Metal Complexes in Fossil Fuels - American Chemical Society

Chapter 14 Upgrading Studies with Californian, Mexican, and Middle Eastern Heavy Oils 1

Geoffrey E. Dolbear, Alice Tang, and Eric L. Moorehead

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 22, 2015 | http://pubs.acs.org Publication Date: July 6, 1987 | doi: 10.1021/bk-1987-0344.ch014

Unocal Science and Technology Division, Fred L. Hartley Research Center, 376 South Valencia, Brea, CA 92621

Conversions of the ever-increasing amounts of heavy crudes into transportation fuels is a challenge facing all refiners. Hydroprocessing is one route to meet this challenge, and new technology permits the upgrading of heavy oils from a variety of sources. One parameter rarely discussed in studies of heavy oil upgrading is the differences in chemical nature of the various oils. It is usually assumed, incorrectly, that i t is more difficult to treat oils with higher levels of contaminants and lower API Gravities. This paper reports detailed comparisons of the measured chemical compositions of oils from California, Mexico, and the Middle East, along with their reactivities in Unocal's Unicracking HDS process. Reactivity for removing nickel and vanadium was found to be a function of the distribution of metals between asphaltenes and resins, while sulfur reactivity was independent of these patterns. Refiners are all facing the challenge of converting increasing amounts of heavy crude oils into transportation fuels. Many processes have been developed to meet this challenge. The process schemes fall into two categories, those that remove carbon and those that add hydrogen. Unocal's Unicracking/HDS process is an example of the latter. One factor that is often overlooked in these upgrading schemes is the difference in chemical reactivities of heavy oils. Because the chemical makeup of heavy oils is complex, finding out what controls reactivity is not simple. In the past several years we have studied the reactivity of over thirty crude oils. This paper presents some of our results for oils from California, Mexico, and the Middle East. We found that gross properties such as gravity and 'Current address: Engelhard Corporation, Specialty Chemicals Division, Menlo Park, P.O. Box 2900, Edison, NJ 08818-2900 0097-6156/87/0344-0220$06.00/0 © 1987 American Chemical Society

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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sulfur content are not good predictors of hydroprocessing behavior. We will discuss some of the properties that do relate to reactivities.


Heavy Oils - What Are They? Heavy Oil is a non-specific term that applies to crude oils with API gravity less than 20 Within this definition there is a wide range of compositions and physical properties. Heavy oils typically have high levels of sulfur, nitrogen, nickel and vanadium, and are rich in the condensed polyaromatic compounds which react readily to form coke. Many of the processes used in refineries rely on catalysts which are poisoned or destroyed by these components and refiners see them as impurities to be eliminated. Refineries are designed to eliminate these components from the liquids to protect the catalysts as well as to produce environmentally acceptable products. By convention, the fraction of a crude oil which boils above 650°F (343°C) is called atmospheric resid. Refineries typically make their initial distillation cut at this temperature; a second cut is made at about 1050°F (565°C), the 1050°F-plus products called vacuum resid (or vacuum tower bottoms, VTB) and the intermediate boiling range liquids (650-1050°F) called vacuum gas oil (VGO). Refinery conversion units are specifically designed to handle these boiling range liquids. Atmospheric resid can be separated into three components on the basis of polarity or solubility. These components, in roughly increasing polarity and molecular weight, are called oils, resins, and asphaltenes. The combination of oils and resins is also referred to as maltenes. It is generally accepted that the resin portion contains asphaltene-1ike molecules which serve as a solvent for the asphaltenes in the less polar oil portion. Koots and Speight (2) demonstrated this by separating the three fractions and recombining them. The asphaltene was only soluble in the presence of the resin fraction. Number average molecular weights of resins are generally lower than those of asphaltenes and their sulfur, nitrogen, and metals levels are generally lower. Typically resins range from 600-5000 in molecular weight and contain 10-20% of the metals, 50-70% of the sulfur and contain 50-70% of the nitrogen. Up to 50% of the 1050°F-plus (565°C-plus) fraction will be asphaltenes. Asphaltenes have molecular weights ranging from 5000-10000 and contain 80-90% of the metals, 2-20% of the sulfur and nitrogen. Yen (3) has proposed a structural model of asphaltenes which describes tïïem as stacks of sheets or plates, rather than long chains like conventional polymers (Figure 1). More recent work (4,5,6) implies that, under proper solvent and temperature conditions, the sheets can separate from the stacks and move about the solution individually. This behavior further complicates asphaltene chemistry. Unicracking/HDS Technology Unocal's Unicracking/HDS process is a proprietary fixed bed, catalytic hydrotreater designed for upgrading heavy oils. It was



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METAL COMPLEXES IN FOSSIL FUELS

Figure 1.

