- Email: [email protected]
Influence of Nitrogen Feed Content On The Performances of A Zeolite Hydrocracking Catalyst
D.L. Trimm et al. (Editors), Catalysts in Petroleum Refining 1989 0 1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
301
INFLUENCE OF NITROGEN FEED CONTENT O N THE PERFORMANCES OF A ZEOLITE HYDROCRACKING CATALYST P. DUFRESNE, A. QUESADA AND S. MIGNARD Institut Francais du Petrole, BP 311, 92506, Rueil Malmaison, (France) ABSTRACT
Hydrocracker feeds are usually rich in nitrogen compounds. These are partially transformed by the first-step hydrotreatment catalysts, creating an ammonia pressure in the reactors. This paper shows the effects of ammonia pressure on the performances of a zeolite hydrocracking catalyst. Ammonia has a strong negative effect on the activity for n-heptane conversion, especially at low ammonia pressures. Adsorption of ammonia on the acidic sites of the zeolite is quickly reversible for a wide range of pressure. However, at zero pressure, ammonia desorption is slow, meaning that the molecule remains strongly adsorbed on the most acidic sites of the zeolite. Ammonia also modifies strongly the products selectivity. Iso-heptane yield is increased to the expense of cracked products yield. INTRODUCTION
Vacuum gas oil hydrocracking is now a major conversion process in the refining industry. One of its main qualities is flexibility. Different types of feeedstocks can be converted into various types of valuable products (1,2,3). The catalyst is one of the key aspects of the process and the merits of a given hydrocracking process largely depend on the catalyst’s qualities. These are exprressed by three main criteria: (1) selectivity or type of yield obtained, (2) activity or temperature for a given conversion level and (3) stability or aging rate. There is a strong incentive today to process more and more refractory feedstocks in order to improve the economics of the process. The feed’s refractory nature can be described by various parameters. Feed heaviness can be characterized by the molecular weight or the midand final boiling points. Composition is also a significant parameter i.e. paraffins, naphthanes, aromatics content and, for non straight-run feeds such as recycle oil from catalytic cracking or coker gas oil, olefin content. The heteroatom content, sulfur and especially nitrogen are also very important points to be considered in feed evaluation. Metals and asphaltenes are not considered here since they usually must not be present in a conventional hydrocracker feed. The specifications for metal content are often less than 1 ppm. These parameters are interconnected. For example an increase in feed molecular weight often accompanies an increase in the aromatics, especially polyaromatics, content. Heavy feeds are more refractory than light ones because of the greater concentration of poisonous molecules rather than the increase in the mean carbon number.
302
For a given hydrocracker. the only parameter enabling more refractory feedstocks to be processed is often the catalyst type. An improvement in catalyst performance goes via a good understanding of the phenomena involved in processing difficult feeds. This paper deals with the effects of nitrogen feed content on catalyst performance. GENERAL ANALYSIS OF THE PROBLEM OF NITROGEN FEED CONTENT A hydrocracking process usually consists of two different steps, whatever the reactor arrangement is used, i.e. "single-stage'' or "two-stage''process, as shown in Figure 1. Reactions occurring in the first step are mainly hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and aromatics hydrogenation (HDA). They are promoted by a hydrotreatment catalyst. In sore cases, a hydrocracking catalyst can also be placed in the first step in order to initiate' cracking reactions. The most critical reaction in that step is HDN because the outlet effluent must respect a severe nitrogen-content specification. Indeed,the acidic function of hydrocracking catalysts, especially when they contain zeolites, can be strongly poisoned by heavy nitrogen-containing molecules. Depending on the licensors recommandations. the value i s typically between 1 and 30 ppm. Clearly, conditions to obtain for instance, 5 ppm at the first step outlet will be very different when starting from a feed containing 200 or 2000 ppm, which are the two extreme values of standard VGO hydrocracker feeds. A nitrogen-rich feed has to be processed under conditions of high hydrogen pressure and/or low space velocity. For a given unit, the only operating parameter is temperature. However,the HDN reaction, as well as the HDA reaction, is bound to a thermodynamic equilibrium, because the first reaction step is the nitrogen-containing ring saturation. Temperature thus has a limited effect
(4). The main reactions occurring in the second step are hydroisomerization and hydrocracking. They are promoted by a more acidic hydrocracking catalyst, containing amorphous silica-alumina or zeolite. However, depending on the type of process used, the second step conditions may be very different (3.5). In the single-stage arrangement, the liquid and gaseous effluents coming from the first step go to the second step. Thus, a hydrocracking catalyst must deal with the large amount of ammonia produced upstream, with ammonia partial pressure typically being between 5 and 30 kPa. In the two-stage arrangement, the effluents are fractionated after the first step.
303
ONE- STAGE PROCESS FRACTIONATION MAKE UP H2
H2 RECYCLE
_ 1
1 or 2 REACTORS r-- i
v A
FRESH FEED
I
- L,,1 J-
I
1
I
I
I
I I
--L-----L-
---..---SEPARATION
I
RESIDUE RECYCLE
TWO- STAGE PROCESS MAKE UP H2
H2 RECYCLE
d =
PSTAGE
FRACTIONATION
REACTOR
FRESH FEED
i If
b
2ndSTAGE
SEPARATION
1
RESIDUE RECYCLE
Fig.
So with
1 only
. Typical the
configuration.
c)FUEL
OIL
hydrocracking p r o c e s s flow-schemes
unconverted,
ammonia-free
considerably
--
lower
b u t h y d r o t r e a t e d . r e s i d u e e n t e r s t h e second s t e p
hydrogen. ammonia
Hydrocracking partial
c a t a l y s t s can t h u s o p e r a t e a t a
presmre
than
in
the
single-stage
304
The chemistry o f hydrocracking and e s p e c i a l l y t h e r e a c t i o n s of p a r a f f i n s on b i f u n c t i o n n a l hydrocracking c a t a l y s t s have been e x t e n s i v e l y s t u d i e d i n t h e l i t e r a t u r e , and good reviews have a l r e a d y been p u b l i s h e d (6,7,8).The transformation
of
paraffins
i m p l i e s t h e independent a c t i o n o f two t y p e s of
c a t a l y t i c s i t e s , i . e . hydrogenating and a c i d i c s i t e s (9). On t h e f i r s t o n e s , p a r a f f i n s are transformed i n t o o l e f i n i c i n t e r m e d i a t e s , which are s u b s e q u e n t l y adsorbed on a c i d i c sites as carbenium i o n s . These i o n s are t h e n i s o m e r i z e d ,
are
and
desorbed from t h e p r o t o n i c s i t e as an i s o - o l e f i n ( i s o m e r i z a t i o n ) or
as two fragments ( c r a c k i n g )
.
I n most c a s e s , t h e m e c h a n i s t i c ( 1 0 , l l ) as well as t h e k i n e t i c ( 1 2 ) s t u d i e s r e p o r t e d i n t h e l i t e r a t u r e have used c a t a l y s t s made o f z e o l i t e s l o a d e d w i t h a noble m e t a l , such as platinum o r palladium. The r e a c t i o n o f a long-chain on t h i s t y p e o f system r e s u l t s i n a h i g h s e l e c t i v i t y t o isomers a t or medium conversion rates, and t h i s c h a r a c t e r i s t i c f e a t u r e h a s been r e f e r r e d t o as " i d e a l h y d r o c r a c k i n g " ( 8 ) . It is t y p i c a l o f a b i f u n c t i o n a l
paraffin low
c a t a l y s t having noble metal i n
a v e r y h i g h hydrogenation f u n c t i o n , which i s t h e c a s e of a s u l f u r and n i t r o g e n f r e e environment. However, a c t u a l hydrocracking c o n d i t i o n s i n v o l v e h i g h p a r t i a l p r e s s u r e s of s u l f u r and ammonia c o n t a i n i n g s p e c i e s . Despite t h e great i n f l u e n c e o f t h e s e compounds on c a t a l y s t performance, t h e t o p i c h a s n o t been d e s c r i b e d c l e a r l y i n t h e open literature. According t o p r e v i o u s work (13,14), i t is clear t h a t t h i s " i d e a l hydrocracking" mechanism does n o t p r e v a i l anymore i n t h e p r e s e n c e of s u l f u r and
nitrogen
containing
feed.
The
s e l e c t i v i t y o f t h e r e a c t i o n is t o t a l l y modified, and c r a c k i n g r e a c t i o n s predominate under t h e s e c o n d i t i o n s . T h i s phenomenon can be i n t e r p r e t e d by a d r a m a t i c change i n t h e b a l a n c e between t h e two f u n c t i o n s (hydrogenation and c r a c k i n g ) . Another s t u d y (15) d e s c r i b e s t h e influence of hydrogen sulfide pressure on the reactivity of a P a l l a d i u m / z e o l i t e Y system. It also clearly shows t h e m o d i f i c a t i o n of s e l e c t i v i t y . A t around 40% c o n v e r s i o n , n-dodecane can be completly isomerized i n a s u l f u r - f r e e environment when, f o r t h e same conversion i n t h e p r e s e n c e of s u l f u r , t h e isododecane y i e l d i s v e r y low compared t o t h e hydrocracked product y i e l d . S o , t h e q u e s t i o n o f n i t r o g e n f e e d c o n t e n t can be summarized i n t h i s way : t h e nitrogen l e v e l w i l l c o n t r o l t h e o p e r a t i n g conditions o f the f i r s t hydrotreatment s t e p , so t h a t t h e n i t r o g e n s p e c i f i c a t i o n s o f t h e second s t e p
i n l e t f e e d can b e reached. Moreover t h e ammonia p a r t i a l p r e s s u r e o f t h e hydrocracking s e c t i o n w i l l b e a f u n c t i o n o f t h e n i t r o g e n l e v e l and w i l l a l s o depend on t h e t y p e of p r o c e s s used, whether s i n g l e - s t a g e o r two-stage. W e w i l l s t u d y h e r e t h e i n f l u e n c e of ammonia p a r t i a l p r e s s u r e on t h e a c t i v i t y and
s e l e c t i v i t y o f a z e o l i t e hydrocracking c a t a l y s t .
