Temperature effects on solubility of asphaltenes in crude oils

chemical engineering research and design 9 4 ( 2 0 1 5 ) 573–583

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Temperature effects on solubility of asphaltenes in crude oils Zubair A. Chandio, Ramasamy M. ∗ , Hilmi B. Mukhtar Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak D.R., Malaysia

a b s t r a c t Asphaltenes, the heaviest fraction in crude oil, are linked to severe problems in petroleum industry. Asphaltenes are either in dissolved state or in colloidal dispersion form in crude oil. Temperature has a significant role in dissolving the asphaltenes in continuous phase of crude oil. However, there are differing views on the effect of temperature on solubility and precipitation of asphaltenes. In this work, an investigation was carried out on the effect of temperature on solubility of asphaltenes for six different crude oil samples in the temperature range of 20–95 ◦ C, using automated flocculation onset titration. The test results are interpreted in terms of the Heithaus parameters and using Hildebrand’s solubility parameter. The experimental data, the Heithaus parameters and the Hildebrand solubility parameter show that solubility of asphaltenes increased with increase in temperature and possible phase transition of asphaltenes takes place with the increase in temperature. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Asphaltenes; Precipitation; Solubility; Heithaus parameters; Hildebrand solubility parameter; Automated flocculation titrimeter

1.

Introduction

Asphaltenes are the heaviest and most complex molecules in crude oil and are defined by its solubility class as the constituents of oil which are soluble in toluene but insoluble in n-heptane. Presence of asphaltenes in crude oil has been linked to several problems in petroleum industry. Much of the research has been driven by the tendency of asphaltenes to aggregate, precipitate and deposit onto surfaces (Arciniegas and Babadagli, 2014; Hoepfner et al., 2013; Mullins et al., 2007; Spiecker et al., 2003; Maqbool et al., 2011). Deposit formation in crude oil processing can be attributed to various factors but asphaltenes are generally considered as main precursors of deposit formation on heat transfer surfaces in crude oil processing (Hoepfner et al., 2013; Murphy and Campbell, 1992; Wiehe, 2006). The presence of asphaltenes on heat transfer surfaces further favors fouling of equipment because asphaltenes are more prone to attach to already coked

∗

surfaces than a clean metallic alloy surface (Asprino et al., 2005). In order to mitigate problems caused by asphaltenes, a thorough understanding of the process of asphaltenes precipitation, flocculation and deposition and the factors affecting them is necessary (Macchietto et al., 2011; Mohammadi and Richon, 2008; Srinivasan, 2008; Wiehe, 2008). Factors, such as, temperature, pressure and crude oil composition have been observed to effect asphaltenes solubility and precipitation (Crittenden et al., 2009; Deshannavar et al., 2010; Ramasamy and Deshannavar, 2014; Watkinson, 2003). Whether a change in temperature dissolves asphaltenes in oil or it favors precipitation of asphaltenes from oil; and whether the asphaltenes exist in crude oil in dispersed state forming a colloidal suspension of oil or they exist in dissolved state in oil like a true solution, are still debatable. There are different schools of thoughts on relationship between phase behavior of asphaltenes and change in temperature. Some studies (Branco et al., 2001; Chung, 1992) show that a part of asphaltenes in oil is dissolved and the rest of asphaltenes is in colloidal dispersed form (Bartholdy and

Corresponding author. Tel.: +60 53687585; fax: +60 53656176. E-mail addresses: [email protected], ma [email protected] (R. M.). Received 24 May 2014; Received in revised form 21 September 2014; Accepted 25 September 2014 Available online 2 October 2014 http://dx.doi.org/10.1016/j.cherd.2014.09.018 0263-8762/© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Nomenclature C FR P Po Pa Tp T V Vo Vs VT vT W ıas ıF ım ıo ıs ıT o s T

concentration (g/mL) flocculation ratio Heithaus compatibility parameter peptizing power of oil peptizability of asphaltenes time to flocculation peak (s) temperature (◦ C) molar volume (mL/mol) volume of oil (mL) volume of solvent (mL) volume of titrant (mL) volumetric flow rate of titrant (mL/s) weight (g) solubility parameter of asphaltenes (MPa0.5 ) solubility parameter of solvent–titrant mixture (MPa0.5 ) solubility parameter of mixture (MPa0.5 ) solubility parameter of oil (MPa0.5 ) solubility parameter of solvent (MPa0.5 ) solubility parameter of titrant (MPa0.5 ) volume fraction of oil volume fraction of solvent volume fraction of titrant

