Study of transpiring fluid dynamics in supercritical water oxidation using a transparent reactor

J. of Supercritical Fluids 88 (2014) 117–125

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Study of transpiring fluid dynamics in supercritical water oxidation using a transparent reactor Zhong Chen a,b,1 , Guangwei Wang a,b,1 , Zakaria. A. Mirza a,b , Shu Yang a,b , Yuanjian Xu a,b,∗ a b

Chongqing Institute of Green and Intelligent Technology (CIGIT), Chinese Academy of Sciences, Chongqing 400714, PR China Key Laboratory of Reservoir Aquatic Environment, Chinese Academy of Sciences, Chongqing 400714, PR China

a r t i c l e

i n f o

Article history: Received 26 August 2013 Received in revised form 29 January 2014 Accepted 29 January 2014 Available online 14 February 2014 Keywords: Fluid dynamics Transpiring fluid Transpiring wall reactor Gas seal Supercritical water oxidation

a b s t r a c t The transpiring wall reactor (TWR) is considered to be one of the most promising reactors because it minimizes both corrosion and salt precipitation problems that seriously hinder the industrialization of supercritical water oxidation technologies. A transparent reactor is built to study the fluid dynamics of transpiring flow, which are the foundation of reactor design and optimization. The results showed that the transpiring flow is anisotropic with respect to the surface of the transpiring wall due to both the static pressure and viscous resistance. Finally, the novel idea of using air as the transpiring fluid instead of water is presented in an attempt to alleviate current TWR problems such as high energy consumption, high volume of pure water consumption, and temperature fluctuation in the reaction area. A series of experiments and theoretical derivations demonstrate that this novel idea is feasible. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In the 1980s, Modell [1] pioneered the degradation of organic waste in supercritical water, which marked the birth of supercritical water oxidation (SCWO) technology. Modell’s process utilizes the special physical properties of water under supercritical state (T ≥ 374 ◦ C, P ≥ 22.1 MPa), such as the non-polar character, high diffusivity, and excellent transport properties [2,3]. The SCWO is hailed as the greatest potential end-of-pipe technology, but its commercialization is hindered by corrosion and salt precipitation problems. Some plants have been interrupted or shut down due to these two problems [4,5]. In the last three decades, many kinds of reactors have been designed and studied to solve these problems [5,6]. The transpiring wall reactor (TWR), which was first reported by McGinness in 1993 [7], is undoubtedly one of the most promising technologies because it can simultaneously handle both corrosion and salt precipitation problems. The typical TWR structure is shown in Fig. 1, and the key design principles are described as follows. First, a TWR consists of an outer

∗ Corresponding author at: Environmentally-Benign Chemical Process Research Center, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, No.266 Fangzheng Avenue, Shuitu Hi-tech Industrial Park, Shuitu Town, Beibei District, Chongqing, P. R. China, 400714. Tel.: +86 23 65935819. E-mail address: [email protected] (Y. Xu). 1 Zhong Chen and Guangwei Wang contributed equally to this work. http://dx.doi.org/10.1016/j.supflu.2014.01.020 0896-8446/© 2014 Elsevier B.V. All rights reserved.

non-porous wall bearing high pressure and an inner porous wall. All reactions occur inside the porous tube. Second, the annulus between the two walls is filled with either subcritical or supercritical pure water; therefore, the fluid prevents the pressure-bearing wall from contacting the corrosive reaction solution. Third, water flows through the transpiring wall and forms a protective water film on the inner wall surface against the corrosive reaction mixture, salt deposition, scaling, and high temperatures [5]. More than 10 organizations have researched TWRs, and three types of transpiring walls have been designed. A summary of the different types of transpiring walls is provided in Table 1. In 1993, Mueggenburg et al. [8] designed the first transpiring wall type, called platelets. Their design was based on Aerojet platelets technology. In order to provide better wall protection, Daman [9] and Ahluwalia [10] from Foster Wheeler Development Corporation patented a better performance platelet tube, which is a seven-layer structure that can disperse a single inlet stream into 1134 flow paths. This platelet reactor was widely used in SCWO in the 1990s [11–13]. The second transpiring wall type is more efficient at forming an isotropic protective film and is well known as the sintered porous tube. The ITC-CPV [14] group takes the lead in adopting this type of transpiring wall. Since 2001, various materials have been studied to improve the corrosion resistance of the transpiring wall, such as stainless steel [14], ceramic [17,24], Inconel alloy [20,22], and 316L low-carbon stainless steel [26]. Furthermore, both the pore diameter of the porous tube [15,17,20,23,24,26] and the reactor design [15,17,20,22,26] have been studied. Wellig et al. [20] and

