Coke and Deactivation in Catalytic Cracking - Advances in Chemistry

32 Coke and Deactivation in Catalytic Cracking THOMAS M. JOHN, ROMAN A. PACHOVSKY, and BOHDAN W. WOJCIECHOWSKI

Downloaded by COLUMBIA UNIV on July 6, 2012 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch032

Department of Chemical Engineering, Queen's University, Kingston, Ontario, Canada

Catalyst decay and the mode of coke formation in the catalytic cracking of extracted neutral distillates were studied. Feedstocks differed only in wax content and ranged from~0to ~27 mole % wax. This study unequivocally shows that coke is the product of secondary cracking reactions. Coke is suspected to be produced by the olefin reactions which arise in the primary cracking step. This view is supported by the total lack of high boiling condensed products in the cycle stock. We have also shown that the catalyst decays purely as a function of the time-on-stream whereas coke is not such a function. We thus conclude that in this case the relationship between coke-on-catalyst and catalyst activity is at best not a simple one and in fact may not exist.

T

he loss of catalytic activity during cracking reactions has long been attribbuted entirely to the formation of "coke" on the catalyst. Although no clear definition of coke exists, the term usually embraces all hydrogen-deficient carbonaceous materials remaining on the catalyst after post reaction stripping. X-ray diffraction studies by Appleby et al. (1) and infrared work by Eberly et al. (2) indicated that coke deposits are at least partly composed of condensed aromatic structures of low hydrogen content. The existence of such putative aromatic structures in coke deposits arising from the cracking of pure aliphatic hydrocarbons has led to much speculation on the mechanism of formation of coke deposits. In these considerations, attention has centered on mechanisms of coke formation which involve aromatics and olefins as the principal coke-forming fractions. One such mechanism arises from the consideration of olefin saturation by successive hydrogen transfers (2, 3, 4):

+

C H — C H — C H = C H + acid site -> (CH —CH —CH—CH ) adsorbed + (CH —CH2—CH—CH ) adsorbed -f R H —> C H C H C H C H + (R+) adsorbed (olefin) (olefin ion) (R+) adsorbed -> Acid Site + R (diolefin) where R H is the donor species which eventually becomes coke. 3

3

2

2

2

3

3

3

2

3

2

2

3

1

422

In Chemical Reaction Engineering—II; Hulburt, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.


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Thomas (3), Greensfelder and Voge (4), and Blue and Engle (5) have presented data which show that naphthenes are a valuable source of hydrogen for olefin saturation and can thus be considered as donor species. This observation has led to a mechanism involving the formation of aromatics via the successive transfer of hydride ions, followed by the loss of a proton from the resulting carbonium ion. In this way, depending on the structure of the naphthene, hydrogen-transfer reactions could lead to the production of highly unsaturated or aromatic species, which finally result in coke. On the other hand, Thomas (3) and Petrov and Frost (6) obtained high coke yields and considerable product saturation when pure octene was passed over an acidic catalyst at 375°C. They concluded that hydrogen transfer from one olefin to another and polymerization or aromatization of the alkyl carbonium ion were responsible for coke formation. This postulate represents the other proposed mechanism of coke formation—viz., polymerization on the catalyst surface. To determine the role of aromatics in coke formation, Appleby et al. (I) compared the product yields from cracking recycle oil with a high concentration of polyaromatic compounds and the yields from cracking the same stock after it had undergone hydrogénation. The results show that at 50% feed conversion, coke yield was 13 wt % in the first case and only 4 wt % for the second. The high activity of polycyclic and heterocyclic aromatics in coke formation identified these compounds as one of the main coke-forming species. What was not shown is whether coke is formed directly from the feed constituents or whether it is the product of secondary reactions. In this investigation, feeds from which virtually all polycyclic and heterocyclic aromatic materials had been removed by solvent extraction were cracked over L a - Y at three reaction temperatures in a fixed-bed reactor. The ranges of the properties and molecular compositions of the two feedstocks obtained from the solvent extraction processes are summarized in Table I. [The catalyst was prepared by repeated exchanges of Linde SK40 sieves with a 0 . 1 M solution of L a C l . The exchanges were interspersed with 4-hr calcination at 200°C and continued until no further exchange was occurring as determined by atomic adsorption spectrophotometry of the exchange medium.] 3

Table I.

