Effect of feedstock variability on catalytic cracking yields

Applied

Catalysis, 65 (1990)

189-210

189

Elsevier Science Publishers B.V., Amsterdam

Effect of feedstock variability on catalytic cracking yields I.P. Fisher Petro-Canada

Inc., Research

and Development

Department,

Sheridan

Park, Ontario (Canada),

tel. (+I -426) 896 6823, fax (+ l-416) 896 6740

(Received 13 March 1990, revised manuscript received 21 May 1990)

ABSTRACT Nine compositionally distinct vacuum gas oil feedstocks have been studied to develop a rational basis for understanding the distribution of major products in fluid catalytic cracking (FCC) reactions. Key hydrocarbon and carbon types have been quantitated from mass spectrometric data. Product yields were determined, under standardized conditions, using an FCC microactivity test (MAT ) system. Severity conditions were varied to obtain yield profiles encompassing that MAT conversion where the yield of C5+ gasoline was at maximum. These optimum yield profiles were correlated with the feedstock properties which characterized and quantitated those precursors capable of producing gasoline molecules. The correlations were tested against two further vacuum gas oils. The conclusions are that realistic comparisons between feeds may be made at conversions where the yield of gasoline is at maximum. catalytic cracking, selectivity ( C5+ gasoline ), catalyst characterization (MAT), feed characterization.

Keywords:

INTRODUCTION

The fluid catalytic cracking (FCC) process is, without argument, the most important refinery process in the North American context. The reason for this lies in the ability of the FCC unit to convert more of the crude barrel into fuel than any other process. The chemical composition of the feedstock of the FCC unit is the most important variable in determining the basic yield structure from the unit. This paper is concerned with studying the effect of a wide variety of FCC feedstocks on the FCC yields in general from an FCC microactivity test (MAT) unit and the yield of debutanized gasoline ( C5+ ) in particular. White [l] first showed, experimentally, which constituent hydrocarbon group types in the feed to an FCC unit were responsible for the various products. A definitive paper by Hinds [ 21 dealt with the potential and constraints of non-hydrogenative processes, such as the FCC, on petroleum. Hinds may have been the first to quantitate, on a theoretical basis, the kind of products yielded from vacuum gas oils as a function of the structure and hydrogen con-

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0 1990 -

Elsevier Science Publishers B.V.

190

tent of the constituent hydrocarbon group types. Both the above authors demonstrated the superiority of defining FCC feedstocks by hydrocarbon groups rather than by purely physical/chemical measurements. The ideas and arguments in the references cited above have been applied to the evaluation of the gas oils produced from upgrading Athabasca bitumen [ 31 showing how precursors for the production of gasoline were related to the value of the oil as an FCC feedstock. An extension of these ideas to residuum cracking has also been demonstrated, at least for a limited number of residua [ 41. The hydrocarbon molecular types present in the feed to the FCC unit are responsible for and, to a high degree, determine the subsequent structure of the yielded products. The major reaction which occurs in the FCC unit is fission of paraffmic and cycloparaffinic bonds (this includes dealkylation of aromatic compounds). Aromatic rings are generally not cracked. Other reactions such as olefin cracking, hydrogen transfer, dehydrogenation of cycloparaffins and isomerization of intermediate carbenium ions also play a role in determining the yield structure. Early attempts to characterize FCC feedstocks by physical properties and their correlations were, in reality, simply devices to simulate the hydrocarbons present [5]. Mass spectrometry was able to elucidate many of the unknowns associated with the composition and structure of petroleum and gave a rationale for directly determining the hydrocarbon distributions [6,7]. Today, mass spectrometry is the key element in the molecular level characterization both of petroleum and synfuel derived streams [ 81. Despite the availability of sophisticated means of characterization, little has been reported on the variability of FCC yield structure as a function of the molecular constitution of the feed. Nate and co-workers [9,10] studied the effects of feedstock in the development of their model for FCC yield structures. They showed how characterizing the feed as paraffins, cycloparaffins and total aromatics gave good correlations with kinetic rate constants. Their work showed how the gasoline yield increased from zero at zero conversion to pass through a maximum and decrease to zero once more at 100% conversion. The position of the maximum gasoline point depended on conditions such as contact time and temperature. For example, the maximum moved to higher values and higher conversions as the contact time decreased. They showed, too, that more refractory stocks exhibited maximum gasoline yields at lower conversions than the high quality stocks. The work described here extends the charge stock characterization to allow comparisons to be made. There is a real need for such data because the pioneering work of White [ 11 and Hinds [ 21 and others [ 9,101 was conducted in the era where zeolite cracking was just being started. Modern FCC units crack to high conversion using a wide variety of zeolite catalysts whose formulations are designed to meet often specific refinery needs. Other work has evaluated the effect of feedstock on catalyst performance [ 111, discussed the cracking of heavy oils [ 121 and considered the role of thermal and matrix effects on cracking a heavy vacuum gas oil [ 131 and its con-

