CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble

CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble

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CHERD-1301; No. of Pages 8

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Contents lists available at ScienceDirect

Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd

CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble Shahram Niazi, Meysam Mirarab Razi ∗ , Seyed Hasan Hashemabadi Computational Fluid Dynamics Research Laboratory, School of Chemical Engineering, Iran University of Science and Technology, 16846 Tehran, Iran

a b s t r a c t In acoustic cavitation, high pressure and temperature are generated due to cavitation bubble collapse in the liquid bulk around the bubble which causes physical and chemical changes in the liquid. In this study, pressure distribution in water caused by ultrasonic wave propagation in a sonoreactor was investigated. Active cavitation zones were determined by calculating acoustic pressure threshold for cavitation inception and compared with experimental results. Collapse pressure and temperature were predicted 3000 atm and 3200 K, respectively, for crude oil at temperature of 25 ◦ C by evaluating cavitation bubble dynamics in the exerted acoustic field. As a consequence, the huge amounts of energy generated by this phenomenon can be applied for changes in oil properties and crude oil upgrading. © 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Acoustic cavitation; CFD simulation; Cavitation threshold; Bubble collapse; Sonoreactor; Crude oil

1.

Introduction

Crude oil is the greatest and most widely used natural resource for fuels and chemicals in the world. Today, many researchers attempt to improve and create new refining technologies due to increasing in the price of petroleum products and decreasing in the supply of light sweet crude oil. In recent years, the use of energy generated by cavitation instead of other conventional energy sources has been increased. Using acoustic cavitation energy to break different molecular bonds in the crude oil molecules is one of the most effective and efficient methods for upgrading crude oil. Generally, cavitation impacts caused by acoustic energy can result in growth, oscillating, and collapsing of micro1bubbles in the bulk of crude oil. In such a cavitation process, pressure distribution is produced by ultrasonic wave propagation (16 kHz–100 MHz) in liquid bulk. Furthermore, ultrasound waves are generated by piezoelectric transducers and transmitted by waves, which alternately compress and stretch the molecular structures of the medium through their passes. During ‘stretching’ phase (rarefaction), providing the

strong enough negative pressure to overcome intermolecular binding forces, a fluid medium can be ruptured and create voids (micro1bubbles). In succeeding cycles these cavities can grow and then collapse violently with the release of large amount of energy (He and Zhao, 1981; Ma and Shen, 1983; Shah et al., 1999). Experimental results have shown that during the collapse of cavitation bubbles, high temperatures and pressures (5000 ◦ C and 2000 atm, respectively) are achieved around the bubbles, while pressure and temperature of the liquid bulk remain constant at ambient temperature and pressure (Gogate and Pandit, 2000, 2004; Gogate et al., 2003, 2006; Gong and Hart, 1998; Gordon et al., 2010; Mason and Lorimer, 2002; Mitome, 2001; Prabhu et al., 2004; Putterman et al., 2001; Suslick et al., 1999). Cavitation process is a desired process due to large amounts of local energy generation in liquid bulk, low cost, flexibility, and minimizing the environmental damages. Therefore, using this process in petroleum industry can be very beneficial. If the ultrasonic cavitation occurs in bulk of crude oil, these large amounts of temperature and pressure



Corresponding author. Tel.: +98 172 3330782. E-mail address: meysam [email protected] (M.M. Razi). Received 7 July 2012; Received in revised form 18 June 2013; Accepted 1 July 2013 0263-8762/$ – see front matter © 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cherd.2013.07.002 Please cite this article in press as: Niazi, S., et al., CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.07.002

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Nomenclature

t T0 Tg

probe tip surface area (m2 ) van der Waals hard core of gas (m) velocity of sound in medium (m/s) probe diameter (m) sonoreactor diameter (m) frequency (Hz) sonoreactor height (m) ultrasound intensity of sound generator (W/m2 ) wave number (–) gas polytropic index (rad2 /m) pressure (N/m2 ) threshold acoustic pressure amplitude (N/m2 ) collapse pressure (N/m2 ) minimum acoustic pressure for rectified diffusion (N/m2 ) hydrostatic pressure (N/m2 ) minimum acoustic pressure amplitude (N/m2 ) ambient pressure (N/m2 ) power of sound generator (W) vapor pressure (N/m2 ) bubble radius (m) equilibrium radius of the bubble (m) bubble radius at equilibrium conditions (m) velocity of the cavity wall (m/s) acceleration of bubble wall (m/s2 ) time (s) liquid bulk temperature (K) instantaneous temperature (K)