Properties and Composition of the Wax-Bearing (A0W1) and Wax-Free (A1W0) Feedstocks A1W0 A0W1 Average mulecular weight 387 400 Sulfur, wt % 0.5 0.17 Conradson carbon, wt % 0.01 0.018 Aniline point, °F 210.0 118.0 Bromine number 0.0 0.0 Pour point, °F +5 +87 Mass Spectrometry Paraffins, wt % 15 26 Naphthenes, wt % 69.6 65.1 Aromatics, wt % 14.6 8.4 0

« A2W1 is a blend of 70% A1W0 and 30% AOWL A1W2 is a blend of 70% A0W1 and 30% A1W0. Both are on a volume basis.

In each experimental run coke yield was determined quantitatively, and the effects of various parameters on the coke yield and catalyst decay were observed. The results on conversion, selectivity, and others are discussed elsewhere; we confine our discussion to the relationship between coke and catalyst activity.


424

CHEMICAL REACTION ENGINEERING

II


Treatment of Experimental Results Catalytic cracking occurs at temperatures which make thermal cracking reactions unavoidable. In this study coke is one product of these thermal reactions. This fact was deduced from the results obtained in the absence of any catalyst when the reactor was packed with crushed glass. Thus the overall coke formed, and in general the overall catalytic yield of any product, consists of two components: thermal and catalytic. Our experiments were done in a packed bed reactor preceded by a heater-vaporizer, and the catalyst was purged and then regenerated in situ. Thus, to obtain purely cata lytic yields, the thermal yields obtained in blank runs using no catalyst were subtracted from the results of runs of similar time-on-stream where catalyst was used. The assumption that the thermal conversion during a blank run was approximately the same as that during catalytic reaction was satisfactory within the limits of our conditions, and its validity is improved under conditions important in this study—viz., at low times-on-stream (short space time). —τ

1

EXPERIMENTAL CAT/OIL RATIO Ο -

0·25

WT/WT

Δ - 6-05

"

•

M

- 001

-Q-

-B-

-Χ ΙΟ

20 CATALYST

30

40

TIME-ON-STREAM

50

60

(MIN)

Figure 1. Coke-on-feed vs. catalyst time-on-stream at 937° F for A1W0. Similar plots at 900°F and 975°F produce morphologically identical results. Discussion A discussion on coke formation is difficult because the "coke" is poorly defined. The most common definition is "the material which remains on the catalyst after a post reaction stripping purge with an inert gas for a specific period of time and at the reaction temperature." This definition is inadequate because the above conditions represent important, flexible parameters. We used nitrogen gas for purging the 300-cc reactor at the reaction temperature and at a flow rate of 200 cc/min for 30 minutes. Since some question may arise concerning the validity of separating the total coke yield into catalytic and thermal fractions, we offer the following observation. Most of the thermal coke produced during a blank run was



32.

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425

formed in the preheater-vaporizer zone of the reactor and not on the inert packing replacing the catalyst. Thus, it seems reasonable to assume that the additional coke formed thermally does not plug or coat the catalyst particles to any significant degree. Our data are consistent in their morphological behavior with those of Eberly et al. (2). Figure 2 shows that as the cat/oil ratio is increased at a constant time-on-stream, the weight percent coke on catalyst decreases. Since data were not collected at a sufficiently low cat/oil ratio, we did not observe the maximum in coke on catalyst yield for a constant time-on-stream observed by Eberly et al. However, the fact that the difference between our lower two cat/oil ratios is less than the difference between the highest and middle ratios suggests that the trend may be toward a maximum. Relationship of Coke to Catalyst Decay. Figures 1 and 2 show weight percent catalytic coke based on feed and on catalyst as a function of time-onstream for A 1 W 0 at one reaction temperature. These figures are representative of the results for all temperatures and feeds studied. Figure 3 shows the thermal coke yield for the same stock and at the same temperature. The dashed lines are the thermal coke yields at the two other temperatures we studied. A suitable summation of the values from Figures 2 and 3 will produce composite curves for total coke yield that behave in some n order fashion. On the other hand Figure 2 shows that the catalytic coke-on-catalyst curves at all catalyst to oil ratios reach an early maximum and remain there for all longer times-onstream. At the same time the catalyst activity (Figure 4 ) , as measured by Θ, continues to decline (7). Pachovsky and Wojciechowski (8) showed that catalyst decay with this particular stock follows an exponential decay function t h

ft = e " M 50 EXPERIMENTAL CAT/OIL RATIO O - 0 25 40

Δ - 0

WT/WT

05

O - 0 0I

"

Ο

-

Ο

•

-Δ

_Ι_

_ι_

20

30

40

50

60

TIME - ON - STREAM (MIN)

Figure 2. Coke-on-catalyst vs. catalyst time-on-stream at 937°F for A1W0. More coke is produced per unit weight of catalyst as the absolute amount of catalyst is decreased (that is, cat I oil ratio is decreased). Simûar plots result at 900°F and 975°F.


