Incompatibility Determination of Crude Oil Blends from Experimental

Article pubs.acs.org/EF

Incompatibility Determination of Crude Oil Blends from Experimental Viscosity and Density Data José L. Mendoza de la Cruz,*,† Juan C. Cedillo-Ramírez,‡ Adriana de J. Aguirre-Gutiérrez,† Fernando García-Sánchez,‡ and Marco A. Aquino-Olivos† †

Programa de Aseguramiento de la Producción de Hidrocarburos, and ‡Laboratorio de Termodinámica, Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, 07730 Mexico City, Distrito Federal, Mexico ABSTRACT: In this work, a study to determine the incompatibility phenomenon in blends of crude oils mixed with a liquid hydrocarbon from experimental viscosity and density information is presented. The effect of the temperature and composition on asphaltene aggregates and asphaltene precipitation onset was investigated for two crude oil blends mixed with a refinery stream liquid hydrocarbon containing high isopentane and n-pentane concentrations. Dynamic viscosities of the system (crude oil containing asphaltenes and toluene) + n-heptane were measured to assess the changes of aggregation occurring in a crude oil under the influence of the temperature to determine the incompatible asphaltenes in the oil fraction of the liquid hydrocarbon. The method to detect the incompatibility of crude oil blends was based on experimental observations of an increase in the viscosity and density of the crude oil blends mixed with a liquid hydrocarbon, in which asphaltene particle aggregation occurs. From experimental determination of viscosities and densities, it was possible to determine an incompatibility region for the crude oil blends.

1. INTRODUCTION Many of the problems encountered in oil production, transportation, and refining operations are related to the presence of asphaltene aggregates in the crude oils. In the refining industry, for instance, the fouling of process equipment is a severe and costly problem.1−5 Also, it is well-known that crude oils and other liquid hydrocarbons (i.e., paraffinic hydrocarbons, liquefied petroleum gas, liquefied natural gas, etc.) are mixed before being sent to the refinery for their conversion to more valuable products.6 However, such processes involve a series of variables that must be taken into account when the mixing is realized. For example, the fouling in crude oil process equipment and heat exchangers depends upon the oil composition, asphaltene content, inorganic materials, and temperature and pressure conditions. Although there are various mechanisms that contribute to the fouling process, some laboratory tests have shown that the incompatibility phenomenon of asphaltene−crude oil systems is a major contributing factor.7−9 Asphaltenes are considered the most aromatic and polar fraction in the crude oil and have been defined in terms of a solubility classification.10,11 Major issues of interest in the oil industry, especially in refineries, are when and how much asphaltene will precipitate under certain thermodynamic conditions, knowing that the degree of dispersion of the asphaltene aggregates in crude oils depends upon their chemical composition. In heavy and highly aromatic crude oils, the asphaltenes are well-dispersed, but in the presence of a low-molecular-weight liquid hydrocarbon in excess (e.g., npentane or n-heptane), they precipitate.6 This problem has motivated various investigators to carry out experimental studies to understand the properties of asphaltene aggregates as a function of several thermodynamic conditions. Laboratory experiments reported in the literature have shown © XXXX American Chemical Society

how the changes in the pressure, temperature, and/or composition can provoke asphaltene precipitation. However, most of the works published concern the addition of a precipitant in excess (n-pentane, n-heptane, isooctane, etc.) to the crude oil for determining the incipient precipitation region.2,12−16 Specifically, the addition of n-heptane, which is a known precipitant for petroleum products, produces an aggregation process between asphaltene particles. Petroleum products are characterized by the existence of a precipitation region that corresponds to the required amount of n-heptane to induce asphaltene phase separation.17 On the other hand, there are few studies reported in the literature about the incompatibility of crude oil blends or liquid hydrocarbon blends. These studies have shown that the blending process of different crude oils can lead to the asphaltene precipitation phenomenon known as crude oil incompatibility.1,8,9 However, for crude oils and other unprocessed oils, it is generally believed that all blends are always compatible,18 although there is not at present clear evidence that the crude oil blends used in refinery are compatible or not. Therefore, the development of new experimental procedures for determining the asphaltene precipitation onset in crude oil blends is of considerable interest to avoid fouling in the petroleum industry. Toward this end, we present in this work a procedure for determining the incipient precipitation region in crude oil blends through viscosity and density measurements. The main objective of the work is to evaluate the capacity and sensitivity of the techniques based on the constant-force electromagnetic viscometer and vibrating tube densimeter to determine the mechanisms of the Received: July 4, 2014 Revised: December 11, 2014

