The Influence of Catalyst in Gas Oil Hydrotreating

K.J. Smith, E.C.Sanford (Editors), Progress in Catalysis 0 1992 Elsevier Science Publishers B.V. All rights reserved.

247

THE INFLUENCE OF CATALYST IN GAS OIL HYDROTREATING

M. C. Oballa, P. S. Herrera, A. F. Somogyvari, M. Wilson' NOVA HUSKY Research Corporation, Calgary, Alberta, CANADA 'CANMET, Energy Research Laboratories, Ottawa, Ontario, CANADA Abstract Six industrial hydrotreating catalysts - Ni-Mo/Al,O, were evaluated for their activity towards hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and polynuclear aromatics (PNA) reduction. The feedstock was gas oil (249 - 524°C) from a hydrocracked 50/50 mixture of Cold Lake/Lloydminster atmospheric residue. The reactions were carried out in a continuous fixed-bed reactor operating in an upflow mode. Results from the experimental runs are presented. HDN, PNA reduction and HDS correlated with molybdenum surface coverage, while chemical hydrogen consumption depended on the phosphorus content of the catalyst. Coke deposition on the catalyst surface showed an inverse dependence with phosphorus loading per unit area. The influences of the Ni-Mo ratio, phosphorus content, pore size, pore size distribution and surface area of the fresh catalysts on their performance are examined. Catalyst performance as measured by the reduction of aromatics, sulfur and nitrogen is discussed. A choice of the most active catalyst was based on results from 1000 hour time-onstream runs and the properties of the spent catalysts. 1. INTRODUCTION

The upgrading of heavy oils to synthetic crude involves a series of processing steps. These include atmospheric distillation, hydrocracking of residuum from the atmospheric distillation tower, coking of bottoms from hydrocracking, hydrotreating distillates from hydrocracking and recombining both hydrotreated materials and straight run cuts to obtain synthetic crude -"custom designed crude oil". The quality of this synthetic crude depends to a great extent on the properties of the original feedstock, and on the extent of processing (hydroprocessing and coking severity). Husky Oil of Calgary, in collaboration with the governments of Alberta, Saskatchewan and Canada, is constructing a Bi-Provincial Upgrader (BPU) in Lloydminster. This grassroot facility has an upgrading objective of producing synthetic crude from a feedstock made up of a 50/50 volumetric blend of Lloydminster/Cold Lake heavy oils. The BPU is planned for start up in the fall of 1992. The BPU has made a conscientious effort during the design stage to ensure that specifications on gas oil, diesel and jet fuel are met. More than this, the Bi-

248

Provincial Upgrader wants to ensure that the best catalyst in the market for its particular feedstock is used. This paper discusses the results of a program geared towards the selection of a "proper" catalyst. The effect of the catalysts in hydrotreating the gas oil fraction from hydrocracked atmospheric residuum is discussed. The activity of the different catalysts (six) is measured in terms of reduction of polynuclear aromatics, sulfur removal, hydrogen consumption, nitrogen removal and 343"C+ conversion.

2. EXPERIMENTAL

Reactor: The hydrotreating experiments were performed in a 316 stainless steel fixed-bed reactor 1.64m long and 1.9cm ID. To overcome the problems of flow maldistribution, poor catalyst wetting, low catalyst utilization efficiency and poor test r e p r ~ d u c i b i l i t y ~we ~ ~ operated ~ ~ ~ ~ , the reactor under the following conditions: upflow mode, diluted bed, isothermal and plug flow, high catalyst contacting and once through (no off gas recycle). Catalysts: The catalysts were all of the NiMo/Al,O, commercial catalyst vendors.

