Effect of Flow Improvers on Rheological and Microscopic Properties of

Article pubs.acs.org/IECR

Effect of Flow Improvers on Rheological and Microscopic Properties of Indian Waxy Crude Oil Shivanjali Sharma, Vikas Mahto,* and Virender Parkash Sharma Department of Petroleum Engineering, Indian School of Mines, Dhanbad, 826004, Jharkhand, India ABSTRACT: This research article investigates the effect of flow improvers on the rheological and microscopic properties of a waxy crude oil of the western oil field of India, since the application of flow improvers is the most economic way for controlling the flow assurance problems associated with the production, handling, and processing of waxy crude oils. In this work, hexatriethanolamine mono oleate, which is a flow improver, was synthesized in the laboratory. The effect of this synthesized flow improver and a commercial pour point depressant on the waxy crude oil was studied thoroughly and their experimental results were compared. It was observed that both flow improvers have the potential to improve the flow behavior of this crude oil significantly, and the synthesized flow improver has shown better performance than the commercial flow improver.

1. INTRODUCTION Waxy crude oils have a high pour point and higher viscosity, and they cause severe problems during the production, separation, transportation, and refining of these crude oils.1 Thermal and shear history, as well as the amount and redistribution of saturates, resins, and asphaltenes of high-pour-point waxy crude sample affects the rheological properties of crude oil. Asphaltenes, resins, and wax are the main components of the heavy fraction, which participate in the formation of the solid phase. Petroleum fluids undergo phase changes during production, transportation, and processing.Variation in temperature is the dominant factor affecting the waxy crude oil properties. At higher temperature, all paraffins are in dissolved state in the crude oil and in balanced state. As soon as this balance is broken, paraffins crystallizes out and precipitates. Precipitation of the wax components out of the oil is responsible for the changes in the waxy crude oil properties. When the fluid temperature falls below the wax appearance temperature (WAT), there is the possibility of wax deposition on the tubing/pipelines. With further decreases in temperature, a stage is reached when crude oil ceases to flow; this is called the pour point. Wax deposition reduces the effective flow area and may lead to complete blockage of a pipeline.2,3 Deposited wax will also increase the roughness of the solid/liquid interface and, therefore, increase the pressure drop. The structure needs to be broken to cause the flow of oil; for this to occur, a certain yield stress is needed, which becomes restricted by the permissible stress of the flow line material. Therefore, it is important to understand the behavior of waxy crude oil and determine accurate properties so that the design of the subsea system may be optimized. Polymeric additives known as flow improvers or pour point depressants are generally used to lower the pour point, viscosity, and yield stress of crude oil. These additives improve the fluidity of waxy crude in the pipeline or flowline and reduce extra pumping costs.3 The flow improvers and the shear rate act in the same sense in the rheological behavior of waxy crude oils but in a different manner. The flow improvers changes the flow properties by adsorption/nucleation/co-crystallization, and/or thermodynamic change of solublity of wax constituent in the © 2014 American Chemical Society

crude oil, and the shear rate acts through the breakdown of secondary interparticle bonds formed among flocculated wax crystals during cooling.4 Keeping these in view, the present work consists of the effect of hexatriethanolamine mono oleate, which is a flow improver that was synthesized in the laboratory, and commercial pour point depressants (PPDs) on the rheological behavior of the waxy crude oil at different shear rates and changes in microscopic property of waxy crude after addition of flow improvers at different temperatures.