Andersen, 2000; Gharfeh et al., 2004). Chung (1992) stated that asphaltenes can either precipitate or dissolve in crude oil on changing thermodynamic conditions such as temperature. According to Mansoori (1997), asphaltenes appears in crude oil both as soluble phase and as well as colloidal one and asphaltenes deposition from petroleum fluids is partly due to solubility effect and partly due to colloidal phenomenon. The asphaltenes colloids are considered as molecules in a true solution and are also reported to be thermodynamically a liquid phase at higher temperature, forming liquid–liquid equilibrium with oil phase (Akbarzadeh et al., 2005; Chung, 1992; Ganeeva et al., 2014; Mansoori, 1997; Pan and Firoozabadi, 2000). Mathematical modeling has been used in predicting asphaltenes precipitation and solubility. Some of the models are based on colloidal dispersed state of asphaltenes and some are based on the existence of asphaltenes in liquid state in crude oil. Firstly, Hirschberg et al. (1984), by using Flory–Huggins polymer solutions theory and definition of asphaltene solubility as a reversible phenomenon, presented a new model for asphaltenes precipitation. Leontaritis and Mansoori (1987) proposed a model predicting the onset of asphaltene flocculation in which asphaltenes exist in the oil as solid particles in colloidal suspension, stabilized by resins adsorbed on their surface. Rassamdana et al. (1996) proposed scaling equation based on partial aggregation and dissolution phenomenon of asphaltenes. Akbarzadeh et al. (2005) used a generalized regular solution model for their data to estimate asphaltenes precipitation from n-alkane diluted heavy oils and bitumen. Mehranfar et al. (2014) used atomic force microscopy to observe the structural changes in asphaltene nano-aggregates as a function of temperature (25–80 ◦ C), demonstrating the transition of asphaltenes to liquid phase with increase in temperature above 70 ◦ C. Maqbool et al. (2011) used optical microscopy to evaluate the effect of temperature on

asphaltenes precipitation kinetics. They presented a hypothesis that destabilized asphaltenes aggregate more at 50 ◦ C than at 20 ◦ C but precipitation rate is lower due to lesser number of particle collisions because of the higher aggregate size. Hu and Guo (2001) studied the effect of temperature and molecular weight of n-alkanes on asphaltenes precipitation using flocculation onset titration method along with light scattering technique, and their data show that amount of asphaltenes precipitation was higher at 20 ◦ C than at 65 ◦ C. Laux et al. (1997) used flocculation onset titration coupled with light transmission technique to investigate the influences of different factors on stability of colloid disperses in crude oils. They studied the effect of temperature on flocculation onset point using polar and non-polar titrants in the temperature range of 0–50 ◦ C and concluded that there was no significant effect of temperature on flocculation onset point. Andersen and Stenby (1996) precipitated and then redissolved asphaltenes in n-heptane-toluene solution to study thermodynamics of asphaltenes precipitation in the temperature range of 24–80 ◦ C and observed that asphaltenes solubility increased with the increase in temperature and increase in concentration of toluene in n-heptane-toluene solution. According to a study by Lambourn and Durrieu (1983), dependence of asphaltenes solubility on temperature was observed, where asphaltenes dissolved between 100 and 140 ◦ C temperature range but re-precipitated above 200 ◦ C. Storm et al. (1996) established similar conclusions in their rheological and small-angle x-ray scattering study on asphaltenes flocculation that asphaltenes flocculate at 150–200 ◦ C and the flocculation is purely a physical phenomenon not a chemical one. But Wiehe’s (1997) hot-stage microscopy results are somewhat differing from the above two, according to which insoluble asphaltenes re-dissolve in residue on heating from room temperature to 200 ◦ C. This was in agreement with Hong and Watkinson’s (2004) study on solubility and precipitation of asphaltenes carried out in the bulk temperature range of 60–300 ◦ C. In the study it was observed that the concentration of dissolved asphaltenes increased at higher temperature and it was suggested that, for deposition and fouling studies, the concentration of suspended asphaltenes is important than the dissolved ones. Evdokimov et al. (2003) diluted virgin crude oil in toluene to observe asphaltenes aggregation in their viscosity, optical and NMR studies. Based on their results of the studies, the authors concluded that the molecular aggregation in crude oil solutions is a sequential process, which leads to a phase transformation to liquid. A suitable approach to investigate influences of different factors on the stability of colloidal dispersion is to determine flocculation onset point in crude oil (Andersen and Stenby, 1996; Cimino et al., 1995; Mofidi and Edalat, 2006). Solubility of asphaltenes depends on the solvating power of the solvent (Chung, 1992). This is in agreement with Hildebrand and Scott’s (1962) theory of regular solution which is a widely used theory to interpret asphaltenes precipitation. According to the theory, the solubility of one component in a mixture affects the solubility of other components. This is further substantiated by Wiehe’s (1996) investigation on asphaltenes solubility which shows that asphaltenes are more soluble in the solvent having higher field force solubility parameter. Several types of laboratory units and techniques have been reported to be used in the study of asphaltenes solubility and precipitation. Some studies involved manual methods and others employed automated ones. The following

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Table 1 – Properties of crude oils. Properties Origin API gravity @ 60 ◦ F (◦ API) Density @ 15 ◦ C (g/mL) Kinematic viscosity (cST) @ 15 ◦ C Asphaltenes (%) Pour point (◦ C) Flash point (◦ C)

