Experimental studies of ignition and explosion in a water column bubbling with hydrogen and oxygen

Journal of Loss Prevention in the Process Industries 22 (2009) 7–14

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Experimental studies of ignition and explosion in a water column bubbling with hydrogen and oxygen Jenq-Renn Chen*, Zhao-Sheng Huang, Chun-Der Liu, Pavel A. Fomin Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science & Technology, 1 University Rd, Yenchau, Kaohsiung 824, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 August 2008 Received in revised form 30 September 2008 Accepted 30 September 2008

A bubble column is constructed and fitted with five sets of igniters and pressure sensors to study the ignition and explosion of hydrogen/oxygen bubbles in water. Two pressure-resistant glass windows are also fitted in the central section of the column for visualization of the ignition. A loop is built to circulate water through the column. A double concentric tube is used to feed oxygen, hydrogen, and water. Ignitions at different locations, oxygen/hydrogen molar ratios and flow rate are also performed. The results find that significant overpressure is generated when large oxygen/hydrogen bubbles or slugs contacted the fusing Nichrome wire igniter. The explosion overpressure also causes strong pulsation of the liquid in the whole column. Finely disperse bubbles are however prone to ignition and do not produce any overpressure. Co-current flow of water helps to minimize the formation of large bubbles or slugs and provides a safer means for reacting hydrogen and oxygen. The results are expected to be useful for scaling up the direct synthesis of hydrogen and oxygen into hydrogen peroxide. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Bubble explosion Hydrogen Hydrogen peroxide

1. Introduction Hydrogen peroxide is an important chemical for a wide range of application including pulp bleaching and environmental protection. Currently, it is almost produced exclusively with the anthraquinone oxidation process in which a two-step cycle with hydrogenation followed by air oxidation of anthraquinone is employed. The one-step synthesis of hydrogen peroxide through direct reaction of oxygen and hydrogen has been proposed for more than 90 years ago (Henkel & Weber, 1914). Oxygen and hydrogen are bubbled through an aqueous media with a supported Pd catalyst. As the requirement for cheaper and more environmental friendly bleacher grows, the direct synthesis process of hydrogen peroxide has been a major focus for researches in recent years (Campos-Martin, Blanco-Brieva, & Fierro, 2006). One major challenge for commercializing the direct process is to resolve the potential explosion problem, in particular the explosion from oxygen/hydrogen bubbles. As oxygen and hydrogen entering the reacting liquid, it is almost inevitable for the oxygen and hydrogen to mix and form explosive bubbles. Nitrogen inerting is possible but has no economic feasibility owing to the low minimum oxygen concentration (MOC) of hydrogen (Crowl & Louvar, 2002). Ignition of these potentially explosive bubbles may still give rise to overpressure that endanger the reaction system.

* Corresponding author. Tel.: þ886 7 6011000x2355. E-mail address: [email protected] (J.-R. Chen). 0950-4230/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jlp.2008.09.010

The potential bubble explosion problems have been studied extensively for oxygen or air bubbles in flammable hydrocarbon, in particularly for cyclohexane, for the safety of liquid-phase hydrocarbon oxidation. As oxygen or air enters the hydrocarbon liquid, the oxygen or air bubbles will be rapidly saturated by the hydrocarbon. It is certain that these bubbles must be in the flammable regime before the oxygen is depleted. Barfuss et al., (1993) performed explosion tests for a single bubble in water at ambient and elevated pressures. Under ambient pressure, they found no significant overpressure during bubbles ignition. Under initial pressures higher than 0.5 MPa, significant overpressure was observed to reach 2–3 times as high as the initial pressure. The pressure wave dissipated rapidly. It is not clear how the exploded bubbles affect neighboring bubbles. Williams, Mahoney, Baker, and Thistlethwaite (1998) performed preliminary tests on the propagation of blast wave in air bubbles in cyclohexane liquid at ambient conditions. No extra energy was observed when the blast wave, which was initiated by C-4 explosives, passed through the bubbling liquid. They argued that bubble explosion was probably not a likely scenario because the energy from an individual bubble was too small and/or the bubbles exploded individually rather than simultaneously. The results were said to be in consistent with those of Franke, Burnett, Danly, and Howard (1975) who also found that blast wave would be dissipated by bubbling liquid. Menon and Lal (1998) conducted experimental studies on the dynamics of an exploding fuel–oxygen bubble. The exploding bubble underwent pulsation and lost energy in each pulsation cycle.

