Hydrocracking and Hydrotreating

9 Consideration of Catalyst Pore Structure and Asphaltenic Sulfur in the Desulfurization of Resids RYDEN L. RICHARDSON and STARLING K. ALLEY

Downloaded by CORNELL UNIV on September 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0020.ch009

Union Oil Co. of California, Union Research Center, Brea, Calif. 92621

The residuum f r a c t i o n of a full range crude is that f r a c t i o n remaining a f t e r all of the distillate i s taken overhead. As noted in Figure 1, t h i s residuum f r a c t i o n or r e s i d may be obtained with e i t h e r atmospheric or vacuum f r a c t i o n a t i o n , y i e l d i n g e i t h e r long or short r e s i d . Table I shows t h a t , upon distillation of a Kuwait crude i n t o s e v e r a l f r a c t i o n s , s u l f u r components concentrate mostly i n the r e s i d f r a c t i o n . Upon h y d r o t r e a t i n g each f r a c t i o n at f i x e d r e a c t o r c o n d i t i o n s , the r e s i d s u l f u r i s more difficult to remove than the gasoline s u l f u r or the gas oil s u l f u r . CHARACTERIZATION OF RESIDS The d i s t i n g u i s h i n g features of r e s i d feedstocks are (1) the presence of asphaltenes or p e n t a n e - i n s o l u b l e s , (2) high carbon r e s i d u e s , (3) the presence of metals, mainly n i c k e l and vanadium, and (4) unknown endpoints i n s o f a r as b o i l i n g range is concerned. Table II compares two atmospheric r e s i d s , West Coast and K u w a i t , i n a t r a d i t i o n a l manner. The obvious d i f f e r e n c e s i n c l u d e s u l f u r , n i t r o g e n , asphaltenes, t o t a l metals and m i d - b o i l i n g p o i n t . Apart from s u l f u r content, one might surmise a greater c a t a l y s t demand by the West Coast feedstock i n that the boxed values suggest heavy coke laydown and metals d e p o s i t i o n . Neither of the s u l f u r values i s boxed because there i s no i n d i c a t i o n as to (1) what f r a c t i o n of the s u l f u r i s r e f r a c t o r y or "hard" s u l f u r , nor (2) the degree of d e s u l f u r i z a t i o n to be achieved. Table I I I extends the comparison of these r e s i d s with an emphasis on r e a c t i v i t y , asphaltene c h a r a c t e r i s t i c s , compound types and the r e f r a c t o r y forms of s u l f u r , such as benzothiophenes and asphaltenic s u l f u r . The lower r a t e constant of the Kuwait feed i n d i c a t e s i t s greater r e f r a c t o r i n e s s or r e s i s t a n c e toward d e s u l f u r i z a t i o n . These second order r a t e constants were measured at 6 7 5 ° F w i t h a p r e s u l f i d e d c o b a l t moly c a t a l y s t having a c a t a l y s t age of 10 days. Conversion l e v e l s d i d not exceed 75% and hence d i d not go deeply

136 Ward and Qader; Hydrocracking and Hydrotreating ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

9.

RICHARDSON

Desulfurization of Resids

A N D A L L E Y

137

TABLE I

FRACTIONATION

AND

HYDROTREATING

HYDROTREATED SULFUR

PRODUCT

SULFUR

VOL. %

WT. %

S,%

REMOVAL, %

25

0.5

0.02*

96

400-650

25

1.8

0.5 *

72

650 +

50

3.7

1.6·

57

1050 +

(20)

(5)

BOILING FRACTION

RANGE, °F

GASOLINE


GAS

X-400

OIL

LONG

SHORT

RESID

RESID

e

• 780 F ,

1000 psig, 1 LHSV,

COMPARISON

OF

4000

TABLE

II

WEST

COAST AND

ATMOSPHERIC

FEED

DESIGNATION

SCF/B

KUWAIT

RESIDS

WEST

COAST

KUWAIT

11.5

17.4

IBP/10

536/725

447 / 6 3 4

50/60

996/1051

937/1020

GRAVITY, *API

DISTILLATION, °F

SULFUR, %

1.73

3.66

NITROGEN, %

0.90

0.203

ASPHALTENES,%

1 1.2

6.3

CONRADSON

10.7

8.1

CARBON,% e

POUR POINT, F

75

30

METALS, ppm COPPER

1

IRON

53

2

NICKEL

75

11

VANADIUM

63

38

Ward and Qader; Hydrocracking and Hydrotreating ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

