Energy & Fuels 2009, 23, 1575–1582
1575
Effect of Asphaltene Dispersants on Aggregate Size Distribution and Growth Kriangkrai Kraiwattanawong,†,‡ H. Scott Fogler,*,† Samir G. Gharfeh,§ Probjot Singh,§ William H. Thomason,§ and Sumaeth Chavadej‡ Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109, Petroleum and Petrochemical College, Chulalongkorn UniVersity, Bangkok 10330, Thailand, and ConocoPhillips Company, BartlesVille, Oklahoma 74004 ReceiVed August 25, 2008. ReVised Manuscript ReceiVed December 1, 2008
A crude oil system can become unstable because of changes in hydrocarbon composition, pressure, or temperature during normal production from the reservoir or during commingling with dissimilar crude oils. These changes can generate asphaltene particles that can result in significant production and refining problems. The generation of these particles is a two-step process: phase separation and asphaltene particle growth. Phase separation occurs when nanosize asphaltene particles from the crude oil precipitate and grow into large aggregates. Our study on asphaltene precipitation shows that large asphaltene particles are aggregates consisting of very small (sub-micrometer) size asphaltene particles. One mechanism to control asphaltenes is to kinetically inhibit the phase separation of asphaltenes by adding a small amount of a chemical that interferes with the phase separation processes. Another mechanism to control asphaltenes is to inhibit growth by stabilizing the colloidal suspension of the sub-micrometer asphaltene particles to significantly slow the flocculation and settling processes. Asphaltene chemical additives of known molecular structures as well as proprietary chemical blends were selected for this study. None of chemicals studied inhibited phase separation; however, some of the dispersants did slow or stop flocculation and growth. Four different analytical techniques have been used to study the effect of chemical additives on asphaltene aggregation and settling and to evaluate the effectiveness of different asphaltene chemicals in keeping asphaltene particles suspended/dispersed in crude oils. From the turbidity measurement, asphaltene dispersants can be classified into three categories based on their performance. The particle size distribution measurement showed three different types of asphaltenes: stable asphaltenes, colloidal asphaltenes, and flocculated asphaltenes, on the basis of aggregate sizes. Asphaltene dispersants can stabilize colloidal asphaltenes and slow the growth and formation of flocculated asphaltenes. These results can be used to select the best chemical treatment plan for preventing/reducing asphaltene settling and deposition.
1. Introduction Asphaltenes are usually classified as the most polar fraction of crude oil that are soluble in aromatic solvents but insoluble in normal alkanes.1 Asphaltenes have been identified as molecules having polyaromatic and polycyclic rings with short aliphatic chains and heteroatoms, such as nitrogen, oxygen, sulfur, and metals (e.g., nickel, vanadium, and iron).1-4 The asphaltene molecule has a diameter between 10 and 20 Å5-7 with the presence of asphaltene nanocolloids or asphaltene nanoaggregates.6-11 Mullins and co-workers12 summarized asphaltene molecular weight measured by different techniques (FIMS, FDMS, ESI FT-ICR MS, APPI MS, APCI MS, FD MS, * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Michigan. ‡ Chulalongkorn University. § ConocoPhillips Company. (1) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1999. (2) Bestougeff, M. A.; Byramgee, R. J. Chemical constitution of asphaltenes. In Asphaltenes and Asphalts; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science: Amsterdam, The Netherlands, 1994; pp 67-94. (3) Yen, T. F. Asphaltenes: Types and sources. In Structure and Dynamic of Asphaltenes; Sheu, E. Y., Ed.; Plenum Press: New York, 1998; pp 120. (4) Wattana, P.; Fogler, H. S.; Yen, A.; Carmen Garcı`a, M. D.; Carbognani, L. Energy Fuels 2005, 19 (1), 101–110.
