BLENDING PROPERTIES OF MTBE AND OTHER OXYGENATES IN GASOLINE

Mohammad Ashraf Ali and Halim Hamid

Center for Refining & Petrochemicals, The Research Institute,King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia.

INTRODUCTION

Methyl Tertiary Butyl Ether (MTBE) has been accepted worldwide as an octane booster and it is

being blended with gasoline upto 15 volume percent. The demand for MTBE is growing rapidly

and it is the fastest growing chemical nowadays because it has replaced lead alkyl compounds in

gasoline. The use of lead and other metal containing compounds e.g., tetraethyl lead (TEL),

tetramethyl lead (TML) and methylcyclopentadienyl manganese tricarbonyl (MMT) as gasoline

additives for octane boosting is being discouraged. The emission of their combustion products

from the vehicle exhausts creates atmospheric pollution causing serious health hazards. United

States and some European countries have imposed a complete ban on the use of such compounds.

Consequently other blending agents are required to replace the metal based agents presently in

use in order to increase the octane of gasoline. To have lead free high octane gasoline, various

components such as methanol, tertiary butyl alcohol (TBA), secondary butyl alcohol (SBA),

tertiary amyl ethyl ether (TAME) and MTBE can be used. Among these possibilities, MTBE

appears to be the most effective choice because its physical, chemical and thermal properties are

compatible with that of gasoline, especially in the boiling range where gasoline typically show

lowest antiknock characteristics.

MTBE has exhibited highest growth over the past decade among all the oxygenates which are

being blended in gasoline as octane improvers. World capacity of MTBE has increased

approximately over ten-fold in this period and expected to increase further in this decade.

Currently, MTBE has a world capacity of 12 million tons per year with a projection that this

figure will increase to 20 million tons per year by 1994. This increased use of the MTBE is due

to the phasing out of lead from gasoline mandated by the Environmental Protection Agency.

EPA has permitted the addition of MTBE in the gasoline up to 15 volume percent which

corresponds to 2.7 percent oxygen. The addition of MTBE not only increases the octane number

but also reduces the toxic exhaust of the vehicles. Contrary to the leaded gasoline, catalytic

converters can be used in the vehicles operating on gasoline containing MTBE.

MTBE is an effective octane booster and volume extender for unleaded gasoline. However, it

should be made clear that MTBE is not as efficient as tetraalkyl lead compounds as far as

specific octane number improvements are concerned. An advantage of its use is that it enables

the amount of gasoline produced from a refinery to be increased for a given crude run, firstly by

adding volume to the gasoline pool and secondly be reducing the severity of the naphtha

reforming and conversion operations. MTBE also allows the low octane components such as

straight run gasoline and raffinates to remain in the pool and reduce the amount of expensive

petrochemical aromatics that would be required to boost octane number.

The addition of MTBE affects the properties of gasoline. The change in properties depends not

only upon the characteristics of the base gasoline but also on the concentration of MTBE. The

behavior of MTBE varies when blended with different gasolines at different concentrations. The

exact amount of MTBE required for a particular lead content reduction or octane number

improvement depends on the properties of the MTBE and the composition of the gasoline base

stock.

PROPERTIES OF MTBE

The physical, chemical and thermal properties of MTBE are given in Table I [1-37]. MTBE is

an excellent high-octane gasoline component and it is combustible, non-oxidizable, clear,

colorless liquid of low viscosity with a distinct odor which is neither pleasant nor nauseating.

Octane Number

The output of an engine is determined by its knocking. Excess of knocking could damage the

engine. Low engine speed knock is usually audible to the driver but is notdamaging to the

engine. High engine speed knock, however, is often inaudible above the engine, road and wind

noise. The most severe knock, which can be very damaging, often occurs at motor way cruising

speeds of 4000 to 5000 rpm and modern high compression engines increase the tendency to

knock. Many engines will fail in less than 50 hours under conditions of heavy knock and the

damaging effect of knock is cumulative [38]. The same study also concludes that the maximum

engine speed associated with knock is greatly reduced with MTBE. Laboratory Research and

Motor Octane rating procedures such as ASTM methods D-2699 and D-2700 are not suitable for

use with neat oxygenates such as MTBE. Octane values obtained by these methods are not

useful in determining knock-limited compression ratios for vehicles operating on neat oxygenates

when blended with gasoline [36].

