Fluidization of Particles in Supercritical Water: A Comprehensive

Subscriber access provided by Iowa State University | Library

Thermodynamics, Transport, and Fluid Mechanics

Fluidization of Particles in Supercritical Water: a Comprehensive Study on Bubble Hydrodynamics Jikai Huang, Youjun Lu, and Hao Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05103 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 44 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

Industrial & Engineering Chemistry Research

Fluidization of Particles in Supercritical Water: a Comprehensive Study on Bubble Hydrodynamics By

Jikai Huang1,2, Youjun Lu1,*, Hao Wang1

1, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China 2, Energy Research Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250014, Shandong, China

Submitted to

INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH

* Corresponding author. Tel.:+86-29-82664345; fax: +86-29-8266-9033 E-mail: [email protected] (Youjun Lu) 1

ACS Paragon Plus Environment

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

ABSTRACT: Supercritical water fluidized bed (SCWFB) is a new concept of reactor for coal and biomass gasification without releasing pollutants. In this paper, a special dual-capacitance probe measurement system is developed and a comprehensive study on bubble hydrodynamics in SCWFB is carried out. Four groups of particles with different mean diameters were fluidized by supercritical water with the pressure ranges from 20 to 27 MPa and temperature ranges from 410 to 570 °C. The effect of operating parameters on bubble size, frequency and rising velocity is discussed in detail. Special rules of bubble hydrodynamics are observed and the theoretical mechanisms are revealed. New predicting correlations of bubble diameter and bubble rising velocity are proposed. The results in this paper expand the research of fluidization under extreme operating conditions and also provide useful guidance for the optimization of the reactor.

2


Page 2 of 44

Page 3 of 44 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 traditional combustion of coal releases large amounts of pollutants including SOx，NOx，PM and greenhouse gases inevitably. Developing new, clean and efficient technology to utilize coal is an important solution to the present problems. Supercritical water gasification is a newly developed technology to convert coal and biomass into fuels without releasing pollutants (Guo and Jin,1 Lu et al.,2 Jin et al.,3 Matsumura et al.4). Reactor is the core component of supercritical water gasification and supercritical water fluidized bed (SCWFB) is the most promising reactor. However, the present design criterions of SCWFB are mainly based on the traditional theories of gas solids fluidized bed, most of which are actually inappropriate in a SCWFB. As a result, some undesired phenomenon occurred such as uneven distribution of temperature, reactants overflowing from the reactor and so on (Lu et al.2). The important fluidization properties in SCWFB are still unclear which greatly limits the development of supercritical water gasification technology. Therefore a comprehensive investigation of the fluidization properties in SCWFB is very necessary for the development of supercritical water gasification technology. In a supercritical fluidized bed, the fluidization is different from that in an atmospheric gas solids fluidized bed due to the special physical properties of the supercritical fluid and has received widely attentions in recent years. So far in literatures, the research works about fluidization properties in supercritical fluidized bed mainly includes supercritical carbon dioxide fluidized bed and supercritical water fluidized bed, and the former was more frequently studied. Liu et al.5 studied the fluidization properties of different kinds of particles including quartz sands, stainless beads, resin grain, active carbon granules, alumina particles and so on which belong to Geldart A, B and D types in a supercritical carbon dioxide fluidized bed. The bed expansion and fluctuations of local volume fraction of solids are analyzed to acquire the fluidization quality. The authors found that with the increase of system pressure there is an increase of fluidization quality. The authors proposed a Dn number to describe the transition from particulate fluidization to aggregative fluidization. The authors also proposed a heterogeneity index and a non-ideal index to describe local and integral fluidization properties. Vogt et al.6 carried out a comprehensive study about the fluidization behavior with supercritical carbon dioxide at pressures up to 30 MPa for various solids which behave as Geldart type A and B powders respectively under ambient conditions. It was found that Ergun’s equation is also applicable to the flow of supercritical fluid 3



