Evolution and Size Distribution of Solid CO2 Particles in Supercritical

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Thermodynamics, Transport, and Fluid Mechanics

The evolution and size distribution of solid CO particles in supercritical CO releases 2

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Lin Teng, Yuxing Li, Datong Zhang, Xiao Ye, Shuaiwei Gu, Cailin Wang, and Jinghan Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00178 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Industrial & Engineering Chemistry Research

The evolution and size distribution of solid CO2 particles in supercritical CO2 releases Lin Teng, Yuxing Li*, Datong Zhang, Xiao Ye, Shuaiwei Gu, Cailin Wang, Jinghan Wang Shandong Provincial Key Laboratory of Oil & Gas Storage and Transportation Security， China University of Petroleum (East China), Qingdao 266555, China *Corresponding author at Science Hall D320, China University of Petroleum (East China), Qingdao 266555, China Tel.: +86 053286981818; fax: +86 053286981822. E-mail address: [email protected] (Y. Li).

Abstract CO2 transportation safety is important for the successful implementation of Carbon Capture and Storage (CCS) projects. The formation of solid CO2 is a unique phenomenon in high pressure CO2 release. The evolution and size distribution of dry ice particles have a significant impact on assessment of the consequences of accidental release of supercritical CO2 from a pipeline. The motivation is to investigate the particle behavior and size distribution and understand how the impact factors affect them. An experiment with various measurement methods was developed to carry out controllable CO2 release from a high pressure vessel. The macro parameters such as jet configuration, temperature and velocity were recorded. Meanwhile, motion of the micro particles was also analyzed. In addition, the effects of initial pressure and temperature on the particle size distribution were investigated. The results showed the agglomeration influenced the size distribution at different positions of the jet.

Keywords Supercritical CO2 pipeline; Leakage; Multiphase CO2 jet; Particle size distribution; Carbon Capture and Storage (CCS)

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 Introduction The most of hydrocarbons over the world are being transported via pressurized pipelines.1 Despite the pipeline is considered as the safest mode of hydrocarbon transportation, globally over 3.6 million kilometers pipelines have an average of 250 pipeline rupture incidents per year.2,3 Long-distance pipelines are also expected to play an important role in combating the global warming by transporting the captured CO2 at large point emission sources to other spots for use (e.g. oil fields for enhanced oil recovery (EOR)).4,5 Currently, there are over 6200 km of CO2 pipelines in the United States.6,7 In more-stringent environmental policy, up to 200,000-360,000 km of CO2 pipelines by 2050 could be built and operated in the U.S., China and Europe.7 Supercritical CO2 has a density close to liquid, but has a viscosity of magnitude more frequently associated with gases.8 Thus, most of CO2 have been transported in supercritical state due to its economic efficiency.9 However, due to significant Joule-Thomson cooling, the complex phenomena such as phase transition, multiphase flow, particle deposition and adhesion can occur upon a leakage.10 The unusual phase transition behavior and physical properties make the modeling of CO2 fluid dynamics pose a unique set of problems.8 The dry ice forms due to the rapid pressure drop to the ambient and the solid particles will sublime at ambient atmospheric conditions. These phenomena are not common in the depressurization of other gases,8,11 but of great importance in the assessment of the hazards of supercritical CO2 leaks. In view of multiphase CO2 jet, the near-field characteristics in supercritical CO2 release need to be investigated.

