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Experimental and Computational Approaches for CH4 and C2H4 Flammability Zones Shenghui Qin, XingXin Sun, Wei Cheng Lin, Chi-Min Shu, Fei You, and Sing Cheng Ho Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00457 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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Energy & Fuels

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Experimental and Computational Approaches for CH4 and C2H4 Flammability Zones

3

Sheng-Hui Qin,† Xing-Xin Sun,† Wei-Cheng Lin,‡ Chi-Min Shu,*,§,ǁ Fei You,*,† Sing-Cheng

4

Ho§

5

† College of Safety Science and Engineering, Nanjing Tech University, Nanjing 210009, Jiangsu Province,

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China

7

‡ Graduate School of Engineering Science and Technology, National Yunlin University of Science and

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Technology (YunTech), 123, University Rd., Sec. 3, Douliou, Yunlin 64002, Taiwan, ROC

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§ Department of Safety, Health, and Environmental Engineering, YunTech, 123, University Rd,. Sec. 3, Douliou,

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Yunlin, Taiwan, ROC

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ǁ Center for Process Safety and Industrial Disaster Prevention, School of Engineering, YunTech, 123,

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University Rd., Sec. 3, Douliou, Yunlin, Taiwan, ROC

13 14 15 16 17 18 19 20 21 22 23 24 25

*

26

E-mail address: [email protected] (C.-M. Shu), [email protected] (F. You).

Corresponding author.

1

ACS Paragon Plus Environment

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ABSTRACT: Flammability limit and combustion severity are vital and indispensable in

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process loss prevention. Combustion of gaseous C2H4/O2, C2H4/air, CH4/O2, and CH4/air

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mixtures at various concentrations with initial conditions of 1 bara and 273 K were

30

experimentally investigated in a 20-L apparatus. The two objectives of this study were (1) to

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originally predict the gaseous mixture flammability limit by the calculated adiabatic flame

32

temperature (CAFT) and to analyze CAFT influential factors, and (2) to creatively predict the

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experimental gas explosion pressure by the equilibrium-calculated combustion pressure

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values. The CAFT values and equilibrium pressure were obtained by a chemical equilibrium

35

model.

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The results revealed that the CAFT criterion of 1530 and 1230 K effectively predicted the

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CH4 and C2H4 flammability zones. Moreover, selecting the adiabatic flame temperature

38

corresponding to the upper flammable limit of O2 in the fuel as a CAFT criterion effectively

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predicted the flammability zone (as verified by the experimental and reference values for

40

simple hydrocarbons). The experimental and equilibrium-calculated pressure values indicated

41

that the combustion along the stoichiometric line increased linearly with increasing C2H4

42

concentration (>5.0 vol%). Moreover, the equilibrium-calculated pressure is always higher

43

than the experimental value considering loss in energy. In short, this study can help predict

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the flammability zone and combustion severity for a flammable gas while the measurements

45

are unlikely to be implemented.

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2


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1. INTRODUCTION

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In an adiabatic process, energy is transferred in the form of work in a system, and this energy

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shift from the initial to final state is consistent and is determined solely by the initial and final

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states.1

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Theoretically, the adiabatic flame temperature is an upper limit of the temperature

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produced by a combustion process without heat loss. The maximum flame temperature is

53

mainly determined by the fuel–oxidant ratio and inert gas concentration. Pure O2 gas when

54

used as an oxidant engenders the maximum flame temperature, and the temperature falls by

55

air. In addition, the lean or diluted fuel inevitably causes incomplete combustion, resulting in

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lower temperature. The flame temperature is determined on the basis of the energy balance of

57

the reaction at equilibrium.2–4

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The calculated adiabatic flame temperature (CAFT) has been used as an effective tool to

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estimate the lower flammable limit (LFL), upper flammable limit (UFL), and flammability

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zones in several studies: Shebeko et al.5 demonstrated an analytical method for evaluating the

61

ﬂammability limits of ternary gaseous mixtures; Razus et al.6 obtained the CAFT at the LFL

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(CAFTLFL) as well as at LOC (CAFTLOC) and derived an empirical linear correlation between

63

the CAFTLFL and CAFTLOC; Xin et al.7 presented an approach for predicting the UFL of

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hydrocarbons in air using the CAFT with an acceptable relative error. Vidal et al.8 adopted an

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algebraic methodology for estimating the LFL with CAFT. Mashuga and Crowl3 proposed a

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reliable prediction of the flammable zone using a CAFT criterion of 1200 K. Melhem9

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evaluated the flammable envelopes on the basis of the chemical equilibrium, in which the

68

experimental flammability limits at corresponding CAFT were 1000–1500 K. Palucis et al.10 3


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used the CAFT value of 1200 K to reasonably predict the flammable envelope, and Du et al.11

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forecasted the flammability zone of CH4 at a CAFT threshold of 1450 K.

