BACKGROUND OF THE INVENTION
[0001] The present invention is directed at a process for catalytic cracking of hydrocarbon
feedstocks. More specifically, the present invention is directed at a method for reducing
the detrimental effects of metal contaminants such as nickel, vanadium and/or iron,
which typically are present in the hydrocarbon feedstock processed and are deposited
on the cracking catalyst.
[0002] In the catalytic cracking of hydrocarbon feedstocks, particularly heavy feedstocks,
nickel, vanadium and/or iron present in the feedstocks become deposited on the cracking
catalyst promoting excessive hydrogen and coke makes. These metal contaminants are
not removed by conventional catalyst regeneration operations, which convert coke deposits
on the catalyst to CO and C0
2. As used hereinafter, the term "passivation" is defined as a method for decreasing
the detrimental catalytic effects of metal contaminants such as nickel, vanadium and/or
iron which become deposited on the cracking catalyst.
[0003] Several patents disclose the use of a reducing atmosphere to passivate cracking catalyst.
U. S. Patent No. 2,575,258 discloses the addition of a reducing agent to regenerated
catalyst at a plurality of locations in the transfer line between the regeneration
zone and the cracking zone for countercurrent flow of the reducing gas relative to
the flow of the regenerated catalyst. This patent also discloses the addition of steam
to the transfer line downstream of the points at which reducing gas is added to the
transfer line to assist in moving regenerated catalyst from the regeneration zone
to the reaction zone. Countercurrent flow of the reducing gas relative to the catalyst
flow is not desirable, particularly at relatively high catalyst circulation rates,
since the catalyst and reducing gas will tend to segregate into two oppositely flowing
phases. This would result in poor catalyst contacting. Moreover, it is possible that
bubbles of countercurrently flowing reducing gas intermittently could interrupt the
recirculation of the catalyst.
[0004] International Patent Application (PCT) No. WO 82/04063 discloses in the processing
of metal-contaminated hydrocarbons, the addition of reducing gas to a stripping zone
disposed between the regeneration zone and the reaction zones to strip the catalyst.
This patent also discloses the addition of reducing gas to a separate vessel and/or
to the riser downstream of the flow control means to reduce at least a portion of
the oxidized nickel contaminants present.
[0005] European Patent Publication No. 52,356 also discloses that metal contaminants can
be passivated utilizing a reducing atmosphere at an elevated temperature. This publication
discloses the use of carbon monoxide, hydrogen, propane, methane, ethane, and mixtures
thereof as reducing gases for passivating regenerated catalyst before the catalyst
is returned to the reaction zone. This publication also discloses that the contact
time of the reducing gas with the catalyst may range between 3 seconds and 2 hours,
preferably between about 5 and 30 minutes. This patent publication further discloses
that the degree of passivation is improved if antimony is added to the cracking catalyst.
[0006] U. S. Patent No. 4,377,470 discloses a process for catalytic cracking of a hydrocarbon
feed having a significant vanadium content. Reducing gas may be added to the regenerator
and to the transfer line between the regenerator and the reactor to maintain the vanadium
in a reduced oxidation state.
[0007] U. S. Patent Nos. 4,280,895; 4,280,896; 4,370,220; 4,372,840; and 4,372,841 disclose
that cracking catalyst can be passivated by passing the catalyst through a passivation
zone, having a reducing atmosphere maintained at an elevated temperature for a period
of time ranging from 30 seconds to 30 minutes, typically from about 2 to 5 minutes.
[0008] U. S. Patent No. 4,298,459 and 4,280,898 describe processes for cracking a metals-containing
feedstock where the used cracking catalyst is subjected to alternate exposures of
up to 30 minutes of an oxidizing zone and a reducing zone maintained at an elevated
temperature to reduce the hydrogen and coke makes. This patent describes the use of
a transfer line reaction zone disposed between a regeneration zone and a stripping
zone.
[0009] U. S. Patent No. 4,268,416 also describes a method for passivating cracking catalyst
in which metal contaminated cracking catalyst is contacted with a reducing gas at
elevated temperatures to passivate the catalyst.
