Effect of Demulsification for Crude Oil-in-Water Emulsion: Comparing

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Article

Effect of Demulsification for Crude Oil-In-Water Emulsion: Comparing CO and Organic Acids 2

Dongfang Liu, Yuxin Suo, Jihe Zhao, Peiyao Zhu, Jiang Tan, Baogang Wang, and Hongsheng Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03334 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Energy & Fuels

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Effect of Demulsification for Crude Oil-In-Water Emulsion:

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Comparing CO2 and Organic Acids

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Dongfang Liu, † Yuxin Suo, †Jihe Zhao, † Peiyao Zhu, † Jiang Tan, † Baogang Wang †

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and Hongsheng Lu *, †, ‡

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†

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*Corresponding author: College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P. R. China. Email: [email protected]; Fax: +86-28-83037330; Tel: +86-28-83037330.

College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P. R. China ‡ Engineering Research Center of Oilfield Chemistry, Ministry of Education, Chengdu 610500, P. R. China

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ACS Paragon Plus Environment

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ABSTRACT

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Acidic substances (naphthenic acids) were found in crude oil, which provides

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favorable conditions for the formation of surfactants. N, N-Dimethylcyclohexylamine

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(DMCHA) can activate the naphthenic acid in the crude oil to form the surfactant by

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noncovalent bonding. This process has an unusual meaning for the crude oil pipelines

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transportation by the method of forming a low viscosity oil-in-water emulsion. At the

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same time, CO2 can be used as a demulsifier to separate crude oil and water at the end

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of pipeline transportation because of the surfactant has a CO2 stimulus response

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characteristic. In our study, it was found that crude oil-in-water emulsion can be

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formed and has very good stability. CO2 has a high demulsification efficiency in

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emulsions with lower crude oil content. For high oil content conditions, the emulsion

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cannot be demulsified completely by CO2. Since sulfuric acid, hydrochloric acid and

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the like are liquid and corrosive, this is not conducive to its application as a

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demulsifier in crude oil pipeline transportation. Organic acids in this area have a

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stronger advantage obviously, so citric acid, oxalic acid, acetic acid and lactic acid

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were used as demulsifiers in this article, all of them have good demulsification

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performance in high crude oil content emulsions, especially citric acid. Meanwhile,

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DMCHA can be recycled and reused by adding NaOH to the lower aqueous solution

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after demulsification.

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Keywords: DMCHA; citric acid; crude oil emulsion; demulsification.

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1. INTRODUCTION

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In general, reducing the viscosity of the crude oil is necessary during its pipeline

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transportation1. Many methods to reduce the viscosity of crude oil have been studied,

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such as heating2, dilution3, emulsification4, 5 and so on. However, a green, low energy

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consumption and low-cost solution are always the pursuits of goals. Thus, how to

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achieve these goals in the field of crude oil pipeline transportation? We have provided

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a very good solution based on our research about stimuli-responsive materials6-9.

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The study of stimulus-response has been extensively studied. In general, the

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stimulus-response is mainly focused on pH10, 11, CO212, 13, light14, 15, temperature16, 17,

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magnetic18, 19 and redox20, 21 response. CO2 has been extensively studied because it is

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considered to be an ideal switchable trigger factor, including its green, abundant, does

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not accumulate in the system and easy to remove22. Recently, CO2 stimuli-responsive

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materials have received extensive attention and research. For example, CO2

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responsive micelles23, 24, emulsions25, 26, polymers27, 28, etc. In 2006, Jessop group

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reported the study of switchable surfactants29. The long-chain alkyl amidine

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compound can be reversibly converted to a positively charged surfactant in the

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presence of CO2. Hexadecane, water, and long-chain alkyl amidine can form stable

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emulsions in the presence of CO2 and the demulsification occurs in the presence of

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argon. Emulsification is an effective means of reducing the viscosity of crude oil. If

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the crude oil emulsions with the characteristics of the CO2 switch can be prepared, we

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can solve the problems encountered in the pipeline transportation. At the beginning of

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transport, the crude oil emulsions with lower viscosity were prepared by using CO2 3


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switchable surfactants and the crude oil and water can be separated at the end of

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transport under CO2 stimulation. In addition, naphthenic acids are common in crude

