Article pubs.acs.org/EF
Contribution to a More Reproductible Method for Measuring Yield Stress of Waxy Crude Oil Emulsions Carla Barbato,† Bruno Nogueira,† Marcia Khalil,‡ Roberto Fonseca,‡ Marcelo Gonçalves,‡ José C. Pinto,§ and Márcio Nele*,†,§ †
Departamento de Engenharia Química, Escola de Química, Universidade Federal do Rio de Janeiro, Cidade Universitária, Rio de Janeiro, Rio de Janeiro 21941-909, Brazil ‡ Centro de Pesquisa Leopoldo Américo Miguez de Mello (CENPES), Petrobras, Cidade Universitária, Rio de Janeiro, Rio de Janeiro 21941-598, Brazil § Programa de Engenharia Química da COPPE, Universidade Federal do Rio de Janeiro, Cidade Universitária, CP 68502, Rio de Janeiro, Rio de Janeiro 21945-970, Brazil ABSTRACT: This study looked to improve reproducibility in the procedure to determine the yield stress of water-in-oil (W/O) emulsions prepared with waxy crude oil. The influence of various experimental variables was studied: measurement geometry, emulsion cooling rate, shear stress during the cooling step, gap reduction (parallel plates), final gap (parallel plates), and conditioning steps. The measured yield stress varied significantly depending upon the measuring geometry used (from 100 to 500 Pa). The geometries cross-hatched parallel plate (D = 60 mm) and grooved coaxial cylinder measured the highest yield stress, and it was observed that the shear stress during cooling was the most important measurement variable in the emulsion yield stress. where ΔP, τ, L, and D represent the applied pressure drop, yield stress of the gel, and pipeline length and diameter, respectively. The factors that influenced the strength of a wax gel network has been investigated by several researchers.2,10,13−15 It is reported that a number of factors influence the yield stress, including the cooling rate, composition of the oil, shear history, thermal history, and aging time.5,16−18 Kané et al.19 showed that the waxy structure varies gradually from a pure liquid (Newtonian behavior) via a viscoplastic gel to a solid-like structure (shear thinning behavior) when the sample is statically cooled. When the sample is quiescently cooled, according to Zhao et al.,16 Lin et al.,17 and Kané et al.,19 the yield stress decreases with the increase of the cooling rate because of the formation of smaller wax crystals.5,17 However, when cooling under shear, the yield stress increases with the increase of the cooling rate. A lower cooling rate results in a longer time that the sample is subjected to shear/stress, which destroys the structure formed by the waxy crystals, resulting in a weak wax gel. Visintin et al.20 and Chang et al.10 researched the influence of the temperature holding time on the wax gel strength. Visitin et al.20 showed that the gel strength increases with the increase of the holding time and obtained an asymptotic value after 4 h. In contrast, in the results by Chang et al.,10 the waxy crude oil was isothermally and statically held for different times (from 0 to 30
1. INTRODUCTION Paraffin particles can adsorb on the surface of emulsified water droplets by interacting with natural surfactants of crude oil as asphaltenes, resins, organic acids and bases.1 Upon cooling, wax crystals can overlay the water droplet surface and build a gel network in the continuous phase. In this condition, the free movement of water droplets is hindered, contributing to high values of viscosity and yield stress.2,3 Furthermore, these emulsions are very stable as a result of the resistant film formation at the crude oil/water interface, resulting in a physical barrier between the dispersed and continuous phases, which contributes to a decrease of the water drop coalescence rate.4−7 The exploitation of waxy crude oils poses important challenges. During crude oil transportation, temperature and pressure decrease, which may cause operational problems, including pipeline blockage. The transportation of waxy crude oil emulsions through submarine pipelines is interrupted for normal operational or emergency procedures. In this case, a temperature gradient between the crude oil and the environment leads to wax crystallization. Wax particles can deposit on the tube inner wall, forming a solid layer that reduces the oil flow rate.8,9 During a prolonged shutdown, wax particles interact by van der Waals forces, forming a high-strength gel that can cause serious operational problems.1,10 To restart a waxy crude oil emulsion flow, the network structure of the gel must be destroyed by a large pressure drop across the pipeline (eq 1).2,11,12 To estimate the pressure drop, it is necessary to determine the minimum stress to produce the shear flow, i.e., yield stress13 ΔP =
4τoL D
Special Issue: 14th International Conference on Petroleum Phase Behavior and Fouling Received: October 3, 2013 Revised: January 27, 2014 Published: January 28, 2014
(1) © 2014 American Chemical Society
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emulsion to 50 mL of cyclohexane). The average diameter of the particles was determined as a function of the total volume of the droplets. The average water droplet size in W/O emulsions prepared using oil A was measured in an Axiovert 40 inverted microscope to ensure emulsion preparation reproducibility. The samples were diluted spindle in a ratio of 1:30 (1 g of emulsion to 30 g of spindle). The diameter of 100 droplets (magnification of 1000 times) was measured to calculate the average diameter. 2.4. Yield Stress Measurement. 2.4.1. Experimental Procedure. The working procedure to measure the yield stress (Figure 1) of a
min) and the wax gel strength was similar for different holding times. Tinsley et al.18 and Venkatesan et al.21 studied the influence of the asphaltene concentration on the yield stress of waxy crude oils. It was verified that the asphaltene concentration did not influence the yield stress when the waxy concentration is high and the concentration of asphaltenes is small. However, when the asphaltene concentration is high, the wax gel is weak and the yield stress of the oil is small. In the literature, there is not a standard test to estimate the yield stress of waxy crude oils; the reasons are poor repeatability in any equipment and poor reproducibility between the tests. 10,13 The possible causes for poor reproducibility are different thermal and shear history of the oil, distinct methods of measuring yield stress because of the lack of understanding of yielding phenomena, and the usual instrumental effects, such as wall slip, instrument inertia, and damping characteristics of the rotation body. In this study, the main objective is to investigate different experimental procedure to determine the yield stress of waxy water-in-oil (W/O) emulsions. The influence of various variables was studied: measurement geometry, emulsion cooling rate, shear stress cooling step, gap reduction (parallel plates), final gap (parallel plates), and conditioning steps. The conditioning steps are required to stabilize the sample temperature and eliminate the shear history before the cooling step as well as to allow for complete precipitation of wax crystals before the step in which the yield stress will be determined.