71

Therefore, the CAFT has been conclusively proved to be valuable in estimating the

72

flammable zones, LFL, and UFL of gaseous mixtures. In the CAFT section, this study

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focused on two aspects: Flammability zone prediction using appropriate CAFT criterion and

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influential factors of CAFT.

75

Combustion severity not only predicts the flammability zone of the gases contributing to

76

the explosion, but also is a reliable hazard index in practical applications. The maximum

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explosion pressure (Pmax) and maximum pressure rise rate ((dP/dt)max) in enclosures are

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critical for gas explosion; they are fundamental parameters for risk assessment and in the

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design of emergency vents.12,13 For safety consideration, very few studies have discussed the

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experimental pressure situation along the stoichiometric line, and characteristic explosion

81

parameters (Pmax and (dP/dt)max) data are still scarce. The pressure and pressure rise rate of

82

C2H4 along the stoichiometric line and at 60.0 vol% N2 were experimentally measured in this

83

study. The corresponding equilibrium-calculated combustion pressure and experimentally

84

obtained combustion pressure were compared, discussed, and elucidated.

85

The aim of this study was to delve into the combustion characteristics of flammable gases

86

based on a comparison between experimental and simulated values. For a given flammable

87

gas, the CAFT criterion can be applied for flammability zone prediction; the

88

equilibrium-calculated combustion pressure can be used to speculate on the real combustion

89

pressure. By the combination of these two predictions with a creative approach, in a way, the

90

hazard of a flammable gas can be adequately estimated. 4


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2. EXPERIMENTAL SETUP AND MODEL SIMULATION

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2.1. Experimental setup. The experimental setup (Fig. 1) is consisted of a 20-L closed

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spherical vessel (Adolf Kuhner AG, Basel, Switzerland), which was employed to determine

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the essential combustion characteristic parameters, such as explosion limits, LOC, Pmax, and

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deflagration index.14 The spherical vessel is composed of stainless steel and can withstand a

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high pressure of ca. 24.0 bara. The periphery of the sphere contains a 1.5-L interlayer for

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thermal oil circulation to facilitate temperature regulation. The condenser discharging ignition

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device is governed by a KSEP 320 unit of the vessel. The KSEP 332 unit together with a

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piezoelectric pressure sensor transmits the pressure data to the computer.15 To mitigate energy

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losses, the spherical vessel was thermally insulated.16 Experiments were performed according

101

to ASTM E 681-09 and ASTM E 918-83 standards at an initial ambient temperature and

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pressure of 273 K and 1 bara, respectively.17,18 KSEP 6.0 was used to ensure safer operation

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and to compute the explosion test results. The fuel–oxygen–inert mixtures were prepared

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using the partial pressure method to attain a total pressure of 1.0 bara in the 20-L apparatus.

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In each experiment, the fuel–oxygen–inert mixtures were left to stand for 3.0 min for

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thorough mixing before ignition.

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The experimental procedure of the present study was as follows: First, the 20-L apparatus

108

was purged three times with N2 to eliminate residual combustion products and was evacuated

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to 0.08 bara. Second, the vessel was filled with fuel–oxygen–inert mixtures until a total

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internal pressure of 1.0 bara was achieved. After the gases were mixed thoroughly, the

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mixtures were ignited at an ignition energy of approximately 10.0 J. Finally, a pressure rise of

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≥0.1 bar was considered an explosion. 5


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During the purge process, the apparatus was evacuated to 0.08 bara and filled with N2 to

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1.05 bara. After purging three times, the residual combustion product concentration can be

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expressed as eq 1:

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(0.08/1.05)3 ≈ 0.045 vol%

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The error due to the residual combustion product is less than 0.045 vol%. CH4 (99.9 vol%),

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C2H4 (99.9 vol%), N2 (99.9 vol%), and O2 (99.9 vol%) fuels were obtained from a local

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gasholder station and were used without further refinement.

(1)

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2.2. Model simulation. The first law of thermodynamics states that alteration in the

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internal energy of a thermodynamic system is equivalent to the heat transferred to the system

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and the work conducted by the system. For an equilibrium reaction under adiabatic process, , T ,P= p,j (Tad ,P) i

(2)

j

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where and are the enthalpy of the reactant and product, T and Tad are the initial

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temperature and adiabatic flame temperature, and P is system pressure.

