The present work has main target to study the effect of additives molecular weight and composition on the flow characteristics of wax crude oil at low temperature below pour point temperature. In this respect, maleic anhydride ester-co-vinyl acetate copolymers with varied monomers feed ratios and different alkyl ester lengths, namely, dodecyl, stearyl, and behenyl alkyl chains, were prepared. These polymeric materials were characterized by FTIR, 1HNMR, and GPC. The performance of these additives as pour point depressants and flow improver for Egyptian waxy crude oil was evaluated through measurements of pour point and rheological parameters (viscosity and yield stress). It was observed that stearyl maleate-vinyl acetate copolymer with 1 : 2 feed ratio shows the best efficiency as pour point depressant even at low concentration while octadecyl maleate-vinyl acetate copolymers with 2 : 1 feed ratio are effective as flow improver.
The paraffin deposition formed during production and transportation of light crude oil and natural gases and condensates is one of the main problems that affect the oil productivity especially at low temperature [
Copolymerization is of great interest in synthesizing polymers with desired physical and chemical properties through controlling monomers ratio, their concentrations, and polymerization procedure. PPD is synthesized with two essential parts: oil soluble paraffinic chain and a polar moiety. The usefulness of paraffin chain is to cause nucleation and cocrystallization while the polar part controls the crystal growth and limits the size of wax crystals [
Vinyl acetate, maleic anhydride, dodecyl alcohol (DA), stearyl alcohol (SA), behenyl alcohol (BA), benzoyl peroxide (BP), and P-toluene sulfonic acid monohydrate (PTSA) were purchased as analytical grade from Aldrich Chemicals Co., Germany. Benzene, dimethylformamide (DMF), and xylene were delivered from Adweic Chemicals Co., Egypt.
Egyptian waxy crude oil (Norpetco, Egypt) was delivered without treatment from Fardous field. The physicochemical characteristics and composition of Fardous mixed crude oils are listed in Table
The physicochemical properties of Norpetco crude oil.
Test | Method | Result |
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API gravity at 60 F | ASTM D-1298 | 41.1 |
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Specific gravity at 60/60 F | ASTM D-1298 | 0.820 |
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Wax content (wt%) | UOP 46/64 | 8.4 |
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Asphaltene content (wt%) | IP 143/84 | 3 |
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Water content (vol%) | IP 74/70 | 0.23 |
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Kinematic viscosity (cSt) at | ||
50°C | ASTM D-445 | 7 |
60°C | 4.3 | |
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Pour point °C | QPC procedure | 30 |
Maleic anhydride-vinyl acetate copolymer was prepared by copolymerizing vinyl acetate (VA) and maleic anhydride (MA) in different molar feed ratios of VA : MA, namely, 1 : 1, 1 : 2, and 2 : 1, in reaction flask using dry benzene as solvent and 1% (wt/wt) benzoyl peroxide (BP) as initiator. The reaction proceeds for 6 hours at 60–70°C with constant stirring under nitrogen atmosphere. After completion of polymerization, benzene was distilled off under vacuum. The copolymer was purified using benzene as solvent and petroleum ether as nonsolvent. The purified copolymer was dried at 60°C under vacuum.
The reaction mixture containing 0.01 mol VA-MA copolymer solution in DMF, with one of the previously described molar ratios, and 0.02 mol of each alcohol (dodecyl, stearyl, or behenyl) was refluxed separately in presence of 0.1 (wt%) PTSA as a catalyst. The reaction was carried out at the refluxing temperature until theoretical amount of water was collected azeotropically in the Dean Stark trap that contains small amount of toluene to determine the amount of produced water. The resulting esters were washed out with water to remove the catalyst and any unreacted materials.
The prepared esters, dodecyl maleate-vinyl acetate (VADM) copolymers, stearyl maleate-vinyl acetate (VASM) copolymers, and behenyl maleate-vinyl acetate (VABM) copolymers were purified and used as additives for crude oil.
The carbon distribution number of separated wax was determined using GC-Mass spectrometer.