[0010] U. S. Patent No. 3,408,286 discloses the addition of a liquid hydrocarbon to regenerated
catalyst under cracking conditions in a transfer line before the regenerated catalyst
is recharged to the cracking zone. The cracking of the liquid hydrocarbon prior to
entering the cracking zone operates to displace entrained regenerator gases from the
regenerated catalyst entering the cracking zone.
[0011] In many existing catalytic cracking systems it may not be desirable to add a separate
free standing passivation or reduction zone due to space limitations particularly
where the passivation zone is to be retrofitted in an existing cracking facility.
[0012] It also may be desirable to avoid the expense associated with the construction and
installation of a separate free-standing passivation zone.
[0013] It also may be desirable to provide a process which regulates the rate of addition
of reducing gas to the passivation zone.
[0014] It also may be desirable to provide a process which does not have a significantly
adverse effect on the catalyst circulation rate.
[0015] The present invention is directed at the incorporation of the reduction or passivation
zone into a transfer line. The passivation zone preferably is disposed in a transfer
line between the regeneration zone and the reaction zone, such that at least a portion
of the metal contaminated catalyst circulating between the regeneration zone and the
reaction zone passes through the passivation zone. The passivation zone preferably
is located in the transfer line through which regenerated catalyst passes from the
regeneration zone to the reaction zone. Depending upon the particular operating conditions
in the reaction zone and regeneration zone and the desired operating efficiencies,
it may be desirable to dispose a stripping zone before the passivation zone to minimize
the amount of oxygen carried into the passivation zone by the regenerated catalyst.
SUMMARY OF THE INVENTION
[0016] The present invention is directed at an improved method for passivating catalyst
used to crack hydrocarbon feedstock to lower molecular weight products in a reaction
zone where the feedstock contains a metal contaminant selected from the group consisting
of nickel, vanadium, iron and mixtures thereof, where at least some of the metal contaminant
becomes deposited on the catalyst and where coke becomes deposited on the cracking
catalyst. Coke and metal contaminated catalyst is transferred from the reaction zone
to a regeneration zone for removal of coke therefrom. Regenerated catalyst is circulated
from the regeneration zone to the reaction zone through a transfer zone. The improvement
comprises adding a H
2-containing reducing gas to the transfer zone to thereby provide a passivation zone
wherein cracking catalyst passing therethrough is at least partially passivated.
[0017] The H
2-containing reducing gas (hereinafter referred to as "reducing gas") preferably is
selected from the group consisting of hydrogen and mixtures with CO and/or hydrocarbons.
A stripping gas may be added at the point where the catalyst enters the transfer zone,
the stripping gas removing at least a portion of the oxygen and/or oxygenated compounds
present on the catalyst. To maximize the contact time of the reducing gas in the transfer
zone, reducing gas preferably is added to the transfer zone upstream of the flow control
means. Preferably the reducing gas is added to the standpipe from the regeneration
zone. The rate of addition of reducing gas to the passivation zone preferably is regulated.
This may be accomplished by monitoring the composition of the cracked product stream
from the reaction zone and/or the catalyst circulation rate. When the reducing gas
added to the passivation zone comprises hydrogen, the rate of reducing gas added to
the passivation zone may be regulated to minimize the hydrogen concentration in the
cracked product. The reducing gas addition rate may be regulated to maintain the catalyst
circulation rate within a predetermined range.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018]
Figure 1 is a simplified schematic drawing of one embodiment for practicing the subject
invention.
Figure 2 is a simplified schematic drawing of an alternate embodiment for practicing
the subject invention.
Figure 3 presents plots of hydrogen yield and coke yield as a function of the residence
time of metals contaminated catalyst in a passivation zone.
Figure 4 presents a plot of the Gas Producing Factor as a function of catalyst residence
time in a passivation zone, the reducing gas utilized and the temperature of the passivation
zone.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Referring to Figure 1, one method for practicing the subject invention is shown.
In this drawing pipes, valves, instrumentation, etc. not essential to an understanding
of the invention have been deleted for simplicity. Reaction or cracking zone 10 is
shown containing a fluidized catalyst bed 12 having a level at 14 in which a hydrocarbon
feedstock is introduced into the fluidized bed through line 16 for catalytic cracking.