125 126 127 128

Taking C2H4 complete combustion in air under constant pressure and constant enthalpy conditions for example, the chemical equation can be expressed as follows: C2H4 + 3(O2 + 3.76N2) = 2CO2 + 2H2O + 11.28N2

(3)

Here, the total enthalpy of the initial state is: H1 = Cp C2 H4 dT + ∆hf C2 H4 T1

T0

+ 3 Cp O2 dT + 11.28 Cp N2 dT T1

T1

T0

129

T0

The total enthalpy of state 2 is:

6


(4)

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Energy & Fuels

H2 = 2 Cp CO2 dT + ∆hf CO2 + 2 Cp H2 OdT + ∆hf H2 O T2

T2

T0

T0

+11.28 Cp N2 dT T2

T0

(5)

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Energy balance leads to H1 equaling H2; considering that all pure species have zero formation

131

enthalpies, the following formula was deduced as eq 6: 2 Cp CO2 dT + 2 Cp H2 OdT + 11.28 Cp N2 dT T2

T2

T0

T2

T0

T0

− Cp C2 H4 dT + 3 Cp O2 dT + 11.28 Cp N2 dT = Qm T1

T1

T0

T1

T0

T0

(6)

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where T2 is the maximum flame temperature or the adiabatic flame temperature, T0 is the

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room temperature, Cp is the specific heat capacity at constant pressure, and Qm is the molar

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heat of reaction.

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Cp can be expressed as follows: Cp = a0 + a1 T + a2 T 2 + a3 T 3 + a4 T 4 R

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(7)

Qm is shown below: Qm = ∆hf C2 H4 − ∆hf CO2 − 2∆hf H2 O

(8)

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The adiabatic flame temperature, T2, can be obtained by using eq 5. Although applying

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databases, such as the GRI-Mech 3.0, permits the use of variable Cp with different

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temperatures and components, the aforementioned calculation equation is unreliable because

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dissociation occurs frequently at high temperatures, which disrupts the molecular bond and

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reduces the adiabatic flame temperature.19 Therefore, to receive highly precise results, a 7


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chemical equilibrium calculation model that considers dissociation is required for obtaining

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CAFT.

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The CAFT and equilibrium pressure of C2H4 combustion were calculated using a chemical

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equilibrium reactor model named CHEMKIN adopting GRI-Mech 3.0 reaction mechanism.

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This program is appropriate for modeling and simulating complex gas phase reactions, and

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GRI-Mech 3.0 is an optimized reaction mechanism designed to model natural gas combustion,

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which contains 53 species and 325 reactions.20 The element-potential method for attaining

149

equilibrium is based on the minimization of Gibbs free energy. In this equilibrium model, it is

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assumed that the gas phase is a mixture of ideal gases and no heat loss occurs. Gas-phase

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kinetics and the thermodynamics package of GRI-Mech 3.0 were selected for preprocessing

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the related data. The initial temperature and initial pressure were 298 K and 1.0 bara,

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respectively. The CAFT data were gained under constant enthalpy and constant pressure

154

conditions, and the equilibrium calculated pressure data were acquired under constant

155

enthalpy and constant volume conditions.

8


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Energy & Fuels

3. RESULTS AND DISCUSSION 3.1. Flammable zone prediction using CAFT.

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3.1.1. Flammable zone and CAFT diagram. A typical flammability diagram can be used to

159

represent all possible mixtures of a three-component system.21,22 Figure 2 shows a ternary

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diagram of the flammability zone of the C2H4-O2-N2 mixture. The red grid triangular

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envelope represents the flammable zone; any mixture within this triangle is considered

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flammable. The air line represents all possible mixtures of C2H4 and air; therefore, the UFL

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and LFL are located on this line. The stoichiometric line indicates that the stoichiometric

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mixtures (Φ = 1.0) of C2H4 and O2 are present on this line. Ternary CAFT diagrams of the

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C2H4-O2-N2 and CH4-O2-N2 mixtures are shown in Figures 3 and 4, respectively. In Figure 3,

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the short horizontal red and blue lines represent the CAFT criterion of 1230 and 1480 K,

167

respectively.

