Determination of asphaltene content in oil samples by UV-visible spectrophotometer - Master's thesis - Dissertation

Determination of asphaltene content in oil samples by UV-Vis spectrophotometer

Key words: UV-visible spectrophotometer; asphaltene; oil sample; US analytical instrument ; UV-1100 Abstract: A method for the determination of asphaltene content in oil by spectrophotometry is presented. The n-heptane asphaltene content in the sample was obtained by measuring the absorbance of the sample suspension at 750 nm and 800 nm. The measurement principle of the method was described. The influence of the sampling range and asphaltene particles on the method was discussed. The results show that the method can be applied to the analysis of various oils (such as crude oil, heavy distillate, residual oil and asphalt), and has the advantages of simplicity and rapidity, and the relative standard deviation is less than 3%. Key words: UV-visible spectrophotometry; asphaltene; n-heptane solute; suspension; petroleum This method has the advantages of simplicity and rapidity, and the relative standard deviation is less than 3%, which reduces the harm of benzene to the human body. The definition of asphaltenes in this method is the same as the weight method, so the results have a good correspondence with the gravimetric method. It can be used to determine the content of n-heptane asphaltenes in oil samples instead of gravimetric method. This method is applicable to the determination of asphaltenes such as crude oil, heavy distillate, residual oil, bitumen and catalytic or thermal cracking oil. 1 Experimental part 1.1 Instruments and reagents analysis of China UV-1100 ultraviolet spectrophotometer; toluene, analytical purity; n-heptane, analytically pure, and dearomatized. 1.2 Test procedure Weigh accurately 0.100-1.000g oil sample, add 1.0 00 mL toluene to dissolve the sample, measure 100 mL of n-heptane, heat to 85 ^o C, add to the above toluene solution, shake vigorously to obtain sample A uniform suspension; after cooling to room temperature, using n-heptane as a reference solution, the absorbance of the suspension was measured at 750 nm and 800 nm using a 1 cm quartz bath, and the asphaltene content in the oil sample was calculated. 2 Results and discussion 2.1 Basic principle of the method The absorption spectrum of the sample suspension at 700-800 nm is determined by spectrophotometry. As shown in Fig. 1, the absorbance of the sample, n-heptane dissolved matter and asphaltene suspension with wavelength Increase and monotonically decrease. The oily suspension is a complex dispersion system comprising both a molecularly dispersed phase and a coarsely dispersed phase. In the ultraviolet region, the absorbance of heptane solute is mainly caused by the molecular valence electron excitation transition of aromatic hydrocarbons, colloids and other components, while the scattered light is relatively weak, but it is not considered; at 750 nm, 800 nm, due to lower energy The probability of molecular valence electron transition is very small (this can be confirmed by the zero absorption of toluene or gasoline at 750 nm and 800 nm). At this time, the absorbance of the solution mainly comes from the light scattering caused by the colloid of macromolecules, according to Rayheigh. The scattering formula explains the decrease in the absorbance of the heptane solute solution from 700 nm to 800 nm as the wavelength increases. 5%(w)。 The content of the gum is not more than 0.5% (w). Compared with the conventional dilute solution, the sample suspension is far from the molecular or ion in the true solution, and there is no essential difference. Therefore, within the above content range, the sample system complies with Lamber-Beer's law. 2.2 Calculation of asphaltene content Although the absorption spectra of n-heptane solute of different types of oil samples will be different, according to Rayheigh's formula, the ratio of absorbance at 750 nm and 800 nm of the same n-heptane lysate solution is constant ( See Table 1). For the same system content C and tank thickness l is the same, the ratio of absorbance is equal to the ratio of absorbance, as shown in Table 1: A ⁷⁵⁰ _m / A ⁸⁰⁰ _m = 1. 32 (1) The absorption of asphaltene suspension is refracted by particles Or produced by reflection, regardless of the source of crude oil. It is also possible to derive from the theory of geometric optics that the shorter the wavelength, the stronger the reflection or refraction. It is known from Table 2 that the ratio of the absorbance of the asphaltene suspension at 750 nm to 800 nm is a constant. A ⁷⁵⁰ _m / A ⁸⁰⁰ _m = 1. 