191

stituent distillation fractions. Dhulesia [ 141 has proposed a detailed characterization based on correlation of physical test data. Direct evaluations of FCC feedstocks have been made by comparing the MAT unit yields [ 151. Comparisons were made between MAT, pilot plant and commercial yield data. Catalyst-to-oil severity profiles derived for each feedstock demonstrated how product yields changed with conversion. It is generally acknowledged that MAT unit yields can be correlated with commercial unit yields, with the exception of the coke yield. This last yield is determined by heat balance requirements for the commercial unit. The effects of nitrogen, contaminant metals, catalyst activity etc. on the product yields are not considered here since the values of the feeds are determined on a common basis by comparing the yield structures obtained at the conversion where the yield of C5+ gasoline is at maximum value. It is recognized that catalyst poisons will require increasing the catalyst-to-oil ratio to the conversion level where the gasoline yield maximizes. The need to provide a more rational definition of an FCC feedstock has been recognized [ 121 and this reference provides an excellent review of historical and current practices with particular reference to feedstocks containing residua. The purpose of this study is to provide some quantitative experimental evidence to show how the hydrocarbon compositions of a wide variety of feedstocks may be related to the yields at a specific point in the yield severity profile, the point of maximum gasoline yield. EXPERIMENTAL

Nine vacuum gas oils are selected here to represent a variety of compositionally distinctive feedstocks. The gas oils, in FCC terms, vary from excellent (BC Light) to poor (Visbroken VGO) . A general description follows. This superior crude is from Northern British Columbia BC LIGHT and was distilled from the reduced crude. This gas oil was distilled from a pipeline sample of the PEMBINA Albertan crude oil. These gas oils were distilled from crudes from tanker EKOFISK, BRENT shipments received from the Norwegian and British sectors of the North Sea respectively. This gas oil was distilled from a sample of crude reCOLD LAKE ceived by steam stimulation of heavy oil in the Cold Lake reservoir in Northern Alberta. This sample was received from a Petro-Canada refmBOW RIVER ery. This crude is a typical asphaltic crude. This gas oil was received from the CANMET heavy oil CANMET VGO demonstration plant [ 161 at the Montreal Petro-Canada refinery. This gas oil was derived from upgrading

192

COLD LAKE vacuum residuum and is hydrotreated before processing. COKER VGO This vacuum gas oil was obtained from the fluid coker unit at the Fort McMurray, Alberta Synthetic Crude plant. The gas oil was derived from coking Athabasca bitumen and is hydrotreated before processing. VISBROKEN VGO This vacuum gas oil was obtained from the thermal visbreaker, processing pipelined conventional vacuum residua at the Petro-Canada Montreal refinery. The physical and chemical properties of these feedstocks are shown in Table 1.Standard ASTM tests were used throughout [ 121. The distillation data were derived from gas chromatographic simulated distillations. The data in this table are useful in generating average structural parameters such as % aromatic, naphthenic and paraffinic carbon atoms using the appropriate correlations, for example see refs. 12 and 14. The hydrocarbon group types present in these streams were determined by mass spectrometry using the modified low resolution method, ASTM 2786. The highly aromatic and high sulphur feeds were also analyzed by high-resolution mass spectrometry using updated and expanded matrix calibration coefficients [17]. The results of mass spectrometric analyses of these feeds are shown in Table 2. The analyses in Table 2 indicate also the precursors in the feedstocks which will predominantly contribute to the production of molecules boiling in the gasoline or potential gasoline range i.e. C, to Cl1 hydrocarbons. The chemistry of FCC reactions [2,18] precludes scission of aromatic bonds and the standard boiling range of gasoline precludes diaromatic molecules. As a result, the first six hydrocarbon groups are classified as gasoline precursors. Furthermore, as will be shown later, the carbon types associated with these six precursor hydrocarbon types were found to correlate well with FCC yields. In calculating the carbon types associated with the feedstock hydrocarbon composition it is necessary to know the average molecular weight of each hydrocarbon type, to assume an average structure of each hydrocarbon type and to adopt some basic rules governing general or predominant feed decomposition routes. The average molecular weight of the group types is readily available from mass spectrometric analyses. Hydrocarbon rings may be considered predominantly trisubstituted with two short chains and one long chain [ 71. Dealkylation takes place mainly p to ring systems. Low-molecular weight monosubstituted alkyl benzenes (e.g. cumene) exhibit cleavage (Y to the ring. The average aromatic molecule in a vacuum gas oil will have a molecular weight in. the 300-400 range. Such molecules which are multisubstituted on the aromatic ring are reported to decompose mainly p to the ring [ 19-211. Scission of alkyl chain p to the aromatic ring is a predominant feature of the gas phase decomposition of the positive molecular ions of,alkyl benzenes [22].Methyl and ethyl groups are generally not cracked from the aromatic ring. The carbon

_--

367.0

250 306 346 376 409 464 514

0.5 0.1

302 340 384 428 470 522 623

0.2 0.1

575.0

-.