Figure 1. Schematic of supercritical CO2 discharge and dispersion 2


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In recent years, some research including experiments and numerical simulations related to near-filed characteristics of high-pressure CO2 release have been carried out. The thermo-hydraulic experimental data in CO2 releases has been obtained.12 A highly under-expanded jet flow also was observed in the supercritical CO2 releases.13, 14 Webber15 developed a two-phase flow model for flashing jet of CO2. It revealed that two-phase homogeneous equilibrium flow models may be generalized to simulate such a release. Liu et al.16 simulated highly under-expanded single-phase CO2 jets using CFD software Fluent with Peng−Robinson (PR) equation of state (EoS) to account for the real gas behavior. Zheng et al.1, 17 developed an efficient integral multiphase jet expansion model in consideration of real fluid behavior to predict the accidental release of two-phase CO2 following the puncture of a high pressure containment. Hulsbosch-Dam et al.18 developed a complicated model considering aerodynamic break-up and thermodynamic break-up to calculate the size distribution of dry ice particles in such a release. Unfortunately, due to a lack of experimental data of evolution of solid CO2 in such supercritical CO2 releases, currently it is hard to validate the models, and the development of more complex models is limited. Notably Wareing and Woolley8, 11, 19-24 at the University of Leeds have done some pilot experiments for the measurement of dry ice particles in liquid CO2 release and developed a multiphase model to describe particle-laden sonic CO2 jet. A 20 milliliter capacity canister filled with CO2 in saturated conditions is released to an atmosphere, mimicking the release from a saturated CO2 pipeline. A laser Doppler anemometer (LDA) was used to measure the size of particles. They found a large-diameter-skewed lognormal distribution in the initial expansion and concluded that the agglomeration of the particles has to be considered. Their work plays a pilot role in the research on solid-gas CO2 jet in high-pressure CO2 pipeline release. In addition, in surface cleaning area the size distribution of dry ice particles has also been measured experimentally.25,26 Liu et al. investigated the state of primary dry ice particles produced by expanding liquid carbon dioxide. The result showed that the primary particles ejected from the nozzle were about 1 µm in mass median diameter. 3



As there is little research on the evolution of dry ice particles in the expansion of supercritical CO2, it is worthy to obtain a better understanding of the physics of dry ice particles produced by the expansion of liquid CO2. This is important to the assessment of CO2 pipeline failure,27 surface cleaning and rock breaking28 with supercritical CO2 jet. In this paper, an experiment setup with a high speed camera and a laser particle size analyzer is designed to study the macro and micro parameters in the near-field CO2 jet. The work focuses on (1) the macro parameters, such as jet structure, jet temperature and velocity; (2) the motion of solid CO2 particles; (3) the particle distribution of dry ice. The scale-independence would indicate the results of the experiments can be applied to industrial-scale supercritical CO2 pipelines, which will benefit the simulation of CO2 pipeline rupture or puncture.

2. Experiments 2.1. Experimental apparatus and uncertainty To investigate the evolution of the dry ice particles in supercritical CO2 released from pressurized pipeline, an experimental setup, which is shown in Figure 2, was designed and constructed. The experimental apparatus consists of a gas source, a refrigerating unit, a CO2 pump, a high pressure vessel, a thermostatic water bath, nozzles (orifices), a high speed camera system, a laser particle size analyzer and data acquisition system. The design pressure of the vessel is 15.0 MPa and the material is 316L stainless steel. The vessel was charged with liquid CO2 cooled by refrigerating unit. The temperature inside the vessel can be controlled accurately using thermostatic water bath. The orifices have an inner diameter of 1 mm and 2 mm. Some sensors were placed in near-field jet region to obtain temperature and velocity, as shown in Table 1.

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Figure 2. Schematic of the experimental setup. 1 – CO2 gas cylinders, 2 –Refrigerating unit, 3 – CO2 pump, 4 – high pressure vessel, 5 – Safety valve, 6 – Base, 7 – Load cells, 8 – Pressure and temperature sensors, 9 – Visible window, 10 – Pneumatic valve, 11 – Nozzles, 12 – LED light, 13- High speed camera, 14 – Data acquisition system, 15 –Thermostatic water bath, 16 – Measure points in the near-field, 17 – Laser particle size analyzer (1) In the vessel and pipe The equipment and sensors in this part are key components of whole experimental apparatus. It is necessary to give their capabilities and limitations, though data monitored by these sensors are out of the scope of this paper. Four high frequency pressure sensors and four T-type thermocouples were inserted into the vessel and pipe. The high frequency pressure sensors with a frequency response of 10 kHz are used to measure the pressure variation within the vessel. The accuracy of high frequency pressure sensor is 0.25%FS of full scale and the maximum uncertainty in experimental conditions is ±20kPa. The thermocouples were used to measure fluid temperature change. The uncertainty of T-type thermocouples is ±1 ℃. Four load sensors were installed under the base of apparatus to measure the mass variation during the release. The range of the weighing sensors is 0-800 kg and the accuracy is 0.05%FS of full scale. The maximum uncertainty in experimental conditions is ±0.4 Kg.

