Crude oil contains a wide range of hydrocarbon components with extremely different physical properties. The major constituent of these hydrocarbon streams is generally described as paraffin wax. These paraffins are relatively insoluble and they precipitate when crude oil is cooled below a certain temperature. At sufficiently low temperatures, oil may even solidify completely. The presence of crystallised wax impairs the flow of crude oil. Other constituents in crude oil, such as asphaltene, resin, lighter distillates and polar aromatics, also affect its flow properties.1
Asphaltenes are very large heterogeneous molecules with condensed aromatic nuclei, which may associate to form colloidal-sized particles that strongly influence the viscosity of the oil medium and affect the crystallisation of the wax.2–4
The lowest temperature at which crude oil can still flow is generally known as the pour point. Crude oil production under deepwater (and in cold weather) encounters severe operational problems; the main complication is paraffin blockage, with serious economic consequences due to possible obstruction of flow pipes or production lines.5,6 In general, wax deposition can cause problems in production, storage, transportation and consumption.
Highly waxy crude oils are characterised by high pour point, viscosity and yield stress, and exhibit non-Newtonian flow behaviour below pour point temperature.7–10 The flow properties of crude oil play a large role in its production, storage, transport and refining.9 Therefore, it is very important to minimise the adverse effects of wax on the flow properties of oil. Several options are available, including stream heating, blending with lighter cutter stocks, mechanical scraping and the use of chemical additives.11 The preferred option is the use of chemical additives referred to as wax crystal modifiers, also known as pour point depressants (PPDs), flow improvers (FIs) and paraffin inhibitors. PPDs modify the size and shape of crystals and inhibit the formation of large wax crystal lattices.12,13 They typically have a wax-like paraffinic part that co-crystallises with wax-forming components of oil, and a polar component limiting the degree of co-crystallisation.
Polymers with these properties are homo- and co-polymers: ethylenevinyl acetate co-polymers,14 polyalkyl acrylates and methacrylates,15 alkyl esters of styrene-maleic anhydride co-polymers16,17 and alkyl fumarate– vinyl acetate co-polymers.18 Flow improvers are very selective; that is, not all additives are sufficiently effective for every crude oil.8 For example, some additives that considerably reduce viscosity and gel strength are poor PPDs.
Recently, PPDs and flow improvers have been developed that, in small concentrations, affect crystal growth and as a result improve flow properties.
Typical Examples of Polymeric Pour Point Depressants
Crystal modifier surfactants were developed by esterification of hexatriethanolamine with oleic acid to prepare mono-, di- and tri-esters with a structure shown in Figure 1 (molecular weight 1,124–1,568g/mol). In this work, the prepared surfactants were evaluated as PPDs/FIs for waxy crude oils with different wax and asphaltene contents.19 The decreases in the pour point temperature and viscosities of the crude oils on using these modifiers were found to depend on the concentration of the additive and the nature of the crude composition.
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