type obtained from

Feedstock: The feedstock was an enriched gas oil fraction (249 to 524"Celsius) of hydrocracked 50150 (volumetric blend) LLoydminster/Cold Lake residue (BPU-GO). Analysis: Specific gravities were determined at 155°C on a Paar DMA 48 instrument while dynamic viscosities were determined at 25°C on a Brookfield DV II instrument. Sulfur in wt% was determined on a Leco SC-132 sulfur analyzer while trace sulfur in wppm was determined on a Dohrmann instrument. Trace nitrogen was determined by chemiluminescence method on an Antek analyzer while trace metals in the oils were determined by X-ray fluorescence using a Horiba MESA 710 analyzer. Both simulated distillation and vacuum distillation were performed according to ASTM D- 2887 and ASTM D-1160 respectively. Both Proton and Carbon-13 NMR analyses were obtained on a Bruker ACE200 instrument. Low resolution mass spectrometric analyses for aromatic types were obtained commercially. The method of Robinson and Cook? which has been adopted as ASTM D-3239, was used with minor modifications. As well, the weight percent of total aromatics was determined by a column chromatographic technique as described by WatsonG. The method of Fitzgerald et a17, for the determination of aromatic components by UVNis spectroscopy, was adapted inhouse to account for expected differences in the sample composition. Pore area of catalysts, pore volume and pore size distribution were measured using an Autoscan 60 Mercury Porosimeter. Coke on catalyst after a run was determined by coke burn-off in a muffle furnace at 550°C. Elemental analysis was also carried out on spent catalysts in order to determine the carbon, nitrogen, sulfur and hydrogen contents. Metals content of both the fresh and spent catalysts were determined by Proton Induced X-Ray Emission (PIXE) method.

249

Reaction Unit: The reaction unit including a simplified scheme of the experimental system have been described and illustrated elsewhere8 and shall not be described here.

3. RESULTS A N D DISCUSSION During initial runs, an attempt was made to optimize process conditions so as to attain given specifications for the hydrotreated gas oil. The comparison of activity for the six catalysts were therefore based on the process conditions selected from the initial runs, i.e.,: : 375 Reaction Temperature ("C) Reaction Pressure (kPa) : 12,066 Liquid Hourly Space Velocity (hr -1) : 0.5

The gas oil product specificationsg set up for the Bi- Provincial Upgrader (BPU) are : Sulfur Content Nitrogen content Density @ 15.5"C Total Aromatics Content

: 400 wppm : 500 wppm : 893.3 kg/m3 : 45 wt.% (Watson Method)

Because of the tightness of hydrogen supply at the plant site, hydrogen consumption was of grave concern. These targets above were then used to rank the catalysts. Total liquid product properties as well as the properties of the feed are shown in Table 1. Table 1 Feedstock and Hydrotreated Product Properties Feed Cat. Cat. Cat. Cat. A B C D 923.3 872.8 890.2 890.2 886.3 Density (kg/ma) Viscosity (cP) 38 13 22 21 19 Carbon (wt%) 87.09 86.60 87.04 87.02 86.91 Hydrogen (wt"/,) 11.61 12.74 12.15 12.26 12.47 Sulphur (wppm) 14123 795 984 517 393 Nitrogen (wppm) 1799 102 276 204 62 Simulated Distillation (wt%): IBP-177°C -4.1 1.5 1.4 1.8 3.8 4.7 177-249°C 1.6 7.4 4.3 36.3 36.5 39.2 35.7 249-343°C 31.4 57.0 58.5 343-524°C 49.3 58.5 65.4 ----524+"C 1.6 Note: A, B, C, D, E, F are commercially available catalysts

Cat. E 889.2 22 86.78 12.38 391 129

Cat. F 880.4 16 86.50 12.65 247 30

1.5 4.2 35.8 58.5

2.7 5.9 37.4 54.0

--

--

250

3.1 Density and Viscosity Reduction

The density and viscosity of the hydrotreated oil were significantly less than those of the feedstock and comparable to those set for the commercial product. Catalysts A & F however showed higher selectivity to both density and viscosity reduction than the other catalysts. Some studies showed1O that liquid product density values correlate with aromatic content (degree of hydrogenation). Our results on Table 1 also show that catalysts A & F which have the best density reduction also indicate the least aromatic content of all samples collected during the program. Figure 1 shows the dependence of viscosity and density of the product oil on hydrogen consumption. The figure confirms that the catalyst with the highest aromatic reduction capability also consumed the highest amount of hydrogen. 0.805