HYDROCRACKING AND HYDROTREATING

TABLE

III

EXTENDED COMPARISON OF WEST COAST AND KUWAIT RESIDS KUWAIT

DESIGNATION

RATE

CONSTANT

52

38

SULFUR,%

1.73

3.66

ASPHALTENIC SULFUR, %

0.22

0.38

60

70

0.72

0.78

TOTAL

GEL.


WEST COAST

FEED

PERM. CHROMO. ASPHALTENE RELATIVE

MODE

AMOUNT

DlAM., A OF

ASPHALTENES AT D - 150 LIQUID

λ

CHROMO

C O M P O U N D TYPE

FEED

SATURATES, Jo

20.6

26.7

AROMATICS, %

41.2

50.4

POLAR AROMATICS, %

22.9

10.4

ASPHALTENES, %

10.7

6.3

WEST COAST

KUWAIT

DESIGNATION SULFUR

CONT ENT

SATURATES, ppm

22

34

AROMATICS, %

0.94

2.36

POLAR

0.4 1

0.59

0.22

0.38

AROMATICS, %

ASPHALTENES, % HIGH MASS O N X-1100 °F Q.H. THIOPHENES

2.9

2.3

BENZOTHIOPHENES

7.4

15.0

TRI BENZOTHIOPHENES

0.0

0.0

0.43

0.33

0.38

0.42

LOW

HIGH

IR ABSORBANCE AROMATIC ALKYL RATIO

GROUPS

OF NMR

CH /CH 2

RINGS

3

PEAK HEIGHTS



9.

RICHARDSON

A N D A L L E Y


139

into asphaltenic s u l f u r . F i g u r e 2 shows the GPC curves of the c i t e d r e s i d s and t h e i r asphaltene f r a c t i o n s . Note the s l i g h t p r e dominance of very l a r g e asphaltenes i n the Kuwait r e s i d . A l s o note that very l a r g e molecules e x i s t i n the non-asphaltenic f r a c t i o n of both r e s i d s . For the conversion l e v e l s under c o n s i d e r a t i o n , we regard the l a r g e p r o p o r t i o n of s t e r i c a l l y hindered aromat i c s u l f u r i n the Kuwait r e s i d l a r g e l y r e s p o n s i b l e f o r i t s greater refractoriness. Returning to Table I I I , one notes the h i g h values for aromatic s u l f u r (2.36 v s . 0.94%) and benzothiophenes (15.0 v s . 7.4%) i n the Kuwait feed. A l s o , the IR and NMR s p e c t r a i n d i c a t e d that the Kuwait aromatics are more h i g h l y s u b s t i t u t e d , the R groups being naphthenes and/or long p a r a f f i n s . A p e r t i n e n t example of s t e r i c hindrance on the d e s u l f u r i z a t i o n of thiophenes has been reported by a Gulf i n v e s t i g a t o r (1). A l k y l or r i n g a d d i t i o n at the 2 p o s i t i o n can reduce the r e a c t i o n r a t e by a f a c t o r of 100. REFRACTORINESS OF ASPHALTENES When s u l f u r conversion l e v e l s are pushed above 90%, the unique r e f r a c t o r i n e s s of asphaltenes becomes dominant. The tendency of a f i n e pore c a t a l y s t to p a r t i a l l y exclude asphaltenes and the complete s t e r i c hindrance or "burying" of s u l f u r i n a s p h a l tenes c o n t r i b u t e to t h i s r e f r a c t o r i n e s s . However, before c o n s i d e r i n g the f a t e of a s p h a l t e n i c s u l f u r at high r e a c t o r s e v e r i t i e s , some patent aspects of c a t a l y s t pore s t r u c t u r e would be of i n t e r e s t . Table IV shows a divergence of o p i n i o n as to the d e s i r a b i l i t y of asphaltene e x c l u s i o n . I t i s noted that patent (2) favors the e x c l u s i o n of a s p h a l tenes by maximizing surface area contained by pore diameters of 30-70 Â. Patent (3) d i s c l o s e s an upper l i m i t on the amount of macropore volume represented by pore diameters greater than 100 A . Patents (4) and (5) suggest that c a t a l y s t s c o n t a i n i n g mostly m i c r o pores w i l l be poisoned soon; asphaltenes p e n e t r a t i n g the l a r g e r pores subsequently w i l l block entrance to the smaller p o r e s . Patent (6) claims the d e s i r a b i l i t y of intermediate pores (100-1000 Â) p l u s channels (>1000 Â) "to take up p r e f e r e n t i a l l y adsorbed l a r g e molecules without causing blockage, so that the smaller s i z e pores can d e s u l f u r i z e ' s m a l l e r molecules." Patent (7) a l s o p r e f e r s the open s t r u c t u r e for c o l l e c t i o n of coke and metals. It s p e c i f i e s 0.3 c c / g of pore volume i n diameters l a r g e r than 150 Â and "many pores from 1,000-50,000 A . " With t h i s b r i e f c o n s i d e r a t i o n of v a r i a t i o n s i n c a t a l y s t pore s t r u c t u r e , l e t us examine the pore s t r u c t u r e of two c a t a l y s t s used i n t h i s r e f r a c t o r i n e s s study. One observes i n Table V only s l i g h t d i f f e r e n c e s between the two c o b a l t moly c a t a l y s t s , Τ and R. They are t y p i f i e d by h i g h surface a r e a , small micropore mode diameters and low macropore volumes. Upon processing Kuwait r e s i d over these c a t a l y s t s , a s i m i l a r trend i n product d i s t r i b u t i o n i s shown i n Table V I . Saturates i n 1


