HIGH PRESSURE PHASE EQUILIBRIUM IN CARBON DIOXIDE+ETHANOL SYSTEM

Transcription

1 U.P.B. Si. Bull., Series B, Vol. 75, Iss. 1, 2013 ISSN HIGH PRESSURE PHASE EQUILIBRIUM IN CARBON DIOXIDE+ETHANOL SYSTEM Viorel FEROIU 1, Sergiu SIMA 2, Dan GEANĂ 3 Sunt prezentate date de ehilibru lihid-vapori (ELV) la şi K la presiuni între 1 şi 14 MPa şi date ritie (presiune-temperatură-ompoziţie) pe intervalul de presiune între 9 şi 14 Mpa, pentru sistemul dioxid de arbon + etanol. Metoda experimentală folosită în aeastă lurare a fost o metodă analitiă statiă u luare de probe din faza lihidă folosind un injetor on line uplat u un gaz romatograf pentru analiză. Datele de ELV măsurate pentru sistemul dioxid de arbon + etanol au fost modelate u o euaţie ubiă generală de stare, folosind regulile lasie de amesteare van der Waals. Un set uni de parametri, ales astfel înât să reprezinte bine maximul în presiunea ritiă, a fost utilizat preditiv pentru alularea datelor de ehilibru şi a puntelor ritie. Rezultatele arată o onordanţă satisfăătoare între model şi datele experimentale. Vapor liquid equilibria (VLE) data at and K and pressures between 1 and 14 MPa, and ritial data (pressure - temperature - omposition) at pressures between 9 MPa and 14 MPa for the arbon dioxide + ethanol system are reported. The experimental method used in this work was a stati analytial method with liquid phase sampling using a rapid on-line sampler injetor oupled to a gas hromatograph for analysis. Measured VLE data for arbon dioxide + ethanol system were modeled with a general ubi equation of state using lassial van der Waals mixing rules. A single set of interation parameters, representing well the ritial pressure maximum was used in this work to alulate the VLE data and ritial points. The results show a satisfatory agreement between the model and the experimental data. Keywords: equation of state, VLE, arbon dioxide, ethanol. 1. Introdution Superritial fluid extration is an important alternative to onventional methods, suh as extration, partile mironization, material proessing, hromatography or rystallization/purifiation. 1 Prof., Dept. of Inorgani Chemistry, Applied Physial Chemistry and Eletrohemistry, University POLITEHNICA of Buharest, Romania, v_feroiu@atedra.hfiz.pub.ro 2 PhD. student, Dept. of Inorgani Chemistry, Applied Physial Chemistry and Eletrohemistry, University POLITEHNICA of Buharest, Romania 3 Prof., Dept. of Inorgani Chemistry, Applied Physial Chemistry and Eletrohemistry, University POLITEHNICA of Buharest, Romania

2 54 Viorel Feroiu, Sergiu Sima, Dan Geană High-pressure vapor liquid equilibrium measurements of arbon dioxide + alohol systems are of interest due to their importane in the superritial extration of thermal labile ompounds, dehydration of alohols using superritial arbon dioxide, and extration of natural produts using near ritial solvents. The arbon dioxide + ethanol mixture is very important industrially and the system has reeived muh attention [1,2]. The goals of this work were to add new experimental VLE data and ritial points measured using a rapid on-line sampler injetor (ROLSI) [3], and to represent the global phase behavior of arbon dioxide + ethanol system with a simple ubi EOS model, based on a single set of interation parameters. Therefore, in this work we made new VLE measurements using a statianalytial method, in a high-pressure visual ell with variable volume, for arbon dioxide + ethanol. New ritial points (pressure - temperature - omposition) were also measured in the same visual ell. In our previous papers [1,2], the global phase behavior of arbon dioxide + ethanol system was modeled with the ubi general equation of state (GEOS) [4-6], oupled with lassial van der Waals mixing rules (two parameters onventional mixing rule 2PCMR). This ubi equation is a generalized form with four parameters for all ubi equations of state with two, three and four parameters. A single set of interation parameters, representing well the ritial pressure maximum (CPM) was used to model the global phase behavior of the arbon dioxide + ethanol system [1]. The same set of parameters is used in this work to represent the new VLE data and ritial points. 2. Apparatus and proedure A detailed desription of the experimental apparatus was presented in earlier papers [2,7]. The apparatus is based on a high pressure visual ell with variable volume, to whih a rapid on-line sampler injetor (ROLSI) was added and oupled with a gas hromatograph (GC). The set-up is shown in fig. 1. The ROLSI is onneted by a apillary to the equilibrium ell. A heating resistane is used to heat the expansion hamber of the sampler injetor in order to have liquid samples vaporized rapidly. The transferring line between ROLSI and the GC is heated by means of a linear resistor oupled to an Armines /CEP/TEP regulator. The GC (Perihrom 2100) uses a thermal ondutivity detetor, TCD, and a 30 m long and mm diameter olumn HP-Plot/Q. The GC arrier gas is helium at a flow rate of 30 ml/min. The apparatus was ompleted with a syringe pump Teledyne ISCO model 500D.

