High-efficiency separation and extraction of naphthenic acid from high acid oils using imidazolium carbonate ionic liquids

Fenghua Geng,Rui Zhang,Luo Wu,Zheng Tang,Han Liu,Haiyan Liu,Zhichang Liu,Chunming Xu,Xianghai Meng

State Key Laboratory of Heavy Oil Processing,China University of Petroleum,Beijing 102249,China

Keywords: Extraction Ionic liquid Naphthenic acid Recovery Computational chemistry

ABSTRACT N-alkyl imidazolium carbonate ionic liquids were employed to separate and recover naphthenic acid from model oils.The effects of the cationic and anionic structures of ionic liquids and operating conditions on the deacidification performance were investigated.The deacidification performance of traditional organic solvents was also investigated for comparison.The results indicated that the naphthenic acid could be completely removed from the model oil with a small mass ratio of ionic liquid to oil.The extracted naphthenic acid was regenerated with a recovery of up to 92%.In addition,imidazolium carbonate ionic liquids could be successfully regenerated and recycled.The mechanism of interaction between imidazole ionic liquids and the naphthenic acid molecules were explained by Gauss calculation.

1.Introduction

Global demand growth for high acid crudes increases owing to its low price.The crude is highly acidic when the total acid number(TAN)of crude is above 1.0 mg KOH·(g oil)-1by titration.The naphthenic acid(NA)accounts for about 90%of the total acidic oxygencontaining compounds in crude oil,which mainly exists in the middle fraction (boiling range 250–400 °C),such as kerosene,diesel and vacuum gas oil[1].NA is a complex mixture containing cyclic and aromatic monocarboxylic acids present in crude oil.Its common formula is CnH2n+zO2,where n represents the number of carbon atoms and z represents the lack of hydrogen due to the attendance of a naphthenic or aromatic ring.The value of z is less than or equal to zero [2–5].

The presence of NA will cause corrosion to metal equipment,such as pipelines,distillation units,heat exchangers,storage and transportation equipment,and even affect the properties of petroleum[6–8].NA also affects the refining process even if at an extremely low TAN,for instance,causing the foaming in various operation units [9].In addition,NA,as a natural surfactant,can form a stable water emulsion due to the interaction between oil and water.It has been reported that NA is one of the main substances of water pollution in the petroleum refinery[10].However,NA is a valuable byproduct,which can be widely used in metal,machine-building,agriculture and military industries [11].Hence,there is a great prospect for the separation and regeneration of NA from crude and distillate oils.

NA can be removed from oils by thermal decomposition [12],catalytic decarboxylation[13–17],esterification[18–21]and other destructive methods.However,these methods have such disadvantages as catalyst poisoning,complex technology,high operating costs and high operating temperature.In particular,the yield of NA is low since the structure of NA is destroyed during the above removal processes.

The removal of NA can also be accomplished by non-destructive processes such as neutralization [22,23],extraction [24,25] and adsorption [26],but these techniques still have some shortcomings.The traditional solvent extraction process is not friendly to the environment due to the generation of volatile organic solvent waste,but the adsorption technique is only appropriate for light fractions.Currently,caustic washing is the most frequent industrial technology for NA removal from oils.However,this method has the potential environmental pollution problem.To solve these problems,the development of a simple,environmentally sound and industrially feasible process has become the research focus.

Ionic liquids (ILs) are considered as environmentally friendly solvents because of their promising and particular characteristics such as nonvolatility,nonflammability,thermally stability and designability.In recent years,ILs have been widely used in extraction,purification,electrochemistry,catalysis,and so on [27–30].Currently,significant progress signs have been made in the extraction and separation of NA from crude oil by ILs [31–38].Imidazole ILs are reported to remove NA from oils and achieved good deacidification performance [32,34,37,38].However,these studies mainly focus on the oil with the TAN below 1.5 mg KOH·(g oil)-1,and few studies pay attention to the deacidification of acid from oils with higher acid value.Moreover,these ILs have high viscosity at room temperature,which is not conducive to mass transfer.Therefore,the optimal extraction temperature is generally high in the deacidification process,leading to high energy consumption.Shah et al.[36]successfully used hydroxide-based IL aqueous solution to remove NA from model oils and efficiently recovered NA.However,the optimal deacidification temperature of this process is still high(50–70 °C).

