Evaluation of density-based models for the solubility of solids in supercritical carbon dioxide and formulation of a new model

Many models have been developed to calculate supercritical solubility behavior and most can be either a semi-empirical relationship or based on an equation of state. In this work, density-based, semi-empirical models were evaluated in terms of their ability to accurately correlate solid solubility in supercritical carbon dioxide. The models considered were the methods of Chrastil, del Valle and Aguilera, Adachi and Lu, Méndez-Santiago and Teja, and Bartle. Six binary systems (solid + supercritical carbon dioxide), each with three isotherms, were selected for this evaluation. The average error was calculated for all 18 isotherms with each of the models evaluated. The solid compounds used in this study were naphthalene, anthracene, fluorene, hydroquinone, 1,5-naphthalenediamine, and cholesterol. The solubility data were obtained from literature. Of the previously mentioned models, the Adachi-Lu and del Valle-Aguilera equations provided, in general, lower average error than the other models. Since the Adachi-Lu equation and the del Valle-Aguilera equation correct for different effects, a new model is proposed in this work as a combination of the previous two methods. The proposed equation provided the least overall average error compared to all other models considered in this study. The new model is particularly useful when the reduced density of the solvent is below 1 where previous models tend to fail. This work also emphasizes on the advantages of expressing density-based models in dimensionless form to avoid dimensional inconsistencies in Chrastil-type models. One of the benefits, for example, is that parameters obtained by different authors can be readily compared, regardless of the units used.     

Phase equilibria of alcohols in supercritical fluids Part II. The effect of side branching on C8 alcohols in supercritical carbon dioxide

This paper is a continuation of a previous study and investigated the phase equilibria of six C₈ alcohols (2,2,4-trimethyl-1-pentanol, 2,4,4-trimethyl-1-pentanol, 2-ethyl-1-hexanol, 2-propyl-1-pentanol, 4-methyl-3-heptanol and 6-methyl-2-heptanol) in supercritical carbon dioxide. Data has been measured between 308 and 348 K for alcohol mass fractions between 0.660 and 0.0162. The results show that the position, size and quantity of side chains have a significant effect on the phase behaviour by changing the shape of the molecule and the effect of the hydroxyl group on the polarity of the molecule. The pressure required for total solubility increases in the following sequence: 4-methyl-3-heptanol < 2,2,4-trimethyl-1-pentanol < 6-methyl-2-heptanol < 2,4,4-trimethyl-1-pentanol < 2-propyl-1-pentanol < 2-ethyl-1-hexanol < 1-octanol. The difference in phase behaviour is believed to be a result of a difference in shielding of the hydroxyl group. Greater shielding of the hydroxyl group results in a less asymmetric system, and this, in turn, results in higher solubility of the molecule.     

超临界CO_2萃取大豆油的实验研究

采用超临界CO2 萃取技术进行了大豆油的萃取实验。在压力为 2 0～ 30MPa、温度为 30 8～ 32 3K的范围内 ,考察了萃取压力、萃取温度、流体流量和物料预处理方式等条件对出油率的影响。在本文实验范围内 ,大豆油的最佳萃取工艺条件为 :压力 30MPa ,温度 313K ,物料状态为约 0 .4mm厚的大豆片。流体流量只影响萃取速率 ,而不影响最终的出油率     

超临界二氧化碳和醇类体系的相平衡计算

应用Peng-Robinson(P-R)状态方程对超临界CO2系统进行了相平衡模拟。对超临界CO2和醇类二元系统进行了汽液相平衡计算,结果表明,P-R状态方程模拟高压下CO2系统的相平衡具有较高的精度。     

Drying processes for preparation of titania aerogel using supercritical carbon dioxide

Supercritical carbon dioxide drying was performed for the preparation of titania aerogels from sol–gel routes. The conditions of supercritical carbon dioxide drying were 313–323 K and 7.8–15.5 MPa. The solvents in titania wet gels obtained from the sol–gel routes were replaced by acetone. The titania aerogels obtained from supercritical carbon dioxide drying form needle-like structures. In supercritical carbon dioxide drying, the extraction rates of acetone from the wet gels were measured by using an on-line Fourier transform infrared spectroscope. It was found that the titania aerogels with lower cohesion were induced from the formations of homogenous phase for carbon dioxide + acetone system and the lower extraction rates of acetone. Furthermore, titania films were prepared by the depositions of the titania aerogels on ITO-coated PET substrates. The needle-like aerogels with lower cohesion derive the titania film with high surface area.     

