| dc.description.abstract | Calcium naphthenates in crude oil causes operational problems in refineries and lowers the market price for oil production companies. This Master s thesis aims to study the naphthenic acids who form calcium naphthenates in crude oil. These acids have previously been investigated by Christiansen (2014). The conclusion from this work was that a wide range of naphthenic acids had the capability to form calcium naphthenates. The calcium naphthenate forming acids were found to have a heavier average molecular weight than other naphthenic acids.
The present Master s thesis addresses itself to further elaboration on the solubility of calcium in crude oils. Specifically to assess whether the conclusions from previous work are valid also for other crude oils.
The TAN measurements consistently predicted lower average molecular weights for free naphthenic acids than the MS. This indicates the presence of polyacids in the free naphthenic acids.
Furthermore the TAN measurements differed from MS measurements by consistently predicting a higher average molecular weight for naphthenic acids from calcium naphthenates. This is attributed to impurities in the polar fraction containing the naphthenic acids from calcium naphthenates.
Of the results obtained from MS and TAN, TAN measurements of the average molecular weight of napthenic acids were considered the more reliable of the two, due to few sources of error and good accuracy. The average molecular weights of free naphthenic acids in the two oils were found to be 544 g/mole and 490 g/mole. The average molecular weights of the naphthenic acids from calcium naphthenates were estimated to be slightly lower than the values of 970 g/mole and 934 g/mole as estimated by TAN.
According to TAN measurements the oils in this Master s thesis have the same difference in average molecular weight between naphthenic acids from calcium naphthenates and other naphthenic acids as reported in previous work by Christiansen (2014). It can thus be established that there is a general trend of naphthenic acids who form calcium naphthenates to have a heavier average molecular weight than other naphthenic acids.
The MS spectra show that the isolated naphthenic acids range over a broad molecular weight distribution with no specific peaks. Naphthenic acids from calcium naphthenates seem to have roughly the same distribution as other naphthenic acids, but with a lower proportion of the lower molecular weight acids. Calcium naphthenate formation appears to be controlled by the total amount of acids in the oil, their molecular weight and pH. No specific structures seem to be involved although, structural components like multiple aromatic rings might explain the higher average molecular weight found in naphthenic acids from calcium naphthenates.
High TAN was measured on non-polar fractions after all Acid-IER isolations, indicating that the Acid-IER method did not extract all the naphthenic acids from the crude oil. However, FT-IR analysis showed that this TAN was not caused by naphthenic acids due the fact that the carbonyl functional group did not appear in the FT-IR spectra. Mercaptans, phenols and asphaltenes were considered as alternative TAN sources. No firm explanation was found for the high TAN in the non-polar fractions. Similar results have been reported in previous work Christiansen (2014) which point to a trend for heavy high acid crudes, i.e. there are acidic components present in the crudes which are not extractable by the Acid-IER method.
The Ca-exchange between oil and water is described by the simple model.
CaA_2 (oil)+2H^+ (aq) ↔2HA(oil)+〖Ca〗^(2+) (aq) (8)
K=([〖Ca〗^(2+) ] [HA]^2)/([〖CaA〗_2 ] [H^+ ]^2 ) (10)
This model explains well the observed increase in Ca-content in the oil phase when exposing high TAN crude oils to formation waters, high in Ca-contents and pH levels above 6.5.
The model also explains why the observed Ca-content in the oil phase are reduced during the standard washing procedure with neutral Ca-free waters. If crude oil is to be washed for impurities, but retain its calcium naphthenate content, high pH, low water cut and low-medium temperature should be applied to limit the loss of calcium naphthenates.
Based on the current experiments, individual equilibrium constants were calculated for each oil. The equilibrium constant varied in the range 1 x 1012-2 x 1013, Due to inaccurate pH- measurements it is not possible to conclude whether these variations reflect real crude oil characteristics or if they are solely caused by experimental uncertainties. | en |