The hydrocarbon feedstock may comprise naphthas, light gas oils, heavy gas oils, residual
fractions, reduced crude oils, cycle oils derived from any of these, as well as suitable
fractions derived from shale oil kerogen, tar sands, bitumen, processing, synthetic
oils, coal hydrogenation, and the like. Such feedstocks may be employed singly, separately
in parallel reaction zones, or in any desired combination. Typically, these feedstocks
will contain metal contaminants such as nickel, vanadium, and/or iron. Heavy feedstocks
typically contain relatively high concentrations of vanadium and/or nickel. Hydrocarbon
gas and vapors passing through fluidized bed 12 maintain the bed in a dense, turbulent,
fluidized condition.
[0020] In reaction zone 10, the cracking catalyst becomes spent during contact with the
hydrocarbon feedstock due to the deposition of coke thereon. Thus, the terms "spent"
or "coke-contaminated" catalyst as used herein generally refer to catalyst which has
passed through a reaction zone and which contains a sufficient quantity of coke thereon
to cause activity loss, thereby requiring regeneration. Generally, the coke content
of spent catalyst can vary anywhere from about 0.5 to about 5 wt.% or more. Typically,
spent catalyst coke contents vary from about 0.5 to about 1.5 wt.%.
[0021] Prior to actual regeneration, the spent catalyst is usually passed from reaction
zone 10 into a stripping zone 18 and contacted therein with a stripping gas, which
is introduced into the lower portion of zone 18 via line 20. The stripping gas, which
is usually introduced at a pressure of from about 10 to 50 psig, serves to remove
most of the volatile hydrocarbons from the spent catalyst. A preferred stripping gas
is steam, although nitrogen, other inert gases or flue gas may be employed. Normally,
the stripping zone is maintained at essentially the same temperature as the reaction
zone, i.e., from about 450°C to about 600°C. Stripped spent catalyst from which most
of the volatile hydrocarbons have been removed, is then passed from the bottom of
stripping zone 18 through U-bend 22 and connecting vertical riser 24, which extends
into the lower portion of a regeneration zone. Air is added to riser 24 via line 28
in an amount sufficient to reduce the density of the catalyst flowing therein, thus
causing the catalyst to flow upward into regeneration zone 26 by simple hydraulic
balance.
[0022] In the particular configuration shown, the regeneration zone is a separate vessel
(arranged at approximately the same level as reaction zone 10) containing a dense
phase catalyst bed 30 having a level indicated at 32, which is undergoing regeneration
to burn-off coke deposits formed in the reaction zone during the cracking reaction,
above which is a dilute catalyst phase 34. An oxygen-containing regeneration gas enters
the lower portion of regeneration zone 26 via line 36 and passes up through a grid
38 in the dense phase catalyst bed 30, maintaining said bed in a turbulent fluidized
condition similar to that present in reaction zone 10. Oxygen-containing regeneration
gases which may be employed in the process of the present invention are those gases
which contain molecular oxygen in admixture with a substantial portion of an inert
diluent gas. Air is a particularly suitable regeneration gas. An additional gas which
may be employed is air enriched with oxygen. Additionally, if desired, steam may be
added to the dense phase bed along with the regeneration gas or separately therefrom
to provide additional inert diluents and/or fluidization gas. Typically, the specific
vapor velocity of the regeneration gas will be in the range of from about 0.8 to about
6.0 feet/sec., preferably from about 1.5 to about 4 feet/sec.
[0023] In regeneration zone 26, flue gases formed during regeneration of the spent catalyst
pass from the dense phase catalyst bed 30 into the dilute catalyst phase 34 along
with entrained catalyst particles. The catalyst particles are separated from the flue
gas by a suitable gas-solid separation means 54 and returned to the dense phase catalyst
bed 30 via diplegs 56. The substantially catalyst-free flue gas then passes into a
plenum chamber 58 prior to discharge from the regeneration zone 26 through line 60.
