Synthesis, characterization and application of branched polymers as middle distillate fuels cold flow additives

Abstract

The research described in this thesis entails the synthesis of comb-like polymeric materials with long alkyl methacrylates side chains and their cold flow properties evaluations in crude derived as well as Gas-To-Liquid (GTL) diesel fuels produced by Sasol’s proprietary Fischer Tropsch technology. Statistical copolymers of styrene with methacrylate pendant alkyl chain lengths ranging from C18 to C22, as well as blend alkyl chains of C16 – C24 were selected for this study, and prepared via Atom Transfer Radical Polymerization (ATRP) as the chosen controlled radical polymerization technique. The design of the polymeric materials of interest was based on obtaining polymers with two major constituents viz. crystallizable fractions and the amorphous/non-crystalline fractions. The crystallizable fraction would co-crystallize with the fuel’s paraffin wax fractions thus preventing the paraffins from forming unfavorable large 3-D crystal matrix, while the non-crystalline part would aid in disrupting crystal growth and modifying the crystal structures. The first part of the study focused on using ATRP to synthesize various statistical copolymers of interest in a controlled manner. The second part of the study dealt with applications of the synthesized polymeric additives in diesel fuels’ cold flow applications. Within the applications, two application areas were looked at viz. cold flow improvers and wax dispersants or wax antisettling applications. The synthesized statistical copolymers were evaluated for their cold flow properties for selected diesel fuels, with the focal aim of gaining fundamental understanding into factors influencing crystallization responses of the selected middle distillate fuels and the respective polymeric additives. The physicochemical properties of the copolymers were followed via crystallization interactions of both the diesel fuel and the comb-type polymeric additives. These interactions were assessed through Differential Scanning Calorimmetry (DSC), low temperature optical microscopy, Cold Filter Plugging Point (CFPP) and Pour Point (PP) tests. The influence of varying styrene contents of the statistical copolymer was investigated. The crystallization response in GTL diesel fuels demonstrated the significant influence in the structure/property activity of the additive as well as the fuel. Based on crystallization modifications observed, the polymeric additives illustrated solvent matrix selectivity in different fuels studied. Copolymers with too high, too low or no crystalline components promoted the unfavorable wax crystallization, as observed in the case of homopolymers of alkyl methacrylates for example. These polymeric types did not facilitate good solubility of the polymeric additive in the diesel matrix therefore leading to minimal interactions with the diesel matrix. Obtaining polymeric additives with just the appropriate balance of the two components was therefore crucial. Such polymers in the diesel fuels evaluated demonstrated a lowering of Wax Appearance Temperature (WAT) also known as Cloud Point (CP) by 5 degrees and also managed to obtain ?CFPP of 6 degrees. These results allowed an insight into the crystallization operating mechanisms of the statistical styrene-alkyl methacrylate copolymers. The results illustrated that the statistical copolymer additives indeed displayed both cloud point depressants and crystal growth inhibitors qualities. These results were however diesel fuel specific since the polymeric additive demonstrated different crystallization activities in different GTL diesel fuels used in the study. In the second application area, wax antisettling additives (WASA) or wax dispersing (WD) additives were investigated. Compounds that contained long alkyl chains and ionic charges were evaluated as wax dispersants for crude derived as well as GTL diesel fuels in combination with the synthesized ATRP statistical copolymers. The WD included quaternary salts, ionic liquids and gemini surfactants. Gemini surfactants were synthesized and in a novel application they were investigated for wax dispersing properties in crude oil derived diesel. Together with the evaluated statistical copolymers the surfactants were found to be efficient wax dispersants. These systems demonstrated the ability to facilitate a further size reduction and modifications in wax crystals, as evidenced by low temperature microscope from 10µm needles to smaller dot-like structures. A further lowering of the CFPP (?CFPP ˜ 12 °C) was achieved with some of the evaluated gemini surfactants.