Petroleum Cracking Catalyst Characterization - ACS Symposium

Chapter 17

Petroleum Cracking Catalyst Characterization Secondary Ion Mass Spectrometry Imaging Processing Methods 1

D. P. Leta, W. A. Lamberti, M. M. Disko, E. L. Kugler , and W. A. Varady

Downloaded by COLUMBIA UNIV on May 27, 2014 | http://pubs.acs.org Publication Date: January 23, 1991 | doi: 10.1021/bk-1991-0452.ch017

Exxon Research and Engineering Company, Annandale, NJ 08801

The characterization of petroleum cracking catalysts, with which a third of the world's crude oil is processed, presents a formidable analytical challenge. The catalyst particles are in the form of microspheres of 60-70 micron average diameter which are themselves composites of up to five different micron and submicron sized phases. In refinery operation the catalysts are poisoned by trace concentrations of nickel, vanadium and other contaminant metals. Due to the replacement of a small portion of equilibrium catalyst each day (generally around 1% of the total reactor inventory) the catalyst particles in a reactor exist as a mixture of differing particle ages, poisoning levels and activities. Direct imaging SIMS has been shown to be capable of viewing both the phases and contaminants within individual catalyst particles, and differences from particle to particle. Multiple image processing techniques now allow the rapid analysis of the relative concentrations of several elements (normally 6-8) on a large number of particles (50-100) using multiple low magnification SIMS images. Using the fact that nickel concentrations increase systematically with a particle's age permits the analysis of poisoning and elemental composition as a function of a particle's "time in the reactor." Within individual particles, SIMS views coupled with elemental image ratios and overlays have also allowed the measurement of the quantity of each phase (such as zeolite and alumina) and the distribution of nickel and vanadium poisons in relationship to those phases.

The t e c h n i q u e o f Imaging Secondary Ion Mass S p e c t r o m e t r y (SIMS) has proven t o be v e r y w e l l s u i t e d t o t h e c h a r a c t e r i z a t i o n o f fluidized petroleum c r a c k i n g c a t a l y s t s ( F C C ) . U " 4 ) The a b i l i t y t o view 1

Current address: West Virginia University, Morgantown, W V 26506

0097-6156/91/0452-0269$07.00/0 © 1991 American Chemical Society

In Fluid Catalytic Cracking II; Occelli, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

270

FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN

elemental d i s t r i b u t i o n s with 0.5 micron spatial resolution at c o n c e n t r a t i o n s i n the ppm range i s well s u i t e d to the a n a l y s i s of the submicron phases and low c o n c e n t r a t i o n c o n t a m i n a n t s p r e s e n t in commercial m u l t i - c o m p o n e n t FCC p a r t i c l e s . The use o f ultra-low light level imaging systems(S) with the intrinsically sensitive SIMS t e c h n i q u e makes r e a l time v i e w i n g o f many o f t h e elements important in FCC c a t a l y s t s p o s s i b l e . Aluminum, s i l i c o n and the r a r e e a r t h elements s e r v e t o d e f i n e the major phases p r e s e n t w i t h i n each c a t a l y s t p a r t i c l e , w h i l e the t r a n s i t i o n row elements and all o f t h e a l k a l i and a l k a l i n e elements may be seen a t t r a c e c o n c e n t r a tions. Of p a r t i c u l a r importance i s t h e use o f the technique to study the d i s t r i b u t i o n s of n i c k e l and vanadium which a r e the most d e l e t e r i o u s o f t h e contaminant m e t a l s . ( ) Modern image p r o c e s s i n g computers and s o f t w a r e now a l l o w the r a p i d q u a n t i t a t i v e a n a l y s i s o f SIMS e l e m e n t a l images i n o r d e r t o more c l e a r l y r e v e a l t h e l o c a t i o n s o f the c a t a l y s t phases and the quantitative distributions of the c o n t a m i n a n t m e t a l s on t h o s e p h a s e s . A l t h o u g h the a n a l y s i s techniques discussed in t h i s s t u d y may be applied to any o f the c o n t a m i n a n t e l e m e n t s , f o r s i m p l i c i t y we w i l l l i m i t our examples to the major catalyst elements, and the nickel and vanadium contaminants.


6

EXPERIMENTAL METHODS Catalyst Preparation. A l l o f the FCC c a t a l y s t samples used i n t h i s s t u d y were prepared by embedding the as-received particles in thermosetting c o p p e r doped epoxy (commercially available from m e t a l l u r g i c a l supply d i s t r i b u t o r s ) to provide e l e c t r i c a l c o n d u c t i v ity. The hardened epoxy b l o c k s were then p o l i s h e d t o expose the i n t e r i o r s of the c a t a l y s t p a r t i c l e s (approximate cross-sections) u s i n g s i l i c o n c a r b i d e paper t o 600 g r i t . The p o l i s h i n g i s carried out w i t h o u t water t o a v o i d the p o s s i b i l i t y o f e l e m e n t a l r e d i s t r i b u tion. P r i o r to data c o l l e c t i o n i t i s necessary to presputter areas o f interest for 10-15 m i n u t e s , which atomically removes s e v e r a l m i c r o n s o f m a t e r i a l , c l e a n i n g away embedded p o l i s h i n g grit and e x p o s i n g t h e u n d i s t u r b e d p a r t i c l e i n t e r i o r s . SIMS I n s t r u m e n t a t i o n . A CAMECA IMS-3f i o n m i c r o p r o b e / m i c r o s c o p e , m o d i f i e d by the a d d i t i o n o f a h i g h s e n s i t i v i t y v i d e o based imaging system(5) was used f o r elemental viewing o f the FCC p a r t i c l e s . Oxygen as 0 2 was used as the p r i m a r y bombarding i o n , a t an impact energy o f 10.5 keV and p o s i t i v e s e c o n d a r y i o n s were used f o r data collection. Generally, integration times o f 8-30 seconds were s u f f i c i e n t to obtain good s i g n a l - t o - n o i s e c h a r a c t e r i s t i c s in the images. All o f the images in t h i s s t u d y were c o l l e c t e d using e i t h e r a nominal 150 o r 400 micron f i e l d - o f - v i e w . The t y p e of image i n t e g r a t i o n used s e t s the f i n a l i n t e n s i t y o f the brightest p i x e l i n each image t o a s t a n d a r d v a l u e , e n s u r i n g v i e w a b i l i t y . As s u c h , the brightness within one image may be used t o gauge an element's r e l a t i v e c o n c e n t r a t i o n from region to r e g i o n , but no c o n c l u s i o n s s h o u l d be drawn between d i f f e r e n t images w i t h o u t using t h e mass s p e c t r a l count r a t e s and comparison t o s u i t a b l e s t a n d a r d s . For t h i s s t u d y , the d i g i t i z e d brightness within t h e images were +


17.

LETAETAL.

