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The high pressure phase behaviour of detergent range alcohols and alkanes

Fourie, Frederick Christiaan van Niekerk (2018-03)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

ENGLISH SUMMARY: Supercritical fluids fulfil specialised roles in the food and beverage, petroleum, natural gas, polymers, pharmaceuticals and novel materials development industries. Supercritical fluid extraction (SFE) represents a separations niche and utilises the tuneable properties of a high pressure solvent to execute challenging extractions. One such example is the purification of detergent alcohol product streams. These streams often exhibit a range of carbon backbone lengths and contain significant residual alkanes, linear and branched alcohol isomers. The boiling and crystallisation points of these species are often narrowly distributed or overlap and SFE is considered as an alternative means of product purification. Robust thermodynamic models are key to designing SFE processes but model development is partially reliant on access to accurate and comprehensive high pressure phase equilibria data. Vapour-liquid equilibria (VLE) data of ternary solvent + solute A + solute B systems are particularly useful because the solute A-solute B interaction, a determining factor in mixture phase behaviour, is inherently incorporated. Unfortunately these data remain scarce because the requisite analytical equipment is complex and costly. A high pressure multi-component static analytic phase equilibria setup capable of operation at 150 °C and 300 bar was developed. It features a visual, variable volume equilibrium cell incorporating two ROLSITM samplers. Simultaneous sampling from two phases coupled with concurrent parallel online analysis via gas chromatography (GC) is possible. Additional features include vertical adjustment of both samplers whilst maintaining system pressure, reduced cell dead volume and a high definition camera enabling the observation of internal phenomena not visible with the naked eye. Heating is achieved using jacketed fluid circulation coupled with a forced convection oven. The GC contains two inlets, four columns, two switch valves and three detectors arranged in parallel pathways and is equipped to handle mixture constituents from volatile gases to mid-length hydrocarbons. Adapted manual injection techniques enabled quantitative GC calibration methods that are decoupled from the equilibrium cell. The setup was validated by sampling from a one-phase ternary of known composition and comparison with binary and ternary literature data. Observation of the cell contents has proven essential for accurate vapor phase sampling. Visuals were used to indicate that sampling can disrupt a high-pressure equilibrium system even if pressure and temperature remain constant to within 0.1 bar and 0.01 °C. Such disruptions may manifest in one of three phenomena: global mist formation, localized mist formation, or no-warning droplet formation. In a similar fashion, the impact of temperature gradients and particularly the small scale of the resultant disruptions, are conveyed. The new apparatus was used to study nine ternary mixtures comprising the solvent CO2 and different combinations of the solutes n-dodecane (nC12), 3,7-dimethyl-1-octanol (37DM1O) and 1-decanol (C10OH). Three ternary mixtures – [CO2 + nC12 + 37DM1O], [CO2 + 37DM1O + C10OH] and [CO2 + nC12 + C10OH] – were each investigated at bulk CO2-free solute A:solute B mass ratios of 75:25, 50:50 and 25:75. Phase compositions were measured at 35, 55 and 75 °C and pressures between 68 and 237 bar. The experimental outline and target reduced pressures in particular enable a comparative study using ethane as solvent. Phase behaviour in the two alkane-containing mixtures – nC12 + 37DM1O and nC12 + C10OH – displayed similarities and deviated from behaviour in the alcohol + alcohol mixture, 37DM1O + C10OH. The presence of nC12 led to enhanced solubility in the ternary mixture and, in the high-nC12 region, co-solvency. On Gibbs diagrams this presents as s-shaped convex-to-concave liquid curvature, concave vapour curvature and, at intermediate pressures, a pinched two-phase band. In the CO2 + 37DM1O + C10OH system enhanced solubility