Ionic liquid performance in pilot plant contactors for aromatics extraction

S.A.F. Onink

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    The main objectives of this study were an investigation into the applicability, in this case extraction capacity and equipment performance, of room temperature ionic liquids as solvent in the extraction of aromatics from aliphatics and a comparison of three types of contactors (a rotating disc contactor (RDC), a Kühni contactor and a pulsed disc and doughnut column (PDDC)) for this extraction. The separation of aromatic hydrocarbons (benzene, toluene, ethylbenzene and xylenes) from C4 – C10 aliphatic hydrocarbon mixtures is challenging since these hydrocarbons have boiling points in a close range and several combinations form azeotropes, ruling out conventional distillation processes. The contactors, RDC, PDDC and Kühni were selected since they are amongst the most commonly used extractors, or are most promising for (aromatics) extraction in the (petro)chemical industry. Early research showed that ionic liquids are promising solvents for the extraction of aromatics from aliphatics, but these studies are mainly based on a thermodynamic approach and only a conceptual process design was suggested. However, successful introduction of RTILs into extraction operations also requires knowledge on their physical properties, hydrodynamics, mass transfer characteristics, stability after long term usage and, since the costs of replacement of lost solvent plays an important role, recovery of the RTIL from the raffinate stream. Model To fully understand the behavior of RTILs as solvent for aromatics extraction, a theoretical model was developed using existing theory on rotating disc contactors. This model described the operational and mass transfer characteristics of an RDC for the extraction of toluene from n-heptane with Room Temperature Ionic Liquids. Mass transfer characteristics were modeled with a differential axial dispersion model, with correlations for the mass transfer coefficient and the axial dispersion coefficient. The modeled hydraulic characteristics covered the Sauter mean diameter, the hold-up of the dispersed phase and the operational window, all correlated using physical properties, operational parameters, and geometrical characteristics of the column.. Since all applied equations were originally derived for conventional solvents, corrections might be necessary, which need to be confirmed by pilot experiments. All equations are based on the physical properties and operational parameters of the system, geometrical characteristics of the column and internals as well as certain fit parameters, describing the influence of each variable. Physical properties and Liquid-Liquid Extraction Equilibiria (LLE) The input of above mentioned model consisted of physical properties and data on the liquid-liquid extraction equilibrium. Density data were correlated well using a linear relation for the influence of the concentration of solute, whereas the volumetric thermal expansion coefficient is used to describe the temperature influence on the system. The absolute values of the relative error (AARE’s) varied between 0.10 and 0.76%. The density of sulfolane was the largest, followed by that of [4-mebupy]BF4 and followed by that of [3-mebupy][DCA]. Viscosity data were correlated using the Nissan and Grunberg equation for the influence of concentration, taking into account the influence of temperature on the binary interaction parameter. The Vogel equation was used to accurately describe the influence of temperature on the viscosity of the pure solvents, leading to AARE’s of 0.30 to 4.03%. The viscosities of both investigated ionic liquids exceeded the viscosity of the commonly used solvent sulfolane, but with increasing toluene content the viscosities of both RTILs decreased dramatically. Interfacial tension data were fitted with the Szyzkowski equation and the Jasper equation for the temperature influence on the binary system. The influence of concentration on the ternary systems was described via the concentration of toluene in the organic phase, which resulted in good fits. An influence of the temperature on the Szyzkowski coefficients for [3-mebupy][DCA] was observed, but for [4-mebupy]BF4, this influence was not clearly present. The resulting AARE’s were between 2.5 and 3.2%, hence indicating that a good representation of the interfacial tension for all systems was obtained using this approach. LLE phase compositions for the the systems n-heptane + toluene + [4-mebupy]BF4 and n-heptane + toluene + [3-mebupy][DCA] were determined at 313 K. Regarding extraction capacity, [3-mebupy][DCA] outperformed [4-mebupy]BF4 and sulfolane when applied as solvent for the extraction of toluene from a mixture of toluene and n-heptane with a toluene content below 40 wt%. The weight based distribution coefficient decreased from 0.45 to 0.34 and from 0.32 to 0.25 for [3-mebupy][DCA] and [4-mebupy]BF4, respectively and increased from 0.26 to 0.34 for sulfolane for concentrations of 6 to 40 weight percent toluene in n-heptane. The selectivity decreased from 65 to 29, from 30 to 13 and from 25 to 11 for [3-mebupy][DCA], sulfolane and [4-mebupy]BF4, respectively. Solvent comparison The use of the ionic liquids [4-mebupy]BF4 and [3-Mebupy][DCA] as solvents for the extraction of toluene from n-heptane on a pilot plant rotating disc contactor has been investigated and benchmarked against the conventional solvent sulfolane. It has been found that both RTILs could be applied as solvent for the extraction of toluene from n-heptane. Furthermore, the studied hydrodynamic parameters, drop size, hold-up and operational area, indicated that the use of the ionic liquids [4-mebupy]BF4 and [3-mebupy][DCA] as extraction solvents was not limited by their higher viscosity of 80 mPa s and 20 mPa s for pure [4-mebupy]BF4 and [3-mebupy][DCA], respectively. The pure IL viscosities decreased with increasing toluene content in the ionic liquid phase to 25 mPa s and 12 