I.FLUID CATALYTIC CRACKING:
B.CATALYST/ADDITIVES:
Q-26:Pl.Review the development of FCC Cataysts
A-26:
Development of FCC Catalysts
At the heart of FCC units are the catalysts themselves. The development of active and
stable FCC catalysts went parallel with the FCC design development .It was known, that
for cracking of C-C bonds, the acid catalysts are needed. The first acid catalyst, tested for
cracking of heavy petroleum fraction, was aluminium chloride. But the problems with
corrosion and the waste treatment were greater than its positive action.
In the 1940’s, silica-alumina catalysts were created and greatly improved over the
natural clay catalysts. It was Houdry, who for the first time used acid-activated bentonite as
active acid catalyst for catalytic cracking. But the most significant advance came in 1962
when zeolite catalysts were incorporated into the silica-alumina structures. Advances in
catalysts have produced the greatest overall performance of FCC units over the last fifty
years.
After natural alumino silicates, synthetic aluminoosilicates were prepared with
outstanding cracking properties. Both natural and synthetic aluminosilicates (silica-alumina
catalysts) were known for their Lewis acid sites. The early synthetic amorphous alumosicate
catalysts contained about 13 % wt. of Al2O3 (low alumina), in about 1955 the content of
Al2O3 increased to about 25%. But when the zeolites were put into their structure, strong
Bronsted acid sites resulted, with very easily accessible Lewis acid sites also present. After
experimentation, it was found that these new catalysts possessed all of the properties required
of a successful catalyst: activity, stability, selectivity, correct pore size, resistance to fouling,
and low cost.
In 1962 a catalyst known as Zeolite-Y was added to the active alumina catalyst.
Researchers from Mobil Oil found that by adding small amounts of zeolite into the matrix of
the older silica-alumina catalyst structures, a new catalyst was produced which performed
notably better than any catalyst before. The zeolite catalyst vastly improved gasoline yield
and quality. The first commercial zeolite catalysts were introduced in 1964, and zeolite
catalysts are still in use today.
Not only quality of acid component of cracking catalysts has a great importance for
the use in FCC-process. The very important properties of the catalysts are size and shape of
catalyst particles. In Houdry´s fixed-bed catalytic cracking unit the catalyst – activated
bentonite was probably in the form of pellets. For Thermofor catalytic cracking unit the
catalysts were of spherical shape with the diameter of about 1-2 mm.
For FCC technology, the finely powdered catalyst was originally obtained by grinding
the catalyst material. In 1948, the first spray-dried catalyst was introduced, examples are in
The microspherical particles (50-100 μm) were produced with the similar particle-size
distribution as the ground catalysts before. However, the spherical particles showed both
improved fluidization properties as well as significant reduction of attrition losses.
Zeolites as acid component of FCC catalysts:
Natural zeolites were known from 1765, when the first natural zeolite was discovered
by Swedish mineralogist Croensted. Since that time, more naturally-occuring zeolites as
mineralogical rarities were discovered in volcanic rocks. Because of the very small quantities
of zeolite supply, their use was impossible. Only after the discovery of huge resources of
some types of natural zeolites in sedimentary rocks, great applications were opened in ionexchange
and sorption areas. At the same time, their catalytic properties were studied, and the
great effort to prepare synthetic zeolites started.
As first prepared synthetic zeolite by Union Carbide in 1949 was A-type zeolite (LTA)
having Si/Al=1 with very high ion-exchange capacity but it was not possible to convert
it to acidic form,to remain stable in the Process conditions of the FCC-technology. The
next synthetic zeolite prepared in 1950 was zeolite X with faujasite structure (FAU), having
Si/Al=1.2 is synthesized in Na-form, that is inactive in acid catalysis. It is impossible to
ion-exchange sodium cation into ammonium cation and to calcine them to obtain H-form
, because such form is not stable and after such treatment the crystalline zeolite structure collapses
. To obtain acid catalytic activity of X-zeolite, the only possibility was the ion-exchange with
multivalent cations, partially with calcium, but predominantly with rare earths, mainly
Lantanum and/or Cerium (REX). The rare-earth cations increased the hydrothermal
stability of X-zeolite in FCC process, because the La3+ cations eliminated the negative charge
of three aluminium atoms in zeolite structure at the same time.
Very soon after the X-zeolite first synthesis, the next synthetic zeolite with faujasite
structure (FAU) was prepared: Y-zeolite with Si/Al=2.5-3. The higher Si/Al ratio was found
as more stable against acid and hydrothermal dealumination. It was possible by ion exchange
of NaY zeolite to prepare ammonium form, and by calcinations convert into H-form,
possessing strong Broensted acid sites.But, such prepared HY-zeolite was still not stable in the Operating condtions in FCC - the presence of 100% steam at high temperature in riser, reactor and stripper about 550 °C, in regenerator more than 700 °C. At such “hydrothermal” conditions, the hydrolysis of framework aluminium occurs and the frameworks strongly dealuminates, causing partial and even total collapse of zeolite Y structure.