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oil30, and even high acid crude oil will cause serious corrosion of equipment. The

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naphthenic acids can form a surfactant when combined with a CO2 stimuli-responsive

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tertiary amine or amidine. Not only can emulsify the crude oil to form emulsions but

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also can reduce the corrosion of equipment in the pipeline transportation. In the crude

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oil pipeline transportation process, the amount of crude oil in the emulsion determines

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the transport costs directly. In general, the crude oil content in the emulsion should be

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above 70 wt%31 during the pipeline transportation process. However, the increase of

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oil content in emulsions also affects the stability of the emulsion and demulsification

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performance. It cannot break the emulsion effectively with high oil content by CO2

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because with the introduction of CO2, a large amount of crude oil will cover the upper

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layer, which makes CO2 difficult to escape. This will undoubtedly increase the

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difficulty of the operation. Due to the strong acid, such as hydrochloric acid and

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sulfuric acid are corrosive and not suitable for use as a demulsifier. On the contrary,

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organic acids are undoubtedly the best choice, especially citric acid. Solid acid citric

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acid can greatly reduce transportation costs and improve demulsification efficiency at

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room temperature.

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In this paper, hydrophobic tertiary amine N, N-Dimethylcyclohexylamine

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(DMCHA) with CO2 response was used to emulsify the crude oil. CO2 is an effective

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demulsifier for emulsions with low oil content. On the contrary, if the crude oil

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content in the emulsion increases, the crude oil cannot be effectively separated with 4


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CO2 bubbling. At the meanwhile, organic acids such as acetic acid, oxalic acid, lactic

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acid and citric acid play the vital role in demulsifier. DMCHA can emulsify crude oil

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to form an emulsion that facilitates pipeline transportation. Organic acids can be used

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as demulsifiers to achieve separation of crude oil and aqueous solutions at the end of

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transport. This process has important guiding significance for crude oil transportation

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industry.

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2. EXPERIMENTAL SECTION

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2.1 Chemicals and materials

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Tertiary amines N, N-Dimethylcyclohexylamine (DMCHA, 98%) was purchased

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from Aladdin Reagents of China. Oxalic acid (99.8%) was purchased from Shanghai

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Macklin Biochemical Co., Ltd. Acetic acid (99.5%), lactic acid (90%) and citric acid

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(99.8%) were purchased from Chengdu Kelong Chemical Factory. CO2 (>99%) were

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purchased from Chengdu Jinli gas Co., Ltd. The crude oil sample was obtained from

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the Xinjiang China. The parameters of the crude oil are shown in Table 1. All the

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chemicals without further purification before use.

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Table 1. The specifications of crude oil Entry Amount Unit o Density (20 C) 0.98 g/cm3 Viscosity (50oC) 113.42 Pa·s Acid number 3.78 mg KOH/g Asphaltene 1.27 wt% Resin 11.48 wt% Saturates 60.14 wt% Aromatics 25.63 wt% Water content 13.10 wt%

5


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2.2 Preparation of crude oil-in-water emulsion

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At room temperature, weigh the crude oil, DMCHA, and water, respectively, into

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the 50 mL plug cylinder. Then put the plug cylinder in the ultrasonic environment for

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30min at 50oC. After the full action of DMCHA and naphthenic acids, crude oil

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emulsion was formed. The type of emulsion was judged by drop test6, a drop of crude

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oil emulsion was dropped into the water. If the droplets were dispersed immediately in

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water, the emulsion was an oil-in-water emulsion and vice versa was a water-in-oil

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emulsion.

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2.3 The demulsification process of crude oil emulsions

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The stability of the crude oil emulsion was illustrated by observing the rate of

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separation of the aqueous phase. The volume of the separated aqueous solution was

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recorded as V1. The total volume of water (contains the water content of the crude oil

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itself) and DMCHA were recorded as V. =

× 100% (1)

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S represents the demulsification rate. Under the same conditions, the smaller of

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the S value corresponds to the more stable emulsions; on the contrary, the larger of the

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S value corresponds to the unstable emulsions. For the demulsification process, the

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greater the value of S per unit time corresponds to the higher the demulsification

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efficiency.

