Article pubs.acs.org/EF
Use of CO2-Triggered Switchable Surfactants for the Stabilization of Oil-in-Water Emulsions Chen Liang,†,§ Jitendra R. Harjani,† Tobias Robert,† Estrella Rogel,‡ Donald Kuehne,‡ Cesar Ovalles,‡ Vasudevan Sampath,‡ and Philip G. Jessop*,† †
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802, United States
‡
ABSTRACT: Pipelining of heavy crudes can be facilitated by preparing oil-in-water (O/W) emulsions of the crude, but separation of the oil from the water after pipelining is problematic if conventional surfactants are used. Long-chain acetamidines are CO2-triggered switchable surfactants, being surface-active when CO2 is present but not when CO2 is absent. Unfortunately, in the presence of CO2, they stabilize water-in-oil (W/O) emulsions of heavy crude rather than the desired O/W emulsions. However, in the absence of added CO2, several compounds (Na2CO3, three of the long-chain acetamidines, and two other amidines) stabilize O/W emulsions. These low-viscosity emulsions can later be broken by the addition of CO2. The residual oil content in the recovered water is lowest if the compound used to stabilize the original emulsion was a long-chain acetamidine.
1. INTRODUCTION Over the next decade, the supply of conventional crude oil is expected to diminish. The need to tap into unconventional crude oil reserves will increase as production from major wells shrinks and the search for new deposits yields fewer smaller deposits.1−3 Unconventional reserves, such as heavy oil, very heavy oil, oil sands, and oil shale, will provide a growing portion of future petroleum, as both an energy source and chemical feedstock. The Canadian oil sand deposits are estimated to hold an amount of oil equivalent to the oil fields of Saudi Arabia. Venezuelan deposits of heavy crude oil are estimated to be large. However, heavier crude oils have been degraded over millions of years by microbes into a viscous product with a greater proportion of high-molecular-weight molecules.2,4,5 The extremely high viscosity makes transport difficult. Pipeline transport of heavy oil is often assisted by heating, dilution, or emulsification.6 Heating of pipelines requires specialized equipment and significant energy consumption. Dilution with low-viscosity solvents is effective but requires large amounts of valuable diluents.7,8 Finally, the viscosity of heavy crude oil can also be reduced by suspending oil droplets into a low-viscosity fluid, such as water.6 Significant viscosity reduction can be achieved with an oil-inwater (O/W) emulsion containing approximately 70−80% crude oil.6,9−13 The heavy crude oil emulsion must be stabilized by surfactants, which lower the interfacial tension and delay phase separation. Emulsion stability during transport is essential to avoid phase separation inside a pipeline, which would lead to blocking, stratified flow, and other problems. The presence of water is also a major drawback because water is incompatible with downstream operations and cannot be readily removed from a stable emulsion. In other words, once the crude oil is transported to its destination, emulsion stability is no longer desired. Therefore, a surfactant that can be “switched off” would be preferable. The literature has many examples of switchable surfactants, including some that are switched by light,14,15 pH,16,17 or redox reagents.18−26 We reported that acetamidines containing a long © 2011 American Chemical Society
hydrophobic chain could serve as switchable surfactants; they can be reversibly and repeatedly switched between the neutral amidine and the amidinium bicarbonate simply by the addition and then removal of CO2 at 1 atm (Figure 1).27 The amidinium
Figure 1. Application of N′-hexadecyl-N,N-dimethylacetamidine as a CO2-switchable surfactant.
bicarbonate is a competent surfactant, while the neutral amidine is a demulsifier of at least some crude oil/water mixtures. In our earlier study, emulsions of light crude oil were successfully stabilized by N′-hexadecyl-N,N-dimethylacetamidinium bicarbonate (1).27 The emulsion was broken when CO2 was flushed out of the stabilized mixture by argon gas. The removal of CO2 caused the surfactant to convert to the neutral amidine, which was found to increase the rate of oil/water phase separation when compared to an identical emulsion prepared without the amidine. N′-Hexadecyl-N,N-dimethylacetamidine was less effective with heavy crude oil samples; only partial phase separation was achieved when the switchable surfactant was converted to the neutral form. The type of surfactant used to stabilize an emulsion is instrumental in determining the properties of the emulsion. Important properties, such as the nature of the continuous phase and emulsion stability, are influenced by the surfactant. For example, water-soluble surfactants are likely to stabilize O/W emulsions, while oil-soluble surfactants are likely to stabilize water-in-oil (W/O) emulsions. For pipelining applications, only the O/W emulsions are useful because only they Received: May 12, 2011 Revised: November 30, 2011 Published: December 27, 2011 488
dx.doi.org/10.1021/ef200701g | Energy Fuels 2012, 26, 488−494
Energy & Fuels
Article
either the water or the toluene depending upon the continuous phase of the emulsion. For example, if the continuous phase was water, then the drop dispersed instantly when added to water. The identification of the continuous phase was also confirmed under light microscopy, where oil is opaque and water is transparent. 2.3.2. Emulsion Stability. The stability of each emulsion was evaluated visually. Samples were prepared and transferred into Corning 15 mL centrifuge tubes. The centrifuge tubes were then stored at ambient conditions and checked periodically for phase separation. 2.3.3. Emulsion Viscosity. Viscosity was measured using factorycalibrated Cannon Zeitfuchs cross-arm glass viscometers. The emulsion sample and a glass viscometer were preheated to a specific temperature inside a Cannon CT-500 constant temperature bath (±0.01 °C). A small amount of sample was poured into the glass viscometer. The time required for the sample to flow between markings in the glass viscometer was used to calculate the viscosity using factory-calibrated viscometer constants. 2.3.4. Water Content. The water content of the removed heavy crude oils was determined gravimetrically. Samples were heated in an oven at 100 °C overnight in an open beaker. The mass lost during this treatment was assumed to be equal to the mass of water plus the mass of volatiles in the oil. A minimum of three replicates were measured for each sample. The mass of volatiles in the oil was determined by similarly heating virgin heavy crude oil samples (before exposure to water) at 100 °C overnight. The results of those tests are presented in Table 2.
have low viscosity. Surfactants can be designed to specification by tuning its physical properties. The relative and absolute solubility of a surfactant is an important factor in determining its hydrophilic−lipophilic balance (HLB) and, thus, behavior. The following work describes the synthesis of a series of amidine surfactants with varying degrees of water solubility to find one that will reversibly stabilize an O/W emulsion for transport of heavy crude oil.
2. EXPERIMENTAL SECTION 2.1. Materials. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), IGEPAL CO-720, and Na2CO3 were purchased from Sigma-Aldrich, while the acetamidines were prepared by literature methods.28 Three crude oil samples were provided by the Chevron Energy Technology Company (Table 1), including two Venezuelan extra heavy crude oils (samples I and II) and a sample of North Sea crude oil (sample III).
Table 1. Analysis of Crude Oil Samples crude oil sample properties
I
II
III
API gravity (deg) specific gravity viscosity at 40 °C (cSt) viscosity at 100 °C (cSt) water (ppm) TANa (mgKOH/g) TBNb (mgHCl/g) asphaltene content (wt %) carbon content (wt %) hydrogen content (wt %) nitrogen content (wt %) sulfur content (wt %)
9.4 1.004 18925 653.1 0.46 1.26 2.31 13.6 83.2 10.4 0.7 4.9
7.7 1.017 65689 687.2 453 3.49 3.15 8.7 84.2 10.4 0.7 4.2
19.5 0.937 129 11.22 542 1.65
