impact of gasoline and diesel specifications on the refining industry
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Impact of Gasoline and Diesel Specifications on theRefining IndustryJAMAL ADLI ANABTAWI a , SYED AHMED ALI a & MOHAMMED ASHRAF ALI aa Petroleum and Gas Technology Division The Research Institute King Fahd University ofPetroleum and Minerals , Dhahran, Saudi ArabiaPublished online: 16 May 2007.
To cite this article: JAMAL ADLI ANABTAWI , SYED AHMED ALI & MOHAMMED ASHRAF ALI (1996) Impact of Gasoline and DieselSpecifications on the Refining Industry, Energy Sources, 18:2, 203-214
To link to this article: http://dx.doi.org/10.1080/00908319608908759
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Impact of Gasoline and Diesel Specifications on the Refining Industry
JAMAL ADLI ANABTAWI SYED AHMED ALI MOHAMMED ASHRAF ALI
Petroleum and Gas Technology Division The Research Institute King Fahd University of Petroleum and Minerals Dhahran, Saudi Arabia
Future gasoline specifications demand reduction in aromatics, benzene, sulfur, uolatility, and boiling point. In response, refiners must modify their processing conditions, catalysts, and mode of operation of catalytic reformers and isomerization units. Diesel fuel is abo facing tighter specifications for cetane number, aromatic content, sulfur, and color. To meet these specificntions, operational modifications include use of higher actiuity catalysts, higher hydrotreating seuerily, and two-stage processing. This paper reuiews fuel legislation adopted recently worldwide, and refining technology solutions practiced a n d / o r planned to meet the new specifi- cations.
Keywords aromatic reduction, cetane improvement, deep HDS, gasoline and diesel specification, hydrogen balance, oxygenate production, refining solution, Reid vapor pressure reduction
The concern about health and environmental effects from the processing and consumption of fuels is increasing. Issues such as air pollution, acid rain, atmo- spheric warming, and toxic waste release will push refiners to generate cleaner fuels through process modifications with minimal hazardous waste by-products (Pittas, 1990; Ali & Anabtawi, 1993)l. As in the past, US. legislators have taken the lead in establishing future regulations. Canadian, Western European, and Japanese lawmakers have followed with modified versions to reflect local requirements and practicality. The need to improve gasoline and diesel quality will be the largest influence on the refining industry and technology in the decade ahead.
Efforts to reduce the effects of using fuels on the environment started in the 1970s, and the U.S. refining industry responded by adopting new and improved refining processes to produce lead-free gasoline. This enabled automakers to install catalytic converters, which resulted in 96% reduction in hydrocarbon and carbon monoxide (CO) emissions and 76% reduction in NO, emissions from automotive exhaust. The Clear Air Act Amendments (CAAA) of 1990 represent the greatest challenge to the U.S. refining industry. The new requirements on
Received 16 January 1995; accepted 26 February 1995. The authors wish to acknowledge the support of the Research Institute of King Fahd
University of Petroleum and Minerals in publishing this article. Address correspondence to Jamal Adli Anabtawi, Petroleum and Gas Technology
Division, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia. E-mail: [email protected].
Energy Sources, Volume 18:203-214, 1996 Copyright O 1996 Taylor & Francis
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gasoline would reduce hydrocarbon emissions by 15% starting in 1995, and further reduction is mandated by the year 2000. The U.S. legislation for reformulated gasoline and diesel fuel can be taken as a sign of what is in store for the rest of the world. The refining industry will be under enormous pressure because in order to produce these mandated clean fuels, they have to modify, design, and construct new units (Ragsdale, 1994). The investment required in the U.S. alone has been estimated at $30 to 65 billion. The costs can be reduced through better catalysts, operational changes, and revamping (Pittas, 1990; Ali & Anabtawi, 1993).
This article reviews present and proposed legislation regarding gasoline and diesel fuel specifications adopted in the United States, Europe, Japan, and Saudi Arabia. The impact of these regulations on the refining industry and recent process developments to comply with the legislation are highlighted.
Gasoline Specifications
Gasoline contributes about 70% of carbon monoxide and 40% of total hydrocar- bon emissions. The toxic and carcinogenic chemicals, originating from combustion engines, include benzene (72.5%), aldehydes (16.5%), butadienes (5%), and poly- cyclics (3%). Measures to control these chemicals include fuel economy, catalytic converters, carbon canisters, reformulated gasoline, and alternative fuels.
