(688e) Improved Simulation of BTEX Emissions from a Natural Gas Dehydration Plant

Natural gas dehydration process is typically employed to reduce the water content of the natural gas at high pressure to prevent hydrate formation and corrosion in natural gas processing. To reduce the water content of natural gas, typically glycol solvents such as triethylen glycol (TEG) is utilized to remove water from natural gas through a high pressure absorption column. The natural gas includes some volatile organic compounds (VOCs) as well as benzene, toluene, ethylbenzene and xylens, collectively named BTEX. The environmentally hazardous compounds (VOCs and BTEX) are absorbed in TEG in the absorption column of the natural gas process resulting in emission of such pollutants from a flash unit and stripping column to the atmosphere.  Therefore, it is imperative to estimate the true emission rate of such pollutants to the atmosphere to perform an accurate exposure analysis for the workers of such plant. An accurate steady state simulation of a natural gas dehydration plant enables us to obtain emission rates of BTEX, VOCs and CO₂ for human exposure and global warming analyses. In this study, steady state simulation of one of the existing natural gas dehydration units operating in the United Arab Emirates (UAE) was performed using Aspen Plus (V7.2) process simulator, and the simulation results were compared with available field data to ensure the accuracy of the simulation package. The natural gas dehydration plant includes an absorption column operating at 618 psia, a flash unit, heat exchangers and atmospheric regenerator and stripper. To develop the best simulation package, two major approaches were considered for the simulation of the plant. In the first approach (single model), one single thermodynamic model including a standard equation of state (EOS) or a predictive EOS was used for all unit operations involved in the plant. In an alternate solution (double-model), NRTL-RK model was utilized for unit operations operating under atmospheric pressure while a predictive equation of state model was used for the absorption column operating at 618 psia. The binary interaction data in the simulator were updated to improve the accuracy of the simulation. The standard EOS and predictive EOS models included in this study were PRMHV₂, RKSMHV₂, PRWS, RKSWS, RKS-Aspen, PSRK, SRPOLAR, PR and RKS. Among different scenarios, single model approach employing RKS-MHV₂ model showed a much better agreement with filed data. The simulation results show that reboiler duty in the regenerator column plays a key role in controlling the BTEX emissions by changing the solvent (TEG) circulation rate. The BTEX and CO₂ emission rates from the plant were found to be 44.86 lb/h and 59.28lb/h, respectively.