Energy Fuels 2010, 24, 6501–6505 Published on Web 11/10/2010
: DOI:10.1021/ef101094p
Surface Properties of Basic Components Extracted from Petroleum Crude Oil Andreas L. Nenningsland,* Sebastien Simon, and Johan Sj€ oblom Ugelstad Laboratory, Department of Chemical Engineering, the Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway Received August 16, 2010. Revised Manuscript Received October 22, 2010
The properties of the entire separated basic fraction extracted from a crude oil have been investigated, by utilizing a Langmuir trough to determine surface pressure (SP) isotherms and a ring tensiometer for interfacial tension (IFT) measurements. The IFT between oil and water indicated that the interfacial activity is governed by the bases in the lower pH range (pH e 5) and by the nonbases at higher pH values. However, the results do not imply any interactions between the two separated fractions, but rather that the activity of the crude oil is a combination of the individual components. The SP isotherms did not appear to depend on the pH of the subphase, except at pH 1. This was later found to be a result of higher electrolyte concentration rather than a pH effect. An addition of salt to the aqueous subphase increased the surface pressure by screening the electrostatic repulsions in the surfactant monolayer. Salting-in or salting-out effects were not observed in these systems. The SP isotherm for the bases was compared with those for maltenes and asphaltenes, where the similarities with the former support previous statements of bases being a subfraction of maltenes.
crude oil are partly to blame for the lack of available information but also are the fact that the nitrogen compounds have high boiling points and complex molecular structures.9 Although indigenous basic molecules from crude oil have not been subject to extensive interfacial studies, some authors have noticed how basic model molecules may influence the colloidal properties of a system. Spildo et al.10 used octylamine and octanoic acid to show how the interfacial tension (IFT) and the surface tension (ST) are lowered upon adding the amine to the acid, compared with the values for a system containg acid alone. To overcome the problem of having a low concentration of basic nitrogen molecules in crude oil, it was absolutely vital to have access to an efficient extraction procedure. The various developed procedures can be regarded as either liquid/liquid (L/L), solid/liquid (S/L), or precipitation as hydrochloric salts. Results from other publications indicate a preference for the S/L extraction method, where the extraction yield is highest among the three methods.6,11 The procedure that will be used in the work covered by this article is the S/L extraction method developed by Simon et al.12 The method consists of extracting the basic molecules with a cation-exchange sorbent, before using methyl amine to recover them. The goal of this article is to investigate the surface and interfacial properties of the extracted basic molecules from a petroleum crude oil sample, by determining their ability to influence both the IFT in an oil/water system and the film stability of a monolayer on an aqueous surface. The intention is to lay a foundation for future studies on the molecular interactions between the basic molecules in crude oil and other
1. Introduction The composition of crude oil is extremely complex. The vast amount of molecules includes alkanes, aromatics, cycloalkanes, polycyclic aromatics, sulfur-containing components, nitrogen-containing components, etc.1 The nitrogen compounds are generally present at low levels in crude oil (from 0.05 to 0.9 wt % of elemental nitrogen),2 compared with for instance sulfur compounds, but they are still worthy of attention. Problems associated with nitrogen compounds are related to major catalytic processes, such as catalytic cracking and hydrocracking.1 They have also proven to contribute to gum formation in fuel oil3 and to inhibit hydrodesulfurisation (HDS) reactions.4 The nitrogen compounds can generally be separated into basic and neutral molecules, or more specifically, titrable and nontitrable by a mineral acid.5 The subject of this article is focused exclusively on the basic molecules. The structures of the basic molecules are predominantly derived from pyridine,6 also called azaarenes. As for their surface and interfacial properties, much less is known about them compared with for instance the naphthenic acids, which are responsible for stabilizing petroleum emulsions and forming calcium naphthenates.7,8 The low levels of nitrogen in *Corresponding author. E-mail address: andreas.nenningsland@ chemeng.ntnu.no. (1) Moulijn, J. A.; Makkee, M.; Van Diepen, A. Chemical Process Technology; John Wiley & Sons Ltd.: New York, 2001. (2) Thompson, K. F. M. Org. Geochem. 1994, 21, 877. (3) Dahlin, K. E.; Daniel, S. R.; Worstell, J. H. Fuel 1981, 60, 477. (4) Laredo, G. C.; Leyva, S.; Alvarez, R.; Mares, M. T.; Castillo, J.; Cano, J. L. Fuel 2002, 81, 1341. (5) Speight, J. G. The Chemistry and Technology of Petroleum, 4th ed.; CRC Press: Boca Raton, FL, 2007. (6) Merdrignac, I.; Behar, F.; Albrecht, P.; Briot, P.; Vandenbroucke, M. Energy Fuels 1998, 12, 1342. (7) Nordgard, E. L.; Magnusson, H.; Hanneseth, A.-M. D.; Sj€ oblom, J. Colloids Surf., A 2009, 340, 99. (8) Ese, M.-H.; Kilpatrick, P. K. J. Dispersion Sci. Technol. 2004, 25, 253. r 2010 American Chemical Society
(9) Roussis, S. G.; Proulx, R. Energy Fuels 2004, 18, 685. (10) Spildo, K.; Blokhus, A. M.; Andersson, A. J. Colloid Interface Sci. 2001, 243, 483. (11) Wallace, S.; Crook, M. J.; Bartle, K. D.; Pappin, A. J. Fuel 1986, 65, 138. (12) Simon, S.; Nenningsland, A. L.; Herschbach, E.; Sj€ oblom, J. Energy Fuels 2010, 24, 1043.