.

_- -

_. ___..._..,

“GCD = Gas chromatographic distillation.

GCD” (“C) IBP 10% 30% 50% 70% 90% FBP

(&g) Sulphur (% ) Conradson carbon (wt.-%)

1.4936

1.4786

(&g) Total nitrogen

5.0

2.6

92.7 154.0

30.6

0.8932

PEMBINA

9.9

87.8 103.0

0.8640

Kinematic viscosity. 40°C (&) Kinematic viscosity, 100°C (cSt) Refractive index, 20°C Aniline point ( ‘C) Basic nitrogen

(kddm”)

Density, 15°C

BC LIGHT

305 342 392 444 496 535 577

0.3 0.0

790.0

87.2 325.0

_.

1.5013

5.0

Waxy

0.9019

EKOFISK

Physical and chemical properties of the FCC feedstocks

TABLE 1

,

300 322 363 406 448 502 555

0.5 0.3

938.0

86.2 413.0

---

1.4990

5.3

35.6

0.8937

BRENT

309 342 381 421 459 504 570

2.7 0.1

989.0

66.7 253.0

1.5171

5.9

47.3

0.9315

__.

BOW RIVER

219 314 375 415 448 485 501

-

2.0 0.1

748.0

70.0 241.0

.I.._

1.5154

5.1

35.2

0.9283

COLDLAKE

215 282 333 369 414 480 557

0.4

2.9

4122.0

58.5 1543.0

I.,

1.5297

3.6

20.3

0.9438

CANMET

213 306 361 388 413 444 503

1.9 0.1

1537.0

50.5 346.0

I

1.5588

3.4

19.1

0.9688

.

VISBROKEN

234 333 381 429 468 514 595

4.6 2.2

2910.0

33.1 1144.0

1.5695

11.4

Solid

1.0077

COKER

Paraffins Monocycloparaffins Condensed cycloparaffins Alkylbenzenes Benzocycloparaffins Benzodicycloparaffins Diaromatics Triaromatics Tetraaromatics Aromatic sulphur Polar hydrocarbons Precursors for gasoline

30.2 16.8 24.7

3.4 3.9 3.5 8.4 3.5 2.2 1.1 2.2 82.5

4.7 3.4 3.2 7.1 2.8 2.2 1.7 1.2 85.0

PEMBINA

35.2 18.2 20.3

BCLIGHT

5.6 5.2 4.3 10.6 4.5 2.7 1.4 3.9 77.0

28.3 13.4 20.2

EKOFISK

Hydrocarbon compositions of the FCC feedstocks (wt.-% )

TABLE 2

5.4 4.0 3.5 10.2 4.6 4.5 1.9 3.0 75.8

24.6 13.5 24.8

BRENT

6.2 5.5 5.9 14.0 5.9 4.3 7.1 3.0 65.8

15.2 10.6 22.4

BOW RIVER

5.1 5.1 5.2 14.6 6.1 5.0 6.7 2.2 65.4

16.1 11.1 22.8

COLD LAKE

4.2 5.4 4.9 16.0 5.3 3.0 7.6 6.2 61.9

19.5 10.7 17.2

CANMET

2.4 3.8 3.4 16.6 15.1 8.2 7.3 2.9 49.9

16.4 9.5 14.4

VISBROKEN

3.9 6.3 6.9 19.6 10.0 9.2 9.5 9.3 42.4

2.4 4.5 18.4

COKER

195

types associated with the hydrocarbons which can potentially convert to gasoline are defined below: Paraffinic carbon: Paraffmic carbons are associated with paraffins in the feed and with alkyl substituents of ring systems. Thus all hydrocarbon group types will contribute. As an example, consider a vacuum gas oil containing 15% paraffins of average carbon number C,, as well as 15% alkyl naphthalenes of average carbon number CZZ. The paraffmic carbon content of the 15% paraffin is given by 25~12x15 (12X25 j+(2x26)+2°r12q8%offeed' the denominator being the molecular weight. The paraffmic content of the 15% alkyl naphthalenes is calculated assuming that the naphthalene nucleus is substituted by one CH,, one C&H, group and, by difference, one C$H,, alkyl group. If we further assume that p scission of a C,H,, moiety is the primary dealkylation process, then the paraffinic carbon contribution is given by 8x12~15 (12x22)+(2x22)-12

or 4.9% of feed.

