Dimethylcyclohexylamine Switchable Solvent Interactions with

introduced for different industries.14-17Solvents in the oil industry are used .... profile analysis tensiometer (PAT1, Sinterface Technology, Berlin,...
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Dimethylcyclohexylamine Switchable Solvent Interactions with Asphaltenes towards Viscosity Reduction and In-Situ Upgrading of Heavy Oils Armin Mozhdehei, Negahdar Hosseinpour, and Alireza Bahramian Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01956 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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Dimethylcyclohexylamine Switchable Solvent Interactions with Asphaltenes towards Viscosity Reduction and In-Situ Upgrading of Heavy Oils Armin Mozhdeheia, NegahdarHosseinpoura,b,*, Alireza Bahramiana aInstitute

of Petroleum Engineering, School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box: 11155/4563, Tehran, Iran bHeavy

Oil & Residue Upgrading Center of Excellence, School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box: 11155/4563, Tehran, Iran ABSTRACT The ever-increasing depletion rate of conventional oil resources makes it essential to design processes for heavy oil recovery from petroleum reservoirs. Heavy oils contain high quantities of large molecules, such as asphaltenes, giving rise to high viscosities during production. Solvent extraction processes are capable candidates for heavy oil recovery; however, the separation of the conventional solvents is energy-intensive. Therefore, a solvent with not only a high capacity for heavy oil dissolution but also an easy and environmentally friendly separation is in demand. In this work, N,N-dimethylcyclohexylamine switchable hydrophilicity solvent (SHS) was employed for viscosity reduction and upgrading of heavy oils. Optimum weight ratios of the SHS solvent were added to three different dead heavy-oil samples with 11.33 to 17.54 oAPI and the corresponding room-temperature viscosity of more than 4287.0 to 3365.4 cp. To shed light on the interactions of the SHS solvent with asphaltenes, the solvent was also added to heavy oil model solutions of asphaltenes in toluene. The solutions containing the solvent and the recovered oil were exposed to pressurized CO2 in the presence of de-ionized water to switch the hydrophilicity of the solvent to water-miscible and separate the recovered oil. The thus-obtained aqueous solution was then warmed up to 65oC in the presence of N2 gas bubbling through the liquid to switch the solvent hydrophilicity to water-immiscible and recover the SHS solvent. Results indicate that more than 52 wt% of the oils are recovered at the optimum solvent to oil ratio (SOR). The heavier the oil, the higher is the optimum SOR, resulting in the viscosity reductions of more than 70%. Dynamic light scattering shows that the asphaltene aggregates sizes become smaller and considerably uniform by the SHS solvent, indicating weakening the asphaltene self-association interactions. Dynamic interfacial tension measurements calibrate the amount of impurity in the recovered SHS. Keywords:Switchable hydrophilicity solvent; SHS;CyNMe2; Interfacial rheology; Green chemistry. 1. INTRODUCTION Petroleum reservoirs containing heavy and extra-heavy oils account for more than 70% of the in-place volume of the discovered oils around the world. In addition, the demand for crude oil and petroleum products is growing worldwide. Therefore, developing efficient enhanced oil recovery (EOR) methods for heavy oil and asphaltene-rich fields is essential.1-2Heavy and extra-heavy oils contain high levels of large molecules, such as asphaltenes, leading to a high initial viscosity and a significant viscosity rise during reservoir depletion. Asphaltenes are the most polar and heaviest components of the reservoir oils. The molecular structure of the asphaltenes consists of a polycyclic aromatic sheet with peripheral aliphatic chains. When destabilized, the asphaltenes self-association, mainly through π-π and acid-base attractions, results in the asphaltenes aggregates 1 ACS Paragon Plus Environment

