Note: Descriptions are shown in the official language in which they were submitted.
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K 9306
PRfCESS FOR THE HYDROTRE~TING OF A HE~VY OIL
me invention relates ~o a process for the hydrotreating
of a heavy oil by leading the heavy oil and hydrogen at
elevated temperature and pressure cc~urrently in dcwnward
direction through a reactor which contains at least one bed
of a solid catalyst.
In the context of this specification and claims the term
hydrotreatment is used for conversion processes in which
heavy oils are converted in the presence of hydrogen. These
conversion processes camprise in particular demetallization,
desulphurization, denitrogenation, asphaltene conversion and
hydrocracking.
me term heavy oils is used in this specification and
claims for mQxtures of hydrocarbons which are at least for
the greater part in the lit~lid phase at the conditions of
tt-3nperature and pressure prevailing in the reactor during
normal operation of the hydrotreating process. As examples
of heavy oils may be mentioned crude mineral oils, topped
mineral oils, residues of atmospheric or vacuum distillation
of mineral oils, deasphalted residual oils, asphalts, shale
oils, oils c-~btained from tar sands.
Hydrotreatment of heavy oils is conventionally carried
out by leading the oil together with hydrogen (in this
specification the word hydrogen stands for pure hydrogen as
well as for hydrogen-containing gases) in t~c~wnward direction
thraugh a reactor which contains at least one bed of a solid
catalyst. The heavy oil (also called the feed) trickles
around the catalyst particles at the surface of which the
reaction with hydrogen takes place. Because these reactions
are exothermic, sufficient cooling capacity must be present
during operation to control the temperature so as to avoid
the development of undesired high temperatures which may lead
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to deactivation of the catalyst, coke formation, plugging of
~the catalyst bed and exposing the reactor walls to higher
temperatures than those for which they are designed. This
cooling capacity (also called heat-sink) is provided by the
hydrogen containing gas (which may include recycle gas) and
the heavy oil flawing through the reactor.
Apart frcm controlling the temperature rise over all
individu21 beds in the reactor the tl3mperature at the outlet
of each bed has to be reduced to the desired temperature at
the inlet of the next bed. This cooling is in many cases
accGmplished by injection between the catalyst beds of fresh
hydrogen and/or, hydrogen-containing recycle gas at a temr
perature lower than the temperature prevailing in the re-
actor.
1 5 Hawever it may happen that the forwarding of heavy oil
or the recycle of gas to the reactors is interrupted by
malfunctiomng of equipment. In particular the heavy oil flcw
may be hamçered or completely disrupt~Kd. S mce the heavy oil
present in the reactors will continue to react, the heat sink
as provided by the gas flow on its own might become insuffi-
cient then and temperatures could increase to undesirable
heights (so-called "temperature runaway"). In order to avoid
such a temperature runaway in case e.g. the supply of heavy
oil to the reactor is decreased or interrupted, the hydro-
treatment process has to be brought to a standstill bydepressurizing the reactor and discontinuing the heating of
the gas and oil supply.
It would be attractive to have provisions which enable
avoidance of temperature runaway in the first catalyst bed
u~der all circumstances, even during an interruption of heavy
oil or hydrogen supply to that first catalyst bed. It is
possible to provide sufficient heat sink in the first
catalyst bed - even in case of interruption of feed supply -
by continuous injection of a larger amount of hydrogen and/or
hydrogen containing recycle gas into the reactor upstream of
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the first catalyst bed than what is normally applied for
undisturbed feed flow. However, for such a purpose gas
campressors with extra high capacity would be needed, which
is very unattractive for technological and econamical reasons.
It has now been found that sufficient heat sink for the
first catalyst bed can also be achieved by injection of a
hydrocarbon muxture instead of part of a hydrogen containing
gas into the reactor upstream of the uppermost (also called
first) catalyst bed.
Accordingly the present invention relates to a process
for the hydrotreating of a heavy oil by leading the heavy oil
and hydrogen at elevated temperature and pressure cocurrently
in downward direction through a rPactor which contains at
least one bed of a solid catalyst, in which process also a
hydrocarbon mixture, which is at least for the greater part
in the gaseous phase at the conditions prevailing in the
reactor, is introduced into t~e reactor at a point upstream
of the uppermost bed of solid catalyst.
The hydrocarbon mixture which is at least for the
2a greater part in the gaseaus phase at the conditions pre-
vailing in the reactor is very suitably braught at reactor
pressure in the liquid state with the aid of a pump. In this
way the need for using gas compressors with a high capacity
is avercome. Part of this hydrocarbon mixture may evaporate
between the said pump and its entrance into the reactor owing
to heating or heat exchange with other streams.
