OXYGENATES FOR THE FUTURE-CONCLUSION OXYGENATE/HYDROCARBON SHIFT WILL REWRITE GASOLINE RECIPES

George H. Unzelman
HyOx Inc.
Fallbrook, Calif.

An estimate has been made of reformulated gasoline component requirements for 1995 ozone and carbon monoxide (CO) attainment, as mandated by the amended Clean Air Act of 1990.

The most significant change in reformulated gasoline composition will be an oxygenate-hydrocarbon shift, replacing aromatics with ethers--primarily MTBE early in the decade.

As the decade progresses, the trend will be toward an average fuel that combusts more completely and tends to be more stable with respect to evaporative emissions.

HYDROCARBONS SUBJECT TO REPLACEMENT

Butanes have already been restricted in gasoline by EPA during the summer season, and the influx of MTBE in the gasoline pool has helped balance refinery octane quality. In the future, more n-butane and some alternative light hydrocarbons may be limited. Both octane and Rvp control can be provided by replacing these compounds with ethers.

The amended CAA limits aromatics and includes oxygen in future reformulated gasoline. The law provides a very convenient exchange for high-octane components. Not only are oxygenates the only alternative means of maintaining pool octane, but the exchange is one of fairly low-boiling components for high-boiling components, benzene excepted.

Very recently, the Auto/Oil Air Quality Improvement Research Program implicated heavy hydrocarbons in general as contributors to hydrocarbon emissions. It is useful to have a better understanding of the hydrocarbon composition of the gasoline fraction surrounding the 90% ASTM distillation temperature. Some of that fraction could be restricted and possibly replaced by oxygenates in the 1990s.

AROMATICS IN GASOLINE

The CAA mandate to remove aromatics from reformulated gasoline by 1995 targets total aromatics and benzene. However, the initial phase of the Auto/Oil Research Program found that "reducing the boiling range (T90) from 360 F. to 280 F. lowered hydrocarbon emissions from the current vehicles by 22%." 1

It is the opinion of this writer that heavy aromatics are significant contributors to hydrocarbon emissions. Assuming that future work by the Auto/Oil Research Program points in this direction, it is possible that EPA may specify aromatics removal by boiling range.

Table 1 summarizes data on the aromatics currently in U.S. gasoline. In general, this class of compounds is low in API gravity (compared to average gasoline) and high in octane quality. The C9 and C10 aromatics are those in the "heavy" category and exhibit typical octane blending characteristics of the class. Morris determined the average octane-blending effects of these aromatics by group: 107.5 (R + M)/2 for the C9s and 101 (R + M)/2 for the C10s. 2

While smaller amounts of aromatics having higher carbon numbers are present in the back end of reformed gasolines, as indicated by end points well above 400 F., the C9s and C10s contribute to the distillation curve in the 300-400 F. range and have major impact on T90 (Tables 2 and 3). On the other hand, T90 reduction may not prove optimum for hydrocarbon emission control and end point (EP) could prove equally significant.

The T90 ASTM distillation point is more reproducible in the laboratory and is favored to indicate and control characteristics of motor fuel specifications. However, if very heavy aromatics are implicated in the future as contributors to hydrocarbon emissions, a combination of T90 and EP limits could serve as a means to control future reformulated gasoline in critical areas.

BLEND STOCKS THAT CONTRIBUTE AROMATICS

The Auto/Oil Research Program covered an excellent and typical range of blend stocks contributing aromatics to U.S. gasoline. Key analytical data on these streams are presented in Tables 4 and 5.

The light reformate is high in benzene content, 4.01 vol %. The mid-cut reformate contains most of the toluene, ethylbenzene, and xylenes, plus some C9 aromatics from naphtha reforming. The heavy reformate is primarily C9 and C10 aromatics, plus that interesting distribution of "C10 plus" aromatics in the ASTM distillation tail between T90 and EP. Both the mid-cut and heavy reformates contained over 90 vol % total aromatics.

Two catalytically cracked naphthas were represented in the Auto/Oil Research Program and both contained over 35 vol % aromatics. The cracked streams contained over 1 vol % benzene, and aromatic compounds similar to those in reformate, but distributed among olefins and saturates, The concentration of aromatics is high in the back end of cracked naphtha. The fraction between T90 and EP, ranging approximately from 375-435 F., contains "C10 plus" aromatics.

