REFORMULATED FUELS-CONCLUSION OPTIONS TO MEET 1990S FUEL COMPOSITION RULES LIMITED

George H. Unzelman
HyOx Inc.
Fallbrook, Calif.

Reduction of aromatics in gasoline and diesel fuel, and reducing gasoline vapor pressure below current levels while maintaining fuel quality at current levels required by the consumer, will be tough challenges for U.S. refiners.

A study of the current U.S. gasoline pool composition implies that processing changes are feasible to reduce average summer Rvp to 9 psi, and to reduce gasoline aromatics content from 32 to 30 vol % (close to 28 vol % if MMT is approved for use in gasoline).

It is unlikely that U.S. gasoline pool aromatics can be reduced to 25 vol % during the 1990s without a compromise of marketed octane quality. Special gasolines with 20-25 vol % aromatics, manadated oxygenates, etc., are feasible for critical environmental areas. Such gasolines are possible at the expense of transferring less desirable components to other grades or other market areas.

Low sulfur diesel will require higher severity hydrotreating, and low-aromatics diesel will require more hydrogen to saturate aromatic ring compounds. But lower aromatics gasoline will reduce fluid catalytic cracking (FCC) severity, thus reducing the aromatics content of cracked distillates.

Because there will be a limit to available capital to meet composition rules on all fuels, those on gasoline will take priority over diesel fuel.

The first installment of this two-part series on reformulated transportation fuels (OGJ, Apr. 9, p. 43) examined current refinery gasoline compositions and how the existing and future environmental regulations will affect these compositions.

This concluding installment examines some long-range estimates of the composition of gasoline and diesel fuel, and their alternatives that are possible during the 1990s, and how these can meet regulatory agency environmental targets. Some current refining-industry experience with reformulated gasolines is also presented here.

FUEL RESEARCH PROGRAM

In October 1989, three U.S. domestic auto makers and 14 oil companies announced an extensive joint research and testing program to evaluate a wide range of gasolines. The target is to pinpoint fuel composition to reduce emissions, especially ground-level ozone formation.

The participants emphasize that the program, as designed, has no bias as to which fuels will be best for air quality.

Participants state that the study is in support of the President's Clean Air Act proposal. However, the results may lay the groundwork for a re-evaluation of Executive Branch thinking, and possibly redirect some of the emphasis away from methanol as a transportation fuel.

The first phase of the study is scheduled for completion by mid-1990. More than likely the date will slip to 1991 because the timing of the initial work is already under pressure.

The first phase involves examination of the emissions, air quality effects, and economic relationships of methanol fuels and methanol fuel blends in prototype vehicles, as well as reformulated gasolines in a representation of the current car population.

Methanol-fuel vehicles will be specially designed to handle both gasoline and methanol-fuel compositions. Reformulated gasolines comprise various hydrocarbon and oxygenated blends, the latter including ethanol, MTBE, and ETBE.

All vehicle emissions, exhaust and evaporative, from the program's car/fuel running combinations, will be measured and analyzed chemically. This block of information will constitute input to air-quality models to evaluate the effect on potential ozone formation from each car/fuel combination.

The second phase of the program is in the design stage and will use results from the first phase to establish the final targets. Future production vehicles, alternative fuels, and reformulated gasolines will generate the basis of this echelon of research.

It is entirely possible that the total study will extend well into the 1990s.

EMISSION CONTROL GASOLINE

To date, three companies have announced reformulated gasolines for specific marketing areas to replace leaded gasoline sold through wide-nozzle pumps. ARCO's EC-1 (Emission Control-1) was the first and is marketed in Southern California primarily to relieve ground-level ozone formation.

Diamond Shamrock Refining & Marketing Inc. and Conoco Inc. have introduced special formulations for the Denver area, targeted toward carbon monoxide and ground-level ozone. Seasonal adjustments include reduced vapor pressure during the summer and an increase of oxygenates for winter blends.

The ARCO EC-1 grade meets ASTM D4814 specifications, and typical properties are listed as follows11 (See, OGJ, Apr. 9, p. 48 for list of references):

RVP, SOUTHERN CALIFORNIA CRITERIA MINUS 1 PSI

Aromatics: 20 vol %

Benzene: 1 vol %

Bromine No.: 20

Oxygen: 1 wt % (min.)

Sulfur: 300 ppm (max.)

Lead: below 0.05 g/gal

In general, the EC-1 characteristics are close to expected regulatory agency targets during the 1990s. One exception is the 20% level for total aromatics which is too restrictive for the U.S. gasoline pool.

EPA targets could ultimately fit in the following ranges for U.S. gasoline:

Rvp: 8.0-9.0 psi (summer average)

Aromatics: 25-30 vol % (max.)

