High-pressure phase behaviour of saturated and unsaturated fatty acid esters in supercritical CO2

Moorcroft, Jacobus Johannes (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: A variety of high-value compounds can be found in the oils extracted from fish, plants, and algae. Chief among these are fatty acids. These compounds are frequently processed in the form of fatty acid ester oils, most notably methyl (FAMEs) or ethyl (FAEEs) esters. They are, however, often produced in mixtures of varying chain length and degree of unsaturation as a result of genetic and dietary variation between source organisms. In order to upgrade the quality of the oil, it is desired to alter the chain length distribution and degree of unsaturation distribution. Supercritical fluid fractionation (SCFF) is an attractive potential solution, as supercritical CO2 is a safe and environmentally benign solvent. It has been shown that supercritical CO2 is capable of fractionating fatty acid esters based on their hydrocarbon chain length. Little work has been done to investigate fractionation based on the degree of unsaturation independent of chain length, however, and phase behaviour data available in the literature is inconclusive. This study aimed to investigate the feasibility of the SCFF of FAMEs with CO2 as solvent based purely on degree of unsaturation. To achieve this aim, the study was divided into three objectives: (1) Measure phase behaviour data for binary FAME + CO2 systems to investigate the influence of unsaturation on the phase behaviour; (2) Measure ternary phase behaviour and equilibrium data to investigate the influence of unsaturation on FAME mixture behaviour and the distribution of FAMEs between fluid phases; (3) Conduct thermodynamic modelling on the investigated systems to evaluate the ability of equations of state (EOSs) to describe the influence of unsaturation on FAME phase behaviour. Binary high-pressure bubble and dew point data were measured using a synthetic-visual method for the methyl stearate + CO2, methyl oleate + CO2, and methyl linoleate + CO2 systems. The family of C18 FAMEs was investigated as it includes a variety of industrially relevant compounds with varying degrees of unsaturation. The data showed that there is a difference in the phase transition pressures (and by extension solubility) between the saturated methyl stearate and the two unsaturated esters. However, there is no difference between the solubility of methyl oleate and methyl linoleate exceeding the experimental uncertainty of 1.8 bar. The solubility difference between saturated and unsaturated esters decreases as the temperature increases. The correlation of molecular order (CMO) was presented as an argument to explain the observed phase behaviour. These results indicate that supercritical CO2 can distinguish between saturated and unsaturated FAMEs, supporting the possible application of SCFF. For further investigation, high-pressure bubble and dew point data were measured with a synthetic-visual method for the ternary methyl stearate + methyl oleate + CO2 system. The data showed that a higher methyl stearate content leads to higher phase transition pressures and decreased solubility, following a linear trend. This system thus exhibits no cosolvency or other nonideal interactions. Ternary vapour-liquid equilibrium (VLE) data were also measured for the methyl stearate + methyl oleate + CO2 system. The distribution coefficients of methyl stearate were generally slightly higher than for methyl oleate. The relative solubility of methyl stearate relative to methyl oleate was found to be greater than one and to decrease with an increase in pressure. This data suggests that CO2 is selective for methyl stearate, the less soluble species. A rigorous uncertainty analysis was performed, and revealed that there was considerable uncertainty present in the calculated relative solubilities. The uncertainty in the relative solubility data thus introduces doubt into any conclusions drawn from the data. Thermodynamic modelling was conducted on the measured data with the predictive Peng-Robinson (PPR) and modified Sanchez-Lacombe (MSL) equations of state. While the PPR and MSL (with a regressed binary interaction parameter) could give reasonable descriptions of general C18 FAME phase behaviour, they were not able to describe, either qualitatively or quantitatively, the influence of unsaturation on the binary phase behaviour of C18 FAME + CO2 systems. The models were also unable to predict both the measured data for the ternary system and the experimental relative solubility data. These results highlight the relevance of experimental phase equilibria measurements, particularly for systems that do not exhibit large differences in solubility, since the shortcomings of predictive modelling can potentially overshadow phase behaviour features that occur over small ranges of pressure or temperature.

