Equilibrated Vapor Concentrations for Bicomponent Organic Solvents

J Occup Health 1998; 40: 13 136 Journal of Occupational Health Equilibrated Vapor Concentrations for Bicomponent Organic Solvents Hajime HORI 1 and Isamu TANAKA 1 Department of Environmental Management I, School of Health Sciences and Department of Environmental Health Engineering, Institute of Industrial Ecological Sciences, University of Occupational and Environmental Health, Japan Abstract: Equilibrated Vapor Concentrations for Bicomponent Organic Solvents: Hajime HORI, et al. Department of Environmental Management I, School of Health Sciences, University of Occupational and Environmental Health, Japan The equilibrated vapor concentrations for bicomponent organic solvents of toluene o-xylene, methanol toluene and methanol o-xylene mixtures were measured at various compositions. The equilibrated vapor concentration of each component for the toluene o-xylene system was almost proportional to its liquid phase molar fraction. For the methanol toluene and the methanol o-xylene systems, however, methanol vapor occupied 80 90% of the total organic vapor in the gas phase, even the molar fraction of methanol in the liquid phase was less than 5%. For the toluene o-xylene system, because the solution was regarded as an ideal solution, the equilibrated vapor concentrations could be estimated from the saturated vapor concentrations of pure solvents and their liquid molar fractions. On the other hand, the methanol toluene and the methanol o-xylene systems could not be regarded as ideal solutions. For these systems, the equilibrated vapor concentrations could be estimated by introducing the activity coefficients calculated with the Margules equation. (J Occup Health 1998; 40: 13 136) Key words: Organic solvent, Vapor-liquid equilibrium, Toluene, Methanol, Xylene, Binary mixture More than 400 kinds of organic solvents are used in the work environment 1). Because all of the organic solvents are more or less volatile, they are easily Received May 1, 1997; Accepted Oct 8, 1997 Correspondence to: H. Hori, Department of Environmental Management I, School of Health Sciences, University of Occupational and Environmental Health, Japan, 1 1 Iseigaoka, Yahatanishi-ku, Kitakyushu 807-8555, Japan vaporized in air. The volatility of solvents depends on the saturated vapor pressure. In general, the more volatile solvents have higher vapor pressures, causing higher the vapor concentrations in the work environment. It is therefore very important to know the saturated vapor pressures of organic solvents used in the workplace in order to control the work environment. The saturated vapor pressure of an organic solvent is expressed as a function of the temperature and its value has been obtained for individual solvents ). Therefore, even when an organic solvent used in a workplace is replaced by another solvent without changing working methods and working processes, the environmental vapor concentration can be predicted if only a single component of a solvent is used and the former environmental measurement data are given. Nevertheless, most organic solvents used in the workplace are mixed solvents consisting of several components 1). The saturated vapor pressures of the components (equilibrated vapor pressures) in multicomponent solvents depend on the mole fraction of liquid solvents in the mixture. Because the physical and chemical properties such as the saturated vapor pressure and the polarity of pure components are different from solvent to solvent, the fractions of solvents in the liquid phase and the vapor phase at an equilibrium state are usually different. We have had studied the relationship of the liquid composition and its equilibrated vapor concentration for a methanol toluene mixture experimentally, and have found that the equilibrated methanol vapor concentration was extremely high even when the methanol content in the liquid phase was small 3). In this paper, three couples of two-component solvents were chosen from among popular solvents, that is, toluene, o-xylene and methanol, all of which are frequently used in the work environment. The equilibrated vapor concentrations were then measured at room temperature and atmospheric pressure