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Significant energy dissipated during the first expansion and contraction phase of the oscillation. As a result, bubble size and oscillation amplitude were decreased significantly after the first oscillation. Their study also suggested that the energy dissipation be related to the excitation and amplification of interface instabilities. Mitropetros, Fomin, Steinbach, Plewinsky, and Hieronymus (2004) were engaged in experimental studies on the explosion behavior of pure oxygen bubbles in cyclohexane at ambient temperature. Strong shock waves were used to ignite the bubbles. High-speed photography and pressure measurement were used to monitor the bubble ignition and oscillation behaviors. Interactions between bubbles were however not found. Chen and Chen (2005) carried out direct ignition tests for oxygen bubbles in cyclohexane liquid under actual process oxidation condition. The overpressure was found to travel and dissipate completely in a distance of no greater than 60 cm. The explosion energy also did not lead to the ignition of neighboring bubbles. Thus, the bubbles were confirmed to be flammable, ignitable and may produce appreciable overpressure. Yet the overpressure and the explosion energy did not propagate far away from the ignition source. In summary, the above existing work on bubble explosion suggests that the energy produced from an exploding bubble may be significant but the explosion energy may be rapidly dissipated in bubbly liquid. Contrary to the liquid-phase oxidation processes in which the fuel evaporation is the controlling factor in determining the formation of potential explosive bubble, the fuel (the hydrogen)

in the present direct process for synthesis of hydrogen peroxide is fed as a non-condensable gas and can easily be mixed with the oxidant gas (oxygen) to form an explosive mixture. Thus, bubble formation and mixing will be the most critical processes for controlling the bubble explosion. This paper aims at direct experimental studies on the behavior of ignition and explosion of oxygen/hydrogen bubbles in water. A bubble column is constructed and fitted with five sets of igniters and pressure sensors to study the ignition and explosion of bubbles. In addition, two pressure-resistant glass windows are also fitted in the central section of the column for visualization of the ignition. A loop is built to circulate the water through the column. A double annulus tube is used to feed oxygen, hydrogen, and circulation water. Ignitions at different locations, oxygen/hydrogen ratio and total gas flow rate are performed. Details of the results are presented. A tentative critical bubble size to produce negligible overpressure is determined from the direct visualization of bubbles in contacting the fusing igniter. The results are expected to be useful for scaling up the direct synthesis of hydrogen peroxide from hydrogen and oxygen. 2. Experimental setup The bubble explosion rig design is similar to a previous work for cyclohexane/oxygen bubble explosion test (Chen & Chen, 2005). Fig. 1(a) shows the setup of the explosion rig. The rig

Fig. 1. (a) Setup of the explosion rig. (b) Dimension of the visualization rig. (c) Distributor design.

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Table 1 Summary of ignition results in static liquid at different ignition location, feed combination, feed molar ratio and total flow rate. The number indicates maximum overpressure in MPa. ‘‘X’’ denotes no overpressure. H2/O2 feed molar ratio

Igniter location

1:9

I1 I2 I3 I4 I5

2:8

I1 I2 I3 I4 I5

Inner: Middle: Outer:

H2 O2 -

Inner: Middle: Outer:

H2 O2

Inner: Middle: Outer:

H2 O2

Inner: Middle: Outer:

O2 H2

Inner: Middle: Outer:

