Shenwen Fang *,Manlin Wang Zipei Xu Lei Zhai,Xiujun Wang ,Yan Xiong

1 School of Chemistry and Chemical Engineering,Southwest Petroleum University,Chengdu 610500,China

2 Oil&Gas Applied Chemistry Key Laboratory of Sichuan Province,Chengdu 610500,China

3 Beijing Research Center of China National Offshore Oil Corporation,Beijing 100027,China

ABSTRACT It is not easy for liquid cationic flocculant diallyldimethyl-ammonium chloride(DM)homopolymer to simultaneously exhibit both low viscosity and high polymer content,which limited its application in offshore oil field.In this paper,sodium allylsulfonate(XS)and sodium formate were used in the aqueous solution polymerization of DM.An amphoteric flocculant(PDMXS)with the properties of high polymer content(about 55 wt%)and low viscosity(efflux time measured by an Apply 4 viscometer was less than 100 s)was prepared.The optimum reaction conditions were identified as follows:the mass ratio of XS/DMwas 10 wt%,the concentration of sodium formate was 2000 mg·L-1,reaction temperature was 55 °C,the concentration of KPS was 0.5 wt%and the reaction time was 4 h.The polymerization kinetics was discussed.The results showed that R p∝[M]1.97[I]0.68[CTA]0.71 and the apparent activation energy was 72.55 kJ·mol-1.

1.Introduction

Polymer flooding is an enhanced oil recovery process,during which partially hydrolyzed polyacrylamide(HPAM)is added into the flooding water to improve the mobility ratio in the flooding process[1,2].In China,polymer flooding plays an important role in crude oil recovery process[3].The treatment of oily wastewater produced from polymer flooding(OWPF)is more difficult than that of oily wastewater without HPAM[4].The HPAM residue in OWPF can increase the viscosity of wastewater and the residue absorbed at the oil/water interface making oil drops in the wastewater extremely stable and difficult to remove[5,6].A lot of methods have been developed to treat the OWPF,among which the most applicable and effective one is flocculation[7,8].Until now,cationic flocculant and non-ionic flocculant have been widely used to treat the OWPF in oil fields.Cationic flocculants include diallyldimethyl-ammonium chloride homopolymer(PDM)and acrylamide-based copolymers,such as copolymer of acrylamide and acryloyloxyethyl trimethyl ammonium chloride[9,10].They have the merit of fast flocculation process,but also have two main shortcomings.One is that the viscosity of cationic flocculant is very high and making it difficultto pump especially in winter.The otherone is thatthe produced flocs are very viscous and may adhere to the pipelines or filter clothes.As for non-ionic flocculant,the most commonly used in oil fields is block polymer of ethylene oxide and propylene oxide,such as polyoxyalkylated and polyethyleneimine[11,12].Neither the nonionic flocculants nor the produced flocs are viscous,but the flocculation process is slow and the dosage is usually very high[13].

Generally,low viscosity and high polymer content cannot be easily obtained by PDM in the meantime.Low viscosity ensures the flocculant can be pumped easily and high polymer content ensures lower flocculant dosage.Especially,the requirements of viscosity and dosage were very high for the offshore oil field because the space of the platform was limited.According to the enterprise standard Q/12TG 4187-2010 of Energy Technology and Services Oil field Technology Services Company of CNOOC,China,a liquid flocculant's efflux time measured by an Apply 4 viscometer should be less than 100 s to ensure that it is easy to pump and the flocculant dosage was usually lower than 300 mg·L-1.To our knowledge,no reports have focus on this problem of PDM for its application in offshore oil field.In this paper,sodium allylsulfonate(XS)and sodium formate were used to controlthe viscosity of PDM.A liquid amphoteric flocculant(PDMXS)with both high polymercontent(about55 wt%)and low viscosity(the efflux time measured by an Apply 4 viscometer was less than 100 s)was prepared successfully.It is worth mentioning that the space required for the storage of resultant PDMXS is much smaller than that of conventional cationic flocculants,making ita promising candidate for OWPF treatmentin narrow offshore platforms.