超临界CO2萃取的研究与应用

介绍了超临界流体的特性及其萃取的基本原则,讨论了超临界流体萃取技术的优点,评述了超临界CO2的特点,概述了国内外超临界CO2萃取技术在医药,食品,香料,石油化工以及环境保护等领域的开发及应用。     

Dyeing of natural and synthetic textiles in supercritical carbon dioxide with disperse reactive dyes

Polyester, nylon, silk and wool were dyed with disperse reactive dyes in supercritical carbon dioxide (scCO₂). The dyes were substituted with either vinylsulphone or dichlorotriazine reactive groups. Since earlier research showed that water, distributed over the scCO₂ and the textile, increased the colouration, experiments were done with the vinylsulphone dye with varying amounts of water in the dyeing vessel, to investigate if there is an optimum water concentration. The amounts were such, that no liquid water was present. The maximum colouration was obtained when both the scCO₂ and the textiles were saturated with water. At the saturation point, deep colours were obtained with the vinylsulphone dye for polyester, nylon, silk and wool, with fixation percentages between 70 and 92% when the dyeing time was 2 h. The positive effect of water was due to its ability to swell fibres or due to an effect of water on the reactivity of the dye–fibre system. Also the dichlorotriazine dye showed more colouration when the scCO₂ was moist. With this dye, experiments were conducted in water-saturated scCO₂, varying the pressure from 225 to 278 bar and the temperature from 100 to 116 °C. The colouration of polyester increased with pressure, the results for silk and wool were not sensitive to pressure. Increasing the temperature had no influence on the dyeing of polyester, silk and wool. The fixations on polyester, silk and wool, being between 71 and 97%, were also independent of pressure and temperature.     

Dyeing of pet textile fibers and films in supercritical carbon dioxide

Polyethylene terephthalate (PET) textile fibers were dyed with a disperse dye in the presence of supercritical carbon dioxide at three temperatures of 333.2, 363.2 and 393.2 K and at pressures between 15 and 25MPa. The PET film was also dyed at 393.2 K. It was found that the dye uptake in the fiber increased 2–5 times when a small amount of acetone was added as a cosolvent. The equilibrium dye uptake increased with increasing pressure at all temperatures. At 393.2 K the pressure effect appeared to be much larger. It was explained with the shift of the glass transition temperature of the polymers at high pressures. These results may be useful in designing and developing the pollution-free supercritical dyeing technique, a potential alternative to the conventional dyeing of polyesters that produces a lot of wastewater.     

Ternary phase equilibria of the iso-propanol+water+carbon dioxide system at high pressure

Ternary phase diagrams are presented for the system: iso-propanol(IPA)+water+carbon dioxide at temperatures from 15 to 70 °C and pressures from 7 to 17 MPa. The distribution coefficients of IPA between the dense phase carbon dioxide and water changed dramatically with temperature and pressure. In the vicinity of the critical point, distribution coefficients was low, yet at liquid-like densities carbon dioxide had a high affinity for IPA. Selectivity reversal was observed at differing pressures. High selectivity of CO₂ for IPA was achieved in the near-critical liquid and in supercritical carbon dioxide at high pressure.     

Solubility of non-ionic poly(fluorooxetane)-block-(ethylene oxide)-block-(fluorooxetane) surfactants in carbon dioxide

The impact of side chain and midblock length on the solubility of ABA triblock copolymers of fluorooxetane-(ethylene oxide)-fluorooxetane in carbon dioxide is examined. The use of short fluorinated side chains instead of long fluorinated chains prevents issues with bioaccumulation of the degradation products of the surfactant. At 40 °C for the same degree of polymerization, increasing the number of perfluoro-units from one to four results in a non-monotonic change in the cloud point pressure; the cloud point pressure decreases as the side chain is increased from perfluoromethyl to perfluoroethyl. However, further increasing the fluorinated side chain to perfluorobutyl results in a significant increase in the cloud point. However when the temperature is increased to 60 °C, the cloud point pressure for the surfactants with perfluoroethyl and perfluorobutyl side chains is statistically similar, while the perfluoromethyloxetane based surfactant requires a substantially larger pressure to obtain comparable solubility. An increase in cloud point pressure is observed when increasing the hydrophilic ethylene oxide segment. These results illustrate that commercially available fluorooxetane-(ethylene oxide)-fluorooxetane surfactants are highly soluble in CO₂.     