Table 1 shows the gasoline specifications expected in the United States and Europe (Pittas, 1990; UOP, 1993; Hydrocarbon Publishing Company, 1993). To a large extent, lead has been phased out already. The mandated gasoline reformula- tion will include a reduction in aromatics and light olefins. Currently, US. gasoline on average contains 32% aromatics, 12% olefins, 1.53% benzene, and 2.0 wt% oxygen. The CAAA requires reformulated gasoline by January 1995 in areas with extreme pollution problems. The limit includes a minimum oxygen content of 2 wt%; a maximum benzene content of 1 vol%; a maximum aromatic content of 25 vol%; and a 15 vol% reduction in volatile organics compared with 1990 levels. Stricter measures are planned to be implemented in 1999 (Hadder, 1992). The
Table 1 New gasoline specifications in the United States and Europe
United States
1994 California Europe
Specification Lower Upper 1995 1996 1994 1996
RVP, psi 8.7 11.5 7.4 7.0 8.5 14.5 Benzene, vol% 1.53 1.64 1.0 1.0 2.0 5.0 Aromatics, vol% 32 26.4 32 29.2 25 Olefins, vol% 9.2 11.9 10.6 6.0 9.9 - Sulfur, ppm 338 339 338 40 150 500 Oxygen, wt% 0 0 > 2 2.0 0 - 90% distilled PC) 170 170 170 149 166 180
*85% distilled.
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Impact of Gasoline Specifications on Refining 205
proposed standards represent 10% reduction in aromatics with a more stringent reduction of benzene from about 5% to 1%. Although not as specific as for the United States, Europe's new regulations are under way. Canada has banned the sale of leaded gasoline for automotive use since 1990, and the new standards for cars and light and heavy trucks were introduced in 1987. Efforts to reduce car emissions are also emphasized in Western Europe, the Asian Pacific, and Japan (Ali & Anabtawi, 1991; UOP, 1993; HPC, 1993). In Saudi Arabia, premium gasoline contains about 0.4-0.6 g/L of lead at present. The presence of lead and the severe climatic conditions increases the evaporative and refueling emissions. Saudi Arabia, which is in the process of eliminating lead additives by 1996 for domestic use, has to abide by international specifications for export quality gasoline.
Options to Meet Gasoline Specifications
The important elements in the gasoline specifications are reduction in vapor pressure, aromatics, and benzene and mandatoly increase in oxygen content. The vapor pressure specification will necessitate reduction of butanes in gasoline. Major sources of aromatics and benzene are catalytic reformers and fluid catalytic cracking (FCC) units, contributing 70% and 20-27%, respectively. The reduction in aromatics will cause reduction in the severity and capacity of catalytic reformers. This will run reformers into a hydrogen-deficient situation and lower octane operation. The FCC products, which do not meet the new octane requirement, can be improved by increasing operation severity and by using the modified catalysts. The following are options to meet the new specifications in the period 1995-1998.
Reid Vapor Pressure
To meet the new Reid vapor pressure (RVP) regulations, refiners have to reduce the volume of high-octane, high-volatility, and low-cost butane that presently makes up 7% of the gasoline pool. Butane removal from the gasoline pool eliminates a high-octane component and forces the refiner to find another use for this material. The RVP can be reduced by using the following options (Pittas, 1990).
Butane Fractionation. Most gasoline stabilizers are operating above design and may not meet the typical design parameter of 1 mol% butane in the bottom fraction. Higher butane levels in the stabilized gasoline component make RVP control of the finished gasoline more difficult. Modem fractionation and heat exchange technology may be used to improve the debutanizer efficiency and minimize the RVP in the gasoline components. When butane control is difficult or costly, pentane removal is usually preferred.
Other Alternatives to Control R W . RVP of gasoline can also be reduced by operating reformers at lower pressure, which causes a reduction in C, content in the reformate. The remaining n-butane can be isomerized and dehydrogenated to isobutylene, or alkylated with Cj olefins to produce liquids with superior stability and antiknock quality. The need for cost-effective butane conversion technologies
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and the drawbacks of the existing processes have led to significant developments in the area of isomerization, dehydrogenation, and alkylation (Haldor Topsoe, 1994).
