Effective utilization of residual type feedstock to middle distillates by hydrocracking technology

Studies in Surface Science and Catalysis 142 R. AieUo, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

771

Effective utilization of residual type feedstock to middle distillates by hydrocracking technology and D. Biswas b. S.K. Saha ~*, G.K.Blswas, " aChemical Engineering Department, Jadavpur University, Calcutta-700032, India bChemical Technology Department, Calcutta University, Calcutta-70009, India Hydrocracking is an attractive technique among th~secondary conversion processes. The processing problem, however, goes up markedly as the crude oil .quality decreases such like ~

gravity while on the other hand increases the conradson carbon, sulfur and

metal contents are due to excessive consumption of petroleum products. Hydrocracking is the most flexible in respect to change in feed quality that handles poor quality feeds easily to produce lighter products. We studied a case using 60:40 combination of reduced crude oil and cycle oil containing 50% aromatics with 1.15% S, and 0.1% N having pour point +24~

Temperature, pressure, and residence time were studied as a process

parameters. Catalytic parameters were also studied. The maximum yield of middle distillates was found to be 49.51% under the following condition: temperature = 623 K, pressure = 7.0 MPa, initial hydrogen partial pressure = 6.0 MPa, residence time = 900 see, feed = 250 g, and catalyst = 10 g 20:80 ratio of A:Z (A- amorphous silica-alumina, Zmolecular sieve 13X). Palladium metal was chosen for hydrogenation site. 1. INTRODUCTION In the modem refinery, catalytic hydrocracking is an attractive among the secondary conversion processes to get more valuable products as well as clean atmosphere from heavier petroleum fraction. The versatility of this process makes it easy to equilibrate the supply and demand of fuels such as gasoline, diesel, and jet fuel. The main goal of hydrocraeking conversion is the reduction of the average carbon number, and the production of branched isomerization of linear paraffins is desirable to improve *Correspondence should be addressed to: S.K Saha Department of Chemistry, Faculty of Engineering, Gifu University, C_hfia501-1193, Japan. E-mail: ksshyama168 @hotmail.com

772 the quality of the different petroleum fractions. Demand patterns of petroleum products have been changed from gasoline to middle distillates and the change continues at present, all over the world [1]. In this perspective, hydrocracking is considered to be the best economic way of converting heavy ends to quality fuels, particularly to middle distillates. A recent report suggests that hydrocracking of polyaromatic compounds proceeds via initial hydrogenation of peripheral ring to naphthenic ring, [2] cleavage into aliphatic substitutes and isomerise to a branched naphthenic compound and finally undergoes into dealkylafion. Another report studied on the role of dispersed phase Mo catalyst in hydrocracking of Guado H [3] revels that cracking reaction occurs essentially through the normal cracking pathway, and that Mo catalyst can considerably inhibit coke formation and enhance desulfurisation. Evidence complemented~y the works on hydrocracking of vacuum gas oil assembled in studies with using highly dispersed metals such as W, Mo, Co and Ni [4,5,6] explored that higher temperature favours more coke whereas lower pressure gives rise to middle distillate with mild acidity. Refractory cycle oil feed could be easily hydrocracked over SiO2-A1203-Ce exchanged Y containing Ni and Mo to jet fuels [7]. Ultrastable Y zeolite catalyst has been found more active to increase middle distillates compared to commercial LZY-82 catalyst [8]. Omega zeolite containing catalyst [9] has also been reported to afford high conversion and selectivity to middle distillates. Studies conducted by Saha et al. [10] on refinery waste to middle distillates reports hydrogen partial pressure plays a vital role for the hydrocracking of refinery waste mainly refractory type of compounds. Various catalyst types viz. zeolites, amorphous SiO:z-A1203, ZrO2-SiO2, USY-zeolite, ZSM-5 etc. were tried as cracking site while Ni, Mo, W, Pt and Pd etc. were studied as hydrogenation site by a number of researchers [ 11]. Still, better catalyst is in search for economic process technology as well as quality products. In this work large pore molecular sieve 13X and SIO2-A1203 amorphous supports were chosen for cracking site and palladium metal for hydrogenation site. 13X molecular sieve adsorb critically larger diameter molecules, such as aromatics and branched chain hydrocarbon and offer very good mass transfer rate in parallel, palladium metal has higher hydrogenation capacity. Our present work designed with the mixed feed is so far the first report on catalyst support variation for middle distillate yield. The present paper deals with hydrocraeking of residual feed (mixed feed) with catalyst support variation from amorphous SiO2-A1203 to zeolite 13X and their combination at different proportion. Various parameters were also studied for maximum middle distillate yield.