Where the regeneration zone is operated for substantially complete combustion, the
flue gas typically will contain less than about 0.2, preferably less than 0.1 and
more preferably less than 0.05 volume % carbon monoxide. The oxygen content usually
will vary from about 0.4 to about 7 vol.%, preferably from about 0.8 to about 5 vol.%,
more preferably from about 1 to about 3 vol.%, most preferably from about 1.0 to about
2 vol.%.
[0024] Regenerated catalyst exiting from regeneration zone 26 preferably has had a substantial
portion of the coke removed. Typically, the carbon content of the regenerated catalyst
will range from about 0.01 to about 0.6 wt.%, preferably from about 0.01 to about
0.1 wt.%. The regenerated catalyst from the dense phase catalyst bed 30 in regeneration
zone 26 flows through the transfer zone comprising standpipe 42 and U-bend 44 to reaction
zone 10.
[0025] In Figure 1, passivation zone 90 extends for substantially the entire length of standpipe
42 and U-bend 44 to the point where hydrocarbon feedstock enters through line 16 to
gain substantially the maximum possible residence time. If a shorter residence time
is desired, passivation zone 90 could comprise only a fraction of the length of standpipe
42 and/or U-bend 44. Conversely, if a greater residence time were desired, the cross-sectional
area of standpipe 42 and/or U-bend 44 could be increased. Stripping gas streams optionally
may be added at the inlet of passivation zone 90 to minimize the intermixing of regeneration
zone gas with the passivation zone reducing gas. The stripping gas may be any which
will not adversely affect the passivated catalyst and which will not hinder the processing
of the feedstock in the reaction zone. A preferred stripping gas is steam although
reducing gas and other gas also may be satisfactory. In this embodiment, line 92 is
disposed upstream of passivation zone 90, to minimize intermixing of the reducing
atmosphere in passivation zone 90 with the gas stream from regeneration zone 26 by
stripping out entrained oxygen from the regenerated catalyst.
[0026] Since the catalyst residence time in standpipe 42 and U-bend 44 typically may range
only from about 0.1 to about 2 minutes, often it may be advantageous to maximize the
effectiveness of the catalyst residence time in passivation zone 90 by injecting increasing
quantities of reducing gas into the passivation zone until the additional reducing
gas ceases to produce benefits in the cracking process. This may occur if the addition
of reducing gas adversely affects the catalyst flow rate through the passivation zone.
This also may occur when the incremental increase in the rate of reducing gas addition
to the passivation zone does not result in a decrease in the hydrogen and/or coke
make in reaction zone 10. In Figure 1, the reducing gas flow rate through line 70
is regulated by a control means, such as control valve 72. Reducing gas passing through
control valve 72 in line 70 subsequently passes through a plurality of lines such
as 74, 76, 78, 80, and 96 to distribute the reducing gas into passivation zone 90.
Control valve 72 is shown being regulated by a cracked product monitoring means, such
as analyzer 82. Analyzer 82 may be adapted to monitor the content of one or more products
in stream 52. Since the hydrogen content of the cracked product is a function of the
degree of catalyst metals passivation, in a preferred embodiment, analyzer 82 may
be a hydrogen analyzer. Alternatively, since the rate of coke production also is a
function of the degree of catalyst metals passivation, the rate of reducing gas addition
also could be regulated by monitoring the rate of coke production. This may be accomplished
by monitoring the heat balance around reaction zone 10 and/or regeneration zone 26.
[0027] The rate of addition of reducing gas to passivation zone 90 also must be maintained
below the point at which it will cause a significant fluctuation in the catalyst circulation
rate. In the embodiment shown in Figure 1, the rate of catalyst circulation through
passivation zone 90 may be monitored by a sensing means, such as sensor 84, shown
communicating with regeneration zone 26, standpipe 42 and control valve 72.
[0028] In the commercial operation of this embodiment, the concentration of hydrogen in
product stream 52 may be monitored by analyzer 82, which adjusts the rate of addition
of reducing gas through control valve 72 to minimize the hydrogen content in stream
52. Sensor 84 operates as a limit on control valve 72, by decreasing the rate of addition
of reducing gas to passivation zone 90, when the rate of addition of reducing gas
begins to adversely affect the catalyst circulation rate.
