1

No. 1998.078

Effect of Different Vacuum Gas Oils Obtained from Heavy Oils on FCC Products Selectivities

Jorge Ancheyta-Juárez and Enrique Aguilar-Rodríguez, Instituto Mexicano del Petróleo, Col. San Bartolo Atepehuacan, México; and Felipe López-Isunza, Universidad Autónoma Metropolitana, Iztapalapa, México

Abstract

The kinetic parameters in the 3-lump model, proposed byWeekman for the catalytic cracking process, have been corre-lated as a function of aromatic carbons and paraffinic/napth-thenic ratio obtained by the n-d-M method, and total sulfurand nitrogen contents. Six gas oils, obtained from heavy oils,were selected in order to cover a wide range of feedstocks.Good predictions of gasoline yields were obtained from thekinetic model, with average absolute deviation less than 1.5%with respect to experimental data. The effect of gas oil proper-ties on products selectivities are discussed using the kineticmodel and experimental results obtained in a microactivityreactor at typical operating conditions (reaction temperatureof 520°C, WHSV of 15 and catalyst-to-oil ratio of 5).

Introduction

Fluid Catalytic Cracking, better known as FCC, is one of themost important and complex processes in petroleum refining.Since its first commercial advent in 1942, various papers havebeen written dealing with different aspects of this technology.For many refiners, the FCC process is the key to profitability,since the successful operation of the unit can determinewhether or not a refinery can remain competitive in today’smarket (Sadeghbiegi, 1995).

Any changes in the feed quality impact the operation of theFCC unit and will thus be reflected immediately in the opera-tion and economics of the overall refining processes. It is thennecessary to have the ability to quantify these effects inadvance in order to predict changes in the yields and quality ofthe products and modify operating conditions accordingly.

As in the case of Mexico (Figure 1), the crude slates insome countries are increasingly being integrated with heavyoils with high sulfur, metals and carbon contents, and thereexists a policy towards processing higher rates of heavy oils.These changes adversely affect the operations of the FCC unitin terms of both products yield distribution and catalyst life.

The FCC process can be applied to process many differenttypes of feedstocks, and its chemical composition has a strongeffect on the rates of cracking reactions and catalyst decay

(Nace, et at, 1971). The most significant effect in productyields is estimated by properties such as ASTM distillation,density, refractive index, sulfur, conradson carbon, nitrogenand metal contents, molecular weight and carbon distribution(Venuto and Habib, 1978).

One of the first studies about the effect of feedstock proper-ties on the kinetics of cracking reactions was performed byNace, et al, (1971). Voltz, et al, (1971) correlated this informa-tion in order to calculate the rate constants of the 3–lumpkinetic model proposed by Weekman (1968) as a function ofaromatics/naphthenes ratio obtained by mass spectrometry.Yen (1983) showed that the kinetic parameters can be corre-lated as a function of aromatic carbons and paraffinic/napth-thenic ratio obtained by n-d-M method (ASTM, 1982).Recently, Ancheyta (1998) reported new correlations includ-ing carbon distribution and sulfur and nitrogen contents.

Some important aspects concerning the effect of gas oilcomposition are still to be answered in order to define thecost-effectiveness of processing heavier crudes slates in exist-ing facilities and this requires to build up kinetic models ableto predict yields and changes to carry out for a proper unitoperation.

In this paper we analyze the effect of different vacuum gasoils on FCC products selectivities using microactivity (MAT)experimental data and a kinetic model that includes the effectof sulfur and nitrogen contents and carbon distribution (paraf-fins, aromatics and naphthenes) on the kinetic parameters.

Cracking Kinetic Model

The reaction scheme for gas oil cracking developed by Week-man (1968) involves parallel cracking of gas oil to gasolineand gas plus coke, with consecutive cracking of the gasolineto gas plus coke (Figure 2).

For gas oil cracking the rate is assumed to be second orderand first order for gasoline cracking (Blanding, 1953). Gaso-line yield (

y

2

) is obtained using the following equation asfunction of unconverted gas oil yield (

y

1

) (Weekman, 1968).The gas plus coke yield (

y

3

) is determined by mass balance.