Theoretical investigation of the phase behaviour of mixtures of a novel family of perfluoroalkyl-polyoxyethylene ether diblock surfactants in aqueous solutions of carbon dioxide

The accurate knowledge of the thermodynamic properties, with special emphasis on phase equilibria, of aqueous solutions of carbon dioxide is essential from practical and theoretical points of view. These aqueous mixtures are of great interest in different industrial processes, including its use as supercritical solvents, CO₂ sequestration technologies and oil enhanced recovery, among many different applications. From a theoretical point of view, this mixture exhibits a variety of interactions, including dispersive forces, hydrogen bonding between water molecules, and also dipolar and quadrupolar electrical interactions, which determine its phase behaviour. In particular, the mixture exhibits type III phase behaviour, which leads to large regions of liquid-liquid immiscibility. In a number of practical situations, it is important to make miscible this mixture. This can effectively be done by adding some amount of a selected surfactant that stabilizes the mixture. An ideal candidate would be a surfactant formed by hydrophilic and CO₂-philic parts that interact favourably with water and carbon dioxide molecules, respectively. In this work we study a novel family of diblock amphiphiles, the perfluoroalkyl-polyoxyethylene ether non-ionic diblock surfactants, which have the general formula F(CF₂)_i(OCH₂CH₂)_jOH or simply F_iE_j. We employ the sophisticated and versatile Statistical Associating Fluid Theory for potentials of variable range, the so-called SAFT-VR approach, to study the phase behaviour of aqueous solutions of carbon dioxide and the novel surfactants. All molecules are modeled under the united-atom approach, in which different chemical groups are treated as hard-sphere attractive segments. The F_iE_j surfactant is modelled as a diblock compound, which consists of a perfluorinated alkane chain and a polyoxyethylene ether chain. The optimized molecular parameters for H₂O, CO₂, and the perfluoroalkyl, polyoxyethylene ether and the rest of chemical groups forming the surfactant are taken from previous works from the literature. Unlike interactions between different chemical groups are carefully chosen to reproduce the phase behaviour of similar associating surfactants previously studied in the literature. In this work we study the effect of temperature, pressure and composition of a selection of mixtures to understand the microscopic conditions that determine the stabilization of aqueous solutions of carbon dioxide when this kind of surfactants are added. To our knowledge, this is the first time the phase behaviour of mixtures containing this kind of surfactants is studied using a microscopic modelling approach.     

Formulation of a new generic density-based model for modeling solubility of polyphenols in supercritical carbon dioxide and ethanol

The aim of this work is to present a first approach in formulating a generic model for polyphenols solubility in ternary mixtures (polyphenol + ethanol + sc-CO₂). Solubility data of six polyphenols were collected from the literature, and six different groups of parameters were proposed for the new generic model in order to evaluate their effects and find the best set for each polyphenol. Likewise, four dimensional groups of factors were proposed to evaluate the effect of dimensions on solubility data calculation. The results show that the originally formulated model and its modifications are particularly useful in calculating polyphenols solubility data; for instance, when resveratrol solubility data was fitted, the AARD decreased from a value of 38.52 to 14.03, upon changing from a simple to a complex model. Additionally, this generic model with a specific modification can estimate solubility maxima occurring in the ternary resveratrol + ethanol + sc-CO₂ system.     

Solubility, swelling degree and crystallinity of carbon dioxide–polypropylene system

Solubilities are reported for carbon dioxide (CO₂) in polypropylene (PP) at temperatures from 313.2 to 483.7 K and at pressures up to 25 MPa. Buoyancy corrections required in the data reduction were estimated with the Sanchez and Lacombe equation of state (S-L EOS). Solubilities of CO₂ in molten state PP could be correlated to within 5% for both the S-L EOS and the group-contribution lattice-fluid equation of state (GCLF EOS). The change in crystallinity of rubbery state PP with CO₂ dissolution into the polymer was predicted with the GCLF EOS and the solubility data under the assumption that CO₂ could dissolve only in the amorphous regions. The trend of the estimated crystallinity with CO₂ dissolution into the polymer was consistent with results in the literature.     

