China Petroleum Processing and Petrochemical Technology 2011,Vol. 13, No. 1, pp 6-15

Review

March 30, 2011

Review and Comprehensive Analysis of Composition and Origin of High Acidity Crude Oils Cai Xinheng; Tian Songbai (Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China) Abstract: High acidity crude oils have an advantage over normal oils in terms of their price, but can cause corrosion and refinery problems. They are the so-called opportunity crudes and likely to be important reserved resources in the 21st century. Researches on high acidity crude oils are becoming more and more profound. Based on the existing research achievements, this article has given an overview of the chemical composition and acid distribution of high acidity oils, and also analyzed their origin types and potential influence factors. Key words: high acidity oils; total acid number; acid composition; origin; influence factors

1　Introduction Petroleum acids occur widely in crude oils, and have become one of the most concerned chemical species by researchers and refiners. Generally, acidic constituents in crude oils consist of organic acids, inorganic acids and some other compounds which could influence the oil acidity such as esters, phenols, amines and pyrrole series[1-2]. Though many analytical methods have been established and reported[3], at present, the acidity of a crude oil is most commonly expressed by its total acid number (TAN), which is the number of milligrams of KOH determined by non-aqueous titration (ASTM D 664-1989) needed to neutralize the acidity in one gram of oil[4]. In petroleum industry, oils with a TAN value of higher than 0.5 mg KOH/g are considered as high acidity crude oils, the global reserves of which are quite considerable[5-6], and they can be probably utilized as one of the significant reserved resources in the 21st century. However, for the present time, high acidity crude oils are less desirable than normal oils because of the corrosion problems they cause in storage, transport and refining processes, so correspondingly these oils have a relatively lower price[6-8]. As a result, high acidity oils provide an economic opportunity, as well as a technical challenge for the petrochemical enterprises. With the worldwide upsurging demand for crude oil resources, supply of con·

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ventional crude oils would decrease year by year, and in the meantime the trend of deterioration of crude supply would be aggravated gradually. On this background, the proved reserves and annual production of high acidity crudes are increasing steadily, and they have become an important target of exploration, exploitation and utilization[9-10]. Consequently, it is quite necessary to study the composition and origin of high acidity crude oils. A lot of researches and reviews have been reported concerning some classes of acid species in crude oils or their origin, while there appears to be relatively little work published relating to not only the investigation of full range of composition and distribution of acidic fractions in high acidity crude oils, but also analysis on the influence factors and possible origin of acidity. For instance, petroleum acids have been inappropriately termed as naphthenic acids since the earliest identified acids were saturated cyclic carboxylic acids[4], whereas now it is known that there are potentially many other acid species existing in crude oils. Several researchers[11-12] have further analyzed the carboxylic acid fraction of crude oils, but they did not work on the detailed relationship between these acidic compounds and oil acidity. Others, such as Meredith, et al. [8], focused on the influence of biodegradation in controlling crude oil Corrresponding Author: Dr. Cai Xinheng, Telephone: +8610-82368443; E-mail: [email protected].

Cai Xinheng, et al. Review and Comprehensive Analysis of Composition and Origin of High Acidity Crude Oils

acidity and carboxylic acid composition, while finding out that not all their studied oils originated from the process of biodegradation. Based on these references and some other research achievements, this paper aims to offer an overview on chemical composition, acid distribution and origin of high acidity crude oils.

2　 Chemical Composition and Acid Distribution of High Acidity Oils Since the acidity in crude oils has long been a problem for refining, the knowledge of detailed chemical composition and distribution of the acids can facilitate the identification of inferior crude oils and potentially lead to improved processing options for acidic oils[13].

