Molecular sieve abstract
Molecular sieve catalyst compositions having decreased catalytic
activity as a result of accumulating more than 2 weight-% carbonaceous
coke deposits are restored essentially to their pre-coked activity
by heating an essentially homogeneous mixture of particles thereof
with particles of an inert refractory material in the weight ratio
of from 1:2 to 1:10 in air at a temperature of from 500.degree.
C. to 725.degree. C. for a period of time sufficient to decrease
the carbonaceous coke to less than two weight percent.
Molecular sieve claims
What is claimed is:
1. Process for regenerating a mass of coked zeolite molecular sieve-containing
particles which comprises providing a homogeneous mixture of particles
of zeolitic molecular sieve composition containing greater than
2 weight per cent based on the weight of the molecular sieve particle
of a hydrocarbonaceous coke formed thereon, with from 2 to 10 times
the weight thereof of particles of an inert refractory material
essentially free of coke formation the particles of said mixture
having a minimum particle size of 200 mesh and having a maximum
dimension along any axis of 13 millimeters, and heating said mixture
to a temperature of from 500.degree. C. to 725.degree. C. in air
and the reaction products of air and the hydrocarbonaceous coke
on said molecular sieve particles in a rotary kiln for a period
of time to reduce the coke deposit to less than 2 weight per cent.
2. Process according to claim 1 wherein the coked zeolitic molecular
sieve-containing particles being treated contain initially at least
8 weight per cent coke, and the particles of refractory material
are size separable from the molecular sieve-containing particles.
3. Process according to claim 2 wherein the refractory material
is quartz.
4. Process according to claim 3 wherein the refractory material
is quartz and is present in an amount of from 5 to 10 times the
weight of the molecular sieve-containing particles.
Molecular sieve description
This invention relates in general to oxidative regeneration of
zeolitic molecular sieve-based catalyst compositions employed in
hydrocarbon conversion processes having thereon a hydrocarbonaceous
coke deposit whereby the bulk of the coke is reacted to form water
and the oxides of carbon. More particularly the invention relates
to an improved method for rapidly removing coke deposits from particles
of a molecular sieve composition in which the said composition particles
are homogeneously admixed with from 2 to 10 times their weight of
size-separable particles of a refractory substance and heated in
air at a temperature of from 500.degree. C. to 725.degree. C. This
method is especially suitable for large-scale continuous thermal
treatment of spent catalyst masses, such as for hydrocracking and
alkylation, returned in large quantities from commercial petroleum
refineries.
The physical and chemical properties of zeolitic molecular sieves
and their uses as adsorbents and catalysts are well known and treated
in detail in the literature. In many of these processes of use organic
compounds are in contact with the molecular sieves, and as a result
hydrocarbonaceous material, called coke, which is non-volatile at
the process operating conditions, is formed on the surface and within
the pores of the molecular sieve. This coke formation causes a reduction
in the adsorptive capacity and catalytic activity of the molecular
sieve. Consequently the molecular sieve must be periodically reactivated
by removal of the coke deposit.
The periodic reactivation of a molecular sieve mass by removal
of coke deposits must be carried out in such a manner that high
selective adsorptive capacity of the sieve is retained and no substantial
damage is done to the crystal structure of the sieve. The adsorptive
capacity must be retained not only on the surface of the molecular
sieve crystals but especially throughout the entire pore volume
of the crystals. Further, the selective adsorption properties of
crystalline zeolitic molecular sieves depend on the uniformity of
the pores in the crystal lattice. Therefore any substantial damage
to the essential crystal structure destroys the selective properties
of the sieve.
Crystalline zeolitic molecular sieves may also be loaded, within
the pores of the crystal structure, with a variety of metals such
as nickel, platinum and palladium. Efficient use of the metal component
of the catalyst composition is achieved only when it is finely dispersed.
In use at elevated temperatures the metal particles tend to agglomerate
and cause a substantial loss of available surface area and activity.
A number of methods are available to redistribute the agglomerated
metal periodically, as for example in U.S. Pat. No. 3647717 issued
to A. P. Bolton on Mar. 7 1972.
