aimed to obtain products, which are quite similar
to catalytic cracking products, but in hydrocracking products are much less
aromatized, refined from heteroatoms and not containing olefinic
hydrocarbons.
Since the depth of hydrogenation processes depends on the process
52
conditions, under more severe conditions during hydrocracking hydrocarbon
cracking reactions occur.
Task 8.1.
Write the following conversion reactions of organosulfur
compounds during hydrotreating process:
n-butyl mercaptan;
ethyl, isobutyl sulfide;
dipyridyl disulfide;
hexa thiophane;
3-ethyl thiophane;
2-methyl thiophene;
thia tetralin;
dibenzothiophene.
Task 8.2.
Write the following conversion reactions of organonitrogen
compounds during hydrotreating process:
aniline;
pyrrole;
pyridine;
quinoline;
indole (benzpyrrol);
benzokarbazol.
Task 8.3.
Write the following conversion reaction of oxygen-containing
compounds in the hydrotreating process:
meta-methylcyclohexane carboxylic acid;
furan;
benzofuran.
Task 8.4.
Write the mechanism of n-heptane conversion of in the
hydrocracking process.
Task 8.5.
Write the mechanism of cycloalkanes hydrocracking based on the
example of 1,2,3,4 - tetra-methyl-cyclohexane and n-butyl-cyclohexane.
53
Task 8.6.
Write the mechanism of decalin transformation during the
hydrocracking process.
Task 8.7.
Write the conversion reaction of phenanthrene in hydrocracking.
54
9. HYDROCARBON PROCESSING MATERIAL
aТ
15
15
2
15
15
liq
ρ
1
)3.334762.0Т(0.0017
ρ
1
I
15
15
ρ
308.99)ρ(4)00059.0134.058.129(
v
I
15
15
2
TT
308.99)ρ(4
15
15
a
AND HEAT BALANCES CALCULATION
9.1 Heat balance calculation of a catalytic cracking reactor
The amount of heat carried by any technological stream can be
calculated using the following formula:
Q = G · I, (9.1)
where G is a flow rate, kg/h; I is enthalpy, kJ/kg.
Enthalpy of liquid petroleum products can be defined according to
the following formula:
, kJ/kg, (9.2)
where
is relative density (specific gravity) of the petroleum product; Т
is temperature, К; a is the coefficient from reference table (Appendix 3).
Enthalpy of vapours of petroleum products can be defined according
to the following formula:
, kJ/kg . (9.3)
Relative density (specific gravity) of petroleum products can be
defined according to the Craig formula:
where М is the molar mass of petroleum product, found by Voinov's
formula:
55
М = 60 + 0.3 · t + 0.001· t2, kg/kmol, (9.5)
87.6016.579
927.0
1
ρ
1
I
15
15
liq
a
where t is an average boiling point of petroleum fraction, °С.
Enthalpy of petroleum product decreases with the increase of
pressure. If total pressure in a technological system is more than 0.4 MPa,
pressure correction should be applied to the value of enthalpy.
Example 9.1
Calculate the heat flow which enters to the reactor of catalytic
cracking by feedstock the with the temperature boiling range 350–500 °С).
Unit capacity is 2.5 mln. t/year. The number of unit operation days equals
332 days per year. The temperature of feedstock is 270 °С. The pressure in
the reactor is 180 kPa.
Solution:
The average boiling point of vacuum gas oil will be:
The molar mass of vacuum gas oil:
М = 60 + 0.3·t + 0.001·t2 = 60 + 0.3·450 + 0.001·4502 = 398 kg/kmol.
Relative density (specific gravity) of vacuum gas oil:
Enthalpy is calculated using the formula for liquid petroleum
products (9.2):
kJ/kg .
The pressure in the reactor is less than 0.4 MPa, so there is no need
to introduce pressure correction to the value of enthalpy.
56
Unit capacity in kg/h:
Feedstock heat flow will be:
Qf = 313755 · 601.87 = 188.84 GJ/h .
Task 9.2.
Calculate the heat flow which comes out of the reactor of catalytic
cracking by gasoline fraction with the temperature boiling range 80–180 °С.
Unit capacity is 2.1 mln. t/year. The number of unit operation days equals
332 days per year. The temperature at the output point of product flow is
500 °С. The pressure in the reactor is 180 kPa. Gasoline yield on feedstock
is 54 %.
Example 9.3
Calculate the heat balance of the catalytic cracking reactor and the
circulation multiplicity of the catalyst.
The feedstock in the unit is hydrotreated vacuum gas oil with the
temperature boiling range 320 – 545 ° C, density 0.888.
The pressure in the reactor is 180 kPa. The temperature of feedstock
is 265 °C.
Along with feedstock, 4100 kg/h of superheated steam with the
temperature of 350 °C is introduced into the reactor. An additional amount
of steam for stripping the catalyst (for the circulated catalyst) is 0.5% wt.
Specific heat of the catalyst equals 1.1 kJ / kg ∙°C, the specific heat
of coke is 2.0 kJ / kg ∙°C.
Temperatures in the reactor zones are the following: t1 = 510 °C, t2
= 500 °C, t3 = 490 °C. The regenerated catalyst temperature is 670 °C. The
average molecular weight of the products Msr. = 110 kg/kmol, the
molecular weight of the dry gas is 32 kg/kmol.
The enthalpy of the acid gas at 490 °C is 1728 kJ / kg.
Heat loss is 5% of the total heat introduced into the reaction zone.
Material balance of the unit is given in table 9.1.
57
Table 9.1 – Material balance of the CC unit
Material stream
Specific gravity
% wt.
kg/h
Income:
1. Vacuum gas oil (F)
2. Recycled product (R)
3. Steam
0.888
1.050
100.00
5.55
?