2

(1)

where

The correlations reported by Ancheyta (1998) to calculatethe kinetic constants of the 3–lump model at 482°C, as a func-tion of aromatic carbons (

C

A

) and paraffinic/napththenic ratio(

C

P

/C

N

) obtained by n-d-M method (ASTM, 1982), and sulfur(

S

) and nitrogen (

N

) contents are:

(2)

(3)

(4)

The kinetic parameters used in the correlations wereobtained by non-linear regression, their validity are within therange of experimental data from which they were determined.

Experimental

Experimental runs were performed at reaction temperature of520°C, constant catalyst-to-oil ratio (C/O) of 5 and WHSV of15 in a fixed-bed microactivity reactor (MAT) described byASTM D3907 Method (ASTM, 1992). A REUSY commercialequilibrium catalyst taken from the circulating inventory of acatalytic cracking plant, and six different feedstocks, wererecovered from a FCC industrial unit and used in this study.

Feedstocks were selected in order to cover a wide range ofproperties as indicated in Table 1. They are classified in anincreasing amount of aromatic contents (18.9–27.33 wt% forA and F gas oils respectively). The following values wereobtained:

API gravity : 19.8-26.5

Sulfur : 1.5-2.6 wt%

Nitrogen : 0.10-0.13 wt%

Metals contents : 0.55-1.11 ppm of Ni+V

Molecular weight : 330-440

Aromatics carbons : 18.9-27.3 wt%

Paraffinic carbons : 56.2-62.1 wt%

Naphthenic carbons : 12-16.5 wt%

Experimental results are shown in Table 2. It can be seenthat conversions and gasoline yields were obtained in therange of 61–74 wt% and 42–52 wt% respectively, which aresimilar of those reported in commercial units.

Results and Discussion

Kinetic constants of the 3-lump model (

k

1

,

k

2

and

k

3

) werecalculated at 482°C using equations (2–4), and properties ofgas oils reported in Table 1. Because the experiments werecarried out at 520°C, it was necessary to include the tempera-ture effect on the kinetic parameters, using the Arrhenius Lawwith the following activation energies (Pope and Ng, 1990):

E

A0

= 51.5 KJ/mol

E

A1

= 55.2 KJ/mol

E

A2

= 51.0 KJ/mol

Figure 3

shows a comparison between experimental andcalculated yields for gasoline (equation 1) and gases+coke(mass balance) at the experimental conversions. It can beobserved that yield predictions are acceptable with errorssmaller than 1.5%, with respect to the experimental informa-tion.

Predicted gasoline selectivities (conversion versus yieldcurves) for each feedstock are presented in Figure 4. Experi-mental gasoline yields for each conversion level are alsoincluded. It can be seen that the model using the correlationsfor kinetic parameters, accurately predicts gasoline yield foreach gas oil.

The conversion range with the most significant differencesin gasoline yields is from 60–85 wt%, which is in the range ofindustrial FCC units. Feedstock A, with high naphthenics andlow aromatics carbon contents, showed the best crackingcapability (74.03 and 52.11wt% conversion and gasolineyield, respectively). The smaller crackeability was observed infeedstocks E and F due to low naphthenics and high aromaticscontents respectively. This behavior agree with experimentaland industrial results reported in the literature (Nace, et al,1971; Sadeghbiegi, 1995).

Aromatics are not normally used as FCC feedstocks,because most of the molecules will not crack. The cracking ofaromatics mainly involves breaking off the side chains, andthis can result in excess of fuel gas yield. In addition, some ofthe aromatic compounds contain various rings that will end up

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as coke on catalyst.

Table 2 shows that feedstock A producesless coke that the others and feedstock F presents the highestcoke yield due to their low and high aromatic contents,respectively.