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pubs.acs.org/EF
Energy Fuels 2010, 24, 6501–6505
: DOI:10.1021/ef101094p
Nenningsland et al.
Table 1. Properties of Oil Samples Used for Base Extraction SARA analysisd sample
origin
crude oil NB fraction B fraction
North sea North sea North sea
density, 15 °C [g/cm3]
water contenta [%]
particle contentb [%]
viscosity, 15 °C [mPa 3 s]
TANc [mg/g]
TBNc [mg/g]
saturates [wt %]
aromatics [wt %]
resins [wt %]
asphaltenes [wt %]
0.939
0.11
0.04 ( 0.003
471.6
2.15 2.07 nde
2.81 nde 37.5
37
44
16
2.5
a Determined by Karl Fischer titration. b Determined by calcination. c Determined as described by Simon et al.12 d Determined by HPLC as described by Hannisdal et al.18 e Not determined.
Table 2. Aqueous Buffers Used in IFT and SP Measurements pH
solutions
1 2 3 4 5 6 8 10
0.1 M HCl 0.01 M HCl 0.001 M HCl 0.1 M CH3COOH adjusted with 0.1 NaOH 0.1 M CH3COOH adjusted with 0.1 NaOH 0.1 M KH2PO4 adjusted with 0.1 M NaOH 0.025 M Borax adjusted with 0.1 M HCl 0.025 M Borax adjusted with 0.1 M NaOH
Prior to all experiments, the surface tension of pure water was measured to ensure that the system was clean. If the value was within 72.8 ( 1 mN/m, the system was considered to be ready for use. When starting the interfacial tension measurements, an aqueous buffer (Table 2), about 15-20 mL, was initially added to a vessel. Subsequently, the same amount of oil sample was gently added on top before the experiment was launched. IFT was measured every 300 s for a total of 24 h to ensure that equilibrium was reached. All experiments were repeated to ensure a good reproducibility within 5% of the initial results. For both the crude oil and the NB fraction, the sample was used without any modifications. The basic molecules extracted from the crude oil, however, had to be dissolved in p-xylene with a mass concentration of 0.5 g/L, since that fraction appeared almost solid-like. To minimize the evaporation of the solvent, the vessel was partly covered by aluminum foil. In addition, a few glass vials containing pure p-xylene was put next to the sample vessel to saturate the atmosphere inside the apparatus. All experiments were carried out at room temperature, and the pH of the aqueous phase was measured after the experiment to make sure that it was relatively constant throughout the 24 h. The maximum pH variation was observed at pH 3, where it increased by 0.23 units. A hydrochloric acid solution at pH 3 has a lower number of protons and, consequenly, does not have the same capability of withstanding a change in pH. For all other aqueous solutions, the variation was within 0.1 units. 2.4. Surface Pressure Measurements. The instrument used to obtain Langmuir isotherms was a KSV Minitrough (Finland). All the subphases used are presented in Table 2. The setup consists of a trough made of solid PTFE (Teflon) mounted on an antivibrational table. The area of the trough was 242.25 cm2. A paper probe was utilized instead of a platinum Wilhelmy plate, due to the fact that the surfactants in the sample adsorbed to the platinum plate. Over the trough are two movable barriers made of Delrin. Prior to each experiment, the purity of the subphase was tested, by compressing the surface without actually adding any surfactants. The subphase was considered to be clean if the change in surface pressure did not exceed 0.5 mN/m. Then 35 μL of a stock solution of the separated fraction (0.75 g/L in chloroform) was carefully applied on top of the subphase. For the maltene fraction, the concentration had to be increased to 3.0 g/L to get measurable results. The solvent was evaporated for 5 min before the experiment was launched, enabling the formation of a monolayer on top of the aqueous subphase. The software (Windows LB software) moved the barriers at a constant rate of 5 mm/min, while simultaneously measuring
Figure 1. Interfacial tension between the oil phase and water phase as a function of pH. The IFT for both the crude oil and the NB fraction at pH 10 was below the detection limit for the instrument (