The distribution of paraffinic carbon atoms is similarly calculated for all the hydrocarbon types, making the same assumptions regarding ring substitution and /3 scission. Monocycloparaffinic carbon: These carbon atoms are associated with monocycloparaffins in the feed and some contribution from naphthenic/aromatic groups such as benzodicycloparaffins. Similar assumptions are made for substitution and p scission. Condensed cycloparaffinic carbon: Mass spectrometry provides a distribution of condensed cycloparaffins from two to six rings, The average degree of condensation is first calculated from the distribution and the average structure written with similar substituents as suggested earlier. The condensed cycloparaffinic carbon is considered to be those carbons remaining after p scission of the long alkyl side chain. Monoaromatic carbon: These carbons derive from the carbons associated with the three monoaromatic group types alkyl benzenes, benzocyclo-

Paraffinic carbon Monocycloparaffinic carbon Condensed cycloparaffinic carbon Monoaromatic carbon Diaromatic carbon Triaromatic carbon Tetraaromatic carbon Aromatic carbon in sulphur types Aromatic carbon in polars Precursors for gasoline

0.7

1.0

0.8

89.6

2.2

2.5 84.5

1.5

87.7 80.4

1.3

0.8

6.4 4.4 5.4

5.7 3.7 2.8

4.7 3.0 2.4

2.3 2.3

3.9

5.2

5.3

3.9

4.0

46.6 8.5

75.1

2.1

4.6

8.1 5.3 4.8

6.6

18.2

42.2 8.1

BOW RIVER

75.0

1.7

4.3

8.3 5.4 5.3

5.6

17.8

43.5 8.1

COLDLAKE

(wt.-% ) of carbon atoms

BRENT

20.1

18.3

14.8

56.7 7.7

EKOFISK

14.8

57.0 8.5

62.3 8.5

PEMBINA

calculated from Table 2 for the feedstocks

BC LIGHT

Carbon type distributions

TABLE 3

5.2 57.2

66.8

5.1

10.5 14.9 10.2

3.9

12.5

32.2 8.6

VISBROKEN

5.4

6.2

11.4 5.7 4.5

6.7

16.9

32.7 10.5

CANMET

57.3

6.3

6.6

11.3 8.5 10.0

6.4

15.6

28.0 7.3

COKER

s

197

paraffins and benzodicycloparaffins. The values are calculated by analogous reasoning for the previous groups. Thus the aromatic ring carbon together with the methyl and ethyl substituents and the -CH,- (IIto the ring after dealkylation are considered to be monoaromatic carbon species, at least for FCC purposes. In this way all the carbon atom types associated with the gasoline precursor molecules were calculated and are noted in Table 3. The diaromatic and multiaromatic carbons associated with the more highly hydrogen deficient molecules were calculated in a similar manner. The data in Table 3 are renormalized values, expressed as percentages of the sum of carbon atoms. The calculation examples, above, give values which are percentages of all the atoms in the feed and consequently sum to 100% - ( % H ) - % heteroatoms. The MAT unit used to derive the yields is the standard somewhat simplified version of that suggested in ASTM 3907. Co-purging with nitrogen gas (15 ml/ min) during the run was practised to reduce axial dispersion as suggested by Carter [ 23 1. Axial dispersion is often characterized by values of coke on catalyst which vary with catalyst-to-oil ratio. This was not observed in the catalystto-oil surveys carried out here. The reactor effluent gas was analyzed for all components by mass spectrometry. The reactor effluent liquid was sampled through a fine glass capillary and analyzed by gas chromatography for its simulated distillation profile (ASTM 2887 ). The contact time was held constant at 100 s, the catalyst temperature was 490°C. The severity of cracking was varied to obtain the product yields as a function of conversion by changing the catalyst-to-oil weight ratio. The space velocity, WHSV, h-l, is related to t,, the length of time the oil and catalyst are in contact, by the equation: 3600 WHSV=

(Cat-to-Oil) t,

The catalyst-to-oil ratio was varied by changing the amount of catalyst charged to the reactor. For a constant t, the space velocity will thus vary. Coke on catalyst after the MAT run was determined by a LECO CR12 Combustion apparatus with infra-red determination of the carbon dioxide produced. Only those results are reported where mass balance closures of better than 95% were calculated. The standard catalyst sample used in these studies was a moderate octane catalyst Davison, Nova D taken from a commercial unit. This catalyst was decoked to less than 0.02% carbon on catalyst. The properties of this catalyst are, briefly: Surface area 105 m2/g Pore volume 0.27 cm3/g

198

Bulk density Average particle size Alumina Sodium (as oxide ) Nickel Vanadium Rare earth (as oxide ) Conversion activity

0.92 g/cm3 63 pm 42.2% 0.26% 315 Pg/g 533 pug/g 2.1 wt.-% 70 vol.-% (Davison standard activity)

