Ci a ion: Vela, F.J.; Palos, R.; Bilbao,
J.; A andes, J.M.; Gu ié ez, A.
Hyd ogen P essu e as a Key
Pa ame e o Con ol he Quali y o
he Naph ha P oduced in he
Hyd oc acking o an HDPE/VGO
Blend. Ca alys s 2022,12, 543.
h ps://doi.o g/10.3390/
ca al12050543
Academic Edi o s: Hugo de Lasa and
Mohammad Mozaha Hossain
Recei ed: 22 Ap il 2022
Accep ed: 13 May 2022
Published: 16 May 2022
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ca alys s
A icle
Hyd ogen P essu e as a Key Pa ame e o Con ol he Quali y o
he Naph ha P oduced in he Hyd oc acking o an HDPE/
VGO Blend
F ancisco J. Vela 1, Robe o Palos 1,2 , Ja ie Bilbao 1, JoséM. A andes 1and Alazne Gu ié ez 1,*
1Depa men o Chemical Enginee ing, Uni e si y o he Basque Coun y UPV/EHU, P.O. Box 644,
48080 Bilbao, Spain; anciscoja ie [email p o ec ed] (F.J.V.); [email p o ec ed] (R.P.);
ja ie [email p o ec ed] (J.B.); [email p o ec ed] (J.M.A.)
2Depa men o Chemical and En i onmen al Enginee ing, Uni e si y o he Basque Coun y UPV/EHU,
Plaza Ingenie o To es Que edo 1, 48013 Bilbao, Spain
*Co espondence: alazne.gu ie [email p o ec ed]
Abs ac :
The hyd oc acking o high-densi y polye hylene (HDPE) blended wi h acuum gas oil
(VGO) has been s udied o assess he e ec o H
2
p essu e on he yield and composi ion o he
p oduc s and wi h he aim o ob aining comme cial uels, mainly naph ha. The expe imen s ha e
been pe o med using a P Pd/HY ca alys in a semiba ch eac o unde he ollowing condi ions:
H
2
p essu e, 20–110 ba ; 440
◦
C; ca alys o eed a io, 0.1 g
ca
(g
eed
)
−1
; HDPE o o al eed a io,
0.2 g
HDPE
(g
eed
)
−1
; and eac ion ime, 2 h. The composi ion o he main ac ions p oduced (gas,
naph ha, and ligh cycle oil) e eals he in e es in ca ying ou he p ocess a 110 ba . Thus, con e -
sions o 96 and 79% o he emo al o hea y hyd oca bons and he emo al o HDPE molecules ha e
been ob ained, espec i ely, oge he wi h a yield o naph ha o 53.4 w %. This naph ha is mainly
pa a inic, and i has a RON o 91.5 (wi hin he comme cial s anda ds). Fu he mo e, h ee ac ions
ha e been obse ed in he analysis ( empe a u e-p og ammed oxida ion) o he coke. This analysis
e eals ha a 110 ba , he coke e ained in he HY zeoli e cages is less de eloped and bu ns a a
mode a e empe a u e.
Keywo ds:
hyd oc acking; HDPE; VGO; p essu e; hyd ogen; uel; naph ha; medium dis illa es; coke
1. In oduc ion
Hyd oc acking is a key p ocess in e ine ies o he con e sion o hea y oil ac ions
in o low-molecula -weigh s eams, a he same ime undesi able molecules a e emo ed,
in pa icula S, N, and me als. Thus, a e subsequen e o ming and condi ioning s ages,
hyd oc acked s eams a e sui able o being used in he blending o comme cial naph ha
and medium dis illa es [
1
,
2
]. In addi ion, he e sa ili y o he hyd oc acking p ocess
places i a he o e on o he in ensi ica ion p ocesses ha aim o he di ec p oduc ion o
uels wi h he composi ion es ablished by legal equi emen s [
3
]. This way, hyd oc acking
p ocesses ha e been widely used in he alo iza ion o di e en seconda y e ine y s eams,
such as acuum esidue [
4
], ligh cycle oil [
5
], and acuum gas oil [
6
]. In addi ion, i s
use has also been p oposed o ea ing bio-oil [
7
], which is a liquid p oduc ob ained
in he as py olysis o lignocellulosic biomass, as well as was es om he consume s’
socie y (mainly was e plas ics and end-o -li e i es) and he oil ob ained in he py olysis o
hese was es [
8
,
9
]. The co- eeding o hese was es o e ine y uni s (Was e-Re ine y) is an
inno a i e managemen s a egy ha is ecei ing inc easing a en ion in he li e a u e [
10
].
The Was e-Re ine y p oposal has he a ac ion o b inging he Ci cula Economy s a egy
close o he oil indus y, which will en ail impo an sa ings in aw ma e ials and inancing
he in en o y equi ed o he alo iza ion o was es. In pa allel, i will con ibu e o sol ing
he en i onmen al issues de i ed om land illing [
11
] o incine a ion [
12
] o hese was es.
Ca alys s 2022,12, 543. h ps://doi.o g/10.3390/ca al12050543 h ps://www.mdpi.com/jou nal/ca alys s
Ca alys s 2022,12, 543 2 o 17
Howe e , he indus ial scale-up o his s a egy equi es es ablishing he app op ia e
ope a ing condi ions and adap ing he ca alys s o his speci ic goal.
The hyd oc acking o was e plas ic in o uels has ecei ed less a en ion han o he
he mochemical p ocesses, such as he mal and ca aly ic c acking. The mal c acking is pa -
icula ly in e es ing o he eco e y o monome s om s y ene [
13
], polyme hyl me hac y-
la e (PMMA) [
14
], and polyole ins [
15
]. In con as , ca aly ic c acking o e zeoli e-based
ca alys s (HZSM-5 being he mos s udied one) is in e es ing o he selec i e p oduc ion o
ligh ole ins [
16
]. None heless, unde he condi ions equi ed o he p oduc ion o uels
(high empe a u es and high con en s o s ong acid si es in he zeoli e), he composi ion o
he liquid p oduc ob ained p esen s high con en s o ole ins and a oma ics ha impede
he di ec use o his liquid as au omo i e uel [
17
], equi ing a subsequen adap a ion o
i s composi ion by hyd op ocessing.
