Vapor-Liquid Equilibria of Water + Triethylene Glycol (TEG) and Water + TEG + Toluene at 85 kPa Ali Khosravanipour Mostafazadeh, Mohammad Reza Rahimpour,* and Alireza Shariati School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran In this study, vapor-liquid equilibria data have experimentally been measured for systems water + triethylene glycol (TEG) and water + TEG + toluene at 85 kPa and various temperatures. The VLE data were determined in a modified Othmer still, and the samples were analyzed using gas chromatography and titration methods. The NRTL, UNIQUAC, and Van Laar models were used to correlate the data. The results demonstrate the enhancement of volatility of water in water + TEG solutions and increasing the purity of the dehydrated TEG by the addition of toluene. Using this method, the more concentrated TEG can be obtained in the stripping columns of the natural gas dehydration systems. Introduction Natural gas dehydration is an important operation in the gas processing and conditioning industry. In this process, water vapor is eliminated from natural gas streams for domestic usage or other downstream gas processes. The level of water vapor in natural gas should be maintained below a certain value to prevent hydrate formation and minimization of corrosion in transportation pipelines. 1–3 The standard method for natural gas dehydration is by the absorption of water using triethylene glycol. The glycols are effective liquid desiccants because of their high hygroscopic property, low vapor pressure, high boiling point, and low solubility in natural gas. The four types of glycols that have been used for natural gas dehydration are ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (T 4 EG). 4 TEG is used in about 95 % of the glycol dehydration units for natural gas streams. Glycol dehydration units conventionally consist of a contactor and a regenerator. An alternate approach for the enhancement of reconcentrator performance is the use of stripping agent. In this approach a volatile hydrocarbon liquid is added to the glycol regeneration system. The hydrocarbon increases the volatility of water in the solution of water + TEG. Smith and Skiff 5 reported that this type of process can achieve compositions of over 99.99 % with triethylene glycol, resulting in potential dry gas with the water dew points range in (-73.3 to -95.5) °C. Toluene and isooctane (2,2,4-trimethyl pentane) are used as entrainers in the stripping columns of natural gas dehydration units. 6,7 Knowledge of the vapor-liquid equilibrium (VLE) data is necessary for accurate design and simulation of stripping columns. Therefore, vapor-liquid equilibria for the systems water + TEG and water + TEG + toluene (one of the stripping agents) have been studied in this work. It is common in industry, such as in extraction and distillation units, to add a third component or extract an agent to alter relative volatilities in the interest of facilitating extraction or distillation. Morrison et al. 8 investigated the salt effect on the VLE of the water + alcohol system using modified Othmer still. Herskowitz and Gottlieb 9 determined the activity coefficient of water in a water-triethylene glycol solution using an isopiestic method. Gupta et al. 10 determined isobaric vapor-liquid equilibria for the systems TEG-benzene, toluene-TEG and benzene-N-methylpyrrolidone. Scauzillo 11 presented equilib- rium ratios and activity coefficients of water in the systems water + TEG + natural gas. Cartaya et al. 12 evaluated a local composition model (the LCM model) for the VLE of the binary mixtures of triethylene glycol in the presence of methane, water, and benzene. Chung and Luo 13 investigated the vapor pressures of the aqueous desiccants. Wise et al. 14 determined saturated vapor densities and activities of triethylene glycol-water at low pressures. Dingman and Lebas 15 presented dew point data for the binary solution water + TEG. Chen et al. 16 reported solubility of glycol in water-hydrocarbon systems. Twu et al. 17 developed an advanced equation of state for modeling the TEG + water system for the glycol gas dehydration process. * To whom correspondence should be addressed. Tel: +987112303071. Fax: +987116287294. E-mail: rahimpor@shirazu.ac.ir. Table 1. Identifications of Materials Used in This Study chemical purity water F/g · cm -3 supplier TEG > 99 % < 0.3 % 1.123 Merck toluene > 99 % < 0.01 % 0.866 Merck distillated water 1.00 Shiraz University Table 2. UNIQUAC Parameters for Pure Materials parameter TEG toluene water r 5.59 3.92 0.92 q 4.89 2.97 1.40 Table 3. Vapor Pressure Constants for Pure Components 27,28a comp c 1 c 2 c 3 c 4 c 5 water 73.649 -7258.2 -7.3037 4.1653e -6 2 toluene 80.877 -6902.4 -8.7761 5.8034e -6 2 TEG 29.36842 -8897.103 -1.467521 2.126367e -6 2 a ln P i ) c 1 + c 2 /T + c 3 ln T + c 4 T c 5 (T in K and p in Pa except for TEG in kPa). Table 4. Physical Properties for Pure Components 17,25,28,29 T c p c · 10 -6 V c comp M K Pa m 3 · kmol -1 z c ω TEG 150.2 769.5 3.320 0.5347 0.2462 1.254 H 2 O 18.01 647.1 21.94 0.056 0.228 0.343 toluene 92.14 591.8 4.10 0.314 0.262 0.262 J. Chem. Eng. Data 2009, 54, 876–881 876 10.1021/je800675u CCC: $40.75  2009 American Chemical Society Published on Web 01/29/2009