Extractive distillation with ionic liquids as solvents : selection and conceptual process design

Extractive distillation technology is widely used in the chemical and petrochemical industries for separating azeotropic, close-boiling and low relative volatility mixtures. It uses an additional solvent in order to interact with the components of different chemical structure within the mixture. The activity coefficients are modified in such a way that the relative volatility is increased. Therefore, the choice of the solvent determines the effectiveness of this process. Several solvent selection methodologies had been developed in the literature. They are based on one-way interaction parameters, meaning interactions of the components to be separated with the solvent. It has been widely accepted to consider as a promising solvent the one which is able to increase the relative volatility the most. However, the total annual cost (TAC) and the energy demand influence the final selection. Ionic liquids (ILs) are promising replacements of existing volatile solvents in extractive distillation. However, at this moment insufficient knowledge exists on the optimal properties for ionic liquids to be employed and their implementation in actual process systems. The main goal of this research was to analyze the selection and performance of ionic liquids in extractive distillation processes for three separation cases which differ from each other in polarity and chemical structures: 1-hexene/n-hexane, methylcyclohexane/toluene and water/ethanol. Theoretical ionic liquid design and selection for each mixture is done using COSMOtherm software (version C2.1, release 01.11a) by predicting activity coefficients at infinite dilution. Experimental selectivities and relative volatilities of real solutions were measured in order to choose the most suitable ionic liquids. At last, different extractive distillation processes using ionic liquids were proposed and analyzed. 1-Hexene/n-hexane separation Olefins are important base chemicals used for the manufacture of poly(olefins), plasticizers, etc. and according to Sasol, the projected demand for 2012 of C6-C8 olefins is around 0.85x106 ton. Due to the small differences in boiling temperatures and to the low relative volatility of the system, the separation of olefins and paraffins is energy intensive, meaning that all the commercially available processes for the production of olefins use several fractional distillation columns. In this work, 1-hexene and n-hexane were chosen as representative olefin and paraffin components. Extractive distillation using N-methyl-2-pyrrolidone (NMP) has been used to separate this mixture. COSMOtherm activity coefficients at infinite dilution were used to select suitable ionic liquids for this case study. Non-cyclic, cyclic and aromatic-like cations were tested in this software in combination with 27 different anions. According to the activity coefficients predicted with COSMOtherm, the solubility and selectivity of ionic liquids in 1-hexene and n-hexane is very low. This was confirmed experimentally for the selected ionic liquids using vapor – liquid equilibrium data. None of the ILs studied in this work is able to significantly increase the relative volatility in comparison with the conventional solvent NMP. Only the ionic liquid 1-hexyl-3-methyl-imidazolium tetracyanoborate [HMIM][TCB] reached a slightly higher relative volatility (1.63) than the conventional solvent NMP (1.55). However, the increase is not large enough to consider this solvent as a suitable replacement. Besides this, the ionic liquids have solubility constraints which force the use of large solvent to feed ratios to avoid the formation of two liquid phases. Methylcyclohexane/toluene separation Aromatics are among the most important chemical raw materials for the manufacture of plastics, synthetic rubber and synthetic fiber. The total production in 2009 in Western Europe of benzene, toluene and p-xylene was about 7.2x106, 1.6x106 and 1.7x106 tons, respectively. Because of the low relative volatility, external agents are used to increase the economic feasibility of the distillation units, e.g. N-methyl-2-pyrrolidone, sulfolane. The activity coefficients at infinite dilution for the mixture methylcyclohexane (MCH) and toluene with ionic liquids predicted with COSMOtherm showed a clear compromise between selectivity (easiness of separation) and solubility. Aromatic-like cations in combination with bis((trifluoromethyl)sulfonyl)imide (BTI) and tetracyanoborate (TCB) anions were selected and experimentally investigated. The relative volatility of the mixture MCH + toluene increased when any of the selected solvents (including the conventional solvent NMP) was added. The results showed that the TCB anion performed better than BTI. Therefore, the ILs 1-hexyl-3-methyl imidazolium tetracyanoborate [HMIM][TCB] and 1-butyl-3-methyl imidazolium tetracyanoborate [BMIM][TCB] seem to be the most promising replacements of NMP in the extractive distillation of methylcyclohexane and toluene. Binary and ternary liquid-liquid experimental data for the systems methylcyclohexane + toluene + [HMIM][TCB] and [BMIM][TCB] were collected and correlated with the NRTL and UNIQUAC thermodynamic models. The binary correlations were less satisfactory than ternary correlations. The results showed that UNIQUAC represented the experimental data better than the NRTL model, with a root mean square error below 0.02. The parameters obtained from the regressions of liquid-liquid equilibrium data were used to predict the vapor-liquid equilibrium (VLE). These were compared with experimental data taken by a headspace technique which showed that UNIQUAC and its parameters are able to predict the VLE of the ternary systems with a maximum error of 0.2. The non-aromatic/aromatic selectivities and relative volatilities of the ionic liquids make them suitable solvents to be used in extractive distillation processes. After obtaining the parameters for the thermodynamic model, process simulations for the extractive distillation technology using the IL [HMIM][TCB] were performed and compared with the benchmark solvent NMP. The process variables (reflux, solvent flow and number of stages) are obtained such that the energy requirements of the process are minimized. Just in the extractive distillation column, the process using the ionic liquid requires 43% less energy than the conventional solvent. Several recovery technologies were analyzed (e.g. flash evaporation, stripping with hot nitrogen, supercritical CO2, and stripping with hot MCH). The most energy efficient process (using [HMIM][TCB]) saves up to 50% of the energy requirements compared to the conventional solvent. This optimized process requires an extractive distillation column of 22 equilibrium stages, using a molar reflux ratio of 0.2 and a solvent to feed mass ratio of 2.03. The recovery of [HMIM][TCB] is done in a stripping column using part of the distillate product of the extractive distillation column as the stripping agent. Ethanol/water separation Ethanol is an important base chemical which is produced from petrochemical streams or bioprocesses. It has been used as solvent, in cosmetic and food industry, among others. However, ethanol as a (partial) replacement of gasoline has influenced its worldwide demand. Just in USA, 42x106 m3 (33x106 tons) of ethanol were added to gasoline in 2009 accounting for about 8% of gasoline consumption by volume. Water is involved in the ethanol production chain. This mixture forms an azeotrope with an ethanol mass composition of 0.956 and its challenging energy-efficient separation has been widely reported. Extractive distillation using ethylene glycol (EG) is commonly used to separate this mixture. Selectivities and activity coefficients at infinite dilution were predicted using COSMOtherm. In this case, the activity coefficients showed high attractive forces among the most promising ionic liquids and water, meaning that these ionic liquids are highly hydrophilic. The experimental relative volatility can be increased up to 23% when the conventional solvent is replaced by 1-ethyl-3-methyl-imidazolium acetate [EMIM][OAc] or 1-ethyl-3-methyl-imidazolium dicyanamide [EMIM][DCA]. These ionic liquids seem to be promising solvents for the extractive distillation of water and ethanol. Ternary VLE data were collected for the systems ethanol + water + [EMIM][OAc] and [EMIM][DCA]. In this case, the NRTL model correlates the data better than UNIQUAC, with a value for the root mean square error below 0.02. The ionic liquids are able to increase the relative volatility of the mixture ethanol – water by strongly attracting the water and making it "less volatile" which makes the recovery of the solvent rather challenging and energy intensive. Only after heat integration, the use of ILs appeared to be more attractive, yielding 16% of energy savings compared to the heat integrated conventional process. The recovery conditions and the relatively low energy savings might limit the applicability of ILs for the separation of ethanol – water by extractive distillation. Overall Finally, it can be concluded that, although ionic liquids can be suitable extractive distillation solvents, special attention should be paid to the solvent recovery technology and its heat integration with the extractive distillation column. In this study the most successful case was the separation of toluene/methylcyclohexane, where tetracyanoborate-based IL [HMIM][TCB] yielded 50% energy savings compared to the conventional solvent