2. EXPERIMENTAL WORK 2.1. Material. A crude oil sample was collected from a western Indian oil field and two commercial PPDs (one was obtained from the chemical supplier and one was synthesized in the laboratory) to show their effects on the waxy crude oil. Different chemicals used for the present investigation were procured from various companies. Triethanolamine, n-pentane, n-hexane, p-toluene sulfonic acid, chloroform, and methanol was obtained from RFCL, Ltd. (New Delhi, India). Oleic acid was obtained from Loba Chemie Pvt. Ltd. (Mumbai, India). Toluene and acetone were obtained from CDH, New Delhi, India. nHeptane was obtained from Otto Kemi Pvt. Ltd. (Mumbai, India). Sodium hydroxide was obtained from Finar Chemicals (Ahmedabad, India). 2.2. Experimental Procedure. 2.2.1. Characterization of Crude Oil. Initially, different crude oil characteristics, such as water content, API gravity, pour point, and cloud point were determined using standard ASTM/IP methods. Bottom sediments and water determination was performed using the centrifuge method (ASTM Standard D 96-58 T).5 Two layers (water and oil) were formed and the volume of water is measured at the bottom of calibrated centrifuge tube. The API gravity is used to reveal the quality of the crude oil. The API gravity values for each crude oil were estimated using specific gravity values at Received: Revised: Accepted: Published: 4525

September 15, 2013 February 18, 2014 March 5, 2014 March 5, 2014 dx.doi.org/10.1021/ie403050y | Ind. Eng. Chem. Res. 2014, 53, 4525−4533

Industrial & Engineering Chemistry Research

Article

40 °C. The density of the crude oil was determined with the use of a pycnometer.6 Pour point temperatures of the crude oil were measured using the standard test method for the pour point (ASTM Standard D97-06.11).7 In this ASTM standard, a pour point apparatus that includes a test jar, bath, jacket, and thermometer is used for the pour-point determinations. The pour point was obtained by heating the sample to ∼45−48 °C and then cooled and checked at intervals of every 3 °C. When the flow stops upon tipping the test jar horizontally, that temperature was noted as the pour point.8,9 To determine the cloud point, the viscosity of the crude oil was determined at different temperatures. The viscosity was measured with a Redwood viscometer, using method IP 70/62. The samples were heated above the WAT, to ensure that wax particles are fully dissolved before performing viscosity measurement. 2.2.2. Wax Content. To determine the wax content, 2 g of crude oil was dissolved in 40 mL of n-pentane and stirred during 30 min. To this mixture, 120 mL of acetone (acetone/n-pentane ratio of 3:1) was added and the mixture was cooled to −20 °C for 24 h. The solid phase was separated via filtration, using a Buchner funnel with a glass microfiber Whatman filter (No. 934). The solid phase was redissolved in n-hexane to remove asphaltenes. After solvent removal, the final product was weighed.10,11 2.2.3. SARA Distribution. SARA separation is a group-type analysis that is used to separate crude oils on the basis on differences in solubility and polarity into saturates (S), aromatics (A), resins (R), and asphaltenes (A). The chemical composition of the crude oil is determined based on the SARA fractions. The purpose of this method is to determine the content of saturated, aromatics, resins, and asphaltenes in a crude oil. It is based on the ASTM D2007-93 standard test method. In the first stage, asphaltenes and insoluble resins are separated by precipitation

with n-hexane (crude oil/solvent ratio equal to 1/30 (by volume)). The mixture is cooled to −30 °C and the precipitated asphaltenes are filtered out. The filtered sample (maltene fraction) is later split in a chromatographic column to obtain saturated compounds, aromatics, and polar resins. The solid asphaltenes were washed with n-heptane before drying and then weighed. The chromatographic separation of maltenes is carried out in an installation that is composed of glass columns packed with silica gel (Macherey-Nagel 0.071−0.160 mm/100−200 mesh). The different fractions are separated, depending on their affinity to the solvent being used at each step of extraction. Trichloromethane is used to recover resins, n-hexane is used for saturates, and hot toluene is used to extract aromatics. The solvents used are extracted using a Soxhlet apparatus and the percentage of each fraction (weight percentage of crude oil) is calculated.12,13 2.2.4. Synthesis of Pour Point Depressant (PPD). One mole (1 mol) of triethanolamine was reacted with 0.04 mol of sodium hydroxide at 240 °C under constant stirring in a three-necked flask equipped with a Dean and Stark trap to remove water azeotropically as it is formed. The reaction mixture was then washed with acetic acid (5%) and dissolved in petroleum ether (bp 40−60 °C). After separating the organic layer, the solvent was distilled off. The pale yellow viscous liquid of hexatriethanolamine was then reacted with 1 mol of oleic acid using p-toluene sulfonic acid (0.005 mol) as a catalyst. The reaction was conducted at 150 °C under constant stirring. The reaction was carried out until a theoretical amount of water (18 g) is collected. The reaction mixture was washed with sodium carbonate (5%) to remove the catalyst and then extracted with petroleum ether (bp 40−60 °C). The prepared product was then purified by distilling of the solvent at the end of reaction using a Soxhlet apparatus.14,15 The product obtained is hexatriethanolamine mono oleate. The reaction involved is as follows:

H[−OCH 2CH 2NCH 2CH 2OHCH 2CH 2−]6 + [CH3(CH 2)7 CHCH(CH 2)7 COO] hexatriethanolamine

oleic acid

⇄ [CH3(CH 2)7 CHCH(CH 2)7 COO]CH 2CH 2NCH 2CH 2OHCH 2CH 2O]6 hexatriethanolamine mono oleate

2.2.4.1. Characterization of Hexamethylenetriamine Mono Oleate. The synthesized oleate ester was further characterized. The molecular weight of synthesized ester was determined using a Waters gel permeation chromatography (GPC) system and functional groups were determined using fluorescence tube infrared spectroscopy, using a Perkin−Elmer FTIR spectrophotometer. Spectrum-10 software was used to analyze the data. The elemental analysis of the PPD (carbon, hydrogen, nitrogen, and sulfur) was performed using an ELEMENTAR Vario Micro Cube CHNS analyzer. 2.2.5. Rheological Studies. Flow properties of an oil containing crystallized wax are distinctly non-Newtonian. A yield stress can be detected that, under some circumstances, can be many times greater than the normal pumping pressure. Rheological studies of crude oil were carried out using a Model MC-1 rheometer at various temperatures with and without an additive. It is a rotational viscometer based on the Searle principle and it has a temperature regulator, ranging from −40 °C to 180 °C, which controls the temperature, using water or oil as the heat-transfer medium, and it is controlled using Physica US 200 software, such that measurements are performed under controlled shear stress or controlled shear rate.

2.2.6. Microscope Studies. The utilization of polarization microscopy for the study of wax crystallization is based on the fact that all crystalline materials with noncubic geometry are optically anisotropic, meaning that the crystals rotate the plane of polarization of transmitted, polarized light. Hence, by crossing two Nicol prisms on opposite sides of the sample, all light is initially blocked. Upon cooling, the crystallizing material appears as spots against a black background. A cross-polarized light microscope (Olympus, Model UC-30) was used to study the wax crystal structure in virgin and additive-treated crude oil at room temperature.

3. RESULTS AND DISCUSSION 3.1. Characterization of Crude Oil and the Synthesized Pour Point Depressant. 3.1.1. Discussions on the Chemical Characteristics of Crude Oil. The API gravity of the crude oil sample under present study is 18.97, so it is a heavy crude oil. The wax content, viscosity, and water content of the crude oil are listed in Table 1. The wax content is 13.5%, which is found in highly waxy crude oil. The water content is also high, and water needs to be removed before performing further experiments. The 4526

dx.doi.org/10.1021/ie403050y | Ind. Eng. Chem. Res. 2014, 53, 4525−4533


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Table 1. Characterization of Crude Oil sample

parameter

observed value

method

1 2 3 4 5

API gravity water content pour point Redwood viscosity wax content

18.97 2.5% (v/v) 29 °C 232 s 13.5% (w/w)

IP 160/64 IP 74/64 IP 15/65 IP 70/62

viscosity is also high. This could be attributed to the presence of wax in the crude oil. Table 2 shows the SARA distribution of the crude oil, and it is found that the sample is predominantly rich in saturated fraction and the asphaltenes content is low. Table 2. SARA Distribution of Crude Oil Sample component

amount (% (w/w))

saturates aromatics resins asphaltenes

59.5 14 7 0.13

Figure 1. FT-IR spectra of oleate ester (a synthesized pour point depressant (PPD)).

