SCIENCE CHINA Chemistry • ARTICLES • · SPECIAL TOPIC · Chemistry of Heavy Petroleum Fractions and Its Impacts on Refining Processes
July 2013 Vol.56
No.7: 848–855
doi: 10.1007/s11426-013-4897-6
Thermal transformation of acid compounds in high TAN crude oil YANG BaiBing, XU ChunMing, ZHAO SuoQi, HSU Chang Samuel, CHUNG Keng H. & SHI Quan* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Received March 14, 2013; accepted April 22, 2013; published online June 3, 2013
The Liaohe crude oil with high total acid number (TAN) was subjected to thermal reaction at 300 °C to 500 °C. Reaction products were collected and analyzed by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to determine acid compounds in the crude oil. The double-bond equivalence (DBE) versus carbon number was used to characterize the oxygenated components in the feed and reaction products. The O2 class which mainly corresponds to naphthenic acids decarboxylated at 350–400 °C, resulting in a sharply decrease in TAN. Phenols (O1 class) are more thermally stable than carboxylic acids. Carboxylic acids were also thermally cracked into smaller molecular size acids, evidenced by the presence of acetic acid, propanoic acid, and butyric acid in the liquid product. These small acid species are strong acids likely responsible for corrosion problems in refineries. naphthenic acid, TAN, ESI, FT-ICR MS, thermal cracking
1 Introduction Petroleum contains a small amount of organic acidic components, such as naphthenic acids and phenols, which are known to cause various operational problems, such as corrosion/fouling of process units and water/oil emulsification [1–5]. Petroleum industry uses total acid number (TAN) as a measure of crude oil corrosivity [2]. TAN is defined as milligrams of potassium hydroxide (KOH) required to neutralize all acid compounds in 1 g of oil sample [6]. The higher the TAN, the more corrosive the crude is [7]. However, it is known that some crude oils with relatively low TAN exhibit high corrositives which are comparable to those of high TAN crudes [8]. The corrosivity of crude oil is not linearly correlated to the TAN [9] and is dependent on the size and structure of naphthenic acids and/or other acidic components in crude oil [10]. Naphthenic acid corrosion [3, 4] is commonly known in refinery to describe corrosion problems in heaters, distilla*Corresponding author (email:
[email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2013
tion units, and pipes. Refinery corrosion was also related to other feedstock properties such as sulfur content and process operating conditions: temperature and flow rate [11]. In the atmospheric distillation operation, corrosion of side-stream gas oil product line was attributed to liquid-phase corrosion at 250–400 °C [12]. In recent years, the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) [13, 14] which provides ultra-high resolving power and mass accuracy, has become a desired technique used for characterizing highly complex hydrocarbon mixtures. In coupling with the electrospray ionization (ESI) which has high selectivity for ionizing trace polar compounds [15], FT-ICR MS has been widely used to characterize acidic and basic compounds in crude oil [2, 12, 16–25]. Stanford et al. [22] reported the acidic polar heteroatomic molecular class composition of three distillate fractions of vacuum gas oil (VGO), showing different compositions of carboxylic acids and oxygen-sulfur (SxOy) species in light, middle and heavy distillates. Shi et al. [2] investigated the distribution of acids in a kind of Chinese crude oil and its fractions. The composition chem.scichina.com
www.springerlink.com
Yang BB, et al.
Sci China Chem
of acids varied from each distillate fractions, 4–5 rings naphthenic acids likely make a vital contribution for the TAN of heavy distillates. Smith et al. [9] showed the thermal stability of acids in Athabasca bitumen heavy vacuum gas oil at 350–525 °C. Even though the acid compounds in crude oil are susceptible to thermal decomposition, systematic and comprehensive study on thermal transformation of petroleum acid compounds is lacking. Since the amount of organic acids is in ppm level, it is difficult to determine all reaction products using the conventional analytic techniques. In this paper, ESI FT-ICR MS analyses were used to characterize the feed and products of the high TAN crude oil which was subjected to thermal reactions. The class (number of oxygen containing heteroatoms), type (number of rings plus double bond equivalence (DBE)), and carbon number of the acid compounds were determined. The findings validated the proposed thermal reaction mechanisms of petroleum acids. More importantly, trace amount of small molecular acidic compounds which are likely the cracking products of large petroleum acids, were detected. These small acids are likely causing the refinery corrosion problems.
2 Experimental 2.1
Materials
A kind of high TAN heavy crude oil was obtained from Liaohe oilfield in Bohai Basin, China, with its properties showed in Table 1. 2.2
849
July (2013) Vol.56 No.7
drop of condensed reaction product was collected in the glass flask. After heating, heavy liquid product was obtained from the reactor which was naturally cooled to room temperature. The liquid products obtained from the reactor and glass flask were collected and subjected to simulated distillation and ESI FT-ICR MS analyses. The gas product in the air collector was analyzed by gas chromatography. Thermo reactions at 400 °C were carried out twice, one of the liquid products (about 60 mL) was distilled to obtain