through a fixed bed and the Wen and Yu relationship can be used for the determination of the minimum fluidization velocity. After initial fluidization, homogeneous bed expansion was formed and the averaged bed voidage increases with superficial velocity in an exponential relationship. The authors identified the onset of bubbling fluidization based on bed expansion method. Marzocchella and Salatino7 studied the fluidization properties of Geldart A and B type solids in supercritical carbon dioxide fluidized bed and found that the bed pressure drop decreases with the increasing of system pressure under fluidization state. The authors found homogeneous bed expansion phenomenon after initial fluidization and a predicting correlation of the increase of bed voidage with superficial velocity was acquired based on Richardson and Zaki model. Niu and Subramaniam8 studied the fluidization of glass beads and resin particle with the mean diameter ranging from 0.18 mm to 1.0 mm in supercritical carbon dioxide and found that the bed voidage increase with the superficial velocity exponentially. Due to the extreme operating condition of high pressure and high temperature supercritical water, the detailed fluidization properties in SCWFB are more difficult to acquire compared with supercritical carbon dioxide fluidized bed. Matsumura and Minowa9 designed a continuous biomass gasification process using a supercritical water fluidized bed. Alumina particles with a diameter of 0.1 mm and density of 3400 kg/m3 were used as the fluidizing particles. The authors predict a bubbling fluidized bed by the help of Dn number proposed by Liu et al.5. The effect of operating conditions on the minimum fluidization velocity of the SCWFB in supercritical water is discussed. Potic et al.10 introduced a micro-fluidized bed made up of quartz with the internal diameter of the reactor is only 1.0 mm. The maximum operating condition is 244 bar and 500 °C. Properties of the micro-fluidized bed such as the minimum fluidization velocity, bed expansion, fluidization regime transition are investigated by visual inspection. The authors found that the empirical correlation of minimum fluidization velocity is applicable in supercritical water fluidized bed. In order to form a homogeneous fluidization, the diameter of the reactor must be more than 12 times of particle diameter. Slugging fluidization was observed, but bubbling fluidization was not observed due to the minor diameter. Koda et al.11 studied the oxidation of a packed bed of carbon particles in supercritical water. The fluidization of the packed carbon particles was observed and it is found that the minimum fluidization velocity agrees well with conventional prediction method. 4


Page 4 of 44

Page 5 of 44 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


In our previous work (Lu et al.12), an experimental research work of the fluidization properties in a SCWFB was conducted with the system pressure ranging from 23 to 27 MPa and the system temperature ranging from 633 to 693 K. The results showed that the Ergun correlation can be also used in SCWFB to calculate the pressure drop of fixed bed. Also, a predicting correlation for the minimum fluidization velocity (umf) in a SCWFB was proposed based on the experimental results. The fluidization behaviors of Geldart type A and B particles in high pressure, sub-critical and supercritical water were investigated experimentally in our subsequent work (Wei and Lu,13 Wei and Lu14). The maximum operating condition was 27.2 MPa and 482 °C. Homogeneous expansion fluidization was found in SCWFB after initial fluidization although the particles are classified as Geldart type B under ambient conditions. A new predicting correlation of the increase of averaged bed voidage with superficial velocity was proposed based on Richardson and Zaki model. Besides, the deviation point from the homogeneous expansion was considered as the onset of bubbling fluidization. The Dn number proposed by Liu et al.5 was used in the work to determine the transitions of fluidization regimes and predict whether bubbling fluidization will occur. Besides the experimental study, numerical simulation studies are also carried out in our previous work to acquire detailed information of fluidization properties in SCWFB. The effects of feeding types and feeding rate on residence time of particles in a SCWFB are studied by via of TFM simulation method (Wei et al.15). It is found that particle residence time is extended and there is a more uniform distribution of particles when 45° incline double-symmetry feeding pipes are used. A DEM simulation study about the fluid hydrodynamic characteristics in a SCWFB was conducted (Lu et al.16) and it was found that there is a non-bubbling fluidization when superficial velocity exceeds the minimum fluidization velocity although the particles are categorized as Geldart B types. Internal circulation and global circulation of particles are found in SCWFB which benefit the solids mixing process significantly. The distribution of local voidage, particle and fluid velocity, granular temperature, and particle Reynolds number in the SCWFB is also discussed in detail. Compared with gas fluidized bed, the standard deviation of bed pressure drop keeps almost constant after the initial fluidization in a SCWFB and the effect of single particle behavior has a more significant influence on the fluidization phenomenon (Lu et al.17). There is a better solids mixing in a SCWFB than that in a gas solids fluidized bed due to the particles moving more randomly in SCWFB. Besides, it is found that the bubbles in SCWFB are much smaller and the 5



