The influence of feedstocks and catalyst formulation on thedeactivation of FCC catalysts

Catalysts in PetroleumRefining and PetrochemicalIndustries 1995 M. Absi-Halabi et al. (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

313

THE INFLUENCE OF FEEDSTOCKS AND CATALYST F O R M U L A T I O N ON THE D E A C T I V A T I O N OF FCC CATALYSTS

R. Hughes 1, G. Hutchings 2, C.L. Koon 1, B. McGhee 3 and C.E. Snape 3

l Department of Chemical Engineering, University of Salford, M5 4W~, U.K. 2 Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K. Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, U.K. ABSTRACT The effect of quinoline and phenanthrene additions to a n-hexadecane feedstock has been determined for a model four-component FCC catalyst by means of a MAT reactor with analysis of all products and characterisation of the coke produced. Both additions lead to an overall loss in conversion; quinoline is considered to act as a poison while phenanthrene participates strongly in coke formation and the resultant coke becomes more aromatic in nature. The cracking propensity and associated coke formation have been measured for a series of FCC catalysts with differing compositions. Increasing amounts of zeolite in a matrix lead to increasing extents of conversion but with little effect on the extent of coke production. However, a pure zeolite gave a very high coke content. I. I N T R O D U C T I O N The catalytic upgrading of petroleum fractions in fluid bed/riser reactors (FCC) is a major refinery operation. Because of its industrial significance, this process has been the subject of much study, yet there remains a lack of detailed knowledge concerning the mechanisms of product formation and the deposition of coke during FCC operations. In addition, there have been relatively few studies concerning the role of poisons and feedstock additives in this complex process (1). Deactivation of catalysts, particularly by coke deposition (the main means of reversible FCC catalyst deactivation) has been the subject of intensive study over the past 50 years (2-4). Initially, the loss of activity was correlated with the time on stream, but it is now generally accepted that a more appropriate approach to understanding the effect of deactivation by coke is to relate deactivation to the deposited coke concentration (5). Furthermore, few studies on the effect of catalyst formulation on both the product distribution and coke formation have appeared in the open literature. In recent years, attention has tended to be focused on coke deposition in zeolites (6, 7) in order to characterise the coke formed. In one specific study Groten et al (8) carded out a study of coke formation using zeolite USHY with n-hexane as reactant, but in this case, as in others (6, 7) it was necessary to deposit excessive amounts of coke (> 5%) to enable characterisation of the coke deposits to be achieved. However, if demineralisation of the catalyst is used to concentrate the coke as in the present work the inherently quantitative single pulse excitation (SPE) ~3C NMR procedure may be used to characterise coke deposits on FCC catalysts at realistic levels of ca 1% by weight.

314 In this paper we present our results on a study of the deactivation and characterisation of FCC catalysts, together with product yields at realistic coke levels (0.5 to 1.0%), that are typically found on FCC catalysts during industrial operation. In particular, the effect of quinoline and phenanthrene as additives to the n-hexadecane feedstock has been studied at two concentration levels and the relative roles of these additives as catalyst poison and coke inducer are discussed. A further aspect investigated is the influence of catalyst formulation. Pure zeolites are seldom used as FCC catalysts: instead, catalysts comprise a number of components, which apart from the zeolite, may include matrix, binder and clay. In the present work, catalyst formulations ranging from 100% matrix to 100% zeolite have been examined and the influence of the various catalyst compositions on product distribution and coke formation is assessed. 2. EXPERIMENTAL A number of FCC catalysts was used in the present study. For comparison of the effects of quinoline and phenanthrene additions to the n-hexadecane feedstock a model catalyst of composition, zeolite US-Y (30%), silica binder (25%), Kaolin (25%) and pseudo Boehmite matrix (20%) was used. Quinoline and phenanthrene additions to the n-hexadecane amounted to 1% and 10%. The catalysts used to assess the effect of composition on product yields varied from a basic matrix material through a variety of zeolitic catalysts containing 20% zeolite and 15% silica binder, the remainder being clay, to a pure zeolite catalyst. Data for all the catalysts used are presented in Table 1. In addition for the 13CNMR analysis a sample of coked refinery catalyst obtained from a unit processing heavy feedstock was obtained. The coke level on this catalyst was 0.9% and 30g. of this catalyst was demineralised by standard HF treatment to produce a 250 mg sample of coke concentrate containing 65% carbon. Reactivity and coking were determined using a standard MAT test reactor (9). The products from the MAT reactor were analysed by gas chromatography (GC)and peak identifications for the liquid products were made with the aid of GC-MS used in conjunction with the concentration of the aromatic species by open column chromatography on alumina. To give a clear indication of the boiling point distribution of the products, the peaks in the chromatograms have been grouped using successive n-alkanes, although specific quantification of individual isomers was also obtained. This procedure could not be used as precisely for the n-hexadecane/quinoline mixtures because of the overlap of the quinoline and product peaks close to C12. Coke levels were measured by combustion and by the weight gain of the catalyst; both methods gave good agreement. Mass spectrometric (MS) analysis was conducted on the deactivated catalysts from the MAT reactors using a Vacuum Generators instrument in which the probe was heated from ambient to 500~ at a rate of 200~ minl and spectra over the mass range 50 to 600 were recorded every 5 s. Spectra were recorded in both electron impact (El) and chemical ionisation (CI, with ammonia) modes. A number of deactivated samples have also been analysed after extraction in chloroform to remove physically-trapped molecular species. Solid statel3C NMR analysis of the coke concentrate was carried out using a Brake MS4 100 spectrometer. The single pulse excitation procedures described elsewhere (10,11), were used to derive carbon aromaticity and the proportion of bridgehead aromatic carbon.

