The amount of acidic crude oils produced worldwide is increasing. These oils introduce problems throughout the hydrocarbon value chain due to their high content of carboxylic acids, examples of which are naphthenate deposition in production facilities and refinery corrosion. In order to deal with these problems, it is often necessary to isolate the carboxylic acids in different fluid streams or solids to allow further analysis without interference from other compounds.
This paper presents and verifies an ion-exchange method that can be used for selective isolation of carboxylic acids from crude oils, distillate fractions and other organic solvent. The efficiency and selectivity of the method have been demonstrated as >98 mol-% using two synthetic carboxylic acids. Its usefulness for isolation of carboxylic acids from solids and non-organic solvents has also been demonstrated, exemplified by calcium naphthenate deposits and water.
Introduction
Among the oilfields found and developed around the world, an increasing fraction contains naphthenic acids and has a high TAN-value (Total Acid Number). Producing and refining high-TAN crude oils introduces a number of problems, e.g. calcium naphthenate deposition in process facilities offshore,1–4 and corrosion in refinery equipment.5
In order to understand the mechanisms behind these and other naphthenic acid-related problems, it is of utmost importance that the acids can be isolated selectively, thereby allowing for thorough characterization of the acids. Through the years, a number of works dealing with isolation and characterization of acids in crude oils, base oils, oil shale, etc. have been published. The different methods are most easily grouped into the categories solid/liquid extraction,6–12 liquid/liquid extraction,13 and chromatography.14,15 Methods for direct extraction of acids from water are also described.16–18
The idea of using a sugar-based QAE Sephadex A-25 ion exchange resin (Acid-IER) was presented to Statoil by private consultant Bjørn A. Ardø, who also worked with Statoil to develop the idea into a working procedure for isolating carboxylic acids from crude oils. The fact that it is sugar-based makes the IER hydrophilic, and, therefore, more selective towards carboxylic acids than hydrophobic IERs.
The present paper describes the Acid-IER method in detail, and gives examples of its use for isolating acids from a crude oil and its distillates as well as from water and calcium naphthenate deposits. The high efficiency and selectivity of the Acid-IER method has been demonstrated using two synthetic acids dissolved in an acid-free crude oil.
Laboratory equipment
The experimental set-up and equipment used during carboxylic acid isolation are ordinary laboratory equipment, e.g. filter funnels, filtering flasks, and round flasks. During the filtering process two filter paper qualities were used - 5891 Black Ribbon and 5893 Blue Ribbon from Schleicher & Schuell - hereafter referred to as coarse and fine filter paper, respectively.
Solvent evaporation was done using a rotavapor (Labrota 4003 equipped with rotacool and rotavac, Heidolph). The rotation and temperature set points were 120 rpm and 60°C, respectively. The pressure was reduced gradually to 35 mbar. The acids were dried further in a heating chamber (60°C) until the weight was stable.