RESULTS

The yield structures determined on the MAT unit are summarized below. HZ-C2 i.e. Ha, H2S, CH,, C2Hs, C,H,; these components are minor and indicate thermal reactions. Nickel on the catalyst will contribute at this level a small amount of dry gas arising from catalytic dehydrogenation reactions. i.e. C3H8, C3Hs, C4H1,,, C4H8, these are primary products imC&* portant as feedstocks to other units. Cg+ gasoline A primary product quantitated as the amount of Cg+ material in the reactor effluent gas plus the amount of liquid boiling up to 221 aC on the distillation curve of the reactor effluent liquid. LCO Light cycle oil product, consisting mainly of diaromatic molecules and conventionally regarded as an unconverted product. The yield is determined from the distillation curve within the boiling range of 221°C to 344°C. DO Decant oil. This product consists mainly of three-, four- and five-membered fused aromatic rings. It is regarded as an unconverted product and quantitated as the amount of material on the distillation curve boiling above 344°C. Coke This represents the amount of hydrocarbon material produced on the catalyst and is quantitated as the catalyst-to-oil ratio multiplied by the percent coke on the catalyst, C,, assuming that there is no coke on catalyst before reaction. Each of the feedstocks was cracked in the MAT unit at several catalyst-tooil ratios The WHSV varied between about 4 and 20 h-l at a constant contact time of 100 s. These space velocities represent liquid flows. In fact, all the vacuum gas oils vaporized prior to contact with the catalyst and the vapour contact time may be computed to be of the order of 1 second. Vapour residence times in commercial riser units are several times higher based on feed flow. The purpose of carrying out this survey was to determine the yield structures at the point of maximum gasoline. To illustrate this, the data for cracking EKOFISK vacuum gas oil are given in Table 4. The data are also shown plotted

199 TABLE

4

MAT unit yield data for EKOFISK

vacuum gas oil Calculated yields at max. C&+

14.5

11.4

10.3

Catalyst-to-oil 2.5 Product yields, wt.-% feed

3.2

3.5 1.5 14.7 53.4

WHSV,

h&l

Hz-C, C&

1.3 13.9

1.5 14.2

Cg+ gasoline

51.9

53.6

LCO

20.2

19.0

DO Coke

9.5 3.1

7.9 3.8

70.3 1.20

73.1 1.18

Conversion C, (% catalyst)

19.5 6.3 4.5 74.1 1.28

9.5 3.8 1.9 16.4

7.1 5.1

3.9

2.4

2.0 16.6

54.5

18.9 54.2

54.7

17.0

14.6

16.5

5.6 4.6

3.7 6.3 81.8 1.22

5.5 4.7

77.4 1.19

78.0 1.21 (average)

in Fig. 1 sothat interpolation of the yields at the point of maximum gasoline is readily made. In this case, the maximum gasoline yield occurring at 78% conversion was interpolated from the curves. The catalyst-to-oil ratio was also determined at this conversion (3.9) and this value allowed the coke yield to be calculated by multiplying by the average coke on catalyst (1.21% ). The renormalized yields at the conversion where the maximum C6+ gasoline occurs are shown in the final column of Table 4. The process of interpolation to find the maximum gasoline yield is, of necessity, subjective but has been attempted by others e.g. [ 241. Nate et al. [9] showed how the gasoline yield changed with conversion. The general shape of this curve reduces somewhat the difficulties in locating the optimum conversion. In general the yield increases from zero at zero conversion, maximizes at the optimum conversion and (theoretically, as modelled by Nate) falls to zero again at 100% conversion. Factors other than the catalyst affect the maximum value of gasoline. These authors showed that lower contact times served to increase the yield and the optimum conversion. Their results showed that the values both of the maximum gasoline yield and the optimum conversion decreased with poorer quality feedstocks. The fundamental nature of this selectivity curve dictates that the conversion range over which the gasoline yield maximizes is narrower for better feedstocks and, conversely, wider for poorer feeds. In fact, for the lowest quality feedstocks this range was ca. 10 conversion numbers. A greater uncertainty is consequently associated with interpolations for the poorer feedstocks. The interpolation procedure is similar to that reported by Mauleon et al. [ 241. In their paper the authors choose the maximum gasoline selectivity point as that yield value where the line drawn from the origin (zero yield, zero conversion) exhibits a decrease in selectivity. The yield of gasoline maximizes at

200

52 /

PRODUCT YIELDS Wt.-% FEED

20

18 16 14 12

i

8

-

6

-

4

-

2

1

70 IXNVERSION

80

90



Fig. 1. MAT product yields for EKOFISK gas oil, wt.-% feed. 0 =H&&., 0 =decant oil, n = C&C,, A = light cycle oil, A = coke, V =catalyst-to-oil.