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Table. 1 Measure point locations in the near field Distance away from Temperature

Differential pressure the orifice/cm

T5

DP

3

T6

6

T7

11

T8

19

T9

29

T10

49

T11

69

T12

99

T13

129

T14

169

T15

209

T16

259

(2) In the near field Four T-type thermocouples were set along the centerline of the jet to measure the temperature. The uncertainty of T-type thermocouples is ±1 ℃. One differential pressure sensor was used to obtain the dynamic pressure of jet flow. The differential pressure sensor has an accuracy of 0.25%FS. A high speed camera (Fastcam SA-X2, Photron Ltd.) with a zoom SLR (single lens reflex) lens (Nikon) was used to observe the structure of near-field jet flow of supercritical CO2. A microscope lens (Zoom 6000, Navitar Inc.) was used to record the motion of solid CO2 particles. The maximum frame rate of high speed camera is 200,000 FPS (frames per second). The size distribution of the solid CO2 particles ejected from the rupture was measured in situ with a He-Ne laser particle size analyzer (DP-02, Omec Ltd.) which based 6


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on the laser diffraction principle. The solid particles have different refractive index compared to the gases, which can cause laser beam to be defocused. The repeatability error of particle size analyzer is below 3%. The size of the particles may be below 50 µm, thus the range of measure focused on several micrometers level. Data was recorded by the National Instruments data acquisition system. 2.2. Experimental procedures Prior to experiments, some prevention measures have been done to ensure experimental safety. The reinforcing measures were taken to prevent the counterforce during the jet of supercritical CO2 and noise level was controlled within acceptable limit. According to pipeline transportation conditions, most of the initial conditions in experiments are in supercritical region. To ensure the repeatability of the experimental results, experiments have been carried out for three times. The experimental procedures were as follows: (1) The physical integrity of the experimental setup was checked to ensure the instruments and data acquisition system were in good condition; (2) The air in the vessel was evacuated to eliminate impurities; (3) Liquid CO2 which was cooled by refrigerator was fed into the vessel using CO2 pump; (4) All valves were shut down when the appropriate mass of CO2 had been fed into the vessel, and the water-bath heating jackets were used to manipulate the condition in the vessel; (5) The pneumatic valve in the pipe was open quickly when the experimental conditions inside the vessel reached conditions designed; (6) The experimental data were recorded. For the movement process of dry ice particles, the images recorded by the high speed camera were processed using ProAnalyst software (Xcitex Ltd).

3. Results and discussion 3.1 The evolution of macro parameters 3.1.1 Jet structures Figure 3 shows the jet shapes of releases from 1 mm and 2 mm diameter orifices. As can be seen, the whole jet structure is a barrel shape. It is typical a highly under-expanded jet.13,14 In the trial with 1 mm diameter orifice, the Mach disk can be observed clearly in fully developed flow, however it cannot be seen in the trial with 2 mm diameter orifice. The reason is that the phase transition occurs in such a jet and 7



plenty of micro-level dry ice particles will generate in this case. The bigger orifice may cause higher concentration of dry ice particles, which may make the Mach disk invisible.

Figure 3. The jet configuration in the developed flow 3.1.2 The temperature of multiphase jet Figure 4 presents the evolution of the temperature along the jet for the 1 mm and 2 mm diameter orifice releases. It indicates that the temperature along the jet reduced sharply initially and reached the lowest temperature very soon. After that, it would gradually increase to the ambient temperature. The lowest temperatures in the 1 and 2 mm diameter orifice cases are -41.9 ℃ and -45 ℃, respectively. The time 8


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within which the 1 and 2 mm diameter orifice cases reached the lowest temperature has a difference of 3 seconds, which indicates the bigger leakage orifice can cause the faster temperature drop. Note that solid CO2 can only exist at and below the sublimation temperature (-78 oC) at atmospheric pressure, but gas temperature recorded is not at this level. The explanation may be explored regarding the thermal equilibrium.11,22 For CO2 particles releasing from a high-pressure pipeline through the small

orifices are neither in dynamical equilibrium nor thermal equilibrium with the flow. For small orifices, such as 1 mm in this paper, the particles do not reach thermal equilibrium with the flow. Thus, even though the temperature of solid CO2 is below -78 oC, the gas temperature measured is higher than that and it is above -50 oC. It validates the thermal non-equilibrium in this multiphase CO2 flow.