A

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Energy & Fuels Table 1. Compositional Analysis of H-Oil, L-Oil, and P-Rs Liquid Hydrocarbon component

H-oil (mol %)

L-oil (mol %)

P-Rs (mol %)

component

H-oil (mol %)

L-oil (mol %)

C3 iso-C4 n-C4 iso-C5 n-C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15

0.57 0.34 1.74 1.38 2.61 5.72 7.10 9.11 5.06 3.91 1.86 1.22 1.15 0.74 0.65

1.50 0.79 3.13 2.37 3.93 7.06 9.73 10.18 7.96 6.10 4.20 2.95 2.39 1.87 1.34

0.00 0.01 0.94 28.94 36.73 21.74 9.07 2.44 0.03

C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C30+

0.40 0.38 0.32 0.32 0.18 0.14 0.12 0.10 0.10 0.07 0.07 0.05 0.04 0.04 54.52

1.04 0.79 0.68 0.58 0.46 0.37 0.27 0.21 0.17 0.14 0.10 0.09 0.08 0.06 29.47

asphaltene aggregation on the incompatibility process of crude oil blends.

Table 2. SARA Analysis (wt %) for H-Oil and L-Oil H-oil

2. EXPERIMENTAL SECTION 2.1. Characterization of Crude Oils and the Hydrocarbon of a Refinery Stream. Two crude oils referred to as H-oil (heavy oil) and L-oil (light oil) and a refinery stream liquid hydrocarbon containing high concentrations of pentanes (isopentane and npentane) referred in this work to as P-Rs hydrocarbon were characterized in terms of the composition by means of gas chromatography (GC), saturates, aromatics, resins, and asphaltenes (SARA) analysis, American Petroleum Institute (API) gravity, and paraffins, isoparaffins, olefins, naphthenes, and aromatics (PIONA) analysis. The crude oils were filtered to remove any suspended solids and measure the “true” asphaltene properties, defined as the crude oil constituents that are insoluble in a liquid low-molecular n-alkane but soluble in toluene and benzene. Yarranton and Musliyah19 found that, when asphaltenes are precipitated from a crude oil, other materials insoluble in toluene, such as ashes, fine clays, and some adsorbed hydrocarbons, also precipitate. All laboratory analyses were performed using filtered crude oils. 2.1.1. Compositional Analysis. Samples of H-oil, L-oil, and P-Rs hydrocarbon were analyzed by means of the high-temperature gas chromatography (HTGC) technique. The two crude oils were analyzed up to C30+ with a high-temperature gas chromatograph (HP, model 6890) that includes a flame ionization detector and a capillary column, whereas the sample of the P-Rs hydrocarbon was analyzed up to C9. Table 1 presents the compositional analysis of the two oils and the liquid hydrocarbon analyzed. As seen in this table, the sample of the refinery stream (P-Rs) contains high concentrations of iso-C5 (28.94 mol %) and n-C5 (36.73 mol %). 2.1.2. SARA Analysis. For SARA analysis, the asphaltene fraction was extracted from the two crude oils (H-oil and L-oil) by adding an excess of n-heptane (ASTM D3279-97) and n-pentane to the crude oils. The maltene fraction was separated into saturates, aromatics, and resins using a procedure based on the high-performance liquid chromatography (HPLC) technique, which is described in detail elsewhere.20 Table 2 shows the different fractions for the two crude oils. As seen, the asphaltene content is higher in the H-oil than in the L-oil for the two different solvents. From the SARA analysis, it is also possible to determine the colloidal instability index (CII).21 Results based on the CII value indicate that both crude oils are stable. However, this result does not guarantee that the asphaltenes contained in such crude oils do not precipitate when they are blended under certain thermodynamic conditions, as will be shown below. 2.1.3. API Gravity. The H-oil is a heavy crude oil with 20.95° API gravity and specific gravity of 0.9282 determined at 288.8 K. The viscosity and density for this oil are 397.005 cP and 971.3 kg/m3 determined, respectively, at 293.2 and 293.7 K. The L-oil is a light