HYDROCRACKING A N D

140 ATMOS

HYDROTRLATING

FRACTIONATION GASOLINE

TURBINE FUEL DIESEL

Λ

FUEL

20 mm Hg VAC COLUMN

CRUDE OIL LGO HEATER


HG Ο

Λ

LVGO

HVGO HEATER

A T M O S . RESID OR L O N G RESID 650 F + e

V A C RESID OR SHORT RESID. 1050 F + e

Figure 1. Distillation of crude oil

DIAMETER, Â

Figure 2. GPC molecular size distribution


RICHARDSON

141


A N D A L L E Y

TABLE IV

SOME OF

PATENT


CODE

NUMBER

(2)

PATENTS

RESID

O N PORE

HYDROTREATING

CoMo,

CATALYSTS

SPECIAL

CATALYST COMPOSITION

STRUCTURE

PORE VOLUME

FEATURES PORE

DIAMETER

SURFACE

NiMo

AREA (SA)

OF 3 0 - 7 0 Â PORES 2

> 100 m / g .

(3)

CoMo,

NiMo,WMo.

ALUMINA WITH 1-6%

150 m V g .

PORES > 1 0 0 Â

SA OF 3 0 - 7 0 Â PORES 2

> 100 m / g .

SILICA.

APPROX. 85 7. OF PORE

(4)

VOLUME

IN 5 0 - 2 0 0 A

RANGE.

PATENT CODE

NUMBER

(5)

SPECIAL

CATALYST COMPOSITION

N i C o M o . NO SILICA

PORE VOLUME

0.46 c c / g

CoMo

(+ Ni)

PORE

DIAMETER

REGULAR DISTRIBUTION 0 - 2 4 0 A. A V E R A G E =

WITH

1 4 0 - 1 8 0 A.

ALUMINA.

(6)

FEATURES

0.45-0.50 cc/g

6 0 - 7 0 A PLUS 100-1000 Â PLUS CHANNELS > 1000 A. S A * 260-355 m /g. 2

(7)

CoMo, NiW. NO

0.3 c c / g

SILICA WITH

PORES >150 A

IN

MANY

PORES

1,000-50,000 Â

ALUMINA


142

HYDROCRACKING A N D H Y D R O T R E A T I N G

TABLE V PORE

STRUCTURE USED

CATALYST SURFACE MODE

IN

OF

COBALT

MOLY

REFRACTORINESS

STUDY

DESIGNATION

Τ 2

AREA (SORPT.), m / g


TOTAL PORE VOL., m l / g 100 A)

