60
4. Hydrocracking Irreversible
5. Hydrocracking Irreversible
50
2. Saturation/ dehydrogenation Reversible
+ H
+ H
40
30
20
± 3 H
Polynaphthene
1. Saturation/ dehydrogenation Reversible
10
± 2 H
0
3. Condensation Irreversible
Temperature, ˚C
Etc.
Chrysene
Decalin
Tetralin
Naphthalene
– 3 H
Polyaromatic
HPNA Precursor
Figure 5 Simple representation of the competition between saturation and condensation of polyaromatics
Figure 6 Thermodynamic calculations are for reactions shown in Figure 5
hydrocracking decreases precursor concentrations, which r educes the formation of HPNA. As shown in Figure 5 and Figure 6 , there is competition between hydrocracking and HPNA formation, a competition that is strongly dependent on temperature and pressure. 7 Reaction 1 : reversible saturation of naphthalene to tetralin. Reaction 2 : irreversible hydrocracking of tetralin to 1,2-diethyl benzene. Reaction 3: irreversible condensation of naphthalene (C 10 H 8 ) with o-xylene (C 8 H 10 ) to give chrysene (C 18 H 12 ) For simplicity, we do not show related reactions, such as the reversible saturations of tetralin and 1,2-diethyl ben- zene and other subsequent hydrocracking reactions. In Figure 7 , we see the effect of temperature and pres- sure on the formation of chrysene, the product of the con- densation of naphthalene with o-xylene. At 400°C and 124 bar, the equilibrium concentration of chrysene is about 6 wt ppm. At first glance, the figure implies that, at this pres - sure, keeping the reaction peak temperature below 420°C Reaction 4: irreversible ring opening of tetralin. Reaction 5: irreversible ring opening of decalin.
should avoid exponential growth of chrysene. But this is misleading. In operating units, chrysene would grow into larger molecules due to subsequent condensation reactions. Figure 8 shows how thermodynamics drive the produc- tion of larger HPNA at higher and higher temperatures. There are many ways to do this, but for illustration purpose we calculated ΔG R (Gibbs free energy of reaction) for HPNA formation via condensation of naphthalene. ΔG F (Gibbs free energy of formation) for the reactants and arbitrary prod- ucts were obtained from the NIST PAH Database. 8 The assumed reactions are as follows: 1.8 naphthalene (C 10 H 8 ) chrysene (C 18 H 12 ) + 1.2 H 2 2 naphthalene (C 10 H 8 ) perylene (C 20 H 12 ) + 2 H 2 2.4 naphthalene (C 10 H 8 ) coronene (C 24 H 12 ) + 3.6 H 2 2.8 naphthalene (C 10 H 8 ) benzo(a)coronene (C 28 H 14 ) + 4.2 H 2 3.2 naphthalene (C 10 H 8 ) ovalene (C 32 H 18 ) + 3.8 H 2
Note that coronene is especially stable due to its uniquely symmetrical structure. Part of the driving force behind
0
-50
0.0000 0.0002 0.0001 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 0.0009 0.0010
-100
-150
-200
-250
-300
-450 -350 -400
100 150
200 250
300
350
400 450 500
550 600
Temperature, ˚C
C H
C H
C H
C H
C H
350
370
390
410
430 450
470
490
Temperature, ˚C
Figure 8 Condensation of naphthalene to form various polynuclear aromatic products as a function of temperature. ΔG F data come from the NIST PAH Database (https://pah.nist.gov/?q=pah315). More negative ΔG R indicates a more favourable reaction
207 bar 103 bar
186 bar 83 bar
166 bar 62 bar
145 bar 41 bar
124 bar 21 bar
Figure 7 Chrysene production vs temperature at various pressures
33
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