Figure 4. Variation of temperature along the jet centerline with time

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Figure 5. The lowest temperature at different position along the jet It can be seen from Figure 5 that bigger orifice can result in lower temperature along the jet centerline. The lowest temperature measured for two orifices occurred at 6 cm away from the release point. The temperature increases sharply after 6 cm and the rate of increase reduces gradually. Note that the temperature at 3 cm is higher than that at 6 cm. There may be two reasons explaining this phenomenon: (1) after 3 cm, the CO2 particles (relatively low temperature) and the flow (relatively high temperature) are closer to thermal equilibrium, though they cannot reach the equilibrium; (2) as we know, the sublimation of dry ice can cause gas temperature drop and the heat transfer between cool flow and surrounding can cause CO2 temperature rise. From 3 to 6 cm, the effect of sublimation on the temperature change surpassed the effect of heat transfer from the surrounding. Thus, the temperature at 3 cm was higher than that at 6 cm. With the increase of distance away from the orifice, the effect of heat transfer from the surrounding on the jet temperature surpassed that of sublimation, which led to the jet temperature rise continuously.

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3.1.3 The velocity of multiphase jet DP is the dynamic pressure at jet centerline at a distance of 3 mm away from orifice with a diameter of 1 mm. The instantaneous velocity of jet flow at this position can be obtained by Eq. (1). It can be assumed that the pressure at 3 mm is equal to atmospheric pressure. The instantaneous temperature (T5) outside the vessel was also measured. Note that in this case the measured differential pressure is gas pressure, and the temperature measured is above the triple point temperature. Meanwhile, considering that dynamic equilibrium may be achieved at this location,11,22 the flow can be assumed as the homogeneous flow. Thus fluid density ρ can be predicted using Peng-Robinson EoS (Eq. (2)).29

p=

= 2 ∙ ∆ ⁄

(1)

a (T ) RT − v − b v (v + b ) + b (v − b)

(2)

Where a (T ) = acα (T ) ac = 0.45724

RTc R 2Tc2 , b = bc = 0.07780 pc pc

α (T ) = 1 + k (1 − Tr0.5 )  , k = 0.37464 + 1.54226ω − 0.26992ω 2 2

Figure 6 (a) shows the dynamic pressure and the velocity in 1 mm and 2 mm diameter orifices release. For 1 mm diameter orifice, the velocity of jet at this position rose to the critical maximum value (ca. 250 m/s). The velocity remained around 230 m/s for 50 seconds, which indicates the leakage process was at quasi-steady stage. Subsequently the velocity near the orifice decreased gradually. For 2 mm diameter orifice, the DP rose almost vertically and then declined immediately due to the blockage of dry ice particles in the gas tube of the differential pressure sensor. Subsequently, dry ice sublimates and the velocity rose to the maximum value (approximately 250 m/s). Compared with the release from 1 mm diameter orifice, the maximum velocity is closed, however the time of quasi-steady stage differs and the smaller orifice has longer time of duration.