315 In this paper we present our results on a study of the deactivation and characterisation of FCC catalysts, together with product yields at realistic coke levels (0.5 to 1.0%), that are typically found on FCC catalysts during industrial operation. In particular, the effect of quinoline and phenanthrene as additives to the n-hexadecane feedstock has been studied at two concentration levels and the relative roles of these additives as catalyst poison and coke inducer are discussed. A further aspect investigated is the influence of catalyst formulation. Pure zeolites are seldom used as FCC catalysts: instead, catalysts comprise a number of components, which apart from the zeolite, may include matrix, binder and clay. In the present work, catalyst formulations ranging from 100% matrix to 100% zeolite have been examined and the influence of the various catalyst compositions on product distribution and coke formation is assessed. 2. EXPERIMENTAL A number of FCC catalysts was used in the present study. For comparison of the effects of quinoline and phenanthrene additions to the n-hexadecane feedstock a model catalyst of composition, zeolite US-Y (30%), silica binder (25%), Kaolin (25%) and pseudo Boehmite matrix (20%) was used. Quinoline and phenanthrene additions to the n-hexadecane amounted to 1% and 10%. The catalysts used to assess the effect of composition on product yields varied from a basic matrix material through a variety of zeolitic catalysts containing 20% zeolite and 15% silica binder, the remainder being clay, to a pure zeolite catalyst. Data for all the catalysts used are presented in Table 1. In addition for the ~3CNMR analysis a sample of coked refinery catalyst obtained from a unit processing heavy feedstock was obtained. The coke level on this catalyst was 0.9% and 30g. of this catalyst was demineralised by standard HF treatment to produce a 250 mg sample of coke concentrate containing 65% carbon. Reactivity and coking were determined using a standard MAT test reactor (9). The products from the MAT reactor were analysed by gas chromatography (GC)and peak identifications for the liquid products were made with the aid of GC-MS used in conjunction with the concentration of the aromatic species by open colunm chromatography on alumina. To give a clear indication of the boiling point distribution of the products, the peaks in the chromatograms have been grouped using successive n-alkanes, although specific quantification of individual isomers was also obtained. This procedure could not be used as precisely for the n-hexadecane/quinoline mixtures because of the overlap of the quinoline and product peaks close to C~2. Coke levels were measured by combustion and by the weight gain of the catalyst; both methods gave good agreement. Mass spectrometric (MS) analysis was conducted on the deactivated catalysts from the MAT reactors using a Vacuum Generators instrument in which the probe was heated from ambient to 500~ at a rate of 200 ~ min1 and spectra over the mass range 50 to 600 were recorded every 5 s. Spectra were recorded in both electron impact (EI) and chemical ionisation (CI, with ammonia) modes. A number of deactivated samples have also been analysed after extraction in chloroform to remove physically-trapped molecular species. Solid statel3C NMR analysis of the coke concentrate was carried out using a Brake MS4 100 spectrometer. The single pulse excitation procedures described elsewhere [ 10,11 ], were used to derive carbon aromaticity and the proportion of bridgehead aromatic carbon.