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growth leading to a significant viscosity rise as a big challenge in the production of heavy oils.3-6In order to overcome the production challenges, many thermal and non-thermal recovery methods in conjunction with polar and non-polar solvents injection have been employed to maximize the economic recovery from heavy oil-bearing reservoirs. Steam-assisted gravity drainage (SAGD) is one of the conventional thermal EOR methods used for recovery of heavy oils. Paraffinic solvents such as n-hexane and n-heptane are employed in combination with SAGD, known as solvent-assisted SAGD, to enhance the performance of the recovery process.7The injection of the solvents with the steam in the steam-based recovery methods is aimed at improving the propagation of the steam chamber in the reservoir, leading to a higher oil recovery factor. However, a high fraction of the in-situ oil precipitates out during the paraffinic solvent-assisted SAGD processes in heavy oil-bearing reservoirs since asphaltenes are insoluble in these solvents.8-10The asphaltenes precipitation and deposition not only alter the wettability of the reservoir rock pores to oil-wet but also have the potential to block the pore throats, resulting in a detrimental formation damage and a high residual oil saturation. On the other hand, the separation of the paraffinic solvents from the produced oil in the surface facilities for recycling is energy-intensive. Therefore, finding solvents with not only a high capacity for dissolution of heavy oil components but also an easy separation in the surface facilities is in demand. In addition, the solvents need to meet the green chemistry principles where reduced operational risk, material waste, and detrimental environmental impacts are anticipated.11-13 Recently, switchable hydrophilicity solvents (SHS) are introduced for environmentally-friendly separation processes in food, drug, and oil industries. The properties of the SHS solvents are possible to be changed reversibly from water-immiscible to water-miscible by exposing to a stimulus and the solvents become ultimately water-immiscible and recovered by heating.14 Therefore, the SHS solvents in water-immiscible form have the capability to dissolve hydrocarbon components following by easy separation via switching them to water-miscible form.Since 2004, many types of SHS solvents with diverse sets of properties have been introduced for different industries.14-17Solvents in the oil industry are used usually to increase the recovery factor of heavy oils through in-situ upgrading processes.18Generally, heavy oil upgrading is categorized into carbon rejection and hydrogen addition processes.19 The in-situ oil upgrading by the SHS solvents is classified as a carbon rejection process since lighter oil components and medium to light asphaltenes are dissolved in the solvents and recovered while the heavy asphaltenes and coke are rejected.20Thus, the hydrogen-to-carbon (H/C) ratio of the recovered oil is higher than that of the reservoir oil since heavy asphaltenes and coke have low H/C ratios.21-23The SHS solvents capable of in-situ oil upgrading and easy separation need to have specific characteristics, as shown by definite range of Log(Kow) and pKaH in the literature.16-17 In addition, inexpensive and easily accessible stimuli for the reversible changing of the properties of the SHS solvents are required. Furthermore, the degree of impurities in the recovered SHS solvent needs to be monitored to assess the performance of the recycled SHS solvents. Interfacial tension and elasticitymeasurements are reported to be effective methods for evaluating the level of impurities in fluids.7, 9, 24 In this work, N,N-dimethylcyclohexylamine (CyNMe2) is used as a SHS solvent for viscosity reduction and upgrading of three types of stock tank oil samples with different viscosity, API gravity, and asphaltene content. To assess the interactions of the SHS solvent with asphaltene, model oils of different concentrations of asphaltene in toluene are also prepared and the impacts of the CyNMe2solvent are studied. CyNMe2is waterimmiscible in its neutral form and becomes water-miscible in its ionic form obtained by exposure to pressurized CO2. Distinct weight ratios of the SHS solvent are added to the oil samples and left to approach equilibrium, followed by injection of pressurized CO2 in the presence of de-ionized water to separate the recovered oil. The aqueous phase is then heated in the presence of N2 inert gas to switch the SHS solvent to water-immiscible and recover the solvent. Interfacial tension and elasticity measurements are employed to 2 ACS Paragon Plus Environment

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monitor the impurity level of the recovered solvent. The higher the asphaltene content of the oil, the greater is the reduction in the oil viscosity and improvement in the oil oAPI using CyNMe2 as the SHS solvent. 2. EXPERIMENTAL 2.1. Materials N,N-dimethylcyclohexylamine (CyNMe2, purity: 98%), toluene (purity> 99.9%), and n-heptane (purity: 99%) were purchased from Merck and used without further purifications. The density, boiling point, Log(Kow), and pKaH of the CyNMe2 switchable hydrophilicity solvent (SHS) are 0.85 g/cm3, 159oC, 2.04, and 10.48, respectively. Carbon dioxide (CO2, purity: 99.99%) and nitrogen (N2, purity: 99.9%) gases were provided by Ideal Gas Company, Tehran, Iran. Three types of dead oil samples from Iranian heavy-oil fields were used in this work. The API gravity of the heavy oil samples was 17.5, 16.0, and 11.3. The heavy oil with the API gravity of 17.5, 16.0, and 11.3 is designated as HO1, HO2, and HO3, respectively. Therefore, HO1 is the lightest and HO3 is the heaviest among the oil samples studied. 2.2. Methods 2.2.1. Characterization of the heavy oil samples The saturate, aromatic, resin, and asphaltene (SARA) content of the dead oil samples was measured following the ASTM D4124 and IP-143 standards.25 Elemental analysis of the oil samples was done by a Vario MaxCHNSO element identification device using the ASTM D5291 and ASTM D4249 standards.26-27 SVM 3000 apparatus (Anton Paar Company, UK) was employed to measure the viscosity of the oil samples before and after upgrading. The SVM apparatus eliminates the influence of the bearing friction considerably and measures both the dynamic and kinematic viscosity of liquids with a high precision.28 In order to evaluate both the content and surface activity of the surface active components of the oil samples, dynamic surface tension of air-oil, dynamic interfacial tension of deionized water-oil, and dilationalvisco-elasticity of the air-oil and deionized water-oil interfaces were measured following the procedure described in section 2.2.2. 2.2.2. Dynamic surface/interfacial tension and elasticity measurements The dynamic surface tension (ST), the dynamic interfacial tension (IFT), and elasticity of the liquids were measured at 25oC by a drop profile analysis tensiometer (PAT1, Sinterface Technology, Berlin, Germany), the schematic representation of the PAT1 setup is shown in Fig. S1, Supporting Information. The details of the PAT1 experimental set-up and the procedures to calculate ST, IFT,and elasticity have been described in the literature.29-30 Briefly, the method of measurement is the temporal image processing of a liquid droplet that is formed by a microsyringe pump (0.01 mm3 accuracy)at the tip of a stainless steel capillary tube within the continuous phase filling a glass cuvette, an example of which is illustrated in Fig. S2, Supporting Information. Next step is the digitization of the droplet profile and then fitting the profile to the Gauss-Laplace equation for ST and IFT calculations.31-33For measuring the ST and IFT, the continuous phase is air and DI water, respectively. This precise dosing pump allows performing pre-designed assessments with different procedures. For instance, measurements of the dynamic IFT/STcan be done where the drop volume is kept fixed in a predetermined value.31 The elasticity of the interface layer is measured by applying sinusoidal oscillations with certain amplitudes, i.e. 7 to 8% of the initial interface area, and frequencies (from 0.01 to 0.1 Hz) on the drop. Then, the IFT or ST response function is converted to the dilationalvisco-elasticity (ε), named elasticity, according to eq. 1, where σ is the IFT/ST, and A and Ao are the instantaneous and initial interface area, respectively. Interfacial elasticity is a representation of the competition between surface-active components of a system for participation in the interface when its interfacial area changes with time. In surfactant solutions, 3 ACS Paragon Plus Environment