From Marginal Deformations to Confinement

We consider type IIB supergravity backgrounds which describe marginal deformations of the Coulomb branch of N=4 super Yang-Mills theory with SO(4) x SO(2) global symmetry. Wilson loop calculations indicate that certain deformations enhance the Coulombic attraction between quarks and anti-quarks at the UV conformal fixed-point. In the IR region, these deformations can induce a transition to linear confinement.Comment: 14 pages, 4 figures, minor corrections, comments and references adde

High Temperature Electron Localization in dense He Gas

We report new accurate mesasurements of the mobility of excess electrons in high density Helium gas in extended ranges of temperature $[(26\leq T\leq 77) K ]$ and density $[ (0.05\leq N\leq 12.0) {atoms} \cdot {nm}^{-3}]$ to ascertain the effect of temperature on the formation and dynamics of localized electron states. The main result of the experiment is that the formation of localized states essentially depends on the relative balance of fluid dilation energy, repulsive electron-atom interaction energy, and thermal energy. As a consequence, the onset of localization depends on the medium disorder through gas temperature and density. It appears that the transition from delocalized to localized states shifts to larger densities as the temperature is increased. This behavior can be understood in terms of a simple model of electron self-trapping in a spherically symmetric square well.Comment: 23 pages, 13 figure

Gravitational anomalies signaling the breakdown of classical gravity

Recent observations for three types of astrophysical systems severely challenge the GR plus dark matter scenario, showing a phenomenology which is what modified gravity theories predict. Stellar kinematics in the outskirts of globular clusters show the appearance of MOND type dynamics on crossing the $a_{0}$ threshold. Analysis shows a ``Tully-Fisher'' relation in these systems, a scaling of dispersion velocities with the fourth root of their masses. Secondly, an anomaly has been found at the unexpected scales of wide binaries in the solar neighbourhood. Binary orbital velocities cease to fall along Keplerian expectations, and settle at a constant value, exactly on crossing the $a_{0}$ threshold. Finally, the inferred infall velocity of the bullet cluster is inconsistent with the standard cosmological scenario, where much smaller limit encounter velocities appear. This stems from the escape velocity limit present in standard gravity; the ``bullet'' should not hit the ``target'' at more than the escape velocity of the joint system, as it very clearly did. These results are consistent with extended gravity, but would require rather contrived explanations under GR, each. Thus, observations now put us in a situation where modifications to gravity at low acceleration scales cease to be a matter of choice, to now become inevitable.Comment: 10 pages, 5 figures, Astrophysics and Space Science Proceedings 38, 4

