Measurement of Wax Appearance Temperature under Simulated

May 30, 2008 - temperature (WAT) under a simulated pipeline (dynamic) condition provides ... WAT values of blends of Alaska North Slope (ANS) crude oi...
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Energy & Fuels 2008, 22, 2437–2442

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Measurement of Wax Appearance Temperature under Simulated Pipeline (Dynamic) Conditions Cajetan E. Ijeomah, Abhijit Y. Dandekar,* Godwin A. Chukwu, Santanu Khataniar, Shirish L. Patil, and Arthur L. Baldwin† Department of Petroleum Engineering, UniVersity of Alaska Fairbanks, Fairbanks, Alaska 99775-5880 ReceiVed December 8, 2007. ReVised Manuscript ReceiVed March 20, 2008

Fluid flow in petroleum pipelines is a dynamic phenomenon. Therefore, measurement of the wax appearance temperature (WAT) under a simulated pipeline (dynamic) condition provides more valuable WAT data than the conventional procedures, which are performed under static (atmospheric) conditions. In this work, the WAT values of blends of Alaska North Slope (ANS) crude oil and gas-to-liquid (GTL) products were determined under dynamic (flowing) conditions using an indigenously designed and developed flow loop. The WAT values of the samples were also determined under static conditions using the viscosity method and verified using the American Society for Testing and Materials (ASTM) D3117 method. The WAT values decreased with an increase in the line pressure and GTL content. The results reported in this paper are part of a major project on studying the operational challenges in GTL transportation through the Trans-Alaska Pipeline System (TAPS).

1. Introduction Petroleum wax is an organic, plastic substance that is solid below a certain temperature, called the wax appearance temperature (WAT), and becomes liquid when heated above the WAT. Waxes are thermoplastic, combustible, and insoluble in water. The WAT of a fluid is the temperature at which the first smallest amount of wax is formed in the fluid. Although petroleum waxes are of three general categories: paraffin, microcrystalline, and petrolatum, the major constituent is paraffin. Paraffin waxes contain predominantly straight-chain hydrocarbons with an average chain length of 20-30 carbon atoms. However, the presence of other hydrocarbon structures in paraffin waxes makes it a complex mixture. Generally, paraffin waxes are nonreactive, nontoxic, and colorless. Pressure has a significant effect on the temperature at which wax appears in crude oils. Weingarten et al.1 and Pan et al.2 used experimental data and model predictions to show that WAT generally decreases as the saturation pressure of live oils increases. However, the work of Brown et al.3 showed that a decrease in WAT as the pressure increases is limited to pressures below the bubble point pressure. Above the bubble point pressure, WAT increases linearly with pressure. They supported the former trend with experimental data and model predictions, while the later trend was supported with model predictions only (without experimental data). We discuss the WAT and pressure trend in details in the Results and Discussion. Wax in a petroleum pipeline adheres to the walls of the pipe and, as a result, reduces the pipe flow diameter. Wax deposition * To whom correspondence should be addressed. Telephone: (907) 4746427. Fax: (907) 474-5912. E-mail: [email protected]. † Current address: U.S. Department of Energy, National Energy Technology Laboratory, Pittsburgh, PA 15236. (1) Weingarten, J. S.; Euchner, J. A. SPE Prod. Eng. 1988, 3, 121– 126. (2) Pan, H.; Firoozabadi, A.; Fotland, P. SPE Prod. Facil. 1997, 12, 250–258. (3) Brown, T. S.; Niesen, V. G.; Erickson, D. D. SPE 1994, 28505, 415–430.

