Pour-Point Depressant (Crystal Modifiers)
PPDs are designed to be cost-effective method to improve cold-flow properties of crude and other fuel
oils to remain primarily as a fluid (Wang et al. 1999).PPDs accomplish this task by modifying the size and
shape of wax crystals and inhibit the formation of large wax crystal lattices (Wang et al. 1999).PPDs
typically have three characteristics:
1. A wax-like paraffinic part, typical mixture of linear alkyl chains of 14 to 25 carbon atoms long,
which co-crystallizes with the oil’s wax-forming components (Wang et al. 1999)
2. A polar component, to limit the degree of co-crystallization (Wang et al. 1999)
3. Primarily consist of polymers, when attached to the wax crystal will sterically hinder growth of
large crystals (Wang et al. 1999)
Figure 1 PPD Chemical Structure (Hemant et al. 2008)
Generally the polar component of the additive creates the barrier to the formation of the interlocking
crystal wax network (Wang et al. 1999).As a result, the altered shape and smaller size of the wax crystals
reduce the formation of the interlocking networks and reduces the pour point (Hemant et al. 2008).
Mechanism:
The mechanism of prevention of agglomeration primarily involves the structure of the PPDs to disrupt
the crystal habit of wax crystals (Hemant et al. 2008).The structure of PPDs primarily involved in this
process are; the pendant chains to co-crystallize with the wax and the polar end groups which are
responsible for disrupting the orthorhombic crystal structure into a compact pyramidal form (Hemant et
al. 2008).This process is responsible to prevent the crystals from agglomerating and forming a gel-like
structure to deposit on the pipeline surface (Hemant et al. 2008).
Figure 2 PPD Inhibition mechanism of wax modification 2a) Chemical structure of wax 2b) Crystal shape
of wax structure 2c) Crystal structure of growing wax lattice 2d) Polymeric Additive with wax-like
components 2e) Co-crystallization of wax and PPD 2f) sterically hindered wax structure (Wang et al.
1999)
Figure 3 Prevention Mechanism of Interlocking of Wax Crystals by polymer Additives through a)
nucleating sites(brown) interaction to asphaltene and wax molecules (blue) b) Polar component of
additive (green) hinder co-crystallization of asphaltenes and wax(Hemant et al. 2008)
Figure 4 Pictomicrographs of Waxes of KS and ND crude oils. A) KS crude without additive, B) KS crude
with additive , C) ND crude without additive D) ND crude with additive (Hemant et al. 2008)
The efficacy of the additive generally depends on the rheological properties on the crude oil (Hemant et
al. 2008).As such crude containing comparable amount of asphaltenes may not react in the same
manner as paraffin predominate crudes (Hemant et al. 2008). Asphaltenes in crude behaves as a natural
PPD, and can hinder the growth the growth of wax crystals by attaching to the surface of the wax crystals
(Herman et al. 2008). And as a result, by incorporating an additive into the crude can diminish the
interaction between the additive and the wax crystals resulting in a poor performance of the PPD
(Hermant et al. 2008). By the experimental data collected by hermant et al., a single PPD cannot
effectively depress the pour point of all types of crudes (Hermant et al. 2008) (Wang et al. 1999).And as
such, specifically tailored PPDs must be designed to match the crude’s paraffin chain length and
composition of the crude, to participate efficiently in the crystallization of wax crystals to depress the
pour point (Wang et al. 1999).
Trichloroethylene -Xylene binary system :
Xylene-based PPDs forms structures with segments which interact with the developing wax crystals to
inhibit crystal formation (Bello et al. 2005).A specific xylene based PPD which exhibits a substantial
effect as a pour-point depressant is trichloroethylene-xylene (TEX) binary system (Bello et al .2006).This
additive utilizes one or more several postulated mechanisms, nucleation, adsorption, co-crystallization ()
and improved wax solubility in the reduction of large wax crystals (Bello et al .2006).Trichloroethylene
compound contains an ionic pair of electrons which facilitates greater adsorption of the inhibitor
molecules onto the wax crystal networks and preventing the interlocking of the wax network (Bello et al.
2005).
TEX binary system has the ability to form a stable suspension in the crude oil which results in the
inhibition of wax crystal formation in the parts per million ranges in comparison to the concentration of
wax crystals (Bello et al. 2005).As a result this TEX additive has a greater application on various different
crudes and a tremendous economic benefit in the inhibition of wax-crystal formation (Bello et al. 2006).
Hemant PS, Kiranbala, Bharambe DP.2008.Performance Based Designing of Wax Crystal Growth
Inhibitors. Energy & Fuels 22 (6):3930–3938.
Wang SL, Flamberg A, Kikabhai T.1999.Select the Optimum Pour Point Depressant. Hydrocarbon
Processing 78:2
Bello OO, Fasesan SO,Teodoriu C, Reinicke KM.2006. An Evaluation of the Performance of Selected
Wax Inhibitors on Paraffin Deposition of Nigerian Crude Oils. Petroleum Science and Technology 24:195206.
Bello OO, Ademodi BT, Akinyemi PO.2005. Xylene-based inhibitor solves crude oil wax problems in Niger
Delta pipeline. Oil & Gas Journal.103:10-56.