The hyd oc acking o polyole ins (LDPE, HDPE, and PP) o e a P /Be a ca alys a
330
◦
C and 20 ba H
2
esul ed in an excessi e p oduc ion o gases (C
3
–C
4
) [
18
], which is
a ibu ed o he di usional limi a ions and o e c acking o he mac omolecules. Ne e he-
less, he use o P ca alys s suppo ed on USY zeoli es inc eased ema kably he yield o
naph ha (C5–C12) and medium dis illa es (C13–C20) [19].
The co- eeding o plas ic oge he wi h a e ine y s eam, such as VGO (benchma k
eed o hyd oc acking uni s), shows impo an ad an ages [
9
]. Fi s ly, i acili a es he
ex en o he hyd oc acking eac ions, inc easing he le el o con e sion a ained. Secondly,
he p esence o HDPE in he eac ion medium leads o he p oduc ion o a less a oma ic and
mo e pa a inic naph ha ac ion. Mo eo e , a p e ious wo k [
20
] in es iga ed he e ec o
he empe a u e on he pe o mance o a P Pd/HY ca alys (ac i i y and selec i i y) in he
hyd oc acking o HDPE blended wi h VGO (20/80 in mass) and ound ha his ca alys was
app op ia e o maximizing he yield o naph ha a 420
◦
C. In addi ion, his empe a u e
was app op ia e o achie ing high a es in he eac ions ha lead o he o ma ion o ole ins
om he HDPE (by means o a ee adical mechanism) and o hei subsequen p o ona ion.
The syne gis ic e ec s be ween he hyd oc acking mechanisms en ailed an inc ease in he
con e sion achie ed wi h espec o ha ob ained in he hyd oc acking o nea VGO. The
composi ion o he naph ha lump ob ained in he hyd oc acking o he HDPE/VGO blend
was also mo e a ou able o being added o he lump o comme cial gasoline since i had
an impo an concen a ion o pa a ins and a low concen a ion o a oma ics. Equally, in
he LCO ac ion ob ained in he hyd oc acking o he blend a 420
◦
C, he concen a ion
o pa a ins is highe han ha ob ained in he hyd oc acking o VGO, whe eas ha o
a oma ics is lowe . I means a highe in e es in being alo ized wi hin he diesel pool.
In his con ex , his wo k assesses he e ec o he H
2
p essu e used in he hyd oc-
acking o a blend o HDPE and VGO since his pa ame e will undoub edly condi ion he
economy o he p ocess. E en hough he ele an e ec o his a iable on he yields and
p oduc dis ibu ion has been widely demons a ed [
21
], ew au ho s ha e ackled i s di ec
s udy in he hyd oc acking o was e plas ics. The aim o he wo k is o ob ain de ailed
in o ma ion abou he e ec o H
2
p essu e on he yields, selec i i ies, and composi ion o
he ac ions wi h comme cial in e es ob ained in he hyd oc acking o he HDPE/VGO
blend. Fo his pu pose, he p oduc s ha e been lumped in o di e en ac ions as is cus-
oma y in he oil indus y: hea y cycle oil (HCO), ligh cycle oil (LCO), naph ha, lique ied
pe oleum gas (LPG), d y gas, and coke. Fu he mo e, in iew o he impo ance o ca alys
deac i a ion by coke deposi ion, he e ec o H
2
p essu e on he con en and na u e o he
coke has also been s udied.
2. Resul s and Discussion
2.1. Ca alys P ope ies
The main cha ac e iza ion esul s ha e been summed up in Table 1. A mo e de ailed
desc ip ion o he echniques, equipmen , and expe imen al p ocedu es used in ca alys
cha ac e iza ion can be ound in p e ious wo k [
20
]. The channel s uc u e o he Y zeoli e
Ca alys s 2022,12, 543 3 o 17
can be seen in he TEM image a 100 nm (Figu e 1a), while in he image a 50 nm (Figu e 1b),
he good dispe sion o P and Pd pa icles (da k spo s) on he suppo can be obse ed.
Table 1. Tex u al p ope ies, chemical composi ion, and acidic p ope ies o he P Pd/HY ca alys .
P ope y P Pd/HY
SBET (m2g−1)620
Smic opo e (m2g−1)543
Smesopo e (m2g−1)77
Vpo e (cm3g−1)0.39
dpo e (nm) 8.44
P (w %) 1.19
Pd (w %) 0.53
AT(mmol -BA g−1)11.69
AS(kJ mol -BA−1)2135
B/L a io 1.53
1To al acidi y; 2a e age acidic s eng h.
Ca alys s 2022, 12, 543 3 o 18
2. Resul s and Discussion
2.1. Ca alys P ope ies
The main cha ac e iza ion esul s ha e been summed up in Table 1. A mo e de ailed
desc ip ion o he echniques, equipmen , and expe imen al p ocedu es used in ca alys
cha ac e iza ion can be ound in p e ious wo k [20]. The channel s uc u e o he Y zeo-
li e can be seen in he TEM image a 100 nm (Figu e 1a), while in he image a 50 nm
(Figu e 1b), he good dispe sion o P and Pd pa icles (da k spo s) on he suppo can be
obse ed.
Table 1. Tex u al p ope ies, chemical composi ion, and acidic p ope ies o he P Pd/HY ca alys .
P ope y P Pd/HY
SBET (m2 g−1) 620
Smic opo e (m2 g−1) 543
Smesopo e (m2 g−1) 77
Vpo e (cm3 g−1) 0.39
dpo e (nm) 8.44
P (w %) 1.19
Pd (w %) 0.53
AT (mmol -BA g−1) 1 1.69
AS (kJ mol -BA−1) 2 135
B/L a io 1.53
1 To al acidi y; 2 a e age acidic s eng h.
(a) (b)
Figu e 1. T ansmission elec on mic oscopy (TEM) a 100 nm (a), and a 50 nm (b) o esh P Pd/HY
ca alys .
2.2. Hyd oc acking Yields and Con e sion
The e ec o H2 p essu e on he yields o p oduc s and he ex en o con e sion a -
ained is depic ed in Figu e 2. O e all, an inc ease in he H2 p essu e a ou s he con-
e sion o bo h HDPE and HCO, as well as he o ma ion o ligh p oduc s, mainly
naph ha and gases. In addi ion, an impo an educ ion in he yield o coke should be
highligh ed ha passes om 2.4 w % a 20 ba o a yield o 1.1 w % a 110 ba .
