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Kittiwake Compatibility Test is included as part of the Power Plant Laboratory .

Whilst every fuel is manufactured to be stable within itself, in that it does not have the tendency to produce asphaltenic sludge, it does not necessarily follow that two stable fuels are compatible when blended or mixed together. Incompatibility is the tendency of a residual fuel to produce a deposit on dilution or on blending with other fuel oils.

Sediment

Sediment by extraction defines the insoluble residues remaining after extraction by toluene. These insoluble residues are contaminants such as sand, dirt and rust scale, and are not derived from the fuel. Such a definition and test method are suitable for clear distillate fuels, but are not applicable to residual fuels. What is of greater importance is the total sediment content of the fuel, including hydrocarbon material which relates to stability.

Info Centre >> Fuel and Lube Oil Technical Manual >> 2. Fuel Oil - Characteristics >> 2.12 Compatibility & Sediment

2.12 Compatibility & Sediment

Residual Fuel Compatibility

Problems of incompatibility between residual fuels are rare but when they happen the results are severe. Typical problems are sludging and blockage of bunker and service tanks, pipe runs, filters and centrifuge bowls. In extreme circumstances, the only remedy is manual removal of the sludge build up. It is impossible to give precise advice on the probability of compatibility problems between two fuels but the risk of incompatibility can be ranked.

A blend is regarded as being stable if it is homogeneous immediately after preparation, remains so in normal storage and at no time produces or tends to produce sludge on a significant scale. Under these circumstances, the fuels forming the blend can be considered as compatible with each other. By definition, residual fuels are the remainder of the crude oil after the more valuable components have been extracted for the manufacture of petroleum products.

The chemical composition of residual fuels is difficult to define as it depends upon the source of the crude oil and the manufacturing processes used in the extraction of the petroleum products. However, by considering the chief constituents of residual fuels, an appreciation can be made of the sludge-forming mechanism. These constituents of a residual fuel include asphaltenes, resins and liquid hydrocarbons. The generic term 'asphaltenes' covers a wide range of heavier hydrocarbon structures. Besides being of high molecular weight and high carbon/hydrogen ratios, they may also contain small amounts of other elements, depending on the source of the crude oil.

Asphaltenes are believed to exist in residual fuel as 'micelles'. The resins can be considered as low molecular weight asphaltenes and these resins produce true solutions (i.e. molecular dispersions). The resin molecules in an oily medium of a residual fuel are known as 'maltenes' whilst the liquid hydrocarbons are an oily medium of still lower carbon/hydrogen ratio and molecular weight than the resins, which acts as a solvent for the other constituents. Thus a residual fuel oil is generally considered to contain a disperse phase of asphaltenes complexed with high molecular weight components of the maltenes (resins) and liquid hydrocarbons in the form of a micelle. The continuous or intermiceller phase consists of low molecular weight constituents of the maltenes. In this way, one can visualise a general decrease in carbon/hydrogen ratio and molecular weight from the centre of the micelle through to the continuous phase. This is shown diagrammatically in the figure. Although the hypothetical zones are shown separately, in fact they merge with each other, so there is no distinct interface between the micelle and intermicellar phases.

In the conditions shown, a state of equilibrium exists and the 'micelles' are considered to be 'peptised' (i.e. colloidally dispersed). If, however, the carbon/hydrogen ratio of the maltenes is lowered, say by the introduction of a paraffinic diluent, the resins which are absorbed in the asphaltenes are to a certain extent desorbed. This results in the asphaltene particles not being completely surrounded by resins and they are mutually attracted. This leads to a precipitation which appears as sludge.

Residual Fuel Stability

Stability of residual fuel may be defined as the ability of a fuel to remain in an unchanged condition despite circumstances which may tend to cause change; or more simply, as the resistance of an oil to breakdown. Conversely, instability would be the tendency of a residual fuel to produce a deposit of asphaltenic sludge as a function of time and/or temperature.

In the 1980s, three sediment test methods were developed which each relate to various aspects of sediment. They are the Total Sediment Existent (TSE), Total Sediment Accelerated (TSA) and Total Sediment Potential (TSP) methods. Each method is that of filtration through a double filter paper and each differs in the treatment of the fuel sample prior to filtration. In the case of the TSE procedure, there is no specific sample preparation, and the sediment result relates to the dirt in the sample. For the TSA procedure, the sample is mixed with 10% cetane and heated for 1h at 100°C before filtration. In the case of the TSP test, the sample is heated for 24h at 100°C to simulate thermal ageing of the fuel.

In the event of lack of stability of a fuel, it is likely that filter blockage and combustion problems will be experienced. Should there be any difficulty in identifying the nature of this material, a small portion should be placed in an open container and allowed to float in a vessel containing water at a temperature of 60-70°C. A waxy material will melt but an asphaltenic sludge will not.