Article pubs.acs.org/EF
Modified Maleic Anhydride Co-polymers as Pour-Point Depressants and Their Effects on Waxy Crude Oil Rheology Yumin Wu,*,† Guangdi Ni,† Fei Yang,‡ Chuanxian Li,‡ and Guoliang Dong† †
College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China College of Storage and Transportation and Architectural Engineering, China University of Petroleum, Qingdao 266555, People’s Republic of China
‡
ABSTRACT: To investigate the influence of structure variables of polymeric additives on the pour-point depression and rheological behavior of waxy crude oil, maleic anhydride co-polymer and its derivatives with different polar and/or aromatic pendant chains were designed and synthesized. All prepared additives were characterized by Fourier transform infrared (FTIR) spectroscopy and gel permeation chromatography (GPC). The pour-point and rheological properties of Changqing (CQ) crude oil with a low asphaltene content before and after additive beneficiation were studied in detail. Differential scanning calorimetry (DSC) and polarizing light microscopy were employed to gain insight on the interactions between such additives and wax crystals. The results are encouraging and showed that all four polymeric additives exhibited good efficiency as flow improvers in CQ crude oil. The reduction of pour-point and rheological parameters after additive addition largely related to the polymer structure. The polymer containing aromatic units showed the best performance, which could depress the pour point by 19 °C and decrease the yield stress as well as viscosity to a large extent.
1. INTRODUCTION Crude oils containing a large amount of wax often exhibit a high pour point, viscosity, and yield stress and follow nonNewtonian flow behavior below the cloud point.1,2 As the temperature of crude oil is lowered, wax will precipitate to form an interlocking network and increase the viscosity of the oil, thus impeding flow. Crystallized wax that deposits from crude oil at sufficiently low temperatures possesses severe problems in oil production, storage, and transportation.3 If its flow in pipelines is halted temporarily or over a period of time, the sufficiently viscous waxy crude oil will be very difficult to restart flow, which requires overcoming the yield stress to initiate. One well-recognized and economically viable solution of this problem is the employment of polymeric additives to improve the flow behavior of crude oil at low temperatures. Polymeric additives act as wax crystal modifiers, pour-point depressants (PPDs), or flow improvers and are capable of building into wax crystals and modifying the crystal morphology and growth characteristics to reduce the tendency to interconnect into three-dimensional networks.4 A combination of these effects depresses the pour point, viscosity, and yield stress to a large extent, thus facilitating the flow of waxy crude oil.5 Nevertheless, such additives are very selective, commonly exhibiting the problem of having non-universal application to every waxy crude oil because of different oil compositions and their contents. In general, a particular additive can be effective only in oils sharing certain physical characteristics and prove mostly ineffective in other oils; thus, experimental tests are needed to evaluate suitable improvers for each oil type. Various types of additives are being investigated or have been used as flow improvers for crude oils, model oils, and/or refined middle-distillate products. The most widely used types among them are linear polymers and comb-shaped polymers. The former include crystallizable domains in the polymeric © 2012 American Chemical Society
backbone, such as ethylene−vinyl acetate co-polymers (EVA)6−8 and ethylene−butene co-polymers (PE-PEB),9,10 while the latter generally have long alkyl chains (crystallizable appendages) appended to the backbone of polymers, such as alkyl acrylate homo-polymers,11 alkyl esters of styrene−maleic anhydride co-polymers,3 alkyl fumarate−vinyl acetate copolymers,12 unsaturated carboxylic ether−maleic anhydride derivatives,13,14 maleic acid alkylamide−α-olefin−styrene terpolymers,15 maleic anhydride−alkyl acrylate terpolymers,16 etc. Such polymers share in common the interactions with the wax crystallization and aggregation processes to moderate the flow properties of waxy crude oils. Other constituents in crude oil, i.e., asphaltenes and resins, should also be considered as important factors