H2/O2 -

8 slm

10 slm

8 slm

10 slm

8 slm

10 slm

8 slm

10 slm

8 slm

10 slm

X X X X X

X X X X X

X X X X 0.11

X X X 0.12 X

X X X X X

0.09 X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X 0.12 X X

X 0.24 0.05 0.04 0.12

0.24 0.23 0.08 X X

0.07 0.16 0.22 0.23 X

0.16 X 0.1 X 0.1

0.16 0.26 X 0.07 X

X 0.09 0.16 0.08 X

X X 0.37 X 0.1

0.09 X X 0.14 0.10

0.19 0.33 0.15 X X

consists of a 3-in schedule 40 pipe (ID ¼ 3.068-in, OD ¼ 3.5-in) welded onto a 2-in ANSI/ASME B16.5 Class 1500 flange. The length of the pipe is 1.3 m, including top and bottom flanges. A blind flange is bolted to the bottom of the pipe. The pipe and flanges are made of Stainless Steel 304L. Two sets of pressure-resistant glass 1.6 cm in width and 32 cm in length is welded onto the middle section of pipe as shown in Fig. 1(b) for visualization of bubble flow. A MotionMeter 1000 highspeed camera is used to video the bubble flow at 1000 frame per second. Two mass flow controllers (MFCs) with flow rate up to a maximum of 10 standard liters per minute (slm) are used to provide the hydrogen and oxygen flow. Five sets of pressure/temperature sensors and fused wire igniters are installed on the pipe with spacing of 0.2 m each. Each fused wire igniter is fitted with a K-type thermocouple and two Nichrome wires each of 5 cm in length and 0.1 mm in diameter. The use of two Nichrome wires is to increase the probability of ignition in flowing bubbles. A 110-V AC voltage can be applied to fuse the Nichrome wire as the ignition source. Takahashi et al. (1998) suggested that a metal wire with a high melting point such as Tungsten or Molybdenum be suitable for explosion measurement in vessels of a few to tens dm3. They found that fusing of Nichrome wire produced the lower energy compared with other metals and was still usable but produced scattered data in their overpressure measurement. In the present case with limited bubble size, the energy from fusing of Tungsten or Molybdenum would be too large and fusing of Nichrome is preferable to be used in the present work. The Nichrome wire is expected to be fused in the first cycle of AC voltage. The energy produced from the fusing is expected to be larger than 20 J which is sufficient to ignite any flammable bubbles. Visualization of fusing Nichrome wire in ambient, static water

reveals that the fusing is mostly limited to several localized points in the wire rather than uniform melting, and frequently fragments such as broken Nichrome wire or small fused Nichrome ball generated. The hot fragments have been found to sustain and glow for at least 0.2 s. The water in contact with the hot fragment does not show to have localized boiling possibly either rapid condensation is occurred in the boiling or the water is localized superheated. Efforts have also been made to find an alternative, clean, fragment-free igniter. The first trial is to replace the Nichrome wire with a thin rod heater. However, the heater creates film boiling and prevents any bubble from contacting the hot surface. Another trial is the glow-stix which is a very high heat flux heater. The glow-stix either creates a hot glow with a film boiling or simply breaks down and creates significant fragments. Thus, the present selection of fusing Nichrome wire is still the most viable igniter for bubble ignition studies. All pressure and temperature signals are unfiltered and sampled at a rate of 5000 Hz through a 16-bit Adventech PCL-1716 data acquisition system. A synchronous actuation of the data acquisition and ignition is made through a solid state relay circuit that controls the applied voltage to the igniter. Normally, the column is operated at the liquid full condition. All liquid expelled from the column is recycled to a buffer vessel in which gas and liquid is separated. The liquid is then pumped back to the column through bottom flange while the gas is vented directly into a safe location with pressure controlled by a backpressure regulator. The recycle of liquid allows a continuous, co-current gas/liquid flow in the column. Temperature of the system is maintained at ambient condition, i.e. 25  2  C. Pressure for static liquid condition is also at ambient pressure while, for co-current liquid flow, the pressure is 0.4–0.6 MPa depending on the liquid flow channel.

Fig. 2. High-speed video clips for the case of inner tube flow of hydrogen, middle tube flow of oxygen, hydrogen/oxygen molar ratio of 2:8, total flow rate of 8 slm, and ignited at I3. Timing at bottom of each clip indicates the time after actuation of wire fusing.

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0.35

Pressure (MPa)

0.3 0.25 0.2 0.15

P1 P2 P3 P4 P5

0.1 0.05

0

10

20

30

40

50

60

70

Time (ms) Fig. 3. Pressure transient for the test in Fig. 2. Each pressure is differed by 0.05 MPa. The timing in the x-axis indicates the time after activation of wire fusing.