Hypothetical Asphaltene Partial Molecule.


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originally developed for producing low sulfur fuel oil from low metals-containing oils, but UK/HDS has evolved into a flexible process capable of upgrading high metals feedstocks. Depending on the individual refiner's needs, the process can operate in either a high or low conversion mode. Conversion refers to the extent of 1050°F-plus (565°C) converted into distillates. In either mode the product oil has substantially reduced levels of sulfur and metals.


Experimental In this study a pilot plant unit using an experimental demetallization catalyst was used. The operation was carried out under moderate pressure but at relatively low temperatures. Each test feed was alternated with our reference o i l , and reactivity was calculated relative to i t . We chose Heavy Arabian as our reference because i t is widely available. This method accounts for changes in catalyst activity caused by metals, sulfur, or carbon depositing on the catalyst. We verified this by testing a blend of California and Alaska oils at the beginning of our feed study and again later in the run. We found that the reactivity remained constant relative to Heavy Arabian. The study tested more than thirty oils. We will discuss the results for five of these oils: Heavy Arabian, California-Alaska blend, consisting of 50% North Slope, 35% Santa Maria, and 15% LA Basin o i l s , Hondo, a large California offshore field north of Santa Barbara, Maya, a Mexican heavy crude processed by a number of U.S. refiners, and Gach Saran, a heavy Iranian crude processed almost as widely as Heavy Arabian. The discussion will center on these oils because, except for the Blend, they are generally available. The conclusions are supported by the results from the other crudes in the study. All measurements and reactions were done with 650°F-plus (343°C-plus) fractions (Table I) except for Hondo, where high viscosity forced us to use 400°F-plus (204°C-plus) liquids. Wherever appropriate, ASTM methods were used in measuring compositions. Asphaltenes were isolated by pentane precipitation, resins by absorption on a column of attapulgas clay and elution with dichloromethane. Reactivity is defined as the rate constant under constant conditions for sulfur or metals removal. The values we report are relative to our reference Heavy Arabian, defined as 100 in each case.


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Table I.


f

Gravity, °API Total Mi + V, wppm Ni, wppm V, wppm Total Sulfur, wt% Total Nitrogen, wt% 1050°F-plus, w « Conradson Carbon, wt% Asphaltenes (C-5), wt%

Properties of Heavy Oils

. Arab.

Hondo

12.6 115 28 87 4.23 0.26 51 12.6 12.6

13.4 372 92 280 5.10 0.70 46 10.8 13.9

Maya 9.4 496 83 413 4.42 0.52 59 15.3 25.2

Gach Saran 15.6 144 36 108 2.60 0.41 50 8.8 6.8

Blend 11.3 222 67 155 2.77 0.67 50 11.0 11.4

Properties and Reactivities The key to the experiment was a thorough and detailed set of characterization data for each feed and treated product. Properties. There is a great variation in the bulk chemical and physical properties. If we look at these oils closely, we can see differences in the relative amounts of o i l s , resins, and asphaltenes (Table II and Figure 2). The biggest differences between the samples is in the relative amounts of o i l s , and asphaltenes. Table II.

Oils, Resins and Asphaltenes in Heavy Oils

Hvy, , Arab. Oil Fraction, wt % 59.9 27.5 Resin Fraction, wt% 12.6 Asphaltenes (C-5), wt%

Hondo 43.9 40.2 13.9

Maya 48.9 25.9 25.2

Gach Saran 64.7 28.5 6.8

Blend 52.6 36.0 11.4

We can go farther and examine the distribution of sulfur and metals (nickel plus vanadium) among the o i l s , resins and asphaltene fractions. The data (Table III and Figures 3 and 4) show that the 5 Table III.

