QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:                           www,wissenschaftler-avh.in

B.CATALYST/ADDITIVES(Contd.):                     

Q-53

How Heavy Metals contaminate the FCC Catalyst?

A-53

Heavy Metal contamination on FCC Catalyst

All cat cracker feedstock contain heavy metal contaminantsin the ppm range, the most common metals being nickel (Ni),vanadium (V), iron (Fe) and copper (Cu), all of which promote dehydrogenation and condensation reactions.

Extensive investigation of their relative activity for coke and hydrogen production indicate that nickel is four times as active as vanadium (4Ni + V) in producing hydrogen and coke[1] as studied by Arco back in the 1970s. Iron was not considered significant since this metal is usually present as “tramp” iron and is not catalytically active.

A distinction must be made between tramp Fe and Fe deposited on the cracking catalyst. Tramp Fe is composed of Fe particles in the catalyst stream that originate from erosion of pipes, vessels and other hardware. To the extent that these particles do not break up in very fine particles that can attach
themselves to the cracking catalyst, they have little effect on catalyst activity and selectivity. However, they could affect CO oxidation and SOx emissions. Iron deposited on the catalyst is in most cases the result of organic, colloidal or other finely dispersed Fe in feed. It has been recently recognized that this latter form of Fe is an important factor causing FCC catalyst deactivation. Most often, loss of activity and bottoms
cracking has been observed. Decreases in average bed density (ABD) have also been reported. In general, the more finely dispersed the depositing Fe is, the more effective it is in causing catalyst deactivation. Fe compounds present in crude oil, and as minute impurities in cracking catalyst, have some dehydrogenation activity but at orders of magnitude less than Ni or V.

Grace Davison generally uses a catalyst Ni equivalent index of Ni + Cu + V/4, which assumes that Cu and Ni are equal in dehydrogenation activity. It should be noted that an NPRA paper dating back to 1979 by Ashland /UOP  has indicated the Ni or V equivalent factors may not be accurate in assessing the relative contribution of metals, especially at high concentrations, because of a non-linear response of the ontaminants. It should be stressed that the previously noted Ni equivalent indices do not necessarily reflect the relative
effects on catalyst activity, but rather only the effects on coke and gas make.

Gas Production & Compressor Limits

Both pilot plant and commercial data have shown a number of undesirable yield shift changes occurring as metals accumulate on the catalyst. The most obvious result of metals poisoning is a sharp increase in hydrogen production. Although the wt% increase is not usually significant, the volume of gas can
rise dramatically and limit compressor capacity A common means of tracking H2 make is via the H2/C1,
or H2/(C1 + C2) molar ratio. With this method the ratio remains fairly constant with conversion as long as reactor temperature and feedstock remain constant, so changes should reflect differences in metal activity. A relatively metals-free commercial operation may operate with a 0.2-0.3 H2/CH4 ratio, whereas resid operations encounter ratios well above 1.0. Levels up to 3.0 have been reported to Grace Davison.

At typical FCC catalyst addition rates, less than one-third of the total metals on commercial equilibrium catalyst are active dehydrogenation catalysts themselves. It should be understood that the activity of metals deposited on the catalyst in a commercial FCCU is much less than an equal amount of artificially deposited metals due to the passivation effect brought about by the continuous reaction regeneration cycles
encountered in the commercial unit Concurrent with the increased hydrogen, metals increase contaminant coke yield. The metals that catalyze H2 formation also catalyze condensation/polymerization reactions,which form coke. .

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:                     www.wissenschaftler-avh.in

B.CATALYST/ADDITIVES(Contd.)

Q-52:

Pl .Highlight the role of LOW RARE EARTH CATALYSTS in FCC operation?

A-52:


For decades, rare earth elements have performed vital roles in refinery catal-ysis and their availability, at reasonable prices, has been taken for granted. However, when the global supply became restricted, refiners faced spiral- ling costs and were forced to re-examine how they used rare earth elements.

Uses of rare earth elements

The rare earth elements are lanthanum, cerium, praseodym- ium, neodymium, promethium, samarium, europium, gadolin- ium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. Yttrium and sometimes scandium are often grouped as rare earth elements because of their similar chemical properties. These  elements  have  a  widerange of industrial applications. They have taken an important role in oil refining catalysts, additives and processes for many years, and also  make vital contributions to other applications. Selected end uses for rare earths are shown in Table 1.Global demand for rare earth elements    is    currently    about 134 000 t/y, but worldwide annual  production  amounts  to 124 000 tonnes, the difference being drawn from previously mined stocks.