The direct hydrogen/oxygen synthesis of hydrogen peroxide requires three separate feeds – hydrogen gas, oxygen gas, and reacting liquid which contain mostly water and some minor amount of catalyst and additives such as hydrogen bromide and sulfuric acid. For simplicity, the reacting liquid is replaced by water. The design of feed distributor is crucial in terms of both safety and mass transfer. In the present hydrogen/oxygen reacting system, safety will be the dominant factor in determining the distributor design. It is always desired to have small but evenly distributed bubbles. To allow three feeds, a special double annulus distributor is proposed. A 1-in tube (OD ¼ 1 in or 25.4 mm, ID ¼ 20.6 mm) is inserted to the blind flange as the distributor through a 1-in borethrough NPT connector. The 1-in tube is further connected to a 1-in reducing union tee and a 1/2-in to 1-in bore-through reducer to allow a 1/2-in tube (OD ¼ 1/2 in or 12.7 mm, ID ¼ 9.4 mm) to be inserted into the 1-in tube. The 1/2-in tube is again connected to a 1/2-in reducing union tee and a 1/2-in to 1/4-in bore-through reducer to allow a 1/4-in tube (OD ¼ 1/4 in or 6.35 mm, ID ¼ 4 mm) to be inserted into the 1/2-in tube. The result is a double annulus distributor as shown in Fig. 1(c) with three separate feeds from the inner tube, middle annulus, and the outer annulus. The gas/liquid flow will be arranged through different combinations of the annulus flow. 3. Results 3.1. Static liquid ignition Ignitions were first performed for static liquid tests, namely tests with no liquid circulation. These tests represented the worst cases in which the gas holdup and bubble size were expected to be the largest. These tests also resembled the abnormal conditions of a failure in the liquid circulation pump or liquid feed pump.

Table 1 summarizes the ignition results at different ignition location, feed combination, feed molar ratio and total flow rate. The number indicates maximum overpressure in MPa. ‘‘X’’ denotes no overpressure. A typical ignition result is shown in Fig. 2 for the case of inner tube flow of hydrogen, middle tube flow of oxygen, hydrogen/oxygen molar ratio of 2:8, total flow rate of 8 slm, and ignition at I3. The I3 igniter is located near the center of the glass window enabling the direct observation of the ignition. The pressure transient for this test is shown in Fig. 3. Figs. 2 and 3 show that explosion is initiated at the time of 15 ms after activation of wire fusing when a large gas slug contacts the fusing igniter. The ignition generates a significant overpressure and an upward movement. The overpressure reaches its peak at the time of 17 ms. The bubbly liquid in the view section is however continuously to move upward like frozen until 19 ms. After 20 ms, the frozen bubbly liquid moves downwards. The result is a vertical oscillating movement. The movement also decays in time. The oscillating movement is ended after 38 ms. The timing of start and end in the vertical movement is roughly consistent with the pressure transient at P3. Thus, the vertical oscillation is likely to be a result of pressure wave movement. Although the pressure transient in Fig. 3 is very similar to the results in cyclohexane/oxygen system as reported in Chen and Chen (2005), only the present work with visualization rig allows the direct observation of the impact from overpressure. The vertical oscillation seems to differ significantly from the symmetric oscillation of a single, isolated bubble as observed in Menon and Lal (1998) and Mitropetros et al. (2004). However, the gas slug is larger than normal bubbles and has a size probably comparable with the column diameter. Thus, expansion and contraction of the slug may cause a movement of the whole section of the gas–liquid mixture in the column. The acceleration and deceleration of the gas–liquid mixture during the oscillatory movement are also expected to contribute a major part of the energy dissipation from the explosion. The nearly flat pressure transient at P1 and P5 in Fig. 3 also suggests that the vertical movement be probably limited to 40 cm above and below the I3 igniter. Maximum overpressure in Fig. 3 is 0.12 MPa with a pressure rise ratio of 2.2. Such large pressure rise has exceeded the generally used criterion of 7% pressure rise in defining the flammability of gases mixture (ASTM, 1999). This result clearly demonstrates that hydrogen and oxygen do mix in the column and form flammable bubbles even though that they were fed separately. For the same feed condition, ignitions in other locations however do not produce any noticeable overpressure and oscillating flow. When the total flow rate is increased to 10 slm, overpressures are observed at I2 to I5 but not I1. Reducing the hydrogen/oxygen feed molar ratio to 1:9 results in no overpressure in all locations and for both 8 and 10 slm flow rates. In fact, Table 1 shows that most ignitions in low hydrogen/oxygen feed molar ratio produce no overpressure. For

Fig. 4. High-speed video clips for the case of inner tube flow of hydrogen, outer tube flow of oxygen, hydrogen/oxygen molar ratio of 1:9, total flow rate of 10 slm, and ignited at I3.