LDI MS, TRFD, TD, NMR, and FCS) to be about 750 Da ((200 Da) with full width at half-maximum of 500-1000 Da. Furthermore, Kaminski and co-workers13 and Wattana and coworkers4 used methylene chloride and n-pentane to further fractionate asphaltenes into different fractions based on their polarity and solubility. They also showed that different asphaltene fractions contain different amounts of heteroatoms and have different dissolution rates. (5) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103 (50), 11237–11245. (6) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14 (3), 677–684. (7) Mullins, O. C.; Bentancourt, S. S.; Cribbs, M. E.; Dubost, F. X.; Creek, J. L.; Andrews, A. B.; Venkataramanan, L. Energy Fuels 2007, 21 (5), 2785–2794. (8) Groenzin, H.; Mullins, O. C. Asphaltene molecular size and weight by time-resolved fluorescence depolarization, In Asphaltenes, HeaVy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer Science-Business Media: New York, 2007; Chapter 2. (9) Andreatta, G.; Goncalves, C. C.; Buffin, G.; Bostrom, N.; Quintella, C. M.; Arteaga-Larios, F.; Pe´rez, E.; Mullins, O. C. Energy Fuels 2005, 19 (4), 1282–1289. (10) Gawrys, K. L.; Kilpatrick, P. K. J. Colloid Interface Sci. 2005, 288, 325–334. (11) Barre´, L.; Simon, S.; Palermo, T. Langmuir 2008, 24 (8), 3709– 3717. (12) Mullins, O. C.; Martı´nez-Haya, B.; Marshall, A. G. Energy Fuels 2008, 22 (3), 1765–1773. (13) Kaminski, T. J.; Fogler, H. S.; Wolf, N.; Wattana, P.; Mairal, A. Energy Fuels 2000, 14 (1), 25–30.
10.1021/ef800706c CCC: $40.75 2009 American Chemical Society Published on Web 01/21/2009
1576 Energy & Fuels, Vol. 23, 2009
Asphaltene precipitation can occur at any step of crude oil production, transportation, and refinery, where significant changes in hydrocarbon composition, pressure, and temperature take place. For example, asphaltenes can be destabilized when (1) the pressure in the reservoir or any production stage reaches the precipitation onset pressure of the unstable crude oil,14-16 (2) the oil-based drilling mud enters the reservoir and mixes with the crude,17 (3) enhanced oil recovery is carried out with light hydrocarbons (mostly normal alkanes) or carbon dioxide (CO2),18,19 or (4) commingling of dissimilar crude oils occurs, especially with naphtha and condensate liquids.17 During crude oil production, asphaltene precipitation can cause formation damage in near-wellbore regions20 by partially or completely plugging of the pore space. Asphaltene precipitation followed by deposition can plug the subsurface and surface tubing.21 During crude oil transportation, asphaltenes precipitation and deposition can plug transportation pipelines and settle in transportation tankers. In the refineries, asphaltenes can deposit in distillation columns, foul heat exchangers, and also cause catalyst deactivation.22 These problems lead to a significant loss of productivity and a high cost of remediation. To remove asphaltene deposits, aromatic-based solvents (i.e., toluene and xylene) have been used to dissolve asphaltene deposits. However, because of the large quantity of solvent required, this procedure will only be economical in a refinery where solvent recovery is plausible or periodically applied in the production facility. While this study focuses only on asphaltene precipitation, ongoing work in our laboratory on asphaltene deposition will be presented at a later time. Because of the economic concerns about asphaltene precipitation and deposition, chemicals for asphaltenes control (asphaltene inhibitors) have become important in the oil industry. If asphaltene inhibitors can kinetically inhibit or delay the phase separation process of asphaltenes then crude oils can be processed and transferred without asphaltene separation. Even if asphaltene inhibitors do not inhibit or delay the phase separation process of asphaltenes completely, they can act as dispersants to stabilize small asphaltene particles (sub-micrometer size) and keep the particles dispersed in the crude oil. Chang and Fogler23-25 used different alkyl-benzene-derived amphiphiles to investigate the effectiveness of amphiphiles to (14) Kraiwattanawong, K.; Fogler, H. S.; Gharfeh, S. G.; Singh, P.; Thomason, W. H.; Chavadej, S. Energy Fuels 2007, 21 (3), 1248–1255. (15) Hischberg, A.; deJong, L. N. J.; Schipper, B. A.; Meijer, J. G. Influence of temperature and pressure on asphaltene flocculation. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, LA, Sept 1982; SPE 11202. (16) Burke, N. E.; Hobbs, R. E.; Kashou, S. F. J. Pet. Technol. 1990, 42, 1440–1446. (17) Gharfeh, S.; Singh, P.; Kraiwattanawong, K.; Blumer, D. A general study of asphaltene flocculation prediction at field conditions. Presented at the Offshore Technology Conference, Houston, TX, May 2006; SPE 112782. (18) Novosad, Z.; Costain, T. G. Experimental and modeling studies of asphaltene equilibria for a reservoir under CO2 injection. Presented at the Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, New Orleans, LA, Sept 1990; SPE 20530. (19) Ibrahim, H. H.; Idem, R. O. Energy Fuels 2004, 18, 743–754. (20) Leontaritis, K. J.; Amaefule, J. O.; Charles, R. E. SPE Prod. Facil. 1994, 157–164. (21) Leontaritis, K. J.; Mansoori, G. A. J. Pet. Sci. Eng. 1988, 1, 229– 239. (22) Leontaritis, K. J. Asphaltene deposition: A comprehensive description of problem manifestations and modeling approaches. Presented at the SPE Production Operations Symposium, Oklahoma City, OK, March 1989; SPE 18892. (23) Chang, C. L.; Fogler, H. S. Asphaltene stabilization in alkyl solvents using oil-soluble amphiphiles. Presented at the SPE International Symposium on Oilfield Chemistry, New Orleans, LA, March 1993; SPE 25185. (24) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10 (6), 1749–1757. (25) Chang, C. L.; Fogler, H. S. Langmuir 1994, 10 (6), 1758–1766.
Kraiwattanawong et al.
Figure 1. General structure of additives.
stabilize asphaltenes in aliphatic solvents. Permsukarome and co-workers26 also used different amphiphiles to investigate their effect on the dissolution kinetics of asphaltenes and found that the asphaltene dissolution rate follows the Langmuir-Hinshelwood kinetics with respect to the concentration of amphiphiles. In the early stage, chemical additives were designed to stabilize fuel and lubricating oil, prevent sludging, prevent oxidation,27,28 preventgumforming,29 andpreventdeterioration.29-31 Chemicals in the above references shared a general structure as shown in Figure 1, where n is an integer from 0-13, M is an alkaline earth metal, R is preferably a straight or branched alkyl group containing 4-24 carbon atoms, X is a sulfur, methylene group, and/or branched alkyl group, and T is a hydrogen or methylol, ethylol, propylol, or butylol group. This chemical is a salt of hydrocarbyl-substituted phenol-aldehyde condensate. Later, Forester32,33 used polyalkenylthiophosphonic acid or ester as an antifoulant in petroleum hydrocarbons and petrochemicals. Stephenson and Kaplan34,35 and Stephenson and co-workers36,37 used an alkyl-substituted phenol-formaldehyde liquid resin with a hydrophilic-lipophilic vinylic polymer as asphaltene dispersants/inhibitors. Sung and co-workers38 used a mixture of oligomeric aliphatic ethers as asphaltene dispersants, while Comer and Stephenson39 used an R-olefin/maleic anhydride co-polymer. Manek and Sawhney40 also used alkylsubstituted phenol-polyethylenepolyamine-formaldehyde resins as asphaltene dispersants. Forester and Malik41 used a salt of hydrocarbyl-substituted linked hydroxyl aromatic compound as an antifoulant. Recently, Miller and co-workers42 used ethercarboxylic acids as asphaltene dispersants in crude oils. Miller and co-workers43,44 also used mixtures of alkylphenol-formaldehyde resins with oxalkylated amines and mixtures of phosphoric esters with carboxylic acids or carboxylic derivatives as asphaltene dis(26) Permsukarome, P.; Chang, C. L.; Fogler, H. S. Ind. Eng. Chem. Res. 1997, 36 (9), 3960–3967. (27) Stevens, D. R.; Fareri, E. L. U.S. Patent 2,760,852, 1956. (28) Bradley, J. S.; Otto, F. P.; Seger, F. M. U.S. Patent 