.
B.CATALYST/ADDITIVES:
Q-26:Pl.Review the development of FCC Cataysts
A-26:
Development of FCC Catalysts
At the heart of FCC units are the catalysts themselves. The development of active and
stable FCC catalysts went parallel with the FCC design development .It was known, that
for cracking of C-C bonds, the acid catalysts are needed. The first acid catalyst, tested for
cracking of heavy petroleum fraction, was aluminium chloride. But the problems with
corrosion and the waste treatment were greater than its positive action.
In the 1940’s, silica-alumina catalysts were created and greatly improved over the
natural clay catalysts. It was Houdry, who for the first time used acid-activated bentonite as
active acid catalyst for catalytic cracking. But the most significant advance came in 1962
when zeolite catalysts were incorporated into the silica-alumina structures. Advances in
catalysts have produced the greatest overall performance of FCC units over the last fifty
years.
After natural alumino silicates, synthetic aluminoosilicates were prepared with
outstanding cracking properties. Both natural and synthetic aluminosilicates (silica-alumina
catalysts) were known for their Lewis acid sites. The early synthetic amorphous alumosicate
catalysts contained about 13 % wt. of Al2O3 (low alumina), in about 1955 the content of
Al2O3 increased to about 25%. But when the zeolites were put into their structure, strong
Bronsted acid sites resulted, with very easily accessible Lewis acid sites also present. After
experimentation, it was found that these new catalysts possessed all of the properties required
of a successful catalyst: activity, stability, selectivity, correct pore size, resistance to fouling,
and low cost.
In 1962 a catalyst known as Zeolite-Y was added to the active alumina catalyst.
Researchers from Mobil Oil found that by adding small amounts of zeolite into the matrix of
the older silica-alumina catalyst structures, a new catalyst was produced which performed
notably better than any catalyst before. The zeolite catalyst vastly improved gasoline yield
and quality. The first commercial zeolite catalysts were introduced in 1964, and zeolite
catalysts are still in use today.
Not only quality of acid component of cracking catalysts has a great importance for
the use in FCC-process. The very important properties of the catalysts are size and shape of
catalyst particles. In Houdry´s fixed-bed catalytic cracking unit the catalyst – activated
bentonite was probably in the form of pellets. For Thermofor catalytic cracking unit the
catalysts were of spherical shape with the diameter of about 1-2 mm.
For FCC technology, the finely powdered catalyst was originally obtained by grinding
the catalyst material. In 1948, the first spray-dried catalyst was introduced, examples are in
The microspherical particles (50-100 μm) were produced with the similar particle-size
distribution as the ground catalysts before. However, the spherical particles showed both
improved fluidization properties as well as significant reduction of attrition losses.
Zeolites as acid component of FCC catalysts:
Natural zeolites were known from 1765, when the first natural zeolite was discovered
by Swedish mineralogist Croensted. Since that time, more naturally-occuring zeolites as
mineralogical rarities were discovered in volcanic rocks. Because of the very small quantities
of zeolite supply, their use was impossible. Only after the discovery of huge resources of
some types of natural zeolites in sedimentary rocks, great applications were opened in ionexchange
and sorption areas. At the same time, their catalytic properties were studied, and the
great effort to prepare synthetic zeolites started.
As first prepared synthetic zeolite by Union Carbide in 1949 was A-type zeolite (LTA)
having Si/Al=1 with very high ion-exchange capacity but it was not possible to convert
it to acidic form,to remain stable in the Process conditions of the FCC-technology. The
next synthetic zeolite prepared in 1950 was zeolite X with faujasite structure (FAU), having
Si/Al=1.2 is synthesized in Na-form, that is inactive in acid catalysis. It is impossible to
ion-exchange sodium cation into ammonium cation and to calcine them to obtain H-form
, because such form is not stable and after such treatment the crystalline zeolite structure collapses
. To obtain acid catalytic activity of X-zeolite, the only possibility was the ion-exchange with
multivalent cations, partially with calcium, but predominantly with rare earths, mainly
Lantanum and/or Cerium (REX). The rare-earth cations increased the hydrothermal
stability of X-zeolite in FCC process, because the La3+ cations eliminated the negative charge
of three aluminium atoms in zeolite structure at the same time.
Very soon after the X-zeolite first synthesis, the next synthetic zeolite with faujasite
structure (FAU) was prepared: Y-zeolite with Si/Al=2.5-3. The higher Si/Al ratio was found
as more stable against acid and hydrothermal dealumination. It was possible by ion exchange
of NaY zeolite to prepare ammonium form, and by calcinations convert into H-form,
possessing strong Broensted acid sites.But, such prepared HY-zeolite was still not stable in the Operating condtions in FCC - the presence of 100% steam at high temperature in riser, reactor and stripper about 550 °C, in regenerator more than 700 °C. At such “hydrothermal” conditions, the hydrolysis of framework aluminium occurs and the frameworks strongly dealuminates, causing partial and even total collapse of zeolite Y structure.