3.1.2. Discussions on the Physical and Chemical Characteristics of Synthesized Pour Point Depressants. The physical state of oleate ester (a synthesized PPD) is semisolid and it is brown in color. The specific gravity of this PPD is found to be 0.902 at 25 °C. The functional groups of the oleate ester determined by FT-IR spectroscopy are given in Table 3. Methyl, esters, tertiary butyl

Table 4. Pour Point for Virgin and Additive-Treated Crude Oil concentration of additive (ppm)

pour point (°C)

difference in pour point, ΔT

(a) for Phoenix

Table 3. Functional Groups in Oleate Ester (a Synthesized Pour Point Depressant (PPD)) observed value (cm−1)

range (cm−1)

functional group

3007.75 2927.10 2854.96 1465.25 1406.80 1160 780

3000−3100 2850−2960 2850−2960 1430−1470 1350−1500 1050−1380 690−800

CC−H in olefins, aromatics C−H in CH3, CH2 C−H in CH3, CH2 C−H C(CH3)3 tertiary butyl ester (RCOOR) C−H

0 500 750 1000 1250 1500 0 500 750 1000 1250

groups, olefins, and aromatics are the main groups that are detected in oleate ester. The FT-IR spectra is shown also in Figure 1. The molecular weight of mono oleate ester, as determined by GPC, is determined to be 437 430, and the polydispersity index is 1.160978. Therefore, we can conclude that the synthesized PPD in the present study possesses higher molecular weight. From the elemental analysis, carbon (66.83%), hydrogen (12.36%), nitrogen (1.64%), and sulfur (0%) are found in the synthesized sample. 3.2. Pour Point. The pour point of both crude oil samples beneficiated with and without PPD are listed in Table 4. All of the samples became nonfluid upon cooling. This is due to precipitation of wax crystals as thin needles and plates.5 The pour point was decreased up to differences as large as 20 °C with the help of additives. This happens because they form a layer on the wax crystals and do not allow them to interlock with each other. It was observed that the synthesized PPD was highly effective in decreasing the pour point (see Table 4). It was also observed that the synthesized PPD was much better in performance than commercial PPD. For every additive, there is

29 27 24 21 24 24 (b) for Mono Oleate 29 27 16 14 13

2 5 8 5 5

2 13 15 16

an optimum concentration of additives up to which the pour point decreases; after that, the pour point increases as the concentration increases. This happens because, after a certain point, PPD crystals themselves form linkages with wax. Therefore, it is very essential to determine the optimum concentration of PPD. The optimum concentration is the one at which the PPD is maximum in the crude oil. As the structure of the PPD matches with the wax component of crude oil, it is able to depress the pour point up to a certain level. After this point, as the concentration of PPD increases above the optimum concentration, the concentration of polymer and its pendant chain length increases, thereby deteriorating its performance. This is because the increased chain length makes the polymer more bulky and less soluble, eventually making it less effective. As a result, the pour point again increases. 3.3). Rheological Study. Rheological studies were done with two PPD compounds at different temperatures, and the results are shown in Figures 2−13. Waxy-paraffinic-crude oils 4527



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Figure 2. Rheology of virgin crude oil at various temperatures above the pour point.

the reduced temperatures, non-Newtonian behavior starts to appear. The initial parts of curves almost coincide with the zeroshear-rate axis. At 25 °C (i.e., 4 °C below the pour point), the flow curve clearly displays non-Newtonian behavior such as shear thinning. The shear thinning can be demonstrated by the nonlinearity of the curve. Upon adding the PPDs, even at low temperature, there is a decrease in the viscosity of the crude oil sample. 3.3.1. Effect of Temperature on the Flow Curve. Flow curves are obtained at three different temperatures. Lines at higher temperature show that crude oil behaves as a Newtonian fluid at these temperatures. With reduced temperatures, non-Newtonian behavior started to appear. For most crudes, at sufficiently high temperature, the viscosity at a given temperature is constant and the crude is a simple nonNewtonian liquid. As the temperature is reduced, however, the flow properties of a crude oil readily change from Newtonian to very complex flow behavior, because of crystallization of the waxes. 3.3.2. Effect of Shear Rate on Flow Curve. A reduction in viscosity is noticed at all temperatures as the shear rate is increased. At high shear rate, the viscosity remained constant; this may be due to stabilization of the wax structure. 3.4. Microscopic Study. A micrograph of wax crystals in virgin crude oil is shown in Figure 14, while Figures 15 and 16