[0029] Referring to Figure 2, an alternate embodiment for practicing the subject invention
is disclosed. The operation of this embodiment is generally similar to that previously
described in Figure 1. In this embodiment, riser reaction zone 110 comprises a tubular,
vertically extending vessel having a relatively large height in relation to its diameter.
Reaction zone 110 communicates with a disengagement zone 120, shown located a substantial
height above regeneration zone 150. The catalyst circulation rate is controlled by
a valve means, such as slide valve 180, located in spent catalyst transfer line 140,
extending between disengagement zone 120 and regeneration zone 150. In this embodiment
hydrocarbon feedstock is injected through line 112 into riser reaction zone 110 having
a fluidized bed of catalyst to catalytically crack the feedstock. Steam may be injected
through lines 160 and 162 in a second transfer zone, such as return line 158, extending
between regeneration zone 150 and reaction zone 110 to serve as a diluent, to provide
a motive force for moving the hydrocarbon feedstock upwardly and for keeping the catalyst
in a fluidized condition.
[0030] The vaporized, cracked feedstock products pass upwardly into disengagement zone 120
where a substantial portion of the entrained catalyst is separated. The gaseous stream
then passes through a gas-solid separation means, such as two stage cyclone 122, which
further separates out entrained catalyst and returns it to the disengagement zone
through diplegs 124, 126. The gaseous stream passes into plenum chamber 132 and exits
through line 130 for further processing (not shown). The upwardly moving catalyst
in reaction zone 110 gradually becomes coated with carbonaceous material which decreases
its catalytic activity. When the catalyst reaches the top of reaction zone 110 it
is redirected by grid 128 into stripping zone 140 in spent catalyst transfer line
142 where it is contacted by a stripping gas, such as steam, entering through line
144 to partially remove the remaining volatile hydrocarbons from the spent catalyst.
The spent catalyst then passes through spent catalyst transfer line 142 into dense
phase catalyst bed 152 of regeneration zone 150. Oxygen containing regeneration gas
enters dense phase catalyst bed 152 through line 164 to maintain the bed in a turbulent
fluidized condition, similar to that in riser reaction zone 110. Regenerated catalyst
gradually moves upwardly through dense phase catalyst bed 152 eventually flowing into
overflow well 156 communicating with return line 158. Return line 158 is shown exiting
through the center of dense phase catalyst bed 152, and communicating with riser reaction
zone 110.
[0031] Flue gas formed during the regeneration of the spent catalyst passes from the dense
phase catalyst bed 152 into dilute catalyst phase 154. The flue gas then passes through
cyclone 170 into plenum chamber 172 prior to discharge through line 174. Catalyst
entrained in the flue gas is removed by cyclone 170 and is returned to catalyst bed
152 through diplegs 176, 178. Regenerated catalyst is returned to reaction zone 110
from regeneration zone 150 through a transfer zone comprising overflow well 156 and
return line 158.
[0032] As previously indicated for the embodiment of Figure 1, a passivation zone, such
as passivation zone 190, may be disposed in or may comprise substantially all of overflow
well 156 and/or return line 158. Additional reducing gas may be added to passivation
zone 190 through lines 160 and 162 into return line 158. If the quantity of reducing
gas added through lines 160, 162 to passivate the catalyst is not sufficient to adequately
aerate the regenerated catalyst particles, it may be desirable to dilute the reducing
gas added through lines 160, 162 with steam or other diluent. As shown for the embodiment
of Figure 1, it may be desirable to add a stripping gas, such as steam, through line
192'to overflow well 156 to remove entrained oxygen from the regenerated catalyst.
[0033] The reducing gas preferably is added to passivation zone 190 at a plurality of locations
through branched lines, such as lines 202, 204, 206, 208, 210, and 212 extending from
reducing gas header 200. As previously described in Figure 1, a control means, such
as control valve 220 is disposed in reducing gas header 200 to regulate the rate of
addition of reducing gas to passivation zone 190. A cracked product monitoring means,
such as analyzer 230 is shown communicating with cracked product line 130 and with
control valve 220 to maintain the sampled cracked product component within the desired
limits by regulation of the rate of addition of reducing gas to passivation zone 190.
