Don P. Hollrah, Allen M. Burns
Ethyl Petroleum Additives Inc.
St. Louis
Tighter emissions standards and lower aromatics specifications are focusing attention on new blending agents and MMT to replace lost octane quality. MMT is methylcyclopentadienyl manganese tricarbonyl.
REFORMULATION OPTIONS
Ethers will play a prominent role as components of U.S. gasoline in the 1990s. Their blending characteristics are a "good fit" with the probable requirements for reformulated gasoline.
Methyl tertiary butyl ether (MTBE) has already become an important gasoline blending agent in the U.S. It is being used in many of the reformulated gasolines that have already been introduced into the U.S. market. Refiners have found that MTBE is relatively inexpensive to produce and is competitively priced with other octane blending agents currently available.
Gasoline containing MTBE can also be shipped in pipelines as a fungible product.
Current production capacity of MTBE in North America is approximately 126,500 b/d.1 Another 76,000 b/d is scheduled to be on stream by January 1992, while construction plans have been announced for an additional 96,000 b/d.
A majority of the current MTBE production is being done at petrochemical plants, but many refineries have announced plans to construct small MTBE units at their facilities.
Ethyl tertiary butyl ether (ETBE), a reaction product of ethanol and isobutylene, provides similar blending attributes to MTBE. In fact, its volatility characteristics are even lower than MTBE.
ETBE should become popular now that the U.S. Treasury Department has interpreted the federal alcohol fuel tax credit regulations to include ethanol used for producing ETBE. The most rapid growth of ETBE will likely be in the Midwest, where a majority of the ethanol is produced.
Currently, ETBE is not produced in the U.S., due to poor economics without an ethanol tax credit. Now that the tax credit has been approved, and will assuredly be continued beyond 1993, manufacturers of MTBE may convert their production to ETBE.
In support of its waiver application to the Environmental Protection Agency (EPA) for its MMT performance additive HiTEC 3000, Ethyl evaluated the performance of MTBE, ETBE, and other oxygenates in conjunction with HITEC 3000. Five commercial unleaded premium and five commercial unleaded regular gasolines were used in the evaluation. The average blend qualities for the premium and regular gasolines are given in Table 1.
MTBE and ETBE were added to each of the 10 fuels at concentrations of 5 and 15 vol %. Research (R) and motor (M) octane numbers were determined for each blend. Except where noted, all octane numbers quoted in this article are (R+M)/2 values.
The average octane number gains (Table 2) were similar for MTBE and ETBE. The octane number gains for 5% MTBE and 5% ETBE both averaged 1.4 for regular gasoline and 1.0 for premium.
Octane gains for 15% MTBE blends averaged 3.1 for regular fuel and 2.2 for premium, while the 15% ETBE blends averaged 3.7 and 2.2, respectively. For comparison, the octane gains for 10% ethanol blends averaged 2.4 for regular gasoline and 1.8 for premium.
Average octane blending values were calculated for the two concentrations of MTBE and ETBE in the regular and premium fuels. The blending values for MTBE and ETBE are similar at an equal concentration of ether; e.g., 114 for 5% MTBE vs. 115 for 5% ETBE, in regular gasoline (Table 3).
The octane blending values for the two ethers also decrease as their concentrations increase. For example, the octane blending value for 5% MTBE in premium gasoline is 113, but it decreases to 107 at a concentration of 15% MTBE (Table 4).
ETBE provides an advantage over MTBE in its volatility blending behavior. The average Reid vapor pressure (Rvp) blending value for the 10 commercial fuels ranged from -4 psi for 5% ETBE to 2 psi for 15% ETBE. The corresponding blending values for MTBE ranged from 12 psi at 5% MTBE to 7 psi at 15% MTBE (Table 5).
This advantage can be very important as Rvp specifications continue to decrease. For example, an Rvp blending value difference of 5 psi could change the Rvp of the finished blend by 0.75 psi at a concentration of 15% ether.
The blending values mentioned here should be viewed with discretion. The octane gains and changes in volatility were derived from a small data set: five regular and five premium gasolines. The average blending values should be directionally correct, and they are consistent with other published data. But they could differ significantly from comparable data obtained from a different set of blends.
MMT ADDITIVE
HITEC 3000, Ethyl's manganese-based octane improver, has the potential of being an important option to the refiner for the production of reformulated gasoline in the 1990s. Better known as MMT, its octane improving capability has been recognized in the refining industry for over 20 years. But based on a recent fleet test program conducted by Ethyl, the use of MMT in gasoline can also help reduce automotive tailpipe emissions.