Densitv

I

25

Hydrogen Consumption (L H2/L Product)

FIGURE I: Dependence of Density and Viscosity of Hydrotreated Gas Oil on Hydrogen Consumption

3.2 Hydrodesulfurization and Hydrodenitrogenation

Reports in the literature indicate that for MoS, based catalysts, the concentration of a "Co-Mo-S" phase correlates well with HDS activity11.12. A similar "Ni-Mo-S" is postulated to exist for Ni-Mo/Al,O, catalysts'3.14. An optimum interaction between Ni and Mo was found to occur at a Ni/(Ni+Mo) atomic ratio of 0.314. While our data are limited, we found an inverse relationship for HDS, HDN, as well as for Polynuclear Aromatics (PNA) reduction and an increase in chemical hydrogen content in the total liquid product with the atomic ratio of Nils. The catalysts which produced the highest HDN activities also had the highest conversions for the 343"C+ fraction and the highest PNA conversions (Figure 2).

25 1 60 0

5 500 r 55 40 0 iNz30 0 C

I

$ 3 20

L o s

0 0

0 0

A

B

C

D

Catalyst Name

E

F

0 343+ Conv. Aromatics Conv. tSS PNA Conv. Lz3 HDS B HDN I Hydrogenation

FIGURE II:

Area Independent Conversions for Fresh Catalysts

Kinetic evaluations carried out with only one of the catalysts were in good agreement with other a ~ t h o r s ~who ~ , ~postulate ~, that HDN of gas oil can be adequately fitted with first order kinetics, i.e.: kN

=

In [N/N,]. LHSV

On the other hand, HDS was best fitted with a second order rate equation (see Figure 3):

k,

= [1/S,

N,, S, N, S, ks kN LHSV

= = = = =

- l/S,]- LHSV,

whereby

Nitrogen and Sulfur content of the feed respectively, and Nitrogen and Sulphur content of the product. reaction rate constant for hydrodesulfurization, and reaction rate constant for hydrodenitrogenation Liquid Hourly Space Velocity [Vol/hrNol].

Studies on hydrodesulfurization carried out with single sulfur containing compounds showed that the reaction is first order with respect to sulfur'8,'9. However, petroleum fractions containing different sulphur compounds have been found to deviate from first order kineticsZov2'.The orders range from 1.2 to 3 for HDS of oil fractions and it has been documented that the order increases with the boiling pointz.

252

3.5. 3

-

HON 181 Order

0 HOS 2nd Order

3.5

-

3 cn e 2.5y

2

2

.?

1.5 *

1

0,

0.5 0

0 0.2 0.4 0.8 0.8 1 1.2 1.4 1.6 1.8 2 2.2

IAHSV [hr]

FIGURE 111:

Kinetlc Plots for HDN and HDS of BPU-GO

3.3 Hydrogen Consumption The liquid product properties were closely related to the chemical hydrogen consumption. An increase in hydrogen uptake resulted in a decrease in the parameters that characterize the liquid product (density, viscosity and aromatics content). The chemical hydrogen consumption included removal of sulfur and nitrogen, olefin saturation and polyaromatics saturation. Figures 4 and 5 show the dependence of HDN, HDS and aromatics content on hydrogen consumption. The hydrogen consumption, for the same experimental conditions and feedstock, was a function only of the nature of the catalyst used. This nature of the catalyst was well depicted by the phosphorus content of the catalyst. Figure 6 presents the dependence of hydrogen consumption on the phosphorus content per unit surface area of catalyst. The hydrogen consumption increases, as well as HDN and HDS, as the phosphorus content of the catalyst is higher. This observation agrees with other claim^@^^^) that the effect of phosphorus, added as a promoter to Ni-Mo/Al,O, catalysts, is to enhance catalyst activity by improving nickel dispersion and decreasing coke deposition. I YU

i

85

H

I

0

=

80

70

80 00 100 110 120 130 140 Hydrogen Consumptlon ( L HA Product)

