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Fundamental behaviour and micronisation of Fischer-Tropsch waxes via supercritical CO2 PGSS processing
Thesis (PhD)--Stellenbosch University, 2026.
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Stellenbosch : Stellenbosch University
2026
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| _version_ | 1867613859451240448 |
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| access_status_str | Open Access |
| author | Barnard, Andri |
| author2 | Schwarz, Cara Elsbeth |
| author_browse | Barnard, Andri Schwarz, Cara Elsbeth |
| author_facet | Schwarz, Cara Elsbeth Barnard, Andri |
| author_sort | Barnard, Andri |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Thesis (PhD)--Stellenbosch University, 2026. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/135606 |
| institution | Stellenbosch University (South Africa) |
| language | English |
| last_indexed | 2026-06-10T12:42:50.594Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2026 |
| publishDateRange | 2026 |
| publishDateSort | 2026 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/135606 Fundamental behaviour and micronisation of Fischer-Tropsch waxes via supercritical CO2 PGSS processing Barnard, Andri Schwarz, Cara Elsbeth Labuschagne, Philip Wouter Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Thesis (PhD)--Stellenbosch University, 2026. Barnard, A. 2026. Fundamental behaviour and micronisation of Fischer-Tropsch waxes via supercritical CO2 PGSS processing. Unpublished doctoral dissertation. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/48446a6c-e551-42fe-b244-3ab43825b02d Synthetic wax particles are routinely employed in formulations of inks and coatings amongst others. Particle characteristics such as size and shape are critical to the quality and effectiveness of the final product. The particles from gas saturated solution (PGSS) process, via use of supercritical CO₂ (sc-CO₂) as plasticiser, offers a promising alternative to harsher traditional methods used to produce particles in the micron size range (“micronisation”). PGSS micronisation with sc-CO₂ is a safe and cost-effective process, thus well-suited to produce high volume low value products such as synthetic wax particles. Currently no literature exists on the PGSS micronisation of Fischer-Tropsch (FT) waxes via sc-CO₂. The primary aim of this project was to assess the viability of sc-CO₂ PGSS micronisation of FT waxes, and to obtain an in-depth understanding of the interdependencies between PGSS processing conditions (temperature, pressure, composition and nozzle size) and the resulting FT wax particle size and shape. To assist in elucidating these dependencies thermophysical and fluid properties of wax + CO₂ mixtures can be utilised. As there is very little literature available on the behaviour of Fischer-Tropsch (FT) waxes in sc-CO₂, this will add to the existing body of knowledge. The secondary aim was to determine the impact of PGSS processing on the crystal structure of the wax particles produced. Three FT waxes with increasing melting temperatures (Tm) were utilised (Wax 1 < Wax 2 < Wax 3), with Wax 3 an oxidised wax. To achieve these aims, new data on the solid-gas (S-G) and liquid-gas (L-G) equilibrium of binary mixtures of the three FT waxes with increasing melting temperatures and CO₂ were measured. The density and viscosity of the mixtures were also measured. The melting temperatures of the waxes increased in CO₂ due to the domination of hydrostatic pressure effects over interaction between wax and CO₂. Wax 1 displayed pressure induced crystallisation and a temperature inversion. Phase transition pressure increased with increasing wax Tm. The phase behaviour of Wax 1 and Wax 2 compared favourably to the phase behaviour of the corresponding pure n-alkanes in CO₂. The viscosity of Wax 1 increased with decreasing temperature and increased wax content, while the viscosities of Wax 2 and Wax 3 could not be measured. The solubility of CO₂ in the waxes could be correlated to within 1 % of the measured data with a modified Chrastil model. PGSS micronisation of Wax 1 and Wax 2 could be performed successfully. Processing conditions where optimum mass yields of free-flowing wax particles could be obtained were identified. For Wax 1, optimal conditions for micronisation were between 40 °C to 45 °C and 150 bar to 200 bar with the smallest diameter nozzle. The optimal operating region of Wax 1 was restricted by the pressure induced crystallisation observed and the high viscosity of the wax melt at low temperatures. The highest Wax 2 yields were also obtained with the smallest diameter nozzle, between 70 °C and 90 °C and 200 bar to 300 bar. Median particle size generally varied between 30 μm and 50 μm for both waxes. Particle size was mainly impacted by temperature due to the importance of viscosity and heat transfer requirements during particle formation as well as limited solubility of CO₂ in the waxes. Statistical modelling of particle size data corroborated this. Particle size distributions (PSD) were mostly unimodal, with some bimodal PSDs recorded. This is attributed to particle formation via atomisation and crystallisation as well as agglomeration. Bimodal distributions occurred at either low pressure and/or higher wax loadings as these conditions favoured segregation of particles formed via the different mechanisms into discreet distributions. The theory was corroborated with SEM imaging indicating the presence of both crystallite structures as well as spherical particles for both waxes. Delayed solidification of Wax 2 particles at high temperature and low pressure resulted in the formation of highly spherical solid particles to a large enough extent to increase bulk density markedly at these conditions. Otherwise, bulk density was not greatly impacted by changes in temperature and pressure, for both Wax 2 as well as Wax 1. Bulk density of Wax 1 particles varied between 279 kg/m ³ and 560 kg/m ³ and ranged from 159 kg/m ³ to 528 kg/m ³ for Wax 2. Nozzle size did not greatly impact wax particle size, but nozzle shape did. The use of a tapered nozzle as opposed to a straight nozzle resulted in increased agglomeration, especially for Wax 1 with its increased oil content. Particle shape was mainly impacted by changes in temperature, as confirmed by statistical modelling. PGSS micronisation resulted in reduced lamellar structures observed for both Wax 1 and Wax 2 as opposed to the unprocessed waxes. Secondary crystallisation or chain migration occurred over time for both waxes, less so for Wax 2 than Wax 1 due to the greater chain mobility of the lower Tm Wax 1. Future modification of the PGSS equipment, including improved heating capacity, directed cooling of the spray and monitoring of the temperature in the product chamber will allow for greater experimental scope and improved control of the particle formation process. Possible further reductions in particle size via the use of smaller diameter nozzles (< 1 mm) can also be investigated. Doctoral 2026-04-02T08:45:37Z 2026-04-02T08:45:37Z 2026-03 Thesis https://scholar.sun.ac.za/handle/10019.1/135606 en Stellenbosch University 349 pages : ill. application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Barnard, Andri Fundamental behaviour and micronisation of Fischer-Tropsch waxes via supercritical CO2 PGSS processing |
| title | Fundamental behaviour and micronisation of Fischer-Tropsch waxes via supercritical CO2 PGSS processing |
| title_full | Fundamental behaviour and micronisation of Fischer-Tropsch waxes via supercritical CO2 PGSS processing |
| title_fullStr | Fundamental behaviour and micronisation of Fischer-Tropsch waxes via supercritical CO2 PGSS processing |
| title_full_unstemmed | Fundamental behaviour and micronisation of Fischer-Tropsch waxes via supercritical CO2 PGSS processing |
| title_short | Fundamental behaviour and micronisation of Fischer-Tropsch waxes via supercritical CO2 PGSS processing |
| title_sort | fundamental behaviour and micronisation of fischer tropsch waxes via supercritical co2 pgss processing |
| url | https://scholar.sun.ac.za/handle/10019.1/135606 |
| work_keys_str_mv | AT barnardandri fundamentalbehaviourandmicronisationoffischertropschwaxesviasupercriticalco2pgssprocessing |