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CAFT gradually decreased along the stoichiometric line, and it lessened on either side of

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this line. Notably, the CAFT criterion of 1230 K was suitable for predicting the flammable

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zone. The flammable envelope at the LFL for the CAFT criterion of 1230 and 1480 K was

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slightly different in pure O2 and air. However, the adiabatic flame temperature values at the

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UFL were similar between the CAFT criterion in air and pure O2. Figure 4 presents the CAFT

173

diagram of CH4 combustion. The CAFT criterion of 1525 K fitted well with the flammable

174

envelope, and the UFL in oxygen corresponding to adiabatic flame temperature was close to

175

the CAFT criterion of 1525 K. Accordingly, at a given condition and mixture composition, an

176

adiabatic flame temperature lower than 1525 K was considered nonflammable. By using the

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CAFT criterion, unknown flammable zones can be predicted, the explosion hazard could be

178

demonstrated vividly, and such CAFT diagrams could have several potential applications in 9


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industrial productions.

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3.1.2. CAFT criterion model. As described in the previous sections, the adiabatic flame

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temperature corresponding to the fuel–oxygen equivalence ratio at UFL in oxygen is an

182

appropriate CAFT criterion. This finding was verified by comparing the predicted flammable

183

limits with the reference and experimental values of several simple hydrocarbons, and the

184

results are shown in Table 2. The flammable limits of the gaseous mixture of CH4/C2H4

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(50/50) were also predicted. The predicted values were in sound agreement with the reference

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and experimental values, confirming the feasibility and applicability of this method in

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flammable zone prediction.

188 189

Suzuki and Koide24 proposed a correlation between the heat of combustion (∆Hc) of the fuel and the UFL: UFL = 6.30∆H○c + 0.567∆H○ 2 c + 23.5

190 191

(9)

This correlation is in accordance with the experimental data for hydrocarbons. Chen et al.25,26 predicted the UFL of the fuel in oxygen by their UFL in air as follows:

U=

p + 0.79(1 − U1 )C p U1 C f N

p − C p ) + (0.21C p + 0.79C p ) 0.79U1 (C f N f N

(1

2

2

2

0)

192

p is the mean molar heat capacity from 298 to 1400 K of the fuel and N2 gases. C p where C

193

can be determined using GRI-Mech 3.0. U1 and U are the UFLs in air and oxygen,

194

respectively. Le Chatelier27 proposed the empirical mixture rule for computing the LFL of

195

hydrocarbon mixtures. The rule is expressed as follows: LFLmix =

1

∑N i=1

yi LFLi

(11)

10


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where yi is the mole fraction of the ith combustible component, and LFLi is the LFL of the ith

197

combustible component in vol%. Moreover, Le Chatelier’s rule indicates that the flammable

198

limit of a mixture lies between the UFL and LFL of the pure component. Furthermore, Kondo

199

et al.28 used Le Chatelier’s rule for UFL evaluation of some gaseous mixtures with acceptable

200

accuracy as follows: UFLmix =

∑N i=1

1 yi UFLi

(12)

201

For some explosive gases, the UFLmix of these fuels in pure O2 can be reckoned by

202

adopting the foregoing equations, and this fuel–oxygen equivalence ratio can be used to

203

determine the corresponding adiabatic flame temperature. According to the aforementioned

204

method, this CAFT criterion is suitable for predicting the flammability zones, UFL, and LFL

205

for gaseous mixtures of interest.

206

3.2. Influential factors of CAFT.

207

3.2.1. Equivalence ratio effect. Figure 5 demonstrates the adiabatic flame temperature as a

208

function of fuel–oxygen equivalence ratio at different N2 concentrations. The CAFT

209

decreased gradually with increasing N2 concentration. The slope was moderate on the rich

210

side because of the definition of the equivalence ratio Φ. With decreasing N2 concentration,

211

the maximum value of CAFT shifted slightly to the rich side of the stoichiometric line. Under

212

no dilution effect, the maximum CAFT was at Φ = 1.28. The maximum CAFT was

213

determined by the maximum heat release, and dissociation may be the main reason for the

214

shift from the equivalence ratio to the rich side. Dissociation is an endothermic reaction,

215

which occurs both on the lean and rich sides of the equivalence ratio. These simulation results 11


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216

indicated that reduced N2 concentration results in increased dissociation on the lean side.

217

3.2.2. Dilution effect. Dilution has a conspicuous effect on fuel combustion.29 In addition,

218

safety standards in numerous situations during industrial processes and maintenance can be

219

satisfied only through inerting. Therefore, extensive research has been conducted on the inert

220

gas dilution effect on gas explosions, mainly N2, CO2, and water vapor.30–32 Several studies

221

have revealed that CO2 is more efficient than Ar or N2 in explosion inerting.33–36 In this

222

simulation procedure, N2, Ar, or CO2 was employed to explore the dilution effect on C2H4/O2

223

combustion. The CAFT for isochoric combustion of C2H4/O2/inert versus inert gas

224

concentrations is presented in Figure 6. The CAFT declined considerably with the addition of

225

inert gas, particularly when the inert gas concentration exceeded 50.0 vol%, indicating that

226

inert gas inhibited combustion. Moreover, CO2 was most effective for combustion inerting,

227

followed by N2. According to Le Chatelier’s rule,37 change in the product concentration shifts

228

the equilibrium to the side that inhibits the change in concentration. This phenomenon was

229

demonstrated by the equilibrium states of C2H4 and O2 gas reacting to form H2O and CO2.