2.Experiments

2.1.Chemicals

Diallyldimethyl-ammonium chloride(DM,60 wt%)was supplied by Green Earth Environment Technology(Beijing)Co.,and sodium allylsulfonate(XS,90 wt%)was purchased from Beijing Jintong Letai Chemicals Co.,Ltd.Potassium persulfate(KPS)was supplied by Chengdu Kelong Chemicals Co.,Ltd.All the chemicals are of analytical grade.The OWPF was obtained from an offshore oil field in China with a polymer(HPAM)content of 60 mg·L-1,oil content of 7012 mg·L-1and solid content of 229 mg·L-1.The concentrations of ions in OWPF are listed in Table 1.

Table 1 The ion concentrations in OWPF

2.2.Synthesis of amphoteric flocculant

The detailed synthesis process was as follows:A 250 ml four-neck flask equipped with a re flux condenser and a stirring rod was installed in a thermostatic water bath.A given amount of DM,XS,chain transfer agent(sodium formate)and deionized water was added into the flask and the total concentration of monomers was kept at 55 wt%.The solution was stirred at a rate of 120 r·min-1and purged with nitrogen gas for 30 min at 60°C.After that,KPS was added into the solution and the mass ratio ofitto monomers was keptat0.5 wt%.Then,the mixture was stirred at 60°C for 8 h under nitrogen atmosphere.Ultimately,the amphoteric flocculant product with a polymer content of 55 wt%was obtained.The copolymerization reaction process is illustrated in Fig.1.

The conversion ratio was determined by using mass method.Brie fly,amphoteric flocculant aqueous solution was firstly precipitated in the mixture of acetone and ethanol.Then the precipitated copolymer was vacuum-dried thus the conversion ratio could be determined.(Note:According to refs.[14],[15],and[16],and,there is a little branching and hexatomic ring structure in the DM polymerization.)

2.3.Characterization

FT-IR spectra were recorded on a Thermo Fisher Nicolet 6700 FTIR spectrometer.Polymer samples evenly mixed with KBr were pressed into thin slices and scanned at 4000–400 cm-1.

The thermal stability of the polymer powders was characterized by thermogravimetric analysis (TGA) using a thermal analyzer(STA449F3,NETZSCH instruments,Germany).10 to 15 mg of the samples was heated from 40 to 800 °C at a heating rate of 10 °C·min-1in air atmosphere.

2.4.Ef flux time measurement

After the polymerization,the efflux time of amphoteric flocculant was measured by using an Apply 4 viscometer according to the China National Standard(GB/T1723-93).With the same conversion ratio,the viscosity and molecule weight of the product can be evaluated by monitoring the efflux time.The relationship between efflux time and viscosity-average molecular weight of the product was shown in Table S1(see Supplementary Material).

2.5.Flocculation test

In this paper,the prepared amphoteric flocculant was in a liquid state and could be used directly after the polymerization.The flocculation test was performed as follows.Firstly,10 ml of OWPF was added into a colorimeter tube at 65°C for 10 min.Then amphoteric flocculant(200 mg·L-1)was added and the mixture was shaken for 30 s by hand.The mixture was heated at 65°C for another 5 min.The reason why chose 65°C is that the temperature of OWPF produced from the SZ36-1 offshore oil field in China is 65°C.Thereafter,the solution at the bottom of the colorimeter tube was removed and oil content in water(OiW)was measured by an OilTech121A(Environ Lab&Tech Inc.,USA).As shown in Fig.2,different phenomena were observed after the addition of PDMXS with different properties.For easier comparison of the flocculating performances,we defined that if the OiW removal was higher than 95%,then the result is noted by “Y”;if the OiW removal was lower than 95%,then the result is noted by “N”.

Fig.2.The results of OWPF treated by different amphoteric flocculants.

Fig.1.Schematic reaction of PDMXS.

2.6.Polymerization kinetics

The polymerization kinetics was studied by dilatometer method.The principle of dilatometer method is to take advantage of the linear relationship between the volume shrinkage in the polymerization process and the conversion.A special polymerizer with a capillary is equipped at the top of the dilatometer,and the volume variation of the system can be read directly from the liquid level drawdown of the capillary.The conversion is calculated as follows:

where C is the conversion.V′is the volume shrinkage of the system at different reaction time t which can be read from the capillary,and V is the volume shrinkage of the monomers which 100%converse into polymers under the given volume.After the polymerization,the C–t curve can be obtained.Rpwas the slope of the C–t curve when the C was lower than 10%.

