Application and mechanism of ultrasonic static mixer in heavy oil viscosity reduction

Accepted Manuscript Application and Mechanism of Ultrasonic Static Mixer in Heavy Oil Viscosity Reduction Chunwei Shi, Wei Yang, Jianbin Chen, Xiaoping Sun, Wenyi Chen, Huiyong An, Yili Duo, Mingyuan Pei PII: DOI: Reference:

S1350-4177(17)30078-0 http://dx.doi.org/10.1016/j.ultsonch.2017.02.027 ULTSON 3564

To appear in:

Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

14 October 2016 17 February 2017 17 February 2017

Please cite this article as: C. Shi, W. Yang, J. Chen, X. Sun, W. Chen, H. An, Y. Duo, M. Pei, Application and Mechanism of Ultrasonic Static Mixer in Heavy Oil Viscosity Reduction, Ultrasonics Sonochemistry (2017), doi: http://dx.doi.org/10.1016/j.ultsonch.2017.02.027

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Application and Mechanism of Ultrasonic Static Mixer in Heavy Oil Viscosity Reduction Chunwei Shi1,2, Wei Yang1, Jianbin Chen2,Xiaoping Sun1, Wenyi Chen1, Huiyong An1, Yili Duo1,Mingyuan Pei3 1.Liaoning Shihua University, Fushun 113001,China 2. Administration Center of the Yellow River Delta Sustainable Development Institute of Shandong Province, Dongying 257001, China 3.Department of Applied Organic Materials Engineering, Inha University, 100 Inharo, Nam-gu Incheon 402-751, Korea.

Abstract:In the present study, heavy oil viscosity reduction in Daqing oil field was investigated by using an ultrasonic static mixer. The influence of the ultrasonic power on the viscosity reduction rate was investigated and the optimal technological conditions were determined for the ultrasonic treatment. The mechanism for ultrasonic viscosity reduction was analyzed. The flow characteristics of heavy oil in the mixer under the effect of cavitation were investigated using numerical modeling, and energy consumptions were calculated during the ultrasonic treatment and vis-breaking processes. The experimental results indicated that the ultrasonic power made the largest impact on the viscosity reduction rate, followed by the reaction time and temperature. The highest viscosity reduction rate was 57.34%. Vacuole was migrated from the axis to the wall along the fluid, accelerating the two-phase transmission and enhancing the radial flow of the fluid, which significantly improved the ultrasonic viscosity reduction. Compared to the vis-breaking process, the energy consumption of ultrasonic treatment process was 43.03% lower when dealing with the same quality heavy oil. The optimal process conditions were found to be as follows: ultrasonic power of 1.8 kW, reaction time of 45 min and reaction temperature of

360 °C. The dissociation of the molecules of heavy oil after ultrasonication has been checked. After being kept at room temperature 12 days, some light components were produced by the cavitation cracking, so the viscosity of the residual oil could not return to that of the original residual oil, which meant that the “cage effect” was not reformed. Keywords:Ultrasonic; Heavy oil; Viscosity; Orthogonal Test; Energy consumption;Mechanism

Introduction Properties of heavy oil, such as high viscosity, high density and poor fluidity at room temperature lead to increased cost for its exploitation and transportation as well as frictional lossesin the heavy oil pipelines. Various new methods,including hydrothermal

catalytic

cracking[1-2],

microbiological

method[3],

magnetic

treatment[4] and ultrasonic treatment[5] have been investigated for reducing the viscosity[6].However, the reduction in viscosity using hydrothermal catalytic cracking and microbiological methodshas disadvantages,such aslow activity and poor adaptability of the catalyst, as well as incomplete microbial degradation of polymers, including asphaltene and colloid, which lead to poor viscosity reduction[7]. For magnetic viscosity reduction technology, the physical effect of viscosity reduction decreased along with the time owing to the short interaction time between the heavy oil and magnetic field, which increased the complexity of the operation. Comparatively, ultrasonic viscosity reduction method has the advantages of easy operation, mild conditions, high efficiency, low cost, and environment friendliness,