2,916,454, 1959. (29) Gottshall, R. I.; Peters, J. G.; Swain, H. W. U.S. Patent 3,035,908, 1962. (30) Greenwald, R. U.S. Patent 3,256,183, 1966. (31) Miller, J. R. U.S. Patent 3,657,133, 1972. (32) Forester, D. R. U.S. Patent 4,775,458, 1988. (33) Forester, D. R. U.S. Patent 4,775,459, 1988. (34) Stephenson, W. K.; Kaplan, M. U.S. Patent 5,021,498, 1991. (35) Stephenson, W. K.; Kaplan, M. U.S. Patent 5,073,248, 1991. (36) Stephenson, W. K.; Mercer, B. D.; Comer, D. G. U.S. Patent 5,100,531, 1992. (37) Stephenson, W. K.; Mercer, B. D.; Comer, D. G. U.S. Patent 5,143,594, 1992. (38) Sung, R. L.; Derosa, T. F.; Storm, D. A.; Kaufman, B. J. U.S. Patent 5,202,056, 1993. (39) Comer, D. G.; Stephenson, W. K. U.S. Patent 5,214,224, 1993. (40) Manek, M. B.; Sawhney, K. N. U.S. Patent 5,494,607, 1996. (41) Forester, D. R.; Malik, B. B. U.S. Patent 5,821,202, 1998. (42) Miller, D.; Vollmer, A.; Feustel, M.; Klug, P. U.S. Patent 6,063,146, 2000. (43) Miller, D.; Feustel, M.; Vollmer, A.; Vybiral, R.; Hoffman, D. U.S. Patent 6,180,683, 2001. (44) Miller, D.; Vollmer, A.; Feustel, M.; Klug, P. U.S. Patent 6,204,420, 2001.
Asphaltene Dispersants on Aggregate Size and Growth
Energy & Fuels, Vol. 23, 2009 1577
Table 1. Crude Oil Properties properties
N2 crude oil
Q1 crude oil
K2 crude oil
API gravity saturates (wt %) aromatics (wt %) resins (wt %) asphaltenes (wt %)
30 55.8 23.9 17.5 2.7
30 59.6 26.5 10.1 3.8
27 49.5 28.4 12.4 9.7
persants. Gochin and Smith45,46 used hydrocarbyl-substituted aromatics for asphaltene control. Breen47 used (1) esters formed from the reaction of polyhydric alcohols with carboxylic acids, (2) ethers formed from the reaction of glycidyl ethers or epoxides with polyhydric alcohols, and (3) esters formed from the reaction of glycidyl ethers or epoxides with carboxylic acids as asphaltene inhibitors. Mukkamala and co-workers48,49 used compounds containing carbonyl, thiocarbonyl, or imine and compounds containing amide and carboxyl groups as asphaltene dispersants in crude oil. Wilkes and Davies50 used co-polymers as asphaltene dispersants. Asphaltene aggregation in hydrocarbon solutions was previously studied by Anisimov and co-workers.51 They found the basic aggregates in the order of 1 µm, with the ultimate floc size of 4-5 µm. Burya and co-workers52 studied asphaltene aggregation using light scattering. They observed diffusionlimited aggregation (DLA) and reaction-limited aggregation (RLA), as well as a crossover between two regimes. From the diffusion coefficient, they calculated the corresponding particle size to be about 20 nm. They also found an intrinsic colloidallike structure, with particles of approximately 0.02 and 1 µm in diameter. Joshi and co-workers53 used light scattering and the sedimentation rate to calculate asphaltene aggregate size and came out to be about 1-3 µm. In this study, the effect of asphaltene chemical additives of known molecular structure as well as proprietary blends of several chemical agents (similar to ones discussed above) on asphaltene aggregation, growth, and settling was investigated. If these chemicals are inhibitors, they will prevent asphaltene precipitation and prevent all asphaltene problems. However, if these chemicals are dispersants/surfactants for asphaltenes, they will disperse asphaltenes in the solution and prevent or reduce settling. An automatic titration system was used to determine the precipitation onset of asphaltenes; an optical microscope was used to verify the titration data; a dynamic turbidity measurement device was used to study the flocculation and settling of asphaltenes; and a particle size distribution measurement device was used to study the effect of asphaltene chemicals on aggregate size distribution. 