Phase Behavior at High Pressure of the Ternary System: CO2, Ionic Liquid and Disperse Dye

1 Department of Chemical Engineering, State University of Maringá, Avenue Colombo 5790, 87020-900 Maringá PR, Brazil 2 Institute for Research and Technology, ITP, PEP/UNIT, Avenue Murilo Dantas 300, 49032-490 Aracaju SE, Brazil 3 Department of Chemistry, State University of Maringá, Avenue Colombo 5790, 87020-900 Maringá PR, Brazil 4 Department of Chemical Process, School of Chemical Engineering, State University of Campinas, Avenue Albert Einstein 500, 13081-970 Campinas SP, Brazil


Introduction
In the past years the development of new technologies to minimize water consumption and wastewater emissions received more attention.In this context, the textile industry has a significant stake in the generation of effluents with highly pollution potential, using large amounts of water with chemical additives [1].The process of dyeing using supercritical fluids is an alternative method that does not use water and any additive is frequently not required.In addition, such process allows the recovering and the reusing of the dyes.According to van der Kraan et al. [2], in dyeing of textiles consisting of nonpolar materials such as polyester, supercritical CO 2 acts as a blowing agent and increases the rate of diffusion of the dye molecules inside the fiber.Several studies have been reported on this textile-dyeing technology [3][4][5][6].The literature describes applications of supercritical technology in combination with the ionic liquids in both the extraction and the separation processes and as an alternative solvent for organic and inorganic compounds in reaction mediums [7][8][9].The ionic liquids are described as "designer solvents," where their characteristics can be adjusted to make the change either of cations or anions, or both.In this way, dyeing process involving association of supercritical technology with ionic liquids (ILs) can be used to improve the solubility of the dyes in the dyeing process of textile fibers and minimize the amount of energy demand.
In this context, the objective of this work is to report new experimental data of CO 2 (1) + 1-butyl-3-methylimidazolium hexafluorophosphate [bmim] [PF 6 ] + (2) and CO 2 (1) + [bmim] [PF 6 ] (2) C.I. Disperse Red 60 systems in the temperature range of 323 to 353 K. C.I. Disperse Red 60 is indeed a popular dye and one of the widely used dyes in dyeing process of synthetic textile fibers [2,10,11].[bmim] [PF 6 ] is one of the most studied ionic liquids (ILs) in the literature with great availability of physical and chemical data as well as phase equilibrium data [12][13][14].Keskin et al. [7], Huddleston et al. [15], and Jaitely et al. [16] have reported that the ionic liquid consisting of imidazole as a cation and [PF 6 ] − as an anion presents hydrophobic characteristics absorbs the moisture during the synthesis process due to its hygroscopic property [17].A small amount of water in the ionic liquid can have a dramatic effect on the phase equilibrium behavior [13] ).The analyses using NMR were performed in a Varian spectrometer, model Mercury Plus BB using CDCl 3 as a solvent.

Equipment and Procedure.
Phase equilibrium experiments (cloud points) were performed through the synthetic static method [19] in a high-pressure variable-volume view cell.The experimental apparatus and experimental procedure are similar to those used in previous studies of phase equilibrium carried out by our group [20][21][22].Briefly, the experiment begins with the introduction of a predetermined amount of compounds (CO 2 , IL, and dye) in the equilibrium cell so that the global composition of the mixture is known.In the sequence, the system was stirred for 12 h at room temperature for complete solubilization of CO 2 in [bmim] [PF 6 ].The procedure used here follows the work of Aki et al. [23].After the initial procedure, pressure and temperature are adjusted to form a homogeneous solution.After reaching the thermal equilibrium of the system, pressure is slowly decreased until the appearance of a second phase is visually detected.
After the measurements of the phase transitions, the IL + Dye mixture was submitted to a process to recover the IL.The process of IL recovery consisted of the addition of a dichloromethane solution and active coal in the mass ration of 1 : 3 into the mixture of IL + Dye.Afterwards, the solution was stirred at room temperature for 48 h.Then, the mixture was filtrated using qualitative filter paper in a chromatographic column with silica gel (silica gel 60 Å; mesh 0.035-0.70mm).The purified solution was evaporated under vacuum at 343 K for 10 h.
The values of water content refer to values obtained before and after the phase equilibrium measurements.For the binary system CO 2 + [bmim] [PF 6 ] the water content was 0.10 ± 0.02% (after phase equilibrium) and 0.23 ± 0.02 (before measurement).For the ternary system CO 2 + [bmim] [PF 6 ] + C.I. disperse Red 60 the water content corresponded to 0.43 ± 0.04 and 0.26 ± 0.02, after and before measurements, respectively.For the binary system with IL recovered the measurement were 0.19 ± 0.01 and 0.66 ± 0.02, after and before, respectively.[24] of state was used to model the vapor-liquid equilibrium data:

Thermodynamic Modeling. Peng-Robinson equation
The parameters a and by were calculated through the following mixing rules: where y i is the molar fraction of the component i and a i and b i are the Peng-Robinson parameters for the pure component i.The binary interaction parameters, k i j , were determined through the correlation of experimental data obtained in this work.The optimization procedure employed for fitting k i j was based on the Nelder and Mead simplex method [25].
Figure 2 and Table 2 show the experimental phase behavior for the ternary system CO 2 (1) + [bmim] [PF 6 ] (2) + disperse dye Red 60 (3).In this case, it is possible to notice the increase of CO 2 solubility with increasing pressure, an analogue behavior to that of the binary system.Moreover, CO 2 solubility decreases as temperature increases.The IL addition in supercritical environment induces a significant solubilization of the dispersed dye Red 60, which is almost insoluble in supercritical CO 2 in the temperature and pressure range investigated.The solubility of the dye Red 60 in CO 2 is around 10 −6 in the investigated range [28].On the other hand, in this range the solubility of the dye Red 60 in CO 2 and IL has an average value of 10 −4 .For all experimental conditions (Figures 1 and 2), a good agreement of the Peng-Robinson equation with the experimental data is observed.(2)) are similar.Solubility differences between these two systems can be attributed to the water content present in each situation.Furthermore, the RMN spectra of IL after removal of the disperse dye Red 60 were consistent with values found for pure LI produced in our laboratory and with those reported in literature [29].
The PR-EoS with the quadratic mixing rules was used to modeling the phase transition experimental data for all systems.The binary interaction parameters in the mixing rules were optimized using the P, x data for each system.The critical properties and acentric factors of the IL were obtained of the Valderrama and Robles [30], and the critical properties, and acentric factors of the disperse dye Red 60 were obtained of the Mishima et al. [31].The critical properties are presented in Table 4 and Table 5 shows the interaction parameters for each system.

Conclusions
It was verified that CO 2 solubility in the ionic liquid decreased by the presence of dye.Such phenomenon results in higher vapor-liquid pressure transitions for the ternary system when compared to the binary system.Water content (impurities) in the systems studied considerably change in the pressure values of phase transition, corroborating literature observations [19,31,32].Therefore, water content in IL synthesis should be carefully controlled.IL's purification protocol for dye removal presented good results without IL physical chemical changes, which enables IL to be reused.Peng-Robinson equation of state presented a good performance in describing the experimental data observed.The results obtained might be useful in the development of dyeing processes using supercritical fluids in association with ionic liquids.Finally, the results of partition of the undisclosed dye composition between SCCO 2 and an ionic liquid demonstrated that it is possible to increase significantly the disperse dye solubility in SCCO 2 using ionic liquids as an addictive.This effect can be used to improve the color aspects in the textiles, one of the most desirable characteristics in the dyeing of textiles.
[27]s possible to notice that CO 2 solubility in the IL increases continually with pressure, reaching 0.50 at 333 K and 9.83 MPa.Such behavior is also reported by Bermejo et al.[27],Kamps et al.

Table 4 :
Critical properties and acentric factor of carbon dioxide, ionic liquid and disperse Red 60.

Table 5 :
PR-EoS binary interaction parameters for the binary and ternary systems.