bubble frequency is much higher than that in gas solids fluidized. In SCWFB, when the superficial velocity exceeds the minimum bubbling fluidization velocity, the excess fluid begins to flow through the bed in the form of “bubble”. As a result, bubbling fluidization was formed. Bubbling fluidization is a very important phenomenon in fluidized bed as the bubbles have a significant influence on the efficiency of the reactor. On one hand, bubbles result in that part of the reactant gas flows through the bed in short cut, which is bad for the chemical reaction. On the other hand, the rising bubbles will stir the reactants in the reactor, which improves solids mixing efficiency and gas-solids contact efficiency. So far in literatures, the investigations about the fluidization properties in SCWFB mainly concentrated on the transition of fluidization regime of the bed, such as the bed pressure drop, the initial fluidization velocity and bed expansion. The previous research works are carried out based on differential pressure measurement method. However, a detailed understanding about the bubble hydrodynamics is still lacked at present due to a lack of measurement method of local voidage in the extreme environment of high-pressure and high-temperature supercritical water. To solve this problem, a new special dual-capacitance probe measurement system in SCWFB is developed in this paper. A comprehensive study about the bubble hydrodynamics in a SCWFB is carried out by experiment. The bubble size, frequency and rising velocity in a SCWFB are successfully acquired by the specially designed capacitance probe measurement system. The effect of operating parameters on bubble hydrodynamics is analyzed. At last, new predicting correlations of bubble size and bubble rising velocity in SCWFB are proposed. The results in this paper provide useful theoretical guidance for the optimization of the SCWFB reactor.

2. EXPERIMENTAL SETUP AND METHOD 2.1. Supercritical water fluidized bed experimental system. The high-temperature and high-pressure supercritical water fluidized bed fluidization experimental system is shown in Figure 1a. The superficial velocity of supercritical water is acquired by a mass flow meter (SIEMENS mass6000) mounted at the exit of experimental section pipeline. The operating condition in the experimental section is monitored by a pressure sensor (Rosemount, 3051) and two sheathed thermocouples. The inner diameter of the fluidized bed is 35 mm with the initial bed height is about 50 cm. Capacitance probe method is used in a SCWFB for the first time. In order to validate the measurement result of the specially designed capacitance probe measurement 6