Figure 3. Rheology of virgin crude oil at various temperatures at and below the pour point.

exhibit a high pour point and possess non-Newtonian flow characteristics at temperatures equal to or less than the pour point, because of wax crystallization. For this reason, a yield stress arises and an increase in viscosity takes place. The rheological properties (shear-stress/shear-rate relationship) become no longer constant and viscosity varies as a function of shear rate.16 Since the pour point of the crude oil sample used is 29 °C, rheological studies have been done at two temperatures: at the pour point (29 °C) and below the pour point (i.e., 25 °C) (see Figure 3). From the rheograms, it could be concluded that, with

Figure 4. Rheology of crude oil with Phoenix at 25 °C. 4528



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Figure 5. Rheology of crude oil with Phoenix at 29 °C.



However, in Figures 15 and 16, the single wax crystals become bigger and have a tendency to aggregate as clusters, which increase the zone unoccupied by the particles. Furthermore, because the formation of a 3D network structure requires more wax to be precipitated, the low-temperature flow property of treated oil is improved. 3.4.1. Microscopic Properties at Different Concentration of Additive. At low concentrations of additive, wax crystals appear

display the wax crystal aggregates in oil beneficiated with four different PPD compounds. In Figure 14, wax crystals have a corresponding homogeneous size distribution and a high degree of dispersity of the particles. This will impart high surface energy to virgin crude oil and, thus, upon intense cooling or application of high shear, wax crystals easily tend to interconnect into a threedimensional (3D) network structure, which will affect the flow behavior of waxy crude oil. 4529



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Figure 9. Rheology of crude oil with hexamethylene triethanolamine mono oleate at 20 °C.


oil, which contains various paraffins and other strong polarity large molecule materials, microscopic observations give evidence of malcrystalline masses rather than regular crystals. Wax particles increase in size upon decreasing the temperature of the crude oil. Thus, the flow becomes more viscous at reduced temperature.

in needle form, whereas upon increasing the concentration of additive, they form circular-shaped wax crystals, which make the crude oil flowable within the pipeline. 3.4.2. Microscopic Properties at Different Temperature. The shape of the particles was found to be neither platelike nor needlelike when cooled. Instead, the particles are irregularly shaped spherulites. When the temperature is decreased, the spherulites tend to agglomerate and form various random chain structures, which ultimately form a network. For this waxy crude

4. CONCLUSIONS The following conclusions can be drawn from this study: 4530



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Figure 12. Rheology of crude oil with mono oleate at 45 °C.

Figure 13. Rheology of crude oil with mono oleate at 55 °C.

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Figure 14. Microscopic properties of virgin crude oil at various temperatures.

Figure 16. Microscopic properties of crude oil beneficiated with mono oleate at various temperatures.

(1) The crude oil under present study is waxy in nature, and it is predominantly rich in saturate fractions. (2) The shear rate has a noticeable effect, with regard to decreasing the viscosity, particularly at low temperatures, and the viscosity tends to stabilize at higher shear rates. (3) The efficiency of flow improvers increases as their concentration in the solution increases. (4) The microstructures of crude oils is influenced strongly by both temperature and cooling rate. (5) Cooling leads to more and bigger wax particles and more agglomeration between particles. (6) The hexatriethanolamine mono oleate, which is a synthesized flow improver, has more potential to improve the flowability of waxy crude oil than the commercial pour point depressant.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-326-5498. Fax: +91-326-2296563. E-mail: vikas. [email protected] Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Sharma, S.; Mahto, V.; Sharma V. P. To Study the Effect of Flow Improvers on Indian Waxy Crude Oil. In Proceedings of the Second International Conference on Drilling Technology 2012 (ICDT-2012) and First National Symposium on Petroleum Science and Engineering 2012 (NSPSE-2012), Chennai, India, Dec. 6−8, 2012.

