Efficient Demulsification of Diesel-in-Water Emulsions by Different

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Efficient Demulsification of Diesel-in-Water Emulsions by Different Structural Dendrimer-Based Demulsifiers Li Hao,†,‡,§ Bin Jiang,†,‡,§ Luhong Zhang,† Huawei Yang,† Yongli Sun,† Baoyu Wang,† and Na Yang*,† †

School of Chemical Engineering and Technology, Tianjin University, Tianjin, P. R. China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin, P. R. China § National Engineering Research Center of Distillation Technology, Tianjin University, Tianjin, P. R. China ‡

S Supporting Information *

ABSTRACT: A series of amine-based dendrimer polyamidoamine (PAMAM) demulsifiers with different initial cores were synthesized and investigated in the demulsification process of diesel-in-water emulsions. With the aim of systemic evaluation of their demulsification performance, some important factors of the demulsification processes were investigated including demulsifier dosage, settling time, temperature, oil content, and kinds of diesel. The demulsifier with the triethylenetetramine (TETA) initial core provided excellent demulsification performance by removing oil with less dosage and at relatively low temperature in short periods and reached 96.66% demulsification efficiency for catalytic cracking diesel emulsion. The results showed its good application prospects. In order to gain insight into the demulsification process and mechanism, some measurement methods were adopted. Micrograph and droplet size distribution of emulsions illustrated that the PAMAM demulsifier could lead to the breakup of diesel-in-water emulsions by flocculation and coalescence. The surface tension and interfacial tension gave a basic understanding of the demulsification mechanism. Zeta potential indicated that emulsion had been broken up. The conductivity measurement explained the demulsification mechanism from the aspect of the electrostatic interactions of moving droplets. The dendrimer and SDS had strong aggregation interactions in the system according to the results of hydrodynamic radium.

1. INTRODUCTION Emulsification plays a significant role in a wide range of industrial processes such as food, pharmaceutical, material coating, cosmetics, drug delivery mechanisms, and petroleum processing.1 With the application of polymer-flooding technology becoming more widely used than ever in enhanced oil recovery, demulsification of oil−water emulsions is a major issue that received sustained attention for decades in oil recovery and oil-spill remediation.2 Interfacial active substances resulting from polymer-flooding technology form interfacial film and make emulsions more stable to break up by reducing the interfacial tension and creating steric repulsion between the dispersed liquid droplets.3 Oil emulsions are deleterious to the environment and aquatic life. Thus, it is a challenge to efficiently remove residual oil from wastewater before discharge to meet the requirements of environmental protection and energy conservation. There is a variety of oil emulsions in industry. Oils that are found in contaminated water can be heavy hydrocarbons such as tars, grease, crude oils, diesel oils, and light hydrocarbons, viz., kerosene, jet fuel, and gasoline.4 Among them, research on diesel oil emulsions are not enough. Unlike free or floating oil spilled at sea, most of the petroleum wastewaters contain oil-in-water emulsions among their basic contaminants, which usually have a high oil content and small oil droplet size. The concentration of oil in effluents from different industrial sources is found to be as high as 40,000 mg/L.5 In the early stages, researchers mainly focused on the demulsification of water-in-oil emulsions due to the exploitation of primarily developed oil fields in which less water was contained. As an economical and convenient method, chemical © 2016 American Chemical Society

demulsification has been widely employed in the petroleum industry to break up water/oil (W/O) emulsions. Demulsifiers are amphiphilic compounds, which can destabilize emulsions by changing the interfacial film properties, such as interfacial tension, mechanical strength, elasticity, and thickness of interfacial regions to promote coalescence, or through flocculation of water droplets.6−9 For W/O emulsions, the demulsifiers are exclusively oil-soluble to allow them to access the oil/water interface through the continuous oil phase. Comparing with W/O emulsion, studies about O/W emulsions have attracted much more attention in recent years and research has not been sufficient and profound until now. Therefore, treatment of various O/W emulsions has become one of the most serious issues in the petroleum industries. Several separation methods have been used in field applications, namely, gravity separation, chemical, thermal, mechanical, membrane, absorption, biological, and microwave methods.10−14 However, limitations, such as high operating cost, high energy consumption, low breaking efficiency, and long settling time, mean that research still requires much improvement. So it is necessary to develop a fast and high efficiency method to remove residual oil from O/W emulsions. Numerous chemical demulsifiers were suggested for demulsification, such as pluronic block copolymers, biodegradable polymeric nanoparticles, ionic liquids, dendrimers, and Received: Revised: Accepted: Published: 1748

November 21, 2015 January 12, 2016 January 26, 2016 January 26, 2016 DOI: 10.1021/acs.iecr.5b04401 Ind. Eng. Chem. Res. 2016, 55, 1748−1759

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Industrial & Engineering Chemistry Research graphene oxide nanosheets. Feng et al.15 used a nontoxic and biodegradable polymer, ethylcellulose, to break up emulsified water from naphtha-diluted bitumen. The addition of ethylcellulose also assisted the removal of fine solids with the water. Diego et al.16 synthesized surface-active ionic liquids that were evaluated for the first time as demulsifier agents for water-incrude oil emulsions of light, heavy, and ultraheavy Mexican crude oil under conventional and microwave dielectric heating. The use of microwave irradiation accelerated and increased significantly the efficiency of demulsification of ultraheavy crude oil emulsion. Liu et al.17 introduced an amphiphilic material, graphene oxide nanosheets, as a versatile demulsifier to break up the oil-in-water emulsion at room temperature. Efficient as they are, these demulsification techniques have their own limitations, which are especially obvious in treating polymerflooding oil-extraction wastewater. Also, there is a lack of studies on using demulsifiers to treat emulsions with small oil droplets. Besides, most previous studies were based on crude oil emulsions, and studies on other emulsions have been quite rare until now. Dendrimers are specially designed macromolecules with certain size, shape, and reactivity. Generally, they are branched from a central core, with numerous terminal groups surrounding this core so as to produce an empty interior. This novel kind of dendrimer was developed by Tomalia and Newkome in the middle of the 1980s.18,19 Dendrimers are considered as nanometer-sized containers, with the relatively empty interior as the cage and the crowded terminal groups as the steric wall. The solubilization mechanism is one common to demulsificaiton mechanisms. It typically means that natural surfactants are dissolved into the bulk phases of the demulsifier.19 Encouraged by the excellent performance of dendrimers as nanocontainers for drug delivery, catalysis, etc., dendrimers with properly designed structures might also be used as efficient demulsifiers. They can dissolve or adsorb the original interfacial substances into or onto the nanocontainers rapidly and lead to the destruction of emulsion. Polyamidoamine (PAMAM) is a kind of dendrimer, which has a polar but hydrophobic interior with polar terminal groups on the outer surface.20 Experimental results revealed that the structure of the terminal group contributed most to the demulsification process. Only the amine-based dendrimer proved to be an effective demulsifier. In addition, demulsificaiton became more efficient as the generation number of dendrimers increased. The results seemed to be consistent with the solubilization mechanism of the surfactants onto the surface of the amine-terminated dendrimers through ionic interactions with the negatively charged surfactants, although encapsulation of surfactants into dendrimers cannot be completely ruled out.19 Even though numerous research papers on PAMAM have been published, research on its demulsifier characteristics and demulsification ability has not been conducted much. According to the previous studies, O/W emulsions are stabilized in the aqueous phase by either one or both of the following mechanisms:21 (i) steric interactions or structural barriers and (ii) electrostatic repulsion between oil droplets. Steric interactions are short-range forces due to the presence of fine solid particles or macromolecules adsorbed onto the droplet surface. Electrostatic interactions arise from the electrical potential difference between two nonmiscible phases and are important at relatively long distances. Most O/W emulsions are stabilized by emulsifying agents (generally surfactants) that have affinity for the oil/water interface, with

an ionized hydrophilic group oriented toward the aqueous phase and a hydrophobic chain immersed in the oil phase.22 However, studies on demulsification mechanisms have not been quite enough. In our previous work, Yao et al.23 synthesized an amine-based dendrimer PAMAM demulsifier with 1,3-propanediamine as an initial core and investigated the efficiency of the demulsifier in dealing with diesel-in-water emulsions with ultrafine oil droplets (average diameter of oil droplets less than 2 μm). As a continuation of this work, we try to improve the drawbacks in the demulsification, such as relatively high demulsifier dosage and lone settling time. In addition, we pay more attention to the demulsifier characteristics and aggregation behavior of the PAMAM molecules. In this study, we synthesized two amine-based dendrimer PAMAM demulsifiers with different initial cores and different chain lengths, taking the demulsifier with the 1,3-propanediamine core as the control, to form a series of PAMAM demulsifiers and study their demulsifier characteristics. This study investigated the influence of structural variations of demulsifiers on demulsification efficiency. The demulsification performance was evaluated in diesel-in-water emulsions with different oil contents, and the influences of demulsifier dosage, settling time, and temperature on the performance of the demulsifier were discussed in detail. Among the PAMAM demulsifiers, the demulsifier with the triethylenetetramine (TETA) core performed the best with less dosage, shorter settling time, and relatively lower temperature. The demulsifier with the TETA core was applied to both catalytic cracking diesel-in-water emulsion and petroleum coking diesel-in-water emulsion, and the demulsification performance was measured. In order to explain the demulsification mechanism, micrograph and droplet size distribution of emulsions were taken to confirm the coalescene process. Surface tension and interfacial tension were measured to give a basic understanding of the demulsification mechanism. Zeta potential and conductivity of the demulsification process were also characterized to illustrate the demulsification mechanism from the view of electrostatic interactions of moving droplets. The aggregation behavior of a demulsifier and sodium dodecyl sulfate (SDS) was deduced from the hydrodynamic radium point of view. The results provided an improved understanding of the correlation between demulsifier characteristics and demulsifier performance.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were used directly without further purification. Methyl acrylate (AR grade, ≥ 0.98), ethanediamine (AR grade, ≥ 0.98), and methanol (AR grade, ≥ 0.98) were all purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. Diethylenetriamine (DETA) (AR grade, ≥ 0.98), triethylenetetramine (TETA) (AR grade, ≥ 0.98), and 1,3-propanediamine (AR grade, ≥ 0.98) were purchased from Aladdin Reagents. Sodium dodecyl sulfate (SDS) (CR grade) was purchased from Tianjin Guangfu Fine Chemical Research Institute. Deionized water used throughout this study was prepared with a Nanopure II water purification system. Diesel was purchased from a local gas station in Sinopec (Tianjin, China). Catalytic cracking diesel and petroleum coking diesel were obtained from the Dagang oilfield (Tianjin, China) and were directly used without further treatment. 2.2. Synthesis of Dendrimer-Based Demulsifiers. Two PAMAM dendrimer-based demulsifiers with different central initial cores (DETA and TETA) were prepared. 1749

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Industrial & Engineering Chemistry Research Demulsifiers with a DETA core had five functional groups (Nc = 5), and the PAMAM dendrimer and demulsifiers with a TETA core had Nc = 6, where Nc represents the core functional groups’ number to form dendrimers. In order to evaluate the demulsification performance of PAMAM dendrimer-based demulsifiers with different Nc and branch cells, we also synthesized the demulsifiers with a 1,3-propanediamine core (Nc = 4) as the control. The PAMAM demulsifiers were synthesized in a two-step method with three cycles, and one cycle of the synthesis procedure is shown in Figure1. The molecular structures of the final product demulsifiers are shown in Figure 2.

The PAMAM dendrimer family initiated from the DETA core or TETA core. A portion of 12.5 mL of DETA or TETA was dissolved in 100 mL of methanol, and then 130 mL of methyl acrylate was added into the flask. The mixture was magnetically stirred for 24 h at 25 °C. The solvent and unreacted methyl acrylate were removed in a rotary evaporator, and then the resultant was dried overnight under vacuum at 50 °C for further purification. Subsequently, the resultant was then dissolved in 100 mL of methanol at 25 °C, and then 130 mL of ethylenediamine was added into the flask. The mixture was stirred for 24 h at this temperature. The solvent and unreacted ethylenediamine were removed in a rotary evaporator, and then the resultant was put into a vacuum oven for further purification. The above process was then repeated twice. Then the final product was obtained. The obtained PAMAM dendrimers were characterized as follows. The molecular structures were analyzed by 1H NMR and 13C NMR spectrometer (Varian INOVA 500 MHz). To further identify the structure of demulsifiers, Fourier transform infrared spectroscopy (FTIR) was also recorded on a Nicolet Nexus FT-IR spectrometer. The samples were prepared as KBr pellets. 2.3. Preparation of Diesel-in-Water Emulsions. The O/W emulsions were prepared with diesel, SDS, and deionized water. Samples of diesel (50 g) and SDS (1 g) were added into a volumetric flask. Then deionized water was added into the flask until the volume of the mixture reached one liter. Then the mixture was stirred using a homogenizer (Fluke homogenizer, FA25, 500W) operated at 10,000 rpm for 5 min, which caused disruption of the droplets by a combination of turbulence and intense shear flow. The emulsions of catalytic cracking diesel and petroleum coking diesel were also prepared using the above process. The emulsions were prepared fresh each day using the procedure above and were found to be stable within the experimental time frame. They were used for the demulsification tests. The resulting emulsions were referred to as diesel-in-water emulsions, catalytic cracking diesel-in-water emulsions, and petroleum coking diesel-in-water emulsions, and the diesel content in the O/W emulsion was 5 wt %. In this work, we also prepared emulsions with oil content of 0.3 wt % as a comparison. 2.4. Demulsification Test. To evaluate the effectiveness of demulsification, 25 mL of freshly prepared emulsion and 1 mL of demulsifier aqueous solution with a certain concentration were thoroughly mixed in a 25 mL colorimeter tube. The mixture was then placed in a water bath under different temperatures for gravity settling in different periods of time. Subsequently, a water sample was taken from the bottom of the colorimeter tube and further analyzed using an ultraviolet spectrophotometer (UNIC, UV-4802) to determine the residual oil content. The demulsification efficiency was calculated by using eq 1. Blank tests were performed for diesel-in-water emulsions without demulsifier addition as the control. Each sample was repeated three times, and the oil content reported is the average of the three repetitions. The sizes of oil droplets in the diesel-in-water emulsions before and after the demulsification process were both measured by a Malvern Mastersizer 3000.

Figure 1. One cycle of synthesis procedure of the PAMAM demulsifiers: (a) demulsifier with DETA initial core and (b) demulsifier with TETA initial core.

E= 1750

C0 − C × 100% C0

(1) DOI: 10.1021/acs.iecr.5b04401 Ind. Eng. Chem. Res. 2016, 55, 1748−1759

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Figure 2. Molecular structure of PAMAM demulsifiers: (a) demulsifier with DETA initial core and (b) demulsifier with TETA initial core.

where E is the demulsification efficiency (%), C0 is the initial oil content (mg/L) of the emulsion, and C is the oil content after the demulsifier solution was added. Micrographs of the emulsions were obtained using an optical microscope equipped with a digital video camera. The emulsion

sample was put on an object slide and then covered with a cover glass. A lower oil content and larger oil droplet size indicated a higher demulsification efficiency. 2.5. Surface Tension and Interfacial Tension Measurement. The surface tensions were measured using an interfacial 1751

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120 min verified the high emulsion stability. By comparing the demulsification efficiency of the two demulsifiers and the similar demulsifier with a 1,3-propanediamine initial core,23 it is found that the TETA core is obviously superior to the DETA and 1,3-propanediamine cores since the oil removal rate of the demulsifier with the 1,3-propanediamine initial core was only 70.32% at 60 min at the same conditions. The order of demulsification efficiency before 60 min is consistent with the order of branches number. However, when the settling time increases to 90 min, the demulsifiers with the DETA and TETA cores have efficiency of 87.72% and 94.24%, while the demulsification efficiency of the demulsifier with the 1,3propanediamine core reaches up to 91.15%, which is higher than that with the DETA core. It can be concluded that the molecular weight of the dendrimer-based demulsifiers also plays a significant role in demulsification. In general, lower molecular weight polymeric demulsifiers possess high interfacial activity and adsorb irreversibly at the oil/water interface and are easier to diffuse to the oil/water interface, causing film rupture and coalescence of oil droplets, leading to higher oil removal rate. High molecular weight, which is caused by more branches numbers, is more effective in flocculation oil droplets, thereby destabilizing emulsions. In addition, a dendrimer performs better with more branches in the aspect of demulsification. Therefore, the structure (length of chains) and molecular weight of dendrimer-based demulsifiers play collaborative roles in demulsification. Under the comprehensive action of these effects, the demulsifier with more compact surface amino groups (with a TETA core) performs the best. To clarify the effect of zeta potential on O/W emulsions stability, zeta potential before and after demulsification was characterized, and the results are shown in Table 1. The fresh

tensiometer (KINO, SL200 KS). The interfacial tensions of the diesel/water interface with the dendrimer-based demulsifiers with different concentrations in the water phase were also measured using an interfacial tensiometer (KINO, SL200 KS). The interfacial tensions of the diesel/water interface without demulsifiers in the water phase and that with only SDS were measured as the control. 2.6. Zeta Potential Measurements. The zeta potentials of the dendrimer-based demulsifiers and diesel-in-water emulsions were measured with a zeta potential analyzer (Zetasizer Nano ZS90). Each measurement was repeated 10 times at room temperature. The average values and standard deviation are reported. 2.7. Conductivity Measurements. The conductivities of the dendrimer-based demulsifiers aqueous solution and three kinds of diesel-in-water emulsions (diesel-in-water emulsion, catalytic cracking diesel-in-water emulsion, petroleum coking diesel-in-water emulsion) were measured with a conductivity meter (Tianjin Shengbang, DDS-302B).

3. RESULTS AND DISCUSSIONS 3.1. Characterizations of PAMAM Demulsifiers. The structures of two dendrimer-based demulsifiers have been identified by 1H NMR, 13C NMR, and FTIR. The results are shown in Figure S1 of the Supporting Information. The results of 1H NMR, 13C NMR and FT-IR analyses confirmed that PAMAM demulsifiers were successfully synthesized with extremely high integrity. 3.2. Demulsification Performance of PAMAM Demulsifiers with Different Initial Cores. The demulsification experiments using demulsifiers with a DETA initial core (G3) and TETA initial core (G3) were conducted simultaneously under the conditions of T = 50 °C and demulsifier dosage = 1500 mg/L, and the results are shown in Figure 3. The oil

Table 1. Zeta Potential Measurement of Fresh Emulsion and After Demulsification Emulsion sample name

zeta potential/mV

fresh diesel-in-water emulsion separated oil layer after demulsification separated water layer after demulsification

−94.6 −79 0.064

emulsion’s zeta potential is −94.6 mV, which shows extreme stability. This stabilization may be due to the fact that diesel-inwater emulsion has a high charge density generating from ionization, adsorption, and friction between oil droplets. The electrostatic repulsion by the negatively charged groups contributes to the stable dispersion of oil in water. After the addition of 150 mg/L of TETA core demulsifier at 40 °C for 60 min, the separated oil phase’s zeta potential becomes a little smaller than the fresh emulsion in absolute value. The smaller absolute zeta potential of the oil phase means that it is easier to coalesce and also implies the attraction force beyond the repulsive force. Enforcing the van der Waals attraction between the oil droplets increases the probability of oil aggregation, so the dispersed phase tends to be destroyed. After demulsification, the zeta potential of the water phase becomes a positive charge 0.064 mV, which shows that the diesel-in-water emulsion has been broken up. This phenomenon can be explained by the double layer theory of emulsions. Thin double layers often lead to loss of electrostatic stabilization. 3.3. Effect of PAMAM Demulsifiers Dosage and Temperature on Demulsification. Figure 4(a) illustrates the effect of the demulsifier dosage on the demulsification

Figure 3. Demulsification efficiency of PAMAM demulsifiers with different initial cores as a function of settling time at 50 °C and with 1500 mg/L dosage.

removal rates of both grow over time, and the demulsification efficiency of the demulsifier with the TETA core is obviously higher than that with the DETA core. The oil removal rate of the demulsifier with the TETA core reaches 83.24% and 93.19% at 30 and 60 min, respectively, while the oil removal rate of the DETA core demulsifier is 75.58% and 83.36%, respectively. In addition, a blank test without a demulsifier was also conducted, and the obtained 9.1% oil removal rate at 1752

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further reduces the diffusion of PAMAM demulsifier in the diesel-in-water emulsions. 3.4. Effect of Settling Time on Demulsification. Figure 5 illustrates the effect of settling time on demulsification

Figure 4. Effect of TETA core demulsifier dosage on demulsification efficiency for different diesel content in emulsions: (a) for 50g/L of diesel content in diesel-in-water emulsion at different temperatures and (b) for 3g/L diesel content in diesel-in-water emulsion at 50 °C.

efficiency at different temperatures for 50g/L of diesel content emulsion. The oil removal rate reaches up to 90% with a demulsifier dosage of 200 mg/L at three temperature circumstances, which indicates its excellent demulsification performance. Therefore, the optimum dosage of a demulsifier with a TETA core is 200 mg/L, a relatively less dosage. The demulsification efficiency increases with an increase in temperature in the range of 30−50 °C. The stronger motion of demulsifier molecules at relatively higher temperature accelerates their transportation toward the surface of oil droplets, thus greatly enhancing the demulsification process. The demulsification efficiency of 3g/L of diesel content emulsion at 50 °C as a function of demulsifier dosage is shown in Figure 4(b). The performance is good but not outstanding. The oil removal rate with 500 mg/L and 1500 mg/L of TETA core demulsifier dosage is 72.12% and 76.76%, respectively, similar to that of the 1,3-propanediamine core demulsifier. Higher dosage of a demulsifier to obtain excellent demulsification performance is less cost-effective. Hence, the dendrimerbased demulsifiers are not well suited to demulsify low oil content emulsions. This can be explained as follows. When oil content is low, dendrimer-based demulsifiers tend to selfassociate at the oil/water interface, leading to the formation of demulsifier domains, which causes steric stabilization and

Figure 5. Effect of TETA core demulsifier settling time on demulsification efficiency in diesel-in-water emulsion at (a) 30 °C and (b) 50 °C.

efficiency in diesel-in-water emulsion with demulsifier concentration ranging from 50 to 250 mg/L at 30 °C (Figure 5a) and 50 °C (Figure 5b). When settling time is 30 min, the demulsification efficiency of the demulsifier with the TETA core is 80.06% and 83.50% under 30 and 50 °C, respectively, with a dosage of 50 mg/L. The demulsification efficiency increases more significantly at 50 °C than at 30 °C. When settling time is extended to 60 min, the demulsification efficiency increases to 91.56% under 30 °C and 97.74% under 50 °C with a demulsifier concentration of 250 mg/L, which shows excellent performance and perfectly meets the industrial requirements. The optimal demulsification settling time could be 60 min because the demulsification efficiency increases slightly after that, which is caused by the dynamic equilibrium of the molecular motion at the interface. Whereas using demulsifier with a 1,3-propanediamine core in the previous literature, the oil removal rate of 1000 mg/L dosage at 30 °C is extremely low, only 29.39%, when settling time is 60 min. The low demulsification efficiency of a 1,3-propanediamine core demulsifier can be attributed to its relatively short molecular 1753

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3.5. Demulsification Performance on Industrial Diesel-in-Water Emulsions. In order to accurately test the applicability of a demulsifier with a TETA initial core, the demulsifier was applied to both catalytic cracking diesel and petroleum coking diesel emulsions under the condition of 50 g/L diesel content, settling time of 60 min, and demulsification temperature of 40 °C. The results are shown in Figure 8. It is obviously shown that almost no demulsification happens with a demulsifier dosage of 50 mg/L for the petroleum coking diesel-in-water emulsion. Although the dosage extends to 250 mg/L, the demulsification efficiency is still not satisfactory, just 45.29%. However, the demulsifier is much more suitable for catalytic cracking dieselin-water emulsion because the demulsification efficiency reaches 96.66% with a demulsifier concentration of 250 mg/L, which perfectly meets the industrial requirements. According to the diesel producing process, both catalytic cracking diesel and petroleum coking diesel are affluent with aromatic, colloid, and asphaltene, unsaturated. Higher content of carbon residue or coke powder in petroleum coking diesel makes the emulsions much too stable to break. Carbon residue or coke powder may adsorb on the oil/water interface and form a layer of stable protective film, which hinders the coalescence of oil droplets. As shown in Figure 9, the average size of catalytic cracking diesel-in-water emulsion and petroleum coking diesel-in-water emulsion is 5 and 4 μm, respectively. The size distribution ranges are both narrow in two fresh emulsions (2−6.5 μm). In the presence of a TETA core demulsifier with dosage of 150 mg/L and settling time of 60 min at 40 °C for emulsion of catalytic cracking diesel, the addition of a demulsifier leads to flocculation and coalescene of oil droplets to a larger droplet size (average size 8 μm) and a wider distribution range (2−16 μm) in the separated oil phase. According to the experimental result, the oil removal rate in the separated layer is 85.66%. The micrographs of industrial catalytic cracking diesel emulsion without and with TETA core demulsifier addition are shown in Figure 10. Without demulsifier addition, the oil droplets are mostly 2−6 μm in diameter (Figure 10a). After the process of demulsification under settling of 60 min at 40 °C with 150 mg/L dosage, oil droplets coalesce to become larger. The average oil drops size is 8 μm, and some oil droplets size is up to 16 μm. It is also interesting to note the presence of small oil droplets attaching to large oil droplets when a demulsifier is added, indicating effective flocculation but incomplete coalescence of oil droplets (Figure 10b). With the addition of a demulsifier, only a few fine oil droplets remained in the water phase (Figure 10c). 3.6. Surface Tension and Interfacial Tension. To further understand the effect of a PAMAM demulsifier initial core on its interfacial activity and its demulsification efficiency, Figure 11 shows the surface tension of demulsifier aqueous solutions and the interfacial tension of the diesel/water interface with two different core demulsifers in the water phase. As shown in Figure 11(a), a TETA core PAMAM demulsifier leads to a slight increase in surface tension of pure water. A DETA core PAMAM demulsifier cannot siginificantly lower the surface tension of pure water; thereby, DETA and TETA core demulsifiers do not behave like typical surfactants for the air/water interface. As shown in Figure 11(b), DETA and TETA core demulsifiers are both effective in decreasing interfacial tension. The interfacial tension at the diesel/pure water interface is 20.04 mN/m. With a small amount of DETA and TETA core demulsifier, the interfacial tension is dramatically

chains, which are ineffective in bridging nearby oil droplets, leading to low flocculation ability. All of these results suggest that the TETA initial core dendrimer-based demulsifier can be used as a fast and highperformanced demulsifier to separate diesel-in-water emulsion, presenting the characteristics of a small amount of dosage, short settling time, and low temperature in the demulsification process. Figure 6 describes oil droplet size distribution before and after demulsification. It is shown that the average droplet size of

Figure 6. Oil droplet size distribution of different stages of diesel-inwater emulsion.

a fresh emulsion is 2.5 μm, which means that the fresh diesel-inwater emulsion is very stable. When measuring droplet size distribution of the fresh emulsion without demulsifiers after 24 h via only gravity, the average size of the emulsion is 6 μm, which means that the emulsion is still relatively stable. Through demulsification with 150 mg/L of TETA core demulsifier addition for settling of 60 min at 40 °C, the oil droplet size range of the separated oil layer becomes wider dramatically, from 14 μm for the fresh emulsion to 28 μm for the separated oil phase. In the meantime, the average size of the separated oil layer is 8 μm, larger than the fresh emulsion and the 24 h emulsion. Besides, for the separated water phase, the oil droplet size distribution range becomes narrower than the fresh emulsion. The results are consistent with the oil removal rate, which is 90.02% in the separated water phase. Therefore, it can be concluded that the TETA core demulsifier destroys dieselin-water emulsion stability and accelerates oil droplet flocculation and coalescence, so the oil/water separation process is greatly sped up. The micrograph in Figure 7 further reveals the demulsification mechanism of a TETA core demulsifier addition. Without any demulsifier addition, most oil droplets are less than 3 μm in size (Figure 7a). In the presence of 150 mg/L of demulsifier for settling of 60 min at 40 °C, the size of the oil droplets becomes much larger. The average size of the oil drops increases to about 10−20 μm, and some oil drops size is more than 20 μm (Figure 7c). Besides, in Figure 9c, the oil droplet distribution becomes more intensive, and most of the oil droplets moved upward to the oil phase of the emulsion. Hardly any oil droplets remain at the water phase of the emulsion (Figure 7d). In contrast, only a few oil droplets coalesce, and most oil droplets maintain the same size as the fresh emulsion without any demulsifier after 24 h (Figure 7b). 1754

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Figure 7. Micrograph images of diesel-in-water emulsion: (a) without any demulsifier addition, (b) without demulsifier after 24 h via gravity at room temperature, (c) oil phase after demulsification of 150 mg/L of TETA core demulsifier for 60 min at 40 °C, and (d) water phase after demulsification of 150 mg/L of TETA core demulsifier for 60 min at 40 °C.

Figure 9. Oil droplet size distribution of different stages for catalytic cracking diesel-in-water emulsion and petroleum coking diesel-in-water emulsion.

Figure 8. Demulsification efficiency for catalytic cracking diesel emulsion and petroleum coking emulsion as a function of TETA core demulsifier dosage.

surfactants and SDS at the interface could be dissolved into the bulk phases of demulsifiers. The two demulsifiers demulsification efficiency is quite different in Figure 3. Therefore, the difference in demulsification performance between the DETA and TETA core demulsifiers could be contributed to their length of chains and molecular weights, which control their flocculation ability. Additional parameters other than interfacial tension should also be considered when selecting a demulsifier.

lowered from 9.39 mN/m (at diesel/water interface with 1g/L SDS) to nearly 1 mN/m. With an increase in demulsifier concentration, no obvious interfacial tension variance is observed, and the two demulsifiers have nearly the same interfacial activity. Once demulsifier molecules reach the surface of the oil droplets, they rapidly adsorb at the oil/water interface and tend to form nanoaggregates, which lead to reorientation of the interfacial substances at the interface, and the natural 1755

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Figure 10. Micrograph images of catalytic cracking diesel-in-water emulsion: (a) without any demulsifier addition, (b) oil phase after demulsification of 150 mg/L of TETA core demulsifier for 60 min at 40 °C, and (c) water phase after demulsification of 150 mg/L of TETA core demulsifier for 60 min at 40 °C.

3.7. Conductivity in Demulsification Process. To further illustrate the effect of electrostatic charge transfer on the demulsification process, the conductivity of DETA and TETA core demulsifier aqueous solutions and the conductivity of three emulsion types were measured at room temperature, and the results are shown in Figure 12. It is shown in Figure 12(a) that both the DETA and TETA core demulsifier aqueous solutions’ conductivities ascend dramatically with an increase in demulsifier dosage. Moreover, the conductivity of the TETA core demulsifier aqueous solution is larger than that of the DETA core demulsifier, especially when the dosage is less than 500 mg/L. The results illustrate that charge diffusion of the TETA core demulsifier is easier than the DETA core demulsifier at low concentration. Figure 12(b) shows the conductivity of the diesel-in-water emulsion, catalytic cracking diesel-in-water emulsion, and petroleum coking diesel-in-water emulsion after demulsification with a TETA core demulsifier at 40 °C for 60 min. Without any demulsifier, the initial conductivity of three kinds of emulsion is 253, 316, and 275 μS/cm, respectively. After adding a demulsifier, the conductivities all increase significantly, and the slope of the diesel-in-water emulsion is largest among the three emulsions. The results are also consistent with oil removal rate, where a larger change in conductivity indicates better demulsification

efficiency. A larger conductivity scope means a less stable emulsion. With an increase in demulsifier dosage, oil droplets in the emulsions gradually rise upward and coalesce, which lead to decreasing spacing between oil droplets. Thus, the hindrance of ion movement in the emulsion is weakened, and the conductivity gradually increases. After a certain concentration, about 250 mg/L, the whole emulsion system reaches equilibrium, so the conductivity changes slightly. The PAMAM demulsifier is hydrophilic, which can ionize a cationic amine group. The cationic group may transfer in the emulsion and form conductive chains. This can cause an increase in the emulsion’s conductivity, thus resulting in an excellent ability to rupture the interface film. The conductivity curves reflect oil droplet aggregation, accumulation, and change process of the oil content in the emulsions. 3.8. Aggregation Behavior of Mixture of Demulsifier and SDS. To further understand the demulsification process, the interaction between the TETA core PAMAM demulsifier and SDS was investigated. With the addition of SDS, there is an increase in hydrodynamic radius (Rh), shown in Figure 13. It is noticeable that even at extremely low concentrations of SDS (0.25 mmol/L of SDS far from the critical micelle concentration), the Rh of aggregates increases from 396.1 nm (without SDS) to 615.1 nm. It may be concluded that the 1756

DOI: 10.1021/acs.iecr.5b04401 Ind. Eng. Chem. Res. 2016, 55, 1748−1759

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Industrial & Engineering Chemistry Research

Figure 12. (a) Conductivity of DETA and TETA core PAMAM demulsifier aqueous solutions and (b) conductivity of different types of diesel-in-water emulsions as a function of TETA core demulsifier dosage.

Figure 11. (a) Surface tension of DETA and TETA core PAMAM demulsifier aqueous solutions and (b) interfacial tension of the diesel/ water interface with DETA and TETA core PAMAM demulsifiers in the water phase as a function of demulsifier dosage.

dendrimer−SDS complex forms and leads to the enlargement of aggregate size. This also indicates that SDS molecules can insert into the cationic amine groups of the TETA core PAMAM demulsifier. The aggregation behavior of the dendrimer−SDS complex is beneficial to the demulsification process. A possible demulsification process and mechanism with the PAMAM dendrimer is illustrated in Figure 14. Because of the strong bridging interaction and charging neutralization between the PAMAM dendrimer and SDS, the protective film was partially destroyed. The partial destruction of the interfacial film provides a site for the aggregation of the small oil droplets. Because of their close contact, the flocculated oil droplets finally coalesce to form big ones.

4. CONCLUSION A series of PAMAM dendrimer-based demulsifiers with different initial cores (1,3-propanediamine, DETA, and TETA) were synthesized and investigated in the demulsification process of diesel-in-water emulsions. The demulsifier with the TETA initial core showed a better demulsification performance due to the collaborative roles of structure (length of chains) and molecular weight. The effects of demulsifier dosage, settling time, and temperature on demulsifier performance were also conducted. In the demulsification process, the

Figure 13. Hydrodynamic radius (Rh) of demulsifier aggregates without SDS and with 0.25 mmol/L of SDS.

optimum combinations of parameters were the TETA core demulsifier dosage of 200 mg/L, settling time of 60 min, and demulsification temperature of 30 °C. Thus, the TETA initial core PAMAM can be used as a highly efficient demulsifier in breaking up the diesel-in-water emulsion. However, the 1757

DOI: 10.1021/acs.iecr.5b04401 Ind. Eng. Chem. Res. 2016, 55, 1748−1759

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Industrial & Engineering Chemistry Research

Figure 14. Schematic representation of a possible demulsification process and mechanism using PAMAM dendrimer for diesel-in-water emulsion breakup.

emulsion had been broken with the addition of the TETA core demulsifier. The conductivity results illustrated that the charge diffusion of the TETA core demulsifier solution was easier than the DETA core demulsifier at low concentration, and conductivity curves of emulsions can reflect the oil droplet aggregation and change the process of oil content in emulsions. There was strong molecular aggregation interaction in the dendrimer−SDS system, and the aggregates became larger with the addition of SDS. These measurements explained the demulsification mechanism of the PAMAM demulsifiers in different aspects.

demulsification efficiency was affected by the oil content of the emulsion and the properties of the diesel. When the oil content was low, the dendrimer-based demulsifier tended to selfassociate, which led to poor demulsification performance. The TETA core demulsifier’s performance was excellent for catalytic cracking diesel emulsion but not satisfactory for petroleum coking emulsion due to the existence of carbon residue or coke powder. Micrograph images and oil droplet size distributions showed that the TETA core PAMAM demulsifier can successfully add to the flocculation and coalescence of oil droplets in the system, which finally led to the breaking of diesel-in-water emulsion and catalytic cracking diesel-in-water emulsion. The surface tension and interfacial tension data were given to partially uncover the mechanism of this demulsification process, which showed that DETA and TETA core demulsifiers were both effective in reducing interfacial tension. From the electrostatic interactions point of view, the zeta potential in the demulsification process also proved that the diesel-in-water

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04401. Information as mentioned in the text. (PDF) 1758

DOI: 10.1021/acs.iecr.5b04401 Ind. Eng. Chem. Res. 2016, 55, 1748−1759

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(18) Parvasi, P.; Hesamedini, A. K.; Jahanmiri, A.; Rahimpour, M. R. A Comparative Study on Droplet Coalescence in Heavy Crude Oil Emulsions Subjected to Microwave and Ultrasonic Fields. Sep. Sci. Technol. 2013, 48, 1591−1601. (19) Tomalia, D. A.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P.; Baker, H. A New Class of Polymers: Starburst-Dendritic Macromolecules. Polym. J. 1985, 17, 117−132. (20) Esfand, R.; Tomalia, D. A. Poly(Amidoamine) (PAMAM) Dendrimers: From Biomimicry to Drug Delivery and Biomedical Applications. Drug Discovery Today 2001, 6, 427−436. (21) Tadros, T. F.; Vincent, B. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1983; Vol. 1, Chapter 3. (22) Allende, D.; Cambiella, Á .; Benito, J. M.; Pazos, C.; Coca, J. Destabilization-Enhanced Centrifugation of Metalworking Oil-inWater Emulsions: Effect of Demulsifying Agents. Chem. Eng. Technol. 2008, 31, 1007−1014. (23) Yao, X.; Jiang, B.; Zhang, L.; Sun, Y.; Xiao, X.; Zhang, Z.; Zhao, Z. Synthesis of a Novel Dendrimer-Based Demulsifier and Its Application in the Treatment of Typical Diesel-in-Water Emulsions with Ultrafine Oil Droplets. Energy Fuels 2014, 28, 5998−6005.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86 22 27400199. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from Tianjin Oceanic Administration R&D Program (No. 19-3BC2014-01) and National Natural Science Foundation of China (No. 21336007).

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