Survey of Petroleum Refining Technology for High-quality (Ultra-low Sulfur Content) Diesel Fuel

01 Survey 4-3 Survey of Petroleum Refining Technology for High-quality (Ultra-low Sulfur Content) Diesel Fuel Yutaka Mukai, Toshiaki Hisamitsu, Japan...
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01 Survey 4-3

Survey of Petroleum Refining Technology for High-quality (Ultra-low Sulfur Content) Diesel Fuel Yutaka Mukai, Toshiaki Hisamitsu, Japan Energy Research Center Co., Ltd. Yoshihiro Mizutani, Technology Department, Petroleum Energy Center (PEC)

1.

Background and Goals

The United States and Japan as well have been strengthening their quality requirements for automotive fuels such as diesel fuel in order to prevent air pollution and comply with environmental regulations. In 1994 a uniform European standard for the sulfur content of diesel fuel was established, with the maximum allowable level being 0.2 wt.%. The maximum level was reduced to 0.05 wt.% in 1996, and plans call for the introduction of new regulations that will further lower the allowable sulfur content to 350 wt.ppm in 2000 and 50 wt.ppm in 2005. There is also a trend toward the implementation of quality regulations covering properties other than sulfur, such as polycyclic aromatic compounds, cetane rating, and distillation characteristics. Regulations that took effect in 1993 in the United States mandated a maximum sulfur content of 500 wt.ppm, a cetane index of 40 or greater, and a maximum aromatic content of 35 vol.%. In addition to the sulfur limitation less than 500 wt.ppm, in California, where the air pollution problem is acute, diesel fuel regulations specified a maximum aromatic content of 10 vol.%. The EPA has suggested lowering the maximum acceptable sulfur content for diesel fuel to 15 ppm in June 2006, and this proposal is being hotly debated. In Japan the maximum acceptable sulfur content for diesel fuel has been set at 500 ppm since 1997. When the JIS (Japanese Industrial Standard) is updated in 2004 this is expected to be lowered to 50 ppm, to become effective in January 2005. In the years ahead measures to deal with air pollution from sources such as automobile exhaust are likely to include calls for higher quality diesel fuel with lower sulfur and aromatic content. It is necessary to assume that in future acceptable sulfur content levels will fall significantly below 50 ppm. With this in mind, this survey examines the present state of deep desulfurization technology in Japan and oversea as well as the current status of high-quality diesel fuel production technology. An attempt is made to sketch the future of high-quality diesel fuel production technology and to identify key issues related to the development of such technology.

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2.

Survey Content and Findings

2.1

Trend in Sulfur Content Regulations on Diesel Fuel

Regulations in Japan, the United States, and Europe mandating the maximum sulfur content of diesel fuel are listed below. The “next stage” maximum value listed for Europe was decided in 1998, and that for Japan was proposed in 2000. Japan (government proposal) U.S.A. (government proposal) Europe

2.2

Present 500 ppm 500 ppm 350 ppm

Next stage 50 ppm 15 ppm 50 ppm

Takes effect 2005 2006 2005

Present State of Japanese Deep Desulfurization Technology and Ultra-deep Desulfurization

The deep desulfurization process generally used in Japan for diesel fuel is a single-stage process in which hydrodesulfurization reaction takes place in a fixed bed reactor. There is also a two-stage process in use, in which desulfurization reaction takes place at relatively higher temperature in the first stage reactor, and color is improved at a lower temperature in the second stage reactor. The typical feedstock consists of straight-run diesel fuel produced from Middle Eastern crude oil and cracked diesel fuel (mainly LCO) in the ratio of approximately 80% to 20%. The sulfur content of the feedstock is between 1.0 wt.% and 1.5 wt.%, and its T90 (90% distillation temperature) is between 350 and 360°C. The characteristics of the various diesel fuel feedstocks used are summarized in Table 2.2.1.