Fractions that boil above 482°C provide an indication onthe coke-making tendency of a given feed (90 % vol. is com-monly used instead of EBP, because experimentally it is morereproducible). Associated with this fraction is a higher level ofcontaminants, such as metals and nitrogen, and as it is wellknown, these contaminants deactivate the catalyst and causethe production of less liquid products and more coke and gas.It can be seen from Table 1 that feedstocks A and B presentboiling points at 90 vol% of 448 and 469°C and both have lesscoke yield, as compared with the other gas oils (Table 2).

Gasoline selectivity, calculated as gasoline yield/conver-sion ratio, showed a linear relationship with respect to gas oilaromatic content. This can be evaluated as a gasoline selectiv-ity/aromatics ratio as follow:

Feedstock A B C D E FAromatics content (

C

A

) 18.92 < 24.04 < 24.97 < 25.81 >26.59 >27.33

Gasoline selectivity (

GS

) 0.7039 0.7017 0.6966 <0.7086 0.6982 0.6635 > > > >

GS/C

A

X 100 3.72 > 2.92 > 2.78 > 2.74 > 2.63 > 2.43

Similar behavior was found with gases plus coke yield, sincethe yield is calculated by mass balance and the selectivity asthe difference of gasoline selectivity with unity.

Conclusions

Kinetic parameters for the 3-lump model for catalytic crack-ing proposed by Weekman (1968) have been correlated as afunction of aromatic carbons (C

A

) and paraffinic/napththenicratio (

C

P

/C

N

) obtained by n-d-M method (ASTM, 1982), andtotal sulfur (

S

) and nitrogen (

N

) contents. These correlationsgive good predictions of gasoline and gas plus coke yieldsusing data from microactivity reactors within experimentalaccuracy.

The correlations and the kinetic model have been used topredict gasoline and gases plus coke selectivities for six differ-ent gas oils obtained from different heavy oils. A comparisonwith experimental data from MAT at 520°C, WHSV of 15 andC/O of 5, showed little deviations with errors less than 1.5%.

Feedstocks with high naphthenics and low aromatics car-bon contents showed the best cracking capability. The oppo-site behavior was found in feedstocks with low naphthenicsand high aromatics contents. In addition, high aromatic feed-stocks and fractions that boil above 482°C presented the high-est coke yield.

It was found that gasoline selectivity showed a linear rela-tionship with respect to aromatic content in gas oil and thistendency can be evaluated as gasoline selectivity/aromaticsratio.

From these results, it can be stated that long straight-chainparaffins are important to the economics of an FCC unit,because these molecules are catalytically cracked, and the unitconversion is increased. Increases in gasoline and LPG yieldsare due to decreases in less undesirable by-products such asDecanted Oil (DO) and fuel gas. Economics and process con-figuration in each refinery dictate whether or not to includefractions that boil above 482°C in the FCC feed. Every effortshould be made to minimize this fractions.

References

1. Ancheyta, J.J., Ph.D. Thesis (In Spanish). UniversidadAutónoma Metropolitana (UAM-I), 1998.

2. ASTM Standard Methods. Carbon Distribution andStructural Group Analysis of Petroleum Oils by the n-d-mMethod. ASTM D–3238–82 Method. 1982.

3. ASTM Standard Methods. Testing Fluid Cracking Cata-lyst by Microactivity Test. ASTM D–3907–92 Method.1992.

4. Blanding, F.H. Reaction rates in Catalytic Cracking ofPetroleum. Ind. Eng. Chem. 1953, 45, 1186–1197.

5. Nace, D.M., Voltz, S.E., and Weekman, V.W. Applicationof a Kinetic Model for Catalytic Cracking, Effects ofCharge Stocks, Ind. Eng. Chem. Proc. Des. Dev. 1971, 10(4), 530–537.

6. Pope, A.E.; Ng, S.H. Evaluation of deasphalted heavy oilresidues as catalytic cracking feed using a riser kineticmodel. Fuel, 69, 1990, 539–546

7. Sadeghbeigi, R. Fluid Catalytic Cracking Handbook:Design, Operation and Troubleshooting of FCC Facili-ties. Gulf Publishing Co., Houston, Texas, 1995.