316 TABLE 1 Physical Properties of Catalysts Catalyst

Type

Surface Area m2.g -1

Alumina Wt.%

Rare Earth Oxides Wt.%

Unit Cell Size A~

MAT16

4 Component

298

37.5

BPM1

Matrix + Clay/Silicabinder

102

45.6

0.0

Z-A2

Zeolite + Clay/Silica binder

125

24.1

0.6

24.26

Z-A4

Zeolite + Clay/Silica binder

134

24.3

1.1

24.28

Z-A6

Zeolite + Clay/Silica binder

143

24.2

2.7

24.33

LZY1

Zeolite Only

24.53

26.0

3 RESULTS AND DISCUSSION 3.1 Influence of additions to n-hexadecane feed.

Using the standard model four component catalyst, experiments were carded out in the MAT reactor for a n-hexadecane feed containing 1% and 10% of quinoline and phenanthrene additions. The results obtained are presented in Figs. 1 and 2 respectively in terms of a normalised yield, defined as the weight of product divided by the weight of injected feed. In all cases a feed rate of 2.7 ml/min was used with a catalyst charge of 4 g in the reactor. The temperature of operation was 530~ Analyses of the gaseous product were made for the C1 to C5 range, while the liquid product distribution was examined in the C5 to C15 range. Liquid products were characterised using GC-MS and a range of aromatic compounds were identified in which the concentrations of alkybenzenes are greater than those of alkylindans and naphthalenes while polynuclear aromatic compounds (PACs) were only minor constituents. The prominent group of constituents eluting between n-pentadecane and n-hexadecane are mixtures of alkenes, alkylbenzenes and naphthalenes. Phenanthrene addition had no significant effect on the overall liquid product distribution. The product distribution shows a maximum for C3, C4 and C5 products for n-hexadecane and for both additives. In general, the effect of increased additive is to decrease the extent of individual product formation. This effect is most marked for quinoline where even the addition of

317

4.5

N-HEXADECANE

F/77/] I WT~. QUINOLINE

I.-

3.5 C~ ._.I t.~

I0 WT% QUINOLINE

3

,.,,.,,

>" 2.5 E3 l.t.l N 2 ._I

~E 1.5 r'Y 0 Z 1

0.5

Cl C2 C3 C4 C5 C6 C7 C8 C9C10CllC12C13C14C1~0KE

CARBON NUMBER FIGURE 1. EFFECT OF QUINOLINE ADDITIONS ON YIELD OF N-HEXADECANE FEED.

N-HEXADECANE

F/7~ I WT% PHENANTHRENE

I--

):

4.

I0 WT% PHENANTHRENE

...J I.i >'3 C3 bJ N ,==,. _.J ~;2 rY O z

I

I

I

I

I

1

I

I

h h _ LtJ l

I

I

I

I

I

I

C1 C2 C3 C4 C5 C6 C7 C8 C9C10CllC12C13C14C15C0KE

CARBON NUMBER FIGURE 2. EFFECT OF PHENANTHRENEADDITIONSON YIELD OF N-HEXADECANE FEED.