L-oil

fraction

n-C5

n-C7

n-C5

n-C7

saturates aromatics resins asphaltenes CII

19.66 35.69 26.18 18.47 0.62

18.66 31.68 34.74 14.92 0.51

30.80 41.80 21.61 5.79 0.58

29.33 40.65 25.56 4.46 0.51

crude oil with 33.23° API gravity and specific gravity of 0.8590 determined at 288.8 K. For this oil, the viscosity and density are 18.818 cP and 899.9 kg/m3 determined, respectively, at 293.2 and 293.7 K. The viscosity and density of the P-Rs hydrocarbon with a high-pentane content are 0.289 cP and 655.8 kg/m3 respectively, determined at 293.2 and 293.7 K. In this case, the API gravity of the crude oils was determined using an Anton Paar DMA 4500 M vibrating densitometer. 2.1.4. PIONA Analysis. A gas chromatograph equipped with a flame ionization detector (GC−FID) was used to determine the amount of paraffins, isoparaffins, olefins, naphthenes, and aromatics contained in the P-Rs hydrocarbon. In this case, the sample of this hydrocarbon was analyzed without further preparation. The results obtained from this analysis are 44.55 wt % paraffins, 41.22 wt % isoparaffins, 0.13 wt % olefins, 7.09 wt % naphthenes, and 1.77 wt % aromatics. 2.2. Preparation of Crude Oil Blends. The samples of heavy and light crude oils were first filtered using a vacuum system (Millipore filter) with a 5 μm membrane to remove any suspended material that could distort the measurements of viscosity and density. The heavy crude oil was then diluted at different ratios with the light crude oil; i.e., the L-oil was added over the H-oil. All of the crude oil blends were prepared on a vol/vol basis. For each concentration, a total volume of 25 mL was prepared to perform the measurements of viscosity and density. Samples of the crude oil blends were agitated for at least 12 h in an ultrasonic bath to reach the thermal equilibrium. The experiments were performed over the temperature range from 293 to 393 K with increments of 20 K, at a constant pressure of 0.1 MPa. At least two different measurements with fresh samples were carried out for each crude oil blend to ensure reproducibility of the experimental data. 2.3. Electromagnetic Viscometer. A viscometer (model SPL 440), designed and manufactured by Cambridge Applied Systems (CAS, Medford, MA) to perform viscosity measurements of liquid fluids over the viscosity range from 0.02 to 10000 cP and at a wide range of temperatures and pressures was used in this work. However, because the viscosity range of the crude oil blends studied is not greater than 200 cP, only two pistons in the viscosity range of 1−200 B

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Energy & Fuels cP were used. The operation principle and specifications of this viscometer has been described in detail elsewhere.22,23 2.3.1. Viscometer Calibration. Viscosity standards supplied by Cannon Instruments Company were used to calibrate the two pistons that cover the viscosity range of 1−200 cP; these pistons were calibrated in the measuring range of 1−200 cP (two pistons: 1−20 and 10−200 cP) with the viscosity fluids (S20, N4, and S200), obtaining a maximum standard deviation of 0.5%. Viscosity measurements with two viscosity standards (S6 and S60) were carried out at various temperatures to verify the accuracy of each piston used; the differences between our results and those reported by the supplier (Cannon) were less than ±5% over the whole temperature range of interest. 2.4. Vibrating Tube Densitometer (VTD). Measurements of density were carried out using a commercial density measurement apparatus Anton Paar DMA 512P vibrating densitometer. The VTD is known to be stable, reliable, fast, and quite simple to set up and use. The basic theory and operation principle to determine the density of a fluid using a VTD have been described elsewhere.24 2.4.1. General Setup of the VTD. The DMA 512P cell is thermoregulated by means of a circulating liquid pump from a liquid bath, in the temperature range from 295 to 423 K with a stability of ±0.01 °C. A Pt-100 Ω/0 °C resistance thermometer is inserted inside a thermometric well of the DMA cell. The platinum resistance thermometer and the F250 thermometer were calibrated in accordance with the ITS-90 with an overall uncertainty of ±0.02 K in the working temperature range. The thermal stability of the apparatus is reached typically after about 2 h in an air-controlled room (291 K). A high-pressure positive displacement pump was used to transfer the crude oil blend in the whole system. A pressure transducer was connected to the measurement circuit to measure the pressure. 2.4.2. Densitometer Calibration. Nitrogen (purity of 99.99%, supplied by Praxair) and water (analysis-grade bidistilled) were used as calibration fluids. The measured vibrating frequencies were translated into densities through the calibration equations.25 To validate the densitometer calibration, the densities of toluene (HPLC grade) and benzene (assay by GC, corrected for water 100%, J.T.Baker) were measured over the temperature range from 313.60 to 413.62 K, all at 0.1 MPa. The maximum deviation in density of 0.33% was found in comparison to the measurements by Vargaftik et al.26 at temperatures from 313.15 to 373.15 K; the densities of benzene at 313.15 K show good agreement with a maximum deviation of 0.17% in comparison to the measurements by Lagourette et al.24 and Sun et al.27 at 0.1 MPa. The experimental densities for toluene and benzene are not reported here.