Benzene: 0.5-1.0 vol % (max.)

Oxygen: mandatory for some areas

The ultimate concentration for oxygenates in gasoline is probably 3.5 wt % oxygen. Gasohol is an exception, and is legal to the 3.7-wt % limit to allow downstream blending flexibility.

It is unlikely that a higher limit would be approved by EPA in the future because nitrogen oxide emissions tend to increase from catalytic converter-equipped vehicles beyond the 3.5-wt % level. Based on 7.5 million b/d U.S. gasoline pool production, the limit would allow close to 1.5 million bbl of ethers in gasoline.1

DIESEL SULFUR, AROMATICS

During the mid-1980s, there was a good deal of EPA concern about diesel fuel sulfur and aromatics.12 Regulatory action is pending because sulfur oxides, nitrogen oxides, particulates, and unburned hydrocarbon emissions can all be related to diesel fuel composition.

Distillate sulfur and aromatics increased during the past 20 years because of the influx of heavier crudes, more conversion in FCCU's to make higher-octane gasoline, and the increasing demand for distillates that required blending of cracked stocks. Table 1 shows the limits that have been considered by EPA: 0.05 wt % sulfur and 20 vol % aromatics.

It is more likely that aromatics will be controlled via cetane index which might allow some flexibility above the 20 vol % limit. Further, EPA may consider control of distillates a local issue (for example the Los Angeles basin) and postpone regulatory action on a national scale.

Any restriction on diesel fuel aromatics would be capital intensive because of hydrogen requirements to saturate aromatic rings.12

Because oil-industry capital to handle regulatory controls has some limit, action on gasoline composition will top the list. At the same time, mandatory reduction of gasoline aromatics would tend to lower FCC conversion, which in turn, would lower cracked distillate aromatics.

ALTERNATIVE FUELS

Alternative fuels have been in and out of the news since the oil embargo in the early 1970s. Assuming the definition implies alternatives for traditional gasoline, the list includes:

Gasoline blended with oxygenates
Methanol
Ethanol
Natural gas
Electricity
Solar

Gasoline blended with oxygenates, specifically ethers, is the front runner for 1990. Reformulated gasoline will rely heavily on ethers as the so-called clean-up agent.

Methanol has been touted by the Bush administration because hydrocarbon emissions are lower than gasoline, whether as M-100 (neat methanol) or M-85 (85% methanol and 15% gasoline). However, any significant replacement of traditional gasoline by methanol would require massive outlays of capital for plant construction.

Because plants could be situated outside the U.S., there is concern about added U.S. reliance on imported energy. Further, there are questions about formaldehyde emissions and safety. (Methanol burns with a colorless flame.)

Nevertheless, it is the most likely candidate for fleet operation, etc., to gain relief from air pollution in critical areas such as the Los Angeles basin.

Ethanol could play a similar role as methanol with E-100 or E-85, but subsidies would be required for the alcohol to compete economically with current transportation-fuel prices. Ethanol has the unique advantage of being renewable from U.S. agriculture.

Natural gas is currently plentiful in the U.S. and worldwide. Emissions are much lower when it is burned than from gasoline, but disadvantages would be difficult to overcome. Vehicle tanks must be under relatively high pressure, and vehicles would require more frequent fueling.

The most likely application is for fleet operation in critical areas where supply is readily available.

Both electricity and the solar fuel cell based on hydrogen are long-range prospects for heavily polluted areas after the turn of the century.

In general it is unlikely that alternatives, aside from oxygenates blended to gasoline, will have much effect on crude oil-derived transportation fuels during the 1990s.

QUALITY WITH RESTRICTIONS

Two major issues, volatility reduction and aromatics reduction, can be estimated in conjunction with gasoline pool octane quality. While each individual refining situation will vary, technical feasibility can be judged by manipulating components in the pool while retaining octane quality.

Table 3 in the first installment is a representation of the composition, octane quality, and aromatic content of the 1989 U.S. gasoline pool. By maintaining average octane of 88.4, a reasonable grade mix is available to satisfy the car fleet of the 1990s.

Table 2 represents a hypothetical U.S. gasoline composition that has been adjusted to reduce summer gasoline Rvp to 9.0 psi and aromatics to a maximum of 30 vol %. Pool gasoline demand is assumed to average 7.5 million b/d.

Table 2 was generated from Table 3, and some assumptions and adjustments were made.