The octane value of MTBE is measured by its BOV (blending octane value) [39]. This value is

calculated from the difference between the octane value of a base gasoline with a known amount

of MTBE and the base gasoline without MTBE. The formula for BOV calculation is given

below:

BOV ON ON base( 1 x )

x ON baseON ON basex

where

ON = RON or MON of base gasoline– MTBE blend

ONbase = RON or MON of base gasoline

x = Volume fraction of MTBE

The range of MTBE blending octane numbers is given below [6,13]. This range is determined as

a result of the large amount of experimental data obtained in the formulation of gasolines within

the specification limits.

Blending RON 115 – 135

Blending MON 98 – 110

Blending (RON + MON)/2 106.5 – 122.5

The blending octane numbers of MTBE are very sensitive to the composition and octane

numbers of the unleaded gasoline base [40]. The MTBE blending octane number generally rises

under the following cases: when base gasoline octane number decreases, MTBE concentration in

the gasoline decreases or the saturate content of the gasoline increases.

Addition of MTBE increases the RON and MON of a gasoline. The effect of MTBE on the

antiknock properties of the three types of base gasolines were determined. The RON of the

gasolines were 84.6, 90.5 and 93.7 whereas the MON were 79.0, 83.0 and 84.0 respectively.

MTBE in the concentration levels of 5, 10 and 15 volume percent was added. An increase in

RON and MON was found for all gasoline blends. The gasoline samples having higher RON and

MON were found to have less increase in their octane numbers as compared to gasolines with

lower octane numbers. The sensitivity (RON-MON) was higher for gasoline having higher

octane numbers (Table II) [6].

A-380 is a gasoline produced by Saudi Aramco. The RON of A-380 lead–free gasoline increases

from 83.7 to 85.6 for adding 5 volume percent MTBE and to 95.5 for 30 volume percent MTBE

(Table III). The increase in RON ranges from 1.9 to 11.8 with the addition of MTBE to A-380

gasoline by 5 to 30 volume percent, respectively [3]. In case of A-380 leaded gasoline having

lead (Pb) concentration 0.28 g/liter, only 10 volume percent of MTBE is needed to increase the

RON from 83.7 to 95.5. When 0.4g Pb/liter of gasoline is present, the RON increases to 97.7.

MTBE acts as a high octane blending stock and not as a lead antiknock agent [41]. Up to 15

volume percent of MTBE was added to a base gasoline with RON and MON of 93 and 83,

respectively. The concentration of antiknock compounds (lead alkyls) in gasolines is much

lower than MTBE blends. It has been reported [42] that the average octane number,

(RON+MON)/2 also abbreviated as (R+M)/2 increases by 2.3 by the addition of 11 volume

percent MTBE to base gasoline having 90 (R+M)/2. Hence, the blending value, (R+M)/2, of

MTBE is 110.9. The addition of 10 volume percent MTBE into gasoline having RON 98.1 and

MON 80.1 increases both the RON and MON by 2–3 points [43-45].

It has been observed that the octane number of a 90 RON base fuel can be increased by

using different concentration of MTBE and secondary butyl [12]. The results clearly showed

that the pure MTBE provide more octane to the gasoline as compared to secondary butanol. A

chart has been formulated comparing the incremental gain of average octane number, (R+M)/2,

in base gasoline resulting from the addition of each volume percent oxygenates including MTBE

[46]. Based on some of the studies [2,12], 15 percent represented a reasonable concentration of

MTBE in gasoline in terms of octane number increase, change in fuel stoichiometry (air/fuel

ratio) and commercial availability of MTBE.

Fuel sensitivity is defined as the difference between RON and MON. It has been reported

that the fuel sensitivity is a function of MTBE blending octane numbers and it increases with

decrease in the blending octane numbers [27]. This has been shown in Figure 1. The high octane

properties of MTBE are particularly effective in blending with low octane unleaded gasoline

components [32]. Supporting this observation, BOV of MTBE is highest in a low octane

unleaded gasoline. For example, MTBE has a blending octane number of 122 when 15 volume

percent is added to 82 octane unleaded gasoline [32].