of 308.1, 313.2, and 318.2 K, all at a constant pressure of 0.1 MPa. Figure 1 presents the measured viscosity data for this test system at 308.1 K and 0.1 MPa as a function of the n-heptane

Figure 1. Viscosity as a function of the n-heptane volume percent for the system crude oil containing asphaltene−toluene at 308.1 K and 0.1 MPa.

volume percent. An examination of this figure indicates that the viscosity of the system under study (crude oil containing asphaltenes and toluene + n-heptane) decreases in a regular trend as the amount of precipitant increases from about 2 to 35 vol % n-heptane. After this point, the viscosity of the crude oil increases with further n-heptane addition until reaching a precipitation region point, a point so-called the asphaltene precipitation onset, according to the criterion by Escobedo and Mansoori.12 Thus, the onset of asphaltene precipitation for this crude oil diluted with n-heptane at 308.1 K and 0.1 MPa is believed to occur at about 35.0 vol % n-heptane. With still further n-heptane addition to the crude oil, the viscosity of the suspension decreases and then follows a regular trend as the concentration of n-heptane increases. Similar behaviors of viscosity as a function of the solvent for the suspension (asphaltene−crude oil−toluene−solvent) is obtained at the different levels of the temperature studied. Figure 2 shows the viscosity curves determined at 308.1, 313.2, and 318.4 K, all at 0.1 MPa, for this suspension. An inspection of this figure shows that the onset of asphaltene precipitation occurs at about the

3. RESULTS AND DISCUSSION 3.1. Viscosity Measurements. 3.1.1. Test System: Crude Oil + n-Heptane. To assess the onset of asphaltene precipitation in crude oils from viscosity information, we measured dynamic viscosities of the system crude oil containing asphaltenes and toluene + n-heptane by following the changes of aggregation occurring in the crude oil, according to the procedure by Escobedo and Mansoori.12 Although toluene is considered to be a “good solvent” for asphaltenes because the aggregates of asphaltene remain stable in the solution and do not grow,28 it is observed that there exists a strong tendency of the asphaltenes to self-aggregate even in diluted solutions.11,29−31 Therefore, to guarantee the detection of aggregate asphaltenes via this technique, toluene was added in excess (about 48.1 wt %) to a light crude oil (32.1° API gravity at 298.2 K). The SARA analysis for this crude oil is 23.64 wt % saturates, 9.21 wt % aromatics, 64.38 wt % resins, and 2.77 wt % asphaltene. From this SARA analysis, the calculated CII21 value is 0.36, which is less than those values obtained for the H-oil and L-oil. Experimental viscosity data for this system were determined at different ratios of n-heptane for the temperatures

Figure 2. Viscosity as a function of the n-heptane volume percent for the system crude oil containing asphaltene−toluene at different temperatures and 0.1 MPa. C

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Energy & Fuels same concentration of n-heptane in the crude oil for the three temperatures studied, i.e., at about 35 vol % n-heptane. On the other hand, to validate the results obtained from viscosity measurements, the light transmission technique along with a fully visual variable volume PVT apparatus was used to determine the onset of asphaltene precipitation of the system crude oil containing asphaltenes and toluene + n-heptane. In this case, every concentration of n-heptane added to the crude oil was analyzed under a microscope for visual detection of the asphaltene precipitation onset. The results showed that the asphaltene precipitation onset was around 35 vol % n-heptane, which is in good agreement with the viscosity measurements. The experimental results of these techniques were reported elsewhere.32 3.1.2. Heavy and Light Crude Oils and a Refinery Stream Hydrocarbon. Two different combinations of crude oil blends and crude oil blends with a refinery stream hydrocarbon were made to determine the asphaltene precipitation onset via viscosity and density measurements, namely, H-oil + L-oil and H-oil + L-oil + P-Rs hydrocarbon. First, the H-oil was diluted at different ratios with the L-oil to cover the entire range of L-oil concentrations from 0 to 100 vol %. Then, to evaluate the effect of adding the P-Rs hydrocarbon to this crude oil blend, a specific concentration of L-oil + H-oil was prepared by assuming that asphaltene aggregates do not yet occur. For these blends, viscosity and density measurements were carried out over the temperature range from 293 to 393 K with intervals of 20 K and at a pressure of 0.1 MPa. All blends were prepared on a vol/vol basis, and then they were stirred during at least 12 h at room temperature before their use. Figure 3 shows the viscosity curves for H-oil, L-oil, and P-Rs hydrocarbon over the temperature range from 293 to 393 K