In this study,low viscosity,strong basic imidazolium carbonate IL aqueous solutions were used to remove NAs from acidic model oils.The investigated imidazolium cations ([Cnmim]+) had varying chain lengths.The deacidification mechanism was investigated by means of Gaussian quantum chemistry calculation.Carbonate ions can snatch hydrogen protons from NA and convert the NA into water-soluble carboxylate anions because of their strong alkalinity.In addition,other imidazole ILs with different anions and traditional organic solvents were selected for comparative experiments.

2.Experimental

2.1.Materials

NA (98%),n-dodecane (98%),N-methylpyrrolidone (99.5%),tetraethylene glycol (99%),sulfolane (99%),N-methylformamide(99%),1-ethyl-3-methylimidazolium acetate ([Emim]Ac) (98%),1-ethyl-3-methylimidazolium nitrate ([Emim]NO3) (98%),1-ethyl-3-methylimidazolium hydrogen sulfate ([Emim]HSO4) (98%),toluene (99%) and isopropyl alcohol (99%) were purchased from Shanghai Aladdin Biochemical Technology Co.,Ltd,China.1-Ethyl-3-methylimidazolium carbonate ([Emim]2CO3) (30% aqueous solution),1-butyl-3-methylimidazolium carbonate ([Bmim]2-CO3) (30% aqueous solution),1-hexyl-3-methylimidazolium carbonate ([Hmim]2CO3) (30% aqueous solution) and 1-octyl-3-methylimidazolium carbonate ([Omim]2CO3) (30% aqueous solution) were purchased from Merfu Technology (Shanghai) Co.,Ltd,China.

2.2.Preparation of model oil

The acidic model oil employed in this work was obtained by mixing n-dodecane with NA.The TAN of the model oil was 2.60 mg KOH·(g oil)-1.n-Dodecane was chosen since it was a component of kerosene and diesel fractions which usually had high TAN.

2.3.Deacidification reaction

IL and model oil with a constant mass ratio were added to a conical flask (50 ml) with a stopper.A thermostatic magnetic agitator was used to stir the mixture with the stirring speed of 500 r·min-1and to control the temperature unchanged during the deacidification process.The deacidification process was described by Eq.(1).

And then,the mixture was moved to a separating funnel and remained standing for two hours at room temperature for phase separation.The upper phase was the raffinate oil,and the bottom phase was the IL aqueous solution with NA.The raffinate oil and pure n-dodecane were measured by FT-IR and NMR.The results(Fig.S1 and Fig.S2,see Supplementary Material) showed that the spectrum of raffinate oil were consistent with that of pure ndodecane,indicating that the IL did not interact with n-dodecane and was insoluble in n-dodecane.

An automatic potentiometer(Metrohm Titrino Plus model 877)was used to determine the TAN according to the ASTM (American Society for Testing and Materials) D664 method.

The model oil (～4.00 g) was added to a 200 ml beaker and diluted with a 100 ml mixture of isopropyl alcohol,toluene and water in a volume ratio of 9:10:1.Titrant was 0.1032 mol·L-1of potassium hydroxide methanol solution.The removal percentage of NA can be calculated by TAN using the Eq.(2):

Where TANDand TANOare the TAN after deacidification and the original TAN of the model oil,respectively.

2.4.Recovery of naphthenic acid

It was worthy of studying the recovery of NA from the IL aqueous solutions (bottom phase).The NA was recovered by adding hydrochloric acid to the bottom phase.The reaction of the recovery process was described by Eq.(3).Anhydrous ether was added to the mixture when the reaction finished.This recovery process was easy to perform because the solubility of NA and imidazolium chloride ILs in anhydrous ether and water was different.And then transferred the two-phase mixture to a separating funnel and remained standing for 40 min at room temperature for phase separation.The upper layer was the ether solution of NA,and the under layer was the imidazolium chloride IL aqueous solution.NA could be obtained from the upper layer by the removal of the ether with distillation or evaporation.