As a result of this study, Ethyl petitioned the EPA on May 9, 1990, to allow the use of HITEC 3000 in unleaded gasoline at a concentration of 0.03125 g manganese (Mn)/U.S. gal. But Ethyl withdrew its waiver application for HiTEC 3000 on Oct. 30, 1990.
The application sought permission for the company to sell this additive in unleaded gasoline in the United States. The withdrawal was made when the EPA raised questions about the health, environmental, and performance characteristics of the product that the company had covered in its filings with the agency.
The additional questions were raised near the end of the 180-day statutory review period. Ethyl's consultants could not perform the analyses necessary to adequately address these concerns within the brief period available to them.
MMT has been used in leaded gasoline in the U.S. for many years and was used quite extensively in unleaded gasoline in the mid-1970s. In Canada, MMT has been used in almost all unleaded gasoline since 1978. Canada has found MMT to be a safe, economical, and beneficial component of gasoline. Canadian automobiles have accumulated more than 400 billion miles using gasoline treated with MMT without experiencing any significant problems relating to vehicle exhaust emission systems, or to air quality.
FLEET TEST
In 1988, Ethyl initiated a comprehensive automobile testing and analysis program designed to assess the effect of MMT on automotive exhaust emissions, automotive emission control systems, automotive fuel system components, refinery operations, and performance with oxygenates.
In all, this has been the most extensive evaluation of a fuel additive ever undertaken by a private company.
The core of the program is a 48-car test fleet designed in consultation with the EPA and the automobile industry. Ethyl compared exhaust emissions at 5,000-mile intervals for 75,000 miles from paired sets of vehicles fueled on clear fuel and fuel containing MMT. These emissions data were then subjected to rigorous statistical analyses to determine the additive's effect on exhaust emissions and vehicle performance.
Analysis of the Ethyl fleet data confirms that MMT causes a significant reduction (7.8%) in overall vehicle tailpipe emissions. This translates to a total emissions reduction from automobiles of 1.6 billion lb/year in the United States, based on projections for 1999.
Most significant is the reduction of nitrogen oxides emissions (NOx) by 0.11 g/mile; approximately 20%, as compared to the control gasoline. The NOx emissions improvement with MMT began early in the mileage program and grew progressively larger with increasing mileage (Fig. 1). Carbon monoxide (CO) emissions are reduced by 0.22 g/mile, while hydrocarbon (HC) emissions show a slight increase of 0.018 g/mile (Fig. 2).
Ethyl evaluated the effect of MMT on various components of the automobile emission and fuel systems. Components that were tested or evaluated were oxygen sensors, fuel injectors, and catalytic converters for plugging and conversion efficiency of catalysts.
MMT did not have an adverse effect on these components. There was no evidence of catalyst plugging on any of the automobiles run on fuel containing the additive. In fact, the NOx conversion efficiency for the catalysts on cars fueled with gasoline containing MMT was much higher; 80.4% vs. 76.7% at 75,000 miles (Fig. 3).
Ethyl's tests indicate that the use of MMT in reformulated gasoline may reduce the reactivity of the various organic compounds emitted from the tailpipe. That is, it may reduce the amount of highly reactive tailpipe hydrocarbons that readily form ozone.
Ethyl conducted tests using two automobiles from the 48-car test fleet after they had accumulated 65,000 miles. For each test, one car was run on control gasoline while the other was run on control gasoline plus MMT. Three fuels were tested in the reactivity evaluation program: Howell EEE (fleet test control gasoline), a commercial unleaded regular, and a commercial "reformulated" gasoline.
MMT was tested in each fuel at a concentration of 0.03125 g Mn/U.S. gal. For each control gasoline without the additive, a small amount of xylene was added to equalize the octane number to that of the gasoline with additive.
Tailpipe emissions were collected for each of the fuels from the two cars. The HC emissions from the fuels were broken down into the various organic compounds, and the reactivity was compared using Carter reactivity factors.
Results of the test show that the reactivity of the fuels containing MMT ranges from 23 to 30% lower than the fuels without the additive (Fig. 4).
FUTURE WAIVER PLAN
Since the Oct. 30 voluntary withdrawal, Ethyl has met with EPA technical and research staff members several times to determine a reasonable course of action that will result in the refiling of the application.
Ethyl believes that it has proved conclusively that the additive will not "cause or contribute to the failure" of emission control systems or components in U.S. automobiles, as required by Section 211 of the Clean Air Act. Under normal circumstances, based on past waiver approvals, meeting this requirement would be sufficient for approval of HITEC 3000.
