Alkane Solutions

3960

Ind. Eng. Chem. Res. 1997, 36, 3960-3967

Kinetic Study of Asphaltene Dissolution in Amphiphile/Alkane Solutions Pornruedee Permsukarome,† Chialu Chang,‡,§ and H. Scott Fogler*,‡ Petrochemical College, Chulalongkorn University, Bangkok, Thailand, and Department of Chemical Engineering, University of Michigan, 3168 H. H. Dow, 2300 Hayward Street, Ann Arbor, Michigan 48109-2136

The kinetics of dissolution of pentane-insoluble solid asphaltene precipitates by amphiphile/ alkane solutions were investigated using a differential reactor flow system. Two amphiphiles, dodecylbenzenesulfonic acid and nonylphenol, and five alkane solvents, ranging from hexane to hexadecane, were used. Results showed that the rate of asphaltene dissolution in amphiphile/ alkane fluids could be approximated with a first-order kinetics with respect to the undissolved asphaltene mass in solution. The specific dissolution rate constant, k, varied with the concentration of amphiphiles, the type of alkane solvents, the temperature, and the fluid flow rate. The rate of asphaltene dissolution displayed a Langmuir-Hinshelwood kinetics with respect to the concentration of amphiphiles. Increasing the temperature of amphiphile/alkane fluids also enhanced the rate of asphaltene dissolution. The apparent activation energy for asphaltene dissolution was approximated to be 4-7 kcal/mol. The rate of asphaltene dissolution was also greater in amphiphile solutions containing lighter alkanes, such as hexane, with lower viscosities. These trends suggest that both surface reaction and mass transfer processes are important to the rate of asphaltene dissolution in amphiphile/alkane fluids. Introduction Asphaltenes are usually referred to as the most polar part of the heavy end of crude oil. Asphaltenes are comprised of natural organic species with a virtually continuous variation in chemical structure and the associated physical properties (Speight, 1991). Owing to the difficulty of isolating and characterizing asphaltenes individually, they are often defined as the fraction of crude oil that is insoluble in light normal alkanes (e.g., n-pentane) but soluble in aromatic solvents (e.g., toluene) (Mitchell and Speight, 1973; Burger and Li, 1981). Under this operational definition, asphaltenes have been identified as poly-aromatic (and polyene) condensed rings with short aliphatic chains and polar hetero-atoms, such as nitrogen, oxygen, sulfur and metals (e.g., nickel and vanadium). The poly-aromatic rings and hetero-atoms of asphaltenes tend to associate together through electron donor-acceptor and hydrogenbonding interactions as to bring about the formation of asphaltene micellar particles in crude oil (Yen, 1974; Burger and Li, 1981; Strausz et al., 1992; Sheu and Mullins, 1995). Resin molecules, the lighter neutral polar components in crude oil, usually surround and peptize these asphaltene particles with their polar head groups oriented toward the surfaces of asphaltenes. As a result, asphaltene particles are dispersed in crude oil with an extremely polar core of asphaltene polyaromatics and a slightly polar shell of resins (Pfeiffer and Saal, 1940). Hirschberg et al. (1984) proposed an alternative approach utilizing the Flory theory for polymer solutions and treating the asphaltenes as solvated, monodisperse macromolecular solutes. The resin fraction of crude oil is treated as an undifferentiated part of the solvent * Author to whom correspondence should be addressed. Fax: (313) 763-0459. E-mail: [email protected]. † Chulalongkorn University. ‡ University of Michigan. § Currently at International Paper, Long Meadow Rd., Tuxedo, NY 10987. S0888-5885(97)00177-2 CCC: $14.00