Page 6 of 44

Page 7 of 44 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


system, a differential pressure sensor is used to acquire the averaged bed voidage and compared with the results acquired by capacitance probe measurement system. More information about the experimental setup can be referred to our previous work (Wei and Lu11). The detailed arrangement of the measuring points is illustrated in Figure 1b. The system pressure ranges from 20 MPa to 27 MPa, system temperature ranges from 410 °C to 570 °C. Four groups of quartz sands with different mean diameters i.e. 0.124 mm (solids#1), 0.198 mm (solids#2), 0.235 mm (solids#3) and 0.398 mm (solids#4), were fluidized by supercritical water respectively. The density of quartz sands used in this paper is measured about 2616 kg/m3. Due to the extreme flow environment of high pressure and high temperature supercritical water in SCWFB, the local volume fraction of solids inside the bed has not been acquired so far. To solve this problem, a self-designed capacitance probe measurement system in SCWFB of is developed in this paper. The problems including insulation, seal, and high strength fixation when the capacitance probe method is applied in supercritical water have been successfully solved. The structure and measuring principle of the capacitance probe method is illustrated in Figure 1c. The measurement system is mainly made up of two parts: the capacitance probe sensor and the measuring circuit. The capacitance probe sensor consists of three coaxial electrodes from inside out: central electrode, active guarded electrode and grounded electrode, with insulating layers mounted between them. The diameter of the central electrode is 0.6 mm and protrudes 5 mm from the top of grounded electrode, forming the conical measuring volume of the capacitance probe sensor. A standard BNC connector is formed at the end of the capacitance probe sensor and then connected to the measuring circuit through a low-noise coaxial-cable. The central electrode of the capacitance probe is connected to the central electrode of the BNC connector and the active guarded electrode is connected to the outer electrode of the BNC connector. The grounded electrode of the capacitance probe is connected to the ground to ensure zero potential. The voltage of the active guarded electrode is kept exactly the same with the central electrode both in amplitude and phase, which eliminates the influence of the stray capacitance of cable and improve the stability of capacitance probe signal effectively. A customized capacitance measurement instrument (MTI Accumeasure 9000/2 SP) is used to measure the small capacitance of the probe sensor in this work. The output signal of the capacitance probe measurement system is recorded 7



Page 8 of 44

with a frequency of 1 kHz. Linear correlation is used to convert the output signal of the capacitance probe measurement system into local volume fraction of solids in SCWFB. The linear correlation is expressed as:

cv  cv , fb

U  U 0' U fb  U 0'

(1)

where cv is the volume fraction of solids in the mixtures, cv,fb is the volume fraction of solids in fixed bed, U is output signal of capacitance probe, U 0' is the output signal of capacitance probe in pure supercritical water, Ufb is the output signal when capacitance probe is immersed in fixed bed in SCWFB. The bubble hydrodynamics are acquired by dual-capacitance probe measurement method and cross-correlation method. Two capacitance probe sensors were mounted parallel to the axial line of the bed with a certain distance dc and the centers of measurement volume of the capacitance probe sensors are on the axial line of the SCWFB. More details about the measurement of bubble hydrodynamics are given in section 2.3.

2.2. Validation of SCWFB capacitance probe measurement system. A comparative experiment is carried out in this paper to validate the accuracy of the specially designed capacitance probe measurement system. The averaged bed voidage of SCWFB is acquired by both capacitance probe method and differential pressure method. Three capacitance probes are mounted in SCWFB with different immerged depth and the voidage acquired by three capacitance probes are averaged to be the averaged bed voidage. Figure 2 shows the comparisons between the averaged bed voidage acquired by differential pressure method and by capacitance probe measurement system under different operating conditions. The averaged relative error between averaged voidage acquired by capacitance probe method and differential pressure method in Fig. 2a-d is 3.54%, 2.45%, 1.11%, 3.78% respectively. It can be concluded that the averaged bed voidage in SCWFB acquired by capacitance probe method agrees quite well with the averaged bed voidage acquired by differential pressure method, which proves the accuracy of the specially designed SCWFB capacitance probe measurement system.

2.3. Calculation of bubble hydrodynamics. In fluidized bed, bubble is the local region in which fluid accumulates. The bubble boundary voidage is mainly determined based on empirical 8


Page 9 of 44 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


values which vary from 0.7 to 0.9 in literatures, such as 0.85 (Patil et al.18), 0.8 (Hulme et al.,19)， 0.75 (Cao et al.21) and 0.7 (Olaofe et al.20). Considering that there is a more particulate distribution of particles in SCWFB compared with gas fluidized bed and there may be particles inside bubbles, a bubble boundary voidage of 0.7 is used to detect the bubbles more effectively. In dual-capacitance probe measurement system, when a rising bubble passes through the two capacitance probe sensors one by one, two time-series of local voidage with a certain time delay are formed. The cross-correlation method is used in this paper to acquire the time delay Δtb between the two time-series of local voidage. The cross-correlation function of the two time-series of the local voidage can be expressed as follows:

Rxy (m) 

1 Nm

N 1 m



x ( k ) y ( k  m)

0  m  N 1

(2)

k 0

where N is the length of analyzed signal, which is 2 s in this paper; x, y are the time-series of local voidage acquired by two capacitance probes. The input value mmax corresponding to the maximum Rxy(m) was used to calculate the time delay between two time-series of local voidage: Δtb=mmax×dt

(3)

where Δtb is the time delay between the two time-series of local voidage, which is also the time that bubble spent in traveling from the under capacitance probe sensor to the upper one; dt is the sampling interval, which is 1×10-3 s. Then the bubble rising velocity ub can be calculated as follows: ub 

dc tb

(4)

where dc is the distance between the central electrodes of two capacitance probe sensors, which is 10 mm in this paper. The bubble chord length lbc is the length that pierced by the capacitance probe and can be calculated as follows: lbc   t2  t1  ub

(5)

where t1, t2 is the start time and end time of bubble in the time-series of local voidage. Usually the minimum bubble diameter should be one order of magnitude higher than the diameter of particles. In order to standardize the minimum bubble size when different groups of particles are fluidized, the minimum bubble chord length is 4.0 mm in this paper.

3. RESULTS AND DISCUSSIONS 9



The hydrodynamics including chord length, frequency and rising velocity of a large number of bubbles in SCWFB for each operating condition are acquired by the dual-capacitance probe measurement system in this paper. The effect of operating parameter on the bubble hydrodynamics is studied. Special rules of bubble hydrodynamics are found and the theoretical mechanisms are revealed.

3.1 Bubble chord length. 3.1.1 Effect of superficial velocity. Figure 3 shows the effect of superficial velocity on the probability density distribution of bubble chord length. It can be seen from the figure that generally there is a mono-peak distribution of bubble chord length in SCWFB. For example when the mean particle diameter is 0.384 mm, the pressure is 23.9 MPa and the temperature is 547 °C as shown in Figure 3a. It is observed that when the superficial velocity is 0.085 m/s, the chord length of most bubbles in SCWFB are less than 30 mm and the probability density decreases with the increase of bubble size. With increasing the superficial velocity to 0.098 m/s, a small group of bubbles with chord length bigger than the inner diameter of SCWFB are found, which indicates that slugs are formed in the bed. When the superficial velocity is increased to the highest value in this work, it can be observed that the major part of bubbles become slugs. When the fluid flows in excess of that required to maintain the solids at homogeneous fluidization state flows through the bed, bubbles are formed in SCWFB. With the increase of superficial velocity, there is more excess fluid (fluid in excess of that needed to maintain minimum bubbling fluidization) and more bubbles are generated in the bed. Bubbles growth will be enhanced due to the frequent coalescence during the way up and when bubble diameter is bigger than the inner diameter of the bed, slugs are formed as a result. Usually in fluidized bed, the slugs are also referred to as “elongated bubble” or “bubble” by many researchers22-25. Considering this, “bubble” is used to represent the accumulation of fluid in SCWFB, i.e. both dispersed bubbles and slugs, in discussing the evolution of their hydrodynamics with operating parameters. Figure 4 shows the variation of averaged bubble chord length with superficial velocity in different operating conditions. It can be observed from the figure that the averaged bubble chord length mainly demonstrates an increase with the superficial velocity, which is due to the enhanced coalescence between bubbles at higher superficial velocity. After the superficial velocity is 10