Greeks  ω   

density of the liquid (kg/m3 ) angular frequency (rad/s) liquid surface tension (N/m) liquid viscosity (Pa s) specific heat ratio

A a c d D f H IUS k K p Pa Pcollapse PD Ph Pt P∞ PUS PV R R0 Re R˙ R¨

reductions can cause physical and chemical changes in crude oil such as breakdown of large molecules to smaller ones. Nesterenko and Berlizov (2007) showed that the energy released in collapse of cavitation bubbles can break hydrocarbon bonds in the molecules of compounds contained in petroleum feedstock. They found that for hydrocarbons with molecular weight range of 100–300 g/mol and density range of 700–900 kg/m3 , if 360 l of petroleum product is pumped through the cavitator, approximately 0.1–0.3 kg of hydrocarbons can be cracked. Sawarkar et al. (2009) studied different vacuum residues (Arabian mix vacuum residue (AMVR), Bombay high vacuum residue (BHVR) and one asphalt (Haldia asphalt (HA)) in acoustic cavitation reactor for considering the influence of reaction times from 15 min to 120 min at ambient temperature and pressure. The ultrasonic horn and ultrasonic bath systems (with 20 kHz frequency and 150 W power) were employed for feedstock upgrading. They reported reduction in asphaltene content of AMVR, BHVR and HA for the irradiation time of 60 min, in the ranges of 35–48%, 40–59% and 30–48%, respectively. Dalai et al. (2006) investigated the use of ultrasonic energy to treat the heavy gas oil (HGO). They employed a cylindrical sonoreactor (with diameter of 0.076 m, height of 0.17 m, the ultrasonic processor power of 1.5 kW and frequency of 20 kHz) and reported a maximum nitrogen and

sulfur conversion of 11% and 7%, respectively, and a 5% reduction in the viscosity of about 0.2 l of heavy gas oil (HGO) at the optimized sonochemical conditions. In recent years, many attempts have been done to determine and predict the distribution of the pressure and cavitational activity in the sonoreactors (Chivate and Pandit, 1995; Contamine et al., 1994; Gogate et al., 2011; Kim et al., 2009; Klima et al., 2007; Moholkar et al., 2000; Saez et al., 2005). Moreover, extensive studies have also been performed on the cavitation bubble dynamics under different conditions (Bubulis et al., 2010; Colussi et al., 1998; Gogate et al., 2003; Gong and Hart, 1998; Hilgenfeldt et al., 1998, 1999; Neppiras, 1968, 1980; Putterman et al., 2001; Tomita and Shima, 1986; Vichare et al., 2000; Young, 2005). The majority of these previous studies investigated the hydrodynamics of the water-filled sonoreactors and its effect on dynamics behavior of the cavitation bubbles. The aim of this study is to simulate a sonoreactor filled with crude oil and investigate dynamics of acoustic cavitation bubbles in the bulk of crude oil. Moreover, the work presented follows two important objectives; first, active cavitation zones, where cavitation bubbles grow and collapse, are simulated by using computational fluid dynamics (CFD) technique and compared with experimental data. Then, collapse pressure and temperature of a cavitation bubble is determined on the basis of CFD simulation results of acoustic cavitation.

2. The governing equations on acoustic cavitation 2.1.

Acoustic cavitation threshold

Gas bubbles need a surface to form nuclei for acoustic cavitation inception. This surface may be container wall, fluid contamination and micro-bubbles that have not been dissolved in fluid. Small bubbles can act as nuclei for other cavitation bubbles. Bubbles grow as exposure to the pressure lower than threshold pressure, which is a criterion for stability or growth of bubbles. Threshold pressure depends on certain factors such as the existence of gases and fine particles in the liquid, applied frequency and ultrasound intensity. A general equation for acoustic cavitation threshold as a function of frequency with the given radius of nucleus is given by (Crum, 1982),