230

With an increase in CO2 concentration, the reaction favored the side opposing CO2 addition,

231

that is, the reaction was decelerated because of product enrichment.

232

3.2.3. Effects of initial pressure and initial temperature. Figure 7 (a) shows the initial

233

temperature versus the CAFT at different N2 concentrations. The CAFT remained almost

234

unchanged with increasing initial temperature and slightly increased linearly with increasing

235

N2 concentration. Figure 7 (b) shows the initial pressure versus the CAFT at different N2

236

concentrations. At the first stage, initial pressure had a considerable effect on the CAFT. The

237

CAFT increased rapidly as initial pressure increased from 0 to 1.0 bara, after which the 12


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238

Energy & Fuels

CAFT only increased slightly with increasing initial pressure.

239

3.3. Combustion pressure study. Explosion pressure and pressure rise rate of gaseous

240

mixtures have been discussed in a few publications. Movileanu et al.38 measured the

241

explosion pressure of ethylene-air in closed cylindrical vessels and found Pmax and (dP/dt)max

242

are linearly correlated; Mitu et al.39 discussed the temperature and pressure influence on

243

ethane-air deflagration pressure and pressure rise rate parameters. Huang et al.40 studied

244

dimethyl ether-air combustion and found that the mixtures around the stoichiometric

245

equivalence ratio render the maximum pressure and pressure rise rate. It is known that around

246

the stoichiometric ratio gives the maximum value of pressure and pressure rise rate. Further

247

studies are still necessary. The aim of this part was to discuss the combustion characteristics

248

(Pmax and (dP/dt)max) along the stoichiometric line, which is seldom mentioned in previous

249

researches.

250

The experimentally obtained and equilibrium-calculated data for C2H4 combustion

251

pressure are shown in Figure 8. These data points were all present at the stoichiometric line

252

(Φ = 1.0); the concentration of O2 was three times that of C2H4. In the first stage (C2H4

253

concentration < 5.0 vol%), the theoretical pressure increased linearly; however, the slope was

254

higher than that in the second stage (C2H4 concentration > 5.0 vol%). Experimental values

255

increased exponentially with C2H4 concentration from 3.5 to 5.0 vol%. In the second stage,

256

both the theoretical and experimental pressure values increased linearly with increasing C2H4

257

concentration, and the two fitting lines presented a similar straight slope (0.61 and 0.52).

258

Accordingly, the maximum pressure of the combustion process within the triangle was

259

determined. Furthermore, the practical combustion pressure can be predicted using the 13


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260

theoretical value, and on the basis of these two approximate parallel lines, the explosion

261

hazard can be appropriately predicted.

262

Figure 9 presents the experimental and calculated data of pressure and pressure rise rate

263

versus C2H4 concentration at 60.0 vol% N2. The maximum pressure was obtained at Φ = 1.28,

264

and a similar phenomenon occurred for the CAFT, which was caused by the increased

265

dissociation of the products on the lean side, thus reducing heat loss. Pressure rise rate shared

266

a similar variation tendency with pressure. The experimental pressure value was closer to the

267

calculated value at C2H4 concentration of 8.0–18.0 vol%, and the pressure rise rate exceeded

268

500 bara/s. The equilibrium calculated pressure is valuable for predicting the actual

269

combustion pressure near the stoichiometric line. The pressure rise rate showed a similar

270

variation tendency with pressure for C2H4 combustion: the higher the pressure, the higher the

271

pressure rise rate.

272

In this way, the maximum pressure and pressure rise rate for a flammable gas explosion

273

can be properly predicted, and a safety related criterion for safer process design and

274

manufacturing can be made according to this method.

14


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275

4. CONCLUSIONS

276

This study proposed an approach for predicting the flammability zones using CAFT that

277

validates

278

equilibrium-calculated combustion pressure values were compared and discussed.