or pinched two-phase bands were not detected and liquid curves remained convex throughout. Relative solubility (ij) and separation potential (SPij) were used to evaluate solvent efficacy. The former is an indicator of attainable fractionation sharpness whilst the latter incorporates ij coupled with vapour phase loading of the more soluble solute. The suitability for fractionation using high pressure CO2 decreased in the order [nC12 + C10OH] > [nC12 + 37DM1O] > [37DM1O + C10OH]. In both nC12-containing systems, ij’s and to a large extent also SPij’s were positively correlated with bulk fraction of the less soluble solute species. In CO2 + 37DM1O + C10OH, ij’s were insensitive to solute ratio and, as a result, SPij’s were positively correlated with bulk fraction of the more soluble species. The alkane + alcohol ij compositional dependency may enable optimisation of a SFE process with a variable or manipulable feed composition. Four thermodynamic models were evaluated for their ability to correlate the experimental vapour and liquid phase equilibrium pressures and compositions. The models, presented in order of decreasing performance, were RK-ASPEN, SR-POLAR, PR-BM and PC-SAFT. In the two C10OH-containing systems at 35 °C, all four models essentially failed. Regardless the mixture, RK-ASPEN and PC-SAFT respectively produced the best and worst pressure correlations with percentage average absolute deviations of 3.1, 4.0 and 3.1 % (RK-ASPEN) and 8.5, 18.3 and 12.9 % (PC-SAFT). RK-ASPEN and SR-POLAR were superior at capturing the s-shaped liquid phase complexity and co-solvency pinch in the high-nC12 regions. In general, however, RK-ASPEN produced the best composition estimates, particularly for the vapour phase. Model-predicted relative solubilities and separation potentials were also evaluated. RK-ASPEN and PC-SAFT correctly ranked the systems in terms of suitability for SFE with CO2 and, on average, produced the smallest absolute deviations in ij. Regardless the mixture, SPij estimates via PC-SAFT, surprisingly, were superior. Density inversions or barotropy was detected in the high-C10OH region of both C10OH-containg systems at 35 °C and pressures in the approximate range 180 to 190 bar. Interesting images illustrating surface tension effects at the inverted conditions and a pressure reduction sequence passing through the inversion point are presented. A number of valuable academic contributions have emanated from this work. Content from the equipment design review was published as a chapter in an edited volume and serves as a valuable point of departure for those who intend to construct similar equipment. In: M.R. Belinsky (Ed.), Supercritical Fluids, Nova Publishers, New York, 2010, Ch.6. The apparatus, design considerations and unique aspects of the equilibrium cell were discussed in a journal publication. This expands the body of knowledge related to high-pressure research. Chemical Engineering & Technology 38 (2015) 1165-1172. A second methodology paper covered GC design criteria and quantitative calibration methods, and presented novel visuals illustrating the challenges associated with high-pressure sampling. Chemical Engineering & Technology 39 (2016) 1475-1482. New phase composition data and modelling results for CO2 + nC12 + 37DM1O were presented in a journal publication. Given the scarcity of and challenges associated with producing high-pressure ternary VLE data, this represents a valuable contribution. The Journal of Supercritical Fluids 130 (2017) 105-117. New phase composition data and modelling results for CO2 + 37DM1O + C10OH, and visuals illustrating density inversion behaviour, were in September 2017 submitted for publication. New phase composition data were measured for CO2 + nC12 + C10OH and the data were used to evaluate four thermodynamic models. A manuscript presenting these results and a comparative discussion on phase behaviour of the three ternaries is currently in preparation.