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Two methods of demulsification were used in this paper. (1) At room temperature,

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CO2 was bubbled into the crude oil emulsion at a rate of 100 mL·min-1 for 10 min,

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and then the volume of the separated water was recorded. (2) The organic acid 6


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aqueous solution was added as a demulsifier to the crude oil emulsion. After fully

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mixed in an ultrasonic environment at 50oC, the volume of the separated aqueous

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phase was recorded.

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2.4 The recovery of DMCHA after emulsion breaking

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DMCHA can be recovered by adding NaOH to the aqueous solution after

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demulsification. The volume change of the DMCHA was recorded with the addition

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of NaOH. When the volume of DMCHA no longer changes, it is the maximum

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volume of the recovered. The ratio of the recovered volume of DMCHA to the total

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volume added is the recovery rate of DMCHA.

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3. RESULTS AND DISCUSSION

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3.1 Preparation of crude oil-in-water emulsions with low oil content

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Tertiary amine DMCHA and naphthenic acid self-assembled to form a surfactant,

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which plays an important role in the formation of crude oil emulsions. In order to find

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the optimum concentration of DMCHA, the following emulsions were prepared

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(Table 2).

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Table 2. The proportion of each component in the crude oil emulsion Entry

Crude oil/g

Water/g

DMCHA/g

T/min

Oil content/wt%

Emulsion type

A

2

40

0.2

30

4.74

O/W

B

2

40

0.5

26

4.71

O/W

C

2

40

1

17

4.65

O/W

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D

2

40

1.5

13

4.60

O/W

E

2

40

2

12

4.55

O/W

T: The minimum time required to form the emulsion.

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As showed in Table 2, Emulsions A, B, C, D, and E can be formed, but the time

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to form the emulsion was different. The lower the DMCHA concentration, the longer

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it takes to form the emulsion. The higher the concentration of DMCHA means that the

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higher the concentration of surfactant that promotes emulsion formation and the

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shorter the time required for emulsion formation. What about the stability of the five

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emulsions prepared? Freshly prepared crude oil emulsions A, B, C, D, and E were

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placed at room temperature to observe and record the volume changes with time.

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Since the demulsifier was not added, the oil and water phases cannot be clearly

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separated. The single phase O/W emulsion began to be divided into two phases, and

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the volume of the upper phase gradually increased with the prolongation of time. The

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ratio of the volume of the upper phase to the total volume of the emulsion was used to

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indirectly represent the stability of the emulsion. The demulsification efficiency of the

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emulsion was shown in Figure 1.

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Figure 1. Stability of crude oil emulsion with various DMCHA concentration.

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It can be seen from Figure 1 that the stability of the emulsions was closely

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related to the concentration of DMCHA. It is clear see that the stability of the

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emulsions D and E was better than A, B, and C. The higher the surfactant

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concentration will not only contribute to the rapid formation of the emulsion but also

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help to improve the stability of the emulsion. The curves D and E in Figure 1 were

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very close, indicating that the concentration of DMCHA can only affect the stability

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of the emulsions within a certain range and not at all of the concentration range.

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3.2 CO2 as a demulsifier for low crude oil content emulsions

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Before refinery, it is necessary to separate the crude oil from the crude oil

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emulsions at the end of the pipeline transportation. Whether the emulsion can quickly

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and thoroughly demulsify is an important factor affecting the transport efficiency of

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crude oil pipelines. At room temperature, CO2 was bubbled into the five emulsions at

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a rate of 100 mL·min-1 for 10 min, then the demulsification process was observed for 9


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standing and the volume of the aqueous phase was recorded. Figure 2 shows

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photographs of emulsions A, B, C, D and E after 10 min of CO2 treatment.

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Figure 2. Demulsification of emulsions A, B, C, D and E with CO2 treatment.

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It can be seen clearly from Figure 2 that emulsions A, B, C, D, and E have been

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demulsified after treatment with CO2, but the final state after demulsification was

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obviously different. Emulsions D and E have almost completely separated the crude

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oil and water, and demulsification efficiency was over 90% (Table 3) in the case of

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CO2 for ten minutes. Emulsion C also has an obvious two-phase separation, but the

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separation efficiency of the emulsion was significantly lower than that of the emulsion

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D and E at the same time, and the lower phase of the emulsion C was more turbid.