Since hydrogen is one of the products produced by the adverse catalytic properties
of the metal contaminants, hydrogen may be the preferred component to be regulated.
Since the metal contaminant also catalyzes the formation of coke, the rate of reducing
gas addition also could be regulated by the monitoring of the rate of coke production,
such as by monitoring the heat balance around regeneration zone 150, as previously
described. As in the embodiment of Figure 1, the rate of catalyst circulation may
be monitored by a sensing means, such as sensor 240, communicating with valve 220,
to control the maximum rate of addition of reducing gas to passivation zone 190. The
commercial operation of this embodiment would be substantially similar to that previously
described for the embodiment of Figure 1. A component in the product stream such as
hydrogen is monitored by analyzer 230, which directs control valve 220 to adjust the
rate of addition of reducing gas to passivation zone 190 to minimize the hydrogen
content in stream 130. Sensor 240, communicating with regeneration zone 150 and line
158, monitors the catalyst circulation rate and operates as an over-ride on control
valve 220, to reduce the rate of addition of reducing gas if the reducing gas has
or is about to have an adverse effect on the catalyst circulation rate.
[0034] The metals concentration deposited on the catalyst is not believed to differ significantly
whether the embodiment of Figure 1 or the embodiment of Figure 2 is used. Thus, the
amount of reducing gas which is consumed in passivation zone 90, 190 of the embodiments
of Figure 1, 2, respectively should not differ greatly.
[0035] While the reducing gas consumption rate in passivation zones 90, 190, of Figures
1, 2, respectively, will be a function, in part, of the metal contaminant levels on
the catalyst, the desired degree of passivation and the amount of reducing gas infiltration
into the regeneration zone, it is believed that the overall rate of consumption of
the reducing gas will range from about 0.5 to about 260 SCF, preferably from about
1 to about 110 SCF for each ton of catalyst passed through passivation zones 90, 190,
if hydrogen is utilized as the reducing gas.
[0036] In the embodiments of Figures 1 or 2 it is believed that the combustion of coke in
regeneration zones 26 or 150, respectively, will heat sufficiently the cracking catalyst
subsequently passed through passivation zones 90, 190, respectively. A temperature
of at least 500°C, preferably above about 600
0C, is necessary for adequate passivation of the catalyst. If the temperature of the
catalyst entering passivation zones 90 or 190 is not sufficiently high, additional
heat may be added to the passivation zone either directly, such as by the preheating
of the reducing gas, or by adding steam, or indirectly, such as by the addition of
a heat exchange means prior to, or within the passivation zone.
[0037] Reaction zones 10, 110 and regeneration zones 26, 150, of Figures 1, 2, respectively,
may be of conventional design and may be operated at conditions well-known to those
skilled in the art. Regeneration zones 26, 150 may be operated in either a net oxidizing
or a net reducing mode. In a net oxidizing mode, oxidizing gas in excess of that required
to completely combust the coke to C0
2 is added to the regeneration zone. In a net reducing mode insufficient oxidizing
gas is added to completely combust the coke to C0
2. It is believed that the regeneration zones 26 and 150 preferably should be operated
in a net reducing mode, since carbon monoxide is a reducing gas which will decrease
the adverse catalytic properties of the metal contaminants on the catalyst prior to
the catalyst entering passivation zones 90, 190.
[0038] The required residence time of the catalyst in passivation zones 90, 190 may be dependent
upon many factors including the metal contaminant content of the catalyst, the degree
of passivation required, the con- cenration of reducing gas in the passivation zone,
and the passivation zone temperature. If the residence time required is greater than
that available, certain changes may be made to increase the passivation zone capacity
and/or increase the rate at which the catalyst is passivated. This may be accomplished
by the addition to the catalyst of effective amounts of passivation zone rate enhancers,
such as cadmium, germanium, indium, tellurium, zinc, and tin or by the addition of
passivation promoters such as antimony, tin, bismuth and manganese.