305 a
For
more
a n a l y s i s of t h e phenomenon from n s c i e n t i f i c p o i n t o f
accurate
view, w e performed t h i s s t u d y w i t h t h e n-heptane molecule. EXPERIMENTAL The
catalyst
used
was
the
industrial
catalyst,
HYC
642.
from
P r o c a t a l y s e . I t s hydrogenation f u n c t i o n c o n s i s t s o f Nickel and Molybdenum, and
i s provided by an U l t r a - S t a b l e Y z e o l i t e . Experiments a c a t a l y t i c u n i t w i t h t h e f o l l o w i n g r e a c t o r dimensions: i n s i d e diameter 19 mm and l e n g t h 550 mm. A c a t a l y s t l o a d i n g o f 13 g , d i l u t e d with s i l i c o n c a r b i d e , was p l a c e d i n t h e middle o f t h e reactor. P r e h e a t i n g o f its
acidic
were
function
performed
in
f e e d was performed by a 150 mm long s i l i c o n c a r b i d e packing. Temperature
the
was c o n t r o l l e d by t h r e e thermocouples p l a c e d i n s i d e t h e r e a c t o r . In
order
to
keep t h e NiMo phase i n a s u l f i d e d s t a t e , a c o n s t a n t p a r t i a l
p r e s s u r e of hydrogen s u l f i d e was maintained i n t h e r e a c t o r by adding of
dimethyl
d i s u l f i d e t o t h e feed
. Ammonia p a r t i a l
1.5 w t %,
p r e s s u r e was c r e a t e d by
a known q u a n t i t y o f a n i l i n e t o t h e f e e d which was r e a d i l y transformed
adding
i n t o ammonia. A q u a n t i t y o f 1000 w t ppm of n i t r o g e n c o r r e s p o n d s t o an ammonia pressure
of
stabilization
6.1 kPa. Every change i n o p e r a t i n g c o n d i t i o n s was followed by a period
of
around
two
hours.
Under t h e c o n d i t i o n s u s e d , no
c a t a l y s t d e a c t i v a t i o n was v i s i b l e , even a f t e r s e v e r a l days. The which
different also
series o f
experiments
are b r i e f l y d e s c r i b e d i n T a b l e 1 ,
i n d i c a t e s t h e f i g u r e s o f t h e r e s u l t s r e p o r t e d . Others c o n d i t i o n s
were a t o t a l p r e s s u r e o f 6 MPa and a hydrogen-to-n-heptane molar r a t i o o f 6. TABLE 1 D e s c r i p t i o n o f t h e main experimental parameters Run number
Temperature
NH3 p r e s s u r e
cont.time
("C)
(kPa)
(h)
260-340
0
0.5
360-425
6.1
0.5
390
0 t o 18
390
29 t o
370 380 390 390
o
6.1 then 0 6.1 then 0 6.1 then 0 6.1
0.5
0.5 0.5 0.5 0.5 0.21-0.71
Figure
306
EVOLUTION OF N-HEPTANE CONVERSION WITH TEMPERATURE
The
variation
can
It
on
conversion
with
temperature
was
followed
for
an
f e e d and f o r a f e e d c o n t a i n i n g 0 . 1 w t % n i t r o g e n ( r u n s 1 and 2 ) .
aniline-free be
seen
in
Figure
that
2
the
reaction
t e m p e r a t u r e ranges are
completely d i f f e r e n t . The poisonous e f f e c t o f ammonia must be compensated f o r by a t e m p e r a t u r e i n c r e a s e of around 100 'C.
-
LOO
5
90
Ef,
:
80
g
E 70 I3O
0 50
u
+ u
40
'
30
z
20 10
0
TEMPERATURE ( C )
Fig.
. Evolution
2
of
n-heptane c o n v e r s i o n w i t h t e m p e r a t u r e w i t h or without
ammonia.
The a p p a r e n t a c t i v a t i o n e n e r g i e s c a l c u l a t e d from Arrhenius e q u a t i o n s are 32 kcal/mole
for
the
nitrogen-containing
nitrogen-free feed.
following
hypothesis:
containing
feed,
increase
is
a
with
partial
liberates
some
This
when
two
feed
slight
temperature
phenomena
occur.
and
36
kcal/mole
for
the
d i f f e r e n c e c o u l d b e e x p l a i n e d by t h e
is The
increased
with
the
nitrogen
f i r s t one is a normal a c t i v i t y
temperature as i n t h e ammonia f r e e environment. The second one ammonia
desorption
from
the
zeolite's
acidic
sites. This
new a c t i v e s i t e s which are a v a i l a b l e f o r t h e r e a c t i o n and can
c o n t r i b u t e t o an i n c r e a s e i n t h e a p p a r e n t a c t i v a t i o n energy.
INFLUENCE OF AMMONIA PRESSURE
W e s t u d i e d t h e i n f l u e n c e o f ammonia p a r t i a l p r e s s u r e on z e o l i t e c a t a l y s t a c t i v i t y a t c o n s t a n t c o n d i t i o n s . Feed A n i l i n e c o n t e n t was v a r i e d i n o r d e r t o generate
different
increased
from
ammonia
pressures.
During
run 3,
ammonia
pressure
0 t o 18 kPa, w i t h a f r e s h c a t a l y s t b e i n g u s e d . During run 4,
p r e s s u r e d e c r e a s e d from 29 t o 0 kPa. The r e s u l t s are g i v e n i n F i g u r e 3.
307
-'
100
INCREASING PRESSURE = 0 DECREASING PRESSURE =O
k
2
2
80 70
::
ul
0
V
40
30 20 10
0
3
Fig.
. Variation
dramatic
A
especially n-heptane
in
5
10
15
20
25
30
AMMONIA PRESSURE (kPa)
of conversion with ammonia p r e s s u r e .
d r o p i n a c t i v i t y was observed when ammonia p r e s s u r e i n c r e a s e d , the
low
p r e s s u r e r e g i o n . Beyond a p r e s s u r e o f around 10 W a ,
conversion reached a pseudo-plateau. Another s t r i k i n g p o i n t i s t h a t
curves o b t a i n e d from an i n c r e a s e or a d e c r e a s e i n t h e a n i l i n e c o n t e n t are
the
nearly
i d e n t i c a l . Thus, t h e poisonous e f f e c t of ammonia i s r a t h e r r e v e r s i b l e .
There
must be an a d s o r p t i o n - d e s o r p t i o n e q u i l i b r i u m between ammonia i n t h e g a s
phase and t h e p r o t o n i c s i t e s o f t h e z e o l i t e . However, It
total
at
390 'C
never
been
n-heptane the
is
this
pressure.
can in
feed
true
over
the
whole
pressure
range e x c e p t a t z e r o
be i n f e r r e d from Figure 2 t h a t conversion of n-heptane i s i n a n i t r o g e n f r e e environment and f o r a c a t a l y s t t h a t has
contact
is
with
nitrogen
p r o d u c t s . Now, when a n i t r o g e n - f r e e
processed on a c a t a l y s t t h a t has a l r e a d y been working i n
presence of ammonia, n-heptane conversion i s o n l y 69 w t
%.The
poisonous
e f f e c t t h u s does n o t seem t o be completely r e v e r s i b l e under t h e s e c o n d i t i o n s .
W e w i l l d i s c u s s t h i s p o i n t i n more d e t a i l l a t e r on. The r e a c t i o n o r d e r with r e s p e c t t o ammonia p r e s s u r e was a l s o c a l c u l a t e d . For t h e set of experiments i n run 3. e x c l u d i n g z e r o ammonia p r e s s u r e , t h e o r d e r was -0.39 The r e s u l t s can be expressed i n a n o t h e r way. L e t u s c a l l "k " t h e f i r s t o r d e r rate c o n s t a n t t h a t would be o b t a i n e d a t a temperature of 390 'C w i t h a n i t r o g e n - f r e e f e e d . T h i s c o n s t a n t was e v a l u a t e d by e x t r a p o l a t i n g r e s u l t s p r e v i o u s l y r e p o r t e d and by u s i n g an a p p a r e n t a c t i v a t i o n energy o f 32 kcal/mole. The r a t e c o n s t a n t s , k. corresponding t o t h e c o n d i t i o n s with v a r i o u s ammonia p r e s s u r e s were a l s o c a l c u l a t e d . The k /k r a t i o r e p r e s e n t s a d e a c t i v a t i o n c o e f f i c i e n t . Its e v o l u t i o n with ammonia p r e s s u r e i s shown i n Figure 4.
.
308 110
100 90
2
24 80 70 60
50
:: y 40 SO
0
Fig.
4
. Deactivation
0
,
,
5
, 10
,
,
.