Simultaneous prediction of the critical and sub-critical phase behavior in mixtures using equations of state II. Carbon dioxide-heavy n-alkanes

In the present study we present the final development of the Global Phase Diagram-based semi-predictive approach (GPDA), which requires only 2-3 key data points of one homologue to predict the complete phase behavior of the whole homologues series. The ability of GPDA to predict phase equilibria in CO₂-heavy n-alkanes is compared with the equations of state LCVM and PSRK. It is demonstrated that both LCVM and PSRK are more correlative rather than predictive because their parameters are evaluated by the local fit of a considerable amount of VLE experimental data. In addition, these models fail to predict accurately the VLE of systems, which have not been considered in the evaluation of their parameters. They are also particularly inaccurate in predicting LLE and critical lines. In contrast, GPDA is reliable in the entire temperature range and for all types of phase equilibria. It yields an accurate prediction of the global phase behavior in the homologues series and their critical lines. Moreover, increasing asymmetry does not affect the reliability of GPDA; it predicts very accurately even the data of the heaviest homologues of the series.     

Continuous countercurrent deterpenation of lemon essential oil by means of supercritical carbon dioxide: Experimental data and process modelling

A thermodynamic model based on the Peng-Robinson equation of state was developed in order to perform high-pressure phase equilibria calculations for the system carbon dioxide-lemon essential oil. The multicomponent natural oil was simulated by a mixture of three key components, one for each relevant class of compounds (monoterpenes, monoterpene oxygenated derivatives and sesquiterpenes). Firstly the proposed model was validated on semi-batch experimental data and then it was used to simulate the behaviour of a continuously operated countercurrent column. Experiments on deterpenation process on a packed column, operating as a stripping section, were carried out in the temperature range 50-70 °C and pressure range 8.7-11.2 MPa. The comparison between experimental results and process simulation demonstrated that the proposed model is capable of reliable predictions on the behaviour of the countercurrent continuous deterpenation process. Furthermore, an average height equivalent to a theoretical plate of about 40 cm was estimated, for the stated packing and operating conditions. A case study for the production of 10-fold high quality oil, with strict specifications for the recovery of oxygenated compounds (99%), was investigated by simulations of a continuous countercurrent process with an external reflux. The linkage between the number of theoretical stages, the reflux ratio and the solvent to feed ratio was investigated throughout the above-mentioned pressure and temperature ranges. Operating conditions at higher pressure and temperature proved to be more favourable. As an example, operating a 20 theoretical stage column at 70 °C and 11.2 MPa, it is possible to attain process specifications with a solvent to feed ratio of about 63.     

Solubility and demixing of polyethylene in supercritical binary fluid mixtures: Carbon dioxide–cyclohexane,carbon dioxide–toluene,carbon dioxide–pentane

Solubility of polyethylene in three different supercritical binary solvent systems—carbon dioxide–cyclohexane, carbon dioxide–toluene, and carbon dioxide–pentane—have been studied. Solvent compositions that lead to complete dissolution at pressures below 70 MPa have been identified. Demixing pressures have been determined for a range of polymer concentrations at temperatures up to 200°C. It is shown that the behavior of the solutions depends strongly on the fluid composition. In the composition ranges studied, solutions in cyclohexane–carbon dioxide mixtures were found to show lower critical solution temperature (LCST) behavior. It is found that the behavior of solutions in carbon dioxide–toluene and carbon dioxide–pentane mixtures shifts from upper critical solution temperature (UCST) to LCST with increasing toluene or pentane content. © 1993 John Wiley & Sons, Inc.     

High-crystallization polyoxymethylene modification on carbon nanotubes with assistance of supercritical carbon dioxide: Molecular interactions and their thermal stability

As a typical engineering plastic and high-crystallization polymer, polyoxymethylene (POM) has been successfully wrapped on single-walled carbon nanotubes (SWCNTs) using a simple supercritical carbon dioxide (SC CO₂) antisolvent-induced polymer epitaxy method. The characterization results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal that the SWCNTs are coated by laminar POM with the thicknesses of a few nanometers. The polymer adsorption on CNTs via multiple weak molecular interactions of CH groups with CNTs has been identified with FTIR and Raman spectroscopy. The experimental results indicate that the decorating degree of POM on the surface of CNTs increases significantly with the increase of SC CO₂ pressure, and accordingly the dispersion of SWCNT modified by POM at higher pressure are more excellent than that of obtained at lower pressure. Further the processing stability of POM/CNTs composites are investigated by differential scanning calorimetry and thermogravimetric analysis. The experimental results obtained show that their thermal stability behavior is closely related to surface properties of CNTs. Apparently, the composites with POM-decorating SWCNTs as the filler shows higher melting points compared to the POM composites with pristine SWCNTs as the filler. Therefore, we anticipate this work may lead to a controllable method making use of peculiar properties of SC CO₂ to help to fabricate the functional CNTs-based nanocomposites containing highly crystalline thermoplastic materials such as POM.     