Page 10 of 44

Page 11 of 44 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


increased to a higher value, the growth of bubble in SCWFB slows down, which may be caused by that at higher superficial velocity, the stability of big bubble is reduced and big bubbles are more readily to creak. 3.1.2 Effect of system pressure. The variations of probability density distribution of bubble chord length with the increasing of system pressure are shown in Figure 5. It can be seen from the figure that with the increase of system pressure, the peak of probability density curve is shifted to the left side, indicating that smaller bubbles are formed at higher system pressures. Besides, the variation of averaged bubble chord length with system pressure is also given in Figure 6, from which it can be confirmed that bubble growth is restrained with the increase of system pressure in SCWFB. The theoretical mechanism of the variation of bubble size with system pressure may be that with the increase of system pressure in SCWFB, the density of supercritical water is increased and as a result there is a more particulate fluidization in the bed. 3.1.3 Effect of system temperature. The variations of probability density distribution of bubble chord length with the system temperature in SCWFB are shown in Figure 7. It can be observed from the figure that with the increase of system temperature, the distribution curve is shifted to the left side, indicating that the bubble growth is effectively restrained. This is also reflected in Figure 8 that the averaged bubble chord length demonstrates a decrease with the increase of system temperature in SCWFB. The variation of bubble chord length may due to that with the increasing of system temperature in SCWFB, the density of supercritical water is decreased and as a result, there is a higher initial bubbling fluidization velocity. Then the excess fluid is decreased with the system temperature when the superficial velocity remains unchanged, which lead to a decrease of the bubble size in SCWFB. 3.1.4 Effect of particle diameter. The probability density distributions of bubble chord length when different groups of solids are fluidized are shown in Figure 9 to acquire the effect of particle diameter on bubble size. On one hand when smaller particles are fluidized, the minimum bubbling fluidization velocity is lower and thus there is more excess fluid in the bed under the same operating condition. As a result, smaller particles will lead to an increase of bubble size. It can be seen from Figure 9 that with decreasing the particle diameter from 0.384 mm to 0.198 mm, the probability density distribution of bubble chord length is shifted to the right side, indicating that more big bubbles or slugs are generated and the overall bubble size is increased. This is also 11



Page 12 of 44

confirmed in Figure 10. On the other hand, it can be seen from Figure 10 that when decreasing the particle diameter to 0.124 mm, however, there is a decrease of bubble chord length as the superficial velocity reaches the maximum value. This may due to that when smaller particles are fluidized in SCWFB, the fluid-solid interaction is more dominant in determining particle motion. The distribution of particles is more dispersed in the bed and bubbles are more readily to creak. As a result, the bubble growth is effectively restrained when the smallest particles are fluidized. 3.1.5 Predicting correlation of bubble diameter in SCWFB. In above discussions, bubble size is reflected in bubble chord length. Werther26 concluded that in a large fluidized bed, the bubble equivalent diameter is about 1.5 times the averaged bubble chord length. In this paper, due to the high pressure and high temperature operating condition in SCWFB, the inner diameter of the bed is limited to 35 mm, which is rather small compared to the gas solids fluidized bed in literatures. The conclusion of Werther17 may be inappropriate in SCWFB. In fact, it is found that in the experiments bubbles tend to rise up along the axial line of the bed. Considering that the capacitance probe actually pierces the major regions of most bubbles, it is reasonable to take the averaged bubble chord length as the bubble equivalent diameter in SCWFB. In literatures about fluidized bed under ambient conditions, several predicting correlations of the variation of bubble diameter along with the bed height were proposed (Busciglio et al.,27 Hulme et al.,28 Shen et al.29). However, in this paper, it is found that the averaged bubble chord length at different bed height (20 cm, 30 cm and 40 cm) keeps almost constant in SCWFB. This may due to that there is a quick bubble growth near the distributer and a maximum bubble size is reached below 20 cm. The distribution of the averaged bubble diameter in the major part of the SCWFB can be considered uniform and thus a predicting correlation of the averaged bubble diameter is very necessary. After taking the effect of operating parameters including superficial velocity, particle diameter, system pressure and temperature into consideration, a new predicting correlation of the averaged bubble diameter in SCWFB is proposed as follow: 1.18

u  db  0.79  f   umb 

(0.0023 Ar 0.178 

14.85 ) ，1900

Fluidization of Particles in Supercritical Water: A Comprehensive

Recommend Documents