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Figure 6. Variation of dynamic pressure and velocity with time for different orifice diameters of (a) 1 and (b) 2 mm 3.2 Size distribution of solid CO2 particles 3.2.1 Comparison of size measurement Figure 7 shows the comparison of size distribution in experimental and theoretical methods. In Lin’s experiment,30 the initial condition is same as what used in our validation experiment, but the orifice diameter in Lin’s experiment is 250 µm and the diameter in our experiment is four times of that. As shown in Figure 7, the range of the particle distribution for the two experiments is 2.5 – 5 µm and D50 is 12


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ca. 4 µm, which indicates that our method of the measurement is appropriate for dry ice particles. The tiny difference between the two experimental results may be caused by the difference in the orifice diameters. Hulsbosch-Dam et al.18 developed a model considering the aerodynamic break-up and thermodynamic break-up to predict the size distribution of dry ice particles, as shown in Supporting Information. In Figure 7, we can see that all results are unimodal, but there is big difference between experimental and theoretical results. The reason may be that the initial condition in the calculation is 100 bar and 88 ℃, which is different from that in the experiment. However, on the other hand, it indicates that the pressure and temperature may have an impact on the size distribution of the particles, which would be analyzed in the following parts.

Figure 7. The cumulative distribution compared with the results in the literature18, 30 3.2.2 Size distribution at different positions along the jet To investigate the particle size at different positions along the jet, the particle size distribution was measured at 8 mm, 12 mm, 30 mm and 100 mm from the release point along the jet centerline. Figure 8 shows the cumulative distribution and D50 at different positions for 1 mm diameter orifice. The D50 does 13



not change with the position linearly. However, the particle size reduced firstly and increased after the initial drop, maintained stable for a while and finally reduced again. The factors impacting particle size are mainly tiny particle evaporation and agglomeration. From 5 mm to 8 mm, the size distribution moves left (shown in Figure 8 (a)) and the D50 goes down (shown in Figure 8 (b)), which indicates that the evolution of the particles gave priority to evaporation along the jet. Notice that the Mach disk was located at about 6 mm from the release point in this case. The temperature across the Mach disk will rise sharply. This is the reason that the size of the particles reduces abruptly. From 8 mm to 12 mm, the rightwards of the size distribution and the increase of the D50 are the evidence of the particle agglomeration. Behind 12 mm, the evaporation becomes dominant again with the increase of the temperature along the jet. Thus, the size of the dry ice particles reduces continuously until it reaches around 2 µm at 100 mm away from the release point.

(a) the cumulative distribution

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(b) the mean particle size Figure 8. The particle size distribution at different positions for 1 mm diameter orifice The size distribution of solid CO2 particles is required for accurately predicting the source strength of a high-pressure CO2 pipeline leakage. At the pseudo-source of dispersion, the jet pressure was supposed to reduce to the ambient pressure at the distance of 10xm.16 The result in this experiment indicates that for the source strength of supercritical CO2 pipeline leakage, the size range of the dry ice particles is 1 - 3 µm, which may be important for the simulation of CO2 dispersion. 3.2.3 Effect of pressure on the size distribution Figure 9 shows the effect of the initial pressure inside the vessel on the particle size distribution in 1 mm diameter orifice jet. All cases are in supercritical region, the initial temperature is 313 K and the pressure is in the typical CO2 pipeline condition. In the center of the jet, the range of the particle size is 2 – 10 µm. And the median size (D50) of the particles is ranging from 3.4 – 5.0 µm. It can be found that the D50 would increase with the decrease of the initial pressure. The higher initial pressure can cause a wider size distribution.

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Figure 9. Effect of pressure on the size distribution for the supercritical release of d = 1 mm, L = 5 mm 3.2.4 Effect of temperature on the size distribution Figure 10 (a) shows the particle size distribution in the jet for the 1 mm diameter orifice case. The initial pressure is 8 MPa and the initial temperature is ranging from 301 K to 313 K. It can be seen that there is little difference in the size distribution. Figure 10 (b) shows the Sauter mean diameter (SMD) with the change of initial temperature. In the view of the SMD calculated,18 the SMD would decrease with the increase of the initial temperature. However, we can find the variation of particle size with the temperature is small that the average increment of the SMD is about 0.1 µm/K with initial pressure of 10 MPa. On the other hand, in the view of the experimental data, there is little difference in the SMD from T – Tsublimation = 100 to 120 K. Overall in supercritical region, the initial temperature has little impact on the solid CO2 particle size.