Table 2.2.1 Characteristics of Diesel Fuel Feedstocks Sample Feedstock Sulfur content Nitrogen content Aromatic content 90% distillation point Cetane index ASTM color

(wt%) (wtppm) (vol%) (°C)

Straight-Run Diesel Fuel 1.22 130 36 348.5 58.0 L0.5

RFCC Diesel Fuel 0.09 290 55 328.0 37.0 L1.5

Thermal Cracked Diesel Fuel 1.38 310 44 365.5 53.0 L5.5

Differences in the type of crude oil used and the distillation characteristics can affect properties such as desulfurization reactivity. The diesel fuel feedstocks used in Japan are mainly straight-run diesel fuel mixed with LCO, derived from Middle Eastern crude oil. In comparison with low-sulfur feedstock produced from North Sea crude oil used in Europe and the United States, the feedstock produced from Middle Eastern crude contains greater amounts of sulfur, total aromatics, and polycyclic aromatics, even for distillates within the same distillation temperature range. Therefore, Middle Eastern feedstock is relatively difficult to process, requiring reactor capacity twice or more that for North Sea feedstock.

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Table 2.2.2 Crude Oil Types and Reactor Capacity Product Sulfur Content (wt.ppm) 50

30

30

Feedstock Crude Oil Type Fraction

North Sea

Middle Eastern

Straight-run diesel fuel

Straight-run diesel fuel/20% LCO

0.2

1.5

Sulfur content (wt%) Relative reactor capacity Hydrogen partial pressure:

1.0

1.25

3.4

30 kg/cm2

Differences in the feedstock’s distillation temperature (T90) correspond with large differences in the reactivity. Figure 2.2.1 provides the relationship between reaction temperature and product sulfur content for feedstocks with different cut temperatures: SR-LGO (T90: 359°C, sulfur content: 1.17 wt%), 335°C cut fraction (T90: 316°C, sulfur content: 1.01 wt%), and 305°C cut fraction (T90: 287°C, sulfur content: 0.81 wt%). Under the same conditions (LHSV: 2, hydrogen partial pressure: 5 MPa) at which the sulfur content of the SR-LGO is reduced to approximately 350 ppm, the sulfur content of the 335°C cut fraction is reduced to less than 100 ppm.

Figure 2.2.1

Effect of Feedstock Cut Temperature on Product Sulfur Content

In order to reduce product sulfur content from 500 ppm to 50 ppm, the reactor capacity has to be increased by 2.9 times when processing heavy straight-run diesel fuel, however, the increase in reactor capacity becomes as little as 1.3 times when 95% distillation points reduced from 360°C to 340°C by substituting feedstock from heavy diesel fuel for summer season to light diesel fuel for winter season (Table 2.2.3).

Table 2.2.3 T95 of Feedstock and Reactor Capacity Feedstock 95% distillation point Sulfur content Product sulfur content Relative reactor capacity Hydrogen partial pressure:

Light Straight-run Diesel Fuel 340 1.0

Heavy Straight-run Diesel Fuel (°C) (wt%) (wt.ppm)

360 1.5 500 1.0

50 2.9

50 kg/cm2

3

1.3

2.3

Technical Approaches to Deep and Ultra-deep Desulfurization

It is anticipated in Japan that ultra-deep desulfurization for reducing the sulfur content of diesel fuel to 50 ppm or less can be accomplished through the extension of current hydrodesulfurization technology. Some refineries succeeded in satisfying the regulations by modifying their existing equipment, which limited sulfur content of 500 ppm or less and took effect in 1997, are planning next steps such as the construction of additional reactors. And refineries that have already installed new deep desulfurization equipment will be able to cope with by introducing newly developed catalysts without major modifications of facility. Technical approaches to deep and ultra-deep desulfurization that are likely to emerge in the years ahead include; (1) (2) (3) (4)

introduction of severer reaction conditions improvement of dispersion of the reaction fluid improvements in catalyst performance development of new processes

2.3.1 Severer Reaction Conditions In addition to increasing the reaction temperature, there are ways to reaction conditions more rigorous as follows; (1) reducing the feedstock/catalyst ratio (2) increasing the hydrogen partial (1)

Reducing the Feedstock/Catalyst Ratio The feedstock/catalyst ratio is reduced by decreasing the feed rate, increasing the amount of catalyst loaded in reactors, and building additional reactors. With regard to the case of reducing the product sulfur by building of additional reactors, in order to reduce the sulfur contents to 50 ppm, 30 ppm, and 10 ppm from 500 ppm, reactor capacity has to be increased by 1.9 times, 2.3 times, and 3.4 times, respectively, for processing a feedstock with a sulfur content of 1.5 wt.%, a mixture of straight-run diesel fuel and LCO derived from Middle Eastern crude oil (Table 2.3.1).