The boiling point of MTBE is low. For this reason, MTBE provides much higher front end

octane numbers (FEON) to gasolines. FEON is the octane number of gasoline fraction that boils

below 100 oC. It is reported as RON at 100 °C. It becomes important in cold start conditions

when the low boiling parts of gasoline gets a chance to vaporize. When there are no lower

boiling point lead additives to increase FEON, MTBE effectively boost the front end octane.

MTBE gives exceptionally high FEON blending numbers, generally in the range of 135 RON.

The FEON of MTBE is higher than the other gasoline blending components such as butane,

reformate, alkylate and aromatics [34,47]. FEON increases engine efficiency during low speed

acceleration stage.

When MTBE is added to an unleaded gasoline with RON = 88, MON = 81 and RON @

100 oC = 77 [28,30], its FEON is increased drastically. e.g. when 15 volume percent MTBE is

added, the FEON reaches to 93 while the RON and MON increased to 93 and 86 respectively

(Figure 2). The FEON characterizing the knocking during acceleration shows an unparalleled

octane boost. FEON advantage of MTBE has been reported for a gasoline containing 11 volume

percent MTBE for which the average octane number, (R+M)/2, was increased by 8 [48]. MTBE

has a very favorable effect on FEON as compared to refinery low boiling components at IBP–

100 oC, which show considerably lower octane properties. e.g. the gasoline having RON 98.5

has 88.5 FEON as compared to a gasoline containing 10 vol% MTBE which has similar RON

but much higher FEON (95.5).

The effects of MTBE on the antiknock properties of a large variety of gasolines and

gasoline stocks have been reported. Since the improvement of octane number by an MTBE

addition depends on the composition of the base fuel which contains hundreds of components,

accurate values can only be determined by testing the particular gasoline. For this reason, it is

important to know the hydrocarbon composition of gasoline. That 5, 10 and 20 volume percent

MTBE increase the RON of the premium unleaded gasoline from 91.5 to 92.4, 94.0, and 96.2,

respectively [48]. The front end octane quality improvement has been reported for a typical

premium gasoline with 98/99 RON, 50/55 % distillate at 100 oC containing 0.4g Pb/liter in the

form of tetraethyl lead [6].

MTBE does not decrease the lead susceptibility of the lead alkyl compounds, tetraethyl lead

(TEL), tetramethyl lead (TML) or their blends. MTBE is not affected by the lead level in

gasolines as shown in Figure 3. This study gives information on the production possibility of 93

RON gasoline using base gasolines with given RON, MTBE and lead alkyls. It is possible to

produce unleaded 93 RON gasoline using 88 RON base gasoline and 15 volume percent MTBE.

Leaded 93 RON gasoline can also be produced using 88 RON base gasoline, 10 volume percent

MTBE and 0.1 g Pb/liter as TML. The possibility of blending low leaded or lead-free gasoline

of 93 RON using MTBE or lead alkyls can be determined. Since the octane number improving

effect of MTBE concentrated in the low boiling fraction due to its low boiling point, front end

octane improvement of gasolines is increased significantly. The difference between RON and

FEON values drops from 6 to less than 2 when there is 10 volume percent MTBE addition to

this gasoline containing 0.6 g Pb/liter [5].

The TEL response of a typical commercial gasoline containing various amounts of MTBE

has been studied [24]. The RON of a gasoline of RON 92 can be increased to 99 by adding to

vol% MTBE and 0.6 g Pb/liter of gasoline. It has been reported that a reduction in the lead

content of gasoline from 0.6 to 0.15 g/liter will increase the consumption of crude oil in gasoline

production by amounts of 1.73, 2.36, and 4.03 % for gasolines with RON of 94, 96 and 98,

respectively [49]. The use of MTBE permits a more effective utilization of petroleum raw

material in gasoline production, thus increasing the gasoline output by 2.6-4% without increasing

the volume of crude oil processed [50]. High aromatic and low olefinic gasolines reduce the

blending octane value of MTBE [9] .