A and A-2 Malaysia

B Malaysia

C Middle East

D Australia

E Middle East

28.21 0.789 5.2 0.5 −6 11.2

25.27 0.9 446 0.28 45.61 156.06

28.3 0.886 44 1.7 −45 −5

18.7 0.941 64.81 0.2 −27 75

27.67 0.887 83.7 7 −45 6.5

laboratory techniques have been reported in the literature on study of asphaltenes flocculation and precipitation, Oliensis spot test, microscopic investigation, optical transmission, light scattering by particles, conductivity measurements, fluorescence spectroscopy, particle size analysis and heat transfer analysis. The reviewed literature suggests that so far the flocculation onset titration test of crude oils and asphaltenes have been carried out in the range of 0–65 ◦ C only. The majority of the experiments were carried out on asphaltenes–solvent–precipitant mixture prepared in laboratory; a few researchers have used crude oil in asphaltenes solubility and precipitation studies. In order to understand the effect of temperature on solubility and precipitation of asphaltenes, it would be rational to use the actual crude oil so that results can be used to mitigate the real asphaltenes precipitation and deposition problems in the field. In conjunction with it, the details of the experimental equipment and procedure are described in Section 2 followed by Section 3.

2.3.

Sample preparation

Before carrying out flocculation onset titration experiments, the crude oils were concentrated to 50% of the weight using Fischer’s True Boiling-Point distillation unit following ASTM D-2892-11a standard (ASTM, 2011). Andersen (1999) also used true boiling point distillation to concentrate asphaltenes in crude oil for asphaltenes flocculation studies. The test samples for the AFT experiments were prepared at three successively increasing concentrations (in the range of 0.5–0.9 g/mL) in 30 mL test reaction vials. For each test, concentrated crude oil and 2.00 mL of toluene were transferred into the vials. After addition of toluene, the three reaction vials are closed tightly and were kept in dark and cold room for a minimum of 4 h to allow crude oil to dissolve in toluene properly prior to titration. The flocculation onset titration experiments of 87 samples were performed in the range of 20–95 ◦ C.

2.4.

Properties of crude oils

J.J. Heithaus (Heithaus, 1960, 1962) introduced flocculation onset titration method to estimate asphaltenes precipitation and stability in crude oil. The method is a widely used technique to investigate the oil properties in terms of flocculation onset and solubility parameters. Automated methods of titration for detecting flocculation onset of asphaltenes give more accurate results than the manual investigation of the asphaltenes flocculation. The data from automated flocculation titration is used to calculate the Heithaus parameter values and the Hildebrand solubility parameter of asphaltenes and maltene.

The properties of the crude oils used in this study are given in Table 1. The properties were obtained from crude oil assays and a processing refinery. The automated flocculation titration was conducted on two specimen of crude oil A. The crude oil A was subjected to fouling experiments in a custom made Annular Flow Fouling Research Unit (AFFRU), prior to its true-boiling point distillation. This post-AFFRU oil is termed, hereafter, as crude oil A-2 and the same composition is assumed for A-2 as that of A. The other specimen of crude oil A was used as neat and without any treatment prior to true-boiling distillation, which is simply termed as crude oil A. The oil B is a low sulfur waxy residue (LSWR), a 500+◦ C product of Crude Distillation Unit (CDU) of refinery with a blend of several crude oils as feed.

2.1.

2.5.

2.

Experimental

Automated flocculation titrimeter

The automated flocculation titrimeter (AFT), with the model number K47190, was supplied by Koehler Instrument Company, Inc. Titrations can be performed in the range of 20–100 ◦ C, with the light transmittance intensity of the spectrophotometer at 380–1050 nm.

2.2.

Chemicals

The ASTM standard D6703-01 for automated Heithaus titration method recommends HPLC grade chemicals to be used in AFT tests and reagent grade chemicals for cleaning purpose. HPLC grade toluene (Methylbenzene, C6 H5 CH3 ) obtained from Tedia Company Inc. was used as a solvent for preparing crude oil solution. HPLC grade iso-octane (2,2,4-trimethyl pentane) used as titrant, was supplied by Merck KGaA.

Procedure of operation

The spectrometer is calibrated to output 100% transmittance when HPLC grade toluene is circulated through it. One of the two vessels of the AFT is half-filled with water to hold the reaction vial containing the solution; the second vessel is filled with iso-octane. The high flow rate pump circulates the solution through the spectrometer and the low flow rate pump doses the titrant. The light transmittance through the sample is recorded in the form of a graph via a computer program run by a computer connected to the AFT unit.

3.

Results and discussion

3.1.