2.1　Elemental composition and molecular formula As already described in many papers, almost all petroleum oils consist of C, H, O, N and S elements without the exception of high acidity crude oils. In the work of Tomczyk, et al.[13], the San Joaquin Valley crude oil was extracted by the modified method of Seifert and Howells[14], and the elemental composition of the SJV oil and its acid extract were analyzed for C, H, N and S elements, with the results shown in Table 1. From these data, by comparing the percentage of each element between the extract and crude, it is not difficult to conclude that heteroatoms such as sulfur, nitrogen and oxygen are mainly concentrated in acidic compounds, especially the element of oxygen. Further study indicated that over 60 percent of the acidic compounds in crude oils contained two or more oxygen atoms, and compounds containing only one oxygen atom accounted for less than 10%. Meanwhile, approximately 25% of the acidic species were sulfur-containing compounds and about 50% of acidic compounds contained nitrogen [13]. Upon investigating component types in the acidic fraction by high resolution mass spectrometry (HRMS), a broad distribution of heteroatom species were discovered, with the observed results illustrated in Figure 1. It can be seen from Figure 1 that each stripe represents the mole numbers (quantities referred to carbons) of each type of heteroatom groups per 10 000 mol of carbon in the whole crude, and the notation used here, such as N2O3, does not indicate that all five atoms are in

one functional group, but just implies that a compound contains two nitrogen atoms and three oxygen atoms. Eight different molecular ions are identified with their quantities greater than 20 moles per 10 000 mol of carbon in the whole crude, including O2, O4, S, N2, NO, NO2, N2O, and N2O2. These data manifests a far greater diversity in acid types than previously reported O2-only species as implied by the term “naphthenic acids”. Parallel research was carried out by Qian, et al.[15], and analytical results demonstrated that many acidic compounds have bimodal distribution of elemental association, suggesting the presence of at least two different core structures in the acids. Although it is evident that carboxylic functionality is a prominent feature, still a large proportion (40%) is not carboxylic acids, or even the carboxylic acids do not simply contain two oxygen atoms just to constitute their carboxyl group, but actually 85% of them contain more heteroatoms[13]. Therefore, the elemental composition of acids in crude oils is far more diverse than expected. Table 1　Elemental analysis of SJV acid extract[13] Elements

Mass fraction in extract, %

in whole crude, %

Carbon

73.6

83.5

Hydrogen

9.0

11.0

Sulfur

2.4

1.2

Nitrogen

1.3

0.75

Oxygen

14.0

0.66

Figure 1　Distribution of heteroatom species types in acid extract from SJV crude[13]

The classical description of petroleum acids refers to compounds with multiple saturated ring systems containing an attached partially oxidized aliphatic side chain. ·

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Then it has been found that this simplified combination of C, H and O is not adequate to explain the broad range of acidities and widely existing corrosivity of oils[13, 16]. In recent papers, Hughey, et al.[17] described acids in crude oils with CcH2c+zO2 (mainly naphthenic and aromatic acids, with z representing the hydrogen deficiency index) and acidic NSO compounds, and Cheng, et al.[18] applied CnHmOxNySz as the molecular formula of petroleum acids. Meanwhile, Qian, et al.[15] found that the negative ion ESI high field FT-ICR-MS can selectively ionize and distinguish petroleum acids without interference from the hydrocarbon background and demonstrate compositional trends of acids as a function of oil TAN values. As shown by them, collectively more than 3 000 chemically different elemental compositions were resolved and identified. On the basis of this work, Li, et al.[19] found that organic compounds and related components in crude oils can be expressed by a general formula of CcH2c+zNnOoSs, where c, n, o, and s stand for the respective numbers of C, N, O and S elements in the molecule, and z represents the number of double bond equivalents (or DBE value). These conclusions are in accordance with the work of Tomczyk, et al.[13], who tested and examined the elemental composition and most of the possible formulas.

