High pressure phase equilibria of the system CO2 + N-dodecane + 3,7-dimethyl-1-octanol + 1-decanol

Latsky, Carla (2019-12)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: In the detergent manufacturing industry, the product streams containing the detergent range alcohols (C8-C20) are often contaminated with unreacted alkanes. Due to similarity in the boiling and melting points of these alkanes and alcohols, traditional separation techniques such as distillation and crystallization are ineffective. Previous work has revealed that supercritical CO2 fractionation is a viable alternative to separate detergent range alcohols and alkanes. It was however found that significant solute-solute interactions exist in mixtures containing CO2 with detergent range alcohols and alkanes, which influences the phase behaviour and complicates thermodynamic modelling of such mixtures. The primary aim of this study was to experimentally characterise the solute-solute interactions which occur in mixtures containing CO2 with detergent range alkanes and alcohols, particularly in the quaternary system containing CO2 with n-dodecane, 3,7-dimethyl-1-octanol and 1-decanol. The secondary aim was to select and fit thermodynamic models within the Aspen Plus® software to determine and compare their ability to account for the interactions which occur in this system to allow prediction of phase equilibrium data for the quaternary system. Evaluation of the effect that fitting the model parameters using different data types has on the model’s ability to predict phase equilibrium data forms part of the secondary aim. The solute-solute interactions which exist in mixtures containing CO2 with n-dodecane, 3,7-dimethyl-1-octanol and 1-decanol were characterised by analysing high pressure bubbleand dew-point and vapour-liquid-equilibrium (VLE) data of systems containing these components. To allow comprehensive analysis of the solute-solute interactions, additional phase behaviour data were required. New bubble- and dew-point data were generated for the ternary CO2 + 3,7-dimethyl-1-octanol + n-dodecane and CO2 + 3,7-dimethyl-1-octanol + 1-decanol systems as well as for the quaternary system. The data were measured at temperatures ranging from 308 K to 358 K and solute mass fractions ranging from 0.015 to 0.630, using the synthetic visual phase detection method. New VLE data were measured for the quaternary system. The data were measured at temperatures ranging from 308 K to 348 K and pressures up to 19.2 MPa, using an analytic-sampling method. The phase equilibrium data revealed that the solute-solute interactions which occur in these mixtures result in complex phase behaviour phenomena. The bubble- and dew-point data indicated the occurrence of co-solvency in mixtures consisting largely of n-dodecane. The relative solubility analysis revealed that the co-solvency which occurs in n-dodecane rich mixtures likely results in pinches in separation, but this can be resolved by incorporating a pressure-temperature swing setup. The mixtures consisting largely of 1-decanol were found to exhibit a temperature inversion. The RK-SOAVE, RK-ASPEN, CPA and PSRK models in Aspen Plus® were selected for the thermodynamic modelling performed in this work. These models are all based on the SRK equation of state, but they differ with regards to mixing rules and approaches to accounting for solute interactions. The RK-ASPEN model with solvent-solute and solute-solute BIPs (no 1-decanol + 3,7-dimethyl-1-octanol BIP) regressed from bubble- and dew-point data (HPBDP) was found to be the best suited model to predict bubble- and dew-point data and approximate VLE data for the quaternary system. The 1-decanol + 3,7-dimethyl-1-octanol BIP was excluded from the model as the incorporation is suspected to exaggerate the interaction between the two polar components, resulting in a decrease in model accuracy. The fact that the optimum model for the prediction of quaternary bubble- and dew-point and VLE data is based on HPBDP data is desired as the measurement technique used to obtain HPBDP data is much easier, cheaper and faster than the method used to measure VLE data. The PSRK model was found to be one of the less suited models for the prediction of quaternary phase equilibrium data and the results highlighted the need for BIPs regressed from high pressure data in regions where the solute-solute interactions significantly impacted the phase behaviour. As the system is fully characterised experimentally, future work should be focussed on thermodynamic modelling of the system. It is suggested that the CPA model which includes a quadrupole term be investigated and other modelling tools such as the thermodynamic software package, VLXE, be evaluated.