Distribution of Nickel, Vanadium and Sulfur in Heavy Oils Blend

Hondo

Maya

Gach Saran

2.76 0 0 0

2.97 2.4 2.4 0

2.59 1 1 0

1.74 0 0 0

1.86 0 0 0

Resin Fraction Sulfur, wt% Ni + V, wppm Ni, wppm V, wppm

5.92 83 25 58

6.70 238 83 155

5.39 233 45 188

4.19 241 64 177

3.51 200 70 130

Asphaltene Fraction Sulfur, wt% Ni + V, wppm Ni, wppm V, wppm

6.53 656 158 498

7.73 1547 417 1130

6.44 1887 317 1570

5.11 1466 366 1100

3.76 1235 345 890

Oil Fraction Sulfur, wt% Ni + V, wppm Ni, wppm V, wppm

Hvy, , Arab.


DOLBEAR ET AL.

•

Upgrading Studies with Heavy OUs

OILS

H

RESINS

ASPHALTENES

100 π


80

A

60 Η 40

20

HVY ARAB

HONDO

Figure 2.

120

I

MAYA

GACH SARAN

BLEND

Oils, Resins, and Asphaltenes.

I OILS

υ ] RESINS

ASPHALTENES

100 Η 80 60 40 20 Η

HVYARAB

HONDO

MAYA

GACH SARAN

BLEND

Figure 3. Sulfur Distributions in Oils, Resins, and Asphaltenes.


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oils have significantly different bulk properties, and the distribu tions of the contaminants among the o i l s , resins, and asphaltenes are also different. Reactivities. With this wide range of properties, i t is not surprising that reactivities vary widely. Sulfur relative reactivities vary by more than a factor of two (Table IV, Figure 5). Gach Saran, the Blend, and Hondo are about twice as reactive as Heavy Arabian, and Maya is slightly more reactive. Both Maya and Hondo have more sulfur than Heavy Arabian. One may suppose that metals reactivities are similar to sulfur reactivities. But that is not true (Table IV and Figure 6). Hondo and Gach Saran are s t i l l much more reactive than Heavy Arabian, the Table IV. HDS HDM

Hvy. Arab. ~ΤϋΙΓ 100

Reactivities of Heavy Oils Hondo ΤΤΓ 157

Maya UE 45

Gach Saran" — m 168

Blend ~T38 85

Blend is about the same, and Maya is considerably less. Not only do sulfur and metals reactivities not correlate (Figure 7), but the metals and sulfur content cannot predict reactivities. When we tested for reactivity effects in the distribution of metals, we found that demetallization reactivities increase with increasing fraction of metals in the resins fractions (Figure 8). However, we found no correlations with the distribution of sulfur among the o i l s , resins, and asphaltenes fractions. We also measured reductions of asphaltenes and Conradson Carbon. The results are presented as percent conversion. In general (Table V, Figure 9) the feeds behave similarly, probably because these conversions are very sensitive to temperature, which was the same for all five oils. Table V.

Conversion of Asphaltenes and Carbon Residue

Asphaltene Conv. t CCR Conv. %

Hvy. Arab. 57 41

Hondo 63 52

Maya 52 32

Gach Saran 54 47

!Blend 55 32

While conversions of the asphaltenes and associated carbon residue are similar, conversions of 1050°F-plus material to distillates (Table VI) shows a much wider variation. When the Table VI. 1050°F-plus, wt% Feed Product Conv. % 650-1050°F, wtX Feed Product

VGO and Vacuum Resid in Feed and Product Oils Hvy. Arab.

Hondo

Maya

Gach Saran

Blend

51 46 10

46 34 27

59 58 0

50 34 31

50 47 5

44 48

36 40

37 33

44 54

47 45


14.

DOLBEAR ET AL.

HI

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100

w

80

227

Upgrading Studies with Heavy Oils

RESINS

•

ASPHALTENES

< 60


UJ

>

< UJ

cc

188 179

180

160

(/) Q Ζ UJ

> < UJ

140

120 100

116 100 HVY. ARAB

Figure 5.

HONDO

MAYA

GACH SARAN

BLEND

Relative Sulfur Reactivities.


228


200 180

>

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168 157

160

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140-|


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Metal Complexes in Fossil Fuels - American Chemical Society

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