and therefore, has become a promising, highly efficient new method for viscosity reduction of heavy oil[8]. However, at present, most of the ultrasonic devices are used at low temperatures, and the reported ultrasonic methodsare conducted with low-powered horizontal or vertical uni-directional sonic waves. The viscosity reduction mechanism of heavy oil using ultrasonic treatment has seldom been performed to explain the cavitation phenomenon through establishing a fluid model. In this work, a self-developed autoclave ultrasonic reactor was utilized to investigate theultrasonic viscosity reductionofheavy oilat high temperature with horizontal and vertical bi-directional sonic waves. The flow characteristics of heavy oil in the mixer were also studied under the effect of cavitationby undertaking numerical modeling, while the energy consumption was calculated during the ultrasonic treatment and the visbreaking process, which provided practical foundation and theoretical support for the utilization of ultrasonic technology to large-scale industrial production. 1. Experimental 1.1. Properties of raw material and experimental devices Daqing heavy oil was used as the raw material for the experiments. The heavy oil was dehydrated for the subsequent ultrasonic viscosity reduction experiments. The sound frequency is 28kHz. The properties of the dehydrated heavy oil are given in Table 1. ZNHW-II-type intelligent PID temperature controller was obtained from Shanghai Yuzheng Equipment Co., Ltd., China, while NDJ-79-type rotating viscometer was

obtained from Shanghai Precision Instrument Co., Ltd., China. The experimental apparatus consisted of an autoclave ultrasonic static mixing reactor (self-developed), which was composed of five parts, namely the ultrasonic generator, PID, autoclave reactor, ultrasonic transducer and a SK static mixer.The experiment can be performed upto temperatures of around 500 °C and pressures of 15 MPa. The experimental apparatus is shown in Figure 1.The equipment used the ultrasonic waves produced by the ultrasonic equipment, and introduced cavitation phenomenon in the static mixer, thus increasing the turbulent intensity of the raw materials in the device, which realizes fast and highly efficient viscosity reduction. The wall of the autoclave was in contact with the heat conducting oil bath to control the reaction temperature within the autoclave. The maximum power of this apparatus was 200 W, which worked in the range of 220 V/50 Hz±10% with the surrounding temperature of 0 - 40 °C, humidity of less than 85% (T=20 °C) and transducer temperature of 0 - 120 °C. The apparatus was capable of reducing the acoustic energy consumption and distributing the acoustic field uniformly. 1.2. Experimental methods A certain amount of heavy oil was added to the autoclave reactor and heated to a specific temperature using the temperature controller. After ultrasonic treatment for a specified time, the oil precipitated and after 30 min of precipitation, it was separated. The sample was cooled to 50 °C and then, the dynamic viscosity was measured using a viscometer. The viscosity reduction rate (V) was calculated according to Equation (1).


= (
 −
 )⁄
 × 100%

(1)

where V1 (Pa·s) is the viscosity at the beginning of the reaction, and V2 (Pa·s) is the viscosity at the end of the reaction. 2. Numerical simulation 2.1. Model establishment The SK static mixer was placed in the ultrasound field. Fluent 12.1 has cavitation simulation module, so there was no need to establish the model for ultrasound device. The model establishment was only necessary for the SK static mixer. 2.2. Control equation Due to the existence of cavitation, the fluid in the ultrasonic static mixer exhibited a complex state of unsteady cavitation, which belonged to the vapor-liquid two-phase turbulent flow. Using mixture model and whole cavitation model for the simulation, the basic control equations were obtained as Equations (2-4) [9]. Steam generation rate is given by Equation (2).

 = 

√ 

   

  [

 

] 1 − ! − !" # (2)

Continuity equation for the mixed phase is given by Equation (3). % %

() + ' ∙ (
) = 0

(3)

Continuity equation for vapor cavitation is given by Equation (4). ) )

*+ # =  − , (! ) + ' ∙ !


(4)

In these equations, ρ (kg/m3) is the mass density of mixed fluid of the flow and cavitation phase, v (m/s) is the velocity vector of the mixed fluid, fv is the volume fraction of the cavitation,