3.Results and Discussion

3.1.The characterization of amphoteric copolymer

Fig.3 shows the FTIR result of the amphoteric copolymer.The peaks at3013 cm-1and 952 cm-1were assigned to stretching vibrations and deformation vibrations of C--H in--N--CH2--group.The vibration bonds at 2939 cm-1and 2862 cm-1were attributed to the methyl group of--N+(CH3)2[17].The peaks at 1188 cm-1and 1034 cm-1were according to the--SO32-group[18].

Fig.3.FTIR spectrum of amphoteric copolymer.

The TGA results of homopolymer of DM(PDM),homopolymer of XS(PXS)and amphoteric copolymer are shown in Fig.4.PDM exhibits two stages of distinct mass losses and the copolymer has three stages.For both polymers,the first stage ranges between 0 and 200°C which was caused by the mass loss of bound water.The mass loss during the second stage was due to the decomposition of DM parts.However,when the temperature was higher than 320°C,the XS part began to decompose like PXS.Therefore,there was a third stage for amphoteric copolymer.The difference between amphoteric copolymer and PDM at 500°C may be caused by their different molecular weights.PDM had higher molecularweightthan thatofamphoteric copolymer.Its molecularconformation was more crimp and contracted.Therefore,the air contact of PDM was less suf ficient than that of amphoteric copolymer and it had lower mass loss at 500°C.The results indirectly confirm that the copolymers of DM and XS have been successfully synthesized.

Fig.4.TGA results of different polymers(the mass ratio of XS/DM was 8 wt%for the amphoteric copolymer).

3.2.The effects of different factors on the viscosity and performance of PDMXS product

3.2.1.The effect of mass ratio of XS/DM

XS has the inhibitive self-polymerization effect because of the allyl groups[19],which could reduce the molecular weight of the amphoteric polymer.In addition,the amphoteric polymer contains both cationic and anionic functional groups.Its molecular extension may be much smallerthan thatof cationic polymer due to the electrostatic neutralization between DMand XS groups.Therefore,the PDMXS with both high polymer content and low viscosity may be obtained.The effect of the mass ratio of XS/DM on the polymerization results was studied under the following conditions:the temperature was 60°C,the concentration ofKPS was 0.5 wt%,the reaction time was 4 h and no chain transfer agent was added into the system.The results are shown in Fig.5.It can be seen that all the samples(mass ratio of XS/DM from 0 to 11 wt%)had high amount of oil removal and the efflux time decreased with the increase of XS content.When the mass ratio of XS/DM was higher than 10 wt%,the efflux time barely changed.However,the efflux time of DMS was still longer than 100 s even when the mass ratio of the XS/DM was 10 wt%.Therefore,the chain transfer agent was added to control the molecular weight of the copolymer for lower viscosity.

Fig.5.The effect of mass ratio of XS/DM on the viscosity and performance of amphoteric flocculants.

3.2.2.The effect of chain transfer agent

The effects of two water soluble chain transfer agents on the viscosity and performance of amphoteric flocculants were studied under the following conditions:the temperature was 60°C,KPS concentration was 0.5 wt%,the reaction time was 4 h and the mass ratio of XS/DM was 10 wt%.As shown in Fig.6,the viscosity of PDMXS decreased with the increase of the chain transfer agent concentration and sodium formate exhibited a better performance than sodium hypophosphite.When the concentration of sodium formate was higher than 1000 mg·L-1,the efflux time of PDMXS can be decreased down to 100 s.Furthermore,the resultant PDMXSstillexhibited satisfactory flocculation performance with the increasing sodium formate concentration.

Fig.6.The effect of chain transfer agent on the viscosity and the flocculation performance.

3.2.3.The effect of polymerization temperature

The effect of temperature was discussed under the following conditions:the KPS concentration was 0.5 wt%,the reaction time was 4 h,the mass ratio of XS/DM was 10 wt%and the concentration of sodium formate was 2000 mg·L-1.As shown in Fig.7,the resultant PDMXS exhibited poor oil removal performance when the reaction temperature was 50°C.This mightbe attributed to the fact that the conversion of copolymerization was too low(the conversion at this condition was 72%)as well as the effective copolymer content in this PDMXS product.When the temperature was higher than 50°C,the polymer conversions were more than 95%and all the resultant PDMXS samples exhibited satisfactory flocculation performances.In addition,the increase of viscosity might be attributed to the increasing molecular weight.

