OXYGENATES FOR THE FUTURE-1 U.S. CLEAN AIR ACT EXPANDS ROLE FOR OXYGENATES

George H. Unzelman
HyOx Inc.
Fallbrook, Calif.

The passage of the amended Clean Air Act (CAA) on Nov. 15, 1990, established a permanent role for oxygenates in U.S. gasoline for the foreseeable future.

This status for oxygenates was confirmed approximately a month later by analytical results from the first phase of the Auto/Oil Air Quality Improvement Research Program.

In view of these events, an evaluation of the potential of the currently acceptable fuel oxygenates is in order. Further, the acute demand for oxygen in gasoline in the 1992-1995 period may reawaken studies of more obscure oxygen compounds and cosolvents for methanol.

A broader perspective indicates the mandated presence of oxygen and the exclusion of aromatics from reformulated gasolines will impact all gasoline blend design and composition. While aromatics as a class (including benzene) are the primary targets to be replaced by oxygenates, high-boiling compounds may cause higher vehicle hydrocarbon (HC) emissions.

Individual hydrocarbons that boil above 300 F. have therefore been placed in perspective as potential contributors to HC emissions. Also, the light hydrocarbons that are related to evaporative emissions, and may be replaced by oxygenates, are characterized in this article.

As the decade progresses, average-marketed summer gasoline will gradually shift from "baseline" composition to a lighter formulation. The trend will be to an average fuel that combusts more completely and tends to be more stable with respect to evaporative emissions.

BACKGROUND

As a result of the amended CAA, ozone nonattainment areas will require reformulated gasoline containing 2 wt % oxygen by Jan. 1, 1995.1

Those are Chicago, Houston, Los Angeles, New York, Baltimore, Milwaukee, Philadelphia, San Diego, and metropolitan Connecticut, representing about 22 vol % of U.S. gasoline. Also, cities that are marginal in ozone compliance may "opt in" and require reformulated motor fuel.

In addition, 44 carbon monoxide (CO) nonattainment areas, accounting for 27 vol % of the U.S. gasoline market, will require at least 2.7 wt % oxygen during critical winter months effective Nov. 1, 1992.

Oxygen requirements for CO nonattainment areas may be subject to waivers which could influence implementation. For example, inadequate supply based on capacity or distribution problems could result in up to a year's extension by the Environmental Protection Agency (EPA).

Also, EPA can waive the oxygen requirement for CO nonattainment if a state can demonstrate that oxygenated gasoline does not reduce CO levels in the area. On the other hand, an area that is considered serious with respect to CO pollution could be required to increase oxygen content of gasoline to 3.1 wt %.

In the final analysis, it is not expected that these potential deviations from basic oxygen requirements for CO nonattainment areas will significantly change the overall oxygenate demand situation; namely 27 vol % of U.S. gasoline requiring 2.7 wt % oxygen during four critical months. On the other hand, shortages of oxygenates could delay timing of implementation in some areas.

OXYGENATES AND RESEARCH

On Dec. 18, 1990, three major auto companies and 14 oil companies jointly released the findings of their program, initiated in October 1989, to develop data on the influence of gasoline composition on air quality. The early results indicated that oxygenates can play a significant role in improving air quality.

The release is quoted as follows with respect to three major findings:2

• "Adding oxygenates reduces the amount of exhaust emissions (hydrocarbons and carbon monoxide), and the benefits have been quantified."

• "Changing the level of olefins in gasoline does not have much of an impact on vehicle exhaust emissions."

• "Reducing aromatics and/or the boiling range of gasoline (referred to as T90) can either reduce or increase exhaust emissions, depending upon vehicle type."

The early work also indicated considerable difference among vehicle types. Reducing aromatics, which in reformulated gasoline is generally accomplished by application of oxygenates, was more effective in reducing emissions from current vehicles than older vehicles. While changing the level of olefins did not impact vehicle exhaust emissions, it remains to be seen whether light olefins influence evaporative emissions.

The Auto/Oil Research Program is scheduled to run through 1993 and is an exhaustive effort to understand air quality effects of motor fuels. While much is yet to be learned, it has not yet shown any conflict with the beneficial role of oxygenates in meeting the demands of the amended CAA.