is the velocity vector, Re is the steam production rate, Rc is

the steam condensation rate, Ce is the evaporation coefficient, Cc is the condensation coefficient and σ (N/m) is the surface tension. The parameters with constant values are given as follows: fv=3540 Pa; density of H2O ρl=1000 kg/m3; density of water vapor pv=0.02558 kg/m3. 3. Results and discussion 3.1. Influence of the ultrasonic power on viscosity reduction rate Figure 2 shows the influence of reaction time on viscosity reduction rate for different values of ultrasonic power at reaction temperature of 100 °C. The viscosity reduction efficiency of heavy oil increased with the increase in ultrasonic power. The highest viscosity reduction rate was 47.61%, which was observed for ultrasonic power of 1.8 kW. The cavitation effect of ultrasonic produced extreme conditions, such as instantaneous high temperature and high pressure, which broke the heavy macro-molecules in the viscous oil and damaged the molecular structure, thus reducing the oil’s viscosity. Ultrasonic power showed a direct relationship with the intensity of the ultrasonic cavitation effect. Higher the power, stronger is the energy being supplied to the system, and more vigorous will be the ultrasonic cavitation phenomenon. This will produce relatively high number of hydrocarbon radicals, which increase the probability of heavy oil cracking and viscosity reduction, thus significantly promoting the viscosity reduction process. 3.2. Orthogonal analysis of the ultrasonic viscosity reduction conditions Orthogonal analysis was designed according to the obtained experimental results to investigate three parameters, namely the reaction temperature (A/°C), reaction time

(B/h) and ultrasonic power (C/kW). Three levels were chosen for each parameter and the results were presented in Table 3. The orthogonal experimental results and analysis are given in Table 4. According to the values of R in Table 4, it can be seen that the ultrasonic power had the greatest influence on viscosity reduction rate, followed by the reaction time and temperature. The 19th experiment showed the best viscosity reduction rate of 57.34% with a level combination of A3B2C3, which represents ultrasonic power of 1.8 kW, reaction time of 45 min and reaction temperature of 360 °C. 4. Mechanism analysis of the ultrasonic viscosity reduction The molecules of heavy oil were surrounded by long-chain radicals, resin and asphaltene, such that the "cage effect" of heavy oil was easily formed for the generally applied heavy oil vis-breaking technology. The collision frequency of the molecules in "cage" increased, and hence initiated a series of chemical reactions, which led to a mild pyrolysis of the oil to produce light oil and gas with a certain mass fraction. This increased unnecessary energy consumption and decreased the utilization efficiency of the energy. The technology commonly used in petroleum processing is the heavy oil viscosity reduction technology. Although most of the modified methods could reduce the occurrence probability of "cage effect", the disadvantages of "cage effect" towards theproperties of oil and energy consumption cannot be completely avoided. Ultrasonic viscosity reduction technology prevented the formation of "cage effect" and increased the viscosity

reduction effect through mixing, homogenization and stirring effectsdue to vigorous cavitation interaction and mechanical vibration [10].Ultrasonic viscosity reduction technology conducted depolymerization and chain scission of big molecules in heavy oil, including long chain colloid and asphaltene, and simultaneously decreased the energy consumption during the process. Before ultrasonic interaction, molecules of heavy oil were surrounded by larger molecular chains, including long chain free radicals, colloid, asphaltene and so on, which were easy to twist, and formed the“cage structure”. After ultrasonic interaction, the ultrasonic wave spread within the elastic media, and significantly increased the amplitude, velocity and accelerated velocity of the elastic particles. Mechanical vibration effect led to a strong relative movement between the smaller molecules and big molecular chains of great inertness in heavy oil, enhanced the frictional force among molecules, broke C-C bond, destroyed the “cage effect” formed by big molecular clusters and played a role in viscosity reduction. 4.1. Analysis of the influence of cavitation on the static mixer Steam rate (vapor phase volume fraction) is an important parameter for cavitation investigation and can intuitively reflect the whole process, including primary cavitation position of the fluid in the static mixer, growth and expansion of the cavity with the fluid flow, and ultimate shrinkage and disappearance with increased pressure [11]. According to the characteristics of cavitation and fluid, it (the cavitation) belonged to travelling cavitation. The travelling cavitation is referred to the cavitation phenomenon which is accompanied by the formation of an unstable cavity within the