Benzene Reduction
As per legislation, the benzene content of reformulated gasoline should not exceed 1 vol% after 1995. The major sources of benzene in the gasoline pool are catalytic reformate and FCC gasoline. Most strategies focus on reducing benzene produc- tion from C,C naphtha streams. The best option is the reformate because this stream typically contributes 50-75% of the benzene in the gasoline pool. The following are proposed approaches to reduce net benzene production (Pittas, 1990).
Precursor Removal. The first approach in reduction of benzene consists of re- moval of methylcyclopentane, cyclohexane, and benzene from the overhead of the naphtha splitter and including these materials in the feed to the isomerization unit or, if octane is not critical, blending this C,-C, stream directly into the gasoline pool. Preventing the formation of benzene rather than converting it, after it is formed, makes good engineering sense. However, in certain instances, practical and economic constraints may limit the ability of refiners to implement this strategy (Pittas, 1990).
Reformer Operation. Since the reformate contains up to 70 vol% aromatics, of which 5 vol% is benzene, the reformer's operation has been investigated by refiners to deduce the optimum operating conditions. Aromatics tend to increase as the reformer's severity and octane number increase. Therefore, to reduce aromatics including benzene, the octane barrel in a refinery will definitely de- crease. The reformer severity can be adjusted to minimize further reduction in benzene production by hydrocracking. However, this would cause a decrease in octane enhancement of the light straight run naphtha and lower octane of naphthenes in the isomerization unit. Changes in reformer operation will affect the refinery wide hydrogen balance; therefore the refinery will have to improve on hydrogen production and utilization (Haldor Topsoe, 1994; Gilman, 1990; HPC, 1993).
Prefactionation of Reformer's Charge. Another option is to prefractionate the reformer feed to eliminate benzene precursors and isomerize the light cut, i.e., increase the boiling point of reformer charge, thus lowering the benzene produc- tion by about 1.5 vol%. The optimum cut point between light straight run (as isomerization feed) and heavy straight run (as reformer charge) will depend on the operating objectives and constraints including C: isomerization unit capability, octane response, and benzene concentration (Pittas, 1990).
Light Reformate Processing. When choosing between light reformate processing or precursor removal for benzene reduction, refiners must consider three variables: the level of benzene allowed in the regular pool, the benzene-producing tendency of the reformer, and the refinery flow scheme and crude source. If reducing benzene below 1% is necessary, then precursor removal may not be adequate. At higher pressures, the production of benzene may occur even after precursor removal because the rate of dealkylation of heavier aromatics to benzene increases
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Impact of Gasoline Specifications on Refining 207
with pressure. Finally, light reformate processing may be necessary if the refinery is highly dependent on the reformate in the gasoline pool or if the feed is cyclic in nature. Four options for benzene removal for light reformate can be considered (Pittas, 1990).
1. Benzene can be saturated to cyclohexane in a saturation unit. However, conversion of benzene entails loss of 18 octane numbers and reduces the octane of light reformate stream by 2-3 octane numbers. In addition, it consumes a significant amount of hydrogen.
2. The isomerization unit supplements benzene saturation with isomerization of the unconverted C, paraffins present in the light reformate, resulting in net gain of about 5 octane numbers. However, the process consumes more hydrogen and produces gasoline with a slightly higher RVP.
3. A viable option for benzene removal from an FCC refinery is alkylation of benzene in light reformate fraction with propylene. This process does not require additional hydrogen, and the economics depend on the availability and cost of propylene.
4. The extraction option is the best choice when benzene is used as petro- chemical feedstock. The economics of installing new extraction and frac- tionation facilities is attractive for refiners who can find a market for benzene.