OXYGENATE REVIEW

Prior to 1980, the oil industry had little interest in oxygenated fuels as a component of gasoline. For a time there was a flurry of research at major oil companies and antiknock manufacturers on octane improvers. The lead phasedown was imminent, and the focus for alternatives was on organics rather than organometallics. Alcohols, ethers, acetates, and many other similar oxygen-containing compounds were reviewed as possible components in gasoline to increase octane quality.

The federal government had a different interest in alcohols-energy independence. In 1980, the United States embarked on a subsidy program with the goal of replacing 10% of the nation's gasoline consumption with alcohols manufactured from biomass sources. At that time, technical and economic considerations suggested that almost all of the 10% target would be in the form of ethanol derived from corn and other feed grains.

The goal was for 1 billion gal/year (65,000 b/d) of ethanol by the end of 1982 and more than 10 times that amount by 1990.

For a time, the fuel-ethanol market grew rapidly because the price of crude oil was over $30/bbl. The growth market was primarily for blending and reblending gasoline downstream of the refinery. When the price of crude oil collapsed because of the worldwide glut, the U.S. fuel ethanol market reached a plateau in the range of 50,000-60,000 b/d.

At the beginning of the 1990 decade, the fuel-ethanol market volume was less than 1% of total U.S. gasoline. The market today is still at the target volume set by the Carter administration for the end of 1982.

Early in the 1980s, the oil industry took a serious look at alcohols as octane improvers for gasoline. The lead phasedown reached the critical phase and the octane deficit threatened to be difficult (but not impossible) to meet solely with hydrocarbons.

For a time, methanol and cosolvents were trial marketed by ARCO and Sun Refining & Marketing Co. (Cosolvents, when used with methanol in gasoline blends, decrease the possibility of phase separation upon exposure to water.) The only economic cosolvent was tertiary butyl alcohol (TBA), a coproduct from ARCO's propylene oxide operations.

Market problems developed for fuel methanol because of a few irresponsible downstream blending situations that resulted in a rash of stalled vehicles. While the instances were not related to ARCO or Sun gasolines, the problems seriously damaged the market for methanol and cosolvents for gasoline blending.

ARCO, the only source of commercial TBA and "Oxinol" (mixtures of methanol and TBA), decided to abandon the business in favor of MTBE (methyl tertiary butyl ether). Subsequent processing moves were to crack TBA to isobutylene as feed for MTBE operations. It was not long before MTBE essentially superseded Oxinol in the marketplace.

Some of the problems with methanol as a gasoline component also became associated with ethanol. The oil industry never seriously considered ethanol as a refinery-level gasoline component because of the water tolerance problem. It was also not accepted by common-carrier pipelines; therefore, gasoline exchange programs involving gasohol were impractical. Nevertheless, some major companies were involved in downstream marketing of gasohol because of the very attractive subsidy economics.

However, as a general pattern, U.S. refiners moved decisively in the direction of MTBE as the preferred oxygenate for gasoline blending. The first MTBE plant was placed on stream in the U.S. in 1979. Since that time, over 100,000 b/d capacity has been added to U.S. petroleum refining operations. Table 1 lists U.S. MTBE plants and capacities.'

The application of oxygenates in gasoline in the 19808 was primarily for octane improvement and economic reasons. In this decade, emphasis will be on the environment and fuel reformulation to eliminate problem hydrocarbons. Obviously, octane improvement and economics will remain an essential part of the equation.

BASELINE GASOLINE

The motor fuel requirements of the CAA amendments include the definition of baseline gasolines.

SUMMER BASELINE

Table 2 lists hydrocarbon-type composition for summer baseline average fuel and the CAA mandated changes for reformulated gasolines. Aromatics average 32 vol %, and benzene contributes slightly over 1.5 vol % of the total.

The "summer baseline" does not indicate an oxygen, content. However, the combination of MTBE and ethanol now blended into U.S. gasoline, if averaged over the total, would equate to about 0.5 wt % oxygen. In 1989, downstream blending of ethanol (50,000-60,000 b/d) as gasohol actually contributed as much oxygen as refinery MTBE blending.

