The high-boiling constituents of petroleum are influencing the physical properties of crude oil, especially in the case of filtration. In particular, asphaltenes and resins are characterised, as high-molecularweight materials, with the tendency to form aggregates in the crude oil and intermediates.
The aggregates are produced by the presence of intermolecular hydrogen bonding between the asphaltenes and the resins. It appears that when resins and asphaltenes are present together, hydrogen bonding is one of the mechanisms by which resin-asphaltenes aggregates are achieved and resin-asphaltene interactions are preferred over straight asphaltene-asphaltene interactions. As a result, asphaltenes and resins are forming a complex mixture of constituents of higher molecular weight.
The interactions between polar aromatics, resins and non-polar are leading to the formation of micelles. The micelle center is formed from polar molecules of asphaltene with pronounced aromatic nature, which are then progressively surrounded by other more soluble molecules - of a less aromatic mature - places between the center, and the periphery.
It has been observed that the resins play an important function in stabilising the asphaltenes in the crude oil. Since the asphaltenes are incompatible with the oil fractions, asphaltenes dispersion is attributable mainly to the resins. Under unfavourable conditions the asphaltenes species might aggregate into clusters that are unstable and can precipitate or maybe easily become stuck to any surface during filtration or other separation process.
The model requires that the asphaltene micelles are composed of an insoluble molecular core that associates with the resins is providing steric stabilisation against flocculation and precipitation. The elementary micelles units can further cluster into larger particles similar to flocs.
An important corollary of petroleum composition is that the mole fraction of the resins is always larger than that of the asphaltenes and hence the micelles are expected to be richer in resins.
The addition of non-polar solvents to crude oil can procure the reduction of solubility parameters or polarity of hydrocarbon medium. The micellar solubilisation of the resins is leading to dissociation of the resin-asphaltene complexes, resulting in destabilisation of the asphaltenes, and relative precipitation and phase separation. The size of the asphaltene agglomerates can vary as a function of the temperature, the asphaltene concentration, as well as the presence of the resins. This establishes that the self-association of the asphaltenes is reversible and that the molecular size in the solution state results from a thermodynamic equilibrium between primary particles and reversible aggregates.
It has been recorded that the formation of micelles and aggregates is highly probable in the presence of asphaltenes and resins. The amount and size of the aggregates are influenced by the temperature and the total storage time in tanks. For temperature over 150°C the aggregates are reduced and generally dissolved, and for temperature below 80-125°C, the formation of aggregates might be consistent, depending on the origin of the oil, procuring a fast clogging of the filter media.
The temperature and process conditions are even influencing the average size of the aggregates. Generally, it has been registered that the specific filtration design adopted by BEA Technologies is less sensible to the clogging by retention of ashaltene and resin conglomerates due to the fact that when the pressure drop, through the filter media, increases the aggregates are partially disrupted and the asphaltenes are able to be extruded on the other side of the filter media - something that is not possible for hard solid particles and contaminants which are plugging catalysers.
The situation to be avoided is the formation of a compact layer of asphaltenes and resin aggregates on the surface of filter media, as this can lead to a fast clogging of the filter surface with a pressure drop increase and sudden decrease of the flow. The usual backwashing cannot clean out the compact layer formed and manual cleaning is required to soften and remove the compact layer.