Most of the aromatics in U.S. gasoline are contributed by the combination of two refinery streams--reformed naphtha and fluid catalytically cracked (FCC) gasoline--which represent close to 70 vol % of the U.S. gasoline pool. 3 Further, it is estimated that over 80 vol % of the hydrocarbons making up summer baseline gasoline between the T90 and EP (330-415 F.) are heavy aromatics.

HEAVY PARAFFINS

Heavy paraffins, primarily branched-chain compounds, can contribute to raising the T90 and increasing the EP of gasoline. Therefore, they can also be implicated along with aromatics as contributors to hydrocarbon emissions, based on findings of the Auto/Oil Research Program. Table 6 lists two C9 and two C10 isoparaffins.

API gravity of these compounds is in the 50-60 range, and boiling points are lower than aromatics with equivalent carbon numbers. The branched-chain paraffins that can raise the T90 much above the 300 F. level (and increase EP) are higher in carbon number, possibly in the C10-C15 range.

HEAVY ALKYLATE

Heavy branched-chain paraffins were introduced to the Auto/Oil Research Program blends with a heavy alkylate having a T90 of 427 F. and an EP of 517 F. (Table 5). It is not surprising that a fuel blended with significant amounts of this compound would increase hydrocarbon emissions. Three of the fuels in the study were blended with more than 15% of this component.

However, two points can be made about alkylate in general:

First, the total volume of all alkylate in the U.S. gasoline pool in 1989 amounted to a little over 11 vol %. This limits the percentage of the full boiling component in most finished gasolines.

It is unlikely that any single gasoline would contain a significant volume of heavy alkylate, which is probably less than 10 vol % of the full boiling stream.

On the other hand, it does imply that the high-boilingpoint paraffins in the back end of some alkylates could cause hydrocarbon emission increases. Examination of typical industry heavy alkylates pinpointed several with EPs above 500 F.

Second, the (R+M)/2 octane quality of heavy alkylate is generally 75 and below. This further limits the amount that would be blended to most marketed gasoline. The exception could be a reformulated gasoline blended with a high percentage of high-octane oxygenate.

Also, it should be pointed out that some high-boilingpoint saturates can be introduced in the heavy fraction of FCC gasoline. Presumably these compounds are cyclic in nature. For example, one C9 compound--1,2,5-trimethylcyclohexane (trans)--boils at 286 F. and has an API gravity of 49.5. One C10 compound--1-methyl-4-iso-propylcyclohexane (trans)--boils at 339 F. and has an API gravity of 47.1.

Higher carbon cycloparaffins exist within the gasoline boiling range and can contribute to the fraction of cracked naphtha between the T90 and EP of typical streams (Table 5). However, similar to the heavy isoparaffins of alkylate, these compounds do not contribute substantial volumes to the "T90 to EP" fraction of finished gasoline.

HEAVY OLEFINS

The initial phase of the Auto/Oil Research Program removed some of the pressure from olefins in gasoline as contributors to emissions. As quoted earlier from the news release, the overall conclusion was that olefin concentration "does not have much of an impact on vehicle exhaust emissions."'

Heavy olefins are present in the back end of FCC gasoline and, while the concentration can vary, they are estimated to range between 10 and 20 vol % in the boiling fraction from 380 F. to 430 F. Because FCC gasoline makes up about 35 vol % of the U.S. gasoline pool, average marketed gasoline would contain 3.5-7 vol % of olefins in that boiling range. Similar to the saturates in the heavy-cracked-gasoline fraction, the olefins can exist as straight and branched chains and as cyclo-olefins.

HYDROCARBONS RELATED TO EVAPORATIVE EMISSIONS

Table 7 lists four light hydrocarbons that contribute to evaporative emissions. There is little question that additional butanes, primarily n-butane, will be removed from gasoline.

Even though n-butane is low in atmospheric reactivity, it is high in vapor pressure. Escape from gasoline during shipment, at service stations, and from under-the-hood of cars is accompanied by other light hydrocarbons. Concern centers around the olefins that are high in photochemical reactivity, and even benzene, which boils at 176 F.

Excellent process technology is available to convert the light olefins, but so far economics have not been favorable. For example, C5 olefins can be removed from the front end of cracked gasoline and alkylated. However, operating costs are high because of excessive acid consumption. On the other hand, the higher boiling alkylate, comprised of longer branched-chain paraffins, may not be the best exchange with respect to hydrocarbon emissions from vehicles.

As pointed out earlier, the highly reactive C5 olefins can be converted to TAME. Special catalyst technology exists to convert unreactive 3-methyl-1-butene to the reactive form. The following is quoted from Reference 6 in Part 1 on the Etherol process:

"The isomerization activity of the trifunctional catalyst allows normally unreactive olefins to be converted into a reactive form. In the presence of hydrogen, the trifunctional catalyst isomerizes the normally unreactive 3-methyl-butene-1 into the reactive 2-methyl-butene-2 by a double bond shift reaction."

At some future point the photochemically reactive C5 isoamylenes may be limited in gasoline by regulatory action. Also, it may become economic for refiners to convert these compounds to ether because of the need to meet oxygen requirements.

OXYGENATE-HYDROCARBON SHIFT

The gasoline properties for summer baseline gasoline are shown in Table 4. General comments can be made about how average summer gasoline properties will change as the decade progresses and the percentage of reformulated gasoline increases in total U.S. marketed motor fuel.

HIGHER API GRAVITY/LOWER BTU CONTENT

The most significant oxygenate-hydrocarbon shift will be aromatic replacement with ethers, primarily MTBE early in the decade. This exchange will reduce the carbon-to-hydrogen ratio and make gasoline lighter, i.e. increase the API gravity. At the same time, loss of aromatics will tend to force more butane from gasoline.

Aromatic hydrocarbons have very low Rvp and can absorb a higher percentage of n-butane than MTBE and alternative ethers. Conversely, because n-butane has an API gravity over 100, a lower concentration in gasoline would tend to make it heavier. Nevertheless, spot calculations show that the overall net effect will be lighter average gasoline and higher API gravity.

The trend holds regardless of which oxygenates are involved because all prospective candidates have higher API gravities than aromatics. In the case of ethanol blending, the trend to lighter gasoline is even more pronounced because higher vapor pressure is allowed in the marketplace. Assuming that future regulations specifically target heavy aromatics for removal, the exchange with oxygenates would move API gravity even higher.

As API gravity of average summer gasoline increases, the BTU content will drop. Assuming other factors are not involved, this would tend to reduce the mpg of future gasoline. However, a lighter gasoline should tend to combust more completely, and this would counteract some of the effect of lower BTU content.

RVP AND ASTM DISTILLATION

Summer Rvp of gasoline is currently subject to EPA regulations, and indications are that future rules will call for more restrictions. Very likely the agency will be guided by the forthcoming Auto/Oil Research Program evaluation of evaporative emissions, which should provide more definitive background on the impact of light hydrocarbons on ozone formation.

Average summer gasoline will gradually decline in Rvp as the decade progresses. Concurrently, petroleum processors will use maximum butanes in gasoline during the winter period, to the extent regulations will allow. Average winter gasoline Rvp will decline more slowly, barring new regulations.

The exchange of MTBE for aromatics will gradually raise the initial boiling point (IBP) of average summer gasoline from baseline. In fact, the IBP is increased slightly when MTBE is simply added to a 9 Rvp base fuel. When the exchange is made with aromatics, less butane can be blended, and the effect is to further increase IBP.

The rest of the ASTM distillation points are lower and the greatest impact on a fuel similar to summer baseline would be to lower the curve in the vicinity of T50. In spite of the overall trend to lighter average fuel, the end result with ether blending should be greater stability with respect to evaporative emissions.

On the other hand, gasolines blended with ethanol as gasohol contain 10 vol % of the alcohol. Generally, ethanol blending is "on top of" specification gasoline and the Rvp increase is in the order of 1 psi. Ethanol also will increase the IBP and lower ASTM distillation points above T10. However, the major impact is in the vicinity of the boiling point of ethanol (170 F.) because the alcohol tends to form constant boiling mixtures with hydrocarbons.

By 1995, the majority of gasolines requiring oxygen will be blended with ethers. Ethanol blending should have only a minor impact on the characteristics of average summer gasoline.

