Phase equilibrium measurements of glycol + water + natural gas systems for the design of subsea dehydration units

Description

Presentation at ESAT 2018 Conference
ABSTRACTOver the past 25 years, offshore gas processing operations have seen a proliferation in the development of subsea installations with complete subsea processing facilities soon to be a reality. Novel technologies – including subsea dehydration and compression units – are aimed at gas-dominated wells with the purpose of production and export directly from the seabed. Subsea production presents opportunities for improved efficiencies and increased profitability. To achieve the required dew point specification, water is absorbed from the gas stream through contact with a glycol such as ethylene glycol (MEG). The design and operation of these dehydration units require a thorough understanding of the phase equilibrium between the gas, water and glycol, at the process conditions observed in subsea installations. This understanding is achieved through a combination of accurate phase equilibrium data and thermodynamic models.

For this purpose we have performed phase equilibrium measurements for two systems: MEG + H₂O + CH₄ and MEG + H₂O + Natural Gas. Experimental temperatures range from
15 – 50 °C, with measurements at pressures of 60 and 125 bar. The experiments were performed in a purpose-built rig at Statoil’s Research and Development Center in Trondheim, Norway. Glycol-rich mixtures have been prepared (>90 wt% in the liquid phase) in order to replicate dehydration applications. Glycol content has been determined using adsorption tubes which are analysed in a GC-MS, while water content was measured using Karl Fischer titration. For the natural gas systems, component analysis for C₁-C₆, N₂ and CO₂ has been performed with the use of gas chromatography. The experimental uncertainty is shown to be ~10% depending on the type of measurement.

Thermodynamic modelling is done using the Cubic-Plus-Association (CPA) equation of state. The model provides a good qualitative description for the vapour phase content for MEG and H₂O, which are critical product specifications for dehydrated natural gas. For the description of MEG, we evaluate the performance of our new 4F association scheme against the literature 4C scheme. The use of uncertainty analysis in the parameterization of the models allows for the determination of operating ranges for process designs. This transference of uncertainty, from the data to the model and to the design, provides greater insight for the decision-maker regarding the process feasibility of a given design.

Period	10 Jun 2018 - 13 Jun 2018
Held at	Institute of Chemical Process Fundamentals of the CAS, Czech Republic
Degree of Recognition	International