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Nomenclature A Din Dout din dout dp Ftw Fcw Fww H P P1 P2 P Qv Qv,radial Qv,axial Rtw Rcw R Vannulus Vtube

test area of transpiring wall (m2 ) inner diameter of pressure bearing wall (m) outer diameter of pressure bearing wall (m) inner diameter of transpiring wall (m) outer diameter of transpiring wall (m) pore diameter of transpiring wall (m) mass flow rate of transpiring water (kg h−1 ) mass flow rate of cooling water (kg h−1 ) mass flow rate of waste water (kg h−1 ) height of the transpiring tube (m) fluid pressure (Pa) pressure drop across the transpiring flow inlet and the reactor outlet with porous tube (Pa) pressure drop across the transpiring flow inlet and the reactor outlet without porous tube (Pa) pressure drop due to porous tube (Pa), P = P1 − P2 volume flow rate (m3 s−1 ) volume flow rate of radial flow (m3 s−1 ), radial flow and transpiring flow are the same in this paper volume flow rate of axial flow (m3 s−1 ) ratio of transpiring water to waste water, Rtw = Ftw /Fww ratio of cooling water to waste water, Rcw = Fcw /Fww ratio of total water to waste water, R = (Fcw + Ftw )/Fww volume of annulus (m3 ) volume of transpiring tube (m3 )

Greek Symbols viscous permeability of transpiring wall (m2 ) ˛  viscosity (Pa s)  density (kg m−3 ) ı thickness of transpiring wall (m) 1 residence time (s) defined by Eq. (4) 2 residence time (s) defined by Eq. (5) 3 residence time (s) defined by Eq. (6)

Zhang et al. [26] separately designed a special system for TWRs that can control both the temperature and flow rate at individual transpiring flow inlets. Bermejo et al. [22,23] innovatively used a partly porous tube in their reactor that achieved good results with a comparatively low transpiring ratio (Rtw ≈ 2). Xu et al. [28] reported the latest transpiring wall type in 2010. The sintered wire netting transpiring wall was manufactured by the sintering metal wire at high temperatures to produce a wire mesh sheet. This kind of transpiring wall combines the strengths of the two previous types. In particular, the sintered wire netting transpiring wall possesses good mechanical strength, which is similar to platelets, and the pores have a uniform distribution, which is similar to the sintered porous tube. After careful reflection, there is still a critical problem with TWRs that must be resolved. In TWRs (Table 1), a large quantity of pure water is injected into the reactor at a high transpiring ratio (Rtw ) and low temperature (Ttw ) in order to protect the reactor from corrosion and salt precipitation. However, this will cause temperature fluctuations in the reaction area and requires high energy consumption [23,27]. The reactor design can be improved if the flow behavior of the transpiring fluid can be directly determined and optimized, but the extreme chemical and physical environment in the SCWO reactor limits the measurements needed for current research. Fauvel et al.

Fig. 1. Typical structure of transpiring wall reactor.

[18] identified a pressure drop across the porous wall, which is governed by Darcy’s law under supercritical conditions. However, the local flow condition inside TWRs has not been studied. In recent years, computational fluid dynamics (CFD) have been applied to describe the velocity and temperature profiles inside the reactor and around the transpiring wall in SCWO [15,30]. Although CFD can be used to describe the flow conditions in TWRs, there are still some inaccuracies in the simulated results due to the limitations of the CFD software and the theoretical models’ inability to predict state of a real, complex system [30]. Sapphire window installation [31,32] has been used to visually observe what happened in a SCWO reactor. This method is the only direct way to make visual observations; unfortunately, the field of vision is too small for a TWR. In this paper, a simplified and custom designed glass transparent reactor was developed to understand the fluid dynamics of the transpiring flow. In particular, a tracing technique (a colored dye) was adapted to show the dynamic processes. In order to avoid the high pressure and high temperature associated with subcritical water, which is commonly used as a transpiring fluid (Table 1), either ethanol or acetone was used as a substitute based on their similar physical properties. A series of experiments were performed at 25 ◦ C and 0.1 MPa. The pressure drop across the porous tube was measured. Finally, a novel idea was introduced that uses air as the transpiring fluid instead of water, and its feasibility was assessed. 2. Experimental sections 2.1. Reactor design The general design of the transparent reactor is shown in Fig. 2. The transparent reactor (11) consists of a glass outer tube (L = 0.500 m, Din = 0.050 m) and a porous inner tube (Alumina, dout = 0.040 m, din = 0.0265 m, dp = 8 × 10−5 m). There is an inlet (0.005 m i.d.) for the transpiring flow in the middle of the glass tube. The annulus between the two tubes are sealed by rubber O-rings at both ends. The effective length of the porous tube is 0.460 m. The glass transparent reactor enables visual observation of the dynamic processes of transpiring fluid. In most cases, subcritical water is used as transpiring fluid (Table 1); however, the glass