medium. Cimino et al. (1995) have used this approach as the basis for developing a new thermodynamic model. Asphaltene precipitation and deposition can occur during petroleum production, transportation, and refinery. This deposition of asphaltenes causes plugging of transportation pipelines, loss of tank capacity, and malfunction of refining equipment. During petroleum production, asphaltene precipitation is a known cause of formation damage in the near-wellbore regions (Leontaritis et al., 1994). Unstable asphaltenes flocculate, precipitate, and deposit onto the pore space of oil formations as to eventually block the flow of crude oil in the near-wellbore region. The stability of asphaltenes in crude oil is influenced by the composition, temperature, and pressure of crude oil. For example, asphaltenes can be destabilized as the pressure approaches toward the bubble point of crude oil (Hirschberg et al., 1984; Burke et al., 1990). Asphaltene precipitation also occurs during enhanced-oil-recovery flooding with light hydrocarbons and carbon dioxide (CO2) (Kokal et al., 1991) and during matrix acidizing where ferric ions (Fe3+) dissolved from pipes and instruments induce the “polymerization” of asphaltene molecules (Jacobs, 1989). With the increasing trend toward the utilization of heavier crude oil and the use of secondary and tertiary methods in oil recovery, more attention has been directed to the research on asphaltene deposition and removal. Various methods have been used to solve asphaltene problems. For example, chemical treatments of asphaltenes usually involves the use of aromatic-based cleaning fluids, such as toluene and xylene, to dissolve asphaltene precipitates and deposits. To improve the efficiency of asphaltene dissolution, cosolvents such as amine and sulfonic acid compounds have been added to aromatic solvents (Sutton, 1975; Newberry and Barker, 1983). Removal of asphaltenes by mechanical means becomes necessary when deposited asphaltenes are tightly condensed and as a result become difficult to be dissolved by chemical solvents. Aromatic-based liquids have been used as solvents for asphaltene removal in the oilfield. However, they are © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3961

not universally effective; also, due to environmental concerns, there is a need to develop effective nonhazardous substitutes for the chemical treatment of asphaltene precipitates and deposits. Studies on the stabilization of asphaltenes using nonaromatic-based fluids has been undertaken in recent years. Gonzalez and Middea (1991) studied the dispersion of asphaltenes in heptane using different oil soluble amphiphiles and found that alkylphenols were effective in peptizing asphaltenes. This study also showed that the effectiveness of amphiphiles on the stabilization of asphaltene was influenced by the head group of amphiphiles. Chang and Fogler (1993, 1994a,b) used a series of alkylbenzenederived amphiphiles to investigate the stability behavior of asphaltenes in amphiphile/alkane solutions and the asphaltene-amphiphile interactions. It was found that asphaltenes can be completely stabilized in alkanebased solutions containing a sufficient amount of amphiphiles. Alkylbenzenesulfonic acid amphiphiles appeared to be the most effective asphaltene stabilizer in this study. A minimum alkyl chain length was also found to be necessary for an amphiphile to adequately stabilize asphaltenes in solutions; for example, the minimum chain length for alkylphenols is about six carbons. On the basis of these observations, it was concluded that the criteria for amphiphile molecules to effectively stabilize asphaltenes in apolar media were the association of amphiphiles to asphaltene surfaces by the head groups of the amphiphiles and the establishment of a steric layer around the asphaltenes by the tail group of amphiphiles. In a later study, de Boer and co-workers (1995) also found that alkylbenzenesulfonic acid with a sufficiently long alkyl tail could effectively reduce the precipitation of asphaltenes in the nearwellbore regions. In this study, the kinetics of dissolution of asphaltene precipitates with amphiphile/alkane solutions were investigated using a differential reactor. To identify the factors affecting the rates of asphaltene dissolution, the following variables were included in this study: namely, the type and concentration of amphiphiles, the type of solvents, the fluid temperature, and the flow rates. From the results of this study, a simple kinetic model was proposed to describe the process of asphaltene dissolution. Experimental Section Sample Preparation. The asphaltene sample used in this study was a pentane-insoluble fraction of crude oil provided by Mobil Research and Development. This asphaltene sample was prepared from crude oil according to a modified procedure described in ASTM 2007D (1983). In brief, 1 volume of crude oil was first mixed with 10 volumes of warm pentane solvent for approximately 1 h to precipitate asphaltenes out of solution. Afterward, asphaltene precipitates were collected from solution by passing the solution media through a fretted glass filter with Whatman NO1 filter papers and were dried at 60 °C. This precipitated Mobil asphaltene sample appeared as fine powders with a dark brown to black color. The solubility of this asphaltene sample was found to be 8.4 (cal/cm3)0.5 in the scale of Hildebrand solubility parameters. The elemental composition and chemical characterization of this asphaltene sample was also reported previously (Chang and Fogler, 1993, 1994a,b). Two alkylbenzene-derived amphiphiles, dodecylbenzenesulfonic acid (DBSA, n-C12H25-C6H4-SO3H) and