fluid and flows with the fluid. It is generally generated around the wall boundary, vortex center or the low-pressure region in the fluid. The relevant experimental parameters were substituted in Equations (1) - (3) as given in Section 2.2.The results were substituted in the model given in Section 2.1 for fitting. A changing nephogram of vapor phase volume fraction along the axis was observed (Figure 3). According to Figure 3(a), the cavitation of ultrasonic static mixer occurred at the axis. After the heavy oil flowed into the static mixer, due to the higher fluid velocity in axis, cavitation firstly occurred to form the cavity. According to Figure 3(b), the location of the highest vapor volume fraction shifted towards the wall, indicating that the cavity was shifted along the radial direction of the component due to the influence of fluid flow. It was the most significant feature of the travelling cavitation [12]. Figure 3(c) shows the shrinkage phenomenon of the cavity with the increase in pressure. Therefore, the reason for significant increase in the viscosity reduction effect by ultrasonic can be described as follows: in travelling cavitation model, the cavity shifted from axis to the wall along the fluid, accelerating the two-phase transmission and enhancing the radial flow of the fluid [12],which resulted in the increase in viscosity reduction. 4.2. Thermodynamic energy consumption analysis of ultrasonic treatment Ultrasonic treatment not only depolymerized and broke the long chains of macro-molecules, such as resin and asphaltene in the heavy oil, but also significantly reduced the thermal energy consumption in the process. In order to verify the energy saving advantage of ultrasonic viscosity reduction technology, the thermal energy

consumptions of two processes were compared. (1) Heavy oil viscosity reduction through ultrasonic treatment According to the 15th experiment, ultrasonic power of 1.2 kW, reaction time of 45 min and reaction temperature of 240 °C were selected for energy consumption calculations. The measured viscosity reduction was 37.68% during the process. The enthalpy values for the oil at 20 °C and 240 °C were 14 kcal· kg-1 and 158 kcal· kg-1, respectively [13].Let the energy needed for heating 1 kg oil be Q, energy consumption for 45 min ultrasonic treatment be WU and total absorbed heat by heat transfer through oil bath be Qo. Then, the total energy consumption for ultrasonic heavy oil treatment W can be given by Equation (2-4). (2) Vis-breaking process Vis-breaking of heavy oil was conducted in the device without using ultrasonic.1 kg oil sample was heated from 20 °C to 380 °C. The enthalpy value forthe oil at 380 °C was 220 kcal· kg-1[14]. The oil was cooled to 50 °C after the reaction and the measured viscosity reduction rate was 37.35%. Let the energy needed for heating 1 kg oil be Q', and energy consumed for maintaining the heating equipment at 380 °C for 3 h be WH. Then, the total energy consumption for the vis-breaking process W' can be given by Equation (2-4). The energy consumption of 1 kg heavy oil vis-breaking process was 3021.08 kJ with the viscosity reduction rate of 44.23%. Comparatively, the energy consumption of ultrasonic treatment for heavy oil having the same quality was 1721.12 kJ with the viscosity reduction rate of 44.69%. Although the viscosity reduction rate was similar,

the energy consumption was reduced by 43.03%. The two processes were conducted in the same autoclave. Due to this, the material balance was accurately calculated and the experimental data was reasonably reliable. 5 Stability of the viscosity reduction effect by ultrasonication After being kept at room temperature, the viscosities of both the oil samples(one treated by ultrasonication with output voltage of 250 V, reaction temperature of 360 °C and reaction time of 1 h, and that without the ultrasonic treatment) were measured at 40 °C. The obtained curve of the viscosity reduction ratiois shown in Figure 4. It can be seen from Figure 4 that the viscosity reduction ratio of both the oil samples with and without the ultrasonic treatment at 40 °C decreased gradually with the increase in time. After 12 days, the viscosity reduction ratio decreased slowly and remained almost constant, which was probably due to the dual function of ultrasonic, whereas the heat showed dissolution effect towards the smaller molecules in residual oil micelle[15]. As a result, the intrinsic colloidal equilibrium of the system was broken, weakening the block of mobility by colloidal and asphaltene molecules. At 40 °C, the initial viscosity reduction ratio of the residual oil was high, and the residual oil system was in dynamic equilibrium. Along with the prolongation of storage time, some of the original dissolving small molecules were gradually absorbed by the asphalt colloidal, and promoted gradual increase of the viscosity. However, some light components were produced by the cavitation cracking, so the viscosity of the residual oil could not return to that of the original residual oil, which meant that the “cage