Table 2.3.1 Product Sulfur Content and Relative Reactor Capacity Product sulfur content (ppm) Catalyst Relative reactor capacity Feedstock:

Feedstock sulfur content:

500

50

30

10

2.3

3.4

Co-Mo 1.0

1.9

1.5wt%, reaction pressure:

4

32 kg/cm2

(2)

Increasing the Hydrogen Partial Pressure The hydrogen partial pressure is increased by increasing the reaction pressure, the hydrogen/oil ratio, and the purity of the hydrogen. In the United States the NPC has publicly released the responses of five companies—UOP, IFP, Akzo, Criterion, and Topsoe—to questions regarding the modifications of existing desulfurization unit, producing diesel fuel with a sulfur content of 500 ppm, for reducing the product sulfur content. The modification was estimated based on the use of feedstock with a sulfur content of 0.9 wt.%, a specific gravity of 0.861, and a T90 value of 321°C. These properties suggest that this feedstock can be desulfurized at milder reaction condition than that for the feedstocks used in Japan. The modifications required to reduce the sulfur content to 30 ppm are summarized in Table 2.3.2. All of them regard an amine scrubber as indispensable facility. UOP and IFP propose to minimize the cut down of LHSV by increasing the purity of circulating hydrogen. On the other hand, the other companies propose to decrease LHSV to the greater extent without increasing the hydrogen purity.

Table 2.3.2 Modification for Reducing the Sulfur Content to 30 ppm from 500 ppm LHSV Amine scrubber installed Purity of circulating hydrogen (mol%) Ratio of circulating hydrogen (scf/bbl) Catalyst packing method Hydrogen partial pressure: 46 kg/cm2

Current 2 No 75 1,000 Sock

UOP 1.5 Yes 90 1,900 Dense

IFP 1.45 Yes 91.3 3,649 Sock

Akzo 1.08 Yes 75 1,000 Sock

Criterion 0.5 Yes 75 1,600 Sock

Topsoe 1 Yes 75 1,160 Sock

The modifications for reducing the sulfur contents further to 10ppm are summarized in Table 2.3.3. They require the significant sacrifice of LHSV. In addition, IFP says that a sulfur content level of 10 ppm can not be achieved at existing unit pressure and that new unit must be constructed. Criterion also says that although the data is tentatively shown, new unit is strongly recommended.

Table 2.3.3 Modification for Reducing the Sulfur Content to 10 ppm from 30 ppm

LHSV Ratio of circulating hydrogen Hydrogen partial 2 pressure (kg/cm )

UOP 1.5 ↓ 0.9 1,900 ↓ 2,000

IFP 1.45 ↓ 1 3,694 ↓ ?

46

58

Akzo 1.08 ↓ 0.45 1,000 46

5

Criterion 0.5 ↓ 0.4 1,600 ↓ 1,850

Topsoe 1 ↓ 0.7

46

46

1,160

2.3.2 Uniform Dispersion of Reaction Fluid To ensure efficient utilization of all the catalyst packed into the reactor it is essential to attain a uniform distribution of the feedstock and hydrogen throughout the entire catalyst layer. The maldistribution may also cause problems, such as occurrence of abnormally high temperature area (hot spots) due to uneven reaction temperature distribution or a rise in pressure differential across the reactor, that force sometimes discontinuance of operation. If the reaction fluid once begins to become deflected within the catalyst layer the normal state is hardly restored. Effective measures to avoid the maldistribution of reaction fluid include improvements of the reactor interior and the catalyst loading method. (1)

Distribution Tray When designing a high-performance liquid distribution tray, the following requirements should be borne in mind. • The distributor nozzles should be spaced close together. • Pressure loss should be minimized. • The tray should be configured so as to avoid deflection due to tilting. • Operation at a turn down ratio with a high liquid flow volume should be supported. • Operation with a wide range of gas-liquid volume ratios should be supported. • Good gas-liquid mixture performance is essential. • The design should incorporate measures to prevent clogging due to scale, etc. Topsoe has developed a dense pattern flexible tray designed to ensure uniform dispersion. In tests using Topsoe’s TK 554 catalyst the apparent relative desulfurization activity achieved with it was more than 2.5 times that with a simple chimney tray, and the average reaction temperature was reduced by 25°C (Table 2.3.4).