Rt =

⎧   

1/3 0.13 P∞ 1/2 P − 1 2 ⎪ ⎪ 1 + − 1) for . . . P ≤ 11 (P √ ⎨ f  3 P  1/2 2

1/2 ⎪ ⎪ ⎩ 0.3 P∞ for . . . P ≥ 11 (P − 1) f



3

(1) where Rt is the radius of the free bubble acting as a nucleus, P∞ is the ambient pressure,  is the density of the liquid, f is the driving frequency of the acoustic field, P = Pt /P∞ , and Pt is the minimum acoustic pressure amplitude required to initiate cavitation, i.e., the threshold pressure. Fig. 1 shows the variation of acoustic pressure amplitude threshold obtained from Eq. (1) at frequency of 20 kHz for saturated crude oil with various radiuses of bubble nuclei. According to Fig. 1, the negative pressure required for the acoustic cavitation inception enhances with increasing the size of cavitation bubble nuclei which means greater negative pressures should be applied on the bulk of crude oil.

Please cite this article in press as: Niazi, S., et al., CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.07.002

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pressure and temperature of the bubble collapse, can be provided (Hilgenfeldt et al., 1999; Tomita and Shima, 1986): Pcollapse =



P0 +

2 R0

  R 3 0 ×

R



2 4 − R R

 dR  dt

(3)

3(−1)

Tg (R) =

T0 R0

(R3 − a3 )

−1

(4)

where T0 is liquid bulk temperature, R0 is initial radius of bubble,  is ratio of specific heat capacity of the gas inside the bubble, and a is van der Waals hard nuclei radius of gas inside bubble. The pressure and temperature produced by bubble collapse can be estimated for different liquids at various operational conditions using Eqs. (3) and (4). Fig. 1 – Acoustic pressure amplitude threshold obtained by Eq. (1) for saturated crude oil with various gas bubble nuclei radius.

2.2.

Cavitation bubble dynamics

Generally, growth and collapse phase of bubble can be determined well by evaluating motion of bubble wall in liquid bulk due to exerted acoustic field. The principal governing equation of the motion of the cavity interface is given by the Rayleigh–Plesset equation (Eq. (2)). Solving Eq. (2) gives the variation of the bubble radius in the liquid bulk (Hilgenfeldt et al., 1998; Young, 2005),

PR¨ +

3 2

R˙ 2 =

 1 

Ph +

2 − PV Re

2.4. The governing equation of acoustic wave propagation In this numerical simulation, the 2D geometry was used. The propagation of acoustic waves through the liquid medium is characterized by wave equation. The acoustic wave equation is a second order partial differential equation that quantifies acoustic pressure as a function of space and time. The wave propagation is assumed linear and the shear stress (which is correct for liquids and gases) is ignored (Klima et al., 2007; Saez et al., 2005). The acoustic wave equation can be written as follows (Kim et al., 2009; Klima et al., 2007; Saez et al., 2005; Younggyu et al., 2009): ∇

  R 3K e

R

2 R˙ − − 4 − (Ph − Pa ) R R

(2)

2.3.

Temperature and pressure of bubble collapse

Cavitation bubble violently collapses due to reaching maximum size in the rarefaction acoustic wave cycles. Pressure and large instantaneous temperature is created in liquid bulk around bubble due to bubble collapse, which can lead to the production of free radicals at liquid bulk. The collapse time of the cavitation bubbles is in the order of micro-seconds; therefore, the heat will not be able to escape from the bubble, and the bubble collapses adiabatically (Bubulis et al., 2010; Colussi et al., 1998; Gogate et al., 2003; Gong and Hart, 1998; Hilgenfeldt et al., 1999; Putterman et al., 2001; Tomita and Shima, 1986; Young, 2005). As a result, the collapse of bubbles can be considered adiabatic. Considering the motion of bubble wall in an adiabatic collapse, the following equations for calculation of the





∇P −

1 ∂2 P =0 c2 ∂t2

(5)

where c is sound speed. In time harmonic case (P(r, t) = p(r)·eiωt ), Eq. (5) has the following form: ∇

where R˙ is speed of bubble wall, R¨ acceleration of bubble wall, Re bubble radius at equilibrium conditions,  liquid surface tension,  liquid viscosity, Pv liquid-vapor pressure,  liquid density, Ph atmospheric pressure (hydrostatic), Pa exerted acoustic pressure (Pa = PA sin ωt) and K gas polytropic index, which varies between specific heats ratio () and 1 (for adiabatic and isothermal conditions). This differential equation can be solved numerically to estimate the maximum bubble radius during expansion cycles and also collapse time.

