How polymer additives reduce the pour point of hydrocarbon solvents containing wax crystals

Abstract

We have investigated how four different pour point depressant (PPD) polymers affect the pour point transition in mixtures of a single pure wax in a solvent. We used either n-eicosane (C20), CH3(CH2)18CH3, n-tetracosane (C24), CH3(CH2)22CH3 or n-hexatriacontane (C36), CH3(CH2)34CH3 as the wax component with either n-heptane or toluene as the solvent component. For all wax-solvent combinations, the measured variation of wax solubility with temperature is well predicted by ideal solution theory. The variation of pour point temperature as a function of the overall wax concentration is quantitatively modelled using the idea that, for each overall wax concentration, the pour point occurs at a temperature at which a critical volume fraction ϕ* of wax crystals has precipitated. Close to the pour point temperature, extraction and examination of the wax crystals show they consist of polydisperse, irregularly-shaped platelets with axial ratios (h/d, where h is the plate thickness and d is the plate long dimension) in the range 0.005-0.05. It is found that the measured ϕ* values corresponding to the pour point transitions are weakly correlated with the wax crystal axial ratios (h/d) for all wax-solvent-PPD polymer combinations. These results indicate that the pour point transition occurs at a volume fraction larger than the value at which the volumes of rotation of the platelet crystals overlap, i.e., 2.5(h/d) < ϕ* < 11(h/d). PPD polymers work, in part, by increasing the wax crystal axial ratio (h/d), thereby increasing ϕ* and reducing the pour point temperature. Since the PPD's ability to modify the wax crystal shape relies on its adsorption to the crystal-solution surface, it is anticipated and observed experimentally that optimum PPD efficacy is correlated with the difference between the wax and the polymer solubility boundary temperatures. This finding and the mechanistic insight gained here provide the basis for a simple and rapid screening test to identify candidate species likely to be effective PPDs for particular wax systems.

Visual Measurements of Solid–Liquid Equilibria and Induction Times for Cyclohexane + Octadecane Mixtures at Pressures to 5 MPa

“…The time was then recorded until the first crystals were visually observed on the copper tip. The results are presented in and show that increased subcooling exponentially shortened the induction time, halving approximately every 0.25 K. These observations are consistent with those generally observed in the nucleation experiments 78 where formation of a new condensed phase requires to overcome an activation energy barrier associated with the interfacial energy between the new and parent phases; no such energy penalty exists for the melting process where the energy bound in the interfacial region is released [79][80][81][82][83].…”

Oleic acid‐based poly(alkyl methacrylate) as bio‐based viscosity control additive for mineral and vegetable oils

“…MPO generally have poor cold flow properties due to crystallization of their wax content (hydrocarbon compounds) at low temperature [28,30,[40][41][42]. This leads to a minimum operating point below which oil flow would be disturbed and will require more energy (i.e.…”

Stearyl Methacrylate-Based Polymers as Crystal Habit Modifiers for Triacylglycerols

“…waxes), and these interactions may not occur in a melt. 40 Omitting OOO from DSC measurements enabled the direct effect of polymer additives on SSS crystals to be observed, independent of solubility in OOO. Mixtures of SSS and polymers prepared with the same ratio as used in SAXD experiments (i.e.…”