1H NMR analysis were recorded on a Varian Gemini 2000 at 300 MHz and Fourier transform infrared (FTIR, Perkin-Bhaskar-Elmer Co., USA) spectrometers were used to determine the chemical structures of copolymers.
The molecular weight data of the prepared copolymers, such as the weight average molecular weights (
Pour points measurements were determined using modified ASTM D-97 method without reheating to 45°C using different concentrations of the prepared additives, namely, 1000, 2000, 3000, 4000, and 5000 ppm. The effect of additive on the wax crystal morphologies was observed using an Olympus BX51 polarized-light microscope with a Linkam THMS 600 hot stage. The images were taken after transferring a small quantity of treated or untreated crude oil to glass slide inside a copper stage which has central window.
Viscosity and flow curves (rheogram) were measured using Brookfield viscometer equipped with thermostated cooling system for temperature adjustment [
The yield stress measurements were determined from the relationship between shear stress and shear rate valued measured using Brookfield viscometer. Oil samples with or without additives were heated to 80°C, with the temperature maintained for 5 min to adopt their thermal history. The temperature decreased by 10°C/min cooling rate to the experimental test temperature. The test temperature was determined from pour point measurements for both the crude oils and treated crude oils. The viscosity values were measured, after annealing the sample at the measurement temperature without shear for 5 min, by applying stress and incrementally increased every 10 s (100 stress increments per decade). The yield stress is defined as the stress below which no flow occurs.
Polymers including vinyl acetate (VA) and alkyl acrylate are used mainly as additives to improve the flow ability of waxy crude oil at low temperature. It is presumed that effective additives should match crude wax in structure, composition, and content. In this respect, we select vinyl acetate and maleic anhydride to prepare polymeric additives for Egyptian waxy crude oil. The main idea depends on variation of copolymer compositions by changing the monomer feed composition of VA : MA as 1 : 1, 1 : 2, and 2 : 1 followed by esterification with different types of n-alkanol such as dodecyl, stearyl, or behenyl alcohol. The chemical structure was designed on the previous results showing that VA copolymers containing from 20 to 40% w/w of vinyl acetate performed well when applied in some petroleum samples [
1HNMR spectra of maleic anhydride-vinyl acetate copolymer with different monomer feed composition are presented in Figure
1HNMR spectra of VA : MA having monomer feed compositions (a) (1 : 2) and (b) (1 : 1).
Molecular weights of the prepared polymers with the different feed ratios were determined using gel permeation chromatography (GPC) using THF as eluent and the results are summarized in Table
The average molecular weight of the prepared copolymer at different mole ratios.
Polymer composition | Molecular weight (g/mol) | ||
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PD | |
VA-MA (1 : 1) | 1.13 |
3.5 |
3.22 |
VA-MA (1 : 2) | 1.7 |
4.7 |
3.61 |
VA-MA (2 : 1) | 9.65 |
6.03 |
1.6 |
Through this paper, three VA-MA copolymers were reacted with alkanol having different alkyl length using esterification reaction as illustrated in Scheme
Synthesis of VA-MA ester copolymers.
FTIR spectra of (a) VA-MA and (b) VADM copolymers.
In addition, the chemical structure of esterified VA-MA copolymers was confirmed by 1HNMR. Figure
1HNMR spectra of (a) VASM, (b) VADM, and (c) VABM copolymers.
It is common that all waxy crude oils eventually become nonfluid on chilling [
Chromatogram of paraffins extracted from Norpetco crude oil.
From data represented in Figure
The evaluation of the prepared copolymer esters as PPD was studied through preparation of crude oil samples treated with different concentrations from each additive, namely, 1000, 2000, 3000, 4000, and 5000 ppm. The results of pour point measurement are given in Tables
The pour point of untreated crude oil and treated crude oil with VADM.
Copolymer |
Pour point temperature (°C) | |||||
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Nil | 1000 | 2000 | 3000 | 4000 | 5000 | |
VADM (1 : 1) | 30 | 27 | 24 | 21 | 21 | 18 |
VADM (1 : 2) | 30 | 30 | 27 | 24 | 24 | 21 |
VADM (2 : 1) | 30 | 30 | 27 | 24 | 24 | 21 |
The pour point of untreated crude oil and treated crude oil with VASM.
