1 Introduction. Wang Zhenhua 1,2 ; Tu Yongshan 2

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1 Scientific Research China Petroleum Processing and Petrochemical Technology 2011,Vol. 13, No. 4, pp December 30, 2011 Distribution of Carboxylic Acids in Sudanese Dar Crude Oil: Characterized by Negative-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Wang Zhenhua 1,2 ; Tu Yongshan 2 (1. China Tianchen Engineering Corporation, Tianjin ; 2. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao ) Abstract: The Dar crude oil produced in Sudan was distilled into several fractions. The heteroatom class species in crude and its fractions were characterized by the negative-ion ESI FT-ICR MS. The major emphasis was put upon the study on the O 2 class species (petroleum carboxylic acids). The test results revealed that aliphatic acids and monocyclic naphthenic acids accounted for a large proportion in petroleum carboxylic acids of the Dar crude. The relative abundance of aliphatic acids and monocyclic naphthenic acids decreased with an increasing boiling point of fractions. Meanwhile, the relative abundance of bicyclic and tricyclic naphthenic acids increased at first and then decreased, whereas the relative abundance of petroleum carboxylic acids with higher double-bond equivalence (DBE) values increased. The high abundance of aliphatic acids in the Dar crude and its fractions covered the carbon numbers in a range of 16 to 18 which had important geochemical significance. The O 2 class species were distributed in a broad range of DBE values and carbon numbers with increasing boiling points of fractions. Key words: O 2 class species; petroleum carboxylic acids; DBE; aliphatic acids; naphthenic acids. 1 Introduction Organic acids in crude mainly consist of aliphatic acids, naphthenic acids, phenols, mercaptans, etc. And naphthenic acids commonly comprise 85 m% of acidic species in crude [1]. So organic acids in crude oil are generally called the naphthenic acids. The general formula of acidic species in crude can be represented as C n H 2n+Z O [2] 2, in which n represents the number of carbon atoms, and Z=0, -2,, or -12 represents the aliphatic acid, monocyclic, bicyclic, tricyclic,, or hexacyclic naphthenic acids, respectively, while Z=-14, -16, or -18, respectively, represents monoaromatic and polycyclic aromatic naphthenic acids [1]. The total acid number (TAN) is a key criterion in crude specifications used to measure the corrosivity of crude oil. TAN is defined as the amount of potassium hydroxide (in milligrams of KOH) required to neutralize all acidic species in one gram of oil sample [3]. The field experimental tests have showed that the corrosivity of naphthenic acids is significantly increased when the TAN is greater than 0.5 mgkoh/g [4] at a test temperature in the range of between 220 and 440. The correlation research work [5] has found that the TAN of crude has certain correlation with its corrosivity, but does not form a direct proportional relationship. And further studies have revealed that the crude corrosivity is dependent upon the size and structure of acidic species in crude [6]. Generally speaking, the corrosivity of small molecules of acidic species is higher than that of macromolecular acidic species [7]. By the way, the corrosivity of different types of acidic species in crude varies remarkably. Moreover, the corrosivity of acidic crude also depends on the interaction of acids with other compounds, which are present in the crude oil [8], the operating temperature [9-10], the material of construction of equipment [11], as well as the flow velocity and flow pattern of the process stream [12]. The corrosion problem is not the only difficulty caused by acidic species in crude. Naphthenic acids form naphthenate deposits which cause Corrresponding Author: Mr. Wang Zhenhua, wangzhenhua@cntcc.cn. 15

2 China Petroleum Processing and Petrochemical Technology 2011,13(4):15-22 emulsion problems in oil production facilities [13]. Therefore, it is very important to study in depth the distribution of acidic species in crude oil. The investigation of acidic species in the crude commonly includes the separation and characterization of relevant acids. The separation methods consist of the liquid-liquid extraction, the solid-liquid extraction, the membrane separation, etc [1]. Thousands of acidic species exist in the crude. So it is too difficult to completely characterize the acidic species by one method alone. Ultraviolet (UV) spectroscopy, infrared (IR) spectroscopy, chromatography, nuclear magnetic resonance (NMR), and