Changes of Petroleum Acid Distribution Characterized by FT-ICR MS in Heavy Acidic Crude Oil after True Boiling Point Distillation

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1 Scientific Research China Petroleum Processing and Petrochemical Technology 2014, Vol. 16, No. 1, pp 8-12 March 30, 2014 Changes of Petroleum Acid Distribution Characterized by FT-ICR MS in Heavy Acidic Crude Oil after True Boiling Point Distillation Liu Yingrong; Zhang Qundan; Wang Wei; Liu Zelong; Zhu Xinyi; Tian Songbai (Research Institute of Petroleum Processing, China Petroleum & Chemical Corporation, Beijing ) Abstract: The molecular transformations of carboxylic acids in heavy acidic SL crude before and after true boiling point distillation were examined by ultra-high resolution negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The acid class (heteroatom number), type (z numbers) and carbon number distributions were positively characterized. It was found out that the total acid number (TAN) of SL crude decreased after true boiling point distillation, and the abundance of O 2 class in mass spectra was also found to be reduced from 67.6% to 34.5% in SL TBP mixed crude as measured by MS spectra, indicating to a potential carboxylic acid decomposition. However, it was interesting that the carboxylic acids type distribution in both oils was almost the same although their relative abundance in SL TBP mixed crude turned to be much lower, suggesting that various petroleum carboxylic acid types have the similar thermal decomposition reaction behavior. Furthermore, for each O 2 type of acids in SL TBP mixed crude, the abundance of carboxylic acids with carbon number higher than 35 was reduced greatly, especially for those with carbon number higher than 60, the mass peaks of which were nearly totally removed, indicating that the large carboxylic acid molecules in heavy fractions decomposed more significantly because of longer heating time during the true boiling point distillation process. As a result, the reduction of TAN may be caused by the thermal decomposition of carboxylic acids especially those with high carbon number, suggesting that quick distillation or much lower pressure is required to avoid the thermal decomposition. Key words: true boiling point distillation; carboxylic acid; ESI; FT-ICR MS; acidic crude; total acid number (TAN) 1 Introduction As the worldwide crude reserves tend to be increasingly heavier presently, the acidic crude production has been rising in recent years [1-2]. The proportion of acidic crudes has accounted for almost 30% of the total China s petroleum output since 2010 [2]. Because the discounted price of heavy acidic crudes [3], the Sinopec Corp. is also increasing the purchase of opportunity crudes from the global market. In order to evaluate the quality of acidic crudes to formulate a reasonable processing scheme, crude assay is a reliable way to characterize the crude and its narrow and wide fractions. Among these measures, the true boiling point (TBP) distillation in compliance with the ASTM methods D2892 [4] and D5236 [5] is the first step to obtain the narrow cuts for measuring the boiling point distribution of petroleum acids. However, it has been found out that the total acid number (TAN) of acidic crude reduces severely after TBP distillation, which may hinder the as- sessment of their processing properties. Blum, et al. [6-7] in 1990 reported that the naphthenic acids decomposed into products with low TAN and low viscosity at high temperature. However, the detailed type and composition of naphthenic acids were not characterized to explain the thermal stability of naphthenic acids due to the limitation of analytical method at that time. With the development of selective ionization and ultra-high resolution mass spectrometry, it is possible to characterize the naphthenic acids without separating them from the crude oils [8]. FT-ICR MS with ultra-high mass power resolution and mass accuracy plays a key role in the characterization of complex petroleum samples [8-18]. Donald, et al. [18] measured the thermal reaction of carboxylic acids in HVGO Recieved date: ; Accepted date: Corresponding Author: Ms Liu Yingrong, Telephone: ; liuyr.ripp@sinopec.com. 8