Road Octane Number (RdON) is difficult to obtain, since it is affected by cars and test

conditions. The general equations of RdON and laboratory measured antiknock properties have

been published in the literature [51]. The European Fuel Oxygenates Association (EFOA)

carried out a RdON performance testing of European unleaded gasoline containing MTBE [52].

Oxygenate blends with methanol, cosolvent and MTBE gave superior road octane performance

under accelerating conditions and at low constant speeds compared to reference gasolines. At

high speeds, 3500 to 4500 rpm, RdON advantage of oxygenates diminished giving similar

performance compared to hydrocarbon only gasolines. This shows that MTBE can increase

gasoline FEON by approximately 4 to 7 numbers in contrast to methanol/cosolvents which have

a FEON increase by 3 to 4 numbers.

Vapor Pressure

In addition to the effect of MTBE on gasoline octane numbers, there are other properties

that MTBE influence the performance of gasolines. Most notable are the Reid vapor pressure

(RVP) and distillation temperatures, which are used to control both hot and cold driveability

performance.

Petroleum refiners have been using increased amount of butanes in the United States.

Butane addition increases both the octane number and RVP of the gasoline. Increased use of

butane in the U.S. is the main reason for a 2 to 2.5 psi increase in the RVP over the last decade

[4]. The U.S. Environmental Protection Agency (EPA) is expected to lower gasoline RVP by 2

in order to reduce the ground level ozone. Table IV is a summary of the blending Reid vapor

pressure of typical gasoline octane components. The blending vapor pressure of MTBE is lower

than typical commercial gasolines [27].

The RVP of MTBE is within the specifications of the gasolines produced by Saudi Aramco

(specification of A-380 gasoline). The RVP of gasoline gradually increases with the addition of

MTBE but remains within acceptable limits (7.11–9.24) [3]. For example addition of 5 to 30

value percent MTBE increase the RVP from 9.20 to 9.24 as shown in Table V. The direction of

RVP change is either up or down, depending on the original vapor pressure of the base gasoline.

Most authors agree that there is only a small butane loss with MTBE and depending upon the

volatility of the base gasoline, butane may be added to the blend to increase the cost effectiveness

of MTBE [39].

Distillation

MTBE is soluble in any ratio with gasoline and, it boils in the same temperature range like

any other light refinery component. Unlike alcohols, its hydrocarbon compatibility feature does

not permit it to create azeotropic effects on the distillation curve of gasoline. A comparison of

distillation data given in Table VI shows that upon addition of MTBE in gasoline, there is

generally a big decrease in the 50% boiling temperature. Addition of more MTBE produced a

further decrease in the 50% boiling temperature. Ten percent of the distillation temperature of

gasoline is not usually affected by addition of MTBE. For this reason, good driveability

performance is maintained in hot weather. The 50 percent temperature of the gasoline is

decreased by addition of MTBE and it usually improve cold engine operation. It has been

reported that addition of MTBE in gasoline has no effect on 10% distillation point, but provide a

decrease of 8, 3 and 30_ C on 50%, 90% and end point (EP) temperature [34,47].

Only butane and MTBE have 50 percent of their temperatures below 93.3 oC but addition

of butane increases the RVP of gasoline. Most of the high octane components produced in a

refinery are usually high boiling point components. Therefore, 50 percent temperature of the

gasoline is more difficult to adjust. ASTM distillation results of the base fuel and the MTBE–

gasoline blends (5, 10, 15 and 20) have shown that there is a decrease in percent boiling

temperature for all blends especially a sharp decrease in 50% boiling temperature [3]. This has

been shown in Figure 4. The distillation characteristics of several gasolines and 7 volume percent

MTBE-gasoline blends were determined and the data were presented. Addition of the 7 percent

MTBE had rather small effects on the gasoline distillation temperatures [53].

The ASTM data reported also show that the addition of MTBE changes the distillation

curves of the base gasolines. Indolene is a standard gasoline used in engine testing. The

distortion of these curves caused by MTBE is very small when compared to the distortion

obtained with alcohol-gasoline mixtures [23]. The effect of adding different amounts of MTBE

to a typical gasoline distillation has been reported [6]. MTBE-gasoline blend curves lie below the

gasoline distillation curve. The greatest distortion in gasoline distillation occurs with methanol

and to a lesser extent, with ethanol. The effect of MTBE is moderately distributed [46]. The

effects of 15 percent MTBE on the distillation characteristics of two different gasolines were

also studied recently [54]. The data for the distillation characteristics of gasolines and MTBE

blends were reported in this article.