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The emulsions A and B have the lowest demulsification efficiency under the same

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conditions.

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Scheme 1. A schematic diagram of the chemical principle based on the DMCHA and naphthenic acid (RCOOH).

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As depicted in Scheme 1, DMCHA can be self-assembled with naphthenic acid

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in crude oil to form surfactants. This process was an emulsification process where

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crude oil emulsions can be prepared. As the acidity of carbonic acid is stronger than

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naphthenic acid, in the presence of CO2, the assembled surfactant was destroyed and

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the emulsion was demulsified. Naphthenic acid was dissolved in the oil phase and the

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aqueous phase was ammonium bicarbonate solution. At the same time, DMCHA can

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be recovered by treating the lower aqueous phase with N2, which can greatly reduce

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the cost of consumption. It can be seen from Figure 2 that the concentration of

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DMCHA has an effect not only on the formation of the emulsion but also the

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demulsification process. At the moment, when CO2 was bubbled into the emulsion A

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and B, a portion of the crude oil in the emulsions A and B accumulates in the upper

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layer first. With the continuous entry to CO2, the crude oil and water were mixed and

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cannot be effectively separated. This may be caused by incomplete emulsification due

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to the low concentration of the surfactant. The demulsification process of the

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emulsions D and E was different from those of the emulsions A and B, the O/W

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droplets of emulsions D and E gradually disintegrate with the CO2 continuous bubbles. 11


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The crude oil was separated from the emulsion in the form of small droplets and

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finally gathered in the upper layer, so the separation effect was more obvious.

218 219

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Table 3. The demulsification efficiency of emulsions A, B, C, D and E after CO2 treatment. Entry

A

B

C

D

E

V1/mL

-

-

-

40.5

41.0

V/ mL

40.2

40.6

41.2

41.7

42.3

S/%

-

-

-

97.12

96.93

3.3 Organic acids as demulsifier for high crude oil content emulsions

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For crude oil emulsions with low oil content, CO2 can be used as an efficient

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demulsifier for effective separation of crude oil and water from emulsions. However,

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the oil content of the emulsions was much higher than the oil content in section 3.1 in

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practice. So what about the formation and demulsification process for high oil content

225

emulsions?

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Crude oil emulsion (7g crude oil + 3g H2O + 2g DMCHA,) can be formed and

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can last more than a week without breaking. The oil content of this emulsion is

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58.33%. The type of the emulsion was O/W determined by the drop test. The effects

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of CO2 and organic acids on the demulsification performance of emulsions were

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investigated. Figure 3 shows the results of the demulsification of crude oil emulsions

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with CO2 and organic acids as demulsifiers.

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Figure 3. The picture of crude oil emulsions was standing for 24h after the demulsification. (a) Crude oil-in-water emulsion, (b) The emulsion was treated with CO2 for 10 min at a rate of 100 mL·min-1. Organic acids are used as demulsifiers to treat crude oil-in-water emulsions, (c) Citric acid solution 10 mL (5wt%), (d) Oxalic acid solution 10 mL (5wt%), (e) Acetic acid solution 10 mL (5wt%), (f) Lactic acid solution 10 mL (5wt%).

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As showed in Figure 3, the phase separation of the crude oil emulsion was very

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clear in the presence of CO2 and organic acids. Naphthenic acid is a mixture of weak

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acids. It will become an uncharged molecular state in the presence of strong acids, and

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then the electrostatic interaction between DMCHA was disappeared. Based on this

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principle, strong acid, such as carbonic acid, citric acid, oxalic acid, acetic acid, and

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lactic acid can play the role of demulsifier. However, citric acid and acetic acid have

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the highest demulsification efficiency and the lowest demulsification efficiency of

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CO2 at the same time (Figure 4).

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Figure 4. Demulsification efficiency of crude oil emulsion under different demulsifiers.

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It is well known that weak acids correspond to larger pka values. Table 4 lists the

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pka values for all acids32. The organic acids mentioned in this article have stronger

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acidity than carbonic acid, which means higher demulsification efficiency than CO2.

