Isolation and Characterization of Oil-Soluble Calcium Naphthenates in North Sea Heavy Crude Oil
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Naphthenic acids are a diverse group of saturated mono- and polycyclic carboxylic acids that may account for as much as 10-12 weight % of crude oil. During production at several newer oil fields, there have been discoveries of oil that have very high acid and calcium content. This indicates that there are some naphthenic acids bound to calcium as calcium naphthenates present in the oil, and that these are oil-soluble. Oil-soluble calcium naphthenates can cause problems during refinery, as the calcium present can lead to fouling and catalytic deactivation. The coke and fuel quality will also be affected. The removal of calcium in the refinery can be an expensive and comprehensive process, which results in the suppliers getting a lower price on their crude oil.In this master thesis, naphthenic acids from non-soluble calcium naphthenates have been isolated by using an ion exchanger method and a Solid Phase Extraction-method. The work also consisted of neutral and acidic washing of the samples, measurements of water-content, Ca-content and total acid numbers. The different phases from the isolations have also been analyzed by using FT-IR Spectroscopy, APPI-MS and ESI-MS for characterization to see if the naphthenic acids bound to calcium differ from other acids.The naphthenic acids that are bound to calcium in the crude oil have a higher average molecular weight than the other naphthenic acids, at 875 and 580 g/mole respectively. This result was confirmed by TAN-measurements and APPI-MS of samples after isolation with the Acid-IER method. The correlation between the TAN-measurements and the results from APPI-MS suggest that the acids can be characterized as monoacids. They do not have a particular molecular weight, but have a broad distribution between 150 and 2000 g/mole. It seems that the calcium naphthenate formation is controlled by the total amount of acids in the oil, their molecular weight and pH, and not as much on structure as first were presumed.