Ethyl Tertiary Butyl Ether

The potential for significantly increased ethanol use in the future may be in its application as a feedstock for Ethyl Tertiary Butyl Ether or ETBE.

ETBE is an oxygenated fuel that can be blended with gasoline to make it burn more cleanly and thus improve overall air quality. ETBE is produced by mixing ethanol and isobutylene and reacting them with heat over a catalyst. The promise of ETBE is that it eliminates many of the historical impediments to the greater use of ethanol such as increased volatility of gasoline and incompatibility with gasoline pipelines. This would allow ETBE to be used at the refinery level and economically transported to areas that previously had not been able to utilize ethanol.

ETBE attains the overall goals of reducing the health impacts of motor fuel because it can utilize excess butanes in the refining industry resulting from the phase-out of lead. These butanes can be converted to isobutylene which, when combined with ethanol, produce an environmentally superior motor fuel. The State of Nebraska's Ethanol Board, Arco Chemical Company, Sun Refining Company and others conducted in-depth research and development of ETBE commercialization processes. Recently, many U.S. etherification plants have been modified to allow for production of fuel ether from ethanol.

Many experts in the motor fuel industry believe the problem has been lack of acceptance of ethanol by the petroleum refining industry and common carrier pipelines. The automobile manufacturers, while they accept alcohol and gasoline blends, have done so with reservations and have expressed a strong preference for ETBE.

Fundamental issues detrimental to the acceptance of ethanol at the refinery operating and marketing levels are:

• Ethanol represents a blending agent not manufactured at the refinery site.
• Ethanol-hydrocarbon blending requires the elimination of water from the refinery tank farm and product delivery systems. This includes common carrier pipelines which have not accepted alcohol/gasoline blends.
• In cases where ethanol-gasoline blends are accepted by one oil company, they are usually not accepted by that company's exchange partners. Therefore marketing is restricted.

The problem can be simply stated as a lack of approval of ethanol as a universally accepted blending agent in gasoline. One solution is to convert ethanol to ETBE, a blending agent that would be manufactured at the refinery. ETBE is water insoluble and thereby eliminates objections attributed to ethanol. This would compliment the existing ethanol blend market while opening up new market opportunities since each gallon of ETBE contains 42 percent ethanol.

An analogy can be made with the petroleum industry's rejection of methanol and cosolvents. By contrast, the industry has widely accepted Methyl Tertiary Butyl Ether, or MTBE, for gasoline blending on the same terms as a hydrocarbon. MTBE plant capacity in the United States is now approaching 215,000 barrels per day and represents a dramatic increase over previous levels.

The MTBE and ETBE processes are similar although reaction rates and operating conditions vary. MTBE units now in operation in the U.S. petroleum industry could probably be readily converted to ethanol feedstock for ETBE production.

Ethanol could become a cost competitive feedstock for ethers currently manufactured with methanol. ETBE, because of its ethanol base, has better blending properties, thus making it superior to MTBE for both octane improvement and motor fuel volatility control. Methanol is currently dependent on natural gas as a feedstock leading some analysts to believe that methanol demand could approach effective capacity by the late 1990s. Ethanol based on grain and other biomass feedstocks is renewable as well as being available worldwide and could help fill the demand  that may result from dramatic increases in methanol production. Projections suggest an increasing supply of worldwide grain surpluses in the future.

Common Carrier And Refiner
Acceptance Of Ethers

MTBE is manufactured by several major oil companies and MTBE/gasoline blends are completely accepted by the product pipeline systems.

MTBE represented the petroleum refiners' first step away from total hydrocarbon operation within the refinery itself. MTBE is a high-octane, water tolerant ether made by combining refinery isobutylene and methanol supplied from sources outside the refinery. Isobutylene is currently derived from two major sources: steam cracking for ethylene manufacture; and fluid cat cracking for gasoline and distillate manufacture. Additional isobutylene supplies can be made available by isomerizing surplus n-butane to isobutane, then dehydrating the isobutane to isobutylene.

Because the basic process chemistry is the same, ETBE can be manufactured with the same catalytic reaction and essentially the same process unit as MTBE. Reaction rates will differ, but modest operational changes should be able to accommodate the ethanol feedstock in essentially the same refinery equipment.

Importance Of Greater Volatility
Control in Refinery Gasoline Blending

A serious problem facing petroleum refiners in the near term involves gasoline volatility. Refinery operations have increasingly generated more light material with the shift from lead antiknocks to processing to meet gasoline-pool-octane requirements. Adding to the need for more unleaded octane quality is the growing demand for premium fuel. The basic refining problem relates to a creeping imbalance between conversion and condensation processing.

Butanes generated in petroleum refining are traditionally either blended to gasoline or used as feed for condensation or petrochemical operations. Additional n-butane purchased from natural gas processors also is absorbed in gasoline. The increasing surplus of light ends which is symbolized by n-butane the "swing-hydrocarbon" for volatility control is being absorbed in the gasoline pool. The economic incentive to absorb a gallon of butane in gasoline ranges from ten to 20 cents depending on geography and season. The octane incentive is equally attractive and, as a gasoline component, butane is several blend numbers above regular unleaded gasoline.