Petroleum Cracking Catalyst Characterization


used w i t h o u t attempt to convert to actual concentration, such a l l v a l u e s r e p r e s e n t " r e l a t i v e c o n c e n t r a t i o n . "

271 and

as

Image P r o c e s s i n g . In our image p r o c e s s i n g / c o l l e c t i o n system the SIMS i n s t r u m e n t ' s i n t e r n a l m u l t i c h a n n e l p l a t e / f l u o r e s c e n t screen i s viewed u s i n g an u l t r a h i g h g a i n v i d e o camera c a p a b l e o f viewing the b u r s t s of l i g h t g e n e r a t e d by individual ions. This analog video signal i s d i g i t i z e d and i n t e g r a t e d using a " C r y s t a l " image p r o c e s s o r (MCI C o n s u l t a n t s ) and the i n t e g r a t e d images s t o r e d as a n a l o g s i g n a l s u s i n g an O p t i c a l L a s e r D i s k which p r o v i d e s inexpens i v e h i - f i d e l i t y image s t o r a g e . Post a n a l y s i s image p r o c e s s i n g is a c c o m p l i s h e d by r e - d i g i t i z i n g the images w i t h a M i c r o v a x II hosted G o u l d / D e A n z a 8500 image p r o c e s s i n g computer system and u s i n g a c o m b i n a t i o n o f i n - h o u s e and commercial s o f t w a r e r o u t i n e s . P r i o r to t h e use o f any image c o m p a r a t i v e c a l c u l a t i o n s , s e t s o f images are accurately registered with each o t h e r using c r o s s - c o r r e l a t i o n techniques. For all steps of image handling and p r o c e s s i n g , r e s o l u t i o n s o f a p p r o x i m a t e l y 512x512 w i t h 256 b r i g h t n e s s l e v e l s are maintained.

RESULTS AND DISCUSSION D i s t r i b u t i o n s Within

Single Catalyst

Particles

Silicon/Aluminum Ratio Images. Many types of FCC catalyst p a r t i c l e s a r e heterogeneous m i x t u r e s o f z e o l i t e , c l a y , a l u m i n a and silica/alumina binder phases. Each o f these phases contains s i l i c o n and aluminum as the major elements, with varying Si/Al r a t i o s i n each p h a s e . It i s p o s s i b l e t o g e n e r a t e a "phase map" of single catalyst particles by t a k i n g advantage of these differing Si/Al ratios. Division o f the SIMS S i i o n image by t h e A l ion image produces a S i / A l i n t e n s i t y r a t i o image which o f t e n clearly shows t h e p o s i t i o n s o f the p h a s e s . Image d i v i s i o n a l s o provides the a d d i t i o n a l b e n e f i t o f e l i m i n a t i n g most o f t h e i n t e n s i t y variat i o n s caused by topography o r by nonuniform p r i m a r y beam illuminat i o n o f the sampled r e g i o n . F i g u r e l a and l b show 150 m i c r o n f i e l d - o f - v i e w S i and A l ion images, r e s p e c t i v e l y , t a k e n o f a f r e s h FCC c a t a l y s t p a r t i c l e o f a t y p e which c o n t a i n s no r a r e e a r t h exchanged z e o l i t e s . When rare e a r t h elements are present (see later discussion) they greatly s i m p l i f y the i d e n t i f i c a t i o n o f the z e o l i t e phase. Inspection of the S i and A l images shows t h a t a l t h o u g h b r i g h t n e s s v a r i a t i o n s are p r e s e n t i n both images i t i s very d i f f i c u l t to " s e e " where the v a r i o u s phases r e s i d e . Division o f the S i image by the A l image y i e l d s f i g u r e l c which shows the S i / A l i n t e n s i t y r a t i o image, renormalized to a viewable b r i g h t n e s s . In this ratio image the v a r i o u s grey l e v e l s (brightnesses) c o r r e s p o n d i n g t o the different phases may be much more clearly discerned. Silica (as silica i m p u r i t i e s o r as aggregated b i n d e r phase) g i v e s t h e h i g h e s t Si/Al r a t i o s and i s t h e r e f o r e the b r i g h t e s t i n t h e i n t e n s i t y r a t i o image, f o l l o w e d by z e o l i t e > c l a y > added b u l k a l u m i n a . The use o f l i n e p r o f i l e methods t o d e t e r m i n e t h e d i g i t a l grey l e v e l s (0-255) p r e s e n t a c r o s s the r a t i o image i s demonstrated in



272


f i g u r e Id. At t h i s point in the analysis it is necessary to d e t e r m i n e the " c u t o f f " g r e y l e v e l s which demark each o f the p h a s e s . Our e x p e r i e n c e has shown t h a t a l t h o u g h the eye seems t o be r e a s o n ably able to " s e e " the position of the phases in the Si/Al i n t e n s i t y r a t i o image, perhaps because o f the b r a i n ' s a b i l i t y to see " o b j e c t s " i n s t e a d o f "grey l e v e l s " , i t is often d i f f i c u l t to a c c u r a t e l y p i c k the grey l e v e l c u t o f f s f o r each p h a s e . Absolute comparison o f the g r e y l e v e l s p r e s e n t i n a s e t o f images w i t h t h o s e d e t e r m i n e d u s i n g s t a n d a r d s i n g l e phase samples i s d i f f i c u l t due to " d a y - t o - d a y " changes i n i n s t r u m e n t t r a n s m i s s i o n , primary ion beam s t a b i l i t y and u n i f o r m i t y , and t o the " a u t o r a n g i n g " o f the image c o l l e c t i o n system (which i n c r e a s e s t h e s y s t e m ' s g a i n f o r weaker s i g n a l s to make the f i n a l images v i e w a b l e ) . For comparison to s e p a r a t e s t a n d a r d s , n o r m a l i z a t i o n o f the b r i g h t n e s s l e v e l s u s i n g a s e p a r a t e measurement o f the i o n c o u n t s - p e r - s e c o n d f o r each element a r i s i n g from the entire f i e l d o f view must be d o n e . For these r e a s o n s i t i s b e s t and most c o n v e n i e n t t o use known phases within the a n a l y s i s f i e l d of view i t s e l f , either within the particles being analyzed or added as "internal standards" t o the mounting m a t e r i a l , to determine the grey l e v e l s representative of each phase. After the grey level representative of each phase is d e t e r m i n e d from l a r g e and w e l l d e f i n e d r e g i o n s the d i f f i c u l t y s t i l l remains as t o how t o a s s i g n the " p i x e l s " whose v a l u e s f a l l between t h e g r e y l e v e l s d e t e r m i n e d as " a v e r a g e " f o r each p h a s e . Due t o the l i m i t s of resolution o f the technique (about 0 . 5 micron), the intimate mixing of the p h a s e s , and the u b i q u i t o u s presence of binder, there e x i s t s a range o f measured g r e y l e v e l s for each phase. Normally the selection of a grey level evenly spaced between the average o f two phases may be used as a c u t o f f l e v e l to d e l i n e a t e the b o u n d a r i e s o f each p h a s e . F i g u r e l e shows the grey l e v e l c u t o f f s chosen t o i n d i c a t e the h i g h s i l i c a (clumped binder), z e o l i t e , c l a y and alumina phases p r e s e n t i n t h i s c a t a l y s t p a r t i c l e . Experience has shown that the average area concentrations d e t e r m i n e d on s t a n d a r d m a t e r i a l s with known c o n c e n t r a t i o n s of z e o l i t e and a l u m i n a phases agrees w i t h i n +20-30% r e l a t i v e once the c o n v e r s i o n from weight concentration to volume c o n c e n t r a t i o n has been made. Improvement, a u t o m a t i o n , and mathematical j u s t i f i c a t i o n o f t h i s p r o c e s s w i l l be a goal o f f u t u r e s t u d y . Once the g r e y l e v e l s r e p r e s e n t i n g each o f the phases have been d e t e r m i n e d , a " s e m i - q u a n t i t a t i v e phase map" such as t h a t shown in f i g u r e I f may be g e n e r a t e d . The w h i t e a r e a s i n t h i s image show the h i g h s i l i c a p h a s e , the l i g h t g r e y shows the z e o l i t e p h a s e , the d a r k g r e y the c l a y phase and the v e r y d a r k g r e y the added b u l k alumina. From such an analysis both the position and the "area c o n c e n t r a t i o n " f o r each phase may be d e t e r m i n e d . It must be kept i n mind t h a t the area concentrations o f each phase d e t e r m i n e d by viewing c r o s s - s e c t i o n e d c a t a l y s t p a r t i c l e s are p r o p o r t i o n a l to the o c c u p i e d volume f r a c t i o n s but must be c o r r e c t e d both f o r a p h a s e ' s absolute and "packed" d e n s i t i e s before comparing the "area c o n c e n t r a t i o n s " t o c o n v e n t i o n a l weight f r a c t i o n s . D i s t r i b u t i o n o f N i c k e l and Vanadium Between P h a s e s . A l o n g w i t h the d e t e r m i n a t i o n o f the amount o f each phase p r e s e n t i n individual catalyst particles it i s very useful t o be a b l e to quantify the