Figu e 1.
T ansmission elec on mic oscopy (TEM) a 100 nm (
a
), and a 50 nm (
b
) o esh P Pd/
HY ca alys .
2.2. Hyd oc acking Yields and Con e sion
The e ec o H
2
p essu e on he yields o p oduc s and he ex en o con e sion a ained
is depic ed in Figu e 2. O e all, an inc ease in he H
2
p essu e a ou s he con e sion o
bo h HDPE and HCO, as well as he o ma ion o ligh p oduc s, mainly naph ha and gases.
In addi ion, an impo an educ ion in he yield o coke should be highligh ed ha passes
om 2.4 w % a 20 ba o a yield o 1.1 w % a 110 ba .
The yield ob ained a 20 ba o he HCO ac ion was 25.2 w %, which co esponds
o a con e sion le el o 67%. Wi h ega d o he yield o p oduc s, a balanced dis ibu ion
has been ob ained o he yield o gas, naph ha, and LCO ac ions (16, 18, and 23 w %,
espec i ely). An inc ease in he H
2
p essu e up o 50 ba has no a ec ed he HCO
con e sion, bu i has a ec ed p oduc dis ibu ion. The yields o naph ha and gas ac ions
ha e inc eased (up o 31 and 17 w %, espec i ely) o he de imen o LCO (12 w %).
A u he inc ease in he H
2
p essu e o 80 ba has e ealed he impo an ole o his
pa ame e . This way, he HCO con e sion has inc eased up o 73%, and he yields o bo h
HCO and LCO ac ions ha e dec eased down o 9 and 21 w %, espec i ely. Consequen ly,
Ca alys s 2022,12, 543 4 o 17
he yields o he ligh ac ions ha e con inued inc easing, eaching a alue o 37 w % o
he naph ha and 25 w % o he gas. Finally, a he highes wo king H
2
p essu e (110 ba ),
he lowes yields o LCO and HCO ac ions ha e been ob ained, which co espond o
almos o al HCO con e sion (94%). A ending o he yields o p oduc s, naph ha is by
a he main one (53 w %), e en hough an impo an p oduc ion o gases has also been
ob ained (30 w %).
Ca alys s 2022, 12, 543 4 o 18
Figu e 2. E ec o H2 p essu e on p oduc yields and con e sions.
The yield ob ained a 20 ba o he HCO ac ion was 25.2 w %, which co esponds
o a con e sion le el o 67%. Wi h ega d o he yield o p oduc s, a balanced dis ibu ion
has been ob ained o he yield o gas, naph ha, and LCO ac ions (16, 18, and 23 w %,
espec i ely). An inc ease in he H2 p essu e up o 50 ba has no a ec ed he HCO con-
e sion, bu i has a ec ed p oduc dis ibu ion. The yields o naph ha and gas ac ions
ha e inc eased (up o 31 and 17 w %, espec i ely) o he de imen o LCO (12 w %). A
u he inc ease in he H2 p essu e o 80 ba has e ealed he impo an ole o his pa-
ame e . This way, he HCO con e sion has inc eased up o 73%, and he yields o bo h
HCO and LCO ac ions ha e dec eased down o 9 and 21 w %, espec i ely. Conse-
quen ly, he yields o he ligh ac ions ha e con inued inc easing, eaching a alue o 37
w % o he naph ha and 25 w % o he gas. Finally, a he highes wo king H2 p essu e
(110 ba ), he lowes yields o LCO and HCO ac ions ha e been ob ained, which co -
espond o almos o al HCO con e sion (94%). A ending o he yields o p oduc s,
naph ha is by a he main one (53 w %), e en hough an impo an p oduc ion o gases
has also been ob ained (30 w %).
Addi ionally, a ema kable posi i e e ec o he H2 p essu e on he HDPE con e -
sion a ained can be obse ed in Figu e 2. Thus, an inc ease in he H2 p essu e en ails a
linea inc ease in he HDPE con e sion. This way, i has gone om a ela i ely mode a e
alue o 24% o 20 ba o a e y a ac i e con e sion le el o 79% a 110 ba .
The p oduc dis ibu ion ob ained is e lec ed in he e olu ion o he selec i i y o
uel index (Figu e 3). The e o e, an inc ease in he H2 p essu e om 20 o 80 ba b ings a
s epwise inc ease in his index om 0.7 o 0.8 because o he inc ease in he yield o
naph ha and he dec ease in he yields o HCO and wax. In addi ion, he ab up change
ha has aken place in p oduc dis ibu ion when inc easing he H2 p essu e om 80 o
110 ba (Figu e 2) has caused a sha p inc ease in he selec i i y o uel index eaching i s
maximum alue o 1.5.
20 50 80 110
0
20
40
60
80
100
0
20
40
60
80
100
Con e sion (%)
Yield (w %)
P essu e (ba )
Gas Naph ha
LCO X
HCO
HCO
Wax Coke X
HDPE
Figu e 2. E ec o H2p essu e on p oduc yields and con e sions.
Addi ionally, a ema kable posi i e e ec o he H
2
p essu e on he HDPE con e sion
a ained can be obse ed in Figu e 2. Thus, an inc ease in he H
2
p essu e en ails a linea
inc ease in he HDPE con e sion. This way, i has gone om a ela i ely mode a e alue o
24% o 20 ba o a e y a ac i e con e sion le el o 79% a 110 ba .
The p oduc dis ibu ion ob ained is e lec ed in he e olu ion o he selec i i y o
uel index (Figu e 3). The e o e, an inc ease in he H
2
p essu e om 20 o 80 ba b ings a
s epwise inc ease in his index om 0.7 o 0.8 because o he inc ease in he yield o naph ha
and he dec ease in he yields o HCO and wax. In addi ion, he ab up change ha has
aken place in p oduc dis ibu ion when inc easing he H
2
p essu e om 80 o 110 ba
(Figu e 2) has caused a sha p inc ease in he selec i i y o uel index eaching i s maximum
alue o 1.5.