[0039] Laboratory tests presented in the examples below have demonstrated that it may be
possible to achieve effective metals passivation in continuous operation utilizing
a transfer zone for metals passivation.
Example I
[0040] These tests demonstrated that effective metals passivation could be achieved in a
passivation zone having a relatively low hydrogen partial pressure. These tests were
conducted in a continuous circulatory pilot unit with an integral passivation zone
operated at 1300°F (704°C) using an equilibrium Super DX cracking catalyst manufactured
by Davison Chemical Co. a division of W. R. Grace and Co. The catalyst, which had
190 wppm nickel, 220 wppm vanadium, 50 wppm copper, and 5500 wppm iron was impregnated
with an additional 500 wppm nickel and 1500 wppm vanadium. The impregnated catalyst
had been utilized for several hours in a reaction zone followed by regeneration prior
to these tests. The test results are presented in Table 1 below.
[0041] In these tests the effectiveness of varying hydrogen-nitrogen gas mixtures on metal
passivation was measured. The samples first were exposed to a simulated net oxidizing
regeneration zone atmosphere having about 2.5 to about 3.5 vol.% excess oxygen. The
samples subsequently were exposed to the indicated passivation atmosphere maintained
at 1300°F (704°C) for about 3 minutes.

[0042] From Table I it may be seen that even the use of a reducing gas having a hydrogen
partial pressure of only 0.20 atmosphere was able to significantly passivate the adverse
catalytic effects of the metal contaminants present on the catalyst. This demonstrates
that significant passivation may be realized even at relatively low reducing gas concentrations
in the passivation zone.
Example II
[0043] These tests demonstrated that passing metal contaminated catalyst through a passivation
zone maintained at an elevated temperature and having a relatively short residence
time was effective in passivating the catalyst. These tests were conducted in a continuous
circulating pilot unit with an integral passivation zone operated at 1300°F (704°C)
using the Super DX equilibrium cracking catalyst which had been impregnated as before
to the same metal contaminant content. The results are presented in Figure 3.
[0044] From Figure 3 it may be seen that the hydrogen make and coke makes may be reduced
by passing of the catalyst through a passivation zone for even relatively short periods
of time, such as the residence time typically available in regenerated catalyst transfer
zones.
Example III
[0045] Additional tests were conducted in a micro catalytic cracking (MCC) unit to determine
if a significant amount of the passivation achieved on one pass through the passivation
zone is retained, or whether circulation through the reaction and/or regeneration
zones reactivates the catalyst. Table 2 below indicates that the degree of passivation
of metal contaminated catalyst is, to some extent, cumulative. This further demonstrates
that relatively short residence periods in a passivation zone will be effective for
catalyst passivation. These tests were conducted in a batch gas treatment vessel operated
at 1300OF using an equilibrium Super DX catalyst impregnated with 1000 wppm nickel
and 4000 wppm vanadium. Changes in the catalytic activity of the metal contaminants
were monitored with the microactivity test gas producing factor (MAT GPF).
[0046] In Table 2 test data are shown in which the catalyst was exposed to only a pure hydrogen
atmosphere for the indicated period. Also shown are test data in which the catalyst
alternately was exposed to a pure hydrogen atmosphere for 30 seconds and to a blend
of gases comprising 8% CO, 12% C0
2 and 80% N
2 for 9 minutes. This latter atmosphere was designed to approximate the conditions
in a regeneration zone operated in a net reducing manner. It may be seen that for
comparable hydrogen treat times, the pure hydrogen atmosphere produced a catalyst
having a lower gas producing factor than the catalyst exposed to the alternate passivation
zone - regeneration zone atmospheres. However, it should be noted that as the cumulative
hydrogen treat time increased, the gas producing factor declined with time. This further
indicates that a short residence time passivation zone such as a passivation zone
disposed in a transfer zone may be effective particularly over a prolonged period
of operation.

[0047] In a typical commercial cracking system such as that shown in Figure 1 catalyst residence
time in the regenerated catalyst transfer zone, comprising standpipe 42 and
U-bend 44, typically is about 0.1 to about 2 minutes. Similarly, for a typical commercial
cracking system similar to that shown in Figure 2, average catalyst residence time
in second transfer zone 190 typically ranges between about 0.1 and about 1.0 minutes.