in the production tubing, processing facilities, and transportation lines is one of the most worrisome problems that have serious consequences, such as a loss of production, reduced pipeline flow capacity because of the build-up of wax, and change in the overall fluid composition. Therefore, wax deposition problems ought to be addressed whenever oil production or transportation operations are considered. One of the best examples of the potential wax deposition problems is the 1287.5 km long Trans-Alaska Pipeline System (TAPS). The pipeline carries crude oil produced on the Alaska North Slope (ANS) to the port of Valdez through an arctic environment where temperatures in the winter period could be as low as -45.6 °C. The oil enters the pipeline at the ANS at about 76.7 °C and exits at Valdez at about 21.1 °C. Because the insulation of the pipeline is the sole protection that the transported fluid has against the arctic cold, a shutdown of the pipeline during the winter period will lead to a gradual cooling of the stagnated fluid, resulting in a 1287.5 km long candle. However, the formation of this candle will depend upon the wax deposition characteristics of the transported fluid. Therefore, an interesting consideration is the transportation of other petroleum liquids, such as gas-to-liquid (GTL), a synthetic crude oil (syncrude) through the pipeline, in addition to the ANS crude oil in a commingled mode. This will result in a hybrid liquid (blend of the two liquids) whose wax deposition characteristics will depend upon the percentage of each of the parent liquids. Our research group at the University of Alaska Fairbanks (UAF) has been studying the transportability of GTL products through the TAPS for the last several years, in both batch and commingled modes with the ANS crude oil: Amadi et al.,4 Sharma et al.,5 Nerella et al.,6 Ramakrishnan et al.,7 and (4) Amadi, S. U.; Dandekar, A. Y.; Chukwu, G. A.; Khataniar, S.; Patil, S. L.; Haslebacher, W. F.; Chaddock, J. Energy Sources 2005, 27, 831– 838.

10.1021/ef7007453 CCC: $40.75  2008 American Chemical Society Published on Web 05/30/2008

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Akwukwaegbu et al.8 The work presented in this paper is part of these ongoing investigations. Fluid flow in petroleum pipelines simultaneously involve pressure and flow rate. A simulation of the pipeline condition does not require a separation of these two parameters. Also, the wax deposition characteristics of a GTL-crude oil blend is a function of the amount of GTL in the blend. As a result, a study of wax deposition in the blend under dynamic conditions involves two independent variables: the dynamic parameters (pressure plus flow rate) and percentage of GTL content in the blend, while the dependent variable determined is the WAT. It is, nonetheless, worthy of further clarification that this study is focused on WAT. It is neither concerned with gel formation, wax disappearance temperature (WDT), which is the temperature at which formed wax disappears (becomes liquid) as the sample is heated, nor the thermodynamics of wax formation. 2. Experimental Section 2.1. Measurement of WAT under Static Conditions. Two techniques were employed in the determination of WAT values of the samples under static conditions. The Arrhenius principle for the viscosity-temperature relationship of Newtonian fluids is the basis of the first method. The second method, American Society for Testing and Materials (ASTM) D3117,9 is the standard ASTM method for determining WAT. It is based on visual identification of the smallest visible wax in the sample. The Arrhenius principle states that the viscosity of a Newtonian fluid is an exponential function of temperature.10 The equation, in logarithmic form, is given as follows: Ea (1) RT From eq 1, a plot of the logarithm of the viscosity of a Newtonian fluid against the reciprocal of its absolute temperature is a straight line. A deviation from this straight line signifies that the fluid has become non-Newtonian. Therefore, for a Newtonian and waxy fluid that is cooled gradually, wax precipitation at the WAT would make the fluid become non-Newtonian and thus cause a deviation from the Arrhenius straight line. Note that the rheological behavior of a fluid with solid suspensions, such as suspensions of wax, is generally non-Newtonian. The viscosity-temperature data were measured using a programmable Brookfield viscometer.7,11 The measurement of WAT requires that the sample is cooled at a certain rate to determine the temperature of wax formation. In the literature, cooling rates used differ significantly (by an order of magnitude in some cases). For example, Pan et al.2 used a cooling rate of 0.5 °C/min; Brown et al.3 used a cooling rate of 0.2 °C/min; Lindeloff et al.12 used a cooling rate of 0.08 °C/min; Daridon et al.13 used a cooling rate of 0.008 °C/min; Monger-McClure et al.14 used a cooling rate of 0.28 ln(µ) ) ln(A) +