2 Liu Yingrong, et al. Changes of Petroleum Acid Distribution Characterized by FT-ICR MS in Heavy Acidic Crude Oil after True Boiling Point Distillation of oil sands bitumen by a homemade FT-ICR MS in The TAN of product after thermal treatment was found to be lower after heating at for 1 hour under high pressure. However, the compound type and carbon number distribution of the carboxylic acids remained almost the same and the reason for the reduction of TAN was suspected as the consequence of decarboxylation of carboxylic acids at high temperature. In this study, the compound type and carbon number distribution of carboxylic acids in crude oils before and after TBP distillation were characterized by the Bruker 9.4T FT-ICR MS. The thermal stability and reaction behaviors of carboxylic acids under true boiling point distillation were discussed to explain the reduction of TAN in heavy acidic crude after TBP distillation. 2 Experimental 2.1 Bulk properties of the samples The bulk properties of Shengli (SL) crude and its true boiling point distillation product (SL TBP mixed crude) are listed in Table 1. The SL TBP mixed crude was obtained by mixing the distilled narrow cuts according to their TBP yields. The yield of wide fractions and their corresponding TAN are listed in Table 2. Table 1 Properties of SL crude and SL TBP mixed crud Specification SL crude SL TBP mixed crude Density (20 ), g/cm Total acid number, mgkoh/g Sulfur content, % Nitrogen content, % Table 2 The yield of wide fractions and their corresponding TAN number Samples Fraction yield (in crude oil), % Total acid number, mgkoh/g IBP > True boiling point distillation [4-5] TBP distillation of SL crude oil was carried out in an AUTODEST 800AC Fisher distillation system operating according to АSTM D2892 for the atmosphere part of the test and according to АSTM D5236 for the vacuum one. The TBP distillation was performed in the AUTODEST 800 Fisher distillation system operating at a pressure drop ranging from 101 to 27 kpa and in the AUTODEST 860 Fisher distillation system operating at a pressure drop ranging from 133 to 26.6 Pa. 2.3 Mass spectrometry analysis The samples were dissolved in a toluene/methanol (volume ratio of 50:50) solution at a concentration of 1 mg/ml and measured by an apex-qe 9.4T FT-ICR MS (made by the Bruker Daltonics Corp.) coupled with a negative mode electrospray ion source (ESI - ). The m/z range covered from 150 to v% of ammonia was added to promote the ionization of carboxylic acids. 2.4 Data processing The mass spectrometer was firstly calibrated by sodium formate dissolved in methanol (0.1 m%). The mass spectra were further calibrated internally according to a known series of naphthenic acid compounds that have already been identified before. Mass peaks with signal-to-noise ratios which were higher than 6 were selected for further data processing. Measured masses were expressed as a general chemical formula, C c H 2c+z N n O o S s, in which z is often referred to as the hydrogen deficiency index and c, n, o and s are referred to the atom numbers of C, N, O and S, respectively. The z value is directly related to the number of rings plus double bonds in the molecules. The MS data were processed by means of a homemade software [19]. 3 Results and Discussion 3.1 The bulk physical properties The narrow fractions obtained from TBP distillation of crude oil were mixed according to their corresponding yields of each cuts and the obtained SL TBP mixed crude was used for comparison with SL crude oil. The properties of SL crude oil and SL TBP mixed crude are listed in Table 1. It can be seen that the density, sulfur content, and nitrogen content were almost the same between the two oils while the TAN of SL TBP mixed crude was decreased sharply. The TAN distribution in various fractions is listed in Table 2. The highest TAN was found in VGO fraction and its value was 0.58 mgkoh/g, while the TAN of vacuum residue (>540 ) was even lower than 0.05 mg KOH/g. The TAN of all fractions was lower than that of original crude. So ESI - FT-ICR MS was employed 9

3 China Petroleum Processing and Petrochemical Technology 2014,16(1):8-12 to characterize the molecular transformation of carboxylic acids occurred during this process. 