The said hydrocarbon mixture will act as a heat sink in
the first reactor bed due to its heat capacity and heat of
evaporation, and it may replace part of the hydrogen-contain-
3o ing gas in this respect during normal aperation. It ispreferred that such an amount of said hydrocarbon mixture is
introduced into the reactor that temperature runaway in
the uppermost catalyst bed does not occur when the supply of
heavy oil or hydrogen is interrupted.
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Althcugh the presence of (part of the) said hydrocarbon
mixture is needed when malfunctioning occurs, lt is of
advantage and preferred to introduce continuously said
hydrocarbon mlxture into the reactor , in order to avoid any
risk of malfunctioning of instrumentation or equipment which
might occur in the absence of this ~uxture.
It is of advantage to use independent means (e.g.
separate liquid pumps with different energy sources) for the
introduction into the reactor of the said hydrocarbon mixture
and the feed respectively. In that way the supply of the said
hydrocarbon mixture is ascertained, even if the feed pump
malfunctions or falls out completely. It is of course pos-
sible to m~x the feed and the said hydrocarbon mlxture
downstream of the feed pump and injecting the mixture thus
obtained upstream of the first catalyst bed, provided the
said hydrocarbon mixture is transported with the aid of a
separate pump with a different en~rgy source.
The hydrocarbon mixture which is at least for the
greater part in the gaseous phase at the conditions prevai-
ling in the reactor and which is introduced into the reactor
at a point ~pstream of the uppermost bed of solid catalystvery convemently consists of a fraction of the reactor
effluent.
In general the effluent of the reactor which consists
of hydrotreated heavy oil and a hydrogen-containing gas is
separated in high temperature ("hot") separators and low
temperature ("cold") separators conæ cutively, yielding
gaseous and liquid products. Liquid product from the cold
separators (which consists of condensable compcunds of the
3a gaseous product from the hot separators) is very suitable to
be used as the said hydrocarbon mixture.
The amLunt of said hydrocarbon mixture, preferably
liquid product from the cold separators, which is to be
introduced in order to have available sufficient cooling
3S capacity to avoid temperature runaway in the uppermost
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catalyst bed under all circumstances, even in case the feed
supply or the hydrogen supply is interrupted, will depend on
the type of feed, the type and degree of feed conversion to
be achieved dNring normal cperation, the reaction conditions
and the catalyst. For each specific case the munimum amount
of the said hydrocarbon mixture which is to be introduced
into the reactor must be determined by experiments on a small
scale and/or calculations.
In most cases it is also desirable to introduce fresh
hydrogen or fresh hydrogen-containing gas and recycle gas
(which consists for the greater part of molecular hydrogen)
into the reactor between the catalyst beds in order to reduce
the inlet temperature of the next bed and thereby avoiding
temperature runaways in subsequent catalyst beds. Part or all
of these gases may be replaced by the said hydrocarbon
mixture, which can be introduced in the liquid phase between
the catalyst beds. In general about 10% - 85% vol. of recycle
gas can be replaced by a hydrocarbon mixture. Preference is
given to the use of hydrocarbon mixtures of which about 70%
wt. is in the vapour phase at the conditions prevailing in
the reactor.
Because of the much larger molecular weight of the
vaporized part of the hydrocarbon mixture rel~tive to the
recycle gas, the total volume flow of the gas flcw through
the catalyst bed(s) is reduced markedly. The part of the
hydrocarbon mixture which is still in the liquid phase in the
reactor has an advantageous viscosity reducing effect on the
heavy oil. The reduction in pressure drop achieved by these
effects is very pro unced. Consequently in the process
acco-ding to the invention use can be made of recycle gas
compressors with a lcwer capacity (in terms of gas rate) and
a lower differential pressure, as compared with a process in
which as a heat sink use is made of feed and hydrogen exclu-
sively.
~230571
m e composition of the catalyst will be adapted to the
reaction desixed. In general supported catalysts will be
used, the supports very conveniently being amorphous refrac-
tory oxides (or mixtures thereof) of elements of Group II,
III and IV of the Periodic Table of Elements e.g. magnesia,
silica, alumina, zirconia, silica-all~nina, silica-zirconia.
Supports consisting of crystalline materials, such as zeolites
may also be used.