Figure 1. (a) Schematic illustration of the experimental setup used in this study. (b) Enlarged view of a differential reactor. Table 1. List of the Experimental Conditions Conducted in This Study variable evaluated

amphiphiles

concentration NP solvent

flow rate temperature

solvents C7, C12

amphiphile temperflow concentration ature rate (wt %) (°C) (mL/min) 2, 4, 7, 10, 20, 22 30 1, 3, 5, 10, 20 22

DBSA C7, C12 NP C6, C7, C10, C12, C16 DBSA C6, C7, C10, C12, C16 NP C7 20 NP C12 20 DBSA C12 5

22 6, 22, 58 6, 22, 58

1 1

0.1, 1 1 1

nonylphenol (NP, n-C9H19-C6H4-OH), and five alkane solvents, ranging from hexane to hexadecane, were used in this study. These two oil-soluble amphiphiles have been proven to be effective in the stabilization of asphaltenes in alkane media (Chang and Fogler, 1993, 1994a,b). All chemicals are commercially available and used directly for study without further purification. Experimental Procedure. To assess the rate of asphaltene dissolution, a differential reactor, shown in Figure 1, was used to carry out the experiments that are listed in Table 1. A syringe pump was used to inject the amphiphile/alkane micellar solutions (usually at 1 mL/min) through the differential reactor to dissolve asphaltene deposits. A water bath was used to maintain a controlled temperature, ranging from 6 to 58 °C, for the flow system. In each experiment, 0.025 g of asphaltene powders was first loosely and uniformly placed in the reactor. The differential reactor was modified from a filter holder with a diameter of 25 mm. The edges of the reactor were made of an O-ring while both of the front and rear faces were made of a pair of 0.45 µm Teflon filter membranes. These inert filter membranes were permeable to the amphiphile/alkane fluid and the dissolved asphaltenes, but impermeable to the undissolved asphaltene powder. Consequently, when the amphiphile/alkane fluid was injected upward, only the dissolved asphaltenes were carried with the fluid out of the reactor and collected by glass vials. This upward flow ensured that air trapped in the reactor could be completely displaced from the reactor by the fluid at the initial stage of the experiment. Afterward,

3962 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997

Figure 2. Profile of asphaltene dissolution by dodecane-based fluids containing different concentrations of NP amphiphile.

the concentration of asphaltenes in the effluent collected at different elution times was measured by the absorbance of effluents at wavelength 400 nm using a UV/ vis spectrophotometer. Kinetics Analysis. In order to further analyze the kinetics and mechanism of asphaltene dissolution, we assumed that the rate of dissolution of asphaltene precipitates, rD, was first order with respect to the undissolved asphaltene mass, i.e.,

-rD )

dM ) -kM dt

(1)

Integrating eq 1 gives

ln

M ) -kt M0

(2)

where k is the apparent specific rate constant for asphaltene dissolution (min-1). M0 and M are the mass of asphaltene precipitates initially placed and that remaining undissolved at time t, respectively. Results Effect of the Type and Concentration of Amphiphiles. The influence of the type and concentration of amphiphiles on the rate of dissolution of solid asphaltene precipitates in alkane-based fluids was studied using two types of amphiphiles, dodecylbenzenesulfonic acid (DBSA) and nonylphenol (NP), and two types of alkane media, heptane and dodecane. Figure 2 shows the time-evolution profile of asphaltene dissolution in dodecane at different NP concentrations. It is clear that the rate of asphaltene dissolution in general increases with an increase in the NP concentration. However, the trend of asphaltene dissolution with respect to the NP concentration appears quite different at the concentrations of NP above and below 7 wt %. When the NP concentration is less than 7 wt %, asphaltene precipitates are not completely dissolved. The percentage of asphaltene dissolved in the first 50 mL of injected fluid increases gradually from 15%, to 30%, and then to 80% as the NP concentration increases from 0 wt %, to 2 wt %, and then to 4 wt %. This result indicates that the asphaltene sample used in this study has a distribution of solubilities, and the less soluble asphaltenes with higher degrees of polarity can only be dissolved in the fluid with higher NP concentrations. When the NP concentration is greater than 7 wt %,