The Heithaus parameters

The Heithaus test method is based on the colloidal suspension model of crude oil in which asphaltenes are in colloidal

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dispersion state in the continuous phase of oil. The extent to which these two would remain in a given state of peptization was thought to be a measure of the compatibility of the suspension (Heithaus, 1960, 1962). In a typical Heithaus titration test, an oil sample is dissolved in a solvent and titration of oil sample is carried out using a precipitant to estimate the onset point of asphaltene flocculation in the oil. From the test, the three parameters of Heithaus titration method, Pa , Po and P are calculated in order to predict the colloidal stability of the oil. The parameter P is the measure of overall colloidal stability of individual oil, asphalt or a blend system, which is determined by calculating the former two parameters, Pa and Po. The typical value of P lies from 2.5 to 10. The oils having a P value lower than 2.5 are considered to be less stable and those having a value above 2.5 are designated to be more stable (Andersen, 1999; Heithaus, 1960). The overall stability parameter P is expressed as: Po P= 1 − Pa

(1)

The parameter Pa is the measure of peptizability of asphaltenes, the higher value of Pa indicates that asphaltenes are more peptized in a colloidal dispersion form; the lower value shows less asphaltenes are peptized in colloidal form in crude oil. The asphaltenes peptizability parameter Pa is expressed as: Pa = 1 − FRmax

(2)

The parameter Po represents the solvating power of oil, the higher values of Po represents increase in solvency characteristics of maltene part (the dispersing phase) of oil and the parameter Po is determined as: Po = FRmax ×

 1  Cmin



+1

(3)

As expressed above, the parameters Pa and Po are calculated as functions of the two quantities Cmin and FRmax and these two are obtained by plotting the values of flocculation ratio FR and dilution concentration C of the solution. The values of FR and C are derived from the other experimental parameters, which are, weight of oil (Wo ), volume of solvent (Vs ) and volume of titrant (VT ) added up to flocculation onset point. Flocculation ratio, FR, is the fraction of volume of solvent to the total volume of solvent–titrant mixture at flocculation onset point and it is expressed as: FR =

Vs Vs + VT

Fig. 1 – Flocculation ratio vs. Concentration for a stable and a less stable asphalt (ASTM, 2001).

The X-axis intercept and Y-axis intercept in the graph is defined by linearly extrapolating the FR versus C points. The Y-axis intercept is termed as FRmax and it can be defined as the minimum amount of solvent in the solvent–titrant–oil solution necessary to keep the asphaltenes dissolved in the solution and is used to measure the peptizability of asphaltenes Pa . The higher values of FRmax indicate less asphaltenes peptizability. The X-axis intercept is Cmin and it is used to determine the solvating power of the dispersing phase in the oil Po . The higher value of Cmin represents lower stability of oil. The automated flocculation titrimeter yields light transmittance data in the form of a graph of percent light transmittance in the solution against time. A typical light transmittance graph is shown in Fig. 2 which shows light transmittance trend of three solutions of successively increasing concentrations. The light transmittance trend increases to a maximum point and starts to decline when flocculation of asphaltenes in the solution takes place. The continuous increase in light transmittance up to the flocculation peak takes place due to dilution of the solution by addition of the titrant. Since flocculation results in increase in the opacity of the solution hence it leads to poor light transmittance, therefore, the % light transmittance trend goes downwards after the flocculation peak. The flocculation onset peaks are illustrated by the intercepts of dotted lines in the light transmittance trends of the three samples as shown in Fig. 2. Using the light transmittance graph various AFT parameters can be determined. As illustrated in the figure, the time from the start of the experiment up to flocculation point is marked as Tp . The

(4)

Dilution concentration, C, is the ratio of mass of oil to the volume of titrant and solvent at flocculation onset point and it is expressed as: C=

Wo Vs + VT

(5)

In order to obtain Cmin and FRmax , the values of flocculation ratio, FR, and dilution concentration, C, are calculated at three successively increasing concentrations of samples in a flocculation onset experiment and the values are plotted on a FR versus C graph. A typical graph of FR versus C is illustrated in Fig. 1.

Fig. 2 – Light transmittance in oil C at 40 ◦ C.

chemical engineering research and design 9 4 ( 2 0 1 5 ) 573–583

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time to flocculation peak is used to calculate the volume of titrant consumed up to flocculation peak, expressed as: VT = Tp × vT

(6)

where, VT is volume of titrant, Tp is time to flocculation peak and vT is volumetric flow rate of titrant. From the transmittance graph, the flocculation point interpretation can be done by assuming the phenomenon that initially the asphaltenes particles, covered by a solvating cover of resins, are dispersed in oil. The addition of titrant starts dissolving the resins solvate cover to a point where the stronger self-association forces between asphaltenes overcome the weaker association of resins cover on asphaltenes particles. This leads to the formation of primary particles which become more stable aggregates by further addition of titrant. The increase in aggregates size and concentration in the solution results in poor light transmittance after the flocculation onset due to scattering of light by the particles. As shown in Fig. 2, the light transmittance is comparatively higher in less concentrated sample than that of the concentrated ones. This is because of the presence of relatively higher concentration of asphaltenes in the successive samples than the preceding ones. It is also noticed that in every experiment the time taken up to flocculation point in the less concentrated samples is comparatively less than the successive higher concentration samples. It is assumed that the concentration of resins is higher in the sample having higher weight of oil. In order to dissolve the resins in higher concentration, more volume of titrant is required, which results in higher time for more concentrated sample. The titrant disturbs the association between resins and asphaltenes resulting in precipitation of asphaltenes.

