Asphaltene Stability Characteristics in Crudes and Residues

Abstract

Heat exchanger networks are provided to recover heat from the hot products and transfer the same to the colder feed to be processed. But Deposit formation on heat transfer surfaces during processing of crude oils is a well documented problem (foul- ing) in the re neries. Due to this, maximization of this heat recovery in the heat exchangers networks is the main interest to the re ners. The actual heat transfer coe cient is a function of fouling characteristics, which in turn are function of feed characteristics. Fouling during the processing of heavy and asphaltenic crudes is a well documented problem. The increased use of heavy crude oils, which are rich in asphaltenes, has increased the severity of this problem. As- phaltenes are the main contributor to fouling problem. Crude oil can be broadly separated in four main classes, which are saturates (S), aromatics (A), resins (R), and the asphaltenes (A) commonly known as SARA-fractions. During processing of heavy crudes, the major fouling is encountered in crude distilla- tion unit (CDU) preheated trains. The physical and chemical mechanisms associated with crude-oil fouling are very complex, which make e ective application of mitiga- tion methods di cult for re ners. First we must recognize that there are multiple causes of fouling both organic and inorganic and depends on the composition of the crude oil in addition to operating conditions within the heat exchangers. In present study we concentrate on the composition of the crude oil. Blending of crude oils in re neries is common practice but it requires utmost care as certain blends are incompatible and cause precipitation of asphaltenes that can rapidly foul process equipment. Once an incompatible blend is obtained, the rapid fouling that results usually requires shutting down the re ning process in a short time. Two alternative but similar methods have been independently developed for predicting the fouling propensity of crude oil and its blends based on crude oil chem- ical composition. The rst method is Wiehe's method, which requires two laboratory tests to deter- mine Insolubility number (IN) and Solubility Blending number (SBN). Based on these numbers it is possible to predict the likelihood of fouling of crude oil blends and op- timization of blending ratios so as to ensure compatibility. The second method is Colloidal Instability Index (CII) determination by SARA analysis. Present work is based on the above two methods mentioned. Firstly for SARA analysis, we used for three methods, rst one is the Combination of ASTM D1319, ASTM D6560 and ASTM D2007, second method is the combination of ASTM D1319, TLC-FID method, third method is analysis using only TLC-FID. Compared the results of all the three methods and tried to address quick SARA anal- ysis of crude that can be attained by only TLC-FID method without any laborious sample preparation. Secondly, using Wiehe's method for the same eight crude oils compatibility is dis- cussed. Blends of two crudes at di erent proportions were considered to compare both method namely SARA analysis (CII) and Wiehe's method to predict the likeli- hood of fouling of crude oil blends and optimization of blending ratios so as to ensure compatibility. Additionally the reasons why correlation of lab scale and eld data for crude oil foul- ing is not possible is discussed and it is emphasized that in re neries SARA analysis (CII) should be considered as one of the parameters at the crude blend window which shows impact on re nery margins.