773 2. EXPERIMENTAL

2.1 Feed and catalyst preparation Reduced crude oil (RCO) blended with cycle oil in the proportion of 60:40 ratio, having characteristics listed in Table 1. Feed was characterized using standard method. F.or the catalyst preparation, molecular sieve 13X support was procured from the market while the amorphous silica-alumina support was made in the laboratory. Silica-alumina ratio was maintained as to 70:30 for both catalysts. The ammonium form of molecular sieve 13X as prepared by ion-exchange of sodium form, then dried and calcined to give the protonated H-form by a treatment with a m m o n i u ~ i t r a t e solution. 0.5% palladium metal was loaded as palladous chloride in both supports by impregnation method. The detail method of preparation of the catalyst have been described elsewhere [12]. The stability of catalyst was checked by DT-TGA. The characteristics of the catalysts have been shown in Table 2.

2.2 Reactor set-up Experiments were carried out in a rocking type batch reactor of laboratory scale (1 dm3 capacity). Details of the reactor and the assembly of other parts were described elsewhere [5]. The reactor was charged with requisite amount of feed and catalyst, and closed. Purging was done with nitrogen gas to ensure an oxygen free environment inside the reactor. Initially, desired pressure was maintained with hydrogen or nitrogen or both. Purity of hydrogen and nitrogen used here was 99.6% and 99.5% respectively. The total pressure was maintained by only nitrogen. The pressure reading was obtained from the pressure gauge, and the valve was properly closed and checked with soap solution for any leakage. Heating was applied and the temperature was regulated by variac. After attaining desired temperature, rocking of the reactor was started and continued for a definite residence time. At the end of residence time, gas and vapor originated inside the reactor was allowed to pass through an ice-cooled spiral condenser. The liquid product was condensed while non-condensable gaseous product was allowed to pass through the scrubbing system for H2S absorption. The scrubber contained 10% NaOH solution. AKer H2S absorption rest of the gas was passed through a wet gas meter and escape to the atmosphere. The liquid product was analyzed by standard methods for petroleum products (IS/ASTM).

774 Table 1 Properties of feedstock at 60:40 combination of reduced crude oil and cycle oil Parameters

Values

Specific gravity, 60~176

0.8874

Viscosity at 100~ cSt

7.70

Sulfur wt., %

1.15

Nitrogen wt., %

0.10

Ramsbottom carbon residue wt., %

0.911

Carbon to hydrogen ratio

7.55

Pour point, ~

+24 240-576

Boiling range, ~

,,,

,

Table 2 Catalyst properties of palladium loaded molecular sieve 13X and amorphous silica-alumina _

|

,

Items

,

i

,,

Molecular sieve 13X

Amorphous silica-alumina

With palladium

With Palladium

Surface area (m2/g)

336.30

133.60

Total pore volume (cc/g)

0.3973

0.088

0.327 0.0870

0.3302 0.0840

81.64

16.02

Acidity (retool/g) Brrnsted acidity Lewis acidity Pore size distribution (%) > 1000A < 1000 A

18.56

,,,,,

83.98 . . . .

,

, .

.

.

.

3. RESULTS AND DISCUSSION

The process parameters studied were the temperature (573 to 683 K), partial pressure of hydrogen (2.0 to 6.0 MPa) and residence time (420 to 1800 sec). During process parameter study 250 g feed and 25 g catalyst of palladium metal loaded with at a combination of 80:20 A:Z were used. Table 3 reveals that the percentage of conversion at 573 K was only 58.60%, which increased to 93.53% at 663 K beyond which percentage conversion slowly decreased to 83.76% at 683 K. However, the yield of middle distillates was the highest, which was 33.80%, at temperature 623 K within the temperature range

