“How Reliable CPA is for the Process Design of Industrial Glycol Processes”
Abstract
There are three major industrial applications of glycols in oil and gas industries. First, glycols (mostly TEG) are used as absorbents in natural gas dehydration units to remove the water from the natural gas before it is transported through pipelines. With more natural gas fields being explored, there will be more TEG dehydration units online. In addition to water, glycol also absorbs the small amount of methane, volatile organic compounds and hazardous air pollutants, which are ultimately emitted into the atmosphere from the glycol regenerator unit. Larger TEG recirculation rate will result in more emissions, higher operating cost and more heat energy for regeneration. The thermodynamic model should predict accurate results ranging from absorber conditions (low temperature and high pressure) to regenerator conditions (high temperature and low pressure) in order to design the dehydration unit accurately. Second, MEG is used as a thermodynamic hydrate inhibitor, which reduces hydrate formation temperature by changing the chemical potential of water in aqueous phase. Loss of MEG to the gas phase is a critical operating problem. Therefore, an accurate amount of MEG should be predicted from a thermodynamic model. Third, glycols are used as an extractive solvent in liquid-liquid extractor unit to extract the aromatic hydrocarbons from reformed naphtha and pyrolysis gasoline.
The cubic-plus-association (CPA) equation of state (EoS) has already been proven to be a successful model for phase equilibrium calculations for the systems containing glycols. In the present work, we interface the thermodynamic property package (ThermoSystem), based on CPA EoS, with Aspen HYSYS through the CAPE-OPEN interface. We, then, simulate certain multicomponent systems where experimental data are available in the literature and which are critical for the process design of natural gas dehydration units, hydrate inhibition units and liquid-liquid extraction of aromatics by glycols. Comparisons between simulation and experimental results are presented in order to illustrate the reliability of ThermoSystem CPA while used in a process simulator for industrial applications. A simulation case study of typical natural gas dehydration process is also presented.