2.5.Regeneration of ionic liquids

The used ILs could be regenerated by anion-exchange resin.The aqueous solution of imidazolium chloride IL obtained after the recovery of NA was added to the chromatography column containing pretreated anion-exchange resin.The aqueous solution of imidazolium chloride IL slowly flowed through the resin and the effluent was collected to obtain the aqueous solution of imidazolium carbonate IL.The regeneration process is shown in the Eqs.(4) and (5):

2.6.Computational details

In order to study the mechanism of the extraction process,Gaussian 09 software [39] was used to analyze and calculate the interaction between ILs and NAs.The optimization and frequency analysis of all the structures were performed on the level of PBE0-D3 functional and custom basis set [40,41],and the structures obtained were minimal point structures with no virtual frequencies.The custom basis set was as follows:6-31+G* basis set was selected [42],all molecular structure diagrams were drawn by Gaussian View 5.0.9,and all isosurface maps and plane graphs were drawn by Multiwfn 3.8 (Dev) [43] and VMD 1.9.3 [44].More specifically,(3-methyl)-cyclopentyl propionic acid was selected as model NA.The structures of ions and molecules involved in the calculation are shown in Fig.1.

Fig.1.Structures of ions and molecules used in the calculation.

3.Results and Discussion

3.1.Effect of IL/oil mass ratio

The mass ratio of IL to oil (IL/oil mass ratio) is a considerable influencing factor for the extraction of NA.IL aqueous solutions were used in the experiment,so the IL/oil mass ratio only considered the mass of ILs in the solution.The extraction process was carried out at 40 °C for 2 h with a stirring speed of 500 r·min-1.And the standing time was 2 h.The effect of IL/oil mass ratio on the removal percentage of NA was investigated and the results are shown in Fig.2.The removal percentage of NA enhanced significantly with the increase of the IL/oil mass ratio.NA could be removed entirely for each IL as long as the IL/oil mass ratio was large enough.The complete removal of NA with [Emim]2CO3,[Bmim]2CO3,[Hmim]2CO3and [Omim]2CO3was achieved when the IL/oil mass ratio was 0.010,0.012,0.014 and 0.016,respectively.

Fig.2.Removal percentage of NA as a function of IL/oil mass ratio (stirring temperature,40 °C;stirring speed,500 r·min-1;stirring time,2 h;standing time,2 h).

The results showed that [Emim]2CO3had the best separation efficiency among the investigated ILs.The deacidification performance of the ILs followed the order of[Emim]2CO3＞[Bmim]2CO3＞[-Hmim]2CO3＞ [Omim]2CO3,which indicated that the NA deacidification efficiency of IL decreased with the growth of the alkyl side chain.This result was contrary to the reported results of NA deacidification by imidazole ILs.It was reported that the separation efficiency of NA increased with the increase of alkyl side chain length in ILs.The literature indicated that the reason for this result was that the interaction between anions and cations weakened,and the interaction between ILs and NAs increased as the length of the alkyl side chain increased[32,34,38].Meanwhile,Holbrey et al.[45] proposed that IL molecules could form cage structures by specific chemical bonds,which was beneficial to the removal of NA.Moreover,the long-chain alkyl group was conducive to the formation of the cage structure,which also explained why the deacidification efficiency increased with the length of the side chain of imidazolium cationic alkyl group.However,the conditions of this study were different from the above situation.ILs aqueous solutions with strong polarity and alkalinity were used in this work.Since the solvent water prevented the formation of cage structures in the IL molecules,it eliminated the advantages of long-chain alkyl IL extraction.Moreover,it was well known that the shorter the alkyl side chain of the IL,the stronger the polarity of the IL.The increase in polarity of IL promoted its interaction with NA,thereby improving the removal efficiency [36].