mass spectrometry (MS) are often used to completely characterize the crude samples. Thanks to the development of MS, the technology for characterization of acidic species in the crude has been developing rapidly. In particular, the negative-ion ESI FT- ICR MS has raised the technology for the oil characterization to a higher rung. In this paper the negative-ion ESI FT-ICR MS has been used to study the carboxylic acidic species in the Sudanese crude oil. In recent years, FT-ICR MS which has ultra-high resolving power and satisfactory accuracy has been used to characterize the acidic and basic components of crude oils. Three vacuum gas oil fractions were analyzed by Fu, et al. [14] using an external EI 7T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). The studies showed that gas oils and atmospheric residue of crude oils had broad molecular-weight distribution, high molecular weight, and double-bond equivalence (DBE) value. The extent of alkylation in hydrocarbons increased with an increasing boiling point. Shi Quan, et al. [15-18] analyzed the heteroatoms, S 1, S 2, S 3, N 1, N 2, N 1 O 1, N 1 O 2, O 1, and O 2 -class species of some crude oils and their fractions with the ESI FT-ICR MS. They characterized the distribution of acidic species, nitrogen compounds, thiophene class of sulfur compounds in crude oils, and their factions. Especially, the results showed that most abundant O 1 and O 2 class species had DBE values and carbon numbers corresponding to biological skeleton structures, such as hopanoic acid, secohopanoic acid, and sterol. With the help of Shi Quan, et al., this paper mainly analyzed petroleum carboxylic acids (the O 2 class species) in crude oil and its fractions, such as aliphatic acids, naphthenic acids, etc. The number of monomeric compounds is so big that the molecules in crude can just be classified based on elemental composition identified by FT-ICR MS. A good method is to divide the compounds in the crude into class and type. If the compounds have the same heteroatoms, such as O 2, they are categorized into the same class of species. If the compounds have the same DBE value, they are classified in the same type of species [19]. The main form of the acidic species existing in the crude is the monocarboxylic acid. So the O 2 class species identified by the negative-ion FT-ICR MS that are regarded as the carboxylic acids having the same DBE value are classified as the same type of petroleum carboxylic acids. For example, the O 2 class species whose DBE value is one belongs to the aliphatic acid. 2 Crude and Properties The object of research in this paper was the Dar crude, which was produced in the 3/7 area oilfield of Melut Basin in southern Sudan. The pertinent data demonstrated that the major causes leading to a high TAN of the crude in the 3/7 area oilfield were assigned to biodegradation and CO 2 dissociation. The basic properties of the Dar crude are listed in Table 1. Figure 1 shows the distribution of TAN and the accumulated yield of fractions. All the data evinced that the Dar crude was a low-sulfur, highlyacidic, and paraffinic-base crude oil with a low yield of light fractions and a high yield of heavy fractions. Table 1 Basic properties of Dar crude Items Data API gravity Crude fraction fraction 31.5 TAN, mgkoh/g 3.68 Elementary analysis, m% C H S 0.20 N 0.37 Wax content, m% 23.8 Kinematic viscosity, mm 2 /s

3 Wang Zhenhua, et al. Distribution of Carboxylic Acids in Sudanese Dar Crude Oil Figure 1 TAN and accumulated yield of various fractions in the Dar crude 3 Experimental Instrument: The MS analysis was performed using a Bruker Apex ultra FT-ICR MS equipped with a 9.4T superconducting magnet. The operating procedure for the negative ESI analysis by FT-ICR MS has been described in the reference [15]. Pretreatment of samples: A certain amount of crude oil and its distillate fractions were dissolved in toluene to prepare a 10 mg/ml solution for negative-ion ESI FT- ICR MS analyses. A total of 20 μl of the solution was further diluted with a mixed toluene/methanol (1:3, v/v) solution to form a 0.2 μg/l solution. Then the 0.2 μg/l solution was mixed and jiggled well with 15 μl of ammonia water. All solvents of analytically-pure grade were distilled twice and kept in a glass bottle. Experimental conditions: The main experimental conditions are listed in Table 2. Table 2 Main experimental conditions Experimental conditions Parameters Experimental conditions Parameters Injection flow rate 180 μl/h Collision energy 1.5 V Flow rate of atomizing gas 1.2 L/min Mass range (m/z ) Flow rate