2. MATERIALS AND METHODS 2.1. Materials. Three different waxy crude oils supplied by Petrobras identified as A, B, and C were used to prepare the W/O emulsions used to carry out the experiments. Milli-Q water was used in all experiments. NaCl (pro analysis) was supplied by Vetec. S.A. 2.2. Crude Oil Properties. The density values of waxy crude oils A, B, and C, at 25 and 4 °C, were determined using Anton Par SVM 3000 Stabinger densimeter and viscosimeter. The water content of crude oil was measured by potentiometric Karl Fisher (KF) titration using a Metrohm 836 KF titration equipment. KF titrations were repeated 3 times for each crude oil, and the results were averaged. The wax appearance temperature (WAT) was measured in a Pyris Diamond differential scanning calorimeter from Perkin-Elmer. The temperature scale was calibrated using a two-point calibration, obtaining the onset temperature from indium and zinc standards. Measurements of flow heat in the crude oils were obtained in the temperature range between 80 and 0 °C. The cooling rate used was 1 °C/min. 2.3. Preparation of the W/O Emulsion and Droplet Size Distribution Measurement. W/O emulsions were prepared using synthetic brine consisting of 5.0 wt % NaCl in Milli-Q water, at aqueous volume fractions of 50%. The crude oil was pretreated to erase the thermal history of previous wax formation. The pretreatment was carried out for a 50 mL crude oil at 80 °C for 2 h in a sealed 100 mL glass bottle. Then, the sample was cooled naturally, without temperature control (Tinitial = 80 °C and Tfinal = 25 °C), in an airconditioned laboratory for 22 h. The emulsification procedure was shearing 50 mL of oil and 50 mL of brine using a Polytron PT 3100 homogenizer at 8000 rpm for 3 min, at 25 °C in a 150 mL beaker. The droplet size distribution of the W/O emulsion was measured using a dispersion analyzer lumisizer from LUM GmbH. This equipment is a multi-sample analytical centrifuge that measures the intensity of transmitted light as a function of time, position, and rotational speed over the entire sample length simultaneously. The emulsion was diluted in the proportion 1:50 in cyclohexane (1 mL of
Figure 1. Procedure for measuring yield stress of waxy W/O crude oil emulsions. “X” represents the values of experimental variables used in the experimental design. waxy crude oil emulsion consisted of five steps: first step, emulsion conditioning in the rheometer at 25 °C, to equilibrate the sample temperature; second step, sample cooling to promote wax precipitation and formation of the gelled waxy crude oil network; third step, gap reduction, if parallel plate geometry was used; fourth step, sample conditioning at 4 °C, to favor a more complete precipitation of wax crystals; and fifth step, yield stress determination of the W/O emulsions prepared with waxy crude oil. Experimental runs were performed according to the procedure of Figure 1, but a low disturbing rheological measurement was included before the yield stress measurement to monitor the sample during the steps that precede the yield stress measurement by the oscillatory sweep. In the first stage (sample equilibration at room temperature), an oscillatory test was performed, with low values of frequency ( f = 0.2 Hz) and oscillation stress (τ = 0.1 Pa). The second step performed a quiescent cooling condition. The gap speed reduction was 10 μm/s. In the fourth stage (sample equilibration at 4 °C), an oscillatory test was performed for a period of 15 min, with low values of frequency ( f = 0.2 Hz) and oscillation stress (τ = 0.1 Pa). In the last step, the yield stress was determined by the oscillatory test (stress sweep, f = 1 Hz, τ = 0.1− 1000 Pa, and 60 points per decade). The sample equilibration at room temperature and 4 °C stages was performed with low values of frequency and oscillation stress to obtain lower strain, near 5 × 10−4%, which was reported by Jia and Zhang,22 which avoids perturbation of the structure formed in the sample. The experimental variables chosen to investigate the influence of the measurement procedure on the yield stress of W/O emulsion were measuring geometry, average cooling rate (X2), shear stress applied 1718