Separation of fish oils ethyl esters by means of supercritical carbon dioxide: Thermodynamic analysis and process modelling

A semicontinuous fractionation process of fish oil ethyl esters was carried out in this work by means of supercritical carbon dioxide. The process focused on the separation of ethyl esters on chain length basis. Experimental temperature ranged from 42 to , whereas pressure ranged from 10.1 to 17.2 MPa. Optimal operating conditions were found for solvent density in the range 570-. A thermodynamic model based on the Peng-Robinson equation of state, proposed to represent high-pressure phase equilibria for this complex system, was validated on the experimental data. The model assumes that the multicomponent natural mixture can be considered as composed of five major components, defined on chain length basis. The proposed thermodynamic model was then used in the modellization of a continuous multistage fractionation process. The continuous process simulations that were carried out allowed to investigate the effect of number of theoretical stages, reflux ratio and solvent to feed ratio on the distribution of the components between extract and raffinate. A case study was considered, in order to find out process operating conditions to attain 95% by weight of heavy components (acid chain length 20 and 22) in the raffinate together with a 95% recovery. Results show that the stated separation is possible with a solvent to feed ratio and a number of theoretical stages in the range 90-150 and 11-30, respectively. The simulations were also employed to study the effect of the operating conditions of the separator used for solvent recycling.     

Enhanced electrical properties of polycarbonate/carbon nanotube nanocomposites prepared by a supercritical carbon dioxide aided melt blending method

Polycarbonate/carbon nanotube (CNT) nanocomposites were generated using a supercritical carbon dioxide (scCO₂) aided melt blending method, yielding nanocomposites with enhanced electrical properties and improved dispersion while maintaining the aspect ratio of the as-received CNTs. Baytubes^® C 150 P CNTs were benignly deagglomerated with scCO₂ resulting in 5 fold (5X), 10X and 15X decreases in bulk density from the as-received CNTs. This was followed by melt compounding with polycarbonate to generate the CNT nanocomposites. Electrical percolation thresholds were realized at CNT loading levels as low as 0.83 wt% for composites prepared with 15X CNT using the scCO₂ aided melt blending method. By comparison, a concentration of 1.5 wt% was required without scCO₂ processing. Optical microscopy, transmission electron microscopy, and rheology were used to investigate the dispersion and mechanical network of CNTs in the nanocomposites. The dispersion of CNTs generally improved with scCO₂ processing compared to direct melt blending, but was significantly worse than that of twin screw melt compounded nanocomposites reported in the literature. A rheologically percolated network was observed near the electrical percolation of the nanocomposites. The importance of maintaining longer carbon nanotubes during nanocomposite processing rather than focusing on dispersion alone is highlighted in the current efforts.     

A significant approach to dye cotton in supercritical carbon dioxide with fluorotriazine reactive dyes

A series of fluorotriazine reactive dyes have been synthesized and applied to dye cotton in supercritical carbon dioxide (scCO₂) with good dyeing results. The pieces of cotton to be dyed were previously presoaked in a protic solvent and cosolvents were applied during dyeing. The colour strength of the dyeings was evaluated by K/S measurements. The K/S values achieved on cotton dyed were up to 35.8 ± 4.2. Even after the cotton was subjected to a Shoxlet extraction at 358 K for 1 h, a maximum K/S value of 20.2 ± 1.8 was measured. The percentage of dye molecules chemically fixed to the cotton was on average 85%. The excellent reactivity of fluorotriazines allowed a reduction of 3 h on the dyeing time. It is noticeable that a dye concentration of 10% on weight of the fibre (owf) can be applied to dye cotton with fluorotriazines, since no damage of the cotton fibres occurred, as observed for the chlorotriazines at this high dye concentration.Dyes with fluorotriazine as reactive group were found to be the most preferable dyes for dyeing cotton in scCO₂, as they were able to exceed the limitation of the reaction with the cotton.