of drying gas 5 L/min Sampling number 4 M Temperature of drying gas 200 Scan times 128 Emitter voltage V Deflection voltage -6 V Voltage at capillary inlet V Compensation voltage of deflection 1.95 V Voltage at capillary outlet -320 V Excitation attenuation 12 db Reflective plate voltage -300 V Front plate voltage V Quadrupole rods Q1= 250 Da Backplate voltage V Radiofrequency 300 Vpp 4 Results and Discussion 4.1 Heteroatom class species in Dar crude identified by negative ESI FT-ICR MS Figure 2 and Figure 3 show the negative ion ESI FT- ICR MS spectra of the crude oil and its fractions. The distribution peak of the crude oil was centered at m/z 400, and its range of mass distribution covered m/z Except vacuum residue (>540 ), the fractions of Dar crude have narrower mass distribution than the crude itself (m/ z ). Table 3 is the heteroatom class (number of heteroatoms) and type (DBE) of distribution derived from negative-ion ESI FT-ICR mass spectra of the crude oil. And the data represented the MS peak number. The test Figure 2 Negative-ion ESI FT-ICR MS of Dar crude 17

4 China Petroleum Processing and Petrochemical Technology 2011,13(4):15-22 Figure 3 Negative-ion ESI FT-ICR MS of the fractions of Dar crude results indicated that the heteroatom class species contained many types, including the N 1, N 1 O 1, N 1 S 1, N 1 O 2, O 1, O 2, and O 3 -class species. be related to the paraffinic nature of the Dar crude oil. 4.2 O 2 class species in Dar crude This paper casts emphasis upon the O 2 class species (petroleum carboxylic acids) which have an important relationship with the corrosivity of crude oil. Figure 4 shows the distribution of the O 2 class species in Dar crude. Most of the DBE values are 1 2, with the compounds corresponding to aliphatic acids and monocyclic naphthenic acids. Meanwhile, the data indicate that the relative abundance of aliphatic acid reaches up to 35.4 m%, which might Figure 4 Distribution of the O 2 class of species in Dar crude oil 18

5 Wang Zhenhua, et al. Distribution of Carboxylic Acids in Sudanese Dar Crude Oil Table 3 MS peak number distribution of Dar crude DBE N 1 /10 7 N 1 O 1 /10 7 N 1 O 2 /10 7 N 1 S 1 /10 7 O 1 /10 7 O 2 /10 7 O 3 / Total Figure 5 is the plot of DBE value versus the carbon number for the O 2 class species determined by the negativeion ESI FT-ICR MS of Dar crude. The O 2 class species are distributed in a broad range of DBE values and carbon numbers. The dominant O 2 class species are found at C 14 C 40. Most aliphatic acids (O 2 -class species, DBE=1) are identified at C 15 C 20. Except C 16 and C 18, the carbon number of aliphatic acids is distributed in a nearly normal distribution pattern centered on C [20] 25. So C 16 and C 18 have obvious advantages among aliphatic acids of Dar crude. The geochemical significance of this phenomenon denotes that the low-maturity oil source or oil source with low degradation degree are interfused with each other [21]. When the DBE value is 2, the c usually is considered to be monocyclic naphthenic acids as mentioned in the literature. Interestingly, C 16 and C 18 also have obvious advantages in the monocyclic naphthenic acids (DBE=2) Figure 5 Plot of DBE value versus carbon number for the O 2 class species of Dar crude determined by the negative-ion ESI FT-ICR MS of Dar crude, which suggests that the crude might be polluted or was interfused with unsaturated fatty acids. The distribution of carbon numbers for another O 2 -class species (DBE>2) is different. And their rules of distribution are still not well understood. 4.3 O 2 class species in the fractions of Dar crude Figure 6 shows the plot of relative abundance versus fractions for O 2 class species determined by the negative-ion ESI FT-ICR MS of Dar crude s fractions. The < 200 fraction was easily vaporized under the experimental condition, so its accurate data could not be obtained. Since the < 200 fraction had a low yield and a low TAN value, then it had little effect on the analysis of Dar oil fractions. Figure 6 Plot of relative abundance versus fractions of Dar crude for the O 2 class species determined by the negative-ion ESI FT-ICR MS 1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17 19