Petroleum Cracking Catalyst Characterization 273


17. LETA ET AL.

F i g u r e 1. Phase i d e n t i f i c a t i o n u s i n g S i / A l r a t i o images, a) s i l i c o n b) aluminum c) Si/Al ratio image d) intensity profile e) s e l e c t i o n o f i n t e n s i t y c u t - o f f s f ) " s e m i - q u a n t i t a t i v e " phase map


Si/Al


274


d i s t r i b u t i o n o f t r a c e p o i s o n s such as n i c k e l and vanadium between those phases. Of p a r t i c u l a r importance i s t h e degree to which vanadium r e s i d e s on the z e o l i t e phase o f a c a t a l y s t . It has been found t h a t vanadium d e a c t i v a t e s c r a c k i n g c a t a l y s t s by d e c r y s t a l l i z i n g t h e z e o l i t e component.(7) In g e n e r a l , we have seen t h a t the g r e a t e r t h e f r a c t i o n o f vanadium which r e s i d e s on t h e z e o l i t e phase the g r e a t e r the a c t i v i t y loss suffered due t o vanadium attack. T h i s i s t r u e f o r both l a b o r a t o r y doped and e q u i l i b r i u m catalysts. Although l a b o r a t o r y methods o f vanadium d o p i n g may not quantit a t i v e l y mimic t h e d i s t r i b u t i o n s o f vanadium found i n equilibrium c a t a l y s t s , f i n a l vanadium d i s t r i b u t i o n s as d i s c e r n i b l e by imaging SIMS i n e i t h e r s i t u a t i o n seem t o c o r r e l a t e w e l l w i t h t h e degree of d e a c t i v a t i o n experienced (in that s i t u a t i o n ) . We have a l s o seen that while vanadium has a s i g n i f i c a n t degree o f mobility both w i t h i n and between p a r t i c l e s under normal FCC c o n d i t i o n s , nickel t e n d s t o be i m m o b i l i z e d once d e p o s i t e d . As s u c h , n i c k e l distribut i o n s may be used t o both d e t e r m i n e the approximate time a p a r t i c l e has been r e c i r c u l a t i n g i n an FCC u n i t ( ) , as w e l l as t o show on which phases the c r a c k i n g occurs f o r the n i c k e l porphyrin type m o l e c u l e s p r e s e n t i n FCC f e e d s . 8

At t h i s p o i n t i t may be a p p r o p r i a t e t o b r i e f l y discuss the mechanism o f s i g n a l f o r m a t i o n i n the SIMS p r o c e s s . A primary ion beam impinges upon the sample s u r f a c e and c a u s e s t h e s p u t t e r i n g of atoms and small m o l e c u l a r c l u s t e r s , some o f which a r e i o n i z e d by the energy o f c o l l i s i o n , u s u a l l y s e v e r a l thousands o f e V . The i o n s thus created are available for analysis by c o l l e c t i o n using electrostatic acceleration and mass s p e c t r o m e t r i c separation f o l l o w e d by s e q u e n t i a l i o n c o u n t i n g o r d i r e c t image f o r m a t i o n (the t y p e o f i n s t r u m e n t used i n t h i s s t u d y a c t u a l l y keeps the sputtered i o n s f o c u s s e d as t h e y are mass s e p a r a t e d through t h e use o f ion lenses). In the s p u t t e r i n g event i t s e l f a s i g n i f i c a n t amount of atomic m i x i n g o f the o u t e r 20-30 angstroms of a material occurs, c r e a t i n g an amorphous and w e l l - m i x e d r e g i o n . It i s from the top few atomic l a y e r s o f t h i s r e g i o n , which i s i n s t e a d y s t a t e w i t h the s e v e r a l angstroms per second s p u t t e r i n g r e c e s s i o n o f t h e surface, t h a t the c o l l e c t e d ions are c r e a t e d . The r a p i d sputtering and m i x i n g s i t u a t i o n p r e v a l e n t i n the "dynamic SIMS" p r o c e s s here has several b e n e f i c i a l characteristics when a p p l i e d to the charact e r i z a t i o n o f FCC c a t a l y s t p a r t i c l e s . F i r s t , the r a p i d sputtering a l l o w s the top s e v e r a l m i c r o n s o f p r e p a r e d c r o s s - s e c t i o n a l surface t o be removed i n a reasonable period of time, thus atomically removing m i x i n g and smearing effects which might have occurred during p o l i s h i n g . S e c o n d l y , the atomic m i x i n g o f t h e s u r f a c e t e n d s t o e l i m i n a t e " o v e r l a y e r " e f f e c t s which might cause one element to "conceal" another. T h i r d l y , i n t h e c a s e o f o x i d e c a t a l y s t s such as t h e s e , the atomic mixing phenomena tends to distribute oxygen t h r o u g h o u t the s p u t t e r i n g region in a r e l a t i v e l y uniform manner, where i t has the e f f e c t of " b u f f e r i n g " any m a t r i x effects on elemental ion y i e l d s . A l t h o u g h the ion y i e l d s f o r the elements ( i . e . the i o n s c r e a t e d r a t i o e d t o the atoms s p u t t e r e d ) v a r y g r e a t l y from element t o e l e m e n t , f o r any one element t h i s o x i d e buffering e f f e c t m i n i m i z e s the changes i n i o n y i e l d from phase t o phase thus g r e a t l y r e d u c i n g the d i f f i c u l t i e s normally encountered in making SIMS r e s u l t s even " s e m i - q u a n t i t a t i v e . "