1

1 



∇P −

ω2 P=0 c2

(6)

where ω = 2f, is the angular frequency. The acoustic pressure distribution in the computational domain was obtained by numerical solution of Helmholtz equation (Eq. (6)). According to Eq. (6) variation of temperature changes the sound speed in fluid (c) and fluid density which leads to changing the sound pressure distribution through the reactor.

2.4.1.

Boundary conditions and numerical simulation

The simulated sonoreactor was a glass cylindrical reactor (diameter = 150 mm, height = 435 mm), filled with water at 25 ◦ C. A probe type of ultrasonic generator (UIP2000, 20 kHz, 2 kW) was used as ultrasound source (diameter = 85 mm and tip diameter = 20 mm) that was immersed into the water (L = 245 mm) from the top. Fig. 2 sketches the computational domain for this sonoreactor. In computational domain, the following boundary conditions were implemented (Kim et al., 2009; Klima et al., 2007; Saez et al., 2005; Sutkar et al., 2010; Younggyu et al., 2009): (a) The sonoreactor walls: total reflection boundary condition (p = 0), this boundary condition is called sound-soft boundary condition. Corresponding to a sound-soft obstacle with the total pressure, i.e., the excess pressure over the static pressure P0 , is vanishing on the boundary. (b) The sides of the sonicator horn: wall (∂p/∂n = 0).

Please cite this article in press as: Niazi, S., et al., CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.07.002

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Table 1 – Physical properties of the saturated crude oil (Guangtian et al., 2006). Speed of sound (m/s) 1406 1352.6 1295.5

Density (kg/m3 )

Temperature (◦ C)

863.5 853.23 843.11

25 40 55

According to Fig. 3, it is visualized well that the cavitation bubbles are only formed in places with minimum pressures, as predicted by numerical simulation. Moreover, it can be seen that simulation results have great agreement with experimental data.

3.2.

Fig. 2 – Computational domain for sonoreactor. (c) Sonicator horn tip: pressure boundary condition (p = p0 ). At horn tip, the boundary condition p0 relies on the power of sonicator and is calculated from: IUS =

P2 PUS = 0 2c A

(7)

where IUS (W/m2 ), PUS (W) and A (m2 ) are the ultrasound intensity of sound generator, sonicator power and horn tip surface area, respectively. The governing equation (Eq. (6)) can be solved by low-order finite element method using FEMLAB MultiPhysics Software version 2.3. In numerical solution of Helmholtz equation, accuracy significantly depends on the wave number, k (k = ω/c). The solution at high wave numbers is extremely oscillatory. Consequently, the discretisation step size of the numerical method (h) is refined so that the product of the wave number (k) and step size discretisation is constant for resolving the oscillations (Klima et al., 2007; Saez et al., 2005). Moreover, in order to eliminate the effect of mesh size on the results, different meshes were used with different increasing densities (13286, 22336 and 62600) until it was found that a further increase in mesh density had negligible effect on the solution values. The minimum and maximum size of optimized mesh was 0.08 and 4 mm2 , respectively. For simulation of the mentioned reactor (Fig. 2), the number of elements is 22336. In this study, the time harmonic analysis, stationary and linear system solver were used. To numerically solve the set of the equations, the direct UMFPACK (using the nonsymmetricpattern, multi-frontal method and direct LU factorization) with relative tolerance of 1E-6 was used.

3.

Results and discussion

3.1.

Experimental verification

Table 1 lists the physical properties of saturated crude oil (Guangtian et al., 2006) which were used in the simulations. Fig. 3 shows comparison of the simulated spatial variation of acoustic pressure with distribution of cavitation bubbles that has been captured (website, 2012) for this sonoreactor.

Active cavitation zones in the sonoreactor

According to the distribution of cavitaion nuclei in water, reported in several studies (Chahine et al., 2008; Franklin, 1992; Hoˇsek, 2008; Liu et al., 1993; Safar, 1969; Sedláˇr et al., 2008), in this study the initial size of bubble nuclei was considered 10 ␮m. For crude oil, cavitation nuclei vary in size from 100 nm to a few millimeters in diameter (Gordon et al., 2010). 10 ␮m was considered as the initial size of bubble nuclei due to the lack of experimental data for thorough physical properties of crude oil (specially the distribution of cavitaion nuclei). Using Eq. (1), acoustic pressure threshold required for acoustic cavitation inception in water, extreme oscillation of the bubbles and bubbles collapse was estimated −1.52 MPa. Therefore, cavitation phenomenon will occur in the zones, which calls active cavitation zones, where the pressure is less than −1.52 MPa. Fig. 4 presents comparison between simulated active cavitation zones by CFD technique and experimental data. According to Fig. 4, simulation results match well with experimental data.