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during cooling (X3), gap reduction size (X4) (parallel plates), final gap (parallel plates), and duration of the conditioning steps (X1 and X6). The method chosen to measure the yield stress, in the last step of the procedure (Figure 1), was the stress amplitude sweep because of its simplicity and the short time required to evaluate this rheological property. The yield stress was defined as the point where the storage modulus is equal to the loss modulus. The experiments were carried out with the frequency equal to 1 Hz, and the shear stress range was 0.1−1000 Pa. The rheological tests were carried out at AR-G2 from TA Instruments. Temperature control was provided by Peltier elements. The sample was loaded into the rheometer using a syringe. This study was split into two steps: performance of a reproducible procedure to measure the yield stress of waxy crude oil emulsion and achievement of experiments with other samples of waxy crude oil emulsion to lend credibility to the procedure. In the first step, the W/ O emulsion prepared with crude oil A was used, and the second step was performed with the W/O emulsion prepared with the waxy crude oils A, B, and C. 2.4.2. Influence of Measuring Geometries. It is important to evaluate different geometries in rheological tests to verify if the wall slip effect is present. The influence of the following geometries was verified: coaxial cylinder (Dinner = 13 mm, Douter = 14 mm and H = 44 mm), grooved coaxial cylinder (Dinner = 15 mm, Douter = 17 mm and H = 44 mm), cross-hatched parallel plates (D = 60 mm), sandblasted parallel plates (D = 40 mm), parallel plates (D = 40 mm) with smooth surfaces, parallel plates (D = 40 mm) with the top and bottom surfaces covered by sandpaper (240 grains/cm2) and the top surface covered by different sandpapers (180, 220 and 240 grains/cm2). Table 1 shows the values of the variables X1, X2, X3, X4, X5, and X6 used to determine, with different geometries, the yield stress of W/O
Table 3. Experimental Conditions According to a Factorial Experimental Design 2(4−1) with Center Point experimental condition
dT/dt (°C/min)
τ (Pa)
gap reduction (μm)
initial gap (μm)
final gap (μm)
1 2 3 4 5 6 7 8 9
1.0 0.1 0.1 1.0 1.0 0.1 0.1 1.0 0.55
1 1 10 10 1 10 1 10 5.5
493 205 493 205 307 307 739 739 417
1493 1205 1493 1205 1807 1807 2239 2239 1667
1000 1000 1000 1000 1500 1500 1500 1500 1250
each experimental condition, five replicas were run to calculate the average and confidence interval using Student’s t distribution. The experimental data from rheological measurements were treated using regression analysis based on the existing methodology for analyzing the design of experiments.23 The yield stress (τ0) of W/O emulsion was fitted by a polynomial equation (eq 2). The independent variables (Xi) are final gap (FG), gap reduction (GR), shear stress applied during the cooling step (SS) and cooling rate (CR). The estimated coefficients are represented by the indexed variable a, where ai is the linear coefficient of the variable i (final gap, gap reduction, shear stress and cooling rate) and aij is the coefficient of the interaction between the variables i and j. 4
τo = a0 +
i
Table 1. Experimental Conditions (Figure 1) Used To Investigate the Influence of the Geometry on the Measured Yield Stress variable
value
initial gap (parallel plates) duration of sample equilibration at the 25 °C step (X1) cooling rate (X2) shear stress (X3) gap reduction (X4) gap reduction speed (X5) duration of sample equilibration at the 4 °C step (X6)
1200 μm 15 min 0.8 °C/min 0 Pa 200 μm 10 μm/s 15 min
Table 4. Experimental Conditions (Figure 1) Used To Investigate the Influence of the Cooling Rate on the Measured Yield Stress
level 0
+1
final gap (μm) gap reduction (%) cooling rate (°C/min) shear stress (Pa)
1000 17 0.10 1.0
1250 25 0.55 5.5
1500 33 1.0 10
variable
value
initial gap duration of sample equilibration at the 25 °C step (X1) shear stress (X3) gap reduction (X4) gap reduction speed (X5) duration of sample equilibration at the 4 °C step (X6)
1205 μm 15 min 1 Pa 205 μm 10 μm/s 15 min
To verify the influence of the gap reduction speed on the yield stress, tests were carried out with the following gap reduction velocities (X5): 1, 10, and 100 μm/s. The geometry used was cross-hatched parallel plates (D = 60 mm). Table 5 shows the values of X1, X2, X3, X4 and X6 of the procedure (Figure 1) used to determine the yield stress of W/O emulsion. To evaluate the influence of the sample equilibration duration at 25 °C (first step of the procedure; Figure 1), tests were performed with the following conditioning times: 0, 15, 30, 60 and 90 min. Table 6 shows the values of the cooling rate (X2), shear stress applied during the cooling step (X3), initial gap, final gap and gap reduction speed (X5). To verify the influence of the sample equilibration duration at 4 °C on the yield stress of W/O emulsion, tests were performed with the following conditioning times: 5, 15, 30 and 45 min. Table 7 shows the
Table 2. Experimental Design Variables
−1
(2)
i