Figure 15. Microscopic properties of crude oil beneficiated with Phoenix at various temperatures.

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(2) Mahto, V.; Verma, D.; Singh, H. Kinetic study of wax deposition in the flow lines due to Indian crude oil. In Proceedings of the First International Conference on Drilling Technology, IIT Madras, Chennai, India, Nov. 18−21, 2010. (3) Khan, A. R.; Mahto, V.; Fazal, S. A.; Laik, S. Studies of Wax Deposition Onset in the Case of Indian Crude Oil. J. Pet. Sci. Technol. 2008, 26, 1706−1715. (4) EI Gamal, I. M. Combined effects of shear and flow improvers: The optimum solution for handling waxy crude below pour point. Colloids Surf., A 1998, 135, 283−291. (5) Nemirovskaya, B. G.; Emel’yanova, S. A.; Ashmyan, D. K. Methods of Analysis of High-Wax Crude Oils, Resins, Asphaltenes, Paraffin Waxes. Chem. Technol. Fuels Oils 2005, 41 (3), 236−240. (6) Layla, M. A.; Mohamed, M. E.; Jean, L. D.; Joao, A. P. C. Characterization of Libyan Waxy Crude Oils. Energy Fuels 2010, 24, 3101−3107. (7) Standard Methods of Test for Pour Point of Petroleum Oils. In 1974 Annual Book of ASTM Standards; American Society for Testing and Materials (ASTM): Philadelphia, PA, 1974; pp 81−84. (8) Taraneh, B. J.; Rahmatollah, G.; Hassan, A.; Alireza, D. Effect of wax inhibitors on pour point and rheological properties of Iranian waxy crude oil. Fuel Process. Technol. 2008, 89, 973−977. (9) Kök, M. V.; Letoffe, J. M.; Claudy, P. Comparative Methods in the Determination of Wax Content and Pour Points of Crude Oils. J. Therm. Anal. Calorim. 2007, 90 (3), 827−831. (10) Baudilio, C.; Carmen, M.; José, L. P.; Juan, J. E.; María, D. R. A New Method for the Determination of Wax Precipitation from NonDiluted Crude Oils by Fractional Precipitation. Fuel 2008, 87, 2090− 2094. (11) Hongying, L.; Zhang, J. A generalized model for predicting nonNewtonian viscosity of waxy crudes as a function of temperature and precipitated wax. Fuel 2003, 82, 1387−1397. (12) Speight, G. J. The Chemical and Physical Structure of Petroleum: Effects on Recovery Operations. J. Pet. Sci. Eng. 1999, 22, 3−15. (13) Coto, B.; Coutinho, J. A. P.; Martos, C.; Robustillo, M. D.; Espada, J. J.; Pena, L. J. Assessment and Improvement of n-Paraffin Distribution Obtained by HTGC to Predict Accurately Crude Oil Cold Properties. Energy Fuels 2011, 25, 1153−1160. (14) Hafiz, A. A.; Khidr, T. T. Hexa-triethanolamine oleate esters as pour point depressant for waxy crude oils. J. Pet. Sci. Eng. 2007, 56, 296− 302. (15) Deshmukh, S.; Bharambe, D. P. Synthesis of polymeric pour point depressants for Nada crude oil (Gujarat, India) and its impact on oil rheology. Fuel Process. Technol. 2008, 89, 227−233. (16) Marchesini, F. H.; Alicke, A. A.; De Souza Mendes, P. R.; Ziglio, C. M. Rheological Characterization of Waxy Crude Oils: Sample Preparation. Energy Fuels 2012, 26, 2566−2577.

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Effect of Flow Improvers on Rheological and Microscopic Properties of

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