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Table 1 Detailed structural parameters of TWRs and operational conditions of transpiring water, adapted from Bermejo et al. [5,23] and updated. Organism

Pressure bearing wall

Transpiring wall

Transpiring water

Ref.

FWDC (USA)

Material: inconel 625 L(upper) = 0.406 m L(lower) = 1.219 m Material: 1.498 steel L = 0.950 m Dout = 0.120 m Din = 0.080 m

Material: inconel 600 Type: platelets (seven floors) din = 0.152 m Material: 1.4404 steel Type: porous dout = 0.066 m din = 0.060 m dp = 3.5 × 10−5 m

Type: two transpiring inlets for transpiring water and one transpiring inlet for quench water

[12,13]

Type: two transpiring inlets for transpiring water and two transpiring inlets for quench water

[14–16]

Material: 316 L(reaction) = 0.500 m L(cooling) = 0.300 m Din = 0.024 m

Material: ␣-alumina Type: porous dout = 0.019 m din = 0.015 m dp = 8 × 10−5 m and 5 × 10−6 m

Material: inconel 625

Material: inconel 625

L(reaction) = 0.177 m L(cooling) = 0.104 m Dout = 0.095 m Din = 0.034 m

Type: porous dout = 0.0295 m din = 0.022 m dp = 3 × 10−6 m and 5 × 10−6 m

Material: steel L = 1.500 m

Material: inconel 600 Type: only the central part of the wall is porous dout = 0.080 m din = 0.074 m dp = 2.4 × 10−5 m Material: ceramic Type: porous din = 0.009 m dp = 2 × 10−6 m Material: 316 L Type: porous dout = 0.060 m din = 0.055 m

ITC-CPV (Germany)

CEA (France)

ETH (Switzerland)

Uva (Spain)

Dout = 0.145 m Din = 0.100 m DHU (China)

Material: C-276 L = 0.500 m Din = 0.015 m

SDU (China)

Material: 321 steel L = 0.750 m Dout = 0.114 m Din = 0.080 m

XJTU (China)

Material: 316 L(upper) = 1.180 m L(lower) = 0.245 m

dp = 2.08 × 10−5 m Material: C-276 Type: wire netting din = 0.300 m dp = 3 × 10−5 m

Qtw = 6.5 kg h−1 Ttw = 550 ◦ C Rtw = 1.18 Qqw = 13.0 kg h−1 Tqw = 30 ◦ C Rqw = 2.36 R = 3.54 Type: two transpiring inlets for transpiring water and an external jacket for cooling Qtw = 0.6–1.8 kg h−1 Ttw = 380 ◦ C Rtw = 1–3 Type: three independent transpiring inlets for transpiring water and one transpiring inlet for quench water Qtw = 3.6 kg h−1 Ttw = 35–250 ◦ C Rtw = 2.26 Qqw = 216–252 kg h−1 Tqw = 28–35 ◦ C Rqw = 60–70 R = 62.26–72.26 Type: one top inlet for transpiring water

Qtw = 10–72 kg h−1 Ttw = 25–250 ◦ C Rtw ≈ 2 Type: one axial inlet for transpiring water

Type: three independent transpiring inlets for transpiring water Qtw = 20–85 kg h−1 Ttw = 200–350 ◦ C(upper) 100–250 ◦ C(middle) Room temperature (lower) Rtw = 2.5–5 (approximate) Type: six transpiring inlets for transpiring water Ttw < 374 ◦ C R = 10

