Article pubs.acs.org/EF
Using the Gas Pycnometer to Determine API Gravity in Crude Oils and Blends Adelitza Strubinger,*,† Ursula Ehrmann,† and Vladimir León‡ †
Universidad Simón Bolívar, Departamento de Procesos y Sistemas, Caracas, Venezuela Emeritus Researcher, Unidad de Biotecnología del Petróleo, Centro de Biotecnología, Fundación Instituto de Estudios Avanzados (IDEA), Caracas, Venezuela
‡
ABSTRACT: In the petroleum industry, the implementation of new methodologies requires a validation procedure in order to guarantee compliance with the specifications. This study presents a comparison between the methodology proposed for the determination of the API gravity using a gas pycnometer with a standard and an internationally recognized method such as the one described in ASTM D1298. One of the main advantages of the gas pycnometer technique is related to the use of a hydrometer, which requires a minimum amount of sample. This makes it an ideal method to monitor API gravity in research laboratory-scale processes. In addition, it is a simple and reasonably fast methodology for small samples and therefore attractive for processes with limited sample availability like biotechnological applications. Although the validation parameters reported in the literature are varied, only the results obtained by using the new method as compared with those obtained by using the standard methodology, such as those related to precision and accuracy, are presented. For the API gravity values obtained by using gas pycnometer to be equivalent to the results obtained by using the ASTM D1298 standard method, which would allow the use of both methodologies without this resulting in a deviation of the API gravity specification value of the sample, it is necessary to carry out a correction using the linear correlation data obtained. However, taking into account the precision data, reproducibility limits specifically, both methodologies are comparable.
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asphaltene contents increase as API gravity decreases.18 Several research studies have studied quantitative correlations to allow more accurate predictions of some properties. In this manner, other properties, which would usually be determined by more expensive, complex and slow techniques and methodologies, could be extrapolated from an easy and rapid determination of the API gravity. Because the experimental determination of the molecular weight of hydrocarbon fractions is difficult, this is one physical characteristic that, in the case of pseudocomponents, is often calculated by using a correlation.16,17 Several mathematical relationships have been developed to predict the molecular weight of pseudocomponents. These correlations are usually based on gravity data, boiling point, viscosity, and UOP K.16,17 Viscosity is another parameter that has been determined from correlations that use the API gravity.5,12−15 Abedini et al.13 proposed a new correlation for the estimation of undersaturated oil viscosity, which is based on real data of the different types of Iranian oils. Input parameters for this correlation are the oil’s API gravity, saturation pressure, and reservoir temperature and pressure, all of which are easily measured in oil fields. On the other hand, Faruk Civan5 presented a practical correlation of the viscosity of typical crude oils with temperature and gravity. The temperature dependence of oil viscosity is described using an Arrhenius-type asymptotic exponential function. The parameters of this equation are correlated with the API gravity. It is demonstrated that this
INTRODUCTION The American Petroleum Institute implemented the API gravity scale in 1921 due to the modification of the Baume scale, established in 1916 by the U.S. National Bureau of Standards. Equation 1 shows the relationship between the API gravity of a given fluid and its specific gravity:1−3 API gravity at 60 °F =
141.5 − 131.5 specific gravity at 60 °F (1)
The API gravity scale has been widely used in the petroleum industry to establish quality and property correlations of crude oils, fractions, and products thereof. Indeed, during the 12th World Petroleum Congress,4 a research group established a crude oil classification based on the API gravity, as follows: (a) “extra heavy” for oils with an API gravity of less than 10°, (b) “heavy” for oils with API gravities between 10 and 22.3°, (c) “medium” for oils with API gravities between 22.3 and 31.1°, and (d) “light” for oils with API gravities greater than 31.1°. Today, this remains the most widely used classification scheme for crude oils. Composition of light crude oil resembles the most valuable refinery products such as gasoline and diesel fuel. Consequently, in general, the commercial value and the price of crude oil increases with its API gravity. In fact, the single most valuable property that determines a crude oil’s price relative to the reference oils (Brent and West Texas Intermediate) is the API gravity. On the other hand, it has been shown that API gravity correlates with a variety of crude oil properties.5−17 For instance, as a general trend, sulfur, nitrogen, metal, and © XXXX American Chemical Society
Received: July 17, 2012 Revised: September 13, 2012
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Figure 1. Venezuelan crude oil samples employed in the study.