3.3.

Effect of fluid properties

To evaluate effectiveness of medium properties on sonoreactor hydrodynamics, the simulations for saturated crude oil at 25, 40, and 55 ◦ C were repeated after validation of simulation results with reported experimental data. Table 1 presents the

Fig. 3 – Comparison of the simulated spatial variation of acoustic pressure and experimental distribution of cavitation bubbles (Hielscher Ultrasound Technology website) in the sonoreactor filled with water at 20 kHz and 2 kW.

Please cite this article in press as: Niazi, S., et al., CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.07.002

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Table 2 – Sound pressure (P) producing cavitations in various liquids under a hydrostatic pressure of 1 atm (Mason and Lorimer, 2002). Liquid Castor oil Olive oil Corn oil Linseed oil CCl4

 (poise) 6.30 0.84 0.63 0.38 0.01

P (g cm−3 ) 0.969 0.912 0.914 0.921 1.60

c (km s−1 )

PA (atm)

1.477 1.431 1.463 1.468 0.926

3.90 3.61 3.05 2.36 1.75

Fig. 4 – Comparison between predicted active cavitation zones by CFD simulation and experimental data (Hielscher Ultrasound Technology website) for the sonoreactor filled with water at 20 kHz and 2 kW.

physical properties of saturated crude oil (Guangtian et al., 2006) which were used in the simulations. Since it is necessary for the negative pressure in the rarefaction cycle to overcome the natural cohesive forces acting in the liquid, any increase in these forces will increase the threshold of cavitation. Increasing the liquid viscosity increases these forces. Table 2 presents the influence of viscosity on the pressure amplitude (PA ) at which cavitation begins at 25 ◦ C, at hydrostatic pressure of 1 atm. Taking corn and castor oils as examples, a ten-fold increase in viscosity only led to a 30% increase in the acoustic pressure needed to bring about

Fig. 6 – The maximum magnitude of negative pressures for water and saturated crude oil at different temperatures (at 20 kHz and 2 kW). cavitation (Table 2) (Mason and Lorimer, 2002). Therefore, the influence of crude viscosity was ignored in the present work. Fig. 5 shows the obtained pressure distribution for water and saturated crude oil at different temperatures. The magnitude of maximum negative pressure is shown as well in reactor. Fig. 6 shows the effect of different medium temperature on maximum negative pressure. According to these results, the magnitude of negative acoustic pressure applied

Fig. 5 – Pressure distribution for water and saturated crude oil at different temperatures at 20 kHz and 2 kW (a: water 25 ◦ C, b: crude oil 25 ◦ C, c: crude oil 40 ◦ C, d: crude oil 50 ◦ C). Please cite this article in press as: Niazi, S., et al., CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.07.002

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Table 3 – Physical properties of the saturated crude used in this study. Properties

Value

T (◦ C)  (Pa s)  (kg/m3 ) Pv (psi)  (N/m)

25.0 2.0 863.5 6.0 0.03

in the reactor for crude oil is higher than water (at 25 ◦ C). According to Eq. (6), it can be explained by this fact that sound pressure distribution is dependent on the density and speed of sound. Speed of sound in fluids is calculated as follows:

 c=

dp d

(8)

We know that density and sound velocity in a bubbly liquid are smaller than those of liquid without bubbles, but like the other references (Chivate and Pandit, 1995; Contamine et al., 1994; Kim et al., 2009; Klima et al., 2007; Moholkar et al., 2000; Saez et al., 2005) in this study, the bubble movement effects were ignored on reactor hydrodynamics. Therefore, in a fluid with lower density, that the speed of sound is lower, (compared to a fluid with higher density) operating pressure in liquid bulk can cause a fluid’s density to change more in comparison with higher density one (see Eq. (8)). Consequently, the pressure distribution in the reactor significantly changed with shifting the medium from water to crude oil. In addition, the maximum magnitude of negative pressure decreased with increasing the temperature (or decreasing the wavelength). For greater values of the maximum magnitude of negative pressure along the largest region of the reactor, probability of cavitation occurrence and its intensity is higher. Owing to this fact, it can be found that the higher intensity of cavitation causes the larger amount of physical and chemical changes in crude oil. Acoustic pressure threshold for acoustic cavitation estimated −0.153 MPa for crude oil with 10 ␮m initial size of bubble nuclei. Consequently, cavitation bubbles grow violently in the reactor zones where pressure is less than −0.153 MPa. Fig. 7 shows simulated active cavitation zones in the reactor filled with saturated crude oil at temperature of 25 ◦ C. As shown, cavitation occurs at the large volume of reactor in the operational conditions. Energy produced by this phenomenon can be used for physical and chemical changes in crude oil.

