The micronisation of synthetic waxes

Koen, Louis (Stellenbosch : University of Stellenbosch, 2003-04)

Thesis

ENGLISH ABSTRACT: Spherical wax particles with diameters of 5 μm and less are known to enhance the properties of powder and solvent-based coatings. Conventional methods, including spray freezing and jet milling, yield only 15 – 20 % of particles within this desired size range. Recently, supercritical fluids have been used in the micronisation of a variety of polymers, pharmaceuticals and other inorganic solids. Of these processes, the Rapid Expansion of Supercritical Solutions (RESS) involves the expansion of a solid dissolved in a supercritical fluid over a nozzle. In the process the solubility of the solid is drastically decreased to yield high supersaturated conditions, resulting in a multitude of small and stable wax nuclei. The technique is known to produce small, solvent free particles with a narrow size distribution. In this project, the RESS technique was applied to two commercial Fischer-Tropsch waxes: Paraflint C80 and Paraflint C105. Propane was selected as supercritical solvent due to its increased solvent power for paraffin wax compared to other conventional supercritical solvents such as carbon dioxide and ethane. From theoretical studies, nozzle geometry, as well as nozzle inlet temperature, pressure and concentration was identified as the parameters most likely to influence the size and shape of the wax particles. Porous plate nozzles were chosen as expansion device due to lower pressure drop and higher flow rate of the expanding solution compared to conventional capillary nozzles. Pre-expansion conditions of 125 to 160 ºC and 125 – 180 bar were investigated. Wax concentrations up to 5 weight % were expanded through porous plate nozzles with pore sizes of 15, 25 and 60 μm. It was found that small particles, less than 5 μm in size could readily be produced for the whole range of parameters investigated. Average particle sizes of less than 2 μm were found in virtually all cases. The C80 particles tended to be less smooth and of more angular shape than C105 particles. It is argued that the higher fraction of low weight compounds softens the C80 wax particle surface, making them more susceptible to deformation and coagulation. Experimental studies confirmed that a combination of lower temperature and higher pressure (175 bar and 130 ºC) increases the chances for the formation of spherical particles. This combination corresponded to conditions furthest away from the dew point of the solution. The 25 μm porous plate nozzle gave better results than the other nozzles, indicating that the characteristics of the nozzle could play a more important role than the pore size. SEM images indicated that this nozzle has a larger porosity, which could have resulted in lower pressure drops compared to the other nozzles. Higher wax concentration did seem to give smaller individual particles, but particle shape visibly deteriorated as the concentration increased. From experiments with C105 wax, at these optimal conditions, it seems as if a maximum concentration between 4 and 5 % is achievable, above which the particle shape became flak-like with rough edges. Due to limitations of the batch experimental set-up, especially regarding the reaching of steady state, it is strongly recommended that the obtained results be verified on pilot plant scale, where longer spray times can be achieved. The RESS process for wax micronisation on industrial scale needs to be critically examined and higher pressure and solvent recycling costs would need to be weighed up against the lower wax recycling required to obtain the desired particles.

AFRIKAANSE OPSOMMING: Sferiese was partikels met ’n deursnee van 5 μm en minder is bekend daarvoor dat dit die eienskappe van ’n verskeidenheid van poeier en oplosmiddel-gebaseerde deklae verbeter. Alledaagse metodes, waaronder sproeivries- en maalprosesse soos straalmaling, lewer slegs sowat 15 – 20 % van die partikels binne die gewenste partikelgrootte. In die afgelope tyd is superkritiese vloeistof mikroniseringsprosesse gebruik om ’n verskeidenheid van polimere, farmaseutiese en ander anorganiese stowwe te mikroniseer. Die sogenaamde “Rapid Expansion of Supercritical Solutions (RESS)”– proses behels die ontspanning van ’n oplossing van die vaste stof en ’n superkritiese oplosmiddel oor ’n spuitstuk. In die proses verlaag die oplosbaarheid van die vaste stof drasties, sodat hoë superversadigde vlakke in ’n baie kort tydperk bereik word om ’n menigte klein en stabiele was kerne te vorm. Die tegniek is bekend daarvoor om fyn, oplosmiddelvrye partikels met ’n nou partikelgrootteverspreiding te lewer. In hierdie projek is die RESS-proses ondersoek om twee Fischer-Tropsch wasse, C80 en C105, te mikroniseer. Propaan is as oplosmiddel gekies weens die verhoogde oplosbaarheid van paraffienwasse daarin vergeleke met ander konvensionele superkritiese oplosmiddels soos koolsuurgas en etaan. Poreuse skywe is as spuitstukke verkies weens die feit dat hoër vloeitempos en laer drukvalle oor hierdie spuitstukke verkry kan word. Na ’n teoretiese ondersoek is spuitstukgeometrie, asook die temperatuur, druk en konsentrasie by die spuitstukinlaat geïdentifiseer as die prosesparameters wat die partikelgrootte en –verspreiding die waarskynlikste sal beïnvloed. Daar is besluit om die spuitstukinlaatkondisies van 125 tot 160 ºC en 125 tot 180 bar te ondersoek. Waskonsentrasie tot en met 5 massa % is deur poreuse skywe met gemiddelde poriegroottes van 15, 25 en 60 μm ontspan. Klein waspartikels, met gemiddelde partikelgroottes van minder as 2 μm is in feitlik al die gevalle verkry. Die C80 waspartikels het geneig om minder glad en meer hoekig as die C105 partikels te wees. Hier word vermoed dat die hoër hoeveelheid lae massa komponente die C80 waspartikeloppervlak versag en dit meer vatbaar maak vir vervorming en koagulasie. Eksperimentele studies het getoon dat ’n kombinasie van laer temperature en hoër drukke (175 bar en 130 ºC) by die spuitstukinlaat die kanse vir sferiese partikels vergroot. Hierdie temperatuur/druk kombinasie val saam met toestande verder weg van die doupunt van die oplossing. Sover dit die spuitstukke aanbetref, het die 25 μm poreuse skywe beter resultate as die ander spuitstukke gelewer. Hier word vermoed dat die fisiese eienskappe van die spuitstuk ’n meer prominente rol as die poriegrootte speel. SEM foto’s het getoon dat hierdie spuitstuk ’n groter porositeit, wat ’n laer druikval tot gevolg kon gehad het, in vergelyking met die ander spuitstukke. Dit blyk asof hoër waskonsentrasies lei tot kleiner individuele partikels, maar die vorm en oppervlak daarvan het sigbaar verswak by hoër konsentrasie. Vanuit eksperimente met C105 was, by die bovermelde optimale kondisies, wil dit voorkom asof ’n maksimum konsentrasie tussen 4 en 5 % haalbaar is, voordat die partikels ’n vlokkierige vorm met ruwe oppervlak aangeneem het. Weens die beperkinge van die eksperimentele opstelling, word dit ten sterkste aanbeveel dat die resultate op loodsaanlegskaal, waar langer sproeitye moontlik is, bevestig word. Die RESS-proses behoort krities ondersoek te word en die hoër druk en oplosmiddel hersirkuleringskoste moet opgeweeg word teenoor die voordele van die laer was hersirkulering benodig om die gewenste partikels te lewer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/16367
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