(5) Sharma, A.; Dandekar, A. Y.; Chukwu, G. A.; Khataniar, S.; Patil, S. L.; Haslebacher, W. F.; Chaddock, J. Energy Sources 2005, 27, 709– 718. (6) Nerella, S.; Das, D. K.; Chukwu, G. A.; Dandekar, A.; Khataniar, S.; Patil, S. L. Pet. Sci. Technol. 2003, 21, 1275–1294. (7) Ramakrishnan, H.; Khataniar, S.; Dandekar, A.; Patil, S. L.; Chukwu, G. A.; Kamath, V. A.; Haslebacher, W. F.; Hackworth, J. H. Pet. Sci. Technol. 2003, 21, 301–314. (8) Akwukwaegbu, C. F.; Chukwu, G. A.; Dandekar, A.; Khataniar, S.; Patil, S. L.; Kamath, V. A. Pet. Sci. Technol. 2002, 20, 819–830. (9) American Society for Testing and Materials (ASTM). Standard test method for wax appearance point of distillate fuels. ASTM D3117, 1996. (10) Hamouda, A. A.; Viken, B. K. SPE 1993, 25189, 385–396. (11) Ijeomah, C. E. Study of solid deposition phenomena and fluid properties of Alaska north slope crude oil, gas-to-liquid products and their blends for transportation through the Trans-Alaska Pipeline System. M.S. Thesis, University of Alaska Fairbanks, Fairbanks, AK, 2005. (12) Lindeloff, N.; Pauly, J.; Andersen, S. I.; Daridon, J. L.; Stenby, E. H. Proceedings of the AIChE National Spring Meeting, Houston, TX, March 1999.

Ijeomah et al. °C/min; whereas Roenningsen et al.15 used a cooling rate of 0.15 °C/min. The effect of the cooling rate was not studied in this work, and a constant rate was used in all tests (including those under dynamic conditions), which was 0.14 °C/min (1 °C/7 min). However, the cooling rate used in this work falls within the ranges used by other researchers. The samples were cooled at a constant rate of 1 °C/7 min and the viscosity, corresponding to every 1 °C decrease in temperature recorded. For each sample, a plot of the log of viscosity against the reciprocal of absolute temperature was prepared and the WAT was appropriately identified on the plot. The shear rate for the tests ranged from as low as 75 s-1 to as high as 1500 s-1 depending upon the viscosity of the blend in question prior to the start of the test. The GTL and the crude oil were the least and most viscous, respectively, while the viscosity of the blends decreased with an increase in the amount of GTL present. Two factors guided the choice of shear rate for each blend. The first factor is the need to obtain a viscometer torque of at least 10% of the viscometer maximum torque capacity as required by the equipment manufacturer for reliable viscosity results. The second factor is the viscometer maximum torque capacity. Marrying these two factors together demands that, at the higher temperature (start of the test), the viscometer torque is at least 10% of the viscometer maximum torque capacity and remains below the viscometer maximum torque capacity until the wax is formed, leading to an abrupt increase in viscosity. Because the ASTM D3117 method is based on visual identification of the smallest visible wax in the sample, only the GTL sample was used for the test because all of the other samples are opaque. The procedure involved cooling the sample gradually while being stirred by an electric motor-driven stirrer. The sample was closely observed, and the temperature at which the smallest visible wax appeared was recorded as the WAT of the sample.4 2.2. Measurement of WAT under Dynamic Conditions. The principle employed in the dynamic test is the ability of precipitated solids in a flowing fluid to clog a filter element of appropriate pore size installed in the flow line. A sudden rise in the differential pressure across the filter element results from this restriction to flow and is indicative of the appearance of wax. The apparatus, which was indigenously designed and developed, is schematically illustrated in Figure 1. Its primary components are a floating piston sample cylinder, E; an inline filter with a 50 µm pore size filter element, H; a pressure gauge of 68.95 MPa capacity, J; a differential pressure transducer with its signal (∆P) indicator, K; a circulation pump, M; and 3.175 cm outside diameter and 1.588 cm inside diameter flow lines. A temperature probe and gauge, L, connected to the inline filter are employed for instantaneous measurement of the sample temperature. The sample cylinder accommodates the sample at its sample end, while a precision high-pressure syringe pump provides the desired line pressure. As the circulation pump circulates the sample through the loop, the differential pressure transducer measures the differential pressure across the inline filter element on a continuous basis. The Laboratory Alliance circulation pump that was used is equipped with a fluorocarbon pulse dampener diaphragm to eliminate pressure pulsations. The flow loop, including the inline filter and the differential pressure transducer, is housed in an environmental chamber, while the pressure and temperature gauges, the circulation pump, the syringe pump, and the sample cylinder are outside the chamber. The reason for placing the sample cylinder outside the environmental chamber was to avoid freezing the pressurizing fluid of the precision high-pressure syringe pump. The circulation pump head and fluid flow lines that are outside the environmental chamber are insulated to avoid thermal communication between the circulating fluid and the ambient conditions. (13) Daridon, J. L.; Pauly, J.; Coutinho, J. A. P.; Montel, F. Energy Fuels 2001, 15, 730–735. (14) Monger-McClure, T. G.; Tackett, J. E.; Merrill, L. S. SPE Prod. Facil. 1999, 14, 4–16. (15) Roenningsen, H. P.; Bjoerndal, B.; Hansen, A. B.; Pedersen, W. B. Energy Fuels 1991, 5, 895–908.

