Theses and dissertations (Engineering and Built Environment)

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Infinite dilution activity coefficient measurements for limonene as a green solvent for separation
(2021-12-01) Mbatha, Banzi Patrick; Ramsuroop, Suresh; Ngema, Peterson Thokozani
There is an increasing call from the international communities for the replacement of traditional petrochemical solvents used by the chemical and allied industries in the separation processes. This has led to the new interest in finding alternative “green” solvents, which can be used to optimize the separation processes of non-ideal or close boiling mixtures for better separation. This study focuses on investigating limonene as a “green” solvent to be utilized as a separating agent for separation processes. Limonene is a non-polar monoterpene solvent extracted from essential oils of the citrus peels. The extraction and distillation of this biomass extracted solvent releases fewer toxic pollutants and volatile gases, and as a result it has minimal impact to the environment. The infinite dilution activity coefficients (IDACs) for various solutes, which include alkanes, alkenes, alkynes, cycloalkanes, heterocycles, alcohol, aromatics, ketones, ethers, nitrile and water in the limonene solvent were measured using gas-liquid chromatography at (303.15, 313.15, 323.15 and 333.15) K. Through the experimental infinite dilution activity coefficients (IDACs), the values of partial molar excess enthalpy at infinite were obtained using the Gibbs-Helmholtz equation. To evaluate its potential of limonene as a mass transfer separation agent, its selectivity and capacity were calculated from the experimental limiting activity coefficients and were compared with ionic liquids and conventional solvents. From the results of this study, it was generally observed that for all solutes the activity coefficient at infinite dilution decreased with the increase of temperature and increased with the increase of alkyl chain length of the solute. The triple bond alkyl solutes had a strong interaction with the limonene, due to their low values of activity coefficients at infinite dilution. In some selective test cases, the selectivity and capacity for the separation of hexane/hex-1-ene and ethanol/water showing promising results when compared with ILs. The selectivity and capacity for the separation mixture of heptane/benzene, octane/ethylactetate, heptane/pyridine, octane/pyridine, and octane/thiophene indicated that the limonene was not suitable as the extraction solvent when compared with other ILs and conventional solvents. However, more investigation of limonene must be conducted through measurements liquid-liquid equilibrium and vapour-liquid equilibrium. Such data would provide useful information and understanding into the separation of hexane/hex-1-ene and ethanol/water mixtures. Green solvents extracted from biomass which have high boiling temperatures also be studied and compared with limonene solvent.
Infinite dilution activity coefficient measurements of organic solutes in selected deep eutectic solvents by gas-liquid chromatography
(2018) Nkosi, Nkululeko; Ramsuroop, Suresh; Tumba, Kaniki Armel; Osman, Khalid
Many separation processes in the chemical and petrochemical industries are energy intensive, and unfortunately, involve a range of solvents that are environmentally harmful and destructive. Alternative, sustainable separation techniques are desired to replace these conventional methods used in the separation of azeotropic as well as close-boiling mixtures, with the intention of reducing energy costs and adverse impact on the environment. In the present study, a new class of solvents called deep eutectic solvents (DESs) of Type III were investigated as alternatives to conventional solvents currently employed in separation processes. DESs are classified as ‘green’ solvents because of a range of favourable properties including lower cost, desirable solubility properties and reduced environmental impact (Abbott et al., 2003b; Smith et al., 2014). The infinite dilution activity coefficients (IDACs) values of 24 solutes – including alk-1-anes, alk-1-enes, alk-1-ynes, cycloalkanes, alkanols, alkylbenzenes, heterocyclics, esters, and ketones – were measured at 313.15, 323.15, 333.15 and 343.15 K by gas-liquid chromatography (GLC) in DESs. The four investigated DESs were as follows: 1) Tetramethylammonium chloride + Glycerol (DES1); 2) Tetramethylammonium chloride + Ethylene Glycerol (DES2); 3) Tetramethylammonium chloride + 1,6 Hexanediol (DES3); and 4) Tetrapropylammonium bromide + 1,6 Hexanediol (DES4). This work focused on the performance of DESs as extractive solvents for selected azeotropic and close-boiling binary mixtures. The two key performance criteria for these extractive solvents – selectivity and capacity – were determined from experimental infinite dilution activity coefficients (IDACs) of various solutes. The effect of solute molecular structure on IDAC values was investigated. Moreover, the effect of varying the hydrogen bond donors (HBDs) in DESs on IDAC values was examined. Partial excess molar enthalpies at infinite dilution were determined from the experimental IDAC data. Moreover, common industrial separation problems were selected to investigate DES potential to separate various mixtures by determining selectivity and capacity at infinite dilution. The results obtained in this study indicate that the use of a long carbon chain HBDs greatly decreases miscibility of DESs with organic solutes. For systems such as n-heptane - toluene, acetone - ethanol, cyclohexane - benzene and n-hexane - benzene systems, DES4 was the best solvent regarding the separation performance index. However, further investigation for DES4 by measurements of vapour-liquid equilibria (VLE) and liquid-liquid equilibria (LLE) data is suggested, as these data would provide additional pertinent information regarding the separation of such mixtures using DES4. The data produced from this study can be used to extend the applicability range of predictive models such as Universal Quasi- Chemical Functional Group Activity Coefficients (UNIFAC) and modified UNIFAC (Do) which are already incorporated in some chemical engineering process simulators.