2. Materials and Methods 2.1. Crude Oil Properties. The crude oils used in this research were obtained from the North Slope of Alaska, named N2 crude oil, Q1 crude oil, and K2 crude oil. Crude oil properties are summarized in Table 1. (45) Gochin, R. J.; Smith, A. U.S. Patent 6,270,653, 2001. (46) Gochin, R. J.; Smith, A. U.S. Patent 7,438,797, 2008. (47) Breen, P. J. U.S. Patent 6,313,367, 2001. (48) Mukkamala, R. U.S. Patent 7,097,759, 2006. (49) Mukkamala, R.; Banavali, R. M. U.S. Patent 7,122,112, 2006. (50) Wilkes, M. F.; Davies, M. C. U.S. Patent Application 2008/0096772, 2008. (51) Anisimov, M. A.; Yudin, I. K.; Nikitin, V.; Nikolaenko, G.; Chernoutsan, A.; Toulhoat, H.; Frot, D.; Briolant, Y. J. Phys. Chem. 1995, 99 (23), 9576–9580. (52) Burya, Y. G.; Yudin, I. K.; Dechabo, V. A.; Kosov, V. I.; Anisimov, M. A. Appl. Opt. 2001, 40 (24), 4028–4035. (53) Joshi, N. B.; Mullins, O. C.; Jamaluddin, A.; Creek, J.; McFadden, J. Energy Fuels 2001, 15 (4), 979–986.
Figure 2. Molecular structures of DR, DP, and DBSA.
2.2. Asphaltene Chemicals. Known chemical compounds, namely, 4-dodecyl resorcinol (DR) and 4-dodecyl phenol (DP), were obtained from Aldrich. The chemical 4-dodecyl benzene sulfonic acid (DBSA) was obtained from Fluka. The molecular structures of these known chemical compounds are shown in Figure 2. In addition, 11 proprietary blends (X1, X2, X3, Y1, Y2, Y3, Y4, Z1, Z2, Z3, and Z4) were obtained from different oil field chemical companies. 2.3. Sample Preparation. Stock solutions containing 500 parts per million (ppm) of the chemicals were prepared by adding 25 µL of the chemical to 50 mL of crude oil. The samples were placed in the oven at 55 °C for 2 h to promote mixing and to make asphaltene chemicals more effective because some of these chemicals were designed to be active at higher temperatures. The samples were then mixed for 1 h. The lower concentration samples were prepared by dilution of the 500 ppm stock solution with neat crude oil prior to the test. 2.4. Experimental Setup. 2.4.1. Automatic Titration. The titration procedure was based on American Society for Testing and Materials (ASTM) standard D6703-01.54 The titration system consisted of a 30 mL titration vessel connected to a flow cell with a 0.2 mm optical path length. A circulating bath and water-jacketed vessels for the titrant and the sample were used. The titrants (nalkanes) were added at a controlled flow rate of 0.50 mL/min using a precision pump. The titrated sample was circulated through the flow cell by a high-flow (10 mL/min) circulating pump. A UV-vis spectrophotometer was set at 740 nm, and the transmittance signal through the cell was monitored as a function of time. The unit was interfaced to a PC for system control and data acquisition. The schematic diagram of the titration system is shown in Figure 3. 2.4.2. Optical Microscopy. An optical microscope was used to confirm the titration results. In this experiment, either 10 mL of crude oil or treated crude oil was placed in a vial. Next, n-heptane was gradually added at a rate of 0.5 mL/min under rigorous mixing until the specific n-heptane/crude oil ratio was achieved. The sample was withdrawn and visually observed under a 400× optical microscope. If no asphaltenes were observed, more n-heptane was added until asphaltene particles were observed. 2.4.3. Dynamic Turbidity. For the turbidity measurements, the asphaltenes were precipitated by adding 150 µL of crude oil sample to 7.5 mL of precipitant (e.g., n-pentane, n-heptane, and n-decane) in a test tube. The blend was agitated briefly, and the transmittance of the blend was measured as a function of time at 1 min intervals over a 1 h period. The turbidity measurements were obtained using a Turbiscan MA2000 from Formulaction that used a pulsed nearinfrared light source (850 nm) to measure the average transmittance. Initially, the entire tube was dark, and transmittance approached zero. As asphaltenes settled, the upper portion of the solution became clearer and more light was transmitted, as shown in Figure 4. In the case of well-dispersed asphaltenes, the transmitted light was constant and the transmittance remained close to zero. A dynamic turbidity testing scheme has been developed, and it is illustrated in Figure 5. (54) American Society for Testing and Materials (ASTM). ASTM D6703-01. ASTM, Philadelphia, PA, 2001.
