Crude oil characteristics, chemical compounds containing in crudes and their corrosiveness are described. Physico-chemical characteristics of petroleum products, such as liquefied petroleum gas (LPG), naphtha, gasoline, kerosene ( jet fuel ), gas oil (diesel fuel ), and fuel oil obtained from crude oil also are analysed. Differentiation between the terms fuel and petroleum product is given. It is shown which components in crudes and petroleum products are corrosive. Corrosiveness of petroleum products is explained by the presence of water and dissolved oxygen. Water can be present as dissolved, emulsion water-in-fuel and free water in petroleum products. Solubility of water depends on temperature, relative humidity of air with which fuels contact, and fuel composition. Water solubility in fuels is greatly influenced by the presence and concentration of aromatic and olefin compounds. The free water is most dangerous in the occurring corrosion. Experimental data of solubility of oxygen in liquid petroleum products, fuels, alcohols, biofuels, their components, and, for comparison, in water are given. The methods of removing dissolved oxygen from fuels are described. Formation of aggressive compounds to metals and polymers as a result of oxidation of hydrocarbons containing in fuels also is described. Definition of corrosiveness of petroleum products is given. In order to understand why corrosion can occur in petroleum product systems, we begin by defining corrosion and petroleum products. Corrosion is an interaction between a material, usually a metal, and its environment that results in deterioration of the material and the environment. The term environment refers to crude oils, petroleum products, fuels, fuel additives, biofuels, and other components (oxygenates and organic solvents). Below we describe their physico-chemical properties and interaction with metals and polymers. First, let us differentiate between the terms fuel and petroleum product. Fuel is any material that stores energy that can later be extracted to perform mechanical work or provide heat. Many types of gaseous (natural gas, and hydrogen gas), liquid (petroleum products, liquid hydrogen, liquid alcohols and esters, substances produced from coal and shale by pyrolysis) and solid fuels (wood, coal, peat, shale, lignite, radioactive metals) exist. 2 1 Physico-Chemical Properties and Corrosiveness of Crude Oils ... Crude oil, a mixture of different liquid hydrocarbons that exist in the Earth’s crust, undergoes distillation, whereby the liquid homogenous mixture is separated into fractions based on differences in boiling points of its components. As a result of the distillation process, the following petroleum products are produced: liquefied petroleum gas (LPG), naphtha, gasoline, kerosene, gas oil (diesel fuel), fuel oil, and bitumen ( asphalt). These petroleum products obtained immediately after distillation are not yet the fuels that are used in cars, ships, and aircraft. Some of these, fuel oil and bitumen, can be used in furnaces and for road pavement without undergoing any other processing. Bitumen, in fact, is not fuel at all. Only after treatment, purification and other processes carried out in oil refineries, such as cracking, catalytic reforming, isomerization, hydrogenation, hydrocracking, sweetening, and clay treatment, do petroleum distillates become fuels. Each petroleum product undergoes a different process. LPG is washed by an alkali solution. Naphtha is not used as a fuel, but is very similar to gasoline in content. It is an intermediate petroleum product used as the feedstock for obtaining high octane gasoline and olefins. Kerosene is treated and transformed into jet fuel; gasoline is also treated and transformed partly into gasoline for motor vehicles and aviation gasoline (avgas) for aircraft; gas oil is processed into diesel fuel. For convenience, when using the term fuels here we also mean petroleum products. Corrosion of metals (as well resistance of polymeric materials) in liquid fuels and biofuels will be discussed in this book. We now describe the chemical compounds comprising crude oils, some of which cause corrosion in fuels. 