However, gasohol is marketed only in key areas where ethanol is readily available, which emphasizes the oxygen supply/distribution problem. Today, MTBE has edged into the position of the leading oxygen contributor to U.S. gasoline.

WINTER BASELINE

According to the amended CAA, the EPA will establish the specifications for winter baseline gasoline. The Agency will most likely follow the specification pattern adopted by the Auto/Oil Research Program when data are available for the 1990-91 winter season.

Winter gasoline is most critical with respect to oxygen because by Jan. 1, 1992, close to 2 million b/d of gasoline will require at least 2.7 wt % oxygen in CO nonattainment areas. It is questionable that the oil industry can respond fully to oxygen needs for CO nonattainment on schedule.

By Jan. 1, 1995, there will be considerable overlap between oxygen requirements for ozone and CO nonattainment. By this time, industry should be in a better processing and distribution position to cover market demand. Therefore, this article focuses on the changes scheduled for summer gasoline composition in 1995.

OXYGENATE CANDIDATES

Tables 3 and 4 list data on oxygenate candidates most likely to be included in future U.S. gasoline based on manufacturing capabilities and technology currently available. It is possible that economic-alternative compounds could surface in the future, primarily as cosolvents for methanol. Some of these compounds are discussed later in this article.

MTBE

MTBE is currently the only ether blended in U.S. gasoline and, as noted in Table 1, capacity has reached 118,000 b/d in U.S. refineries. Projections indicate capacity could reach over 200,000 b/d by 1995.1 3

The compound has a blending octane quality of 109 (R+M)/2, a blending Rvp of 8-10 psi, and a boiling point of 131 F. As a replacement for the aromatics that must be removed from reformulated gasoline, it is the blending component that could make the greatest change in the ASTM distillation curve of fungible motor fuel.

The substitution of MTBE for aromatics will also force additional butane from gasoline. MTBE blending Reid vapor pressure (Rvp) is much higher than the heavy aromatics likely to be replaced. For example, toluene has a blending Rvp of 0.5 psi and can absorb a significant quantity of n-butane before the mixture reaches the blending Rvp of MTBE. The heavy gasoline aromatics have greater capacity for butane absorption.

ETHYL TERTIARY BUTYL ETHER

Ethyl tertiary butyl ether (ETBE), Tables 3 and 4, has the potential to replace aromatics. The ether qualifies for the 600/gal tax credit based on "ethanol content" when blended in gasoline. (it requires 0.4275 gal of ethanol to manufacture one gal of ETBE.) The subsidy places ethanol-feedstock economics close to that of methanol for fuel-ether manufacture.

Currently, American Eagle Fuels of Lincoln, Neb., operates the only known pilot-ETBE production facility, and the unit capacity is about 10 gal/hr. No commercial units are in production at this writing. On the other hand, it is likely that current MTBE units could be revamped to handle either ethanol or a mixture of ethanol and methanol feed.

ETBE has some distinct advantages over MTBE as a future component of reformulated gasoline. Blending (R + M)/2 octane is about one number higher, and blending Rvp is 3-5 psi which allows greater n-butane absorption.

With respect to boiling points, ETBE and MTBE are complementary and could best be used as mixtures in tailoring gasoline blends. Compounding gasoline with a mixture of two or more ethers would facilitate better control of the ASTM distillation curve for oxygenate blending. In other words, the impact of adding oxygen-bearing agents with hydrocarbons would be distributed over a wider part of the gasoline boiling range.

The first commercial ETBE plant will probably be constructed in one of the Midwestern states where ethanol is readily available and local economic incentives are paramount. National interest in ETBE could surface if crude supplies from the Middle East continue to be questionable.

TERTIARY AMYL METHYL ETHER

Tertiary amyl methyl ether (TAME) has an average blending (R+M)/2 octane number of 104.5, blending Rvp of 3-5 psi, and a boiling point of 187 F. Except for lower octane quality, it has many of the same blending advantages of ETBE. The ether is formed by the reaction of 2-methyl-2-butene and 2-methyl-1 -butene with methanol.