influencing the flow behavior.17−19 Although the interplay between additives and wax has been investigated by many academic laboratories, the action mechanisms are still incompletely understood,6,20 which require further in-depth research by means of modern instrumental analysis. It has been observed that the efficacy of the polymeric additive as a flow improver depends upon many factors, such as the composition and quantity of wax and the type of polymer backbone and pendant chains.21,22 Previous studies have revealed that chemical structures (backbone and pendant chains) of polymers play a distinct role in enhancing the flowability of crude.23 The higher similarity between the polymer structure and wax constituents results in the better performance, which can be controlled through proper polymer design. Soni et al.24−26 systematically synthesized a series of additives based on maleic anhydride co-polymers as rheological modifiers for paraffinic crude oils. These studies directly Received: September 25, 2011 Revised: January 2, 2012 Published: January 3, 2012 995
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associated the performance with the structure of PPD and established a correlation between the pour-point depressing power and the polymer structure, which opened a new avenue to develop universal flow improvers. Maleic anhydride co-polymers can react readily with other compounds that contain various functional groups to enhance their performance in a good number of applications around the world. In the present work, co-polymers of octadecyl acrylate and maleic anhydride were synthesized and selected and then different polar and/or aromatic units were incorporated into anhydride parts of the polymer backbone. The efficiency of modified polymers with different functional appendages was evaluated through pour-point tests and rheological measurements. The objective of the present work is to study the influence of varied chemical structures of resultant polymers on the flow behavior of tested waxy crude oil and gain a better understanding of interaction mechanisms between additives and wax crystals, thus providing a direction to develop more effective and economical improvers.
2. EXPERIMENTAL SECTION 2.1. Materials. Acrylic acid, maleic anhydride, and p-toluene sulfonic acid were purchased from TianJin Bodi Chemical Holding Co., Ltd. Octadecanol, dodecanol, dodecylamine, benzyl alcohol, dibenzoyl peroxide, and hydroquinol were obtained from Aladdin. The above substances were used without further purification. Changqing (CQ) waxy crude oil was selected to evaluate the efficiency of polymeric additives as flow improvers. The physicochemical characteristics of CQ crude oil are listed in Table 1.
Figure 1. Chemical structures of anhydride polymer and its derivatives: (a) POM, (b) POMO, (c) POMN, and (d) POMB.
Table 1. Physical Characteristics of Tested Crude Oil properties
CQ
density at 20 °C (g/cm3) WAT (°C) pour point (°C) wax content (wt %) resin content (wt %) asphaltene content (wt %) IBP (°C)
0.8127 42 30 20.78 1.27 0.13 51.5
extracted for 8 h using ether as an extracting agent to remove unreacted alcohol or amine. The purified polymer derivative was finally dried under vacuum at 60 °C for 24 h, and its yield (degree of esterification or amidation) was calculated by adopting the weighing method. It means that the actually reacted amount of alcohol or amine divided by the theoretical amount of it after complete esterification or amidation is the yield. The structural formulas of anhydride polymer and its derivatives are plotted in Figure 1, where the derivatives are designated as POMO (Figure 1b), POMN (Figure 1c), and POMB (Figure 1d). 2.3. Polymer Characterization. The weight-average molecular weights (Mw) of resultant polymers were determined using gel permeation chromatography (GPC), Waters 1515. Tetrahydrofuran (THF), at a flow rate of 1 mL/min at 30 °C, was used as the mobile phase. Polystyrene was used as the standard. Molecular weights and yields of these polymers are given in Table 2. The chemical structures