WAT under Simulated Pipeline (Dynamic) Conditions

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Figure 1. Apparatus for the measurement of WAT under simulated pipeline (dynamic) conditions: A, nitrogen cylinder; B, constant pressure cylinder; C, vacuum source; D, syringe pump; E, sample cylinder; F, environmental chamber; G, inlet coils; H, inline filter; I, differential pressure transducer; J, line pressure gauge; K, differential pressure gauge; L, temperature gauge; M, circulation pump.

The line connecting the sample cylinder and the loop is coiled inside the environmental chamber to avoid thermal shock that might result from an inflow of additional sample from the sample cylinder to the loop during the test. Hence, the coil serves as a fluid storage, supplying the loop with additional fluid necessitated by a temperature decrease during the test. One important issue about the dynamic test apparatus pertains to the appropriateness of the 50 µm pore size: whether it is too large or too small. The standard ASTM method for the WAT measurement involves visual observation of the first visible wax that appears as the transparent sample is simultaneously cooled and stirred. Logically, the formed wax particle has to be “large enough” to be detectable by the human eye, and this would obviously involve the formation of more than one wax crystal of just 5 µm. Then, the question that follows is “can the human eye see through a 50 µm pore?” Without much thought, the answer to that question is “no”. Therefore, it is obvious that the 50 µm filter element is very appropriate for the dynamic test and adequately meets the ASTM standard. Moreover, as far as the filter size for the filter plugging WAT determination technique is concerned, there are no industry standards. Although, Coutinho and Daridon16 have stated that 0.5 µm filter sizes are commonly used, researchers have used a wide variety of filter sizes; for example, Lindeloff et al.12 used a 0.5 mm (500 µm) filter, whereas Daridon and Dauphin17 used a 3 µm filter in the wax filtration cell. Note that, although pressure (compression) and addition of GTL affect the viscosity of the blends, the dynamic test was not concerned with the measurement of the viscosity of the blends. Rather, the parameter of interest is the trend of pressure drop (∆P) across the inline filter as the sample is simultaneously cooled and circulated in the loop. It should be noted that WAT is independent of the viscosity of the fluid. Whatever the viscosity of the fluid is, the wax in the fluid will appear at its characteristic WAT value. This is because WAT is a property of the wax itself and the fluid. The fact that wax appearance in a fluid affects the viscosity of the fluid should not be confused with the established fact that WAT is a property of the wax and not the fluid. WAT depends upon the wax type and configuration.

3. Sample Preparation The ANS crude oil sample, obtained directly from the TAPS at a facility near North Pole, Alaska, was collected in a constant(16) Coutinho, J. A. P.; Daridon, J. L. Pet. Sci. Technol. 2005, 23, 1113– 1128. (17) Daridon, J. L.; Dauphin, C. Meas. Sci. Technol. 1999, 10, 1309– 1314.