The first solution of the Y-zeolite instability was ion-exchange with rare-earths cations similarly as in the case of X-zeolites. REY zeolites were for long-time the most used zeolites in FCC-catalysts. Later, the method of stabilitation of zeolite Y structure without RE-cations was developed: mmonium-exchanged zeolite was treated in special conditions by 100% steam at the temperature up to 800 °C, causing partial dealumination of framework, creation of extraframework aluminium (weak Lewis acid centers), but in the framework still rest strong Broensted acid sites necessary for cracking. Such treated Y-zeolites were extremely stable, and were called as “ultrastable zeolites Y” – USY. This kind of Y-zeolites represented the new generation of acid component in FCC-catalysts.The great advantage of the USY zeolites was that during hydrothermal dealumination with steam at the high temperature the secondary mesoporous structure was created in zeolite crystals.The created mesopores in Y.zeolites increase its cracking activity from the point of view of improving the diffusivity of greater molecules into zeolite crystals to acid catalytic centers.
Generally, the philosophy of the FCC-catalyst preparation is: to have weak acid
centers in macroporous part of catalyst particles to insure pre-crack the great molecules of
residue to smaller molecules which could enter to the mesopores with stronger acidity.
Product of cracking in mesopores could finally enter the zeolite micropores to crack over
strongest zeolite acid centers into the smaller molecules, mainly gasoline fraction .
The mesopores created in zeolite crystals after hydrothermal dealumination improve
the diffusivity of greater molecules to strong acid centers and increase the overall conversion
into gasoline in comparison with zeolite without mesopores.
The above-mentioned overall pore structure of FCC-catalyst is insured by a complex
catalyst composition that is generally “know-how” of each company producing FCCcatalysts
.
.
Generally, the FCC-catalyst contains the active macroporous matrix (maybe the
activated clay), mesoporous synthetic silica-alumina, and zeolite. The activated clay after
calcinations also plays role as a binder and gives to FCC spheres strong resistance against
attrition. As zeolite component, REY, USY and also combination – REUSY are used. Beside
of these components, FCC-catalyst could contain also so-called metal traps for the elimination
of poisoning of acid centers by V and Ni from feed. Moreover, each FCC catalyst could be
“taylor-made” for certain feed composition and desired products spectra.
The future in the FCC-catalysts is in the looking for new materials, maybe special kind
of microporous alumosilicates – zeolites, or even mesoporous molecular sieves with more
opened pore structure allowing the diffusion even greater molecules into pores, but with
geometry and strength of acid sites similar to the strongest acid sites known in zeolites.
One of the results of the effort of Mobil researchers was the synthesis of so-called
“mesoporous molecular sieves” of M41S family, that are marked as MCM-41 and MCM-48.
These materials are prepared via mechanisms of the creation of hexagonal micelles
structures from surfactants (e.g.trimethyl hexadecyl ammonium bromide). Depending on the
alkyl chain of surfactant and the conditions of synthesis, the prepared mesoporous material
could have regular uniform mesoporous structure with pore size 3-10 nm
Such material could be very suitable as acid component of FCC catalyst under
assumption to preserve its strong acidity, similar as in zeolites, even after steam treatment at
elevated temperatures. Unfortunately, up to now, the effort for the preparation of enough
stable acid component of FCC catalyst from mesoporous molecular sieves was not successful;
aluminium in its structure is not as stable as in Y-zeolite
Special types of FCC catalyst is zeolite ZSM-5 containing catalyst, used as additive to
standard FCC catalyst to increase RON of produced gasoline and/or to increase the propylene
yield. This ZSM-5 zeolites has strong “reactant selectiviy” and crack predominantly n alkanes. Because the cracking over acid centers proceeds via cracking of beta C-C bond to,
created secondary (or terciary) carbenium ions, the main product of cracking of longer n-
alkanes is propene (or butene.)
Special types of FCC catalyst is zeolite ZSM-5 containing catalyst, used as additive to
standard FCC catalyst to increase RON of produced gasoline and/or to increase the propylene
yield. This ZSM-5 zeolites has strong “reactant selectiviy” and crack predominantly n alkanes. Because the cracking over acid centers proceeds via cracking of beta C-C bond to,
created secondary (or terciary) carbenium ions, the main product of cracking of longer n-
alkanes is propene (or butene.)
.
Conclusions
FCC-technology represents one of the most expanded processes producing motor fuels
from heavy distillates and residues. On the one side, the FCC technology from the
engineering point of view is one of the most sophisticated equipment in the chemical industry.
On the other side, the key factor in such technology is good active, stabile and selective
“tailor-made” catalyst, converting specific feed of heavy distillates and residues into desired
products. Market with FCC-catalysts is the greatest one in all kinds of catalysts, and at the
same time it is the greatest consumer of zeolites (Y, partially ZSM-5) besides of zeolite A in
detergents. There is still great effort to look for more active, selective and stabile materials as
Wow.. such an informative article. Thanks for the post. i like to read especially about zeolite catalyst
ReplyDelete