FIGURE IV: Dependence of HDN and HDS on Hydrogen Consumption

FIGURE V: Aromatics Content as a Functlon of Hydrogen Consumption

253

FIGURE VI: Dependence of Hydrogen Consumption on Phosphorus Content of Catalyst

3.4 Aromatics Hydrogenation The aromatics content of the feed as well as those of the hydrotreated products using catalysts A to F, were measured using four different methods: UVNis, 13C NMR, Watson and Mass Spectrometric Methods. These methods for analyzing aromatics in gas oils are documented6J,15 and shall not be described here. The various methods are compared for total aromatics content in Table 2. Table 2 Total Aromatics (wt%) by Method Method Feed Cat. A 13C NMR 27.0 9.8 Watson 55.1 25.7 UVIVis 32.4 18.1 Mass Spectrometry 50.45 18.94

Cat. B

Cat. C

Cat. D

Cat. E

13.9 13.9 12.2 13.6 35.2 37.3 33.8 36.3 30.7 33.8 28.0 30.6 29.92 33.72 27.45 30.55

Cat.

F

11.1 29.3 24.0 23.73

The table shows that the aromatics content for all methods increases in the order: catalyst A c catalyst F c catalyst D with catalyst C giving the highest total aromatics content and catalysts B & E changing positions according to analytical method. Among the various methods, the best agreement for total aromatics content appears to be between the UVNis and MS methods. Even if the UVNis method is quick and relatively inexpensive, it is believed that the reliability of the MS method is far superior to those of other methods used here. While the absolute values for total aromatics vary considerably according to analytical method, there seems to be a relationship between the different methods. The above relationship was explored for the conversions obtained by each method. The total aromatics conversion obtained via NMR, UVNis and Watson chromatographic method are plotted against the conversions obtained from

254

mass spectrographic analysis in Figure 7. Linear regression analysis, resulting in the equations below, was carried out. Correlation coefficients of 0.968, 0.988 and 0.997 respectively were obtained. NMR = 0.58. MS + 27.4 Watson = 0.77 . MS + 5.1 UVIVis = 1.65 . MS - 60.5 IU

00

10

20 30 40 50 Converslon (X) by MS

60

70

FIGURE VII: Effect of Analysis Method on Aromatics Conversion

PNA Conversion results follow the same trend as hydrogen consumption (see Table 3) implying some relationship. PNA conversion, as well as sulfur and nitrogen conversion, correlated well with molybdenum coverage (Figure 8). Coke deposit on the catalysts showed an inverse dependence with phosphorus coverage as depicted in Figure 9. Table 3 Properties Dependent on Catalyst Cat. A 50.6 80.1

PNA Conv. (%) (UVNis Method) PNA Conv. ("/.) (MS Method) H, Consumption: (L HdL Feed) 140.6 789.5 (SCF HJBBL Feed)

Cat. B 45.7 61.0

Cat. C 45.7 58.5

Cat. 46.9 65.1

Cat. E 45.7 63.9

Cat. F 49.4 74.3

79.6 447.2

91.0 511.2

112.7 633.1

103.4 580.7

131.3 737.3

D

255 6.0 r

5

N -

5.5 5.0

I

1 Nitrogen

+ Sulphur

iJ - \

c

8-

0

9

3.0 2.5 6.0

.

