## 5 Wax Chemical Inhibitors
### Pour Point Depressant (PPD)
The disadvantage of utilizing comb polymers is that the series of comb polymers must be compatible with the paraffin chain length to prevent the wax crystals from interlocking. This means that the outcome of the wax inhibition effectiveness will depend on the crude oil composition and the paraffin chain length [75]. High molecular weight copolymers are suggested if a crude oil contains a high range of normal paraffins and low carbon number. Meanwhile, for a crude oil that contains a narrow range of normal paraffins and high carbon number, low molecular weight copolymers are recommended [84; 94; 95]. Due to challenges in dealing with waxy crude oil, these polymeric PPDs showed limited wax inhibition performance. Recent findings showed that nanotechnology had contributed in the development of wax deposition inhibitors. Examples of polymeric nano-hybrids which have showed potential include polyoctadecyl acrylate, EVA co-polymers, and methacrylate [46; 89]. The ability of nanoparticles is that they can alter the nucleation site and hence form dispersed nucleation sites, alter the wax crystals growth (co-crystallization) and consequently depress the pour point better than utilizing EVA alone [96; 97]. It was reported that the pour point of crude oil was reduced by 25% when EVA was added, while the pour point was further reduced by up to 50% when a nano-hybrid was added together with EVA [96]. Another effective PPD that has been widely used includes the addition of organic solvents prior to polymeric wax inhibitors, which reduces the viscosity of the crude oil and enhances the oil flowability [98]. Examples of effective organic solvents include benzene-, carbon disulphide-, chlorinated hydrocarbons-, xylene- and toluene-based chemicals. For example, when trichloroethylene-xylene (TEX) was added to Nigerian crude oil in a ratio of 0.01 to 0.1 wt %, TEX was able to depress WAT, reduce the total wax deposition from 0.050 g/g to 0.0015 g/g (at 0.1 wt %), and further increase the wax deposition inhibition performance by adding a corrosion inhibitor [99]. However, there are safety concerns around the storage and handling of these organic solvents due to the toxicity of the solvents and their low flash point [100; 101]. Non-ionic surfactants have been reported to behave in a similar manner to polymeric PPDs by altering the wax crystals' morphology, impeding the three-dimensional wax formation [102; 103]. Non-ionic surfactants are generally viscous sticky liquids, which have extra surface-active properties and can act as a durable emulsifier. Non-ionic surfactants do not ionize in aqueous solution due to the presence of hydrophilic groups (alcohols, phenol, ester, and ether). In addition, non-ionic surfactants are better than anionic surfactants, as they are more reactive at higher temperature, hence performing better at reservoir temperature. Also, the low viscous emulsions make the non-ionic surfactants easier to recover at the refinery. Presently, natural surfactants are also being investigated under the category of PPDs and have been found to be much more promising than conventional surfactants. This is mainly due to their lower toxicity, ability to biodegrade, wide structural variety, and the fact that they can be obtained from cheap renewable materials as well as being stable under wide range of pH values [104; 105]. The natural surfactants that show promising results include _sapindus mukorossi_ sp. and _halomonas xianhensis_ sp. _nov._ bacteria. For example, _halomonas xianhensis_ sp. _nov._ bacteria were able to reduce the pour point of crude oil by up to 24 \({}^{\circ}\)C and reduce the crude oil viscosity by up to 70% [66,72]. Recently, polyamine amide from canola oil showed potential as an excellent pour point reducer with small concentration [106]. [PERSON] et al. (2018) also reported that Jatropha seeds and castor seed oil were able to reduce the pour point (from 24 \({}^{\circ}\)C to 8 \({}^{\circ}\)C) and the viscosity of waxy crude oils when a low dosage was used (0.1-0.3 vol %) [18].