Selection of a Flocculant to Assist in Divalent Cation Removal in a MEG Pre-Treatment Process

Abstract

When monoethylene glycol (MEG) is used to provide hydrate protection for gas condensate production, MEG pre-treatment, reconcentration and reclamation systems are generally employed to recover and reuse the MEG. Prior to reconcentration, low solubility salts of divalent cations such as calcium, iron, strontium and magnesium, that may be present in the Rich MEG, are removed in a MEG pre-treatment process. This process involves the addition of a base, such as NaOH or KOH, to the Rich MEG at elevated temperatures to convert dissolved carbon dioxide to carbonate ions and so precipitate the cations, as their respective insoluble carbonate or hydroxide salts. When enough residence time is available within the process these precipitated salts are removed from the Rich MEG stream through physical separation. For onshore based MEG systems, this is usually accomplished via settling tanks. However, in offshore systems the residence time for crystallization and settling becomes limited due to vessel sizes imposed by facility space limitations so precipitated salts are actively removed using mechanical equipment such as centrifuges. Centrifuges are only effective when crystals reach threshold particle sizes. Contaminants in MEG such as dissolved hydrocarbons and magnesium ions can inhibit crystal growth of calcium and iron carbonate. This study details the development of testing methodologies to screen chemistries to assist in particle agglomeration and led to the identification of a promising class of chemistries that could be applied in MEG Pre-treatment for the flocculation of cation salts.