Realizing an obligation to respond to other reasonable concerns expressed by the EPA, Ethyl will refile its waiver following some additional research and review.
First, the company will analyze data showing differences between its findings and those of the EPA in the emissions from 6 of the 48 fleet test automobiles. The differences are more apparent than real, Ethyl believes, because automobiles used in the EPA tests had been sitting idle for several months prior to testing. Ethyl expects that further analysis of the data from two independent laboratories will explain the minor discrepancies between the EPA and Ethyl test results.
Second, Ethyl will investigate differences in the EPA's findings regarding emissions of particulates. There is no standard for particulates released from gasoline engines and no EPA-approved tests for measuring particulate emissions from gasoline-powered vehicles.
The EPA tests on Ethyl's vehicles were performed on equipment designed and used for diesel vehicles. Ethyl has subsequently run tests in an independent laboratory using the six automobiles loaned to the EPA for testing. These latest tests again showed that the additive does not contribute to any increases in particulate emissions.
Ethyl also intends to utilize new testing equipment at a laboratory known for particulate testing, in a further attempt to identify the reasons for any differences in results. This new equipment will match that used by the EPA in all aspects, except that no diesel vehicles will have been tested by it.
Finally, Ethyl has met with EPA officials in the Office of Research and Development (ORD) to discuss the health and environmental effects of the use of HITEC 3000 performance additive in gasoline.
ORD raised several points in response to Ethyl's waiver application and subsequent filings throughout the summer and early fall of 1990.
The EPA has indicated an interest in an extensive list of studies related to the product; in particular, a study on the effects of the inhalation of manganese.
Ethyl has hired a major environmental studies firm, used by the EPA in other risk assessment research, to review all data and conduct a risk assessment of the additive's use in gasoline. Part of this assessment will be a measure of manganese tailpipe emissions and a comparison of these emissions to data collected by the EPA and other world organizations, from both cities and point-source emitters such as steel plants.
Ethyl has submitted results of air quality studies in Canada, where MMT has been in continuous use for over a decade. These data show no difference in manganese air concentrations between such major cities as Toronto, where there is extensive MMT use in unleaded gasoline, and London, where the additive has never been used.
The additional testing of particulates and the new risk assessment under way are expected to confirm the review already undertaken by Ethyl and other consultants. The company believes that the reductions in major pollutants resulting from the use of MMT (amounting to almost 1.7 billion lb/year by 1999 in the U.S. alone) will provide substantial health benefits.
REFINERY OPERATIONS
The use of MMT in unleaded gasoline not only increases gasoline blending flexibility for the refiner, but also provides other positive benefits.
To examine the effect of MMT on refinery operations, Ethyl retained the services of Turner, Mason, & Co. (TM&C). The study involved TM&C's projections of refinery requirements for 1994, including Phase 11 Rvp limits, and a 0.05% sulfur limit on diesel fuel. Two of TM&C's eight refinery LP models, comprising about 45% of U.S. gasoline production, were used. The results were then extrapolated to the entire U.S. market.
Based on the LP study by TM&C, MMT can be expected to provide the following benefits to the refining industry:
- Lower reformer severity
- Reduced furnace emissions
- Lower aromatic content of gasoline
- Reduced crude throughput.
Because MMT raises the octane quality of gasoline, its use allows the refiner to operate his reformer at less severe conditions. TM&C estimates a reduction of approximately 1.0-1.5 research octane number clear (RONC). This lower severity reduces fuel gas requirements and, as a result, lowers U.S. refinery emissions by an estimated 15 million lb/year.
The lower reformer severity also produces a gasoline with lower aromatic content. TM&C estimates the reduction to be between 1.0 and 1.5%. This characteristic of MMT will enable the refiner to produce a reformulated gasoline with a reduced aromatic content.
MMT provides the refiner with an economical octane improver. It can increase the octane quality of gasoline at a fraction of the cost associated with processing or blending agents. Fig. 5 presents ranges of octane improvement costs that may be associated with the use of MMT, processing, and MTBE. During the second quarter of 1990, when crude oil was priced at $15-20/bbl, octane improvement costs for MMT were roughly one third as high as for processing and oxygenates. At higher crude prices, MMT should be even more attractive.
OCTANE RESPONSE
The use of MMT is an effective means of increasing octane quality. At a concentration of 0.03125 g Mn/U.S. gal in unleaded gasoline, it provides an average octane gain of approximately 0.9 in regular gasoline and 0.5-0.6 in premium gasoline, as shown in Fig. 6.