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the elasticity indicates the speed and intensity of the surface-active components in transferring to or from the interface.31, 34-35 ε=

∂σ A ∂(ln ) A0

()

Eq. 1

The higher the amount of the highly surface-active components in a system, the lower is the change in the elasticity of the system with an increase in the cycle period.31, 36Briefly, elasticity measures the heterogeneity of the surface-active components in a mixture. This means that the elasticity for all pure substances in all cycle periods is almost the same since there is no competition between the components in the system.33-34, 37 The experimental data of the ST, IFT, and elasticity presented in this work are the average results of at least three sets of the tests conducted. 2.2.3. Evaluation of the switchability of the CyNMe2 hydrophilicity N,N-dimethylcyclohexylamine (CyNMe2) exhibits a switchable hydrophilicity, as reported in the literature.14-17, 38-39 In order to reprove the switchable hydrophilicity characteristic of the CyNMe solvent, 5 mL of the solvent 2 were added to the same volume of deionized (DI) water in a glass beaker to form a two-phase liquid system, as illustrated in Fig. 1. This shows that the CyNMe2 solvent is naturally water-immiscible.

Figure 1. Switching the hydrophilicity of the CyNMe2 solvent to (a) water-miscible and (b) water-immiscible.

The beaker was then placed in a stainless-steel autoclave equipped with a thermo-well to measure the inside temperature. The autoclave was placed in an ice-water bath and its temperature was fixed at 10oC. Then the autoclave was purged by CO2 and then pressurized to 2 bar by CO2 injection. As reported in the literature, CO2 is dissolved in DI water to form carbonated water which reacts with the CyNMe2solvent to form water-soluble bicarbonate salt [CyNMe2H]+[HCO3]- structure, according to equation S1, and as shown in Fig. S3, Supporting Information. Under no mixing, the protonation of the CyNMe2 solvent is complete after 24 h, as displayed in 4 ACS Paragon Plus Environment

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Fig. 1a.14-16, 38-40 The next step is the deprotonation of the CyNMe2solvent to be water immiscible. Therefore, the single-phase CyNMe2-carbonated water system was heated to 65oC under the bubbling of the N2 gas at atmospheric pressure to remove the liberated CO2 and the system becomes two-phase. Nitrogen went across a perforated disc produced fine bubbles into the liquid. After 12 h, the CyNMe2 solvent becomes water immiscible and the solvent-water separation is complete, as shown in Fig. 1b.14-15, 38-39, 41 As a result, N,Ndimethylcyclohexylamine (CyNMe2) is a switchable hydrophilicity solvent (SHS) and is denominated as the SHS solvent in this work. In order to investigate the fraction of the solvent remained water-miscible after the heating, dynamic surface tension and elasticity measurements were employed, as discussed in section 2.2.2. These analyses show the surface activity of the SHS solvent. The surface tension and elasticity of air/pure SHS solvent, air/DI water, and air/solutions of the SHS and water were measured. Definite quantities of the SHS solvent were made water-miscible, following the procedure described above,to provide SHS-water solutions with specified concentrations for the surface tension and elasticity measurements. 2.2.4. Upgrading of the heavy oil samples by the SHS solvent Prior to the upgrading process, the heavy oil samples were dissolved in toluene and subjected to ultrasonic irradiation for 30 min, followed by centrifugation at 5000 rpm. No solid minerals were deposited, indicating that the dead oil samples contain no minerals. For upgrading, different mass ratios of the SHS solvent were added to the heavy oil samples in sealed bottles. The bottles were left at room temperature under no continuous mixing for 75 h. A solid deposit was formed in the bottles, the amount of which is a function of the solvent to oil mass ratio, oil API gravity, and contact time. The liquid phase, which contains the SHS solvent and the dissolved fraction of the initial oil sample, was then decanted into a glass beaker filled already with the same volume of DI water. The sample was exposed to pressurized CO2 at 2 bar in the autoclave at 10oC for 24 h to make the SHS solvent water-miscible and to be separated from the recovered oil.17, 42 The thus-obtained oil, called recovered oil, was subjected to API gravity, viscosity, and ST/IFT measurements. The recovered oil was mixed with n-C7 at the ratio of 1 g of oil to 40 mL of n-heptane, followed by ultrasonic irradiation for 30 min. It is found that asphaltene is deposited, indicating that the asphaltene is dissolved in the SHS solvent.17 Finally, the aqueous phase, mainly containing the solvent and water, was heated to 65oC under the bubbling of the N2 to become two-phase and the thus-recovered SHS solvent was analyzed via surface/interfacial tension measurements to assess its hydrocarbon content. Therefore, recovery efficiency of the solvent was obtained. Figure S4, Supporting Information, illustrates the experimental steps followed for heavy oil upgrading by the SHS solvent. In order to find the optimum mass ratio of the SHS solvent to the dead oil samples, sealed bottles containing different mass ratios of the solvent to oil were shaken at 70 rpm and room temperature for 24 h in an incubator (Fan AzmaGostar Ltd, Iran). Then, the affinity of the solvent to the oils and thus the optimum solvent to oil ratio (SOR) were obtained by plotting the values of the solvent per recovered oil versus the solvent per original oil mass ratio, as discussed in section 3.3. The oil recovery factor (RF) is defined as the mass ratio of the recovered oil to the original oil. The recovery factor at the optimum SOR is the highest and remains almost constant at the SOR values higher than the optimum SOR. The optimum SOR and maximum RF vary with the API gravity of the dead oil. In addition, the quasi-equilibrium contact time, defined under no continuous mixing at the optimum SOR, was determined by comparing the recovery factor at each contact time with the maximum values obtained under the shaking for 24 h in the incubator. 2.2.5. Interactions of the asphaltene-SHS solvent 5 ACS Paragon Plus Environment