AFRIKAANSE OPSOMMING: Superkritiese vloeiers word benut binne gespesialiseerde toepassings in die voedsel-en-drank, petroleum, aardgas, polimere, farmaseutiese en gevorderde materiale industrieë. Superkritiese vloeier ekstraksie (SVE) verteenwoordig ‘n skeidingsnis en maak gebruik van die manipuleerbare eienskappe van ‘n hoë-druk oplosmiddel om sodoende uitdagende ekstraksies te bewerkstellig. Een voorbeeld hiervan behels die suiwering van alkohol produkstrome in die sogenaamde ‘wasmiddel’ industrie. Dié strome vertoon dikwels ‘n reeks van koolstofkettinglengtes en bevat noemenswaardige hoeveelhede alkaanreste tesame met lineêre en vertakte alkoholisomere. Die kook en vriespunte van dié komponente is dikwels nou-verspreid of oorvleuel en SVE word oorweeg as ‘n alternatiewe tegnologie vir produksuiwering. Robuuste termodinamiese modelle staan sentraal tot SVE prosesontwerp, maar modelontwikkeling berus deels op akkurate en omvattende hoë-druk fase-ewewigsdata. Damp-vloeistofewewigsdata van ternêre oplosmiddel + opgeloste stof A + opgeloste stof B sisteme is veral nuttig, want die wisselwerking tussen opgeloste stof A en opgeloste stof B – ‘n faktor wat mengselgedrag bepaal – word inherent inaggeneem. Ongelukkig bly data van dié aard skaars, want die nodige analatiese toerusting is ingewikkeld en duur. ‘n Hoë-druk, multi-komponent fase-ewewigsopstelling is ontwerp, met dié ontwerp gebaseer op die statiese-analitiese beginsel. Die opstelling kan bedryf word by maksimum temperature en drukke van 150 °C en 300 bar. Die ewewigsel maak voorsiening vir veranderbare volume en besigtiging van die totale binneruim, en is toegerus met twee ROLSITM monstertoestelle. Dié konfigurasie ondersteun gelyktydige monsterneming uit twee fases gevolg deur gelyktydige parallelle aanlyn analises via gaschromatografie (GC). Bykomende eienskappe van die opstelling sluit in die vertikale verstelling van beide monstertoestelle terwyl die sisteem onder druk verkeer en verminderde dooie-volume. ’n Hoë-definisie kamera stel die operateur in staat om fisiese verskynsels, maar ook fynere detail wat die akkuraatheid van monsterneming en analises beïnvloed, waar te neem. Vloeistofsirkulasie deur ‘n mantel tesame met ‘n konveksieoond word gebruik om die ewewigsel te verhit. Die aanlyn gaschromatograaf is toegerus met twee split-splitlose inlate, vier kolomme, twee skakelkleppe en drie detektors. Dié hardeware is rangskik in twee parallelle paaie en onderskraag die gelyktydige analise van twee monsters met beide vlugtige gasse en medium-lengte koolwaterstowwe. Aangepaste handspuittegnieke het die daarstelling van kwantitatiewe GC kalibrasiemetodes, wat ontkoppel is van die ewewigsel, moontlik gemaak. Die opstelling is geverifieer deur monsterneming vanuit ‘n homogene ternêre mengsel van bekende samestelling en vegelyking met binêre en ternêre verwysingsdata vanuit literatuur. Visuele waarneming van die selinhoud het geblyk noodsaaklik te wees vir akkurate monsterneming van die dampfase. Beeldmateriaal toon dat monsterneming ‘n hoë-druk ewewigsisteem kan versteur selfs al bly die druk en temperatuur stabiel tot binne 0.1 bar en 0.01 °C. Versteurings van dié aard manifesteer in een van drie fenomene: globale misvorming, lokale misvorming, of geen-waarskuwing druppelvorming. Op ‘n soortgelyke wyse word die impak van temperatuurgradiënte, en veral die klein skaal van die versteurings wat volg, oorgedra. Die nuwe apparaat is gebruik vir eksperimentele metings van nege mengsels bestaande uit die oplosmiddel CO2 en verskillende kombinasies van die opgeloste stowwe n-dodekaan (nC12), 3,7-dimetiel-1-oktanol (37DM1O) en 1-dekanol (C10OH). Drie ternêre mengsels – [CO2 + nC12 + 37DM1O], [CO2 + 37DM1O + C10OH] en [CO2 + nC12 + C10OH] – is elk bestudeer by CO2-vrye algehele opgeloste stof A:opgeloste stof B massaverhoudings van 75:25, 50:50 en 25:75. Fasesamestellings is gemeet by 35, 55 en 75 °C en drukke tussen 68 en 237 bar. Die eksperimentele raamwerk en veral teiken-gereduseerde drukke ondersteun ‘n vergelykende studie met etaan as oplosmiddel. Fasegedrag in die twee alkaan-bevattende mengsels – nC12 + 37DM1O en nC12 + C10OH – het ooreenkomste getoon en afgewyk van gedrag in die alkohol + alkohol mengsel, 37DM1O + C10OH. Die teenwoordigheid van nC12 het gelei tot verhoogde oplosbaarheid in die ternêre mengsel en in die hoë-nC12 gebied is ko-solvensie waargeneem. Op Gibbs fasediagramme vertoon dié as s-vormige konvekse-na-konkawe vloeistoffase krommings, konkawe dampfase krommings en, by intermediêre drukke, ‘n geknypte tweefaseband. In die CO2 + 37DM1O + C10OH sisteem