17. LETA ET AL.


275

SIMS i o n images taken o f the A l , S i , L a , V and Ni distrib u t i o n s on s e v e r a l p a r t i c l e s o f e q u i l i b r i u m c a t a l y s t a r e shown in Figure 2a-2e, r e s p e c t i v e l y . T h i s c a t a l y s t i s o f the type designed t o c r a c k vacuum gas o i l s and c o n t a i n s o n l y rare earth exchanged zeolite, clay and b i n d e r . In this catalyst t y p e , much o f the z e o l i t e i s p r e s e n t i n 5-10 micron diameter clumps. This clumping makes the v i s u a l i z a t i o n o f t h i s phase p o s s i b l e e i t h e r as d a r k grey a r e a s i n t h e aluminum image o r as the b r i g h t a r e a s i n t h e lanthanum image which d i r e c t l y shows the r a r e e a r t h doping in the zeolites. Q u a l i t a t i v e l y , vanadium may be seen t o be p r e s e n t a t h i g h e r c o n c e n t r a t i o n s ( b r i g h t e r ) i n the r e g i o n s o f the p a r t i c l e s which contain z e o l i t e , while nickel shows a s t r o n g " r i m " d i s t r i b u t i o n on these c r o s s - s e c t i o n a l views. C l o s e r i n s p e c t i o n o f the n i c k e l image also r e v e a l s t h a t w h i l e much o f the i n t e r i o r o f the p a r t i c l e s has a low, but v i s i b l e amount o f n i c k e l , the r e g i o n s where z e o l i t e i s present show almost no n i c k e l signal. As p r e v i o u s l y stated nickel's a p p a r e n t l a c k o f m o b i l i t y , here demonstrated even w i t h i n i n d i v i d u a l catalyst particles, has caused the nickel t o be found in the l o c a t i o n s where the p o r p h y r i n m o l e c u l e s were c r a c k e d . Very r e a s o n a b l y , i n t h i s c a s e where the z e o l i t e c r y s t a l s a r e clumped together and a l l o w l i t t l e a c c e s s t o t h e i r e x t e r n a l s u r f a c e a r e a s , t h e n i c k e l may be seen t o be e x c l u d e d from t h e s e r e g i o n s because o f t h e large porphyrin molecules' i n a b i l i t y to d i f f u s e into z e o l i t i c channels. F i g u r e 3a-3e demonstrates an image p r o c e s s i n g methodology u s e f u l f o r t h e g e n e r a t i o n o f " b i n a r y phase masks" which may be used to define a r e a s from which the integrated i n t e n s i t i e s of other e l e m e n t s , such as n i c k e l and vanadium, may be d e t e r m i n e d . Figure 3a shows a b i n a r y image g e n e r a t e d by u s i n g e i t h e r t h e S i o r t h e Al image and setting all i n t e n s i t y values above a c u t o f f v a l u e to w h i t e , t o d e f i n e the r e g i o n s o f the images r e p r e s e n t i n g t h e entire catalyst particles. Vanadium and n i c k e l integrated intensities (summed g r e y l e v e l s ) from w i t h i n t h i s r e g i o n , d i v i d e d by t h e number o f p i x e l s i n t h e r e g i o n , i . e . the mean p i x e l i n t e n s i t y v a l u e , g i v e s the t o t a l " r e l a t i v e c o n c e n t r a t i o n " o f the elements on t h e catalyst particles. G e n e r a t i o n o f a b i n a r y map o f the z e o l i t e phase i s the next s t e p in determining the t r a c e element phase distributions. S i n c e the b i n d e r phase i s v e r y homogeneously d i s t r i b u t e d (as is normally the case) i t w i l l not be c o n s i d e r e d as a s e p a r a t e phase ( p r i m a r i l y due t o our i n a b i l i t y to separately view i t ) . Thus the d e t e r m i n a t i o n o f the z e o l i t e d i s t r i b u t i o n w i l l subtend t h e images i n t o two p h a s e s , z e o l i t e and c l a y . As p r e v i o u s l y d i s c u s s e d , the f o r m a t i o n o f a Si/Al intensity r a t i o image g i v e s a u s e f u l map o f the phases w i t h i n t h e catalyst particles. Regions o f high S i / A l such as z e o l i t e and clumped b i n d e r phases show up as b r i g h t a r e a s i n t h e r a t i o image. Figure 3b shows such an image f o r t h e s e "gas o i l " c a t a l y s t particles. C o n s e r v a t i v e s e l e c t i o n o f a high c u t o f f v a l u e t o i n c l u d e both the z e o l i t e and b i n d e r r e g i o n s l e a d s t o the g e n e r a t i o n o f the binary image shown i n figure 3c. Since t h i s type of catalyst contains r a r e e a r t h exchanged z e o l i t e s , we may a l s o use t h e La i o n image to d e f i n e regions of z e o l i t e phase. G e n e r a t i n g a b i n a r y mask from the La image l e a d s t o figure 3d. We have found t h a t a l t h o u g h the La image i t s e l f s h o u l d p r o v i d e a good mask o f t h e z e o l i t e p h a s e , due t o the weak s i g n a l s o f the r a r e e a r t h e l e m e n t s , each o f t e n present




276

Figure 2. particles, e) n i c k e l

Ion images a) aluminum

of equilibrium "VGO t y p e " b) s i l i c o n c ) lanthanum d)

catalyst vanadium




LETA ET AL.

Figure 3. Binary phase masks o f equilibrium VGO catalyst images. a) p a r t i c l e l o c a t i o n mask b) S i / A l i n t e n s i t y ratio c) h i g h S i / A l r a t i o mask d) h i g h lanthanum mask e) zeolite mask ( l o g i c a l AND o f c & d)


277

278


at tenths of percent c o n c e n t r a t i o n s , t h e images may n o t show sufficient clarity to properly define the z e o l i t e regions. P e r f o r m i n g a " l o g i c a l and" o p e r a t i o n on t h e two b i n a r y masks, 3c and 3 d , e l i m i n a t e s both t h e r e g i o n s o f high binder concentration shown i n t h e S i / A l b i n a r y image and t h e r e g i o n s i n t h e La b i n a r y image which a r e caused by i m p e r f e c t i o n o f f o c u s . Such an image, which d e f i n e s t h e z e o l i t e p h a s e , and by d i f f e r e n c e t h e c l a y phase, i s shown i n f i g u r e 3 e .

TABLE 1


EQUILIBRIUM "GAS OIL" CATALYST Phase A r e a Cone.

% of V (RO

% o f Ni (RC)

13.4%

17.4% ( 1 . 3 0 )

6.2% ( 0 . 4 6 )

86.4%

82.6% ( 0 . 9 6 )

93.8% ( 1 . 0 9 )

EQUILIBRIUM "RESID RESISTANT" CATALYST Phase A r e a Cone.

