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Motor gasoline is one of the key products obtained from a petroleum (crude oil) refinery. It is a complex mixture of light hydrocarbons containing 5 to 10 carbon atoms and having a boiling range of 40 oC to 190 oC.
Gasoline at the consumer level is called petrol, benzol, motor spirit or gas, depending on the country where it is sold. At the refinery level, in early stages of crude oil processing, most gasoline components are called naphthas. To be precise, modern reformulated gasoline is a blend of several refinery streams namely Catalytic Reformate, Straight Run Naphtha(SRN), FCC Gasoline, Visbreaker / Coker Naphtha, Isomerate, Alkylate, Oxygenate etc. The product application and customer acceptance set detailed specifications for various gasoline properties which in turn determine which refinery streams are suitable for a specific blend.
Today most petroleum refineries are facing the challenge of producing motor gasoline having all the desirable properties and also comply with the ever increasing environmental regulations & health restrictions on automotive emissions. The environmental regulations were created to guard against high levels of lead, aromatics (benzene in particular), olefins, and sulfur in the gasoline. Reduction in volatile organic compounds (VOCs), toxic and nitrogen oxides (TOx & NOx) in automotive tailpipe emissions, and evaporative and refueling emissions have also assumed high priority. The European Union had announced its dedication to lower the sulfur content of gasoline to 50 ppm by 2005. California and Germany are both contemplating a move to a 10 ppm sulfur specification in the near future.
Novel technological options are evolving that will improve the quality of gasoline pool streams. The focus is on the two most important refinery streams: FCC gasoline and catalytic reformate. FCC gasoline is a major contributor of sulfur and olefins while reformate is the main source of benzene and aromatics.
We'll explore the properties of gasoline, their impact on the environment, and options for improving the quality of the streams.
Impact of Gasoline on the Environment
The impact of gasoline on the environment is directly related to its fuel properties and contents. Table 1 gives a brief account of gasoline properties, their desirable impact on engine performance and their undesirable impact on the environment.
Table 1: Summary of Gasoline Properties
BIS(Bureau of Indian Standards ) , WWFC (World Wide Fuel Charter) , Jan ,
2000 & EURO III.
In view of strong environmental restrictions on gasoline , the main thrust of technological research has been in the field of finding different options for:
Options for Lead Phase and Octane Enhancement
In atmospheric distillation
(Fig 1), the fractions identified as light naphtha, medium naphtha and heavy naphtha are
the potential gasoline components.
Toward Total Lead (TEL) Phase Out
In 1922, Thomas Midgley Jr. discovered that, when added to gasoline in small quantities, Tetra Ethyl Lead (TEL), is an excellent anti-knock material (see Table 2 below). Alkyl lead added to gasoline interferes with hydrocarbon chain branching in the intermediate temperature ranges where HO2 is the most important radical species. Lead oxide, either as solid particles, or in the gas phase, reacts with HO2 and removes it from the available radical pool. This deactivates the major chain branching reaction sequence that results in undesirable, easily-auto ignitable hydrocarbons and thus prevents knocking. But if you keep adding alkyl lead compounds, the lead response of the gasoline decreases, and so there are economic limits to how much lead should be added.
From 1922 until the 1970s, TEL had
been added to conventional gasoline for octane boosting. During the 1970s in the
United States, catalystic emission control systems were fitted into new cars to reduce
Another drawback to TEL addition was that the lead additive decomposed and was emitted as unconverted particulate lead. In fact gasoline combustion accounted for 94.8% of atmospheric lead emissions. These lead emissions were toxic and caused slow poisoning of the nervous system, mental retardation in children and other health related problems.
Over the past three decades, many countries, have passed legislation to decrease the amount of lead emissions with the target of elimating it because of potent health problems in the population. In the U.S. in 1973, the Environmental Protection Agency (EPA) ordered a gradual reduction in gasoline lead content with a total lead phase out by 1990. As per WWFC lead content has to be lowered to non-detectable levels. At the same time, demand for higher-octane gasoline increased with the development of more efficient automobile engines.
Other Options for Octane Enhancement
In view of lead phase out schedules adopted, various options for octane enhancement have been explored.
Table 4: Options for Octance Enhancement
Typical modern refinery processes for producing gasoline blending components are given below:
Among all the options for lead phase out, Catalytic Naphtha Reforming and Fluidized Catalytic Cracking have been the most commonly employed processes in refineries to provide gasoline blending high-octane components.