0 =C5+ gasoline,

a somewhat higher but identifiable conversion value. This latter point they call the overcracking point. Provided a sufficient number of data points are gathered around the maximum, a reasonable estimate of the conversion associated with it may be made. The errors associated with the conversion values are usually quoted at -t 2 conversion numbers. C,+ gasoline yield values are generally repeatable to better than one number. The results of cracking all the feedstocks are consolidated in Table 5. Here, the interpolated optimum yields at the point of maximum gasoline are reported. Gasoline yields can be seen to vary by a factor of two for the best feedstock (BC LIGHT) giving 61% gasoline, to the worst (COKER) giving 28% gasoline. No attempts were made to break out the individual gaseous product yields from the C,C, stream. While it may be important to know if, for example, isobutane exhibits wide variability as a function of feedstock type such an approach was not intended as part of this program. A detailed breakdown of the gaseous and gasoline molecules is, in principle, possible and might be usefully

201

applied in other applications such as octane number determination, hydrogen transfer studies or the chemistry of FCC naphthas, e.g. [ 251. DISCUSSION

The purpose of this work has been to establish a quantitative basis for characterizing FCC feedstocks. The underlying concept envisages certain hydrocarbons in the feedstock which predominantly determine the concentration of gasoline types of molecules in the reactor effluent. Furthermore, the optimum yield structure provides a focus for rationalizing the conversion of these precursor hydrocarbons. Modern analytical methods are used in studying FCC technology but the need for quantitative rational information is evident. The work of White [ 11 and Hinds [ 21 applied here to commercial feedstocks provides a start in quantitating the precursor hydrocarbons and in correlating the yield selectivities as a function of feedstock variability. The manipulation of mass spectrometric data to provide carbon type quantitation is based on reasonable assumptions relating to predominant hydrocarbon structures in conjunction with a knowledge of the major FCC conversion pathways leading to gasoline molecules. The carbon type derivation, in principle, could be determined by 13C NMR [ 121. These procedures too are subject to their own assumptions. These carbon type values should not be confused with conventional techniques such as ASTM 3238 which determine C,, CN, CA. This designation does not differentiate between aromatic carbon types neither does it consider that, for example, some paraffinic carbon must be considered as part of the aromatic moiety to which it remains bonded after conversion. The hydrogen deficiency of hydrocarbons is readily quantitated by mass spectrometry. Those hydrocarbons which can form gasoline have the highest hydrogen content at a given carbon number, The lower the average molecular weight of the feedstock the lower the hydrogen contents of monoaromatic gasoline precursors and the higher the hydrogen content of the paraffins. These precursors which will potentially crack totally to gasoline are shown below for typical feedstock molecules. CH, Paraffin e.g. CH3 ( CH2) IoCH- (CH,) ,,-CH, Monocycloparaffins

e.g.

Condensed cycloparaffins, e.g.

C2H5

C2H5

ICH,),

&H,

202

Alkyl benzenes, e.g.

Benzocycloparaffins,

Benzodicycloparaffins,

CH3CH2

&d-L

e.g.

e.g.

Dealkylation fragments of diaromatic and higher aromatic hydrocarbons undoubtedly produce gasoline molecules but, because of the boiling range of the gasoline, some aromatic molecules, diaromatic or more condensed, are precluded. These average structural types, together with average molecular weights also allow calculation of the carbon types described earlier. A strong correlation exists between the optimum MAT yields and the hydrocarbon precursors for gasoline. Inspection of the data in Table 2, defining the feedstock hydrocarbons and the yields in Table 5, strongly indicates that the compositions set the yield pattern. The higher the precursor amounts the higher the gasoline yield. Being able to quantitate the precursor hydrocarbons will further indicate the limits in both conversion and gasoline yield. The lower the precursor level the lower the conversion achieved or indeed necessary at the maximum gasoline yield. Thus each value of maximum gasoline in Table 5 represents the maximum point on a curve passing through the origin (zero yield at zero conversion) characterizing each of the feedstocks. This fact allows estimates to be made of relative gasoline yields at conversions lower than that at the maximum gasoline point. Beyond the maximum, the non-aromatic molecules in the product gasoline begin to crack and the gasoline yield decreases. Since low-molecular weight molecules crack more slowly than feedstock molecules it seems likely that the point of maximum gasoline represents the point where all the feedstock precursors have been effectively converted. Further conversion increments accrue largely to coke and reduce valuable liquid volume recovery. The only reasons for overcracking would be for higher alkylation feed yields and higher octane numbers in the gasoline. Fig. 2 shows the relation between hydrocarbon precursors and the conversion at the maximum gasoline level. The equation for this line is MAT conversion = 11.8 + 0.86. (hydrocarbon precursors)

(I)

MAT repeatability for conversion activity is about two numbers (reproducibility = 7 numbers ) . The data in this graph indicate that calculated conversion for the very aromatic stocks is no better than about five numbers. A more

203 100 t

80 i

MAT CONVERSION AT MAX C5+ GASOLINE
6.