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At the same time, three pka values of citric acid were 3.13, 4.76, and 6.40 (Table 4),

254

respectively. These three values are less than or close to the carbonic acid pka1, which

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means that citric acid has more carboxyl groups can participate in the reaction,

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demulsification efficiency higher than other acids. This is why citric acid and oxalic

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acid have the highest efficiency of demulsification.

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Table 4. Dissociation constants of different acids (298k). Entry

pKa1

pKa2

pKa3

Carbonic acid

6.35

10.33

-

Citric acid

3.13

4.76

6.40

14


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Oxalic acid

1.25

4.27

-

Acetic acid

4.76

-

-

Lactic acid

3.86

-

-

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The above experiment mentions that CO2 can be used as a demulsifier for crude

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oil-water separation. In practice, the efficiency of CO2 as a gas participating in the

261

reaction is limited, and some of the CO2 is not directly involved in the reaction but is

262

released directly into the air. At the CO2 access time increases, the separated crude oil

263

will cover the surface in the first time, which increases the CO2 gas release pressure.

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It may cause the demulsification operation cannot proceed smoothly. Organic acids do

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not have these problems as demulsifiers and citric acid can be used as an excellent

266

demulsifier. Citric acid is a kind of material that is inexpensive, non-irritating odor,

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non-toxic, environmentally friendly and present in the solid state at room temperature.

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The tricarboxylic structure of citric acid gives it a higher demulsification efficiency

269

than CO2 (Scheme 2).

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Scheme 2. Schematic diagram of the demulsification efficiency of citric acid and CO2 as demulsifier Table 5. Composition of crude oil-in-water emulsion Emulsions Crude oil/g Water/mL DMCHA/mL Oil content/wt% (a) 10 3 3.5 62.50 (b) 12 3 3.5 66.67 (c) 14 3 3.5 70.00 (d) 16 3 3.5 72.73 (e) 18 3 3.5 75.00

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In order to study the effect of citric acid on the demulsification efficiency of high

275

oil content emulsions, five crude oil emulsions were prepared with different oil

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content in Table 5. The stability and viscosity reduction of crude oil-in-water

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emulsions in Table 5 were showed in supporting information (Fig.S2, Fig.S3, and

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Tab.S1).Citric acid solution (20 mL, 11.63wt %) was added to the five stable

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emulsions. The demulsification was carried out after thorough mixing, and then the

280

volume change of the oil phase and the water phase was observed and recorded. After

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demulsification for 24 hours, the photographs of the five emulsions were shown in

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Figure 5.

283 284 285

Figure 5. The picture of demulsification after 24h with 11.63 wt% citric acid solution (20mL) as a demulsifier. The composition of emulsions was shown in Table 5.

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Figure 6. Effect of concentration of demulsifier (citric acid) on separation efficiency of crude oil.

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It can be seen from Figure 5 that citric acid has a very good demulsification

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performance for crude oil emulsions. Figure 6 shows the effect of different

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concentrations of the citric acid solution on demulsification efficiency. When the

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concentration of citric acid increased from 11.63wt% to 17.39wt%, the

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demulsification rate has been significantly improved. The crude oil and water were

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almost completely separated in the Emulsion (a). Obviously, the higher the crude oil

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content of the emulsion requires a higher concentration of citric acid to achieve

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complete demulsification. Crude oil content of 70wt% in Emulsion (c),

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demulsification efficiency up to 91.19%. This provides a viable option for the

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separation of crude oil at the end of pipeline transportation.

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3.4 Analysis of DMCHA recovery mechanism

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DMCHA plays a vital role in emulsifying and demulsifying processes. It is

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highly necessary to recycle and reuse of the DMCHA. DMCHA has a pka value of

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10.5033, which is capable of switching between ionic and molecular states at different

303

pH conditions as shown in Figure 7.

304 305 306

Figure 7. Molecule state distribution of DMCHA in aqueous solution with different pH value.

