Environmental Modelling & Software 19 (2004) 957–965 www.elsevier.com/locate/envsoft Computer simulation of BTEX emission in natural gas dehydration using PR and RKS equations of state with different predictive mixing rules Naif A. Darwish a, , Reyadh A. Al-Mehaideb a , Ahmad M. Braek b , R. Hughes c a Department of Chemical and Petroleum Engineering, College of Engineering, United Arab Emirates University, PO Box 17555, Al-Ain, United Arab Emirates b ADCO, Abu Dhabi, United Arab Emirates c Chemical Engineering Unit, University of Salford, Salford, UK Received 15 May 2002; received in revised form 23 July 2003; accepted 22 October 2003 Abstract A typical natural gas dehydration plant, which employs triethylene glycol (TEG) as the dehydrating agent, is simulated using a steady state flowsheeting simulator (Aspen Plus). All major units were included in the flowsheet, that is: absorption column, flash unit, heat exchangers, regenerator, stripper, and reboiler. The base case operating conditions are taken to resemble field data from one of the existing dehydration units operating in the United Arab Emirates (UAE). To explore effects of the thermodyn- amic model employed in the simulator on the reliability of the whole simulation process, different predictive mixing rules applied to two cubic equations of state (EOS), as programmed by the simulator, have been investigated. The EOS used in the simulation is the Redlich–Kwong–Soave (RKS) and the Peng–Robinson (PR), both with Boston–Mathias (BM) alpha function. In addition to the classical empirical mixing rules, the following ones are investigated: Predictive Soave–Redlich–Kwong–Gmehling (PRKS), Wong–Sandler (WS), and Modified-Huron–Vidal (MHV2) mixing rules. These mixing rules are all predictive in nature. The plant performance criteria that have been studied for their response to changes in the solvent circulation rate include: BTEX (benzene, toluene, ethyl benzene, and xylenes) emissions rate, desiccant losses (makeup), water content in the dehumidified natural gas, purity of the regenerated TEG, and reboiler heat duty. Comparison with the field data is done. Very diverse results have been obtained from the different models and mixing rules. No one single model gives the best results for all criteria. # 2003 Published by Elsevier Ltd. Keywords: Natural gas; Dehydration; Emission; BTEX; Simulation; Mixing rules; Equations of state 1. Introduction and background Gas dehydration is the process of removing water vapor (moisture) from natural gas streams to meet sales specifications and to prevent hydrate formation and corrosion in transmission pipelines (Campbell, 1992; Manning and Thompson, 1991; Pearce and Sivalls, 1984; Grizzle, 1993). The flowsheet for the natural gas dehydration facility that has been simulated here is shown in Fig. 1. This represents a typical glycol dehy- dration unit and resembles the existing dehydration units operating in the United Arab Emirates (UAE). Details of the operating conditions in this facility has been presented elsewhere (Break et al., 2001). Glycol dehydration involves the absorption of water vapor using a liquid desiccant (e.g., glycol) in an absor- ber (also called contactor) and the regeneration of this rich (water-laden) desiccant in a still column (stripper) using steam or stripping gas at high temperatures and preferably low pressures (Pearce and Sivalls, 1984). The rich desiccant leaving the bottom of the absorber is throttled into low pressure in a flash tank before being sent to the stripper/regenerator unit, where the absorbed species are stripped off the solvent. The major sources of air and water pollution in dehydration units  Corresponding author. Tel.: +962-2-7201000 Ext 22361; fax: +962-2-7095018. E-mail address: naif@just.edu.jo (N.A. Darwish). 1364-8152/$ - see front matter # 2003 Published by Elsevier Ltd. doi:10.1016/j.envsoft.2003.10.008