Figure 4. Viscosity as a function of the L-oil volume percent for the Hoil + L-oil blend at 333.2 K and 0.1 MPa.

viscosities were measured at different H-oil + L-oil ratios, as aforementioned. An examination of this figure shows that viscosity decreases as the L-oil volume percent increases in a regular trend up to about 26.6 vol % L-oil. After this concentration of the L-oil, the viscosity of the blend increases up to about 36 vol % L-oil. At this point, the viscosity of the blend start to decrease smoothly as the amount of L-oil increases. Therefore, on the basis of the criterion previously described, the onset of asphaltene precipitation (i.e., the incompatibility region of the blend) occurs at about 26.6 vol % L-oil. In other words, it was possible to identify the beginning of the asphaltene instability (or crude oil incompatibility) at a definite concentration of the L-oil in the blend. Thus, after this beginning, the crude oil blends are in an unstable regime because of the presence of asphaltene aggregates. Hereafter, we refer to this beginning of the incompatibility process as the onset of asphaltene precipitation, as shown in Figure 4. Figure 5 shows the viscosity results of the H-oil + L-oil blends measured at temperatures from 293.2 to 393.1 K as a

Figure 3. Viscosities of the H-oil, L-oil, and P-Rs hydrocarbon at different temperatures and 0.1 MPa.

and 0.1 MPa. As seen in this figure, the viscosities determined from the filtered H-oil and L-oil are higher than those obtained from the unfiltered H-oil or L-oil. This is due to the amount of released light hydrocarbon from the crude oil during the filtered process. This figure shows that the viscosity differences between H-oil and L-oil are considerable to each other, and both viscosity values are much higher than the viscosities obtained for the P-Rs hydrocarbon. Viscosities of the P-Rs hydrocarbon were measured up to 353 K because it is fairly volatile. 3.1.3. System H-Oil + L-Oil. Figure 4 shows the viscosity results obtained for the blends of H-oil and L-oil at 333.2 K and 0.1 MPa as a function of the L-oil volume percent. The

Figure 5. Viscosity as a function of the L-oil volume percent for the Hoil + L-oil blend at different temperatures and 0.1 MPa.

function of the L-oil volume percent. From this figure, it is noted that most of the viscosity curves display a similar viscosity behavior. That is, for the temperatures of 293.2, 313.3, 333.2, and 353.2 K, the viscosity decreases as the L-oil volume percent increases up to a value of about 26.6 vol % L-oil. With a further amount of L-oil added to the blend, the viscosity increases slightly, and at about 36 vol % L-oil, the viscosity decreases in a regular trend as the concentration of L-oil D

