Weng Zhan; Zhang Pengyuan; Chu Guangwen; Zou Haikui; Jimmy Yun; Chen Jianfeng,

(1. Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029; 2. State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029; 3. Suzhou Research Institute, Beijing University of Chemical Technology, Suzhou 215000)

Synthesis of Petroleum Sulfonate Surfactant with Ultra-Low Interfacial Tension in Rotating Packed Bed Reactor

Weng Zhan1; Zhang Pengyuan1; Chu Guangwen1; Zou Haikui1; Jimmy Yun3; Chen Jianfeng1,2

(1. Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029; 2. State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029; 3. Suzhou Research Institute, Beijing University of Chemical Technology, Suzhou 215000)

Petroleum sulfonate is one of the most important surfactants used in surfactant flooding for enhanced oil recovery, which is mainly obtained by treating high-boiling petroleum fractions in a stirred tank reactor (STR) or in a fallingfilm reactor (FFR). The synthesis of petroleum sulfonate with ultra-low interfacial tension from viscous petroleum fractions was carried out in a rotating packed bed (RPB) reactor using dilute liquid sulfur trioxide as the sulfonating agent in this study. The effects of various experimental conditions on components content and oil-water interfacial tension (IFT) were investigated. Under the optimum conditions, the active matter content could reach up to 50.3% and the IFT could be equal to 4.7×10−3mN/m. Compared with the traditional reactor, the active matter content is by 14.12% higher in the RPB as compared to that obtained in the STR. The uneven change of the test oil droplets during the IFT measurement was also discussed. The increase of heavy components content not only can eliminate the contraction phenomenon, but also can reduce the IFT to a minimum. This can be conducive to explaining the reason for producing IFT and the preparation of proper formulations for practical application.

sulfonation; petroleum sulfonate surfactant; RPB reactor; ultra-low interfacial tension

1 Introduction

Nowadays, oilfields have entered into a tertiary oil recovery stage, and surfactant flooding as the main technology is widely used in China[1]. Surfactants for Enhance Oil Recovery (EOR) should have ultra-low oil-water interfacial tension (IFT≤10-3mN/m), low adsorption capacity, good compatibility and low cost[2]. Alkylbenzene sulfonates are the main surfactants which have been already extensively applied in major oilfields. However, these kinds of synthetic sulfonates, which are obtained most commonly via sulfonation of alkyl aromatic hydrocarbons or olefinic polymers, do not meet the requirements for large-scale application. Especially, both of these two surfactants do not reduce the IFT to an ultralow level when they are used alone and must be used in combination with alkali[3]. Taber[4]and Stegemeir[5]investigated the relationships between residual oil and capillary number and drew a conclusion that when oil-water IFT reached ultralow level, oil displacement efficiency became apparent. Furthermore, the addition of alkali not only increases the EOR cost, but also increases the difficulty in oil-water separation[6]. Even more seriously, alkali may corrode the stratigraphic texture to block stratigraphic reservoir channels. Therefore, development of low-priced and adequately abundant raw materials for surfactant production with high efficiency is urgently needed.

Petroleum sulfonate surfactant is attracting more attention thanks to its strong interfacial activity, low adsorption capacity, good compatibility with reservoir fluid and relatively low cost[7]. Petroleum sulfonate is a mixture obtained by sulfonation of high-boiling petroleum fractions with sulfuric acid, fuming sulfuric acid, or sulfur trioxide followed by neutralization of reaction products. The reaction usually takes place in a stirred tank reactor (STR) with fuming sulfuric acid or dilute sulfur trioxideor in a falling-film reactor (FFR) with a mixed gas of air and sulfur trioxide. As it is known to all, the sulfonation reaction is highly exothermic and is often described as a diffusion-controlled process, especially when sulfur trioxide is used as the sulfonating agent. The side reaction is mainly caused by localized non-homogeneous concentration and temperature distributions. Therefore, a reactor with efficient mixing and heat transfer is very important for realizing sulfonation reaction. Although various types of reactors have been running in mass production for a period of time, some shortcomings have never been overcome[8]. For example, the STR, which is characteristic of long residence time and low heat and mass transfer efficiency, can usually lead to low content of active substances. The FFR, which mainly dominates in preparation of laundry products, can easily lead to coking problem when high-boiling alkylbenzene is used as the feedstock. Additionally, the petroleum fractions generally are highly viscous liquids with poor fluidity. It is necessary to dilute them with organic solvent. However, the high speed of sulfonating agent mixture in the FFR might cause a large amount of solvent volatilization loss leading to exhaust gas treatment problem. Besides, the viscosity of sulfonic acid is relatively high making the agitation more difficult and heat removal less efficient, which is the main reason for formation of side reactions[9]. Therefore, it is meaningful to develop a new type of reactor with rapid mixing efficiency for such viscous material system.