Hajime HORI, et al.: Vapor Concentrations of Binary Solvent Mixtures 133 in order to investigate in detail the relationship between the liquid composition and the equilibrated vapor concentrations. Moreover, theories of the vapor-liquid equilibria 4) were applied for both ideal and non-ideal solutions in order to arrange the experimental data. Method 1. Experimental Two kinds of solvents (total volume: 1 to 1.5 ml) were put into vials (5 ml) at various mixing ratios, and placed in a thermostatic bath for 4 hr. The 0 µl of air in the headspace of vials was removed with a gas tight syringe, and injected into a gas chromatograph (Shimadzu, GC- 15A) equipped with a flame ionization detector (FID). An integrator (Shimadzu, Chromatopac R4-C) was used to determine the vapor concentration. Analytical conditions were as follows: Column: DB-WAX capillary column (W&G, 0.3 mm I.D. and 30 m long), column temperature: 70 C, injection port temperature: 00 C, and carrier gas: helium, 40 ml/ min. The saturated vapor concentration increases with temperature, so that if the temperature in the gas tight syringe is lower than that in the vial, the measured vapor concentration may be lower than actual because some of the sampled vapor might be condensed in the syringe. On the other hand, if the temperature is too high, the sample of vapor in air may also be found to be lower than actual because the air in the syringe expands due to high temperature, so that a smaller amount of vapor will be sampled. Therefore, in order to reduce sampling errors, the syringe was placed in an oven at 40 C before the sampling.. Method of data arrangement In case of an ideal solution, the partial (equilibrated) vapor pressures of the solvents are proportional to their saturated vapor pressures (Raoult s law): P i = x i P i, sat (1) where, P i is the equilibrated vapor pressure (mmhg), x i is the molar fraction of component i in the liquid phase, and P i, sat is the saturated vapor pressure of a pure component i (mmhg). In this study, P i, sat is estimated by the Antoine equation ) : B i log P i, sat = A i () C i + t where, A i, B i and C i are constants, and t is the temperature ( C). At atmospheric pressure, the vapor concentration, c (ppm), is expressed as Eq.(3). c = P 10 6 760 (3) By substituting Eq.(3) for Eq.(1), the equilibrated vapor concentration of component i is: c i = x P i i, sat 10 6 (ppm) (4) 760 Eq.(4) indicates that the equilibrated vapor concentration of each component (c i ) is proportional to the molar fraction (x i ) in the liquid phase if the mixture is an ideal solution. In general, however, most solutions cannot be regarded as ideal solutions. In such systems, the equilibrated pressure of component i is expressed as Eq.(5) instead of Eq.(1) 5) : P i = γ i x i P i, sat (5) where γ i is the activity coefficient. For an ideal solution, γ i = 1. Results Figure 1 shows the relationship between the molar fractions of the solvent components in the liquid phase and the equilibrated vapor concentrations for the toluene o-xylene system. Experiments were repeated at least 5 times for each point. The average values and their standard deviations are shown in the figure. The toluene vapor concentration increased and the o-xylene vapor concentration decreased almost linearly with the increasing molar fraction of toluene in the liquid phase. This result indicates that this system can be regarded as an ideal solution. Solid lines in Fig. 1 are the values calculated by Eq.(1). The calculated values are in good agreement with the experimental ones. Figure shows the vapor liquid equilibria, that is, the relationship between the liquid molar fraction and the average values for the equilibrated vapor molar fraction for the toluene o-xylene system. The vapor phase molar fraction was not linearly increased with the liquid phase molar fraction because the equilibrated vapor pressure was different. Figure 3 shows the relationship between the liquid fraction and the equilibrated vapor concentration for the methanol toluene system. The methanol vapor concentration did not linearly increase but remarkably increased with the increasing methanol fraction in the liquid, especially when the methanol fraction was low. This figure indicates that the methanol vapor concentration can become much higher than expected when the methanol content is small. When the liquid methanol molar fraction was greater than 0% (8% in the volume fraction) the methanol vapor concentration was always kept over 100,000 ppm (10%). The lines calculated by means of Eq.(1) are also shown in Fig. 3. The experimental values were much higher than the calculated ones in the low methanol fraction region. The vapor liquid equilibria for the methanol toluene