Oxygenate Production
The oxygenated fuel program calls for a minimum oxygen content of 2.7% to achieve the CO standard. The U.S. gasoline pool currently contains an average oxygen concentration of 1.7%. Ethers and alcohols are high-octane components and can be added to the gasoline pool at the legislated 2 wt% oxygen level. In an effort of the refining industry to be independent of outside suppliers for this critical component, production of MTBE and ETBE from butenes and the produc- tion of TAME from C, olefins is under evaluation by the refining industry [Sarathy & Suffridge, 1994; Dunn & Patton, 19941. MTBE, which has an average octane blending value of approximately 108 and a blending RVP of 8 psi, would continue to be very attractive oxygenate. Projects based on butane dehydrogenation for use in MTBE production are under construction (HPC, 1993). Isopropyl alcohol can be made by C, olefins, adding 1.3 octane numbers at the 2.0 wt% oxygen level. It also has a blending octane number of 108, higher oxygen content (26.6 versus 18.2 wt% for MTBE), but higher RVP (14 versus 8 psi for MTBE at 2.7 wt% oxygen). The total amount of MTBE and TAME that can be produced from FCC isobutylenes and tertiary amylenes in an average refinery is approximately 0.6 wt% of the overall gasoline pool.
The FCC unit, which produces more than 50% of the gasoline pool, will be critical in producing reformulated gasoline. The production of oxygenates in a refinery can be increased by optimizing FCC unit operation to boost its light olefin content by selecting a suitable catalyst, changing operating severity, or revamping. The addition of oxygenates to the gasoline pool would affect the operation of a refinery and product yield. The addition of MTBE to meet the oxygenate require- ments may increase gasoline yield by 11-15%. If crude oil rate is reduced to compensate for this increase in gasoline production, other refined products such as middle distillate decrease. To rebalance the product slate, refiners may consider
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adjusting cut point, increasing reforming severity levels, and/or charging feeds to heavy oil cracking.
Hydrogen Balance
Hydrogen management will be a critical issue for refiners as a result of the CAAA. Reformulation of gasoline could require reduction in aromatics (32-25%) and heavy ends and increase in oxygenates (11-15 ~01%) to the gasoline pool. The reduction in aromatics accomplished by a decrease in reformer severity could result in an equivalent 20-30% drop in production of by-product hydrogen. Consequently, the increase in heavy ends from the reformer may have to be hydrotreated, which would require additional hydrogen. The reduction of gasoline RVP from 9.6 to below 8.5 psi would result in a large surplus of heavy ends from an FCC unit. Furthermore, the reduction in the T , boiling point from 350•‹F to 300•‹F would require 10 vol% of the heavy-cut-catalytic naphtha to be hydrotreated or hydrocracked, which would result in an increase in the hydrogen requirement.
A reduction in reformer pressure can significantly improve hydrogen balance. For example, lowering the reformer pressure from 300 to 50 psi increases hydrogen production by 8-11 MMscfb in a 100,000 bpsd refinery. A reduced pressure reformer operation will provide higher reformate yield and lower benzene content of the reformate. Other ways of overcoming the hydrogen deficit include new hydrogen plants and hydrogen recovery from refinery streams and from the dehydrogenation processes (Pittas, 1990). The overall effect of lower hydrogen production and increased consumption may mean a deficiency of 1-2 MMscfd of crude oil refining capacity.
Diesel Specifications
The market for diesel fuel is growing steadily, due to increasing number of diesel powered vehicles and tax incentives for using diesel. Automobile manufacturers are pressing for improved combustion properties, higher cetane number (CN), and reduced engine emission, while the environmental lobby is stressing the need to reduce sulfur and aromatics in automotive diesel (Haldor Topsoe, 1994).
New diesel specifications are being introduced around the world, as shown in Table 2 (UOP, 1993; Brown & Lambourn, 1994; Soggard-Anderson et al., 1992). Sulfur limit of 0.05 wt% in diesel will be the lower limit in the mid-1990s, with 0.005 wt% imposed in some urban areas. Environment Canada is developing a regulation to lower diesel sulfur to 0.05 wt% by 1995, from the current standard of 0.5 wt%. The European Community (EC) has adopted rules to limit sulfur from the current 0.2 wt% to 0.05 wt% by 1996. The cost to European refiners in meeting the reduced sulfur levels in diesel is approximately $5-70 billion. In California, the regulations are 0.05 wt% sulfur, 10% aromatics for refineries larger than 50 thousand barrels per day (Mbpd) and 20% aromatics for refineries smaller than 50 Mbpd. The aromatic content of 22% in diesel fuel is expected to be widely accepted as the standard for many EC countries in the near future. Sweden has introduced quite severe limitations and coupled them with tax incentives to make these specifications economically attractive for producers and consumers. Current legislation in the United States calls for diesel fuel having CN above 40 after October 1993, while in California, it calls for CN above 48. In the EC and Sweden
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Impact of Gasoline Specifiations on Refining 209
Table 2 New diesel fuel specifications in the United States, Europe, and Japan
United States Sweden 1994
California EEC Japan Specification 1994 1994 1996 Class 1 Class 2 1997
Density, kg/m3 < 876 830 840 810 810 810 Sulfur, wt% 0.05 0.05 0.05 0.01 0.05 0.05 Aromatics, vol% 35 10-20 - 5 20 - Cetane index 40 48 46 50 47 - % distilled/"C 90/338 100/349 85/350 100/300 100/310 -
the CN ranges from 46-50. In Saudi Arabia the sulfur specification for domestic diesel is 1 wt%. The situation is uncertain with respect to aromatic content. However, the general trend will be toward reduction in aromatics. Export-quality diesel from Saudi Arabian refineries has to meet international specifications.