6 China Petroleum Processing and Petrochemical Technology 2011,13(4):15-22 By analyzing the curves, many rules were discovered in this paper: (1) With an increasing boiling point of fractions, the relative abundance of aliphatic acids (DBE=1) and monocyclic naphthenic acids (DBE=2) decreased. (2) With an increasing boiling point of fractions, the relative abundance of bicyclic and tricyclic naphthenic acids (DBE=3, 4) increased at first and then decreased. And the turning points were identified at 300 and 400, respectively. (3) The relative abundance of petroleum carboxylic acids having higher DBE values (DBE>4) increased with an increasing boiling point of fractions. (4) Aliphatic acids, monocyclic and bicyclic naphthenic acids were the main forms of petroleum carboxylic acids in the diesel fraction ( ). (5) Aliphatic acids, monocyclic and bicyclic naphthenic acids were still the main forms of petroleum carboxylic acids in vacuum gas oil ( ). Furthermore, tricyclic and tetracyclic naphthenic acids also accounted for a high proportion in vacuum gas oil. The plots of DBE value versus the carbon number of the O 2 class species in the fractions of Dar crude determined by the negative-ion ESI FT-ICR MS are presented in Figure 7. C 16 and C 18 had obviously a dominant ratio in Figure 7 Plots of DBE value versus the carbon number of the O 2 class species in the fractions of Dar crude determined by the negative-ion ESI FT-ICR MS 20

7 Wang Zhenhua, et al. Distribution of Carboxylic Acids in Sudanese Dar Crude Oil aliphatic acids of the fractions, just like the Dar crude. The O 2 class species were distributed in a broad range of DBE values and carbon numbers as the boiling point of fractions increased. With an increasing boiling point of fractions, the DBE values of the O 2 class species were distributed at 1 6, 1 9, 1 9, 1 12, 1 13, 1 16, and 1 17, respectively. Meanwhile, the dominant O 2 class species were identified at carbon numbers of C 9 C 20, C 9 C 20, C 9 C 21, C 14 C 25, C 14 C 30, C 14 C 35, and C 14 C 40, respectively. 5 Conclusions The heteroatoms N 1, N 1 O 1, N 1 S 1, N 1 O 2, O 1, O 2, and O 3 class species were identified in the Dar crude by the negative-ion ESI FT-ICR MS. Distribution of the O 2 class species (petroleum carboxylic acids) in Dar crude and its fractions were discussed in the paper. The relative abundance of aliphatic acids in petroleum carboxylic acids of the paraffinic-base Dar crude was the highest (35.4 m%). The total content of aliphatic acids and monocyclic naphthenic acids was 53.3 m%. The high abundance of aliphatic acids in the Dar crude and its fractions were found at carbon numbers of 16 and 18. The geochemical significance of the crude indicated that the low maturity oil source was interfused with low-degradation oil source. The distribution of petroleum carboxylic acids in the fractions of the Dar crude showed that the relative abundance of different types of carboxylic acids changed regularly with an increasing boiling point. The relative abundance of aliphatic acids and monocyclic naphthenic acids decreased with an increasing boiling point of oil fractions. Meanwhile, the relative abundance of bicyclic and tricyclic naphthenic acids increased at first and then decreased, whereas the relative abundance of the petroleum carboxylic acids with higher DBE values increased. The O 2 class species were distributed in a broad range of DBE values and carbon numbers with an increasing boiling point of fractions. References [1] Zhang Qundan, Tian Songbai, Huang Shaokai, et al. Resesrch in separation,characterization and corrosion of nanhthenic acid[j]. Chinese Corrosion & Protection in Petrochemical Industry, 2010, 27(1): 12 (in Chinese) [2] Clemente J S, Fedorak P M. A review of occurrence, analyses, toxicity, and biodegradation of naphthenic acids[j]. Chemosphere, 2005, 60(5): [3] American Society for Testing and Materials (ASTM). ASTM D664: Standard Test Method for Acid Number of Petroluem Products by Potentiometric Titration[S]. ASTM International: West Conshohocken, PA [4] Zhou Jianlong, Li Xiaogang, Cheng Xuequn, et al. Progress in research on mechanisms and controlling of naphthenic acid corrosion at high temperatures[j].chinese Corrosion & Protection, 2009, 30(1): 1-6, 15(in Chinese) [5] Elizabeth B K, Craig H K, Rusk G L, et al. Naphthenic acids corrosion settings[j]. Materials Performance, 1993, 32(4): [6] Hsu C S, Dechert G. J, Robbins W. K, et al. Naphthenic acids in crude oils characterized by mass spectrometry[j]. Energy Fuels, 1999,14(1): [7] Laredo G C, Lopez C R, Alvarez R E, et al. Naphthenic acids, total acid number and sulfur content profile characterization[j]. Fuel, 2004, 83(11/12): [8] Ye pez O. Influence of different sulfur compounds on corrosion due to naphthenic acid[j]. Fuel, 2005, 84(1): [9] Slavcheva E, Shone B, Tumbull A. Review of naphthenic acid corrosion in oil refining[j]. British Corrosion Journal, 1999, 34(2): [10] Johnson D, McAteer