To explain hese esul s, he e ec o he H
2
p essu e on he hyd oc acking mechanism
mus be analysed in de ail. I is well es ablished ha in he hyd oc acking o hyd oca -
bons and polyole ins, he c acking akes place ia ca benium ions. Mo eo e , unde he
eac ion condi ions employed (420
◦
C), he ex en o he mal c acking eac ions, wi h ee
adicals as in e media es, also has o be conside ed [
21
]. A e wa ds, he hyd oca bons a e
dehyd ogena ed o e he me al si es p oducing ole ins, which a e subsequen ly p o ona ed
and isome ised o e he acid si es. In addi ion, i he ca alys has a su icien densi y o
s ong acid si es (as occu s wi h HY zeoli es), he c acking akes place by
β
-scission o he
p o ona ed and b anched ole ins, o ming unsa u a ed species ha a e, in u n, sa u a ed
o e he hyd ogena ion si es [
22
,
23
]. In he hyd oc acking o polyole ins, he p esence o
s ong acidic si es a ou s he o ma ion o e ia y ca benium ions ha a e c acked by
β
-scission eac ions esul ing in ole ins and ca benium ions o lowe molecula weigh ,
which a e also c acked in subsequen s ages. The ole ins o med a e p o ona ed o pa a ins
Ca alys s 2022,12, 543 5 o 17
by me a hesis eac ions in he p esence o H
2
in he eac ion medium [
24
,
25
]. The e is also
e idence o he exis ence o syne gis ic e ec s be ween he he mal and ca aly ic hyd oc-
acking mechanisms because he o me leads o he o ma ion o molecules ha a e mo e
eac i e in he la e [26,27].
Ca alys s 2022, 12, 543 5 o 18
Figu e 3. E ec o H2 p essu e on selec i i y o uel index.
To explain hese esul s, he e ec o he H2 p essu e on he hyd oc acking mecha-
nism mus be analysed in de ail. I is well es ablished ha in he hyd oc acking o hy-
d oca bons and polyole ins, he c acking akes place ia ca benium ions. Mo eo e , un-
de he eac ion condi ions employed (420 °C), he ex en o he mal c acking eac ions,
wi h ee adicals as in e media es, also has o be conside ed [21]. A e wa ds, he hy-
d oca bons a e dehyd ogena ed o e he me al si es p oducing ole ins, which a e sub-
sequen ly p o ona ed and isome ised o e he acid si es. In addi ion, i he ca alys has a
su icien densi y o s ong acid si es (as occu s wi h HY zeoli es), he c acking akes
place by β-scission o he p o ona ed and b anched ole ins, o ming unsa u a ed species
ha a e, in u n, sa u a ed o e he hyd ogena ion si es [22,23]. In he hyd oc acking o
polyole ins, he p esence o s ong acidic si es a ou s he o ma ion o e ia y ca be-
nium ions ha a e c acked by β-scission eac ions esul ing in ole ins and ca benium
ions o lowe molecula weigh , which a e also c acked in subsequen s ages. The ole ins
o med a e p o ona ed o pa a ins by me a hesis eac ions in he p esence o H2 in he
eac ion medium [24,25]. The e is also e idence o he exis ence o syne gis ic e ec s
be ween he he mal and ca aly ic hyd oc acking mechanisms because he o me leads
o he o ma ion o molecules ha a e mo e eac i e in he la e [26,27].
The a o emen ioned cha ac e is ics o he mechanisms highligh he ole o he H2
p essu e, which a enua es he o ma ion o ee adicals and a ou s he ex en o he
hyd oc acking o hyd oca bons and polyole ins. S udies o he e ec o H2 p essu e on
he hyd oc acking o di e en polyole ins ha e shown ha , in gene al, inc easing he
p essu e o H2 inc eases he con e sion and he yield o liquid p oduc s. Howe e , his
e ec is p og essi ely smalle as he p essu e o H2 inc eases [24,28–30]. The e ec on he
gas yield is une en, al hough i dec eases wi h p essu e. The explana ion is ha an in-
c ease in he H2 p essu e a ou s he ca aly ic hyd oc acking mechanism wi h espec o
he he mal one, o which he a enua ion o he o ma ion o ee adicals in he p es-
ence o H2 con ibu es.
In addi ion, he a es o he ole ins cycliza ion and polya oma ics condensa ion e-
ac ions a e also a enua ed a high H2 p essu es, leading o a lesse coke o ma ion [31].
A simila esul was ob ained by Akah e al. [30] in he hyd oc acking o pos consume
plas ics. Consequen ly, he lowe ca alys deac i a ion may explain some o he esul s
displayed in Figu e 2 ha can be conside ed unexpec ed. Some o hem would be he
cons an gas p oduc ion obse ed when inc easing he H2 p essu e om 20 o 50 ba
[28], as well as he p oduc dis ibu ion ha has emained unal e ed [32]. Apa om he
20 50 80 110
0.6
0.8
1.0
1.2
1.4
1.6
P essu e (ba )
Selec i i y o uel index
Figu e 3. E ec o H2p essu e on selec i i y o uel index.
The a o emen ioned cha ac e is ics o he mechanisms highligh he ole o he H
2
p essu e, which a enua es he o ma ion o ee adicals and a ou s he ex en o he
hyd oc acking o hyd oca bons and polyole ins. S udies o he e ec o H
2
p essu e on he
hyd oc acking o di e en polyole ins ha e shown ha , in gene al, inc easing he p essu e
o H
2
inc eases he con e sion and he yield o liquid p oduc s. Howe e , his e ec is
p og essi ely smalle as he p essu e o H
2
inc eases [
24
,
28
–
30
]. The e ec on he gas yield
is une en, al hough i dec eases wi h p essu e. The explana ion is ha an inc ease in he H
2
p essu e a ou s he ca aly ic hyd oc acking mechanism wi h espec o he he mal one, o
which he a enua ion o he o ma ion o ee adicals in he p esence o H2con ibu es.
In addi ion, he a es o he ole ins cycliza ion and polya oma ics condensa ion e-
ac ions a e also a enua ed a high H
2
p essu es, leading o a lesse coke o ma ion [
31
].
A simila esul was ob ained by Akah e al. [
30
] in he hyd oc acking o pos consume
plas ics. Consequen ly, he lowe ca alys deac i a ion may explain some o he esul s dis-
played in Figu e 2 ha can be conside ed unexpec ed. Some o hem would be he cons an
gas p oduc ion obse ed when inc easing he H
2
p essu e om 20 o 50 ba [
28
], as well
as he p oduc dis ibu ion ha has emained unal e ed [
32
]. Apa om he a enua ion
o he deac i a ion, he side e ec s o he pa ial p essu e o H
2
ha s ongly a ec he
composi ion o he p oduc s will con ibu e o he di icul in e p e a ion o he esul s.