Figure 3. Profile of asphaltene dissolution by dodecane-based fluids containing different concentrations of DBSA amphiphile.

asphaltene precipitates can be dissolved completely in the fluid. In addition, the rate of asphaltene dissolution increases only slightly with increasing the NP concentration, indicating that the concentrated NP solution does not expedite asphaltene dissolution significantly. Figure 3 shows the time-evolution profile of asphaltene dissolution in dodecane as a function of the concentration of dodecylbenzenesulfonic acid (DBSA). Similar to the results of the NP amphiphile shown in Figure 2, the rate of asphaltene dissolution is faster at higher DBSA concentrations. When the DBSA concentration is less than 3 wt %, asphaltene precipitates can only be partially dissolved. For example, the percentage of asphaltenes dissolved in the first 50 mL of the injected fluid increases gradually from 15% to 33% as the concentration of DBSA increases from 0 wt % to 1 wt %. Above 3 wt % of DBSA, asphaltenes are dissolved completely and the rate of asphaltene dissolution increases slightly with an increase in the DBSA concentration. Comparing both Figures 2 and 3, one observes that both amphiphiles effectively dissolve asphaltene precipitates in dodecane solutions. However, the minimum concentrations of NP and DBSA for completely dissolving asphaltene deposits are 7 wt % and 3 wt %, respectively. These values are consistent with the stability of asphaltenes in the amphiphile/alkane solutions reported previously by Chang and Fogler (1994a,b). The NP-asphaltene association is significantly weaker than the DBSA-asphaltene association. Therefore, a higher concentration of NP amphiphiles is needed to provide a sufficient attractive interaction with asphaltene molecules to sterically stabilize them in the solution. Figures 2 and 3 also show that when the concentration of amphiphiles is higher than the minimum value for completely dissolving asphaltenes, the increment in the rate of asphaltene dissolution with an increase in the amphiphile concentration appears more pronounced with DBSA solutions than with NP solutions. The data in Figures 2 and 3 were replotted in Figures 4 and 5 in terms of the logarithm of undissolved asphaltene mass fraction [ln(M/M0)] versus time (see eq 2). As shown in these figures, for most of dissolution curves, [ln(M/M0)] decreases essentially linearly with time in the early reaction period, indicating the adequacy of the first-reaction-order assumption. However, for low amphiphile concentrations in which asphaltenes are not completely dissolvable, [ln(M/M0)] deviates from the linear decay trend as the reaction time is increased.


Figure 4. Kinetic analysis of asphaltene dissolution in NP/ dodecane solutions containing different concentrations of NP amphiphiles.

Figure 7. Specific dissolution rate constant, k, as a function of the DBSA concentration in solutions. Table 2. List of Ks and k∞ Values

Figure 5. Kinetic analysis of asphaltene dissolution in DBSA/ dodecane solutions containing different concentrations of DBSA amphiphiles.

amphiphile/ alkane soln

k∞ (mol of amphiphile/dm3)

Ks (min-1)

NP/heptane NP/dodecane DBSA/heptane DBSA/dodecane

0.263 0.296 0.110 0.037

0.25 0.11 0.12 0.11

off at about 0.11 min-1 when the DBSA concentration in heptane and dodecane reaches 0.06 and 0.1 M, respectively. Figure 7 shows that the k value for both NP/heptane and NP/dodecane solutions also increases with an increase in the NP concentration. Above 0.3 M of NP, the k value becomes virtually constant at 0.25 and 0.1 min-1 for heptane and dodecane, respectively. Overall, the k value of each amphiphile solution increases steadily at lower amphiphile concentrations and levels out at higher amphiphile concentrations. This trend suggests that the variation of k with respect to the amphiphile concentration can be described by Langmuir-Hinshelwood kinetics (Fogler, 1992):

k)

Figure 6. Specific dissolution rate constant, k, as a function of the NP concentration in solutions.