Several methods have been proposed for oxidatively regenerating
the coked catalyst. Essentially these methods comprise burning the
carbonaceous coke from the zeolite using low controlled amounts
of oxygen to avoid the development of destructively high temperatures
(with respect to the zeolite). Precautions are also taken to avoid
the creation of too high a concentration of water which otherwise
would tend to effect noble metal agglomeration and deteriorate the
crystal structure by hydrolysis. A particularly effective oxidative
regeneration procedure for coked molecular sieve bodies is set forth
in U.S. Pat. No. 3069362 issued Dec. 18 1962 to R. L. Mays, et
al. The process treats the catalyst in situ using a forced supply
of gas containing carefully controlled amounts of oxygen and can
require as long as 160 hours.
If a procedure such as the Mays, et al. process is carefully executed,
it is frequently found that a noble metal-loaded zeolite catalyst
coked during service in a hydrocracking process is regenerated essentially
completely as evidenced by its hydrocracking activity with respect
to a sweet and/or nitrogen-containing petroleum feedstock. This
is true despite the fact that agglomerates of the noble metal persist
in the regenerated catalyst which are clearly visible using an electron
microscope. It is surprisingly found, however, that if the regenerated
catalyst composition is evaluated using a sour feedstock, the degree
of restoration is apparently much less complete. A possible explanation
of this phenomenon is that the presence of agglomerated noble metal
on the regenerated catalyst significantly reduces the effective
concentration of the hydrogenation component and that under sour
conditions this concentration is further reduced by conversion to
noble metal sulphide.
It is the objective of the present invention to provide an improved
oxidative regeneration process for coked zeolitic molecular sieve
compositions which requires a remarkably short period of residence
time, typically less than 2 hours, and causes a minimum amount of
hydrothermal abuse to the crystal structure. It also can be utilized
immediately after a rejuvenation treatment whereby catalytic metal
loadings are redispersed without harmful effect on the dispersed
metal.
The revivification process of the present invention which achieves
the aforesaid objective is based on the surprising discovery that
when refractory particles are admixed with coked molecular sieve
particles and heated in air, there is a significant relationship
between the relative proportions of the two materials and the temperature
employed which increases the rate of coke removal while maintaining
an operating temperature that is consistent with consequent prevention
of crystal lattice deterioration and contributes to essentially
complete restoration of catalytic activity.
The process comprises providing a uniform mixture of first particles
of zeolitic molecular sieve composition containing greater than
2 preferably greater than 8 weight-% of a hydrocarbonaceous coke
formed thereon, with from 2 to 10 times the weight thereof of second
particles of an inert refractory material preferably size-separable
from said first particles, the particles having a minimum particle
size of 200 mesh (U.S. Sieve Series) and having a maximum dimension
along any axis of about 13 millimeters, and heating said mixture
in air and the reaction products of air and the hydrocarbonaceous
coke on said molecular sieve particles at a temperature of from
500.degree. C. to 725.degree. C. for a period of time to reduce
the coke deposit to less than 2 weight-%. By the expression size-separable
is meant that the particles of refractory material are sufficiently
larger or smaller than the particles of molecular sieve-containing
particles that conventional separation methods, such as screening,
based on differences in particle size can be employed.
The class of zeolitic molecular sieves involved in the present
invention is limited only by the requirement that the crystal structure
be capable of withstanding original dehydration and subsequent heating
to 725.degree. C. The vast majority of the known synthetic and naturally-occurring
zeolites are thus included within the suitable class, especially,
erionite, faujasite, mordenite, zeolite X, zeolite L, zeolite Y,
zeolite "omega" and the so-called family identified as
"ZSM-5 type" zeolites well documented in the patent literature.
The coke deposits to be removed from the zeolite-containing catalyst
or adsorbent masses comprises mainly carbon and hydrogen, in which
the ratio of hydrogen atoms to carbon atoms are frequently as high
as 2 to 1. The chief precursors of the coke are polycyclic aromatics
and highly unsaturated aliphatic compounds which are either adsorbed
by or decomposed on the zeolite. The weight of coke formed on the
zeolite base at the time the regeneration procedure is begun is
not a critical factor, but typically the adsorbent or catalyst masses
to be regenerated carry from 5 to 17 weight per cent coke based
on the overall weight of the coked particle.
The conformation of the molecular sieve-containing particles can
be regular or irregular, but most commonly will be spherical or
cylindirical as a result of being formed by extrusion, tableting,
spray-drying or prilling. Regardless of their shape, the maximum
dimension along any axis of those particles does not usually exceed
about 13 millimeters. Typically, the particles would be cylindrical
extruded shapes from about 3 to 13 millimeters in length and 3 to
13 millimeters in diameter, or spherical particles of from about
0.8 to 13 millimeters in diameter. It will be understood that the
molecular sieve-containing particles are typically composites which
may also contain materials such as alumina, silica-alumina, silica,
clays, lubricants, binders and diluents, and possibly also metal
hydrogenation components distributed on such materials.