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In order to investigate the interactions of the SHS solvent with asphaltene, model oil solutions of asphaltene in toluene were prepared. Asphaltene was extracted from the HO2 oil sample following the modified ASTM D6560 standard, as detailed in our previous work.43-45 Definite amounts of the asphaltene were added to toluene to prepare model oils with the asphaltene concentration of 100, 500, 1000 and 3000 mg/L. The model oils were subjected to ultrasonic irradiation in an ultrasonic bath for 30 min in order to approach the equilibrium size of the asphaltene aggregates. A specified amount of the SHS solvent was added to the asphaltene-toluene solutions to prepare model oils with 0.1 M of the CyNMe2solvent, followed by shaking at 70 rpm and room temperature for 30 min in the incubator. The size distribution of the asphaltene aggregates in the model oils was determined by dynamic light scattering (DLS) using a Malvern ZS Nanoanalyser (Malvern Instrument Inc, UK). The SHS solvent interacts with the aromatic sheet of the asphaltenes, resulting in weakening the π-π stacking of the asphaltene molecules and thus smaller aggregates sizes. In addition to the DLS analysis, IFT and elasticity of DI water with both the CyNMe2-free model oils and CyNMe2-containing model oils were measured. Asphaltene is the most polar component of oil and is expected to be surface-active. The DI water/model oils interfacial elasticity and dynamic IFT are acceptable techniques for determining the effects of the SHS solvent on the asphaltene aggregates sizes and the configuration of the asphaltene molecules in the aggregates.46-47 3. RESULTS AND DISCUSSION 3.1. The dead oil samples characterization The results of the SARA and CHNSO analyses of the dead oil samples are summarized in Table 1. Detailed CHNSO elemental analyses results are presented in Table S1, Supporting Information. The HO3 sample has the highest asphaltene content as well as the lowest API gravity and H/C molar ratio. The API gravity and molar H/C ratio improve as the asphaltene content decreases. In addition, the higher resin/asphaltene weight ratio, which is an indication of the smaller asphaltene aggregates size in the oil, confirms that HO1 has the smallest and HO3 the largest size of the asphaltene aggregates. Furthermore, the colloidal instability index (CII) of the asphaltene, which is defined as the weight ratio of (saturate + asphaltene)/(aromatic + resin), is the same for the HO1and HO3 sample and lower than that of HO2. This indicates the higher instability of the asphaltene in the HO2 sample. Table 1. The SARA analysis, API gravity, H/C molar ratio, and CII of the heavy oil samples.

HO1 HO2 HO3

Saturate (wt%) 10.66 11.23 5.16

Aromatic (wt%) 55.71 53.26 58.20

Resin (wt%) 24.03 24.21 21.54

Asphaltene (wt%) 9.60 11.30 15.10

Resin/Asphaltene weight ratio 2.50 2.14 1.43

CII

oAPI

0.25 0.29 0.25

17.5 16.0 11.3

H/C molar ratio 1.39 1.44 1.27

The dynamic viscosity of the HO1 and HO2 sample at 25oC is 3365.4 and 3587.7 cp. The HO3 dead oil is semi-solid at 25oC; therefore, its viscosity was measured at higher temperatures. For the sake of comparison, the dynamic viscosity of HO3 is 4287.0 cp at 60oC. Considering all the upgrading indices, i.e. the API gravity, viscosity, and molar H/C ratio, the HO3 is the heaviest and the HO1 is the lightest dead oil among the samples. 3.2. The surface activity of the CyNMe2SHS solvent The dynamic IFT and ST of the pure SHS solvent along with the optical photo of a droplet of the solvent formed in the DI water are illustrated in Fig. S5, Supporting Information. Similar to the behavior of every pure substance, the IFT and ST profile of the pure CyNMe2solvent are constant with time. The DI water-solvent IFT 6 ACS Paragon Plus Environment