3.2 The composition of carboxylic acids before and after TBP distillation It can be found out that the relative abundance of O 1 compounds increased while that of O 2 compounds reduced after TBP distillation, suggesting that the potential carboxylic acids decomposition was the main cause for loss of TAN. The ESI - FT-ICR MS spectra of SL crude and SL TBP mixed crude are shown in Figure 1. It can be seen clearly that the molecular weight of the acid compounds in the crude oils turned to be smaller after TBP distillation. Figure 2 The heteroatom class distribution (a) and its calculated value (b) of the acid compounds in SL crude oil and SL TBP mixed crude SL Crude; SL TBP mixed crude Figure 1 The ESI - FT-ICR MS spectra of SL crude and SL TBP mixed crude The heteroatom class distribution of the acid compounds in SL crude oil and SL TBP mixed crude is shown in Figure 2a. The N 1 compounds (mainly acid and neutral nitrogen-containing compounds) were ionized simultaneously with acid oxygen-containing compounds including O 1 (mainly phenol compounds) and O 2 (mainly carboxylic acid) compounds. Since the nitrogen content was found to be almost the same after TBP distillation as listed in Table 1, the acid and neutral nitrogen-containing compounds were also expected to be unchanged. As a result, the relative abundance of N 1 compounds in ESI - FT-ICR MS spectra can be used as a reference for estimating the change of the O 1 and O 2 compounds. The calibrated heteroatom class distribution is then illustrated in Figure 2b. The carboxylic acid type and carbon number distribution of SL crude oil and SL TBP mixed crude are illustrated in Figure 3 and Figure 4, respectively. The size of the circles indicates the relative abundance of O 2 class compounds in Figure 4. Figure 3 The carboxylic acid type distribution for SL crude oil and SL TBP mixed crude SL Crude; SL TBP mixed crude 10

4 Liu Yingrong, et al. Changes of Petroleum Acid Distribution Characterized by FT-ICR MS in Heavy Acidic Crude Oil after True Boiling Point Distillation Figure 4 Plots of z number versus carbon number for O 2 class of SL crude oil and SL TBP mixed crude determined by ESI - FT-ICR MS Figure 5 The carboxylic acid total carbon number distribution of SL crude and SL TBP mixed crude SL crude; SL TBP mixed crude The compound type distribution for SL crude oil and SL TBP mixed crude was almost the same although the relative abundance of SL TBP mixed crude turned to be much lower, as shown in Figure 3, which suggests that various petroleum acid types have the similar thermal decomposition reaction behavior. However, the carbon number distribution of the two oils was quite different. The carbon number of O 2 compounds in SL crude oil in Figure 4a varied from 15 to 70, while the carbon number of SL TBP mixed crude reduced within the range of 15 to 60 in Figure 4b. Furthermore, by the abundance summation of each type of carboxylic acids with the same carbon number, the reduction of the carbon number can be further observed in Figure 5 and the share of carboxylic acids with carbon number higher than C 35 reduced sharply after TBP distillation. This result was consistent with the TAN distribution of SL crude oil, in which the TAN of residue fraction was much lower than the TAN of VGO fraction. However, Long, et al. [2] reported that the chemical reaction behaviors of carboxylic acids were independent of the compounds type and their molecular size as well as the carbon number based on the molecular simulation technique and quantum chemistry theory. The difference between the results in this study and the literature report may be attributed to the different reaction conditions. In fact, TBP distillation separates the crude oil into narrow cuts depending on the differences in boiling points of the individual components. Generally, boiling points of big molecules with high carbon number in heavy fraction are higher than those of small molecules with low carbon number in relatively light fraction. Therefore, it takes a longer time to evaporate carboxylic acids with high carbon number during TBP distillation and eventually the heating time for these acids is much longer. Even though the heavy fraction is treated under reduced pressures in compliance with the ASTM method, sometimes the temperature of boiling liquid in distilling flask is close to 300, under which conditions there are no significant cracking signs detected but the carboxylic acids may decompose. As a result, the carboxylic acids with high carbon numbers may decompose much more than those with low carbon numbers because of their longer heating time in distilling flask. Consequently, it was found out that for each type, the abundance of carboxylic acids with carbon number higher than C 35 reduced sharply, especially for those acids with carbon number higher than 60, the mass peaks of which are almost totally removed. According to these observations, it is known that in order to avoid the thermal decomposition of carboxylic acids, lower pressure or fast distillation operation may be applied in TBP distillation. 4 Conclusions ESI - FT-ICR MS is suitable for the characterization of 11