19 (2) According to the principle of absorbance additivity, the absorbance As of the sample solution is equal to the absorbance of the n-heptane solvate Am plus the absorbance Aa of the asphaltene, ie As = Am + Aa ( 3 Therefore, at 750 nm and 800 nm, respectively, the following formula holds: A ⁸⁰⁰ _s = A ⁸⁰⁰ _m + A ⁸⁰⁰ _a ( 4) A ⁷⁵⁰ _s = A ⁷⁵⁰ _m + A ⁷⁵⁰ _a ( 5) From ( l), ( 2), (3), (4), (5) can be introduced A ⁸⁰⁰ _a = 7. 692 3 × (l. 32 A ⁸⁰⁰ _s - A ⁷⁵⁰ _s ) (6) The asphaltene content can be known by Lamber - Beer law Relationship with absorbance Ca = A ⁸⁰⁰ _a ·l/ K ⁸⁰⁰ _a (7 ) where: Ca is the asphaltene content (g / L) in the sample solution; A ⁸⁰⁰ _a is the absorbance of the asphaltene at 800 nm; K ⁸⁰⁰ _a It is the absorption coefficient of asphaltene at 800 nm; l is the thickness of the sample cell of 1 cm. It is known from Table 2 that K ⁸⁰⁰ _a = 365 (8 ) from (6), (8), and: C ⁸⁰⁰ _a = 0. 02l l × ((l. 32 A ⁸⁰⁰ _s - A ⁷⁵⁰ _s ) (9 That is, the absorbance of the sample solution at 750 nm and 800 nm is measured for A ⁷⁵⁰ _s , A ⁸⁰⁰ _s ), and the asphaltene in the sample solution is calculated by the formula (9), and the asphaltene content (wa ) in the sample can be obtained by the following formula. Wa = l0 Ca / m × l00% where m is the sample weight (g).
2.3 Influencing factors The size, uniformity and measurement wavelength of the particles determine the absorption coefficient of the light absorbing medium. Therefore, the size of the asphaltene particles in the sample solution must be distributed within a small range. When the sample solution is directly prepared with n-heptane, the asphaltene particle size distribution is very wide (generally in the range of l ~ l00um). This makes the asphaltene absorption coefficient somewhat affected. If a small amount of toluene solvent is added to the oil sample in advance, the asphaltene will absorb the solvent and swell, and then uniformly disperse into a colloidal solution. When hot n-heptane is added, the equilibrium of the sol system is broken, and the asphaltene colloidal particles are quickly dispersed and re-dispensed. It is polymerized into solid particles of a certain particle size, and the particle size thus obtained is distributed in a narrow range (generally 1 to 5 um). From a thermodynamic point of view, the higher the temperature of the system, the more intense the micelle movement and the easier it is to disperse, so hot n-heptane (85 ^o C) was used in this experiment. After vigorous addition of hot n-heptane for 5 min, ensure that the particles are more evenly dispersed. The sample suspension is a thermodynamically unstable system, and there is a tendency for the particles to be agglomerated to reduce the surface energy. The more the particles per unit volume, the more the particles are more likely to agglomerate into larger particles. 0 g, The asphaltene content in the sample solution is between 0.05 and 0. 20 g / L; too high or too low will cause large deviation. Of course, with the extension of the standing time, the asphaltene particles will slowly sink under the action of gravity. The longer the time, the more the sinking, the greater the concentration gradient formed by the dispersed phase in the solution, which directly affects the accuracy of the measurement. It is generally required to complete the measurement within 5 to 20 minutes after cooling to room temperature, and it is necessary to shake vigorously for 5 minutes before measurement. 2.4 Analysis Accuracy Two samples (Iran crude oil > 450 ^o C and Nanyang crude oil > 500 ^o C) were measured by spectrophotometry. The parallel determination was 6 times, and the relative standard deviations were 2.7% and 2.9%, respectively. The results show that the spectrophotometric method has good repeatability in the determination of heptane asphaltene content in oil samples. 2.5 Comparison with gravimetric method The gravimetric data in Table 3 was measured according to the RIPPI0-90 method of the Institute of Stone Science. It can be seen from Table 3 that the results are almost the same, but for individual oil samples, the results of the gravimetric method are low. We believe that it is caused by the following two reasons: (2) When the gravimetric method is refluxed with benzene, the carbonaceous material insoluble in benzene, oil coke, etc. (which is produced during the thermal processing of crude oil) is removed; (2) Impurities such as catalysts mixed with a small amount of smaller particles in the sample are suspended in the solution, resulting in high photometric results, of which (2) is the main factor, which is consistent with the reported law of photometry and ASTM. Because the ASTM method does not have benzene reflux. Key words: UV-visible spectrophotometer; asphaltene; oil sample; US analytical instrument ; UV-1100