mPa s for a toluene content of 15 wt%, whereas the viscosity of sulfolane only decreased from 8 to 6 mPa s. At fluxes of 3 to 8 m hr-1, the resulting drop sizes ranged from 1.1 to 0.6 mm, for the ILs as well as sulfolane at rotor speeds between 200 to 800 rpm. At first sight it seemed that unexpected behavior for the hold-up was observed in experiments when the RTILs were applied as solvent: the hold-up decreased with increasing rotor speed, whereas an increase was expected, because of formation of smaller droplets (and consecutively a smaller down going velocity) with increasing energy input. This phenomenon could be explained by the existence of three operational regimes for a rotating disc contactor; which clearly depend on flux and on rotor speeds. The operational window is largest for [4-mebupy]BF4 and smallest when [3-mebupy][DCA] is used as solvent. This is caused by a higher interfacial tension, smaller viscosity and lower density difference when [3-mebupy][DCA] is applied as solvent. The mass transfer capacities, expressed in HETS, were comparable for [3-mebupy][DCA] and sulfolane with HETS of 1.5 m and 4.5 m for [4-mebupy]BF4 for the extraction of 10 wt% toluene from n-heptane at 313K. It was concluded that the overall performance of [3-mebupy][DCA] exceeded the extraction capacities of sulfolane and [4-mebupy]BF4, because the highest interfacial area was obtained when [3-mebupy][DCA] was used as solvent. After optimization of the hydrodynamic behavior parameters, which were selected on basis of a sensitivity analysis, by minimizing the AARE’s and with a correction for the diffusion coefficients of the RTILs, the resulting model consisted of equations capable to describe the Sauter drop diameter, hold-up, operational regimes and mass transfer coefficients resulting in a good qualitative description of the hydrodynamic behavior and overall separation performance. Solubilities of RTILs in aromatic/aliphatic mixtures Since also recovery of the solvent plays an important role in the design of an extraction process, because of costs and environmental aspects, an ion chromatographic (IC) method for the quantification of the RTILs concentrations in aromatic solvents has been developed. With an analysis time of only 20 minutes, linear standard curves for both RTILs (R2 varying from 0.9980 to 0.9998), precisions (relative standard deviations) being better than 2.9%, accuracy ranging from 98.5 to 105.4% and finally a limit of detection for both RTILs in all solvents varying between 0.7 and 2.6 mg kg-1, the method was proven to be simple and efficient with excellent accuracy and precision. Results for the solubilities of two RTILs, [4-mebupy]BF4 and [3-mebupy][DCA], in four aromatic solvents and in mixtures of n-heptane and toluene showed that with an increase of the aromatic character of the solvent (ethylbenzene ¿ o-xylene <toluene <benzene) more RTIL dissolves. In every solvent or mixture of n-heptane and toluene used, but most notable for benzene, more [4-mebupy]BF4 is dissolved than [3-mebupy][DCA]. This latter effect is primary caused by the nature of the anion. Below concentrations of 50 weight percent toluene in n-heptane for both RTILs, the solubilities were below the detection limits (±1 mg kg-1). Recovery and regeneration When applying ionic liquids as solvent for the extraction of toluene from n-heptane also the recovery of these solvents from the raffinate, n-heptane stream, and the long-term stability play a major role. Although the solubility of [3-mebupy][DCA] in toluene/n-heptane mixtures was found below the detection limit, in the raffinate stream higher concentrations were found due to entrainment. For the recovery of the ionic liquid from the raffinate phase, a mixer settler with a coalescer with water as solvent was suggested, because of the high affinity for ionic liquids indicated by a distribution coefficient above 56,000. However, water has a negative influence on the extraction performance of [3-mebupy][DCA] and should therefore be totally removed. Except for some colorization, the physical properties and extraction capacity of [3-mebupy][DCA] after extensive usage (approximate 2000 hrs) and regeneration for more than 1200 hrs at elevated temperatures (up to 400 K) were not changed. This was verified by comparing the density, viscosity and interfacial tension of new, fresh, [3-mebupy][DCA] and after extensive usage/regeneration. Furthermore, hydrodynamic and mass transfer profiles ‘before’ and ‘after’ were compared. It was shown that an improvement of the extraction capacity after extensive usage was due to a removal of an impurity, present in ‘fresh’ [3-mebupy][DCA]. Contactor comparison In industry, the most commonly used extractor for the separation of aromatics from aliphatics is the rotating disc contactor. Other contactors like the pulsed disc and doughnut column and the Kühni contactor are, to a lesser extent, also used for this type of separation. It had been found that the extraction of toluene from n-heptane is successful for all three contactors. Furthermore, the studied hydrodynamic behavior indicated that for the PDDC the largest operational window and the largest hold-up were obtained. The mass transfer capacity, expressed in HETS, was also largest for the PDDC (HETS = 1.2), since for this column the highest specific interfacial area (0.69 mm-1) was achieved as result of a larger hold-up. Hence, the overall performance of the PDDC exceeded the extraction capacities of the RDC and Kühni contactor, which could be explained by the fact that in the PDDC the energy input was distributed more homogeneous than when the RDC or Kühni were applied.
    Originele taal-2Engels
    KwalificatieDoctor in de Filosofie
    Toekennende instantie
    • Chemical Engineering and Chemistry
    • de Haan, André, Promotor
    • Meindersma, Wytze, Co-Promotor
    Datum van toekenning31 aug 2011
    Plaats van publicatieEindhoven
    Gedrukte ISBN's978-90-386-2538-6
    StatusGepubliceerd - 2011

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