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Energy & Fuels increases; this viscosity behavior is simply an indication of when the incompatibility region occurs. After this point, all of these blends were unstable. Figure 5 shows the measured incompatibility region for the crude oil blends at these four temperatures, whose beginning of the incompatibility region is about 26.6 vol % L-oil, as indicated by a dashed vertical line over these viscosity curves. At the higher temperatures of 373.2 and 393.1 K, however, the crude oil blends present a short incompatibility region, starting at 15.3 vol % L-oil (see Figure 5). Again, the incompatibility region is indicated by a dashed vertical line over these viscosity curves. It should be mentioned that the experiments covered the immediate vicinity and the vicinity of the incompatibility region beginning for all of the crude oil blends studied. As a result of these experiments, it is clear that asphaltenes are held in crude oils in a delicate balance,8 which can be easily disturbed, for instance, by adding saturates (see Table 2) or removing resins or aromatics,10,33 so that the overall concentration of the crude oil blends is modified, and consequently, the balance could be altered, thus provoking the precipitation of asphaltenes. From the experiments, it is observed that the effect of the temperature also causes the asphaltene aggregation; however, this effect on asphaltene solubility is somewhat controversial in the literature.3 For instance, Maqbool et al.34 reported that the time for determining the asphaltene precipitation onset is shorter at high temperatures than a lower temperatures,8 despite the asphaltene solubility being greater at high temperatures. More recently, Gonzalez et al.16 pointed out that a greater amount of precipitant is needed to precipitate asphaltenes at high temperatures. 3.1.4. System H-Oil + L-Oil + P-Rs Hydrocarbon. To understand the nature of the incompatibility/compatibility phenomenon of crude oil blends and the control of asphaltene precipitation when they are mixed with a liquid hydrocarbon, a specific blend of H-oil and L-oil containing 25.0 vol % L-oil was prepared before starting the viscosity measurements as a function of the P-Rs hydrocarbon concentration at a given pressure and temperature. A SARA analysis was then carried out for the H-oil + L-oil blend, giving the following composition: 22.73 wt % saturates, 40.70 wt % aromatics, 24.29 wt % resins, and 12.25 wt % asphaltenes. According to Table 2, the content of saturates and aromatics in this crude oil blend is greater than that obtained for the original H-oil. As a consequence, the content of resins and asphaltenes in the crude oil blend is lower than that reported for the original H-oil. Figure 6 shows the viscosity results obtained for the crude oil blend mixed with the P-Rs hydrocarbon at 333.2 K and 0.1 MPa. In this case, the viscosities were determined by adding the P-Rs hydrocarbon to the crude oil blend containing 25.0 vol % L-oil over the P-Rs volume percent range from 0 to 48 vol %. Here, the crude oil blend was diluted with n-pentane (see Table 1), so that it is expected that the incompatibility phenomenon or asphaltene precipitation onset occurs at low concentrations of the P-Rs hydrocarbon. An inspection of this figure shows that the viscosity of the suspension (crude oil blend + P-Rs hydrocarbon) decreases smoothly with the addition of the P-Rs hydrocarbon until reaching a value of 3.0 vol % P-Rs. With further addition of the P-Rs hydrocarbon, the viscosity slightly increases from 30 to about 32 cP for a concentration of P-Rs hydrocarbon ranging from 3.0 to 4.0 vol % P-Rs (i.e., the beginning of the incompatibility region). From this point, the viscosity decreases again as the concentration of the P-Rs hydrocarbon increases. This viscosity determined at the

Figure 6. Viscosity as a function of the P-Rs volume percent for the Hoil/L-oil + P-Rs blend at 333.2 K and 0.1 MPa.

concentration of 3.0 vol % P-Rs corresponds to the incompatibility region of the crude oil blend at 333.2 K and 0.1 MPa. Thus, the crude oil blend presents an incompatibility region at low concentrations of the P-Rs hydrocarbon. Beneath this incompatibility region beginning, the crude oil blends are in an unstable regime when the viscosity decreases again. From Figure 6, we can observe that, at higher ratios of P-Rs in the blend (>10 vol %), the decrease in viscosity is less pronounced and it seems to reach a constant value, thus indicating that there exists an optimum P-Rs/crude oil ratio range at which precipitation occurs, such as observed in titration experiments with n-pentane or n-heptane. Figure 7 shows the viscosity curves for the suspension (crude oil blend + P-Rs hydrocarbon) determined at the temperatures

Figure 7. Viscosity as a function of the P-Rs volume percent for the Hoil/L-oil + P-Rs blend at different temperatures and 0.1 MPa.

of 333.2, 353.2, 373.2, and 393.1 K and 0.1 MPa. For the temperatures of 333.2 and 353.2 K, the viscosity of the crude oil blend decreases from 2.0 to 3.0 vol % P-Rs and then increases as the concentration of the P-Rs varies from 3.0 to 4.0 vol %. In this case, the incompatibility region of the crude oil blend for these two temperatures starts at the concentration of 3.0 vol % P-RS; after this point, the viscosity decreases as the concentration of the P-Rs hydrocarbon increases, as described above. In this figure, the incompatibility region beginning of the crude oil blend at the temperatures of 333.2 and 353.2 K is indicated by a dashed line over these two viscosity curves. The viscosity behavior of the crude oil blend as a function of the P-Rs hydrocarbon concentration at 373.2 and 393.1 K and 0.1 MPa presented in Figure 7 shows that the incompatibility E