% o f V (RC)

% o f Ni (RC)

24.3% Z e o l i t e

28.1% ( 1 . 1 6 )

24.4% ( 1 . 0 0 )

3.8% B u l k A l u m i n a

5.0% ( 1 . 3 2 )

5.9% ( 1 . 5 5 )

71.9% C l a y

66.9% ( 0 . 9 3 )

69.7% ( 0 . 9 7 )

M e a s u r i n g t h e t o t a l s i g n a l i n t e n s i t i e s o f vanadium and n i c k e l w i t h i n t h e t o t a l p a r t i c l e mask and t h e z e o l i t e mask r e g i o n s allows t h e q u a n t i f i c a t i o n o f t h e phase d i s t r i b u t i o n s o f these elements. The t o p h a l f o f T a b l e 1 p r e s e n t s t h e r e s u l t s o f such an a n a l y s i s on t h e s e images o f gas o i l catalyst. The number o f p i x e l s contained i n t h e z e o l i t e mask d i v i d e d by t h e number o f p i x e l s i n t h e p a r t i c l e mask d i r e c t l y g i v e s t h e " a r e a p e r c e n t " o f t h e z e o l i t e p h a s e , and by d i f f e r e n c e t h a t o f t h e c l a y ( t h e v a l u e s f o r c l a y i n c l u d e most of the c o n t r i b u t i o n s o f the binder phase). The f r a c t i o n o f the vanadium and n i c k e l i n t h e d e f i n e d z e o l i t e r e g i o n s d i v i d e d by t h e s i g n a l s from t h e p a r t i c l e mask r e g i o n s g i v e s t h e numbers reported i n t h e t a b l e as "% o f V" ( o r N i ) . This f i g u r e i s the percentage o f the t o t a l quantity o f the trace element which r e s i d e s on t h e z e o l i t e phase. "RC" i n t h e t a b l e i s used t o i n d i c a t e t h e " r e l a t i v e c o n c e n t r a t i o n " f a c t o r , which i s t h e p e r c e n t a g e o f t h e element on a phase d i v i d e d by t h e a r e a f r a c t i o n o f t h a t p h a s e . Here an RC v a l u e o f 1.00 would mean t h a t t h e phase c o n t a i n s a c o n c e n t r a t i o n o f t h e element which i s equal t o t h e average c o n c e n t r a t i o n o f t h e element on t h e t o t a l p a r t i c l e . RC f a c t o r s g r e a t e r than 1.0 mean t h a t the phase has a h i g h e r than average c o n c e n t r a t i o n (and c o n v e r s e l y f o r v a l u e s o f l e s s than 1.0). It may be seen t h a t i n t h i s case,



17. LETAETAL.


279

vanadium i s s i g n i f i c a n t l y accumulated on the z e o l i t e p h a s e , w i t h an RC f a c t o r o f 1.3. As was seen q u a l i t a t i v e l y however, n i c k e l is m o s t l y e x c l u d e d from the z e o l i t e r e g i o n s w i t h an RC f a c t o r o f 0 . 4 6 . SIMS e l e m e n t a l images taken o f a second t y p e o f equilibrium commercial c a t a l y s t , marketed as " r e s i d r e s i s t a n t " , a r e shown in f i g u r e 4. C a r e f u l i n s p e c t i o n o f the A l and S i images r e v e a l s the presence of dark regions i n the S i image and c o r r e s p o n d i n g light r e g i o n s i n the A l image. These r e g i o n s a r e b u l k a l u m i n a which has been added t o the c a t a l y s t during manufacture. The use o f image s u b t r a c t i o n , as demonstrated i n the S i - A l image ( f i g . 4 c ) , quickly r e v e a l s the l o c a t i o n s o f t h i s alumina p h a s e . In the s u b t r a c t e d i o n images the alumina appears as black regions. In our s y s t e m , as w e l l as many commercial image p r o c e s s i n g s y s t e m s , image s u b t r a c t i o n (as opposed t o the more calculation intensive image division o p e r a t i o n s ) may be performed at v i d e o r a t e s which makes the i d e n t i f i c a t i o n o f added alumina i n FCC p a r t i c l e s possible in real time w h i l e c o l l e c t i n g the SIMS d a t a . The lanthanum i o n image ( f i g . 4d) shows t h a t i n t h i s c a t a l y s t type the d i s t r i b u t i o n o f the zeolite phase i n t h i s c a t a l y s t i s much more homogeneous than the p r e v i o u s l y c i t e d example. The vanadium i o n image ( f i g . 4e) shows t h a t the a l u m i n a phase has g e t t e r e d the vanadium, which shows t h e brightest areas in the locations of the alumina. The alumina in the catalyst, in addition to likely providing increased "bottoms cracking", is making the c a t a l y s t somewhat "resid resistant" by g e t t e r i n g a f r a c t i o n o f the vanadium. It must be n o t e d , however, t h a t t h e z e o l i t e phase ( f i g . 4d) i s a l s o " t r a p p i n g " a significant portion of the vanadium. The n i c k e l concentrations (fig. 4f) v a r i e s somewhat from p a r t i c l e to p a r t i c l e due t o the particles' d i f f e r e n t residence times (ages) i n the u n i t , but within each p a r t i c l e n i c k e l may be seen t o r e s i d e t o a s i g n i f i c a n t degree on t h e alumina phase and " c l o s e r t o the o u t s i d e " o f the particles. Once a g a i n , the nickel distribution is likely indicating the locations within the p a r t i c l e s where the organometallic nickel s p e c i e s were decomposed. Image p r o c e s s i n g methodology, similar to that described e a r l i e r , may be used t o q u a n t i f y the phase d i s t r i b u t i o n s o f the n i c k e l and vanadium i n t h e s e images. The q u a n t i t a t i v e r e s u l t s of such an a n a l y s i s may be found i n the bottom o f T a b l e 1. Here, the RC f a c t o r f o r vanadium on the z e o l i t e has been lowered from the 1.30 o b s e r v e d i n the gas o i l c a t a l y s t example t o 1.16. T h i s may be p o s t u l a t e d t o be due t o t h e t r a p p i n g o f some o f t h e vanadium by the a l u m i n a p h a s e , w h i c h , w i t h an RC o f 1.32 i s a c t i n g as a s i n k for vanadium. Even if the trapping effect of the alumina is a temporary o n e , i t d e c r e a s e s the e f f e c t i v e a c t i v i t y o f the vanadium i n the r e g e n e r a t o r . A l t h o u g h the q u a n t i t y o f vanadium t r a p p e d by the small q u a n t i t y o f alumina i n t h i s c a t a l y s t i s o n l y 5% o f the t o t a l , i t may be t h a t t h i s mechanism h e l p s reduce t h e a c t i v i t y of the m o b i l e vanadium i n the r e g e n e r a t o r enough t o make a d i f f e r e n c e . As noted b e f o r e , the nickel is also located on t h e alumina phase and here shows a 1.55 RC f a c t o r . The n i c k e l ' s RC o f 1.00 on the z e o l i t e phase (average n i c k e l c o n t e n t ) , as opposed t o b e i n g r e d u c e d as shown f o r the gas oil c a t a l y s t , may be due to the more accessible external reactive surface of the well-distributed z e o l i t e c r y s t a l s in t h i s c a t a l y s t type.