134 J Occup Health, Vol. 40, 1998 Fig. 1. Equilibrated vapor concentration for toluene o-xylene system. Temperature; 7 C, solid line: Eq.(1). Fig.. Relationship between molar fraction in liquid and equilibrated vapor molar fraction in air for toluene o-xylene system. Temperature; 7 C. Fig. 3. Equilibrated vapor concentration for methanoltoluene system. Temperature; 1 C. system is shown in Fig. 4. The data for toluene are abbreviated because they can be easily calculated by subtracting the methanol vapor fraction from unity. Even when the methanol molar fraction in the liquid phase was only 1% (5% in the volume fraction), it occupied more than 75% of the vapor phase, and when the methanol molar fraction in the liquid phase was 0%, the vapor phase molar fraction was around 80%. This value was almost constant when the methanol fraction was between 0 and 80%. Figure 5 shows the vapor liquid equilibria for the methanol o-xylene system. The methanol molar fraction was greater than 90% in the vapor phase when the liquid Fig. 4. Vapor-liquid equilibria for methanol-toluene system. Temperature; 1 C, solid line is the calculated value by Eq.(6) with A 1 =.096, A 1 =.0438. phase molar fraction was only 3%. The vapor phase molar fraction of methanol was almost independent of the liquid phase molar fraction when the liquid phase molar fraction was up to 80%. Discussion When two kinds of solvents are put into a sealed vessel, the vapor concentration increases due to evaporation of the liquids. When the temperature is constant, the vapor

Hajime HORI, et al.: Vapor Concentrations of Binary Solvent Mixtures 135 Fig. 5. Vapor-liquid equilibria for methanol o-xylene system. Temperature; 5 C, solid line is the calculated value by Eq.(6) with A 1 =.050, A 1 =.75. Fig. 6. Vapor-liquid equilibria for methanol toluene system. Temperature; 1 C, solid line is the calculated value by Eq.(6) with A 1 =.7374, A 1 =1.9168. concentration of each solvent approaches to a certain value, and finally an equilibrium state is established between the liquid and vapor phases. The equilibrated molar fraction in the vapor phase is not usually the same as that in the liquid phase because physical and chemical properties such as the saturated vapor pressure and the polarity are different from solvent to solvent 6). Equation (1) can be applied when the interaction between the solvent molecules is the same. In such a case, the equilibrated vapor concentration can be predicted easily because the equilibrated vapor concentration increases linearly with the liquid molar fraction. Such a solution is called an ideal solution, and is applicable to solutions that consist of solvents with similar chemical structures. In this study, the toluene oxylene system, which consists of aromatic hydrocarbons, can be regarded as an ideal solution because the equilibrated vapor concentration is proportional to the liquid molar fraction as shown in Fig. 1, but Eq.(1) cannot be applied to the methanol toluene and methanol oxylene systems. In general, a bicomponent system that consists of component with different chemical structures cannot be regarded as an ideal solution 5). In order to estimate the equilibrated vapor concentration in such a case, Eq.(5) should be used instead of Eq.(1). The vapor liquid equilibria, that is, the relationship between the liquid molar fractions and the equilibrated vapor concentrations of multicomponent systems have been investigated for the design of column stills. For a two-component system, several equations are proposed Fig. 7. Vapor-liquid equilibria for methanol-o-xylene system. Temperature; 5 C, solid line is the calculated value by Eq.(6) with A 1 =.843, A 1 =1.5556. to estimate the activity coefficient 6). One of the simplest equations is the following Margules equation: In γ 1 = [A 1 + (A 1 A 1 ) x 1 ] x (6) In γ = [A 1 + (A 1 A 1 ) x ] x 1 } where, A 1 are constants. For the methanol toluene system, the vapor-liquid equilibria data have been reported, and A 1, A 1 are obtained experimentally at equilibrated temperatures