[17–19]

[20,21]

[5,22,23]

[24,25]

[26,27]

[28,29]

transparent reactor cannot sustain the high temperature and high pressure associated with supercritical conditions. Therefore, it is essential to find suitable fluids whose physical properties at room temperature and atmospheric pressure are close to those of transpiring water. In our study, ethanol and acetone were selected as substitute fluids because their physical properties are similar to subcritical water. The primary physical properties of ethanol and acetone are compared with water under a pressure of 25 MPa at different temperatures (Table 2). Because both ethanol and acetone

Table 2 Physical properties of typical fluids, data collected from [33,34].

Fig. 2. Schematic of the simulation experiment system. (1) air compressor, (2) pure feed tank, (3) feed with tracer tank, (4) pressure reducing valve, (5) valve, (6) airmass flow meter, (7) globe valve, (8) pump A, (9) pump B, (10) pressure meter, (11) transparent reactor, (12)collection tank.

Fluids

Viscosity (10−6 Pa s)

Density (kg m−3 )

Surface tension (10−3 N m−1 )

Ethanol at 0.1 MPa and 25 ◦ C Acetone at 0.1 MPa and 25 ◦ C Water at 0.1 MPa and 25 ◦ C Water at 25 MPa and 95 ◦ C Water at 25 MPa and 270 ◦ C Water at 25 MPa and 400 ◦ C

1074.0 306.0 902.9 304.0 103.1 29.2

787.2 786.0 997.0 973.0 792.4 116.5

22.0 23.5 73.4 59.9 21.3 Inexistence

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are transparent, the tracing technique was adapted with a colored dye to show the dynamic processes. The color change in the annulus shows the flow behavior of transpiring fluid. As shown in Fig. 2, pure liquids with and without the tracer are stored in tanks (2) and (3), respectively, which are pumped into the reactor (11) through valve (5). Valve (5) is used to switch between different liquids. Two plunger-metering pumps (2J-X, Hangzhou Zhejiang Petrochemical Equipment CO., LTD.) are used to deliver the fluids with a flow rate ranging from 1.3 × 10−7 to 1.3 × 10−6 m3 s−1 . The axial flow delivered by pump A (8) induces an upward stream, which ensures that the reactor is fully filled with liquid. The transpiring flow delivered by pump B (9) flows through the porous wall, mixes with the axial flow, and exits the top of reactor into the collection tank (12). Air, as an alternative transpiring fluid, is compressed and dewatered by the air compressor (1) from Atlas (GX4FF-10), and the air is pressure regulated to about 0.1 MPa. The flow rate is controlled by an air-mass flow meter (6) from SEVENSTAR (Beijing, China), which is accurate up to the second decimal place and has a maximum flow rate of 1.67 × 10−4 m3 s−1 under standard conditions. Because the outlet of the transparent reactor is open to the atmosphere, the pressure measured at the inlet of the transpiring flow is the same as the pressure drop across the transpiring flow inlet to the reactor outlet (P1 in Fig. 2). Two precision pressure meters (10) from SYCIF (Shanghai, China) were used to measure the pressure. A pressure meter with a measuring range of 0.0 to 16.0 kPa is utilized when the transpiring fluid is liquid, while another pressure meter with a measuring range of 0 to 160 kPa is utilized when the transpiring fluid is air. All photos are taken by a Nikon D7000. 2.2. Reagents Ethanol (99.7%) and acetone (99.7%) are supplied by Chongqing Chuandong Chemical (Group) Co., Ltd. De-ionized water with a resistivity approximately 18.25 M cm is obtained from a MOLATOM 1850D ultra-pure water machine. Methyl orange (Ind) and methyl red (AR) are purchased from Sinopharm; the former is used as a tracer in water, while the latter is used as a tracer in both ethanol and acetone. The tracer concentration is 0.01 g L−1 . All liquids are filtered through a membrane filter with a pore size of 2 × 10−5 m. 3. Results and discussion 3.1. Viscous permeability coefficient of transpiring wall As a porous medium, the viscous permeability of the transpiring wall plays an important role in the dynamic process of the transpiring fluid. For a laminar flow regime, the incompressible fluid flow in a porous medium is governed by the Darcy’s law [18], in which ˛ is the viscous permeability coefficient. P =

ı Qv ˛A

(1)

The viscosity of fluid () is available from Table 2. Both the thickness (ı) and the test area (A) of the porous cylindrical tube can be calculated from the following relationships [34], using the measured H, din , and dout :



ı=

dout lndout /din



2 

2 dout /din − 1



A=

dout H lndout /din





dout /din − 1

Fig. 3. Measure results of the viscous permeability using air as the test fluid. (Exp. 1 in Table 3).