The refractory particles suitably employed are those which are
relatively resistant to grinding attrition in admixture with the
aforesaid molecular sieve particles and which withstand temperatures
of 750.degree. C. without melting or otherwise deteriorating or
reacting with air, water, the oxides of carbon or the coke deposit
generally. Typical materials of this class are quartz (silica) and
its numerous varieties such as flint and agate in the form of sand,
pellets, spheres, natural pebbles, gravel and rock crystal, as well
as fused quartz in a similar variety of shape: ceramic-ware pebbles
and spheres; commercial refractory aluminas (calcined, sintered
or fused) and natural corundums in the form of chips, tablets, granules
and spheres; refractory oxides such as those of zirconium, magnesium,
calcium and chromium; refractory carbides such as silicon carbide,
tungsten carbide and boron carbide; also spinel (MgO . Al.sub.2
O.sub.3) and mullite (3Al.sub.2 O.sub.3 . 2SiO.sub.2), again in
suitable sizes and shapes.
It is understood, of course, that the particles of refractory material
do not contain any appreciable coating of carbonaceous coke or other
material which will burn in air at the temperature conditions of
the process. In most instances, therefore, the refractory particles
are admixed with the molecular sieve particles after the latter
have become coked, and after treatment by the present process the
two types of particles are separated before the molecular sieve
particles are returned to service. For this mode of operation it
is highly advantageous to select the particle size of the refractory
material with reference to that of the molecular sieve particles
so that simple mechanical size-selective separation means can be
employed. Following such separation step the refractory particles
are recycled to be used with the next charge to be treated. In some
instances, however, the refractory material is much less susceptible
to coking than molecular sieve particles and the two types can,
therefore, remain admixed after the revivification process when
the molecular sieve is returned to the coking site.
More specifically, a typical procedure for revivifying a large
supply of spent hydrocracking catalyst containing palladium on Type
Y is as follows:
The coked catalyst (12% coke, 2% graphite) in the form of 1/8-in.
tablets is first rejuvenated by previously established chemical
procedures. After the rejuvenated material has been washed, it is
sent through a dryer at the rate of 500 lbs. per hour to achieve
an LOI of 10 - 12 percent. The dried material is then blended with
quartz sand (10 - 16 mesh) in the ratio of 2 lbs. of sand per lb.
of catalyst pellets and 1500 lbs. of the mixture is transferred
to an externally heated rotary kiln, which is open to the atmosphere
but no forced draft of air or other purge gas is passed through
the kiln. The maximum bed temperature is 650.degree. C. At the end
of the run (approximately one hour duration) the charge is expelled
from the kiln and sent to a screen separator to remove the treated
catalyst pellets from admixture with the quartz particles. The latter
are recycled to the kiln feed area.
The inter-relation between the process temperature and the relative
proportions of inert refractory material and molecular sieve catalyst
particles is evident from the data presented in Table A. The data
concerning the post-regeneration residual loading of carbon on the
catalyst includes at least a portion of the graphite originally
present in the catalyst tablets. This graphite is an extrusion aid
in the preparation of the tablets.
In obtaining the data shown in Table A the molecular sieve particles
treated were a catalyst composition used in hydrocracking of a petroleum
feedstock and which comprised a low-sodium zeolite Y loaded with
palladium as a hydrogenation agent. The particles were essentially
cylindrical in shape, having a height of 1/8 inch and a base diameter
of 1/8 inch. The particles had become coked in service in a commercial
hydrocracking unit and contained 14 weight-% carbon in the form
of hydrocarbonaceous coke. The refractory particles were quartz
in the form of (essentially spherical) sand particles of average
size of 12 .times. 16 mesh (U.S. Sieve Series). The various mixtures
of refractory material and molecular sieve particles specified were
heated at the temperatures and for the time periods indicated in
a rotary kiln which is open to the atmosphere. In contradistinction
to usual regeneration method no forced draft of air or diluted oxygen
purge gas was passed through the kiln. The preferred mode of operation
permits the handling of large quantities of materials under conditions
which are uniform for all particles in the mass being treated, and
no supply of compressed gas or flow rate control thereof is necessary.