FLUID CATALYTIC CRACKING H: CONCEPTS IN CATALYST DESIGN


280

F i g u r e 4. Ion images o f e q u i l i b r i u m "Resid type" p a r t i c l e s , a) aluminum b) s i l i c o n c) s i l i c o n minus d) lanthanum e) vanadium f ) n i c k e l

catalyst aluminum


17. LETA ET AL.


Distributions

Petroleum Cracking Catalyst Characterization From P a r t i c l e t o

281

Particle

One o f the primary benefits i n the use o f the imaging SIMS t e c h n i q u e t o view FCC c a t a l y s t p a r t i c l e s l i e s in its ability to i d e n t i f y p a r t i c u l a r c a t a l y s t p a r t i c l e s or types in mixtures. This is particularly important in characterizing equilibrium catalyst p a r t i c l e s where t h e r e e x i s t s a range o f p a r t i c l e ages ( i . e . , time i n the u n i t ) as w e l l as the p o s s i b i l i t y o f c a t a l y s t s from d i f f e r e n t vendors a n d / o r s e p a r a t e a d d i t i v e - c o n t a i n i n g p a r t i c l e s . Used at low m a g n i f i c a t i o n s and working with p a r t i c l e s embedded at reasonably high d e n s i t i e s , i t i s p o s s i b l e t o s i m u l t a n e o u s l y view as many as one hundred particle cross-sections. Although at low m a g n i f i c a t i o n s i t may not be p o s s i b l e t o view t h e phases within each c a t a l y s t p a r t i c l e , c o l l e c t i n g images o f a l l o f t h e elements o f interest in a single region followed by q u a n t i t a t i v e image p r o c e s s i n g p r o v i d e s the e q u i v a l e n t o f as many as one hundred m u l t i element a n a l y s e s . Q u a ! i t a t i v e O b s e r v a t i o n s . As an example o f m u l t i - p a r t i c l e imaging SIMS a n a l y s i s , e q u i l i b r i u m c a t a l y s t sampled from an FCC u n i t which had r e c e n t l y begun adding f r e s h c a t a l y s t from a d i f f e r e n t vendor has been c h o s e n . In this s i t u a t i o n , we have a handle on the maximum l e n g t h o f time which a new t y p e c a t a l y s t p a r t i c l e has been i n the reactor, as w e l l as the minimum time f o r an o l d type catalyst particle. (Although mixing in the fresh catalyst feed s i l o adds u n c e r t a i n t y t o the s i t u a t i o n , as w i l l be seen i n the following analysis.) A s e t o f ~400 m i c r o n f i e l d - o f - v i e w SIMS images o f an equilibrium sample taken a p p r o x i m a t e l y f o u r weeks into a catalyst type change i s shown i n F i g u r e 5. It may be seen t h a t t h e lanthanum (5b) and c e r i u m (5d) d i s t r i b u t i o n s are not i d e n t i c a l . Using the mass s p e c t r a l mode o f SIMS a n a l y s i s on s e p a r a t e samples o f the old and new c a t a l y s t t y p e s i t was found t h a t the o l d c a t a l y s t (longer r e s i d e n c e time i n the u n i t ) had a lanthanum t o cerium r a t i o of a p p r o x i m a t e l y t w i c e the new t y p e . T h i s d i f f e r e n c e may be used to r a p i d l y i d e n t i f y the vendor o f each i n d i v i d u a l p a r t i c l e . Figure 5f shows the " c e r i u m minus lanthanum" image i n which the o l d p a r t i c l e s appear v e r y d a r k and the new p a r t i c l e s appear w h i t e . The nickel i n t e n s i t i e s (5e) a r e , q u a l i t a t i v e l y , the i n v e r s e o f t h i s s u b t r a c t e d image. That i s , the p a r t i c l e s known t o be o l d e r may be seen to c o n t a i n more n i c k e l . The vanadium image (5c) a l s o shows a c o r r e l a t i o n w i t h a g e , but the c o n t r a s t from o l d t o new p a r t i c l e s i s much l e s s than f o r n i c k e l . T h i s may be i n t e r p r e t e d t o demonstrate that vanadium has e x h i b i t e d a significant, but finite, degree of p a r t i c l e t o p a r t i c l e m o b i l i t y i n the FCC u n i t . Quantitative Multi-particle Analysis. In o r d e r to determine the r e l a t i v e c o n c e n t r a t i o n s o f each o f t h e e l e m e n t s - o f - i n t e r e s t (here t a k e n t o mean a d i m e n s i o n l e s s measure o f the variations of each element's concentration from p a r t i c l e to particle within the f i e l d - o f - v i e w ) , the s i l i c o n image i s used t o g e n e r a t e a b i n a r y mask which d e f i n e s the p o s i t i o n o f each o f the c a t a l y s t p a r t i c l e s . In cases where the particles are touching, it is additionally n e c e s s a r y t o m a n u a l l y draw a l i n e o f s e p a r a t i o n between them. Once




282

F i g u r e 5. Low m a g n i f i c a t i o n i o n images o f e q u i l i b r i u m c a t a l y s t p a r t i c l e s sampled f o u r weeks i n t o a c a t a l y s t t y p e c h a n g e , a) s i l i c o n b) lanthanum c) vanadium d) c e r i u m e) n i c k e l f ) c e r i u m minus lanthanum



17.

LETA ETAL.


283

t h i s p a r t i c l e mask i s g e n e r a t e d , the mean i n t e n s i t y o f each o f the e l e m e n t s - o f - i n t e r e s t i s determined w i t h i n each o f the "particle" regions (proprietary software). In t h i s c a s e , t h e r e l a t i v e c o n c e n t r a t i o n s o f S i , A l , L a , C e , Ni and V on each p a r t i c l e were determined. To show the p o t e n t i a l o f such a n a l y s i s , the r e l a t i o n s h i p o f vanadium t o the other elements was d e t e r m i n e d i n order to gain i n s i g h t i n t o the f a c t o r s c o n t r o l l i n g i t s f i n a l d i s t r i b u t i o n . F i g u r e 6 shows the r e l a t i o n s h i p o f vanadium t o n i c k e l f o r the sample mentioned above, taken f o u r weeks a f t e r the c a t a l y s t type change. Note t h a t the u n i t s on t h i s , and the f o l l o w i n g figures, a r e t h e mean g r e y l e v e l s from each p a r t i c l e , r a n g i n g from 0-255, which were d i g i t i z e d from the SIMS i o n images. The new catalyst p a r t i c l e s , as determined by t h e i r lower c a l c u l a t e d La/Ce ratios, are i d e n t i f i e d by a " * " . In f i g u r e 6 the n i c k e l c o n c e n t r a t i o n , shown on t h e X - a x i s , is intended to indicate the "age" of the individual particles. A l t h o u g h the nickel concentration o f the particles is certainly p r o p o r t i o n a l to t h e i r age, it is l i k e l y t h a t the r e l a t i o n s h i p is not e x a c t , and may be a f f e c t e d by p a r t i c l e s i z e . It has been shown that nickel i s often, but not a l w a y s , found to r e s i d e in a "rim" distribution on the cross-sectioned catalyst particles. This i n d i c a t e s t h a t the n i c k e l - c o n t a i n i n g m o l e c u l e s o f t e n c r a c k r a p i d l y , r e l a t i v e t o t h e i r speed o f d i f f u s i o n i n t o the c a t a l y s t particles. T h i s d i f f u s i o n l i m i t a t i o n l e a d s t o the p o s s i b i l i t y t h a t the " q u a n t ity of nickel on a p a r t i c l e per the p a r t i c l e ' s external surface a r e a " (here meaning the a r e a o f the s u r f a c e o f the ~30-90 micron s p h e r e s , not the " s u r f a c e a r e a " o f the c a t a l y s t ) would be a more c o r r e c t d e t e r m i n a n t o f age than would the n i c k e l p e r volume, which i s the c o n c e n t r a t i o n . Two f a c t o r s have p r e v e n t e d us from a p p l y i n g t h e e f f e c t s o f t h i s d i f f u s i o n l i m i t a t i o n , which may l e a d t o larger p a r t i c l e s showing lower c o n c e n t r a t i o n s o f n i c k e l a t equal age than smaller ones, in the c a l c u l a t i o n o f the p a r t i c l e s ' age distributions. F i r s t , we have seen t h a t the degree o f " r i m " distribution for nickel v a r i e s among d i f f e r e n t e q u i l i b r i u m catalyst samples, p r o b a b l y due t o d i f f e r e n c e s in the d i f f u s i o n limitations of the catalyst types. S e c o n d l y , a l t h o u g h we do d e t e r m i n e the viewed a r e a o f each o f the p a r t i c l e s , we have no way of determining to what point in its cross-section an individual particle has been polished. Therefore, only a s t a t i s t i c a l analysis of large numbers of p a r t i c l e s , combined w i t h quantitative determination of the radial d i s t r i b u ti o n functions f o r n i c k e l , would a l l o w the proper use o f a f a c t o r e x p r e s s i n g the mix between N i / v o l u m e and N i / s u r f a c e a r e a t o be u s e d . Combined w i t h the n e c e s s a r y assumption t h a t the concentration of nickel i n the FCC u n i t ' s f e e d has been very c o n s t a n t o v e r a y e a r - l o n g p e r i o d ( o f t e n known t o be f a l s e ) l e a d s us t o m e r e l y use t h e n i c k e l c o n c e n t r a t i o n as the indicator of age. Returning to f i g u r e 6, we may note t h a t even w i t h a l l of these c a v e a t s , the use o f mean n i c k e l i n t e n s i t y as the age i n d i c a t o r has given a very good s e p a r a t i o n o f the new p a r t i c l e s ("*") on the l e f t , from the old particles on the right. Note the one "new" p a r t i c l e on the extreme r i g h t o f the p l o t . Although i t appears that this p a r t i c l e i s m i s p l a c e d on the p l o t , an analysis of its S i / A l r a t i o shows i t to be d i f f e r e n t from the o t h e r new particles. A check o f the r e f i n e r y ' s records revealed that although they had