Ethanol could have a greater impact on volatility characteristics of average winter gasoline during the period between 1992 and 1995. Ethers will be in short supply, and the industry will be required to rely more heavily on alcohols to meet oxygen requirements in CO nonattainment areas during the cold season. Later in the decade, however, the ethers will take over the major impact of the oxygenate-hydrocarbon shift.

Methanol cosolvent blending will have an impact similar to ethanol but will depend on the availability of cosolvents. However, gasolines blended with methanol and cosolvents must meet the ASTM-Rvp specifications.

HYDROCARBON TYPE AND OCTANE QUALITY

The ozone nonattainment areas will require reformulated gasoline with a maximum of 25 vol % aromatics for 22% of U.S. gasoline by 1995. This will result in a drop of about 2 vol % aromatics in average summer gasoline from "baseline," and the downward trend will continue throughout the decade.

Benzene removal will be at low concentration, and the impact on average gasoline characteristics will be imperceptible. Also, the concentration of light olefins will fall off slightly as more of these light compounds are processed to branched-chain paraffins and ethers, to minimize evaporative emissions problems.

Octane quality of average summer gasoline should remain about the same as the 87.3 baseline with the exchange of aromatics for oxygenates. The petroleum processing industry may experience some geographical surplus of octane quality during the winter season in CO nonattainment areas.

During this period, refiners will be using maximum n-butane along with the minimum of 2.7 wt % oxygen in CO-critical areas, However, processing moves and selective blending for unregulated areas should balance octane quality for most refining and marketing situations.

OXYGENATES AND COMPLIANCE

Based on a recent forecast of U.S. fuel requirements, gasoline demand should reach 7.6 million b/d by 1995. 4 Further, ethers are projected to comprise about 3.5 vol % of U.S. gasoline or about 260,000 b/d. 5

Over 200,000 b/d are projected to be MTBE, generated at U.S. refineries. The balance will be from imports with possibly some contribution from alternative ethers such as TAME and ETBE.

PROJECTION AND DISCUSSION 1995

In the U.S. during the period to 1995, small U.S. MTBE plants will absorb much of the remaining isobutylene feed from fluid catalytic cracking. The additional feed will come from n-butane rejected in order to control Rvp. Current momentum indicates the oil industry will push ether plant construction and utilize imports to minimize the use of alcohols to fulfill oxygen requirements.

By 1995, the 3.5 vol % ethers could supply about 90% of the oxygen requirements for ozone and CO nonattainment areas, assuming almost perfect distribution. The balance will be supplied by downstream blending of alcohols, primarily ethanol, blended as gasohol (e.g., 10% ethanol at 3.7 wt % oxygen). The current ethanol-blending rate of 50,00060,000 b/d will be expanded considerably.

PROJECTION AND DISCUSSION 2000

Any projection to the year 2000 must be categorized as "blue sky." Nevertheless, based on the rules of the amended CAA and the estimated growth of gasoline demand, approximations can be made. Obviously, factors such as world crude supply and new technology are not predictable.

One recent forecast of gasoline demand indicated 8 million b/d for the U.S. by 2000. 4 Ethers are projected to make up 7 vol % of the gasoline blended at U.S. refineries. 5 Combined ethers from refinery operations, the majority MTBE, would total 485,000 b/d. The balance, about 75,000 b/d, to meet the 7 vol % pool requirement would come from MTBE imports. 5

Reformulated gasoline and special winter blends to meet ozone and CO nonattainment are estimated to represent 65% of U.S. gasoline in the year 2000. Assuming that calculations are based on MTBE (for combined ethers), the 7 vol % could come close to meeting annual oxygen requirements for ozone nonattainment, assuming ideal distribution. Supplementary oxygen would be needed from downstream alcohol blending to cover the logistics of geographical distribution and CO nonattainment during winter months.

REFERENCES

Auto/Oil Air Quality Improvement Research Program News Release, Dec. 18, 1990.
Morris, W.E., "Octane Blending Effects of Aromatics," NPRA Paper AM-80-43, NPRA Annual Meeting, New Orleans, Mar. 22-24, 1980.
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).
Unzelman, G.H., "Environmental Impact on Fuel Composition 1990 Decade," AKZO Catalyst Seminar, Philadelphia, Oct. 2, 1990.
"The Impact of the Clean Air Act on Motor Fuels," Revised Edition, Information Resources Inc., December 1990.