Fig. 3 – Dilution concentration versus temperature. onset at higher temperature indicates that there is lesser amount of asphaltenes in colloidal form because much of the asphaltenes are dissolved in the oil at higher temperature. Therefore, the volume required to flocculate the available asphaltenes aggregates at higher temperature is relatively less. This is illustrated by the schematic diagram of precipitated, suspended and dissolved asphaltenes in oil as shown in Fig. 4. In the figure, precipitated asphaltenes are shown in (a), the fully dispersed asphaltenes in oil making a stable colloidal suspension is illustrated in part (b) of the figure, partially dissolved and partially dispersed asphaltenes are represented in the part (c), and the (d) represents a solution of oil and asphaltenes. So, it can be assumed that the oils contain more dissolved asphaltenes and less colloidal asphaltenes, at higher temperature.

3.3. 3.2.

Effect of temperature on flocculation ratio

Effect of temperature on dilution concentration

The average values of three samples of each experiment are presented in graphical form to interpret the AFT results. The graph of all the oils for dilution concentration at flocculation point against temperature is given in Fig. 3. Experiments with the crude oil residue B were not performed at 20 ◦ C because it solidified at the temperature. It was observed that for all the oils the dilution concentration, C, of the samples increased with the increase in temperature. The increase in the dilution concentration is due to the reason that titrant volume decreased with temperature. The lower volume of titrant required to initiate flocculation

The plot of FR versus C of oil A is shown in Fig. 5. As seen in the figure, the values of FR increased with the temperature and a higher value (0.74) of FR is seen at 95 ◦ C as compared to the value (0.15) at 20 ◦ C. Minor change in FR is observed between 20 and 40 ◦ C, but significant increase is seen at 60 ◦ C. As FR is defined as the ratio of solvent in the solvent–titrant mixture, the increase in the FR value shows that the available amount of solvent in the mixture at the flocculation point is higher. It is due to the reason that the volume of the titrant decreased with the increase in temperature which makes the solvent fraction higher. The FR versus C graphs of all the samples show

Fig. 4 – Schematic diagram of asphaltenes in oil.

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Fig. 5 – Flocculation ration versus concentration graph of crude oil A.

an increasing FR trend with the increase in temperature. The data show that it is possible to obtain reproducible results for FR versus concentration.

3.4.

Effect of temperature on the Heithaus parameters

As discussed earlier, the Heithaus parameter P is calculated as a function of two other designated parameters that are Pa and Po . Fig. 6 shows the graphs of Pa of all the oils. In the present study, it was observed that asphaltene peptizability parameter, Pa , decreased with increase in temperature suggesting less stable asphaltenes in the oil while the peptizing power of oil, Po , increased with temperature suggesting more soluble asphaltenes. So, it can be concluded that the asphaltenes that precipitated out from the colloidal suspension at higher temperatures dissolved in the oil with a net effect of reaching an equilibrium with less asphaltenes in colloidal form and more asphaltenes in soluble form. In stable oils, the dispersed asphaltenes colloids covered by resins are well peptized by the oils (maltene). At higher temperature, the colloidal asphaltenes are small in amount and tend not to form strong associations, because the resins effectively disperse the colloids (Branco et al., 2001; Hoepfner et al., 2013; Maqbool et al., 2011). This is because the part of asphaltenes is dissolved leaving enough mass of the resins and oil to keep the remaining smaller aggregates of asphaltenes well peptized in the oil at higher temperature. Fig. 6 shows that peptizability of asphaltenes decreased with the increase in temperature because of possible phase

Fig. 6 – Peptizability of asphaltenes versus temperature.

Fig. 7 – Solvating power of oil versus temperature.