Stability

The stability of the gasolines can be evaluated by the formation of peroxides during

storage. Long-term oxidation stability tests of three gasolines with 10 percent MTBE blends

were carried out at storage temperatures of up to 43.3 oC [44,45]. Storage at 43.3 oC for a

period of six months can be considered to be equal to approximately two years of storage at

ambient temperature. The gasoline containing MTBE did not produce any peroxides whereas

gasoline alone yielded substantial amount of peroxides. MTBE gasoline blends could be stored

for a minimum of two years under proper antioxidant protection even when prepared with

unstable LCCG gasolines. Oxidation stability of a gasoline with 10 percent MTBE was

performed according to ASTM D-525 test procedure and found no gum formation in excess of

1000 minutes at 100 °C [34].

It has been reported that MTBE is stable during handling and storage, both as a pure

compound and after addition to gasoline [13]. Storage stability was tested after 180 days [6] and

no significant difference of potential gums were found between the base gasoline (98/99 RON,

15 vol% olefins) and base gasoline plus 15 vol% MTBE. Laboratory studies confirmed that

peroxides are not formed with MTBE [2]. Experiments conducted at 60 psig oxygen and 90 oC

temperature showed no titratable peroxide after 15 hours. An extended test performed over a

period of two years indicated no peroxides formation in the MTBE sample exposed to light and

air.

Water Tolerance

MTBE gasoline blends show no phase separation in distribution systems in the presence of

water. Solubility of water in MTBE (1.5 wt% water) is very low as compared to alcohols [3]. It

has been indicated that water tolerance is not a problem with MTBE-gasoline blends. Clouding

was observed during the course of preparing MTBE-gasoline blends for testing [23]. However,

clouding was cleared entirely after 24 hours and no other problems such as separation and

residue were observed in the samples. Analysis of a reproduced sample showed that the

precipitate was primarily water and MTBE and the amount was very small.

Water tolerance of gasolines has been studied at 20 volume percent MTBE and 10 volume

percent water. The water was settled in the mixture and there was no haziness. MTBE losses

due to water contact were negligible (200 to 300 ppm in the water). Phase separation in MTBE-

gasoline blends is not expected to cause problems as in the case of alcohol-gasoline blends [3].

The water solubility of gasolines containing MTBE and secondary butyl alcohol has been studied

[12]. The water solubility of these gasolines is increased considerably with the increase of

secondary butyl alcohol concentration in the blends [12]. There is no hazing problem in a 15

percent MTBE-gasoline blend up to 300 ppm water [35]. Considering the commercial MTBE

containing less than 500 ppm water, a 15 percent MTBE-gasoline blend containing normally 75

ppm water is well below hazing condition. The water holding capacity of MTBE is very high.

e.g. A gasoline containing 15% MTBE was found to have 520 ppm of water compared to 190

ppm water for a gasoline sample having no MTBE. MTBE-gasoline blends are not as

hygroscopic as alcohol-gasoline blends [66].

IMPORTANCE OF MTBE AND LEAD PHASEDOWN

Regulations regarding reduction in the use of lead in gasolines have had significant impact

on the octane requirements of the gasoline pool. As a result, MTBE received worldwide interest

to supplement refinery-octane quality. The use of MTBE has several advantages. These are high

blending octane number, improvement of engine efficiency low speed acceleration, solubility in

hydrocarbons, No additional precautions in the existing gasoline distribution system, insensitive

to lead level in the gasoline, water tolerance, gasoline like toxicity, no adverse health effects,

storage stability, no unusual problems in driveability, vapor lock tendency, fuel consumption,

corrosion and fuel system material compatibility, reduction of catalytic reforming severity and

reduction of carbon monoxide and hydrocarbon emissions.

Saudi Basic Industries Corporation (SABIC) predicts that demand for MTBE could account

for as much as 10 percent of the gasoline pool by 1995 [55]. Worldwide growth of MTBE

capacity will average more than 20 percent per year during 1989-94 [56,57]. MTBE appears is

one of the most economical ways to add octane while accomplishing environmentally desirable

goals [58].