PORE VOL., ml/g

R

284

307

67

65

0.44

0.52

0.012

0.035

DIAMETER, Â

MACRO ( D >

CATALYSTS

T A B L E VI

CHANGE

IN

PRODUCT

ASPHALTENIC

C O M P O U N D TYPE A N D

SULFUR

CONVERSION

REACTOR

CONDITIONS

FEED

CONTENT

WITH

LEVEL

CATALYST

Τ

CATALYST R

TEMPERATURE, *F

65 5

655

709

665

670

710

PRESSURE,

800

1292

1292

800

1292

1292

0.5

0.3

0.3

0.5

0.3

0.3

74

83

92

74

85

94

psig

LHSV, V O L . / ( V O L . ) CONVERSION COMPOUND

(HR)

LEVEL, TYPE,'/.

SATURATES

30.6

36.8

42.8

43.1

37.5

39.7

45.5

AROMATICS

45.8

47.6

46.8

43.5

47.6

46.4

44.3

POLAR

17.3

10.7

6.2

10.3

10.8

11.3

8.2

6.3

4.9

4.2

3.1

4. 1

2.6

2.0

3.66

0.95

0.63

0.29

0.96

0.55

0.23

AROMATICS

ASPHALTENES TOTAL

SULFUR, %

ABSOLUTE RELATIVE

ASPHALTENIC ASPHALTENIC

SULFUR, % SULFUR, %

0.45 12

0.32 34

0.26 41

0.18 62

0.24 25

0.15 27


0.09 39


9.

RICHARDSON

A N D A L L E Y


143

crease, aromatics remain f a i r l y constant and both p o l a r aromatics and asphaltenes decrease. Adsorption chromatography i s the b a s i s f o r type s e p a r a t i o n : pentane e l u t e s the s a t u r a t e s , ether e l u t e s the aromatics, and benzene-methanol e l u t e s the p o l a r aromatics. Quite c l e a r l y , the r e l a t i v e amount of a s p h a l t e n i c s u l f u r increases as the product t o t a l s u l f u r decreases. This i s depicted i n F i g u r e 3. Whereas the r e l a t i v e amount of aromatics remained f a i r l y constant as s u l f u r conversion l e v e l was increased to 92-94%, the r e l a t i v e amount of s u l f u r i n the aromatic f r a c t i o n decreased markedly. T h i s a l s o i s depicted i n F i g u r e 3. P o l a r aromatics are intermed i a t e to the aromatics and asphaltenes i n regard to t h i s behavior. These d i f f e r e n t d i s t r i b u t i o n s of s u l f u r with conversion l e v e l resemble those reported by Drushel f o r Safaniya r e s i d (8). The removal of metals with the a s p h a l t e n i c s u l f u r i s observed i n F i g u r e 4. T h i s response i s c o n s i s t e n t with an asphaltene model i n which vanadium and n i c k e l are b u r i e d as porphyrins or sandwich compounds (9). The s l i g h t l y higher removal of vanadium r e f l e c t s a general tendency f o r vanadium to deposit on the c a t a l y s t more r e a d i l y than n i c k e l . EFFECT OF COKE DEPOSITION ON PORE SIZE DISTRIBUTION A f i r s t step toward c a t a l y s t design i s to r e l a t e pore s t r u c ture roughly to asphaltene dimensions. Another step i s to c o n s i der pore s t r u c t u r e of the used c a t a l y s t and the probable s i z e of asphaltenes at r e a c t o r temperature. Figure 5 i s a histogram showing the d i s t r i b u t i o n of pore v o l ume v s . pore diameter f o r alumina c a r r i e r , f r e s h c o b a l t molybdenum c a t a l y s t and used cobalt molybdenum c a t a l y s t . There was a s l i g h t change i n mode diameter when the c a r r i e r was loaded with about 20% a c t i v e metal oxides. The pore volume was reduced from 0.60 to 0.53 m l / g . However, accumulation of about 17% coke during the p r o c e s s i n g of West Coast r e s i d g r e a t l y s h i f t e d the mode downward and reduced the t o t a l pore volume from 0.53 to 0.30 m l / g . ( A l l of these pore volumes have been normalized to 1.0 gram of alumina). A d d i t i o n a l comparisons of f r e s h v s . used c a t a l y s t are shown i n Table V I I . The coke laydown occurred mainly i n the micropore r e g i o n , causing a s u b s t a n t i a l l o s s of surface a r e a . Coke laydown a l s o was observed i n the macropore r e g i o n of the bimodal c a t a l y s t shown at the bottom of the t a b l e . ASPHALTENE EXCLUSION AS A FUNCTION OF TEMPERATURE The e x c l u s i o n of asphaltenes f i r s t was approached by the simp l e method of immersing a given amount of c a t a l y s t i n a volume of r e s i d equal to 3 times the pore volume of the c a t a l y s t . If asphaltene e x c l u s i o n were to occur to a s i g n i f i c a n t degree, then there