In addition,we speculated that the deacidification efficiency would increase because ILs with short alkyl groups had high molar concentrations at the same IL/oil mass ratio.In order to verify the above speculation,the IL/oil mass ratio was converted into mass molar concentration of IL.The results are presented in Fig.3.The removal percentage of NA raised with the increase of mass molar concentration.There was little difference in the removal percentage of NA among these four ILs at the same mass molar concentration.Obviously,NA was just completely removed when the mass molar concentration of all ILs was about 0.035 × 10-3mol·g-1.It was sufficient to show that the interaction between the imidazolium carbonate ILs and NA followed the stoichiometric relationship.Therefore,the short side chain of the cation showed favorable performance for NA extraction from oil.Therefore,the optimal IL was [Emim]2CO3and the optimal IL/oil mass ratio was 0.01 with the overall consideration.

Fig.3.Removal percentage of NA as a function of mass molar concentration of IL(stirring temperature,40°C;stirring speed,500 r·min-1;stirring time,2 h;standing time,2 h).

3.2.Effect of stirring temperature

The influence of stirring temperature(20–80°C)on deacidification was studied.The IL/oil mass ratio,stirring speed,stirring time and standing time were constant at 0.010,500 r·min-1,2 h and 2 h,respectively.The results are shown in Fig.4.The removal percentage of NA firstly increased and then decreased with the increase of stirring temperature.The optimal stirring temperature using[Emim]2CO3was 40 °C,while the optimal temperature was 50 °C for [Bmim]2CO3,[Hmim]2CO3and [Omim]2CO3.As the stirring temperature increased,the viscosity of both the model oil and the IL decreased,which enhanced the mass transfer of NA from the oil phase to the IL aqueous solution.Moreover,the solubility of NA in ILs increased at a higher temperature [36].All these factors were favorable for the polar part of NA molecules to contact with ILs [46].In addition,proton transfer reaction is an endothermic process,which was the main factor that the removal percentage of NA increased with the increase of temperature [47].At the same time,the number of activated molecules increased with increased temperature,which enhanced the probability of effective collision and improved the separation efficiency.

Fig.4.Removal percentage of NA as a function of stirring temperature(IL/oil mass ratio,0.010;stirring speed,500 r·min-1;stirring time,2 h;standing time,2 h).

As the temperature exceeded the optimum value,the removal percentage of NA decreased with the increase of temperature.The main reason for this result was that the hydrogen bonds between bicarbonate ion and carboxylic acids weakened with the increased temperature.Meanwhile,it was speculated that the reason might be that the bicarbonate IL was not stable at high temperature and decomposed as the temperature increased [48].

3.3.Effect of stirring time

The effect of stirring time on deacidification using [Emim]2CO3was studied,keeping stirring temperature,stirring speed,IL/oil mass ratio and standing time constant at 40 °C,500 r·min-1,0.010 and 2 h,respectively.The results are displayed in Fig.5.It was found that the removal percentage of NA raised with the increase of stirring time in the beginning and remained unchanged afterward.The removal percentage of NA rapidly reached 90% in 2 min,then increased slowly and finally reached 100% at the stirring time over 40 min.Under normal circumstances,the proton transfer reaction occurred rapidly.Therefore,NA could be converted into carboxylic anions with relatively high solubility in water and entered into the aqueous phase in a very short time.As a result,the removal percentage of NA was as high as 90%within two minutes.However,part of NA that did not participate in the proton transfer reaction entered into the aqueous solution mainly by hydrogen bonding interaction,and this process took more time.Consequently,the stirring time was recommended to be above 40 min to ensure complete reaction of NA and ILs.

3.4.Recovery of NA from ionic liquid phase

NA(1.00 g)was put into n-dodecane to obtain a TAN of 2.60 mg KOH·(g oil)-1model oil,0.92 g NA was recovered by this method.The yield of the recovery of NA was 92%.The Fourier Transform Infrared (FT-IR) and13C NMR spectra for pure and recovered NA are shown in Fig.6 and Fig.7,respectively.The FT-IR and13C NMR spectra of pure NA and recovered NA were almost the same,indicating that the recovery of NA by this method was reasonable.The recovery process was simple,and the TAN of the recovered NA(238 mg KOH·(g oil)-1)was very close to that of the purchased NA(265 mg KOH·(g oil)-1).