Copolymer |
Pour point temperature (°C) | |||||
---|---|---|---|---|---|---|
Nil | 1000 | 2000 | 3000 | 4000 | 5000 | |
VASM (1 : 1) | 30 | 27 | 27 | 27 | 27 | 24 |
VASM (1 : 2) | 30 | 15 | 15 | 12 | 12 | 12 |
VASM (2 : 1) | 30 | 15 | 15 | 15 | 15 | 12 |
The pour point of untreated crude oil and treated crude oil with VABM.
Copolymer |
Pour point temperature (°C) | |||||
---|---|---|---|---|---|---|
Nil | 1000 | 2000 | 3000 | 4000 | 5000 | |
VABM (1 : 1) | 30 | 30 | 30 | 27 | 27 | 27 |
VABM (1 : 2) | 30 | 30 | 30 | 27 | 27 | 24 |
VABM (2 : 1) | 30 | 30 | 27 | 27 | 27 | 24 |
The microscopic images of untreated and treated Norpetco crude oil with 5000 ppm of VASM (1 : 2) at the pour point temperature are shown in Figures
Polarized microscopic image morphologies of (a) untreated and (b) treated Norpetco crude oils with 5000 ppm of VASM (1 : 2) at their pour point temperatures.
The rheology is used to evaluate the crude oil flow ability in the absence and presence of the prepared polymeric additives. The rheological parameters for untreated and treated crude oil with 5000 ppm of VASM were determined at different temperatures, namely, 12°C, 15°C, and 21°C. Figures
Yield value of untreated and treated crude oil with 5000 ppm concentration of the additives of different composition.
Oil sample |
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Yield value (Dyne/cm2) |
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Untreated | 12 | 4.89 |
15 | 3.93 | |
21 | 3.52 | |
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VASM (1 : 1) | 12 | 1.57 |
15 | 1.36 | |
21 | 1.34 | |
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VASM (1 : 2) | 12 | 1.83 |
15 | 1.56 | |
21 | 1.19 | |
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VASM (2 : 1) | 12 | 1.99 |
15 | 1.50 | |
21 | 1.30 |
Rheogram of untreated and treated crude oil with 5000 ppm of different mol ratios at 12°C.
Rheogram of untreated and treated crude oil with 5000 ppm of different mol ratios at 15°C.
Rheogram of untreated and treated crude oil with 5000 ppm of different mol ratios at 21°C.
Moreover, the long-chain alkyl grafts in VASM (2 : 1) have the same effect on the long-chain paraffins in the distribution of wax and paraffin in the crude oil. The possible reason for lowering the yield stress and pour point temperature is attributed to match of side alkyl chain length with the paraffin length of the tested crude oil. Moreover, the molecular weight of alkyl substituent has strong effect on the solubility of the additives in the crude oil. It is found that the VA-MA (2 : 1) ester copolymers have low polydisperse molecular weights and have moderate high molecular weight (Table
The apparent viscosities (mPaS) of the untreated and treated crude oils with VASM (2 : 1) were determined at different temperatures to evaluate the effect of the polymer on the viscosities of Norpetco crude (representative samples are shown in Figure
Effect of VASM (2 : 1) on the apparent viscosity on Norpetco crude oil at 15°C.
The values of the plastic viscosity (mPaS) and yield shear stress values (Pa) decreased by the addition of VASM (2 : 1) additives even at high concentrations (10,000 ppm). Figure
The new hydrophobically modified VASM copolymers achieve efficient PPD for the Egyptian waxy crude oil. The effective concentration of PPD to inhibit the wax deposition, to decrease the pour point temperature, and to improve the rheological characteristics of crude oils was found to be 100–10000 ppm. The composition of VASM greatly affects the performance of the additive with the copolymer VASM with mole ratio 1 : 2 which is the most efficient additive in pour point depression, while the copolymer VASM with mole ratio 2 : 1 was the best additive in improving the crude oil yield shear stress and improving the flow properties of crude oil.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through Research Group no. RGP-VPP-235.