318 1% of this compound causes a considerable reduction in product yield. At the 10% level the effect is even more marked and is considerably stronger than the effect of phenanthrene. Thus, addition of 10% quinoline caused a 30 fold reduction in C5 products, whereas the addition of the same amount of phenanthrene reduced the C5 product yield to 40% of the pure n-hexadecane feed. Coke levels were surprising constant for all these experiments, averaging about 0.7% by weight, but the 10% phenanthrene gave a value for about 1.0%, as might be expected, due to its aromaticity. From these results, the quinoline appears to act as a severe catalyst poison. However, while hydrocarbon products are drastically reduced, coke levels remain relatively unaffected and are comparable to those of the pure n-hexadecane, suggesting that quinoline acts as a coke inducer as well as a catalyst poison. An important factor in commercial operation is the relative amounts of alkene produced, relative to alkanes. Alkene/alkane ratios for the C1 to C5 range are presented in Fig. 3 for nhexadecane and for 1% and 10% additions of quinoline and phenanthrene to the n-hexadecane feedstock. In all cases the ratio was greater than unity, with 1% addition of additives having relatively little effect on this ratio. However, at 10% addition, phenanthrene enhanced this ratio, whilst quinoline showed a corresponding decrease. Thus, although these additives diminished the individual yields of the gaseous components, with a marked reduction in the case of quinoline, small concentrations had little effect on the alkene/alkane ratio. Coke deposits were studied using mass spectra obtained from the probe E1 and CI analyses of the deactivated catalysts arising from the various feed streams. Alkane and alkene fragments were observed to dominate the individual mass spectra (particularly, m/z 57, 71 and 55, 69, respectively, in the E1 mode). Although alkylaromatics were evident for the catalyst from the tests with n-hexadecane and the n-hexadecane/phenanthrene mixture PACs are only present in trace quantities. Quinoline addition gave rise to much less intense ions from the deactivated catalyst due to its lower carbon content and the reduced sensitivitymade it difficult to observe the aromatic fragments. Indeed, the most intense peak was from quinoline itself(m/z 129 El, 130 CI). Phenanthrene addition would not appear to significantly increase the amounts of aromatic fragments evolved from the deactivated catalyst. These are primarily alkylbenzenes as observed for 100% n-hexadecane. However, leaving the catalyst at reaction temperature for 15 min. gave rise to a significant increase in the abundance of the aromatics fragments with naphthalenes (m/z 128) evolving in much larger quantities. Chloroform extraction appeared to reduce the concentrations of aromatic fragments observed indicating that the actual coke forms is highly aliphatic in character with alkene groups accounting for most of the sp2 hybridised carbons. All the above experiments were based on a n-hexadecane feedstock. In order to characterise coke deposits using ~3CNMR, a catalyst deactivated from the processing of a heavy oil feedstock was used. The carbon skeletal parameters obtained are summarised in Table 2. The present technique uses the single pulse excitation (SPE) analysis and a comparison with the more conventional cross polarisation (CP) technique, shows that CP significantly underestimates the carbon aromacticity (0.92 compared with 0.96). The fact that over 80% of the carbon has been observed by SPE indicates that the procedure is quantitively reliable for catalyst cokes and that graphitic layers are not present in significant amounts. If present, their paramagnetism would have detuned the probe resulting in little of the carbon being observable. As for the aromaticity, CP also

319

2.5 N~ADECANE

I PHENANTHRENE

2

QUINOLINE 1.5

.5 < 0.5

0 0

1

10

WT% OF POISONIN N-HEXADECANE. FIGURE 3. ALKENE/ALKANE RATIO FOR MAT16 CATALYST WITH QUlNOLINE AND PHENANTHRENE. 14 i

12-

:::

+

LZY 1

F7777A Z-A6

I.-

~:io-

BPM1 ILl >a N ...J .< ]E

QC

8

6-

4.

0 z

tkl ,I

I

--

l

I

I

I

i

I

I

I

!

C1 C2 C3 C4 C5 C6 C7 C8 C9C10CllC12C13C14.C15COKE

CARBON NUMBER FIGURE 4. PRODUCTYIELD OF LZY1, Z-A6 AND BPM1.

320 TABLE 2 13C~ Results for Coke FCC Concentrate. ~3C T~ (aromatic): 0.5 and 10 s (two components of similar proportions)

SPE

CP

Carbon aromaticity:

0.96

0.91

Quaternary aromatic C

0.72

0.51

(Cqa/Car) CH3/aliphatic C: 0.75 Fraction of bridgehead aromatic C: 0.65 (.'. highly condensed).

grossly underestimates the fraction of quaternary aromatic carbon. From the value of 70% derived by SPE (Table 2), it is estimated that bridgehead aromatic carbons account for ca 65% of the total aromatic carbon. The only assumption needed is that each aliphatic carbon is bound to one aromatic carbon which is not unreasonable in view of the fact that arylmethyl groups account for 75% of the aliphatic carbon (Table 2). The aromatic structure is dearly highly condensed corresponding and the proportion of bridgehead aromatic carbon corresponds to 15-20 fiased aromatic tings.

3.2 Effect of Catalyst Formulation As indicated above MAT experiments were made to assess the influence of catalyst composition for a number of materials with zeolite contents ranging from 0% zeolite (matrix only) through various rare earth additions Z-A2, Z-A4, Z-A6 to 100% zeolite (LZY1). The product yield for BPM1 (matrix), Z-A6 and LZY1 are illustrated in Figure 4. As expected the matrix material BPM1 gave the lowest overall product yield, while the zeolite LZY1 gave the highest gas product yield, but the 20% zeolite catalyst Z-A6 gave the highest yield for the liquid products range. The most remarkable of Fig. 4 is the extremely large amount of coke obtained using the zeolite LZY1, which produced approximately 12-15 times as much coke as the other catalysts including the MAT 16 catalyst. A plot of the alkene and alkane yields and the alkeneYalkane ratio forthese three catalysts and the MAT 16 catalyst (the model for component material) is given in Fig. 5. Again the 100% zeolite catalyst LZY1 produces by far the greatest yield ofalkane whereas all the other materials produce more alkene than alkane and thus producing values of the alkene/alkane ratio in excess of unity. On the basis of product yield the catalyst Z-A6 is seen to be superior. Fig. 6 shows the effect of rare earth additions on product yield. Increase of rare earth content (together with the associated increase in surface area), results in a significant increase in product yields.