268 176
14 875
4100 + ?
In total:
287 151 + ?
Outcome:
1. Dry gas
2.Н 2 S
3. Propane fraction
4. Propylene fraction
4. Butane - butylene fraction
5. Gasoline
6.Light gas oil
7.Heavy gas oil
8. Coke
9. Recycled product
10. Losses
11. Steam
0.741
0.973
1.050
-
1.050
-
-
6.06
0.02
1.1
6.61
14.35
53.5
10.36
2.72
4.51
5.55
0.77
?
16 251
53.64
2 949.94
17 726.4
38 483.3
143 474.2
27 783
7 294.4
12 094.7
14 875
2 064.96
4100 + ?
In total:
110.65
287 151 + ?
Solution:
Q
= Q
, MJ/h (9.6)
out
+ Q
rec.in
cir.cat.in
Q
= QF + Q
in
in
+ Q
steam in,
MJ/h
(9.7)
Q
= Q
out
gas
+ Q
where QF is heat flow which is carried in to the reactor by feedstock; Q
is heat flow which is fed to the reactor by the recycled product; Q
heat flow, which is carried into the reactor by the regenerated catalyst; Q
in
which is carried off the reactor by dry gas; Q
carried off the reactor by propane-propylene fraction; Q
is heat flow, which is carried into the reactor by steam; Q
+ Q
ppf
steam out
+ Q
+ Q
+ Q
bbf
cir.cat.out
+ QLG+ QHG+ Q
gasoline
+ Q
+ Q
H2S
58
reac
+ Q
rec.out
+ Q
, MJ/h
losses
is heat flow, which is
ppf
coke
.
is heat flow,
gas
is heat flow,
bbf
+
cir.cat.in
(9.8)
rec.in
is
steam
which is carried off the reactor by butane-butylene fraction; Q
6.16131865.566
0.888
1
268176
35.822365.566
1.050
1
14875
gasoline
is heat
flow which is carried off of the reactor by gasoline; QLG is heat flow which
is carried off the reactor by light gas oil; QHG is heat flow, which is carried
off the reactor by heavy gas oil; Q
reactor by the recycled product; Q
reactor by coke; Q
steam; Q
cir.cat.out
is heat flow, which is carried off the reactor by the
circulated catalyst; Q
acid gas; Q
acid thermal effect of the reactions during process; Q
reac
is heat flow which is carried off the reactor by
steam out
is heat flow which is carried off the reactor by
H2S
is heat flow which is carried off the
rec.out
is heat flow, which is carried off the
coke
losses
acid
heat losses.
The unknown quantity is the amount of circulating catalyst G
cir.cat
.
The amount of heat coming into the reactor can be calculated using
the formulas (9.1), (9.2). The coefficient a can be found in Appendix 3.
QF = GF · I
F(265)
=
MJ/h.
Q
rec.in
= G
rec.in.
· I
rec.in.(265)
=
MJ/h.
The enthalpy of steam and a catalyst can be found by the formula:
I = c · t, kJ/kg, (9.9)
where с is heat capacity, kJ/kg·°С; t is temperature, °С.
Q
cir.cat.in
= G
cir.cat.
· I
cir.cat.(670)
= G
· 1.1· 670 = 0.737·G
cir.cat.
, MJ/h.
ц.к.
Q
steam in
= G
steam
· I
steam in (350)
= (0.005 · G
+ 4100) · 2.1 · 350 =
cir.cat.
= 0.003675 · G
+ 3013.5 MJ/h,
cir.cat.
where 4100 kg/h is the amount of steam introduced to the reactor with
feedstock; 0.005 · G
is the additional amount of steam for stripping the
cir.cat.
catalyst.
Q
= 172555.45 +0.7407 · G
in
cir.cat.,
MJ/h.
59
The enthalpies of dry gas, propane, propylene, and butane-butylene
308.990.741)(4574.95
308.990.973)(4574.95
308.991.050)(4574.95
308.991.050)(4574.95
fraction can be defined by applying the graphs (Appendix 4), using
molecular weight and temperature data.
Q
= G
gas
gas
· I
= 16251 · 2382.6 = 38719.63 MJ/h.
gas(490)
Q
H2S
= Q
H2S
· I
H2S (490)
= 53.64 . 1728 = 92.69 MJ/h.
Q
= G
ppf
ppf
· I
= 20676.34 · 2340.8 = 48399.18 MJ/h.
ppf (490)
Q
= G
bbf
bbf
· I
= 38483.3 · 2299 = 88473.11 MJ/h.
bbf (490)
The amount of heat coming off the reactor is calculated using the
formulas (9.1), (9.3). Coefficient a can be found in Appendix 3.
Q
gasoline
= G
gasoline
· I Q
gasoline (490)
= 143474.2∙
= 224504.12 MJ/h.
QLG = GLG · Q
LG(490)
= 27783·
=39768.03 MJ/h.
QHG = G
HG
· I
HG (490)
= 7294.4 ·
=10118.13 MJ/h.
Q
rec.out
= G
rec.out
· I
rec.out (490)
= 14875 ·
=20633.26 MJ/h.
Q
steam out
= G
steam
· I
steam(490)
= (0.005 · G
+ 4100)· 2.11 · 490 =
cir.cat.
=0.00517 · G
+ 4239.4 MJ/h.
cir.cat.
Q
coke
= G
coke
· I
= 12094.7·2.0 · 500 = 12094.7 MJ/h.
coke(500)
Q
cir.cat.out.
= G
cir.cat.
· I
cir.cat.out.(500)
= G
· 1,1· 500 = 0.550 · G
cir.cat.
60
cir.cat.
, MJ/h.
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