pressure Welker cylinder at the pipeline conditions (about 5.27 MPa and 32.2 °C). At any instant, the composition of the crude oil in the TAPS depends upon the relative contributions from the different ANS oil reservoirs that feed the pipeline. The overall composition of the crude oil used in this study was not determined. It was reported in the work of Ramakrishnan et al.7 However, that composition reflects the relative contributions from the different ANS reservoirs at that time. On average, the ANS oil contains less than 1 mol % of light components (methane, ethane, and propane), whereas C6-C25+ content ranges from 2 to 15 mol %. The GTL was produced from a pilot plant, which is located in Nikiski, Alaska, and uses natural gas from the Alaska Cook Inlet as a feed stock to produce 300 barrels of GTL per day. British Petroleum Alaska, Inc. (BP) owns and operates the plant. At the time that the work was carried out, the primary objective was to only experimentally determine the WAT for the studied mixtures. Hence, the composition of the TAPS oil blend was not compositionally characterized. In the past, we have reported the composition of TAPS oil as well as GTL (different than the one used in this work) when it was feasible for us to do so; for example, see Ramakrishnan et al.7 However, in the present case, we were bound by the confidentially agreement with British Petroleum (BP) Exploration Alaska, Inc. that we will not analyze the composition of the supplied GTL. Therefore, to comply with the requirements of the agreement, the supplied GTL was also not compositionally characterized. The GTL sample was received from BP in 1 gallon stainless-steel containers. According to its material safety data sheets (MSDS), the sample has a weighted melting/freezing temperature of -106.32 °C (-157.6 °F) but it may start to solidify at -56.7 °C (-70.1 °F) based on data for n-octane. The ANS crude oil sample was reconditioned to pipeline conditions before being used in the study, while no reconditioning was necessary for the GTL sample because it was received at ambient conditions. Because the presence of light ends in crude oil reduces the WAT of the crude oil,3 preserving the composition of the ANS crude oil sample was of paramount importance. To achieve this goal, a special procedure was adopted for blending the crude oil and the GTL samples using a modified version of the apparatus shown in Figure 1. The modifications are (refer to

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Figure 1) closure of the valve immediately to the left of the inlet coils (G), placing of sample cylinder (G) inside the environmental chamber, and replacement of the syringe pump (D) with a positive displacement pump. Initially, the sample cylinder was charged with GTL. The floating piston of the sample cylinder was then moved up by means of the positive displacement pump to displace any air in the system. The volume of GTL in the cylinder was noted. The environmental chamber was set to the reconditioning temperature (32.2 °C), and some time (about 20 min) allowed for the GTL temperature to stabilize at this value. The GTL sample was subsequently pressurized to about 0.69 MPa higher than the pressure of the crude oil in the constant-pressure Welker cylinder. The crude oil in the constant pressure cylinder was also pressurized to the same value. The raised pressure ensured that the crude oil sample did not flash into two phases during the transfer process. Once pressure and temperature were stabilized, the GTL and crude oil samples were brought in contact and the positive displacement pump was moved at a slow rate in the negative direction. The nitrogen pressure that was in contact with the other end of the floating piston in the constant-pressure Welker cylinder instantaneously compensated the slight drop in pressure that took place. By this process, transfer of crude oil from the constant-pressure Welker cylinder into the sample cylinder at constant-pressure conditions was achieved. The negative displacement of the positive displacement pump was noted, from which the volume of crude oil transferred, hence, the volumetric blend ratio of the two fluids, was determined. For the static tests, the ANS crude oil and the GTL were blended directly by volume at atmospheric pressure because the static tests were performed at atmospheric pressure. This blending method implied that the crude oil was flashed from the pipeline to ambient conditions. 4. Experimental Procedure for WAT Measurement 4.1. Static Tests. The procedure for obtaining WAT values at static conditions using the ASTM D3117 and the viscometer methods have already been described in a previous section (2.1). Repeating it here is superfluous. 4.2. Dynamic Tests. Prior to measuring the dynamic WAT values of each sample, the sample was first subjected to constant composition expansion using the positive displacement pump, with the sample cylinder placed inside the constant temperature environmental chamber and the valve immediately to the right of the vacuum source, C (see Figure 1), closed. From the generated pressure-volume (P-V) diagram, the bubble point pressure of the sample was determined. The dynamic test measurements were then made at pressures that are above the bubble point pressure of the sample (compressed liquid state) to ensure the light ends in the sample remained in solution during the test. During the dynamic WAT measurement, the syringe pump provided and maintained the sample at the desired test pressure (1.48, 4.24, and 6.99 MPa). The environmental chamber was initially set to 25 °C, and the system allowed time to equilibrate at this temperature. Meanwhile, the circulation pump continued to circulate the fluid at a rate of 3 cm3/min. After the system had stabilized thermally, the environmental chamber was programmed for a temperature ramp of 1 °C/7 min. As the sample was simultaneously cooled and circulated, the differential pressure (∆P) across the inline filter increased as a result of an increase in fluid viscosity. Each increase in ∆P was recorded with the corresponding temperature of the sample, read from the temperature gauge that was connected to the inline