These deviations could be attributed to the fact that the undissolved asphaltene fraction is taken into account in eq 2, leading to positive deviations in the dissolution data from the linear curve. The specific dissolution rate constant, k, calculated from the slope of linear fitting to the initial [ln(M/M0)] versus time data points, was plotted in Figures 6 and 7. Figure 6 shows that for both DBSA/heptane and DBSA/dodecane solutions, the k value increases significantly at low DBSA concentrations and gradually levels

k∞[S] KS + [S]

(3)

where [S] is the concentration of amphiphile in solution. This Langmuir-Hinshelwood equation shows that k is first and zero order with respect to [S] at low and high concentrations, respectively. k∞ and KS are the adjustable parameters in eq 3. For data points of Figures 6 and 7, the fitted values of k∞ and KS are listed in Table 2. Effect of Temperature. The effect of temperature on the rate of asphaltene dissolution was carried out using amphiphile/alkane solutions which contain sufficient amphiphiles to dissolve asphaltene completely. The profiles of asphaltene dissolution with 5 wt % DBSA/dodecane fluids at a flow rate of 1 mL/min are illustrated in Figure 8. It is clear that the rate of asphaltene dissolution increases significantly with an increase in fluid temperature. For example, when temperature increases from 6 °C, to 22 °C, and then to 58 °C, the volume of the injected fluid required to dissolve 80% asphaltene deposits decreases form 40 mL, to 16 mL, and then to 8 mL. The data in Figure 8 are replotted in Figure 9 using the parameter [ln(M/M0)]. One observes that [ln(M/M0)] decreases rapidly with the reaction time as temperature is increased. The k∞ value obtained from the slope of linear fitting to the initial


Figure 8. Profile of asphaltene dissolution at different temperatures by the dodecane-based fluid containing 5 wt % of DBSA amphiphile.

Figure 9. Kinetic analysis of asphaltene dissolution in 5% DBSA/ dodecane solutions at different temperatures.

data points was then plotted according to the Arrhenius equation as shown below:

( )

k∞ ) A exp -

EA RT

(4)

where A is the pre-exponential constant, R is 1.987 cal/ mol K, the ideal gas constant, and EA is the activation energy. Figure 10 shows the logarithm of k∞ as a function of 1/T (K). One observes that the experimental data was linearly fitted reasonably well, suggesting that the dependence of the k∞ value on temperature could be represented by an Arrhenius kinetics. The activation energy, EA, calculated from the slope of the linear fitting was approximated to be 3.99, 6.92, 3.98, and 3.81 kcal/ mol for the dissolution of asphaltene by NP/heptane, NP/dodecane, DBSA/heptane, and DBSA/dodecane solutions, respectively. These measured activation energies are fairly close to the typical values of intermolecular hydrogen bonding (in the range of 3-7 kcal/mol) and charge transfer associations (generally less than 8 kcal/ mol) that may occur between asphaltene molecules or between asphaltenes and polar molecules, e.g., phenol (Pimental and McClellan, 1960; Slifken, 1971; Barbour and Peterson, 1974). Hence, it is inferred that the rate of asphaltene dissolution is significantly affected by reactions on the surface of asphaltene precipitates which involve the transition from asphaltene-asphaltene as-

Figure 10. Specific dissolution rate constant, k, as a function of the solution temperature.

Figure 11. Time evolution profile of asphaltene dissolution in 5% DBSA amphiphile solutions containing different types of alkane solvents.

