Gas-Liquid Chromatography를 이용한

무한희석 활동도계수 측정

박소진, 오종혁, 한규진, 원동복

충남대학교 공과대학 화학공학과

Determination of Infinite Dilution Activity Coefficient

using Gas-Liquid Chromatography

So-Jin Park, Jong-Hyeog Oh, Kyu-Jin Han, Dong-Bok Won

Department of Chemical Engineering, Chungnam National University

INTRODUCTION

Activity coefficient at infinite dilution(□∞) is very useful in practical and theoretical aspect like of following two reasons. First, It has practically direct application in industrial problems where objective component or impurity is exist in very low concentration. Second, theoretically, constants in empirical equations for excess functions and fluid phase equilibrium are relate to thermodynamic property at infinite dilution. Besides, infinitely dilute activity coefficient should be very important parameter to estimate aqueous solubilities and Henry's law constants and octanol/water partition coefficient, which is an important parameter widely used to characterize chemical's behavior in the environment. This octanol/water partition coefficient(Kow) also starting from equality of fugacity of a solute in water and octanol phases. Again relating with activity coefficient, Kow can be written as

[eqn: K_ow`={x_i`^o}over{x_i`^w}``= {gamma _i`^w}over {gamma_i`^o}]	(1)

Where [eqn: x_i`^o] and [eqn: x_i`^w] are mole fractions of the solute i in octanol and water phases and the [eqn: gamma _i`^o` and `gamma _i`^w] are activity coefficients of component i in relate phases, respectively. Usually, environmental pollutants will be present in very small concentrations; activity coefficient in eq. 1 should be replaced with their infinitely dilute activity coefficients[1].

There are two general determination method for activity coefficients at infinite dilution (□∞). The first is to extrapolate activity coefficient, which obtained usually from vapor-liquid equilibria in the whole or high dilution composition region. This gives sometimes great inaccuracy data and is not available to strongly hydrophobic chemicals such as volatile organic pollutants. The second is the direct measurement of infinite dilution behavior. This method can avoid the drawback of interpolation method. However, the direct determination of □∞ involves the treatment of mixtures containing a very low amount of one component, since the normal experimental techniques are not used without particular care. For this reason, some special experimental methods are suggested. These can be classified as a method based on the flow of an inert third component (gas-liquid chromatography (GLC), diluter techniques, etc) and the methods based on the measurement of differential vapor-liquid equilibria (differential ebulliometry, different static technique). Among these methods, GLC method is probably the most time saving measuring method while the ebulliometric method is a conventional method and delivers a reliable data[2].

In this work, therefore, steady state GLC method under isothermal condition is introduced. Experimentally we have focussed on the □∞ for the binary mixtures of n-formylmorpholine(N.F.M) as a solvent with some various solutes (alkane, alkene, aromatic compounds, etc.). Because N.F.M derivatives are of great interest in the chemical industry because of their high extraction of aromatic hydrocarbons from petro-chemical products, however the fluid phase equilibria and thermodynamic properties of the mixtures containing N.F.M are not readily available [3].

EXPERIMENTAL SECTION

Material   The purity of all used chemicals, delivered as a commercial grade from Merck, Riedel de Haen and Fluka were better than 99.8% by GLC analysis. The solid support for stationary phases(selective solvent) was Chromosorb P-AW-DMCS(60-80 mesh).

Measurement   GLC method can be the fastest and most reliable measuring technique of □∞, especially for the high boiling solvents[5]. The coating of the predried Chromosorb with the N.F.M was carried out with methanol as a solubilizer in a rotary evaporator. After removal of methanol, the stationary column(length 200mm, ID 4.1mm) was carefully filled with the N.F.M coated solid support and the liquid loading of N.F.M was determined gravimetrically. Fig. 1 shows a schematic diagram of apparatus used in this investigation. The measurement procedure is the same as the normal GLC analysis method. We need only the true retention time of the solute. Experimental procedure and the theoretical backgrounds are given in more detail in ref.[4].

RESULTS AND DISCUSSION

Table 1 contain the result of experimental □∞ values of solutes for the N.F.M by GLC method. The exact measuring temperatures are given also in Table. 1. The determination of □∞ require the net retention time of the solute. The limit of ebulliometric technique was found by relative volatility at infinite dilution,□12∞＠15. However GLC method has no problem in the realm of high relative volatility.