279

Furthermore, the maximum pressure and pressure rise rate can be inferred using the

280

simulation tool. By combining the CAFT and the explosion pressure section helps establish

281

the flammability zone and explosion severity of a combustible gas, when direct

282

measurements are seldom available. Therefore, the explosion hazard of a combustible gas can

283

be estimated using the aforementioned method, and it might be prominent for designing

284

better devices and setting stricter criterion to avoid fire or explosion accidents. The main

285

conclusions are as follows:

286

A. Flammable diagrams of CH4 and C2H4 were experimentally obtained using the 20-L

287

apparatus. The CAFT criterion of 1530 and 1230 K were found to be effective in

288

predicting the CH4 and C2H4 flammability zones.

the

applicability

of

this

method.

The

experimental

and

chemical

289

B. Selection of the CAFT criterion at the UFL in oxygen of the fuel predicted the UFL, LFL

290

in air, and the flammability zones were consistent with the experimental and reference

291

values for simple hydrocarbons.

292

C. The CAFT was found to be profoundly influenced by the equivalence ratio. The maximum

293

CAFT was received at the rich side of the equivalence ratio because of increased

294

dissociation.

295

D. CO2 exerted a stronger inerting effect than N2 and Ar, and addition of all three inert gases

296

resulted in reduced CAFT. By contrast, the initial temperature and initial pressure had a 15


Energy & Fuels

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297

relatively small effect on CAFT.

298

E. A linear relationship was identified between the experimental and calculated combustion

299

pressure and the C2H4 concentration (5.0–10.0 vol%) along the stoichiometric line.

300

Furthermore, the pressure rise rate increased exponentially in this concentration range.

301

F. At constant N2 concentrations, both the calculated and experimental pressure values

302

formed an inverted U curve with variable C2H4 concentrations. In addition, the

303

experimental combustion pressure were similar to the calculated pressure near the

304

equivalence ratio.

305

16


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AUTHOR INFORMATION

307

Corresponding Authors

308

Telephone/Fax: 886-5-534-2601/886-5-531-2069

309

*E-mail: [email protected], [email protected]

310

Notes

311

The authors declare no competing financial interest.

312

ACKNOWLEDGMENTS

313

The authors are grateful to the experimental support from the Process Safety and Disaster

314

Prevention Laboratory at YunTech in Taiwan. The authors thank the laboratory staff for

315

providing their time and expertise.

316

REFERENCES

317

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(2) Moran, M. J.; Shapiro, H. N.; Boettner, D. D.; Bailey, M. B., Fundamentals of

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Engineering Thermodynamics. John Wiley & Sons: New York, USA, 2010.

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(3) Mashuga, C. V.; Crowl, D. A. Process Saf. Prog. 1999, 18, 127–134.

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(4) Poinsot, T.; Veynante, D., Theoretical and Numerical Combustion. RT Edwards, Inc.:

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2005.

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(5) Shebeko, Y. N.; Fan, W.; Bolodian, I. A.; Navzenya, V. Y. Fire Safety J. 2002, 37, 549–

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(6) Razus, D.; Molnarne, M.; Movileanu, C.; Irimia, A. Chem. Eng. Process. 2006, 45, 193–

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(7) Wan, X.; Zhang, Q.; Lian, Z. Ind. Eng. Chem. Res. 2016, 55, 8472–8479.

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(8) Vidal, M.; Wong, W.; Rogers, W. J.; Mannan, M. S. J. Hazard. Mater. 2006, 130, 21–27.

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(9) Melhem, G. A. Process Saf. Prog. 1997, 16, 203–218.

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(10) Palucis, M.; Glowienka, T.; Van Brunt, V.; Ervin, R.; Chastain, W.; Kline, R.; Lodal, P.

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(11) Du, J. G.; Ma, H. H.; Qu, Z. W.; Wang, L. Q. Process Saf. Prog. 2015, 34, 31–35. 17


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(12) Feldgun, V. R.; Edri, I.; Karinski, Y. S.; Yankelevsky, D. Z. In the 23rd International

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Symposium on Military Aspects of Blast and Shock (MABS23), Oxford, UK, 2014.

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(13) Razus, D. M.; Krause, U. Fire Safety J. 2001, 36, 1–23.

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(14) Shu, C. M.; Wen, P. J. J. Loss Prevent. Proc. 2002, 15, 253–263.

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(15) Shen, S. Y.; Chen, J. C.; Liu, S. H.; Chang, R. H.; Liaw, H. J.; Shu, C. M. Therm. Anal.

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Calorim. 2013, 113, 1619–1624.

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(16) Liaw, H. J.; Chen, C. C.; Lin, N. K.; Shu, C. M.; Shen, S. Y. Process Saf. Environ. Prot.

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2016, 100, 150–162.