Fig.5.Removal percentage of NA as a function of stirring time (IL/oil mass ratio,0.010;stirring temperature,40 °C;stirring speed,500 r·min-1;standing time,2 h).

Fig.6.FT-IR spectrum of pure NA and recovered NA.

Fig.7.13C NMR spectrum of pure NA and recovered NA.

3.5.Recycle of ionic liquids

It is necessary to study the reuse of ILs.The initial and regenerated ILs were characterized by NMR (the13C NMR and1H NMR spectra of ILs are shown in Figs.S3–S10).There were almost no changes in13C NMR and1H NMR of the IL before use and after recycling,which indicated that ILs could be regenerated completely without changing their structure.Then the recycled [Emim]2CO3was used to separate NA from model oil at the same conditions five times.The removal percentage of NA remained at 100%.Although the price of ILs was higher than that of traditional solvents,ILs used in this study were renewable and reusable,and the NA with high value-added could be recovered,which would reduce the process cost and be economically feasible.

3.6.Contrast experiments

Three ILs ([Emim]Ac,[Emim]NO3,[Emim]HSO4) and four traditional organic solvents were selected as contrast solvents for deacidification.The contrast experiments were carried out under the same conditions:ILs (30% aqueous solutions) or pure traditional organic solvents,stirring temperature was 40 °C,solvent/oil mass ratio was 0.010,stirring speed was 500 r·min-1,stirring time was 1 h,and standing time was 2 h.The results are displayed in Table 1.

Table 1 Contrast experiments of other solvents

Results showed that the removal efficiency of [Emim]2CO3was much higher than that of other solvents.The deacidification efficiency of ILs followed the rule of [Emim]2CO3?[Emim]Ac ＞[Emi m]NO3≈[Emim]HSO4.The difference in extraction performance was caused by the anion of the ILs,while the anionic alkaline follows the order ofSo this result was consistent with that reported in the literature that the deacidification efficiency increased with the increasing alkalinity of the anions[34].Furthermore,the deacidification efficiency of traditional organic solvents followed the rule of N-methylformamide ＞N-me thylpyrrolidone ＞tetraethylene glycol ＞sulfolane.The extraction capacity difference between traditional organic solvents was not significant.This might be related to the similar polarity of the organic solvents,and the slightly higher deacidification efficiency of N-methylformamide was due to its obvious alkalinity.

3.7.Quantum chemical calculations

Due to the diversity of structures and functions of ILs,most particle interactions between ILs and other molecules involve dispersion,π-π stacking,n-π transition,hydrogen bonds,polarization and static electricity [49].In order to investigate the mechanism of the deacidification process,the interaction process between the carbonate anion and the NA molecule was investigated by quantum chemical calculation,and the results are shown in Fig.8.The O-H bond length in NA increased from 0.1038 nm to 0.1231 nm with the approaching of the carbonate ion,and then broke,forming a carboxylate anion.Meanwhile,the distance between the H atom in the O-H bond of the carboxylic acid molecule and the O atom of the carbonate ion gradually decreased to 0.0970 nm,forming a new O-H bond,and generating HCO3-ion.In other words,a proton transfer reaction occurred between carboxylic acid molecules and carbonate ions in the deacidification process,carboxylic acid protons in NAs were taken away by carbonate ions in ILs,forming carboxylate anions.While the proton transfer reaction was more conducive to the removal of NAs,which was consistent with the experimental results.