The viscous permeability coefficient (˛) can be determined using Eq. (1) by measuring P and Qv . Previously, water [18] and water–glucose syrup [35] were used as the test fluid to measure ˛. The particulates in liquid have the potential to cause pore blockage in the transpiring wall. Some non-condensable gases are likely sealed in the micro pores of the porous medium, which can lead to gas locks caused by the surface tension between liquids and gases. When liquid is used as the test fluid, these two phenomena make the accurate measurement of ˛ difficult. Consequently, according to BS EN ISO 4022-2006 [36], gases are more suitable than liquids for the determination of ˛. In this paper, air is used as the test fluid at 27 ◦ C and 0.1 MPa (Exp. 1 in Table 3). The average pore diameter and length of the test tube are 8 × 10−5 m and 0.107 m, respectively. The pressure drop due to the porous tube (P) [18] is defined as the difference between the measured pressure with (P1 ) and without the tube (P2 ). The plot of P as a function of Qv is linear and can be fit with a straight line (Fig. 3, square of related coefficient is 0.9995). This linearity indicates that the flow is in the laminar flow regime and that Darcy’s law is applicable to our measurement system. The viscous permeability coefficient (˛) of our porous tube was determined to be 1.15 × 10−13 m2 according to Eq. (1). 3.2. Dynamic process of liquids flowing through the transpiring wall In the Exp. 2–4 (see Table 3), the dynamic processes of the three fluids flowing through the porous tube were photographed. The images at several characteristic times (in s) are chosen to show the dynamic processes (Fig. 4). The theoretical residence time  1 is the time of transpiring flow (radial flow) in annulus space and can be calculated according to Eq. (4). The theoretical residence time  2 is the time of both the transpiring flow (radial flow) and axial flow in the overall reactor space (including the annulus and tube space, Fig. 1). The theoretical residence time  3 is the time of transpiring flow (radial flow) in the overall reactor space. Both  2 and  3 were defined by Fauvel et al. [17] and are expressed by Eqs. (5) and (6), respectively.

(2)

 (3)

1 =

Vannulus Qv,radial

(4)

2 =

Vannulus + Vtube Qv,radial + Qv,axial

(5)

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Table 3 Experimental conditions. No.

1 2 3 4 5 6 7 8 9 10 *

Axial flow

Transpiring flow

Fluid

Flow rate (m3 s−1 )

Fluid

Flow rate (m3 s−1 )

Air Water with tracer Ethanol with tracer Acetone with tracer Water with tracer Ethanol with tracer Acetone with tracer Water with tracer Ethanol with tracer Acetone with tracer

0.0 1.9 × 10−7 1.9 × 10−7 1.9 × 10−7 1.9 × 10−7 1.9 × 10−7 1.9 × 10−7 1.9 × 10−7 1.9 × 10−7 1.9 × 10−7

Air * Switching from water with tracer to water without tracer * Switching from ethanol with tracer to ethanol without tracer * Switching from acetone with tracer to acetone without tracer Water without tracer Ethanol without tracer Acetone without tracer Air Air Air

1.67 × 10−7 –1.67 × 10−4 8.1 × 10−7 8.1 × 10−7 8.1 × 10−7 1.6 × 10−7 –1.3 × 10−6 1.6 × 10−7 –1.3 × 10−6 1.6 × 10−7 –1.3 × 10−6 1.67 × 10−7 –1.67 × 10−4 1.67 × 10−7 –1.67 × 10−4 1.67 × 10−7 –1.67 × 10−4

Switch when the reactor is full filled with colored fluid.