Let's Twist Again: General Metrics of G(2) Holonomy from Gauged Supergravity

We construct all complete metrics of cohomogeneity one G(2) holonomy with S^3 x S^3 principal orbits from gauged supergravity. Our approach rests on a generalization of the twisting procedure used in this framework. It corresponds to a non-trivial embedding of the special Lagrangian three-cycle wrapped by the D6-branes in the lower dimensional supergravity. There are constraints that neatly reduce the general ansatz to a six functions one. Within this approach, the Hitchin system and the flop transformation are nicely realized in eight dimensional gauged supergravity.Comment: 31 pages, latex; v2: minor changes, references adde

Semiclassical strings in Sasaki-Einstein manifolds and long operators in N=1 gauge theories

We study the AdS/CFT relation between an infinite class of 5-d Ypq Sasaki-Einstein metrics and the corresponding quiver theories. The long BPS operators of the field theories are matched to massless geodesics in the geometries, providing a test of AdS/CFT for these cases. Certain small fluctuations (in the BMN sense) can also be successfully compared. We then go further and find, using an appropriate limit, a reduced action, first order in time derivatives, which describes strings with large R-charge. In the field theory we consider holomorphic operators with large winding numbers around the quiver and find, interestingly, that, after certain simplifying assumptions, they can be described effectively as strings moving in a particular metric. Although not equal, the metric is similar to the one in the bulk. We find it encouraging that a string picture emerges directly from the field theory and discuss possible ways to improve the agreement.Comment: 44 pages, LaTeX, 9 figures. v2: References adde

Wrapped branes with fluxes in 8d gauged supergravity

We study the gravity dual of several wrapped D-brane configurations in presence of 4-form RR fluxes partially piercing the unwrapped directions. We present a systematic approach to obtain these solutions from those without fluxes. We use D=8 gauged supergravity as a starting point to build up these solutions. The configurations include (smeared) M2-branes at the tip of a G_2 cone on S^3 x S^3, D2-D6 branes with the latter wrapping a special Lagrangian 3-cycle of the complex deformed conifold and an holomorphic sphere in its cotangent bundle T^*S^2, D3-branes at the tip of the generalized resolved conifold, and others obtained by means of T duality and KK reduction. We elaborate on the corresponding N=1 and N=2 field theories in 2+1 dimensions.Comment: 32 pages, LateX, v2: minor changes, reference added, v3: section 3.5.2 improve

Wide binaries as a critical test of Classical Gravity

Modified gravity scenarios where a change of regime appears at acceleration scales $a<a_{0}$ have been proposed. Since for $1 M_{\odot}$ systems the acceleration drops below $a_{0}$ at scales of around 7000 AU, a statistical survey of wide binaries with relative velocities and separations reaching $10^{4}$ AU and beyond should prove useful to the above debate. We apply the proposed test to the best currently available data. Results show a constant upper limit to the relative velocities in wide binaries which is independent of separation for over three orders of magnitude, in analogy with galactic flat rotation curves in the same $a<a_{0}$ acceleration regime. Our results are suggestive of a breakdown of Kepler's third law beyond $a \approx a_{0}$ scales, in accordance with generic predictions of modified gravity theories designed not to require any dark matter at galactic scales and beyond.Comment: accepted for publication in EPJ

Active vibration isolation system for CLIC final focus

International audienceWith pinpoint accuracy, the next generation of Linear Collider such as CLIC will collide electron and positron beams at a centre of mass energy of 3 TeV with a desired peak luminosity of 2*1034 cm-2s-1. One of the many challenging features of CLIC is its ability to collide beams at the sub-nanometer scale at the Interaction Point (IP). Such a high level of accuracy could only be achieved by integrating Active Vibration Isolation systems (AVI) upstream of the collision to prevent the main source of vibration: Ground Motion (GM). Complementary control systems downstream of the collision (Interaction Point FeedBack (IPFB), Orbit FeedBack (OFB)) allow low frequency vibration rejection. This paper focuses on a dedicated AVI table designed for the last focusing quadrupole (QD0) where the specifications are the most stringent. Combining FeedForward (FF) and FeedBack (FB) techniques, the prototype is able to reduce GM down to 0.6 nm RMS(4Hz) experimentally without any load. These performances couldn't be achieved without cutting edge-technology such as sub-nanometer piezo actuators, ultra-low noise accelerometers and seismometers and an accurate guidance system. The whole AVI system is described in details. Further developments concern the integration of the final focusing magnet above the AVI table, first as part of the simulation with its dynamical model, and finally, as a realistic prototype