Feb 27, 2014 - The 8th International Chemical Engineering Congress & Exhibition ... ghotbi@ sharif.edu ... By definition, asphaltenes are the fraction of a crude oil that ... aggregation equilibrium and the standard free energy of micellization.
The 8th International Chemi cal Engineering Congress & Exhibition (IChEC 2014)
Kish, Iran, 24-27 February, 2014
Thermodynamic Modeling of Asphaltene Precipitation from Iranian crude oil by using micellization model T. Jafari Behbahani, C . Ghotbi*, V. Taghikhani, A. Shahrabadi Department of Ch emical and Petroleum Engineering, Sharif University of Technology, Tehran , Iran ghotbi@ sharif.edu
Abstra ct Asphaltenes are the h eaviest and most polarizable components o f petrol eu m. Asph altene p recipitation is an i mpo rtant problem du ring oil production, b ecause it can result in formation damage and the plugging of wellbore and surface facilities. A th ermodynamic micellization mod el is used in this work to describe asphaltene precipitation from Iranian crude oil. The mod el is based on asphaltene and resin micelle fo rmation and then destruction by addition of an alkane. A coin-like aggregate of asphalten e surrounded by resin molecul es constitutes the mi cellar phase. Th e rest o f the asphalten e and resin are dissolved in the bulk phase. A group contribution method w as used to predict the critical prop erties for the asphaltenes, resin, and oil. The Peng-Robinson EOS was us ed to predict the ons et point and to perform flash calculations. Th e onset point and the amount o f asphalten e precipitated were measured . Pentan e, hexane, heptane, nonan, and dod ecan e in different volu mes and temperatures were used. Th e results show that the model could describ e the exp erimental d ata reasonably well with a mean square erro r o f 0.48 %. Keywo rds :Asphalten e precipitation; Micellization model, Thermodyn ami c mod eling;At mospheri c titration.
Introductio n Asph altenes an d resins are polar fractions of crude oil that can be separ ated by the addition of lo w molecular weight n-alkanes. By definition, asphaltenes are the fraction of a cr ude oil that is soluble in toluene an d insoluble in an n- alkan e; typically pentane or heptane [1]. When pressure in the reservoir is reduced or when light hy drocar bon or other gaseo us com po unds are intro duced, the colloidal suspen sion m ay become destabilized, r esulting in asphaltene and resin molecules precipitating o ut of the o il [2]. A literat ure review in dicates that models of asphaltene precipitation can be classif ied into solubility, so lid, co lloidal, micellization and SAFT. One of the best m odels of solubility in m odelin g asphaltene precipitation is the solubility m odel, suggested by Hirsch ber g et al. [3]. In this m odel, a Vapor– Liquid– Liquid Equilibr ium (VLLE) m odel is employed, usin g separate VLE an d LLE calculation s to predict the phase sp lits (vapor, asphaltene an d cr ude oil fractions). As asph altene is assum ed to be a polymeric substance [4, 5], the theory of a polymeric solution is used for the prediction of asphaltene precipitation in crude oil. The sim plest m odel for the precipitated asphaltene is the single-component Solid mo del. The Solid model can requir e m any em pirical p arameters and
Thermodynamic Modeling of Asphaltene Precipitation from Irani an crude oil...
excessive tunin g to match experim ental data [6]. Nghiem et al. [7] proposed their m odel by considering the precipitated asphaltene as a p ur e den se phase, while the heav iest com ponent in the oil can be split into two parts; non-precipitating an d pr ecip itating. Ch un g [8] dev eloped his pr ecipitation m odel by treating asphaltene as a lumped pseudo com ponent and the other com ponents as solv ents. The first colloidal model used to describe asph altene precipitation was developed by Leontaritis [9]. In this m odel, a VLE calculation is first performed, usin g an EOS to estimate the liquid com position in which asphaltene can flo cculate. Then, the critical chem ical potential, which is calculated usin g the Flory– Huggin s theory [4,5], is used to predict the onset of precipitation. In the m icellization m odels, the asphaltenes are assumed to be aggregate, formin g a micelle cor e with resin m olecules adsor bed on the surface of the core, to stabilize the m icelle. Victorov and Firrozabadi [10] form ulated this model through aggr egation equilibrium an d the standar d free energy of micellization. The polydispersity of asphaltene aggregates was taken into acco unt by Victorov and Sm irnova [11]. The results of the micellization m odel agr ee well with experim ental data [12]. Ch apm an et al. [13] derived the SAFT equation of state by applying an d exten ding Wertheim ’s first-order pertur bation theory [14] to chain molecules. Gro ss and Sadowski [15] developed the pertur bed chain m odification (PC- SAFT) to SAFT, by extending the perturbation theory of Barker and Hen der son [16] to a hard- chain reference. P C-SAFT em ploys a h ar d sphere refer ence fluid, descr ibed by the Mansoor i et al. [17] equation of state.