2.2　F orms and distribution of acidic compounds The prerequisite for designing practical and highly efficient deacidification catalysts and technology is to find out the existing forms and rules for distribution of petroleum acids[20]. Numerous research activities have been carried out on detailed analysis of the petroleum acid fractions of crude oils, with compounds identified including linear fatty acids, isoprenoid acids, and monocyclic, polycyclic and aromatic acids[8]. Other groups of compounds which can influence the acidity of an oil include inorganic acids, such as some compounds of calcium and magnesium which are difficult to be desalted[21-22], and low molecular weight alkylphenols (C 0—C3 alkylphenols), which occur widely in crude oils[23]. For instance, Samadova, et al.[24] studied a low wax content and high TAN crude oil from Azerbaijan, and found that the content of phenols was between two and seven times higher than those of the carboxylic acids. Generally, through ·

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FT-IR/ATR spectrographic analyses, the methyl esters of the low TAN oils are more aliphatic in general, whereas the high TAN oils tend to show higher absorbance from carboxylic groups, ketones, polycyclic quinones and phenols[19]. Mckay, et al.[25], by investigating the N-containing compounds (carbazoles, and amino compounds) and the S-containing compounds, noted that in the Wilmington crude 28% of acidic compounds were carboxylic acids, along with 28% phenols, 28% pyrroles and 16% amino compounds. The information from above researchers indicates that the major forms of acidic components in crude oils are most likely the carboxylic acids for species with two or more oxygen atoms, phenols for the single oxygen species, pyrroles, carbazoles or indoles for the N-species, and thiols, thiophenes or benzothiophenes for the sulfur species. Although there is a diversity of acid species, it is a longstanding belief that aliphatic acids and naphthenic acids are the major components of petroleum acids, and especially the naphthenics account for the dominating part. Lochte and Littman[26] first studied the structures of naphthenic acids, indicating that naphthenics were a series of compounds with high molecular weight ranging from 100 to 1000, and carbon numbers ranging from 7 to 70. In addition, researches[15,19] further confirmed that the crude acids dominantly consist of a mixture of structures ranging from C15 to C55 with cyclic (0—6 rings) and aromatic (1— 3 rings) structure, and it appears that the acid composition is even simpler than that of the corresponding hydrocarbon analogues. Liu, et al.[27] analyzed the composition of petroleum acids in a cut fraction in the range between IBP—350 ℃ of the Penglai crude, finding that naphthenics accounted for 85.6% of total acids, aromatic acids (alkyl aromatic, monocyclic aromatic and bicyclic aromatic acids) accounted for 10.2% and aliphatic acids —4.2%. Several researchers[20, 28-29] have investigated the distribution of these complex mixtures, discovering that the distribution of acids in each distillation cut accords with its boiling range. With the boiling point rising, the total content of petroleum acids and content of naphthenic acids in distillation cut both increase, while the content of aliphatic acids decreases gradually. To be specified, it is shown in Table 2 that as the boiling point of a fraction raises, the average molecular weight of petroleum acids increases,

Cai Xinheng, et al. Review and Comprehensive Analysis of Composition and Origin of High Acidity Crude Oils

and the distribution of naphthenic acids gets wider with its carbon number range increasing as well. Table 2　Distribution of acids in each distillation cut

[20,28-29]

Boiling range,℃

Yield, %

Content of naphthenic acids (w) ,%

< 230

0.68

0.005

150

C7—C19

230—300

3.60

0.08

200

C7—C21

300—350

4.50

1.03

175—420

270

C19—C28

350—400

5.31

2.53

177—470

300

C19—C31

400—450

3.72

2.19

160—560

310

C18—C36

450—500 10.54

11.86

240—660

460

C15—C45

500—550

4.56

6.41

300—710

470

C20—C48

> 550

66.69

66.99

350—900

750

C21—C70

Distribution Average Range of of molecular molecular carbon weight weight numbers

3　Origins of High Acidity Crude Oils According to the mechanisms for formation of oil accumulations, the origin of high acidity crude oils can be assorted into the primary type, the secondary biodegradation type and the mixed type.