1578 Energy & Fuels, Vol. 23, 2009
Kraiwattanawong et al.
Figure 5. Dynamic turbidity testing scheme.
Figure 3. (a) Schematic diagram of the titration system and (b) flow cell.
Figure 6. Titration results of the neat and treated N2 crude oil with 500 ppm.
applied to the crude oil/solvent (e.g., cyclohexane and toluene) system to determine the stable asphaltene size. Figure 4. Dynamic turbidity measurement.
The asphaltene particles will settle out of the solution, and we know from Stokes law55 that the size of the particle dictates how fast a particle will settle out of solution. An asphaltene particle in the order of 1 µm in n-heptane has the settling velocity of 7.25 × 10-5 cm/s and will settle 5 cm within 19 h. Therefore, if one can keep asphaltenes under 1 µm, it may allow enough time for the particle to be transported and removed properly. 2.4.4. Particle Size Measurements. A Malvern Mastersizer S was used to measure the particle size distribution. The asphaltene particles were produced by adding 800 µL of crude oil sample to 400 mL of a precipitant yielding a 1:500 crude oil/precipitant ratio. The mixture was shaken very briefly to avoid shearing effects on the asphaltene particles. The particle size distribution of asphaltenes in the mixture was detected in the range of 0.05 (50 nm) to 900 µm. The same preparation method as the other measurements was
(55) Geankoplis, C. J. Transport Processes and Unit Operations, 3rd ed.; Prentice Hall: Upper Saddle River, NJ, 1993.
3. Results and Discussion This section is divided into four parts in accordance with the equipment used in the study: an automatic titration system, an optical microscope, a dynamic turbidity measurement device, and a particle size distribution measurement device. 3.1. Automatic Titration Results. The neat (untreated) N2 crude oil and the N2 crude oil treated with DR, DP, DBSA, and 11 proprietary blends were titrated with n-heptane. The titrations were conducted to study the effect of different asphaltene chemicals on the precipitation onset of asphaltenes. Figure 6 shows the volume of n-heptane consumed per volume of crude oil at the onset point for the different chemicals studied. It is clearly seen that using 500 ppm of the chemical additives to the N2 crude oil (North Slope, Alaska) delays (i.e., requires more titrant to reach the onset) the precipitation onset of asphaltenes. The onset of the neat crude oil occurred after 1.73 mL of n-heptane was added per 1 mL of crude oil, while for the treated oil, the onset point ranges between 1.89 and 2.07 mL of n-heptane added per 1 mL of crude oil. The delay on the precipitation onset could be either inhibition of asphaltene aggregation or dispersion of asphaltene aggregates. This point
Asphaltene Dispersants on Aggregate Size and Growth
Figure 7. Titration results of the neat and treated N2 crude oil with 25 ppm.