G, Zuk H. Naphehenic Acid corrosion field evaluation and mitigation studies[c]. ERTC (European Refining Technology Conference) 7th Annual Meeting, Paris, 2002: 1-17 [11] Chen Bifeng, Yang Qiming. Kinetics analysis of naphthenic acid corrosion of alloy steels for atmospheric and vacuum distillation equipment[j]. Chinese Corrosion Science and Protection Technology, 2007, 27(1): (in Chinese) [12] Zetlmeis M. A laboratory and field investigation of naphthenic acid corrosion inhibition[c]. Corrosion 95, Houston NACE International, 1995, 334 [13] Mohammed M A, Sorbie K S. Naphthenic acid extraction and characterization from naphthenate field deposits and crude oils using ESMS and APCI-MS[J]. Colloids Surf, A 2009, 349 (1): 1-18 [14] Fu J M, Kim S, Rodgers R P, et al. Nonpolar compositional analysis of vacuum gas oil distillation fractions by electron ionization Fourier transform ion cyclotron resonane mass spectrometry[j]. Energy Fuels 2006, 20(2):

8 China Petroleum Processing and Petrochemical Technology 2011,13(4):15-22 [15] Shi Quan, Zhao Suoqi, Xu Chunming, et al. Fourier transform ion cyclotron resonance mass spectrometry and its application in petroleum analysis[j]. Journal of Chinese Mass Spectrometry Society, 2008, 29(6): (in Chinese) [16] Liu Yingrong, Liu Zelong, Hu Qiuling, et al. Characterization of sulfur aromatic species in vacuum gas oil by Fourier transform ion cyclotron resonance mass spectrometry[j]. Acta Petrolei Sinica (Petroleum Processing Section), 2010, 26(1): (in Chinese) [17] Shi Quan, Zhao Suoqi, Xu Zhiming, et al. Distribution of acids and neutral nitrogen compounds in a Chinese crude oil and its fractions: characterized by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry[j]. Energy & Fuels, 2010, 24(7): (in Chinese) [18] Liu Peng, Shi Quan, Chung K H, et al. Molecular characterization of sulfur compounds in Venezuela crude oil and its SARA fractions by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry[j]. Energy & Fuels, 2010, 24(9): (in Chinese) [19] Shi Q, Hou D, Chung K H, et al. Characterization of heteroatom compounds in a crude oil and its saturates, aromatics, resins, and asphaltenes (SARA) and non-basic nitrogen fractions analyzed by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry[j]. Energy & Fuels, 2010, 24(4): (in Chinese) [20] Shi Quan, Hou Dujie, Lu Xiaoquan, et al. Detailed molecular characterization of naphthenic acids in Liaohe crude oils by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry[j]. Chinese Journal of Instrumental Analysis, 2007, 26(S1): 86 (in Chinese) [21] Hughey C A, Galasso S A, Zumberge J E. Detailed compositional comparision of acidic NSO compounds in biodegraded reservoir and surface crude oils by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry[j]. Fuel, 2007, 86(5/6): The Demonstration Unit for Synthesis of DME from Biomass Passed Acceptance Tests The International Energy Cooperation Project Key Technology and Demonstration Project for Gasification of Biomass into Synfuel sponsored by the Ministry of Science and Technology of PRC and performed by the CAS Guangzhou Energy Research Institute has passed the acceptance tests. The uninterrupted and stable operation of the said demonstration unit has revealed that 6 to 7 metric tons of biomass can produce one ton of dimethyl ether (DME), with the composite biomass gasification efficiency reaching over 80 %, the once-through DME conversion equating to 70%, and DME selectivity reaching 90%. The Guangzhou Energy Research Institute after having assimilated and absorbed the imported and advanced Italian technology for biomass gasification with oxygen-enriched air has constructed a pilot-scale demonstration unit for gasification of biomass into synfuel following the completion of the optimized design of biomass gasification system and fuel gas purification system. Construction of this project was commenced in 2008 and was completed in June In the course of R&D work over the past three years this institute has made a series of innovative achievements such as a system for hydroreforming and adjusting 300 Nm3/h of biomass-based gas capable of implementing high-temperature precipitation by inertia, reforming of tar in the catalyst bed and adjusting gas composition, and utilizing waste heat of hightemperature gas, low-temperature filtration of gas after purification of tar, and high-efficiency dust recovery. The research team has made breakthrough in the design concept to create a bifunctional homogeneous catalyst consisting of CuZnAl/AlOOH instead of the traditional composite catalyst for DME synthesis. 22