Industrialized countries regulate the lead content in gasolines. In the United States, the

Environmental Protection Agency (EPA) called for a lead reduction in gasoline beginning from

1973 [7,59,60]. In early March 1985, the EPA ordered the reduction in gasoline lead content to

0.5 g lead/gal by July 1, 1985 and 0.1 g lead/gal by January 1, 1986 [61]. Addition of lead was

regulated by EPA according to the schedule given below:

Step 1. Late 1973, 0.45 g/liter (1.5 g/gal)

Step 2. Mid 1974, introduction of unleaded gasoline

Step 3. July 1, 1985, 0.13 g/liter (0.5 g/gal)

Step 4. January 1, 1986, 0.03 g/liter (0.1 g/gal)

Step 5. January 1, 1988, 0.01 g/liter (0.04 g/gal)

Step 6. After 1990, total elimination of lead in gasolines.

The European Economic Community (EEC) is also phasing out lead. The tendency

towards the reduction of maximum lead content in gasoline is shown in Table VII [6]. EEC

requirements approved in June, 1985 [62,63] are (1). All EEC member states must make

unleaded gasoline available in their territory by October 1989 and (2). All member states are

invited to reduce maximum limit on lead to 0.15 g/liter by October 1989. Western Europe plans

to eliminate the use of lead, but the timing is uncertain. This will probably be completed by the

turn of the century [62]. No leaded gasoline is being used in Japan now [17,64]. Only unleaded

92 RON grade gasoline is sold in Japan.

The gasoline currently produced in Saudi Arabia is 95 RON. The average lead level in

Saudi gasoline can be reduced from 0.60 to 0.37 g/liter only by operational changes and to 0.25

g/liter by MTBE blending [7]. Table VIII shows the ability of Saudi refineries to reduce lead

content. In this table, Riyadh and Jeddah refineries are considered to produce regular (83 RON)

gasoline in addition to premium (95 RON) gasoline. Regular grade represents 15 percent of the

total gasoline market.

Table IX shows the lead levels when production of regular grade gasoline is eliminated.

The average lead level, in this case, will increase from 0.37 g/liter to 0.40 g/liter with operational

changes and from 0.25 g/liter to 0.31 g/liter with MTBE blending. The lead phase down schedule

for gasolines in the U.S. is much more advanced than in Western Europe. The reason for this is

the application of catalytic converters in the

U.S. in a short period of time. This movement brought increased demand for unleaded gasoline.

In Western Europe, automobiles with smaller engines can satisfy emission standards without

catalytic converter systems [65].

CONCLUSIONS

The improvement of blending properties by the addition of MTBE depends upon the composition of the base gasolines which contain hundreds of components in different concentrations. MTBE blending octane number generally rises with a decrease in the octane number of base gasoline, a decrease in MTBE concentration of gasoline blends, and an increase in saturated hydrocarbons of base gasoline. MTBE provides much higher front end octane numbers (FEON) to gasolines. FEON increases engine efficiency during the low speed acceleration stage. MTBE does not decrease the lead susceptibility of lead alkyl compounds, tetraethyl lead (TEL), tetramethyl lead (TML) or their blends.

MTBE has little effect on the distillation and vapor pressure characteristics of gasoline. There is no evidence of significant azeotrope formation, as is the case when alcohols (methanol or ethanol) are blended with gasolines. Pure MTBE forms azeotropes with water, but no MTBE–gasoline azeotropes have been reported. MTBE–gasoline blends, even in the presence of water, show no separation problems in the distribution system. Phase separation problems cause driveability problems in addition to corrosion. MTBE is stable during handling and storage. No difference in potential gums was found between base gasoline and MTBE blends after an extended storage.

MTBE appears to be the most economical way to increase octane number while accomplishing environmentally desirable goals. The average lead content in Saudi Arabian gasoline is 0.72 g/liter with only premium gasoline production. The United States and Japan eliminated lead in gasoline after 1990. The EEC is also reducing lead to 0.15 g/liter after 1990 with the exception of Spain, Portugal and Greece where the lead content was reduced to 0.40 g/liter.