0

20

40

60

60

100

120

140

160

160

200

220

0

I

20

I

I

40

I

I

I

I

60

I

I

100

Nickel Intensity (Cone. - Age)

60

I

I

120

I

I

140

I

I

160

Figure 6. V vs N i - four weeks into a catalyst type change. The "*" always indicates new catalyst type.

I

CP



17. LETA ET AL.


285

been u s i n g t h e " o l d " t y p e o f c a t a l y s t f o r about a y e a r , p r i o r to t h a t time t h e y had been u s i n g a c a t a l y s t from t h e same vendor as t h a t o f the "new" t y p e . T h i s " o l d o l d " c a t a l y s t was o f a d i f f e r e n t binder type, explaining i t s d i f f e r e n t S i / A l r a t i o , but b e i n g from t h e same v e n d o r , happens t o have the same L a / C e r a t i o as our new catalyst particles. From t h e vanadium c o n c e n t r a t i o n s shown i n the figure, we i m m e d i a t e l y d i s c e r n two f a c t s . F i r s t , even at v e r y s h o r t t i m e s in t h e u n i t t h e p a r t i c l e s have become impregnated w i t h a significant q u a n t i t y o f vanadium. Secondly, this rapid d i f f u s i o n of vanadium does not lead to a total homogeneity o f vanadium concentration r e g a r d l e s s o f age. It would seem l i k e l y t h a t s e v e r a l mechanisms o f vanadium b i n d i n g o r t r a n s p o r t are o c c u r r i n g , leading to a mixture o f s h o r t and l o n g vanadium i m p r e g n a t i o n k i n e t i c s . F i g u r e 7 shows a s i m i l a r a n a l y s i s o f a n o t h e r sample t a k e n from t h e same u n i t f o u r weeks l a t e r . A l t h o u g h not as t i g h t l y separated i n t o o l d and new p a r t i c l e s , the same g e n e r a l o b s e r v a t i o n s still hold. It i s b e l i e v e d t h a t some o f t h e m i x i n g o f o l d and new type particles is r e a l , o c c u r r i n g due t o the m i x i n g of old and new c a t a l y s t types i n the "fresh c a t a l y s t feed s i l o " leading to a g r a d u a l change t o the new c a t a l y s t t y p e , and t h a t some o f the mixing i s due to the difficulties encountered with correctly determining a p a r t i c l e ' s age. The large number of particles sampled i n t h i s case (71) as w e l l as t h e more even m i x t u r e of s e v e r a l c a t a l y s t t y p e s , makes t h i s d a t a b e t t e r s u i t e d f o r use as a d a t a base f o r an e m p i r i c a l view o f the f a c t o r s a f f e c t i n g vanadium c o n c e n t r a t i o n s on i n d i v i d u a l p a r t i c l e s . In o r d e r t o e m p i r i c a l l y remove t h e e f f e c t s o f a g e , as determined by the n i c k e l c o n c e n t r a t i o n , from t h e vanadium distribution, a regression of the d a t a was performed to determine a best fit f u n c t i o n a l form r e l a t i n g vanadium w i t h n i c k e l . With t h e f o r c e d use o f a z e r o i n t e r c e p t , f o r t h i s case (and s e v e r a l o t h e r s , not shown) b e s t r e s u l t s were o b t a i n e d f o r f u n c t i o n s o f the t y p e : V =

A*(NiV3).

No fundamental explanation for this empirically derived relat i o n s h i p has y e t been p o s t u l a t e d . The g e n e r a l shape o f t h i s type o f f u n c t i o n i s r e a s o n a b l e i f one c o n s i d e r s the m o b i l i t y o f vanadium and the l a c k o f m o b i l i t y o f n i c k e l . The c o n s t a n t "A" must be such t h a t young p a r t i c l e s have a g r e a t e r V / N i r a t i o than that of the f e e d and o l d p a r t i c l e s have a lower V / N i r a t i o than t h a t o f the feed. Given q u a n t i t a t i v e d e p o s i t i o n o f both m e t a l s from t h e feed t o t h e c a t a l y s t ( S ) , the average V / N i r a t i o on t h e c a t a l y s t must be equal t o t h a t i n the f e e d , but due t o the t r a n s f e r o f vanadium, o l d e r p a r t i c l e s a r e l o s i n g vanadium t o younger o n e s . Eventually, i t appears that the oldest particles approach the equilibrium s i t u a t i o n o f l o s i n g vanadium as f a s t as they a q u i r e i t , and their vanadium c o n c e n t r a t i o n a s y m p t o t i c a l l y approaches a c o n s t a n t while t h e i r nickel concentration r i s e s . F i g u r e 8 presents the c a l c u l a t e d c u r v e and t h e residual values f o r vanadium as a f u n c t i o n o f the cube r o o t o f n i c k e l . The f i t i s q u i t e r e a s o n a b l e , w i t h an R of 0.70 and a f a i r l y even s c a t t e r o f t h e r e s i d u a l s . 2



,

F i g u r e 7.