, 20
15
. , . 25
3
AMMONIA PRESSURE &Pa)
factor as a function of ammonia pressure.
also calculated the temperature required t o achieve an n-heptane conversion of 22%. This isoconversion temperature is plotted as a function of ammonia pressure i n Figure 5 . W e
w
$
420 410 ‘0°
2W 390 380 a
970
i- 360 350 940
0
330
N-C7 CONMRSION
I N C R W C PRESSURE
320
= 22 W U =0
DkCRMSMC PRESSURE = [I
fi 910 900 290
.
0
5 Temperature needed function of ammonia pressure.
Fig.
10
5
20
15
25
31
AMMONIA PRESSURE (kPa)
to
obtain
22
w t % n-heptane conversion as a
309 The appears
poisonous that,
pronounced.
e f f e c t of ammonia can c l e a r l y be s e e n on t h e s e f i g u r e s . It
a
For
conditions
at
even
to
a
very
low
as
pressure
nitrogen
d e a c t i v a t i o n i s very
ammonia p r e s s u r e ,
as
low
content
1 kPa,
which corresponds i n o u r
164 w t ppm i n t h e f e e d , a c t i v i t y i s
of
by a f a c t o r o f 10 and temperature must be i n c r e a s e d by 60 'C t o g e t same conversion. T h i s f a c t o r becomes around 40 a t a p r e s s u r e o f 6.1 kPa, which corresponds t o a n i t r o g e n f e e d c o n t e n t o f 1000 w t ppm. Such a v a l u e is t y p i c a l of a number of hydrocracker f e e d s . A t h i g h e r ammonia p r e s s u r e s , F i g u r e 5 shows t h a t t h e v a r i a t i o n i n i s o c o n v e r s i o n t e m p e r a t u r e i s l e s s , probably because o f a s a t u r a t i o n e f f e c t o f t h e a c i d i c sites. divided the
REVERSIBILITY OF AMMONIA ADSORPTION
W e
have
previously
seen
that
worked
in
at
ammonia p r e s s u r e and f o r a c a t a l y s t having
zero
t h e presence o f ammonia, n-heptane c o n v e r s i o n i s much
than observed with a f r e s h c a t a l y s t under t h e same c o n d i t i o n s . Ammonia
lower
is
adsorption
t h u s p a r t l y i r r e v e r s i b l e . T h i s p o i n t w i l l be s t u d i e d now i n t o
more d e t a i l with t h e f o l l o w i n g experiments ( r u n s
5. 6, 7 ) .
The
c a t a l y s t was exposed, f o r f o u r h o u r s , t o an n-heptane f e e d c o n t a i n i n g
1wt %
a n i t r o g e n - f r e e f e e d was f e d i n ,
and
s u l f u r and 0.1 w t % n i t r o g e n conversion was followed as a
. Then
function
o f time-on-stream
f o r three
d i f f e r e n t s temperatures. R e s u l t s are g i v e n i n F i g u r e 6.
5
g
80
l-t
m 70 50
2
0
V
Fig.
6
60
0
, I
.
I
0
6
. Recovery
of
.
10
,
.
I
.
15
n-heptane
1
20
.
I
26
.
I
.
90
1
95
.
I
40
RUN TIME
conversion
.
I
46
(a)
.
I
.
60
I
.
56
60
as a f u n c t i o n o f time on stream
a f t e r a s t a b i l i z a t i o n p e r i o d a t an ammonia p r e s s u r e o f 6 . 1 kPa. The v a l u e s o b t a i n e d a t z e r o run time concern c a t a l y s t s i n e q u i l i b r i u m with
a
pressure
o f 6.1 kPa, and t h e r e s u l t s are similar t o t h o s e i n F i g u r e 2 .
310 appears
t h a t a c t i v i t y is p r o g r e s s i v e l y r e s t o r e d when t h e c a t a l y s t i s t o t h e n i t r o g e n - f r e e f e e d . T h i s r e a c t i v a t i o n i s much q u i c k e r a t high temperature. T h i s is a s t r o n g i n d i c a t i o n t h a t t h e phenomenon o c c u r r i n g i s a
It
exposed slow
desorption
that
this
of
from t h e a c i d i c sites. However, i t can be seen
ammonia
desorption
is
rather
slow.
For
i n s t a n c e , a t a t e m p e r a t u r e of
60 h o u r s , whereas a f r e s h same c o n d i t i o n s . W e can conclude t h a t ammonia a d s o r p t i o n i s q u i c k l y r e v e r s i b l e for a wide range of ammonia p r e s s u r e . However, a t z e r o p r e s s u r e , d e s o r p t i o n is r a t h e r slow, 370 'C.
conversion
catalyst
would
is
yield
60% a f t e r
only
100% conversion
around
under
the
probably because ammonia remains f i r m l y bound on t h e sites having t h e h i g h e s t acidity
.
SELECTIVITY BETWEEN ISOMERIZED AND CRACKED PRODUCTS
W e w i l l now focus on t h e p r o d u c t s from t h e r e a c t i o n , and e s p e c i a l l y on t h e s e l e c t i v i t y between i s o m e r i z a t i o n and c r a c k i n g . W e
will
first
consider
the
experiments
performed
i n t h e presence of
ammonia.
The t o t a l i s o - h e p t a n e y i e l d is p l o t t e d as a f u n c t i o n o f t h e cracked
products
yield
figure
as two s e p a r a t e c u r v e s f o r b e t t e r c l a r i t y : Run 2, 3 and
7 and run 3 , 4 and 8 i n F i g u r e 8.
4
in
5 0
HYDROCRACKING (WtX) Fig. It
7
. Isomerization
v e r s u s c r a c k i n g f o r Runs 2.
3. 4.
a p p e a r s t h a t , whatever t h e c o n d i t i o n s may b e , s e l e c t i v i t y i s t h e same
.
The same p r o d u c t d i s t r i b u t i o n between isomers and c r a c k e d p r o d u c t s i s o b t a i n e d when
conversion
varies,
p r e s s u r e o r c o n t a c t time.
the
varying
parameter
b e i n g t e m p e r a t u r e , ammonia
31 1
5
40
= CONSTANT
5 35
-------
+
= INCREASING = DECREASING ---do
5
0
8
Fig.
is
This
catalyst where
no
40
50 60 70 eo HYDROCRACKING (Wtz)
v e r s u s c r a c k i n g f o r Runs
longer
the
case
90
100
3 , 4, 8.
f o r experiments performed a t z e r o ammonia
t h e p r e s e n c e of ammonia ( c u r v e A ) ; Runs 5, 6 and 7 performed on a by ammonia but w i t h a n i t r o g e n - f r e e f e e d ( c u r v e B ) ; Run 1
poisoned
no
30
Figure 9 g i v e s t h e s e l e c t i v i t y o b t a i n e d for d i f f e r e n t experiments:
in
2
20
10
. Isomerization
pressure. Run
o
ammonia
is
p r e s e n t and on a f r e s h c a t a l y s t ( c u r v e C ) .
= -8-
n-C7:S
n-ms
(AMMONU DKSORPTION)
0
0
Fig.
9
.
10
20
SO
40
50
80
70
80
HYDROCRACKING (Wtz)
90
I s o m e r i z a t i o n v e r s u s c r a c k i n g f o r Runs 1. 2 , 5, 6 , 7.
100
312 appears
It
that
isomerization experiments
and
ammonia cracking.
performed
s t r o n g l y modifies p r o d u c t d i s t r i b u t i o n between The
cracking
r e a c t i o n i s more pronounced f o r
w i t h o u t n i t r o g e n . The a c t u a l parameter g o v e r n i n g t h i s
s e l e c t i v i t y i s i n f a c t t h e s u r f a c e s t a t e o f t h e c a t a l y s t . The v a l u e s r e l a t i n g t h e d e s o r p t i o n experiments ( c u r v e B) are s i t u a t e d between c u r v e s A and C,
to
despite
the
zero
ammonia p r e s s u r e and some d o t s c a t t e r i n g can b e observed.
T h i s i s due t o t h e f a c t t h a t measurements were made w i t h c a t a l y s t s undergoing ammonia
desorption,
and
hence c o n t a i n i n g d i f f e r e n t p r o p o r t i o n s o f poisoned
sites. ISOMER SELECTIVITY Figure pressure
10
compares
isoheptanes
s e l e c t i v i t y f o r Runs 1 and 2 , f o r which
r e s p e c t i v e l y 0 and 6 . 1 kPa.
was
1.00
c E
!i w
rn
0.00 I
F i g . 10 The
. I s o h e p t a n e s s e l e c t i v i t y as a f u n c t i o n
first
di-branched when yield
of conversion.
o b s e r v a t i o n i s t h a t , f o r each r u n , t h e p r o p o r t i o n s o f mono- and isomers are f a i r l y s t a b l e as a f u n c t i o n o f conversion. Secondly,
t h e r e a c t i o n i s performed i n t h e p r e s e n c e o f ammonia, t h e methylhexanes increases
selectivity
remains
to
the
detriment
of
dimethyl
pentanes.
Ethylpentane
roughly unmodified. F i g u r e 11 shows t h a t t h e p r o p o r t i o n
o f i s o b u t a n e i n t h e cracked p r o d u c t s d e c r e a s e s i n t h e p r e s e n c e o f ammonia.