3.1　Primary type High acidity oils of the primary type are those which did not suffer any biodegradation and epigenetic reformation (including mixing effect) in the course of their formation, migration and accumulation processes. Being immune to the influence of biodegradation and water washing, the distribution of saturated n-alkanes in the primary oils is relatively intact. In general, the TAN value of this type of crudes is below 2.0 mg KOH/g and aliphatic acids usually account for above 15% of total acids[1]. There is no significant difference in density between this type of crudes and conventional crude oils. Taking oil of the Anbor reservoir in Melut Basin as an example, it contains 1.3 mg KOH/g as the TAN value, with its density equating to 0.874 0 g/cm3, its aliphatic acids/total acids percentage being equal to 15.31%, and its saturated alkanes/aromatics ratio equating to 4.36[30].

3.2　Secondary biodegradation type Most crude oils in the world have been affected by secondary alteration in reservoirs, such as processes of oxidation, biodegradation, sulfidation, water washing, deasphalting, hydrothermal alteration, gravity differentiation, and evaporative fractionation [31]. Many researchers have demonstrated that biodegradation is the dominant factor which causes high acidity in crude oils. Some of them[13, 32] have further illustrated wide occurrence of biodegradation in oil reservoirs on the whole earth, and it was a process that occurs where temperature remained below around 75—80 ℃ , resulting in a gradual increase in density, sulfur content, acidity and viscosity of crude oils. Meanwhile, over the geologic age, the microorganisms could selectively remove alkanes, branched alkanes, and cycloalkanes, and might also attack aromatics according to the order of ring number (smaller rings degraded first). As for the high acidity crude oils, gas chromatograms of saturated hydrocarbon fractions are characterized by a large hump consisting of an unresolved complex mixture, indicating to various levels of biodegradation, and besides, the presence of abundant 25-norhopanes in the high acidity oils also differentiate themselves from oils not suffering from biodegradation[19]. In the past, it has been generally believed that biodegradation is an aerobic process requiring the import of oxygenated meteoric water, while anaerobic process is now confirmed to be dominant[33]. Besides, the origin of this type of biodegradation is influenced greatly by the depositional environment of the concerned petroleum systems, such as factors related with the reservoir temperature, the burial depth, and the availability of nutrients bearing formation waters and salinity.

3.3　Mixed type Mixed type oils are formed from mixing of high or ultrahigh acidity crude oils which have accumulated and suffered the effect of biodegradation and water washing in the early stage, followed by being recharged later with oils of normal acidity. After being mixed, the TAN value of these oils is still greater than 0.5 mg KOH/g. The geochemistry characteristics of this type are intact in terms of the distribution of n-alkanes and the existence of 25-norhopane in oils. For example[34], the TAN value of ·

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crude oil from the Palogue field in Sudan ranges from 3.0 to 10.6 mg KOH/g, and their preservation of n-alkanes is relatively complete, with the pristane/n-C17 ratio being equal to 0.17—0.29, and the phytane/n-C18 ratio equating to 0.05—0.21. The GC-MS (m/z 177) data of the Palogue crude indicated the existence of 25-norhopane, with its biodegradation level being up to 6. So the oil from the Palogue field is a typical mixed type of high acidity crude oil.

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ratio is apparent. This conclusion is supported by the work of Olsen[38], who discovered a strong correlation between TAN value and phytane/n-C18 ratio for a batch of Norwegian oils. In conclusion, it is strongly suggested that the extent of biodegradation is a significant controlling factor on the ultimate TAN value.

4　Influence Factors of Oil Acidity Since the acidity of a crude oil is measured by its total acid number, this paper intends to investigate the influence factors of oil acidity through analyzing the relationship between these factors and TAN values.