sociations to asphaltene-amphiphile associations through the redistribution of intermolecular hydrogen bonding and charge transfer interactions. At higher temperatures, asphaltenes and amphiphiles not only possess higher kinetic energies to overcome the reaction activation energy barrier but also have larger diffusivities to undergo higher molecular collisions to bring about asphaltene dissolution. In addition, the mass transfer of asphaltenes and amphiphiles between the precipitated asphaltene surface and the contact fluid may also be a cause of the temperature-dependent asphaltene dissolution rate. This mass transfer process usually has an apparent activation energy of 2-3 kcal/mol due to the temperature-dependent viscosity of solvent and diffusivity of reacting species (Levich, 1962). Effect of Alkane Solvent. The effect of the alkane solvent of amphiphile solutions on the rate of asphaltene dissolution was investigated using a homologue of alkanes, from hexane (C6) to hexadecane (C16). The profile of asphaltene dissolution by 5 wt % of DBSA alkane solutions is shown in Figure 11. These dissolution profiles were replotted in Figure 12 using eq 2. The values of k∞ calculated from the rate of asphaltene dissolution were then plotted in logarithm as a function of the viscosity (µ) of alkane solvents. As shown in Figure 13, for both NP and DBSA amphiphiles, the rate of asphaltene dissolution decreases significantly with an increase in the chain length of alkane solvent molecules. From the slope of the plot, we found that k∞ ≈ µ-0.5 where µ is the viscosity of the alkane solvent.


Figure 12. Kinetic analysis of asphaltene dissolution in 5% DBSA amphiphile solutions containing different types of alkane solvents.

Figure 14. Profile of asphaltene dissolution by the heptane-based fluid containing 20 wt % of NP amphiphile at different flow rates. The dissolution profile is plotted as a function of both the time of dissolution (up) and the accumulated volume of effluent (bottom).

Figure 15. Schematic illustration of dissolution of asphaltene deposits by amphiphile/alkane fluids.

to be considered in designing and evaluating new chemical treatments for alleviating damages resulting from asphaltene precipitation and deposition in oil production reservoirs and pipelines. Discussion of First-Order Model Figure 13. Specific dissolution rate constant, k, as a function of the viscosity of alkane solvents used in amphiphile solutions.

Therefore, the rapid rate of asphaltene dissolution occurred in light alkane solvents, such as hexane, can be attributed to the low solvent viscosity compared to that of heavy alkanes, such as dodecane and hexadecane. In the less viscous light alkanes, asphaltenes and amphiphiles have larger diffusivities to undergo faster mass transfer and intermolecular collisions to facilitate the process of asphaltene dissolution. Effect of Flow Rate of Fluids. The effect of the flow rate of amphiphile/alkane fluids on the rate of asphaltene dissolution was investigated under two different flow rates, 0.1 and 1 mL/min, for a 20 wt % of NP/heptane-based fluid. As shown in Figure 14, the rate of asphaltene dissolution increases slightly with an increase in the flow rate of fluids. This result further suggests that the convective mass transfer of species between bulk fluids and the surface of asphaltene deposits is also a factor in the rate of asphaltene dissolution. Increasing the flow rate reduces the thickness of the fluid boundary layer around the asphaltene precipitates and therefore enhances the rate of mass transfer between fluids and asphaltene surfaces. In addition, higher flow rates may enhance the movement of fluids within porous asphaltene deposits and expedite the mobilization of asphaltene deposits. Overall, the results of this study demonstrate that the rate of dissolution of asphaltene precipitates and deposits can be affected significantly by the type and concentration of amphiphiles, the type of solvents, fluid temperature, and flow rate. All of these variables have

The dissolution of solid asphaltene precipitates and deposits by amphiphile/alkane fluids involves both mass transfer and surface reaction processes. As shown in Figure 15, the dissolution of asphaltenes involves the following steps (Fogler, 1992). For asphaltene dissolution, the amphiphile is first transported from the bulk liquid to the asphaltene surface:

Sbk f Ssf

(5)

where Ssf and Sbk are amphiphiles at the asphaltene surface and in the bulk fluid, respectively. Subsequently, the amphiphile near asphaltenes adsorbs to the functional site on asphaltene surfaces: kA

} A‚Ssf A + Ssf {\ k

(6)

-A

where A represents the functional site on the asphaltene surface to which the amphiphile can adsorb. A‚Ssf denotes the amphiphile molecule adsorbed to the surface. After being adsorbed, the amphiphile may cause the formation of the amphiphile-asphaltene transition state complex, (AS)* sf, as shown below: kr

A‚Ssf 98 (AS)* sf

(7)

Afterward, this asphaltene-amphiphile complex desorbs from the asphaltene surface