Ijeomah et al.

Figure 2. Determination of static WAT using the viscometric method (blend: 25% ANS oil plus 75% GTL).

filter. At the wax appearance point, there was a sharp rise in the gradient of the plot of ∆P against the sample temperature. The instantaneous plot of ∆P versus the sample temperature enabled identification of the WAT of the sample and gave a good indication as to when to terminate the test. At the end of the test at a particular pressure, the temperature was raised to a high value to melt the wax and return the blend to its original condition. The test pressure was then raised to the next value, and the procedure was repeated. The blend containing the least amount of crude oil was tested first using the procedure described above. The other blends were then produced by increasing the percentage of crude oil (reconditioned) in the sample and tested in the same fashion. The flow was laminar in all of the dynamic tests. Values of the Reynolds number (Re) were in the order of 10-2-10-1. The very small value of Re is not unexpected given the fact that Re is defined as follows: FVD (2) µ In the dynamic test, the values were density ranging from 856.18 to 858.90 kg/m3, velocity of 0.000 25 m/s, pipe internal diameter of 0.0159 m, and viscosity ranging from 26.33 to 177.39 mPa s. In comparison to the 1.588 cm inside diameter of the flow loop tubing, the TAPS inside diameter is 1.2192 m. The average flow rate was 900 000 barrels per day (1.656 m3/s), and prevailing density and viscosity result in a Reynolds number of 15 000. WAT is independent of the shear rate or Reynolds number. As mentioned earlier, it depends upon wax type and configuration. The shear rate or Reynolds number can only affect the deposition of the formed wax on the pipe wall: the higher the Reynolds number of the flow, the less the deposition on the pipe wall because of deviation from the streamlined flow pattern. Thus, whatever the shear rate is, the wax will always appear at its characteristic WAT value. Re )

5. Results and Discussion Figure 2shows an example plot of the linearized Arrhenius equation (eq 1) for obtaining WAT values under nonflowing conditions (atmospheric pressure). As seen in this figure, at the WAT, the plot starts to deviate from the Arrhenius straight line when the abscissa has a value of 0.003 92 K-1. Therefore, the WAT for this blend, in degrees Celsius, is obtained as -18.0 °C. From measured viscosity-temperature data, plots for pure ANS crude oil, GTL, and blends containing 25, 33.33, 50, 66.66,

WAT under Simulated Pipeline (Dynamic) Conditions

Figure 3. Static WAT test result.

Figure 4. Determination of dynamic WAT (blend: 50% ANS oil plus 50% GTL; line pressure of 4.24 MPa).

and 80% of GTL were also prepared and the static WAT values were determined. The overall results are shown in Figure 3. As seen in Figure 3, the ANS crude oil and the GTL had the highest and lowest WAT values, respectively, while the WAT of the blends varied linearly with the percentage of GTL content. The WAT results are obviously reflective of the chemical composition of the ANS crude oil and the GTL, respectively, with the former containing a significant proportion of the high-molecularweight components, which are conducive to wax formation, whereas the latter is comprised of relatively lighter components. The result of the test performed following the ASTM D3117 procedure showed that the GTL sample has a WAT of -33.0 °C. The same value was also obtained using the viscometer method. This perfect agreement validates the accuracy of the WAT results obtained using the viscometer method. It has to be further emphasized that the ASTM test is only possible for transparent samples, such as the pure GTL. Thus, the method was used for the pure GTL sample for the purpose of validating the results obtained for the opaque samples (pure crude oil and the crude oil-GTL blends). All of the studied blends had a bubble point pressure less than 1.48 MPa, which was the lowest line pressure at which dynamic WAT tests were carried out. Figure 4 shows a typical graph of differential pressure across the inline filter versus the inline filter temperature for obtaining the WAT under dynamic or simulated pipeline conditions. The result shown in Figure 4 is for a blend that contains 50% ANS oil and 50% GTL and at a line pressure of 4.24 MPa. As the sample simultaneously cools and flows, the differential pressure across the inline filter increases linearly in response to the increasing blend viscosity. However, at the WAT, the differential pressure begins to