775 studied. The decrease of percentage of middle distillates at higher temperature might be due to secondary cracking reactions occurred beyond temperature of 623 K, thereby augmenting yield of light distillate and gaseous product. It is, therefore, expected that the endothermic cracking reaction predominated over exothermic hydrogenation reaction, and the fact was supported by the increasing tendency of % aromatics at higher temperature. Partial pressure of hydrogen was studied with predetermined total pressure, which was 7.0 MPa at 623 K. The effect of hydrogen partial pressure has been shown in Table 4. It has been observed that effect of hydrogen partial pressure plays a significant role during hydrocracking reaction. The experimental data reveal that there is an increase in the production of middle distillates with corresponding increase of light distillate, and an increase in hydrogen partial pressure up to 6.0 M P ~ i t h correspondingly decreases in heavy distillate. It clearly indicates that initially hydrogenation of higher hydrocarbons makes cracking easier for yielding lighter products. The maximum middle distillate was found at 6.0 MPa hydrogen partial pressure. At higher hydrogen partial pressure, product quality was better and also coke deposition was minimum. Smoke point and octane index of the middle distillate cuts were higher. Table 5 shows the effect of residence time. To investigate the influence of residence time on hydro cracking of residual type feedstock, increasing reaction time from 420 see to 1800 sec resulted in conversion from 65.95 to 78.42%. However, it has been observed that percentage yield of middle distillate is increased with increment of residence time up to 900 sec reaching maximum value of 41.55% which was the summation of MDL-middle distillate light (150-250~ 17.95% and MDH- middle distillate heavy (250-320~

cut of

cut of 23.6% after which the

percentage yield of middle distillates falls. These results indicate that longer reaction time like 1800 see is not beneficial to hydrocrack, rather 900 see might be better choice. This is probably due to the fact residence time less than 900 see is not sufficient to complete the reaction while a longer residence time results in undesirable side reactions, such as partial polymerization and condensation, thus decreasing middle distillate. Catalytic parameters were studied at predetermined process condition and optimum feed to catalyst ratio. For the study of catalyst cracking site variation, 10 g of catalyst was used. Effect of cracking site variation has been shown in Table 6. The study was conducted with catalyst support varying from amorphous silica-alumina to zeolite 13X and their combination at different proportion viz. 80:20, 50:50 and 20:80. Palladium was the metallic support in all the cases for hydrogenation site. It was revealed from the study that neither amorphous silica-alumina nor zeolitel3X was suitable as cracking site when used individually for hydrocracking of residual feed to lighter products especially middle distillate. Their combination, however, was more effective for this purpose resulting in high conversion

776 and more yields of middle distillates of good quality. Again, zeolite rich A:Z of 20:80 combination was far better than amorphous rich combination. The result showed more middle distillate production having lower aromatic content, thus an improved burning characteristics (higher smoke point) and better engine performance (higher Cetane Index). Coke deposition was also minimal. 100% amorphous or zeolite-based catalyst alone was not effective. This implies that there must be some synergistic effect when amorphous-zeolite combination was used. This may be due to the fact that in one hand, amorphous catalyst has good stability against sulfur compounds present in the feed and high selectivity for middle distillate. On the other hand, zeolitic catalyst may have difficulty in converting some of the larger and higher boiling component to lighter product. Hence, presence of certain percentage of a~orphous catalyst in the zeolite matrix would be beneficial in hydrocracking of residual type feedstock. Table 3 Effect of temperature on hydrocracking of mixed feed oil (total pressure: 4.5 MPa, hydrogen partial pressure: 4.5 MPa, residence time: 900 see, feed: 250 g, catalyst: 25 g, A:Z = 80:20) ,,

Items

,

..

,,i

,.,,

i

,

,

Temperature (K) 573

623

663

683

Percentage conversion

58.60

92.44

93.53

83.76

Gas Light distillate (IBP-150~

20.00 11.50

38.75 18.55

39.68 21.43

60.32 7.14

MDL (150-250~

10.20

15.25

15.00

5.25

MDH (250-320~ Heavy distillate (320~

14.90 41.40

18.18 7.55

15.01 6.46

8.24 16.24

Coke

2.00

2.00

2.40

2.80

% Aromatics in MDL (Vol.)

35.00

25.00

26.00

27.00

Smoke point ofMDL, mm

14.00

18.00

18.00

17.00

% Aromatics in MDH (Vol.)