4.1　Influence of biodegradation on oil TAN Biodegradation is mainly a process of oxidation of hydrocarbons, with generation of carbon dioxide, organic acids and other related products. Notwithstanding the reaction condition is aerobic or anaerobic, as long as the condition of nutrient, water, temperature and salinity is appropriate, alkanes, naphthenes and benzenes would be oxidized to oxygen-containing acids by bacterial activities resulted from the catalysis of enzyme[35]. According

Figure 2　Correlation between TAN and degree of biodegradation[8]

to the research performed by Berth, et al.[36], biodegradation can cause an increase of carboxylic acids, and through comparing the IR spectra and GC-MS chromatogram of biodegraded oil with the non-degraded one, they confirmed some acidic compounds in biodegraded oil were reliably generated in the process of biodegradation. Much has also been reported in Meredith and coworkers’ work[8], showing that biodegradation is the main process responsible for the enrichment of carboxylic acids, with their concentration increased with an increasing level of biodegradation. The direct correlation between the degree of biodegradation and oil TAN value was also investigated, and Figure 2 shows a trend of increasing TAN with an increase in biodegradation level (i. e. the concept of biodegradation level defined by Peters and Moldowan[37]), though there is some degree of scatter (r2=0.74). Dou, et al.[33] studied the relationship between biodegradation (assessed by the pristane/n-C17 ratio) and oil TAN value, as is shown in Figure 3. Though there is little fluctuation in this ratio, a trend of increasing TAN with an increasing pristine/n-C17 ·

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Figure 3　Correlation between Pristane/n-C17 ratio and TAN[33

4.2　Influence of acid composition on oil TAN On the whole, petroleum acids mainly exist in the nonhydrocarbon components of crude oil, so there is a positive correlation trend between the non-hydrocarbon components and oil TAN[39], as shown in Figure 4. The carboxylic acid fraction of oils has a vital contribution to oil TAN, and Figure 5 shows a strong correlation (r2=0.91) between the concentration of carboxylic acids and TAN value[8]. This relationship appears to be applicable to oils from different regions and various geological settings with different tectonic circumstances and maturation histories. It can be seen from Figure 5 that the relevance between carboxylic acid concentration and TAN value is not

Cai Xinheng, et al. Review and Comprehensive Analysis of Composition and Origin of High Acidity Crude Oils

as obvious regarding the oils with TAN 1800 m. In contrast to the oil TAN-depth trend, the API gravity of oil generally correlates inversely with the measured TAN as is shown in Figure 8. Thus, it appears that the high acidity crude oils are primarily of low API gravity and tend to exist in shallow reservoirs which are less than 1500 m in depth and below temperatures of around 80 ℃. Another important geological condition is the water-oil contact. To a great extent, the variation of oil TAN is attributed to biodegradation occurring at or near the water-oil contact. Besides, water washing of the oil would selectively remove most of the carboxylic acids, especially those of lower molecular weight which are more water soluble[43]. Recharge and further migration of oils have their influence on TAN as well, because diffusive mixing of biodegraded oil with non-biodegraded oil through either single/ episodic recharge or continuous charging of reservoirs and chromatographic effect of the accompanying rock materials in the process of migration would affect the composition and distribution of acid species in crude oils[13, 44]. Therefore, extensive variation of oil acidity at different sites in reservoirs is probably influenced additionally by local geochemical factors, such as the sedimentary environment, the movement of oils, the presence and thickness of formation waters, and the availability of nutrients and salinity to the oil reservoirs.

Figure 8　Correlation between API gravity and oil TAN[19]

Cai Xinheng, et al. Review and Comprehensive Analysis of Composition and Origin of High Acidity Crude Oils