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(a) effect of temperature on the cumulative distribution

(b) effect of temperature on the Sauter mean diameter (SMD) Figure 10. Effect of temperature on the size distribution for 1 mm diameter orifice case 17



3.3 The visualization on the motion of dry ice particles Figure 11 shows the trajectory of the particles released from 1 mm diameter orifice at the edge region of the jet (region A, as shown in Figure 11). Due to the high concentration of dry ice particle in the center of the jet, it is difficult to record the motion of the particles. It can be seen that the motion of the particles is unpredictable. It may move along the jet (a), or move to the center of the jet (d, e), or get out of the jet (a). Overall, the motion of tiny particles at the edge of the jet is turbulent motion.

Figure 11. The pattern of motion of solid CO2 particles

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Table 2. The diameters and velocities of the particles at the edge region of the jet Index of the particles Diameter (µm) Maximum velocity (m/s) 1

36

29.92

2

41

10.98

3

32

17.52

4

32

24.57

5

23

9.71

6

18

24.47

7

23

31.19

8

23

10.76

9

32

12.41

10

27

10.06

11

23

11.46

12

36

13.26

13

27

11.23

14

27

9.65

15

63

13.65

16

32

24.68

17

32

14.23

18

45

14.8

19

23

12.45

20

32

9.59

The images that recorded by the high-speed camera were processed to obtain the velocity of the solid particles. The image processing method is that the movement distance of one particle can be calculated 19



based on the position change of the same particle in adjacent frames. And the velocity can be obtained using v = distance / time. Table 2 presents the diameters and maximum velocities of 20 particles in the 1 mm orifice case. It can be seen the solid particles with a diameter of roughly 20 – 60 µm were observed, and the velocity is around 10 – 30 m/s. However, the velocity in the center of the jet can reach 250 m/s, as shown in Figure 6. It indicates that the velocity decay is very high from the jet centerline to the edge of the jet. In addition, the size of dry ice particles in the jet center is around 1 – 10µm, and many particles agglomerated at the edge region of the jet. The overall particle size distribution at the edge of the jet was obtained by processing the visible images. The image processing software ProAnalyst was used to choose the particles randomly to gain their equivalent diameters, as plotted in Figure 12. It indicates a log-normal size distribution on the edge of the jet and the medium diameter of the particles is about 32 µm.

Figure 12. The size distribution of solid CO2 particles

4 Conclusions This paper presents the experimental results of the evolution and size distribution of solid CO2 particles when supercritical CO2 is released from the vessel. Some conclusions can be drawn as follows: 20


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(1) The structure of the supercritical CO2 jet shows that it is a typical highly under-expanded jet.13,14 The lowest temperatures measured in the 1 and 2 mm diameter orifice cases were -41.9 ℃ and is -45 ℃, respectively. However, larger leakage orifice can cause faster temperature drop. The maximum velocity along the jet centerline is ca. 250 m/s. (2) The evaporation and agglomeration of particles will lead to the difference in particle size in different position of the jet. The results indicate that for the supercritical CO2 pipeline leakage, the size range of the dry ice particles is 1 – 3 µm. (3) The median size (D50) would increase with the decrease of the initial pressure. Higher initial pressure can cause a wider size distribution. However, the initial temperature has little impact on the size of dry ice particles in supercritical CO2 releases. (4) The motion of tiny particles at the edge of the jet is turbulent motion. The solid particles with a diameter of roughly 20 – 60 µm were observed and a log-normal size distribution can be found in the edge region of the jet. The velocity of the particles is around 10 – 30 m/s.

Acknowledgments The present work is supported by National Science Foundation of China (Grant No.51374231) and the Fundamental Research Funds for the Central Universities (Grant No.16CX06005A). Scholarship from China Scholarship Council (CSC) for the first author is highly appreciated. The first author sincerely appreciates Dr. Xiong Liu and Prof. Cheng Lu from University of Wollongong for their useful advice.

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100

80

flow

Pipeline

Cumulative distribution (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

Solid CO2 particle distribution

L = 5 mm L = 8 mm L = 12 mm L = 30 mm L = 100 mm

40

20

0 2

3

4

5

Particle diameter (m)

release


6

7

8

9 10

Evolution and Size Distribution of Solid CO2 Particles in Supercritical

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