2.2. Preparation of Polymeric Additives. 2.2.1. Esterification. Octadecyl acrylate was prepared by directly reacting 1.2 mol of acrylic acid with 1 mol of octadecanol in the presence of p-toluene sulfonic acid as a catalyst and hydroquinol as a polymerization inhibitor. The melt esterification was carried out using a Dean−Stark apparatus until the calculated amount of water was separated. After completion of the reaction, the catalyst, inhibitor, and unreacted materials were removed by washing first with 5% sodium hydroxide solution and then with excess distilled water, and the pure ester was obtained. 2.2.2. Co-polymerization. Octadecyl acrylate−maleic anhydride copolymer (POM; Figure 1a) was synthesized by free-radical polymerization of octadecyl acrylate with maleic anhydride under a nitrogen atmosphere with constant stirring. The polymerization was conducted in toluene solution at 80 °C for 6 h using dibenzoyl peroxide as an initiator (1 wt %). The co-polymer was precipitated in an excess volume of methanol, filtered, and dried under vacuum at 60 °C. 2.2.3. Synthesis of Anhydride Co-polymer Derivatives. The anhydride co-polymer prepared above was allowed to reflux individually with dodecanol, dodecylamine, and benzyl alcohol in twice the moles of anhydride groups for 12 h in the presence of ptoluene sulfonic acid as a catalyst and toluene as a solvent. The crude product was neutralized by 5% sodium hydroxide solution and then washed repeatedly with distilled water. The organic layer was separated and poured with an excess of methanol for several times. After filtration, the obtained filter cake was shifted into Soxhlet extractor and
Table 2. Molecular Weights and Yields of Polymeric Additives additive
Mw (g/mol)
yield (%)
POM POMO POMN POMB
35518 43640 43080 39857
90 81 84
of polymeric additives were confirmed by means of Fourier transform infrared (FTIR) spectroscopy using a BRUKER TENSOR-27 infrared spectrometer. The FTIR spectra of representative additives are shown in Figure 2. It is clear that the spectrum of POM shows characteristic absorption bands at 2920 and 2852 cm−1 [assigned to νs(CH3) and νs(CH2) of the alkyl group of ester], 1851 and 1780 cm−1 (CO stretching of the anhydride group), 1730 cm−1 (CO stretching in ester), and 1175 cm−1 (C−O−C stretching). When the spectrum of POMN is taken as an example, the disappearance of the CO band at 996
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rate and viscosity versus shear rate curves were measured at constant temperatures below and above the pour point of the crude. Yield stress values were also determined using the same system. In this measurement, the sample was cooled to the test temperature from 60 °C at a cooling rate of 0.5 °C/min and held at this temperature for 30 min before a stress ramp was initiated. The stress was continuously increased from 0 Pa in 5 Pa/min increments with the data collected at 50 points/min. The rheometer was programmed to stop the test when a high rotation speed was reached, and then the sample was reheated to 60 °C for 20 min and retested twice. Figure 6 shows the principle scheme for yield stress determination with a log γ−log τ graphing method. When applied stress is below the yield stress, log γ varies slightly with log τ, which can be seen from line 1. While on line 2, the variation of log γ with log τ becomes dramatic, indicating the start of oil flow. The corresponding value of the intersection point of the two lines is then the yield stress under the test temperature.
3. RESULTS AND DISCUSSION The paraffinic contents of CQ crude oil were analyzed using gas chromatography to determine the average molecular-weight distribution of wax. The data in Figure 3 indicate that the
Figure 2. FTIR spectra of representative polymeric additives. 1851 and 1780 cm−1 confirms that all of the anhydride groups in the polymer were reacted. The results indicate that POM and its derivatives were successfully synthesized in accordance with the proposed structures. 2.4. Pour-Point Determination. To remove the previous shear and thermal history, oil samples were conditioned at 60 °C for 2 h and then left to cool statically. The pour points of samples in the absence and presence of polymeric additives were determined according to the Chinese Standard Petroleum Test Method SY/T 0541-2009. Different concentrations of the prepared additives, namely, 50, 100, 200, 300, and 500 ppm, were added into the oil, and pour-point depression results are presented in Table 3.
Table 3. Pour Points of CQ Crude Oil Treated with Polymeric Additives and Commercial EVA pour-point depression (°C) additive concentration (ppm)
POM
POMO
POMN
POMB
EVA
50 100 200 300 500
3 5 6 7 9
5 8 8 10 12
5 9 10 13 13
9 12 16 18 19
8 12 9 9 8
Figure 3. Carbon number distribution of n-paraffin in CQ crude oil.