transition, whereby, asphaltenes tend to dissolve in oil at higher temperature. Increase in peptizability means associations among asphaltenes colloids is more extensive and they are not sufficiently solvated by the solvent, hence tendency of asphaltenes to aggregate at lower temperature is higher which could result in precipitation and deposition on surfaces. Resins have strong tendency to associate with asphaltenes, this reduces the aggregation of asphaltenes which determine to large extent their solubility in crude oil. Resins are also reported to have a self-association tendency like asphaltenes and it can be assumed that at lower temperature the resins self-associate strongly hence their tendency to associate with asphaltenes is reduced (Pereira et al., 2007). As a result of it, asphaltenes molecules form aggregates due to their stronger polarity and self-association to form aggregates resulting in higher precipitation at lower temperatures. The relatively higher values of peptizability at lower temperature could be caused by dissociation of asphaltenes-resin molecules and aggregate formation. Oil E has comparatively higher concentration of asphaltenes. Therefore, part of asphaltenes is still in colloidal form at higher temperature despite asphaltenes dissolution. Due the higher concentration of asphaltenes the change in Pa is relatively less. The peptizing power of the continuous medium of oil (maltene phase) can be defined as the tendency of oil to keep the asphaltenes solvated in oil. It is represented by Po and the higher value of Po indicates greater solvating power of the maltene phase of oil. Precipitation is governed by weak dispersion forces between asphaltenes and oil. At higher temperature, the association between resins and asphaltenes particles is stronger which keep the asphaltenes dissolved in oil. The affinity of soluble asphaltenes toward aggregates becomes lower with the increase in temperature due to higher solvating tendency of oil. As shown in Fig. 7, the increasing values of Heithaus parameter Po indicate that the solvency characteristics of the oils increased with the increase in bulk temperature from 20 to 95 ◦ C. The value of Po for crude oil B is in the range of 0.9–4.52, for oil C, the Po is in the range of 0.53–4.03 and for oil D it is in the range 1.16–3.61, corresponding to the bulk temperature from 20 to 95 ◦ C. The values for Po of oils B, C and D show that the oils have higher solvating power in comparison to oil A, A-2 and E. Below the temperature of 40 ◦ C, the values of Po of these oils have slightly increased in the range of 0.34–1.40. But between 60 and 95 ◦ C, the solvating power Po of the oils has noticeably increased in the range of 0.847–4.52. The Heithaus parameter P represents the overall stability of colloidal suspension of oil. It is a measure of the extent to which asphaltenes and oil would remain in a stable state

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Fig. 8 – Heithaus parameter P, versus temperature.

Fig. 9 – Comparison of the Heithaus parameters of the oils A and A-2.

3.6. in colloidal system. Fig. 8 shows that the Heithaus parameter, P, increased with the increase in temperature showing an increase in the stability of oil with the increase in bulk temperature from 20 to 95 ◦ C. The AFT experiment on oil B could not be performed at 20 ◦ C because of high wax content in it. At 20 ◦ C temperature, the sample got solidified and could not be pumped through the circulation loop. Therefore, the trend for oil B starts from 40 ◦ C in Fig. 8. The P value of crude oil A is 5.49 at 95 ◦ C and the values of P of oils A-2, B, C, D and E are 4.58, 4.60, 4.11, 4.04, and 3.99, respectively, at 95 ◦ C. The P value of the oils shows that crude oil A is more stable than the rest of the oils. This is possibly because the crude oil A is lighter than the rest of the crude oils and the maltene part of crude oil is making a stronger solution with asphaltenes. Among all the oils, the oil E has lowest overall profile (2.59–3.99) of the P value; this may be because of the highest concentration of asphaltenes in the oil E, making it less stable oil. It is known that at lower stability of oil, the propensity of asphaltenes to precipitate is higher, which can lead to disposition on heat transfer surfaces. At higher temperature, asphaltenes are also reported to behave like liquid and strongly associate with resins which keep them dissolved in oil (Akbarzadeh et al., 2005; Ganeeva et al., 2014; Hirschberg et al., 1984; Mansoori, 1997; Pan and Firoozabadi, 2000; Sirota and Lin, 2007). The intermolecular associations between the oil, the resins and the asphaltenes become stronger at higher temperature hence the overall stability of oil is higher.

3.5.

Comparison of the oils A and A-2 results

As mentioned earlier, the crude oil A was subjected to fouling experiments in a custom made Annular Flow Fouling Research Unit (AFFRU). In the AFFRU, the oil was circulated at a temperature of 120 ◦ C for several h. The AFT results of the post-AFFRU oil, termed as A-2, were compared with neat oil A as shown in Fig. 9. As show in the figure, the asphaltenes peptizability parameter Pa was higher for A-2 than that of A, which indicates that instability of asphaltenes in crude oil A-2 increased after the fouling experiments in AFFRU. The Heithaus parameters P and Po for crude oil A were comparatively higher than those for A-2. The relatively lower values of the Heithaus parameters for crude oil A-2 also indicate that oil A-2 has become slightly unstable after the heat treatment in the AFFRU experiment.