0

0 -|

220

20

!

40

,

V v s . Ni -

,

,

,

80

!

,

100

,

1

120

,

1

140

,

|

160

|

e i g h t weeks i n t o a c a t a l y s t t y p e c h a n g e .

Nickel Intensity (Cone. - Age)

60

,


|

180

I


Figure 8. Ni 1/3.

V vs

Ni - showing an

empirical curve f i t based


on

S3

fi

i

288


Searching for the next most critical determinant of the vanadium d i s t r i b u t i o n r e v e a l e d a r e l a t i o n s h i p between vanadium and the q u a n t i t y o f z e o l i t e in each p a r t i c l e , as measured u s i n g the La+Ce i n t e n s i t y v a l u e . F i g u r e 9 p r e s e n t s t h i s r e l a t i o n s h i p f o r the measured vanadium v a l u e s ( b o x e s ) , as w e l l as f o r the r e s i d u a l vanadium v a l u e s (+) a f t e r s u b t r a c t i o n o f the c a l c u l a t e d c o n t r i b u t i o n s o f t h e p a r t i c l e s ' age. Both s e t s o f v a l u e s show a weak t r e n d f o r p a r t i c l e s w i t h more z e o l i t e t o c o n t a i n more vanadium, an o b s e r v a t i o n made q u a l i t a t i v e l y e a r l i e r i n t h i s s t u d y by v i e w i n g the i n d i v i d u a l p h a s e s . Including a term i n our " f i t " t o account f o r t h i s r e l a t i o n s h i p i n the form o f V = A*(NiV3)

+ B*(La+Ce)


2

l e a d s t o improvement o f t h e R term t o 0 . 7 9 . Further i n s p e c t i o n o f the data showed a t h i r d , but even more tenuous, r e l a t i o n s h i p of vanadium w i t h the Si/Al ratio of the particles. F i g u r e 10 p l o t s t h i s r e l a t i o n s h i p f o r t h e measured d a t a and f o r the residual values after subtraction o f the calculated effects of both age and z e o l i t e content. Here, correction for t h e s e t e r m s , p r i o r t o l o o k i n g f o r a r e l a t i o n s h i p , becomes even more i m p o r t a n t due t o t h e l a c k o f independence o f the S i / A l r a t i o s from t h e age and t h e z e o l i t e levels. From the "measured" vanadium v a l u e s shown i n f i g . 10 i t can be seen t h a t the new c a t a l y s t type p a r t i c l e s have, in g e n e r a l , higher S i / A l r a t i o s than do t h e old. It i s v e r y r e a s o n a b l e f o r them t o have l e s s vanadium m e r e l y because t h e y have been i n the u n i t for less time. Additionally, since z e o l i t e has a h i g h e r S i / A l r a t i o than do t h e o t h e r phases i n the c a t a l y s t p a r t i c l e s , some s o f t e n i n g o f the apparent r e l a t i o n s h i p is l i k e l y due t o t h e changes i n z e o l i t e c o n t e n t (which we have found l e a d s t o more vanadium i n h i g h e r S i / A l c a s e s ) . Given these l i m i t a t i o n s , the residual values do show a tendency f o r there to be somewhat more vanadium i n p a r t i c l e s w i t h lower S i / A l r a t i o s . This i s p r o b a b l y an i n d i c a t i o n o f the " t r a p p i n g " e f f e c t o f alumina in the c a t a l y s t p a r t i c l e s , as demonstrated earlier. Including this term i n our f i t t o account f o r t h i s r e l a t i o n s h i p as V = A*(NiV3)

+ B*(La+Ce) +

C*(Si/Al)

2

l e a d s t o improvement o f the R term t o 0 . 8 4 . The r e l a t i v e s t a n d a r d errors of the A, B and C c o n s t a n t s are 5%, 18%, and 28%, r e s p e c t i v e l y , which s e r v e s as an i n d i c a t i o n both o f t h e q u a l i t y of t h e f i t as w e l l as showing t h e r e l a t i v e importance o f each o f the parameters to the final vanadium d i s t r i b u t i o n . We have not i n c l u d e d the a c t u a l v a l u e s f o r t h e A , B, and C terms s i n c e t h e y a r e r e l a t e d o n l y t o the "grey l e v e l " measurements, and we wish t o a v o i d any confusion with actual concentration related empirical relationships. A presentation of the final calculated "curve" for the vanadium v a l u e s , p l o t t e d a g a i n s t n i c k e l (age) i s shown i n Figure 11. A l t h o u g h we have a d m i t t e d l y taken more l i b e r t i e s with our empirical f i t than the p r o b a b l e data quality permits, we have i n t e n d e d an example o f the t y p e s o f a n a l y s i s t h a t become p o s s i b l e using the combination of modern image processing techniques and



-

120

-40

-20

0

20

40 H

60

80

100

-

140

160

180 H

200

220

•

_+_

+

•

+

+

• H

m

•

Figure

9.

100

1—

V vs.

^7v V measured

n

• „gifs

•

— i 200

S3

r

• •

300

Zeolite

Concentrations.

Zeolite Level (La+Ce) + V(meas.)-V(calc.)

H

• •

Residual After Cube Root Ni Fit


+

400

+

+ +

8

3



290

oin!peuB/\



V measured

•

V calculated

Nickel Intensity (Cone. = Age)

Fit to Cube Root Ni, Zeolite, and Si/Al

F i g u r e 11. V vs Ni - showing an empirical curve f i t N i V 3 , z e o l i t e c o n c e n t r a t i o n and S i / A l r a t i o .

-i

220

A

based

on

V(meas.) - V(calc.)


292


imaging SIMS. In more controlled (but perhaps less realistic) sample p r e p a r a t i o n s done i n the l a b o r a t o r y , such an a n a l y s i s might be e x p e c t e d t o r e v e a l more dependable r e l a t i o n s h i p s .

REFERENCES

1. 2. 3.


4.

5. 6. 7. 8.

Jaras, S., Appl. Catal., 2, 207 (1982). Kugler, E. L., and Leta, D. P., J . Catal., 109, 387 (1988). Leta, D. P., and Kugler, E. L., "Imaging SIMS in Catalysis", SIMS VI, Secondary Ion Mass Spectrometry, A. Beninghoven, A. M. Huber, H. W. Werner, eds., 373 (1987). Leta, D. P., and Kugler, E. L., American Chemical Society Symposium Series 411, "Characterization and Catalyst Development," S. A. Bradley, M. J. Gattuso, R. J . Bertolacini, Eds., ACS, Washington, D.C., 1989, pp. 354-364. Leta, D. P., "Springer Series in Chemical Physics 44", A. Benninghoven, R. J . Colton, D. S. Simons and H. W. Werner, eds., Springer-Verlay, Berlin, 1986, p. 232. Mills, G. A . , Ing. Eng. Chem. 42, 182 (1950). Ritter, R. E., Rheaume, L., Welsh, W. A . , and Magee, J . S., Oil and Gas J., 103, (July 6, 1981). Palmer J . L., and E. B. Cornelius, Journal of Applied Catalysis, vol. 35 (1987), pages 217-235.

RECEIVED June 8, 1990


Petroleum Cracking Catalyst Characterization - ACS Symposium

Recommend Documents