5 g 10 I

p 3.5 $

- 12

I

s 6.5 7.0 7.5 8.0 Molybdenum Coverage x 1E04 (o/rn*)

8.5

FIGURE VIII: Conversion as a Function of Molybdenum Coverage

6

I

I

I

Phosphorus Coverage x 1E04 (g/m')

FIGURE IX: Coke on Catalyst as a Function of Phosphorus Coverage

3.5 Catalyst Selection

For the hydrotreating of BPU gas oil, our most important parameters are ranked as follows: HDN > PNA Reduction > HDS > Hydrogen Consumption. While seemingly contradictory objectives, the best catalyst would maximize the first three parameters and minimize hydrogen consumption. Other parameters of less importance are 343"C+ conversion and total aromatics conversion. It may be seen from Figure 2 that the best combination of HDN, PNA removal and HDS are achieved for catalysts F and A followed closely by catalyst D. All three catalysts produced liquid products meeting the specifications. However, based on other considerations peculiar to the BPU, catalysts A and D were selected for catalyst deactivation runs. Each of the selected catalysts was used for deactivation runs under the same operating conditions for a period of 1000 hours. Liquid product samples were taken at intervals and analyzed. At the end of the runs, the results on HDN, HDS, PNA reduction, 343"C+ and aromatics conversion were compared. There was no conclusive evidence of the superiority of one of the catalysts over the other. Typical deactivation profiles for both catalysts represented by 343"C+ conversion are presented in Figure 10. In the figure, catalyst D showed a very strong higher conversion level which dropped off very rapidly within the first 100 hours and remained almost constant for the rest of the run. Catalyst A seemed to have the same trend but had an initial lower 343"C+ conversion activity. Around 1000 hours, both catalysts showed the same activity towards 343"C+ conversion.

256

I

“.I,, 0

0

200

,

,

400 800 800 Tim. on Stream (Hrs)

, 1000

FIGURE X: 343+ Converslon with Time Stream During Hydrotreating of BPU-GO

1

00

on

Average hydrogen consumption during the latter periods of the runs for both catalysts were calculated based on the average concentration of hydrogen in the liquid product as well as its average density. Results show that catalyst D has a higher hydrogen consumption than catalyst A at about the same conversions. The aged catalysts after 1000 hours runs were analyzed. Coke content was determined by coke burn-off. Pore Volume, Pore Volume distribution as well as Surface area of the fresh and aged catalysts were determined by mercury porosimetry. Results from the examination of the properties of the aged catalysts are summarized in Table 4. From these results, i.e., mercury porosimetric data (reduction in pore volume and surface area), coke content of the catalysts and hydrogen consumption, it became obvious that catalyst A is the catalyst of choice for the BPU. Table 4 Aged Catalyst Properties Coke Content (wt%) Percentage Reduction in Surface Area Percentage Reduction in Pore Volume Average Hydrogen Consumption: (L Hydrogen/L Feedstock) (SCF HdBBL Feed)

Catalyst A 13.30 19.73 37.61

Catalyst D 14.32 24.20 55.77

122.5 688.0

140.5 789.0

4. CONCLUSIONS

Six commercial catalysts were tested for activity and ability to meet the specifications set for the gas oil fraction of the BPU. Only three of the catalysts met the required specifications. Two of these were selected for longer term runs because the proof of a catalyst rests on its performance. This performance is

251

made up of initial activity as well as its performance over time in comparison with other catalysts. We demonstrated that the phosphorus content of the catalyst has a role to play in the catalyst's activity, especially its hydrogen consumption. We showed that aromatics content of gas oils can be quantified using different methods, and even if absolute values are different, other methods correlate well with the MS method. Our results indicate that even after long term runs the choice of one catalyst over the other might still pose a problem, which of course could be solved by taking a closer look at the properties of the aged or spent catalysts. It is to be noted that the choice of a catalyst with high selectivity/ activity towards a given feedstock, and meeting stipulated specifications, is a long and expensive task. But once a "proper" catalyst is chosen based on performance, the returns are great in terms of longer run length, meeting of specifications, less fouling of equipment etc. 5. ACKNOWLEDGEMENTS

