Anal. Chem. 2008, 80, 849-855
Measurement of Total Acid Number (TAN) and TAN Boiling Point Distribution in Petroleum Products by Electrospray Ionization Mass Spectrometry Kuangnan Qian,*,† Kathleen E. Edwards,† Gary J. Dechert,† Stephen B. Jaffe,‡ Larry A. Green,† and William N. Olmstead†
Corporate Strategic Research, ExxonMobil Research and Engineering Company, Clinton, New Jersey 08801, and Process Research, ExxonMobil Research and Engineering Company, Paulsboro, New Jersey 08066
We report a new method for rapid measurement of total acid number (TAN) and TAN boiling point (BP) distribution for petroleum crude and products. The technology is based on negative ion electrospray ionization mass spectrometry (ESI-MS) for selective ionization of petroleum acid and quantification of acid structures and molecular weight distributions. A chip-based nanoelectrospray system enables microscale ( 0.9 mg KOH/g). ESI response leveled at roughly a constant value for low-TAN crudes. This observation may be rationalized as follows. Nonbasic nitrogen compounds (i.e., carbazoles and their benzologues) are weak acids and do not ionize efficiently in the presence of high-concentration acids. Their contributions to ESI signals become significant when crude acid concentration is low. These nitrogen compounds do not react efficiently with KOH in TAN measurement. To correctly quantify acids in low-TAN crudes by ESI, acids and nitrogen compounds must be separated (via odd and even mass split). Only acid mass peaks are used for TAN calculation. TAN measurement by ESI is based on the quantification of all acid species in the sample in reference to an internal standard compound. In this work, stearic acid was used as an internal standard. We assumed uniform response factors for all acid molecules in the TAN calculation.
TAN (mg KOH/g) ) c(56.1/W)(MS/RS)
∑R
A
(3)
where c is a constant instrument factor, W is the weight (g) of sample, MS is the mmol concentration of stearic acid, RS is the ESI response of the stearic acid, and RA is the response of acid molecules in the sample. Table 3 compares the TAN numbers
acid homologues under the framework of SOL. The assignments of BP properties to each of the molecules measured by ESI-MS enabled calculation of TAN values for “virtual” distillation fractions. This is a key advantage of the electrospray-TAN technology. Figure 7 illustrates the TAN BP distributions of a crude oil (crude 18 in Figure 3) as determined by ESI-MS. The distribution pattern matches well with that created by extrapolation of crude assay measurements of selected BP cuts.
Figure 7. TAN boiling point (BP) distributions of 18 crudes (shown in Figure 3) determined by ESI-MS in comparison with BP TAN distributions from extrapolations of assay measurements.
determined by ESI with those by titration for a series of petroleum distillation cuts. The results agree well, suggesting that the assumption of uniform response factor for different acid types is reasonable. Once acid structure and MW are specified, physical properties, such as BPs of the acid molecules, can be calculated using structure-boiling point correlations as described in structureoriented lumping (SOL) by Quann and Jaffe.34 A simpler correlation has been published by Altgelt and Boduszynski involving only H/C ratio and MW.35 Figure 6 shows the calculated BPs for seven (35) Altgelt, K. H., Boduszynski, M. M., Eds. Composition and Analysis of Heavy Petroleum Fractions; Marcel Dekker: New York, 1994.
CONCLUSIONS We have demonstrated that ESI-MS can be used as a means for rapid and microscale measurement of TAN and TAN BP distributions for petroleum crude and products. The latter property can be obtained without physically distilling the sample. Since ESI does not involve thermal processing, decomposition of naphthenic acids is minimized. The use of nanoelectrospray technology greatly enhanced the repeatability and robustness of the method. A mass-dependent CID was developed to eliminate dimers and minimize fragmentation of low MW acids. TAN values determined by the technique agree well with the titration method. The distributed properties compare favorably with the extrapolation of crude assay measurements. ACKNOWLEDGMENT We thank Charles Rebick for the discussions of ESI-MS in petroleum applications and John Wyatt and Leo Brown for providing samples. Received for review September 5, 2007. Accepted October 25, 2007. AC701868V
Analytical Chemistry, Vol. 80, No. 3, February 1, 2008
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