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Figure 5. Dynamic WAT test result.

increase more rapidly because the formed wax clogs the inline filter creating a resistance to the flow of the fluid through the filter. The dynamic WAT results are shown in Figure 5, which clearly indicates a decrease in WAT with an increase in pressure as well as GTL content. As has already been supported in the literature, pressure is one factor that is responsible for the large difference between the WAT values obtained from the static and the dynamic tests. Another factor is the fact that lighter components are preserved in the live oil used for the dynamic tests, while they are absent in the flashed oil used in the static tests. The preserved lighter components actually act as a solvent and retard the appearance of wax, thus prolonging the WAT to significantly lower values. It is also noted that the WAT values show a decrease with an increasing line pressure. This is perceived to be as a result of the fact that the increased line pressure, perhaps, provides an additional driving force for fluid flow, thereby disturbing the equilibrium that facilitates wax formation. As in the static tests, WAT values at dynamic conditions are also influenced by the GTL content in the blend. For each of the studied line pressures, WAT decreases with an increasing GTL content. Although it is largely documented in the literature that the presence of light ends reduces WAT, currently there is no analytical means of determining this extent of reduction in WAT that results from the addition of lighter components. Pan et al.2 state that, on the whole, the effect of pressure and composition on wax precipitation is unclear. However, they state that the cloud point temperature (qualitatively similar to WAT) decreases when a crude is mixed with light hydrocarbons, such as methane, n-pentane, and n-heptane. Below, we review some of the earlier work with regard to the effect of pressure on WAT. In the work of Daridon et al.,18 the liquid-solid transition for four different synthetic multicomponent systems (Figure 3 of their paper) actually show that WAT decreases with an increasing pressure below 10 MPa. Ungerer et al.19 presented WAT for a real high-pressure gas condensate fluid, which showed an unusual trend of WAT versus pressure; i.e., both below and above the convergence of the solid-liquid-vapor phase, the WAT shows an increase as well as a decrease with increasing pressure. However, for the four synthetic gas condensate systems below the solid-liquid-vapor convergence point, WAT increases with increasing pressure, whereas above the convergence point, WAT actually decreases with increasing (18) Daridon, J. L.; Xans, P.; Montel, F. Fluid Phase Equilib. 1996, 117, 241–248. (19) Ungerer, P.; Faissat, B.; Leibovici, C.; Zhou, H.; Behar, E.; Moracchini, G.; Courcy, J. P. Fluid Phase Equilib. 1995, 111, 287–311.