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and air-solvent ST are 4.2 and 24.7 mN/m, respectively.48 In order to evaluate the impacts of the SHS solvent dissolved in carbonated water on the ST of the water, a series of experiments were conducted. 2, 3, and 4 g of the solvent were added to 5 g of DI water and the resulting two-phase systems were subjected to pressurized CO2 at 2 bar and 10oC for 24 h in the autoclave to become single-phase, as described in section 2.2.3. The dynamic ST and elasticity of the thus-obtained single-phase liquids were recorded, the results of which are shown in Figs. 2 and 3. The surface tension decreases with a higher pace as theconcentration of the SHS solvent increases in the carbonated water, as displayed in Fig. 2a. With increase in the SHS content, the equilibrium ST of the aqueous phase approaches that of the pure SHS solvent, confirming the surface activity of the dissolvedCyNMe2 molecules. The initial ST of the aqueous phase containing 2 g of CyNMe2and 5 g of DI water, i.e. 28.6 wt% CyNMe2, is around 37 mN/m, much lower than that of pure DI water, i.e. 72.8 mN/m.49 Therefore, the dissolved CyNMe2 molecules are surface-active, leading to a sharp decrease in the ST of the solution in a very short fraction of time. The decrease in the ST by dissolving additional 1 g of the SHS solvent in the aqueous phase from 2 to 3 g is higher than that observed for the additional 1 g of the solvent from 3 to 4 g. It may imply that the tendency of the dissolved CyNMe2 molecules to participate into the interface is diminished when their concentration in the liquid bulk approaches a critical value, designated as the critical concentration (CC).

Figure 2. The dynamic surface tension of single-phase carbonated water-CyNMe2solution with (a) unsaturated air, and (b) air already saturated with the CyNMe2vapor.

In addition, an unusual hump is observed in the initial seconds of the ST profiles. The hump is diminished with increase in the concentration of the dissolved CyNMe2. This unusual trend is observed when the surface-active molecules transfer from the interface to the bulk of either continuous or dispersed phase, as reported in the 7 ACS Paragon Plus Environment

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literature.30 Since the CyNMe2 molecules are in the dispersed phase, i.e. the droplet, transfer of the SHS solvent molecules from the interface to the bulk of the droplet occurs when the self-association of the CyNMe2 molecules is stronger than their interactions at the interface. On the other hand, the CyNMe2molecules are transferred from the interface to the bulk of the continuous phase, i.e. the air, until the gas phase becomes saturated. The latter seems to be the case for the unusual hump observed in the ST profiles since the intensity of the hump is diminished with the CyNMe2 concentration in the aqueous phase. This means that the concentration of the solvent is getting enough to saturate both the interface and the continuous phase. That is why the hump is not observed when 4 g of the solvent is dissolved in 5 g of water, i.e. 44.4 wt% CyNMe2. In order to assess the validity of the explanation, 1 mL of the SHS liquid was added to the cuvette of the PAT1 setup to saturate the air for almost 200 seconds and then the dynamic ST of the aqueous solutions was repeated, one of the profiles is shown in Fig. 2b. The initial hump does not appear in the ST profiles collected in the saturated air, confirming the validity of the above clarification. After equilibrium is approached, the elasticity of the interface of the air-aqueous solutions was measured at the cycle period of 10, 15, 25, 50, and 100 s, as illustrated in Fig. 3. Generally, the interfacialelasticity decreases with the cycle period.33 The chance of all the molecules for the participation into the interface becomes almost the same and the equilibrium is approached when the length of the cycle becomes infinite.33, 35 The decrease in the elasticity from the cycle period of 10 to 100 s is 5.25, 3.29, and 1.48 for the aqueous solutions of 2, 3, and 4 g of CyNMe2 in the carbonated water, respectively. This indicates that the CC of the SHS solvent in the carbonated water is much higher than 2 g of the SHS in 5 g of water, i.e. 28.6 wt% CyNMe2. Therefore, the elasticity of the Cy 2g-Water 5g sample is not only very high but also decreases rapidly with increasing the cycle period.

Figure 3. Elasticity of the interface of air-aqueous solutions of CyNMe2-carbonated water as a function of cycle period.

As a result, the dynamic ST and elasticity has the potential to be employed as an indication of the level of impurities remained in both the recovered solvent and water during upgrading of the oils by the SHS solvent. For instance, the dynamic ST of the solvent recovered after upgrading of the HO1 sample and that of the pure CyNMe2solvent are presented in Fig. S6, Supporting Information.The surface tension of the recovered solvent decreases with time to approach an equilibrium value, in contrast to that of the pure SHS solvent which is constant with time. This indicates that some of the oil components remain as impurities in the recovered solvent. The presence of the impurities not only leads to the decreasing trend in the ST of the recovered solvent but also results in a lower equilibrium ST value compared to that of the pure solvent. However, the level of the 8 ACS Paragon Plus Environment