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It can be clearly seen from Figure 7 that when pH < 9, DMCHA is a mainly

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hydrophilic cationic state, pH > 12 is a mainly hydrophobic molecular state, 9 < pH

309

9,

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the DMCHA gradually separated from the solution at this time the solution appears to

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be turbid. When pH=10.50, there was divided into two layers, the upper layer was

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DMCHA and the recovery rate of DMCHA was about 50%. When pH=11.94, the

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recovery rate of DMCHA was close to 100%. For ease of observation, the upper layer 19


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of DMCHA was stained with the Nile red in Figure 8. The 1H NMR spectrum of the

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upper layer was found to be the same as that of DMCHA, indicating that DMCHA can

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be recovered from the solution (ESI, Figure S1). In short, it is possible to recover the

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DMCHA by adjusting the pH of the lower layer aqueous solution with the NaOH

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solution.

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3.5 Recycling of DMCHA from high oil content emulsions

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The lower layer of the crude oil emulsion is an ammonium citrate aqueous

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solution in Figure 5. DMCHA can be regenerated by adding NaOH to the aqueous

335

solution. Figure 9 shows the recovery of DMCHA from emulsion (a). When NaOH

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was added, the solution is immediately divided into two layers, the upper layer was

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the recovered DMCHA, and the lower layer was the ammonium citrate solution and

338

the excess NaOH.

339 340

Figure 9. The photograph of DMCHA recovered from emulsion (a).

341

The pH of the lower aqueous solution in Figure 9 was 12.26, and almost all of

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the DMCHA can be recovered in this strongly alkaline environment (Figure 7). 20


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Finally, the recovery rate of DMCHA was obtained by calculating the ratio of the

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volume of recovered DMCHA to the total volume of DMCHA added (Figure 10). The

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highest recovery rate of DMCHA in the five emulsions was 85.71% for Emulsion (a).

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With the increase of crude oil content, the recovery rate of DMCHA gradually

347

decreased (Figure 10). This is associated with the demulsification rate of the emulsion.

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The higher the demulsification rate, the higher the recovery rate of DMCHA.

349

However, the lowest recovery rate of DMCHA in the five emulsions was more than

350

70%.

351 352 353

Figure 10. The recovery of DMCHA in five emulsions after demulsification.

4. CONCLUSION

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DMCHA can emulsify crude oil to form crude oil-in-water emulsion and the

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stability of the emulsion is closely related to the concentration of DMCHA which is

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critical to the stability of crude oil emulsions. It is relatively easy to achieve efficient 21


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separation of crude oil and water in emulsions with low oil content using CO2 as a

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demulsifier. However, the demulsification efficiency of CO2 decreased significantly

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when the oil content was 58.33%. The advantages of organic acids as demulsifiers are

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obvious, especially citric acid. When the oil content of 70%, the use of concentration

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of 17.39% citric acid solution as a demulsifier can make the demulsification

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efficiency of more than 90%. As an important role in the formation of a crude oil

363

emulsion, DMCHA recovery and reuse are very necessary. In this article, the

364

regeneration of DMCHA can be achieved by adding NaOH to the lower aqueous

365

solution after demulsification. The recovery rate of DMCHA can be greater than 80%

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when the oil content of 70% in the crude oil emulsion after breaking. Through the

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emulsification and efficient demulsification of crude oil, we provide a viable solution

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for crude oil pipeline transportation. At the beginning of transport, the crude

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oil-in-water emulsions can be formed by DMCHA. Organic acids, especially citric

370

acid, have excellent demulsification efficiency for high crude oil content emulsions at

371

the end of pipeline transportation. Finally, the DMCHA can be recovered by NaOH.

372

ASSOCIATED CONTENT

373

Supporting Information

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The following contents can be found in the supporting information: (a) The

375

1

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crude oil emulsions without demulsifier.

377

AUTHOR INFORMATION

378

Corresponding author

HNMR spectrum analysis of DMCHA. (b) The stability and viscosity reduction of

22


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*Email: [email protected]; Fax: +86-28-83037330; Tel: +86-28-83037330.

380

ORCID

381

Hongsheng Lu: 0000-0003-3201-0855

382

Notes

383

The authors declare no competing financial interest.

384

ACKNOWLEDGEMENTS

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This work was done with the support of the National Natural Science Foundation of

386

China (NSFC, NO. 21403173), and we would like to express our heartfelt thanks.

387

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Effect of Demulsification for Crude Oil-in-Water Emulsion: Comparing

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