MMT provides better research octane number (RON) response than motor octane number (MON) response. The RON gain in unleaded regular gasoline averages between 1.2 and 1.5 at a concentration of 0.03125 g Mn/gal, while the MON gain averages between 0.4 and 0.6.
Ethyl has recently completed octane response studies with MMT in commercial fuels from the U.S., Germany, and Japan (Table 6). The U.S. fuels used were the five unleaded regular and five unleaded premium gasolines mentioned.
The clear octane number of the five unleaded regular fuels averaged 86.9, while the aromatics content ranged from 22 to 29 vol %. The clear octane number of the five unleaded premium fuels averaged 92.1, while the aromatics content ranged from 22 to 40 vol %.
The German samples used in the study were 13 unleaded premium fuels. Seven of the 13 fuels contained either alcohol, MTBE, or both, while the oxygen content of those blends ranged from 0.43 to 2.16%. MMT was evaluated at 8 mg Mn/l. (equivalent to 0.030 g Mn/gal) and 15 mg Mn/l. (0.057 g Mn/gal).
The clear octane quality of the 13 blends averaged 91.0, while the aromatics content ranged from 28 to 44 vol % and averaged 37 vol % (Table 6). The Japanese fuels used in the octane response study were 10 unleaded regular gasolines. MMT was evaluated at 10 mg Mn/l. (0.038 g Mn/gal) and 20 mg Mn/l. (0.075 g Mn/gal). The clear octane quality of the 10 fuels averaged 86.0 while the aromatics content ranged from 22 to 40 vol % (Table 6).
PERFORMANCE WITH OXYGENATES
Ethyl recognizes the importance of oxygenates in future reformulated gasoline and believes HITEC 3000 must be compatible in performance with oxygenates such as MTBE, ETBE, and others. As part of the waiver application program, Ethyl conducted a 13-week storage stability test with HITEC 3000 in blends of gasoline containing various oxygenates. All blends remained stable throughout the test.
Ethyl also conducted octane response studies with MMT at a concentration of 0,03125 g Mn/gal in several blends containing oxygenates. The fuels used in the study were the five commercial unleaded regular and five commercial unleaded premium gasolines referenced earlier in this article. The oxygenates used were MTBE and ETBE (at 5% and 15%), ethanol (at 10%), and a 5% methanol/3.2% ethanol blend.
MMT and the various oxygenates were added separately to the 10 fuels and the octane gains were measured. These are reported in Table 5. Next, MMT and an oxygenate were added to the fuels and the octane gain was measured. The octane gain for MMT plus the various oxygenates averaged 3.1 for the regular gasoline and 2.1 for the premium gasoline (Table 7).
The results of this study show that MMT performs admirably with oxygenated fuels.
SAFETY AND HANDLING
MMT is classified as a Class B poison. It is toxic to humans if it enters the body by inhalation, ingestion, or absorption through the skin. As a result, it must be handled with care and respect. However, refiners have used toxic chemicals for a long time. The keys to safe handling of hazardous materials are properly designed and well-maintained equipment, trained personnel, and the proper attitude of personnel toward the chemicals. With these, MMT can be handled safely.
MMT is thermally stable, therefore it can be pumped rather than moved with an eductor. This feature not only makes it safer to handle, but also increases a refiner's flexibility in storage and blending.
When compared to TEL, MMT has a very low vapor pressure. This means that if a spill occurs, less of the MMT vaporizes, thus reducing the amount of the hazardous chemical in the air. Further, MMT vapors are extremely sensitive to sunlight. They decompose to form harmless oxides and carbonates of manganese in less than 1 min. The combination of low vapor pressure and rapid reaction in sunlight makes MMT safer to work with than TEL.
Ethyl's lengthy experience in manufacturing, storing, and transporting MMT shows that it is relatively noncorrosive to metals. Carbon steel tanks are recommended for storage, with either stainless steel or carbon steel recommended for valves, meters, etc.
Gaskets and seals used in aromatic component service generally are satisfactory for use with MMT.
MMT can be stored and blended either as a concentrated fluid or as a diluted mixture.
Although using the concentrated compound minimizes the size of a storage tank, it can contribute to certain problems.
MMT has a moderately high freezing point of 30 F. (-1 C.).
Canadian refiners routinely purchase a diluted version of the product to avoid freezing problems.
REFERENCES
- Octane Week, Aug. 27, 1990.
- Corbett, R.A., "Tough air quality goals spur quest for transportation fuel changes," OGJ, June 18, 1990, pp. 33-42.
- Piel, W.J., and Thomas, R.X., "Oxygenates for Reformulated Gasoline," Hydrocarbon Processing, July 1990.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