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impurity is not very high since the ST values do not deviate sharply from that of the pure solvent, in-line with the reports in the literature.15, 35, 42, 50 3.3. Upgrading of the heavy oil samples by the SHS solvent For upgrading, different mass ratios of the SHS solvent were added to the heavy oils. Then, the samples were left at room temperature for 75 h under no continuous mixing. Therefore, the dissolution of the oil components in CyNMe2 is done under no continuous magnetic stirring or agitation, trying to simulate the miscible displacement of the oils in porous media with no dispersion.Finally, the oils recovered were collected following the procedure described in section 2.2.4.Figure 4 illustrates the improvement in the API gravity of the recovered oil with the increase in the solvent/original oil mass ratio (SOR). As the SOR increases from zero to one, the oAPI of the oils recovered from the upgrading of the HO1, HO2, and HO3 samples increases by 14.6, 14.6, and 15.7 API points, respectively. The upgrading potential of the SHS solvent for the HO1 and HO2 is almost the same and lower than that for the HO3 sample. In addition, as will be discussed later on (see Fig. 7), the recovery factor(RF) improves with the increase in the SOR up to the optimum SOR where the RF remains almost constant at the higher SOR values. The optimum SOR for the HO1 and HO2 is similar and lower than that for HO3, resulting in a higher RF for the HO3 compared to the HO1 and HO2 samples. This may imply that the affinity of the SHS solvent for the intermediate and heavy components is higher than lighter compounds, the concentration of which is the lowest in the HO3 sample. Therefore, the higher the concentration of the heavy molecules, the greater is the potential of the solvent for improvement of both oAPI and RF of the oil.

Figure 4. The API gravity of the recovered oils as a function of the solvent/original oil mass ratio.

To shed light on the heavy oil upgrading potential of the SHS solvent, viscosity reduction of the oil as a function of both SOR and temperature was measured, as presented in Fig. 5. The HO3 is semi-solid at 25oC and that is why the viscosity of the original HO3 is not reported in Fig. 5a. The viscosity of the oils recovered from the upgrading decreases sharply at lower SOR values followed by a lower-pace viscosity reduction at higher SOR. At the SOR of 0.16 g/g, a viscosity reduction of around 2500 cp is observed for the oils recovered from the HO1 and HO2 upgrading. The rate of the viscosity reduction with the SOR for HO3 remains high enough even at the SOR values higher than 0.16 g/g. This may imply that, similar to the API gravity, the effect of the SHS solvent on the viscosity of the oil recovered from HO3 is higher than that on the viscosity of the recovered oils obtained from the HO1 and HO2 upgrading. Fig. 5b exhibits the effects of temperature on theviscosity of the original and recovered oils. In the legend of Fig. 5b, the number on the right of the sample 9 ACS Paragon Plus Environment

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name is the amount of the solvent in g added to 6 g of the oils. The dynamic viscosity of the original oils decreases with almost a same trend with temperature. The viscosity of the recovered oils is less affected by temperature, when compared to that of the original oils. In addition, as the SOR increases, i.e. for HO1-3, HO2-3, and HO3-3, the influence of temperature on the viscosity of the recovered oils becomes almost negligible.Therefore, it may be concluded that the addition of the solvent is more effective than temperature rise for the viscosity reduction of the heavy oils. In general, the viscosity of oils arises from their molecular size and intermolecular interactions, i.e. the number and strength of the interactions.19-20 Temperature rise decreases both the number and strength of the intermolecular interactions. In addition, the Brownian motion of the molecules and aggregates in the oil is improved with temperature. Therefore, the asphaltene aggregates become smaller and thus more stable with temperature, leading to a decrease in the oil viscosity.10 It may be inferred, from the results of the oils upgrading, that the SHS solvent interacts with the large molecules and aggregates such as asphaltene, leading to weakening of the asphaltene-asphaltene self-association. The solvent may interact with the aromatic sheet of the asphaltene, resulting in smaller aggregates with lower number of contact points with their neighboring aggregates and large molecules; thus, decreasing the oil viscosity.

Figure 5. Oil viscosity versus (a) the solvent/original oil mass ratio and (b) temperature.

The dynamic surface tension and interfacial tension of the original and recovered oils were collected, as shown in Fig. 6. The ST and IFT of HO3 were not recorded since the viscosity of the HO3 is very high and the sample 10 ACS Paragon Plus Environment

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is almost semi-solid at 25oC. The ST and IFT data are employed to identify the changes in the number and activity of the surface-active components in the oil during the upgrading. The equilibrium IFT of HO1 is higher than that of HO2. However, HO2 has a higher ST compared to the HO1 sample. This may be attributed to the polar molecules of water, compared to those of air, that induce electrostatic forces on the surface-active species, such as asphaltene, imposing a driving force to them to participate into the interface. Therefore, it seems that the HO1 has a higher number and smaller size surface-active components as compared to HO2.30, 51 The equilibrium ST of the recovered oils obtained via the upgrading ofHO1 and HO2 by the SHS solvent is 2 to 3 mN/m lower than that of their original oil. It is worth mentioning that the recovery efficiency of the solvent in the upgrading tests is 80 to 85% depending on the API of the oil and SOR. The entrapment of the solvent in the water and the recovered oil as well as the carryover of the solvent by the N2 gas during switching the solvent hydrophilicity are why the solvent is lost. According to the results obtained by the experiments in section 2.2.3, the solvent loss is associated mainly with the carryover of the solvent by the N2 gas bubbling through the solvent-carbonated water solution at 65oC. The experimental data indicate that around 3% loss of the solvent is ascribed to the entrapment of the solvent in the water, even after heating at 65oC to make the solvent water-immiscible. Therefore, a small fraction of the solvent is entrapped in the recovered oils even after making the solvent water-miscible. The smaller size of the asphaltene aggregates as well as the entrapment of the solvent in the recovered oils may explain the decrease in the equilibrium ST of the recovered oils as compared to the original oils. The interactions of the SHS solvent with the asphaltene will be discussed in section 3.4.