However, the availability of rare earths can no longer be taken for granted. In  2009, China, the largest producer of rare earth elements,( output 97% of the world’s rare earths measured in terms of oxide content.)  cut its export of rare earths from about 50 000 tonnes in 2009 to 30 000 tonnes in 2010. The country also plans to reduce output  further by eliminating prohibited rare earth mining operations. This is likely to restrict the availability of rare earth elements even more. Greater emphasis by the Chinese authorities on the safety and environmental aspects of mining operations are likely to increase operating costs. The much-reduced availability of rare earths means that their price has soared. The price for lanthanum, for example, surged from some $6000/t in May 2010 to about $140 000/t  in  May 2011.

Selected uses of rare earth elements


Light rare earth element                               Major end use                                 Heavy rare earth element                       Major end use Lanthanum             Hybrid engines, metal alloys, refining catalysts                                   Terbium                                       Phosphors, permanent magnets Cerium   Automotive catalysts, refining catalysts, metal alloys                                         Dysprosium       Permanent magnets, hybrid engines Praseodymium                                         Magnets             Erbium                     Phosphors
Neodymium     Automotive catalysts, hard drives in laptops, headphones, hybrid engines                 Holmium                     Glass colouring, lasers Samarium                                         Magnets   Thulium               Medical X-ray units Europium                                                         Red colouration for television and computer screens                   Ytterbium  Lasers, steel alloys Gadolinium                                                                                                   Magnets

Table 1                                                                                                   Adapted from: DOI, US Geological Survey, Circular 930-N


Laboratory comparison of two catalysts with the same formulation but with and without rare earth exchange


Catalyst                                         no rare earth           rare earth exchanged
RE2O3, wt%                                           0.0                             1.0
Conversion at C/O = 4 wt/wt, wt%           Base                         +6.9
Yield structure, wt%
Dry gas                                               Base                         +0.90
Propane                                               Base                         +0.19
Propylene                                           Base                         +0.44
n-butane                                             Base                         +0.15
i-butane                                               Base                         +0.66
Butylenes                                           Base                         +0.28
Gasoline                                             Base                         +4.8
Light cycle oil                                       Base                           -1.1
Bottoms                                             Base                           -5.8
Coke                                                   Base                         +0.9
Selectivities, wt/wt
C3-olefinicity                                         Base                         -0.02
C4-olefinicity                                         Base                         -0.08
Gasoline/conversion                             Base                         +0.01
Coke/second order conversion               Base                         -0.40


Table 2


This comparison at a constant catalyst-to-oil  ratio  (C/O)shows the typical changes through   applying   rare   earth exchange to zeolites:•    The  activity  is  substantially increased, which results in much   higher   liquefied   petroleum gas   (LPG) and particularly, gasoline yield,mainly at the cost of bottoms
•    The   olefinicity   of   the   LPG fraction is decreased •    Owing  to  the  lower  olefinicity,    gasoline    olefinicity    and octanes will also increase.•    Coke   and   delta   coke   are also increased.No  other  elements  have  yet been  found  to  increase  zeolite activity   and   stability   as   efficiently as rare earth  elementsThere  are  limited  sources  of rare    earth   elements    outside China  but   the   quantities currently produced are too low  to  have  any  significant  impact on short-term supply. Initiatives are being taken in, for example,Australia, Brazil, Canada, South Africa,  Greenland  and  the  US, to    find    and    develop    new sources   of   rare   earths   or   to reopen     mines   previously considered uneconomic. But, as demand is projected to increase from 134 000 t/y to 180 000   t/y   in   2012,   it   is unlikely   that   any   new   rare earth production will close the   widening gap in the short term,as   greenfield   mining   projects could  take  10  years  to  reach production.

Rare earth elements in FCC catalysts
Rare earths have found applications in oil refining for FCC catalysts and additives, which use    lanthanum    and    cerium.Lanthanum    and    cerium    are used  in  FCC  catalysts  because they  substantially  increase  the activity   and   stability   of   the zeolite, which is the most active component    in    the    catalysts.Lanthanum  is  most  commonly used   to   increase   the   activity and stability of zeolites.The  effects  of  lanthanum  on the  performance  of  a  zeolite-containing    FCC    catalyst    are shown  in  Table  2.  This  data was  obtained  from  laboratory work   in   which   two   catalysts were  compared  in  the  short-contact- time riser test (SCT-RT)after   a   two-step   cyclic   metal   deactivation  with  5000  ppm  of nickel (Ni), 5000 ppm of vanadium (V)    and     a     residue feed stock.   The   catalysts   have the  same  composition,  but  the first  one  has  not  been exchanged with rare earth and the second one      contains 1 wt% RE2O3.

CONCLUSIONS
Rare earth elements have been of great importance to oil refiners for processing marginal feedstocks in the FCC unit, which is still the preferred unit for converting marginal feed-stocks into LPG and transportation fuels. However, soaring rare earth prices have created turmoil for the refining industry.

QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:                   www.wissenschaftler-avh.in

B.CATALYST/ADDITIVES(Contd.)


Q-51

What are the effects of Rare Earths on FCC Catalysts


A-51

What are Rare Earths?