313
P(NH3) =
0
p(m3)=
6.1 I(PA
---- --
-
YP.4 :IcI/ICI+NCI
=a
._ _ _ _ _ . _ _ _ . _ _
id/ICI+NCI = 0
- - - - -
- - _ _ - - - - - - - - - _-
Thermodynamic equilibrium
I
10
F i g . 11
. Isobutane
'
20
I
~
30
l
40
'
l
50
'
l
60
'
CONVERSION (WTZ)
l
70
~
I
s e l e c t i v i t y a s a f u n c t i o n o f conversion.
DISCUSSION CONCERNING PRODUCT DISTRIBUTION The e f f e c t s of ammonia p r e s s u r e on p r o d u c t d i s t r i b u t i o n can b e summarized as follows: (1) Isomers y i e l d is i n c r e a s e d t o t h e d e t r i m e n t of cracked p r o d u c t y i e l d f o r a g i v e n conversion. ( 2 ) Mono-branched isomers are favored with
respect
t o di-branched ones. ( 3 ) Cracked p r o d u c t s are less isomerized.
An e x p l a i n a t i o n must be found t o account f o r a l l t h e s e e x p e r i m e n t a l r e s u l t s . The
first
temperature
observation increase
of
is t h a t t h e p r e s e n c e o f ammonia r e s u l t s i n 90 t o 100 'C t o o b t a i n t h e same conversion. Hence are d i f f e r e n t between t h e two sets of c o n d i t i o n s .
thermodynamic c o n d i t i o n s The is0 t o normal r a t i o p r e d i c t e d by t h e thermodynamic e q u i l i b r i u m d e c r e a s e s when temperature i n c r e a s e s . However, as shown i n F i g u r e 11, t h e v a r i a t i o n s of t h e e q u i l i b r i u m are t o o small t o e x p l a i n o u r r e s u l t s .
ammonia p r e s s u r e r e s u l t s in a p o i s o n i n g o f a c i d i c s i t e s . Many sites are a v a i l a b l e f o r t h e r e a c t i o n and t h i s e f f e c t is compensated f o r
Obviously,
less
by t h e t e m p e r a t u r e i n c r e a s e . T h i s means t h a t t h e a v e r a g e t u r n o v e r number is g r e a t e r , and t h a t t h e r e s i d e n c e time of carbenium i o n s on a c i d i c sites i s s h o r t e r . On t h e o t h e r hand. i t has been c l e a r l y e s t a b l i s h e d from t h e l i t e r a t u r e t h a t t h e i s o m e r i z a t i o n mechanism proceeds through a carbenium i o n rearrangement. For n-heptane, t h e scheme is (1) a d s o r p t i o n o f n-heptene coming
from
NiMo sites t o produce a normal carbenium i o n . ( 2 ) i s o m e r i z a t i o n
314 to
a
are
isomer and e v e n t u a l l y t o a dibranched isomer. These i o n s
monobranched in
equilibrium
corresponding
between
heptenes.
adsorption/desorption
themselves,
The
and
a l s o i n equilibrium with t h e
h i g h e r t h e t e m p e r a t u r e , t h e more d i s p l a c e d t h e
i s toward o l e f i n s . W e c a n assume t h a t t h e
equilibrium
s t a t e o f t h e average carbenium i o n s l y i n g on t h e s u r f a c e is d i f f e r e n t w i t h or without
ammonia.
t h e l a t e r case, w i t h a l o n g r e s i d e n c e t i m e , t h e a c i d i c
In
s i t e s would b e mostly covered by dibranched i o n s . A t h i g h e r t e m p e r a t u r e s , i n t h e p r e s e n c e o f ammonia, t h e i o n s would be less isomerized. I n t h i s c a s e , t h e cracking would
would be reduced, because c r a c k i n g o f t h e monobranched C7
tendency
carbenium
is
ion
also
isobutane
much
explain
molecule
slower
the
t h a n f o r t h e dibranched i o n . Moreover, t h i s
d e c r e a s e i n i s o b u t a n e i n t h e c r a c k e d p r o d u c t s . The
o r i g i n a t e s e s s e n t i a l l y from di-branched i o n s by a t y p e B
b e t a - s c i s s i o n mechanism , a c c o r d i n g t o t h e c l a s s i f i c a t i o n proposed r e c e n t l y ( 1 6 ) . So, t h e poisonous e f f e c t o f ammonia would r e s u l t i n a lower isomerization
degree
o f t h e carbenium i o n , which i n t u r n would i n d u c e fewer
monobranched heptane i s o m e r s , fewer c r a c k i n g r e a c t i o n s and fewer i s o b u t a n e i n cracked p r o d u c t s . CONCLUSION The
nitrogen
Operating feed
so
content,
feed
level
conditions
can
be
hydrocracking
of
hydrocracker f e e d s l e a d s t o s e v e r a l consequences.
of t h e f i r s t - s t e p h y d r o t r e a t m e n t depend on t h e n i t r o g e n
that
t h e n i t r o g e n s p e c i f i c a t i o n s o f t h e second-step i n l e t
reached. section
Moreover,the
ammonia
partial
pressure
of
the
w i l l depend on t h e n i t r o g e n l e v e l and a l s o on t h e type
o f p r o c e s s used, whether s i n g l e - s t a g e o r two-stage. Ammonia
pressure
hydrocracking
dramatic quantity
A
e f f e c t on t h e a c t i v i t y o f t h e z e o l i t e of
1000 w t ppm
nitrogen
added t o an
f e e d , g e n e r a t i n g a p a r t i a l p r e s s u r e o f 6.1 kPa. i s r e s p o n s i b l e f o r
n-heptane an
a
has
catalyst.
loss
activity
temperature
a
of
increase
reversible
for
desorption
is
factor
of
a wide slow,
of
40,
which can be compensated f o r by a
100 " C . Ammonia a d s o r p t i o n i s q u i c k l y p r e s s u r e . However, a t z e r o p r e s s u r e , t h a t ammonia remains s t r o n g l y adsorbed on t h e to
90
range
of
meaning
most a c i d i c sites o f z e o l i t e . The
presence
selectivity.The products.
isomer
ammonia
of
These
isomers
effects
isomerization
in
modifications
in
product
i n c r e a s e d t o t h e d e t r i m e n t o f cracked are f a v o r e d and cracked p r o d u c t s are less
be r a t i o n a l i z e d by assuming a lower average
can
of
results
is
yield
Monobranched
isomerized. degree
of
carbenium
ions,
due
to
a sharp decrease i n
r e s i d e n c e time a t t h e p r o t o n i c sites. From
a
refractory that
process
point
feedstocks
operating
of
view,
containing
conditions
must
t h e r e i s a problem o f how t o cope w i t h
large
amounts o f n i t r o g e n . I t i s obvious
be a d j u s t e d t o compensate f o r t h e a c t i v i t y
315
loss either by temperature increase or by space velocity decrease. However, the start-of-run temperature must remain inside reasonable limits (e.g. 400 'C), to prevent a quick deactivation of the catalytic system because of thermodynamic limitations for polyaromatics hydrogenation. Of course these limits depend on hydrogen pressure. For very difficult cases, a solution may also be to choose a two-stage process configuration, which makes for a substantial reduction of ammonia pressure in the hydrocracking section. Likewise, the replacement of amorphous type catalyst by more active zeolite catalysts results in a decrease in operating temperature, which is especially appreciated in this case of refractory feeds.