Experim ental An Iranian crude oil sample was collected for m easurin g the onset, bubble point and titration experim ents with n-alkan es. This oil an d its asphaltene/resin fraction s were sep arated and characterized. Experim ents wer e performed to determine the am ounts of asphaltene precipitated when a sam ple of oil was titrated with five diff erent solvents (n- C5, n-C6, n-C7, n-C9 an d n- C12). The crude oil was previo usly filtrated, Teflon mem brane to remove any susp ended m aterial. The titration experim ents were perform ed by addin g a specified volume of -alkan e, corr espondin g to a desired solv ent to oil ratio, to 5 g of crude o il in an appropriate flask. After 15 m in of ultrasonic sh akin g, the mixture was left overnight. Then, the solution of n-alkane an d deasphalted oil was filtered, using a v acuum system with a Teflon membrane (previously weighed). Finally, the f lask an d the m em brane were rinsed with sm all volumes of the corresponding n-alkan e to eliminate residual o il. The membrane with the precipitated m aterial was dried in a vacuum oven an d weigh ed to determ ine the asphaltene m ass precipitated. Model Equations Model Implementation.. The asph altene and resin critical properties calculation s are based on the Joback gro up contribution [18] method. The fugacity coefficient of the asphaltene m onomers in the crude is calculated on the basis of an initial guess for asphaltene m onomer m ole fraction xa. Then, upon addition of a certain am ount of solvent, the fugacity coefficient of the asphaltene m onomers was calculated usin g the critical properties of the added solvent. Then, the m ole fraction of asph altene at onset point in equilibrium with asphaltene monom ers was calculated. Consequently, the fractional coverage of an asphaltene cor e covered by resin is calculated. The numbers of molecules of asph altene an d resin are estimated on the basis of the fractional coverage . Then, the standar d Gibbs free ener gy chan ge an d the m ole fraction of m icelles ar e estimated. The n umbers of m oles of asphaltene m onomers an d resin monom ers were recalculated usin g the material balances. A com parison bet ween the initial guess and the estim ated m ole fraction s is made usin g the technique of error minim ization. The act ual values
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The 8th International Chemi cal Engineering Congress & Exhibition (IChEC 2014)
Kish, Iran, 24-27 February, 2014
for asph altene and resin m onomer m ole fraction are o btained when the normalized m ean square error ( NMSE) is minimized. Finally, the material balance equations are used to calculate the mole fraction of asphaltene pr ecipitated. Parameters of PR EOS. In the present work, the Jo back group contribution method [19] is used to fin d the group frequencies constitutin g the r esin or asphaltene m olecules using elem ental analy sis alon g with the measured molecular weights of aspaltene and resin. Five group s from the Jo back gro up contribution m ethod are assum ed to exist in the asphaltene and resin monom ers, each with a fr equency n um ber. The cr itical prop erties are estim ated by the Joback group contributions. The calculation of the critical properties was tested by estimating the molar volumes of asphaltene, resin, an d the deasphalted oil using the PR EOS and com paring these values with the experimental results. Results a nd Discussion In our calculations, we assumed that the asphaltene micelles co uld be either coinlike (plate), spherical, or ro dlik e in shape. The sam e calculation Procedur e was applied for the three shapes to calculate the weight of asph altene precipitated. Comparison with exper imental results shows that the coin-sh aped m icelles giv e the best results. Therefore, we adopt this shape for all subsequent calculations. The m ole fractions of asphaltene at the on set and the correspondin g m onomeric asph altene mole fractions in the bulk are predicted upon addition of diff erent solvents, at different so lvent -to-crude ratio s . Before precip itation (i.e., before the onset point), as the solvent-to-crude ratio increases, resin s in the m icelle shell dissolve in the solvent, resulting in the destruction of some micelles. Hence, asphaltene an d resin molecules are transported to the bulk. As a result, the m ole fraction of asphaltene in the m icelles decreases, whereas the mole fraction of asphaltene in the bulk ph ase increases. When the onset point is reached and precipitation starts, any addition of solvent leads to larger m icelles and lower bulk asphaltene concentrations. Hence, the m ole fraction of asphaltene m onomer becomes very small. Therefore, the ratio bet ween the fugacity coeff icient of initial m onomer and the f ugacity coeff icient of m onomers at higher dilution ratio s becom es con stant. This clarif ies the flat pattern upon solv ent addition after the onset point was reached. From model parameter estim ation, the m icelles become bigger upon solvent addition. The m aterial balance calculations sho w that, as the so lvent-to-crude ratio incr eases, the bulk concentration of asphaltene decreases an d the asphaltene concentration in the m icelles incr eases, so precipitation in creases. The results show that the critical m icelle concentration is reached at the solubilization r atio na/nr ) 0.2, which agrees with the value reported by Victorov and Firrozabadi[10]. The onset point (volume of solv ent) increases as the n umber of car bon atoms in the correspon din g solvent increases. For the highest so lvent carbon n um ber (C12), we needed a gr eater am ount of solvent (aroun d 7 cm3/g) to start precipitation, whereas for C6, we needed only 4 cm3/g of cr ude. Ear lier, we explained that the onset point depends on the type of solvent, because the f ugacity coefficient depen ds on the solvent type and dilution ratio. The com parison bet ween the m easur ed an d estim ated weight percentages was based on calculations of the normalized m ean square error (NM SE). The aver age NM SE is 0.0033, which in dicates that the estimated points are in good agreem ent with the exper imental values. Note that, at constant dilution ratio, as the solvent density incr eases, precipitation decreases. The results also imply that C5 sho uld result in gr eater precipitation. On e reason for the greater precipitation is that C5 might cause pr ecip itation of resin s in addition to asphaltenes. Conclusions A m icellization/colloidal/aggregation m odel containin g m icelles of asphaltene collo idal aggr egates surro un ded by resin shell is used to describe cr ude oil containing asphaltene and
Thermodynamic Modeling of Asphaltene Precipitation from Irani an crude oil...
resin monom ers, along with the satur ates an d aromatics that constitute the bulk of the cr ude oil. Exp erim ental data agreed well with the assum ption that asphaltene aggregates are present in a coin-like shape. The critical properties of the asphaltene, resin, an d crude oil were estim ated using the gro up contribution technique. The onset points and amounts of asphaltene precipitated were also pr edicted an d fo un d to com pare favor ably with experimental values.
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