8. Venuto, P. and Habib, E. Catalyst Feedstock EngineeringInteractions in Fluid Catalytic Cracking. Cat. Rev.-Sci.1978, 18 (1), 1–150.

9. Voltz, S.E., Nace, D.M., and Weekman, V.W. Applicationof a Kinetic Model for Catalytic Cracking, Some Correla-tions of Rate Constants. Ind. Eng. Chem. Proc. Des. Dev.1971, 10 (4), 538–541.

10. Weekman, V.M. A Model of Catalytic Cracking Conver-sion in Fixed, Moving and Fluid-Bed Reactors. Ind. Eng.Chem. Prod. Res. Dev. 1968, 7, 90–95.

11. Yen, L.C. Reaction Kinetic Correlation for PredictionGasoline Yield in Fluid Catalytic Cracking. World Cong.III of Chem. Eng. Sept. 21–25, 1986, Vol. IV, Tokyo,Japan, 374–377.

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Table 1: Properties of Sample Gas Oils

Feedstock A B C D E F

API Gravity 26.5 22.7 19.8 20.3 19.9 20.1

Sulfur, wt% 1.49 2.49 2.17 2.56 2.53 2.62

Nitrogen, wt% 0.108 0.129 0.136 0.134 0.135 0.127

Metals (Ni+V), ppmw 1.84 0.55 0.81 1.11 1.05 0.85

Molecular Weight 330 440 330 386 403 339

ASTM Distillation, °C

IBP 253 258 383 270 261 260

10 vol% 342 345 426 373 381 322

30 vol% 376 378 450 418 432 382

50 vol% 395 395 470 448 463 427

70 vol% 416 423 497 473 484 457

90 vol% 448 469 531 511 520 502

EBP 480 545 570 557 567 561

Carbon Distribution by

n-d-M Method

Paraffinics, wt% 62.06 59.28 61.23 60.31 61.43 56.20

Naphthenics, wt% 19.02 16.68 13.80 13.88 11.98 16.47

Aromatics, wt% 18.92 24.04 24.97 25.81 26.59 27.33

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Table 2: MAT Experimental Results at 520°C

Figure 1: Heavy and Light Crude Oils Production in Mexico

FeedstockYields (wt%)

A B C D E F

Conversion, wt% 74.03 66.44 66.21 68.08 61.33 67.67

LCO 18.18 20.36 17.56 19.91 19.91 24.12

DO 7.79 13.20 16.23 12.01 18.76 8.21

Gasoline 52.11 46.62 46.12 48.24 42.82 44.90

Gases + Coke 21.92 19.92 20.09 19.84 18.51 22.77

Gases 18.07 15.80 15.44 15.47 13.58 16.67

Dry Gas 1.95 1.82 2.05 1.82 1.74 2.37

LPG 16.12 13.98 16.12 13.65 11.84 14.30

Coke 3.85 4.02 4.65 4.37 4.93 6.10

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997

Year

Vol

%

Heavy Crude Oil

Light Crude Oil

6

Figure 2: 3-Lump Kinetic Model

Figure 3: Experimental and Predicted Cracking Yields for Gasoline and Gases+Coke at 520°C

Gas oil(y1)

Gasoline(y2)

Gases + Coke(y3)

k1

k2k3

15

20

25

30

35

40

45

50

55

15 20 25 30 35 40 45 50 55

Experimental yield, wt%

Pre

dict

ed y

ield

, wt%

Gases + Coke

Gasoline

7

Figure 4: Experimental and Predicted Cracking Selectivities for Gasoline at 520°C

40

42

44

46

48

50

52

54

50 60 70 80 90 100

Conversion (1-y1), wt%

Gas

olin

e yi

led

(y2)

, wt%

Predicted.Gas oil Bo

C

B

AGas oil A

+

F

ED

Gas oil C

Gas oil D

Gas oil E

Gas oil F

X

*-