321

1

~30 I

1"8 !

1

o__16;

t

414 re,

i

i

,- 2 r--,

!

I.--

!

'-20

z

iT

w12 i Z

9

.~

i i

''

-15 i

J

4

~

\0.8

F10

rh L.I.J N ...J

ps

r"F O Z

-~

Z

Ii

'j " ov 69 j 4044

i t

I

i

0.2+i

I

,

~

BPM I

Z-A6

MAT 16

&LKENE/ALKANE RATIO I

LZYI

ALKENE, WT%.

ALKANE, WT%.

FIGURE 5. ALKENE/ALKANE RATIOS FOR LZYI, Z-A6, BPMI AND MAT 1 6.

14

i i

(1.1)

i.-

d

_J Lo >rm L0 N _J 4 rF O Z

(2.7)

Z-A CATALYSTS:

12~

,4-

I

-I-

! iI

ALKANE,

WT%.

ALKENE,

WT%.

/ /,"

/ I

_

// ,,,

RARE EARTH OXIDES, (WT,o) ~ iN BRACKETS.

~

/

m--

(0.6)//./ / /

( 2-

0 100

I()5 --110

~115

120

125

130

155

I~0

_J LIJ >-.

145

SURFACE AREA, m 2GI FIGURE 6. EFFECT OF SURFACE AREA AND RARE EARTH CONTENT ON ALKENE AND ALKANE YIELD.

322 4. CONCLUSIONS An experimental study has shown that the addition of quinoline and phenanthrene to a nhexadecane feedstock in MAT experiments leads to a loss in overall conversion. Characterisation of the coke from this feedstock, indicates that the initial coke formed is highly aliphatic in nature. Quinoline acts primarily as a catalyst poison but also favours coke formation. Solid state 13CNMR was used to characterise the coke formed from a heavy oil feedstock on demineralisation of the deactivated catalyst. The coke was now observed to be aromatic and highly condensed and it was possible to achieve this characterisation at realistic coke levels of ca. 1% without employment of large coke deposits as hitherto. An examination of catalyst formulation on product yield for a number of catalysts of various zeolitic content has shown that the most effective catalyst is of intermediate zeolite content. A catalyst containing 100% zeolite results in a very large amount of coke deposition. REFERENCES .

2. 3. 4. 5. .

7. 8. 9. 10. 11.

J.R. Kittrell, P.S. Tam and J.W. Eldridge, Hydrocarbon Processing 64, No. 8 (1985) 63. J.S. Butt, Catalyst Deactivation, Adv. Chem. Series 109 (1972) 259. R. Hughes, Deactivation of Catalysts, Academic Press, London (1984). E.H. Wolf and F.Alfani, Cat. Rev. Sci. Eng. 24 (1982) 329 and references therein. G.F. Froment in "Progress in Catalyst Deactivation". (J.L. Figueiredo, Ed). NATO Adv. Study Inst. Series-E54, Nijhoff, The Hague, 1982. M. Guisnet and P. Magroux, Appl. Catal. 54 (1989) 1. J. Biswas and I.E. Maxwell, Appl. Catal., 63 (1990). W.A. Groten, B.W. Wojciechowski and B.K. Hunter, J. Catal. 125 (1990) 311. R.W. Mott, Oil and Gas Journal, Jan 26th (1987) 73. G.D. Love, R.V. Law and C.E. Snape, Energy and Fuels, 7 (1993) 639. M.M. Maroto-Valer, G.D. Love and C.E. Snape, Fuel (1994), In Press.

ACKNOWLEDGEMENTS We thank the SERC (UK) for financial support of this work and the SERC Mass Spectrometry service at the University of Swansea for analysis of deactivated samples. We also acknowledge the generous assistance of Dr. N. Gudde at BP Oil and of Crosfield Chemicals for provision of catalyst samples and data on these.