is geen verhoogde oplosbaarheid of geknypte tweefasebande waargneem nie en vloeistoffase kurwes was deurgaans konveks van aard. Relatiewe oplosbaarheid (ij) en skeidingspotensiaal (SPij) is gebruik as parameters om die doeltreffendheid van die oplosmiddel te evalueer. Eersgenoemde omskryf die bereikbare skeidingsskerpte terwyl laasgenoedme ‘n funksie is van beide ij en die dampfasefraksie van die meer oplosbare koolwaterstof. Die geskiktheid vir skeiding deur middel van hoë-druk CO2 het afgeneem in die volgorde [nC12 + C10OH] > [nC12 + 37DM1O] > [37DM1O + C10OH]. In beide die nC12-bevattende sisteme was ij’s, en tot ‘n groot mate ook SPij’s, positief gekorreleerd met die algehele fraksie van die minder oplosbare koolwaterstof. In die CO2 + 37DM1O + C10OH sisteem was daar geen verwantskap tussen ij’s en die verhouding van opgeloste stowwe nie en gevolglik was SPij’s positief gekorreleerd met algehele fraksie van die meer oplosbare koolwaterstof. Die samestelling-afhanklikheid van ij’s in die alkaan + alkohol mengsels kan benut word om ‘n SVE proses met variërende of manipuleerbare voersamestelling te optimeer. Vier termodinamiese modelle is getoets vir hul vermoë om die eksperimentele ewewigsdrukke en samestellings van die damp en vloeistoffases te korreleer. Die modelle, gerangskik volgens afnemende prestasievermoë, was RK-ASPEN, SR-POLAR, PR-BM en PC-SAFT. In die twee C10OH-bevattende mengsels by 35 °C het al vier modelle gefaal om sisteemgedrag te beskryf. Ongeag die mengsel, RK-ASPEN en PC-SAFT het onderskeidelik die beste en slegste drukkorrelasies, met persentasie gemiddelde absolute afwykings van 3.1, 4.0 and 3.1 % (RK-ASPEN) en 8.5, 18.3 and 12.9 % (PC-SAFT), gelewer. Wat betref die beskrywing van die s-vormige vloeistoffase-kompleksiteit asook die ko-solvensie vernouing het RK-ASPEN en SR-POLAR die ander modelle oortroef. RK-ASPEN het egter, in die algemeen, die mees akkurate samestellingskattings gelewer en veral só vir die dampfase. ‘n Evaluasie van die modelle se vermoë om relatiewe oplosbaarheid en skeidingspotensiaal te voorspel, is ook uitgevoer. RK-ASPEN en PC-SAFT het die drie sisteme korrek rangskik in terme van geskiktheid vir SVE met CO2 en het, in die reël, die kleinste absolute afkwykings in ij gelewer. Ongeag die mengsel, PC-SAFT het, verrassend, die mees akkurate SPij skattings gelewer. Digtheidsinversies of barotropie het voorgekom in die hoë-C10OH gebied van beide C10OH-bevattende sisteme by 35 °C en drukke van ongeveer 180 tot 190 bar. Interessante beeldmateriaal wat oppervlakspanningseffekte by die omgekeerde toestande en ‘n druverlagingsreeks wat deur die inversiepunt beweeg uitbeeld, word getoon. ‘n Aantal waardevolle akademiese bydrae het vanuit dié studie voortgevloei: Die hersiening van toerustingontwerpe is gepubliseer as boekhoostuk in ‘n saamgestelde volume en dien as handige vertrekpunt vir andere wat beoog om soortgelyke toerusting te bou. In: M.R. Belinsky (Red.), Supercritical Fluids, Nova Publishers, New York, 2010, Hfst.6. Die apparaat, ontwerpsoorwegings en unieke aspekte van die ewewigsel is bespreek in ‘n joernaalpublikasie wat die kennisbasis rakende hoë-druk navorsing verbreed. Chemical Engineering & Technology 38 (2015) 1165-1172. ‘n Tweede metodologie-publikasie het GC ontwerpskriteria en kwantitatiewe kalibrasiemetodes gedek, en beeldmateriaal wat die uitdagings van hoë-druk monsterneming uitbeeld, getoon. Chemical Engineering & Technology 39 (2016) 1475-1482. Nuwe fasesamestellingsdata en modelleringsresultate vir CO2 + nC12 + 37DM1O is gepubliseer. Gegewe die skaarsheid en uitdagings gepaardgaande die produksie van hoë-druk ternêre damp-vloeistofewewigsdata is dié publikasie ‘n noemenswaardige bydrae. The Journal of Supercritical Fluids 130 (2017) 105-117. Nuwe fasesamestellingsdata en modelleringsresultate vir CO2 + 37DM1O + C10OH, en beeld-materiaal wat die digtheidsinversies uitbeeld, is in September 2017 ingedien vir publikasie. Nuwe fasesamestellingsdata is gemeet vir CO2 + nC12 + C10OH en dié data is gebruik in ‘n evaluasie van vier termodinamiese modelle. ‘n Manuskrip wat dié resultate kommunikeer en ‘n vergelykende bespreking van fasegedrag in die drie ternêre mengsels aanbied, word tans voorberei.

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