Introduction
Crude oil is a complex mixture that consists of paraffins, aromatic hydrocarbons, resins, and asphaltenes. At high temperatures (70°C-150°C), crude oils behave as fluid with low viscosity. However, as the temperature declines, the solubility of long chain paraffins and asphaltenes decreases sharply. The long chain paraffins and asphaltenes will precipitate and deposit at the pipe walls, reducing oil flow and even blocking the pipeline [1]. The problem becomes more serious for offshore deep-sea wells, where the temperature at the sea bed can be 4°C or lower [2,3]. To solve this problem, heating and mechanical pigging are always applied in the petroleum industry [4]. However, these methods consume a lot of energy and are uneconomic. Using the chemical additives to alleviate or solve the wax deposition problem becomes a better choice and an important method that has absorbed intensive research interests recently.
Several kinds of comb-type polymer additives, such as poly(ethylene-butene) (PEB) and poly(maleic anhydride-co-α-olefin) (MAC), have been reported to be effective in improving the cold flow ability of waxy oils [5-8]. In this work, new comb-type copolymers of poly(maleic acid alkylamide-co-α-olefin-co-styrene) (MASC) with different ratios of maleic anhydride (MA) to styrene were designed and synthesized. The introduction of styrene will lead to a better interaction with asphaltenes that contain aromatic substance. 1H NMR and FTIR spectra were used to characterize the chemical structure of the MASC copolymers. The effect of copolymers on the cold flow ability of model waxy oil and crude oil were studied by rheological method.
Experiment
Chemicals and sample preparation
Decane (anhydrous,>99%), hexatriacontane (C36,>98%), maleic anhydride (99%), α-octadecene (95%), benzoyl peroxide (99%), o-xylene (98%), and octadecylamine (97%) were purchased from Alfa-Easer company and used as obtained. Styrene (90%), purchased from Shanghai Lingfeng Medicine Company, was washed by 5% NaOH aqueous solution for 3 times, then washed by excess distilled water until the pH reached 7, and finally dried by anhydrous Na2SO4.
The model waxy oil was prepared by dissolving hexatriacontane (C36) in decane with concentrations of 4 wt-%. The number in MASC0.5, MASC0.75, and MASC1.0 means that the molar ratio of styrene to maleic anhydride is 0.5, 0.75, and 1.0, respectively.
Synthesis of MASC
Poly(maleic anhydride-co-α-olefin-co-styrene) was synthesized by α-octadecene, maleic anhydride, and styrene in o-xylene by free-radical polymerization, using benzoyl peroxide as initiator (Scheme 1(a)). By amidation with excess long chain n-amines, the comb-type terpolymers (MASC) were obtained (Scheme 1(b)).
The terpolymers were precipitated in methanol, washed by distilled water after filtration, and then dried by freeze drying. The chemical structure of terpolymer was confirmed by 1H NMR and FTIR spectroscopy. The molecular weight of terpolymers with different ratios of maleic anhydride (MA) to styrene was determined by GPC with THF as solvent (Table 1).
Tab.1 The molecular weight of MASCs measured by GPC |
| sample | molecular weights /(kg·mol-1) |
|---|---|
| MASC0.5 | 16.9 |
| MASC0.75 | 24.4 |
| MASC1.0 | 21.9 |
Instruments
1H NMR spectra were recorded on a Bruker DRX500 spectrometer for samples dissolved in deuterated acetone. FTIR spectra were recorded on Magna-IR550 (Nicolet, America). Wax crystal morphologies were observed using a Nikon COOLPIX P5100 inverted microscope (Shanghai Changfang 59XA). The molecular weights and molecular weight distributions were determined by GPC (DIONEX Ultimate 3000) using poly(styrene) samples as standards.