In terms of etherification, ETBE would even be more attractive than MTBE for volatility control. Not only would refinery butanes be displaced from the gasoline pool, but ethanol could be more fully integrated in the gasoline pool when converted to ETBE. In addition, refineries may be inclined to incorporate ETBE into the refining operations due to a comparatively low blending Reid vapor pressure in gasoline.

With reformulated gasoline requiring a minimum of 2.0 percent oxygen, ETBE becomes even more effective.

As noted by the above table, ETBE actually lowers gasoline volatility even when blended in low volatility summer grade reformulated gasoline. When blended for oxygen, it also provides 23 percent more octane. With this added flexibility, refiners may significantly reduce the capital investment necessary to meet reformulated gasoline volatility and aromatic requirements. As an example, refiners must decrease their summer grade of gasoline inthe south from 7.8 pounds per square inch to 7.2 pounds per square inch. ETBE will provide 0.4 pounds per square inch of that 0.6 pounds per square inch reduction when blended for oxygen in reformulated gasoline.

In addition, the higher volume of ETBE necessary to meet the oxygen content specification for reformulated gasoline will result in a greater displacement of other gasoline compounds such as benzene, sulphur, olefins and high boiling compounds currently in gasoline. The combination of greater displacement and higher octane will assist the refiners in reducing aromatics such as benzene, toluene and xylene, thereby assisting refiners in meeting the required 15 percent reduction in toxics. This will enable refiners to produce reformulated gasoline without sacrificing gasoline performance or quality.

Finally, it is important to note numerous studies by industry, the EPA and other institutions which conclude that ETBE compares quite favorably to MTBE on both emissions and performance.

ETBE Economics

Given the superior blending, performance and emissions characteristics of ETBE, a refiner's decision to use ethanol as a feedstock for ethers, as opposed to methanol, would appear to be both obvious and simple. Unfortunately, the economic advantage for methanol, which has been imported from foreign energy sources for as little as 25 cents per gallon, makes the manufacture of ETBE produced from American grain-derived ethanol uneconomic. A change to the existing ETBE credit is necessary for commercialization to continue. Although ethanol used in ETBE is eligible for existing tax credits, those credits are then taxed. This serves as a disincentive to use ethanol in this manner. Legislation in the U.S. Senate would address this problem and make the existing tax credit more applicable to ethers.

Environmental Impact of Ethers

The substitution of ethers for alcohols and butanes in gasoline blending would have a positive effect on emissions, in a number of specific areas:

• Reduction of carbon monoxide.
• Reduction of aromatic content of gasoline and resulting toxics.
• Reduction of olefin content of gasoline.
• Reduction of volatile organic compounds.
• Reduction of carbon dioxide.

Aromatics in gasoline have become a concern at both the national and local levels and restrictions on this class of hydrocarbon specifically benzene are being required. The ethers, particularly ETBE, would be well suited for this purpose. ETBE, with a boiling point of 176 degrees Fahrenheit and an octane blending value of over 100 octane {(Research Octane Number + Motor Octane Number)/2}w ould be an ideal replacement with very little variance from the basic American Society for Testing and Materials' distillation curve. ETBE could also replace toluene and heavier aromatics with the necessary adjustments for Reid vapor pressure and other gasoline specifications.

Current octane improvement units now being planned and under construction in the United States will further increase overall gasoline volatility. Considering future EPA volatility controls, it will be necessary to counteract isomerization with more condensation such as etherification, alkylation and polymerization.

Olefins can also be reduced or replaced in gasoline by substituting ethers. Again the necessary volatility adjustments would have to be made.

Summary

The following points summarize the advantages of ETBE:

• Potential Decreases inAuto Emissions from ETBE Blended in Reformulated Gasoline Source: ARCO Chemical Company

  Chemically combines ethanol into the gasoline pool in a manner acceptable for refinery gasoline blending, for gasoline exchange and for transport through common carrier pipelines.
• Increases the octane number of the gasoline pool. Allowing two percent oxygen by weight would accommodate 12.5 volume percent ETBE. Allowing 3.5 percent oxygen by weight same as gasohol would accommodate almost 23 volume percent ETBE.
• Reduces the overall volatility of the gasoline pool. This would be beneficial to the refinery industry in the event Reid vapor pressure restrictions on all gasoline are stipulated by EPA.
• Can reduce overall evaporative emissions and carbon monoxide tailpipe emissions.
• Can reduce aromatic and/or olefin content of motor fuel.
• Opens the door for an acceptable fuel component to absorb the growing surplus of grain and other biomass a fuel component completely compatible with hydrocarbons.

President George Bush
June 1989
Lincoln, Nebraska