according to the definitions in Eqs. (5) and (6), the transpiring fluid will flow through the whole reactor space after 1043 s ( 2 , considering the influence of the axial flow) or 1287 s ( 3 , ignoring the influence of the axial flow). However, the experimental results do not match the theoretical predictions (see Fig. 4). In particular, colored dye is still present in the annulus after 402 s ( 1 ), and the colored dye does not fade away in the lower section of annulus after 1287 s ( 3 ). The colored fluid initially moves away from the inlet, dissipates in top section, and gradually fades away in the lower section. In this study, the color change is used to track the streamline of the transpiring fluid. In general, a high color dissipation rate indicates a high flow speed. The phenomena mentioned above suggests that the majority of the transpiring fluid flows into the porous tube through the middle and upper section of the annulus, while the rest of the transpiring fluid flows down to the lower section of the annulus before entering the porous tube. The visual images (Fig. 4) confirm that the transpiring fluid is anisotropic with respect to the surface of the porous tube. Based on Fig. 4, a 2-D scheme (Fig. 5) was developed to depict the transpiring fluid dynamics from the side and top cross-sectional views of the reactor. After injection into the annulus, the transpiring fluid simultaneously flows around the porous tube (around flow path [37]) and flows through the porous wall into the tube (seepage flow path [37]). The anisotropy of the transpiring fluid is partially caused by the inlet position. However, the primary cause of the transpiring fluid anisotropy is the flow resistance, which can be

Fig. 4. Dynamic processes of water, ethanol and acetone flowing through the transpiring wall. Images at several characteristics moments (s) were captured to show the dynamic processes. (Exp. 2–4 in Table 3).

3 =

Vannulus + Vtube Qv,radial

(6)

Based on the size of the reactor, Vannulus and Vtube are 3.25 × 10−4 m3 and 7.15 × 10−4 m3 , respectively, and the  1 ,  2 , and  3 are 402, 1043, and 1287 s, respectively. According to the definition in Eq. (4), the transpiring fluid will flow through the whole annulus space after 402 s ( 1 ). Additionally,

Fig. 5. Schematic diagram of transpiring flow was visualized from Fig. 4. The transpiring flow was dispersed to around-flow and seepage flow and the process is anisotropy.

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Fig. 6. Pressure drop due to the porous tube as a function of transpiring fluids flow rate. (Exp. 5–7 in Table 3).

attributed to the static pressure (Eq. (7)) and viscous resistance (Eq. (1)). P = gH

(7)

The resistance of the transpiring fluid increases as the height (H) of the transpiring tube increases because the static pressure increases from the top (H = 0.000 m) to the bottom (H = 0.500 m) of the transpiring tube. This explains why the color disappears in upper section before it disappears in the lower section. The pressure drop due to the porous tube (P) was measured for water, ethanol, and acetone (Exp. 5–7 in Table 3). Before each measurement, the air in both the reactor and the porous tube is purged by liquid in order to avoid a gas lock [36]. Fig. 6 shows the plot of P and Qv for both the experimental values and the theoretical values calculated from Eq. (1). All of the experimental values are higher than the theoretical values calculated by Eq. (1). The difference between experimental and theoretical values decreases as the liquid changes from water to ethanol to acetone. On the other hand, for all liquids, the difference between the experimental and theoretical values increases as the transpiring flow rate increases. According to Darcy’s law, P is proportional to Qv and is inversely proportional to the surface area (A) of a fluid. As shown in Figs. 4 and 5, the transpiring fluid is anisotropic, which occurs because the majority of the transpiring fluid (Qv ) flows through the small surface area (A) near the transpiring fluid inlet. This anisotropy may explain why all of the experimental values are higher than the theoretical values (Fig. 6). In addition, it seems that a higher Qv magnifies this effect because the increase in the transpiring flow results in bigger difference between the experimental and theoretical values for P (Fig. 6). In other words, the process is more anisotropic at a high transpiring flow rate, which suggests that increasing the transpiring flow may not be an effective way to form an isotropic protective water film. According to Eq. (1), P is also proportional to the viscosity () of the fluid. The viscosity of acetone and ethanol are 3.06 × 10−4 Pa s and 1.074 × 10−3 Pa s, respectively (Table 2). The viscosity of ethanol is three times greater than that of acetone, and the fluid dynamics of ethanol is more anisotropic than that of acetone (Fig. 4). The difference in viscosity can explain why the difference between the experimental results and the theoretical calculation of P is larger for ethanol than it is for acetone (Fig. 6). However, this does not apply for water. The viscosity of water lies between that of acetone and ethanol at 9.029 × 10−4 Pa s (Table 2),