4.5　Influence of crude maturity on oil TAN When kerogen generates hydrocarbons at the period of low maturity, the O/C ratio will decrease drastically, while the H/C ratio does not change a lot. However, when the process for generating hydrocarbons happens at the period of high maturity of oil source rocks, the H/C ratio will decrease quickly without obvious change of the O/C ratio[45]. It means that plenty of oxygen were released from source rocks during the process of generating hydrocarbons at low maturity, so the content of oxygen embedded into the generated oils is relatively high, and this would be propitious to the formation of oxygen-containing species such as acidic compounds, and, as a result, the acidity of crude oils increases consequently. On the contrary, the acidity of crude oils derived from the high maturity source rocks is liable to be relatively low[39]. Therefore, the oil TAN value is negatively correlated with the crude maturity. For example, the relationship between the oil TAN and the crude maturity assessed by the C29αββ/C29ααα+αββ sterane ratio or the Ts/Tm ratio has been investigated[8, 39], though within the same geological area, and even if there is a slight variation in crude maturity, a large variation in the TAN values would be anticipated. It has also been reported by Seifert[46] that oils of low maturity or immaturity may have high acidities, but in this research the sample selected did not contain sufficient immature oils to verify the inference. Dou, et al.[33], in their study of oils from the Melut Basin of Sudan, also indicated that these high acidity oils were all thermally unmatured or of low maturity, and were indeed generated within a relatively narrow thermal maturity window. In the work of Hughey, et al.[17], they considered that acids were common constituents in young and immature crude oils. In further discussions they inferred that some of the naphthenic acids present in heavy crude oils were original components of the oil and were retained as biomarker skeletons (e.g., the hopane skeleton).

5　Conclusions Based on the existing research achievements, the chemical composition and acids distribution of high acidity oils were summarized. Analyses of the acid composition have revealed that acids in crude oils mainly consist of aliphat-

ic acids and naphthenic acids which account for the dominating share, along with many other acidic species that contain varying amounts of heteroatoms such as nitrogen, oxygen and sulfur. Meanwhile, the distribution of these acids in each distillation cut of crude oils accords with its boiling range. Then, the origin of high acidity oils are studied and assorted into the primary type, the secondary biodegradation type, and the mixed type on the basis of their accumulation mechanisms. Upon further discussions it is concluded that biodegradation is the dominant process that leads to high acidity in crude oils, and carboxylic acids are the primary functional components compared to other composition of oils. In addition, crude maturity, parent materials, geological environment and geochemical interactions, such as water washing, thermal alteration and migration, are also significant factors influencing the ultimate formation of high acidity oils.

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Nano MCM-49 Zeolite Catalyst for Cumene Production Developed by Jilin Petrochemical Research Institute Recently the research institute of Jilin Petrochemical Company

zeolite catalyst in place of the existing catalyst at the 125 kt/a

has successfully developed the nano MCM-49 zeolite catalyst

cumene unit can create a yearly cost saving amounting

for manufacture of cumene. The said MCM-49 zeolite has

to 738 thousand kWh of electricity, 5 250 kt of steam,

a relatively large specific surface area, short and orderly

9 580 kt of water and 60 t of benzene without changing the

arranged channels, and higher intracyrstalline diffusion rate

current operation conditions to bring about an economic

capable of improving catalytic performance and enhancing

benefit of 8.6 million RMB a year. By taking advantage of

the activity and selectivity of catalyst for synthesis of

the low benzene/styrene ratio and 20 ℃ lower value in reaction

cumene.

temperature required by the new zeolite catalyst the annual

The phenol/acetone unit at the Dyestuff Plant of Jilin

incremental economic benefit would be at least 15.36 million

Petrochemical Company used to have problems related

RMB. If this nano MCM-49 zeolite catalyst would be used at

with high reaction temperature, high benzene/styrene molar

the grassroots unit, more economic benefit could be obtained

ratio, and frequent washing with hot benzene, resulting

because of a lower investment cost. Currently the activities

in high energy consumption for cumene production and

targeting the commercial application of this nano MCM-49

high production cost. These problems could be tackled

zeolite catalyst are being pushed forward at full steam. This

by adoption of the said nano MCM-49 zeolite catalyst.

technology will be adopted for the revamp of the cumene

It is estimated that the application of nano MCM-49

unit at the Dyestuff Plant of Jilin Petrochemical Company. ·

15 ·