3.4.

Fig. 7 – Active cavitation zones simulated by CFD technique for the reactor filled with saturated crude oil at temperature of 25 ◦ C, 20 kHz and 2 kW. Figs. 8 and 9 show the collapse pressure and temperature of the cavitation bubble with initial size of 10 ␮m for saturated crude oil. According to Fig. 9, high collapse time occurs for cavitation bubbles located in reactor zones with higher negative acoustic pressure amplitude. Since maximum bubbles radius takes place in these zones, collapse pressure and temperature reach larger values.

Temperature and pressure of the bubble collapse

As mentioned earlier, solving Eq. (2) at the acoustic field generated by ultrasound wave propagation gives changes in size and radius of cavitation bubble. In this equation, Ph − Pa represents resultant of exerting acoustic pressure and hydrostatic pressure at the reactor. This term was calculated for active cavitation zones, where bubbles collapse, based on CFD simulation results for sonoreactor filled with saturated crude oil (Fig. 7). Collapse pressure and temperature can be calculated at any spot of the active activation zones by substituting the term into Eq. (2) and solving it. Hence, Eqs. (3) and (4) solved for zones with pressures of −20 MPa, −15 MPa, −10 MPa, −5 MPa, −1 MPa and −0.5 MPa. Table 3 lists the characteristics of the crude oil.

Fig. 8 – Collapse pressure of a bubble in active cavitation zones of the sonoreactor filled with saturated crude oil (f = 20 kHz, 2 kW,  = 863 kg/m3 ,  = 2 Pa s, T = 25 ◦ C, Pv = 6 psi,  = 0.03 N/m, nuclei radius = 10 ␮m).

Please cite this article in press as: Niazi, S., et al., CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.07.002

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References

Fig. 9 – Collapse temperature of a bubble in active cavitation zones of the sonoreactor filled with saturated crude oil (f = 20 kHz, 2 kW,  = 863 Kg/m3 ,  = 2 Pa s, T = 25 ◦ C, Pv = 6 psi, ␴ = 0.03 N/m, nuclei radius = 10 ␮m).

Fig. 10 – Collapse temperature and pressure of a bubble in active cavitation zones of the sonoreactor filled with saturated crude oil (f = 20 kHz, 2 kW,  = 863 kg/m3 ,  = 2 Pa s, T = 25 ◦ C, Pv = 6 psi,  = 0.03 N/m, nuclei radius = 10 ␮m). According to the CFD simulation results of the sonoreactor, collapse pressure and temperature in the spot of bubble collapse may reach several thousand atmospheres and Kelvins, respectively (Fig. 10). These results are in agreement with the findings of other researchers (Gogate et al., 2003, 2006; Gong and Hart, 1998; Mason and Lorimer, 2002; Mitome, 2001; Putterman et al., 2001; Suslick et al., 1999). These large amounts of collapse temperatures and pressures can lead to physical and chemical changes in crude oil such as breakdown of large molecules to smaller ones.

4.

Conclusion

In this study, acoustic pressure distribution was assessed by numerical solution of acoustic wave propagation equation in a crude oil upgrading sonoreactor. Active cavitation zones, where cavitation bubbles grow and collapse, were determined by calculating negative pressure required to develop cavitation. CFD simulation results of acoustic cavitation and evaluation of the cavitation bubble dynamics at exerted acoustic field showed that collapse pressure and temperature of cavitation bubble reach 3000 atm and 3200 K, respectively, for saturated crude oil at temperature of 25 ◦ C. Accordingly, this huge amounts of energy generated can be applied for changes in crude oil properties and then crude oil upgrading.

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Please cite this article in press as: Niazi, S., et al., CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble. Chem. Eng. Res. Des. (2013), http://dx.doi.org/10.1016/j.cherd.2013.07.002