but the P difference between the experimental results and the theoretical calculation is greater than that of ethanol. Due to the high surface tension between water and air (7.34 × 10−2 N m−1 ), some of the porous tube pores are sealed by air, leading to a reduction of the active area (A). Moreover, these results explain why air is chosen as the test fluid to measure the permeability of the porous tube instead of an aqueous liquid (Section 3.1). For acetone, the experimental P results are close to the theoretical calculations. Under near critical and supercritical conditions, the viscosity of water is less than 1.000 × 10−4 Pa s, which is significantly smaller than that of acetone (Table 2). Therefore, under near critical and supercritical conditions, we can assume that the differences between the experimental results and the theoretical calculations are negligible. As a result, the pressure drop across the porous tube follows Darcy’s law, and these findings are in agreement with a previously published work [18]. As shown in Fig. 4, the colored dye completely disappears in less than 2400 s when acetone is used as the transpiring fluid; however, some colored solution remains in the bottom right corner of the tube after 3600 s when both water and ethanol are used as the transpiring fluid. These behaviors can also be explained by the viscous resistance (Eq. (1)) and the static pressure (Eq. (7)), which essentially correlate with the fluid density and viscosity (see Table 2). The viscosity of ethanol is greater than water by a difference of 0.1711 Pa s, while the density of ethanol is less than water by a difference of 209.8 kg m−3 . The counter-effect of viscosity (Eq. (1)) and density (Eq. (7)) on the fluid flow resistance causes the movement of the dye in the tube to be similar for ethanol and water. Acetone is different from both ethanol and water because its viscosity is approximately one-third that of ethanol, but the densities are comparable. Fig. 4 shows that the fluid dynamics of acetone is more isotropic than those of water and ethanol, which suggests that fluids with low viscosity tend to form an isotropic protective film. During the experiments (Exp. 5–7 in Table 3), the axial fluid was colored, and the transpiring fluid was colorless. The annulus remained colorless regardless of the transpiring fluid type. This indicates that the transpiring fluid flows through the entire surface of the porous tube, and there is no fluid in the porous tube flowing back to the annulus. All of the colored fluid in the annulus gradually fades away (Exp. 2–4, see Table 3 and Fig. 4), which further verifies this conclusion. 3.3. A novel design and the feasibility analysis Because the transpiring flow is anisotropic, most TWRs have more than one inlet for the transpiring flow. Those designs maybe helpful for the formation of an isotropic protective water film; however, these systems have high costs, and a high volume of transpiring water is required. Currently, all TWRs use water as the transpiring fluid, which has some disadvantages such as high energy consumption due to the high heat capacity of water and the corrosive nature [38] of subcritical water. Therefore, a new idea is needed. A new reactor is designed at the Chongqing Institute of Green and Intelligent Technology (CIGIT), Chinese Academy of Sciences, and the transpiring fluid is air instead of water. The feasibility of the proposed design depends on the transpiring air confinement of the corrosive reaction mixtures in the inner space of the porous tube, which we call the dynamic gas seal. A series of experiments (Exp. 8–10 in Table 3) were carried out to test the feasibility of the proposed design using the transparent glass reactor (Fig. 2). The images of the process were captured, and the pressure drop of the porous tube (P1 ) was measured. The results show that none of the test liquids leave the annulus over the whole flow range (Fig. 7). As shown in Fig. 8, the P1 of water

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123

Fig. 7. Images of the reactor. (Exp. 8–10 in Table 3).

is much higher than that of the other two liquids, which correlates with their surface tension values (Table 2). This indicates that the surface tension of a liquid plays a leading role in the wall transpiring process. Because the surface tension of ethanol is close to that of water at 25 MPa and 270 ◦ C, the results suggest that the new design should work at temperatures below 270 ◦ C. The surface tension effects should gradually disappear as the reactor temperature increases. Under supercritical conditions, water and air form a single fluid phase [39], and the gas-liquid interface no longer exists. At this point, the axial fluid and the transpiring fluid can be considered to be one phase. As we showed in Section 3.2, the axial fluid does not enter the annulus when the transpiring fluid and axial fluid are the same. Under supercritical conditions, the resistance of the water flow through a porous tube with pore diameters of 8 × 10−5 m is negligible [18], and the viscosity of air is approximately the same as water (Fig. 9B). Thus, the resistance caused by viscosity may be

Fig. 8. Pressure drop with porous tube as a function of flow rate using air as the transpiring flow. (Exp. 8–10 in Table 3).