(AS)*sf a (AS)sf and is transported to the bulk liquid

(8)


(AS)sf f (AS)bk

(9)

where (AS)sf and (AS)bk are the asphaltene-amphiphile associated complex at the asphaltene surface and in the bulk fluid, respectively. The fact that the rate of asphaltene dissolution follows a Langmuir-Hinshelwood kinetics as a function of amphiphile concentrations (as shown in Figures 6 and 7 and eq 3) indicates that the surface reaction step (eq 7) may be assumed to be rate-limiting with respect to the adsorption and desorption steps (eqs 6 and 8). Hence,

[A‚Ssf] ) KA[A][Ssf]

(10)

with KA ) kA/k-A. For the first-order model, we have assumed the total number of functional sites on asphaltene surfaces is proportional to the mass of undissolved asphaltenes and is the sum of the bound and unbound sites. That is,

M ) [A] + [(AS)sf] ) [A] + KA[A][Ssf] [A] )

M 1 + KA[Ssf]

(11) (12)

Then, the rate of asphaltene dissolution becomes

-

dM ) kr[A‚Ssf] ) krKA[A][Ssf] ) dt M[Ssf] ) kM (13) krKA 1 + KA[Ssf]

which is a Langmuir-Hinshelwood form as that observed experimentally in Figures 6 and 7 and eq 3. This Langmuir-Hinshelwood model indicates that the extent of amphiphile adsorption to asphaltene surfaces dictates the rate of asphaltenes in amphiphile/alkane solutions. At low amphiphile concentrations, the amount of amphiphiles adsorbing to asphaltenes rate increases linearly with an increase in the concentration of amphiphiles; hence, the rate of asphaltene dissolution increases correspondingly. At sufficiently high amphiphile concentrations, most functional groups of hydrogen bonding or charge transfer interactions on asphaltene surfaces are already bound with amphiphiles; therefore, the rate of asphaltene dissolution cannot be further increased with a further increase in the amphiphile concentration. On the other hand, the fact that the asphaltene dissolution rate is influenced by the flow rate and the viscosity of alkane solvents implies that the convective mass transfer of reacting species (asphaltenes and amphiphiles) between bulk fluids and the surface of the asphaltene solid (eqs 5 and 9) may also be important to the rate of asphaltene dissolution. The rate of mass transfer for amphiphiles can be described as

-

dM ) kc([Sbk] - [Ssf]) dt

(14)

and that for dissolved asphaltenes is

-

dM ) kc([(AS)sf] - [(AS)bk]) dt

(15)

where kc is the mass transfer coefficient for amphiphiles and asphaltenes between solid asphaltene surfaces and the bulk fluid. For a spherical solid asphaltene pre-

cipitate, kc can be approximated as

(Ds)2/3(U)1/2 Ds kc ) 2 + 0.6 1/6 dA (ν) (dA)1/2

where ν )