Table 2.3.4 Distribution Tray Effectiveness Average reaction temperature (°C) Feedstock sulfur content (wt%) Product sulfur content (wt%) Relative desulfurization activity

(2)

Simple Chimney Tray 346 0.7 0.05 1.0

Dense Pattern Flexible Tray 321 0.9 0.035 2.5

Catalyst Loading Systems If the catalyst is loaded in such a way that the top surface of layer is at an incline on the way of the loading, the catalyst particles become slanted along the inclination. It is known that the oil tends to flow along the inclination and may be deflected by the slanted portions, even if the top surface of the catalyst is packed flat.

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In the past, to keep the surface flat during the catalyst loading, the work was halted periodically and the surface was leveled by manpower. However, there were limits to the effectiveness of such techniques, and the frequent interruption of the loading work lowered the efficiency. In order to get around these problems a catalyst loading system has recently been developed that incorporated monitors which allow real-time measurement of the flatness of the catalyst surface without interrupting the work and a catalyst loading machine which is able to constantly control the incline of the surface. 2.3.3 Improvements in Catalyst Performance (1)

Improvement of Base Metal Catalysts The performance of molybdenum catalysts has been successively improved up to date. In addition to the desulfurization performance, well-balanced activities related to nitrogen removal, aromatics hydrogenation, cracking, isomerization are demanded to the hydrorefining catalysts for diesel fuel, in order to meet additional requirements for the performance of diesel fuel, such as stability, combustibility, lubricity, etc. other than sulfur content. R&D on the improvement of catalyst performance currently focuses mostly on the properties of carrier such as the porous structure and acidity, as well as the impregnation of metals regarding to the amount of metals and the impregnation methods, including the promoter.

(2)

Improvement of Precious Metal Catalysts Precious metal catalysts, particularly catalysts incorporating platinum or platinum and palladium, are used in the latter stages of deep-desulfurization process. They have excellent performance in hydrogenation of monocyclic aromatic hydrocarbons and are likely to become more and more important in the years ahead. Work on improvement focuses on increasing hydrogenation activity and resistance to sulfur and nitrogen poisoning while balancing these characteristics. In addition, work is being done to assign appropriate cracking activity to match specific applications. Techniques include the use of inorganic (composite) oxides, such as amorphous alumina, silica alumina, or crystalline silica alumina, as carriers and optimization of the amount of the precious metals and highly-dispersed metal impregnation methods.

(3)

Development of Catalysts for Deep Desulfurization and Aromatics Hydrogenation The Co-Mo, Ni-Mo, and precious metal catalysts on the market for use in deep desulfurization and aromatics hydrogenation are shown in Table 2.3.5. The performance of the up-to-date molybdenum catalysts is 1.5 times or more that of the catalysts available when deep desulfurization was first introduced. This means that some refineries will be able to produce diesel fuel with a sulfur content of 50 ppm or less only by substituting catalysts.

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Table 2.3.5 Catalysts for Deep Desulfurization and Aromatics Hydrogenation Company Akzo Criterion Topsoe

Co-Mo Type KF-757 DC-160, DC-185, DC-2000 TK-574

UOP

N-40, N-108, N-200

IFP CCIC OCC

HR-416 CDS-LX6 HOP-467

Ni-Mo Type KF-848 DN-200 TK-573, TK-525, TK-555 HC-H, HC-K, HC-P, HC-R, HC-T HR-448

Precious Metal Type KF-200 TK-907, TK-908 AS-250 Platinum type

Hop-414

2.3.4 Development of New Processes New processes under development other than hydrodesulfurization include oxidative desulfurization, bio-desulfurization, and adsorption separation. (1)

Oxidative Desulfurization An oxidative desulfurization process called CED (Conversion/Extraction Desulfurization) developed by Petro Star Inc. has been selected as one of projects for development of ultra-clean fuels by the DOE in the United States. In this process sulfur compounds in diesel fuel are selectively oxidized and then extracted using a solvent. Sulfur compounds with lower reactivity in hydrodesulfurization tend to show greater reactivity toward oxidation, and can be eliminated easily by oxidative desulfurization. If feedstock with high sulfur content is directly processed using oxidative desulfurization, large quantities of sulfinyl and sulfonyl compounds are produced as byproducts. However, employment of the process after hydrodesulfurization process holds promise as ways to achieve deep and ultra-deep desulfurization while limiting the hydrogen consumption and the production of the oxidized sulfur compounds. Still, finding ways to dispose of or utilize these oxidized sulfur compounds is an issue.