YIELD

‘Ol 40

PRECURSORS

-

60

80

HYDROCARBON

100 GROUPS


Fig. 2. Effect of feedstock hydrocarbon precursor levels on conversion at maximum yield of gasoline. l =BC LIGHT, A =BRENT, A =CANMET, 0 =PEMBINA, V =COLD LAKE, 0 =VISBROKEN, 0 =EKOFISK, n =BOW RIVER, v =COKER.

100

80

GASOLINEOLD (Wt.-%) 40

20

40 PRECURSORS

60 -

80

100

CARBON TYPE.(W-t-%>

Fig. 3. Effect of calculated carbon type precursors on conversion at maximum yield of gasoline. Symbols as in Fig. 2.

12.6 2.4 3.2 85.0 0.84

LCO DO Coke Conversion C, (% catalyst)

13.5 4.3 4.5 82.2 1.06

1.4 16.0 60.3

1.5 19.3 61.0

C,C, Cg+ gasoline

I-kc,

PEMBINA

BCLIGHT

16.5 5.5 4.7 78.0 1.21

16.6 54.7

1.9

EKOFISK

18.0 6.0 4.3 76.0 1.13

1.7 15.0 55.0

BRENT

21.0 10.0 4.0 69.0 1.30

2.3 14.6 48.2

BOW RIVER

Product yield profiles for all feedstocks at the maximum Cg+ gasoline point (wt.-% feed)

TABLE 5

20.8 7.2 4.2 72.0 1.24

2.6 15.2 50.0

COLDLAKE

28.0 22.0 6.4 50.0 1.63

26.0 11.0 10.6 63.0 1.63

_..

2.0 10.6 31.0

VISBROKEN

2.3 12.1 38.0

CANMET

30.0 20.0 8.7 50.0 2.64

3.8 9.5 28.0

COKER

205

80

MAX. YIELD OF C5+ GASOLINE wt %)

6.

_

40

-

20

-

L 20

40 MAT.

Fig. 4. Effect of feedstock in Fig. 2.

60

CONVERSION

on the optimum

80 Wt

100

%)

gasoline yield showing constant

selectivity.

Symbols

as

precise correlation is observed between the carbon types associated with the hydrocarbon precursors and optimum conversion as shown in Fig. 3. The equation for this line is MAT conversion = 1.06. (carbon precursors) - 10.0

(2)

Combining these two equations we have, MAT conversion = 0.53 - (carbon precursors ) + 0.43. (hydrocarbon precursors) + 0.9

(3)

The agreement between calculated and observed conversions for the data set in Table 5 is two numbers, which is close to the repeatability for MAT conversions. The major product yield of interest is the C,+ gasoline yield. It was found that, for this data set, all the product yield selectivities (yield/conversion) were independent of feedstock. Yield selectivities are strongly influenced by catalyst design; for example, octane catalysts usually yield lower C,+ gasoline at a given conversion than higher unit cell size, rare earth stabilized catalysts. Fig. 4 shows the C5+ gasoline yields from Table 5 plotted against conversion for all the feedstocks. The data in Table 5 are an example of one of the constant selectivities, in this case C5+ gasoline selectivity, over the range of feedstocks. The equation for this line is: yield of C5+ gasoline = 0.97 (conversion) - 19.9 Similar equations for the other products are given by:

(4)

yield of LCO = 56.0 - 0.5 (conversion)

(5)

yield of DO = 100 - LCO - conversion (by definition)

(6)

yield of C, C, = 0.2 (conversion)

(7)

The relative coking propensities of these feedstocks are indicated by the values of the coke on catalyst (C,) under the conditions prevailing during the standardized MAT experiments. However for predicting MAT coke yields it was found that the Cs+ gasoline yield correlated with the conversion-coke as shown in Fig. 5. From this line the equation for coke yield was, yield of COKE=conversion-

1.21 (yield of C&,+gasoline) -6.1

(8)

Finally, the difference between the sum of all the calculated yields and 100 is the yield of Hz-C,. This last yield depends on the state of the catalyst, the amount of non-thiophenic sulphur in feedstock and the extent of thermal reactions. It is not regarded as being specifically feedstock dependent. The equations derived above which describe the experimental yields and conversions infer that the product yields are directly attributable to the composition of the feedstock. The implication from the form of these equations is that the C,+ gasoline yield is determined by the optimum conversion independent of feedstocks. This is not an unreasonable observation since gasoline selectivity is so strongly catalyst dependent. The feedstock analysis dictates a value or limit for C&+gasoline availability, the catalyst design (and, to a smaller 100

MAX. YIELD OF C5+ GASOLINE cwt.-z>

I

6.

t 20

40

CONVERSION

60 -

COKE

80

100

(Wt.-%)

Fig. 5. Effect of feedstock on conversion-coke. Symbols as in Fig. 2.