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Fig. 5. High-speed video clips for the case of inner tube flow of hydrogen, outer tube flow of oxygen, hydrogen/oxygen molar ratio of 2:8, total flow rate of 10 slm, and ignited at I3.

hydrogen/oxygen feed molar ratio of 2:8, more than 50% of the ignitions produce overpressures, regardless of the feed arrangement and total flow rate. Fig. 4 shows a series of high-speed video clips for the case of inner tube flow of hydrogen, outer tube flow of oxygen, hydrogen/ oxygen molar ratio of 1:9, total flow rate of 10 slm, and ignited at I3. This ignition does not produce any noticeable overpressure as

summarized in Table 1. However, Fig. 4 clearly shows that the fusing wire does contact a bubble, about 5 mm in diameter, and results in rupture and disintegration of the bubble. Such a result of bubbles being ignited but produced no overpressure is not a single event. In fact, most ignitions without overpressure show similar behavior of bubble rupture. For example, Fig. 5 shows a series of high-speed video clips for the case of inner tube flow of hydrogen, outer tube

pressure(MPa)

0.3

0.2 0ms

0.1

0

26ms

36ms

0.02

37ms

0.04 31ms

0.06

51ms

62ms

0.08

0.1

time after ignition(sec)

45ms

116ms

114ms

122ms

0.12 120ms

0.14

130ms

0.16

127ms

0

Fig. 6. Comparison of pressure transient and P3 and the corresponding high-speed video clips for the case of premixed hydrogen/oxygen, hydrogen/oxygen molar ratio of 2:8, total flow rate of 10 slm, and ignited at I3.

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Table 2 Summary of ignition results in flowing liquid at different ignition location, feed combination, feed molar ratio and total flow rate. The number indicates maximum overpressure in MPa. ‘‘X’’ denotes no overpressure. H2/O2 feed molar ratio

Igniter location

1:9

I1 I2 I3 I4 I5

2:8

I1 I2 I3 I4 I5

Inner: Middle: Outer:

H2 O2 water

Inner: Middle: Outer:

water H2 O2

Inner: Middle: Outer:

H2 water O2

Inner: Middle: Outer:

O2 water H2

Inner: Middle: Outer:

H2/O2 water -

8 slm

10 slm

8 slm

10 slm

8 slm

10 slm

8 slm

10 slm

8 slm

10 slm

X X X 0.36 X

0.35 X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

0.25 X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X

flow of oxygen, hydrogen/oxygen molar ratio of 2:8, total flow rate of 10 slm, and ignition at I3. At 0 ms, a bubble with diameter of 7 mm flows through the igniter and ruptures at 22 ms. The igniter remains hot even at 112 ms so that a swarm of bubbles is again ruptured and produced various small bubbles. One possible reason for the bubble being ignited produced no overpressure is that the gases mixture inside the bubble is outside the flammable range owing to poor mixing of hydrogen and oxygen. To further assure that the gases mixture is flammable, a series of tests with premixed oxygen and hydrogen is performed. The tests are performed by premixing oxygen and hydrogen in the feed tube and feed directly through a 2 mm hole at the bottom flange. The results, as indicated in Table 1, show that all ignitions for hydrogen/oxygen feed molar ratio of 1:9 produce no overpressure while 40% of the ignitions for hydrogen/oxygen feed molar ratio of 2:8 produce no overpressure. Fig. 6 shows a comparison of pressure transient and the corresponding high-speed video clips for the case of premixed hydrogen/oxygen with molar ratio of 2:8, total flow rate of 10 slm, and ignition at I3. Overpressure and strong vertical oscillation are generated at about 114 ms after the ignition. However, the igniter clearly ruptures a bubble with a diameter of 5 mm into smaller bubbles at 26–51 ms with no overpressure at all. This result suggests that bubble size be the dominant factor in determining the overpressure from bubble ignitions. The result also suggests that a minimum size of bubble be required to produce noticeable overpressure upon ignition. Hydrogen/oxygen feed ratio is another dominant factor in determining the flammability of the resulting bubbles. Unfortunately, most catalysts under development require hydrogen/oxygen feed molar ratio to be 1:1, i.e. the stoichiometric ratio for synthesis of hydrogen peroxide, to gain the