Limits on the amount of oxygen allowed in MTBE blends are 2.0 wt% (11 vol% MTBE in the blends based on 0.737 average specific gravity gasoline) according to the 1981 EPA rule in the United States and 2.5 wt% (14 vol% MTBE in the blends) according to the EEC directives in Europe. It is unlikely that more than 3.5 wt% oxygen limit for ethers will be approved, because nitrogen oxides increase beyond this limit.

ACKNOWLEDGEMENT

The authors wish to acknowledge the support of SABIC and Research Institute of the King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia for this work under Project No. 21097.

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FIG. 1. Effect of base fuel sensitivity on the blending octane numbers of MTBE.

FIG. 2. Range of octane number improvement by adding MTBE to an unleaded gasoline.

FIG. 3. Lead susceptibility of MTBE-gasoline blends (production of 93 RON gasoline using MTBE and TML.

FIG. 4. Effect of MTBE blending on boiling range distribution of gasoline A-380 produced by Saudi Aramco.

TABLE I

Physical, Chemical and Thermal Properties of MTBE.Molecular weight 88.15

C/H/O contents, wt% 68.1/13.7/18.2

C/H ratio 5.0

Density, g/cm3

@ 15/4 oC 0.7456

@ 20/4 oC 0.7404

@ 25/4 oC 0.7352

@ 30/4 oC 0.7299

Reid vapor pressure, psi

@ 25 oC 4.7

@ 37.8oC 7.8

Boiling point, oC 55.0

Freezing point, oC -108.6

Vapor density, calculated, (air = 1), g/cm3 3.1

Solubility @ 25 oCMTBE in water, wt% 5.0Water in MTBE, wt% 1.5

Viscosity @ 37.8 oC, cSt 0.31

Stoichiometric air/fuel ratio 11.7

Refractive index @ 20 oC 1.3694

Surface tension, din/cm2 19.4

Latent heat of vaporization, Cal/g, @ 25 oC, 81.7

Specific heat, Cal/g. oC, @ 25 oC, 0.51

Lower heating value, Cal/g 8,400

Flammability limits in AirLower limit, vol% 1.65Upper limit, vol% 8.4

Auto ignition temperature, oC 435

Flash point, closed cup, oC -25.6

TABLE II

Octane Improvement by the Addition of MTBE to Base Stocks.Base Gasoline Composition(vol%) A B C

Straight run light gasoline 10 – –C5-C6 Isomerate – 50 –Reformate, 86/87 RON 90 -- -Heptanes plus reformate, 94 RON – 50 –Reformate, 94/95 RON – – 59.6Light cat, crack gasoline – – 22.9Heavy cat, crack gasoline – – 6.1C3-C4 alkylate – – 11.4

PropertiesRVP, psi 5.26 7.96 4.55

Specific gravity @ 15/4 oC 0.751 0.740 0.749

% distilled at 70 oC 14 36 21

% distilled at 100 oC 50 55 53Olefins, vol% – – 12Aromatics, vol% 40 36 34Lead content nil nil nilMTBE Addition(vol%) 0 5 10 15 Gasoline A RON 84.6 87.0 88.9 90.8

_ RON - 2.4 4.3 6.2MON 79.0 80.6 82.4 83.8_ MON – 1.6 3.4 4.8Sensitivity 5.6 6.4 6.5 7.0

Gasoline B RON 90.5 92.2 93.7 95.2_ RON – 1.7 3.2 4.7MON 83.0 84.0 85.1 86.4_ MON - 1.0 2.1 3.4Sensitivity 7.5 8.2 8.6 8.8

Gasoline C RON 93.7 94.9 96.0 97.2_ RON - 1.2 2.3 3.5MON 84.0 84.6 85.4 86.5_ MON – 0.6 1.4 2.5Sensitivity 9.7 10.3 10.6 10.7

TABLE III

MTBE Addition to Reformate and Gasolines.Base Stock Base MTBE MTBE Blends(Composition Stock Addedvol%) RON vol% RON MON (R+M)/2