426


Π

with time-on-stream (t). It is clear from the above that catalyst decay is not a function of catalyst-to-oil ratio whereas coke-on-catalyst is. In view of these fundamental disagreements in the behavior of θ and of coke we must say that coke-on-catalyst is not a measure of catalyst decay.


16

20

30

40

50

60

T I M E - O N - S T R E A M (MIN)

Figure 3. Thermal coke vs. run time for the three temperatures studied. Solid and dashed lines are least-squares fittings, and experimental points ob tained at 937°F are shown.

TIME - ON - S T R E A M (HR)

Figure 4.

Effect of temperature on the catalyst decay as calculated from conversion data


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Froment and Bischoff (9, 10) have postulated that the fraction of unpoisoned active sites remaining on a catalyst is a function of the coke-on-catalyst 9 = e~aC

c

where α is a constant and C is a coke-on-catalyst. To reconcile this concept with the time-on-stream approach we would have to accept that coke on catalyst is related to time-on-stream by a linear relationship. Figure 2 shows that this would only be approximately true at low times-on-stream. It should not, however, be immediately assumed that our results contradict Froment and Bischoff (9, 10) or Ruderhausen and Watson (II) since they based their correlations on total coke yield (thermal plus catalytic). There is little doubt that total coke yield which is an n order function of time-onstream can be correlated with catalyst deactivation in some systems over certain conditions. However, the use of total coke rather than catalytic coke, we feel, leads to erroneous conclusions and certainly does not give a realistic picture of the relationship between coke and catalyst decay. From our treatment of the data, we conclude that coke is not directly responsible for decay but should be treated merely as an undesirable side product. c


t h

EXPERIMENTAL CAT/OIL 4

-

RATIO

Ο - 0-25

WT/WT

Δ - 0 05

ω

3

"

-

MCE

ο

ο υ

40

50

60

70

80

CONVERSION (WT %)

Figure 5. Coke selectivity plot. MCE is the minimum coke envelope and represents the results that would be observable in a moving orfluidizedbed reactor. Sources of Coke in Catalytic Cracking. Figures 5 and 6 show the rela tionship between percent coke based on feed and percent conversion for the different feedstocks used. Feeds A2W1 and A1W2 are prepared from blends of A1W0 and A0W1. In Figure 5 the actual cat/oil loops are shown, and the minimum coke envelope is inferred from these. In Figure 6, however, only the four minimum coke envelopes have been shown. The slopes of the coke curves at the origin are all zero. This is important and shows that whereas at initial conditions the cracking reaction is proceeding


428


II


4 -

CONVERSION ( W T % )

Figure 6. Coke selectivities for the four charge stocks studied. The MCE decreases as the wax content is increased; all MCE's have zero slope at the origin. at a finite rate, the rate of catalytic coke production is zero. At the same time the catalyst is decaying at a finite rate (Figure 4). This result underlines our main point—i.e., coking is not directly responsible for catalyst decay. Furthermore, Figures 5 and 6 show that coke is most certainly not produced in primary cracking reactions but is solely a product of the secondary reactions of primary products. Since the only primary products which are qualitatively different from the feed are olefins, we ascribe coke formation in this case to the reactions of olefins. The curves in Figures 5 and 6 are not those for a constant time-on-stream. They are those which we call the minimum coke envelopes, and it has been shown elsewhere (12) that these would be the results observed if data were obtained for catalytic coke in moving or fluid bed reactors. Such curves can be obtained from static bed reactor data by tracing an optimum envelope as shown in Figure 5. Figure 7 is a theoretical case and shows that interpreting coke results at various cat/oil ratios but at a constant time-on-stream is likely to lead to erroneous results. The dashed curve is the locus of runs at constant time-on-stream and various cat/oil ratios. Such data could lead to the erroneous conclusion that the coke-us.-conversion curve has a finite initial slope and hence that coke is a result of primary cracking reactions. Using such considerations, we see that the data reported by Plank et ai. (13) and Eberly et al. (2) for cracking of methylcyclohexane and n-hexadecane respectively all show the absence of coke formation in the primary reaction step. For methylcyclohexane and n-hexadecane cracking, accounting for thermal coke was unnecessary since these pure, low molecular weight, saturated hydrocarbons do not yield any significant amount of thermal coke. On the other hand, without accounting for thermal coke, the data at constant time-on-stream reported by Plank and