3 High pressure phase equilibrium in arbon dioxide+ethanol system 55 The proedure is similar to that used in our previous work [2,7]. The entire internal loop of the apparatus inluding the equilibrium ell was rinsed several times with arbon dioxide. Then, the equilibrium ell was evauated with a vauum pump. The ell was filled with alohol, whih was degassed by using a vauum pump and vigorously stirring. The lighter omponent (in this ase CO 2 ) is introdued with the syringe pump into the equilibrium ell to set the pressure to the desired value. Then the ell was heated to the experimental temperature. To failitate the approah to an equilibrium state, the mixture in the ell was stirred for a few hours. Then the stirrer was swithed off, and the mixture was allowed for about 1 h to ompletely separate the oexisting phases. Samples of liquid phase are withdrawn by ROLSI and analyzed with GC. Normally at least six samples of the liquid phase were analyzed at the equilibrium temperature and pressure, in order to hek the repeatability. The sample sizes being very small, the equilibrium pressure in the ell remains onstant. The alibration of the TCD for CO 2 and ethanol is done by injeting (using gas hromatographi syringes) known amounts of eah omponent. Calibration data are fitted to quadrati polynomials to obtain the mole number of the omponent versus hromatographi area. The orrelation oeffiients of the GC alibration urves were for arbon dioxide and for ethanol. The ritial points were obtained in this work following the proedure of Sheidgen [8]. At a fixed temperature (for example 373 K), the pressure in the ell was inreased by introduing CO 2 with the syringe pump. The transition from heterogeneous (two phases) to the homogeneous range is visually observed. Then, by slowly ooling (1-3 K) the pressure in the ell dereases and the inverse transition from homogeneous to heterogeneous range an be observed. The temperature and the pressure of the opalesene point are onsidered as the ritial point oordinates. The omposition orresponding to the ritial point is obtained by sampling from the homogeneous mixture. The proedure is then repeated by introduing new amounts of CO 2, and slowly ooling.

4 56 Viorel Feroiu, Sergiu Sima, Dan Geană Fig. 1. High pressure phase equilibrium set-up

5 High pressure phase equilibrium in arbon dioxide+ethanol system GEOS3C equation of state List of symbols a, b,, d - parameters in GEOS AAD - absolute average deviation B - dimensionless parameter in GEOS3C, eq. (5) C 1, C 2 and C 3 - parameters in GEOS3C temperature funtion H - enthalpy M - molar mass P, P S - pressure, saturation pressure R - universal gas onstant S - entropy T - temperature V, V L, V V - molar volume, liquid volume, vapor volume Y - thermodynami funtion (general notation) Z - ompresibility fator Greeks α β Ω a, Ω b, Ω, Ω d ω - Riedel's riterium (parameter in GEOS) - redued temperature funtion in GEOS - parameters of GEOS - aentri fator Subsripts r - ritial property - redued property As pointed out previously [4-6], the ubi GEOS equation is a general form for all the ubi equations of state with two, three and four parameters. This is the meaning of the statement general ubi equation of state used for GEOS. P = RT a (1) V b (V d) 2 + The four parameters a, b,, d for a pure omponent are expressed by: R T RT a = a β (T r ), a = Ω a, b = Ωb (2a) P P 2 2 R T RT = Ω, d = Ω 2 d (2b) P P The temperature funtion used is: m β ( T ) = T (3) r r