carbon number distribution of the total wax is broad and the average carbon number of n-paraffin is 18.25. It can be found that the waxy crude oil contains 55.66 wt % content of nparaffins, whose presence permits strong gelled networks. All of these factors are known to contribute to the high pour point of the oil. The prepared polymeric additives served as wax crystal modifiers are specially designed substances having hydrophobic moieties (long alkyl chains) and hydrophilic moieties (polar groups). Yang et al.27 have concluded that the pendant alkyl chain made from octadecanol is more effective in pour-point depression of oils from the CQ region. Therefore, in our previous work, a series of octadecyl acrylate−maleic anhydride co-polymers with different monomer molar ratios and molecular weights were synthesized and a certain molar ratio (3:1) with the best efficiency as the crystal modifier was selected for use. To further improve the performance and investigate the influence of different pendant chains on pourpoint depression, anhydride polymer derivatives with different polar and/or aromatic side chains were prepared. 3.1. Pour Point. The effects of the synthesized polymers as well as commercial EVA on the pour point of CQ oil are reported in Table 3. It can be observed that every polymeric additive is effective in reducing the pour point and the effectiveness increases with increasing its concentration added
2.5. Differential Scanning Calorimetry (DSC) Analysis. Wax precipitated significantly influences the flow behavior of waxy crude oil. To investigate the interactions of polymeric additives with wax crystals in crude oil, thermal analysis via DSC was conducted to determine the wax appearance temperature (WAT) and enthalpy of the oil sample. DSC measurements were performed using DSC821e (Mettler-Toledo, Greifensee, Switzerland) in the temperature range from 60 to −20 °C at a cooling rate of 10 °C/min. When POMB is taken as an example, the thermogram of virgin and POMB-treated crude oil is plotted in Figure 4. 2.6. Optical Microscopy. Microscopic observation of wax crystal morphology was carried out using a cross-polarized light microscope Olympus BX5 (Olympus, Japan) fitted with an automatic camera in transmission mode. The oil samples untreated and treated with polymeric additives were initially heated to 60 °C for 30 min to dissolve all wax and additives, and then one droplet was taken on a glass slide covered by a coverslip. The samples viewed with the microscope were cooled at a rate of 0.5 °C/min in the temperature range from 60 to 0 °C with the images recorded manually. 2.7. Rheological Measurements. All rheological measurements were performed on a HAAKE RheoStress 75 rheometer equipped with a coaxial cylinder system and thermostatted cooling system for temperature control. The preconditioned samples before and after additive beneficiation were heated to 60 °C for 30 min and then loaded on the rheometer to start the tests. Shear stress versus shear 997
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difference in the WAT (42 °C) of virgin and additive treated oil, but the second peak has shifted to a lower temperature after POMB beneficiation. It indicates that the addition of POMB may change the process of wax crystallization and extend wax precipitation time, thereby improving the low-temperature flowability of treated oil. The concentration of precipitated wax before and after additive addition is 20.78 and 21.12%, respectively, which was measured according to the report by Yi and Zhang.29 The results exhibit that there is little influence of modifier POMB on the total amount of wax crystals precipitated out from the oil. It can be demonstrated that the investigated additive has no effect on WAT and the amount of crystals formed; therefore, it may not act as a nucleating agent precipitated before wax crystals. A further microscopic investigation was used to gain a better understanding of action mechanisms between the additive and waxy crude oil. 3.3. Microscopic Study. The microscopic images of wax crystals in the absence and presence of POMB captured at 25 °C are presented in Figure 5. Figure 5a shows that wax crystals
