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Depressor Additives. The first suggestions about the mechanism in which depressor additives affect fuel were first made in the 1930s-1950s. The modern theory explains the efficiency of the depressors by the presence of n-akanes in the fuel. These components crystallize when the temperature drops. These crystals cause clouding of diesel fuel. The crystals grow in size. At a certain point, a spacial framework forms. The fuel loses fluidity and cannot be pumped through filters and piping.

If a pour point depressor is mixed into the fuel, the process of crystal formation changes. The additive accumulates on the surface of the crystals, preventing their further growth. However, the exact nature of the process is not entirely known. There are two competing theories. One suggests that co-crystallization of paraffin and the additive occurs, integrating the depressor molecule (its neutral part) in the crystal. The polar part of the molecule remains outside the crystal and prevents other paraffin molecules from attaching to the crystal. The other theory stipulates that the polar part of the molecule is adsorbed on the surface of the crystal, while the neutral part remains outside and isolates the crystal preventing their flocculation and formation of a larger structure.

Both theories confirm the interaction of the depressor additive with the growing crystal. Therefore, the depressor only acts when crystals start to form. This is why this type of additives has no effect on fuel cloud point. Without the additive, paraffin crystal size grows to tens of microns. When the additive is used, crystal size is reduced by an order of magnitude.

With time, depressor additives which can lower the cloud point of the fuel have been developed. Their principle of operation involves integration of polymer into paraffins with the subsequent formation of accosiations, which is possible due to similar solubility of paraffins and the additive. The additive in the paraffins prevents clouding, delays crystallization and reduces the cloud point.

Depressant-Dispersion Additives. The low temperature properties of diesel fuel have always been a focus of the oil refining industry and motorists alike. They are especially important in cold climates, where the demand for arctic and winter diesel fuel is high.

The main method of adjusting low temperature properties of diesel is the use of special depressor and depressor-dispersion additives. This approach is considered the most economical, and it increases oil processing efficiency and flexibility.

Traditional pour point depressor additives reduce the pour point and the filter point of diesel. In most cases, these additives are injected into fuel at refineries. However, the end used can also use depressors to improve fuel quality.

The development of depressor additives for fuel began forty years ago, while the same additives for oil were first developed 80 years ago. So why did they come into use relatively late? The main reason is that these additives do reduce the pour point, but have little effect on cloud point. For a long time, the latter was considered the main parameter of diesel usability in the cold seasons. Depressor additives do not prevent formation of initial n-alkane crystals, but limit their growth. The fact that filtration temperature is the main parameter for the use of diesel fuel in winter became the beginning of active development of depressor additives.

At this time, the most commonly used depressors are:

• ethylene copolymers with polar monomers (ethylene polymer with vinyl acetate, ethylene copolymer with acrylic acid ether);
• alkyl methacrylate and polyalkyl methacrylate copolymers;
• polyolefin type copolymers (ethylene-propylene and ethylene-propylene-diene copolymers and the products of their destruction, α-olefin copolymers and modified polyolefins);
• maleic anhydride copolymers;
• polymers of vinyl acetate with fumaric acid;
• aromatic hydrocarbon copolymers, which consist of two or three monomeres;
• non-polymer chemicals (alkyl naphthalene; polyacid ethers and alcohols, amides containing long alkyles).

Most depressor additives are polymers of ethylene with vinyl acetate.

In long storage, small crystals precipitate in diesel fuel. The fuel then consists of two visible layers: clear at the top and clouded at the bottom. It is at the bottom where small paraffin crystals deposit. Both layers are quite fluid, but taking fuel from the top layer does significantly complicate engine start and running, while taking fuel from the bottom layer means the engine will not start at all.

Pour point depressors cannot prevent fuel stratification, so dispersing additives are also used. This is a relatively new type of additive, first introduced by Exxon Chem in 1989. Disperser additives prevent fuel stratification in cold storage. Good paraffin dispersers are high molecular amides and imides of carboxylic acids, quaternary ammonium salts  and polyalkylene polyamine type amines.

A depressor-dispersion additive is a mix of components each improving a certain aspect of the fuel. Thus, the depressor determines the filtration and cloud point, the disperser prevent flocculation of paraffins at low temperatures, preserving fuel stability. Therefore, a depressor-disperser additive has two functions:

• improvement of diesel fuel cold performance;
• improvement of diesel stability at low temperature.