The e o e, hei eac i i y in c acking and hyd ogena ion eac ions will be di e en . O he
ac o s ha will condi ion he esul s depic ed in Figu e 2will be (i) he di e en di usi i y
o he eac ion mix u e in he di e en hyd oc acking s ages, which will be c ucial o he
HDPE-de i ed mac omolecules and he hea y molecules wi hin he HCO ac ion o he
VGO, and (ii) he syne gis ic e ec s in he hyd oc acking mechanisms be ween he VGO
and he HDPE-de i ed molecules.
Ca alys s 2022,12, 543 6 o 17
2.3. Composi ion o he Gas F ac ion
The e ec o he H
2
p essu e on he composi ion o he gas ac ion is depic ed in
Figu e 4. The gas ac ion is composed o hyd oca bons be ween one and ou ca bon
a oms (CH
4
, C
2
H
4
, C
2
H
6
, C
3
H
6
, C
3
H
8
, i-C
4
H
10
, C
4
H
8
, and n-C
4
H
10
). One should no e
ha an H
2
p essu e o 20 ba is no high enough o sa u a ing all he ligh ole ins. Indeed,
hyd ogena ion is an exo he mic eac ion a ou ed a low empe a u es and high H
2
p es-
su es. The e o e, small concen a ions o e hylene, p opylene and bu ylenes (0.4, 6.9 and
4.7 w %, espec i ely) ha e been de ec ed in he gas ac ion. Spli ing he gas ac ion
in o d y gas (C
1
–C
2
) and LPG (C
3
–C
4
), he concen a ion o he la e is by a highe
as i accoun s o 68.9 w %, i s main compounds being p opane and n-bu ane (25.5 and
23.6 w %, espec i ely). Rega ding he composi ion o he d y gas ac ion, i is almos
o ally composed o me hane and e hane (13.5 and 17.1 w %, espec i ely).
Ca alys s 2022, 12, 543 6 o 18
a enua ion o he deac i a ion, he side e ec s o he pa ial p essu e o H2 ha s ongly
a ec he composi ion o he p oduc s will con ibu e o he di icul in e p e a ion o he
esul s. The e o e, hei eac i i y in c acking and hyd ogena ion eac ions will be di -
e en . O he ac o s ha will condi ion he esul s depic ed in Figu e 2 will be (i) he
di e en di usi i y o he eac ion mix u e in he di e en hyd oc acking s ages, which
will be c ucial o he HDPE-de i ed mac omolecules and he hea y molecules wi hin
he HCO ac ion o he VGO, and (ii) he syne gis ic e ec s in he hyd oc acking mech-
anisms be ween he VGO and he HDPE-de i ed molecules.
2.3. Composi ion o he Gas F ac ion
The e ec o he H2 p essu e on he composi ion o he gas ac ion is depic ed in
Figu e 4. The gas ac ion is composed o hyd oca bons be ween one and ou ca bon
a oms (CH4, C2H4, C2H6, C3H6, C3H8, i-C4H10, C4H8, and n-C4H10). One should no e ha an
H2 p essu e o 20 ba is no high enough o sa u a ing all he ligh ole ins. Indeed, hy-
d ogena ion is an exo he mic eac ion a ou ed a low empe a u es and high H2 p es-
su es. The e o e, small concen a ions o e hylene, p opylene and bu ylenes (0.4, 6.9 and
4.7 w %, espec i ely) ha e been de ec ed in he gas ac ion. Spli ing he gas ac ion
in o d y gas (C1–C2) and LPG (C3–C4), he concen a ion o he la e is by a highe as i
accoun s o 68.9 w %, i s main compounds being p opane and n-bu ane (25.5 and 23.6
w %, espec i ely). Rega ding he composi ion o he d y gas ac ion, i is almos o ally
composed o me hane and e hane (13.5 and 17.1 w %, espec i ely).
Figu e 4. E ec o H2 p essu e on he composi ion o he gas ac ion.
An inc ease in he p essu e up o 50 ba b ings a ma ked change in he composi ion
o he gas ac ion, led by an almos o al ole ins disappea ance and a boos ing o he
isome iza ion eac ions. Fu he mo e, he yield o he d y gas ac ion is also educed a
he same ime as C3 and C4 p oduc s became he main ones. Indeed, he lowes concen-
a ion o he d y gas ac ion has been a ained a his p essu e eaching jus a alue o
20.8 w %. Rega ding LPG ac ion, p opane con inues o be he main p oduc (34.5 w %),
ollowed by iso-bu ane (27.3 w %), whe eas he concen a ion o n-bu ane has been e-
duced down o 16.9 w %. Mo awski and Mosio-Mosiewski [33] also obse ed he same
ends o he d y gas and LPG ac ions when inc easing he H2 p essu e om 20 o 50
ba in he hyd oc acking o acuum esidue o e a NiMo/Al2O3 ca alys .
20 50 80 110
0
20
40
60
80
100
Concen a ion (w %)
P essu e (ba )
CH
4
C
2
H
4
C
2
H
6
C
3
H
6
C
3
H
8
C
4
H
8
i-C
4
H
10
n-C
4
H
10
Figu e 4. E ec o H2p essu e on he composi ion o he gas ac ion.
An inc ease in he p essu e up o 50 ba b ings a ma ked change in he composi ion
o he gas ac ion, led by an almos o al ole ins disappea ance and a boos ing o he
isome iza ion eac ions. Fu he mo e, he yield o he d y gas ac ion is also educed a he
same ime as C
3
and C
4
p oduc s became he main ones. Indeed, he lowes concen a ion
o he d y gas ac ion has been a ained a his p essu e eaching jus a alue o 20.8 w %.
Rega ding LPG ac ion, p opane con inues o be he main p oduc (34.5 w %), ollowed
by iso-bu ane (27.3 w %), whe eas he concen a ion o n-bu ane has been educed down
o 16.9 w %. Mo awski and Mosio-Mosiewski [
33
] also obse ed he same ends o
he d y gas and LPG ac ions when inc easing he H
2
p essu e om 20 o 50 ba in he
hyd oc acking o acuum esidue o e a NiMo/Al2O3ca alys .