Figure 6. (a) Dynamic interfacial tension of the original oil samples, and (b) dynamic surface tension of the original and recovered oils.

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A series of upgrading experiments was conducted to obtain the optimum solvent/original oil mass ratio (SOR). Different doses of the solvent were added to the heavy oil samples in sealed bottles followed by shaking at 70 rpm and room temperature for 24 h to approach equilibrium. The oil recovery factor is the highest at the optimum SOR and does not change significantly with the SOR as the solvent doses increase to the values higher than the optimum SOR. Figure 7a gives the solvent/recovered oil mass ratio as a function of the dose of the solvent. The SOR was adjusted in the range of 0.07–1.20 (g/g). The mass of the recovered oil was obtained from the mass difference between the original oil and the deposited solid in the upgrading tests. As depicted in Fig. 7a, the curves of the recovered oil versus the original oil for all the samples approach the line of symmetry asymptotically as the SOR increases. This indicates that the mass of the recovered oil continually approaches that of the original oil. This implies that the active sites of all the solvent molecules are becoming involved and saturated in the upgrading process. Compared to that of the other heavy oil samples, the upgrading of the HO3 requires a higher dose of the solvent since the curve depicting the recovered oil from the HO3 upgrading gives the highest distance to the line of symmetry.52 The saturation dose, designated as the optimum SOR, is defined as the point at which the slope of the curves approaches the slope of the symmetry line; thereafter the distance to the line remains almost constant. The optimum SOR for the HO1, HO2, and HO3 sample is at around 0.32, 0.36, and 0.51 g/g. The optimum SOR increases with the asphaltene content of the original oil, indicating the interaction of the solvent with the large molecules. The RF at the optimum SOR is 53.3, 52.2, and 59.5% for the HO1, HO2, and HO3, respectively. The RF obtained for the HO1 and HO2 is almost similar and lower than that obtained for the HO3. The higher RF obtained for the HO3 is ascribed to the higher optimum SOR for the HO3 compared to the other oil samples. For the sake of clarity, as observed in Fig. 7a, the RF obtained at the constant SOR of 0.32 g/g is 53.3, 51.1, and 32.7% for the HO1, HO2, and HO3, respectively. Therefore, at a constant SOR of 0.32 g/g, the RF decreases with the asphaltene content of the original oil.

Figure 7. Oil recovery as a function of (a) the solvent/original oil mass ratio, and (b) contact time without mixing at the optimum solvent/original mass ratio at 25oC.

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The quasi-equilibrium contact time of the solvent with the heavy oil samples was determined by obtaining the RF with time under no mixing at the optimum SOR. As illustrated in Fig. 7b, the RF increases sharply at the initial contact time and then begins to level off and approach an ultimate value. The quasi-equilibrium contact time is 50, 75, and more than 100 h for the upgrading of the HO1, HO2, and HO3, respectively. The higher the asphaltene and thus the viscosity of the oil samples, the longer is the quasi-equilibrium contact time at the optimum SOR. This implies that the diffusion of the solvent into the oils is associated with the weakening of the intermolecular interactions of the large molecules, such as asphaltene. The disintegration of the large molecules and their dissolution in the solvent require a longer time as the amount and size of the asphaltene aggregates become larger.4, 8, 52 This is confirmed by the gradual growth of the RF curve of the HO3 sample after the contact time of 75 h. Therefore, the contact time of 75 h was considered as the batch time of the upgrading experiments, the results of which are displayed in Fig. 4. 3.4. Interactions of theCyNMe2 solvent with the asphaltene The interactions of the SHS solvent with the asphaltene were determined by the addition of the solvent to the model oils of asphaltene in toluene. The asphaltene was extracted from the HO2 sample and dissolved in toluene at the concentration of 100, 500, 1000, and 3000 mg/L, denominated as the model oil. A distinct amount of the SHS solvent was added to the model oils to obtain 0.1 M of the solvent. Figure 8 presents the dynamic IFT of the model oils with and without the solvent. The presence of the asphaltene decreases the equilibrium IFT of the original model oils compared to that of pure toluene, reported to be 36 mN/m. The IFT of original model oils decreases with a slow pace, as observed in Fig. 8a. The dynamics of the IFT is accelerated and the equilibrium IFT is diminished with the increase in the concentration of the asphaltene in the original model oils. This implies that the asphaltene aggregates become more surface-active at higher concentrations. The surface activity of the asphaltene is not only a function of the asphaltene aggregate size but also the configuration of the asphaltene molecules in the aggregates.6, 53-54

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Figure 8. Dynamic interfacial tension of the asphaltene/toluene model oils in the (a) absence and (b) presence of 0.1 M of solvent.