207

extent MAT conditions) determines the distribution of C3 to Cl1 products as a function of conversion. The constraints of constant catalyst and MAT conditions thus favour constant product selectivities independent of feedstock. The results and discussion have demonstrated and argued the importance of comparing feedstocks, when cracked under standard conditions, at the optimum yield point. The feed precursor levels determine this point. It follows, therefore, that comparisons between feedstocks at any point other than that at the optimum conversion may miss important information. Comparisons made at constant conversion, for example, may be suspect because, as the data show, each feedstock has its own characteristic optimum conversion level. Similar reasons can be invoked against comparisons made at constant coke yield or constant catalyst-to-oil ratio. It has been common practice to evaluate catalysts at, for example, constant coke yield or at constant conversion. Mott [ 261 however has shown convincingly how a MAT catalyst-to-oil survey using a constant feed can be used to rate catalysts in a variety of scenarios. The same rationale has been used here to derive a yield structure, via a catalyst-to-oil survey, of a variety of feeds using a constant catalyst [ 131. Whether or not a particular FCC unit will achieve an identical yield structure as determined by the MAT will depend on unit constraints and heat balances. The important point is that the results here address feed differentiation which will add to the refiner’s base in selecting and planning for feed variability. The absolute values of the yields, conversions and selectivities cannot be applied directly to commercial units; nevertheless their relative ratings are significant. The equations generated here were assessed against two feedstocks not included in the data set of Table 5. One of the feedstocks, VGl, furthermore was a fairly severely hydroprocessed stream. VGl was a product from hydrogenation of the COKER gas oil. This gas oil is sufficiently different from conventional gas oils to offer a severe test of the feedstock definition. This gas oil is relatively aromatic (54.4% ) and conventional definitions would greatly underestimate conversion and gasoline yield and overestimate coke production. In fact, over 43% of the total aromatics are monoaromatics capable, as has been argued, of being converted into gasoline. The second example is Gullfaks gas oil, a second product from the Norwegian sector of the North Sea. The agreement between calculated and observed yields, Table 6, is satisfactory for both the Gullfaks feedstock and VGl. The result for VGl is somewhat surprising since the dehydrogenation of hybrid naphtheno-aromatics to fully condensed aromatic groups is favoured under FCC conditions (low pressure, high temperature ), Certain feedstock compositions, resulting from extensive aromatic hydrogenation, might behave in such a way that experimental yields much different from predictions based on conventional streams can be produced. This scenario warrants further study. The usefulness of determining precursor levels in FCC feedstocks is also evident when commercial unit surveys are carried out. A component balance

208 TABLE 6 Prediction of MAT yields for VGl and Gullfaks VGO (wt.-%) VG1

Gullfaks

Hydrocarbon types Paraffins Monocycloparaffins Condensed cycloparaffins Alkylbenzenes Benzocycloparaffins Benzodicycloparaffins

7.0 9.5 29.1 8.4 8.1 7.0

17.2 14.9 24.8 6.8 6.1 4.8

Hydrocarbon precursors Carbon precursors

69.1 78.8

74.6 82.9

MAT yields (optimum)

Calculated

Observed

Calculated

Observed

H,-C, C3C4 C,+ gasoline

2 14 50

2 13 50

2 15 54

2 14 56

LCO DO Coke Conversion

20 8 6 72

22 9 4 69

18 6 5 77

18 6 4 76

of precursors between the feed and unconverted products through the reactor shows the efficiency with which these molecules are converted to gasoline. These cracking efficiencies are a powerful tool in optimizing FCC operations and are often necessary for devising strategies for improving the yield structures from commercial units. Finally, it is recognized that more sophisticated analytical procedures are being developed to characterize hydrocarbons with the aim of correlating with processability [ 271. Such research can only serve to increase the flexibility and operational efficiencies of modern FCC units [ 241. Furthermore it should be possible to provide more analytical detail of the broad classification of precursors developed here. Such a breakdown could provide a basis for pursuing more fundamental studies relating to FCC technology. CONCLUSIONS

An improved means of characterizing feedstocks to FCC units has been proposed. Identifying and quantitating those feedstock hydrocarbon (and carbon) types which can be converted to gasoline has been shown to give valuable correlative and predictive meaning to the proposal. A variety of virgin and processed streams have been cracked in a MAT unit.

209

The yield patterns obtained over a severity profile have been evaluated to provide, for each feedstock, an optimum yield structure. This structure, describing the yields at the point of maximum yield of gasoline, has been shown to be a sound quantitative reflection of the comparative value of the feedstock. The predictive equations, whilst being limited to the standard MAT conditions described here, gave reasonable estimates of the yield structures for two test gas oils It is recognized that some limitations to these arguments may be needed for severely hydrogenated feeds. Nevertheless this procedure offers a rational guide for predicting MAT yields, offering substantial improvements over existing schemes. REFERENCES 1

2 3 4 5 6 7 8 9 10 11 12

13 14 15 16 17 18 19 20 21 22 23 24

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