Many research and development projects related to heavy and extra heavy crude oils, especially in the biotechnology area, employ limited sample amounts, precluding conventional API gravity determinations. The use of the gas pycnometer to assess API gravity could be a suitable solution when only very small amounts of sample are available. Technological advances have introduced significant improvements in the analytical sciences, either reducing costs, increasing speed, simplifying procedures, or enhancing recoveries, precision, and accuracy, among others. However, it is essential to know if the results of the new methodologies are in agreement with the established ones. The selection of the acceptance criteria, experimental design and statistical analysis of comparative studies has been widely discussed.31−35 Analytical method comparisons require careful statistical analysis since it is easy to choose the wrong statistical test for this task. Borman et al.31 employ different statistical tests including the TOST (two one-sided tests)34 approach to compare methods through equivalence tests. The test is based on the comparison of the confidence interval (CI) of the difference between means to predefined acceptance criteria (θ). On the other hand, Astrua et al.32 discussed and compared several techniques to assess the conformity between two methods and/or instruments that measure the same quantity. In general, method comparison studies are traditionally summarized by estimates of between-method bias or by a number of regression techniques.32,35−37 All cases consider aspects such as experiment design and data collection, acceptance criteria, an d preliminary data assessment (outliers, normality, etc.). In this study, tests statistics used in the literature31−39 are evaluated to assess the agreement between API gravity values obtained with the gas pycnometer and the traditional hydrometer test (ASTM D129820). This work presents a comparative study of the gas pycnometer with the standard ASTM D129820 method to assess the scope and limitations of the technique applied to crude oil samples.
approach yields an accurate correlation of oil viscosity through a simple equation. Al-Maamari et al.8 developed a new correlating parameter or corrected API (CAPI) for heavy crude oils, for which the API gravity was corrected using a factor that comprised saturate, aromatic, resin, and asphaltene content of the heavy crude oil. The authors found that relating viscosity to CAPI was more representative than relating the viscosity to the API measurement alone. Asif et al.10 used the correlation between the pristane/n-C17 alkane (Pr/n-C17) and/or phytane/n-C18 alkane (Ph/n-C18) ratios and API gravity in order to characterize the degree of biodegradation of different crude oils of similar source and thermal maturity from the Upper Indus Basin. From the above-mentioned examples, it is quite evident that API gravity is extensively used; therefore, it is of utmost importance to have reliable methods to determine its value. A variety of extensively evaluated standardized methodologies, most of them published by ASTM International (American Society for Testing Materials,) is available, and routinely used in the petroleum industry. These standardized methods include the use of hydrometers (ASTM D287,19 ASTM D129820), digital density meters (ASTM D4052,21 ASTM D500222), and thermohydrometers (D682223). These methods either require large sample amounts or are not suitable for heavy and extra heavy crude oils. Pycnometer (ASTM D70,24 ASTM D1480,25 ASTM D148126) and, specifically, gas pycnometer techniques (ASTM D4892,27 ASTM D5550,28 ASTM D263829) are generally used for solid samples and have not been evaluated for crude oil samples. Use of these standardized methodologies in highly viscous or solid samples, such as heavy and extra heavy oil, vacuum residua, and asphaltenes, has the disadvantage of requiring sample heating for its suitable application (except for asphaltenes, where density cannot be measured by standardized methods). Therefore, it is interesting to search for simple density determination methods for these cumbersome samples. Carbognani et al.30 studied the solution pycnometry (using toluene as solvent) and demonstrated that it is a feasible, relatively simple, and reliable technique, applicable to a wide variety of petroleum materials, including asphaltenes and vacuum residua.
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EXPERIMENTAL SECTION
Samples. Thirteen (13) crude oils from different Venezuelan oilfields located throughout the country, including extra-heavy, medium and light oils, were tested (see Figure 1). Because of the B
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nonavailability of medium and heavy oil samples with an API gravity between 14 and 26°, six blends of Boscan crude oil (11.1°API) and diesel (33.9°API) were also prepared. Thus, the study included the complete range from 8 to 30°API. Gas Pycnometer. This common analytical technique uses a gas displacement method to accurately measure volumes. Inert gases such as helium or nitrogen are used as the displacement medium; in this work, nitrogen was used (AGA de Venezuela, 99.995% purity). The experiments were performed with a Micromeritics AccuPyc II 1340 Gas Displacement Pycnometer System with temperature control and a 10 cm3 nominal cell volume containing 1 cm3 cups. For each analysis, approximately 0.4−0.5 g (to the nearest 0.1 mg) were weighed directly into the cup without applying heat or solvent. The cup with the sample was placed inside the cell, which thereafter was placed inside the sample compartment and sealed. The system was allowed to reach the work temperature (15.5 ± 0.3 °C, equivalent to 60.0 ± 0.5 °F). Nitrogen was admitted into the sample compartment until reaching an equilibrium rate of 0.0050 psig/min. After pressure stabilization, gas was allowed to expand into the reference compartment (a second compartment of precisely known volume). The pressure before and after expansion was measured automatically and used to compute the sample volume. The ratio of the known sample mass and volume yields the density at the work temperature, which thereafter allows the calculation of the API gravity. Each analysis took between 30 and 40 min, with the pressure equilibration being the most time-consuming step. Each sample was analyzed in duplicate under intermediate reproducibility conditions (where mutually independent test results are obtained with the same test method in the same laboratory by different operators with the same equipment at different times, using test specimens taken at random from a single sample of material40,41). Specific gravity (60/60 °F) was calculated, dividing the experimental density value at 15.5 °C (60 °F) by the water density at the same temperature, obtained from literature. Finally, API gravity was calculated from eq 1. Hydrometer. API gravities of crude oils and blends were determined in accordance with the ASTM D129820 standard method. An ERTCO API Model 141.5 glass hydrometer (A-21 for 9−21°API and ASTM 3H for 19−31°API) and all apparatus required by the method were employed. Depending on the sample viscosity, measurements were performed between 29 and 60 °C. The Petroleum Measurement Tables, Table 5A referenced in the ASTM D1250 Guide,42,43 were used to obtain the correction factors of observed API gravity to API gravity at 60 °F. According to ASTM D129820 Precision Data for Opaque Liquids (Crude Oil and Blends), the repeatability is 0.2 API (i.e., the difference between two test results, obtained under repeatability conditions, exceeds this value in only one out of 20 cases) and reproducibility is 0.5 API (that is, the difference between two test results, obtained under reproducibility conditions, exceeds this value only in one out of 20 cases). These precision data will be set as acceptance criteria to state that the results obtained by the hydrometer and gas pycnometer are comparable.