REFERENCES N.Choudary and D.N.Saraf, Ind.Eng.Chem.,Prod.Res.Dev., 1 4 ( 2 ) , 74, 1975. 1 2 A.Billon, J.P.Franck and J.P.Peries, Hydroc. Proc., 139, Sept 1975 C.E.Maier. P.H.Biaeard. A.Billon and P.Dufresne. NPRA Annual Meeting. 3 -. San Antonio, March 19g8. . P.Dufresne. P.I!.Bineard and A. Billon, Catalysis Today, 1. 367. 1987 4 C .Marcilly and JTP. Franck, Catalysis by-Zeolites; .B.Imelik et a1 Eds , 5 Elsevier. Amsterdam, 93. 1980. 6 G.E.Lang1oi.s and R.F.Sulivan, Adv.Chem.Ser. 97, 38, 1969 M.Guisnet and 3.Perot. Proc. NATO Advanced Study Institute on Zeolites, 7 Portugal, 39, May 1983. J.Weitkamp. P.A.Jacob and J.A.Martens, Appl. Catal., 20, 239 and 283, 8 1986 H.L.Coonradt and W.E.Garwood,Ind. Eng. Chem. Proc Des. Dev., 3, 38, 1964 9 10 J.Weitkamp, Ind. Eng. Chem. Prod. Res. Dev. 21, 550, 1982 11 G.Giannetto, G.Perot and M.Guisnet, Ind. Eng. Chem. Prod Res. Dev., 25, 481, 1986. 12 M.Steijns and G.F.Froment, Ind. Eng. Chem. Proc Des. Dev., 20, 660, 1981. and A.Billon, Franco-Venezuela Conference, 13 P.Dufresne, C.Marcilly Rueil-Malmaison, France, 1985. 14 F.Y. El-Kady, Egypt. J. Chem., 21, 5, 349, 1978. 15 H.Dauns, S.Ernst and J.Weitkamp, Proc. 7th Int. Zeol. Conf.. Murakami et a1 Eds., 787, Kodansha, Tokyo and Elsevier, Amsterdam, 1986. 16 J.Weitkamp, P.A.Jacob and J.A.Martens. Appl. Catal., 8, 123, 1983
301
INFLUENCE OF NITROGEN FEED CONTENT O N THE PERFORMANCES OF A ZEOLITE HYDROCRACKING CATALYST P. DUFRESNE, A. QUESADA AND S. MIGNARD Institut Francais du Petrole, BP 311, 92506, Rueil Malmaison, (France) ABSTRACT
Hydrocracker feeds are usually rich in nitrogen compounds. These are partially transformed by the first-step hydrotreatment catalysts, creating an ammonia pressure in the reactors. This paper shows the effects of ammonia pressure on the performances of a zeolite hydrocracking catalyst. Ammonia has a strong negative effect on the activity for n-heptane conversion, especially at low ammonia pressures. Adsorption of ammonia on the acidic sites of the zeolite is quickly reversible for a wide range of pressure. However, at zero pressure, ammonia desorption is slow, meaning that the molecule remains strongly adsorbed on the most acidic sites of the zeolite. Ammonia also modifies strongly the products selectivity. Iso-heptane yield is increased to the expense of cracked products yield. INTRODUCTION
Vacuum gas oil hydrocracking is now a major conversion process in the refining industry. One of its main qualities is flexibility. Different types of feeedstocks can be converted into various types of valuable products (1,2,3). The catalyst is one of the key aspects of the process and the merits of a given hydrocracking process largely depend on the catalyst’s qualities. These are exprressed by three main criteria: (1) selectivity or type of yield obtained, (2) activity or temperature for a given conversion level and (3) stability or aging rate. There is a strong incentive today to process more and more refractory feedstocks in order to improve the economics of the process. The feed’s refractory nature can be described by various parameters. Feed heaviness can be characterized by the molecular weight or the midand final boiling points. Composition is also a significant parameter i.e. paraffins, naphthanes, aromatics content and, for non straight-run feeds such as recycle oil from catalytic cracking or coker gas oil, olefin content. The heteroatom content, sulfur and especially nitrogen are also very important points to be considered in feed evaluation. Metals and asphaltenes are not considered here since they usually must not be present in a conventional hydrocracker feed. The specifications for metal content are often less than 1 ppm. These parameters are interconnected. For example an increase in feed molecular weight often accompanies an increase in the aromatics, especially polyaromatics, content. Heavy feeds are more refractory than light ones because of the greater concentration of poisonous molecules rather than the increase in the mean carbon number.
302
For a given hydrocracker. the only parameter enabling more refractory feedstocks to be processed is often the catalyst type. An improvement in catalyst performance goes via a good understanding of the phenomena involved in processing difficult feeds. This paper deals with the effects of nitrogen feed content on catalyst performance. GENERAL ANALYSIS OF THE PROBLEM OF NITROGEN FEED CONTENT A hydrocracking process usually consists of two different steps, whatever the reactor arrangement is used, i.e. "single-stage'' or "two-stage''process, as shown in Figure 1. Reactions occurring in the first step are mainly hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and aromatics hydrogenation (HDA). They are promoted by a hydrotreatment catalyst. In sore cases, a hydrocracking catalyst can also be placed in the first step in order to initiate' cracking reactions. The most critical reaction in that step is HDN because the outlet effluent must respect a severe nitrogen-content specification. Indeed,the acidic function of hydrocracking catalysts, especially when they contain zeolites, can be strongly poisoned by heavy nitrogen-containing molecules. Depending on the licensors recommandations. the value i s typically between 1 and 30 ppm. Clearly, conditions to obtain for instance, 5 ppm at the first step outlet will be very different when starting from a feed containing 200 or 2000 ppm, which are the two extreme values of standard VGO hydrocracker feeds. A nitrogen-rich feed has to be processed under conditions of high hydrogen pressure and/or low space velocity. For a given unit, the only operating parameter is temperature. However,the HDN reaction, as well as the HDA reaction, is bound to a thermodynamic equilibrium, because the first reaction step is the nitrogen-containing ring saturation. Temperature thus has a limited effect
(4). The main reactions occurring in the second step are hydroisomerization and hydrocracking. They are promoted by a more acidic hydrocracking catalyst, containing amorphous silica-alumina or zeolite. However, depending on the type of process used, the second step conditions may be very different (3.5). In the single-stage arrangement, the liquid and gaseous effluents coming from the first step go to the second step. Thus, a hydrocracking catalyst must deal with the large amount of ammonia produced upstream, with ammonia partial pressure typically being between 5 and 30 kPa. In the two-stage arrangement, the effluents are fractionated after the first step.
303
ONE- STAGE PROCESS FRACTIONATION MAKE UP H2
H2 RECYCLE
_ 1
1 or 2 REACTORS r-- i
v A
FRESH FEED
I
- L,,1 J-
I
1
I
I
I
I I
--L-----L-
---..---SEPARATION
I
RESIDUE RECYCLE
TWO- STAGE PROCESS MAKE UP H2
H2 RECYCLE
d =
PSTAGE
FRACTIONATION
REACTOR
FRESH FEED
i If
b
2ndSTAGE
SEPARATION
1
RESIDUE RECYCLE
Fig.
So with
1 only
. Typical the
configuration.
c)FUEL
OIL
hydrocracking p r o c e s s flow-schemes
unconverted,
ammonia-free
considerably
--
lower
b u t h y d r o t r e a t e d . r e s i d u e e n t e r s t h e second s t e p
hydrogen. ammonia
Hydrocracking partial
c a t a l y s t s can t h u s o p e r a t e a t a
presmre
than
in
the
single-stage
304
The chemistry o f hydrocracking and e s p e c i a l l y t h e r e a c t i o n s of p a r a f f i n s on b i f u n c t i o n n a l hydrocracking c a t a l y s t s have been e x t e n s i v e l y s t u d i e d i n t h e l i t e r a t u r e , and good reviews have a l r e a d y been p u b l i s h e d (6,7,8).The transformation
of
paraffins
i m p l i e s t h e independent a c t i o n o f two t y p e s of
c a t a l y t i c s i t e s , i . e . hydrogenating and a c i d i c s i t e s (9). On t h e f i r s t o n e s , p a r a f f i n s are transformed i n t o o l e f i n i c i n t e r m e d i a t e s , which are s u b s e q u e n t l y adsorbed on a c i d i c sites as carbenium i o n s . These i o n s are t h e n i s o m e r i z e d ,
are
and
desorbed from t h e p r o t o n i c s i t e as an i s o - o l e f i n ( i s o m e r i z a t i o n ) or
as two fragments ( c r a c k i n g )
.
I n most c a s e s , t h e m e c h a n i s t i c ( 1 0 , l l ) as well as t h e k i n e t i c ( 1 2 ) s t u d i e s r e p o r t e d i n t h e l i t e r a t u r e have used c a t a l y s t s made o f z e o l i t e s l o a d e d w i t h a noble m e t a l , such as platinum o r palladium. The r e a c t i o n o f a long-chain on t h i s t y p e o f system r e s u l t s i n a h i g h s e l e c t i v i t y t o isomers a t or medium conversion rates, and t h i s c h a r a c t e r i s t i c f e a t u r e h a s been r e f e r r e d t o as " i d e a l h y d r o c r a c k i n g " ( 8 ) . It is t y p i c a l o f a b i f u n c t i o n a l
paraffin low
c a t a l y s t having noble metal i n
a v e r y h i g h hydrogenation f u n c t i o n , which i s t h e c a s e of a s u l f u r and n i t r o g e n f r e e environment. However, a c t u a l hydrocracking c o n d i t i o n s i n v o l v e h i g h p a r t i a l p r e s s u r e s of s u l f u r and ammonia c o n t a i n i n g s p e c i e s . Despite t h e great i n f l u e n c e o f t h e s e compounds on c a t a l y s t performance, t h e t o p i c h a s n o t been d e s c r i b e d c l e a r l y i n t h e open literature. According t o p r e v i o u s work (13,14), i t is clear t h a t t h i s " i d e a l hydrocracking" mechanism does n o t p r e v a i l anymore i n t h e p r e s e n c e of s u l f u r and
nitrogen
containing
feed.
The
s e l e c t i v i t y o f t h e r e a c t i o n is t o t a l l y modified, and c r a c k i n g r e a c t i o n s predominate under t h e s e c o n d i t i o n s . T h i s phenomenon can be i n t e r p r e t e d by a d r a m a t i c change i n t h e b a l a n c e between t h e two f u n c t i o n s (hydrogenation and c r a c k i n g ) . Another s t u d y (15) d e s c r i b e s t h e influence of hydrogen sulfide pressure on the reactivity of a P a l l a d i u m / z e o l i t e Y system. It also clearly shows t h e m o d i f i c a t i o n of s e l e c t i v i t y . A t around 40% c o n v e r s i o n , n-dodecane can be completly isomerized i n a s u l f u r - f r e e environment when, f o r t h e same conversion i n t h e p r e s e n c e of s u l f u r , t h e isododecane y i e l d i s v e r y low compared t o t h e hydrocracked product y i e l d . S o , t h e q u e s t i o n o f n i t r o g e n f e e d c o n t e n t can be summarized i n t h i s way : t h e nitrogen l e v e l w i l l c o n t r o l t h e o p e r a t i n g conditions o f the f i r s t hydrotreatment s t e p , so t h a t t h e n i t r o g e n s p e c i f i c a t i o n s o f t h e second s t e p
i n l e t f e e d can b e reached. Moreover t h e ammonia p a r t i a l p r e s s u r e o f t h e hydrocracking s e c t i o n w i l l b e a f u n c t i o n o f t h e n i t r o g e n l e v e l and w i l l a l s o depend on t h e t y p e of p r o c e s s used, whether s i n g l e - s t a g e o r two-stage. W e w i l l s t u d y h e r e t h e i n f l u e n c e of ammonia p a r t i a l p r e s s u r e on t h e a c t i v i t y and
s e l e c t i v i t y o f a z e o l i t e hydrocracking c a t a l y s t .