Howe e , a u he inc ease in he H
2
p essu e o 80 ba has caused a mode a e
inc ease in he concen a ion o he d y gas ac ion, p incipally o he de imen o he C
4
compounds and, o a lesse ex en , he C
3
compounds. Indeed, p opane is s ill he main
compound (35.9 w %). In addi ion, no ole ins ha e been de ec ed a his p essu e because
he hyd ogena ion o he componen s wi hin he LGP ac ion has been g ea ly a ou ed.
A he highes H
2
p essu e (110 ba ), he concen a ion o d y gas has inc eased up o
30.9 w %— he concen a ions o e hane and me hane a 16.2 and 14.7 w %, espec i ely.
Conce ning he LPG, he la ges concen a ion co esponds o p opane (35.9 w %), ollowed
by iso- and n-bu ane (21.4 and 11.8 w %, espec i ely). The inc ease obse ed in he
Ca alys s 2022,12, 543 7 o 17
concen a ion o d y gas a high H
2
p essu es esul s om a high le el o he mal c acking
being main ained.
2.4. Composi ion and RON o he Naph ha F ac ion
The composi ion o he naph ha ac ion, oge he wi h i s esea ch oc ane numbe
(RON), is collec ed in Figu e 5. A low H
2
p essu e (20 ba ), he naph ha is mainly a oma ic
because o he a oma ics al eady p esen in he VGO (Table S1) and he limi ed ex en o
he hyd ogena ion eac ions ha can be achie ed unde hese condi ions. A oma ics a e
he p edominan compounds (59.3 w %), he alkylbenzenes (A
1
) being he mos abundan
compounds (56.2 w %). The concen a ion o 2- ing a oma ics (A
2
) is 3.1 w %, and naph-
henic compounds a e in a ela i ely low amoun (10.5 w %). A small quan i y o ole ins,
0.7 w %, has also been de ec ed. Linea pa a ins accoun o almos 3 w % o naph ha,
whe eas ami ied pa a ins accoun o 21.9 w %.
Ca alys s 2022, 12, 543 7 o 18
Howe e , a u he inc ease in he H2 p essu e o 80 ba has caused a mode a e in-
c ease in he concen a ion o he d y gas ac ion, p incipally o he de imen o he C4
compounds and, o a lesse ex en , he C3 compounds. Indeed, p opane is s ill he main
compound (35.9 w %). In addi ion, no ole ins ha e been de ec ed a his p essu e because
he hyd ogena ion o he componen s wi hin he LGP ac ion has been g ea ly a ou ed.
A he highes H2 p essu e (110 ba ), he concen a ion o d y gas has inc eased up o 30.9
w %— he concen a ions o e hane and me hane a 16.2 and 14.7 w %, espec i ely.
Conce ning he LPG, he la ges concen a ion co esponds o p opane (35.9 w %), ol-
lowed by iso- and n-bu ane (21.4 and 11.8 w %, espec i ely). The inc ease obse ed in
he concen a ion o d y gas a high H2 p essu es esul s om a high le el o he mal
c acking being main ained.
2.4. Composi ion and RON o he Naph ha F ac ion
The composi ion o he naph ha ac ion, oge he wi h i s esea ch oc ane numbe
(RON), is collec ed in Figu e 5. A low H2 p essu e (20 ba ), he naph ha is mainly a o-
ma ic because o he a oma ics al eady p esen in he VGO (Table S1) and he limi ed
ex en o he hyd ogena ion eac ions ha can be achie ed unde hese condi ions. A o-
ma ics a e he p edominan compounds (59.3 w %), he alkylbenzenes (A1) being he
mos abundan compounds (56.2 w %). The concen a ion o 2- ing a oma ics (A2) is 3.1
w %, and naph henic compounds a e in a ela i ely low amoun (10.5 w %). A small
quan i y o ole ins, 0.7 w %, has also been de ec ed. Linea pa a ins accoun o almos 3
w % o naph ha, whe eas ami ied pa a ins accoun o 21.9 w %.
Figu e 5. E ec o H2 p essu e on he composi ion o he naph ha ac ion. Key: n-P—n-pa a ins;
i-P—isopa a ins; N—naph henes; O—ole ins; A1—1- ing a oma ics; A2—2- ing a oma ics.
An inc ease in he H2 p essu e p omo es he ex en o he hyd ogena ion eac ions
educing he concen a ion o a oma ics and inc easing ha o naph henes [21]. Howe -
e , he boos ing o he a e o hyd ogena ion eac ions is no di ec ly p opo ional o he
inc ease in p essu e. This way, an inc ease in he H2 p essu e om 20 o 50 ba has a
sub le e ec on he composi ion o he naph ha ac ion. The o al concen a ion o a o-
ma ics has ba ely dec eased o 57.9 w %, causing a sligh inc ease in he alipha ic com-
pounds (up o 41.9 w %). I should also be highligh ed ha he concen a ion o ole ins
has been no ably educed (0.12 w %), exposing ha an H2 p essu e o 50 ba is no high
20 50 80 110
0
20
40
60
80
100
Concen a ion (w %)
P essu e (ba )
n-P
i-P
N
O A
1
A
2
RON
89
90
91
92
93
RON
Figu e 5.
E ec o H
2
p essu e on he composi ion o he naph ha ac ion. Key: n-P—n-pa a ins;
i-P—isopa a ins; N—naph henes; O—ole ins; A1—1- ing a oma ics; A2—2- ing a oma ics.
