Until the 1970s there was little or no use of additives in automotive diesel fuel. The product manufactured at most refineries around the world was generally a blend of straight run atmospheric distillate components and, apart from sulphur content, the specification points could be met without the need for further processing or the use of additives. In the U.S., where the enormous gasoline market had necessitated a high level of downstream conversion to yield more gasoline components, some cracked gas oils went into diesel fuel.
Routine use of diesel fuel additives effectively started in the late 1960s in Europe, with the introduction of cold flow improvers. With the largest proportion of diesel-powered road vehicles of any world region, the growth in demand for diesel fuel was starting to pose problems for the refining industry. The supply situation was further aggravated by the crude oil price rises during the 1970s. Although total demand for petroleum products went down, refiners had to increase the yield of diesel fuel while reducing crude throughput. The use of flow improvers enabled the refiner to produce more diesel fuel by cutting deeper into the crude oil and using the additive to restore the cold properties of the fuel.
Other additive types are now being used in diesel fuel, as more refineries have been obliged to move towards the typical pattem in the U.S., with downstream conversion units to increase the yield of “clean” products by cracking the fractions used for heavy fuel oil, for which there is a decreasing demand.
More low-cetane material is being diverted into automotive diesel fuel because it can no longer be absorbed by the shrinking market for domestic heating oil. This necessitates occasional use of an ignition improver to bring the cetane number on specification.
An additional factor influencing the trends in additive use is a growing awareness of the need for fuel product differentiation in the market. lt is common practice in many countries for oil companies to exchange and re-brand products to keep down the costs of fuel transportation, the exchanged product being accepted on the basis of an agreed specification and marketed as such. Nowadays, further additive treatment may be made before an exchanged fuel is sold, in order to support the marketing company's advertising claims for a product of superior quality to those of its competitors. This practice has been widely adopted in Europe and other parts of the world.
Additive treatment of diesel fuels is usually by weight and expressed either in parts per million (ppm) or as a percentage, where 0. 1 % is equal to 1 000 ppm.
When formulating a package in the laboratory, the most viscous product is added first, followed by the next most viscous. The solvents are added last. This is done for practical reasons. If too much of a viscous product is added, it can easy be removed. If solvent is added first, then too much of a viscous product is added, the chemist has to start again. However, in manufacturing, solvent would be added first (a).
Antifoams are used in most diesel additive packages to help speed up or to allow more complete filling of vehicle tanks. Their use also minimizes the likelihood of fuel splashing on the ground or onto clothing, avoiding the nuisance of stains and unpleasant odour, and reducing the risk of spills polluting the ground and the atmosphere (2). Diesel fuel packages do not appear to be used by supermarkets selling diesel fuel because it increases the cost of the fuel.
Antifoams are dosed at a rate of 1-4% in packages, with the smallest amount used in winter and the largest in summer(d). In testing our antifoams, we used 7.5 ppm to 30 ppm calculated on diesel fuel. The "dilution effect" is not an important consideration because most packages only contain about 2% antifoam. Therefore, the dilution effect is already present. The antifoam package is dosed in diesel fuel at about 800 ppm.
Antioxidants & Stabilisers:
These are free radical scavengers. They are sterically hindered phenols and they are used to interrupt the radical oxidation process.
Antioxidants used in diesel fuels are usually hindered phenols that prevent high-temperature gum-forming reactions. Stabilizers are amines or other nitrogen-containing basic compounds that prevent sediment formation at ambient temperature by interfering with acid/base reactions. These additive types are not normally used in diesel fuel prepared from straight-run components but, if it contains cracked gas oil, protection may be desirable, especially if the fuel is likely to be in storage for a lengthy period.
The same types of antioxidant are used in diesel fuels and gasolines to prevent high-temperature reactions, but stabiliser additives are more specific in their action and need to be selected to suit the particular fuel to be treated.
Antioxidant/stabiliser additives react with peroxy radicals in unstable fuel, thereby suppressing the radical propagation reaction that would normally occur.
The effectiveness of additive treatment will depend very much on the dominant fuel characteristics that determine the degradation reactions. The choice of additive is generally decided by trial-and-error to find out which is best for the particular fuel. Treating levels are usually in the range of 25 to 200 ppm(2).
Cetane improvers are used to increase the cetane number of a diesel fuel by reducing the delay between injection and ignition when fuel is sprayed into the combustion chamber. Several types of chemicals - alkyl nitrates, ether nitrates, nitroso compounds and certain peroxides - have been identified as effective cetane improvers. They are all materials that decompose readily and, at elevated temperatures, generate free-radicals that accelerate oxidation of the fuel and initiate combustion (2). Commercial and safe handling considerations have resulted in most attention being given to primary alkyl nitrates. However, the RCEP believes that alkyl nitrates may contribute to the formulation of nitrogenated PAH’s which are known carcinogens.
In refineries, cetane improvers are used mainly to give fairly modest improvements of 2 or 3 numbers, to bring off-grade fuel blends in specification. This would require additive treat levels in the 500 to 1000 ppm range. This type of additive is used in some multifunctional additive packages, where the proportions will give a cetane improver treat level of around 500 ppm.
Some multifunctional additive packages contain corrosion inhibitor to protect the vehicle fuel system. Corrosion inhibitors are surfactant materials having a polar group at one end and an oleophilic/hydrophobic group at the other. The polar group attaches itself to metal surfaces in the system, while the other group repels water and provides an oily layer to prevent rust formation. Some corrosion inhibitors contain chlorine, while some do not as chlorine is banned in some countries. There is a possible relationship between corrosion inhibitors and demulsifiers such that more demulsifier has to be used in chlorine containing corrosion inhibitors. A wide range of chemical types are used as anticorrosion additives. They include esters or amine salts of alkenyl succinic acids, alkyl orthophosphoric acids, alkyl phosphoric acids and aryl sulphonic acids(2).