(2)

Bio-Desulfurization Enchira Biotechnology (formerly Energy Bio System) has licensed bio-desulfurization technology for diesel fuel to Petro Star’s Valdez Alaska refinery. In addition, Enchira Biotechnology has announced that it will be engaged in a three-year project, subsidized by the DOE, that will involve everything from the development of microorganisms to the design of a plant with a capacity of 5,000 barrels per day.

(3)

Adsorption Separation Another project to develop ultra-clean fuels, subsidized by the DOE, is an adsorption separation process. It is adaptation to diesel fuel from a process for gasoline developed by Phillips (S Zorb). A pilot plant have already succeeded in producing diesel fuel with a sulfur content of 15 ppm or less using this process. In this process sulfur compounds are removed from feedstocks in the form of adsorbate on a specially designed adsorbent, then, decomposed.

8

Like the above oxidative desulfurization process, this process can be combined with hydrodesulfurization process, and may show promise as deep and ultra-deep desulfurization processes for the future. 2.4

High-quality Diesel Fuel Production Technology

Although diesel fuel with a sulfur content or 10 ppm or less is already commercially available in some particular regions such as urban district in Sweden, the T95 of the fuels is 285°C or below, which is as low as the specification for kerosene in Japan. In California in the United States the maximum sulfur content is presently 500 ppm. In addition, regulations stipulate a maximum aromatic content of 10 vol.% and T90 between 290°C and 320°C. The leading hydrorefining technologies capable of meeting these strict standards on sulfur content and aromatic content are shown in Table 2.4.1.

Table 2.4.1 High-quality Diesel Fuel Production Technology Process SynSat Ultra Deep HDS MAK Fining MQD Unionfining Deep HDS, HDAr

(1)

Licensers Criterion, ABB Lummus, Shell Topsoe Mobil, Akzo, Kellogg, Fina UOP IFP

SynSat Process The SynSat process is one of hydrorefining processes included in Syn-technology. The SynSat process uses either one reactor or two reactors in series and divides the reaction zone into two stages. In the first stage both feed oil and hydrogen flow downward, while in the second stage direction of hydrogen flow can be set upwardly or downwardly depending on conditions. The Syn-technology consist of the following four hydrorefining processes. 1)

SynSat HDS:

Deep hydrodesulfurization

2)

SynSat HDS/HDA: Deep hydrodesulfurization/Hydrogenation of aromatics

3)

SynShift:

Improvement of cetane number/Shift of boiling point range

4)

SynFlow:

Improvement of cold-flow properties

The SynSat process is being used commercially in Sweden, Germany, and the United States. Examples of commercial applications of the SynSat process are listed in Table 2.4.2. In the second stage reactor of the units in Scanraff and Preem refineries in Sweden a precious metal catalyst is used, and oil and hydrogen flow countercurrently. They produce two types of product containing different amount of sulfur, 100 ppm and less than few ppm. It is likely that the second stage reactors are used for production of the latter ultra-low sulfur fuel with low aromaticity from feedstocks with lower T95 and sulfur content.

9

Meanwhile at Lyondell-Citgo refinery in the United States an existing hydrodesulfurization unit was revamped to two-stage system. The unit employs base metal catalysts and the cocurrent system for feedstock and hydrogen in the second stage. This unit achieved reduction of product sulfur to a level of 5 ppm or less from a feedstock with relatively high sulfur content of 1.38 wt. However, this figure is based on a test operation over a short period of time, and no data concerning catalyst life has been reported.

Table 2.4.2 Commercial Applications of SynSat Process Company

Processing Volume BPSD

Scanraff

48,000

Preem

37,000

LyondellCitgo

50,000

(2)

Feedstock Characteristics Sulfur Aroma T95 Density Content Content 3 (wt%) (°C) g/cm (vol%) 0.54 323 0.841 22.1 0.34 286 0.827 21.5 0.14 338 0.866 — 0.1 277 0.827 21.8 1.38

367

0.902

Product Characteristics Sulfur Aroma Density Content Content 3 (ppm) g/cm (vol%) 100 0.835 11.3 1 0.812 4.4 100 0.858 — 2 0.815 3

58.2

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