For the calculation of □∞ values by GLC method, following eq.(2) was used.

[eqn: 273.15R over {v_g^o □_1^s` P_1^s M_L } ~=~□_1^∞]	(2)

where  Pis  is the saturation vapor  pressure,  □is  the saturation fugacity coefficient,  R gas constant and ML the molar mass of solvent. The fugacity coefficient was calculated with SRK equation and the saturation vapor pressure was calculated using the Antoine equation. In eq. (2), the specific net retention volume, vog, is related to experimentally determined net retention time. The systematic error in the GLC method will be larger in more volatile solvents. Fig. 2 shows the experimental□∞ values for benzene and toluene in sulfolane with literature values. This comparison demonstrates good agreement between the experimental data and those taken from the literature. The □∞ were decreased with increasing the temperature in this study. Fig. 3 shows this tendency and an linear relationship between logarithmic activity coefficient at infinite dilution(ln□∞) and the inverse of the temperature for some solutes in N.F.M. The increase in the activity coefficients on going from relatively low molecular weight n-alkane to high molecular weight n-alkane is visible; however, this is not always the case.

ACKNOWLEDGEMENT

This work was supported by grant  No. 2000-1-30700-011-2 from the Basic Research Program of the Korea Science & Engineering Foundation.

REFERENCES

[1] G. Tse, H. Orbey and S.I. Sandler : Environ. Sci. Tenol., 26, 2017 (1992)

[2] L. Dallinga, M. Schiller and J. Gmehling: J. Chem. Eng. Data, 38, 147 (1993)

[3] S. J. Park and J. Gmehling : J. Chem. Eng. Data, 34, 399 (1989)

[4] C. Knoop, D. Tiegs and J. Ggmehling : J. Chem. Eng. Data, 34, 240 (1989)

[5] B. S. Rawat, I. B. Gulati, and K. L. Millik : J. Appl. Chem. Biotechol, 26, 247 (1976)

[6] A. A. Galie, V. N. Parizheva, and V. A. Proskuryakov : Zh. Prikl. Khim. (Leningrad), 47,191 (1974)

[7] M. Schiller : Unpublished data, Oldenburg (1989)

A : carrier gas, B : needle valve, C : heating coil

D, G, K : saturators, E : thermal conductivity detector

F, J : tempering coil, H : injection block, I : column

L : soap bubble flowmeter, TH1, TH2 : thermostat

Fig. 1. Schematic diagram of the GLC system.

Table 1.  Experimental activity coefficient at infinite dilution by gas-liquid

chromatography for the solvent n-Formylmorpholine(N.F.M)

solute	□∞			solute	□∞
	303.21K	313.13K	323.12K		303.21K	313.13K	323.12K
n-pentane	25.679			cyclohexane	17.804	16.115	15.624
n-hexane	34.230	33.250	32.193	iso-butyl- aldehyde	2.320	2.024	1.908
n-heptane	40.904	43.211	41.378	methylcyclo- pentane	18.849	16.785	14.809
n-octane	63.476			methylcyclo- hexane	24.782	23.487	22.151
1-octene	32.146	28.624	24.202	ethylcyclo- hexane	33.340
benzene	1.935	1.831	1.715	1-hexene	17.711	15.970	14.331
toluene	3.219	2.986	2.896	cyclohexene	9.761

Fig. 2. Activity coefficients at infinite dilution, [eqn: gamma_i^inf `], for benzene and toluene in the selective solvent sulfolane as a function of temperature T: benzene in sulfolane(■) this work, (◇) Rewat et al.(1976), (▽) Galie et al.(1974); toluene in sulfolane(▲) this work, (□) Rewat et al.(1976), (○) Schiller(1989); (―) linear regression of the present data.	Fig. 3. Activity coefficients at infinite dilution, [eqn: gamma_i^inf `], for ten hydrocarbons in the selective solvent N.F.M. as a function of temperature T: (●) n-hexane, (■) n-heptane, (□) 1-hexene, (◆) 1-octene, (▽) MCP, (▲) MCH, (○) cyclohexane, (□) benzene, (▼) toluene, (◇) iso-butyl aldehyde; (―) linear regression of the present data.