Options to Meet Diesel Specifications
In order to meet the new specifications, refiners have several options. The impact on each refinery's flow scheme varies considerably, depending upon the crude properties, existing processing facilities, and future product slate. The combination of factors involving increased relative demand of diesel fuel, very low sulfur and aromatics, and higher CN challenges the refiner to upgrade middle distillate material in the refinery, especially straight run gas oils and light cycle oils. The various options available and their limitations are discussed below.
Deep Desulfurirntrrntron
To achieve the significant reduction of sulfur in diesel fuel from the 0.2 to 0.05 wt% level, considerable modifications in the exis!ing units and significant increase in hydrotreating capacity are required (McCulloh et a]., 1987). Available options include revamping of existing units by increased severity and improved catalysts and installation of new units.
Revamping the Eristing Units. The revamping route offers refiners an alternative to meet the diesel specifications at a lower cost than building a new unit. Increasing severity of hydrotreater operation (higher temperature, higher pressure, and lower space velocity) using conventional hydrodesulfurization (HDS) catalysts can meet the lower sulfur specification (0.05 wt%) but will result in significantly shorter cycle lengths (about half) and is hence uneconomical. Some existing units cannot be operated at higher temperatures due to design considerations (Haldor Topsoe, 1994). Furthermore, increasing temperatures may lower aromatic satura- tion due to thermodynamic limitations. Hence this option is viable for low-sulfur, low-aromatic feedstocks only.
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To achieve the same cycle length and production rate, it will be necessary to increase catalyst volume by a factor of 3.3 for a change of product sulfur from 0.3 to 0.05 wt% (Haldor Topsoe, 1994; Brown & Lambourn, 1994). In recent years, new high-activity hydrotreating catalysts are introduced that are especially tailored to meet the deep HDS requirement of middle distillate applications and are 20-30% more active than their predecessors. An example of such a CoMo/Al,O, catalyst is Akzo KF-752 (Rautianen, 1991). The use of an NiMo/Al,O, catalyst, such as Haldor Topsoe TK-525, is favored to achieve the twin objectives of dearomatization and deep HDS. The use of dense loading instead of sock loading will give an additional 15-20% desulfurization capacity. However, this will be at the expense of the reactor pressure drop. %on has developed a diesel oil deep-HDS technology and demonstrated it in pilot plant and commercial units (Cavanaugh et al., 1994).
New Grassroot Units. Growing demand of low-sulfur, low-aromatic diesel from high-sulfur crudes will force refiners to build new grassroots units. In fact, future refineries will have multiple HDS units for mild, high-capacity application, and separate high-pressure units for deep HDS (Sweeney, 1993). In addition to invest- ment and operating costs, the design of these units must take into account an eventual requirement for future aromatic limits. To meet the current 0.05% sulfur limit, most units must operate at hydrogen partial pressures of 40-60 bafs. Provision for higher pressure operation will be helpful for future, more severe requirements. Other considerations include allowance for additional reactor vol- ume, compressor capacity, and recycle hydrogen purification (Cavanaugh et al., 1994).