305 a
For
more
a n a l y s i s of t h e phenomenon from n s c i e n t i f i c p o i n t o f
accurate
view, w e performed t h i s s t u d y w i t h t h e n-heptane molecule. EXPERIMENTAL The
catalyst
used
was
the
industrial
catalyst,
HYC
642.
from
P r o c a t a l y s e . I t s hydrogenation f u n c t i o n c o n s i s t s o f Nickel and Molybdenum, and
i s provided by an U l t r a - S t a b l e Y z e o l i t e . Experiments a c a t a l y t i c u n i t w i t h t h e f o l l o w i n g r e a c t o r dimensions: i n s i d e diameter 19 mm and l e n g t h 550 mm. A c a t a l y s t l o a d i n g o f 13 g , d i l u t e d with s i l i c o n c a r b i d e , was p l a c e d i n t h e middle o f t h e reactor. P r e h e a t i n g o f its
acidic
were
function
performed
in
f e e d was performed by a 150 mm long s i l i c o n c a r b i d e packing. Temperature
the
was c o n t r o l l e d by t h r e e thermocouples p l a c e d i n s i d e t h e r e a c t o r . In
order
to
keep t h e NiMo phase i n a s u l f i d e d s t a t e , a c o n s t a n t p a r t i a l
p r e s s u r e of hydrogen s u l f i d e was maintained i n t h e r e a c t o r by adding of
dimethyl
d i s u l f i d e t o t h e feed
. Ammonia p a r t i a l
1.5 w t %,
p r e s s u r e was c r e a t e d by
a known q u a n t i t y o f a n i l i n e t o t h e f e e d which was r e a d i l y transformed
adding
i n t o ammonia. A q u a n t i t y o f 1000 w t ppm of n i t r o g e n c o r r e s p o n d s t o an ammonia pressure
of
stabilization
6.1 kPa. Every change i n o p e r a t i n g c o n d i t i o n s was followed by a period
of
around
two
hours.
Under t h e c o n d i t i o n s u s e d , no
c a t a l y s t d e a c t i v a t i o n was v i s i b l e , even a f t e r s e v e r a l days. The which
different also
series o f
experiments
are b r i e f l y d e s c r i b e d i n T a b l e 1 ,
i n d i c a t e s t h e f i g u r e s o f t h e r e s u l t s r e p o r t e d . Others c o n d i t i o n s
were a t o t a l p r e s s u r e o f 6 MPa and a hydrogen-to-n-heptane molar r a t i o o f 6. TABLE 1 D e s c r i p t i o n o f t h e main experimental parameters Run number
Temperature
NH3 p r e s s u r e
cont.time
("C)
(kPa)
(h)
260-340
0
0.5
360-425
6.1
0.5
390
0 t o 18
390
29 t o
370 380 390 390
o
6.1 then 0 6.1 then 0 6.1 then 0 6.1
0.5
0.5 0.5 0.5 0.5 0.21-0.71
Figure
306
EVOLUTION OF N-HEPTANE CONVERSION WITH TEMPERATURE
The
variation
can
It
on
conversion
with
temperature
was
followed
for
an
f e e d and f o r a f e e d c o n t a i n i n g 0 . 1 w t % n i t r o g e n ( r u n s 1 and 2 ) .
aniline-free be
seen
in
Figure
that
2
the
reaction
t e m p e r a t u r e ranges are
completely d i f f e r e n t . The poisonous e f f e c t o f ammonia must be compensated f o r by a t e m p e r a t u r e i n c r e a s e of around 100 'C.
-
LOO
5
90
Ef,
:
80
g
E 70 I3O
0 50
u
+ u
40
'
30
z
20 10
0
TEMPERATURE ( C )
Fig.
. Evolution
2
of
n-heptane c o n v e r s i o n w i t h t e m p e r a t u r e w i t h or without
ammonia.
The a p p a r e n t a c t i v a t i o n e n e r g i e s c a l c u l a t e d from Arrhenius e q u a t i o n s are 32 kcal/mole
for
the
nitrogen-containing
nitrogen-free feed.
following
hypothesis:
containing
feed,
increase
is
a
with
partial
liberates
some
This
when
two
feed
slight
temperature
phenomena
occur.
and
36
kcal/mole
for
the
d i f f e r e n c e c o u l d b e e x p l a i n e d by t h e
is The
increased
with
the
nitrogen
f i r s t one is a normal a c t i v i t y
temperature as i n t h e ammonia f r e e environment. The second one ammonia
desorption
from
the
zeolite's
acidic
sites. This
new a c t i v e s i t e s which are a v a i l a b l e f o r t h e r e a c t i o n and can
c o n t r i b u t e t o an i n c r e a s e i n t h e a p p a r e n t a c t i v a t i o n energy.
INFLUENCE OF AMMONIA PRESSURE
W e s t u d i e d t h e i n f l u e n c e o f ammonia p a r t i a l p r e s s u r e on z e o l i t e c a t a l y s t a c t i v i t y a t c o n s t a n t c o n d i t i o n s . Feed A n i l i n e c o n t e n t was v a r i e d i n o r d e r t o generate
different
increased
from
ammonia
pressures.
During
run 3,
ammonia
pressure
0 t o 18 kPa, w i t h a f r e s h c a t a l y s t b e i n g u s e d . During run 4,
p r e s s u r e d e c r e a s e d from 29 t o 0 kPa. The r e s u l t s are g i v e n i n F i g u r e 3.
307
-'
100
INCREASING PRESSURE = 0 DECREASING PRESSURE =O
k
2
2
80 70
::
ul
0
V
40
30 20 10
0
3
Fig.
. Variation
dramatic
A
especially n-heptane
in
5
10
15
20
25
30
AMMONIA PRESSURE (kPa)
of conversion with ammonia p r e s s u r e .
d r o p i n a c t i v i t y was observed when ammonia p r e s s u r e i n c r e a s e d , the
low
p r e s s u r e r e g i o n . Beyond a p r e s s u r e o f around 10 W a ,
conversion reached a pseudo-plateau. Another s t r i k i n g p o i n t i s t h a t
curves o b t a i n e d from an i n c r e a s e or a d e c r e a s e i n t h e a n i l i n e c o n t e n t are
the
nearly
i d e n t i c a l . Thus, t h e poisonous e f f e c t of ammonia i s r a t h e r r e v e r s i b l e .
There
must be an a d s o r p t i o n - d e s o r p t i o n e q u i l i b r i u m between ammonia i n t h e g a s
phase and t h e p r o t o n i c s i t e s o f t h e z e o l i t e . However, It
total
at
390 'C
never
been
n-heptane the
is
this
pressure.
can in
feed
true
over
the
whole
pressure
range e x c e p t a t z e r o
be i n f e r r e d from Figure 2 t h a t conversion of n-heptane i s i n a n i t r o g e n f r e e environment and f o r a c a t a l y s t t h a t has
contact
is
with
nitrogen
p r o d u c t s . Now, when a n i t r o g e n - f r e e
processed on a c a t a l y s t t h a t has a l r e a d y been working i n
presence of ammonia, n-heptane conversion i s o n l y 69 w t
%.The
poisonous
e f f e c t t h u s does n o t seem t o be completely r e v e r s i b l e under t h e s e c o n d i t i o n s .
W e w i l l d i s c u s s t h i s p o i n t i n more d e t a i l l a t e r on. The r e a c t i o n o r d e r with r e s p e c t t o ammonia p r e s s u r e was a l s o c a l c u l a t e d . For t h e set of experiments i n run 3. e x c l u d i n g z e r o ammonia p r e s s u r e , t h e o r d e r was -0.39 The r e s u l t s can be expressed i n a n o t h e r way. L e t u s c a l l "k " t h e f i r s t o r d e r rate c o n s t a n t t h a t would be o b t a i n e d a t a temperature of 390 'C w i t h a n i t r o g e n - f r e e f e e d . T h i s c o n s t a n t was e v a l u a t e d by e x t r a p o l a t i n g r e s u l t s p r e v i o u s l y r e p o r t e d and by u s i n g an a p p a r e n t a c t i v a t i o n energy o f 32 kcal/mole. The r a t e c o n s t a n t s , k. corresponding t o t h e c o n d i t i o n s with v a r i o u s ammonia p r e s s u r e s were a l s o c a l c u l a t e d . The k /k r a t i o r e p r e s e n t s a d e a c t i v a t i o n c o e f f i c i e n t . Its e v o l u t i o n with ammonia p r e s s u r e i s shown i n Figure 4.
.
308 110
100 90
2
24 80 70 60
50
:: y 40 SO
0
Fig.
4
. Deactivation
0
,
,
5
, 10
,
,
.