Blending butanes have been reduced 1.5 vol % from the 1989 gasoline pool, equivalent to about 112,500 b/d. In theory, the reduction to 9.0 psi would require the removal of 150,000 b/d of n butane from gasoline.1

The additional pool Rvp reduction accrued from reduced naphtha reformer severity and less conversion in the FCCU (less butane make and lower gasoline Rvp from these units which account for over 70% of total gasoline to the pool).

Light-straight-run gasoline has been essentially eliminated as a gasoline component because of low octane, which averages 65 (R + M)/2. Disposition was to isomerization where the average octane is increased to 85 (R + M)/2.

The higher volatility of the C5-C6 isomers is balanced by additional condensation processing (etherification).

Aromatics were lowered from 32 to 30 vol % in the pool, primarily by reducing naphtha reforming severity. This required reducing the octane quality of the reformed gasoline by more than 2 (R + M)/2 octane, but, at the same time, gasoline yield increased.

Pool octane also was lowered incrementally by the loss of 1.5 vol % blending butanes.

Compensating octane was generated by conversion of light straight run to isomerate, the elimination of direct coker gasoline blending and, most significantly, by the addition of 1.1 vol % ethers. Ethers to the pool (in exchange for aromatics) preserve the gasoline-blending flexibility required from high-octane components.

Lower conversion on the FCCU reduced gasoline aromatics marginally. It was assumed that the octane quality of the gasoline stream was maintained by taking advantage of both existing and new FCC catalyst technology.

Some conversion of isoamylenes to TAME gasoline was assumed to help retain the octane quality of the gasoline stream originating from FCC operations. Alkylate volume to the gasoline pool decreased incrementally due to routing of more FCC isobutylene to etherification.

The exercise to restrict aromatics in the U.S. gasoline pool indicates that levels below 30 vol % will be far more difficult to achieve. Aromatics emanate from two sources: reformed and FCC gasolines, which make up over 70% of total gasoline.

Within the limits of existing technology, high-octane isoparaffins cannot be generated in sufficient volume to replace aromatics. If the integrity of finished gasoline octane is to be maintained, the reduction of aromatics below 30 vol % can be accomplished only by the introduction of additional ether, or by the use of an antiknock compound for unleaded gasoline.

MMT, at 1/32 g Mn/gal could add close to 1.0 octane to the unleaded gasoline pool. Eventually this could facilitate the reduction of pool aromatics about 2 additional volume percent (28 vol % in pool) without loss of octane quality.

MMT response is less effective with aromatic hydrocarbons. The octane return from a given amount of MMT should increase as aromatics are withdrawn from the gasoline pool.

It is unlikely that U.S. gasoline pool aromatics can be reduced to 25 vol % during the 1990s without some compromise of marketed gasoline octane quality. Gasoline grades with 20 and 25 vol % aromatics are being marketed in critical environmental pockets, such as Southern California and the Colorado front range, but it is at the expense of adjusting other grades and/or exporting aromatics to other marketing areas.

The various refining strategies that seem to be technically feasible to control aromatics in gasoline during the 1990s are given below:

Reduction of total aromatics in the U.S. gasoline pool to 30 vol %, or to 28 vol % with an EPA waiver for MMT, is possible. Octane quality could be maintained, and the target might be achieved by 1995.
Further reduction of total aromatics while holding current quality (Table 3 in Part 1) would require an extensive petroleum refining move to etherification by the latter part of the decade.
Assuming all marketed grades were reduced to an average of 87 octane (Table 3, all unleaded regular) aromatics could be reduced to about 27 vol % or to 25 vol % with an EPA waiver for MMT.
Special gasolines containing 20-25 vol % aromatics could be blended for critical pollution areas. Excess aromatics would either appear in other grades or be exported for use outside the regulated area. Current quality could be maintained. This appears to be the present oil industry approach with today's emission control fuels.

An alternative to the control of total aromatics in all U.S. gasoline would be to shift regulatory targets to specific aromatics such as benzene and the photochemically reactive heavy aromatics such as xylenes.

While this would require extensive processing and separation facilities within petroleum refining, it is possible that a higher level of environmental benefit could be achieved while maintaining current octane quality. Further study is indicated.

The oil industry will require extensive capital investment for processing as well as adequate lead time to further reduce volatility and limit aromatics in U.S. gasoline. Aromatic reduction, whatever course it takes by regulatory agencies, has the greatest potential effect on gasoline octane quality.

The outlook is for continued attrition of small refineries and independent refiners in the U.S., a trend that commenced in the early 1970s with the oil embargo and accelerated with lead phase-down. Because of the need for capital to handle fuel regulatory requirements, foreign interest in U.S. refining will continue to rise.

Further, if gasoline quality cannot be maintained from domestic refining sources, high-octane blendstocks and feedstocks will be imported in increasing quantities.