During the extraction process,the generated carboxylate ion had strong polarity due to its negative charge,which increased its solubility in the water phase and simultaneously reduced its solubility in the oil phase,realizing the separation of NA from the oil.Moreover,under the same IL/oil mass ratio,the molar concentration ofin the IL aqueous increased with the shorted alkyl side chain,and the removal percentage of NA also increased.It was reported that the polarity of the IL reduced with the increase of the alkyl side chain,which was conducive to the entry of the IL into the non-polar oil phase,enhancing the interaction of IL with NA and the deacidification efficiency [50].However,IL aqueous solutions were used in this study,and the proton transfer reaction between anions and NA complied with the stoichiometric relation-ship.Therefore,the effect of the length of the alkyl side chain was almost negligible.

Fig.8.Interaction pathway of carbonate and NA molecule.The white,silver and red spheres represent H,C and O atoms,respectively.

Anions played a leading role (Fig.S11),and hydrogen bonds provided a driving force during the deacidification process.Through Atoms in Molecules(AIM)analysis[51],it was found that there was a(3,-1)bond critical point between the carboxyl group H in carboxylic acid and the highly electronegative O in the IL or organic solvent molecule.The interaction energies of IL anions or organic solvent molecules with NA molecules were calculated based on the electron density at the bond critical point (BCP) ρ and formula(6)reported by Emamian[52].The calculation results are shown in Fig.9.The bind energies ΔE were in the range of 10.46–58.60 kJ·mol-1,which fully indicated that the deacidification process was mainly caused by hydrogen bonding.

Where ρBCPwas the electron density at the bond critical point.

Further,the independent gradient model (IGM) [53] was performed to analyze and simplify the weak interactions between molecules.In Fig.9,the isosurface of the hydrogen bond region was obviously blue,so it was believed that the hydrogen bond was indeed formed,which was consistent with the results of the AIM analysis.However,as for ILs with anions of Ac-,and HSO4--,there were some areas of green flat sheets near the hydrogen bonds.This explained that the electron density was small,and the interaction intensity was weak in these regions,indicating that it was the dispersion effect.Moreover,it can be seen from the calculation results that the strength of the hydrogen bond was consistent with the deacidification efficiency of ILs.However,the calculation results of the hydrogen bond energy of organic reagents were not wholly consistent with the deacidification efficiency.These results showed that the hydrogen bond energy of tetraethylene glycol was slightly higher than that of n-methylpyrrolidone,while the extraction efficiency of n-methylpyrrolidone was higher than that of tetraethylene glycol.This might be related to the steric effect of the long-chain structure of tetraethylene glycol and the weak alkalinity of n-methylpyrrolidone.As mentioned above,the deacidification process by organic reagents was affected by various factors although it was related to hydrogen bonding.

Fig.9.Intermolecular interactions of(3-methyl)-cyclopentyl propionic acid(isosurfaces:0.01 a.u.)with different models((a)nitrate;(b)acetate;(c)hydrogen sulfate(d)Nmethylformamide;(e)tetraethylene glycol;(f)sulfolane;(g)N-methylpyrrolidone)using independent gradient model(IGM)analysis.The white,silver,red,blue and yellow spheres represent H,C,O,N and S atoms,respectively.

4.Conclusions

Strong alkaline imidazolium carbonate IL aqueous solution could effectively remove NA from high acid model oil.The removal percentage of NA declined with the growth of the alkyl side chain.The optimal deacidification conditions of [Emim]2CO3were as follows:stirring temperature was 40 °C,IL/oil mass ratio was 0.010,stirring speed was 500 r·min-1and stirring time was 1 h.In the deacidification process,IL and NA followed the stoichiometric relationship,and the NA was just completely removed when the molar ratio of IL/oil was about 0.76.Temperature was an essential factor in the deacidification process,and the removal percentage of NA showed maximum with the increase of temperature.The anion played a leading role and the proton transfer reaction occurred in the deacidification process.

For imidazolium carbonate ILs,NA could be removed entirely with a low IL/oil mass ratio,and the extracted NA could be successfully recovered.Moreover,the aqueous solutions of imidazole carbonate ILs after extraction could be effectively recovered and recycled.These results revealed the potential feasibility of this method.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (22078359,21276275).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.12.012.