maximum yield in hydrogen peroxide. Thus, bubble explosion should be considered as an important issue in scaling up the process. 3.2. Ignitions in co-current flowing liquid For ignition in flowing liquid, a circulation pump with a fixed flow rate of 24.2 L/min is used to generate co-current flow of water with the gases. Ignitions are then performed at different feed combinations and locations. These tests represent the typical conditions in a continuous process of direct synthesis of hydrogen peroxide. Table 2 summarizes the ignition results in flowing liquid at different ignition location, feed combination, feed ratio and total flow rate. It is clear that only the tests with water flowing from outer annulus show overpressures. All other tests show no overpressure. Detailed reviews of the video clips find that the flow patterns for tests with and without co-current water flow differ significantly as shown in Fig. 7. The co-current flow of water helps to break up large bubbles or gas slugs and gives a more uniform distribution of smaller bubbles. However when the water flows from the outer annulus, the inner and middle gases feeds mix together and form large bubbles or slugs as shown in Fig. 8. Fig. 9 shows a series of high-speed video clips for the case of inner tube flow of oxygen, middle tube flow water, outer tube flow of hydrogen, hydrogen/oxygen molar ratio of 2:8, total flow rate of 10 slm, and ignition at I3. It clearly shows that the bubbles flow more uniformly and the igniter does contact a flat bubble, about 6 mm in diameter, and result in rupture and disintegration of the bubble into much smaller bubbles. This ignition does not produce any noticeable overpressure as summarized in Table 2.

Fig. 7. Video clips for comparison of some typical flow patterns for the same tests with and without co-current water flow. A1/A2: inner: without/with water flow, middle: hydrogen, outer: oxygen. B1/B2: inner: hydrogen, middle: without/with water flow, outer: oxygen. C1/C2: inner: oxygen, middle: without/with water flow, outer: hydrogen.

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Fig. 8. Video clips for comparison of some typical flow patterns for tests with water flow at outer annulus and with water flow at inner and middle annuluses. A1: inner: hydrogen, middle: oxygen, outer: water, total flow rate ¼ 8 slm, H2:O2 ¼ 1:9. B1: inner: water, middle: oxygen, outer: hydrogen, total flow rate ¼ 8 slm, H2:O2 ¼ 1:9. A2: inner: hydrogen, middle: oxygen, outer: water, total flow rate ¼ 10 slm, H2:O2 ¼ 1:9. B2: inner: hydrogen, middle: water, outer: oxygen, total flow rate ¼ 10 slm, H2:O2 ¼ 1:9. A3: inner: hydrogen, middle: oxygen, outer: water, total flow rate ¼ 10 slm, H2:O2 ¼ 2:8. B3: inner: oxygen, middle: water, outer: hydrogen, total flow rate ¼ 10 slm, H2:O2 ¼ 2:8.

3.3. Summaries The ignitability and flammability bubbles are subjected to various factors. First, the bubbles must have a sufficient degree of mixing of the two gases to form flammable mixture. Second, the bubbles must contact a proper ignition source to initiate the combustion. Furthermore, the bubble must be large enough to produce enough energy upon ignited. It is clear from Table 1 that the ratio of hydrogen to oxygen strongly affects the flammability of the bubbles. When the molar ratio of hydrogen to oxygen is 1:9, the

bubbly mixtures are less likely to be ignited and produce overpressure. Increase of the hydrogen feed molar ratio from 10% to 20% of hydrogen significantly raises the likelihood of overpressure. The lower flammability of hydrogen in oxygen has been known to be around 4%. However, it has been reported by Cashdollar, Zlochower, Green, Thomas, and Hertzberg (2000) that flame propagates only upwards and generates very weak overpressure (<0.03 MPa) for hydrogen concentration between 4% and 8%. It is expected that the mixing of hydrogen and oxygen bubbles from 10% hydrogen feed will likely have hydrogen concentration in the bubbles to be less

Fig. 9. High-speed video clips for the case of inner tube flow of oxygen, middle tube flow of water, outer tube flow of hydrogen, hydrogen/oxygen molar ratio of 2:8, total flow rate of 10 slm, and ignited at I3.