Fig. 9. Physical properties of air and water at a pressure of 25 MPa versus temperature, data collected from [33,40,41].

neglected under supercritical conditions. Because the densities of air and water are different under supercritical conditions, density plays the major role in the fluid flow resistance. In order to achieve the dynamic gas seal effect, the static pressure in annulus must be higher than that in reaction area (Fig. 1) at every point along the porous tube. In other words, the density of the transpiring fluid (air) should be greater than that of axial fluid (water) according to Eq. (7). When the temperature is greater than 422 ◦ C (Fig. 9C), the density of compressive air is greater than that of supercritical water, and the dynamic gas seal effect should be achieved. Hence, the new design is feasible. The interfacial tension plays the leading role for the fluid flow resistance under subcritical conditions, while the density difference between compressive air and water plays the leading role under supercritical conditions. Based on the above analysis, a novel supercritical water oxidation reactor was developed at CIGIT [42], named the dynamic gas seal wall reactor (DGSWR). Sewage sludge with a solid concentration of 5 g L−1 was degraded at 400 ◦ C and 25 MPa with the flow rate of 2.0 × 10−7 m3 s−1 . The chemical oxygen demand (COD) removal efficiency reached 99.7%. After one month of intermittent operation, no obvious corrosion in the pressure bearing wall or salt precipitation were observed (Fig. 10), which shows that the DGSWR effectively avoids the corrosion and salt precipitation problems seen with TWRs. In order to achieve a better dynamic gas seal effect, the inlet temperature should be greater than 422 ◦ C for waste water (axial fluid) and room temperature for air (transpiring fluid). On one hand, the density of compressive air is greater than that of supercritical water (Fig. 9C). On the other hand, the gas–liquid interface will appear due to the cooling effect of the cold air. Both the density

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cstc2012gg-sfgc20001 and cstc2011ggC20014) and the 100-talent program of the Chinese Academia of Sciences. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.supflu.2014. 01.020. References

Fig. 10. Inner surface of pressure bearing wall after one month intermittent operations under 25 MPa and 400 ◦ C.

difference and surface tension work in synergy to ensure an effective gas seal under this condition. Because of the special physical properties of compressive air (Fig. 9), the DGSWR possesses some advantages compared with current TWRs. First, the consumption of high quality water is reduced. Current TWRs use pure water as transpiring fluid. Table 1 shows that the consumption of transpiring water is about 2 to 10 times that of treated wastewater. However, in DGSWR, air functions as transpiring fluid, and no pure water is required. At the same time, air serves as the oxidant; therefore, no additional oxidant delivery system is required. Second, because the isobaric heat capacity of compressive air is only one-fifth of the average value of water (Fig. 9A), the energy consumption for DGSWR should be much less than current TWRs. Therefore, less energy is required to heat the transpiring fluid. This will also reduce the temperature fluctuation in the reaction area and improve the reactor stability. Third, the DGSWR has enhanced reactor protection. The analysis in Section 3.2 suggests that fluids with low viscosity tend to form an isotropic protective film, Fig. 9B shows that the viscosity of air is much lower than that of water. 4. Conclusions and future work The development of TWRs was reviewed and their problems were discussed. The permeability coefficient of the porous tube was determined using air to be 1.15 × 10−13 m2 . A transparent glass reactor was used to visually study the dynamics of the transpiring fluid in TWRs, which are anisotropic due to the static pressure and viscous resistance. The DGSWR is designed to solve both the corrosion and salt precipitation problems found in TWRs, while minimizing the energy and high-quality water consumption associated with current TWRs. Its feasibility was demonstrated by the fact that there were no obvious corrosion in the pressure bearing wall or salt precipitation observed after one month of intermittent operation. The methodology used in this paper provides an easy and effective way for performing fundamental fluid dynamics research in very harsh chemical or physical environments. However, the effects of the temperature profile on the fluid dynamics are not explored in this paper and should be investigated in the future. Acknowledgments This work was financially supported by two Key Technology R&D Programs of Chongqing (grant numbers:

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