to the oil. These additives achieve good performance at 500 ppm dose, while doses higher than 500 ppm may further depress the pour point to a small extent and are not economically advisible. In the mean time, the efficiency of EVA can be found lost at concentrations higher than 100 ppm. This loss may be attributed to its high molecular weight that prompts itself to precipitate and form three-dimensional networks. The selected POM can lower the pour point by 3 °C at the concentration of 50 ppm and up to 9 °C at 500 ppm. Alkyl side chains with different lengths can provide more favorable adsorption sites for wax crystals; therefore, dodecanol was used to react with the anhydride group in the POM backbone as a shorter pendant chain, and POMO was obtained. The data in Table 3 verify that POMO can decrease the pour point by 12 °C at 500 ppm concentration, the same depression power with the best effectiveness of EVA (at 100 ppm dose). Previous work has shown that a higher dispersing activity of the polymeric improver can be obtained by exerting a N-containing group in the structure;13 therefore, POMN was prepared and tested in comparison to POMO. The data indicate that a 500 ppm dose of POMN can decrease the pour point by 13 °C, which has a little better efficacy than that of POMO. To further improve the efficiency, aromatic rings were incorporated into the anhydride groups of POM. It is obvious that POMB containing aromatic units is most effective in reducing the pour point by 19 °C (from 30 to 11 °C) at 500 ppm dose and by 9 °C at 50 ppm. This conclusion is in agreement with a previous observation.25 It can be explained that the benzene ring matches well with the asphaltenes and aromatic resins, which can increase the solubility of the additive and enhance the interactions between the additive and paraffins in crude oil. Considering the pour-point test results, POMB with the best performance was selected for further studies. 3.2. DSC. WAT is defined as the temperature at which the first wax crystal appears in oil,28 while according to DSC analysis, it is the temperature at which the curve deviates from the baseline. The thermogram of CQ crude oil undoped and doped with 500 ppm concentration of POMB is given in Figure
Figure 5. Photomicrographs of (a) virgin crude oil and (b) crude oil with 500 ppm concentration of POMB at 25 °C.
formed from virgin crude oil are needle-shaped, which have a large crystal−liquid oil surface area and high surface energy. As the temperature decreases, wax crystals will easily interlock with each other to form gel structures; hence, the low-temperature fluidity of crude oil will be greatly deteriorated. However, in the presence of POMB at 500 ppm dose (Figure 5b), it is obvious that large spherical crystal aggregates have been formed, which
Figure 4. Thermogram of CQ crude oil without and with 500 ppm concentration of POMB.
4. From the thermogram, we can see that both of the curves have two distinct peaks representing two crystallization regions because of the broad distribution of wax molecules. There is no 998
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are in accordance with the previous studies.20,30 There may be two molecular forces working together for wax accumulation to form aggregates. First, the aromatic rings of the pendant chains can interact with each other or with asphaltenes by a π−π stacking attractive force. Second, there exists inter- and intramolecular hydrophobic association of alkyl chains. The alkyl chains of the polymer that have not fully interacted with long-chain paraffins will continue to provide adsorption sites for short-chain paraffins to form large aggregates at lower temperatures. Therefore, the liquid oil trapped by the gels was released, and the zone unoccupied by wax crystals was increased, resulting in the improvement of flow properties of treated oil. 3.4. Rheological Properties. Paraffins crystallized out from waxy crude oil cause serious problems, such as a blockage of the pipeline in the petroleum industry. Pretreatment with polymeric improvers is an effective method to enhance its flowability at low temperatures. The performance of improvers largely depends upon the rheological behavior of treated crude oil. Panels a−c of Figure 7 give the rheograms of neat CQ crude oil taken at temperatures below and above its pour point (24,
Figure 6. Log γ−log τ graphing method for yield stress determination.
30, and 36 °C). It can be seen that virgin oil at 36 °C (above the pour point) follows the Newtonian behavior, where the shear stress increases linearly with the shear rate and the yield stress is even 0. While at 30 °C (pour point) and below, it shows the non-Newtonian yield pseudo-plastic rheological behavior with yield stress required for initiating flow. As the shear rate increases, viscosity continuously decreases until it comes to a constant at higher shear rates, which reveals the shear thinning behavior of crude oil. For example, as the shear rate increases from 10 to 500 s−1, the viscosity reduces from 0.869 Pa s to a nearly constant value of 0.028 Pa s at 24 °C. Shear stress versus shear rate and viscosity versus shear rate curves of crude oil treated with 500 ppm concentration of POMB taken at temperatures from 24 to 36 °C are presented in Figures 8 and 9, respectively. The relationship between the temperature and yield stress of crude oil without and with prepared additives and commercial EVA at 500 ppm dose is exhibited in Figure 10. The data from Figure 8 reveal that the tested oil shows Newtonian characteristics that it can flow easily with negligible yield stress (