Options for Benzene and Aromatics Control
Benzene is one of the
key aromatic hydrocarbons desirable in gasoline for its high octane (~100). It is
present in automotive evaporation and refueling vapors as well as in their exhausts.
The positive octane impact has been offset by its tendency to induce severe health
hazards as it is a known carcinogenic.
In view of such severe health impact, the European Union has set the safe limit of benzene in air at 10 ppm. To attain such low benzene levels, the World Wide Fuel Charter (WWFC), Jan 2000 has set the maximum limit of benzene content in gasoline at a mere one percent.
Because 60-70 percent of the total benzene in gasoline is produced in the reforming unit, benzene reduction in the reformate has a major effect in meeting the specification imposed on the component.
Refineries world wide have adopted three basic approaches for benzene reduction:
Pre-fractionation of benzene precursors combined with low pressure reformer operation ( < 100 psig ) will usually produce less than 1 % vol benzene in the reformate regardless of the feed composition.
The WWFC has proposed a restriction of the total aromatic content in gasoline to about 35 % vol . Aromatics are the most desirable hydrocarbons for octane improvements but they are also responsible for disproportionate amounts of CO and HC exhaust emissions that lead to higher levels of smoke and smog.
The aromatics in reformate from Catalytic reforming processes cannot be reduced below certain levels without compromising octane levels. The development of new reforming catalysts, which isomerise the lighter C6 and C7 hydrocarbons to the branched isomers with higher octane numbers, are very much in demand.
FCC gasoline is another major contributor of aromatics to the gasoline pool as it contains about 10-20% by volume aromatics on average. Refiners have several options for lowering aromatics in FCC gasoline: process more naphthenic or paraffinic feedstocks; lower unit conversion ; undercut full distillation range gasoline. Undercutting is probably the most desirable choice because the FCCU can then be run to maximize desirable light olefins (C3=, C4=, and C5=) while controlling aromatics. The light olefins can be converted to excellent blending components for gasoline, e.g. polygasoline, dimate, alkylate, MTBE, tertiary butyl alcohol (TBA), ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME), and tertiary amyl ethyl ether (TAEE). Isobutylene and isoamylenes are the olefins in greatest demand. They are made into MTBE and TAME, the preferred oxygenate blending components for gasoline. This is called the "aromatic substitution effect".
Oxygenates are just pre used hydrocarbons having structure that provides a reasonable antiknock value, thus they are good substitutes for aromatics. For example, MTBE (Methyl Tertiary Butyl ether) works by retarding the progress of the low temperature or cool-flame reactions, consuming radical species responsible for uncontrolled combustion and thus reduce knocking. Also, as they contain oxygen, fuel combustion is more efficient, reducing hydrocarbons in exhaust gases. The only disadvantage is that oxygen in the fuel cannot contribute energy; consequently the fuel has less energy content. For the same efficiency and power output, more fuel has to be burned. Oxygenates are also being evaluated for carcinogenicity, and even ethanol and ETBE may be carcinogens.
Options for Sulfur and Olefins Control
Sulfur in the fuel contributes to corrosion, and when combusted, will form corrosive gases that attack the engine, exhaust and environment. Sulfur also adversely affects the alkyl lead octane response and will adversely affect exhaust catalysts. However, monolithic catalysts will recover when the sulfur content of the fuel is reduced, so sulfur is considered an inhibitor rather than a catalyst poison.
In view of the undesirable effects of sulfur, various options have been explored to reduce sulfur concentrations to undetectable levels. The sources of sulfur contribution to gasoline include mainly; FCC gasoline ,Visbreaker / Coker naphtha, and straight run naphtha from high sulfur crude.
As FCC gasoline generally makes up 40-60% of the total gasoline pool, it contributes as much as 85-95% of the sulfur in motor gasoline and 100% of the olefins. The sulfur in FCC gasolines in mainly concentrated in the heavier fractions in the form of compounds such as thiophenes, benzo thiophenes, etc. The lighter fractions contain primarily mercaptans. Mainly, there are three options for sulfur reduction in FCC gasoline:
Olefins can also be removed by selective saturation and by isomerisation processes. With suitable alkylation reactions olefins can be converted to high octane alkylates.