An inc ease in he H
2
p essu e p omo es he ex en o he hyd ogena ion eac ions
educing he concen a ion o a oma ics and inc easing ha o naph henes [21]. Howe e ,
he boos ing o he a e o hyd ogena ion eac ions is no di ec ly p opo ional o he
inc ease in p essu e. This way, an inc ease in he H
2
p essu e om 20 o 50 ba has a sub le
e ec on he composi ion o he naph ha ac ion. The o al concen a ion o a oma ics
has ba ely dec eased o 57.9 w %, causing a sligh inc ease in he alipha ic compounds
(up o 41.9 w %). I should also be highligh ed ha he concen a ion o ole ins has been
no ably educed (0.12 w %), exposing ha an H
2
p essu e o 50 ba is no high enough
o sa u a e all he ole ins, as a o emen ioned o he gas ac ion (Figu e 4). Ne e heless,
an inc ease in he H
2
p essu e o 80 ba signi ican ly modi ies he composi ion o he
naph ha ac ion. The concen a ion o all he sa u a ed compounds has inc eased, eaching
concen a ions o 9.8, 27.8, and 14.0 w % o he n-pa a ins, i-pa a ins, and naph henes,
espec i ely. Consequen ly, he concen a ion o he a oma ic compounds has been educed
o 48.4 w %, he mono-a oma ics being he clea p edominan ones (46.6 w %). No e ha
ole ins a e no longe de ec ed in he naph ha ac ion. An addi ional inc ease in he H
2
p essu e o 110 ba has exposed he key ole o his pa ame e in he hyd oc acking o
he HDPE/VGO blend. Thus, he concen a ion o bo h ami ied and linea pa a ins has
inc eased signi ican ly (43.8 and 22.6 w %, espec i ely). Indeed, isopa a ins ha e clea ly
Ca alys s 2022,12, 543 8 o 17
become he p edominan compounds. In he same line, he concen a ion o naph henes
has inc eased, eaching a alue o 20.4 w %, which is he maximum concen a ion ob ained
o his amily o compounds in he whole ange o H
2
p essu e s udied. Wi h ega d o
a oma ic compounds, he concen a ion o 2- ing a oma ics has been signi ican ly educed
o an almos symbolic alue (0.3 w %), and ha o 1- ing a oma ics has eached i s minimum
alue (13.0 w %). A simila end has been epo ed in he li e a u e in he hyd oc acking
o di e en eeds [
33
,
34
], in which a subs an ial H
2
p essu e is equi ed o o ally emo e
ole ins and achie e ema kable concen a ions o alipha ic compounds.
As he RON s ongly depends on he composi ion o he naph ha ac ion, he alue
ob ained o his pa ame e has emained almos simila o H
2
p essu es o 20 and 50 ba
(89.9 and 90.2, espec i ely). Howe e , when he p essu e is aised o 80 ba , he e is a con-
side able inc ease in he RON, eaching a alue o 92.5. This esul lies in he inc ease in he
concen a ions o isopa a ins since his amily o compounds has a ma kedly posi i e e ec
on his index. None heless, an inc ease in he p essu e o alues abo e 80 ba dec eases
he alue ob ained o he RON, as he concen a ion o a oma ic compounds has been
impo an ly educed. Thus, a he highes p essu e (110 ba ), he RON dec eases sligh ly
o 91.5. A posi i e e ec o he H
2
p essu e has also been epo ed in he hyd oc acking
o hea y oils and pe oleum esidues ega ding RON o he naph ha ac ion ope a ing a
p essu es below 75 ba [35].
2.5. Composi ion and Ce ane Index o he LCO F ac ion
The composi ion o he LCO ac ion o he di e en H
2
p essu es s udied is displayed
in Figu e 6. In addi ion, he ce ane index (CI) has been calcula ed in o de o compa e he
e ec o he p essu e on he quali y o he LCO ac ion. O e all, a clea in luence o he H
2
p essu e on he composi ion o he LCO ac ion can be obse ed. This way, high p essu es
inc ease he concen a ion o sa u a ed compounds, mainly ha o isopa a ins, whe eas
he concen a ion o 1-, 2-, and 3+- ing a oma ics dec eases.
Ca alys s 2022, 12, 543 9 o 18
Figu e 6. E ec o H2 p essu e on he composi ion o he LCO ac ion. Key: n-P—n-pa a ins;
i-P—isopa a ins; N—naph henes; A1—1— ing a oma ics; A2—2— ing a oma ics; A3+—3+— ing a o-
ma ics.
A a p essu e o 20 ba , a low concen a ion o o al pa a ins has been ob ained (30.5
w %), whe e isopa a ins a e in a g ea e amoun han no mal pa a ins (24.2 s. 6.2 w %,
espec i ely). Naph henes a e he less impo an compounds accoun ing o jus a 1.9
w %. The e o e, a oma ics a e he p edominan compounds wi h a o al concen a ion o
67.7 w %, he di-a oma ics being he main ones (51.9 w %), ollowed by poly- and
mono-a oma ics (10.1 and 5.6 w %, espec i ely).
An inc ease in he H2 p essu e modi ies he composi ion o he LCO ac ion in he
same way ha has modi ied he composi ion o he naph ha ac ion (Figu e 5). Looking
mo e closely a he concen a ion o he di e en amilies, i can be seen ha he concen-
a ion o isopa a ins has inc eased exponen ially wi h H2 p essu e, whe eas ha o
n-pa a ins has isen mo e linea ly. Fu he mo e, naph henes ha e dec eased o p es-
su es below 80 ba , bu a 110 ba , hey ha e achie ed hei maximum concen a ion (4.4
w %). Rega ding he ends o he di e en a oma ic compounds, he concen a ion o
2- ing and 3+- ing a oma ics has dec eased wi h p essu e, causing an inc ease in he
mono-a oma ics when inc easing H2 p essu e. This esul shows ha an inc ease in he
H2 p essu e p omo es he hyd odea oma iza ion eac ions, bu i also shows ha 2- ing
and 3+- ings a oma ics a e pa ially hyd ogena ed, leading o he o ma ion o 1- ing
a oma ics. Subsequen ly, hese mono-a oma ics will be hyd ogena ed, o ming o ally
sa u a ed naph henes, which a e easily c acked in o ami ied pa a ins. This mechanism
was also obse ed by Palos e al. [36] in he hyd o ea ing o a highly a oma ic s eam
ob ained as a by-p oduc o he FCC uni . Mo eo e , he signi ican ly high acidi y o he
ca alys (Table 1) p omo es he isome iza ion and skele al ea angemen eac ions be-
ween he pa a ins and o a la ge ex en as he H2 p essu e is inc eased, leading o he
o ma ion o isopa a ins. In addi ion, he acidic p ope ies o he ca alys will also boos
he hyd ogenolysis eac ions as he p essu e is inc eased, ob aining e en a highe dehy-
d oa oma iza ion deg ee [37].