Olefin feed is from fluid catalytically cracked (FCC) gasoline and is estimated to range between 70,000 and 90,000 b/d from U.S. refining.4

Commercial plants operating in England and France manufacture "TAME gasoline" rather than an ether blending component.

The front end of FCC gasoline is used as the olefin-feed stream, and the reactive olefins are converted to ether.

Because the isoamylenes have an (R+M)/2 octane quality in the low 90s and blending-vapor pressure of about 15 psi, the net result of etherification is a greatly improved cracked gasoline stream for blending. The presence of TAME significantly increases motor and (R+M)/2 octane quality as well as lowering ASTM distillation curve temperatures.

The importance of TAME in U.S. gasoline will probably await further results from the Auto/Oil Research Study. As pointed out earlier, one conclusion from the initial analysis of data was that changing the level of olefins in gasoline did not have much influence on vehicle exhaust emissions. However, the primary negative environmental effect of the volatile olefins is most likely related to evaporative emissions during fuel transport and handling, as well as from under-the-hood of vehicles.

FUEL ALCOHOLS

While ethers are generally preferred for refinery blending, alcohols will be needed in the 1990s for downstream operations to meet oxygen requirements of the amended CAA. Alcohols have one outstanding advantage-they can supply higher levels of oxygen at lower concentrations. Also, depending on the mixture, the octane values can be equal or higher than ethers.

Whether methanol can find a place in gasoline blending in the future is dependent on an adequate commercial cosolvent. As pointed out earlier, TBA is no longer available from ARCO. However, with a shortage of oxygen-blending components for downstream operations, there may be some future economic incentive to redirect TBA to the marketplace.

A 50:50 mixture of methanol and TBA provides 35.7 wt % oxygen, close to twice as much as MTBE. Blending octane quality of the mixture is about the same as MTBE. The great disadvantage is a vapor pressure increase which must be controlled to the ASTM-Rvp class of the marketplace.

Ethanol has almost the same value as an oxygen contributor; 34.7 wt %. It has a higher blending octane number than the 50:50 mixture of methanol and TBA and can be added to specification gasoline at 10 vol % as gasohol without a vapor pressure adjustment.

Ethanol will have a tremendous advantage if it continues to be set apart from EPA's future rulings with respect to gasoline volatility. In fact, because it is the only renewable component blended in U.S. gasoline, favorable treatment is likely to continue through the decade and possibly beyond.

Because ethanol has an established position as a downstream blending agent for reformulated gasoline, it is the most likely candidate to provide oxygen in gasoline outside the refinery gate.

Also, because it is generally blended at 10 vol % and does not require vapor pressure adjustment, the overall impact on gasoline characteristics will be more pronounced than for alternative oxygenate blending.

ALTERNATIVE CANDIDATES

While the pattern in U.S. petroleum processing and gasoline blending is currently directed toward the use of the oxygenates listed in Tables 3 and 4, it is possible that alternative oxygenates could find their way into the gasoline pool.

The potential incentives are a shortage of oxygen components, special feedstock or coproduct economics, and new technology.

A shortage of oxygen components could bring more alcohols into marketed gasoline. Table 5 lists four alcohols-two propanols and two butanols-that could serve as cosolvents for methanol or as blending components in their own right. Currently, economics are not favorable and supplies of these compounds are limited. Higher carbon number alcohols are not practical because of low-octane quality. For example, n-hexanol has (R+M)/2 octane quality of 50.

Alcohol mixtures based on oil field flare gas or synthesis gas have been suggested as possible gasoline-blending components.5 Most of these mixtures incorporate enough heavy alcohols to reduce octane quality to an unattractive level.

However, there could be a resurgence of studies in this area as the demand for oxygenates increases.

Another concept that has been discussed is to convert these mixtures of alcohols to ethers to make them attractive for refinery blending. Probably none of the alcohols of Table 5 could compete with TBA economics (ARCO) as a cosolvent for methanol.

Five alternative ethers are also listed in Table 5. Assuming they were available in the marketplace at a competitive price, they could be acceptable components to add oxygen to gasoline. While none have been approved by EPA, the three "tertiary" ethers are chemically similar to MTBE and would probably be waivered by the EPA. The exceptions are methyl phenyl ether, a ring compound, and diisopropyl ether. Both would probably require extensive testing for approval as gasoline components.