effect” was not reformed. Conclusions (1) The best viscosity reduction effect for Liaohe heavy oil was obtained with the ultrasonic power of 1.8 kW, reaction temperature of 360 °C and reaction time of 30 min. The viscosity reduction rate was 57.34%. (2) With the extension of ultrasonic treatment time, the oil’s viscosity reduction rate firstly increased and then became constant. Under the same processing temperature, higher viscosity reduction was obtained for higher ultrasonic power, and vice versa. Under a certain ultrasonic power, appropriate increase in the reaction temperature led to increased viscosity reduction rate. (3) The cavity shifted from the axis to the wall along the fluid, accelerating the two-phase transmissionand enhancing the radial flow of the fluid, which were the main reasons for significantlyincreasedviscosity reduction effect by ultrasonic method. (4) Compared with the vis-breaking technology, the ultrasonic viscosity reduction technology was more energy-efficient. In comparison to the vis-breaking technology, the energy consumption for the ultrasonic viscosity reduction technology was less by 43.03% for processing oils of the same quality. (5) At 40 °C, the initial viscosity reduction ratio of the residual oil was high, and the residual oil system was in dynamic equilibrium after standing for12 days. Acknowledgments: The authors acknowledge the financial support from the Research Fund for the Doctoral Program of Higher Universities (20100042110008), Fund for Research Project of Education and Teaching Reform of Liaoning Shihua University (20165230080023), Fund for Liaoning Shihua

University Students Innovation and Entrepreneurship (2016008) and Talent Scientific Research Fund of LSHU(2016). Author Contributions: Chunwei Shi, Xiaoping Sun and Wenyi Chen conceived and designed the experiments. Chunwei Shi, Jianbin Chen and Wenyi Chen performed the experiments and analyzed the data. Chunwei Shi, Wei Yang, Jianbin Chen, Xiaoping Sun, Wenyi Chen, Huiyong An, Yili Duoand and Mingyuan Pei wrote the manuscript.

Conflicts of Interest:None of the authors has any competing conflicts of interest.

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Figure 1 The experimental device of ultrasonic static mixer for viscosity reduction of heavy oil

Figure 2 Effect of ultrasonic time on viscosity reduction rate under different ultrasonic power

Figure 3 The volume fraction of vapor phase changes with the axis

Figure 4 Effect of holding time on viscosity reduction rate

Figure 1 The experimental device of ultrasonic static mixer for viscosity reduction of heavy oil

50

Viscosity reducing rate/%

40

30

20

0.2kW 1.0kW 1.8kW

10

0 0

5

10

15

20

25

30

time/min

Figure 2 Effect of ultrasonic time on viscosity reduction rate under different ultrasonic power

(a) The central part of the ultrasonic generator component in Figure 1

(b)The central part of the ultrasonic transducer component in Figure 1

(c)The central part of the SK static mixer component in Figure 1 Figure 3 The volume fraction of vapor phase changes with the axis

56

Viscosity reducing rate/%

48

40

32

oil sample with ultrasonic oil sample without ultrasonic

24

16

8

0 0

5

10

15

20

T/d

Figure 4 Effect of holding time on viscosity reduction rate

Table 1 The properties of the Daqing heavy oil Table 2 Structural parameters of SK static mixer Table 3 Factor level design Table 4 Results of orthogonal experiments

Table 1 The properties of the Daqing heavy oil Project Number Density（20℃）/(g/cm3) Viscosity（50℃）/(mPa·s) ω(Four component content)/% Saturation component Aromatic component Colloid Asphaltene ω(Element)/% C H S N O

0.9222 2830 24.27 26.04 40.88 8.81 85.82 10.93 0.54 0.87 1.84

Table 2 Structural parameters of SK static mixer Pipe diameter (D)/mm

Pipe length to diameter

Number of units (n)

Length of units ( L)/mm

10

85

ratio (h)

60

8.0

Table 3 Factor level design Factor

Level 1

2

3

A

120

240

360

B

15

30

45

C

0.6

1.2

1.8

Table 4 Results of orthogonal experiments The serial number

Factors

V/%

A

B

C

11 12 13 14 15 16 17 18 19

1 1 1 2 2 2 3 3 3

1 2 3 2 3 1 3 1 2

1 2 3 1 2 3 1 2 3

k1 k2 k3

73.07 76.73 82.50

66.93 71.87 76.50 83.03 88.93 77.40

R

6.14

6.53

11.53

24.38 37.12 46.61 25.94 37.68 36.03 41.74 32.68 57.34