Chen Yao, Liu Xueen, Xiong Zihao
(Department of Chemistry, Jinan University, Guangzhou, 510632)

Granted by the Ministry of Education of China (No. 200652) and Scientific Research from Guangdong Province of China (No. 2002C32404)

Abstract Experimental tie-line compositions for one quaternary system of water + 2-propanol + dimethyl carbonate + octane and one relevant ternary system of water + dimethyl carbonate + octane were measured at temperature of 298.15 K and ambient pressure. The experimental liquid-liquid equilibria results were correlated by using a modified UNIQUAC model with ternary and quaternary interaction parameters in addition to binary ones.
Keywords Fuel additive, Dimethyl carbonate, Liquid-liquid equilibrium, Modified UNIQUAC model

1. INTRODUCTION
Dimethyl carbonate (DMC) may be an ideal gasoline additive to replace 1,1-dimethylethyl methyl ether (MTBE) because of its excellent characteristics as a gasoline additive: high octane, low RVP, reduced CO and NOx emissions, and a very high density ^[1,2]. DMC has about 3 times the oxygen content of MTBE. It has both low toxicity and relatively quick biodegradability, and can be considered to be an option for meeting the oxygenate specification on gasoline and as a means of converting natural gas to a liquid transportation fuel ^[3]. To understand the mutual solubility of DMC with hydrocarbon in gasoline and assess the effect of DMC addition on hydrocarbon-water mutual solubility, we measured liquid-liquid equilibrium (LLE) data for quaternary mixtures of water + 2-propanol + DMC + octane, and relevant ternary mixtures of water + DMC + octane at 298.15 K. The experimental results were correlated by means of the modified UNIQUAC model ^[4] including ternary and quaternary parameters, in addition to binary parameters. The binary vapor-liquid equilibria (VLE), mutual solubility and ternary LLE relevant to the quaternaries have been available from the literatures: (2-propanol + octane) ^[5], (2-propanol + DMC) ^[6], (2-propanol+ water) ^[7], (DMC +octane) ^[8], (Octane +water) ^[9], (DMC +water) ^[10], (water +2-propanol + octane) ^[11], (water +2-propanol + DMC) ^[12], and (water + DMC + octane) measured in this work.

2. EXPERIMENTS
2.1 Materials                      
2-Propanol was provided by Tianjin Chemical Reagent Factory with nominal minimum mass fraction of 0.997. Octane was supplied by the Guangzhou Chemical Reagent Factory with nominal minimum mass fractions of 0.997. The DMC was obtained from Tianjin Guangfu Chemical Reagents Factory with nominal minimum mass fraction of 0.992. The gas chromatograghy analysis gave mass fractions of 0.991 for DMC, 0.997 for 2-propanol and 0.998 for octane. Water was distilled twice and had a mass fraction of 0.999. All chemicals were used directly in this work.
2.2 Apparatus and Procedures
Ternary and quaternary LLE measurements were carried out at the temperature (298.15 ± 0.01) K. The experimental apparatus and procedures were described in detail previously ^[13]. The quaternary mixtures whose volume is about 80 cm³ were loaded in the equilibrium glass cell placed in a thermostated water bath, stirred for 3 h, and then left to equilibrate for 3 h. The liquid samples about 5 cm³, withdrawn from both upper and lower phases in the cell by using a milliliter syringe without changing the phase equilibria between two layers, were analyzed by a gas chromatograph (Shimadzu, GC-14C) equipped with a thermal conductivity detector. The accuracy of the experimental measurements was estimated to be within ±0.001.
The quaternary mixtures were prepared by mixing binary mixtures of DMC + octane whose compositions are M1, M2, and M3 with water then 2-propanol stepwise to cover the two-phase region shown in Figure 1. The values of M1, M2, and M3 are approximate 0.25, 0.50, and 0.75, respectively, indicating the mole fraction of DMC in the binary mixtures of DMC + octane. Figure 1 shows schematically a tetrahedron to depict three planes of the quaternary mixtures of water + 2-propanol + DMC + octane. Experimental LLE data for the ternary mixtures of water + DMC + octane and quaternary mixtures of water + 2-propanol + DMC + octane are list in Tables 1 and 2.

Table 1 Equilibrium phase compositions in mole fraction (x) for the ternary mixtures of water + DMC + octane at 298.15 K

(1)

and ( I, II = two liquid phases ) (2)

F= (3)

where min denotes minimum values, i = 1 to 3 for ternary mixtures or i = 1 to 4 for quaternary mixtures, j = 1, 2 (phases), k = 1, 2, ...，M (number of tie lines), M = 2ni, and x is the liquid-phase mole fraction.

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