3.2.4.The effect of initiator concentration

The effectof initiator concentration was studied under the following conditions:the reaction temperature was 55°C,the reaction time was 4 h,the mass ratio of XS/DM was 10 wt%and the concentration of sodium formate was 2000 mg·L-1.As shown in Fig.8,it can be seen that the PDMXS displayed relatively low amount of oil removal when the concentration of initiator was 0.2 wt%.This also might be mainly caused by the low conversion(the conversion at this condition was 78%)of copolymerization and the low effective copolymer content in the PDMXS product at this condition.When the initiator concentration was 0.3 wt%,the conversion can reach to 94%and the resultant PDMXS products began to exhibit satisfactory performance again.The decrease of the viscosity may be due to the loss of molecular weight.

Fig.7.The effect of polymerization temperature on the viscosity and performance of amphoteric flocculants.

Fig.8.The effectofinitiatorconcentration on the viscosity and performance ofamphoteric flocculants.

3.2.5.The effect of reaction time

The effect of reaction time was studied under the following conditions:the reaction temperature was 55°C,the mass ratio of XS/DM was 10 wt%,the concentration ofKPS was 0.5 wt%and the concentration of sodium formate was 2000 mg·L-1.As shown in Fig.9,both of the efflux time and conversion increased with the reaction time.When the reaction time was less than 3 h,the conversion was smaller than 90%and the product exhibited relatively low amount of oil removal.When the reaction time was more than 3 h,the conversion can reach up to 90%and the oil removal amount was very high.Furthermore,the efflux time of all the samples was less than 100 s.

In summary,the optimum reaction conditions for the preparation of PDMXS with both excellent flocculation performance and low viscosity were identified as follows:the mass ratio of XS/DM was 10 wt%,the concentration of sodium formate was 2000 mg·L-1,the reaction temperature was 55°C,the concentration of KPS was 0.5 wt%and the reaction time was 4 h.Moreover,the effects of dosage and temperature on the resultantamphoteric flocculantwere also studied and the results were shown in Figs.S1 and S2(see Supplementary Material).

Fig.9.The effect of reaction time on the conversion and performance of amphoteric flocculants.

3.3.Polymerization kinetics

When the temperature was 55°C,[I]was 0.5 wt%and the concentration of chain transfer agentwas 2000 mg·L-1,the relationship between ln[M]and ln Rpwas shown in Fig.10.It can be seen that ln Rpshowed a linear correlation to ln[M]and the slope was 1.97.

Fig.10.The relationship between ln[M]and ln R p.

When temperature was 60°C,[M]was 55 wt%and chain transfer agent was 2000 mg·L-1,the relationship between ln[I]and ln Rpwas shown in Fig.11.The results showed that ln Rpwas also linearly related to ln[I],and the slope was 0.68.

When temperature was 60°C,[M]was 55 wt%and[I]was 0.5%.The relationship between ln[CTA]and ln Rpwas shown in Fig.12.It was found that ln Rpwas also linearly related to ln[CTA]and the slope was 0.71.

Moreover,the relationship between 1/T and ln Rpwhen[M]was 55 wt%,[I]was 0.5%and chain transfer agent was 2000 mg·L-1was shown in Fig.13.The ln Rpdecreased with 1/T and the slope was-8.73.According to the Arrhenius equation,the apparent activation energy of this polymerization was calculated to be 72.55 kJ·mol-1.

Fig.11.The relationship between ln[I]and ln R p.

Fig.12.The relationship between ln[CTA]and ln R p.

Fig.13.The relationship between 1/T and ln R p.

4.Conclusions

An amphoteric flocculant(PDMXS)with both high polymer content and low viscosity was prepared by using diallyldimethyl-ammonium chloride(DM)and sodium allylsulfonate(XS)as monomers.The structure of amphoteric copolymer was confirmed by IR and TGA.The polymerization conditions were optimized by taking oil removal amount and viscosity as the assessmentparameters.The optimum reaction conditions were as follows:the concentration ofmonomers was 55 wt%,the mass ratio of XS/DM was 10 wt%,the concentration of sodium formate was 2000 mg·L-1,the reaction temperature was 55 °C,the concentration of KPS was 0.5 wt%to monomers and the reaction time was 4 h.In addition,the results of polymerization kinetics experiments showed that Rp∝[M]1.97[I]0.68[CTA]0.71and the apparent activation energy was 72.55 kJ·mol-1.

Supplementary Material

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.cjche.2018.03.002.