32.00

22.00

24.00

26.00

Cetane Index of MDH

33.00

48.00

46.00

46.00

777 Table 4 Effect of hydrogen partial pressure (temperature: 623 K, total pressure: 7.0 MPa, residence time: 900 see, feed: 250 g, catalyst: 25 g, A:Z = 80:20) ,

,,,

--

Items

=

,

,,

, , ,

,

,,

,,,

Hydrogen partial pressure (MPa) 2.0

4.5

6.0

Percentage conversion

57.20

73.20

76.61

Gas

14.03

19.64

24.00

Light distillate (mP-150~

4.00

6.82

8.26

MDL (150-250~

14.85

18.50

17.95

MDH (250-320~

19.10

22.24

23.60

Heavy distillate (320~

42.80

26.80

23.39

Coke

5.20

6.00

2.80

% Aromatics in MDL (Vol.)

28.00

26.00

22.00

Smoke point ofMDL, mm

18.00

18.00

20.00

% Aromatics in MDH (Voi.)

25.00

25.00

20.00

Cetane Index of MDH

44.00

47.00

,,

48.00 ,,,,,

.

.

.

.

.

.

.

.

Table 5 Effect of residence time (temperature: 623 K, total pressure: 7.0 MPa, hydrogen partial pressure: 6.0 MPa, feed: 250 g, catalyst: 25 g, A:Z = 80:20) .

.

.

.

,,,

,,

,

,,

,

,

,

.

.

.

.

Items

Residence time (see) 420

900

1800

Percentage conversion

65.95

76.61

78.42

Gas

11.05

24.00

26.00

Light distillate (IBP-150~

9.36

8.26

6.60

MDL (150-250~

17.75

17.95

18.52

MDH (250-320~

23.00

23.60

21.30

Heavy distillate (320~

34.04

23.39

21.58

Coke

4.80

2.80

6.00

% Aromatics in MDL (Vol.)

26.00

22.00

26.00

Smoke point ofMDL, mm

19.00

20.00

20.00

% Aromatics in MDH (Vol.)

25.00

20.00

25.00

Cetane Index of MDH

47.00

48.00

46.00

778 Table 6 Effect of catalyst cracking site variation (temperature = 623 K; total pressure = 7.0 MPa; hydrogen partial pressure = 6.0 MPa; feed = 250 g; catalyst = 10 g, all catalysts are loaded with palladium metal) ,,,,,,

,,

,,

i

J

,

Items

,

|

i,,l|l

,,

,, i

i

,

,

A

A'Z

A'Z

A'Z

Z

(100%)

(80:20)

(50:50)

20:80

(100%)

Percentage conversion

79.21

83.91

85.48

85.78

61.43

Gas

30.27

29.60

29.18

21.13

18.51

Light distillate (IBP-150 ~

4.26

7.41

10.08

12.34

2.75

MDL (150-250 ~

19.01

20.46

22.12

19.68

17.85

MDH (250-320 ~

23.67

24.04

22.74

29.83

20.31

Heavy distillate (320 ~ +)

20.79

16.10

1.4.52

14.22

38.58

Coke

2.00

2.40

2.00

2.80

2.00

% Aromatics in MDL (Vol.)

26.00

25.00

25.00

20.00

22.00

Smoke point ofMDL, mm

18.00

20.00

19.00

22.00

20.00

% Aromatics in MDH (Vol.)

20.00

20.00

21.00

16.00

16.00

47.00

47.00

50.00

Cetane index of MDH

45.00 ,

,

,

,

,

,,

49.00 ,

,

i

i

m,,

4. CONCLUSION Higher catalytic activity was observed with larger external surface area, due to the greater number of pore opening. Greater surface area, high pore volume and presence of majority of pores in the macro pore regions were the positive result for hydro cracking of residual type feedstock with zeolytic rich catalyst. The above result showed that hydrocracking reaction was not suitable at higher temperature and higher residence time but higher hydrogen partial pressure was favorable for middle distillate yield. Palladium metal based catalyst showed lower stability in presence of high sulfur containing feeds though properties of middle distillate was better.

The maximum yield of middle

distillates was found 49.51% under the following reaction condition: temperature = 623 K, Pressure - 7.0 MPa (hydrogen partial pressure 6.0 MPa), residence time = 900 sec and feed to catalyst ratio = 25:1. ACKNOWLEDGEMENTS We are greatly indebted to Prof. Y. Sugi, Department of Chemistry, Faculty of Engineering, Gifu University, Japan for helpful discussion.

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