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(17) ASTM, E. 681. Standard Test Method for Concentration Limits of Flammability of

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Chemicals (Vapors and Gases). West Conshohocken, PA, USA, 1998.

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(18) ASTM, E. 918-83. Standard Practice for Determining Limits of Flammability of

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Chemicals at Elevated Temperature and Pressure. West Conshohocken, PA, USA, 1999.

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(19) Atkins, P.; De Paula, J. Atkins’ Physical Chemistry. Oxford University Press: London,

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UK, 2006.

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(20) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriarty, N. W.; Eiteneer, B.; Goldenberg,

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M.; Bowman, C. T.; Hanson, R. K.; Song, S.; Gardiner Jr., W. C. GRI-Mech 3.0. URL:

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http://www. me. berkeley. edu/gri_mech, Berkeley, California, USA, 1999, 51, 55.

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(21) Crowl, D. A. Understanding Explosions. John Wiley & Sons: New York, USA, 2010.

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(22) Zabetakis, M. G. Flammability Characteristics of Combustible Gases and Vapors.

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Bulletin 627, U.S. Bureau of Mines: 1965.

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(23) Razus, D.; Molnarne, M.; Fuß, O. Chemical Engineering and Processing: Process

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Intensification. 2004, 43, 775–784.

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(24) Suzuki, T.; Koide, K. Fire Mater. 1994, 18, 393–397.

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(25) Chen, C .C. Ind. Eng. Chem. Res. 2011, 50, 10283–10291.

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(27) Le Chatelier, H. Ann. Mines. 1891, 19, 388–395.

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(34) Rubtsov, N. M.; Troshin, K. Y.; Borisov, A. A.; Seplyarskii, B. S.; Chernysh, V. I.;

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Tsvetkov, G. I. Russ. J. Phys. Chem. B. 2011, 5, 57–66.

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Mater. 2014, 278, 520–528.

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(36) Wu, S. Y.; Lin, N. K.; Shu, C. M. Process Saf. Prog. 2010, 29, 349–352.

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(37) Le Chatelier, H.; Boudouard, O. Bull. Soc. Chim. (Paris) 1898, 19, 483–488.

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(38) Movileanu, C.; Gosa, V.; Razus, D. J. Hazard. Mater. 2012, 235, 108–115.

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(39) Mitu, M.; Giurcan, V.; Razus, D.; Oancea, D. Energy Fuels. 2012, 26, 4840–4848.

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(40) Huang, Z. H.; Wang, Q.; Miao, H. Y.; Wang, X. B.; Zeng, K.; Liu, B.; Jiang, D. M.

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Energy Fuels. 2007, 21, 2013–2017.

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382

FIGURE CAPTIONS

383

Figure 1. Graphical representation of the experimental setup and control system.15

384

Figure 2. Experimental flammable zone for C2H4 received from 20-L apparatus tests.

385

Figure 3. CAFT diagram for C2H4.

386

Figure 4. CAFT diagram for CH4.

387

Figure 5. CAFT versus CH4/O2 equivalence ratio with different N2 concentrations.

388

Figure 6. CAFT versus concentration of different inert gases.

389

Figure 7. CAFT versus (a) initial temperature and (b) initial pressure with different N2

390

concentrations.

391

Figure 8. Pressure and pressure rise rate versus C2H4 concentration along the stoichiometric

392

line (Φ = 1.0).

393 394 395 396 397 398 399 400

Figure 9. Pressure and pressure rise rate versus C2H4 concentration at 60.0 vol% N2.

20


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Energy & Fuels

401 402

Figure 1. Graphical representation of the experimental setup and control system.15

403

21


Energy & Fuels

0

100

80

75

e len

Flammable mixtures

hy

Ox

60

50

Et

yg en

25

ine A ir l

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

Page 22 of 32

40 UFL

Stoich iomet ric lin e

20 LFL LOC

100 0

404 405

0 20

40

60

80

100

Nitrogen

Figure 2. Experimental flammable zone for C2H4 received from 20-L apparatus tests.

406

22


Page 23 of 32

Unit: K 0

290.0

100

591.0 892.0

CAFT = 1230 K

1193

CAFT = 1480 K

1494

80

25

1795 2096

50

2999 3300

ne

Ox

2698

yle

yge n

2397

60

Eth

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

Energy & Fuels

Flammable zone of C2H4

40

75 20

Stoich iomet r ic lin e 100 0

407 408

0 20

40

60

80

100

Nitrogen Figure 3. CAFT diagram for C2H4.