1.1 Crude Oil Characteristics 1.1.1 Chemical Compounds in Crude Oils Crude oil (often used shortly as crude) was formed from organic matter (planktonic plants and animals) in aquatic deposits over a period of million of years. The chemical composition of crude oils from different producing regions, and even from within a particular formation, can vary tremendously. Crude oils are complex mixtures of different chemical compounds mostly hydrocarbons over wide boiling range. The detailed analysis of chemical composition of crude oils and petroleum products is given in Appendix A. Following chemical compounds are contained in crudes: a. Alkanes (called also aliphatic hydrocarbons or paraffins) are saturated hydrocarbons, non-cyclical, n-alkanes (normal alkanes) and iso-alkanes (branched alkanes). b. Cycloalkanes (called also cycloparaffins or naphthenes). c. Crude oils generally contain no alkenes. Alkenes (called also olefins) are unsaturated hydrocarbons (e.g., R-C = C-R). R is radical CnHm ( n and m are amounts of carbon C and hydrogen H atoms respectively). Alkenes are formed in oil refining units and are contained in gasoline (to 25 %) and kerosene (to 5 %). d. Aromatic hydrocarbons (called shortly aromatics) contain one or more aromatic (benzene) rings. Like alkenes, they are unsaturated hydrocarbons. Usually aromatics are less abundant than the saturated hydrocarbons. 3 1.1 Crude Oil Characteristics e. Waxes (un-branched n-alkanes with up to C30 carbon atoms). f. Heteroatomic organic compounds containing sulphur, nitrogen, and oxygen atoms. g. Dissolved hydrocarbon gases (methane CH4, ethane C2H6, propane C3H8, and butane C4H10) and hydrogen sulphide (H2S). h. Metals: generally vanadium (V), nickel (Ni); sometimes also iron (Fe), aluminum (Al), sodium (Na), potassium (K), calcium (Ca), and copper (Cu). Not all compounds containing in crudes are corrosive and aggressive to metals, alloys, and polymers (Appendix B). We will discuss in the next section which compounds in crudes are responsible for their corrosiveness and aggressiveness. 1.1.2 Corrosive Characteristics of Crude Oils Because crude oil is a mixture of widely varying constituents and proportions its physico-chemical properties also vary widely. Most organic compounds containing in crude oils are not corrosive to metals and alloys (see Appendix B). Crude oils are not corrosive at ambient temperatures. Even certain crudes can inhibit corrosion of metals because of organic constituents adsorbing on metal surface, modifying the corrosion products and forming a protective layer. Crude oils can become corrosive when they are heated in refineries. Crude oils can contain water, inorganic salts dissolved in water, hydrogen sulphide, organic compounds containing nitrogen, oxygen and sulphur, small amounts of metals, solid particles, and microorganisms. Corrosion characteristics of crudes are defined by total sulphur (S) content, total acid number (TAN), salt and water content, and microorganisms. These species influence corrosion by different manner and at different stages of preparation, transportation, storage, and distillation of crudes. Water has limited solubility in hydrocarbons and the presence of free (undissolved) water is necessary for corrosion. For instance, limiting amount of salt and water is < 1 % (often < 0.5 %) in crude oils in transmission pipelines. Otherwise, the solid particles tend to be encapsulated by a layer of water on the pipe surface. Water drop-out and accumulation can occur at low velocities and under stagnant conditions. Total sulphur content is used to characterize potential corrosion by various organic sulphur-containing compounds and hydrogen sulphide. The latter is the main corrosive agent among sulphur-containing compounds that are present in crude oils and influences corrosion at all stages of output, transportation, storage and treatment of crudes. Total acid number (called also neutralization number) is a measure of the numbers of milligrams of potassium hydroxide (KOH) needed to neutralize 1 g of crude or its distillate fraction. TAN values are used to characterize corrosiveness of crudes and their distillate fractions because of the presence of organic acids (including also naphthenic acids) which mostly corrosive during distillation and further transportation of some petroleum distillates (gas oil and fuel oil) at temperatures 190–360 °C in refineries. Inorganic Compounds in Crudes Inorganic chloride salts (NaCl, MgCl2, CaCl2), hydrogen sulphide (H2S) and elemental sulphur (S8) are main corrosive species

