Resolving Operational Challenges of a MEG System

Abstract Although continuous Monoethylene glycol (MEG) injection is found as the most reliable and cost-effective method of hydrate inhibition on numerous gas/condensate field developments, a MEG system is complex and represents a number of chemical, physical and hence operational challenges. MEG is used in an Argentinian gas field as hydrate inhibitor for wet gas transport through the pipeline. MEG has been chosen because it is a good thermodynamic hydrate inhibitor and because of its low volatility, low losses, relatively low toxicity and low flammability. The system is characterized by a reconcentration of MEG in reboilers. There is no reclaimer or salt removal system other than filters. Several problems must be resolved such as filter blockage due to emulsions and solid particles, fouling of equipment as well as MEG losses. In this paper, scale prediction has been carried out at different process units of the MEG loop with the help of a commercial thermodynamic scale prediction software with the glycol add-in (MEGScale); the electrolyte model in the software is a "Pitzer model" for dissolved species in addition to a complete PVT model. The model has been specially developed to handle distribution of hydrocarbons, CO2 and H2S between gas, oil and aqueous phases. The probability of precipitation is not only governed by the divalent cation concentration; the alkalinity that reacts to HCO3- and CO32- is at least as important. To give an indication of the divalent cation concentration that can be tolerated in the lean MEG to keep the saturation Ration (SR) below 1 anywhere in the glycol loop, simulations were performed with 0.1, 1 and 10 mmol/kg alkalinity in form of OH- in the lean MEG. Measurements indicate that Ca2+ has reached a steady state concentration of ca. 100 mg/l. The simulations indicate that this happens when the strong alkalinity in the lean MEG approaches 1 mmol/l. For scale mitigation strategy, a scale inhibitor needs to be at the well heads; however, this chemical cannot prevent scaling in the heat exchanger and reboiler. If the divalent cation concentration continues to increase, the MEG must be partly replaced or reclaimed before reaching the limit. The other alternative is the use of ion exchange resin in order the remove the divalent ions.