6 58 Viorel Feroiu, Sergiu Sima, Dan Geană with the redued temperature Tr = T / T. The expressions of the parameters Ω a, Ω b, Ω, Ω d are: 3 Ωa = ( 1 B ) Ωb = Z B (4) 2 Ω = (1 B ) ( B 0.25 ) Ω d = Z 0.5(1 B ) 1+ C1 B = αc - Riedel s riterion (5) α + C1 As observed, the a, b,, d oeffiients of the ubi GEOS equation are finally funtion of ritial data (T P and V ), m and α parameters. In this work, the oeffiients a, b,, d were obtained for mixtures using the lassial van der Waals mixing rules: a = xixjaij b xx i jbij i j i j i j = = xx i jij ; d = xd i i i b 1/2 i + bj aij = ( aa i j ) (1 kij ); bij = (1 lij ) 2 (7) 1/2 ij =± ( i j ) (with + for i, j > 0 and for i or j < 0). Generally, negative values are ommon for the parameter of pure omponents. The GEOS parameters m and α of eah omponent were estimated by onstraining the EOS to reprodue the experimental vapor pressure and liquid volume on the saturation urve between the triple point and the ritial point. The values of ritial data and GEOS parameters of the pure omponents are given in Table 1. The binary interation parameters, k 12 and l 12, (k 12 = 0.06, l 12 = 0.01) were estimated in our previous paper [1], to represent well the ritial pressure maximum and to derease the temperature of the UCEP (upper ritial end point) at lower temperature (~270 K). The arbon dioxide + ethanol system is a Type I phase behavior, aording to the lassifiation of van Konynenburg and Sott [9], but the EOS model an predit a type II behavior with an UCEP. In this ase the temperature of UCEP must be dereased at low temperature values. The region of the Type I (II) phase behavior an be obtained by traing the triritial and double ritial end point boundary urves in the k 12 l 12 global phase diagram of Polishuk et al. [10]. These types of phase behavior are loated on the left side of the triritial boundary. The values of the interation binary parameters (k 12 and l 12 ) fulfilling these requirements are k 12 = 0.06, l 12 = (6)

7 High pressure phase equilibrium in arbon dioxide+ethanol system 59 The alulations were made using the software pakage PHEQ, developed in our laboratory [11]. 4. Results and disussion The equilibrium ompositions for the arbon dioxide + ethanol binary system were measured at and K and pressures between 1 and 14 MPa and the results are shown in Figs. 2 and 3. GEOS preditive alulations with the single set of parameters (k 12 = 0.06, l 12 = 0.01) were done for the new experimental data of this work. Critial data [12] and GEOS parameters for arbon dioxide and ethanol [13] Component T /K P /MPa V /(m 3 /mol) α m CO C 2 H 5 OH Table 1 Fig. 2 and 3 show a omparison of our experimental data (symbols) at T = 343 K, and T = 363 K with the predition results by GEOS equation (line). It an be seen, that both sets of data are satisfatorily represented by the urve obtained with the GEOS equation. Taking into aount the semipreditive approah used in our previous work [1], the single set of interation parameters leads to satisfatory predition of VLE for the arbon dioxide + ethanol system P /MPa X (CO 2 ) Fig. 2. Phase equilibrium data for the arbon dioxide (1) + ethanol (2) system at T = K; experimental data; GEOS/2PCMR predition

8 60 Viorel Feroiu, Sergiu Sima, Dan Geană P /MPa X (CO 2 ) Fig. 3. Phase equilibrium data for the arbon dioxide (1) + ethanol (2) system at T = K; experimental data; GEOS/2PCMR predition The ritial properties (P, T, X 1 ) for the arbon dioxide + ethanol binary system, measured in this work at pressures between 9 MPa and 14 MPa, are shown in Figs. 4 and 5. The ompositions of ritial points were obtained as those for VLE by gas hromatographi analysis P /MPa T /K Fig. 4. P-T fluid phase diagram of arbon dioxide + ethanol system., ritial points measured in this work; GEOS/2PCMR predition