A specific additive is selected for each specific fuel type. This is the only way to mutually amplify the performance of each component.

A basic criteria for the efficiency of a depressor-disperser additive is the sedimentation stability of fuel at temperatures below the cloud point.

Antiwear Additives. Fuel system parts wear down with time, so antiwear additives are mixed into diesel fuel.

Originally, diesel fuel has certain anti-wear properties due to the presence of naphthenic hydrocarbons. Many sulfuric compounds in diesel fuel also have such properties (benzothiophenes and sulfides). However, environmental considerations dictate limitations of sulfur content, making it impossible to ensure the required degree of wear protection by sulfur only. Special additives are required.

The most common element of such additives is carboxylic acid. These are usually fatty acids of tall oil and their fractions, which are obtained from wood. However, the acid contains resinous acids, which degrade stability and performance of diesel. Beside the fatty acids, additives also contain corrosion inhibitor and a demulsifier. Alkyl salicylic acids are also quite efficient.

Antiwear additives create a layer on the surface of metal, distributing the loads more evenly and thus reducing the possible wear. The layer contains products of mechanical and chemical transformation of the additive. The lubricity of the additives is due to adsorption on the metal surface as well as chemical activity of the additive to the material of the friction parts.

Additives by BASF, Clariant, Lubrizol are based on tall oil acids with various supplementary components, additives by Infinеum are based on complex esters of mono and diglycerol and fatty acids with С12-С18 chains.

Anticorrosion Additives. Anticorrosion additives are special substances mixed with fuel to give it protective properties. The result is the formation of a protective film on metal surfaces, preventing corrosion.

There are two types of anticorrosion additives. The first type includes alkyl sulfonate and nitrate oil. These promote the creation of a strong chemisorption layer on the protected surface, isolating air and moisture. The second type are the salts of organic acids and esters, which reduce the interfacial tension on the boundary between fuel and water and improve the affinity of metal and fuel.

The corrosive properties of diesel fuel are determined by the presence of chemicals, which cause electrochemical and chemical corrosion of fuel system components. Fuel can also become contaminated by the products of corrosion, reducing  system throughput, as well as lubricity of the fuel.

In standard diesel fuel the amount of contaminants which may cause corrosion of metal parts is strictly limited. Such contaminants as bases, water-soluble acids and hydrogen sulfide are prohibited.

Electrochemical corrosion is usually caused by water and electrolytes. When oil products are shipped from the refinery, their copper strip test must be negative. This is only possible in the absence of hydrogen sulfide or free sulfur or their presence in such concentrations which cause no chemical corrosion of metal surfaces in the fuel system.

Modern oil refining technologies do not involve separate mixing of anticorrosion additives into diesel fuel. This is due to the fact that such additives are already present in lubricating and multifunctional additives.

Cetane Improver Additives. One of the ways to improve diesel fuel cetane number is to mix it with cetane improver additives. These additives reduce ignition lag of the fuel mixture.

There are two types of cetane improver additives:

• alkyd
• peroxide.

The additives cause homolytic molecule breaking and formation of free radicals, which provoke fuel ignition. The additives are only effective in the first stages of combustion.

Adding of up to 1% cetane improver raises the cetane number by 10-12 units. Previously, alkyl nitrates were used for this purpose, but their efficiency was accompanied by significant drawbacks, such as corrosive properties, reduction of wear-protective additive efficiency and lubricity of the fuel. Storage of fuel with nitrate additives for over six months reduces additive concentration due to its oxidation reactions with hydrocarbons, dropping the cetane number by 4-6. The above limitations are why research into the development of cetane improvers is ongoing.

Organic compounds based on peroxides, such as diaryl and dialkyl peroxides, are of particular interest. Some of the advantages of these substances is stability in storage and at higher temperatures, as well as stability in contact with water and other materials present in diesel fuel in the market.

Today, the most popular peroxide is ditertiary butyl peroxide. The general advantages of peroxides are compatibility with antiwear additives, lower toxicity and explosion hazard, as well as no corrosion effects. Diesel fuel in the United States are expected to be produced with peroxides, due to the stricter requirements to nitric compound content in diesel fuel.