Our thanks go to Mr. Leon Neumann, who performed the hydrotreating runs, Mr. Antonio Nicola, who was instrumental in obtaining the Watson aromatics, and Ms. Nadia Hamza, who performed the distillations and obtained the densities and viscosities, for their technical assistance. As well, the authors thank the Analytical Services Group at NHRC for adapting the UVNis technique for internal use and for providing those, NMR, and simulated distillation data. Part of this work was supported by a 50/50 cost-shared program between Husky Oil Ltd. and CANMET, Energy Mines and Resources Ottawa under contract number 23440-8-9261/01-SQ. The useful technical discussions with W. Vandenhengel and G. Dennis of Husky Oil are gratefully acknowledged. We appreciate the cooperation of the commercial catalyst vendors who cannot be named here in order to protect the identify of the catalysts used. 6. LITERATURE 1

2 3

9

J. Van Klinken and R.H. Van Dongen, Chem. Eng. Sci., 35 (1980) 59. M. De Wind, F.L. Plantenga and J.J.L. Heinerman, Applied Catalysis, 43 (1988) 239. A. De Brujin, 6th International Congress on Catalysis, Paper 834, London, (1976). C.N. Satterfield, AlChE Journal, 21 (1975) 209. C.J. Robinson and G.L. Cook, Anal. Chem., 41 (1969) 1548. A.T. Watson, Anal. Chem., 24 (1952) 507. M.E. Fitzgerald eta/, Appl. Spectros., 24 (1970) 106. P.S. Herrera, M.C. Oballa and A.F. Somogyvari, Paper presented at the 41st Annual Canadian Chemical Engineering Conference, Vancouver, B.C., Oct. 6-9, (1991). S. Chase, Paper presented at the 6th Annual Heavy Oil and Oil Sands Technical Symposium, Calgary, March, 8, (1989).

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10 D.W. Soveran, M.V.C. Sekhar and F.H. Wu, Paper presented at the

Technical Meeting of the South Saskatchewan Section of the Petroleum Society of CIM, Regina, Oct. 6-8,(1 987). 1 1 H. Topsoe, B.S. Clausen, R. Candia, C. Wivel and S. Morup, J. Catal., 68

(1981)433. 12 C. Wivel, R. Candia, B.S. Clausen, S. Morup, and H. Topsoe, J. Catal., 68 (1981)453. 13 B.S. Clausen, W. Niemann, P. Zeuthen and H. Topsoe, Prepr. Am. Chem. SOC.Div. Petr. Chem., 35 (1 990) 208. 14 Louwens, S.P.A., and Prins, R., Prepr. Am. Chem. SOC., Div. Petr. Chem., 35 (1 990)308. 15 P.S. Herrera, L. Neumann, M. Oballa and A.F. Somogyvari, Final Report Part 1, Energy Mines and Resources Canada, Contract File No. 23440-89261/01-SQ, Dec., (1990). 16 S.Kolbe, Can. J. Chem., 47 (1969)352. 17 C.I. Chu, I. Wang, Ind. Eng. Chem. Proc. Des. Dev., 21 (2)(1982)338. 18 B.C.Gates, J.R. Katzer, G.C.A. Schuit, Chemistry of Catalytic Processes, McGraw Hill Inc., (1979). 19 C.G. Frye, J.F. Mosby, Chem. Eng., Prog., 63 (1967)66. 20 R.P. Kirchen, E.C. Sanford, AOSTRA J. of Res., 5 (1989)287. 21 A.A. Mohammed eta/,J. Petr. Res., 8 (1)(1989)25. 22 H. Christensen and B.H. Cooper, AlChE Paper No. 44b, Presented at the AlChE Spring National Meeting, March 18-22,(1 990). 23 C.W. Fitz, Jr., H.F. Rase, Ind. Eng. Chem. Prod. Res. Dev., 22,(1983)41. 24 S. Eijsbouts et a/,in Hydrotreating Catalysts, Preparation Characterization and Performance, edited by M.L. Ocelli and R.G. Anthony, Elsevier Publishers, Amsterdam, (1989)79.