, 20
15
. , . 25
3
AMMONIA PRESSURE &Pa)
factor as a function of ammonia pressure.
also calculated the temperature required t o achieve an n-heptane conversion of 22%. This isoconversion temperature is plotted as a function of ammonia pressure i n Figure 5 . W e
w
$
420 410 ‘0°
2W 390 380 a
970
i- 360 350 940
0
330
N-C7 CONMRSION
I N C R W C PRESSURE
320
= 22 W U =0
DkCRMSMC PRESSURE = [I
fi 910 900 290
.
0
5 Temperature needed function of ammonia pressure.
Fig.
10
5
20
15
25
31
AMMONIA PRESSURE (kPa)
to
obtain
22
w t % n-heptane conversion as a
309 The appears
poisonous that,
pronounced.
e f f e c t of ammonia can c l e a r l y be s e e n on t h e s e f i g u r e s . It
a
For
conditions
at
even
to
a
very
low
as
pressure
nitrogen
d e a c t i v a t i o n i s very
ammonia p r e s s u r e ,
as
low
content
1 kPa,
which corresponds i n o u r
164 w t ppm i n t h e f e e d , a c t i v i t y i s
of
by a f a c t o r o f 10 and temperature must be i n c r e a s e d by 60 'C t o g e t same conversion. T h i s f a c t o r becomes around 40 a t a p r e s s u r e o f 6.1 kPa, which corresponds t o a n i t r o g e n f e e d c o n t e n t o f 1000 w t ppm. Such a v a l u e is t y p i c a l of a number of hydrocracker f e e d s . A t h i g h e r ammonia p r e s s u r e s , F i g u r e 5 shows t h a t t h e v a r i a t i o n i n i s o c o n v e r s i o n t e m p e r a t u r e i s l e s s , probably because o f a s a t u r a t i o n e f f e c t o f t h e a c i d i c sites. divided the
REVERSIBILITY OF AMMONIA ADSORPTION
W e
have
previously
seen
that
worked
in
at
ammonia p r e s s u r e and f o r a c a t a l y s t having
zero
t h e presence o f ammonia, n-heptane c o n v e r s i o n i s much
than observed with a f r e s h c a t a l y s t under t h e same c o n d i t i o n s . Ammonia
lower
is
adsorption
t h u s p a r t l y i r r e v e r s i b l e . T h i s p o i n t w i l l be s t u d i e d now i n t o
more d e t a i l with t h e f o l l o w i n g experiments ( r u n s
5. 6, 7 ) .
The
c a t a l y s t was exposed, f o r f o u r h o u r s , t o an n-heptane f e e d c o n t a i n i n g
1wt %
a n i t r o g e n - f r e e f e e d was f e d i n ,
and
s u l f u r and 0.1 w t % n i t r o g e n conversion was followed as a
. Then
function
o f time-on-stream
f o r three
d i f f e r e n t s temperatures. R e s u l t s are g i v e n i n F i g u r e 6.
5
g
80
l-t
m 70 50
2
0
V
Fig.
6
60
0
, I
.
I
0
6
. Recovery
of
.
10
,
.
I
.
15
n-heptane
1
20
.
I
26
.
I
.
90
1
95
.
I
40
RUN TIME
conversion
.
I
46
(a)
.
I
.
60
I
.
56
60
as a f u n c t i o n o f time on stream
a f t e r a s t a b i l i z a t i o n p e r i o d a t an ammonia p r e s s u r e o f 6 . 1 kPa. The v a l u e s o b t a i n e d a t z e r o run time concern c a t a l y s t s i n e q u i l i b r i u m with
a
pressure
o f 6.1 kPa, and t h e r e s u l t s are similar t o t h o s e i n F i g u r e 2 .
310 appears
t h a t a c t i v i t y is p r o g r e s s i v e l y r e s t o r e d when t h e c a t a l y s t i s t o t h e n i t r o g e n - f r e e f e e d . T h i s r e a c t i v a t i o n i s much q u i c k e r a t high temperature. T h i s is a s t r o n g i n d i c a t i o n t h a t t h e phenomenon o c c u r r i n g i s a
It
exposed slow
desorption
that
this
of
from t h e a c i d i c sites. However, i t can be seen
ammonia
desorption
is
rather
slow.
For
i n s t a n c e , a t a t e m p e r a t u r e of
60 h o u r s , whereas a f r e s h same c o n d i t i o n s . W e can conclude t h a t ammonia a d s o r p t i o n i s q u i c k l y r e v e r s i b l e for a wide range of ammonia p r e s s u r e . However, a t z e r o p r e s s u r e , d e s o r p t i o n is r a t h e r slow, 370 'C.
conversion
catalyst
would
is
yield
60% a f t e r
only
100% conversion
around
under
the
probably because ammonia remains f i r m l y bound on t h e sites having t h e h i g h e s t acidity
.
SELECTIVITY BETWEEN ISOMERIZED AND CRACKED PRODUCTS
W e w i l l now focus on t h e p r o d u c t s from t h e r e a c t i o n , and e s p e c i a l l y on t h e s e l e c t i v i t y between i s o m e r i z a t i o n and c r a c k i n g . W e
will
first
consider
the
experiments
performed
i n t h e presence of
ammonia.
The t o t a l i s o - h e p t a n e y i e l d is p l o t t e d as a f u n c t i o n o f t h e cracked
products
yield
figure
as two s e p a r a t e c u r v e s f o r b e t t e r c l a r i t y : Run 2, 3 and
7 and run 3 , 4 and 8 i n F i g u r e 8.
4
in
5 0
HYDROCRACKING (WtX) Fig. It
7
. Isomerization
v e r s u s c r a c k i n g f o r Runs 2.
3. 4.
a p p e a r s t h a t , whatever t h e c o n d i t i o n s may b e , s e l e c t i v i t y i s t h e same
.
The same p r o d u c t d i s t r i b u t i o n between isomers and c r a c k e d p r o d u c t s i s o b t a i n e d when
conversion
varies,
p r e s s u r e o r c o n t a c t time.
the
varying
parameter
b e i n g t e m p e r a t u r e , ammonia
31 1
5
40
= CONSTANT
5 35
-------
+
= INCREASING = DECREASING ---do
5
0
8
Fig.
is
This
catalyst where
no
40
50 60 70 eo HYDROCRACKING (Wtz)
v e r s u s c r a c k i n g f o r Runs
longer
the
case
90
100
3 , 4, 8.
f o r experiments performed a t z e r o ammonia
t h e p r e s e n c e of ammonia ( c u r v e A ) ; Runs 5, 6 and 7 performed on a by ammonia but w i t h a n i t r o g e n - f r e e f e e d ( c u r v e B ) ; Run 1
poisoned
no
30
Figure 9 g i v e s t h e s e l e c t i v i t y o b t a i n e d for d i f f e r e n t experiments:
in
2
20
10
. Isomerization
pressure. Run
o
ammonia
is
p r e s e n t and on a f r e s h c a t a l y s t ( c u r v e C ) .
= -8-
n-C7:S
n-ms
(AMMONU DKSORPTION)
0
0
Fig.
9
.
10
20
SO
40
50
80
70
80
HYDROCRACKING (Wtz)
90
I s o m e r i z a t i o n v e r s u s c r a c k i n g f o r Runs 1. 2 , 5, 6 , 7.
100
312 appears
It
that
isomerization experiments
and
ammonia cracking.
performed
s t r o n g l y modifies p r o d u c t d i s t r i b u t i o n between The
cracking
r e a c t i o n i s more pronounced f o r
w i t h o u t n i t r o g e n . The a c t u a l parameter g o v e r n i n g t h i s
s e l e c t i v i t y i s i n f a c t t h e s u r f a c e s t a t e o f t h e c a t a l y s t . The v a l u e s r e l a t i n g t h e d e s o r p t i o n experiments ( c u r v e B) are s i t u a t e d between c u r v e s A and C,
to
despite
the
zero
ammonia p r e s s u r e and some d o t s c a t t e r i n g can b e observed.
T h i s i s due t o t h e f a c t t h a t measurements were made w i t h c a t a l y s t s undergoing ammonia
desorption,
and
hence c o n t a i n i n g d i f f e r e n t p r o p o r t i o n s o f poisoned
sites. ISOMER SELECTIVITY Figure pressure
10
compares
isoheptanes
s e l e c t i v i t y f o r Runs 1 and 2 , f o r which
r e s p e c t i v e l y 0 and 6 . 1 kPa.
was
1.00
c E
!i w
rn
0.00 I
F i g . 10 The
. I s o h e p t a n e s s e l e c t i v i t y as a f u n c t i o n
first
di-branched when yield
of conversion.
o b s e r v a t i o n i s t h a t , f o r each r u n , t h e p r o p o r t i o n s o f mono- and isomers are f a i r l y s t a b l e as a f u n c t i o n o f conversion. Secondly,
t h e r e a c t i o n i s performed i n t h e p r e s e n c e o f ammonia, t h e methylhexanes increases
selectivity
remains
to
the
detriment
of
dimethyl
pentanes.
Ethylpentane
roughly unmodified. F i g u r e 11 shows t h a t t h e p r o p o r t i o n
o f i s o b u t a n e i n t h e cracked p r o d u c t s d e c r e a s e s i n t h e p r e s e n c e o f ammonia.