5 China Petroleum Processing and Petrochemical Technology 2014,16(1):8-12 the compound type and carbon number distribution of the carboxylic acids in SL crude before and after TBP distillation. The TAN of crude decreased after TBP distillation, and the FT-ICR MS results proved that the compound type of the carboxylic acids reduced without exception, indicating that the reaction is similar for different compound types. However, the ratio of carboxylic acids with carbon number higher than C 35 reduced sharply after TBP distillation because of the longer heating time. As a result, the reduction of TAN may be due to the thermal decomposition of carboxylic acids especially with regard to high carbon number, denoting that quick distillation or much lower pressure is suggested to avoid the thermal decomposition of carboxylic acids. Acknowledgment: This work was financially supported by the National Key Basic Research Development Program 973 Project (2006CB202501). Reference [1] Wang Xieqing, Fu Xiaoqin, Tian Songbai, et al. Preliminary research of acidic crude oil FCC decarboxylation technology[j]. Petroleum & Petrochemical Today, 2006, 14(10): 7-13 (in Chinese) [2] Long Jun, Mao Anguo, Tian Songbai, et al. Development and commercial application of packaged technology for catalytic decarboxylation and cracking of high acid crude oil[j]. Petroleum Processing and Petrochemicals, 2011, 42(3): 1-6 (in Chinese) [3] Stratiev D, Dinkov R, Petkov K, et al. Evaluation of crude oil quality[j]. Petroleum & Coal, 2010, 52 (1): [4] ASTM D2892: Standard Test Method for Distillation of Crude Petroleum (15-Theoretical Plate Column)[S] [5] ASTM D5236: Standard Test Method for Distillation of Heavy Hydrocarbon Mixtures (Vacuum Potstill Method)[S] [6] Blum S C, Olmstead W N. Viscosity reduction by heat soak induced naphthenic acid decomposition in hydrocarbon oils: The United States, US [P], [7] Blum S C, Olmstead W N, Jr Bearden R. Thermal decomposition of naphthenic acids: The United States, US [P], [8] Qian K, Robbins W K, Hughey C A, et al. Resolution and identification of elemental compositions for more than 3000 crude acids in heavy petroleum by negative-ion microelectrospray high-field Fourier transform ion cyclotron resonance mass spectrometry[j]. Energy Fuels, 2001, 15 (6): [9] Hsu C S, Dechert G J, Robbins W K, et al. Naphthenic acids in crude oils characterized by mass spectrometry[j]. Energy Fuels, 2000, 14 (1): [10] Hughey C A, Rodgers R P, Marshall A G, et al. Identification of acidic NSO compounds in crude oils of different geochemical origins by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry[j]. Org Geochem, 2002, 33 (7): [11] Christine A, Rodgers R P, and Marshall A G. Resolution of compositionally distinct components in a single electrospray ionization Fourier transform ion cyclotron resonance mass spectrum of crude oil[j]. Anal Chem, 2002, 74: [12] Stanford L A, Kim S, Rodgers R P, et al. Characterization of compositional changes in vacuum gas oil distillation cuts by electrospray ionization Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry[j]. Energy Fuels, 2006, 20(4): [13] Stanford L A, Rodgers R P, Marshall A G, et al. Detailed elemental compositions of emulsion interfacial material versus parent oil for nine geographically distinct light, medium, and heavy crude oils, detected by negative- and positive-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry[j]. Energy Fuels, 2007, 21(2): [14] Purcell J M, Hendrickson C L, Rodgers R P, et al. Atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry for complex mixture analysis[j]. Anal Chem, 2006, 78 (16): [15] Smith D F, Schaub T M, Rahimi P, et al. Self-association of organic acids in petroleum and Canadian bitumen characterized by low- and high-resolution mass spectrometry[j]. Energy Fuels, 2007, 21 (3): [16] Marshall A G, Rodgers R P. Petroleomics: The next grand challenge for chemical analysis [J]. Acc Chem Res, 2004, 37(1): [17] Rogers R P, Marshall A G. Petroleomics: MS returns to its roots[j]. Anal Chem, 2005, 77(1): 20A-27A [18] Smith D F, Rodgers R P, Rahimi P, et al. Effect of thermal treatment on acidic organic species from Athabasca bitumen heavy vacuum gas oil, analyzed by negative-ion electrospray Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry[j]. Energy Fuels, 2009, 23: [19] Liu Yingrong, Wang Wei, Hu Qiuling, et al. Characterization of basic nitrogen aromatic species obtained during fluid catalytic cracking by Fourier transform ion cyclotron resonance mass spectrometry[j]. China Petroleum Processing and Petrochemical Technology. 2012, 14 (2):