The yield stress measurements were performed on a Physica MCR101 controlled stress rheometer with a 25 mm parallel plate. Oil samples were heated to 70°C, kept at this temperature for 5 min to erase their thermal history, and then cooled to the experimental test temperature at a rate of ca. 10°C/min. Both the model waxy oil and crude oil tests were made at 0°C. After allowing the samples to anneal at constant temperature with no stress for 5 min, stress was applied and incrementally increased every 10 s (100 stress increments per decade), and the viscosity was measured. The yield stress (τy) is defined as the stress below which no flow occurs. An operational definition of the yield stress is adopted as the stress at the transition between the creep and liquid-like viscosity regimes, where the yield stress can be identified as the stress for which the derivative is a maximum [6]. The initial applied stress was chosen well below the stress at which creep began.
Results and discussion
Characterization of MASC
Figure 1 shows the 1H NMR spectrum of MASC sample after purification. The resonance peaks of protons in phenyl group (-C6H5) of styrene unit, –CH of maleic anhydride unit, and –CH3 of octadecene unit can be found at 7.2, 3.5, and 0.89 ppm, respectively. In Fig. 2, the stretching of the carbonyl group (C=O) from maleic anhydride unit at 1720 cm-1 can be found in the FTIR spectrum. The characteristic absorptions of the styrene appear at 1613 and 705 cm-1.
The solubilities of the new copolymer MASC in various solvents are compared with those of the possible homopolymer and dipolymers: polystyrene, poly(maleic anhydride-styrene), and poly(maleic anhydride-co-olefin). MASC does not dissolve in cyclohexane, while poly(maleic anhydride-styrene) and polystyrene do. MASC also shows different solubility from poly(maleic anhydride-co-olefin) in ethyl acetate.
The new copolymer was also confirmed by comparing its melting point with other copolymers using the melting point apparatus. The melting point of the new copolymer MASC is between 125°C - 130°C, which is higher than poly(maleic anhydride-co-olefin) (85°C - 90°C) but is much lower than poly(maleic anhydride-styrene) (200°C - 210°C). Therefore, we consider that the obtained copolymer MASC should be a random terpolymer of maleic anhydride, α-olefin, and styrene.
Effect of MASCs on the model oil and crude oil
As shown in Fig. 3, the yield stress of model waxy oil and crude oil decreased significantly upon the addition of 0.1 wt-% MASC0.5. Upon the addition of 0.1% MASC0.5, the yield stress of crude oil with asphaltene reduced from 1.2 to 0.2 Pa.
The relative yield stress is defined as the ratio of the yield stress with and without MASC. With the increase of the concentration of MASCs from 0 to 100 ppm, the yield stress of the model waxy oil decreased significantly. However, further increase of the MASC concentration to 200 ppm results in the enhancement of the yield stress of model waxy oil due to the “bridge effect” of extra polymers [3]. Obviously, 100 ppm (0.10 wt-%) of MASCs is the best concentration for the reduction of yield stress of model waxy oil (Fig. 4).
Comparing the effect of MASC0.5, MASC0.75, and MASC1.0, the copolymer with less styrene shows a better result to reduce the yield stress of model waxy oil, and MASCs containing more styrene units show a better result of reducing the yield stress of crude oil with asphaltenes mainly consisted of aromatic components (Fig. 5) due to enhanced interactions between styrene and asphaltenes.
Effect of MASCs on the morphology of paraffin crystal
With the addition of 0.10% MASCs, the crystal of model waxy oil became smaller, and the shapes changed significantly from large plate-like crystals to small pieces (Fig. 6). The reduced size of waxy crystals may contribute to the reduction of yield stresses and the improvement of cold flow ability.
Conclusions
Novel comb-type poly(maleic acid alkylamide-co-α-olefin-co-styrene)s (MASCs) were synthesized successfully by free-radical copolymerization. They significantly reduce the yield stress and improve the cold flow ability of model waxy oil and crude oil at low temperature. For model waxy oil without asphaltenes, the MASCs with less styrene units show a better result to reduce the yield stress and the crystal size as observed by optical microscope. For crude oil with asphaltenes, MASCs with more styrene units work better due to the enhanced interactions with aromatic component in asphaltenes.