Options for Reduction in Tail Pipe and Evaporative Emissions
Automobiles emit several pollutants as combustion products out the tailpipe (tailpipe emissions) and as losses due to evaporation (evaporative emissions, refueling emissions). The volatile organic chemical (VOC) emissions from these sources, along with nitrogen oxides (NOx) emissions from the tailpipe, will react in the presence of ultraviolet (UV) light (wavelengths of less than 430nm) to form ground-level (tropospheric) ozone, which is one of the major components of photochemical smog. Also from the auto exhausts additional CO2 is added to the atmospheric burden. More and more scientific evidence is accumulating that global warming is occurring due to the effect of the additional CO2 in the global environment.
Along with the tail pipe emissions, the hydrocarbons produced by evaporation of the gasoline during distribution, vehicle refueling, and from the vehicle, have become more and more significant. A recent European study found that 40% of man-made volatile organic compounds came from vehicles. The health risks to service station workers, who are continuously exposed to refueling emissions remain a concern .
Exhaust catalysts have offered a post-engine solution that could ensure pollutants are converted to more benign compounds. As engine management systems and fuel injection systems have developed, the volatility properties (Reid vapor pressure) of the gasoline have been tuned to minimize evaporative emissions, and yet maintain low exhaust emissions.
The design of the engine can also have very significant effects on the type and quantity of pollutants. Unburned hydrocarbons in the exhaust originate mainly from combustion chamber crevices, such as the gap between the piston and cylinder wall where the combustion flame can not completely use the HCs.
In a nutshell, the type and amount of unburned hydrocarbon emissions are related to the fuel composition (volatility, olefins, aromatics, final boiling point), as well as state of engine condition, tuning and condition of the engine lubricating oil.
Future automoblies will be required to trap the refueling emissions in large carbon canisters. Carbon canister systems can reduce evaporative emissions by 95% from uncontrolled levels.
Conclusions1. Options are available to meet new gasoline specifications. The solution will be site specific depending upon the refinery configurations and governmental regulations.
2. Besides considering operational changes in Catalytic Reforming & FCC, the technologies to be considered for the future include:
Alkylation : Alkylate is an excellent blending component for a high octane component free of lead, benzene and olefins.
Isomerisation : Isomerate is a sulfur free, high octane, non-aromatic gasoline blending stock.
Selective gasoline desulfurisation / Olefins saturation processes.
Oxygenate blending : Methyl tertiary butyl ether (MTBE) and Tertiary amyl methyl ether (TAME) are main oxygenates compounds used in gasoline in addition to their high octane numbers .
To summarize, the drive towards making Green Gasoline is full of challenges. A joint global effort by refiners, automobile manufacturers and governments can alone provide the desired momentum to make gasoline a really clean burning, non-toxic, and non-polluting fuel and pave the way for cleaner and safer motoring in this "environmental" millennium.
References:Little , Donald M ; Catalytic Reforming , Penn Well Publishing Company,Oklahoma ,1985, ISBN 0-87814-281-9 .
George J. Antos ,Abdullah M. Aitani , Jose M. Parera ; Catalytic Naphtha Reforming , Marcel Dekker Inc., New York ,1995, ISBN 0-8247-9236-X
A.J.Pahnke and W.E.Bettoney , Role of Lead Antiknocks in Modern Gasolines, SAE Paper 710842 (1971) 32p.
I.S. Al-Mutaz , How to implement a gasoline pool lead phase down , Hydrocarbon Processing , February 1996 , 63-69p.
Michela montesi , Filippo Trivella , Alessandro Brambilla , Massimiliano Dell Agnello , Luciano Paolicchi ; Reduction of benzene precursors in the reformer feed , Petroleum Technology Quarterly , Winter 1998/99 , 107-111p.
H.L.Hoffman , Petroleum and Its Products , Riegels Handbook of Industrial Chemistry.
Indian Institute Of Petroleum , Lecture Notes.
Reduction of Sulfur, Aromatics, and Olefins from the Gasoline Pool - A Review of Technological Options, M. Bhaskar, G. Valavarasu, V. Selvavathi, B. Sairam, Chennai Petroleum Corporation Ltd. (R & D Centre)
Case Study Petroleum Modern Refining http://www3.cems.umn.edu/~aiche_ug/history/h_toc.html