Hildebrand solubility parameter

Regular solution theory has been widely used to interpret asphaltenes precipitation and solubility data. Hildebrand’s solubility parameter can be interpreted as the relative solvency behavior of a particular solvent. The theory states that the maximum solubility is experienced when cohesive energy densities of the solute and solvent are identical (Hildebrand and Scott, 1962). The solubility parameter is derived from the cohesive energy density of a material and calculated as:

ı=

 E 1/2

(7)

V

where, ı is the solubility parameter, E is cohesive energy density of material and V is molar volume of the material. The conventional unit of the Hildebrand’s solubility parameter is (calories/mL)1/2 and can be converted to MPa1/2 by multiplying with 2.0455. The solubility parameter of the mixture of mutually soluble liquids is proportional to the volume fraction and solubility parameter of each liquid in the mixture that can be determined as: ım = o ıo + s ıs + T ıT



i = 1

(8)

where, ım is the solubility parameter of the oil–solvent–titrant mixture at the onset of flocculation in the mixture, o , s and T are the volume fractions of oil, solvent and titrant, respectively, and ıo , ıs , and ıT are the solubility parameters of oil, solvent and titrant, respectively. The value of solubility parameters of toluene is 18.2 MPa0.5 and isooctane is 14.3 MPa0.5 which are obtained from the literature. Solubility parameter of oil and asphaltenes are determined by estimating the solubility parameter of the solvent–titrant mixture. Seifried et al. (2013) estimated the solubility parameter of oil–solvent–titrant mixture using Confocal Laser Scanning Microscopy. They stated that asphaltenes aggregation and precipitation in crude oil can change solvent properties of the oil and to evaluate these changes the Hildebrand solubility parameter ı can be used. They concluded that aggregation is faster at lower Hildebrand solubility parameter. To investigate asphaltene precipitation, Arciniegas and Babadagli (2014), used Hildebrand solubility parameter measured by refractive index method to explain asphaltenes flocculation data, and the data show that asphaltenes precipitation was significantly lower at 120 ◦ C relative to 40 ◦ C.

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3.7. Solubility parameter of the solvent–titrant mixture The solubility parameter of solvent–titrant mixture is needed to be determined in order to obtain the solubility parameter of oil. Asphaltenes precipitation initiates at a certain dilution ratio of the solvent–titrant mixture. The value of solubility parameter of the solvent–titrant mixture at the flocculation onset point at the dilution ratio is often referred to as critical solubility parameter. The critical solubility parameter (ıF ) of the solvent–titrant mixture can be obtained at the infinite dilution conditions when o ≈ 0. Eq. (8) becomes ıF = s ıs + T ıT



i = 1

(9)

Fig. 10 – Solubility parameter of solvent–titrant mixtures.

Under the infinite dilution conditions, s becomes equal to (Vs /(Vs + VT )) which in-turn is defined as FR in Eq. (4). Therefore, Eq. (9) with FR = FRmax can be written as:

The solubility parameters the oils, asphaltenes and solvent–titrant mixtures are presented in Table 2.

ıF = FRmax ıs + (1 − FRmax )ıT

3.8.

(10)

The average critical solubility parameter of solvent–titrant mixture in each oil solution is given in Fig. 10. The graph shows that for all the 6 oil samples the critical solubility parameter of solvent–titrant mixtures increased with the increase in temperature. The increase in the solubility parameter of solvent–titrant mixture corresponds to increased solubility of asphaltenes and oil in the solution.

Effect of temperature on solubility parameter of oil

Solubility parameter of oil (ıo ) can be calculated after obtaining the critical solubility parameter of the solvent–titrant mixture at flocculation onset point, by rearranging Eq. (8) replacing ım with ıF as: ıo =

ıF − s ıs − T ıT o

(11)

Table 2 – Solubility parameter of solvent–titrant mixture, oil and asphaltenes. Temperature ◦C

Solubility parameter of oil ıo MPa0.5

Critical solubility parameter ıF MPa0.5

Solubility parameter of asphaltenes ıas MPa0.5

A

20 40 60 80 95

16.17 16.25 18.51 21.31 30.38

14.91 14.93 15.19 15.72 17.23

18.91 18.93 19.19 19.72 21.23

A2

20 40 60 80 95

15.63 18.16 18.36 18.8 23.43

15.18 15.52 15.56 15.64 16.29

19.18 19.52 19.56 19.64 20.29

B

40 60 80 95

17.86 20.74 27.19 32.25

15.57 16.04 17.2 18.2

19.57 20.04 21.2 22.2

C

20 40 60 80 95

16.41 18.64 19.48 26.19 30.3

15.42 15.75 15.88 17.26 18.19

19.42 19.75 19.88 21.26 22.19

D

20 40 60 80 95

18.92 19.84 21.62 25.65 28.62

15.86 16.06 16.39 17.27 17.84

19.86 20.06 20.39 21.27 21.84

E

20 40 60 80 95

16.84 17.07 17.66 18.63 19.72

15.28 15.3 15.38 15.51 15.66

19.28 19.3 19.38 19.51 19.66

Oil

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Fig. 12 – Solubility parameter of asphaltenes versus temperature.

Fig. 11 – Solubility parameter of oil versus temperature.