Aromatics Reduction
Processes for aromatics reduction in middle distillates have received considerable attention in recent years (Stanislaus & Cooper, 1994). At present, conventional hydrotreating technology is adapted for aromatic saturation. However, it should be noted that aromatic saturation is more difficult than HDS and hydrodenitrogena- tion under conditions normally used for hydrotreating. In addition, there are thermodynamic equilibrium limitations on the aromatic hydrogenation within the normal operating range of hydrotreating (Anabtawi & Ali, 1991). The aromatics contents of diesel fuels vary widely (20-70 vol%), depending on origin (straight run or cracked) and processing. Among the types of aromatics present in straight run gas oil, diaromatics constitute a major portion. However, in hydrotreated oils, monoaromatics are present in larger quantities (Asim et al., 1990).
The new specification of lower aromatics in diesel would require moderate to severe treatment, depending on the overall refinery design. The existing middle distillate hydrotreaters, designed to reduce sulfur level, can reduce aromatics content only marginally. These units can be revamped by increasing operational severity, utilization of new catalysts, or two-stage processing.
lncreasing Seuerity of Operation. Conventional hydrotreating catalysts, designed for HDS and/or hydrodenitrogenation, can be used for aromatic saturation by increasing the severity of operation. These catalysts, namely, Co-Mo, Ni-Mo, and Ni-W support on gamma-alumina, would require higher temperatures (about
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Impact of Gasoline Specifications on Refining 211
37SoC), leading to an unfavorable thermodynamic equilibrium. Very high hydrogen pressures (about 12 MPa) are required at such temperatures to overcome the thermodynamic limitation and achieve acceptable aromatic reduction (Anabtawi & Ali, 1991). Such severe conditions will result in much shorter cycle length and significantly higher operational costs (McCulloh et al., 1987). However, if the aromatic content is not very high, conventional catalysts can still be applied. In such cases, dense loading instead of sock loading of catalysts will be advantageous.
New Catalysts. Since the reaction is primarily controlled by kinetics at lower temperatures, it is possible to achieve high conversion by using highly active catalysts that could promote aromatic hydrogenation reactions at relatively lower temperatures and pressures. Recently, high-activity hydrogenation catalysts were developed utilizing support group VIII metals such as Pt, Pd, Ru, and Ni (Wilson et al., 1987). With these high-activity catalysts, aromatic saturation reactions can be carried out under thermodynamically favorable conditions. However, the stability of these catalysts in processing sulfur-containing feedstocks is limited, as they are rapidly deactivated by adsorption of sulfur-containing species. The sulfur and nitrogen contents in the feedstocks should be reduced to certain minimum levels by hydrotreating prior to processing over these noble metal catalysts. Moreover, the stability of these noble metal catalysts can be improved by the use of sulfur-tolerant supports, such as Y-zeolite. Zeolite-supported noble metal catalysts that tolerate several hundred parts per million (ppm) sulfur have been demon- strated to achieve required performance for aromatic saturation (Suchanek, 1990).
Two-S&ge Processing
From the above discussion, it is clear that achievement of the twin objectives of low aromatics and ultralow sulfur in diesel fuel in a single reactor is extremely difficult, if not impossible. As a result, two-stage processing technologies have been devel- oped, and their performance has been demonstrated (Suchanek, 1990; Peries et al., 1991). In these processes, the first stage reduces sulfur and nitrogen, and the second stage saturates the aromatics. The catalyst used in the first stage is generally NiMo/Al,O,, while the second stage applies a noble metal (Pt or Pd) supported on silica or zeolite. The advantage of two-stage processing is the operation at lower temperature, lower pressure, and higher space velocity.
Haldor Topsoe studied several catalyst systems to find suitable candidates for diesel upgrading (Haldor Topsoe, 1994; Cooper et al., 1993). The four catalyst systems studied were
1. conventional Ni-Mo in a single-stage process, 2. Ni-Mo/Ni-W in a two-stage process, 3. Ni-Mo/Pt/Al,O, in a two-stage process, and 4. Ni-Mo/S-tolerant noble-metal catalyst in a two-stage process.
It is reported that the nickel-molybdenum-tungsten (system 2) system is pre- ferred at moderate levels of saturation, while for deep aromatics saturation, the sulfur-tolerant noble-metal catalyst (system 4) is preferred. The removal of hydro- gen sulfide and ammonia between stages is necessary; the catalyst choice for the second stage is dependent on the target product sulfur level (Cooper et al., 1993).