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Energy & Fuels region of the crude oil blend starts at the concentration of 2.0 vol % P-Rs, this region being indicated by the dashed vertical line over these viscosity curves. After the concentration of P-Rs hydrocarbon ranging from 3.0 to 4.0 vol % P-Rs is reached, the viscosity curves decrease smoothly as the concentration of the P-Rs hydrocarbon increases. At 393.1 K, the viscosity of the crude oil blend increases abruptly at 2.0 vol % P-Rs, finding the incompatibility region beginning almost immediately as the PRs is added to the blend. Then, the viscosities of the crude oil blend decrease in a regular trend as the concentration of the PRs hydrocarbon increases. The maximum uncertainty in viscosity over the temperature range from 293.2 to 393.1 K was 0.32 cP. It should be mentioned that, once the onset of asphaltene precipitation is reached and after passing the incompatibility region, when a decrease in viscosity is observed at higher ratios of L-oil or P-Rs hydrocarbon in the blend, the blends were unstable. 3.2. Density Measurements. 3.2.1. Heavy and Light Crude Oils and a Hydrocarbon Refinery Stream. Figure 8

Figure 9. Density as a function of the L-oil volume percent for the Hoil + L-oil blend at 333.7 K and 0.1 MPa.

Then, at about 36.0 vol % L-oil, the density of the crude oil blends starts to decrease in a regular trend as the concentration of the L-oil increases. On the basis of the fact that the viscosity behavior as a function of the L-oil concentration can be visualized as a mechanism of asphaltene aggregation, it can be said that the incompatibility region of the crude oil blend at a given L-oil concentration corresponds to a determined density value before it begins to increase. Thus, the onset of asphaltene precipitation of the crude oil blend is assumed to be 26.6 vol % L-oil, which is the same concentration found in the viscosity measurements at the temperature of 333.2 K and 0.1 MPa, as also seen in Figure 9. Here, as in the viscosity measurements, after the onset of asphaltene precipitation has been determined and when a decrease in density is observed at higher ratios of the L-oil in the blend, the blends are in a regime completely unstable because of the presence of asphaltenes. Figure 10 shows the density curves as a function of the L-oil concentration for the crude oil blends determined over the

Figure 8. Density as a function of the temperature for the H-oil, L-oil, and P-Rs hydrocarbon at 0.1 MPa.

shows the trend of the density measurements for filtered and unfiltered H-oil and L-oil over the temperature range from 293 to 393 K at 0.1 MPa. As seen, the density measurements produce straight lines that decrease as the temperature increases. Also observed in this figure is that the densities of the filtered crude oils are greater than those of the unfiltered crude oils, maybe because of the removal of the light hydrocarbons from the crude oils during the filtered process. Similar to the viscosity measurements, the density differences between H-oil and L-oil are significant. Also presented in Figure 8 are the density measurements of the P-Rs hydrocarbon plotted as a function of the temperature. In this case, the temperature range of measurement was limited to 353 K because the volatility of the P-Rs hydrocarbon did not allow for the performance of density measurements at higher temperatures. An examination of this figure shows that the densities of the P-Rs hydrocarbon are much lower than the densities of the two crude oils. 3.2.2. System H-Oil + L-Oil. Figure 9 shows the density measurements of the H-oil + L-oil blends at 333.7 K and 0.1 MPa. An inspection of this figure shows that the density decreases rapidly as the amount of the L-oil added is increased, until reaching a given value (about 26.6 vol % L-oil), where the density slightly increases with further addition of the L-oil.

Figure 10. Density as a function of the L-oil volume percent for the Hoil + L-oil blend at different temperatures and 0.1 MPa.

temperature range from 293 to 393 K and 0.1 MPa. As seen in this figure, for all temperatures studied, the density of the crude oil blends rapidly decreases as the L-oil is added, until reaching the density value of the asphaltene precipitation onset. Then, with further addition of L-oil, the density of the crude oil blend increases, until reaching a maximum density value at a given Loil concentration. After this maximum density value is passed, the density curves tend to decrease as the concentration of the L-oil increases, as shown in Figure 10. It is worth mentioning that the onset of asphaltene precipitation of the crude oil blends F

DOI: 10.1021/ef501512b Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels was found at the same concentration of 26.6 vol % L-oil, irrespective of the level of the temperature. This same value of L-oil concentration was obtained for the viscosity measurements at the same temperature range, as indicated by a dashed vertical line over the density curves. Therefore, the density range from 26.6 to 36.0 vol % L-oil is assumed to be the beginning of the incompatibility region of the H-oil and L-oil blends; after this concentration range, a decrease in density is observed again and the blends are at a unstable regime, as aforementioned. 3.2.3. System H-Oil + L-Oil + P-Rs Hydrocarbon. Figure 11 presents the densities of the H-oil + L-oil blend containing 25.0