2-ethylhexanol is used as a co-solvent to dissolve some antifoams before putting them into the package. If too much needs to be used, then the finished product is priced out of the market. The use of 2-ethylhexanol could be a problem at some companies because it is considered to be toxic.
Dehazer treatment may occasionally be needed if the fuel becomes hazy due to the presence of finely dispersed droplets of water. Contamination with water can occur at almost any stage, as the fuel passes from the refinery and through the distribution network until it reaches the vehicle tank. It can be the result of dissolved water coming out of solution or condensing from the air when the temperature falls, leakage of rain water into the tank, or entrainment of water accumulated in storage tank bottoms. The situation may be aggravated by the characteristics of the fuel or the type of additives it contains, and by excessive turbulence in the pumping system.
If the haze persists after the normal 1 or 2 days settling time, additive treatment may be necessary to accelerate clearance and meet the usual “clear and bright” requirement. Effective dehazer additives include quaternary ammonium salts, typically used at dose rates between 5 and 20 ppm.
As hazy fuel tends to be a spot problem, the practical approach is for alternative additives to be tested on-site, in cold samples drawn directly from the affected tank. Samples taken away for testing will usually have cleared by the time they reach the laboratory because of a temperature change or contact with the sample container.
A demulsifier may be included when detergent/dispersant additives are used, to avoid problems due to pick-up of storage tank bottoms. Entrainment of water and debris in pipelines and during product transfer might result in the formation of stable emulsions and suspended matter which could plug filters or otherwise make the fuel unacceptable(2).
Demulsifiers are highly surface-active chemicals selected for their limited solubility in oil and water. They are usually prepared by reacting a hydrophobic molecule such as a long chain alkylphenol with ethylene or propylene oxide. The effectiveness of different additive types and treat rates can be checked using the 10-cycle multiple contact test in which a small amount of water is successively agitated with ten portions of fuel to represent repeated filling and emptying of a storage tank. The assessment is based on the amount of emulsion and suspended matter at the oil/water interface. Typical treating levels are generally not more than 10 ppm(2).
Detergents (surfactants, dispersants):
Detergent additives are considered to be of growing importance in controlling the formation of fuel deposits where they can have a detrimental effect on combustion. Gummy deposits in the fuel injection system can cause sticking of injector needles, resulting in misfires, power loss and increased smoke. The build up of lacquer and carbonaceous deposits on injector tips can affect the amount of fuel injected and the spray pattern, causing problems of reduced power and higher smoke. Starting may also become more difficult.
Detergents for diesel fuels are of the same chemical type as those used in gasolines - amines, amides, imidazolines, etc. These are surfactant additives with a polar group at one end that forms a barrier film on metal or particulate surfaces while a nonpolar, oleophilic group at the other end dissolves in the fuel. Particulates are effectively solubilised and prevented from agglomerating by the film formed around them. In the same way, metal surfaces are protected against deposit formation. The function of the detergent additive also gives some antirust protection. Polymeric dispersants are sometimes used in conjunction with detergents to help in the dispersion of particulate matter.
The choice of additive type and treat rate will be determined by the characteristics of the fuel and also whether the requirement is “keep clean” or “clean up” performance. A higher treat rate or a more effective additive is usually needed to clean up dirty injectors. Treat rates to control deposits on new or cleaned injectors are in the 100 to 200 ppm range.
Polymeric dispersants are often used to complement the role of the detergent additive. Chemically the dispersants are relatively high molecular weight materials, generally either alkenyl succinimides or hydrocarbyl amines. Recommended treat rates are around 200 ppm for the succinimides and up to three times that amount for the hydrocarbyl amines(2).
Flow improvers are used to improve the flow of fuel especially in winter. They are usually added at a rate of 50 to 700 ppm. In diesel fuel wax crystals form macrocrystalline structures on cooling that permeate through the whole of the fuel, causing it to form a solid gel. Flow improvers, such as ethylenevinylacetate copolymer, are designed to co-crystalize with the paraffin wax on cooling, disrupting the crystalline structure of paraffin wax.. The effect is that the wax crystals remain very small and well dispersed within the distillate fuel. That means diesel remains fluid to temperatures below the original operability and pour point temperatures(a).
Lubricity additives are acid type components such as alkenyl carboxylic acids and amino esters. The process of reducing the amount of sulphur in diesel fuel to 0.05% removes other polar compounds which had lubricating properties. This has lead to increased wear in injection systems with significant deterioration in fuel pump performance resulting in poor drivability, increased emissions and, in some cases, pump failure. Lubricity additives are therefore used to replace the lubricating properties and prevent wear in the diesel fuel pumps.
As the odour of diesel fuel is considered objectionable by many people, odour masks are occasionally employed to improve the market acceptability of some branded diesel fuels. Because diesel fuel is less volatile than gasoline, the stain and smell of spills will persist, which can be very annoying, particularly if clothing is contaminated. Attitudes to smells vary widely and are subjective, but odour panel tests suggest that the market preference is for a neutral rather than a positive odour, which puts the emphasis mainly on odour-masking effects. Various products, with a choice of fragrances, are commercially available for use at treating rates of 1 0 to 20 ppm(2).
Solvents are used in all packages to dissolve the ingredients.