Within the useful operating temperature range according to the
present invention, the weight ratio of refractory material to catalyst
mass (R/C) and the residence time are inter-related. In the lower
portion of the temperature region, i.e., approximately 500.degree.-
600.degree. C. using an R/C value of about 2 the coke deposit can
be reduced to below 2 wt.-% by increasing the residence time. By
operating at temperatures up to about 725.degree. C. and by using
higher R/C values, i.e., up to about 10 residence times as short
as about 30 minutes are sufficient to achieve excellent removal
of the coke deposit. Thus, those operating the regeneration unit
in accordance with the invention are able to select temperature
and R/C values to meet a particular time schedule for processing
the coked charge; this is especially important if virtually continuous
operation is required for handling large quantities of coked catalyst.
Additional flexibility is offered by selection of particle size
of the refractory material. For timely processing, operating temperatures
in the range of about 625.degree. to 725.degree. C. at the higher
R/C values (5-10) are preferred. Typically, surface areas of the
thus-processed catalyst materials exceed about 460 M.sup.2 /gram.
In a specific example of the operation of the present process,
a hydrocracking catalyst which had previously been in service in
a commercial hydrocracking unit for at least four years was revivified
by (a) regeneration to remove coke deposits and (b) rejuvenation
to redisperse the palladium metal component. The catalyst had been
initially prepared by exchanging a sodium zeolite Y having a SiO.sub.2
/Al.sub.2 O.sub.3 molar ratio of 4.8 with ammonium cations to a
degree of about 85 equivalent-per cent. Thereafter the zeolite was
back-exchanged with 40 equivalent-% magnesium cations and then loaded
with 0.5 wt.-% palladium by the ion-exchange technique using Pd(NH.sub.3).sub.4
Cl.sub.2. The zeolite was tableted with 20 wt.-% alumina and fired
at about 520.degree. C. for 3/4 hours. A one-pound lot of this coked
(14 wt.-%C.) zeolite catalyst in the form of 1/8 inch .times. 1/8
inch tablets (containing 3.3 wt.-% MgO, 1.3 wt.-% Na.sub.2 O and
0.49 wt.-% Pd) was premixed with ten pounds of 6 .times. 12 mesh
quartz chips. This mixture was charged to a rotary kiln and fired
therein under static conditions, i.e., no additional air purge was
introduced. The total retention time for the charge in the kiln
was about 1.5 hours, and the residence time in the hot zone was
approximately 30 minutes. The temperature of the bed was 500.degree.
C. At the end of the regeneration step, the charge was expelled
from the kiln and the tablets were separated from the quartz chips
by screening. Analysis of a sample of the regenerated material showed
a carbon residue of 1.3weight per cent. Zeolite crystallinity was
maintained, as evidenced by a surface area of 516 square meters
per gram. Following a calcination at 500.degree. C. to remove water
inadvertently adsorbed during the preceding separation step, a test
sample was rehydrated to achieve a residual LOI of 8.4% and tested
for catalytic activity by a procedure which simulates first-stage
hydrocracking conditions. In accordance with the test procedure
the sample was used to catalyze the conversion of a gas oil feedstock
boiling in the range of 400.degree. to 850.degree. F. containing
about 74 volume-% saturated hydrocarbons and about 26 volume-% aromatic
hydrocarbons. The conversion temperature is that value at which
the catalyst exhibits a 55% conversion to below 400.degree. F. end-point
product after 100 hours on stream. For the sample here involved
a conversion temperature of 733.degree. F. was measured. This value
is comparable to that typically obtained with the same coked catalyst
after regeneration by a convention oxidative fixed-bed method. Another
portion of the regenerated material was given a rejuvenation treatment
in aqueous NH.sub.4 NO.sub.3 -- NH.sub.4 OH, dried and calcined
at 500.degree. C. In the same activity test, as described above,
a conversion temperature of 702.degree. F. after about 100 catalyst
age hours was measured. This is to be compared with an activity
value of 707.degree. F. measured on a batch of the same coked catalyst
regenerated under fixed-bed conditions and thereafter rejuvenated
by the same procedure as was used for the kiln-regenerated lot.
In a test procedure which simulates second-stage hydrocracking
activity, satisfactory activity was exhibited by a sample of coked
(14 wt.-% C) catalyst that had been chemically rejuvenated by ammonium
ion treatment and thereafter regenerated in a rotary kiln at 550.degree.
C. for one hour using a refractory material:catalyst ratio of 10:1.
The refractory material was 200-mesh (U.S. Sieve Series) silica
particles. Residual carbon was 0.9 wt.-%. |