144

HYDROCRACKING

A N D HYDROTREATING


CATALYST R -CATALYST Τ

SATURATES

, 0

20 7.

•^ • 40

60

80

DESULFURIZATION

100 Figure 3. Distribution of sulfur with conversion level


9.

RICHARDSON

20



A N D A L L E Y

145

140

60 80 100 PORE DIAMETER, Â

40

Figure 5. Histograms of pore volumes vs. pore diameter for carrier, fresh catalyst, and used catalyst

TABLE VII

CHANGE

IN

PORE

DUE TO COKE

SAMPLE AND

DESCRIPTION DESIGNATION

PORE

STRUCTURE

DEPOSITION

VOLUME, ml/g

MODE

SURFACE

DIAMETER

AREA

A

m /g

MACRO

MICRO

TOTAL

S - 0293 - A

0.03

0.3 6

0.39

90

207

USED S - 0 2 9 3 - A

0.04

0.13

0.17

55

164

V-8477-Β

0.02

0.35

0.37

85

136

USED

0.02

0.15

0.17

60

102

X - 02 34 - A

0.02

0.41

0.43

90

167

USED X - 0234 - A

0.02

0.21

0.23

60

140

H-6013

0.70

0.4 5

1.15

95, 350

350

USED

0.39

0.30

0.69

80, 400

174

V-8477-Β

H-6013


2


146

HYDROCRACKING A N D H Y D R O T R E A T I N G

would be an enrichment of asphaltenes i n the e x t e r n a l l i q u i d . Using a 24-hour e q u i l i b r a t i o n p e r i o d of 2 1 2 ° F , the four samp l e s shown i n Table V I I I a l l give an enrichment of asphaltenes. The e x t e r i o r l i q u i d s contained 12.0-13.8% asphaltenes compared to 6.3% i n the Kuwait feed. These r e s u l t s q u a l i t a t i v e l y agree with observations of Drushel who used a d i f f e r e n t technique w i t h Safani y a r e s i d at room temperature (8). The e x c l u s i o n of asphaltenes as a f u n c t i o n of temperature subsequently was approached w i t h the a i d of g e l permeation chromotography (GPC) i n a manner s i m i l a r to that described by D r u s h e l . C a t a l y s t was e q u i l i b r a t e d 4 hour at constant temperatures w i t h a volume of r e s i d equal to 3 times the pore volume of the c a t a l y s t . During t h i s time, the system was n i t r o g e n blanketed and a g i t a t e d every 1/2 hour. The e x t e r n a l l i q u i d subsequently was drained through a screen and submitted f o r GPC analyses and metals content. The i n t e r n a l l i q u i d was e x t r a c t e d from the c a t a l y s t pores f i r s t by benzene and then 50/50 methanol-benzene. A f t e r evaporation of these s o l v e n t s , the i n t e r n a l l i q u i d was submitted f o r i d e n t i c a l analyses. The GPC curves i n Figure 6 show asphaltene or "large molecule" e x c l u s i o n at the nominal temperatures of 200, 400 and 6 0 0 ° F . The e f f e c t decreases with i n c r e a s i n g temperature: planimeter areas f a l l i n the r a t i o of 1.00, 0.65, and 0.40. Metals d i s t r i b u t i o n , which i n some degree should r e l a t e to asphaltene d i s t r i b u t i o n , are shown f o r the a c t u a l e q u i l i b r a t i o n temperatures i n Table IX. N i c k e l showed the expected enrichment at a l l temperatures. Vanadium responded s i m i l a r l y except at 403 and 6 0 3 ° F . Progressive s u l f i d i n g (0.6, 0.7, 1.4% S) and coke l a y down (0.8, 1.5, 7.8% C) were observed f o r the used c a t a l y s t s and hence represent an experiment c o m p l i c a t i o n . CONCLUSIONS The p o i n t s to be emphasized i n c l u d e the f o l l o w i n g : 1. The r e f r a c t o r i n e s s of r e s i d feeds has been considered from the standpoint of process s e v e r i t y and d i v i s i o n between "hard" and "easy" s u l f u r . 2. For lower conversions l e v e l s , where a s p h a l t e n i c s u l f u r removal i s not deep, r e f r a c t o r i n e s s appears to be l a r g e l y i n f l u enced by a l a r g e p r o p o r t i o n of s t e r i c a l l y hindered aromatic s u l fur compounds. S u b s t i t u t e d thiophenes, benzothiophines and d i benzothiophenes are r e p r e s e n t a t i v e compounds. 3. A s p h a l t e n i c s u l f u r i s the most r e f r a c t o r y specie i n r e s i d s and the removal of metals, p a r t i c u l a r l y n i c k e l , c o r r e l a t e s w e l l with removal of a s p h a l t e n i c s u l f u r . 4. Coke d e p o s i t i o n a l t e r s c a t a l y s t pore s i z e d i s t r i b u t i o n s s i g n i f i c a n t l y and i s an e f f e c t to be followed i n regard to c a t a l y s t aging. 5. The e x c l u s i o n of asphaltenes i n Kuwait r e s i d i s observed by an autoclave technique at 2 1 2 ° F with s e v e r a l c a t a l y s t s having