9 High pressure phase equilibrium in arbon dioxide+ethanol system P /MPa X (CO 2 ) Fig. 5. P-X fluid phase diagram for arbon dioxide + ethanol system., ritial points measured in this work; GEOS/2PCMR predition In these figures we have illustrated also the predition with GEOS/2PCMR for k 12 = 0.06 and l 12 = As an be seen in the Figs. 2-3, the liquid phase data are well represented by the predited urves, with GEOS equation. This fat leads to a reasonable predition of the vapor phase urve, whih is also shown in the figures. It an be observed that the ritial pressure maximum is a little overestimated. 5. Conlusions In this work we report new measured VLE data with a stati-analytial method using a rapid on-line sampler injetor oupled to a GC for analysis, in a high-pressure visual ell with variable volume, for arbon dioxide + ethanol at and K and pressures between 1 and 14 MPa. New ritial points (pressure - temperature - omposition) were measured in the same visual ell at pressures between 9 MPa and 14 MPa. Measured VLE and ritial data for arbon dioxide + ethanol system were modeled with a general ubi equation of state (GEOS) using lassial van der Waals mixing rules. A single set of interation parameters, representing well the ritial pressure maximum was used in this work to represent the new VLE data and ritial points. The preditions show a satisfatory agreement between the model and the experimental data.

10 62 Viorel Feroiu, Sergiu Sima, Dan Geană Aknowledgments The work has been funded by the Setorial Operational Programme Human Resoures Development of the Romanian Ministry of Labour, Family and Soial Protetion through the Finanial Agreement POSDRU/88/1.5/S/61178 R E F E R E N C E S [1]. Seuianu, C., Feroiu, V., Geana, D. Phase behavior for arbon dioxide + ethanol system: Experimental measurements and modeling with a ubi equation of state. J. Superrit. Fluids 2008, vol. 47, pp [2]. S. Sima, V. Feroiu, D. Geană, New high pressure vapor - liquid equilibrium and density preditions for the arbon dioxide + ethanol system, J. Chem. Eng. Data, 2011, vol. 56 pp [3]. Guilbot, P., Valtz, A., Legendre, H., Rihon, D. Rapid on-line sampler-injetor: a reliable tool for HT-HP sampling and on-line GC analysis. Analusis, 2000, vol. 28, pp [4]. Geana, D. A new equation of state for fluids. I, II. Appliations to phase equilibria, Rev. Chim. (Buharest) 1986, vol. 37, pp ; and vol. 37, pp [5]. Geana, D., Feroiu, V. Thermodynami properties of pure fluids using the GEOS3C equation of state, Fluid Phase Equilib. 2000, vol. 174, pp [6]. Feroiu, V., Geana, D. Volumetri and thermodynami properties for pure refrigerants and refrigerant mixtures from ubi equations of state, Fluid Phase Equilib. 2003, vol. 207, pp [7]. Seuianu, C., Feroiu, V., Geana, D., High-pressure vapor liquid equilibria in the system arbon dioxide and 2-propanol at temperatures from K to K, J. Chem. Eng. Data, 2003, vol. 48, pp [8]. Sheidgen, A. Fluidphasengleihgewihte binärer und ternärer Kohlendioxidemishungen mit shwerflühtingen organishen Substantzen bis 100 MPa. Cosolveny effet und Misibility windows. Ph.D. Thesis, Ruhr-Universitat Bohum, Bohum, Germany, [9]. van Konynenburg, P. H,; Sott, R. L., Critial lines and phase equilibria in binary van der Waals mixtures. Philos. Trans. R. So. London, Ser. A 1980, vol. 298, pp [10]. Polishuk, I; Wisniak, J.; Segura, H.; Yelash, L. V.; Kraska, T., Predition of the ritial lous in binary mixtures using equation of state II. Investigation of van der Waals-type and Carnahan-Starling-type equations of state. Fluid Phase Equilib. 2000, vol. 172, pp [11]. Geana, D.; Rus, L., Phase equilibria database and alulation program for pure omponents systems and mixtures. Pro. Romanian Int. Conf. Chem. Chem. Eng. (RICCCE XIV), Buharest, Romania 2005, vol. 2, pp [12]. Reid, R.C., Prausnitz, J.M., Poling, B.E. The properties of gases and liquids. (fourth edition), MGraw-Hill 1988, Appendix A. [13]. D. Geană, V. Feroiu, Euaţii de stare. Apliaţii la ehilibre de faze (Equations of State. Appliations to Phase Equilibria), Ed. Tehniă, Buureşti, 2000.