was proven in the optical microscopy section. At a concentration of 500 ppm, it is difficult to differentiate the effectiveness of different chemicals. To study the effect of the chemical dosage on the asphaltene onset point, the neat N2 crude oil and treated samples with 25 ppm of the different chemicals (DR, DP, DBSA, and 11 proprietary blends) were titrated with n-heptane. The titration results giving the amount of n-heptane required at the onset point can be seen in Figure 7. For the neat crude oil, 1.73 mL of n-heptane was required per 1 mL of crude oil, which corresponds to a volume fraction of n-heptane of 0.63. Upon treatment with 25 ppm, no difference was observed among the neat and treated N2 crude oil with asphaltene chemicals. 3.2. Optical Microscopy. Optical microscopy experiments were carried out with the N2 crude oil treated with 500 ppm of the chosen chemicals. Small numbers of the asphaltene particle in the range of 1-10 µm were detected for selected additives when the n-heptane/crude oil ratio was above 1.73 but below the onset ratio observed by automatic titration. This result suggests that the asphaltene chemicals at a concentration of 500 ppm are unable to stop the formation of particles with size in the range of 1-10 µm but rather dispersed them in smaller size compared to untreated samples. Consequently, at these concentrations, the asphaltene chemicals are not true inhibitors because they do not stop the formation of asphaltene particles and they do not completely stop the flocculation and growth of small asphaltene particles. These chemicals are dispersants for asphaltenes. We will discuss this issue further in the section of particle size measurements. 3.3. Turbidity Measurement Results. The neat and N2 crude oil samples treated with 500 ppm of different chemicals (DR, DP, DBSA, and 11 proprietary blends) were tested using a dynamic turbidity instrument. An aliquot of 150 µL of the crude oil sample was added to 7.5 mL of n-heptane in a test tube and scanned every minute for a period of 1 h, at 40 µm increments along the height of the test tube. The mean transmittance (average transmittance along the tube height) was recorded as a function of time as the particles settled. Particles in the size range of 1-10 µm will settle 5 cm between 19 and 0.19 h, respectively, and all particles above 4.4 µm will settle 5 cm in 1 h. If the chemical additives are effective, they will keep the asphaltenes dispersed (in the size range of 0.1-1 µm) in the solution and prevent them from settling, and as a result, the mean transmittance will remain unchanged. If the asphaltene chemical additives are ineffective, the asphaltene particles will agglomerate and settle out, resulting in an increase in the mean
Energy & Fuels, Vol. 23, 2009 1579
Figure 8. Turbidity measurement results of the neat and treated N2 crude oil with 500 ppm.
Figure 9. Turbidity measurement results of the neat and treated N2 crude oil with 25 ppm.
transmittance. The results from the dynamic turbidity measurements on N2 crude oil are shown in Figure 8. Figure 8 shows that, by treating the N2 crude oil with DR, DP, and DBSA even at 500 ppm, these chemicals cannot keep the asphaltenes in solution; thus, they are called “ineffective chemicals”. However, by treating the N2 crude oil with 500 ppm of the proprietary blends, the results show that the mean transmittance remains essentially the same because the asphaltene particles remained dispersed in solution. Most of the proprietary chemicals kept the asphaltenes dispersed in solution, and as such, they are called “effective chemicals”. The stock samples of N2 crude oil with 500 ppm of the effective chemicals (X1-X3, Y1-Y4, and Z1-Z4) were diluted with the neat N2 crude oil to obtain a chemical concentration of 25 ppm. The dynamic turbidity of these samples was then measured, and the results are shown in Figure 9. Figure 9 shows that, while the chemicals X1, Y3, Y4, Z2, Z3, and Z4 were effective at concentrations of 500 ppm, at a 25 ppm dosage, they were unable to keep asphaltenes dispersed in the solution throughout a 1 h period of testing. These chemicals are classified as “less effective chemicals”. On the other hand, the chemicals X2, X3, Y1, Y2, and Z1 were effective in keeping asphaltenes dispersed in the solution over the 1 h period of testing even at 25 ppm concentrations, and thus, these chemicals are classified as “more effective chemicals”. To our knowledge, some of the less effective chemicals are high-molecular-weight (30 000-50 000 Da) co-polymer and some of the more effective chemicals are low-molecular-weight
1580 Energy & Fuels, Vol. 23, 2009
Figure 10. Mean transmittance at a 1 h period of N2 crude oil treated with different chemical concentrations.
Kraiwattanawong et al.
Figure 12. Turbidity measurement results of the neat and treated K2 crude oil with 25 ppm. Table 2. Effectiveness of Asphaltene Chemicals by Turbidity Measurement
chemical
Figure 11. Turbidity measurement results of the neat and treated Q1 crude oil with 25 ppm.
(