A N D

A L L E Y



RICHARDSON

Figure 6. GPC molecular size distributions at various temperatures


148

HYDROCRACKING AND HYDROTREATING

TABLE VIII

ASPHALTENE

EXCLUSION EXPERIMENTS

AT 212 *F A N D Ί ATMOS. .

CATALYST


RESID

WEIGHT, g

WEIGHT, g

ASPHALTENE

24.00

23.00

25.50

26-60

36.49

35.00

35.07

47.00

CONTENT

INITIAL, % FINAL, OBS., % PORE

CATALYST

6.3

6.3

6.3

6.3

13.8

12.8

12.0

13.4

V O L U M E , ml/g

0.50

0.52

0.47

1.15

MICROPORE VOLUME, ml/g

0.42

0.50

0.44

0.45

MODE

DIAMETER,

65

λ

60

75

TABLE IX

METALS DISTRIBUTION IN RESID FRACTIONS Ni

e

V (PPM)

9

40

INTERNAL

4

18

FEED 197 F

(PPM)

EXTERNAL

15

58

e

INTERNAL

2

48

e

EXTERNAL

12

35

INTERNAL

5

49

EXTERNAL

10

27

e

197 F 403 F 403 F e

603 F e

603 F


95

9.

RICHARDSON AND A L L E Y


149

mode diameters of 60-95 Â. 6. Exclusion of asphaltenes and/or large molecules i s also observed at 200, 400 and 600°F by similar technique involving GPC. The effect diminishes moderately with increasing temperature due to thermal dissociation into smaller particles. 7. The exclusion of asphaltenes is matched by the distribution of nickel inside and outside the catalyst pore structure. Vanadium distribution is inconsistent, probably influenced by coke deposition at the higher temperatures. ACKNOWLEDGEMENT


The authors acknowledge Dr. Dennis L . Saunders for the GPC analytical work and many useful discusssions. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9)

Larson, O. Α . , Pittsburgh Catalysis Society, Spring Symposium, (April, 1972). US 3,509,044 (Esso) US 3,531,398 (Esso) US 3,563,886 (Gulf) UK 1,122,522 (Gulf) NPA 6,815,284 (Hydrocarbon Research) German 1,770,996 (Nippon Oil) Drushel, Η. V., Preprints, Div. Petrol. Chem., ACS, 17, No. 4, F-92 (1972). Dickie, J. P . , Yen, T. F., Preprints, Div. Petrol. Chem., ACS, 12, No. 2, B-117 (1967).


Hydrocracking and Hydrotreating

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