AFRIKAANSE OPSOMMING: ‘n Verskeidenheid waardevolle komponente word in geëkstraheerde olies afkomstig van visse, plante en alge gevind. Vetsure vorm 'n groot deel van sulke olies, en word dikwels in die vorm van vetsuuresterolies, hoofsaaklik as vetsuurmetielesters (VSMEs) of vetsuuretielesters (VSEEs) geprosesseer. Die olies word vervaardig in mengsels met ‘n verspreiding van koolstofkettinglengtes en graad van onversadigdheid as gevolg van genetiese variasie en dieetverskille tussen die organismes waaruit die olie geëkstraheer word. Die kwaliteit van die olie kan vir ‘n spesifieke toepassing opgradeer word deur die verspreiding van kettinglengtes en oversadigdheid aan te pas. Superkritiese vloeierfraksionering (SKVF) is ‘n aanloklike oplossing, aangesien superkritiese CO2 ‘n veilige en omgewingsvriendelike oplosmiddel is. Dit is reeds bewys dat vetsuurester mengsels op grond van hul koolstofkettinglengtes geskei kan word deur superkritiese koolstofdioksied. Die fraksionering van vetsuur esters op grond van hul onversadigdheid ongeag koolstofkettinglengte is nog nie ondersoek nie, en die fasegedragsdata in die ope literatuur is onbeslis. Die mikpunt van hierdie studie was dus om die vatbaarhied van die SKVF van VSMEs met CO2 as oplosmiddel alleenlik op grond van die graad van onversadigdheid te ondersoek. Derhalwe is die studie in drie doelwitte verdeel: (1) Meet fasegedragsdata vir binêre VSME + CO2 stelsels om die invloed van onversadigdheid op die fasegedrag te ondersoek; (2) Meet ternêre fasegedrags- en ewewigsdata om die invloed van onversadigdheid op VSME mengselgedrag en die verspreiding van VSMEs tussen vloeierfases te ondersoek; (3) Voer termodinamiese modellering op die gebestudeerde stelsels uit ter evaluering van die vermoë van toestandsvergelykings om die invloed van onversadigdheid op VSME-fasegedrag te beskryf. Binêre hoëdruk borrel- en doupuntdata is vir die metiel stearaat + CO2, metiel oleaat + CO2, en linoleiensuur metiel ester + CO2 stelsels gemeet. Die C18 familie VSMEs is ondersoek aangesien dit ‘n verskeidenheid industrieel relevante komponente met verskillende grade van onversadigdheid bevat. Die data het getoon dat daar ‘n verskil in die fase-oorgangsdrukke (en dus ook die oplosbaarheid) tussen die versadigde metiel stearaat en die twee onversadigde esters is. Daar is ook geen verskil tussen die oplosbaarheid van metiel oleaat en linoleiensuur metiel ester wat die eksperimentele onsekerheid van 1.8 bar oorskry nie. Die oplosbaarheidsverskil tussen versadigde en onversadigde esters neem af namate die temperatuur toeneem. Die korrelasie van molekulêre orde (KMO) is as ‘n argument voorgestel om die waargenome fasegedrag te verduidelik. Hierdie resultate dui aan dat superkritiese CO2 tussen versadigde en onversadigde VSMEs kan onderskei, wat die moontlike toepassing van SKVF ondersteun. Verdere ondersoek is ingestel deur ternêre hoëdruk borrel- en doupuntdata vir die metiel stearaat + metiel oleaat + CO2 stelsel te meet. Die data het getoon dat ‘n hoër metiel stearaat inhoud tot hoër fase-oorgangsdrukke en verlaagde oplosbaarheid lei, wat ‘n meestal lineêre tendens volg. Die sisteem toon dus geen verhoogde oplosbaarhied of ander nie-ideale interkasies nie. Ternêre damp-vloeistof ewewigsdata (DVE) is ook vir hierdie stelsel gemeet. Beide esters het lae verspreidingskoëffisiënte gehad, wat aandui dat daar ‘n beperkte lading van die dampfase is. Die verspreidingskoëffisiënte vir metiel stearaat was in die algemeen effens hoër as vir metiel oleaat. Gevolglik is die relatiewe oplosbaarheid van metiel stearaat met betrekking tot metiel oleaat as groter as een bevind, en dat dit met toenemende druk afneem. Hierdie data stel voor dat CO2 selektief vir metiel stearaat, die minder oplosbare spesie, is. ‘n Omvattende onsekerheidsanalise is uitgevoer, en het aansienlike onsekerheid in die berekende relatiewe oplosbaarhede getoon. Daar bestaan dus redelike twyfel oor enige gevolgtrekkings wat oor die DVE data gemaak word. Termodinamiese modellering is op die gemete data met die voorspellende Peng-Robinson (VPR) en die gewysigde Sanchez-Lacombe (GSL) toestandsvergelykings uitgevoer. VPR en GSL (met ‘n geregresseerde binêre interaksie parameter) kon redelike beskrywings van die algemene C18 VSME-fasegedrag gee, maar kon nie kwalitatief of kwantitatief die invloed van onversadigdheid op die fasegedrag van binêre VSME + CO2 stelsels beskryf nie. Die modelle kon ook nie die data vir die ternêre stelsel voorspel nie. Hierdie resultate beklemtoon die belang van eksperimentele fase-ewewig metings, spesifiek vir stelsels wat nie ‘n groot oplosbaarheidsverskil toon nie, aangesien die tekortkominge van modellering moontlik die fasegedragseienskappe wat in klein druk- en temperatuurintervalle plaasvind, kan verbloem.

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