Figure 12. Density as a function of the P-Rs volume percent for the Hoil/L-oil + P-Rs blend at different temperatures and 0.1 MPa.

measurements, who point out that native crude oils of different sources exhibit sharp viscosity and density peaking at phase boundaries, defined by specific asphaltene contents. As mentioned earlier, this study focused on the compatibility/incompatibility phenomenon in crude oil blends (H-oil/ L-oil systems) and blends of crude oils (H-oil/L-oil) with a refinery stream hydrocarbon (P-Rs). Although it is known that there is not a reliable standardized method to determine the incompatibility of asphaltenes in crude oil blends, we used potentially incompatible blends to investigate the application of the constant-force electromagnetic viscometer and vibrating tube densimeter techniques, finding that both techniques were capable of identifying the onset of asphaltene precipitation in crude oil blends.

Figure 11. Density as a function of P-Rs volume percent for the H-oil/ L-oil + P-Rs blend at 333.7 K and 0.1 MPa.

vol % L-oil as a function of the P-Rs hydrocarbon concentration, which were determined at 333.7 K and 0.1 MPa. These density measurements were carried out over the concentration range of the P-Rs hydrocarbon from 0 to 48 vol %. This figure shows that, initially, the density of the crude oil blend slightly decreases until reaching the incompatibility region point at 4.0 vol % P-Rs. After this point, the density of the crude oil blend increases, until reaching a maximum density point at P-Rs concentration of 5.0 vol %. Then, with further addition of the P-Rs hydrocarbon, the density curve decreases in a regular trend. It is also observed in this figure that the onset of asphaltene precipitation determined at 333.7 K is 4.0 vol % P-Rs, which is 1% greater than that obtained from viscosity measurements at 333.2 K (i.e., 3.0 vol % P-Rs), as shown in Figure 6. At the temperatures of 353.7, 373.7, and 393.7 K, the incompatibility region beginning was determined at the concentration of 3.0 vol % P-Rs, as seen in Figure 12 and indicated by a dashed vertical line over the three density curves, whereas for the viscosity measurements, this region was displaced at a concentration of 2.0 vol % P-Rs at the temperatures of 373.2 and 393.1 K (see Figure 7). The maximum uncertainty in density over the temperature range from 293.7 to 393.7 K was 0.4 kg m−3. It is well-known that, when an n-alkane is added to the crude oil, asphaltenes and asphaltene aggregates form clusters and flocculate. The incompatibility region, therefore, corresponds to the n-alkane concentration, leading to the precipitation of asphaltene aggregates.29 Because the incompatibility region determined from density measurements is in good agreement with that found from viscosity measurements, then the density technique is another possibility to determine the incompatibility region in crude oil blends mixed with liquid hydrocarbons. These results are in accordance with Evdokimov’s35

4. CONCLUSION The techniques of electromagnetic viscometry at a constant force and vibrating tube densimetry can be used for measuring the onset of asphaltene precipitation in crude oil blends. The results of viscosity and density obtained with these techniques are indicators of the compatibility/incompatibility of asphaltenes in crude oil blends mixed with a liquid hydrocarbon. In these techniques, the incompatibility region was detected by visual observation of a sharp deviation in the viscosity and density behavior. It was showed that viscosity and density measurements can be used to assess the changes of asphaltene aggregation in crude oils and crude oil blends under the influence of liquid hydrocarbons and temperature. Finally, it was confirmed that the incompatibility region determined from viscosity measurements compares favorably to that obtained from density measurements, so that these two techniques can be used with confidence to determine the beginning of the incompatibility region of crude oil blends and crude oil blends mixed with liquid hydrocarbons.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +52-55-9175-6503. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their appreciation for the financial support of the Sectorial Fund CONACyT−SENER−Hydrocarbons G

DOI: 10.1021/ef501512b Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels under Grant 144396 (IMP-Project Y.00118). The Laboratory of Well Productivity at the Mexican Petroleum Institute is also acknowledged. The authors thank the Mexican Petroleum Company PEMEX Refinación for providing the samples of the two crude oils and the hydrocarbon from the refinery stream.

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REFERENCES

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DOI: 10.1021/ef501512b Energy Fuels XXXX, XXX, XXX−XXX