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Rosinski (14) for cracking gas oil leads to the potentially wrong conclusion that coke is a primary reaction product. Plank et al. (13) and more recently Butt (15) have speculated that polymerization and hydrogen transfer reactions might be involved in coke formation. The cycle stock obtained in this study did not reveal any compound with either a higher carbon number or higher boiling point that the feed (see Figures 8 and 9). Any polymerization which might occur would have to involve substances adsorbed on the catalyst surface which do not subsequently desorb. We are not denying the suggestion that coke can be produced from aromatics. In fact, Appleby et al. (1) and White (16) have shown that aromatics, especially polynuclear aromatics, produce a large amount of coke for a particular degree of conversion. However, their runs were not corrected for thermal conversion; this leaves two possibilities: (1) that the complex aromatics may adsorb readily, leading to high "coke" values, or (2) that the high coke values are the result of thermal conversion. In either case the coke resulting from complex aromatics would represent a material present in the feed and should not be seen in the same light as coke which is a product of the catalytic cracking reaction. Unfortunately their definition of coke does not allow this distinction at this time. Since we were interested in studying coke formation during reaction, we chose a feed that was very low in polynuclear aromatics (0.6-3.0%). In the absence of large quantities of polynuclear aromatics, coke was formed strictly through secondary reactions. Since these involve olefins, which were not present in our original feed, it must be argued that olefins are the source of coke formation.

MCE

CONVERSION (WT %)

Figure 7. Theoretical coke selectivity plot, illustrating the danger of trying to obtain results from constant time-on-stream data. The locus of points at a fixed time-on-stream will show a finite slope at the origin whereas the true behavior of coke as derived from the MCE indicates zero slope at the origin and hence zero rate of coke production under initial conditions.



430


0

200

400

600

800

TEMPERATURE

Figure 8.

1

I

I

I

30

(°F)

ι

CYCLE

jy

1200

Distillation curves for AOWI feedstock and cycle stock at—'65% conversion

FEED

36

1000

Π

ι 24

V—^ ι

(AOWI)

STOCK

ι

18 CARBON

STOCK

12

8

NUMBER

Figure 9. Gas chromatogram for AOWI feedstock and cycle stock. Cycle stock shows no species higher in molecular weight than the feed.


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431

Conclusions On the basis of the success of the time-on-stream theory in describing catalyst activity as a monotonically decreasing function of time-on-stream and on the basis of the observed behavior of coke-on-catalyst we conclude that the relationship between coke-on-catalyst and activity is not direct. We also con clude that coke in catalytic cracking of extracted neutral distillate arises solely from secondary reactions and not directly from the feed. Furthermore, we infer that coke in this system arises from the reactions of product olefins but not from their polymerization to a range beyond the final boiling point of the feed.


Nomenclature a C k t

c

d

θ

— t= = =

empirical constant coke on catalyst first-order decay rate constant time-on-stream

= fraction of active sites remaining unpoisoned

Acknowledgment We thank Imperial Oil for the chargestocks and for assistance with some of the analytical work.

Literature Cited 1. Appleby, W. G., Gibson, J. W., Good, G. M., Ind. Eng. Chem., Process Design Develop. (1962) 1, 102. 2. Eberly, P. E., Kimberlie, C. N., Miller, W. H., Drushel, Η. V., Ind. Eng. Chem., Process Design Develop. (1956) 5, 193. 3. Thomas, C. L.,J.Amer. Chem. Soc. (1944) 66, 1586. 4. Greensfelder, B. S., Voge, H. H., Ind. Eng. Chem. (1945) 37, 1038. 5. Blue, R. W., Engle, C. J., Ind. Eng. Chem. (1951) 43, 494. 6. Petrov, Α. Α., Frost, Α. V., Dokl. Akad. Nauk SSSR (1949) 65, 851. 7. Wojciechowski, Β. W., Can.J.Chem. Eng. (1968) 46, 48. 8. Pachovsky, R. Α., Wojciechowski, Β. W., in press. 9. Froment, G. F., Bischoff, R. B., Chem. Eng. Sci. (1961) 16, 189. 10. Ibid., (1962) 17, 105. 11. Ruderhausen, C.G.,Watson, C.C.,Chem. Eng. Sci. (1954) 3, 110. 12. Campbell, D. R., Wojciechowski, B. W., Can. J. Chem. Eng. (1971) 48, 224. 13. Plank, C. J., Sibbett, D. J., Smith, R. B., Ind. Eng. Chem. (1957) 49, 742. 14. Plank, C. J., Rosinski, E. J., Chem. Eng. Progr., Symp. Ser. (1967) 63, 26. 15. Butt, J. B., ADVAN. CHEM. SER. (1972) 109, 259. 16. White, P. J., Petrol. Refiner (1968) 47, 103. RECEIVED January 2, 1974.


Coke and Deactivation in Catalytic Cracking - Advances in Chemistry

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