Figure 8b shows that the addition of the SHS solvent accelerates the IFT dynamics and decreases the equilibrium IFT values compared to that of the original model oils. In the presence of 0.1 M of the solvent, the equilibrium IFT is diminished by around 6 to 8 mN/m for the model oils. This is inferred that the addition of the solvent increases the amount of the surface-active components in the model oils since the solvent molecules are surface-active as discussed in section 3.2. The solvent molecules interact with the aliphatic and aromatic moieties of the asphaltene, leading to a decrease in the aggregates size of the asphaltene. In addition, the solvent is considered as a co-surfactant for asphaltene aggregates, pushing them towards the oil-water interface, resulting in a fast dynamics and lower equilibrium of IFT. Dynamic light scattering (DLS) technique was employed to monitor the change in the aggregates size of the asphaltene by the addition of the SHS solvent to the model oils, as depicted in Fig. 9. The size distribution of the asphaltene aggregates in the original model oil with 3000 mg/L asphaltene concentration exhibits two peaks at around 30 and 2200 nm. The addition of the SHS solvent increases the intensity and decreases the width of the peak observed at around 30 nm, as shown in Fig. 9b. This may confirm that the large-size aggregates of the asphaltene are broken-up by the solvent to form smaller size and considerably uniform aggregates in the solution.

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Figure 9. The size distribution of the asphaltene aggregates in the model oil with the concentration of 3000 mg/L in (a) the absence and (b) the presence of 0.1 M CyNMe2 solvent.

In order to shed light on the surface activity of the asphaltenes at the interface of the DI water-model oils, interfacial elasticity was measured after the equilibrium was approached. Figure 10 exhibits the elasticity of the interface of the model oils-DI water. The model oil with 3000 and 100 mg/L of the asphaltene shows the highest and the lowest elasticity, respectively, at each cycle period, as depicted in Fig. 10a. In addition, the interfacial elasticity decreases with increase in the cycle period.29, 37 The highest and the lowest drop in the elasticity with the cycle period are associated with the model oil of 3000 and 100 mg/L asphaltene concentration, respectively. This indicates that the asphaltene aggregates with different range of surface activity are present in the model oil with 3000 mg/L of asphaltene. Since the surface activity of the asphaltene aggregates is associated with their size and molecular configuration,53 it is inferred that the aggregate size and configuration of the asphaltene at the concentration of 3000 mg/L have a wide range. The interfacial elasticity decreases by the addition of the SHS solvent to all the model oils, as displayed in Fig. 10b. The drop in the elasticity by increasing the cycle period from 10.08 to 100.08 s is around 3.3 to 3.6 mN/m, indicating the presence of the surface-active species with almost similar activity for participation in the interface. Consequently, the SHS solvent interacts with the asphaltene aggregates to form smaller and considerably uniform aggregates in the model oils.

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Figure 10. Elasticity of the interface of DI water-model oils as a function of cycle period in (a) the absence and (b) the presence of 0.1 M CyNMe2 solvent.

4. CONCLUSIONS Dimethylcyclohexylamine was employed as a switchable hydrophilicity solvent (SHS) for upgrading of heavy oils. The switchability of the hydrophilicity of the solvent was confirmed. The dynamic surface tension (ST), dynamic interfacial tension (IFT), and interfacial elasticity measurements reveal that the dissolved CyNMe2 molecules are more surface-active than the water molecules. The upgrading potential of the SHS solvent on three different heavy oil samples with the API gravity of 11.33 to 17.54 was studied. The addition of the solvent to the oil samples improves the API gravity of the recovered oils by 14.6 to 15.7 API points. The solvent loss is found 15-20% during the heavy oil upgrading and the subsequent switching the solvent hydrophilicity. In addition, the dynamic viscosity of the recovered oils is much lower than that of the original oils. A reduction of more than 2500 cp in the viscosity of the original oils is observed upon the addition of the SHS solvent at the solvent to oil ratio (SOR) of 0.16 g/g. The higher the amount of the large molecules in the oil, the greater is the API gravity improvement and the viscosity reduction by the addition of the solvent. Furthermore, it is found that the addition of the solvent is more effective than temperature rise for the viscosity reduction of the heavy oils. The optimum SOR and ultimate recovery factor (RF) was obtained for the oils. Finally, a small amount of the SHS solvent was added to the model oils of the asphaltene in toluene in order to reveal the effects of the SHS solvent on the asphaltene aggregates size and molecular configuration. The results of dynamic interfacial tension and elasticity measurements as well as dynamic light scattering indicate that the asphaltene aggregates become smaller and considerably uniform in the presence of the SHS solvent. 16 ACS Paragon Plus Environment

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As a conclusion, considering the great upgrading potential of the dimethylcyclohexylamine SHS solvent, enhanced oil recovery and in-situ oil upgrading are expected when the solvent is employed in the solventassisted steam-assisted gravity drainage. ASSOCIATED CONTENT Supporting Information Available: The schematic representation of the PAT1 apparatus, optical image of the oil droplet in DI water, reaction of N,N-dimethylcyclohexylamine (CyNMe2) solvent with carbonated water, the experimental procedure followed in this work, the dynamic surface and interfacial tension of the pure SHS solvent, the comparison of the dynamic surface tension of the pure and recovered solvent, and the CHNSO elemental analysis results of the dead oil samples. This material is available in the online version. AUTHOR INFORMATION *Corresponding Author E-mail address: [email protected] (N. Hosseinpour) Tel: +98 (21) 61114712, Fax: +98 (21) 88632976 ACKNOWLEDGMENTS The authors would like to thank Prof. Aliyar Javadi and Mr. Saeid Dowlati at the Institute of Petroleum Engineering of the University of Tehran for both interesting discussions and useful advices.

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