One or more metals (and/or ccmpounds thereof) with
hydrogenating activity are very suitably present onto the
supports, in particular metals of Group VB, VIB, VIIB and/or
VIII of the Periodic Table of Elements. For example, in case
hydrodesulphurization is the most desired reaction to take
place, catalysts which contain compounds of cobalt and/or
nickel together with compounds of molybdenum and/or tungsten
on alumina as a support are very suitable. In case hydrode-
metallization is the most desired reaction, catalysts based
on silica as a support and containing only compounds of
molybdenum, or a combination of ccmpounds of nickel and
vanadium, respectively, are very convenient.
The catalyst particles present in the beds may have any
suitable form, e.g. powders, spheres, pellets, cylindrical
ex~trudates, multilobed extrudates, rings and the like.
Cylindrical extrudates with a diameter from 0.5 to 2.5 mm are
very suitable in general.
The reaction conditions prevailing in the reactor will
be adapted to the hydrotreating reaction desired. In general
temperatures from 300-450C, total pressures from 25-300 bar,
hydrogen partial pressures from 25-250 bar, and space veloci-
ties of 0.l-l0 kg feed per kg catalyst per hour will be very
suitable.
EXWMPLE
Four hydrotreatment experiments are carried out with a
short residue of a Middle East crude as feed. This feed is
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led in all cases in downw æd direction through two reactors
in series, each of which contains three beds of catalyst.
The catalyst (in the form of extrudates with 0.8 mm
diameter) consists of an alumina support onto which nickel
oxide and molybdenum oxide have been applied; the cata]yst is
sulphided before use.
me feed of fresh hydrogen cont~;n;ng gas (95% vol. pure
hydrogen, 5% vol. methane) is 225 nm3/ton feed. The off-gas
of the reactors is (after removal of H2S) recycled and
introduced into the reactor upstream of the first catalyst
bed. The gases æe led over the catalyst cocurrently with the
feed. The reactor pres Æ es æe adapted so as to have an
average hydrogen partial pressure of 150 bar in all cases.
In experiments 2 and 4 half of the recycle gas is
replaced by a hydrocarbon mixture of which abaut 70% wt. is
in the vapour phase at the conditions prevailing in the
reactors. This hydrocarbon mixture is brought to reactor
pressure in the liquid phase and introduced into the first
reactor upstream of the first catalyst bed after heating.
In experiments 1 (a ccmp æ ative experiment) and 2
(experiment according to the invention) the average reactor
temperature is 380C. The amount of recycle gas in experiment
1 as well as the amount of recycle gas together with the
amount of hydroc æbon mixture in experiment 2 are sufficient
to avoid temperature runaway in case the feed flcw is inter-
rupted. From the table it can be seen that the pressure drop
~p) in experiment 1 (73 b æ) is much higher than that in
experiment 2 (14 bar). In order to overcome the pressure drop
the pressure at the inlet of the first reactor m~st be higher
3Q in experiment 1 than in experiment 2. Accordingly the equip-
ment of experiment 1 must be designed to withstand higher
pressures than that of experiment 2, which of course is
unattractive from an economical point of view. Moreover, the
gas ccmpressor for the recycle gas can be much smaller in
experiment 2 than in experiment 1.
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In experiments 3 (comparative) and 4 (according to the
invention), in which the average reactor temperature is
390C, the amount of recycle gas and the amounts of recycle
gas plus hydrocarbon mixture, respectively, are not suffi-
cient to avoid temperature runaway ~n case of feed flcwinterruption. The advantages of experiment 4 in comparison
with experiment 3 as far as pressure drop (~p) and recycle
gas co~,pressor capacity are concerned are similar to those of
experiment 2 in ccmparison with experiment 1. Temperature
runaway can be avoi~ed when operating the reactors at 390C
by increasing the hydrocarbon mixture/feed ratio to 1.83. The
temperature rise in the first bed with feed flow amounts to
26C; in the absence of feed flow to 46C.
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g
,~BLE
Exp. 1 2 3 4
Space- ton feed/ 0.37 0.37 0.37 0.37
velocity m3 cat. hr
Fresh gas Nm3/ton feed 225 225 225 225
flow
Recycle Nm3/ton feed3640 1820 3640 1820
gas flow
hydrocarbon ton/ton feed - 0.98 - 0.98
mixture flow
average C 380 380 390 390
reactor temp.
temp. rise in C 23 23 44 44
first bed with
feed flow
temp. rise in C 46 46 runaway
first bed no
feed flow
H2 in reactor gas %vol. 86 80 86 80
pressure drop bar 73 14 69 14
over reactors
average reactor bar 175 188 175 188
pressure
inlet pressurebar 212 195 209 195
first reactor