Fig. 11 shows that the solubility parameter of oil increased with the increase in temperature, confirming an increase in solvency characteristics of oil at higher temperature. A dominance of solubility parameter of dispersion phase in oil–solvent–titrant mixture at higher temperature is recognized from the data as shown in Table 2. The higher values of solubility parameters of the oils at higher temperature show that asphaltenes solubility is dominated by increase in solubility parameter of oil which is, thus, increase in solvency tendency of oil due to increase in temperature. Aguiar et al. (2013) reported solubility parameters of two oils to be in the range of 17.5–24.6 MPa0.5 . Solubility parameters were also reported by Arciniegas and Babadagli (2014) as 19.85 MPa0.5 and 19.607 MPa0.5 at ambient temperature for two different oils. The results obtained in this study for solubility parameters of the oils at 40 ◦ C in the range of 16.25–19.85 MPa0.5 are in agreement with the values reported in literature.

3.9. Effect of temperature on asphaltenes solubility parameter Solubility parameter of asphaltenes (ıas ) can be calculated once the critical solubility parameter of the solvent–titrant mixture, ıF , is known. Empirically, it is estimated as (Andersen, 1999; Laux, 1992): ıas = ıF + 4 MPa1/2

(12)

The increasing trend of asphaltenes solubility parameter as a function of temperature can be seen in Fig. 12. It was observed that the solubility parameters of oil and asphaltenes increase with the increase in temperature. The difference between solubility parameters of oil and asphaltenes is smaller at lower

temperatures and observed to be increasing with temperature. According to the Hildebrand’s regular solution theory, solubility of one component in a mixture affects the solubility of other components. From the solubility parameters data obtained from the AFT experiments, it can be concluded that asphaltenes which are present in colloidal form in oil at lower temperature, tend to dissolve in oil with the increase in temperature due to increase in the solubility parameter of both asphaltenes and oil. The higher solubility parameter values of the oils at 95 ◦ C indicate that asphaltenes solubility is favored by the solubility parameter of oil. An identical trend among solubility parameters of asphaltenes, oil and solvent–titrant mixture has been observed. A fairly linear trend up to 60 ◦ C is observed but a significant rise in the solubility parameters has been seen at temperatures above 60 ◦ C, it is presumed to occur because of a possible phase transition of asphaltenes above 60 ◦ C. This observation is in agreement with Mehranfar et al. (2014), who demonstrated that asphaltenes transform from being purely amorphous phase at temperature below 70 ◦ C to a liquid crystalline phase above 70 ◦ C. It is also observed that average asphaltenes solubility parameter has largely remained in between 18.5 and 22.5 MPa0.5 . A comparison of the values of solubility parameters of asphaltenes reported in the literature and average values obtained in this study are presented in Table 3. An average of the solubility parameter values obtained at each temperature was taken to compare with single values of other studies because the solubility parameter data of asphaltenes with respect to change in temperature are not mentioned in the cited literature. The values of asphaltenes solubility parameter obtained from the AFT experiments are fairly in agreement with data of solubility parameter of asphaltenes reported in the literature, as shown in Table 3. But the data by Hirschberg et al. (1984), show a decreasing trend of asphaltenes solubility parameter

Table 3 – Comparison of asphaltenes solubility parameters. This study

(Laux et al., 1997)

(Hirschberg et al., 1984)

(Aguiar et al., 2013)

Oil

Avg. ıas (MPa0.5 )

Oil

ıas (MPa0.5 )

Oil

ıas (MPa0.5 )

Oil

ıas (MPa0.5 )

A A-2 B C D E

19.59 19.63 20.75 20.49 20.68 19.42

Arabian Heavy Black Minnel Laguna Maya

21.65 20.75 21.17 21.13

Tank oil

19.5

A B

19.1–23.8 17.9–23.8

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with increase in temperature from 20 to 60 ◦ C. Aguiar et al. (2013), calculated the solubility parameters of asphaltenes at ambient temperature, therefore, the data could not be compared for effect of temperature on the solubility parameter.

4.

Conclusion

In this research work, the effect of bulk temperature on solubility of asphaltenes in oil was investigated by determining the flocculation onset point for 6 different crude oil samples using automated flocculation titrimeter. The results were interpreted based on the classical Heithaus titration parameters, P, Pa and Po and using Hildebrand solubility parameter. A significant change in all the AFT parameters of all the crude oil samples was observed above 60 ◦ C. It shows that significant change in solubility and phase transition of asphaltenes takes place above the threshold temperature of 60 ◦ C. The peptizability of asphaltenes, Pa , had a decreasing trend while the peptizing power, Po , of maltenes (oil) increased with the increase in temperature. A possible phase transition causing asphaltenes to dissolve in oil at higher temperature has been observed. The value of the Heithaus parameter, P, indicates that overall stability of oil increased with the increase in temperature. The results obtained for Hildebrand solubility parameters of oil and asphaltenes using regular solution theory also show that solubility of asphaltenes increased with the temperature.

Acknowledgments This research is funded by Yayasan-UTP Petroleum Research Fund (PRF) through project No. 015-3AA-A25. The authors express their thanks for the financial support.

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