409

23


Energy & Fuels

Unit: K 290.0

0

100

571.0 852.0

CAFT = 1525 K CAFT = 1960 K

1133 1414

80

25

1695 1976

Ox yge n

2257

60

2819

Stoic h

iome t

3100

40

Flammable zone of CH4

75

2538

e an

50

th Me

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

Page 24 of 32

20 r ic li

ne

100 0

0 20

40

60

80

100

Nitrogen 410 411

Figure 4. CAFT diagram for CH4.

412

24


Page 25 of 32

3500

N2 concentration

3000

0 vol% 20 vol% 40 vol% 60 vol% 80 vol%

2500

CAFT (K)

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

Energy & Fuels

2000

1500

1000

500

Φ = 1.28

0.0 0.5 1.0 1.5 2.0 2.5 3.0

20

40

60

Equivalence ratio

413 414

Figure 5. CAFT versus CH4-O2 equivalence ratio with different N2 concentrations.

415

25


Energy & Fuels

3500

CO2 Ar N2

3000

CAFT (K)

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

Page 26 of 32

2500

2000

1500

1000 0

20

40

60

80

100

Inert gas concentration (vol%)

416 417

Figure 6. CAFT versus concentration of different inert gases.

418

26


Page 27 of 32

3200

3000

CAFT (K)

2800 N2 concentration

2600

0 vol% 20 vol% 40 vol% 60 vol% 80 vol%

2400

2200

2000 300

350

400

450

500

550

(a) Initial temperature (K)

419

3600 3400 3200

CAFT (K)

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

Energy & Fuels

3000 2800 N2 concentration 0 vol% 20 vol% 40 vol% 60 vol% 80 vol%

2600 2400 2200 2000 0

2

4

6

8

10

(b) Initial pressure (bar)

420 421

Figure 7. CAFT versus (a) initial temperature and (b) initial pressure with different N2

422

concentrations.

27


Energy & Fuels

10

y1 = 0.60893x + 4.17505

3500

2

R = 0.98865 3000

8

2500

6

2000

y2 = 0.52x + 3.21273 2

R = 0.97496

1500

4 y3 = 28.01977*e(x/2.22479) − 124.40499

2

1000

Pressure rise rate (bar/s)

Calculated pressure Experimental pressure Pressure rise rate Linear fit of experimental pressure Linear fit of calculated pressure Exponential fit of pressure rise rate

12

Pressure (bar)

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

Page 28 of 32

2

R = 0.9858 500 0 0

2

4

6

8

10

0 12

C2H4 concentration (vol%)

423 424

Figure 8. Pressure and pressure rise rate versus C2H4 concentration along the stoichiometric

425

line (Φ = 1.0).

426

28


Page 29 of 32

4000 Calculated pressure Experimental pressure Pressure rise rate

10

3500 3000

8 2500 6

2000

4

1500

2

1000 500

Φ = 1.28

0

Pressure rise rate (bar/s)

12

Pressure (bar)

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

Energy & Fuels

0 2

427 428 429

4

6

8

10

12

14

16

18

20

22

24

C2H4 concentration (vol%)

Figure 9. Pressure and pressure rise rate versus C2H4 concentration at 60.0 vol% N2.

430

29


Energy & Fuels

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

431

Table captions

432

Table 1. Experimental flammability properties of CH4 and C2H4.

433

Table 2. Flammability limit comparison between predicted and existing results. 3,23

434 435

30


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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

436

Energy & Fuels

Table 1. Experimental flammability properties of CH4 and C2H4.

CH4 C2H4

LFL in air (vol%)

CAFT (K)

UFL in air (vol%)

CAFT (K)

LFL in O2 (vol%)

CAFT (K)

UFL in O2 (vol%)

CAFT (K)

LOC (vol%)

CAFT (K)

5.0 2.6

1480 1340

13.0 30.0

1960 1276

5.0 3.0

1440 1431

60.0 80.0

1526 1232

10.0 10.0

1488 1479

437 438

31


Energy & Fuels

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439

Page 32 of 32

Table 2. Flammability limit comparison between predicted and existing results.3,23 UFL in O2 (vol%)

CAFT (K)

Predicted LFL in air (vol%)

LFL in air (vol%)

Predicted UFL UFL in air in air (vol%) (vol%)

CH4exp C2H4exp

60.0

1405

4.7

5.0

13.0

14.0

80.0

1232

2.3

2.6

35.3

30.0

C2H6 C3H8 50/50 CH4/C2H4

66.0 55.0

1139 1242

1.9 2.2

2.4 1.8

14.6 10.1

14.8 10.5

78.0

1277

3.1

3.6

440 441 442 443

32


19.7

21.5

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