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Table 3 Summary of overpressure and size of bubble being ignited at ignition location I3, feed combination, feed molar ratio and total flow rate. Upper part is for static water and lower part is for co-current flowing of water. H2/O2 feed molar ratio

Overpressure and size of bubble ignited

Inner: Middle: Outer: 8 slm

1:9

O.P. (MPa) Size (mm) O.P. (MPa) Size (mm)

X – 0.12 >16

H2/O2 feed molar ratio

Overpressure and size of bubble ignited

Inner: Middle: Outer:

H2 O2 water

8 slm 1:9

O.P. (MPa) Size (mm) O.P. (MPa) Size (mm)

X – X 9.8

2:8

2:8

H2 O2 -

Inner: Middle: Outer:

H2 O2

Inner: Middle: Outer:

H2 O2

Inner: Middle: Outer:

O2 H2

Inner: Middle: Outer:

H2/O2 -

10 slm

8 slm

10 slm

8 slm

10 slm

8 slm

10 slm

8 slm

10 slm

X – 0.05 NA

X 10.2 0.08 >16

X 3.0 0.22 >16

X 8.8 0.1 >16

X 6.4 X 8.2

X 3.1 0.16 >16

X 9.8 0.37 NA

X 3.8 X 0.7

X 3.5 0.15 >16

Inner: Middle: Outer:

water H2 O2

Inner: Middle: Outer:

H2 water O2

Inner: Middle: Outer:

O2 water H2

Inner: Middle: Outer:

H2/O2 water -

10 slm

8 slm

10 slm

8 slm

10 slm

8 slm

10 slm

8 slm

10 slm

X 5.4 X –

X 3.7 X 7.0

X – X 5.0

X 3.9 X 6.1

X – X 6.0

X 6.6 X 3.4

X 4.9 X 5.7

X – X 3.4

X 5.7 X 3.5

‘‘X’’ denotes no overpressure. ‘‘–’’ denotes no bubble ignited. ‘‘NA’’ denotes bubble being ignited was not fully covered in the glass window. ‘‘>16’’ denotes bubble being ignited covered all the cross section of the glass window.

than 10%. This might partly explain that some ignitions produce no overpressure. Increase of the feed molar ratio to higher than 20% of hydrogen drastically increases the flammability of the resulting bubbles. For the sake of safety, these feed ratios are not tested in the present rig with visualization window. The effect of bubble size on the explosion overpressure is important but difficult to be assessed. The bubble size distribution is affected by various factors, including gas holdup, distributor design, shear force, etc. It is difficult to study all factors. Instead, the visualization videos at all I3 ignitions where the igniter is in the center of the glass window are analyzed to measure the bubble size upon in contact with the hot, fusing igniter. The bubble size measurements are done by scaling the bubble shade to the horizontal scale of the glass window of 16 mm. The results are shown in Table 3 in which the corresponding overpressures are also presented. Table 3 reveals that all ignitions which produce positive overpressure have bubble sizes greater than 16 mm. The largest bubble being ignited but produced negligible overpressure is 10.2 mm. All other ignitions with bubble size smaller than 10.2 mm produce no overpressure. Thus, 10.2 mm can be tentatively assumed to be the critical bubble size for bubble explosion in the current test configurations. Further studies are certainly needed to determine the critical bubble size for different pressure, temperature, gas composition, and the potential effects of catalyst. Effects of reactor geometries such as column height, diameter, and other possible reactor configuration to be used in practice should also be tested to confirm the potential hazards of bubble explosion. 4. Conclusions A bubble explosion rig is constructed to study the ignition and explosion of hydrogen/oxygen bubbles in static and flowing water. Ignitions initiated at different locations, oxygen/hydrogen ratios and flow rates are also performed. The results find that bubble coalescence forming flammable bubbles is unavoidable even when oxygen and hydrogen are fed separately. Significant overpressure is generated when large oxygen/hydrogen bubbles or slugs with diameter greater than 16 mm are ignited. All other ignitions with bubble size smaller than 10.2 mm produce no overpressure. The explosion overpressure also causes strong pulsation of the liquid in the whole column. Increase of the hydrogen/oxygen feed molar ratio from 10% to 20% of hydrogen strongly raises the likelihood of

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