µ (16) F

µ is the viscosity of fluid, F is the density of fluid, Ds is the diffusivity of reacting species, U is fluid velocity, and dA is the size of asphaltene precipitates. Equation 16 shows that the rate of asphaltene dissolution increases with an increase in the velocity of fluid, U, and the diffusivity of reacting species, Ds, but decreases with an increase in the solvent viscosity, µ. The higher rates of asphaltene dissolution at higher temperatures result from the enhancement of mass transfer processes. At higher temperatures, the viscosity of solution is decreased and the diffusivity of amphiphiles and asphaltenes is increased, leading to a higher rate of transport of amphiphiles toward the asphaltene precipitates and a higher rate of transport of the dissolved asphaltenes toward the bulk fluid. In summary, the results of this experimental study reveal that pentane-insoluble asphaltene precipitates are comprised of asphaltene molecules aggregating together through fairly weak hydrogen bonding or charge transfer associations. As a consequence, dissolution of asphaltenes occurs under a LangmuirHinshelwood surface reaction and a mass transfer process. Summary This study demonstrated that pentane-insoluble solid asphaltene precipitates could be dissolved by alkanebased fluids containing two alkylbenzene-derived amphiphiles, dodecylbenzenesulfonic acid, and nonylphenol. The factors influencing the rate of asphaltene dissolution by amphiphile/alkane fluids are summarized as follows: (1) Effect of type and concentration of amphiphiles: The rate of asphaltene dissolution appears to follow Langmuir-Hinshelwood kinetics with respect to the concentration of amphiphiles; that is, the dissolution rate increases steadily at low amphiphile concentrations and reaches a plateau at higher amphiphile concentrations where asphaltenes are completely dissolved. (2) Effect of temperature: The rate of asphaltene dissolution increases with an increase in the fluid temperature and follows the Arrhenius temperature dependence, yielding an activation energy of approximately 4-7 kcal/mol. On the basis of this result, it is deduced that asphaltene dissolution is rate-controlled by the reactions involving the transition from asphaltene-asphaltene associations to asphaltene-amphiphiles associations through the redistribution of intermolecular hydrogen bonding and charger transfer interactions as well as by the mass transfer of reacting species between the precipitated asphaltene surface and bulk fluids. (3) Effect of type of solvent: The rate of asphaltene dissolution becomes higher in lighter alkane solvents, perhaps due to the faster intermolecular collisions between asphaltenes and amphiphiles. (4) Effect of flow rate: Higher flow rates of amphiphile/alkane fluids can also increase the rate of asphaltene dissolution by enhancing the penetration of fluids into porous asphaltene precipitates as well as reducing the boundary layer that control the mass transfer of reacting species between asphaltenes and fluids.


Acknowledgment This research was supported by the U.S. Department of Energy, under Subcontract BDM-OKL (BDM-Oklahoma Inc.) G4S40807. The financial support from industrial affiliates sponsors, ARCO, Chevron, Conoco, Halliburton Services, Mobil, Schlumberger-Dowell, and Unocal, is also appreciated. Literature Cited ASTM Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1983; Vol. 05.02, p 158. Barbour, R. V.; Petersen, J. C. Molecular Interactions of Asphalt: An Infrared Study of the Hydrogen-Bonding Basicity of Asphalt. Anal. Chem. 1974, 46, 273-277. Burger, J. W.; Li, N. C., Eds. Chemistry of Asphaltenes; Advances in Chemistry Series 195; American Chemical Society: Washington, DC, 1981. Burke, N. E.; Hobbs, R. E.; Kashou, S. F. Measurement and modeling of asphaltene precipitation. J. Pet. Technol. 1990, 42, 1440-1446. Chang, C. L.; Fogler, H. S. Asphaltene stabilization in alkyl solvents using oil-soluble amphiphiles. SPE International Symposium on Oilfield Chemistry, New Orleans, LA, March 1993; Society of Petroleum Engineers: Richardson, TX, 1993; Paper SPE 25185. Chang, C. L.;Fogler, H. S. Stabilization of Asphaltenes in Aliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 1. Effect of the Chemical Structure of Amphiphiles on Asphaltene Stabilization. Langmuir 1994a, 10, 1749-1757. Chang, C. L.; Fogler, H. S. Stabilization of Asphaltenes in Aliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 2. Study of the Asphaltene-Amphiphile Interactions and Structures Using FTIR and SAXS Techniques. Langmuir 1994b, 10, 1758-1766. Cimino, R.; Correra, S.; del Bianco, A.; Lockhart, T. P. Solubility and phase behavior of asphaltenes in hydrocarbon media. In Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995. de Boer, R. B.; Leerlooyer, K.; Eigner, M. R. P.; Van Bergen, A. R. D. Screening of crude oils for asphalt precipitation: theory, practice, and the selection of inhibitors. SPE Prod. Fac. 1995, February, 55-61. Fogler, H. S. Elements of Chemical Reaction Engineering, 2nd ed.; Prentice Hall: Englewood Cliffs, NJ, 1992. Gonzalez, G.; Middea, A. Peptization of asphaltene by various oil soluble amphiphiles. Colloids Surf. 1991, 52, 207-217.

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Received for review February 25, 1997 Revised manuscript received April 29, 1997 Accepted May 2, 1997X IE970177A

X Abstract published in Advance ACS Abstracts, June 15, 1997.

Alkane Solutions

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