Acco ding o he desc ibed end, a an H2 p essu e o 110 ba , he concen a ion o
all he amilies o alipha ic compounds achie es i s maximum alue. Thus, he concen-
a ion o no mal pa a ins and naph henes has inc eased up o 12.3 and 4.4 w %,
whe eas isopa a ins ha e become by a he p edominan compounds (53.3 w %) in he
20 50 80 110
0
20
40
60
80
100
Concen a ion (w %)
P essu e (ba )
n-P
i-P
N
A
1
A
2
A
3+
CI
30
35
40
45
50
55
Ce ane Index
Figu e 6.
E ec o H
2
p essu e on he composi ion o he LCO ac ion. Key: n-P—n-pa a ins; i-P—isopa a ins;
N—naph henes; A1—1— ing a oma ics; A2—2— ing a oma ics; A3+—3+— ing a oma ics.
A a p essu e o 20 ba , a low concen a ion o o al pa a ins has been ob ained
(30.5 w %), whe e isopa a ins a e in a g ea e amoun han no mal pa a ins (24.2 s.
6.2 w %, espec i ely). Naph henes a e he less impo an compounds accoun ing o jus a
1.9 w %. The e o e, a oma ics a e he p edominan compounds wi h a o al concen a ion
Ca alys s 2022,12, 543 9 o 17
o 67.7 w %, he di-a oma ics being he main ones (51.9 w %), ollowed by poly- and
mono-a oma ics (10.1 and 5.6 w %, espec i ely).
An inc ease in he H
2
p essu e modi ies he composi ion o he LCO ac ion in he
same way ha has modi ied he composi ion o he naph ha ac ion (Figu e 5). Looking
mo e closely a he concen a ion o he di e en amilies, i can be seen ha he con-
cen a ion o isopa a ins has inc eased exponen ially wi h H
2
p essu e, whe eas ha o
n-pa a ins has isen mo e linea ly. Fu he mo e, naph henes ha e dec eased o p essu es
below 80 ba , bu a 110 ba , hey ha e achie ed hei maximum concen a ion (4.4 w %).
Rega ding he ends o he di e en a oma ic compounds, he concen a ion o 2- ing and
3
+
- ing a oma ics has dec eased wi h p essu e, causing an inc ease in he mono-a oma ics
when inc easing H
2
p essu e. This esul shows ha an inc ease in he H
2
p essu e p o-
mo es he hyd odea oma iza ion eac ions, bu i also shows ha 2- ing and 3
+
- ings
a oma ics a e pa ially hyd ogena ed, leading o he o ma ion o 1- ing a oma ics. Subse-
quen ly, hese mono-a oma ics will be hyd ogena ed, o ming o ally sa u a ed naph henes,
which a e easily c acked in o ami ied pa a ins. This mechanism was also obse ed by
Palos e al. [
36
] in he hyd o ea ing o a highly a oma ic s eam ob ained as a by-p oduc o
he FCC uni . Mo eo e , he signi ican ly high acidi y o he ca alys (Table 1) p omo es he
isome iza ion and skele al ea angemen eac ions be ween he pa a ins and o a la ge
ex en as he H
2
p essu e is inc eased, leading o he o ma ion o isopa a ins. In addi ion,
he acidic p ope ies o he ca alys will also boos he hyd ogenolysis eac ions as he
p essu e is inc eased, ob aining e en a highe dehyd oa oma iza ion deg ee [37].
Acco ding o he desc ibed end, a an H
2
p essu e o 110 ba , he concen a ion o all
he amilies o alipha ic compounds achie es i s maximum alue. Thus, he concen a ion o
no mal pa a ins and naph henes has inc eased up o 12.3 and 4.4 w %, whe eas isopa a ins
ha e become by a he p edominan compounds (53.3 w %) in he LCO ac ion. A he
same ime, he concen a ion o o al a oma ics has been dec eased down o 30.1 w %, wi h
speci ic alues o 11.4, 17.8, and 0.87 w % o mono-, di-, and poly-a oma ics, espec i ely.
Rega ding he ce ane index, he e is a clea e ec o H
2
p essu e on i because o he
composi ional changes ob ained when aising he p essu e. A he lowes H
2
p essu e
(20 ba ), he LCO ac ion has he poo es ce ane index (31.6). Howe e , when p essu e
ises, he ce ane index also inc eases, achie ing i s maximum alue o 53.1 a 110 ba . This
esul can be co ela ed wi h he con en s o pa a ins and a oma ics. Thus, he g ea e he
pa a in concen a ion, he highe he ce ane index. Howe e , a ending o he LCO ac ion
ope a ing a such high p essu es is no equi ed. The quali y o he LCO ac ion ob ained
a bo h 50 and 80 ba is high enough o enable he use o his ac ion in he blending o
comme cial diesel in e ine ies.
2.6. Coke Deposi ion
The e ec o H
2
p essu e on he deposi ion o coke on he ca alys has been s udied by
empe a u e-p og ammed oxida ion (TPO) analysis, and ob ained esul s a e depic ed in
Figu e 7. This analysis is sui able o quan i ying he amoun o coke deposi ed bu also o
iden i ying i s na u e and loca ion in he ca alys pa icle [
38
]. The e o e, he empe a u e
co esponding o he maximum combus ion a e (T
Max
) is a pa ame e ha can be used
as an index o assessing he ease o combus ion. I depends on [
39
] (i) he na u e o he
coke because combus ion akes place a a lowe empe a u e in a poo ly s uc u ed coke
(wi h a high H/C a io), and (ii) he loca ion o he coke, since he coke loca ed on he
ou side o he ca alys pa icle and on he ou side o he zeoli e mic opo es will be bu n
as e (combus ion akes place wi hou di usional limi a ions). Likewise, combus ion
occu s a a lowe T
Max
when he coke is deposi ed on a me allic pa icle ha ca alyzes
i s combus ion. The e o e, he decon olu ion o he TPO p o ile allows o iden i ying
di e en coke ac ions loca ed in di e en posi ions o he ca alys pa icles acco ding
o hei di e en combus ion a es [
40
]. Wi h he aim o cha ac e ising he di e en coke
ac ions, he TPO p o iles (colou ed lines in Figu e 7) ha e been decon olu ed. The
esul s o he decon olu ion, oge he wi h he empe a u e a which he combus ion a e is
Ca alys s 2022,12, 543 16 o 17
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