Compared with the conventional reactors, the rotating packed bed (RPB) as a highly ef ficient reactor has been applied to the process intensi fication[10-11]and successfully applied in polymerization[12], adsorption[13], and polymer devolatilization processes[14]. The enhanced process is realized through the centrifugal force produced by the rotation of packing. When the liquid is sprayed into the packing, the fluids are split into films and droplets in nano- or micro-size level through the strong shearing action. The fluids can coalesce and be broken many times during the rotation of packing. The size of fluids is much smaller and the frequency of coalescence and breaking is much faster than that in the traditional reactor, resulting in a signi ficant intensi fication of the phase interface renewal and micro-mixing[15]. Therefore, the mass transfer coefficient in a RPB is much higher than that in the conventional equipment. The whole process in RPB takes place very fast, less than one second. It is indicated that RPB is suitable for intensified mixing to cope with a fast reaction involving a high viscosity liquid.

In this paper, two kinds of local petroleum fractions were used as the feedstock and a RPB was used as the sulfonation reactor. The goal was to prepare petroleum sulfonate surfactant with high active component content and good interfacial performance for EOR. The product performance was characterized in terms of the product quality and oil/water IFT.

2 Experimental

2.1 Material

Two kinds of petroleum fractions (JSQ and RA) employed in this work were obtained from the Daqing Oilfield. The oil fractions were mixed to form the feedstock at different mass ratios before sulfonation. JSQ was a broad boiling range distillate with an average molecular weight of 527 and a kinematic viscosity of 1.101 Pa·s, and RA was a refined oil with a concentrated molecular weight of around 300 and a kinematic viscosity of 0.0491 Pa·s. The composition of the mixture (JSQ: RA=9:1) is shown in Table 1. The sulfonating agent was distilled with fuming sulfuric acid (55%—60% by weight) and mixed with 1,2-dichloroethane at a mass ratio of 1:3 in order to reduce the reaction intensity[13]. 1,2-Dichloroethane (1,2-CH2ClCH2Cl, with a purity of >99%, manufactured by the Beijing Chemicals Ltd.) was used as the solvent thanks to its stability and easy recovery. Addition of the solvent could increase the mobility and improve the mass transfer and heat exchange rate. Finally, the solvent was recycled via vacuum distillation.

Table 1 Composition of the feedstock (JSQ: RA=9:1)

All the solvents, such as ethanol, petroleum ether, isopropanol and n-pentane, used in this study were of analytically pure grade supplied by the Beijing Chemicals Ltd. without any further treatment. The test oil employed was obtained from the No. 2 Oil Production Site of the Daqing Oilfields with anacid number of 0.05 mgKOH/g, a density of 0.8452 g/mL and a kinematic viscosity of 23.79 mm2/s at 45 ℃. The oilfield produced water was used for oil/water IFT measurements. The polymer used was a partially hydrolyzed polyacrylamide (HPAM) with an effective mass content of 91.94%, a hydrolysis degree of 25 mol%, and a molecular weight of 1.6×107. If there were no special instructions, the concentration of polymer was set at 1 600 mg/g.

2.2 Experimental procedures

The RPB reactor mainly consisted of a rotator provided with a stainless wire mesh packing, a fixed casing, and different inlets to introduce liquid or gas. The inner and outer diameter of the rotator was 50 mm and 142 mm, respectively, and the axial length was 35 mm. The porosity and surface area of the packing was 0.90 and 500 m2/m3, respectively. The inner diameter of the sulfonating agent pipeline and oil fraction pipeline was 4.8 mm and 6 mm, respectively. A structure diagram of the moving part is shown in Figure 1. Prior to the experiments, all the pipelines and equipment were swept by 1,2-dichloroethone in order to remove the moisture.

Figure 1 Schematic diagram of the RPB reactor used in the sulfonation reaction1, 2—Liquid inlets; 3—Products outlet; 4 —Gas outlet; 5—Packing and rotator; 6—Jacket for cooling water; 7—Spindle

The experimental setup for sulfonation is shown schematically in Figure 2. The sulfonation experiment was performed as follows. The sulfonation reaction was a semicontinuous process. A total amount of 2 000 g of diluted petroleum fractions was firstly added into the stirred tank and then circulated between the tank and the RPB reactor several times at a rate of 300 mL/min in liquid inlet 1. Meanwhile, the sulfonating agent was sprayed into the packing through one hole with a diameter of 1 mm in the liquid inlet 2 and the rate was adjustable by a peristaltic pump. The amount of sulfonating agent was adjusted to obtain the desired ratio. The liquid hold-up in the RPB was calculated about 4.9% to 12.3% under the operating conditions according to Burns[16]. The average residence time in the RPB ranges from 4 s to 11s, and the time for all the material recycling once is about 7 min. The cooling water was circulated in the jacket between RPB and the stirred tank in order to maintain the temperature at a given value. The aging step was carried out in RPB with the same circulation volume and rotating speed (N) when the sulfonating agent was used up. Finally, the petroleum sulfonate surfactant was obtained after aging and neutralization procedures.

Figure 2 Scheme of experimental setup1—Sulfonating agent storage tank; 2—RPB reactor; 3—Circulating stirred tank; 4—Neutralization and separation tank

2.3 Analytical method

The petroleum sulfonate could be mainly divided into four components including active ingredients, unsulfonated oil, inorganic salts and volatiles. The separation and gravimetric determination methods were adopted[17-18]. The volatiles portion was determined by weight-loss analysis, and the inorganic salts were measured by dissolving the salts in petroleum ether and heated anhydrous ethanol alternately followed by vacuum filtration. Finally the active ingredients and unsulfonated oil were separated with pentane and aqueous isopropyl alcohol solution by multiple solvent/anti-solvent extractions. For the rationality andconvenience of comparison, the contents of active matter and unsulfonated oil were converted to values based on 0% volatiles content in the following data. The IFT between oil and water was determined by a spinning drop tensiometer (model 500C made by the University of Texas). The petroleum sulfonate surfactant concentration was 0.1% and the tests were performed at 45±0.1 ℃ for at least two hours. Before the measurement, the amplification factor corresponding to different concentrations of the surfactant solutions should be revised.

3 Results and Discussion

3.1 Effect of JSQ/RA mass ratio

Based on the principle of “Likes dissolve Likes”, the local oil fraction was the first choice. However, petroleum oil is a very complicated mixture with a broad molecular weight distribution and one type of oil fraction is generally difficult to meet the requirements. The material with different JSQ/RA mass ratios was sulfonated in a jar test and the effect on IFT is shown in Figure 3. It can be clearly seen that the ability of petroleum sulfonate mixture to lower the IFT was much better than that of JSQ alone. It was believed that there was a synergistic effect between the two kinds of sulfonate. When the ratio ranged from 7:3 to 9:1, the IFT decreased from 3.1×10−2mN/m to 6.3×10−3mN/m. On one hand, the property of JSQ was more similar to crude oil, so more JSQ in the mixture meant a better intermiscibility with crude oil. On the other hand, JSQ was a heavy distillate relative to RA. It was understood that heavy components usually had good lipophilicity and could be adsorbed on crude oil interface more closely. Thus, petroleum sulfonate with high molecular weight was more effective in lowering IFT[19]. It was worth noting that there was a contraction phenomenon making the IFT drop to a minimum of 5×10−3mN/m and then rise to a constant value of 1.4×10−2mN/m, when the ratio was 8:2. The contraction phenomenon was discussed later. For comparison, a ratio of JSQ: RA=9:1 which lowered the IFT to an ultra-low level was chosen for further study.

3.2 Effects of rotating speed

Figure 3 Effect of JSQ/RA mass ratio on IFT (surfactant concentration = 0.1%, without HPAM)■—JSQ;●—JSQ: RA=7:3;▲—JSQ: RA=8:2; ▼—JSQ: RA=9:1

Figure 4 Effect of rotating speed on components content (a) and oil/water IFT (b)Reaction teneperature (T=35℃, reaction time (t)=20 min, solvent/petroleum fractions mass ratio =1;1, petroleum fractions/sulfonating agent mass ratio =1:0.4)

Figure 4 shows the effect of rotating speed on components content (a) and oil/water IFT (b). Figure 4a indicates that the active matter content obviously increased from 42.38% to 48.62% when the rotating speed increased from 400 r/min to 800 r/min, indicating that the rapid anduniform mixing was a control factor for sulfonating such a viscous material at lower rotating speed. However, when the rotating speed was further increased to 1400 r/min, the active matter had a slight increase. It is indicated that the mixing was no longer a main factor influenaing the reaction in the high rotating speed phase. Usually, the higher rotating speed means the shorter average residence time. While, the RPB provided a good mixing condition, and the recycle operation extended the reaction time. Therefore, the active matter content didn’t reduce under the high speed ratation corditions. At the same time, the content of free oil and inorganic salt decreased from 48.24% to 42.73% and 6.57% to 3.48%, respectively, which was attributed to the intensified micromixing performance of RPB. Figure 4B presents the effect on oil/water IFT. Clearly, the IFT between oil and water remained almost unchanged. It was easy to understand that the IFT was mainly related to the structure of the surfactant. Sulfonation of petroleum fractions in a high-gravity environment would not change the location of the sulfonic groups, but only increased the yield. According to the above explanation, the rotating speed was specified at 800 r/min.

3.3 Effects of solvent content

As mentioned above, the sulfonation reaction was a diffusion-controlled and highly exothermic process. It was a good measure to improve mixing and heat removal by adding organic solvent to reduce the material viscosity. Besides, volatilization of solvent could take away part of the reaction heat. Figure 5 displays the influence on product composition (a) and interfacial properties (b) at different ratios of solvent/petroleum fractions. On a whole, the active matter increased with an increase of solvent content and then leveled off when the ratio was over 1.0. Without solvent, the active matter was only 17.6%, and the free oil and inorganic salt content was as much as 67.2% and 11.8%, respectively. Meanwhile, the products had no interfacial activity because the test oil droplets were not lengthened in the tensiometer during the 2-hours measurement. The reason was that the raw material oil with high viscosity and poor liquidity could hardly mix well with the sulfonating agent, which was prone to local over-sulfonation and oxidation reaction, resulting in formation of by-products such as sulfuric acid and sulfone. When the ratio was greater than 1.0, the product content and the IFT generally plateaued off thereafter. Upon taking the economic factor into account, a mass ratio of 1.0 was the optimum choice.

3.4 Effects of sulfonating agent dosage

Figure 5 Effect of solvent/petroleum fractions mass ratio on components content (a) and oil/water IFT (b)(T=35℃, t=20 min, N=800 r/min, petroleum fractions/sulfonating agent mass ratio =1:0.4).

Figure 6 shows the effect of petroleum fractions/sulfonating agent mass ratio on components content (a) and oil/water IFT (b). When the ratio ranged from 1:0.3 to 1:0.5, the active matter content increased from 37.94% to 53.66% and the free oil content decreased from 55.19% to 36.04%. With a further increase of the said mass ratio to 1:0.6, the active matter content decreased rapidly to 44.53% and the free oil content increased to 37.62%. Meanwhile, a small amount of acid sludge was also generated and the color of free oil changed from reddishbrown to black. The inorganic salt content showed a sharp increase because excessive amount of sulfur trioxide had formed sulfate in the neutralization step. The IFT reached an ultralow level when the ratio ranged from 1:0.3 to 1:0.5. The IFT first decreased to 4.7×10−3mN/m, and then increased to 9×10−3mN/m. Although sulfonation by sulfur trioxide was an equimolar reaction stoichiometrically, an appropriate excess over the equimolar ratio was required practically. This was because some sulfur trioxide could be volatilized to the air and some were consumed by the side reactions inevitably[20-21]. Besides, other components such as cycloparaffins could also be sulfonated[22-23]. Although the active matter content was the highest when the ratio was 1:0.5, the efficiency for lowering the IFT was not the greatest. When the reaction was carried out at a high mass ratio of sulfonating agent/petroleum fractions, it could easily form multi-sulfonic acids, which destroyed the hydrophile-lipophile balance, and as a result, the IFT value increased. So, the mass ratio of petroleum fractions/ sulfonating agent was recommended as 1:0.4.

3.5 Effects of reaction temperature

Figure 6 Effect of petroleum fractions/ sulfonating agent mass ratio on components content (a) and oil/water IFT (b)(T=35 ℃, t=20 min, N=800 r/min, solvent/petroleum fractions mass ratio=1:1).

Figure 7 Effect of reaction temperature on components content (a) and oil/water IFT (b)(t=20 min, N=800 r/min, petroleum fractions/sulfonating agent mass ratio =1:0.4, solvent/petroleum fractions mass ratio =1:1).

Temperature had a dual effect on sulfonation, because lower temperature could avoid the occurrence of adverse reactions, while higher temperature could decrease viscosity of the reactants and reduce the mass transfer resistance. The effect on sulfonation and IFT at temperatures ranging from 25 ℃ to 45 ℃ is shown in Figure 7. There was an optimum temperature for sulfonaton at which the active matter content was the highest, and IFT was ultralow. The active matter content continuously increased significantly from 37.63% to 48.62% with the temperature rising from 25 ℃ to 35 ℃, and then went up gently to 50.27% when the temperature increased to 45 ℃. The free oil content remained almost unchanged whentemperature was over 35 ℃. It was indicated that the aromatic compounds were sulfonated completely and the increase of the temperature could only promote the side reaction. The IFT reached its ultra-low value when the temperature increased from 25 ℃ to 35 ℃. Specifically, the IFT decreased from 6.1×10−3mN/m to 4.7×10−3mN/m with the reaction temperature increasing from 25 ℃ to 35 ℃. However, with the further increase in temperature to 45 ℃, the IFT went up to 1.28×10−2mN/m. Because sulfonation was highly exothermic and the side reactions such as over-sulfonation and oxidization would be aggravated at high temperature, 35 ℃ was determined as the optimum reaction temperature on account of the quality of the products as well as the expected IFT.

3.6 Effects of reaction time

The effect of reaction time on components content (a) and oil/water IFT (b) is shown in Figure 8. The reactants circulated between the RPB reactor and the agitator tank. Therefore, the reaction time was adjusted by the rate of introduction of sulfonating agent. The material could pass through the RPB reactor 3 to 7 times when the reaction time ranged from 0 to 40 min. The reaction time of“zero” meant that the reactants passed through the reactor once without recycling. The experiments indicated that the active matter content increased sharply with an increase of reaction time and then went down gradually when the time was more than 20 minutes. On the contrary, the free oil and inorganic salt content decreased at first and then rose mildly. Additionally, when the reaction occurred without recycling, 47.5% of active matter increment (23.9% when t=0 and 50.3 % when t=20 min) was attributed to reaction in the RPB. This result indicated that the RPB plays not only a role of efficient mixer, but also is a location for reaction to occur. The IFT decreased from 1.36×10−2mN/m to 4.7×10−3mN/m when the reaction time ranged from 0 to 20 minutes and then increased to 8.43×10−3mN/m. On one hand, it was necessary to increase the reaction time for sulfonation of some cycloalkanes. On the other hand, sulfonation of aromatic compounds was a fast reaction, and a too long reaction time usually led to side reactions. Furthermore, petroleum sulfonate was not readily dissolved and its aqueous solution was susceptible to stratification when the reaction time was 0 min because of its high free oil content, while at an extended reaction condition, the sulfonate solution was aqueous with a tea-like color but not galactoid. It was indicated that the surfactant was more hydrophilic because of the increase of multi-sulfonic acid content, which was unfavorable to reduction of the oil/water IFT.

3.7 Effects of aging time

Although sulfonation reaction was very rapid, it did not complete in RPB reactor and required a further residence time of 30—50 min called as the ageing step. The usual explanation for this phenomenon was that the sulfur trioxide reacted on arenes at different stoichiometric coefficients. And the by-products, anhydride, sulfuric acid and even the sulfone, together acted as a slow sulfonating agent to sulfonate the remaining aromatic hydrocarbons[24].Figure 9 shows the effect of aging time on active matter, and the free oil and inorganic salt content. The active matter content increased with an increase in aging time and then leveled off 30 minutes later. The free oil content and the inorganic salt content decreased at first and then plateaued thereafter.

Figure 8 Effect of reaction time on components content (a) and oil/water IFT (b)(T=35 ℃, N=800 r/min, petroleum fractions/sulfonating agent mass ratio =1:0.4, solvent/petroleum fractions mass ratio =1:1).

Figure 9 Effect of aging time on components content

3.8 Comparison of the product performance between the RPB and STR

A 2000-mL, three-necked, round bottom flask with a mechanical stirrer was used to simulate a STR. Table 2 gives a comparison of the product performance in the RPB reactor and the STR under similar operating conditions. The active matter content was measured to be 50.3% for RPB and 36.18% for STR. The former was by 14.12% higher than the latter. The IFT between oil and water was reduced. Thus it could be envisioned that the RPB had exhibited a great potential for sulfonation applications, especially for the sulfonation of high boiling alkylated benzene such as petroleum fractions.

Table 2 Comparison of the product performance in the RPB reactor and the STR

3.9 Some phenomena observed in IFT measurement

Some interesting phenomena were observed during the measuring process. The oil drop was prolonged first and then contracted to a stable value or sometimes, the oil drop was wide on both sides, but narrow in the middle (as shown in Figure 10).

There was much discussion in the literature that the process at the liquid/liquid interface was diffusion-controlled, and the transport of particles from the bulk to the interface played an important role in IFT change with respect to time[25-26]. Figure 11 shows the effect of polymer concentration on IFT of two kinds of petroleum sulfonate surfactants. The polymer had little effect on final IFT but could greatly increase the viscosity of the aqueousphase[27-28]. The increase of viscosity led to the increase of diffusion resistance of the petroleum sulfonate monomers, which would have a significant impact on the instantaneous change in IFT value.

Figure 10 The retraction phenomena (a) and uneven change (b) of the test oil during the IFT measurement

Sample A was the sulfonate mentioned above, and sample B was another sulfonate with different formulation in which the content of heavy components was less than that of sample A. For sample B, there was a retraction phenomenon leading to the drop in IFT at first and then its rise without HPAM. However the retraction phenomenon disappeared in the presence of HPAM and the final IFT was almost unchanged. Furthermore, upon comparing the DIT (dynamic interfacial tension) curves between sample A and sample B, it was found out that the retraction phenomenon disappeared as well and the final IFT reduced, which indicated that the increase of heavy components not only eliminated retraction but also reduced IFT value to a minimum. The reason may be that the heavy components usually had good lipophilicity and were adsorbed on crude oil interface more closely, so the petroleum sulfonate with high molecular weight was more effective in lowering the IFT. This phenomenon is very meaningful for practical application, because the oilfields are entering into a high water-cut stage and the composition of crude oil is becoming heavier. So, the surfactant used for EOR should also increase its molecular weight simultaneously.

Figure 11 Effect of HPAM concentrations on DIT■—Sample A;●—Sample B; —Sample A containing 800 μg/g of HPAM; ▼—Sample A containing 1 600 μg/g of HPAM;▲—Sample B containing 1 600 μg/g of HPAM

4 Conclusions

The petroleum sulfonate with an ultralow IFT was obtained by sulfonation of two kinds of local petroleum fractions because local petroleum fractions could be better matched with the crude oil and there was a synergy between these two kinds of surfactants. Furthermore, on recognition of conventional reactors’ deficiency in micro mixing for rapid sulfonation reaction, RPB was employed for the synthesis of petroleum sulfonate surfactant via sulfonation of petroleum fractions with diluted liquid sulfur trioxide. Qualified petroleum sulfonate was obtained by virtue of the intensified micro-mixing performance of RPB. The active matter in the RPB reactor was by 14.12% higher than that obtained in the STR. The optimum experimental conditions were specified at a rotating speed of 800 r/min, a solvent/petroleum fractions mass ratio of 1:1, a petroleum fractions/sulfoanting agent mass ratio of 1:0.4, a reaction time of 20 min, a reaction temperature of 35 ℃ and an aging time of 30 min, under which the active matter content was as high as 50.3% and the IFT could reach 4.7×10−3mN/m.

The uneven change of the test oil droplet during the IFT measurement was also discussed. The contraction phenomenon was related to the diffusion rate and petroleum sulfonate structure. The former could not change the IFT equilibrium value but could eliminate the contraction phenomenon only, while in the latter case, for example, increasing the heavy components content of petroleum sulfonates not only could eliminate the contraction phenomenon, but also could reduce the IFT to a minimum.

Acknowledgements:This work was financially supported by Project of the National Twelfth Five-Year Research Program of China (Grants. 2014BAE03B02)

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date: 2014-08-20; Accepted date: 2014-10-08.

Professor Chen Jianfeng, E-mail: chenjf@mail.buct.edu.cn; Zhang Pengyuan, zhangpy@mail. buct.edu.cn.