Table 1. Summary of Means for Different Crude Oil and Blend Samples Obtained by Both Methods (Hydrometer and Pycnometer) API gravity 60/60° crude and blend
hydrometer
pycnometer
difference, Δ hydrometer − pycnometer
Carabobo Zuata C Zuata D Zuata A Boscan-1 Zuata B Bachaquero Tiá Juana Blend 1 Boscan-2 Blend 2 Blend 3 Blend 4 Sinco D Blend 5 Blend 6 Guafita Mesa La Victoria
8.09 8.40 8.80 9.50 11.10 11.29 11.60 11.60 13.10 15.50 15.52 18.80 20.80 22.60 22.86 24.80 28.60 28.80 30.80
8.35 7.85 8.28 9.08 10.26 9.47 10.26 10.84 13.03 14.54 16.54 18.33 19.98 21.91 22.74 25.66 28.76 27.65 29.30
−0.26 0.55 0.52 0.42 0.84 1.82 1.34 0.76 0.07 0.96 −1.02 0.47 0.82 0.69 0.12 −0.86 −0.16 1.15 1.50
outlier. In addition, the data, deviations from the mean of the measurements on both days in intermediate reproducibility conditions are plotted versus the crude oil and blend samples, as shown in Figure 2. The random scattering of the data points
Figure 2. API gravity deviations from the mean of the measurements on both days plotted as a function of the crude oil and blend samples.
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RESULTS AND DISCUSSION Table 1 presents the mean results for each crude oil and different blend, using the gas pycnometer and the hydrometer, respectively. In addition, the differences between the reported values are indicated. As is outlined in the Experimental Section, gas pycnometer analyses were carried out at least twice for each sample under intermediate reproducibility conditions. The Cochran test was applied to the complete set of results to assess the uniformity of variance and to detect outliers. In this test, the hypothesis of equal variances is rejected if g > gα,n,k, where the gα,n,k value is obtained from the critical value table for the Cochran test.44 In this case, the test statistic g is 0.187, whereas the g0,05,2,19 value for a 95% confidence level is 0.403. This shows that the variance is uniform and does not identify any result as an
and the application of Cochran’s test indicate that there are no outliers present in the API gravity results obtained when using the gas pycnometer.45 The guidelines established in ISO 5725-445are used to obtain the reproducibility limit in the precision intermediate conditions. The intermediate precision standard deviation was estimated for two different factors, this is for n = 2 (i.e., two independent test results for each material sample under intermediate precision conditions). The simplified formula used for the calculation is shown in eq 2. SI = C
1 2t
t
∑ (Yj1 − Yj2)2 j=1
(2) dx.doi.org/10.1021/ef301193x | Energy Fuels XXXX, XXX, XXX−XXX
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The intermediate precision standard deviation obtained was 0.182. Furthermore, the reproducibility limit, defined as ″the value less than or equal to which the absolute difference between two test results obtained under reproducibility conditions may be expected to be with a probability of 95%”, was calculated as 2.8 SI.45,46 Therefore, the reproducibility limit was 0.51 for the determinations of API gravity for crude oil and blend samples by gas pycnometer. The ASTM D1298 standard hydrometer test establishes a reproducibility limit of 0.5, meaning that the difference between two test results, obtained under reproducibility conditions, exceeds this value only in 1 out of 20 cases. Therefore, it can be established that the intermediate reproducibility limit for the determination of the API gravity in crude oil and blend samples by a gas pycnometer is comparable to a reproducibility limit reported by the ASTM standard. Additionally, the intermediate precision standard deviation obtained with gas pycnometry technique of ±0.182 API is comparable to that reported by Carbonagni et al.30 for toluene solution pycnometry (±0.15 API) for similar samples. However, gas pycnometry has the advantages of requiring less sample amount (