Rare earth is a generic name used for the 14 metallic elements of the Lanthanide series which contain the atomic numbers from 57 through 71 plus Scandium (At. #21) and Yttrium (At. #39). These elements occupy a unique place in the periodic chart. They are the first elements where the increasing atomic number results in the filling of the inner electron shell after an outer shell has been filled, causing a high similarity in chemical properties. This similarity complicates the separation of one rare earth from another. They are, therefore, often supplied as a mixture of oxides as extracted from ores such as bastnaesite or monazite.

The typical rare earth consists of 46% Cerium Oxide, 20% Lanthanum Oxide, 15% Neodymium Oxide, as well as other oxides of the series. Processors also supply a form of rare earth in which the greater part of the Cerium has been removed. This mixture, known as "Lanthanum rich", contains 6-10% Ce 2O3, up to 80% La 2O3, 15% Nd 2O3 and other oxides. Extensive laboratory tests indicate minimum difference in catalytic performance, and FCC catalysts are accordingly treated with either of the two mixtures.

Why Rare Earths?

. . . Hydrothermal Stability

Lack of hydrothermal stability is the primary reason for adding rare earth oxides to catalytic cracking catalysts. Upon exposure to the high temperatures observed in commercial FCC regenerators the original X-type zeolite cracking catalysts had a tendency to sinter and become amorphous in nature, resulting in a loss of catalyst activity. Addition of rare earth oxides to the cracking catalysts improved their hydrothermal stability; providing stable operations at regenerator temperatures as high as 1300° F.

The subsequent development of Y-type zeolites improved the catalyst's inherent hydrothermal stability. In fact, these newer FCC catalysts without rare earth could withstand temperatures comparable to their earlier counterparts  with rare earth. These Y-type zeolites with rare earth remain stable at temperatures as high as 1400° F. Figure 1 illustrates differences in catalyst hydrothermal stability with and without rare earth stability.        

Higher Gasoline Yield and Reduced Gas Make

Rare earth oxides are added to zeolite cracking catalysts through a process known as ion exchange. During this process a portion of the acidic protons and sodium located within the zeolite crystal are exchanged with rare earth ions.

Because rare earth inhibits the dealumination of a zeolite, a higher concentration of acid sites will be found in a rare earth exchanged catalyst. This improves both the activity and the hydrothermal stability of the catalyst. On average, these sites are weaker and in closer proximity to each other than those found in a more highly dealuminated catalyst characterized by lower unit cell size measurements. 

As a result of the greater number of active sites, both the primary cracking and primary hydrogen transfer reactions that occur   within the zeolite are enhanced. Primary cracking reactions involve the initial scission of the carbon-carbon bond to form higher valued liquid products such as gasoline. Primary hydrogen transfer reactions are those that occur between cracked products to terminate the cracking reactions in the gasoline range, thus, reducing the overcracking of gasoline to C 3's and C 4's. The hydrogen transfer reactions are greatly increased with the addition of rare earth to the zeolite.

Thus, rare earth in catalytic cracking catalysts enhances their gasoline yield. 

Lower Octane and Reduced Cetane Index

The addition of rare earth into the zeolite inhibits the degree of unit cell size shrinkage during 
equilibration in the regenerator. Steam in the FCC regenerator removes active acidic alumina from the zeolite. Rare earth inhibits the extraction of aluminum from the zeolite's structure (dealumination) which in turn increases the equilibrium unit cell size for FCC catalysts. Since reducing the equilibrium unit cell size of an FCC catalyst has the effect of improving octane, adding rare earth decreases the octane. 

In addition to the above mentioned reactions, the rate of secondary hydrogen transfer reactions is also increased by rare earth. For  a rare earth exchanged cracking catalyst the hydrogen transfer reaction of interest is as follows:

Naphthenes(LCO) + Olefins(Gasoline) -> Aromatics(LCO) + Paraffins(Gasoline)

Thus hydrogen transfer reduces the amount of olefins found in the product. These reactions also influence the molecular weight distribution of the product by terminating carbonium ions before they crack to shorter chain fragments. As hydrogen transfer reactions increase relative to cracking reactions, olefin yield, light gas yield and octanes decrease, while gasoline yield increases.

Because rare earth oxides promote hydrogen transfer, the yield of C 3 and C 4 olefins in the LPG fraction will be lower.

 The resulting reduction in the total LPG yield results in a reduction in the wet gas yield. This reduction in wet gas can have a major effect on plant operations, as compressor capacity is often the limiting factor for FCC unit throughput.

A refiner's own product requirements determine whether a rare earth or non rare earth catalyst is used. If gasoline is desired, a rare earth cracking catalyst should be used; if higher gasoline octane is required, a minimum rare earth catalyst or partial rare earth catalyst should be the catalyst choice. For a catalyst containing reduced levels or no rare earth, the use of an Ultrastable (USY) Zeolite Catalyst is recommended for improved hydrothermal stability.