AFRIKAANSE OPSOMMING: In die skoonmaakmiddel vervaardigings-industrie word die alkohol (C8 – C20) produkstroom dikwels gekontamineer met ongereageerde alkane. Die C8-C20 alkane en alkohol-isomere in die produkstrome het ooreenstemmende smelt- en kookpunte en dus kan effektiewe skeiding nie met tradisionele metodes soos distillasie en kristallisasie bewerkstellig word nie. Vorige studies het gevind dat superkritiese CO2 fraksionering ‘n lewensvatbare alternatief is om die alkane en alkohol-isomere te skei. Die studies het wel ook gevind dat daar in mengsels wat bestaan uit, superkritiese CO2 en C8-C20 alkane en alkohol-isomere, sterk interaksies tussen die komponente is. Hierdie interaksies beïnvloed die fase-gedrag en bemoeilik termodinamiese modellering. Die primêre doelwit van hierdie projek was om eksperimenteel die interaksies wat plaasvind in mengels wat bestaan uit superkritiese CO2 + C8-C20 alkane en alkohol-isomere te karakteriseer, deur te fokus op die CO2 + n-dodekaan + 3,7-dimetiel-1-oktanol + 1-dekanol sisteem. Die sekondêre doelwit was om termodinamiese modelle in Aspen Plus® te kies en te verfyn en om die vermoë van die modelle om die interaksies wat plaasvind in die sisteme in ag te neem, om sodoende akkurate fase-gedrag vir die kwatinêre sisteem te voorspel, te bepaal en te vergelyk. Evaluering van die effek wat die tipe regressie data wat gebruik word om die model parameters te bepaal het op die vermoë van die model om ewewigsdata te voorspel vorm ook deel van die sekondêre doelwit. Die interaksies wat plaasvind in die CO2 + n-dodekaan + 3,7-dimetiel-1-oktanol + 1-dekanol sisteem was ondersoek deur hoë-druk oplosbaarheidsdata en damp-vloeistof-ewewigsdata vir sisteme, wat bestaan uit hierdie komponente, te analiseer. Addisionele hoë-druk ewewigsdata was benodig om ‘n omvattende analise van die interaksies wat in die mengsels plaasvind, te doen. Nuwe hoë-druk oplosbaarheidsdata was gemeet vir die ternêre CO2 + n-dodekaan + 3,7- dimetiel-1-oktanol en CO2 + 3,7-dimetiel-1-oktanol + 1-dekanol sisteme, sowel as die kwatinêre sisteem. Die sintetiese visuele fase-opsporingsmetode was gebruik om die oplosbaarheidsdata van die sisteme te meet tussen 308 K en 358 K met totale alkaan en/of alkohol massafraksies tussen 0.015 tot 0.630. Nuwe damp-vloeistof-ewewigsdata was gemeet vir die kwatinêre sisteem deur gebruik te maak van ‘n analitiese metode wat damp en vloeistof monsters aanlyn geanaliseer het met gaschromatografie. Die data was gemeet tussen temperature van 308 K en 348 K en met drukke tot 19.2 MPa. Die ewewigsdata het aangedui dat die interaksies wat plaasvind tussen die komponente komplekse fase-gedrag veroorsaak. Die oplosbaarheidsdata het aangedui dat mengsels wat grootliks uit n-dodekaan bestaan, verhoogde oplosbaarheid in CO2 toon in vergelyking met die suiwer komponente. Die relatiewe oplosbaarheidsanalise het aangedui dat die verhoogde oplosbaarheid wat plaasvind in die n-dodekaan ryk mengsel moontlik lei tot skeidingsknyppunte, die probleem kan wel opgelos word deur die implementering van ‘n druk-temperatuur omkeer stelsel. Die fasegedrag van mengsels wat grootliks uit 1-dekanol bestaan het ‘n temperatuur inversie getoon. Die Aspen Plus® modelle wat ondersoek is in die werk is die RK-SOAVE, RK-ASPEN, CPA en PSRK modelle. Hierdie modelle is almal gebaseer op die SRK toestandsvergelyking, maar verskil ten op sigte van mengreëls en die manier waarop hul interaksies benader/beskryf. Die RK-ASPEN model met oplosmiddel-opgeloste stof interaksie parameters en opgeloste stofopgeloste stof (geen 1-dekanol + 3,7-dimetiel-1-oktanol interaksie parameter) interaksie parameters bepaal deur die regressie van hoë-druk oplosbaarheidsdata, was gevind om die mees gepaste model te wees vir die voorspelling van oplosbaarheidsdata en die benadering van damp-vloeistof-ewewigsdata vir die kwatinêre sisteem. Die 1-dekanol + 3,7-dimetiel-1-oktanol interaksie parameter was nie in die model ingesluit nie, want dit word vermoed dat dit die interaksie tussen die twee polêre komponente oordryf wat model akkuraatheid verlaag. Die feit dat die optimum model vir die voorspelling van die kwatinêre ewewigsdata (oplosbaarheidsdata en damp-vloeistof-ewewigsdata) gebaseer is op interaksie parameters wat bepaal is deur die regressie van hoë-druk oplosbaarheidsdata is gewens aangesien die eksperimente om oplosbaarheidsdata te meet veel makliker, goedkoper en vinniger is as die eksperimente om damp-vloeistof-ewewigsdata te meet. Die PSRK model was een van die minder akkurate modelle vir die voorspelling van die kwatinêre ewewigsdata en die modelleringsresultate het die nodigheid van interaksie parameters, bepaal deur die regressie van hoë-druk data, uitgewys in areas waar die interaksies die fase-gedrag beduidend beïnvloed het. Aangesien die sisteem ten volle eksperimenteel beskryf is, is aanbevelings vir toekomstige werk slegs gerig op termodinamiese modellering van die sisteem. Dit word voorgestel dat die CPA model wat ‘n kwadrupool term insluit ondersoek word en ander modelleringsprogramme soos byvoorbeeld die termodinamiese sagteware, VLXE, evalueer word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107264
This item appears in the following collections: