Synthesis of a Novel Dendrimer-Based Demulsier and ItsApplication in the Treatment of Typical Diesel-in-Water Emulsionswith Ultrane Oil DropletsXing Yao, Bin Jiang,, Luhong Zhang,*, Yongli Sun, Xiaoming Xiao, Zhiheng Zhang,

and Zongxian Zhao

School of Chemical Engineering and Technology and National Engineering Research Center for Distillation Technology, TianjinUniversity, Tianjin 300072, P. R. China

ABSTRACT: Waste water resulted from polymer ooding oil recovery generally has a bad impact on the subsequent process ofenhanced oil recovery. Separating residual oil from oil/water (O/W) emulsion with suitable kinds of demulsier is one strategygenerally adopted by oil companies. Because of the existence of large amounts of ultrane oil droplets with the average diameterless than 2 m, the emulsions can be extremely dicult to break up. To solve this problem, an amine-based dendrimerdemulsier PAMAM (polyamidoamine) was synthesized in this study, and the eciency of the demulsier in dealing with O/Wemulsions with ultrane oil droplets was investigated. Because of its strong interfacial activity and relatively good solubility inwater, the dendrimer-based demulsier can easily attach to emulsied oil droplets in a stable diesel-in-water emulsion. Theinuences of temperature, settling time, and concentration of the demulsier used on the eciency of the demulsier wereinvestigated in detail. The optimal operating condition under which more than 90% oil was removed from the original emulsionby the demulsier was found. In contrast, less than 2% oil was removed from the emulsion without applying the demulsierunder the same conditions. Micrographs showed that the PAMAM demulsier could lead to the breakup of diesel-in-wateremulsions with ultrane oil droplets by occulation and coalescence. The surface tension and interfacial tension at the dieselwater interface were also measured to give a basic understanding of the demulsication mechanism. Though not perfect indealing with emulsions with the average oil droplets less than 2 m due to the relatively high demulsier dosage, its relativelysimple synthetic procedure and mild operating conditions showed a great promise in industrial applications with uniqueadvantages over traditional physical methods.

INTRODUCTIONWith the application of polymer ooding technology becomingmore widely than ever in enhanced oil recovery, wastewatertreatment has become a stubborn problem in the oil-extractionindustry.1 Interfacial active substances resulted from thistechnology has made the wastewater more dicult to handlewith traditional methods. Because of the amphiphilic propertyof certain molecules aggregating at the interface, the resultingemulsion can be extremely dicult to break for furthertreatment process.2 Thus, it is urgent to remove residual oilfrom the wastewater so that water can be recycled into thereinjection well for second usage. It is obvious that this kind ofwastewater can be categorized into an oil-in-water (O/W)emulsion, which usually has a high oil content and small oildroplet size. Because certain kinds of interfacial activesubstances aggregate at the surface of oil droplets, theemulsions can be very stable. Furthermore, with water-solublepolymers such as HPAM adsorbing at the oil droplet surface,the aqueous phase become more viscous, which makes thedemulsication operation more dicult.3 Without propertreatment, the wastewater could do great harm to theenvironment if directly ejected into rivers and lakes. Manydemulsication techniques have been developed, including bothphysical and chemical methods.4,5 Recently, biodegradablepolymers with amphiphilic properties and complex structureshave attracted the attention of many scientists due to theirenvironmental friendly properties. Feng et al. (2009) found a

nontoxic and biodegradable polymer, ethylcellulose, and used itto break up emulsied water from naphtha-diluted bitumen.The ethylcellulose polymer not only showed great dewateringperformance and could also assist the removal of ne solidswith the water.6 Feng et al. (2011) then investigated the eectof hydroxyl content and molecular weight of the biodegradableethylcellulose on dewatering rate in water-in-diluted bitumenemulsions. Their results showed that the performance of thedemulsier can also be linked with the molecular structure.7 Toenhance the performance of current demulsiers and developnovel recyclable demulsiers, scientists have tried to graftamphiphilic polymers onto nanoparticles in order to takeadvantage of the unique properties of nanoparticles. Peng et al.(2012) developed a novel interfacial active nanoparticles, whichcan remain highly stable in the organic phase and can attach tothe surface of water droplets. Once given a strong magneticeld, the attached water droplets would respond to themagnetic eld accordingly; thus the demulsication processoccurred.8 Peng et al. (2012) then investigated the separationeciency of the demulsier on heavy naphtha diluted bitumenemulsions. Their investigation showed the recyclability of thedemulsier is amazingly well.9 Li et al. (2014) also synthesizeda novel magnetic demulsier and investigated its application in

Received: July 12, 2014Revised: August 10, 2014Published: August 19, 2014

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pubs.acs.org/EF

2014 American Chemical Society 5998 dx.doi.org/10.1021/ef501568b | Energy Fuels 2014, 28, 59986005

the treatment of oil-charged industrial wastewater. They thendemonstrated the recyclability of the demulsier through someexperiments as well.10 Apart from demulsiers with traditionalstructure mentioned above, demulsiers with novel structureshave also been synthesized and tested. The relations betweenstructure and performance are investigated to a great extent aswell. Wang et al. (2006) synthesized a series of structurallydierent dendrimer-based demulsiers and investigated theperformance of these demulsiers in treating crude oilemulsion. From their research results, they concluded thatproperly structurally designed dendrimer macromolecules canact as an eective demulsier.11 Wang et al. (2008) synthesizeda novel broom molecule and investigated its demulsicationperformance in treating the oilwater emulsion.12 Wang et al.(2010) then reasoned that dendrimers with more brancheswould demonstrate a better demulsication performance. Toprove this, they synthesized a series of dendrimer-baseddemulsier with the same basic structure and investigated theamount of PEO and PPO within the demulsier molecularstructure on the performance of the demulsier.13 Zhang et al.(2005) synthesized several kinds of polyether demulsier with atypical PEOPPO copolymer as the branch structure. Theyalso concluded that dendrimers with more branches woulddemonstrate better demulsication performance and thedierent amounts of PPO and PEO aect the interfacialactivity and thus has a great inuence on the performance ofthe demulsier.14

Ecient as they are, these demulsication techniques havetheir own limitations, which is especially obvious in treatingpolymer ooding oil-extraction wastewater. In most cases, theaverage diameter of droplets in the emulsion system to betreated is around 5 m, and numerous research papers indealing with emulsions with the average diameter of around 5m have been published.614 However, when dealing withemulsions with much smaller oil droplets, the abilities of mosttechniques are far from satisfactory from an industry point ofview, which is also common in the oil recovery industry. Spethet al. (2002) once used ber-bed coalescers to deal withemulsions with the average diameter of oil droplets around 2m and developed a physically founded model describing thecoalescence process.15 Apart from that, no research on usingdemulsiers to treat emulsions with such small oil droplets hasbeen reported yet. As a result, it is urgent to develop a properkind of demulsier, which could result in a high oil removal rateas well as rapid oilwater separation for emulsions with theaverage diameter of oil droplets less than 2 m.Dendrimers are specially designed macromolecules with a

certain size, shape, and reactivity. Generally, they are branchedfrom a central core, with numerous terminal groupssurrounding the core so as to produce an empty interior.This novel kind of dendrimers was developed by Tomalia andNewkome in the 1980s.11 Because specially designeddendrimers with certain interfacial activity can dissolve theoriginal interfacial substances on the surface of the oil dropletsrapidly, their potential to break the O/W emulsion is extremelystrong.11

Polyamidoamine (PAMAM) is a kind of dendrimer, whichhas a polar but hydrophobic interior with polar terminal groupson the outer surface. Experimental results showed that thestructure of the terminal group contributes most to thedemulsication process. Only the amine-based dendrimerproved to be an eective demulsier.11

Even though numerous research papers on PAMAM havebeen published, research on its demulsication ability has notbeen conducted much. Only Wang et al. investigated thedemulsication ability of PAMAM.11,12 So far, no researchusing PAMAM to break up diesel-in-water emulsions with theaverage diameter of oil droplets less than 2 m has beenreported yet.In this study, 1,3-propanediamine were rst reacted with

methyl acrylate and then with ethanediamine. The resultantswere treated with the same synthetic procedure twice (rstreacted with methyl acrylate and then with ethanediamine), andthen the nal products PAMAM were obtained. The prepareddemulsier was then applied to typical diesel-in-water (O/W)emulsions with the average diameter of oil droplets less than 2m, which were referred to as the ultrane oil droplets. Theinuences of temperature, settling time, and demulsierconcentration on the performance of the demulsier wereinvestigated in detail. This study showed that, under certainconditions, the oil removal rate could reach more than 90%,which perfectly meets the industrial requirements. Micrographsof the emulsions with and without treatment of the demulsierwere taken and compared to conrm the occulation andcoalescence process during the demulsication process. Surfacetension and interfacial tension of the demulsier were measuredto give a basic understanding of the demulsication mechanism.This is the rst report on the synthesis of PAMAM-baseddemulsier applied to diesel-in-water emulsions with theaverage diameter of oil droplets less than 2 m. Though notperfect in dealing with emulsions with ultrane oil droplets, thisstudy shed light upon a novel chemical method in dealing withthis kind of emulsion with obvious advantages over physicalmethods like ber-bed coalescers developed by Speth.15

EXPERIMENTAL DETAILSMaterials. All chemicals were used directly without further

purication. Methyl acrylate (AR grade, 0.98), ethanediamine (ARgrade, 0.98), and methanol (AR grade, 0.98) were all purchasedfrom Tianjin Jiangtian Chemical Technology Co., Ltd. 1,3-Propane-diamine (AR grade, 0.98) was purchased from Aladdin Reagents.Sodium dodecyl sulfate (SDS) (CR grade) was purchased fromTianjin Guangfu Fine Chemical Research Institute.Synthesis of an Amine Dendrimer-Based Demulsier. A

portion of 12.5 mL of 1,3-propanediamine was dissolved in 100 mL ofmethanol; then 130 mL of methyl acrylate was added into the ask.The mixture was stirred for 24 h at 25 C. The solvent and theunreacted methyl acrylate were removed in a rotatory evaporator, andthen the resultant was put into a vacuum oven for further purication.The resultant was then dissolved in 100 mL of methanol; then 130 mLof ethylenediamine was added into the ask. The mixture was stirredfor 24 h at 25 C. The solvent and unreacted ethylenediamine wereremoved in a rotatory evaporator, and then the resultant was put into avacuum oven for further purication. The above process was thenrepeated twice. Then the nal product was obtained.11

Preparation of Diesel-in-Water Emulsions with Ultrane OilDroplets. The emulsions were prepared with deionized water anddiesel. A sample of 50 g of diesel and 1 g of SDS were added into avolumetric ask with the volume of 1 L. Then deionized water wasadded into the ask until the volume of the mixture reached one liter.Then the mixture were treated with a homogenizer (Flukehomogenizer, 500W) operated at 10 000 rpm for 5 min. The resultingemulsion contained 5 wt %% diesel and was referred to as diesel-in-water emulsions. The emulsions obtained as such are very stablewithin the experimental time frame and are extremely complex withaverage drop sizes typically less than 2 m measured by MalvernMastersizer 3000.

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Demulsication Test. The ability of the demulsier was tested bymeasuring the oil content and the size of oil droplets in diesel-in-wateremulsions after the demulsication process nished. In each test, 25mL of freshly prepared emulsion and 1 mL of demulsier solution witha certain concentration were thoroughly mixed in a 25 mL colorimetertube by shaking the mixture 200 times by hand.8,9 Then the mixturewas put in a water bath (Shanghai Yijing, YQ-120C) under dierenttemperatures for dierent periods of time. Subsequently, the solutionat the bottom of the colorimeter tube was taken out, and the oilcontent in it was measured using an ultraviolet spectrophotometer(UNIC, UV-4802). Each sample was repeated three times, and the oilcontent reported is the average of the three repetitions. The blank testswere performed for diesel-in-water emulsions without demulsieraddition as a control. The demulsication performance is derived fromthe oil removal rate, which can be calculated from the equation:

= R C CC

100%00

where R (%) is the oil removal rate, C0 (mg L1) is the initial oil

content, and C (mg L1) represents the oil content after thedemulsier solution was added.10

After settling in the water bath of 70 C for 90 min, micrographs ofthe emulsion sample without any demulsier addition and that with2000 mg L1 demulsier addition were recorded using an opticalmicroscope equipped with a digital video camera linked with acomputer. The emulsion sample was put on an object slide and thencovered with a cover glass. The image was taken under halogen light.Surface Tension and Interfacial Tension Measurement. The

surface tension of the dendrimer-based demulsier solution withdierent concentrations was measured using an interfacial tensiometer.The interfacial tension of dieselwater interface with the dendrimer-based demulsier with dierent concentrations in the water phase wasalso measured using an interfacial tensiometer. The interfacial tensionof dieselwater interface without the dendrimer-based demulsier inthe water phase and that with only sodium dodecyl sulfate of certainconcentration in the water phase were measured as a control.

RESULTS AND DISCUSSIONSynthesis of PAMAM Demulsier. According to Wang et

al.s research, the demulsication rates increased as thedendrimer generation increased.11 However, when the numberof generation increases to some certain extent, the densely piledsurface groups bring great diculty to the next step of reactionprocess, which causes insucient further development of thedendrimer, thus making the molecular structure defect.11 Thus,the demulsication eciency of the amine-based dendrimer ofthe third generation was systematically studied in this paper.Wangs study on the inuence of the ratio of reactants on the

yield of the product also showed that, when the ratio of methylacrylate and ethanediamine to the 1,3-propanediamine or theresultants from the previous reaction reaches much more thanthe molar ratio according to the chemical equation, the yieldreaches more than 99.9%.11 Thus, during the synthesisprocedure, the amounts of methyl acrylate and ethanediamineused were much more than the molar ratio required by thechemical equation.The PAMAM demulsier were synthesized in two-step

method with three cycles, and one cycle of the synthesisprocedure is shown as Figure 1a. The molecular structure of thenal product demulsier is shown in Figure 1b.To identify the structure, 1H NMR spectra were recorded for

the demulsier. CDCl3 was used as the solvent.11 Experimental

results (Figure 2a) indicate that the NMR spectra of thepuried product totally matches those reported by ref 16. Thehydrogen-1 chemical shift of the demulsier showed in Figure 1is as follows: (a) 2.39, (b) 2.46, (c) 2.68, (d) 2.59, (e) 3.20, (f)

2.73, (g) 2.68, (h) 2.59, (i) 3.20, (j) 2.73, (k) 2.68, (l) 2.59,(m) 3.20, (n) 2.73, and (o) 1.33. The unmarked chemical shiftbelongs to the unreacted ethanediamine which is extremelyhard to remove from the nal product due to the nano-container structure of the molecule.11 The characteristicprotons of amino groups appeared at 1.33 ppm as a broadsingle peak. The reason why the chemical shift of the aminogroups on the surface of the dendrimer molecule is relativelysmall is that the nitrogen atom linked with the hydrogen atomis not a strong electrophilic atom. The characteristic protons ofother groups are also shown in Figure 2a, in which some of thegroups in dierent parts of the molecule shared nearly the samechemical shift (c, g, and k at 2.68 ppm; d, h, and l at 2.59ppm; f, j, and n at 2.73 ppm; e, i, and m at 3.20 ppm). Thiscan be explained by their highly similar positions inside themolecule as shown in Figure 1b.To further identify the structure of the demulsier, FTIR

spectra were also recorded for the demulsier. Figure 2 showsthe FTIR spectra of PAMAM demulsier. Typical bandsassociated with NH2 vibration are visible at around 3269.60cm1. For CONH, the bands were observed at around1645.80 and 1544.38 cm1 with the former referred to as thestretching of CO and the latter referred to as the couplingband combined with the bending of NH and the stretching ofCN. Typical bands associated with CN vibration at around1195.50 cm1 conrmed the existence of NCH2. No typicalbands being observed at around 1740 cm1 showed that hardlyany ester-terminated intermediate products existed in the nalproduct. For CNCH2, the typical band at 1032.39 cm1

Figure 1. (a) One cycle of the synthesis procedure of the PAMAMdemulsier. (b) Molecular structure of PAMAM demulsier.

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was observed. For CH2, the bands were observed at around2927.56 and 2849.17 cm1, which were referred to as theasymmetric and symmetric stretching vibration of CH2,respectively. Furthermore, typical bands of COO-C at1195.50 cm1 and CC at 929.98 cm1 were observed,although the area is extremely small, which indicated that smallamount of ester-terminated intermediate products still existedin the nal products.Demulsication of PAMAM Demulsier. Figure 3 shows

the relationship between oil removal rates measured by the oilcontent at the bottom of the emulsion and the demulsierconcentration applied under two certain circumstances. Whenno demulsier was added into the emulsion, which can beregarded as a blank test, the oil removal rate under pure gravitywas just 1.7% after 90 min at the temperature of 30 C that isvery near to room temperature. This explains that theemulsions prepared are very stable under normal conditions.To further prove the high stability of the emulsions, anotherexperiment was conducted, in which the settling time wasextended to 120 min and the temperature was set to 50 C.Again, without any demulsier addition, the nal oil removalrate by natural gravity under the temperature of 50 C after 120min was just 9.1%. The above two blank test shows that the

emulsions prepared are extremely stable within a relatively longperiod of time.From Figure 3, it is clear that when the temperature and

settling time increased, the oil removal rate increased as wellunder the same demulsier concentration. To further prove thehigh eciency of the demulsier, systematically investigationon the inuences of temperature, settling time as well as thedemulsier concentration on the performance of thedemulsier was conducted as follows.The eect of the demulsier concentration on the oil

removal rate at dierent temperatures is shown in Figure 4. For

this purpose, the demulsier concentration in the diesel-in-water emulsion under study was set as 500 mg L1, 1000 mgL1, 1500 mg L1, and 2000 mg L1 in ascending order. Apartfrom that, the settling time was kept constant at 60 min, whichis relatively much shorter than the settling time set in anotherresearch treating oileld wastewater with typical O/Wemulsions involved.11 Three pairs of experiments with thetemperature ranging from 30 to 70 C were conducted to give aqualitative impression of the demulsier eciency. It is obviousthat, under each temperature, the oil removal rate increased

Figure 2. (a) 1H NMR spectra of PAMAM demulsier. (b) FTIRspectra of PAMAM demulsier.

Figure 3. Eect of demulsier concentration on the oil removal rate inO/W emulsions at two certain circumstances.

Figure 4. Eect of demulsier concentration on the oil removal rate indiesel-in-water emulsions.

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with the increase of demulsier concentration. When thetemperature is 30 C, the oil removal rate is very low even if thedemulsier concentration reaches as high as 2000 mg L1. Lessthan 50% oil was removed from the emulsion system under thiscondition, which is undoubtedly far from satisfactory from anindustry point of view due to its low ecacy at operatingcondition near room temperature. To further investigate thefactors aecting the performance of the demulsier, thetemperature was raised to 50 C, which is also easy to achievewithout much energy in industry, the oil removal rates showeda signicant increase, with more than 70% oil being removedwhen the demulsier concentration are relatively high. Andeven when the concentration of the demulsier is just 500 mgL1, the oil removal rate is near 50%, which has shown muchbetter performance than that in low temperature. From the twoexperiments shown above, it can be concluded that temperatureplays a signicant role in improving the performance of thedemulsier. However, even when the temperature reaches 50C and the demulsier concentration reaches as high as 2000mg L1, the oil removal rate is just a little more than 80%,which still cannot meet the requirement standards of industry.Thus, experiment with the temperature of 70 C was conductedto further investigate the inuence of temperature on theperformance of the demulsier. From the experimental results,it can be seen that at 70 C more than 90% oil was removedfrom the emulsion system even when the demulsierconcentration is just 500 mg L1, which perfectly meets theindustrial requirements.Satisfactory as it is, the inuence of demulsier concentration

on the eciency of the demulsier was still unclear, with only aslightly increasing trend being observed. Thus, another fourpairs of experiments were conducted to describe this trend indetail. Figure 5 shows the eect of temperature on oil removal

rate in diesel-in-water emulsions. The general trend is the sameas previously investigated that the oil removal rate increaseswith the demulsier concentration. However, it is interesting tosee that, even though all four curves showed an increasingtrend, the dierence of demulsier concentration also plays apart in it. When the demulsier concentration is as low as 500mg L1, the increasing trend of the curve is rather small at lowtemperatures, with the slope of the curve being just 1.3. Butwhen at high temperatures, the increasing trend of the curve

becomes much higher, with the slope of the curve increasing to2.1. This can be explained by the molecular motion of thedemulsier molecules. According to the molecular dynamictheory, when at high temperature, the motion of demulsiermolecules becomes much stronger than at room temperature,which accelerates their transportation toward the surface of oildroplets thus greatly adds to the demulsication process.From Figure 5, it can also be clearly seen that when the

demulsier concentration reaches as high as 2000 mg L1, theincreasing trend of oil removal rate is relatively high at lowtemperatures, with the slope of the curve being 1.95. But whenthe temperature ranges are set from 50 to 70 C, the increasingtrend of oil removal rate slows down a lot with the slope of thecurve being 0.85. With the oil removal rate increasing to 99%,nearly transparent water phase is got with hardly any oil in it.This could be the contribution of high demulsier concen-tration to improving the transportation rate of demulsiermolecules. When the demulsier concentration is relativelyhigh at high temperature, much more demulsier molecules aretrying to reach the oil droplets in the water phase, which greatlyadds to the viscosity of the water phase. This behavior ofdemulsier molecules in turn slows down the rate of theirtransportation toward the oil droplets.6 In another perspective,when the oil removal rate is already as high as 80%, whichmeans lots of demulsier molecules have aggregated onto thesurface of oil droplets to replace the original natural surfactantsand SDS, the residual positions for other demulsier moleculesare not enough. Thus, some of the residual demulsiermolecules would tend to aggregate by themselves due to thehigh concentration, which further slows down the trans-portation rate of demulsier molecules in the water phase, thusleading to such experimental results.6

Another interesting phenomenon could also be seen inFigure 5. When the demulsier concentration was varied from1000 mg L1 to 1500 mg L1, the oil removal rate onlyincreased slightly at each temperature. Although the variance ofincreasing trend of oil removal rate with respect to temperatureis not clearly observed, it can still be seen that the inuence ofdemulsier concentration plays a signicant part in thedemulsication process. An obvious plateau was observedwhen the demulsier concentration ranged from 1000 to 1500mg L1. Thus, new pairs of experiments were conducted toinvestigate the eect of demulsier concentration on thedemulsication performance in detail.Figure 6 shows the eect of settling time on oil removal rate

in diesel-in-water emulsions at the temperature of 30 C witheach curve indicating a certain demulsier concentration. FromFigure 6, it can be obviously seen that, when the settling time is60 min, the oil removal rates of the four experiments at 30 Care all extremely low. Even when the demulsier concentrationreaches more than 2000 mg L1, the oil removal rate is stillslightly more than 40%. However, when the settling time wasjust extended to 90 min, the oil removal rates increased rapidlyfor all four experiments with dierent demulsier concen-trations. Especially when the demulsier concentration was setto as low as 500 mg L1, the oil removal rate increased from22% to 76% within the period of 30 min set previously, whichhas shown excellent performance of the demulsier. However,when the settling time was extended to 120 min, the increasingrate is not as high as before. Even when the demulsierconcentration reached as high as 2000 mg L1, the oil removalrate only increased a little, from 85% to 98%. Although oilremoval rate of 98% means that the performance of the

Figure 5. Eect of temperature on the oil removal rate in diesel-in-water emulsions.

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demulsier is extremely excellent, it can be clearly seen that thehigh demulsier concentration also made some contribution.Thus, experimental results with the demulsier concentrationbeing set as 1500 mg L1 and 1000 mg L1 were compared forthe purpose of nding the optimal concentration at 30 C.When the demulsier concentration was 1500 mg L1, the oilremoval rate at 30 C was 92.6%, and when the demulsierconcentration was 1000 mg L1, the oil removal rate at 30 Cwas 82.8%. Thus, it is clearly that the optimal demulsierconcentration at 30 C was 1500 mg L1 for at this operatingcondition because industry requires more than 90% oil removalrate.Again, when the demulsier concentration was 500 mg L1,

only a slightly increase in oil removal rate was observed whenthe settling time was extended from 90 to 120 min. With oilremoval rate increasing from 72.5% to 75.6%, it can beconcluded that the increase of demulsication eciency hasslowed down a lot at 30 C with the demulsier concentrationof 500 mg L1. According to the increasing trend shown byother three curves of Figure 6, the maximum oil removal rateare obviously higher when oil demulsier concentration reachesa higher level although no clear plateau was observed in Figure6.In Figure 7, when the temperature was raised to 50 C,

obvious plateaus were observed for the two curves with thedemulsier concentration being 2000 mg L1 and 1500 mg L1.With the demulsier concentration of 1500 mg L1, the oilremoval rate increased from 91.1% to 92% when the settlingtime was extended from 90 to 120 min, being rather a slightlyincrease. While the demulsier concentration reached 2000 mgL1, the oil removal rate increased from 93.1% to 94.4% whenthe settling time was extended from 90 to 120 min. With anincrease of nearly 1%, it can be regarded that plateau has beenreached. When the demulsier concentration was as low as1000 mg L1, the oil removal rate increased from 86.6% to 91%when the settling time was extended from 90 to 120 min. Eventhough with only a slightly increase of nearly 5%, the oilremoval rate had reached more than 90%, which perfectlymeets the industrial requirements.It is also interesting to see that the curve with the demulsifer

concentration of 500 mg L1 showed a slightly decrease whenthe settling time was extended from 90 to 120 min. With the oil

removal rate decreased from 74% to 70% when the settlingtime was extended from 90 to 120 min, it can be assumed thatat the concentration of 500 mg L1, more settling time wouldnot contribute to improving the performance of the demulsier.This can be explained as follows. At the temperature of 50 C,the molecular motion at the interface, which actually means thesurface monolayer of the oil droplets, has reached equilibriumto some extent. With nite amounts of demulsier molecules inthe water phase competing with the original free surfactantssuch as SDS to get to the interface, the nal state has beenachieved at the settling time of 90 min. As to the slightlydecrease when the settling time is extended, it can be assumedthat the equilibrium state has not been totally stable, whichmight result in errors in measurement.From the comparison between Figure 6 and Figure 7, it can

be seen that the plateau tends to shift to higher demulsiferconcentration band as the temperature rises (from 500 mg L1

to 2000 mg L1). This can be explained as follows. When thetemperature is as low as 30 C, with the concentration ofdemulsier being only 500 mg L1, the amount of freedemulsier molecules existing in the water phase is extremelysmall. Thus, increasing temperature to 50 C could notsignicantly cause more free demulsier molecules to get to thesurface of oil droplets thus leading to demulsication. It can beclearly seen from the comparison between Figure 6 and Figure7 that, when the temperature is raised from 30 to 50 C, thecurve indicating the demulsier concentration of 500 mg L1 atthe section between 90 and 120 min basically remained thesame height. However, when a higher demulsier concentrationis applied, the amount of free demulsier molecules existing inthe water phase is relatively much larger. Thus, when thetemperature is raised from 30 to 50 C, more free demulsiermolecules would try to get to the surface of oil droplets thusleading to demulsication due to stronger molecular motioncaused by the increase of temperature. So no obvious plateau isobserved for the curve indicating the demulsier concentrationof 1000 mg L1. Nevertheless, when the demulsierconcentration is as high as 1500 mg L1 or 2000 mg L1,even within 90 min at 50 C, lots of demulsier molecules haveaggregated onto the surface of oil droplets to replace theoriginal natural surfactants and SDS, the residual positions atthe surface for other demulsier molecules are not enough.

Figure 6. Eect of settling time on the oil removal rate in diesel-in-water emulsions at 30 C.

Figure 7. Eect of settling time on the oil removal rate in diesel-in-water emulsions at 50 C.

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Thus, extending settling time to 120 min would not cause morefree demulsier molecules to get to the surface of oil dropletsthus leading to demulsication. So obvious plateau is observedfor the curve indicating the demulsier concentration of 1500mg L1 or 2000 mg L1 at the section between 90 and 120 min.Micrographs of typical diesel-in-water emulsions without and

with 2000 mg L1 demulsier addition after settling for 90 minat 70 C are shown separately in Figure 8a, b, and c. As shown

in Figure 8a, the oil droplet size in the emulsion system withoutany demulsier addition is actually less than 2 m, which is inperfect agreement with the average oil droplet diametermeasured by Malvern Mastersizer 3000. In contrast, whentreated with the demulsier with the concentration of 2000 mgL1 at 70 C for 90 min, the oil droplet size increasedsignicantly, with the diameter of most oil droplets variedbetween 20 and 30 m. And most of the oil droplets hadmoved to the upper level of the emulsion (Figure 8b). Hardly

any oil droplets remained at the lower level of the emulsion(Figure 8c). It can be obviously seen that the demulsiergreatly improved occulation and coalescence of emulsied oildroplets in diesel-in-water emulsions.Compared with other similar studies studying the demulsi-

cation process with larger droplets (the average diameterbeing around 5 m) in the emulsions,614 it can be clearly seenthat the demulsier dosages they used are relatively lower thanthis study with other operating conditions more or less thesame. This in some sense conrms the great diculty intreating emulsions with ultrane droplets (the average diameterbeing less than 2 m). Though the demulsier synthesized inthis study might not be regarded as the perfect demulsier intreating emulsions with ultrane oil droplets for the highdemulsier dosage, it showed strong potentials in dealing withthis kind of emulsion, which can be seen from the excellentperformance under a certain temperature and settling time.Combined with its simple synthetic procedure as well as mildoperating conditions, it showed obvious advantages over ber-bed coalescers developed by Speth15 from an industrial point ofview.Surface Tension and Interfacial Tension Study of

PAMAM Demulsier. To get a further understanding of thedemulsication mechanism, the surface tension of the aqueoussolution of PAMAM demulsier and the interfacial tension ofthe PAMAM demulsier at the dieselwater interface weremeasured and compared. Table 1 shows the surface tension of

the PAMAM demulsier aqueous solution, and Table 2 showsthe interfacial tension of the dieselwater interface with thePAMAM demulsier in the water phase. It can be clearly seenfrom Table 1 that the PAMAM demulsier cannot signicantlylower the surface tension of pure water no matter what theconcentration is. This result perfectly matches Wangs researchthat amine terminated dendrimers cannot reduce the surfacetension of pure water and thus do not behave like typicalsurfactants for airwater interface.11 According to the datashown in Table 2, it can be clearly seen that, with a certainamount of demulsier in the water phase, the interfacial tensionof the dieselwater interface get signicantly lowered from38.12 mN m1 to less than 8 mN m1. Even no clearlyinterfacial tension variance with the increase of demulsierconcentration was observed, it can still be concluded thatlowering the interfacial tension is the precondition for thedemulsication process to take place. To further investigate therelationship between interfacial tension and demulsicationprocess, the interfacial tension at diesel-water interface withonly SDS of 1 g L1 in the water phase was also measured. The

Figure 8. Micrographs with of typical diesel-in-water emulsionswithout demulsier addition after settling for 90 min at 70 C (a), at aupper level with oil most occupied (b), and at a lower level with watermost occupied (c).

Table 1. Surface Tension of PAMAM Demulsier AqueousSolution

demulsier concentration(mg L1)

0.0 1000.0 1500.0 2000.0

surface tension k(mN m1) 77.81 76.98 77.38 76.8

Table 2. Interfacial Tension of the DieselWater Interfacewith the PAMAM Demulsier in the Water Phase

demulsier concentration(mg L1)

0.0 1000.0 1500.0 2000.0

interfacial tension (mN m1) 38.12 7.89 7.99 7.75

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dx.doi.org/10.1021/ef501568b | Energy Fuels 2014, 28, 599860056004

measured interfacial tension is 3.26 mN m1, which alsoconrms the high stability of the emulsion system from the lowinterfacial tension perspective. Although this value was still alittle lower than the interfacial tension at diesel-water interfacewith demulsier in the water phase, the demulsication processstill occurred. This can be explained by the properties of theinterfacial monolayers with demulsier molecules in it. Oncethe demulsier molecules reach the surface of oil droplets, theytend to form nanoaggregates at the interface, which lead toreorientation of the interfacial substances at the interface. Thisin turn makes the interfacial monolayer more compressible.17

When the amount of demulsier molecules at the interfaceincreases, the stability of the interfacial monolayer gets weakerbecause the nanosize aggregates formed at the interface canreduce the mechanical strength of the monolayer, which hasbeen conrmed by AFM images of deposited monolayer.17

From another perspective, the natural surfactants and SDS atthe interface could be dissolved into the bulk phases ofdemulsier once the demulsier molecules reached the surfaceof oil droplets.16

CONCLUSIONAccording to the study reported above, the followingconclusions can be made:(1) Generally, the oil removal rate increases with the increase

of temperature, settling time, and demulsier concentration.From the experimental data, it can be seen that with the settlingtime being 120 min, when the temperature was set as 30 Cand the demulsier concentration was set as 1500 mg L1, theoil removal rate could reach 92.6%, which perfectly meets theindustrial requirements. Increasing the temperature or thedemulsier concentration would not do much more contribu-tion to improving the oil removal rate, which can be seen fromFigure 6 and Figure 7. Micrograph images showed that thePAMAM demulsier can successfully add to the occulationand coalescence of oil droplets in the system, which nally leadsto the breaking of typical diesel-in-water emulsions.(2) Among the several factors leading to the demulsication

process, the most signicant factor is temperature, which can beseen from Figure 4 and Figure 5. The least signicant factor isdemulsier concentration, which can be seen from Figure 6 andFigure 7 due to the high similarity of the four curves. Theinuence of settling time depends on the variance of timeperiod, which can be seen in Figure 6 and Figure 7.(3) Though not perfect in dealing with emulsions with

ultrane oil droplets due to its high demulsier dosage, thisstudy shed light upon a novel method in dealing with this kindof emulsion. The simple synthetic procedure and mildoperating conditions give its unique advantages in dealingwith emulsions with ultrane oil droplets over ber-bedcoalescers developed by Speth.15

(4) The surface tension and interfacial tension data weregiven to partially uncover the mechanism of this demulsicationprocess, which showed that the precondition of demulsicationprocess is the ability of the demulsier to lower the interfacialtension of the dieselwater interface and that there are otherfactors contributing to this process such as the properties of theinterfacial monolayer changed by the demulsier molecules,which remains to be studied.

AUTHOR INFORMATIONCorresponding Author*E-mail: zhanglvh@tju.edu.cn.

NotesThe authors declare no competing nancial interest.

ACKNOWLEDGMENTSWe are grateful for the nancial support from the NationalNatural Science Foundation of China (No. 21336007).

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Energy & Fuels Article

dx.doi.org/10.1021/ef501568b | Energy Fuels 2014, 28, 599860056005