Reformate 90 5 92.0 87.2 89.6*P=41.0 10 93.5 89.5 91.5 N=2.0 15 96.0 90.5 93.3

A=57.0 20 97.5 91.5 94.5

A-380 Blend 83.7 5 85.6 83.1 84.4(Unleaded) 10 88.0 84.0 86.0P=59.7 15 89.6 84.9 87.3N=2.4 20 91.5 86.1 88.8A=37.9 30 95.5 90.2 92.9

A-380 Blend 87.0 5 89.5 83.5 86.5+ 0.15g Pb/liter 10 92.6 86.2 89.4

15 95.0 89.5 92.3

A-380 Blend 89.5 5 91.6 85.9 88.8+0.28 g Pb/liter 10 95.5 89.7 92.6

A-380 Blend 90.2 5 92.5 87.5 90.0+ 0.40 g Pb/liter 10 97.0 90.5 93.8* P= Paraffins, N= Naphthenes and A= Aromatics.

TABLE IV

Vapor pressure of Typical Octane Boosters.BlendingRVP(psi)

Normal butane 60.0

Isopentane 21.0

Normal pentane 14.0

Commercial gasoline 11.5

Base gasoline (No butanes) 8.5

TABLE V

The RVP of MTBE and MTBE-Gasoline Blends.

Base fuel A-380 + MTBE 5 % 10% 15 % 20 % 30 %

MTBE MTBE MTBE MTBE MTBE

RVP, psi 8.70 9.20 9.20 9.20 9.23 9.24

TABLE VI

Effect of MTBE Addition on ASTM Distillation Curve.% Evaporated Base Base fuel Base fuel

Fuel +5% MTBE +10% MTBE

(°C) (°C) (°C)IBP 29 27 27

10 45 45 4420 58 58 5650 108 104 9670 147 144 14790 191 189 18895 206 205 204EP 218 217 217

TABLE VII

Maximum Lead Content (g/liter) and RON Values in European Countries. 1980 1990 Lead RON Lead RON

Norway Premium 0.40 98 0.15 96

Regular 0.15 90 0.15 91

Denmark Premium 0.40 97 0.15 96

Regular 0.40 91 0.15 91

Sweden Premium 0.40 99 0.15 96

Medium 0.40 97Regular 0.15 90 0.15 91

Finland Premium 0.70 98 0.15 96

Regular 0.45 91 0.15 91

Netherlands Premium 0.40 98 0.15 96

Regular 0.40 91 0.15 91

Belgium Premium 0.45 98 0.15 96

Regular 0.45 90 0.15 91

W. Germany Premium 0.15 98 0.15 96

Regular 0.15 91 0.15 91

Switzerland Premium 0.40 98 0.15 96

Regular 0.15 90 0.15 91

Austria Premium 0.40 96 0.15 96

Regular 0.40 86 0.15 91Four stars 0.45 97 0.15 96

UK Three Stars 0.45 94 – –Two stars 0.45 90 0.15 91

France Premium 0.50 97 0.15 96

Regular 0.50 91 0.15 91

Italy Premium 0.64 98 0.15 96

Regular 0.64 84 0.15 91

Spain Premium 0.60 96 0.40 96

Regular 0.48 90 0.40 91

Portugal Premium 0.64 98 0.40 96

Regular 0.64 85 0.40 91

Greece Premium 0.50 96 0.40 96

Regular 0.50 90 0.40 91

TABLE VIII

Capacities of Saudi Refineries to Reduce Lead Contents with Existing Structure.

Lead content, g/literRefinery Production Lead Operational With MTBE

(Barrels/day) content changes Blending

Ras Tanura 64900 0.84 0.60 0.42Jeddah 13600 0.84 0.74 0.28Riyadh 43400 0.30 0.10 –Yanbu 37600 0.49 0.15 0.12Total: 159500 0.60 0.37 0.25

TABLE IX

Capacities of Saudi Refineries to Reduce Lead Contents with only Premium Gasoline Production.

Lead content, g/literLead Operational With MTBE

Refinery content changes Blending

Ras Tanura 0.84 0.60 0.42

Jeddah 0.84 0.35 0.15Riyadh 0.72 0.33 –Yanbu 0.49 0.15 0.12Total: 0.72 0.40 0.31