313
P(NH3) =
0
p(m3)=
6.1 I(PA
---- --
-
YP.4 :IcI/ICI+NCI
=a
._ _ _ _ _ . _ _ _ . _ _
id/ICI+NCI = 0
- - - - -
- - _ _ - - - - - - - - - _-
Thermodynamic equilibrium
I
10
F i g . 11
. Isobutane
'
20
I
~
30
l
40
'
l
50
'
l
60
'
CONVERSION (WTZ)
l
70
~
I
s e l e c t i v i t y a s a f u n c t i o n o f conversion.
DISCUSSION CONCERNING PRODUCT DISTRIBUTION The e f f e c t s of ammonia p r e s s u r e on p r o d u c t d i s t r i b u t i o n can b e summarized as follows: (1) Isomers y i e l d is i n c r e a s e d t o t h e d e t r i m e n t of cracked p r o d u c t y i e l d f o r a g i v e n conversion. ( 2 ) Mono-branched isomers are favored with
respect
t o di-branched ones. ( 3 ) Cracked p r o d u c t s are less isomerized.
An e x p l a i n a t i o n must be found t o account f o r a l l t h e s e e x p e r i m e n t a l r e s u l t s . The
first
temperature
observation increase
of
is t h a t t h e p r e s e n c e o f ammonia r e s u l t s i n 90 t o 100 'C t o o b t a i n t h e same conversion. Hence are d i f f e r e n t between t h e two sets of c o n d i t i o n s .
thermodynamic c o n d i t i o n s The is0 t o normal r a t i o p r e d i c t e d by t h e thermodynamic e q u i l i b r i u m d e c r e a s e s when temperature i n c r e a s e s . However, as shown i n F i g u r e 11, t h e v a r i a t i o n s of t h e e q u i l i b r i u m are t o o small t o e x p l a i n o u r r e s u l t s .
ammonia p r e s s u r e r e s u l t s in a p o i s o n i n g o f a c i d i c s i t e s . Many sites are a v a i l a b l e f o r t h e r e a c t i o n and t h i s e f f e c t is compensated f o r
Obviously,
less
by t h e t e m p e r a t u r e i n c r e a s e . T h i s means t h a t t h e a v e r a g e t u r n o v e r number is g r e a t e r , and t h a t t h e r e s i d e n c e time of carbenium i o n s on a c i d i c sites i s s h o r t e r . On t h e o t h e r hand. i t has been c l e a r l y e s t a b l i s h e d from t h e l i t e r a t u r e t h a t t h e i s o m e r i z a t i o n mechanism proceeds through a carbenium i o n rearrangement. For n-heptane, t h e scheme is (1) a d s o r p t i o n o f n-heptene coming
from
NiMo sites t o produce a normal carbenium i o n . ( 2 ) i s o m e r i z a t i o n
314 to
a
are
isomer and e v e n t u a l l y t o a dibranched isomer. These i o n s
monobranched in
equilibrium
corresponding
between
heptenes.
adsorption/desorption
themselves,
The
and
a l s o i n equilibrium with t h e
h i g h e r t h e t e m p e r a t u r e , t h e more d i s p l a c e d t h e
i s toward o l e f i n s . W e c a n assume t h a t t h e
equilibrium
s t a t e o f t h e average carbenium i o n s l y i n g on t h e s u r f a c e is d i f f e r e n t w i t h or without
ammonia.
t h e l a t e r case, w i t h a l o n g r e s i d e n c e t i m e , t h e a c i d i c
In
s i t e s would b e mostly covered by dibranched i o n s . A t h i g h e r t e m p e r a t u r e s , i n t h e p r e s e n c e o f ammonia, t h e i o n s would be less isomerized. I n t h i s c a s e , t h e cracking would
would be reduced, because c r a c k i n g o f t h e monobranched C7
tendency
carbenium
is
ion
also
isobutane
much
explain
molecule
slower
the
t h a n f o r t h e dibranched i o n . Moreover, t h i s
d e c r e a s e i n i s o b u t a n e i n t h e c r a c k e d p r o d u c t s . The
o r i g i n a t e s e s s e n t i a l l y from di-branched i o n s by a t y p e B
b e t a - s c i s s i o n mechanism , a c c o r d i n g t o t h e c l a s s i f i c a t i o n proposed r e c e n t l y ( 1 6 ) . So, t h e poisonous e f f e c t o f ammonia would r e s u l t i n a lower isomerization
degree
o f t h e carbenium i o n , which i n t u r n would i n d u c e fewer
monobranched heptane i s o m e r s , fewer c r a c k i n g r e a c t i o n s and fewer i s o b u t a n e i n cracked p r o d u c t s . CONCLUSION The
nitrogen
Operating feed
so
content,
feed
level
conditions
can
be
hydrocracking
of
hydrocracker f e e d s l e a d s t o s e v e r a l consequences.
of t h e f i r s t - s t e p h y d r o t r e a t m e n t depend on t h e n i t r o g e n
that
t h e n i t r o g e n s p e c i f i c a t i o n s o f t h e second-step i n l e t
reached. section
Moreover,the
ammonia
partial
pressure
of
the
w i l l depend on t h e n i t r o g e n l e v e l and a l s o on t h e type
o f p r o c e s s used, whether s i n g l e - s t a g e o r two-stage. Ammonia
pressure
hydrocracking
dramatic quantity
A
e f f e c t on t h e a c t i v i t y o f t h e z e o l i t e of
1000 w t ppm
nitrogen
added t o an
f e e d , g e n e r a t i n g a p a r t i a l p r e s s u r e o f 6.1 kPa. i s r e s p o n s i b l e f o r
n-heptane an
a
has
catalyst.
loss
activity
temperature
a
of
increase
reversible
for
desorption
is
factor
of
a wide slow,
of
40,
which can be compensated f o r by a
100 " C . Ammonia a d s o r p t i o n i s q u i c k l y p r e s s u r e . However, a t z e r o p r e s s u r e , t h a t ammonia remains s t r o n g l y adsorbed on t h e to
90
range
of
meaning
most a c i d i c sites o f z e o l i t e . The
presence
selectivity.The products.
isomer
ammonia
of
These
isomers
effects
isomerization
in
modifications
in
product
i n c r e a s e d t o t h e d e t r i m e n t o f cracked are f a v o r e d and cracked p r o d u c t s are less
be r a t i o n a l i z e d by assuming a lower average
can
of
results
is
yield
Monobranched
isomerized. degree
of
carbenium
ions,
due
to
a sharp decrease i n
r e s i d e n c e time a t t h e p r o t o n i c sites. From
a
refractory that
process
point
feedstocks
operating
of
view,
containing
conditions
must
t h e r e i s a problem o f how t o cope w i t h
large
amounts o f n i t r o g e n . I t i s obvious
be a d j u s t e d t o compensate f o r t h e a c t i v i t y
315
loss either by temperature increase or by space velocity decrease. However, the start-of-run temperature must remain inside reasonable limits (e.g. 400 'C), to prevent a quick deactivation of the catalytic system because of thermodynamic limitations for polyaromatics hydrogenation. Of course these limits depend on hydrogen pressure. For very difficult cases, a solution may also be to choose a two-stage process configuration, which makes for a substantial reduction of ammonia pressure in the hydrocracking section. Likewise, the replacement of amorphous type catalyst by more active zeolite catalysts results in a decrease in operating temperature, which is especially appreciated in this case of refractory feeds.
REFERENCES N.Choudary and D.N.Saraf, Ind.Eng.Chem.,Prod.Res.Dev., 1 4 ( 2 ) , 74, 1975. 1 2 A.Billon, J.P.Franck and J.P.Peries, Hydroc. Proc., 139, Sept 1975 C.E.Maier. P.H.Biaeard. A.Billon and P.Dufresne. NPRA Annual Meeting. 3 -. San Antonio, March 19g8. . P.Dufresne. P.I!.Bineard and A. Billon, Catalysis Today, 1. 367. 1987 4 C .Marcilly and JTP. Franck, Catalysis by-Zeolites; .B.Imelik et a1 Eds , 5 Elsevier. Amsterdam, 93. 1980. 6 G.E.Lang1oi.s and R.F.Sulivan, Adv.Chem.Ser. 97, 38, 1969 M.Guisnet and 3.Perot. Proc. NATO Advanced Study Institute on Zeolites, 7 Portugal, 39, May 1983. J.Weitkamp. P.A.Jacob and J.A.Martens, Appl. Catal., 20, 239 and 283, 8 1986 H.L.Coonradt and W.E.Garwood,Ind. Eng. Chem. Proc Des. Dev., 3, 38, 1964 9 10 J.Weitkamp, Ind. Eng. Chem. Prod. Res. Dev. 21, 550, 1982 11 G.Giannetto, G.Perot and M.Guisnet, Ind. Eng. Chem. Prod Res. Dev., 25, 481, 1986. 12 M.Steijns and G.F.Froment, Ind. Eng. Chem. Proc Des. Dev., 20, 660, 1981. and A.Billon, Franco-Venezuela Conference, 13 P.Dufresne, C.Marcilly Rueil-Malmaison, France, 1985. 14 F.Y. El-Kady, Egypt. J. Chem., 21, 5, 349, 1978. 15 H.Dauns, S.Ernst and J.Weitkamp, Proc. 7th Int. Zeol. Conf.. Murakami et a1 Eds., 787, Kodansha, Tokyo and Elsevier, Amsterdam, 1986. 16 J.Weitkamp, P.A.Jacob and J.A.Martens. Appl. Catal., 8, 123, 1983