136 J Occup Health, Vol. 40, 1998 (A 1 =.096, A 1 =.0438) 6). But these data were obtained under different experimental conditions from those in this study, that is, under the equilibrated temperatures (64.6 C 110.6 C) at atmospheric pressure, and the system consisted of solvents only, and air was not included in the vapor phase, so that it is unclear whether the values for A 1 are applicable to our experimental data obtained under atmospheric air. The values for γ 1 and γ were calculated by substituting the above A 1 for Eq.(6), and the equilibrated vapor pressure was calculated with Eq.(5). The results are shown by a solid line in Fig. 4. The calculated values are generally in good agreement with the experimental ones. This fact suggests that, for the methanol toluene system, the relationship between the liquid molar fraction and the equilibrated vapor pressure was only slightly affected by the temperature and the existence of air in the vapor phase. Therefore, the equilibrated vapor concentration of each component for the methanol toluene system can be estimated even under conditions of room temperature and atmospheric pressure such as are found in the work environment. For the methanol o-xylene system, the values for A 1 are also available (A 1 =.050, A 1 =.75) 7). The values calculated with these values are shown in Fig. 5. The calculated values are generally in good agreement with the experimental ones. Nevertheless, in Figs. 4 and 5, the experimental values tend to be higher than the calculated ones at low methanol molar fraction. This fact means that the methanol concentration will become higher than expected. The reason for this may be that the above parameters (A 1 and A 1 ) were obtained under different experimental conditions from this work. The most suitable values for A 1 were obtained by non-linear least squares (Gauss-Newton method) based on our experimental data. The equilibrium curves with A 1 obtained from our data are shown in Figs. 6 and 7. The calculated lines are in good agreement with the experimental data even at low methanol molar fractions. Conclusion The present study shows that when we use a thinner consisting of methanol and toluene or xylene the methanol concentration in air can become extremely high even if the methanol fraction in the liquid is very small. Current Japanese law states that a solvent which exceeds 5% by weight of the product should be indicated on the vessel. This means that a solvent which is less than 5% of the weight need not be indicated on the vessel. But, as shown in Fig. 5, if xylene is only 3% in methanol, the methanol concentration in air can become almost the same as a thinner that is 90% methanol and 10% o-xylene. If the methanol content is small, the methanol fraction in the liquid phase decreases quickly with time because of the high vapor generation rate of methanol, but when handling thinners that consist of non-ideal solutions such as the methanol o-xylene system, one should be careful because a worker can be exposed to a high concentration of vapor even if the liquid fraction is quite small. We should therefore be informed about all the components of the organic solvents that are used in the work environment including small components. References 1) Inoue T, Ikeda M, Ogata M, et al. A nationwide survey on the use of organic solvents in Japan. Jpn J Ind Health 1984; 6: 518 538. ) The Chemical Society of Japan (ed.). Kagaku Binran Kisohen II. 4th ed. Tokyo: Maruzen, 1993: II-117 II- 135. 3) Kohriyama K, Hori H, Yamada N, Inoue N, Kohno K. Vapor-liquid equilibria for a methanol toluene system: high vapor concentration of methanol. J UOEH 1991; 13: 5 8. 4) Arai Y, Miyamoto A, Kameyama H, Yamaguchi K. Keisanki Kagaku Kogaku. Tokyo: Ohm-sha, 199: 19. 5) Liley PE, Gambill WR, Perry RH, Green DW, eds. Chemical Engineers Handbook, 6th ed. New York : McGraw-Hill, 1984: Chapter 3. 6) Ishikawa T, Iizuka Y, Hirata M. Vapor-liquid equilibria of 1,-dichloromethane methyl butyl ketone and methanol toluene binary systems at 760 mm of Hg. Kagaku Kogaku 197; 36: 566 569. 7) Gmehling J, Onken U, Arlt W. Aromatic hydrocarbons. In: Vapor-liquid equilibrium data collection. Chemistry Data Series Vol. 7. Frankfurt. DECHEMA, 1980.