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Refiners with the traditional FCC/visbreaker need to consider a range of options. Possible strategies are arranged in order of increasing investment (UOP, 1993):
1. Segregate L C 0 exclusively for heating oil. 2. Build a wet treatment until (mercaptan oxidation) for jet kerosene to
release HDS capacity. 3. Revamp existing HDS units for higher hydraulic capacity, to meet the
0.05 wt% sulfur specification for diesel fuel. 4. Build a grassroots refining unit to reduce aromatics.
Celnne Improvement
Cetane number is a measure of the performance obtained in a diesel engine.' Automobile manufacturers are currently recommending a higher CN (40-50) in order to achieve higher engine efficienj. CN generally-incre&es with decreasing aromatic content, and the presence of iso-hydrocarbons, olefins, and polynuclear aromatics contributes to the lowering of CN. The variation of CN'wiih other variables, such as distillation cut points and conversion process severities, is not well defined (Gross & Murphy, 1979). In general, cracked stocks from FCC units, visbreakers, and cokers exhibit lower CN.
The CN of cracked middle distillates can be improved significantly by hydro- treating at conditions that favor aromatic saturation (Martin, 1983). Hydrotreating can also saturate olefins and convert polynuclear aromatics to mono-aromatics. However, if the CN is low primarily due to iso-hydrocarbons, hydrotreating will not improve diesel quality. In such a case, blending diesel with higher cetane feedstock or adding cetane enhancers will be required. These additives are low-cost altema- tives for improving the quality of the diesel. They act as ignition improvers, cold flow improvers, or antioxidants without changing the aromatic content or the final boiling point of the fuel (Martin, 1983).
Color Degradation and Improvement
When diesel fuel is desulfurized, the yellow color of straight run diesel disappears at first up to a conversion level of 80%. It then becomes colored again, with yellow-green fluorescence, when the conversion is increased beyond 95%. The color of diesel fuel is affected by the reaction conditions of hydrotreating and is quite an important consideration in the design of a deep HDS unit. In Chiyoda, Japan, Takatsuka (1993) investigated the effects of hydrotreating on sulfur removal and the mechanism of color degradation. It was concluded that low temperatures and higher pressure suppress color degradation of diesel fuel during deep desulfur- ization.
Hydrogen Balnnce
The reduction of sulfur levels from about 0.35 to 0.05 wt% will require less additional hydrogen of about 30-50 N m3 ~ , / m ~ for straight run feed and 50-100 N m3 H2/m3 for cracked stocks. Reducing the aromatics to 10 vol% would require an additional 1000 standard cubic feet per barrel (scf/b) and 600 s d / b for reduction to 20 wt%. The need for lighter products and higher CN will also
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Impact of Gasoline Specifications on Refining 213
increase hydrogen demand. In order to meet the 40 cetane index specification, the additional hydrogen needed would be in excess of 250 scf/b. The availability of hydrogen on he refinery should be evaluated, and improvement in hydrogen utilization and production exercised as well (Simenson et al., 1993).
Conclusions
In order to meet the proposed gasoline and diesel specifications and demand, the industry must implement new technologies, making investments and offering long- term flexibility without jeopardizing the economic incentives. The reformulation of gasoline must be achieved without lead additives while meeting the low aromatic, benzene, vapor pressure, and olefin limits. Isomerization and alkylation can be used to produce nonaromatic and high-octane components, thus reducing aromat- ics and benzene in reformulated gasoline. Options available to reduce benzene in the reformate include reducing reformer severity, prefractionation of the feed to eliminate benzene precursors, and optimizing the FCC unit operation. To satisfy the proposed oxygen content, ethers such as MTBE or alcohols could also be used.
Diesel fuel sulfur, aromatic content, and CN are important, while limitations on distillation end point, color, and stability are also in sight. Forecasters in the United States, Europe, and Japan all envisage a progressive reduction in sulfur from 0.3-0.5% typical levels to 0.05%. To meet the ultralow sulfur specification, operational modification may involve one or a combination of the following actions: use of higher activity catalysts; higher catalyst volume; hydrogen purifica- tion to improve HDS and cracking of less desirable diesel blending stocks; lower hydrotreater throughput; higher temperature; or lower blending rates for higher sulfur components. There are several methods for reducing aromatics content in diesel fuels. These include reduction in diesel fuel end point, using hydrocracked components, and deep desulfurization followed by de-aromatization using a noble- metal catalyst.
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