The temperature of the regenerated catalyst in the regenerated catalyst transfer zones
of Figures 1 and 2 typically ranges between about 600°C and about 790
0C. Thus, the regenerated catalyst transfer zones of Figures 1 and 2 typically have
sufficient residence time and catalyst at a sufficiently high temperature to passivate
catalyst upon the introduction of reducing gas.
[0048] It is believed that commercial grade CO and process gas streams containing H
2 and/or CO can be utilized as the reducing agent in passivation zone 90. Hydrogen
or a reducing gas stream comprising hydrogen is preferred, since this produces the
highest rate of metals passivation and achieves the lowest levels of metal contaminant
potency. This is shown by the MCC unit data presented in Figure 4. Preferred reducing
gas streams containing hydrogen include catalytic cracker tail gas streams, reformer
tail gas streams, spent hydrogen streams from catalytic hydroprocessing, synthesis
gas, steam cracker gas, flue gas, and mixtures thereof. The reducing gas content in
the passivation zone should be maintained between about 2% and about 100%, preferably
between about 10% and about 75% of the total gas composition depending upon the hydrogen
content of the reducing gas and the rate at which the reducing gas can be added without
adversely affecting the catalyst circulation rate.
[0049] The stripping gas, if any, added through line 92 of Figure 1 and line 192 of Figure
2 will be a function in part of catalyst flow rate. Typically, the stripping gas flow
rates through each of these lines may range between about 0.1 SCF and about 80 SCF,
preferably between about 8 and about 25 SCF per ton of catalyst circulated.
[0050] Passivation zones 90, 190 may be constructed of any chemically resistant material
capable of withstanding the relatively high temperature and the erosive conditions
commonly associated with the circulation of cracking catalyst. The materials of construction
presently used for transfer piping in catalytic cracking systems should prove satisfactory.
[0051] The pressure in passivation zones 90, 190, of Figures 1, 2, respectively, will be
substantially similar to or only slightly higher than the pressures in the regenerated
catalyst transfer zones of existing catalytic cracking systems. When the embodiment
of Figure 1 is used, the pressure in passivation zone 90 may range from about 5 to
about 100 psig, preferably from about 15 to about 50. When the embodiment of Figure
2 is used the pressure may range from about 15 psig to about 100 psig, preferably
from about 20 psig to about 50 psig.
[0052] In general, any commercial catalytic cracking catalyst designed for high thermal
stability could be suitably employed in the present invention. Such catalysts include
those containing silica and/or alumina. Catalysts containing combustion promoters
such as platinum also can be used. Other refractory metal oxides such as magnesia
or zirconia may be employed and are limited only by their ability to be effectively
regenerated under the selected conditions. With particular regard to catalytic cracking,
preferred catalysts include the combinations of silica and alumina, containing 10
to 50 wt.% alumina, and particularly their admixtures with molecular sieves or crystalline
aluminosilicates. Suitable molecular alumino-silicate materials, such as faujasite,
chabazite, X-type and Y-type aluminosilicate materials and ultra stable, large pore
crystalline aluminosilicate materials. When admixed with, for example, silica-alumina
to provide a petroleum cracking catalyst, the molecular sieve content of the fresh
finished catalyst particles is suitably within the range from 5-35 wt.%, preferably
8-20 wt.%. An equilibrium molecular sieve cracking catalyst may contain as little
as about 1 wt.% crystalline material. Admixtures of clay-extended aluminas may also
be employed. Such catalysts may be prepared by any suitable method such as by impregnation,
milling, co-gelling, and the like, subject only to the provision that the finished
catalysts be in a physical form capable of fluidization.
[0053] In this patent specification, the following apply:
Length expressed in feet is converted to m by multiplying by 0.3048.
Gauge pressure in pounds per square inch gauge (psig) is converted to equivalent kPa
by multiplying by 6.895.
SCF is an abbreviation for standardized cubic feet. 1 standardized cubic foot is 28.316
liters referred to 0°C and 0 kPA gauge pressure.