Corrosion in Systems for Storage and Transportation of Petroleum Products and Biofuels: Identification, Monitoring and Solutions

The Oxidation of Hydrocarbons in the Liquid Phase Edited by N. M. Emanuel. Translated from the Russian by K. R. Dobson and B. J. Hazzard. Pp. xv + 407. (Oxford, London and New York: Pergamon Press, 1965.) 140s.

Liquid-Phase Oxidation of Hydrocarbons

The diesel fuel cold filterability test has been developed to predict fuel filter plugging by diesel fuels containing flow improver additives, and to understand how flow improvers interact with diesel fuels to affect low-temperature operation of diesel vehicles. Mechanisms are proposed to explain the effectiveness of flow improvers on the basis of these factors. 19 refs.

Understanding of the effectiveness of diesel fuel flow improvers

Two Oldsmobiles with 5.7 L diesel engines and one with a 4.3 L diesel engine were tested to determine the effect of the low temperature properties of diesel fuel on operability (cold-start and warm-up driverability). Startability limits ranged from -18 degrees C to -24 degrees C and were unaffected by cetane quality of the fuel within the range of 39 to 50 cetane numbers. With untreated fuels, the cars would not operate when the fuel temperature fell below the cloud point of the fuel because wax crystals precipitated from the fuel and plugged the fuel tank filter. Low temperature operability was extended as much as 10 degrees C by the use of fuel flow improver additives and a modified vehicle fuel systems.

Cold weather fuel requirements of oldsmobile diesels

The oxidative stability of synthetic fuels and fuel blends with monoaromatic blending components

1.

Straight-chain alkanes, when added to diesel fuel in amounts of 5% or more, harm the low-temperature characteristics of the fuel. The presence of these hydrocarbons will produce sharp increases in the cloud point, filterability temperature, and solid point of diesel fuels, and it is found that these three temperatures tend to converge when the n-alkanes are present.




2.

The most effective pour-depressant additives are those containing such structural elements as an ester group in an aliphatic chain with olefinic bonds. The total molecular weight of such additives must be several times the molecular weight of the diesel fuel.




3.

A given pour-depressant additive manifests identical effectiveness in fuels of a given hydrocarbon composition, including fuels with similar contents of n-alkanes.

Pour-depressant additives for diesel fuels

1.

Extraction of oxygen compounds will decolorize middle-distillate fuels, improve the thermal stability and cleanliness level, and reduce the charging tendency, and will also lead to a considerable reduction of the tendency to form emulsions with water.




2.

After removing the oxygen compounds, the antiwear properties of the fuels are poorer, and they are more susceptible to oxidation.




3.

Before the accumulation of oxidative-condensation products (this accumulation requiring a prolonged period), the oxidation of the purified fuels proceeds with the formation of oxygen compounds that are for the most part readily soluble in the fuel.

Service properties of middle-distillate petroleum fuels as influenced by oxygen compounds present in fuels

Oxidizability of aromatic hydrocarbons present in jet fuels

1.

The components of the hetero-organic impurities which very greatly improve anti-wear properties of fuels are naphthenic acids in an amount of 0.002–0.01% by wt. and products from auto-oxidation of unstable hydrocarbons - olefinic-aromatic alcohols - in an amount of 0.02–0.04% by wt.




2.

With increase in the temperature of fuel which is in contact with atmospheric oxygen, up to a limit, when liquid-phase oxidation of hydrocarbons is possible, the anti-wear properties do not become worse, but improve, due to the formation and accumulation of positively acting oxygenated compounds.

Ya. B. Chertkov

Papers

Pour-depressant additives for diesel fuels

Service properties of middle-distillate petroleum fuels as influenced by oxygen compounds present in fuels

Oxidizability of aromatic hydrocarbons present in jet fuels

Effect of hetero-organic compounds of middle distillate fuels on their anti-wear properties

Jet aircraft fuel additives (a review)