To date, the ethers have been considered only as blending components for gasoline . However, they also can serve as cosolvents for methanol.

This writer has some personal background with respect to combinations of methanol and MTBE. Logically, the heavier "tertiary ethers" of Table 5 should be more effective as cosolvents for methanol.

It also has been pointed out that the heavier methyl ethers can be produced from the C6 and C7 olefins of FCC gasoline along with TAME .6 These ethers probably have (R + M)/2 octane quality close to 90.

As pointed out earlier, the etherification route could be more practical than alkylation to eliminate light olefins from cracked gasoline.

TAEE (ethyl t-amyl ether) can be considered as a substitute for TAME, assuming ethanol feedstock economics are more cost effective than methanol. This is likely to be the case in some areas where fuel ethanol has a state as well as federal subsidy.

The octane characteristics of the acetates have been studied extensively both as possible substitutes for lead antiknocks7 and as cosolvents for methanol. Esters as a class of compounds deserve further study in the 1990s as possible gasoline components. The three shown in Table 5 have octane quality over 100. Assuming cosolvent action is equivalent or better than butyl alcohols, they could be superior gasoline-blending components in mixture with methanol.

The second of this two-part article will present an estimate of oxygenate requirements for 1995 ozone and carbon monoxide attainment.

REFERENCES

1. Information Resources Inc., The Impact of the Clean Air Act on Motor Fuels, Revised Edition, December 1990.

2. Auto/Oil Air Quality Improvement Research Program News Release, Dec. 18, 1990.

3. Unzelman, George H., "Maintaining Product Quality in a Regulatory Environment," NPRA Paper AM-90-31, San Antonio, Mar. 2527, 1990 (also OGJ, Apr. 9,1990, pp. 43-48; OGJ, Apr. 23, 1990, pp. 91-93).

4. Miller, David J., "Ether Options: MTBE[TAME and ETBE," NPRA Paper AM-89-58, San Francisco, Mar. 19-21, 1989.

5. Elsawy, Abdel, and Gray, David, "Evaluation of Synthesis Gas Based High Octane Oxygenates," DOE/PETC Indirect Liquefaction Contractor's Review Meeting, Nov. 15-17, 1988.

6. Hargrove, J.D., and Tapley, S.A., "The BP Etherol Process: Flexible Technology for Gasoline Enhancement," Japan Petroleum Institute, Tokyo, Oct. 19-21, 1988.

7. Unzelman, G.H., Forster, E.J., and Burns, A.M., "Are There Substitutes for Lead Antiknocks?" API Division of Refining 36th Midyear Meeting, San Francisco, May 14, 1971.

BIBLIOGRAPHY

1. Beuther, Harold, and Kobylinski, T.P., "The Chemistry of Oxygenates Suitable for Use in Gasoline," American Chemical Society, Kansas City, Sept. 12-17, 1982.

2. Chase, J. D., and Galvez, B.B., "Process for Blending Ethers--TAME and MTBE," NPRA Paper AM-80-46, 1980 NPRA Annual Meeting, New Orleans, Mar. 23-25, 1980.

3. Unzelman, G.H., and Michalski, George W., "Octane Improvement Update-Refinery Processing, Antiknocks, and Oxygenates," NPRA Paper AM-84-43, San Antonio, Mar. 25-27, 1984.

4. American Petroleum Institute, "Alcohols and Ethers," API Publication 4261, Second Edition, July 1988.

5. American Petroleum Institute Research Project 45, "Knocking Characteristics of Pure Hydrocarbons," ASTM Special Technical Publication No. 45.

6. Pahl, R.H., and McNally, M.J., "Fuel Blending and Analysis for the Auto/Oil Air Quality Improvement Research Program," SAE Technical Paper 902098, Tulsa, Oct. 22-25, 1990.

7. Technical Bulletin No. 1, "Initial Mass Exhaust Emission Results from Reformulated Gasolines." Auto/Oil Air Quality Improvement Research Program.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.