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pressure. Their results thus indicate that the system will have the same WAT at two or even three different pressures. Unusual trends in WAT are also reported in the paper by Daridon et al.13 for two different gas condensate fluids from the North Sea. Starting from a low-pressure region, the WAT increases with increasing pressure (note that the models actually predict the reverse); however, after crossing the liquid-vapor transition line, the WAT decreases with increasing pressure and then reverts to the original trend. This means that above a certain pressure WAT will be the same for two different pressure conditions. Peters et al.20 reported WAT data as a function of the pressure for various fixed composition ethane and eicosane mixtures. The majority of the low eicosane concentration mixtures exhibited a decrease in WAT with increasing pressure. Mei et al.21 reported WAT versus pressure (both above and below the saturation pressure) for a Chinese reservoir fluid. The reported data shows a decrease in WAT with increasing pressure. Alboudwarej et al.22 presented WAT data as a function of pressure for a West African waxy oil, in which they reported a decrease in WAT with increasing pressure from stock tank conditions (atmospheric pressure) to pressure just below the bubble point. In the case of the studied oil, they state that most of the dissolved gas is released as the fluid pressure is lowered below the bubble point and WAT increases; however, the amount of gas released within a pressure depletion of 4 MPa is insignificant. Riazi23 stated that pressure behaves as an inhibitor for wax precipitation for live oils, gas condensates, or natural gases, which is not the case for heavy liquid oils, such as crude oils or dead oils. Similarly, on the basis of the results reported by Pan et al.,2 Riazi23 has restated the conclusions that for heavy oils at low pressure or live oils (where light gases are dissolved in oil) the increase in pressure decreases the cloud point temperature. It should be noted that the WAT measurements under dynamic conditions were carried out using live oil samples; i.e., TAPS oil under representative pipeline conditions was used. This means that light components typically intermediates (n-pentane being one of them among others), albeit small, were still maintained in a dissolved state in the oil for which special precaution was taken (transfer under live pipeline conditions). Our measurements for the studied conditions indicate a decrease in WAT with an increasing pressure. Therefore, in view of the various results discussed above, we believe that our measured WAT are representative for the tested conditions and not unusual per se.

Ijeomah et al.

pipeline (dynamic) conditions. The static WAT values were measured using the ASTM D3117 technique (for pure GTL) and the viscometric method. An indigenously designed and developed flow loop was used to determine the dynamic WAT values for different ANS crude oil and GTL blends at various system pressures, which is considered as a valuable technique suitable for the determination of the WAT of almost any highpressure hydrocarbon system. In general, at the simulated pipeline condition, the samples had significantly lower WAT values compared to atmospheric pressure because the lighter components were retained in the former condition, while they were flashed in the later. The addition of GTL to the ANS crude oil was found to decrease WAT. An increase in the line pressure was also found to decrease the WAT of the samples. The results obtained in this study indicate that, for all of the samples, WATs are well below the minimum TAPS operating temperature range. Because flow and pressure significantly reduced the WATs, for all of the studied samples, the possibility of wax formation during the normal operation of TAPS can be safely precluded. However, wax deposition is a potential major problem during prolonged pipeline shutdown in the peak winter months when ambient temperatures could drop to values significantly lower than the measured WATs. In summary, on a much broader scale, the measured WAT data will provide significant support to the project upon evaluating the transportability of GTL through the existing TAPS. Similarly, other projects that are potentially considering such type of pipeline transportation can also draw useful parallels from the present work. For example, the developed experimental apparatus, the measurement techniques, and the results at least qualitatively are applicable/valuable to any pipeline systems that transport hydrocarbon liquids (crude oils and/or their blends with diluents) under various pressure and temperature conditions, such as, long production tubings, subsea oil gathering umbilicals, networks of surface pipeline installations, long distance pipelines, or in summary, any solid paraffin deposition-related flow assurance investigations. Acknowledgment. The authors gratefully acknowledge Alyeska Pipeline Service Company (APSC) for providing TAPS oil samples. They also thank British Petroleum Exploration Alaska, Inc. for providing GTL samples.

Nomenclature 6. Conclusions The WATs of blends of ANS crude oil and GTL products were successfully measured at static as well as simulated (20) Peters, C. J.; de Roo, J. L.; Lichtenthaler, R. N. Fluid Phase Equilib. 1991, 65, 35–43. (21) Mei, H.; Zhang, M.; Li, L.; Li, S. Proceedings of the International Petroleum Conference, Canadian Petroleum Society, Calgary, Alberta, Canada, June 2002. (22) Alboudwarej, H.; Huo, A.; Kempton, E. SPE 2006, 103242. (23) Riazi, M. R. Characterization and Properties of Petroleum Fractions; American Society for Testing and Materials (ASTM): Philadelphia, PA, 2005.

A ) material constant (mPa s) D ) pipe internal diameter (m) Ea ) energy required to overcome the internal friction (J/mol) R ) gas constant (J mol-1 K-1) Re ) Reynolds number (dimensionless) T ) absolute temperature (K) V ) velocity (m/s) Greek Letters F ) density (kg/m3) µ ) viscosity (mPa s) EF7007453